The Variation of Functional Characteristics of a Euro VI Selective Catalytic Reduction Reactor after Ageing 2020-01-2205
The selective catalytic reduction (SCR) of nitrogen oxides by ammonia is commonly applied as a method of exhaust aftertreatment for lean burn compression ignition (CI) engines. The catalytic reactor of an SCR system, like all catalytic emission control devices, is susceptible to partial deactivation as its operating time progresses. Long-term exposure of an SCR reactor to exhaust gas of fluctuating temperature and composition results in variations of the characteristics of its catalytically active layer. The aim of this study was to observe and investigate the variation of parameters characterizing the SCR reactor as a result of its ageing. Attention was paid to changes in ammonia storage capacity, selectivity of chemical reactions and maximum achievable NOx conversion efficiency. The experimental setup was a heavy duty (HD) Euro VI-compliant engine and its aftertreatment system (ATS). The setup was installed on a transient engine dyno instrumented with emission measurement devices. The experimental test plan consisted of execution of World Harmonized Steady Cycle (WHSC) on the fresh and aged SCR reactor, with an ageing protocol run between. The ageing protocol was of novel type, being repetitive execution of type-approval cycles with frequent cold start events. The obtained emission results indicated a significant increase in NOx emission as a result of SCR ageing. Post-SCR NOx concentration traces were analyzed in detail to indirectly evaluate the shift in the reactor’s characteristics. The ammonia storage capacity was found to be one of the contributors of overall NOx conversion efficiency reductions, especially under transient conditions. A further contributor to elevated NOx emission was a decrease of reactor selectivity towards N2O formation. The reactor at both fresh and aged conditions featured a similar NOx conversion efficiency at low and moderate space velocities. The decrease in the reactor’s performance was found to be proportional to the space velocity.