The critical point marks a significant change in the thermophysical properties of water (Figure 7.2). There is a sharp rise in the specific heat near the critical temperature followed by a similar drop. The thermal conductivity of water drops from 0.330 W/m. K at 400 °C to 0.176 W/m. K at 425 °C. The drop in molecular viscosity is also significant, although the viscosity starts rising with temperature above the critical value. Above this critical point, water experi­ences a dramatic change in its solvent nature primarily because of its loss of hydrogen bonding. The dielectric constant of the water drops from a value of about 80 in the ambient condition to about 10 at the critical point. This changes the water from a highly polar solvent at an ambient condition to a nonpolar solvent, like benzene, in a supercritical condition.

FIGURE 7.2 Specific heat of water above its critical pressure shows a peak at its pseudo-critical temperature. Dielectric constant at 22.1 MPa, also plotted on this graph, shows rapid decline closer to the critical temperature.

The change in density in supercritical water across its pseudo-critical tem­perature is much more modest, however. For example, at 25 MPa it can drop from about 1000 to 200 kg/m3 while the water moves from a liquidlike to a vaporlike state. At subcritical pressure, however, there is an order of magnitude drop in density when the water goes past its saturation temperature. For example, at 0.1 MPa or 1 atm of pressure, the density reduces from 1000 to 0.52 kg/m3 as the temperature increases from 25 to 150 °C (refer to Table 7.1).

The most important feature of supercritical water is that we can “manipu­late” and control its properties around its critical point simply by adjusting the temperature and pressure. Supercritical water possesses a number of special properties that distinguish it from ordinary water. Some of those properties relevant to gasification are as follows:

• The solvent property of water can be changed very strongly near or above its critical point as a function of temperature and pressure.

• Subcritical water is polar, but supercritical water is nonpolar because of its low dielectric constant. This makes it a good solvent for nonpolar organic compounds but a poor one for strongly polar inorganic salts. SCW can be a solvent for gases, lignin, and carbohydrates, which show low solubility in ordinary (subcritical) water. Good miscibility of intermediate solid organic compounds as well as gaseous products in liquid SCW allows single-phase chemical reactions during gasification, removing the interphase barrier of mass transfer.

• SCW has a high density compared to subcritical steam at the same tempera­ture. This feature favors the forward reaction between cellulose and water to produce hydrogen.

• Near its critical point, water has higher ion products ([H+][OH-]~10-11 (mol/l)2) than it has in its subcritical state at ambient conditions (~10-14 (mol/l)2) (Figure 7.3). Owing to this high [H+] and [OH] ion, the water can be an effective medium for acid — or base-catalyzed organic reactions (Serani et al., 2008). Above the critical point, however, the ion product drops rapidly (~10-24 (mol/l)2 at 24 MPa), and the water becomes a poor medium for ionic reactions.

• Most ionic substances, such as inorganic salts, are soluble in subcritical water but nearly insoluble under typical conditions of SCW gasifiers. As the temperature rises past the critical point, the density as well as the ionic product decreases (Figure 7.3). Thus, highly soluble common salt (NaCl) becomes insoluble at higher temperatures above the critical point. This tunable solubility property of SCW makes it relatively easy to separate the salts as well as the gases from the product mixture in an SCW gasifier.

• Gases, such as oxygen and carbon dioxide, are highly miscible in SCW, allowing homogeneous reactions with organic molecules either for oxida­tion or for gasification. This feature makes SCW an ideal medium for destruction of hazardous chemical waste through SCWO.

• SCW possesses excellent transport properties. Its density is lower than that of subcritical water but much higher than that of subcritical steam. This, along with other properties like low viscosity, low surface tension (surface tension of water reduces from 7.2 x 10-2 at 25 °C to 0.07 at 373 °C), and high diffusivity greatly contribute to the SCW’s good transport property, which allows it to easily enter the pores of biomass for effective and fast reactions.

• Reduced hydrogen bonding is another important feature of SCW. The high temperature and pressure break the hydrogen-bonded network of water molecules.

Table 7.1 compares some of these water properties under subcritical and super­critical conditions.

Biomass feed in fluidized-bed gasifiers needs special considerations, which are discussed in the following sections. For bubbling fluidized beds, we have the choice of two types of feed systems: (1) overbed (Figure. 8.20a) and (2) under­bed (Figure 8.20b).

Gasification is a relatively slow process compared to combustion, so the rapid mixing of fuels is not as critical as it is in a combustor. Table 8.1 compares the characteristics of the two types of feeder as used in a boiler. Such a com­parison may be valid for fluidized-bed gasifiers but only on a qualitative basis.

Overbed feeders can handle coarse particles; underbed feeders need fine sizes with less moisture. An underbed system consists of crushers, bunkers, gravimetric feeders, air pumps, a splitter, and small fuel-transporting lines. An overbed feed system, on the other hand, consists of crushers, bunkers, gravi­metric feeders, small storage bins, a belt conveyor, and spreaders.

Simulation, or mathematical modeling, of a gasifier may not give a very accu­rate prediction of its performance, but it can at least provide qualitative guid­ance on the effect of design and operating or feedstock parameters. Simulation allows the designer or plant engineer to reasonably optimize the operation or the design of the plant using available experimental data for a pilot plant or the current plant.

Simulation can also identify operating limits and hazardous or undesirable operating zones, if they exist. Modern gasifiers, for example, often operate at a high temperature and pressure and are therefore exposed to extreme operating conditions. To push the operation to further extreme conditions to improve the gasifier performance may be hazardous, especially if it is done with no prior idea of how the gasifier might behave at those conditions. Modeling may provide a less expensive means of assessing the benefits and the associated risk.

Simulation can never be a substitute for good experimental data, especially in the case of gas-solid systems such as gasifiers. A mathematical model, however sophisticated, is useless unless it can reproduce real operation with an acceptable degree of deviation (Souza-Santos, 2004). Still, a good mathematical model can

• Find optimum operating conditions or a design for the gasifier.

• Identify areas of concern or danger in operation.

• Provide information on extreme operating conditions (high temperature, high pressure) where experiments are difficult to perform.

• Provide information over a much wider range of conditions than one can obtain experimentally.

• Better interpret experimental results and analyze abnormal behavior of a gasifier, if that occurs.

• Assist scale-up of the gasifier from one successfully operating size to another, and from one feedstock to another.

To find the biomass feed rate, Mf, the required power output is divided by the LHV of the biomass (LHVbm) and by the gasifier efficiency, Hgef.

The LHV may be related to the higher heating value (HHV) and its hydrogen and moisture contents (Quaak et al., 1999) as

ШУЪт = ННУаау — 20,300 x Hd¥ — 2,260 x Md¥ (6.7)

Here, Hdaf is the hydrogen mass fraction in the fuel, Mdaf is the moisture mass fraction, and HHVdaf is the HHV in kJ/kg on a moisture-ash-free basis. By using the definition of these one can relate the HHV on moisture-ash-free basis to that on only dry-basis value as

HHVdaf = HHVd ——— ^—M—— | (6.8)

F d К1 — ash — M J

where the subscripts d and daf refer to dry and moisture-ash-free basis, respec­tively; M is the moisture fraction; and ASH is the ash fraction in fuel on a raw-fuel basis.

On a dry basis, the HHV, HHVd, is typically in the range 18 to 21 MJ/kg (Van Loo and Koppejan, 2003, p. 48). It may be calculated from the ultimate analysis for the biomass using the following equation (Van Loo and Koppejan, 2003, p. 29):

HHVd = 0.3491 C +1.1783 H + 0.1005 S — 0.0151 N (69)

— 0.1034 О — 0.0211 ASH 1 . )

where C, H, S, N, O, and ASH are the mass fraction of carbon, hydrogen, sulfur, nitrogen, oxygen, and ash in the fuel on a dry basis.

A longer residence for the reactants in the reactor gives a better yield. Lu et al. (2006) experimented with 2% (by weight) sawdust and 2% carboxymethyl cellulose (CMC) in a flow reactor at 650 °C and 25 MPa. Mettanant et al. (2009) experimented with 2% rice husk in a batch reactor under the same conditions. Both found a steady increase in hydrogen and a moderate increase in methane (Figure 7.8) when the residence time was increased by three times and six times, respectively. Total organic carbon in the liquid product decreases with residence time, whereas carbon and hydrocarbon gasification efficiencies increase. This implies that a longer residence time is favorable for SCW biomass gasification. The optimum residence time, beyond which no further improve­ment in conversion efficiency is possible, depends on several factors. At a higher temperature, the residence time required for a given conversion is shorter.

7.4.1 Solid Concentration in Feedstock

Unlike in other gasification methods, solids in the feed have an important effect on the gasification in supercritical water. Thermodynamic calculations suggest that the conversion of carbon to gases in SCW declines rapidly when the solid content in a liquid feed exceeds 50% (Prins et al., 2005), but experimental results show this to occur for a much lower concentration. Experimental data (Mettanant et al., 2009; Schmieder et al., 2000) indicate that gasification
efficiency starts to decline when the solid concentration exceeds a value as low as 2%.

Table 7.3 presents data (Mozaffarian et al., 2004) that show the effect of solid content in feed. Although experimental conditions and feedstock vary, we can broadly classify these results into groups of low, medium, and high solid feedstock. For a lower feed concentration (<2%), carbon conversion efficiency is in the range 100 to 92% and reduces to 60 to 90% for an intermediate con­centration (2-10%) and to 68 to 80% for a >10% concentration. An SCW gasifier thus needs a very low solid concentration in the feed for high carbon conversion efficiency. This requires higher pumping costs and liquid effluent disposal, which may be a major impediment in commercialization of SCW gasification.

The reactor type also influences how solid concentration affects gasifi­cation efficiency. For example, Kruse et al. (2003) noted that a stirred reactor shows opposite results—that is, higher gasification efficiency at higher solid content (1.8 to 5.4%) in feed. This contrasts with data from Schmieder et al. (2000) from tumbling and tubular reactors that indicate a decrease in gasification efficiency with solid content (0.2-0.6 M). In stirred reactors, reactants are very well mixed, resulting in a heating rate that is faster than achieved in other reactor types. This may be the explanation for the higher gasification efficiency where there is a higher solid content. The exact reason for this decrease is not clear and is a major issue in the development of com­mercial SCW gasifiers. Catalysts, high gasification temperatures, and high heating rates can avoid the drop in conversion of a high-solid-content feedstock (Lu et al., 2006).

Bio-oil is renewable and cleaner than nonrenewable mineral oil extracted from the ground (petroleum). Thus, it offers a “green” alternative in many applica­tions where petro-oil is used. Bio-oil is mainly an energy source, but it may also be used as a feedstock for the production of “green chemicals.”

Energy Production

Bio-oil may be fired in boilers and furnaces as a substitute for furnace oil in energy production. This allows a rapid and easy switchover from fossil fuels to biofuels, as it does not call for complete replacement or any major renovation of the firing system as would be needed if raw biomass were to be fired in a furnace or boiler designed for furnace oil. The combustion performance of a bio-oil-fired furnace should be studied carefully before such a switchover is made, because furnace oil and bio-oil have varying combustion characteristics, including significant differences in ignition, viscosity, energy content, stability, pH, and emission level. In many cases we can overcome these differences through proper design (Wagenaar et al., 2009).

Tar remains vaporized until the gas carrying it is cooled, when it either con­denses on cool surfaces or remains in fine aerosol drops (<1 micron). This makes the product gas unsuitable for use in gas engines, which have a low tolerance for tar. Thus, there is a need for tar reduction in product gas when the gas is to be used in an engine. This can be done through appropriate design of the gasifier and the right choice of operating conditions, including reactor temperature and heating rate. Even these adjustments may not reduce tars in the gas to the required level, necessitating further downstream cleanup.

Standard gas cleaning involves filtration and/or scrubbing, which not only removes tar but also strips the gas of particulate matters and cools it to room temperature. These practices clean the gas adequately, making it accept­able to most gas engines. However, they result in a great reduction in overall efficiency in the production of electricity or mechanical power using a gas engine. Furthermore, gas cleaning greatly adds to the capital investment of the plant.

Biomass gasification is at times used for distributed power generation in remote locations in small — to medium-capacity plants. For such plants, the addi­tion of a scrubber or a filtration system significantly increases the overall plant costs. This limitation makes biomass-based distributed power-generation proj­ects highly sensitive to the cost of tar cleanup.

The presence of tar in the product gas from gasification can potentially decide the usefulness of the gas. The following are the major applications of the product gas:

• Direct-combustion systems

• Internal-combustion engines

• Syngas production

• Pipeline transport over long distances

In direct-combustion systems, the gas produced is burnt directly in a nearby unit. Co-firing of gasified biomass in fossil-fuel-fired boilers is an example. Industrial units like ovens, furnaces, and kilns are also good examples of direct firing. In such applications, it is not necessary to cool the gas after production. The gas is fired directly in a burner while it is still hot, in the temperature range of 600 to 900 °C. Thus, there is little chance of tar condensation. However, the pipeline between the gasifier exit and the burner inlet should be such that the gas does not cool down below the dewpoint of tar. If that happens, tar deposi­tion might clog the pipes, leading to hazardous conditions.

In applications where the raw gas is burnt directly without cooling, there is no need for cleaning. Such systems have no restrictions on the amount of tar

and particulates as long as the gas travels freely to the burner, and as long as the burner design does not impose any restrictions of its own. However, the flue gas produced after combustion must meet local emission requirements.

Internal-combustion engines, such as diesel or Otto engines, are favorite applications of gasified biomass, especially for distributed power generation. In such applications the gas must be cooled, but there is a good chance of condensation of the tar in the engine or in fuel-injection systems. Furthermore, the piston-cylinder system of an internal-combustion engine is not designed to handle solids, which imposes tighter limits on the tar as well as on the particu­late level in the gas. Particulate and tar concentrations in the product gas should therefore be below the tolerable limits, which are 30 mg/Nm3 for particulates and 100 mg/Nm3 for tar (Milne et al., 1998, p. 41). The gas turbine, another user of biomass gas, imposes even more stringent restrictions on the cleanliness of the gas because its blades are more sensitive to deposits from the hot gas passing through them after combustion. Here, the particulate concentration should be between 0.1 and 120 mg/Nm3 (Milne et al., 1998).

The limits for particulates and tar in syngas applications are even more stringent, as tar poisons the catalyst. For these applications, Graham and Bain (1993) suggest an upper limit as low as 0.02 mg/Nm3 for particulates and 0.1 mg/Nm3 for tar. Interest in fuel cells is rising, especially for the direct production of electricity from hydrogen through gasification. The limiting level of tar in the gas fed into a fuel cell is specific to the organic constituents of the gas. Table 4.1 presents data on the tolerance levels of tar and particulate con­tents for several applications of gas.

The amount of tar in product gas depends on the gasification temperature as well as on the gasifier design. Typical tar levels in gases from downdraft and updraft biomass gasifiers are 1 g/Nm3 and 50 g/Nm3, respectively (Table 4.2). Tar levels in product gas from bubbling and circulating fluidized-bed gasifiers are about 10 g/Nm3. Table 4.2 also shows that the amount of tar

produced varies from 1 to 20% of the feed of the biomass. For a given gasifier, the amount of tar reduces with temperature, as shown in Figure 4.1.

Extensive work on the modeling of entrained-flow gasifiers is available in the literature. Computational fluid dynamics (CFD) has been successfully applied to this gasifier type. This section presents a simplified approach to entrained — flow gasification following the work of Vamvuka et al. (1995).

The reactor is considered to be a steady-state, one-dimensional plug-flow reactor in the axial direction and well mixed radially—similar to that shown in Figure 5.12. Fuel particles shrink as they are gasified. Five gas-solid reactions (R1-R5 in Table 5.2) can potentially take place on the char particle surface. The reduction in the mass of char particles is the sum of these individual reac­tions, so if there are Nc char particles in the unit gas volume, the total reduction, Wc, in the plug flow is as shown in the equation that follows the figure.

dWc = -(NcAdz)£rk(Ts, Lr) (5.85)

К=1

where rk(Ts, Lr) is the surface reaction rate of the kth reaction (one of R1-R5) at the reactor’s surface temperature, Ts, and length, Lr. A is its cross-section area.

Gaseous reactants diffuse to the char surface to participate in k reactions. Thus, if ajk is the mass of the jth gas, required for the kth reaction, the overall diffusion rate of this gas from free stream concentration, yj, to the char surface, yjs, may be related to the total of all reactions consuming the jth gas as follows:

where yjs and yj are mole fractions of gas on the char surface and in the bulk gas, respectively; P is the reactor pressure; and Dgj is the diffusion coefficient of the jth gas in the mixture of gases.

The surface reaction rate, rk(Ts, Lr), may be written in nth-order form as

rk(Ts, Lr ) = 4nrc2Ksk(Ts) (Pyjs) mols

where n is the order or reaction, and Ksk(Ts) is the surface reaction rate constant at temperature Ts.

For conversion of gaseous species, we can write dF 5

-Z = ±NcA£ аіЛ(Ts, Lr) (5.88)

k=l

where ajk is the stoichiometric coefficient for the jth gas in the kth reaction.

The total molar flow rate of the jth gas is found by adding the contribution of each of nine gas-solid and gas-gas reactions:

F = Fo + X ajA (5.89)

where Fgj0 is the initial flow rate of the gas.

Although virgin biomass contains little or no sulfur, some waste biomass fuels do. For these, limestone is fed into the fluidized-bed gasifier for in-bed sulfur removal. The height of the gasifier (freeboard and bed) should be adequate to allow the residence time needed for the desired sulfur capture.

The tar produced should be thermally cracked inside the gasifier as far as possible. Therefore, the depth of the gasifier should be such that the gas resi­dence time is adequate for the desired tar conversion/cracking.

The deeper the bed, the higher the pressure drop across it and the higher the pumping cost of air. Because bubble size increases with bed height, a deeper bed gives larger bubbles with reduced gas-solid mixing. Furthermore, if the bubble size becomes comparable to the smallest dimension of the bed cross­section, a highly undesirable slugging condition is reached. This imposes another limit on how deep the dense section of a fluidized bed can be.

Some biomass char, like that from wood, is fine and easily undergoes attri­tion in a fluidized bed. In such cases a deeper bed may not guarantee a longer residence time (Barea, 2009). Here, special attention must be paid to capturing the char and either combusting it in a separate chamber to provide heat required by the gasifier, or reinjecting it at an appropriate point in the bed where solids are descending.

A kinetic model (nth-order, shrinking particle, and shrinking core) may also be used to determine the residence time, the net solid holdup, and therefore the height of the dense bed.

Freeboard Height

Entrainment of unconverted fine char particles from the bubbling bed is a major source of carbon loss. The empty space above the bed, the freeboard, allows entrained particles to drop back into it. A bubbling, turbulent, or spouted fluid­ized bed must have such a freeboard section to help avoid excessive loss of bed materials through entrainment and to provide room for conversion of finer entrained char particles. The freeboard height must be sufficient to provide the required residence time for char conversion. It can be determined from experi­ence or through kinetic modeling.

A larger cross-sectional area and a taller freeboard increase the residence time of gas/char and reduce entrainment. From an entrainment standpoint, the freeboard height need not exceed the transport disengaging height (TDH) of a bed because no further reduction in entrainment is achieved beyond this.

In an SCWG or SCWO, where the temperature can go as high as 600 °C and the pressure can be in excess of 22.089 MPa, water becomes highly corrosive. SCWG and SCWO plants work with organic compounds, which react with oxygen in supercritical water oxidation to produce mostly CO2 and H2O, or hydrolyze in SCWG. Halogen, sulfur, and phosphorous in the feed are con­verted into mineral acids such as HCl, H2SO4, or H3PO4. High-temperature water containing these acids along with oxygen is extremely corrosive to stain­less steels and nickel-chromium alloys (Friedrich et al., 1999).

After oxidation of neutral or acidic feeds, the pH of SCWO solutions is low, making it as corrosive as hydrochloric acid (Boukis et al., 2001). Chlorine is especially corrosive in SCW. Interestingly, a supercritical steam boiler, which is one of the most common uses of supercritical water, is relatively free from corrosion because the water used in the boiler is well treated and contains no corrosive species such as salts and oxygen or only very low concentrations.

The following sections briefly describe the mechanism and the prevention of corrosion in biomass SCWG plants. More details are available in reviews presented by Kritz (2004) and Marrone and Hong (2008).