Experimental approach for reducing nitrogen oxides emissions from ammonia–natural gas dual-fuel spark-ignition engine

Abstract

In this study, characteristics of nitrogen oxides (NOx) emissions from an ammonia–natural gas dual-fuel operation were investigated by conducting spark-ignition engine experiments under the maximum load and a low engine speed (∼1,100 rpm). The feasibility of using a conventional selective catalytic reduction (SCR) system to reduce the NOx emissions was also investigated. For the experiments, an 11-L six-cylinder spark-ignition engine was used with appropriate modifications to use natural gas and ammonia together as the fuel. Several species comprising NOx emissions (NO, NO2, and N2O) and ammonia were measured together using a Fourier transform infrared analyzer (FT-IR). An aftertreatment system, including an SCR system and an ammonia oxidation catalyst (AOC), for an original natural gas-fueled engine was used to reduce engine-out NOx using the unburned ammonia as a reducing agent.From the experimental results, it was found that the NO concentration first increased on the addition of ammonia, as the fuel NOx emissions were produced. However, the total NO concentration became saturated with increasing ammonia fraction, owing to the increased amount of unburned ammonia. Specifically, considering the trend of the burned gas temperature and its value at exhaust valve opening (EVO) timing, it is inferred that nitrogen oxide was reduced owing to unburned ammonia after the exhaust valve opening timing, where the temperature range of combustion product was suitable to trigger selective non-catalytic reduction (SNCR). The remaining ammonia, after the SNCR, was then virtually oxidized to reduce the remaining NO while passing SCR system, showing an almost zero level (<10 ppm) at the catalyst-out level. N2O and NO2 were produced in the order of tens and hundreds of ppm, respectively. Thus, despite the high level of global warming potential (GWP) of N2O, its contribution to global warming effect was not significant. On the other hand, the unburned methane was particularly increased at some point with poor combustion efficiency to offset the effect of decrement in carbon dioxide due to its high level of GWP.