Zhedian Zhang

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

Effect of Fuel Dilution on the Stability Characteristics of Syngas Diffusion Flames

In order to investigate the effects of fuel dilution on flame stability characteristics, open syngas diffusion flames are established and H2 O, N2 and CO2 are employed respectively as diluents. The burner configuration used in this study consists of a bluff body with a central jet flow of the fuel and a surrounding coflow of the air. The syngas is composed of 50% of H2 and 50% of CO (by volume). The experiments are conducted at 1 atmospheric pressure, and the temperatures of the fuel and the air are kept constant at about 400 K. The results show that the flame tapers inward and becomes more cylindrical in the shape as after the dilution, the flame becomes unstable due to dilution effects. It has been found that there is a maximum flow rate of diluents responsible for the flame extinction. Among these three dilutions, H2 O diluted flames exhibit a highest stability, while CO2 diluted flames have the lowest one due to its large specific heat. Planar Laser-Induced Fluorescence (PLIF) measurements of the OH radical are applied to study the behavior of the OH radical in the flames. The results show that as the diluents introduced into the flame increases, the overall OH mole fraction significantly decreases, and the flame width also decreases. The structures of flame bases are also studied to obtain a better understanding of fuel dilution effects on the flame stability. The radial stabilization distance is decreased and the local flame extinctions in the reaction zones are found as dilution increases. For operating conditions close to the flame extinction limit, the flame reaction zones in the flame bases take on a more intermittent, shredded appearance.© 2008 ASME

Comparative Study of Syngas Mild Combustion Characteristics in Swirl Diffusion and Coflow Diffusion Staged Combustor

MILD combustion is a promising combustion technology for the future gas turbine combustor due to its high combustion efficiency, low exhaust emissions and enhanced combustion stability. It utilizes the concept of exhaust gas recirculation to achieve combustion at reduced temperature and flat thermal field. To examine the role of gas recirculation level on MILD combustion performance, a laboratory-scale axially staged combustor constituted of gas generation zone, mixing zone and MILD combustion zone is presented. To realize ultra-low NOx emissions for syngas characterized by high flame temperature, it is necessary to select an appropriate combustion mode for the gas generation zone. This study compared combustion performance and gas/fuel/air mixing feature between two configurations, gas generation zone of which are based on swirl diffusion combustion and coflow diffusion combustion, respectively. The results are compared on flow field with numerical simulation, and global flame signatures and exhaust emissions with experiment. Both numerical simulation and experiment are performed at equivalence ratio of 0.4, heat load of 24.4 kW, using 10 MJ/Nm3 syngas as the fuel at atmospheric pressure and normal temperature fuel and air. More uniform oxidizer, lower flame temperature and less NOx production are observed in coflow diffusion staged combustion. MILD combustion zone is beneficial for the reduction of NOx and oxidation of CO exit from the gas generation zone.Copyright

Three-Dimensional CFD Analysis of a Gas Turbine Combustor for Medium/Low Heating Value Syngas Fuel

Gas turbine is one of the key components for integrated gasification combined cycle (IGCC) system. Combustor of the gas turbine needs to burn medium/low heating value syngas produced by coal gasification. In order to save time and cost during the design and development of a gas turbine combustor for medium/low heating value syngas, computational fluid dynamics (CFD) offers a good mean. In this paper, 3D numerical simulations were carried out on a full scale multi-nozzle gas turbine combustor using commercial CFD software FLUENT. A 72 degrees sector was modeled to minimize the number of cells of the grid. For the fluid flow part, viscous Navier-Stokes equations were solved. The realizable k-e turbulence model was adopted. Steady laminar flamelet model was used for the reacting system. The interaction between fluid turbulence and combustion chemistry was taken into account by the PDF (probability density function) model. The simulation was performed with two design schemes which are head cooling using film-cooling and impingement cooling. The details of the flow field and temperature distribution inside the two gas turbine combustors obtained could be cited as references for design and retrofit. Similarities were found between the predicted and experimental data of the transition duct exit temperature profile. There is much work yet to be done on modeling validation in the future.Copyright

Influence of Air Humidity on the Oxidation of CO in Syngas Diffusion Flames

Combustion of syngas with humid air is important for integrated coal gasification humid air turbine cycle for power generation. In contrast to hydrocarbon fuels, CO exists in bulky content in syngas. Thus the influence of air humidity on the oxidation of CO in syngas flames should be well understood. In the present study, the influence of air humidity on CO oxidation in syngas diffusion flames was investigated both experimentally and numerically. In the experiment, CO, CO2 , O2 and temperature profiles in a model combustor of a syngas turbulent jet diffusion flame were measured. It was found that as the mass flow rate of dry air and the exhaust temperature kept constant, the profile of CO was influenced by air humidity, but CO exhaust did not exhibit a monotone increase over humidity. There exist an inflexion at the case of absolute humidity of the air X = 6.0. In addition, CO emission decreases with the increase of the thermal load. And the influence of the humidity on CO oxidation was not obviously when the thermal load is high. In the numerical simulation, flow field in the combustor was calculated by applying the composition PDF transport model and Flamelet model respectively. The numerical results were compared with the experiment and the PDF model was verified that it is more suited to simulate the CO oxidation. From the numerical analysis, it was found that the concentration of OH wasn’t monotonously increased with the added H2 O and this directly affect the oxidation of CO via CO + OH → CO2 + H. In general, the influence of air humidity on the oxidation of CO in syngas diffusion flames depends on the coupling and competition of chemistry and fluid mechanics aspects. This helps to interpret the discrepancy of the results of gas turbine combustor test.Copyright

Influence of Humid Air on Gaseous Combustion of Gasturbines

Combustion with humid air is a key process of humid air turbine (HAT) cycle. In the present study, the influence of humid air on gas turbine combustion was studied both experimentally and numerically. Performance of a full-scale can-type combustor equipped with a diffusion burner was investigated when burning propane and syn-gas with various humidity of intake air. The results indicate that the effect of humid air on pollutant emission depends on fuel type due to the difference of chemical mechanisms. For the syn-gas flames, moisture addition can effectively reduce NO emission without increasing CO. A numerical model was developed to simulate the 3D flow field in the combustor when burning syn-gas. The mixture fraction approach and the laminar flamelet model were applied to simulate the diffusion flame. The thermochemical quantities of the flamelets were computed by adopting a detailed chemical reaction mechanism for the H2 -CO-N2 -O2 system. The numerical results show that an oval hot zone above 2100 K is formed near the axis of the combustor due to flow recirculation. The hot zone mainly accounts for the thermal NO in the syn-gas flames. With the moisture addition into intake air, the volume of this zone is substantially decreased, and, therefore, the NO production is suppressed. This explains the NO reduction due to humid air observed in the experiment.© 2005 ASME