Method for purification of an exhaust gas from a diesel engine

A method for removing impurities in exhaust gas from a diesel engine, where the impurities comprise nitrogen oxides, carbon monoxide, particulate matter and incompletely combusted hydrocarbons,

• which method comprises the steps of
• (a) injection in excess compared to stoichiometric ratio of a reductant comprising urea or ammonia into the exhaust gas from the engine;
• (b) reduction of the nitrogen oxides in the exhaust gas by the reductant in the presence of a catalyst active in selective reduction of nitrogen oxides to nitrogen;
• (c) intermittent injection of a hydrocarbon into the effluent from step (b);
• (d) oxidation of carbon monoxide, particulate matter, incompletely combusted hydrocarbons and injected hydrocarbon in the presence of a catalyst active in oxidising carbon monoxide, particulate matter and hydrocarbons, to carbon dioxide and water, and in selectively oxidising excess of reductant to free nitrogen;
• (e) filtration of the effluent from step (d) by passing the gas through a catalysed filter, wherein the remaining particulate matter is retained in the filter, and wherein the catalyst is active in oxidising carbon monoxide, particulate matter and hydrocarbons to carbon dioxide and water, and in selective oxidising reductant to nitrogen, creating a purified exhaust gas;
• and
• (f) withdrawal of the purified exhaust gas.