Photos: Coskata's celluslosic ethanol conversion process
The announcement from General Motors at this year's Detroit auto show that it had invested in Coskata, a producer of cellulosic ethanol, created buzz across the automotive world. Coskata says its proprietary process, which relies on a combination of gasification and fermentation by microorganisms, is up to six times more efficient than the production of ethanol from corn. The process can handle everything from switchgrass to industrial waste to household garbage, produces fewer emissions and noxious byproducts than comparable systems, and will result in ethanol with a production cost of around one dollar per gallon.
While the world scrambles to prepare for alternatives to petroleum, concerns with corn-based ethanol--including its effect on rising food prices and its relative inefficiency--have become increasingly apparent. A promising alternative is cellulosic ethanol, which can be made from a variety of biomass feedstocks that place less demand on agricultural land and food-based crops. The amount of energy derived from cellulosic ethanol relative to the energy needed to harvest (known as the wells-to-wheels measurement) is also many times greater than that of corn-based ethanol. In December 2007, the Federal Government mandated the production of 36 billion gallons of ethanol annually by 2022, 21 billion of which is to come from cellulosic (non-corn-based) feedstock.
Coskata's ethanol conversion plants can handle a wide range of carbon-based feedstocks, including woodchips, switchgrass, corn stover, landfill waste, and even old tires. The first step in the process is gasification through combustion, which "cracks" the carbon-hydrogen bonds of the feedstock, turning it into synthetic gas or "syngas," a combination of carbon monoxide, hydrogen, and carbon dioxide.
The combustion process, which is largely self-sustaining, generates heat in the range of 1,700 degrees Fahrenheit. The gas must then be cooled to 100 degrees Fahrenheit before the next stage of the conversion process. According to Coskata, the excess heat can be recovered and used to create steam for co-located corn-based ethanol production facilities.
After being "scrubbed" to remove any particulate debris, the syngas is fed into a bioreactor, where the chemical conversion takes place. Tiny anaerobic microorganisms go to work on the syngas, digesting the hydrogen, carbon monoxide, and carbon dioxide, while producing a mixture of ethanol and water. The microorganism strains used by Coskata were developed at Oklahoma and Oklahoma state universities.
Coskata claims that its microorganisms can extract "almost the entire energy value available in the incoming syngas stream," producing more than 100 gallons of ethanol per dry ton of input material. It says that the use of microorganisms to break down the syngas is more cost-effective than using chemical catalysis (the other major means of converting syngas into ethanol) as the latter requires a purer stream of syngas and relies on expensive catalytic reactors.
The final stage of the process involves separating the ethanol from water, which yields 99.7 percent pure ethanol. Another benefit of the microorganism process is that it requires less water than either corn-derived ethanol production or cellulosic ethanol production using enzymatic pretreatment. Coskata's process uses less than one gallon of water per gallon of ethanol produced. According to the Department of Energy, cellulosic ethanol provides between 4.4 and 6.1 times the amount of energy used to produce it, compared with between 1.3 and 1.7 times for corn-based ethanol.
At present, only around 1,400 of the United States' 170,000 filling stations are ethanol-equipped. Earlier this year, GM's CEO Rick Wagoner said that between 15,000 and 20,000 more are needed. Research from Argonne National Laboratories shows that on a per-gallon basis, greenhouse-gas emissions from cellulosic ethanol are 85 percent lower than those from gasoline.
GM has 11 2008 models that can run on a mixture of 85 percent ethanol and 15 percent gas--known E85--including the Chevy Impala (pictured). The automaker has committed itself to making half of its new cars ethanol-compatible "flex-fuel" vehicles by 2012. Other companies that produce flex-fuel vehicles include Ford, Chrysler, Nissan, and Mercedes.