Thermodynamics, of combustion

Thermodynamics of Combustion of Various Pyrotechnic Compositions , NAD-RDTR No 277 (1974) 81) D.C. Izod R.F. Eather,... 

Ya.B. s studies of combustion and detonation are diverse and multidirectional. They include the chemical thermodynamics of combustion, propagation of exothermic chemical transformation fronts, deflagration and detonation theory, thermo-diffusion and chemo-kinetic processes in combustion and at high temperatures in general, and gasdynamics of flows in the propagation of non-uniform flame fronts and in detonation. 

R. W. Smith, Jr., H. E. Edwards, and Stuart R. Brinkley, Jr., The Thermodynamics of Combustion Gases Temperatures of Methane-Air and Propane-Air Flames at Atmospheric Pressure, Bureau of Mines, U.S. Department of the Interior, Report of Investigations 4938, Pittsburgh, 1953, 3 pp. 

Ribaud has presented tabulations of Gibbs energy for saturated hydrocarbons and some other organic compounds, and Brinkley and Lewis have considered the thermodynamics of combustion gases. Latimer s book contains an extensive collection of redox potentials. 

S. R. Brinkley and B. Lewis, The Thermodynamics of Combustion General Considerations , U.S. Bureau of Mines, Bulletin 4806, 1952. 

Fluoroacetic acid [144-49-OJ, FCH2COOH, is noted for its high, toxicity to animals, including humans. It is sold in the form of its sodium salt as a rodenticide and general mammalian pest control agent. The acid has mp, 33°C bp, 165°C heat of combustion, —715.8 kJ/mol( —171.08 kcal/mol) (1) enthalpy of vaporization, 83.89 kJ /mol (20.05 kcal/mol) (2). Some thermodynamic and transport properties of its aqueous solutions have been pubHshed (3), as has the molecular stmcture of the acid as deterrnined by microwave spectroscopy (4). Although first prepared in 1896 (5), its unusual toxicity was not pubhshed until 50 years later (6). The acid is the toxic constituent of a South African plant Dichapetalum i mosum better known as gifirlaar (7). At least 24 other poisonous plant species are known to contain it (8). 

When the partial pressures of the radicals become high, their homogeneous recombination reactions become fast, the heat evolution exceeds heat losses, and the temperature rise accelerates the consumption of any remaining fuel to produce more radicals. Around the maximum temperature, recombination reactions exhaust the radical supply and the heat evolution rate may not compensate for radiation losses. Thus the final approach to thermodynamic equiUbrium by recombination of OH, H, and O, at concentrations still many times the equiUbrium value, is often observed to occur over many milliseconds after the maximum temperature is attained, especially in the products of combustion at relatively low (<2000 K) temperatures. 

Flame Temperature. The adiabatic flame temperature, or theoretical flame temperature, is the maximum temperature attained by the products when the reaction goes to completion and the heat fiberated during the reaction is used to raise the temperature of the products. Flame temperatures, as a function of the equivalence ratio, are usually calculated from thermodynamic data when a fuel is burned adiabaticaHy with air. To calculate the adiabatic flame temperature (AFT) without dissociation, for lean to stoichiometric mixtures, complete combustion is assumed. This implies that the products of combustion contain only carbon dioxide, water, nitrogen, oxygen, and sulfur dioxide. 

Compiled from Daubert, T. E., R. R Danner, H. M. Sibiil, and C. C. Stebbins, DIPPR Data Compilation of Pure Compound Properties, Project 801 Sponsor Release, July, 1993, Design Institute for Physical Property Data, AlChE, New York, NY and from Thermodynamics Research Center, Selected Values of Properties of Hydrocarbons and Related Compounds, Thermodynamics Research Center Hydrocarbon Project, Texas A M University, College Station, Texas (extant 1994). The compounds are considered to be formed from the elements in their standard states at 298.15 K and 101,325 P. These include C (graphite) and S (rhombic). Enthalpy of combustion is the net value for the compound in its standard state at 298.15K and 101,325 Pa. 

To make the necessary thermodynamic calculations, plausible reaction equations are written and balanced for production of the stated molar flows of all reactor products. Given the heat of reaction for each applicable reaction, the overall heat of reaction can be determined and compared to that claimed. However, often the individual heats of reaction are not all readily available. Those that are not available can be determined from heats of combustion by combining combustion equations in such a way as to obtain the desired reaction equations by difference. It is a worthwhile exercise to verify this basic part of the process. 

Computes thermodynamic properties of air, argon, carbon monoxide, carbon dioxide, hydrogen, nitrogen, oxygen, water vapor, and products of combustion for hydrocarbons. Computes all properties from any two independent properties. 

The thermodynamic properties of thiophene,2-methylthiophene, ° and 3-methylthiophene have been computed from careful measurements of the heat capacity of the solid, liquid, and vapor states, the heat of fusion, the heat of vaporization, and the heat of combustion. From the heat of combustion of thiophene and from thermochemical bond energies, the resonance energy of thiophene has been re-estimated to be only 20 kcal/mole. 

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... 

A discussion with 14 refs on expls and proplnts considering the thermodynamic characteristics of expl substances, the kinetics of combustion of powders and the effects of catalysts, corrosion, and instability on the kinetics, the occurrence of deflagration on detonation, and forms of solid mixts in view of the augmentation of their performance and the extension of conditions used in their mixts. The importance of modern methods of calcn is stressed... 

Fundamental research in pyrotechnics is published in the US in Combustion and Flame by the Combustion Institute, and in the UK in Combustion Science and Technology and in Fuel . Germany has the new, journal, Propellants and Explosives (German Chemical Society), which is the successor to the discontinued Explosivstoffe . A necessary caveat is that these journals are strongly oriented toward combustion or propulsion so that only rarely do they yield pyrotechnic information. Likewise, the various publications of the learned societies contain much data on thermodynamics, spectroscopy, and instrumental analysis which are useful in the study of pyrotechnics. In the USSR the situation is somewhat better as Physics of Combustion and Explosion (Fizika Gorenia i Vzryva) of the Siberian Branch Academy of Sciences USSR is exclusively oriented toward subjects of interest, as several scientific institutes are primarily devoted to research in pyrotechnics. The same authors do publish also, however, in the journals of the Academy of Science USSR (of which there are several) as well as in the corresponding journals of the academies of the various republics, so that the impression is created of a high level of activity... 

Almost all of the directly measured thermochemical data for the sulfoxides, sulfones, sulfites and sulfates are due to the work of Busfield and Mackle and their coworkers at the University of Leeds and The Queens University, Belfast1-14. This work involved measurement of enthalpies of combustion, fusion and vaporization. It is the basis of the subsequent compilations of Benson and coworkers15, Cox and Pilcher16 and Pedley, Naylor and Kirby11. The data given by the latter are used as the basic data set in the present work. Corrections and omissions are noted in the next section. Data on additional compounds were sought by searching the IUPAC Bulletin of Thermochemistry and Thermodynamics for the years 1980 198318, and by searches of Chemical Abstracts. 

For values of heats of combustion of large numbers of organic compounds hydrocarbons and others, see Cox, J.D. Pilcher, G. Thermochemistry of Organic and Organometallic Compounds, Academic Press NY, 1970 Domalski, E.S. J. Phys. Chem. Ref. Data, 1972,1, 221. For large numbers of heats of formation values (from which heats of combustion are easily calculated) see Stull, D.R. Westrum Jr., E.F. Sinke, G.C. The Chemical Thermodynamics of Organic Compounds, Wiley NY, 1969. 

To calculate the heat release from combustion and the temperature of the products of combustion, the thermodynamic path shown in Figure 15.20 can be followed18. The actual combustion process goes from reactants at temperature T to products at temperature T2. However, it is more convenient to follow the alternative path from reactants at temperature T that are initially cooled (or heated) to standard temperature of 298 K. The combustion reactions are then carried out at a constant temperature of 298 K. Standard heats of combustion are available for this. The products of combustion are then heated from 298 K to the final temperature of 7i. The actual heat of combustion is given by18 ... 

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. ... 

Smith, N.K., Stewart, Jr., R.C., Osbom, A.G. (1980) Pyrene vapor pressures, enthalpy of combustion, and chemical thermodynamic properties. J. Chem. Thermodyn. 21, 919-926. 

In marked contrast to the n-alkanes, the cycloalkanes exhibit thermodynamic properties where such regularities are no longer present. Heats of formation (AH ) for a substantial number of cycloalkanes are available from heats of combustion. With the exception of cyclohexane, AH°f is always more positive than the quantity — 4.926n. The difference between the two quantities leads to a quantitative assessment of the important notion of ring strain. The AH -values and strain energy data listed in Table 1 were taken from Skinner and Pilcher (1963). Other references give different but usually comparable... 

It follows that the efficiency of the Carnot engine is entirely determined by the temperatures of the two isothermal processes. The Otto cycle, being a real process, does not have ideal isothermal or adiabatic expansion and contraction of the gas phase due to the finite thermal losses of the combustion chamber and resistance to the movement of the piston, and because the product gases are not at thermodynamic equilibrium. Furthermore the heat of combustion is mainly evolved during a short time, after the gas has been compressed by the piston. This gives rise to an additional increase in temperature which is not accompanied by a large change in volume due to the constraint applied by the piston. The efficiency, QE, expressed as a function of the compression ratio (r) can only be assumed therefore to be an approximation to the ideal gas Carnot cycle. 

Up to now we have only considered prescribed reactions. Given a reaction, the tools of thermodynamics can give us the heat of combustion and other information. In a combustion reaction we could impose conditions of chemical equilibrium, or ideally complete combustion. While these approximations can be useful, for actual fire processes we must rely on experimental data for the reaction. The interaction of turbulence and temperature variations can lead to incomplete products of combustion. For fuels involving CxHyOz we might expect that... 

Determine the heat of combustion of toluene and express it in proper thermodynamic form. The heat of formation is 11.95 kcal/mole. 

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