SFB 686 - Model-based Control of Homogenised Low-Temperature Combustion

The distribution of energy plays an important part in the domestic sector, for process heat and for the transportation of people and goods. For the foreseeable future, the disposability of fossil fuels and especially their effective use will remain a prerequisite for it. Fossil fuels basically consist of hydrocarbons which are gained from coal, petroleum or natural gas and processed according to their technical applliance. Even alternative concepts, e. g. the fuel cell, rely on fossil fuels for the extraction of the hydrogen needed. Especially in transportation, the use of fluid hydrocarbons seems indispensable, due to their high energy density.

But the combustion of hydrocarbons brings along a row of well known disadvantages. These are, for one, the emission of pollutants like nitrogen oxide (NOx) and soot, which greatly add to communal and regional air pollution; secondly, the production of carbon dioxide (CO2) which as a greenhouse gas is held responsible for the rise in global temperature and the following change in the world's climate.

The decrease of emissions is therefore an important research objective in these fields.

Thus, the emission of pollutants could be diminished in stationary gas turbines by the changeover from diffusion-controlled to premixed combustion made several years ago, for peaks in temperature were avoided through homogenisation. Unfortunately, unwanted self-induced thermo-acoustic instabilities occured.

With engines, new combustion procedures that satisfied the demand for lower emissions while maintaining high efficiency, have been developed on a wide scale as well. These are the processes known as HCCI (Homogenous Charge Compression Ignition) and CAI (Controlled Auto-Ignition) that, like modern gas turbines, avoid high peaks in temperature through homogenisation and exhaust-gas recirculation and can thus lower emissions perceivably. However, instabilities in combustion appear here as well, in the shape of spatially and temporally randomly distributed self-ignitions.

Thus, in different technical applications, such as gas turbines and engines, we see a turning away from mixture-controlled high-temperature combustion and a turning towards a homogenised combustion with averagely low temperatures. In both cases, however, this leads to an occurrence of instabilities in combustion.

As one cannot expect these instabilities to be repaired by combustion scientific measures alone, they are to be controlled by intervening in the process conduct. This demands an analysis of the processes, based on the physical and chemical basis, with the aim of controlling them. The application of such a control is to be carried out on the basis of findings made for a respective area of application and the physical models developed for them; thus, it will be a model-based control. The development of such model-based controls is the SFB's medium-term aim.