Reducing the emission of nitrogen oxide when starting up systems for producing nitric acid
A method of reducing NO x in tail gas obtained during startup of a plant for preparing nitric acid may involve heating the tail gas from a starting temperature T 0 , through a threshold temperature T G , to an operating temperature T B at which steady-state operation of the plant can occur (T 0 <T G <T B ). NO x -containing tail gas may be passed through a storage medium and at least partially stored while the temperature of the tail gas is lower than the threshold temperature T G . The NO x may be released, preferably when the temperature of the tail gas has attained the threshold temperature T G . The NO x may be combined with a reducing agent in the presence of an SCR catalyst after the temperature of the tail gas has exceeded the threshold temperature T G , but not before, resulting in catalytic reduction of at least a portion of the NO x .
1. A method, comprising:
generating nitrogen oxides (NO x ) in a nitric acid production plant;
directing the NOx into an absorption tower of the nitric acid production plant and removing tail gas comprising unabsorbed NO x from the absorption tower; and
reducing the concentration of NO x , in the tail gas obtained during startup of the nitric acid production plant,
wherein the tail gas is heated from a starting temperature, through a threshold temperature, and to an operating temperature at which steady-state operation of the nitric acid production plant for preparation of nitric acid can be effected,
passing the tail gas through a storage medium for NO x and storing at least a portion of the NO x in the storage medium while the temperature of the tail gas is below the threshold temperature, wherein a selective catalytic reduction (SCR) catalyst serves as the storage medium;
combining, when the temperature of the tail gas exceeds the threshold temperature, the NO x in the storage medium with a reducing agent for NO x in the presence of the SCR catalyst, which results in catalytic reduction of at least a portion of the NO x in the storage medium;
measuring, when the temperature is below the operating temperature, a concentration of the NO x in the tail gas before the tail gas contacts the SCR catalyst; and
metering, based on said measuring of the concentration of the NO x , an amount of the reducing agent sufficient to degrade the NO x already adsorbed on the SCR catalyst and the measured concentration of the NO x .
2. The method of claim 1 wherein at least one of
the starting temperature is less than 120° C.,
the threshold temperature is greater than or equal to 120° C. and less than 300° C., or
the operating temperature is greater than or equal to 300° C.
3. The method of claim 1 wherein the SCR catalyst comprises a catalytically active material selected from a group consisting of iron, compounds of iron, cobalt, compounds of cobalt, copper, and compounds of copper.
4. The method of claim 1 wherein the SCR catalyst comprises aluminum silicate.
5. The method of claim 4 wherein the aluminum silicate comprises a zeolite.
6. The method of claim 1 further comprising adjusting a level of oxidation of the NO x in the tail gas.
7. The method of claim 1 wherein the reducing agent for NOx comprises ammonia.
8. The method of claim 1 further comprising reducing a concentration of the NO x and N 2 O in tail gas obtained after the startup of the nitric acid production plant during the steady-state operation of the nitric acid production plant by way of a gas cleaning system of the nitric acid production plant for preparation of nitric acid.
9. The method of claim 8 wherein the SCR catalyst is disposed in the gas cleaning system.
10. The method of claim 1 comprising the steady-state operation of the nitric acid production plant, the steady-state operation comprising:
breaking down N 2 O catalytically to O 2 and N 2 and reducing NO x catalytically in the presence of the reducing agent for NO x ; or
reducing N 2 O catalytically in the presence of a reducing agent for N 2 O and reducing NO x catalytically in the presence of the reducing agent for NO x ; or
breaking down N 2 O catalytically to O 2 and N 2 , further reducing residual N 2 O catalytically in the presence of the reducing agent for N 2 O, and reducing NO x catalytically in the presence of the reducing agent for NO x .
11. The method of claim 10 wherein the nitric acid production plant includes a gas cleaning system for reducing the concentration of the NO x and N 2 O in the tail gas obtained during the steady-state operation, wherein
the gas cleaning system includes a two-stage construction, wherein the N 2 O is broken down catalytically to O 2 and N 2 in a first stage in a flow direction of the tail gas during the steady-state operation of the nitric acid production plant, after the first stage the reducing agent for NOx is mixed with the tail gas and then, in a second stage, the NOx is reduced catalytically in the presence of the reducing agent; or
during the steady-state operation of the nitric acid production plant the tail gas is mixed in the gas cleaning system with the reducing agent for the NOx and a hydrocarbon, carbon monoxide, hydrogen, or a mixture thereof as the reducing agent for the N 2 O, wherein the reducing agent is added in such an amount that is sufficient at least for complete reduction of the NOx, and that a gas mixture is passed through at least one reaction zone comprising the SCR catalyst for reduction of the NOx and for reduction of the N 2 O; or
the gas cleaning system includes a two-stage construction, wherein during the steady-state operation of the nitric acid production plant in a flow direction of an offgas, in a first stage, the N 2 O is broken down catalytically to O 2 and N 2 , and the offgas, after the first stage, is mixed with a reducing agent for the NOx and a hydrocarbon, carbon monoxide, hydrogen, or a mixture thereof as a reducing agent for the N 2 O, wherein the reducing agent is added in such an amount that is sufficient at least for complete reduction of the NOx, and that a gas mixture is passed through at least one reaction zone comprising a catalyst for reduction of the NOx and for reduction of the N 2 O.