Selective catalytic reduction (SCR) offers tremendous potential for reducing nitric oxides in the exhaust gas from power plant and marine diesel engines. With this method, the exhaust gas is treated with ammonia or urea and fed through a catalytic converter at a temperature of 300 to 400 degrees Celsius. The chemical reaction that takes place there is selective. This means that the unwanted nitric oxides are reduced first, by more than 80 per cent, and the side-effects, such as the oxidation of sulphur dioxide to sulphur trioxide, are suppressed.
SCR means that, instead of NOx, only nitrogen and water vapour exit the chimney. This only works, however, if the exhaust gas temperature is correct. If it is too high, the ammonia burns rather than forming a compound with nitric oxide. If it is too low, it forms ammonium hydrogen sulphate and gradually blocks the catalytic converter. The same happens if the sulphur content of the exhaust gas is too high. The minimum temperature required depends on the fuel’s sulphur content.
Thanks to SCR, the NOx problem in the exhaust gas can largely be resolved. The method can be optimised further through modifications to the engine. However, it requires increased investment for procurement, storage space and the large quantities of ammonia or urea that are needed. As a result, the annual expenditure for the use of SCR increases by around five per cent of the usual fuel costs.