Ammonium-Salt Formation and Catalyst Deactivation in the SCR System for a Marine Diesel Engine

Abstract

Due to the low temperature and complex composition of the exhaust gas of the marine diesel engine, the working requirements of the selective catalytic reduction (SCR) catalyst cannot be met directly. Moreover, ammonium sulfate, ammonium nitrate, and other ammonium deposits are formed at low temperatures, which block the surface or the pore channels of the SCR catalyst, thereby resulting in its reduction or even its loss of activity. Considering the difficulty of the marine diesel engine bench test and the limitation of the catalyst sample test, a one-dimensional simulation model of the SCR system was built in this paper. In addition, the deactivation reaction process of the ammonium salt in the SCR system and its influencing factors were studied. Based on the gas phase and the surface reaction kinetics, the models of the urea decomposition, the surface denitrification, the nitrate deactivation, and the sulfate deactivation were both constructed and verified in terms of accuracy. Moreover, the formation/decomposition reaction pathway and the catalytic deactivation of ammonium nitrate and ammonium bisulfate, as well as the composition concentration and the exhaust gas temperature range were correspondingly clarified. The results showed that within a certain range, the increase of the NO2/NOx ratio was conducive to the fast SCR reaction and the NH4NO3 formation’s reaction. Increasing the exhaust gas temperature also raised the NO2/NOx ratio, which was beneficial to both the fast SCR reaction and the NH4NO3 decomposition reaction, respectively. Furthermore, the influence of the SO2 concentration on the denitrification efficiency decreased with the increase of the exhaust gas temperature because of increasing SCR reaction rate and reversibility of ammonia sulfate formation, and when the temperature of the exhaust gas was higher than 350 °C, the activity of the catalyst was almost unaffected by ammonia sulfate.

A Case Study for the Deactivation and Regeneration of a V2O5-WO3/TiO2 Catalyst in a Tail-End SCR Unit of a Municipal Waste Incineration Plant

“…Therefore, from the onset temperature of desorption, it can be argued that the last temperature the catalyst experienced in the SCR reactor was around 210 • C. The total amount of desorbed ammonia was lower (0.075 mmol/g) than the amount desorbed from the fresh sample due to the lack of NH 3 pre-adsorption step at 100 • C. Nevertheless, in the high-temperature range (above 300 • C), the NH 3 desorption peak for the aged catalyst exceeded that of the fresh catalyst. This result strongly suggests the decomposition of ammonium salts, like sulfate and bisulfate, probably formed by the reaction between NH 3 and SO 2 /SO 3 or other superficial sulfates which were eventually deposited on the catalyst surface due to the low temperature of operation [7,11,18,[20][21][22]. Notably, the content of gaseous SO 2 in the exhaust gas after the desulfurization unit is quite low (in the single digit ppmv range) [15].…”

Fuel Consumption and Emission Reduction for Non-Road Diesel Engines with Electrically Heated Catalysts

“…This is attributed to the increase in NH₄NO₃ formation due to the increase in NO₂ conversion efficiency caused by excessive use of the oxidation catalyst. The increased NO₂ formed NO₃-by Cu(OH)+ on the SCR catalyst and the NH₄NO₃ formed by combining the injected/absorbed NH₃ with NO₃-resulted in N₂O emission due to the decomposition of NH₄NO₃ with increasing exhaust temperature [39][40][41][42]. The NRTC cold start results when the UWS injection quantity was increased by 5% overall in NSR map (III) are shown in Figure 10.…”

“…The remaining NH 3 slips through the SCR catalyst and is removed by the ASC located downstream of the SCR [11,28]. In this process, various deposits and particles formed by urea polymerization and N 2 O are generated through sub-reactions depending on the composition of the catalyst and the injection method of UWS [11,[38][39][40][41][42][43]45]. The following shows representative reactions in SCR and ASC.…”

Reaction mechanism and chemical kinetics of NH3-NO/NO2-SCR system with vanadium-based catalyst under marine diesel exhaust conditions

“…Zhu et al. 15–19 has been working on SCR for years and have obtained some satisfactory results. However, detailed change processes, especially in the exhaust flow of marine diesel, cannot be described only using overall reaction pathways, so elementary gas–solid reaction pathways should be coupled to analyze the reaction kinetics of the catalytic process.…”