31. SETKÁNÍ KATEDER MECHANIKY TEKUTIN A TERMOMECHANIKY

26. – 28. června 2012, Mikulov

Efficiency ofcombined cyclewithacondensing extractionsteam turbine and backpressure steam turbine

František Urban1, Peter Fodor2, Ľubor Kučák3, Viera Peťková4

1Faculty of Mechanical Engineering of the Slovak University of Technology in Bratislava, Institute of thermal power engineering, Námestie slobody 17, Bratislava,
2Faculty of Mechanical Engineering of the Slovak University of Technology in Bratislava, Institute of thermal power engineering, Námestie slobody 17, Bratislava,
3Faculty of Mechanical Engineering of the Slovak University of Technology in Bratislava, Institute of thermal power engineering, Námestie slobody 17, Bratislava,
4Eustream a.s., Votrubova 11/A, Bratislava,

Abstract Analysis ofnewly constructedcombined-cycle(CCGT)supplyingheatto thehousing and communalarea. CCGT with combustion turbine, combustion boiler and condensing extraction turbine may be supplemented byback-pressuresteam turbineofthe original heating plant. Based on mathematical models of CCGT machines, equipment and defined optimization criteria, this paper optimizes the cooperation of the combustion turbine and steam turbines to the needs of district heating system (DHS) heat throughout the year.

1Introduction

In source and distribution system of district heating (DH) occurs after a period need of modernization, expansion of heat and electricity production and more efficient production and distribution of heat. The present paper analyze CCGT co-operation with existing operation of condensing extraction turbine and backpressure turbine of original heating plant.

The heat source is connected to the primary DH network which was extended by 11 km of new routes. Working substance in distribution network is hot water (HW) with calculated pressure of 1,6 MPa and heat gradient in the winter of 120/60 – 80°C and in summer 75/40 – 50°C. To the primary hydraulic balanced network with the existing heat exchanger stations, was newly installed 103 compact object heat exchanger stations.

The main source of heat in the analyzed system is CCGT Fig.1, with an installed heat output of 63,2 MW and 60 MW electrical output. In CCGT is installed combustion turbine (1) GE LM6000 PD Sprint in double-shaft arrangement. Flue gases from combustion turbine flow into double-pressure combustion boiler (2) with supplementary burner (4) with heat output 10 MW. Condensing steam turbine Siemens SST-300 (6) with one controlled supply or extraction of steam, can be operated from fully condensing mode (electric power generator produces 12,8 MW) to maximum DH operation, where electric power generator creates 6,1 MW and heat exchanger station 32 MW of heat. If heat demand for consumers (3) connected to DH is greater than the heat output supplied by CCGT, the lack of power is added by hot water boilers (5). Available are HW boilers LOSS UNIMAT UT-M 64x16 with power output 20 MW and two packaged boilers Bresson with output 2 x 11,63 MW.

To CCGT shall be in addition to condensing extraction turbine Siemens SST-300 (6) integrated backpressure turbine PBS R6 - 3,5/0,6 (7) with capacity 68 t/h steam with maximal electric power generator output 6 MW. Characteristic data of machines and equipment installed in combined heat and power (CHP) plants are summarized in tab.1.

Fig.1 Schematic of the CCGT with condensing extraction steam turbine and backpressure steam turbine

2Operation of CCGT with condensing extraction and back pressure steam turbine

Operational data of the existing system CCGT with condensing – extration steam turbine are gathered in the kontrol system. The operation of the CCGT was analysed in the periode from 1.8.2011 till 23.4.2012, with cumulated operational data in the 1 hour steps. For the final evaluation of the CCGT operation the terminal power of the gas turbine P GT_CST and condensing – extraction turbine P CST, natural gas consumption in the combusting chamber of the GT m GT and in additional burner m CB from the measured data of mass flows, temperatures and pressures of water and steam were combined with calculated values of the heat outputs of the heat exchanging center from primary P HES and secondary side P DH, condenser P Cond , energy in the fuel in the combustion chamber P F GT and additional burner P F CB and of internal power of the condensing – extraction turbine P th CST. Thermal efficiency of the CCGT with condensing – extraction turbine is calculating from the equation

C_CST = eta C_CST = (P GT_CST + P CST + P HES ) / (P F GT + P F CB )

For the further analyses of the gasturbine and flue gas boiler cooperation the regression relations were compilled:

Terminal power of the gas turbine P GT_CSTas afunction of low pressure steam mass flow m LP_CBproduced in the flue gas boiler and air temperature t air P GT_CST (m LP_CB , t air) ,

Natural gas consumption m GTin the GT burner as afunction of GT electrical output P GT_CST and air temperature t air m GT ( P GT_CST , t air) ,

High pressure steam mass flow m HP_CBfrom the flue gas boiler as afunction of low pressure steam mass flow m LP_CB and air temperature t air m HP_CB (m LP_CB , t air) .

From the needed thermal power of the heat exchanging center P DH the steam mass flow m HES is calculated. By the GT, boiler and back pressure turbine cooperation the low pressure steam m LP_CBand steam m HP_CB, is determined as follows m HES = m LP_CB + HP_CB. From the steam parameters and from efficiences given in the turbine PBS R6 -3,5/0,6characteristics the terminal power is P ST calculated. For the CCGT with back pressure turbine is the thermal efficiency, C_ST = eta C_ST = (P GT_ST + P ST + P HES ) / P F GT .

Tab. 1 Basic technical parameters of equipment
Combustion turbine (GE LM6000PD Sprint)
Power output / 46,5 / MW / Mass flow / 476 / t/h
Electrical efficiency / 40,9 / % / Outlet temperature / 452 / °C
Fuel consumption / 11890 / m3/h / Nominal gas overpressure / 2,5 / MPa
Condensing extraction steam turbine (Siemens SST-300)
Power output / 13,0 / MW / Extraction pressure / 0,4 / MPa
Inlet steam pressure / 5,5 / MPa / Condensing outlet pressure / 0,01 / MPa
Inlet steam temperature / 429 / °C / Outlet temperature / 46 / °C
Mass flow / 44,4 / t/h
Combustion boiler
Power output / 50,7 / MW / Boiler inlet temperature / 92 / °C
Combustion flow / 457 / t/h / Boiler outlet temperature / 455 / °C
Supplementary burner
Power of burner / 10 / MW / Natural gas flow / 1 100 / m3/h
Heat exchanger station
Primary site (construction pressure 0,6 MPa, temperature 300°C)
Inlet pressure / 0,35 / MPa / Outlet pressure / 0,35 / MPa
Inlet temperature / 180 / °C / Outlet temperature / 75 / °C
Secondary site (construction pressure 2,5 MPa, temperature 180°C)
Inlet pressure / 1,7 / MPa / Outlet pressure / 1,65 / MPa
Inlet temperature / 70 / °C / Outlet temperature / 130 / °C
Backpressure steam turbine (PBS R6 -3,5/0,6)
Power output / 6,0 / MW / Mass flow / 68 / t/h
Inlet steam pressure / 3,5 / MPa / Outlet steam pressure / 0,4 / MPa
Inlet steam temperature / 435 / °C / Outlet steam temperature / 212 / °C

3Conclusion

By comparing of the CCGT operation with the condensing – extraction turbine and with back pressure turbine issues (Fig. 2 and 3):

Fig. 2CCGToperationwith condensing extraction turbine and backpressure turbine in the period from 1. do 14.2.2012
Fig. 3CCGToperationwith condensing extraction turbine and backpressure turbine in the period from 10. till 22.4.2012
In periode from 1. till 14.2.2012, when the air temperature t airwas in interval from -18,9 to -2,2 °Cthe heat demand wasP DHfrom6,84 to 48,56 MW and CCGT delivered 12 345 MWh of heat. The maximal power of the GT with the condensing turbine were P GT_CST = 46,60 MW, resp. P CST = 6,31 MW and the total electricity production was 11081 MWh. Total NG consumption was 2 911125 m3. Thermal efficiency eta C_CSTwas 70,6 to 90,9 %. By the operation with back pressure turbinethe maximal electrical outputs were P GT_ST = 43,77 MW and P ST = 5,14 MW and total electricity production was 12536 MWh. The NG consumption was 2 903 802 m3 . Thermal efficiency was in interval 88,7 to 90,3 %. The operation with back pressure turbine was mor proifitable then with condensing one.
In periode from10. till 22.4.2012 the air temperature t airwas from -4,8 to 18,5 °C, heat demand wasP DHfrom5,18 to 24,52 MW and CCGT delivered 4 425 MWh of heat. Maximum terminal power of the GT and ST were P GT_ST = 41,65 MW, resp. P CST = 9,55 MW and total electricity production was 10 748 MWh . Total NG consumption was 2 486 028 m3. Thermal efficiency eta C_STwas from 49,6 to 73,6 %. Operation with back pressure turbine have given maximal power P GT_ST = 33,68 MW and ST P ST = 2,35 MW. Total electricity production was 5 499 MWh. Total NG consumption was 1 180 663 m3 . Thermal efficiency was between 89,5 to 90,1 %. Operation with back pressure turbine was more advantageous from the energy piont of view. The described comparison needs an economical evaluation.

4Literature

[ 1 ] Klíma, J.: Optimalizace venergetických soustavách. ACDEMIA, Praha, 1985.

[ 2 ] Urban, František; Malý, Stanislav; Kučák, Ľubor; Pulman, Marián; Muškát, Peter; Belko, Ján : Návrh opatrení pre zvýšenie efektívnosti prevádzky teplárne. Výskumná správa SjF STU vBranislave. Bratislava 2007. 87 s.

[ 3 ] Urban, František: Optimálne radenie a zaťažovanie zdrojov tepla a turbín vteplárenskej sústave. Habilitačná práca. Katedra tepelnej energetiky, Strojnícka fakulta SVŠT vBratislave, Bratislava , 1991, 67 s.