The sequence of gasification reactions depends to some extent on the type of gas-solid contacting reactors used. A brief description of this process as it occurs in some principal reactor types follows.

Moving-Bed Reactor

To explain the reaction process in moving-bed gasifiers, we take the example of a simple updraft gasifier reactor (Figure 5.5).

In a typical updraft gasifier, fuel is fed from the top; the product gas leaves from the top as well. The gasifying agent (air, oxygen, steam, or their mixture), is slightly preheated and enters the gasifier through a grid at the bottom. The gas then rises through a bed of descending fuel or ash in the gasifier chamber.

The air (the gasifying medium), as it enters the bottom of the bed, meets hot ash and unconverted chars descending from the top (Figure 5.5). The

Fuel

Product gas

Ash

FIGURE 5.5 Stages of gasification in an updraft gasifier.

temperature in the bottom layer well exceeds the ignition temperature of carbon, so the highly exothermic combustion reaction (Eq. 5.24) takes place in the presence of excess oxygen. The released heat heats the upward-moving gas as well as the descending solids.

C + O2 ^ CO2 — 394 kJ/mol (5.24)

The combustion reaction (Eq. 5.24), being very fast, rapidly consumes most of the available oxygen. As the available oxygen is reduced further up, the combustion reaction changes into partial combustion, releasing CO and a mod­erate amount of heat.

C + I/2O2 ^ CO — 111 kJ/mol (5.25)

The hot gas, a mixture of CO, CO2, and steam (from the feed and the gas­ifying medium), moves further up into the gasification zone, where char from the upper bed is gasified by Eq. (5.26). The carbon dioxide concentration increases rapidly in the first combustion zone, but once the oxygen is nearly depleted, the CO2 enters the gasification reaction (Eq. 5.26) with char, resulting in a decline in CO2 concentration in the gasification zone.

C + CO2 ^ 2CO +172 kJ/mol (5.26)

C + H2O ^ CO + H2 +131 kJ/mol

Sensible heating of the hot gas provides the heat for the two endothermic gasification reactions in Eq. (5.26): R1 and R2 (Table 5.2). These are respon­sible for most of the gasification products like hydrogen and carbon monoxide. Because of their endothermic nature, the temperature of the gas reduces.

The zone above the gasification zone is for the pyrolysis of biomass. The residual heat of the rising hot gas heats up the dry biomass, descending from above. The biomass then decomposes (pyrolyzed) into noncondensable gases, condensable gases, and char. Both gases move up while the solid char descends with other solids.

The topmost zone dries the fresh biomass fed into it using the balance enthalpy of the hot product gas coming from the bottom. This gas is a mixture of gasification and pyrolysis products.

In an updraft gasifier biomass fed from the top descends, while air injected from the side meets with the pyrolysis product, releasing heat (see Chapter 6). Thereafter, both product gas and solids (char and ash) move down in the down­draft gasifier. Here, a part of the pyrolysis gas may burn above the gasification zone. Thus, the thermal energy required for drying, pyrolysis, and gasification is supplied by the combustion of pyrolysis gas. This phenomenon is called flaming pyrolysis.

In downdraft gasifiers, the reaction regions are different from those for updraft gasifiers. Here, steam and oxygen or air are fed into a lower section of the gasifier (Figure 5.6) with the biomass. The pyrolysis and combustion prod­ucts flow downward. The hot gas then moves downward over the remaining

FIGURE 5.6 Gasification reactions in a downdraft gasifier.

hot char, where gasification takes place. Such an arrangement results in tar-free but low-energy-content gases.

Fluidized-Bed Reactor

In a bubbling fluidized bed, the fuel fed from either the top or the sides mixes relatively fast over the whole body of the fluid bed (Figure 5.7). The gasifying medium (air, oxygen, steam, or their mixture) also serves as the fluidizing gas and so is sent through the bottom of the reactor.

In a typical fluidized-bed gasifier, fresh solid fuel particles are brought into contact with hot bed solids that quickly heat the particles to the bed temperature and make them undergo rapid drying and pyrolysis, producing char and gases.

Though the bed solids are well mixed, the fluidizing gas remains generally in plug-flow mode, entering from the bottom and leaving from the top. Upon entering the bottom of the bed, the oxygen goes into fast exothermic reactions (R4, R5, and R8 in Table 5.2) with char mixed with bed materials. The bed materials immediately disperse the heat released by these reactions to the entire fluidized bed. The amount of heat released near the bottom grid depends on the oxygen content of the fluidizing gas and the amount of char that comes in contact with it. The local temperature in this region depends on how vigorously the bed solids disperse heat from the combustion zone.

Subsequent gasification reactions take place further up as the gas rises. The bubbles of the fluidized bed can serve as the primary conduit to the top. They are relatively solids-free. While they help in mixing, the bubbles can also allow gas to bypass the solids without participating in the gasification reactions. The pyrolysis products coming in contact with the hot solids break down into

noncondensable gases. If they escape the bed and rise into the cooler freeboard, tar and char are formed.

A bubbling fluidized bed cannot achieve complete char conversion because of the back-mixing of solids. The high degree of solid mixing helps a bubbling fluidized-bed gasifier achieve temperature uniformity, but owing to the intimate mixing of fully gasified and partially gasified fuel particles, any solids leaving the bed contain some partially gasified char. Char particles entrained from a bubbling bed can also contribute to the loss in a gasifier. The other important problem with fluidized-bed gasifiers is the slow diffusion of oxygen from the bubbles to the emulsion phase. This encourages the combustion reaction in the bubble phase, which decreases gasification efficiency.

In a circulating fluidized bed (CFB), solids circulate around a loop that is characterized by intense mixing and longer solid residence time within its solid circulation loop. The absence of any bubbles avoids the gas-bypassing problem of bubbling fluidized beds.

Fluidized-bed gasifiers typically operate in the temperature range of 800 to 1000 °C to avoid ash agglomeration. This is satisfactory for reactive fuels such as biomass, municipal solid waste (MSW), and lignite. Since fluidized-bed gasifiers operate at relatively low temperatures, most high-ash fuels, depending on ash chemistry, can be gasified without the problem of ash sintering and agglomeration. Owing to the large thermal inertia and vigorous mixing in flu­idized-bed gasifiers, a wider range of fuels or a mixture of them can be gasified. This feature is especially attractive for biomass fuels, such as agricultural resi­dues and wood, that may be available for gasification at different times of the year. For these reasons, many developmental activities on large-scale biomass gasification are focused on fluidized-bed technologies.

Entrained-Flow Reactor

Entrained-flow gasifiers are preferred for the integrated gasification combined cycle (IGCC) plants. Reactors of this type typically operate at 1400 °C and 20 to 70 bar pressure, where powdered fuel is entrained in the gasifying medium. Figure 5.8 shows two entrained-flow gasifier types. In the first one, oxygen, the most common gasifying medium, and the powdered fuel enter from the side; in the second one they enter from the top.

In entrained-flow gasifiers, the combustion reaction, R5 (Eq. 5.24), may take place right at the entry point of the oxygen, followed by reaction R4 (Eq. 5.25) further downstream, where the excess oxygen is used up.

Powdered fuel (< 75 micron) is injected into the reactor chamber along with oxygen and steam (air is rarely used). To facilitate feeding into the reactor, especially if it is pressurized, the fuel may be mixed with water to make a slurry. The gas velocity in the reactor is sufficiently high to fully entrain the fuel particles. Slurry-fed gasifiers need additional reactor volume for evaporation of the large amount of water mixed with the fuel. Furthermore, their oxygen

Gas

Gasification

Biomass

Combustion

Steam, air, or oxygen

Ash

(a)

Steam, air, or oxygen

Biomass

Combustion

Height

Gasification

Temperature (°C)

Ash

Gas

(b)

FIGURE 5.8 Two main types of entrained-flow gasifiers: (a) side-fed entrained-flow reactor, and (b) top-fed entrained-flow reactor.

consumption is about 20% greater than that of a dry-feed system owing to higher blast requirements (Higman and van der Burgt, 2008).

Entrained flow gasifiers are of two types depending on how and where the fuel is injected into the reactor. Chapter 6 discusses several types. In all of these designs, oxygen, upon entering the reactor, reacts rapidly with the volatiles and char in exothermic reactions. These raise the reactor temperature well above the melting point of ash, resulting in complete destruction of tar or oil. Such high temperatures should give a very high level of carbon conversion.

An entrained-flow gasifier may be viewed as a plug-flow reactor. Although the gas is heated to the reactor temperature rapidly upon entering, solids heat up less slowly along the reactor length because of the reactor’s large thermal capacity and plug-flow nature, as shown in Figure 5.8. Some entrained-flow reactors are modeled as stirred tank reactors because of the rapid mixing of solids.