Catalytic Aftertreatment Systems for Combustion Exhaust Gases from Future Hydrogen, Ammonia and e-HC Engines

Abstract

Heavy-duty and marine transport will continue to rely on robust, high energy–density, combustion engine technology. Sustainable fuels, fuel-flexible engines and stricter future emission standards all call for further development of catalytic aftertreatment systems (ATSs). This paper reviews progress and evaluates emission removal methods, based on synthetic gas bench (SGB) experiments simulating characteristic lean and stoichiometric conditions in hydrogen, ammonia, methanol and methane engine exhaust gases. The oxidation reactivity of fuel compounds on tailored catalysts showed the following light-off temperatures (T50, °C) in lean conditions: hydrogen(140) < methanol(170) ~ CO < diesel-hydrocarbons, reference(180) < ammonia(250) < methane(380). NOx removal in mobile applications will be challenging, due to NOx limits and varying fuel types/mixtures. Urea/NH3-SCR will remain the main NOx removal method, with an option of double SCR widening its temperature window. NOx storage catalysts based on metal oxides or zeolites, increased NOx removal at 100–200 °C by passive adsorption–desorption. NOx reduction by hydrogen (H2-SCR) showed NOx reduction of up to 60–70% on platinum catalysts at 100–160 °C, before NH3-SCR reactions at higher temperatures. Selective ammonia slip catalysts (ASCs) were effective to cut NH3 emissions. High selectivity to N2 with low N2O formation was challenging with H2-SCR and ASC. Three-way catalysts, applied with stoichiometric hydrogen combustion, were operating below 200 °C. More efficient catalytic methods for methane and N2O removal are required to improve the feasibility of methane and ammonia as future fuels. Integration of different properties in the same units is essential to minimise ATS volume and costs. The flexible use of green fuels requires flexible ATSs too.