For some applications, such as gas turbine fuel, complete de-oxygenation may not be necessary to give a suitable fuel with acceptable chemical and physical properties. This concept of partial hydrotreating has not yet been properly explored. For example, raising the LHSV (liquid hourly space velocity) from around 0.1 to 0.2 or 0.3 will give an oxygen level of around 4 or 15% respectively compared to 0.5% oxygen at 0.1 LHSV (116). The typical oxygen content of a flash pyrolysis oil is about 35-40%. The uncertainty lies in the selectivity of hydrotreating and hydrogenation of different classes of compounds in the crude bio-oil and the properties of the resultant partially upgraded oil. Some exploratory work has, however, been carried out (96) including studies on model compounds (75, 101).

A 15% oxygen content product would have a similar oxygen content as pressure liquefaction products which have very high viscosity and limited water miscibility under ambient conditions. This is, therefore, relatively difficult to use in most conventional applications. A partially hydrotreated pyrolysis oil has not been produced, however, so it’s characteristics cannot be defined. If there is a separate water phase, this would be highly contaminated with soluble organics and would require extensive and costly treatment. However, if this approach were successful in producing a stable and directly usable product, the hydrogen consumption could be reduced by 60% and the capital cost reduced by about 30%. In addition, the product may either retain the water in a solubilised form, or a relatively clean water could be separated. The concept of stabilisation by partial hydrotreatment deserves further consideration.