University of Northern Iowa releases study of biomass feedstocks

By Katie Fletcher | December 30, 2015

The Tallgrass Prairie Center at the University of Northern Iowa recently released a final report for its Prairie Power Project. The project studied biomass production of four different native species seed mixes over three different soil types on non-prime agricultural land on a production scale over five years. Additionally, wildlife habitat and soil carbon sequestration were studied at the site, and in 2012 all of the material at the site was harvested, processed and test burned in the Cedar Falls Utilities power plant.

The site where the study took place, the Cedar River Ecological Research Site, is located within Black Hawk County’s Cedar River Natural Resource Area near La Porte City, Iowa, on the floodplain near the Cedar River. The soil types studied on the site were distributed so that biomass production and animal responses to habitat of four diversity treatments on three soil types could be studied. The four treatments of differing diversity were: 1-species, a switchgrass monoculture; 5-species, a mix of C4 grasses; 16-species, a mix of grasses, forbs (including legumes); and 32-species, a mix of grasses, forbs (including legumes), and sedges. According to the report, these mixtures were all specifically designed to assess their potential value as biomass feedstocks.

During the study period of five years, the research plots on the floodplain were exposed to a wide range of climatic conditions: 1) an establishment year-normal weather, 2) a normal weather year, 3) a drought year, 4) an extended flooding and inundation year, and 5) a moderate flooding year. The study found that the high diversity mixtures (16- and 32-species) were as productive as the switchgrass monoculture, but the 5-species grass mixture was less productive. Collectively, the report found that these productivity and ecosystem service results indicate that a diverse prairie mixture would be an effective bioenergy feedstock in the Midwestern U.S. It was also found that soil type influences the relative performance of the four feedstocks. The results suggest biomass seed mixes should be specifically tailored to site characteristics for maximum productivity and stand success.

Changes in soil profile carbon and nitrogen in the five-year period following planting of the perennial cropping systems was studied. According to the final report, the 16- and 32-species mixes fostered greater increases in soil C concentration than the 5-species mix and the switchgrass monoculture. The research team observed an increase of approximately 0.4 tons C per acre, averaged across the four perennial cropping systems, for the five-year period from May 2008 to May 2014.

The viability of the prairie biomass for electrical generation was also tested. A standardized test-burn was conducted by Cedar Falls Utilities. The process of harvesting, pelletizing and burning of the prairie biomass provided valuable information, according to the final report. Combustion of the prairie biomass yielded about 57 percent of the energy value of coal; however, stack emissions air pollutants and the potential for slagging and fouling were lower.

The final report included a summary of technical results and management considerations for biomass production. It was found that all four diversity mixtures are productive on marginal farmland. Five year averages for clip plots were: switchgrass 3.67 tons per acre, 5-grass mixture 3.2 tons per acre, 16-species mixture 3.58 tons per acre and 32-species mixture 3.53 tons per acre. The study stated that literature suggests regular harvest yield would be approximately 18 percent less. It was found that switchgrass monocultures are drought tolerant, recover well from flooding and are not subject to changes in species composition. In contrast, flooding drastically altered the species composition of the high-diversity mixtures suggesting that the additional cost of these seed mixes may not be worth the risk in a floodplain.

Another management consideration offered in the final report was that fall harvests capture more biomass than spring harvests (more foliage on stems). Slagging potential for fall-harvested biomass is equivalent to spring harvest, however, the fouling potential for fall harvests is medium compared to low for spring harvests.

The last point made in the summary of technical results is that it’s easier to form smaller pellets (one-fourth inch) from prairie biomass. According to the report, densification of pellets larger than one-fourth inch was difficult due to stems that resisted grinding. The smaller pellets remained intact through several stages of handling, various forms of transport and storage.

The test burn at Cedar Falls Utilities, originally scheduled for August of 2012 with five-eighth inch pellets was delayed due to various impeding factors until December and only a one-fourth inch pellet was able to be pelletized. Pellet Technology was hired to pelletize corn stover for the project. Forty tons of the densified prairie biomass was burned in a stoker furnace at 5 MW output. Particulate matter compared to coal was 0.007 pounds per million Btu (MMBtu) versus 0.562 pounds per MMBtu. Sulfur and mercury levels were both found to be dramatically lower than coal.

The full report and a summary brochure can be accessed from the Tallgrass Prairie Center’s website.