Combustion E-Book

Maximilian Lackner

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 365

Veröffentlichungsjahr: 2013

Beschreibung

Combustion, the process of burning, is defined as a chemical reaction between a combustible reactant (the fuel) and an oxidizing agent (such as air) in order to produce heat and in most cases light while new chemical species (e.g., flue gas components) are formed. This book covers a gap on the market by providing a concise introduction to combustion. Most of the other books currently available are targeted towards the experienced users and contain too many details and/or contain knowledge at a fairly high level. This book provides a brief and clear overview of the combustion basics, suitable for beginners and then focuses on practical aspects, rather than theory, illustrated by a number of industrial applications as examples. The content is aimed to provide a general understanding of the various concepts, techniques and equipment for students at all level as well as practitioners with little or no prior experience in the field.

The authors are all international experts in the field of combustion technology and adopt here a clear didactic style with many practical examples to cover the most common solid, liquid and gaseous fuels. The associated environmental impacts are also discussed so that readers can develop an understanding of the major issues and the options available for more sustainable combustion processes.

With a foreword by Katharina Kohse-Hoinghaus

Leseprobe

Contents

Cover

Related Titles

Title Page

Copyright

Foreword

Preface

Chapter 1: History of Combustion

1.1 Introduction

1.2 Timetable

1.3 Outlook

1.4 Web Resources

References

Chapter 2: Fuels

2.1 Introduction

2.2 Gaseous Fuels

2.3 Liquid Fuels

2.4 Solid Fuels

References

Chapter 3: Combustion Principles

3.1 Basic Combustion Calculations

3.2 Heat-, Mass- and Momentum Transport and Balance

3.3 Elementary Reactions and Radicals

3.4 Ignition

References

Chapter 4: Environmental Impacts

4.1 Pollutants: Formation and Impact

4.2 Combustion and Climate Change

References

Chapter 5: Measurement Methods

5.1 Introduction

5.2 In Situ versus Ex Situ Measurements

5.3 Fuel Characterization

5.4 Investigation of Combustion Processes

References

Chapter 6: Applications

6.1 Burners

6.2 Industrial Boilers

6.3 Industrial Technologies

References

Chapter 7: Safety Issues

7.1 Introduction

7.2 Fundamentals

7.3 Fire Classes

7.4 Working Mechanism of Fire Extinguishing Media

7.5 Fire Detectors

7.6 Deflagrations and Detonations

7.7 Dust Explosions

7.8 Legal Framework: Example of ATEX in Europe

7.9 Preventing and Mitigating the Effect of Explosions in Industry

7.10 Aspects of Preventive Fire Protection

7.11 Fire Suppression by Oxygen Reduction

7.12 Safety by Process Design

7.13 Other Important Terms Related to Fire Safety

References

Index

Related Titles

Lackner, M., Winter, F., Agarwal, A. K. (eds.)

Handbook of Combustion

5 Volumes

2010

ISBN: 978-3-527-32449-1

Koch, E.-C.

Metal-Fluorocarbon Based Energetic Materials

2012

ISBN: 978-3-527-32920-5

Stolten, D., Scherer, V. (eds.)

Transition to Renewable Energy Systems

2013

ISBN: 978-3-527-33239-7

Fricke, J., Borst, W. L.

Essentials of Energy Technology

Sources, Transport, Storage, Conservation

2013

ISBN: 978-3-527-33416-2

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Print ISBN (Hardcover): 978-3-527-33376-9

Print ISBN (Softcover): 978-3-527-33351-6

ePDF ISBN: 978-3-527-66721-5

ePub ISBN: 978-3-527-66720-8

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Foreword

Combustion is a fascinating process which has been quite instrumental in civilization and industrialization. From the hearth fire and cooking stove, via techniques for ore smelting, glass blowing and porcelain making, to steam engines, cars and power plants, combustion has accompanied the history of humankind. Now, in a situation where global warming and air quality deterioration are associated with combustion-generated carbon dioxide and pollutants, it is at the same time important to provide access to affordable but clean and sustainable energy. Combustion as mature technology still dominates today's power generation and transportation, and it is used in a number of important industrial processes and applications. Many professions, trades and businesses are linked to combustion, and many people world-wide depend on their work for car and aircraft manufacturers, for the petroleum, cement or steel industry or even as safety engineers and firefighters. This global situation is not likely to be changed rapidly, in spite of considerable effort to replace fossil by renewable energy, regarding the increase in world population and the desire to raise living standards and productivity accordingly. Combustion thus shows its Janus face today with promises of high-density fuels for an energy-hungry and mobile society on the one hand and threatening pictures of smog-polluted megacities without blue skies on the other.

It is thus time for the present volume as a summary and introduction to combustion fundamentals and applications for the more generally interested reader, including students and practitioners. With a fundament in physics and chemistry, modern concepts of combustion are presented in the necessary detail for a broad overview, without an excess of detail, in coherent and comprehensible fashion. The book provides a clear structure with seven Chapters, starting with some historical facts and interesting details in Chapter 1. The second Chapter introduces fuels with respect to their important properties and physic-chemical characteristics, accompanied by useful tables and literature. With Chapter 3, the fundamental principles of combustion are provided in an instructive form with some illustrative and facile calculation examples. The reader is introduced to the concepts of stoichiometry, the conservation equations and transport processes as well as to the basics of chemical reaction mechanisms and ignition processes. Pollutants are characterized in Chapter 4, which mainly gives some classifications and describes the main sources for specific emissions. For a more in-depth understanding of the different categories of pollutants and their chemical formation and destruction pathways, readers are referred to relevant original literature. The very important aspect of carbon dioxide formation from combustion processes and concepts for carbon dioxide management are presented in Chapter 5. The next Chapter is very instructive regarding the typical technical environments in which combustion is encountered, and it explains many interesting features of combustion devices including those found in heating, power generation, transportation, and in certain industries. As a modern concept, combustion and gasification are seen as somewhat related subjects, with a short access also to gasification strategies. The book concludes with important safety aspects, especially also regarding industrial-scale applications.

In a timely manner, the book offers an overview on an introductory level, and in this respect, it will be useful to a broad community. Certainly, huge tomes could be written for each subject treated in these Chapters, and substantial reviews and literature exists on individual facets of combustion – as for example, conventional and bio-derived fuels, combustion kinetics or specific combustion systems and applications. For a field that is as complex as that of combustion, a guided tour – as in the present case – is helpful to not lose orientation! I wish that you, the reader, may find combustion, not only at a candle-light dinner or for a barbecue, a fascinating object for study, in spite or because of the many challenges presented by its use. I also hope that by understanding the fundamental principles and limitations of combustion better, the community might find suitable replacement strategies for the systems in use today to contribute to a more efficient and cleaner energy use in the near future.

Bielefeld, April 2013

Prof. Dr. Katharina Kohse-Höinghaus

Preface

Combustion, the source of comfort and fear, warmth as well as devastation, has always fascinated mankind. It has been and still is one of the most important and most widely used technologies. In 2010, the authors published the “Handbook of Combustion” [1], a five-volume reference work that was very well received by the scientific community. Soon the idea was born to distill the knowledge from the approximately 3200 pages into a digestible textbook for students.

This book is designed to be a compilation of up-to-date knowledge in the field of combustion in a way that even a reader from a different field of expertise can understand the basic principles and applications. The purpose of this textbook is to provide an introduction to combustion science and technology, spanning from fundamentals to practical applications. It deliberately does not dwell too much on the details, although the book aims at providing the necessary knowledge for those wishing to move further into the various sub-disciplines, such as energy efficiency, oxyfuel combustion, gasification, pollutant reduction, or combustion diagnostics.

This book is written not only for undergraduate and graduate students of chemistry, chemical engineering, materials science, engineering and related disciplines, but also for practitioners in the field.

Topics covered are:

History of combustionFuelsCombustion principlesEnvironmental impactsMeasurement methodsApplicationsSafety issues

Each chapter can be studied independently. For further reading, web resources are suggested at the end of each chapter.

The authors are proud to present this textbook and hope that it will serve many technicians, scientists and engineers throughout their studies and careers.

Vienna, June 2013

M. Lackner, Á. B. Palotás, F. Winter

Reference

1. Lackner, M., Winter, F., and Agarwal, A.K. (eds) (2010) Handbook of Combustion, Wiley-VCH Verlag GmbH, Weinheim, 978-3527324491.

1

History of Combustion

1.1 Introduction

Combustion can be considered the oldest technology of mankind, and one of our most important discoveries/inventions. There are numerous legends around fire, for example, how it was handed to mankind by Prometheus. In Greece and India, there are stories and myths about eternally burning fires, often in relation to religious or supernatural phenomena. Legend has it that the Oracle of Delphi is located at the site of just such a fire [1]. Our ancestors used fire for various purposes, and today it accounts for the major share of primary energy production, approximately 85%. The science of combustion has a long history. Fire was one of the four elements in alchemy, and combustion processes were used for many transformations. Figure 1.1 depicts an alchemist's laboratory from ~ 1600, where combustion plays a pivotal role.

Figure 1.1 Alchemist's laboratory, around 1600.Source: Deutsches Museum, Munich, Germany.

Fire has been used by man for a long time for various purposes, such as cooking, metal production and warfare. However, as combustion phenomena are complex, significant advances in the understanding of combustion theory were only made in the last decades by a close collaboration between experimentalists and theoreticians.

An early observation made by the Flemish alchemist Johann Baptista van Helmont (1580–1644) was that a burning material results in smoke and a flame. From this, he concluded that combustion involved the escape of a “wild spirit” (spiritus silvestre) from the burning substance. Van Helmont's theory was further developed by the German alchemist Johann Becher (1635–1682) and his student Georg Ernst Stahl (1660–1734) into the phlogiston theory, according to which all combustible materials contain a special substance, the so-called phlogiston, which is released during combustion. This theory stayed in place for two centuries and was strongly defended by Joseph Priestley (1733–1804), who, with Carl Wilhelm Scheele, is also credited with the discovery of oxygen. Figure 1.2 shows Priestley's laboratory.

Figure 1.2 Joseph Priestley's laboratory, around 1773.Source: Deutsches Museum, Munich, Germany.

A classic textbook on combustion history is Michael Faraday 's “The Chemical History of a Candle” [2]. Other names associated with the development of combustion technology are James Watt (1736–1819), the inventor of the steam engine (more precisely, he made improvements to the Newcomen steam engine), plus RudolfDiesel (1858–1913) and Nikolaus Otto (1832–1891), the inventors of their homonymous engines.

During the Industrial Revolution (~1750–1850), combustion of fossil fuels began to be used on a large scale for energy production, raw materials, the manufacture of various goods, mainly from steel, and transportation, for example, via steam engines, see Figure 1.3.

Figure 1.3 Industrial Revolution (Vintage engraving from 1878 of the Shipyards and shipping of the Clyde, Scotland).

The black smoke coming out of factory chimneys during the Industrial Revolution (compare Figure 1.3) was seen as a sign of progress and prosperity.

In archeology and physical anthropology, one uses the so-called three-age system for the periodization of human prehistory into three consecutive time periods, which are named for their respective tool-making technologies: Stone Age, Bronze Age and Iron Age. The latter two are intimately associated with combustion. One can draw an analogy to energy and speak about the “coal age”, “nuclear age” and “renewables age”, while some observers are talking about a “second coal age” with the renewed interest in energy production from coal. Historically, the coal age – also termed the carboniferous period – is the time when coal was formed: 360 to 290 million years ago. Without doubt, mankind has lived in a “combustion age” for the last centuries.

1.2 Timetable

The following section lists key milestones in man's “taming” of fire, compiled from [3–9].

The process started 1–2 million years ago with natural fires, for example, triggered by a flash of lightning, that man could grab, and progressed to primitive ways of creating fire, for example, by friction (Figure 1.4) or flint stones (Figure 1.5) almost 10 000 years ago, followed by a rapid evolution of technologies:

First 500 000 years and beyond…1–2 million years BC:Man discovers fireBefore our time…~7000 BC:Man uses flint stones~3000 BC:Egyptians invent candles, made from beeswax~1000 BC:Chinese use natural gas for lamps [1]500 BC:Greeks describe combustion with the 4 elements earth, water, air and fire450 BC:Herodotus describes oil pits near BabylonApproaching the middle ages…~100 AD:Chinese invent gunpowder and fireworks1242:Roger Bacon, an English friar, publishes gunpowder formula [9]fifteenth century:Candles are used for street lighting500 years ago…1556:Georgius Agricola publishes “De re metallica ”, a book cataloging the state of the art in mining, refining, and smelting of metals.400 years ago…1627:First recorded use of black powder for rock blasting in Hungary [9]1650:Otto von Guericke demonstrates that a candle does not burn in a vacuum1678:Abbé Hautefeuille describes an engine for raising water, powered by burning gun powder1698:Thomas Savery builds a steam-powered water pump for removing water from mines~1700:Christiaan Huygens comes up with the idea of an internal combustion engine300 years ago…1712:Thomas Newcomen builds a piston-and-cylinder steam-powered water pump for use in mines (Figure 1.6). It is the first practical device to harness the power of steam to produce mechanical work [10]1772:Carl Wilhelm Scheele carries out experiments to split air and discovers oxygen (Figure 1.7).1778:Alessandro Volta discovers the analogy between the ignition of combustible gases and fen fire (ghost light) in swamps (Figure 1.8).1750–1850:Industrial Revolution1769:James Watt (Figure 1.9) patents a steam engine1790s:Steam ship pioneer Samuel Morey invents a steam-powered paddle wheel1791:John Barber obtains a patent for a gas turbine1792:William Murdoch discovers the distillation of gas from coal and its use for lighting purposes1794:Concept for the first internal combustion engine by Robert Street1800:Phillippe Lebon patents an engine that uses compressed air and electricity for ignition1801:First coal powered steam engine1806:François Isaac de Rivaz invents a hydrogen-powered internal combustion engine200 years ago…1814:George Stephenson builds a steam locomotive1815–1819:Sir Humphry Davy discovers catalytic combustion1816:Robert Stirling invents his hot air Stirling engine1824:Nicolas Léonard Sadi Carnot publishes that the maximum efficiency of a heat engine depends on the temperature difference between an engine and its environment1834:Joseph Morgan develops a machine that allowed the continuous production of molded candles1837:First American patent for an electric motor (US Patent 132)1850:Rudolf Clausius describes the first and second law of thermodynamics~1855:Robert Bunsen builds the Bunsen burner1855:Johan Edvard Lundstrom (Sweden) patents his safety match1857:Development of the kerosene lamp1859:John Tyndall discovers that some gases block infrared radiation. He suggests that changes in the concentration of the gases could bring climate change [4]1860:Étienne Lenoir and Nikolaus Otto build an internal combustion engine1860:Invention of fire extinguishers1863:J.D. Rockefeller opens an oil refining company in Cleveland1863:Julius Bernhard Friedrich Adolph Wilbrand invents trinitrotoluene (TNT) [9]1866:Swedish chemist Alfred Nobel invents dynamite by mixing kieselguhr with nitroglycerine [9]1877:Nikolaus Otto patents a four-stroke internal combustion engine (US Patent 194 047)1880:Thomas Alva Edison opens an electric power plant1882:James Atkinson invents the Atkinson cycle engine, which is used in some hybrid vehicles1884:Charles Parsons develops the steam turbine1885:Karl Benz builds a gasoline-powered car1885:Gottlieb Daimler patents the first supercharger1892:Rudolf Diesel patents the Diesel engine (US Patent 608 845)1896:Svante Arrhenius publishes the first calculation of global warming from human emissions of CO21899:Ferdinand Porsche creates the first hybrid vehicle1903:First flight by the Wright Brothers, Orville and Wilbur, at Kitty Hawk1906:Frederick Gardner Cottrell invents the electrostatic smoke precipitator100 years ago…1913:René Lorin invents the ramjet1915:Leonard Dyer invents a six-stroke engine, now known as the Crower six-stroke engine named after its reinventor Bruce Crower1920:Robert H. Goddard develops the principle of a liquid-fueled space rocket1923:Fritz Pregl receives the Nobel Prize for combustion analysis1929:Felix Wankel patents the Wankel rotary engine (U.S. Patent 2 988 008)1930s:Global warming trend since late nineteenth century reported [4]1936:Maiden flight of the “Hindenburg” LZ 129 airship (volume 200 000 m3, hydrogen-filled)1954:Foundation of the “Combustion Institute”1957:Russia launches “Sputnik I”, the first artificial Earth satellite50 years ago…1969:The USA land men on the moon, propelled by the Saturn V booster rocket developed by Wernher von Braun1970s:Electronically controlled ignition appears in automobile engines1973:Oil embargo and price rise bring first “energy crisis“1975:Catalytic converters are introduced on production automobiles in the US, co-invented by Carl Donald Keith1979–1981:Oil prices rise from $13.00 to $34.00/barrel1981–2011:The Space Shuttle, a partially reusable launch system and orbital spacecraft, is operated by the US National Aeronautics and Space Administration (NASA) for human spaceflight missions1980s:Electronic fuel injection appears on gasoline automobile engines1986:Word car population exceeds 500 million vehicles [11]25 years ago…1990:First IPCC (Intergovernmental Panel on Climate Change) report says world has been warming and future warming seems likely1990s:Market introduction of hybrid vehicles that run on an internal combustion engine and an electric motor charged by regenerative braking1990s:CFD (computational fluid dynamics) is widely used as a tool for combustion simulation1997:Toyota introduces the Prius in Japan, the first mass-market electric hybrid car1997:The Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC) sets binding obligations on industrialized countries to reduce emissions of greenhouse gases1998:50 year moratorium on mining and oil exploration in Antarctica approvedCurrent century…2002:Güssing's biomass gasification (demonstration plant with 8 MWth combined heat and power) in operation [12]2003:The US government announces its plan to build a near zero-emission coal-fired power plant for hydrogen and electricity production using carbon capture and storage (CCS)2007:European Union introduces new environmental regulations to reduce GHG (greenhouse gases) emissions by 20% by 20202008:Wärtsilä builds the world's largest reciprocating engine, a two-stroke turbocharged diesel engine designed for large container ships with a power of 80 000 kW2009:Level of CO2 in the atmosphere reaches 385 ppm [4]2010:1 000 000 000 cars on the road [11]2013:World's largest fluidized bed boiler (600 MWel) is expected to be in operation in ChinaIn future…2020–2030:Peak oil expected [5]

Figure 1.4 Making fire (Image from the Dutch stone age theme park Archeon).

Figure 1.5 Flintstone, found in the Cliffs of Dover/UK.

Figure 1.6 Newcomen engine [10].

Figure 1.7 Experiments by C.W. Scheele to investigate the composition of air and to explain combustion and breathing processes.Source: Deutsches Museum, Munich, Germany.

Figure 1.8 The gases which are formed in swamps are comparable to combustible gases from the lab.Source: Deutsches Museum, Munich, Germany.

Figure 1.9 James Watt [10].

1.3 Outlook

Combustion has been a science and engineering discipline for several hundred years, and it has driven the industrial revolution. Today, combustion plays an important role in our lives, from transportation to energy production, and it will continue to do so.

The focus of combustion research in the last decades has moved to pollution abatement, energy efficiency and alternative fuels (green combustion, zero emission combustion and near-zero emission combustion) [13]. Biomass – a renewable fuel – is expected to gain an increasing share over fossil fuels, as well as hydrogen as a clean energy carrier [14].

Today, energy is largely used inefficiently, because we deploy it as heat, where the limitations of Carnot apply, which are far from the thermodynamic limits. The Carnot efficiency is defined as

where Th is the absolute temperature of the hot body and Tc that of the cold body for a heat engine. So instead of talking about an “energy crisis” we could talk about a “heat crisis” of technical processes. Organisms, on the contrary, do not thermalize their energetic compounds, but rather utilize their chemical energy in a more efficient way: in a cascade of molecular-scale mechanisms that approach the reversible limit set by the difference in free energy. As these processes occur around room temperature, the irreversible losses (T ΔS) are significantly smaller. A short-term technical realization of this concept is the fuel cell [15]. It is expected that fuel cells will gain importance as an efficient and clean “special” mode of combustion. There are various concepts and technologies, for example, proton exchange membrane (PEM) and molten carbonate (MC) fuel cells. Fuel cells can be combined with on-board fuel processing. Gasification can be used to obtain the feed for fuel cells from solid fuels.

When combustion processes are miniaturized, one talks about “microscale combustion ”. This technology has attracted the interest of researchers recently. Microscale combustion differs fundamentally from “normal” combustion processes, since the vessel walls are closer than the quenching distance, so catalytic combustion processes are applied. Microscale combustion [16] might soon replace conventional batteries, as liquid fuels such as hydrocarbons have a high energy density, see Figure 1.10.

Figure 1.10 Comparison of specific energy densities of lithium ion batteries with hydrocarbon and oxygenated hydrocarbon fuels as well as different engines. SSME = space shuttle main engine.Reproduced with permission from [16].

Figure 1.11 depicts a micro-gas turbine.

Figure 1.11 Scheme of a micro-gas turbine (a) and image of the turbine blade (b).Reproduced with permission from [16].

As mentioned, pollutant formation and reduction has become an important field of study in combustion in the recent decades as environmental awareness has increased. The study of health effects from combustion products [17] is also gaining importance.

Concerning fossil fuels, oil sands [18] will receive increased research attention. With respect to engines, homogeneous charge compression ignition (HCCI) [19] and various alternative ignition systems [20] are promising concepts, see Figure 1.12.

Figure 1.12 Concept of HCCI combustion with the aim of reducing soot and NOx emissions. In HCCI and petrol (gasoline) engines, the fuel and air are mixed prior to combustion, which prevents soot emissions as found with diesel engines. The lean burn operation prevents the formation of NOx [21].

HCCI is difficult to control. The use of two fuels with different reactivities (such as gasoline and diesel) can help to improve this situation. Such a fuel is called gasoline/diesel blend fuel (GDBF) [22].

Microdiagnostics and combustion modeling are other promising fields for future research. Another area of significant future research in the context of combustion is energy storage.

Today's burners and engines are mature, they will not solve the issues at hand. Major challenges in combustion research over the next decades, with a never-changing focus on safety and affordability, are sustainability (CO2 reduction, energy efficiency, less pollutants) and depletion of fossil fuels (change to renewable resources).

There is additional potential in knowledge exchange and the sharing of bestpractices among combustion equipment operators.

It is also expected that combustion synthesis [23] will gain importance for the manufacturing of certain materials, see Figure 1.13 for an example.

Figure 1.13 Combustion synthesis of Al2O3 in stoichiometric conditions at 600 °C. The number in the bottom right-hand corner refers to the time elapsed since the time the dish was placed in the oven (expressed in minutes and seconds) [24].

Future research in combustion will help overcome several engineering limitations of today, as they are not fundamental limits imposed by nature.

We will see a transition from “conventional” fossil fuels to renewable fuels and also to “unconventional fossil fuels” (oil shale, tar sands, clathrates [25]), and eventually to all-renewable fuels, compare Figure 1.14 for the distribution of organic carbon on Earth.

Figure 1.14 Distribution of organic carbon on earth (excluding dispersed organic carbon such as kerogen and bitumen) [25].

The structure of a gas hydrate (clathrate) is shown in Figure 1.15.

Figure 1.15 Unit cell of a gas hydrate [25].

Figure 1.16 depicts the location of hydrates for possible exploration on the world map.

Figure 1.16 Map of in situ hydrate locations [25].

Renewable fuels can be not only traditional fuels, such as wood, but also so-called energy crops, which can be burned directly or after conversion to a gaseous or liquid fuel by various technologies.

Figure 1.17 shows a projection of the oil and gas production in the US until 2035. One can see that unconventional oil and gas production will increase both in absolute numbers and relative contribution.

Figure 1.17 Trends in the US oil and gas production from “conventional” to “unconventional” according to the IEA Energy Outlook 2012 [26].

Other emerging trends in combustion are:

Algae for biomass combustion [27,28]Gasification for fuel cell technologyRevival of the external combustion engine (Stirling), also in other areas such as solar power [29]Increasing significance of methanol as a transportation fuel [30].

In [31], a vision for process steam and process heat as “Combustion 2020” is formulated. An outlook on combustion research is provided in [32] and the IEA World Energy Outlook 2012 [5,26,33].

1.4 Web Resources

There are good resources on the internet to learn more about the history of combustion. A few of them are listed here for further exploration by the interested reader:

History Of Fire Milestone, One Million Years Old, Discovered In Homo Erectus' Wonderwerk Cavehttp://www.huffingtonpost.com/2012/04/02/history-fire-million-homo-erectus_n_1397810.htmlAlchemists, ancient and modernhttp://www.economist.com/node/18226821University of Bristol: Alchemyhttp://www.chm.bris.ac.uk/webprojects2002/crabb/history.htmlIEA World Energy Outlook 2012http://www.worldenergyoutlook.org/media/weowebsite/2012/factsheets.pdf

References

1. Vattenfall (2013) http://www.vattenfall.co.uk/en/file/Natural_gas-ENG.pdf_16619005.pdf (accessed May 28, 2013).

2. Faraday, M. (2007) The Chemical History of a Candle, Book Jungle, ISBN: 978-1604241129.

3. Russum, D. (2013) http://www.geohelp.net/world.html (accessed May 28, 2013).

4. Spencer Weart & American Institute of Physics (2013) The Discovery of Global Warming, February 2013 http://www.aip.org/history/climate/timeline.htm (accessed May 28, 2013).

5. World Energy Outlook 2012, IEA (2013) http://www.worldenergyoutlook.org/publications/weo-2012, http://www.worldenergyoutlook.org/docs/weo2010/weo2010_es_german.pdf (accessed May 28, 2013).

6. International Society of Explosives Engineers (2013)http://www.explosives.org/index.php/component/content/article?id=69 (accessed May 28, 2013).

7. Cobb, C. and Goldwhite, H. (2001) Creations of Fire: Chemistry's Lively History from Alchemy to the Atomic Age, Basic Books, ISBN: 978-0738205946.

8.http://www.britannica.com/EBchecked/topic/127367/combustion/285206/History-of-the-study-of-combustion (accessed May 28, 2013).

9. Gregory, J.C. (1934) Combustion from Heracleitos to Lavoisier. Edward Arnold & Co, London.

10. Spear, B. and Watt, J. (2008) The steam engine and the commercialization of patents. World Patent Information, 30 (1), 53–58.

11. Sousanis, J. (2013) (15 August 2011) “World Vehicle Population Tops 1 Billion Units”, Wards Autohttp://wardsauto.com/ar/world_vehicle_population_110815 (accessed May 28, 2013).

12. Hofbauer, H., Rauch, R., Bosch, K., and Koch, R., and Aichernig, C. (2002) Biomass CHP plant Güssing – A success story, in Pyrolysis and Gasification of Biomass and Waste: Proceedings of an Expert Meeting, Strasbourg, 30 September–1 October (ed. A.V. Bridgwater), CPL Press.

13. Chen, W.Y.Seiner, J.Suzuki, T. and Lackner, M. (eds.) (2012) Handbook of Climate Change Mitigation, Springer, ISBN: 978-1441979902.

14. Rifkin, J. (2003) The Hydrogen Economy, 1st edn, Tarcher, ISBN: 978-1585422548.

15. O'Hayre, R., Colella, W., Cha, S.-W., and Prinz, F.B. (2009) Fuel Cell Fundamentals, 2nd edn, John Wiley & Sons Inc., Hoboken, ISBN: 978-0470258439.

16. Ju, Y. and Maruta, K. (2011) Microscale combustion: Technology development and fundamental research. Progress in Energy and Combustion Science, 37 (6), 669–715.

17. Di Lorenzo, A. and D'Alessio, A. (1988) Trends in combustion technology in relation to health risk. Annals of the New York Academy of Sciences, 534, 459–471.

18. Banerjee, D.K. (2012) Oil Sands, Heavy Oil & Bitumen: From Recovery to Refinery, Pennwell Corp., ISBN: 978-1593702601.

19. Zhao, F. (2003) Homogeneous Charge Compression Ignition (HCCI) Engines: Key Research and Development Issues, Society of Automotive Engineers, ISBN: 978-0768011234.

20. Lackner, M. (ed.) (2009) Alternative Ignition Systems, Process Eng Engineering GmbH, Vienna, ISBN: 978-3902655059.

21. Eco Gas (2013)http://www.eco-gas.com.au/wp-content/uploads/2009/06/homogenous_mix1.jpg (accessed May 28, 2013).

22. Yu, C., Wang, J.-xin., Wang, Z., and Shuai, S.-jin. (2012) Comparative study on Gasoline Homogeneous Charge Induced Ignition (HCII) by diesel and Gasoline/Diesel Blend Fuels (GDBF) combustion, Fuel, in press.

23. Lackner, M. (ed.) (2010) Combustion Synthesis: Novel Routes to Novel Materials, Bentham Science, eISBN 978-1-60805-155-7 http://www.bentham.org/ebooks/9781608051557/ (accessed May 28, 2013).

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2

Fuels

Abstract

In this chapter the most important fuels are discussed. Gaseous, liquid and solid fuels are described from the point view of their combustion and emission characteristics as well as fuel handling. Typical values for their main features are presented.

2.1 Introduction

Combustion is a series of exothermic chemical reactions between substances, including a combustible reactant, known as fuel, and an oxidizer, usually air or oxygen. The heat release is accompanied in most cases by light in the form of glowing or flame. Fuel in this context can be any material storing chemically bonded energy that can be extracted by the oxidation process after (self or external) ignition at a suitable temperature. The efficiency of the combustion process and the temperature to be maintained are affected by a number of parameters, including those of the environment, construction and operation; however, fuel characteristics are of major importance. In the following the most important properties of fuels are discussed for gaseous, liquid and solid fuels, respectively.

2.2 Gaseous Fuels

Gaseous fuels are simpler to ignite, handle and control than liquid or solid fuels. The molecular mixing of gaseous fuels and oxygen is very fast, especially when compared to the time of mass transport needed for the heterogeneous reactions of solid fuels. The combustion process of gaseous fuels is only limited by the velocity of mixing and the kinetics of the combustion reactions allowing for compact and intense burning. At atmospheric pressure a very short reaction time (∼1 ms) can be achieved.

Burning stability means that the flame ignites easily and thereafter burns continuously and steadily. Burning stability depends on the burner geometry, the control of air and fuel flow that is maintained by the re-ignition points

1. above the fuel's minimum ignition temperature,

2. between the fuel's ignition limits, and

3. when the feeding-velocity equals the flame-velocity in the full operation range of the burner and under all circumstances.

Flame stabilization can be achieved through various means, including a continuously lit pilot flame(s), swirl, bluff body or other techniques that prevent flame extinction or lift off.

Gaseous fuels can be classified by their origin as

1. gases found in their final form in nature (e.g., natural gas, coal bed methane)

2. gases produced directly from various sources (e.g., hydrogen, biogas, landfill gas, synthesis gas)

3. by-product gases, where the primary aim of the process is not the generation of the gaseous fuel in question (e.g., coke oven gas, blast furnace gas, chemical processes)

The characterization as well as the combustion chemistry of the gaseous fuels can be generalized independently of the classification shown above, therefore we will not make a distinction in the following according to the origin of the fuel.

The most common gaseous fuel is natural gas, of which the main component is methane (CH4). Valuable characteristics of natural gas are the relatively steady composition, high calorific value and combustion temperature, non-sooting flame, non-toxic nature and compressibility. Negative characteristics are: it is colorless, burns with a relatively low velocity, produces a non-luminescent flame, its exhaust is oxidative due to the water content, and it is highly explosive. The nominal heating value of natural gas is 35 MJ m−3, although the exact value is a function of the actual composition. Certain sources are rich in inert components (CO2, N2), causing a reduced calorific value as low as 15 MJ m−3. The composition of natural gas varies considerably between regions of origin, as shown in Tables 2.1 and 2.2.

Table 2.1 Typical compositions of natural gas [1]

NameFormulaAmount (vol.%)MethaneCH470–90EthaneC2H60–20PropaneC3H8ButaneC4H10Carbon dioxideCO20–8OxygenO20–0.2NitrogenN20–5Hydrogen sulfideH2S0–5Rare gasesAr, He, Ne, Xetrace

Table 2.2 Variations in composition of natural gas in Europe, vol. % [2–9]

Another important gaseous fuel in municipal use is PB (propane–butane) gas, obtained from raw natural gas by extracting the heavier components. PB gas can be stored and transported in liquid form. The chemical energy bound in PB gas is much higher than that of the natural gas: its calorific value is in the range 92–146 MJ m−3. PB gas is used by home and business consumers lacking natural gas pipelines. PB gas is also utilized for enriching natural gas for high temperature technologies (e.g., glass production).

The physical properties and combustion characteristics of PB gas components as well as other technological and technical gases are listed in Table 2.3.

Table 2.3 Density, molar mass and volume, gas constant of technical gases.

2.2.1 Density

The density of the gas mixture (ρmixt) can be calculated from the density of each component (ρi) and their volume fraction φi (Vi/V):

(2.1)

With the exception of extreme conditions, gaseous fuels and combustion products can be assumed to behave according to the law of ideal gases. Therefore the density of a gas mixture as a function of temperature, T, and pressure, p, can be calculated as:

(2.2)

2.2.2 Specific Heat Capacity

The specific heat capacity of the gas mixture (cp mixt) can be calculated from the components' specific heat capacity values (cpi) and the corresponding weight ratios of the components (mass fraction, wi):

(2.3)

In practice, specific heat capacity per unit volume of gas mixture is commonly used.

(2.4)

2.2.3 Molar Weight

The molar (molecular) weight of a gas mixture (Mmixt) can be calculated from the molar weights of the constituents (Mi):

(2.5)

2.2.4 Gas Constant

For calculations, for example, related to burner design the gas constant of the gas mixtures (Rmixt) might be an important parameter. One can calculate the gas mixture's gas constant from the molar weight of the mixture (Mmixt) and the universal gas constant (R):

(2.6)

The gas constant of gas mixtures can also be calculated from the weight ratios and the gas constant values (Ri) of the components:

(2.7)

2.2.5 Thermal Conductivity

The thermal conductivity of gases is a feature depending on the material properties, temperature and pressure.

For gas mixtures, the thermal conductivity (λmixt) can be calculated based on the individual thermal conductivities (λi) according to the following equation:

(2.8)

2.2.6 Viscosity

During the flow of real gases shear stress (internal friction) cannot be neglected.

Shear stress arising between adjacent gas layers:

(2.9)

In the correlation η is dynamic viscosity in Pa s; w is the flow velocity in m s−1; y is the distance coordinate perpendicular to the direction of flow in m. The dynamic viscosity of perfect gases in a pressure range from 103 to 106 Pa is independent of the pressure.

The viscosity of gas mixtures (ηmixt) cannot be calculated according to the simple mixing rules, especially in the case of high hydrogen content. The dynamic viscosity of gas mixtures can be determined according to the Herning–Zipperer equation:

(2.10)

The accuracy of the calculation conducted with this equation decreases with increase in the mixture's hydrogen content (up to 10% hydrogen content the error is about 2%, above 25% the equation is not reliable).

Kinematic viscosity (v) can be expressed as a ratio of dynamic viscosity and density – both of which are strongly dependent on pressure and temperature.

(2.11)

The dynamic viscosity dependence on temperature can be taken into consideration with the Sutherland equation:

(2.12)

where p0 = 101 325 Pa and T0 = 273.15 K (= 0 °C), and η0 is the corresponding dynamic viscosity, C is the Sutherland constant in K. Some of the common gases and their viscosity and conductivity data are listed in Table 2.4.

Table 2.4 Heat conductivity and viscosity of gases (at 0 °C and 101.3 kPa)

2.2.7 Heating Values

The higher heating value or HHV (also known as gross calorific value) is the heat released by burning a unit of dry fuel, when the starting temperatures of both the fuel and the air used for the combustion and the final temperature of the combustion product are 20 °C, the water content of the combustion product is in liquid form. The definition of lower heating value or LHV (also known as net calorific value) only differs in one aspect: the water content of the combustion product stays in the gaseous form. Thus the difference between the HHV and the LHV is equal to the heat of evaporation. The higher the water content of the flue gas, the greater the difference between the HHV and the LHV. Accordingly, there is no difference between the HHV and the LHV if the combustion products do not contain water, for example, if the fuel is pure CO.

The higher and/or lower heating value of a dry gas mixture can be calculated using the mixing rules, that is, from the composition of the mixture and the corresponding heating values of the components:

(2.13)

(2.14)

Table 2.5 summarizes the lower and higher heating values of various combustible gases.

Table 2.5 Heating values of pure gases.

When both experimental and theoretical (calculated) heating values are available, generally the measured (experimental) data are preferred.

2.2.8 Ignition Temperature

During combustion the reaction is initiated by either an external ignition source or autoignition. They are both the result of energy transfer (see also Chapter 3, Section 3.4).

Autoignition occurs when the slow oxidation reactions across the whole volume accelerate suddenly and the slow oxidation process transforms into a sudden explosion, like burning. The autoignition temperature in a given system is defined as the lowest temperature at which the gas mixture in that given system exhibits self-acceleration of the reaction as described above. In order to prevent autoignition, the container (the enclosing wall) of the fuel–oxidant mixture should be at a lower temperature than the autoignition temperature.

The ignition temperature is a feature of the fuel type and the igniting system. The gas–oxidizer mixture that has the lowest ignition temperature is called by various names, such as the minimum autoignition temperature, the minimum spontaneous ignition temperature and the self-ignition temperature. Usually the autoignition temperature reported in the literature is the minimum one.

The autoignition temperature depends on many factors, such as

pressure,oxygen content,ignition delay,energy of the ignition source,flow conditions,volume of the combustion chamber.

Increasing the oxygen content and the presence of dust decreases the autoignition temperature [10]

2.2.9 Ignition Limits

Limits of ignition (at a given pressure) refer to concentration values, outside of which range ignition will not be possible. In other words, if the fuel concentration is above the upper limit or below the lower limit, the mixture cannot be ignited.

The ignition concentration limits of combustible gases and gas mixtures (Zc mixt) are affected by the initial pressure and temperature, and their inert gas content and can be calculated as follows:

(2.15)

where Zi is the lower (or upper) ignition concentration limit of the components, and φci is the volume ratio of combustible constituents to the combustible material of the mixture.

Table 2.6 lists ignition concentration limits of various gases in air at atmospheric pressure and ambient temperature (20 °C).

Table 2.6 Ignition limits of combustible gases (in 20 °C air) and their laminar flame velocities.

2.2.10 Laminar Flame Velocity

Experimental data confirm that, upon ignition of a combustible mixture, the reaction front travels with a definite velocity. The actual chemical reaction occurs in a thin layer separating the still unburned combustible gas mixture and the combustion product at any given moment. This layer is called the burning zone or flame front. The velocity of the combustion reaction can be characterized by the laminar flame velocity, that is, the velocity of the flame front, in the normal direction (perpendicular to the front itself).

The laminar flame velocity should not be mistaken for the velocity of burning. Laminar flame velocity is the physico-chemical property of the combustible gas for a given oxidizing agent, temperature and pressure and is independent of the actual equipment in which the gas is burned. On the other hand, the burning velocity is affected by the conditions of combustion (the construction of the actual equipment). Laminar flame velocity values are usually determined experimentally. The maximum laminar flame velocity of gases in air is shown in the last column of Table 2.6.

Among practical gaseous fuels, hydrogen has the highest laminar flame velocity while methane shows the lowest. Their maximum value corresponds to slightly fuel rich mixtures, that is, at a concentration below the stoichiometric ratio. Flame velocity is also dependent on the system's initial temperature: increasing temperature markedly increases flame velocity. The effective flame velocity has a great significance, especially for burner design (see Chapter 3, Section 3.1).

2.2.11 Wobbe Index

Although, most burners are designed for combusting a certain type of fuel, there are situations when a burner should operate efficiently with more than one fuel.

For both transport and the combustion itself, it is essential that the various fuels used in a specific burner be similar in nature. This interchangeability is best described by the Wobbe index (Wo) in J m−3 (or Wobbe number) that is calculated from the fuel's HHV in J m−3 and its specific gravity v, which is the ratio of the density of the fuel gas to the density of the dry air under the same conditions.

(2.16)

The lower Wobbe index (WoL) can be calculated similarly from the LHV:

(2.17)