Method and a device for removing nitrogen oxides and sulphur trioxide from a process gas

A method of removing, at least partially, nitrogen oxides from a hot process gas through the use of a catalytic reactor, said method comprising:

• causing said hot process gas with an ammonia or urea additive to flow through at least a first catalyst bed segment of at least two catalyst bed segments of said catalytic reactor with catalytic material of vanadium pentoxide, tungsten trioxide or a combination thereof, said hot process gas being at a first temperature in the range of 180-300° C. at which sulphur trioxide entrained with the hot process gas is at least partially precipitated out as solid ammoniumbisulfate onto said catalytic material of the first catalyst bed segment;
• periodically operating a closing device of said first catalyst bed segment in order to thereby isolate said first catalyst bed segment from the flow of hot process gas, while at least a second catalyst bed segment of said at least two catalyst bed segments remains operative for purposes of removal of sulphur trioxide and nitrogen oxides from said hot process gas;
• regenerating using a regenerating gas said catalytic material of said first catalyst bed segment at a second temperature less than 400° C. when said first catalyst bed segment is isolated from the flow of the hot process gas;
• and
• passing said regenerating gas, after said regenerating gas has flowed through said first catalyst bed segment, with said hot process gas through a sulphur trioxide removal device, said sulphur trioxide removal device being separate from said catalytic reactor.