Although fast pyrolysis of biomass has achieved commercial status, there are still many aspects of the process which are largely empirical and require further study to improve reliability, performance, product consistency, product characteristics and scale-up. This section summarises these topics.

3.3.1 Reactor configuration

A variety of reactor configurations have been investigated as listed in Table 3.1. Pyrolysis, perhaps more than any other conversion technology, has received considerable creativity and innovation in devising reactor systems that provide the essential ingredients of high heating rates, moderate temperatures and short vapour product residence times for liquids.

Table 3.1 Pyrolysis Reactors, Heating Methods and Heating Rates

NOTES

* Used for solid waste processing, not liquids production 1 Used for gas production

§ Operational mode unclear

There are two important requirements for heat transfer in a pyrolysis reactor:

1 to the reactor heat transfer medium (solid in ablative reactor, gas and solid in fluid and transport bed reactors, gas in entrained flow reactors),

2 to the pyrolysing biomass.

Two main ways of heating biomass particles in a fast pyrolysis system can be considered: gas-solid heat transfer as in an entrained flow reactor where heat is transferred from the hot gas to the pyrolysing biomass particle by primarily convection (for example the Egemin process), and solid-solid heat transfer with mostly conductive heat transfer. Fluid bed pyrolysis utilises the inherently good solids mixing to transfer approximately 90% of the heat to the biomass by solid — solid heat transfer with a probable small contribution from gas-solid convective heat transfer of up to 10%. Circulating fluid bed and transport reactors also rely on both gas-solid convective heat transfer from the fluidising gas and solid-solid heat transfer from the hot fluidising solid although the latter may be less significant than fluid beds due to the lower solids bulk density. Some radiation effects occur in all reactors.

The important feature of ablative heat transfer is that the contact of the biomass and the hot solid abrades the product char off the particle exposing fresh biomass for reaction. This removes particle size limitations in certain ablative reactors (e. g. the NREL vortex reactor). Attrition of the char from the pyrolysing particle can also occur in both fluid and circulating fluid beds, due to contact of the biomass with in­bed solids where solids mixing occurs. In fluid bed reactors however, attrition of the product char is relatively low and it has been observed that the char particles have the original particle shape, but are slightly reduced in size by char layer shrinkage and attrition.

Char removal is an essential requirement for large particles (> 2 mm) to avoid slow pyrolysis reactions from the low thermal conductivity of biomass giving low heating rates through larger particles which leads to increased char formation. Hot char is known to be catalytically active. It cracks organic vapours to secondary char, water and gas both during primary vapour formation and in the reactor gas environment, therefore it’s rapid removal from the hot reactor environment and minimal contact with the pyrolysis vapour products is essential.

Since the thermal conductivity of biomass is very poor (0.1 W/mK along the grain, ca 0.05 W/mK cross grain), reliance on gas-solid heat transfer means that biomass particles have to be very small to fulfil the requirements of rapid heating to achieve high liquid yields. Claimed temperature increases of 10,000’C/s may be achieved at the thin reaction layer but the low thermal conductivity of wood will prevent such temperature gradients throughout the whole particle. As particle size increases, liquid yields reduce as secondary reactions within the particle become increasingly significant (2). Union Fenosa are using particle sizes of smallest dimension 2 mm in their 200 kg/h fluid bed to achieve total liquid yields of around 55% wt. on dry feed with a 15% water content (3). Ensyn claim that particle sizes of up to 5-6 mm

in their reactor will still give total liquid yields of up to 75% wt. on feed with 10% moisture which is equivalent to 83% wt. on dry feed basis. Based on the original vortactor work at the University of Western Ontario, it seems likely that attrition of particles at the base of the Ensyn RTP reactor would be a feature of their system to remove char from the particle surface with some degree of solid-soiid heat transfer. Egemin found with their entrained reactor that particle sizes of 6 mm caused a large proportion to be expelled from the reactor substantially unreacted due to poor heat transfer and no char removal from ablation. This resulted in total liquids yields of less than 40% wt on dry feed (4), while GTRI claimed yields of over 60% total liquids with an up flowing entrained flow reactor using feed sizes up to 6 mm (5).

A consistent method of expressing product yields is required to remove ambiguities in the comparison of product yields. It is recommended that the water in the feed should be discounted in the final pyrolysis products with only the water of pyrolysis being quoted and the product yields expressed on a dry feed basis. As a rule of thumb, the water of pyrolysis is typically 11 wt% of dry feed.