High-Temperature Pyrolysis of Blended Animal Manures for Producing Renewable Energy and Value-Added Biochar

Abstract

In this study, we used a commercial pilot-scale pyrolysis reactor system to produce combustible gas and biochar at 620 °C from three sources (chicken litter, swine solids, mixture of swine solids with rye grass). Pyrolysis of swine solids produced gas with the greatest higher heating value (HHV) followed by the mixture of swine solids with rye grass and chicken litter. Relatively high S-containing gases were produced; dimethyl sulfide and methyl mercaptan concentrations were higher than the OSHA PEL limits. Biochar yield ranged from 43 to 49% based on dry weight with about 53% of carbon recovery. Whereas the HHV of the chicken litter biochar was slightly below that of low rank coals, swine-based biochars had HHVs between high and low rank coals. Approximately 50% of the feedstock energy was retained in biochar and 25% in produced gases. Manure biochars contained higher concentrations of P and K than that of original manure feedstocks. Consequently, these could be used as a low-grade fertilizer to improve soil fertility and crop yields. Extremely high energy (232.3 MJ/kg) was required to make 1 kg of biochar from wet swine manure with 97% MC. However, dewatering of the wet swine manure to 75% MC substantially reduced the external energy requirement by 19 folds. Mixing of dried biomass such as rye grass with the dewatered swine solids almost eliminated the need for external energy. If one can copyrolyze wet animal wastes with additional feedstock that are drier and more energy dense than rye grass such as waste plastic pellets, it may be possible to produce both valuable biochar and extra power.