Abstract: A system of selective catalytic reduction units for reducing nitrogen oxides by operating a plurality of parallel combustion units in parallel with a spare selective catalytic reduction reactor is disclosed. A primary selective catalytic reactor with spent catalyst is isolated for maintenance while the flue gas is diverted to the spare selective catalytic reduction reactor while continuing to discharge flue gas essentially free of nitrogen oxides from the spare selective catalytic reduction reactor. The spare selective catalytic reduction reactor can include a spare induced draft fan to provide an alternate means for operation and maintenance.

Abstract: The present invention utilizes radiation reflectors on the refractory wall of a fired furnace opposite the spaces between adjacent tubes. The radiation reflectors focus the reflected radiation from the flame onto the dark side of the tubes. The invention increases the overall heat transfer of the tube by increasing the heat flux rate for the backside of the tube, and also decreases the flux and temperature differentials between the front and rear sides of the tubes.