EERC Update

More Biomass Power Options for Ethanol Plants
By Bruce Folkedahl
Last month, I discussed the search for opportune biomass fuels. Once an ethanol plant has located a source of biomass fuel, including its own distillers dried grains with solubles upon market saturation, the conversion technology must be considered. The four basic ways to use biomass for heat and power are 1) direct combustion of the biomass as a sole fuel source, 2) cofiring biomass with fossil fuels, 3) gasification of biomass to produce a synthetic natural gas, or syngas, and 4) fast pyrolysis to produce a syngas and combustible liquid. All of these technologies have been commercialized, but fast pyrolysis is the only one not being used at ethanol facilities.

Direct combustion with biomass as the sole fuel source is fairly straightforward and has been practiced for years. Traditionally, stoker boilers have been this technology's workhorse. However, fluid-bed combustors (FBCs) have begun to supplant stokers because of the increased emission control inherent in FBC systems, which are more fuel flexible and have slightly higher overall efficiencies. The steam generated by both systems can be used in the ethanol process and for electrical production.

Cofiring biomass with coal or other fossil fuels allows one to get around the typical seasonal shortages that plague biomass fuel sources. In some situations, biomass cofiring has reduced overall emission levels compared to straight fossil fuel firing. The combustor configuration and fuel types need to be evaluated before this can be quantified.

Gasification may soon be the technology of choice for biomass utilization at ethanol facilities. Gasification systems can be robust enough to handle multiple fuel types and produce a combustible gas of comparable value for different fuels. Several companies have installed systems to deliver syngas that can be combusted to generate a plant's heat and power. Lignin from the lignocellulosic ethanol process can be gasified when implementing this technology at fuel ethanol plants. The syngas produced can be used in catalytic Fischer-Tropsch-type reactions to produce fuels and chemicals providing more value than simply combusting the gas. Several applications of this technology are being demonstrated under programs funded by the U.S. DOE.

In fast pyrolysis systems, the fuel is quickly heated in excess of 400 degrees Celsius (752 degrees Fahrenheit) in low-oxygen environments. Fast pyrolysis of biomass produces a char, an organic gas stream and pyrolysis gases that are condensed to a liquid. The char and gas are typically recycled to produce the heat required to drive pyrolysis. The condensed pyrolysis gases, or bio-oil, are generally 70 percent of the feedstock weight. The bio-oil can be fired in boilers or used in other ways. Unlike syngas, which must be used immediately, bio-oil can be easily transported and stored.

It's recommended that test burns of a selected fuel using the mentioned technologies be performed to ensure satisfactory compatibility between fuel, conversion technology and plant needs. Whatever the fuel or technology, biomass is one of the keys to self-sustaining biorefineries that reduce our carbon.
