Thermal Power Plant Boiler Efficiency Improvement
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Transcript of Thermal Power Plant Boiler Efficiency Improvement
Boiler Efficiency
Improvement
ANKUR GAIKWAD
B.E (MECHANICAL), BITS-PILANI
Boilers are widely used in power
generation, chemical & process industries
They’re used mainly for generating steam
at high pressures & temperatures for a
variety of purposes
Its development began in the 18th century
Industrial Revolution
Being fairly mature technology, today’s
boilers have become very efficient at
converting the thermal energy in coal, oil
or natural gas to heat water & form steam
at high pressure & temperature
This presentation seeks to explore the
methods available to maintain & improve
the boiler efficiency
Introduction
Main Areas for Improvement
Combustion Management
Makeup, Feedwater, Condensate & Blowdown Management
Steam Distribution Management
Combustion Efficiency Management
Boiler burns fuel efficiently if following 3 conditions are met:
It burns fuel completely
It uses as little excess air as possible to do it
It extracts as much heat as possible from the combustion gases
The first 2 conditions are met by careful control of excess air in the boiler
Control of Excess Air
In general, boiler efficiency decreases for excess oxygen above 2-3% or excess air
above 10-15%
Optimum excess air is recommended for each type of boiler on the basis of fuel
used
Combustion Efficiency Indicators
Oxygen Test
Smoke Opacity Test
Carbon Monoxide Test
Combustion Control
Usual causes of deficient combustion:
Improper Draft: Remedied by Draft Control
Improper Air-Fuel Mixture
Draft Control
Pitfalls of Improper Draft:
Insufficient Draft: Prevents adequate air supply for combustion; Results in smoky,
incomplete combustion
Excessive Draft: Larger volume of air & flue gas moves quickly through the furnace;
Less time for heat transfer, High flue gas exit temperature; Contributes to maximum
heat loss
Ideal Draft: Controlled such that boiler operates at 2-4% excess oxygen
Close Draft Regulation difficult due to burners’ requirement of proper air-fuel
mixture
Air-Fuel Mix Control
Stoichiometric air-fuel mix depends on masses
Fact to be considered: Density of air & gaseous fuels changes with ambient
temperatures
Control challenging due to:
Inadequate tolerance of burner controls
Faulty burners
Improper Fuel Delivery system
Reclaiming Boiler Heat Losses
Residual heat in flue gas is the main heat loss
Residual heat used in following ways:
Economisers: Feedwater preheated
Flue gas condensing by water: Water absorbs flue gas heat
Combustion Air Preheat: Combustion air preheated for better combustion
Flue Gas Recirculation: Recirculated with incoming air-fuel mix; decreases NOx
emissions
Heat Cascading: Exhaust heat used in lower temperature applications
Makeup, Feedwater & Blowdown
Management
Necessary to monitor & control scale formation in water tubes in boiler
Effects of Scale Formation on boiler operation:
Reduces heat transfer
Impedes proper fluid flow
Boiler tubes subjected to failure due to overheating
Fuel Wastage
Monitoring scale formation in boiler tubes
Directly during boiler maintenance shutdowns
Indirectly through monitoring flue gas exit temperatures for longer periods of time
Boiler Water Treatment
2 types of boiler water treatment methods:
Internal
External
Internal Water Treatment
Chemical dosage converts scale-forming compounds to free-flowing sludge
Sludge removed by blowdown
Corrosion inhibitors (e.g. amines) form protective film for corrosion protection of
boiler internals
Common Internal Treatment chemicals:
Polyphosphates & Sodium Meta Phosphate for scale control
Sodium Sulphite, Hydroquinone Hydrazine, Diethylhydroxyamine (DEHA), Methyl
ethyl ketoxime for dissolved oxygen
Neutralizing and filming amines for corrosion control due to CO2
External Water Treatment
Two Stages:
Remove only hardness salts; also called ‘water softening’
Remove Total Dissolved Salts (TDS); also called ‘de-mineralisation’
Water Softening
Done when hardness alone is a limiting factor
Cation-exchange zeolite resin exchanges all hardness ions to reduce hardness to
zero
Techniques:
Cold lime softening: Addition of hydrated lime at ambient temperature; can reduce
hardness to 35-50 ppm of calcium carbonate
Hot lime softening: Addition of lime at 227-240 F
Hot lime soda: Addition of soda with ash in hot lime process; reduces hardness to 8
ppm calcium carbonate & 2-5 ppm magnesium content
De-alkalisation
Done when hardness & alkalinity are limiting factors
Two types:
Split-stream de-alkalisation:
2 cation exchange units in parallel
2 sodium zeolite softener resins; one regenerated with salt, the other regenerated with acid
Lowers hardness to zero, reduces alkalinity, removes dissolved solids
Chloride de-alkalisation:
2 ion exchange units in series
1 sodium zeolite cation exchanger resin; other is anion exchange resin
Doesn’t remove dissolved solids,silica
De-silicisation
Done when silica is the limiting factor
Removes silica using strongly basic anion exchange resin regenerated with caustic
soda
2 systems:
Sodium zeolite softener, followed by strongly basic anion resin unit. Reduces hardness,
anions, silica
Cation exchanger regenerated with acid, followed by strong base anion exchanger
regenerated with caustic soda. Removes all dissolved solids (including silica)
De-mineralisation
Done when dissolved solids (TDS) is a limiting factor
Consists of ion exchange resin columns-a strong cation unit & strong anion unit
Hydrogen cation exchange converts dissolved salts to their corresponding acid
forms, removed in anion exchanger
De-mineralised water approaches distilled water in purity
High cost of operation makes it difficult for low to moderate pressure boilers
Feedwater Management
Boiler feedwater consists of:
Returned condensate
Make-up water
Make-up water is the main source of contaminants, making condensate recovery
important
Condensate recovery important due to the following:
Losing hot condensate results in heat loss of fuel
More the condensate recovery, lesser will be the make-up water, lesser the need for
water treatment
More condensate recovery implies lower blowdown & associated losses
Flash Steam Recovery
Flash steam formed when the condensate’s pressure is suddenly reduced
Flash steam used for low-pressure heating
Flash steam formed in a flash vessel, a vertical vessel in which there’s
considerable pressure drop of condensate while it falls down
Steam leaves from the top part of the vessel
Condensate Water Treatment
Common chemicals in condensate:
Dissolved CO2
Suspended Iron
Carbonic Acid
Elaborate condensate treatment not needed
Soft measures like condensate polishing or conditioning are required to ensure reliability of equipment
2 types of condensate water treatment:
Amines for neutralising carbonic acid (e.g. Cyclohexylamine)
Amines for filming a protective barrier against carbonic acid & oxygen (e.g. Octyldecylamine)
De-aeration of Boiler Feedwater
Oxidative corrosion, due to dissolved oxygen in feedwater, accelerates at high
temperatures in boiler
De-aeration necessary to remove dissolved oxygen from feedwater
De-aeration:
Live steam heats feedwater upto 105 C
Feedwater is mechanically agitated simultaneously to drive off dissolved oxygen
Dissolved oxygen, along with tiny amount of live steam, is vented to atmosphere
Higher the proportion of make-up water in circulation, greater the need for de-
aeration
Blowdown Water
Dissolved solids are left behind in the boiler water when water is converted into steam
As makeup water is circulated & converted to steam, the amount of solids in the boiler increases
till the water can’t dissolve all of the solids
This ‘saturated’ water is then discharged as ‘bottom blowdown’
Amount of solids in water
Double when, Amount of make-up water = Amount of water originally used in boiler. Also called
‘2 cycles of condensation’
Triple when, Amount of make-up water = 2*(Amount of water originally used in boiler). Also called
‘3 cycles of condensation’
Effects of Blowdown:
Insufficient Blowdown: Formation of deposits
Excessive Blowdown: Wastage of energy, water & chemicals
Blowdown Water Regulation Tests
2 Tests:
Chloride Test
Specific Conductance Test
Chloride Test:
Chloride is chosen since it’s inert to chemicals, heat; and is always present in make-up
water
If chloride doubles, it implies that amount of solids in water has also doubled
Specific Conductance Test:
Conductivity of make-up water is measured against that of boiler water.
Cycles of concentration = Conductivity of (make-up water)/(boiler water)
Blowdown Control
Manual Control:
Mostly used for mud/bottom blowdown for a few seconds after periodic intervals of
several hours
Designed to remove suspended solids that settle out of boiler water & form a heavy
sludge
Automatic Control:
Electronic sensors & controllers sense boiler water TDS
Open & close surface blowdown lines to maintain boiler water TDS at a minimum
Surface Blowdown: Removes dissolved solids concentrated near liquid surface
Blowdown Heat Recovery
2 Methods:
Flash Steam Recovery
Blowdown Heat Recovery
Flash Steam Recovery:
Blowdown water sent to a flash tank to give flash steam at low pressure
Flash steam at low pressure used in de-aerator, etc.
Blowdown Heat Recovery:
Hot blowdown heats boiler make-up water to recover blowdown heat
Steam Distribution Management
Steam distribution equipment must supply high quality steam at required pressure
& flow rate with minimum heat loss
Key Components of Steam Distribution System:
Steam distribution piping
Valves & Flanges
Insulation
Steam Traps
Air Vents
Drip Legs
Strainers
Important Concerns of Steam Distribution Management
Optimum Pipe Sizing
Proper Insulation
Plugging Leaks
Steam Traps & Associated Pipelines
Steam Use in Heating
Steam Distribution Management
Optimum Pipe Sizing
Affected by Steam Velocities:
Superheated: 50-70 m/s
Saturated: 30-40 m/s
Wet/Exhaust: 20-30 m/s
Velocities lesser than 15 m/s at shorter pipe bends
Standard data tables available to help selection of appropriate pipe sizes
Steam piping size based on ‘permissible velocity’ & ‘available pressure drop’
considerations
Condensate piping size designed based on the assumption of only water flow at
starting conditions, despite mostly carrying two-phase flow in practice
Proper Piping Design & Maintenance
Ensure right sizing of pipes
Oversized Pipes: Increase capital, maintenance & insulation costs; Increase surface heat losses
Undersized Pipes: Require higher pressure & pumping energy; Have higher rates of leakage
Get rid of redundant & obsolete pipework
Fix Steam Leaks
Keep track of facility-wide & individual process-unit steam balances
Piping at equipment connections should accommodate thermal responses during system start-ups & shutdowns
Steam separators should be installed to ensure dry steam throughout the process equipment & branch lines
Done to avoid excessive heat loss to atmosphere
Important Insulation Properties: Thermal conductivity, Strength, Abrasion
resistance, Workability, and Resistance to water absorption
Common Insulating Materials:
Steam Piping: Calcium Silicate, Fiberglass, Perlite, Cellular Glass
Steam Distribution Components/Attachments: Fiberglass, Fabric Insulation Blankets
Smaller the pipe diameter, thinner the insulation
Higher the temperature of the insulated pipe, higher the return on investment
Running pipes in groups reduces heat losses
Air movement & Draft increase heat losses of un-insulated pipes
Proper Insulation of Steam Piping
Plugging Leaks
Steam leaks commonly develop around valve stems, pressure regulators & pipe
joints
Leaks are easy to detect
Even a small leak amounts to significant costs over the year, as shown
Steam Traps & Associated Pipelines
Steam traps distinguish condensate from steam & remove the condensate
Types of steam traps, classified based on:
Density difference: Known as mechanical traps; Include float traps & bucket traps
Temperature difference: Known as thermostatic traps; Include Balanced-pressure traps,
Bimetal traps & Liquid expansion traps
Flow characteristics: Known as thermodynamic traps
Steam Traps Maintenance:
Periodic Cleaning & Checking for wear
Fixing strainers ahead of the steam traps to avoid damage by scale & dirt
Steam traps handling more air require more frequent inspection & proper venting
Steam Use in Heating
Steam can be used in various ways as follows:
Providing Dry steam for Process
Using steam at lowest pressures required by end-user
Heating by Direct Injection
Proper Air Venting: Done to avoid reduced heat transfer performance due to air
films
Summary
The three important phases of operation to be managed for high boiler efficiency
are:
Combustion
Feedwater, Make-up & Blowdown
Steam Piping
Good draft control, air-fuel mixture control results in high boiler efficiency
Maintaining low amount of dissolved solids & acids helps in maintaining high
efficiency & prolonging equipment life
Proper piping design & maintenance helps in increasing boiler efficiency
THANK YOU!