Abstract: A method of reducing NOx in tail gas obtained during startup of a plant for preparing nitric acid may involve heating the tail gas from a starting temperature T0, through a threshold temperature TG, to an operating temperature TB at which steady-state operation of the plant can occur (T0<TG<TB). NOx-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 TG. The NOx may be released, preferably when the temperature of the tail gas has attained the threshold temperature TG. The NOx 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 TG, but not before, resulting in catalytic reduction of at least a portion of the NOx.

Abstract: An apparatus is provided for carrying out a process for removing N2O and NOx from offgases by catalytic decomposition of N2O by means of iron-containing zeolite catalysts and catalytic reduction of the NOx by means of reducing agents, the deNOx stage connected downstream of the deN2O stage being operated at inlet temperatures of T<=400° C., and the inlet gas for the deN2O stage comprising water and having a selected N2O/NOx ratio, and the operating parameters of temperature, pressure and space velocity of the deN2O stage being selected so as to result in an N2O degradation of 80 to 98%. Under these conditions, a degree of NOx oxidation of 30-70% is established at the outlet of the deN2O stage, which is defined as the ratio of the molar amounts of NO2 to the total molar amount of NOx. The result of this is that the downstream deNOx stage can be operated under optimal conditions.

Abstract: A process for removing N2O and NOx from offgases by catalytic decomposition of N2O by means of iron-containing zeolite catalysts and catalytic reduction of the NOx by means of reducing agents, the deNOx stage connected downstream of the deN2O stage being operated at inlet temperatures of T<=400° C., and the inlet gas for the deN2O stage comprising water and having a selected N2O/NOx ratio, and the operating parameters of temperature, pressure and space velocity of the deN2O stage being selected so as to result in an N2O degradation of 80 to 98%. Under these conditions, a degree of NOx oxidation of 30-70% is established at the outlet of the deN2O stage, which is defined as the ratio of the molar amounts of NO2 to the total molar amount of NOx. The result of this is that the downstream deNOx stage can be operated under optimal conditions. Also provided is an apparatus for carrying out the process.

Abstract: A method of reducing the concentration of NOx nitrogen oxides in tail gas obtained during the startup of a plant for preparation of nitric acid may involve heating the tail gas as a result of measures for preparation of nitric acid from a starting temperature T0, through a threshold temperature TG, and to an operating temperature TB at which steady-state operation of the plant can be effected (T0<TG<TB). The method may comprise passing the NOx-containing tail gas through a storage medium for NOx and storing at least a portion of the NOx in the storage medium for NOx while the temperature of the tail gas is lower than the threshold temperature TG. In some cases, the NOx stored in step may be released, preferably when the temperature of the tail gas has attained the threshold temperature TG.

Abstract: Disclosed is a method and system for reducing the nitrogen oxide off-gas concentration in a nitric acid plant operated under pressure and equipped with a residual gas purification first reactor configured to remove nitrogen oxides from the off-gas during steady-state operation of the plant, and a second reactor configured to remove nitrogen oxides from the off-gas during a start-up and/or shut down of the plant. The method includes, during start-up and/or shut-down of the nitric acid plant, passing pressurized nitrogen-oxide-containing off-gas from the nitric acid plant and a gaseous reducing agent for the nitrogen oxides into the second reactor charged with a catalyst, to reduce the NOx content in the off-gas by at least catalytic reduction. Using the process and system, a colorless start-up and shut-down of nitric acid plants is possible and the nitrogen oxide content in the off-gas during start-up and/or shutdown can be substantially lowered.

Abstract: Disclosed is a method and system for reducing the nitrogen oxide off-gas concentration in a nitric acid plant operated under pressure and equipped with a residual gas purification first reactor configured to remove nitrogen oxides from the off-gas during steady-state operation of the plant, and a second reactor configured to remove nitrogen oxides from the off-gas during a start-up and/or shut down of the plant. The method includes, during start-up and/or shut-down of the nitric acid plant, passing pressurized nitrogen-oxide-containing off-gas from the nitric acid plant and a gaseous reducing agent for the nitrogen oxides into the second reactor charged with a catalyst, to reduce the NOx content in the off-gas by at least catalytic reduction. Using the process and system, a colourless start-up and shut-down of nitric acid plants is possible and the nitrogen oxide content in the off-gas during start-up and/or shutdown can be substantially lowered.

Abstract: A process for removing N2O and NOx from offgases by catalytic decomposition of N2O by means of iron-containing zeolite catalysts and catalytic reduction of the NOx by means of reducing agents, the deNOx stage connected downstream of the deN2O stage being operated at inlet temperatures of T<=400° C., and the inlet gas for the deN2O stage comprising water and having a selected N2O/NOx ratio, and the operating parameters of temperature, pressure and space velocity of the deN2O stage being selected so as to result in an N2O degradation of 80 to 98%. Under these conditions, a degree of NOx oxidation of 30-70% is established at the outlet of the deN2O stage, which is defined as the ratio of the molar amounts of NO2 to the total molar amount of NOx. The result of this is that the downstream deNOx stage can be operated under optimal conditions. Also provided is an apparatus for carrying out the process.