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Rethinking thermochemical conversion of biomass into biofuel.

All fossil fuels found in nature--petroleum, natural gas, and coal, based on biogenic hypothesis--are formed through processes of thermochemical conversion (TCC) from biomass buried beneath the ground and subjected to millions of years of high temperature and pressure. In particular, existing theories attribute that petroleum is from diatoms and deceased creatures, and coal is from deposited plants.

TCC is a chemical reforming process of biomass in a heated and usually pressurized, oxygen-deprived enclosure, where long-chain organic compounds (solid biomass) break into short-chain hydro-carbons such as syngas or oil. TCC is a broad term that includes gasification (Fisher-Tropsch process), liquefaction (hydrothermal process), and pyrolysis (anaerobic burning). Gasification of biomass produces a mixture of hydrogen and carbon monoxide, commonly called syngas. The syngas is then reformed into liquid oil with the presence of a catalyst. Pyrolysis is a heating process of dried biomass to directly produce syngas and oil. Both gasification and pyrolysis require dried biomass as feedstock, and the processes occur in an environment higher than 600[degrees]C. The hydrothermal process involves direct liquefaction of biomss, with the presence of water and perhaps some catalysts, to directly convert biomass into liquid oil, with a reacting temperature of less than 400[degrees]C.


Historically speaking

The development of TCC technologies has always been linked to the energy shortage. Gasification of coal was first invented in 1923 by Franz Fischer and Hans Tropsch (consequently the name for the process) at the Kaiser Wilhelm Institute in the oil-poor, but coal-rich, Germany. By the end of World War II, Germany had produced more than 6 million metric tons (6.6 million tons) per year of synthetic oil from coal gasification. Then the 1970s oil crisis arrived. The Bureau of Mines in the United States investigated TCC of coal and wood to produce liquid fuel, and several pilot plants were developed. However, the economics of the technology was not sustained, largely due to the continuing low price of petroleum. The Fischer-Tropsch-type plants are in operation today in only a few countries including Germany, South Africa, and the United States.

$$$ today

Today, petroleum prices have reached a historic high. It is time to rethink the TCC technology for biomass conversion--not only for the sustainability of energy, but also for the protection of the environment.

From biomass to biofuel

TCC may have two pathways from biomass to biofuel: (1) direct conversion of biomass or (2) pretreatment of biomass and then fermentation. The biomass with little lignocellulosic fraction--such as waste streams from animal, human, and food processing--can be directly converted into biofuel thermochemically. Researchers at the University of Illinois have successfully developed TCC processes that converted 70 percent of swine manure volatile solids (dry mass) into crude oil, with heating values between 32 to 37 MJ/kg (13,800 to 15,900 BTU/lb). Similar energy production in waste streams for human wastes, lower but still significant oil yields, were also obtained with crop residues. When this technology is fully developed, the annual human and animal waste in the United States could yield more than 200 million barrels of crude oil, with a net zero carbon dioxide emission to the environment. This "green" energy potential and the avoided cost of waste disposal could further enhance the value of the fuel produced from TCC.

Gasification has been a widely applied process with lignocellulosic feedstocks. A direct hydrothermal process may be possible, but more research is needed. Pretreatment is currently a bottleneck in the conversion of cellulosic feedstock. TCC may hold a substantially greater potential to shorten the fermentation time of lignocellulose.

Traditionally, acid hydrolysis was commonly used to convert lignocellulosic materials to monosaccharides, but the high concentration of acids used in hydrolysis requires extensive waste treatment or recovery costs. The use of acid feedstock also requires a large capital investment for the necessary corrosion-resistant equipment. Relatively high temperatures are required to break down the crystalline cellulose to glucose by using dilute-acid hydrolysis, which causes decomposition of sugars at a significant rate and the degradation products, such as acidic acid, hydroxymethylfurfural, and furfural, significantly inhibit ethanol fermentation. Other commonly used pretreatment technologies include controlled pH, aqueous ammonia recycle, flow-through, and lime methods.

Back to Mother Nature

Lignocellulosic materials are the most abundant biomass available on earth. In the United States, one study showed that 1.2 billion metric tons (1.3 billion tons) of crop residue could be collected without decarbonating the soil. By natural design, lignocellulose has evolved to have some crystalline structures with closed rings, lacking open bonds to react with enzymes, thus resisting biological degradation. While scientists are engineering new enzymes that could break down lignocellulosic faster, the thermochemical conversion still has something important to offer: It can break down the long carbon chain much faster than biological means, typically on the order of minutes instead of days at present. The conversion of lignocellulosis biomass into biofuel requires a comprehensive approach employing science and engineering in biology, chemistry, and physics.

ASABE member Yuanhui Zhang is a professor in the Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, Ill., USA;
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Author:Zhang, Yuanhui
Publication:Resource: Engineering & Technology for a Sustainable World
Date:Apr 1, 2008
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