Energy Fuels 2010, 24, 4560–4570 Published on Web 08/05/2010
: DOI:10.1021/ef100681u
Tar Formation and Destruction in a Fixed-Bed Reactor Simulating Downdraft Gasification: Equipment Development and Characterization of Tar-Cracking Products Fadimatu Dabai, Nigel Paterson,* Marcos Millan, Paul Fennell, and Rafael Kandiyoti Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom Received June 2, 2010. Revised Manuscript Received July 26, 2010
The aim of the present study is to examine operating parameters that would reduce the residual tar content in the fuel gas in downdraft gasifiers and eventually eliminate it altogether. A two-stage fixed-bed reactor has been employed to simulate elements of tar cracking in a downdraft gasifier. In this reactor, tar is generated by pyrolysis in the first stage and cracking and gasification take place in the second stage. Modifications to a previous configuration of this reactor are described, which have enabled the use of smaller char particle sizes in the second stage and the generation of a more complete inventory of the reaction products. In this work, the effect of the temperature and the presence of char on product distributions are reported. Increasing the temperature from 700 to 1000 °C resulted in a decrease in the quantity of tar recovered and an increase in the total amount of CO released. The amount of CH4 released increased between 700 and 800 °C before remaining steady up to 1000 °C. The CO2 content of the gas was relatively constant between 700 and 800 °C and increased as the temperature increased from 800 to 1000 °C. The amount of water and light hydrocarbons (C2-C5 alkanes and alkenes) sharply decreased at 1000 °C. The presence of char in the second stage had significant effects on tar cracking and product distributions. These effects were more obvious on the concentrations of CO, CO2, and H2O, which may be a result of reduction reactions taking place with the carbon in the packed char bed. These reactions appear to be more significant at temperatures between 900 and 1000 °C, where the rates of gasification are expected to increase.
Biomass can be converted into a fuel gas by gasification, which can be efficient, flexible, and environmentally friendly. However, gasification involves numerous complex parallel reactions, leading to many potential byproducts and impurities, which can cause operating problems downstream of the gasification stage. One undesirable product is “tar”, defined as “material in the product stream that is condensable in the gasifier or in downstream processing steps or conversion devices”.5 Tar is considered to be an obstacle to clean electricity production from biomass gasification, because cleaning and conditioning combustible gas containing tar carries energy and economic penalties.6 Downdraft gasification lends itself to the production of a fuel gas from biomass at the small scale, which is particularly suitable for decentralized heat and power generation, especially in developing countries in areas not well-covered by a centralized electricity generation system. Downdraft gasifiers can use fuels (e.g., wood) that release large amounts of tars, because they have more efficient mechanisms of tar destruction than most other biomass gasifiers.6 Tars released in the pyrolysis zone go through the combustion zone, where they can be oxidized at least partially, and then through the reduction zone, where they come in contact with chars and can crack to form gases or undergo condensation reactions to form chars. Further advantages of downdraft gasification are
Introduction Biomass is seen as a viable source of renewable energy and is often a low-value byproduct of agriculture, which can be sustainably transformed into fuel for the generation of heat and power or for the production of liquid fuels and chemicals, using a variety of technology options.1 Biomass use is considered “environmentally friendly” compared to the use of fossil fuels because it has an almost closed carbon cycle.2 Therefore, its use does not necessarily contribute to the net release of carbon dioxide to the atmosphere and represents an attractive alternative to fossil fuels, particularly when agricultural byproducts are used. Currently, the energy demand of the world is estimated at 474 EJ/year,3 and biomass makes up about 10% of the primary energy consumption.4 In recent years, economic, environmental, and energy security issues have renewed interest in bioenergy, making it worthwhile to further develop the technologies to improve the quality of the products and performance of processes using biomass. Improved process economics can encourage the development and use of bioenergy, so that a higher percentage of the primary energy consumption of the world can be derived from sustainable biomass. *To whom correspondence should be addressed. E-mail: n.paterson@ ic.ac.uk. (1) Hall, D. O.; Moss, P. A. Biomass for energy in developing countries. GeoJournal 1983, 7 (1), 5–14. (2) Faaij, A. P. C. Bio-energy in Europe: Changing technology choices. Energy Policy 2006, 34 (3), 322–334. (3) British Petroleum (BP). Statistical Review of World Energy; BP: London, U.K., June 2009; www.bp.com/statisticalreview. (4) International Energy Agency (IEA). World Energy Outlook; IEA: Paris, France, 2008; ISBN 978-92-64-04560-6. r 2010 American Chemical Society
(5) Milne, T. A.; Evans, R. J.; Abatzoglou, N. Biomass Gasifier “Tars”: Their Nature, Formation, and Conversion; National Renewable Energy Laboratory (NREL): Golden, CO, Nov 1998; NREL/TP-570-25357. (6) Reed, T. B.; Das, A. Handbook of Biomass Downdraft Gasifier Engine Systems; The Biomass Energy Foundation Press: Franktown, CO, 1988.
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Energy Fuels 2010, 24, 4560–4570
: DOI:10.1021/ef100681u
Dabai et al.
its relative simplicity and low investment costs. However, it requires biomass of less than 20% moisture, and the amount of tars released, although smaller than in other gasifier configurations, may still affect the performance of downstream equipment.1 It is desirable to completely eliminate the low residual tar content in the fuel gas from this type of gasifier. Minimization of tars released in downdraft gasification is the aim of this research. An existing laboratory-scale twostage fixed-bed reactor has been employed to study tar destruction in conditions relevant to downdraft gasifier operation. The first stage acts as a pyrolysis tar generator, while the second stage is used either empty or packed with a char bed to study cracking of these tars. Previous work using this system focused on determining the impact that varying operating conditions in the tar-cracking section of the reactor have on the tar content of the product gas.7,8 That work established that significant reductions in tar content can be achieved by partial combustion of the tars and cracking on a hightemperature char bed. It was shown that tar release decreased with an increasing temperature, a decreasing bed particle size, and an increasing residence time in the second stage. This paper builds on that research and presents a more thorough characterization of the tar-cracking products, identifying the compounds present in the gas and in the light-end fraction of the tars. How the composition of gas and light tars varies, when operating parameters of the reactor are changed, has been determined. Identification of these components leads to insight into some of the fundamental chemical reactions that take place in a downdraft gasifier. It also enables the investigation of reactor operating conditions to optimize the performance of downdraft gasifiers. To achieve the fuller characterization of the products, a series of modifications to the system used in previous work were introduced. The modifications have improved the scope of experiments that can be performed and the quality of the data obtained. Also, the range of species that are collected and analyzed has been increased. Therefore, a more valid understanding of the tar-cracking behavior has been obtained. The initial configuration is described in the Experimental Section of this paper. The modifications to the reactor and experimental procedures introduced are presented in the Results and Discussion, together with a set of experiments on cracking with and without a char bed obtained with the new configuration. A further paper is in preparation, and this will extend the scope of the experiments to include different biomasses, the effect of the addition of O2 to the flange between the first and second stages, and the impact of particle size in the secondstage bed. It will also review the implications of this study on the operation of commercial-scale downdraft, fixed-bed gasifiers.
Figure 1. Diagram of the reactor before modifications introduced in the present work, reproduced from Monteiro Nunes.18
described by O’Brien et al.9,10 Modifications were successively made by G€ onenc- et al.,11,12 Pindoria et al.,13,14 and Collot et al.15,16 Successive stages in the development of this reactor have been previously summarized.17 The version of the reactor used in the present study as the starting point in this work was described by Monteiro Nunes et al.7,8,18 and is described in more detail below. The first stage of the reactor has an internal diameter of 12 mm, a wall thickness of 2 mm, a length of 250 mm and is made from American Iron and Steel Institute (AISI) 316-grade stainless steel. It is fitted with a “T” piece at the top to allow for a thermocouple to be placed inside the bed and gas to be supplied to the reactor in a downdraft configuration. A fixed mass of biomass is pyrolyzed in the first stage under set conditions to generate a repeatable quantity of tar and char. The char produced is retained in this first stage and does not interact with products in the second stage of the reactor. The bottom of the (10) Bolton, C.; Riemer, C.; Snape, C. E.; O’Brien, R. J.; Kandiyoti, R. Effect of carrier gas flow and heating rates in fixed-bed hydropyrolysis of coal. Fuel 1987, 66, 1413–1417. (11) G€ onenc-, Z. S. Coal pyrolysis in a fixed bed reactor. Ph.D. Thesis, University of London, London, U.K., 1989. (12) G€ onenc-, Z. S.; Bartle, K. D.; Gaines, A. F.; Kandiyoti, R. Effect of secondary reactions on molecular mass distributions of coal pyrolysis tars produced in a hot-rod reactor. Erdoel, Erdgas, Kohle 1990, 2, 82–85. (13) Pindoria, R. V. Thermochemical upgrading of biomass to gaseous and liquid fuels. Ph.D. Thesis, University of London, London, U.K., 1997. (14) Pindoria, R. V.; Lim, J. Y.; Hawkes, J. E.; Lazaro, M. J.; Herod, A. A.; Kandiyoti, R. Structural characterization of biomass pyrolysis tars/oils from eucalyptus wood waste: Effect of H2 pressure and sample configuration. Fuel 1997, 76, 1013–1023. (15) Collot, A. G. Co-processing of coal and biomass in a fixed bed reactor: Product distribution and trace element partitioning. Ph.D. Thesis, University of London, London, U.K., 1999. (16) Collot, A. G.; Zhuo, Y.; Dugwell, D. R.; Kandiyoti, R. Copyrolysis and co-gasification of coal and biomass in bench-scale fixedbed and fluidised bed reactors. Fuel 1999, 78, 667–679. (17) Kandiyoti, R.; Herod, A. A.; Bartle, K. D. Solid Fuels and Heavy Hydrocarbon. Thermal Characterisation and Analysis; Elsevier, Amsterdam, The Netherlands, 2006. (18) Monteiro Nunes, S. Investigation of tar destruction reactions in a downdraft gasifier using biomass and waste feedstock. Ph.D. Thesis, University of London, London, U.K., 2007.
Experimental Section Reactor. The two-stage reactor employed for this research (Figure 1) was developed from an earlier single-stage design (7) Monteiro Nunes, S.; Paterson, N.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Tar formation and destruction in a fixed bed reactor simulating downdraft gasification: Optimization of conditions. Energy Fuels 2008, 22 (3), 1955–1964. (8) Monteiro Nunes, S.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Tar formation and destruction in a simulated downdraft, fixed-bed gasifier: Reactor design and initial results. Energy Fuels 2007, 21 (5), 3028–3035. (9) O’Brien, R. J. Tar production in coal pyrolysis;The effect of catalysts, pressure and extraction. Ph.D. Thesis, University of London, London, U.K., 1986.
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: DOI:10.1021/ef100681u
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Table 1. Proximate and Ultimate Analyses of the Biomasses Used in This Studya ultimate analysis (wt %, as received)
silver birch pine sawdust beechwood
b
proximate analysis (wt %, as received)
C
H
N
S
Cl
ash
moisture
volatiles
52.0 46.5 43.4
7.0 5.1 4.8
0.1 0.22 0.26
