COAL COMBUSTION COMBUSTION AND FUELS

Transcription

1 COAL COMBUSTION

2 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.

3 BASIC COAL FURNACES b) fumes coal drying air pyrolysis gasification combustion ash air secondary air System for coal burning: a) Moving grate, b) Fixed bed, c) Fluidized bed d) Pulverized coal flame d) pulverized coal

4 COMBUSTION PROCESS COAL SIZING REQUIREMENTS Grate furnaces (stoker firing systems): fixed beds: mud, dust, tiny, peas, nut, thick, Fluidized bed: moving grate: peas, nut (10-30 mm), bubling <25 mm, circulating: 6 mm, Pulverized coal furnaces: bituminous coal: R 90 = 25-30%, R 200 <8%, lignite: R 90 = 48-55%, R 200 = 25-32%, R 1000 <2-3%

5 BASIC PROCESSES OF COAL COMBUSTION - evaluation and combustion of volatile matter, - char combustion Char Ash Ash Ash Coal particle COMBUSTION Volatile matter AND FUELS

6 SINGLE COAL PARTICLE COMBUSTION

7 SINGLE COAL PARTICLE COMBUSTION Zapłon Zapłon Cząstka Coal Ignition Ignition części pozostałości Ash Popiół węgla particle of lotnych v.m. of koksowej char Nagrzewanie Heating Wydzielenie Volatile i spalenie części matter lotnych combustion Spalanie Char pozostałości combustion koksowej Combustion Spalanie

8 HISTORY OF SINGLE COAL PARTICLE COMBUSTION T Temperatura Particle temperature cząstki Temperatura spalin Flue gas temperature d Średnica cząstki Particle diameter t d 0 Stage of Stage Faza Faza zapłonu heat. spalania of & części v.m. ignit. lotnych comb. Faza spalania koksu Stage of char combustion t

9 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

10 VOLATILE MATTER CONTENT Component Lignite Montana, % 6.9 MPa He 1000 C, 3 10 s Bituminous coal Pittsburgh, % 6.9 MPa He C, 2 10 s CO CO 2 H 2 O CH 4 C 2 H 4 C 2 H 6 Liquid HC Tar Char COMBUSTION 59.9 AND FUELS

11 MECHANISMS OF VOLATILE MATTER EVALUATION Dyfuzyjny płomień Diffusion flame Części Volatile lotne matter Produkty Products spalania of combustion Ciepło heat Rotation Bubbles Koks Coal O 2 Volatile matter

12 THERMAL DECOMPOSITION OF COKING COAL 1 st STAGE (coal/metaplast) 2 nd STAGE primary pyrolysis 3 rd STAGE secondary pyrolysis tar soot RAW COAL primary gases secondary gases char char

13 DYNAMICS OF VOLATILE MATTER EVALUATION Lignite Drying Young hard coal Hard coal Anthracite Coke intensity of v. m. evaluation Temperature C

14 CHAR STRUCTURE Pores Microscopic photo of char surface after coal devolatilization

15 CHAR COMBUSTION

16 MAJOR FACTORS INFLUENCING RATE OF CHAR PARTICLE COMBSTION C O 2 a) chemical: chemical kinetic of oxidation reaction (temperature, O 2 concentration) b) physical: transport of oxygen to the surface of char particle

17 Simple model of char particle combustion Oxygen Char particle Oxygen Reaction occurs here Carbon dioxide Model of reaction: C + O 2 = CO 2 Rate of reaction: W c = Ω Y s k, g/(m 2 s)

18 CHEMICAL MECHANISM OF CHAR COMBUSTION Basic chemical reactions of char combustion : C + O 2 = CO kj/mol (I) C + 0.5O 2 = CO kj/mol (II) C + CO 2 = 2CO kj/mol (III) 2CO + O 2 = 2CO kj/mole (IV)

19 COMBUSTION OF A SINGLE PARTICLE OF CHAR CO O 2 CO 2 2CO + O 2C 2 O 2 C + CO 2 2CO Distribution of primary and secondary products of oxidation of char

20 RATE OF CHAR OXIDATION Assumption: 1. Single chemical reaction of oxidation is considered C + O 2 CO 2 2. Char particle is considered spherical and smooth. C + O 2 CO 2

21 FORMULA RATE OF CHAR OXIDATION, Wc W C = Y 0 /(1/k + 1/α D ), kg/(m 2 s) Rate constant: k = k 0 exp[ E/(RT)], Stoichiometry coefficient: Ω (Ω = when burning is to CO 2 ), Mass transfer coefficient: α D = (D/d)Sh of (Sh = CRe n Pr k Units: W c [kg/(m 2 s)]

22 COMPETITION OF TEMPERATURE AND DIFFUSION

23 REGIMES TEMPERATURE AND DIFUSSION REGIMES OF CHAR REACTION Overal rate:w C = Y 0 /(1/k + 1/α D ) Remember: Arrhenius rate: k = k 0 exp[ E/(RT)], temperature dependent Mass transfer coefficient: α D = (D/d)Sh, temperature independent Two regimes of char burning Kinetic (reaction) control regime (low temperature): 1/k >> 1/α D W C = ΩY O k 0 exp[ E/(RT)] Diffusion control regime (high temperature): 1/k << 1/α D W C = ΩY O α D

24 LOW AND HIGH TEMPERATURE PROFILES AROUND CHAR PARTICLE Profiles of oxygen concentration Y Y s = O Y O Y O Y O Y s r r Y s = 0 r a) b) c) Low temperature - reaction control Transition High temperature diffusion control

25 REGIMES OF COMBUSTION OF POROUS CHAR PARTICLE III II I Reaction rate Szybkość reakcji Zewnętrzny obszar dyfuzyjny Wewnętrzny obszar dyfuzyjny Obszar kinetyczny 1/T c k=k o exp(-e/(rt) Bulk surface diffusion Pore diffusion Reaction control

26 Influence of temperature on the course of combustion of solid particle of coal 650 C 450 C

27 INFLUENCE OF AERODYNAMICS ON THE RATE OF COMBUSTION OF SINGLE COAL PARTICLE

28 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 in the rate of AND burning. FUELS

29 COAL COMBUSTION AND SLAGING PROCESSES

30 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 SiO 2 and hematite Fe 2 O 3, carbonates calcite CaCO 3, syderyt FeCO 3 and dolomite CaCO 3 MgCO 3, sulphur compounds pyrite FeS 2 i gipsium CaSO 4 2H 2 O.

31 SOLID RESIDUES AFTER COAL COMBUSTION MINERAL MATTER COMBUSTION FLY ASH, BOTTOM ASH

32 MEMBRANE WALLS DEPOSITS IN PULVERIZED COAL FIRED BOILERS Fly ash Wall deposits

33 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.

34 BASIC PROCESSES OF MINERAL MATTER CONVERSION IN THE FURNACE coal particles with bound mineral matter vaporasing of inorganic matter heterogenic condensation homogenic nucleation and coalescence surface covering small particles coalescence coal particles with bound and free mineral matter swelling cenospheres char disintegration

35 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).

36 TYPES OF DEPOSITS loose bound COMBUSTION sintered AND FUELS melted

37 SLAG DEPOSIT ON THE WALL OF A FURNACE

38 PULVERIZED COAL FLAME

39 STRUCTURE OF PULVERIZED COAL FLAME Pyrolysis v.m. Basic part of flame Char End of flame recirculation Flame front Unburned coal particles

40 JET OF PULVERIZED COAL FLAME

41 PULVERIZED COAL FLAME IN THE BOILER FURNACE

42 UNBURNED CARBON (UBC) IN FLY ASH

43 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

44 INFLUENNCE OF MINERAL MATTER IN COAL ON UBC W t ash content Ud ział p o p io łu 20% 10% 5% 40% 35% 30% 25% 20% 15% 10% 5% 0% 0,00% 90,00% 99,00% 99,90% 99,99% degree of burnout Sto p ień wy p alen ia High content of mineral matter in coal decreases UBC UBC UNITS: % Accepted level of UBC: 5%

45 BURNERS AND BOILER FURNACES

46 SELECTION OF PULVERIZED COAL FURNACES Antr. Hard coal Lignite Peat Biomass LCV, MJ/kg (daf) Vertical furnace, jet burners Volatile matter content, % Wall front furnace, swirl burners Tangencial furnace, jet burners

47 SWIRL BURNERS external recirculation secondary air rosette b ) oil lance external recirculation internal recirculation swirler vanes pulverised coal / air mixture Swirl number: S = Outlet velocity of pulverized coal/air mixture: Secondary air velocity: m/s m/s

48 FRONT OF SWIRL BURNER (40-50 MWth)

49 BACK OF THE SWIRL BURNER AIR BOX VANES OF AIR SWIRLER vanes

50 SWIRL BURNERS ON FRONT WALL OF THE OP-650 BOILER

51 STRUCTURE OF SWIRLED PULVERIZED COAL FLAME

52 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

53 JET BURNERS FOR BITUMINOUS COAL pulverized coal mazout pulverized coal air pulverized coal mazout pulverized coal

54 JET BURNERS FOR BITUMINOUS COAL

55 JET BURNERS FOR BITUMINOUS COAL

56 JET BURNER IN BOILER FURNACE OP 430

57 JET BURNERS FOR PULVERIZED LIGNITE Outlet velocities from the burner: Pulverized lignite/gas m/s, Upper and lower air m/s, Middle air m/s, Cooling and core air m/s. nozzle for ceramics cooling air nozzle for upper air 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

58 HOW PULVERIZED COAL IS TRANSPORTED FROM THE MILL TO BURNERS? Media transporting pulverized coal: Bituminous coal: Lignite: hot air, flue gas.

59 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, m 3 /h (flue gas) No. of pulverized lignite nozzles in the burner do No. of nozzles in a jet burner depending on the ventilation of a mill

60 FLAME PATTERN OF TANGENTIALLY FIRING FURNACE Corner furnace

61 BOILER DUCTS IN TANGENTIALLY FIRING FURNACES

62 FLUIDIZED COMBUSTION

63 BURNING IN CIRCULATING FLUIDIZED BED BOILERS

64 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: o C Optimal temperature wth respect desulfurization: 850 o C

65 COAL BURNING IN FLUIDIZED BED FREE BOARD ZONE CaSO 4 CaO ½ O 2 CO 2 CO 2 HO 2 CO 2 SO 2 CO 2 HO 2 C n H m CO C n H m CO 2 CaO coal ½ O 2 particle CO 2 CO ½ O 2 CaO fines CO 2 BED CO 2 sorbent CO char CO O 2 CO 2

66 LIGNITE BURNING IN LABORATORY FLUIDIZED BED

67 COAL COMBUSTION IN FREE BOARD ZONE OF FB

68 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.

69 SPECIAL BOILER FURNACES

70 RETORT FURNACE FOR SMALL BOILERS

71 RETORT FURNACE: COAL COMBUSTION

72 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

73 BOILER WITH CYCLONE COMBUSTION CHAMBER postcombustion zone air pulverised coal reburning zone secondary air coal air Primary air main combustion zone cyclone burner water liqiud slag