Mole Fraction Of Ammonia Research Articles

Investigation on self-excited thermoacoustic instability and emission characteristics of premixed CH4/NH3/air flame

Ammonia is one of the promising carbon-free alternative fuels for low-carbon power generation. This study investigated the self-excited thermoacoustic instability of premixed CH4/NH3/air flame in a swirl burner. The work aims to determine the effects of ammonia mole fraction (χNH3), equivalence ratio (Ф) and combustion chamber length (CL) on flame morphology and self-excited flame dynamics. In addition, the emission characteristics of CH4/NH3/air flame were also investigated. Results show that the thermoacoustic instability of the 1/4 wave mode is excited when 0.8≤Ф≤1.2 and χNH3 ≤ 0.3. There are apparent pressure fluctuations and heat release fluctuations in the combustion chamber. The fluctuating frequency decreases with increasing CL. The maximum fluctuating frequency and oscillation intensity appear near Ф=1.0. When χNH3 increases, the amplitude of fluctuating pressure and heat release rate decrease obviously, and the instability range narrows to Ф=1.0–1.1 simultaneously. The low-order acoustic network model successfully predicted the instability transition and frequency decreasing trend caused by increasing ammonia. The increase in the convective time delay is responsible for this transition. Emission analysis indicates that NOx emission can be controlled within 100 ppm under fuel-rich conditions, but the CO emission increases sharply. Thermoacoustic instability and nitrogen oxides can be simultaneously controlled under fuel-rich conditions when χNH3>0.3. However, this will be limited by unburned reactants.

Effect of flue gas recirculation on combustion instability and emission characteristics of premixed CH4/NH3/air flame

The hydrogen carrier ammonia is considered a prominent carbon-free alternative fuel. This paper examines the impact of flue gas recirculation (FR) on combustion instability and emission behavior of premixed CH4/NH3/air swirl flame at different equivalence ratios (Φ) and ammonia mole fractions (χNH3). Experimental results show that FR is beneficial to suppressing the self-excited pulsating pressure and pulsating heat release rate of CH4/NH3/air flame, and the pulsating frequency decreases monotonically with the increase of FR. Under fuel-lean and stoichiometric conditions, FR is beneficial to suppressing NOx formation but will increase CO and NH3 emissions. Under fuel-rich conditions, FR reduces CO emissions but increases NOx emissions. The NO emissions obtained from the chemical reactor network simulation are in qualitative agreement with the experimental results. The results show that the elementary reactions related to HNO and NHi radicals play a crucial role in the influence of FR on NO formation.

Numerical investigation of premixed methane-ammonia combustion in a mesoscale porous combustor

The present study focuses on the numerical modeling of the premixed methane-ammonia combustion in a mesoscale porous combustor. The primary objective is to reduce carbon dioxide emissions by using ammonia as a carbon-free fuel while maintaining constant the combustor wall temperature compared to pure methane. Ammonia addition in the mixture increases nitrogen oxide compared to pure methane-oxygen combustion, which is undesirable. On the other hand, porous material improves fuel and air mixing and leads to more appropriate fuel distribution which could lower the combustion temperature and as well NOx emission. So, simultaneous using ammonia and porous media in the combustion is the effective combination to reduce the consumption of hydrocarbon fuels. In the current study, porosity of porous media is changed within the range of 50 %–90 %, molar fraction of ammonia is varied from 0.1 to 0.9, and equivalence ratios ranging from 0.8 to 1. Also, to find the optimal combustion parameters, the response surface methodology has been employed. The results indicated that to achieve satisfactory conditions for methane-ammonia-oxygen combustion in contrast to the combustion of pure methane-oxygen, it is the best condition to employ a porosity of 63 %, an ammonia mole fraction of 0.9, and a stoichiometric equivalence ratio. Because of the utilization of porous media, the mean wall temperature reached to 1543 K, indicating a 3.5 % reduction compared to pure methane combustion. Simultaneously, nitrogen oxide emission drops to 2270 ppm, showcasing a 10 % reduction compared to the non-porous media ammonia-methane combustion. Additionally, the use of the 0.9 M fraction of ammonia leads to a noteworthy 75 % decrease in the mass fraction of carbon dioxide emissions compared to pure methane combustion. These findings accelerate the optimal use of carbon-free fuels such as ammonia as alternatives to hydrocarbon fuels.

Effect of ammonia addition on nanostructure of soot in laminar coflow diffusion flames of ethylene diluted with nitrogen

The effect of ammonia addition on the nanostructure of soot particles was studied experimentally for ethylene diffusion flames. To compensate the thermal effect and nitrogen-containing species production when ammonia was added, the total mole fraction of ammonia and nitrogen was fixed in the fuel stream. Soot particle size, fringe length, and fringe tortuosity were measured through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). A complementary X-ray photoelectron spectroscopy (XPS) analysis provided information about the chemical bonding of soot. A significant delay in soot growth was observed and the particle size increased with the addition of ammonia. While there was no obvious correlation between the fractal dimension of soot and ammonia mole fraction (XNH3) or sampling location. With the addition of ammonia, the mean fringe length increased reasonably linearly with XNH3 and the fringe tortuosity increased up to XNH3 ≈ 0.17 and then decreased with XNH3, which suggested that ammonia addition led to higher graphitization and lower oxidative activity. The soot from ammonia diluted flame exhibited lower reactivity, implying the delay of soot surface growth. With the addition of ammonia, the value of sp2/sp3 (indication of the graphitization degree of soot particles) did not change much for XNH3 from 0 to 0.17 then increased significantly, which indicated the degree of graphitization of soot particles significantly increased with ammonia addition. The intensity of the N1s peak (indication of the N-containing species in soot) increased with the addition of ammonia. This study confirmed that the addition of NH3 promotes the graphitization of soot.

Nitrogen-origin-determination in NOX formation under ammonia/methane/air co-combustion using a nitrogen-tagged reaction model

This paper proposes a chemical reaction mechanism using the nitrogen element-tracking method for ammonia combustion to obtain the fuel-origin nitric oxides formation characteristics under the co-combustion of ammonia/methane/air diffusion flames. The proposed mechanism was verified by comparing the reaction rates of nitric oxide, nitrous oxide of the element-tracking method with those of the original reaction mechanism (non-tracking). Subsequently, the effects of the mole fraction of ammonia, strain rate of the flow, and preheating temperature of the oxidizer on the emission characteristics of nitric oxides in co-combustion of ammonia/methane/air diffusion flame were investigated. The results showed that the emission index of fuel-originated nitric oxide (N*O) increased slightly, whereas that of oxidizer-originated nitric oxide (NO) decreased with an increase in the ammonia content in the fuel. The emission index of N*O overcame that of NO with an increase in the ammonia concentration in the fuel, and N*O became the dominant molecule in the emission of ammonia/methane co-combustion. Additionally, the reaction path analysis highlighted that a decrease in flame temperature under high strain rate conditions causes an increase in N*O formation as well as the suppression of N2*O decomposition.

High-throughput screening of suitable nitrogen carriers for chemical looping ammonia synthesis

Chemical looping ammonia synthesis is a promising pathway to synthesise ammonia with lower energy and economic costs than the current Haber-Bosch process, for example, by lowering the operating temperature and/or pressure, whilst maintaining comparable NH3 yield. A solid nitrogen carrier is used to mediate the reaction and form ammonia in consecutive steps. In contrast to the Haber-Bosch process, the thermodynamics and kinetics of the reaction are dependent on the properties of the nitrogen carrier. This introduces additional opportunities for novel designs, though finding suitable materials becomes the challenge. Herein, nitrides from the Materials Project database are screened based on their equilibrium nitrogen pressures and nitrogen exchange capacities. The equilibrium nitrogen pressure was used as a proxy to determine the equilibrium ammonia mole fraction of the reaction, a key performance indicator for chemical looping ammonia synthesis. Using high-throughput gas-solid equilibrium calculations, 2515 nitrides were screened, and 111 nitrides were found to achieve sufficiently high ammonia mole fraction at equilibrium to compete with the Haber-Bosch process. Due to large inaccuracies associated with theoretical material properties, the viability of screened candidates cannot be adequately ascertained. The quality and quantity of theoretical and experimental data for nitrogen carriers, respectively, must be improved further to identify suitable materials for chemical looping ammonia synthesis.

Study on the laminar combustion characteristics and kinetic of IC8H18/NH3 premixed flames

To promote the transition to a carbon neutral society, it is necessary to conduct a comprehensive research on the utilization of ammonia contained in gasoline engine. In the present study, the laminar burning velocity (SL), Markstein length (Lb) and flame instability of isooctane/ammonia (IC8H18/NH3)-air mixtures were analyzed at the initial temperatures (T) of 400 K and 470 K, initial pressure (P) of 1–4 atm, ammonia mole fraction (XNH3) of 0%-70% and an equivalence ratio (ϕ) ranging from 0.8 to 1.3. Also, the kinetic mechanism of IC8H18/NH3 as proposed by Cai et al. [Combust. Flame, 162, 2015] was further optimized and compared with the mechanism of CRECK and Cai. The results show that the difference between the Cai and present mechanisms is largely attributable to the variation in sensitivity of NH3 oxidation-related reaction to SL. In addition, it is widely known that SL is dependent on the initial temperature and pressure. In order to improve the applicability of IC8H18/NH3 fuel mixtures, their temperature (α) and pressure (β) coefficients were determined given varying ϕ and XNH3. Meanwhile, a mathematical correlation for SL was also conducted. The analysis of Lb was also conducted to obtain the results showing that Lb decreases with ϕ increasing, except for NH3-air flame. Moreover, as XNH3 increases, Lb varies between lean and rich flames. For lean-flames, NH3 addition leads to a significant decrease in Lb, while for rich-flames, NH3 addition causes a marginal increase in Lb. All these are attributed to the variation in Zel’dovich number (Ze), effective Lewis number (Leeff) and flame thickness (δ). Furthermore, the instability of flame suggests that the addition of ammonia enhances the stability of the flame due to the reduced molecular diffusion effect. Moreover, an increase in XNH3 contributes only to a slight decrease of critical Peclect number (Pecr), and a sharp rise in the critical flame radius (Rcr) results from the increase of flame thickness (δ).

A comprehensive experimental and kinetic modeling study of laminar flame propagation of ammonia blended with propene

Enhancement of ammonia reactivity is crucial for potential applications of ammonia as an engine and gas turbine fuel. A common strategy for improving ammonia's poor reactivity is blending it with more reactive fuels like hydrogen and methane. However, fundamental studies of ammonia combustion with higher hydrocarbons and key intermediate oxidation species of higher alkanes such as propene do not exist. Thus, this work presents an effort to study the laminar flame propagation of ammonia blended with propene. Laminar burning velocity (SL) of NH3/C3H6/air mixtures was measured at 298 K, pressures up to 5 bar, equivalence ratios of 0.7 to 1.3, and various propene to ammonia ratios (i.e.,% propene to ammonia mole fraction, xC3H6 = 10 to 50) in a high-pressure spherical propagating flame vessel. A kinetic model was developed based on our previous work to characterize the combustion behavior of NH3/C3H6/air mixture. The model reasonably agrees with the experimental data and follows the observed trends very well. The results showed that blending NH3 with C3H6 positively enhanced SL of NH3 by promoting the formation of key radicals e.g., O, OH, and H. Relative to a neat ammonia/air mixture, co-firing ammonia with propene leads to a reduced pressure dependence of the laminar burning velocity. However, the reaction H + O2(+M)=HO2(+M) leads to strong pressure dependency of lean NH3/C3H6 mixtures compared to rich mixtures. The model reveals that besides fuel-NO coming from NH3, prompt NO also actively contributes to NO formation. It is seen that N2O formation is significantly suppressed with increasing pressure or increasing C3H6 content in the fuel blend. In contrast to NO and N2O, NO2 concentration increases slightly with an increase in pressure. The reported experimental data and model will be useful in understanding the interaction between NH3 and alkenes.

Investigation of soot suppression by ammonia addition to laminar ethylene flames at varying pressure

Blending ammonia with hydrocarbon fuels is a promising pathway to expedite its use in different industries, as it might offer a good compromise between energy output and minimizing the emissions of both NOx and carbonaceous combustion products, including soot. This work describes the effects of pressure on soot formation in ethylene/ammonia laminar co-flow diffusion flames. A high-pressure chamber was used to study the flames at pressures between 1 to 7 atm, and ammonia addition mole fractions between 0 and 50% (38% in mass fraction). Soot volume fraction and soot temperature measurements using a Spectral Soot Emissions technique show that adding a 50% mole fraction of ammonia at atmospheric pressure reduces soot concentration below the detection limit of the technique and between 70% and 80% for all the remaining studied pressures; that is, the ammonia soot suppression characteristics are demonstrated when increasing pressure. Maximum soot yield and total volumetrically integrated soot volume fraction results are investigated to reveal the contrasting effects of pressure and ammonia addition on the flame structure and the soot formation mechanisms. The results showcase the potential of NH3 in high-pressure applications when mixed with hydrocarbon fuels to reduce carbon emissions.

Chemical kinetic analysis to study the potential of fuel staging in reducing the emissions from NH3/CH4-air combustion at different pressures

The combustion characteristics of ammonia/methane blends to enhance the reactivity of ammonia are investigated successfully for carbon–neutral energy applications. However, commercializing ammonia blends demands techniques to retrench the superfluous NOX emissions under fuel-lean conditions. The present study is a chemical kinetic analysis investigating a new ammonia-methane combustion strategy of injecting ammonia into hot methane combustion gases (ammonia fuel staging) to reduce the NOX propensity without ammonia slip. The combustion kinetics of NH3/CH4 mixtures are studied in chemical reactor networks (CRNs), depicting typical burner flow conditions with and without fuel staging. A comparison of the emission chemistry in the fuel-staged CRN model with the single-stage CRN model is carried out for the ammonia mole fraction (XNH3) in the fuel mixture ranging from 20 to 60 % and the equivalence ratio ranges (ϕ) of 0.6–0.9 at both low and high-pressures of 1 and 10 atm, respectively. It is observed that in the single-stage reactor, the entire reactor is an active reaction zone due to the reduced chemical kinetics of methane in the presence of ammonia. Recognizing the need of raising the radical concentration and temperature in the flame zone to enhance ammonia chemistry, a fuel-staged reactor model is proposed. In the fuel-staged model, staged combustion is achieved by injecting NH3 into the lean methane combustion products for the constant combustor power output of 25 kW. Results show that fuel staging enhances the ammonia chemistry at low and high-pressure conditions of 1 and 10 atm. In the fuel-staged reactor, the flame zone of the second stage is observed to be the active reaction zone with maximum NH3 consumption. At low-pressure conditions, fuel staging leads to a reduction in both CO and NOX emissions without ammonia slip. The NOX reduction is due to the stable availability of NO-consuming NHi radicals for the entire duration of combustion in the flame zone via the reactions: 2NH2→NH3+NH and NH3+OH→NH2+H2O. The CO-consuming reactions: NH2+CO→HNCO+H and OH+CO→H+CO2 further reduces the CO emissions from the second stage. However, the augment in ammonia chemistry adversely affected NOX reduction at high-pressure operating conditions due to the lack of NHi radicals for NO consumption. The study contributed to the NOX reduction from ammonia combustion by providing an active high-temperature combustion zone with a self-sustaining NHi environment.

Spatially resolved broadband absorption spectroscopy measurements of temperature and multiple species (NH, OH, NO, and NH[formula omitted]) in atmospheric-pressure premixed ammonia/methane/air flames

The temperature and multi-species concentration distributions, including NH, OH, NO, and NH3, are of great importance to the research on ammonia/methane combustion mechanisms. However, there is scarce research simultaneously measuring these parameters in atmospheric-pressure ammonia/methane flames. In this work, we proposed a measurement method based on the ultraviolet broadband absorption spectroscopy (BAS) to realize the simultaneous measurement of temperature and multiple species concentrations for the first time (absolute concentrations for OH, NH, and NO, and relative concentrations for NH3). Due to the strong absorption of the electronic transitions in the ultraviolet range, the proposed method offers low detection limits for concentration measurements. Taking NH radicals as an example, the detection limit is as low as 31 ppb at 1700 K. High sensitivity for temperature measurements is also achieved due to the multi-line spectra of OH broadband absorption. Moreover, a fitting strategy was proposed to separate the NO and NH3 spectra near 225 nm, enabling a simultaneous determination of both species. We established an optical system based on the proposed BAS method with a spatial resolution of approximately 0.1 mm to perform spatially resolved measurements of temperature and multiple species on atmospheric ammonia/methane/air flames. The ammonia/methane flames were investigated at different equivalence ratios (0.9, 1.0, and 1.1) and different ammonia mole fractions in fuels (10%, 30%, and 50%). As investigated, NH and NH3 concentrations increased linearly with increasing ammonia mole fractions, while OH and NO concentrations decreased in similar trends with increasing equivalence ratios. The experimentally measured temperature and multi-species concentrations were compared with computational fluid dynamics (CFD) simulations using the mechanism of the CRECK modeling group. Good agreements were achieved in absolute values, spatial profiles, and variations with equivalence ratios and ammonia mole fractions.

Using Ammonia as Future Energy: Modelling of Reaction Mechanism for Ammonia/Hydrogen Blends

To utilize ammonia-based fuels, it is fundamental to understand chemical mechanisms of combustion process, in which reaction characteristics of such a chemical are described in detail. Detailed chemical-kinetics mechanism of ammonia was modelled by an automatic reaction mechanism generation program to investigate characteristics of premixed combustion for ammonia/hydrogen fuel mixture. To develop an accurate model for practical combustion applications, validation of the reaction mechanism was carried out in terms of laminar flame speed under different conditions. Results suggested that the established mechanism model has satisfying performance under different ammonia/hydrogen ratio conditions. Moreover, comparison with other mechanism models demonstrated that the developed model can be used to describe flame propagation of ammonia/hydrogen fuels. Then characteristics of laminar flame speed were predicted under various ammonia concentration and equivalence ratio conditions. Sensitivity analyses showed that ammonia mole fraction has a prominent impact on kinetics of flame speed for ammonia/hydrogen blends. Flame structure analyses showed that hydrogen can enhance ammonia flames with higher light radical concentrations whilst deteriorate NOx emission in exhaust gases.

The influence of ammonia on the laminar burning velocities of methylcyclohexane and toluene: An experimental and kinetic modeling study

Laminar burning velocities of methylcyclohexane and toluene blended with ammonia have been determined using the heat flux method at atmospheric pressure and initial temperature of 338 K, over equivalence ratios ranging from 0.7 to 1.3 and ammonia blending fractions in the binary fuel mixtures from 0 to 90%. It was observed that the addition of ammonia to methylcyclohexane and toluene leads to a decrease in laminar burning velocity that is not proportional to the ammonia mole fraction. Such a burning velocity reduction is due to synergistic thermal and kinetic effects. In addition, ammonia has a slightly higher impact on the burning velocities of toluene due to fuel structure effects. The CRECK detailed kinetic model has been used to interpret the experimental measurements and minor modifications on methylcyclohexane, toluene, and methyl-phenoxy radical chemistry allowed even improved agreement. New experimental results have been compared with predictions of this refined kinetic mechanism. The model provided good predictions of the measurements capturing the effect of equivalence ratio and ammonia fraction ranges investigated. Finally, a mass fraction-based mixing rule was shown to be predictive for binary blends of NH3 with methane and several hydrocarbons typically used to formulate surrogates for practical fuels.

Preparation of Activated Coke by One-Step Activation Method, Ammonization, and K2CO3 Modification of Coal and Biomass

Abstract Coal and biomass are regularly used in the preparation of activated coke. In this paper, coal and biomass (poplar bark) were co-pyrolyzed, and activated coke was prepared by physical “one-step activation,” followed by ammonization and additional activation by potassium carbonate (K2CO3). The activation temperature was set at 800 °C, the time was 60 min, and the activation atmosphere contained 10% steam and 20% CO2 by volume in nitrogen. The physical and chemical properties of activated coke prepared by “one-step activation method” and “two-step activation method” were compared. The mole fraction of ammonia was 5%, 10%, 15%, 20%, respectively, and the concentration of K2CO3 solution was 3%, 6%, 9%, 12%, respectively. It is found that the addition of ammonia can improve the physical and chemical properties and adsorption capacity of activated coke, but the increase of ammonia mole fraction has little effect on the activation reaction. The physical and chemical structure of activated coke impregnated with 3% K2CO3 solution was found to be well developed. The surface of the produced activated coke prepared by the above activation methods has abundant microporous structure and organic functional groups; hence, the final product is suitable to be used as an adsorbent in practical applications.

Applying heat flux method to laminar burning velocity measurements of NH3/CH4/air at elevated pressures and kinetic modeling study

Combustion of ammonia (NH3) blended fuels under elevated pressure conditions is critical for adopting this non-carbon fuel in the energy system for decarbonization. In the present work, laminar burning velocities of ammonia/methane(CH4)/air mixtures were measured using the heat-flux method at the pressure from 1 to 5 atm with the mixture equivalence ratios ranging from 0.6 to 1.6 and the mole fraction of NH3 ranging from 0 to 1.0. The relatively completed results obtained at elevated pressures were then used for validating and modifying the kinetic mechanisms (CEU-NH3-Mech 1.0) leading to a new version (CEU-NH3-Mech-1.1). Experimental results of NH3/H2/air in the present work, NH3/H2/CO/air mixtures measured on the same setup and reported in our previous works were also considered in the development of the kinetic mechanism. It was found that the CEU-NH3−Mech-1.1 can predict well the laminar flame speed, ignition delay time and species concentration in the ammonia oxidation at high temperatures for both NH3/CH4/air and NH3/H2/CO/air mixtures in a wide range of equivalence ratios and elevated pressures, including oxygen-enriched combustion conditions. The present experimental results also show that the value of pressure exponent (β) varies with the mole fraction of ammonia and behaves differently for the mixtures of ammonia blending into CH4 and H2. The kinetic and sensitivity analyses show that the sensitive reactions for β are weakly correlated to those for the laminar burning velocity, indicating that β can also work as a potential parameter for validating kinetic mechanisms. Ammonia content in the NH3/CH4/air mixtures determines the pressure exponent variation at over-rich equivalence ratios and reaction pathway variation in the post-flame zone. This work also clarifies the utilization of ammonia containing fuels in rich-lean combustion strategies.

Modeling and performance improvement of an industrial ammonia synthesis reactor

Ammonia production from synthesis gas is one of the most important processes in the petrochemical industry. A heterogeneous model is used to improve the efficiency of an industrial ammonia synthesis reactor including three adiabatic catalyst beds. In this model, the effectiveness factor was determined by considering the diffusion reaction equation. Reasonable agreement was achieved between simulated results and industrial data in terms of reactor component mole fraction and temperature. This mathematical model was modified to use for improving the ammonia converter performance and predict the effectiveness factors, nitrogen fractional conversion, temperature, and hydrogen and ammonia mole fraction profiles. The competency of the modified model has been investigated for industrial application by manipulating the reactor operation conditions and observing their effects on reactor output and results revealed the reliability of the developed model.The model is employed to obtain the optimum temperature profiles of each catalyst bed in the reactor based on equilibrium curve of ammonia synthesis reaction. Operating condition of the reactor was changed to achieve the optimum temperature profile according to model results by manipulating the reactor quench valves. After implementing changes in the reactor, the reactor performance and efficiency has improved with increasing ammonia conversion from 15.26% to 15.45% that increases ammonia production by 3t/d and energy saving by 1.66 Gj/h. The differential temperature of the first catalyst bed significantly increased by 6 °C and overall differential temperature through ammonia converter is increased by around 4 °C. This increases steam production by 2 t/h through loop boiler.

Comparative Effect of Ammonia Addition on the Laminar Burning Velocities of Methane, n-Heptane, and Iso-octane

Adiabatic laminar burning velocities for methane, n-heptane, and iso-octane blended with ammonia were experimentally determined using the heat flux method. The flames were stabilized at atmospheric pressure and at an initial temperature of 338 K, over equivalence ratios ranging from 0.7 to 1.4 and ammonia blending fractions in the binary fuel mixtures from 0 to 90%. These experiments are essential for the development, validation, and optimization of chemical kinetic models, e.g., for the combustion of gasoline-ammonia fuel mixtures. It was observed that the addition of ammonia to methane, n-heptane, and iso-octane leads to a decrease in the laminar burning velocity that is not proportional to the ammonia mole fraction. In addition, ammonia has the same impact on the burning velocities of n-heptane and iso-octane but a slightly higher effect on those of methane. Such a burning velocity reduction is due to synergistic thermal, kinetic, and indirect transport effects. New experimental results were compared to predictions of the POLIMI detailed chemical kinetic mechanism. An overall good agreement between the measurements and simulated results was observed for the laminar burning velocities over the equivalence ratio and ammonia fraction ranges investigated. (Less)

Experimental study and kinetic analysis of the laminar burning velocity of NH3/syngas/air, NH3/CO/air and NH3/H2/air premixed flames at elevated pressures

Mixing ammonia with syngas can be a promising way to overcome the low reactivity of ammonia, allowing it to find usage in IGCC (Integrated Gasification Combined Cycle) systems and gas turbines for power generation. However, fundamental experimental data on laminar burning velocity of NH3/syngas/air are rather scarce, especially at elevated pressures. This information is critical for the development and validation of reaction mechanisms and advances in combustor design. In the present work, measurements of the laminar burning velocities (SL) of NH3/syngas/air, NH3/CO/air, and NH3/H2/air premixed flames were performed by the heat flux method at pressures up to 5 atm, equivalence ratios ranging from 0.7 to 1.6, ammonia mole fractions in the fuel mixture from 0.2 to 1.0 in the NH3/syngas/air mixtures and 0.03–1.0 in the NH3/CO/air mixtures. Several recently published ammonia oxidation mechanisms were tested against the present experimental data. The measurements and predictions of SL exhibit discrepancies especially for NH3/H2/air flames at elevated pressures. The pressure exponent factors, β, characterizing burning velocity at elevated pressure via empirical power-law correlation SL/SL0 = (P/P0)β are extracted from the measured SL and compared with the numerical results. The thermal, diffusion, and chemical effects of blending syngas with ammonia on SL of the mixtures are distinguished, and the dominant role of the adiabatic flame temperature on the variation of the pressure exponent β is discussed. Kinetic modeling and sensitivity analyses showed that reactions of NHi to N2Hi (i = 0–4) species affect the predicted SL under rich conditions. At elevated pressures, these reactions also affect the NO formation via third-body collision reactions and NHi + NO reactions. Even for rich flames, the ammonia consumption is favored with the addition of syngas which also promotes NO formation by enriching the H and OH radical pools and increasing the flame temperature. The addition of hydrogen or carbon monoxide has equally promoting effect on the ammonia decomposition and NOx formation although their flame speed differs a lot.

Thermodynamic potential of a novel plasma-assisted sustainable process for co-production of ammonia and hydrogen with liquid metals

In the present article, the thermodynamic potential of a sustainable plasma-assisted nitrogen fixation process for co-production of ammonia and hydrogen is investigated. The developed process takes advantage of chemical looping system by using a liquid metal such as gallium to drive nitrogen fixation reaction using three reactors including reactor R1 to produce gallium nitride from gallium and nitrogen, reactor R2 to produce ammonia and hydrogen from gallium nitride, and plasma reactor R3 to convert gallium oxide to pure gallium. The results of the thermodynamic assessments showed that the proposed reactions are spontaneous and feasible to occur in the reactors. Likewise, the first two reactions are exothermic with ΔH=-230kJmol and ΔH=-239kJmol in the reactors R1 and R2, respectively with an equilibrium chemical conversion of 100%. The plasma reactor requires thermal energy to drive an endothermic reaction of gallium oxide dissociation withΔH=+870kJmol. Thermochemical equilibrium analysis showed that the molar ratio of steam to GaN, as well as the operating pressure and temperature of reactor R2 are the main operating parameters identifying the product composition in the reactor such that by increasing the temperature, the molar ratio of hydrogen to ammonia increases. However, by increasing the molar ratio of steam/GaN (φ value) from 0.1 to 1, the hydrogen content of the reactor increases from 45% to 70% at 400 °C. For φ > 1.0, the hydrogen content decreases while more hydrogen participate in the formation of NH3 thereby increasing the mole fraction of ammonia in the reactor. The equilibrium chemical conversion of all three reactors is expected to reach the completion point (χ = 100%) due to the highly negative Gibbs free energy of the liquid metal-based reactions together with a large thermal driving force supported by thermal plasma reactor. Finally, a scalability study points at a possible use of the new disruptive process design at small scale, and possible industrial transformation scenarios for a distributed production at a local site of consumption are depicted.

Effects of concentration and pressure on the aqueous solvation structure of ammonia and composition dependent ion solvation scenario in water-ammonia mixtures

The hydrogen bonding structure of water and its dynamics in aqueous ammonia solution is investigated by using classical molecular dynamics simulations. We have considered eight different mole-fraction of ammonia, ranging from xNH3 = 0.02 to 0.30, and the pressure variation is carried out on the concentrated ammonia solution (xNH3=0.30), ranging from P = 0.1 MPa to 800 MPa, at 293K. The radial distribution function indicates that the addition of ammonia facilitates the structural organization of water, which is further supported by increasing values of asphericity parameter. But, the tetrahedral order parameter values suggest an angularly disordered arrangement of water molecules. The self-diffusion coefficient values of water pass through shallow minima, whereas it increases for ammonia with ammonia concentration in the solution. The application of pressure causes a sharp decline in translation mobility of ammonia compared to water, but at very high pressure (above 600 MPa), both the species diffuse at the same rate. The rotational relaxation of ammonia is approximately three times faster than water in the solution, which further increases along with concentration as well as pressure. A similar trend is also observed in the case of structural reorganization of water-water and water-ammonia hydrogen bonds. The water-water and water-ammonia hydrogen-bond lifetimes are increasing with ammonia concentration, whereas it passes through shallow maxima with the application of pressure. In this study, we have compared two different Lennard-Jones (LJ) combination rule, which causes negligible differences in the calculated properties. The solvation scenario of different ionic (Na+, Cl−) and neutral (Cl0) solutes are also analyzed along with the coordination number. It is observed that the hydration shell of ion is mainly dominated by water molecules in the solution.