Modelling of the gas-turbine colorless distributed combustion: An application to hydrogen enriched – kerosene fuel

This study examines hydrogen-enriched kerosene combustion under distributed regime in a gas turbine combustion chamber. With hydrogen enrichment, it is aimed at increasing combustion performance of those fuels. However, in this circumstance, it is obvious to increase the flame temperature with taking place hydrogen enrichment. Thus colorless distributed combustion (CDC), which is one of the advanced combustion techniques, can be suggested to control flame temperature with slowing down the reaction rate, resulting in ultra-low NO_X emissions and more uniform temperature distribution with a broadened flame. For this purpose, the hydrogen-enriched kerosene fuels were examined by modeling a CFD code using the eddy dissipation concept, the radiation model (P-1) and the turbulence model (standard k-ε). In this way, the thermal fields and the NO_X distributions have been obtained. The results showed that hydrogen enrichment increased the flame temperatures from about 2490 K to 2605 K at air-combustion conditions until 30% H₂, resulting in the NO_X levels predicted increased in the combustor. With reducing oxygen percentage the flame started to be the broadened one. The flame temperatures decreased, for instance, from about 2605 K to 2230 K at 15% O₂ for the 30% H₂ containing fuel. As a result of this, the NO_X levels reduced from about 30 ppm to the values lower than 1 ppm in the combustor. It is concluded that increments in temperature and NO_X levels with hydrogen can be suppressed with distributed regime, which enables that gas turbines can be operated at wider flammability limits with hydrogen enrichment.     

Characterization of confined hydrogen-air jet flame in a crossflow configuration using design of experiments

Hydrogen combustion has many industrial applications and development of new hydrogen burners is required to fulfil new demands. A novel configuration of hydrogen burner utilizing crossflow injection of fuel jets into swirling combustion air is characterized empirically in this work. It is intended as a first step in the development of new burner technologies having reduced emission levels and improved efficiency. Experiments were designed using the full factorial design method. Operating parameters were varied simultaneously and the NO_X emissions from the flame stabilized on the burner were measured. Statistical analysis of the experimental data showed that overall equivalence ratio is the dominant factor and lower NO_X emissions are observed at low equivalence ratios, irrespective of the burner power level. The analysis yielded an empirical relationship among NO_X emission, overall equivalence ratio, and power level that is useful in the design activity for a future combustion system based on the proposed configuration.     

Influence of nozzle design parameters on exhaust gas characteristics in practical-scale flameless hydrogen combustion

Considering the trend toward decarbonization, hydrogen is expected to be used as a fuel in industrial furnace burners. One of the challenges in using hydrogen as a fuel is the increase in thermal-NOx emission compared to hydrocarbon fuel owing to its high flame temperature. This study experimentally evaluated the combustion characteristics of flameless combustion, which is a low-NO_x combustion technology, with hydrogen as a fuel in a practical-scale experimental furnace as well as the effect of nozzle design parameters on the combustion characteristics. Through comparative tests with city gas by considering parameters, such as the fuel gas velocity, combustion air velocity, and air nozzle pitch, the low-NO_x effect of flameless combustion was confirmed in hydrogen combustion with appropriate nozzle design parameters. The optimal nozzle design parameters to achieve this effect differ from those for city gas, and the design guidelines are summarized.     

Effect of hydrogen addition on combustion and emission characteristics of methane fuelled upward swirl can combustor

The present research aims to assess the potential of hydrogen in the form of a supplementary fuel to accelerate combustion chemistry and reduce CO emissions of methane fuelled upward swirl gas turbine combustor. Effects of hydrogen enrichment on flame characteristics and chemical kinetics are analysed using Large Eddy Simulations (LES). Flame visualization is performed and measurements of temperature and emissions at the exit of combustor are reported. For the same energy input, flames are relatively broader and shorter at higher hydrogen concentrations. Augmentation of hydrogen is advantageous in terms of flame velocity, temperature, rate of chemical reactions and CO emissions. Higher flame temperature favours NO_x emissions at higher hydrogen content. At a constant volumetric fuel flow, reduction in carbon-generated species is attributed to hydrocarbon substitution and chemical kinetic effects are less. Hydrogen addition increases flame temperature, decreases flame dimensions and reduces CO emissions with marginal increase in NO_x emissions.     

The role of heat recirculation and flame stabilization in the formation of NOX in a thermo-photovoltaic micro-combustor step wall

The health and durability of micro thermophotovoltaic systems are contingent upon the level of gaseous emissions of micro combustors regarding their small size, thickness, and compactness. In small combustion devices, the flame stabilization is achieved via conjugated heat transfer from the stabilized flame to the fresh reactant via the step of the micro-combustors. The step could also create a recirculation of products, and a stagnation zone for the fluid, as a result leading to the accumulation of pollutants. In turbulent H₂ flame, the main attention is given to the NO_X as no other noxious emission, especially carbon emission (CO, CO₂, PAH, and VOC), form during the combustion of hydrogen. The existence of NO_X in the presence of water, as in the combustion of hydrogen is prevalent, could lead to corrosion in combustor interior walls and other detrimental impacts for the ecosystem. In the presented work, micro-combustion of H₂ flame in a cylinder with a step is simulated and the formation of nitrogen oxides is analyzed. The influence of different combustor specifications (equivalence ratio, solid materials) NO_X species are discussed and evaluated. Results revealed nitrogen oxides form and accumulate in the vertical step of the microchannel and that the microchannel walls are more prone to the high concentrations of nitrogen oxides. The application of cavity promotes the two-dimensionality of flow, resulting in effective heat transfer from the hot gas to the cavity walls. This not only leads to flame anchoring to the cavity walls but also results in significant NO_X.     

Design and performance of a pressurized cyclone combustor (PCC) for high and low heating value gas combustion

The combustion difficulties for low heating value (LHV) gases derived from biomass fuels via a gasification process have led to more investigations into LHV gas combustors. Cyclone combustors provide good air/fuel mixing with long residence times. In this study, a small-scale pressurized cyclone combustor (PCC) was designed and optimized using computational fluid dynamics (CFD) simulation. The PCC, along with a turbocharger-based, two-stage microturbine engine, was first characterized experimentally with liquefied petroleum gas (LPG) fuel and then with both LPG and LHV gas derived from biomass in dual-fuel mode. The combustor achieved ultra-low CO and NO_x emissions of about 5 and 7 ppm, respectively, for LPG fuel and of about 55 and 12 ppm, respectively, in dual-fuel mode at the maximum second-stage turbine speed of 26,000 rpm with stable turbine operation.     

An overview on dry low NOx micromix combustor development for hydrogen-rich gas turbine applications

The paper presents a survey of the interactive optimization cycle at Aachen University of Applied Sciences, used for the development of a new low emission Micromix combustor module for application in hydrogen fueled industrial gas turbines. During the development process, experimental and numerical methods are applied to optimize a given baseline combustor with 0.3 mm nozzles with respect to combustion efficiency, combustion stability, higher thermal power output per nozzle and reduced manufacturing complexity.Within the described research cycle combustion and flow simulations are used in the context of parametric studies for generating optimized burner geometries and the phenomenological interpretation of the experimental results. Experimental tests, carried out on an atmospheric combustion chamber test stand provide the basis for validation of simulation results and proof of the predicted combustion characteristics under scaled down gas turbine conditions.In the presented studies, an integration-optimized Micromix combustor with a nozzle diameter of 0.84 mm is tested at atmospheric pressure over a range of gas turbine operating conditions with hydrogen fuel. The combustor module offers an increase in the thermal power output per nozzle by approx. 390% at a significant reduced number of injectors when compared to the baseline design. This greatly benefits manufacturing complexity and the robustness of the combustion process against fuel contamination by particles.During atmospheric testing, the optimized combustor module shows satisfactory operating behavior, combustion efficiency and pollutant emission level. Within the evaluated operating range, which correlates to gas turbine part-, full- and overload conditions, the investigated combustor module exceeds 99% combustion efficiency. The Micromix combustor achieves NO_x emissions less than 2.5 ppm corrected to 15 Vol% O₂ at the design point.Based on numerical analyses and experimental low pressure testing, a full-scale gas turbine combustion chamber is derived. High pressure testing in the auxiliary power unit Honeywell/Garrett GTCP 36–300 shows stable operation during acceleration of the engine, during IDLE and during load variations between IDLE and Main Engine Start (MES) mode. Throughout the investigated operating range, the combustion chamber generates low NO_x emissions under full-scale gas turbine conditions.     

Flameless combustion role in the mitigation of NOX emission: a review

Nitrogen oxide formation is one source of pollution emission in fossil fuel combustion. Numerous technologies, such as flameless combustion, have been introduced to prevent such formation. This paper presents a panorama of flameless combustion and its principles. The influence of this technology on air preheats and gas diluents is discussed, as well as the effect of the firing mode on thermal NO_X. The benefits of flameless combustion, including its energy conservation capability and capability to reduce pollutant emissions such as NO_X emission to overcome environmental dilemmas in combustion, are discussed in this paper. Copyright © 2014 John Wiley & Sons, Ltd.     

A computational study on the combustion of hydrogen/methane blended fuels for a micro gas turbines

To understand the combustion performance of using hydrogen/methane blended fuels for a micro gas turbine that was originally designed as a natural gas fueled engine, the combustion characteristics of a can combustor has been modeled and the effects of hydrogen addition were investigated. The simulations were performed with three-dimensional compressible k-ε turbulent flow model and presumed probability density function for chemical reaction. The combustion and emission characteristics with a variable volumetric fraction of hydrogen from 0% to 90% were studied. As hydrogen is substituted for methane at a fixed fuel injection velocity, the flame temperatures become higher, but lower fuel flow rate and heat input at higher hydrogen substitution percentages cause a power shortage. To apply the blended fuels at a constant fuel flow rate, the flame temperatures are increased with increasing hydrogen percentages. This will benefit the performance of gas turbine, but the cooling and the NO_x emissions are the primary concerns. While fixing a certain heat input to the engine with blended fuels, wider but shorter flames at higher hydrogen percentages are found, but the substantial increase of CO emission indicates a decrease in combustion efficiency. Further modifications including fuel injection and cooling strategies are needed for the micro gas turbine engine with hydrogen/methane blended fuel as an alternative.     

Experimental investigation of dynamic and emission characteristics of a DLE gas turbine combustor

The DLE (dry low emission) technology has already been used on industrial gas turbine combustor and the NO X emission can be limited to 25 ppmv (@15% O 2 ), but one of the destructive effects is combustion instability. In this paper, the dynamic and emission characteristics of a DLE gas turbine combustor have been researched in the authors’ laboratory, and the results show that the key source of combustion instability is the non-uniformity of fuel in the flame zone. Two main fuel supply methods have been used to form different fuel distribution types; it is shown that in the perfectly premixed case the emission level is low and combustion process is stable. The PPF also has an obvious effect on the combustor’s emission and dynamic characteristics.     

Hydrogen assisted diesel combustion

Hydrogen assisted diesel combustion was investigated on a DDC/VM Motori 2.5L, 4-cylinder, turbocharged, common rail, direct injection light-duty diesel engine, with a focus on exhaust emissions. Hydrogen was substituted for diesel fuel on an energy basis of 0%, 2.5%, 5%, 7.5%, 10% and 15% by aspiration of hydrogen into the engine's intake air. Four speed and load conditions were investigated (1800 rpm at 25% and 75% of maximum output and 3600 rpm at 25% and 75% of maximum output). A significant retarding of injection timing by the engine's electronic control unit (ECU) was observed during the increased aspiration of hydrogen. The retarding of injection timing resulted in significant NO_X emission reductions, however, the same emission reductions were achieved without aspirated hydrogen by manually retarding the injection timing. Subsequently, hydrogen assisted diesel combustion was examined, with the pilot and main injection timings locked, to study the effects caused directly by hydrogen addition. Hydrogen assisted diesel combustion resulted in a modest increase of NO_X emissions and a shift in NO/NO₂ ratio in which NO emissions decreased and NO₂ emissions increased, with NO₂ becoming the dominant NO_X component in some combustion modes. Computational fluid dynamics analysis (CFD) of the hydrogen assisted diesel combustion process captured this trend and reproduced the experimentally observed trends of hydrogen's effect on the composition of NO_X for some operating conditions. A model that explicitly accounts for turbulence–chemistry interactions using a transported probability density function (PDF) method was better able to reproduce the experimental trends, compared to a model that ignores the influence of turbulent fluctuations on mean chemical production rates, although the importance of the fluctuations is not as strong as has been reported in some other recent modeling studies. The CFD results confirm that temperature changes alone are not sufficient to explain the observed reduction in NO and increase in NO₂ with increasing H₂. The CFD results are consistent with the hypothesis that in-cylinder HO₂ levels increase with increasing hydrogen, and that the increase in HO₂ enhances the conversion of NO to NO₂. Increased aspiration of hydrogen resulted in PM, and HC emissions which were combustion mode dependent. Predominantly, CO and CO₂ decreased with the increase of hydrogen. The aspiration of hydrogen into the engine modestly decreased fuel economy due to reduced volumetric efficiency from the displacement of air in the cylinder by hydrogen.     

Characterization and challenge of development of producer gas fuel combustor: A review

Producer gas, which derived from a biomass gasification process, is considered as one of the alternative fuels, which is suitable for the heating process and power generation. Due to low heating density and impurities, combustion in an external combustion chamber constitutes an obvious option for the utilization of producer gas via the combustion process. This paper reviews the technical challenges and the development of the producer gas combustor. Various combustion techniques are reviewed. A stable flame combustion with low emissions (both CO and NO_x) constitutes a main requirement of the producer gas combustion. Flame stabilization techniques such as swirl-vane coupled with bluff-body, swirl flow configuration, and staging combustion were successfully employed to enhance the stability and performance of the producer gas combustion. As shown in the results of the studies, the combustion process can operate in a wide range of equivalence ratios with the exhaust gas temperature >600 °C. This temperature is sufficiently hot for the power generation and heating applications. Overall, NOx and CO emissions were below 700 ppm and 1.3%, respectively. In the flameless combustion mode, ultra-low emission for both CO and NOx were recorded. However, higher emission can be found when operated at a higher thermal load combustor. Homogeneity of the thermal field and low polluting emissions make flameless combustion a promising lean and clean combustion technology. Integration of the benefits of flameless combustion and producer gas fuel is an outstanding contribution in reducing emissions and enhancing the efficiency of the combustion systems.     

Hydrogen addition effects in a confined swirl-stabilized methane-air flame

The effect of hydrogen addition in methane-air premixed flames has been examined from a swirl-stabilized combustor under confined conditions. The effect of hydrogen addition in methane-air flame has been examined over a range of conditions using a laboratory-scale premixed combustor operated at 5.81 kW. Different swirlers have been investigated to identify the role of swirl strength to the incoming mixture. The flame stability was examined for the effect of amount of hydrogen addition, combustion air flow rates and swirl strengths. This was carried out by comparing adiabatic flame temperatures at the lean flame limit. The combustion characteristics of hydrogen-enriched methane flames at constant heat load but different swirl strengths have been examined using particle image velocimetry (PIV), micro-thermocouples and OH chemiluminescence diagnostics that provided information on velocity, thermal field, and combustion generated OH species concentration in the flame, respectively. Gas analyzer was used to obtain NO_x and CO concentration at the combustor exit. The results show that the lean stability limit is extended by hydrogen addition. The stability limit can reduce at higher swirl intensity to the fuel-air mixture operating at lower adiabatic flame temperatures. The addition of hydrogen increases the NO_x emission; however, this effect can be reduced by increasing either the excess air or swirl intensity. The emissions of NO_x and CO from the premixed flame were also compared with a diffusion flame type combustor. The NO_x emissions of hydrogen-enriched methane premixed flame were found to be lower than the corresponding diffusion flame under same operating conditions for the fuel-lean case.     

Study of non-premixed turbulent flame of hydrogen/air downstream Co-Current injector

For three decades, hydrogen has been identified as a versatile potential fuel concurrent to the conventional fuel such as gasoline. In order to fully implement it and to develop the combustion based power devices that may supply much higher energy density, it is very essential to understand the mechanism of Hydrogen/Air combustion. In this work, Computational Fluid Dynamics (CFD) numerical simulations have been performed to study the combustion of non-premixed turbulent hydrogen-air mixture with different equivalence ratios and different mass flow rates and its effect on different species formation, peak temperature and NO_x formation. The performance of the combustor is evaluated by using FLUENT software under adiabatic wall condition. Generalized finite rate chemistry model was used to analyze the hydrogen-air combustion system. The combustion is modeled using multi-step reaction mechanism with 14 species, until complete conversion of fuel to H₂O. Through such a systematic analysis, a proper controlled operation condition for the combustor is suggested which may be used as a guideline for combustor design. Results reported in this work illustrate that the CFD simulation can be one of the most powerful, beneficial and economical tool for combustor design and for optimization and performance analysis. They are more sensitive to the model of the transport properties while the reasonable results can be achieved even with the use of global reaction mechanism and a simple turbulence model as k- ε, which are not excessively time and memory consuming. From an environmental point view, this study shows that the radical production (OH and NO) is very small although maximum temperature reached exceeded 2000 (K). The mass fraction of NO is much lower if we increase the air inlet velocity, which makes the cold reaction mixture do not promote the NO formation by dissociation.     

Hydrogen combustion test in a small gas turbine

Details of the gaseous hydrogen combustion test in a can-type conventional gas-turbine combustor and the operating performance of a 275 PS (202 kW) small gas turbine are provided.Initially, experiments were conducted to determine the configuration of the hydrogen fuel nozzles on a combustor test facility. The kerosene fueled gas turbine combustor was used without modification of the original configuration and dimensions.Secondly, the operation performance of the gas turbine was investigated when the gaseous hydrogen was used as a substitute fuel for kerosene fuel. The kerosene fuel supply system was removed or rendered inoperative and a hydrogen flow metering system was newly installed. The high pressure storage cylinders were used to supply hydrogen to the fuel metering system.Data was obtained on pressure losses of the fuel nozzles, ignition performance, temperature distributions at the combustor outlet, combustion efficiency, liner wall temperature distributions, NO_x emission levels, noise levels, operating performance, etc.     

Experimental study of fuel‐lean reburn system for NOX reduction and CO emission in oxygen‐enhanced combustion

A fuel‐lean reburn system is found here to be able to replace a conventional reburning technique in terms of increasing efficiency. In the fuel‐lean reburn system, the amount of injected reburn fuel into the reburning zone is low enough to maintain the overall fuel‐lean condition in the furnace, so that no additional air system is required, and CO emission can be maintained at almost zero level. In this study, an experimental study has been done to examine the reduction characteristics of NO_X in a lab scale combustor (15 kW) with various oxygen‐enhanced combustion conditions. Liquefied Petroleum Gas (LPG) was used as a main fuel and reburn fuel. Finally, the current fuel‐lean reburn system, even with only an amount of reburn fuel of 13% of total heat input, was observed to achieve a maximum of 48% in NO_X reduction. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydrogen addition effects on methane-air colorless distributed combustion flames

In this investigation the role of hydrogen addition in a reverse flow configuration, consisting of both non-premixed and premixed combustion modes, have been examined for the CDC flames. In the non-premixed configuration the air injection port is positioned at combustor exit end while the fuel injection port is positioned on the side so that the fuel is injected in cross-flow with respect to air injection. The thermal intensity of the flames investigated is 85 MW/m³ atm to simulate high thermal intensity gas turbine combustion conditions. The results are presented on the global flame signatures, exhaust emissions, and radical emissions using experiments and flowfield using numerical simulations. Ultra low NO_x emissions are found for both the premixed and non-premixed combustion modes. Addition of hydrogen to methane fuel resulted in only a slight increase of NO emission, significant decrease of CO emission and extended the lean operational limit of the combustor.     

Catalytic combustion of hydrogen for residential heat supply application

Hydrogen can be converted to thermal energy by combustion or to electricity energy by fuel cells. Considering the stringent requirements for safety from fire hazards and elimination of pollutants, the flameless catalytic combustion of hydrogen is favorable over conventional flame combustion for residential heat supply application. This paper reported an industrial‐scale heat acquisition system based on hydrogen catalytic combustion. The 1 wt% Pt‐loaded glass fiber felts prepared by an impregnation process were used as the combustion catalyst, and a catalytic combustion burner with a capacity of 1 kW was designed. It was found that 100% hydrogen conversion rate could be obtained during the stable combustion stage, and the stable combustion could be achieved by adjusting hydrogen flow rate. The change in H₂/air ratio would influence the initial combustion stage but has little impact on the stable combustion stage. A heat efficiency of 80% for hot water supply was obtained based on the present catalytic hydrogen combustion burner. Copyright © 2016 John Wiley & Sons, Ltd.     

Investigation of colorless distributed combustion regime using a high internal recirculative combustor

This study aims at investigating the combustion characteristics of methane and a hydrogen-rich fuel on distributed regime in a combustor with high internal recirculation as distributed regime can achieve with highly internal entrainment. The model validation was first taken place with the existing experimental results under non-reacting conditions, and it is demonstrated that the mean velocity profiles predicted are in satisfactorily good agreement with the existing data in the combustor. Then, the methane and the hydrogen-rich fuel were consumed at an equivalence ratio of 0.8 and a thermal power of 10 kW under distributed conditions which means that oxygen concentration in the oxidizer is reduced from 21% to 15% along with internal entrainment for each fuel used. The results showed that the velocity magnitudes increased in the combustor with decrease in oxygen concentration because of diluent introduction. Moreover, distributed regime enabled uniformly temperature field inside the combustor together with the ultra-low NO_X values with introducing the diluent. In addition, one can say that the maximum temperature and the outlet NO_X values (1800 K and 27.60 ppm at 21% O₂ – 1450 K and 0.05 ppm at 15% O₂ for methane, 2000 K and 32.47 ppm at 21% O₂ – 1700 K and 0.19 ppm at 15% O₂ for the hydrogen-rich fuel) were relatively higher while combusting the hydrogen-rich fuel compared to those of methane due to the presence of hydrogen. However, It is concluded that distributed regime provided uniformly temperature fields and ultra-low NO_X levels even the hydrogen-rich fuel is used.     

Frontiers in combustion techniques and burner designs for emissions control and CO2 capture: A review

The use of fossil fuel is expected to increase significantly by midcentury because of the large rise in the world energy demand despite the effective integration of renewable energies in the energy production sector. This increase, alongside with the development of stricter emission regulations, forced the manufacturers of combustion systems, especially gas turbines, to develop novel combustion techniques for the control of NO_x and CO₂ emissions, the latter being a greenhouse gas responsible for more than 60% to the global warming problem. The present review addresses different burner designs and combustion techniques for clean power production in gas turbines. Combustion and emission characteristics, flame instabilities, and solution techniques are presented, such as lean premixed air‐fuel (LPM) and premixed oxy‐fuel combustion techniques, and the combustor performance is compared for both cases. The fuel flexibility approach is also reviewed, as one of the combustion techniques for controlling emissions and reducing flame instabilities, focusing on the hydrogen‐enrichment and the integrated fuel‐flexible premixed oxy‐combustion approaches. State‐of‐the‐art burner designs for gas turbine combustion applications are reviewed in this study, including stagnation point reverse flow (SPRF) burner, dry low NO_x (DLN) and dry low‐emission (DLE) burners, EnVironmental burners (including EV, AEV, and SEV burners), perforated plate (PP) burner, and micromixer (MM) burner. Special emphasis is made on the MM combustor technology, as one of the most recent advances in gas turbines for stable premixed flame operation with wide turndown and effective control of NO_x emissions. Since the generation of pure oxygen is prerequisite to oxy‐combustion, oxygen‐separation membranes became of immense importance either for air separation for clean oxy‐combustion applications or for conversion/splitting of the effluent CO₂ into useful chemical and energy products. The different carbon‐capture technologies, along with the most recent carbon‐utilization approaches towards CO₂ emissions control, are also reviewed.