Gas Detarring Following Biomass Gasification

Transcription

1 Gas Detarring Following Biomass Gasification Marek Balas, Martin Lisy, Zdenek Skala Abstract The significance of energy use of biomass as a renewable energy source keeps on growing and so does the significance of finding new ways of transformation. Being one of the forms of biomass conversion, thermal gasification has for years been in the focus of interest of expert public. The gas produced in the process of gasification can be used either by burning it directly in a steam boiler, combustion engine, or a gas turbine, or it can be used for separation of hydrogen and consequent use in fuel cells. Use of the gas generated in the course of biomass gasification in other processes is limited by the efficiency of its cleaning from impurities and admixtures. Track is kept, above all, of the content of dust, tar and sulphur and chlorine compounds. The methods of gas cleaning are numerous, starting from those applied directly in the fluidized bed of the gasification generator via wet scrubbing up to thermal and catalytic methods in secondary reactors. The present article deals with the results of research into the cleaning of gas coming from atmospheric fluidised bed gasifier. Our institute has experience from both the primary measures reducing tar production and the methods using lime and metallic catalysts to reduce tar in gas in a secondary bed. It also includes methods and results of measurements taken during long-term activities involving pilot atmospheric fluidised bed gasifier BIOFLUID 100. Investigation was undertaken in particular into the impact of operating conditions of individual catalysts on the composition and heat value of the gas and also on the residual amounts of tar in the gas and its make-up. Keywords Biomass, tar reduction, catalysts 1 Introduction Worldwide, biomass is one the most significant renewable energy sources. Using it has many benefits, such as a generally lower impact of the technology on environment, and utilization of idle agricultural land that is not fit or needed for food production. Of late, the Czech Republic has been generating almost 4 % of its electricity from renewable sources where biomass plays an irreplaceable role. There is multitude of ways to use biomass to produce heat and power, starting with esterification of oils via generation and use of biogas up to thermal processes such as pyrolysis, combustion, and gasification. The currently most widely used method of energy use of biomass is direct combustion. Direct combustion is the oldest method with best-mastered technology. However, with smaller outputs, it is preferably

2 suitable for generation of heat, not so much for the generation of the so much valued electric power. Another way of using biomass to generate energy is fuel gasification.[1] with consequent use of the producer gas in a cogeneration unit. Technologies based on gasification can be divided into downdraft gasifiers (co-current and counter-current) and gasifiers with fluidized bed (with fixed bed or circulating bed). Each of these gasifiers has its benefits and its drawbacks. The downdraft technology has a simpler design and high level of conversion. On the other hand, fluidized bed technology is more tolerant to fluctuations in fuel parameters and is easy to control []. There is a multitude of criteria to compare individual technologies. Focusing on the biggest problem of the generated gas, which is tar, and then the most advantageous technology is that of co-current gasifier with inclined bed where the generated gas contains tars amounting to hundreds of mg/m. The disadvantage of this technology is lack of possibility to take primary measures to fight tars and the fact that it can only be used for small output up to one MW. Another suitable technology is use of gasification in fluidized bed. The difference between gasification in circulating or fixed bed does not consist so very much in tar content but rather in the efficiency of fuel conversion and the complexity of design and control of the facility. The amount of tar in the gas generated in fluidized bed is 1 g/m n 5 g/m n. This amount, however, can be limited right in the generator by taking some of the primary measures. Gasification is a relatively new technology offering in comparison with combustion a number of benefits. The main benefit of gasification is increasing the efficiency of using biomass energy, particularly in relation to generation of power. Combustion of producer gas is a process easier to control than combustion of solid biomass, which reduces production of harmful emissions. Using gas in gas turbines and gas-steam cycle makes us achieve higher efficiency in power generation. Compared with combustion, gasification also has lower heat losses and better energy use of the fuel. Gasification is a thermochemical conversion of organic mass under oxygen deficiency into low heating value gas (LHV gas, 4 MJ/m n 15 MJ/m n ). The process consists of a series of simple reactions and occurs at elevated temperatures, typically from 750 C to 1000 C. The generated gas is then combusted in boilers or in combustion engines (combustion turbines). Use of air as gasification medium results in low heating value of the gas (4 MJ/m n 7 MJ/m n ) due to dilution of the gas by nitrogen (in excess of 50 %). Gasification with a mixture of air and oxygen or possibly even the use of steam as gasification medium give rise to medium level calorific values amounting to 10 MJ/m n 15 MJ/m n [7]. The heat needed for endothermic reactions mostly comes from partial oxidation of the gasified material (gasification using air or oxygen) or is supplied from outer sources. The main effort in the process of gasification is to transform the largest amount of energy contained in the fuel into the largest possible amount of energy in the gas [4]. The product of gasification is gas, the main constituents of which include CO, CO, H, CH 4, higher hydrocarbons, N, and impurities. Focus in the gas is mainly on its quality (heat value, composition), yield, and amount and composition of its impurities.

3 The potential of the gas generated in the process of biomass gasification to be used in power generation is mainly hampered by the problems connected with purification of this product. The content of impurities in the gas causes operational problems due to blocking of inlets and tar deposits on working surfaces of engines and turbines, which may result in serious defects of the operating units. Impurities mainly include dust (air-borne solids), alkali compounds, nitrogen, sulphur and halogen (Cl, F) compounds, and tar (which involves all organic substances with boiling point above that of benzene (80.1 C) [5]. Tar and dust are the main culprits behind the limited use of fuel gas. The last link of a CHP plant is the cogeneration unit with compression ignition or spark ignition engine. Combustion engine is extended on a significantly larger scale mainly because it can be used to combust fuel gas without major modifications, which results in low purchasing costs and low modification cost of the cogeneration unit [6]. In addition, the operating properties of this engine are suitable. Using an engine originally designed for liquid fuels to combust fuel gas with lower heat value obviously will result in output reduction. Output reduction is, above all, given by heat value of the inflammable mixture fuel/air, amount of the combustible mixture that enters the combustion cycle, and engine revolutions [7]. It is obvious from the above that the limiting factor in the operation of a combustion engine running on gas generated from gasifier units are the quantities of impurities, particularly tar, solid particles and alkali compounds. Removal of alkali compounds from the gas is very complicated and goes beyond the frame of the present article. Removal of tar and dust from the gas is achievable by taking primary and secondary measures that will the subject of the following chapters. Gas Cleaning Methods As already mentioned above, removal of tar from the fuel gas is a pivotal issue to use this gas in combustion engines. There are several methods of tar removal; we shall focus on the catalytically ones..1 Primary Methods of Tar Degradation Primary methods include measures applied within the reactor. They are very significant from operational point of view as they increase overall efficiency of energy conversion and, at the same time, reduce the need to remove tar by subsequent measures. One of the ways is application of catalyst straight to the fluidized bed of the generator. Using suitable material with catalytic and adsorption properties may result not only in reduced tar content, but also in reduced concentration of the undesirable compounds of sulphur and chlorine in the gas. Olivine and dolomite seem to be the most suitable materials. An intensive abrasion of particles of adsorbent occurs in the fluidized bed, small particles emerge with large surface and considerable adsorption activity.

4 . Secondary Methods of Gas Cleaning Out of the host of secondary measures for elimination of tar content in the gas, our team focused on methods using catalytic reactions of tar cracking. The most frequently used catalysts include either natural catalysts (dolomite, olivine, zeolite, calcite), or metal-based (Ni, Mo, Co, etc.) catalysts. Both have their pros and cons. Non-metallic catalysts and the metallic ones as well are used to reduce the amount of tar contained in the product of gasification (gas) in separate reactors with fixed bed. They are referred to as secondary catalysts. The most frequently used nonmetallic (natural) catalyst is dolomite calcium magnesium carbonate. Calcinated dolomite used as catalyst for tar destruction following biomass gasification is subject of long-term examination. A number of institutions in the world carry out research into its application and its industrial application have already been devised. It is obvious from a majority of studies that dolomite is a very effective catalyst in tar cracking and its benefits also include the fact that it is relatively cheap and available. Dolomite is a calcium magnesium ore with chemical formula CaMg(CO ). At elevated temperatures, calcination occurs, carbonates turn to oxides, which results in CO release. The course of calcination depends on the following factors: temperature, grain size, dolomite composition, speed of heating and ambient atmosphere, particularly partial pressure of CO [8, 9]. At temperatures exceeding 600 C and at the presence of CO the less stable MgCO gets decomposed. It is obvious that calcination temperature of MgCO as constituent of dolomite is higher than that of pure magnezite [10]. At higher temperatures, even CaCO gets calcinated. With growing content of CO, the calcination temperature of CaCO moves towards higher values. Calcinated dolomite exhibits high activity in tar cracking, particularly at temperatures in excess of 850 C. A mechanism of reforming of tar on dolimite is described in (1) and (). ( CO ) CaCO + MgO CO (1) CaMg + CaCO CaO + CO () To remove tar from gas, one can also apply metallic catalysts that are used in petrochemical industry for oil and natural gas reforming to syngas, for CO removal, for production of hydrogen for fuel cells and so forth. The commonest metal-based catalyst is nickel coated on a variety of carriers (alumina or alumina silicates). Usage of nickel catalysts results in higher content of H and CO in the gas, methane and higher hydrocarbons are reduced or removed. Some studies show that nickel also reduces amounts of NH in the gas. Commercial nickel catalysts can be divided into two groups: Pre-reforming catalysts operating at lower temperatures (around 450 C 500 C) Reforming catalysts operating in the range 750 C 900 C.

5 A simplified mechanism of catalytic reforming of tar is described in detail in literature [11]. First, methane, or other hydrocarbon, is separated by adsorption on the active centres of nickel where hydrogen fission begins. Water is separated by adsorption on a ceramic carrier, hydroxylating the surface. At suitable temperature, OH radicals migrate to the side of the metal; take control over the oxidation of fragments of medium hydrocarbons and surface carbon to CO and H. We can complete proces describe equations () to (7): C H n n m x m + * CnH x * + H () C H * CH x * + n* Cn 1H x * + CH x n (4) x * n + O * CO + H + ( n + 1)* x (5) H + O + * O * H (6) H + * H * (7) * is active centre of catalyst. The main advantages of metallic catalysts consist in the possibility to use them even at lower temperatures and their several times higher activity, which makes it possible to use smaller amounts or catalysts to clean the gas and thus to use smaller and more compact facilities (in comparison with e.g. dolomite). The disadvantage of metallic catalysts is their relatively great tendency to get deactivated. De-activation can be caused by: Blocking of active centres of the catalysts due to coking an effect of reactions on the surface of the catalyst with high tar content in the gas. The speed of deactivation can be reduced by raising the ratio steam/carbon in the fuel or by modification of surface reactions due the presence of some other metal [1, 1]. Catalytic poisons (H S) or substances blocking the porous system of the catalyst (alkali metals, SiO ). Temperature is an important factor of the rate of deactivation by sulphur catalysts get more tolerant to sulphur compounds with growing temperatures [14]. Irreversible changes in the system carrier/catalyst (caking, sintering) nickel crystals grow in size, which results in reduced surface area and consequently reduced activity of the catalyst. Resistance to sintering is increased by proper selection of carrier [15].

6 Experimental Unit Biofluid 100 Fig. Experimental unit Biofluid 100 Caption: 1 - fuel storage - gasifier - hot filter Obr. 1 Fig. 4 Diagram of the experimental unit BIOFLUID 100 Starting from 000, research is carried out at Institute of Power Engineering, Faculty of Mechanical Engineering, Brno University of Technology into fluidized gasification of biomass and sorted municipal waste. Experiments are carried out on fluidized atmospheric gasification reactor with stationary fluidized layer called Biofluid 100 (Fig. 1). Starting-up the fluidized gasification generator to steady state is carried out by way of combustion mode. Temperature of the process is controlled within the 750 C 900 C range by change in the fuel/air ratio. Average heating value of the generated gas ranges within 4 MJ/mn - 7 MJ/mn, the content of solids is within the range 1.5 g/mn to g/m n and tar content is between 1 g/m n to 5 g/m n in relation to the fuel used and based on operating conditions. Fuel is supplied from the bin equipped with rake and is dosed to the reactor by worm conveyor. Air compressed by blower is supplied to the reactor under the grate as primary air providing partial oxidation of fuel and maintaining the fluidized layer. Air can also be supplied at two different height levels as secondary air and tertiary air. The produced gas is rid of particulates in a cyclone and then burned up in a burner fitted with a small flame holder burning natural gas and having its own air supply. Ash from the reactor is intermittently discharged into ashbin by way of specially designed travelling grate. To be able to investigate impact of air preheating, electric heater has been installed at blower outlet. Simplified diagram of the

7 experimental unit is shown in Fig.. A more detailed description is given in literature [16]. The following are reactor specifications: Output (in generated gas) 100 kw t Input (in fuel) 150 kw t Fuel consumption max 40 kg/h Air flow max 150 m n /h The form, in which fuel can be supplied is limited mainly by worm conveyor dimensions and fuel moisture content. Optimum fuel moisture content is 0 % to 0 %. In woody biomass, it is mostly wood shavings and small wood chips, size some cm to cm. In herbaceous biomass, the fuel is either fine chopped matter, or smaller-size pellets. Chopped matter was not a big success as there are significant problems with fuel dosing, above all vaulting in fuel storage bin. Neither is this type fuel optimal for fluid layer formation. For secondary gas cleaning, hot catalytic filter with dolomite filling and optionally also filter for metallic catalysts have been installed at the outlet of the cyclone..1 Hot Catalytic Filter The design of the hot catalytic filter (HCF) has been aimed from its onset to the development of a functional industrial technology. A series of design and operating problems had to be eliminated to that end. The most complicated task was to ensure long-term operation with replacement of passivated catalyst. A specially designed two-part rotary grate serves that purpose. The operation of the grate is intermittent. With increased pressure loss part of the filling is removed which disturbs the deposits in the lower part. The filter is heated electrically with 5 kw input. This system of heating is currently being replaced by partial burning of the fuel gas right in the filter. Air intake has three entry points in the area of the grate. Ongoing oxidation is scheduled to remove the deposits of trapped sooth and fly ash. Overall concept of the HCF is given in [17]. The filter mainly serves elimination of tar, but it also traps dust particles contained in the generated gas. Various forms of dolomite are used as filter medium. All tested materials showed good efficiency in eliminating tar, but some of the materials had low abrasion resistance.. Nickel Catalyst Filter The research into the use of metallic catalysts to clean fuel gas is in a budding stage in the Institute of Power Engineering. On the basis of an extensive literature search, it was decided for reasons of lower working temperatures - to focus oneself on pre-reforming catalysts. The filter with nickel catalysts is installed behind HCF, which is used to remove dust (the HCF charge then consists of chippings). Moreover, installed before the filter with metallic catalysts is a protective bed to reduce the content of sulphur compounds in the gas, as these might be the cause of rendering the nickel catalysts useless. This

8 filter is charged with iron oxides. The nickel filter and the protective bed are fitted with electric elements to be able to set optimum working conditions. Both filters are fitted with thermo couples both in the bed and on the walls (to control electrical heating) and with pressure difference sensors. 4 Research Results To verify theoretical knowledge, experiments were carried out with Biofluid 100. The aim was to establish efficiency of primary measures and operating conditions of catalysts in add-on filters. Constant operating conditions have been established during the experiments and gas heating value and tar contents were monitored afterwards. 4.1 Methods of measurement at experimental unit Biofluid 100 Gas quality measurement is usually carried out in two methods. One consists of an on-line monitoring of gas composition with simultaneous gas sampling to gastight glass sample containers. The samples are subsequently analyzed using gas chromatograph. Tar sampling is carried out in line with IEA methodology [18] by capturing tar in a solution that is subsequently analyzed by gas chromatograph with mass spectrometer. Presence of HCl, HF and NH in the gas is examined by their trapping in an NaOH solution. Operating parameters are monitored during operation and continuously recorded by the control computer. They include, in particular, mass flow of fuel, temperatures at various points of the unit, pressure difference in the fluidized bed, gas flow and pressure, and the temperature and flow of primary air. 4. Primary Measures In experiments carried out in our Institute, dolomite and quartz sand were used that are easily available in the Czech Republic. Optimum temperature conditions for the use of these catalysts are somewhere around 850 C. Efficiency of quartz sand is low at these temperatures; however, the presence of sand in the fluidized layer resulted in its high stability and higher gas quality. Application of dolomite in the fluidized bed of Biofluid 100 resulted in up to 60 % drop of original output concentration of raw gas from the generator. Though this is a positive result, it involves the necessity of taking secondary measures to fully clean the gas to the cogeneration unit. Mean values can be found in Table I. Table I: Tar content table prior to primary measures and following their deployment dolomite silica sand without inert 0 mg/m n 1894 mg/m n with inert 805 mg/m n 114 mg/m n 60.5 % 9.5 %

9 4. HCF Content of tars at the outlet of dolomite filter ranges within 10 mg/m n - 70 mg/m n and the content of dust within 10 mg/m n - 80 mg/m n based on operating conditions of the unit and operating temperature of the HKF (see Table II). However, the dust caught at the outlet of the dolomite filter contains more than 90 % of particulates from the dolomite and can be easily removed from the gas rid of tars using ordinary cloth filters. Table II: Tar content at dolomite filter inlet and outlet operating temperature 790 C 850 C 905 C before HCF 17 mg/mn 1408 mg/mn 144 mg/mn after HCF 68 mg/mn 1 mg/mn 0 mg/mn 94.5 % 98.0 % 98.5 % 4.4 Nickel catalysts It is obvious from experiments on metallic catalysts that tar reduction in fuel gas is possible with high level of efficiency. Experiments were carried out for different types of catalysts and temperatures under constant conditions (amount of catalyst, volume of passing gas, HCF, and generator mode) and gas and tar sampling was carried out using the same methods. Occupation tar removal efficiency for each of the three catalysts is shown in Fig. 1. Further increase of efficiency is subject of further research. Gas composition is show in Table III. Fig. 1 Depending on the efficiency of tar removal with temperature, catalyst: ShiftMax 80, G1-80 catalyst, KATALCO 46-Q

10 Table III: Average gas composition before and after nickel filter for KATALCO 46-Q (space velocity 000 h -1, average temperature in bed 550 C) Component Before filter After filter Content % vol. CO [%] 1,1 7,67 H [%] 16,94 7,56 CO [%] 7, 16,0 CH 4 [%] 4,08 0,15 N [%] 55,95 46,4 C x H y [%] 1,96 0,04 HHV [MJ.m - ] 6, 6,5 Total Tar [mg.m - ] 1017,1 56,5 5 Conclusion The Czech Republic pays considerable attention to the development of renewable energy sources. The research work undertaken at the Institute of Power Engineering of The Brno University of Technology in the area of fluidized bed gasification of biomass and waste is aimed at higher use of modern units with electric output in excess of about 1MW in decentralized supply of heat and power to regions. The main technical problem can be seen in the quality of the gas for combustion engine. Experimental researches, the results of which have been summarized in the present paper, give prerequisites for early introduction of this promising technology in practice. Use of gas from gasified biomass for generation of electricity in cogeneration units is a promising technology. For industrial applications, however, the gas has to be rid of its main impurities, i.e. dust and tar. As is obvious from the results of the research, primary measures and secondary catalytic reduction using natural or metallic catalysts are suitable for conditioning the gas. It is only a matter of further research to find optimum composition of the catalysts and to set optimum operating conditions for individual constituents of the facility. References: [1] Pospisil, J., Fiedler, J., Skala, Z.: Cool Producing Systems Based on Burning and Gasification of Biomass, [online] Available: [] Bridgwater, A. V.: The Technical and Economic Feasibility of Biomass Gasification for Power Generation. Energy Research Group, Aston University, Birmingham, (1995) Fuel Vol.74 No. 5 [] Baláš, M., Lisý, M.: Vliv vodní páry na proces zplyňování biomasy, Acta Metallurgica Slovaca 1/005, Roč. 11, Košice, 005, ISSN [4] Ochrana L., Skála Z., Dvořák P., Kubíček J., Najser J.: Gasification of Solid Waste and Biomass, VGB PowerTech, 004, vol. 84, no. 6, p ISSN [5] Neft, J. P. A. at al: Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases. Energy project ERK6-Ct

11 [6] Pospisil, J., Fiedler,J., Applicability of Tri-generation Energy Production for Air-conditioning Systems in Czech Republic, [online] Available: [7] FAO Forestry Department: Wood gas as engine fuel, 1986 [online] [cit ] Dostupné z WWW: ISBN [8] Shorter Communications: Predicting the rate of thermal decomposition of dolomite, Chemical Engineering Science, Vol. 51, No., pp. 59-5, (1996) [9] Wiedemann H.G., BAYER, G.: Note of the thermal decomposition of dolomite, Thermochimica Acta, 11, , (1987). [10] Boyton R.S.: Chemistry and Technology of Lime and Limestone, pp , Willey, N.York, USA, (1980). [11] Ross, J. R. H., Steel, M. C. F., Zeni-Isfahani, A.: Evidence for the Participation of Surface Nickel Aluminate Sites in the Steam Reforming of Methane over Nickel/alumina Catalysts. Journal Catalyst 5 (1978) 80 90, Elsevier [1] Trimm, D. L.: Coke Formation and Minimization During Steam Reforming Reactions. Catalysis Today 7 (1997) 8, Elsevier [1] Rostrup-Nielsen, J. R.: Industrial Relevance of Coking. Catalysis Today 7 (1997) 5, Elsevier [14] Zhang, Y., Draelants, D. J., Engelen, K., Baron, G. V.: Improvement of Sulphur Resistance of a Nickel-modified Catalytic Filter for Tar Removal from Biomass Gasification Gas. Journal of Chemical Technology & Biotechnology, 78, 65-68, 00. ISSN , Online ISSN: [15] Bengaard, H. S. at al: Steam Reforming and Graphite Formation on Ni Catalyst. Journal of Catalysis 09 (00) p. 09 [16] Ochrana L., Dvořák P., Nguyen Van Tuyen: Zplyňování biomasy a tuhých odpadů v atmosférické fluidní vrstvě. Energetika č. 4/00, str , ISSN [17] Skála, Z., Ochrana, L., Lisý, M., Baláš, M., Kohout, P., Skoblia, S.: Research into Biomass and Waste Gasification in Atmospheric Fluidized Bed. Sborník 0th World Energy Congress, říjen 007, Řím, Itálie, [18] Van Paasen, S.V.B et al.: Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases. Final report documenting the guideline, R&D work and dissemination. 00, ECN-C-0-090