Table 1- An overview of the different options of repowering. %-point Boiler Boiler

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1 Proceedings of the International Gas Turbine Congress 23 Tokyo Noveber 2-7, 23 DRAFT IGTC23Tokyo-TS-11 Repowering of Lowshan Power Plant Mohaad R. Shahnazari,Dariush Foroughi,Haid Fakharian Niroo Research Institute Departent of Energy End of Ponak-e-Bakhtari, Shahrak-e-Ghods,P.B: 14665/517 Phone: , Fax: , ABSTRACT Nowadays, repowering is considered as the ost coon ethods for iproving status of current power plants. Each ethods of repowering fro "para repowering" to "full repowering" shall probably be the best choice for special national and econoical power plant. In this paper different repowering ethods have been introduced. Moreover, application of those ethods were investigated for Lowshan power plant. Technical calculation including parts designing as well as calculations concerning to heat cycle changes have been done for each ethods. Additionally, by paying attention to the technical liitations an econoical study have been perfored for coparing the said two ethods. INTRODUCTION Repowering existing fossil stea generating units with gas turbines and cobined cycles or with other new technology options is eerging as a centerpiece of copetitive corporate strategies aied for transforing relatively unproductive assets into ore efficient, low cost producers. A repowering strategy can siultaneously address load growth, environental copliance and technological obsolescence. Using already established sites and existing facilities can give repowering projects substantial cost savings (2-4%) over new construction at a green-field site and offers environental, peritting and other advantages as well. As a result repowering is expected to account for a ajor share of the increase in generating (i.e. the renant life) on one side and typical needs of the utility. On the other side] in repowering existing plants the size and the quality of the existing boiler and stea turbine (Fig. 1.) deterine the ain choice.( Plouen, Veenea, 1996) An overview of the different options of repowering is given in table 1. In general existing power plants in the size between 5 and 2 MW are ost suitable for repowering with a gas turbine and a new heat recovery stea generator, which delivers the stea to the existing stea turbine. The required gas turbine size is roughly twice the size of the stea turbine and therefore the power increase for this option is very high (2%). FEED WATER REPOWERING Lowshan stea power plant was selected as a case study in order to repowering ethods application investigation. Lowshan power plant is located in Manjil and includes a stea and gas cycle. The capacity of existing boiler is 12 MW and stea enters to stea turbine in 53 C and 121 at. The available energy of gas turbine flue gas is sufficient for Feed Water Heating Repowering and there is no need to purchase the additional gas turbines. Figure 2 shows a scheatic of Lowshan Option Description Power Increase % A Cobined Cycle (GT+HRSG) B Hot Windbox C (HWB) Suppl.Boiler+ Windbox (SB+WB) capacity over the next decade. (taylor, 1998) There are several different options for repowering existing plants with gas turbines. A choice for one of the repowering options is based on the size and the technical condition of the existing plant Table 1- An overview of the different options of repowering Efficiency Liiting Investent Iproveent Factor %-point % 1) NO x Decrease % 2) Outage Tie onths Boiler Boiler D Feed water Stea Heating Turbine (FWH) E IP-Stea Stea Repowering Turbine 1) Relative investent copared to investent for a new Cobined Cycle of the sae capacity 2) Relative decrease of NOx-eissions of total plant after repowering stea cycle before repowering. In this option, turbine extractions are eliinated and two new gasliquid heat exchangers are designed in order to use energy of flue gas caused by gas turbine to heat feed water-entering boiler. Copyright (c) 23 by GTSJ Manuscript Received on April 9, 23

2 This option could be executed in different ways as stea turbine has six extractions, which deliver stea to feed water heaters and deaerator. Due to eliination of turbine extractions, ore stea will pass through the turbine blades and therefore the output power will increase. The ost applicable ethods are discussed below: A) Eliination of all extractions save the one that delivers stea to deaerator. Figure 3 shows a scheatic of this ethod. As shown, feed water leaving condenser passes through the lowpressure gas-liquid heat exchanger and then enters to deaerator. In high-pressure gas-liquid heat exchanger, feed water leaving deaerator is heated up to 229 C and then lead to boiler. In these heat exchangers hot fluid is flue gas fro gas turbine. For this option, increase in power reaches to 17% of noinal capacity. Extraction to 2 nd HP Heater Extraction to 1 st HP Heater Extraction to Condenser Extraction to 3 rd LP Heater Extraction to 2 nd LP Heater Extraction to 1 st LP Heater Stea Leaving LP Turbine Output Power (MW) Power Increase (MW) B) Eliination of high-pressure turbine extractions saves deaerator extraction. A scheatic of this ethod is shown in figure 4. Feed water leaving condenser passes through low-pressure heaters and then enters to deaerator. High-pressure gas-liquid heat exchanger, heats feed water leaving deaerator up to 229 C before it enters boiler. In this option power increase is about 1% of noinal capacity. Table 2 shows the characteristics of required hat exchangers for both A and B ethods. Table 2- Characteristics of Heat Exchangers Properties Low-Pressure HE High-Pressure HE* Shell Side Tube Side Shell Side Tube Side Fluid Type Flue Gas Feed Water Flue Gas Feed Water Operating Press (bar) In/Out Tep. 43/19 6/ / /188.8 ( C) Allowable Press Drop (bar) Fluid Flow-Rate (kg/hr) *: The characteristics of high-pressure heat exchanger for both options (A) and (B) are the sae Feed water flow rate and enthalpy of water and stea for both ethods are shown in table 3. Plant Restriction for FW Repowering Eliination of turbine extraction causes an increase in stea passing rate through the turbine blades and as a result the aount of water in the condenser ay exceed fro perissible liit (2-25% of noinal load). Also there is a restriction for stea passing rate through turbine blades, which should not exceed 2% of noinal load. In option (A), the aount of condensed water is about 422-ton/ hr, which is 18% ore than noinal load. But as it is still less than 2% of noinal load, it consists of no technical proble for condenser. Table 3-Feed water flow-rate and enthalpy of water and stea for ethods (A) and (B) Properties Option (A) Option (B) Fluid Flow-Rate (kg/hr) Enthalpy (kj/kg) Fluid Flow- Rate (kg/hr) Enthalpy (KJ/kg) Main Stea Increase of stea passing rate through turbine blades is fro 19% in high pressure cylinder to 41% in low-pressure one. Therefore these cylinders will have technical probles. In option (B), the increase of water rate in condenser is about 14 % of noinal load. (349 ton/ hr), which is less than 2 %. Stea increase in turbine cylinders is fro 14% in high pressure cylinder to 16 % in low-pressure turbine. Therefore perforing this

3 option consists of no basic change in the cycle and this option could be considered as a suitable ethod for feed water heating repowering. Econoic Evaluation for FW Repowering In order to defining specifications of the plant utilities. Always requires for econoic evaluation of a repowering option. The iproveents are: Decrease in heat rate, NOx eission decrease and capacity increase. ( Bazzini, 1992) For a specified repowering project, ost of the following repowering paraeters have to be defined. These paraeters are plant heat-rate, plant eission before and after repowering, plant capacity, plant dispatch, plant availability, plant O&M cost, total investent cost and total tie for non-availability during odification. The econoic evaluation is done with the following assuptions: Fuel price 2.5 $/GJ, NOx value 1 $/ton, interest rate 8%, O&M cost 1.5 $/kwh for stea cycle and 3 $/kwh for cobined cycle and Table 4- Econoic evaluation for option (B) full repowering option. ( Liudy etal, 21) The econoic evaluation has been done for option (B) and the results are shown in table 4. HOT WINDBOX REPOWERING Repowering an existing unit using Hot Windbox repowering could be considered as an option with the following advantages: increase in unit capacity efficient and better based on environental laws. Technical restrictions of the unit and boiler conditions ust be taken into account before choosing a gas turbine. In this option the capacity of the suitable gas turbine is roughly twice the capacity of stea turbine. Power increase for this option could be between 2 to 3 higher than the present capacity. In HWB repowering the flue gas exiting gas turbine could be used as a source of Oxygen to iprove cobustion in boiler. Lowshan s Boiler inforation is shown in table 5. The first proble is due to change in Oxygen to fuel flow-ratio, which is depended on both air and flue gas fro gas turbine. Therefore it could be equal or ore than Oxygen to fuel ratio in design condition( Plouen and Veenea,1996) O fuel Properties Option B Efficiency (%) 35 Investent cost (USD/kW) 44.3 O&M cost ( c / kwh) 1.5 Present Worth ( c / kwh).56 Total investent (MMUSD) Investent rate ( c / kwh).46 Fuel cost ( c / kwh) 1.8 Total generating cost 3.76 ( c / kwh) Power increase (MW) 12 O, 2 2 (1) If we consider and F as air and fuel ratios respectively: air ; F air, fuel (2) Then iniu required flue gas could be calculated by the following equation: g,in air or g O2, fuel a ( F ) b b reains constant while coposition of flue gas does not change. Considering that stea flow-rate ust not exceed 12% of design condition, axiu allowable flue gas flow-rate could be calculated. This restriction could be explained by the following equation: 1.2( air D, D air, D (1.2 ) 1. 2 F g, ax (5) D Furnace analysis and heat transfer calculations would be the first step. Adiabatic flae teperature will be changed due to utilization of cobustion products instead of cobustion air. The change in adiabatic flae teperature is negligible while the difference in the flue gas teperature-leaving furnace is large coparing to the one for present conditions of boiler. Due to soe change in radiation flux, teprature profile and cobustion products flow, the aboveentioned changes occur. Considering the increase in cobustion product s volue and teperature leaving furnace, ass velocity and etal teperature in heating surface area ust be calculated Hot Wind box repowering ethod need to ake so changes in size and arrangeent of heating surfaces. Heat transfer calculation in furnace and heating surface area is done because of deterined teperature profile, heat flux in furnace and circulation rate for three following conditions. The results are shown in table 6. A: Noinal cycle load (44 ton/hr) B: Maxiu boiler loads (49 ton/hr and introducing 576 ton/hr of flue gas fro gas turbine to boiler) C: Operation of boiler decrease in 44-ton/hr load, in fuel consuption. And it introducing 576 ton/hr of flue gas to boiler in 6 43 C in option B required extra heat is about kj/hr 6 while the available heat in flue gas is about kj/hr. Therefore presence of a duct burner would be necessary in boiler. In option C, decrease of fuel consuption by using available energy of flue gas, is the ain purpose. In both options B and C, FD Fan will be eliinated. Due to radiation decrease in furnace, flue gas teperature leaning furnace is increased and therefore etal teperature calculations in heating surface area for option B are necessary in order to avoid increase in local teperatures. Decrease in heat absorption inside furnace causes higher flue gas teperature in heating surface area and therefore having a larger econoizer is necessary in order to have ore energy absorption inside boiler and also avoid high teperatures in stack. Required additional surface in econoizer is shown in table 6. as deterined in option B ore additional surface is required. Increase in stea generation up to 5 ton/hr in boiler causes 9 to 1 MW increase in output power. ) (4) (3)

4 The ost iportant paraeters to quantitative evaluation of the iproveents are flue cost, interest rate, capacity increase, Nox eissions decrease, additional power and cost of outage tie during perforing the project. Total generation cost could be considered as a basic paraeter in order to copare different options Equation 6 is used to calculate power generation cost for a HWB repowering project: ( Werner etal, 1999) Cu ( RlCt M C f ) / W (6) W K Where: Cap F n y C u : Total generation cost C t : Total investent cost Table 5- Boiler characteristics for lowshan power plant [3] Properties (Unit) Aount Maxiu Stea Flow-Rate (Kg/hr) 49 Miniu Stea Flow-Rate (kg/hr) 11 Pressure (bar) 147 HP Output Pressure 126 Dru pressure (bar) 143 Stea Output Teperature ( C ) 535 Feed Water Teperature ( C ) 28 Econoizer Output Teperature (oil/gas) ( 275/285 C ) Fuel Properties Fuel Flow-Rate in Load of 1% (kg/hr) 3112 Gas Flow-Rate in Load of 1% ( ) Cobustion Properties N / hr Air Teperature Entering Burner ( C ) 29 Air Flow-Rate Entering Burner (oil/gas) /34164 (Kg/hr) Fan Output Pressure (WC) Air Teperature Entering Fan ( C ) Flue G as Teperature Leaving Econoizer ( C ) 295/37 C f : Flue cost M: Operation and aintenance cost R l : Pay back rate W: Energy n per year K F : Capacity coefficient Cap: Unit noinal capacity Table 7- Total generation cost for option B and C using different econoic paraeters Ite Interest rate Fuel cost ($/GL) Unit life (year) Option B Option A

5 n : Operation hours of plant in a year y Table7 show total generation cost for option B and C using different econoic paraeters CONCLUSION For repowering an existing plant the final choice depends on unit capacity, energy, plant reained life and environental concerns. Econoic evaluation shows that full repowering is the ost suitable option for Lowshan power plant. Considering total generation cost and the fact that gas turbine already exists, Feed Water heating repowering and Hot Windbox repowering sre the least suitable ethods, respectively. Especially HWB repowering is the least suitable ethod for Lowshan power plant due to coplication of ethod and econoic concerns. In all repowering projects by gas turbine power increase fro 8 to 12 percent and decrease in fuel consuption is also at the sae range. Table 6- Boiler paraeters for options A to C Paraeter Option A Option B Option C Fuel flow-rate (kg/hr) Air flow-rate (kg/hr) Flue gas flow-rate(kg/hr) - 3 Adiabatic teperature of Flue gas ( C ) Flue gas teperature Leaving furnace( C ) Heat absorption in Furnace(kW) Flue gas teperature leaving heating surface area( C ) Heat absorption in heating surface area (kw) Flue gas teperature leaving econoizer( C ) Additional required surface in econoizer to obtain desired output teperature (%)

6 REFERENCES Taylor Moore, "Repowering as a copetitive strategy", Internet (EPRI), plouen P.J., Veenea J.J., "Dutch experience with hot windbox repowering", Publication ASME 96-GT-25 Lowshan Power Plant Docuents Bazzini, H.A., "Repowering a Stea Turbine-Generator Power Plant through Heat Recovery type Cobine-Cycle-Selection of the Cycle- a Case Study", ASME TURBO Expro, Birangha-UK, June 1992 Liudy J., Werner K.H., Gysel B., "Knowing when repowering is copetitive", Noveber 21 Werner Karl- Heinz, Strlcabosko Rudolf, Jordan Thorstel, the Copetitive Edge of Repowering ALSTOM, 1999 Figure 1- Capacity of gas turbine vs. capacity of stea turbine for different repowering options Fig 2- Scheatic of ain cycle

7 Figure 3- Scheatic of option A Fig 4- Scheatic of option b