A study of the importance of secondary reactions in char formation and pyrolysis : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Process Engineering at Massey University, Manawatū, New Zealand
Anthropogenic climate change, caused primarily by excessive emissions of carbon dioxide, has led to a renewed interest in char, the solid product of pyrolysis. When applied to soil as biochar it can both sequester carbon and improve soil function. To make its manufacture environmentally friendly and economically viable it is important to maximise char yield, which can be done by promoting secondary reactions.
This research shows that secondary reactions, which are enhanced by prolonged vapour-phase residence time and concentration, not only increase the char yield but are the source of the majority of the char formed. All four biomass constituents (extractives, cellulose, hemicellulose and lignin) undergo secondary reactions concurrent with primary reactions over the entire pyrolysis range ≈ 140 to 500 °C, which makes it practically impossible to separate them. Secondary char formation was confirmed to be exothermic which affects the overall heat of pyrolysis. Impregnating the feedstock with the elements K, Mg and P, which are plant macro-nutrients naturally present in biomass, resulted in the catalysis of secondary char formation. The results reveal that a first order reaction model does not describe pyrolysis accurately when char formation is enhanced by catalysis and secondary reactions.
Secondary char can be enhanced by increasing the particle size but there is a limit due to increased cracking and fracturing of the pyrolysing solid. This limitation is overcome by pyrolysis in an enclosed vessel, termed autogenous pressure pyrolysis, which was discovered to cause significant changes in the volatile pyrolysis products; indicating the co-production of a high quality liquid. This process, however, negatively affects the char properties relevant for biochar like the surface area, similar to self-charring and co-carbonisation of condensed volatile pyrolysis products. To increase research capabilities a unique high temperature/ high pressure reactor (600 °C at 20 MPa) was designed to allow the detailed characterisation of all three pyrolysis product classes under extreme pyrolysis conditions. This was demonstrated to be invaluable for understanding the underlying pyrolysis mechanism and physical processes at play.