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

•Ammonia addition lowers the maximum combustion temperature and thermal NOx.•Total NOx is reduced by SNCR process during expansion and exhaust strokes.•Conventional SCR system reduces unburned ammonia and NOx emissions.•N2O emissions increases as the excess air and ammonia fraction increase.•NO oxidation in the SCR system increases tail-pipe NO2 emissions.