Simultaneous Control of Unburned NH₃ and NO_x Emissions From High Load Dual-Fuel Ammonia Operation on a High-Speed Diesel Engine Using a Cu-SCR System

Dual-fuel ammonia strategies are being investigated as a promising way to utilize NH₃ as an alternative fuel for internal combustion engines in the maritime sector. One of the remaining barriers to implementing dual-fuel NH₃ combustion strategies is understanding ways to minimize unburned NH₃ and nitrogen oxide (NO_x) emissions from these engines, both of which are elevated relative to a conventional diesel baseline. Selective catalytic reduction (SCR) systems are widely used for lean NO_x emission controls for engines across transportation and stationary energy applications. SCR systems use a reducing agent, such as NH₃, to react with NO_x in the exhaust, converting it into nitrogen and water. Typically, NH₃ is injected into the exhaust as a urea solution. In dual-fuel NH₃ engines, where unburned NH₃ is present in the exhaust, an SCR system could be used to mitigate both NH₃ and NO_x emissions. The presented work evaluates a commercial copper-zeolite SCR and ammonia slip catalyst system, designed for on-road diesel engine applications, for controlling unburned NH₃ and NO_x emissions from a dual-fuel NH₃ combustion engine. The aftertreatment system was installed downstream of a single-cylinder four-stroke diesel engine that has been modified for dual-fuel ammonia use. The emissions were characterized by using a Fourier transform infrared spectrometer for both late- and early-injection diesel pilot strategies over three air–fuel equivalence ratios spanning from 1.6 to 1.0 at 1200 rpm and 12.6 bar IMEP_g condition (with greater than 95% ammonia energy fraction). Initial findings indicate that the SCR achieves more than 99% NO_x conversion with less than 50 ppm NH₃ slip at air–fuel equivalence ratios greater than 1.4 at the operating conditions investigated. However, these benefits are accompanied by additional N₂O emissions that are formed over the Cu-SCR.