3.7.1 Introduction

A summary of the opportunities for using pyrolysis liquids is included as Figure 1. Within Europe the most promising application is seen as electricity production due to the anticipated ability to use raw bio-oil as produced in an engine or turbine without the need for extensive upgrading as well as the ability to de-couple fuel production from electricity generation with storage and/or transport of the liquid fuel which is not possible for gasifier products and IGCC systems. There are believed to be substantial longer term opportunities for chemicals from either catalytic pyrolysis or extraction and recovery of chemicals from the liquid products which would enhance the overall economics of conversion.

This liquid product may be readily burned (3) and has been employed for this purpose (13), provided the viscosity is not too high (Huffman Interlaken). Preheating to reduce viscosity is not usually favourable due to thermal degradation of the bio-oil, although short term exposure to temperatures up to 90°C have not had any adverse effects. The water content can be considered an advantage both for the combustion process and because it reduces the viscosity of the liquid. Therefore the oil can be considered an outlet or disposal route for the pyrolysis water. Phase separation is only likely to occur at water concentrations greater than 50% in the case of flash pyrolysis oils which is unlikely to arise. Some precautions may be needed in handling, storage and combustion due to the water and high oxygen content as described above.

Pyrolysis liquids are immiscible with any form of hydrocarbon liquid, and cannot, therefore, be expected to be assimilated into a conventional fuel marketing infrastructure without some conversion or upgrading to give a product that is compatible with conventional fuels. One alternative is to feed to crude pyrolysis liquid into a refinery for upgrading in orthodox refinery operations, utilising the hydrogen availability and blending opportunities (23). The basic problem with this approach is that bio-oil is immiscible with crude oil and creates handling problems. The alternative is to create a discrete pyrolysis liquids storage, distribution and utilisation system, that is managed by experts who understand the special problems of this fuel.

3.7.2 Combustion

Liquid products are easier to handle and transport in combustion applications and this is important in retrofitting existing equipment. Existing oil fired burners cannot be fuelled directly with solid biomass without major reconstruction of the unit, which may not be attractive in uncertain fuel markets, however bio-oils are likely to require only relatively minor modifications of the equipment or even none in some cases. Problems have been reported in handling the fuel with high viscosity and suspended char causing atomisation difficulties and incomplete combustion with some fuels. With recognition of the problems, solutions are unlikely to prove onerous.