From elementary reactions to technical combustion systems
Whilst combustion has become a cornerstone of our civilisation through its manifold practical applications in traffic, transport and energy supply, its side-effects such as damage to the biosphere by air pollutants are clearly noticeable today. Is it possible to utilise combustion without its negative effects, and how can this be realised on a large scale? Whereas humanity’s relationship with combustion for a million years has consisted solely in its utility, over the past century its systematic research has become an added decisive aspect. This is the field of activity of the Institute of Technical Thermodynamics.
Physical-chemical foundations
Combustion is a chemical reaction that typically involves significant heat release, in which the fuel and the oxidant are converted to the reaction products. A prominent example of such a reaction is the oxidation of methane (CH4):
CH4 + 2 O2 → CO2 + 2 H2O .
In this reaction, chemical energy is converted into thermal energy, such that the waste gas (with the same total energy) exhibits a much higher temperature than the initial substances. The resulting high temperature difference relative to the environment can be utilised for technically important processes.
This highly simplified description of combustion, already highlights several essential aspects: it requires a fuel (in this case CH4), i.e. a substance that stores a great deal of energy in its molecules. As fuel prices show only too clearly, such substances are not available in infinite amounts. Oxygen is consumed also, which although available in the air ‘for free’ is still limited in principle, and as a foundation for life is extremely precious. In addition to the desired heat, the process also generates ‘waste products’ (reactants, waste gas) which mostly are of no use and occasionally harmful, as here the greenhouse gas CO2.
Interestingly, there is a process in nature that in a sense is the opposite of the combustion reaction, namely photosynthesis in plants: by utilising sunlight, CO2 and H2O are used to build very energy-rich substances that can be used as fuels. In principle, then, these two processes of ‘combustion“ and ‘photosynthesis“ can form a closed loop, on the basis of which a permanent, ecologically sustainable and economically sensible utilisation of combustion is conceivable.
Unfortunately, however, this simple model is far too imprecise. One of the greatest problems with combustion, namely the formation of pollutants such as carbon monoxide (CO) and the nitrogen oxides generated during combustion with air such as NO and NO2, does not even form part of this description. The same is true for the formation of soot, and for the well-known problem of incomplete combustion leading to the emission of unburned hydrocarbons. One key to an understanding of these problems lies in the detailed treatment of the chemical reactions that occur during a combustion process.
Elementary chemical kinetics – a crucial detail
In reality, the methane combustion shown above does not take place in a single step, but as a sequence of many hundreds of elementary reactions in which a large number (typically several hundreds or thousands) of chemical species take part. A small section of the reaction flowchart for methane/air is shown in Fig. 1 on the left: Starting with methane, many elementary reactions produce numerous chemical species which in turn can react with each other. The main combustion products, CO2 and H2O (the latter is not shown in the diagram), are at the bottom of this chain.
Fig. 1 illustrates the crucial detail, that it is mainly chemical species such as OH, H and O that occur very often in the reaction chain, therefore playing an important role in it. These substances are radicals, that is to say, molecules with unpaired electrons, which as a result are very reactive. It is just these substances, eminently important for the reaction, that in general are present during combustion only in vanishingly small concentrations. Analyses of the reaction flowchart show that it is mainly steps such as
H + O2 → OH + O
where two radicals are formed from one radical and one ‘stable’ molecule, that are very important for the further progress of the combustion reactions.
One approach for describing combustion is to regard it as a sequence of chemical reactions in a spatially homogeneous system. Mathematically, this model of the homogeneous chemical reactor is described by a system of ordinary differential equations, which describes the temporal rate of the formation or consumption of each individual chemical species. Since the numerous chemical species react with each other in manifold ways (Fig. 1, left), the formation or consumption velocity of a species depends in a very complication fashion on the concentrations of all the species. Therefore, usually the equations need to be solved by numerical methods running on computers.
Fig. 1 (right) shows as an example the computed temporal course of the temperature and the concentrations of a few chemical species during the ignition of a methane/air mixture. The presence of two prominent pollutants, CO and NO, is predicted correctly by the simulation. It also shows correctly that the oxygen is not used up completely during the reaction. Finally, the conversion of the reactants into products does not take place instantaneously, but needs a certain length of time that is, in fact, observed in reality as a so-called ignition delay. At the end, the gas mixture is in a state of chemical equilibrium (waste gas) that exhibits not only CO2 and H2O as end-products but also substances such as CO and NO. Note also that OH occurs only at very low concentrations, but nevertheless is a key-species for the reaction’s progress. Also notable is the fact that the temporal course of the profiles is mostly quite even, with the exception of the time around approx. 0.3773 s where all the quantities change almost abruptly. This behaviour too, is an important feature of many ignition and combustion processes.
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| A small section of the reaction flowchart for the combustion of methane. The thickness of the arrows denotes the share of each reaction step in the total consumption or formation of a substance. | Numerically computed, spatially homogeneous methane-air mixture at a pressure of 7 bar and an initial temperature of 900 K, shown as a function of time for the concentrations of various species and for the temperature. Note the logarithmic scale on the ordinates. |
The modelling of combustion as a homogeneous reaction system whose behaviour is determined solely by chemical reactions, therefore, already yields important data. A more detailed analysis, however, shows that considering the chemical kinetics alone fails to provide a complete picture of combustion. This is due to the spatial non-homogeneity of most of the combustion processes that occur in nature and in technical applications, which results in strongly dissipative processes (heat conduction, diffusion). A complete description, therefore, needs to include the interaction between chemical reactions and physical processes (transport phenomena).