Integrated SSFR Process Could Make Cellulosic Biobutanol More Competitive with Ethanol

Jose Michael

Researchers at the US Agricultural Research Service (ARS) are refining an integrated method of producing cellulosic biobutanol that could make it more competitive with ethanol as an alternative to gasoline. (Earlier post.)

Research team leader Nasib Qureshi began investigating the production of cellulosic biobutanol from wheat straw in 2003. His initial fermentation-based process involved the usual four sequential steps in such an approach (pre-treatment, enzymatic hydrolysis, fermentation using Clostridium bacteria, and recovery). Qureshi and colleagues then devised a way to consolidate three of the four steps into a simultaneous saccharification, fermentation and recovery (SSFR) process.

Dilute acid pretreated wheat straw in a bioreactor is simultaneously saccharified by a combination of three commercial enzymes and fermented by a culture of C. beijerinckii P260, which produces a combination of acetone, biobutanol and ethanol (ABE). There is no expensive detoxification step (overliming) typically required for dilute acid pretreated feedstock, thus offering significant cost savings.

A gas stripping procedure (the fourth step) is used to remove the acetone, biobutanol, and ethanol as they are produced. Gas stripping also solves the problem of product inhibition caused by the toxic effect of an accumulation of butanol on the fermenting organism, C. beijerinckii.

In early trial runs, the method increased biobutanol productivity by twofold above traditional glucose-based fermentation, but the pace of fermentation outran the pace of hydrolysis. Qureshi added small batches of additional sugar in a process dubbed “fed-batch feeding”. This resulted in a significant increase in biobutanol production.

During a 22-day fed-batch operating period, a culture of C. beijerinckii P260 converted nearly 430 grams of sugar (glucose, xylose, arabinose, galactose, and mannose) into 192 combined grams of acetone, biobutanol, and ethanol.

If scaled up further, the process could yield 307 combined kilograms, or 99 gallons, of acetone, biobutanol, and ethanol from 1 ton of wheat straw. According to Dr. Richard Bain at the National Renewable Energy Laboratory, the current expected yield for cellulosic ethanol from a biochemical process is about 90 gallons per ton of biomass. Dry mill corn ethanol yields about 102 gallons per ton of grain.

C. beijerinckii produces a specific ratio of the ABE chemicals (about 3:16:1), but efforts are under way at ARS to develop genetically modified bacteria that will make only biobutanol.

The SSFR process reduces the production cost of butanol significantly and can make the commercial production of butanol from lignocellulosic feedstock much more economical.

Qureshi and his colleagues are also investigating the use of other cellulosic feedstocks for the process.

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