Combustion and flame phenomena in context
Creators: name: Chen, Junjie
Publisher: name: Zenodo
Year: 2022
Subjects: Combustion phenomena; Flame phenomena; Chemical reactions; Redox reactions; Reaction mechanisms; Redox reactions; Premixed flames; Diffusion flames

description: <strong>Combustion and flame phenomena in context</strong> Junjie Chen Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China Contributor: Junjie Chen, ORCID: 0000-0002-5022-6863, E-mail address: koncjj@gmail.com A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur. The substance (or substances) initially involved in a chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants [1]. Reactions often consist of a sequence of individual sub-steps, the so-called elementary reactions, and the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, and sometimes intermediate products and reaction conditions. Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. Reactions may proceed in the forward or reverse direction until they go to completion or reach equilibrium. Reactions that proceed in the forward direction to approach equilibrium are often described as spontaneous and reduce the free energy if they occur at constant temperature and pressure. Non-spontaneous reactions require input of energy to go forward. A reaction may be classified as redox in which oxidation and reduction occur or non-redox in which there is no oxidation and reduction occurring. Most simple redox reactions may be classified as combination, decomposition, or single displacement reactions. Chemical reactions must be distinguished from physical changes [2]. Physical changes include changes of state, such as ice melting to water and water evaporating to vapor. If a physical change occurs, the physical properties of a substance will change, but its chemical identity will remain the same. No matter what its physical state, water is the same compound, with each molecule composed of two atoms of hydrogen and one atom of oxygen. However, if water, as ice, liquid, or vapor, encounters sodium metal, the atoms will be redistributed to give the new substances molecular hydrogen and sodium hydroxide [2]. By this, a chemical change or reaction has occurred. In reactions under normal laboratory conditions, matter is neither created nor destroyed, and elements are not transformed into other elements [2]. Therefore, equations depicting reactions must be balanced; that is, the same number of atoms of each kind must appear on opposite sides of the equation. The balanced equation for the iron-sulfur reaction shows that one iron atom can react with one sulfur atom to give one formula unit of iron sulfide. The ratio of reactants and products in a chemical reaction is called chemical stoichiometry [2]. Stoichiometry depends on the fact that matter is conserved in chemical processes, and calculations giving mass relationships are based on the concept of the mole. One mole of any element or compound contains the same number of atoms or molecules, respectively, as one mole of any other element or compound. By international agreement, one mole of the most common isotope of carbon (carbon-12) has a mass of exactly 12 grams (this is called the molar mass). Energy plays a key role in chemical processes [2]. According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved on making new bonds, and the net result is the absorption of energy. Such a reaction is said to be endothermic if the energy is in the form of heat. The opposite of endothermic is exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic (energy evolved) and endoergic (energy required) are used when forms of energy other than heat are involved. A great many common reactions are exothermic [2]. The formation of compounds from the constituent elements is almost always exothermic [2]. Formation of water from molecular hydrogen and oxygen and the formation of a metal oxide such as calcium oxide from calcium metal and oxygen gas are examples. Among widely recognizable exothermic reactions is the combustion of fuels (such as the reaction of methane with oxygen). The formation of slaked lime when water is added to lime is exothermic. This reaction occurs when water is added to dry Portland cement to make concrete, and heat evolution of energy as heat is evident because the mixture becomes warm. Not all reactions are exothermic (or exoergic) [2]. A few compounds, such as nitric oxide and hydrazine, require energy input when they are formed from the elements [2]. The decomposition of limestone to make lime is also an endothermic process; it is necessary to heat limestone to a high temperature for this reaction to occur [2]. The decomposition of water into its elements by the process of electrolysis is another endoergic process [2]. Electrical energy is used rather than heat energy to carry out this reaction. Generally, evolution of heat in a reaction favors the conversion of reactants to products [2]. However, entropy is important in determining the favorability of a reaction. Entropy is a measure of the number of ways in which energy can be distributed in any system. Entropy accounts for the fact that not all energy available in a process can be manipulated to do work. A chemical reaction will favor the formation of products if the sum of the changes in entropy for the reaction system and its surroundings is positive [2]. An example is burning methane. Methane has a low entropy. When methane burns, it produces ash as well as the high-entropy substances carbon dioxide gas and water vapor. The entropy of the reacting system increases during combustion. Just as important, the heat energy transferred by the combustion to its surroundings increases the entropy in the surroundings. The total of entropy changes for the substances in the reaction and the surroundings is positive, and the reaction is product-favored. When hydrogen and oxygen react to form water, the entropy of the products is less than that of the reactants. Offsetting this decrease in entropy, however, is the increase in entropy of the surroundings owing to the heat transferred to it by the exothermic reaction [2]. Again because of the overall increase in entropy, the combustion of hydrogen is product-favored. Chemical reactions commonly need an initial input of energy to begin the process [2]. Although the combustion of methane is an exothermic process, a burning match or a spark is needed to initiate this reaction. The energy supplied by a match arises from an exothermic chemical reaction that is itself initiated by the frictional heat generated by rubbing the match on a suitable surface. For a reaction to occur, it is not sufficient that it be energetically product-favored [2]. The reaction must also occur at an observable rate. Several factors influence reaction rates, including the concentrations of reactants, the temperature, and the presence of catalysts. The concentration affects the rate at which reacting molecules collide, a prerequisite for any reaction. Temperature is influential because reactions occur only if collisions between reactant molecules are sufficiently energetic [2]. The proportion of molecules with sufficient energy to react is related to the temperature. Catalysts affect rates by providing a lower energy pathway by which a reaction can occur. Among common catalysts are precious metal compounds used in automotive exhaust systems that accelerate the breakdown of pollutants such as nitrogen dioxide into harmless nitrogen and oxygen. Chemists classify reactions in a number of ways: (a) by the type of product, (b) by the types of reactants, (c) by reaction outcome, and (d) by reaction mechanism. Often, a given reaction can be placed in two or even three categories. Oxidation-reduction (redox) reactions involve the transfer of one or more electrons from a reducing agent to an oxidizing agent. This has the effect of reducing the real or apparent electric charge on an atom in the substance being reduced and of increasing the real or apparent electric charge on an atom in the substance being oxidized [2]. Simple redox reactions include the reactions of an element with oxygen. For example, magnesium burns in oxygen to form magnesium oxide. The product is an ionic compound. The reaction occurs with each magnesium atom giving up two electrons and being oxidized and each oxygen atom accepting two electrons and being reduced. Another common redox reaction is one step in the rusting of iron in damp air. Here iron metal is oxidized to iron dihydroxide; elemental oxygen is the oxidizing agent. Redox reactions are the source of the energy of batteries [2]. The electric current generated by a battery arises because electrons are transferred from a reducing agent to an oxidizing agent through the external circuitry [2]. In a common dry cell and in alkaline batteries, two electrons per zinc atom are transferred to the oxidizing agent, thereby converting zinc metal to the zinc ion [2]. In dry-cell batteries, which are often used in flashlights, the electrons given up by zinc are taken up by ammonium ions present in the battery as ammonium chloride [2]. In alkaline batteries, which are used in calculators and watches, the electrons are transferred to a metal oxide such as silver oxide, which is reduced to silver metal in the process. Reaction mechanisms provide details on how atoms are shuffled and reassembled in the formation of products from reactants [2]. Chain and photolysis reactions are named on the basis of the mechanism of the process [2]. Chain reactions occur in a sequence of steps, in which the product of each step is a reagent for the next. Chain reactions generally involve three distinct processes: an initiation step that begins the reaction, a series of chain-propagation steps, and, eventually, a termination step. Polymerization reactions are chain reactions, and the formation of Teflon from tetrafluoroethylene is one example [2]. In this reaction, a peroxide (a compound in which two oxygen atoms are joined together by a single covalent bond) may be used as the initiator [2]. Peroxides readily form highly reactive free-radical species (a species with an unpaired electron) that initiate the reaction. Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion does not always result in fire, because a flame is only visible when substances undergoing combustion vaporize, but when it does, a flame is a characteristic indicator of the reaction. While the activation energy must be overcome to initiate combustion, the heat from a flame may provide enough energy to make the reaction self-sustaining. Combustion is often a complicated sequence of elementary radical reactions [3]. Solid fuels first undergo endothermic pyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that incandescent light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into water vapor, a reaction which is commonly used to fuel rocket engines. Uncatalyzed combustion in air requires relatively high temperatures. Complete combustion is stoichiometric concerning the fuel, where there is no remaining fuel, and ideally, no residual oxidant. Thermodynamically, the chemical equilibrium of combustion in air is overwhelmingly on the side of the products. However, complete combustion is almost impossible to achieve, since the chemical equilibrium is not necessarily reached, or may contain unburnt products such as carbon monoxide, hydrogen and even carbon. Thus, the produced smoke is usually toxic and contains unburned or partially oxidized products. Any combustion at high temperatures in atmospheric air, which is 78 percent nitrogen, will also create small amounts of several nitrogen oxides, since the combustion of nitrogen is thermodynamically favored at high, but not low temperatures. Since burning is rarely clean, fuel gas cleaning or catalytic converters may be required by law. In complete combustion, the reactant burns in oxygen and produces a limited number of products. When a hydrocarbon burns in oxygen, the reaction will primarily yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yield carbon dioxide, sulfur will yield sulfur dioxide, and iron will yield iron (III) oxide. Nitrogen is not considered to be a combustible substance when oxygen is the oxidant. Still, small amounts of various nitrogen oxides form when the air is the oxidative. Combustion is not necessarily favorable to the maximum degree of oxidation, and it can be temperature-dependent. For example, sulfur trioxide is not produced quantitatively by the combustion of sulfur. Nitrogen oxides species appear in significant amounts at temperatures above about 1,540 °C, and more is produced at higher temperatures [4]. The amount of nitrogen oxides is also a function of oxygen excess. In most industrial applications and in fires, air is the source of oxygen. In the air, each mole of oxygen is mixed with approximately 3.71 mol of nitrogen. Nitrogen does not take part in combustion, but at high temperatures some nitrogen will be converted to nitrogen oxides. On the other hand, when there is insufficient oxygen to combust the fuel completely, some fuel carbon is converted to carbon monoxide, and some of the hydrogens remain unreacted. A complete set of equations for the combustion of a hydrocarbon in the air, therefore, requires an additional calculation for the distribution of oxygen between the carbon and hydrogen in the fuel. Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink, such as a solid surface or flame trap. As is the case with complete combustion, water is produced by incomplete combustion; however, carbon, carbon monoxide, and hydroxide are produced instead of carbon dioxide. For most fuels, pyrolysis occurs before combustion [5]. In incomplete combustion, products of pyrolysis remain unburnt and contaminate the smoke with noxious particulate matter and gases. Partially oxidized compounds are also a concern; partial oxidation of ethanol can produce harmful acetaldehyde, and carbon can produce toxic carbon monoxide. The designs of combustion devices can improve the quality of combustion, such as burners and internal combustion engines. Further improvements are achievable by catalytic after-burning devices (such as catalytic converters) or by the simple partial return of the exhaust gases into the combustion process. Such devices are required by environmental legislation for cars in most countries. They may be necessary to enable large combustion devices, such as thermal power stations, to reach legal emission standards. Carbon monoxide is one of the products from incomplete combustion. Carbon is released in the normal incomplete combustion reaction, forming soot and dust. Rapid combustion is a form of combustion, otherwise known as a fire, in which large amounts of heat and light energy are released, which often results in a flame. This is used in a form of machinery such as internal combustion engines and in thermobaric weapons. Such a combustion is frequently called a rapid combustion, though for an internal combustion engine this is inaccurate. An internal combustion engine nominally operates on a controlled rapid burn [6]. When the fuel-air mixture in an internal combustion engine explodes, that is known as detonation. Spontaneous combustion is a type of combustion which occurs by self-heating (increase in temperature due to exothermic internal reactions), followed by thermal runaway (self-heating which rapidly accelerates to high temperatures) and finally, ignition. For example, phosphorus self-ignites at room temperature without the application of heat. Organic materials undergoing bacterial composting can generate enough heat to reach the point of combustion. Assuming perfect combustion conditions, such as complete combustion under adiabatic conditions, the adiabatic combustion temperature can be determined. The formula that yields this temperature is based on the first law of thermodynamics and takes note of the fact that the heat of combustion is used entirely for heating the fuel, the combustion air or oxygen, and the combustion product gases. The adiabatic combustion temperature (also known as the adiabatic flame temperature) increases for higher heating values and inlet air and fuel temperatures and for stoichiometric air ratios approaching one. In industrial fired heaters, power station steam generators, and large gas-fired turbines, the more common way of expressing the usage of more than the stoichiometric combustion air is percent excess combustion air. For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air is being used. All flames can be classified either as premixed flames or as flames that burn without premixing [7]. Flame combustion is most prominent with fuels that have been premixed with an oxidant, either oxygen or a compound that provides oxygen, for the reaction. The temperature of flames with this mixture is often several thousand degrees [7]. The chemical reaction in such flames occurs within a narrow zone several micrometers thick. This combustion zone is usually called the flame front. Dilution of the burning mixture with an inert gas, such as helium or nitrogen, lowers the temperature and, consequently, the reaction rate. Great amounts of inert gas extinguish the flame, and the same result is achieved when substances that remove any of the active species are added to the flame [7]. Conditions must be such that the flame is fixed at the burner nozzle or in the combustion chamber; this positioning is required in many practical uses of combustion [7]. Various devices, such as pilot flames and recirculation methods, are designed for this purpose. The principal quantitative characteristic of a flame is its normal, or fundamental, burning velocity, which depends on the chemical and thermodynamic properties of the mixture and on pressure and temperature, under given conditions of he
description: Contributor: Junjie Chen, ORCID: 0000-0002-5022-6863, E-mail address: koncjj@gmail.com, Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China
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