5.13.1 Summary

The original aim at the beginning of the 1980s was to convert biomass into non­equilibrium gases in a low pressure process without the addition of a catalyst in an analogous concept as the work at the University of Western Ontario which lead to the formation of Ensyn. This aim was modified in 1984 to produce liquid fuels in high yield, and more recently it has been modified again to consider production of hydrocarbon fuels such as gasoline, aromatics such as benzene, xylene and toluene, and chemical fractions such as polyphenols.

Interchem attempted to adapt the concept of the vortex reactor in their first pilot/demonstration plant at Springfield, MO, USA, (q. v.) but this was not successful. The second attempt was based on an NREL designed ablative vortex reactor and is shown below. While the reactor was built, the completed plant was never operated and the project is understood to have been abandoned.

NREL are continuing to operate their original pilot plant in Golden and has commissioned a second unit. There is more recent work on fluid bed fast pyrolysis (56), on MBMS fast pyrolysis (57) and on close coupled zeolite upgrading (see Chapter 6).

The description below focuses on the ablative vortex reaction system as the major R&D activity in fast pyrolysis.

5.13.2 Description

Initially a smooth walled vortex reactor with a gas recycle loop, made from Inconel 800H in order to withstand temperatures of 1000°C, was used. However, early experimentation demonstrated that severe coke deposits were formed at wall temperatures much above 625°C (58). Since a lower reactor temperature of 625°C was needed, a vortex reactor made from stainless steel was designed, constructed and tested. Very high organic vapour yields resulted from this lower wall temperature operation. The design capacity of the vortex reactor is 50 kg/h biomass but the maximum throughput achieved to date is 36 kg/h (59, 60, 61).

Figure 5.11 shows the current configuration of the reactor system. Biomass, with a particle size of about 5 mm, is metered into the system, where it is entrained and mixed with the recycle stream. The biomass particles, entrained in the carrier gas, enter the vortex reactor tangentially at speeds of over 400 m/s so that the particles are forced to the reactor wall by high centrifugal forces.

The reactor is made from 316 stainless steel with a diameter of 13.4 cm and a length 70 cm. The reactor is heated externally by three electric cylindrical furnaces. To force the particles into tighter helical paths than would naturally occur, a helical rib having a pitch of 25 mm and width and height of 3 mm was machined from the wall of the reactor. An insulated recycle loop is also added tangentially at the exit of the reactor to recycle partially pyrolysed feedstock and any large char particles.

GAS at 5°C

The fine char, gases and vapours in the reactor leave through the axial exit which extends part way into the reactor. The reactor has a very high specific capacity and can in principle be easily scaled up. Very high heat transfer rates are achieved between the hot wall and biomass particles centrifuging against the hot reactor wall.

The wall temperature has to be limited to a maximum of about 625°C to ensure production of a liquid film between the wall of the reactor and the particle which then vaporises and leaves the reactor. The product stream then passes through a char cyclone where the char is removed. The diameter is 4 in. (10 cm) and operates at 475-500°C. The vapours pass to the first heat exchanger which is a 38 cm diameter cyclone. The condensed liquids and water are retained in the receiver. The cooled gas stream at about 80°C is then passed to a series of heat exchanger before passing through an orifice meter, and then to flare.