Exhaust gas aftertreatment systems for ammonia-fueled engines

Ammonia ( NH 3 ) is a carbon-free fuel with considerable potential to defossilize internal combustion engines (ICEs). Its application is particularly promising for the maritime sector, where large volumes of NH 3 are already transported for the fertilizer industry and a well-established safety infrastructure exists on ships and in ports. NH 3 -fueled ICEs can operate under both stoichiometric and lean conditions, resulting in distinct exhaust gas compositions and varying levels of NO x and unburned NH 3 . As part of preliminary investigations, raw emissions were additionally investigated on a modified four-stroke diesel-based single-cylinder engine (SCE), adapted to a spark-ignited configuration and equipped with port fuel injection (PFI) and a passive pre-chamber. The results confirm a strong dependence of the exhaust gas composition on the relative air–fuel ratio λ , showing increasing NO x emissions and decreasing NH 3 concentrations with rising λ . To address these challenges, suitable exhaust aftertreatment concepts need to be developed. This study comprises investigations conducted not only on an engine test bench to obtain information on exhaust gas composition, but also on a synthetic gas test bench (SGB) for catalyst characterization under reproducible conditions. Selective catalytic reduction (SCR) catalysts are investigated, which have proven effective in minimizing NO x emissions in different ICE applications under both lean and stoichiometric conditions. However, excess NH 3 that is not utilized for NO x reduction nor oxidized continues to occur as slip. In addition, the use of an oxidation catalyst (OC) and a three-way catalyst (TWC) is explored. Especially the TWC demonstrates improved conversion of both NH 3 and NO x at higher temperature ranges under stoichiometric conditions. Nevertheless, substantial amounts of secondary emissions are generated at lower temperatures with both catalysts. When employing ammonia-slip catalysts (ASCs) under comparable exhaust gas conditions, a similar trend in NO x reduction to that observed with both SCRs can be achieved, while NH 3 conversion tends to be slightly more efficient. However, the issue of high secondary emission formation persists with varying catalyst loadings, which is also known from OC and TWC. This study demonstrates that achieving low tailpipe emissions requires simultaneous optimization of both the combustion process and the exhaust gas aftertreatment system. For catalyst development, in addition to ensuring a high NH 3 conversion efficiency, the selectivity toward nitrous oxide ( N 2 O ) is identified as a critical target parameter.