Disclosed are a process and apparatus for selective catalytic reduction of NO.sub.x. The process is enabled by passing a heat exchanger section, such as an economizer, of the boiler in advance of an SCR unit at low load conditions to enable NO.sub.x reduction even at low loads using urea instead of ammonia. In a preferred form, under high load conditions, the bypass can be almost fully closed and the economizer can be operated normally without excessively cooling the combustion gases, using only a portion of bypassed gases which are hot enough to decompose the urea into its active components including ammonia.