Ion current and exhaust gas composition measurements for combustion monitoring

The efficiency of combustion processes is assuming nowadays a huge importance, since the energy production, many industrial processes, as well as building heating systems are still mainly based on the combustion of hydrocarbons. The performance of the combustion process depends on many factors and it is a crucial point for the reliability and the efficiency of a plant or a thermal machine that exploits combustion as a primary source of energy. Moreover, the constant increasing of carbon dioxide concentration in atmosphere makes more and more important reducing the emission of this gas as well as the other pollutant/toxic chemical compounds that are produced during combustion. An optimized combustion process allows reducing dramatically the production of chemical compounds like carbon monoxide or nitrogen oxides, and also to releasing in the atmosphere the minimum amount of carbon dioxide per unit of energy produced. There are many studies related to the optimization of the internal combustion of the engines, especially for automotive applications, whereas the literature is less exhaustive for burner combustion optimization. The focus of this work is the study and the development of measurement systems allowing to get information about the combustion characteristics in gas turbines, with the aim of providing tools for monitoring/controlling the combustion parameters and keeping the combustion efficiency as high as possible over time. This activity has been developed in collaboration with Beker Huges (Nuovo Pignone Tecnologie - Florence), one of the world leaders in the design and development of gas turbines. Two different sources of information on the state of the combustion process have been considered in this thesis, namely the density of ions produced by the flame in the combustion chamber and the composition of the exhaust gases. The measurement of the ionic density due to the flame has been used since several years, particularly in the automotive sector, to obtain information about the combustion process: from the postprocessing of the signal obtained using ionization sensors (or ionic current sensors), it is possible to determine, for example, the onset of the combustion, the air–fuel ratio (and therefore the pollutant concentration at the exhaust), as well as to get information about the flame stability and the occurrence of periodic pressure variations in the combustion chamber. On this basis, even if the relationship between combustion parameters and flame induced ion density is highly dependent on the type of fuel, there is room to exploit the information of the ion sensors also with gas turbines, to optimize the operation of the combustor (e.g. reducing instability) and to monitor the polluting emissions. Ion or ionization sensors, which are usually used to measure the ion density in a burning gas, are essentially conductive electrodes capable of generating signals for either the charge transferred to/from the ionized gas and/or the charge induced on the electrodes themselves. The challenging issue concerns the choice of the materials for the sensor (electrodes and electrical insulators) which, being placed in the combustion chamber, must operate in extreme conditions, i.e., for example, in presence of very high temperatures. On the other hand, the conditioning front-end electronics for this kind of sensors is not critical. As far as the measurement of the concentration of toxic/pollutant compounds in exhaust gases is concerned, the most relevant compounds to be considered are carbon monoxide (CO) and nitrogen oxides (NOx). Monitoring CO and NOx in the exhaust gases is important not only from the point of view of environmental pollution, but also because their concentrations are useful and reliable indicators about the combustion efficiency. The drawback is that, due to the measurement procedure, they cannot be used for a timely feedback control of the combustion process, the reason is that the exhaust gases must be sampled from the chimney and pumped to the measurement instrument (gas analyser), and this procedure introduces a significant delay between the instant in which the gases are produced by the combustion and the time at which they are analysed. From the standpoint of the measurement instruments, exhaust gas analysers with different accuracies and costs (which are usually relevant) are available on the market. These devices can be portable or fixed and can exploit different measurement principles. Besides cost, an issue of these devices is that accurate gas sensors need frequent calibration exploiting reference gas tanks, which can be a problem in specific industrial plants such as power generation or oil and gas plants. The possibility to use a more flexible gas analyser, with a better trade-off among cost, measurement accuracy, the calibration intervals and robustness, is a deeply felt need in the oil & gas sector, considering also that these instruments are required to operate in environments that can be severely harsh, especially in terms of temperature and humidity. In this thesis, the developed and tested, in laboratory and in actual real test rigs of two measurement instruments, one for ion current measurements and one for exhaust gas composition measurement is discussed. For the first instrument, a theoretical model of the ion sensor used was also developed, which significantly helped in interpreting the experimental data.