Pre-heating the air before it enters the combustion chamber allows for a more homogeneous combustion at lower temperatures, which makes NOx emissions practically negligible.
Researchers from the Polytechnic School, connected to the Research Centre for Gas Innovation (RCGI), have studied modifications in the natural gas combustion process to reduce emissions of nitrogen oxides (NOx), considered air pollutants. The idea of “flameless” combustion is associated, in this case, to a more homogeneous fuel burning process, at lower temperatures, which would prevent NOx from forming.
“One sees the flame because, in a conventional burning, there are regions in which the reactions are more intense, thin regions of very high temperature. Around these regions, reactions do not occur with the same intensity. We thus only see the flame because the ambient in which the combustion occurs is not homogeneous. NOx is formed exponentially at high temperatures. Therefore, a drop in temperature also exponentially reduces its formation”, explains Guenther C. Krieger Filho, who coordinates the project Development of an Advanced Natural Gas Burner using the Flameless Oxidation Concept.
Krieger and his team (two other engineers involved in the project) aim to build two prototypes of flameless burners and, by using them, generate data that may later feed computational models to design new burners and new combustion chambers. “To perform combustion at lower temperatures, the air has to be pre-heated before it enters the combustion chamber and to promote, within the combustion chamber, re-circulations of the gases, so that the whole chamber becomes a large region of low-temperature reaction. That is, instead of a region in which the reactions are concentrated, the whole content of the chamber reacts in the same manner, allowing a homogeneous combustion, at lower temperatures, which makes the NOx emissions to be practically negligible.”
The engineer lists one more advantage of flameless combustion: it prevents other polluting gases from escaping into the atmosphere, such as methane, a natural gas compound. “In the case of conventional combustion, a pollutant passing the region of the flame, the most intense region, that has not reacted for some reason, usually associated to turbulence, will no longer burn; it will be emitted into the atmosphere. However, if we can ensure a more homogeneous combustion, we greatly reduce this possibility.”
The team intends to apply the technique to gas turbines. “We are going to build a demonstration burner, at laboratory level, and I am trying to assemble it in the micro-gas turbine we made here in our laboratory at the Mechanical Engineering Department of the Escola Politécnica (Engineering School) of the USP. The idea is not merely obtaining the burner, but rather to know how it works so as to be able to design a larger one, if that is the case.”
Krieger advances that, despite the lack of gas turbine manufacturers in Brazil, it is important to contribute to this issue. “I have noticed there is a demand for investigating innovative combustion processes, which are especially interesting to the power generation sector (thermoeletric plants) – a regulated marked that has to reduce emissions. If we study these processes, it will raise interest and new partnerships may emerge.”
The first prototype will be delivered in one year. “It is a model without optical access (quartz windows), which will serve for us to be sure that we will really manage to operate this type of burner.” Between the second and the third years, another prototype will be assembled, with quartz or sapphire windows to allow the penetration of laser beams to determine what happens during the combustion.
The whole experimental part will be developed at the Laboratory of Advanced Combustion Diagnosis; its assemblage is the goal of one of the projects conducted within the RCGI ambit. “The laboratory will be a multi-user centre, that is, research teams having a demand for using it will be able to do it, even not being connected to the RCGI or to Poli, as long as they present their proposal and this is also interesting to us. We want more people to use it, the demands to appear”. The laboratory is being equipped with different types of laser diagnostics, which allow accurate analyses of the combustion processes studied.