3.1 INTRODUCTION

This chapter shows how the underlying scientific principles and requirements can be applied to the design and operation of pyrolysis processes. Included are the key product liquid characteristics that result from practical application of pyrolysis.

3.2 PYROLYSIS CONCEPTS

It has already been explained that biomass is a mixture of hemiceflulose, cellulose, lignin and minor amounts of other organics which each pyrolyse or degrade at different rates and by different mechanisms and pathways. Lignin decomposes over a wider temperature range compared to cellulose and hemicellulose which rapidly degrade over narrower temperature ranges, hence the apparent thermal stability of lignin during pyrolysis. The rate and extent of decomposition of each of these components depends on the process parameters of reactor (pyrolysis) temperature, biomass heating rate and pressure. The degree of secondary reaction (and hence the product yields) of the gas/vapour products depends on the time-temperature history to which they are subjected to before collection which includes the influence of the reactor configuration. Although some research has been carried out on the individual components of biomass, most applied and larger scale work has focused on whole biomass as the cost of pre-separation is considered too high. In addition, the separation and recovery of pure forms of lignin and hemicelfufose are difficult due to structural changes in their processing, although pure cellulose is relatively easy to produce.

Research has shown that maximum liquid yields are obtained with high heating rates, at reaction temperatures around 500’C and with short vapour residence times to minimise secondary reactions. Fast pyrolysis processes have been developed for production of food flavours (to replace traditional slow pyrolysis processes which had much lower yields) and speciality chemicals which utilise very short vapour residence times of typically 100-300 ms and reactor temperatures around 500*C. Both residence time and temperature control is important to "freeze" the intermediates of most chemical interest in conjunction with moderate gas/vapour phase temperatures of 400-500’C before recovery of the product to maximise organic liquid yields.

Liquids for use as fuels can be produced with longer vapour residence times [up to ~6 s] and over a wider temperature range although yields might be affected in two ways: secondary gas decomposition at temperatures above 500’C and condensation reactions at gas/vapour product temperatures below 400’C. Most woods give maximum liquid yields of up to 80% wt% dry feed basis [64 wt% organics and 16 wt% water] at 500-520*C with vapour residence times not more than 1 second. Very short residence times result in incomplete depolymerisation of the lignin due to random bond cleavage and inter-reaction of the lignin macromolecule resulting in a less homogenous liquid product, while longer residence times can cause secondary cracking of the primary products, reducing yield and adversely affecting bio-oil properties. Evidence from SEC analysis of the

liquids would suggest that the reactor configuration and the dominant mode of heat transfer strongly influences the average molecular weight of the products (1). This is discussed further below.