Biochar Production Process Optimisation and Product Characterisation

Biochar is a carbonaceous solid material produced through heating organic material in a reduced oxygen environment. The impact of feedstock and pyrolysis conditions (temperature and time) on biochar properties was assessed using a pilot plant fluidized bed pyrolyser. Three feedstocks; macadamia nutshell, eucalyptus woodchip and hemp stem where chosen for their range of woodiness, and as locally available waste materials. Apatite was used as the fluid bed, given potential for use as a phosphorus (P) source after roasting. Three different temperatures (350, 450 and 600 ℃) and three residence times (5, 10 and 20 min) were employed. Residence time had almost no influence on biochar physical and chemical properties, indicating that the pyrolysis process was completed within five minutes, a result ascribed to the efficiency of heat transfer in the fluid bed system. For all feedstocks, increasing pyrolysis temperature was associated with increasing biochar pH, EC, liming potential, ash content, carbon content, calorific value, water holding capacity, surface area and aromaticity along with decreasing bulk density, volatile matter, cation exchange capacity (CEC), zeta potential and functional groups. For example, pH of nutshell significantly increased from 5.7 to 9.12 and bulk density decreased from 0.36 to 0.22 g/cm3 when temperature increased from 350 to 600 ℃. Feedstock significantly influence biochar properties, with the two woody materials (nutshell and woodchip) separating from hemp stem in properties and in a principal component analysis of infrared spectra. For example, total P, K and Ca was higher in hemp stem (1.2, 2.04 and 3.12 % w/w) as compared to macadamia nutshell (0.071, 0.084 and 0.146 %) and eucalyptus woodchip (0.81, 0.008, 1.93 %). Infrared spectroscopy is recommended as a potential tool for quality control of the production process. Thus, assessed biochar properties were used to match feedstock and pyrolysis conditions to potential uses, such as improvement of soil water holding capacity, soil CEC, animal feed and wastewater treatment. After 5 minutes at 600 ℃ under reducing conditions, apatite P extractability (in 2 % formic acid) decreased from 1.21 to 0.25 % of dry weight, for apatite that contained 12.7 % of total P. However, further work is recommended to consider the effect of repeated oxidation and reduction cycles on P extractability, as will occur in a full-scale pyrolysis plant.