Reactions combustion

Combustion—the rapid reaction of a fuel with oxygen—is perhaps more important than any other class of industrial chemical reactions, despite the fact that combustion products (CO2, H2O, and possibly CO and SO2) are worth much less than the fuels burned to obtain them. The significance of these reactions lies in the tremendous quantities of energy they release— energy that is used to boil water to produce steam, which is then used to drive the turbines that generate most of the world s electrical power. 

The job of designing power generation equipment usually falls to mechanical engineers, but the analysis of combustion reactions and reactors and the abatement and control of environmental pollution caused by combustion products like CO, CO2, and SO2 are problems with which chemical engineers are heavily involved. In Chapter 14, for example, we present a case study involving the generation of electricity from the combustion of coal and removal of SO2 (a pollutant) from combustion products. 

In the sections that follow, we introduce terminology commonly used in the analysis of combustion reactors and discuss material balance calculations for such reactors. Methods of determining the energy that can be obtained from combustion reactions are given in Chapter 9. 

Most of the fuel used in power plant combustion furnaces is either coal (carbon, some hydrogen and sulfur, and various noncombustible materials), fuel oil (mostly high molecular weight hydrocarbons, some sulfur), gaseous fuel (such as natural gas, which is primarily methane), or liquefied petroleum gas, which is usually propane and/or butane. 

When a fuel is burned, carbon in the fuel reacts to form either CO2 or CO, hydrogen forms H2O, and sulfur forms SO2. At temperatures greater than approximately 1800 C, some of the nitrogen in the air reacts to form nitric acid (NO). A combustion reaction in which CO is formed from a hydrocarbon is referred to as partial combustion or incomplete combustion of the hydrocarbon. 

About 82% of the energy the United States uses is produced by combustion reactions. Source U.S. Energy Information Administration. Annual Energy Review. 2012. (Numbers may not sum to 100% because of independent rounding.) 

As you learned in Section 4.1, combustion reactions are characterized by the reaction of a substance with O2 to form one or more oxygen-containing compounds, often including water. Combustion reactions also emit heat. For example, as you saw earlier in this chapter, natural gas (CH4) reacts with oxygen to form carbon dioxide and water  

In this reaction, carbon is oxidized and oxygen is reduced. Ethanol, the alcohol in alcohohc beverages, also reacts with oxygen in a combustion reaction to form carbon dioxide and water  

Compounds containing carbon and hydrogen—or carbon, hydrogen, and oxygen—always form carbon dioxide and water upon complete combustion. Other combustion reactions include the reaction of carbon with oxygen to form carbon dioxide  

Write a balanced equation for the combustion of liquid methyl alcohol (CH3OH). SOLUTION 

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When a suitable reaction involving the analyte does not exist it may be possible to generate a species that is easily titrated. Eor example, the sulfur content of coal can be determined by using a combustion reaction to convert sulfur to sulfur dioxide. 

In the reaction phase, hydrocarbons react with oxygen according to the highly exothermic combustion reaction. Practically all of the available oxygen is consumed in this phase. 

Waste Destruction by Combustion Reaction. At the simplest level, waste destmction can be thought of as a first-order process involving the thermally excited mpture of a chemical bond. The time to achieve a given extent of reaction, T, can be found from... 

Common combustion reactions and heat releases for 0.454 kg of reactant under ideal combustion conditions are as follows, where Btu represents British thermal unit ... 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

The three T s of combustion—time, temperature, and turbulence—govern the speed and completeness of the combustion reaction. For complete combustion, the oxygen must come into intimate contact with the combustible molecule at sufficient temperature and for a sufficient length of time for the reaction to be completed. Incomplete reactions may result in the generation of aldehydes, organic acids, carbon, and carbon monoxide. 

The referenced table for A and E is a summary of first-order HC combustion reactions. 

Stoichiometric Concentration (Used by permission of Frank T. Bodurtha, Inc., New London, New Hampshire). In a combustion reaction in air, the stoichiometric concentration, Cjt, of any reac tant is the concentration theoretically required for complete conversion by reacting completely with oxygen. For example, for the combustion of propane in air ... 

Design of explosion suppression systems is clearly complex, since the effectiveness of an explosion suppression system is dependent on a large number of parameters. One Hypothesis of suppression system design identifies a limiting combustion wave adiabatic flame temperature, below which combustion reactions are not sustained. Suppression is thus attained, provided that sufficient thermal quenching results in depression of the combustion wave temperature below this critical value. This hypothesis identifies the need to deliver greater than a critical mass of suppressant into the enveloping fireball to effect suppression (see Fig. 26-43). 

Confirmation of the formation of the radicals during combustion reactions has been made by inuoducing a sample of dre flames into a mass spectrometer. The sample is withdrawn from a turbulent flame which is formed into a thin column, by admitting a sample of the flame to the spectrometer drrough a piidrole orifice, usually of diameter a few tenths of a millimetre. An alternative procedure which has been successful in identifying the presence of radicals, such as CHO, has been the use of laser-induced fluorescence. 

The results of combustion reactions are normally products in which the htral density is somewhat lower than tire theoretical value, normally about 70%. Increased densihcation can be achieved if dre hot product, which is still relatively plastic, is compressed. This procedure combines tire thermal... 

The problems with the combustion reaction occur because the process also produces many other products, most of which are termed air pollutants. These can be carbon monoxide, carbon dioxide, oxides of sulfur, oxides of nitrogen, smoke, fly ash, metals, metal oxides, metal salts, aldehydes, ketones, acids, polynuclear hydrocarbons, and many others. Only in the past few decades have combustion engineers become concerned about... 

When visualizing a combustion process, it is useful to think of it in terms of the three Ts time, temperature, and turbulence. Time for combushon to occur is necessary. A combustion process that is just initiated, and suddenly has its reactants discharged to a chilled environment, will not go to completion and will emit excessive pollutants. A high enough temperature must exist for the combustion reaction to be initiated. Combushon is an exothermic reachon (it gives off heat), but it also requires energy to be inihated. This is iUustrated in Fig. 6-5. 

Since the oxygen is contained in air, which also has nitrogen, the combustion reaction can be written as follows ... 

In catalytic incineration, organic contaminants are oxidized to carbon dioxide and water. A catalyst is used to initiate the combustion reaction, which occurs at a lower temperature than in thermal incineration. Catalytic incineration uses less fuel than the thermal method. Many commercial systems have removal efficiencies eater than 98%. 

Lunn, G. A. 1982b. The Influence of Chemical Structure and Combustion Reactions on the Maximum Experimental Safe Gap of Industrial Gases and Liquids./. Hazardous Materials, 6, 341-359. 

If such a process continues to accelerate, the combustion mode may suddenly change drastically. The reactive mixture just in front of the turbulent combustion zone is preconditioned for reaction by a combination of compression and of heating by turbulent mixing with combustion products. If turbulent mixing becomes too intense, the combustion reaction may quench locally. A very local, nonreacting but highly reactive mixture of reactants and hot products is the result. 

Heat of combustion (Section 2.18) Heat evolved on combustion of a substance. It is the value of — A//° for the combustion reaction. 

Thus, the heat of a reaction is obtained by taking the difference between the heat of formation (AHi) of the products and reactants. If the heat of reaction is negative (exothermic), as is the case of most combustion reactions, tlien energy... 

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