1Vorlesung WKA

1. Lecture Industrial Energy Management

Rankine Cycle Optimisation

Jun-Prof. B. Fond

benoit.fond@ovgu.de

G10/R119

2Vorlesung WKA

Objectives :

• Apply thermodynamic principles to improve the Rankine cycle, via additional components (feedwater heaters, reheating, combined cycle)

• Understand heat transfer limitations

• Understand the process diagram of large scale steam power plant

Rankine Cycle Optimisation

3Vorlesung WKA

Ideal Rankine cycle

Four processes :

• 1 – 2 : Isentropic

compression – Pump

• 2 – 3 : Constant

pressure (and

temperature) heat

addition – Boiler

• 3 – 4 : Isentropic

expansion – Turbine

• 4 – 1 : Constant

pressure (and

temperature) heat

rejection – Condenser

How to increase efficiency ?

𝑡ℎ < 1 −𝑇3𝑇4

4Vorlesung WKA

Efficiency improvement

1. Simple Rankine cycle : Increase maximum temperature (superheat)Increase boiler pressure (or maximum average temperature)Decrease condenser temperature (and pressure)

2. Regenerative feed water heating 3. Reheating 4. Combined cycle 5. Heat and Power (Cogeneration)

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• Increase maximum temperature

3-3’-4’-4 has higher efficiency than 1-2-3-4 so improvement

Limited by temperature of steel to

be used (~600 °C)

Increase steam quality at 4, decreasing erosion damage of blades

• Increase boiler pressure at fixed max. T -> increases average temperature of heat addition

Will decrease steam quality if no increase in max. T Supercritical rankine cycle

Simple cycle

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Simple cycle

• Decrease minimum temperature

Less heat rejected -> higher efficiency

Limited by available cold source. For heat exchange, Q ~ hA x TT4-Tcw > 10 K for reasonable size of heat exchanger

Adverse effect: Lower steam quality

Water cooled

condenser

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• Regeneration : Heat exchange between the fluid in one process during the power cycle and the working fluid in another process of the cycle

• Idea : -> Carnotisation of process.

• Heat transfer during expansion recovered during boiling. For infinite number of heat exchange steps : -> Carnot cycle

• With only heat exchange between steam and water, the main problem is decrease in steam quality during expansion

• Solution : Bleed steam -> The steam is extracted at intermediate pressure and condensed. Latent heat of condensation converted into sensible heat of condensate.

Regenerative Heat exchange

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• On the board …

Regenerative heat exchange

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• The more feedwater heaters the closer the cycle to Carnot cycle.

• Non contact heaters (NCFWH): Only need one pump on condensate loop

• Direct contact heaters: Pressure must be equal : additional pump. However it allows deaeration of dissolved gas, e.g. O2 that leads to corrosion.

Feedwater heaters (FWH)

Unit size (MW) Number of

heaters

0-50 3-5

50-100 5 or 6

100-200 5-7

Over 200 6-8

Thermopedia

Powerplant Engineering

Black & Veath

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• NCFWH

Feedwater heaters (FWH)

DCFWH

Heat feedwater temperature to remove

dissolve gases. (Dalton’s law)

• Trays promote mass exchange

• Equal pressure of steam and water

Drain cooler

approach

Terminal

temperature

difference

Efficiency increases with decreasing TTD

and DCA but surface area increases

DCA and TTD design points : ~ 6 and 3 K

Thermopedia

ISA transactions

Industrialsteam.com

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Po

we

rpla

ntE

ngin

eering

Bla

ck &

Ve

ath

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• Steam is sent back to boiler after first expansion

• On the board…

• Can increase efficiency depending on Pint

• High steam quality after reheating

• Single stage reheat in normal power plant, and double stage reheat in supercritical power plant

Reheating

Source Ohio.edu

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Top and bottom cycleHeat rejected from top cycle at Tmin is used as high temperature heat source of bottom cycle

• Water/steam and organic fluid

• Joule cycle + Rankine cycle

Combined cycle Gas

turbine

Steam

Plant

Combined

cycle

Tmax

(K) 1500 700 1500

Tmin

(K) 800 300 300

carnot

General Electrics

http://www.zeroco2.no/

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• Pinch point temperature difference.

• Compromise on size of heat exchanger and efficiency of the cycle

• The lower the temperature difference, the lower the irreversibility in heat transfer

• CC cycle efficiency can be as high as 55-60 % compared to 45-48 % for supercritical steam cycle.

• Sometimes HRSG have gas burners as boosters

Heat recovery steam generator

15Vorlesung WKA

• Heat rejection typ. >40 %

• Can be used for heating, or processes (hospital, industry)

𝐸𝑛𝑒𝑟𝑔𝑦 𝑈𝑡𝑖𝑙𝑖𝑠𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 𝐸𝑈𝐹 =𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦+𝑢𝑠𝑒𝑓𝑢𝑙 ℎ𝑒𝑎𝑡 𝑜𝑢𝑡𝑝𝑢𝑡

𝑒𝑥𝑡𝑒𝑟𝑛𝑎𝑙 ℎ𝑒𝑎𝑡 𝑎𝑑𝑑𝑖𝑡𝑖𝑜𝑛

• Industrial site : Local CHP to replace electricity purchase and on-plant boiler means less transmission distribution loss

Combined Heat and Power - Cogeneration

Imperial College CHP plant

US

Energ

y I

nfo

Adm

inis

tration

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• Combined Cycle Heat and Power (CCHP)

Combined cycle + Cogeneration

Cornell university

2 x 15 MW gas turbine

Double stage HRSG

As compared to on-site

(central heating) and off-

site (electricity sources) :

-20 % CO2, -55% NOx, -

55% SO2.

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Conclusions

Typical steam power plants have multiple feedwater heaters including

a de-aerator, and at least a reheater

Additional utility gain by combined cycles, or combined heat and

power

Next lecture : focus on steam generators and combustor from heat

and mass transfer standpoint