Two ethanol plants under construction in the Midwest are apparently vying (indirectly) to become the first coal-fueled ethanol plants in the country.
Each plant has a 50-million gallon per year production capacity, uses corn as the feedstock and will use coal combustion as the direct energy source for steam and drying. The decision to go with coal was based strictly on economics; the groups calculate they will thereby reduce their production energy costs up to 70% compared to using natural gas.
Heron Lake plans to use Powder River Coal (lower in mercury and sulfur), mined from the North Antelope Rochelle Mine in Campbell County, Wyoming. Red Trail will use North Dakotan lignite coal.
Planned combustion and emissions management is conventional. The Heron Lake plant, for example, will use fluidized bed combustion, a technology that Interior Department’s Office of Coal Research, one of the forerunners of the Energy Department, began studying in the early 1960s.
Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective chemical reactions and heat transfer.
Fluidized-bed combustion evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers). The technology burns fuel at temperatures of 1,400º to 1,700º F, well below the threshold where nitrogen oxides form (at approximately 2,500º F, the nitrogen and oxygen atoms in the combustion air combine to form nitrogen oxide pollutants).
The mixing action of the fluidized bed results brings the flue gases into contact with a sulfur-absorbing chemical, such as limestone or dolomite. More than 95% of the sulfur pollutants in coal can be captured inside the boiler by the sorbent.
The coal combustion will, however, still produce mercury, dioxin and other emissions and require mitigation.