Reversible reactions combustion processes

Thermochemistry—The foundations of the branch of physical chemistry dealing with thermochemistry was laid by Lavoisier and Laplace, who through their collaborative efforts showed that the heat evolved in a reaction is equal to the heat absorbed in the reverse reaction. They investigated the specific and latent heats of a number of substances, as well as the amount of heat generated in combustion. It was Hess who in 1840 enunciated the law that now bears his name—Hess s law —that the evolution of heat in a reaction is the same whether the process is accomplished in one step or in a number of stages. As an illustration, the heat of formation of CO2 is the sum of the heat of the formation of CO and the heat of oxidation of CO to C02. ... 

As shown in Fig. 13.4, after the initial low temperature reactions, which are mainly due to chain-branching, there is a temperature region, called the cool-flame region, where reverse reactions lead to a very slow burning process. This can result in a small temperature increase and hence in an increase of the ignition delay. Subsequently, after the cool-flame regime, other reaction paths dominate the chemical processes and release sufficient heat such that the associated temperature increase leads to the high-temperature reactions, that is, the actual combustion process. 

One possible implementation of a chemical reaction as a thermodynamically reversible process was proposed by van t Hoflf. This method involves applying semi-permeable membranes that allow only one of the reactants involved in the reaction to pass. The van t Hoflf chamber, the device in which there is a thermodynamically reversible reaction, is presented in Fig. 2.1. It will be considered in the example that hydrogen combustion occurs in the gas phase reaction given by Eq. 2.2. 

CO2. Besides, the reverse reaction is necessary to establish the water-gas equilibrium (Warnatz, 1978). Accordingly, variations of the rate coefficients both of CO + OH - CO 2 + H and of its reverse reaction strongly influence flame propagation, as demonstrated in Fig. 4. Ignition is not influenced by these steps because of their late occurrence during the combustion process (Fig. 1). 

The maximum work output of any thermodynamic system or process can be obtained, if the material in the system or the working fluid in the process is brought into equilibrium with the environment reversibly. The actual work output of a technical process with combustion is much smaller because the combustion is highly irreversible. The work losses in a continuous combustion can be evaluated if the exergy (or available energy) before and after the reaction is calculated. This exergy is described by the equation ... 

MCFCs and intermediate-temperature SOFCs can incorporate catalysed reform at their anodes, where the hydrogen electrochemical oxidation proceeds simultaneously, and heats the non-Faradaic and endothermic reform and shift reactions The latter process is immediately superior to a separate reformer, because it eliminates combustion reaction irreversibility. Heat produced at such an anode is given, in Appendix A, the title reversible heat , that is heat produced without the thermal degradation which occurs in the combustion reaction. 

These reactions increase the heating value of the gas product, since methane has a high heat of combustion. However, these reactions are very slow except under high pressure and in the presence of a catalyst. Another source of the methane in the syngas is the pyrolysis process. Reaction R-4.11 is the reverse steam methane reforming reaction. All reactions that produce methane are exothermic reactions. 

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