Water Chemistry Practices in Thermal Power Stations

Boiler Chemistry, Boiler Engineering Books 9,623 Views

Water Chemistry Practices in Thermal Power Stations

Water chemistry plays a critical role in the safe, efficient, and reliable operation of thermal power stations. Proper chemical control not only maximizes equipment life and thermal efficiency but also minimizes corrosion, scaling, fouling, and environmental impact. This article provides an in-depth overview of water treatment and testing practices within thermal power plants.

Table of Contents

1. The Role of Chemistry in Power Plant Operations

In thermal power plants, water chemistry governs the health of the entire feedwater-steam-condensate cycle and the cooling water systems. Proper chemistry ensures:

Reduced corrosion and scaling
Optimal heat transfer
Minimal downtime
Compliance with environmental standards

2. Water Treatment Chemistry

Water entering the plant contains impurities like dissolved salts, oxygen, silica, and organics that can damage boiler tubes, turbines, and condensers. Treatment processes are designed to remove these impurities before water enters critical components.

Demineralization (DM)

Demineralization involves the removal of dissolved salts through ion exchange columns:

Cation exchanger removes Ca²⁺, Mg²⁺, Na⁺
Anion exchanger removes Cl⁻, SO₄²⁻, NO₃⁻
Mixed bed exchanger provides final polishing for high-purity water

3. Boiler Feed and Steam Cycle Chemistry

Boiler feedwater requires precise conditioning to prevent corrosion and scale:

Oxygen scavengers (e.g., hydrazine) to eliminate dissolved O₂
pH control agents like ammonia or morpholine to maintain alkaline conditions
Phosphate treatment for internal scale control
Condensate polishing using resins or filtration to remove corrosion products

4. Cooling Water Chemistry

Cooling systems use large volumes of water and are susceptible to:

Scaling from calcium and silica
Corrosion due to dissolved oxygen or low pH
Biological fouling from microbial growth

Treatment includes:

Antiscalants
Biocides
Corrosion inhibitors (e.g., molybdates, phosphonates)

In seawater-cooled plants, special materials and antifouling measures are necessary to resist high chloride levels and marine growth.

5. Chemistry of Water Pollution

Power plants must manage effluents and meet regulatory limits on:

Chemical Oxygen Demand (COD)
Biochemical Oxygen Demand (BOD)
Heavy metals
Oil and grease

Proper water chemistry management reduces the environmental footprint and supports sustainable operations.

6. Water Testing Methods

Regular monitoring is essential. Parameters tested include:

pH – Measured using pH meters (glass electrode method)
Alkalinity/Acidity – Via titration with acid/base
Total and Calcium Hardness – Using EDTA titration
Dissolved Oxygen – Using Winkler’s method or online DO meters
COD/BOD – Chemical or biological oxidation tests
Total Organic Nitrogen – Kjeldahl method or online analyzers

7. Reverse Osmosis (RO) in Water Treatment

RO systems are widely used in power stations for producing high-purity water. Working principle:

High pressure forces water through semi-permeable membranes
Salts and impurities are retained, producing permeate (pure water) and concentrate (reject stream)

Pre-treatment with filters and antiscalants is essential for membrane longevity.

8. Instrumental Methods for Water Analysis

Direct Readout Instruments

Online conductivity meters for ionic contamination
Online silica analyzers to prevent steam turbine damage
Online sodium analyzers for early leak detection
Online TOC analyzers for organic content

Colorimetric Methods

Test kits with color change indicators for:
- Chlorine
- Iron
- Phosphate
- Ammonia
- Nitrate

These are quick, easy to use, and suitable for field applications.

9. Case Studies from Thermal Power Stations

Several plants have improved reliability through disciplined water chemistry:

A 500 MW unit in India reduced condenser fouling by switching to film-forming amines
A coastal plant minimized seawater corrosion using advanced titanium condenser tubes
A North American plant improved boiler efficiency by adopting online silica and sodium monitoring

These examples show how good chemistry translates into better performance and reduced downtime.