Paper Mill Sludge Biochar to Enhance Energy Recovery from Pyrolysis: A Comprehensive Evaluation and Comparison

Bio-oil and pyrolysis gas (py-gas) are two pyrolysis products available for potential energy recovery. Crude bio-oil, however, is typically corrosive and unstable, requiring special combustion equipment or catalytic upgrading to produce drop-in-grade fuel. In contrast, py-gas is readily useable in standard equipment for energy recovery. Previous research revealed that Ca-impregnated biochar catalyst improved bio-oil to py-gas conversion. Biochar produced from paper mill sludge (p-sludge) has very high Ca content. In this study, the catalytic ability of p-sludge biochar was systematically evaluated for the first time in pyrolysis. P-sludge biochar resulted in higher py-gas yield (40 wt% of total pyrolysis products) and py-gas energy (8400 kJ of py-gas per biosolids pyrolyzed) than other biochar catalysts (e.g. wood and corn stover biochars) and mineral catalysts (e.g. calcined dolomite). Under some conditions (e.g. high temperature and catalyst loading), catalysis completely eliminated the nonaqueous phase condensate. A lower catalyst-to-feedstock ratio was required using p-sludge biochar compared to other biochars for similar performance. P-sludge biochar also had a longer catalyst lifetime based on the effectiveness over five reuse cycles. Bio-oil catalyzed by p-sludge biochar contained fewer organic constituents based on GC-MS and GC-FID analyses (e.g. toluene, ethylbenzene, styrene, phenol, cresol, and indole were not identified after catalysis)