7.1 INTRODUCTION

In the mid-1970s Sanjay Amin, a graduate student working at the Massachu­setts Institute of Technology (MIT), was studying the decomposition of organic compounds in hot water (steam reforming):

While conducting an experiment in subcritical water, he observed that in addi­tion to producing hydrogen and carbon dioxide, the reaction was producing much char and tars. Herguido et al. (1992) also made similar observations in the steam gasification of biomass at atmospheric pressure.

Sanjay interestingly noted that when he raised the water above its “critical state,” the tar that formed in the subcritical state disappeared entirely (Amin et al., 1975). This important finding kick-started research and development on supercritical water oxidation (SCWO) for disposal of organic waste materials (Tester et al., 1993), which has now become a commercial option for disposal of highly contaminated organic wastes (Shaw and Dahmen, 2000).

Biomass in general contains substantially more moisture than do fossil fuels like coal. Some aquatic species, such as water hyacinth, or waste products, such as raw sewage, can have water contents exceeding 90%. Thermal gasifica­tion, where air, oxygen, or subcritical steam is the gasification medium, is very effective for dry biomass, but it becomes very inefficient for a high — moisture biomass because the moisture must be substantially driven away before thermal gasification can begin; in addition, a large amount of the extra energy (~2260 kJ/kg moisture) is consumed in its evaporation. For example, Yoshida et al. (2003) saw the efficiency of their thermal gasification system reduce from 61 to 27% while the water content of the feed increased from 5 to 75%. So, for gasification of very wet biomass, some other means such as anaerobic digestion (see Section 2.2) and hydrothermal gasification in high — pressure hot water are preferable because the water in these processes is not a

liability as it is in thermal gasification. Instead it serves as a reaction medium and a reactant.

The efficiencies of these processes do not decrease with moisture content. For anaerobic digestion and supercritical gasification, Yoshida et al. (2003) found the gasification efficiency to remain nearly unchanged, at 31% and 51%, respectively, even when the moisture in the biomass increased from 5 to 75%.

A major limitation of anaerobic digestion is that it is very slow, with a rela­tively low efficiency and, most important, it produces methane only, no hydro­gen. If hydrogen is the desired product, as is often the case, an additional step of steam reforming the methane (CH4 + H2O = CO + 3H2) must be added to the anaerobic digestion process.

Hydrothermal gasification involves gasification in an aqueous medium at very a high temperature and pressure exceeding or close to its critical value. While subcritical water has been used effectively for hydrothermal reaction, supercritical water has attracted more attention owing to its unique features. Supercritical water offers rapid hydrolysis of biomass, high solubility of inter­mediate reaction products, including gases, and a high ion product near (but below) the critical point that helps ionic reaction. These features make super­critical water an excellent reaction medium for gasification, oxidation, and synthesis.

This chapter deals primarily with hydrothermal gasification of biomass in supercritical water. It explains the properties of supercritical water and the biomass conversion process in it. The effects of different parameters on SCW gasification and design considerations for the SCW gasification plants are also presented.

A typical biomass gasification plant comprises a large number of process units, of which the biomass-handling unit is the most important. Unlike coal-fired boiler plants, an ash-handling system is not a major component of a biomass gasification plant because biomass contains a relatively small amount of ash. Normally it does not produce a large volume of spent catalysts or sorbents. Transportation, feed preparation, and feeding are more important for biomass than they are for coal — or oil-gas-fired units.

The biomass-handling system can be broadly classified into the following components:

• Receiving

• Storage and screening

• Feed preparation

• Conveying

• Feeding

The design of the handling system is very similar to that of a biomass-fired steam plant. Figure 8.2 illustrates the layout of a typical plant showing receiv­ing, screening, storage, and conveying.

Major considerations for the design of feeding and handling systems are transportation, sealing, and injection. The feed should be transported smoothly

from the temporary storage to the feed system, which must be sealed against the gasifier’s pressure and temperature. The fuel is then injected into the gas­ifier. Metering or measurement of the fuel feed rate is an important aspect of the feed system, as it controls the entire process.

The following subsections discuss the individual components of a solids — handling system for biomass. They assume the biomass to be solid, although some biomass, such as sewage sludge, is in slurry or semisolid form.

Ethanol is the most extensively used biofuel in the transportation industry. Ethanol can be mixed with gasoline (petroleum) or used alone for operating spark-ignition engines, just as biodiesel can be mixed with petrodiesel for operating compression-ignition engines. In most cases engine modifications may not be needed for substitution of mineral oil with bio-oil-derived fuels. Ethanol is produced mainly from food crops, but, less commonly, it can also be produced from nonfood ligno-cellulosic biomass.

For the gasification reaction to take place within the char’s pores, the reacting gas must enter the pores. If the availability of the gas is so limited that it is entirely consumed by the reaction on the outer surface of the char, gasification is restricted to the external surface area. This can happen because of the limita­tion of the mass transfer of gas to the char surface. We can illustrate using the example of char gasification in CO2:

C + CO2 ^ 2CO (5.58)

Here, the CO2 gas has to diffuse to the char surface to react with the active carbon sites. The diffusion, however, takes place at a finite rate. If the kinetic rate of this reaction is much faster than the diffusion rate of CO2 to the char surface, all of the CO2 gas molecules transported are consumed on the external surface of the char, leaving none to enter the pores and react on their surfaces. As the overall reaction is controlled by diffusion, it is called the diffusion — or mass-transfer-controlled regime of reaction.

On the other hand, if the kinetic rate of reaction is slow compared to the transport rate of CO2 molecules, then the CO2 will diffuse into the pores and react on their walls. The reaction in this situation is “kinetically controlled.”

Diffusion rate >> kinetic rate [Kinetic control reaction] (, ,„)

Diffusion rate << kinetic rate [ Diffusion control reaction ]

Between the two extremes lie intermediate regimes. The relative rates of chemical reaction and diffusion determine the gas concentration profile in the vicinity of the char particle; how the reaction progresses; and how char size, pore distribution, reaction temperature, char gas relative velocity, and so forth,

influence overall char conversion. Figure 5.9 shows how the concentration profile of CO2 around the particle changes with temperature. With a rise in the surface temperature, the kinetic rate increases and therefore the overall reaction moves from the kinetic to the diffusion-controlled regime, resulting in less reaction within the pores.

The overall gasification rate of char particles, Q, when both mass transfer and kinetic rates are important, may be written as

p

Q = — ^^kg Carbon/m2.s (5.60)

—— 1—-

fym Rc

where Pg is the concentration in partial pressure (bar) of the gasifying agent outside the char particle, hm is the mass transfer rate (kg carbon/(m2bar. s)) to the surface, and Rc is the kinetic rate of reaction: kg carbon/(m2bar. s).

For any design, specification of the plant is very important. The input includes the specification of the fuel, gasification medium, and product gas. A typical fuel specification will include proximate and ultimate analysis, operating tem­peratures, and ash properties. The specification of the gasifying medium is based on the selection of steam, oxygen, and/or air and their proportions. These parameters could influence the design of the gasifier, as follows:

• The desired heating value of the product gas dictates the choice of gasifica­tion medium. Table 6.4 gives typical ranges of heating value for different mediums.

• Hydrogen can be maximized with steam, but if it is not a priority, oxygen or air is a better option, as it reduces the energy used in generating steam and the energy lost through unutilized steam.

• If nitrogen in the product gas is not acceptable, air cannot be chosen.

• Capital cost is lower for air, followed by steam. A much larger investment is needed for an oxygen plant, which also consumes a large amount of auxiliary power.

• Equivalence ratio

For the product gas, the specification includes:

• Desired gas composition

• Desired heating value

• Desired production rate (Nm3/s or MWth produced)

• Yield of the product gas per unit fuel consumed

• Required power output of the gasifier, Q

The design outputs of process design include geometry and operating and per­formance parameters.

Basic size includes reactor configuration, cross-section area, and height (hardware design). Important operating parameters are: (1) reactor temperature; (2) preheat temperature of the steam, air, or oxygen; and (3) amount (i. e., steam/ biomass ratio) and relative proportion of the gasifying medium (i. e., steam/ oxygen ratio). Performance parameters of a gasifier include carbon conversion and cold-gas efficiency.

A typical process design starts with a mass balance followed by an energy balance. The following subsections describe the calculation procedures for these.

Temperature has an important effect on the conversion, the product distribution, and the energy efficiency of an SCW gasifier, which typically operates at a maximum temperature of nearly 600 °C. The overall carbon conversion increases with temperature; at higher temperatures hydrogen yield is higher while methane yield is lower. Figure 7.7 shows the temperature dependence of gasification efficiency and product distribution in a reactor operated at 28 MPa (30-s residence, 0.6-M glucose) (Lee et al., 2002). We see that the hydrogen yield increases exponentially above 600 °C, while the CO yield, which rises gently with temperature, begins to drop above 600 °C owing to the start of the shift reaction (Eq. 5.52).

Gasification efficiency is measured in terms of hydrogen or carbon in the gaseous phase as a fraction of that in the original biomass. Carbon conversion efficiency increases continually with temperature, reaching close to 100% above 700 °C. Hydrogen conversion efficiency (the fraction of hydrogen in glucose converted into gas) also increases with temperature. It appears strange that at 740 °C, the hydrogen conversion efficiency exceeds 100%, reaching 158%. This clearly demonstrates that the extra hydrogen comes from the water, confirming that water is indeed a reactant in the SCWG process as well as a reaction medium.

Hydrothermal gasification of biomass has been divided into three broad temperature categories: high, medium, and low with their desired products (Peterson et al., 2008). Table 7.2 shows that the first group targets production of hydrogen at a relatively high temperature (>500 °C); the second targets production of methane at just above the critical temperature (~374.29 °C) but below 500 °C; and the third gasifies at subcritical temperature, using only simple organic compounds as its feedstock. The last two groups, because of their low-temperature operation, need catalysts for reactions.

Temperature (°C)

Hydrogen — — ■ Methane

— Carbon dioxide ——— Carbon monoxide

(a)

450 500 550 600 650 700 750 800

Temperature (°C)

Hydrogen —— Oxygen —— Carbon

(b)

FIGURE 7.7 Effect of temperature on gas yield (a), and effect of temperature on gasification efficiency (b). (Source: Adapted from Lee et al, 2002.)

For a wide dispersion of fuel over the bed, spreader wheels are used (Figure 8.19). The spreader throws the fuel received from a screw or other type of metering feeder over a large area of the bed surface. Typically it comprises a pair of blades rotating at high speed; slightly opposite orientation of the blades helps throw the fuel over a larger lateral area. This is not a metering device. A major problem with the spreader is that it encourages segregation of particles in the bed.

Pneumatic Injection Feeder

A pneumatic injection feeder is not a metering device; rather, it helps feed already metered biomass into the reactor. This works well for gravity feeding, and it is especially suitable for fine solids. Pneumatic injection is preferred for less reactive fuels, which must reside in a gasifier bed longer for complete conversion. It transports dry fuel particles in an air stream at a velocity higher than their settling velocity. The fuel is typically fed from underneath a bubbling fluidized bed. The maximum velocity of air in the fuel transport lines may not
exceed 11—15 m/s to avoid line erosion. The air for transporting constitutes part of the air for gasification.

Splitting of the fuel-air mixture into multiple fuel lines is a major problem with pneumatic injection. A specially designed feed splitter, like the one dis­cussed in Basu (2006, p. 355), can be used.

In a underbed pneumatic system, air jets that carry solid particles with high momentum to enter into the fluidized bed, forming a plume that could punch through the bed. To avoid this, a cap sits at the top of the exit of each feeder nozzle. This cap reduces the momentum of the jets breaking into the freeboard of a bubbling fluidized bed. A highly erosive zone may be formed near each outlet nozzle of the feeders, which might corrode the tubes nearby.

Another innovative, but one that is less common, feed system uses pulsed air. Controlled-air pulses push the biomass into the gasifier, avoiding pyrolysis of feed in the gasifier feed line. A very small amount of air minimizes dilution of the product gas with nitrogen. The University of Western Ontario in Canada applied this design with success in a commercial mobile pyrolyzer.

1.1 INTRODUCTION

Tar is a major nuisance in both gasification and pyrolysis. It is a thick, black, highly viscous liquid that condenses in the low-temperature zones of a gasifier, clogging the gas passage and leading to system disruptions. Tar is highly unde­sirable, as it can create the following problems:

• Condensation and subsequent plugging of downstream equipment

• Formation of tar aerosols

• Polymerization into more complex structures

Nevertheless, tar is an unavoidable by-product of the thermal conversion process. This chapter discusses what tar is, how it is formed, and how to influ­ence its formation such that plants and equipment can live with this “necessary evil” while minimizing its detrimental effects.

The dense zone (assumed to be the bubbling bed) is modeled according to the modified two-phase theory. Bubble size is calculated as a function of bed height (Darton and LaNauze, 1977), and it is assumed that all bubbles at any cross­section are of uniform size:

or

where A/Nor is the number of orifices per unit of cross-section area of the bed.

The interphase mass transfer between bubbles and emulsion, essential for the gas-solid reactions, is modeled semi-empirically using the specific bubble surface as the exchange area, the concentration gradient, and the mass-transfer coefficient. The mass-transfer coefficient, KBE, based on the bubble-emulsion surface area (Sit and Grace, 1978), is

Umf 4£mfDr Ub

“ = + ПВ

where Umf and emf are, respectively, fluidization velocity and voidage at a minimum fluidizing condition, Dr is the bed diameter, and UB is the rise velocity of a bubble of size dB.

The axial mean voidage in the freeboard is calculated using an exponential decay function.

Since gasification involves only partial oxidation of the fuel, the heat released inside a gasifier is only a fraction of the fuel’s heating value, and part of it is absorbed by the gasifier’s endothermic reactions. Thus, it is undesirable to extract any further heat from the main gasifier column. For this reason, the height of a fluidized-bed gasifier is not determined by heat-transfer consider­ations as for fluidized-bed boilers. Instead, gas and solid residence times are major considerations.

The total height of the gasifier is made up of the height of the fluidized bed and that of the freeboard above it:

Total gasifier height = bubbling bed height (depth) + freeboard height (6.31)