5.9.1 Summary

This project was initiated in 1980 based on early research which showed that higher oil yields could be obtained from a rotating tube furnace with moving particles (typically 28 wt% liquid yield) rather than a stationary reaction bed (typically 17 wt% liquid yield) (38). From this work, an entrained flow bed reactor was designed for the production of pyrolysis oil. This operated successfully until around 1989. Although apparently successful, the process was never scaled up. The only other entrained flow system for fast biomass pyrolysis was built by Egemin (q. v.). GTRI also developed the tech-Air process which similar to that of Bio- Alternative (q. v.) in that the principal product was charcoal with appreciable quantities of secondary oil derived as a by-product. This material was used extensively in early upgrading work.

5.9.2 Description

A process research unit was built and completed in 1983 (39, 40, 41). In 1985, modifications were made so that optimisation of the oil yield could be further investigated, based on experience gained in the operation of the pilot plant and the results of the commissioning runs. The major changes were the replacement of the 8 in. diameter (20.32 cm) reactor tube with a 6 in. (15.24 cm) tube, the addition of a quench vessel and a second demister (42, 43, 44). The nominal operating feed rate was then 56.8 kg/h of dry biomass. Figure 5.9 is a flowsheet of the process.

The feed is dried, hammer-milled to about 1.5 mm and fed from a loss-in-weight feeder into the reactor via a rotary valve. The reactor used is a 6 in. inner diameter vertical tube made of stainless steel. The initial feed point was the refractory lined mixing section, located below the reactor tube. However, by introducing the feed into feed ports higher up the reactor, the effective length of the reactor could be reduced which in turn reduced the residence time. The wood particles are entrained in a stream of hot combustion gas (927°C) obtained by burning propane gas and air stoichiometrically. Gas and wood flow cocurrently upwards through the reactor tube in which pyrolysis takes place; the resulting mixture consists of non­condensable gases, water vapour (moisture plus pyrolytic reaction water), pyrolysis oil vapours and char.

A cyclone separator is used to remove most of the char particles. The exiting gas stream consists of non-condensable gases, water vapour, pyrolysis oil vapours and some char fines. The hot effluent enters a water-sprayed quench vessel where it is rapidly cooled. Following the quench vessel, the mixture enters an air-cooled condenser in which the pyrolysis vapours are condensed with some water vapour. Early problems were reported with accumulation of tarry material in the first stages of the air-cooled condenser.

The condensed phases are removed via sumps and receivers and the gaseous product is passed through two demisters connected in series. Most of the aerosols present in the gaseous product are removed in the demisters. The remaining effluent, consisting of non-condensable gases, water vapour and remaining
aerosols, enters a flare where it is burnt and the combustion products are exhausted to the atmosphere.

In the scaled up process, it was intended that waste water production would be minimised or eliminated by controlled cooling and condensation of oil to retain the water in the vapour phase with subsequent combustion of the water laden by­product gas for process heat. There was, however, no experience of internal gas recycling in the pilot plant.

5.9.3 Product

Table 5.10 shows some of the last results with liquid yields approaching 60% wt on feed (45). Modelling and optimisation studies produced predictive models which indicated that yields of 70 wt% would be achievable with a well designed reactor and system. The oils are highly oxygenated with no phase separation as shown in Table 5.12. They have an initial boiling point range from 70°C to 90°C. They are heat sensitive and will decompose when heated to temperatures greater than 185°C-195°C, The oils are acidic, have an acrid odour and also exhibit corrosive properties with some metals. A typical bio-oil analysis is shown in Table 5.11. The product was upgraded by hydrotreating at Battelle PNL (46) which is discussed in more detail in that chapter. The liquid was also upgraded in the liquid phase over zeolite cracking catalyst — the only known application of this approach to zeolite cracking which is usually carried out on freshly produced vapours. The results have not been published, but extensive coking is understood to have occurred.

INETI

Fluid bed pyrolysis was investigated by LNETI (now INETI) at two scales of operation using hot gas to effect heat transfer. Initially work was carried out on a 10 cm fluid bed with subsequent research on a 30 cm square fluid bed with top feeding. Several parameters were investigated including pressure, temperature and bed additives or secondary reactants including zeolites, zinc chloride, carbonates and alkali (48). The results were not very satisfactory in that pyrolysis yields were relatively low and the catalysts showed little activity.