Life cycle assessment of torrefied cornstalk pellets combustion heating system

Biomass combustion for heating is an important method to utilize raw biomass resources, especially stalks with low calorific value. Torrefaction and compression molding are the effective and common pretreatments for biomass upgrading. In this study, the resources, energy consumption and environmental impact of torrefied cornstalk pellets combustion heating system were studied using the Life Cycle Assessment (LCA) method. The scope of the LCA includes 5 stages: biomass planting and collection, transportation, plant construction, pretreatment, and combustion heating. The resource consumption assessment showed a total energy output of the life cycle system corresponding to 19.7 GJ (The utilization of by-products was not considered), while the total energy input was 2671.3 MJ/t, and the net energy was 17028.69 MJ/t. The estimated energy input–output ratio was 7.3. The increased value of fuel energy of cornstalk pellets after torrefaction pretreatment was greater than the energy consumption during the same process. The results of energy consumption showed that the maximum energy input was identified as the planting and collection stage (1246.54 MJ/t), among which the top two are the nitrogen fertilizer energy input (56%) and the agricultural machinery fuel consumption (27%). The nitrogen fertilizer input represents 84.17% of the total energy input of fertilizer. The environmental impact assessment showed that the total GWP of the system is −175.806 kg CO2eq, indicating that the whole life cycle of the system can reduce greenhouse gas emissions. Acidification and photochemical oxidation potentials become the main factors influencing the ecological footprint of the system. Thus, considering the comprehensive impact of energy consumption and environmental emission, it is feasible to use torrefied cornstalk pellets as fuel for heating.