
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13715-2
10.1016/j.heliyon.2024.e37684
e37684
Research Article
Investigation on the reduction in unburned ammonia and nitrogen oxide emissions from ammonia direct injection SI engine by using SCR after-treatment system
Park Cheolwoong cwpark@kimm.re.kr
a⁎
Choi Yonghyun b
Park Gyeongtae c
Jang Ilpum c
Kim Minki a
Kim Yongrae a
Choi Young a
a Korea Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon, 34103, Republic of Korea
b Korea Construction Equipment Technology Institute, 36 Sandan-ro, Soryong-dong, Gunsan-si, Jeollabuk-do, 54004, Republic of Korea
c Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, 34134, Republic of Korea
⁎ Corresponding author. Department of Mobility Power Research, Korea Institute of Machinery and Materials, Republic of Korea. cwpark@kimm.re.kr
12 9 2024
30 9 2024
12 9 2024
10 18 e3768411 7 2024
24 8 2024
8 9 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Currently generated nitrogen oxides (NOx) and unburned ammonia (NH3) can be converted into nitrogen and moisture that are harmless to the human body and environment using selective catalytic reduction (SCR). The concentrations of NOx and unburned NH3 emitted from the ammonia combustion engines are significantly higher than those emitted by engines using existing hydrocarbon fuels. In this study, ammonia, a representative carbon-free fuel, was used in spark ignition engines for existing passenger vehicles to identify the trends in exhaust gases emitted from engines and conduct experiments on after-treatment strategies to reduce NOx and unburned NH3. The addition of oxygen significantly maximized the conversion efficiency of the SCR after-treatment system by changing the concentration of both NOx and NH3 in the exhaust gas.

Keywords

Ammonia
In-cylinder direct injection
SCR after-treatment system
Oxygen addition
Conversion efficiency
Operating condition
==== Body
pmc1 Introduction

In response to the ‘2050 carbon neutral vision’ and to reduce greenhouse gases such as carbon dioxide, research on power sources is actively underway in the fields of power generation and transportation. These power sources use either carbon-free fuels such as ammonia, or fuels with low carbon numbers, such as natural gas [1]. Ammonia has the following advantages: highest hydrogen storage density per unit volume, easily liquefiable, and has been used for producing fertilizers for a long time. Owing to these characteristics, the production process and transportation infrastructure have been secured [2]. In addition, ammonia can be burned and used directly, and therefore it is viewed as a very advantageous hydrogen transport medium in many aspects. Although ammonia is a flammable gas, it has a high lower flammable limit, narrow range, and a minimum ignition energy of 680 mJ, which is significantly higher than that of methane (0.28 mJ) [3]. Whether ammonia will become a precursor technology to accelerate the realization of a hydrogen society and be used directly and consistently in an equal structure with hydrogen must be considered along with its economic feasibility, reliability and efficiency of the ammonia combustion system.

However, research on infrastructure construction and use plans for ammonia is insufficient compared to those of natural gas, which has successfully replaced existing liquid fuels in the automobile and ship fields. In addition, during the combustion of hydrocarbon-based fuels, nitrogen oxides (NOx) are generated only in the high-temperature post-combustion area, whereas in the case of ammonia containing nitrogen, it is generally known that NOx is additionally generated during the combustion process itself [4,5]. Therefore, measures for reducing additional NOx is vital at this point to successfully reduce the greenhouse gases and other harmful exhaust gases that are byproducts during the application of ammonia.

At present, although the technology to reduce NOx is vital, the technological development to reduce unburned ammonia (NH3) is inevitable in order to reduce environmental problems in future fuel use. In particular, NH3 itself causes toxicity and must be handled in an environmentally sensitive manner [4]. NH3 causes bad odor even at a lower concentration of 5 ppm, which pose the need for its removal even at very low concentrations. Several technologies such as absorption, adsorption, condensation, biological filtration, and catalytic combustion are known to remove gaseous ammonia contained in the exhaust gas, but it is difficult to provide a universal solution owing to technical/economic limitations [6,7].

NOx is a fine dust-causing substance that has been designated as a class 1 carcinogen among the air pollutants by the International Agency for Research on Cancer under the World Health Organization [8]. In addition, regulations are being strengthened worldwide as it is a major cause of ozone layer destruction, acid rain, and respiratory diseases. According to a new standard from the U.S. Environmental Protection Agency, all engines that are produced from 2027, must be certified for maintaining the amount of NOx to be 0.035 g/bhp-h during normal operation and 0.050 g/bhp-h during low load. This is an 82.5 % reduction and reinforcement from the current 0.2 g/bhp-h during normal operation [9]. In the case of ammonia, there is a need to discuss not only safety standards, but also emissions regulations related to the environment. The draft interim guidelines for the safety of ammonia-fueled ships were submitted by Japan and Norway, and are being discussed with the participation of member countries based on the document submitted by Japan. The European Union is promoting zero pollution for a toxic-free environment by changing major environmental regulations and has added ammonia as a new regulated ingredient concerning exhaust gas components of passenger cars [10].

At present, there exists a mechanism to convert the generated NOx and unburned NH3 into nitrogen and moisture that are harmless to the human body and environment via selective catalytic reduction (SCR), which is an efficient technology that has been currently commercialized. In the case of after-treatment methods using catalysts, various materials are used depending on the catalyst material and reducing agent, but the most widely used NOx reduction method currently is the SCR technology. The reduction reaction of NOx is the main reaction of SCR reaction, and the reaction in which NO and NH3 proceed 1:1 is called the “standard SCR”, which proceeds relatively quickly [11]. The catalyst for proceeding the abovementioned reaction is V2O5-WO3 (or MoO3)/TiO2 [12,13]. The denitrification reaction exhibits excellent performance at approximately 300–400 °C, and the performance decreases for lower temperatures owing to the decrease in heat energy, owing to the characteristics of the heated catalyst. At higher temperatures, a side reaction occurs: direct oxidation of NH3, where a reducing agent, and NOx is formed. Moreover, the catalytic performance is reduced at high temperatures due to the agglomeration of active metals, and therefore, research on thermal stability is in progress [6,7].

Various research is underway on catalysts for the selective oxidation reaction of NH3, including noble metals, metal oxides, and zeolites [14,15]. Research on noble metal-based catalysts is being conducted to achieve high reactivity at low temperatures. However, the stronger the oxidizing power, the more important it is to evaluate selectivity because it converts NH3 into NOx or nitrous oxide (N2O). In the oxidation reaction of a catalyst, the type of product is determined by the reaction mechanism. The oxidation reaction of NH3 begins with the activation of gaseous oxygen (O2) [16]. Activated O2 reacts with NH3, and the reaction path changes as various intermediate species are formed [17,18]. As suggested in many existing literature, it is important to design the catalyst to follow a reaction mechanism that ultimately proceeds to nitrogen (N2) [19].

To change the catalyst method, the concentration of NOx and unburned NH3 contained in the exhaust gas emitted from the engine must be considered [20]. Previous researches have confirmed that the concentrations of NOx and unburned NH3 emitted from ammonia combustion engines are significantly higher than those emitted by engines that use existing hydrocarbon fuels. Accordingly, in this study, ammonia, a representative carbon-free fuel, was used in spark ignition engines for existing passenger vehicles to identify the trends in exhaust gases emitted from these engines and conduct experiments on after-treatment strategies to reduce NOx and unburned NH3. The exhaust gas compositions at the up- and down-stream were measured and analyzed using the SCR catalytic post-treatment system. Because the selective oxidation reaction of NH3 was affected by the concentration and ratio of NOx, additional oxygen was supplied to the engine's intake manifold to change the concentration of NOx and unburned NH3 in the exhaust gas, thereby changing the oxygen concentration in the intake manifold to reduce NOx and the reduction rate of unburned NH3 was confirmed. To determine the adequacy of the catalyst capacity for NOx and unburned NH3 emitted at relatively high concentrations, the reduction performance was observed by varying the engine speed and load operating conditions.

2 Experimental setup and method

2.1 Experimental apparatus

Table 1 lists the specifications of the passenger liquified petroleum gas (LPG) engine that was used to examine the effect of variations in the concentration of NOx and unburned NH3 in the exhaust gas on the performance of the SCR catalyst after-treatment system in an NH3 combustion engine using a direct injection fuel supply system. This 2.5 L supercharged LPG engine uses a direct injection fuel injector capable of responding to a fuel injection pressure of 15 MPa. However, to supply the liquid ammonia fuel with high injection pressure, it is equipped with an injector with changed materials of the O-ring (ethylene-propylene diene monomer) and orifice (stainless steel) applied to the fuel injector. Because ammonia is highly corrosive to copper, brass, and zinc alloys including aluminum brass, it forms cracks or green ammonia-corrosive compounds on the surface of the material. In the case of NBR material called nitrile rubber, its resistance to ammonia is low and deformed, which implies that the rubber of appropriate material should be used. Although it is based on the existing LPG engine, a piston with a relatively high compression ratio was applied, and various devices for high-pressure ammonia supply and control were added based on the engine. For example, to secure the flow rate of liquid ammonia, a low-pressure pump (G10X, Hydra-cell) was used to prevent the evaporation due to a decrease in the supply pressure upstream of the high-pressure pump. Afterward, the high-pressure fuel pump for the LPG installed in the engine was pressurized to a level of 15 MPa and supplied.Table 1 Specifications of test engine.

Table 1Item	Specification	
Displacement volume [cc]	2,497	
Number of cylinders [−]	4	
Compression ratio [−]	10.5	
Bore X Stroke [mm]	88.5 × 101.5	
Max. Torque [Nm]	26.0 @ 3,800 rpm (w/LPG)	
Max. Power [kW]	101 @ 3,800 rpm (w/LPG)	

In the case of pure ammonia, the ammonia discharged from the liquid low-pressure gas container must be supplied in a liquid state so that it can be sufficiently pressurized by the high-pressure fuel pump attached to the engine and supplied to the fuel injector. The supply of O2 for mixing O2 in the intake air is stored by an O2 gas tank at a pressure of 12 MPa. O2 was introduced in the laboratory through an O2 line connected to the engine laboratory, depressurized by a regulator, and supplied through the engine's intake manifold at a supply pressure of 0.8 MPa. The flow rate of supplied O2 was controlled by a thermal mass flow controller (M3400V, Linetech), and the flow rate was measured using a Coriolis-type mass flowmeter (CMFS010M, Micro-Motion). A schematic diagram of the engine and experimental equipment used is depicted in Fig. 1.Fig. 1 Schematic of the experimental setup.

Fig. 1

Because the supplied ammonia fuel is controlled by the fuel injection period, which in turn is controlled by the engine control unit, the air-fuel ratio was controlled to enable the operation in a steady state, and a Coriolis-type mass flowmeter (CMFS010P, Micro-Motion) was used to measure the flow rate. By monitoring the engine control state, data such as the fuel injection duration and ignition timing, which are combustion control variables that directly affect engine performance, were acquired. The engine speed and load were controlled using an AC dynamometer (INDY S33-2/0700-1BS-1). Combustion performance was identified by measuring combustion pressure data through a spark plug-type pressure sensor (6115CF, Kistler) and combustion analyzer (DEWE211, DEWETRON co.). In addition, temperature and pressure sensors were installed to measure the temperature and pressure of the key parts of the engine. The airflow rate was measured using a flow meter (FSA 100, AVL). Emissions of NOx, N2O, and unburned NH3, which are emitted without combustion among gases emitted from the engine, were measured using an exhaust gas analyzer (SESAM FTIR, AVL). The SCR after-treatment system used in this study is a prototype developed for an ammonia engine and is a Cu-Zeolite-based catalyst with specifications of closed-coupled SCR of 400/3, 1.5 L and under-floor SCR of 400/4, 6.03 L.

The conversion efficiency is the quantification of the amount of NOx and NH3 reduced in the SCR system and is expressed as a percentage using the concentration before and after treated by the SCR system, which is expressed as follows:conversionefficiency=100(xin−xoutxin)

where xin and xout are the NOx or unburned NH3 concentrations before and after treated by the SCR system, respectively.

2.2 Experimental procedure

From previous research results [21,22], we confirmed areas, where driving with ammonia alone is possible under various engine speeds and load conditions. Under an engine speed condition of 1500 rpm, stable operation was possible with ammonia alone without the addition of other fuels such as hydrogen, except under low load conditions. In this study, changes in exhaust gas from an ammonia combustion engine were confirmed under low and high load operation conditions of 1000 rpm and 1500 rpm. Gas hourly space velocity (GHSV) values, exhaust gas/intake air temperatures, ammonia fuel and air flow rates, and excess air ratios for each operating condition are summarized in Table 2. The conversion efficiency of the SCR after-treatment system, which is a reduction device according to changes in the O2 concentration, was experimentally confirmed by additionally supplying pure O2 to the intake manifold.Table 2 GHSV values, exhaust gas/intake air temperatures, fuel and air flow rate, and excess air ratio under each operating condition.

Table 2Item	GHSV [h-1]	Exhaust gas temperature [° C]	NH3 fuel flow rate [kg/h]	Intake air flow rate [kg/h]	Excess air ratio [−]	Intake air temperature [° C]	
1000 rpm BMEP 0.4 MPa	3935	321	5.0	32.7	1.1	23.5	
1000 rpm BMEP 1.2 MPa	5207	373	12.2	79.8	1.1	31.1	
1500 rpm BMEP 0.4 MPa	5509	369	9.1	60.0	1.1	24.2	
1500 rpm BMEP 1.2 MPa	8111	432	17.5	114.7	1.1	35.2	

Under high-load operating conditions, changes in the concentration of unburned NH3 and NOx and conversion efficiency of the after-treatment system were observed by increasing the exhaust gas flow rate without adding hydrogen. Under low-load operating conditions, the operation was possible only with the addition of O2 owing to the slow-burning rate of NH3. By supplying O2, operability was confirmed and the performance of the after-treatment system was compared. For operations using O2 with different loads under an engine speed condition of 1500 rpm, the O2 concentration was increased by changing it in 1 % increments, and the maximum conversion efficiency conditions of the after-treatment system were confirmed according to the change in the concentration of unburned NH3 and NOx in the exhaust gas. The concentration of O2 was measured using an exhaust gas analysis device (i60R1, AVL) by sampling the intake air mixed with O2 from the intake manifold. Ignition timing optimization was performed in each operating condition to operate under the minimum advance for the best torque ignition timing condition. Because it is difficult to secure combustion stability even when the engine operating conditions are constant and the engine coolant and engine oil temperatures are low, the experiment was conducted in a sufficiently warmed-up state (90 ± 2.5 °C).

3 Experimental results and discussion

3.1 After-treatment system performance under operating conditions

First, to check the concentration of NOx and unburned NH3 in the exhaust gas of an ammonia combustion engine according to engine speed and load conditions, and the conversion efficiency of the SCR after-treatment system, the concentration of each exhaust gas at the engine outlet and down-stream of the SCR after-treatment device was examined under operating conditions of engine speeds of 1000 rpm and 1500 rpm, and break mean effective pressure (BMEP) of 0.4 and 1.2 MPa of the engine load. From Fig. 2, it is evident that the NOx emitted from the engine through combustion was high under conditions of low rotational speed and high load with high thermal efficiency. After passing through the SCR after-treatment device, NOx emissions were significantly reduced through denitrification reaction, reducing to a level where NOx emissions were almost nonexistent under low-load operating conditions, regardless of the engine speed. Under high-load operating conditions, the concentration of NOx emitted from the engine was high even after undergoing the denitrification reaction in the SCR after-treatment device, and several hundred ppm of NOx was emitted.Fig. 2 NOx and unburned NH3 in the exhaust gas of an engine-out and downstream of the SCR after-treatment system according to various engine speeds and load conditions of the ammonia direct injection engine.

Fig. 2

In the case of unburned NH3 emitted via the combustion in the engine, the level was similar regardless of load under low engine speed conditions. At a relatively high rotation speed of 1500 rpm, the emission of unburned NH3 increased under low-load conditions with low thermal efficiency and decreased under high-load conditions with high thermal efficiency. It increased as the load decreased, thereby resulting in high emissions under low-efficiency conditions, showing the opposite trend to NOx emissions. Due to the selective oxidation reaction in the SCR after-treatment system, the concentration of unburned NH3 decreased at downstream of the after-treatment device, but the concentration of unburned NH3 emitted from the engine was very high, so it was still at a high level.

Fig. 3 depicts the results of N2O and O2 concentrations in exhaust gas measured under the same operating conditions. The nitrogen component in fuel is an intermediate species in the process of NOx formation during ammonia combustion and also acts as a cause of N2O generation. N2O is known to have a global warming potential 265 times that of CO2, so the need to reduce N2O is recognized as important in power sources that use ammonia as a fuel [23]. Because N2O is very stable in molecular structure, it is known that a decomposition catalyst to decompose it into N2 and O2 requires a temperature of approximately 300 °C or more [[24], [25], [26]]. Because huge amounts of heat energy is required to decompose N2O independently, it is important to fundamentally reduce it by changing the control factor during ammonia combustion. As can be seen in Fig. 3, N2O emissions are higher under low load conditions compared to high-load conditions. Under low-load conditions, the engine speed appears low at a low 1000 rpm, thereby indicating a tendency that is exactly opposite to NOx emissions.Fig. 3 N2O and O2 in the exhaust gas of an engine-out and downstream of the SCR after-treatment system according to various engine speeds and load conditions of the ammonia direct injection engine.

Fig. 3

The copper-based SCR post-treatment device used in this study has excellent redox characteristics and can be applied to the oxidation reaction of ammonia; however, it is known to generate N2O with a strong oxidation tendency [19,27]. The results depicted in Fig. 3 also indicate that the concentration of N2O increases downstream of the SCR after-treatment system, thereby exhibiting the same trend as the previous study. Because the oxidation reaction of NH3 begins with the activation of O2 in the gas phase in the after-treatment system, sufficient excess O2 in the exhaust gas is required. Under each operating condition, the O2 concentration in the exhaust gas is more than 3 %, which can be sufficiently utilized for an oxidation reaction. A decrease in the O2 concentration of 0.3–0.5 % was observed at the rear of the SCR after-treatment system owing to the O2 used in the selective oxidation reaction.

3.2 After-treatment system performance according to changes in oxygen concentrations

The main mechanism of the selective oxidation reaction that occurs in the catalyst of the SCR after-treatment system is as follows [28]: by considering that the ratio of NOx and NH3 is the same. SCR-related reactions can be divided into NOx reduction reaction, NH3 oxidation reaction, and additional reaction. The NOx reduction reaction is greatly affected by the concentration of reactants in the exhaust gas. On most SCR catalysts, the NH3 oxidation reaction progresses more actively as the reaction temperature increases and competes with the main reaction, which is the NOx reduction reaction. Because the exhaust gas temperature of the engine used in this study was sufficiently high, it was determined that the changes in NOx and NH3 emission concentrations could affect the improvement of the conversion efficiency of the SCR system.

Under the above operating conditions, the emission concentration of unburned NH3 emitted from the engine through combustion of ammonia was relatively high, and as a result, NOx was insufficient even under conditions with sufficient O2; therefore, the reduction of unburned NH3 emissions was not effective.4NH3+4NO+O2→4N2+6H2O

2NH3+NO+NO2→2N2+3H2O

The SCR reaction proceeds by continuous adsorption and desorption of NO and NH3 and the efficiency was determined by the transfer rate of substances to the active site, which directly affects the reaction and the reaction rate of each active site. Owing to these characteristics, there may be differences in the efficiency depending on the concentration of NOx, which is a reactant [29], and under conditions where the concentration of unburned NH3 is relatively high, the emission of NH3 is not sufficiently reduced.

From previous research results [30], the authors confirmed that adding O2 to the intake air increases the combustion rate of ammonia and consequently improves combustion stability and efficiency. The improvement in combustion speed by adding O2 also affects the concentration of components in the exhaust gas, and as the O2 concentration increases, NOx increases while unburned NH3 decreases. When considering the standard SCR mechanism [11], the trend of NOx and NH3 emissions is preferable for improving the SCR efficiency. Focusing on the possibility of changes in the concentration of NOx and unburned NH3 in the exhaust gas as described above, we attempted to confirm the operating conditions that can maximize the conversion efficiency in the SCR after-treatment system by changing the ratio of NOx and unburned NH3 through changes in the O2 concentration.

Fig. 4, Fig. 5 compare the change in concentration of NOx and unburned NH3 emitted from the engine and that in the concentration downstream of the SCR after-treatment system when the O2 concentration increases thereby adding O2 to the intake air, and exhibit the conversion efficiency for each. As the O2 concentration in the intake air increases, the NOx emitted from the engine increases linearly due to the increase in the combustion temperature owing to the increase in the combustion speed of ammonia. Although it reduced the downstream of the SCR after-treatment system due to the NOx reduction reaction in the SCR after-treatment system, the conversion efficiency tends to decrease as the O2 concentration increases.Fig. 4 Change in the concentration of NOx emitted from the engine and concentration downstream of the SCR after-treatment system and conversion efficiency when the O2 concentration increases at 1500 rpm and a BMEP of 1.2 MPa condition.

Fig. 4

Fig. 5 Change in the concentration of unburned NH3 emitted from the engine and concentration downstream of the SCR after-treatment system and conversion efficiency when the O2 concentration increases at 1500 rpm and a BMEP of 1.2 MPa condition.

Fig. 5

In the case of unburned NH3 emitted through the combustion of ammonia, unlike the trend of NOx, it maintains an almost constant level despite the increase in the O2 concentration in the intake air. As the O2 concentration in the intake air increases, the concentration of unburned NH3 discharged from the downstream of the SCR after-treatment system decreases. When the O2 concentration exceeds 25.5 %, it decreases to less than 1000 ppm and no significant additional reduction was observed. Under conditions of relatively low O2 concentration, the conversion efficiency increases as the NOx required for the reduction reaction of NH3 increases. When the O2 concentration reaches 25.5 %, the concentrations of NOx and unburned NH3 are reversed, and although most of the unburned NH3 is reduced under conditions of higher O2 concentration, it is believed that NOx that was not used in the reduction reaction of NH3 was discharged as is and had an effect on reducing the conversion efficiency of NOx. Although the conversion efficiency of unburned NH3 is lower than the NOx conversion efficiency under conditions of relatively lower oxygen concentrations, the absolute concentration difference reduced through the SCR reaction was similar.

The NOx conversion efficiency changes from 77.9 %, when no O2 is added to the intake air, to 62.6 % at an O2 concentration of 28 %. On the other hand, the conversion efficiency of unburned NH3 varied from 21.6 % to 91.8 %, thereby showing differences by component. The changes in the efficiency of NOx and unburned NH3 according to changes in the O2 concentration exhibit opposite trends, and when considering each exhaust gas component, the fact that a reduction effect close to 100 % cannot be achieved indicates that the SCR after-treatment system needs to be improved. Even under an O2 concentration condition of 25.5 %, where the concentrations of NOx and NH3 were the most similar, the conversion efficiencies were 70.8 % and 85.6 %, respectively. It should be noted that under low-speed operation conditions where the GHSV value was not high, the conversion efficiency was lower than that of a general SCR after-treatment system owing to the high concentration of NOx and unburned NH3 emissions.

To confirm the change in GHSV due to the increase in O2 concentration and its influence, the total flow rate of intake air and O2 added under the same operating conditions satisfy with the excess air ratio measured from the exhaust pipe depicted in Fig. 6. Under operating conditions with a constant engine load, the amount of intake air required decreased as O2 was added to the intake air and the total flow rate of O2 and fresh air slightly increased as the O2 concentration increased. However, it can be seen that the increase is at a level of 4.58 kg/h, which is similar to the GHSV under base conditions. In the case of excess air ratio, the excess air ratio was converted based on the oxygen sensor mounted on the exhaust pipe; therefore, it increased significantly to 1.9 for an O2 concentration condition of 30 %. As the result is different from the tendency exhibited during the lean combustion in which excess air containing N2 increases, an indicator to exhibit the effect of O2 concentration is required in this case.Fig. 6 Change in the total and fresh air mass flow rate and excess air ratio when the O2 concentration increases at 1500 rpm and a BMEP of 1.2 MPa condition.

Fig. 6

Fig. 7 depicts the variation in the N2O concentration upstream and downstream of the SCR after-treatment system when the O2 concentration in the intake air increases under the same operating conditions. Even as the O2 concentration increases, N2O emissions from the engine through ammonia combustion remain at a low level of 18 ppm or less, thereby showing a similar trend to that exhibited by unburned NH3. However, unlike the tendency of NOx and unburned NH3 in which conversion efficiency increases or decreases with an increase in O2 concentration, N2O emissions increased linearly downstream of the SCR after-treatment system. This is believed to be the result of the effect of the high O2 concentration in the exhaust gas on the oxidation of nitrogen components. Because N2O is very stable in its molecular structure, a catalyst to decompose it requires a lot of heat energy, and moisture and oxygen present in the exhaust gas are known to reduce the decomposition ability of N2O [31]. Therefore, the development of an after-treatment system using other metal compounds to fundamentally suppress the N2O generated in the SCR after-treatment system is realistically promising.Fig. 7 Change in the concentration of N2O emitted from the engine and concentration downstream of the SCR after-treatment system when the O2 concentration increases at 1500 rpm and a BMEP of 1.2 MPa condition.

Fig. 7

The SCR system is a technology that can remove more than 90 % of NOx and is commercialized as the optimal reduction technology in terms of price competitiveness and stability of catalytic activity [32]. However, considering the relatively high concentrations of NOx and NH3 emitted and the need to meet strengthened emission regulations, it is expected that a conversion efficiency of more than 95 % will be required. We attempted to maximize the conversion efficiency of the SCR after-treatment system by changing the concentration ratio of NOx and NH3 in the exhaust gas by changing the O2 concentration in the intake air. However, a low conversion efficiency of less than 85 % was confirmed for both components at 1500 rpm and a BMEP of 1.2 MPa operating conditions. To confirm that this was a problem due to the capacity of the after-treatment system, the same experiment was conducted under low-load operation conditions with low GHSV, and the results were compared.

From Fig. 8, Fig. 9, it is evident that when the O2 concentration increases with the addition of O2 to the intake air under the operating conditions of 1500 rpm and BMEP of 0.4 MPa, the graph also shows the conversion efficiency for each by comparing the concentration changes of NOx and unburned NH3 emitted from the engine and the concentration changes downstream of the SCR after-treatment system. When the O2 concentration in the intake air is 22.0 %, although the NOx concentration emitted from the engine is more than 4000 ppm due to increased combustion speed, it was mostly reduced downstream of the SCR after-treatment system. In the case of unburned NH3, most of it was reduced to less than 100 ppm downstream of the SCR after-treatment system under conditions where the O2 concentration in the intake air was 24.0 % or more. As in high load conditions, under conditions where the O2 concentration is relatively low, the NOx required for the reduction reaction of NH3 was not sufficient, so the conversion efficiency of unburned NH3 was low. Conversely, under conditions where the O2 concentration was relatively high, the conversion efficiency of NOx was low because NH3 was insufficient compared to NOx.Fig. 8 Change in the concentration of NOx emitted from the engine and concentration downstream of the SCR after-treatment system and conversion efficiency when the O2 concentration increases at 1500 rpm and a BMEP of 0.4 MPa condition.

Fig. 8

Fig. 9 Change in the concentration of unburned NH3 emitted from the engine and concentration downstream of the SCR after-treatment system and conversion efficiency when the O2 concentration increases at 1500 rpm and a BMEP of 0.4 MPa condition.

Fig. 9

Because it was a relatively low load condition with low GHSV, the maximum conversion efficiency of NOx and unburned NH3 was high at 97.1 % and 99.5 %, respectively. While the O2 concentration condition that can maximize the simultaneous reduction of NOx and unburned NH3 is 25.5 % under the relatively high load condition of 1500 rpm and BMEP of 1.2 MPa, it is 23 % based on NOx and 24 % based on unburned NH3, which shows that the optimal O2 concentration required when operating the engine considers the after-treatment system and varies depending on the operating conditions under low load conditions. Under low GHSV conditions, the maximum conversion efficiency of NOx and unburned NH3 increases, but it can be seen that under maximum conversion efficiency conditions, the emission levels are 67.3 ppm and 25.3 ppm, respectively, which are higher than the emission levels required for general commercial engines. Table 3 lists the O2 concentration in the intake air that can maximize the simultaneous reduction efficiency of the SCR after-treatment system under the operating conditions of engine speeds of 1000 rpm and 1500 rpm, and BMEP of 0.4 and 1.2 MPa, and the conversion efficiencies of NOx and unburned NH3.Table 3 O2 concentration of intake air that can maximize the simultaneous reduction efficiency of the SCR after-treatment system and the conversion efficiency of NOx and unburned NH3.

Table 3Item	O2 concentration [%]	Conversion efficiency of NOx [%]	Conversion efficiency of NH3 [%]	
1000 rpm BMEP 0.4 MPa	24.4	96.0	97.1	
1000 rpm BMEP 1.2 MPa	26.2	83.5	75.6	
1500 rpm BMEP 0.4 MPa	24.0	86.1	98.7	
1500 rpm BMEP 1.2 MPa	25.0	74.2	83.0	

4 Conclusions

In this study, we examined the concentrations of NOx and unburned NH3 in exhaust gas according to changes in operating conditions in a 2.5 L-class ammonia engine and the reduction rate through an SCR after-treatment system. When O2 was added to the intake air, the ratio of NOx to unburned NH3 and the effect of exhaust flow rate on the conversion efficiency of the SCR after-treatment system were experimentally compared and analyzed for each operating condition. The summary of the results is as follows.1. NOx appeared high under conditions of low rotation speed and high load, which is highly efficient and was reduced significantly after going through the SCR after-processing system. Although the concentration of unburned NH3 decreased downstream of the after-treatment system due to the selective oxidation reaction, it was still high because the concentration of unburned NH3 discharged from the engine was significantly high.

2. N2O emissions were higher under low load conditions compared to high load conditions. In low load conditions, the engine speed appeared low at low conditions, which was the opposite case for NOx emissions, and the concentration of N2O increased downstream of the copper-based SCR after-treatment system.

3. As the O2 concentration in the intake air increased, NOx emissions increased significantly. This reduced the downstream of the SCR after-treatment system owing to the NOx reduction reaction, but the conversion efficiency decreased with an increase in the O2 concentration.

4. In the case of unburned NH3, even if the O2 concentration in the intake air increases, it remains at an almost constant level, and the concentration of unburned NH3 discharged downstream of the SCR after-treatment system decreases.

5. Under high GHSV conditions, the conversion efficiency of NOx and unburned NH3 was low, and the O2 concentration in the intake air that can maximize the simultaneous reduction efficiency of the SCR after-treatment system was 25.5 % under high load conditions, whereas under low load conditions, it was 23 % based on NOx and 24 % based on the unburned NH3, which varied depending on the operating conditions.

5 Limitation of the present study

As the oxygen concentration increases, the conversion efficiency of NOx decreases, whereas the conversion efficiency of unburned NH3 increases; therefore, it was impossible to obtain results that simultaneously reduce NOx and unburned NH3 to near-zero levels.

6 Future works

By applying an improved SCR after-treatment system that can sufficiently reduce NOx and unburned NH3 emitted at a high level in the future, experiments on the strategy to confirm low concentration in the tail-pipe for commercialization of ammonia engines and maximize reduction under conditions without adding oxygen to the intake air should be conducted.

Funding

This research was a part of the project titled ‘Development of 2100 PS LNG-Ammonia dual fuel engine’, funded by the Korean 10.13039/501100003566 Ministry of Oceans and Fisheries (Project No. 1525011796 ) and “Development of core parts technology for non-carbon fuel main propulsion engine”, funded by the 10.13039/501100003052 Ministry of Trade, Industry and Energy , Republic of Korea (Project No. 20017612 ).

CRediT authorship contribution statement

Cheolwoong Park: Writing – original draft, Supervision. Yonghyun Choi: Formal analysis. Gyeongtae Park: Validation. Ilpum Jang: Data curation. Minki Kim: Resources. Yongrae Kim: Methodology. Young Choi: Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Nomenclature

BTDC Before top dead center

BMEP Brake mean effective pressure

CAD Crank angle degree

COVIMEP Coefficient of variation for indicated mean effective pressure

LPG Liquid petroleum gas
==== Refs
References

1 Huang Q. Liu J. Preliminary assessment of the potential for rapid combustion of pure ammonia in engine cylinders using the multiple spark ignition strategy Int. J. Hydrogen Energy 55 2024 375 385 10.1016/j.ijhydene.2023.11.136
2 Wijayanta A.T. Oda T. Purnomo C.W. Kashiwagi T. Aziz M. Liquid hydrogen, methylcyclohexane, and ammonia as potential hydrogen storage: comparison review: comparison review Int. J. Hydrogen Energy 44 2019 15026 15044 10.1016/j.ijhydene.2019.04.112
3 Kobayashi H. Hayakawa A. Somarathne K.D.K.A. Okafor E.C. Science and technology of ammonia combustion Proc. Combust. Inst. 37 2019 109 133 10.1016/j.proci.2018.09.029
4 Yang R. Liu Z. Liu J. The methodology of decoupling fuel and thermal nitrogen oxides in multi-dimensional computational fluid dynamics combustion simulation of ammonia-hydrogen spark ignition engines Int. J. Hydrogen Energy 55 2024 300 318 10.1016/j.ijhydene.2023.09.105
5 Yang R. Liu J. Liu Z. Liu J. Applying separate treatment of fuel- and air-borne nitrogen to enhance understanding of in-cylinder nitrogen-based pollutants formation and evolution in ammonia-diesel dual fuel engines Sustain. Energy Technol. Assessments 69 2024 103910 10.1016/j.seta.2024.103910
6 Kwon D.W. Choi J. Nam K.B. Ha H.P. New insight into the role of Mo–Sb addition towards VMoSbTi catalysts with enhanced activity for selective catalytic reduction with NH3 Chem. Eng. J. 428 2022 132078 10.1016/j.cej.2021.132078
7 Kwon D.W. Kim D.H. Lee S. Kim J. Ha H.P. A dual catalytic strategy by the nature of the functionalization effect as well as active species on vanadium-based catalyst for enhanced low temperature scr Appl. Catal., B 289 2021 120032 10.1016/j.apcatb.2021.120032
8 de la Salud O.M. World Health Organization, Health ECE. WHO Global Air Quality Guidelines: Particulate Matter (PM2.5 and PM10), Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide 2021 World Health Organization
9 Control of Air Pollution from New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards 2023 Environmental Protection Agency https://www.regulations.gov/docket/EPA-HQ-OAR-2019-0055
10 Regulation (EU) OF the European Parliament and of the COUNCIL of 24 April 2024 2024/1257 2024 Eur Parliam Counc Eur UNION http://data.europa.eu/eli/reg/2024/1257/oj
11 Dumesic J.A. Topsøe N.-Y. Topsøe H. Chen Y. Slabiak T. Kinetics of selective catalytic reduction of nitric oxide by ammonia over Vanadia/Titania J. Catal. 163 1996 409 417 10.1006/jcat.1996.0342
12 Lian Z. Deng H. Xin S. Shan W. Wang Q. Xu J. He H. Significant promotion effect of the rutile phase on V 2 O 5/TiO 2 catalysts for NH 3-scr Chem. Commun. 57 2021 355 358 10.1039/D0CC05938B
13 Kwon D.W. Park K.H. Hong S.C. Enhancement of SCR activity and SO2 resistance on VOx/TiO2 catalyst by addition of molybdenum Chem. Eng. J. 284 2016 315 324 10.1016/j.cej.2015.08.152
14 Raja S. Alphin M.S. Sivachandiran L. Promotional effects of modified TiO 2- and carbon-supported V 2 O 5-and MnO x-based catalysts for the selective catalytic reduction of NO x: a review Catal. Sci. Technol. 10 2020 7795 7813 10.1039/D0CY01348J
15 Marberger A. Ferri D. Elsener M. Sagar A. Artner C. Schermanz K. Kröcher O. Relationship between structures and activities of supported metal vanadates for the selective catalytic reduction of NO by NH3 Appl. Catal., B 218 2017 731 742 10.1016/j.apcatb.2017.06.061
16 Shan W. Yu Y. Zhang Y. He G. Peng Y. Li J. He H. Theory and practice of metal oxide catalyst design for the selective catalytic reduction of NOx with NH3 Catal. Today 376 2021 292 301 10.1016/j.cattod.2020.05.015
17 Xiong S. Chen J. Huang N. Yang S. Peng Y. Li J. Balance between reducibility and N2O adsorption capacity for the N2O decomposition: CuxCoy catalysts as an example Environ. Sci. Technol. 53 2019 10379 10386 10.1021/acs.est.9b02892 31380634
18 Gao F. Liu Y. Sani Z. Tang X. Yi H. Zhao S. Yu Q. Zhou Y. Advances in selective catalytic oxidation of ammonia (NH3−SCO) to dinitrogen in excess oxygen: a review on typical catalysts, catalytic performances and reaction mechanisms J. Environ. Chem. Eng. 9 2021 104575 10.1016/j.jece.2020.104575
19 Wang Z. Qu Z. Quan X. Li Z. Wang H. Fan R. Selective catalytic oxidation of ammonia to nitrogen over CuO-CeO2 mixed oxides prepared by surfactant-templated method Appl. Catal., B 134–135 2013 153 166 10.1016/j.apcatb.2013.01.029
20 Jeon S.W. Song I. Lee H. Kim D.H. Enhanced activity of vanadia supported on microporous titania for the selective catalytic reduction of NO with NH3: effect of promoters Chemosphere 275 2021 130105 10.1016/j.chemosphere.2021.130105
21 Park C. Jang Y. Min C. Kim Y. Choi Y. Kim M. Experimental investigation of operating conditions on performance and emissions of spark ignition ammonia direct injection engine Appl. Therm. Eng. 241 2024 122382 10.1016/j.applthermaleng.2024.122382
22 Park C. Jang Y. Kim S. Kim Y. Choi Y. Influence of hydrogen on the performance and emissions characteristics of a spark ignition ammonia direct injection engine Gas 3 2023 144 157 10.3390/gases3040010
23 Pachauri R.K. Allen M.R. Barros V.R. Broome J. Cramer W. Christ R. Contribution of working groups I, II and III to the fifth assessment report of the intergovernmental panel on climate change Ipcc, Clim. Change. 2014 2014 synthesis report
24 Galle M. Agar D.W. Watzenberger O. Thermal N2O decomposition in regenerative heat exchanger reactors Chem. Eng. Sci. 56 2001 1587 1595 10.1016/S0009-2509(00)00386-9
25 Konsolakis M. Recent advances on nitrous oxide (N2O) decomposition over non-noble-metal oxide catalysts: catalytic performance, mechanistic considerations, and surface chemistry aspects ACS Catal. 5 2015 6397 6421 10.1021/acscatal.5b01605
26 Amrousse R. Tsutsumi A. Bachar A. Lahcene D. N2O catalytic decomposition over nano-sized particles of Co-substituted Fe3O4 substrates Appl. Catal., A 450 2013 253 260 10.1016/j.apcata.2012.10.036
27 Zhang X. Wang H. Wang Z. Qu Z. Adsorption and surface reaction pathway of NH3 selective catalytic oxidation over different Cu-Ce-Zr catalysts Appl. Surf. Sci. 447 2018 40 48 10.1016/j.apsusc.2018.03.220
28 Liu C. Malta G. Kubota H. Toyao T. Maeno Z. Shimizu K. Mechanism of NH3–selective catalytic reduction (SCR) of NO/NO2 (fast SCR) over Cu-CHA zeolites studied by in situ/operando infrared spectroscopy and density functional theory J. Phys. Chem. C 125 2021 21975 21987 10.1021/acs.jpcc.1c06651
29 Xiong S. Xiao X. Liao Y. Dang H. Shan W. Yang S. Global kinetic study of NO reduction by NH3 over V2O5–WO3/TiO2: relationship between the SCR performance and the key factors Ind. Eng. Chem. Res. 54 2015 11011 11023 10.1021/acs.iecr.5b03044
30 Park C. Jang I. Park G. Min C. Kim M. Kim Y. Choi Y. Effect of oxygen concentrations in intake air on combustion characteristics of ammonia direct injection SI engine Fuel 376 2024 132643 10.1016/j.fuel.2024.132643
31 Han J. Wang A. Isapour G. Härelind H. Skoglundh M. Creaser D. Olsson L. N2O formation during NH3-SCR over different zeolite frameworks: effect of framework structure, copper species, and water Ind. Eng. Chem. Res. 60 2021 17826 17839 10.1021/acs.iecr.1c02732
32 Koebel M. Elsener M. Kleemann M. Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines Catal. Today 59 2000 335 345 10.1016/S0920-5861(00)00299-6
