Wet electrostatic precipitators (ESPs) are used in some gasification plants. The gas is passed through a strong electric field with electrodes. High voltage charges the solid and liquid particles. As the flue gas passes through a chamber containing anode plates or rods with a potential of 30 to 75 kV, the particles in the flue gas pick up the charge and are collected downstream by positively charged cathode collector plates. Grounded plates or walls also attract the charged particles and are often used for design simplicity. Although collection efficiency does not decrease as particles build up on the plates, periodic mechanical wrapping is required to clean the plates to prevent the impediment of the gas flow or the short-circuiting of the electrodes through the built-up ash.

The collected solid particles are cleaned by mechanical means, but a liquid like tar needs cleaning by a thin film of water. Wet electrostatic precipitators have very high (> 90%) collection efficiency over the entire range of particle size down to about 0.5 micron, and they have very low pressure drop (few inches water gauge). Sparking due to high voltage is a concern with an ESP, especially when it is used to clean highly combustible syngas. Thus, the savings from lower fan power due to low pressure drop is offset by a higher safety cost. Additionally, the capital cost for ESP is three to four times higher than that for a wet scrubber.

Lurgi, a process development company, developed a pressurized dry-ash updraft gasifier. It is called dry ash because the ash produced is not molten. One that produces molten ash is called a slagging gasifier.

Though the peak temperature (in the combustion zone) is 1200 °C, the maximum gasification temperature is 700 to 900 °C. The reactor pressure is in the neighborhood of 3 MPa, and the residence time of coal in the gasifier is between 30 and 60 minutes (Ebasco, 1981). The gasification medium is a mixture of steam and oxygen, steam and air, or steam and oxygen-enriched air. It uses a relatively high steam/fuel carbon ratio (~1.5).

The coal is first screened to between 3 and 40 mm (Probstein and Hicks, 2006, p. 162) and then fed into a lock hopper. The gasifying agent moves upward in the gasifier while the solids descend. The reactor is a double-walled pressure vessel. Between the two walls lies water that quickly boils into steam

under pressure, utilizing the heat loss from the reactor. As the coal travels down the reactor, it undergoes drying, devolatilization, gasification, and combustion. Typical residence time in the gasifier is about an hour (Probstein and Hicks, 2006, p. 162). In a dry-ash gasifier, the temperature is lower than the melting point of ash, so the solid residue dries and is removed from the reactor by a rotating grate.

The dry-ash technology has been used at SASOL in South Africa, the world’s biggest gasification complex. SASOL produces 55 million Nm3/day of syngas, which is used to produce 170,000 bbl/day of Fischer-Tropsch liquid fuel.

Slagging Gasifier

The British Gas/Lurgi consortium developed a moving-bed gasifier that works on the same principle as the dry-ash gasifier, except a much higher tempera­ture (1500-1800 °C) is used in the combustion zone to melt the ash (hence its name, slagging gasifier). Such a high temperature requires a lower steam-to — fuel ratio (~0.58) than that used in dry-ash units (Probstein and Hicks, 2006, p. 169).

Coal crushed to 5 to 80 mm is fed into the gasifier through a lock hopper system (Minchener, 2005). The gasifier’s tolerance for coal fines is limited, so briquetting is used in places where the coal carries too many of them. Gasifica­tion agents, oxygen and steam, are introduced into the pressurized (~3 MPa) gasifier vessel through sidewall-mounted tuyers (lances) at the elevation where combustion and slag formation occur.

The coal introduced at the top gradually descends through several process zones. The feed is first dried in the top zone and then devolatilized. The descending coal is transformed into char and then passes into the gasification (reaction) zone. Below this zone, any remaining carbon is oxidized, and the ash content is liquefied, forming slag. Slag is withdrawn from the slag pool through an opening in the hearth plate at the bottom of the gasifier vessel. The product gas leaves from the top, typically at 400 to 500 °C (Minchener, 2005).

A large number of operational issues confront a biomass gasifier. Universal to all gasifier types are problems related to biomass handling and feeding. Bridging of biomass over the exit of a hopper is common for plants that use low-shape-factor (flaky) biomass such as leaves and rice husk. This problem is discussed in more detail in Chapter 8.

Fixed-Bed Gasifier

Charcoal particles become porous and finer during their time in the gasification zone. Thus, in a downdraft gasifier, when fine charcoal drops into the ash pit, the product gas can easily carry the particles as dust. Escaping particles can be a source of carbon loss, and they often plug downstream equipment.

The movement of solids in any layer of a moving-bed gasifier should be equal to the feed rate of the fuel at the top. Even with that balance, if the fuel is dry, the pyrolysis zone may, in an updraft gasifier, travel upward faster, thus consuming the layer of fresh fuel above and leading to premature pyrolysis. The gas lost in this way may result in lower gasification efficiency.

On the other hand, if the fuel is moist, its pyrolysis may be delayed. This may move the pyrolysis zone downward. In the extreme case, the cooler pyroly­sis zone may sink sufficiently to extinguish the gasification and combustion reaction. Clearly, a proper balance of rates of fuel flow and air flow is required for stabilization of each of these zones in respective places.

8.1 INTRODUCTION

Liquids and gases are relatively easy to handle because they continuously deform under shear stress—they easily take the shape of any vessel they are kept in and flow easily under gravity if they are heavier than air. For these reasons, storage, handling, and feeding of gases or liquids do not generally pose a major problem. On the other hand, solids can support shear stress and do not flow freely. This problem is most evident when they are stored in conical bins and are withdrawn from the bottom. Because they do not deform under shear stress, solids can form a bridge over the cone and cease to flow.

Biomass is particularly notorious in this respect, because of its fibrous nature and nonspherical shape. The exceptionally poor flow characteristic of biomass poses a formidable challenge to both designers and operators of biomass plants. The cause of many shutdowns in these plants incidents can be traced to the failure of some parts of the biomass-handling system.

This chapter describes the design and operating issues involved in the flow of biomass through the system. It discusses options for the handling and feeding of biomass in a gasification plant.

Biodiesel from fat or oil produces a large amount (about 10%) of glycerol (HOCH2CH[OH]CH2OH) as a by-product. Large-scale commercial production of biodiesel can therefore bring a huge amount of glycerol into the market. For example, for every kg of biodiesel, 0.1 kg of glycerol is produced (86% FAME, 9% glycerol, 4% alcohol, and 1% fertilizer) (www. biodiesel. org). If produced in the required purity (>99%), glycerol may be sold for cosmetic and pharma­ceutical production, but that market is not large enough to absorb it all. There­fore, alternative commercial uses need to be explored. These include:

• Catalytic conversion of glycerol into biogas (C8-Ci6 range) (Hoang et al.,

2007)

• Liquid-phase or gas-phase reforming to produce hydrogen (Xu et al., 1996)

A large number of other chemicals may potentially come from glycerol. Zhou et al. (2008) reviewed several approaches for a range of chemicals and fuels. Through processes like oxidation, transesterification, esterification, hydrogenolysis, carboxylation, catalytic dehydration, pyrolysis, and gasifica­tion, many value-added chemicals can be produced from glycerol.

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

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

Gasification

Biomass

Combustion

Steam, air, or oxygen

Ash

(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.

For thermal gasification of the refractory components of biomass (those diffi­cult to gasify) such as lignin, the minimum temperature requirement is similar to that for coal (~900 °C) (Higman and van de Burgt, 2008, p. 147). Entrained- flow gasification of biomass is therefore rather limited and has not been seen on a commercial scale for the following reasons:

• The residence time in the reactor is very short. For the reactions to complete, the biomass particles must be finely ground. Being fibrous, biomass cannot be pulverized easily.

• Molten ash from biomass is highly aggressive because of its alkali com­pounds and can corrode the gasifier’s refractory or metal lining.

Given these shortcomings, entrained-flow gasifiers are not popular for biomass. However, there is at least one successful entrained-flow biomass gasifier, known as the Choren process.

Choren Process

The Choren entrained-flow biomass gasifier is comprised of three stages (Figure 6.18). The first stage receives biomass in a horizontal stirred-type low-temper­ature reactor for pregasification at 400 to 500 °C in a limited supply of air. This produces solid char and a tar-rich volatile product. The latter flows into the second chamber (stage 2), an entrained-flow combustor where oxygen and the product gas from the first stage are injected downward into the reactor. Com­bustion raises the temperature to 1300 to 1500 °C and completely cracks the tar. The hot combustion product flows into the third chamber (stage 3), where the char is gasified.

The solid char received from the first stage is pulverized and fed into the third stage of the Choren process. It is gasified in the hot gasification medium produced in the second stage. Endothermic gasification reactions reduce the

temperature to about 800 °C. Char and ash from the product gas are separated and recycled into the second-stage combustor. The ash melts at the high tem­perature in the combustor and is drained from the bottom. Now molten, the ash freezes, forming a layer on the membrane wall that protects the wall against the corrosive action of fresh molten biomass ash. The product gas is processed downstream for Fisher-Tropsch synthesis or other applications.

A typical SCWG plant includes the following key components:

• Feedstock pumping system

• Feed preheater

• Gasifier/reactor

• Heat-recovery (product-cooling) exchanger

• Gas-liquid separator

• Optional product-upgrading equipment

The feed preheating system is very elaborate and accounts for the majority (~60%) of the capital investment in an SCW gasification plant.

Figure 7.6 describes the SCWG process using the example of an SCWG plant for gasifying sewage sludge. Biomass is made into a slurry for feeding. It is then pumped to the required supercritical pressure. Alternatively, water may be pressurized separately and the biomass fed into it. In any case, the feedstock needs to be heated to the designed inlet temperature for the gasifier, which must be above the critical temperature and well above the designed gasification temperature because the enthalpy of the water provides the energy

required for the endothermic gasification reactions. This temperature is a criti­cal design parameter.

The sensible heat of the product of gasification may be partially recovered in a waste heat-recovery exchanger and used for partial preheating of the feed (Figure 7.6). For complete preheating, additional heat may be obtained from one of the following:

• Externally fired heater (Figure 7.6)

• Burning of a part of the fuel gas produced to supplement the external fuel

• Controlled burning of unconverted char in the reactor system (refer to Figure 7.12 later in chapter)

After gasification, the product is first cooled in the waste heat-recovery unit. Thereafter, it cools to room temperature in a separate heat exchanger by giving off heat to an external coolant.

The next step involves separation of the reaction products. The solubility of hydrogen and methane in water at low temperature but high pressure is considerably low, so they are separated from the water after cooling while the carbon dioxide, because of its high solubility in water, remains in the liquid phase. For complete separation of CO2, the gas may be scrubbed with additional water (refer to Figure 7.14 later in chapter). The gaseous hydrogen is separated from the methane in a pressure swing adsorber. The CO2-rich liquid is depressurized to the atmospheric pressure, separating the carbon dioxide from the water and unconverted salts.

The six main feeder types for biomass are: (1) gravity chute, (2) screw con­veyor, (3) pneumatic injection, (4) rotary spreader, (5) moving-hole feeder, and (6) belt feeder. These are broadly classified as traction, nontraction, and others as shown in Figure 8.16. In the traction type, there is linear motion of the surface carrying the fuel, as with a belt feeder or a moving-hole or drag-chain feeder. In the nontraction type, the motion is rotating and oscillatory screw feeders and rotary feeders belong to this group. Oscillatory feeders are of the vibratory or ram type. Other feeder types move the fuel by gravity or air pressure.

Gravity Chute

A gravity chute is a simple device in which fuel particles are dropped into the bed with the help of gravity. The pressure in the furnace needs to be at least slightly lower than the atmospheric pressure; otherwise, hot gas will blow back into the chute, creating operational hazards and possible choking of the feeder due to coking near its mouth.

FIGURE 8.16 Types of feeders used in biomass plants.

In spite of the excellent mixing capabilities of a fluidized bed, a fuel-rich zone is often created near the outlet of a chute feeder that is subjected to severe corrosion. Since the fuel is not well dispersed in gravity chute feeding, much of the volatile matter is released near the feeder outlet, which causes a reducing environment. To reduce this problem, the chute can be extended into the furnace. However, the extension needs insulation and some cooling air to avoid premature devolatilization of the feed passing through it. Additionally, a pres­sure surge might blow fine fuel particles back into the chute while reducing conditions might encourage corrosion. An air jet can help disperse the fine particles away from the fuel-rich zone.

A gravity feeder is not a metering device. It can neither control nor measure the feed rate of the fuel. For this, a separate metering device such as a screw feeder is required upstream of the chute.

In a typical torrefaction process the biomass is heated gently to the desired torrefaction temperature (6tor), held there for a specified reaction time, and then cooled down. The torrefaction temperature and the reaction time are two of the most important parameters in this process. The torrefaction temperature dtor generally reduces with reaction time theating.

The design norm for torrefaction is

200 °C < вІ0Г < 300 °C

fyor — 200 < і °c/s (3.12)

Theating

where 0tor is the torrefaction temperature in °C, and theating is the heating time above 200 °C. A typical reaction time is about 30 minutes. The properties of torrefied wood depend on (1) the type of wood, (2) the reaction temperature, and (3) the reaction time.

Torrefaction loses more oxygen and hydrogen than carbon. Hence, the H/C and O/C ratios decrease. However, it should not be confused with carboniza­tion, which takes place at a much higher temperature and produces charcoal with even lower H/C and O/C ratios.