Ummel, D. and Price, K., "Performance and Sulfur Effect Evaluation of Tier 4 DOC+SCR Systems for Vanadia, Iron, and Copper SCR," SAE Int. J. Engines 7(3):1244-1251, 2014, doi:10.4271/2014-01-1519.
Non-road Tier 4 Final emissions standards offer opportunities for engines to be certified with DOC + SCR aftertreatment systems (ATS), where particulate matter (PM) emissions will be controlled by engine measures. These non-filter systems will not experience high thermal conditions common for filter regeneration and, therefore, will not have the secondary benefit of thermal events removing sulfur from the DOC and SCR aftertreatment. An experimental program was conducted on DOC + SCR systems in which the DOC was selected for the anticipated NO2 and sulfur management requirements of a fixed volume of 3 SCR types (vanadia, copper and iron). Each system was optimized to NOx conversion levels of 90%+ on NRTC cycles then exposed to accelerated sulfur poisoning and various cycles of increasing temperature after each poisoning to observe the performance recovery of the system. Specific sulfur management strategies are defined, depending on technology. Though each system has its own unique sensitivity to sulfur and conditions where it recovers, all SCR systems are capable of sustaining 90%+ NOx conversion levels. In the case of the copper SCR (Cu-SCR), a new DOC formulation has been developed that noticeably decreases the sensitivity to sulfur, while still allowing the necessary durability for periodic desulfation events.