Roll Press Briquetting and Pelleting of Corn Stover and Switchgrass

Abstract

Corn stover and switchgrass, potential biomass feedstocks for bioenergy and bioproducts industries, are often harvested during a limited harvest season and stored as bales with bulk densities of about 100 to 200 kg m-3. Because of low bulk density, corn stover and switchgrass are difficult to handle, transport, store, and use in their natural forms. One of the solutions to reduce these problems and the associated costs is to densify these biomass feedstocks into pellets or briquettes. In this study, roll press briquetting characteristics of corn stover and switchgrass were studied using a pilot-scale roll press briquetting machine. Almond-shaped briquettes 28.7 to 31.3 mm in length were made. Results showed that high-durability corn stover and switchgrass briquettes with bulk densities of 480 to 530 kg m-3 could be produced. This corresponds to about a three- to five-fold increase in bulk densities compared to those of bales. Briquettes produced with the roll press briquetting machine had bulk densities (351 to 527 kg m-3), durabilities (39% to 90%), and crushing strengths (28 to 277 N) that were somewhat less than, but in a range comparable to, the pellets (9.6 to 9.8 mm diameter) produced with a conventional ring-die pelleting machine. The bulk density, durability, and hardness of the pellets ranged from 528 to 610 kg m-3, 75% to 95%, and 148 to 224 N, respectively. Micro-structural studies (chemical composition analyses, scanning electron microscopy imaging, and UV auto-fluorescence imaging) on grinds, briquettes, and pellets confirmed that highly dense, strong, and durable briquettes and pellets from corn stover and switchgrass could be produced without adding chemical binders (i.e., additives) by activating (softening) the natural binders such as water-soluble carbohydrates, lignin, protein, starch, and fat in the biomass materials by providing moisture and temperature in the range of glass transition of the biomass materials. No steam conditioning was necessary to produce good-quality corn stover briquettes. For switchgrass, a grind temperature of about 75C obtained by steam conditioning was necessary to produce good-quality briquettes. Roll press briquetting appears to be a promising low-cost, low-energy, high-capacity densification approach for commercial production of biomass briquettes.