Recalibration of carbon-free NH3/H2 fuel blend process: Qatar's roadmap for blue ammonia

Due to growing concerns about carbon emissions, using zero-carbon fuels has become an interesting alternative to overcome this problem. The NH₃(70%)-H₂(30%) fuel blend is an innovative fuel example that has the potential to replace conventional hydrocarbon fuels. Studies on the NH₃(70%)-H₂(30%) fuel blend have shown its superior combustion performance and its effect on enhancing cycle efficiency compared to other compositions of the NH₃–H₂ blends. However, without calibrating ammonia plants and simply mixing portions of the produced pure ammonia to hydrogen at the desired molar fraction essentially requires coupling ammonia plants with other hydrogen-producing plants, leading to potential difficulties in commercializing the unused (as fuel in the NH₃–H₂/air gas turbines) hydrogen portions from the hydrogen-producing plan. Therefore, in this paper, as an attempt to utilize the existing ammonia production infrastructure and facility without acquiring major changes that could lead to resisting the adoption of the NH₃(70%)-H₂(30%) fuel blend, the independent parameters of a conventional ammonia plant have been calibrated, and the reactors have been sized to provide a continuous supply of the NH₃(70%)-H₂(30%) fuel blend with the exact molar fraction to run a power plant. Calibrating of the ammonia plant has been performed using an ASPEN PLUS model.     

Novel dual fuel diesel-ammonia combustion system in advanced TDI engines

As ammonia (NH₃) is the best hydrogen carrier for safety and efficiency, transportation engines have been proposed using ammonia. NH₃ may be used as a single combustion fuel for an internal combustion engine. However, as NH₃ is a difficult fuel for what concerns combustion, faster and more complete combustions may be achieved by working dual fuel. The most promising alternatives are offered by using either diesel injection ignition plus port/direct injection of the NH_3, or jet ignition of a gasoline-like fuel (gasoline, CH₄, C₃H₈, H₂) plus port/direct injection of the NH₃. The opportunity of diesel injection ignition plus direct injection of NH₃ is considered here. The simulations show the prospect to achieve Diesel-like power densities and efficiencies, and load control by quantity of fuel injected. Critical component for prototyping is the high pressure fuel injection systems specifically developed for NH₃.     

Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion

To achieve comprehensive prediction of ammonia combustion in terms of flame speed and ignition delay time, an improved mechanism of ammonia oxidation was proposed in this work. The present model (UT-LCS) was based on a previous work [Song et al., 2016] and improved by relevant elementary reactions including NH₂, HNO, and N₂H₂. The model clearly explained reported values of laminar flame speed and ignition delay time in wide ranges of equivalence ratio and pressure. This suggests that NH₂, HNO, and N₂H₂ reactivities play a key role to improve the reaction mechanism of ammonia oxidation in the present model. The model was also applied to demonstrate NH₃/H₂/air combustion. The present model also appropriately predicted the laminar flame speed of NH₃/H₂/air combustion as a function of equivalence ratio. Using the model, we discussed the reduction of NO concentration downstream and H₂ formation via NH₃ decomposition in NH₃/H₂ fuel-rich combustion. The results provide suggestions for effective combustion of NH₃ for future applications.     

Study on the mechanism of the ignition process of ammonia/hydrogen mixture under high-pressure direct-injection engine conditions

As environmental problems and energy crisis become more serious, ammonia is one of the potential alternative fuels. In order to better use ammonia as fuel in power equipment, the ignition process was studied under high-pressure direct-injection engine condition. In the paper, the Homogeneous model in Chemkin package was selected for numerical calculation. In the six cases with different hydrogen mixing ratios, the effect of initial temperature, pressure, equivalence ratio and hydrogen mixing ratio on ignition delay time (IDT) were studied. It conducted that IDT could be effectively reduced when adding 10–50% hydrogen to ammonia. Then, after sensitivity analysis of NH₃/H₂ mixtures, the key equations and free radicals affecting combustion characteristics were found. The rate of production (ROP) of the key radicals were carried out. It was found that the hydrogen provided the initial concentration of H radical before the start fire, which greatly improved the ROP of OH radical of R1(H + O₂=O + OH) compared to the original H needed to break the N–H chemical bond in pure ammonia. And the OH radical was related to the consumption of NH₃ by R31(NH₃+OH=NH₂+H₂O).     

Experimental and numerical study of the laminar burning velocity of NH3/H2/air premixed flames at elevated pressure and temperature

Ammonia (NH₃) is a carbon-free fuel that shows great research prospects due to its ideal production and storage systems. The experimental data of the laminar burning velocity of NH₃/H₂/air flame at different hydrogen ratios (X_H2 = 0.1–0.5), equivalent ratios (φ = 0.8–1.3), initial pressures (P = 0.1–0.7 MPa), and initial temperatures (T = 298–493 K) were measured. The laminar burning velocity of the NH₃/H₂/air flame increased upon increasing the hydrogen ratios and temperature, but it decreased upon increasing the pressure. The equivalent ratio of the maximum laminar burning velocity was only affected by the proportion of reactants. The equivalence ratio value of the maximum laminar burning velocity was between 1.1 and 1.2 when X_H2 = 0.3. The chemical reaction kinetics of NH₃/H₂/air flame under four different initial conditions was analyzed. The less NO maximum mole fraction was produced during rich combustion (φ > 1). The results provide a new reference for ammonia as an alternative fuel for internal combustion engines.     

Uncertainty quantification of the premixed combustion characteristics of NH3/H2/N2 fuel blends

Green ammonia is a candidate fuel to decarbonise shipping and other industries. However, ammonia features a lower reactivity compared to conventional fuels and is therefore difficult to burn. To resolve this issue, thermo-catalytic cracking of ammonia using waste heat is often employed to produce NH₃/H₂/N₂ blends as fuel. However, on-site operational variations in this process can become sources of uncertainty in the fuel composition, causing randomness of the flame's physicochemical properties and challenging flame stability. In the present work, a surrogate model is built using the polynomial chaos expansion (PCE) method to investigate the impact of fuel composition variability on combustion characteristics at different operating conditions. Impacts of 1.5% deviation in the fuel composition on the flame properties for different initial pressures (P_i) and unburnt fuel temperatures (T_u) are investigated for a wide range of equivalence ratios covering lean and rich mixtures. The uncertainty effects defined by the coefficient of variation (COV) fluctuate for equivalence ratios greater than 1.1, while no fluctuation is observed in COV for near stoichiometric combustion conditions. It is shown that H₂ variation in the fuel blend has the strongest effect (over 80%) on the uncertainty of all investigated physicochemical properties of the flame. The least affected property is the adiabatic flame temperature with variations of about 2.5% in richer fuel conditions. The results further show that preheating of the reactants can significantly reduce the COV of laminar flame speed. The consequences of these uncertainties upon different combustion technologies are then discussed and it is argued that moderate and intense low oxygen dilution (MILD) and colourless distributed combustion (CDC) technology may remain resilient.     

Stochastic reactor modelling of multi modes combustion with diesel direct injection or hydrogen jet ignition start of combustion

Recent papers 1, 2, 3, 4 and 5 have proposed two different systems to more efficiently and more rapidly burn the fuel in highly boosted, high compression ratio, directly injected internal combustion engines permitting multi-mode combustion operation. In a first system, a second direct injector is coupled with the standard Diesel direct injector and glow plug. The second direct injector introduces the most of the fuel while the Diesel direct injector only introduces a minimum amount of fuel to control the start of the combustion about top dead centre. The fuel injected before the Diesel ignition injection burns premixed, the fuel injected after the Diesel ignition injection burns diffusion. This design permits combustion premixed gasoline-like if all the fuel is injected before the Diesel ignition injection, diffusion Diesel-like if all the fuel is injected after the Diesel ignition injection (as done in the Westport High Pressure Direct Injection concept [12]), and mixed gasoline/Diesel like injecting the fuel before and after the Diesel ignition injection. The premixed gasoline-like mode is actually a homogeneous charge compression ignition (HCCI)-like mode, where an amount of fuel smaller than the threshold value producing top dead centre auto ignition is then ignited at top dead centre by the Diesel ignition injection in a more robust, stable and repeatable operation unaffected by small changes in properties and composition of the fuel and air mixture. In an alternative design, the glow plug is replaced by a jet ignition devices feed preferably with H₂. In this case, a spark ignition ignites the stoichiometric H₂-air mixture within the jet ignition pre-chamber. The jets of hot reacting H₂-air combusting gases then ignite the main chamber premixed mixture in the gasoline-like operation or create suitable conditions for the fuel subsequently injected to burn diffusion in the Diesel-like operation or perform both duties in the mixed gasoline/Diesel-like operation. A single main chamber direct injector is generally needed (for example with H₂, CH₄ or C₃H₈ fuels). With NH₃, a second main chamber direct injector with H₂ is also used to limit the volume of the jet ignition pre-chamber. In this short communications, the results of detailed chemistry simulations with the SRM (Stochastic Reactor Model) suite, a sophisticated engineering tool combining conventional 1D or 3D fluid dynamics approaches are presented to further support these two engine concepts working with fuels H₂, CH₄, C₃H₈, NH₃, I-C₈H₁₈ and N-C₇H₁₆ and adopting two different mechanisms for chemical kinetics. Within the limits of the present simulations (a very accurate chemical kinetic for combustion of I-C₈H₁₈ and N-C₇H₁₆ but a much less accurate chemical kinetic for the other fuels and especially for NH₃, unavailability of variable composition and variable properties multiple injections), the Diesel injection ignition and the hydrogen jet ignition are proved to permit combustion modes leading to indicated thermal efficiencies up to 10% better than the latest Diesels at high loads within the same peak pressure and peak temperature constraints.     

Novel heavy duty engine concept for operation dual fuel H2–NH3

A prior paper has presented a novel design of a heavy duty truck engine fuelled with H₂. In this design, the customary in-cylinder Diesel injector and glow plug are replaced with a main chamber fuel injector and a jet ignition pre-chamber. The jet ignition pre-chamber is a small volume that is connected to the in-cylinder through calibrated orifices accommodating another fuel injector and a glow or a spark plug that controls the start of combustion. This design permits to operate the engine in four different modes: traditional compression ignition (CI), diffusion, Diesel-like (M₁); mixed gasoline/Diesel-like (M₂); traditional spark ignition (SI), premixed, gasoline-like (M₃); premixed, homogeneous charge compression ignition HCCI-like (M₄). In the mode diffusion with jet ignition (M₁), an injection occurs in the jet ignition pre-chamber before the main chamber fuel is injected and the engine operates therefore mostly Diesel-like. In the mode mixed diffusion/premixed Diesel/gasoline-like (M₂) an injection occurs in the jet ignition pre-chamber after only part of the main chamber fuel is injected and mixed with air. In the mode premixed with jet ignition (M₃), an injection occurs in the jet ignition pre-chamber after the main chamber fuel is injected and mixed with air and the engine operates gasoline-like. Finally, in the mode premixed without jet ignition (M₄), no injection occurs in the jet ignition pre-chamber and the engine operates HCCI-like. Modelling results have already been presented and discussed with H₂ as the main chamber and pre-chamber fuel. This paper considers the option to accommodate a second main chamber injector that will inject the NH₃ that will then burn in air thanks to the hot combusting gases from the combustion of H₂ and air using the modes M₁ and M₂ described above. The mode M₃ also of interest is not considered here. First results of simulations show the opportunity to achieve better than Diesel fuel energy conversion efficiency thanks to the reduced heat losses of the “cold burning” NH₃ and suggest to perform the experiments needed to further support the findings.     

Effects of O2 enrichment on NH3/air flame propagation and emissions

The potential utilization of ammonia as a carbon-free fuel under oxygen (O₂)-enriched condition is demonstrated, suggesting its practically appropriate burning conditions by measuring and predicting the combustion characteristics of outwardly-propagating spherical O₂-enriched NH₃/air premixed flames at normal temperature and pressure. Measured and computed laminar burning velocities and predicted flame structure exhibit that the O₂-enriched ammonia/air flames become thinner and propagate faster with O₂ enrichment. Observed flame morphologies and measured and computed Markstein numbers reveal that all the present O₂-enriched flames are stable in terms of the flamefront cellular instability due to preferential diffusion and the effects of O₂ enrichment on the instability are negligible. Volume-based 35–40% O₂ in the nonfuel mixtures demonstrates the proper burning intensity for practical applications, comparable to the typical hydrocarbon/air flames. In the present flame configuration, however, local nitrogen oxides emissions are found to be high, which should be substantially reduced in the practical systems.     

Theoretical investigation of the combustion performance of ammonia/hydrogen mixtures on a marine diesel engine

Ammonia is a good hydrogen carrier and can be well combined with hydrogen for combustion. The combustion performance of the mixtures of ammonia and hydrogen in a medium-speed marine diesel engine was investigated theoretically. The HCCI combustion mode was selected for reducing thermal-NO_x production. The start fire characteristic of the NH₃–H₂ mixtures was studied under different equivalence ratio, hydrogen-doped ratio, and intake air temperature and pressure. Then, the combustion performance of the NH₃–H₂ mixtures (doping 30% hydrogen) was analyzed at a typical operation condition of engine. The addition hydrogen improved the laminar flame velocity of ammonia, and affected the NO_x emission. For the medium-speed marine engine fueled with NH₃–H₂, reducing combustion temperature, introducing EGR and combining with post-treatment technology would be a feasible scheme to reduce NO_x emission.     

An intelligent control of NH3 injection for optimizing the NOx/NH3 ratio in SCR system

The distribution of nitrogen oxides (NO_x) flux within the cross-section area in front of ammonia injection grid (AIG) under different operating conditions was obtained by computational fluid dynamics (CFD) method. Weight of NO_x flux in the sub-zone corresponding to each of the ammonia (NH₃) injection branch-pipes of AIG system was analyzed and the sensitivity of which against the plant power load was figured out. A number of “critical” ammonia injection branch-pipes were determined with regard to the weight sensitivity analysis. The selected “critical” branch-pipes were changed to be controlled by the automatic valves, and an intelligent tuning strategy was proposed. The NO_x/NH₃ mixing stoichiometry over the cross-section area in front of AIG system was significantly modified for the high utilization ratio of ammonia. A case work was launched on the selective catalytic reduction (SCR) system of a 660 MW plant. As a result, the ammonia consumption rate (ACR) was found to be reduced by 6.44% compared to that under previous control system, and was 9.31% lower than that of the unapplied system. The methodology for determining the “critical” branch-pipes and intelligent tuning strategy of ammonia injection notably saved the ammonia consumption of SCR system, and the formation of ammonium bisulfate (ABS) were greatly confined.     

Burning velocity and flame structure of CH4/NH3/air turbulent premixed flames at high pressure

Ammonia is one of the most promising alternative fuels. In particular, ammonia combustion for gas turbine combustors for power generation is expected. To shift the fuel for a gas turbine combustor to ammonia step-by-step, the partial replacement of natural gas by ammonia is considered. To reveal the turbulent combustion characteristics, CH₄/NH₃/air turbulent premixed flame at 0.5 MPa was experimentally investigated. The ammonia ratio based on the mole fraction and lower heating value was varied from 0 to 0.2. The results showed that the ratio of the turbulent burning velocity and unstretched laminar burning velocity decreased with an increase in the ammonia ratio. The reason for this variation is that the flame area decreased with an increase in the ammonia ratio as the flame surface density decreased and the fractal inner cutoff increased. The volume fractions in the turbulent flame region were almost the same with ammonia addition, indicating that combustion oscillation can be handled in a manner similar to that for the case of natural gas for CH₄/NH₃/air flames.     

Combustion features of CH4/NH3/H2 ternary blends

The use of so-called “green” hydrogen for decarbonisation of the energy and propulsion sectors has attracted considerable attention over the last couple of decades. Although advancements are achieved, hydrogen still presents some constraints when used directly in power systems such as gas turbines. Therefore, another vector such as ammonia can serve as a chemical to transport and distribute green hydrogen whilst its use in gas turbines can limit combustion reactivity compared to hydrogen for better operability. However, pure ammonia on its own shows slow, complex reaction kinetics which requires its doping by more reactive molecules, thus ensuring greater flame stability. It is expected that in forthcoming years, ammonia will replace natural gas (with ^～90% methane in volume) in power and heat production units, thus making the co-firing of ammonia/methane a clear path towards replacement of CH₄ as fossil fuel. Hydrogen can be obtained from the pre-cracking of ammonia, thus denoting a clear path towards decarbonisation by the use of ammonia/hydrogen blends. Therefore, ammonia/methane/hydrogen might be co-fired at some stage in current combustion units, hence requiring a more intrinsic analysis of the stability, emissions and flame features that these ternary blends produce. In return, this will ensure that transition from natural gas to renewable energy generated e-fuels such as so-called “green” hydrogen and ammonia is accomplished with minor detrimentals towards equipment and processes. For this reason, this work presents the analysis of combustion properties of ammonia/methane/hydrogen blends at different concentrations. A generic tangential swirl burner was employed at constant power and various equivalence ratios. Emissions, OH1/NH1/NH₂1/CH1 chemiluminescence, operability maps and spectral signatures were obtained and are discussed. The extinction behaviour has also been investigated for strained laminar premixed flames. Overall, the change from fossils to e-fuels is led by the shift in reactivity of radicals such as OH, CH, CN and NH₂, with an increase of emissions under low and high ammonia content. Simultaneously, hydrogen addition improves operability when injected up to 30% (vol), an amount at which the hydrogen starts governing the reactivity of the blends. Extinction strain rates confirm phenomena found in the experiments, with high ammonia blends showing large discrepancies between values at different hydrogen contents. Finally, a 20/55/25% (vol) methane/ammonia/hydrogen blend seems to be the most promising at high equivalence ratios (1.2), with no apparent flashback, low emissions and moderate formation of NH₂/OH radicals for good operability.     

Effect of ammonia addition on combustion and emission characteristics of hydrogen-fueled engine under lean-burn condition

Hydrogen (H₂) is a carbon-free fuel with many excellent combustion characteristics, but abnormal combustion is one of the main obstacles to the promotion and application of hydrogen-fueled engines. This experimental study aims to investigate the suppression of the heat release rate (HRR) of a hydrogen-fueled engine through the addition of ammonia (NH₃). The engine was run at 1300 rpm, with manifold absolute pressure (MAP) of 61 kPa and NH₃ addition ratio of 0% and 2.2%, under lean-burn conditions. The results showed that the addition of small amounts of ammonia reduced the combustion rate of the fuel mixture, prolonged the flame development period (CA0-10) and propagation durations (CA10-90) of the engine, and reduced the peak in-cylinder pressure and peak HRR under lean-burn conditions. The addition of ammonia increased the peak indicated mean effective pressure (IMEP) and the peak indicated thermal efficiency (ITE) of the engine. The addition of ammonia resulted in increased nitrogen oxides (NOx) emissions.     

Numerical study of experimental feasible heat release rate markers for NH3–H2-air premixed flames

Ammonia (NH₃) and hydrogen (H₂) as carbon-free fuels have attracted much attention for combustion applications in recent years. Co-firing ammonia with hydrogen provides a solution to overcome the extremes in the reactivities of both pure ammonia and hydrogen fuels. Heat release rate (HRR) is one of the most important quantities in the study of turbulent combustion, but direct measurement of local HRR is not experimentally feasible. In this study, we explored several quantities, [NH], [O], and the gradient of [OH] (Grad [OH]) as potential experimentally feasible HRR markers for NH₃–H₂-air premixed flames using numerical simulations. The performance of these quantities over a wide range of equivalence ratios and H₂ blending ratios have been examined, and some key reactions have been identified to explain the corresponding variations of the correlation for [NH] and [O]. It is concluded that the [NH] and Grad [OH] can be used in general as a suitable HRR marker for NH₃–H₂-air premixed flames, and the use of [NH] is especially recommended for lean flame conditions. A strategy that slightly shifts the [NH] and Grad [OH] profiles to overlap the corresponding HRR shows a further improvement on the performance of [NH] and Grad [OH]. The use of [O] can be considered for rich flame conditions while cautions are needed for conditions with high H₂ blending ratios.     

An experimental and modelling study of dual fuel aqueous ammonia and diesel combustion in a single cylinder compression ignition engine

The ability of ammonia to act as a hydrogen carrier, without the drawbacks of hydrogen gas-storage costs and low stability-renders it a potential solution to the decarbonisation of transport. This study combines both modelling and experimental techniques to determine the effect of varying the degree of aspiration of ammonium hydroxide (NH₄OH) solution, at different engine loads, in the combustion of a compression ignition engine. Ignition delay was extended as ammonia injection increased, causing an increase in peak in-cylinder temperature, but generally lower combustion quality-increasing incomplete combustion products, while decreasing particle size. The higher peak in-cylinder temperatures generally correlated with higher nitrous oxide (NO_x) emissions in the exhaust, though a fuel-bound nitrogen effect was apparent. Chemical kinetic modelling at equivalent conditions found increasing levels of unburnt ammonia with greater aspiration. Moreover, the ignitability of NH₄OH was found to improve in simulations substituting diesel with hydrogen peroxide direct injection.     

Assessment of the co-combustion process of ammonia with hydrogen in a research VCR piston engine

The presented work concerns experimental research of a spark-ignition engine with variable compression ratio (VCR), adapted to dual-fuel operation, in which co-combustion of ammonia with hydrogen was conducted, and the energy share of hydrogen varied from 0% to 70%. The research was aimed at assessing the impact of the energy share of hydrogen co-combusted with ammonia on the performance, stability and emissions of an engine operating at a compression ratio of 8 (CR 8) and 10 (CR 10). The operation of the engine powered by ammonia alone, for both CR 8 and CR 10, is associated with either a complete lack of ignition in a significant number of cycles or with significantly delayed ignition and the related low value of the maximum pressure p_max. Increasing the energy share of hydrogen in the fuel to 12% allows to completely eliminate the instability of the ignition process in the combustible mixture, which is confirmed by a decrease in the IMEP uniqueness and a much lower p_max dispersion. For 12% of the energy share of hydrogen co-combusted with ammonia, the most favorable course of the combustion process was obtained, the highest engine efficiency and the highest IMEP value were recorded. The conducted research shows that increasing the H₂ share causes an increase in NO emissions, for both analyzed compression ratios.     

Study on using hydrogen and ammonia as fuels: Combustion characteristics and NOx formation

This paper evaluates the potential of hydrogen (H₂) and ammonia (NH₃) as carbon‐free fuels. The combustion characteristics and NO_x formation in the combustion of H₂ and NH₃ at different air‐fuel equivalence ratios and initial H₂ concentrations in the fuel gas were experimentally studied. NH₃ burning velocity improved because of increased amounts of H₂ atom in flame with the addition of H₂. NH₃ burning velocity could be moderately improved and could be applied to the commercial gas engine together with H₂ as fuels. H₂ has an accelerant role in H₂–NH₃–air combustion, whereas NH₃ has a major effect on the maximum burning velocity of H₂–NH₃–air. In addition, fuel‐NO_x has a dominant role and thermal‐NO_x has a negligible role in H₂–NH₃–air combustion. Thermal‐NO_x decreases in H₂–NH₃–air combustion compared with pure H₂–air combustion. NO_x concentration reaches its maximum at stoichiometric combustion. Furthermore, H₂ is detected at an air‐fuel equivalence ratio of 1.00 for the decomposition of NH₃ in flame. Hence, the stoichiometric combustion of H₂ and NH₃ should be carefully considered in the practical utilization of H₂ and NH₃ as fuels. H₂ as fuel for improving burning performance with moderate burning velocity and NO_x emission enables the utilization of H₂ and NH₃ as promising fuels. Copyright © 2014 John Wiley & Sons, Ltd.     

Improvement of H2/O2 chemical kinetic mechanism for high pressure combustion

An updated H₂/O₂ kinetic mechanism was proposed by incorporating carefully selected reaction rate coefficient and great progress in radical chain mechanisms, in which the uncertainties of rate coefficient were discussed. The performance of the current mechanism was compared to other H₂ mechanism and validated against a wide range kinetic targets, including oxidation, decomposition in shock waves, ignition, flame speed and flame structure. Results show that the current mechanism obtains an overall improvement of performance, especially for the flame speed. By using the updated binary diffusion coefficient from ab initio calculations and the chemically termolecular reactions, the current mechanism presents better agreement with the new experimental flame speed at atmospheric pressure and obtains the improved performance with respect to the negative pressure dependence of high-pressure H₂ flame. Furthermore, the flame speed predictions are strongly sensitive to the H₂O third body efficiency in the H₂ mechanism, affecting the water-contained H₂ flame. The modeling results of rapid compression machine ignition show that present mechanism can more accurately predicts the ignition delay under engine-like conditions. However, all three mechanisms cannot accurately reproduce the negative pressure dependence behavior of mass burning rate in high-pressure H₂ flame, which may be attributed to the fact that the important reaction O + OH(+M) = HO₂(+M) that significantly affects lean high-pressure H₂ flame is not included in current mechanism. Consequently, continuous works should be emphasized on the reactions that are important but neglected in H₂ mechanism. All these not only develop an improved H₂ reaction mechanism for high-pressure combustion, but also point out the direction for refining the H₂ mechanism.     

Experimental and numerical study on laminar premixed NH3/H2/O2/air flames

Adding the product of water electrolysis (i.e. 2:1 volume of H₂ and O₂) is an effective strategy to enhance the combustion intensity of NH₃/air mixtures. In this work, the laminar burning velocity (LBV) of the obtained NH₃/H₂/O₂/air mixtures was measured at 303 K, 0.1 MPa and compared with the values predicted by seven mechanisms. To improve the prediction performance, a new mechanism is developed based on the existing mechanism and adopted for numerical simulation. The results of this study show that the LBV of NH₃ is significantly increased by additional H₂ and O₂. By comparison, it is found that H₂ shows a more significant promoting effect on LBV when the volume ratio of additional H₂ and O₂ is 2. The concentration of key radicals and the flame temperature increase remarkably due to the addition of H₂ and O₂, which promote the flame propagation. Furthermore, the experimental results also indicated that the additional H₂ and O₂ make the burned gas Markstein length decrease on the lean side and increase on the rich side.