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Energy & Fuels 1996, 10, 652-658
High-Yield Biomass Charcoal† Michael Jerry Antal, Jr.,* Eric Croiset, Xiangfeng Dai, Carlos DeAlmeida, William Shu-Lai Mok, and Niclas Norberg Hawaii Natural Energy Institute and the Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii 96822
Jean-Robert Richard and Mamoun Al Majthoub Laboratoire de Combustion et Syste` mes Re´ actifs, Centre National de la Recherche Scientifique, 1C Avenue de la Recherche Scientifique, 45071 Orle´ ans, Cedex 2, France Received September 25, 1995. Revised Manuscript Received January 4, 1996X
The theoretical yield of charcoal from biomass lies in the range 50-80% on a dry weight basis. In spite of the fact that mankind has been manufacturing charcoal for about 6000 years, traditional methods for charcoal production in developing countries realize yields of 20% or less, and modern industrial technology offers yields of only 25-37%. Moreover, reaction times for the batch process in an industrial kiln are typically 8 days. In this article we describe a practical method for manufacturing high-quality charcoal from biomass that realizes near-theoretical yields of 4262% with a reaction time of about 15 min to 2 h, depending on the moisture content of the feed. Because of its high efficiency, this technology can help to reduce worldwide deforestation and pollution, while providing greater amounts of a desirable, renewable fuel and chemical resource to mankind.
Introduction “Founder’s hoards” of the Bronze age scattered throughout Europe indicate that shallow pits of charcoal were used to smelt tin before the dawn of recorded history.1 Today, charcoal continues to be used as a reductant in the iron and steel industry. For example, over one million tonnes of wood charcoal were consumed in Brazil during 1991 to smelt iron ore.2 Other uses for charcoal include the production of chemicals (e.g., carbon disulfide, calcium carbide, silicon carbide, sodium cyanide, soil conditioners, and various pharmaceuticals), the refining of metals (e.g., copper, bronze, silicon, nickel, aluminum, and electro-manganese), the manufacture of fireworks, the production of activated carbon, and domestic cooking and heating. The FAO estimates worldwide charcoal production during 1991 to be 3.8 million tonnes.2 Remarkably, methods for manufacturing charcoal have not changed greatly during the past six millennia. Here in Hawaii, the ever popular Kiawe wood charcoal is produced on the island of Niihau by piling Kiawe logs in a pit, setting the pile on fire, covering it with dirt, and returning some days later to dig out the charcoal. * Author to whom correspondence should be addressed. Email:
[email protected]. † We dedicate this paper to our colleague Dr. Woraphat Arthayukti. X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Agricola, G. De Re Metallica; Dover Publications: New York, 1950; pp 354-355. (2) Quarterly Bulletin of Statistics; Food and Agriculture Organization of the United Nations, 1992; Vol. 5 (4). Surprisingly, the Associacao Brasileira de Carvao Vegetal3 estimates Brazilian production during 1989 to be about 9.3 million tonnes. Such gross disagreements in estimates of regional charcoal production are not uncommon and point to the difficulty in assessing the role of this renewable commodity in national economies. (3) Statistical Yearbook; Associacao Brasileira de Carvao Vegetal, Brazil, 1990.
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On the mainland, Missouri kilns4-7 are used to carbonize biomass with a typical yield of about 25% (by weight charcoal from wood on an oven dry basis) during a 7-12 day operating cycle. However, in other countries the cycle time can be 1-2 months.8 Improved Beehive kilns in Brazil achieve charcoal yields from Eucalyptus and other woods as high as 31-35% on an 8 day operating cycle.9 Prior to 1991 the highest yield of charcoal from biomass reported in the literature was 38%.7,10 Contrary to common opinion, charcoal is expensive throughout much of the world. In developing countries its price varies from $0.09 to $0.18/kg, but can be as high as $0.40 in Africa.11 Barbecue charcoal retails in grocery stores in the USA and Europe at prices between $0.55 and $2.20/kg. But in the USA coal, plastics and other non-biomass material are often used to manufacture the lower priced briquettes, which cannot be sold as charcoal in Europe. The recognition that a common form of charcoal in the USA cannot be marketed as “charcoal” in Europe prompts the question: “What is charcoal?”. Emrich4 (4) Emrich, W. Handbook of Charcoal Making; Reidel: Dordrecht, 1985. (5) Foley, G. Earthscan Technical Report No. 5; International Institute for Environment and Development; Russell Press: Nottingham, U.K., 1986. (6) Baker, A. J. Charcoal Industry in the U.S.A. In Symposium on Forest Products Research International-Achievements and the Future; National Timber Research Institute: Pretoria, Republic of S. Africa, 1985; Vol. 5. (7) Slocum, D. H., McGinnes, E. A., Beall, F. C. Wood Sci. 1978, 11, 42-47. (8) Bhattacharya, S. C.; Shrestha, R. M.; Sivasakthy, S. State of the Art for Biocoal Technology; Asian Institute of Technology: Bangkok, 1988. (9) DeAlmeida, M. R.; Magalhaes, J. G. R.; de Melo e Souza, R. M. Some Considerations about Charcoal Production in Brazil, 1982. (10) Antal, M. J.; Mok, W. S. L.; Varhegyi, G.; Szekely, T. Energy Fuels 1990, 4, 221-225. (11) Mendis, M. S. World Bank, unpublished material.
© 1996 American Chemical Society
High-Yield Biomass Charcoal
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proposes the following definition: “charcoal is the residue of solid non-agglomerating organic matter, of vegetable or animal origin that results from carbonization by heat in absence of air at a temperature above 300 °C”. Emrich notes that the volatile matter content of a good-quality charcoal should not exceed 30%; whereas Foley5 indicates that the optimum volatile matter content depends upon the end use. For example, Foley states that metallurgical grade charcoal should have a fixed-carbon content of 85-90%, whereas charcoal intended for domestic cooking should have a minimum volatile matter content of 20-30%, and a maximum of 40%. In this article the word charcoal refers to carbonaceous material derived from biomass with a volatile matter content below 30%. Ten years ago Dr. Woraphat Arthayukti of Chulalongkorn University in Bangkok asked one of the authors (M.J.A.) to speak to a USAID meeting on methods to improve the yields of charcoal from biomass. Arthayukti justified this request by explaining that deforestation in Thailand was a serious problem and that much of the wood taken from Thai forests was used to produce charcoal by traditional methods with very low yields. He also observed that excessive pollution, which always accompanies charcoal manufacture, is a concomitant result of the low yield.12,13 Responding to Arthayukti’s request, we were led to ask the question: “What is the stoichiometric limit to the yield of charcoal from biomass?” As cellulose is the principal component of woody biomass, the stoichiometry of charcoal formation
C6H10O5 f 6C + 5H2O
(1)
indicates a potential charcoal yield of 44.4 wt %. Accounting for the fact that a high-quality charcoal contains about 82% fixed carbon, the maximum charcoal yield from cellulose is about 54%. More detailed calculations, based on the actual composition of potential substrates, led to the conclusion that the theoretical yield of charcoal from most biomass feeds should be in the approximate range 55 (corn cobs with a carbon content of 45%) to 71% (Macadamia nut shells with a carbon content of 58%). Thus it became evident that Arthayukti’s request unveiled an opportunity to dramatically improve a process that has been in continuous use by mankind for the past 6000 years. With a small grant from the State of Hawaii, we explored the influences of thermal pretreatments, heating rate, peak temperature, catalysts, feedstock composition, and other conditions on charcoal yields.10 Only one condition was found to significantly increase the yield: elevated pressure combined with a prolonged vapor phase residence time. The reason for this increase is easy to understand. Thermolysis of the (12) For example, charcoal production is estimated to release over 30 Tg of carbon to the atmosphere each year. See: Levine, J. S.; Cofer, W. R.; Cahoon, D. R.; Winstead, E. L. Environ. Sci. Technol. 1995, 29, 120-124. (13) Recent work has shown that nearly 35% of the carbon in the wood feedstock of a traditional charcoal kiln is released as products of incomplete combustion (PIC’s), which contribute more to global warming than the atttendant carbon dioxide emissions. See: Smith, K. R.; Thorneloe, S. A. Household Fuels in Developing Countries: Global Warming, Health, and Energy Implications. In Proceedings from the 1992 Greenhouse Gas Emissions and Mitigation Research Symposium; Air and Energy Engineering Research Laboratory: Washington, DC, 1992.
biopolymer creates monomeric (e.g., levoglucosan) and oligomeric (e.g., cellobiosan) species, as well as their degradation products (e.g., glycolaldehyde), which immediately enter the vapor phase.14 These species are extremely reactive. If permitted to quickly escape the reactor (as in a fluidized bed), they form condensable oils and tars. When held in contact with solid biomass undergoing pyrolysis within the reactor, the vapors degrade further to form charcoal, water vapor, and various light gases. In 1989 a small bomb type reactor, fabricated from pipe and heated in a furnace, first demonstrated charcoal yields from wood as high as 44%. This reduction to practice became the basis of a patent application by the University of Hawaii, which was recently granted.15 In this paper we describe the design and operation of practical, large-scale reactors which effect conditions that offer near-theoretical yields of charcoal from biomass. Special attention is given to the influences of elevated pressure and the moisture content of the feedstock on the charcoal yield. Apparatus and Experimental Procedures Following the successful outcome of the pipe bomb experiments, a process development unit (PDU) was designed and constructed to demonstrate the technology of high-yield (HY) charcoal manufacture on a larger scale.16,17 Figure 1 captures many of the details of the PDU’s operation. Moist wood logs (15 cm diameter × 30 cm length), chips, nut shells, or other biomass are loaded into a 25 cm I. D. × 163 cm cylindrical steel canister, whose top is subsequently covered by a lid. The canister is then lifted into a cylindrical pressure vessel capped by a pressure-tight hinged closure. Larger reactors can employ mechanized hinged closures to speed the loading and unloading process. The PDU was heated by four 4 kW Tempco electrical resistance heaters, which projected through the bottom flange of the pressure vessel into a 61 cm tall annular space within the canister. Power consumption was measured by two Westinghouse watt-hour meters. Recently, the PDU was refurbished with an Eclipse 11.4 cm (4.5 in.) Auto-Recupe gas-fired radiant burner, which is a much cheaper and more practical source of process heat. Some of the results presented below were obtained with the gas-fired burner. If the feedstock contains large, moist logs and the reactor is cold, about 150 min of heating are required to produce a high-quality, HY charcoal. From hot-start only about 60 min are needed to carbonize moist logs, and less than 30 min are required if the feedstock is dry (
