Flameless oxidation technology (FLOX®) has already demonstrated great potential in reducing thermal nitrogen oxides while burning natural gas. Due to the high stability of the combustion process, the FLOX® technology has been further developed to burn various low calorific value gases (LCVG).
The work presented in this thesis focuses on the optimization of such flameless burners for LCVG as well as on a profound analysis of the combustion processes, with a special emphasis on the fuel-NOx formation and reduction mechanisms.
The test results have shown that when combusting NH3 doped synthetic gases, the NH3 to NOx conversion is highly dependent on the CH4 concentration in the fuel. It significantly increases when increasing the CH4 content. The CO/H2 ratio has no measurable influence on the NH3 conversion. When gasifying solid biomass and combusting the product gases in the flameless burner, it has been observed that the final NOx emission depends on the initial N content in the fuel, the gasifier parameters, and the final stoichiometry of the burner. For the fuels with lower N content, the NOx emissions, thus N to NOx conversion ratios, are comparable to other common combustion technologies. However, for high-N content fuels, the conversion ratios are higher. It has been observed that the higher temperature and air ratio in the gasifier, the better NOx emission can be achieved in the flameless burner. Due to very good mixing conditions, it is possible to operate the flameless burners for LCVG at very low excess oxygen, simultaneously achieving CO emissions at the level of a few ppm.
Numerical modeling using CFD software has been performed to optimize the design of various burners. The most important parameter when optimizing the geometry for LCVG is the internal recirculation rate, which stabilizes the combustion process. The methodology to calculate the recirculation has been introduced to the software code by the calculation of the local recirculation rate for quantitative analysis of the flue gas flow pattern.
It has been shown that the CFD analysis with a global chemistry model is a sufficient tool for the proper geometry design to optimize the fluid flow in a flameless burner. However, for the NOx modelling a two-stage numerical model has been developed comprising the reactive flow modelling using CFD and detailed chemistry modeling using reactor network model for pollutant formation analysis. Two different detailed chemistry schemes have been validated, identifying both suitable to predict NOx emissions for methane containing gases. Three different mixing approaches have been applied and validated, showing that the proper mixing representation is crucial when modelling gases with lower CH4 content.
It has been shown that CH4 influences the kinetics of combustion causing significant delay in the fuel oxidation and it simultaneously blocks ammonia decomposition. The later the decomposition is taking place the more air is entrained in the jet, the more nitrogen is converted to NOx. For methane rich fuels the mixing is faster than the combustion reactions. The combustion is kinetically controlled. For methane free fuel the combustion occurs immediately – what is mixed is also burned, causing the ammonia to react immediately after introducing into the combustor under fuel rich conditions. The process is diffusion controlled. Therefore, for the methane free gases the precise representation of mixing processes is deciding for correct NOx predictions.