Low-Temperature SCR NOx Removal via External NO2 Injection
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Solution Overview
Problem
Existing SCR catalysts exhibit low efficiency in reducing nitrogen oxides (NOx) at flue gas temperatures below 250°C, necessitating the splitting of the HRSG unit and re-heating the gas to enhance NOx removal efficiency.
Innovation Solution
Injecting nitrogen dioxide (NO2) formed externally through the oxidation of ammonia into the flue gas upstream of the SCR catalyst at low temperatures to promote the 'fast' SCR reaction, which requires equimolar amounts of NO and NO2, thereby enhancing NOx reduction efficiency at temperatures below 250°C.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If SCR catalyst is used at low flue gas temperatures below 250°C, then the system can maintain simple HRSG configuration without splitting, but the NOx removal efficiency is low
Solution Approach 1:
The invention generates NO2 in advance through an external oxidation unit where ammonia is oxidized to NO and then to NO2. This pre-generated NO2 is stored and injected into the flue gas upstream of the SCR catalyst when low temperature conditions are detected, enabling the fast SCR reaction to occur efficiently at temperatures below 250°C without modifying the HRSG configuration.
2Productivity
If flue gas is re-heated to higher temperatures, then the SCR catalyst efficiency is improved, but additional energy consumption and equipment complexity increase
Solution Approach 1:
The invention changes the chemical composition parameters of the flue gas by injecting pre-generated NO2, which enables the fast SCR reaction mechanism. This chemical parameter change allows the SCR reaction to proceed efficiently at the existing low flue gas temperature, eliminating the need for thermal energy input through re-heating systems.
3Productivity
If NO2 is injected to promote fast SCR reaction, then low temperature NOx removal efficiency is significantly improved, but the system complexity increases due to additional NO2 generation and injection equipment
Solution Approach 1:
The invention introduces an external oxidation unit as an intermediary system that converts ammonia to NO2. This intermediary unit acts as a bridge, transforming readily available ammonia into the required NO2 species that enables fast SCR reaction. The generated NO2 is then injected into the flue gas stream, facilitating efficient NOx removal at low temperatures without directly modifying the SCR catalyst or HRSG structure.
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
This approach significantly raises the low-temperature activity of the SCR reaction, allowing for efficient NOx removal without the need for re-heating the gas, thus enabling tail-end SCR installations and reducing the duty of the flue gas reheater unit, while maintaining efficiency at higher temperatures.
Implementation Method 1
catalytically oxidizing ammonia or a precursor thereof with an oxygen containing atmosphere to an effluent gas comprising nitrogen monoxide and oxygen in presence of an oxidation catalyst
Implementation Method 2
cooling the effluent gas to ambient temperature and oxidizing the nitrogen monoxide in the cooled effluent gas to nitrogen dioxide
Implementation Method 3
passing the flue gas through a catalyst for selective reduction of nitrogen oxides in presence of ammonia added to the flue gas
Implementation Method 4
In Selective Catalytic Reduction (SCR) of NOx, nitrous oxide compounds are selectively reduced to harmless nitrogen and water by reaction with a reduction agent, e.g. ammonia, over a catalyst
Data Source
AI summary
Method and system for the removal of nitrogen oxides, flue gas at low temperatures.