Milad Zarghami-Tehran. A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Transcription

1 Experimental Investigation of the Effects of Fuel Aging on Combustion Performance and Emissions of Biomass Fast Pyrolysis Liquid-Ethanol Blends in a Swirl Burner by Milad Zarghami-Tehran A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Mechanical and Industrial Engineering University of Toronto Copyright by Milad Zarghami-Tehran 2012

2 ii Abstract Experimental Investigation of the Effects of Fuel Aging on Combustion Performance and Emissions of Biomass Fast Pyrolysis Liquid-Ethanol Blends in a Swirl Burner Milad Zarghami-Tehran Master of Applied Science Graduate Department of Mechanical and Industrial Engineering University of Toronto 2012 Biomass fast pyrolysis liquid is a renewable fuel for stationary heat and power generation; however degradation of bio-oil by time, a.k.a. aging, has an impact on combustion performance and emissions. Moreover, the temperature at which bio-oil is stored has a strong effect on the degradation process. In this study, the same biooil-ethanol blends with different storage conditions are tested in a pilot stabilized spray burner under the same flow conditions. Measurements were made of the steady state gas phase emissions and particulate matter, as well as visual inspection of flame stability. The results confirm a relationship between room temperature storage time and storage at higher temperatures (accelerated aging). They also show that fuel aging increases the emissions of carbon monoxide, unburned hydrocarbon and the organic fraction of particulate matter. These emissions increase more rapidly as more time is allocated for aging. NOx emission shows a slight decrease with fuel aging.

3 iii Acknowledgements I would like to thank Professor Murray J. Thomson for his guidance and supervision during the course of this project. My mother and sister also deserve a special appreciation for their support during this project. Dr. Tommy Tzanetakis was my mentor, and I appreciate his endless support during various stages of this project. I am thankful to Sina Moloodi, because of his generous advice whenever a problem came up and his patience in teaching me how to operate the setup. I am also grateful for all the support and help received from Brian Nguyen during the reconstruction of the lab and running the experiments. Many thanks to Umer Khan and Babak Borshanpour for their help during the experiments. I would also like to thank R. Rizvi and Professor H. Naquib for their help with the thermogravimetric analyzer. The help and support of other combustion research group members, especially, Meghdad, Armin and Reza is gratefully appreciated. Finally, I would like to acknowledge Natural Sciences and Engineering Research Council (NSERC) of Canada, as well as Advanced Biorefinery Innovation Network (ABIN) for their financial support and funding.

4 iv Table of Contents Abstract... ii Acknowledgements... iii Table of Contents iv List of Tables... viii List of Figures... ix Nomenclature... xii 1. Introduction Motivation Objective Literature Review Bio-oil Production Bio-oil Properties Acidity Oxygen & Water Content vs. Heating Value Viscosity Solids Content Ash Content Evaporative Residue Storage Stability Aging Mechanisms Esterification Polymerization Air Oxidation Gas Forming... 10

5 v 2.4 Effects of aging on bio-oil properties Visual & Structural Changes Viscosity Water Content Average Molecular Weight Volatility Phase Separation Methods to Slow Down Aging Solvent Addition Very mild hydrogenation Minimizing the exposure to air Droplet Combustion of Bio-oil Effect of Bio-oil Properties on Spray Combustion Viscosity TGA Residue Water Content Solids & Ash Content Experimental Methodology Spray Burner Overall Setup Variable Swirl Generator Air-Blast Atomizing Nozzle Pilot Flame Fuel Analysis Fuel Composition & Heating Value Viscosity... 37

6 vi Thermogravimetric Analysis Gas Phase Emissions Measurement Oxygen Concentration Unburned Hydrocarbons Detailed Speciation of Pollutants Particulate Matter Emissions Measurement Isokinetic Sampling System Gravimetric & Loss on Ignition Analysis Flame Visualization Aging Procedure Combustion Test Procedure Results and Discussion Experimental Test Plan Experimental Results Summary Mechanism of Pollution Formation from Bio-oil Combustion Analysis on the First Batch: Natural vs. Accelerated Aging Fuel Properties Gaseous Emissions PM Emissions Flame Visualization Analysis on the Second Batch: Long Term Accelerated Aging Fuel Properties Gaseous Emissions PM Emissions Flame Visualization NO x Emissions... 76

7 vii 4.7 Acetaldehyde, Formaldehyde and Methane Emissions Conclusions and Recommendations Conclusions Recommendations Future Works Bibliography Appendix A- Theoretical Isokinetic Sampling System Calibration Appendix B- Liquid and Gaseous Flow Calibration Appendix C- FTIR Calibration Validation Appendix D- Example of the TGA Curve Appendix E- Data Acquisition System... 94

8 viii List of Tables Table 2.1 Effect of Aging Time and Temperature on Water Content [27] Table 2.2 Effect of Aging on Average Molecular Weight of Bio-oil [27] Table 3.1 Fuel properties measurement standards Table 3.2 Detection limits and uncertainty levels of the FTIR calibration model Table 3.3 Calculation methods of each PM fraction Table 4.1 Base point Operating Condition Table 4.2 Basic Fuel Properties and Emissions of Aged Pure Bio-oil Table NO x Emissions of the Aged Bio-oil Table 4.4 Emissions for Selected Blends... 78

9 ix List of Figures Figure 2.1- Fast Pyrolysis Process [10]... 4 Figure 2.2 Viscosity vs. Temperature and Methanol Addition [19]... 6 Figure 2.3 TGA curves for batches with different solid content [22]... 8 Figure 2.4 Changes of Waxy material morphology during Aging [38] Figure Viscosity of bio-oils and bio-oils with solvent addition at 60 C [39] Figure 2.6 Viscosity of bio-oil from softwood bark aged at 80 C [40] Figure 2.7 Effect of Storage Time and Measurement Temperature on Viscosity [25] Figure 2.8 Rate of viscosity increase vs. time of storage [37] Figure 2.9 Effect of Aging and Solvent Addition on Water Content [39] Figure 2.10 Effect of Water Content and Methanol on Aging Rate [32] Figure 2.11 Effect of Aging on Bio-oil Constituents [37] Figure 2.12 Effect of Aging on Different Molecular Weight Compounds [25] Figure 2.13 Effect of Aging on Volatility [25] Figure 2.14 Effects of additives on aging [32] Figure 2.15 Four Stages of Bio-oil Droplet Combustion (left to right) [43] Figure 2.16 Solid State Combustion of the Cenospheric Residue [43] Figure 2.17 Effect of SMD on Combustion [46] Figure 2.18 Effect of TGA Residue on Combustion [22] Figure 2.19 Effect of Water Content on Combustion [22] Figure 2.20 Effect of Solids Content on Combustion [22] Figure 3.1 Bio-oil Burner Assembly [46] Figure 3.2 Overall schematic of experimental setup [46] Figure 3.3 Schematic of Movable Block Swirl Generator [46] Figure CRZ of swirling flows in confined geometry [50] Figure 3.5 Atomizing Nozzle Tip Assembly [52] Figure 3.6 Schematic of Nozzle Cooling System [22] Figure 3.7 Alignment of Pilot Flame [46] Figure 3.8 Schematic of the gas phase emissions measurement system [46] Figure 3.9 Gas Streamlines around sampling probe (V S < V) Figure 3.10 Schematic of PM sampling system [46]... 44

10 x Figure 3.11 Geometry and position of sampling probe and pressure taps [46] Figure 3.12 Gravimetric analysis and Loss on ignition procedure [22] Figure 3.13 Borescope Assembly [46] Figure 3.14 Temperature vs Time for Natural Aging Figure 4.1 Pollutant Formation Mechanisms of Bio-oil Combustion [22] Figure 4.2 Calculated Viscosity of Batch 1 Pure Bio-oil (via handheld viscometer) over Aging Time Figure 4.3 Measured Viscosity of Batch 1 Pure Bio-Oil (based on ASTM D445) over aging time Figure 4.4 Solids Content of Batch 1 Pure Bio-oil vs. Aging Time Figure 4.5 Effect of Aging on the first batch blends TGA residue Figure 4.6 CO emissions for Batch 1 Blends vs. Aging Time Figure UHC emissions for Batch 1 Blends vs. Aging Time Figure 4.8 CR Emissions of Batch 1 Blends over the Aging Period Figure 4.9 Comparison of Calculated and Measured CR for Batch 1 Blends Figure 4.10 Borescopic Photos of batch 1 bio-oil blend combustion Figure Calculated Viscosity of Batch 2 (via handheld viscometer) over Aging Time Figure Measured Viscosity of Batch 2 (based on ASTM D445) over Aging Time Figure Solids Content of Batch 1 vs. Aging Time Figure Effect of Aging on the second batch blends TGA residue Figure CO emissions for Batch 2 vs. Aging Time Figure CO emissions for Batch 2 vs. Aging Time Figure CR Emissions of Batch 2 Blends over the Aging Period Figure Comparison of Calculated and Measured CR for Batch 2 Blends Figure 4.19 Borescopic Photos of batch 2 bio-oil blends combustion Figure 4.20 NOx emissions of Batch 1 Blends over Aging Period Figure NOx emissions of batch 2 over Aging Period Figure A.1 Velocity profile for the exhaust flow Figure C ppm CO spectrums before and after changing the FTIR He-Ne laser Figure D.1 TGA curve for bio-oil blend 1N Figure E.1 Front View of the Labview Program Figure E.2 Logged Temperatures for Bio-oil blend 1A

11 Figure E.3 oxygen sensor voltage for bio-oil blend 1A xi

12 xii Nomenclature P r R S U W Pressure Radial distance from the center of the burner Inner radius of the combustion chamber inlet Swirl number Axial gas velocity Tangential gas velocity Axial flux of the tangential momentum in the combustor Axial flux of the axial momentum in the combustor Fuel mass flow rate Absolute pressure as measured by the gauge in condenser exit Gas temperature at the condenser exit Dry gas flow rate though the PM sampling line Wet gas flow rate though the PM sampling line Total exhaust flow rate from the burner Molar fraction of water in the wet exhaust based on mixture stoichiometry Sampling time of the filter

13 xiii Greek Symbols ρ σ α ζ Density Surface tension Fixed Swirl Block Angle Variable Block Angle ζ m Maximum Variable Block Angle Abbreviations and Acronyms ASTM BD CHNO CR EPA FID FTIR HHV HMW LHV LMW NOx American society of testing and materials Below detection Carbon-hydrogen-nitrogen-oxygen content Carbonaceous residue Environmental protection agency Flame ionization detector Fourier transform infrared spectrometer Higher Heating Value High Molecular weight Lower Heating Value Low Molecular Weight Nitrogen oxides

14 xiv PM PPM RMSE SAT SLPM SMD TGA UHC Particulate matter Parts per million Root Mean Square Error Saturated Standard liters per minute Sauter mean diameter Thermogravimetric analysis Unburned hydrocarbons V DC Volts DC

15 1 1. Introduction 1.1 Motivation The concerns over the usage of fossil fuels and their limited supply as well as their adverse environmental impact have intensified over the past decade. Clean renewable energy is being discussed and developed to substitute for fossil fuel usage around the world [1]. Biomass, which has stored the energy it receives from sunlight by photosynthesis, promises the highest potential to contribute to the high energy needs of modern society for both the developed and developing countries. Furthermore, biomass offers carbon neutral energy, which would mitigate the greenhouse gas emissions of burning fossil fuels and contribute towards the emission objectives of the Kyoto protocol and resolving some of the issues related to climate change [2]. Biomass has been utilized for energy purposes since man started a fire with wood. Although the same method is being used today, numerous developments have been made over the millennia. The main reason behind these advances is the difficulty of transportation and storage of biomass, as well as the low efficiency of about 15% to 30% for electricity generation in direct biomass combustion systems [3]. As a result, the tendency to convert biomass into biofuel has risen. Liquid biofuels have lower emissions when combusted and are also more convenient to handle. However, these improvements come with a price. In this case, it s the capital investment and the required energy for upgrading [4]. One type of liquid biofuels is fast pyrolysis liquid, also called bio-oil. This liquid is the product of conversion of biomass through the fast pyrolysis process. Since the process cannot be fully controlled and the feed material varies widely, the fuel product has different physical and chemical specifications each time [5]. Properties such as acidity, high viscosity, low heating value and inherent water, solids and ash make bio-oil difficult to handle and special burner designs are required for stable combustion [6], [7], [8]. One issue with bio-oil which makes it less interesting for the use in industry is the degrading of the fuel when stored for prolonged periods of time. There are some literature on the effects of storage time and condition on bio-oil properties. There is also literature found on the effect of

16 2 each individual property on combustion performance. However, not much has been done to analyze the effect of storage time and conditions on combustion performance and emissions of bio-oil in a spray burner. This analysis would provide some insight on emissions trends and could be advantageous for industry users by providing some timelines on bio-oil storage to minimize their emissions. 1.2 Objective The objective of this study is to examine the effect of storage time and conditions on combustion performance and emissions of wood derived bio-oil-ethanol blends. This study will explain how storage time changes the fuel properties and how these properties were found to influence the combustion quality. The bio-oil was stored at room temperature to age it naturally. The storage time is also mimicked by accelerating the aging process at higher temperatures. Different batches with different aging methods and times are tested in a pilot stabilized swirl burner designed and optimized by Tzanetakis [9]. Combustion quality was determined from the combustion chamber pressure, oxygen sensor and flame photography. Detailed analysis is also carried out on measured gaseous and particulate matter (PM) emissions. Particulate matter emissions are especially critical for usage of bio-oil in diesel engines and gas turbines.

17 3 2. Literature Review 2.1 Bio-oil Production Bio-oil is produced via a process called biomass fast or flash pyrolysis. This process is basically the thermal decomposition of biomass in the absence of oxygen. It is the first step in combustion and gasification processes [2]. Gases, vapors, aerosols and char are the product of fast pyrolysis. If maximizing the liquid yield is of particular interest, achieving both moderate reactor temperature and very short vapor residence time are necessary [2]. The short residence time is to avoid further thermal decomposition into non-condensable gases which are more desirable in the gasification process. On contrary, lower process temperature and longer residence time is referred to as slow pyrolysis with charcoal as the major product [2], [9]. The specifications for fast pyrolysis are high heat transfer rates to the biomass with a reactor temperature at about 500 C and rapid cooling or quenching of the products with vapor residence time less than 2 seconds [2], [10], [11]. The schematic of the fast pyrolysis process is shown in Figure 2.1. Biomass is dried and then ground before entering the reactor where high heat transfer rates are required. The output of the reactor is a mixture of gases, condensable vapors, char particles and entrained sand from the fluidized bed. These products are passed through a cyclone to separate the char and sand particles which are fed back to the reactor. The reactor heat is provided by burning the char and making the sand heated up. The rest of the stream coming out of cyclone heads to the condenser units. Non-condensable gases are collected from the condenser and are burnt to provide the necessary heat for the process and also for drying biomass. The condensed vapors form bio-oil which typically accounts for 75 wt% of the dried feedstock [12]. The main advantage of the fast pyrolysis process is that because the final product is a liquid, it is easier to handle, store and transport. Another benefit of using this process is in the flexibility of the feedstock the can be used. It has been found in the literature that almost 100 different types of biomass including agricultural and forestry residues, energy crops and solid waste have been tested [2].

18 4 Figure 2.1- Fast Pyrolysis Process [10] 2.2 Bio-oil Properties The bio-oil used in this study is derived from woody biomass which mainly consists of cellulose, hemicellulose and lignin. The ratio of these components varies with the type of wood. In addition, inorganic materials are also present in small amounts which appear as ash in bio-oil [11]. Bio-oil is a dark brown viscous liquid which is an aqueous solution of decomposed oxygenated compounds with suspended macro-molecules of lignin [13]. The ratio of the aqueous to tar-like phase is typically 3:1 by weight percent [14]. Several bio-oils studied have shown to contain over 300 individual chemical species, including high molecular weight (HMW) and low molecular weight (LMW) lignin, sugars, aldehydes and ketone, acids and alcohols [11], [15] Acidity

19 5 Bio-oil is acidic with ph value between 2-3 which is mainly due to acetic and formic acids [16]. In order to prevent corrosion, materials such as stainless steel and Teflon should be used for handling, storage, fuel lines and the combustion chamber Oxygen & Water Content vs. Heating Value The oxygen content of bio-oil on dry feed basis is about wt% [17], which is responsible for the difference in properties of bio-oil compared to petroleum fuels. This oxygen is present in almost all compounds of bio-oil, especially water which accounts for wt% of bio-oil [16]. These are the major factors that the heating value of bio-oil is less than that of hydrocarbon fuels. Bio-oil has lower heating value (LHV) of about MJ/kg when petroleum fuels almost have double that amount [17]. The water content of bio-oil make the aqueous phase polar which reduces miscibility of bio-oil with non-polar, hydrocarbon fuels [18]. The presence of water also lowers the viscosity which improves the flow and atomization quality [17] Viscosity Bio-oil has a viscosity at about cp at 40 C [17], which is higher than No.2 fuel oil but lower than No. 6 residual fuel oil. To improve atomization quality, bio-oil can be heated to C [19]. Figure 2.2 shows the effect of temperature and methanol addition on the viscosity. Increasing temperature reduces the viscosity, however it should be noted that higher temperatures may lead to polymerization and agglomeration of bio-oil that end up in a solid-like state. Moreover, solvent addition such as methanol, ethanol and acetone also reduces viscosity Solids Content Oasmaa et al. describe the solid content of bio-oil as the methanol insoluble material left after the procedure described elsewhere [20]. The solid particles contain both organic char and inorganic ash which are entrained in the pyrolysis vapors [21]. The size of the ground biomass and the efficiency of the cyclone have an effect on the percentage and size distribution of the solids content which respectively range from 0.01 to 3 wt% and 1 to 200 microns [16]. The adverse effects that the solid particles have on the fuel quality are seen as agglomeration during storage and formation of a sludge layer at the bottom of the fuel containers [16]. In addition, during the

20 6 pyrolysis process, char particles encourage the cracking of vapor molecules which in turn decreases the liquid yield [5]. High concentration of large solid particles in bio-oil can lead to nozzle clogging and erosion in the combustion system. This physical property of bio-oil increases the particulate matter (PM) emissions as solid particles become larger [22] Ash Content Figure 2.2 Viscosity vs. Temperature and Methanol Addition [19] Biomass feedstock usually contains mineral elements that show up as the ash composition in biooil. Entrained fluidizing material which is usually sand also contributes to the amount of ash. The procedure for measuring ash residue in bio-oil is to heat it to 775 C in the presence of oxygen in order to burn off all other materials [13]. It has been found that wood-derived bio-oil contains some amount of alkali metals like calcium, sodium and potassium. Other metals like iron, zinc and aluminum are also present in smaller fractions [21]. Char particles in bio-oil carry most of the ash [21], however over time due to the polarity of alkali metal ions, some ash might leach out towards the aqueous phase bio-oil and get dissolved there [23].

21 7 In combustion systems, ash is considered to be a major reason for corrosion and erosion of heat transfer surfaces and rotor blades. Compounds formed by alkali metals like sodium and potassium have low melting point, which would solidify on heat transfer surfaces at lower temperatures and will decrease the contact area, therefore decreasing the efficiency [13]. Bio-oil, having passed through the pyrolysis process and filtration, contains much less ash than biomass feedstock, 0.1 wt% for bio-oil vs wt% for biomass. Therefore, compared to direct combustion of biomass in some power plants, using bio-oil will reduce the ash in the combustion system [23] Evaporative Residue Bio-oil contains many compounds with different molecular weights. The lighter compounds, including water, evaporate from bio-oil within a lower temperature range of C [18]. However, evaporation stops at about 320 C and a solid residue is formed, typically at wt% of the original sample [16], [22]. In other studies, thermo-gravimetric analysis (TGA) has been performed which heats up the sample at a constant rate and continually measures the mass. Figure 2.3 shows the stages of evaporation of bio-oil batches with different solid content as temperature is increased. Initially, LMW compounds and water are evaporated below 115 C, then cracking and evaporation of thermally unstable compounds takes place in C, and finally HMW, water-insoluble materials devolatilize till about 500 C [22], [24]. The carbonaceous residue (CR) is the amount of organic matter left after all these stages, which is typically accompanied with some inorganic ash content.

22 8 Figure 2.3 TGA curves for batches with different solid content [22] Because bio-oil is not fully distillable, it cannot be used when complete evaporation is required for combustion, such as in gasoline engines. The fuel is also difficult to burn in spray combustors due to its high molecular weight compounds. However, studies have shown that lighter miscible fuels such as methanol or ethanol, decrease the average molecular weight of bio-oil and increase its volatility. These effects improve ignition and help stabilize combustion of this liquid fuel Storage Stability The oxygenated chemical compounds that are present in bio-oil make it an unstable liquid fuel even at room temperature. These instabilities are mainly caused by polymerization and esterification [19]. The process of changes in properties over time is referred to as aging in the literature [19], [25], [26]. The aging rate changes when bio-oil is exposed to different temperatures; which is very important for fuel applications [27]. The single phase bio-oil can also separate into a sludgy phase and a thin aqueous phase due to aging. The main reason for this phenomenon is the shift in molecular weight distribution which changes the solubility of aqueous

23 9 and non-aqueous phases [28]. Details on aging mechanisms and the effect on bio-oil properties will be discussed in the following sections. 2.3 Aging Mechanisms Bio-oil contains more than 300 compounds, and considering all the reactions that take place in this mixture is beyond the scope of this study. However, an overview of the important chemical reactions could provide a better understanding of aging Esterification Equation 2.1 shows the reaction between alcohols and organic acids which yields esters and water: Equation 2.1 Where R & R are alkyl groups. This reversible ester-forming reaction can take place over a course of several years. However, this time might be shortened in the presence of mineral acid catalysts, which are abundant in bio-oil with ph value of 2-3. The formation of esters from organic acids and alcohols is thermodynamically favored, because the equilibrium constant is greater than unity. The heat of reaction for esterification is relatively small; therefore the equilibrium constant is independent of temperature [29]. Radlein et al showed that by adding methanol, ethanol or propanol to bio-oil in the presence of a mineral acid, esters and acetals were formed after 2-5 hours of reaction at room temperature [30] Polymerization Aldehydes and water react with each other to form polyacetal oligomers. Equation 2.2 shows this reaction:

24 10 Equation 2.2 Methanol in aqueous formaldehyde solution decreases the value of n. This has commercially been used to stabilize formaldehyde. During prolonged aging, these solutions produce noticeable quantity of methylal [31]. The higher molecular weight compounds produced by polymerization, lead to formation of tarlike precipitates. This is the main cause of viscosity increase when bio-oil is stored for prolonged periods [32] Air Oxidation Bio-oil content such as alcohols and aldehydes can be oxidized when exposed to air, which produces carboxylic acids. Moreover, another reaction that affects the storage of pyrolysis oil is the formation of alkylperoxides and hydroperoxides when autoxidation with air takes place. The stability of these peroxides is low and their decomposition into free radicals is almost spontaneous. They might even become an explosion hazard, if the concentration is high enough [33]. The free radicals formed by peroxides can catalyze the polymerization reactions. Thus, exposure of bio-oil to air will increase the formation of polymers. Oasmaa et al performed a series of tests to show the effect of air in bio-oil containers. Half of the containers were purged with nitrogen and the other half were left with oxygen above the liquid. Accelerated aging at 80 C for 25 hours showed no significant difference in viscosity increase between the two set. The results suggest that purging of air in a nearly full and tightly sealed container is not necessary for storage of bio-oil [20]. However, if the amount of available oxygen is increased and some mixing occurs, the polymerization reactions are favored and the bio-oil would be aged Gas Forming Some di-carboxylic acids are not stable and tend to form mono-acids and CO 2 at moderate temperatures.

25 11 Equation 2.3 shows this reaction: Equation 2.3 Where R & R could be hydrogen or alkyl groups. For example if both R & R are H, the reaction proceeds at 150 C [34]. In addition, keto-acids form ketone and CO 2 at low temperatures. Equation 2.4 demonstrates this path: Equation 2.4 If R is CH 3 group, the reaction takes place readily at 25 C [34]. The other CO 2 forming path in bio-oil is from ferulic acid, which is mainly due to the presence of lignin in pyrolysis process. Equation 2.5 explains this reaction: Equation 2.5 It has been observed that single carbon-carbon bond has lower breakage activation energy than a normal bond and in the presence of oxygen, the decomposition rate is much more rapid which introduces the idea of free radicals being involved. The free radicals that are present in the aqueous phase of bio-oil can catalyze this reaction at low storage temperatures [35]. Peacocke et al analyzed the headspace of a bio-oil container which was sealed and stored for 6 months at ambient temperature. The gas in the positive pressure build up headspace was run through a GC and contained 29 vol % CO 2, 1% CO, 1% CH 4, 61% N 2, and 6% O 2. It can be assumed that all nitrogen was from the trapped air and the 10% missing oxygen could have

26 12 contributed to about 10% of 29% CO 2, the rest of the oxygen present in CO 2 coming from the oxygenated compounds [36]. 2.4 Effects of aging on bio-oil properties Visual & Structural Changes Oasmaa reported the visual changes in one month stored bio-oil as a change in colour from reddish-brown to dark brown and an increase in dimness of the liquid. Formation of some flaky sediments occurred after a few months of storage at 9 C [37]. Garcia-Perez et al. analyzed the structural changes that take place during aging. Figure 2.4 shows cross-polarized pictures, taken at 30 C, of bio-oil at different aging times. The aging process was accelerated by exposing the sample to 80 C. Figure 2.4 Changes of Waxy material morphology during Aging [38] It is observed that the morphology of waxy materials change with time, suggesting polymerization reactions happening between these materials and formation of new crystal structures [38].

27 Viscosity There are several literatures that suggest the viscosity of bio-oil increases with time of aging. Yu et al. analyzed the aging of microwave pyrolysis liquid of corn stover and showed that accelerated aging of bio-oil at 60 C would result in a rapid increase in viscosity of pure (original) bio-oil. Adding a solvent would decrease both the viscosity and rate of viscosity increase, however there is still an increasing trend with time. Figure 2.5 demonstrates all these information graphically [39]. Figure Viscosity of bio-oils and bio-oils with solvent addition at 60 C [39] Boucher et al conducted a study on bio-oil obtained from vacuum pyrolysis of softwood bark. They measured the viscosity of different samples aged at 40, 50 and 80 C for 1, 6, 24 and 168 hours. They also found that viscosity of the bio-oil grows as the time of heating increases. In addition, higher temperature of heating causes greater increase in viscosity. Figure 2.6 presents the viscosity data for bio-oil aged at 80 C, measured at 30 C [40]. Chaala et al. considered the natural aging of vacuum pyrolysis liquid of softwood bark at room temperature. Their result showed the same trend, however they also demonstrated that viscosity of the bio-oil decreases at each point in time if the temperature is increased. Figure 2.7 presents this information. The viscosity increase rate was also calculated for accelerated aging tests, assuming a linear relationship between the viscosity and heating time. The data suggests that bio-

28 14 oil experiences a rate of increase in viscosity when heated at 80 C that is almost an order of magnitude larger than when bio-oil is heated at 60 C. This information suggest strong temperature dependence for aging [25]. heated at 80 C Viscosity (cst) Aging Time (h) Figure 2.6 Viscosity of bio-oil from softwood bark aged at 80 C [40] Figure 2.7 Effect of Storage Time and Measurement Temperature on Viscosity [25] Oasmaa and Kuoppala studied one year storage of forestry residue pyrolysis liquid at room temperature in tightly sealed glass containers. The increase in viscosity over time is observed here as well. In addition, the results indicate that the rate of increase in viscosity diminishes after the first month, and starts to retard significantly after 6 months. Figure 2.8 depicts this observation, as the time of storage becomes greater, the slope of the tangent (k) decreases [37].

29 Water Content Figure 2.8 Rate of viscosity increase vs. time of storage [37] As mentioned earlier, the by-product of the main reactions occurring during aging is water. Therefore, an increase in the amount of water present in bio-oil is expected over time. Yu et al. performed accelerated aging experiments at 60 C on bio-oil from corn stover and their results showed an increase in water content for pure (original) bio-oil. It is also understood from the data that addition of solvents would both decrease the water content of bio-oil and slow the water concentration increase. The results are summarized in Figure 2.9 [39]. Figure 2.9 Effect of Aging and Solvent Addition on Water Content [39] Czernik et al. also found that water content of the bio-oil increases with time. This increase was greater and faster when bio-oil was stored at a higher temperature. Their data is presented in Table 2.1 [27].

30 16 Table 2.1 Effect of Aging Time and Temperature on Water Content [27] 37 C 60 C 90 C Time (Days) Water (wt%) Time (Days) Water (wt%) Time (Days) Water (wt%) Diebold and Czernik studied the effect of water content on the aging rate of bio-oil. Figure 2.10 shows that increasing the water content from 20% to 25% slows down the aging rate by almost 16% [32]. This could be an explanation for the previous result on increasing water content over time flattening out after a certain time. As more water is produced over time, the aging rate is slowed which in turn reduces the production of water. The addition of methanol also decreases the aging rate. Figure 2.10 Effect of Water Content and Methanol on Aging Rate [32]

31 Average Molecular Weight Oasmaa and Kuoppala suggest that there is a direct correlation between average molecular weight of bio-oil and its viscosity. Therefore, they showed that over time the average molecular weight of bio-oil increases which is the reason for the viscosity increase. Figure 2.11 presents the changes in constituents of bio-oil over time when stored in sealed glass containers at 9 C. It can be seen that the only components with major changes are ether-soluble (ES) materials and High Molecular Weight (HMW) lignin materials which are the dichloromethane-insoluble fraction of water-insolubles. Ether-solubles decreased gradually over time, mainly in aldehyde and ketones. The oxygen content of ES decreased and that of Low Molecular Weight (LMW) lignin increased. However, since the amount of LMW did not change significantly, some of it might have converted to HMW lignin fraction [37]. Figure 2.11 Effect of Aging on Bio-oil Constituents [37] Czernik et al. performed measurements on naturally and accelerated aged samples of bio-oil and realized that the proportion of the low molecular weight material decrease over time while that of high molecular weight material increased. Table 2.2 presents the data that shows an increase in average molecular weight for stored bio-oil [27].

32 18 Table 2.2 Effect of Aging on Average Molecular Weight of Bio-oil [27] 37 C 60 C 90 C Time (Days) Molecular Weight Time (Days) Molecular Weight Time (Days) Molecular Weight Chaala et al. divided the molecular weight range of compounds present in bio-oil into categories and analyzed each category. Figure 2.12 shows a decrease in the fraction of lower molecular weight compounds while the fraction of higher molecular weight materials increases with the heating time at 80 C. This translates into an increase in average molecular weight of bio-oil with time of storage [25].

33 19 Figure 2.12 Effect of Aging on Different Molecular Weight Compounds [25] Volatility Thermogravimetric analysis (TGA) under nitrogen performed on fresh bio-oil and heat treated bio-oil for 168 hours at 80 C in a sealed container, shows a loss in volatility and an increase in the non-evaporative residue. Figure 2.13 demonstrates the effect of aging on the volatility of biooil and the residue [25]. It is obvious that if the aging happens in a container that is not properly sealed, almost all the volatile material of bio-oil will be lost and a thick, viscous tar-like material will be left. Figure 2.13 Effect of Aging on Volatility [25]

34 Phase Separation The reasons for phase separation are substantial polarity, density and solubility difference between the hydrophobic extractives and hydrophilic compounds present in bio-oil [28]. Yu et al. performed aging tests on bio-oil from microwave pyrolysis of corn stover and reported phase separation. A water-rich layer appeared on the top and a tar-rich layer appeared on the bottom after 30 days at 40 C and 15 days at 60 C. When solvent (methanol or ethanol) was added to bio-oil, no phase separation was observed after 30 days at both 40 C and 60 C. This suggests that solvent addition has another benefit which is prohibiting the bio-oil from phase separation. This is especially valuable for bio-oil storage [39]. 2.5 Methods to Slow Down Aging Aging has a negative effect on the fuel quality of bio-oil and hence preventing it or at least slowing down the process has potential advantages for both manufacturers and end users. The usual methods involve solvent addition, mild hydrogenation to reduce the inherent oxygenated compounds and proper sealing to minimize the exposure to air Solvent Addition One of the earliest recommendations to add water, methanol or acetone to pyrolysis oils was made by Polk and Phingbodhippakkiya. However, they did not present any data to illustrate the usefulness of solvent addition to prohibit some the chemical reactions and avoid large increase in viscosity [41]. Water addition effects analysis show that increasing the water content from 17 wt% to 30 wt% reduces the bio-oil viscosity measured at 25 C from 1127cP to 199 cp. Moreover, the rate of biooil aging after 4 months of storage at room temperature with 20 wt% water was 3.3 cp/day. This value was decreased when water was added to reach 25 wt% and 30 wt% to lower rates of 0.9 cp/day and 0.05 cp/day, respectively [42]. The effect of ethanol addition was investigated by Oasmaa et al. Ethanol was added to hardwood bio-oil at 2 wt%, 5 wt%, 10 wt% and 20 wt%. The samples were aged for 4 months and the rate of the viscosity increase was measured. The sample with no ethanol added showed an increase

35 21 with a rate of 0.12 cst/day, while the sample with 20% ethanol experienced only a minute aging rate of 0.01 cst/day. Accelerated aging tests were also performed. Bio-oil was aged for 7 days at 50 C and the rate of viscosity increase reduced from 3.5 cst/day for pure bio-oil to 0.4 cst/day for the mixture of bio-oil and 5 wt% ethanol [20]. Diebold and Czernik investigated the influence of different solvents on the aging rate of bio-oil. As much as 10 wt% solvent was added including methanol, ethanol, acetone, ethyl acetate, 50/50 mixture of methanol and acetone and a 50/50 mixture of methyl iso-butyl ketone and methanol. The experiments were carried out by accelerated aging of bio-oil at 90 C. The variation of viscosities of pure bio-oil and the mixtures with time proved to be somewhat linear. The pure bio-oil exhibited a rate of 60 cp/day of viscosity increase. This value was reduced to 12 cp/day and 3.4 cp/day when 5 wt% and 10 wt% methanol was added, respectively. The samples with 10 wt% ethyl acetate, ethanol and acetone, measured a viscosity change rate of 8.6 cp/day, 5.3 cp/day and 4.6 cp/day, respectively. The rate of increase in viscosity of the mixture of 5% methanol and 5% methyl iso-butyl ketone was 6 cp/day and the other mixture of 5% methanol and 5% acetone was 4.8 cp/day. Methanol proved to be the most promising of these solvents because of both its lower price and effectiveness in slowing the aging process [32]. Figure 2.14 summarizes all these changes over time.

36 22 Figure 2.14 Effects of additives on aging [32] The timing of the solvent addition was demonstrated to be an important factor as well. Diebold and Czernik used accelerated aging method to age two samples for 20.5 hours at 90 C. The first one was pure bio-oil and the second one contained a mixture of bio-oil and 10 wt% methanol. The viscosities of the sample measured at 40 C were 80 cp and 17 cp. After these measurements, 10 wt% methanol was added to aged pure bio-oil to recover its viscosity, however it only got reduced to 25 cp. This observation suggests that adding solvents right after bio-oil production is the best way to prevent it from aging; however storage volume might become an issue [32]. It has also been mentioned elsewhere that a mixture of fresh bio-oil and 5 wt% methanol showed a 15% increase in viscosity when stored for 3 months at room temperature, while the same bio-oil aged at the same condition and added the same amount of methanol after the storage time experienced a viscosity increase at about 28% [33] Very mild hydrogenation Hydrogenation has been used in unsaturated vegetable oils to make it stable. The result is a greasy semisolid material used sometimes instead of butter. However, this increase in viscosity is

37 23 not desirable for bio-oil fuel applications although the process would saturate the reactive compounds and slow the aging of bio-oil. A hydrogenation study was performed on bio-oil with a Palladium catalyst on carbon. After 8 months of aging at room temperature, the viscosity of the bio-oil diluted with 20% m-cresol increase 72%, while the diluted hydrogenated bio-oil experienced only 21% increase in viscosity. However, adding the solvent suggests that the viscosity increase after hydrogenation has been significant although no data has been provided on the actual value [41] Minimizing the exposure to air As previously mentioned, the minor volume of air trapped on top of the bio-oil in a container does not have any major effect on aging. However, if more air is available to the bio-oil, the outcomes might not be the same. Mixing bio-oil with a medium to high speed blender in an open container would result in bubble formation and the entrained air would saturate the bio-oil. This augmented level of available oxygen could be responsible for production of peroxides that would act as a catalyst for polymerization reactions. Limiting the access of air to bio-oil is necessary to reduce the likelihood of peroxide formation that catalyzes the polymerization of olefins. Adding some antioxidant such as hydroquinone also stabilizes the olefins, and seems to be more cost effective than hydrogenation [33]. 2.6 Droplet Combustion of Bio-oil Bio-oil spray combustion can be explained with the aid of analysis on single droplet burning studies. There are typically four stages in the droplet combustion of bio-oil: (1) surface burning of volatiles, (2) droplet micro-explosion and a burst of fuel vapor, (3) sooty combustion of micro-explosion droplets, (4) solid state combustion of the residues [43]. Figure 2.15 shows these stages.

38 24 Figure 2.15 Four Stages of Bio-oil Droplet Combustion (left to right) [43] After ignition, evaporation of volatiles takes place from the surface of the droplet and a spherical blue flame is produces by the quiescent combustion of these volatile compounds. During this process, the outer crust is mostly left with viscous HMW material due to the loss of evaporative compounds and the surface is exposed to heat and oxygen which leads to polymerization of these heavier materials. This phenomenon causes the formation of a hardened shell on the surface of the droplet which prevents the volatiles from escaping outside of the droplet [44]. As more heating is provided to the droplet, more material evaporates inside the restricted shell and pressure is build up there, leading eventually to a micro-explosion to take place. This microexplosion produces many more droplets with a reduced effective diameter. Right after this stage that is recognized by a yellow flame attributed to soot burning, the droplet contracts to almost the original diameter. This is accompanied by a reduction in flame size around the droplet, indicating a substantial decrease in fuel evaporation. The last stage of droplet combustion begins when the flame extinguishes completely [45]. The porous cenosphere left from the previous combustion stages, burns in a non-volatile solid-state, fuel-rich combustion mode. The details of heterogeneous burning of this carbonaceous residue are depicted in Figure The nonevaporative fraction of bio-oil that should be burned heterogeneously is usually about 20-30%. At the end, the only material remaining should be ash, if complete combustion takes place [24]. Figure 2.16 Solid State Combustion of the Cenospheric Residue [43]

39 25 Thermogravimetric analysis of bio-oil can also give insight to the combustion stages [24]. Although heating rates and time scales are totally different between TGA and droplet combustion studies, the stages can be described similarly based on the results. TGA suggests that very high temperatures and long residence time is required for complete burnout of carbonaceous residue. This implies that bio-oil may not be well suited to combustion devices that require full evaporation of the fuels such as internal combustion engines and jet engines. 2.7 Effect of Bio-oil Properties on Spray Combustion Viscosity Tzanetakis et al. have used a correlation to estimate the bio-oil droplet size or Sauter Mean Diameter (SMD) coming out of an air-blast nozzle. One of the many parameters in this correlation is the dynamic viscosity of bio-oil (µ L ). Equation 2.6 presents this correlation. [ ] Equation 2.6 After performing spray combustion test with biooil-ethanol blends, they concluded that as the calculated droplet size increases, the combustion quality becomes inferior and emissions rise. This effect is shown in Figure 2.17 [7].

40 26 Figure 2.17 Effect of SMD on Combustion [46] As a result, one could conclude that since increasing viscosity tends to increase the droplet size, it will deteriorate the combustion performance and increase the emissions TGA Residue Thermogravimetric analysis provides insight on the fuel`s volatility. The TGA residue is a measure of non-volatile matter present in the liquid. As this residue increases in bio-oil, combustion becomes more challenging. Moloodi analyzed the effect of TGA residue on combustion and emissions of bio-oil by adding different amounts of ethanol to bio-oil in order to vary the TGA residue of the fuel. It was concluded that more volatile matter helps ignition and sustainable combustion. This led to lower emission for the batch with lower TGA residue. The effect of TGA residue on CO emission of bio-oil combustion is presented in Figure 2.18 [22].

41 CO (PPM) CO TGA Residue 0 14% 15% 16% 17% 18% Figure 2.18 Effect of TGA Residue on Combustion [22] Water Content Water removal from bio-oil causes the adiabatic flame temperature to increase owing to higher heating value, and droplet evaporation time to decrease due to the large latent heat of evaporation of water [47]. Moreover, removing water increases the viscosity of bio-oil and lowers the probability of having micro-explosions in droplet combustion [48]. Moloodi investigated the effect of water content on bio-oil spray combustion performance and concluded that more water decreases the emission. Two batches of bio-oil with similar solids and ash content and same ethanol addition were tested in a spray burner. The main difference was the TGA residue because of water dilution. Figure 2.19 shows that emission decreases as water content increases [22]. However, it needs to be mentioned that the water content of bio-oil has an upper limit for combustion purposes. More water will decrease the heating value and rate of combustion reactions, which in turn would cause instability and termination of combustion. In addition, too much water would accelerate phase separation of bio-oil when stored.

42 28 Figure 2.19 Effect of Water Content on Combustion [22] Solids & Ash Content Effects of solids & ash content on combustion performance of bio-oil have been examined in the literature [22]. To show the isolated effect of solids, three batches with similar ethanol addition, water content and SMD were chosen. Author has labeled them S2, S3 & S4 with 0.089%, 0.839% and 2.217% solids content. Figure 2.20 shows that the emissions increase with increasing solids content of the fuel. The large jump from S3 to S4 is mainly due to the almost doubling of solids content as well as a substantial increase in char particle size which requires more time for complete heterogeneous combustion [45]. The effect of ash is considered when two batches with similar solids, water and ethanol content but different ash content were tested. The first batch had 0.027% and the second batch had 0.223% ash, and the CO emissions were ppm and ppm, respectively. The increase in emission corresponding to an increase in ash content may be due to catalytic effect of alkali metals in the gasification of char particles [49].

43 CO [mg/mj] CO S S2 S3 200 Solids[%] 0 0.0% 0.5% 1.0% 1.5% 2.0% 2.5% Figure 2.20 Effect of Solids Content on Combustion [22] A linear correlation between carbonaceous residue emissions with the amount of solids content and the TGA residue of the fuel has been suggested by Moloodi [22]. It shows that the effect of the TGA residue is greater than the solids content.

44 30 3. Experimental Methodology 3.1 Spray Burner Figure 3.1 shows the 10 kw spray burner used in this study which was designed by Tzanetakis [46]. The main sections are variable swirl generator, air-blast atomizing nozzle and pilot flame. Primary combustion air is heated by passing it through a 1.5 kw electric heater before entering the moveable block swirl box installed above the combustion chamber. The fuel is atomized in the nozzle by compressed air in order to achieve spray combustion. All the burner sections downstream of the nozzle are built from 316 stainless steel to prevent corrosion. A pilot flame from a methane-oxygen torch is used to stabilize the combustion. In addition, a 3.2 mm thick quartz window enables us to directly see the flame and monitor the combustion quality during the tests. Figure 3.1 Bio-oil Burner Assembly [46]

45 Overall Setup Figure 3.2 presents the schematic of the overall experimental system. The main inputs of the burner are fuel and atomizing air through nozzle, primary combustion air and methane-oxygen mixture for pilot flame. Two parallel peristaltic pumps are used to deliver ethanol or biooil/ethanol mixture to the nozzle. All fuel lines are chosen from 316 stainless steel or Teflon tubing for corrosion resistance. Fuel is atomized in the nozzle by compressed air that is passed through a pressure regulator and rotameter. The primary combustion air is provided by a stack fan downstream of all instruments in the system. The air is heated to between C by a heater mounted on top of combustion chamber and the flow is controlled by changing the voltage on stack fan using a variac. The whole system operates at a slight negative pressure of Pa provided by the stack fan in order to contain the exhaust gases and discharging them to the roof stack and preventing them from spreading into the room. Exhaust is cooled to room temperature using a spiral heat exchanger and most of particulate matter is collected in the water traps below the heat exchanger. However before the heat exchanger, a sample stream is tapped from the exhaust line downstream of combustion chamber using a 6.4mm stainless steel heated line that avoids condensation. The sample passes through a filter to remove PM before entering the gas phase emissions measurement instruments. PM is also isokinetically sampled and collected on a filter for analysis. Details of gas phase and PM emissions measurements will be discussed in the following sections.

46 32 Figure 3.2 Overall schematic of experimental setup [46] Variable Swirl Generator Swirling flows introduce tangential velocity that causes angular momentum to the streamlines of the main combustion air. The movable block swirl generator shown in Figure 3.3 can change the ratio between axial and angular momentum of the flow by changing ζ [46]. The fixed swirl block angle is represented by α and is set to 60 and the maximum operating angle shown by ζ m is 12. If angular momentum is made large enough compared to axial momentum, a pressure gradient in the opposite direction of the bulk flow is created. This causes the fluid to flow back toward the upstream region, creating a central recirculation zone (CRZ) which is especially beneficial for combustion systems in terms of mixing and stability. This concept is depicted in Figure 3.4.

47 33 Figure 3.3 Schematic of Movable Block Swirl Generator [46] Figure CRZ of swirling flows in confined geometry [50] In swirling flows, the conservation of axial flux of momentum applies for both radial (G Φ ) and axial (G x ) directions [51]: Equation 3.1 Equation 3.2 where W, U and p are tangential and axial components of velocity and local static pressure, respectively. Based on these parameters, the non-dimensional swirl number S is introduced, which characterizes the ratio between angular and axial momentum fluxes and enables to

48 34 compare the swirl intensity of different flows. In order to obtain CRZ, a minimum swirl number between 0.5 < S < 0.6 is required [50]. The movable block swirl generator used in this study is capable of achieving S = 5.41 when swirl is set to 100% Air-Blast Atomizing Nozzle An internal mix, air-blast nozzle from BEX Engineering Ltd. (model 1/4 JX6BPL11 with a 152 mm long extension tube and 2X2JPL back-connect body) is used in this study. The internal construction and assembly of the nozzle is depicted in Figure 3.5. All the components are made from stainless steel in order to avoid erosion and corrosion. The nozzle is placed along the centerline of the burner and there is only 15.9 mm clearance between the tip of the nozzle and the centerline of the pilot flame in order to spray the fuel-air mixture as close as possible to the ignition source. Figure 3.5 Atomizing Nozzle Tip Assembly [52] Fuel is carried through a 1.0 mm single liquid cap orifice to the internal mixing chamber, where it is mixed with air. After that, the spray exits through six symmetrically spaced, 0.89 mm air cap discharge orifices. The six individual jets make an angle of 60 with the centerline of the burner and create a hollow cone pattern. This is important because the center of the spray does not introduce any significant axial momentum along the centerline of the flow compared to central discharge orifice. This benefits the formation of a CRZ that helps combustion stability. Another factor that affects stability is fuel boiling. If this phenomenon is controlled for atomization, it is called flash boiling atomization and it can benefit the combustion by reducing

49 35 the droplet size and widening the spray angle [53]. However, in the system used for this study, the boiling and bubble forming causes instability which sometimes ends in flame blow-out. Therefore, a water cooling system for the nozzle was designed by Moloodi in order to prevent the fuel from boiling and avoid the instabilities. Figure 3.6 depicts the details of this cooling system which consists of a 1/16 stainless steel tube enveloped helically around the nozzle body [22]. Fuel temperature is monitored and whenever the boiling is about to happen, a ball valve is manually opened to allow water to run through the system and cool down the fuel Pilot Flame Figure 3.6 Schematic of Nozzle Cooling System [22] An oxy-fuel torch body and standard No. 7 tip with a 1.2 mm orifice diameter (Hoke Model No ) is inserted vertical to the axis of the burner to produce a fully premixed stoichiometric methane-oxygen flame. The tip has a hexagonal slit which allows multiple flames to be issued from the small openings, producing a wider overall flame. The energy throughput of the pilot flame is 5% of the total 10 kw input from bio-oil blends, which corresponds to 0.88 standard liters per minute (SLPM) of methane and 1.8 SLPM of oxygen. The pilot is always kept running during a test, in order to sustain and stabilize the combustion. Although the relative position of the pilot flame to the burner is fixed, nozzle orifices and the pilot flame need to be aligned. The procedure is described by Tzanetakis [46]. Pure ethanol is burnt in the combustion chamber and the nozzle is rotated until an evenly distributed spray flame is observed and the pilot flame does not impinge on any of the fuel jets. Figure 3.7 describes the difference between a good and poor alignment. After the correct configuration is found, the nozzle is locked into place by four set-screws and won t be repositioned unless a major nozzle cleaning is required.

50 36 Figure 3.7 Alignment of Pilot Flame [46] 3.2 Fuel Analysis Fuel Composition & Heating Value Fuel composition of the bio-oil is reported by measuring the carbon, hydrogen and nitrogen and assuming that the rest is oxygen. The solids, ash and water content of the fuel are also important for analyzing and predicting the behaviour of the combustion. The higher heating value (HHV) needs to be determined in order to calculate the flow rate required for a 10 kw operation. Table 3.1 lists the measurement methods used to obtain these properties which are outsourced for measurements. Table 3.1 Fuel properties measurement standards Property Test Method Water Content ASTM E203 Solids Content MeOH-DCM insoluble Ash Content ASTM 482 C-H-N-O ASTM 5291 HHV ASTM 4809

51 Viscosity Viscosity has an effect on atomization quality and pumping of liquid fuels. The viscosity of pure bio-oil was measured at room temperature using a handheld dip viscometer (Zahn Cup) before each combustion test. The time required for the liquid to be drained from the cup is recorded and using a correlation provided by the manufacturer, the time is converted to viscosity. This measurement method is used 3 times for each sample and the arithmetic average is presented as the data. However, the result is not very accurate and is only used for comparison between different samples. More accurate measurement is outsourced to be done according to standard ASTM D445 at 40 C for pure bio-oil and 80 C for bio-oil/ethanol blend which is closer to the fuel temperatures exiting the spray nozzle. This measurement method uses a calibrated glass capillary tube and is suitable for opaque fluids. For the higher temperature, slow heating rate is required to avoid bubble formation from evaporation of volatiles in order to achieve more accurate results [54] Thermogravimetric Analysis Two important pieces of information are obtained when TG analysis is performed: fuel s volatility distribution and its tendency for formation of a solid residue [44]. For this study, a TA Instrument Q50 Analyzer is used. The bio-oil and ethanol blend is shaken to achieve a thoroughly mixed and homogenized fluid. Then, a mg sample is picked and placed on an aluminum pan, which is put on the sample holder of the TG analyzer. Throughout the test, the sample is kept under a 100 ml/min atmospheric flow of nitrogen. Heating starts at a rate of 10 C/min after 5 minutes of running nitrogen through the lines in order to deplete the system from any Oxygen. The heating continues for an hour to reach a maximum temperature of 600 C. The percentage of the material left at the end of the test is a representative of fuel s tendency for char formation and is named as TGA residue hereafter.

52 Gas Phase Emissions Measurement Exhaust emissions were analyzed for carbon monoxide (CO), nitrogen oxides (NO x ), unburned hydrocarbons (UHC) and oxygen (O 2 ) percent. Different instruments have been used for measurement, which have been described in detail elsewhere [46]. The schematic of sampling and analysis system is depicted in Figure 3.8. All the sampling lines and chambers are heated via either heated tapes or feedback controlled heaters, and the temperature of the gas is maintained at C in order to avoid any condensation in the lines. Figure 3.8 Schematic of the gas phase emissions measurement system [46] Oxygen Concentration The fraction of the exhaust that is composed of oxygen can be determined continuously using a Zirconia (ZrO 2 ) model OXY6200 oxygen sensor. The accuracy of this sensor is ±0.1 %O 2 and it is calibrated by running ambient room air through it to set the 21 vol% mark. The output is a 0-5 V DC that is linearly proportional to the 21% concentration. A stream of exhaust sample is passed through the oxygen sensor at a flow rate of 1.8 SLPM provided by a vacuum pump. A separate sampling line is used for this sensor as seen in Figure 3.8, because it operates at a high

53 39 temperature which can oxidize hydrocarbons and CO of the exhaust and affect the detection results. The voltage output of the sensor is logged by a data acquisition system and is used to back calculate the equivalence ratio of the combustion. The voltage is translated into the percent oxygen in exhaust, and by using the fuel s atomic composition from CHNO analysis and assuming complete combustion, real-time equivalence ratio and the flow of primary combustion air required for that equivalence ratio is calculated. Since, the amount of CO present in exhaust is in most cases below 0.2%, the complete combustion assumption only has minor effect on the results and the error can be neglected Unburned Hydrocarbons Unburned Hydrocarbon (UHC) emissions are measured using a flame ionization detector (FID) from California Analytical Instrument. The detection mechanism of this device is that a sample of exhaust is passed through a small hydrogen flame which produces a current proportional to the number of carbon atoms in the sample [55]. The flow rate of this sample is automatically kept at 1.5 SLPM by an internally built vacuum pump. The temperature of this exhaust sample is kept at C to prevent the heavier hydrocarbons from condensing. The current produced by the flame is outputted as a voltage of 0-5 V DC and is fed into a data acquisition system connected to a computer. Before each combustion test, the FID is calibrated by setting two values. The first one is zero, using purified air and the second point is set to 90.2 ppm of methane in air. The range is extrapolated to a maximum of 300 ppm with an uncertainty of ±3 ppm Detailed Speciation of Pollutants The concentration of carbon monoxide (CO), NO x, methane (CH 4 ), formaldehyde (CH 2 O), acetaldehyde (C 2 H 4 O), carbon dioxide (CO 2 ) and water (H 2 O) emissions in the exhaust sample are measured using a Nicolet 380 Fourier Transform Infrared Spectrometer (FTIR). This spectrometer compares the absorption spectrum of a gas sample in the mid infrared region (500 to 4000 cm -1 ) against known standards [56].

54 40 A two way valve is used to switch the gas sampling between FID & FTIR. The FTIR gas cell has a sample volume of 0.19 L and gas is continuously drawn into it at a flow rate of 10.3 SLPM and a pressure of 86.3 kpa when spectra are taken. Each spectrum is obtained by scanning the sample 24 times over 1 minute at a resolution of 1 cm -1, which translates into about 50 refills of the gas cell per spectrum. A total of 5 spectra are taken for each combustion test and the arithmetic average of the measured emission values are presented. The calibration is done via a partial least square model using different gas mixtures of the compounds considered in the study. Details of the FTIR calibration experimental setup and method are provided in a previous work [57]. The minimum concentration of each species that outputs a signal to noise ratio of 4 is set as the detection limit. The root mean square error (RMSE) determines how accurate the model is by representing the deviation between the predicted value by the model and actual concentration of the employed standard gas sample mixture. Table 3.2 summarizes the detection limits and the accuracy of the models. Two separate models have been used in this study, corresponding to different ranges of CO concentration. This is due to the nonlinearity of CO absorption versus concentration curve. Each spectrum is compared with the standard gas spectra to determine which of the two models is more suitable for quantifying the spectrum. Table 3.2 Detection limits and uncertainty levels of the FTIR calibration model Species Units Detection Limits RMSE CO (Low) ppm CO (High) ppm NO x ppm CH 4 ppm CH 2 O ppm C 2 H 4 O ppm CO 2 % vol H 2 O % vol

55 Particulate Matter Emissions Measurement There are two main factors involved in PM measurements of an exhaust stream. First, a sample that accurately represents the exhaust PM needs to be obtained. Isokinetic sampling is the method used in this study. Second, the sample has to be analyzed and the concentration of the PM should be measured. For this matter, gravimetric analysis and loss on ignition methods have been employed Isokinetic Sampling System PM sampling of an exhaust stream is usually done by inserting a small diameter probe with an opening that faces upstream in a way that minimum disturbance is imposed on the flow. Under frictionless flow conditions, isokinetic velocities are achievable when gas enters the tube without changing velocity [58]. In other words, isokinetic condition is obtained if the sampling nozzle is set parallel to the flow stream and the suction velocity of the nozzle is equal to the exhaust gas velocity [59]. It is important to satisfy the isokinetic sampling conditions because it provides an unbiased size distribution of exhaust PM. Controlling the sampling flow rate via a needle valve is the most common method of practice for ensuring closest approximation to isokinetic sampling. Inertial effects make the larger and heavier particles deviate more from the gas streamline, while smaller and lighter particles follow the streamline closely. Figure 3.9 shows that when the exhaust flow velocity is more than the sampling velocity, flow streamlines tend to deflect outwards when they encounter the nozzle. As discussed above, heavier particle don t follow the streamlines and tend to continue on their straight path through the nozzle. This phenomenon results in a change in distribution of particle size in a way that larger particles of the flow get accumulated on the filter and the sample will not be an accurate representative of the flow [60].

56 42 Figure 3.9 Gas Streamlines around sampling probe (V S < V) In order to find the sampling flow rate for isokinetic sampling condition, two methods are recommended. First method is using a pitot tube to measure the gas velocity as described by EPA [61]. The other method, called null method, matches the velocities of main stream and sampling stream by matching their static pressure [58]. In this study, the pitot tube method was not used due to large amount and size of PM which would have ended in clogging of opening hole of the tube that faces upstream. As an alternative, the null method with two static pressure measurement openings parallel to the streamlines was used. If Bernoulli s equation is used along a streamline that enters the probe, assuming the gas is incompressible and frictionless (Mach number << 0.3), it can be shown that the velocities inside the sampling tube and the exhaust streamline are the same when the static pressure difference at these points is zero. In reality, friction of the gas and incompressibility effects can cause a deviation from the assumptions made for isokinetic conditions. Therefore, a calibration method is required to validate the accuracy of these conditions based on theoretical considerations. Some of these details are briefly discussed in Appendix A. The outcome of these calculations is that the ideal isokinetic sampling flow rate is determined to be 10.4 % of the total exhaust flow rate. Figure 3.10 shows the schematic of the isokinetic PM sampling system. The gas mixture inside the combustion chamber is highly swirling. Therefore, the angular momentum of the flow can be seen far downstream of the swirl generator [62]. This causes non-uniformity in PM distribution

57 43 as particles tend to accumulate near walls. For solving this issue, a flow straightener is placed in the outlet pipe of the burner. It is constructed of 0.25 mm thick stainless steel sheet metal arranged into a 24 x 24 mm checkerboard pattern. The purpose of the flow straightener is to ensure an axial pipe flow velocity profile is achieved and the PM distribution is uniform before entering the isokinetic sampling probe. The isokinetic sampling probe uses the pressure gradient between fully stagnated and free stream flow, so the dynamic pressure inside the main exhaust pipe needs to be detectable. For this matter, a pipe size reduction is placed in the system from the 102 mm i.d. elbow to 38 mm i.d. elbow. This change in diameter causes the gas velocity to increase, which in turn, increases the dynamic pressure to about 21 Pa. This pressure can be readily detected by a manometer with an accuracy of 1.2 Pa. This manometer is used during the combustion tests to make sure the pressure difference between the two streams is always zero by controlling the flow rate. The sampling probe is placed in a way that flow disturbances such as bends and size reductions have minimum effect on uniformity of PM distribution within the gas. It is recommended in literature that the probe should be located 8-10 duct diameters downstream and 3-5 diameters upstream of any disturbance in order to minimize the effect [63]. In the system used for this study, the probe is positioned 10.4 diameters (394 mm) downstream of the elbow and 1.7 diameters (64 mm) upstream of any flow disturbance due the lab space restrictions.

58 44 Figure 3.10 Schematic of PM sampling system [46] Figure 3.11 shows the geometry and position of the sampling probe and the static pressure taps. The sampling probe is made of 11.3 mm i.d., 12.7 mm o.d. seamless stainless steel tube. The diameter size was selected based on recommendations made by EPA [61]. Some other considerations included the maximum flow rate in the probe that the vacuum pump could sustain during sampling time and a reasonable amount of PM mass collected on the filter during the time that pump could draw flow. For this matter, 5 minutes of sampling was chosen in order to optimize both requirements. In addition, small diameter tubes were inserted in the sampling tube and mounted perpendicular to the exhaust stream, in order to measure the static pressure at these points.

59 45 Figure 3.11 Geometry and position of sampling probe and pressure taps [46] The temperature of the gas running through the sampling probe is about 250 C, which would damage some of the gaskets in the assembly. For this reason, gas temperature fed into the filter is controlled to be between C according to EPA methods [61]. To do so, a cooling jacket that uses compressed air at room temperature surrounds the sampling tube as seen in Figure In addition to the main sampling line and filter, an auxiliary line and filter is installed with ball valves required to switch the gas between them. This auxiliary line is used during a filter change in the main line; the purpose is to keep the sampled gas flow at a steady rate. This switching prevents any changes in the combustion chamber pressure and also keeps the sampling line passages warm. A 47 mm diameter Tissuquartz filter (Product No. 7221) provided by Pall Life Sciences is placed in an Advantec MFS Inc. stainless steel filter holder body (Model LS47, Part No ), in order to collect exhaust PM. The filters are made of quartz fibers which can endure high temperatures in the range of 1100 C without decomposing. They have a 99.9 % aerosol retention efficiency for 0.3 micron diameter particles (measured by ASTM D A) [64]. As shown in Figure 3.10, a J-type thermocouple with a 1.6 mm diameter stainless steel sheath measures the gas temperature just downstream of the filter. In addition, a shut-off ball valve is located prior to the holder body on both the main and auxiliary lines, in order to make a sudden start/stop in the

60 46 flow and accurately time the sampling process. When the valves are fully open, the diameter matches the sampling line in order to minimize any obstruction and PM loss. A shell and tube heat exchanger provided by Seakamp Engineering Inc. (Part No ), is placed between filter holder bodies and the sampling pump to condense the water vapor and prevent liquid accumulation inside the line and the pump. Water is run in the tubes to cool down the exhaust gas that runs in the shell. The flow rate of water is about 0.25 GPM and the temperature increase is about 2 C for all tests. In order to calculate the PM concentration, the total flow rate through the sampling line should be quantified. However, the gas rotameter which is calibrated at the test pressure, measures the dry exhaust flow rate. It cannot be used to measure the flow rate before the heat exchanger because of condensation problems. These two flow rates show a relationship when it is assumed that the gas coming out of the condenser is at equilibrium and saturated with water vapour. By performing a mass balance over the condenser, the following equation is found between the standard volumetric gas flow rates: Equation 3.3 where P sat is the saturation pressure at the temperature of the gas exiting the condenser, P total is the absolute pressure as measured by the gauge in the condenser exit and X water vapor is the molar fraction of water in the exhaust before heat exchanger, based on mixture stoichiometry calculated from oxygen sensor Gravimetric & Loss on Ignition Analysis There are two successive sampling filters for each bio-oil test. Each collection on a filter takes 5 minutes. Depending on the pressure drop over the filter, this time might change. For high PM loading, 3 minutes is used and for depositions that has minimum effect on pressure and might be detectable, 10 minutes of collection is used. The filters are desiccated and cleaned from any contamination prior to each combustion test, by placing them in an oven at 750 C for two hours. They are then weighed 3 times using a Scientech SM-128D Microbalance, and placed in a Petri dish and caped. In order to calculate the amount of unburned carbonaceous residue (CR) and ash of the PM, a standard method called loss

61 47 on ignition described by ASTM code number D was followed for each of the filters after combustion tests [65]. The filter is placed in the Petri dish and capped immediately after collecting it from the filter holder body. After the test, the collected filter is weighed 3 times again to measure the total amount of PM. There is a high vapor content in the exhaust which gets accumulated on the porous filter. A drying stage is done by placing the filter in the oven at 150 C for two hours. Once more, the filter is weighed and the mass difference between this stage and previous one, measures the water content present on the filter. Lastly, the filter is positioned in the oven for an hour at 750 C for burning the CR fraction of the PM. The weight of the filter containing only ash is measured 3 times. All the weights that are used for calculation are the arithmetic average of the 3 measurements. Figure 3.12 shows these steps and Table 3.3 provides the calculation method for each fraction of the PM based on the nomenclature of Figure Figure 3.12 Gravimetric analysis and Loss on ignition procedure [22] Table 3.3 Calculation methods of each PM fraction Total PM(mg) Water(mg) CR(mg) Ash(mg) M2 - M1 M2 M3 M3 M4 Total-Water-CR

62 Flame Visualization Combustion quality is evaluated by observing the flame through the viewport, monitoring pressure fluctuations in the combustion chamber and using a borescope assembly to take pictures and videos of the flame. These photographs qualitatively describe the trends in mixing and atomization quality for each test. Figure 3.13 shows a schematic of the flame visualization system used in this study. The probe is a 9 mm diameter Lenox Instrument Co. direct view borescope with a tube length of 35 mm. It is a rigid fiber-optic member that has a 90 angle mirror integrated to its body at one end and is coupled to a 10 mega pixel camera at the other end. The mirror, when inserted in the burner, provides an unobstructed, upward-looking view of the flame along the central axis of the burner. In order to avoid very high temperatures on the optics, the optical assembly can be easily inserted and removed from the burner via a rail-guide system. A 19 mm insulated tube surrounds the fiber optic and lets compressed air at a flow rate of 250 SLPM pass through it, in order to cool the optics and protect the mirror from PM impingement. This extra air is introduced far downstream of the flame and in order to compensate for the fluid dynamic effects, stack fan draw is increased until the same burner pressure as before these changes is achieved. Figure 3.13 Borescope Assembly [46] During a combustion test, the probe is inserted through a tube fitting and remains there for only 5-10 seconds to a take a photograph or a short video. Several photographs are taken for each test

63 49 while the aperture opening (f-stop), shutter speed and ISO value (light sensitivity) of the camera are manually adjusted to achieve the best picture quality possible. No post process image treatment (besides cropping and resizing) is applied to any of the photographs. 3.6 Aging Procedure For the purpose of this study, two separate batches of bio-oil have been used. Although manufactured by the same provider and from the same feedstock, they had slightly different properties and characteristics. First batch was poured into several bottles and sealed with a plastic cap and separated into two categories. Some of the sealed bottles were stored in a ventilated cabinet located in a room with a central climate control system which kept the temperature at around 20 C. The cabinet s door was kept closed and only opened when temperature measurements were done, in order to keep the bio-oil out of any direct lighting. This procedure is referred to as Natural Aging in this study, because the temperature is a close approximate of room temperature. Figure 3.14 demonstrates the temperature fluctuations measured on the glassware with a type K thermocouple over the period of the storage. The frequency of the measurements was higher at the beginning to ensure consistency over different times of the day.

64 Temp vs Time /08/2012 0:00 30/07/2012 0:00 20/07/2012 0:00 10/07/2012 0:00 30/06/2012 0:00 20/06/2012 0:00 10/06/2012 0:00 31/05/2012 0:00 21/05/2012 0:00 11/05/2012 0:00 01/05/2012 0:00 21/04/2012 0:00 11/04/2012 0:00 01/04/2012 0:00 22/03/2012 0:00 12/03/2012 0:00 02/03/2012 0:00 21/02/2012 0:00 11/02/2012 0:00 01/02/2012 0:00 22/01/2012 0:00 12/01/2012 0:00 02/01/2012 0:00 23/12/2011 0:00 13/12/2011 0:00 03/12/2011 0:00 23/11/2011 0:00 13/11/2011 0:00 03/11/2011 0:00 24/10/2011 0:00 14/10/2011 0:00 04/10/2011 0:00 24/09/2011 0:00 14/09/2011 0:00 04/09/2011 0:00 25/08/2011 0:00 15/08/2011 0:00 05/08/2011 0:00 Figure 3.14 Temperature vs Time for Natural Aging The rest of the bottles were stored in a fridge at 5 C, in order to slow down the aging of bio-oil to a point that for the purpose of this study, it was considered no changes associated to aging occurred during this storage. These bottles were taken out one by one from the fridge and placed in a fume hood for five hours to reach the room temperature before accelerated aging procedure could begin. In order to a mimic the natural aging in a shorter period of time, literature has suggested that bio-oil can be heated up to higher temperatures and equivalent changes in viscosity and average molecular weight are achieved after some time. A correlation is provided by Diebold and Czernik for the aging rate as a function of temperature: ( ) Equation 3.4 [32] The aging rate here is defined as the change in viscosity over time and has a unit of cp/day and T is the storage temperature in kelvin [32]. In order to calculate the time required to heat the bio-oil

65 51 and achieve an equivalent viscosity increase to the one obtained when stored at room temperature, Equation 3.4 was manipulated to obtain the following equation: Equation 3.5 ( ) ( ) The inputs for this equation are the temperature of the heated storage and time of natural aging that is to be mimicked by accelerated aging. The outcome is the time of storage required at higher temperature. The bottles from the first batch that were taken out from the fridge to reach room temperature were kept at 80 C in a vacuum oven from Precision Scientific Inc. (Catalog No ); therefore T was replaced by 353 in Equation 3.5. The second batch was provided at a later date, and was poured into bottles with plastic caps sealing the bio-oil so that the evaporative fraction won t be loss during storage. The bottles were immediately place into the fridge to inhibit the aging. Before each accelerated aging, one bottle was taken out and left at room temperature for 5 hours. The required time of heating at 80 C was calculated using Equation 3.5. Then, the bottle of bio-oil was placed in the oven and the time was recorded as the start of the accelerated aging. The calculated time was added to the recorded start time, in order to achieve the time that bio-oil had to be taken out of the oven. The bio-oil was then left outside at room temperature overnight, in order to prepare it for viscosity measurements. 3.7 Combustion Test Procedure The combustion test procedure for the two different batches was the same. The heated lines, including the ones with heating tape leading the sample to oxygen sensor, FID and FTIR, start to warm up three hours before the official beginning of the test. The FTIR cell is heated to a temperature of C and a vacuum is drawn into it to take a background spectrum. Then, the borescope assembly is set up and aligned such that the mirror sees the centerline of the burner the cooling tube can be easily inserted in and out of the fitting on the burner, without any extra force. The stack fan is then ramped up until the main combustion air flow reaches 250 SLPM and the atomizing air is set to 25 SLPM. Swirl is positioned at 50% before the air preheater is started. Two hours before the test, cooling water begins to run through the system

66 52 and the air preheater is fully powered on. After one hour, the background spectrum for FTIR is taken and validated against pure nitrogen spectrum and also the FID is calibrated. In addition, the oxygen sensor is turned on and when it reaches steady state after 20 minutes of sampling heated room air, the voltage is used to calibrate it and calculate the mole fraction of oxygen which is used for back calculating the equivalence ratio during a test. The 15/85 vol.% mixture of ethanol and bio-oil is prepared and the fuel flow calibration starts 30 minutes before the test. The volumetric flow rate based on 10kW operation, density and heating value of the fuel is calculated and the peristaltic pumps are calibrated for this flow rate using a graduate cylinder with accuracy of 1 ml and a stopwatch. The pilot flame is ignited and put into position right before the test. All the temperatures, oxygen sensor and FID outputs are recorded via the Labview program, from the moment that all six spray jets are ignited. The burner is warmed up by running ethanol for the first 20 minutes while the swirl is set to 50% of its full capacity in order to reduce the excessive recirculation of combustion products and avoiding fuel boiling in the nozzle. At this swirl intensity, some of the combustion tests exhibit aerodynamic instability, which need immediate attention or they have the potential to cause great pressure fluctuations in the burner to the extent that the flame blows out completely. These instabilities have been reported in swirl combustor and the prediction before the combustion begins is almost impossible [66]. One solution to this situation is to change the swirl to 100% for a few seconds and when the fluctuations are dampened, set it back to 50%. The reason behind this phenomenon might be the fact that there are no instabilities observed when the system starts and operates at 100% swirl. After the ethanol combustion period and when the temperature of the outer side of the combustion chamber flange is above 300 C, the two-way valve shown in Figure 3.8, is switched to let bio-oil/ethanol blend run through the fuel lines and the swirl is changed to 100%. Diagnostics follow the steady state combustion of bio-oil in the same order and the same timing for all the test; first the PM sampling takes place, then data are taken from FID and FTIR and lastly photographs and videos are taken using the borescope assembly. The combustion of bio-oil blend with the desired fuel to air ratio, usually becomes stable after 20 minutes, and PM sampling procedure starts at the 45 minute mark. Before the procedure begins, the ball valves on both the main and auxiliary sampling lines are shut off and the filter holders contain unofficial dummy filters. In order to warm up the lines to the desired temperature of C, the main line s ball valve is opened and exhaust sample runs through the dummy

67 53 filter. The sampling line is switched to the auxiliary line after 3-5 minutes before significant PM accumulation and water condensation on the filter cause huge pressure drop. At this time, a second dummy filter is placed in the filter holder of the main line. The second dummy filter allows the line to reach the temperature that no significant water condensation occurs. Now that the line is hot enough, the official filter replaces the second dummy. When the second dummy filter is exposed to exhaust, the operator has to find the flow rate that makes the static pressure difference between the sampling probe and the exhaust stream, shown on the manometer equal to zero. This flow rate satisfies the isokinetic sampling condition and is an approximate of the theoretical isokinetic flow rate of 10.4 % of the total exhaust flow rate (Appendix A). This condition has to always be satisfied during sampling while the oxygen sensor shows the correct equivalence ratio. The two official filters are consecutively positioned in the filter holder to take samples of PM loading and are placed into their Petri dishes and capped immediately after removal. During the official sampling, some parameters are recorded manually or automatically by the data acquisition system and are later used for calculation of emissions. These parameters include start/stop time of sampling on each filter, average dry gas temperature, sampling line rotameter value, the exhaust oxygen concentration and condenser output pressure after each 1 minute mark. When the PM sampling is done, the combustion air flow rate is adjusted and 5 minutes is allocated for the system to reach steady state again. After this time, during a 5 minute period before taking FTIR spectrum, the average amount of UHC is reported, and if a minor blow out occurs during this time, data gets filtered such that the values that are at saturation point of FID get discarded. The FTIR sampling begins at the hour mark and 5 spectrums are taken till minutes. The pressure and temperature of the FTIR gas cell remains constant during the sample for each test. The last instrument used for diagnostics is the borescope. Before opening the fitting to place the borescope in, the pressure in the combustion chamber is recorded in order to bring the system back at this pressure after insertion of the borescope to match the flame dynamics. The tip of the tube is inserted such that cooling air is guided into the combustion chamber, and meanwhile the stack fan and cooling air are ramped up slowly to avoid changing the chamber pressure. The tube is then fully inserted to take photos and videos and taken out after each photo. When all the data collection is completed, nozzle and fuel lines are cleaned by switching the fuel back to ethanol. Since the system is already at a very high temperature, the air preheater is turned

68 54 off, the swirl is set back to 50 % and the nozzle cooling water is set to maximum in order to avoid fuel boiling inside the nozzle. The system is shut down when the UHC reading reaches below 20ppm which is usually after 20 minutes of running ethanol. After letting the system to cool down, the deposits are scrubbed down from the nozzle tip and the combustion chamber. Fuel lines and nozzle holes are fully cleaned from any bio-oil contamination by running acetone through them. The PM sampling system is cleaned after each test by shooting compressed air backwards through the sampling lines. All the water traps are also emptied after each test.

69 55 4. Results and Discussion 4.1 Experimental Test Plan To study the effect of natural and accelerated aging on the combustion performance and emissions of bio-oil, a total of 14 official tests were carried out over a 10 months period. There are two separate batches used for this project, both of which provided by the same manufacturer and through the same production process. However, they possessed slightly different properties and characteristic. The first batch was divided into two portions, one being aged naturally and the other being aged in an accelerated fashion. Because of the limit in supply, the number of tests for each method of aging was limited to 3 plus a base point at time zero. This batch served two purposes: showing the effect of aging and validating the accelerated aging correlation. However, in order to produce more data points and observe over an extended time period the effect of aging, a second batch was ordered. Due to the time constraints of this project, the bio-oil from this batch only underwent accelerated aging. A total of 7 experiments with different aging times were performed with this second batch. Small burners usually experience some problems with stability of the combustion. In order to minimize this issue, all bio-oil batches were mixed with 15 % ethanol on a volumetric basis. The burner parameters were optimized for bio-oil blend combustion in a previous study [46]. This condition is called the base operating point which is presented in Table 4.1. The ranges that are reported for equivalence ratio, air preheat temperature and primary combustion air flow rate are due to the difference of basic properties in the two batches and also the limit in control accuracy of the system. However, the fluctuations in these parameters are minute among all tests. In most combustion applications, especially for heat and power generation, the power input is fixed. Therefore, in all the tests, the fuel flow rate is adjusted to provide a power input of 10 kw based on the heating value of the fuel. These changes in the flow rate of the bio-oil blend causes a change in the air to fuel flow rate ratio in the nozzle which translates into variation in SMD. In order to keep the SMD constant, the atomizing air flow rate needs to be modified for each experiment. However, experience with pure ethanol as well as literature suggest that the combustor flow field and mixing, flame stability and recirculation configuration is extremely

70 56 sensitive to the atomizing air [46]. Therefore, it was decided that a fixed atomizing air would be used for all tests, in order to avoid any significant variation in flow characteristics between tests. Table 4.1 Base point Operating Condition Swirl number 5.41 Power input from the blend 10 kw Pilot power input 0.5 kw Primary air preheater power 1.5 kw Primary air temperature at the swirl box C inlet Primary air flow rate SLPM Equivalence ratio Atomizing air flow rate 23.2 SLPM 4.2 Experimental Results Summary The fuel properties, calculated SMDs and emissions of the combustion tests under the base point are presented in Table 4.2 and Table 4.3. The two different sections in the table are dedicated to each batch and the labels show the number of months that the aging occurred and whether it was natural (N) or accelerated (A). For example, 2A6 is the label used for the bio-oil from the second batch that has underwent accelerated aging equivalent to 6 months of storage. The reason that NO x is presented in a separate table here is that the main mechanism responsible for emissions shown in Table 4.2 is incomplete combustion of bio-oil, while the mechanism behind NO x formation is not related to incomplete combustion of the fuel [45]. This mechanism is described in a separate section of this chapter.

71 57 Table 4.2 Basic Fuel Properties and Emissions of Aged Pure Bio-oil Batch Label Viscosity (cst) Solids (%mass) TGA Residue 1 SMD (µm) CO (ppm) UHC (ppm) CR (mg/mj) 1N N N A N A A N A A A A A This measurement is done on the 85/15 vol.% mixture of bio-oil and ethanol

72 58 Table NO x Emissions of the Aged Bio-oil Batch Label Fuel Nitrogen (mass%) NO x (ppm) 1N N N A N A A N A A A A A Mechanism of Pollution Formation from Bio-oil Combustion As described in section 2.6, there are four primary stages in droplet combustion of bio-oil. In the first stage, volatile gases evaporate from the surface of the droplet and produce a blue flame in a homogenous combustion. Some of the gaseous emissions are formed in this stage, if incomplete combustion occurs. When the volatiles tend to decrease, the flame extinguishes and only a char particle remains which mostly consists of non-evaporative HMW molecules and solids and ash particles. This char particle continues to burn in a solid state combustion mode, however char burning can be distinguished in lower burner section because of its low temperature. The temperature may even be so low that CO and UHC produced during char burnout cannot be oxidized. The ash and carbonaceous residue that are left after the residence time are considered

73 59 the PM emissions. Figure 4.1 demonstrates the emissions produced at each stage of bio-oil combustion. Figure 4.1 Pollutant Formation Mechanisms of Bio-oil Combustion [22] High energy volatile materials, like alcohols, evaporate quickly and burn rapidly such that complete combustion can be achieved. This fact can be used to reduce the gaseous emission such as CO and UHC, by increasing the volatile content of bio-oil. These emissions are also formed during the heterogeneous combustion of the char particle. Therefore, avoiding the formation of char particles by reducing the solids content and HMW fraction of bio-oil, leads to a reduction in CO and UHC emissions. Ash is mostly present in the solids content, and acts as catalyst for char gasification, hence aiding the formation of CO and UHC in the heterogeneous combustion. The particulate matter emissions mainly consist of CR and ash. Moloodi et al. concluded that despite the complicated chemical composition of bio-oil, CR emission can be predicted by the solids content and TGA residue of the fuel. Equation 4.1 demonstrates this linear correlation [22]. As mentioned in section 3.2.3, TGA residue is a measure of the char formation potential of the fuel and consists of HMW molecules, ash and solids. However, since the mass fraction of HMW molecules is much larger than that of solids and ash, TGA residue can be treated as a variable independent of solids and ash content.

74 60 ( ( ) ) ( ) ( ) Equation 4.1 [22] In order to do this linear regression analysis, the assumption was made that both of the independent variables, solids content and TGA residue, vary in a small range. This analysis sheds light on the effectiveness of each parameter in CR formation. The coefficient for TGA residue is an order of magnitude larger than that of solids content, which means for small burners, reducing TGA residue can help a lot in terms of reduction in PM emissions. Another importance of this analysis is that an estimate of the CR emissions can be predicted without having to perform an actual combustion test and by only doing a TGA, which is a much easier test. 4.4 Analysis on the First Batch: Natural vs. Accelerated Aging Fuel Properties The fuel specification sheet from the producer of first batch reported a water content of 26.5 wt% based on ASTM E203 test method and an ash content of 0.24 wt% based on ASTM D482 test method. The other properties that follow were outsourced to Alberta Innovates-Technology Futures. The carbon, hydrogen, nitrogen and oxygen mass fraction of the fuel are 40.78, 7.67, 0.12 and 51.43%, respectively. The gross heat of combustion measured at 25 C was reported to be MJ/kg. The atom balance of the fuel was assumed to remain constant with aging while the viscosity, solids content and TGA residue were measured after each aging period. All the results presented in this section except the TGA residue data are for pure bio-oil. Figure 4.2 and Figure 4.3 demonstrate the viscosity of the fuel calculated with the handheld viscometer and measured according to ASTM D445 at 40 C, respectively over the aging period. The values in Figure 4.2 and Figure 4.3 are very different because the viscosity that is calculated from the handheld viscometer uses a correlation that was initially meant for water. Therefore, these calculated values are not accurate and are just used to check the trend of viscosity before each combustion test. As discussed in section 2.4.2, the viscosity is expected to increase with time of storage,

75 61 mainly due to the polymerization reactions that take place among the larger molecules of the fuel; the same upward trend is observed in this study. The close values of natural and accelerated aging are noteworthy which validates that the accelerated aging which was based on the viscosity increase can actually imitate the natural aging process Calculated Viscosity (cst) Aging Time (Months) Natural Accelerated Figure 4.2 Calculated Viscosity of Batch 1 Pure Bio-oil (via handheld viscometer) over Aging Time The dip that occurs from time zero to 3 months might be due to an experimental error. One reason could be that the measurement might have been done when the core temperature of the liquid has not yet reached the room temperature Measured Viscosity (cst) Aging Time (Months) Natural Accelerated

76 Solids Content (% mass) 62 Figure 4.3 Measured Viscosity of Batch 1 Pure Bio-Oil (based on ASTM D445) over aging time The trend is evident here as well, however the increase in viscosity is very slow over the first 6 months and from this time forth, the rate of increase starts to climb. The solids content of the bio-oil has shown an increase over time as well. This is mainly due to the polymerization reactions discussed in section and agglomeration of HMW molecules into solid particles. The solids were measured through a non-conventional method (MeOH DCM insoluble) where the fuel is dissolved in methanol and the fraction that is not soluble is the solids content. Figure 4.4 demonstrates the increase in solids content for both naturally and accelerated aged samples over the aging period Aging Time (Months) Natural Accelerated Figure 4.4 Solids Content of Batch 1 Pure Bio-oil vs. Aging Time The TGA residue is described as a measure of the tendency of the fuel to evaporate in section and the effect of aging on fuel s volatility is explained in section It is expected that TGA residue increases over time mainly because of the polymerization reactions that take place in the fuel over time and increase the HMW molecular fraction of the fuel. In addition, the gas forming reactions mentioned in section cause the volatiles to exit the liquid fuel and accumulate on top of the bottle and escape as soon as the cap is opened. Figure 4.5 demonstrates the effect of aging on the TGA residue of 85/15 volumetric bio-oil/ethanol blends.

77 TGA Residue (% mass) Aging Time (Months) Natural Accelerated Figure 4.5 Effect of Aging on the first batch blends TGA residue As can be seen in all these graphs, the effect of aging on physical properties start to kick in some time after the first three months of aging, and for the two years of aging case, the increase is substantial suggesting a somewhat exponential relationship between time of storage and the increase in values of each physical property Gaseous Emissions The main gaseous emissions that were considered in this aging study were CO & UHC that were caused by incomplete combustion of the fuel spray. The other important gaseous emission is NO x which is discussed in section 4.5. Figure 4.6 shows the effect of aging on CO emissions for this batch. There is almost no change in the first 3 months corresponding to the small changes in physical properties over this period, but the major increase occurs after this time through the 9 months data point. However, the value for the 2 year accelerated emission is not following the trend and is actually lower that the 6 months value.

78 UHC (ppm) CO (ppm) Aging Time (Months) Accelerated Natural Figure 4.6 CO emissions for Batch 1 Blends vs. Aging Time Previous studies have shown that CO and UHC trends follow each other closely [46], [22]. Error! Reference source not found. confirms this observation, where a slight increase is etected for the first 3 months and after that the jumps in values for 6 and 9 months are observed, followed by the decline for the 24 months Aging Time (Months) Natural Accelerated Figure UHC emissions for Batch 1 Blends vs. Aging Time

79 65 The behavior that is seen for 2 years of aging can be explained in terms of the PM emissions of the fuel which is discussed in section The fuel quality and the combustion performance is so poor that the fuel droplets cannot be burned fully in the first stage of combustion and the abundance of the char particles that are produced after this stage, don t have adequate residence time to burn the carbon fraction in a heterogeneous combustion. Therefore, as aging progresses more of the fuel is not even burnt to produce any gaseous emissions and more of the fuel just escapes in the form of particulate matter. The close values of gaseous emissions of naturally and accelerated aged bio-oil suggests that the accelerated aging method produces a very similar liquid to the one that is naturally aged. Therefore, in order to save time for a study, natural aging can be substituted with the heating of bio-oil at a higher temperature and the corresponding calculated time base on the correlation PM Emissions Figure 4.8 shows the effect of aging on carbonaceous residue emissions of the bio-oil blends. It is observed that the PM emissions go up as more time is allocated for storage. This is attributed to the loss of volatiles during aging that makes the combustion of the fuel more difficult because the high energy volatile material that help stabilize and enhance the combustion are decreasing in volume over time. Also, the polymerization reactions increase the HMW molecules fraction of the fuel which makes the droplets denser and thus heavier. These denser droplets are more difficult to penetrate with the free radicals for complete combustion. For the purpose of this study, because the energy throughput of the burner is fixed to 10 kw for all tests, the carbonaceous residue emission of combustion of bio-oil blends is normalized by the energy of the fuel that goes through the system and is presented in units of mg/mj.

80 CR (mg/mj) Aging Time (Months) Natural Accelerated Figure 4.8 CR Emissions of Batch 1 Blends over the Aging Period The inconsistency of the 3 months data might be because of a small instability and partial blowout during the PM sampling period. There are many parameters involved in the combustion of bio-oil blends in the confined geometry of the burner, some of which are not controllable with the current setup. One example is the pressure build-up in the combustion chamber which extinguishes the flame momentarily. The very high value of CR for 24 months of aging can be explained with Equation 4.1. Both the TGA residue and the solids content of the 2 years accelerated aged bio-oil are much higher compared the data for earlier times. The TGA residue has jumped 150% compared to base point and solids content has increased 81%. As mentioned before, the effect of TGA residue is an order of magnitude higher than solids content and the substantial increase in CR emissions is not surprising. Figure 4.9 compares the CR values obtained from experiments with the values predicted by Equation 4.1. Although the values are not matching closely, the trend is captured well by the equation. The main reason is that the linear regression was performed on a set of data that was obtained from a different batch with different fuel properties.

81 CR (mg/kg fuel) Experiment Natural Experiment Accelerated Calculated Natural Calculated Accelerated Aging Time (Months) Figure 4.9 Comparison of Calculated and Measured CR for Batch 1 Blends Flame Visualization Photographs taken during each combustion test, using the borescope assembly are presented in Figure The combustion performance can be examined visually in these pictures. As the time of aging increase, the flame quality becomes poorer and more glowing particles burning is detected. 1N0 1N3 1N6

82 68 1A6 1N9 1A9 1A24 Figure 4.10 Borescopic Photos of batch 1 bio-oil blend combustion 4.5 Analysis on the Second Batch: Long Term Accelerated Aging Fuel Properties After confirming that the method of accelerated aging produces a similar liquid to natural aging in terms of physical properties and combustion performance, and in order to fill in the gap between 9 months and 24 months of aging, a second batch was ordered. Therefore batch 2 was provided at a later time than batch 1 and although it was produced in the same pyrolysis plant as the first batch, it had different physical characteristics. The manufacturer reported the water content and ash content to be 23 wt% and 0.26 wt% respectively. However the solids content is much higher than the previous batch at the value of 1.9 wt%. The Carbon, Hydrogen, Nitrogen

83 Calculated Viscosity (cst) 69 and Oxygen mass fractions are 43.45, 7.38, 0.27 and 48.90%. The gross of heat of combustion is reported as 18.2 MJ/kg which is higher compared to the first batch; therefore a smaller fuel flow rate is required for the operation of the system at 10 kw. Figure 4.11 shows that the viscosity of the second batch bio-oil calculated with the time data obtained from the handheld viscometer, increases with aging time. There is no data point presented for 2 years of aging in this graph, because the liquid formed solid gel-like chunks that would restrict the flow through the handheld viscometer orifice. The 1 year aging was first produced and a full set of physical and emissions measurement was done on it, however the results did not seem to be correct. All other tests were done after that to make sure that this data point is the one that needs to be repeated. When confirmed, a new bottle of bio-oil was taken out of the fridge and the complete aging and testing process was repeated. The discrepancy of the first point must have been due to the aging process and some malfunctioning of the oven over night. Another point that can be seen in this graph is that the rate of viscosity increases in the first 9 months of aging and reduces after that Aging Time (Months) Accelerated Figure Calculated Viscosity of Batch 2 (via handheld viscometer) over Aging Time The measured viscosity via ASTM D445 test method of the fuel after each aging period is presented in Figure The viscosity measurement after 24 month of aging was possible with this method. The same trend is seen here as well, the slope of the line segments between each consecutive data points decrease after the 6 months. This means that as time passes by, the viscosity increases more slowly. The same is trend is seen in Figure 2.8 [37]. The same issue

84 Measured Viscosity (cst) 70 discussed for the 1 year of aging shows up here as well. An inconsistent dip was suspicious and proved to be an error in the aging process of the fuel Aging Time (Months) Accelerated Figure Measured Viscosity of Batch 2 (based on ASTM D445) over Aging Time The solids content of this new batch was almost 2 orders of magnitude larger than the first batch. The changes made to the amount of solids over the aging period are demonstrated in Figure The abundance of solids accelerates the polymerization reactions and enhances the agglomeration of the particle to an extent that visual inspection of the 24 months aged bio-oil concluded the presence of gel-like solid chunks in the liquid. The solid content was also visibly flowing in the fuel lines during the tests. The solids content of batch 2 bio-oil increases steadily with the aging time and the trend can almost be approximated by a linear function.

85 TGA Residue (%mass) Solids Content (%mass) Aging Time (Months) Accelerated Figure Solids Content of Batch 1 vs. Aging Time TGA residue of this batch shows an upward trend over time as well; however the value at each point of time is less compared to batch 1. The rate of increase in TGA residue shows a different behavior than viscosity over the course of aging time for this fuel. The increase starts slower and picks up after a year of aging, with the biggest jump being for 18 months to 24 months. These results are presented in Figure Aging Time (Months) Accelerated

86 72 Figure Effect of Aging on the second batch blends TGA residue The addition of ethanol before conducting the TG analysis broke apart the gel-like solids that were present after 24 months of aging and there was no sign of them after pouring the blend into another container. However, as the very high TGA residue value suggests, the polymerization of the HMW weight molecules have made the evaporation of them much more difficult Gaseous Emissions Figure 4.15 presents the CO emissions of the combustion of bio-oil blend as the aging time is varied. As the fuel quality and evaporation decreases over time, complete combustion of the volatiles in the first stage of combustion and burning of the char in the third stage become more challenging. This results in increasing CO emissions over the aging period. The discrepancy between the base and 6 months points can be discussed. One possibility is that the value of CO for the base point is overestimated and the other option is that the 6 month might be underestimated. If the fuel properties are taken into consideration, all three show an increase over this period. However, the slope of the line from base to 6 months is less than that of the line from 6 to 9 months of aging. Based on previous studies performed on the effect of fuel properties on combustion, we can conclude that the base point data is an off point and has to be slightly lower than the 6 months data point [22]. The reason for this could be explained in terms of partial blow-outs that occur in the burner from time to time during the test. When a blow-out occurs fuel is sprayed on the walls as well without undergoing combustion. This deposited fuel starts to gasify in the very hot environment of the combustion chamber and emits carbon monoxide as one the gasification products. Another reason could be that the only possible way for having an underestimated CO value, considering the sampling method and 5 consecutive spectrums is that the sampling lines are not hot enough such that condensation occurs in the FTIR. However, this was not the case for the 6 months experiment, and all the temperatures looked normal during the FTIR sampling period.

87 CO (ppm) Aging Time (months) Accelerated Figure CO emissions for Batch 2 vs. Aging Time Also, if we take a look at the effect of aging on the UHC emissions for the second bio-oil batch, which is demonstrated in Figure 4.16, and keeping in mind that UHC and CO emissions usually follow each other, we can confirm that the base data point is overestimated in these results. The increasing trend can be seen here as well, with the rate of emission increase becoming larger after 12 months of aging UHC (ppm) Aging Time (Months) Accelerated PM Emissions Figure CO emissions for Batch 2 vs. Aging Time The solids content and TGA residue of this fuel suggest that an increasing CR emissions trend should be expected based on the prediction of Equation 4.1. Figure 4.17 demonstrates that this

88 CR (mg/mj) 74 expectation is reasonable and in fact the CR emission for the second batch shows an increase with the aging time. Also the trend in rate of increase in CR follows the same trend as the rate of increase in TGA residue, where the slope between the successive points becomes larger after the first year of aging. When compared to the first batch, the TGA residue for both of them is in the same range but the solids content of batch 2 is almost two orders of magnitude larger than that of batch 1. This is the reason fo the higher values of CR emission of batch 2. However, the effect of solids on the CR emission is much less than the TGA residue and this explains why the increase in CR values between the two batches is not as high as the jump in the solids content. The main driving force for repeating the 12 months data point was the huge dip in value from the previous experiment which was the 6 months of aging. After that, when the 9 months experiment was conducted and showed a higher value than both 6 months and 12 months, it was confirmed that not only the aging process of the one year point had some issues because of discrepancies in physical properties values, but also the PM sampling of this point was faulty because it provided the same value as the base point in spite of the increase in TGA residue Time (months) Accelerated Figure CR Emissions of Batch 2 Blends over the Aging Period Figure 4.18 show a comparison between the calculated CR using Equation 4.1 and the measured CR during experiments. Although the values do not match, the same trend is seen in both sets of data. The difference in values between them is mainly because of linear regression coefficients which were optimized for another batch of fuel with different properties.

89 CR (mg/kg fuel) Experiment Accelerated Calculated Accelerated Aging Time (Months) Figure Comparison of Calculated and Measured CR for Batch 2 Blends Flame Visualization The qualitative comparison between the flames is carried out visually by taking photographs during the combustion test and inspection the changes in flame dynamics that contribute to the emissions. Figure 4.19 presents these pictures in the chronological order. As the time of aging is increased, the combustion quality of the flame is lowered and as can be seen in the pictures, more PM char burnout occurs which is identified as the glowing particles.

90 76 2A0 2A6 2A9 2A12 2A18 2A24 Figure 4.19 Borescopic Photos of batch 2 bio-oil blends combustion 4.6 NO x Emissions NO x is the sum of nitrogen oxide and nitrogen dioxide in the exhaust. There are three major mechanisms of NO x formation that are dependent on operating conditions and fuel chemistry: prompt, thermal and fuel NO x [45]. Prompt NO x formation is described as the scission of the nitrogen molecules in air via their reactions with free radicals of hydrocarbons that are mostly present in flame fronts. This mechanism usually has a minor contribution to the overall NO x. The thermal NO x formation is very temperature sensitive and occurs at high temperatures. It is mainly due to the reactions of air nitrogen with oxygen and OH radicals. Fuel NO x forms from fuel bound nitrogen during combustion and is mainly controlled by the stoichiometry. The formation of NO x is weakly dependent on temperature in this mechanism. The literature that has studied the NO x formation in bio-oil combustion has shown that the dominant mechanism of NO x formation is fuel NO x [46], [67]. Figure 4.20 shows the effect of aging on NO x for the first batch. A slight decreasing trend is seen for both cases.

91 NO x (ppm) Aging Time (months) Natural Accelerated Figure 4.20 NOx emissions of Batch 1 Blends over Aging Period This trend is more obvious in the NO x emissions of the second batch. Figure 4.21 presents the NO x data for this batch as the time of aging is increased. The best fit line shown on the graph has a slight negative slope, which can be described in terms of CO and UHC emissions. It has been shown in previous section that as the aging time is increased, these emissions go up because of lower burning quality and heavier incomplete combustion. These phenomena cause the flame temperature and as a result the burner temperature to decrease. As described earlier, thermal NO x is formed at high temperatures and as the temperature is lowered, this value decrease too. However, the dominance of thermal NO x formations is less than the fuel NO x formation and therefore, the decrease in value is much less than the total value. In addition, another reason for this decrease could be the increase in PM emission. As more PM is produced, more of the fuel escapes without fully oxidizing. Therefore, more fuel nitrogen which is still bound to the PM does not burn in order to form any NO x.

92 NO x (ppm) y = x Aging Time (Months) Accelerated Figure NOx emissions of batch 2 over Aging Period 4.7 Acetaldehyde, Formaldehyde and Methane Emissions The emissions of acetaldehyde, formaldehyde and methane for most of the aged bio-oil blends were below detection (BD) limits of the FTIR. These emissions are mainly the product of poor and unstable combustion conditions. Therefore, they are detected when the UHC values are high. Table 4.4 lists the values for these emissions. Table 4.4 Emissions for Selected Blends Batch Label CH 4 (ppm) CH 2 O (ppm) C 2 H 4 O (ppm) 1N9 BD 16 BD 1A9 BD 13 BD 1A A12 BD A A

93 79 5. Conclusions and Recommendations 5.1 Conclusions The aging process is very sensitive to temperature and the kinetics of the chemistry is accelerated as the liquid temperature increases. The results show that the fuel quality of bio-oil becomes poorer as it is stored for longer periods of time. Viscosity, which is a crucial factor in atomization quality, increases with time. Also, solids content and TGA residue of the fuel, which is a measure of fuel polymerization, show an increase with aging. The amount of particulate carbonaceous residue was calculated. There is an upward trend in CR emission with aging time which follows the shape of the graph of TGA residue. In addition, the batch with much higher solids content produced more CR emission while the TGA residues were close but still the dominating factor. The trend was predicted by an equation relating CR emissions with solids content and TGA residue. This suggests that aging increases the solids content and polymerization (TGA residue) of the fuel. As a result, there is more char formation during burnout which causes more CR emissions. The major gaseous emissions considered in this study are CO and UHC. They follow each other s increasing trend and reach a higher rate of increase as more time passes. They also follow the increasing trend of CR emissions. The results show a slight decrease in NO x emission over the aging period. This is due to both thermal NO x formation mechanism and the amount of fuel nitrogen trapped in the PM emission. The higher CO and UHC emissions prove a more incomplete combustion with lower temperature inside the burner. Thermal NO x is decreased when the temperature is lowered. Moreover, NOx formation in bio-oil combustion is dominated by fuel bound nitrogen and as more PM emissions is formed, less fuel nitrogen is available for combustion and thus the decreasing trend is observed. To summarize, aging has a detrimental effect on the fuel quality of bio-oil which results in higher combustion emissions. Therefore, a few solutions can be sought to minimize this effect. The bio-oil should be kept in a cool place above the freezing point in order to delay the aging process. Bio-oil should also be stored in sealed containers in order to avoid any air exposure.

94 80 Leaving the fuel in an open container, would let the high energy volatiles escape and also accelerate the polymerization reactions. Another effective approach is to add a solvent such as ethanol, as soon as the bio-oil is produced. The solvent retards the aging reactions significantly and also improves the quality of the fuel for combustion. However, the best way to handle a problem is to eliminate it. The best option is to use bio-oil as a renewable source to produce heat and power, near the production plant and with the minimum storage time required. 5.2 Recommendations 1. In order to improve the combustion efficiency and reduce THC and CO emissions, a refractory lining could be added to the burner system. The insulation would mitigate the heat losses through walls and bring their temperatures higher such that wall quenching effects are minimized. 2. If the bio-oil is burned under rich conditions, fuel bound nitrogen is more favored to convert to N 2 [45]. Therefore, if this could happen in the first stage of staged combustion, NO x reduction could be achieved. More air would be provided for the second stage to reach complete combustion. 3. The axial momentum of the atomizing air jets is so high that it penetrates through the central recirculation zone and negatively impacts the stability of combustion. Therefore, designing a nozzle that can atomize bio-oil at very low flow rates of atomizing air would be beneficial. The best option for such a design would output an axisymmetric hollowcone spray pattern. 4. In the present setup, FID and FTIR sampling cannot be performed simultaneously. The connections used for the heated line can be replaced by a T-section to enable these two systems to draw from the sampling line at the same time. 5.3 Future Works 1. Atomization quality has a great impact on combustion performance and emissions. Droplet size and velocity measurements would provide insight into interpretation of the results.

95 81 2. Another parameter that has a significant effect on emission trends is the complicated flow pattern in the combustion chamber. Details of velocity vector field in the burner could be measured which would shed light on the flame dynamics and central recirculation zone. 3. In order to reduce the consumption of fossil fuels such as diesel and No. 4 fuel oil and replace it with renewable energy source such as bio-oil, the first step could be to use a blend of them in small scale combined heat and power units. The blending would be achieved by mechanical or chemical emulsification of the two fuels. Blends with different ratios of diesel and bio-oil could be burned in the burner and emissions would be analyzed to choose the best mixture. 4. The effect of different light intensity exposure to bio-oil during storage time can be investigated by comparing the emission trends of combustion in this spray burner. 5. Heating rate of the accelerated aging method could be varied and the results of the combustion tests could be examined to analyze the impact of different heating rates.

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102 88 Appendix A Theoretical Isokinetic Sampling System Calibration When the sampling flow rate and theoretical isokinetic flow rate match each other at ΔP = 0 by varying the depth of both static pressure taps, the system is calibrated. Therefore, the theoretical flow rate is needed to be calculated first. Within the main duct, the Reynolds number may be estimated using the properties of air at 250 C (based on prior measurements at the probe cross section during combustion tests) and a total exhaust mass flow rate of 6.06 g/sec (under average base operating conditions). This corresponds to a Reynolds number of approximately 7400 and an average main duct velocity of 8 m/s, verifying that the flow in the main duct is turbulent. The probe is also placed about 10 diameters downstream of the elbow, which is enough distance that hydrodynamic entry length has insignificant effect on turbulent flow. Therefore, the following velocity profile may be used to describe the duct velocity [58]: ( ) (A.1) Here, v max is the maximum centerline velocity, y is the coordinate distance from the wall and R is the inner radius of the main duct. Figure A.1 shows this theoretical velocity profile in relation to the straight pipe section and sampling probe. Figure A.1 Velocity profile for the exhaust flow In order to find the total exhaust flow rate, equation (A.1) should be integrated across the duct radius (R): (A.2) ( )

103 89 The flow through a stream tube with diameter size of sampling probe (r) can be derived from the velocity profile: (A.3) ( ) The ratio of sampled over total gas can be readily found to be: (A.4) This ratio only depends on geometry and is independent of velocity (total flow rate). For the current geometry, this number is 10.4 % and represents the fraction of the total flow that should be drawn through the sampling probe in order to maintain isokinetic conditions. Knowing this flow rate, the system was calibrated during a 100 % pure ethanol combustion test under well controlled conditions. While repositioning the static pressure taps to show ΔP = 0, sampling flow rate was kept at 10.4 % of the total exhaust flow rate using a needle valve and a calibrated rotameter. The total exhaust flow rate was calculated based on the output of the oxygen sensor and knowing the mass flow rate of fuel, assuming complete combustion.

104 90 Appendix B Liquid and Gaseous Flow Calibration Fuel Flow Calibration 1. Start the pump at some initial speed (RPM 1 ) 2. Using the graduate cylinder and a stopwatch, measure the changes in volume (ΔV) and time (Δt) 3. Calculate the fuel flow: 4. Linear interpolation gives the pump speed that provides the desired flow rate 5. Verify the new flow rate and repeat if necessary

105 91 Gas Rotameter Calibration 1. Run the cooled gas through the system and record both ball value and inlet pressure during operation 2. When the digital flow meter is hooked up, set the pressure to the recorded value and adjust the needle valve to achieve the same ball reading as recorded

106 92 Appendix C FTIR Calibration Validation During the period that experiments were carried out, the Helium-Neon laser in the FTIR collapsed and needed to be replaced. In order to check if the calibration method developed by Farra [57] was shifted, some standard gas mixtures were prepared and tested in the FTIR. The resulting spectrums were paired and compared in the regions that showed each gas. In addition, each pair of spectra was quantified using the calibration method in order to check for any major discrepancies. Figure C.1 shows two spectrums of 1500 ppm carbon monoxide, one taken before and the other after the replacement of the He-Ne laser. It has been zoomed to show the region of wavelength that corresponds to the CO emission. Figure C ppm CO spectrums before and after changing the FTIR He-Ne laser The close match of the two spectra demonstrated that the calibration has not shifted. Also when the two spectra are quantified with the calibration method, the one for before changing the laser reads 1489 ppm and the one for after show 1465ppm CO. Therefore, calibration is validated for further use in this study.

107 93 Appendix D Example of the TGA Curve Figure D.1 TGA curve for bio-oil blend 1N3

108 94 Appendix E Data Acquisition System Two separate data acquisition cards were used in this project. First one was a National Instrument model NI USB-9213 for logging the temperatures from the thermocouples and the other was a National Instrument model NI USB-6229 BNC for recording the DC voltage signals. The temperature that were recorded are at the swirl box inlet, fuel in the nozzle, the nozzle sheath, port flange, exhaust flange, main exhaust heat exchanger outlet, main heat exchanger cooling water inlet and outlet, swirl air direct, PM line gas before and after the PM heat exchanger and nozzle cooling water outlet. The DC voltages that were recorded include oxygen sensor and FID outputs. The frequency of data acquisition was 1 Hz. Figure E.1 shows a snap shot of the front panel of the Labview program just before a combustion test. Figure E.2 demonstrates the logged temperatures during the testing of the 24 months old bio-oil blend from the first batch, while Figure E.3 presents the DC voltage signals during the same experiment.

109 Figure E.1 Front View of the Labview Program 95