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Chemical catalysis uses an added—but not consumed—substance to augment a chemical reaction. Catalytic conversion will be a primary tool for industry to produce valuable fuels, chemicals, and materials from biomass platform chemicals. The National Corn Growers Association corn fiber biorefinery project, for example, will make glycols from sugars and Biomass Program researchers are looking at catalysis of pyrolysis oil as a renewable way of producing hydrogen. Catalytic conversion of biomass is best developed, however, for synthesis gas or syngas.
Syngas (a mixture of carbon monoxide and hydrogen) produced by gasification of fossil fuels or biomass can be converted into a large number of organic compounds that are useful as chemical feedstocks, fuels and solvents. At the center of this transformation is a selective catalyst that works under heat and pressure to convert the carbon monoxide into larger, more useful compounds. Many of the conversion technologies were developed for natural-gas-derived syngas, but would apply similarly to biomass syngas.
Franz Fischer and Hans Tropsch first studied conversion of syngas into larger, useful organic compounds in 1923. Using syngas made from coal, they were able to produce liquid hydrocarbons rich in oxygenated compounds in what was termed the Synthol process. Succeeding these initial discoveries, considerable effort went into developing improved and more selective catalysts for this process. Collectively, the process of converting CO and H2 mixtures to liquid hydrocarbons over a transition metal catalyst has become known as the Fischer-Tropsch (FT) synthesis.
The first FT plants began operation in Germany in 1938 but closed down after the Second World War. Then in 1955, Sasol, a world-leader in the commercial production of liquid fuels and chemicals from coal and crude oil, started Sasol I in Sasolburg, South Africa. Following the success of Sasol I, Sasol II and III, located in Secunda, South Africa, came on line in 1980 and 1982, respectively. The syngas at these three plants as well as at several other plants abroad is converted to more than 200 fuel and chemical products. In the early 1990's, two other FT plants came on line. The Mossgas plant which converts natural gas to FT products using a high temperature process and an iron catalyst started up in South Africa in 1992. Additionally, Shell commissioned a plant in 1993 in Bintuli, Malaysia using the Shell Middle Distillate Synthesis process, which is essentially, enhanced FT synthesis. Currently, Syntroleum is building a 10,000 BPD specialty chemicals and lube oil plant located in Northwestern Australia.4
Common problems of Fisher-Tropsch synthesis (FTS) are the unavoidable production of a wide range of hydrocarbon products (olefins, paraffins, and oxygenated products) and the sensitivity of the catalyst to contamination in the syngas that "poison" or inactivate the catalyst. Research to improve both the selectivity of catalysts to make purer, high value products and better resistance to "poisons" both contribute to lowering the cost of production.
A second, important, conversion processes for syngas (usually made from natural gas) is to methanol and has become an important industry world-wide. Methanol is a commodity chemical, one of the top ten chemicals produced globally and is an important chemical intermediate used to produce a number of chemicals, including: formaldehyde, dimethyl ether, methyl tert-butyl ether, acetic acid, and olefins, to name a few. Methanol can also be used directly or blended with other petroleum products as a clean burning transportation fuel. Overall, as of January 1, 2002 worldwide annual capacity of methanol was 12.8 billion gallons.2
Process Description
Raw syngas from the gasifier needs to first have contaminants removed that would inactivate the catalyst. This includes sulfur compounds (e.g. H2S, mercaptans), nitrogen compounds (e.g. NH3, HCN), halides (e.g. HCl), and heavy organic compounds that are known collectively as "tar". Next, depending on the catalyst being used and the product being made, the ratio of hydrogen to carbon monoxide may need to be adjusted and the carbon dioxide byproduct may also need to be removed. For methanol synthesis a ratio of 2:1 hydrogen to carbon monoxide is common but research is ongoing to allow lower ratio hydrogen:carbon monoxide syngas to be used.1,3 This allows syngas to be produced from a wider variety of carbon containing materials, like biomass, to be used more economically. This clean gas is then compressed to the required operating pressure. For methanol synthesis pressures of 50 — 100 bar is common, for FTS the pressures can vary from atmospheric (1 bar) up to 150 bar. The clean compressed gas is then passed through the reactor containing the catalyst at the appropriate temperature. The reactor temperature is typically around 200ºC for methanol synthesis and 150 — 250ºC for FTS reactors. Temperature control is very important since the reaction is very exothermic (heat releasing) and temperature changes of as little as 10ºC can have drastic changes in the efficiency of conversion and the range of products produced. At the exit of the reactor, the products are separated from the partially converted gas, usually by condensation and the unconverted gas is recycled to the entrance of the reactor. The products are then further processed using technology that is commonly use in all major petroleum refineries, to separate the different fractions, depending on their best use, and are then upgraded into their final products.
Commercial Status
The conversion of syngas to products has only been economic when inexpensive natural gas is available and generally has been limited to the manufacture of methanol. Commercial FTS in South Africa, based on coal derived syngas, was driven partially by the world-wide oil embargo on South Africa in response to their system of apartheid. Without access to inexpensive petroleum, the higher cost of synthetic fuel production was acceptable in the face of having to do completely without.
However, as the cost of liquid hydrocarbon fuels continues to increase with a rise in crude oil prices as well a concerns about depleting fossils fuel resources mount, interest in the production of high value fuels from low cost and renewable feedstocks has increased.
References
1Bhatt, B. L., Heydorn, E. C., Tihm, P. J. A., Street, B. T., and Kornosky, R. M. (1999). "Liquid phase methanol (LPMEOH) process development." Preprints - American Chemical Society, Division of Petroleum Chemistry, 44(1), 25-27.
2Davenport, B. (2002). "Methanol." Chemical Economics Handbook Marketing Research Report, SRI International, Menlo Park, CA.
3DOE, U. S. (1992). "Commercial-Scale Demonstration of the Liquid Phase Methanol (LPMEOHTM) Process." DOE/FE-0243P, U.S. Department of Energy.
4Spath, P. and Dayton, D. (2002). "Fisher-Tropsch Synthesis Technology Brief." NREL Milestone Report ID# FY03-6985
For Further Reading
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