Kejin Mu

Effect of Fuel Dilution on the Structure and Pollutant Emission of Syngas Diffusion Flames

Combustion with diluted syngas is important for integrated gasification combined cycle (IGCC) system that attains high efficiency and low pollutant emissions. In syngas diffusion flames, peak flame temperature is higher than that in nature gas flames, so NOx emission is more significant. To achieve low NOx emission, fuel dilution is an effective way. In the present study, Flame structure and emission characteristics were experimentally and numerically studied in various fuel diluted syngas diffusion flames, and H2 O, N2 and CO2 were employed as diluents respectively. The purpose of this paper is to better understand the behavior and mechanism of fuel diluted combustion and to provide fundamental data base for the development of syngas combustion techniques. Experiments were conducted by using jet diffusion flames in a model combustor. Flame size, exhaust temperature and emission concentration were measured. It was found that by introducing diluents into fuel stream, the stoichiometric surface was brought inward, namely the flame envelope shrunk due to a relatively low fuel concentration. The exhaust temperature was decreased. The results also indicated that with diluted fuel stream, there was an increase of CO emission and an apparent decrease of NO emission. For the same exhaust temperature, H2 O had the most significant influence on NO emission among the three diluents, while CO2 affected CO emission most by inhibiting its oxidation thermally and chemically. Numerical simulations were performed in counterflow diffusion flames by applying Chemkin software. To reveal the mechanisms of various diluents in flames, the detailed chemistry of H2 -CO-N2 system was employed. It was found that the concentration of OH radical is important for both NO and CO emissions. The OH concentration is affected not only by the type of diluents but also by the flame temperature, therefore it is determined by the coupling and competition of diluents’ chemical and thermal effects.Copyright

Combustion Characteristics of Hydrogen-Methane Hybrid Fuels in Coflow Jet Diffusion Flames

As the development and increasingly widespread use of IGCC and zero emission energy system, the development of advanced combustion capabilities for gaseous hydrogen and hydrogen rich fuels in gas turbine applications is becoming an area of much great concern. The combustion characteristics of hydrogen rich fuel is very different from nature gas in aspects such as flame stability, flame temperature, combustor acoustics, pollutant emissions, combustor efficiency, and some other important quantities. However, few of these issues are clearly understood by far. The purpose of this paper is to compare in detail the combustion performance of hydrogen-methane hybrid fuels with various volumetric H2 fractions ranging from 0% to 100%. Meanwhile, the comparison of pure H2 , pure CH4 , and 80%H2 +20%CH4 was the emphasis. 80%H2 +20%CH4 hybrid gas is selected expressly because its component is approximately equal to the outcome of a hydrogen production test bed of our laboratory, and it is considered by the team to be a potential transition fuel of gas turbines between nature gas and pure hydrogen. Detailed experimental measurements and numerical simulations were conducted using a coflow jet diffusion burner. It was found that in the extent of experiments, when under equal general power, the flame length of hydrogen contained fuels wasn’t much shorter than methane, and didn’t get shorter with the increase of H2 fraction as expected. That was because the shortening tendency caused by the increase of H2 fraction was counteracted partially by the increase of fuel velocity, results of which was the extending of flame length. Maximum temperature of H2 flame was 1733K, which was 30K higher than 80%H2 +20%CH4 and 120K higher than CH4 . All of the highest temperatures of the three fuels were presented at the recirculation zone of the flame. Although it seemed that the flame of CH4 had the longest dimension compared with H2 contained fuels when observed through photos, the high temperature region of flames was getting longer when increasing H2 fractions. Curves of temperature distribution predicted by all the four combustion models in FLUENT investigated here had a departure away from the experimental data. Among the models, Flamelet model was the one whose prediction was comparatively close to the experimental results. Flame of H2 and 80%H2 +20%CH4 had a much better stability than flame of CH4 , they could reach a so called recirculating flame phase and never been blew out in the extent of experiments. On the contrary, CH4 flames were blew out easily soon after they were lifted up. Distribution of OH concentration at the root of flames showed that the flame boundary of H2 and 80%H2 +20%CH4 was more clearly than CH4 . That is to say, at the root of the flame, combustion of H2 was the most intensive one, 80%H2 +20%CH4 took the second place, while CH4 was the least. NOx emissions didn’t show a linear relationship with the volumetric fraction of H2 , but showed an exponential uptrend instead. It presented a fairly consistent tendency with flame temperature, which proved again there was a strong relationship between flame temperature and NOx emissions in the combustion of hydrogen contained fuels. If adding CH4 into pure H2 , NOx concentration would have a 17.2ppm reduction with the first 20% accession, but only 11.1ppm with the later 80% accession. Hence, if NOx emission was the only aspect to be considered, 80%H2 +20%CH4 seemed to be a better choice of transition fuel from pure CH4 to pure H2 .Copyright