Sulfite Control for Mercury Re-emission in WFGD
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Solution Overview
Problem
Existing methods for reducing mercury emissions in coal combustion flue gas during wet scrubbing operations face challenges in controlling mercury re-emission due to the conversion of oxidized mercury back to elemental mercury, which has high vapor pressure and can re-enter the environment, making it difficult to achieve stringent emission standards like 0.3 μg/Nm3 without increasing costs or sulfur emission issues.
Innovation Solution
The method involves using a sulfite sensor to control sulfite concentration in the aqueous alkaline slurry within the wet flue gas desulfurization (WFGD) system by adjusting the oxidation air stoichiometry, thereby maintaining the oxidation reduction potential and minimizing mercury re-emission, while optimizing alkalizing agent use and oxidation air consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If oxidizing agents are added to convert elemental mercury to ionic mercury for removal by wet scrubbing, then mercury removal efficiency is improved, but mercury re-emission occurs when ionic mercury is reduced back to elemental mercury form
Solution Approach 1:
The patent controls the sulfite concentration parameter in the aqueous alkaline slurry to prevent mercury re-emission. By adjusting sulfite levels, the system maintains oxidation conditions that prevent reduction of ionic mercury back to elemental form, while still allowing effective mercury removal through the wet scrubbing process
Solution Approach 2:
The patent uses sulfite sensors to continuously monitor sulfite concentration in the slurry and provides feedback control. This feedback mechanism adjusts oxidation air supply to maintain optimal sulfite levels, preventing mercury re-emission while ensuring effective removal
2Reliability
If stringent emission standards of 0.3 μg/Nm3 are achieved through enhanced mercury removal, then environmental compliance is improved, but system complexity and operational difficulty increase
Solution Approach 1:
The patent makes the aqueous alkaline slurry serve multiple functions: it continues to remove sulfur oxides as before, while also removing mercury through oxidation. The sulfite-controlled oxidation process handles both desulfurization and demercurization functions within the same wet scrubbing system, avoiding the need for separate complex systems
Solution Approach 2:
By adjusting sulfite concentration as a key parameter, the system achieves stringent emission standards without requiring complex additional equipment. The parameter control allows the existing wet scrubbing system to meet 0.3 μg/Nm3 standards through optimized chemical conditions
3Reliability
If oxidation air is increased to enhance mercury oxidation, then mercury removal is improved, but oxidation air power consumption increases
Solution Approach 1:
The patent changes the sulfite concentration parameter to control oxidation air requirements. By optimizing sulfite levels, the system achieves effective mercury oxidation with reduced oxidation air consumption, as sulfite acts as an intermediate that facilitates oxidation without requiring excessive air supply
Solution Approach 2:
Sulfite acts as an intermediary substance that facilitates mercury oxidation. Instead of directly oxidizing mercury using large amounts of oxidation air, sulfite mediates the oxidation process, reducing the energy input required while maintaining effective mercury removal
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 effectively reduces mercury emissions and re-emissions in the cleaned flue gas, achieving stringent emission standards while minimizing costs and ensuring sulfur emission compliance.
Implementation Method 1
contacting the flue gas with an aqueous alkaline slurry to absorb the sulfur oxides from the flue gas
Implementation Method 2
dissolving at least a portion of gaseous ionic mercury species present in the flue gas in the aqueous alkaline slurry to remove the gaseous ionic mercury species from the flue gas
Implementation Method 3
adjusting an amount of oxidation air supplied to the WFGD system thereby modifying the system's oxidation air stoichiometry
Data Source
AI summary
A method for reducing mercury emission and/or re-emission in cleaned flue gas through control of sulfite concentration within a wet flue gas desulfurization (WFGD) system is disclosed. One method for reducing mercury emission and/or re-emission through control of sulfite concentration is to measure the sulfite concentration of an aqueous alkaline slurry used in a WFGD system and comparing the same to a predetermined sulfite concentration value. If the comparison reveals the measured sulfite concentration is above the predetermined values, the amount of oxidation air supplied to the system is increased. If the comparison reveals the measured sulfite concentration is below the predetermined values, the amount of oxidation air supplied to the system is decreased.