Reducing NO.sub.x emissions from an engine by temperature-controlled urea injection for selective catalytic reduction

A method for reducing the emissions of NO.sub.x from a lean-burn engine having associated therewith an exhaust system having an exhaust passage for carrying exhaust gases containing NO.sub.x to a reactor effective for selective catalytic NO.sub.x reduction, comprising:

• feeding an aqueous urea solution from a storage vessel through a line to an injector;
• returning 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 for NO.sub.x reduction and maintain the temperature of the urea solution sufficiently low that it is not permitted sufficient time to hydrolyze the urea to such an extent that solids precipitate prior to injection of the urea solution into the exhaust gases;
• injecting urea solution into the exhaust gases at an exhaust gas temperature sufficient for SCR;
• and
• passing the exhaust gas through an SCR reactor.