Experimental investigation on gas emission characteristics of ammonia/diesel dual-fuel engine equipped with DOC + SCR aftertreatment

Abstract

After-treatment device diesel oxidation catalyst (DOC) and selective catalytic reduction (SCR) were installed in the exhaust system of ammonia/diesel dual-fuel engine to remove harmful pollutants such as hydrocarbon (HC), carbon monoxide (CO), ammonia (NH3), nitrogen oxide (NOx), nitrous oxide (N2O) and other pollutants produced by the dual-fuel combustion mode. The results indicate that the HC, CO and N2O emissions of ammonia/diesel dual-fuel engine increase with the increment of ammonia fraction, whereas NOx emissions decrease. At 50 % load, the CO conversion efficiency of DOC decreases from 100 % at diesel-only mode to 18.9 % at 40 % ammonia fraction, while the HC conversion efficiency is independent of ammonia fraction. NOx can be reduced by NH3 in the exhaust, while NH3 will be oxidized to N2O and NOx in DOC. Moreover, at 50 % load, more NH3 emission is oxidized to N2O compared to 75 % load. After SCR, the NOx, N2O and NH3 emissions in the exhaust are further reduced, but due to the NH3/NOx ratio, there are still a large amount of NOx and NH3 emissions at SCR outlet. The N2O conversion efficiency of SCR is relatively low, with a maximum value of only 47.8 % at 50 % load and 30 % ammonia fraction. Owing to the high N2O emission from dual-fuel engine, the direct introduction of ammonia cannot significantly reduce greenhouse gas (GHG) emissions. Furthermore, the massive amount of unburnt ammonia in the exhaust is oxidized to N2O by DOC, leading to a sharp increase in GHG emissions after DOC + SCR aftertreatment, and the GHG emissions at 50 % load and 40 % ammonia fraction are 6.2 times higher than those before the aftertreatment, and 9.2 times higher than those of diesel-only mode.