2 CONTENTS PULVERISATION FUELS COAL COAL APPLICATIONS COAL COMBUSTION COMBUSTION OF FUELS- SOLID, LIQUID & GASEMERGING TRENDSNox FORMATION AND CONTROL
3 PULVERSIATION Coal dust is a fine powdered form of coal More surface area per unit weight than lumps of coalMore susceptible to spontaneous combustionPulverised coal has significant dust explosion hazard
5 Evolution Time, Pres.&Temp. COALHeterogeneous organic fuel formed mainly from decomposed plant matter.Over 1200 coals have been classified.40-90%C2-7%H<18-30%O<6%S1-5%N20-70%Char5-45%Ash2-20%H2O15-30%VMUltimate AnalysisCoalification forms different Coal Types: Peat Lignite Bituminous coal AnthraciteEvolution Time, Pres.&Temp.Coal RankProximate Analysis
7 COAL APPLICATIONS Home – heating and cooking Transportation – steam enginesIndustry – Iron & steelElectricity – power plants
8 COMBUSTIONOxidationoxygen combines with other elements and forms oxidies.Combustion,A special form of oxidationOxygen combines with fuels; coal, oil, gassubstantial amounts of heat is liberated.The Degree of Flammability dependsconvertibility to a gas,-nothing truly burns until it is a gas.Nature of fuelQuantity of the fuel,
9 Stages of Combustion Ignition Combustion Stability Completion of CombustionCombustion process, ignition occurs in vapour phase.Solid and liquid fuels get ignited from their vapours.9
10 Combustion Stability: Ignited flame shall be sustained further so that the process of combustion would be continuous.Completion of Combustion:Fuel staying for the minimum period (residence time)Completely oxidizing the combustible.Considerations of combustion:Safety consideration during fuel handlingGeneration of Pollutants viz. Nox and Sox .
13 Converting the complex fuel into elementary fuels; Combustion of Fuels in furnacePreparing the fuel and air ;Converting the complex fuel into elementary fuels;Right fuel and air mixtureTransferring heat from the products of combustion to the boiler or other surfaces.
15 The physical processes influencing pulverized coal combustion Turbulent/Swirling flow of air and coal.Turbulent/Convective/molecular diffusion of gaseous reactants and products.Convective heat transfer through the gas and between the gas and coal particles.Radiative heat transfer between the gas and coal particles and between the coal/air mixture and the furnace walls.
16 COMBUSTION OF DIFFERENT FUELS Solid FuelVolatile matter is released over a temp. of deg. C.The volatile matter is first ignited.The coal particle upon releasing the volatile matter become a char. The char slowly burns out.Liquid FuelBoils and releases volatile matter and gets ignitedThe balance char completes the combustion.7
17 COAL - AIR BALANCING IN FUEL PIPING Un balance in Coal- air flow into the furnace resultsuneven heat releaseuneven distribution of excess airunpredictable Nox formationRemediesSelection of Orifices for Coal air two phase flow regime.Provision of on -line adjustment dampers in coal air flow path.On-line measurement of coal -air flow using microwave techniques
20 OIL FIRING SYSTEM FEATURES Fuel Oil PreparationPumping the oil and heating it are the major preparatory functions.Filtration of oil to remove any dust, dirt, sediments, sledge etc.This renders long trouble free service life to pumps, valves, atomisers etcMaintaining the HFO temperature constant, corresponding to the atomizing viscosity of 15 to 20 centistokes, is essential for better fuel oil atomization.A lower temperature of fuel oil impairs the burner performance and a higher temperature causes oil cracking.
21 Fuel Oil AtomisationAtomisationspraying the fuel oil into fine mistBetter mixing of the fuel with the combustion airPressure and viscosity influence atomisationPressure energy of the steam to velocity energy, which breaks up the oil stream into fine particlesPoorly atomisation results inBigger spray particlesLonger burning timeCarryovers of carbon andflame instability due to low rate of heat liberationIncomplete combustion and smoke
22 Oil RecirculationTo warm up the oil supply linesTo maintain correct atomising temperatureSystem VentsFuel oil heatersOil strainersoil & steam lines - get rid of air locks.System DrainsOil lines with a drain valve at the lowest point.
23 BURNERS:To deliver coal , oil and air in a proper proportionTo facilitate ignition energy to the coal air streamTo sustain the ignitionTo provide a stable flame during the operationTypes of BurnersTangential BurnersWall Burners, Ex: Low Nox R burnersDown shot or fan tail burners
24 Modern Burners are equipped with: Separate flame envelope ports for coal, oil and gasSecondary air control to adjust the flame envelopsIgnitorsFlame Scanners - detect the distinct flames in an enclosureFlame StabilisersFlame Analysers
25 Burner ArrangementTangential firing:Four tall windboxes (combustion air boxes) one at each corner of the furnace.The oil and gas burners are located at different levels or elevations of the windboxes.The coal , oil and gas burners are sandwiched between air nozzles or air compartments.That is, air nozzles are arranged between gas spuds, one below the bottom gas spud and one above the top gas spud.
27 Burner Tilt:The burners are tiltable +/- 30o about horizontal,To shifts the flame zone across the furnace heightTo control over steam temperatureCombustion Air DistributionThe Combustion airPrimary Air (PA) and Secondary Air (SA)Secondary Air(SA) provided from FD FansPrimary Air(PA) provided from PA Fans
29 IgnitorsOil and gas are ignited by a pilot flame.Type of IgnitorsOil ignitorGas ignitorHigh Energy Arc ( HEA) ignitor
30 Flame SensingDevicesFlame sensing devices are broadly grouped in toInfrared flame sensorsUV flame scannersVisible light scannersRecent developmentFlame analysers for multiple fuels
31 Emissions of Combustion - Pollutants 1. Nox emission2. Sox emission3.CO 2 emission (Green House Gases)4. CO emission5. Particulate emission
32 combustion system Design Emerging Trendsincombustion system Design
33 Emerging trends in combustion system Design Multiple fuel BurnersLow Emission Burner- Technology developmentLonger guarantee period for high ash coals- Material selection,- Improved design featuresNew devices such as thermal analysers.,
34 Emerging trends in combustion system Design Cont.……Micro processor based on line measurement and controlComputer simulations using software tools viz., ANSYS, CFD.Virtual assembly using CAD tools
36 NOX FORMATIONThermal NOx FormationNox Formation from Fuel Nitrogen
37 NOx Control options Control Technique NOx Reduction Potential(%) Over fire air (OFA)20-30Low Nox Burners (LNB)35-55LNB + OFA40-60Re-burn50-60SNCR ( Selective Non Catalytic Reduction)30-60SCR (Selective Catalytic Reduction)75-85LNB with SCR50-80LNB with OFA and SCR85-95
41 OVER FIRE AIR (OFA)Through additional air compartments, to handle 15 percent of total wind box air flow.Inhibits formation of both fuel Nox and thermal Nox as an Oxygen Deficient environment is established in the primary combustion zone20 to 30% reduction in Nox formation
44 While Nox emission decrease linearly with increasing over fire air, Excess air rises (i.e., More Air Is Needed To Complete The Combustion).If over fire air is increased beyond 15%. This decreases boiler efficiency due to the heating of extra air
45 Nox Reduction Techniques STAGED COMBUSTION:Fuel Bound Nitrogen to be regulatedHigh-Temperature Formation must be curtailed.Withholding of Some O2 from primary flame zoneAir staging
46 LOW NOx BURNERsWall burners (oil and gas firing )Low NOx R-burner is capable of emitting NOx at a level of 150 ppm on oil firing.Tangential firing ( Coal firing)CCOFA(Close Coupled Over Fire Air) feature.Separate Over Fire Air( SOFA ) .