Coal Based Thermal Power Plant Water Management.pdf

KaushikKaushikKaushikKaushik ChaudhuriChaudhuriChaudhuriChaudhuri

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Global distribution of Water

2% 1%

% Distribution

97%

Ocean

Ice

Fresh water

Use of Fresh Water

10%5%

3%

Fresh Water Distribution

70%

12% Evaporation

Power Plants

Irrigation

Industries

Domestic

Sources of Raw Water for Power

Plants� Ocean

� River

� Dam

Lake� Lake

� Canal

Raw water contains� Suspended solids

� Dissolved solids

� Dissolved gases

Organics� Organics

� Industrial effluents

Types of Water used in a Power

Plant� Raw

� Clarified

� De-mineralized

Points of UseRaw Water

� Mainly Condenser cooling (once through)

Washing/Cleaning� Washing/Cleaning

Points of UseClarified Water

� Condenser cooling

� Auxiliary cooling (Turbine Lub oil cooler, Vacuum pump � Auxiliary cooling (Turbine Lub oil cooler, Vacuum pump cooler, DMCW HEX etc)

� Service water system (Cleaning, washing, gardening, Ash handling system, dust suppression, water fogging, ESP hopper cooling, Air washery , Sludge conveying, Intake pump gland cooling)

� Hydrant/Mulsifier system

�Drinking water (after proper chlorination)

Points of UseDe-mineralized Water

� Boiler makeup through RFW tank� Condenser emergency makeup� DMCW system makeup � DMCW system makeup � Stator coolant makeup� Oil Centrifuge sealing� Chemical preparation� Laboratory� Boiler cold filling� Deaerator cold filling

Raw Water TreatmentTypical impurities in River Water (like Ganga)

1. Non-ionic & undissolved impurities (Suspended Solids)

e.g. mud,dirt,slime,clay and other suspended matter,organicmatters etc.matters etc.

2. Ionic & Dissolved impurities (Dissolved Solids)

e.g. Salts of calcium, magnesium, sodium etc.

Fulvic , Humic acids,Tannins,Lignins as organics.

3. Gaseous impurities (Dissolved Gases)

e.g. Oxygen , carbon dioxide, hydrogen sulphide,

Pre-treatment of Raw Water1. Clarification

a) Coagulation

b) Flocculation

c) Sedimentationc) Sedimentation

2. Filtration

3. Chlorination

RAW WATER TREATMENT PLANT

R A W W A T E R C I R C U I T

Clarification Process� Flash Mixture Tank

� Flocculation Tank/Clariflocculator

� Inclined Surface Settler

Effect : Lowering of Turbidity (NTU)

Filtration� Pressurized Sand Filter

or

� Gravity Sand Filter

Effect : Further lowering of Turbidity (NTU)

FILTRATIONClarified Water (Turbidity: 2 NTU)

Sand

Pebbles

Gravels

Filtered Water (Turbidity: 0.2 NTU)

CLARIFICATION• Al2(SO4)3 Al(OH)3 + H2SO4

• Fe2(S04)3 Fe(OH)3 + H2SO4

Chemical reactions in water is as followsChemical reactions in water is as follows.

•Coagulation Mechanism : Lowering of ZETA POTENTIALZETA POTENTIALZETA POTENTIALZETA POTENTIAL.

CrabingCrabingCrabingCrabing action as well as reduction of Z PZ PZ PZ P found in PE.It also increases sludge density and increases efficacy of alum.

Poly electrolyte : Cationic form (0.2Poly electrolyte : Cationic form (0.2Poly electrolyte : Cationic form (0.2Poly electrolyte : Cationic form (0.2————0.5 %. Alum soln. : 20.5 %. Alum soln. : 20.5 %. Alum soln. : 20.5 %. Alum soln. : 2------------5% Is 5% Is 5% Is 5% Is generally used as generally used as generally used as generally used as coagulantcoagulantcoagulantcoagulant

Jar Test Apparatus

CHLORINATION

Na(OCl)H2O HOCl

HOCl H+ + Ocl-HOCl H+ + Ocl-HOCl as well as Ocl enters into the cells of

bacteria and destroy the protein structures

of living beings

Typical River Water Parameters

pH COND.

(ms/cm)

TURB.

(NTU)

ALK.

(ppm)

TH.

(ppm)

Ca-H

(ppm)

Mg-H

(ppm)

NaCl

(ppm)

SiO2

(ppm)

7.7-8.3 200-500 50-1200 70-170 55-175 30-100 25-75 12-35 7-14

RWTP GSF FWR DMSP

ACFSACDegasserDGP

WBA SBA MB DMStorage

DM Water Production

Storage

RWTPRWTP Removes Suspended & Colloidal particlesRemoves Suspended & Colloidal particles

GSFGSF Reduces TurbidityReduces Turbidity

ACFACF Removes Oil, grease, colour, odour, ChlorineRemoves Oil, grease, colour, odour, Chlorine

SACSAC Removes Cations like Ca,Mg,NaRemoves Cations like Ca,Mg,Na

DegasserDegasser Removes CO2Removes CO2

WBAWBA Removes strong acids like Chloride, Removes strong acids like Chloride, SulphatesSulphates

SBASBA Removes weak acids like Silica, carbonatesRemoves weak acids like Silica, carbonates

MBMB Final polishingFinal polishing

Some Facts about DM Water ProductionRegeneration Requirement (sample)

� SAC : Requires about 1 Te of HCL (33% bulk conc), resins are regenerated using 5% HCL

� WBA & SBA : Require approx 400 Kg of NaOH (46.5% bulk conc), 4% NaOHfor SBA & 2% for WBAfor SBA & 2% for WBA

� MB : Requires 260 Kg of HCL & 170 Kg of NaOH� Regeneration waste goes to Neutralizing Pit where it is neutralized before

discharging

High Pressure Boiler Water Treatment

Methods

� Co-ordinated Phosphate –Ph control

� Congruent control

� All volatile treatment

Coordinated Phosphate -Ph control method.

•In this method only TSP solution is used.

•PO4 content : 10 ppm•PO4 content : 10 ppm

•pH : 9.8---10.0

Congruent control Method

In this process TSP & DSP

are mixed 4:1 ratio

externally and dosed to externally and dosed to

boiler. Na:PO4=2.66:1

All Volatile Treatment (AVT) � Hydrazine Hydrate : 40—100 ppb

(in Economizer inlet water)

� Ammonia : 1.5---2.00 ppm� Ammonia : 1.5---2.00 ppm(in Main Steam sample)

� Advantage : Zero blowdown

AVT Control of Power Cycle Water

Drum Water

Parameters

Range

PH 9 – 9.5

Conductivity (ms/cm) 5 - 15

Silica (ppm) 0.2 (max)

Chloride (ppm) 0.13


Main Steam

Parameters

Range

PH 9.3 – 9.7



Ammonia (ppm) 1.4 – 2.5


Economizer Inlet

Water Parameters

Range

PH 9.3 – 9.7


Hydrazine (ppb) 40 - 200


CEP Discharge Water

Parameters

Range

PH 9.3 – 9.7



Ammonia (ppm) 1.4 – 2.5


Deaerator Water

Parameters

Range

PH 9.3 – 9.7

Conductivity (ms/cm) 6 - 15Conductivity (ms/cm) 6 - 15

Chemical Control of Stator Coolant Water

Stator Coolant Water

Parameters

Range

PH 6 – 7.5

Conductivity (ms/cm) 2 (max)Conductivity (ms/cm) 2 (max)

Chemical Control of Closed Circulating

Condenser Cooling Water

Forebay/CW Water

Parameters

Range

PH 8.5 – 8.7

Turbidity (NTU) < 20Turbidity (NTU) < 20

M- Alkalinity 150 - 200

Ca - Hardness < 650

Chloride (ppm) 500

Corrosion Limits

Type of Water Control

Power Cycle Iron < 10 ppb

DMCW Iron < 50 ppbDMCW Iron < 50 ppb

Stator Coolant Copper < 80 ppb

Cooling Tower < 4 MPY

Problems faced with closed loop

Condenser cooling water

�Scaling

�Corrosion�Corrosion

�Biofouling

Scale is a dense coating of predominantly inorganic material formed from the precipitation of water-soluble constituents. Some common scales are

Scaling

� Calcium phosphate� Calcium phosphate� Magnesium salts� Calcium Carbonate� Silica

Calcium Carbonate deposition in

tube

Corrosion

Some common types of Corrosion

1. General Corrosion

Corrosion is the mechanism by which metals are reverted back to their natural“oxidized” state

1. General Corrosion2. Galvanic Corrosion 3. Localized Pitting Corrosion

• General Corrosion

The metal loss is uniform from the surface. It is often combined with high-velocity fluid erosion, with or without abrasives.

• Galvanic Corrosion :-Occurs when two different metals are in the same system. Can occur when two different metals are in contact. The more active metal corrodes rapidly.

• Localized Pitting Corrosion :- exists when only small area of the metal corrodes. Pitting may perforate the metal in short time. The main source for pitting attack is dissolved oxygen.

Bio-fouling� Biofouling or Biological fouling is the deposition &

growth of micro-organisms on wet surface.

� Biofouling severely impede heat exchange in Condenser, HEX and Cooling towerCondenser, HEX and Cooling tower

Bio-fouling in BBGS Unit-3 CT

Microbiological Growth

Mainly three kinds of troublesome micro-organisms found in Cooling tower water

Algae , Fungi , Bacteria

Classifications of Bacteria

Planktonic:Free-floating bacteria in bulk water.

Sessile:Sessile:Bacteria attached to surfaces.Over 95% of bacteria in a cooling system are sessile and live in Biofilms.

Biofilms can generally be described as a physically coordinated community ofbacteria and other microorganisms, embedded in a protective glycocaylx withentrained organic and inorganic debris attached to a surface.

MIC (Microbiologically influenced corrosion)

o Corrosion caused or promoted by micro-organisms.

o MIC does not involve direct attack of bacteria on metal. Rather, MIC refers to corrosion that is induced or accelerated by the presence of products of microbiological metabolism.

o The most commonly seen cases of MIC are caused by Sulphate-o The most commonly seen cases of MIC are caused by Sulphate-reducing bacteria (SRB).

o Other bacteria can also cause MIC to occur - Acid-producing bacteria (APB).

o Anaerobic bacteria, specially Sulfate Reducing Bacteria (SRB) and Acid Producing Bacteria (APB) can accumulate under-deposits. Metabolic reactions of these bacteria produce acids, dropping the pH low enough to cause serious localized or pitting attack.

Outer surface of the Fire Water & Service water line at Turbine house

The common types of bacteria can form gelatinous masses in pipes. they adsorb suspended matter and form a physical obstacle to the water flow. In addition, they create local conditions favorable to the growth of ferruginous and sulfate reducing bacteria.

Inside the Fire water

pipe at pipe at Turbine house

Condenser

CW bus

Fo

rbay

Vent

Controls Microbiological growth by reducing nutrient food source

Air for backwash

Chemical Management of Closed Circulating

Condenser Cooling Water

� Dosing of Sulphuric acid

� Dosing of antiscalant & corrosion inhibitor (Organophosphonate)(Organophosphonate)

� Dosing of Bio-dispersant

� Dosing of Oxidizing Biocide (Sodium hypochlorite/ClO2)

� Shock dosing of Non-oxidizing Biocide followed by Blowdown

In Forebay

Ca(HCO3)2 + H2SO4 = CaSO4 +CO2 + 2H2O

NaClO2 + Cl2 + HCl � ClO2 + NaCl + NaoHNaClO2 + Cl2 + HCl � ClO2 + NaCl + NaoH

For Recirculated water

Ca(HCO3)2 + NaOH = CaCO3↓ + NaHCO3+ H2O

CaSO4+ 2NaHCO3 = CaCO3 ↓ +Na2SO4+H2O+CO2

Oxidizing BiocidesBacteria Fungi Algae PH range

Chlorine (Cl2) Excellent Good Good 5 - 8

Chlorine-di-oxide (ClO2)

Excellent Good Good insensitive

Bromine Excellent Good Poor 5 - 10Bromine Excellent Good Poor 5 - 10

Ozone Excellent Good Good 7 - 9

� When chlorine is added to water, the initial chemical reaction creates a mixture ofhypochlorous acid (HOCl) and hydrochloric acid (HCl):

Cl2 + H2O → HCl + HOCl� HOCl is the actual oxidant that attacks the microorganism. As pH increases, HOClstarts to dissociate into hypochlorite (OCl-):

HOCl ↔ H+ + OCl-

Chlorine is the most popular oxidizing Biocide

HOCl ↔ H+ + OCl-� OCl- is also an oxidant, but a much weaker one than HOCl. Together these chlorine species are known as free chlorine. At a pH of 5.5, the HOCl concentration in the solution is near 100%. As the pH of the solution increases to 8.5, the HOCl concentration drops to near 10% and the OCl- concentration is now 90%.

Chlorine Dissociation Curve

Non-oxidizing BiocidesNon-oxidizing Biocides are Organic Biocides, normally slow acting and are applied periodically in high concentration for maximum efficacy

Measurements & Implications� Langelier Saturation Index (LSI)

� Corrosion rate (CR)

� Oxidation Reduction Potential (ORP)

Total Bacterial Count (TBC)� Total Bacterial Count (TBC)

� Sulphate Reducing Bacteria (SRB)

Langelier Saturation Index� It is used to predict the calcium carbonate stability of water

LSI = pH - pHs

Where:� pH is the measured water pH� pH is the measured water pH� pHs is the pH at saturation in calcite or calcium carbonate and is

defined as:

pHs = (9.3 + A + B) - (C + D)Where:� A = (Log10 [TDS] - 1) / 10� B = -13.12 x Log10 (oC + 273) + 34.55� C = Log10 [Ca2+ as CaCO3] - 0.4� D = Log10 [alkalinity as CaCO3]

Corrosion RateCorrosion Rate (MPY) =

22.3 X Difference of weight of coupon in gm X 1000

-------------------------------------------------------------------------

Surface area of the coupon (sq inch) X Density (gm/cc) X no.

of days exposed

Corrosion Rack

Total Bacterial Count

Before Shock Dozing After Shock Dozing

Sulphate Reducing Bacteria

Day 2 sample Day 4 sample

Day 5 sample

Coal Based Thermal Power Plant Water Management.pdf

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