Pyrolysis of crop residues in a mobile bench-scale pyrolyser: Product characterization and environmental performance

Pyrolysis of four different crop residues (wheat straw, maize straw, rice straw and rice husk) was carried out at 400 °C in a designed mobile bench-scale pyrolyser. Proximate analysis and elemental analysis were performed together with determination of pH, nutrients, surface area, and functional groups of the produced biochars. Significant variations of the biochar characteristics were related to type and source of the feedstock. Biochar yield was found to vary from 33.4% to 43.8%, whereas the liquid and syn-gas yield varied from 21.2% to 27.5% and 30.8% to 39.1%, respectively. Carbon and major nutrients of N, P and K were enriched but hydrogen, oxygen and sulfur were lower in the biochars due to the pyrolysis process. All of the biochars were alkaline, ranging from a pH 7.3 to pH 10.7. Among the feedstock sources, biochar from rice straw was high in pH, cation exchange capacity as well as in nutrient contents, indicating a greater potential use of rice straw biochar as amendment to acid, nutrient-poor rice soils. As tested in this study, the crop residues available in China currently could potentially produce 160.2 Mt of biochar per year, with a potential to sequester 0.30 Pg CO2 year−1. In addition, syngas produced via pyrolysis process could be used to provide process heat.