Fast and microwave-induced pyrolysis bio-oil from Eucalyptus grandis : possibilities for upgrading

Abstract

The hardwood Eucalyptus grandis has been shown to be an important commodity for forestry-related industries as it has significantly faster specific growth rates per annum when compared with other types of tree species. It has therefore been suggested that residues from E. grandis may be a useful source of biomass for use in the production of biofuels for the transportation industry. Notably, E. grandis plantations within the Southern Hemisphere have some of the fastest growth rates worldwide. Due to the inherent nature of biomasses, such as lignocellulosic types having a significant amount of oxygen present, upgrading of biofuels produced from E. grandis is necessary. Several approaches were therefore evaluated to upgrade pyrolysis oils produced from E. grandis so as to increase their calorific values by decreasing oxygen content and subsequently increasing the hydrogen ratio. The hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios may be used successfully to evaluate the performances of catalyst-based upgrading techniques for either in situ or ex situ pyrolysis. In this regard the van Krevelen diagram, in which biofuels can be compared for their suitability as transportation fuels, along with their respective calorific values, is useful. The pyro-gas chromatography/mass spectroscopy (GC/MS) equipment is useful for the rapid and accurate evaluation of different catalysts for fast pyrolysis applications, and it was used here to evaluate the performances of the catalysts bentonite and ZSM-5 zeolite for upgrading pyrolysis oil produced from E. grandis biomass. A van Krevelen diagram was used to evaluate the performance of these catalysts, in conjunction with calorific values, based on the higher heating values v for the pyrolysis oils. Further studies were completed for microwave pyrolysis as it is a less harsh form of pyrolysis based on energy-transfer mechanisms. Mass balances were done and demonstrated good repeatability, with more stable pyrolysis oils being produced. This stability may be attributed to similarities between microwave pyrolysis and hydrothermal liquefaction as microwave pyrolysis induces conditions comparable to those of hydrothermal liquefaction within the wood cells, and both methods produce a stable product called bio-crude. Furthermore, it was found that these pyrolysis oils could be distilled so as to remove some of the water content and improve the higher heating value (HHV) from 13.80 to 23.30 MJ/kg. However, this was not as high as the theoretical yield of 26.70 MJ/kg, and better performance was obtained for fast pyrolysis catalysed with ZSM-5 zeolite at 300 °C, which achieved an HHV of 34.54 MJ/kg. It is recommended that ZSM-5 zeolite catalysis be used in microwave-assisted vacuum pyrolysis to determine whether a similar improvement may be realised. Microwave-assisted pyrolysis should also be investigated as a possible technology for inducing conditions similar to hydrothermal liquefaction processes within the cells that make up the biomass.