NOx Storage Medium for Nitric Acid Startup Emissions
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Solution Overview
Problem
Current methods fail to effectively reduce nitrogen oxide emissions during the startup of nitric acid production plants, leading to temporary emissions exceeding usual limit values due to residual gases and unbleached nitric acid outgassing, resulting in brown-colored exhaust and increased environmental concerns.
Innovation Solution
A method involving passing NOx-containing residual gas over a storage medium to store and release NOx at specific temperatures, followed by reduction with a reducing agent in the presence of an SCR catalyst, optimizing the storage and reduction process to minimize NOx emissions during startup.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional gas purification systems are used during startup, then NOx can be removed during steady-state operation, but temporary emissions exceed limits during startup due to insufficient temperature for catalyst operation
Solution Approach 1:
The patent applies preliminary action by pre-heating the residual gas to at least 200°C using a heater before it enters the SCR catalyst during startup. This ensures the catalyst is already at its minimum operating temperature when the gas flows through, enabling immediate NOx reduction without temporary emissions exceeding limits. The heater is activated before the main ammonia oxidation to prepare the system for effective catalytic reduction from the outset.
Solution Approach 2:
The patent changes the temperature parameter of the residual gas from ambient or low temperatures during startup to at least 200°C by using a heater. This parameter change enables the SCR catalyst to function effectively during startup, transforming the gas conditions to match the catalyst's operational requirements and achieving emission compliance throughout the entire startup process.
2Stability of the object's composition
If the plant is cooled down during shutdown, then NOx is predominantly present as NO2 at lower temperatures, but this results in brown-colored exhaust gas during restart due to visible NO2 emissions
Solution Approach 1:
The patent converts the harmful effect of brown-colored NO2 emissions into a benefit by using the presence of NO2 at lower temperatures as an indicator to activate the heater. The brown exhaust, rather than being merely an unwanted side effect, triggers the pre-heating action that subsequently eliminates the visibility issue. The heater raises the temperature to convert NO2 back to colorless NO, which is then effectively reduced by the SCR catalyst.
Solution Approach 2:
The patent utilizes the phase transition or transformation of NO2 to NO through heating. By raising the gas temperature to at least 200°C before it contacts the SCR catalyst, the brown-colored NO2 is converted to colorless NO, eliminating the visible exhaust. This thermal transformation occurs as the gas passes through the heater, changing its physical state and color properties.
3Stress or pressure
If external energy sources are used to pressurize the plant during startup, then the plant reaches operating pressure, but the residual gas temperature remains insufficient to activate the SCR catalyst
Solution Approach 1:
The patent applies preliminary action by implementing a heater that activates during the pressurization phase to pre-heat the residual gas to at least 200°C before it reaches the SCR catalyst. This preliminary heating action ensures that when pressurization is complete and ammonia oxidation begins, the catalyst is already at the required temperature for effective NOx reduction, eliminating the temperature insufficiency problem.
Solution Approach 2:
The patent introduces a heater as an intermediary device between the pressurization system and the SCR catalyst. This intermediary component bridges the temperature gap by heating the residual gas during pressurization, enabling the catalyst to receive gas at the appropriate temperature without requiring external energy sources beyond what the heater provides.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Significantly reduces NOx emissions during plant startup, achieving colorless emissions and compliance with environmental regulations by storing and reducing NOx using a storage medium and SCR catalyst, ensuring efficient gas purification even at lower temperatures.
Implementation Method 1
passing the NOx-containing residual gas produced during start-up of the plant through a storage medium for NOx, by which at least a portion of the NOx is stored
Implementation Method 2
passing the NOx-containing residual gas produced during start-up of the plant through a storage medium for NOx, by which at least a portion of the NOx is stored
Implementation Method 3
combining NOx and a reducing agent for NOx in the presence of an SCR catalyst, by which at least a part of the NOx is catalytically reduced, preferably to N2 with simultaneous formation of H2O
Implementation Method 4
combining NOx and a reducing agent for NOx in the presence of an SCR catalyst, by which at least a part of the NOx is catalytically reduced
Implementation Method 5
heating the residual gas from a starting temperature T0, passing through a limit temperature TG, to an operating temperature TB by means of measures for the recovery of the reaction energy generated during the ammonia oxidation process
Implementation Method 6
recovery of the reaction energy generated during the ammonia oxidation process in the nitric acid production process
Data Source
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AI summary
The invention relates to a method for reducing the concentration of NOx nitrogen oxides in residual gas which accumulates when starting up a system for producing nitric acid. The residual gas contains NOx, and the residual gas is accumulated when starting up the system and is heated from a starting temperature T0 to a final operating temperature TB, passing through a threshold temperature TG, as the result of the measures taken for producing nitric acid (T0<TG<TB). A stationary operation of the system is then carried out at the operating temperature. The method has the following steps: (a) conducting the residual gas containing NOx over a storage medium for NOx and storing at least one part of the NOx in the storage medium for NOx as long as the temperature of the residual gas is lower than the threshold temperature TG; (b) optionally releasing the NOx stored in step (a) preferably when the temperature of the residual gas has reached the threshold temperature TG; and (c) combining the NOx with a reduction agent for NOx in the presence of an SCR catalyst after the temperature of the residual gas exceeds the threshold temperature TG but not before, whereby at least one part of the NOx is catalytically reduced, preferably to N2.