Effect of the Carbonization Temperature on the Properties of Biochar Produced from the Pyrolysis of Crop Residues

Biochar, a carbon-rich product, can be obtained from crop residues via pyrolysis. Its properties may vary widely depending upon the pyrolysis conditions and feedstock type. Physicochemical properties were studied for biochars produced from rice straw, wheat straw, corn stover, rape stalk, and cotton stalk pyrolyzed at 300 degrees C to 700 degrees C. At higher pyrolysis temperatures, the carbon content, pH, and electrical conductivity of the biochars slightly increased, while the O/C and H/C ratios decreased. The pH values had a strong negative linear correlation with the H/C ratio. Higher carbonization temperatures resulted in larger pores and increased the aromatic/aliphatic carbon ratio in the biochars. The oxygen functional groups in the biochars, such as -COOH and -OH, decreased with an increasing carbonization temperature. The combustion performance of the biochars varied with the carbonization temperature because of the differences in the physicochemical compositions of the biochars. Additionally, the crop residue types also influenced the physicochemical properties. The cotton stalk biochar had the highest fixed carbon content and lowest H/C ratio, and thus can be used as a solid biofuel. The rice straw biochar, which had the highest N and O contents, may be a potential soil amendment.