industrial boilers

BOILERS

What is a Boiler?Vessel that heats water to become hot water or steamAt atmospheric pressure, water volume increases 1,600 times when it gets converted to steam

Industrial BoilersSteam generatorsClosed vesselsmade of steel and used for generation of steam by vaporizing watercombustion of fuels like coal, coke, oil, wood, saw dust Steam used for producing powerindustrial process work for heating proposes.

BURNER

WATER SOURCEBRINESOFTENERSCHEMICAL FEEDFUELBLOW DOWN SEPARATORVENTVENTEXHAUST GASSTEAM TO PROCESSSTACKDEAERATORPUMPSFigure: Schematic overview of a boiler room

BOILER

ECO-NOMI-ZER

Typical Coal-Fired Power Plant

Typical Pulverized Fuel Boiler

Typical Coal-Fired Power Plant

Requirements of a Perfect Steam BoilersShould be absolutely reliable,capable of producing maximum steam at required pressure for minimum of fuel consumptionminimum initial cost Minimum operating costSimple constructionQuick starting from coldCan withstand stresses & strainsWell designed combustion chamberLess floor area, space & light weightBest gauges, safety valves and other mountingsEasily accessible for inspection,cleaning and repairs

Coal Rank

What is Peat?Peat coal, but is the initial stage in coal formation

A dark colored, brown to black, spongy substance formed from partial decay of marsh vegetation by moisture and bacteria

Classification of Steam BoilersVertical or HorizontalStationary, Portable, Locomotive or MarineInternally Fired (Locomotive & Lancashire)Externally Fired (Babcock & Wilcox)Solid,Liquid or Gas Fired Fire (Smoke)TubeWater is outside the tubes while the hot gases are inside the tubes Eg.Vertical,Cochran,Lancashire,Cornish,Locomotive Boiler Water TubeContain a large number of small tubes through which water circulates,the fire and hot gases being outside of the tubesEg.Babcock and Wilcox, Stirling and High pressure Boilers

Vertical BoilersVertical Boilers are simple (having one to three cross tubes)Fire tube or water tubeUsed in small factoriesRequired minimum floor spaceNo elaborate foundationPortableCheapEasy to start Inefficient due to large wastage of fuel and less heating surface.

Fire Tube Boiler (~ 1800)Relatively small steam capacities (12,000 kg/hour)Low to medium steam pressures (18 kg/cm2)Operates with oil, gas or solid fuels

The fire tube boiler, the oldest design, is made so the products of combustion pass through tubes surrounded by water in a shell.

Fire Tube Boiler

Fire Tube BoilerThe fire tube boiler, the oldest design, is made so the products of combustion pass through tubes surrounded by water in a shell.The furnace/flame volume can either be inside or external to the shell that contains the water.The upper steam capacity of fire tube boilers is about 20,000 lbm/hr, and the peak pressure obtainable is limited by their large shells to about 300 psi.Fire-tube boilers are used for heating systems.

Water Tube Boiler (1867)Used for high steam demand and pressure requirements Capacity range of 4,500 120,000 kg/hourCombustion efficiency enhanced by induced draft provisionsLower tolerance for water quality and needs water treatment plantA water-tube boiler is one in which the products of combustion pass around the outside and heattubes containing the water.

Water Tube Boiler

Water Tube BoilerA water-tube boiler is one in which the products of combustion pass around the outside and heat tubes containing the water.The water tube diameter is much smaller than the shell diameter of a fire-tube boiler, so much higher pressures can be obtained, well over 2000 psi.The furnace and boiler tube area must be surrounded by a heavily insulated refractory wall to prevent heat transfer through the boiler walls.The refractory lining is a high maintenance item.

Simple Cross tube vertical boilerconsists of a cylindrical shell surrounding a cylindrical fire box.heating surface is about 8-10 times the grate area.50% efficiency

Cochran BoilerIt is well designed Vertical fire tube (multitube) boilerImprovement over the simple vertical boiler as it provides greater heating surface.Total heating surface area is about 10-25 times the grate areaEfficiency 70-75%Ranges from 1m diameter x 2m high (evaporation 20kg/hr) to 3m diameter x 6m high (evaporation 3000kg/hr)

Cochran Boiler contd.

Packaged Boiler (Cochran)Comes in complete packageFeaturesHigh heat transferFaster evaporation Good convective heat transferGood combustion efficiencyHigh thermal efficiencyClassified based on number of passes

Lancashire BoilerIs a horizontal fire tube boiler Size ranges from a shell 2m diameter x 6m long to 3m diameter x 10m longWorking pressure range are up to 20 barRatio of heating surface to grate area is 24-30Efficiency is about 56% without economizer and 75% with economizerSimilar to Cornish boiler ; in Lancashire Boiler two flue tubes but in Cornish boiler only one flue tube is present

Lancashire Boiler contd

Locomotive BoilerIs a internally fired fire tube (multitube) boiler

Babcock and Wilcox water tube BoilerWater tube boiler having larger pressure ranges and larger sizes.Three main parts : steam and water drum, water tubes and furnace.

Stirling BoilerExample of water tube boiler where bent tube are used instead of straight tube which are used in Babcock and Wilcox boiler

Waterwall BoilerAll large and many intermediate-sized boilers are water-tube boiler with a boiler section that consists of closely-spaced water tubes covering the furnace wall.The waterwall boiler design allows much lighter, less expensive walls by having the waterwalls form an integral part of the boiler wall so that the wall is water cooled.If so equipped, the superheater and reheater are separate sections hanging above the main furnace volume.

Membrane wall of waterwall boiler

Waterwall under construction

High Pressure BoilersTwo types- Natural circulation & Forced circulation Boilers

High Pressure BoilersLa-Mont Boiler (Forced circulation Boiler)

Once-Through Forced Circulation BoilerDoes not required a steam and water drum,saving in weightSupercritical Pressure BoilerTwo types- Benson and Ramzin

Benson Boiler

Ramzin Boiler

Loeffler BoilerIndirect heating is combined with forced circulation

Velox boiler (Fire tube forced circulation Boiler)

Fire tubes Vs water tubes BoilersFire tubes boilers has a large volume of water,therefore more flexible and can meet the sudden demand of steam without much drop of pressure.Fire tubes boiler is rigid and of simple mechanical construction, so greater reliability and low in first cost.Fire tube boilers can be made in smallest sizes; therefore simple to fabricate and transport, occupies less floor space but more height.As water tube boilers are externally fired, furnace can be altered considerably to meet the fuel requirements.Water tubes boilers are more readily accessible for cleaning, inspection and repairs,compared to the fire tube boilers.Modern trend is in the favors of water tube boiler due to continuous increase in capacities and steam pressures.

What Type of Boilers Are There?Fire Tube BoilerWater Tube BoilerPackaged BoilerStoker Fired BoilerWaste Heat BoilerPulverized Fuel BoilerFluidized Bed (FBC) Boiler

COAL/SOLID FUEL BURNINGThere are a considerable number of ways to feed coal in use, including :hand-fired boilerschain or traveling grate stokersvibrating grate stokersunderfeed stokersspreader stokerpulverized coal boilerscyclone boilersfluidized bed boilers

Fire Tube Boiler (~ 1800)Relatively small steam capacities (12,000 kg/hour)Low to medium steam pressures (18 kg/cm2)Operates with oil, gas or solid fuels

The fire tube boiler, the oldest design, is made so the products of combustion pass through tubes surrounded by water in a shell.

Water Tube Boiler (1867)Used for high steam demand and pressure requirements Capacity range of 4,500 120,000 kg/hourCombustion efficiency enhanced by induced draft provisionsLower tolerance for water quality and needs water treatment plantA water-tube boiler is one in which the products of combustion pass around the outside and heattubes containing the water.

Packaged Boiler (Cochran)Comes in complete packageFeaturesHigh heat transferFaster evaporation Good convective heat transferGood combustion efficiencyHigh thermal efficiencyClassified based on number of passes

Stoker BoilersThe term stoker implies a boiler that automatically feeds (or " stokes) the boiler. Stoker coal size is typically 1.25 inches maximum with less than 30% under 0.25 inches.

Stoker Fired Boilersa) Spreader stokersCoal is first burnt in suspension then in coal bedFlexibility to meet load fluctuationsFavored in many industrial applications

Spreader Stokers

Fed by a rotating bladed wheel that throws the coal out over the grate.Spreaders stokers are more expensive than other stokers in small sizes, and are more expensive than pulverized coal boilers in large sizes (over 500,000 lbm/hr of steam) but are very common in the intermediate (large industrial) size range.Compared to previous stokers, more fuel burning occurs in suspension- that is, in the air as the fine particles are slung out over the fuel bed.

Stoker Fired Boilersb) Chain-grate or traveling-grate stokerCoal is burnt on moving steel grateCoal gate controls coal feeding rateUniform coal size for complete combustion

Traveling or Chain GrateStokers

Traveling Grate

Traveling or Chain GrateStokersTraveling or chain grate stokers feed coal out onto a rotating metal belt that supports the fire.Coal is fed from a hopper.Grate speed is automatically controlled to maintain desired steam pressure.Burning progresses as the belt moves from front to back of furnace.Combustion is essentially complete at the back end of belt, and ash is dumped off into an ashpit there.

c) Underfeed Stokers

So named because they use rams to force the coal up underneath the burning fuel bed.Grates are designed to flex up and downs to break up fuel bed and prevent "clinker" formation.Action of feed rams and fuel bed flexing cause fuel to move from front to back of furnace.Underfeed stokers range in size from small home heating boilers to large industrial size.Underfeed stokers are very good at burning high volatile coal with a high turn-down ratio.

Screw fed Single Retort Stoker

Screw Conveyor feeds coal from hopper to retort

Ram fed Single Retort Stoker

Ram fed Stoker

Multiple Retort Underfeed Stoker

d) Water-Cooled Vibrating Grate Boiler

Vibrating Grate Stokers

Waste Heat BoilerUsed 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 engines

Pulverized Fuel Boiler

Pulverized coal powder blown with combustion air into boiler through burner nozzlesCombustion temperature at 1300 -1700 CBenefits: varying coal quality coal, quick response to load changes and high pre-heat air temperatures

Pulverized Fuel Boiler

Pulverized Fuel Boilers

Pulverized coal boilers fire finely powdered coal, typically with an average particle size of about 25m (0.001 in). Coal burns in suspension, like the combustion in an oil- or gas-fired boiler.Coal is pulverized in some type of large millPulverized coal is fired out into the furnace volume using burners that look somewhat like oil or gas burners.

PC vs. Stoker Boilers:Advantages/DisadvantagesAdvantages of PC vs. stoker boilers:much quicker response to changing loadslower excess air/higher efficiencyeasily adaptable to automatic controlcan burn wide variety of coalsDisadvantages of PC vs. stoker boilers:more expensive (at least for smaller capacities)require more skilled personnelrequire better emission control (particulates)require more energy to pulverize fuel

Fluidized Bed Combustion (FBC) BoilerParticles (e.g. sand) are suspended in high velocity air stream: bubbling fluidized bedCombustion at 840 950 CCapacity range 0.5 T/hr to 100 T/hrFuels: coal, washery rejects, rice husk, bagasse and agricultural wastesBenefits: compactness, fuel flexibility, higher combustion efficiency, reduced SOx & NOx

Type of Fluidized Bed BoilersAtmospheric Fluidized Bed Combustion (AFBC) BoilerMost common FBC boiler that uses preheated atmospheric air as fluidization and combustion airPressurized Fluidized Bed Combustion (PFBC)BoilerCompressor supplies the forced draft and combustor is a pressure vesselUsed for cogeneration or combined cycle power generation

Typical Circulating Fluidized-Bed Power Plant

Atmospheric Circulating Fluidized Bed Combustion (CFBC) Boiler(Thermax Babcock and Wilcox)Solids lifted from bed, rise, return to bedSteam generation in convection sectionBenefits: more economical, better space utilization and efficient combustion

Boiler Mountings and AccessoriesFitting and devices which are necessary for the safety and control are knows as boiler mountingsFitting or devices which are provided to increase the efficiency of the boiler and help in the smooth working of the plant are knows as boiler accessories.Fittings which are essential from the safety point of view are as follows:Water level indicatorsSafety valvesCombined high steam and low water safety valveFusible plug

Boiler Mountings and Accessories contd.Fittings which are essential from the control point of view are as follows:Pressure gaugeJunction or stop valveFeed check valveBlow-off cockMan hole and Mud BoxThe important accessories areSuperheaterEconomiserAir-preheaterFeed pump

AIR PREHEATERAir preheaters are installed on all of the worlds fossil fired utility boilers, used particularly in coal-fired power stations

They reduce fuel consumption by around 10%

This equates to savings of around 250MT/yr of coal and of 750MT/yr of CO2 emissions

AIR PREHEATER

FANSCentrifugal fanAxial fan

ROTARY AIR PREHEATER

TUBULAR AIR PREHEATER

Boiler Blow DownControls 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:

Boiler Blow DownTwo types of blow downIntermittentManually operated valve reduces TDSLarge short-term increases in feed waterSubstantial heat lossContinuousEnsures constant TDS and steam purityHeat lost can be recoveredCommon in high-pressure boilers

Boiler Blow DownBenefitsLower pretreatment costs Less make-up water consumption Reduced maintenance downtime Increased boiler life Lower consumption of treatment chemicals

Boiler Feed Water TreatmentQuality of steam depend on water treatment to control Steam purity Deposits CorrosionEfficient heat transfer only if boiler water is free from deposit-forming solids

Boiler 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 scales

Boiler 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 recommended

Boiler Feed Water TreatmentExternal water treatment:Removal of suspended/dissolved solids and dissolved gasesPre-treatment: sedimentation and settlingFirst treatment stage: removal of saltsProcessesIon exchangeDemineralizationDe-aerationReverse osmosis

External 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 ions

External 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 oxygen

External Water TreatmentMechanical de-aerationO2 and CO2 removed by heating feed water Economical treatment processVacuum type can reduce O2 to 0.02 mg/lPressure type can reduce O2 to 0.005 mg/l

External Water Treatment

Chemical de-aerationRemoval of trace oxygen with scavengerSodium sulphite:Reacts with oxygen: sodium sulphateIncreases TDS: increased blow downHydrazineReacts with oxygen: nitrogen + waterDoes not increase TDS: used in high pressure boilers

External Water Treatmentd) Reverse osmosisOsmosisSolutions of differing concentrationsSeparated by a semi-permeable membraneWater moves to the higher concentrationReversed osmosisHigher concentrated liquid pressurizedWater moves in reversed direction

External water treatmentd) Reverse osmosis

Heat BalanceBalancing total energy entering a boiler against the energy that leaves the boiler in different formsHeat in Steam

BOILERHeat loss due to dry flue gas Heat loss due to steam in fuel gas

Heat loss due to moisture in fuelHeat loss due to unburnts in residueHeat loss due to moisture in airHeat loss due to radiation & other unaccounted loss12.7 %8.1 %1.7 %0.3 %2.4 %1.0 %73.8 %100.0 % Fuel73.8 %

U.S. Generation (~ 4000 X 106 MWh in 2003) by Fuel Source

U.S. Capacity Installations (MW)from 1940 to 2003

Technology StatusPulverized CoalSubcritical CyclesMature TechnologyThermal Efficiency ~ 36% (HHV)Little Opportunity for Improvement in EfficiencySupercritical CyclesMature TechnologyThermal Efficiency ~ 39% (HHV)

Technology Status Circulating Fluidized-BedTechnology is Proven & Mature up to 350 MWThermal Efficiency ~ 36% Suppliers Proposing Units >350 MW Supercritical Cycles for Improved Efficiency and Reduced Emissions are Under Development Same Boiler Can Burn Wide Variety of CoalSO2/NOX Control Internal to CFB

Air Pollution ControlsPC BoilersWet or Dry Scrubbers for SO2 ControlSCR (Selective Catalytic Reduction) and SNCR/Low NOX Burners for NOX ControlSorbent Injection for Mercury ControlCFB BoilersSO2 Control with Limestone InjectionLower Temperature Combustion Reduces NOX FormationSNCR for Additional NOX ControlSorbent Injection for Mercury Control

Emissions Comparison500MW Unit; 3% Sulfur coal; 95% Sulfur rem. for PC & CFB;98% Sulfur rem. for IGCCNOX at 0.07 lbs/mmBtu for PC & CFB; 2 ppmvd for IGCC

FUEL COMBUSTIONFuel combustion time is mainly dominated by the combustion reaction velocity and the rate at which oxygen is supplied into the reaction zone.The combustion reaction velocity depends on chemical characteristics of the fuel.Main technical factors that affect the combustion time are:Combustion characteristics of the fuel.Mixing characteristics.Fluid flow characteristics of the furnace.The combustion velocity of an oil fuel droplet is generally less than 0.1 msec.In the case of coal combustion time is much longer.

Coal pulverizersCoal pulverizers are essentially volumetric devices, because the density of coal is fairly constant, are rated in mass units of tons/hr. A pulverizer accepts a volume of material to be pulverized which is dependent on the physical dimensions of the mill and the ability of coal to pass through the coal pulverizing system. The furnace volume and mill capacity in a specific power station may dictate the need to purchase coals which are reactive and easily grind.The common measure of mass in tons enables matching of energy requirements with available coal properties and mill capacity.Increased combustible loss can occur if the furnace volume or mill capacity is less than desirable for a particular coal. There are a number of possible remedial actions. Operators can correct some deficiencies in the combustion system : Biasing the performance of the coal pulverizing for variable coal qualities. Use the spare mill into service for peak periods to ensure full output.

Coal pulverizersSize reduction is energy intensive and generally very inefficient with regard to energy consumption. In many processes the actual energy used in breakage of particles is around 5% of the overall energy consumption. Pulverizing coal is no exception to this.There are basically four different types of pulverizing mills which are designed to reduce coal with a top particle size of about 50 mm to the particle size range necessary for fairly complete combustion in a modern pulverized coal fired boiler. Each type has a different grinding mechanism and different operating characteristics. There are four unit operations going concurrently within the mill body, coal drying, transport, classification and grinding.For coal pulverizers the capacity of a mill is normally specified as tonnes output when grinding coal with a HGI of 50, with a particle size of 70% less than 75 micron and 1 % greater than 300 micron and with a moisture in coal of less than 10%. A few manufacturers specify 55 instead of 50 with respect to HGI.This standardization enables selection of an appropriate mill for a specific duty.

Ball & Tube MillThe oldest pulverizer design still in frequent use.25% to 30% of cylinder volume is filled with wear resistant balls of 30 60mm.The cylinder is rotated at a speed of about 20 rpm.Specific power consumption 22 kWh per Ton.Suitable for hard coals.Highly reliable in requires low maintenance.Bulky and heavy in construction.

mw2RmgaPulverization due to ATTRITION

mw2RmgaPulverization due to Impact

Bowl MillThe most widely used mill for grinding coal.The raw coal is fed into the center of the mill.This is an intermediate speed pulverizer.The vertical shaft rotates at a speed 30 50 rpm.Specific power consumption 12 kWh/ton.

Hot Air~ 2500CCoal 10 to 25 mm SizeBowl Mill

Aerodynamic Lifting of Coal Particles

Schematic of typical coal pulverized systemA Inlet Duct; B Bowl Orifice;C Grinding Mill;D Transfer Duct to Exhauster; E Fan Exit Duct.

Schematic of typical coal pulverized system

Combustion in BoilersThere are four important factors that control combustion in boiler furnace:1. Air supply- Need adequate air for completecombustion.The rating (capacity) of a boiler can be increased by supplying additional air (think of the effect of bellows on a small fire).Too much air can result in excessive stack losses.

Combustion factors2. Mixing of fuel and air- fuel and air molecules must be brought into close proximity in order for combustion to occur.

The larger the fuel "particles" the greater the difficulty in achieving good mixing-easiest for gaseous fuels,more difficult for liquid fuels and pulverized solids,most difficult for stoker coal, bark or large trash clumps.

Combustion factors3. Temperature- all combustion reactions proceed exponentially more rapidly with increasing T

Temperatures too low: incomplete combustion, waste fuelunburned hydrocarbons and soot emissions greatly increasedTemperatures too high:equipment failure, metal strength drops off quickly at high TNOx emissions greatly increased.

Combustion factors4. Combustion time- fuel "particles" must be given sufficient time (residence time) in the furnace to achieve complete combustion.Like fuel/air mixing, the required residence time is least for gases and most for large solid fuels:Gases and fine liquid sprays- 10 - 20 msPulverized fuel (coal, sawdust)- 1 s

Coal Residues Produced in U.S.More than 120 Million Tons (2005) of coal related residues are generated in the U.S. by coal-burning power plants each year

Boilers & Ash By-Products

In general, there are 3 types of coal-fired boiler furnaces used in the electric utility industryThey are referred to as:1. Dry-bottom boilers2. Wet-bottom boilers3. Cyclone furnaces

Boilers & Ash By-ProductsThe most common type of coal burning furnace is the dry-bottom boiler with fly ash constituting the major ash component at 80-90% with bottom ash in the range of 10-20%

Wet bottom boilers yield molten ash, or slag from furnace bottom which drops into a water-filled hopper

Mercury in CUBsAbout 75 Tons of Hg are found in coal delivered to U.S. power plants/year & about two thirds is emitted to the air, resulting in 50 Tons of Hg emissions. The 25 Ton reduction is achieved in the power plant through existing pollution controls such as:Scrubbers for SO2SCRs (Selective catalytic reduction) for NOxPM fabric filters

Pelton TurbinePelton turbines are suited to high head, low flow applications

Kaplan TurbineKaplan turbines are well suited to situations in which there is a low head and a large amount of discharge.

The adjustable runner blades enable high efficiency even in the range of partial load, and there is little drop in efficiency due to head variation or load.

Kaplan Turbine

Francis TurbineA type of hydropower reaction turbine that contains a runner that has water passages through it formed by curved vanes or blades. The runner blades, typically 9 to 19 in number, cannot be adjusted.

The Francis turbine has a wide range of applications and can be used for fall heights of 2800 meters.

The largest Francis turbines have an output of 750 MW.

Francis Turbine

Nuclear Steam Generator

Furnace or Radiant Section

This is a schematic overview of a boiler room:As 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).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. To 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.

To 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.

In 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


Does anyone recognize what type of boiler this is? (Click once and name will appear) This is a packaged boiler. More specifically, it is a typical 3 pass, oil fired packaged boiler.The packaged boiler is so called because it comes as a complete package. Once delivered to a site, it requires only the steam, water pipe work, fuel supply and electrical connections to be made to become operational. 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. These boilers are classified based on the number of passes - the number of times the hot combustion gases pass through the boiler.


In 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

Does anyone recognize what type of boiler this is? (Click once and name will appear) This is a packaged boiler. More specifically, it is a typical 3 pass, oil fired packaged boiler.The packaged boiler is so called because it comes as a complete package. Once delivered to a site, it requires only the steam, water pipe work, fuel supply and electrical connections to be made to become operational. 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. These boilers are classified based on the number of passes - the number of times the hot combustion gases pass through the boiler.

Stoker fired boilers are classified according to the method of feeding fuel to the furnace and by the type of grate. The main classifications of stokers are spreader stoker and 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.

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.

A 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. It is widely used in the heat recovery from exhaust gases from gas turbines and diesel engines.

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 1300-1700 degrees Celsius depending mainly on the coal grade. The particle residence time in the boiler is typically 2 to 5 seconds an dthe particles has to be small enough to be completely combusted during this time period.This system has many advantages such as ability to fire varying quality of coal, quick responses to changes in load, use of high pre-heat air temperatures etc. One 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.

When 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 include coal, washery rejects, rice husk, bagasse & 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 design, fuel flexibility, higher combustion efficiency and reduced emission of noxious pollutants such as SOx and NOx. Three types of FBC boilers are explained on the next slides.

Most operational boiler of this type is of the Atmospheric Fluidized Bed Combustion. (AFBC). In this boiler, atmospheric air, which acts as both the fluidization and combustion air, is delivered at a pressure, after being preheated by the exhaust fuel gases.In Pressurized Fluidized Bed Combustion (PFBC) type, a compressor supplies the Forced Draft (FD) air and the combustor is a pressure vessel. A deep bed is used to extract large amounts of heat. This will improve the combustion efficiency and sulphur dioxide absorption in the bed. The steam is generated in the two tube bundles, one in the bed and one above it. Hot flue gases drive a power generating gas turbine. The PFBC system can be used for cogeneration (steam and electricity) or combined cycle power generation

This figure illustrates another type of fluidized bed combustion, the atmospheric circulating fluidized bed combustion boiler.(Click once) In a circulating system the bed parameters are maintained to promote solids elutriation from the bed. They are lifted in a relatively dilute phase in a solids riser, and a down-comer with a cyclone provides a return path for the solids. (Click once) There are no steam generation tubes immersed in the bed. Generation and super heating of steam takes place in the convection section, water walls, at the exit of the riser. (Click once) Benefits:CFBC boilers are generally more economical than AFBC boilers for industrial application requiring more than 75 100 T/hr of steam. For large units, the taller furnace characteristics of CFBC boilers offers better space utilization, greater fuel particle and sorbent residence time for efficient combustion and SO2 capture, and easier application of staged combustion techniques for NOx control than AFBC steam generators.

When 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 water / maximum permissible TDS in boiler water.

There 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.

Good boiler blow down control can significantly reduce treatment and operational costs that include: Lower pretreatment costs Less make-up water consumption Reduced maintenance downtime Increased boiler life Lower consumption of treatment chemicals

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.

Deposits 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.

There are two major types of boiler water treatment, namely internal and external water treatment: internal and external. 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.

External 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 colour 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. The external treatment processes we will explain next are: Ion exchangeDe-aeration (mechanical and chemical)Reverse osmosisThere are two major types of boiler water treatment, namely internal and external water treatment: internal and external. 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.

In 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.

In 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.

Mechanical de-aeration is typically utilized prior to the addition of chemical oxygen scavengers. Mechanical de-aeration is based on that the removal of oxygen and carbon dioxide can be accomplished by heating the boiler feed water as it reduces the concentration of oxygen and carbon dioxide in the atmosphere surrounding the feed water. Mechanical de-aeration can be the most economical external water treatment.Mechanical de-aeration can be of two types: vacuum or pressure type. The vacuum type of de-aerator operates below atmospheric pressure can reduce the oxygen content in water to less than 0.02 mg/liter The pressure-type de-aerators operates by allowing steam into the feed water through a pressure control valve to maintain the desired operating pressure. This type can reduce the oxygen content to 0.005 mg/litre. While the most efficient mechanical deaerators reduce oxygen to very low levels (0.005 mg/liter), even trace amounts of oxygen may cause corrosion damage to a system. Consequently, good operating practice requires removal of that trace oxygen with a chemical oxygen scavenger such as sodium sulfite or hydrazine. Sodium sulphite reacts with oxygen to form sodium sulphate, which increases the TDS in the boiler water and hence increases the blow down requirements and make-up water quality. Hydrazine reacts with oxygen to form nitrogen and water. It is invariably used in high pressure boilers when low boiler water solids are necessary, as it does not increase the TDS of the boiler water.

When solutions of differing concentrations are separated by a semi-permeable membrane, water from a less concentrated solution passes through the membrane to dilute the liquid of high concentration. This is called osmosis.If the solution of high concentration is pressurized, the process is reversed and the water from the solution of high concentration flows to the weaker solution. This is known as reverse osmosis.

The figure illustrates a continuously operating reversed osmosis system with feed water and concentrated reject streamThe quality of water produced depends upon the concentration of the solution on the high-pressure side and pressure differential across the membrane. This process is suitable for waters with very high TDS, such as sea water.

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.

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