Reducing emissions of nitrogen oxide(s) from lean-burn engine with associated exhaust system - by feeding aqueous urea solution from storage vessel through line to injector, returning solution to storage vessel, injecting solution into exhaust gases and passing gas to selective catalytic reduction reactor
A method for reducing the emissions of NOx from a lean-burn engine with an associated exhaust system, including an exhaust passage linked to a reactor effective for selective catalytic NOx reduction, comprises feeding an aqueous urea solution from a storage vessel through a line to an injector; returning the urea solution from the injector to the storage vessel, the rates of feed and return being sufficient to supply urea as needed to the exhaust gases and maintain the temperature of the urea solution sufficiently low that it is not permitted sufficient time at elevated pressure to hydrolyse the urea to such an extent that solids precipitate; injecting the urea solution into the exhaust gases at a temperature sufficient for selective catalytic reaction (SCR); and passing the exhaust gas through the SCR reactor. Also claimed are: (i) a method in which a heat exchange fluid is passed in heat exchange contact with the line and/or injector, but otherwise the same as above; and (ii ) an apparatus for reducing the emissions of NOx from a lean-burn engine with an associated exhaust system, including an exhaust passage linked to a reactor effective for selective catalytic NOx reduction, comprising means for feeding an aqueous urea solution form a storage vessel through a line to an injector; means for returning the solution to the storage vessel, means for injecting the solution into the exhaust gas and exhaust passage means leading from the injector to an SCR reactor. Preferably the urea solution contains at least 25 wt.% urea. The temperature of the exhaust gases is 180-650 [deg]C. The urea solution is maintained at less than 140 [deg]C. The urea solution is injected into the exhaust gases at at least 30 psi. The urea solution is heated in whole or in part by an auxiliary heating means or by heat transfer from the exhaust. The urea solution is introduced into the exhaust between exhaust valves of the engine and a turbocharger exhaust turbine. The urea is introduced into the exhaust close-coupled relation with the SCR catalyst. Introduction of the urea solution into the exhaust is controlled by a controller integrated with an engine management system. The temperature of the urea solution is allowed to rise above 100 [deg]C and the solution is maintained at a pressure above the saturation vapour pressure of the temperature of the solution.
