Overview of Dioxins
Dioxins are persistent organic pollutants (POPs) that include 210 compounds of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). These substances are highly toxic, chemically stable, resistant to photodegradation, and bio-degradation, making them persist in the environment for extended periods. Known for their teratogenic, carcinogenic, and mutagenic properties, they are referred to as the “poison of the century.”
Dioxins are primarily formed in exhaust gases containing chlorinated organic compounds, either as impurities introduced from raw materials or as by-products of combustion processes. Their formation mechanisms include:
- High-Temperature Gas-Phase Reactions (500°C–800°C): Relatively minor contribution, accounting for less than 15% of total dioxin production when temperatures exceed 600°C.
- Low-Temperature Reactions (<500°C): This pathway contributes significantly to dioxin formation, with an optimal formation temperature range of 280°C–450°C.
Dioxins primarily form through the dimerization of free chlorine and chlorobenzene oxy radicals on carbon rings in specific temperature ranges.
Strategies for Controlling Dioxin Formation and Treatment
1. Source Control
- Optimize Oxygen Supply: Maintain an excess air ratio of at least 1.2 to ensure complete oxidation of carbon-based compounds into stable CO₂ structures at sufficiently high temperatures.
- Control Chlorine Content: Minimize the introduction of chlorine-containing organic compounds in raw materials.
2. Process Control
- Ensure Sufficient Combustion Temperature: Maintain the RTO combustion chamber at a minimum of 850°C with a residence time of over 1 second.
- Enhance Turbulence: Use positive pressure controls in the RTO system to achieve high turbulence, ensuring uniform oxidation of exhaust gases.
- Prevent Reformation During Cooling:
- Design the RTO ceramic heat storage system to reduce the gas residence time between 500°C and 200°C to approximately 0.5 seconds, below the critical threshold of 1 second for dioxin reformation.
- Apply a dioxin decomposition catalyst on the heat storage layer within specific temperature ranges to break down precursors during the cooling phase.
3. End-Stage Control
- Active Carbon Adsorption: Install an active carbon adsorption system downstream of the RTO to capture residual dioxins that may have formed during combustion.
Key Benefits
- Comprehensive Management: Controls dioxin formation from all stages of the process.
- High Efficiency: Ensures compliance with environmental standards for dioxin emissions.
- Sustainability: Mitigates long-term environmental and health risks associated with dioxins.
Implementing these strategies ensures effective dioxin control, safeguarding public health and the environment.