ISU to study biomass gasification for ethanol, process heat

By Susanne Retka Schill | February 04, 2009
Iowa State University's research on gasification technologies for the ethanol industry got a boost this winter in the form of a $2.37 million grant from the Iowa Power Fund. The research aims to create systems that produce process heat from clean biomass-derived synthesis gas. Ethanol plants could use these sytems to replace natural gas usage. Another dimension of the research aims to improve the process of making ethanol from the syngas produced by biomass gasification.

Song-Charng Kong and Robert Brown, professors in mechanical engineering at ISU, are leading the project with Frontline BioEnergy LLC and Hawkeye Renewables LLC as partners. "This is a commercially driven project," Brown said. It seeks to refine gasification technologies in order for ethanol plants to reduce energy costs and improve their carbon footprints. A rule of thumb is that replacing natural gas with syngas for process heat in ethanol plants will reduce greenhouse gas emissions (GHG) by 90 percent, he said.

Although wood chips are easier to use in biomass gasification, the targeted biomass feedstock in this project is corn stover, whose nitrogen content presents a challenge, Brown said. The nitrogen in the feedstock itself, along with nitrogen from the air in some processes, increases nitrogen oxide emissions (NOx)—gases with a far greater GHG impact than carbon dioxide. Distillers grains would be another candidate for gasification, although its protein content creates other issues. One goal of the ISU project is to demonstrate a low NOx burner developed by Kong to reduce those GHG emissions. NOx burners were originally developed for coal and natural gas, and aren't yet widely available for the mixture of gases found in syngas.

The project will use a pilot-scale fluidized-bed gasification unit, developed by Frontline and ISU, at the Iowa Energy Center's Biomass Energy Conversion facility in Nevada, Iowa. It will also demonstrate a Frontline process to clean tar out of the syngas produced from gasifying biomass. While the tar burns well, Brown explained, it can precipitate out when syngas cools as it's piped from the gasifier.

Another dimension of the project will test catalysts that convert syngas into ethanol. These are being developed by ISU chemistry professor Victor Lin. The advantage to using gasification, a thermochemical process, is the potential to convert all the biomass components into ethanol. Companies working on the other broad category of cellulosic ethanol technology, biochemical conversions, are targeting the cellulose and hemicellulose content of biomass by using pretreatments, enzymes and microbial fermentation. This approach, however, leaves more than one-fourth of the biomass bound in lignin. Gasifying biomass converts it all with a theoretical maximum ethanol yield of 187 gallons per ton, significantly higher than the other cellulosic ethanol technologies.

While the theoretical yield is promising, the actual conversions being reported by companies working on the thermochemical gasification process are closer to 120 gallons per ton, Brown said. That compares with the theoretical maximum yield of 110 gallons per ton for corn-based ethanol, which the industry is approaching, he added. "Remember, the cost is around $150 per ton for corn, whereas biomass is expected to cost between $50 and $100 per ton," he said.

The ultimate goal of the collaboration partners is to develop cost-effective technologies that can be adapted in the existing corn-based ethanol industry within a reasonable payback time. Frontline BioEnergy, located in Ames, Iowa, has installed a gasification unit at Chippewa Valley Ethanol Co. LLLP in Benson, Minn., which is using wood chips to displace 90 percent of its natural gas. In production since April, the gasifier burns approximately 380 tons of wood waste per day. Long-term plans include the use of grasses, corn cobs and other ag residues.