The Chemistry Behind Thermal Power Plant Processes
Part 1: Water Chemistry Fundamentals
Thermal power plants are marvels of engineering, where chemistry plays a pivotal role in ensuring efficiency, safety, and environmental compliance. This section covers raw water treatment, boiler feedwater conditioning, and cooling tower water chemistry — shared processes across both coal- and gas-fired plants.
Raw Water Treatment
Before water can be used in a power plant, it must undergo treatment to remove impurities that could cause scaling, corrosion, or fouling:
Coagulation and Flocculation: Chemicals like aluminum sulfate (alum) or ferric chloride are added to destabilize and aggregate suspended particles. These form flocs with the help of long-chain polymers.
Sedimentation: Flocs settle out in clarifiers as sludge.
Filtration: Multimedia or sand filters remove remaining solids.
Softening: Lime or soda ash precipitates calcium and magnesium ions. For example:
Ca(OH)₂ + Ca(HCO₃)₂ → 2 CaCO₃ (precipitate) + 2 H₂O
These steps reduce hardness, turbidity, and biological contaminants to protect downstream equipment.
Boiler Feedwater Demineralization and Chemistry
High-pressure boilers require ultra-pure water to prevent tube scaling and corrosion:
Ion Exchange: Removes ions using cation and anion exchange resins, producing demineralized water.
Deaeration: Mechanical deaerators remove O₂ and CO₂.
Oxygen Scavengers: Hydrazine (N₂H₄), sodium sulfite, or alternatives like DEHA consume residual oxygen. Example:
N₂H₄ + O₂ → 2 H₂O + N₂ (gas)
pH Conditioning: Ammonia or volatile amines raise pH and buffer condensate lines.
Phosphate Treatment: Trisodium phosphate reacts with calcium to form non-adherent sludge:
3 Ca²⁺ + 2 PO₄³⁻ → Ca₃(PO₄)₂ (precipitate)
These treatments ensure metal passivation, minimize scale, and extend equipment life.
Cooling Tower Chemistry
Cooling towers recirculate large volumes of water and must control:
Scale Formation: Evaporation concentrates calcium carbonate, which can precipitate. Phosphonates inhibit crystal growth.
Corrosion: Zinc or phosphate-based inhibitors form protective films. Alkalinity is maintained to avoid acidic attack.
Biological Growth: Chlorine or bromine kills microbes; non-oxidizing biocides and dispersants help control slime and biofilms.
Managing these parameters prevents heat exchanger fouling, microbial corrosion, and ensures consistent thermal performance.
Boiler Tube Protection
Even trace impurities can cause long-term damage in high-temperature environments. Key strategies include:
Oxygen Scavenging: Hydrazine and other reducing agents remove residual O₂ and passivate steel surfaces.
Alkalinity Control: pH is maintained using ammonia or phosphate systems to prevent acidic corrosion.
Phosphate & Polymer Programs: Precipitate hardness ions as non-adherent sludge, preventing scale.
Filming Amines: Apply hydrophobic coatings to steam and condensate lines, reducing corrosion.
Part 2: Emissions and Fuel Chemistry
NOₓ Reduction in Gas Plants: Selective Catalytic Reduction (SCR)
SCR systems are common in gas-fired plants for removing nitrogen oxides:
Ammonia or Urea Injection: Ammonia (NH₃) or thermally decomposed urea is introduced upstream of the catalyst.
Catalyst Reaction: At around 300–400°C, NH₃ reacts with NOₓ over a catalyst (typically vanadium-based):
4 NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O
Ammonia Slip Control: Ammonia feed is tuned to minimize unreacted NH₃ in the exhaust.
SCR systems achieve up to 90% NOₓ reduction, critical for regulatory compliance.
Coal Pre-Combustion Chemistry
Coal often contains impurities like sulfur, ash, and trace metals. Treatment before combustion includes:
Washing: Physical separation methods (e.g., dense media cyclones, jigs) remove rocks and pyritic sulfur.
Froth Flotation: Fine coal is separated from mineral matter using surfactants.
Drying and Pulverizing: Enhances burn rate and consistency in furnaces.
Additives: Certain additives bind sulfur or mercury in combustion, reducing emissions.
Pre-cleaning coal improves combustion efficiency and reduces SO₂ and ash loads on downstream scrubbers.
Part 3: Flue Gas Scrubbing and Wastewater Treatment
Flue Gas Desulfurization (FGD)
Coal-fired power plants emit sulfur dioxide (SO₂) from burning sulfur-containing fuel. To capture it before release into the atmosphere, most plants use a process called wet flue gas desulfurization:
Absorption: SO₂ is absorbed into a slurry of limestone (CaCO₃) or lime (Ca(OH)₂) sprayed into the flue gas stream:
CaCO₃ + SO₂ → CaSO₃ + CO₂
Ca(OH)₂ + SO₂ → CaSO₃ + H₂O
Oxidation to Gypsum: The CaSO₃ (calcium sulfite) is then oxidized with air to form gypsum (CaSO₄·2H₂O):
CaSO₃ + ½ O₂ + 2 H₂O → CaSO₄·2H₂O
Gypsum can be dewatered and sold for use in drywall manufacturing.
pH Control: The scrubber pH is carefully controlled (typically 5–6) to optimize SO₂ absorption and minimize scaling.
Scrubber Discharge and Effluent Water Treatment
Wet FGD systems generate large volumes of slurry containing dissolved and suspended solids, which must be treated before discharge:
Solid-Liquid Separation: Clarifiers or centrifuges separate gypsum solids from the slurry.
Metal Removal: Chemicals like lime or caustic soda raise the pH to precipitate heavy metals (e.g., arsenic, selenium, mercury).
Example: As³⁺ + 3 OH⁻ → As(OH)₃ (precipitate)
Sulfide Treatment: Sulfide reagents (e.g., Na₂S) bind certain metals as insoluble metal sulfides.
Coagulation and Flocculation: Ferric chloride or polymers gather fine precipitates into removable flocs.
Filtration: Multimedia or sand filters capture remaining solids.
Biological Treatment: Anoxic bioreactors reduce selenate to elemental selenium and nitrate to nitrogen gas.
Polishing Steps: Activated carbon and dechlorination ensure water is safe for discharge.
Waste Management and Environmental Compliance
Gypsum Reuse or Disposal: Dewatered gypsum is often reused commercially or sent to lined landfills.
Zero Liquid Discharge (ZLD): Some plants use evaporators and crystallizers to recover water and produce solid waste only.
Monitoring: Final effluent is tested for pH, total dissolved solids (TDS), and trace metals to comply with EPA limits.
Conclusion
While gas plants focus on controlling NOₓ, coal plants must manage SO₂, ash, and metal-laden wastewater. Chemistry enables power plants to capture pollutants, treat water, and protect air and water quality. These processes illustrate applied chemistry in action — making the grid cleaner, safer, and more efficient.
Link to my full paper on this (I am still working on the formats of the chemical reactions; this is a work in progress): https://docs.google.com/document/d/12QMDWj_HJ9QQZMPSZyrMtsLcsDYeCCZrUUzDJK90uwQ/edit?usp=sharing
