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Biochar as a Fuel: 1. Properties and Grindability of Biochars Produced from the Pyrolysis of Mallee Wood under Slow-Heating Conditions Hanisom Abdullah 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 May 20, 2009. ReVised Manuscript ReceiVed June 20, 2009
Biomass as a fuel suffers from its bulky, fibrous, high moisture content and low-energy-density nature, leading to key issues including high transport cost and poor biomass grindability. This study investigates the possibility to pretreat biomass to produce biochar as a solid biofuel to address these issues. Biochars were produced from the pyrolysis of centimeter-sized particles of Western Australia (WA) mallee wood in a fixedbed reactor at 300 to 500 °C and a heating rate of 10 °C/min. The data show that, at pyrolysis temperatures g320 °C, biochar as a fuel has similar fuel H/C and O/C ratios compared to Collie coal that is the only coal being mined in WA. Converting biomass to biochar leads to a substantial increase in fuel mass energy density from ∼10 GJ/ton of green biomass to ∼28 GJ/ton of biochars prepared from pyrolysis at 320 °C, in comparison to 26 GJ/ton for Collie coal. However, there is little improvement in fuel volumetric energy density, which is around 7-9 GJ/m3 in comparison to 17 GJ/m3 of Collie coal. Biochars are still bulky and grinding is required for volumetric energy densification. Biochar grindability experiments show that the fuel grindability increases drastically even at pyrolysis temperature as low as 300 °C. Further increase in pyrolysis temperature to 500 °C leads to only a small increase in biochar grindability. Under the grinding conditions, a significant size reduction (34-66% cumulative volumetric size below 75 µm) for biochars can be achieved after 4 minutes grinding (in comparison to only 19% for biomass after 15 minutes grinding), leading to a significant increase in volumetric energy density (e.g., from ∼8 to 19 GJ/m3 for biochar prepared from pyrolysis at 400 °C). Whereas grinding raw biomass typically results in large and fibrous particles, grinding biochars produces short and round particles. The results in this article indicate that biochar has desired fuel properties and potentially a good solution to address the key issues including high transport cost and poor grindability associated with the direct use of biomass as a fuel.
1. Introduction Coal is responsible for the majority (over 70%) of electricity generation in Australia and will continue to provide cheap and secure electricity supply to the Australian economy in the foreseeable future.1 However, stationary coal-fired power generation is one of the biggest contributors to various emissions including carbon dioxide, leading to global warming and the problems related to climate change.2 In the transition to future sustainable development, renewable energy sources are becoming increasingly important in the global energy supply mix. The International Energy Agency predicts that renewable energy will increase to 15% of the global energy supply by 2030, of which biomass will be the single most important renewable energy source, accounting for over 70% of the total renewable energy supply.3 Australia is projected for significant expansion of a biomassbased power-generation industry especially from short rotation * To whom correspondence should be addressed. E-mail: h.wu@ curtin.edu.au. Telephone: +61-8-92667592. Fax: +61-8-92662681. (1) ABARE Energy update 2008; Australian Bureau of Agricultural and Resource Economics., 2008. (2) IPCC, In Intergovernmental Panel on Climate Change, http:// www.ipcc.ch: 2009. (3) IEA World energy outlook 2006; International Energy Agency: Paris, 2006.
crops of the genus Eucalypt or mallee.4,5 As a byproduct of managing dryland salinity to prevent the loss of quality agricultural land, mallee biomass does not compete with but complements to food production. Mallee is typically grown in conventional agricultural land in the form of alley farming and has the potential to produce large quantities of biomass.4,6-8 For example, in western Australia alone, the potential annual production of mallee biomass can be ∼10 million dry tons per annum.6,8 Life cycle analysis has demonstrated that mallee biomass production in Australia is close to carbon neutral9 and achieves an energy ratio of 41.7 and an energy productivity of 206.3 GJ/(ha year) far exceeding the performance of other energy crops, for example canola, which has an energy ratio below 7.0 and energy productivity below 40.0GJ/(ha year).10 (4) Sochacki, S. J.; Harper, R. J.; Smettem, K. R. J. Biomass and Bioenergy 2007, 31 (9), 608–616. (5) Raison, R. J. Biomass and Bioenergy 2006, 30 (12), 1021–1024. (6) Bartle, J.; Olsen, G.; Don, C.; Trevor, H. Int. J. Global Energy Issues 2007, 27 (2), 115–137. (7) Clark, C. J.; George, R. J.; Bell, R. W.; Hatton, T. J. Aus. J. of Soil Research 2002, 40, 93. (8) Cooper, D.; Olsen, G.; Bartle, J. Aus. J. of Experimental Agriculture 2005, 45, 1369. (9) Yu, Y.; Wu, H. Life cycle greenhouse gas emission from mallee biomass production. In CHEMECA Newcastle, Australia, 2008. (10) Wu, H.; Qiang, F.; Rick, G.; Bartle, J. Energy Fuels 2008, 22, 190– 198.
10.1021/ef900494t CCC: $40.75 2009 American Chemical Society Published on Web 07/24/2009
Grindability of Biochars
Energy & Fuels, Vol. 23, 2009 4175 Table 1. Comilling Test Experience Using Ball Mill Systems
plant Australia15
biomass type
biomass blend ratio (wt %)
woodchips
5
Shawville Generating Station, USA16
sawdust, tree trimming, hybrid poplar
3
Plant Hammond, USA17
Wood
9.7-13.5
Georgia Power Company, USA18
Bark
5-20
Macquarie Generation,
Therefore, mallee biomass can contribute significantly to local energy security and regional development in Australia. However, there are several undesired characteristics of using biomass as a fuel. Biomass is bulky and has high moisture content and low energy density. In the case of mallee, the green biomass contains 45% moisture and has a very low energy density of ∼10 GJ/ton.11 Long distance transport of mallee biomass is not economic and the capacity of dedicated bioenergy plants utilizing mallee biomass as feedstock is constrained.12 These undesired characteristics also impose significant limitations on biomass utilization such as biomass cofiring, which is considered to be a key strategy to increase biomass utilization in the vast existing coal-based power stations. For example, the uptake of biomass for cofiring in coal-fired power stations is limited13 and the average cost of power generation is also increased.14 Furthermore, biomass as a fuel also has a poor grindability due to their bulky and fibrous nature, leading to significant costs for size reduction of biomass materials as well as other operating and maintenance costs. In fact, poor grindability of biomass is another key factor limiting the uptake of biomass as a fuel for cofiring in conventional coal-based power plants. As results of poor grindability of biomass, coarse biomass particles may result in incomplete burn out, blockage/bridging to the feeding system, sedimentation, and poor mixing. Large biomass particles also increase milling costs and require expensive storage. For example, a previous study found that the comilling of coal and biomass in a pilot scale vertical spindle mill is limited by a maximal 5 wt % biomass blend.15 Although widely used in coalfired power stations, ball mills are less suitable to grind biomass for its gravity impacts and tumbling actions only flattened biomass fiber rather than cutting. Several previous trials15-18 of biomass grinding in ball mill systems or combinations of ball-race and ball-bowl systems were unsuccessful (Table 1). Attempts have been made to address the above problems associated with the direct use of biomass as a fuel. This includes (11) Olsen, G.; Cooper, D.; Huxtable, D.; Carslake, J.; Bartle, J. DeVeloping multiple purpose species for large scale reVegetation, search project final report (nht project 973849); Department of Conservation and Land Management: Perth, Western Australia, 2004. (12) Yu, Y.; Bartle, J.; Li, C.-Z.; Wu, H. Energy Fuels 2009, 23, 3290. (13) Sami, M.; Annamalai, K.; Wooldridge, M. Prog. Energy Combust. Sci. 2001, 27, 171–214. (14) NAFI Wood waste bioenergy information sheet no.9 - cost considerations in using wood waste to produce renewable energy; National Association of Forest Industries: Deakin, ACT, Australia, 2006. (15) Zulfiqar, M. H.; Moghtaderi, B.; Wall, T. F. Co-milling of coal and biomass in pilot-scale Vertical spindle mill; CooperatiVe Research Centre for Coal in Sustainable DeVelopment (CCSD), QCAT, Technology Transfer Centre, Pullenvale, Quensland, 2006. (16) Prinzing, D. E.; Hunt, E. F. Fuel Process. Technol. 1998, 54, 143– 157. (17) Hughes, E. E.; Tillman, D. A. Fuel Process. Technol. 1998, 54, 127–142. (18) Boylan, D. M. Biomass and Bioenergy 1996, 10 (2-3), 139–147.
summary Ball mill. Unsuccessful, fibrous biomass flattened but particle size remains large. Different densities contribute to biomass build up problem on coal and ball charge bed. Ball-race mill and bowl mill. Grindability index reduced by 6 points. Feeder limitation causes 8-10 MW loss of boiler capacity, milling power increase 4-5%, mill outlet temperature increase. Ball-race mill. Larger particle size, mill power increase, unburned combustibles higher compared to firing coal although associated boiler operates at full capacity. Ball-Race mill. Unsuccessful, fibrous bark material formed “bird nests”, blockage of fuel flow.
compaction methods such as biomass briquette/pelletizing19,20 or light upgrading methods such as torrefaction.21 Another approach is biomass pyrolysis, through which the bulky green biomass can be converted to biomass-derived fuels such as biochar and/or biooil. Pyrolysis is a low-cost technology, which is flexible to process a wide variety of feedstocks.22 Recent developments in slow or fast pyrolysis studied within a low to moderate temperature window and investigated product yield, product compositions, and applications.22-28 Pyrolysis of mallee biomass for the production of biochar and bio-oil is one of the possible routes for establishing a malleebased bioenergy industry in WA. The main objectives of this study are to investigate the properties of biochar produced from the pyrolysis of mallee wood (species Eucalyptus polybractea) under slow-heating conditions and the possibility to use biochar as a solid fuel, addressing key issues associated with the direct use of biomass as a fuel, including being bulky, of high moisture, low-energy density and poor grindability. Experimental work was focused on the key biochar properties that are important to fuel applications, including fuel chemistry, grindability, bulk density, mass and volumetric energy densities, and particle shape, and so forth, benchmarking against Collie coal in Western Australia. 2. Experimental Section 2.1. Samples and Pyrolysis Experimental Setup. The mallee wood sample was prepared by separating the wood component from the mallee tree (species: E. polybractea), which was obtained from the Narrogin area in Western Australia. The lignocellulosic compositions of biomass sample, on a dry basis, are 41.5% cellulose, 27.5% hemicellulose, 29.7% lignin, and 1.3% ash. The wood sample was then cut to about 1 × 1 cm2 size, 3 mm thick, and stored in freezer under -4 °C before experiments. As-mined (19) Finney, K. N.; Sharifi, V. N.; Swithenbank, J. Energy Fuels 2009, 23, 3203. (20) Finney, K. N.; Sharifi, V. N.; Swithenbank, J. Energy Fuels 2009, 23, 3195. (21) Bergman, P. C. A.; Boersma, A. R.; Kiel, J. H. A.; Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. Torrefaction for entrained-flow gasification of biomass; Energy Research Centre of the Netherlands: Petten, The Netherlands, 2005. (22) Bridgewater, A. V. Thermal Science 2004, 8 (2), 21–49. (23) Chiaramonti, D.; Oasmaa, A.; Solantausta, Y. Renewable and Sustainable Energy ReViews 2007, 11 (6), 1056–1086. (24) Garcia-Perez, M.; Adams, T. T.; Goodrum, J. W.; Geller, D. P.; Das, K. C. Energy Fuels 2007, 21 (4), 1827–2774. (25) Hu, S.; Xiang, J.; Sun, L.; Xu, M.; Qiu, J.; Fu, P. Fuel Process. Technol. 2008, 89 (11), 1096–1105. (26) Mohan, D.; Pittman, C. U.; Steele, P. H. Energy Fuels 2006, 20, 846–889. (27) Calabria, R.; Chiariello, F.; De Bellis, V.; Massoli, P. In Combustion fundamentals of pyrolysis oil based fuels. 4th Mediterranean Combustion Symposium; Lisbon, Portugal, 2005. (28) 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.
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Collie coal sample was collected from the Collie coal field and then also cut to a similar size. Collie coal is the only coal currently being mined for power generation in Western Australia so that it makes sense to use this coal as a benchmark. In this study, the biomass samples were dried in an oven at 40 °C and Collie coal sample was dried at 105 °C in an oven to reduce the moisture contents of all fuels to be around 4-5%. These samples are referred as “dried wood biomass” sample and “Collie” coal sample. The pyrolysis experiments were carried out in a fixed-bed reactor (length, 136 mm; ID, 102 mm), similar to the ones used in our previous studies on the pyrolysis of Collie coal at slow heating rates.29,30 The reactor was externally heated by an electric furnace with a thermocouple inserted in the sample bed for temperature control. Briefly, a biomass sample (∼80 g) was charged into the reactor at room temperature and the reactor was heated at 10 °C/ min to 105 °C and held for 20 min for drying before being further heated to a desired pyrolysis temperature and maintained at the temperature for 30 min. Nitrogen was used as a carrier gas throughout the experiment at a flow rate 2.00 L min-1. Biochar samples were prepared at pyrolysis temperatures of 300, 320, 330, 350, 400, 450, and 500 °C. These biochar samples are referred as “WCxxx” chars, where “xxx” indicates the pyrolysis temperature in degrees Celsius. After each experiment, the biochar sample was cooled, collected, and stored in a desiccator. Moisture in the char was also determined. The produced biochars along with the biomass sample and the Collie coal sample were then subjected to various physical and chemical characterizations. It should be noted that, different from the raw biomass and coal samples, no drying was carried out on the biochar samples. 2.2. Grindability Experiments. A series of experiments for biochar grindability investigation was carried out using a laboratory ball mill (Retsch Mixer Mill MM400). Briefly, a biochar sample was charged into the grinding cell and the ball mill operated at a grinding frequency 15 Hz with a 15 mm ball size. During grinding, the same volume of samples was charged in the cells for all fuel samples. Various grinding times (1, 2, and 4 min) were considered. Grinding experiments were also carried out for the dried wood biomass and Collie coal samples under the same grinding conditions. Grinding at longer grinding times (8 and 15 min) was done for the dried wood biomass and the WC300 char for comparisons. Multiple milling experiments were carried out to produce sufficient amounts of ground samples for subsequent analysis. The milling energy consumption for grinding was also estimated based on electricity usage during sample milling, serving as an indicator for the potential energy saving in grinding, benchmarking against different fuel samples. Bulk and Volumetric Energy Density. Bulk density of all fuel samples (unground and ground) was determined using a filling and tapping procedure.31 Briefly, a fuel sample is loaded in discrete portions into a glass column with known volume. After filling of each portion, the cylinder was tapped onto a bench until no volume change was observed. The final volume and sample weight were recorded. Multiple measurements were done for each sample and the results are reported in the unit of kg/m3. For bulk density measurements, the standard error is 1.5%. 2.3. Particle Size Distributions and Particle Shape. Fuel samples were sieved into two fractions. One is the size fraction of size >1.8 mm and the other fraction of size
