Preface

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 concen­trated 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 gas­ification 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 gasifica­tion 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

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 reduc­tion in tar in the product gas. Chapter 5 concerns the basics of the gasification of biomass. It explains the gasification process and important chemical reac­tions 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 gas­ification 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 bio­gasoline 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 col­leagues, 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 encour­agement of my wife, Rama Basu.

About the Author

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 develop­ment 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 Univer­sity in Canada.

Chapter 1

Introduction

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 impor­tant 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; pro­duction 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 H2 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.

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

Copyright © 2010 Prabir Basu. Published by Elsevier Inc. All rights reserved.