Ammonium bisulphate formation in marine SCR
Ammonium bisulphate forms below a sulphur-dependent temperature threshold in an SCR reactor, fouling the catalyst and setting the minimum operating temperature.
Ammonium bisulphate is the reason a marine selective catalytic reduction system carries a minimum operating temperature rather than only a maximum. Ammonia from the urea reagent reacts with sulphur trioxide and water to form a sticky salt that deposits on the catalyst surface and on downstream heat exchange surfaces. The formation temperature rises with the sulphur content of the fuel.
That single mechanism is what links the sulphur regime to the NOx regime. It is why resolution MEPC.291(71) requires the maximum allowable fuel oil sulphur content to be documented for an approved SCR system, and why an operator burning residual fuel behind an exhaust gas cleaning system has to think about SCR inlet temperature at low load.
The full article will cover the formation and decomposition chemistry, the temperature thresholds and their sulphur dependence, soot blowing and thermal regeneration, the effect on catalyst life, and the same salt’s role in cold corrosion and economiser fouling.