Experimental investigation of urea-water injection-borne particles in selective catalytic reduction systems of hydrogen combustion engines

Selective catalytic reduction (SCR) systems using urea water solution (UWS) are widely applied to reduce NO x emissions. However, active UWS injection leads to particle formation, raising concerns about particulate number (PN) emissions. This study investigates particle formation in an SCR system coupled to a hydrogen combustion engine. For the first time, a clear linear correlation between UWS injection rate and PN emissions is established across a wide range of operating conditions, demonstrating a direct quantitative relationship between urea injection and particle output. Parallel measurements at two cut-off sizes (PN10 and PN23) confirm the high number of urea-induced particles and show that the particle size peaks below 23 nm, highlighting the dominance of ultrafine particles. Additionally, UWS atomization and temperature in the mixing section are identified as critical factors. Fine spray and elevated temperatures facilitate rapid urea decomposition, promoting particle formation. The use of a static mixer enhances secondary atomization, further increasing PN levels. Chemical analyses reveal that deposits consist mainly of cyanuric acid, indicating solid polymerization processes rather than ammonium nitrate formation as a primary contributor to particle formation. Furthermore, a targeted variation of the NO x composition confirms that the formation of ammonium nitrate does not contribute to PN under the examined conditions. Overall, this study provides new insights into the mechanisms of particle formation due to UWS injection. The findings highlight the need for further investigation of the urea decomposition and associated side reactions to fully elucidate the particle formation mechanism in SCR systems. • A linear correlation is observed between urea injection and particle formation. • Spray atomization and droplet evaporation strongly affect particle number. • Mixing-section temperature significantly impacts UWS particle formation. • Particle formation is governed primarily by urea-derived intermediates. • The exhaust gas composition has negligible influence on particle formation.