The Chemical and Physical Effect of Diluent H2O on NO and CO Emissions in Computational CH4 / Air Laminar Diffusion Flames (original) (raw)
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Journal of Engineering for Gas Turbines and Power, 2008
A numerical study was carried out to understand the effect of CO enrichment on flame temperature and NO formation in counterflow CH 4 /air diffusion flames. The results indicate that when CO is added to the fuel, both flame temperature and NO formation rate are changed due to the variations in adiabatic flame temperature, fuel Lewis number, and chemical reaction. At a low strain rate, the addition of carbon monoxide causes a monotonic decrease in flame temperature and peak NO concentration. However, NO emission index first slightly increases, and then decreases. At a moderate strain rate, the addition of CO has negligible effect on flame temperature and leads to a slight increase in both peak NO concentration and NO emission index, until the fraction of carbon monoxide reaches about 0.7. Then, with a further increase in the fraction of added carbon monoxide, all three quantities quickly decrease. At a high strain rate, the addition of carbon monoxide causes increase in flame temperature and NO formation rate, until a critical carbon monoxide fraction is reached. After the critical fraction, the further addition of carbon monoxide leads to decrease in both flame temperature and NO formation rate.
Effect of Hydrogen and Carbon Monoxide Addition to Methane on Laminar Burning Velocity
2019
Exhaust gas recirculation (EGR) in spark-ignited engines is a key technique to reduce in-cylinder NOx production by decreasing the combustion temperature. The major species of the exhaust gas in rich combustion of natural gas are hydrogen and carbon monoxide, which can subsequently be recirculated to the cylinders using EGR. In this study, the effect of hydrogen and carbon monoxide addition to methane on laminar burning velocity and flame morphological structure is investigated. Due to the broad flammability limit and high burning velocity of hydrogen compared to methane, this addition to the gaseous mixture leads to an increase in burning velocity, less emissions production, and a boost to the thermal efficiency of internal combustion engines. Premixed CH4-H2-CO-Air flames are experimentally investigated using an optically accessible constant volume combustion chamber (CVCC) accompanied with a high-speed Z-type Schlieren imaging system. Furthermore, a numerical code is applied to q...
Combustion and Flame, 1998
Flue gas recirculation (FGR) is a well-known method used to control oxides of nitrogen (NO x) in industrial burner applications. Recent small-and large-scale experiments have shown that introducing the recirculated flue gases with the fuel results in a much greater reduction in NO x , per unit mass of gas recirculated, compared to introducing the flue gases with the combustion air. At present, however, there is no definitive understanding of why introducing the recirculated gases with the fuel is more effective than conventional FGR. The objective of the present investigation is to ascertain to what degree chemical kinetics and/or molecular transport effects can explain the differences in NO x reduction observed between fuel-side and air-side introduction of flue gases by studying laminar diffusion flames. Numerical simulations of counterflow diffusion flames using full kinetics were performed and NO x emission indices calculated for various conditions. Studies were conducted in which N 2 diluent was added either on the fuel-or air-side of the flame for conditions of either fixed initial velocities or fixed fuel mass flux. Results from these simulation studies indicate that a major factor in diluent effectiveness is the differential effect on flame zone residence times associated with fuel-side versus air-side dilution. Experiments using laminar jet flames were conducted in which either the air or fuel stream was diluted with N 2. The experiments showed that fuel-side dilution results in somewhat greater NO x emission indices than air-side dilution. The higher flame temperatures measured with fuel dilution appear to be the principal cause of the higher emissions. The results of both the numerical simulations and the experiments suggest that, although molecular transport and chemical kinetic phenomena are affected by the location of diluent addition depending on flow conditions, the dramatically greater effectiveness of fuel-side over air-side introduction of recirculated flue gases in practical applications likely results also from differences in turbulent mixing and heat transfer.
A numerical investigation of NOx formation in counterflow CH4/H2/air diffusion flames
Proceedings of …, 2006
A detailed numerical study was carried out for the effect of hydrogen enrichment on flame structure and NO X formation in counterflow CH 4 /air diffusion flames. Detailed chemistry and complex thermal and transport properties were employed. The enrichment fraction was changed from 0 (pure CH 4) to 1.0 (pure H 2). The result indicates that for flames with low to moderate stretch rates, with the increase of the enrichment fraction from 0 to 0.5~0.6, NO emission index keeps almost constant or only slightly increases. When the enrichment fraction is increased from 0.5~0.6 to about 0.9, NO emission index quickly increases, and finally NO formation decreases again when pure hydrogen flame condition is approached. However, for flames with higher stretch rates, with the increase of hydrogen enrichment fraction from 0 to 1.0, the formation of NO first quickly increases, then slightly decreases and finally increases again. Detailed analysis suggests that the variation of the characteristics in NO formation in stretched CH 4 /air diffusion flames is caused by the change of flame structure and NO formation mechanism, when the enrichment fraction and stretch rate are changed.
Combustion and Flame, 2007
Axisymmetric co-flowing acetylene/air laminar diffusion flames have been experimentally investigated to study the effect of hydrogen addition on soot formation and soot morphology. An acetylene-hydrogen jet burning in co-flowing air at atmospheric pressure has been studied under different flow arrangements, i.e., premixed and with separate addition of acetylene and hydrogen. Thermophoretic sampling and analysis by transmission electron microscopy are employed for soot diagnostics. Soot microstructure, primary particle size, soot volume fraction, and fractal geometry results are reported. The effect of hydrogen addition on the temperature field is moderate (maximum increase ∼100 K), the effect being greater when hydrogen is premixed with acetylene. Soot volume fraction decreases with hydrogen addition. A shift was noted in the soot volume fraction peak with change in the Reynolds and Froude numbers at the burner exit. The primary soot particle diameter is in the range of 20-35 nm. Soot particles are larger in size close to the burner for the pure acetylene flame. A reverse trend is observed with hydrogen addition. The fractal dimension of the soot aggregates is about 1.7-1.8. It is unaffected by hydrogen addition and location in the flame. Soot aggregate size tends to decrease with hydrogen addition. The results of the present study on the effect of hydrogen addition on soot volume fraction and mean primary particle size are in good correlation with the results of other investigators for ethylene-, propane-, and butane-air flames, which have been described with regard to the HACA mechanism of soot nucleation and growth and enhanced soot oxidation in fuel-rich flames by increased OH radical concentration.