Effects of temperature on the physicochemical characteristics of fast pyrolysis bio-chars derived from Canadian waste biomass

Bio-chars are produced by means of a mobile pyrolysis unit from fast pyrolysis of different types of Canadian waste biomass including agricultural waste (wheat straw and flax straw), forest residue (sawdust) and animal manure (poultry litter). They were analyzed for their physicochemical changes with pyrolysis temperature (400–550 °C). To study the chemical nature of bio-char samples, analyses such as XRD, FTIR, Raman spectroscopy, XPS, SEM, ICP, TGA and electrical conductivity measurements were performed. ICP-MS analysis showed that poultry litter-derived bio-char had the largest concentration of inorganic elements (∼200,000 ppm) followed by wheat straw, flax straw and sawdust derived bio-chars. In addition, the alkaline elements were 4–14 times that of essential elements (Fe and P) and 18–57 times that of heavy elements. Electrical conductivity of bio-chars, a measure of their salinity, was maximum for all samples prepared at 400 °C. SEM showed that sawdust derived bio-chars retained relatively less dissociated surfaces compared with other bio-chars. XRD confirmed the presence of sylvite, dolomite and quartz in the bio-chars. The deconvoluted XPS spectra indicated that for all precursors except poultry litter, aromatic/aliphatic carbon portion increased in the corresponding bio-char with the pyrolysis temperature. For all precursors, O/C mass ratio decreased with an increase in the pyrolysis temperature due to the development of compact aromatic structure in bio-char. This result was confirmed by a drastic increase in ID/IG (defect to graphitic carbon) ratio of bio-char samples produced at 550 °C from the deconvolution results of Raman spectroscopy. Thermogravimetric analysis showed that biomass decomposition started at lower temperatures for the following order: poultry litter, wheat straw, flax straw and sawdust.