Energy Fuels 2010, 24, 1972–1979 Published on Web 02/08/2010
: DOI:10.1021/ef901435f
Biochar as a Fuel: 2. Significant Differences in Fuel Quality and Ash Properties of Biochars from Various Biomass Components of Mallee Trees Hanisom Abdullah, Kun Aussieanita Mediaswanti, and Hongwei Wu* Curtin Centre for Advanced Energy Science and Engineering, Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia Received November 25, 2009. Revised Manuscript Received January 9, 2010
This study shows the significant differences in the fuel quality and ash properties of biochars produced from the slow pyrolysis of various biomass components (leaf, wood, and bark). The objective is to identify which component is likely to cause problems in subsequent utilization processes if biochar produced from various components of mallee trees is used as a fuel. It is found that the pyrolysis of different biomass components produced biochars with distinct characteristics, largely because of the differences in the biological structure of these components. Leaf biochar showed the poorest grindability, possibly because of the presence of abundant tough oil glands in leaf. Even for the biochar prepared from the pyrolysis of leaf at 800 °C, the oil gland enclosures remained largely intact after grinding. Biochars produced from leaf, bark, and wood components also have significant differences in ash properties. Even with low ash content, wood biochars have low Si/K and Ca/K ratios, suggesting that these biochars may have a high slagging propensity, in comparison to bark and leaf biochars. It appears that, in the utilization of biochar prepared from mallee biomass, the grindability is likely to be limited by the leaf fraction while ash-related problems could be due to the wood and bark components.
production of liquid transport fuels,5,6,13,14 biochar may be a good alternative solid fuel for bioenergy production, addressing the key issues associated with the use of bulky biomass as a direct fuel, including high moisture content, low energy density, and poor grindability. However, a practical supply chain of mallee biomass to a bioenergy plant will be based on the utilization of biomass from the whole mallee trees.2,11 Typically, whole mallee trees are harvested from the field and the total biomass consists of mainly wood, leaf, and bark. It is known that fuel characteristics are broadly diverse among various biomass or biomass components.15,16 A previous review by Nordin17 compared the compositions and characteristics of various biomass fuels and concluded that biomass fuels are very heterogeneous and have significant different elemental characteristics, compared to coal. Specifically, for the genus Eucalyptus, the variation of chemical compositions and heating value is shown, not only in different species and tree age but also of different tree parts.15,18 Similar results are reported from other biomass parts, such as the leaves, stems, and reproductive parts of Cyanara, fibrous and sweet sorghum, as well as Miscanthus, which show that the quality of biomass may be drastically altered with biomass partition.19 In the case of the whole mallee biomass, which contains mainly wood, leaf, and bark, there are various possible scenarios for the utilization of the entire biomass. For example, a typical scenario is to separate the leaf component from
Introduction Mallee biomass is considered to be the most important source for bioenergy development in Australia, and it has attracted significant research and development.1-12 The first part4 of this series has demonstrated that biochars prepared from the low-temperature pyrolysis of mallee wood have good fuel chemistry, excellent grindability, and volumetric energy density, benchmarking against a local sub-bituminous coal in western Australia. Grinding of biochars is of low energy consumption and produces fuel particles with favorable shapes.4 This provides a potentially good biomass utilization strategy based on pyrolysis. Via pyrolysis, biomass can be converted to biochar and bio-oil while light gases can be used to supply the energy requirement of pyrolyzer operations. While the bio-oil may be further upgraded and refined for the *Author to whom correspondence should be addressed. Tel.: þ61-892667592. Fax: þ61-8-92662681. E-mail:
[email protected]. (1) Bartle, J.; Olsen, G.; Don, C.; Trevor, H. Int. J. Global Energy Issues 2007, 27, 115. (2) Wu, H.; Qiang, F.; Rick, G.; Bartle, J. Energy Fuels 2008, 22, 190. (3) Yun, Y.; Wu, H., Life cycle greenhouse gas emission from mallee biomass production. Presented at CHEMECA 2008 Conference, Newcastle, Australia, Sept. 28-Oct. 1, 2008. (4) Abdullah, H.; Wu, H. Energy Fuels 2009, 23, 4174. (5) Garcia-Perez, M.; Wang, X. S.; Shen, J.; Rhodes, M. J.; Tian, F.; Lee, W.; Wu, H.; Li, C. Ind. Eng. Chem. Res. 2008, 47, 1846–1854. (6) Garcia-Perez, M.; Wang, S.; Shen, J.; Rhodes, M.; Lee, W. J.; Li, C. Z. Energy Fuels 2008, 22, 2022. (7) Bartle, J. R.; Abadi, A. Energy Fuels 2010, 24 (1), 2. (8) Harper, R. J.; Sochacki, S. J.; Smettem, K. R. J.; Robinson, N. Energy Fuels 2010, 24 (1), 225. (9) Wu, H.; Yip, K.; Tian, F.; Xie, Z.; Li, C.-Z. Ind. Eng. Chem. Res. 2009, 48, 10431. (10) Yip, K.; Tian, F.; Hayashi, J.-i.; Wu, H. Energy Fuels 2010, 24 (1), 173. (11) Yu, Y.; Bartle, J.; Li, C. Z.; Wu, H. Energy Fuels 2009, 23, 3290. (12) Mulligan, C. J.; Strezov, L.; Strezov, V. Energy Fuels 2010, 24, 46. (13) Bridgewater, A. V. Therm. Sci. 2004, 8 (2), 21. r 2010 American Chemical Society
(14) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044. (15) Senelwa, K.; Sims, R. E. H. Biomass Bioenergy 1999, 17, 127. (16) Rothpfeffer, C.; Karltun, E. Biomass Bioenergy 2007, 31, 717. (17) Nordin, A. Biomass Bioenergy 1994, 6, 339. (18) Perez, S.; Renedo, C. J.; Ortiz, A.; Ma~ nana, M.; Sili o, D. Thermochim. Acta 2006, 451 (1-2), 57–64. (19) Monti, A.; Di Virgilio, N.; Venturi, G. Biomass Bioenergy 2008, 32, 216.
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pubs.acs.org/EF
Energy Fuels 2010, 24, 1972–1979
: DOI:10.1021/ef901435f
Abdullah et al.
significant roles which the AAEM and other inorganic species play in fuel combustion. This study continues the previous work in the first part of this series, focusing on investigating the key differences in the fuel quality and ash properties of biochars produced from the slow pyrolysis of various mallee biomass components and identifying adverse fuel characteristics and undesirable issues associated with the application of mallee biochars as fuel. Key biochar properties that are important to bioenergy applications include fuel chemistry, grindability, energy densities, and ash-forming species in the biochars are investigated systematically in the experimental work of this study.
the entire biomass for the production of eucalyptus oil, which is a value-added product. This was part of the integrated wood processing concept,20 which produces multiple products, including eucalyptus oil, electricity, and activated carbon. In other cases, the total biomass may also be used without separation, which is more likely in a future large-scale application of mallee biomass, based on a continuous supply chain.11 In considering the deployment of pyrolysis in different application scenarios, essential information on the fuel property of biochar from different components is scarce in the public domain. Therefore, it is important to have a fundamental understanding of the fuel properties of the biochars produced from individual mallee components for the optimized utilization of the biomass. The outcomes of this project, which identify specific characteristics and possible limitations that biochars of individual biomass components may impose in power generation facilities, are noteworthy for process design and techno-economic evaluation of different application scenarios of mallee biomass. The fuel quality of biomass is one of the important considerations in determining the choice of appropriate technology, energy output, and effectiveness of a power generation plant. Key fuel physical properties (i.e., grindability, bulk and volumetric energy densities) strongly influence fuel handling characteristics, process control, and transportation/storage cost. Concerning chemical properties, evaluation of fuels ash and mineral compositions is crucial. The degree of slagging, fouling, and corrosion in a power plant mainly is dependent on ash and mineral volatilization.21 In the case of mallee biochars, biochars gasification and combustion are some of the possible applications.9,10 For example, the major obstacles for biomass combustion technologies are ash-related problems (including sintering, agglomeration, deposition, erosion, and corrosion) and the key responsible ash-forming species are the alkali and alkaline-earth metallic (AAEM) species, which can form alkali silicates or alkali sulfates.21 The alkali silicates and sulfates have lower melting points (
