Chemistry Super Critical.pdf

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OBJECTIVE 0F POWER PLANT CHEMICAL TREATMENT

• Minimizing corrosion in the boiler, steam, condensate and feed water systems.

• Prevention of deposit formation on heating surfaces.

• Maintenance of high level of steam quality

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Deposition is detrimental

• Reduce efficiency

• Increase heat rate

• Under deposit corrosion –tube failure

• Overheating – tube failure

• Demands Chemical cleaning –unit availability.

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• To minimize corrosion and deposition, the concentration of required chemical constituents and impurities, must be in agreement with chemistry guidelines

• Deviation from guidelines results in corrosion-deposition

• Analysis of deposits indicate 80-85% consists of Iron, the corrosion products

• Corrosion Deposition

Depositor scale

Deposits are mostly porous in nature.

Scale / Deposit

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Corrosion, Deposition and Impurity Ingress in Power Plant Cycle

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Why hydrazine dosing?• Corrosive components O2 and CO2 to be removed (by deaerator

in subcritical boilers).

• Hydrazine (N2H4 ) is a colorless liquid which boils at 113°C

and soluble in water.

• N2H4 is a powerful oxygen scavengers for boiler feed water and

reduce oxygen to form nitrogen and water as follows:

N2H4 + O2 -------------> 2 H2O + N2

• Hydrazine volatilizes out of the boiler with the steam and breaks down in passing through the super heater above 204°C(400°F) decomposes forming nitrogen and ammonia which increases the feed water pH level, reducing the risk of acidic corrosion

3 N2H4 → 4NH3 + N2

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Why Ammonia dosing?• Feed water has to be alkalized (pH of 9 or higher), to

reduce oxidation and to support forming of stable

layer of magnetite, protecting the material from

further corrosion.

• This is usually done by dosing alkalic agents, like

Phosphate, sodium hydroxide (caustic soda) or

volatile ammonia.

• NH3 a colorless gas that is about one half as dense as

air.

• The solution is alkaline because dissolved NH3 reacts

with water, to form ammonium hydroxide, NH4OH, a

weak base.

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Why Phosphate dosing?• Tri-sodium phosphate provides the needed alkalinity in boiler

systems as follows :

(TSP)Na3PO4+ H2O NaOH (increases pH) + Na2HPO4

• TSP also provide the sufficient chemical buffering capacity to cope with small system upset e.g. a small condenser leaks.

• Absorption of contaminants : Ca & Mg salts removes as sludge's

10Ca2+ + 6PO43-- + 2OH-- 3Ca3(PO4).Ca(OH)2

calcium hydroxyapetite

3 Mg2+ + 2SiO32-+ 2OH-- + H2O 3MgO.2SiO2.2H2O

serpentine

Calcium hydroxyapetite and serpentine exist as soft sludge's and much easier to remove ; typically settle in the drum and removed by blow down

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Ammonia Vs TSP• NH3 & Na3PO4 (TSP) their purpose is same: to

increase pH level.

• NH3 dosing carried out in feed water side i.e. pre

boiler side (before Eco inlet). TSP dosing carried

out in boiler (mostly in drum).

• Two chemicals for same purpose.

• If NH3 goes into drum 70% of NH3 get vaporize &

escape with steam. Hence in water wall circuit it is

difficult to maintain pH by NH3 so we add TSP.

• NH3 dosing carried out at BFP suction

• TSP dosing carried out at Drum

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Iron has tendency to go back to original state of ore, Fe2O3

Corrosion can not be avoided but we can minimize it

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Magnetite formation Steam/ water above 100°C reacts with the iron ions on

the water side, to form a uniform layer of iron oxide (Fe3 O4) magnetite

3Fe + 4H2O ( 100°C+) = Fe3O4 + 4H2

Iron + Water/Steam » Magnetite + Hydrogen gas

The magnetite layer is black color iron oxide.

In a new /clean boiler, the initial corrosion process produces this magnetite film as a tenacious layer at the steel surface. This magnetite layer prevents any further contact with the steel or water surface

Magnetite (Fe3O4) is a mixture of FeO & Fe2O3. FeO is susceptible to Flow accelerate corrosion

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Mechanism Of FAC

• A process whereby the normal protective magnetite layer on carbon steel get dissolves into stream of flowing water or wet steam

• FAC reduces or eliminates oxide layer and leads to a rapid removal of base metal

• Wall thinning rate as high as 3mm/year observed due to FAC

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FEED WATER CORROSION• Carbon steel or low alloyed steel form protective

oxide layers due to corrosion in water and steam.

• In high temperature regions (>200oC) a dense uniform magnetite Fe3O4 protective layer is formed spontaneously on surfaces.

• But in low temp regions (<200oC) steel surfaces in contact with water remain active

• Rate of Corrosion of Carbon Steel is Max. at 150oC and Sharp Drop at > 150oC

• Pre boiler area or comparatively cooler surfaces like condensate line & Feed water line remains active and vulnerable to corrosion.

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Solubility of Magnetite in water with Ammonia

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• With AVT the protective layer in the low temp region (<200oC) of condensate and feed systems consists almost exclusively of magnetite.

• Because of porous nature of magnetite there is erosion-corrosion in AVT.

• Corrosion product transported and deposits.

• It causes Pressure drop, required chemical cleaning, causes tube failure

FEED WATER CORROSION contd..

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Corrosion Product Transport During Start-up

After light up when temp increases

After heater charging

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Rise in Pressure Drop & CleaningIntervals for SCB –with AVT

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Magnetite Growth Under AVT

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DISADVANTAGE OF MAGNETITE

• POROUS NATURE

• LOW RANGE OF STABILITY

• LESS ADHERENCE

• NOT RESISTANT TO FLOW ACCELERATED CORROSION and EROSION ---CORROSION

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Magnetite Solubility as a function of pH

Minimum solubility and thus minimum magnetite transport occur when the cycle pH is maintained at around 9.5

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Porous internal deposit layer in water wall

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Oxygenated Treatment• OT chemistry gain popularity as it prevents flow accelerated

corrosion (FAC) and reduces iron content in the feed water.

• In OT, the High purity Oxygen is injected into the outlet of Condensate polishing unit.

• In presence of Oxygen, the pores in the magnetite protective layer are blocked and covered with lower solubility ferric hydrate oxide (FeOOH) and/or ferric oxide (Fe2O3 ) Hematite.

• In this way, the lesser iron concentration in the feed water is achieved by OT

• Plants with OT demonstrated superiority over AVT for long term protection of pre-boiler system and improved boiler performance using stringent water quality control

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HEMATITE LAYER

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OT AN ALTERNATIVE• Provides long term protection of pre boiler system

by forming Hematite layer.

• CPU operating period will be more due to low condensate Ammonia content

• Iron transport will be reduced by 90%

• Less generation of CRUD

• Reduction in chemical cleaning frequency

• Reduction in outage time

• Leads to very less BTF

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HEMATITE LAYER

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COMPARISON OF PROTECTIVE LAYERS UNDER AVT & OT

• Hematite Blocks the pores of magnetite layer.

• Hematite Coherent and adherent.

• Solubility Product Constant (Ksp) of hematite is 1037, and that of magnetite is 1044. (It represents the level at which

a solute dissolves in solution. A more a substance dissolves, the higher the Ksp value it has)

• Hematite is resistant to FAC at least 2 orders of magnitude

• BTF DUE TO CYCLE WATER CHEMISTRY WILL BE MINIMUM.

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CONDENSER

LPHeater

Boiler CPU

CEP

D/AHP

Heater BFP

Chemical AdditionPoint ofOxygenated Treatment

All Volatile treatment [AVT (R)]

Ferrous or mixed feed water train

Cation conductivity < 0.2 µs / cm

pH 9.2 – 9.6

Fe > 5 ppb; O2 < 5ppb

Oxygenated Treatment (OT)

All Ferrous feed water train

Cation conductivity < 0.15 µs / cm

pH 8.0 – 8.5 for once through units

pH 9.0 – 9.4 for drum units

Fe ~ 0.5 ppb

N2H4

NH3

O2 30-50 ppb for drum units

30-150 ppb for once throu unit

NH3

Additional injection point

only for conversion

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OXIDATION RESISTANCE• The most beneficial form of iron oxide is

Fe3O4 (magnetite), which forms at normal operating ranges and results in a stable, thin, protective layer on the water/steam side of boiler tubing.

• A less stable form is FeO (wustite), which forms at high temperatures(>560ºC) and can lead to rapid oxidation of the tube wall.

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Electric Power Research Institute (EPRI) conducts research on issues of interest to the electric power industry in the USA

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Cation Conductivity ExcursionCC µS/cm Action Required

< 0.15 Normally desired level, continue normal

operations. Less than 0.1 µS/cm is preferred and

easily attainable in optimized units.

0.15 -0.3 Increase system pH to AVT levels (~9.2 for all-

ferrous system). Continue feed of O2 and monitor

to determine the source of the contaminant.

> 0.3 Terminate oxygen feed, continue operating with

AVT without the use of hydrazine. Use Guideline

for Once-through Units using AVT(O)

> 1 Immediate shutdown

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Corrosion Product Transport Vs Cation Conductivity

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Influence of Water Quality on Corrosion in Steel

If conductivity is low: rate of corrosion decreases with increase in O2 %. However if conductivity is high rate of corrosion increases sharply with increase in O2 %.

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Corrosion Rate of Steel as a Function of PH & Oxygen

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Oxygen Dosing System

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Specification for Oxygen

• Purity 99.893 %

• Moisture 5 ppm by vol

• Carbon di-oxide 0.5 ppm by vol

• Carbon Monoxide 0.5 ppm by vol

• Nitrogen 200 ppm by vol

• Argon 800 ppm by vol

• Total Hydrocarbon 60 ppm by vol

• Hydrogen Nil

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Vent Operation• The venting of feed water and deaerator are

critical with O.T. These should be capable of easy opening & closing

• During Normal operation i.e. with OT-all the vents of Deaerator and heaters are in closed condition.

• Feed water Cation conductivity must be maintained below 0.15 μS/cm

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Vent Operation• During startup-similar to AVT, the Oxygen

dosing system is isolated and the deaerator vent was kept open and the pH of the feed water will be maintained between 9 -9.5. Oxygen is recommended only when the Cation conductivity is below 0.15 μS/cm

• During shutdown -oxygen can be stopped manually up to an hour prior to shutdown, and the Deaerator vents are kept open. The pH can be raised above 9.0 from 8.0 -8.5

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LP Heater

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Hot Well

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When to start O.T.

• Either immediately after the acid cleaning of boiler till new magnetite layer becomes too heavy, or,

• Within one year of acid cleaning

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Process for Oxygenated Treatment• Hydrazine feed is terminated: This allows the

conversion of magnetite to hematite even before allowing oxygen dosing

• Oxygen is introduced: This leads to oxidation of ferrous ions in existing oxide layer i.e. consuming the oxygen. So no oxygen concentration seen initially in samples taken up stream of the injection points

• As oxidation continues, anions bound to oxide film are released, causing Cation conductivity to rise.

• Over the next several hours or days dissolved oxygen begins to appear at sample points farther and farther downstream

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Process for Oxygenated Treatment

• The feed water heaters vents are closed and oxygen feed is reduced to normal operating level when its concentration in main or reheat steam samples reaches 90% of that in upstream samples

• Oxygen is fed at a calculated rate to produce only 60 ppb oxygen in the beginning

• Slowly the oxygen concentration may be increased to tolerate dissolved oxygen concentration excess of 140 ppb at eco inlet.

• Dissolved oxygen and Cation conductivity in the unit should be monitored at CPU outlet, deaerator inlet & outlet, eco outlet, primary super heater, down comer and in main steam

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Unit Shut down, start up unchanged

• Unit shutdown is much the same as with AVT. Several hours before a unit is schedule to come down, ammonia feed is initiated to reach the pH to AVT levels- about 9.0

• Oxygen feed is terminated and deaerator vents are opened to drop oxygen concentration below 5 ppb

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Limitation with O.T.• The Copper based alloys should be avoided in

heaters including condensers as these are more prone to corrosion in presence of high concentration of oxygen and ammonia. Hence All Ferrous metallurgy is required

• Need of 100% condensate polishing Unit (CPU).

• Feed water Cation conductivity to be meticulously maintained around 0.15 us/cm

• For making oxygen an effective corrosion inhibiter, pH of feed water to be maintained around 8 – 8.5

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Thanks

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