Because gasification and pyrolysis reactions are endothermic, heat from some external source is required for operation of the reactor. In thermal gasification systems, the reaction temperature is very high (800-1000 °C), so a large amount of energy is required for production of fuel gases from biomass or other feed­stock. This heat is generally provided by allowing part of the hydrocarbon or carbon in the feed to combust in the gasifier, but then a part of the energy in the feedstock is lost.

A SCW gasifier operates at a much lower (450-650 °C) temperature and thus requires a much lower but finite amount of heat. Thermodynamically, the heat recovered from the gasification product is inadequate to raise the feed to the gasification temperature (450-600 °C) and provide the required reaction heat. This shortfall is made up either by an external source or by combustion of part of the product gas in a heater.

Both options are expensive. For example, a study of an SCWG design for gasification of 120 t/day (5000 kg/h) of sewage sludge with 80% water showed that 122 kg/h of natural gas is required to provide the gasification heat. This, along with an electricity consumption of 541 kW, constitutes 23% of the total revenue requirement for the plant (Gasafi et al., 2008). A better alternative would be controlled combustion of the unconverted char upstream of the gas­ifier, which would make SCWG energy self-sufficient.

Although SCWG is known for its low char and tar production, in practice we expect some char formation. Furthermore, as shown previously in Figure 7.7, gasification efficiency is low at lower temperatures. A low gasification temperature is thermodynamically more efficient, but raises the char yield. If this char can be combusted in SCW, it can provide the extra heat needed for preheating the feed, thereby improving the efficiency of the overall system.

Combustion of char offers an additional benefit for an SCWG that some­times uses solid catalysts, which are deactivated after being coated with uncon­verted char in the gasifier. A combustor can burn the deposited carbon and regenerate the catalyst. The generated heat is carried to the gasifier by both solid catalysts and the gasifying medium (SCW and CO2).

Recycling of solid catalysts is an issue for plug-flow reactors. Special devices such as fluidized beds may be used for these, as shown in Figure 7.12. Here, the catalysts or their supports are granular solids, which are separated from the product fluid leaving the reactor in a hydrocyclone operating in an SCW state. The separated solids drop into a bubbling fluidized-bed combustor,

where oxygen or air is injected to facilitate burning of the deposited carbon. The bed is fluidized by pressurized water already heated above its critical tem­perature in a heat-recovery heat exchanger.

Under supercritical conditions, oxidation or combustion reactions occur in a homogeneous phase where carbon is converted to carbon dioxide.

C + O2 = CO2 — 393.8 kJ/mol (7.16)

Because these reactions are exothermic, the process can become thermally self­sustaining with the appropriate concentration of oxygen. Heated water from the combustor carries the regenerated catalysts to the gasification reactor, into which the biomass is fed directly.

Under supercritical conditions, water acts as a nonpolar solvent. As a result, the supercritical water fully dissolves oxygen gas. The mass transfer barrier that is between dissolved oxygen and solid char may be lower than that between gas and char. This, along with its high-density feature, may allow the SCW to conduct the combustion reaction quickly and efficiently. Another advantage of low-temperature combustion is that it avoids formation of toxic by-products.

Bio-oil (or biofuel) is any liquid fuel derived from a recently living organism, such as plants and their residues or animal extracts. In view of its importance, a detailed discussion of bio-oil is presented next.

9.3.1 What Is Bio-Oil?

Bio-oil is the liquid fraction of the pyrolysis product of biomass. For example, a fast pyrolyzer typically produces 75% bio-oil, 12% char, and 13% gas. Bio-oil is a highly oxygenated, free-flowing, dark-brown (nearly black) organic liquid (Figure 9.1) that contains a large amount of water (~25%) that is partly the

original moisture in the biomass and partly the reaction product. The composi­tion of bio-oil depends on the biomass it is made from as well as on the process used.

Table 9.2 presents the composition of a typical bio-oil. It shows that water, lignin fragments, carboxylic acids, and carbohydrates constitute its major com­ponents. When it comes from the liquid yield of pyrolysis, bio-oil is called pyrolysis oil. Several other terms are often used to describe bio-oil or are asso­ciated with it, including:

• Tar or pyroligneous tar

• Bio-oil

• Biocrude

• Wood liquid or liquid wood

• Liquid smoke

• Biofuel oil

• Wood distillates

• Pyrolysis oil

• Pyroligneous acids

Note that there is an important difference between pyrolysis oil and biocrude. The former is obtained via pyrolysis; the latter can be obtained via other methods such as supercritical gasification.

Bio-oil may be seen as a two-phase microemulsion. In the continuous phase are the decomposition products of hollocellulose; in the discontinuous phase are the pyrolytic lignin macromolecules. Hollocellulose is the fibrous residue that remains after the extractives, lignin, and ash-forming elements have been removed from the biomass. The same as crude petroleum oil, which is extracted from the ground, pyrolysis liquid and biocrude contain tar as their heaviest component.

Bio-oil is a class-3 substance falling under the flammable liquid desig­nation in the UN regulations for transport of dangerous goods (Peacocke and Bridgwater et al., 2001, p. 1485).

5.1 INTRODUCTION

The design and operation of a gasifier require an understanding of the gasifica­tion process and how its design, feedstock, and operating parameters influence the performance of the plant. A good comprehension of the basic reactions is fundamental to the planning, design, operation, troubleshooting, and process improvement of a gasification plant, as is learning the alphabet to read a book. This chapter introduces the basics of the gasification process through a discus­sion of the reactions involved and the kinetics of the reactions with specific reference to biomass. It also explains how this knowledge can be used to develop a mathematical model of the gasification process.

5.2 GASIFICATION REACTIONS AND STEPS

Gasification is the conversion of solid or liquid feedstock into useful and con­venient gaseous fuel or chemical feedstock that can be burned to release energy or used for production of value-added chemicals.

Gasification and combustion are two closely related thermochemical pro­cesses, but there is an important difference between them. Gasification packs energy into chemical bonds in the product gas; combustion breaks those bonds to release the energy. The gasification process adds hydrogen to and strips carbon away from the feedstock to produce gases with a higher hydrogen-to — carbon (H/C) ratio, while combustion oxidizes the hydrogen and carbon into water and carbon dioxide, respectively.

A typical biomass gasification process may include the following steps:

• Drying

• Thermal decomposition or pyrolysis

• Partial combustion of some gases, vapors, and char

• Gasification of decomposition products

Pyrolysis is a thermal decomposition process that partially removes carbon from the feed but does not add hydrogen. Gasification, on the other hand,

Biomass Gasification and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00005-2

Copyright © 2010 Prabir Basu. Published by Elsevier Inc. All rights reserved. 117

requires a gasifying medium like steam, air, or oxygen to rearrange the molecu­lar structure of the feedstock in order to convert the solid feedstock into gases or liquids; it can also add hydrogen to the product. The use of a medium is essential for the gasification process.

Because an open-top, or a throatless, gasifier is simple in construction, it is used to describe the gasification process in the downdraft gasifier (Figure 6.5). The throatless process can be divided into four zones (Reed and Das, 1988, p. 39). The first, or uppermost, zone receives raw fuel from the top that is dried in air drawn through the first zone. The second zone receives heat from the third zone principally by thermal conduction.

During its journey through the first zone, the biomass heats up (zone I in Figure 6.5). Above 350 °C, it undergoes pyrolysis, breaking down into charcoal, noncondensable gases (CO, H2, CH4, CO2, and H2O), and tar vapors (condens­able gases). The pyrolysis product in zone II receives only a limited supply of air from below and burns in a fuel-rich flame. This is called flaming pyrolysis. Most of the tar and char produced burn in zone III, where they generate heat for pyrolysis and subsequent endothermic gasification reactions (Reed and Das, 1988, p. 28).

Zone III contains ash and pyrolyzed char produced in zone II. While passing over the char, hot gases containing CO2 and H2O undergo steam gasification and Boudouard reactions, producing CO and H2. The temperature of the down­flowing gas reduces modestly, owing to the endothermic gasification reactions, but it is still above 700 °C.

The bottommost layer (zone IV) consists of hot ash and/or unreacted char­coal, which crack any unconverted tar in this layer. Figure 6.5 shows the reac­tions and temperature distribution along the gasifier height. In one version of the throatless downdraft gasifier, the open-core type, the air enters from the top along with the feed. This type is free from some of the problems of other downdraft gasifiers.

Throated Gasifier

The cross-sectional area of a throated (also called constricted) gasifier is reduced at the throat and then expanded, as shown in Figure 6.4. The purpose is for the oxidation (combustion) zone to be at the narrowest part of the throat and to force all of the pyrolysis gas to pass through this narrow passage. Air is injected through nozzles just above the constriction. The height of the injection is about one-third of the way up from the bottom (Reed and Das, 1988, p. 33).

The movement of the entire mass of pyrolysis product through this hot and narrow zone results in a uniform temperature distribution over the cross-section and allows most of the tar to crack there. In the 1920s, a French inventor, Georges Imbert, developed the original design, which is popularly known as an Imbert gasifier (Figure 6.6).

The fuel, fed at the top, descends along a cylindrical section that serves as storage. The air pyrolyzes the biomass and burns the pyrolysis product or some charcoal. The hot char and the pyrolysis product pass through the throat, where most of the tar is cracked and the char is gasified. Figure 6.6 showed a flat-type throat construction, but it can be a V-type like in Figure 6.4.

Throated downdraft gasifiers are not suitable for scale-up to larger sizes because they do not allow for uniform distribution of flow and temperature in the constricted area. Beyond 1 MWth capacity, an annular constriction can be employed, but this has not been the practice to date.

Water above its critical temperature (374.29 °C) and pressure (22.089 MPa) is called supercritical (Figure 7.1). Water or steam below this pressure and tem­perature is called subcritical. The term water in a conventional sense may not be applicable to SCW except for its chemical formula, H2O, because above the critical temperature SCW is neither water nor steam. It has a waterlike density but a steam like diffusivity. Table 7.1 compares the properties of subcritical water and steam with those of SCW, indicating that SCW’s properties are intermediate between the liquid and gaseous states of water in subcritical pressure; descriptions of each follow the table.

Figure 7.1 shows that the higher the temperature, the higher the pressure required for water to be in its liquid phase. Above a critical point the line sepa­rating the two phases disappears, suggesting that the division between the liquid and vapor phases disappears. Temperature and pressure at this point are known as critical temperature, and critical pressure, above which water attains super­critical state and hence is called supercritical (SCW).

Critical point

Subcritical water (T < Tsat; P < Pc). When the pressure is below its critical value, Pc, and the temperature is below its critical value, Tc, the fluid is called subcritical. If the temperature is below its saturation value, the fluid is known as subcritical water, as shown in the lower left block of Figure 7.1.

Subcritical steam (T > Tsat; P <Pc. Note: T may be above Tc). When water (below critical pressure) is heated, it experiences a drop in density and an increase in enthalpy; this change is very sharp when the temperature of the water just exceeds it saturation value, Tat. Above the saturation temperature, but below the critical value, the fluid (H2O) is called subcritical steam. This regime is shown below the saturation line in Figure 7.1.

Supercritical water (T > Tc; P > Pc). When heated above its critical pres­sure, Pc, water experiences a continuous transition from a liquidlike state to a vapor like state. The vaporlike, supercritical, state is shown in the upper right block in Figure 7.1. Unlike in the subcritical stage, no heat of vaporization is needed for the transition from liquidlike to vaporlike. Above the critical pressure, there is no saturation temperature separating the liquid and vapor states. However, there is a temperature, called pseudo-critical temperature, that corresponds to each pressure (>Pc) above which the transition from liquidlike to vaporlike takes place. The pseudo­critical temperature is characterized by a sharp rise in the specific heat of the fluid.

The pseudo-critical temperature depends on the pressure of the water. It can be estimated within 1% accuracy by the following empirical equation (Malhotra, 2006):

T * = (P*)F

F = 0.1248 + 0.01424P* — 0.0026 (P*)2 (7.2)

T P

T* = sc • p* =

T ’ p

c c

where Tsat is the saturation temperature at pressure P; Psat is the saturation pressure at temperature T; Pc is the critical pressure of water, 22.089 MPa; Tc is the critical temperature of water, 374.29 °C; and Tsc is the pseudo­critical temperature at pressure P (P > Pc).

Biomass is brought to the plant typically by truck or, sometimes, by rail car. For large biomass plants, unloading from the truck or rail car is a major task. Manual unloading can be strenuous and uneconomical except in very small plants. This is why large plants use truck hoisters, wagon tipplers, or bottom — discharge wagons. Figure 8.3 shows a typical system where a truck hoister unloads the biomass. The truck drives onto the hoist platform and is clamped down. The hoister tilts to a sharp angle, allowing the entire load to drop into the receiving chute under gravity. This method is fast and economical.

A bottom-discharge wagon may be used for rail cars. The wagon drops its load into a large bin located below the rail. An alternative is a standard open — top wagon and a tippler to rotate it 180 degrees to empty its contents into a bin underneath. Such units are procured from the suppliers of various bulk material­handling equipment. Their capacities depend on a number of factors, including plant throughput and frequency of truck and/or rail arrival.

Ethanol (C2H6O) is presently produced primarily from glucose from grain (com, maize, etc.), sugar (sugarcane), and energy crops using the fermentation — based biochemical process. A typical process, as shown in Figure 9.3, com­prises the following major steps:

Milling: Corn is ground to a fine powder called cornmeal.

Liquefying: A large amount of water is added to make the cornmeal into a solution.

Hydrolysis: Enzymes are added to the solution to break large carbohydrate molecules into shorter glucose molecules.

Fermentation: The glucose mixture is taken to the fermentation batch reactor, where yeast is added. The yeast converts the glucose into ethanol and carbon dioxide as represented by the equation

C6Hi2O6 (glucose) + yeast Fermentalion > 2C2H6O (ethanol) + 2CO2 (9.8)

Distillation: The product of fermentation contains a large amount of water and some solids, so the water is removed through distillation. Distillation purifies ethanol to about 95 to 96% purity. The solids are pumped out and discarded as a protein-rich stock, which may be used only for animal feed. Dehydration: The ethanol produced is good enough for car engines in countries like Brazil, but further purification is needed if it has to be blended with mineral gasoline for ordinary cars. In this stage, a molecular sieve is used for dehydration. Small beads with pores large enough for water but not for ethanol absorb the water.

A large amount of energy is consumed in distillation and other steps in this process. By one estimate, for the production of 1 liter of purified ethanol, about

12,350 kJ of energy is needed for processing, especially for dehydration. An additional 7440 kJ/L of energy consumed in harvesting the corn is required (Wang and Pantini, 2000). Although a liter of ethanol releases 21,200 kJ of energy when burnt, the farming and processing of the corn consumes about 19,790 kJ of energy. The net energy production is therefore a meager 1410 kJ (21,200 — 19,790) per liter of ethanol.

The shortcoming of this process is that it uses a valuable food source— indeed, a staple food in many countries. The search for an alternative is there­fore ongoing. Though not fully commercial yet, some methods are available using either the biochemical or the thermochemical process.

Optimal conversion of chemical energy of the biomass or other solid fuel into the desired gas depends on proper configuration, sizing, and choice of gasifier operating conditions. In commercial plants, optimum operating conditions are often derived through trials on the unit or by experiments on pilot plants. Even though expensive, experiments can give more reliable design data than can be obtained through modeling or simulation. There is, however, one major

limitation with experimental data. If one of the variables of the original process changes, the optimum operating condition chosen from the specific experimen­tal condition is no longer valid. Furthermore, an experimentally found optimum parameter can be size-specific; that is, the optimum operating condition for one size of gasifier is not necessarily valid for any other size. The right choice between experiment and modeling, then, is necessary for a reliable design.

Basic mass and energy balance is common to all types of gasifiers. It involves calculations for product gas flow and fuel feed rate.

Product Gas Flow Rate

The gasifier’s required power output, Q (MWth), is an important input param­eter specified by the client. Based on this, the designer makes a preliminary estimation of the amount of fuel to be fed into the gasifier and the amount of gasifying medium. The volume flow rate of the product gas, Vg (Nm3/s), from its desired lower heating value, LHVg (MJ/Nm3), is found by

The net heating value or lower heating value (LHV) can be calculated from its composition. The composition may be predicted by the equilibrium

calculations, described later, or by sophisticated kinetic modeling of the gas­ifier, as discussed in Chapter 5. In the absence of these, a reasonable guess can be made. either from published data on similar fuels in similar gasification conditions or from the designer’s experience.

For example, for air-blown fluidized-bed biomass gasifiers, the LHV is in the range 3.5 to 6 MJ/Nm3 (Enden and Lora, 2004). For oxygen gasification, it is in the range 10 to 15 MJ/Nm3 (Ciferno and Marano, 2002). So, for an air — blown gasifier, we start with a value of 5 MJ/Nm3 as a reasonable guess (Quaak et al., 1999).

An effective degradation of biomass and the gasification of intermediate prod­ucts of thermal degradation into lower-molecular-weight gases like hydrogen require the SCW reactor to operate in the high-temperature range (>600 °C). The higher the temperature, the better the conversion, especially for production of hydrogen, but the lower the SCW’s energy efficiency. A lower gasification temperature is therefore desirable for higher thermodynamic efficiency of the process.

Catalysts help gasify the biomass at lower temperatures, thereby retaining, at the same time, high conversion and high thermal efficiency. Additionally, some catalysts also help gasification of difficult items like the lignin in biomass. Watanabe et al. (2003) noted that the hydrogen yield from lignin at 400 °C and 30 MPa is doubled when a metal oxide (ZrO2) catalyst is used in the SCW. The yield increases four times with a base catalyst (NaOH) compared to gasification without a catalyst. The three principal types of catalyst used so far for SCW gasification are: (1) alkali, (2) metal, and (3) carbon-based.

An important positive effect of catalysts in SCWG is the reduction in required gasification temperature for a given yield. Minowa et al. (1998) noted a significant reduction in unconverted char while gasifying cellulose with an Na2CO3 catalyst at 380 °C. Base catalysts (e. g., NaOH, KOH) offer better performance, but they are difficult to recover from the effluent. Some alkalis (e. g., NaOH, KOH, Na2CO3, K2CO3, and Ca(OH)2) are also used. They, too, are difficult to recover.

The special advantage of metal oxide catalysts is that they can be recovered, regenerated, and reused. Commercially available nickel-based catalysts are effective in SCW biomass gasification. Among them, Ni/MgO (nickel sup­ported on an MgO catalyst) shows high catalytic activity, especially for biomass (Minowa et al., 1998).

Metal catalysts have a severe corrosion effect at the temperatures needed to secure high yields of hydrogen. To overcome this problem, Antal et al. (2000) used carbon (e. g., coal-activated and coconut shell-activated carbon and maca — damia shell and spruce wood charcoal). The carbon catalysts resulted in high yields of gas without tar formation.

18 16

14

9>

12

о

10

T3

8

>.

6

CO

4 2 0

H2 —ch4 — A — CO — X — CO2

FIGURE 7.8 Effect of residence time on the gasification of 2% rice husk in supercritical water at 650 °C and 30 MPa in a batch reactor.