Simplified Mechanisms of Nitrogen Migration Paths for Ammonia-Coal Co-Combustion Reactions

Abstract

Ammonia–coal co-combustion has emerged as a promising strategy for reducing carbon emissions from coal utilization, although its underlying reaction mechanisms remain insufficiently understood. The Chemkin simulation of zero-dimensional homogeneous reaction model and entrained flow reaction model was employed here, and the ROP (rate of production) and sensitivity analysis was performed for analyzing in-depth reaction mechanisms. The nitrogen conversion pathways were revealed, and the mechanisms were simplified. Based on simplified mechanisms, molecular-level reaction pathways and thermochemical conversion networks of nitrogen-containing precursors were established. The results indicate that NO emissions peak at a 30% co-firing ratio, while N₂O formation increases steadily. The NH radical facilitates NO reduction to N₂O, with NH + NO → N₂O + H identified as the dominant pathway. Enhancing NNH formation and suppressing NCO intermediates are key to improving nitrogen conversion to N₂. This paper quantifies the correlation between NO_x precursors such as HCN and NH₃ and intermediates such as NCO and NNH during ammonia–coal co-firing and emphasizes the important role of N₂O. These insights offer a molecular-level foundation for designing advanced ammonia–coal co-combustion systems aimed at minimizing NO_x emissions.

1. Introduction

Coal remains China’s dominant primary energy source, with national commercial coal consumption reaching approximately 4.85 billion tons in 2024, marking a 4.3% year-on-year increase [1]. Amid the implementation of dual-carbon goals, reducing carbon emissions has become a critical research focus. Ammonia energy, as one of the green renewable energy sources, holds significant development potential. Co-firing ammonia with coal powder represents an effective approach to mitigate carbon emissions. The primary objective of ammonia–coal co-combustion is to enhance combustion efficiency by utilizing ammonia as a supplementary fuel, optimizing combustion characteristics, and reducing pollutant emissions such as CO₂ and NO_x, thereby achieving more efficient and cleaner energy utilization. China successfully conducted a 40-megawatt ammonia–coal co-firing boiler demonstration test for the first time in 2022 [2], which demonstrated significant reductions in coal-fired carbon emissions. The synergistic effect of ammonia, coal char, and volatiles improved combustion efficiency. However, in high-temperature oxygen-rich atmospheres, nitrogen tends to convert into NO_x, posing challenges for boiler flue gas treatment [3]. Boiler experiments have demonstrated the feasibility of ammonia–coal co-firing in reducing carbon emissions. However, there is currently little research on nitrogen emissions during the ammonia–coal co-firing process, especially on the mechanism of nitrogen element migration and transformation pathways. Additionally, due to the extreme complexity of the ammonia–coal co-combustion process, the underlying combustion mechanisms remain poorly understood.

To better control emissions and optimize boiler performance, it is essential to investigate the fundamental reaction mechanisms underlying ammonia–coal co-combustion. During the ammonia–coal co-combustion process, a series of chemical reactions occur between ammonia gas, coal volatiles, and char, leading to NO_x formation. The nitrogen sources in NO_x are highly complex, including gaseous N₂ and NH₃, as well as fuel-bound nitrogen in the solid phase [4]. The concentrations of ammonia and oxygen directly alter the types and quantities of radicals in both gas and solid phases, thereby modifying the reaction pathways.

On one hand, homogeneous oxidation serves as a major pathway for NO_x formation. Wang et al. [5] showed that NO derived from homogeneous oxidation of ammonia accounted for over 70%. Chen et al. [6] further revealed through high-temperature experiments and quantum chemical simulations that NO generation results from the competition between oxidation of nitrogen species (NHi) and their reduction with NO on char surfaces; below 1573 K, oxidation dominates, causing higher NO emissions. Wargadalam et al. [7] found that volatiles such as CH₄ and CO promote NH₃ oxidation but suppress HCN oxidation, highlighting that homogeneous NO formation reflects a balance between oxidation and reduction. Similarly, Hong et al. [8] demonstrated via molecular dynamics that competition between NH₃–O₂ and char–O₂ reactions enhances O and OH radical levels, weakens NO reduction, and underscores the importance of NNH in this process. Complementary studies have also shown that both homogeneous and heterogeneous reductions contribute to NO abatement, with char often displaying superior performance [9,10,11]. In summary, existing research has revealed some of the mechanisms of homogeneous oxidation and nitrogen intermediates in ammonia–coal combustion. However, the coupling effect of homogeneous and heterogeneous reactions, the quantitative role of key intermediates such as NNH, and the specific migration and transformation pathways of nitrogen elements still need to be further explored.

In addition, heterogeneous reactions play a vital role in ammonia–coal co-combustion, involving char-mediated NO reduction, char oxidation, and pyrolysis of small molecules. Chen et al. [12] compared gas-phase and surface reactions, showing that NHi species on char exhibit lower activation barriers than NH3 in the gas phase, with NH radicals highly effective in both. However, NNH formation on char surfaces may inhibit reduction. The relative advantage of heterogeneous reduction also becomes more pronounced at elevated temperatures. Jiao et al. [13] identified synergistic effects between ammonia and char, where moderate O₂ further enhances reduction. Coal rank also influences the reaction pathways, with anthracite tending toward heterogeneous routes while bituminous coal favors homogeneous reactions [14]. Moreover, mineral components catalyze these processes: Ca lowers the barrier for NNH formation and promotes high-temperature NO reduction, though it can hinder CO release at lower temperatures [15]. Overall, volatile–char interactions significantly affect NO evolution, and a mechanistic understanding of homogeneous reactions helps clarify the complexities of heterogeneous pathways. In summary, existing research has revealed some of the mechanisms of homogeneous oxidation and nitrogen intermediates in ammonia–coal combustion. However, the coupling effect of homogeneous and heterogeneous reactions, the quantitative role of key intermediates such as NNH, and the specific migration and transformation pathways of nitrogen elements still need to be further explored.

Chemkin, based on detailed chemical reaction mechanisms and thermodynamic data, can fully simulate complex gas-phase, surface, and multiphase chemical reactions. It helps identify NO_x formation pathways and evaluate the impact of different combustion conditions on NO_x emissions, thereby analyzing temperature distribution and flame characteristics during combustion to optimize combustion efficiency and pollutant control. Compared to other simulation software, Chemkin offers similar accuracy to Cantera and OpenSMOKE++, all relying on the accuracy of the mechanism. The three softwares provide generally consistent results. FactSage and NASA CEA are suitable for combustion equilibrium but lack dynamic simulation. CFD coupled simulations offer strong spatial distribution predictions, but the differences in chemical reaction accuracy primarily stem from the turbulence-chemical reaction coupling model. Therefore, Chemkin is still the most widely used chemical reaction path analysis software. Ding et al. [16] extracted 15 fundamental reactions from the complex ammonia–coal co-combustion process and established a highly adaptable and accurate dual-fuel co-combustion model using Chemkin, analyzing NO and unburned NH₃ contents under varying injection positions and mixing ratios. Further application to 330 MWe subcritical pressure boiler combustion model with a wall-swirling burner in Fluent revealed that a 20% nitrogen blending ratio reduced NO emissions at the cost of lower furnace outlet temperature and combustion efficiency. Chen et al. [17] investigated reaction pathways, which were based on the pulverized coal combustion mechanism developed by Hashemi, under wide temperature ranges and variable ammonia blending ratios in a zero-dimension perfectly stirred reactor. The Chemkin results align well with experiments. Through ROP (rate of production) analysis, sensitivity analysis, and reaction pathway analysis, temperature and ammonia blending ratios were identified as critical NO_x influencers, with 1200 °C as a key inflection point. The NH₂ → HNO → NO pathway dominated, suggesting that reducing OH radical concentrations could mitigate NO formation. Dai et al. [18] simulated nitrogen conversion characteristics in ammonia/coal co-firing under deep air staging using Chemkin, employing two plug flow reactors (PFR) to simulate the deeply air-staged combustion model, which is composed of the primary combustion area and the burnout zone of the boiler, respectively. Within 1200–1500 °C, higher temperatures improved combustion efficiency but increased NO conversion rates. Elevated ammonia blending ratios raised NO conversion rates but lowered volume fraction. Air staging reduced NO generation, with minimum NO_x conversion achieved when excess air coefficient of the overfire air ranged between 0.35 and 0.45. Cai et al. [19] employed the premixed freely propagating flame model in Chemkin to study ammonia premixed combustion, which consists of a one-dimensional premixed combustion model and a zero-dimensional reactor model, elucidating combustion characteristics across temperatures. Initial temperature and pressure were found to fundamentally influence NO emissions due to their impact on H + NH₂ and OH radicals. Additionally, the functional relationship between laminar flame speed and ignition delay was temperature- and pressure-dependent. Zero-dimensional model calculations are simpler and are often used to quickly evaluate reaction mechanisms, but their simulation accuracy is not as good as that of multi-dimensional models. How to find the best balance between accuracy and speed in zero-dimensional and multi-dimensional simulation calculations is also a key point. While these studies focused on extracting key homogeneous reactions by converting solid coal into gaseous radical groups, they partially addressed sensitivity to temperature, radical concentrations, and initial ammonia/coal ratios to propose NO_x control strategies. However, they inadequately revealed mechanisms and pathways for other NO_x precursors (e.g., N₂O, NO₂).

This study advances the field by conducting Chemkin-based chemical kinetic simulations for ammonia–coal co-combustion employing zero-dimensional model for fixed-bed and PSR (perfectly stirred reactor) model for homogenous entrained-flow conditions, analyzing NO, N₂O, and NO₂ production via ROP analysis. This study deeply analyzes the gas-phase homogeneous reaction mechanism during the ammonia–coal co-combustion process, clarifies the migration path of fuel nitrogen to products such as HCN and NH₃, and quantitatively studies the important role of intermediates such as NCO and NNH and gaseous products such as N₂O, constructing a molecular-level nitrogen migration and transformation network. This study aims to simplify reaction mechanisms and identify key intermediates for NO_x reduction.

2. Materials and Methods

2.1. Parameters for Zero-Dimensional Reaction Model for Ammonia–Coal Co-Combustion

The zero-dimensional homogeneous reaction model for ammonia–coal co-combustion is established in Chemkin Pro 2022 R1 using a closed homogeneous reactor (closed homogeneous). The zero-dimensional homogeneous reaction model assumes that the gas is completely uniform throughout the reaction space. This model is simple and facilitates mechanism sensitivity analysis. It clearly shows the impact of kinetics on the overall reaction rate and is suitable for rapid evaluation of reaction mechanisms.

The simulation conditions were set based on our commonly used experimental parameters. The combustion temperature was kept at 1273 K, with the model volume of 584.022 cm³ (corresponding to corundum tube dimensions: 26 cm inner diameter × 1100 cm length). The system pressure was set as 101,325 Pa, with the reaction duration of 0.2 s. Following references [20,21], coal volatile gases (focusing on the main carbon and nitrogen species) were equivalently represented by CO, CH₄, C₂H₂, NH₃, and HCN, with conversion formulas given by Equations (1)–(5). The calculated fuel gas composition for ammonia–coal co-combustion with different co-firing ratios is summarized in

Table 1. Molar ratio of gas components of homogeneous reaction model at different co-firing ratios.

Co-Firing Ratios	Ar	C2H2	CH4	CO	HCN	NH3	O2
0%	0.95	0.0033	0.011	0.0037	0.00018	0.000076	0.030
10%	0.95	0.0030	0.010	0.0034	0.00016	0.0033	0.029
30%	0.95	0.0023	0.0081	0.0026	0.00012	0.0098	0.028
50%	0.95	0.0016	0.0058	0.0019	0.000090	0.016	0.027

.

n(CO) = O_ar/16,

(1)

n(CH₄) = (C_ar/12 − n(HCN)) × (2/3),

(2)

n(C₂H₂) = (C_ar/12 − n(HCN)) × (1/6),

(3)

n(HCN) = N_ar/14 × (0.7),

(4)

n(NH₃) = N_ar/14 × (0.3),

(5)

where n(CO), n(CH₄), n(HCN), n(C₂H₂), and n(NH₃) represent the molar percentages of CO, CH₄, HCN, C₂H₂, and NH₃ respectively; and O_ar, C_ar, and N_ar represent the percentages of oxygen, carbon, and nitrogen in the coal element analysis, in %.

The comprehensive GRI-Mech 3.0 mechanism, which includes 53 species and 325 reactions, demonstrates high sensitivity to small-molecule reactions involving nitrogen (N) and carbon (C) elements. This makes it particularly suitable for predicting nitrogen oxide (NO_x) emissions during ammonia (NH₃) and hydrocarbon combustion processes [22], which is adopted here for in-depth analyzing the reaction mechanisms and model simplification.

2.2. Parameters for Entrained Flow Reaction Model for Ammonia–Coal Co-Combustion

The zero-dimensional reaction model can only represent a transient system, which deviates from the actual engineering equipment. On the other hand, PSR allows for continuous feeding and discharging, allowing for the study of the effects of equivalence ratio and residence time on product distribution and pollutant formation, which is closer to the actual experimental process. The model for ammonia–coal co-combustion in a carrier flow reactor was established using the Chemkin Pro 2022 R1 software, based on the perfectly stirred reactor (PSR) configuration, which is suitable for the rapid homogeneous reactions highlighting the kinetic influences over thermodynamic factors. The inlet and outlet sections were correspondingly added before and after the reactor, as illustrated in Figure 1.

Based on the experimental conditions, the simulation parameters were set as follows: The temperature was set at 1273 K with a duration of 70 s, and the volume of the PSR model was set to 584.022 cm³ (using a corundum tube with an inner diameter of 26 cm and a length of 1100 cm for the reaction) with the pressure of 101,325 Pa. Since a transport solver was employed, no residence time was specified. At the start of the reaction, the PSR was filled with argon. The fuel gas composition for the ammonia–coal co-firing ratio calculation is shown in

Table 2. Molar ratio of gas components of entrained flow reaction model at different co-firing ratios.

Co-Firing Ratios	Ar	C2H2	CH4	CO	HCN	NH3	O2
0%	0.95	0.0033	0.011	0.0037	0.00018	0.000076	0.035
10%	0.94	0.0030	0.010	0.0034	0.00016	0.0033	0.035
30%	0.94	0.0023	0.0081	0.0026	0.00012	0.0098	0.034
50%	0.94	0.0016	0.0058	0.0019	0.000090	0.016	0.033

, with the oxygen content determined by the equivalence ratio and an excess air coefficient of 1.2. The GRI-Mech 3.0 mechanism was used as the foundational reaction mechanism.

3. Results

3.1. Nitrogen Containing Products of Ammonia–Coal Co-Combustion Based on Closed 0D Reactor

The molar fraction variations of NO_x in the closed homogeneous reactor under different co-firing ratios are presented in

Table 3. Molar fraction of NO_x in products generated by homogeneous reactions at different co firing ratios.

Co-Firing Ratios	NO	NO2	N2O	Conversion Efficiency of NOx
0%	0.000203	5.39 × 10−8	9.23 × 10−7	0.804
10%	0.00226	4.15 × 10−6	0.000766	0.499
30%	0.00202	8.93 × 10−6	0.00218	0.233
50%	0.00152	1.92 × 10−5	0.00346	0.178

. All NO_x concentration curves exhibit an instantaneous peak formation.

As can be seen from Table 3, with the increase in the co-firing ratio, the total amount of NO_x produced increases, whereas the total conversion ratio of NO_x generated from nitrogen sources in the reactants gradually decreases. Additionally, when the co-firing ratio increases from 0% to 10%, although the molar concentration of ammonia does not increase significantly, it leads to a sharp rise in NO_x concentration. Compared to NO, the molar concentration of NO₂ changes more noticeably, differing by several orders of magnitude, and continues to increase with the rise in the co-firing ratio. The molar concentration of N₂O increases with the co-firing ratio, while the molar concentration of NO first rises and then declines as the co-firing ratio increases. Furthermore, under co-firing ratios of 30% and 50%, the molar concentration of N₂O exceeds that of NO. During the co-combustion reaction at 1273 K, when the co-firing ratio increases from 10% to 30%, the release of NO decreases while the increase in N₂O is more pronounced. This is mainly due to the reducing effect of NH and NH₂ radicals on NO [23,24], leading to an increase in N₂O and a certain reduction in NO content as the co-firing ratio rises. Notably, in the experiments, when the co-firing ratio reaches 50%, the amount of N₂O further increases, while NO peaks at a 30% co-firing ratio. This phenomenon may be attributed to the inherent ability of coke to reduce NO, whereas the Chemkin simulation overlooks the heterogeneous reduction process.

3.2. ROP Analysis of Ammonia–Coal Reaction Products Based on Closed 0D Homogeneous Reactor

The ROP of NO_x produced in the zero-dimensional homogeneous reaction of ammonia–coal co-combustion with different co-fired ratios was analyzed. The evolution of nitrogen-containing products with time is shown in Figure 2, and the reactions involved and the corresponding frequency factors, temperature indexes and activation energies are shown in

Table 4. Elementary reactions involved in the generation of NO_x through homogeneous reactions at different co-firing ratios.

Reaction Number	Free Radical Reaction	A	B	E (cal)
178	N + NO = N2 + O	2.70 × 1013	0	355
179	N + O2 = NO + O	9.00 × 109	1	6500
180	N + OH = NO + H	3.36 × 1013	0	385
181	N2O + O = N2 + O2	1.40 × 1012	0	10,810
182	N2O + O = 2NO	2.90 × 1013	0	23,150
183	N2O + H = N2 + OH	3.87 × 1014	0	18,880
184	N2O + OH = N2 + HO2	2.00 × 1012	0	21,060
185	N2O(+M) = N2 + O(+M)	7.91 × 1010	0	56,020
186	HO2 + NO = NO2 + OH	2.11 × 1012	0	−480
187	NO + O+M = NO2 + M	1.06 × 1020	−1.41	0
188	NO2 + O = NO + O2	3.90 × 1012	0	−240
189	NO2 + H = NO + OH	1.32 × 1014	0	360
190	NH + O = NO + H	4.00 × 1013	0	0
191	NH + H = N + H2	3.20 × 1013	0	330
195	NH + O2 = NO + OH	1.28 × 106	1.5	100
198	NH + NO = N2 + OH	2.16 × 1013	−0.23	0
199	NH + NO = N2O + H	3.65 × 1014	−0.45	0
212	H + NO + M = HNO + M	4.48 × 1019	−1.32	740
213	HNO + O = NO + OH	2.50 × 1013	0	0
214	HNO + H = H2 + NO	9.00 × 1011	0.72	660
215	HNO + OH = NO + H2O	1.30 × 107	1.9	−950
216	HNO + O2 = HO2 + NO	1.00 × 1013	0	13,000
222	NCO + O = NO + CO	2.35 × 1013	0	0
228	NCO + NO = N2O + CO	1.90 × 1017	−1.52	740
246	CH + NO = HCN + O	4.10 × 1013	0	0
274	HCCO + NO = HCNO + CO	9.00 × 1012	0	0
281	CN + NO2 = NCO + NO	6.16 × 1015	−0.752	345
282	NCO + NO2 = N2O + CO2	3.25 × 1012	0	−705

.

For NO, with the increase in co-firing ratio, the main radical reaction of NO production changes from N + OH = NO + H to HNO + H = H₂ + NO. The increase in ammonia concentration increases the concentration of H radical, which reduces the reaction rate of ammonia to NH₂ and NH. In addition, H radical is susceptible to combining with N, resulting in the decrease in free N radical. Therefore, the generation of NO prefers HNO as the precursor. The main reaction consuming NO changes from H + NO + M = HNO + M to NH + NO = N₂O + H. In the stage of raw coal combustion, the H free radical is less, and the main reaction consuming NO is the reverse reaction of HNO dehydrogenation, the reaction rate of which is lower, and NO is basically not consumed. With the increase in co-firing ratio, the rate of NH groups being oxidized to HNO and NO is slower, resulting in the binding reaction between unreacted NH groups and NO, consuming NO and generating additional N₂O. It is worth noting that the temperature factor of the reaction is negative, and the reaction activity is strong at low temperatures. The amount of N₂O can be effectively reduced by increasing the reaction temperature. In addition, the reaction of NH + NO = N₂ + OH can be promoted by the control of temperature and catalyst, which can promote the conversion of N radical to N₂ and realize the reduction in NO [25].

As for NO₂, it is mainly the oxidation reaction of HO₂ and O free radicals on NO. While during the reduction process of ammonia–coal combustion, HO2 free radicals are unstable in the environment rich in H free radicals. At the beginning of the reaction, less H and more O₂ are dissociated, and NO is oxidized after combining with O₂, as can be seen from Figure 2. The reaction HO₂ + NO = NO₂ + OH is active only at the beginning of the reaction and is subsequently replaced by NO + O + M = NO₂ + M. This reaction rate is slow, resulting in NO₂ not being significant in the NO_x production process.

As for N₂O, it comes from the reduction in NH radical to NO following the reaction NH + NO = N₂O + H, whilst the consumption of N₂O comes from the reduction in H radical as dominated by N₂O + H = N₂ + OH. The consumption and production of N₂O increase with the growth of NH and H radicals, that is, the production and consumption rates of N₂O increase with the increase in co-firing ratio. In the experiment, the formation of N₂O is limited at the center of the corundum tube at high temperature, because the consumption of N₂O is more significant under the catalysis of coke. In addition, the increase in the co-firing ratio enhances the local reducibility, resulting in the rise in the reducing intermediate NCO. Thus, the reduction in NO₂ by NCO further increases the content of N₂O where the reaction NCO + NO₂ = N₂O + CO₂ prevails.

3.3. Nitrogen Containing Products of Ammonia–Coal Co-Combustion Based on Entrained Flow Reactor

The influence of co-firing ratio on the mole fraction variations of NO_x is shown in Figure 3. Since the molar fraction of NO₂ is very small, the variation in its molar concentration is not shown.

It can be seen from Figure 3 that a NO_x emission peak is formed after the fuel gas enters the PSR at the beginning. This is because the argon filled in the PSR affects the gas transport process. As the reaction gas fills the entire PSR reaction chamber, the NO_x generated by the reaction reaches the peak within 10 s. In addition, the NO_x emission of 10% co-combustion condition is much higher than that of 0% situation. With the increase in co-firing ratio, the total emission of NO_x increases. It can be further observed that the stable emission concentration of NO increases first and then decreases, and reaches the maximum at 30% co-firing ratio, while the N₂O emission increases continuously. Due to the influence of transport and mass transfer processes, considering the mixing degree of reaction substances, the production of NO and N₂O in the entrained flow reaction is lower than that in the zero-dimensional reactor. In addition, with the increase in co-firing ratio, the release of NH increases, which leads to the increase of nitrogen source that induces the increase in NO [26]. At the same time, the increase in NH can also lead to the reduction in NO, which on the other hand further increases the formation of N₂O. When the co-firing ratio increases to 30%, NH groups can accumulate in the reaction stage. The consumption rate of NO has exceeded the production rate, which leads to a decrease in NO, resulting in the concentration of NO at 50% co-firing ratio being lower than 30%.

3.4. ROP Analysis of Ammonia–Coal Reaction Products Based on Entrained Flow Reactor

The ROP analysis of NO_x produced by co-combustion of ammonia and coal with different co-firing ratios is analyzed, and the evolution rates of products with time are shown in Figure 4.

For NO, with the increase in co-firing ratio, the main radical reaction producing NO changes from NO₂ + H = NO + OH to HNO + H = H₂ + NO, which then changes back to NO₂ + H = NO + OH at 50% co-firing ratio, resulting in the highest NO concentration at 30% co-firing ratio. The formation of NO takes HNO as the precursor. When the co-firing ratio is 30%, the ratio of coal and ammonia just makes the formation rate of intermediate HNO maximum, which eventually leads to an increase in the reaction rate of NO formation. The main NO-consuming reaction changes from the oxidation of NO to NO₂ to the reduction in NO to N₂O. In addition, the reduction ability of C₂H₂ and CH₄ is not as good as that of NH₃. When the co-firing ratio is small, the oxidizing atmosphere is stronger. With the increase in the ratio, the NH group enhances the reduction in NO, resulting in the binding reaction between the unreacted NH group and NO, which leads to the consumption of NO and generation of additional N₂O.

For NO₂, the oxidation reaction of HO₂ radical towards NO dominates the process, and the oxidation ability of O radical on NO is far less than that of HO₂. The H-rich environment also plays a part here. However, the HO₂ group is not stable in the environment with high NH group and thus the production of NO₂ is less.

For N₂O, it basically comes from the reduction in NH radical to NO following the reaction of NH + NO = N₂O + H, whilst the consumption of N₂O is mainly the thermal decomposition effect at high temperature. For the 10% co-firing ratio case, the reduction of N₂O by the H radical prevails as N₂O + H = N₂ + OH. It can be concluded that the addition of a small amount of ammonia leads to an increase in H radicals, which changes the consumption path of N₂O. Furthermore, the consumption and production of N₂O increase with the rise in NH and H radicals; that is, with the increase in co-firing ratio, the production and consumption rates of N₂O both increase.

3.5. Simplified NO_x Formation Mechanisms of Ammonia–Coal Co-Combustion

The mechanisms of NO_x formation are simplified based on reduction in GRI-Mech 3.0 comprehensive mechanisms, taking ammonia-coal co-combustion under 30% mixed ratio at 1273 K as the analysis object, and the results are shown in Figure 5, in which the ROP values are shown in relative quantity.

NNH [27], NCO [28] and N are important intermediates for the formation of N₂. In addition, O₂ can be formed by the thermal decomposition of N₂O. NNH is an important intermediate [29] for the reduction in NO to N₂. The elementary reaction NH + NO = NNH + O consumes NO. At the same time, the selectivity of NNH is very strong, and only N₂ is produced. However, the amount of NH produced is quite low, and only a small part of it can react with NO to produce NNH. According to the pathway of NH₃ reaction to produce NNH, in order to increase the content of NNH intermediate and thus promote the production of N₂, increasing the content of H radical is an important way to promote the production of NNH. In addition, NCO can be converted from HCN. In the process of ammonia–coal co-combustion, NH₃ reacts with coal char and volatile matter to give birth to HCN. Furthermore, the volatile matter itself also contains certain HCN, which can generate NCO through various intermediates. As a result, the amount of NCO is much greater than that of N and NNH. However, the selectivity of NCO to N₂ is very poor, and instead, N₂O is more likely to be produced. Therefore, in order to increase the conversion rate of nitrogen source to N₂, the treatment of N₂O should be considered while also increasing NCO. The formation of free N is relatively difficult, requiring NH₃ to remove three H radicals, so this transformation path is not significant.

NO mainly comes from the oxidation of intermediate HNO [30] and NCO [31], as well as the reduction in NO₂. Due to the influence of reducing atmosphere, the conversion rate of NO₂ to NO is very high, indicating that NO₂ is not stable in the case of ammonia–coal co-combustion, which will be reduced by H radical. Based on the ROP analysis of NO₂ in Figure 4, although the final concentration of NO₂ is low, its rates of generation and consumption are very high. Hence, it can be concluded that NO primarily originates from the reduction in NO₂. On the other hand, both HNO and NCO can be oxidized to NO. In the process of oxidation, H₂O and OH groups play the role of oxidants. In fact, the introduction of H₂O in the process of ammonia–coal co-combustion will affect the reaction rate of the coal char/NH₃/NO ternary system. H₂O leads to a significant increase in the conversion of NO [32], and the OH groups produced by its decomposition further promote the formation of NO [33]. In addition, H₂O inhibits catalytic performance by competing the catalytic site, resulting in the inhibition of the heterogeneous reduction in NO [34]. Therefore, the additional H₂O induced by coal-ammonia co-combustion should be taken into account for developing low-NO_x co-combustion technologies.

N₂O comes from the reduction in NO by NCO and NH, and the oxidation of N also contributes to its increase. Since N radicals are very difficult to reduce, almost all of N₂O comes from the reduction in NO. Therefore, the production of N₂O can be inhibited simultaneously while the generation of NO is also inhibited. On the other hand, the production of N₂O can be reduced by decreasing the content of NCO. Therefore, the formation of HCN in the reaction process should be controlled as much as possible. The decoupling combustion method through separating the volatile and char combustion phases is a promising way to abate the volatile-ammonia interaction and thus alleviate the NO_x emissions.

Based on the above analysis, in order to increase the conversion efficiency of nitrogen source to N₂ and inhibit the production of NO_x, the production of NNH intermediate should be promoted and the generation of NCO intermediate should be reduced. In addition, H radical is beneficial to the formation of N₂, while H₂O and OH groups can promote the formation of NO_x. Furthermore, in order to reduce the formation of HCN, i.e., the dominant precursor of NO_x, the decoupling combustion method can be used to reduce the contact between volatiles and ammonia.

4. Conclusions

In this paper, the Chemkin platform was used to carry out the reaction kinetics analysis of ammonia–coal co-combustion processes, and the situations of fixed bed and entrained flow reactors with different temperatures and blending ratios were studied in detail. The nitrogen conversion paths and its simplified mechanisms in the ammonia–coal co-combustion process were revealed. More importantly, the molecular reaction mechanisms of ammonia–coal co-combustion and the thermochemical conversion reaction network of nitrogen-containing precursors were constructed. The main conclusions are as follows:

(1)

The co-firing ratio shows a significant effect on the formation of NO_x in the ammonia–coal co-firing processes. In the zero-dimensional homogeneous reaction process, with the increase in co-firing ratio, the molar concentrations of NO₂ and N₂O increase, whereas the molar concentration of NO increases first and then decreases. In the entrained flow reaction process, the total emission of NO_x increases with the rise in co-firing ratio, where the stable emission concentration of NO increases first and then decreases, reaching the maximum at 30% co-firing ratio. However, the stable emission concentration of N₂O continues to increase monotonously.

(2)

The formation of N₂O mainly depends on the reduction in NH and NO. With the increase in co-firing ratio, the unreacted NH promotes the reduction in NO to N₂O, while the H radical produced by the reaction hinders the formation of NO. At 1273 K, N₂O is mainly derived from the reduction in NO, where NH + NO = N₂O + H is the primary formation reaction of N₂O.

(3)

The formation of intermediates such as NNH and NCO is essential to the transformation of NO_x. In order to increase the conversion efficiency of nitrogen source to N₂ and inhibit the formation of NO_x, the formation of NNH intermediates should be promoted as much as possible, while the formation of NCO intermediates should be reduced. In addition, in order to reduce the formation of HCN, the dominant precursor of NO_x, the decoupling combustion method can be used to reduce the contact between volatiles and ammonia.

For the first time, the simplified mechanism model constructed in this paper quantifies the correlation between the precursors of NO_x such as HCN and NH₃ and the intermediates such as NCO and NNH during the ammonia–coal co-combustion, emphasizing the important roles of N₂O. These findings provide a design basis at the molecular level for the development of a new generation of low-NO_x control technology for ammonia–coal co-combustion. However, the current work mainly focuses on homogeneous reactions, and the analysis of heterogeneous reaction and its interaction with homogeneous reaction lacks sufficient depth. Coal char contains a large number of active sites and gas adsorption sites due to its rich micro-pores. At the same time, the C element in char can be used as a reducing agent. Therefore, the heterogeneous reduction in NO_x and the heterogeneous and homogeneous synergistic reaction of coal char cannot be ignored. In the future, further research on the complex reaction environment of coal char surface will be carried out. The functional group characteristics, lattice structure, micro-pore distribution and other reaction environments on the surface of coal char determine the effect of heterogeneous reduction on NO_x reduction. Studying the relationship between heterogeneous reactions and coal char, and integrating homogeneous reactions to determine the optimal operating conditions for synergistic homogeneous–heterogeneous reactions that minimize NO_x emissions, will be a major focus of future research.