Densification characteristics of corn stover and switchgrass

Corn stover and switchgrass are two important biomass feedstocks being considered for commercial production of renewable fuels (ethanol), combined heat and power, chemicals, and bio-products in the U.S. These biomass feedstocks currently are collected as low-density bales involving high cost and energy for handling, transportation, storage, and use. Densification of biomass feedstocks into pellets/briquettes is a cost- and energy-effective strategy for a consistent, year-round supply of these biomass feedstocks for biobased and bioenergy industries. In this research, densification characteristics of corn stover and switchgrass were studied using a piston-cylinder densification apparatus to understand the densification and binding mechanisms, and to determine the optimum densification conditions for commercial production of pellets/briquettes from these biomass materials. The effects of densification process variables such as pressure (100 to 150 MPa), moisture content (8% to 20% wet basis), particle size (0.34 to 0.80 mm), and preheating temperature (75°C to 200°C) on the densification characteristics of corn stover and switchgrass were studied. At room temperature (about 25°C), corn stover briquettes (about 19.2 mm diameter) with relaxed densities (i.e., individual briquette densities measured one week after forming) of 745 to 1013 kg m-3 and switchgrass briquettes (about 19.4 mm diameter) with relaxed densities of 417 to 825 kg m-3 were produced. Densification at 25°C resulted in briquette durability of 50% to 96% for corn stover and 0% for switchgrass. Preheating corn stover and switchgrass to 75 C to 100 C increased the density further and resulted in briquettes with maximum durability of 97% for corn stover, and 67% for switchgrass. The measured glass transition (i.e., softening) temperature of both corn stover and switchgrass averaged 75°C. Densification conditions causing glass transition in the biomass was essential to fully activate the natural binding components such as lignin, protein, starch, and water-soluble carbohydrates in the biomass to help effective particle binding. The optimum densification condition to produce highly durable corn stover pellets/briquettes in commercial-scale systems is either conditioning the grind to a moisture content of 15% to 20% (wet basis) at room temperature (25°C) or preheating the grind to 75°C at a moisture content of 10% to 15% (wet basis). The optimum densification condition for switchgrass is preheating the grind to >75°C at a moisture content of 8% to 15% (wet basis).