Life cycle assessment of corn stover torrefaction plant integrated with a corn ethanol plant and a coal fired power plant

A life cycle assessment (LCA) study was conducted to understand and assess potential greenhouse gas (GHG) emissions reduction benefits of a biomass torrefaction business integrated with other industrial businesses for the use of the excess heat from the torrefaction off-gas volatiles and biocoal. A torrefaction plant processing 30.3 t/h (33.4 ton/h) of corn stover at 17% wet basis (w.b.) moisture content was modeled. The torrefaction plant produced 136,078 t/year (150,000 ton/year) of torrefied material (i.e., biocoal) at 1.1% (w.b.) moisture content and 28.1 MW_th (96 million Btu/h) of excess heat energy in the torrefaction off-gas volatiles. At the torrefaction plant gate, the life-cycle GHG emission for the production of biocoal from corn stover (including corn stover logistics GHG emissions) is 11.35 g CO₂e/MJ biocoal dry matter (229.5 kg CO₂e/t biocoal at 1.1% w.b. moisture content). The excess heat from the torrefaction plant met about 42.8% of the process steam needs (excluding the co-products dryer heat demand) of a 379 million liter per year (100 million gallon per year) natural gas-fueled dry-grind corn ethanol plant, which results in about 40% reduction in life-cycle GHG emissions for corn ethanol compared to gasoline. Co-firing 10%, 20%, and 30% (energy basis) of biocoal at a coal-fired power plant reduced the life-cycle GHG emissions of electricity generated by 8.5%, 17.0%, and 25.6%, respectively, compared to 100% coal-fired electricity. A sensitivity analysis showed that adding a combined heat and power (CHP) system at the torrefaction plant to meet 100% electricity demand of the torrefaction plant (i.e., 2.5 MW_e) would result in lower GHG emissions for biocoal, corn ethanol, and co-fired electricity than for the case where the torrefaction plant purchased electricity from the grid.