Review on the Interaction Mechanism of Nitrogen/Sulfur Pollutants During Fuel Combustion

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 (NO_x) and sulfur oxides (SO_x) 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 SO_x 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 NO_x 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 NO_x [12,13]. Furthermore, numerous studies have confirmed that the interaction between the two alters their respective evolution mechanisms [14–16], highlighting the importance of understanding the interaction mechanism between them for predicting the concentration distribution of NO_x and SO_x 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.

In the combustion system, the formation of NO_x 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 SO_x is more significantly governed by fuel characteristics and typically remains unaffected by combustion conditions [2,5]. Considering this, the suppression of NO_x 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 NO_x emissions by altering fuel properties. Additionally, optimizing combustion organization through adjusting injection strategies also contributes to NO_x reduction. Research on this control strategy may also provide ideas for reducing SO_x 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 NO_x and SO_x in combustion and analyzes the mechanism of pollutant formation.

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 NO_x 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 NO_x, and Fig. 1 summarizes these three mechanisms of NO_x formation.

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

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.

The sulfur content can have an impact on the formation of NO_x during fuel combustion, and this process depends on factors such as equivalence ratio and N/S ratio [12,39–41]. For thermal NO_x, sulfur components inhibit NO_x formation across all equivalence ratios. However, for fuel NO_x, sulfur components facilitate NO_x 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 NO_x 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 NO_x is often overlooked.

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

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 NO_x 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 NO_x emissions, and the sulfur component led to a 5–10% reduction in NO_x produced at the outlet of the combustion chamber. Consistent with Wendt's conclusion, the reduction in NO_x 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 NO_x.

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.

The combustion experiments described earlier all indicate that in the presence of sulfur components, the formation of thermal NO_x 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 NO_x.

Fuel NO_x 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 NO_x 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.

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 NO_x and SO_x formation.

Under lean fuel conditions, the impact of sulfur components on NO_x 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 [48–50].

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.

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.

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 NO_x 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 NO_x formation. The fuel NO_x content also decreases under lean fuel conditions for the same reason as thermal NO_x. However, under rich fuel conditions, the direct reaction of sulfur components with intermediate nitrogen-containing species promotes fuel NO_x 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.

While much research has focused on the impact of interactions on the formation of NO_x, 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 SO_x.

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.

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 SO_x and NO_x [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 NO_x and SO_x, 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.

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.

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).

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 NO_x formation. Furthermore, in examining direct interactions, they did not adequately consider reactions between nitrogen-containing intermediates such as NO_x/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 NO_x/HCN/NH_i (i = 0–3) and SO_x/H₂S, with specific reaction mechanisms depending on the combustion conditions.

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].

Against the backdrop of addressing climate change and achieving sustainable development goals, the formation and control of key pollutants, namely, nitrogen oxides (NO_x) and sulfur oxides (SO_x), 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 NO_x, while under lean fuel conditions, it inhibits NO_x 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/NH_i (where i ranges from 0 to 3) and H₂S/SO_x, 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.

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

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

There are no conflicts of interest.

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