Emissions of nitrogen oxides from partially premixed flames stabilized on a conical bluff body
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Carbon capture and storage (CCS) technologies allow for the reduction of carbon dioxide (CO2) emissions from the combustion sources of fossil fuels. One such technology is based on the use of hydrogen or hydrogen-enriched fuels and allows for a significant reduction of CO2 emissions. All regulated pollutant emissions, except for harmful nitrogen oxides (NOx), can also be reduced using hydrogen. Hydrogen’s unique properties are challenging for burner designers. High hydrogen-air flame temperatures and flame speeds result in increased NOx emissions and significant changes in flame shape compared with conventional fuels. A novel burner concept employing partially premixed combustion and flame stabilization on a conical bluff body is here proposed to address some of these challenges. In this study a laboratory-scale burner which can operate with methane, hydrogenenriched methane and pure hydrogen has been studied. The burner performance and pollutant emissions were found to depend on factors such as fuel composition, thermal load, excess air and the burner design parameters. Particular burner operation settings affected near-burner aerodynamics and consequently NOx emissions. In more detail, it was found that the flow past the bluff body burner head exhibited vortex shedding contributing to enhanced fuel-air mixing and formed a recirculation zone behind the burner head. Flame presence increased turbulent kinetic energy production near the shear layer of the recirculation zone. The burner operated within the highly turbulent flow regime and significant turbulent kinetic energy was contained at the small scales of the flow. The burner operation was controlled by two parameters: position of the burner head and the fuel split between primary and secondary fuel ports. When the burner head was shifted upstream, NOx emissions were reduced for all fuels tested and the position of the burner head was an important parameter for the reduction of NOx emissions. This was attributed to shortened residence time in the high-temperature zone, enhanced fuel-air mixing and an increase in the amount of furnace flue gas entrained into the combustion zone. The burner head shifted upstream also resulted in lower flame stability and increased carbon monoxide emissions. Influence of the secondary fuel stream on the flow field and NOx emissions was relatively complex and affected by the remaining burner operation settings. Generally, NOx emissions increased when more fuel was provided to the burner through the secondary fuel ports. Some exceptions were observed and the secondary fuel fraction helped in reduction of NOx emissions from hydrogen combustion at low burner thermal loads. Depending on the mixing and the velocities in the burner, the secondary fuel may have penetrated the recirculation zone and the hot combustion products in this zone, what could have led to flame extinction. NOx emissions were largely dependent on the fuel composition. More hydrogen in the methane-hydrogen mixture resulted in higher NOx emissions. This is attributed to higher peak flame temperatures enhancing NOx formation via the thermal mechanism. Another factor was the furnace temperature. At higher furnace temperatures the radiative heat losses from the flame were reduced and consequently NOx emissions increased. Excess air could be used to suppress increase of NOx emissions, but it was only effective for methane and methane-hydrogen mixtures containing low fractions of hydrogen. It is believed that this effect was associated with the flame shape, which changed significantly between methane and hydrogen at given burner operation settings. Shorter and wider flames were characteristic for hydrogen combustion. The burner was capable of firing methane, hydrogen and mixtures of both of these fuels, but its operational settings had to be carefully chosen for each of these fuels in order to ensure safe burner operation and low NOx emissions. Empirical models developed on the basis of statistically cognizant experimental design allowed for NOx emissions prediction in a wide range of operating conditions. At the most promising burner operation settings and furnace temperatures of 1050-1150 °C, NOx emissions equivalent to 26 and 66 ppmvd at 3% O2 were achieved for methane and hydrogen, respectively. Wide fuel flexibility capacity and the low NOx emissions achieved by the burner make it worth considering as a technical solution in deployment of CCS technologies or retroffiting of existing industrial combustion systems.