Effect of Ammonia on Laminar Combustion Characteristics of Methane-Air Flames at Elevated Pressures.

In this paper, laminar combustion characteristics of methane/ammonia/air flames are numerically investigated using the Chemkin/Premix code. The initial temperature is set as 298 K; the initial pressures are set as 1, 2, 5, 10, and 20 atm; and the equivalence ratios are set as 0.8-1.6. Laminar burning velocity (LBV); adiabatic flame temperature (AFT); net heat release rate (NHRR); and the mole fractions of H, NH2, NO, NO2, and HCN at stoichiometric ratio are studied with ammonia (NH3) addition. Meanwhile, temperature sensitivity and rate of production (ROP) are analyzed. The results show that with the increase of the initial pressures, LBV decreases and AFT and NHRR increase. With the increase of ammonia doping ratios, LBV, AFT, and NHRR decrease. From temperature sensitivity analyses, the main reactions that promote temperature rise are R39 (H + O2 < = > O + OH), R100 (OH + CH3 < = > CH2(S) + H2O), R102 (OH + CO < = > H + CO2), and R122 (HO2 + CH3 < = > OH + CH3O). The main reactions that inhibit temperature rise are R53 (H + CH3(+M) < = > CH4(+M)), R36 (H + O2 + H2O < = > HO2 + H2O), and R46 (H + HO2 < = > O2 + H2). For the rate of production of the free radical pool, the trends of H and NO are consuming first and then producing, and the trends of NH2, NO2, and HCN are the opposite. The pathway from methane to carbon dioxide is CH4 → CH3 → CH3O → CH2O → HCO → CO → CO2, and the pathway from ammonia to nitrogen is NH3 → NH2 → NH/HNO → NO/NO2 → N2.

Introduction

Coal, oil, and natural gas are the three traditional fossil fuels, and their combustion produces CO2. Among conventional energy, natural gas is considered to be one of the most potential clean alternative energies.[1−4] It has the advantages of high energy utilization efficiency and thermal efficiency, convenient transportation and storage, low emission, good antistorm performance (octane number is generally between 115 and 130), and low price. Therefore, it is widely used in industries and transportation. The main component of natural gas is CH4, which is one of the important greenhouse gases and responsible for 20% of global warming effects in the world (excluding water vapor).[5]

In recent years, the energy crisis and the greenhouse effect have become hot topics. The energy crisis has forced us to solve the future energy problem. The world is striving to control greenhouse gas emissions to achieve the Paris Agreement’s goal of limiting the global average temperature rise to less than 2 °C based on the preindustrial global average temperature level. To achieve this goal, the applications of carbon capture, utilization, and storage (CCUS) technology and using carbon-free fuel are the key to a low-carbon future.[6,7]

Hydrogen emitted during the combustion process is not a pollutant, but it is inconvenient to carry and store. The use of ammonia as a hydrogen transport carrier is highly anticipated. NH3 has high hydrogen energy and therefore can be used as a carrier of hydrogen energy. It is also a potential carbon-free alternative fuel, with no carbon in its structure and no CO2 generated, contributing much less to global warming than conventional fuels. In addition, NH3 has the added attraction that it can be sold in the international market; therefore, it has a wide range of applications. However, pure ammonia has the disadvantages of low combustion reactivity,[8−13] poor flame stability, very low burning velocity,[13] and high NOx emission; therefore, it is difficult for it to burn effectively in the combustion chamber and engine. One way to address these shortcomings is to burn NH3 with a mixture of hydrocarbons (such as natural gas), which does not completely eliminate but reduces CO2 emission and can serve as a springboard for pure ammonia combustion. Especially in recent years, the mixture of NH3 and CH4 as fuel for gas turbines has attracted increasing attention.[14−18] Many scholars have carried out a series of numerical and experimental studies on the combustion of methane and ammonia. Montgomery et al.[19] experimentally studied the volume fraction of soot and mole fraction of gaseous matter in a CH4–NH3 flame and analyzed the addition of NH3, which had a strong inhibitory effect on soot formation. Tian et al.[20] reported the experimental and model study of NH3/CH4/O2/Ar premixed flame under low pressures at the stoichiometric ratio. Xiao et al.[21] simulated NH3/CH4 combustion, and the results showed that the ammonia component and equivalence ratio had important effects on the ignition delay time. Honzawa et al.[22] found that the NH3/CH4/air flame was very sensitive to the concentrations of H and OH free radicals and gas temperature. Liu et al.[23] experimentally measured the laminar flame velocity of mixtures with different NH3/CH4 ratios. Ichikawa et al.[24] investigated the burning velocity of the CH4/NH3/air turbulent premixed flame at high pressure. Hayakawa et al.[25−27] determined the laminar burning velocity (LBV) and the Markstein length of the NH3/air premixed flame under different pressures and analyzed the production of NO and the laminar burning velocity. Okafor et al.[28] experimentally measured the unstretched laminar burning velocity and proposed an optimized reduction reaction mechanism. Mathieu et al.[29] measured the laminar burning velocity of the H2/NH3/air jet flame through experiments and upon investigation found that the flame velocity and the concentrations of H, O, and OH free radicals had the greatest difference. Han et al.[30] experimentally studied the laminar burning velocity of NH3/air, NH3/H2/air, NH3/CO/air, and NH3/CH4/air mixtures and carried out related kinetic modeling. Kumar and Meyer[31] experimentally determined the laminar burning velocity of a H2/NH3/air flame, taking the heat loss of the flame into account. Li et al.[32] experimentally studied the combustion characteristics and NOx generation of the H2/NH3 laminar flame under different equivalence ratios and hydrogen concentrations.

In this paper, the combustion characteristics of the CH4/NH3/air laminar premixed flame under the initial pressures of 1, 2, 5, 10, and 20 atm are numerically simulated, which is a good supplement to the lack of simulation data in a high-pressure environment. Ammonia doping ratios vary from 0 to 80%, and methane and ammonia occupy the main fuel positions. Laminar burning velocity (LBV), adiabatic flame temperature (AFT), rate of production (ROP), temperature sensitivity, and reaction pathway analysis are expressed in detail with the increase of ammonia blend ratios, which has certain enlightening significance for emission reduction and the trend of laminar burning characteristics.

Calculation Method

The simulation used one-dimensional premixed laminar freely propagating flames in the Chemkin/Premix code to simulate methane/ammonia/air combustion under different conditions and used the Okafor mechanism,[28] whose reaction kinetics is based on the complete carbon chemistry of GRI Mech 3.0[33] and important ammonia oxidation, NO production, and reduction of the mechanism by Tian et al.[34] In the Okafor mechanism, it was found that GRI Mech 3.0 overpredicted NO production from NH3 oxidation, as the impact of the reaction NH + H2O < = > HNO + H2 may not be significant in fuel NO chemistry. The maximum initial pressure is set as 20 atm. The ammonia doping ratios are set as 0, 0.2, 0.5, and 0.8. The equivalence ratios are from 0.8 to 1.6. In this numerical simulation, the Soret effect has been considered.

Table gives numerical simulation examples. During the numerical simulation, the grid converges to 500, and the gradient and curvature are both 0.04. To meet the simulation accuracy, the relative and absolute errors are set to 10–4 and 10–6, respectively.

Table 1

Simulation Conditions

		mole fraction of reactant
P (atm)	T (K)	α	O2	N2	CH4	NH3
1–20	298	0	0.193727–0.179795	0.728782–0.67637	0.077491–0.143836	0
		0.2	0.191606–0.176174	0.720803–0.662752	0.070073–0.128859	0.017518–0.032215
		0.5	0.187135–0.168761	0.703986–0.634863	0.038979–0.068994	0.090951–0.160986
		0.8	0.179795–0.157186	0.67637–0.591317	0.028767–0.050299	0.115068–0.201198

The equivalence ratio (Φ) in the mixture is defined as[35]where F and A denote the volume fractions of fuel and air in the mixed system, respectively. F/A denotes the actual ratio of fuel and air, and (F/A)st denotes the ratio when fuel and air in the theoretical state are completely combusted.

We define the ammonia doping ratio in the mixture aswhere XNH and XCH denote the volume fractions of ammonia and methane, respectively.

The formula[28] for the mixed combustion of methane and ammonia is

Results and Discussion

Effects of the Ammonia Doping Ratio and Elevated Pressure on LBV and AFT

The study of laminar premixed combustion of CH4/NH3/air under different conditions is helpful to understand the effects of the ammonia doping ratio and initial pressure for combustion characteristics. LBV and AFT are the most important parameters to characterize combustion and control flame propagation.

The simulation results are obtained using the Okafor mechanism[28] with 59 species and 356 elementary reactions. Figure compares the simulation results of the Okafor mechanism with other mechanisms and experimental results. As can be seen, the simulation results are not very different from the experimental results, which further verifies the feasibility of the Okafor mechanism used in this paper. In Figure a, the simulation results are compared with the experimental results of Han et al.[30] under different ammonia doping ratios at 1 atm and 298 K. LBV is well predicted at lean combustion and overpredicted at rich combustion. With the increase of the ammonia doping ratios, the difference becomes larger and appears at a smaller equivalence ratio. In Figure b, the simulation results are compared with the experimental results of Okafor et al.[28] under different ammonia doping ratios and initial pressures at 298 K. The tendency of the simulation results is consistent with that of the experiment, while LBV is underpredicted when the ammonia doping ratio is 0 and overpredicted when the ammonia doping ratio is 0.2. In Figure c, the simulation results are compared with the simulation results of Li et al.[36] under different ammonia doping ratios at 1 atm and 298 K, and it can be seen that the trend of LBV is highly consistent, but LBV of the Okafor mechanism is lower than that of the Konnov mechanism.

Figure 1

Comparison of the simulation results of the Okafor mechanism with other mechanisms and experimental results.

Figure exhibits LBV at different initial pressures and the same ammonia doping ratios. With the increase of the initial pressure, LBV gradually decreases. The initial pressure has an inhibiting effect on LBV. The reasons for the decrease of LBV are as follows. On the one hand, the initial pressure changes the combustible fuel volume, resulting in a change of density; on the other hand, the initial pressure has an impact on chemical reactions, thereby decreasing LBV. With the increase of equivalence ratios, LBV increases first and then decreases. The peak appears at Φ = 1.05. This is because the condition changes from lean combustion to rich combustion, and the CH4/NH3 mixture cannot fully burn, which leads to the decrease of AFT and the inhibiting effect on LBV. At different ammonia doping ratios and the same initial pressures, with the increase of the ammonia doping ratios, LBV gradually decreases. Because the molecular weight of NH3 is similar to that of CH4, increasing the proportion of NH3 has a greater impact on the volume of the main body occupied by CH4. The combustion efficiency of ammonia is lower than that of methane.

Figure 2

LBV at different ammonia doping ratios and initial pressures.

Figure a illustrates AFT at different initial pressures and different ammonia doping ratios. It can be clearly seen that with the increase of initial pressures, AFT increases. The increase range of AFT at the nearby peak is greater than that at rich combustion and lean combustion. AFT keeps decreasing with increasing ammonia doping ratios. With the increase of equivalence ratios, AFT increases first and then decreases, which is because the adiabatic temperature of NH3 is lower than that of CH4. The peak of AFT appears at Φ = 1. Figure b shows the maximum AFT of the methane/ammonia/air combustion process. It can be seen that with an increasing ammonia doping ratio, the reduction of AFT is around 5% at different pressures. The biggest drop occurred at P = 10 atm, and the smallest drop occurred at P = 2 atm.

Figure 3

AFT at different conditions. (a) Initial pressures and ammonia doping ratios and (b) the maximum AFT.

Effects of the Ammonia Doping Ratio and Elevated Pressure on NHRR

Figure reports NHRR at different ammonia doping ratios and different initial pressures at the stoichiometric ratio. It can be seen that with the increase of the initial pressures, the peaks of NHRR are accompanied by exponential growth and they move toward the high-temperature region, indicating that more reaction heat is released in the high-temperature region because the increase of initial pressures strengthens the collision rate of activated molecules and increases the intensity of reactions. It is worth noting that with the increase of the ammonia doping ratios, NHRR decreases and the peaks move toward the low-temperature region. The heat capacity of ammonia is large, and the flame speed and adiabatic temperature are lower than those of methane. As the ammonia doping ratio increases, a part of ammonia absorbs heat, which reduces the peak of NHRR and moves toward the low-temperature region. From the view of the peak, the effects of initial pressure and ammonia doping ratio on NHRR are very obvious.

Figure 4

NHRR at different ammonia doping ratios and different initial pressures.

Effects of the Ammonia Doping Ratio and Elevated Pressure on Radical Pool

Free radical H plays an important role in the change of LBV. For ammonia mixing in methane, there are numerous H free radicals released in the combustion process. Figure exhibits the change curve of H mole fraction at different initial pressures and different equivalence ratios. It can be seen that with the increase of the equivalence ratios, the peak of H from lean to rich combustion increases first and then decreases. The peak first moves upstream and then downstream. This is because when CH4 is fully combusted at the stoichiometric ratio, a large number of H free radicals produced at the initial stage of combustion react with related free radicals; therefore, the peak appears downstream. With the increase of initial pressures, the peak of H decreases and moves upstream.

Figure 5

Mole fraction of H at the same ammonia doping ratio and different initial pressures.

Figure depicts the change curve of NH2 mole fraction at different initial pressures and different equivalence ratios. It can be seen that with the increase of equivalence ratios, the peak of NH2 from lean to rich combustion increases first and then decreases. The peak first moves upstream and then downstream. With the increase of initial pressures, the peak of NH2 decreases and moves upstream.

Figure 6

Mole fraction of NH2 at the same ammonia doping ratio and different initial pressures.

Because CH4 mixes with NH3 for combustion, it may produce high-temperature NOx. Thus, it is necessary to analyze the emission of NOx. In this paper, the main emissions NO and NO2 were analyzed. Figure shows the change curve of NO mole fraction at different initial pressures and different equivalence ratios. It can be seen that with the increase of equivalence ratios, the peak of NO from lean to rich combustion increases first and then decreases. This is because when the ammonia content is larger, the unreacted ammonia recombines with NO, resulting in the decrease of NO. The peak first moves upstream and then downstream. With the increase of initial pressures, the peak of NO decreases and moves upstream, indicating that the initial pressure has an inhibiting effect on NO formation. This has a certain enlightening significance for emission reduction. Similar conclusions can be found in previous studies on ammonia combustion with CH4/NH3/Air[37,38] and NH3/H2/Air[39,40] flames.

Figure 7

Mole fraction of NO at the same ammonia doping ratio and different initial pressures.

Figure reports the change curve of NO2 mole fraction at different initial pressures and different equivalence ratios. It can be seen that with the increase of equivalence ratios, the peak of NO2 decreases at the atmospheric pressure but remains flat from 1.0 to 1.2 with increasing initial pressure. The peak first moves upstream and then downstream. With the increase of initial pressures, the peak of NO2 increases and moves upstream.

Figure 8

Mole fraction of NO2 at the same ammonia doping ratio and different initial pressures.

Figure provides the change curve of HCN mole fraction at different initial pressures and different equivalence ratios. It can be seen that with the increase of equivalence ratios, the peak of HCN increases. The peak first moves upstream and then downstream. With the increase of initial pressures, the peak of HCN decreases and moves upstream. It is worth noting that the peak at Φ = 1.2, P = 1 atm is much larger than those at other conditions.

Figure 9

Mole fraction of HCN at the same ammonia doping ratio and different initial pressures.

Figure S1 provides the change curve of H, NH2, NO, NO2, and HCN mole fraction at different ammonia doping ratios under the condition of the stoichiometric ratio (in the Supporting Information).

Temperature Sensitivity Analysis

The temperature sensitivity analysis uses the following formulawhere i denotes the ith component, c denotes the component concentration, T denotes AFT, l denotes the distance from the jet, and ∂c/∂l denotes the first-order sensitivity coefficient. Temperature and laminar combustion characteristics have a very strong correlation. The temperature sensitivity can be analyzed to understand the influence of the equivalent ratio and initial pressure change on the laminar combustion speed.

Figure analyzes temperature sensitivity at the same initial pressures and different ammonia doping ratios. It can be seen that when the initial temperature is 298 K and α = 0.5, the main reactions that promote temperature rise are R39 (H + O2 < = > O + OH), R100 (OH + CH3 < = > CH2(S) + H2O), R102 (OH + CO < = > H + CO2), and R122 (HO2 + CH3 < = > OH + CH3O). With the increase of initial pressures, the ammonia doping ratio corresponding to the maximum temperature sensitivity coefficient of R39 becomes smaller. The main reactions that inhibit temperature rise are R53 (H + CH3(+M) < = > CH4(+M)), R36 (H + O2 + H2O < = > HO2 + H2O), R88 (OH + HO2 < = > O2 + H2O), and R46 (H + HO2 < = > O2 + H2). With the increase of the ammonia doping ratios, the maximum temperature sensitivity coefficient of R53 decreases. For enhancing effect R39 (H + O2 < = > O + OH), the enhancing effect keeps increasing at the atmospheric pressure. But the enhancing effect increases first and then decreases as the pressure increases.

Figure 10

Temperature sensitivity at the same initial pressures and different ammonia doping ratios.

Figure analyzes the temperature sensitivity at the same ammonia doping ratio and different initial pressures. With the pressure increasing, the maximum promoting temperature sensitivity coefficient of R39 is at P = 10 atm. With the increase of the ammonia doping ratios, the maximum temperature sensitivity coefficient of R39 is at P = 5 atm. With the increase of initial pressures, the maximum temperature sensitivity coefficient of R53 increases. For the temperature sensitivity, the main promoting reactions are R39 (H + O2 < = > O + OH), R102 (OH + CO < = > H + CO2), R100 (OH + CH3 < = > CH2(S) + H2O), and R122 (HO2 + CH3 < = > OH + CH3O) and the main inhibiting reactions are R53 (H + CH3(+M) < = > CH4(+M)), R36 (H + O2 + H2O < = > HO2 + H2O), R88 (OH + HO2 < = > CH3 + H2O), and R101 (OH + CH4 < = > CH3 + H2O). It is worth noting that the inhibiting effects R46 (H + HO2 < = > O2 + H2), R54 (H + CH4 < = > CH3 + H2), and R88 (OH + HO2 < = > O2 + H2O) appear under elevated pressure.

Figure 11

Temperature sensitivity at the same ammonia doping ratio and different initial pressures.

Figure analyzes the temperature sensitivity at different equivalence ratios at atmospheric pressure. With the increase of equivalence ratios, the main promoting and inhibiting effects are both strengthened. But it is worth noting that the maximum sensitivity coefficient of R39 (H + O2 ≤> O + OH) appears in α = 0.5, Φ = 1.2, which is different from lean combustion and complete combustion. As the ammonia doping ratio increases, the sensitivity coefficient of R39 gradually increases except at an equivalence ratio of 1.2 and when the ammonia doping ratio increases from 0.5 to 0.8.

Figure 12

Temperature sensitivity at different equivalence ratios.

Effects of the Ammonia Doping Ratio and Elevated Pressure on the Rate of Production

Figure displays the total rate of production (ROP) and the first five ROPs of free radicals under the conditions of Φ = 1.0, α = 0.5, and P = 1 atm. Figure a gives the ROP of H. R39 (H + O2 < = > O + OH) and R53 (H + CH3(+M) < = > CH4(+M)) are the main consuming reactions, while R3 (O + H2 < = > H + OH), R10 (O + CH3 < = > H + CH2O), and R11 (O + CH3 < = > H + H2 + CO) are the main producing reactions. Figure b shows the ROP of NH2. R245 (NH2 + H < = > NH + H2), R246 (NH2 + O < = > HNO + H), R247 (NH2 + O < = > NH + OH), R248 (NH2 + OH < = > NH + H2O), and R249 (NH2 + HO2 < = > NH3 + O2) are the main consuming reactions. Figure c represents the ROP of NO. R220 (N + NO < = > N2 + O) and R228 (HO2 + NO < = > NO2 + OH) are the main consuming reactions, while R221 (N + O2 < = > NO + O), R222 (N + OH < = > NO + H), and R231 (NO2 + H < = > NO + OH) are the main producing reactions. Figure d displays the ROP of NO2. R231 (NO2 + H < = > NO + OH) and R230 (NO2 + O < = > NO + O2) are the main consuming reactions, while R228 (HO2 + NO < = > NO2 + OH), R229 (NO + O + M < = > NO2 + M), and R243 (NH + HONO < = > NH2 + NO2) are the main producing reactions. Figure e displays the ROP of HCN. R310 (HCN + M < = > H + CN + M), R311 (HCN + O < = > NCO + H), and R312 (HCN + O < = > NH + CO) are the main consuming reactions, while R301 (CN + H2 < = > HCN + H) and R299 (CN + H2O < = > HCN + OH) are the main producing reactions. For rate production of the free radical pool, the trend of H and NO is consuming first and then producing, and the trend of NH2, NO2, and HCN is the opposite.

Figure 13

Rate of production of the free radical pool.

A detailed rate of production of the free radical pool can be found in the Supporting Information. Figure S2 gives the total ROP and the first five ROPs of free radicals at different equivalence ratios under the conditions of P = 1 atm and α = 0.5. Figure S3 shows the total ROP and the first five ROPs of free radicals at different ammonia doping ratios under the conditions of P = 1 atm and Φ = 1.0. Figure S4 demonstrates the total ROP and the first five ROPs of free radicals at different initial pressures under the conditions of α = 0.5 and Φ = 1.0.

Combustion Reaction Pathways

CH4 → CO2 and NH3 → N2 reaction pathways in α = 0.5, T = 298 K, P = 1 atm, and Φ = 1.0 are shown in Figure . The selected species are the top 10 maximum. The pathway from methane to carbon dioxide is CH4 → CH3 → CH3O → CH2O → HCO → CO → CO2. In the reaction process, CO2 is mainly produced from CO. The pathway from ammonia to nitrogen is NH3 → NH2 → NH/HNO → NO/NO2 → N2. The NNH reacts with O2 to produce plenty of N2. N2O is mainly produced by NO2 reacting with NH2. NO can also be converted to N2 by reacting with NH2. The main inhibition reactions of NO are reactions with NH2 and HO2. The biggest difference between ammonia and methane combustion is that CO2 is relatively stable and does not easily participate in chemical reactions and can be used as the final product. However, NO2 has a strong oxidizing property and is easily reduced to NO. Both NO and NO2 can react with NH2 and directly produce more stable precursors NNH and N2O of N2 or N2. Therefore, the combustion of ammonia usually takes H2O and N2 as the final products.

Figure 14

Combustion reaction pathways of CH4 → CO2 and NH3 → N2.

Conclusions

The simulation used one-dimensional premixed laminar freely propagating flames in the Chemkin/Premix code to simulate methane/ammonia/air combustion under different conditions and the Okafor mechanism. During the numerical simulation, the grid converges to 500, and the gradient and curvature are both 0.04. To meet the simulation accuracy, the relative and the absolute errors are set as 10–4 and 10–6, respectively. The combustion conditions are at equivalence ratios (0.8–1.6) and ammonia doping ratios (0–0.8). The initial pressure, equivalence ratio, and ammonia doping ratio are used to analyze the combustion characteristics of LBV, AFT, NHRR, free radical pool, temperature sensitivity, ROP, and reaction pathways. The major conclusions are summarized as follows.

Using Chemkin to simulate CH4/NH3/air combustion, as the proportion of ammonia increases, LBV, AFT, and NHRR continuously decrease, and with the initial pressure increasing, the LBV decreases and AFT and NHRR increase. For the trend of NHRR with the ammonia doping ratio increasing, the peaks of NHRR continually move to the high-temperature area.

For radical pool, the peaks of H, NH2, NO, NO2, and HCN continuously decrease with the initial pressure increasing; the peaks of H, NH2, and NO increase first and then decrease with the equivalence ratio increasing, but the peak of NO2 decreases continuously and the peak of HCN increases.

In the CH4/NH3/air combustion, the most important temperature-enhancing reaction is H + O2 < = > O + OH, and the most significant temperature-inhibiting reaction is H + CH3(+M) < = > CH4(+M).

For the ROP of the free radical pool, the H and NO behave as consuming first and then producing, and the NH2, NO2, and HCN exhibit an opposite trend.

In the combustion reaction pathways, the pathway from methane to carbon dioxide is CH4 → CH3 → CH3O → CH2O → HCO → CO → CO2, and the pathway from ammonia to nitrogen is NH3 → NH2 → NH/HNO → NO/NO2 → N2.