Effects of pyrolysis temperature and residence time on physicochemical properties of different biochar types

Here we selected eight types of feedstocks to assess the effects of pyrolysis temperature (300°C, 400°C, 500°C and 600°C) and residence time (0.5, 1, 2, 4, 8 and 24 h), respectively, on the physicochemical properties. The fixed-carbon content, pH value and amount of basic functional groups in biochars increased as the pyrolysis temperature increased from 300°C to 600°C; the opposite trend was found in the biochar yield, adsorption capacity and amount of acidic functional groups. Increasing the residence time at low pyrolysis temperature (300°C) resulted in a gradual reduction in the biochar yield and progressive increase in the pH and iodine adsorption number of biochars. However, increasing the residence time at high pyrolysis temperature (600°C) had little effect on the biochar yield or pH, while it decreased the iodine adsorption number of biochars. Given the effects of pyrolysis conditions on the pH and iodine adsorption number of biochars, low-ash agricultural wastes (e.g. wheat straw) can be pyrolysed at 300°C, 2 h to produce biochar for improving alkaline soils; high-ash agricultural wastes (e.g. sweet potato vine) and forest litter (e.g. fresh leaves of apricot tree) are preferably pyrolysed at 300°C, 4 h to produce biochar for use in acidic soils.