Torrefied Agri-Food Residues as a Coal Substitute: How the Supply Chain and the Allocation Rule Determine the Carbon Footprint

Agri-food processing leaves large volumes of lignocellulosic residues for which their energy is rarely recovered, while coal-fired heat and power in Europe carries a rising carbon price. This study quantifies the carbon footprint of five food-processing residues—spent coffee grounds (SCGs), spent tea leaves (TEAs) and walnut, pistachio and peanut shells—with spruce wood chips as a non-food benchmark, torrefied at 250–550 °C and burned in place of Czech lignite. Fuel properties were measured for all six materials at five temperatures and their replicate variances propagated. Emissions were quantified for nine process stages—from cultivation of the crop to combustion of the fuel—per tonne of dry residue under a fuel-first allocation that charges the residue with the agricultural chain and credits the food co-product by system expansion; cut-off, mass and economic allocation were compared. All six materials save emissions at their selected temperature, from −557 ± 22 kg CO2-eq t−1 for SCG to −1214 ± 23 for spruce, avoiding 36–77% of the emissions of the lignite they replace. The ranking is set by the supply chain rather than by the fuel: SCG has the highest calorific value of the set yet saves least, because 89% of its residual burden comes from roasting, brewing and intercontinental freight. The result is conditional: Under mass allocation, four of the five food residues become net emitters, and so do SCG and TEA when collected wet. At 65 EUR tCO2−1, the avoided emissions are worth 36–79 EUR t−1 against a fuel-cost advantage over lignite of 116–200 EUR t−1.