Method for reducing nox in a nitric acid plant during transitory events

A method for processing absorption tail gas in of a nitric acid plant, wherein:

• said nitric acid plant comprises a synthesis section and an absorption section wherein in said synthesis section ammonia is catalytic oxidised to obtain a nitrogen-oxides containing gas and wherein in said absorption section said nitrogen-oxides containing gas is absorbed in water to yield a concentrated nitric acid a tail gas containing NOx and N2O,
• said nitric acid plant comprises a treatment section suitable for removing N2O and NOx from said tail gas before it is discharged to atmosphere, said treatment section including a first catalytic bed and a second catalytic bed arranged to be traversed in sequence by the tail gas,
• wherein, during a condition of normal operation of the plant. the tail gas processing includes passing the tail gas through said first catalytic bed and second catalytic bed in sequence, wherein one of said first catalytic bed and second catalytic bed operates as N2O-removing bed and the other one of said first catalytic bed and second catalytic bed operates as—NOx-removing bed, and said tail gas processing includes adding a reducing agent to said NOx-removing bed so that removal of NOx is performed in the presence of said reducing agent,
• wherein, during a transitory event of said nitric acid plant, which is a startup or a shutdown of said nitric acid plant, the tail gas processing is modified to include:
• feeding an additional amount of said reducing agent to one catalytic bed selected among said first catalytic bed and second catalytic bed;
• wherein the selected catalytic bed, receiving said additional amount of reducing agent during the transitory event, is the N2O-removing catalytic bed used for removing N2O from the tall gas during normal operation outside the transitory event;
• wherein, during the transitory event, the tail gas traverses said selected catalytic bed in the presence of said additional amount of reducing agent,
• wherein said step of feeding the additional amount of reducing agent to the selected catalytic bed is carried out only during the transitory event, so that during the transitory event said selected catalytic bed acts temporarily as an additional bed for removing NOx from the tail gas due to the presence of said additional amount of reducing agent.