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

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chapter

8 Biomass Handling

system (especially with moist fuels), but it does achieve high char conversion

efﬁciency.

Fuel entering at a feed point disperses over a much smaller area than it does

in overbed feeding, so the feed points are more numerous and more closely

spaced. Spacing greatly affects gasiﬁcation. Because a deeper bed allows wider

dispersion of the fuel and hence works with wider feed-point spacing, increased

spacing with no sacriﬁce of char conversion efﬁciency can be achieved, but

only if it is compensated by a corresponding increase in bed height. A decrease

in bed height must be matched by increased feed-point spacing; otherwise, the

conversion efﬁciency can drop. Coarser particles take longer to gasify and are

less prone to entrainment. Therefore, wider spacing is preferred for them; ﬁner

particles require closer spacing.

The freeboard can provide room for further reaction of particles entrained

from the bed. Freeboard design is important, especially when wide feed-point

spacing is used.

Feed-Point Allocation

The excellent solids–solids mixing in a ﬂuidized bed helps disperse the fuel

over a small bed area of 1 to 2

. A single feeder is adequate for a small bed

having a cross-sectional size of less than 2

. Larger beds need multiple

feeders. The number required for a given bed depends on factors such as quality

of fuel, type of feeding system, amount of fuel input, and bed area. Highly

reactive fuels with high volatiles need a larger number of feed injection points

because they react relatively fast; less reactive fuels require fewer feed points.

Industrial designs often call for redundancy. For example, if a reactor needs

two overbed feeders, designers will provide three, each with a capacity that is

at least 50% of the design feed rate. In this way, if one feeder is out of service,

the plant can still maintain full output on the other two. The number of redun-

dant feeders depends on the capacity and reliability required of the plant.

symbols and nomenclature

A = area of the cross-sectional area of silo (m

)

B = parameter, depending on the silo (m)

C = parameter, depending on the silo (–)

= particle size (m)

D = diameter of the silo (m)

= diameter of the screw (m)

= shaft diameter (m)

dh = height of a differential element in the silo (m)

g = acceleration due to gravity (9.81

m/s

)

H = height of the silo (m)

= wall friction coefﬁcient (–)

K = Janssen coefﬁcient (–)

= a constant, depending on D

or P/D

(–)

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8.4 Biomass Feeders

m = mass-ﬂow rate (kg/s)

P = pitch of the screw (m)

= normal pressure on the wall in the silo (Pa)

= vertical pressure on the biomass in the silo (Pa)

= pressure at the base of the silo (Pa)

T = torque of the screw (Nm)

= average solid velocity through outlet (m/s)

τ = wall friction (Pa)

ρ, ρ

= density of solids (kg/m

)

= density of air (kg/m

)

= vertical stress for the ﬂow (Pa)

µ = viscosity of air (kg/m.s)

θ = semi-included angle of hopper (degree)

Production of Synthetic Fuels

and Chemicals from Biomass

Chapter 9

Biomass Gasiﬁcation and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00009-X

301

9.1 IntroductIon

Earlier chapters discussed methods of converting solid feedstock into gases and

their use in energy production. Besides energy production, gasiﬁcation and

pyrolysis have important application in the production of chemicals and trans-

port fuels. Many of our daily necessities like plastic, resin, and fertilizer can

potentially come from biomass.

Syngas, a mixture of H

and CO, is an important product of gasiﬁcation. It

is a fuel as well as a basic building block for many hydrocarbons. Transport

fuel and a large number of chemicals are produced from different syntheses of

CO and H

. These products can be divided into three broad groups: (1) energy

feedstock (e.g., methane, carbon monoxide); (2) transportation fuels (e.g.,

biodiesel, biogas); and (3) chemical feedstock (e.g., methanol, ammonia). Pres-

ently, syngas is produced primarily from natural gas, but it can also be produced

from

 Biomass

 Solid fossil fuels (e.g., coal, petcoke)

 Liquid fuels (e.g., reﬁnery wastes)

Interest in biomass as a chemical feedstock is rising given that it is renew-

able and carbon-neutral. There is a growing shift toward “green chemicals”

and “green fuels,” which are derived from carbon-neutral biomass. Gasiﬁcation

and pyrolysis are effective and powerful ways to convert the biomass (or

another fuel) into energy, chemicals, and transport fuels. This chapter discusses

different ways to convert biomass-derived syngas into such useful products.

9.2 SyngaS

The following sections discuss syngas—its physical properties and uses, as well

as its production and its cleaning and conversion.

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9 Production of Synthetic Fuels and Chemicals from Biomass

9.2.1 What Is Syngas?

As mentioned earlier, syngas is a mixture of hydrogen and carbon monoxide

gases. It should not be confused with SNG (the abbreviation for “synthetic (or

substitute) natural gas”), which is primarily made of methane gas. Syngas is an

important feedstock for the chemical and energy industries. A large number of

hydrocarbons traditionally produced from petroleum oil can also be produced

from syngas.

Syngas may be produced from many hydrocarbons, including coal and

petroleum coke, as well as from biomass. To distinguish syngas generated from

biomass from that produced from fossil fuel, the former is sometimes called

biosyngas. Here, syngas implies that derived from biomass unless speciﬁed

otherwise.

One of the major applications of syngas is the production of liquid transport

fuel. South African Synthetic Oil Limited (SASOL) has for many years been

producing a large amount of liquid fuel from coal using Fischer-Tropsch syn-

thesis of syngas produced from the gasiﬁcation of coal. The same liquid fuel

may be produced from biomass-derived syngas.

The typical product gas of biomass gasiﬁcation contains hydrogen, mois-

ture, carbon monoxide, carbon dioxide, methane, aliphatic hydrocarbons,

benzene, and toluene, as well as small amounts of ammonia, hydrochloric acid,

and hydrogen sulﬁde. From this mixture, the carbon monoxide and hydrogen

must be separated to produce syngas.

9.2.2 applications for Syngas

As mentioned, syngas is an important source of valuable chemicals. These

include

 Hydrogen, produced in reﬁneries

 Diesel gasoline, using Fischer-Tropsch synthesis

 Fertilizer, through ammonia

 Methanol, for the chemical industry

 Electricity, generated through combustion

It should be noted that a major fraction of the ammonia used for fertilizer

production comes from syngas and nitrogen (see Section 9.4.3).

9.2.3 Production of Syngas

Gasiﬁcation is the preferred route for the production of syngas from coal or

biomass. The low price of natural gas is currently encouraging syngas produc-

tion from it, but the situation may change when the price rises. A steam refor-

mation reaction is used to produce syngas from natural gas that is mainly CH

This reaction is also the most widely used commercial method for bulk produc-

tion of hydrogen, which is one of the two components of syngas.

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9.2 Syngas

In the steam reforming method, natural gas (CH

) reacts with steam at high

temperatures (700–1100 °C) in the presence of a metal-based catalyst (nickel).

CH H O CO H kJ mol

Catalyst

4 2 2

3 206+  → + + (9.1)

If hydrogen production is the main goal, the carbon monoxide produced is

further subjected to a shift reaction (see Eq. 9.2), as described in the next

section, to produce additional hydrogen and carbon dioxide.

The ratio of hydrogen and carbon monoxide in the gasiﬁcation product gas

is a critical parameter in the synthesis of the reactant gases into desired products

such as gasoline, methanol, and methane. The product desired determines that

ratio. For example, gasoline may need the H

/CO ratio to be 0.5 to 1.0, while

methanol may need it to be ~2.0 (Probstein and Hicks, 2006, p. 124). In a com-

mercial gasiﬁer the H

/CO ratio of the product gas is typically less than 1.0, so

the shift reaction is necessary to increase this ratio by increasing the hydrogen

content at the expense of CO. The shift reaction often takes place in a separate

reactor, as the temperature and other conditions in the main gasiﬁer may not

be conducive to it.

9.2.4 gasiﬁcation for Syngas Production

The two main routes for production of syngas from biomass or fossil fuel are

low-temperature (~<1000 °C) and high-temperature gasiﬁcation (~>1200 °C).

Low-temperature gasiﬁcation is typically carried out at temperatures below

1000 °C. In most low-temperature gasiﬁers, the gasifying medium is air, which

introduces undesired nitrogen in the gas. To avoid this, gasiﬁcation can be

carried out indirectly by one of the following means:

 An oxygen carrier (metal oxide) is used to transfer the oxygen from an air

oxidizer to another reactor, where gasiﬁcation takes place using the oxygen

from the metal oxide.

 A combustion reaction in air is carried out in one reactor and heat-carrier

solids carry the heat to a second reactor, where this heat is then used in

gasiﬁcation.

 Dilution of the product gas by nitrogen is avoided by the use of steam or

oxygen as the gasifying medium.

Low-temperature gasiﬁcation produces a number of heavier hydrocarbons

along with carbon monoxide and hydrogen. These heavier hydrocarbons are

further cracked, separated, and used for other applications. High-temperature

gasiﬁcation is carried out at temperatures above 1200 °C, where biomass is

converted mainly into hydrogen and carbon monoxide. Primary gasiﬁcation is

often followed by the shift reaction, as described in the next section, to adjust

the hydrogen-to-carbon monoxide ratio to suit the downstream application.

In any case, the product gas must be cleaned before it is used for synthesis

reactions. Special attention must be paid to clean the syngas of tar and other

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9 Production of Synthetic Fuels and Chemicals from Biomass

catalyst-poisoning elements before it is used for Fischer-Tropsch synthesis,

which uses iron- or cobalt-based catalysts.

Shift Reaction

For a reaction like Fischer-Tropsch synthesis that produces various gaseous and

liquid hydrocarbons, a deﬁnite molar ratio of CO and H

in the syngas is neces-

sary. This is done through the shift reaction that converts excess carbon mon-

oxide into hydrogen:

CO H O CO H kJ mol

Catalyst

+  → + −

2 2 2

41 1. (9.2)

The reaction can be carried out either at higher temperatures (400–500 °C)

or at lower temperatures (200–400 °C). For high temperatures, the shift reaction

is often catalyzed using oxides of iron and chromium; it is equilibrium limited.

At low temperatures, the shift reaction is kinetically limited; the catalyst is

composed of copper, zinc oxide, and alumina, which help reduce the CO con-

centration down to about 1%.

9.2.5 cleaning and conditioning of Syngas

For synthesis reaction, a high degree of gas purity is needed, so the gas must

be cleaned of particulates and other contaminating gases. The raw syngas may

contain three principal types of impurity: (1) solid particulates (unconverted

char, ash); (2) inorganic impurities (halides, alkali, sulfur compounds, nitro-

gen); and (3) organic impurities (tar, aromatics, carbon dioxide).

At high temperatures, the equilibrium shifts toward hydrogen-producing

hydrogen-rich gas. The ash in the biomass appears as slag. At low temperatures,

the ash remains in the product gas as dry ash. Cleaning has two aspects: remov-

ing undesired impurities and conditioning the gas to get the right ratio of H

and CO for the intended use. This use determines the level of cleaning and

conditioning. Table 9.1 presents examples of product-gas speciﬁcations for

different end uses.

Cleanup Options

For cleaning the gas of dust or particulates, there are four options: (1) cyclone,

(2) fabric or other barrier ﬁlter, (3) electrostatic ﬁlter, and (4) solvent scrubber.

Among organic impurities, tar is the most undesirable. The three main options

for tar removal are

 Scrubbing with an organic liquid (e.g., methyl ester)

 Catalytic cracking by nickel-based catalysts or olivine sand

 High-temperature cracking

Inorganic impurities are best removed in sequence because some removal

processes produce other components that need to be removed as well. First,

305

9.3 Bio-Oil

water quenching removes char and ash particles. Next, hydrolysis removes

COS and HCN by converting them into H

S and NH

. The ammonia and halides

can be washed with water, followed by adsorption of H

S, which can be

removed with the wash water. Solid or liquid adsorbents are used to remove

carbon dioxide from the product gas.

9.3 BIo-oIl

Bio-oil (or biofuel) is any liquid fuel derived from a recently living organism,

such as plants and their residues or animal extracts. In view of its importance,

a detailed discussion of bio-oil is presented next.

9.3.1 What Is Bio-oil?

Bio-oil is the liquid fraction of the pyrolysis product of biomass. For example,

a fast pyrolyzer typically produces 75% bio-oil, 12% char, and 13% gas. Bio-oil

is a highly oxygenated, free-ﬂowing, dark-brown (nearly black) organic liquid

(Figure 9.1) that contains a large amount of water (~25%) that is partly the

TABLE 9.1 Product-Gas Speciﬁcations for Various Applications

Speciﬁcation

Hydrogen or

Reﬁnery Use

Ammonia

Production

Methanol

Synthesis

Fischer-Tropsch

Synthesis

Hydrogen content

>98%

75% 71% 60%

Carbon monoxide

content

<10–50

ppm(v)

[CO + CO

]

<20

ppm(v)

19% 30%

Carbon dioxide

content

<10–50

ppm(v)

4–8%

Nitrogen content

<2%

25%

Other gases N

, Ar, CH

Ar, CH

, Ar, CH

, CO

Balance As low as

possible

As low as

possible

Low

ratio ~3

/CO ratio 0.6–2.0

/[2CO + 3CO

]

ratio

1.3–1.4

Process

temperature

350–550 °C 300–400 °C 200–350 °C

Process pressure

>50 bar

100–250 bar 50–300 bar 15–60 bar

Source: Adapted from Knoef, 2005, p. 224.

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original moisture in the biomass and partly the reaction product. The composi-

tion of bio-oil depends on the biomass it is made from as well as on the process

used.

Table 9.2 presents the composition of a typical bio-oil. It shows that water,

lignin fragments, carboxylic acids, and carbohydrates constitute its major com-

ponents. When it comes from the liquid yield of pyrolysis, bio-oil is called

pyrolysis oil. Several other terms are often used to describe bio-oil or are asso-

ciated with it, including:

 Tar or pyroligneous tar

 Bio-oil

 Biocrude

 Wood liquid or liquid wood

 Liquid smoke

 Biofuel oil

 Wood distillates

 Pyrolysis oil

 Pyroligneous acids

Note that there is an important difference between pyrolysis oil and biocrude.

The former is obtained via pyrolysis; the latter can be obtained via other

methods such as supercritical gasiﬁcation.

FIgurE 9.1 Bio-oil is a thick, black, tarry liquid.

307

9.3 Bio-Oil

Bio-oil may be seen as a two-phase microemulsion. In the continuous phase

are the decomposition products of hollocellulose; in the discontinuous phase

are the pyrolytic lignin macromolecules. Hollocellulose is the ﬁbrous residue

that remains after the extractives, lignin, and ash-forming elements have been

removed from the biomass. The same as crude petroleum oil, which is extracted

from the ground, pyrolysis liquid and biocrude contain tar as their heaviest

component.

Bio-oil is a class-3 substance falling under the ﬂammable liquid desig

nation in the UN regulations for transport of dangerous goods (Peacocke and

Bridgwater et al., 2001, p. 1485).

9.3.2 Physical Properties of Bio-oil

As we can sense from Figure 9.1, bio-oil is free-ﬂowing. Its low viscosity is

due to its high water content. Also, it has an acrid, smoky smell that can irritate

eyes with long-term exposure. With a speciﬁc gravity of ~1.2, bio-oil is heavier

than water or any oil derived from petroleum. A comparison of its physical

and chemical properties with those of conventional fossil fuels is given in

Table 9.3.

An important feature of bio-oil not reﬂected in Table 9.3 is that some of

its properties change with time. For example, its viscosity increases and its

volatility decreases (Mohan et al., 2006) with time. Some phase separation and

TABLE 9.2 Composition of Bio-Oil

Major Group Compounds Mass (%)

Water 20–30

Lignin fragments Insoluble pyrolytic lignin 15–30

Aldehydes Formaldehyde, acetaldehyde, hydroxyacetaldehyde,

glyoxal, methylglyoxal

10–20

Carboxylic acids Formic, acetic, propionic, butyric, pentanoic,

hexanoic, glycolic

10–15

Carbohydrates

Cellobiosan, α-D-levoglucosan, oligosaccharides,

1.6 anhydroglucofuranose

5–10

Phenols Phenol, cresols, guaiacols, syringols 2–5

Furfurals 1–4

Alcohols Methanol, ethanol 2–5

Ketones Acetol (1-hydroxy-2-propanone), cyclopentanone 1–5

Source: Adapted from Bridgwater et al., 2001, p. 989.

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9 Production of Synthetic Fuels and Chemicals from Biomass

deposition of gums may also occur with time, primarily because of polymeriza-

tion, condensation, esteriﬁcation, and etheriﬁcation. This feature distinguishes

bio-oil from mineral oils, the properties of which do not change with time.

Bio-oil is not soluble in water, although it contains a substantial amount

of water. However, it is miscible in polar solvents, such as methanol and

acetone, but immiscible with petroleum-derived oils. Bio-oil can accept

water up to a maximum limit of 50% (total moisture). Any more water results

in phase separation. Table 9.3 shows that bio-oil has a heating value nearly

half that of conventional liquid fuels but has comparable ﬂash and pour

points.

TABLE 9.3 Comparison of Physical and Chemical Properties of Bio-Oil

and Three Liquid Fuels*

Property Bio-Oil Heating Oil Gasoline Diesel

Heating value (MJ/kg) 18–20 45.5 44

Density at 15 °C (kg/m

) 1200 865 737

820–950

Flash point (°C) 48–55 38 40

Pour point (°C) –15 –6 –60 –29

Viscosity at 40 °C (cP) 40–100

(25% water)

1.8–3.4 cSt 0.37–0.44

2.4

pH 2.0–3.0 –

Solids (% wt)

0.2–1.0 – 0 0

Elemental

analysis

weight)

Carbon 42–47 86.4 84.9 87.4

Hydrogen 6.0–8.0 12.7 14.76 12.1

Nitrogen

<0.1

0.006 0.08 392 ppm

Sulfur

<0.02

0.2–0.7 1.39

Oxygen 46–51 0.04

Ash

<0.02 <0.01

*Except as indicated, all values are excerpted from www.dynamotive.com.

Note: cP—centipoise; cSt—centipoise. Values for gasoline and diesel are for a representative

sample and can vary.

http://www.engineeringtoolbox.com.

Hughey and Henerickson, 2001.

Bridgwater et al., 2001, p. 990.

Mohan et al., 2006.

Maples, 2000.

Sources: