SCR technology can be used to reduce NOx emissions on lean-burn gas engines. Normally this approach is reserved for large stationary engines, where the cost of an SCR system is more than offset by the fuel savings which arise from running the engine lean. On smaller engines, where low NOx emissions are important it may work out less expensive to run the engine with a stoichiometric mixture and use a three-way catalytic converter.
As with diesel engines, the reductant is ammonia. This can be derived from Adblue or alternatively ammonia (either anhydrous or aqueous) can be stored in a tank. For more details please refer to the section headed ‘reductants‘.
Engines running on biogas may have significant levels of sulphur present in the exhaust gas. In these cases, catalysts based on vanadia are preferable to the zeolite type as they are less susceptible to poisoning by sulphur. Sulphur levels also have an impact on the temperature at which the reductant is injected into the exhaust, since below a certain temperature threshold it reacts with ammonia to produce solids which may block the catalytic converter.