Bio-oil from fast pyrolysis contains up to 40wt.% oxygen; around 25wt.% water; it has a heating value of around 18 MJ/kg; and is not miscible with hydrocarbons. It also has a number of other unattractive properties such as low pH (high acidity) and relative instability causing viscosity increases and potentially phase separation.
While bio-oil can be used directly in a number of heat and power applications, some upgrading is often required to meet the requirements of relevant applications, especially for transport fuels. A comprehensive review of bio-oil upgrading has been published (Bridgwater AV. (2011). Upgrading biomass fast pyrolysis liquids. Chapter 6 in: Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power. Brown RC. (ed.) Wiley Series in Renewable Resources. Wiley-Blackwell. ISBN: 978-0-470-72111-7).
One method of upgrading that removes oxygen and thus leads to hydrocarbon fuels and chemicals is cracking where the organic molecules in fast pyrolysis vapours are catalytically cracked and re-formed over a ZSM-5 or modified ZSM-5 catalyst into aromatics. The basic technology was originally developed by Mobil and has been explored by many researchers. Zeolite cracking rejects oxygen as CO2, and produces aromatics by molecular re-arrangement inside the pores of the zeolite. The overall process is summarised in the conceptual reaction below:
C1H1.33O0.43 + 0.26 O2—> 0.65 CH1.2 + 0.34 CO2 + 0.27 H2O
Other zeolites have been studied with different pore sizes including mesoporous zeolites, but with less success.
There are several ways in which this can be carried out as summarised below. Liquid phase catalysis and re-volatilisation of the liquid both result in poor yields and processing problems, so development is focused on integrating catalysts into the fast pyrolysis reactor and/or close coupling a secondary reactor.
Catalytic cracking process configurations
The low hydrogen content of biomass and bio-oil results in substantial deposits of carbon on the catalyst surface which require removal by oxidation. This results in production of CO2 which is how the oxygen is removed as shown in the conceptual reaction scheme above. While yields of hydrocarbons are relatively low at around 20wt.% on dry biomass feed, there is no hydrogen requirement, atmospheric pressure is employed, and capital cost is only marginally increased as the processes are close coupled. Recovery and conversion of byproducts of cracking such as olefins can improve hydrocarbon yields. The resultant high aromatic content product requires further refining into marketable products.