Exothermic reaction combustion

Any chemical reaction that gives off heat is called an exothermic reaction. Combustion reactions are always exothermic because they always give off heat. Sometimes they give off light, too. For example, wood burning in a fireplace is an exothermic combustion reaction, giving off both light and heat. 

Reactions that release energy in the form of heat are called exothermic reactions. Combustion reactions of fossil fuels are familiar examples of exothermic reactions. Hydrocarbons—including methane (CH4), the main component of natural gas, and octane (CgHis), a minor component of gasoline— undergo combustion with an excess of O2 to yield CO2 and H2O. These reactions release heat energy. The amounts of heat energy released at constant pressure are shown for the reactions of one mole of methane and of two moles of octane. 

Heat Recovery and Feed Preheating. The objective is to bring the reactants to and from reaction temperature at the least utihty cost, and to recover maximum waste heat at maximum temperature. The impact of feed preheating merits a more careful look. In an exothermic reaction, preheated feed permits the reactor to act as a heat pump, ie, to buy low and sell high. The most common example is combustion-air preheating for a furnace. 

In the following, only dusts are called combustible in the airborne state if they require oxygen from the air for exothermic reaction. 

Regeneration with air can be done with continuous or periodic addition of small amounts of air. Both must be done carefully because of exothermic reaction. Regeneration is never complete, so the beds must be eventually changed out. This must be done carefully because of the pyrophoric (spontaneously combustible) nature of the iron sulfide. The entire bed is wetted first. 

Hydrochloric acid may conveniently be prepared by combustion of hydrogen with chlorine. In a typical process dry hydrogen chloride is passed into a vapour blender to be mixed with an equimolar proportion of dry acetylene. The presence of chlorine may cause an explosion and thus a device is used to detect any sudden rise in temperature. In such circumstances the hydrogen chloride is automatically diverted to the atmosphere. The mixture of gases is then led to a multi-tubular reactor, each tube of which is packed with a mercuric chloride catalyst on an activated carbon support. The reaction is initiated by heat but once it has started cooling has to be applied to control the highly exothermic reaction at about 90-100°C. In addition to the main reaction the side reactions shown in Figure 12.6 may occur. 

General Incineration (oxidation) is the best-known method for the removal of gaseous industrial waste. Combustible compounds containing carbon, hydrogen, and oxygen are converted to carbon dioxide and water by the overall exothermic reactions [Eq. (13.72)]. When chlorinated or sulfur-containing compounds are present in the effluent, the products of combustion include HCl/CE or S02/S03. ... 

A deflagration is a slow burning exothermic reaction similar to the combustion explosion, but which propagates from the burning gases into the unreacted material at a velocity that is less than the speed of sound in the unreacted material. Most (not all) explosions are deflagrations. 

Detonation explosions are similar to combustion explosions and are exothermic reactions that proceed into the unreacted material at a velocity much greater than the speed of sound in an unreacted material and are accompanied by a flame front shock wave in the material followed closely by a combustion wave that releases the energy and sustains the shock wave at extremely high pressure [39] [40]. In hydrocarbons, the velocity can reach 6,000-9,000 ft/sec. 

The standard heat of combustion (ziH") of a chemical substance (usually an organic compound) is the same as the standard heat of reaction for complete oxidation of 1 mole of the substance in pure oxygen to yield COj(g) and HjO(f) as products. A reference state of 25°C and 1 atm is assumed in quoting standard heats of combustion in cal/g-mole. The value of AH" is always negative because combustion is an exothermic reaction. Note that the standard heats of combustion for carbon and hydrogen are the same as the heats of formation for CO,(g) and HjO(f), respectively. 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

A third alternative has been proposed by Anderson and Brown (A6, A9) as an outgrowth of their research on the ignition of composite propellants. Their ignition studies suggest significant contributions to the overall combustion process from the solid phase. Two exothermic reaction zones contributing to combustion are considered, as shown schematically in Fig. 19. 

In an exothermic reaction, such as the synthesis of ammonia or a combustion reaction, the heat released by the reaction increases the disorder of the surroundings. In some cases, the entropy of the system may decrease, as when a gaseous reactant is converted into a solid or liquid. However, provided that AH is large and negative, the release of energy as heat into the surroundings increases their entropy so much that it dominates the overall change in entropy and the reaction is spontaneous (Fig. 7.18). 

Zel dovich Ya.B., Regime classification of an exothermic reaction with non-uniform initial conditions. Combust. Flame, 39,211,1990. 

Combustion of fuels is an exothermic reaction. Explain how the heat energy from this type of reaction is often used to do useful work. 

When one considers the potential high-energy release on rupture of a carborane unit, together with the thermodynamic stability of combustion products, it is hardly surprising that there is a body of literature that reports on the use of carbo-ranes within propellant compositions. Their use in energetic applications is to be expected when the enthalpy of formation (AH/) data for the products of combustion for boron are compared to those of carbon. Thermodynamic data for the enthalpy of formation of o-carborane and of typical boron and carbon combustion products is shown in Table 4. Measurements of the standard enthalpy of combustion32 for crystalline samples of ortho-carborane show that complete combustion is a highly exothermic reaction, AH = — 8994 KJmol. ... 

Interaction of powdered boron and steam may become violent at red heat [1]. The highly exothermic reactions with water might become combustive or explosive processes at sufficiently high temperatures and pressures [2],... 

A process is described [224] in which an exothermic reaction takes place in a semi-batch reactor at elevated temperatures and under pressure. The solid and liquid raw materials are both toxic and flammable. Spontaneous ignition is possible when the reaction mass is exposed to air. Therefore, the system must be totally enclosed and confined in order to contain safely any emissions arising from the loss of reactor control, and to prevent secondary combustion reactions upon discharge of the materials to the atmosphere. Further, procedures and equipment are necessary for the safe collection and disposal of solid, liquid, and gaseous emission products. 

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