Poster Title: Briquettes, Cubes, or Pellets: GHG Tradeoffs in Bioenergy
Authors: Tom Wilson, Paul Adler, Sabrina Spatari
Poster Abstract: Switchgrass has gained recent attention as an alternative energy resource. To be utilized for energy, switchgrass must be harvested, collected, and transported to the point of utilization. End-use applications range from electricity via co-firing with coal to biofuel conversion through residential heating. Currently, switchgrass is packaged in large square or round bale formats. To further reduce the costs associated with transporting this bulky material, switchgrass may be densified beyond baling into several different forms: pellets, briquettes, or cubes. The purpose of densifying biomass is to increase the energy density of the product so that it can be transported more efficiently, reducing both costs and emissions associated with transportation. The end-use application will largely dictate the appropriate form of densification due to tradeoffs that exist between transport distance, product density, and physical limitations of the equipment. Feedstock preparation is a critical and energy intensive step in the densification process. For quality production, appropriate moisture content and particle size distribution are essential. This requires grinding and drying; both processes consume energy. New technology enables some particle size reduction in the baling process, and this method is compared to the traditional model of particle size reduction downstream. Preliminary data indicate that in-field particle size reduction requires only a fraction (0.5%) of the energy of that downstream. In addition, switchgrass may be harvested in the spring or fall. The fall scenario produces higher yields, but due to stochastic variability in weather, the fall crop is generally wetter and requires energy input due to drying necessary for densification. Preliminary analysis indicates that emissions from biomass pellets for heating applications are 4.7 g CO2e/MJ compared to 59, 51, and 83 for fuel oil, natural gas, and electricity for space heating, respectively. Despite yield losses from overwintering, this number improves by nearly 60% with the spring harvest scenario to 1.9 CO2e/MJ due to the substantial energy input required for drying with the fall scenario. For the case of the fall harvest with in-field particle size reduction, life-cycle emissions improve by 12.7% to 4.1 g CO2e/MJ compared to the base scenario.