A selective catalytic reduction apparatus has at least first and last catalyst layers in series for reducing nitrogen oxides in a flue gas, at least one interstage heat exchanger located after the first layer and before the last layer, that lowers flue gas temperature, and that acts as a mixing body to lower the standard deviation of the NH.sub.3/NO ratio entering catalyst layers after the first catalyst layer, thus providing better consumption of both ammonia and NOx in the reactor than would be achieved in the absence of the at least one interstage heat exchanger.