Abstract
In Part I ideal gases and incompressible liquids have been introduced and the basic thermodynamic correlations have been derived. Part II has shown, that real fluids can be subject to a change of aggregate state and that many thermodynamic cycles are based on phase change, e.g. a steam power plant. Furthermore, Part II covered mixture of fluids, e.g. humid air or mixtures of ideal gases. However, within these mixture each component was stable and not part of a chemical reaction, i.e. decomposition of the present atoms and molecules. In Part III the focus now is on chemical reacting systems: First, the stoichiometry of a chemical reaction is investigated, i.e. the principle of mass conservation is applied to reactants and products of a chemical reaction. In doing so it is possible to predict the composition of the products. This is important for instance, when the composition of an exhaust gas of a combustion process needs to fulfil technical thresholds. Second, a chemical reacting system is investigated in terms of an energy balance. In this chapter the heating-value approach is followed with focus on technical combustions, i.e. combustions based on fossil fuels. In Chap. 24 another energetic approach based on absolute enthalpy respectively entropy is introduced. Major advantage of this method is, that the irreversibility of chemical reactions can be quantified.
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- 1.
Hydrogen in its stable form under atmospheric conditions is molecular.
- 2.
The other components like nitrogen, water and ashes are treated as inert, i.e. not chemically reactive.
- 3.
Thus, each combustible element is fully oxidised. The oxygen on the left hand side of the equation is fully consumed, so that there is no residual oxygen on the right hand side.
- 4.
\({1\,\mathrm{\text {k}{\text {mol}}}\,}\) of \(\text {C}\) has a mass of \({12\,\mathrm{\text {k}{\text {g}}}\,}\). \({1\,\mathrm{\text {k}{\text {mol}}}\,}\) of \(\text {O}\) has a mass of \({16\,\mathrm{\text {k}{\text {g}}}\,}\). \({1\,\mathrm{\text {k}{\text {mol}}}\,}\) of \(\text { CO}_{2}\) has a mass of \(1\cdot {12\,\mathrm{\text {k}{\text {g}}}\,}+2\cdot {16\,\mathrm{\text {k}{\text {g}}}\,}={44\,\mathrm{\text {k}{\text {g}}}\,}\). See also Table 3.2.
- 5.
Such as methane, propane, butane, ethanol or others.
- 6.
Actually, steps 1–5 could be skipped, since the former reactions can be handled by this generic approach as well!
- 7.
The value 1.6022 is taken here, though the standard atmosphere has been reduced to nitrogen and oxygen! The related error shall be ignored.
- 8.
Sure, the mass balance needs to be fulfilled!
- 9.
Applying a simplified standard atmosphere containing purely nitrogen and oxygen.
- 10.
In this section it is the component water.
- 11.
The air is supposed to be dry, so that its water content \(x=0\)!
- 12.
According to their chemical composition: \(\text {CH}_{4} \rightarrow a=1,\, b=4\), \(\text {C}_{2}\text {H}_{6} \rightarrow a=2,\, b=6\) and \(\text {C}_{3}\text {H}_{8} \rightarrow a=3,\, b=8\).
- 13.
Mind, that \(\sigma _{i}=x_{i}\), i.e. molar fractions are identical with volume fractions!
- 14.
Mechanical or electrical energy!
- 15.
The enthalpy differences can not be substituted by temperature differences, as the exhaust gas and fuel/air have different specific heat capacities.
- 16.
Supplied thermal energy increases temperature and thus enthalpy, heat release decreases temperature and enthalpy.
- 17.
Hence, Fig. 23.8 also works out with unsaturated humid air \(\dot{m}_{\text {A}}\). If liquid water would be supplied with the combustion air, thermal energy would be required to vaporise the liquid water and turn it into vapour. Thus, the specific lower heating value would decrease.
- 18.
The first bracket in Eq. 23.231 contains purely fuel, the second purely air.
- 19.
Ashes usually occur for solid fuels! The impact of the ashes on the energy balance disappear for \(a=0\).
- 20.
With its water in vapour state!
- 21.
The enthalpy difference between super-cooled liquid and saturated liquid is assumed to be negligible small. An evaluation regarding this aspect is done in Sect. 23.4.3.
- 22.
E.g. lowering the temperature from \(T_{\text {a}}\) to \(T_{0}\) decreases the capability of air to carry vapour, see Chap. 20. In case \(T_{\text {a}}<T_{0}\) the amount of liquid water sinks and the amount of vapour rises.
- 23.
According to Fig. 23.17 the water results from the fuel: it can contain water or water can be a result of the oxidisation of hydrogen.
- 24.
Mind, that the pressure of the liquid is equal to the pressure of the gaseous atmosphere above the water!
- 25.
The enthalpy difference between saturated liquid and super-cooled liquid is assumed to be negligible small!
- 26.
At standard conditions!
- 27.
No step 4 is required!
- 28.
This case requires a step 4!
- 29.
This equations counts for combustion temperatures below and above the dew point!
- 30.
In case of solid fuels.
- 31.
In case the combustion air is humid but unsaturated before and after step 1 and in case the combustion temperature is above the dew point!
- 32.
Step 3 is supposed to be isobar.
- 33.
And further assuming, that step 3 is isobar.
- 34.
It has been asked for the maximum mass flux of fuel in a.
- 35.
According to the steam table it is \( p_{\text {s}}({25\,\mathrm{{{}^{\circ }\text {C}}}\,})={0.0317\,\mathrm{\text {bar}}\,}\).
- 36.
The exhaust gas temperature is above the dew point.
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Schmidt, A. (2019). Combustion Processes. In: Technical Thermodynamics for Engineers. Springer, Cham. https://doi.org/10.1007/978-3-030-20397-9_23
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DOI: https://doi.org/10.1007/978-3-030-20397-9_23
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