Black Gold from the Fields: Obtaining Pyrolytic Biochar from Agricultural Wastes at Various Pyrolysis Temperatures

Abstract

A considerable amount of agricultural waste residues is generated in India, which is disposed of in a traditional way, i.e., open burning. These practice increases harmful health and environmental issues. To overcome this concern, conversion of waste into biochar through pyrolysis is a sustainable path to enhance soil fertility, sequester carbon, and valorise waste. Although much research is carried out on the effects of pyrolysis temperature on the properties of biochar, the majority of these studies focus on common feedstocks like rice husk and woody biomass, paying little attention to underutilized residues like finger millet hulls. Moreover, regular cross-comparisons of different feedstock under specified pyrolysis conditions while comprehensive multi-level characterizations are lacking. In this study, four agricultural residues (finger millet hulls, rice husk, sawdust, and hardwood) were paralyzed at 300, 400, 500, and 600 °C, respectively, and characterized using proximate, ultimate, and physicochemical analyses, alongside SEM, FTIR, and BET surface area evaluations. Results showed that, increasing pyrolysis temperature reduced biochar yield and volatile matter, at the same time enhancing fixed carbon, pH, organic carbon, porosity, and surface area. However, variations also depend on feedstock composition. Biochar produced from finger millet hull at 300 °C is flaky, has chemically active structures, and is suitable for microbial colonization, while hardwood biochar prepared at 600 °C was highly carbonized, porous, and stable that indicating potential for pollutant adsorption and long-term carbon storage. By integrating different characterization techniques and linking key properties to functional applications, this study not only introduces finger millet hulls as a novel biochar precursor but also identifies optimal feedstock–temperature combinations for sustainable agricultural and environmental uses.