Chemistry
Combustion
Combustion is a chemical reaction between a fuel and an oxidizing agent, typically producing heat and light. It is characterized by the rapid release of energy in the form of heat and often accompanied by the production of flames. This exothermic reaction is essential for many industrial processes and is a key component of various energy production methods.
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10 Key excerpts on "Combustion"
eBook - PDF- James G. Quintiere(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
34 Principles of Fire Behavior The term spontaneous Combustion might be classed as a distinct fire pro- cess, but its occurrence results in either smoldering or a flame. This is simply an ignition process that occurs naturally due to the energy generated by a chemical reaction. Let us examine these forms of Combustion or fire in more detail. 2.2 Fire and Its Ingredients A chemical reaction is a process that involves the change in a molecule by rearranging its atoms into different molecules. Combustion or fire is a chemical reaction involving the release of energy, some of which can be in the form of light. Combustion and fire are synonymous. A Combustion reaction commonly involves the fuel molecule combining with oxygen to produce new molecules as products . A flame is Combustion in which a fuel gas reacts with an oxidizer, commonly oxygen in the air. Smoldering is Combustion in which a fuel reacts as a solid with oxygen in the air. To define a chemical reaction as fire or combus- tion, sufficient perceptible energy must be released. The rate of energy release per unit volume of the chemical reaction determines whether that reaction is fire. The size of the flame is not a factor. For a flame produced by a gaseous fuel combining with oxygen in normal air, the energy release is strongly dependent on the temperature of the molecules. Low temperatures produce an impercep- tible amount of energy. Let us take a deeper look at this process. 2.2.1 Typical Temperatures and Energy Levels to Achieve Combustion Roughly, the power production rate of a gaseous fuel reacting in air can be described in terms of how many kilowatts (kW) are produced in a cubic meter (m 3 ) of the mixture of fuel and air. So, consider such a small volume region composed of a mixture of fuel and air. The power production depends on the temperature of the mixture. The speed of the reaction and the energy released are the study of chemical kinetics, an entire branch of chemistry.
eBook - PDFIndustrial and Process Furnaces
Principles, Design and Operation
- Barrie Jenkins, Peter Mullinger(Authors)
- 2011(Publication Date)
- Butterworth-Heinemann(Publisher)
Chapter contents 2.1 Simple Combustion chemistry 32 2.2 Combustion calculations 33 2.3 Chemical reaction kinetics 36 2.4 The physics of Combustion 47 Nomenclature 63 References 64 Chapter 2 The Combustion process Combustion is a specific group of chemical reactions where a fuel and oxygen burn together at sufficiently high temperature to evolve heat and Combustion products. The fuel can be a gas (e.g. hydrogen, natural gas), a liquid (e.g. oil, alcohol, sulphur), or a solid (e.g. coal, wood, peat). The rate at which these Combustion reactions occur can vary from a very slow decay to an instantaneous explosion. Both the chemical kinetics and physical diffusion processes involved control this rate. The objective of the Combustion engineer and plant opera-tor is to obtain a steady heat release at the rate required to suit the process objectives. 2.1 Simple Combustion chemistry Most industrial fuels are hydrocarbons, so called because their primary elemental constituents are carbon and hydrogen. These are oxidised to release heat during Combustion. The chemistry of this oxidation process involves very complex chain reactions. However, for most engineering design purposes we can reasonably simplify the chemistry to four basic reactions. The following equations define the Simple Chemically Reacting System (SCRS). 2.1.1 The complete oxidation of carbon C O CO kJmol 2 2 1 394 Providing there is sufficient oxygen present in the mixture, the above reaction describes the overall result of the oxidation of carbon. However, this reaction rarely occurs in practical Combustion systems but is the result of a chain of reactions involving carbon monoxide as an intermediate product, as discussed later in this section.
eBook - PDF- Ghazi A. Karim(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
93 6 Chemical Kinetics of Fuel Combustion 6.1 Chemical Reactions Fuel Combustion processes by definition involve chemical reactions that are exothermic and relatively fast� The proper understanding or modeling of such processes needs to define what type and sequence of chemical changes take place and consider their consequent temporal changes in the composi-tion and associated properties of the reactive system� Often, this is no simple task because complex coupled nonlinear differential equations would result with much of the key information about the mode of these reactions, their corresponding properties, and the reactive species may not be sufficiently known� It is only relatively recently, with the continuing increase in the capacity and speed of computing facilities coupled with significant advances made in the solution of complex mathematical systems and continuing prog-ress in the science of chemistry, that much success is achieved in satisfying these objectives� This effort was prompted largely by the need to bring about substantial further improvements to the performance of fuel Combustion systems to enhance efficiencies, reduce emissions, and secure greater safety and reliability while burning a wider range of fuels� Classical thermodynamic consideration of the Combustion process, as shown earlier, can yield useful information as to the total ideal amount of energy changes for the reaction and the associated final temperature of the products and their ultimate composition under ideal equilibrium conditions� However, such approaches cannot provide any indication of the rates of pro-cesses involved or the transient changes in the properties of the reactive sys-tem as it progresses toward the assumed final stage� Such information is of critical importance in practice such as in considering the transfer of energy and extent of associated emissions (Figure 6�1)�
eBook - PDF- F El-Mahallawy, S. E-Din Habik(Authors)
- 2002(Publication Date)
- Elsevier Science(Publisher)
Chapter 1 Combustion Fundamentals I.I Introduction This chapter is intended as introductory text in the fundamentals of Combustion for engineering graduate students, as well as a basis for the next four chapters. Combustion is defined as a rapid exothermic reaction that liberates substantial energy as heat and flames as Combustion reactions with the ability to propagate through a suitable medium. This propagation results from the strong coupling of the reaction with the molecular transport process. The chemistry and physics of Combustion, i.e. destruction and re- arrangement of certain molecules, rapidly release energy within a few millionths of second. Currently, Combustion is a mature discipline and an integral element of diverse research and development programs from fundamental studies of the physics of flames and high-temperature molecular chemistry to applied engineering projects involved with developments such as advanced coal-burning equipment and improved Combustion furnaces, boilers, and engines. These developments are important in controlling the pollutant emissions. Therefore, it is appropriate in this chapter to present two very important practical considerations relative to the Combustion reaction systems, which are the mass and energy balance used to describe such systems. The chapter starts with the energy sources including the energy characteristics of various important fuels resources and their physical and chemical properties. This is followed by introducing some definitions of ideal gases, mass conservation and basic thermodynamic principles, as well as, general energy balance for a chemically reactive medium. Description of the practical stoichiometry and thermochemical requirements, which apply during Combustion processes such as chemical reaction, equilibrium composition and temperature, are also presented.
eBook - PDFExperimental Combustion
An Introduction
- D. P. Mishra(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
1 1 Introduction to Combustion Conscience is the celestial fire that propels us to the heaven of humanity. D. P. Mishra 1 .1 Introduction Combustion is as old as human civilization, and I personally feel that the discov-ery of fire is one of the greatest findings of human civilization. Man’s subsequent mastery over fire has made possible all the developments in science and technol-ogy that we enjoy today. It is believed that Indians were first to recognize the efficacy of the fire even in the ancient Vedic era. The description of fire goes back to the Rig Veda, one of the oldest scripture of human civilization. In modern times, Combustion continues to play a very important role in driv-ing the human race toward the path of prosperity and progress, because 90% of our worldwide energy demand is met by the Combustion of fuel. Hence, I believe that Combustion will remain a very important subject of interest as long as human civilization exists. 2 1. Introduction to Combustion In layman’s terms, Combustion can be thought of as the process of setting fire to a fuel, of course in a controlled manner. It is basically a chemical process in which fuel is burned in the presence of an oxidizer. The chemical reactions involved in the process of Combustion must be exothermic in nature, which lib-erate enough heat to sustain the Combustion process itself. Examples of combus-tion devices are candle flames, lighting of matchsticks, cigarette burning, wood burning, liquefied petroleum gas (LPG) burners for cooking, furnaces, piston engines, gas turbine engines, and rocket motors. 1 .2 Definition of Fuel/Oxidizer Since fuel and oxidizer are the main constituents for the Combustion process to take place, we need to understand the scientific meaning of fuel and oxidizer. In other words, we must ask certain pertinent questions such as, What is a fuel? What is an oxidizer? Chemically, an oxidizer can be defined as an element that accepts an electron.
eBook - PDF- Jr., Charles E. Baukal(Authors)
- 2000(Publication Date)
- CRC Press(Publisher)
29 0-8493-7686-6/97/$0.00+$.50 © 1997 by CRC Press LLC 2 Some Fundamentals of Combustion 2.1 Combustion CHEMISTRY Combustion is usually considered to be the controlled release of heat and energy from the chemical reaction between a fuel and an oxidizer. This is in contrast to a fire or explosion which are usually uncontrolled and undesirable. Virtually all of the Combustion in industrial processes uses a hydro-carbon fuel. A generalized Combustion reaction for a typical hydrocarbon fuel can be written as follows: fuel + oxidizer → CO 2 + H 2 O + Other species (2.1) The “other species” depends on what oxidizer is used and what is the ratio of the fuel to oxidizer. The most commonly used oxidizer is air, which consists of nearly 79% N 2 by volume and is normally carried through in the Combustion process. If the Combustion is fuel rich, meaning there is not enough oxygen to fully combust the fuel, then there will be unburned hydrocarbons in the exhaust products and little if any excess O 2 . If the Combustion is fuel lean, meaning there is more oxygen than required to fully combust the fuel, then there will be excess O 2 in the exhaust products. The exhaust gas composition is very important in determining the heat transfer in the system. Unburned hydrocarbons in the exhaust indicate that the fuel was not fully combusted and therefore not all of the available heat was released. High excess O 2 levels in the exhaust usually indicate that too much oxidizer was supplied. The excess oxidizer carries sensible energy out the exhaust, which again means that some of the available heat of the fuel was not fully utilized to heat the load. If the oxidizer is air, then a large proportion of the available energy in the fuel will normally be carried out the flue with the exhaust products. This is discussed in more detail in Chapter 12. 2.1.1 F UEL P ROPERTIES Table E.2 in Appendix E gives some of the properties for gaseous fuels commonly used in industrial Combustion systems.
eBook - PDF- Scott I. Fairgrieve(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
23 2 Fire and Combustion 2 .1 FIRE In order to be able to interpret the damage done to human tissues by fire, it is imper-ative to have a clear understanding of what fire is and how it is physically altering these tissues. Simply put, fire is a chemical reaction; more specifically, an oxidation reaction that generates heat and light (DeHaan, 2002). This process, known generally as Combustion, involves the release of visible energy in the form of flames (Icove and DeHaan, 2004). Flaming Combustion is, in fact, a gaseous combination in which both fuel and oxidizer are gases. An example of flaming fire would be the flames associated with any active fire such as those seen in a fireplace. Nearly all destruc-tive fires involve flaming Combustion (DeHaan, 2002). Glowing Combustion occurs when the surface of a solid fuel combines with a gaseous oxidizer, typically the oxygen in air (DeHaan, 2002). Glowing Combustion is exemplified by a smoldering fire, such as one would find in a mattress, or even a charcoal fire. The limiting factor between a flaming fire and a glowing or smoldering fire is the nature and condition of the fuel and its availability to oxygen. Most people are aware of the three requirements in order to make a fire: fuel, heat, and air (oxygen). This so-called “fire triangle” needs to be refined and exam-ined in greater detail if an analyst is to ultimately understand Combustion of human tissues. The fuel in the triangle is simply the combustible material. The heat that is required must be of a sufficient level in order to raise the fuel to its ignition temperature and release fuel vapors. The oxidizing agent, oxygen (O 2 ) in air, must be present in a quantity that will sustain Combustion. However, the fire triangle is actually now referred to as the fire tetrahedron. The fourth factor, given that all of the three aforementioned conditions are met, is an uninhibited exothermic chemical chain reaction.
eBook - PDFThermodynamics
A Smart Approach
- Ibrahim Dinçer(Author)
- 2020(Publication Date)
- Wiley(Publisher)
Running power plants at full capacity is not practical due to the fluctuating nature of energy demands during night and day, summer and winter, etc. There is a need for storage as well. Coming back to thermal energy (heat) requirement, the need is huge and is why many countries go around fossil fuels where Combustion becomes the heart of the equation. A Combustion process involves the oxidation of a fuel in order to release thermal energy. Fuels include not only fossil fuels but also many other materials that can be reacted exothermically, such as hydrogen and ammonia. A fuel is defined as any material that can be burned to release thermal energy. Fossil fuels are, nonetheless, the dominant fuel used currently glob-ally, accounting for nearly 78% of the total energy used today. This chapter focuses its discus-sion on Combustion reactions; however, the methodology discussed in this chapter is applicable to all chemical reactions. The main objectives of this chapter are the reader should be able to understand how to balance the chemical reactions in order to be able to understand the amounts of reactants and products, then how to apply the balance equations on chemical reactions, and, finally, to be able to assess the performance of these processes. The aim of this chapter is to first introduce the basic information about fuels and their heating values ( HVs ), forms of chemical reactions, fuel Combustion related parameters and concepts, and to discuss the use of the first law of thermodynamics ( FLT ) and the second law of thermodynamics ( SLT ) for Combustion processes. It is also aimed to provide principles and methodologies about how to write balance equations and analyze the Combustion processes and evaluate their performances. 8.2 Fuels The basic definition of fuel is any material that is combustible and produces heat if com-busted.
eBook - PDF- James G. Speight(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
Thus, Combustion phenomena arise from the interaction of chemical and physical processes. The first requirement, somewhat difficult with coal because of its molecular complexity, is that the overall stoichiometry of the reaction must always be established. For these purposes, coal is usually represented by carbon, which can react with oxygen in two ways, producing either carbon monoxide or carbon dioxide: C O 2CO coal 2 + → C O CO coal 2 2 + → In direct Combustion, coal is burned (i.e., the carbon and hydrogen in the coal are oxidized into car-bon dioxide and water) to convert the chemical energy of the coal into thermal energy after which the sensible heat in the products of Combustion can then be converted into steam that can be external work or directly into shaft horsepower (e.g., in a gas turbine): C O CO coal 2 2 + → H O H O coal 2 2 + → C H O CO H coal 2 2 + → + In fact, the Combustion process actually represents a means of achieving the complete oxidation of coal. 433 Combustion On a more formal basis, the Combustion of coal may be simply represented as the staged oxida-tion of coal carbon to carbon dioxide with any reactions of the hydrogen in the coal being consid-ered to be of secondary importance: C O 2CO coal 2 + → 2CO O 2CO 2 2 + → The stoichiometric reaction equations are quite simple but there is a confusing variation of hypothe-ses about the sequential reaction mechanism, which is caused to a great extent by the heterogeneous nature (solid and gaseous phases) of the reaction. But, for the purposes of this text, the chemistry will remain simple as shown in the earlier equations. Other types of Combustion systems may be rate controlled due to the onset of the Boudouard reaction: CO C 2CO 2 + → In more general terms, the Combustion of carbonaceous materials (which contain hydrogen and oxygen as well as carbon) involves a wide variety of reactions between the many reactants, inter-mediates, and products (Table 14.1).
eBook - PDF- Eugene L. Keating(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
42 Chapter 2 hydrocarbon chemistry, the most common oxidant is O 2 and the most stable oxidized products are CO 2 and H 2 O; i.e., O H CO oxidant Fuel 2 2 b a STP + → + If the water formed exists in the vapor state, the heat of Combustion is termed lower heating value. If the water in the product state exists as a liquid, the heat of Combustion is termed higher heating value of the fuel. This implies that the product mixture gave up its additional latent heat in going from a vapor to a liquid state. Obviously, actual Combustion processes do not occur at STP , and real Combustion processes may not go to completion. It is therefore necessary to develop a means of expressing the energetics of chemical reactions in the more general case of Combustion, which will be done in the following sections. 2.5 FIRST LAW FOR REACTIVE SYSTEMS The first law of thermodynamics leads directly to an energy conservation principle. An extensive development of this concept can be found in most undergraduate thermo-dynamics texts. This fundamental relationship provides a statement, in general terms, of conservation and conversion of various energy forms. From classical thermodynamics and the first law, one can obtain a differential form of the general energy equation for an open system (also termed a fixed or control volume relationship) as ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ Σ − Σ + − = min Btu kW out in i i i i i i e m e m W Q dt dE & & & & (2.25a) or i i i i i i e N e N W Q dt dE & & & & & out in Σ − Σ + − = (2.25b) where E = total system energy in C.V. Q & = rate of heat transfer to/from (+ in/– out) the C.V. W & = rate of work transfer to/from (– in/+ out) the C.V. j i m , = mass flow rate of material transferred across the C.V. boundaries j i N , & = molar flow rate of material transferred across the C.V. boundaries j i e , = intensive total energy per unit mass of material transferred across the C.V. boundaries j i e , = intensive total energy per unit mole of material transferred across the C.V.
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