The catalytic effect of inherent and adsorbed metals on the fast/flash pyrolysis of biomass: A review

Relevant literature which deals with the presence of metals during the catalytic pyrolysis of biomass is reviewed. Only those conditions where the metal was in intimate contact with the biomass components are included. Cellulose, hemicellulose and lignin all fuse during the early stages of pyrolytic heating and the dehydration and decomposition processes begin during this transition. Hemicelluloses such as xylan are more labile and difficult to isolate, whereas both cellulose and lignin produce mostly bio-oils when demineralised and flash pyrolysed. The dominant primary products from ‘pure’ cellulose are anhydrosugars as well as smaller oxygenates. Lignin gives aromatics based on the syringol and guiacol molecules. The alkali and alkaline earth metals are found to curtail the yield of bio-oil and modify product distribution, even in the low concentrations naturally found in biomass. All other metals act to catalyse cross-linking reactions, with the nickel and zinc most studied. The electropositivity of the metal tends to correlate with the extent of catalytic activity. The presence of metals causes little change to the production of gases, but more char is formed at the expense of liquids. If the aim of pyrolysis is bio-oil for fuel, prewashing of biomass to remove metals is beneficial.