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PULVERISED COMBUSTION SYSTEM.  PULVERISATION  FUELS  COAL  COAL APPLICATIONS  COAL COMBUSTION  COMBUSTION OF FUELS- SOLID, LIQUID & GAS  EMERGING.

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Presentation on theme: "PULVERISED COMBUSTION SYSTEM.  PULVERISATION  FUELS  COAL  COAL APPLICATIONS  COAL COMBUSTION  COMBUSTION OF FUELS- SOLID, LIQUID & GAS  EMERGING."— Presentation transcript:

1 PULVERISED COMBUSTION SYSTEM

2  PULVERISATION  FUELS  COAL  COAL APPLICATIONS  COAL COMBUSTION  COMBUSTION OF FUELS- SOLID, LIQUID & GAS  EMERGING TRENDS  Nox FORMATION AND CONTROL

3  Coal dust is a fine powdered form of coal  More surface area per unit weight than lumps of coal  More susceptible to spontaneous combustion  Pulverised coal has significant dust explosion hazard

4 FUELS Solid Fuels Coal  Peat  Lignite  Bituminous  Anthracite Liquid Fuels  HSD  LFO  HFO Gaseous Fuels  BFG  Producer Gas  LPG  Natural Gas

5 COAL40-90%C2-7%H <18-30%O <6%S 1-5%N 20-70%Char 5-45%Ash 2-20% H2OH2OH2OH2O 15-30%VM  Heterogeneous organic fuel formed mainly from decomposed plant matter.  Over 1200 coals have been classified. Evolution Time, Pres.&Temp. Coal Rank Proximate Analysis Ultimate Analysis

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7  Home – heating and cooking  Transportation – steam engines  Industry – Iron & steel  Electricity – power plants

8 Oxidation oxygen combines with other elements and forms oxidies. Combustion,  A special form of oxidation  Oxygen combines with fuels; coal, oil, gas  substantial amounts of heat is liberated. The Degree of Flammability depends  convertibility to a gas,-nothing truly burns until it is a gas.  Nature of fuel  Quantity of the fuel,

9 Stages of Combustion  Ignition  Combustion Stability  Completion of Combustion  Combustion process, ignition occurs in vapour phase.  Solid and liquid fuels get ignited from their vapours.

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 handling  Generation of Pollutants viz. No x and So x.

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12 coal particle p-coal, d=30-70  m devolatilization volatiles char homogeneous combustion heterogeneous combustion CO 2, H 2 O, … t char =1-2sect volatiles =50-100mst devolatile =1-5ms t

13 Combustion of Fuels in furnace  Preparing the fuel and air ;  Converting the complex fuel into elementary fuels;  Right fuel and air mixture  Transferring heat from the products of combustion to the boiler or other surfaces.

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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 Fuel  Volatile 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 Fuel  Boils and releases volatile matter and gets ignited  The balance char completes the combustion.

17 COAL - AIR BALANCING IN FUEL PIPING Un balance in Coal- air flow into the furnace results  uneven heat release  uneven distribution of excess air  unpredictable Nox formation Remedies  Selection 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

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20 OIL FIRING SYSTEM FEATURES Fuel Oil Preparation  Pumping 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 etc  Maintaining 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 Atomisation Atomisation  spraying the fuel oil into fine mist  Better mixing of the fuel with the combustion air  Pressure and viscosity influence atomisation  Pressure energy of the steam to velocity energy, which breaks up the oil stream into fine particles Poorly atomisation results in  Bigger spray particles  Longer burning time  Carryovers of carbon and  flame instability due to low rate of heat liberation  Incomplete combustion and smoke

22 Oil Recirculation  To warm up the oil supply lines  To maintain correct atomising temperature System Vents  Fuel oil heaters  Oil strainers  oil & steam lines - get rid of air locks. System Drains  Oil lines with a drain valve at the lowest point.

23 BURNERS:  To deliver coal, oil and air in a proper proportion  To facilitate ignition energy to the coal air stream  To sustain the ignition  To provide a stable flame during the operation Types of Burners  Tangential Burners  Wall Burners, Ex: Low Nox R burners  Down shot or fan tail burners

24 Modern Burners are equipped with:  Separate flame envelope ports for coal, oil and gas  Secondary air control to adjust the flame envelops  Ignitors  Flame Scanners - detect the distinct flames in an enclosure  Flame Stabilisers  Flame Analysers

25 Burner Arrangement Tangential 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.

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27 Burner Tilt:  The burners are tiltable +/- 30o about horizontal,  To shifts the flame zone across the furnace height  To control over steam temperature Combustion Air Distribution The Combustion air  Primary Air (PA) and Secondary Air (SA)  Secondary Air(SA) provided from FD Fans  Primary Air(PA) provided from PA Fans

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29 Ignitors  Oil and gas are ignited by a pilot flame. Type of Ignitors  Oil ignitor  Gas ignitor  High Energy Arc ( HEA) ignitor

30 Flame Sensing Devices Flame sensing devices are broadly grouped in to  Infrared flame sensors  UV flame scanners  Visible light scanners Recent development  Flame analysers for multiple fuels

31 Emissions of Combustion - Pollutants 1. Nox emission 2. Sox emission 3.CO 2 emission (Green House Gases) 4. CO emission 5. Particulate emission

32 Emerging Trends in combustion system Design

33 Emerging trends in combustion system Design  Multiple fuel Burners  Low Emission Burner - Technology development  Longer guarantee period for high ash coals - Material selection, - Improved design features  New devices such as thermal analysers.,

34 Emerging trends in combustion system Design Cont.……  Micro processor based on line measurement and control  Computer simulations using software tools viz., ANSYS, CFD.  Virtual assembly using CAD tools

35 NOx Formation and Control Strategies

36 NO X FORMATION  Thermal NOx Formation  Nox Formation from Fuel Nitrogen

37 Control TechniqueNOx Reduction Potential(%) Over fire air (OFA)20-30 Low Nox Burners (LNB)35-55 LNB + OFA40-60 Re-burn50-60 SNCR ( Selective Non Catalytic Reduction) SCR (Selective Catalytic Reduction) LNB with SCR50-80 LNB with OFA and SCR85-95 NOx Control options

38 Selective Catalytic Reduction (SCR) Reactions

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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 zone  20 to 30% reduction in Nox formation

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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 regulated  High-Temperature Formation must be curtailed.  Withholding of Some O 2 from primary flame zone  Air staging

46 LOW NOx BURNERs Wall 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 ).

47 THANK YOU ALL


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