Abstract:The bulk density of raw corn stover is a major limitation to its large-scale viability as a biomass feedstock. Raw corn stover has a bulk density of 50 kg/m 3 , which creates significant transportation costs and limits the optimization of transport logistics. Producing a densified corn stover product during harvest would reduce harvest and transportation costs, resulting in viable pathways for the use of corn stover as a biomass feedstock. This research investigated the effect of different process variables (compression pressure, moisture content, particle size, and material composition) on a densification method that produces briquettes from raw corn stover. A customized bench-scale densification system was designed to evaluate different corn stover inputs. Quality briquette production was possible using non-reduced particle sizes and low compression pressures achievable in a continuous in-field production system. At optimized bench settings, corn stover was densified to a dry bulk density of 190 kg/m 3 . Corn stover with a moisture content above 25% wb was not suitable for this method of bulk densification, and greater cob content had a positive effect on product quality.
A modified biomass combine was used in field experiments focused on measuring the productivity of singlepass bulk harvest and single-pass bale harvest systems. These harvesting machines were outfitted with ISOBUS data loggers to track overall in-field performance data. Testing of machine productivity was conducted at .7 ton/ac (1.6 Mg/ha), 1.5 ton/ac (3.4 Mg/ha), and 2.4 ton/ac (5.4 Mg/ha) for each system. The combine was also tested in a conventional configuration to provide baseline productivity data. Testing revealed significant impacts of the harvesting system on overall machine productivity and highlight the need for additional machine development to support the collection and harvest of biomass residues and grain.
Densification of agricultural residues is a key step in a cost-effective, large-scale, biomass feedstock supply system. Lab-scale densification systems exist which can produce large-scale densified corn stover briquettes that measure between 220 and 420 mm in length and with an average bulk density of 190 kg/m3 (dry particle density of 460 kg/m3). MOG (material other than grain) and pure cob briquettes produced similar lengths and dry particle densities of 424 kg/m3 and 421 kg/m3, respectively. Durability testing, utilizing a modified form of ASABE S269.4 (ASABE Standards, 2007) was conducted to determine the overall product quality associated with these densified briquettes. This publication describes experiments which quantified the durability or briquettes produced with different material types (corn stover, MOG, the chaff from a conventional corn harvest, and pure cobs) and material particle sizes (produced from a combine chopper, MOG, and a hammer mill with a 19-mm screen opening size). The durability rating varied with each of the main effect test parameters and produced a maximum 46% durability rating using pure cobs directly from a combine without additional size reduction. The durability rating was quite low for corn stover and MOG briquettes, and it was improved for pure cob briquettes. Biomass preprocessing in a hammermill significantly reduced durability due to a lack of fiber interaction throughout the large briquette.
The bulk density of corn stover poses a major obstruction to its large scale acceptance as a biomass feedstock. The loose bulk density of corn stover is low enough to create large inefficiencies during the harvest, transport, and storage phases of production. Overall production costs of stover could be reduced if a densification method were developed that provided adequate bulk density at a low specific energy during the harvest phase of production. So far, stover densification has been accomplished either by baling, grinding, or briquetting processes. Baling faces a logistical challenge with the handling cost of an individual bale, grinding systems don't achieve high enough bulk densities alone, and briquetting systems generally require stover preprocessing (grinding), and the addition of heat energy. All of these factors for each system drive up the unit cost of corn stover production.
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