Fuel requirements prompt combined-cycle power plant innovation
Essar Power Ltd.`s 515 MW Hazira plant in India has the trend-setting distinction of using naphtha and NGL fuels
By Arun Kumar Srivastava and Prasad Raju
Essar Power Ltd.
Essar Power Ltd.`s 515 MW combined-cycle power plant in Gujarat state was the first independent power project (IPP) to be commissioned after the Government of India (GOI) liberalized its power sector in 1992. The project is one of the largest gas turbine plants operating on naphtha/natural gasoline liquid (NGL) in the world.
The Hazira station supplies baseload power to Gujarat Electricity Board (GEB) and Essar Steel Ltd. Originally commissioned as a 330 MW simple-cycle plant in November 1995, Hazira`s 185 MW combined-cycle capacity became operational in May 1997. The plant is located on 34 hectares of land at Hazira, Surat district, Gujarat state. Gujarat in general and Surat district in particular are one of India`s more highly industrialized areas, but suffer from a lack of coal reserves for power generation. Thus, fuel has to be transported from eastern coal fields by ship. Additionally, Indian coals have a high ash content (45 percent), which makes coal transport cost expensive and also creates ash disposal problems. Due to environmental restrictions in highly industrialized areas such as Surat, the only viable fuel options available for a power plant are natural gas (NG), a liquid fuel such as naptha or NGL.
In the area, NGL is available from the Oil and Natural Gas Commission (ONGC) landfall point at Hazira. The necessary jetty infrastructure is readily available at Hazira for bringing naphtha through to the plant on seagoing vessels. Since natural gas was not immediately available, Essar carried out the necessary techno-economic studies and subsequently decided to install a dual-fuel, combined-cycle plant capable of using a potential combination of fuels such as NG, NGL, naphtha and high speed diesel (HSD).
Essar Projects Ltd., a company of India`s Essar Group, was the turnkey plant constructor and was responsible for plant design, equipment procurement, construction, startup and performance testing. Power production from gas turbines Nos. 1 and 2 commenced in August 1995 and turbine No. 3 in November 1995.
The plant`s gas turbines improved guaranteed heat rates and outputs on NG and NGL fuels. While Hazira`s turbines experienced initial problems on naphtha/NGL liquid fuels, initial design deficiencies were corrected to solve a series of minor problems. To date, the plant`s gas turbines have achieved an average availability of 95 percent.
Essar decided to implement the power project as an IPP and proposed selling of entire power mainly to two customers: GEB, a state utility, and a steel complex named Essar Steel Ltd. (ESL), another company in the Essar group. The government of Gujarat granted IPP status in June 1995, with a capacity allocation to ESL and GEB of 215 MW and 300 MW, respectively.
The Hazira combined-cycle block consists of three dual-fuel GE Frame 9E gas turbine generators nominally rated at 110 MW at an average site ambient temperature of 28 C; three Korean Heavy Industries & Construction Company Ltd. (Hanjung) dual-pressure, unfired heat recovery steam generators (HRSG); and one GE 186 MW condensing steam turbine generator. Gross plant output is approximately 515 MW based on an average site ambient temperature of 28 C (see figure). The gross plant design heat rate is 1,694 kcal/kWh (LCV) or 50.8 percent efficiency.
Each gas turbine generator is rated at a maximum to 123.4 MW at ISO conditions with dual-fuel naphtha firing. The gas turbines are equipped with three spindle-screw fuel injection pumps, radial flow divider and lubricity additive skid for handling naphtha. The gas turbines were also retrofitted with a drain scheme to flush out the naphtha from the system in the event a turbine trips on naphtha. Each gas turbine has a fuel selection skid to enable startup of the unit on HSD and transfer to naphtha after achieving a stable load of 5 MW and vice versa while load shedding.
Naphtha and NGL have a very low viscosity and poor lubricity characteristics. The fuel injection pump and the flow divider, which distributes equal quantity of fuel to the gas turbine combustors, are positive displacement pumps. With the use of poor lubricity fuels like naphtha/NGL, these pumps are susceptible to metal-to-metal contact and early failure. To improve pump lifespan, GE recommended an injection of a lubricity additive (Hitec 580) while operating on naphtha/NGL fuels.
HRSGs, steam turbines
The steam parameters of high pressure (HP) and low pressure (LP) steam from each HRSG has been optimized by the engineering, procurement and construction contractor after duly considering capital and operating cost, HRSG pinch point selection, cooling water inlet temperature and selected turbine cycle parameters (see table). The Hazira HRSGs designed and manufactured by Korea Heavy Industries and Construction (Hanjung) under a collaboration agreement with ABB Combustion Engineering in the US.
HP steam is generated at 80.5 kg/cm2(a), 525 C, with an evaporation rate of 176.2 T/hr at the superheater outlet. The LP steam is generated at 6.3 kg/cm2(a) at 210.4 C, with a rate of 34.16 T/hr at the superheater outlet. The HP selection consists of superheater, attemperator, evaporator and a two-stage economizer. The LP section comprises a superheater, evaporator and an economizer. A condensate preheater of SS 304 material is provided to preheat the condensate feed to the deaerator. The condensate preheater has a bypass provision to prevent acid dew-point corrosion of tubes while operating with high-sulphur fuels.
Three-by-50 percent HP and LP boiler feed pumps were provided to supply feed water to each HRSG. If the steam turbine is down, 100 percent of HP and LP steam from all three HRSGs can be bypassed to the condenser through dump valves. The extent of shop prefabrication of HRSG modules was very high; the total HRSG tube bundles were sectioned into eight blocks. Prefabrication reduced site erection time with all HRSG pressure parts being erected in less than three months.
The steam turbine is a two-casing, non-reheat, induction type, designed and supplied by GE through Hanjung. The steam turbine generator is rated for an output of 186.121 MW with inlet steam parameters of 76 kg/cm2(a), 528.6 T/hr, 520 C and 5.5 kg/cm2, 84.38 T/hr and 205 C. The generator is rated at 240 MVA, 15.56 kV at 0.8 power factor.
The circulating water (CW) system is a recirculation type with induced-draft cooling towers. The water treatment plant is designed for a total productive output of 486 m3/day of demineralized water for HRSG makeup and soft water of 24,000 m3/day for CW system makeup. The plant`s liquid fuel system consists of four, 12,500 m3 capacity floating-roof fuel oil storage tanks; the supply provision is one month`s liquid fuel requirement. A 27 km NGL pipeline has been laid from ONGC Hazira complex; facilities have also been constructed for unloading fuel from road and ship tankers. The fuel oil storage tanks are protected with spray water and foam system fire protection; the plant transformers are equipped with a high velocity spray system. The overall plant area is protected by a hydrant system.
Control and electrical systems
Each gas turbine and the steam turbine has a stand-alone GE Mark-V Speedtronic control system. However, the overall operation, startup and shutdown of the plant is automated from a central control room. An integrated, digital distributed control system using the Siemens Teleperm ME system is linked directly to the control systems of the gas turbines, steam turbine, HRSGs and other plant systems.
The DDC systems continually monitors, alarms and logs process data and also provides operator interface to control the plant processes. It also provides data logging and trending of all important plant parameters and events. On-line management reports are generated covering plant operations and performance, fuel consumption, stack emissions and other pertinent data.
The power generated by the gas turbine generator and steam turbine generator is stepped up to 220 kV level for evacuation by the respective generator transformers. The generator transformer for the gas turbine generator is of 160 MVA each and the generator transformer for steam turbine generator is 250 MVA. A common 220 kV switchyard is provided for the evacuation of power from the power plant and for the steel plant auxiliaries. Since the power plant serves a nearby local source of industrial load (Essar Steel), there is substantial cost savings in the area of transmission loss.
The auxiliary power required for the power plant is taken by two station transformers of 12.5/16 MVA each. The auxiliary power is utilized at 6,600 V and 415 V.
Gas turbine generator protection was provided by GE; steam turbine generator protection was provided by GEC Alsthom. The generator transformers were supplied by BHEL (India) for the gas turbine generators and by Crompton Greaves (India) for the steam turbine generator. The generators are provided with high negative-sequence capability so as to withstand the high negative sequence component produced by Essar Steel`s DC electric arc furnace. All critical plant protection features 100 percent standby so failure does not lead to any generation loss.
Uniqueness of plant
The Hazira station features an ABB islanding scheme that connects it directly with the nearby steel plant, thus enabling power supply to continue even during grid collapse. Furthermore, both the power and steel plants are provided with a real-time energy management system for the effective energy utilization and conservation. The energy management system is implemented using Power Measurement Ltd.`s (Canada) 3720 ACM meter and intellution software.
Since the steel plant has harmonic loads, achieving maximum demand depends on effective operation of electric arc furnaces and rolling mills. A power system study was carried out to ensure that the stop-start operation of heavy duty DC arc furnaces and hot strip mills did not effect the power plant operational effectiveness or the grid.
Swedepower of Sweden finished its power system studies for the complete Essar complex at Hazira, which is comprised of the power plant of Essar Power Ltd., the steel plant of Essar Steel Ltd. and the grid of GEB. The Essar complex is connected to GEB via a transmission line from the 220 kV switchyards at Essar, Hazira and GEB in Ichhapur. Studies were carried out with Essar operating in parallel with the GEB, as well as operations in islanded conditions.
With the system in interconnected mode, periodic load variation caused by the steel plant caused minor dynamic problems. To date, this has been corrected by installing power system stabilizers on the gas turbines; this protection system has been found adequate, along with the negative sequence capability of the gas turbine generators. Additional transmission lines to GEB to evacuate power have been recommended, and a double-circuit line from Essar, Hazira to GEB, Sachin, is being developed for power evacuation.
No combined-cycle power plant using NGL/naphtha in a similar size and capacity is currently operating in the world. Because of the plant`s unique fuel requirements and steel plant loads, considerable changes have been incorporated in the gas turbine generators in association with GE. The three gas turbines were commissioned and operated in simple-cycle mode on naphtha/NGL fuels.
During initial startup and trial operation of the plant, several design deficiencies surfaced. They included the following:
– Fuel injection pump seals failure due to presence of metal chips in the lubricator; difficulties in maintaining differential pressure regulation on the lube oil side while operating on naphtha and HSD. Accumulators of lubricator were replaced and metal chips problem was solved.
– Lube oil supply and return pipe sizes changed to three-fourths inch and orifice sizes were increased.
– Mechanical seals were relocated for achieving adequate compression for sealing.
– Seizure of flow divider of gas turbine flow divider occurred upon opening. Analysis of the divider indicated there was presence of a sticky substance inside critical clearances, which was preventing the flow divider rotation. Corrective measures entailed a reduction of the Hitec 580 injection rate to 75 ppm. Since this action has been implemented, such seizure has not been encountered.
– The location of naphtha drain headers were installed below a workable height, in hot conditions. These drain valves were not accessible. The location of drain headers was changed; several subsequent demonstrations proved it was possible to drain naphtha/NGL from the on-base piping immediately after a tripout, placing the machine on load within 30 minutes.
– Hitec injection flow regulation linearity was not maintainable at low loads due to injection-pump motor-speed limitations. This problem was rectified by providing a bypass line from discharge line to suction (which operates only at lower loads).
– The initial HSD fuel system was sized only to support startup and shutdown of gas turbine generators. For safety reasons, GE has introduced a HSD changeover feature at any load. On one occasion while the gas turbine was operating at baseload, there was disturbance in NGL header pressure and fuel switched to HSD. Since HSD was not adequately sized, the fuel injection pump seized. A separate HSD system with adequately sized pipes and pumps was installed and three pressure switches introduced in the fuel injection suction pump for LP trip. With this modification, fuel changeover at any load has been achieved.
A gas turbine hall at Hazira.
Hazira`s switchyard area.
HRSGs at Hazira.
Mr. Prasad Raju is a post graduate in thermal engineering from Indian Institute of Technology, Madras, India. He worked 11 years at a consulting engineering company before joining Essar Power Ltd. in 1994. Mr. Prasad is a joint general manager and is currently involved in the project development of IPPs.
Mr. Arun Kumar Srivastava is a mech- anical engineering graduate from Allahabad University, India. He is senior vice president in Essar Power Ltd., working there since 1990. He is responsible for power projects development for Essar Group. He has a target of achieving 2,000 MW capacity development by the year 2000.