5.21.1 Summary

This work was instigated under the JOULE programme to develop a new reactor technology for the pyrolysis of biomass by sliding and pressing the particles on a heated surface. Ablative pyrolysis is achieved in a rotating cone where the particles "slide" across a heated metal surface. The process as developed and described has been licensed to BTG and a larger plant of 50 kg/h is being built by Schelde for delivery to China in 1995.

5.21.2 Description

The concept is that biomass particles are fed onto an impeller which is mounted at

the base of the heated rotating cone and are then flung on to the heated surface with the final char and ash residue flung out of the top of the cone. The concept is depicted in Figure 5.18.

An initial cold flow experimental cone was built to investigate the dynamic behaviour of nearly spherical mono-sized PVC particles, diameter 140-780|im (91). This work was carried out in a cone of angle 60° [vertical height 0.31 m and cone top width of 0.5 m] and 90°. Rotational speeds up to 1800 rpm were used to determine the influence of gas flow on the residence time and motion of the particles. Particle motion was recorded using an endoscopic camera. Particles greater than 400pm appeared to be unaffected by gas viscous forces and the residence time of the particles is almost independent of the particle diameter. For particles smaller than 200pm, viscous forces become dominant and the residence time of the particles is strongly dependent on the particle diameter. Derived particle residence times were 0.01-0.3 s (92).

A heat transfer rig was then tested with the same cone dimensions to investigate the heat transfer to particles ablatively "sliding" on the cone surface (93, 94). Measurement of particle temperature as it travels along the wall of the cone has bee achieved by fluoroptic methods. The experimental reactor is depicted in Figure 5.19. Initial experiments showed that the fine biomass particles [-250 pm] adhered to the cone surface, thereby reducing the reactor throughput quickly.

Rice husks with a high ash content [-20 wt%] have flowed successfully through the reactor zone with no surface adhesion. Modifications have been carried out to the cone to add sand to promote the flow of solids in September 1992 (95). Initial experiments were carried out mainly with a cone temperature of 600°C and a cone rotational speed of 900 rpm.

The reactor interior has been modified as shown in shown in Figure 5.20 to reduce

the operational volume from 0.25 m3 to 0.003 m3, otherwise the gas/vapour residence time would be around 80 s giving significant vapour cracking (96). To remove the problem of particle adhesion to the reactor wall, sand is added in a mass ratio of 10 or 20:1 sand to biomass fed. Preheated sand is used on a once through basis. The reactor is to be modified to permit internal sand recycle. The reactor outside the cone quickly becomes filled with sand and char, restricting experimental runs to 10 minutes. The liquids are collected in a series condenser system.

The reactor is to be modified so that the sand is removed from the reactor with the char, the char combusted and the hot sand re-fed to the reactor, i. e. an internal sand recycle. A cold model has been constructed at the beginning of 1995 and this will subsequently be tested as a hot system.

5.21.3 Products

Isothermal reactor operation leads to significant cracking of the product vapours. Typical yields at 1 s residence time and a heated surface temperature of 600°C are: 50 wt% liquids, 30 wt% gases and 15 wt% char. Reactor throughputs are claimed to be 7.2 kg/h of biomass.