NOx Emissions from Ammonia-Fueled SOFC System during Start-up and Normal Operation

Introduction Ammonia is a promising fuel for carbon neutrality because no carbon is contained in ammonia. In particular, ammonia has a high energy storage density of 11.5 MJ/L. Ammonia is also easy for storage and transportation because ammonia is liquefied at ambient temperature when compressed to about 10 barg. Australia and Saudi Arabia are demonstrating the production and export of green ammonia produced using renewable energy. On the other hand, South Korea and Japan are developing technologies such as ammonia-fueled fuel cells, engines, and turbines to utilize the imported green ammonia. Ammonia can be used as a fuel in various energy conversion devices, including fuel cells, engines, and turbines. Ship engine manufacturers are developing ammonia engines with the goal of onboard deployment by 2026. Ammonia has challenges as an engine fuel such as a slow flame speed and high ignition energy. To overcome these combustion characteristics, most ammonia engines use fossil fuels such as diesel or natural gas as pilot fuel. As a result, carbon dioxide emissions can be reduced by about 70% in ammonia engines. However, it is difficult to achieve zero carbon dioxide emissions. On the other hand, solid oxide fuel cell (SOFC) can use ammonia as a single fuel through an electrochemical reaction to achieve zero carbon dioxide emissions. In button cell experiments, solid oxide fuel cells showed equivalent performance when either ammonia or simulated gas of ammonia decomposition was supplied with sufficient temperature for ammonia decomposition. However, SOFC performance degradation due to nitriding of cells, current collectors and interconnectors has been reported in ammonia-fueled SOFC stacks. In particular, rapid performance degradation and cell fracture due to nitriding were observed during temperature cycling between 600°C and 700°C. Nitriding is more prone at around 600°C. Therefore, an operational strategy is required for ammonia-fueled SOFC systems to prevents ammonia exposure within the temperature range. Additionally, unreacted ammonia supplied to the burner can increase NOx emissions because ammonia is readily oxidized to NOx. In this study, NOx emissions during start-up and normal operation of an ammonia-fueled SOFC system were investigated. In particular, an ammonia decomposition reactor was applied to prevent nitriding of the SOFC stack during cold start-up. A catalytic burner was included to remove hydrogen and ammonia from the SOFC off-gases and supply heat to the ammonia decomposition reactor. To reduce NOx emissions during the start-up, ammonia was partially oxidized and fed to the catalytic burner. In particular, the NOx emissions of the catalytic burner were investigated as a function of the O 2 /NH 3 ratio in the partial oxidation reactor. On the other hand, NOx emissions during normal operation were analyzed by thermodynamic equilibrium calculations and experiments with an SOFC off-gases simulator. Experiments To investigate the NOx emissions from the start-up of an ammonia SOFC system, a hot-BOP was configured with a partial oxidation reactor, catalytic combustor and ammonia cracking reactor. The partial oxidation reactor was loaded with 60 ml of a commercial Ru catalyst (2.0 wt.%) in a 1-inch STS tube. A glow plug was installed in the partial oxidation reactor to initiate the reaction. Ammonia and air were supplied to the partial oxidation reactor, and the flow rates were controlled using a mass flow controller (MS3150V, Line Tech Inc.). The gases produced in the partial oxidation reactor were fed to the catalytic burner. A catalytic burner was included in the ammonia decomposition reactor to provide the heat required for ammonia decomposition. Additional air was supplied to the catalytic combustor to burn the hydrogen generated in the partial oxidation reaction. The total air flow rate supplied to the partial oxidation reactor and the catalytic combustor was maintained at an excess air ratio of 3. NOx emissions and ammonia slip were investigated as a function of the O 2 /NH 3 ratio in the partial oxidation reactor from 0.105 to 0.210. When the ammonia decomposition reactor was sufficiently heated by the combustion reaction in the catalytic burner, ammonia was fed to the decomposition reactor. The resulting ammonia decomposition gas was fed through a dummy stack to the catalytic burner. The ammonia slip and NOx emissions from the undecomposed ammonia were measured during the start-up. An off-gases simulator of an ammonia SOFC was used to investigate the emission characteristics under steady-state conditions. The simulated cathode off-gases, consisting of air and nitrogen, were controlled using a mass flow controller (MS3150V, Line Tech Inc.) and heated with an electric heater before being supplied to the catalytic burner. The simulated anode off-gases, containing hydrogen, nitrogen and ammonia, were controlled and supplied using mass flow controllers (MS3150V, Line Tech Inc.). Water was fed using a high-pressure liquid pump (UI-22, FLOM, Inc.) and mixed with the gases after being vaporized. And then the mixture of gases and steam was supplied to the catalytic burner after being heated in an electric furnace. Ammonia slip in the exhaust gas was analyzed using a tunable diode laser absorption spectroscopy gas analyzer (Airwell+7, KINSCO Technology Co., Ltd.). NOx emissions were measured with a chemiluminescence analyzer (VA-3000, Horiba, Ltd). By varying the composition of the off-gases according to the ammonia conversion rate, fuel utilization rate and air excess ratio at the ammonia SOFC stack, ammonia slip and NOx emissions in the catalytic burner were investigated. Results and discussions Ammonia is completely oxidized at an O 2 /NH 3 ratio of 0.75. When ammonia was directly oxidized in the catalytic burner with an O₂/NH₃ ratio higher than 0.75, the NOx emissions exceeded the maximum limit of the NOx analyzer. During the ammonia oxidation, the generated NOx is reduced by the unoxidized ammonia. Therefore, the insufficient ammonia at high O₂/NH₃ ratios increases NOx emissions. When the O₂/NH₃ ratio is below 0.75, some of ammonia is oxidized with heat generation. On the other hand, the unoxidized ammonia is decomposed into hydrogen and nitrogen using the heat. At thermodynamic equilibrium of the ammonia partial oxidation, 99.0% of ammonia was converted at an O₂/NH₃ ratio of 0.15 with the highest hydrogen concentration of 46.4%. When the O₂/NH₃ ratio was higher than 0.15, the produced hydrogen concentration decreases in the partial oxidation reaction. The partial oxidation reactor in this experiment was initiated using a glow plug and started within 10 seconds. The partial oxidation is suitable for fast start-up because its reaction is exothermic and thermally self-sustainable. Therefore, in this study, the gas generated in the partial oxidation reactor was burned in a catalytic burner to heat the ammonia decomposition reactor in order to reduce NOx during the start-up. The NOx emissions in the catalytic burner with the partial oxidation reactor were shown in figure 1. The NOx emissions were corrected to a standard oxygen concentration of 4%. The NOx emissions from the catalytic burner decreased as the O 2 /NH 3 ratio increased from 0.105 to 0.210. In particular, when 7 L/min of ammonia was supplied at an O₂/NH₃ ratio of 0.210, the NOx emissions minimized to 42 ppm. The amount of oxidized ammonia increases as the O₂/NH₃ ratio in the partial oxidation reactor increases. In addition, the high reaction temperature due to the exothermic oxidation reaction increased the conversion rate of the unoxidized ammonia. The amount of ammonia supplied to the catalytic burner decreased due to both oxidation and decomposition at high O 2 /NH 3 ratio. These leaded to reduction of NOx emissions in the catalytic burner. About 20 minutes was needed to reached the reaction temperature of the ammonia decomposition reactor using the heat from the catalytic burner. After then, 10 LPM of ammonia supplied to the ammonia decomposition reactor. The ammonia decomposition rate was stabilized within 30 minutes. This indicates that hydrogen could be supplied to the SOFC stack stably during the start-up. Therefore, it is expected to prevent nitriding of the SOFC and NOx emissions using the partial oxidation reactor during start-up of ammonia-fueled SOFC system. The NOx emissions in the catalytic burner as a function of the ammonia conversion rate after the SOFC stack were shown in figure 2. The NOx emission during normal operation was analyzed using the simulated SOFC off-gases under the conditions of f