Densifying biomass into pellets or briquettes helps overcome raw biomass limitations such as low mass and energy densities. This preprocessing can reduce transportation costs and improve flowability, processing performance and conversion performance for biorefineries. But the cost of such processing has been a persistent concern. INL researchers have developed a pelleting process for various high-moisture feedstocks such as switchgrass, corn stover, lodgepole pine, municipal solid waste and chemically pretreated biomass. These high-moisture pellets retain the advantages of densified biomass while reducing overall pelleting costs. Secondary benefits include reduced volatile organic chemical emissions because of the low temperature drying method associated with the process.
Traditional pelleting processes typically involve drying biomass in a 160 C to 180 C rotary dryer to achieve 8 to 12 percent moisture content before the biomass is processed in the pellet mill. The High-Moisture Pelleting Process eliminates the energy-intensive rotary drying stage. Instead, high-moisture biomass goes through two grinding steps and enters the pellet mill at moisture contents higher than 20 percent. Some high moisture pelleting techniques require a short preheating step or a binder such as corn starch. Frictional heat generated in the pellet die due to compression and extrusion of biomass results in about 5 to 10 percent moisture loss in the biomass. The high-moisture pellets are then further dried to about 9 percent moisture content in a low cost grain dryer.
With the High-Moisture Pelleting Process, INL researchers are able to reduce the overall preprocessing cost of pelletized biomass by 35 to 40 percent using biomass with moisture contents ranging from 20 to 39 percent. Those savings include a roughly 55 percent reduction in operating energy costs and a 35 percent reduction in cap
Applications and Industries
• Timber industry
• Animal feed
• Ethanol byproducts
• Pyrolysis byproducts
- Overall preprocessing cost of pelletized biomass is reduced by 35 to 40 percent.
- Pellets formed using biomasswith moisture contents ranging from 26 to 39 percent.
- Bulk density and durability are in the range of 400-610 kg/m3 and 90 to 98 percent based on the initial moisture content and type of the biomass.
- Process is environmentally friendly due to fewer volatile organic emissions during drying.