Experimental and Numerical Studies on Low Emission Syngas Combustion
The present study concerns experimental and numerical investigation of the combustion of low-calorific value syngas in an optically accessible reverse flow combustion chamber. Several modes of operation are investigated to identify the best strategy for stable operation with low emissions of NOx and CO. The first part of the study investigates the combustion dynamics in the chamber and establishes the range of parameters for stable operation using OH* chemiluminescence (5 kHz), noise (50 kHz), and exhaust emissions measurements (NOx and CO). The combustion dynamics have been investigated as a function of the global equivalence ratio (0.32 - 0.89), O2% in the co-flow (7.6 - 21%), and the oxidizer preheat temperature (~ 400 - 800 K). The variation of these parameters resulted in different operating conditions designated as: conventional (Φglobal = 0.8), ultra-lean (Φglobal = 0.32), transition (Φglobal = 0.47, O2 = 14.3% in oxidizer), and MILD (Φglobal = 0.89, O2 = 7.6% in oxidizer) combustion modes. For all cases, autoignition was observed to be the mode of flame stabilization that indicated the role of H2 in reducing the ignition delay. The conventional mode displayed the highest sound pressure level (SPL) and fluctuations in the reaction zone (OH*). The most stable operation was obtained for the MILD case where the SPL decreased by 6 dB caused by a suppression of the high-frequency (> 800 Hz) longitudinal modes. In the second part of the study, OH concentration and temperature are measured using Planar Laser-induced Fluorescence (PLIF) and Rayleigh thermometry to provide a detailed understanding of the reaction zone structure. The OH radical, which is a marker of the reaction zone, shows maximum intensity for the conventional case and lowest intensity for the MILD case. The instantaneous images show a complex reaction zone with thin structures near the inlet and progressive distribution of OH at the bottom. The temperature measurements reveal a uniform thermal field throughout except very close to the centreline. Such a distribution can provide superior heat transfer characteristics in furnaces. The maximum temperature is measured for the conventional case (~ 1700 K), while the temperature is similar for the ultra-lean, transition, and MILD cases (~ 1300 K) supporting the observations of low NOx emissions. In the third part of the study, we evaluate the performance of the combustor by measuring NOx and CO emissions. The NOx emission is less than 1-ppm for all the cases, while the CO emission is highest for the MILD case (461-ppm) and lowest for the conventional case (32-ppm). In the last two parts of the study, the experimentally generated data is used to validate models that are subsequently used to numerically simulate scaled-up designs of the combustor with power ranging from 3.3 kW to 25 kW. The influence of four different scaling criteria on the performance of the combustor is evaluated. These are constant velocity (CV), constant residence time (CRT), constant volume-to-jet momentum ratio (CM), and constant volume-to-jet kinetic energy ratio (CK). The CV criterion performs the best in terms of pressure drop and CO emissions. Overall, the current investigation establishes that the combustion of low calorific value syngas can be performed in a reverse flow configuration with low emissions and potential for scaling to industrial sizes.