Power Plant Benefits
The Chariton Valley Biomass Project has completed two short-term test burns, cofiring switchgrass with coal, at Ottumwa Generating Station. Each of the previous test burns was conducted over several weeks. During each of these tests, an extensive data collection program on a wide range of power plant performance parameters was executed. In addition, hundreds of samples (coal, switchgrass, fly ash, bottom ash solids and liquids, economizer ash) were collected and sent out for independent laboratory analysis. Air emissions for criteria air pollutants were monitored throughout the tests. During these tests, the average heat input from switchgrass was about 2% of the total heat input for the power plant. The results of that testing showed:
- Overall reductions of sulfur dioxide emissions by more than 4.5%. Sulfur dioxide is a precursor to acid rain. Although the sulfur dioxide emissions from OGS are already low because of the low-sulfur coal used at the power plant, these emissions were significantly reduced by firing switchgrass because switchgrass has almost zero sulfur content.
- Nitrogen oxide emissions remained unchanged.
- Carbon monoxide emissions, which are already extremely low at OGS, remained unchanged.
- Particulates emissions were reduced by about 4%.
- The burn-out of switchgrass in the boiler was excellent, allowing the power plant to maintain its very low unburned carbon content in its fly ash. This is important for maximizing the power plant’s efficiency and maintaining its high-quality fly ash for use in making concrete and other products.
- There was no difference in the power plant’s thermal efficiency when burning switchgrass as compared to coal-only operation.
On the basis of the emissions-related benefits mentioned above (and thoroughly documented in test reports), the Chariton Valley RC&D Inc. and Alliant Energy / Interstate Power & Light have received approval and permits for operating the Chariton Valley Biomass Project on a commercial basis at a switchgrass input rate up to 25 tons per hour (the equivalent of 35 megawatts of electricity generation). This permission was granted by the Iowa Department of Natural Resources and the U.S. Environmental Protection Agency in the Spring of 2005.
In addtition to the benefits mentioned above, from an operational and planning standpoint, firing switchgrass at OGS offers advantages to the electric utility over other renewable energy sources because the output of switchgrass power from the power plant can be predicted and controlled. This is in contrast to power supplied by wind or solar technologies which are dependant on wind and solar conditions, respectively, which can vary significantly (and unpredictably) over a given period of time. From the standpoint of efficient conversion of switchgrass into electricity, cofiring switchgrass at a large-scale coal-fired power plant offers significant advantages over a smaller stand-alone power plant fuel only by switchgrass—due to economies of scale, large-scale power plants are designed to operate more efficiently than smaller plants. Instead of being converted to power in a stand-alone 35 megawatt power plant fueled only by switchgrass, the Chariton Valley Biomass Project’s switchgrass will be converted to power in a very efficient 725 megawatt power plant with all of the advanced controls, features, and emissions control devices of the large-scale power plant.