There are four thermochemical methods of converting biomass: pyrolysis, gasification, liquefaction and direct combustion. Each gives a different range of products and employs different equipment configurations operating in different modes. These are summarised below in Table 1.1.

The basis of a fuel or chemical production system is that the feedstock is converted to a useful primary energy product and either used as such, or further converted, upgraded or refined in subsequent processes to give a higher quality and higher value secondary product as shown in Figure 1.1.

When organic materials are heated in the absence of air, they degrade to a gas, a liquid, and a solid as summarised in Figure 1.1. It is possible to influence the proportions of the main products by controlling the main reaction parameters of temperature, rate of heating, and vapour residence time. For example fast or flash pyrolysis is used to maximise either the gas or liquid products, depending on temperature as summarised below:

• Slow pyrolysis at low temperatures of around 400°C and long reaction times (which can range from 15 minutes to days in traditional beehive kilns) maximises charcoal yields at about 30% wt.

* Flash pyrolysis at temperatures of typically 500°C; at very high heating rates and short vapour residence times of typically less than 1 second or 500 ms; maximises liquid yields at up to 85% wt (wet basis) or up to 70% dry basis.

• Similar flash pyrolysis at relatively high temperatures of above 700°C; at very high heating rates and similarly short residence times maximises gas yields at up to 80% wt. with minimum liquid and char production.

* "Conventional" pyrolysis at moderate temperatures of less than about 500°C and low heating rates (with vapour residence times of 0.5 to 5 minutes) gives approximately equal proportions of gas liquid and solid products.

This study is focussed on pyrolysis for the production of liquid fuels by the relatively novel process of fast pyrolysis, as these are currently viewed as a promising process and a promising product both in Europe and North America. The pyrolysis liquids are variously referred to as "bio-oil", "bio crude oil" or even as "oil" although they share few similarities with any oil products.

In order to appreciate the scientific and technical complexities of flash pyrolysis, the development of understanding of reaction mechanisms and pathways is first reviewed which will explain some of the unusual product properties that have been reported and some of the reasons for the way the technology has developed. Modeling of the complex and interactive physical and chemical processes that occur in pyrolysis has also attracted considerable attention and this area is also reviewed to at least partly explain why technology and scale up is still largely empirical. Developments of some of the process technologies that have been promoted and scaled up are subsequently described to show the underlying principles have been applied.