Flameless Combustion of Natural Gas in the SJ/WJ Furnace

Abstract

Flameless combustion in a 48 kW pilot scale furnace fired with natural gas is studied experimentally and computationally. The burner geometry involved a tunnel furnace with two separate feed streams --- one for a high momentum air jet and the other for a low momentum fuel jet. This burner configuration, called a Strong-Jet/Weak-Jet (SJWJ) burner, together with the jetto- jet interactions generate the flameless combustion mode with relatively uniform furnace gas temperature distributions and low NOX emissions. Experiments were carried out under laboratory conditions for turbulent reactive mixing in order to obtain local temperature and gas concentrations. The experimental findings were used to test the performance of CFD numerical models for turbulence, mixing and chemical reactions.

For the SJWJ furnace operated in flameless combustion mode, 32 different flow cases were examined to assess the effects of the three main parameters (fuel/air momentum flux ratio, fuel/air nozzle separation distance and fuel injection angle) on the furnace wall temperature profile. Three specific flow configurations were selected for detailed near-field temperature measurements. The gas temperature distribution inside the combustion chamber was found to be relatively uniform, a characteristic of flameless combustion. Four flow configurations were studied to examine the effect of the fuel jet injection angle (0 degrees or 10 degrees) and fuel/air momentum flux ratio (0.0300 and 0.0426) on the mixing, combustion performance and NOX emissions. Gas compositions were measured in the flue gas and within the furnace at selected locations to estimate the concentrations of CO2 CO, CH4, O2, NO and NOX. The NOX concentrations in the flue gas were quite low, ranging from 7 - 13 ppm, another characteristic of flameless combustion.

The combusting flow CFD calculations were carried out using the k-ε turbulence model and the eddy-dissipation model for methane-air-2-step reactions to predict the temperature and concentration field. The numerical results for gas temperature and compositions of CH4, O2 and CO2 generally showed good agreement with the experimental data. The predicted CO concentration profiles followed expected trends but the experimental data were generally underpredicted. The NOX concentrations were estimated through post-processing and these results were significantly underpredicted.