Combustion stoichiometric proportions

Pure ammonium nitrate decomposes in a complex manner in a series of progressive reactions having different thermochemical effects (Table 17). Oxygen is Hberated from combination with combustibles only at temperatures above 300°C. When a combustible material such as fuel oil is present in stoichiometric proportions (ca 5.6%) the energy evolved increases almost threefold... 

Because this reaction is highly exothermic, the equiUbrium flame temperature for the adiabatic reaction with stoichiometric proportions of hydrogen and chlorine can reach temperatures up to 2490°C where the equiUbrium mixture contains 4.2% free chlorine by volume. This free hydrogen and chlorine is completely converted by rapidly cooling the reaction mixture to 200°C. Thus, by properly controlling the feed gas mixture, a burner gas containing over 99% HCl can be produced. The gas formed in the combustion chamber then flows through an absorber/cooler to produce 30—32% acid. The HCl produced by this process is known as burner acid. 

On the basis of an extended experimental program described in Section 4.1.3, Harris and Wickens (1989) concluded that overpressure effects produced by vapor cloud explosions are largely determined by the combustion which develops only in the congested/obstructed areas in the cloud. For natural gas, these conclusions were used to develop an improved TNT-equivalency method for the prediction of vapor cloud explosion blast. This approach is no longer based on the entire mass of flammable material released, but on the mass of material that can be contained in stoichiometric proportions in any severely congested region of the cloud. 

Instantaneous images obtained in a turbulent premixed V-shaped flame configuration, methane and air in stoichiometric proportions. (Reproduced from Kobayashi, H., Tamura, T., Maruta, K., Niioka, T, and Williams, F. A., Proc. Combust. Inst., 26,389,1996. With permission. Figure 2, p. 291, copyright Combustion Institute.)... 

In industrial reactions the components are seldom fed to the reactor in exact stoichiometric proportions. A reagent may be supplied in excess to promote the desired reaction to maximise the use of an expensive reagent or to ensure complete reaction of a reagent, as in combustion. 

It has been suggested that powdered metals, e.g. aluminium should be added to the combustible component in the form of a suspension. Stettbacher [57], for example, suggested the following equation for the combustion of a mixture of petrol with aluminium suspended in it in stoichiometric proportions ... 

The optimum combination of a tripropellant system is predictable through the systematic variation of the concentration of each of the propellants. The weight ratio of the oxidizer and metal components are always in stoichiometric proportion to produce the metal oxide or metal halide. The low molecular weight species is then added in an amount determined by the trade-off between decreasing molecular weight and decreasing temperature of the combustion products. The results of such calculations are reported in the literature (47). 

Figure 7.3. The combustion time (the time needed to reach the maximum pressure in a closed vessel) for a methane/air flame with and without turbulence, plotted as a function of the proportion of CH4 in the reactant mixture, corresponding to the equivalence ratio (j> (stoichiometric proportions correspond to about 10 % of methane). Time units correspond to 10-2 seconds ([377 307]).

The extreme case of an inexpensive reactant is air, which is free. Combustion reactions are therefore invariably run with more air than is needed to supply oxygen in stoichiometric proportion to the fuel. The following terms are commonly used to describe the quantities of fuel and air fed to a reactor. 

We saw in Section 9.6b that the highest attainable temperature in a combustion reaction— the adiabatic flame temperature—depends on the fuel-lo-air ratio, and we stated but did not prove that this upper temperature limit is a maximum when the fuel and oxygen are present in stoichiometric proportion. If the mixture is either rich (fuel in excess) or lean (O2 in excess), the adiabatic flame temperature decreases. 

The proposed process uses two parallel columns containing beds of solid particles. The air-acetone stream, which contains acetone and oxygen in stoichiometric proportion, enters one of the beds at 1500 mm Hg absolute at a rate of 1410 standard cubic meters per minute. The particles in the bed have been preheated and transfer heat to the gas. The mixture ignites when its temperature reaches 562°C, and combustion takes place rapidly and adiabatically. The combustion products then pass through and heal the particles in the second bed. cooling down to 350°C in the process. Periodically the flow is switched so that the heated outlet bed becomes the feed gas preheater/combustion reactor and vice versa. 

This technique utilizes an acid solution, a base solution, a dye tracer that fluoresces over a narrow range of pH and a source of coherent radiation (typically an argon ion laser). The acid and base are mixed in strengths such that when they combine in stoichiometric proportions consistent with the combustion system that is under evaluation, the pH will be within the narrow range that the dye tracer fluoresces. For example, when an oxy/natural gas system is studied, the acid and base are mixed such that when two parts of the oxidizer stream mix with one part of the fuel stream, the pH is within the narrow window in which the dye tracer fluoresces. 

As the concentration of fuel in the fuel-air mixture increases, the fuel burned per unit volume of mixture increases, and hence, the heat released per unit volume of mixture increases, until the point is reached when the fuel and air are in stoichiometric proportions Cpstoic- Beyond this point, there is insufficient oxygen to combust all the fuel in the mixture. For fuel concentrations beyond Cpstoicr therefore, it is the quantity of oxygen per unit volume of mixture which dictates the quantity of fuel burned and hence the heat released per unit volume of mixture. 

Stoichiometry in Reactive Systems. The use of molar units is preferred in chemical process calculations since the stoichiometry of a chemical reaction is always interpreted in terms of the number of molecules or number of moles. A stoichiometric equation is a balanced representation that indicates the relative proportions in which the reactants and products partake in a given reaction. For example, the following stoichiometric equation represents the combustion of propane in oxygen ... 

The stoichiometric ratio is the proportion of fuel and oxidizer that results in optimal combustion and maximum heat release. The optimal ratio is deter-... 

Chemical reactions can be represented by an overall stoichiometric equation which indicates the relative molecular proportions with which the reactants combine to form the products of the particular reaction. Some simple examples from combustion chemistry are... 

Characterization may involve simple fingerprinting of compounds already known, or more extensive investigation designed to establish the formula and structure of a new compound. The proportions of each element allow a stoichiometric formula to be obtained. Chemical methods can be used, but instrumental methods are more routine and include combustion analysis (for C, H, N and sometimes S) and methods based on atomic spectroscopy of samples atomized at high temperature. 

Additionally around 70% of N2 1 H2 also appears typically in a proportion of 1/3 with respect to CO.2,6 bDifferences between the lean and stoichiometric condition in this engine affect mainly to the O2 concentration but also to the other components.5 7 cNO + NO2. dA large variety of hydrocarbons can be produced as a function of the type of fuel employed see Ref. 1 for details. emainly SO2, in a vol.% of around 1/20 of the wt% sulphur content in the fuel. fAir-to-fuel mass ratio employed in the fuel combustion. SA = (actual A/F)/(stoichiometric A/F). hOscillating between lean-burn (most of the time) and quasi-stoichiometric condition. 

There are also many high temperature combustion processes that use an oxidizer that contains a higher proportion of oxygen than the 21% (by volume) that is found in normal atmospheric air. This is referred to as OEC and has many benefits, which include increased productivity and thermal efficiency while reducing the exhaust gas volume and pollutant emissions [6]. A simplified global chemical reaction for the stoichiometric combustion of methane with air is given as follows ... 

The normal rate of flame propagation is inversely proportional to the thermal conductivity of the gas mixture. w reaches its maximum not just at the stoichiometric composition but when the gas mixture is somewhat richer in the combustible component. The normal rate of flame propagation increases rapidly with the initial temperature of the gas mixture (Fig. 2.7). 

The sensor (a sensor) is designed to deliver measurements of the [O j in the exhaust gas. P(02) in the exhaust can be taken as a direct measure of the A/F ratio in the combustion chamber s inlet. The sensor is essentially a yttrium-stabilised zirconia electrode whose potential depends on P(02). This electrode (which is composed of the same material as the electrol)i e in a solid oxide fuel cell) transports as O " ions generating an electrical signal whose strength is proportional to P(02). The signal is sent to the fuel injection system which increases or decreases the A/F ratio as desired in order to keep the mixture stoichiometric [12]. 

---