Results of declining coal quality on boiler remnant life T Vosloo/K McIntyre

Transcription

1 Results of declining coal quality on boiler remnant life T Vosloo/K McIntyre July 2014

2 Results of declining coal quality on boiler remnant life Watertube Boilers Package Boilers

3 Water tube Coal size Inhibition of heat transfer Overheating Grate Tubing Phosphorus Sulphur Erosion Corrosion High ash content

4 Water tube coal size Ideal coal size Washed peas 2-8% < 3mm 0-10% < 6.3mm 95% < 25mm 0% 30mm

5 Water tube coal size Problems associated with coal size - thermal Bad particle distribution leads to Uneven burning giving hot spots Excessive bed thickness causing metal burning Fin cracking

6 Water tube -coal size Problems associated with coal size - mechanical Bad particle distribution mechanical damage to tubing. Blockage of spreaders Incompletely burnt coal with ash handling problems

7 Water tube coal size damage grate

8 Water tube coal size Hot spots resulting in heavy scale and metallurgical damage to tubes and grate Heavy scale build up on tubes giving a burned appearance

9 Water tube coal size - grate damage Build up at rear of stoker Resulting in refractory loss and damage Exposure of manifolds Serious damage to stoker carrier keys and side keys

10 Water tube coal size grate damage Thick beds can lead to burning of the grate bars wall blocks and rear wall seals

11 Water tube coal size grate damage Materials degrade at the high temperatures even the high quality nodular irons which can function up to 900 C

12 Water tube coal size - grate damage

13 Water tube coal size - grate damage Damaged wall blocks and rear seals

14 Water tube coal size grate damage Cracked furnace wall fins

15 Water tube- Overheating Formation of scale Scale forms when water converts to steam and leaves impurities behind Scale is a poor conductor of heat To create the same amount of steam more heat is required The steel overheats as the cooling is inadequate

16 Water Tube -Overheating Departure from nucleate boiling

17 Water tube - Overheating Δ T Effects of scale on heat transfer

18 Water tube - Overheating Tube metal temp - Deg C k=0,105 k=0,2 k=0,495 k=0,995 k=1,73 Nature of scale Thermal conductivity (W/mK) Range Av. Silicate 0,08 0,13 0,105 Porous 0,09 0,90 0,495 CaSO 4 0,69 1,3 0,995 CaCO 3 0,86 2,6 1,73 Basis: BCURA Information Circular No. 15 (British Coal) Scale thickness - mm Effects of scale on heat transfer

19 Water tube Overheating Localised overheating of tubes Swelling/bulging of tubes due to metallurgical degradation

20 Water tube - Overheating

21 Water Tube -Overheating Excess scale deposit in mainbank tube

22 Water tube - Overheating Short term overheat

23 Water tube - overheating

24 Water tube Overheating Long term Overheat Tree bark Thick wall fracture Internal and external scale Metal degradation

25 Water tube Overheating

26 Water tube -Overheating Failures - Long term overheat Surface microstructure of long term overheat Loss of pearlite on surface Intergranular cracks

27 Water tube - Overheating 700 C S Spheroidisation / Graphitisation S - spherodising G - graphitising G time 10 5

28 Water tube - Overheating Degradation Degrades with time above a certain temperature Spheroidising or graphitising starts 430ºC for carbon steel The process is irreversible The steel is weakened

29 Water tube - Overheating Increasing time

30 Water tube - Overheating

31 Water tube - Overheating Spheroidising

32 Water tube - Overheating Graphitisation

33 Water tube - Overheating

34 Water tube - Overheating

35 Water tube High ash Fuel ash corrosion Superheater corrosion Coal K SO /Na SO form tri sulphates -- K Fe(SO ) / Na Fe(SO ) -in the presence of SO rapid corrosion 590º -680 ºC due to corrosion of oxide layer. HFO V O.K O; V O.Na O forming low melting V O. K SO ; V O. Na SO 540 º -840ºC. Furnace walls are subject to corrosion from sodium and potassium pyrosulphate Na /K S O which melt below 400ºC High Temp/pressure units

36 Water tube High ash Severe pitting of furnace tubing

37 Water tube Phosphorus Fouling 0.03% Max P in coal Metal temps 320º-360ºC Results in drop in steam superheater performance The higher the temp the harder the deposit

38 Water tube Phosphorus Economiser fouled and corroded

39 Water tube Phosphorus Fouling 0.1% Max P in 0.03% Max P in

40 Water tube High ash Bird nesting

41 Water tube High ash Ash build up in the economiser

42 Water tube - sulphur Sulphur deposition

43 Water tube - sulphur Back end dye point corrosion Airheaters Casings S + O = SO SO + H O = H SO Dew point corrosion at temps less than 140ºC

44 Water tube High ash Typical economiser erosion due to high ash content

45 Water tube High ash

46 Prevention Monitor and control backend temperatures 16Cº increase in outlet temp is equivalent to 1% decrease in efficiency (heat out/heat in)

47 Package Boilers

48 Package Boilers Spiral tubes Designed to EN Fully wet back Reversal Chamber

49 Package Boilers Coal Segregation Larger coal particles separate from the fines into different zones Generally, the larger particles travel to the sides furthest from the feed point Uneven burning rate Low stoker efficiency Reduced boiler load

50 Package boiler coal size Blockage of spreaders Incompletely burnt coal with ash handling problems

51 Package boiler coal size Ash build up Blocking ash chutes and affecting gas flow

52 Package boiler coal size The recommendations from JT are that O 2 must not exceed 12%. Calculations show that increasing the oxygen content will increase the gas velocity. The CV of coal is given as 6300kcal/kg. At 9%O 2 this gives a velocity of approximately 39m/s. Increasing O 2 to 13.5% increases the velocity going up to 64.8m/s)

53 Package boiler coal size Altered gas flow results in Fouling and birdnesting Blocked ash ports can lead to high gas flows

54 Package boiler coal size Over firing Irregular coal and low quality coal can lead to clogging in stoker high O in flue. This leads to high temps which leads to overheating of tube ends resulting in Thermal fatigue. JT tube plate temp is designed to be 380ºC max

55 Package boilers Coal size Over firing Schematic representation of cracking

56 Package boilers Coal size Over firing Thermal fatigue cracks

57 Package boilers Coal size Departure from Nucleate boiling on a fire tube

58 Package boilers Coal size Thermal fatigue of furnace tube end

59 Package boilers- high ash content High ash content with high gas flow Erosion of heat transfer surfaces leading to leakage

60 Water tube High ash Fuel ash corrosion Superheater corrosion Coal K SO /Na SO form tri sulphates -- K Fe(SO ) / Na Fe(SO ) -in the presence of SO rapid corrosion 590º - 680ºC due to corrosion of oxide layer. HFO V O.K O; V O.Na O forming low melting V O. K SO ; V O. Na SO 540º -840ºC. Furnace walls are subject to corrosion from sodium and potassium pyrosulphate Na /K S O which melt below 400ºC High Temp/pressure units

61 Design Parameters Parameter Coal firing Oil and gas firing Requirements to be satisfied Reversal chamber leaving temperature to be 200 o C below initial deformation temperature of ash Reversal chamber leaving temperature up to 1100 o C

62 Package boilers- Phosphorus Heavy deposits on heat transfer surfaces inhibiting heat transfer. High back end temperatures

63 Package boilers- Phosphorus Heavy deposits on heat transfer surfaces inhibiting heat transfer. High back end temperatures. Can also be due to overfiring

64 Package boilers- Sulphur Sulphur deposition and corrosion.

65 Worst case scenario

66 Worst case scenario

67 Questions?

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69 Prevention

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