Pressure effects on NOx and CO emissions of a model syngas combustor were experimentally and numerically studied. The validated numerical method was employed to analyze pressure effects on the combustor emissions such as different mixing levels and extra reactant addition. The model combustor, fueled with 10 MJ/Nm3 coal-derived syngas, was installed in a pressurized test-rig, and emissions were measured within 0.1–0.35 MPa. Based on the flow, temperature and species fields calculated by the CFD method, a chemical reactor network (CRN) model was established. With a detailed chemical scheme, the emissions calculated by the CRN model agreed well with experimental results. The model was then employed to calculate emissions within pressure range from 0.1 to 2.0 MPa, with the adiabatic flame temperature in the primary zone varied from 1477 to 2317 K. The calculated NOx and CO emissions generally showed exponential relationship with the operating pressure, except that the NOx emission decreased at higher pressure when the primary adiabatic flame temperature was lower than 1800 K. Through the analyses of the NOx formation pathways, it was found that the higher pressure would generally enhance the N2O pathway, which dominated NOx formation at low temperature, but NOx emission concentration through N2O pathway decreased with pressure above 1.0 MPa at low temperature, which could explain the NOx emission behavior. The CRN modeling method was applied to analyze the effects ofpressure on emission behaviors under the different mixing modes and with the extra reactant such as H2O and NH3 in fuel. Premixed and steam dilution burning modes showed better performance on reducing emissions at elevated pressures. NH3 could obviously increase the emission levels, especially at low pressure. Comprehensive understanding of the relationship between the emissions and the operating pressure in full pressure range was obtained, which could be valuable for the predicting and analyzing the syngas combustor pollutant emissions.
Conclusions
Pressure effects on pollutant emissions of a syngas model combustor of gas turbine were studied in the pressure range of 0.1–2.0 MPa. The combustor emissions with thermal powers up to180 kW and pressures within 0.1–0.35 MPa were studied using a variety of measurement techniques, and the combustion emission behaviors within the full pressure range were simulated using a CRN model, which showed good agreement with experimental data. NOx emissions kept a power-law with pressure in the full pressure range up to 2.0 MPa when Tad, full was higher than 1800 K. The exponent n varied from 0.09 to 0.51, and n increased with the exhaust temperature. However, when Tad, full was lower than1800 K, NOx emission shows parabolic profiles with pressure. Similar behaviors were shown in premixed combustion. CO emissions also kept a power-law with pressure in general. The exponent avaried from -0.44 to -1.01 with exhaust temperature range from1120 to 1813 K, and seemed to depend on the exhaust temperature. Pressure sensitivity of CO emission increased monotonously with the increase of temperature due to the higher OH production. It was shown that the N2O pathway was enhanced obviously under high pressure. When the Tad was below 1800 K, the NOx emissions increased with pressure mainly through the N2O pathway. When the Tad was higher than 1800 K under richer condition, NOx emission increased with pressure mainly through the thermal pathway. When Tad was lower than 1800 K, the mole fraction of NOx through the N2O pathway showed parabolic profiles with pressure. However, the mole fraction of NOx through thermal pathway monotonously increased with pressure under rich condition. Besides, NOx was produced during syngas combustion mainly through the NNH pathway in lean condition, while through thermal pathway in rich condition under the atmospheric pressure. Both temperature and pressure seemed to suppress the NNH pathway. Air–fuel mixing levels at the primary burning zone influenced pollutant emissions significantly. Premixed combustion was suggested for NOx emissions controlling especially at elevated pressure. As for the syngas, the non-premixed combustion was probably adopted to avoid problems as such flashback. However, it could still be efficient if the mixing level of air and fuel in the primary burning zone was reduced by proper jet design. It was notable that the mixedness deterioration might increase CO emissions. Steam dilution turned out to be an efficient way to reduce NOx formation without increasing CO emission for syngas combustors. Also, steam seemed to suppress pressure sensitivity of NOx emissions. Lower CO emissions might be caused by higher OH concentrations due to the high third body collision efficiency of the steam. NOx emissions of industrial syngas combustor could be much higher with NH3 existence. Converting rate of NH3 was suppressed at high pressure, and NH3 could obviously shift the emission behaviors at elevated pressure. In low temperature condition, NOx showed negative dependence to pressure. In addition, it seemed that NH3 had little influence on CO emissions.
The results have been published onApplied Thermal Engineering 102 (2016) 318–328.
