IPP innovation at Map Ta Phut

IPP innovation at Map Ta Phut

A 2 x 230 MW project is being built at the Map Ta Phut industrial complex in Rayong Province, Thailand. The plant has an unusual hybrid arrangement which allows it to fire natural gas and other fuels efficiently.

S. Castle, R. Curran and A. Soininen,

Foster Wheeler International, Inc.,

New Jersey, USA

Although much of the recent news from the Asia Pacific power generation market has been about the continuing financial problems and the resultant slowdown in new infrastructure projects, many projects that were underway before the onset of the financial crisis have continued unimpeded. And with these projects, the region continues to lead much of the world`s action and innovation in power generation projects. Thailand has been no exception, especially for ground breaking independent power projects. A case in point is the Map Ta Phut Cogeneration Project Phase III currently under construction by the Cogeneration Company (COCO) of Thailand.

COCO, the first IPP to sell electricity to the Electricity Generating Authority of Thailand (EGAT), was established in 1993 as a joint venture between Ban Pu Coal and Nordic Power Invest AB (NPI). Sithe Energies subsequently joined this non-recourse financed project as an investor in 1998.

The 2 x 230 MW Phase III project site is the Map Ta Phut industrial complex in Rayong Province, an important centre of Thailand`s petrochemical and steel industry activity where electric demand continues to increase. When completed, the plant will add to the 300 MW available from Phases I and II of the project, making over 760 MW of total power capacity available. Phase III calls for 300 MW of electricity to be supplied to EGAT as part of their small power producer programme, with the balance of the generation to be supplied as both electricity and steam to the industrial complex. Black & Veatch (UK) Limited and Marubeni Corporation, as consortium partners, are performing the engineering, procurement and construction for the Phase III project.

The hybrid cycle

The Phase III project will utilize a 2 x 230 MW hybrid cycle. Each unit consists of a circulating fluidized bed (CFB) boiler, steam turbine generator set, two combustion turbine generator sets, two gas turbine exhaust heat recovery units (HRUs), and all associated balance of plant equipment. The units are considered hybrid because the exhaust gas from the combustion turbines is used for condensate heating and feedwater heating (normally provided by closed feedwater heaters with extraction steam) as well as steam reheating (typically done with the back-pass of the CFB boiler).

In this hybrid arrangement, the gas turbine and steam turbine work together to generate a high efficiency reheat steam cycle. Main steam from the CFB boiler superheaters is sent to the high pressure steam turbine, expanded, and then sent as cold reheat steam to the reheater section of the heat recovery units. The HRU captures the exhaust heat from the 36 MW (nominal) General Electric Frame 6B combustion turbine and reheats the steam, which is returned to the intermediate pressure and low pressure steam turbine. Following the IP and LP turbines, the steam is exhausted into a condenser.

Each of the two CFB and HRU trains are matched with a 175 MW (nominal), 50 Hz, 300 r/min tandem compound, two-case, condensing steam turbine-generator de-signed by General Electric. The turbine is a single flow, single reheat type machine with steam throttle conditions at maximum continuous rating at 175 bar, 565 degrees C and reheat temperature of 524 degrees C.

The hybrid cycle was selected for its ability to combine the advantages of both solid and gaseous fuels into an overall high efficiency plant. As Thailand`s largest private coal producer and importer, Ban Pu Coal is able to diversify the plant`s fuel sources and take advantage of the continuing stabilization of coal prices and supply.

The resultant plant is highly fuel flexible due to its use of both coal and natural gas. This flexibility is enhanced even further by the selection of a circulating fluidized bed boiler, a combustion technology that has a proven ability to efficiently fire a wide range of fuels.

Steam generating equipment

Foster Wheeler Energy International, Inc. (FWEI) is supplying the circulating fluidized bed boilers and heat recovery units to the Black & Veatch/Marubeni consortium. The CFB equipment is being designed and supplied by Foster Wheeler Energy Corporation (US). Foster Wheeler Energia Oy (Finland) is designing and supplying the heat recovery units.

In both instances, the supply is comprised of both domestic and imported components, and each is supported by technical advisory services for on-site commissioning and start-up services.

With the very aggressive schedule for the Phase III project, on-site presence during erection and commissioning are critical.

The heat recovery units: Each HRU consists of a tube bundle for each of its cycle components: the reheater, high pressure feedwater economizer, and low pressure condensate economizer, together with their associated casing and structural support. The horizontal unfired units (shown in Figure 1) were designed to maximize heat transfer efficiency while minimizing flue gas draft loss. As shown in the water-steam circuit diagram in Figure 2, there are two HRUs for each CFB steam generator.

Each HRU is designed for a cold reheat steam temperature of 347 degrees C and a hot reheat temperature of 524 degrees C at a steam flow of 59 kg/s, with an overall thermal rating of 24.5 MW across the reheater. The HRU receives exhaust gas from the combustion turbine at a temperature of 551.1 degrees C and sends the cooled gas to the stack at a nominal temperature of 97 degrees C. The HRU system includes a combustion turbine exhaust gas bypass duct that is used when the HRU is not in service.

Foster Wheeler`s Kuovala Workshop in Finland fabricated the HRU modules in two phases to match the construction schedule. Much of the steel structure for the HRU, along with stacks and duct work, were sourced from fabricators local to the Map Ta Phut complex. Delivery of all HRU components will be completed in mid-1998, and erection is underway (see Figure 3).

The CFB boilers: The 100 MWe (nominal) CFB boilers incorporate many of Foster Wheeler`s special design features, including a water-cooled cyclone and a horizontal in-furnace superheater panel. These features, along with the overall Foster Wheeler CFB process, provide the excellent temperature control that will allow the plant to operate with the combustion turbines out of service, an important project specification. To provide this flexibility, superheat control in the CFB boiler required careful consideration and the unit was designed for operation with feedwater temperatures ranging from 150 degrees C to 260 degrees C (depending upon whether feedwater heating was available from the gas turbine HRU). The units are designed for a maximum continuous rating (MCR) of 120.6 kg/s main steam at a temperature of 568 degrees C and a pressure of 183 bar.

Shown in Figure 4, the horizontal superheater panels are fabricated from SA213T91 A double-Omega style tubes. These panels penetrate the combustion chamber front and rear walls. This arrangement was selected to minimize tube erosion concerns. T91 was used in several other steam generating components to accommodate the high main steam temperature and pressure requirement.

An advantage of CFB technology is its inherently clean and efficient combustion process. Emissions are controlled using pneumatic limestone injection into several locations in the furnace for in-situ sulphur capture and a fabric filter for collection of fly ash and particulate prior to the stack. NOx formation is limited to a low quantity through air staging and the relatively low temperature of the combustion chamber, and is further reduced through the use of an ammonia injection system.

A further advantage of CFB technology is fuel flexibility, an important consideration for the Phase III project where there is a range of design coals with a very wide range of fuel properties. Heating values, sulphur content, and ash constituencies all vary considerably among the coals, so the CFB required special design considerations. The units shall use a versatile fuel feed system that will include four independent 33 per cent capacity feed trains each with a gravimetric coal feeder. The fuel is fed to the four fuel injection points on the combustor front wall via a feed chute from the gravimetric feeder.

The CFB boiler components were fabricated in several strategic locations, including Foster Wheeler`s workshops in Dansville (USA) and Varkaus (Finland), along with subcontractors in Mexico and China. The airheater, ducts and other plate work are being sourced locally in Thailand. As with the HRUs, on-site presence during erection and commissioning is vital to meeting the project schedule.

With hydro testing scheduled very shortly after setting of the steam drum (within five months), the project schedule has been aggressive and presented a challenge to the equipment supply, especially in consideration of the special aspects of the CFB design for high temperature and pressure operation over the fuel range. Delivery and erection of the CFB boiler components are to be completed in late 1998 and are on schedule to meet their expected mechanical completion milestones.

Operating scenarios

The Phase III plant has been designed for maximum flexibility in the operation of the combustion turbine generator, steam turbine generator, and the CFB steam generator. A variety of specific operating scenarios – with widely varying process steam demands – are anticipated and each required careful design consideration.

In the first scenario, both the CFB and the combustion turbine generators (CTGs) to operate at matching loads, whether full or part load. Generally, the overall thermodynamic balance experienced at MCR is maintained as load is evenly reduced across both systems and no special operating considerations are required.

Alternately, the CTGs may be operated at full load while the CFB load varies. As the CFB load decreases, feedwater is bypassed to the condenser to reduce the feedwater temperature to the cycle.

Similarly, the CFB load may be maintained at 100 per cent of MCR while the load of the combustion turbine generators varies. In this case, as the CTG load decreases the feedwater temperature to the CFB also decreases. With both CTGs operating, the CTG output can drop to as low as 60 per cent load and the CFB will be able to maintain MCR steam flow and operate within the steam turbine temperature limits. An additional concern at these lower CTG loads is the decrease in temperature of the hot reheat steam.

At such lower loads, a portion of the main steam is bypassed to the hot reheat steam flow to maintain the minimum required steam temperature for the given steam turbine load.

In a more extreme scenario, the CTGs may be out of service and only the CFB will be in operation. Due to the low feedwater temperature in this scenario, the maximum attainable CFB load will be restricted to approximately 85 to 90 per cent of MCR. As in several other scenarios, a portion of the main steam shall be bypassed to the hot reheat steam to maintain the minmum required steam temperature.

Project status

The Phase III project is progressing according to schedule, and the plant is expected to begin operation in the Spring of 1999. With the commercial operation of these units and the hybrid Phase III plant, Asia will continue to demonstrate its leadership in the innovation of power generation technology and the ways in which independent power plants can be constructed and operated.

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Figure 1. The horizontal unfired units were designed to maximize heat transfer efficiency while minimizing flue gas draft loss

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Figure 2. Water-steam circuit diagram. There are two HRUs for each CFB steam generator

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Figure 3. Delivery of all HRU components will be completed in mid-1998 and erection is underway

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Figure 4. The horizontal superheater panels are fabricated from SA213T91 double-Omega style tubes

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Figure 5. The cyclone cones. The CFB process provides temperature control that allows the plant to operate with the combustion turbines out of service

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