After-Treatment Technologies for Emissions of Low-Carbon Fuel Internal Combustion Engines: Current Status and Prospects

Abstract

In response to increasingly stringent emission regulations, low-carbon fuels have received significant attention as sustainable energy sources for internal combustion engines. This study investigates four representative low-carbon fuels, methane, methanol, hydrogen, and ammonia, by systematically summarizing their combustion characteristics and emission profiles, along with a review of existing after-treatment technologies tailored to each fuel type. For methane engines, unburned hydrocarbon (UHC) produced during low-temperature combustion exhibits poor oxidation reactivity, necessitating integration of oxidation strategies such as diesel oxidation catalyst (DOC), particulate oxidation catalyst (POC), ozone-assisted oxidation, and zoned catalyst coatings to improve purification efficiency. Methanol combustion under low-temperature conditions tends to produce formaldehyde and other UHCs. Due to the lack of dedicated after-treatment systems, pollutant control currently relies on general-purpose catalysts such as three-way catalyst (TWC), DOC, and POC. Although hydrogen combustion is carbon-free, its high combustion temperature often leads to elevated nitrogen oxide (NO_x) emissions, requiring a combination of optimized hydrogen supply strategies and selective catalytic reduction (SCR)-based denitrification systems. Similarly, while ammonia offers carbon-free combustion and benefits from easier storage and transportation, its practical application is hindered by several challenges, including low ignitability, high toxicity, and notable NO_x emissions compared to conventional fuels. Current exhaust treatment for ammonia-fueled engines primarily depends on SCR, selective catalytic reduction-coated diesel particulate filter (SDPF). Emerging NO_x purification technologies, such as integrated NO_x reduction via hydrogen or ammonia fuel utilization, still face challenges of stability and narrow effective temperatures.

1. Introduction

With the accelerating pace of urban industrialization, countries around the world are confronted with the major issue of global climate change and environmental warming. Reducing greenhouse gas emissions as much as possible has become an essential part of ecological and environmental construction worldwide. Internal combustion engines, as the core power units for transportation, engineering machinery, and marine vessels, consume approximately 70% of the world’s petroleum resources, making them the primary sector of oil consumption and a significant source of greenhouse gas (GHG) emissions. To address the challenge of reducing GHG emissions from internal combustion engines, the utilization of low-carbon alternative fuels—such as methane, methanol, ammonia, and hydrogen—has emerged as a pivotal research focus in ICE technology. These fuels offer significant potential to decarbonize transportation and industrial sectors while maintaining engine performance. However, these alternative fuels still face critical challenges, particularly in pollutants, include nitrogen oxides (NO_x), particulate matter (PM), hydrocarbons (HCs), and carbon monoxide (CO) [1,2,3,4]. Among the various application fields of internal combustion engines, road vehicles face the most stringent exhaust pollutant standards globally [5,6,7]. To better regulate vehicle pollutant emissions, countries around the world have progressively implemented stricter emission standards [8,9], as shown in Figure 1. Since California introduced the first emission regulations in 1966, and up to the present day with the Euro VII emission standards, the limits on harmful emissions have become increasingly stringent, approaching “zero emissions.” Non-road mobile machinery and marine engines currently operate under comparatively lenient emission standards. However, this regulatory gap is narrowing as international bodies (IMO Tier III for ships) and regional policies (EU Stage V for off-road equipment) progressively adopt cleaner fuel specifications and after-treatment technologies. Therefore, in response to these tightening emission regulations and to better protect the atmospheric environment, it is essential to use low-carbon fuels, develop new low-emission engines, and employ after-treatment technologies to purify emission pollutants.

The energy structure of the internal combustion engine industry is gradually transitioning from high-carbon fossil fuels to carbon-neutral and low-carbon energy sources. Currently, commercially deployed low-carbon alternative fuels include biodiesel and methane. Emerging solutions such as renewable hydrogen, methanol, and ammonia (produced via catalytic synthesis of renewable hydrogen and carbon dioxide/nitrogen) demonstrate lifecycle CO₂ reductions of 70–90%. However, due to the high energy demand for their production, these fuels still face challenges for large-scale application. In addition, when fossil fuels are used, engines often produce pollutants like soot and NO_x, which also cause environmental problems. To assess the emission limits of vehicles in use on the road, European legislation has introduced a series of periodic technical requirements. This approach makes it easier to identify vehicles that exceed exhaust emission limits. In addition, several European cities have established low-emission zones and implemented emission limits for vehicles operating within these areas. At the same time, exposure limits for individuals have been set for older engines that are still in service [11]. So finding efficient combustion and low-pollution after-treatment technologies for low-carbon fuel engines is a key focus of current internal combustion engine combustion technology development.

At present, the use of low-carbon fuels is becoming increasingly diverse, and various methods are available for their production. The production process of vehicle-grade natural gas mainly includes extraction, purification, compression, storage, and transportation. Raw natural gas extracted from underground contains impurities such as moisture and sulfur compounds, which must be removed through dehydration, desulfurization, and other treatment processes to meet fuel quality standards [12]. The purified gas is then compressed to high pressure and delivered to refueling stations via pipelines or transport vehicles for vehicle use. In contrast to natural gas, methanol can be produced through a wider range of methods, including biomass-based methanol, fossil fuel-derived methanol, and green methanol [13]. Conventional fossil fuel-based methanol is primarily produced from natural gas. The typical process involves steam methane reforming to generate syngas, which is then converted into methanol over a catalyst under high-temperature and high-pressure conditions. Biomass-based methanol, on the other hand, is derived from raw materials such as agricultural and forestry residues, straw, and wood. These materials are subjected to pyrolysis or gasification to produce syngas, which is subsequently converted into methanol. Green methanol is produced by synthesizing methanol from CO₂ and green hydrogen, the latter being generated from renewable energy sources.

Hydrogen is currently produced primarily from fossil fuels, mainly through two approaches: hydrocarbon reforming and pyrolysis [14]. Among these, steam methane reforming is the most widely used method. In addition, hydrogen can also be produced from renewable energy sources, commonly referred to as green hydrogen. At present, one of the commonly used approaches for producing hydrogen from renewable sources is biomass conversion, which includes both thermochemical and biological pathways. The thermochemical methods involve processes such as gasification, pyrolysis, combustion, and liquefaction to convert biomass into hydrogen [15], while biological methods mainly utilize biophotolysis and biological conversion reactions to generate hydrogen from biomass [16]. The production of ammonia is closely tied to hydrogen, as it is primarily synthesized from nitrogen and hydrogen under high temperature, high pressure, and in the presence of a catalyst. Nitrogen is typically obtained from air, while the hydrogen used for producing green ammonia is mainly generated through water electrolysis—commonly referred to as green hydrogen [13].

2. Low-Carbon Fuels and Corresponding Emission After-Treatment Technologies

2.1. Methane Fuels

Natural gas, as the third-largest global energy source after coal and oil, plays a crucial role in the transition towards low-carbon energy. Its application in internal combustion engines is increasing, highlighting the growing importance of natural gas in this field [17]. Natural gas, as a complex mixture, is composed of 85% to 96% methane [18]. Therefore, the use of natural gas is, to a large extent, essentially the use of methane. However, during actual engine operation, methane slip can occur due to factors such as poor engine design and low fuel flammability [19]. Secondly, methane is primarily derived from natural gas processing. Raw natural gas typically contains various impurities such as water vapor, carbon dioxide (CO₂), and hydrogen sulfide, and therefore must undergo processing to meet usage standards. During this processing stage, methane leakage can easily occur due to issues such as dehydrator venting or damaged casing equipment. As a result, methane emissions are not limited to the usage phase but also exist during the production process [12]. Some researchers conducted a modeling study on the global warming potential (GWP) of methane and found that over a 100-year time horizon, methane accounted for 81.5% of the total GWP, indicating that methane is one of the major contributors to global warming [20]. Several organizations and companies have discarded methane as a fuel option due to concerns over its global warming impact. To address this issue, common strategies include adding hydrogen to the combustion chamber to improve combustion and using after-treatment technologies to process the exhaust gases [21,22]. Nevertheless, compared to other low-carbon fuels such as ammonia and hydrogen, methane offers both advantages and disadvantages.

2.1.1. The Use of Methane Fuels

Firstly, methane benefits from an established infrastructure for transportation, storage, and distribution, allowing it to be directly utilized through existing pipelines, whereas ammonia and hydrogen require new infrastructure. Compared to methane, hydrogen has a wider flammability range, making it more likely to form flammable mixtures with air in the event of a leak, thereby increasing the risk of ignition [23]. When burned, methane emits fewer CO₂ and NO_x, whereas ammonia usage can lead to NO_x emissions, and some hydrogen production methods also involve the use of methane, such as steam methane reforming, which produces hydrogen by reacting methane with high-temperature steam [24]. In terms of cost, methane is relatively inexpensive, and its price is stable, while hydrogen, particularly green hydrogen, has higher production costs. Overall, methane offers advantages in terms of infrastructure, cost, and technological adaptability, but both hydrogen and ammonia hold significant potential for future low-carbon energy transitions.

Methane can be divided into two main types: conventional methane and unconventional methane. Conventional methane is typically found alongside oil, while unconventional methane must be extracted from sources such as shale, tight sandstone, coal seams, deep aquifers, and deep-sea sediments. The extraction of unconventional methane is more challenging compared to conventional methane [25]. Although the sources of unconventional methane vary, its main components are generally consistent, including methane (CH₄), ethane, propane, isobutene, CO₂, and N₂ [26]. Compared to gasoline and diesel, methane produces the lowest pollutant emissions over its entire lifecycle, including extraction, production, storage, transportation, and use [27].

In addition to processing natural gas into methane for direct use, it is also commonly used in combination with diesel in dual-fuel (DF) engines. Compared with conventional diesel engines, DF engines typically emit more CO and HC, but less NO_x. This is because DF engines operate with two combustion modes: non-premixed combustion of diesel and premixed combustion of natural gas. While diesel combustion generates a certain amount of NO_x, its emissions are lower than those of conventional diesel engines due to the small quantity of diesel used [28]. Natural gas combustion, on the other hand, only leads to a noticeable increase in NO_x emissions under high load conditions, when the mixture becomes richer and combustion temperature rises. However, even in such cases, NO_x emissions remain lower than those of conventional diesel combustion [28]. Additionally, DF engines exhibit higher brake thermal efficiency (BTE) under high load, while BTE is relatively lower at medium and low loads [29]. This lower BTE can lead to increased HC and CO emissions under these conditions [30].

2.1.2. After-Treatment System for Methane Engines

Methane engines, recognized as a promising low-carbon fuel alternative. However, similar to conventional gasoline and diesel engines, they continue to emit pollutants during operation, necessitating the adoption of effective emission control strategies. Among these pollutants, unburned hydrocarbons (UHCs), with methane as the primary component, are particularly significant [31]. Since the current emission standards in some regions do not clearly distinguish between methane and non-methane hydrocarbons, regulating UHC emissions remains difficult. Nonetheless, with increasingly stringent environmental regulations, enhancing the control of UHC emissions, especially methane, has become an urgent priority.

At present, the three-way catalyst (TWC) remains one of the primary technologies used to treat emissions from methane engines [32,33]. TWC can effectively remove non-methane hydrocarbons while simultaneously reducing CO and NO_x, all within a relatively compact after-treatment system. However, due to the stable molecular structure and high auto-ignition temperature of methane, it is difficult to oxidize at low temperatures [34]. Moreover, methane engines require stricter lambda control, which also results in fluctuations in the surrounding exhaust gas environment [35]. These make it challenging to achieve effective conversion using TWC alone.

Currently, further reductions in methane and other HC emissions can be achieved by optimizing the mixture formation process and adopting lean-burn combustion strategies in engine operation [36,37]. While enhancing the mixture formation process can help reduce UHC emissions to some extent, it may also lead to increased fuel consumption and higher NO_x emissions. To balance NO_x control and fuel economy, exhaust gas recirculation (EGR) technology is often employed to reduce thermal load by reintroducing a portion of the exhaust gas into the combustion chamber, thereby lowering the combustion temperature. However, when applied to dual-fuel engines such as diesel/methane systems, EGR may contribute to increased PM emissions. In addition, one of the main drawbacks of EGR is that it can significantly lower exhaust temperatures, which in turn further reduces the effectiveness of the TWC in oxidizing methane [34,38]. In contrast, lean-burn combustion offers the advantage of maintaining high thermal efficiency while reducing fuel consumption and significantly lowering the engine’s raw NO_x emissions. At the same time, lean-burn combustion produces an oxygen-rich exhaust, which facilitates oxidation reactions in after-treatment systems. However, due to the lower exhaust temperatures associated with lean-burn operation, additional catalytic systems are still required to effectively remove NO_x. In addition to methane and NO_x, methane engines also emit other pollutants, such as formaldehyde (HCHO) and small amounts of PM [39]. Formaldehyde poses serious health risks due to its neurotoxicity and potential carcinogenicity, and therefore requires strict control under ultra-low emission standards [40]. HCHO formation is primarily attributed to the incomplete oxidation of methane in low-temperature combustion zones. PM emissions may originate from unburned hydrocarbons or partial combustion of lubricating oil in direct contact with the flame. Nevertheless, compared with conventional diesel engines, methane engines generally produce lower levels of PM emissions [41].

In addition to optimizing combustion strategies and employing TWC, exhaust purification in methane engines also relies heavily on noble metal catalysts coated onto carrier materials. Commonly used active metals include platinum (Pt), palladium (Pd), and rhodium (Rh), which are typically supported on alumina or ceria–zirconia composite oxides. Under lean-burn conditions, Pt and Pd generally exhibit better catalytic oxidation performance for methane compared to Rh [22]. However, noble metal catalysts often suffer from poor thermal stability and are susceptible to deactivation due to water vapor interference and sulfur poisoning during high-temperature operation. As a result, enhancing the stability and durability of these catalysts while maintaining high conversion efficiency has become a critical focus in current research on after-treatment technologies.

As previously discussed, the purification of methane relies on noble metal catalysts supported on substrate materials. Therefore, the after-treatment system of methane engines shares the same structural framework used in conventional gasoline or diesel engines while implementing specific design modifications to enhance methane oxidation efficiency. For instance, placing a diesel oxidation catalyst (DOC) at the upstream section of the system can significantly enhance both methane conversion efficiency and catalyst durability [42]. The DOC is generally designed with a flow-through structure and positioned at the front of the after-treatment system, as shown in Figure 2. Catalysts such as Pt and Pd are commonly used in DOC, which function primarily by lowering the activation energy of chemical reactions for exhaust pollutants, thereby enabling oxidation at relatively low temperatures. In addition, the DOC can contribute to the removal of acetaldehyde and certain exhaust odors [43], making it, alongside the TWC, one of the key components in current methane engine after-treatment systems. However, the DOC may also generate byproducts such as ammonia (NH₃) and nitrous oxide (N₂O) during methane oxidation, which pose risks of secondary pollution [42]. Therefore, future efforts should focus on precise control of exhaust gas composition and the optimization of catalytic oxidation strategies in DOC to mitigate these issues.

Kinnunen et al. [45] proposed a combined methane purification strategy based on the TWC configuration by placing the TWC upstream of the methane oxidation catalyst to form a synergistic system with the downstream catalyst. This setup provides a certain degree of sulfur storage capability under lean-burn conditions, thereby improving the sulfur tolerance of the after-treatment system and reducing the risk of sulfur poisoning to some extent. Moreover, under rich-burn conditions, the TWC can promote the formation of reductive gas species, which further enhances the purification performance of downstream catalysts. To efficiently remove methane while reducing cost, some studies have explored the use of ozone instead of oxygen for catalytic oxidation. Keenan et al. [46] conducted ozone-assisted oxidation experiments using an iron-based zeolite catalyst and achieved a methane conversion rate of up to 60% at 220 °C. This approach demonstrated promising potential under low-temperature conditions, particularly for large-scale stationary engine systems. Yasumura et al. [47], using a computational catalyst design approach, developed a proton-type β-zeolite catalyst containing silicon and aluminum, which significantly improved methane combustion efficiency in the presence of ozone. This catalyst outperformed conventional Pd-based catalysts in the temperature range of 100 to 300 °C, and also exhibited good water and sulfur tolerance. However, under higher temperature conditions, traditional catalysts such as Pd still demonstrated superior catalytic activity. Although ozone has demonstrated promising performance in methane purification, the energy consumption involved in its generation and the potential formation of secondary pollutants such as NO_x cannot be overlooked. Particularly in working environments containing water vapor and sulfur species, greater attention must be paid to the low-temperature conversion of methane. Similarly, as after-treatment systems become increasingly complex, thermal management becomes especially important for large-bore engines [48].

Although ozone has unique advantages in assisting methane purification, its energy consumption and the potential for secondary pollution should not be overlooked. To begin with, due to its strong oxidative properties, ozone can cause significant corrosion to metal pipelines and sealing components, requiring the use of ozone-resistant materials, which increases both system costs and technical complexity. Furthermore, under oxygen-rich conditions, ozone readily reacts with NO to form NO₂, further complicating the practical application of this technology.

2.2. Methanol Fuel

2.2.1. The Use of Methanol Fuel

Methanol is widely used as a low-carbon and clean alternative fuel in engine applications. Besides NO_x, formaldehyde is an important component of UHC emitted from methanol engines. As the combustion temperature of methanol increases, methanol reacts with OH radicals to form CH₂OH and CH₃O, both of which subsequently react with oxygen to produce formaldehyde. The specific reactions are shown in Equations (1)–(4). To reduce the emissions of such pollutants, technologies such as low-temperature combustion, water or ethanol injection, and combustion chamber optimization are commonly employed to improve overall engine performance and enhance combustion efficiency, thereby limiting pollutant formation [49]. Under the limitations of current in-cylinder control strategies, additional exhaust after-treatment technologies are typically required to further reduce methanol and formaldehyde emissions. However, there are currently no dedicated after-treatment systems specifically designed for these two pollutants. Instead, pollutant control is generally achieved using devices such as the DOC, TWC, and particulate oxidation catalyst (POC), as previously discussed [50,51,52].

$$\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}+\mathrm{O}\mathrm{H}=\mathrm{C}{\mathrm{H}}_2\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}$$

(1)

$$\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}+\mathrm{O}\mathrm{H}=\mathrm{C}{\mathrm{H}}_3\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(2)

$$\mathrm{C}{\mathrm{H}}_2\mathrm{O}\mathrm{H}+{\mathrm{O}}_2=\mathrm{C}{\mathrm{H}}_2\mathrm{O}+\mathrm{H}{\mathrm{O}}_2$$

(3)

$$\mathrm{C}{\mathrm{H}}_3\mathrm{O}+{\mathrm{O}}_2=\mathrm{C}{\mathrm{H}}_2\mathrm{O}+\mathrm{H}{\mathrm{O}}_2$$

(4)

As engine load increases, the in-cylinder temperature rises, which facilitates the oxidation of formaldehyde. Meanwhile, at different engine speeds, formaldehyde emissions exhibit a trend of first increasing and then decreasing with the rise in the EGR rate. Higher formaldehyde emissions occur at low engine speeds, possibly due to the reduction in combustion temperature caused by EGR. At higher engine speeds, the in-cylinder temperature increases significantly, and the exhaust gas flow rate accelerates, shortening the residence time in the high-temperature zone, thereby affecting formaldehyde emissions [53]. Changes in engine load, speed, and EGR rate influence the temperatures inside the cylinder and exhaust system, thus impacting formaldehyde emissions [54]. Complete oxidation of formaldehyde requires sufficiently high temperatures [55].

Moreover, the methanol substitution rate (MSR) and injection strategy significantly influence the homogeneity of the methanol–air mixture [56], which in turn affects in-cylinder combustion quality and pollutant emissions [57]. Introducing pilot injection helps reduce methanol emissions, and advancing the pilot injection timing further decreases emissions by raising the in-cylinder temperature prior to combustion, thereby promoting methanol vaporization and oxidation [58]. An increase in MSR tends to lower the in-cylinder temperature and raise the methanol concentration, which can lead to incomplete combustion and elevated formaldehyde emissions [59]. Pilot injection facilitates better fuel oxidation, as evidenced by the fact that a single injection typically results in the highest formaldehyde emissions. As the amount of pilot injection increases, the duration of high in-cylinder temperatures is prolonged, promoting further oxidation of formaldehyde and consequently reducing its emission level.

Gong et al. [60] investigated the effects of different equivalence ratios on methanol and formaldehyde emissions. The results indicated that when the equivalence ratio reached 0.5, emissions of formaldehyde and unburned methanol decreased significantly. This reduction is attributed to the improved in-cylinder combustion conditions, which enhance the oxidation of methanol and formaldehyde. Further analysis revealed that when the equivalence ratio exceeds 0.5, the lower in-cylinder temperature helps maintain low levels of both formaldehyde and unburned methanol emissions.

To avoid the formation of formaldehyde, methanol compression ignition technology can be used to raise the in-cylinder temperature and achieve methanol auto-ignition. This approach effectively reduces formaldehyde emissions during methanol use, while the resulting CO and NO_x emissions are comparable to those of conventional diesel engines [61]. To further reduce NO_x emissions, methanol–diesel dual-fuel combustion technology can be employed to control NO_x formation during methanol utilization. This method can suppress up to 50% of NO_x emissions and also reduce soot formation [62]. However, this technology requires precise control of the methanol ratio; an excessively high methanol proportion may deteriorate cold-start performance, leading to increased CO and HC emissions [63]. In addition, due to methanol’s hygroscopic nature, Sileghem et al. [64] investigated the emission differences between neat methanol and water-containing methanol. The results showed that water-containing methanol produced lower NO_x emissions than pure methanol. Therefore, in addition to minimizing formaldehyde formation, addressing NO_x emissions is also a critical issue in methanol fuel applications.

2.2.2. After-Treatment System for Methanol Engines

The purification of methanol and formaldehyde by the DOC primarily relies on the synergistic effect between the catalyst and oxygen under high-temperature conditions. As exhaust gases pass through the DOC, unburned methanol and formaldehyde react with oxygen and are converted into CO₂ and water (H₂O). During this process, the catalyst facilitates the catalytic oxidation of these unburned components. As previously noted, TWC is not only effective in reducing emissions of HC, CO, and NO_x, but also exhibits a certain capability in removing unburned methanol and formaldehyde [65,66]. The POC proposed by Ecocity Oy in 2004 is a semi-flow-through filtration device designed for exhaust gas treatment [67]. Constructed from corrugated stainless-steel mesh and featuring a multi-layer folded design with non-blocking channels, the structure of the POC is illustrated in Figure 3. This device can effectively reduce HC compounds and CO produced from the combustion of methane or methanol, achieving a purification efficiency ranging from 40% to 70%.

Compared with conventional exhaust purification devices, the POC features a simpler structure, lower operating costs, and can function independently without complex system integration. Recent studies have shown that placing the POC downstream of the DOC can further improve the removal efficiency of unburned methanol and formaldehyde [68].

2.3. Hydrogen Fuel

2.3.1. The Use of Hydrogen Fuel

As a representative zero-carbon fuel, hydrogen produces only water vapor upon combustion, with negligible emissions of CO₂, PM, or UHC, making it widely regarded as a key energy carrier for achieving zero-carbon emissions. Hydrogen possesses exceptionally low ignition energy, a wide flammability range, and a high flame propagation speed, which provides excellent combustion reactivity and rapid flame development in engines. These properties make hydrogen well-suited for lean-burn conditions and contribute to improved thermal efficiency. However, hydrogen combustion is still associated with certain emission risks and engineering challenges. Its high flame temperature may substantially promote NO_x generation, especially during high load operation with fuel-rich combustion conditions. Furthermore, hydrogen’s low molecular weight and high diffusivity characteristics can lead to inhomogeneous mixture formation, resulting in localized fuel-rich zones. These zones may induce abnormal combustion phenomena such as knocking, pre-ignition, or backfire, thereby compromising engine operational stability and long-term durability. In summary, hydrogen itself is a clean fuel that does not directly produce pollutant emissions during combustion. The primary challenge lies in the formation of NO_x, which occurs indirectly due to variations in combustion conditions. Therefore, the application of hydrogen in internal combustion engines requires not only the optimization of hydrogen supply and combustion control strategies but also the integration of efficient NO_x after-treatment technologies to achieve low-emission and high-efficiency combustion, in line with increasingly stringent future emission regulations.

To reduce NO_x formation, combustion can be carried out under lean mixture conditions, as the lower combustion temperature in such scenarios helps suppress NO_x emissions. In addition, hydrogen’s excellent diffusivity promotes better air–fuel mixing, which further facilitates efficient combustion [12].

2.3.2. After-Treatment System for Hydrogen Engines

Selective catalytic reduction (SCR) has become a well-established after-treatment technology for engines, extensively employed to mitigate nitrogen oxide (NO_x) emissions. Its primary working principle involves the injection of a urea–water solution through a dosing system, which is then thermally decomposed into ammonia (NH₃) at high temperatures. NH₃ subsequently reacts with NO_x to produce harmless N₂ and H₂O [69]. With a NO_x conversion efficiency exceeding 90%, SCR is currently one of the most effective technologies for controlling NO_x emissions [70]. A schematic diagram of the SCR system is shown in Figure 4.

In the SCR process, adsorption and desorption of NH₃ typically occur first, as shown in Equations (5) and (6). This is followed by the selective catalytic reduction of NO_x. When NO is the dominant component of NO_x, the standard SCR reaction primarily occurs, as shown in Equation (7). When NO and NO₂ are present in approximately equal proportions, the fast SCR reaction takes place, as shown in Equation (8), which features the highest reaction rate. When NO₂ is the dominant component, the slow SCR reaction becomes dominant, as shown in Equation (9). In addition, due to the reversibility between NO and NO₂ and elevated temperatures above approximately 500 °C, side reactions such as the oxidation of NO and NH₃ and the formation of the byproduct nitrous oxide (N₂O) may also occur, as shown in Equations (10)–(12).

$$\mathrm{N}{\mathrm{H}}_3+\mathrm{S}\to \mathrm{N}{\mathrm{H}}_3\left(\mathrm{S}\right)$$

(5)

$$\mathrm{N}{\mathrm{H}}_3\left(\mathrm{S}\right)\to \mathrm{N}{\mathrm{H}}_3+\mathrm{S}$$

(6)

$$4\mathrm{N}{\mathrm{H}}_3\left(\mathrm{S}\right)+4\mathrm{N}\mathrm{O}+{\mathrm{O}}_2\to 4{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}+4\mathrm{S}$$

(7)

$$2\mathrm{N}{\mathrm{H}}_3\left(\mathrm{S}\right)+\mathrm{N}\mathrm{O}+{\mathrm{N}\mathrm{O}}_2\to 2{\mathrm{N}}_2+3{\mathrm{H}}_2\mathrm{O}+2\mathrm{S}$$

(8)

$$8\mathrm{N}{\mathrm{H}}_3\left(\mathrm{S}\right)+{6\mathrm{N}\mathrm{O}}_2\to 7{\mathrm{N}}_2+12{\mathrm{H}}_2\mathrm{O}+8\mathrm{S}$$

(9)

$$4\mathrm{N}{\mathrm{H}}_3\left(\mathrm{S}\right)+{3\mathrm{O}}_2\to 2{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}+4\mathrm{S}$$

(10)

$$2\mathrm{N}\mathrm{O}+{\mathrm{O}}_2\leftrightarrow 2\mathrm{N}{\mathrm{O}}_2$$

(11)

$$2\mathrm{N}{\mathrm{H}}_3\left(\mathrm{S}\right)+2\mathrm{N}{\mathrm{O}}_2\to {\mathrm{N}}_2+{\mathrm{N}}_2\mathrm{O}+3{\mathrm{H}}_2\mathrm{O}+2\mathrm{S}$$

(12)

During the operation of SCR systems, catalyst selection plays a crucial role in determining overall conversion efficiency [71]. Currently, the most commonly used catalysts are copper-based and iron-based zeolites. Copper-based zeolite catalysts offer high NO_x conversion efficiency at low temperatures and exhibit good thermal stability. In contrast, iron-based zeolites demonstrate excellent catalytic performance at higher temperatures, with an effective activity window typically ranging from 458 to 598 °C [72,73].

At present, copper-based zeolites have been successfully applied to NO_x conversion in diesel vehicles in both the European Union and the United States, due to their wide operating temperature window and favorable hydrothermal stability [74]. Chen et al. [75] found that the auto-reduction of isolated copper sites in copper-based zeolites can enhance low-temperature NO_x conversion efficiency in SCR. Shan et al. [76] observed that under fast SCR conditions, hydrothermal aging of copper-based zeolites leads to a reduced inhibitory effect of NO₂ on NO_x conversion. Peng et al. [77] investigated the influence of alkali metal potassium on the physicochemical properties of cerium–copper-based catalysts and reported that cerium modification improves SCR performance and mitigates catalyst deactivation caused by potassium poisoning. Xi and Lee et al. [78,79] demonstrated that prolonged pre-sulfation aging reduces the extent of activity loss in SCR reactions and enhances sulfur resistance at low sulfur loading. They also found that the mobility of copper ions is a key factor determining catalytic activity and, ultimately, SCR conversion efficiency. Shen et al. [80] reported that during steady-state active regeneration, the NO_x conversion efficiency of copper-based SCR catalysts drops to approximately 80.3%.

In addition to copper-based zeolites, Liu et al. [81] proposed that iron-based zeolites, due to their excellent high-temperature NO_x conversion performance and low N₂O selectivity, could serve as a promising complementary catalyst to copper-based systems. Jung et al. [82], through their investigation of N₂O formation during the SCR process, found that N₂O generation is strongly influenced by the NO₂/NO_x ratio at the inlet. They also observed that iron-based zeolites tend to produce N₂O more readily than copper-based ones. Yu and Wang et al. [83,84] explored the roles of iron sites and acidity in the SCR reaction and concluded that the catalytic activity of iron-based zeolites primarily depends on the type and concentration of active iron species. Qiao et al. [85] studied the effect of CO₂ on the SCR performance of iron-based catalysts and found that CO₂ significantly inhibits catalytic activity at temperatures below 300 °C.

Although SCR technology offers significant advantages in NO_x removal, its application in future hydrogen-fueled engines still faces challenges, particularly under low-temperature operation and cold-start conditions, which are difficult to avoid. Under such circumstances, the SCR system often struggles to maintain effective denitrification efficiency due to insufficient reaction temperature. In addition, hydrogen-fueled engines tend to emit a certain amount of unburned hydrogen (H₂) during transient operation conditions [86]. Therefore, achieving efficient NO_x removal under low-temperature conditions while simultaneously utilizing unburned H₂ has become a pressing issue. In response, hydrogen selective catalytic reduction (H₂-SCR) technology has emerged. This approach uses H₂ as a reductant to convert NO_x into harmless N₂ with the help of a catalyst. Notably, it can effectively reduce NO_x even in high-oxygen environments and is less likely to produce secondary pollutants [87,88]. Recent studies have further demonstrated that, with the aid of specific catalysts, the H₂-SCR process can achieve high NO_x conversion efficiency even at temperatures below 200 °C [89].

During the H₂-SCR process, the following main reactions occur [90]:

$$2\mathrm{N}\mathrm{O}+4{\mathrm{H}}_2+{\mathrm{O}}_2\to {\mathrm{N}}_2+4{\mathrm{H}}_2\mathrm{O}$$

(13)

$$2\mathrm{N}\mathrm{O}+3{\mathrm{H}}_2+{\mathrm{O}}_2\to {\mathrm{N}}_2\mathrm{O}+3{\mathrm{H}}_2\mathrm{O}$$

(14)

$$2\mathrm{N}\mathrm{O}+{\mathrm{H}}_2\to {\mathrm{N}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(15)

$$2\mathrm{N}\mathrm{O}+5{\mathrm{H}}_2\to 2\mathrm{N}{\mathrm{H}}_3+2{\mathrm{H}}_2\mathrm{O}$$

(16)

$$2\mathrm{N}\mathrm{O}+{\mathrm{O}}_2\to 2\mathrm{N}{\mathrm{O}}_2$$

(17)

$$2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}$$

(18)

Reaction (13) represents the most desirable pathway, as it produces only N₂ and H₂O. In contrast, reactions (14) and (15) generate N₂O, a potent greenhouse gas with a GWP approximately 300 times that of CO₂, which may lead to secondary environmental pollution. Reaction (16) results in the formation of NH₃ as a byproduct; although NH₃ can participate in NO_x reduction under certain conditions, it is still considered an undesired product. Additionally, the NO₂ produced in reaction (17) is more reactive than NO and plays a significant role in the H₂-SCR process. Reaction (18) involves the combustion of H₂, which competes with NO_x reduction for hydrogen and thus reduces the overall utilization efficiency of the reductant [91].

Compared to other SCR technologies, H₂-SCR offers several advantages: first, the reduction process does not generate CO or CO₂, thereby avoiding secondary pollution; second, the reaction can proceed efficiently under oxygen-rich conditions, with water being the main product; third, hydrogen has diverse sources—it can be directly supplied from onboard hydrogen storage tanks or produced in the exhaust system via the water–gas shift reaction [91]. Additionally, with the advancement of biomass and solar-powered hydrogen production technologies, the cost of hydrogen production is gradually decreasing [92]. In the H₂-SCR process, platinum (Pt) and palladium (Pd) are widely recognized as the primary active metals, capable of significantly enhancing NO_x conversion even at relatively low temperatures [93]. Among them, Pt exhibits high NO_x catalytic activity, while Pd—despite its comparatively lower overall catalytic efficiency—shows better selectivity toward N₂ formation [94].

During the H₂-SCR process, other components in the exhaust gas can also influence catalytic performance. Borchers et al. [95] found that O₂ concentration plays a significant role in NO removal efficiency: at 220 °C with 5% O₂, the NO conversion rate reached nearly 60%, whereas increasing the O₂ concentration to 10% led to a decrease in conversion below 50%. This indicates that proper oxygen control is crucial for enhancing the NO removal efficiency of Pd-based catalysts. Duan et al. [96] reported that moderately increasing the H₂ concentration (from 0.25% to 1%) significantly improves NO_x conversion; however, further increasing the concentration to 2% yields only a slight improvement while reducing N₂ selectivity. This suggests that an appropriate H₂ concentration is beneficial for NO_x reduction. Additionally, their study showed that H₂O can influence the catalytic activity of Pd-based catalysts at different temperatures. Further research by Xu et al. [97] demonstrated that trace amounts of SO₂ can inhibit the H₂-SCR reaction: introducing just 0.01% SO₂ into the gas stream reduced the NO conversion rate from 97% to 78.1%.

2.4. Ammonia Fuel

2.4.1. The Use of Ammonia Fuel

Ammonia has emerged as a promising zero-carbon alternative fuel, garnering significant research interest in recent years. Its synthesis from hydrogen and atmospheric nitrogen, coupled with existing large-scale production infrastructure, guarantees reliable supply chain continuity [98]. NH₃ is easy to liquefy, convenient to store, transport, and offers a higher volumetric energy density than liquid hydrogen. Moreover, its strong pungent odor provides a “self-alarming” characteristic that facilitates early detection and risk prevention in the event of a leak [99,100]. In addition, its high hydrogen content and compatibility with renewable energy sources make NH₃ one of the most promising candidates for future clean energy systems [101,102].

However, the practical application of NH₃ still faces dual challenges related to poor combustion characteristics and significant pollutant emissions. NH₃ exhibits a low laminar burning velocity, along with a high auto-ignition temperature and ignition energy requirement, which makes it difficult to ignite and results in slow flame propagation. These factors contribute to incomplete combustion, ultimately affecting energy efficiency and system stability [103]. More critically, NH₃ combustion leads to substantial NO_x emissions, including both thermal NO_x and fuel NO_x, with emission levels significantly higher than those of conventional hydrocarbon fuels. Experimental studies have shown that the volume fraction of NO in ammonia flames can reach up to 2000 ppm, far exceeding the levels observed in methane and other hydrocarbon fuels [104,105], and in practical applications, this value may be even higher. Beyond NO_x, the emission of residual/unburned NH₃ constitutes another critical environmental challenge. This compound exhibits dual hazards: direct toxicity and corrosivity endangering human respiratory systems, and material degradation risks for engine/exhaust components through chemical attacks on metal surfaces [98]. Compared with conventional hydrocarbon fuels, ammonia exhibits low reactivity, a high auto-ignition temperature, a slow laminar flame propagation speed, and a high minimum ignition energy, all of which limit its application in internal combustion engines [13]. As a result, current approaches often involve using ammonia–diesel or ammonia–hydrogen dual-fuel engines. This means that the exhaust pollutants from ammonia-fueled engines originate not only from ammonia itself, but also from conventional fuels.

Compared with gasoline-fueled internal combustion engines, ammonia-fueled engines exhibit significantly higher NO_x emissions. A similar trend is observed for UHC, while CO emissions show an opposite pattern [106]. In addition, ammonia engines not only emit NO_x during operation but also release considerable amounts of unburned NH₃, which requires further treatment by the after-treatment system [107]. Westlye et al. [108] fixed the hydrogen–ammonia fuel ratio and found that NO is the dominant component of NO_x emissions from hydrogen–ammonia-fueled engines, with NO₂ typically accounting for less than 4%; therefore, the focus should be on NO formation. It is also noteworthy that the exhaust of hydrogen–ammonia-fueled engines may contain N₂O, a greenhouse gas with a global warming potential approximately 300 times that of CO₂. This gas is mainly produced by the reaction between NO₂ and NH₂ radicals derived from unburned ammonia [107].

Ammonia has a high auto-ignition temperature of 651 °C, and relying solely on compression ignition requires a very high compression ratio exceeding 35:1 [109]. Currently, a more feasible approach to using ammonia in compression ignition engines is the dual-fuel mode, where a fuel with a lower auto-ignition temperature is used to initiate ammonia combustion [107]. In ammonia-fueled compression ignition engines, the introduction of ammonia helps reduce PM emissions but also results in increased NO_x and unburned ammonia emissions [110]. When the ammonia fraction is kept below 60%, NO_x emissions tend to decrease compared to conventional diesel engines [111], likely due to the lower flame temperature and the reduction effect of ammonia on NO_x during combustion. However, the lower combustion temperature also leads to increased emissions of UHC and unburned ammonia [107].

2.4.2. After-Treatment System for Ammonia Engines

Similar to NO_x control in hydrogen combustion, SCR has also become one of the primary after-treatment technologies for NH₃ engines due to its high denitrification efficiency [112]. Kurata et al. [113] demonstrated effective NO_x reduction using an SCR system combined with an ammonia–kerosene co-firing strategy. Further research indicated that integrating SCR with low-NO_x combustion methods not only enhances purification efficiency but also allows for downsizing the SCR unit, thereby reducing its impact on the overall system layout—an advantage particularly relevant for high-power NH₃-fueled gas turbines [114]. For the large-scale deployment and commercialization of NH₃ as a fuel, the denitrification after-treatment system must be considered a critical component, especially under operating conditions that demand both high power output and low emissions. Only through the coordinated optimization of combustion and after-treatment systems can NH₃ truly serve as a clean and viable substitute in the ongoing energy transition.

Although SCR is highly effective in reducing NO_x emissions, the use of NH₃ as a fuel introduces the potential risk of ammonia slip. To address this issue, an ammonia slip catalyst (ASC) is typically incorporated into the system to suppress NH₃ release. Additionally, conventional SCR systems only achieve optimal NO_x conversion efficiency at temperatures above 473 K, making it challenging to control NO_x emissions during low-temperature operation or cold-start conditions [103]. Ammonia-fueled engines exhibit notably poor combustion and emission characteristics under low-temperature and low-load conditions, so engines typically adopt a dual-fuel combustion strategy when utilizing NH₃ as fuel, where diesel serves as the ignition source to initiate ammonia combustion. In this case, PM emissions become unavoidable. Against this backdrop, SDPF technology has gained significant attention due to its strong potential for low-temperature NO_x removal and effective PM capture. The core concept of SDPF technology lies in coating the SCR catalyst onto the porous filtration walls of the DPF. As exhaust gases enter through the inlet channel and pass through the filter walls, PM is trapped on the wall surfaces, while in the presence of NH₃, the SCR catalyst facilitates the reduction of NO_x in the exhaust to harmless N₂ [115]. A schematic diagram of the SDPF working principle is shown in Figure 5.

In addition to a series of SCR reactions, soot oxidation reactions also occur in the SDPF. At around 260 °C, NO₂ can oxidize soot, as shown in Equations (19) and (20). When the exhaust temperature is increased above 550 °C using in-cylinder post-injection techniques, oxygen becomes the primary oxidant for soot, as shown in Equations (21) and (22).

$$\mathrm{C}+{\mathrm{N}\mathrm{O}}_2\to \mathrm{C}\mathrm{O}+\mathrm{N}\mathrm{O}$$

(19)

$$\mathrm{C}+{2\mathrm{N}\mathrm{O}}_2\to \mathrm{C}{\mathrm{O}}_2+2\mathrm{N}\mathrm{O}$$

(20)

$$2\mathrm{C}+{\mathrm{O}}_2\to 2\mathrm{C}\mathrm{O}$$

(21)

$$\mathrm{C}+{\mathrm{O}}_2\to \mathrm{C}{\mathrm{O}}_2$$

(22)

Tan et al. [117] investigated the emission characteristics of SDPF under different catalyst coating loadings. The experiments showed that as the catalyst coating loading increased, both the NO_x conversion efficiency and particulate matter filtration efficiency of the SDPF changed. However, excessively high coating loadings led to an increase in exhaust backpressure, which affected performance. When the coating loading was 90 g/L, the SDPF exhibited the best balance between NO_x conversion and particulate matter filtration. Chen et al. [118] investigated the impact of catalyst coating on the emission characteristics and performance of SDPF under different soot loadings. The study found that as the soot loading increased, both the pressure drop and inlet temperature of the SDPF gradually rose, while the NO_x conversion efficiency significantly decreased. When the soot loading reached 9.97 g/L, the SDPF lost its ability to reduce NO_x emissions. The study also showed that in nucleation-mode particle emissions, SDPF could reduce emissions by 2 to 3 orders of magnitude, while the reduction in accumulation-mode particle emissions was less significant. Furthermore, increasing the soot loading improved the particulate matter filtration efficiency of the SDPF, especially at low-speed conditions, where the filtration efficiency for both nucleation-mode and accumulation-mode particles was significantly enhanced.

Kim et al. [119] investigated the impact of different after-treatment systems (SCR, Lean NO_x Trap (LNT) + SCR, and SDPF) on NO_x emissions from Euro 6 compliant light-duty diesel vehicles. By analyzing diesel vehicles sold in South Korea since 2018, the study found that the LNT + SCR after-treatment system is applied to most vehicles meeting the Euro 6 standard. The NO_x emission characteristics were tested under various driving modes, such as the new European driving cycle and the world light vehicle test procedure. The study revealed that NO_x emissions are closely related to the signals from the Engine Control Unit and the Dosage Control Unit. Furthermore, control strategies such as urea injection quantity and timing have a significant impact on emissions, particularly during cold starts and high load conditions. The study also showed that the SDPF system plays a key role in simultaneously reducing both PM and NO_x emissions. Karamitros et al. [120] studied the impact of model-based catalyst partition optimization on the performance of SDPF. The study simulated different coating partition strategies to investigate the effects of coating quantity on SCR reactivity, pressure drop, and NO_x conversion efficiency. The results showed that regionally partitioned coating significantly improved NO_x conversion efficiency under different soot loads, particularly under soot-loaded conditions, where regionally partitioned coatings outperformed uniform coatings. The study also highlighted that regionally partitioned coatings help reduce the consumption of NO₂ in the contaminated front regions, thus enhancing passive regeneration rates and improving SCR reactivity.

In addition, the SDPF system also exhibits certain limitations under frequent cold-start conditions. When the temperature drops below 200 °C or the proportion of NO₂ in NO_x exceeds 0.5, a white crystalline compound known as ammonium nitrate (NH₄NO₃) tends to accumulate on the catalyst surface. This compound can cover the active sites of the SDPF catalyst and inhibit the progress of catalytic reactions [121,122]. To address this issue, increasing the soot content has been proposed as an effective strategy to suppress the formation of NH₄NO₃, thereby enhancing the performance of the SDPF system during cold-start operation [123].

Compared with conventional NO_x treatment technologies such as SCR and SDPF, which rely on external reductants, electrochemical NO_x decomposition has attracted increasing attention in recent years. This approach directly reduces NO_x to harmless products using an externally applied electric current, thereby eliminating issues related to reductant storage, transportation, and potential secondary pollution caused by leakage. The operating principle involves applying an external voltage to an electrolytic cell, which polarizes the solid electrolyte and generates additional oxygen vacancies. At the three-phase boundary on the cathode, NO is reduced to N₂, while the resulting O₂− ions migrate to the anode under appropriate temperature conditions, release electrons, and form O₂ [124]. Although this technology eliminates the need for external reductants and holds great potential for efficient NO_x removal, its high energy consumption limits its application in ammonia-fueled engines. Therefore, this technology is more suitable for ammonia-fueled hybrid vehicles equipped with multiple power sources.

In addition, direct ammonia injection for NO_x reduction is regarded as a novel and promising NO_x control approach. This technology utilizes NH₃ directly as a reductant to react with NO_x, converting it into harmless N₂ and H₂O. It is particularly suitable for ammonia-fueled engines, as it leverages the ammonia already present in the fuel system, eliminating the need for additional reductant supply. Recent studies have proposed the application of this method in high-pressure direct injection (HPDI) ammonia/diesel dual-fuel combustion modes. By carefully managing the timing of post-injection of ammonia, NO_x formation can be effectively suppressed. However, excessive delay in post-injection may lead to a decrease in indicated thermal efficiency and significantly increase emissions of N₂O and unburned NH₃. Research indicates that maintaining the post-injection ammonia ratio within the range of 20% to 30% is generally optimal [125].

In addition, during the combustion of ammonia fuel, not only are NO and NO₂ emitted, but also the byproduct N₂O and unreacted ammonia. N₂O is a potent greenhouse gas with a global warming potential approximately 310 times that of CO₂ and is also considered an ozone-depleting substance [126]. Therefore, it is necessary to implement measures to control N₂O emissions generated during ammonia combustion as well as in SCR and SDPF processes.

At the current stage of thermal decomposition treatment of N₂O, the following four reactions are particularly important [126]:

$${\mathrm{N}}_2\mathrm{O}\to {\mathrm{N}}_2+\mathrm{O}$$

(23)

$${\mathrm{N}}_2\mathrm{O}+\mathrm{O}\to {\mathrm{N}}_2+{\mathrm{O}}_2$$

(24)

$${\mathrm{N}}_2\mathrm{O}+\mathrm{O}\to 2\mathrm{N}\mathrm{O}$$

(25)

$${3\mathrm{N}}_2\mathrm{O}+2\mathrm{N}{\mathrm{H}}_3\to {4\mathrm{N}}_2+3{\mathrm{H}}_2\mathrm{O}$$

(26)

Due to the strong thermal stability of N₂O itself, its thermal decomposition into N₂ and O₂ generally requires high-temperature conditions. Equation (23) only occurs at temperatures exceeding 800 °C. Therefore, current research focuses on lowering the temperature required for the decomposition reaction to enable Equations (24) and (25). Because N₂O thermal decomposition demands considerable energy, some studies suggest that introducing NH₃ as a reductant can improve conversion efficiency, as shown in Equation (26). During N₂O decomposition, catalysts donate electrons to the anti-bonding orbitals of N₂O molecules, weakening the N–O bond and causing N₂O to break down into gaseous N₂ and oxygen atoms adsorbed on the catalyst surface [127].

Pekridis et al. [128] evaluated the catalytic performance of various metal catalysts supported on γ-Al₂O₃ for the conversion of N₂O to N₂, and found that Rh could achieve complete N₂O decomposition at temperatures below 450 °C. Building on this, Haber et al. [129] investigated the effect of doping Rh/Al₂O₃ catalysts with different levels of alkali metals. Their results showed that the introduction of alkali metal cations alters the dispersion of Rh, thereby influencing its catalytic activity, with an optimal performance achieved at approximately 0.08 mol% Cs loading. Although Rh-based catalysts exhibit excellent catalytic performance at low temperatures, their activity can be significantly inhibited when dealing with combustion exhaust containing O₂ and H₂O. This inhibition is primarily due to the competitive adsorption of O₂ and H₂O with N₂O on the active sites of the catalyst surface [130]. Huang et al. [131] investigated the effects of O₂ and H₂O on the catalytic decomposition of N₂O and found that the N₂O removal efficiency of the Rh/Zn-Al₂O₃ catalyst declined markedly when either or both gases were introduced. Therefore, the proper selection of catalysts and temperature control is particularly critical for effective N₂O mitigation. Moreover, to achieve better control of N₂O emissions. Guo et al. [132] proposed a dual-stage SCR strategy employing a vanadium-based catalyst in the upstream position and a copper-based catalyst downstream, aiming to balance NO_x reduction efficiency and N₂O generation under low-temperature conditions. By constructing a simulation model and incorporating N₂O reaction kinetics, they systematically analyzed the effects of urea injection quantity and NO₂ ratio on NO_x and N₂O emissions. The results demonstrated that this combination effectively enhances low-temperature NO_x removal while suppressing N₂O formation, achieving coordinated control of both pollutants.

3. Conclusions

To better achieve the dual goals of energy transition and environmental protection, the promotion and application of low-carbon fuels in internal combustion engines has become a key research focus. This paper provides a systematic summary of the emission characteristics and corresponding after-treatment technologies of representative low-carbon fuels, including methane, methanol, hydrogen, and ammonia. The main conclusions are as follows:

• Methane as a fuel results in relatively low CO₂ and PM emissions, but tends to produce significant amounts of unburned hydrocarbons such as methane and formaldehyde. Due to the chemical stability and low reactivity of methane, traditional TWCs are generally ineffective at converting these compounds at low temperatures. Therefore, current strategies rely on integrating DOC with methane oxidation catalysts, implementing zoned catalyst designs, or applying ozone-assisted oxidation to improve low-temperature methane conversion efficiency.
• Methanol combustion under low-temperature conditions tends to generate unburned methanol and formaldehyde, yet no dedicated after-treatment systems have been developed specifically for methanol-fueled engines. As a result, general-purpose devices such as DOC, TWC, and POC are commonly used for emission control. Among them, POC has gained attention for its simple structure, low cost, and high purification efficiency. Furthermore, the combination of DOC and POC demonstrates significant potential for improving the removal efficiency of methanol-derived pollutants.
• Hydrogen combustion produces only water vapor, making it a zero-carbon fuel in terms of direct emissions. However, the high combustion temperature easily leads to the formation of thermal NO_x. To achieve ultra-low emissions, hydrogen-fueled engines require an integrated approach combining optimized hydrogen injection/combustion strategies with advanced NO_x after-treatment technologies such as SCR, to ensure low emissions.
• Ammonia has become a promising low-carbon alternative fuel due to its stable combustion performance, its ability to be produced from renewable energy sources, and its compatibility with existing storage and transportation infrastructure, and it offers significant advantages, including wide availability and ease of storage/transportation, positioning it as a promising low-carbon alternative. However, its practical application is hindered by inherent combustion challenges—notably low flame propagation speed and high minimum ignition energy—which often result in incomplete fuel oxidation and increased NO_x emissions. Moreover, the toxic and corrosive nature of ammonia raises concerns over its unburned slip. SCR remains the dominant after-treatment technology for ammonia-fueled engines, and its combination with ASC and SDPF can significantly improve system stability and emission control. Electrochemical NO_x decomposition, a novel reductant-free technology, also shows promise, though its high energy consumption currently limits its application to ammonia-based hybrid power systems.
• To enable the widespread application of low-carbon fuels in internal combustion engines, it is necessary to develop fuel-specific after-treatment routes that strike an optimal balance between emission reduction efficiency, thermal management, catalyst durability, and cost-effectiveness. And to provide a reference for future research, we have briefly presented information on some after-treatment devices, as shown in

  Table 1. Information on after-treatment devices.

  Name	Primary Pollutants Treated	Purification Efficiency	Catalyst	Optimal Temperature Range
  TWC	CO/HC	80–90%	Pt\Pd\Rh	250–500 °C
  DOC	CO/HC	90%	Pt\Pd	220–350 °C
  POC	CO/HC (Methane or methanol formation)	40–70%	Pt\Pd	250–400 °C
  SCR	NOx	90%	Cu-SSZ-13 (250–400 °C)\Fe-SSZ-13 (400–600 °C)	200–500 °C (N2O tends to form at temperatures above 500 °C)
  SDPF	NOx\PM	90% (PM) 70–90% (NOx)	Cu/ZSM-5\Fe/ZSM-5 (The coating amount is approximately three times that of SCR)	350–450 °C

  .

4. Prospects

With the tightening of global emission regulations and the ongoing pursuit of carbon neutrality, the research and application of low-carbon fuels in internal combustion engines are steadily advancing. However, the significant differences in physical and chemical properties and combustion characteristics among various fuels pose complex challenges for emission control technology development. Future research should focus more on developing customized after-treatment systems based on the combustion and emission characteristics of different fuels, particularly those capable of maintaining high pollutant conversion efficiency under low-temperature conditions.

For pollutants such as methane and formaldehyde, future efforts should prioritize enhancing the hydrothermal stability and sulfur resistance of catalysts. For hydrogen engines, although hydrogen is a clean energy source, the NO_x emissions indirectly generated during its use must be taken seriously. In the case of ammonia engines, attention should be paid to the risk of ammonia leakage. Since ammonia is commonly used in SCR systems for NO_x reduction, future development could theoretically focus on integrating after-treatment systems based on the characteristics of both hydrogen and ammonia. Existing mature engine emission control technologies remain valuable references, particularly the continued optimization of NO_x reduction techniques such as SCR and SDPF, which will continue to be key areas of future research. In the selection and development of catalytic materials, it is essential to consider factors such as the reaction temperature window, reaction rate, and byproduct formation of different catalysts. A more comprehensive evaluation should be conducted through a combination of experimental studies and simulation modeling, in order to optimize the design of the after-treatment system. In addition, emerging technologies such as electrochemical NO_x decomposition offer new pathways for reductant-free and energy-efficient pollutant removal. For SDPF technology, both catalyst coating and cold-start performance must be carefully considered. Compared with standalone DPF or SCR systems, SDPF presents a competitive relationship in the utilization of NO₂, requiring a well-balanced reaction strategy to ensure effective purification of both NO_x and PM. Under cold-start conditions, it is crucial to prevent the formation of NH₄NO₃. If soot is introduced to suppress its formation, the amount must be carefully controlled—too little may fail to inhibit NH₄NO₃ formation, while excessive soot could lead to secondary pollution.

Finally, emerging NO_x purification technologies such as H₂-SCR still face challenges, including limited water resistance of catalysts and low reaction efficiency at lower temperatures. Future research should focus on developing catalytic materials with higher selectivity and enhanced stability, while integrating optimized thermal management strategies to improve practical applicability. In contrast, the direct utilization of NH₃ from ammonia fuel for NO_x reduction enables synergistic integration between fuel combustion and emission control systems, eliminating the need for external reductants while streamlining overall system design. However, this approach also faces issues such as ammonia slip, difficulty in controlling injection precision, and a narrow effective temperature window. These challenges become more pronounced under variable operating conditions and high power output scenarios, imposing stricter demands on system safety and response performance.