• The product gas of SCWG automatically separates from the liquid contain­ing tarry materials and char if any.

7.3 BIOMASS CONVERSION IN SCW

There are three major routes for SCW-based conversion of biomass into energy as follows:

Liquefaction: Formation of liquid fuels above critical pressure (22.1 MPa) but near critical temperature (300-400 °C).

Gasification to CH4: Conversion in SCW in a low-temperature range (350­500 °C) in the presence of a catalyst.

Gasification to H2: Conversion in SCW with or without catalysts at higher (>600 °C) temperatures.

Here we discuss only the last two gasification options.

Biomass received from its source cannot be fed directly into the gasifier for the following reasons:

• Presence of foreign materials (e. g., rocks and metals)

• Unacceptable level of moisture

• Too large (or uneven in size)

Such undesirable conditions not only affect the flow of solids through the feeder, but they also affect operation of the gasifier. It is thus necessary to eliminate them and prepare the collected biomass appropriately for feeding. Foreign materials pose a grave problem in biomass-fired plants. They jam feeders, form arches in silos, and affect the gasifier operation, so it is vitally important to remove them as much as possible. The three main foreign materials are: (1) stones, (2) ferrous metals (e. g., iron), and (3) nonferrous metals (e. g., aluminum).

Some of the equipment used to remove foreign materials from the collected biomass are as follows:

De-stoner. The basic purpose of a de-stoner is the separation of heavier- than-biomass materials such as glass, stones, and metals. Typical de-stoners

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use vibration in tandem with suitable air flow to stratify heavy materials according to their specific gravity.

Nonferrous metal separators. Separation of nonferrous metals like alumi­num has always been a challenge. One solution is an eddy current separa­tor—essentially a rotor with magnet blocks—which, depending on the application, is made of either standard ferrite ceramic or a more powerful rare-earth magnet. The rotors are spun at high revolutions (more than 3000 rpm) to produce an “eddy current,” which reacts differently with dif­ferent metals according to their specific mass and resistivity to create a repelling force on the charged particle. If a metal is light yet conductive, such as aluminum, it is easily levitated and ejected from the normal flow of the product stream, making separation possible (Figure 8.11). Separation of stainless steel is also possible depending on its grade. Particles from mate­rial flows can be sorted down to a minimum size of 3/32 in. (2 mm) in diameter. Eddy current separators are crucial in the recycling industry because of their ability to separate nonmagnetic materials.

Magnetic metal separation. The use of powerful magnets to separate iron and other magnetic materials from the feed is a standard procedure in many plants. Magnets are located at several places along the feed stream. They are generally suspended above the belt to attract magnetic materials, which are then discharged away.

Use of food cereals, such as wheat and corn for the production of biodiesel or ethanol, has been commercially successful; however, it has had a major impact on the world’s food market, driving up prices and creating shortages. Alterna­tive sources of biodiesel are being researched. Instead of sugar beets or rape seed, cellulosic biomass like wood may be used as the feedstock. With cellu — losic materials, the industry can significantly increase the yield of fuel per unit of cultivated area.

There are two options for production of ethanol or gasoline from nonfood sources: thermal and biochemical.

Thermal Process

In the thermal process, cellulosic feedstock is subjected to fast pyrolysis (Chapter 3). The liquid produced is refined and upgraded to gasoline or ethanol. Since cellulose is the feedstock, the ethanol from it is often referred to as cel­lulosic ethanol. An alternative thermal process involves gasification of the biomass to produce syngas and synthesis of the syngas into diesel oil using the FT process. This process was described in Section 9.4.2 and is illustrated in Figure 9.6.

Reaction kinetics must be solved simultaneously with bed hydrodynamics and mass and energy balances to obtain the yields of gas, tar, and char at a given operating condition.

As the gasification of a biomass particle proceeds, the resulting mass loss is manifested either through reduction in size with unchanged density or reduc­tion in density with unchanged size. In both cases the rate is expressed in terms of the external surface area of the biomass char. Some models, where the reac­tion is made up of char alone, can define a reaction rate based on reactor volume. There are thus three ways of defining the char gasification reaction for biomass: (1) shrinking core model, (2) shrinking particle model, and (3) volu­metric reaction rate model.

Reactor Hydrodynamics

The kinetic model considers the physical mixing process and therefore requires knowledge of reactor hydrodynamics. The hydrodynamics may be defined in terms of the following types with increasing sophistication and accuracy:

• Zero dimensional (stirred tank reactor)

• One dimensional (plug flow)

• Two dimensional

• Three dimensional

Unlike other models, the kinetic model is sensitive to the gas-solid contact­ing process involved in the gasifier. Based on this process, the model may be divided into three groups: (1) moving or fixed bed, (2) fluidized bed, and (3) entrained flow. Short descriptions of these are given in Section 5.6.

An equilibrium calculation ideally predicts the product of gasification if the reactants are allowed to react in a fully mixed condition for an infinite period of time. There are two types of equilibrium model. The first one is based on equilibrium constants (stoichiometric model). The specific chemical reactions used for the calculations have to be defined, so this model is not suitable for complex reactions where the chemical formulae of the compounds, the reaction path, or the reaction equations are not known. This requires the second model type, which involves minimization of the Gibbs free energy (nonstoichiometric model). This process is more complex but it is advantageous because the chemi­cal reactions are not needed.

Stoichiometric Model

The stoichiometric model requires a selection of appropriate chemical reactions and information concerning the values of the equilibrium constants. Chapter 5 explains the calculation procedure, so it is not repeated here.

Waste treatment is another SCWG application. As explained in Section 7.3.3, in supercritical water even highly toxic wastes can be oxidized to harmless disposable residues. The agricultural industry produces large volumes of non­toxic but unhealthy products such as animal extracts and farm wastes that need to be disposed of productively. Many of these contain so much moisture that economical combustion or thermal gasification is not possible. Anaerobic digestion is a widely used alternative, especially in developing countries for production of useful gas (mostly methane) from animal extracts. Along with methane, anaerobic digestion produces fermentation sludge, which can be used as fertilizer.

Nevertheless, anaerobic gasification is orders of magnitude slower than thermal and other gasification processes, even with the use of catalysts. As a result, this makes large-scale commercial operation of anaerobic digesters dif­ficult. Furthermore, the attractiveness of this method depends on the price of fertilizer, which can vary as a result of over — or undersupply in the market (Matsumura, 2002).

SCWG or SCWO is an alternative suitable for waste treatment because it does not depend on the production of sludge and is much faster than anaerobic digestion. Matsumura (2002) noted that supercritical water gasification has better energy efficiency, cheaper gas production, and faster CO2 payback time (64.8%, 3.05 yen/MJ, and 4.19 years, respectively) in comparison with bio — methanation (49.3%, 3.74 yen/MJ, and 5.05 years, respectively).

The excellent solids-solids mixing in a fluidized bed helps disperse the fuel over a small bed area of 1 to 2 m2. A single feeder is adequate for a small bed having a cross-sectional size of less than 2 m2. Larger beds need multiple feeders. The number required for a given bed depends on factors such as quality of fuel, type of feeding system, amount of fuel input, and bed area. Highly reactive fuels with high volatiles need a larger number of feed injection points because they react relatively fast; less reactive fuels require fewer feed points.

Industrial designs often call for redundancy. For example, if a reactor needs two overbed feeders, designers will provide three, each with a capacity that is at least 50% of the design feed rate. In this way, if one feeder is out of service, the plant can still maintain full output on the other two. The number of redun­dant feeders depends on the capacity and reliability required of the plant.

Symbols and Nomenclature

A = area of the cross-sectional area of silo (m2)

B = parameter, depending on the silo (m)

C = parameter, depending on the silo (-) dp = particle size (m)

D = diameter of the silo (m)

D0 = diameter of the screw (m)

Dc = shaft diameter (m)

dh = height of a differential element in the silo (m) g = acceleration due to gravity (9.81 m/s2)

H = height of the silo (m) kf = wall friction coefficient (-)

K = Janssen coefficient (-)

K = a constant, depending on D0/D0 or P/D0 (-)

m = mass-flow rate (kg/s)

P = pitch of the screw (m)

Pw = normal pressure on the wall in the silo (Pa)

Pv = vertical pressure on the biomass in the silo (Pa) P0 = pressure at the base of the silo (Pa)

T = torque of the screw (Nm)

V0 = average solid velocity through outlet (m/s)

T = wall friction (Pa)

p, pp = density of solids (kg/m3)

pa = density of air (kg/m3)

av = vertical stress for the flow (Pa)

U = viscosity of air (kg/m. s) в = semi-included angle of hopper (degree)

The tar in coal gasification comprise benzene, toluene, xylene, and coal tar, all of which have good commercial value and can be put to good use. Tar from biomass, on the other hand, is mostly oxygenated and has little commercial use. Thus, it is a major headache in gasifiers, and a major roadblock in the com­mercialization of biomass gasification. Research over the years has improved the situation greatly, but the problem has not completely disappeared. Tar removal remains an important part of the development and design of biomass gasifiers.

Several options are available for tar reduction. These may be divided into two broad groups: (1) in-situ (or primary) tar reduction, which avoids tar forma­tion; and (2) post-gasification (or secondary) reduction, which strips the product gas of the tar already produced. They are shown in Figure 4.3.

In-situ reduction is carried out by various means so that the generation of tar inside the gasifier is lessened, thereby eliminating the need for any removal to occur downstream. As this process is carried out in the gasifier, it influences the product gas quality. Post-gasification reduction, on the other hand, does not interfere with the process in the reactor, and therefore the quality of the product gas is unaffected.

At times it may not be possible to remove the tar to the desired degree while retaining the quality of the product gas. In such cases a combination of in-situ and post-gasification reduction can prove very effective. The tar reduced is removed after the product gas leaves the gasifier. Details of these two approaches are given in the following sections.

Gasifiers are classified mainly on the basis of their gas-solid contacting mode and gasifying medium. Based on the gas-solid contacting mode, gasifiers are broadly divided into three principal types (Table 6.1): (1) fixed or moving bed, (2) fluidized bed, and (3) entrained flow. Each is further subdivided into specific types as shown in Figure 6.1. Major western technology providers, as listed in the figure, supply their gasification technologies as per one of these.

One gasifier type is not necessarily suitable for the full range of gasifier capacities. There is an appropriate range of application for each. For example, the moving-bed (updraft and downdraft) type is used for smaller units (10 kWth — 10 MWth); the fluidized-bed type is more appropriate for intermediate units (5 MWth-100 MWth); entrained-flow reactors are used for large-capacity units (>50 MWth). Figure 6.2 shows the overlapped range of application for different

FIGURE 6.1 Gasification technologies and their commercial suppliers.

^ Fluid bed w

M————————- ►

Updraft

________ Downdraft Entrained flow ^

I________ I________ I________ I_______ I_________ I

10 kW 100 kW 1 MW 10 MW 100 MW 1000 MW

Thermal input

FIGURE 6.2 Range of applicability for biomass gasifier types.

types of gasifiers developed with data from Maniatis (2001) and Knoef (2005). Crossdraft gasifiers are for the smallest size while entrained flow are the largest size gasifiers.

The following subsections discuss the design of auxiliary systems in fluidized — bed gasifiers.

Position of Biomass Feeding Position

The feed points for the biomass should be such that entrainment of any particles in the product gas is avoided. This can happen when the feed points are located too close to the expanded bed surface of a bubbling fluidized bed. If they are in close proximity to the distributor plate, excessive combustion of the volatiles in the fluidizing air produced can occur. To avoid this, they should be some distance further above the grate.

Nascent tar is released close to the feed point, so tar cracking can be impor­tant for some designs. If tar is a major concern, the feed port should be close to the bottom of the gasifier so that the tar has adequate residence time to crack (Barea, 2009).

Distributor Plate

The distributor plate of a fluidized bed supports the bed materials. It is no dif­ferent from that used for a fluidized combustor or boiler. The ratio of pressure drop across the bed and that across the distributor plate must be estimated to arrive at the plate design. More details are available in books on distributor plate design, including Basu (2006, Chapter 11). The typical open area in the air distributor grate is only a few percentage points.

Bed Materials

For the process design of a fluidized-bed gasifier, the choice of bed materials is crucial. These comprise mostly granular inorganic solids and some (<10%) fuel particles. For biomass, sand or other materials are used (as explained next);

coal gasification requires granular ash produced from the gasification process. Sometimes limestone is added with coal particles to remove sulfur. At different stages of calcination and sulfurization, the limestone can also form a part of the bed material.

Biomass has very little ash (less than 1% for wood), so silica sand is nor­mally used as the inert bed material. This is a natural choice because silica is inexpensive and the most readily available granular solid. One major problem with silica sand is that it can react with the potassium and sodium components of the biomass to form eutectic mixtures having low melting points, thereby causing severe agglomeration. To avoid this, the following alternative materials can be used:

• Alumina (Al2O3)

• Magnesite (MgCO3)

• Feldspar (a major component of Earth’s crust)

• Dolomite (CaCO3MgCO3)

• Ferric oxide (Fe2O3)

• Limestone (CaCO3)

Magnesite (MgO) was successfully used in the first biomass-based IGCC plant in Varnamo, Sweden (Stahl et al., 2001).

Tar is a mixture of higher-molecular-weight (higher than benzene) chemical compounds that condenses on downstream metal surfaces at lower tempera­tures. It can plug the passage and/or make the gas unsuitable for use. The bed materials, besides serving as a heat carrier, can catalyze the gasification reaction by increasing the gas yield and reducing the tar. Bed materials that act as a catalyst for tar reduction are an attractive option. Some are listed here (Pfeifer et al., 2005; Ross et al., 2005):

• Olivine

• Activated clay (commercial)

• Acidified bentonite

• Raw bentonite

• House brick clay

Common house brick clay can be effectively used in a CFB gasifier to reduce tar emission and enhance hydrogen production. The alkalis deposited on the bed materials from biomass may potentially behave as catalysts if their agglomerating effect can be managed (Ross et al., 2005).

Tar production can be reduced using olivine. The Fe content of olivine is catalytically active, and that helps with tar reforming (Hofbauer, 2002). Nickel — impregnated olivine gives even better tar reduction as nickel is active for steam tar reforming (Pfeifer et al., 2005).

Bingyan et al. (1994) reported using ash from the fuel itself (sawmill dust) as the bed material in a CFB gasifier. This riser is reportedly operated at a very low velocity of 1.4 m/s, which is 3.5 times the terminal velocity of the biomass

particles. Chen et al. (2005) tried to operate a 1-MWe CFB gasifier with rice husk alone, but the system had difficulty with fluidization in the loop seal because of the low sphericity of the husk ash; however, the main riser report­edly operated in the fast bed regime without major difficulty.