Process for the removal of nitrogen oxides from off-gases

A process for removing nitrogen oxides from a cold off-gas containing said nitrogen oxides, said cold off-gas being cooler than the temperature in a central portion of the reactor, which comprises:

• passing said off-gas in admixture with ammonia or an ammonia precursor through a reactor under conditions so as to reduce the nitrogen oxides to nitrogen gas while heat is liberated, wherein the reactor comprises a plurality of adjacent spaced catalyst beds with an intervening space between said adjacent beds and wherein the off-gas is passed sequentially through said beds and the ammonia or ammonia precursor is first admixed with off-gas in the space between adjacent beds, and resulting admixture passing through at least a portion of the next adjacent bed, said process including
• maintaining a central portion of the reactor at a temperature in the range of 180.degree. C. to 600.degree. C. by periodically reversing the direction of flow of the off-gas through said reactor between respective ends of said reactor by taking a temperature measurement at a distance from the point of introduction of the off-gas of 10% to 90% of the total length of the reactor, and when said temperature reading indicates a temperature of the admixture at said distance, of between 30.degree. C. to 400.degree. C. as a result of the heat of reaction of the nitrogen oxides with the ammonia or ammonia precursor, reversing the direction of flow;
• after a reversal of flow, a first part of the reactor through which off-gas flows has retained heat from a previous reaction with ammonia or ammonia precursor, said retained heat being liberated to heat said off-gas then being introduced into the first part of the reactor.