Advanced Biomass Pyrolysis vs. Advanced Bacterial Fermentation

GCCBiomass to liquid fuels (BTL) conversions will assume increasing importance as the technologies achieve greater yields and economies. Both thermochemical and fermentation approaches are capable of producing high quality substitute fuels and chemical feedstocks. And both technological approaches are receiving a lot of attention as the race for substitute fuels heats up.

UCSB researchers have developed an advanced pyrolytic method of biomass to fuels conversion, using methanol as a super-critical reaction medium.

Researchers at the University of California, Santa Barbara (UCSB) have developed a one-pot process for the catalytic conversions of wood and cellulosic solids to liquid and gaseous products in a reactor operating at 300–320 °C and 160-220 bar. Little or no char is formed during this process.

The reaction medium is supercritical methanol (sc-MeOH) and the catalyst—a copper-doped porous metal oxide—is composed of earth-abundant materials, they report in a paper published in the Journal of the American Chemical Society. The major liquid product is a mixture of C2–C6 aliphatic alcohols and methylated derivatives thereof that are, in principle, suitable for applications as liquid fuels.

There have been two types of approaches conventionally considered for the conversion of woody biomass to liquid fuels, Matson et al. note: (a) acid pretreatment and separation followed by fermentation or liquid phase processing, or (b) high temperature conversion such as gasification or pyrolysis to bio-oils. Each of these process has problems related to their efficiency. _GCC

Meanwhile, dozens of research centers around the world are tweaking the genomes of microbes in the attempt to create self-reproducing factories for the production of high value chemicals and fuels from cheap biomass feedstock.

A new microbe engineering trick could potentially make butanol, a promising biofuel, so cheaply that it could compete with ethanol. By tapping into a highly efficient metabolic pathway, scientists at Rice University engineered E. coli to convert sugars to butanol 10 times more efficiently than any other organism.

...Cobalt Biofuels, a biobutanol startup based in Mountainview, California, uses Clostridium bacteria to break down plant matter and convert the resulting sugars into a mix of butanol, acetone, and ethanol. Gevo, a company based in Englewood, Colorado is working with E. coli that are altered to divert some of their metabolites, which would otherwise be involved in synthesizing amino acids, toward alcohol production. And Butamax, a joint venture between Dupont and BP, is using genetically modified yeast.

...Gonzalez and his colleagues [Rice U.] outlined their new approach in a paper published online in the journal Nature. The researchers tapped into a pathway that microbes use to break down fatty acids, which are hydrocarbon molecules, to generate energy. They modified about a dozen genes in E. coli to reverse this beta-oxidation pathway so that the microbes build fatty acids.

The method is more efficient than others because it adds two carbon atoms at a time, rather than one, to the hydrocarbon molecules being formed. "What makes it really efficient is that the mechanism by which those two carbon atoms are added to the chain doesn't require [energy]," Gonzalez says.

By selectively manipulating genes, the researchers can program the microbes to synthesize many different fuels and chemicals. In addition to butanol, the bacteria can produce various useful fatty acids that existing processes derive from plant and animal oils. _TechnologyReview

Microbial fermentation produces chemicals and fuels at lower temperatures, saving energy. But living organisms are somewhat fragile compared to inorganic catalysts and methods. None of the approaches are ready to compete head to head with the petroleum industry just yet. But things are looking very promising for breakthroughs within the next 5 to 10 years in thermochemical fuels, and the next 10 to 15 years for advanced microbial fermentation, according to Al Fin analysts.