1.6 Pyrolizer types

Modern pyrolyzers of different types have been developed. Based on the gas–solid contacting mode, they can be broadly classified as fixed bed, fluidized bed, and entrained bed, and then further subdivided depending on design configuration. The following are some of the major pyrolyzer designs in use [4]: Fixed or moving bed; bubbling fluidized bed ) BFB; circulating fluidized bed (CFB); ultra-rapid; rotating cone; ablative; vacuum.

Fixed or moving bed reactors. Fixed-bed pyrolysis, operating in batch mode, is the oldest pyrolyzer type. Heat for the thermal decomposition of biomass is supplied either from an external source or by allowing limited combustion as in a beehive oven. Modern continuous downdraft moving-bed reactor using wood chips or pellets is shown in Figure 11.

Figure 11. Lurgi process - charcoal production plant in Bunbury, Western Australia [16]

Fluidized bed reactors. Figure 12a shows a BFB pyrolyzer. Crushed biomass (2–6 mm) is fed into a bubbling bed of hot sand. The bed is fluidized by an inert gas such as recycled flue gas. Intense mixing of inert bed solids (sand is commonly used) offers good and uniform temperature control. The residence time of the solids is considerably higher than that of the gas in the pyrolyzer. The required heat for pyrolysis may be provided either by burning a part of the product gas in the bed or by burning the solid char in a separate chamber and transferring that heat to the bed solids (see Figure 12b). The pyrolysis product would typically contain about 70 to 75% liquid on dry wood feed. The char in the bed solids acts as a vapour cracking catalyst, so its separation through elutriation or otherwise is important if the secondary cracking is to be avoided to maximize the liquid product. The entrained char particles are separated from the product gas using cyclones.

A circulating fluidized-bed (CFB) pyrolyzer, shown in Figure 12b, works on the same principle as the bubbling fluidized bed except that the bed is highly expanded and solids continuously recycle around an external loop comprising a cyclone and loop seal. The bed operates in a special hydrodynamic regime known as fast bed. It provides good temperature control and is uniform around the entire height of the unit. The superficial gas velocity in a CFB is considerably higher than that in a bubbling bed. High velocity combined with excellent mixing allows a CFB to have large throughputs of biomass.

Here, gas and solids move up the reactor with some degree of internal refluxing. As a result, the residence time of average biomass particles is longer than that of the gas, but the difference is not as high as it is in a bubbling bed. A major advantage of this system is that char entrained from the reactor is easily separated and burnt in an external fluidized bed. The combustion heat is transferred to the inert bed solids that are recycled to the reactor by means of a loop seal.

Figure 12. Bubbling (a) and circulating (b) fluidized bed pyrolysers [4]

Rapid thermal pyrolysis (RTP), a commercial process developed by Ensyn, probably originated from the ultra-rapid fluidized-bed pyrolyzer developed at the University of Western Ontario in Canada. RTP uses a riser reactor. Here, biomass is introduced into a vessel and rapidly heated to 500 °C by a tornado of upflowing hot sand; it is then cooled within seconds. The heating rate is on the order of 1000 °C/s, and the reactor residence time is from a few hundredths of a millisecond to a maximum of 5 seconds, which gives a liquid yield as high as 83% for wood [4].

Ultra-Rapid Pyrolyzer. The ultra-rapid pyrolyzer, shown in Figure 13a, developed by the University of Western Ontario provides extremely short mixing (10–20 ms), reactor residence (70–200 ms), and quench (~20 ms) times. As the reactor temperature is also low (~650 °C), one can achieve a liquid yield as high as 90% [4]. The inert gas nitrogen is heated 100 °C above the reactor temperature and injected at very high velocity into the reactor to bombard a stream of biomass injected in the reactor. The reactor can also use a heat-carrier solid like sand that is heated externally and bombarded on a biomass stream through multiple jets. Such a high-velocity impact in the reactor results in an exceptionally high heating rate. The biomass is thus heated to the pyrolysis temperature in a few milliseconds. The pyrolysis product leaves the reactor from the bottom and is immediately cooled to suppress a secondary reaction or cracking of the oil vapour. This process is therefore able to maximize the liquid yield during pyrolysis.

Figure 13. Ulytra-rapid (a) and ablation (b) pyrolysers [4]

Ablative Pyrolyzer. This process, shown in Figure 13b, involves creation of high pressure between a biomass particle and a hot reactor wall. This allows uninhibited heat transfer from the wall to the biomass, causing the liquid product to melt out of the biomass the way frozen butter melts when pressed against a hot pan. The biomass sliding against the wall leaves behind a liquid film that evaporates and leaves the pyrolysis zone, which is the interface between biomass and wall. As a result of high heat transfer and short gas residence time, a liquid yield of 80% high is reported. The pressure between biomass and wall is created either by mechanical means or by centrifugal force. In a mechanical system a large piece of biomass is pressed against a rotating hot plate.

Rotating-Cone Pyrolyzer. In this process, biomass particles are fed into the bottom of a rotating cone (360–960 rev/min) together with an excess of heat-carrier solid particles (see

Figure 14a). Centrifugal force pushes the particles against the hot wall; the particles are transported spirally upward along the wall. Owing to its excellent mixing, the biomass undergoes rapid heating (5000 K/s) and is pyrolyzed within the small annular volume. The product gas containing bio-oil vapour leaves through another tube, while the solid char and sand spill over the upper rim of the rotating cone into a fluidized bed surrounding it, as shown in Figure 14a. The char burns in the fluidized bed, and this combustion helps heat the cone as well as the solids that are recycled to it to supply heat for pyrolysis. Special features of this reactor include very short solids residence time (0.5 seconds) and a small gas-phase residence time (0.3 seconds). These typically provide a liquid yield of 60 to 70% on dry feed. The absence of a carrier gas is another advantage of this process. The complex geometry of the system may raise some scale-up issues.

(a)

(b)

Figure 14. Rotating-cone (a) and vacuum (b) pyrolyzers [4]

Vacuum pyrolyzer, shown in Figure 14b, comprises a number of stacked heated circular plates. The top plate is at about 200 °C while the bottom one is at about 400 °C. Biomass fed to the top plate drops into successive lower plates by means of scrapers. The biomass undergoes drying and pyrolysis while moving over the plates. No carrier gas is required in this pyrolyzer. Only char is left when the biomass reaches the lowest plate. Though the heating rate of the biomass is relatively slow, the residence time of the vapour in the pyrolysis zone is short. As a result, the liquid yield in this process is relatively modest, about 35 to 50% on dry feed, with a high char yield. This pyrolyzer design is complex, especially given the fouling potential of the vacuum pump.