Yunhan Xiao

Modeling HAT cycle and thermodynamic evaluation

The HAT cycle has aroused considerable interest. This paper emphasized the right representation of humidifier that is a key unit in the HAT cycle. An improved superstructure for the HAT cycle was then proposed to improve the thermodynamic performance working at a wide range of pressure ratio. The configuration and parameters were optimized simultaneously. The results revealed that the optimal system can reach the minimum exergy losses match. The artificial assumptions based on intuition and locally thermodynamic analyses by previous researchers are harmful to the efficiency of HAT, and also mislead the frame of HAT. The insights in this paper can provide a basis for system match and unit design for the development of HAT power generation system.

Experimental and Theoretical Studies on Air Humidification by a Water Spray for Humid Air Turbine Cycle

Humidification of compressed air is important for humid air turbine cycle. Earlier studies paid more attention to packed bed towers. However, a spray tower has inherent advantages such as less pressure loss and low cost. In this study, a pressurized model spray tower was established for the experiments. A specially designed air diffuser was installed to achieve uniform air velocity profiles over the cross section and a mist eliminator was used to trap the water droplets in the outlet air. Performance of the tower was tested at different pressures and water/air ratios. Pressure loss was measured and analyzed at different air velocities. A comprehensive analysis of the humidification process in the spray tower was carried out. A mathematical model considering droplet motion and conservation of heat and mass was developed to predict heat and mass transfer in the water droplet-air two-phase flow. Local heat and mass transfer coefficients over height of the tower were calculated. It has been shown that the parameters of outlet air and water can be calculated within a maximal error of 7.3% compared with the experiment results. Droplet size is a main parameter that affects operating performance of the spray tower. It has also been indicated that pressure loss in the spray tower is low and this will benefit its application in HAT.Copyright

System Optimization of Humid Air Turbine Cycle

The humid air turbine (HAT) cycle, proposed by Mori et al. and recently developed by Rao et al. at Flour Daniel, has been identified as a promising way to generate electric power at high efficiency, low cost and simple system relative to combined cycle and steam injection gas turbine cycle. It has aroused considerable interest.Thermodynamic means, such as intercooling, regeneration, heat recovery at low temperature and especially non-isothermal vaporisation by multi-phase and multi-component, are adopted in HAT cycle to reduce the external and internal exergy losses relative to the energy conversion system. In addition to the parameter analysis and the technical aspect of HAT cycle, there is also a strong need for “systems” research to identify the best ways, of configuring HAT cycle to integrate all the thermodynamic advantages more efficiently to achieve high performance.The key units in HAT cycle are analyzed thermodynamically and modelled in this paper. The superstructure containing all potentially highly efficient flowsheeting alternatives is also proposed. The system optimization of the HAT cycle is thus represented by a nonlinear programming problem. The problem is solved automatically by a successive quadratic algorithm to select the optimal configuration and optimal design parameters for the HAT cycle.The results have shown that the configuration of the HAT cycle currently adopted is not optimal for efficiency and/or specific power, and the current pressure ratio are too high to be favorable for highest performance. Based on the current technical practice, the optimal flowsheeting for thermal efficiency can reach 60.33% when TIT=1533K, while the optimal flowsheeting for specific power can achieve 1300kW/kg/s air for TIT at 1533K. The optimal flowsheeting configuration is compared favorably with the other existing ones.Copyright

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

Performance Prediction of a Transport Gasifier–Based IGCC System with CO2 Capture under Uncertainties

AbstractIn this study, a transport gasifier–based integrated gasification combined cycle (IGCC) with 90% CO2 capture rate was analyzed to predict possible net power output and net efficiency distributions in the system under uncertainties. Moreover, the influence of each parameter on the total model output uncertainty was quantified and displayed using Pareto charts. Uncertainties in parameters of the gasifier, Selexol-CO2 capture process, hydrogen-fueled gas turbine, compressors, steam turbines, and pumps were considered and quantitatively represented in uniform, triangle, or normal distributions. Monte Carlo simulation was adopted to propagate the parameters through process modeling. Results showed that the predicted median net plant efficiency of the system was 37.71% with a standard deviation of 0.41%. The unconverted carbon rate in the gasifier and the gas turbine combustor temperature (gtCMBT) exhibited the greatest effect on predicting uncertainties in efficiency and power output, respectively. Thu...

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

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

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

Investigation of Coal Fueled Chemical Looping Combustion Using Fe3O4 as Oxygen Carrier

Chemical-looping combustion (CLC) is a novel combustion technique with CO2 separation. Magnetite (Fe3O4) was selected as the oxygen carrier and Shenhua coal (Inner Mongolia, China) as the fuel for this study. The influences of operation temperatures, and coal to Fe3O4 mass ratios on the reduction characteristics of the oxygen carrier were investigated using an atmosphere TGA. The sample, comprised of 2.25mg coal and 12.75mg Fe3O4, was heated to 1000°C. Experimental results show that the reaction between the coal volatile and Fe3O4 began at 700°C while the reaction between the coal char and Fe3O4 occurred at 800°C and reached a peak at 900°C. Fe3O4 was fully reduced into FeO, while some FeO was further reduced to Fe. As the operation temperature rises, the reduction conversion rate increases. At the temperatures of 850°C, 900°C, and 950°C, the reduction conversion rates were 37.1%, 46.5%, and 54.1% respectively. When the mass ratios of coal to Fe3O4 were 5/95, 10/90, 15/85, and 20/80, the reduction conversion rates were 29.5%,40.8%,46.5%, and 46.6% respectively. With the increase of coal to Fe3O4 mass ratio, the conversion rate increases first and then changes no more. There exists an optimal coal to Fe3O4 mass ratio.