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Abstract

The formation and control of pollutants emitted from fuel combustion have always been a focal point in combustion chemistry. Key pollutants primarily include nitrogen oxides (NOx) and sulfur oxides (SOx), making it crucial to elucidate the formation processes of nitrogen and sulfur components during combustion for pollutant control. Due to the highly coupled evolution processes of both components, independent reaction mechanisms struggle to describe this process; thus, considering the interactions between them is significant for the evolution of nitrogen and sulfur components. This article investigates the promotional or inhibitory effects between nitrogen and sulfur components in fuel combustion experiments, with the magnitude of this interactive effect varying between 2% and 250%, contingent upon the equivalence ratio and the N/S ratio impacts. Additionally, from a microkinetic perspective, two mechanisms underlying N/S interactions are identified: direct and indirect interactions. Direct interaction involves the formation of NS radicals, primarily through direct reactions of nitrogen species (NOx/HCN/NHi, where i ranges from 0 to 3) with sulfur constituents (SOx/H2S). Conversely, indirect interaction alters the radical pool via the intervention of NO or SO2, subsequently influencing each other's reaction pathways. It is noted that the current reaction system is incomplete, lacking key reactions, while the kinetic parameters of some reactions are still contentious. Advanced theoretical calculations are needed to refine the N/S interaction reaction model, to provide more accurate predictions for nitrogen and sulfur pollutant levels.

1 Introduction

Amidst the escalating global climate crisis, attaining sustainable development goals and net-zero emissions targets has emerged as a shared objective for the international community. To achieve these goals, fundamental transformations in both energy production and consumption are imperative, alongside urgent demands for effective management and mitigation of pollution issues arising from traditional energy usage. Among these, fuels, as primary carriers of energy, inevitably generate pollutants such as nitrogen oxides (NOx) and sulfur oxides (SOx) during combustion, posing significant threats to atmospheric environments [1,2]. Nitrogen and sulfur oxides react with water vapor and oxygen in the atmosphere to form nitric and sulfuric acids, which contribute to the formation of acid rain. Acid rain directly damages soil and water bodies, impacting the balance of ecosystems. Additionally, nitrogen and sulfur oxides participate in complex photochemical reactions, leading to the formation of ground-level ozone, which has negative effects on plant photosynthesis and human health. The quality and source of fuel play pivotal roles in determining the level of emitted pollutants. Hoang et al. [3] argue that SOx emissions constitute a major cause of environmental pollution, posing significant threats to both ecological systems and human health. In response, the authors employed ultrasonic-assisted technology to develop a novel diesel blend, a strategy that not only enhances the stability and optimizes the spray characteristics of the blended fuel but also markedly reduces its sulfur content. However, Sharma et al. [4] found in a study that the emissions of NOx actually increased when B20 fuel (20%/80% of biodiesel/diesel fuel) was mixed with 10% H2O2. Therefore, using low-sulfur and low-nitrogen fuels or adopting appropriate fuel-blending strategies has a very positive effect on mitigating environmental pollution [5,6]. Furthermore, fuel produced through clean processes can effectively circumvent impurities introduced during industrial procedures like oil refining [7]. Conversely, inexpensive and subpar fuels may harbor more impurities, leading to heightened pollutant emissions. To reduce the emission of pollutants, the formation mechanisms and inhibition effects of nitrogen oxides and sulfur oxides have been extensively studied [8,9]. Despite significant progress being made, the chemical interactions between nitrogen/sulfur components in combustion remain an area of considerable uncertainty and continue to be a focal point in nitrogen and sulfur chemistry research [10].

The interdependence between nitrogen and sulfur components on each other's evolution has been well established. Jeffries and Crosley [11] first observed the presence of NS radicals in flames of methane mixed with H2S and NH3, demonstrating a direct correlation between nitrogen and sulfur components. Subsequent conclusions from experiments conducted by Hampartsoumian et al. in the combustion of liquid (gasoline) and solid fuels (coal) indicated that the presence of SO2 significantly affects the emission of NOx [12,13]. Furthermore, numerous studies have confirmed that the interaction between the two alters their respective evolution mechanisms [1416], highlighting the importance of understanding the interaction mechanism between them for predicting the concentration distribution of NOx and SOx as well as pollution control.

In general, nitrogen oxides and sulfur oxides produced during the combustion of solid and liquid fuels result from the interaction between the fuel and oxidizer [17]. In contrast, gaseous fuels such as methane, ethane, and other hydrocarbons do not inherently contain sulfur or nitrogen elements and therefore do not produce sulfur oxides during combustion. The generation of nitrogen oxides primarily stems from the oxidation of N2 in the air [18,19]. Based on this fact, it appears that the interaction between nitrogen and sulfur components is more significant in the combustion of solid and liquid fuels compared to gaseous fuels. However, in large fixed facilities (boilers) burning solid fuels (coal), these complex compound fuels rapidly decompose into smaller hydrocarbons and other light gases, leading to the interaction of nitrogen and sulfur components primarily in the gas phase [20]. Therefore, studying the sulfur/nitrogen component interaction mechanism during the combustion of gaseous fuels is also meaningful. This review outlines the impact of nitrogen and sulfur pollutants on each other's evolution under different combustion conditions, summarizes the micro-reaction mechanisms of N/S interaction, and provides guidance and reference for further research on the interaction of N/S during combustion processes.

2 The Formation Mechanism of Nitrogen and Sulfur Pollutants

In the combustion system, the formation of NOx is primarily influenced by fuel characteristics (such as fuel type, nitrogen content, and its form), combustion organization (the mixing mode of fuel and oxygen), and reaction conditions (including equivalence ratio, reaction time, etc.). Conversely, the formation of SOx is more significantly governed by fuel characteristics and typically remains unaffected by combustion conditions [2,5]. Considering this, the suppression of NOx generation can be approached by both optimizing fuel properties and improving combustion organization. For example, Bui et al. [21] enhanced engine performance and reduced greenhouse gas emissions by adjusting the injection strategy of syngas (mainly composed of CO and H2). Compared to pure diesel combustion, the blending of syngas with diesel significantly reduces NOx emissions by altering fuel properties. Additionally, optimizing combustion organization through adjusting injection strategies also contributes to NOx reduction. Research on this control strategy may also provide ideas for reducing SOx emissions. To comprehend this process and the interaction between nitrogen and sulfur components during combustion, this section focuses on the reaction kinetics model of the independent evolution of NOx and SOx in combustion and analyzes the mechanism of pollutant formation.

2.1 NOx Formation Mechanism.

During the combustion of fuel, nitrogen in the air and nitrogen elements in the fuel will be transformed into nitrogen-containing components, such as NO, NO2, N2O, NH3, and HCN. Subsequently, under the action of the oxidant, mainly NO, N2O, and NO2 are formed, with the latter two being present in low concentrations and generally not considered [22,23]. This is due to the fact that in typical NOx emissions from methane or coal powder combustion systems, NO usually accounts for over 90%, NO2 typically ranges between 5% and 10%, while N2O typically constitutes less than 2%. There are three mechanisms for the formation of nitrogen oxides: thermal type, prompt type, and fuel type NOx, and Fig. 1 summarizes these three mechanisms of NOx formation.

Fig. 1
The evolution pathways of thermal, prompt, and fuel NOx. Reproduced with permission from Ref. [16]. © 2007 Elsevier.
Fig. 1
The evolution pathways of thermal, prompt, and fuel NOx. Reproduced with permission from Ref. [16]. © 2007 Elsevier.
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The formation of thermal NOx originates from the oxidation of N2 in the air. Under high temperature (typically above 1500 K) and fuel-lean conditions, N2 in the air reacts with atomic O to form NO, which exhibits strong temperature dependence [24]. This process is widely described using the Zeldovich mechanism [24]:
(N1)
(N2)
(N3)
In the fuel-rich flame region, the chemical bonds of N2 in the air are cleaved by highly reactive hydrocarbon radicals (CHx), leading to rapid oxidation and the generation of NO, known as prompt NOx. The most significant reaction involved is [25]
(N4)

In combustion systems, the contribution of prompt NO to total NO emissions is relatively small and is typically neglected [26].

Fuel NOx is formed by the oxidation of nitrogen bound to solid and liquid fuels, accounting for over 80% of NO emissions and serving as its main source [27]. Nitrogen bound to fuel is typically connected to other species through N–C bonds and N–H bonds, making the formation of fuel NO easier compared to the thermal NO formation process that requires breaking N≡N bonds. Many scholars have studied the formation and reduction mechanisms of fuel NO. Pels et al. [28] constructed a detailed mechanism model for HCN evolution, Xu et al. [29] established a detailed reaction model for NH3 conversion, and Zhou et al. [30] proposed migration and conversion pathways for fuel nitrogen. In numerical simulations, the generation of fuel NO and homogeneous reduction reactions of NO with NH3/HCN are usually calculated using the DeSoete mechanism model [31]. In this model, nitrogen within the fuel is oxidized, taking the form of NH3/HCN, to generate fuel NO. Concurrently, a competing reaction occurs, resulting in a reduction in N2. The pivotal reactions are as follows [31]:
(N5)
(N6)

2.2 SOx Formation Mechanism.

Unlike nitrogen oxides, the formation of sulfur oxides is entirely derived from the sulfur element in the fuel. Therefore, gaseous fuels (such as methane, ethane, and other hydrocarbons) do not produce sulfur oxides during combustion. However, solid fuels, especially coal, produce SO2 during combustion, leading to severe pollution. Hence, the following sections elaborate on the formation of SO2 during the combustion of coal dust.

The evolution of sulfur components is divided into two stages: the precipitation of sulfur components from coal pyrolysis and the gas-phase reactions of sulfur components [32,33]. Since the N/S interaction mainly occurs in the gas phase, this article focuses on the latter stage [12]. In the gas-phase reactions, the main sulfur-containing components are H2S and SO2 produced from coal pyrolysis, which interconvert under different combustion atmospheres, with SO2 being abundantly generated in oxidizing atmospheres. The University of Leeds proposed a detailed reaction mechanism model to describe this process, widely cited by many scholars [34], with the sulfur component evolution mechanisms shown by the black arrows in Fig. 2 of the model. Additionally, although in smaller quantities, COS and CS2 cannot be overlooked, as indicated by the yellow arrows in Fig. 2 outlining their reaction pathways. Glarborg et al. [35] established a kinetic model for CS2 oxidation using ab initio methods, and the author also conducted a detailed study on COS oxidation models [36], both of which are oxidized to SO2. Building on the research of the aforementioned scholars, Ma et al. [37] constructed a detailed kinetic model including four components: H2S/SO2/COS/CS2, which matched well with experimental values. Furthermore, a small portion of SO2 will further oxidize to SO3 under conditions of sufficient oxygen, but this process can lead to severe corrosion issues. Hindiyarti et al. [38] developed a detailed model for this process.

Fig. 2
Detailed schematic diagram of gas-phase reaction mechanisms for sulfur species [34–37]
Fig. 2
Detailed schematic diagram of gas-phase reaction mechanisms for sulfur species [34–37]
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3 The Influence of Sulfur Components on the Formation of NOx

The sulfur content can have an impact on the formation of NOx during fuel combustion, and this process depends on factors such as equivalence ratio and N/S ratio [12,3941]. For thermal NOx, sulfur components inhibit NOx formation across all equivalence ratios. However, for fuel NOx, sulfur components facilitate NOx formation at equivalence ratios greater than 1 while exerting the opposite effect at ratios below 1. Alterations in the N/S ratio intensify or diminish this effect. Due to the different mechanisms of NOx formation during the combustion process, the influence of sulfur components on it will also change accordingly; therefore, they need to be discussed separately. In addition, the impact of sulfur components on the formation of prompt NOx is often overlooked.

3.1 Influence on Thermal NOx Generation.

Thermal NO is formed by the oxidation of N2 in the air, so this section analyzes the combustion results of fuels without nitrogen content.

Wendt and Ekmann [42] found that the NO emissions from methane flames were reduced by up to 36% when doping with a 4.9% volume fraction of SO2 at equivalence ratios of 0.8–1.2, as shown in Fig. 3. Simultaneously, the authors used inert gas N2 as a fuel diluent and observed that the NO emissions caused by the addition of N2 to the fuel were less than 7 ppm. Therefore, based on Fig. 3, the change in NO due to the addition of SO2 to the fuel is not a result of physical dilution, but rather a consequence of chemical kinetic interactions. This chemical interaction is explained as SO2 catalyzing the integration of OH and H radicals, thereby reducing radical concentrations through the following reactions [42]:
(1)
(2)
Fig. 3
The influence of SO2 on thermal NO generation. The equivalence ratio ranges from 0.8 to 1.2. Reproduced with permission from Ref. [42]. © 1975 Elsevier.
Fig. 3
The influence of SO2 on thermal NO generation. The equivalence ratio ranges from 0.8 to 1.2. Reproduced with permission from Ref. [42]. © 1975 Elsevier.
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As the equivalence ratio increases, the extent of this effect decreases. Additionally, when the volume fraction of SO2 is less than 1%, this influence can be considered negligible.

The influence of H2S on thermal NO generation was studied in the combustion of ethane. Pfefferle and Churchill [43] investigated the impact of sulfur components on thermal NOx emissions in premixed ethane–air flames with H2S added to a hot-stable piston combustion chamber at equivalence ratios of 0.8–1.6. The results indicated that the addition of 0.06 wt% and 0.03 wt% of H2S resulted in similar NOx emissions, and the sulfur component led to a 5–10% reduction in NOx produced at the outlet of the combustion chamber. Consistent with Wendt's conclusion, the reduction in NOx emissions is also attributed to the catalytic integration of free radicals by SO2 [42], indicating that H2S needs to be converted to SO2 to achieve the inhibitory effect on NOx.

The combustion conclusions of liquid fuel (kerosene) are essentially consistent with those of gaseous fuel [13,44]. Hampartsoumian and Nimmo [13] increased the sulfur content in the fuel from 0.2% (initial sulfur content in the fuel) to 4.6% by adding tetrahydrothiophene and studied the impact of the primary zone equivalence ratio Φ1 and sulfur content on thermal NO formation under staged air conditions, with a secondary zone equivalence ratio Φ2 maintained at 0.85. The research results, as shown in Fig. 4, indicated a decrease in thermal NO formation at all equivalence ratios studied, with the impact becoming less significant as the equivalence ratio increased. It seems that the primary zone equivalence ratio is the main parameter controlling the effect of sulfur on thermal NO.

Fig. 4
The influence of sulfur component concentrations on NO emissions at various equivalence ratios. The equivalence ratio varies from 0.85 to 1.41, while the sulfur component content ranges from 0.2% to 4.6%. Reproduced with permission from Ref. [13]. © 1995 Taylor & Francis.
Fig. 4
The influence of sulfur component concentrations on NO emissions at various equivalence ratios. The equivalence ratio varies from 0.85 to 1.41, while the sulfur component content ranges from 0.2% to 4.6%. Reproduced with permission from Ref. [13]. © 1995 Taylor & Francis.
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The combustion experiments described earlier all indicate that in the presence of sulfur components, the formation of thermal NOx at various equivalence ratios is inhibited, and this phenomenon can be explained by the catalytic integration of SO2 with active free radicals. Additionally, the sulfur component content can influence this effect, but when the content is below a certain threshold, sulfur components have almost no impact on the generation of thermal NOx.

3.2 Influence on Fuel NOx Generation.

Fuel NOx is formed by the oxidation of nitrogen in the fuel, so this section analyzes the combustion results of hydrocarbon gas fuels and coal doped with both nitrogen and sulfur components.

The influence of different SO2 contents on fuel NOx was studied under rich fuel conditions [45]. C2N2 was added to methane to simulate fuel nitrogen, and the results are shown in Fig. 5. At an equivalence ratio of 2.17, the addition of 630 ppm of SO2 had little effect on NO emissions, but at SO2 addition levels greater than 5528 ppm, there was a significant impact on fuel nitrogen formation. In the absence of sulfur components, the NO content rapidly increased to 10 ppm and then remained constant, while with increasing SO2 content, NO formation became significantly evident.

Fig. 5
The influence of SO2 content on fuel NOx under rich combustion conditions. The respective amounts of SO2 added are 0 ppm, 630 ppm, 5528 ppm, 1.1%. Reproduced with permission from Ref. [45]. © 1979 Elsevier.
Fig. 5
The influence of SO2 content on fuel NOx under rich combustion conditions. The respective amounts of SO2 added are 0 ppm, 630 ppm, 5528 ppm, 1.1%. Reproduced with permission from Ref. [45]. © 1979 Elsevier.
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Corley and Wendt [46] introduced HCN/SO2 into a CH4/He/O2 flame under rich fuel conditions to investigate the influence of fuel sulfur on the distribution of nitrogen-containing components in the flame. The study results indicated that at an equivalence ratio of 1.71, the addition of sulfur promoted the substantial formation of NO and proposed the possible reaction N + SO → NO + S to explain the production of NO under this condition but failed to account for the significant increase in NO. Therefore, this model was considered unsuccessful by Jeffries and Crosley [11] due to the exclusion of important reactions and intermediates from the model. Consequently, the authors conducted a study on rich methane flames with the addition of H2S and NH3 for the first time and detected NS radicals in an atmospheric pressure flame through the laser-induced fluorescence (LIF) technique. These results indicated that NS radicals are important intermediates in the flame and directly interconnect with the mechanisms of NOx and SOx formation.

Under lean fuel conditions, the impact of sulfur components on NOx differs from that under rich fuel conditions. Hughes et al. [47] conducted experiments on methane flames with mixed SO2/NH3 under high percentage argon dilution (lean fuel conditions) to investigate the influence of various SO2 contents on NO in the combustion zone using LIF, as depicted in Table 1. Figure 6 shows the measurement and simulation results. The experimental results indicated a significant reduction in NO emissions after the addition of SO2, with a greater decrease in NO levels observed as the SO2 content increased. Additionally, simulations using the chemkin program were performed to analyze the experiments, and sensitivity analysis identified sulfur-containing species-related reactions important for NS and NO radicals, demonstrating the significance of nitrogen–sulfur direct interaction reactions. However, the aforementioned simulations were based on predicted rates under the experimental conditions, and parameters such as key elementary reactions and reaction pathways in the mechanism were uncertain, lacking relevant data, leading to significant discrepancies between the simulated results and experimental values [4850].

Fig. 6
The measured and simulated values of the impact of added SO2 on NO emissions in flames 1 and 2. Reproduced with permission from Ref. [47]. © 2002 Faraday Division, Royal Society of Chemistry.
Fig. 6
The measured and simulated values of the impact of added SO2 on NO emissions in flames 1 and 2. Reproduced with permission from Ref. [47]. © 2002 Faraday Division, Royal Society of Chemistry.
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Table 1

The flame conditions for the influence of SO2 levels on NO emissions.

FlameΦP (Torr)Mass flow rate (g · cm−2 · s−1)Composition (%)
ArCH4O2NH3SO2
10.7400.00430165.668.7524.880.30–0.3
21.0400.00425265.6811.2322.490.30–0.3
FlameΦP (Torr)Mass flow rate (g · cm−2 · s−1)Composition (%)
ArCH4O2NH3SO2
10.7400.00430165.668.7524.880.30–0.3
21.0400.00425265.6811.2322.490.30–0.3

Note: Reproduced with permission from Ref. [47]. © 2002 Faraday Division, Royal Society of Chemistry.

Building upon the results of gaseous fuel combustion, the combustion process of solid fuel coal has been extensively investigated. Hampartsoumian et al. [12] measured the NO emissions from a 20-kW coal powder furnace with and without the addition of SO2 at various equivalence ratios. Figure 7 illustrates the impact of adding SO2 on NO emissions at excess air ratios of 1.3 and 0.84 (the excess air ratio and the equivalence ratio are reciprocal to each other). It was observed that under rich fuel conditions, the addition of sulfur increased NO emissions by 20%, while under lean fuel conditions, it led to a decrease in NO emissions. Clearly, two distinct mechanisms exist, with the dominant mechanism determined by stoichiometry, a conclusion consistent with previous reports on gaseous fuel combustion [42,43,45,47]. The interactions of sulfur-enhancing NO and reduction mechanisms occur simultaneously in coal powder flames, dependent on local atmospheres, and the overall NO emissions result from the competition between these mechanisms.

Fig. 7
The impact of SO2 addition on NO emissions under different equivalence ratios: (a) fuel-rich condition (excess air ratio λ = 0.84) and (b) fuel-lean condition (excess air ratio λ = 1.3). Reproduced with permission from Ref. [12]. © 2001 Elsevier.
Fig. 7
The impact of SO2 addition on NO emissions under different equivalence ratios: (a) fuel-rich condition (excess air ratio λ = 0.84) and (b) fuel-lean condition (excess air ratio λ = 1.3). Reproduced with permission from Ref. [12]. © 2001 Elsevier.
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Furthermore, Fig. 8 illustrates the impact of SO2 addition on NO emissions in coal-fired furnaces under both rich and lean fuel conditions [12]. Various coal types were utilized in the experiment. It can be observed that under rich fuel conditions, the relationship between SO2 addition and the increase in NO emissions is nonlinear, with the curve gradually flattening. Conversely, under lean fuel conditions, there is a linear negative correlation between SO2 addition and the reduction in NO emissions. This suggests that there may be an upper limit to the promoting effect of increasing SO2 content on NO emissions, while the inhibitory effect seems to be absent. Variations in the N/S ratio under different atmospheres influence the extent of the interaction, thereby resulting in either an increase or decrease in the amount of NO change.

Fig. 8
Influence of SO2 addition quantity from different coal types on NO variation under various equivalence ratios: (a) fuel-rich condition and (b) fuel-lean condition. Reproduced with permission from Ref. [12]. © 2001 Elsevier.
Fig. 8
Influence of SO2 addition quantity from different coal types on NO variation under various equivalence ratios: (a) fuel-rich condition and (b) fuel-lean condition. Reproduced with permission from Ref. [12]. © 2001 Elsevier.
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Table 2 provides a summary of the fuel combustion results discussed earlier, with the equivalence ratio being the primary influencing factor. From Table 2, it can be observed that the thermal NOx content decreases at each equivalence ratio due to the catalytic integration of SO2 on radicals, leading to a reduction in the concentration of active radicals and consequently inhibiting the reactions N1–N4, thereby lowering the thermal NOx formation. The fuel NOx content also decreases under lean fuel conditions for the same reason as thermal NOx. However, under rich fuel conditions, the direct reaction of sulfur components with intermediate nitrogen-containing species promotes fuel NOx formation. This result suggests the existence of two distinct mechanisms, with the equivalence ratio determining which mechanism predominates. Therefore, for combustion systems requiring load variations (such as boilers and gas turbines), changes in the equivalence ratio make the interaction between nitrogen and sulfur components nonnegligible. It necessitates the incorporation of an N/S interaction reaction system to further enhance the nitrogen–sulfur evolution mechanism model.

Table 2

Summary of the impact of sulfur components on NOx formation

FuelSulfur additiveNitrogen additiveEquivalence ratio ΦEffect on thermal NOx emissionEffect on fuel NOx emissionRef.
CH4SO20.8–1.2Decreased by up to 36%[42]
C2H6H2S0.8–1.6Decreased by 5–10%[43]
Gas oilSO20.85–1.41Decreased by up to 21%[13]
CH4SO2C2N22.17Increased by 200%[45]
CH4SO2HCN1.71Increased by up to 250%[46]
CH4SO2NH30.7/1.0Decreased by 12–30%[47]
CoalSO21.20 (λ = 0.84)Increased by 8%[12]
0.77 (λ = 1.30)Decreased by 2–6%
FuelSulfur additiveNitrogen additiveEquivalence ratio ΦEffect on thermal NOx emissionEffect on fuel NOx emissionRef.
CH4SO20.8–1.2Decreased by up to 36%[42]
C2H6H2S0.8–1.6Decreased by 5–10%[43]
Gas oilSO20.85–1.41Decreased by up to 21%[13]
CH4SO2C2N22.17Increased by 200%[45]
CH4SO2HCN1.71Increased by up to 250%[46]
CH4SO2NH30.7/1.0Decreased by 12–30%[47]
CoalSO21.20 (λ = 0.84)Increased by 8%[12]
0.77 (λ = 1.30)Decreased by 2–6%

4 The Influence of Nitrogen Components on the Formation of SOx

While much research has focused on the impact of interactions on the formation of NOx, the interaction between the two is mutual, and therefore, the interaction on the evolution of sulfur components cannot be ignored. This section primarily focuses on coal combustion and explores the interactions in the evolution process of SOx.

To mitigate nitrogen oxide emissions, coal powder is typically burned under rich fuel conditions [51]. Jiang et al. [52] investigated the impact of nitrogen–sulfur interactions on sulfur component evolution in coal powder furnaces under air-staged combustion. The study results indicate that in a reducing atmosphere, during the process of SO2 reduction to H2S, NO facilitates the conversion of SH radicals to SO radicals through the reactions NO + SH → NS + OH and NS + NO → N2 + SO, leading to an increase in SO2 concentration and a decrease in H2S concentration. Additionally, as the reducing atmosphere strengthens, the NO concentration decreases, and the interactions become less significant. Furthermore, there is limited research on the influence of NH3 on the SO2 reduction process; it is speculated that NH3 may promote the reduction of SO2 through the reactions NH + SO → NS + OH and NS + NH → N2 + SH.

In an oxidizing atmosphere, sulfur components typically convert to SO2, and the influence of nitrogen components on this process is rarely reported in the literature reviewed by the author. However, in this atmosphere, some of the SO2 will further oxidize to SO3 [53,54], prompting some scholars to investigate the impact of nitrogen components on this process. Fleig et al. [55] conducted a study on SO3 generation under coal powder oxy-fuel combustion technology, and the results indicate that SO3 formation is favored when the NO concentration is typically greater than 50 ppm, with the enhancing effect reaching a maximum at 100 ppm, while higher NO concentrations tend to decrease SO3 formation. The authors attribute this result to the indirect effect of NO on the radical pool through the following reactions [55]:
(3)
(4)
(5)

Reaction (3) enhances the concentration of OH radicals at lower NO concentrations, promoting the formation of SO3, while reactions (4) and (5) consume available OH and H radicals at higher NO concentrations, reducing the formation of SO3. In subsequent studies, the authors consider the reaction NO2 + SO2 → SO3 + NO to be the most significant direct reaction between SOx and NOx [56]. However, due to considerable controversy surrounding the interaction between nitrogen and sulfur, the direct interaction between nitrogen and sulfur was not considered in the model. Choudhury and Padak [57], after incorporating the direct interaction between NOx and SOx, conducted kinetic simulations and sensitivity analyses of methane oxy-fuel combustion experiments using the chemkin software. Figure 9 illustrates the impact of NO concentration on the outlet SO3 concentration under the conditions of equivalence ratio Φ = 0.85 and SO2 concentration = 2500 ppm. It can be observed that the standalone C/H/O/N/S mechanism simulation exhibits significant deviations from experimental values, but the addition of reactions involving direct interactions between N and S significantly improves the model's predictions.

Fig. 9
The relationship between the simulated and experimental values of outlet SO3 + H2SO4 concentration from the reactor and the concentration of NO. The NO concentration varies from 300 ppm to 1500 ppm, with an equivalence ratio of 0.85 and an SO2 concentration of 2500 ppm. Reproduced with permission from Ref. [57]. © 2017 American Chemical Society.
Fig. 9
The relationship between the simulated and experimental values of outlet SO3 + H2SO4 concentration from the reactor and the concentration of NO. The NO concentration varies from 300 ppm to 1500 ppm, with an equivalence ratio of 0.85 and an SO2 concentration of 2500 ppm. Reproduced with permission from Ref. [57]. © 2017 American Chemical Society.
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5 N/S Interaction Mechanism

The interaction between nitrogen and sulfur components in the combustion process has been summarized earlier, revealing the presence of two distinct mechanisms of nitrogen–sulfur interaction: indirect interaction and direct interaction.

5.1 Indirect Interaction.

The nitrogen and sulfur components alter their respective evolutions by changing the radical pool, a mechanism that has been described by many scholars. For example, Dagaut et al. [58] proposed the following reactions in their study on the effects of SO2 and NO on the oxidation of CO/H2 mixtures:
(6)
(7)
(8)
(9)
The aforementioned reactions result in a decrease in the concentration of H radicals through the interaction of NO and SO2. Cerru et al. [59,60] similarly proposed the integrating effect of SO2 on H radicals in their study of sulfur mechanisms. Additionally, SO2 has a similar integrating effect on O and OH radicals [10,61]. The aforementioned reactions can be summarized as follows:
(10)
(11)

In this reaction, X and Y represent O, OH, or H active radicals. This mechanism predominates in the nitrogen–sulfur interaction under lean fuel conditions, affecting the evolution of each component by reducing the concentration of active radicals (O/OH/H).

5.2 Direct Interaction.

Compared to indirect interactions, direct reactions of nitrogen–sulfur intermediates play a major role under rich fuel conditions. The presence of NS radicals has been shown to directly impact the evolution of nitrogen and sulfur components [11,62]. In early studies, many scholars proposed possible reactions based on flame combustion experiments for qualitative analysis. For example, Pfefferle and Churchill [43] proposed the following reactions:
(12)
(13)
(14)
(15)
(16)

Furthermore, other scholars have proposed similar reactions to simulate the nitrogen–sulfur interaction in flames. However, many of these reactions only provide estimated parameters without specific kinetic parameters [63]. Therefore, it is particularly important to accurately calculate the kinetic parameters for direct nitrogen–sulfur reactions.

Table 3 provides a comparison between this study and selected literature regarding the chemical reaction mechanisms. Some of the literature is limited to investigating the interaction in a single atmosphere [42,46,52,55]. Specifically, under reducing atmospheres, the nitrogen–sulfur interaction manifests as a direct reaction between the two components, while under oxidizing atmospheres, the interaction is primarily characterized by the catalytic integration or consumption of radicals by SO2/NO. It is worth noting that although Hampartsoumian et al. [12] considered both atmospheres and reached conclusions similar to those for a single atmosphere, their focus was solely on the impact of sulfur on NOx formation. Furthermore, in examining direct interactions, they did not adequately consider reactions between nitrogen-containing intermediates such as NOx/HCN, beyond NH radicals, and sulfur components. Due to the complexity and diversity exhibited by nitrogen and sulfur components in actual combustion processes, a comprehensive investigation into the interactions between various nitrogen and sulfur components is crucial. Building upon previous literature, this study proposes new reaction mechanisms for nitrogen–sulfur interactions under different atmospheric conditions. Under oxidizing atmospheres, nitrogen and sulfur components tend to participate in the form of stable oxides through NO and SO2, thereby altering the composition and distribution within the radical pool as indicated by the reaction formulas in the table. Conversely, under reducing atmospheres, nitrogen and sulfur components lean toward direct reactions in the form of NOx/HCN/NHi (i = 0–3) and SOx/H2S, with specific reaction mechanisms depending on the combustion conditions.

Table 3

Comparison of the proposed nitrogen–sulfur interaction mechanisms in selected literature with the results of this study

Nitrogen–sulfur interaction mechanismRef.
Equivalence ratio > 1 (reducing atmosphere)Equivalence ratio < 1 (oxidizing atmosphere)
NOx/HCN/NHi(i = 0–3) + SOx/H2S → NS+…NO/SO2 + X + M → XNO/XSO2 + MThis article
NO/XSO2 + Y + M → NO/SO2 + XY + M
NH + SO → NO + SHSO2 + H + M → HSO2 + M
HSO2 + OH → H2O + SO2
[12]
SO2 + H + M → HSO2 + M
HSO2 + OH → H2O + SO2
[42]
N + SO → NO + S[46]
NO + SH → NS + OH
NS + NO → N2 + SO
[52]
NO + HO2 → NO2 + OH
NO + O + M → NO2 + M
NO + OH + M → HONO + M
[55]
Nitrogen–sulfur interaction mechanismRef.
Equivalence ratio > 1 (reducing atmosphere)Equivalence ratio < 1 (oxidizing atmosphere)
NOx/HCN/NHi(i = 0–3) + SOx/H2S → NS+…NO/SO2 + X + M → XNO/XSO2 + MThis article
NO/XSO2 + Y + M → NO/SO2 + XY + M
NH + SO → NO + SHSO2 + H + M → HSO2 + M
HSO2 + OH → H2O + SO2
[12]
SO2 + H + M → HSO2 + M
HSO2 + OH → H2O + SO2
[42]
N + SO → NO + S[46]
NO + SH → NS + OH
NS + NO → N2 + SO
[52]
NO + HO2 → NO2 + OH
NO + O + M → NO2 + M
NO + OH + M → HONO + M
[55]

With the advancement of computer technology, computational chemistry methods have become a powerful tool for studying the dynamic models of chemical reactions. Hu et al. [64] utilized the Gaussian 09 quantum chemistry software, based on density functional theory and transition state theory, to calculate the rate constants of reactions between NO/NO2 and sulfur-containing small molecules (SH/S/SO/NS) during combustion processes. Hassani et al. [65] employed the same software to compute the kinetic parameters of the reaction NH + SO within the temperature range of 300–3000 K, enhancing the integrity of the N/S direct interaction reaction system and the accuracy of the kinetic parameters. However, there remains debate over certain reaction pathways and kinetic parameters [66], and the precision of reaction kinetic parameters depends on the development of theoretical calculation levels, which may have limitations. Therefore, perfecting the N/S interaction reaction system is of significant importance for the accurate prediction of nitrogen and sulfur pollutants [67,68].

6 Summary and Outlook

Against the backdrop of addressing climate change and achieving sustainable development goals, the formation and control of key pollutants, namely, nitrogen oxides (NOx) and sulfur oxides (SOx), from fuel combustion have emerged as a central focus in combustion chemistry research. Given the highly coupled evolution of nitrogen and sulfur species during the combustion process, single reaction mechanisms fall short in adequately elucidating this complexity. Consequently, this article delves into a detailed exposition of the interplay between nitrogen and sulfur components. The article summarizes experimental combustion studies on different fuels and elucidates the impact of nitrogen and sulfur components on each other's evolution under varying equivalence ratios and N/S ratios. Furthermore, a microkinetic analysis of the reaction mechanisms governing N/S interactions was conducted, leading to the following conclusions.

In rich fuel combustion, the presence of SO2 typically promotes the generation of NOx, while under lean fuel conditions, it inhibits NOx formation. Similarly, the evolution of nitrogen components is also influenced by equivalence ratios. Under rich fuel conditions, NO promotes the formation of SO2, whereas, under lean fuel conditions, small amounts of NO promote the formation of SO3, but high NO concentrations inhibit SO3 formation. Generally, the equivalence ratio determines whether the interaction is promotive or inhibitory, while the extent of change in the N/S ratio can adjust the intensity of this effect. Notably, this effect becomes particularly pronounced when the N/S ratio reaches higher levels, with variations in its impact ranging from 2% to 250%, indicating that the N/S ratio is also a significant influencing factor. However, when the nitrogen or sulfur content is at a low threshold, its intervention in the overall reaction process can be considered secondary and does not constitute a significant impact.

From the perspective of reaction kinetics at the microscopic level, there are two different interaction mechanisms between nitrogen and sulfur, with the actual reaction pathway depending on the equivalence ratio conditions during combustion. Specifically, when the equivalence ratio is greater than 1, the direct reaction of nitrogen–sulfur intermediates plays a dominant role. The primary pathway involves interactions between nitrogen–sulfur intermediates such as HCN/NO/NHi (where i ranges from 0 to 3) and H2S/SOx, with the NS radical being the direct product linking the two. Conversely, when the equivalence ratio is less than 1, nitrogen and sulfur components tend to react in the form of stable oxides like NO and SO2. The specific reactions can be represented as NO/SO2 + X + M → XNO/XSO2 + M and subsequent reactions XNO/XSO2 + Y + M → NO/SO2 + XY + M. These reactions alter the concentration distribution of active radicals such as H, OH, and O in the radical pool, thereby affecting the formation mechanisms of nitrogen and sulfur compounds.

Although significant progress has been made, current research tends to focus more on the effects of interactions on the evolution of nitrogen components, with fewer studies dedicated to sulfur components. Therefore, research on the interactions in the evolution of sulfur components is needed. Additionally, the calculation of kinetic parameters for direct reactions is limited by the computational capabilities at the time, resulting in low accuracy and a narrow temperature range of applicability. This limitation makes it challenging to accurately describe the interactions between nitrogen and sulfur species during combustion processes. Therefore, improvements to the N/S interaction mechanism model are necessary. It is expected that enhancing the understanding of nitrogen–sulfur component interactions will lead to more reliable computational results in numerical simulations, providing stronger theoretical support for predicting and reducing nitrogen and sulfur pollutants in fuel combustion.

Funding Data

  • The Basic Research Program of Shanxi Province (Grant No. 20210302123199).

  • The Major Special Projects of Science and Technology in Shanxi Province (Grant No. 20201102006).

Conflict of Interest

There are no conflicts of interest.

Data Availability Statement

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

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