Hydrogen fuelled IC engine has gained a lot of importance in the recent years, in our journey towards carbon neutral transportation. This is especially relevant for commercial vehicles due to the advantages in total cost of ownership, refuelling time and long term durability.
NOx emission from the H2-ICE remains a challenge, especially with more stringent emission norms like EPA27 and further moving towards zero emission powertrain. Due to leaner operations, engine out specific NOx emissions are of the order of 30 – 40%, compared to a similar modern diesel engine. However, transient NOx peaks, higher space velocities, residual hydrogen, higher oxygen and water content in the exhaust make it quite challenging to design an optimum aftertreatment system.
The application of a complying reaction scheme for the simulation of aftertreatment system with H2-ICE exhaust is presented, including reactions specific to H2 combustion and H2-SCR. The chemical kinetics of standard and fast SCR reactions as well as N2O formation reactions were calibrated for H2-ICE specific exhaust gas composition, based on SGB (Synthetic Gas Bench) test results. H2-ICE exhaust consists of higher O2 and H2O concentrations, in comparison to diesel exhaust. Higher O2 in the exhaust can potentially oxidize more NO to NO2, in the absence of hydrocarbon and soot particles and this can lead to higher NO2 concentration upstream of SCR and result in significant N2O formation via the nitrate route, i.e., NO2 reacts with adsorbed NH3 at the active site to form NH3NO2, which in turn decomposes to form N2O.
NH3* + NO2 --> (NH3NO2)* (1)
2(NH3NO2)* --> N2O + N2 + 3H2O (2)
* indicates adsorbed species
H2-ICE exhaust composition impacts the NH3 storage characteristics of the SCR catalyst. Thus, the NH3 storage profile, target buffer ratios and dosing control strategy were redefined suitably. Water condensation in the monolith and exothermic water adsorption by zeolite in the relevant flow and temperature regimes were also accounted by the addition of respective reactions to the scheme.
For the current study, EO exhaust profiles from the virtual engine model of a 13L heavy duty H2-ICE, with the LPDI combustion system was employed. Different aftertreatment layout configurations and their potential in overall NOx conversion efficiencies in different drive cycles, viz., WHTC, Highway and Urban are discussed. A comparison is made of the effectiveness of these layouts in taking advantage of H2-SCR reactions for NOx conversion in low temperature regime. The role of aftertreatment system in taking H2-ICE towards zero emission powertrain can be inferred.