Chapter 6 Steam Generators MEE 325 Power Plants Engineering

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1 Chapter 6 Steam Generators MEE 325 Power Plants Engineering Atikorn W. Mechanical Engineering Department King Mongkut s University of Technology Thonburi

2 Basics components Main Structure Furnance Superheater Reheater Boiler or Evaporator Economiser Air preheater Auxiliaries Pulverizers Burners Fans (ID &FD) Stokers Dust collectors Precipitators Ash handling equipment Chimney or Stack

3 6.1 Basics type of steam generators Classification by applications Utility steam generators are used for electric-power generating plants. Subcritical or supercritical (221.1 bar) can be used, subcritical range between bar, supercritical is above 240 bar. Generation rate ranges from kg/s of plants output ranging from MW.

4 6.1 Basics type of steam generators Industrial steam generators mostly used in process industries like sugar, paper, chemical and so on, and institutions like hospitals, hotel, residential, commercial, building complexes. They operate at P ranging from bar with steam ranging up to 125 kg/s of saturated steam, wet steam or hot water. Marine steam generators are used in many marine ships and ocean liners driven by steam turbines. They are usually oil-fired to produce superheated steam at about bar and 540 C.

5 6.2 Fire-tube boilers Using in industrial purpose, hauling railway locomotives and river launches to produce sat. steam at the upper limit about 18 bar and 6.2 kg/s (22.32 Tph) Advantages low first cost reliability in operation need of only unskilled labor less draught required quick response of load changes Named Externally fired ( Locomotives, Lancashire boilers, Horizontal Return Tubular (HRT) Internally fired (Scotchmarine boilers, Package boiler)

6 Fusible plug: made of lead-based plays as a safety which will melt due to overheating of the furnace. Safety valve: a spring load device helps to release the over pressure of steam

7 Corrugated flue increase strength, allowing thermal expansion and contraction. Safety valve: a spring load device helps to release the over pressure of steam

8 Advantages of Package boilers Highly compact in size No ID fan is required, stack draught Easy of fabrication and installation Occupies less space Easy to operate Disadvantages Definite size and pressure limitations Limited or maximum operating P. Drum or Vessel design thin-wall pressure vessel with internal pressure P.d/(2t) (Pa)

9 6.3 Water-tube boilers Straight-Tube Boilers Tubes are in size of mm O.D. Upward about 15 to the horizontal Drum is smaller pressure vessel Relies on downcomer-riser circulation (Natural or Foced) Longitudinal drum are limited for low P. (12-23 bar, kg/s) of heating surfaces of m 2. Cross-drum could accommodate for low P. ( bar, kg/s) of heating surfaces of m 2. Disadvantages Less accessibility and inspection, time consume and costly gaskets. Inadequate design of handhole caps resulted in much leakage. Circulating sluggish due to low head reducing steam rate.

12 6.3.2 Bent-Tube Boilers Four-drum Stirling boilers and Two-drum simplification design. Advantages of Package boilers Greater accessibility for inspection, cleaning and maintenance Operate at higher steaming rate Obtained drier steam quality Occupies less space easy to operate

13 6.3.3 Heat absorption in water-tube boilers Heat is typically input at Economiser, Evaporator (Downcomer- Riser circuit) and Superheater. Heat absorption Q Econ = h2-h1 Q Evap = h3-h2 Q Sup = h4-h3 Q Total = h4-h1

15 6.3.4 Circulation A simple downcomer-riser circuit with natural circulation Available head P = gh ( D - m ) D = Density of sat. water in the downcomer m = Mean density of steamwater mixture in the riser H = Height of the riser (about the height of the furnance) m = ( D + top ) /2 top = 1/( f + x fg )

16 Circulation important Higher density difference is more head available. Higher working pressure is risk of circulation block. At critical P is no more natural circulation. If the boiler pressure is less than 30 bar, the downcomers are placed outside the furnace to secure more density difference. If P> 30 bar, both downcomers and riser are placed inside the furnace but the riser placed in the hotter zone. For P>180 bar the forced circulation with pump is needed.

17 Circulation Ratio CR = Flow rate of sat.water in downcomers Flow rate of steam released from the drum CR = m g+m l m g = 1 = 1 m g /(m g +m l ) TDF Natural circulation The hotter riser generate higher steam, has less CR than cooler riser Any riser should not have CR less than 6. The CR should not be higher than 25 for effective utilization of the tube in generation. Too much steaming can cause bubbles forming to vapor film which lead to lower heat transfer of heat through the tube wall. It may rupture the tube in high temperature.

19 Natural circulation is depended on the pressure balance of pressure drop in the loop. P = gh ( D - m ) = P D + P R + P bends + P header Then the number of downcomers is less but larger than the risers tube

20 6.3.5 Steam Drum Mainly function To store water and steam sufficiently to meet varying load requirement To aid in circulation To separate vapor or steam from water-steam mixture, discharged by the risers. To provide enough area for solid-liquid disengagement To maintain a certain desired ppm in the drum water by phosphate injection and blowdown Impurities in = Impurities out fw x ppm fw = BD(ppm) drum + (1-x) s (ppm) carryover TDS control in the drum could be varied from ppm

21 Mechanical steam drum design

22 6.3.6 Modern water tube boilers (describe in p.344)

23 Boiler wall tube Heat absorbed by radiation (T g 4 T w4 ) Tangent touching Tubes embedded Studded tube Membrane tubes Tubes space about of tube diameter.

24 Forced circulation boilers (La Mont type) Steam production ranges for Tph, and working P bar.

25 Once-through boilers (supercritical boiler P> bar) Once-through boilers by mean of supercritical boiler, it is no steam drum since the steam sublimation occurs in supercritical region.

26 6.4 Economizers Hot gas input ( C) Output ( Tsat@P or Sat. mixture up to 20% quality Q Eco = ω g C pg (t g1 t g2 ) = ω fw C pfw t sat t fw = U 0 A 0 t 1,m

27 6.5 Superheaters Convective SH, Radiant SH, Pendent SH (combined Conv. & Rad.)

28 6.5 Superheaters Convective SH

29 6.5 Superheaters Convective Superheater (Gas to Tube) Q CSH = ω g C pg (t g1 t g2 )= ω s h 2 h 1 designed max. steam velocity in CSH; LP-IP = 22 m/s HP = 12 m/s ; Very HP = 10 m/s Radiation Superheater (Flame to Wall) Q RSH = σa T F f w (T f 4 T w 4 ) = ω s h 3 h 2 = Stefan-Boltzmann constant = 5.67 x 10-8 F f-w = View factor T = absolute temp. in Kelvin

30 6.5 Superheaters

31 6.5 Superheaters Pendent SH (combined Conv. & Rad.) ¼ Area Counter flow ¾ Area Counter flow Steam T = 560 C Gas T = 1100 C Some parallel flow could raise T up without Tube failure of over heat

32 6.6 Reheaters Convective Reheaters

33 6.8 Air Preheaters Two types of APH; Recuperative and Regenerative

34 6.8 Air Preheaters Regenerative

35 6.7 Steam generator control Steam temperature control Principal variables of superheat temp.: 1. Furnace Temp. 2. Cleanliness of CSH, RSH and PSH 3. Gas temp. to enter the CSH 4. Mass flow rate of gases through the CSH 5. Feedwater temp. 6. Steam load variation (most important)

36 6.7.3 Steam temperature control; Desuperheating/Attemperation

37 6.7.3 Steam temperature control; Desuperheating/Attemperation Use only high purity water for spray High chrome steel used as thermal sleeve to protect the main pipe Control of mass flow of water spray to vary steam outlet temp.

38 6.7.3 Steam temperature control; Gas by-pass or damper control

39 6.7.3 Steam temperature control; Gas recirculation

40 6.7.3 Steam temperature control; Tilting burners Full load (Steam) Partial load Low load

41 6.9 Fluidized Bed Boilers 1. Bubbling Fluidized Bed (BFB) Boilers

42 6.9 Fluidized Bed Boilers 2. Circulating Fluidized Bed (CFB) Boilers

43 6.9.2 Cyclone Separators F c = m p θ θ 2 r

44 6.9.3 Advantage of CFB boilers Fuel Flexibility Ex. rice husk, wood chips, oil shale, etc. High combustion efficiency as high as 99%, if the furnace height is well designed. Efficient sulphur removal with limestone (CaCO 3 ) and dolomite (CaCO 3,MgCO 3 ) chemical reaction. Low NOx emission Simpler fuel handling and feed system Small furnace cross-section Good turndown and load following capability High availability

45 6.10 Electrostatic Precipitator

46 6.11 Fabric filters and Baghouses

47 6.11 Fabric filters and Baghouses

48 6.14 Deaeration Heat input required, normally steam Decrease in dolubility of dissolved gases, O 2, CO 2

49 6.17 Boilers blowdown % blowdown = quantity of water blowdown quantity of feed water 100 % blowdown = m blowdown m generation 100 = TDS FW TDS control TDS FW 100 Blowdown is a way to control TDS in the desired range There is no one number fits all boilers More blowdown = more steam loss = fuel compensating Blowdown reduction and heat recovery is one effective energy saving

50 End of Chapter 6

Module-3A: Heat Transfer Equipment. Applied Thermo Fluids-II (Autumn 2017) Dr. M. Ramgopal, Mechanical Engineering, IIT Kharagpur

Module-3A: Heat Transfer Equipment Applied Thermo Fluids-II (Autumn 2017) Dr. M. Ramgopal, Mechanical Engineering, IIT Kharagpur Introduction From 1 st law of thermodynamics applied to the combustion reaction;