Pyrolysis of waste biomass: investigation of fast pyrolysis and slow pyrolysis process conditions on product yield and gas composition

High temperature fast pyrolysis of wood, rice husk and forestry wood residue was carried out in a laboratory scale fixed bed reactor. The results were compared with pyrolysis of the biomass samples in a different reactor under slow pyrolysis conditions. There was a marked difference in product yield depending on heating rate, for example the gas yield from slow pyrolysis was 24·7 wt-% for wood, 24·06 wt-% for rice husks and 24·01 wt-% for forestry residue; however, for fast pyrolysis, the gas yields were 78·63, 66·61 and 73·91 wt-% respectively. There were correspondingly significantly lower yields of oil and char from fast pyrolysis, whereas for slow pyrolysis, oil and char yields were higher. The composition of the product gases was also influenced by the heating rate. In additional experiments, the influence of pyrolysis temperature was investigated under fast pyrolysis conditions from 750 to 1050°C. It was found that the increase in temperature increased overall gas yield and also increased hydrogen gas concentration with a decrease in CH4, CO2 and C2–C4 hydrocarbons. High gas yields of ∼90 wt-% conversion of the biomass to gas was obtained during the pyrolysis of biomass at 1050°C. Steam was also added to the fast pyrolysis system to enhance the hydrogen production. The amount of hydrogen produced was found to significantly increase in the presence of added steam.