Compostable Characteristics of Reject Materials produced during Industrial-Scale Corn Stover Biomass to Ethanol Conversion

Abstract.

Compostable characteristics of reject stover and soil stover, in terms of moisture content, total, fixed, and volatile solids, bulk density, free air space (FAS), water holding capacity (WHC), particle density, total carbon, total nitrogen, and carbon to nitrogen ratio were determined. Moisture content for soil stover was 10.4% whereas it ranged from 13.5% to 76.2% for reject stover sample with an average of 38.6% for reject stover. Reject stover samples had fixed solids in the range of 7.5% to 12.5% whereas soil stover had fixed solids content of 23.9%. Presence of significant amount of compostable mass was confirmed by the percent volatility test in which both reject stover and soil stover samples tested above 65% volatility. Bulk density, on dry weight basis, ranged between 37.6 to 69.4 kg/m³ for reject stover. Corn stover had relatively large particle size distribution and plenty of free-air space. It, therefore, had relatively lower bulk density than soil stover which measured at 206.2 kg/m³. Air-filled pore space or free air space (FAS) for reject stover calculated in the range of 80% to 93%, whereas it was 84% for soil stover. Large FAS values can cause the composting mixtures to lose heat easily and may need to be mixed with denser materials. Water Holding Capacity (WHC) for soil stover tested at 1491 kg/m³ due its fine particle size. In comparison, reject stover average WHC was 286 kg/m³ which is six times lower than the soil stover WHC. Carbon to nitrogen ratio of reject stover averaged 27:1 whereas it was 18:1 for soil stover. The materials will likely perform well in the composting process as the C:N ratio of the mixture is likely to be above the lowest acceptable ratio of 20:1. It is not expected that the soil stover, when mixed with reject stover, will significantly lower the air-filled pore space of mixture. Fine soil stover particles are expected to adhere to large reject stover particles upon wetting, thus, having minimal effect on airflow in the mixture. It will need to be further tested in different mixture ratios with reject stover to establish its effectiveness. Fineness of soil stover can potentially pose a challenge during the composting process as particles may become air-borne during mixing if the compost mixture is relatively dry.