A comprehensive review on synthesis,chemical kinetics,and practical application of ammonia as future fuel for combustion

Recently, ammonia has been explored as a potential fuel for internal combustion engines, gas turbines and other industrial purposes. Ammonia consists of 17.6% by weight of hydrogen and is thus considered a carbon-free emission fuel. The synthesis of ammonia for bulk production takes place using the Haber-Bosch process. The production, storage and transportation of ammonia is relatively safe. This paper reports various aspects of ammonia as an alternative fuel for combustors. Several studies reporting the laminar burning velocity of ammonia and its blends are discussed. Recent advances in the development of chemical kinetics for ammonia combustion are presented. The paper explores all experimental and numerical works on ammonia as a fuel for I C engines, gas turbines and other combustion systems.This review further suggests ways to overcome the disadvantages associated with ammonia combustion, such as lower burning velocities and high NOx emissions.     

Multidimensional CFD simulation of syngas combustion in a micro-pilot-ignited dual-fuel engine using a constructed chemical kinetics mechanism

A multidimensional computational fluid dynamics (CFD) simulation of a constructed syngas chemical kinetic mechanism was performed to evaluate the combustion of syngas in a supercharged dual-fuel engine for various syngas initial compositions under lean conditions. The modelled results were validated by comparing predictions against corresponding experimental data for a supercharged dual-fuel engine. The predicted and measured in-cylinder pressure, temperature, and rate of heat release (ROHR) data were in good agreement. The effect of the hydrogen peroxide chain-propagation reaction on the progress of combustion under supercharged conditions was examined for different types of syngas using various initial H₂ concentrations. The effect of the main syngas kinetic mechanism reactions on the combustion progress was analysed in terms of their contribution to the total heat of the reaction. The best results compared with experimental data were obtained in the range of equivalence ratios below about 0.8 for all types of syngas considered in this paper. As the equivalence ratio increased above 0.8, the results deviated from the experiment data. The spatial distribution of the in-cylinder temperature and OH^∗ within this equivalence-ratio range showed the completeness of the combustion. The present CFD model captured the overall combustion process well and could be further developed into a useful tool for syngas-engine combustion simulations.     

Investigation of influence of detailed chemical kinetics mechanisms for hydrogen on supersonic combustion using large eddy simulation

Five detailed hydrogen combustion chemical kinetics mechanisms coupled with a partially stirred reactor (PaSR) combustion model were applied with large eddy simulation (LES) to study the influence of detailed mechanisms on supersonic combustion in a model scramjet combustor. The LES predictions of five detailed mechanisms for velocity, temperature, and combustor wall pressure show reasonable agreement with experimental results. Examining the effects on the distributions of temperature and species in supersonic combustion reveals that the supersonic flame structure is affected by detailed mechanisms. The different detailed mechanisms have a strong influence on the combustion efficiency, volume of the subsonic region, and subsonic combustion heat release rate in the combustor. Moreover, the total heat release in the computational domain for the five detailed chemical kinetics mechanisms is quite different. The subsonic combustion is dominant in the combustor for all detailed mechanisms. An analysis of the important reactions for H₂O, HO2, and OH is performed, revealing the reasons for differences in temperature and species distributions among the different detailed mechanisms in the combustor.     

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.     

The regulation effect of methane and hydrogen on the emission characteristics of ammonia/air combustion in a model combustor

Ammonia, made up of 17.8% hydrogen, has attracted a lot of attention in combustion community due to its zero carbon emission as a fuel in gas turbines. However, ammonia combustion still faces some challenges including the weak combustion and sharp NOx emissions which discourage its application. It was demonstrated that the combustion intensity of ammonia/air flame can be enhanced through adding active fuels like methane and hydrogen, while the NOx emission issue will emerge in the meantime. This study investigates regulation effect of methane and hydrogen on the emission characteristics of ammonia/air flame in a gas turbine combustor. The instantaneous OH profile and global emissions at the combustion chamber outlet are measured with Planar Laser Induced Fluorescence (PLIF) technique and the Fourier Transform Infrared (FTIR), respectively. The flames are also simulated by large eddy simulation to further reveal physical and chemical processes of the emissions formation. Results show that for NH₃/air flames, the emissions behavior of the gas turbine combustor is similar to the calculated one-dimensional flames. Moreover, the NOx emissions and the unburned NH₃ can be simultaneously controlled to a proper value at the equivalence ratio (φ) of approximate 1.1. The variation of NO and NO₂ with φ for NH₃/H₂/air flames and NH₃/CH₄/air flames at blending ratio (Z_f) of 0.1 are similar to the NH₃/air flames, with the peak moving towards rich condition. This indicates that the NH₃/air flame can be regulated through adding a small amount of active fuels without increasing the NOx emission level. However, when Z_f = 0.3, we observe a clear large NOx emission and CO for NH₃/CH₄/air flames, indicating H₂ is a better choice on the emission control. The LES results show that NO and OH radicals exhibit a general positive correlation. And the temperature plays a secondary role in promoting NOx formation comparing with CH₄/air flame.     

氢氧加湿燃烧的数值模拟

介绍了利用CHEMKIN 4.1软件中封闭的全混同性反应器模型模拟加湿情况下氢氧燃烧的方法,同时基于详细的氢氧燃烧化学反应动力学机理分析了初始反应温度、反应压力及水蒸气添加量等对氢氧燃烧的影响。模拟结果表明,初始反应温度、反应压力及水蒸气添加量等因素均会对氢氧燃烧的点火延迟时间和燃烧最高温度产生一定程度的影响,其中水蒸气添加量的变化对氢氧燃烧的最高温度及火焰传播速度的影响较为显著。这对后续在实验室中进行氢氧加湿燃烧研究可提供有益的指导。     

Numerical investigations on tri-fuel chemical kinetics of hydrogen + Methane +LPG/air mixtures using reduced skeletal mechanism

The combustion and ignition characteristics of three fuels with different reactivities have been investigated by a reduced chemical kinetic model. In the present work, the chemical kinetics of conventional single fuel and binary fuel, relevant to gas-turbine engines, are extended and attempted to explore in the tri-fuel (TF) context, with the help of TF blends of LPG + CH₄+H₂ at the pressure and temperature range of 1–20 atm and 900–2000 K, respectively. The blending of hydrogen with hydrocarbon fuels improves flame propagation, reduces emissions, and increases the combustion performance of the engine. A detailed study is conducted to explore the characteristics of TF mixture over a wide range of operating conditions by considering eight different test mixtures (M1-M8). The test mixtures (M2 to M4) contain higher hydrogen content and thus hydrogen kinetics will tend to dominate, while test mixtures (M6 to M8) contain a higher concentration of hydrocarbons, thus the methyl radical chemistry plays a prominent role in the oxidation process. Such contrasting trends were further explored by extensive chemical kinetic modeling with the help of the reduced USC Mech_50 species model from our previous work [1] to analyze the ignition delay time, laminar flame speed, flame temperature, and heat release rate characteristics. In addition, the reaction pathway analysis through sensitivity analysis of OH and CO radical, and flow rate sensitivity analysis has also been conducted to highlight the essential chemical reactions which play a crucial role in auto-ignition, combustion, and emissions characteristics of TF blends.     

End-gas autoignition and knocking combustion of ammonia/hydrogen/air mixtures in a confined reactor

End-gas autoignition and detonation development in ammonia/hydrogen/air mixtures in a confined reactor is studied through detailed numerical simulations, to understand the knocking characteristics under IC engine relevant conditions. One-dimensional planar confined chamber filled with ammonia/hydrogen/air mixtures is considered. Various initial end-gas temperature and hydrogen concentration in the binary fuels are considered. Homogeneous ignition of stochiometric ammonia/hydrogen/air mixtures is firstly calculated. It is found that H₂ addition significantly promotes autoignition, even if the amount of addition is small. For ammonia/air mixtures and ammonia/hydrogen/air mixtures with low hydrogen mole ratios, it is found from chemical explosive mode analysis results that NH₂ and H₂NO are most important nitrogen-containing species, and R49 (NH₂+NO<=>N₂+H₂O) is a crucial reaction during thermal runaway process. When the hydrogen mole ratio is high, the nitrogen-containing species and reactions on chemical explosive mode becomes less important. Moreover, a series of one-dimensional simulations are carried out. Three end-gas autoignition and combustion modes are observed, which includes forcibly ignited flame propagation, autoignition (no detonation), and developing detonation. These modes are identified within wide ranges of hydrogen contents and initial end-gas temperatures. Furthermore, chemical kinetics at the reaction front and autoignition initiation locations are also studied with chemical explosive mode analysis. Finally, different thermochemical conditions on knocking intensity and timing are investigated. It is found that a higher initial temperature or a higher H₂ content does not always lead to a higher knocking intensity, and the knocking timing decreases with the reactivity of end-gas.     

稀薄氨气的燃烧特性及动力学研究

文章通过稀薄氨气在固定床反应器中的燃烧,研究了反应温度、停留时间、氨气浓度和氧气浓度对低浓度氨气燃烧特性的影响,并描述了氨气在氧气过量条件下在陶瓷蜂窝蓄热体中燃烧的动力学过程。研究结果表明:提高反应温度、延长停留时间以及增大氧气浓度和氨气浓度均可以提高NH3转化率,氧气浓度过高会促进NO生成;当反应温度为740~770℃、氨气浓度为1%、氧气浓度为15%时,氨气在陶瓷蓄热体中燃烧的活化能为253.56 kJ/mol;与氨气在自由空间内的燃烧相比,氨气在陶瓷蜂窝蓄热体中主要发生表面燃烧反应。     

Experimental investigation of premixed combustion limits of hydrogen and methane additives in ammonia

In this study, a specially designed premixed combustion chamber system for ammonia-hydrogen and methane-air laminar premixed flames is introduced and the combustion limits of ammonia-hydrogen and methane-air flames are explored. The measurements obtained the blow-out limits (mixed methane: 400–700 mL/min, mixed hydrogen: 200–700 mL/min), mixing gas lean limit characteristics (mixed methane: 0–82%, mixed hydrogen: 0–37%) and lean/rich combustion characteristics (mixed methane: ? = 0.6–1.9, mixed hydrogen: ? = 0.9–3.2) of the flames. The results show that the ammonia-hydrogen-air flame has a smaller lower blow-out limit, mixing gas ratio, lean combustion limit and higher rich combustion limit, thereby proving the advantages of hydrogen as an effective additive in the combustion performance of ammonia fuel. In addition, the experiments show that increasing the initial temperature of the premixed gas can expand the lean/rich combustion limits of both the ammonia-hydrogen and ammonia-methane flames.     

湍流燃烧模型在燃烧室数值计算中的对比分析

对比分析了不同燃烧模型对某型回流式燃气轮机燃烧室流场的影响,建立了描述燃烧室流场的控制方程组,采用Realizablek-ε湍流模型,湍流燃烧模型分别为涡耗散模型(ED)、涡耗散概念模型(EDC)、简单概率密度模型(PDF)和稳态小火焰模型(SFM).对比分析了不同燃烧模型下燃烧室的温度场、速度分布以及NOx排放量,并...     

Estimation of NOx and soot emission from a constant volume n-butanol/n-dodecane blended spray using unsteady flamelet model based on n-dodecane/n-butanol/NOx/PAH chemistry

A 246 species and 1062 reactions containing skeletal n-dodecane/n-butanol/NOx/Polycyclic Aromatic Hydrocarbon (PAH) combustion mechanism (Mix246) was developed for n-dodecane/n-butanol blend combustion under diesel engine-like conditions. The Mix246 mechanism was validated using ignition delay and species concentration profile data from various n-dodecane and n-butanol combustion experiments as well as by comparisons with predictions from the parent mechanisms. Subsequently, the Mix246 mechanism was coupled with a 3D Computational Fluid Dynamic (CFD) solver to simulate turbulent combustion of n-butanol/n-dodecane blended sprays in a constant volume combustor using an unsteady flamelet model. The CFD model for the 100% n-dodecane case was benchmarked against the non-reacting and reacting Spray-A conditions of the Engine Combustion Network (ECN). Finally, a parametric study was conducted to investigate the effects of various diesel engine-like conditions such as fuel injection pressure, initial ambient pressure and temperature conditions and varying n-butanol/n-dodecane blend ratios on NOx and soot emissions. Simulations show that the turbulent spray combustion evolves through distinct temporal stages and that the fully developed flame have spatially separated zones of reactive physics. Combustion properties and emission profiles for pure n-dodecane and blends with up to 20% n-butanol were seen to be nearly identical suggesting that n-butanol is a suitable alternative biofuel for CI engine combustion.     

Study on nonpremixed methane/air combustion from flame structure and NOX emission aspect for different burner head structures

In the present study, the air turbulator, which is a part of a nonpremixed burner, is investigated numerically in terms of its effects on the diffusion methane flame structure and NO_X emissions. A computational fluid dynamics (CFD) code was used for the numerical analysis. At first, four experiments were conducted using natural gas fuel. In the experimental studies, the excess air ratio was taken constant as 1.2, while the fuel consumption rate was changed between 22 and 51 Nm³/h. After the experimental studies, the CFD studies were carried out. Pure methane was taken as fuel for the simulations. The nonpremixed combustion model with the steady laminar flamelet model (SFM) approach was used in the combustion analyses. Methane‐air extinction mechanism with 17 species and 58 reactions was used for the simulations. The results obtained from the CFD studies were confronted with the measurements of the flue gas emissions in the experimental studies. Then, a modified burner head was analysed numerically for the different air turbulator blade numbers and angles. The CFD results show that increasing the air turbulator blade number and angle causes the thermal NO emissions to be reduced in the flue gas by making the flame in the combustion chamber more uniform than the original case. This new flame structure provides better mixing of the fuel and combustion air. Thus, the diffusion flame structure in the combustion chamber takes the form of the partially premixed flame structure. The maximum reduction in the thermal NO emissions in the flue gas is achieved at 38% according to the original case.     

氢气预混合微尺度催化燃烧的数值模拟

通过耦合计算流体力学软件FLUENT和化学反应动力学软件CHEMKIN并采用空间气相和表面催化详细化学反应机理,对氢气和空气的预混合气体在微型管道内的催化燃烧过程进行了数值模拟,讨论了不同反应模型的燃烧特性以及预混合气体入口速度、当量比Φ和管径对催化燃烧反应的影响。计算结果表明：表面催化反应对空间气相反应有抑制作用;随着入口速度的增大,燃烧过程同时存在着表面催化反应和空间气相反应两种控制因素;当量比Φ和管径对氢气的催化燃烧过程有重要的影响。     

Numerical analysis on influence of hydrogen peroxide (H2O2) addition on the combustion and emissions characteristics of NH3/CH4-air (O2/N2)/ H2O2 mixture

In this work, extensive chemical kinetic modeling is performed to analyze the combustion and emissions characteristics of premixed NH₃/CH₄–O₂/N₂/H₂O₂ mixtures at different replacement percentages of air with hydrogen peroxide (H₂O₂). This work is comprehensively discusses the ignition delay time, flame speed, heat release rate, and NOx & CO emissions of premixed NH₃/CH₄–O₂/N₂/H₂O₂ mixtures. Important intermediate crucial radicals such as OH, HO₂, HCO, and HNO effect on the above-mentioned parameters is also discussed in detail. Furthermore, correlations were obtained for the laminar flame speed, NO, and CO emissions with important radicals such as OH, HO₂, HCO, and HNO. The replacement of air with H₂O₂ increases flame speed and decreases the ignition delay time of the mixture significantly. Also, increases the CO and NOx concentration in the products. The CO and NOx emissions can be controlled by regulating the H₂O₂ concentration and equivalence ratios. Air replacement with H₂O₂ enhances the reactions rate and concentration of intermediate radicals such as O/H, HO₂, and HCO in the mixture. These intermediate radicals closely govern the combustion chemistry of the NH₃/CH₄– O₂/N₂/H₂O₂ mixture. A linear correlation is observed between the flame speed and peak mole fraction of OH + HO₂ radicals, and 2nd degree polynomial correlation is observed for the peak mole fraction of NO and CO with HNO + OH and HCO + OH radicals, respectively.     

An experimental study of mild flameless combustion of methane/hydrogen mixtures

This paper presents an experimental study of mild flameless combustion regime applied to methane/hydrogen mixtures in a laboratory-scale pilot furnace with or without air preheating. Results show that mild flameless combustion regime is achieved from pure methane to pure hydrogen whatever the CH₄/H₂ proportion. The main reaction zone remains lifted from the burner exit, in the mixing layer of fuel and air jets ensuring a large dilution correlated to low NOx emissions whereas CO₂ concentrations obviously decrease with hydrogen proportion. A decrease of NOx emissions is measured for larger quantity of hydrogen due mainly to the decrease of prompt NO formation. Without air preheating, a slight increase of the excess air ratio is required to control CO emissions. For pure hydrogen fuel without air preheating, mild flameless combustion regime leads to operating conditions close to a "zero emission furnace", with ultra-low NOx emissions and without any carbonated species emissions.     

Turbulent diffusion combustion model using chemical equilibrium combined with the eddy dissipation concept for reducing detailed chemical mechanisms: An application of H2‐air turbulent diffusion flame

This research aims at building a turbulent diffusion combustion model based on chemical equilibrium and kinetics for simplifying a complex chemical mechanism. This paper presents the combustion model based on chemical equilibrium combined with an eddy dissipation concept model (CE‐EDC); the model is validated by simulating a H₂‐air turbulent diffusion flame. In the CE‐EDC model, the reaction rate of fuels and intermediate species are estimated by using the equations of the EDC model. Then, the reacted fuels and intermediate species are assumed to be in chemical equilibrium; the amounts of the other species are determined by the Gibbs free energy minimization method by using the amounts of the reacted fuels, intermediate species, and air as reactants. An advantage of the CE‐EDC model is that the amounts of the combustion products can be determined without using detailed chemical mechanisms. Moreover, it can also predict the amounts of the intermediate species. The obtained results are compared with Takagi's experimental data and the data computed by the EDC model, which uses the complex chemical mechanisms. The mole fractions of H₂, O₂, H₂O, temperature, and velocity obtained by using our CE‐EDC model were in good agreement with the reference data without taking into account the chemical reaction rates of the O₂ and H₂O. Furthermore, the mole fractions of OH and H are in good agreement with the results of the EDC model at high temperatures. On the other hand, the chemical equations involving OH and H were used for predicting the mole fractions of OH and H, which were similar to those obtained from the EDC model at low temperatures. Using the present CE‐EDC model, amounts of combustion products can be calculated by using a reduced chemical mechanism and the Gibbs free energy minimization theory. The accuracy of this model is in the same order as that of the EDC model. The CE‐EDC model uses reduced chemical mechanism concerning selected species, while the result of the chemical equilibrium calculation was employed to the other species; further, the accuracy of the CE‐EDC model is the same as that of the EDC model as to major species, T, U, and k. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20296     

Studies on properties of laminar premixed hydrogen-added ammonia/air flames for hydrogen production

In order to evaluate the potential of burning and reforming ammonia as a carbon-free fuel in production of hydrogen, fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) for laminar premixed hydrogen-added ammonia/air flames were studied both experimentally and computationally. Freely (outwardly)-propagating spherical laminar premixed flames at normal temperature and pressure were considered for a wide range of global fuel-equivalence ratios, flame stretch rates (represented by the Karlovitz number) and the extent of hydrogen substitution. Results show the substantial increase of laminar burning velocities with hydrogen substitution, particularly under fuel-rich conditions. Also, predicted flame structures show that the hydrogen substitution enhances nitrogen oxide (NO_x) and nitrous oxide (N₂O) formation. At fuel-rich conditions, however, the amount of NO_x and N₂O emissions and the extent of the increase with the hydrogen substitution are much lower than those under fuel-lean conditions. These observations support the potential of hydrogen as an additive for improving the burning performance with low NO_x and N₂O emissions in fuel-rich ammonia/air flames and hence the potential of using ammonia as a clean fuel. Increasing the amount of added hydrogen tends to enhance flame sensitivity to stretch.     

Experimental and numerical study on premixed partially dissociated ammonia mixtures. Part I: Laminar burning velocity of NH3/H2/N2/air mixtures

Ammonia is considered as a promising hydrogen carrier, which is seen as a reliable carbon-free fuel. Improving the combustion properties of ammonia is the focus of current research. The hydrogen could be dissociated from the ammonia in real applications. For purpose of combustion, partially dissociated ammonia could be combusted directly without using extra hydrogen. Laminar burning velocity is an important combustion parameter, but there are only a few data of partially dissociated ammonia are reported. To fill the data gap, the laminar burning velocity was measured at various equivalence ratios and dissociation degrees of ammonia by the constant pressure spherical flame method in this study. Besides, fifteen kinetic models were compared with experimental data, and the model with the best consistency was obtained. The experimental results show that the laminar burning velocity increases monotonically with the increase of the dissociating degree. When ammonia is completely dissociated, the maximum laminar burning velocity increases from 7.9 cm/s to 228 cm/s, and the equivalence ratio corresponding to the peak value also shifts from 1.1 to 1.6. The laminar burning velocity predicted by the model constructed by Stagni is in best agreement with the experimental data. Moreover, data calculated by the five correlations for predicting laminar burning velocity were compared with the numerical data to verify that whether they are suitable for the mixtures with additional nitrogen. The results show that the correlation based on the activation temperature is the most accurate. However, it still has a maximum relative error of ±20% within the calculated range.     

Laminar flame speed correlations of ammonia/hydrogen mixtures at high pressure and temperature for combustion modeling applications

Ammonia/hydrogen mixtures are among the most promising solutions to decarbonize the transportation and energy sector. The implementation of these alternative energy carriers in practical systems requires developing suitable numerical tools, able to estimate their burning velocities as a function of both thermodynamic conditions and mixture quality. In this study, laminar flame speed correlations for ammonia/hydrogen/air mixtures are provided for high pressures (40 bar–130 bar) and elevated temperatures (720 K–1200 K), and equivalence ratios ranging from 0.4 to 1.5. Based on an extensive dataset of chemical kinetics simulations for ammonia/hydrogen blends (0-20-40-60-80-90-100 mol% of hydrogen), dedicated correlations are derived using a regression fitting. Besides these blend-specific correlations, a generalized (i.e., hydrogen-content adaptive) formulation, with hydrogen content used as additional parameter, is proposed and compared to the dedicated correlations.