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2 Chemical Composition of Biomass

2 Chemical Composition of Biomass

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293



Biomass

Cellobiose

H HO





O

O



O



H



H



H



H



H



O H



H O



HO



O H



O



O



H O



HO



O



O



H



H O



H



OH



H O



O



HO



H



H







O

O



H



n



D-glucose



FIGURE 8.4  Cellulose. The dashed lines represent hydrogen bonds.



In contrast to cellulose, hemicellulose (occasionally referred to as polyose,

Figure 8.5) is a diverse polymer made up of a variety of C5 (pentose) and C6 (hexose) sugar units, several of which are acetylated (see Figure 8.6). This branched

heteropolysaccharide is tightly bound to the cellulose bundles through hydrogen

bonding and makes up about 25–33% of most plant materials (Cai and Paszner

1988) (Sun et  al. 2004). Hemicellulose is much smaller than cellulose, with a

degree of polymerization of 80–200 and a relatively low Mw of ≈3 × 104. The general nature of the hemicellulose structure depends on the type of plant with the

result that certain types of lignocellulosic materials are easier to hydrolyze than

others (Agbor et al. 2011).

Lignin (Figure 8.7), like hemicellulose, is a diverse conglomerate of a variety of

polyphenols that is found predominantly in the cell walls of woody plants. Lignin is

embedded within the hemicellulose to provide additional rigidity to the plant. Woody

plants are typified by these cellulosic fibers, whereas the fibers in herbaceous plants are

more loosely bound, indicating a lower proportion of lignin. Lignin is less polar than

cellulose or hemicellulose and is fairly impervious, protecting the polysaccharides

from microbial degradation (Vanholme et  al. 2010). Lignin is biosynthesized from

three basic cinnamyl alcohol units (Figure 8.7) and contains numerous ether linkages,



HO



H O



O



HO



H O

O

HO



O

H3C



CH3O

HO



CH3



HO

O



O

H



HO



H



O



OH



O



O



OH

CH3

O



O



OH

HO

HO



HO



O

OH

H



O



O

OH



O



H O

O



O



O

HO



HO

O



CO2H

O



OH



OH



CH3



FIGURE 8.5  A representative portion of hemicellulose.



HO



O

O

O



294



Chemistry of Sustainable Energy

OH OH



H OH

H



H OH

H



O



OH O



O



H



HO

HO



HO



H



H

H



H



H



OH



Glucose



HO



OH



H



OH



HO



OH



H

HO2C

O



O



H



HO

OH



HO



OH



OH



H



O



HO

OH



HO

OH



H



OH

H



OH

Mannose



HO

H



H



H



Galactose



H



H



H



Xylose



H



H



HO



H



Arabinose



H



Glucoruonic acid



FIGURE 8.6  Structures of the primary sugar components in hemicellulose.



CH2OH



HO



OCH3



OH



Trans-p-coumaryl alcohol



O



CH3O

O



OCH3

HO



O



HO



CH3O



OH



O



CH3O



HOCH2

O



O

OH



OCH3

H3CO

OH



OH



CH3O



HO



HO



CH2OH

HO



OH

OCH3

Trans-sinapyl alcohol



HOCH2

O



Trans-coniferyl alcohol



CH3O



OCH3

O



FIGURE 8.7  Lignin and its constituents.



CH2OH



OCH3



295



Biomass



TABLE 8.1

Typical Percent Composition of Inorganic Elements in Plant

Biomass

Element

Potassium

Sodium

Phosphorus

Calcium

Magnesium



Percent (Dry Matter)

0.1

0.015

0.02

0.2

0.04



Source: Reprinted with permission from Mohan, D., C.U. Pittman, and P.H. Steele.

Pyrolysis of wood/biomass for bio-oil: A critical review. Energy Fuels 20.

p. 854. Copyright 2006, American Chemical Society.



resulting in a polymer with only about 120 phenolic units and an Mw of roughly 20,000

(Lange 2007). When put together in a plant, lignin, cellulose, and hemicellulose knit

together a lignin–carbohydrate complex in which the lignin is bonded to hemicellulose, and hemicellulose bonded to cellulose through extensive hydrogen bonding.

Biomass, while largely organic (in the chemical sense), does contain trace

amounts of inorganics as well. Silicon, aluminum, titanium, iron, calcium, magnesium, sodium, potassium, phosphorus, sulfur, and chlorine may all be present in

the biomass and at different levels depending upon both the environment and the

genome; Table 8.1 gives some typical values. Chloride, in particular, can pose problems as thermochemical conversion processes can convert Cl into HCl, although all

these contaminants present their own unique challenges depending upon the process.

A detailed analysis of the chemical composition of a wide variety of plant materials and their corresponding value as potential feedstocks for biofuel production has

recently been published (Godin et al. 2013).



8.3  REACTIVITY AND CONVERSION OPTIONS

8.3.1  Conversion Options

There are essentially three general methods for converting biomass into energy/fuels:

thermochemical, biochemical, and mechanical. Table 8.2 provides a very general

overview of these processes. The thermochemical processes—pyrolysis, gasification,

and combustion—are generally very fast (seconds to minutes) but nondiscriminating,

often producing a mixture of products whose composition varies depending upon

the particular feedstock and conditions. On the other hand, biochemical methods are

relatively slow, taking days or weeks but usually providing 1–2 major product(s).

The particular conversion option for which the biomass is best suited depends on the

physical and chemical properties of the material. Sewage sludge is more appropriate for digestion than for, say, gasification, due to its moisture content. Each unique

method is examined in detail later in this chapter.



296



Chemistry of Sustainable Energy



TABLE 8.2

Biomass Conversion Options

Type of Conversion

Thermochemical



Method

Combustion (excess air)



Applications

Heat

Steam → boiler → electricity



Gasification (partial air)



Heat (CHP)

Gas turbine → electricity

Syngas → chemicals, fuels



Biochemical

Mechanical



Pyrolysis (no air)



Heat, fuel



Digestion



Biogas → electricity



Fermentation



Fuel (ethanol)



Extraction



Fuel (biodiesel)



Source: Adapted from Turkenburg, W.D. (Ed.), 2000. Renewable energy technologies. In World Energy

Assessment: Energy and the Challenge of Sustainability, edited by J. Goldemberg, New York:

UNDP/UN-DESA/World Energy Council, Chapter 7, Figure 7.1, p. 223.



8.3.2  General Reactivity Patterns

Given the complexity of the various materials being used as feedstocks, the reactions

involved in biomass energy conversions are far ranging and complex. This diversity

is amplified by the huge variability in process conditions, from gas phase to aqueous

phase, room temperature to thousands of degrees, and so on. Nevertheless, some

general patterns emerge and it is worthwhile to engage in an overview of the reactivity of some typical biomass functionality here.

Understandably, sugars and their derivatives (including the celluloses shown

above) play a large role in biomass energy conversion. A simple carbohydrate, such

as sucrose, can be considered to be a “polyhydroxyaldehyde,” a name that is revealing: the hydroxyaldehyde (or hydroxyketone) open-chain form of the sugar is in

equilibrium with the hemiacetal (or hemiketal) closed form (Figures 8.8 and 8.9).

The transformations associated with most biomass energy conversion processes

are directly related to the presence of the carbonyl and hydroxyl functionality. For

example, aldol (Figure 8.10) and conjugate additions (Figure 8.11a and b)—as well

as their associated retro reactions—are commonplace and can lead to unwanted side

products and degraded products.

Biomass energy conversions are not limited to carbohydrates, however. Plant and

animal biomass also contain large amounts of oils (lipids) and proteins. The central

reaction relevant to these biomass components is hydrolysis, as shown in Figure 8.12.

As we will see later in this chapter, these reactions can be readily accomplished biochemically by enzymatic catalysis.

Because of the harsh operating conditions of thermochemical conversions (gasification and pyrolysis), reactivity is much more erratic. Hemicellulose, cellulose,



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