This invention relates generally to the treatment of NOx in combustion flue gas. In certain embodiments, the invention relates to the use of a regenerative heat exchanger (RHE) to convert urea to ammonia in a side stream of flue gas. Ammonia and/or other urea decomposition products exit the heat exchanger, are mixed with the rest of the flue gas, and enter a selective catalytic reduction (SCR) unit for reduction of NOx in the flue gas. The use of an RHE significantly improves the thermal efficiency of the overall process. More particularly, in certain embodiments, the regenerative heat exchanger is a dual chamber RHE.