Applications of multi-phase flow technologies to clean marine engineering are introduced. Large efforts have been made to reduce CO2, SOx, NOx and Particulate matter (PM) emissions from marine diesel engines. Urea Selective Catalytic Reduction (SCR) system is one of the most promising technology to largely reduce NOx emission. PM in exhaust gas may deposit on a honeycomb SCR catalyst, which increases pressure drop, blocks the flow area and decreases NOx reduction performance. Air blow (soot blow) can be effective for removing PM on a catalyst. An experimental investigation confirms that (1) large pore diameter of SCR catalyst and optimum exhaust gas velocity prevent PM accumulation, (2) PM accumulates mainly at the inlet and exit of the catalyst, and (3) high pressure soot blow removes PM.