A circulating fluidized-bed (CFB) gasifier has a special appeal for biomass gasification because of the long gas residence time it provides. It is especially suitable for fuels with high volatiles. A CFB typically comprises a riser, a cyclone, and a solid recycle device (Figure 6.10). The riser serves as the gasifier reactor.

Although the HTW process (Figure 6.9) appears similar to a CFB, it is only a bubbling bed with limited solid recycle. The circulating and bubbling fluid­ized beds are significantly different in their hydrodynamic. In a CFB, the solids are dispersed all over the tall riser, allowing a long residence time for the gas as well as for the fine particles. The fluidization velocity in a CFB is much

Uniflow cyclone

Reactor

Biofuel feed

Product gas at 650-750 °C

Cooling… -…… water

Bottom ash-cooling screw Bottom ash

FIGURE 6.10 Circulating fluidized-bed gasifier. (Source: Adapted from Foster Wheeler.) higher (3.5-5.5 m/s) than that in a bubbling bed (0.5—1.0 m/s). Also, there is large-scale migration of solids out of the CFB riser. These are captured and continuously returned to the riser’s base. The recycle rate of the solids and the fluidization velocity in the riser are sufficiently high to maintain the riser in a special hydrodynamic condition, known as fast fluidized bed. Depending on the fuel and the application, the riser operates at a temperature of 800 to 1000 °C.

The hot gas from the gasifier passes through a cyclone, which separates most of the solid particles associated with it, and the loop seal returns the par­ticles to the bottom of the gasifier. Foster Wheeler developed a CFB gasifier where an air preheater is located in the standpipe below the cyclone to raise the temperature of the gasification air and indirectly raise the gasifier tempera­ture (Figure 6.10).

Many commercial gasifiers of this type have been installed in different countries. At the time of writing, the biggest among these is a 60-MWth unit in a coal-fired and natural-gas-fired power plant in Lahti, Finland, to provide a cheap supplementary fuel by gasifying waste wood and refuse-derived fuels (RDFs). Several manufacturers around the world have developed versions of the CFB gasifier that work on the same principle and vary only in engineering details.

TABLE 6.3 Comparison of Hydrodynamic Operating Conditions of Commercial Transport Gasifier and Circulating Fluidized Bed of Fluid Catalyst Cracking Units
Parameter	Smith et al., 2002	Peterson and Werther, 2005	Zhu and Venderbosch, 2005
Particle size (|rm)	200-350	180-230	20-150
Riser velocity (m/s)	12-18	3.5-5.0	6-28
Circulation rate (kg/m2-s)	730-3400	2.5-9.2*	400-1200
Riser temperature (°C)	910-1010	800-900	500-550
Riser pressure (bar)	140-270 psig	1 bar	150-300 kPa
Reactor	KBR gasifier	CFB gasifier	FCC cracker
^Computed from comparable units.

Transport Gasifier

This type of gasifier has the characteristics of both entrained-flow and fluidized — bed reactors. The hydrodynamics of a transport gasifier is similar to that of a fluid catalytic cracking reactor. A transport gasifier operates at circulation rates, velocities, and riser densities considerably higher than those of a conventional circulating fluidized bed. This results in higher throughput, better mixing, and higher mass and heat-transfer rates. The fuel particles are also very fine (Basu, 2006) and as such it requires a pulverizer or a hammer mill. A comparison of typical hydrodynamic operating conditions in a transport gasifier and in a fluid catalytic cracking unit is given in Table 6.3.

A transport gasifier consists of a mixing zone, a riser, a disengager, a cyclone, a standpipe, and a J-leg. Coal, sorbent (for sulfur capture), and air are injected into the reactor’s mixing zone. The disengager removes the larger carried-over particles, and the separated solids return to the mixing section through the J-valve located at the base of the standpipe (Figure 6.11). Most of the remaining finer particles are removed by a cyclone located downstream, from which the gas exits the reactor. The reactor can use either air or oxygen as the gasification medium.

Use of oxygen as the gasifying medium avoids nitrogen, the diluting agent in the product gas. For this purpose, air is more suitable for power generation, while oxygen is more suitable for chemicals production. The transport gasifier has proved to be effective for gasification of coal, but it is yet to be proven for biomass.

Twin Reactor System

One of the major problems in air gasification of coal or biomass is the dilution of product gas by the nitrogen in the air used for the exothermic combustion

reaction necessary in a self-sustained gasifier. To avoid this, oxygen is used instead, but oxygen gasification is expensive and highly energy intensive (see Example 6.5 later in chapter). A twin reactor (e. g., a dual fluidized bed) over­comes this problem by separating the combustion reactor from the gasification reactor such that the nitrogen released in the air combustion does not dilute the product gas. Twin reactor systems are used for coal and biomass. They are either externally circulating or internally circulating.

This type of system has some limitations; for example, Corella et al. (2007) identified two major design issues with the dual fluidized-bed system:

• Biomass contains less char than coal contains; however, if this char is used for gasification the amount of char available may not be sufficient to provide the required endothermic heat to the gasifier reactor to maintain a tempera­ture above 900 °C. External heating may be necessary.

• Though the gasifier runs on steam, only a small fraction (<10%) of the steam participates in the gasification reaction; the rest of it simply leaves the gas­ifier, consuming a large amount of heat and diluting the gas.

The Technical University of Vienna used the externally circulating system to gasify various types of biomass in an industrial plant in Gussing, Austria.

FIGURE 6.12 Twin reactor (dual fluidized-bed) gasifier.

The system is comprised of a bubbling fluidized-bed gasifier and a circulating fluidized-bed combustor (Figure 6.12). The riser in a CFB operates as a com­bustor; the bubbling fluidized bed in the return leg operates as a gasifier. Pyrolysis and gasification take place in the bubbling fluidized bed, which is fluidized by superheated steam. Unconverted char and tar move to the riser through a nonmechanical valve. The riser is fluidized by air.

Tar and gas produced during pyrolysis are combusted in the riser’s combus­tion zone. Heat generated by combustion raises the temperature of the inert bed material to around 900 °C. This material leaves the riser and is captured by the cyclone at the riser exit. The collected solids drop into a standpipe and are then circulated into the bubbling fluidized-bed reactor to supply heat for its endo­thermic reactions. The char is gasified in the bubbling bed in the presence of steam, producing the product gas. This system overcomes the problem of tar by burning it in the combustor. In this way, a product gas relatively free of tar can be obtained.

The Rentech-Silvagas process is also based on the externally circulating principle. Here, both the combustor and the gasifier work on circulating fluid — ized-bed principles. In the internally circulating design, the fluidized-bed

Upflowing Downflowing FIGURE 6.13 Internally circulating dual fluidized-bed gasifier.

reactor is divided into two chambers and connected by a window at the bottom of the division wall separating them. The chambers are fluidized at different velocities (Figure 6.13), which result in their having varying bed densities. As the bed height is the same in both, the hydrostatic pressure at the bottom of the two chambers is different. The biomass and sand thus flow from the higher — density chamber to the lower-density chamber, creating a continuous circulation of bed materials similar to the natural circulation in a boiler. This helps increase the residence time of solids in the fluidized bed.

Such an arrangement can provide a more uniform distribution of biomass particles in the reactor, with increased gasification yield and decreased tar and fine solids (char) in the syngas (Freda et al., 2008). A special feature of the twin reactor is that more air or oxygen can be added in one part of the bed to encourage combustion, and more steam can be added in another part to encourage gasification.