Effects of various pyrolysis conditions and feedstock compositions on the physicochemical characteristics of cow manure-derived biochar

Pyrolysis, as a thermal decomposition process and environment-friendly technology, can be used for the safe treatment of cow manure. In this paper, the effects of pyrolysis conditions on the properties of biochar derived from different types of cow manure (i.e., fresh cow manure, flushing cow manure, and anaerobic fermentation residues) were studied using slow pyrolysis methods. The properties of feedstocks, thermal decomposition, and biochar were analyzed using elemental analysis, coarse fiber determination, TG–FTIR, ICP–MS, FTIR, XRD, and SEM–EDS. Results showed that the cow manure-derived feedstocks were rich in total organic C (>37%) and had no heavy metal risk. According to the results of TG–FTIR, pyrolysis produced more residual solids, CO, CH4, and NH3 and less CO2 than combustion. The factors affecting alkalinity, calorific value, adsorption capacity, and surface functional group properties of cow manure-derived biochar in order of effectiveness were as follows: pyrolysis temperature > manure characteristics > heating rate > residence time. In general, increasing the temperature from 300 °C to 700 °C increased the pH (8.05–11.74) and C content (45.75%–49.64%) of fresh cow manure biochar and decreased the yield (62.17%–39.82%), nutrient content (N + P + K = 6.90%–5.64%), iodine adsorption capacity (269–96 mg g−1), aromaticity, and polarity. The biochar derived from flushing cow manure and anaerobic fermentation residues had similar C content (>48%), alkalinity (pH > 10), calorific value (>20000 J g−1), stability, and polarity. Fresh cow manure biochar had higher nutrient recovery (6.6%) and iodine adsorption capacity (215 mg g−1) than others. Compared with flushing cow manure and anaerobic fermentation residues, the pyrolysis of fresh cow manure reduced nutrient loss and treatment energy consumption. This study provides a good basis for the optimization of the pyrolysis process of three typical kinds of cow manures in dairy farms. [Display omitted] •Cow manure waste is rich in total organic C (>37%) and has no heavy metal risk.•Pyrolysis produces more residual solids, CO, CH4, and NH3 and less CO2 than combustion.•Increasing temperature decreases the adsorption capacity and aromaticity of biochar.•Fresh cow manure derived biochar has higher yield and adsorption capacity than the others.•Pyrolysis of fresh cow manure reduces nutrient loss and treatment energy consumption.