BoilersIntroduction to Boiler It is an enclosed Pressure Vessel Heat generated by Combustion of Fuel is transferred to water to become steam Process: Evaporation Steamvolume increases to 1,600 times from water andproduces tremendousforce Care is must to avoidexplosion.What is a boiler?Boiler Specification Boiler Make & Year:XYZ & 2003 MCR(Maximum Continuous Rating) :10TPH (F & A 100oC) Rated Working Pressure :10.54 kg/cm2(g) Type of Boiler : 3 Pass Fire tube Fuel Fired: Fuel Oil Heating surface : M2What is F&A 100OC ?Boiler SystemsFlue gas systemWater treatment systemFeed water AND CondensatesystemSteam SystemBlow down systemFuel supply systemAir Supply systemBoiler Types and Classifications Fire in tube or Hot gas through tubes and boiler feed water in shell side Fire Tubes submerged in waterApplication Used for small steam capacities ( upto 25T/hr and 17.5kg/cm2Merits Low Capital Cost and fuel Efficient (82%) Accepts wide & load fluctuations Packaged BoilerFire Tube BoilerBoiler Types and ClassificationsWater Tube Boiler Water flow through tubes Water Tubes surrounded by hot gasApplication Used for PowerPlants Steam capacitiesrange from 4.5- 120 t/hrCharacteristics High Capital Cost Used for high pressure high capacity steam boiler Demands more controls Calls for very stringent water quality Package boilers aregenerally of shell typewith fire tube designMore number of passes-so more heat transferLarge number of small diameter tubes leading to good convective heat transfer.Higher thermal efficiencyPackaged BoilerChain Grate or Traveling Grate Stoker Boiler Coal is fed on one end of a moving steel chain grate Ash drops off at end Coal grate controls rate of coal feed into furnace by controlling the thickness of the fuel bed. Coal must be uniform in size as large lumps will not burn out completelySpreader Stoker BoilerUses both suspension and grate burningCoal fed continuously over burning coal bedCoal fines burn in suspension and larger coal pieces burn on grateGood flexibility tomeet changing load requirementsPreferred over other type of stokers in industrial applicationPulverized Fuel BoilerTangential firingCoal is pulverised to a fine powder, so that less than 2% is +300 microns, and 70-75% is below 75 microns.Coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles. Combustion temperatures from1300-1700C Particle residence time in theboiler is 2-5 seconds Most popular system for firingpulverized coal is the tangentialfiring using four burners corner tocorner to create a fire ball at thecenter of the furnaceAdvantages 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 Highpower demandfor pulverizing Requires more maintenance, flyash erosion and pollution complicate unit operationPulverized Fuel Boiler (Contd..)Fluidized bed Combustion (FBC) boilerFurther, increase invelocity gives rise tobubble formation, vigorousturbulence and rapidmixing and the bed is saidto be fluidized.Coal is fed continuously into a hot air agitatedrefractory sand bed, thecoal will burn rapidly andthe bed attains a uniformtemperatureDistributed air is passed upward through a bed of solid particlesThe particles are undisturbed at low velocity.As air velocity is increased, a stage is reached when the particles are suspended in the airFluidised Bed CombustionFluidized-bed boiler (Contd..)Advantages : Higher rates of heat transfer between combustion process and boiler tubes (thus reduced furnace area and size required), combustion temperature 850oC is lower than in a conventional furnace. The lower furnace temperatures means reduced NOxproduction. In addition, the limestone (CaCO3) and dolomite (MgCO3) react with SO2to form calcium and magnesium sulfides, respectively, solids which do not escape up the stack; This means the plant can easily use high sulfur coal. Fuel Flexibility: Multi fuel firing There are two methods of assessing boiler efficiency.1) The Direct Method: Where the energy gain of the workingfluid (water and steam) is compared with the energy content of theboiler fuel.2) The Indirect Method: Where the efficiency is the differencebetween the losses and the energy input.Boiler Efficiency Evaluation Method1. Direct Method2. Indirect MethodPerformance Evaluation of BoilersExample:Type of boiler: Coal fired BoilerHeat input dataQty of coal consumed :1.8 TPHGCV of coal :3200K.Cal/kgHeat output data Qty of steam gen:8 TPH Steam pr/temp:10 kg/cm2(g)/1800C Enthalpy of steam(sat) at 10 kg/cm2(g) pressure :665 K.Cal/kg Feed water temperature :850C Enthalpy of feed water: 85 K.Cal/kgFind out the efficiency ?Find out the Evaporation Ratio?Efficiency Calculation by Direct MethodBoiler efficiency (q): = Q x (H h)x 100(q x GCV)Where Q =Quantity of steam generated per hour (kg/hr)H = Enthalpy of saturated steam (kcal/kg)h = Enthalpy of feed water (kcal/kg)q = Quantity of fuel used per hour (kg/hr)GCV = Gross calorific value of the fuel (kcal/kg)Boiler efficiency (q)= 8 TPH x1000Kg/Tx (66585) x 1001.8 TPH x 1000Kg/T x 3200=80.0%Evaporation Ratio =8 Tonne of steam/1.8 Ton of coal= 4.4BoilerFlue gasSteam OutputEfficiency =100 (1+2+3+4+5+6+7+8)(by In Direct Method)AirFuel Input, 100%1. Dry Flue gas loss2. H2 loss3. Moisture in fuel4. Moisture in air5. CO loss7. Fly ash loss6. Surface loss8. Bottom ash lossWhat are the losses that occur in a boiler?Feed water Blow downExample:The following are the data collected for a typical oil fired boiler.Find out the efficiency of the boiler by indirect method andBoiler Evaporation ratio.Ultimate analysis of OilC : 84.0 % H2: 12.0 %S: 3.0 % O2: 1.0 %GCV of Oil : 10200 kcal/kgSteam Generation Pressure : 7kg/cm2(g)-saturatedEnthalpy of steam : 660 kCal/kgFeed water temperature : 60oCPercentage of Oxygen in flue gas: 7Percentage of CO2in flue gas: 11Flue gas temperature (Tf) : 2200CAmbient temperature (Ta) : 270CHumidity of air : 0.018 kg/kg of dry airSolutionStep-1: Find the theoretical air requirement= kg/kg of oil=[(11.6 x 84) + [{34.8 x (12 1/8)} + (4.35 x 3)]/100 kg/kg of oil=14 kg of air/kg of oilStep-2: Find the %Excess air suppliedExcess air supplied (EA) = = = 50%100 / )] 35 . 4 ( )} 8 / ( 8 . 34 { ) 6 . 11 [(2 2S x O H x C x + +10021%% 22xOO1007 21% 7xStep-3: Find the Actual mass of air suppliedActual mass of air supplied /kg of fuel = [ 1 + EA/100] x Theoritical Air(AAS) = [1 + 50/100] x 14= 1.5 x 14= 21 kg of air/kg of oilStep-4: Estimation of all lossesi. Dry flue gas lossPercentage heat loss due to dry flue gas =m= mass of CO2+ mass of SO2+ mass of N2+ mass of O2m = 21 kg / kg of oil100) (xfuel of GCVT T x C x ma f p|.|

\| + + + =10023) 14 21 (10077 213264 03 . 01244 84 . 0xx x xm10010200) 27 220 ( 23 . 0 21xx x = 9.14 %Alternatively a simple method can be used for determining the dry flue gas loss as given below.Percentage heat loss due to dry flue gas = Total mass of flue gas (m) = mass of actual air supplied + mass offuel supplied = 21 + 1=22%Dry flue gas loss = 100) (xfuel of GCVT T x C x ma f p% 57 . 9 10010200) 27 220 ( 23 . 0 22=xx xii. Heat loss due to evaporation of water formed due to H2in fuel=Where, H2 percentage of H2in fuel== 7.10% 0.45 =Cp sup heat steam & 584=latent100fuel of GCV)} T - (T C {584 xH x9a f p 2x+10010200)} 27 - (220 0.45 {584 x12 x9x+iii. Heat loss due to moisture present in air= 100) (xfuel of GCVT T x C x humidity x AASa f p== 0.32210010200) 27 220 ( 45 . 0 018 . 0 21xx x x iv Heat loss due to radiation and other unaccounted lossesFor a small boiler it is estimated to be 2%Boiler Efficiencyi. Heat loss due to dry flue gas : 9.14%ii. Heat loss due to evaporation of water formed due to H2in fuel : 7.10 %iii. Heat loss due to moisture present in air : 0.322 %iv. Heat loss due to radiation and other unaccounted loss : 2%Boiler Efficiency = 100- [9.14+7.10+0.322+2]= 100 18.56 = 81 (app)Evaporation Ratio = Heat utilized for steam generation/Heat addition to the steam= 10200 x 0.81/ (660-60)= 14.11Why Boiler Blow Down ?When waterevaporates Dissolved solids gets concentrated and Solids precipitates on tubes. Reduces the heat transfer rateIntermittent Blowdown The intermittent blown down is given by manuallyoperating a valve fitted to discharge pipe at thelowest point of boiler shell to reduce parameters(TDS or conductivity, pH, Silica etc) withinprescribed limits so that steam quality is not likelyto be affected TDS level keeps varying fluctuations of the water level in the boiler. substantial amount of heat energy is lost withintermittent blow down.Continuous Blowdown A steady and constant dispatch of smallstream of concentrated boiler water, andreplacement by steady and constant inflowof feed water. This ensures constant TDS and steampurity. This type of blow down is common in high-pressure boilers.