Is it possible to produce biochar at different highest treatment temperatures in the pyrolysis range? - The exothermic nature of pyrolysis

Biochar, charcoal applied to soil, has been proposed as a means to sequester carbon and concurrently improve soil properties. Typically, it is produced in the temperature range 300 to 700 degreesC. Lower temperatures are desired to favour biochar-soil interactions, and higher temperatures to enhance the adsorptive capacity of the char. To control the temperature during the manufacture of biochar it is essential to understand the pyrolysis heat effects, which were investigated in this study by recording the internal temperature profile in large cylindrical samples (d = 20, 30, 74, and 120 mm; h = 60 mm). Pyrolysis was carried out at 5 degreesC/min in a Macro-thermogravimetric analyser. It was found that with increasing sample size the thermal lag and internal heating rate increases while the number of observed peaks in the measured centre heating rate decreases, revealing the effect of transfer limitations during pyrolysis. In the case of negligible internal heat transfer limitations, d <= 20 mm, four separate peaks in the pyrolysis range ≈ 140 to 500 degreesC were identified and interpreted with respect to the underlying thermochemical events. The results show that pyrolysis is a primarily exothermic process. This was confirmed by heating cylinders with d = 120 mm at ≈ 1 degreesC/min demonstrating that a thermal runaway occurs from the onset of weight loss. The maximum temperature difference was recorded to be 146 degreesC at a heater temperature of ≈ 290 degreesC illustrating the difficulty in controlling the highest treatment temperature below approximately 440 degreesC.