Life cycle assessment of combination of anaerobic digestion and pyrolysis: focusing on different options for biogas use
The combination of anaerobic digestion and pyrolysis technologies could be a novel energy-biochar production system to maximize energy and nutrient recovery from pulp and paper mill sludge. Herein, the life-cycle energy production and emissions reduction of sludge treatment from a typical pulp and paper mill were investigated, in which alternative uses of biogas for industrial or household application, in different regions of the world, were assessed. The three scenarios considered for different end-uses of biogas are: (A) biogas for vehicle fuel in the transportation sector in Sweden, (B) biogas for heat and electricity in the power sector in Brazil, and (C) biogas for cooking in households in China. The results of Environmental Life-Cycle Assessment (E-LCA) show that for all these three scenarios, the use of biogas and pyrolysis gas contributes most to emissions mitigation, while the dewatering and drying processes carried out on the sludge, contribute the most to the environmental emissions. Addition of biochar to the soil, contributes significantly to a reduction in global warming by sequestering carbon in the soil. Compared to scenarios B and C, Scenario A, in which biogas substitutes gasoline in transportation, and heat from combusted pyrolysis gases is used for district heating in Sweden, demonstrates the highest environmental performance for all the evaluated impact categories.