Ammonia Combustion Research Articles

Role of methane in ammonia combustion in air: From microscale to macroscale

Ammonia (NH3) has gained increasing recognition as a carbon-free fuel. To enhance NH3 combustion, reactive gases, like methane (CH4), are usually added to the combustion system. In this work, the role of CH4 in NH3 combustion is systematically studied. A series of reactive force field molecular dynamic (ReaxFF MD) simulations are implemented to investigate effects of CH4 addition on the consumption of NH3 and the yields of nitrogen oxides (NOx) from the atomic perspective: CH4 accelerates the consumption of NH3 by shortening the decomposition time of the first NH3 molecule and increasing the translational kinetic energy of the system; CH4 modifies the yield of NOx by complicating the elementary reactions and introducing additional intermediates. The fuel ratio of CH4 and NH3 between 0.5 and 1 is suggested for a cleaner and enhanced NH3 combustion. By summarising the findings from the latest publications and the present work, the role of CH4 in NH3 combustion is comprehensively analysed from the macroscale and microscale perspectives: CH4 accelerates the progress of NH3 combustion flame, activates chemical reactions, and aggravates NOx emissions at a low CH4 content. Taking the NH3/CH4 combustion as an example, this study provides an exclusive perspective to understand combustion phenomena from the microscale events to macroscale observations.

On ammonia/diesel dual-fuel combustion in optical engine

Ammonia-diesel dual-fuel combustion mode is one of the potential ways to achieve lower carbon emissions in compression ignition engines. In this work, the effects of ammonia addition on diesel combustion characteristics were experimentally investigated in an optical CI engine. The gaseous ammonia is introduced to the engine through a port injection while diesel is directly injection into the cylinder. A color high-speed camera was utilized to detect and differentiate between diffusion burning and lean-premixed ammonia combustion. Additionally, computational models were conducted and the findings were coupled with the experimental data. According to optical investigations, the lift-off distance is created during the diesel spray and progressively grows as the ammonia energy ratio (AER) rises. The findings indicate that, in comparison to pure diesel combustion mode, combustion in an ammonia environment exhibits prolonged ignition delay duration and lesser flame luminosity. Secondly, a multi-injection strategy for diesel fuel was used where the diesel pre-injection timing (SOPI) values adjusted from −30 °CA ATDC to −60 °CA ATDC). Advancing SOPI is an effective approach to suppress diesel diffusion flame and reduce soot production in the cylinder. For advanced SOPI, findings reveal that rich pockets have decreased drastically as the combustion progresses. With the advancement of SOPI, there is a notable a connection between the flame speed and the peak heat release rate, where a high flame speed correlates to a greater heat release rate peak. Finally, the ratio of pre-injection of diesel fuel (ROPI) varied from 0 to 100% as a mean to improve flame propagation. There is a crucial threshold of pre-injection (ROPI) mass ratio of 45%, at which the initial flame features alter considerably. As the ROPI exceeds 45%, the flame kernel appears as a visible flame near to the main injection timing (SOMI), indicating that the ignition delay duration is decreasing.

Exploring NH3 combustion in environments with CO2 and H2O via reactive molecular dynamics

In existing power units such as internal combustion engines and gas turbines, the combustion of ammonia mixed with hydrocarbon fuels can effectively enhance the combustion intensity of ammonia. In the duel-fuel combustion scenario, the effect mechanism of the main components of combustion exhaust gas i.e., CO2 and H2O on ammonia combustion is still unclear and needs to be studied. Therefore, this paper uses reactive molecular dynamics simulations to study the combustion and emission processes of ammonia in environments with CO2 and H2O. The effects of the diverse gas environments (N2/O2, O2/CO2, and O2/CO2/H2O environments), high CO2 levels, varying H2O proportions, and O2 concentrations on ammonia combustion and emission were systematically explored. It was found that ammonia consumption was accelerated in the O2/CO2/H2O environments compared to N2/O2 environments, resulting in more NH2, H2, and H2O production. The effect of high CO2 concentration on ammonia oxidation was mainly related to temperature. The simulations found that the high CO2 concentration inhibited ammonia consumption at T < 2800K while enhancing it at T ⩾ 2800K. In addition, the high CO2 concentration could inhibit NO production under stoichiometric conditions but promote the conversion of nitrogen-containing intermediates to NO with the oxidizing capacity of CO2 enhanced significantly under extremely high temperature conditions. It was found that the increasing content of H2O molecules in the environment promoted NO production and inhibited CO production under stoichiometric conditions. In addition, the reaction paths for the conversion of HNO and HNO2 molecules into NO molecules were significantly enhanced with increasing H2O concentration. The increase in oxygen concentration can promote OH radical production and NO formation at high temperatures.

Ammonia marine engine design for enhanced efficiency and reduced greenhouse gas emissions

Pilot-diesel-ignition ammonia combustion engines have attracted widespread attentions from the maritime sector, but there are still bottleneck problems such as high unburned NH3 and N2O emissions as well as low thermal efficiency that need to be solved before further applications. In this study, a concept termed as in-cylinder reforming gas recirculation is initiated to simultaneously improve the thermal efficiency and reduce the unburned NH3, NOx, N2O and greenhouse gas emissions of pilot-diesel-ignition ammonia combustion engine. For this concept, one cylinder of the multi-cylinder engine operates rich of stoichiometric and the excess ammonia in the cylinder is partially decomposed into hydrogen, then the exhaust of this dedicated reforming cylinder is recirculated into the other cylinders and therefore the advantages of hydrogen-enriched combustion and exhaust gas recirculation can be combined. The results show that at 3% diesel energetic ratio and 1000 rpm, the engine can increase the indicated thermal efficiency by 15.8% and reduce the unburned NH3 by 89.3%, N2O by 91.2% compared to the base/traditional ammonia engine without the proposed method. At the same time, it is able to reduce carbon footprint by 97.0% and greenhouse gases by 94.0% compared to the traditional pure diesel mode.

Computational Investigation of the Influence of Combustion Chamber Characteristics on a Heavy-Duty Ammonia Diesel Dual Fuel Engine

In response to increasingly stringent emissions regulations and the depletion of conventional fuel sources, integrating carbon-free fuels into the transport sector has become imperative. While hydrogen (H2) presents significant technical challenges, ammonia (NH3) could present a better alternative offering ease of transport, storage, and distribution, with both ecological and economic advantages. However, ammonia substitution leads to high emissions of unburned NH3, particularly at high loads. Combustion chamber retrofitting has proven to be an effective approach to remedy this problem. In order to overcome the problems associated with the difficult combustion of ammonia in engines, this study aims to investigate the effect of the piston bowl shape of an ammonia/diesel dual fuel engine on the combustion process. The primary objective is to determine the optimal configuration that offers superior engine performance under high load conditions and with high ammonia rates. In this study, a multi-objective optimization approach is used to control the creation of geometries and the swirl rate under the CONVERGETM 3.1 code. To maximize indicated thermal efficiency and demonstrate the influence of hydrogen enrichment on ammonia combustion in ammonia/diesel dual fuel engines, a synergistic approach incorporating hydrogen enrichment of the primary fuel was implemented. Notably, the optimum configuration, featuring an 85% energy contribution from ammonia, outperforms others in terms of combustion efficiency and pollutant reduction. It achieves over 43% reduction in unburned NH3 emissions and a substantial 31% improvement in indicated thermal efficiency.

Flame characteristics and abnormal combustion of ammonia-diesel dual-fuel engine with considering ammonia energy fractions

Carbon neutrality is a significant challenge but also an essential opportunity for internal combustion engines (ICE), and fueled with low-carbon fuels is the key for ICE to reduce carbon emissions. Ammonia is a carbon-free fuel, whereas ammonia-diesel dual-fuel (ADDF) combustion can provide efficient and clean combustion of ammonia. In this study, the flame characteristics and abnormal combustion of ADDF engines are studied by considering ammonia energy fractions and diesel injection timings. The results show that a high ammonia energy fraction led to an extended period of combustion, while the CA05 first advances and then decreases with the decrease of ammonia energy fraction. This phenomenon indicates that an easily ignitable ammonia-air mixture is also important for ammonia-diesel dual-fuel ignition. Besides, an optimal SOI is needed to maximize the promoting effect of ignited fuel on ammonia combustion, and the optimal SOI changes with the ammonia energy fraction. Flame images confirm that the weightiness of the auto-ignition and flame propagation changes with the SOI, and the auto-ignition kernels gradually reduce and flame propagation plays a more and more important role as the SOI advances. At high loads, knocking combustion will also occur for ADDF engines, although the knock intensity is low. Besides, due to the enhancement effect of the knocking pressure wave, the flames show a strong white light at the knocking onset. The current results can help understand the combustion process and thus find the optimization method of ADDF engines.

NOx formation mechanism of plasma assisted ammonia combustion: A reactive molecular dynamics study

The effects of ionization ratio (α) on the reaction rate of NH3 combustion and the formation characteristics of NOx are studied by ReaxFF MD, and the formation mechanism of NOX in plasma assisted NH3 combustion are analyzed from the molecular level. It is found that the maximum growth rate of NOX gradually slows down with increasing reaction system temperature. The plasma promotes NOX formation at the initial stage of NH3 combustion, and when the nitrogen-containing radicals are consumed, the non-nitrogen-containing components will inhibit NO formation. When the α is 25%, the NH3 reaction rate reduces slightly. This is attributed that the proportion of NH2/H is the larger at this lower α. The plasma effect at lower α = 25% enhances the N2 formation path from NH3 by the primary reaction pathway of NH3→NH2→NH→NNH→N2, and the intermediate NH consumes some of the NO to form N2 by NH + NO↔HN2O and HN2O↔N2+OH. As α increases to 50%, the plasma leads to the direct generation of NO from NH by NH + OH↔H2NO and H2NO + O↔H2O + NO, which makes the maximum of NO increase.

Hybrid rich- and lean-premixed ammonia-hydrogen combustion for mitigation of NOx emissions and thermoacoustic instabilities

The large-scale direct utilization of two carbon-free fuels, ammonia and hydrogen, is currently attracting significant interest in the context of the development of new gas turbine combustion technologies, with paying close attention to the reduction of nitrogen oxides and unburned ammonia emissions. Motivated by recent observations that rich-premixed conditions tend to mitigate excessive NOx emissions from ammonia combustion, and that uniformly blended ammonia-hydrogen fuel/air mixtures tend to increase NOx production exponentially, here we propose a hybrid configuration of hydrogen-doped rich-premixed ammonia-air flames (inner stage) and lean-premixed pure hydrogen-air flames (outer stage) in a radially stratified primary reaction zone. This fuel staging scheme offers a possible mechanism for chemical kinetics-controlled NOx abatement and asymmetry-induced thermoacoustic instability suppression. The relevance and feasibility of the unconventional premixing approach are rigorously evaluated based on detailed measurements of exhaust gas concentrations and self-excited pressure fluctuations, in conjunction with OH*/NH2*/NH* chemiluminescence and OH PLIF imaging measurements. We observe that drastically different non-homogeneous reaction regions can be stably established in a comparatively compact combustion volume without producing negative flame stabilization effects – such as blowoff of less reactive ammonia flames and flashback of more reactive hydrogen flames. As compared with the uniform-blend baseline condition, the hybrid staging method is shown to significantly mitigate total nitrogen oxides emissions, from 7764 to 310 ppmvd (96 % reduction), and achieving an approximately two-fold reduction in dynamic pressure amplitude. Interestingly, hydrogen-enriched rich-premixed ammonia flames are revealed to exhibit anomalous oscillatory states originating from the preferential diffusion of hydrogen molecules and reaction rate-dependent separation of reactive layers, enabling interacting non-homogeneous reaction zones with markedly different characteristic time scales to resist the growth of intense pressure perturbations.

RCCI combustion of ammonia in dual fuel engine with early injection of diesel fuel

Ammonia has gained considerable attention as a valuable, carbon-free alternative fuel for internal combustion engine (ICE) applications. The growing interest in ammonia's potential as an energy source stems primarily from its advantageous storage and transport properties. The reactivity-controlled compression ignition (RCCI) combustion mode presents a promising avenue for overcoming the challenges intrinsic to conventional diesel engines, such as elevated nitrogen oxides (NOx) and particulate matter (PM) emissions. Additionally, it addresses the limitations of both homogeneous charge compression ignition (HCCI) combustion (limited operational range) and premixed charge compression ignition (PCCI) combustion (diminished power output). The application of an ammonia-based RCCI combustion strategy emerges as a viable pathway for harnessing ammonia as a fuel. In this study, a numerical simulation on a single-cylinder heavy-duty engine operated in RCCI mode is conducted utilizing ammonia and diesel as fuels. The engine is operated at medium load and 910 RPM. The simulation results are validated against experimental data sourced from existing literature. The study investigates the impact of varying ammonia energy fractions, injection timing, and intake valve close temperature (TIVC) on key operational characteristics of the engine including in-cylinder pressure, heat release rate (HRR), combustion efficiency (CE), indicated mean effective pressure (IMEP), and emission levels. The results indicated that increasing the ammonia energy fraction within the RCCI combustion mode, transitioning from 30 % to 70 %, leads to a notable enhancement in IMEP, while maintaining low levels of carbon monoxide (CO), hydrocarbons (HC), unburned ammonia, and nitrous oxide (N2O) as greenhouse gases (GHG). Furthermore, increasing ammonia's energy fraction yielded a discernible reduction in NOx emissions and carbon dioxide (CO2). Conversely, the RCCI combustion mode attained superiority over the dual fuel combustion mode by simultaneously elevating CE and diminishing emissions of GHG, (CO2 and N2O) as well as unburned NH3.

Numerical Assessment of a Rich-Quench-Lean Staging Strategy for Clean and Efficient Combustion of Partially Decomposed Ammonia in the Constant Pressure Sequential Combustion System

Abstract In a future energy system prospective, predictably dominated by (often) remote and (always) unsteady, nondispatchable renewable power generation from solar and wind resources, hydrogen (H2) and ammonia (NH3) have emerged as logistically convenient, chemically simple and carbon-free chemicals for energy transport and storage. Moreover, the reliability of supply of a specific fuel feedstock will remain unpredictable in the upcoming energy transition period. Therefore, the ability of gas turbine combustion systems to seamlessly switch between very disparate types of fuels must be ensured, aiming at intrinsically fuel-flexible combustion systems, i.e., capable of operating cleanly and efficiently with novel carbon-free energy vectors like H2 and NH3 as well as conventional fossil fuels, e.g., natural gas or fuel oils (back-up feedstock). In this context, a convenient feature of Ansaldo's constant pressure sequential combustion (CPSC) system, resulting in a fundamental advantage compared to alternative approaches, is the possibility of controlling the amount of fuel independently fed to the two combustion stages, depending on the fuel reactivity and combustion characteristics. The fuel-staging strategy implemented in the CPSC system, due to the intrinsic characteristics of the auto-ignition stabilized reheat flame, has already been proven able of handling fuels with large hydrogen fractions without significant penalties in efficiency and emissions of pollutants. However, ammonia combustion is governed by widely different thermo-chemical processes compared to hydrogen, requiring a considerably different approach to mitigate crucial issues with extremely low flame reactivity (blow-out) and formation of significant amounts of undesired pollutants and greenhouse gases (NOx and N2O). In this work, we present a fuel-flexible operational concept for the CPSC system and, based on unsteady Reynolds-Averaged Navier–Stokes (uRANS) and large eddy simulation (LES) performed in conjunction with detailed chemical kinetics, we explore for the first time full-load operation of the CPSC architecture in a Rich-Quench-Lean (RQL) strategy applied to combustion of partially-decomposed ammonia. Results from the numerical simulations confirm the main features of ammonia-firing in RQL operation already observed from previous work on different combustion systems and suggests that the CPSC architecture has excellent potential to operate in RQL-mode with low NOx and N2O emissions and good combustion efficiency.

Laminar flame stability analysis of ammonia-methane and ammonia-hydrogen dual-fuel combustion

In recent years, dual-fuel combustion of ammonia blended with reactive fuels like methane or hydrogen has gained attention for reduced carbon emissions. This study investigates the stability of NH3-CH4 and NH3-H2 laminar propagating flames, utilizing experimental and numerical methods. It covers an extensive span of equivalence ratios (0.7–1.6), fuel compositions, and a variety of initial temperatures (300–600 K), as well as pressures ranging from 1 to 5 bar·NH3-CH4 blends exhibit stability across all equivalence ratios, with Markstein lengths of 0.02–2.49. In contrast, NH3-H2 flames display significant variations in Markstein lengths, ranging from 3.73 to −3.36 at an equivalence ratio of 0.7, particularly destabilizing fuel-lean flames due to heightened preferential-diffusional instabilities. Increasing ammonia content stabilizes fuel-rich flames but destabilizes fuel-lean ones. As ammonia content rises, both NH3-CH4 and NH3-H2 flames become more hydrodynamically stable due to flame thickening, although this can exacerbate flame instabilities when preferential-diffusional instabilities dominate. Additionally, preheating destabilizes both ammonia-methane and ammonia-hydrogen flames hydrodynamically, while pressure affects NH3-CH4 and NH3-H2 mixtures oppositely, destabilizing the former and stabilizing the latter due to reduced flame thickness. The addition of diluents stabilizes NH3-CH4 flames but destabilizes NH3-H2 flames, influencing Le and Ze numbers. Diluent addition significantly reduces the flame temperature gradient (up to 64 %), resulting in thicker flames and enhanced hydrodynamic stabilities. This research provides insights into the complexities of ammonia-blended dual-fuel combustion and its impact on flame stability under various conditions.

Modeling and chemical kinetic analysis of methanol and reformed gas (H2/CO2) blending with ammonia under lean-burn condition

Hydrogen can be easily produced by methanol steam reforming. This work aims to investigate the effects of methanol addition and its reformed gas of H2/CO2 on ammonia combustion under lean-burn operation. A newly-developed reaction mechanism for NH3, H2, CH3OH, CO and their blends was established based on the highly-precise validations of laminar burning velocity (LBV), nitrogen monoxide (NO) and ignition delay time (IDT). The premixed flame analysis showed that methanol blends and reforming with hydrogen addition can greatly enhance the combustion rate, resulted from the chemical effect. The equivalence ratio (ER) can be extended to 0.45 for a methanol blending ratio of 60 % and reforming ratio of 90 % (M60R90) to achieve the similar level of LBV at pure ammonia at an ER of 1.0 under ambient condition. The methanol blends and reforming lead to the increase of NO, while the lean-burn operation can help to reduce the NO emission.

Ammonia combustion using hydrogen jet ignition (AHJI) in internal combustion engines

The application of ammonia and hydrogen in internal combustion engines (ICE) is a promising zero-carbon technology. This paper investigates ammonia combustion in ICE using hydrogen multiple-injection jet-ignition (MIJI). The experiment was conducted on a single-cylinder heavy-duty engine with a compression ratio of 17 and a cylinder bore of 123 mm. In the passive jet ignition mode, the minimum hydrogen energy ratio for stable operation of the ammonia-hydrogen engine was 15 %. Using H2 active jet ignition could improve the combustion stability significantly. It was found that engine performance was gradually improved by H2 injection strategy in order of single-injection, double-injection to triple-injection. The triple-injection strategy could achieve a more ideal distribution of ammonia-hydrogen mixture: the first injection forms a homogeneous mixture to increase fuel reactivity; the second injection creates a stratified hydrogen mixture, facilitating rapid flame propagation; the third injection forms a relatively rich mixture in the jet chamber conducive to spark ignition. Through the optimization of the hydrogen injection strategy and spark timing, stable combustion (COV = 1 %) and high thermal efficiency (ITE = 42.5 %) in ammonia-hydrogen engines were achieved, with a minimum hydrogen energy ratio less than 3 %.

Optical investigation of the influence of high-reactivity iso-octane turbulent jet ignition on the combustion characteristics of ammonia/air mixtures

Ammonia has attracted considerable attention as a zero-carbon fuel; however, challenges such as ignition difficulty and slow flame propagation persist. These challenges can be addressed by employing a highly reactive fuel in the pre-chamber to initiate the ignition of ammonia. This work aims to experimentally investigate the influence of pre-chamber fuel composition and concentration on ammonia combustion characteristics across various equivalence ratios. The results are as follows: elevating iso-octane equivalence ratios in the pre-chamber enhances stoichiometric ammonia combustion characteristics, reducing ignition delay and burn duration. However, excessive values result in significant delays and a decrease in flame speed. At an iso-octane equivalence ratio of 0.8, the main chamber exhibited the shortest combustion duration. By utilizing the optimal pre-chamber total equivalence ratios of 2.0 and 2.3, the ammonia lean combustion limit was achieved at an excess air ratio of 1.89 under experimental conditions (0.8 MPa, 440 K). As main chamber equivalence ratios decreased, ignition delay shortened. The pre-chamber total equivalence ratios exert a more pronounced impact on the main chamber ignition delay. In contrast, the main chamber equivalence ratios substantially influence the middle and later stages of combustion.

Effect of ammonia-air combustion melting on the color and physical properties of soda lime silicate

Ammonia is a carbon-free fuel; hence, it does not generate CO2 during combustion. However, ammonia has a different combustion reaction and flue gas composition from conventional city gas; therefore, its impact on the glass-melting process and glass quality is unknown. In this study, batches of soda lime silicate glass and cullet raw materials were melted using ammonia or city gas as the fuel. Furthermore, the glass melted in the combustion and exhaust gas fields was analyzed to evaluate the effects on composition and color tone. The type of fuel and combustion atmosphere were found to affect the color and glass transition temperature. The batches and cullet materials that melted in the combustion field were colored because of combustion-atmosphere-induced changes in the valence of the Fe in the raw materials, and the coloration of this glass was more affected by the cullet than by the batches. Meanwhile, the glass melted in the exhaust gas field of ammonia combustion exhibited a lower glass transition temperature owing to the higher presence of H2O in the exhaust gas. The temperature was measured by four thermocouples (TC1–4) installed inside the furnace. The glass transition temperature of the glass at the TC4 position in the exhaust gas field using ammonia as fuel was 24 and 9 °C lower than those of the reference batch and cullet, respectively. Ammonia can be used as a decarbonized fuel for glass-melting furnaces by designing raw materials for soda lime silicate glass. However, the raw materials containing carbonates emitted CO2 in the exhaust gases during ammonia combustion. Therefore, to achieve a carbon-neutral glass manufacturing process, CO2 derived from both raw materials and fuel should be reduced.

Flame evolution and pressure dynamics of premixed stoichiometric ammonia/hydrogen/air in a closed duct

Ammonia is regarded as a promising energy carrier due to its zero-carbon emissions and its suitability for long-distance, large-scale storage, and transportation. Ammonia/hydrogen mixed combustion is an important way to solve the problem of high ignition temperature and low flame speed in the process of ammonia combustion. This study systematically investigates the evolution of flames and pressure dynamics in a closed duct for ammonia/hydrogen/air mixtures with varying hydrogen blending ratio (0–100 %). Simultaneously, the study assesses the impact of flame instability and heat release on the evolution of ammonia/hydrogen/air flames and pressure dynamics. The results show that the flame tip speed and overpressure increase with the increase of hydrogen blending ratio and initial pressure, and when the hydrogen blending ratio is more than 25 %, the growth range of flame tip speed peak and overpressure peak value increases significantly. At the same time, the flame thickness decreases after hydrogen blending, the hydrodynamic instability increases, and the degree of wrinkles on the flame surface increases, which promotes flame acceleration. The Froude number increases significantly, the influence of buoyancy instability gradually weakens, and the flame core does not undergo significant displacement. In the later stage of flame propagation, both the flame front propagation velocity and pressure show significant pulsation phenomena. The heat release calculation results show that hydrogen blending significantly increases the adiabatic flame temperature. Consistent with the evolution law of overpressure, when the hydrogen blending ratio is greater than 25 %, the net heat release increases significantly. The elementary reaction with the highest heat release rate changes from the chain termination reaction R41 to the chain termination reaction R3. The research results can provide theoretical basis and experimental data support for the formulation of safety standards during the regulation and utilization of the combustion structure of ammonia/hydrogen/air mixtures.

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 study of combustion instability and emission characteristics of ethanol/ammonia co-firing swirl flame

Ammonia and ethanol are promising carbon–neutral alternative fuels that can be produced through renewable energy sources. This study investigates the combustion instability and emission characteristics of ethanol/ammonia co-firing flame with ammonia heat fraction up to 40% in a swirl burner. Three different ammonia blending strategies are studied: premixed addition, root injection, and middle injection. The co-firing flame exhibited intense combustion instability under certain operating conditions with a pulsating frequency of about 150 Hz. The instability intensity weakens as the ammonia ratio increases. The pulsating pressure and OH* chemiluminescence intensity first increase and then decrease with the equivalence ratio, reaching a peak value near Ф=0.35. Adding a small amount of ammonia will cause CO and NOx emissions to increase simultaneously, but when αNH3 > 0.15, CO and NOx emissions will decrease. When the Ф increases, NOx emissions of premixed addition gradually decrease, while the NOx emissions of root injection and middle injection do not change significantly. The unburned NH3 significantly decreases with increasing Ф, and ammonia combustion is more efficient with the middle injection method.

Experimental investigation of operating conditions on performance and emissions of spark ignition ammonia direct injection engine

Hydrogen, attracting attention as a future eco-friendly fuel, is difficult to handle and store because of its material characteristics; thus, ammonia has been proposed as an alternative. However, when only ammonia is utilized as the fuel, the operating range in which stable combustion is possible is limited because of the slow combustion speed and high minimum ignition energy. Here, to examine the applicability of ammonia as an engine fuel, an experiment was conducted based on changes in hydrogen energy ratios and operating conditions in a direct-injection ammonia combustion engine with hydrogen addition. The hydrogen energy ratio for stable operation is about 4.5 % at 1500 rpm under low-load operating conditions such as BMEP 0.2 MPa, and hydrogen must be supplied at a high energy ratio of 27.8 % in the case of a rated-power condition of 3800 rpm. The unburned NH3 and NOx emissions were high at 9285 ppm and 4870 ppm, respectively, under rated-power operating conditions.

Enhanced ammonia combustion by partial pre-cracking strategy in a gas turbine model combustor: Flame macrostructures, lean blowout characteristics and exhaust emissions

Cofiring with hydrogen presents a reasonable approach to achieve enhanced ammonia (NH3) combustion without introducing an extra carbon footprint. A promising strategy for NH3/H2 cofiring in gas turbines involves on-site partial pre-cracking of NH3 and burns of NH3/H2/N2 mixtures, eliminating additional hydrogen transportation and storage. This work investigates the effects of the pre-cracking ratio (γ) on flame macrostructures, lean blowout characteristics and exhaust emissions of the partially pre-cracked NH3 flames in a single-swirl gas turbine model combustor. Flow and flame macrostructures were captured using particle image velocimetry (PIV) and OH planar laser-induced fluorescence (OH-PLIF) measurements. Lean blowout limits (ϕLBO) were assessed under varying γ, and emissions at the burner outlet were measured using a Fourier transform infrared spectroscopic (FTIR) gas analyzer. Results show that as γ increases, the flame exhibits a shortened height, strengthened OH fluorescence, amplified core jet velocities and significantly reduced ϕLBO, indicating an effective enhancement of NH3 combustion by partial pre-cracking strategy. Nevertheless, NO and NO2 emissions exhibit a substantial increase with larger γ. Opposite trends of NO and NH3 emissions versus equivalence ratio (ϕ) suggest a trade-off between NO and NH3 emissions, with relatively low NO/NH3 window appearing under slightly-rich (ϕ = 1.0–1.1) conditions. Low NO emissions are also noted under ultra-lean conditions (ϕ = 0.4–0.5) with the penalty of high NH3 and N2O emissions, making it an unacceptable trade-off. Furthermore, the effect of N2 separation from the partially pre-cracked NH3 mixtures was evaluated at γ = 0.4. The results show deteriorating effects on NOx emissions, resulting in 13 % and 21 % increases in peak NO and NO2 emissions, respectively, which implies more feasibility to burn the partially pre-cracked NH3 in a direct manner rather than N2 separation.