Life cycle assessment of densified wheat straw pellets in the Canadian Prairies

Abstract

Densification, a process used to manufacture pellets in order to increase biomass bulk density, plays a crucial role in the economics of biomass utilization. The Canadian Prairies produce large quantities of agricultural residues each year, in particular wheat straw. This study performs life cycle assessment of wheat straw pellets by evaluating environmental effects of the entire pellet production system comprising feedstock production (on-farm wheat straw production), harvesting, baling, transportation, and the industrial processing involving drying, grinding, pelletizing, and packing in the densification plant. The effects of each process on the environmental performance of wheat straw pellets were investigated. This study was conducted using LCA software and incorporating the Ecoinvent database supplemented with literature data for the Canadian Prairies. Wheat straw pellets manufactured from the densification plant are evaluated with respect to their use of resources and energy consumption. Environmental emissions associated with the agricultural processing and manufacturing systems are quantified. Sensitivity analysis is conducted to compare allocation methods and investigate the environmental impact of pelletizing and drying processes. The functional unit is defined as 1 kg wheat straw pellet. The study quantified the environmental impact of producing wheat straw pellets in terms of global warming potential, acidification, eutrophication, ozone layer depletion, abiotic depletion, human toxicity, photochemical oxidation, fresh water aquatic ecotoxicity, and terrestrial ecotoxicity. Drying, pelletizing, and fertilizer are the main contributors to global warming, acidification, abiotic depletion, human toxicity, terrestrial ecotoxicity, photochemical oxidation, and most of the other environmental impacts. Wheat seed has more impact on eutrophication. Transportation has an impact on ozone layer depletion, while grinding has an effect on freshwater aquatic ecotoxicity. The environmental impact of materials and energy fluxes on producing wheat straw pellet in the Canadian Prairies is assessed. The effect of each processing step on the entire manufacturing process is described. Overall, drying and pelletizing processes contribute the most environmental burdens except eutrophication and terrestrial ecotoxicity which are dominated by agricultural fertilizer/seed utilization and harvesting. In order to mitigate the environmental impact of wheat straw pellet production, minimizing energy consumption and machinery burdens from the drying and pelletizing processes are the main intervention points for wheat straw densification. Fertilizer production and utilization are key variables in strategies to lower eutrophication and terrestrial ecotoxicity.