pulverized coal combustion - university of kentucky … coal combustion •rankine ... pre-heats...
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Pulverized Coal Combustion
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•Rankine (steam)•Brayton (gas)•Combined (gas and steam)
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Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.
Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor. Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur.
Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid
Work (area: maximize)
Tem
pera
ture
Entropy
1
2 3
4
Com
pre
ssio
n Heat In
Expansio
n
Heat Out (Condenser)
Thermal Loss (minimize)
Carnot (Perfect) CycleEvery heat cycle isa variation of this
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Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase change.
Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.
Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor.
Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur.
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Every heat cycle begins with a diagram like this.If it won’t work here, it won’t work at all.
Typical fossil plantuses 6 to 8 stages
of water heating
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Tem
pe
ratu
re
Entropy
Condenser
Boiler
Feedwater heater
1
2
3
4
5
6
7
Feed Pump 1
Feed Pump 2
LowPressure
Turbine
HighPressureTurbine
1234
5 67
Boiler
FeedPump 1
FeedPump 2
Feed-WaterHeater
Condenser
GeneratorHigh
Pressure
Turbine
LowPressure
Turbine
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Source: www.netl.doe.gov
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Raymond® Bowl MillAlstom Pulverizer
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Source: www.babcock.com
Type:Vertical, air-swept ball-and-ring, or ball-and-race.
Design features:Compact design; maintains performance with up to 80% weight loss of balls; stationary or variable speed rotating dual stage classifiers.
Capacity:Up to 23 t/h
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Source: navier.engr.colostate.edu
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Heating (cooling) through metal fluid boundaries
Tubes◦ Cu Alloy (Brass)
◦ Carbon steel
◦ Stainless Steel in hottest part of boiler
Sheets Shell and tube heat exchangers Boiler waterwalls, convection passes Condenser tubes Feedwater heater tubes
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www.babcock.com
Superheater
Reheater
Economizer
Superheater and reheateroutlet temperatures:usually in the range of 1000
to 1050oF (538 to 566oC).
Pressure:Subcritical, usually 1800 to 2400 psi (12.4 to 16.5 MPa)
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Source: www.jamarcompany.com
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www.babcock.com
www.mjpil.comwww.tubeweld.com
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Saturated steam is drawn off the top of the drum and re-enters the furnace through the superheater.
The drum stores the steam generated in the water tubes
Acts as phase separator for water/steam mixture by density difference
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www.jansenboiler.com
www.tubeweld.com
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Pre-heats water delivered to boiler◦ Improves thermodynamic efficiency
Reduces operating costs
Avoids thermal shock to boiler metal when feedwateris recycled back to the steam cycle
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Recover heat for pre-heating water and/or air
Located down-stream
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Burner
Firebox
Waterwall
Superheater
Reheater
Economizer
Preheater
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Avoid corrosion buildup◦ Blow tube soot; dynamite (or shotgun) and clean slag
Avoid water deposits on evaporating surfaces◦ Maintain absolute purity in water chemistry
Remove accumulations◦ Clean surfaces with special treatments
Avoid corrosion pitting◦ Tube plugging-induced loss of area
Test Performance◦ ASME/PTC Performance Monitoring
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Water is evaporated to make steam
Water is recycled
Salts build–up and precipitate
Layer forms and hardens on tube walls
Blow-down removes built-up layers
MgCO3, MgCl2, MgSO4 and Ca salts primary culprits
www.basukienergi.com
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To transfer heat◦ Minimize Temperature Delta: DT or ΔT: Th-Tc
Add heat incrementally
◦ Conductive effects
◦ Surface effects
Fundamental Idea: add heat incrementally◦ Reduce metal stresses; aging and burning
◦ Lower thermodynamic losses
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Expanding steamdrives turbine
Turbine drivegenerator
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Bi-directional turbine
Uni-directional turbine
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To transfer “waste” heat to atmosphere“waste” heat is wet heat
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Hyperboloid cooling tower was patentedby Iterson and Kuypers in 1918 Crucas nuclear power plant
in France
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Old answer: efficiency◦ Adding heat incrementally keeping ΔT low
New answer: emissions◦ Managing flue gas, ash, thermal and water
waste streams
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~40%
Heat rate is really a function of efficiency
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Rankine Cycle efficiency is typically 32% to 42%Low end is for typical PCCHigh end is for higher steam pressure
i.e. 600oK and 230 bar
Assume : 38% Rankine Cycle efficiency: 88% boiler and combustion efficiency
Question: What is overall conversion efficiency?
Answer: 38% x 88% = 33.44%
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Heat rate is heat input to produce 1 kWhr of electricity
1 kW = 3600 𝑘𝐽
ℎ𝑟
1 kWhr = 3600 kJ
@100% efficiency, 3600 𝑘𝐽
ℎ𝑟x
1
𝑘𝑊x 100% = 3,600
𝑘𝐽
𝑘𝑊ℎ𝑟
@ 33.44% efficiency, 3600 𝑘𝐽
ℎ𝑟x
1
𝑘𝑊x 33.44% = 10,765
𝑘𝐽
𝑘𝑊ℎ𝑟
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If the feed coal contains 20,000 kJ/kg (8598 Btu/lb)How much coal is needed to produce 100 MW of electricity?
HHV = 20,000 kJ/kgHeat Rate = 10,765 kJ/kWhr (from previous page)
𝑥 =10,765𝑘𝐽
𝑘𝑊ℎ𝑟𝑥
1𝑘𝑔
20,000𝑘𝐽=0.538kg
kwhr
0.538 𝑘𝑔
𝑘𝑊ℎ𝑟𝑥
1000 𝑘𝑊
𝑀𝑊𝑥100𝑀𝑊
1= 53,800
𝑘𝑔
ℎ𝑟=53.8
𝑡𝑜𝑛𝑛𝑒𝑠
ℎ𝑟= 59.2
𝑡𝑜𝑛𝑠
ℎ𝑟
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How much coal is needed to produce 759 MW of electricity (E.W. Brown)?
0.538𝑘𝑔
𝑘𝑊ℎ𝑟𝑥 1000
𝑘𝑊
𝑀𝑊𝑥 759 𝑀𝑊 = 408,342
𝑘𝑔
ℎ𝑟=408.3
𝑡𝑜𝑛𝑛𝑒𝑠
ℎ𝑟= 449.2
𝑡𝑜𝑛𝑠
ℎ𝑟
What if they used better coal, say 29,075 kJ/kg (12,500 Btu/lb)
𝑥 = 10,765𝑘𝐽
𝑘𝑊ℎ𝑟𝑥
1𝑘𝑔
29,075𝑘𝐽=0.370kg
kWhr
0.370𝑘𝑔
𝑘𝑊ℎ𝑟𝑥 1000
𝑘𝑊
𝑀𝑊𝑥 759 𝑀𝑊 = 280,830
𝑘𝑔
ℎ𝑟=280.3
𝑡𝑜𝑛𝑛𝑒𝑠
ℎ𝑟= 308.9
𝑡𝑜𝑛𝑠
ℎ𝑟
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What would it cost for better quality fuel?
12,500 Btu/lb coal = $54.90/ton + $10/ton freight = $64.90/ton delivered
64.90$
𝑡𝑜𝑛𝑥 308.9
𝑡𝑜𝑛𝑠
ℎ𝑟= 20,047
$
ℎ𝑟
20,047$
ℎ𝑟𝑥
1
759𝑀𝑊𝑥
𝑀𝑊
1000𝑘𝑊= 0.0264
$
𝑘𝑊ℎ𝑟= 2.64
₵
𝑘𝑊ℎ𝑟
Current KU electricity rate: $0.08508 /kWh
2.64
8.508= 31.03% for fuel
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