3Objectives1. Describe the theory, construction, and applications of boilers2. Describe the common type of boilers3. Explain how to assess the performance and efficiency of a boiler4. Describe methods to improve boiler efficiency5.List energy efficiency opportunitiesThe objectives with this training session is to learn how a boiler works and what different kinds of boilers that there are.1. Describe the theory, construction, and applications of boilers2. Describe the common type of boilers3. Explain how to assess the performance and efficiency of a boiler4. Describe methods to improve boiler efficiency; blowdowns5. List energy efficiency opportunities
4Objectives Introduction – Purpose? Type of boilers Boiler Assessment Energy efficiency opportunitiesWe will learn how to assess the performance and efficiency of a boiler and how to identify energy efficiency opportunities.Those of you who already have experience from evaluating the performance of a boiler, and from identifying energy efficiency opportunities are of course more than welcome to share your experiences.Uses in a nuclear power plant include:Production of steam for plant piping warm upProduction of steam for auxiliary loads – Air Ejectors, Water Treatment, Heating, gland steamSteam for testing the main turbines while nuclear steam not availableNitrogen inerting of the Containment building
5Introduction What is a Boiler? Vessel that heats water to become hot water or steamAt atmospheric pressure water volume increases 1,600 timesHot water or steam used to transfer heat to a processA boiler is an enclosed vessel that provides a means for combustion heat (chemical energy) to be transferred to water until it becomes heated water or steam (thermal energy), which can be transported to produce usable work or heating.When water at atmospheric pressure is boiled into steam its volume increases about 1,600 times, producing a force that is almost as explosive as gunpowder. This causes the boiler to be an equipment that must be treated with utmost care.The hot water or steam under pressure is then usable for transferring the heat to a process.
6Introduction Thermal Equipment/ Boilers BOILER STEAM TO PROCESSEXHAUST GASVENTSTACKDEAERATORECO- NOMI- ZERPUMPSThermal Equipment/BoilersBOILERAs you can see, the boiler system comprises of a feed water system (click and circle will appear); a steam system (click and 2 circles will appear); as well as a fuel system (click and circle will appear).This is a schematic overview of a boiler room:The feed water system provides water to the boiler and regulates it automatically to meet the steam demand. Various valves provide access for maintenance and repair. The water supplied to the boiler that is converted into steam is called feed water. The two sources of feed water are:(1) Condensate or condensed steam returned from the processes and(2) Makeup water (treated raw water) which must come from outside the boiler room and plant processes. For higher boiler efficiencies, an economizer preheats the feed water using the waste heat in the flue gas.The steam system collects and controls the steam produced in the boiler. Steam is directed through a piping system to the point of use. Throughout the system, steam pressure is regulated using valves and checked with steam pressure gauges.The fuel system includes all equipment used to provide fuel to generate the necessary heat. The equipment required in the fuel system depends on the type of fuel used in the system.VENTBURNERWATER SOURCEBLOW DOWN SEPARATORFUELBRINECHEMICAL FEEDSOFTENERSFigure: Schematic overview of a boiler room
7IntroductionType of boilersAssessment of a boilerEnergy efficiency opportunitiesThermal Equipment/Boilers
8What Type of Boilers Are There? Types of BoilersWhat Type of Boilers Are There?Fire Tube BoilerWater Tube BoilerPackaged BoilerFluidized Bed (FBC) BoilerStoker Fired BoilerPulverized Fuel BoilerWaste Heat BoilerThermal Equipment/BoilersThere are different types of boilers based on different fuels and with various capacities.(Questions to audience) What type of boilers do you know of? What kind of boilers do you use in the industry where you work? (Discussion)(Click once and boiler types will appear) We will look closer at the following types of boilers: Fire Tube Boiler, Water Tube Boiler, Packaged Boiler, Fluidized Bed Boiler, Stoker Fired Boiler, Pulverized Fuel Boiler and Waste Heat Boiler.
9Type of Boilers 1. Fire Tube Boiler Thermal Equipment/ Boilers Relatively small steam capacities (12,000 kg/hour)Low to medium steam pressures (18 kg/cm2)Operates with oil, gas or solid fuelsThermal Equipment/BoilersTo begin with, we will look at the fire tube boiler:This is generally used for relatively small steam capacities and at low to medium steam pressures.The steam rates for fire tube boilers are up to 12,000 kg/hour with pressures of 18 kg/cm2.Fire tube boilers can operate on oil, gas or solid fuels.The figure illustrates how a fire tube boiler works:The fuel is burned and heats up the water to steam which is turn channeled to the process. Today, most fire tube boiler are in a packaged construction for all fuels.(Light Rail Transit Association)
12Type of Boilers 2. Water Tube Boiler Thermal Equipment/ Boilers Used for high steam demand and pressure requirementsCapacity range of 4,500 – 120,000 kg/hourCombustion efficiency enhanced by induced draft provisionsLower tolerance for water quality and needs water treatment plantThermal Equipment/BoilersIn a water tube boiler, boiler feed water flows through the tubes and enters the boiler drum. The circulated water is heated by the combustion gases and converted into steam at the vapour space in the drum. These boilers are selected when the steam demand as well as steam pressure requirements are high as in the case of process cum power boiler / power boilers.Most modern water boiler tube designs are within the capacity range 4,500 – 120,000 kg/hour of steam, at very high pressures. Many water tube boilers are of “packaged” construction if oil and /or gas are to be used as fuel. Solid fuel fired water tube designs are available but packaged designs are less common.The features of water tube boilers are:Forced, induced and balanced draft provisions help to improve combustion efficiency.Less tolerance for water quality calls for water treatment plant.Higher thermal efficiency levels are possible(Your Dictionary.com)
13Type of Boilers 3. Packaged Boiler Thermal Equipment/ Boilers Comes in complete packageFeaturesHigh heat transferFaster evaporationGood convective heat transferGood combustion efficiencyHigh thermal efficiencyClassified based on number of passesOil BurnerTo ChimneyThermal Equipment/BoilersThe packaged boiler is so called because it comes as a complete package. Once delivered to a site, it requires only the steam and water pipe work, fuel supply and electrical connections to be made to become operational.These boilers are classified based on the number of passes - the number of times the hot combustion gases pass through the boiler.More specifically, it is a typical 3 pass, oil fired packaged boiler.This is a packaged boiler.Does anyone recognize what type of boiler this is? (Click once and name will appear)Package boilers are generally of a shell type with a fire tube design so as to achieve high heat transfer rates by both radiation and convection.The features of packaged boilers are:Small combustion space and high heat release rate resulting in faster evaporation.Large number of small diameter tubes leading to good convective heat transfer.Forced or induced draft systems resulting in good combustion efficiency.Number of passes resulting in better overall heat transfer.Higher thermal efficiency levels compared with other boilers.(BIB Cochran, 2003)
144. Fluidized Bed Combustion (FBC) Boiler Type of Boilers4. Fluidized Bed Combustion (FBC) BoilerParticles (e.g. sand) are suspended in high velocity air stream: bubbling fluidized bedCombustion at 840° – 950° CFuels: coal, washery rejects, rice husk, bagasse and agricultural wastesBenefits: compactness, fuel flexibility, higher combustion efficiency, reduced SOx & NOxThermal Equipment/BoilersThe fluidized bed boilersWhen an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – 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”.The fuels burnt in these boilers can include coal, washery rejects; rice husk and bagasse (sugar cane wastes) & other agricultural wastes.The fluidized bed boilers have a wide capacity range- 0.5 T/hr to over 100 T/hr.The fluidized bed combustion (FBC) takes place at about 840oC to 950oC.Fluidized bed combustion (FBC) has emerged as a viable alternative and has significant advantages over a conventional firing system and offers multiple benefits:compact boiler designfuel flexibilityhigher combustion efficiencyreduced emission of noxious air pollutants such as SOx (Sulfur oxides) and Nox (Nitrous oxides), both of which contribute to “acid rain”.Three types of FBC boilers are explained on the next slides,
15Type of Boilers 5. Stoke Fired Boilers a) Spreader stokers Coal is first burnt in suspension then in coal bedFlexibility to meet load fluctuationsFavored in many industrial applicationsThermal Equipment/BoilersA type of FBC boiler: Stoker fired boilers are classified according to the method of feeding fuel to the furnace and by the type of grate.main classifications of stokers are: “spreader stoker”“chain-grate or traveling-grate stoker”.To begin with we will look at spreader stokers. These stokers utilize a combination of suspension burning and grate burning.Spreader stokers utilize a combination of suspension burning and grate burning. The coal is continually fed into the furnace above a burning bed of coal. The coal fines are burned in suspension; the larger particles fall to the grate, where they are burned in a thin, fast-burning coal bed.This method of firing provides good flexibility to meet load fluctuations, since ignition is almost instantaneous when the firing rate is increased. Due to this, the spreader stoker is favored over other types of stokers in many industrial applications.
16Type of Boilers 5. Stoke Fired Boilers b) Chain-grate or traveling-grate stokerCoal is burnt on moving steel grateCoal gate controls coal feeding rateUniform coal size for complete combustionThermal Equipment/BoilersA type of FBC boiler: This picture illustrates a chain grate or traveling grate stoker.Coal is fed onto one end of a moving steel grate. As the grate moves along the length of the furnace, the coal burns before dropping off at the end as ash.The coal-feed hopper runs along the entire coal-feed end of the furnace. A coal gate is used to control the rate at which coal is fed into the furnace by controlling the thickness of the fuel bed.Coal must be uniform in size as large lumps will not burn out completely by the time they reach the end of the grate.(University of Missouri, 2004)
176. Pulverized Fuel Boiler Type of Boilers6. Pulverized Fuel BoilerPulverized coal powder blown with combustion air into boiler through burner nozzlesThermal Equipment/BoilersCombustion temperature at °CBenefits: varying coal quality coal, quick response to load changes and high pre-heat air temperaturesA type of FBC boiler: Pulverized Fuel BoilerOne of the most popular systems for firing pulverized coal is the tangential firing using four burners corner to corner to create a fireball at the center of the furnace. This is shown in the figure.The coal is pulverized to a fine powder until less than 2% of the coal is +300 micro meter and 70-75% is below 75 microns for bituminous coal.The pulverized coal is then blown with part of the combustion air into the boiler plant through a series of burner nozzles.The combustion takes place at temperatures ranging between degrees Celsius depending mainly on the coal grade. The particle residence time in the boiler is typically 2 to 5 seconds and the particles have to be small enough to be completely combusted during this time periodThis system has many advantages :ability to fire varying quality of coalquick responses to changes in loaduse of high pre-heat air temperatures etc.Tangential firing
18Independence Steam Electric Station - Newark, AR Facility DetailsPlant Name: Independence Steam Electric Station City: Newark, ArkansasOperation: Unit 1 - January 1983/Unit 2 - December Fuel: Low-sulfur coal mined near Gillette, WyomingCapability: 1,678 megawattsOperation: Unit 1 - January 1983/Unit 2 - December 1984Fuel: Low-sulfur coal mined near Gillette, Wyoming Capability: 1,678 megawatts
19Type of Boilers 7. Waste Heat Boiler Thermal Equipment/ Boilers Used when waste heat available at medium/high tempAuxiliary fuel burners used if steam demand is more than the waste heat can generateUsed in heat recovery from exhaust gases from gas turbines and diesel enginesThermal Equipment/BoilersA Waste heat boiler can be economically installed wherever waste heat can be available at medium or high temperatures.Wherever the steam demand is more than the steam generated during waste heat, auxiliary fuel burners are also used. If there is no direct use of steam, the steam may be let down in a steam turbine-generator set and power produced from it.widely used in the heat recovery from exhaust gases from gas turbines and diesel engines.Agriculture and Agri-Food Canada, 2001
20Training Agenda: Boiler IntroductionType of boilersBoiler AssesmentEnergy efficiency opportunitiesThermal Equipment/BoilersAssessment of a boiler.
21Boiler Assessment Boiler Boiler blow down Boiler feed water treatment Thermal Equipment/BoilersWe will go through three topics under the assessment of a boiler:Assessment of the boiler itselfBoiler blow downBoiler feed water treatment
22Assessment of a Boiler 1. Boiler performance Causes of poor boiler performancePoor combustionHeat transfer surface foulingPoor operation and maintenanceDeteriorating fuel and water qualityHeat balance: identify heat lossesBoiler efficiency: determine deviation from best efficiencyThermal Equipment/BoilersThe performance parameters of a boiler, like efficiency and evaporation ratio, reduces with time due to poor combustion, heat transfer surface fouling and poor operation and maintenance.Even for a new boiler, reasons such as deteriorating fuel quality and water quality can result in poor boiler performance.We will now discuss heat balance and boiler efficiency, which are important in assessing the boiler performance:A heat balance helps us to identify avoidable and unavoidable heat losses.Boiler efficiency tests help us to find out the deviation of boiler efficiency from the best efficiency and target problem area for corrective action.
23Assessment of a Boiler Heat Balance Thermal Equipment/ Boilers An energy flow diagram describes geographically how energy is transformed from fuel into useful energy, heat and lossesThermal Equipment/BoilersStochiometricExcess AirUn burntFUEL INPUTSTEAM OUTPUTStack GasAsh and Un-burnt parts of Fuel in AshBlow DownConvection & RadiationWe will start with looking at heat balance.The combustion process in a boiler can be described in the form of an energy flow diagram.This shows graphically how the input energy from the fuel is transformed into the various useful energy flows and into heat and energy loss flows.The thickness of the arrows indicates the amount of energy contained in the respective flows
24Assessment of a Boiler Heat Balance Thermal Equipment/ Boilers Balancing total energy entering a boiler against the energy that leaves the boiler in different formsThermal Equipment/Boilers12.7 %BOILERHeat loss due to dry flue gas8.1 %Heat loss due to steam in fuel gas100.0 %1.7 %Heat loss due to moisture in fuelA Heat balance is an attempt to balance the total energy that enters a boiler against the energy that leaves it.This figure illustrates the different typical losses that occurs while generating steam.(Question) Does anyone have any suggestions of what the two major heat losses are? (Discussion)(Click once and answer reveals) They are dry fly gas that represents a heat loss of 12.7% and heat loss as a result of steam in the flue gas of 8.1%.(Click once for other heat losses to appear) Other heat losses are due to moisture in the fuel and in the air, as well as unburnts in residue and radiation.(Click once) This leaves 73.8% of heat that goes to steam generation.Fuel0.3 %Heat loss due to moisture in air2.4 %Heat loss due to unburnts in residue1.0 %Heat loss due to radiation & other unaccounted loss73.8 %73.8 %Heat in Steam
25Assessment of a Boiler Heat Balance Thermal Equipment/ Boilers Goal: improve energy efficiency by reducing avoidable lossesAvoidable losses include:Stack gas losses (excess air, stack gas temperature)Losses by unburnt fuelBlow down lossesCondensate lossesConvection and radiationThermal Equipment/BoilersThe goal of a Cleaner Production and/or energy assessment must be to reduce the avoidable losses, i.e. to improve energy efficiency.The following losses can be avoided or reduced:Stack gas losses: Excess air (reduce to the necessary minimum which depends from burner technology, operation, operation (i.e. control) and maintenance) and Stack gas temperature (reduce by optimizing maintenance (cleaning), load; better burner and boiler technology).Losses by unburnt fuel in stack and ash (optimize operation and maintenance; better technology of burner).Blow down losses (treat fresh feed water, recycle condensate)Condensate losses (recover the largest possible amount of condensate)Convection and radiation losses (reduced by better insulation of the boiler).
26Assessment of a Boiler 1. Boiler Efficiency Thermal Equipment/ Boilers Thermal efficiency: % of (heat) energy input that is effectively useful in the generated steamThermal Equipment/BoilersWe will now look at boiler efficiency:Thermal efficiency of a boiler is defined as: the percentage of heat energy input that is effectively useful in the generated steamWe will look at the direct method of calculating boiler efficiency, using steam characteristics in a future Thermodynamics class.
27Boiler efficiency () = hf /hg Assessment of a BoilerBoiler Efficiency: Direct MethodBoiler efficiency () = hf /hgHeat Input * 100%Heat OutputThermal Equipment/BoilersThere are two different methods to assess boiler efficiency.(Click once) They are direct and indirect methods.(Click once) In the direct method, the energy gain of the working fluid, that is the water and steam, is compared to the energy content of the boiler fuel.(Click once) In the indirect method, the efficiency is calculated as the difference between the losses and energy input.The direct method of determining boiler efficiency is also known as the “input-output” method. This is because it only needs the useful output, which is steam, and the heat input, which is fuel, in order to evaluate the efficiency.The efficiency is evaluated by using this formula where hg is the enthalpy of saturated steam and hf is the enthalpy of feed water.(Click once) The parameters to be monitored for the calculation of boiler efficiency through the direct method are: Quantity of steam generated per hour; the quantity of fuel used per hour; the working pressure and superheat temperature if any; the temperature of feed water; the type of fuel and gross calorific value, GVC, of the fuel.
28Assessment of a Boiler 2. Boiler Blow Down Thermal Equipment/ Boilers Controls ‘total dissolved solids’ (TDS) in the water that is boiledBlows off water and replaces it with feed waterConductivity measured as indication of TDS levelsCalculation of quantity blow down required:Thermal Equipment/BoilersWhen water is boiled and steam is generated, any dissolved solids contained in the water remain in the boiler. Above a certain level of concentration, these solids encourage foaming and cause carryover of water into the steam. The deposits also lead to scale formation inside the boiler, resulting in localized overheating and finally causing boiler tube failure.The control of total dissolved solids (TDS) is achieved by 'blowing down‘: a certain volume of water is “blown off” and is automatically replaced by feed waterSince it is tedious and time consuming to measure TDS in a boiler water system, conductivity measurement is used for monitoring the overall TDS present in the boiler. A rise in conductivity indicates a rise in the "contamination" of the boiler water.(Click once) The quantity of blow down required to control boiler water solids concentration is calculated by using the following formula:Blow down in percentage = feed water TDS x Make up watermaximum permissible TDS in boiler waterBlow down (%) =Feed water TDS x % Make up waterMaximum Permissible TDS in Boiler water
29Assessment of a Boiler Boiler Blow Down Two types of blow down IntermittentManually operated valve reduces TDSLarge short-term increases in feed waterSubstantial heat lossContinuousEnsures constant TDS and steam purityHeat lost can be recoveredCommon in high-pressure boilersThermal Equipment/BoilersThere are two methods for blowing down the boiler, they are intermittent and continuous (click once).Intermittent blow down is given manually by operating a valve that is fitted to discharge pipe at the lowest point of boiler shell. This is to reduce the total dissolved solids as well as conductivity, pH, silica and phosphates without affecting the steam quality.Intermittent blow down requires large short-term increases in the amount of feed water put into the boiler and might therefore require large feed water pumps. It should also be noted that substantial amounts of heat energy are lost during an intermittent blow down.(Click once) A continuous blow down ensures constant TDS and steam purity at given steam load through a steady and constant dispatch of small stream of concentrated boiler water that is replaced by a steady and constant inflow of water. Once blow down valve has been set for certain conditions it does not require further operation interventions. Large quantities of heat is wasted but this can be recovered by blowing into a flash tank and generating flash steam. This type of blow down is common in high-pressure boilers.
30Assessment of a Boiler Boiler Blow Down Benefits Lower pretreatment costsLess make-up water consumptionReduced maintenance downtimeIncreased boiler lifeLower consumption of treatment chemicalsThermal Equipment/BoilersGood boiler blow down control can significantly reduce treatment and operational costs that include:Lower pretreatment costsLess make-up water consumptionReduced maintenance downtimeIncreased boiler lifeLower consumption of treatment chemicals
313. Boiler Feed Water Treatment Assessment of a Boiler3. Boiler Feed Water TreatmentQuality of steam depend on water treatment to controlSteam purityDepositsCorrosionEfficient heat transfer only if boiler water is free from deposit-forming solidsThermal Equipment/BoilersBoiler Feed Water TreatmentProducing quality steam on demand depends on properly managed water treatment to control steam purity, deposits and corrosion. A boiler is the sump of the boiler system. It ultimately receives all of the pre-boiler contaminants.Boiler performance, efficiency, and service life are direct products of selecting and controlling feed water used in the boiler.The boiler water must be sufficiently free of deposit forming solids to allow rapid and efficient heat transfer and it must not be corrosive to the boiler metal.
32Boiler Feed Water Treatment Assessment of a BoilerBoiler Feed Water TreatmentDeposit controlTo avoid efficiency losses and reduced heat transferHardness salts of calcium and magnesiumAlkaline hardness: removed by boilingNon-alkaline: difficult to removeSilica forms hard silica scalesThermal Equipment/BoilersDeposits and corrosion result in efficiency losses and may result in boiler tube failures and inability to produce steam.Deposits also act as insulators and therefore slow heat transfer. Different types of deposits affect the boiler efficiency differently why it may be useful to analyze the deposits for their characteristics.The most important chemicals in water that influence the formation of deposits in the boilers are the salts of calcium and magnesium. These are known as hardness salts.Calcium and magnesium bicarbonate dissolve in water to form an alkaline solution and are therefore known as alkaline hardness that can be removed by boiling.Calcium and magnesium sulphates, chlorides and nitrates etc., when dissolved in water, are chemically neutral and are known as non-alkaline hardness. These are called permanent hardness chemicals and form hard scales on boiler surfaces, which are difficult to remove.Silica in boiler water can rise to the formation of hard silicate scales. Silica can also associate with calcium and magnesium salts and form calcium and magnesium silicates of very low thermal conductivity. Silica can also give rise to deposits on steam turbine blades.
33Boiler Feed Water Treatment Assessment of a BoilerBoiler Feed Water TreatmentInternal water treatmentChemicals added to boiler to prevent scaleDifferent chemicals for different water typesConditions:Feed water is low in hardness saltsLow pressure, high TDS content is toleratedSmall water quantities treatedInternal treatment alone not recommendedThermal Equipment/BoilersThere are two major types of boiler water treatment, namely internal and external water treatment.We will first explain internal water treatment.Internal treatment involves adding chemicals to a boiler to prevent the formation of scale. Scale-forming compounds are converted to free-flowing sludge, which can be removed by blow down.This method is limited to boilers, wherefeed water is low in hardness saltswhere low pressure, high TDS content in boiler water is toleratedwhen only a small quantity of water is required to be treated.If these conditions are not met, then high rates of blow down are required to dispose off the sludge. They become uneconomical considering heat and water loss.Different types of water sources require different chemicals.Internal treatment alone is not recommended.
34Boiler Feed Water Treatment Assessment of a BoilerBoiler Feed Water TreatmentExternal water treatment:Removal of suspended/dissolved solids and dissolved gasesPre-treatment: sedimentation and settlingFirst treatment stage: removal of saltsProcessesIon exchangeDemineralizationDe-aerationReverse osmosesThermal Equipment/BoilersExternal treatment is used to remove suspended solids, dissolved solids (particularly the calcium and magnesium ions which are major a cause of scale formation) and dissolved gases (oxygen and carbon dioxide).Before any of these are used, it is necessary to remove suspended solids and color from the raw water, because these may foul the resins used in the subsequent treatment sections.Methods of pre-treatment include:Simple sedimentation in settling tanks or settling in clarifiers with aid of coagulants and flocculants.Pressure sand filters, with spray aeration to remove carbon dioxide and iron, may be used to remove metal salts from bore well water.The first stage of treatment is to remove hardness salt and possibly non-hardness salts. Removal of only hardness salts is called softening, while total removal of salts from solution is called demineralization.Other external treatment processes used are:Ion exchangeDe-aeration (mechanical and chemical)Reverse osmosis
35External Water Treatment Assessment of a BoilerExternal Water Treatmenta) Ion-exchange process (softener plant)Water passes through bed of natural zeolite of synthetic resin to remove hardnessBase exchange: calcium (Ca) and magnesium (Mg) replaced with sodium (Na) ionsDoes not reduce TDS, blow down quantity and alkalinityb) DemineralizationComplete removal of saltsCations in raw water replaced with hydrogen ionsThermal Equipment/BoilersIn the ion-exchange process, the hardness is removed when the water passes through a bed of natural zeolite or synthetic resin and without the formation of any precipitate. The simplest way to remove hardness through ion exchanges is ‘base exchange’ in which calcium and magnesium ions are exchanged for sodium ions. Since the base exchanger only replaces the calcium and magnesium with sodium, it does not reduce the TDS content, and blow down quantity. It also does not reduce the alkalinity.Demineralization is the complete removal of all salts. This is achieved by using a “cation” resin that exchanges the cations in the raw water with hydrogen ions, producing hydrochloric, sulphuric and carbonic acid. Carbonic acid is removed in degassing tower in which air is blown through the acid water. Following this, the water passes through an “anion” resin which exchanges anions with the mineral acid and forms water. Regeneration of cations and anions is necessary at intervals using, typically, mineral acid and caustic soda respectively. The complete removal of silica can be achieved by correct choice of anion resin.Ion exchange processes can be used for almost total demineralization if required, as is the case in large electric power plant boilers.
36External Water Treatment Assessment of a BoilerExternal Water Treatmentc) De-aerationDissolved corrosive gases (O2, CO2) expelled by preheating the feed waterTwo types:Mechanical de-aeration: used prior to addition of chemical oxygen scavangersChemical de-aeration: removes trace oxygenThermal Equipment/BoilersIn de-aeration the dissolved gases such as oxygen and carbon dioxide are expelled by preheating the feed water before in enters the boiler. As all natural waters contain dissolved gases in solution such as carbon dioxide and oxygen, these are released as gases when heated and combine with water to form carbonic acid. This way, two very corrosive gases are removed.The removal of non-condensable gases from the boiler feed water is essential to the longevity to the boiler equipment and also to the safety of the operation.De-aeration can be done through either mechanical or chemical de-aeration processes or both together.
37Training Agenda: Boiler IntroductionType of boilersAssessment of a boilerEnergy efficiency opportunitiesThermal Equipment/BoilersWe will go through different energy efficiency opportunities for boilers.
38Energy Efficiency Opportunities Stack temperature controlFeed water preheating using economizersCombustion air pre-heatingIncomplete combustion minimizationExcess air controlAvoid radiation and convection heat lossAutomatic blow down controlReduction of scaling and soot lossesReduction of boiler steam pressureVariable speed controlControlling boiler loadingProper boiler schedulingBoiler replacementThermal Equipment/BoilersThere are numerous areas for energy efficiency improvements for boilers. We will briefly explain these 14 areas.
39Energy Efficiency Opportunities 1. Stack Temperature ControlKeep as low as possibleIf >200°C then recover waste heatThermal Equipment/Boilers2. Feed Water Preheating EconomizersPotential to recover heat from 200 – 300 oC flue gases leaving a modern 3-pass shell boilerThe stack temperature should be as low as possible. However, it should not be so low that water vapor in the exhaust condenses on the stack walls. This is important in fuels containing significant sulphur as low temperature can lead to sulphur dew point corrosion. Stack temperatures greater than 200°C indicates potential for recovery of waste heat. It also indicates the scaling of heat transfer/recovery equipment and hence the urgency of taking an early shut down for water / flue side cleaning.(click once) Typically, the flue gases leaving a modern 3-pass shell boiler are at temperatures of 200 to 300 oC. Thus, there is a potential to recover heat from these gases. The potential for energy savings depends on the type of boiler installed and the fuel used.(click once) Combustion air preheating is an alternative to feed water heating. In order to improve thermal efficiency by 1%, the combustion air temperature must be raised by 20 oC. Most gas and oil burners used in a boiler plant are not designed for high air-preheat temperatures.3. Combustion Air PreheatingIf combustion air raised by 20°C = 1% improve thermal efficiency
40Energy Efficiency Opportunities 4. Minimize Incomplete CombustionSymptoms:Smoke, high CO levels in exit flue gasCauses:Air shortage, fuel surplus, poor fuel distributionPoor mixing of fuel and airOil-fired boiler:Improper viscosity, worn tops, cabonization on dips, deterioration of diffusers or spinner platesCoal-fired boiler: non-uniform coal sizeThermal Equipment/BoilersIncomplete combustion can arise from a shortage of air, surplus of fuel or poor distribution of fuel. You can identify it from the color of the smoke. A quite frequent cause of incomplete combustion is the poor mixing of fuel and air at the burner.The root cause can beFor oil fires it can be the result from improper viscosity, worn tips, carbonization on tips and deterioration of diffusers or spinner plates.For coal firing, non uniform fuel size could be one of the reasons for incomplete combustion. In chain grate stokers, large lumps will not burn out completely, while small pieces and fines may block the air passage, thus causing poor air distribution. In sprinkler stokers, stoker grate condition, fuel distributors, wind box air regulation and over-fire systems can affect carbon loss. Increase in the fines in pulverized coal also increases carbon loss.
41Energy Efficiency Opportunities 5. Excess Air ControlExcess air required for complete combustionOptimum excess air levels varies1% excess air reduction = 0.6% efficiency risePortable or continuous oxygen analyzersThermal Equipment/BoilersExcess air is required in all practical cases to ensure complete combustion, to allow for the normal variations in combustion and to ensure satisfactory stack conditions for some fuels.The optimum excess air level varies with furnace design, type of burner, fuel and process variables. It can be determined by conducting tests with different air fuel ratios.Controlling excess air to an optimum level always results in reduction in flue gas losses; for every 1 percent reduction in excess air there is approximately 0.6 percent rise in efficiency.Various methods are available to control the excess air:Portable oxygen analyzers and draft gauges can be used to make periodic readings to guide the operator to manually adjust the flow of air for optimum operation. Excess air reduction up to 20 percent is feasible.The most common method is the continuous oxygen analyzer with a local readout mounted draft gauge, by which the operator can adjust air flow. A further reduction of percent can be achieved over the previous system.The same continuous oxygen analyzer can have a remote controlled pneumatic damper positioner, by which the readouts are available in a control room. This enables an operator to remotely control a number of firing systems simultaneouslyFuelKg air req./kg fuel%CO2 in flue gas in practiceSolid FuelsBagasseCoal (bituminous)LignitePaddy HuskWood3.310.78.54.55.710-1210-139 -1314-1511.13Liquid FuelsFurnace OilLSHS13.814.19-14
42Energy Efficiency Opportunities 6. Radiation and Convection Heat Loss MinimizationFixed heat loss from boiler shell, regardless of boiler outputRepairing insulation can reduce lossThermal Equipment/BoilersThe external surfaces of a shell boiler are hotter than the surroundings. Therefore, the surfaces lose heat to the surroundings depending on the surface area and the difference in temperature between the surface and the surroundings. The heat loss from the boiler shell is normally a fixed energy loss, irrespective of the boiler output. Repairing or augmenting insulation can reduce heat loss through boiler walls and piping.(click once) Uncontrolled continuous blow down is very wasteful. Automatic blowdown controls can be installed that sense and respond to boiler water conductivity and pH.7. Automatic Blow Down ControlSense and respond to boiler water conductivity and pH
43Energy Efficiency Opportunities 8. Scaling and Soot Loss ReductionEvery 22oC increase in stack temperature = 1% efficiency loss3 mm of soot = 2.5% fuel increaseThermal Equipment/Boilers9. Reduced Boiler Steam PressureIn oil and coal-fired boilers, soot buildup on tubes acts as an insulator against heat transfer. Any such deposits should be removed on a regular basis. An estimated 1per cent efficiency loss occurs with every 22oC increase in stack temperature. Therefore, stack temperature should be checked and recorded regularly as an indicator of soot deposits. It is also estimated that 3 mm of soot can cause an increase in fuel consumption by 2.5per cent due to increased flue gas temperatures. Periodic off-line cleaning of radiant furnace surfaces, boiler tube banks, economizers and air heaters may be necessary to remove stubborn deposits.(click once) Reduction of boiler steam pressure is an effective means of reducing fuel consumption by as much as 1 to 2 per cent. Lower steam pressure gives a lower saturated steam temperature and without stack heat recovery, a similar reduction in the temperature of the flue gas is obtained. Steam is generated at pressures normally dictated by the highest pressure and temperature requirements for a particular process.Lower steam pressure= lower saturated steam temperature= lower flue gas temperatureSteam generation pressure dictated by process
44Energy Efficiency Opportunities 10. Variable Speed Control for Fans, Blowers and PumpsSuited for fans, blowers, pumpsShould be considered if boiler loads are variableThermal Equipment/BoilersVariable speed control is an important means of achieving energy savings. Generally, combustion air control is affected by throttling dampers fitted at forced and induced draft fans. Though dampers are simple means of control, they lack accuracy, giving poor control characteristics at the top and bottom of the operating range. In general, if the load characteristic of the boiler is variable, the possibility of replacing the dampers by a VSD should be evaluated.(click once) The maximum efficiency of the boiler does not occur at full load, but at about two-thirds of the full load. In general, efficiency of the boiler reduces significantly below 25per cent of the rated load and operation of boilers below this level should be avoided as far as possible.11. Control Boiler LoadingMaximum boiler efficiency: 65-85% of rated loadSignificant efficiency loss: < 25% of rated load
45Energy Efficiency Opportunities 12. Proper Boiler SchedulingOptimum efficiency: 65-85% of full loadFew boilers at high loads is more efficient than large number at low loadsThermal Equipment/Boilers13. Boiler ReplacementSince, the optimum efficiency of boilers occurs at percent of full load, it is usually more efficient, on the whole, to operate a fewer number of boilers at higher loads, than to operate a large number at low loads.(click once) The potential savings from replacing a boiler depend on the anticipated change in overall efficiency. A change in a boiler can be financially attractive if the existing boiler is old and inefficient; not capable of firing cheaper substitution fuel; over or under-sized for present requirements; and not designed for ideal loading conditions.Financially attractive if existing boiler isOld and inefficientNot capable of firing cheaper substitution fuelOver or under-sized for present requirementsNot designed for ideal loading conditions
46Boilers & Thermic Fluid Heaters Training Session on Energy EquipmentBoilers & Thermic Fluid HeatersTHANK YOUFOR YOUR ATTENTIONThermal Equipment/Boilers