Process for reducing the nitrogen oxide off-gas concentration in a nitric acid plant during shut-down and/or start-up, and nitric acid plant suitable therefor

摘要

Process for reducing the nitrogen oxide off-gas concentration in a nitric acid plant operated under pressure and equipped with residual gas purification during shut-down and/or start-up of the plant, wherein the residual gas purification is designed for removing nitrogen oxides from the off-gas for the steady-state operation. The process is characterized in that, during start-up and/or during shut-down of the nitric acid plant, pressurized nitrogen-oxide-containing off-gas from the nitric acid plant and gaseous reducing agent for the nitrogen oxides are passed into a reactor charged with catalyst, which reactor is provided in addition to the reactor of the residual gas purification, and in that the NOx content in the off-gas is reduced by catalytic reduction or in that the NOx content in the off-gas is reduced by catalytic reduction and the N2O content in the off-gas is reduced by catalytic reduction and/or by catalytic decomposition in the additional reactor. Furthermore, a nitric acid plant for carrying out this process is described. Using the process, or using the plant, a colourless start-up and shut-down of nitric acid plants is possible without problems, and the nitrogen oxide content in the off-gas during start-up and/or shut-down can be substantially lowered.