Factors Influencing Biochar's Properties

Steam explosion of crop straws before pyrolysis changes the surface structure of biochar, making it rougher and less precise than untreated crop straw.

FREMONT, CA: Biochar, or biomass-derived carbon, is a very abundant source of carbon. Biochar produced from waste biomass is gaining popularity as a solution to some of the most pressing environmental issues. Biochar is a means of enhancing their capacity to eliminate pollution. It can aid government bodies and legislators in determining whether or not to utilize it. Biomass is a solid combination of organic, inorganic, and biological elements from living or non-living organisms. Biomass can be woody or non-woody. Forestry and tree residue comprise woody biomass.

Non-woody biomass includes agricultural crop residue, animal waste, and municipal and industrial solid waste. It has low moisture and ash content and high calorific and bulk density values. Biochar physicochemical properties depend on biomass moisture. As maple bark moisture decreased, the charcoal surface became more graphite-like and polyaromatic due to prolonged pyrolysis time. The cellulose component of feedstock helped generate tar during pyrolysis, while a high lignin concentration helped form char. High feedstock lignin increases char formation. Biochar carbon and ash levels increase with plant biomass lignin.

Residence affects feedstock carbonization and biochar output at low temperatures. Pyrolysis residence time increases carbonization, lowering organic matter and biochar sensitivity to microbial attack. Residence time also affects biochar surface area. Biochar made by co-pyrolysis of sewage sludge and cotton stalks increases the surface area from 30 to 90 minutes. The enhanced heat breakdown of organic materials and volatiles released from etching pores over residence time caused biochar pore structures to develop and expand. Pre-treating biomass before pyrolysis affects biochar properties. Physical, physiochemical/thermal, chemical, and biological pre-treatments exist.

Mechanical pre-treatment alters biomass characteristics. Mechanical particle size reduction is the most common physical pre-treatment. Finer feedstocks produce finer biochar. Finer feedstock biochar has lower nitrogen content, higher surface area, electrical conductivity, and pH. Fine apple wood and maize stover-derived biochar had a greater surface area than the feedstock. Steam explosion, HTC, and hot water extraction are the most prevalent thermal pre-treatment procedures that affect biomass characteristics. A steam explosion occurs when biomass is heated to 160-260°C and compressed to 0.69–4.83 MPa, scattering fiber material and disrupting the covalent connections between hemicellulose and lignin.