SCR-Catalysts

As most of the engines powered with gas or diesel the limits of emission can be achieved with an optimum adjustment of the engine plus an oxidation- or 3-way-catalyst. With an oxidation catalyst only CO and HC can be reduced. With a 3-way-catalyst besides CO and HC also NOx can be reduced, but only at gas engines with Lambda=1.

With lean-burn or diesel engines a 3-way-catalyst can not be used. Special applications e.g. green houses, ships etc. where mainly reductions of nitrogens oxides ( NO and NO2 = NOx ) are required only SCR technology can be used.

NO is a colorless gas which oxides in the air to NO2. It causes in a higher dosis central paralytic symptoms. NO2 is a red-brown gas with sharp and pungent odour. In conjuntion with sunlight and hydrocarbons NO2 forms smog.

The nitrogen oxides will be reduced by the SCR – Technology (Selective Catalytic Reduction). This technology must be used at oxygenic exhaust gases. The exhaust gas will be mixed with the reduction media urea and passes through a catalyst. Nitrogen oxides together with urea react at the active surface of the catalyst and are reduced to water and nitrogen. Should it be necessary to reduce CO and HC besides nitrogen oxides, an oxidation catalyst can be placed right after the SCR-catalyst in the same housing.

The reduction media normally is urea. Theoretically also ammonia could be used, however urea has advantages in transport, handling and storage against ammonia and is cheaper.

At diesel engines eventually particles need to be reduced in addition. In such a case a soot filter can be installed in front of the SCR-plant.

Our exhaust purification systems are capable to dramatically reduce the emissions of stationary engines und keep the required emission limits for a longer period.