4.1 INTRODUCTION

In the descriptions of pyrolysis pathways and models in Chapter 3, it has been shown how a high yield of fast pyrolysis liquids can be obtained from an optimal combination of very high heating rates; vapour temperatures below about 600°C; very low vapour residence times; and rapid quenching of the resultant vapours. Although the three main products of gas, liquid and solid are formed, both the gas and char are minimised through careful control of temperature and residence time at typically 15-20% weight of the dry feedstock. There is an trade-off between reaction temperature and residence time that has not yet been explored. Higher temperatures with low residence times gives an olefin rich gas which caused much interest in the early developments of flash pyrolysis until it was found that the upper limit on olefin yield was too low to be of commercial interest.

The high temperature required for pyrolysis can be obtained in several ways:

• heating the gas-solid reacting mix through the wall of the reactor;

• heating with a heat transfer medium that may be gas such as preheated recycle gas, or liquid such as molten metal or molten salt;

• heating through exothermic chemical reactions inside the reactor such as partial oxidation;

• heating the particle directly through the wall of the reactor into an ablative pyrolysis.

In all cases the heat transfer mechanism and controlling step is significant in the design of the reactor, the performance of the reactor and its ability to be scaled up 0).

Since high liquid yields result from very high heating rates, low vapour residence times and moderate temperatures, the method of energy transfer has to be very efficient and well controlled. This imposes constraints on the particle size and method of heat transfer. The thermal conductivity of most biomass is relatively low and thus rapid heating of a sufficiently high proportion of the particle to achieve flash pyrolysis conditions and give high liquid yields imposes an upper particle size of 3 to 5 mm. Above this characteristic dimension, the rate of heat penetration into the particle becomes too slow to achieve high heating rates and also results in severe degradation of primary products as they diffuse back through the hotter outer shell of the pyrolysing particle.

This chapter is devoted to the major pyrolysis processes that have been developed from first principles into working processes, and that in some cases have achieved demonstration scale and commercial operation. Direct thermal liquefaction has been reviewed by Elliott et al. (2), and a review of pyrolysis processes for liquid production carried out by Bridgwater and Bridge (1). A survey of commercial and advanced technologies has also recently been published which includes pyrolysis as well as gasification (3).

4.2.1 introduction

Before providing a detailed review of individual processes, the current situation is Europe is described and compared to that in North America.