Pulverized Coal Combustion

•Rankine (steam)•Brayton (gas)•Combined (gas and steam)

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

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.

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

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

Source: www.netl.doe.gov

Raymond® Bowl MillAlstom Pulverizer

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

Source: navier.engr.colostate.edu

Source: www.babcock.com

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

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)

Source: www.jamarcompany.com

www.babcock.com

www.mjpil.comwww.tubeweld.com

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

www.jansenboiler.com

www.tubeweld.com

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

Recover heat for pre-heating water and/or air

Located down-stream

Burner

Firebox

Waterwall

Superheater

Reheater

Economizer

Preheater

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

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

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

Expanding steamdrives turbine

Turbine drivegenerator

Bi-directional turbine

Uni-directional turbine

To transfer “waste” heat to atmosphere“waste” heat is wet heat

Hyperboloid cooling tower was patentedby Iterson and Kuypers in 1918 Crucas nuclear power plant

in France

Old answer: efficiency◦ Adding heat incrementally keeping ΔT low

New answer: emissions◦ Managing flue gas, ash, thermal and water

waste streams

~40%

Heat rate is really a function of efficiency

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%

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

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

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

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