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

Подождите немного. Документ загружается.

chapter

3 Pyrolysis and Torrefaction

21.6 kJ/kg (LHV as received basis), respectively, it was 5.0, 4.6, 10.5, and

18.4

GJ/m

, respectively, on a volume basis (Bergman, 2005c). Thus, pelletiza-

tion of torreﬁed wood greatly increases the transportation and handling

cost of biomass. Pelletization of torreﬁed biomass is better than torrefaction

of pelletized wood from the standpoint of process energy consumption and

product stability.

symbols and nomenclature

A = pre-exponential factor (s

–1

)

E = activation energy (J/mol)

k = reaction rate (s

–1

)

= mass of biomass at time t (kg)

= mass of char residue (kg)

= initial mass of biomass (kg)

R = universal gas constant (J/mol.K)

T = temperature (K)

pyr

= pyrolysis temperature (K)

heating

= heating time (s)

= reaction time (s)

X = fractional change in mass of biomass

tor

= torrefaction temperature (°C)

Tar Production

and Destruction

Chapter 4

Biomass Gasiﬁcation and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00004-0

4.1 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 unde-

sirable, 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 inﬂu-

ence its formation such that plants and equipment can live with this “necessary

evil” while minimizing its detrimental effects.

4.2 BasIcs of tar

Tar is a complex mixture of condensable hydrocarbons, including, among

others, oxygen-containing, 1- to 5-ring aromatic, and complex polyaromatic

hydrocarbons (Devi et

al., 2003). Neeft et

al. (1999) deﬁned tar as “all organic

contaminants with a molecular weight larger than 78, which is the molecular

weight of benzene.” The International Energy Agency (IEA) Bioenergy Agree-

ment, the U.S. Department of Energy (DOE), and the DGXVII of the European

Commission agreed to identify all components of product gas having a molecu-

lar weight higher than benzene as tar (Knoef, 2005, p. 278).

A common perception about tar is that it is a product of gasiﬁcation and

pyrolysis that can potentially condense in colder downstream sections of the

unit. While this is a fairly good description, a more speciﬁc and scientiﬁc deﬁ-

nition may be needed for technical, scientiﬁc, and legal work. Presently, there

is no universally accepted deﬁnition of tar. As many as 30 deﬁnitions are avail-

able in the literature (Knoef, 2005, p. 279). Of these, that of the IEA’s gasiﬁca-

tion task force, as follows, appears most appropriate (Milne et

al., 1998):

chapter

4 Tar Production and Destruction

The organics, produced under thermal or partial-oxidation regimes (gasiﬁcation) of any

organic material, are called “tar” and are generally assumed to be largely aromatic.

4.2.1 acceptable Limits for tar

Tar remains vaporized until the gas carrying it is cooled, when it either con-

denses 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 accept-

able 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 addi-

tion of a scrubber or a ﬁltration system signiﬁcantly increases the overall plant

costs. This limitation makes biomass-based distributed power-generation proj-

ects 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 deposi-

tion 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

4.2 Basics 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 particu-

late level in the gas. Particulate and tar concentrations in the product gas should

therefore be below the tolerable limits, which are 30 mg/Nm

for particulates

and 100

mg/Nm

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/Nm

(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/Nm

for particulates and

0.1

mg/Nm

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 con-

tents 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/Nm

and 50 g/Nm

, respectively (Table

4.2). Tar levels in product gas from bubbling and circulating ﬂuidized-bed

gasiﬁers are about 10

g/Nm

. Table 4.2 also shows that the amount of tar

TABLE 4.1 Upper Limits of Biomass Gas Tar and Particulates

Application Particulate (g/Nm

) Tar (g/Nm

)

Direct combustion No limit speciﬁed No limit speciﬁed

Syngas production 0.02 0.1

Gas turbine 0.1–120 0.05–5

IC engine 30 50–100

Pipeline transport 50–500 for compressor

Fuel cells

<1.0

Source: Data compiled from Milne et al., 1998.

100

chapter

4 Tar Production and Destruction

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.

4.2.2 tar formation

Tar is produced primarily through depolymerization during the pyrolysis stage

of gasiﬁcation. Biomass (or other feed), when fed into a gasiﬁer, ﬁrst undergoes

pyrolysis that can begin at a relatively low temperature of 200 °C and complete

at 500 °C. In this temperature range the cellulose, hemicellulose, and lignin

components of biomass break down into primary tar, which is also known as

wood oil or wood syrup. This contains oxygenates and primary organic con-

densable molecules called primary tar (Milne et

al, 1998, p. 13). Char is also

produced at this stage. Above 500 °C the primary tar components start reform-

ing into smaller, lighter noncondensable gases and a series of heavier molecules

TABLE 4.2 Typical Levels of Tar in Biomass Gasiﬁer by Type

Gasiﬁer Type

Average Tar Concentration

in Product Gas (g/Nm

) Tar as % of Biomass Feed

Downdraft

<1.0 <2.0

Fluidized bed 10 1–5

Updraft 50 10–20

Entrained ﬂow negligible

200

Yield of condensible liquids

(kg/100 kg dry wood)

400 600 800

Temperature (°C)

1000 1200 1400

fIGurE 4.1 Effect of maximum gasiﬁcation temperature on tar yield.

Source: Data compiled from Milne and Evans, 1998, p. 15.

101

4.2 Basics of Tar

called secondary tar. The noncondensable gases include CO

, CO, and H

At still higher temperatures, primary tar products are destroyed and tertiary

products are produced.

4.2.3 tar composition

As we can see in Table 4.3, tar is a mixture of various hydrocarbons. It may

also contain oxygen-containing compounds, derivatives of phenol, guaiacol,

veratrol, syringol, free fatty acids, and esters of fatty acids (Razvigorova et

al.,

1994). The yield and composition of tar depends on the reaction temperature,

the type of reactor, and the feedstock. Table 4.3 shows that benzene is the largest

component of a typical tar.

Tar may be classiﬁed into four major product groups: primary, secondary,

alkyl tertiary, and condensed tertiary (Evans and Milne, 1997). Short descrip-

tions of these follow.

Primary Tar

Primary tar is produced during primary pyrolysis. It comprises oxygenated,

primary organic, condensable molecules. Primary products come directly from

the breakdown of the cellulose, hemicellulose, and lignin components of

biomass. Milne et

al. (1998) listed a large number of compounds of acids,

sugars, alcohols, ketones, aldehydes, phenols, guaiacols, syringols, furans, and

mixed oxygenates in this group.

TABLE 4.3 Typical Composition of Tar

Component Weight (%)

Benzene 37.9

Toluene 14.3

Other 1-ring aromatic hydrocarbons 13.9

Naphthalene 9.6

Other 2-ring aromatic hydrocarbons 7.8

3-ring aromatic hydrocarbons 3.6

4-ring aromatic hydrocarbons 0.8

Phenolic compounds 4.6

Heterocyclic compounds 6.5

Others 1.0

Source: Adapted from Milne et al., 1998.

102

chapter

4 Tar Production and Destruction

Secondary Tar

As the gasiﬁer’s temperature rises above 500 °C, primary tar begins to

rearrange, forming more noncondensable gases and heavier molecules called

secondary tar, of which phenols and oleﬁns are important constituents.

Tertiary Tar Products

The alkyl tertiary product includes methyl derivatives of aromatics, such as

methyl acenaphthylene, methylnaphthalene, toluene, and indene (Evans and

Milne, 1997).

Condensed tertiary aromatics make up a polynuclear aromatic hydrocarbon

(PAH) series without substituents (atoms or a group of atoms substituted for

hydrogen in the parent chain of hydrocarbon). This series contains benzene,

naphthalene, acenaphthylene, anthracene/phenanthrene, and pyrene.

The secondary and tertiary tar products come from the primary tar.

The primary products are destroyed before the tertiary products appear (Milne

al., 1998).

Figure 4.2 shows that with increasing temperature the primary tar product

decreases but the tertiary product increases. Above 500 °C the secondary tar

increases at the expense of the primary tar. Once the primary tar is nearly

destroyed, tertiary tar starts appearing with increasing temperature. At this stage

the secondary tar begins to decrease. Thus, high temperatures destroy the

primary tar but not the tertiary tar products.

600 700 800 900 950 1000850750

secondaryprimary tertiary-alkyl tertiary-PNA

550500

0.8

0.6

0.4

0.2

Principle component score

650

Temperature (°C)

fIGurE 4.2 Variation in primary, secondary, and tertiary tar products with temperature measured

at 0.3 seconds residence time. (Source: Adapted from Evans and Milne, 1997, p. 804.)

103

4.3 Tar Reduction

4.3 tar rEductIon

The tar in coal gasiﬁcation comprise benzene, toluene, xylene, and coal tar, all

of which have good commercial value and can be put to good use. Tar from

biomass, on the other hand, is mostly oxygenated and has little commercial use.

Thus, it is a major headache in gasiﬁers, and a major roadblock in the com-

mercialization of biomass gasiﬁcation. Research over the years has improved

the situation greatly, but the problem has not completely disappeared. Tar

removal remains an important part of the development and design of biomass

gasiﬁers.

Several options are available for tar reduction. These may be divided into

two broad groups: (1) in-situ (or primary) tar reduction, which avoids tar forma-

tion; and (2) post-gasiﬁcation (or secondary) reduction, which strips the product

gas of the tar already produced. They are shown in Figure 4.3.

In-situ reduction is carried out by various means so that the generation

of tar inside the gasiﬁer is lessened, thereby eliminating the need for any

removal to occur downstream. As this process is carried out in the gasiﬁer,

it inﬂu ences the product gas quality. Post-gasiﬁcation reduction, on the other

hand, does not interfere with the process in the reactor, and therefore the

quality of the product gas is unaffected.

At times it may not be possible to remove the tar to the desired degree while

retaining the quality of the product gas. In such cases a combination of in-situ

and post-gasiﬁcation reduction can prove very effective. The tar reduced is

removed after the product gas leaves the gasiﬁer. Details of these two approaches

are given in the following sections.

Gasifying

agent

(b)

(a)

Gasifier

Product gas

plus tar

Tar-free

product gas

Biomass

Gasifier

with in-situ

tar removal

Dust cleaning

Clean gas

Post-gasification

cleaning,

tar and dust

scrubbing,

catalytic tar

reduction

fIGurE 4.3 (a) In-situ tar reduction. (b) Post-gasiﬁcation tar reduction.

104

chapter

4 Tar Production and Destruction

4.3.1 In-situ tar reduction

In this approach the operating conditions in the gasiﬁer are adjusted such that

tar formation is reduced. Alternately, the tar produced is converted into other

products before it leaves the gasiﬁer. Reduction is achieved by

 Modiﬁcation of the operating conditions of the gasiﬁer

 Addition of catalysts or alternative bed materials in the ﬂuidized bed

 Modiﬁcation of the gasiﬁer design

Biomass type also inﬂuences the tar product. The appropriate choice of one

or a combination of these factors can greatly reduce the amount of tar in the

product gas leaving the gasiﬁer. Reforming, thermal cracking, and steam crack-

ing are three major reactions responsible for tar destruction (Delgado et

al.,

1996). They convert tar into an array of smaller and lighter hydrocarbons as

shown here:

Tar

Reforming

Thermal cracking

Steam cracking

CO CO H CH⇒ ⇒ + + + +

2 2 4

…… +

[ ]

Coke

Tar reforming. We can write the reforming reaction as in Eq. (4.1) by

representing tar as C

. The cracking reaction takes place in steam gasiﬁca-

tion, where steam cracks the tar, producing simpler and lighter molecules

like H

and CO.

C H H O H CO

n x

n n x n+ → +

( )

2 2

2 (4.1)

Dry tar reforming. The dry reforming reaction takes place when CO

the gasifying medium. Here tar is broken down into H

and CO (Eq. 4.2).

Dry reforming is more effective than steam reforming when dolomite is

used as the catalyst (Sutton et

al., 2001).

C H CO H CO

n x

n x n+ →

( )

2 2

2 2 (4.2)

Thermal cracking. Thermal cracking can reduce tar, but it is not as attrac-

tive as reforming because it requires high (>1100 °C) temperature and

produces soot (Dayton, 2002). Because this temperature is higher than the

gas exit temperature for most biomass gasiﬁers, external heating or internal

heat generation with the addition of oxygen may be necessary. Both options

have major energy penalties.

Steam cracking. In steam cracking, the tar is diluted with steam and is

brieﬂy heated in a furnace in the absence of oxygen. The saturated hydro-

carbons are broken down into smaller hydrocarbons.

The following sections elaborate the operating conditions used in in-situ reduc-

tion of tar.

105

4.3 Tar Reduction

Operating Conditions

Operating parameters inﬂuencing tar formation and conversion include reactor

temperature, reactor pressure, gasiﬁcation medium, equivalence ratio, and resi-

dence time.

Temperature

Reactor operating temperature inﬂuences both the quantity and composition of

tar. The quantity in general decreases with an increase in reaction temperature,

as does the amount of unconverted char. Thus, high-temperature operation is

desirable on both counts. The production of oxygen-containing compounds like

phenol, cresol, and benzofuran reduces with temperature, especially below

800 °C. With increasing temperature the amount of 1- and 2-ring aromatics

with substituents decreases, but that of 3- and 4-ring aromatics increases. Aro-

matic compounds without substituents (naphthalene, benzene, etc.) are favored

at high temperatures. The naphthalene and benzene content of the gas increases

with temperature (Devi et

al., 2003). High temperature also reduces the

ammonia content of the gas and improves the char conversion, but has a nega-

tive effect of reducing the product gas’ useful heating value.

An increase in the freeboard temperature in a ﬂuidized-bed gasiﬁer can

also reduce the tar in the product gas. A reduction in tar was obtained by

Narváez et

al. (1996) by injecting secondary air into the freeboard. This may

be due to increased combustion in the freeboard. Raising the temperature

through secondary air injection in the freeboard may have a negative impact

on heating value.

Reactor Pressure

With increasing pressure, the amount of tar decreases, but the fraction of PAH

increases (Knight, 2000).

Gasiﬁcation Medium

Four mediums—air, steam, carbon dioxide, and a steam–oxygen mixture—can

be used for gasiﬁcation; they may have different effects on tar formation and

conversion. The ratio of fuel to medium is an important parameter that inﬂu-

ences the product of gasiﬁcation, including tar. This parameter is expressed

differently for different mediums. For example, for air gasiﬁcation the param-

eter is the equivalence ratio (ER); for steam gasiﬁcation it is the steam-to-

biomass ratio (S/B); and for steam–oxygen gasiﬁcation it is the gasifying ratio

(Table 4.4). Table 4.5 shows the range of tar production for three gasiﬁcation

mediums for typical values of their characteristic parameters.

In general, the yield of tar in steam gasiﬁcation is greater than that in steam–

oxygen gasiﬁcation. Of these, air gasiﬁcation is the lowest tar producer (Gil

al., 1999). The tar yield in a system depends on the amount of gasifying

medium per unit biomass gasiﬁed.