REDUCING THE COST FOR START UP THE HRSG BANG PAKONG COMBINED CYCLE PLANT 3, 4

Transcription

1 REDUCING THE COST FOR START UP THE HRSG BANG PAKONG COMBINED CYCLE PLANT 3, 4 Thanapong Na Ubon Bang Pakong power plant, The Electricity Generating Authority of Thailand Bang Pakong, Chachoengsao 24130, Thailand @egat.co.th Anon Promniyom Bang Pakong power plant, The Electricity Generating Authority of Thailand Bang Pakong, Chachoengsao 24130, Thailand @egat.co.th ABSTRACT Bang Pakong Combined Cycle Power Plant Unit 3 and 4, commissioned in 1991, currently operated in cyclic mode which gas turbines necessitate to start up and shut down every week. During start up process, gas turbine begins at fixed full load that has excess exhaust discharged through bypass stack. This lost exhaust gas consumes energy which increases cost of operation. To reduce this excess exhaust gas, normal start up process will be modified and replaced by step-load start that can saves energy and reduces carbon dioxide emission as well. Key words: Gas turbine start, Step-load start, Exhaust gas loss, Start duration INRODUCTION Bang Pakong Combined Cycle Power Plant Unit 3 and 4 are multi-shaft systems that consist of one steam turbine and two gas turbines with heat recovery steam generator (HRSG) for each unit. Combined cycle starts with first gas turbine, steam turbine and second gas turbine consecutively. After starting first gas turbine at fixed full load, exhaust gas will be released to HRSG by opening bypass stack damper in position of 30, 45, 65, and 90 degree respectively. After passing damper, exhaust gas will heat water to steam which will be delivered to start steam turbine. Step-load start of gas turbine is applied for conserving this valuable exhaust gas. Gas turbine on testing new process started and loaded at 30, 40, 60, 75 and 85 MW respectively and bypass damper will be opened in position of 45, 65 and 90 degree that conforms to exhaust gas volume. Changing to new operation exhibits intact equipment and easy implementation. MAIN CONTENT Data collection Heat consumption & Load output & Exhaust energy curve (HLE curve) Chart in figure 1 is shown heat input of fuel to gas turbine, natural gas as fuel, and exhaust heat that is released from gas turbine. Both of two heats are represented in vary power generation (Megawatts). This curve derived from performance test history of power plant Exhaust flow & Damper degree curve (ED curve) Chart in figure 2 is shown percentage of maximum volume of flow that is delivered to HRSG. This percentage is varied by opening bypass damper that has maximum value of 90 degree. This curve derived from HRSG manufacture. Step-load start The purpose is to find load of gas turbine at different angle of bypass damper. HLE and ED curve will be main tool for finding new load. New start up process will be optimized during testing for finding the suitable operation that concerned operator and machine. New load for startup will be described as following

2 Verify original startup that has 4 steps which is differentiated by angle of bypass damper 30, 45, 65, 90 degree. Find volume of exhaust gas from gas turbine at normal load 85 MW and based load 100 MW from HLE curve. The volume is shown in fourth column of table 1 and table 2. Then find exhaust volume that passes to HRSG at different angle of bypass damper. By multiplying percentage of flow with volume of gas from gas turbine, the passed exhaust is shown in last column of table 1 and table 2. Figure 1 Heat consumption & Load output & Exhaust energy curve Figure 2 Exhaust flow & Damper degree curve

3 Start up with variable Damper at 85Mw (ref. for Heat consumption ) GT MW (Mw) GT HC (Gj) damper degree GT Exh.(Gj) Exh.to bypass Exh.to hrsg Table 1 Exhaust to HRSG at 85 MW Start up with variable Damper at 100Mw (ref. for Exhaust energy ) GT MW (Mw) GT HC (Gj) damper deg GT Exh.(Gj) Exh.to bypass Exh.to hrsg Table 2 Exhaust to HRSG at 100 MW Create new startup table that is based on the same volume of exhaust to HRSG at 85 MW and 100 MW Find new step-load of gas turbine from exhaust to HRSG. In his case, gas turbine has minimum exhaust heat at 300 GJ then fully opening damper at first step will not be possible. New step-load start is shown in table 3 Start up with variable Load output & Damper (ref. for New Start) GT MW (Mw) GT HC (Gj) damper deg GT Exh.(Gj) Exh.to bypass Exh.to hrsg Table 3 New start up with variable load of gas turbine & damper RESULT Step-load start is tested for 8 repetitions. The results confirm that each new step-load start process saves fuel gas that can be converted to energy, which is shown in figure 4, without effect to steam turbine start-up process, damaging equipment and start-up time.

4 Power generation (MW) Power generation (MW) Time Figure 3 Step-start diagram Time Figure 4 Energy comparison between fixed-load and step-load start up CONCLUSIONS AND RECCOMMENDATIONS Gas turbine with step-load start up reduces fuel cost amount 13,000 USD for starting first unit and another 3,200 USD for starting second unit. With no need to pay investment, this improvement has been implemented at Bang Pakong Combined Cycle Unit 3 and 4 efficiently. Due to original installation of bypass damper, position is only fixed at 30, 45, 65, 90 degree then future improvement will be focused on modifying bypass damper that can be varied with gas flow of gas turbine exhaust. The modification can reduce excess exhaust heat closes to zero.

5 REFERENCE [1] GE international power system, Gas turbine (MS9001E) operation training (BPK CC3,4) [2] GE international power system, Gas turbine (MS9001E) performance test (BPK CC3,4) [3] CMI (Cockerill Mechanical Industries), Instruction book for HRSG (BPK CC3,4) [4] EGAT Bang Pakong Training Center, BPK CC3,4 Combined Cycle Plant operation [5] Efficiency Section BPK CC3,4, BPK CC3,4 STEP Factor