Electrostatic precipitators (ESPs) remain critical for mitigating air pollution in coal-fired power generation, but their design requires meticulous engineering to address thermal, mechanical, and aerodynamic challenges. For one of our clients – we identified potential flaws in the ductwork system of a fly-ash ESP, particularly in managing thermal stresses and gas flow dynamics.
Historical Context and Design Imperatives
The shift to pulverized coal combustion in the 1920s introduced fine fly ash particles (≤10 µm) that conventional filtration couldn’t capture efficiently. ESPs emerged as the dominant solution, leveraging electrostatic forces to achieve >99% collection efficiency by charging particles and depositing them on collector plates. However, hot-side ESPs—operating at temperatures exceeding 700°F (371°C)—demand specialized ductwork designs to accommodate thermal expansion (“thermal breathing”). Restrained expansion generates stresses exceeding 50 ksi in carbon steel, risking buckling, weld fractures, or hopper misalignment. Industry standards mandate:
- Sliding supports with low-friction bearings
- Multi-directional expansion joints
- High-temperature alloys (e.g., ASTM A387 Grade 11)
Key Design Flaws Identified
Gas Flow Uniformity: Computational Fluid Dynamics (CFD) simulations revealed inlet flow separation.
Uniformity: Computational Fluid Dynamics (CFD) simulations revealed inlet flow separation
Thermal Stress Mismanagement: Finite element analysis (FEA) showed localized stresses of 72 MPa in fixed-support regions—above ASME Section VIII limits. Missing expansion joints amplified axial loads by 30%.
Inadequate Specific Collection Area (SCA): The original SCA fell below the recommended 60 m²/(m³/s) for sub-3 µm particles, risking efficiency drops during load fluctuations.
Mitigation Strategies
- Flow Conditioning: Adding porous baffles at the inlet reduced TKE and improved plate utilization.
- Thermal Design: Implementing sliding supports with PTFE coatings and adding expansion joints limited peak stresses.
