DOI: 10.3303/CET25120013 Paper Received: 15 May 2025; Revised: 25 August 2025; Accepted: 16 September 2025 Please cite this article as: Ino I., Kansha Y., 2025, Development and Simulation of Capture and Utilization System for NOX Gas from NH3 Combustion, Chemical Engineering Transactions, 120, 73-78 DOI:10.3303/CET25120013 CHEMICAL ENGINEERING TRANSACTIONS VOL. 120, 2025 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Bing Shen How, Viknesh Andiappan, Denny K.S. Ng, Hon Loong Lam, Petar S. Varbanov Copyright © 2025, AIDIC Servizi S.r.l. ISBN 979-12-81206-21-2; ISSN 2283-9216 Development and Simulation of Capture and Utilization System for NOX Gas from NH3 Combustion Isshin Ino, Yasuki Kansha* Organization for Programs on Environmental Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3- 8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan kansha@global.c.u-tokyo.ac.jp Ammonia (NH3) combustion is gaining increasing attention as a zero-carbon fuel due to its advantages in production, storage, and transportation. However, several challenges—such as its low flammability and the risk of nitrogen oxide (NOX) emissions—must be addressed before NH3 can be widely implemented in energy systems. To overcome the NOX emission challenge of NH3 combustion, this study focuses on the development and evaluation of a NOX capture and utilization system. As a case study of NOX utilization, potassium nitrate synthesis via alkaline absorption using potassium hydroxide was selected. A simulation-based analysis was conducted for an NH3-fired combined-cycle power plant integrated with the proposed system. The system's value was assessed based on the combined outputs of electricity and the chemical product. The simulation results demonstrated that utilizing NOX at an equivalence ratio (ER) of 0.5 can increase total profit by 53 % compared to low-NOX combustion at an ER of 1.1. These findings highlight the high feasibility of the proposed system as a chemical recycling approach and underscore its significant potential for future development. 1. Introduction The combustion and utilization of fossil fuels have long provided cheap and abundant energy sources for the development of human society. However, the large-scale emission of pollutants and greenhouse gases from the power sector has led to serious anthropogenic hazards and critical climate change. Governments and organizations worldwide have implemented various countermeasures to mitigate the environmental impact of such human activities. Toxic and hazardous pollutants—such as sulfur oxides (SOX) and nitrogen oxides (NOX)—are strictly regulated. At the same time, emissions of greenhouse gases, particularly carbon dioxide (CO2), are receiving increasing attention and regulation due to the escalating threat of global warming. Current decarbonization strategies for the power sector include the replacement of thermal power with renewable energy sources, the substitution of fossil fuels with decarbonized alternatives, improvements in power generation efficiency, and the deployment of carbon capture, utilization, and storage (CCUS) technologies (Rahimi et al., 2024). Among these strategies, the complete or partial replacement of combustion fuels with decarbonized alternatives, including carbon-neutral and zero-carbon fuels, is regarded as a mid-term transformation approach for existing thermal power plants (Nose et al., 2023). Moreover, such fuels offer the potential for large-scale carbon-free energy storage and conversion in a future decarbonized society. In recent years, ammonia (NH3) has attracted growing attention as a promising zero-carbon fuel (Gao et al., 2017). Unlike hydrogen gas, NH3 has a higher boiling point, enabling easier storage and transportation under lower pressure and higher temperature conditions. Additionally, the industrial-scale production of NH₃ via the Haber–Bosch process is well-established, making it capable of meeting the increasing demand for mid-term decarbonization in thermal power applications. Nevertheless, the adoption of NH3 as a fuel poses several challenges. For instance, its flammability is significantly lower than that of conventional fuels. As a reference, the maximum laminar burning velocity of NH3 is approximately 8 cm/s, which is only about 23 % that of methane, the slowest among all major hydrocarbon fuel gases (Murai et al., 2019). Another challenge arises from the potential for substantial NOX emissions during NH3 combustion. While conventional fuels produce thermal NOX through the oxidation of atmospheric nitrogen at high temperatures, NH3 combustion produces fuel NOX in addition to thermal NOX due to the oxidation of 73 nitrogen present within the fuel itself. This results in elevated NOX concentrations in the exhaust gas of NH3 combustion (Murai et al., 2019). Emissions of NOX pose serious environmental and health risks, contributing to respiratory and mucosal diseases and environmental degradation via acid rain formation (Zhu and Xu, 2022). Consequently, NOX emissions are regulated globally through various environmental policies. For example, the Japanese government has enforced the Air Pollution Control Act since 1968, which includes stringent NOX emission limits. These regulations have become increasingly strict in response to societal and technological advancements, aiming to protect the environment and public health. Therefore, various NOX mitigation technologies have been developed to address the increased NOX emissions from NH3 combustion. These include reducing NOX formation through partial decomposition of NH3 (Nose et al., 2023), optimizing combustion conditions (Skryja et al., 2014), and post-combustion removal technologies. Among these, Selective Catalytic Reduction (SCR) is a widely adopted and commercialized post-combustion de-NOX technology in Japan (Ministry of the Environment(Japan), 2014). SCR employs selective reductants such as NH3 and specific catalysts to convert NOX into nitrogen and water. However, although SCR is simple, stable, and highly efficient, it involves operational costs related to the consumption of reductants and energy, as well as catalyst degradation. These costs become particularly significant when applied to NH3 combustion, which inherently generates more NOX. As a result, current studies focus on developing low-NOX NH3 combustion technologies to reduce the need for post-combustion treatment. However, this often comes at the cost of reduced power generation efficiency per unit of NH3 used. On the other hand, since NOX is the dominant acidic component in the exhaust gas from NH3 combustion, the NOX selectivity character of SCR may not be essential. Moreover, considering that NOX is a raw material in nitric acid production, the dilute but relatively pure NOX-containing exhaust gas from NH3 combustion presents a promising opportunity for value-added utilization rather than mere pollutant treatment. By harnessing the reactivity of NOX through appropriate chemical pathways, it is possible to achieve environmental protection and resource recovery simultaneously. Nevertheless, few studies have explored the direct utilization of emitted NOX for valuable material production, leaving its feasibility largely uncertain. 2. Proposed System To address the existing uncertainty regarding NOX treatment through valuable material production, the authors proposed a novel NOX capture and utilization system that treats NOX-containing exhaust gas as a raw material for producing value-added products. The goal is to offset or surpass the costs associated with conventional NOX treatment through the economic value of the generated products. To evaluate the feasibility of this concept, the present study conducted a preliminary techno-economic analysis of an NH3-fired combined-cycle power plant integrated with the proposed NOX capture and utilization system. In this study, a system was proposed to harness the chemical reactivity of NOX to produce the value-added material with market potential. Potassium nitrate (KNO3) was selected as the target among various possible products due to its commercial value as a chemical fertilizer and raw material in gunpowder production. The system employed an alkaline absorption method, using potassium hydroxide (KOH) as the absorbent to capture NOX and synthesize KNO3. The proposed NOX capture and utilization system was integrated into the exhaust stream of an NH3-fired combined-cycle power plant, as shown in Figure 1. In the integrated system, a premixed gaseous flow of NH3 fuel, dry air, and recirculated exhaust gas (EGR) is first compressed and introduced into a primary combustor, which undergoes high-temperature combustion. This process generates thermal energy for electricity production via a gas turbine. However, the elevated combustion temperature also facilitates the formation of thermal and fuel NOX. If unreacted NH3 remains after the primary combustion stage—particularly under fuel-rich conditions— the remaining NH3 is directed into a secondary combustor, where additional air is introduced to ensure complete combustion. In this secondary stage, combustion occurs at a comparatively lower temperature. Notably, the remaining NH3 can also chemically reduce a portion of the previously generated NOX, resulting in partial NOX neutralization. Following secondary combustion, the high-temperature exhaust gas is utilized to transfer heat to a water-based coolant loop, which produces steam to drive a steam turbine and generate additional electricity. Finally, the cooled exhaust gas with the remaining NOX enters the NOX absorber unit, where NOX is captured and converted into KNO3, thereby detoxifying the exhaust gas. A portion of the treated gas is recirculated as EGR to improve system efficiency and temperature control. Thus, this integrated system co-produces electricity and chemical material, KNO3. 74 Figure 1: The schematic illustration of the proposed co-production system. 3. Method In this research, a simulation study followed by an economic comparison between different NH3-to-air conditions was conducted to demonstrate the difference in value produced from NOX utilization for evaluation of the proposed system. A lab-scale experiment for KNO3 synthesis from nitrogen gas containing 500 ppm NO2 was conducted in a previous study (Ino et al., 2025) to demonstrate the feasibility of the proposed NOX absorber shown in Figure 1. The experimental results provided the necessary data for simulating the performance of the proposed system. The findings indicated that the NOX absorption ratio (𝜂𝐴) —defined as the proportion of NOX removed by the absorbent relative to the NO2 content in the incoming gas—exceeded 75 %. However, 𝜂𝐴 decreased to 43 % as the absorbent became neutralized. Meanwhile, the product conversion ratio (𝜂𝑃) —defined as the mass of KNO₃ formed relative to the theoretical value based on the stoichiometric consumption of KOH—was found to be 96%. In the subsequent calculations, 𝜂𝐴 = 75 % and 𝜂𝑃 = 96 % were adopted as performance parameters. These experimental values were adopted to evaluate the effectiveness of the proposed system in this research. 3.1 Simulation setup An NH3-fired thermal power plant was simulated using AVEVA PRO/II Simulation 2024 software. The system configuration shown in Figure 1 was implemented in the simulation flowchart presented in Figure 2. This configuration was used to assess both the power plant's performance and the amount of NO produced. Figure 2: The simulation flowchart of the proposed co-production system. All gaseous inputs were supplied at a temperature of 25 °C and a pressure of 101.325 kPa. The flow rate of NH3 (𝐹𝐴𝑀𝑀𝑂𝑁𝐼𝐴) as shown Ammonia in Figure 2 was fixed at 100 kmol/h of NH3, while the inflow air to the compressor with NH3 was assumed to be standard dry air, consisting of 78.08 % nitrogen, 20.95 % oxygen, 0.93 % of argon, and 0.04 % of CO2. The air flow rate (𝐹𝐴𝐼𝑅, in kmol/h) to the combustor was calculated based on the equivalence 75 ratio (ER: 𝜖), defined as the ratio of the actual fuel(NH3)-to-air ratio to the stoichiometric fuel(NH3)-to-air ratio. Accordingly, 𝐹𝐴𝐼𝑅 was derived using Eq(1). FAIR = 3 4 × 100 ϵ × 20.95 % = 75 ϵ × 20.95 % (1) where 3/4 is the stoichiometric ratio for complete combustion of NH3, shown as Eq(2) below. 4NH3 + 3O2 → 2N2 + 6H2O (2) The EGR inflow increases the inlet gas temperature while suppressing the peak combustion temperature, thereby preventing excessive thermal stress in the combustor without altering the ER. In the simulation, as the component and thermal property of exhaust gas rely on the NH3 combustion and NOX absorption conditions, the flow of EGR was simulated with pure nitrogen gas at 25  °C and 101.325 kPa corresponding to the conditions in NOX absorber, and its flow rate was adjusted to maintain the combustor temperature at the target value. When ER exceeds 1, the combustion of NH3 becomes incomplete, leading to unburned NH3 that may cause NH3 slip when released into the atmosphere. To prevent such cases, an inflow of standard dry air with sufficient flowrate was introduced to the secondary combustor to ensure complete combustion of the remaining NH3. The circulation of coolant was also simulated with pure water inflow. The water was supplied at 25 °C and 5 kPa, and its flowrate was set so the steam turbine's vapor temperature reached the target temperature. Table 1 summarizes the assumptions made for the remaining system components. The temperatures and pressures for the combustor and steam turbine were based on the gas turbines and the supercritical steam turbines that have already been put into practical use. All flows between components and combustion reactors were assumed to be adiabatic. As NO forms from the combustion of NH3 as Eq(3) and NO2 is produced from the oxidation of NO through the system, the NOX formation ratio from NH3 combustion was referred to the NO conversion ratio (𝜂𝑁𝑂) in the primary combustor under different ER values provided by JST-LCS (2018). 4NH3 + 5O2 → 4NO + 6H2O (3) Table 1: Power generation conditions for the simulation model. Name Value Name Value Combustor temperature 1,627 °C Compressor adiabatic efficiency 85 % Steam temperature 600 °C Turbine adiabatic efficiencies 85 % Compressor compression ratio 25 Pump efficiency 80 % Steam pumped pressure 25 MPa Heat exchanger minimum temperature difference 50 °C Steam final pressure 5 kPa 3.2 Economic evaluation The simulation results were used to evaluate and compare the economic value generated per unit of NH3 consumed, taking into account both electricity generation and material production. The amount of electricity generated was calculated as the network output, defined as the difference between the work (all in kW) produced by the gas turbine (𝑊𝐺) and steam turbine (𝑊𝑆) and the work consumed by the pump (𝑊𝑃) and compressor (𝑊𝐶). It was assumed that no energy losses occurred in power generation. The amount of material (KNO3) produced was estimated based on the referred NOX conversion ratio 𝜂𝑁𝑂 and the previously determined absorption and conversion ratios, 𝜂𝐴 and 𝜂𝑃. Both the electricity and material outputs were then converted into monetary values to enable comparison across different ERs. The revenue from electricity generation (𝑉𝐸, in JPY/kmol-NH3) was calculated using an electricity price (𝑝𝐸) of 30.0 JPY/kWh, corresponding to the home electricity (low voltage) price level in Japan (TEPCO 2023). The net value of the material produced (𝑉𝑃, in JPY/kmol-NH3) was calculated using the commercial prices of KOH (𝑝𝐾𝑂𝐻) at 14.6 JPY/mol (METI, 2023) and KNO3 (𝑝𝐾𝑁𝑂3 ) at 55.1 JPY/mol (Plants Corp., 2023). The calculation methods are presented in Eqs(4) and (5). VE = (WG + WS - WP - WC) p E FAMMONIA (4) VP = η NO η A η P (p KNO3 - p KOH ) × 10 3 (5) 76 4. Results and Discussions 4.1 Simulation outcomes Figure 3 and Table 2 present the simulation results at different ERs. The electricity generated per unit of NH3 combusted was slightly higher at ERs below 1. In contrast, NOX emissions at ERs greater than one were significantly reduced due to the unburned NH3 reacting with NOX within the combustor. For this reason, previous studies have preferred combustion conditions around ER 1.1–1.2 to minimize post-treatment costs associated with NOX emissions. When ER is lower, the presence of excess oxygen in the combustor increases NOX formation. The additional NOX enables greater KNO3 production in the proposed system, thereby generating higher economic returns per unit of NH3 combusted at lower ERs. Figure 3: Simulated results of electricity and NOX produced at different ERs. Table 2: Simulation results of flowrates and works at different ERs. ER(-) 𝜂𝑁𝑂(%) 𝐹𝐸𝐺𝑅(kmol/h) 𝐹𝐶𝑂𝑂𝐿𝐴𝑁𝑇(kmol/h) 𝑊𝑃(kW) 𝑊𝐶(kW) 𝑊𝐺(kW) 𝑊𝑆(kW) 0.5 2.89 104.2 446.8 70.1 3,850.0 6,501.9 2,954.5 0.6 2.37 226.3 447.4 70.2 3,862.8 6,517.1 2,958.2 0.7 1.87 313.9 448.0 70.3 3,873.6 6,530.1 2,961.8 1.1 0.00 411.8 467.4 73.3 3,499.6 5,899.9 3,090.6 1.2 0.00 360.0 481.8 75.6 3,163.3 5,343.8 3,185.7 1.4 0.01 278.7 504.5 79.1 2,635.5 4,469.6 3,335.4 4.2 Economic evaluation Based on the simulation results, 𝑉𝐸 and 𝑉𝑃 were calculated for each ER using Eqs(4) and (5) to allow for direct comparison. Figure 4 illustrates the net economic value per unit of NH3 combusted, derived from electricity generation and KNO3 production. Figure 4: Economic values from electricity and KNO3 produced at different ERs. 77 Compared to low-NOX combustion at ER 1.1, the utilization of NOX in the exhaust gas led to a substantial increase in overall economic value when the proposed system was applied. At ER 0.5, the profit from KNO3 production alone accounted for approximately 52 % of the electricity profit at ER 1.1. In addition, electricity output slightly improved (by 1 %) due to a higher contribution from the gas turbine than from the steam turbine. Overall, a 53 % increase in total economic return was observed for ER 0.5 compared to ER 1.1, underscoring the strong potential of the proposed system as an effective chemical recycling and value-enhancing solution. 5. Conclusions Utilizing NOX-containing exhaust gas to synthesize valuable materials offers a promising approach to offset NOX treatment costs and generate additional profit through chemical recycling. However, the economic potential of the recovered material depends on both the concentration of NOX and the purity of other acidic gases in the exhaust. Therefore, this study conducted a preliminary investigation into the feasibility and economic effectiveness of a proposed KOH-based NOX capture and utilization system, integrated into an NH3-fired combined-cycle power plant. A simulation-based analysis was conducted to evaluate both power generation and NOX production under various operating conditions. The resulting electricity and material outputs were then translated into economic values to assess profitability across different ERs. The results demonstrated that, for the same amount of NH3 combusted, operation at ER 0.5 led to increased electricity output and significant KNO3 production. Under the assumed system and economic conditions, this translated to a 53 % increase in total profit compared to low- NOX combustion at ER 1.1, primarily driven by the value of the coproduced KNO3. 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