Effect of Operating Conditions on Product Distributions and Bio-oil Ageing in Biomass Pyrolysis

Abstract

Alternatives to petroleum-derived fuels are receiving significant interest in order to reduce dependence on finite resources of fossil fuels and to lower fossil-derived CO2 emissions. The present study addresses the production of bio-oil from biomass pyrolysis, one of the potential renewable substitutes to petroleum-derived fuels. The first objective of this work was to investigate the effect of pyrolysis operating parameters, i.e. temperature, heating rate and pyrolysis time, on product distributions in a wire-mesh reactor (WMR) which was designed to minimise secondary reactions. It has been found that high heating rate promotes melting of biomass and this facilitates volatile ejection, thereby resulting in high yield of large bio-oil molecules and high combustion reactivity of residual char. Maximum bio-oil yield is obtained at 500 °C for both rice husk and beech wood whereas a relatively low pyrolysis temperature, e.g. 350 °C, does not allow complete pyrolysis to take place. Chars produced from long holding time and high temperature tests show a decrease in the TGA combustion reactivity which is due to thermal annealing. The comparison between bio-oils obtained from the WMR and Gray-King retort demonstrates the impact of reactor configuration on the variation of bio-oil properties. The unstable nature of bio-oils provided the second objective of this work. The ageing behaviour of bio-oil and the use of organic solvents to improve the bio-oil properties have been investigated. Polymerisation plays a key role in bio-oil ageing and is enhanced by high temperature. Only slight changes in functional groups have been observed by 13C-NMR and FT-IR. UV-F results suggest that phenolic resin formation is one of the polymerisation reactions occurring during bio-oil ageing. With the addition of methanol and acetone to bio-oil, the extent of polymerisation decreases and NMR results indicate the formation of hemiacetals/acetals.