6.2.1 Introduction

Many flash pyrolysis processes have been developed to pilot, demonstration and commercial scale based solely on thermal conversion without any extrinsic catalytic activity, to give a crude fuel product (2, 3). Concerns over utilisation and assimilation into a fuel market infrastructure have caused attention to be paid to in-

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situ or close coupled catalytic upgrading.

The liquid product is currently attracting the most interest in Europe and North America because of its high energy density, easy transportability, ease of use and de-coupling of conversion and utilisation processes (4). The liquid approximates to biomass in elemental composition, and is composed of a very complex mixture of oxygenated hydrocarbons. The complexity arises from the degradation of lignin, cellulose and hemicellulose, resulting in a broad spectrum of oxygenated compounds from uncontrolled and interactive degradation. Its composition is determined intrinsically by the temperature, rate of reaction, vapour residence time, and temperature-time cooling and quenching process which controls the extent of secondary reactions, and extrinsically by the feed composition. The liquid is often referred to as "oil" or "bio-oil" or "bio-crude-оіГ and can be upgraded to liquid hydrocarbon fuels, as indicated in Table 2 above.

Typical properties of flash pyrolysis oil have been reported from analytical programmes sponsored, for example, by the EC JOULE and AIR programmes (5), Energy Mines and Resources Canada (6), the United States Department of Energy (7) and the International Energy Agency Bioenergy Agreement (8), as well as extensively by technology developers. The key feature from a fuel utilisation viewpoint is the high oxygen content which ranges from 30 to 55% wt. depending on the feed, the feed water content, the oxygenated product spectrum, product water content and basis of reporting.

It is important to note that there are two types of liquids produced by pyrolysis of biomass:

• Primary liquid from flash pyrolysis processes. This is produced in high yields of up to 85% weight. It has a relatively low viscosity and high water miscibility tolerance of up to 35-50% wt water and can be readily combusted in most applications such as boilers, kilns and dual fuel engines. It is relatively unstable compared to conventional fuel oils being, for example, very temperature sensitive and non-volatile in distillation due to polymerisation reactions (1, 4).

• Secondary oil or tar from conventional or slow pyrolysis processes. This is produced in low yields of up to 20% weight It is very viscous and can only tolerate up to about 20% wt water before phase separation occurs. While it can also be combusted in many applications, the high viscosity and potential for water separation require careful handling.

The characteristics of both slow and flash pyrolysis oils are summarised in Table 6.2. Both products may be upgraded by any of the processes described in this chapter, but the low yields of slow pyrolysis liquid products will give very poor overall yields of upgraded products, thus adversely affecting the economics.