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

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Chapter

1 Introduction

Torrefaction increases the energy density of the biomass. It also greatly reduces

its weight as well as its hygroscopic nature, thus enhancing the commercial use

of wood for energy production by reducing its transportation cost.

Gasiﬁcation

Gasiﬁcation converts fossil or nonfossil fuels (solid, liquid, or gaseous) into

useful gases and chemicals. It requires a medium for reaction, which can be

gas or supercritical water (not to be confused with ordinary water at subcritical

condition). Gaseous mediums include air, oxygen, subcritical steam, or a

mixture of these.

Presently, gasiﬁcation of fossil fuels is more common than that of nonfossil

fuels like biomass for production of synthetic gases. It essentially converts a

potential fuel from one form to another. There are three major motivations for

such a transformation:

 To increase the heating value of the fuel by rejecting noncombustible com-

ponents like nitrogen and water.

 To remove sulfur and nitrogen such that when burnt the gasiﬁed fuel does

not release them into the atmosphere.

 To reduce the carbon-to-hydrogen (C/H) mass ratio in the fuel.

In general, the higher the hydrogen content of a fuel, the lower the vaporiza-

tion temperature and the higher the probability of the fuel being in a gaseous

state. Gasiﬁcation or pyrolysis increases the relative hydrogen content (H/C

ratio) in the product through one the followings means:

Direct: Direct exposure to hydrogen at high pressure.

Indirect: Exposure to steam at high temperature and pressure, where hydro-

gen, an intermediate product, is added to the product. This process also

includes steam reforming.

Pyrolysis: Reduction of carbon by rejecting it through solid char or CO

gas.

Gasiﬁcation of biomass also involves removal of oxygen from the fuel to

increase its energy density. For example, a typical biomass has about 40 to 60%

oxygen by weight, but a useful fuel gas contains only a small percentage of

oxygen (Table 1.4). The oxygen is removed from the biomass by either dehy-

dration or decarboxylation. The latter process, which rejects the oxygen through

, increases the H/C ratio of the fuel so that it emits less greenhouse gas

when combusted:

Dehydration C H O C H qH O O removal through H O:

m n q m n q

→ +

−2 2 2 2

(1.1)

Decarboxylation C H O C H

qCO

O removal through CO:

m n q m q n

→

− 2

2 2

(1.2)

Hydrogen, when required in bulk for the production of ammonia, is pro-

duced from natural gas (∼CH

) through steam reforming, which produces

1.3 Biomass Conversion

syngas (a mixture of H

and CO). The CO in syngas is indirectly hydrogenated

by steam to produce methanol (CH

OH), an important feedstock for a large

number of chemicals. These processes, however, use fossil fuels, which are not

only nonrenewable but are responsible for the net addition of carbon dioxide

(a major greenhouse gas) in the atmosphere.

Biomass can deliver nearly everything that fossil fuels provide, whether fuel

or chemical feedstock. Additionally, it provides two important beneﬁts that

make it a viable feedstock for syngas production. First, it does not make any

net contribution to the atmosphere when burnt; second, its use reduces depen-

dence on nonrenewable and often imported fossil fuel.

For these reasons, biomass gasiﬁcation into CO and H

provides a good

base for production of liquid transportation fuels, such as gasoline, and syn-

thetic chemicals, such as methanol. It also produces methane, which can be

burned directly for energy production. Gasiﬁcation is carried out generally in

one of the three major types of gasiﬁers:

 Moving bed (also called ﬁxed bed)

 Fluidized bed

 Entrained ﬂow

Downdraft and updraft are two common types of moving-bed gasiﬁer. A

survey of gasiﬁers in Europe, the United States, and Canada shows that down-

draft gasiﬁers are the most common (Knoef, 2000). It shows that 75% are

downdraft, 20% are ﬂuidized beds, 2.5% are updraft, and 2.5% are of various

other designs.

TABLE 1.4 Carbon to Hydrogen Ratio of Different Fuels

Fuel C/H mass ratio Oxygen % Energy density, GJ/t

Anthracite

∼44 ∼2.3 ∼27.6

Bituminous coal

∼15 ∼7.8 ∼29

Lignite

∼10 ∼11 ∼9

Peat

∼10

∼7

Crude oil

∼9

42 (mineral oil)

Biomass/Cedar 7.6 40 20

Gasoline 6 0 46.8

Natural gas (∼CH

)

3 0 56

Syngas (CO

1:3 ratio)

2 0 24

Chapter

1 Introduction

Liquefaction

Liquefaction of solid biomass into liquid fuel can be done through pyrolysis,

gasiﬁcation as well as through hydrothermal process. In the latter process,

biomass is converted into an oily liquid by contacting the biomass with water

at elevated temperatures (300–350 °C) with high (12–20

MPa) for a period of

time. There are several other means including the supercritical water process

(Chapter 7) for direct liquefaction of biomass. Behrendt et al. (2008) present a

review of these processes.

1.4 motivation For Biomass ConvErsion

Gasiﬁcation is almost as ancient as combustion, but it is less developed because

commercial interest in it has not been as strong as in combustion. However,

there has been a recent surge of interest in conversion of biomass into gas or

liquid due to three motivating factors:

 Renewability beneﬁts

 Environmental beneﬁts

 Sociopolitical beneﬁts

1.4.1 renewability

Fossil fuels like coal, oil, and gas are good and convenient sources of energy,

and they meet the energy demands of society very effectively. However, there

is one major problem: Fossil fuel resources are ﬁnite and not renewable.

Biomass, on the other hand, grows and is renewable. A crop cut this year will

grow again next year; a tree cut today may grow up within a decade. Unlike

fossil fuels, then, biomass is not likely to be depleted with consumption. For

this reason, its use, especially for energy production, is rising fast.

We may argue against cutting trees for energy because they serve as a CO

sink. This is true, but a tree stops absorbing CO

after it dies. On the other hand,

if left alone in the forest it can release CO

in a forest ﬁre or release more

harmful CH

when it decomposes in water. The use of a tree as fuel after its

life provides carbon-neutral energy as well as avoids greenhouse gas release

from deadwood. The best option is new planting following cutting, as is done

by some pulp industries. Fast-growing plants like switch grass and Miscanthus

are being considered as fuel for new energy projects. These plants have very

short growing periods that can be counted in months.

1.4.2 Environmental Beneﬁts

With growing evidence of global warming, the need to reduce human-made

greenhouse gas emissions is being recognized. Emission of other air pollutants,

such as NO

, SO

, and Hg, is no longer acceptable, as it was in the past. In

1.4 Motivation for Biomass Conversion

elementary schools and in corporate boardrooms, the environment is a major

issue, and it has been a major driver for gasiﬁcation for energy production.

Biomass has a special appeal in this regard, as it makes no net contribution to

carbon dioxide emission to the atmosphere. Regulations for making biomass

economically viable are in place in many countries. For example, if biomass

replaces fossil fuel in a plant, that plant earns credits for CO

reduction equiva-

lent to what the fossil fuel was emitting. These credits can be sold on the market

for additional revenue in countries where such trades are in practice.

Carbon Dioxide Emission

When burned, biomass releases the CO

it absorbed from the atmosphere in

the recent past, not millions of years ago, as with fossil fuel. The net addition

of CO

to the atmosphere through biomass combustion is thus considered to

be zero.

Even if the fuel is not carbon-neutral biomass, CO

emissions from the

gasiﬁcation of the fuel are slightly less than those from its combustion on a

unit heat release basis. For example, emission from an IGCC plant is 745

kWh compared to 770

g/kWh from a combustion-based subcritical pulverized-

coal (PC) plant (Termuehlen and Emsperger, 2003, p. 23). Sequestration

of CO

is becoming an important requirement for new power plants. On that

note, a gasiﬁcation-based power plant has an advantage over a conventional

combustion-based PC power plant. In an IGCC plant, CO

is more concentrated

in the ﬂue gas, making it easier to sequestrate than it is in a conventional PC

plant. Table 1.5 compares the emissions from different electricity-generation

technologies.

TABLE 1.5 A Comparison of Emissions from Electricity-Generation

Technologies

Emission

Pulverized-Coal

Combustion Gasiﬁcation

Combined

Natural-Gas

Combustion

(kg/1000 MWh) 0.77 0.68 0.36

Water use (L/1000

MWh) 4.62 2.84 2.16

(kg/MWh) 0.68 0.045 0

(kg/MWh) 0.61 0.082 0.09

Total solids (kg/100

MWh) 0.98 0.34

∼0

Source: Recompiled from graphs by Stiegel, 2005.

Chapter

1 Introduction

Sulfur Removal

Most virgin or fresh biomass contains little to no sulfur. Biomass-derived

feedstock such as municipal solid waste (MSW) or sewage sludge does contain

sulfur, which requires limestone for the capture of it. Interestingly, such derived

feedstock also contains small amounts of calcium, which intrinsically aids

sulfur capture.

Gasiﬁcation from coal or oil has an edge over combustion in certain situa-

tions. In combustion systems, sulfur in the fuel appears as SO

, which is rela-

tively difﬁcult to remove from the ﬂue gas without adding an external sorbent.

In a typical gasiﬁcation process 93 to 96% of the sulfur appears as H

S with

the remaining as COS (Higman and van der Burgt, 2008, p. 351). We can easily

extract sulfur from H

S by absorption. Furthermore, in a gasiﬁcation plant we

can extract it as elemental sulfur, thus adding a valuable by-product for the

plant.

Nitrogen Removal

A combustion system ﬁring fossil fuels can oxidize the nitrogen in fuel and in

air into NO, the acid rain precursor, or into N

O, a greenhouse gas. Both are

difﬁcult to remove. In a gasiﬁcation system, nitrogen appears as either N

, which is removed relatively easily in the syngas-cleaning stage.

Nitrous oxide emission results from the oxidation of fuel nitrogen alone.

Measurement in a biomass combustion system showed a very low level of N

emission (Van Loo and Koppejan, 2008, p. 295).

Dust and Other Hazardous Gases

Highly toxic pollutants like dioxin and furan, which can be released in a com-

bustion system, are not likely to form in an oxygen-starved gasiﬁer. (This

observation is disputed by some.) Particulate in the syngas is also reduced

signiﬁcantly by multiple gas cleanup systems, including a primary cyclone,

scrubbers, gas cooling, and acid gas–removal units. These reduce the particulate

emissions by one to two orders of magnitude (Rezaiyan and Cheremisinoff,

2005, p. 15).

1.4.3 sociopolitical Beneﬁts

The sociopolitical beneﬁts of biomass are substantial. For one, biomass is a

locally grown resource. For a biomass-based power plant to be economically

viable, the biomass needs to come from within a certain distance from it. This

means that every biomass plant can prompt the development of associated

industries for biomass growing, collecting, and transporting. Some believe that

a biomass fuel plant could create up to 20 times more employment than that

created by a coal- or oil-based plant (Van Loo and Koppejan, 2008, p. 1). The

biomass industry thus has a positive impact on the local economy.

1.5 Commercial Attraction of Gasiﬁcation

Another very important aspect of biomass-based energy, fuel, or chemicals

is that they reduce reliance on imported fossil fuels. The volatile global political

landscape has shown that supply and price can change dramatically within a

short time, with a sharp rise in the price of feedstock. Locally grown biomass

is relatively free from such uncertainties.

1.5 CommErCial attraCtion oF gasiFiCation

A major attraction of gasiﬁcation is that it can convert waste or low-priced

fuels, such as biomass, coal, and petcoke, into high-value chemicals like metha-

nol. Biomass holds great appeal for industries and businesses, especially in the

energy sector. For example:

 Downstream ﬂue-gas cleaning in a gasiﬁcation plant is less expensive than

that in a coal-ﬁred plant with ﬂue-gas desulphurization, selective catalytic

reducers (SCRs), and electrostatic precipitators.

 Polygeneration is a unique feature of a gasiﬁer plant. It can deliver steam

for process, electricity for grid, and gas for synthesis, thereby providing a

good product mix. Additionally, for high-sulfur fuel a gasiﬁer plant pro-

duces elemental sulfur as a by-product; for high-ash fuel, slag or ﬂy ash is

obtained, which can be used for cement manufacture.

 For power generation, an IGCC plant can achieve a higher overall efﬁciency

(38–41%) than can a combustion plant with a steam turbine alone. Gasiﬁca-

tion therefore offers lower power production costs.

 Carbon dioxide capture and sequestration (CCS) may become mandatory

for power plants. An IGCC plant can capture and store CO

at one-half

of what it costs a traditional PC plant (www.gasiﬁcation.org). Other

applications of gasiﬁcation that produce transport fuel or chemicals may

have even lower CCS costs. Established technologies are available to

capture carbon dioxide from a gasiﬁcation plant, but that is not so for a

combustion plant.

 A process plant that uses natural gas as feedstock can use locally available

biomass or organic waste instead, and thereby reduce dependence on

imported natural gas, which is not only rising sharply in price but is also

experiencing supply volatility.

 Gasiﬁcation provides signiﬁcant environmental beneﬁts, as described in

Section 1.4.2.

 Total water consumption in a gasiﬁcation-based power plant is much less

than that in a conventional power plant (Table 1.5). Furthermore, a plant

can be designed to recycle its process water. For this reason, all zero-

emission plants use gasiﬁcation technology.

 Gasiﬁcation plants produce signiﬁcantly lower quantities of major air pol-

lutants like SO

, NOx, and particulates. Figure 1.8 compares the emission

from a coal-based IGCC plant with that from a combustion-based coal-ﬁred

Chapter

1 Introduction

steam power plant and a natural-gas-ﬁred plant. It shows emissions from

the gasiﬁcation plant are similar to those from a natural-gas-ﬁred plant.

 An IGCC plant produces lower CO

per MWh than a combustion-based

steam power plant.

1.5.1 Comparison of gasiﬁcation and Combustion

With heat or power production, the obvious question is why a solid fuel should

be gasiﬁed and then the gas burned for heat, losing some part of its energy

content in the process. Does it not make more sense to directly burn the fuel

to produce heat? Example 1.1 may provide an answer to this question by

comparing two energy conversion options. The comparison is based, where

applicable, on an IGCC plant and a PC-ﬁred plant, both generating electricity

with coal as the fuel.

 For a given throughput of fuel processed, the volume of gas obtained from

gasiﬁcation is much less compared to that obtained from a direct combustion

system. The lower volume of gas requires smaller equipment and hence

results in lower overall costs.

 A gasiﬁed fuel can be used in a wider range of application than can its

precursor solid fuel. For example, sensitive industrial processes such as

glass blowing and drying cannot use dirty ﬂue gas from combustion of coal

or biomass, but they can use heat from the cleaner and more controllable

combustion of gas produced through gasiﬁcation.

 Gas can be more easily carried and distributed than a solid fuel among

industrial and domestic customers. Transportation of synthetic gas, or the

liquid fuel produced from it, is both less expensive and less energy intensive

than transportation of solid fuel for combustion.

Coal combustion

Relative scale

NOX

SOX

0.2

0.15

0.1

0.05

Integrated

gasification

combined cycle

Natural-gas-

based combined

cycle

igurE 1.8

Comparison of pollutant emissions from a coal-based steam plant, an IGCC plant,

and a natural-gas-ﬁred combined-cycle plant.

1.5 Commercial Attraction of Gasiﬁcation

 The concentration of CO

in the product of gasiﬁcation is considerably

higher than that of combustion, so it is less expensive to separate and

sequestrate the CO

in IGCC.

 SO

emissions are generally lower in an IGCC plant (Table 1.5). Sulfur in

a gasiﬁcation plant appears as H

S and COS, which can be easily converted

into saleable elemental sulfur or H

. In a combustion system sulfur

appears as SO

, which needs a scrubber producing ash-mixed CaSO

, which

has less market potential.

 Gasiﬁcation produces much less NO

than a combustion system (Table 1.5).

In gasiﬁcation, nitrogen can appear as NH

, which washes out with water

and as such does not need a SCR to meet statutory limits. A PC system, on

the other hand, requires SCR for this purpose.

 The total solid waste generated in an IGCC plant is much lower than that

generated in a comparable combustion system (Table 1.5). Furthermore, the

ash in a slagging entrained-ﬂow gasiﬁer appears as glassy melt, which is

much easier to dispose of than the dry ﬂy ash of a PC system.

 For mechanical work or electricity in a remote location, a power pack com-

prising a gasiﬁer and a compression ignition engine can be employed. For

a combustion system, a boiler, a steam engine, and a condenser might be

needed, making the power pack considerably more bulky and expensive.

 The producer gas from a gasiﬁer can be used as a feedstock for the pro-

duction of fertilizer, methanol, and gasoline. A gasiﬁcation-based energy

system has the option of producing value-added chemicals as a side stream.

This polygeneration feature is not available in direct combustion.

 A gas produced in a central gasiﬁcation plant can be distributed to individual

houses or units in a medium-size to large community.

 If heat is the only product that is desired, combustion seems preferable,

especially in small-scale plants. Even for a medium-capacity unit such as

for district heating, central heating, and power, combustion may be more

economical.

Example 1.1

Compare the theoretical thermodynamic efﬁciency of electricity generation from

biomass through the following two routes:

Biomass is combusted in a boiler with 95% efﬁciency (on lower heating value

[LHV] basis) to generate steam, which expands in a steam turbine from

600 °C to 100 °C driving a generator.

Biomass is gasiﬁed with 80% efﬁciency; the product gas is burnt into hot

gas at 1200 °C. It expands in a gas turbine to 600 °C. Waste gas from the gas

turbine enters a heat recovery steam generator to produce steam at 400 °C.

This steam expands to 100 °C in a steam turbine.

Both turbines are connected to electricity generators. Neglect losses in the

generators.

Chapter

1 Introduction

Solution

For a steam power plant, given:

Combustion efﬁciency: η

= 0.95

Inlet steam temperature: T

= 600 °C = 873K

Exhaust steam temperature: T

= 100 °C = 373K

We assume the turbine to be an ideal heat engine, operating on a Carnot cycle.

This makes the efﬁciency simple to calculate.

Steam turbine efficiency: .

T T= −

( )

= −

( )

=1 1 373 873 0 573

2 1

The overall efﬁciency of the ﬁrst route is the combination of the two separate

efﬁciencies.

Overall efficiency: . . ,

η η η η

a c s a

= × = × = =0 95 0 573 0.544 54.4%

For an IGCC plant, given:

Gasiﬁcation efﬁciency: η

= 0.8

Energy supplied to steam: 20%

Inlet steam temperature: T

= 400 °C = 673K

Exhaust steam temperature: T

= 100 °C = 373K

We assume that all of the waste energy from the gas turbine is used to heat the

steam. This means that 20% of the energy input to the gas turbine is used for

steam heating; the remaining is used to generate electricity.

Gas turbine efficiency: .

η η

g g g

T T= −

( )

= −

( )

1 1 873 1473 0 407

2 1

gg cc

× = × =

0 407 0 85. . 0.346

The steam turbine is again considered to be a Carnot heat engine and the efﬁ-

ciency can easily be calculated. From basic thermodynamics the ideal cycle

efﬁciency is written as:

Steam turbine efficiency: .

T T= −

( )

= −

( )

=1 1 373 673 0 446

2 1

Both the steam and gas turbines have been assumed to be ideal, so the ideal

efﬁciency of the combined cycle can be calculated using the expression for

combined cycle efﬁciency given in basic thermodynamics.

Combined efficiency: . .

. .

η η η η η

cc g s g s

= + − × = +

− ×

0 346 0 446

0 346 0 4466

( )

= 0.638

Thus, the IGCC plant has an overall efﬁciency of 63.8% compared to 54.4% for

a PC boiler steam power plant.

1.6 BriEF dEsCriPtion oF gasiFiCation

and rE

latEd ProCEssEs

When a biomass or other carbonaceous material is heated in a restricted oxygen

supply, it is ﬁrst pyrolyzed or decomposed into solid carbon and condensable

and noncondensable gases.

1.6 Brief Description of Gasiﬁcation and Related Processes

1.6.1 Pyrolysis

The solid carbon as well as the condensed liquid enters the gasiﬁcation reaction

with carbon dioxide, oxygen, or steam to produce combustible or synthetic gas.

To illustrate the different reactions we take simple carbon as the feedstock.

C H O H O C H O C

n m p a b c

liquid

x y z

gas solid

Heat C+ ⇒ + +

∑ ∑ ∑

(1.3)

1.6.2 Combustion of Carbon

When 1 kmol of carbon is burnt completely in adequate air or oxygen, it pro-

duces 394

MJ heat and carbon dioxide. This is a combustion reaction. The

positive sign on the right side (+Q kJ/kmol) implies that heat is absorbed in

the reaction. A negative sign (−Q kJ/kmol) means that heat is absorbed in the

reaction.

C O CO kJ kmol+ → −

2 2

393 770, (1.4)

1.6.3 gasiﬁcation of Carbon

Instead of burning it entirely, we can gasify the carbon by restricting the oxygen

supply. The carbon then produces 72% less heat than that in combustion, but

the partial gasiﬁcation reaction shown here produces a combustible gas, CO.

C O CO kJ kmol+ → −1 2 110 530

, (1.5)

When the gasiﬁcation product, CO, subsequently burns in adequate oxygen,

it produces the remaining 72% (283 MJ) of the heat. Thus, the CO retains only

72% of the energy of the carbon, but in complete gasiﬁcation the energy recov-

ery is 75 to 88% owing to the presence of hydrogen and other hydrocarbons.

The producer gas reaction is an endothermic gasiﬁcation reaction, which

produces hydrogen and carbon monoxide from carbon. This product gas mixture

is also known as synthesis gas, or syngas.

Producer gas reaction C H O CO H kJ kmol: ,+ → + +

2 2

131 000 (1.6)

Production of heavy oil residue in oil reﬁneries is an important application

of gasiﬁcation. Low-hydrogen residues are gasiﬁed into hydrogen.

Heavy oil gasification C H O CO H:

n m

n n m+

( )

→ +

( )

2 2

(1.7)

This hydrogen can be used for hydrocracking of other heavy oil fractions into

lighter oils.

The reaction between steam and carbon monoxide is also used for maximi-

zation of hydrogen production in the gasiﬁcation process at the expense of CO.

Shift reaction CO H O H CO kJ kmol: ,+ → + −

2 2 2

41 000 (1.8)