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!