These two upgrading routes offer quite different opportunities for development. Hydrotreating is fairly well established and understood in the context of hydro — desulphurisation although the very high oxygen content and relative instability of many constituent chemicals require careful study for catalyst and process optimisation. Partial hydrotreating to stabilise the pyrolysis liquids and render them more suitable for direct firing into a gas turbine for power generation is an interesting concept if pyrolysis liquids cannot be fired directly. This would require significantly less hydrogen and a lower capital cost plant. The extent of hydrotreating required is dependent on the demands of gas turbine fuel characteristics. An inherent problem in the concept of hydrotreating is the progressive de-oxygenation as conditions become more severe, resulting in total hydrogenation of some components and barely adequate de-oxygenation of others, compared to the cracking and seemingly selective synthesis of the zeolite route summarised below. The major problems to be overcome are in catalyst selection and catalyst support in relation to catalyst life and water resistance; and also selectivity in de-oxygenation.

Zeolite catalysts are interesting as they give a higher value transport fuel product (high aromatic content) and also offer the possibility of deriving even higher value chemical intermediates. There is a wider potential through catalyst design and development including possibilities based on multi-functionality, none of which has been investigated. The nature of wood pyrolysis liquids, however, particularly lignin derived components, creates potential problems through coking as well as by-passing of larger less cracked molecules through the catalyst without reaction. Again, the high oxygen content and relative instability of many constituent chemicals require careful study for catalyst and process optimisation. Such problems might be overcome through careful catalyst design and development as being undertaken by NREL (163); through modification of the reactive environment such as addition of steam or operation at different temperatures; or through technology development for example through the adaptation of processes such as fluid catalytic cracking for catalyst regeneration that is widely employed in oil refining. These possibilities currently remain conjectural, although a number of organisations are known to be active as shown in Table 8 including NREL and the University of Saskatchewan.