Silica Removal in Boiler Makeup Water for Power Plants
Why Silica Matters in Modern Power Generation
Low silica is critical in high purity boiler water for power plants to prevent efficiency losses.
Silica (SiO₂) control is critical in high-purity boiler makeup water for power plants. When not properly removed, dissolved silica can deposit on boiler and turbine surfaces, leading to efficiency losses, and increase maintenance and unplanned outages.
Electrodeionization (EDI), when properly designed and integrated, enables the continuous
High-Purity Water Requirements in Power Plants
Power plants rely on high-purity water to generate steam that drives turbines. This is especially critical in:
- Combined-cycle power plants
- Cogeneration (CHP) facilities
- High-pressure steam turbine systems
These plants achieve high thermal efficiency by operating at increasingly severe temperature and pressure conditions, which significantly increases sensitivity to water impurities.
Evolution of Boiler Operating Conditions
| Era | Steam Temperature | Steam Pressure |
|---|---|---|
| 1960s boilers | ~450 °C (840 °F) | ~4 MPa (40 atm) |
| Modern combined-cycle | ~620 °C (1150 °F) | ~30 MPa (300 atm) |
As operating pressure and temperature increase, silica solubility decreases, and the consequences of silica carryover become more severe.
Silica Deposition Mechanism
Under high-temperature, high-pressure boiler and turbine conditions:
- Dissolved silica volatilizes with steam
- Silica re-deposits downstream as a hard rough layer
- Turbine blades and heat transfer surfaces are affected
Even small amounts of silica can:
- Reduce turbine efficiency
- Increase fouling and erosion
- Require costly cleaning or blade replacement
Boiler Makeup Water Quality Targets
To maintain efficiency, uptime, and equipment life, boiler makeup water must meet stringent purity requirements:
-
Resistivity: ≥ 17 MΩ·cm
-
Silica:
- Typically 5–20 ppb, depending on boiler pressure
- Lower pressures tolerate higher silica
- Ultra-supercritical systems require the lowest levels
Limitations of Mixed Bed Deionization
Traditional mixed bed (MB) ion exchange systems are commonly used for polishing but present operational challenges:
- Batch operation requiring duplex towers
- Periodic resin exhaustion and changeover
- Risk of silica breakthrough or spikes during bed swaps
- Chemical regeneration and associated handling
These transient silica excursions can be unacceptable in modern high-pressure boiler systems.
Advantages of EDI
Electrodeionization (EDI) is a continuous membrane-based process, combining ion exchange resins, ion-selective membranes, and an applied electric field.
Key advantages for power plant applications include:
- Continuous operation (no batch cycling)
- No resin regeneration chemicals
- Stable, low-silica permeate
- Reduced risk of silica spikes
- High product water resistivity
When properly designed, EDI reliably produces ultrapure water with consistently low silica levels.
State-of-the-Art Boiler Makeup System Designs
Modern high-performance boiler makeup systems typically use one of the following configurations:
RO–RO–EDI (typical for high pressure boilers)
- Dual-pass reverse osmosis (RO) for bulk contaminant removal
- Interstage pH should be optimized at 8.4 to 8.7
- EDI for final polishing and continuous deionization
RO–GTM–EDI (typical for low pressure boilers)
- RO followed by GTM (Gas Transfer Membrane) for CO₂ removal
- Reduced ionic load improves EDI efficiency and performance
Note: GTM is a membrane-based degassing technology (aka Liqui-Cel) that lowers dissolved CO₂, reducing bicarbonate loading to the EDI, and improving final EDI water quality.
Achievable Performance
With proper system design, component selection, and operating conditions:
- 1–5 ppb silica in boiler makeup water is achievable
- Continuous compliance without silica excursions
- Improved turbine protection and plant availability
Summary
As power plants continue to push toward higher temperatures and pressures, silica control becomes increasingly critical. Compared to batch mixed bed deionization, EDI-based systems provide continuous, stable, low-silica ultrapure water, making them well suited for modern combined-cycle and high-pressure boiler applications.
Proper integration of RO, GTM, and EDI technologies enables reliable achievement of today’s demanding boiler water quality requirements.