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Boiler-As per “THE INDIAN BOILERS(AMENDMENT ) ACT2007A ‘Boiler’ means a pressure vessel in which steam is generatedfor use external to itself by application of heat which is whollyor partly under pressure when steam is shut off but does notinclude a pressure vessel(1) With Capacity less than 25 ltrs (such capacity being measured from the feed check valve to the main steam stop valve);(2) With less than 1 kilogram per centimeter square design gauge pressure & working gauge pressure; or(3) In which water is heated below one hundred degree centigrade .
‘Boiler component’ means Steam piping , Feed waterpiping, Economizer ,Super heater, any mounting or other fittingand any other external or internal part of a Boiler which issubjected to pressure exceeding one kilogram per centimetersquare gauge.
“Steam Pipe "means any pipe through which steam passes if-(1)The pressure at which the steam passes through such pipeexceeds 3.5kg/cm^2 above atmospheric pressure, or(2)Such pipe exceeds 254 mm in internal diameter and pressureof steam exceeds 1kg/cm^2.above the atmospheric pressure. and includes in either case any connected fitting of asteam pipe.
At atmospheric pressure water volume increases 1,600 times STEAM TO EXHAUST GAS VENT PROCESS STACK DEAERATOR PUMPS ECO- NOMI- ZER VENT BOILER BURNER WATER SOURCE BLOW DOWN SEPARATOR FUEL CHEMICAL FEED SOFTENERSFigure: Schematic overview of a boiler room
Boiler Systems Water treatment system Feed water system Steam System Blow down system Fuel supply system Air Supply system Flue gas system
Fuels used in Boiler S.No Solid Liquid Gaseous AgroWaste 1 Coal HSD NGas Baggase 2 Lignite LDO Bio Gas Pith 3 Charcoal Fur.Oil Rice Husk 4 LSHS Paddy Straw 5 Coconut shell 6 Groundnutshell MSW/RDF
Types of Boilers1. Fire Tube Boiler2. Water Tube Boiler3. Packaged Boiler4. Stoker Fired Boiler5. Pulverized Fuel Boiler6. Waste Heat Boiler7. Fluidized Bed (FBC) Boiler
Type of Boilers • Relatively small steam capacities (12,000 kg/hour)1. Fire Tube Boiler • Low to medium steam pressures (18 kg/cm2) • Operates with oil, gas or solid fuels(Light Rail Transit Association)
Type of Boilers2. Water Tube Boiler • Used for high steam demand and pressure requirements • Capacity range of 4,500 – 120,000 kg/hour • Combustion efficiency enhanced by induced draft provisions • Lower tolerance for water quality and needs water treatment plant (Your Dictionary.com)
Type of Boilers3. Packaged Boiler To Chimney • Comes in complete package • Features • High heat transfer • Faster evaporation • Good convective heat transfer • Good combustion efficiency Oil Burner • High thermal efficiency (BIB Cochran, 2003) • Classified based on number of passes
Type of Boilers 4. Stoke Fired Boilers a) Spreader stokers Uses both suspension and grate burning Coal fed continuously over burning coal bed Coal fines burn in suspension and larger coal pieces burn on grate Good flexibility to meet changing load requirements Preferred over other type of stokers in industrial application
Type of Boilers Uses both suspension and4. Stoke Fired Boilers grate burningb) Chain-grate or traveling- Coal fed continuously over grate stoker burning coal bed Coal fines burn in suspension and larger coal pieces burn on grate Good flexibility to meet changing load requirements Preferred over other type of stokers in industrial(University of Missouri, 2004) application
Type of Boilers 5. Pulverized Fuel BoilerCoal is pulverized to a fine powder, so that less than 2% is +300microns, and 70-75% is below 75 microns.Coal is blown with part of the combustion air into the boiler plantthrough a series of burner nozzles.• Pulverized coal powder blown with combustion air into boiler through burner nozzles• Combustion temperature at 1300 -1700 °C• Benefits: varying coal quality coal, quick response to load changes and high pre-heat air temperatures Tangential firing
Pulverized Fuel Boiler (Contd..)Advantages Its ability to burn all ranks of coal from anthracitic to lignitic, and it permits combination firing (i.e., can use coal, oil and gas in same burner). Because of these advantages, there is widespread use of pulverized coal furnaces.Disadvantages High power demand for pulverizing Requires more maintenance, flyash erosion and pollution complicate unit operation
Type of Boilers6. Waste Heat Boiler • Used when waste heat available at medium/high temp • Auxiliary fuel burners used if steam demand is more than the waste heat can generate • Used in heat recovery from exhaust gases from gas turbines and dieselAgriculture and Agri-Food enginesCanada, 2001
7.Fluidized Bed (FBC) BoilerAn Overview-Fluidized bed combustion has emerged as a viablealternative and has significant advantages overconventional firing system and offers multiple benefits –compact boiler design, fuel flexibility, higher combustionefficiency and reduced emission of noxious pollutantssuch as SOx and NOx. The fuels burnt in these boilersinclude coal, washery rejects, rice husk, bagasse & otheragricultural wastes. The fluidized bed boilers have a widecapacity range.
Mechanism of Fluidised Bed CombustionWhen an evenly distributed air or gas is passed upwardthrough a finely divided bed of solid particles such as sandsupported on a fine mesh, the particles are undisturbed at lowvelocity. As air velocity is gradually increased, a stage isreached when the individual particles are suspended in the airstream – the bed is called “fluidized”. With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid – “bubbling fluidized bed”.
At higher velocities, bubbles disappear, and particles areblown out of the bed. Therefore, some amounts of particleshave to be recirculated to maintain a stable system –“circulating fluidised bed”. Fluidization depends largely on the particle size and the air velocity.If sand particles in a fluidized state is heated to the ignitiontemperatures of coal, and coal is injected continuously intothe bed, the coal will burn rapidly and bed attains a uniformtemperature. The fluidized bed combustion (FBC) takesplace at about 840OC to 950OC.
Since this temperature is much below the ash fusiontemperature, melting of ash and associated problems are avoided.The lower combustion temperature is achieved because of highcoefficient of heat transfer due to rapid mixing in the fluidized bedand effective extraction of heat from the bed through in-bed heattransfer tubes and walls of the bed. The gas velocity is maintainedbetween minimum fluidisation velocity and particle entrainmentvelocity. This ensures stable operation of the bed and avoids particleentrainment in the gas stream.Combustion process requires the three “T”s that is Time, Temperature andTurbulence. In FBC, turbulence is promoted by fluidisation. Improvedmixing generates evenly distributed heat at lower temperature. Residencetime is many times greater than conventional gratefiring. Thus an FBC system releases heat more efficiently at lowertemperatures.
Fixing, bubbling and fast fluidized beds As the velocity of a gas flowing through a bed of particles increases, a value is reaches when the bed fluidises and bubbles form as in a boiling liquid. At higher velocities the bubbles disappear; and the solids are rapidly blown out of the bed and must be recycled to maintainprinciple of fluidisation a stable system.
Since limestone is used as particle bed, control of sulfur dioxide and nitrogenoxide emissions in the combustion chamber is achieved without any additionalcontrol equipment. This is one of the major advantages over conventionalboilers.Types of Fluidised Bed Combustion BoilersThere are three basic types of fluidised bed combustion boilers:1. Atmospheric classic Fluidised Bed Combustion System (AFBC)2. Pressurised Fluidised Bed Combustion System (PFBC).3. Circulating (fast) Fluidised Bed Combustion system(CFBC)
AFBC / Bubbling Bed In AFBC, coal is crushed to a size of 1 – 10 mm depending on the rank of coal, type of fuel feed and fed into the combustion chamber. The atmospheric air, which acts as both the fluidization air and combustion air, is delivered at a pressure and flows through the bed after being preheated by the exhaust flue gases. The velocity of fluidising air is in the range of 1.2 to 3.7 m /sec. The rate at which air is blown through the bed determines the amount of fuel that can be reacted. Almost all AFBC/ bubbling bed boilers use in-bed evaporator tubes in the bed of limestone, sand and fuel for extracting the heat from the bed to maintain the bed temperature. The bed depth is usually 0.9 m to 1.5 m deep and the pressure drop averages about 1 inch of water per inch of bed depth. Very little material leaves the bubbling bed – only about 2 to 4 kg of solids are recycled per ton of fuel burned.
Bubbling Bed BoilersIn the bubbling bed type boiler, a layer of solid particles(mostly limestone, sand, ash and calcium sulfate) iscontained on a grid near the bottom of the boiler. This layeris maintained in a turbulent state as low velocity air is forcedinto the bed from a plenum chamber beneath the grid. Fuelis added to this bed and combustion takes place. Normally,raw fuel in the bed does not exceed 2% of the total bedinventory. Velocity of the combustion air is kept at aminimum, yet high enough to maintain turbulence in thebed. Velocity is not high enough to carry significantquantities of solid particles out of the furnace.
This turbulent mixing of air and fuel results in a residence time of up to fiveseconds. The combination of turbulent mixing and residence time permitsbubbling bed boilers to operate at a furnace temperature below 1650°F. Atthis temperature, the presence of limestone mixed with fuel in the furnaceachieves greater than 90% sulfur removal. Boiler efficiency is the percentageof total energy in the fuel that is used to produce steam. Combustionefficiency is the percentage of complete combustion of carbon in the fuel.Incomplete combustion results in the formation of carbon monoxide (CO)plus unburned carbon in the solid particles leaving the furnace. In a typicalbubbling bed fluidized boiler, combustion efficiency can be as high as92%. This is a good figure, but is lower than that achieved bypulverized coal or cyclone-fired boilers. In addition, some fuels that arevery low in volatile matter cannot be completely burned within theavailable residence time in bubbling bed-type boilers.
Features of bubbling bed boilerFluidised bed boiler can operate at near atmospheric or elevatedpressure and have these essential features:• Distribution plate through which air is blown for fluidizing.• Immersed steam-raising or water heating tubes which extract heatdirectly from the bed.• Tubes above the bed which extract heat from hot combustion gasbefore it enters the flue duct.
2. Pressurised Fluidised Bed CombustionSystem (PFBC).Pressurised Fluidised Bed Combustion (PFBC) is a variation of fluid bedtechnology that is meant for large-scale coal burning applications. InPFBC, the bed vessel is operated at pressure up to 16 ata ( 16 kg/cm2).The off-gas from the fluidized bed combustor drives the gas turbine. Thesteam turbine is driven by steam raised in tubes immersed in the fluidizedbed. The condensate from the steam turbine is pre-heated using wasteheat from gas turbine exhaust and is then taken as feed water for steamgeneration.The PFBC system can be used for cogeneration or combined cycle powergeneration. By combining the gas and steam turbines in this way,electricity is generated more efficiently than in conventional system. Theoverall conversion efficiency is higher by 5% to 8%. .At elevated pressure, the potential reduction in boiler size is considerabledue to increased amount of combustion in pressurized mode and highheat flux through in-bed tubes.
3. Circulating (fast) Fluidised Bed Combustionsystem(CFBC) The need to improve combustion efficiency (which also increases overall boiler efficiency and reduces operating costs) and the desire to burn a much wider range of fuels has led to the development and application of the CFB boiler. Through the years, boiler suppliers have been increasing the size of these high-efficiency steam generators.This CFBC technology utilizes the fluidized bed principle in whichcrushed (6 –12 mm size) fuel and limestone are injected into the furnaceor combustor. The particles are suspended in a stream of upwardlyflowing air (60-70% of the total air), which enters the bottom of thefurnace through air distribution nozzles. The fluidising velocity incirculating beds ranges from 3.7 to 9 m/sec. The balance of combustionair is admitted above the bottom of the furnace as secondary air.
The combustion takes place at 840-900oC, and the fine particles (<450microns) are elutriated out of the furnace with flue gas velocity of 4-6 m/s.The particles are then collected by the solids separators and circulated backinto the furnace. Solid recycle is about 50 to 100 kg per kg of fuel burnt.There are no steam generation tubes immersed in the bed. The circulatingbed is designed to move a lot more solids out of the furnace area and toachieve most of the heat transfer outside the combustion zone - convectionsection, water walls, and at the exit of the riser. Some circulating bed unitseven have external heat exchanges.The particles circulation provides efficient heat transfer to the furnacewalls and longer residence time for carbon and limestone utilization.Similar to Pulverized Coal (PC) firing, the controlling parameters in theCFB combustion process are temperature, residence time and turbulence.
For large units, the taller furnace characteristics of CFBC boiler offersbetter space utilization, greater fuel particle and sorbent residence time forefficient combustion and SO2 capture, and easier application of stagedcombustion techniques for NOx control than AFBC generators. CFBCboilers are said to achieve better calcium to sulphur utilization – 1.5 to 1 vs.3.2 to 1 for the AFBC boilers, although the furnace temperatures are almostthe same.CFBC boilers are generally claimed to be more economical than AFBCboilers for industrial application requiring more than 75 – 100 T/hr ofsteamCFBC requires huge mechanical cyclones to capture and recycle the largeamount of bed material, which requires a tall boiler.A CFBC could be good choice if the following conditions are met.1. Capacity of boiler is large to medium2.Sulphur emission and NOx control is important3.The boiler is required to fire low-grade fuel or fuel with highlyfluctuating fuel quality.
Circulating bed boiler (At a Glance)-At high fluidizing gas velocities in which a fast recycling bed of finematerial is superimposed on a bubbling bed of larger particles. Thecombustion temperature is controlled by rate of recycling of finematerial. Hot fine material is separated from the flue gas by a cyclone andis partially cooled in a separate low velocity fluidized bed heat exchanger,where the heat is given up to the steam. The cooler fine material is thenrecycled to the dense bed.
Advantages of Fluidised Bed Combustion Boilers1. High EfficiencyFBC boilers can burn fuel with a combustion efficiency of over 95% irrespectiveof ash content. FBC boilers can operate with overall efficiency of 84% (plus orminus 2%).2. Reduction in Boiler SizeHigh heat transfer rate over a small heat transfer area immersed in the bedresult in overall size reduction of the boiler.3. Fuel FlexibilityFBC boilers can be operated efficiently with a variety of fuels. Even fuels likeflotation slimes, washer rejects, agro waste can be burnt efficiently. These can befed either independently or in combination with coal into the same furnace.4. Ability to Burn Low Grade FuelFBC boilers would give the rated output even with inferior quality fuel. Theboilers can fire coals with ash content as high as 62% and having calorific valueas low as 2,500 kcal/kg. Even carbon content of only 1% by weight can sustainthe fluidised bed combustion.
5. Ability to Burn FinesCoal containing fines below 6 mm can be burnt efficiently in FBCboiler, which is very difficult to achieve in conventional firing system.6. Pollution ControlSO2 formation can be greatly minimised by addition of limestone or dolomitefor high sulphur coals. 3% limestone is required for every 1% sulphur in thecoal feed. Low combustion temperature eliminates NOx formation.7. Low Corrosion and ErosionThe corrosion and erosion effects are less due to lower combustiontemperature, softness of ash and low particle velocity (of the order of 1m/sec).8. Easier Ash Removal – No Clinker FormationSince the temperature of the furnace is in the range of 750 – 900o C in FBCboilers, even coal of low ash fusion temperature can be burnt without clinkerformation. Ash removal is easier as the ash flows like liquid from thecombustion chamber. Hence less manpower is required for ash handling.
9. Less Excess Air –Higher CO2 in Flue Gas The CO2 in the flue gases will be of the order of 14 – 15% atfull load. Hence, the FBC boiler can operate at low excess air - only 20 – 25%.10. Simple Operation, Quick Start-UpHigh turbulence of the bed facilitates quick start up and shut down. Fullautomation of start up and operation using reliable equipment is possible.11. Fast Response to Load FluctuationsInherent high thermal storage characteristics can easily absorb fluctuation in fuelfeed rates. Response to changing load is comparable to that of oil fired boilers.12. No Slagging in the Furnace-No Soot BlowingIn FBC boilers, volatilisation of alkali components in ash does not take place and theash is non sticky. This means that there is no slagging or soot blowing.13 Provisions of Automatic Coal and Ash Handling SystemAutomatic systems for coal and ash handling can be incorporated, making the planteasy to operate comparable to oil or gas fired installation.
14 Provision of Automatic Ignition SystemControl systems using micro-processors and automatic ignition equipmentgive excellent control with minimum manual supervision.15 High ReliabilityThe absence of moving parts in the combustion zone results in a highdegree of reliability and low maintenance costs.16 Reduced MaintenanceRoutine overhauls are infrequent and high efficiency is maintained for longperiods.17 Quick Responses to Changing DemandA fluidized bed combustor can respond to changing heat demands moreeasily than stoker fired systems. This makes it very suitable for applicationssuch as thermal fluid heaters, which require rapid responses.18 High Efficiency of Power GenerationBy operating the fluidized bed at elevated pressure, it can be used togenerate hot pressurized gases to power a gas turbine. This can becombined with a conventional steam turbine to improve theefficiency of electricity generation and give a potential fuel savingsof at least 4%.
General Arrangements of FBC BoilerFBC boilers comprise of following systems:i) Fuel feeding systemii) Air Distributoriii) Bed & In-bed heat transfer surfaceiv) Ash handling systemMany of these are common to all types of FBC boilers1. Fuel Feeding systemFor feeding fuel, sorbents like limestone or dolomite, usually two methodsare followed: under bed pneumatic feeding and over-bed feeding.Under Bed Pneumatic FeedingIf the fuel is coal, it is crushed to 1-6 mm size and pneumaticallytransported from feed hopper to the combustor through a feed pipepiercing the distributor. Based on the capacity of the boiler, the number offeed points is increased, as it is necessary to distribute the fuel into thebed uniformly.
Over-Bed FeedingThe crushed coal, 6-10 mm size is conveyed from coal bunker to a spreader by ascrew conveyor. The spreader distributes the coal over the surface of the beduniformly. This type of fuel feeding system accepts over size fuel also andeliminates transport lines, when compared to under-bed feeding system.2. Air DistributorThe purpose of the distributor is to introduce the fluidizing air evenly through thebed cross section thereby keeping the solid particles in constant motion, andpreventing the formation of defluidization zones within the bed. Thedistributor, which forms the furnace floor, is normally constructed from metalplate with a number of perforations in a definite geometric pattern. Theperforations may be located in simple nozzles or nozzles with bubble caps, whichserve to prevent solid particles from flowing back into the space below thedistributor.The distributor plate is protected from high temperature of the furnace by:i) Refractory Liningii) A Static Layer of the Bed Material oriii) Water Cooled Tubes.
3. Bed & In-Bed Heat Transfer Surface:a) BedThe bed material can be sand, ash, crushed refractory or limestone, with anaverage size of about 1 mm. Depending on the bed height these are of two types:shallow bed and deep bed.At the same fluidizing velocity, the two ends fluidise differently, thus affecting theheat transfer to an immersed heat transfer surfaces. A shallow bed offers a lowerbed resistance and hence a lower pressure drop and lower fan power consumption.In the case of deep bed, the pressure drop is more and this increases the effectivegas velocity and also the fan power.b) In-Bed Heat Transfer SurfaceIn a fluidized in-bed heat transfer process, it is necessary to transfer heat betweenthe bed material and an immersed surface, which could be that of a tube bundle,or a coil. The heat exchanger orientation can be horizontal, vertical or inclined.From a pressure drop point of view, a horizontal bundle in a shallow bed is moreattractive than a vertical bundle in a deep bed. Also, the heat transfer in the beddepends on number of parameters like (i) bed pressure (ii) bed temperature (iii)superficial gas velocity (iv) particle size (v) Heat exchanger design and (vi) gasdistributor plate design.
4. Ash Handling Systema) Bottom ash removalIn the FBC boilers, the bottom ash constitutes roughly 30 - 40 % of the totalash, the rest being the fly ash. The bed ash is removed by continuous over flow tomaintain bed height and also by intermittent flow from the bottom to removeover size particles, avoid accumulation and consequent defluidization. Whilefiring high ash coal such as washery rejects, the bed ash overflow drain quantity isconsiderable so special care has to be taken.b) Fly ash removalThe amount of fly ash to be handled in FBC boiler is relatively very high, whencompared to conventional boilers. This is due to elutriation of particles at highvelocities. Fly ash carried away by the flue gas is removed in number of stages;firstly in convection section, then from the bottom of air preheater/economizerand finally a major portion is removed in dust collectors.The types of dust collectors used are cyclone, bagfilters, electrostaticprecipitators (ESP’s) or some combination of all of these. To increase thecombustion efficiency, recycling of fly ash is practiced in some of the units.
General Features of our Project(3nos)Installed Capacity : 1 X 15 MWProposed Fuels : 85 % of Bagasse / Biomass, 15 % of Coal, Pet Coke.Boiler Type : Circulating Fluidized Bed Combustion (CFBC)Boiler parameters : Flow – 75 TPHPressure – 87 Kg/cm^2Temperature - 515 ± 5 oCTurbine Type : Two Nos. of uncontrolled extraction type and one no. of controlled extraction cum condensing typeTurbine parameters : Pressure – 84 Kg/cm^2Temperature - 510 ± 5 oCPlant load Factor : 0.85No. of Days of power : 335plant operation in a year
General Features of our Project(1nos)Installed Capacity : 1 X 15 MWProposed Fuels : 85 % of Bagasse / Biomass, 15 % of Coal, Pet Coke.Boiler Type : Circulating Fluidized Bed Combustion (CFBC)Boiler parameters : Flow – 100 TPHPressure – 87 Kg/cm^2Temperature - 515 ± 5 oCTurbine Type : Two Nos. of uncontrolled extraction type and one no. of controlled extraction cum condensing typeTurbine parameters : Pressure – 84 Kg/cm^2Temperature - 510 ± 5 oCPlant load Factor : 0.85No. of Days of power : 335plant operation in a year
Future of CFBC Boiler600 MWeOTU CFB.Using theBENSONVerticaltechnology, FosterWheelerhasdeveloped adesign for a600 MWesupercriticalCFB boiler
FOSTER WHEELER AWARDED CONTRACT FORWORLD’S LARGEST 100% BIOMASS BOILERZUG, SWITZERLAND, April 7, 2010 - Foster Wheeler AG (Nasdaq: FWLT)announced today that its Global Power Group has been awarded a contractby GDF SUEZ, one of the leading energy providers in the world, for thedesign, supply and erection of a 190 MWe (gross megawatt electric) 100%biomass-fired circulating fluidized-bed (CFB) boiler island for the PolaniecPower Station in Poland.Foster Wheeler has received a full notice to proceed on this contract whichwill be executed jointly by its subsidiaries in Finland and Poland. The termsof the agreement were not disclosed and the contract value will be includedin the company’s bookings for the first quarter of 2010. Constructioncompletion and start of operation of the new steam generator is scheduledfor fourth-quarter 2012.Foster Wheeler will design and supply the steam generator and auxiliaryequipment, including biomass yard, and will carry out the civil works,erection and commissioning of the boiler island. Once complete, this will bethe world’s largest biomass boiler burning wood residues and up to 20% agrobiomass.
Performance of a boiler1. Boiler2. Boiler blow down3. Boiler feed water treatment
Performance of a Boiler1. Boiler performance• Causes of poor boiler performance -Poor combustion -Heat transfer surface fouling -Poor operation and maintenance -Deteriorating fuel and water quality• Heat balance: identify heat losses• Boiler efficiency: determine deviation from best efficiency
Performance of a BoilerHeat BalanceAn energy flow diagram describes geographicallyhow energy is transformed from fuel into usefulenergy, heat and losses Stochiometric Excess Air Un burnt Stack Gas FUEL INPUT STEAM OUTPUT Convection & Blow Ash and Un-burnt parts Radiation Down of Fuel in Ash
Performance of a BoilerHeat BalanceBalancing total energy entering a boiler againstthe energy that leaves the boiler in different forms % Heat loss due to dry flue gas % Heat loss due to steam in fuel gas %100.0 % Heat loss due to moisture in fuel BOILER % Fuel Heat loss due to moisture in air 2% Heat loss due to unburnts in residue % Heat loss due to radiation & other unaccounted loss % Heat in Steam
Performance of a BoilerHeat BalanceGoal: improve energy efficiency by reducing avoidable lossesAvoidable losses include:- Stack gas losses (excess air, stack gas temperature)- Losses by unburnt fuel- Blow down losses- Condensate losses- Convection and radiation
Performance of a BoilerBoiler EfficiencyThermal efficiency: % of (heat) energy input that iseffectively useful in the generated steam BOILER EFFICENCY CALCULATION 1) DIRECT METHOD: 2) INDIRECT METHOD: The energy gain of the The efficiency is the working fluid (water and steam) different between losses is compared with the energy and energy input content of the boiler fuel.
Performance of a Boiler Boiler Efficiency: Direct Method Heat Input x 100 Q x (hg – hf) x 100 Boiler efficiency () = = Heat Output Q x GCV hg -the enthalpy of saturated steam in kcal/kg of steam hf -the enthalpy of feed water in kcal/kg of waterParameters to be monitored:- Quantity of steam generated per hour (Q) in kg/hr- Quantity of fuel used per hour (q) in kg/hr- The working pressure (in kg/cm2(g)) and superheat temperature (oC), if any- The temperature of feed water (oC)- Type of fuel and gross calorific value of the fuel (GCV) in kcal/kg of fuel
Performance of a BoilerBoiler Efficiency: Direct MethodAdvantages• Quick evaluation• Few parameters for computation• Few monitoring instruments• Easy to compare evaporation ratios with benchmark figuresDisadvantages• No explanation of low efficiency• Various losses not calculated
Performance of a BoilerBoiler Efficiency: Indirect MethodEfficiency of boiler () = 100 – (i+ii+iii+iv+v+vi+vii)Principle losses:i) Dry flue gasii) Evaporation of water formed due to H2 in fueliii) Evaporation of moisture in fueliv) Moisture present in combustion airv) Unburnt fuel in fly ashvi) Unburnt fuel in bottom ashvii) Radiation and other unaccounted losses
Performance of a BoilerBoiler Efficiency: Indirect MethodRequired calculation data• Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content)• % oxygen or CO2 in the flue gas• Fuel gas temperature in ◦C (Tf)• Ambient temperature in ◦C (Ta) and humidity of air in kg/kg of dry air• GCV of fuel in kcal/kg• % combustible in ash (in case of solid fuels)• GCV of ash in kcal/kg (in case of solid fuels)
Performance of a BoilerBoiler Efficiency: Indirect MethodAdvantages• Complete mass and energy balance for each individual stream• Makes it easier to identify options to improve boiler efficiencyDisadvantages• Time consuming• Requires lab facilities for analysis
Performance of a Boiler2. Boiler Blow Down• Controls ‘total dissolved solids’ (TDS) in the water that is boiled• Blows off water and replaces it with feed water• Conductivity measured as indication of TDS levels• Calculation of quantity blow down required: Feed water TDS x % Make up waterBlow down (%) = Maximum Permissible TDS in Boiler water
Performance of a BoilerBoiler Blow DownTwo types of blow down• Intermittent • Manually operated valve reduces TDS • Large short-term increases in feed water • Substantial heat loss• Continuous • Ensures constant TDS and steam purity • Heat lost can be recovered • Common in high-pressure boilers
Performance of a BoilerBoiler Blow DownBenefits• Lower pretreatment costs• Less make-up water consumption• Reduced maintenance downtime• Increased boiler life• Lower consumption of treatment chemicals
Performance of a Boiler3. Boiler Feed Water Treatment• Quality of steam depend on water treatment to control • Steam purity • Deposits • Corrosion• Efficient heat transfer only if boiler water is free from deposit-forming solids
Performance of a BoilerBoiler Feed Water TreatmentDeposit control• To avoid efficiency losses and reduced heat transfer• Hardness salts of calcium and magnesium • Alkaline hardness: removed by boiling • Non-alkaline: difficult to remove• Silica forms hard silica scales
Performance of a BoilerBoiler Feed Water TreatmentInternal water treatment• Chemicals added to boiler to prevent scale• Different chemicals for different water types• Conditions: • Feed water is low in hardness salts • Low pressure, high TDS content is tolerated • Small water quantities treated• Internal treatment alone not recommended
Performance of a Boiler Boiler Feed Water TreatmentExternal water treatment:• Removal of suspended/dissolved solids and dissolved gases• Pre-treatment: sedimentation and settling• First treatment stage: removal of salts• Processes a) Ion exchange b) Demineralization c) De-aeration d) Reverse osmoses
Performance of a BoilerExternal Water Treatmenta) Ion-exchange process (softener plant)• Water passes through bed of natural zeolite of synthetic resin to remove hardness• Base exchange: calcium (Ca) and magnesium (Mg) replaced with sodium (Na) ions• Does not reduce TDS, blow down quantity and alkalinityb) Demineralization• Complete removal of salts• Cations in raw water replaced with hydrogen ions
Performance of a BoilerExternal Water Treatmentc) De-aeration• Dissolved corrosive gases (O2, CO2) expelled by preheating the feed water• Two types: • Mechanical de-aeration: used prior to addition of chemical oxygen scavangers • Chemical de-aeration: removes trace oxygen
Performance of a Boiler External Water Treatment Mechanical Vent de-aeration SprayBoiler FeedWater Nozzles • O2 and CO2 removed by Stea heating feed water m Scrubber Section • Economical treatment (Trays) process Storage • Vacuum type can reduce Section O2 to 0.02 mg/l De-aerated • Pressure type can Boiler Feed Water reduce O2 to 0.005 mg/l ( National Productivity Council)
Performance of a BoilerExternal Water TreatmentChemical de-aeration• Removal of trace oxygen with scavenger• Sodium sulphite: • Reacts with oxygen: sodium sulphate • Increases TDS: increased blow down• Hydrazine • Reacts with oxygen: nitrogen + water • Does not increase TDS: used in high pressure boilers
Performance of a BoilerExternal Water Treatmentd) Reverse osmosis• Osmosis • Solutions of differing concentrations • Separated by a semi-permeable membrane • Water moves to the higher concentration• Reversed osmosis • Higher concentrated liquid pressurized • Water moves in reversed direction
Performance of a Boiler External water treatment d) Reverse osmosis PressureFeed Water Fresh Water More Concentrated SolutionConcentrate Water FlowFlow Semi Permeable Membrane
IntroductionType of boilersPerformance of a boilerEnergy efficiency opportunities
Energy Efficiency Opportunities1. Stack temperature control2. Feed water preheating using economizers3. Combustion air pre-heating4. Incomplete combustion minimization5. Excess air control6. Avoid radiation and convection heat loss7. Automatic blow down control8. Reduction of scaling and soot losses9. Reduction of boiler steam pressure10. Variable speed control11. Controlling boiler loading12. Proper boiler scheduling13. Boiler replacement
Energy Efficiency Opportunities1. Stack Temperature Control• Keep as low as possible• If >200°C then recover waste heat2. Feed Water Preheating Economizers• Potential to recover heat from 200 – 300 oC flue gases leaving a modern 3-pass shell boiler3. Combustion Air Preheating• If combustion air raised by 20°C = 1% improve thermal efficiency
Energy Efficiency Opportunities4. Minimize Incomplete Combustion• Symptoms: • Smoke, high CO levels in exit flue gas• Causes: • Air shortage, fuel surplus, poor fuel distribution • Poor mixing of fuel and air• Oil-fired boiler: • Improper viscosity, worn tops, cabonization on dips, deterioration of diffusers or spinner plates• Coal-fired boiler: non-uniform coal size
Energy Efficiency Opportunities5. Excess Air Control• Excess air required for complete combustion• Optimum excess air levels varies• 1% excess air reduction = 0.6% efficiency rise• Portable or continuous oxygen analyzersFuel Kg air req./kg fuel %CO2 in flue gas in practiceSolid FuelsBagasse 3.3 10-12Coal (bituminous) 10.7 10-13Lignite 8.5 9 -13Paddy Husk 4.5 14-15Wood 5.7 11.13Liquid FuelsFurnace Oil 13.8 9-14LSHS 14.1 9-14 83
Energy Efficiency Opportunities6. Radiation and Convection HeatLoss Minimization• Fixed heat loss from boiler shell, regardless of boiler output• Repairing insulation can reduce loss7. Automatic Blow Down Control• Sense and respond to boiler water conductivity and pH
Energy Efficiency Opportunities8. Scaling and Soot Loss Reduction• Every 22oC increase in stack temperature = 1% efficiency loss• 3 mm of soot = 2.5% fuel increase9. Reduced Boiler Steam Pressure• Lower steam pressure = lower saturated steam temperature = lower flue gas temperature• Steam generation pressure dictated by process
Energy Efficiency Opportunities10. Variable Speed Control forFans, Blowers and Pumps• Suited for fans, blowers, pumps• Should be considered if boiler loads are variable11. Control Boiler Loading• Maximum boiler efficiency: 65-85% of rated load• Significant efficiency loss: < 25% of rated load
Energy Efficiency Opportunities12. Proper Boiler Scheduling• Optimum efficiency: 65-85% of full load• Few boilers at high loads is more efficient than large number at low loads13. Boiler ReplacementFinancially attractive if existing boiler is• Old and inefficient• Not capable of firing cheaper substitution fuel• Over or under-sized for present requirements• Not designed for ideal loading conditions