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

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319

9.5 Transport Fuels from Biomass

the requirements of ASTM D 6751.” Its characteristics are similar to those of

petrodiesel, but not identical. Biodiesel, which can be mixed with petrodiesel

for burning in diesel engines, has several positive features for use in engines,

as listed in the following:

 Petrodiesel contains up to 20% polyaromatic hydrocarbon, while biodiesel

contains none, making it safer for storage.

 Biodiesel has a higher ﬂash point, making it safer to handle.

 Being oxygenated, biodiesel is a better lubricant than petrodiesel is and

therefore gives longer engine life.

 Its higher oxygen content allows biodiesel to burn more completely.

Biodiesel Production from Methanol

Biodiesel is generally produced from vegetable oil and/or from animal fats with

major constituents that are triglycerides. It is produced by transesteriﬁcation of

vegetable oil or fat in the presence of a catalyst. Biodiesel carries the name

fatty acid methyl (or ethyl) ester, commonly abbreviated as FAME. A popular

production method involves mixing waste vegetable oil or fat with the catalyst

and methanol (or ethanol) in appropriate proportion. A typical proportion is

87% oil, 1% NaOH catalyst, and 12% alcohol. Both acid and base catalysts can

be used, but the base catalyst NaOH is the most common. Because NaOH is

not recyclable, a “nongreen” feed is required to produce “green” biodiesel.

Efforts are being made to produce recyclable catalysts and thereby make the

product pure “green.”

Figure 9.5 shows the reaction for the conversion of triglyceride into bio-

diesel (FAME) and its by-product, glycerol. Glycerol cannot be used as a

transport fuel, and its disposal is a major issue.

An alternative noncatalytic conversion route for biodiesel is under develop-

ment in which transesteriﬁcation of triglycerides is by supercritical methanol

(above 293 °C, 8.1

MPa) without a catalyst (Kusdiana et al., 2006). The metha-

nol can be recycled and reused, but the process for this must be carried out at

high temperatures and pressures. Efforts are also being made to use woody

biomass (ligno-cellulose) instead of fats or oil to produce biodiesel using the

supercritical method (Minami and Saka, 2006). The reaction is carried out in

1,2,3

Triglyceride Fatty acid

methyl ester

Glycerol

O O

+3CH

HO+

FIgurE 9.5 Diesel (fatty acid methyl ester) production from triglyceride.

320

chapter

9 Production of Synthetic Fuels and Chemicals from Biomass

a ﬁxed or ﬂuidized bed. The ﬂuidized bed has the advantage of continuous

catalyst regeneration and efﬁcient removal of the heat of reaction.

9.5.4 transport Fuel Production from nonfood Biomass

Use of food cereals, such as wheat and corn for the production of biodiesel or

ethanol, has been commercially successful; however, it has had a major impact

on the world’s food market, driving up prices and creating shortages. Alterna-

tive sources of biodiesel are being researched. Instead of sugar beets or rape

seed, cellulosic biomass like wood may be used as the feedstock. With cellu-

losic materials, the industry can signiﬁcantly increase the yield of fuel per unit

of cultivated area.

There are two options for production of ethanol or gasoline from nonfood

sources: thermal and biochemical.

Thermal Process

In the thermal process, cellulosic feedstock is subjected to fast pyrolysis

(Chapter 3). The liquid produced is reﬁned and upgraded to gasoline or ethanol.

Since cellulose is the feedstock, the ethanol from it is often referred to as cel-

lulosic ethanol. An alternative thermal process involves gasiﬁcation of the

biomass to produce syngas and synthesis of the syngas into diesel oil using the

FT process. This process was described in Section 9.4.2 and is illustrated in

Figure 9.6.

Biochemical Process

Figure 9.7 illustrates the biochemical process for production of ethanol from

nonfood ligno-cellulosic biomass. To produce alcohol, the long-chain sugar

molecules in the cellulose must be broken down into free sugar molecules. Only

then can the sugar be fermented into alcohol (ethanol), as in the food-based

process (refer to Figure 9.3). This extra step of breaking down to free sugar

molecules is not necessary in the latter process because there the feedstock

(corn, sugarcane, etc.) is already in sugar form.

The breakdown of cellulose into sugar can be carried out by either (1) acid

hydrolysis or (2) enzymatic hydrolysis. The production of cellulosic ethanol

typically takes ﬁve steps:

Feed preparation (i.e., mechanical cleaning and sizing, physico-chemical

preparation)

2. Hydrolysis (conversion into sugar)

Fermentation (conversion of the sugar into ethanol)

Distillation (removal of water and solids)

Dehydration (ﬁnal drying)

The second step is different for the two biochemical processes. All other steps

are the same.

Biomass

Feedstock

processsing

and drying

Gasification

quench

compression

removal

synthesis

Product

recovery/

upgrading

Diesel

Naphtha

Steam

uel gas

recovery

compression

injection gas

Catalyst

Selexol

AGT

Shift

reaction

Air

separation

unit

Ash

Syngas

Oxygen

FIgurE 9.6 Transport fuel production from coal and biomass using Fisher-Tropsch synthesis. (Source: Adapted from White et al.,

2007.)

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chapter

9 Production of Synthetic Fuels and Chemicals from Biomass

Feed Preparation

This step prepares the biomass for processing. It involves cleaning and then

pretreatment. Unlike food grain (e.g., corn, wheat), ligno-cellulose often come

mixed with dirt and debris. These must be cleaned from the delivered biomass,

which is then shredded into small particles. In pretreatment, the hemicellulose/

lignin sheath that surrounds the cellulose in plant material is disrupted. Physi-

cal, chemical, or biological pretreatment, as follows, makes the cellulose more

accessible to the hydrolysis process.

Physical methods: grinding, milling, shearing (energy intensive) and steam

explosion (to produce some inhibitory compounds)

Chemical methods: treatment with acid (for pH neutralization and recovery

of chemicals), treatment with alkalis (for pH adjustment and recycling

of chemicals), and treatment with organic solvents (solvent removal and

recycling is expensive)

Biological method: enzymatic treatment of the cellulose (time consuming)

Hydrolyis

Acid hydrolysis uses dilute acid at high temperature and pressure. Concentrated

acid at lower temperature and pressure may be used, but this produces a toxic

by-product that inhibits fermentation and so must be removed.

In enzymatic hydrolysis, cellulose chains are broken into glucose molecules

by cellulose enzymes, in a process similar to what occurs in the stomach of a

cow to convert grass or fodder cellulose into sugar. Xylanose and hemicellulose

enzymes can convert many cellulosic agricultural residues into fermentable

sugars. These residues include corn stover, distiller grains, wheat straw, and

Pretreatment

Cellulose

hydrolysis

Glucose

fermentation

Biomass

delivery

Biomass

handling

Enzyme

production

Ethanol

shipping

Pentose

fermentation

Ethanol

recovery

gurE

9.7

Biochemical process for producing ethanol from ligno-cellulosic feed.

323

9.5 Transport Fuels from Biomass

sugarcane bagasse, as well as energy crops such as switch grass. Lignin is dif-

ﬁcult to convert into sugar, so it is discarded as waste. Figure 9.7 shows a

process based on cellulose hydrolysis.

Fermentation of Hemicellulosic Sugars

Through a series of biochemical reactions, bacteria convert xylose and other

hemicellulose and cellulose sugars into ethanol. The yeast or other microorgan-

isms for the fermentation of cellulose and that for hemicellulose are not neces-

sarily the same. In any case, they consume sugar molecules and produce ethanol

and carbon dioxide.

Distillation and Dehydration

Dilute ethanol broth produced during the fermentation of hemicellulose and

cellulose sugars is distilled to remove water and concentrate the ethanol. Solid

residues containing lignin and microbial cells can be burned to produce heat

or used to generate electricity consumed by ethanol production. Alternately,

the solids can be converted to co-products, such as animal feed and nutrients

for crops). The last step in the process involves removal of the remaining water

from the distilled ethanol.

Deﬁnition of Biomass

Appendix A

Biomass Gasiﬁcation and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00015-5

325

In the United States, the deﬁnition of biomass has been hotly debated. Cur-

rently, the generally accepted deﬁnition can be found in the American Clean

Energy and Security Act of 2009, HR 2454, excerpted as follows.

The term “renewable biomass” means any of the following:

(A)

Plant material, including waste material, harvested or collected from actively

managed agricultural land that was in cultivation, cleared, or fallow and non-

forested on the date of enactment of this section;

(B)

Plant material, including waste material, harvested or collected from pastureland

that was non-forested on such date of enactment;

(C)

Nonhazardous vegetative matter derived from waste, including separated yard

waste, landscape right-of-way trimmings, construction and demolition debris or

food waste (but not municipal solid waste, recyclable waste paper, painted, treated

or pressurized wood, or wood contaminated with plastic or metals);

(D)

Animal waste or animal byproducts, including products of animal waste

digesters;

(E)

Algae;

(F)

Trees, brush, slash, residues, or any other vegetative matter removed from within

600 feet of any building, campground, or route designated for evacuation by a

public ofﬁcial with responsibility for emergency preparedness, or from within 300

feet of a paved road, electric transmission line, utility tower, or water supply line;

(G)

Residues from or byproducts of milled logs;

(H)

Any of the following removed from forested land that is not Federal and is not high

conservation priority land:

(i)

Trees, brush, slash, residues, inter-planted energy crops, or any other vegeta-

tive matter removed from an actively managed tree plantation established—

(I)

Prior to the date of enactment of this section; or

(II)

On land that, as of the date of enactment of this section, was cultivated

or fallow and non-forested.

(ii)

Trees, logging residue, thinnings, cull trees, pulpwood, and brush removed

from naturally regenerated forests or other non-plantation forests, including

for the purposes of hazardous fuel reduction or preventative treatment for

reducing or containing insect or disease infestation.

326

Appendix

A Deﬁnition of Biomass

(iii) Logging residue, thinnings, cull trees, pulpwood, brush and species that are

non-native and noxious, from stands that were planted and managed after the

date of enactment of this section to restore or maintain native forest types.

(iv)

Dead or severely damaged trees removed within 5 years of ﬁre, blow down,

or other natural disaster, and badly infested trees:

(I)

Materials, pre-commercial thinnings, or removed invasive species from

National Forest System land and public lands (as deﬁned in section 103

of the Federal Land Policy and Management Act of 1976 (43 U.S.C.

1702)), including those that are byproducts of preventive treatments (such

as trees, wood, brush, thinnings, chips, and slash), that are removed as

part of a federally recognized timber sale, or that are removed to reduce

hazardous fuels, to reduce or contain disease or insect infestation, or to

restore ecosystem health, and that are—

(i)

Not from components of the National Wilderness Preservation

System, Wilderness Study Areas, inventoried road-less areas, old

growth or mature forest stands, components of the National Land-

scape Conservation System, National Monuments, National Conser-

vation Areas, Designated Primitive Areas, or Wild and Scenic Rivers

corridors;

(ii)

Harvested in environmentally sustainable quantities, as determined

by the appropriate Federal land manager; and

(iii)

Harvested in accordance with Federal and State law and applicable

land management plans.

Another accepted deﬁnition is that of the Ontario Corporations Tax Act,

excerpted as follows:

The term “biomass resource” means

(a)

organic matter that is derived from a plant and available on a renewable basis,

including organic matter derived from dedicated energy crops, dedicated trees,

agricultural food and feed crops, or

(b)

waste organic material from harvesting or processing agricultural products, includ-

ing animal waste and rendered animal fat, forestry products, including wood waste,

and sewage.

Physical Constants

Appendix B

Biomass Gasiﬁcation and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00017-9

327

Atmospheric pressure

101.325

N/m

101.325 kPa

1.013

bar

Avogadro’s number

6.022 × 10

/mol

Boltzmann’s constant

1.380 × 10

−23

J/K

Gravitational acceleration (sea level), g

9.807

m/s

Planck’s constant

6.625 × 10

−34

J.s

Speed of light in vacuum

2.998 × 10

m/s

Stefan-Boltzmann constant

5.670 × 10

−8

W/m

Universal gas constant, R

8.205 × 10

−2

.atm/kmol.K = 8.314 × 10

−2

.bar/kmol.K

= 8.314

kJ/kmol.K

= 282

N.m/kg.K

= 8.314

kPa.m

/kmol.K

= 1.98

kCal/kmol.K

Selected Design

Data Tables

Appendix C

Biomass Gasiﬁcation and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00018-0

329

Table C.1 Fusibility of Biomass Ash

Type

Temperature (°C)

Initial Deformation Softening Hemispherical Fluid

Corn cob

900 1020

Corn stalk

820 1091

Grape pruning (oxidizing)

1313 1368 1374 1424

Grape pruning (reducing)

1310 1360 1371 1382

Olive pit

850 1480

RDF pellet

890 1130

RDF (oxidizing)

1065 1092 1131 1193

RDF (reducing)

1024 1063 1097 1182

Rice hulls 1439 >1650

Rice straw

1060 1250

Walnut shell

820 1225

Source: Osman and Goss, 1983.

Source: Rossi, 1984, pp. 69–99.

Source: Alter and Campbell, 1979, pp. 127–142.

TABLE C.2 Standard Heating Value of Constituents of Typical Product Gas from Biomass Gasiﬁcation

Gases H

CO CO

HHV (MJ/Nm

)

12.74 12.63 39.82

70.29

63.41 58.06 101.24

LHV (MJ/Nm

)

10.78 12.63 35.88

64.34

59.45 56.07 99.09

Viscosity

(µP)

90 182 150 112

103 104 82

Thermal conductivity

(W/m.K) 0.1820 0.0251 0.0166 0.0343

0.0218

0.0214 0.0213 0.0183

Speciﬁc heat

(kJ/kg.K) 3.467 1.05 0.85 2.226

1.926

1.691 1.775 1.708

Gases C

i-C

n-C

HHV (MJ/Nm

)

93.57 125.08 133.12 134.06 142.89 13.07 25.10

LHV (MJ/Nm

)

87.57 116.93 122.91 123.81 141.41 10.13 23.14

Viscosity

(µP)

180

Thermal conductivity

(W/m.K) 0.026

Speciﬁc heat

(kJ/kg.K) 1.05

Source: Data compiled from Jenkins, in Kitani and Hall, 1989, p. 887.

Source: Data compiled from Waldheim and Nilsson, 2001.