In plasma gasification, high-temperature plasma helps gasify biomass hydro­carbons. It is especially suitable for MSW and other waste products. This process may also be called “plasma pyrolysis” because it essentially involves thermal disintegration of carbonaceous material into fragments of compounds in an oxygen-starved environment. The heart of the process is a plasma gun, where an intense electric arc is created between two electrodes spaced apart in a closed vessel through which an inert gas is passed (Figure 6.19).

Though the temperature of the arc is extremely high (~13,000 °C), the temperature downstream, where waste products are brought in contact with it, is much lower (2700-4500 °C). The downstream temperature is still sufficiently high, however, to pyrolyze complex hydrocarbons into simple gases such as CO and H2. Simultaneously, all inorganic components (e. g., glass, metals,

in

silicates, heavy metals) are fused into a volcanic-type lava, which after cooling forms a basaltic slag. The product gas leaves the gasifier at very high tempera­tures (1000-1200 °C).

A typical plasma reactor provides a relatively long residence time for the gas in the gasifier. This and the high temperature cause the tar products to be cracked and harmful products like dioxin and furan to be destroyed.

Owing to the high reactor temperature and the presence of chlorine in wastes, the life of the reactor liner is an issue. However, an attractive feature is that plasma gasification is relatively insensitive to the quality of the feed­stock. This is the result of an independent energy source run by electricity instead of partial combustion of the gasification product.

6.5PROCESS DESIGN

The design of a gasifier involves both process and hardware. The process design gives the type and yield of the product, operating conditions, and the basic size of the reactor. The hardware design involves structural and mechani­cal components, such as grate, main reactor body, insulation, cyclone, and others, that are specific to the reactor type. This section focuses on gasifier process design.

The product of gasification is defined by its yield and composition, which are influenced by a number of gasifier design and operating parameters. For proper design and operation of an SCW gasifier, a good understanding of the influence of the following parameters is important:

• Reactor temperature

• Catalyst use

• Residence time in the reactor

• Solid concentration in the feed

• Heating rate

• Feed particle size

• Reactor pressure

• Reactor type

A screw feeder is a positive-displacement device. Not only can it move solid particles from a low-pressure zone to a high-pressure zone with a pressure seal; it can also measure the amount of fuel fed into the bed. By varying the speed of its drive, a screw feeder can easily control the feed rate. As with a gravity chute, the fuel coming out of a screw does not have any means for dispersion. An air dispersion jet employed under the screw feeder can serve this purpose.

Plugging of the screw is a common problem. Solids in the screw flights are compressed as they move downstream; sometimes they are packed so hard that they do not fall off the screw. Compaction against the sealed end of the trough carrying the screw is even worse, often leading to jamming of the screw. Plug­ging and jamming can be avoided by one of the following:

• Variable-pitch screw (Figure 8.17a)

• Variable diameter to avoid compression of fuels toward the feeder’s dis­charge end (Figure 8.17b)

• Wire screw

• Multiple screws (Figure 8.18)

A wire screw is suitable for a highly fibrous biomass. It is made of a helical springlike wire with no central shaft or blades. Because there is minimum metal-feed contact, there is less chance of feed buildup even if the feed is cohesive.

Multiple screws are effective especially for large-biomass fuels. Figure 8.18 shows a feeder with two screws. Some feed systems use three, four, or more.

The hopper outlet, to which the inlet of a feeder is connected, needs careful design. Figure 8.9 showed two designs. The first (refer to Figure 8.9a) has a tapered wall hopper. It develops a large stagnant layer on the hopper’s down­stream wall. The second (refer to Figure 8.9b) is a vertical hopper wall toward the discharge end. This is superior to the traditional inclined wall because it develops a smaller stagnant layer and thus avoids formation of rat holes.

A screw feeder typically serves 3 m2 or less area of a bubbling fluidized bed, so several feeders are needed for a large bed. A major and very common operational problem arises when the fuel contains high moisture. It has to be dried first before it enters the screw conveyor to avoid plugging.

Dai and Grace (2008) developed a model of the mechanism of solid flow through a screw feeder. They noted that the torque required by the screw is proportional to the vertical stress exerted on the hopper outlet by the bulk mate­rial in the hopper; it also depends strongly on screw diameter. The choke section

(the part of the screw extending beyond the hopper exit) accounts for more than half of the total torque required to feed the biomass, especially with compress­ible particles. The torque, T, required by a screw of diameter, D0, rotating in a shaft of diameter, Dc, is given as

T = KovD30 (8.9)

where av is the vertical stress for the flow and D0 is the screw diameter. The constant, K, depends on the ratios P/D0 and Dc/D0 (normal stress/axial stress) and the wall friction, where Dc is the shaft diameter and P is the pitch of the screw.

The pelletizing process resolves some typical problems of biomass fuels: trans­port and storing costs are minimized, handling is improved, and the volumetric calorific value is increased. Pelletization may not increase the energy density on a mass basis, but it can increase the energy content of the fuel on a volume basis. For example, while the energy density on a mass basis for raw wood, torrefied wood, wood pellet, and torrefied pellet was 10.5, 19.9, 16.2, and

21.6 kJ/kg (LHV as received basis), respectively, it was 5.0, 4.6, 10.5, and 18.4 GJ/m3, respectively, on a volume basis (Bergman, 2005c). Thus, pelletiza­tion of torrefied wood greatly increases the transportation and handling cost of biomass. Pelletization of torrefied biomass is better than torrefaction of pelletized wood from the standpoint of process energy consumption and product stability.

Symbols and Nomenclature

A = pre-exponential factor (s-1)

E = activation energy (J/mol) k = reaction rate (s-1) mb = mass of biomass at time t (kg) mc = mass of char residue (kg) mo = initial mass of biomass (kg)

R = universal gas constant (J/mol. K)

T = temperature (K)

Tpyr = pyrolysis temperature (K) heating = heating time (s) tr = reaction time (s)

X = fractional change in mass of biomass dtor = torrefaction temperature (°C)

The kinetic modeling of fluidized-bed gasifiers requires several assumptions or submodels. It takes into account how the fluidized-bed hydrodynamics is viewed in terms of heat and mass transfer, and gas flow through the fluidized bed. The bed hydrodynamics defines the transport of the gasification medium through the system, which in turn influences the chemical reaction on the biomass surface. Each of these is subject to some assumptions or involves submodels.

One can use several versions of the fluidization model:

• Two-phase model of bubbling fluidized bed: bubbling and emulsion phases

• Three-phase model of bubbling fluidized bed: bubbling, cloud, and emul­sion phases

• Fluidized bed divided into horizontal sections or slices

• Core-annulus structure

Gas flow through the bed can be modeled as:

• Plug flow in the bubbling phase; ideally mixed gas in the emulsion phase

• Ideally mixed gases in both phases

• Plug flow in both phases (there is exchange between phases)

• Plug flow through the bubble and emulsion phases without mass transfer between phases

• Plug flow of gas upward in the core and solid backflow in the annulus

The following sections present the essentials of a model for a circulating fluidized-bed combustor and one for a bubbling fluidized-bed gasifier (Kaushal et al., 2008). A typical one-dimensional steady-state model of a circulating fluidized-bed combustor, as shown in Figure 5.11, assumes gases as ideal and in the plug-flow regime. The riser is divided into three hydrodynamic zones: lower dense bed zone, intermediate middle zone, and top dilute zone. The solids are assumed uniform in size with no attrition. Char is a homogeneous matrix of carbon, hydrogen, and oxygen.

A bubbling fluidized-bed gasifier is divided into several zones with different hydrodynamic characteristics: dense zone and freeboard zone for bubbling beds; core-annulus for circulating beds. The dense zone additionally deals with the drying and devolatilization of the introduced feed. Superheated steam is introduced at the lower boundary of the dense zone. Each zone is further divided into cells, which individually calculate their local hydrodynamic and thermodynamic state using chosen equations or correlations. The cells are solved sequentially from bottom to top, with the output of each considered the input for the next. The conservation equations for carbon, bed material, and energy are evaluated not in each cell but across the entire zone. Therefore, each

▲

——- Solid

……… Solid + gas

——- Gas/air

FIGURE 5.11 Model of a circulating fluidized-bed gasifier.

zone shows a homogeneous char concentration in the bed material and a uniform temperature. Additional input parameters to the model are geometric data, particle properties, and flow rates.

The range of fluidizing velocity, Ug, in a bubbling bed depends on the mean particle size of the bed materials. The choice is made in the same way as for a fluidized-bed combustor. The range should be within the minimum fluidization and terminal velocities of the mean bed particles. The particle size may be within group B or group D of Geladart’s powder classification (see Basu, 2006, Appendix I). The typical fluidization velocity for silica sand of about 1 mm mean diameter may, for example, vary between 1.0 and 2.0 m/s.

If the gasifier reactor is a circulating fluidized-bed type, the fluidization velocity in its riser (Figure 6.12) must be within the limits of fast fluidization, which favors groups A or group B particles. Typical fluidization velocity for particle size in the range 150 to 350 microns is 3.5 to 5.0 m/s in a CFB. This type of bed has another important operating condition to be satisfied for opera­tion in the CFB regime. Solids, captured in the gas-solid separator at the gasifier exit, must be recycled back to the gasifier at a rate sufficiently high to create a “fast-fluidized” bed condition in the riser. Additional details about this are available in Basu (2006) or Kunii and Levenspiel (1991).

Table 7.5 illustrates the operation of a typical heat-recovery exchanger for supercritical water gasification. The data are taken from a large operating near­supercritical gasification plant. The fluid-to-wall heat-transfer coefficient in clean supercritical water in the tube may be calculated by the correlation of Yamagata et al. (1972):

Nu = 0.0135Re»a85Pr»a8 (7.14)

Based on Yamagata’s experiments with isobutane, Hsu (1979) found that the Sieder-Tate equation, as follows, is in better agreement:

Heat transfer in SCWG may vary because of solids in the fluid. Thus, applica­bility of these equations to SCWG is uncertain. Information on this aspect of heat transfer is presently unavailable.

For synthesis reaction, a high degree of gas purity is needed, so the gas must be cleaned of particulates and other contaminating gases. The raw syngas may contain three principal types of impurity: (1) solid particulates (unconverted char, ash); (2) inorganic impurities (halides, alkali, sulfur compounds, nitro­gen); and (3) organic impurities (tar, aromatics, carbon dioxide).

At high temperatures, the equilibrium shifts toward hydrogen-producing hydrogen-rich gas. The ash in the biomass appears as slag. At low temperatures, the ash remains in the product gas as dry ash. Cleaning has two aspects: remov­ing undesired impurities and conditioning the gas to get the right ratio of H2 and CO for the intended use. This use determines the level of cleaning and conditioning. Table 9.1 presents examples of product-gas specifications for different end uses.

Cleanup Options

For cleaning the gas of dust or particulates, there are four options: (1) cyclone, (2) fabric or other barrier filter, (3) electrostatic filter, and (4) solvent scrubber. Among organic impurities, tar is the most undesirable. The three main options for tar removal are

• Scrubbing with an organic liquid (e. g., methyl ester)

• Catalytic cracking by nickel-based catalysts or olivine sand

• High-temperature cracking

Inorganic impurities are best removed in sequence because some removal processes produce other components that need to be removed as well. First,

water quenching removes char and ash particles. Next, hydrolysis removes COS and HCN by converting them into H2S and NH3. The ammonia and halides can be washed with water, followed by adsorption of H2S, which can be removed with the wash water. Solid or liquid adsorbents are used to remove carbon dioxide from the product gas.

Compared to fossil fuels, biomass is rich in alkali salts that typically vaporize at high gasifier temperatures but condense downstream below 600 °C. Because condensation of alkali salts causes serious corrosion problems, efforts are made to strip the gas of alkali. If the gas can be cooled to below 600 °C, the alkali will condense onto fine solid particles (<5 microns) that can be captured in a cyclone, ESPs, or filters. Some applications do not permit cooling of the gas. In such cases, the hot gas may be passed through a bed of active bauxite main­tained at 650 to 725 °C.

Disposal of Collected Tar

Tar removal processes produce liquid wastes with higher organic compound concentrations, which increase the complexity of water treatment. Wastewater contaminants include dissolved organics, inorganic acids, NH3, and metals. Collected tars are classified as hazardous waste, especially if they are formed at high temperatures (Stevens, 2001). Several technologies are available for treatment of these contaminants before their final disposal. Hasler et al. (1997) presented a description of the available technologies that comprise extraction with organic solvent, distillation, adsorption on activated carbon, wet oxidation, oxidation with hydrogen peroxide (H2O2), oxidation with ozone (O3), incinera­tion, and biological treatment.

Cracking

Cracking involves breaking large molecules into smaller ones. It converts tar into permanent gases such as H2 or CO. The energy content of the tar is thus mostly recovered through the smaller molecules formed. Unlike in physical cleaning, the tar need not be condensed for cracking. This process involves heating the tar to a high temperature (~1200 °C) or exposing it to catalysts at lower temperatures (~800 °C). There are two major types of cracking: thermal and catalytic.

Thermal Cracking

Thermal cracking without a catalyst is possible at a high temperature (~1200 °C). The temperature requirement depends on the constituents of the tar. For example, oxygenated tars may crack at around 900 °C (Stevens, 2001). Oxygen or air may be added to allow partial combustion of the tar to raise its tempera­ture, which is favorable for thermal cracking. Thermal decomposition of biomass tars in electric arc plasma is another option. This is a relatively simple process but it produces gas with a lower energy content.

Catalytic Cracking

Catalytic cracking is commercially used in many plants for the removal of tar and other undesired elements from product gas. It generally involves passing the dirty gas over catalysts. The main chemical reactions taking place in a cata­lytic reactor are represented by Eq. (4.5) in the presence of steam (steam reforming) and Eq. (4.6) in the presence of CO2 (dry reforming). The main reactions for tar conversion are endothermic, so a certain amount of combustion reactions are allowed in the reactor by adding air.

Nonmetallic catalysts include less-expensive disposable catalysts: dolomite, zeolite, calcite, and so forth. They can be used as bed materials in a fluidized bed through which tar-laden gas is passed at a temperature of 750 to 900 °C. Attrition and deactivation of the catalyst are a problem (Lammars et al., 1997). A proprietary nonmetallic catalyst, D34, has been used with success in a fluid­ized bed at 800 °C followed by a wet scrubber (Knoef, 2005, p. 153).

Metallic catalysts include Ni, Ni/Mo, Ni/Co/Mo, NiO, Pt, and Ru on sup­ports like silica-alumina and zeolite (Aznar et al., 1997). Some of them are used in the petrochemical industry and are readily available. A Ni/Co/Mo blend converts NH3 along with tars. Catalysts deactivate during tar cracking and so need reactivation. Typically the catalysts are placed in a fixed or fluidized bed. Tar-laden gas is passed through at a temperature of 800 to 900 °C.

Dolomite (calcined) and olivine sand are very effective in in-situ tar reduc­tion. This type of catalytic cracking takes place at the typical temperature of a fluidized bed. Good improvement in gas yield and tar reduction is noted when catalytic bed materials are used.

This gasifier type is also called open top, or stratified throatless. Here, the top is exposed to the atmosphere, and there is no constriction in the gasifier vessel because the walls are vertical. Figure 6.5 shows that a throatless design allows unrestricted movement of the biomass down the gasifier, which is not possible in the throated type shown in Figure 6.4. The absence of a throat avoids bridging or channeling. Open-core is another throatless design, but here air is not added from the middle as in other types of downdraft gasifiers. Air is drawn into the gasifier from the top by the suction created downstream of the gasifier. Such gasifiers are suitable for finer fuels—for example, lighter biomass such as rice husk.

The following are some of the shortcomings of a downdraft gasifier:

• It operates best on pelletized fuel instead of fine light biomass.

• The moisture in the fuel must not exceed 25%.

• A large amount of ash and dust remains in the product gas.

• As a result of its high exit temperature, it has a lower gasification temperature.