Ch3 Steam Boiler - Sub-critical.pdf

Boiler - Steam Generators Contents: Introduction Types of steam generators Main components Thermodynamic analysis Operation Maintenance Summary

Introduction 1. Function of steam generator: To heat & convert water from liquid phase to superheat steam at the specified pressure by addition of heat

Feedwater

BOILER Heat

Fuel 2. Heat is obtained by burning of fuel i.e. chemical energy => thermal energy (heat)

Steam

Types of Steam Generators (1) 1. Can be classified by its: Capacity Operating pressure Fuel type 2. Capacity:

Application Heat transfer direction Water circulation

Steam generation capacity (tonne/hour, kg/s) Steam thermal energy (MWh)

3. Operating pressure : the pressure at which the boiler is operating 4. Fuel type : coal, oil, gas

Example: Kapar Power Station Phase I 266 kg/s, SH outlet 172 bar, 538°C Oil and gas fired

Types of Steam Generators (2) 5. Applications: UTILITY 130 < P <240 bar T~540°C 125<m<1250 kg/s 125<W<1300 MW

INDUSTRIAL P < 105 bar M < 125 kg/s

6. Heat transfer direction : Fire-tube boiler

Water-tube boiler Heat

Heat Hot gas

Water

Water

Hot gas

Types of Steam Generators (3) a. Fire-tube boiler: * A water-filled vessel with combustion product in tubes * Heat transfer is from hot gas from tube to water in the vessel * Limited in size, steam pressure, & low level of operating safety * No longer used in power plants Still used in industrial plants ( P <18 bar, m <6.3 kg/s)

Heat Hot gas Water

Types of Steam Generators (4) b. Water-tube boiler : * Water flows inside tubes & the combustion gases flow outside * Heat transfer is from hot gas to water in the tubes * Higher in capacity, steam pressure, & high level of operating safety * Widely used in power plants * Various designs: forced circulation, natural circulation, once-through

Heat Water Hot gas

Types of Steam Generators (5) 7. Water circulation: a. Natural circulation: water circulates by virtue of density difference (factor 8) between the water in the downcomer Downcomer and risers

Steam drum

Riser

b. Controlled-circulation: water circulation is helped by pumps; e.g TNB Janamanjung c. Forced circulation : water circulation entirely dependant on external pumps. d. Once-through supercritical boiler: Water-to-superheat steam formation happens in one pass, No steam drum

Heat

Pump

Types of Steam Generators (6) Supercritical Boilers Today many modern steam power plants operate at supercritical pressures (P > 22.06 MPa) and have thermal efficiencies of about 40% for fossil-fuel plants.

Main Components (1) 1. Major components of steam generator: BOILER STEAM DRUM

Drum Superheater Reheater

SUPERHEATER Boiler

Economiser

REHEATER ECONOMISER AIR HEATER

Air heater

Main Components (2) 2. Steam flow:

From boiler feed pump

Economiser Drum

Steam Drum

Superheater Reheater

Boiler: Downcomer Boiler

Boiler: Riser

Economiser

Air heater

Steam Drum Superheater To HP Turbine

Main Components (3) 3. Economiser: a. Raises water temperature to saturation temperature, at the boiler’s operating pressure; (steaming avoided) b. Utilises high temperature gas leaving superheater or reheater (convective) c. Important because increase efficiency: ~1% for every 5.5°C rise in Tfeedwater d. Conditions to prevent internal corrosion of economiser tubes: Exit T > above SOx or acid dew point in flue gas. Dissolved O2< 0.007 ppm 8 < pH < 9

Drum Superheate r

Boiler

Reheater Economiser

Air heater

Main Components (4) 4. Steam drum: a. Chamber that separates water & steam b. Water-steam separation methods: * primary separation: removes water from steam * secondary separation (drying) : removes remaining mist and droplets from steam * methods: baffles, screens, bent plates, cyclones c. Other functions of steam drums: * control water-steam mixture during load changes * chemical dosing point for water treatment * removes particulate matter from steam

SATURATED STEAM CONNECTIONS DRUM SLING

EXTENT OF ILLUSTRATION

METH OD OF ATTACHING CYCLONE SEPARATORS TO CONNECTING BOXES

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ECONOMISER RISER CONNECTIONS

DRAIN POT

FEED WATER FROM ECONOMISER

VORTEX INHIBITOR

KEY

WATER TO BOILER

~STEAM

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Main Components (5) 5. Furnace-Combustion Chamber: a. Converts saturated water to saturated steam b. Also refers to entire steam generator c. Consists of water walls: downcomers and risers d. Water tubes located on the furnace walls and on top of the furnace e. Radiation => primary mode of heat transfer f. Heat source is from fuel combustion via burners

FIG.

1.8

Corrosion of furnace wall tubes at Drakelow C Power Station

FIG. 2.20 Photograph of a panel of co-extruded tubing installed in a furnace sidewall at Eggborough Power Station Note the nearness of the end burner to the sidewall. allowing little room for error in setting up the burner conditions.

Main Components (6) 6. Superheater: Drum

a. Converts saturated steam to superheated steam b. Normally built in 2 stages: primary superheater & secondary superheater c. 2 types (according to source of heat): Convection: - placed in gas passage - heat transfer by convection Radiation : - placed above furnace - heat transfer by radiation

Superheater Reheater

Boiler

Economiser

Air heater

Before

During

After

FIG.

1.3

Typical off-load water cleaning of platen superheater elements

Main Components (7) 7. Superheater (..continued):

Pendant

c. Two types (according to construction): Pendant: - tubes hung from the roof - not drainable Platen : - tubes arranged side by side to form a wall d. Main criterion in superheater tube selection is its temperature strength e. Degree of superheat is determined by: * position of the superheater * amount of superheating surface * velocity of steam through the tubes

Platen

Main Components (8) 8. Reheater: a. Reheat steam from HP turbine

Drum Superheater

b. Used to limit excessive moisture in steam to about 10~16% c. Usually pendant type and placed behind the secondary superheater d. Cold reheat: from HP turbine to reheater Hot reheat: from reheater to LP turbine

Reheater

Boiler

Economiser

Air heater

Main Components (9) 9. Air heater: a. Exchanges heat from outgoing exhaust gas to incoming fresh air

Drum Superheater Reheater

b. Increases system thermal efficiency Boiler

Economiser

c. Two types: * recuperative > heat transfer direct from gas to air across heat-exchanger * regenerative > heat transfer from gas to air via intermediate heat-storage medium

Air heater

Boiler view

Heat transfer area d. Main problems: * corrosion => keep flue gas above acid dew point Tadp * chokage due to fly ash clogging => regular cleaning

Thermodynamic Analysis (1) 1. Steam generator performance is represented by its efficiency ηB 2. Boiler efficiciency calculation: Input/output method

ηB =

Thermal energy transfer to working fluid Thermal energy released by fuel

m& S (h2 − h1 ) + m& RH (h4 − h3 ) = Q& in

Reheater outlet



Reheater outlet



Fuel

 Reheater inlet

Heat



Q

Superheater outlet

 Reheater inlet 

Air

Superheater outlet

Exhaust gases


Feedwater inlet

Simple model

Heat loss




Feedwater inlet

Actual

Thermodynamic Analysis (2) 3. Other method: heat loss method

(HHV) − ∑ Lossi ηB = (HHV)

× 100%

4. Major sources of losses from boiler system: a. Incomplete combustion [2.5~3.0%] b. Unburned carbon [1~2%] c. Sensible heat of dry gas [~10%] d. Evaporation of moisture in fuel [5~6% for coal] e. Evaporation of moisture in air [ 0.5~0.8%] f. Thermal radiation of boiler [~0.2%] 5. Methods to calculate boiler losses: a. approximate: data from fuel analysis & flue gas Orsat analysis b. more accurate: ASME Power Test Code 4.1

Thermodynamic Analysis (3) 6. Thermal loads in steam generators: a. Economiser:

Drum Superheater

qe = m& (heo − hei )

Reheater

b. Boiler: c. Superheater: d. Air heater:

qb = m& (hg − heo ) Boiler

Economiser

q sh = m& (hsh − hg )

qah = m& a (hae − hai ) ≅ m& a C p (Tae − Tai )

7. Heat rate HR: rate of heat added to steam generator Heat Rate = Net plant power output, kW

Air heater

Operation (1) 1. Steam generator is primarily designed to generate steam at rated load, i.e. under specified pressure, temperature and flowrate conditions 2. This is achieved once stable conditions has been established a. correct thermal gradient c. expansions completed b. all clearances are normal d. shaft alignment within limit 3. Types of plant start-up: a. Cold start b. Warm start d. Very hot start

c. Hot start

Operation (2) 4. Typical conditions for cold start: a. plant shut down for long period (> 48 hours)

d. boiler depressurised & drained

b. turbine metal temperature < 298°C

e. turbine shaft at rest

c. feedwater system drained 5. Typical conditions for warm start: a. plant shut down for between 8 to 48 hours b. turbine metal temperature between 298 - 400°C

c. boiler steam between 50°C above turbine metal temperature d. turbine shaft on barring

Operation (3) 6. Typical conditions for hot start: a. plant shut down between 2 to 8 hours b. turbine metal temperature > 400°C c. boiler steam 50°C above turbine metal temperature d. turbine shaft on barring 7. Typical maximum rate of metal temperature increase is 5°C per minute

8. Planning and operational activities required to bring large unit from cold to full load are indicated by its critical path

Dr. Mohd Hariffin Boosroh

Operation (4)

Steam Turbines: Boiler

CRITICAL PATH OF BOILER START-UP Feed pump check

D/A filling pump check

Fill D/A

Prime & start feed pump

Prime HP heaters

Fill boiler

Sootblower system check

Main steam and reheat drains check

Reheat safety v/v check

HP chemical dosing check

Boiler drains & vents check

Boiler dampers & actuator check

Dose prepared

Dose to chemist instruction

Fill boiler

Start air heater

Boiler inspection doors check

Precipitator check

ID fan pre-start check

FD fan pre-start check

Burner pre-start check

PA fan, blowdown vessel, sprays and pumps pre-start check

Boiler recirculation & spray

Start ID fan

Start FD fan

Burner i/s

Raise boiler pressure

Maintenance (1) 1. Typical problems/maintenance: a. Tube scales d. Foaming b. Tube fouling e. Tube corrosion c. Tube slagging f. Caustic embrittlement 2. Corrosion: * metal oxidation which forms “rust” that goes into solution in the boiler water * also due to electrolytic action of two metals * prevention: removal of dissolved O2 via deaeration, Corrosion 1 sacrificial anodes

Maintenance (2) 3. Scale formation: * hard substance created when mineral salts come out of solution as their solubility drops

steam

* typical components: calcium sulphate, calcium and magnesium carbonates, and silicates * adhere directly to heating surfaces => substantially decreases heat transfer efficiency

Tube rupture due to scale

* results in metal fatigue/failure causing overheating, energy waste, high maintenance costs and safety risks * prevention: settling tanks, distillation of water, chemical treatment e.g. slaked lime, soda, phosphates

Maintenance (2) 4. Fouling: * Accumulation of ash on heating surfaces * Occur when volatile matters & Al2O3, SiO2 etc. co-exist

steam

* Prevention: - approach temperature of convection heat surface is limited to under vapor point of volatile constituents (approx. 800oC) - Sooblowing 5. Slagging: * Melting ash that adheres to furnace wall & heating surfaces * Composed of composed of Al2O3,SiO2,Fe2O3,MgO,CaO etc. * Prevention: - Design burner zone heat rate within adequately suitable range - Sootblowing

Maintenance (3) 6. Foaming: * concentrations of soluble salts create bubbles in steam * can cause priming: bubbles break & create liquid that later form slugs of water => destructive to steam blades, valves & piping * prevention: steam traps

Steam traps

7. Caustic embrittlement: * hairline cracks in highly stressed areas due to high concentrations of alkaline salts * alkaline salts liberate hydrogen, absorbed by iron in steel, changing its physical properties

Maintenance (4) 8. Water treatment: * Purpose: to provide plant with properly treated water in sufficient quantities to meet plant needs * Treatment methods: Chemical treatment

Conditioning the water to pre-determined levels by using a variety of chemicals

Demineralization

Replacement of specific inorganic salts by ion exchange

Deaeration

Removal of dissolved oxygen and carbon dioxide by heating and bombarding the water with steam Deaerator

Summary 1. Steam generators are applied for utility and industrial uses 2. Types of steam generators: fire-tube and water-tube 3. Types of water-tube steam generators: natural-circulation, controlled-circulation, and once-through 4. Main components: boiler (furnace), drum, economiser, superheater, reheater, air heater 5. Performance is represented by boiler (steam generator) efficiency 6. Boiler start-up operation is determined by the its initial state: cold, warm or hot start-up 7. Boiler tube maintenance: scaling, fouling, corrosion, foaming and caustic embrittlement

Industrial Fire-tube Boiler

1. Water Level Controls 2. Main Steam Outlet Valve 3. Steam Separator 4. Safety Valve 5. Manhole for Access and Inspection 6. Flue Outlet Flanged Rear or Top 7. Twin Reflex Gauges 8. Feed Water Pump Interconnected to Boiler

9. Reverse Flame 10. Off Inspection Belly Handholes 11. Supporting Beams 12. Steam Space 13. Self-Adjusting Hinges 14. Automatic Burner High/Low Modulating 15. Tubes 16. Opening Door with Ceramic Fibre Insulation 17. No Tubes Above Furnace

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Circu'ia'tion Pump Applicable Pressure Range

Subcritical

Subcritical

Subcritical Supercrltical

Subcritical Supercrltlcal

Steam Condition (CJCampte)

t 7MPa >< 538/538'C

t7MPa x 538/538'C

24MPa >< 538i538'C

Efliclency

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1.00

1.02

1.02

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- 900 MW

-900 MW

- 1300MW

- 1300 MW

24MPa x 538/536'C

Economiser

Economiser tubes with fins

Steam Drum STEAM DRUM INT Cyclon e Se parat or

HORIZONTAL SEPARATOR DRY BOX

Gl RTH BAFFLE CHEVRON DRYER

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405 t/h steam drum

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Boiler furnace

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Air Heater Ljungstrom (ABB) Ljungstrom

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Boiler tube maintenance

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Sootblowers

Sootblower operation

Sootblower gun

Tube fouling

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Tube scaling & corrosion

Tube corrosion Tube scale

Tube split due to corrosion

Steam traps Disc steam trap

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Dr. Mohd Hariffin Boosroh

Boiler furnace

Steam Turbines: Boiler

Types of Flames

FGD Process Flow diagram

DESULFURilED GAS

flUE GAS

STACK

HEAT EXCHANGER

AMBIENT AIR

SEAWATER -----------------'-----~-----o~.-a.

SEAWATER DISCHARGE