Combustion products, equilibrium composition

Equilibrium combustion product compositions and properties may be readily calculated using thermochemical computer codes which minimize the Gibbs free energy and use thermodynamic databases... 

Several techniques are available in the literature for evaluation of the flame temperature, exit temperature, equilibrium composition of combustion products, and performance parameters of energetic composites [11-13]. The optimum combination of the composite ingredients is determined by thermodynamic means, so as to arrive at a composition having maximum performance... 

TABLE 1.5 Equilibrium Combustion Product Composition of Hydrogen-Air ... 

Equilibrium combustion product compositions and properties may be readily calculated using thermochemical computer codes which minimize the Gibbs free energy and use thermodynamic databases containing polynomial curve-fits of physical properties. Two widely used versions are those developed at NASA Lewis (Gordon and McBride, NASA SP-273, 1971) and at Stanford University (Reynolds, STANJAN Chemical Equilibrium Solver, Stanford University, 1987). 

The initial state of the powder or EM is given and quite exactly defined. Under certain assumptions, for example, that complete chemical equilibrium is achieved or that the reaction runs to certain chemical products (nitrogen oxide), we may also determine the state—composition and temperature—of the combustion products. The choice of initial assumptions is controlled by experiment—at least for now, in the absence of sufficient information on the kinetics of chemical reactions at high temperatures. 

Consider an example composition of the mixture 12.1% 02 28.5% H2 59.4% N2 theoretical explosion temperature 2500°K in the first approximation without considering dissociation reactions the composition of the combustion products is as follows 24.2% H20, 4.3% H2 59.4% N2 (the percentages are taken with reference to the initial mixture). The equilibrium amount of oxygen (at the theoretical temperature) is... 

The existence of non-equilibrium combustion products is important to at least two considerations. Firstly, the observed propellant performance may depart substantially from the predicted level. This departure may result in performance either less than or greater than the equilibrium predicted level. A striking example of greater than equilibrium performance is that of hydrazine monopropellant decomposition, table m-A-1. Another is that of ethylene oxide monopropellant, as mentioned in section n. B. 4., in which the equilibrium quantities of condensed carbon never are formed. Secondly, the non-equilibrium composition may have significant effects on the expansion process. In particular, nozzle kinetic calculations based on an assumed equilibrium composition initial condition may diverge significantly from expansions occurring from non-equilibrium initial conditions. 

Direct evidence may be found both in laboratory and rocket engine experiments that the kinetics of the hvHragnnp/nitrogen tetroxide reaction controls the composition of the reaction products. Even early observations of the reaction of hydrazine and nitrogen tetroxide in rocket combustion chambers contain evidence of the role of chemical kinetics in the production of non-equilibrium combustion products (36). 

The following partial equilibrium combustion, PEC, model for the composition of hydrazine and nitrogen tetroxide combustion products and their nozzle expansion was employed to make theoretical predictions of the PEC performance of these propellants, (Table m-A-3). 

The performance of the hydrazine/nitrogen tetroxide propellant combination as based on the PEC model is summarized in figures m-A-3 and m-A-4. Four cases are presented equilibrium combustion with equilibrium and frozen composition expansion and partial equilibrium combustion with partial equilibrium and frozen expansion. At large equivalence ratios, fuel rich, PEC performance is greater than equilibrium combustion performance. At small equivalence ratios, fuel lean, PEC performance is less than equilibrium combustion performance. At equivalence ratios in the region of the stoichiometric value, the PEC performance is less than the equilibrium combustion performance. Combustion product composition and properties are compared in table m-A-4. 

Table m. A. 4 - Hydrazine/Nitrogen Tetroxide Combustion Product Composition and Properties according to Equilibrium Combustion (EC) and Partial Equilibrium Combustion (PEC) (Pc = 1000 psia)... 

The frozen performance is based on the assumption that the composition of combustion products remains constant ( frozen ), while equilibrium performance is based on the assumption of instantaneous chemical equilibrium during the expansion in the nozzle. 

Determination of Combustion Temperature and Equilibrium Composition of Combustion Products... 

Results of equilibrium thermochemical calculations for the thermal destruction of nonplastic and plastic materials show the effect of material composition on the flame temperature, particulate emission, metals, dioxins, and product gas composition. The effect of waste composition has greater influence on adiabatic flame temperature, combustion air requirement, and the evolution of products and intermediate species. The combustion of waste in air produces higher flame temperature for 100% plastic than for nonplastic and mixtures. The 100% plastic requires lower number of moles of oxidant than 100% nonplastic and mixtures. Plastic produces HCl and H2S with concentration levels ranging from 1000 to 10,000 ppm. Emission of NO and NO2 from 100% nonplastic showed an increase with increase in moles of air while that from 100% plastic a slight decrease with increase in moles of air. The higher theoretical flame temperatures predicted with plastic waste corresponds to lower waste feed rate requirement of plastic at constant furnace temperature. This resulted in higher excess air operation with plastic waste and hence lower equivalence ratio. The gas residence time calculated for all the samples was found to be about 1 s. Variation of residence time more or less follows the same trend as excess air for all the samples. 

The determination of the composition of the combustion prothicts is actually performed after cooling the combustion products, i.e., at a moment of freezing the chemical equilibrium. This does not equal the moment w en reactions are finished. The composition and the volume of the combustion products of the propellants may be determined using the closed bomb or the calorimetric bomb. For the determination of the composition of the detonation products, specially designed closed bombs of 20 dm chamber volume are used. Such is the Bichel bomb. 

As mentioned earlier, the final composition of the combustion products depends on many factors and changes during the cooling. Therefore, such tests determine the composition of the combustion products after they are completely cooled down. In order to obtain the composition of the combustion products that would correspond as closely as possible to the composition at the very moment the combustion ended, specially designed closed bombs have been developed. They enable fast cooling, i.e., freezing of the chemical equilibrium, thus retaining as truly as possible the initial composition of the combustion products. 

The second problem that may arise when performing gasdynamic calculations is formation of pollutants, such as nitrogen oxides, sulfur oxides, CO, and unburnt hydrocarbons. Here, a difference between the frozen and equilibrium compositions of the expanding combustion products is much greater than that for the internal energies. 

The error in the work done by the combustion products and in the heat transferred to the surrounding gas calculated using the equilibrium approach may be due to two reasons first, to inaccurate evaluation of the chemical energy evolved, and, second, to incorrect specification of the physical properties of the mixture (coefficients of the equation of state, specific heats, etc.). Calculations show that for explosions of fuel-oxygen and fuel-air mixtures the contribution of the first factor to the error amounts to 80-90%. Thus, a change in the product composition manifests itself, first of all, in the chemical energy evolved. Therefore, one may assume that the equilibrium model... 

Cudzik) investigated the combustion behaviour and obscuration performance of a number of Mg/PTFE-based obscurant compositions [21]. Table 11.3 lists the tested compositions. Table 11.4 lists both mass extinction coefficients and chemical equilibrium compositions of the major products. 

The thermochemical modelling of pyrotechnic compositions has been used as an approach to assess the quality and amount of potential combustion products [1]. However, thermochemical equilibrium calculations fail to give an exact answer to the actual species involved in dynamic processes. Thus it is necessary to take into account reaction kinetics in order to obtain a realistic insight into Magnesium/Teflon/Viton (MTV) combustion [2]. Three typical MTV formulations are given in Table 20.1. 

The parameters essential for the evaluation of combustion systems are the equilibrium product temperature and composition. If all the heat evolved in the reaction is employed solely to raise the product temperature, this temperature is called the adiabatic flame temperature. Because of the importance of the temperature and gas composition in combustion considerations, it is appropriate to review those aspects of the field of chemical thermodynamics that deal with these subjects. 

Each of these dissociation reactions also specifies a definite equilibrium concentration of each product at a given temperature consequently, the reactions are written as equilibrium reactions. In the calculation of the heat of reaction of low-temperature combustion experiments the products could be specified from the chemical stoichiometry but with dissociation, the specification of the product concentrations becomes much more complex and the s in the flame temperature equation [Eq. (1.11)] are as unknown as the flame temperature itself. In order to solve the equation for the n s and T2, it is apparent that one needs more than mass balance equations. The necessary equations are found in the equilibrium relationships that exist among the product composition in the equilibrium system. 

TABLE 1.3 Equilibrium Product Composition of Propane-Air Combustion... 

TABLE 1.4 Combustion Equilibrium Product Composition of Propane-Oxygen ... 

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