Renewable Hydrocarbon Fuels from Catalytic Pyrolysis of Lignocellulosic Biomass

Transcription

1 Renewable Hydrocarbon Fuels from Catalytic Pyrolysis of Lignocellulosic Biomass Recipient Organization: NCSI and AU Principal Investigator: Dr. Sunkyu Park and Dr. Sushil Adhikari Project Location: NCSU and AU Reporting Period: July 1, 2012 September 30, 2012 Date of Report: November 1, 2012 Written by: Dr. Sunkyu Park and Dr. Sushil Adhikari 1. Planned Activities: a. Characterize three torrefied biomass feedstocks (pine wood, switch grass, and sweet gum) b. Determine energy requirement for torrefaction at different moisture levels. 2. Actual Accomplishments: 2.1 Research Methods a. Biomass Preparation Loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) samples were obtained from the Mary Olive Thomas Research Forest (Auburn, Alabama) of Auburn University. The whole trees were chipped with a Morbark brush chipper (Model Beever M12R, Winn, Michigan) and air dried in an open area to a moisture content of 10-15% (w.b.). The chips were then fractionated to remove large (>30 mm) and small chips (<3.00 mm) with the use of a Kason vibrating screen separator (Kason Corporation, Millburn, New Jersey) before the thermal treatment. On the other hand, switchgrass (Panicum virgatum) was acquired from the E.V. Smith Research Center (Shorter, Alabama) of Auburn University. It was then chopped with a heavy- duty cutting mill (Retsch, Model SM2000, Haan, Germany) fitted with a 6 mm screen and the small particles (<0.707 mm) were removed using Kason separator prior to the thermal treatment. The selected samples for the treatment were analyzed for bulk density and particle size distribution. For other analyses, samples were further ground using hammer mill (C.S. Bell Company, Tiffin, Ohio) or Retsch heavy- duty cutting mill (Model SM2000, Haan, Germany) and then a Wiley Mill (Thomas Scientific, Model L10, Swedesboro, New Jersey) fitted with a 40 sieve screen (0.42 mm). b. Treatments Each of the three biomass types were exposed at three temperatures (225, 250 and 275 C) and three times (15, 30, and 45 min) for a total of 9 treatments for each biomass 35

2 type. There were 27 treatments total with 3 samples per treatment, and treatments were randomized. Preliminary tests were performed using a closed, batch reactor, but the torrefied product showed uneven treatment among the particles. It is for this reason that the treatments were conducted in open- top aluminum pans using a batch process in a furnace (Thermo Scientific, Model BF51842PFMC- 1, Asheville, North Carolina), as shown in Figure 1, to achieve better heat transfer through the particle bed. A thermocouple wire connected to a data logger (Omega, Model HH147U, Stamford, CT) was inserted in through the vent of the furnace and into the bed of one of the samples to be treated. The temperature of the biomass was recorded every five seconds for the length of the treatment. Volatiles from the furnace were swept away by a centrifugal fan. Before and after each treatment, the biomass samples were weighed using a balance (A & D Company, Model GX- 4000, San Jose, California), and the moisture content was determined using a moisture analyzer (Ohaus, Model MB45,, Parsippany, New Jersey). The mass of sample used for torrefaction was based on its density and volume of the pan used (approximately 0.3 L). Initial mass of samples were about 65 and 45 g for woody biomasses (pine and sweetgum) and switchgrass, respectively. The small amount of sample was used to prevent uneven heating of the sample bed during treatments. Figure 1. Thermo Scientific furnace (on the left) and treatment pans After the samples were placed inside the furnace chamber (56.6 liters (2 ft 3 )), it was purged with nitrogen gas at the maximum flow rate of the furnace flow meter of 4.7 L/min (10 ft³/h) for 12 min. The flow rate was then decreased to 0.47 L/min (1 ft³/hr), and the furnace was preheated to the desired temperature set point at a heating rate of approximately 22 C/min. Treatment time was said to begin when the furnace reached the desired set point. At the end of the treatment time, the biomass samples were pulled from the furnace and immediately placed in desiccators to prevent further treatment and combustion. Once the pans cooled to room temperature, the samples were weighed and moisture analysis was performed to determine solids retained in each sample. After bulk density of the treated samples was determined, each sample was size reduced with a conventional coffee grinder (Hamilton Beach, Model 80335, Southern Pines, NC) for further characterization. 36

3 c. Biomass Characterization Particle size of the raw samples was determined using a dynamic image analyzer (Retsch, Camsizer, Haan, Germany). Approximately 200 g of each biomass sample was used for the analysis. Bulk density of the samples was measured by filling a 100 ml graduated cylinder with known amount of sample. Equilibrium moisture content was found by placing the biomass inside an insulated environmental chamber with specific temperature and humidity conditions controlled by a conditioner (Parameter Generation and Control, Inc. Model , Black Mountain, North Carolina). The chamber was kept constant at 25 C and 90% relative humidity. Initial moisture content of each sample was measured with an Ohaus moisture analyzer. The samples were put in aluminum pans, and initial mass of each sample was measured with a balance. The samples were placed in the chamber, and the weight of the sample was measured every hour for the first 5 h, then at 24 and 48 h. The change in weight was used to calculate moisture contents for each time. Proximate analysis of the products included moisture, ash, volatile matter, and fixed carbon. Moisture content of each sample was found using an Ohaus moisture analyzer. Ash content was found according to NREL/TP (NREL, 2005). Volatile matter was determined at Hazen Research Laboratory (Golden, Colorado) using ASTM (ASTM, 2011). Fixed carbon (FC) of the samples was calculated indirectly from ash and volatile matter contents. The higher heating value of the biomass was measured using a bomb calorimeter (IKA Works Inc. Model C200, Wilmington, NC). Elemental analysis of the biomass (C, H, and N) was measured with an elemental analyzer (Perkin Elmer Model 2400 CHNS/O, Waltham, MA). Each test was performed in triplicate, and the average values are reported on dry basis unless otherwise stated. d. Differential Scanning Calorimetry The energy required to heat the biomass from ambient temperature to torrefaction temperature (225 o C) at different moisture contents was found using a differential scanning calorimeter (DSC) (TA Instruments Model Q200, New Castle, Delaware). A known amount of samples (~6-7 mg, particle size mm) were heated from 25 C to 225 C at a heating rate of 20 C/min, and nitrogen was used as a carrier gas. Energy requirement for biomass torrefaction was calculated using Equation 1. Q m! =!! m! c! dt dt + m!h! m! 37 dt (1) where Q is the energy requirement for biomass pyrolysis, m 0 is the initial sample mass, t is the run time, m i is the instantaneous mass, c p is the specific heat of the sample, dt/dt is the heating rate, and H! is the reaction heat flow of the biomass. Calorimetry was performed on each biomass type at four moisture contents, and each test was performed in triplicate. Specific moisture contents for the biomass types were achieved by sealing the untreated biomass samples inside desiccators containing saturated salt solutions of lithium chloride (LiCl), magnesium chloride (MgCl 2 ), sodium

4 chloride (NaCl), and potassium nitrate (KNO 3 ) prepared using 50 ml water. Aluminum pans containing raw biomass samples (5 g) were placed above the solutions on an aluminum grate inside each desiccator. The desiccators were placed inside an environmental chamber (ESPEC, Model ESL- 2CA, Hudsonville, Michigan) in order to maintain a constant temperature of 25 C which stabilized the relative humidity and moisture content. The saturated salt solutions provided different relative humidity (LiCl 11%, MgCl 2 33%, NaCl 75%, KNO 3 94%) to achieve different moisture contents (Labuza, 1984). The final (actual) moisture content of each biomass sample was measured using an Ohaus moisture analyzer. e. Data Analysis Microsoft Excel (MS Office 2010, Redmond, WA) was used for data analysis and visuals. Statistical analysis was performed with SAS software (Version 9.2, Cary, NC) and JMP (Version 10, Cary, NC). Analysis was performed on the biomass types (pine, sweetgum, and switchgrass) separately because of their different natures. The switchgrass bulk density was much lower than that of the woody biomass and, as a result, the initial masses of the samples being treated were different. Since the mass to be treated was different, the results were expected to be different. 2.2 Results and Discussion a. Particle Size Figure 2 shows the size distribution of the raw samples. The size range for the untreated woody biomass was between 3 to 28 mm and 1 to 6 mm for switchgrass. 16 Pine 12 Solid retained (%) Size (mm) Figure 2. Size distribution of biomass prior to treatment. b. Treatments Figure 3 shows an actual temperature profile of each biomass type during the 15 and 30 minutes exposure at 225 C. The woody biomass never reached the exposed temperature during any treatment, and the same could be said about switchgrass for treatments of 30 minutes or less with the exception of treatment at 275 C. This would indicate that torrefaction could be performed successfully on all biomass types at even lower temperatures. For all the 45 minutes treatments and the 275 C treatment for 30 38

5 minutes, the temperature of the switchgrass actually rose above the exposure temperature of the furnace in the final stages of treatment, which would indicate an exothermic reaction. A pyrolysis process for a lignocellulosic material is a combination of endothermic and exothermic reactions, and the exothermic reactions typically happen during the final 40% weight loss which represents lignin decomposition (Park, 2008). Temperature ( C) Temperature ( C) Time (min) Figure 3. Sample temperature for torrefaction at 225 C (a) 15 minutes and (b) 30 minutes c. Solid Yield Table 1 shows the total percent solids retained for each set of treatment parameters. The range of values was %, %, and % for pine, sweetgum, and switchgrass, respectively. The open top pans promoted more volatilization which resulted in lower solid yield in this study compared to studies where a closed reactor design was employed (Pimchuai et al., 2010; Prins et al., 2006). Biomass is composed of three major components: hemicellulose, cellulose, and lignin. During torrefaction, biomass undergoes a change in mass which is a result of volatilization of part of the biomass. Hemicellulose is the most volatile portion of biomass, followed by cellulose, and finally lignin. Analysis of variance indicated that the effects of temperature, time and their interaction on the solid yield were highly statistically significant for all three feedstocks accounting for 99% of the total variation in the solid yields. The effect of time on solid yield for pine was even more significant, accounting for 42% of the total variation. However, the interaction effect between temperature and time was not significant. 39 (a) Pine Time (min) 250 (b) 200 Pine

6 Table 1. Solids yield and dry bulk densities for biomass samples Pine Sweetgum Switchgrass Sample ID Solids yield Bulk Density Solids yield Bulk Density Solids yield Bulk Density (wt%) (g/l) (wt%) (g/l) (wt%) (g/l) Untreated N/A N/A N/A a aa 98.9 h ee 94.6 n kk b aa 87.4 i ee 73.9 o ll c bb 79.9 j ff 51.9 p 98.3 mm b aa 90.8 i ee 75.5 o nn d bb 72.4 k ff,gg 53.0 p 87.5 oo e cc 62.6 l hh 40.7 q 88.8 oo d bb 74.0 k gg,hh 60.3 r ll f cc 60.5 l ii 41.3 q 89.7 oo g dd 46.4 m jj 34.3 s 91.6 oo Values followed by the same superscript are not significantly different at the 0.05 level using Tukey HSD multiple comparison. Sample ID = the first and second numbers represent temperature and time of torrefaction, respectively. 40

7 d. Bulk Density The bulk densities of the raw and treated samples are shown in Table 1. The results for dry densities of the raw chips were similar to previous reports (Harris & Phillips, 1986). The densities for treated biomass consistently decreased, with only two exceptions for switchgrass, as solids retained decrease. The decrease in bulk density is mainly due to the increase in pore volume (not measured) in treated biomass. The statistical analysis of variance for pine indicated that neither the main effects of temperature and time nor their interaction on bulk density were statistically significant as the residual error accounted for 83% of the total variability in bulk density. e. Moisture Absorption Initial moisture contents were 3 to 7% for all biomass samples, and absorption rates were the highest within the initial two hours in the chamber for every sample. At hours, the moisture contents appeared to be almost constant for each biomass type. Each of the samples behaved similarly to the curves of pine moisture absorption shown in Figure 4. Figure 4 shows the maximum moisture contents of each biomass sample. Statistical analysis showed that torrefaction did significantly decrease moisture absorbed. Moisture conetnt (% w.b.) Raw 225 o C, 30 min 250 o C, 30 min Time (h) Figure 4. Moisture absorption of pine at 25 C and 90% RH f. Chemical Properties Table 3 shows the results for ultimate and proximate analyses as well as the higher heating values of the samples. Carbon increased while oxygen decreased with increases in temperature and time. Arias et al. saw similar trends in the torrefaction of eucalyptus (Arias et al., 2008). Carbon percentages for each biomass type increased from 48-51% for the raw biomass to 68-74% (depending on the biomass types) for the maximum treatment temperature of 275 C and time of 45 minutes, and oxygen contents for each biomass decreased from 42-45% for raw biomass to 22-27% for the maximum treatment conditions. Hydrogen content slightly decreased as treatment increased while nitrogen very slightly increased. Statistical analyses of variance indicated that the effects of 41

8 temperature, time and their interaction on the elemental composition (C, H, N and O) were statistically very significant with p- values less than in all cases for pine. Furthermore, the effects of temperature, time and their interaction on the content of C, H, N and O of sweetgum and switchgrass were also significant statistically in all scenarios with the exception of the interaction effects on nitrogen content. Furthermore, temperature, time and their interaction adequately explained the variability in elemental composition with the residual error accounting for less than 4% of the total variability in all cases except in the case of temperature and time interaction effects on the nitrogen content for sweetgum and switchgrass where the residual error accounted for 12 and 27%, respectively. Elemental analysis was also performed on a sample of coal from Alabama Power s electrical generating plant in Gorgas, AL, to compare values of the torrefied biomass to the coal. Carbon and oxygen values for the coal were 67% and 28%, respectively. When subjected to torrefaction, biomass elemental composition can become similar to that of coal. 42

9 Table 2. Maximum moisture contents (w.b.) of samples at 25 C and 90% RH Sample ID Pine Sweetgum Switchgrass Initial MC% 48 hr MC% Initial MC% 48 hr MC% Initial MC% 48 hr MC% Pine a f o b g p c h q d h,j q c i q,r d j,l r,s e h,j t d l,m s e m q,r,s e l,m t Values followed by the same superscript are not significantly different at the 0.05 level using Tukey HSD multiple comparison. Sample ID = the first and second numbers represent temperature and time of torrefaction, respectively. 43

10 Table 3. Chemical properties of raw and torrefied biomass Sample ID Elemental Analysis (wt%, dry) Proximate Analysis (dry) HHV C H N O Ash (wt %) Volatiles (wt %) Fixed Carbon (wt %) (MJ/kg) Pine a 5.99 g 0.61 m t y dd ii b 5.70 g,h 0.78 n,o u y dd ii,kk b 4.88 j,k 0.88 p v z ee ll b 5.95 g 0.78 n,o,q u y dd kk,mm c 5.27 i,j 0.84 p,q,r v aa ff ll d 4.68 k 0.87 p w bb gg nn c 5.49 h,i 0.74 s v aa ff kk,ll e 4.81 k 0.81 n,q,r w bb gg nn f 3.47 l 0.92 p x cc hh oo Sweetgum , , , , , , , , , , , , , ,10, , , , Switchgrass

11 α 5.78 η µ 2.77 σ ψ γγ θθ β 5.36 η,θ ν,ξ 3.75 τ ω δδ ϊϊ γ 4.32 ϊ ο 5.39 µ αα εε ϊϊ β 5.33 η,θ ν 3.67 τ ω δδ ϊϊ γ,δ 4.65 ϊ,κ ο 5.37 µ αα εε κκ,λλ ε 3.34λ π 6.62 φ,χ ββ ζζ κκ,µµ δ 4.94 θ,κ ρ 4.21 τ γγ ηη νν ε,ζ 3.55 λ π 6.40 χ ββ ζζ µµ ζ 2.93 λ π 7.24 φ ββ ζζ µµ Values followed by the same superscript are not significantly different at the 0.05 level using Tukey HSD multiple comparison. Sample ID = the first and second numbers represent temperature and time of torrefaction, respectively. 45

12 Ash and fixed carbon contents of the treated samples increased while volatiles decreased as the temperature and time of treatment increased. At the most intense treatment conditions, each biomass type showed a decrease in volatile content of up to ~50%, while ash percentages approximately doubled, and fixed carbon percentages more than doubled. Only volatile matters are driven off the biomass during torrefaction, leaving the ash and fixed carbon portions. These trends agree with other studies of torrefied biomass (Couhert et al., 2009; Pach et al., 2002; Prins et al., 2006). Statistical analysis of variance showed that the effects of temperature and time on means of ash, volatile matter and fixed carbon were very significant for pine, sweetgum and switchgrass with p- values less than at all levels. The interaction effects of temperature and time on the means of all responses (ash, volatile matter and fixed carbon) were also highly significant except for ash content of pine and sweetgum. Energy content for the biomass also increased with treatment. Table 3 shows the energy content of each sample at each set of treatment conditions. Initial energy content for each biomass type was MJ/kg on a dry basis. The energy content for each biomass type was found to consistently increase with increases in treatment temperature and time. It increased by up to ~35% for the most intense treatments. Lignin is the least reactive portion of biomass and also the most energy dense which means it would constitute a large portion of the sample after treatment which would increase the energy content (ASME, 1987). The higher carbon percentage and lower oxygen percentage in each sample also contributes to an increase in energy content. Furthermore, the effects of temperature, time and their interaction were highly significant for all levels for all feedstocks. g. Differential Scanning Calorimetry Figure 5 shows heat flow versus biomass temperature for each biomass type for different moisture contents. A prominent peak was visible for every curve in the temperature range of C, which was attributed to the moisture release. Higher moisture contents clearly increase the height of this peak and, as expected, it has an effect on the energy input. Once the moisture is released, the graphs remain stable up to the torrefaction temperature of 225 C, which would indicate that little to no degradation reactions in the biomass had taken place. The average enthalpy of torrefaction, h, and average moisture content of three tests for each biomass type is shown in Table 4. 46

13 Figure 5. Energy flow versus temperature at different moisture contents for (a) pine, (b) sweetgum, and (c) switchgrass Table 4. Enthalpy of torrefaction Pine Sweetgum Switchgrass MC% (w.b.) h (J/g) MC% (w.b.) h (J/g) MC% (w.b.) h (J/g) a d g a d h b e i c f j Values followed by the same superscript are not significantly different at the 0.05 level using Tukey HSD multiple comparison. 47

14 The effect of energy required for torrefaction at different moisture content was modeled using simple linear regression, and the results are shown in Figure 6 and in Table 5. The enthalpy of pyrolysis was strongly correlated with the moisture content of the samples. 800 Enthalpy (J/g) Moisture content (% w.b.) Pine Sweetgum Switchgrass Figure 6. Enthalpy of torrefaction vs. moisture content Table 5. Enthalpy as a function of moisture content (w.b.) Biomass Enthalpy, J/g R² Pine h = (MC%) Sweetgum h = (MC%) Switchgrass h = (MC%) The values of slope and intercept for pine are significantly different from those of sweetgum and switchgrass, which could be related to the nature of biomass. Energy requirement for removing moisture ( o C) from different biomass samples were also analyzed, and the study found that the higher the moisture content, the lower the energy required to remove unit mass of water. Most of the water at lower moisture content is stored in cell wall and requires more energy for water removal. Table 6. Caloric requirement for moisture removal Pine Sweetgum Switchgrass MC% (w.b.) h (J/g) MC% (w.b.) h (J/g) MC% (w.b.) h (J/g)

15 3. Explanation of Variance: N/A 4. Plans for Next Quarter: 5. Budget: a. Stability measurements for bio- oils derived from raw and torrefied biomass; and b. Characterization of physical and chemical properties of pyrolysis bio- oils derived from raw and torrefied biomass 6. Patents: N/A a. Funds Expended to Date (End of Reporting Period): $28,860 b. Remaining Balance of Funds: $71, Publications / Presentations: Peer- Reviewed a. Vaishnavi Srinivasan, Sushil Adhikari, Shyamsundar Ayalur Chattanathan and Sunkyu Park. Catalytic pyrolysis of torrefied biomass for hydrocarbons production. Energy & Fuels. DOI: /ef301469t b. Suchithra Thangalazhy- Gopakumar, Sushil Adhikari, and Ram B. Gupta, Catalytic pyrolysis of biomass over H + ZSM- 5 under hydrogen pressure. Energy & Fuels DOI: /ef c. Suchithra Thangalazhy- Gopakumar, Sushil Adhikari, Shyamsundar Ayalur Chattanathan and Ram B. Gupta, Catalytic pyrolysis of green algae for hydrocarbon production using H + ZSM- 5 catalyst. Bioresource Technology Vol. 118, pg Conferences and Workshops Presentation a. Sushil Adhikari, Vaishnavi Srinivasan, and Sunkyu Park Catalytic pyrolysis of torrefied biomass for hydrocarbons production presented at 2012 National Conference: Science for Biomass Feedstock Production and Utilization, October 2- October 5, New Orleans, LA. b. Jiajia Meng, David Tilotta, Sushil Adhikari, and Sunkyu Park Study on stability of pyrolysis bio- oil from torrefied loblolly pine will be presented at Annual International Meeting of American Institute of Chemical Engineers (AIChE), October 28- November 2, Pittsburgh, PA. 49

16 c. Sushil Adhikari, Chad L. Carter and Oladiran Fasina, Physicochemical properties and thermal decomposition of torrefied biomass. Biomass and Bioenergy Workshop at Tuskegee University. April 12 and 13, d. S. Park, J. Meng, J. Park, K.H. Lim, D.C. Tilotta, S. Adhikari, and O. Rojas, Properties of pyrolysis bio- oil produced from torrefied biomass, 243rd ACS National Meeting, March 2012, San Diego, CA e. Jiajia Meng, David Tilotta, Sushil Adhikari, and Sunkyu Park, Study on stability of pyrolysis bio- oil from torrefied loblolly pine, AIChE 2012, October 2012, Pittsburg, PA f. Vaishnavi Srinivasan and Sushil Adhikari Catalytic pyrolysis of torrefied biomass for hydrocarbons production presented at Annual International Meeting of American Society of Agricultural and Biological Engineers, July 29- August 1, Dallas, TX. 8. Name of Students Funded on Project, including Department, Institution, Thesis/Dissertation Title (if applicable), Degree Obtained (if applicable), and Program Area: Student Name Department Institution Thesis/Dissertation Title Degree Obtained/Date Program Area Vaishnavi Srinivasan Biosystems Engineering Auburn University Catalytic pyrolysis of pre- treated biomass for hydrocarbon yields MS/Summer 2013 Catalytic Pyrolysis Shyamsundar A. Chattanathan Biosystems Engineering Auburn University Steam reforming and bio- oil upgrading from fast- pyrolysis of pine- wood PhD/Summer 2014 Bio- Oil Upgrading Chad L. Carter Biosystems Engineering Auburn University Physicochemical properties and thermal decomposition of torrefied woody biomass and energy crop. MS/Spring 2012 Torrefaction Jiajia (August) Meng Forest Biomaterials NC State University Fast pyrolysis of torrefied biomass PhD/Fall 2013 Fast pyrolysis 50