Cogeneration. Rangan Banerjee. Department of Energy Science and Engineering. IIT Bombay

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1 Cogeneration Rangan Banerjee Department of Energy Science and Engineering IIT Bombay Lecture in KIC-TEQIP programme on Energy Management and Energy Efficiency - IITG - 24 th May 2016

2 Utility options Fuel Heat BOILER PROCESS Heat Electricity Electricity Heat Cogen Plant Fuel SHP Electricity Power Plant Fuel Cogeneration 2

3 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

4 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 = v 2 /2+ g(z-z 0 )+(h-h 0 )-T 0 (s-s 0 )+ex ch 4

5 Cogeneration Concept Process boiler, sat steam at 180 C 90% (1 st law eff) T u = =453 K, T 0 = 300 K II =Q u (1- T 0 / T u )/ Q in (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

6 Early 19 th century Cogeneration plant Source: Dryden Efficient use of steam 6

7 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 Decreasing X 7

8 Evaluation Criteria Relative Fuel Savings R f Fuel savings over separate heat & power generation R f = ( F nc F c ) / F nc F nc = F boiler + F power plant Fuel Chargeable to Power (FCP) The incremental fuel in cogeneration is charged to the power generation. FCP =( F c F boiler )/ W kg of oil/kwh, kj/kwh, Nm 3 gas/kwh, kg of bagasse/kwh 8

9 Back Pressure Steam Turbine System HP Steam Fuel Air BOILER ST Electricity Water LP Steam to Process 9

10 Steam Turbine Cogeneration Configuration X Boiler BPT with extraction 10 Back Pressure Turbine Condensing Extraction Turbine 3 Condensing Power Plant 0 Decreasing X 10

11 Steam Turbine Calculations Specific Enthalpy h1 1 is = h 1 -h 2 h 1 -h 2i h2 h2i 2i 2 Specific Entropy s 11

12 Back-Pressure with extraction turbine HP MP LP 12

13 GAS TURBINE BASED COGEN Fuel C CC GT Power Steam to Process Air Suppl Fuel WHRB Feed water Stack 13

14 Condensing Extraction Turbine HP LP 14

15 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

16 T-S Diagram for Brayton cycle P 2 T Q sup 3 P 1 2i 2 4i 4 1 Q rej s 16

17 Brayton cycle calculations ( 2) 3 sup T T C m Q P ( 1) T 4 T C m Q P rej 1 sup sup 1 1 p rej r Q Q Q i T T p r T i T P P r p 17

18 Component efficiencies is( comp ) T2 i T 2 T 1 T 1 is( T ) T T 3 3 T 4 T 4i 18

19 Simple back-pressure turbine with reducing and surplus valves Source: D.M.E. DIAMANT, TOTAL ENERGY 19

20 GAS TURBINE BASED COGEN Fuel C CC GT Power Steam to Process Air Suppl Fuel WHRB Feed water Stack 20

21 Schematic of a recuperated micro-turbine based cogeneration uni (Oniovwona and Ugursal) 21

22 Sankey diagram for diesel engine Alternator Stack loss 4% Energy Input Diesel engine 34% 35% Electrical output 24% Surface heat loss 3% Coolant loss 22

23 Source: J. H. HORLOCK 23

24 Source: J. H. HORLOCK 24

25 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

26 Indian Installation: 1 MRPL, Mangalore 45 MW Cogen plant 3 Boilers-Each 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

27 Indian Installation: 2 RPL, Hazira 60 MW Cogen plant 2 GTGs 2 Fired HRSG - Each 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,

28 Indian Installation: 3 Tata Chemicals, Babrala 40 MW Cogen plant 2 GTGs 2 Fired HRSG - Each 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

29 Source: J. H. HORLOCK 29

30 Costa et al (2007) 30

31 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

32 Part-load Characteristics 32

33 33

34 22 ata 330 o C 58 T/hr FEED WATER Feed water BOILER 0.5T/hr 4.5T/hr 27T/hr 26T/hr BAGASSE 0.5T/hr PRDS MILLING PRDS 6 ata ~ 2 ata Process Flashed Condensate STEAM TURBINE 2.5 MW Process Schematic of typical 2500 tcd Sugar factory 34

35 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

36 BOILER Feed water 75 TPH, 65 ata, 480 O C BAGASSE (Alternate fuel) STEAM TURBINE 13 MW ~ 9.5 MW Power export PROCESS 2 ata 6 ata 4.5 TPH 2 ata Condenser CONDENSER ESS 1.0 MW Mill drives BFP PROCESS 2.5 MW Captive load PROPOSED PLANT CONFIGURATION: OPTION 2 36

37 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

38 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

39 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

40 Gas turbine -1 G 1 20 MW WHRB-1 Supp. Firing LSHS 5.6 T/h Stack 136 T/h Fuel, HSD 5.9 T/h Gas turbine -2 G 1 20 MW WHRB-2 Supp. Firing LSHS 5.6 T/h 136 T/h SHP Steam 100 bar,500 o C Grid 7.52 MW BUS Process Load, 60 MW T/h Fuel, LSHS 9.64 T/h Boiler T/h Process Load,150 T/h Process Load,125 T/h PRDS-1 PRDS-2 HP Steam 41b,400 o C 60.6 T/h 20 T/h 76.2 T/h ST G MW MP Steam 20b, 300 o C Process Load 40 T/h 49.5 T/h 40 T/h 16.2 T/h PRDS-3 LP Steam 5. 5 b, 180 o C 53.4 T/h Process Load 40 T/h Deaerator Make up water,357 T/h Condenser 40

41 Import Power from Grid with Cogeneration for a Petrochemical Plant Import power MW flat tariff MW Time hours TOU tariff peak period demand 0 41

42 Export power to the grid with Cogeneration for a Petrochemical Plant Export Power MW flat tariff Time hours TOU tariff Peak period demand 9.7 MW 42

43 CHP Potential in India Major Industries Caustic soda 394 Cement Cotton textile 506 Iron & steel 362 Manmade fibers 144 Potential (MW) Breweries Coke oven batteries Commercial sector Distilleries 2900 Major Industries Potential (MW) Fertilizer Petrochemical Rice mills 1000 Solvent extraction Sponge iron 225 Tyre plants Paper & pulp 850 Refineries 232 Sugar 5200 Sulphuric acid

44 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

45 References J Raghu Ram, R.Banerjee, Applied Thermal Engineering, Vol 23, p , 2003 S. Khurana, R.Banerjee, U.N.Gaitonde, Applied Thermal Engineering, Vol 22, p , 2002 S.Ashok, R.Banerjee, IEEE Trans on Power Systems, Vol 18, May 2003, p Horlock, Cogeneration-CHP-thermodynamics and economics, Pergamon Press, 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 , A. Costa,J. Paris, M.Towers, T.Browne, Energy, 32, 2007, pp Dryden Efficient Use of Steam, Butterworths,

Chapter 2.7: Cogeneration

Chapter 2.7: Cogeneration Part-I: Objective type questions and answers 1. In cogeneration, the system efficiencies can go up to ------ a) 70% b) 80% c) 90% d) 60% 2. Cogeneration is the simultaneous generation