Low NOx Emissions Research Articles

Effects of fuel injection pressure and quantity on low octane gasoline sprays in a simulated high-pressure ambient environment of a gasoline compression ignition engine

Gasoline Compression Ignition technology using low-octane gasoline-like fuel can lead to very low NOx and Soot emissions and enhanced thermal efficiency than conventional diesel engines. The difference in test fuel properties is a significant reason for the difference in the fuel–air mixture formation in Gasoline Compression Ignition engines offering these advantages. In this study, microscopic and macroscopic spray characterisation of baseline diesel, 50 % v/v diesel blended with 50 % v/v gasoline (G50), and 20 % v/v diesel blended with 80 % v/v gasoline (G80) are conducted in a high-pressure ambient environment relevant to Gasoline Compression Ignition engines. The study was performed in a constant volume spray chamber in non-reacting conditions using Phase Doppler Interferometry and Diffused Backlit Illumination techniques. The test fuels were injected into a 30 bar ambient pressure environment at 400, 500, and 700-bar fuel injection pressures. The liquid spray penetration length of gasoline-diesel blends was slightly shorter than baseline diesel, but their spray cone angles and entrained air mass were significantly higher. The average axial droplet velocity was higher, and the Sauter Mean Diameter was lower for gasoline-diesel blends than baseline diesel 40 mm away from the injector nozzle exit, indicating superior mixture formation for the gasoline-diesel blends. The effect of droplet collision and injected fuel quantity on droplet size distribution was assessed in all test fuels. Droplet collisions increased the Sauter mean diameter of the spray at farther distances. Smaller fuel injection quantities decreased the Sauter mean diameter of the spray, increasing the probability of unburnt hydrocarbon emission producing ultra-lean regions in the engine combustion chamber.

An efficient combustion concept for low calorific gases

The main features of low calorific gases as those from biomass gasification processes, landfills, mines and sewage sludge are well known. They can be expressed as low calorific values, high water content, corrosive actions and composition changes. Due to these characteristics there are up to date no combustion systems available which are capable to utilise such gases efficiently. The ordinary combustion concepts frequently do not guarantee sufficient flame stability. The high emissions of harmful substances as CO and NOx are another problem. Because of mentioned difficulties it is necessary and also a challenge to develop an efficient and flexible combustion system which can handle these problems. Through several matching steps of numerical and experimental investigations Gaswärme-Institut e. V. Essen (GWI) developed a progressive combustion system based on the concept of continuous air staging (COSTAIR). Burner tests at a small scale thermal load of 30 kW proved a stable combustion and low NOx and CO emissions for different qualities of low calorific gases. The best design of a small scale burner was transferred to larger thermal loads by applying of scale-up criteria. Already achieved experimental results at 200 kWth confirmed the efficient performance and the high potential of the burner for use of low calorific gases.

Low emission stepped piston engine for marine outboard motor application

Separating the scavenging process from the lubrication methodology in conventional two-stroke engines is an effective method of reducing emissions. Segregation of scavenging by use of stepped pistons is one such method providing significant improvements in engine durability, and reduced lubricant consumption, together with reduced production cost. The study discusses the approaches used together with those applied to conventional engines. This paper focusses on a marine outboard motor application and cross-comparisons with a comparable four-stroke outboard motor are drawn within the study. One-dimensional computational fluid dynamics models of the engine types have allowed performance and emissions simulations. Where possible, data from experimental dynamometer testing is compared with the theoretical models. A reasonable level of correlation in terms of performance has been observed. Two-stroke cycle engines are able to demonstrate reduced oxides of nitrogen emissions when compared with conventional four-stroke engines. Models of the stepped piston engine have confirmed the lower NOx emission evident with operation on the two-stroke cycle.

Ammonia fired gas turbines: Recent advances and future perspectives

Transition to carbon-neutral energy generation is a reality, and great efforts are being made in this direction in the last years. Ammonia is a promising carbon-free fuel, and it can be used as a gas turbine fuel in existing power generation cycles without significant modification. This paper considers various aspects of the use of ammonia as a gas turbine fuel: it provides a review of existing ammonia-fired gas turbines and conducts a study of the prospective technology of using ammonia via thermochemical transformation into hydrogen-rich gas. Moreover, this paper considers on-board hydrogen production technology via thermochemical ammonia transformation. The ammonia fired chemically recuperated gas turbine is analyzed for which the thermal efficiency can be increased up to 5%–7% comparing to traditional gas turbines. Moreover, hydrogen-rich fuel with a hydrogen mole fraction up to 75% is used as a fuel, leading to more stable combustion with lower NOx emission up to 6–10 ppm. Additionally, an approach to on-board hydrogen production from ammonia via the utilization of solar energy is investigated. It is shown that solar energy can replace up to 25% of heat obtained via ammonia combustion. The paper discusses future perspectives in investigations for ammonia-fired gas turbines with on-board ammonia transformation.

Experimental investigation of multi-burner array with lean lifted spray flames in inline and inclined configurations

Lean combustion in gas turbines garners attention for emissions reduction, yet it faces stability challenges demanding careful design considerations. In this study, a novel CHAIRLIFT combustor concept, which integrates low swirl lean lifted spray flames known for substantial nitrogen oxides emissions reduction, into a Short Helical Combustors (SHC) arrangement, is investigated. A comprehensive experimental test campaign explores key parameters, including staggered arrangement offset, equivalence ratios, and air inlet temperature, to evaluate its performance. The results reveal exceptional stability in low swirl, lifted flames compared to moderate swirl flames with larger inner recirculation zones. The unwanted flow deflection of highly swirled flames in the SHC arrangement is effectively avoided with the investigated low swirl, lifted flames. The study emphasizes the significant influence of fuel droplet evaporation time on flame stability near the flame root under low air inlet temperatures. Furthermore, at high air inlet temperatures, the complex flame stability mechanism in inclined configurations is controlled by various parameters, including outer recirculation zone size, air inlet temperature, and additional vortices generated by pressure differences near the wall. OH∗ chemiluminescence results corroborate these stability mechanisms, showing that at lower temperatures, the flame stabilizes closer to the nozzle exit due to local rich pockets near the shear layer. Conversely, at the same tilt angle, higher air inlet temperatures lead to an increased flame lift-off height due to faster evaporation and enhanced mixing. Exhaust gas analyses confirm the potential of the investigated lean lifted spray flames in a multi-burner arrangement to achieve very low NOx emissions over a wide range of operating conditions for all investigated burner inclinations. These findings may facilitate the future development and optimization of lean combustion technology for aircraft engines.

The cellulose hydrolysis into glucose with carbon-based solid acid catalyst via machine learning, life cycle assessment and bibliometric analysis

The cellulose hydrolysis into glucose with carbon-based solid acid catalyst contributes to sustainable and low-carbon development. Determining the quantitative impact of key variables on glucose yield and environmental sustainability is a notoriously time-consuming and labor-intensive process. In this study, a bibliometric analysis was carried out to examine the current research on cellulose hydrolysis into glucose using solid acid catalyst. The application of machine learning (ML) has demonstrated that the random forest (RF) model possesses optimal predictive performance concerning the glucose yield (Training R2 = 0.89, Testing R2 = 0.80, MAE = 6.52, RMSE = 8.29). The SO3H acid densities was emerged as a most crucial influential parameter. The interactions relationship of influenced factors for glucose production was clarified by the partial dependence plot analysis (PDPs). Furthermore, the life cycle assessment (LCA) further indicated that the carbon solid acid catalysts used in cellulose hydrolysis to produce glucose have lower emissions of CO2, NOx, and SO2 compared to traditional acid catalysts, which is 85.5 kg ep of CO2, 0.17 kg ep of NOx, 0.23 kg ep of SO2 for carbon solid acid catalysts, while 91.84 kg ep of CO2, 0.24 kg ep of NOx, 0.61 kg ep of SO2 emission for acid hydrolysis. Thus, carbon solid acid catalysts have a comparably lower negative environmental impact than traditional acid hydrolysis. This study provides crucial insights into improving the prediction accuracy of ML models and reducing the carbon footprint involved in glucose production.

A highly efficient and cost-effective liquid biofuel for agricultural diesel engines from ternary blending of distilled Yang-Na (Dipterocarpus alatus) oil, waste cooking oil biodiesel, and petroleum diesel oil

This study reports a highly efficient and cost-effective liquid biofuel for agricultural diesel engines by a ternary blending of waste cooking oil (WCO) biodiesel, distilled Yang-Na oil, and petroleum diesel oil. Biodiesel oil from WCO synthesized by NaOH-catalyzed pilot plant-scale solar reactor exhibited fuel properties that met ASTM-D6751 and EN-14214 standards. Crude Yang-Na was distilled at 280 oC to obtain distilled Yang-Na oil with major components of α-Gujernene (63.07 wt.%), α-Caryophyllene (19.08 wt.%), γ-Gujernene (5.16 wt.%), and γ-Elemene (5.23 wt.%), along with 7.46 wt.% of other hydrocarbon compounds. The ternary blendings of waste cooking oil (WCO) biodiesel, distilled Yang-Na oil, and petroleum diesel oil were formulated (50:30:20 and 50:25:25) and their physicochemical properties were evaluated and compared with a commercial B10 petroleum diesel oil. The formulated liquid biofuels met standards and closely resembled petroleum diesel with a viscosity of 3.86 cSt, density value of 876 kg m-3, and heating values of 9991 kcal/kg. The performance testing of the blended fuels in a small agricultural diesel engine showed the efficiency levels matched with a commercial B10 petroleum diesel oil and outperformed individual biodiesel and distilled Yang-Na oil. Variances in Brake Power and Brake Specific Fuel Consumption were <1% with low emissions of CO2, CO, HC, and NOx, while Engine Torque and biofuel consumption during cultivation showed differences of 3.42% and 2.54%, respectively. Accordingly, the obtained blended fuel is a promising option for community resource utilization, offering an alternative energy source for future communities.

Achieving high flame stability with low NO And Zero N2O and NH3 emissions during liquid ammonia spray combustion with gas turbine combustors

Liquid ammonia spray direct injection is more desirable than gaseous ammonia injection in large scale gas turbine combustors because it enables a larger volumetric energy density and allows a reduction in cost and size of the fuel supply system. However, previous studies have shown that the large evaporative cooling effects of liquid ammonia droplets vaporization impair flame stabilization, and inhibit efficient control of emissions such as NO, N2O and unburned fuels. This article reports the development and testing of a novel two-stage gas turbine combustor for pure liquid ammonia spray, which achieves approximately zero N2O and unburned fuel emissions with NO emissions as low as 282 ppmv (@16% O2) in a 50 kW micro gas turbine combustor test rig. The combustor design focused on optimization of the secondary injection holes to prevent the dilution of the primary zone (PZ) by air injected into the secondary zone; improving mixture formation using a multi-nozzle twin-fluid atomizer; and elongation of the combustor length to allow sufficient fluid residence time for N2O reduction as well as droplets vaporization and combustion. It was found that the mitigation of PZ dilution resulted in a significant increase in the combustion temperature thereby encouraging efficient combustion of the liquid droplets. On the other hand, the multi-nozzle twin fluid atomizer resulted in a more uniform combustor wall temperature, a more efficient combustion and lower NOx emissions than a single-nozzle pressure swirl atomizer. It is considered that the multi-nozzle injector allowed for an improved droplets dispersion, which is necessary to mitigate large local heat extraction from the flame. These features of the combustor enabled a substantial improvement in liquid ammonia spray flame stability and a better simultaneous control of NO, N2O and unburned fuel emissions never reported before.

Pure ammonia flames with high thermal intensities through fuel and air staging under extreme rich-to-lean conditions

Ammonia combustion has gained significant attention due to its high hydrogen content and zero carbon emissions. It poses a challenge for stabilization due to their low energy content and limited flammability range, with the added concern of fuel NOx emissions. In the current study, a novel burner design equipped with four series of reactors is proposed to achieve stable pure ammonia-air flames with reduced NOx emissions. The impact of thermal intensity (∼0.7 MW/m3 to ∼4 MW/m3), number of stages, equivalence ratio (0.5–1.2), and fuel staging on flame stabilization and emissions were investigated in the proposed burner. Comprehensive emissions analysis is performed at various burner levels. Numerical simulations incorporating Large Eddy Simulation (LES) modeling are employed to enhance understanding of the impact of thermal intensity and equivalence ratios on ammonia dissociation, mixedness, and NO emissions. The results indicated that the present burner design improved reactant mixing, and flame stability, reduced NH3 emissions (∼ 0 PPM), and lowered NOx levels in non-premixed ammonia-air flames. The computational and experimental results demonstrated that the implementation of fuel staging is crucial for reducing NOx emissions for the flames with lean global equivalence ratios. Lower NO emissions were identified at a global equivalence ratio of 1.1 for all the considered flames in the range of 0.5–1.2. In the four-stage rich-lean combustion strategy employed in this study; it was observed that higher thermal intensity (4 MW/m3) with fuel staging resulted in lower NOx emissions per kW of energy input compared to lower thermal intensities 0.7 MW/m3. This finding underscores the significance of achieving uniform mixtures and ensuring local flame in rich conditions for achieving low NOx emissions in ammonia combustion. Exhaust gas analysis is conducted at three stages of the burner to enhance the understanding of emissions at various levels of the burner. The proposed combustor design has achieved a substantial reduction in NOx to ∼1 PPM/kW and ∼2.8 PPM/kW with and without fuel staging, respectively. This impressive outcome is attributed to the controlled ammonia consumption facilitated by uniform mixing generated through the use of tangential air inlets.

Extraction of turbulent flame structures and dynamic modes with corrected OH-PLIF images for a hydrogen micromix burner

Diluted micromix combustion technology is an effective way to realize stable H2 flames with low NOx emission, and the turbulent flame structures of a micromix burner are experimentally revealed in this work with corrected OH-PLIF images.

Reduction of NO[formula omitted] emissions in ammonia combustion using a double-flame premixed co-combustion concept

Ammonia is a carbon-free fuel that can be produced from renewable energy sources and has the potential to replace fossil fuels, exerting a significant impact on the decarbonization of power production and propulsion industries. However, the challenge lies in the high NOx emissions, narrow flammability, and low flame speed of ammonia/air mixtures. In this paper, we study a novel concept of double-flame premixed co-combustion (DFPC) of ammonia and methane in a double-swirl premixed combustion burner, which results in low NOx emissions and high flame stabilization. Large eddy simulations using a detailed chemical kinetic mechanism and planar laser-induced fluorescence imaging of OH and exhaust gas NO emission measurements are employed to investigate the fundamental mechanisms behind flame/flame interactions and NO emissions. The main findings are: (a) NO emissions can be reduced by 90% using the DFPC concept along with a significant broadening of flammability; (b) the outer methane/air flame stabilizes the inner ammonia flame in the shear layer of the two flames; (c) combustion products and excess oxygen leaked across the shear layer decrease the equivalence ratio of the inner ammonia/air mixture, reducing the NO formation of close-to-stoichiometric ammonia/air flame but increasing the NO formation in the fuel-rich ammonia/air flames; (d) mixing of the combustion products from the inner and outer flames reduces the NO emissions in the flue exhaust gas.

Effects of compression ratio on the performance and emission levels of a CI engine fueled with safflower oil methyl ester through an engine simulation approach

In recent years, the research community has shown significant interest in the potential of biodiesel as a renewable alternative to conventional fossil-based fuels. Nevertheless, the experimental investigation of the effects of diverse biodiesel formulations on internal combustion engines demands a significant investment of time and financial resources. Consequently, the numerical alternative methodologies are advocated as a viable substitute for practical experiments. Numerical simulations offer the opportunity for a meticulous examination of the characteristics of internal combustion engines under diverse operational conditions and various biodiesel blends, thereby optimizing efficiency and cost-effectiveness. This study focused on the simulation of performance and emission characteristics of a diesel engine running on safflower (Carthamus tinctorius L.) oil methyl ester (SOME) and traditional diesel fuel using AVL simulation software. Furthermore, the simulation results were compared with a laboratory study carried out under identical conditions. The simulated engine underwent testing across various compression ratios (CRs) (ranging from 12:1 to 18:1) and engine loads (from 25% to full load) while sustaining a consistent speed of 1500 rpm. The simulation findings revealed that the engine exhibited its highest BSFC as 0.495 kg/kWh with SOME fuel, at a CR of 12:1, modestly lower than the corresponding experimental observation of 0.520 kg/kWh. Concurrently, the lowest value of BSFC, recorded as 0.267 kg/kWh with diesel fuel and a CR of 18:1, demonstrated a marginal deviation from the experimental result of 0.281 kg/kWh. Additionally, SOME fuel usage was correlated with diminished CO and HC emissions. The experimental findings indicated the lowest value of CO and HC emissions, as 0.14% and 21.7 ppm, respectively, with SOME fuel at a CR of 18:1, marginally below the simulation-derived values of 0.13% and 20.8 ppm. Conversely, diesel fuel at a CR of 12:1 exhibited maximal CO and HC emissions, registering 0.38% and 199.5 ppm, respectively, in the experimental study. In comparison, the simulation values were slightly lower at 0.36% and 194.1 ppm. Moreover, the experimental investigation identified SOME fuel as yielding the highest CO2 emission, reaching a peak of 11.9% under a CR of 18:1, while the simulation showed a slightly lower value of 11.2%. In contrast, diesel fuel at a CR of 12:1 resulted in the lowest CO2 emission at 3.85% in the experiment, with the simulation reporting a slightly reduced value of 3.77%. Regarding NOx emissions, the experiment recorded the peak at 1687 ppm with SOME fuel and a CR of 18:1, slightly surpassing the simulation’s value of 1643 ppm. Conversely, the experimental data indicated the lowest NOx emission as 103 ppm with diesel fuel and a CR of 12:1, with the simulation suggesting a slightly lower value of 98.2 ppm under identical conditions. The simulation results demonstrated favorable concordance with experimental findings, notably strengthening with an increase in CR.

Analysis of combustion and heat transfer performance in a slit-type fully premixed burner

Since most gas water heaters on the market still use diffusion or partially premixed combustion. Fully premixed combustion, with its advantages of complete combustion and low CO and NOx emissions, offers a promising alternative. In response to this, a slit-type fully premixed gas water heater with a rated power of 20kW was designed. The combustion and heat transfer performance, as well as the combustion temperature and burnout characteristics, were analyzed under ten different load conditions ranging from 100% to 8% using theoretical modeling. The study identified the variations in physical parameters and used numerical simulations to partially validate the theoretical results. Subsequently, by individually modifying structural variables, the optimal structure under single-variable conditions was determined. This optimal structure was then combined in various configurations to form a multi-variable structural design. Further theoretical analysis was conducted to assess whether the changes in the multi-variable structure, as well as the original design, met economic efficiency requirements. The results showed that the slit-type fully premixed burner achieved optimal performance when the number of burner plates was 120, with a length of 99 mm and a height of 10 mm.

NOx Emission Characteristics of a Low NOx Emission Burner for Hydrogen Aircraft under High Pressure and Temperature

NOx Emission Characteristics of a Low NOx Emission Burner for Hydrogen Aircraft under High Pressure and Temperature

Combustion characteristics of low-quality lignite for different bed material sphericities in a circulating fluidized bed boiler: A numerical study

This study delves into the combustion behavior of various lignite types within a circulating fluidized bed boiler (CFBB), with a primary focus on the impact of different bed material sphericity ratios (0.5, 0.7, and 0.9). Utilizing bed material with a sphericity ratio of 0.9 sourced from the Çan power plant and verified through experimentation, the research reveals several key findings. Notably, furnace temperatures tended to rise with higher sphericity ratios, albeit with variations between lignite types, particularly highlighting the complexity of this relationship in the case of GLI-Tunçbilek lignite. Pressure levels in the combustion chamber remained consistent across different sphericity ratios, indicating minimal influence on pressure dynamics. Improved combustion efficiency, especially at the bottom of the boiler, was observed at lower sphericity levels (0.5 and 0.7) for Çan lignite, as reflected in CO2 mole fractions. While NOx emissions generally decreased with lower sphericity, the sensitivity to sphericity varied by lignite type, with Ilgın lignite showcasing low NOx but high SO2 emissions, underscoring the intricate interplay between lignite properties, sphericity, and emissions. Overall, this study advances our understanding of CFBB combustion dynamics, offering insights valuable for optimizing performance and emissions control, particularly in lignite-based power.

On the Demonstration of a Humid Combustion System Performing Flexible Fuel-Switch From Pure Hydrogen to Natural Gas With Ultra-Low NOx Emissions

Abstract To fight global warming the European Union formulated the objective of completely decarbonizing the energy sector, stimulating the advent of unconventional fuels and the adaption of the corresponding energy infrastructure. The decarbonization strategy identified hydrogen to play a key role as an energy storage medium, making systems capable of pure hydrogen operation essential. This requirement can be fulfilled with humid power cycles which offer additional advantages such as highly efficient and fuel-flexible operation with low emissions. As an integral part of such a cycle, a humid combustion system was presented previously showing promising results with respect to complete combustion with low emissions for a variety of fuels. The current work introduces an upgraded version of that combustion system. The new Double Swirler system is capable of stable and safe combustion of low calorific value biosyngas surrogate, hydrogen, and natural gas, from dry to steam-rich conditions within the required pressure drops. The inclusion of dry operation of the system can benefit the startup procedure of the humid cycle. The combustor's fuel switching performance is demonstrated by a fast fuel switch at full load from pure hydrogen to pure natural gas and vice versa, while maintaining a stable performance with low NOx-emissions at otherwise constant operation parameters.

Characterizing CO2 and NOx emission of vehicles crossing toll stations in highway

Electronic toll collection (ETC) has been designed and implemented to improve traffic efficiency and reduce traffic-related air pollution. However, the real-world impact of ETC on these factors is currently under-qualified. With this consideration, we attempted to reveal the effect of ETC on traffic efficiency and traffic pollution using portable emission measurement systems. Firstly, in contrast to the manual toll collection (MTC), ETC offer lower emissions and time savings, and there exist a quadratic relationship between emission rates and vehicle specific power. Secondly, vehicles in acceleration state emit more pollutants than other situations such as deceleration, steady cruising and idling in both ETC and MTC lanes. Thirdly, ETC lanes show 34.25% and 13.63% lower CO2 and NOx emissions, respectively, along with a 46.55% timesaving compared to MTC lanes. Finally, the study identifies engine load percent as the primary factor impacting emissions, accounting for 60–88% of the variance.

A New Generation of Hydrogen-Fueled Hybrid Propulsion Systems for the Urban Mobility of the Future

The H2-ICE project aims at developing, through numerical simulation, a new generation of hybrid powertrains featuring a hydrogen-fueled Internal Combustion Engine (ICE) suitable for 12 m urban buses in order to provide a reliable and cost-effective solution for the abatement of both CO2 and criteria pollutant emissions. The full exploitation of the potential of such a traction system requires a substantial enhancement of the state of the art since several issues have to be addressed. In particular, the choice of a more suitable fuel injection system and the control of the combustion process are extremely challenging. Firstly, a high-fidelity 3D-CFD model will be exploited to analyze the in-cylinder H2 fuel injection through supersonic flows. Then, after the optimization of the injection and combustion process, a 1D model of the whole engine system will be built and calibrated, allowing the identification of a “sweet spot” in the ultra-lean combustion region, characterized by extremely low NOx emissions and, at the same time, high combustion efficiencies. Moreover, to further enhance the engine efficiency well above 40%, different Waste Heat Recovery (WHR) systems will be carefully scrutinized, including both Organic Rankine Cycle (ORC)-based recovery units as well as electric turbo-compounding. A Selective Catalytic Reduction (SCR) aftertreatment system will be developed to further reduce NOx emissions to near-zero levels. Finally, a dedicated torque-based control strategy for the ICE coupled with the Energy Management Systems (EMSs) of the hybrid powertrain, both optimized by exploiting Vehicle-To-Everything (V2X) connection, allows targeting H2 consumption of 0.1 kg/km. Technologies developed in the H2-ICE project will enhance the know-how necessary to design and build engines and aftertreatment systems for the efficient exploitation of H2 as a fuel, as well as for their integration into hybrid powertrains.

Development of a Hydrogen Micro Gas Turbine Combustor: Atmospheric Pressure Testing

Abstract The vision of a carbon-neutral world implies the shift from fossil to clean fuels for combustion-driven processes and machines like gas turbines. Green hydrogen is a promising alternative to substitute natural gas and other fossil fuels. In the H2mGT project, funded by the German BMWK, a microgas turbine (mGT) burner with 100% hydrogen firing is developed and validated. The project is collaboration between Technische Universität Berlin (TUB) and the manufacturer Euro-K GmbH. The project consists of three phases: (1). Atmospheric pressure tests with a fused silica combustion chamber; (2). Atmospheric pressure tests with counterflow-cooled steel combustion chamber and secondary air injection; (3). Validation of the burner in the micro gas turbine at elevated pressure levels. This paper will present the results of Phase 1. The hydrogen burner is based on a swirl-stabilized burner of TUB and was scaled to match the requirements of the mGT with its 130 kW thermal power. The burner design features multiple geometrical parameters to enable the optimization of the flame towards low NOx emissions. Therefore, a variable swirl intensity, additional axial momentum of air in the mixing tube, a movable center-body and different fuel injection locations are implemented. Phase 1 investigates the parameter space in terms of flame stability, operational range, and parameter impact on flame shape and emissions. Therefore, temperature, pressure, and emission measurements as well as OH* imaging are carried out. It is found that the flame can be operated over a large range of equivalence ratios and preheating temperatures up to 500 ∘C for many parameter settings. However, at some configurations, flashback into the mixing tube is triggered. As expected, the NOx emissions are mainly influenced by the equivalence ratio, the fuel distribution, and the swirl intensity. Single-digit emissions are reached up to an equivalence ratio of 0.4 at atmospheric pressure conditions. Furthermore, at low air mass flow, the burner can be operated at 100% natural gas or 100% hydrogen without any geometry changes. The fuel switch, thereby, does not change the NOx emissions significantly if reasonable normalization is used.

Experimental and numerical characterization of hydrogen combustion in a reverse-flow micro gas turbine combustor

This study conducts experimental and numerical investigations on a stagnation-point, reverse-flow combustor to examine the effect of its design features on the combustion behavior and emission characteristics of non-premixed, turbulent hydrogen/air flames. The study covers a wide range of equivalence ratios, from ultra-lean to stoichiometric, and thermal power inputs varying between 5 kW and 20 kW. The main motivation behind this design is to achieve passive recirculation of flue gas within the combustor to enhance dilution of the incoming charge, thus potentially leading to relatively uniform thermal fields, lower temperatures, and suppressed NOx emissions. This design concept is implemented in a micro gas turbine – a practical energy conversion device tapped to play a major role in decarbonizing CHP systems suitable for small-scale industries. Optical chemiluminescence measurements, in addition to thermocouple-based temperature and Quantum Cascade Laser (QCL) spectroscopy-based exhaust emissions measurements, were conducted for all the test cases. Spatial OH* signal intensity is used as heat release signature to identify distinct features of the test flames, allowing for useful information on the nature and dynamics of the combustion process within the combustor. The results show stable hydrogen/air flames for all the operating conditions; however, a more distributed combustion is observed at higher powers for all the given equivalence ratios. Relatively higher NOx emissions at φ >0.2 suggest the lower achieved mixing between burnt and fresh gases, particularly in the non-preheated case, prevent a thorough dilution of the fresh mixture. The latter is necessary to achieve low-temperature, uniform thermal fields – an essential for low thermal NOx emissions in gas turbine combustors. Further, CFD results show that preheating considerably changes flow field dynamics and recirculation pattern for better mixing; however, this also dramatically increases thermal NOx emission, with the lowest levels observed at φ = 0.2, while the peak occurs at φ = 0.4. Finally, a ‘brute-force’ sensitivity analysis shows that thermal NO and NNH pathways contribute the most to NO formation in hydrogen/air flames.