Oxidative pyrolysis of pellets from lignocellulosic anaerobic digestion residues and wood chips for biochar and syngas production

Abstract

Compared to pyrolysis in an inert atmosphere, oxidative pyrolysis for biomass conversion is autothermal and decreases the concentration of tars in the produced gases. The aim of this work is to study a fixed bed reactor for oxidative pyrolysis on a laboratory scale with a capacity of 1.5 kg/h of lignocellulosic anaerobic digestion residues (Pellets-DR) at a temperature range of 600 °C and under a low specific airflow of 0.012 kg/m2.s. Wood chips (WC-P16F05) were used as a reference to evaluate the potential of Pellets-DR conversion to biochar and syngas using oxidative pyrolysis. Oxidative pyrolysis produced only 10% and 6% of tars on a fuel dry and ash-free basis for Pellets-DR and wood chips, respectively, which is equivalent to a concentration of 25 g/Nm3 and 38.6 g/Nm3 in gases. Ultimate and proximate analysis of biochar produced from both types of biomasses showed that the main element present was carbon in the case of wood chips (87.8%). However, carbon is present in only 48.33% of the biochar from Pellets-DR. The main difference between Pellets-DR and wood chips was also found in biochar’s low heating value (LHV), which is estimated to be 31.9 MJ/kg for wood chips and 13.01 MJ/kg for Pellets-DR. The propagation of the oxidation zone was inversely proportional to biomass density, and it plays a key role during biomass oxidative pyrolysis; it was about 0.14 cm/min and 0.5 cm/min for Pellets-DR and wood chips, respectively. Air flux was the most important factor affecting the oxidation zone propagation velocity, but in this study, low values of air flux meant that changes in air flux did not have a significant effect on the yield and the composition of the products. Regardless of its high ash content, Pellets-DR may have the potential to be converted into biochar and syngas by oxidative pyrolysis.