disclosed are a process and apparatus for selective catalytic reduction of no x . the process is enabled by bypassing a heat exchanger section , such as an economizer , of the boiler in advance of an scr unit at low load conditions to enable no 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 .