Combustion stoichiometric equation

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 ... 

For most hydrocarbon combustion explosions in air the change in the number of moles is small. For example, consider the combustion of propane in air. The stoichiometric equation is... 

Complete combustion in air produces carbon dioxide and water. The stoichiometric equation for this oxidation reaction is ... 

Write the stoichiometric equations for the combustion of methane with oxygen to form (a) CO2 and H2O and (b) CO and H2O and for the combustion of ethane with oxygen to form (c) CO2 and H2O and (d) CO and H2O, Then prove that only three of these four reactions are independent. 

If you know the fuel feed rate and the stoichiometric equation(s) for complete combustion of the fuel, you can calculate the theoretical O2 and air feed rates. If in addition you know the actual feed rate of air, you can calculate the percent excess air from Equation 4.8-1. It is also easy to calculate the air feed rate from the theoretical air and a given value of the percentage excess if 50% excess air is supplied, for example, then... 

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... 

In Eqn 3 the gaseous products of combustion are limited to CO2, Oj, N2 and H2O. The CO and CH4 contents in the flue gas can be derived from the empirical correlations derived earlier and when combined with the stoichiometric equation, these can be used to calculate heat losses due to incomplete combustion. So doing allows the performance of different biomass fuel combustion to be described for the tested combustion rig in terms of flue gas composition, combustion temperature and useful heat output. 

We first describe the standard heat of formation (AH/) and then the standard heat of combustion (AH ). The units of both quantities are usually tabulated as energy per mole, such as J/g mol, kJ/g mol, kcal/g mol, or Btu/lb mol. The per mole refers to the specified reference substance in the related stoichiometric equation. 

Tables of enthalpies of formation, combustion, and reaction are available in the literature (particularly thermodynamics texts/reference books) for a wide variety of compounds. It is important to note that these are valueless unless the stoichiometric equation and the states of the reactants and products are included. However, enthalpy of reaction is not always employed in engineering reaction/combustion calculations. The two other terms that have been used are the gross (or higher) heating value and the net (or lower) heating value. These are discussed in the next problem.

Assuming complete combustion, the stoichiometric equation for the combustion of the coal can now be written ... 

From the stoichiometric equation, for every 1 Ibmol of CgH g that is combusted, 8 Ibmol of CO2 are generated ... 

The combustion reaction equation for 1 mol of pentane with stoichiometric air is... 

When a detailed chemical description is not required, a limited set of a few stoichiometric equations can be included into the scheme just to describe the rate of heat evolution and change of total number of gas species in the system. Chemically oversimplified models of this kind are widely used, for instance, to describe heat-transfer and to optimize thermal regimes in reactors (see, e.g., Fukuhara and Igarashi, 2005 Kolios et al., 2001). A similar approach is used to describe the fuel combustion and corresponding dynamic phenomena in engines of different types simplified equations describing kinetic features are solved together with complex equations of heat- and mass-transfer and fluid dynamics (Frolov et al., 1997 Williams, 1997). 

It is observed in several combustion reactions that the reaction rate increases even in isothermal conditions, i. e. the reaction does not follow the Arrhenius law, suggesting that the mechanism of these reactions may deviate from that given by the stoichiometric equations. It has been shown that intermediate products requiring a lower activation energy are formed in these reactions. Most of these intermediates are very active atoms or radicals. Due to their high reactivity, they can easily interact even with molecules saturated with valency electrons. As a result of such interactions, one or more new active radicals are also formed as well as the end-product, so that further reactions can be initiated. Reactions with such a mechanism are called chain-reactions. 

When methane is burned (or combusted) completely, the stoichiometric equation for the reaction is... 

Based on a "stoichiometric" equation for the combustion of biomass and the oxygen consumption during combustion, Ho and Payne arrived at a very simple equation for the combustion enthalpy only depending on the mass fraction of carbon fcT calculated for biomass without ash and water [30] ... 

Where T)is flame temperature in K MC is moisture content of the waste, expressed on a total weight basis SR is defined as stoichiometric ratio or moles O2 avadable/moles O2 required for complete oxidation of the carbon, hydrogen, and sulfur in the fuel, ie, 1/SR = equivalence ratio and is temperature of the combustion air, expressed in K. In Fnglish units, this equation is as follows ... 

Theoretical Oxygen and Air for Combustion The amount of oxidant (oxygen or air) just sufficient to burn the carbon, hydrogen, and sulfur in a fuel to carbon dioxide, water vapor, and sulfur dioxide is the theoretical or stoichiometric oxygen or air requirement. The chemical equation for complete combustion of a fuel is... 

The rate constants (/c[and k]) and the stoichiometric coefficients (t and 1/ ) are all assumed to be known. Likewise, the reaction rate functions Rt for each reaction step, the equation of state for the density p, the specific enthalpies for the chemical species Hk, as well as the expression for the specific heat of the fluid cp must be provided. In most commercial CFD codes, user interfaces are available to simplify the input of these data. For example, for a combusting system with gas-phase chemistry, chemical databases such as Chemkin-II greatly simplify the process of supplying the detailed chemistry to a CFD code. 

The heat of combustion of a stoichiometric hydrocarbon-air mixture is approximately 3.5 MJ/m3, and by multiplying by the confined volume, the resulting total energy is (2094 m3)(3.5 MJ/m3) = 7329 MJ. To apply the TNO multi-energy method, a blast strength of 7 is chosen. The Sachs-scaled energy is determined using Equation 6-25. The result is... 

LFL is the volume percent of fuel in air at the lower flammability limit, and z is the stoichiometric oxygen coefficient from the combustion reaction given by Equation 6-9. 

A useful application of this result is shown in Figure AC-5. Suppose that we wish to find the oxygen concentration at the point where the LFL intersects the stoichiometric line shown. The oxygen concentration in question is shown as point X in Figure AC-5. The stoichiometric combustion equation is represented by... 

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