coal combustion
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COAL COMBUSTION COMBUSTION AND FUELS FURNACES OF COAL-FIRED BOILERS Basic boiler furnaces: - grate (steady and moving grates) - fluidized beds (bubble and circulating beds), - pulverized coal-furnace. Others: - cyclone, - retort furnace, - rotary furnace. COMBUSTION AND FUELS BASIC COAL FURNACES fumes coal b) drying pyrolysis gasification combustion ash air System for coal burning: a) Moving grate, b) Fixed bed, c) Fluidized bed d) Pulverized coal flame air secondary air d) pulverized coal COMBUSTION AND FUELS COMBUSTION PROCESS COAL SIZING REQUIREMENTS Grate furnaces (stoker firing systems): • fixed beds: mud, dust, tiny, peas, nut, thick, • moving grate: peas, nut (10-30 mm), Fluidized bed: • bubling <25 mm, • circulating: 6 mm, Pulverized coal furnaces: • bituminous coal: R90= 25-30%, R200<8%, • lignite: R90= 48-55%, R200 = 25-32%, R1000<2-3% COMBUSTION AND FUELS BASIC PROCESSES OF COAL COMBUSTION - evaluation and combustion of volatile matter, - char combustion Ash Char Ash Coal particle COMBUSTION Volatile matter AND FUELS Ash SINGLE COAL PARTICLE COMBUSTION COMBUSTION AND FUELS SINGLE COAL PARTICLE COMBUSTION Cząstka Coal węgla particle Zapłon Ignition części of v.m. lotnych Zapłon Ignition pozostałości of char koksowej Wydzielenie Volatile i spalenie matter części lotnych combustion Heating Nagrzewanie Ash Popiół Spalanie Char pozostałości combustion koksowej Combustion Spalanie COMBUSTION AND FUELS HISTORY OF SINGLE COAL PARTICLE COMBUSTION T Temperatura cząstki Particle temperature Temperatura spalin Flue gas temperature d Średnica cząstki Particle diameter t d0 Stage Stage of Faza Faza of heat. Faza spalania koksu zapłonu spalania Stage of char combustion v.m. & części COMBUSTION AND FUELS comb. ignit. lotnych t STAGES OF BURNING OF SINGLE PARTICLE OF COAL Induction ignition time: - heating time, - drying time. Time of combustion: - evaluation and combustion of volatile matter, - burning of char. Ratio of time of volatile matter combustion/time of char burning: 1:10 COMBUSTION AND FUELS VOLATILE MATTER CONTENT Lignite Montana, % Bituminous coal Pittsburgh, % 6.9 MPa He 1000 °C, 3–10 s 6.9 MPa He 850–1070 °C, 2–10 s Component CO CO2 H2O CH4 C2H4 C2 H6 Liquid HC Tar Char 9.0 10.6 12.9 2.5 0.6 0.2 – 3.0 59.9 COMBUSTION AND FUELS 2.5 1.7 9.5 3.2 0.5 0.9 0.7 12.0 62.4 MECHANISMS OF VOLATILE MATTER EVALUATION iojnny sy u f f i z DDyffulame ń mie pło Products Produkty of spalania combustion Części Volatile matter lotne Coal Koks Ro Ciepło heat n o i tat O2 Volatile matter COMBUSTION AND FUELS Bubbles THERMAL DECOMPOSITION OF COKING COAL 1st STAGE (coal/metaplast) 2nd STAGE primary pyrolysis 3rd STAGE secondary pyrolysis tar RAW COAL primary gases char COMBUSTION AND FUELS soot secondary gases char DYNAMICS OF VOLATILE MATTER EVALUATION Young hard coal Hard coal Anthracite Coke Drying Temperature °C COMBUSTION AND FUELS intensity of v. m. evaluation Lignite CHAR STRUCTURE Pores Microscopic photo of char surface after coal devolatilization COMBUSTION AND FUELS CHAR COMBUSTION COMBUSTION AND FUELS MAJOR FACTORS INFLUENCING RATE OF CHAR PARTICLE COMBSTION C O2 a) chemical: chemical kinetic of oxidation reaction (temperature, O2 concentration) b) physical: transport of oxygen to the surface of char particle COMBUSTION AND FUELS Simple model of char particle combustion Char particle Oxygen Oxygen Reaction occurs here Carbon dioxide Model of reaction: C + O2 = CO2 Rate of reaction: Wc = Ω Ys k, g/(m2s) COMBUSTION AND FUELS CHEMICAL MECHANISM OF CHAR COMBUSTION Basic chemical reactions of char combustion : C + O2 = CO2 – 393.5 kJ/mol (I) C + 0.5O2 = CO – 110.5 kJ/mol (II) C + CO2 = (III) 2CO + 172.5 kJ/mol 2CO + O2 = 2CO2 – 283.0 kJ/mole (IV) COMBUSTION AND FUELS COMBUSTION OF A SINGLE PARTICLE OF CHAR O2 CO CO2 2CO + O2 → 2CO2 C + CO2 → 2CO Distribution of primary and secondary products of oxidation of char COMBUSTION AND FUELS RATE OF CHAR OXIDATION Assumption: 1. Single chemical reaction of oxidation is considered C + O2 → CO2 2. Char particle is considered spherical and smooth. C + O2 COMBUSTION AND FUELS CO2 RATE OF CHAR OXIDATION, Wc FORMULA WC = Y0/(1/k + 1/αD) , kg/(m2s) Rate constant: k = k0 exp[– E/(RT)], Stoichiometry coefficient: Ω (Ω = 0.375 when burning is to CO2), Mass transfer coefficient: αD = (D/d)Sh –of (Sh = CRenPrk Units: Wc [kg/(m2s)] COMBUSTION AND FUELS COMPETITION OF TEMPERATURE AND DIFFUSION COMBUSTION AND FUELS REGIMES TEMPERATURE AND DIFUSSION REGIMES OF CHAR REACTION Overal rate: WC = Y0/(1/k + 1/αD) Remember: Arrhenius rate: k = k0 exp[– E/(RT)], temperature dependent Mass transfer coefficient: αD = (D/d)Sh, temperature independent Two regimes of char burning Kinetic (reaction) control regime (low temperature): 1/k >> 1/αD → WC = ΩYO k0 exp[– E/(RT)] Diffusion control regime (high temperature): 1/k << 1/αD COMBUSTION → WC = ΩAND YOαD FUELS LOW – AND HIGH TEMPERATURE PROFILES AROUND CHAR PARTICLE Profiles of oxygen concentration Y s = YO YO YO YO Ys r a) Low temperature reaction control r b) Transition COMBUSTION AND FUELS Ys = 0 c) r High temperature – diffusion control Szybkość reakcji Reaction rate REGIMES OF COMBUSTION OF POROUS CHAR PARTICLE III II I Zewnętrzny obszar dyfuzyjny Wewnętrzny obszar dyfuzyjny Obszar kinetyczny 1/ T c Bulk surface diffusion Pore diffusion k=ko exp(-E/(RT) Reaction control COMBUSTION AND FUELS Influence of temperature on the course of combustion of solid particle of coal 650° C 450°C COMBUSTION AND FUELS INFLUENCE OF AERODYNAMICS ON THE RATE OF COMBUSTION OF SINGLE COAL PARTICLE COMBUSTION AND FUELS ROLE OF SECONDARY CHEMICAL REACTIONS IN BURNING OF SINGLE CHAR COMBUSTION Flame zone Flame zone Convection and temperature influence on the chemical reactions participation COMBUSTION FUELS in the rate ofAND burning. COAL COMBUSTION AND SLAGING PROCESSES COMBUSTION AND FUELS MINERAL MATTER OF COAL Mineral matter of coal: - included (a mineral associated with organic matter) - excluded (liberated, like clay, sand and rocks). There are four major groups of minerals in coal: – aluminosilicates – mainly kaolinite, – oxides – quartz SiO2 and hematite Fe2O3, – carbonates – calcite CaCO3, syderyt FeCO3 and dolomite CaCO3MgCO3, – sulphur compounds – pyrite FeS2 i gipsium CaSO4 2H2O. COMBUSTION AND FUELS SOLID RESIDUES AFTER COAL COMBUSTION COMBUSTION FLY ASH, MINERAL MATTER COMBUSTION AND FUELS BOTTOM ASH MEMBRANE WALLS DEPOSITS IN PULVERIZED COAL FIRED BOILERS Fly ash Wall deposits COMBUSTION AND FUELS PHYSICAL AND CHEMICAL PROCESSES OF MINERAL MATTER CONVERSION DURING COAL COMBUSTION During coal combustion mineral matter undergoes conversion: - physical: melting, evaporation, sublimation and condensation, - chemical: dehydratation, carbonates decomposition and oxidation of sulfides. COMBUSTION AND FUELS BASIC PROCESSES OF MINERAL MATTER CONVERSION IN THE FURNACE heterogenic condensation vaporasing of inorganic matter homogenic nucleation and coalescence coal particles with bound mineral matter surface covering small particles coalescence coal particles with bound and free mineral matter swelling cenospheres char disintegration SLAGING PROCESS Slaging a process of covering of walls (tubes) of furnace by deposit being a result of conversion of mineral matter of coal during combustion. Types of deposits: - loose, - bound (sintered, melted). COMBUSTION AND FUELS TYPES OF DEPOSITS loose sintered COMBUSTION AND FUELS bound melted SLAG DEPOSIT ON THE WALL OF A FURNACE COMBUSTION AND FUELS PULVERIZED COAL FLAME COMBUSTION AND FUELS STRUCTURE OF PULVERIZED COAL FLAME Pyrolysis Basic part of flame v.m. Char recirculation Flame front End of flame Unburned coal particles COMBUSTION AND FUELS JET OF PULVERIZED COAL FLAME COMBUSTION AND FUELS PULVERIZED COAL FLAME IN THE BOILER FURNACE UNBURNED CARBON (UBC) IN FLY ASH COMBUSTION AND FUELS UNBURNED CARBON IN FLY ASH: FACTORS INFLUENCING UBC The most important factors determining UBC (LOI): • reactivity of coal • stoichiometric ratio • temperature • quality of grinding Petrographic components of UBC: inertenite, ksylite COMBUSTION AND FUELS INFLUENNCE OF MINERAL MATTER IN COAL ON UBC ashziacontent Ud ł p o p io łu 20% 10% 5% High content of mineral matter in coal decreases UBC 40% 35% 30% Wt 25% 20% 15% 10% UBC UNITS: % 5% 0% 0,00% 90,00% 99,00% 99,90% 99,99% Accepted level of UBC: 5% Sto p ie ńofwy p a le n ia degree burnout COMBUSTION AND FUELS BURNERS AND BOILER FURNACES COMBUSTION AND FUELS SELECTION OF PULVERIZED COAL FURNACES Hard coal Lignite Peat LCV, MJ/kg (daf) Antr. Volatile matter content, % Vertical furnace, jet burners Wall front furnace, swirl burners Tangencial furnace, jet burners COMBUSTION AND FUELS Biomass SWIRL BURNERS secondary air external recirculation rosette b ) oil lance external recirculation internal recirculation swirler vanes pulverised coal / air mixture Swirl number: S = 0.4-0.7 Outlet velocity of pulverized coal/air mixture: Secondary air velocity: 20-22 m/s 26-30 m/s COMBUSTION AND FUELS FRONT OF SWIRL BURNER (40-50 MWth) COMBUSTION AND FUELS BACK OF THE SWIRL BURNER AIR BOX – VANES OF AIR SWIRLER COMBUSTION AND FUELS vanes SWIRL BURNERS ON FRONT WALL OF THE OP-650 BOILER COMBUSTION AND FUELS STRUCTURE OF SWIRLED PULVERIZED COAL FLAME COMBUSTION AND FUELS INFLUENCE OF AIR SWIRL ON THE PULVERIZED COAL STRUCTURE primary 25° secondary 45° tertiary 30° primary 35° secondary 35° tertiary 35° primary 35° secondary 45° tertiary 45° primary 35° secondary 45° tertiary 60° JET BURNERS FOR BITUMINOUS COAL o pulverized c al mazout d coal air e pulveriz e pulveriz d coal ut mazo ed coal pulveriz JET BURNERS FOR BITUMINOUS COAL JET BURNERS FOR BITUMINOUS COAL JET BURNER IN BOILER FURNACE OP 430 JET BURNERS FOR PULVERIZED LIGNITE Outlet velocities from the burner: Pulverized lignite/gas 11-13 m/s, Upper and lower air 45–55 m/s, nozzle for ceramics cooling air Middle air 45–55 m/s, nozzle for upper air Cooling and core air 20–25 m/s. nozzle for pulverised coal with core air nozzle for middle air nozzle for pulverised coal with core air nozzle for lower air nozzle for ceramics cooling air HOW PULVERIZED COAL IS TRANSPORTED FROM THE MILL TO BURNERS? Media transporting pulverized coal: Bituminous coal: hot air, Lignite: flue gas. COMBUSTION AND FUELS JET BURNERS: PULVERIZED LIGNITE BURNING Ventilation ratio of the coal mill: 1.6 kg/kg Volumetric rate of flue gas/ Volumetric rate of air Ventilation, m3/h (flue gas) No. of pulverized lignite nozzles in the burner do 80 000 80 000–200 000 200 000–300 000 300 000–400 000 400 000–500 000 2 3 4 6 8 No. of nozzles in a jet burner depending on the ventilation of a mill COMBUSTION AND FUELS FLAME PATTERN OF TANGENTIALLY FIRING FURNACE Corner furnace COMBUSTION AND FUELS BOILER DUCTS IN TANGENTIALLY FIRING FURNACES COMBUSTION AND FUELS FLUIDIZED COMBUSTION COMBUSTION AND FUELS BURNING IN CIRCULATING FLUIDIZED BED BOILERS COMBUSTION AND FUELS COMPOSION AND TEMPERATURE OF FLUIDIZED BED Composition of fluidized bed: - inert material (sand and ash), - dolomite/calcite ( and products of desulfurization), - 2-6% of coal. Temperature of fluidized bed: 800 – 900 oC Optimal temperature wth respect desulfurization: 850 oC COMBUSTION AND FUELS COAL BURNING IN FLUIDIZED BED FREE BOARD ZONE CaSO4 CaO ½ O2 CO2 CO2 CO2 ½ O2 CO Cn Hm coal particle CaO BED SO2 CO2 HO 2 Cn Hm CaO CO2 HO 2 fines CO2 ½ O2 CO2 CO CO2 CO CO char sorbent O2 CO2 LIGNITE BURNING IN LABORATORY FLUIDIZED BED COMBUSTION AND FUELS COAL COMBUSTION IN FREE BOARD ZONE OF FB FRAGMENTATION OF COAL PARTICLES IN FLUIDIZED BED Coal particles undergo fragmentation in fluidized bed due to:- increase of porosity of char particles (percolative fragmentation), - collision of coal particles with bed particles, - attrition of coal particles in bed. COMBUSTION AND FUELS SPECIAL BOILER FURNACES COMBUSTION AND FUELS RETORT FURNACE FOR SMALL BOILERS COMBUSTION AND FUELS RETORT FURNACE: COAL COMBUSTION COMBUSTION AND FUELS CYCLONE COMBUSTION CHAMBERS igniter secondary air oil burner pulverised coal / air mixture tertiary air igniter secondary air oil burner pulverised coal / air mixture tertiary air Cyclone combustion chambers with tangent (a) and secant (b) injection of pulverised coal / air mixture BOILER WITH CYCLONE COMBUSTION CHAMBER postcombustion zone air pulverised coal reburning zone secondary air coal air Primary air cyclone burner main combustion zone water liqiud slag COMBUSTION AND FUELS
