Cogeneration

Rangan Banerjee

Department of Energy Science and Engineering

IIT Bombay

Lecture in KIC-TEQIP programme on Energy Management and Energy Efficiency - IITG - 24th May 2016

Utility options

PROCESSHeat

Electricity

BOILER

Power Plant

Fuel

Fuel

Cogen Plant

Fuel

Electricity

Electricity

HeatHeat

CogenerationSHP

2

Cogeneration Concept

Boiler 90% Power plant 40%

Where is the scope for improvement?

Cogeneration- Simultaneous generation of heat and power (motive power or electricity) – CHP- Total Energy

Second Law of Thermodynamics –Concept of Exergy

3

Exergy

Quality of energy- 100 kJ of heat equivalent?

27 °C ambient, 127 °C, 227 °C, 327 °C

Available energy/ Exergy – The exergy of a substance is the maximum work that can be obtained by interacting with the environment and bringing it into complete reversible equilibrium with the environment

ex = v2/2+ g(z-z0)+(h-h0)-T0(s-s0)+exch

4

Cogeneration Concept

Process boiler , sat steam at 180 °C 90% (1st law eff)

Tu= 180+273 =453 K, T0 = 300 K

II=Qu(1- T0 / Tu)/ Qin (for fuel 1.0)=0.9(1-300/453) =0.3 (30%)

Increase generation temperature to 400 °C and pass through an expansion turbine

5

Early 19th century Cogeneration plant

Source: Dryden Efficient use of steam6

Selection of Cogen Option

Heat/Power Ratio X (Range of values)

Fuel Availability

Costs

Steam Turbine 5.9 ( 3-7)

Gas Turbine 1.5

Combined Cycle 1.2

D.G. Set 0.7

Decre

asin

g X

7

Evaluation Criteria

Relative Fuel Savings Rf – Fuel savings over separate heat & power generation

Rf = ( Fnc – Fc) / Fnc

Fnc = Fboiler + Fpower plant

Fuel Chargeable to Power (FCP) – The incremental fuel in cogeneration is charged to the power generation.

FCP =( Fc – Fboiler)/ W kg of oil/kWh, kJ/kWh, Nm3 gas/kWh, kg of bagasse/kWh

8

LP Steam to Process

ST

HP Steam

Fuel

Air

Water

BOILER

Electricity

Back Pressure Steam Turbine System

9

Steam Turbine Cogeneration

Configuration X

Boiler

BPT with extraction 10

Back Pressure Turbine

Condensing Extraction Turbine 3

Condensing Power Plant 0

Decre

asin

g X

10

Steam Turbine

Calculations

1

2i

2

h1

h2

h2i

Specific Entropy s

Specific

Enthalpy

is = h1-h2

h1-h2i

11

Back-Pressure with extraction turbine HP

MP

LP

12

C GT

GAS TURBINE BASED COGEN

CC

WHRB

Steam to Process

Suppl Fuel

Fuel

Stack

Air

Feed water

Power

13

Condensing Extraction Turbine HP

LP

14

Gas Turbine Cogeneration

Unfired Heat Recovery Steam Generator (HRSG)

Supplementary Fired (Duct Burners)

Fully Fired HRSG

Steam Injected Gas Turbines (STIG)

Combined Cycle cogen with extractions

15

T-S Diagram for Brayton cycle

3

2i2

T

s

4i4

P1

P2

supQ

rejQ1

16

Brayton cycle calculations

)( 23sup TTCmQ P

)( 14 TTCmQ Prej

1

sup

sup 11

p

rej

rQ

QQ

iT

T

pr

T

iT

4

31

1

2

1

2

P

Prp

17

12

12)(

TT

TT icompis

i

TisTT

TT

43

43)(

Component efficiencies

18

Simple back-pressure turbine with reducing and surplus valves

Source: D.M.E. DIAMANT, TOTAL ENERGY 19

C GT

GAS TURBINE BASED COGEN

CC

WHRB

Steam to Process

Suppl Fuel

Fuel

Stack

Air

Feed water

Power

20

Schematic of a recuperated micro-turbine based cogeneration unit

(Oniovwona and Ugursal)21

Electrical output

Energy InputDiesel

engine

Stack loss

Coolant loss

Surface heat

loss

Alternator

4%

24%

34%

3%

35%

Sankey diagram for diesel engine

22

Source: J. H. HORLOCK 23

Source: J. H. HORLOCK 24

Typical packaged internal combustion engine based (spark

ignited)

cogeneration system (Oniovwona and Ugursal)25

Indian Installation: 1 MRPL, Mangalore

45 MW Cogen plant

3 Boilers-Each 40 TPH @ 103 kg/cm2g, 510C, oil fired

2 STG -Each 22.5 MW Condensing

Fuel: LSHS/ Visbreaker oil/LDO

Steam: HP40kg/cm2, MP

16kg/cm2, LP 4kg/cm2

26

Indian Installation: 2

RPL, Hazira

60 MW Cogen plant

2 GTGs

2 Fired HRSG - Each 125 TPH @ 115 kg/cm2g, 515C

Fuel: Natural Gas / HSD

Power: 60 MW

Steam: 115 kg/cm2g, 515 C

27

Indian Installation: 3 Tata Chemicals , Babrala

40 MW Cogen plant

2 GTGs

2 Fired HRSG - Each 98 TPH @ 115 kg/cm2g, 515C

Fuel: Naphtha/ Natural Gas

Power: 40 MW

Steam: HP-115kg/cm2g, 515C MP-40 kg/cm2g, 380C

LP- 3.5kg/cm2g, 180C

28

Source: J. H. HORLOCK 29

Costa et al (2007)

30

Operating Strategy

Standalone/ Isolated

Grid Interconnection Parallel with Grid – Only Buying from grid

Buying and Selling to Grid

Thermal Load Following

Electrical Load Following

Maximum Cogeneration

31

Part-load Characteristics

32

33

0.5T/hr

Feed water

Process

Process

2 ata

~

STEAM

TURBINE

2.5 MW

6 ata

BAGASSE

58 T/hr 22 ata

330o C

4.5T/hr 27T/hr

26T/hr

Schematic of typical 2500 tcd Sugar factory

Flashed

Condensate

PRDS

PRDS

MILLING

0.5T/hr

FEED

WATER

BOILER

34

Options

A- Replace mill turbines by motors + power turbine by efficient power turbine

B- New Boiler 43 ata 480 C + additional TG

C- HP Boiler 65 ata 480 C + additional TG

D – C+ replace mill turbines with TG

E – similar to D but with condensing extraction turbine

35

Feed water

Con

dens

er

2 ata

PROCESS

75 TPH, 65

ata, 480O

C

Process

Process

4.5 TPH

~

6 ata

BAGASSE (Alternate fuel)

2 ata

BFP

13 MW

BOILER

1.0 MW

Mill

drives

9.5 MW

Power export

2.5 MW

Captive

load

PROCESS

PROPOSED PLANT CONFIGURATION: OPTION 2

STEAM

TURBINE

CONDENSER

ESS

36

Comparison of Options

Case Output Export kWh export /tc

A 5.4 MW 1.9 MW 18

B 7.5 MW+M 5.0 MW 48

C 6.8 MW+M 4.3 MW 41

D 10.7 MW 7.2 MW 69

E 13 MW 9.5 MW 91

37

Optimal Cogeneration Strategy

Decisions Grid Electricity Bought/Sold

Equipment Mass Flow rates

Electric/Steam Drive

Constraints Equipment Characteristics – Min/Max

Process Steam & Electricity Loads

Grid Interconnection

Objective Function Minimise annual operating cost (Maximise

revenue)38

Cogeneration

Process Steam, Electricity load vary with time

Optimal Strategy depends on grid interconnection(parallel- only buying, buying/selling) and electricity,fuel prices

For given equipment configuration, optimal operating strategy can be determined

GT/ST/Diesel Engine – Part load characteristics – Non Linear

Illustrative example for petrochemical plant- shows variation in flat/TOU optimal.

39

LP Steam 5. 5 b, 180 oC

Gas turbine -1

Boiler

ST

PRDS-1

PRDS-3

Condenser

Deaerator

Process Load

Process Load

40 T/h

G

1

G

4

Process Load,

60 MW

BUS

Grid

7.52 MW

SHP Steam 100 bar,500o C

HP Steam 41b,400 oC

Fuel, LSHS

9.64 T/h

WHRB-1

Supp. Firing

LSHS 5.6 T/h

Stack

20 MW

Process Load,125 T/h

Process Load,150 T/h

MP Steam 20b, 300 oC

PRDS-2

Gas turbine -2

G

1

WHRB-2

Supp. Firing

LSHS 5.6 T/h

20 MW

Fuel, HSD

5.9 T/h

136 T/h

136 T/h

131.7 T/h12.5

MW

76.2 T/h60.6 T/h

117.1

T/h

40 T/h 49.5 T/h 16.2 T/h

20 T/h

40 T/h

53.4 T/h

Make up water,357 T/h40

Import Power from Grid with Cogeneration for a Petrochemical Plant

11 MW

17.6

21.6

00

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Time hours

Imp

ort p

ow

er M

W

flat tariff TOU tariff

peak

period

demand

41

Export power to the grid with Cogeneration for a Petrochemical Plant

0

10

20

30

40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Time hours

Exp

ort P

ow

er M

W

flat tariff TOU tariff

9.7 MW

Peak

period

demand

42

CHP Potential in IndiaMajor Industries Potential (MW)

Caustic soda 394

Cement 78-100

Cotton textile 506

Iron & steel 362

Manmade fibers 144

Breweries 250-400

Coke oven

batteries

200

Commercial

sector

175-350

Distilleries 2900

Major Industries Potential (MW)

Fertilizer 850-1000

Petrochemical 250-500

Rice mills 1000

Solvent extraction 220-350

Sponge iron 225

Tyre plants 160-200

Paper & pulp 850

Refineries 232

Sugar 5200

Sulphuric acid 74-125

43

Summing Up

Cogeneration, Tri-generation, Polygeneration –more efficient than separate heat and power

Even in industries with cogen – Retrofits for additional power generation

Grid Agreement –Parallel, Buying/Selling

Optimal operating strategy – can result in significant savings

Significant potential in process industries

44

References J Raghu Ram, R.Banerjee, Applied Thermal Engineering, Vol 23, p 1567-

1575, 2003

S. Khurana, R.Banerjee, U.N.Gaitonde, Applied Thermal Engineering, Vol 22, p 485-494, 2002

S.Ashok, R.Banerjee, IEEE Trans on Power Systems, Vol 18, May 2003, p931-937

Horlock, Cogeneration-CHP-thermodynamics and economics, PergamonPress, 1997

R.M.E. Diamant, Total Energy, 1970

YP Abbi,R K Bhogra,TERI Env Monitor,v10, 1994

p19-25

Onovwiona, Ugursal, ‘Residential Cogeneration systems: review of the current technology’, Applied Thermal Engineering, Vol 27, Issues 5-6,p 848-861, 2007.

A. Costa,J . Paris, M.Towers, T.Browne, Energy, 32, 2007, pp 474-481

Dryden Efficient Use of Steam, Butterworths, 1982

45