Basu P. Biomass Gasification and Pyrolysis: Practical Design and Theory

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4 Tar Production and Destruction

Cracking

Cracking involves breaking large molecules into smaller ones. It converts tar

into permanent gases such as H

or CO. The energy content of the tar is thus

mostly recovered through the smaller molecules formed. Unlike in physical

cleaning, the tar need not be condensed for cracking. This process involves

heating the tar to a high temperature (~1200 °C) or exposing it to catalysts at

lower temperatures (~800 °C). There are two major types of cracking: thermal

and catalytic.

Thermal Cracking

Thermal cracking without a catalyst is possible at a high temperature (~1200

°C). The temperature requirement depends on the constituents of the tar. For

example, oxygenated tars may crack at around 900 °C (Stevens, 2001). Oxygen

or air may be added to allow partial combustion of the tar to raise its tempera-

ture, which is favorable for thermal cracking. Thermal decomposition of

biomass tars in electric arc plasma is another option. This is a relatively simple

process but it produces gas with a lower energy content.

Catalytic Cracking

Catalytic cracking is commercially used in many plants for the removal of tar

and other undesired elements from product gas. It generally involves passing

the dirty gas over catalysts. The main chemical reactions taking place in a cata-

lytic reactor are represented by Eq. (4.5) in the presence of steam (steam

reforming) and Eq. (4.6) in the presence of CO

(dry reforming). The main

reactions for tar conversion are endothermic, so a certain amount of combustion

reactions are allowed in the reactor by adding air.

Nonmetallic catalysts include less-expensive disposable catalysts: dolomite,

zeolite, calcite, and so forth. They can be used as bed materials in a ﬂuidized

bed through which tar-laden gas is passed at a temperature of 750 to 900 °C.

Attrition and deactivation of the catalyst are a problem (Lammars et

al., 1997).

A proprietary nonmetallic catalyst, D34, has been used with success in a ﬂuid-

ized bed at 800 °C followed by a wet scrubber (Knoef, 2005, p. 153).

Metallic catalysts include Ni, Ni/Mo, Ni/Co/Mo, NiO, Pt, and Ru on sup-

ports like silica-alumina and zeolite (Aznar et al., 1997). Some of them are used

in the petrochemical industry and are readily available. A Ni/Co/Mo blend

converts NH

along with tars. Catalysts deactivate during tar cracking and so

need reactivation. Typically the catalysts are placed in a ﬁxed or ﬂuidized bed.

Tar-laden gas is passed through at a temperature of 800 to 900 °C.

Dolomite (calcined) and olivine sand are very effective in in-situ tar reduc-

tion. This type of catalytic cracking takes place at the typical temperature of a

ﬂuidized bed. Good improvement in gas yield and tar reduction is noted when

catalytic bed materials are used.

Gasiﬁcation Theory and

Modeling of Gasiﬁers

Chapter 5

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

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5.1 IntroductIon

The design and operation of a gasiﬁer require an understanding of the gasiﬁca-

tion 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 discus-

sion 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 con-

venient 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 pro-

cesses, 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,

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chapter

5 Gasiﬁcation Theory and Modeling of Gasiﬁers

requires a gasifying medium like steam, air, or oxygen to rearrange the molecu-

lar 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.

5.2.1 Gasifying Mediums

Gasifying agents react with solid carbon and heavier hydrocarbons to convert

them into low-molecular-weight gases like CO and H

. The main gasifying

agents used for gasiﬁcation are

 Oxygen

 Steam

 Air

Oxygen is a popular gasifying agent, though it is primarily used for the

combustion step. It may be supplied to a gasiﬁer either in pure form or through

air. The heating value and the composition of the gas produced in a gasiﬁer are

strong functions of the nature and amount of the gasifying agent used. A ternary

diagram (Figure 5.1) of carbon, hydrogen, and oxygen (see Section 2.4.3)

demonstrates the conversion paths of formation of different products in a

gasiﬁer.

Char

Solid

fuel

H hydrogen S steam O oxygen

P slow pyrolysis F fast pyrolysis

Gaseous

fuel

Biomass

Combustion

products

Coal

fIGure 5.1 C-H-O diagram of the gasiﬁcation process.

119

5.3 The Gasiﬁcation Process

If oxygen is used as the gasifying agent, the conversion path moves toward

the oxygen corner. Its products include CO for low oxygen and CO

for high

oxygen. When the amount of oxygen exceeds a certain (stoichiometric) amount,

the process moves from gasiﬁcation to combustion, and the product is “ﬂue

gas” instead of “fuel gas.” Neither ﬂue gas nor the combustion product contains

residual heating value when cooled. A move toward the oxygen corner (Figure

5.1) leads to a lowering of hydrogen content and an increase in carbon-based

compounds such as CO and CO

in the product gas.

If steam is used as the gasiﬁcation agent, the path is upward toward the

hydrogen corner in Figure 5.1. Then the product gas contains more hydrogen

per unit of carbon, resulting in a higher H/C ratio. Some of the intermediate-

reaction products like CO and H

also help to gasify the solid carbon.

The choice of gasifying agent affects the heating value of the product gas.

If air is used instead of oxygen, the nitrogen in it greatly dilutes the product.

From Table 5.1, we can see that oxygen gasiﬁcation has the highest heating

value followed by steam and air gasiﬁcation.

5.3 the GasIfIcatIon process

A typical gasiﬁcation process generally follows the sequence of steps listed on

the next page (illustrated schematically in Figure 5.2).

TABLE 5.1 Heating Values for Product Gas

Based on Gasifying Medium

Medium Heating Value (MJ/Nm

)

Air 4–7

Steam 10–18

Oxygen 12–28

Gasses

(CO, H

, H

CO, H

, CH

O, CO

unconverted

carbon

CO, H

, CH

O, CO

cracking +5%

products

Liquids

(tar, oil,

naphtha)

Oxyenated

compounds

(phenols, acid)

Pyrolysis

Solid

(char)

Gas-phase reactions

Char gasification reactions

(cracking, reforming,

combustion, shift)

(gasification,

combustion, shift)

Drying

Biomass

fIGure 5.2 Potential paths for gasiﬁcation.

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chapter

5 Gasiﬁcation Theory and Modeling of Gasiﬁers

 Preheating and drying

 Pyrolysis

 Char gasiﬁcation

 Combustion

Though these steps are frequently modeled in series, there is no sharp boundary

between them, and they often overlap. The following paragraphs discuss these

sequential phases of biomass gasiﬁcation.

In a typical process, biomass is ﬁrst heated (dried) and then it undergoes

thermal degradation or pyrolysis. The products of pyrolysis (i.e., gas, solid,

and liquid) react among themselves as well as with the gasifying medium to

form the ﬁnal gasiﬁcation product. In most commercial gasiﬁers, the thermal

energy necessary for drying, pyrolysis, and endothermic reactions comes from

a certain amount of exothermic combustion reactions allowed in the gasiﬁer.

Table 5.2 lists some of the important chemical reactions taking place in a

gasiﬁer.

5.3.1 drying

The typical moisture content of freshly cut wood ranges from 30 to 60%, and

for some biomass it can exceed 90% (see Table 2.9). Every kilogram of mois-

ture in the biomass takes away a minimum of 2260

kJ of extra energy from the

gasiﬁer to vaporize water, and that energy is not recoverable. For a high level

of moisture this loss is a concern, especially for energy applications. While we

cannot do much about the inherent moisture residing within the cell structure,

efforts may be made to drive away the external or surface moisture. A certain

amount of predrying is thus necessary to remove as much moisture from the

biomass as possible before it is fed into the gasiﬁer. For the production of a

fuel gas with a reasonably high heating value, most gasiﬁcation systems use

dry biomass with a moisture content of 10 to 20%.

The ﬁnal drying takes place after the feed enters the gasiﬁer, where it

receives heat from the hot zone downstream. This heat dries the feed, which

releases water. Above 100 °C, the loosely bound water that is in the biomass

is irreversibly removed. As the temperature rises, the low-molecular-weight

extractives start volatilizing. This process continues until a temperature of

approximately 200 °C is reached.

5.3.2 pyrolysis

In pyrolysis no external agent is added. In a slow pyrolysis process, the solid

product moves toward the carbon corner of the ternary diagram, and more char

is formed. In fast pyrolysis, the process moves toward the C-H axis opposite

the oxygen corner (Figure 5.1). The oxygen is largely diminished, and thus we

expect more liquid hydrocarbon.

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5.3 The Gasiﬁcation Process

Pyrolysis, which precedes gasiﬁcation, involves the thermal breakdown of

larger hydrocarbon molecules of biomass into smaller gas molecules (condens-

able and noncondensable) with no major chemical reaction with air, gas, or any

other gasifying medium. For a detailed description of this process, see

Chapter 3.

One important product of pyrolysis is tar formed through condensation of

the condensable vapor produced in the process. Being a sticky liquid, tar creates

a great deal of difﬁculty in industrial use of the gasiﬁcation product. A

discussion of tar formation and ways of cracking or reforming it into useful

noncondensable gases is presented in Chapter 4.

TABLE 5.2 Typical Gasiﬁcation Reactions at 25 °C

Reaction Type Reaction

carbon reactions

R1 (Boudouard)

C + CO

↔ 2CO + 172 kJ/mol

R2 (water-gas or steam)

C + H

O ↔ CO + H

+ 131 kJ/mol

R3 (hydrogasiﬁcation)

C + 2H

↔ CH

− 74.8 kJ/mol

C + 0.5 O

→ CO − 111 kJ/mol

oxidation reactions

C + O

→ CO

− 394 kJ/mol

CO + 0.5O

→ CO

− 284 kJ/mol

+ 2O

↔ CO

+ 2H

O − 803 kJ/mol

+ 0.5 O

→ H

O − 242 kJ/mol

shift reaction

CO + H

O ↔ CO

+ H

− 41.2 kJ/mol

Methanation reactions

R10

2CO +2H

→ CH

+ CO

− 247 kJ/mol

R11

CO + 3H

↔ CH

+ H

O − 206 kJ/mol

R14

+ 4H

→ CH

+ 2H

O − 165 kJ/mol

steam-reforming reactions

R12

+ H

O ↔ CO + 3H

+ 206 kJ/mol

R13

+ 0.5 O

→ CO + 2H

− 36 kJ/mol

Source: Higman and van der Burgt, 2008, p. 12.

Source: Klass, 1998, p. 276.

Source: Higman and van der Burgt, 2008, p. 3.

Source: Knoef, 2005, p. 15.

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chapter

5 Gasiﬁcation Theory and Modeling of Gasiﬁers

5.3.3 char Gasiﬁcation reactions

The gasiﬁcation step that follows pyrolysis involves chemical reactions among

the hydrocarbons in fuel, steam, carbon dioxide, oxygen, and hydrogen in the

reactor, as well as chemical reactions among the evolved gases. Of these, char

gasiﬁcation is the most important. The char produced through pyrolysis of

biomass is not necessarily pure carbon. It contains a certain amount of hydro-

carbon comprising hydrogen and oxygen.

Biomass char is generally more porous and reactive than coke. Its porosity

is in the range of 40 to 50% while that of coal char is 2 to 18%. The pores of

biomass char are much larger (20–30 micron) than those of coal char (~5 ang-

strom) (Encinar et al., 2001). Thus, its reaction behavior is different from that

of chars derived from coal, lignite, or peat. For example, the reactivity of peat

char decreases with conversion or time, while the reactivity of biomass char

increases with conversion (Figure 5.3). This reverse trend can be attributed to

the increasing catalytic activity of the biomass char’s alkali metal constituents

(Risnes et al., 2001).

Gasiﬁcation of biomass char involves several reactions between the char

and the gasifying mediums. Following is a description of some of those reac-

tions with carbon, carbon dioxide, hydrogen, steam, and methane.

Char O CO and CO+ →

2 2

(5.1)

Char CO CO+ →

(5.2)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Char conversion (–)

Reactivity in steam (%/min)

Biomass char, 0.1 Mpa, covered crucible k = 0.013 L/min and k

= 0.56

Fossil fuel char, 0.73 Mpa k = 0.067 L/min and k

= 0.76

Peat char

Hardwood char

fIGure 5.3 Reactivities of peat char for gasiﬁcation in steam decrease with conversion; reactivi-

ties of hardwood char increase with conversion. (Source: Data from Liliedahl and Sjostrom, 1997.)

123

5.3 The Gasiﬁcation Process

Char H O CH and CO+ →

2 4

(5.3)

Char H CH+ →

2 4

(5.4)

Equations (5.1) through (5.4) show how gasifying agents like oxygen,

carbon dioxide, and steam react with solid carbon to convert it into lower-

molecular-weight gases like carbon monoxide and hydrogen. Some of the

reactions are known by the names listed in Table 5.2.

Gasiﬁcation reactions are generally endothermic, but some of them can be

exothermic as well. For example, those of carbon with oxygen and hydrogen

(R3, R4, and R5 in Table 5.2) are exothermic, whereas those with carbon

dioxide and steam (reactions R1 and R2) are endothermic. The heat of reaction

given in Table 5.2 for various reactions refers to a temperature of 25 °C.

Speed of Char Reactions

The rate of gasiﬁcation of char (comprising of mainly carbon) depends primar-

ily on its reactivity and the reaction potential of the gasifying medium. Oxygen,

for example, is the most active, followed by steam and carbon dioxide. The

rate of the char–oxygen reaction (C + 0.5O

→ CO) is the fastest among the

four in Table 5.2 (R1, R2, R3, and R4). It is so fast that it quickly consumes

the oxygen, leaving hardly any free oxygen for any other reactions.

The rate of the char–steam reaction (C + H

O → CO + H

) is three to ﬁve

orders of magnitude slower than that of the char–oxygen reaction. The Boud-

ouard, or char–carbon dioxide, reaction (C + CO

→ 2CO) is six to seven orders

of magnitude slower (Smoot and Smith, 1985). The rate of the water–gas or

water–steam gasiﬁcation reaction (R2) is about two to ﬁve times faster than

that of the Boudouard reaction (R1) (Blasi, 2009).

The char–hydrogen reaction that forms methane (C + 2H

→ CH

) is the

slowest of all. Walker et al. (1959) estimated the relative rates of the four reac-

tions, at 800 °C temperature and 10

Pa pressure, as 10

for oxygen, 10

for

steam, 10

for carbon dioxide, and 3 × 10

−3

for hydrogen. The relative rates, R,

may be shown as

R R R R

C O C H O C CO C H+ + + +

>> > >>

2 2 2 2

(5.5)

When steam reacts with carbon it can produce CO and H

. Under certain condi-

tions the steam and carbon reaction can also produce CH

and CO

Boudouard Reaction Model

The gasiﬁcation of char in carbon dioxide is popularly known as the Boudouard

reaction.

C CO CO reaction R in Table+ ↔

( )

2 1 5 2. (5.6)

Blasi (2009) describes the Boudouard reaction through the following steps. In

the ﬁrst step, CO

dissociates at a carbon-free active site (C

fas

), releasing carbon

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chapter

5 Gasiﬁcation Theory and Modeling of Gasiﬁers

monoxide and forming a carbon–oxygen surface complex, C(O). This reaction

can move in the opposite direction as well, forming a carbon active site and

in the second step. In the third step, the carbon–oxygen complex produces

a molecule of CO.

Step C CO C O CO1

fas

+  →

( )

+ (5.7)

Step C O CO C CO2

( )

+  → +

fas

(5.8)

Step C O CO3

( )

 →

(5.9)

where k

is the rate of the ith reaction.

The rate of the char gasiﬁcation reaction in CO

is insigniﬁcant: below

1000

Water–Gas Reaction Model

The gasiﬁcation of char in steam, known as the water–gas reaction, is perhaps

the most important gasiﬁcation reaction.

C H O CO H R in Table+ ↔ +

( )

2 2

2 5 2. (5.10)

The ﬁrst step involves the dissociation of H

O on a free active site of carbon

fas

), releasing hydrogen and forming a surface oxide complex of carbon C(O).

In the second and third steps, the surface oxide complex produces a new free

active site and a molecule of CO.

Step C H O C O H1

2 2

fas

+  →

( )

+ (5.11)

Step C O H C H O2

2 2

( )

+  → +

fas

(5.12)

Step C O CO3

( )

 →

(5.13)

Some models (Blasi, 2009) also include the possibility of hydrogen inhibition

by C(H) or C(H)

complexes as here:

C H C H

fas

+ ↔

( )

(5.14)

C H C H

fas

+ ↔

( )

0 5

. (5.15)

The presence of hydrogen has a strong inhibiting effect on the char gasiﬁca-

tion rate in H

O. For example, 30% hydrogen in the gasiﬁcation atmosphere

can reduce the gasiﬁcation rate by a factor as high as 15 (Barrio et al., 2001).

So an effective means of accelerating the water–gas reaction is continuous

removal of hydrogen from the reaction site.

Shift Reaction Model

The shift reaction is an important gas-phase reaction. It increases the hydrogen

content of the gasiﬁcation product at the expense of carbon monoxide. This

reaction is also called the “water–gas shift reaction” in some literature (Klass,

1998, p. 277), though it is much different from the water–gas reaction (R2).

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5.3 The Gasiﬁcation Process

CO H O CO H kJ mol reaction R in Table+ ↔ + −

( )

2 2 2

41 2 9 5 2. . (5.16)

This is a prestep in syngas production in the downstream of a gasiﬁer, where

the ratio of hydrogen and carbon monoxide in the product gas is critical.

The shift reaction is slightly exothermic, and its equilibrium yield decreases

slowly with temperature. Depending on temperature, it may be driven in either

direction—that is, products or reactants. However, it is not sensitive to pressure

(Petersen and Werther, 2005).

Above 1000 °C the shift reaction (R9) rapidly reaches equilibrium, but at

a lower temperature it needs heterogeneous catalysts. Figure 5.4 (Probstein and

Hicks, 2006, p. 63) shows that this reaction has a higher equilibrium constant

at a lower temperature, which implies a higher yield of H

at a lower tempera-

ture. With increasing temperature, the yield decreases but the reaction rate

increases. Optimum yield is obtained at about 225 °C.

Because the reaction rate at such a low temperature is low, catalysts like

chromium-promoted iron, copper-zinc, and cobalt-molybdenum are used (Prob-

stein and Hicks, 2006, p. 124). At higher temperatures (350–600 °C) Fe-based

Temperature (°K) 1/T scale

2C + 2H

O ↔ CO

+ CH

(gasification)

C + H

O ↔ CO + H

(gasification)

300

Equilibrium constant (log

–1

–2

–3

–4

–5

–6

3000

C + H

O ↔ CO

+ H

(gas shift)

CO + 2H

↔ CO

OH (methanol)

C + CO

↔ 2CO (Boudouard)

CO + 3H

↔ H

O + CH

(methanation)

fIGure 5.4 Equilibrium constants for selected gasiﬁcation reactions. (Source: Adapted from

Probstein and Hicks, 2006, p. 63.)