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

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Chapter

1 Introduction

1.6.4 syngas Production

Syngas is also produced from natural gas (>80% CH

), using a steam-methane-

reforming reaction, instead of from solid carbonaceous fuel alone. The reform-

ing reaction is, however, not gasiﬁcation but a molecular rearrangement.

Steam reforming reaction CH H O catalyst CO H

4 2 2

206 000

( )

→ +

+ JJ kmol

(1.9)

Partial oxidation of natural gas or methane is an alternative route for produc-

tion of syngas. In contrast to the reforming reaction, partial oxidation is exo-

thermic. Partial oxidation of fuel oil also produces syngas.

Partial oxidation reforming CH O CO H

kJ kmol

4 2 2

1 2 2

36 000

+ → +

−

(1.10)

The hydrogen may be used as fuel in fuel cells or in production of chemical

feedstocks like methanol and ammonia.

1.6.5 methanol synthesis

Syngas provides the feedstock for many chemical reactions, including methanol

synthesis (Eq 1.11). Methanol (CH

OH) is a basic building block of many

products, including gasoline.

CO H catalysts CH OH+

( )

→2

2 3

(1.11)

1.6.6 ammonia synthesis

Ammonia (NH

) is an important feedstock for fertilizer production. It is pro-

duced from pure hydrogen and nitrogen from air.

3 2 92 000

2 2 3

H N catalysts NH kJ kmol+

( )

→ − , (1.12)

1.6.7 Fischer-tropsch reaction

The Fischer-Tropsch synthesis reaction can synthesize a mixture of CO and H

into a range of hydrocarbons, including diesel oil.

2 1

2 2 2 2

n n n

n n

( )

→ +

( )

H CO catalyst C H H O

(1.13)

Here, C

(2n+2)

represents a mixture of hydrocarbons ranging from methane and

gasoline to wax. Its relative distribution depends on the catalyst, the tempera-

ture, and the pressure chosen for the reaction.

1.6.8 methanation reaction

Methane (CH

), an important ingredient in the chemical and petrochemical

industries, can come from natural gas as well as from solid hydrocarbons like

1.6 Brief Description of Gasiﬁcation and Related Processes

biomass or coal. For the latter source, the hydrocarbon is hydrogenated to

produce synthetic gas, or substitute natural gas (SNG). The overall reaction

for SNG production may be expressed as

C H CH kJ kmol+ → −2 74 800

2 4

, (1.14)

More details on these reactions are given in Chapters 5 and 9.

symbols and nomenclature

LHV = lower heating value of gas (kJ/mol)

= combustion efﬁciency (–)

= steam turbine efﬁciency (–)

= efﬁciency of combustion and steam turbine (–)

= gas turbine efﬁciency (–)

= combined efﬁciency of steam turbine and gas turbine (–)

= overall efﬁciency, including gasiﬁcation efﬁciency (–)

= inlet steam temperature in the steam turbine (K)

= exhaust steam temperature in the steam turbine (K)

= inlet temperature in the gas turbine (K)

= exhaust temperature in the gas turbine (K)

Biomass Characteristics

Chapter 2

Biomass Gasiﬁcation and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00002-7

2.1 IntroductIon

The characteristics of biomass greatly inﬂuence the performance of a biomass

gasiﬁer. A proper understanding of the physical and the chemical properties of

biomass feedstock is essential for the design of a biomass gasiﬁer to be reliable.

This chapter discusses some important properties of biomass that are relevant

to gasiﬁcation and related processes.

2.2 What Is BIomass?

Biomass refers to any organic materials that are derived from plants or animals

(Loppinet-Serani et

al., 2008). A generally accepted deﬁnition is difﬁcult to

ﬁnd. However, the one used by the United Nations Framework Convention on

Climate Change (UNFCCC, 2005) is relevant here:

[A] non-fossilized and biodegradable organic material originating from plants, animals

and micro-organisms. This shall also include products, by-products, residues and waste

from agriculture, forestry and related industries as well as the non-fossilized and bio-

degradable organic fractions of industrial and municipal wastes.

Biomass also includes gases and liquids recovered from the decomposition

of nonfossilized and biodegradable organic materials. In the United States,

there has been much debate on a legal deﬁnition. Appendix A gives a recent

legal interpretation of renewable biomass.

As a sustainable and renewable energy resource, biomass is constantly being

formed by the interaction of CO

, air, water, soil, and sunlight with plants

and animals. After an organism dies, microorganisms break down biomass into

elementary constituent parts like H

O, CO

, and its potential energy. Because

the carbon dioxide, a biomass releases through the action of microorganisms

or combustion was absorbed by it in the recent past, biomass combustion does

not increase the total CO

inventory of the Earth. It is thus called greenhouse

gas neutral or GHG neutral.

chapter

2 Biomass Characteristics

Biomass includes only living and recently dead biological species that can

be used as fuel or in chemical production. It does not include organic materials

that over many millions of years have been transformed by geological processes

into substances such as coal or petroleum. Biomass comes from botanical (plant

species) or biological (animal waste or carcass) sources, or from a combination

of these. Common sources of biomass are:

 Agricultural: food grain, bagasse (crushed sugarcane), corn stalks, straw,

seed hulls, nutshells, and manure from cattle, poultry, and hogs

 Forest: trees, wood waste, wood or bark, sawdust, timber slash, and mill

scrap

 Municipal: sewage sludge, refuse-derived fuel (RDF), food waste, waste

paper, and yard clippings

 Energy: poplars, willows, switchgrass, alfalfa, prairie bluestem, corn, and

soybean, canola, and other plant oils

 Biological: animal waste, aquatic species, biological waste

2.2.1 Biomass Formation

Botanical biomass is formed through conversion of carbon dioxide (CO

) in

the atmosphere into carbohydrate by the sun’s energy in the presence of chlo-

rophyll and water. Biological species grow by consuming botanical or other

biological species.

Plants absorb solar energy in a process called photosynthesis (Figure 2.1).

In the presence of sunlight of particular wavelengths, green plants break down

water to obtain electrons and protons and use them to turn CO

into glucose

FIGurE 2.1 Biomass grows by absorbing solar energy, carbon dioxide, and water through

photosynthesis.

2.2 What Is Biomass?

(represented by CH

), releasing O

as a waste product. The process may be

represented by this equation (Hodge, 2010, p. 297):

Living plant CO H O Sunlight CH O

Chlorophyll

+ + +  →

( )

+ −

2 2

m n

00 kJ mol

(2.1)

For every mole of CO

absorbed into carbohydrate or glucose in biomass,

1 mole of oxygen is released. This oxygen comes from water the plant takes

from the ground or the atmosphere (Klass, 1998, p. 30). The chlorophyll pro-

motes the absorption of carbon dioxide from the atmosphere, adding to the

growth of the plant. Important ingredients for the growth of biomass are:

 Living plant

 Visible spectrum of solar radiation

 Carbon dioxide

 Chlorophyll (serving as catalyst)

 Water

The chemical energy stored in plants is then passed on to the animals and to

the human that take the plants as food. Animal and human waste also contribute

to biomass.

2.2.2 types of Biomass

Biomass comes from a variety of sources as shown in Table 2.1. Loosely speak-

ing, these sources include all plants and plant-derived materials, including

TABLE 2.1 Two Major Groups of Biomass and Their

Subclassiﬁcations

Virgin Terrestrial biomass Forest biomass

Grasses

Energy crops

Cultivated crops

Aquatic biomass Algae

Water plant

Waste Municipal waste Municipal solid waste

Biosolids, sewage

Landﬁll gas

Agricultural solid waste Livestock and manures

Agricultural crop residue

Forestry residues Bark, leaves, ﬂoor residues

Industrial wastes Demolition wood, sawdust

Waste oil or fat

chapter

2 Biomass Characteristics

livestock manures. Primary or virgin biomass comes directly from plants or

animals. Waste or derived biomass comes from different biomass-derived prod-

ucts. Table 2.1 lists a range of biomass types, grouping them as virgin or waste.

Biomass may also be divided into two broad groups:

 Virgin biomass includes wood, plants, and leaves (ligno-cellulose); and

crops and vegetables (carbohydrates).

 Waste includes solid and liquid wastes (municipal solid waste (MSW));

sewage, animal, and human waste; gases derived from landﬁlling (mainly

methane); and agricultural wastes.

Ligno-Cellulosic Biomass

A major part of biomass is ligno-cellulose, so this type is described in some

detail. Ligno-cellulosic material is the nonstarch, ﬁbrous part of plant materials.

Cellulose, hemicellulose, and lignin are its three major constituents. Unlike

carbohydrate or starch, ligno-cellulose is not easily digestible by humans. For

example, we can eat the rice, which is a carbohydrate, but we cannot digest the

husk or the straw, which are ligno-cellulose. Ligno-cellulosic biomass is not

part of the human food chain, and therefore its use for biogas or bio-oil does

not threaten the world’s food supply.

A good example of ligno-cellulosic biomass is a woody plant—that is, any

vascular plant that has a perennial stem above the ground and is covered by a

layer of thickened bark. Such biomass is primarily composed of structures of

cellulose and lignin. A detailed description of wood structure is given in Section

2.3.1. Woody plants include trees, shrubs, cactus, and perennial vines. They

can be of two types: (1) herbaceous and (2) nonherbaceous.

An herbaceous plant is one with leaves and stems that die annually at the

end of the growing season. Wheat and rice are examples of herbaceous plants

that develop hard stems with vascular bundles. Herbaceous plants do not have

the thick bark that covers nonherbaceous biomass like trees.

Nonherbaceous plants are not seasonal; they live year-round with their

stems above the ground. Large trees fall in this category. Nonherbaceous peren-

nials like woody plants have stems above ground that remain alive during the

dormant season, and grow shoots the next year from their above-ground parts.

These include trees, shrubs, and vines.

The trunk and leaves of tree plants form the largest group of available

biomass. These are classiﬁed as ligno-cellulosic, as their dominant constituents

are cellulose, hemicellulose, and lignin. Table 2.2 shows the distribution of

these components in some plants. Section 2.3.2 presents further discussions of

ligno-cellulose components.

There is a growing interest in the cultivation of plants exclusively for pro-

duction of energy. These crops are ligno-cellulosic. They typically have a short

growing period and high yields, and require little or no fertilizer, so they

provide quick return on investment. Energy crops are densely planted. For

2.2 What Is Biomass?

energy production, woody crops such as Miscanthus, willow, switchgrass, and

poplar are widely utilized. These plants have high energy yield per unit of land

area and require much less energy for cultivation.

Crops and Vegetables

While the body of a plant or tree (trunk, branches, leaves, etc.) is ligno-

cellulosic, the fruit (cereal, vegetable) is a source of carbohydrate, starch, and

sugar. Some plants like canola also provide fat. The fruit is digestible by humans,

but the ligno-cellulosic body is not. (Because of special chemicals in their

stomach, some animals can digest ligno-cellulosic biomass.) Because they serve

as human food, the use of crops or vegetables for the production of chemicals and

energy must be weighed carefully as it might affect food supplies.

Compared to ligno-cellulosic compounds, carbohydrates are easier to dis-

solve, so it is relatively easy to derive liquid fuels from them through fermenta-

tion or other processes. For this reason, most commercial ethanol plants use

crops as feedstock. There are two types of crop biomass: (1) the regularly

harvested agricultural crops for food production and (2) the energy crops for

energy production.

Natural crops and vegetables are a good source of starch and sugars and

can be hydrated. Some vegetables and crops (coconut, sunﬂower, mustard,

canola, etc.) contain fat, providing a good source of vegetable oil. Animal waste

(from land and marine mammals) also provides fat that can be transformed into

bio-oil. If carbohydrate is desired for the production of biogas, whole crops,

such as maize, Sudan grass, millet, and white sweet clover, can be made into

silage and then converted into biogas.

Wastes

Wastes are secondary biomass, as they are derived from primary biomass (trees,

vegetables, meat) during different stages of their production or use. MSW is

an important source of waste biomass, and much of it comes from renewables

TABLE 2.2 Composition of Some Ligno-Cellulosic Wood

Plant Lignin (%) Cellulose (%) Hemicellulose (%)

Deciduous 18–25 40–44 15–35

Coniferous 25–35 40–44 20–32

Willow 25 50 19

Larch 35 26 27

Source: Adapted from Bergman et al., 2005, p. 15.

chapter

2 Biomass Characteristics

Trash

Methane gas

recovery

system

Clay cap

Leachate collection

system

Landfill

liner

FIG

urE

2.2

Anaerobic digestion of biodegradable waste.

TABLE 2.3 Typical Composition of Some Waste Biomass

Biomass

Moisture

(wt. %)

Organic Matter

(dry wt. %)

Ash (dry

wt. %)

Higher Heating

Value (MJ/dry kg)

Cattle manure 20–70 76.5 23.5 13.4

Sewage 90–98 73.5 26.5 19.9

RDF 15–30 86.1 13.9 12.7

Sawdust 15–60 99.0 1.0 20.5

Source: Adapted from Klass, 1998, p. 73.

like food scraps, lawn clippings, leaves, and papers. Nonrenewable components

of MSW like plastics, glass, and metals are not considered biomass. The com-

bustible part of MSW is at times separated and sold as refuse-derived fuel

(RDF). Sewage sludge that contains human excreta, fat, grease, and food wastes

is an important biomass source. Another waste is sawdust, produced in sawmills

during the production of lumber from wood. Table 2.3 lists the composition

and heating values of some waste biomass products.

Landﬁlls have traditionally been an important means of disposing of

garbage. A designated area is ﬁlled with waste, which decomposes, producing

methane gas. Modern landﬁlling involves careful lining of the containment cell

(Figure 2.2) so that leached liquids can be collected and treated instead of

leaking into groundwater. The containment cells are covered with clay or earth

to avoid exposure to wind and rain.

2.3 Structure of Biomass

An increasing number of municipalities are separating biodegradable wastes

and subjecting them to digestion for degradation. This avoids disposal of leach-

ate and reduces the volume of waste. Two types of waste degradation are used:

aerobic digestion and anaerobic digestion.

Aerobic digestion: This process takes place in the presence of air and so

does not produce fuel gas. Here, the leachate is removed from the bottom

layer of the landﬁll and pumped back into the landﬁll, where it ﬂows over

the waste repeatedly. Air added to the landﬁll enables microorganisms to

work faster to degrade the wastes into compost, carbon dioxide, and water.

Since it does not produce methane, aerobic digestion is most widely used

where there is no additional need for landﬁll gas.

Anaerobic digestion: This process does not use air and hence produces the

fuel gas methane. Here, the land-ﬁlled solids are sealed against contact

with the atmosphere oxygen. The leachate is collected and pumped back

into the landﬁll as in aerobic digestion (Figure 2.2). Additional liquids may

be added to the leachate to help biodegradation of the waste. In the absence

of oxygen, the waste is broken down into methane, carbon dioxide, and

digestate (or solid residues). Methanogenesis bacteria like thermophiles

(45–65 °C), mesophiles (20–45 °C), and psychophiles (0–20 °C) facilitate

this process (Probstein and Hicks, 2006). These biodegradation reactions

are mildly exothermic. The process is represented by Eq. (2.2):

C H O representing wastes bacteria CO

CH digestate

6 12 6 2

( )

+ →

+ +

(2.2)

Methane is an important constituent of landﬁll gas. A powerful greenhouse

gas (∼ 21 times stronger than CO

), it is often burnt in a ﬂare or utilized in a

gas engine or in similar energy applications. Anaerobic digestion is very popular

in farming communities, where animal excreta are collected and stored because

the gas produced can be collected in a gas holder for use in cooking and heating

while the residual solid can be used as fertilizer.

2.3 structurE oF BIomass

Biomass is a complex mixture of organic materials such as carbohydrates, fats,

and proteins, along with small amounts of minerals such as sodium, phospho-

rus, calcium, and iron. The main components of plant biomass are extractives,

ﬁber or cell wall components, and ash (Figure 2.3).

Extractives: Substances present in vegetable or animal tissue that can

be separated by successive treatment with solvents and recovered by evap

oration of the solution. They include protein, oil, starch, sugar, and so on.

Cell wall: Provides structural strength to the plant, allowing it to stand

tall above the ground without support. A typical cell wall is made of

chapter

2 Biomass Characteristics

carbohydrates and lignin. Carbohydrates are mainly cellulose or hemicel-

lulose ﬁbers, which impart strength to the plant structure; the lignin holds

the ﬁbers together. These constituents vary with plant type. Some plants,

such as corn, soybeans, and potatoes, also store starch (another carbohydrate

polymer) and fats as sources of energy, mainly in seeds and roots.

Ash: The inorganic component of the biomass.

Wood and its residues are the dominant form of the biomass resource base.

A detailed discussion of wood-derived biomass is presented next.

2.3.1 structure of Wood

Wood is typically made of hollow, elongated, spindle-shaped cells arranged

parallel to each other. Figure 2.4 is a photograph of the cross-section of a tree

trunk showing the overall structure of a mature tree wood.

Outer

dead bark

Inner

live bark

Sapwood

Heartwood

Wood rays

FIGurE 2.4 Cross-section of a tree trunk showing outer dead bark, inner live bark, sapwood,

heartwood, and wood rays. (Source: Photograph by author.)

Components of

wood biomass

Extractives Ash

Cellulose

Lignin

Hemicellulose

Cell wall

components

FIGurE 2.3 Major constituents of a woody biomass.