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

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Biomass Gasiﬁcation

and Pyrolysis

Practical Design and Theory

Prabir Basu

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Library of Congress Cataloging-in-Publication Data

Basu, Prabir

Biomass gasiﬁcation and pyrolysis : practical design and theory / Prabir Basu.

cm.

Includes bibliographical references and index.

ISBN 978-0-12-374988-8 (alk. paper)

Biomass gasiﬁcation.

Biomass—Combustion.

Pyrolysis.

Gas manufacture and

works.

Title.

TP339.B355

2010

662′.88—dc22

2010010068

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To Mother.

Preface

vii

The art of gasiﬁcation and pyrolysis of biomass is as old as our natural habitat.

Both processes have been at work since the early days of vegetation on this

planet. Flame leaping from forest ﬁres is an example of “ﬂaming pyrolysis.”

Blue hallow in a swamp is an example of methane gas formation through

decomposition of biomass and its subsequent combustion in contact with air.

Human beings, however, learned to harness these processes much later.

First, large-scale application of gasiﬁcation for industry and society concen-

trated on coal as the fuel. It was primarily for lighting of city streets and afﬂuent

people’s houses. Use of gasiﬁcation, though nearly as ancient as combustion

technologies, did not rise with industrialization the same as combustion because

of the abundant supply and low prices of oil and natural gas. Only in the recent

past has there been an upsurge in interest in gasiﬁcation, fueled by several

factors:

 Interest in the reduction in greenhouse gas emissions as a result of energy

production

 Push for independence from the less reliable supply and ﬂuctuating prices

of oil and gas

 Interest in renewable and locally available energy sources

 Rise in the price of oil and natural gas

Several excellent books on coal gasiﬁcation are available, but a limited few

are available about biomass gasiﬁcation and pyrolysis. A large body of peer-

reviewed literature on biomass gasiﬁcation and pyrolysis is available; some

recent books on energy also include brief discussions on biomass gasiﬁcation.

However, there is a dearth of comprehensive publications speciﬁcally on gas-

iﬁcation and pyrolysis; this is especially true for biomass.

Engineers, scientists, and operating personnel of biomass gasiﬁcation

plants clearly need such information from a single easy-to-access source. Better

comprehension of the main aspects of gasiﬁcation technology could help an

operator understand the workings of the gasiﬁcation plant, a design engineer

to size the gasiﬁer, and a planner to evaluate different options. The present book

was written to ﬁll this important need. It attempts to mold available research

results in an easy-to-use design methodology whenever possible. Additionally,

it brings into focus new advanced processes such as supercritical water gasiﬁca-

tion and torrefaction of biomass.

This book is comprised of nine chapters and three appendices, which include

several tables that could be useful for the design of biomass gasiﬁers and their

viii

Preface

components. Chapter 1 introduces readers to the art of gasiﬁcation and its

present state of art. It also discusses the relevance of gasiﬁcation to the current

energy scenario around the world. Chapter 2 presents the properties of biomass

with speciﬁc relevance to gasiﬁcation and pyrolysis of biomass. The basics of

pyrolysis are discussed in Chapter 3, which also covers torrefaction. In addition,

it introduces readers to the design of a pyrolyzer and elements of the torrefac-

tion process.

Chapter 4 deals with an important practical aspect of biomass gasiﬁcation—

the tar issue. This chapter provides information on the limits of tar content in

product gas for speciﬁc applications. It also discusses several means of reduc-

tion in tar in the product gas. Chapter 5 concerns the basics of the gasiﬁcation

of biomass. It explains the gasiﬁcation process and important chemical reac-

tions that guide pyrolysis and gasiﬁcation. Chapter 6 discusses design

methodologies for gasiﬁers and presents some worked-out examples on design

problems. Chapter 7 introduces the new ﬁeld of hydrothermal gasiﬁcation, with

speciﬁc reference to gasiﬁcation of biomass in supercritical water. It covers the

basics of this relatively new ﬁeld.

One of the common, but often neglected, problems in the design of a gas-

iﬁcation plant is the handling of biomass. Chapter 8 discusses issues related to

this and provides guidelines for the design and selection of handling equipment.

The production of chemicals and synthetic fuels is gaining importance, so

Chapter 9 provides a brief outline of how some important chemicals and fuels

are produced from biomass through gasiﬁcation. Production of diesel and bio-

gasoline is also discussed brieﬂy here. Appendix A contains deﬁnitions of

biomass and Appendix B lists physical constants. Appendix C includes several

tables containing design data. The Glossary presents deﬁnitions of some terms

used commonly in the chemical and gasiﬁcation industries.

Acknowledgments

The author is greatly indebted to a large number students, professional col-

leagues, and institutions who helped revise numerous drafts of this book and

provided permission for the use of published materials. Drs. D. Groulux,

I. Ugursal, N. Mahinpey, N. Bakshi, A. Dutta, A. M. Leon, and P. Kaushal read

parts of the manuscript and provided valuable suggestions. Many students

worked tirelessly to support the work on this book. Special efforts were made

by M. Greencorn, S. Rao, V. Mettanant, B. Acharya, A. Dhungana, and

A. Basu. My hope is that is that what is here will beneﬁt at least some students

and/or practicing professionals in making the world around us a little greener

and more habitable.

Finally, this book would not have materialized without the constant encour-

agement of my wife, Rama Basu.

Dr. Prabir Basu is an active researcher and designer of gasiﬁers with a speciﬁc

interest in ﬂuidized-bed gasiﬁcation of biomass. His current research interests

include frontier areas, such as supercritical gasiﬁcation, as well as applied

research dealing with biomass co-ﬁring. He is the founder of the prestigious

triennial International Conference series on Circulating Fluidized Beds, and

founder of Greenﬁeld Research Incorporated, a private research and develop-

ment company based in Canada that specializes in ﬂuidized-bed boilers and

gasiﬁcation.

Professor Basu has been working in the ﬁeld of energy conversion and the

environment for more than 30 years. Prior to joining the engineering faculty at

Dalhousie University (formerly known as the Technical University of Nova

Scotia), he worked with both a government research laboratory and a private

boiler manufacturing company.

Dr. Basu’s passion for the transformation of research results into industrial

practice is well known, as is his ongoing commitment to spreading advanced

knowledge around the world. He has authored more than 200 research papers

and six monographs in emerging areas of energy and environment, some of

which have been translated into Chinese and Korean. He teaches short courses

and seminars in industries and at universities across the globe. Presently, he

leads the Circulating Fluidized-Bed Research Laboratory at Dalhousie Univer-

sity in Canada.

About the Author

Introduction

Chapter 1

Biomass Gasiﬁcation and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00001-5

Gasiﬁcation is a chemical process that converts carbonaceous materials like

biomass into useful convenient gaseous fuels or chemical feedstock. Pyrolysis,

partial oxidation, and hydrogenation are related processes. Combustion also

converts carbonaceous materials into product gases, but there are some impor-

tant differences. For example, combustion product gas does not have useful

heating value, but product gas from gasiﬁcation does. Gasiﬁcation packs energy

into chemical bonds while combustion releases it. Gasiﬁcation takes place in

reducing (oxygen-deﬁcient) environments requiring heat; combustion takes

place in an oxidizing environment giving off heat.

The purpose of gasiﬁcation or pyrolysis is not just energy conversion; pro-

duction of chemical feedstock is also an important application. In fact, the ﬁrst

application of pyrolysis of wood into charcoal around 4000 B.C.E. was not for

heating but for iron ore reduction. In modern days, gasiﬁcation is not restricted

to solid hydrocarbons. Its feedstock includes liquid or even gases to produce

more useful fuels. Partial oxidation of methane gas is widely used in production

of synthetic gas, or syngas, which is a mixture of H

and CO.

Pyrolysis (see Chapter 3), the pioneer in the production of charcoal and

the ﬁrst transportable clean liquid fuel kerosene, produces liquid fuels from

biomass. In recent times, gasiﬁcation of heavy oil residues into syngas has

gained popularity for the production of lighter hydrocarbons. Many large

gasiﬁcation plants are now dedicated to production of chemical feedstock

from coal or other hydrocarbons. Hydrogenation, or hydrogasiﬁcation, which

involves adding hydrogen to carbon to produce fuel with a higher hydrogen-

to-carbon (H/C) ratio, is also gaining popularity. Supercritical gasiﬁcation (see

Chapter 7), a new option for gasiﬁcation of very wet biomass, is drawing

growing interest.

This chapter introduces the subject of biomass gasiﬁcation with a short

description of its historical developments, its motivation, and its products.

It also gives a brief introduction to the chemical reactions that are involved in

gasiﬁcation.

Chapter

1 Introduction

1.1 HistoriCal BaCkground

The earliest known investigation into gasiﬁcation was carried out by Thomas

Shirley, who in 1659 experimented with “carbureted hydrogen” (now called

methane). Figure 1.1 shows some of the important milestones in the progression

of gasiﬁcation.

The pyrolysis of biomass to produce charcoal was perhaps the ﬁrst large-

scale application of a gasiﬁcation-related process. When wood, owing to its

overuse, became scarce toward the beginning of the eighteenth century, coke

was produced from coal through pyrolysis, but the use of by-product gas from

pyrolysis received little attention. Early developments were inspired primarily

by the need for town gas for street lighting. The salient features of town gas

from coal were demonstrated to the British Royal Society in 1733, but the

scientists of the time saw no use for it. In 1798, William Murdoch used coal-gas

(also known as town gas) to light the main building of the Soho Foundry, and

in 1802 he presented a public display of gas lighting, astonishing the local

population. Friedrich Winzer of Germany patented coal-gas lighting in 1804

(www.absoluteastronomy.com/topics/coal gas).

By 1823 numerous towns and cities throughout Britain were gas-lit. At the

time, the cost of gas light was 75% less than that for oil lamps or candles, and

this helped accelerate its development and deployment. By 1859, gas lighting

had spread throughout Britain. It came to the United States probably in 1816,

with Baltimore the ﬁrst city to use it.

The history of gasiﬁcation may be divided into four periods, as described

in the following:

1739

Dean Clayton:

Distilled coal in a

closed vessel

1792

Murdoc: First use

of coal-gas for

interior lighting

1861

Siemens gasifier:

First successful unit

1945–1974

Post-war “oil

glut”

2001

Advanced

gasification biomass

renewable energy

projects

1788

Robert Gardner: First

gasification patent

1659

Thomas Shirley:

Discovered gas

from coal mine

1801

Fourcroy: Water-

gas shift reaction

1920

Carl von Linde:

Cryogenic separation

of air, fully continuous

gasification process

1926

Winkler fluidized-

bed gasifier

1931

Lurgi: Pressurized

moving-bed

process

1974

Arab oil embargo

renewed

gasification interest

1997

First commercial

gasification plant in U.S.

FigurE 1.1 Milestones in gasiﬁcation development.

1.1 Historical Background

1850–1940: During this early stage, the gas made from coal was used

mainly for lighting homes and streets and for heating. Lighting helped along

the Industrial Revolution by extending working hours in factories, espe-

cially on short winter days. The invention of the electric bulb circa 1900

reduced the need for gas for lighting, but its use for heating and cooking

continued. With the discovery of natural gas, the need for gasiﬁcation of

coal or biomass decreased. All major commercial gasiﬁcation technologies

(Winkler’s ﬂuidized-bed gasiﬁer in 1926, Lurgi’s pressurized moving-bed

gasiﬁer in 1931, and Koppers-Totzek’s entrained-ﬂow gasiﬁer) made their

debut during this period.

1940–1975: The period 1940–1975 saw gasiﬁcation enter two ﬁelds of

application as synthetic fuels: internal combustion and chemical synthesis

into oil and other process chemicals. In the Second World War, Allied

bombing of Nazi oil reﬁneries and oil supply routes greatly diminished the

crude oil supply that fueled Germany’s massive war machinery. This forced

Germany to synthesize oil from coal-gas using the Fischer-Tropsch (see Eq.

1.13) and Bergius processes (nC + (n + 1)H

→ C

2n+2

). Chemicals and

aviation fuels were also produced from coal.

A large number of cars and trucks in Europe operated on coal or biomass

gasiﬁed in onboard gasiﬁers. During this period over a million small gasiﬁ-

ers were built primarily for transportation (see Figure 1.2). The end of the

Second World War and the availability of abundant oil from the Middle

East eliminated the need for gasiﬁcation for transportation and chemical

production.

FigurE 1.2 Bus with an onboard gasiﬁer during the Second World War. (Source: http://www.

woodgas.com/history.htm.)