Building for the Future

In March 1998, an international consortium led by Sumitomo signed a contract with Electricity of Vietnam (EVN) for the construction of Pha Lai II – Vietnam’s largest coal-fired power plant. By January 2001, the plant’s first unit will enter commercial operation.

This aggressive fast-track schedule – 34 months from contract signing to commercial operation – is not the only challenge that Pha Lai II presents. The 2 x 300 MWe plant will burn some of the lowest-volatile anthracite fuels ever considered for power generation, and must still achieve high efficiency and availability.

Pha Lai II is part of a wider government programme to add generation capacity to Vietnam’s grid
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These challenges are made more formidable given the importance of Pha Lai II to Vietnam’s economy as the country emerges from the Asian financial crisis. It is a high profile project that will generate much-needed power for the northern area of Vietnam, and will bring balance to a hydropower-dominated generation mix. Supported by the Japan Bank for International cooperation (JBIC), Pha Lai II is part of a wider government programme to add generation capacity to the grid. Almost 4000 MWe of development is underway in line with government aims of doubling Vietnam’s 1995 capacity.

Pha Lai II is located alongside the existing Pha Lai I plant, 65 km northeast of the capital, Hanoi. The site is in the heart of Vietnam’s main anthracite fields, and close to the port of Hai Phong. Pha Lai I comprises eight 220 t/h coal-fired Russian-designed boilers and eight 55 MW steam turbines. The plant was constructed in the 1980s and continues to operate at 75 per cent output during the dry season.

An international challenge

Pha Lai II is being developed by a consortium comprising Japan’s Sumitomo, Stone and Webster of the USA, Korea’s Hyundai Engineering and Construction and UK-based Mitsui Babcock. The consortium leader is Sumitomo, who is also responsible for supplying the GE steam turbines and the Barclay Mowlam coal handling plant to the project.

As the project’s overall technical leader, Stone and Webster is responsible for the engineering and coordination of the project, and will also lead the commissioning phase. In addition, it is responsible for the turbine island balance of plant design and equipment supply. Hyundai Engineering and Construction is the civil and erection contractor, while Mitsui Babcock is supplying the plant’s boilers and boiler auxiliary equipment.

The consortium is developing the plant under a turnkey engineering, procurement and construction (EPC) contract which was negotiated at the height of Asia’s financial crisis. But according to Tim Picken, Mitsui Babcock’s regional director for the Pacific region, the poor economic climate had little impact on project development. This, said Picken, was largely due to the participation of JBIC. “It made us feel slightly more relaxed compared with other projects in the region,” said Picken.

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JBIC, formerly the Overseas Economic Cooperation Fund (OECF), has provided the financing for Pha Lai II under a series of untied loans worth $710 million. An initial loan signed in 1994 was for engineering services, while four other loans signed between 1995 and 1999 are for the procurement of goods and services for the construction of the power plant, the transmission lines, substations and environmental control equipment.

The consortium members have developed ‘localisation strategies’ for the Pha Lai II project, placing contracts locally in Vietnam for equipment and services ranging from site canteen services to the major fabrication of steel components, depending on the schedule and quality requirements. According to Picken, Mitsui Babcock has placed an estimated $6-7 million of contracts in Vietnam for Pha Lai II. Training local personnel to operate and maintain the plant is also a major part of the consortium’s contract.

Pha Lai design

The new plant will comprise two Mitsui Babcock 922 t/h natural circulation boilers, providing 541à‚°C main steam and 541à‚°C reheat steam at a main steam pressure of 174.6 kg/cm2. The guaranteed boiler efficiency is 90.75 per cent (NCV).

Pha Lai II will comprise two Mitsui Babcock 922 t/h natural circulation downshot fired boilers
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GE, through Sumitomo, is supplying two 300 MW GE model D5 tandem compound double flow reheat steam turbines and a Mark V turbine control system. The control and instrumentation for Pha Lai II is a Yokogawa system, while a wet flue gas desulphurization system will be supplied by Marsulex Environmental Technologies. The plant will have to meet JBIC emission standard requirements of 1000 mg/Nm3 NOx, 500 mg/Nm3 SOx, and 100 mg/Nm3 for particulates.

The boilers are refractory lined and have a downshot configuration. Downshot furnaces are specifically geared to the stable and efficient combustion of difficult coals such as the anthracites to be used at Pha Lai II. In order to achieve satisfactory combustion efficiencies and turndown flexibility with these coals, a burner and furnace arrangement is needed which promotes strong ignition as well as prolonged residence time.

In Mitsui Babcock designs, slot burners are provided in specifically arranged groups in the two firing arches. Fuel is supplied by four tube mills which deliver pulverised coal of high fineness and with a high primary air temperature directly to the burners. The burners use large quantities of combustion air through the firing arch which provides the the momentum necessary to force the flames downward.

To maintain high furnace temperatures, a portion of the lower furnace is fitted with pre-fired refractory tiles.

With the creation of such high temperatures and heat fluxes, attention must be paid to the absorption and circulation aspects of the furnace design. Ribbed tubing is provided throughout the bulk of the furnace to extend the range of conditions over which the tubes can operate safely. The furnace also has an octagonal plan to aid circulation by ensuring that all furnace tubes are exposed to sufficient heat in the hopper area at the bottom of the furnace to promote natural circulation. This is especially important at the high steam pressure required for Pha Lai II, where the density differential between steam and water is reduced.

Testing fuels

Achieving good environmental performance, high efficiencies and availability was perhaps the greatest technical challenge for the Pha Lai II partners, Mitsui Babcock in particular. The Vietnamese anthracite that Pha Lai II will burn is probably the most difficult in the world for combustion using pulverised coal boiler technology, and proven experience with anthracite was a major factor in EVN’s selection of boiler supplier.

Mitsui Babcock was therefore required to undertake fuel testing during the tender stage and also after the contract had been awarded in order to demonstrate the behaviour of the Vietnamese anthracite, which poses problems for efficiency and stability, particularly at part load conditions where expensive fuel oil support may be required.

Sumitomo is supplying two GE steam turbines
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The basis of these tests, carried out over a six month period at its Renfrew Technology Centre, was Mitsui Babcock’s experience at the 724 MW Yue Yang power plant in China. Yue Yang, situated on the Yangtze River in Hunan Province, consists of two Mitsui Babcock downshot fired natural circulation units firing a wide range of Chinese anthracite and semi-anthracite fuels. These units entered service in 1991, and represent the latest in proven anthracite firing technology. They have shown that a high combustion efficiency can be maintained, and that turndown to 40 per cent and below without oil support can be achieved.

The Yue Yang anthracites and semi-anthracites, with volatile contents of 4-14 per cent dry mineral matter free (dmmf), have been successfully fired at the plant for seven years, occasionally with ash contents of over 40 per cent. The range of fuels for Pha Lai will also be wide, but will be from the more difficult end of the anthracite spectrum.

Coals for Pha Lai II will be supplied from the Cam Pha, Hon Gai and Uong Bi fields in the Quang Ninh coal basin. The design coal is a blend of up to five individual coals, with a volatile content of 3.75 per cent dmmf. The maximum volatile content of any one of the individual component coals is six per cent dmmf. At the lower end of the scale is the coal from Vang Danh, with just two per cent dmmf. With an ash content of 30 per cent and above, the fuels for Pha Lai II are the most difficult ever to be fired on this scale anywhere in the world.

Mitsui Babcock’s experience with Yue Yang laid the foundation for the Pha Lai II boiler design, but the Vietnamese coals were clearly a more difficult prospect from a combustion perspective. The testing programme at Renfrew was therefore essential for understanding the coals and their behaviour, and was vital in the Pha Lai II furnace design process.

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Samples of Vietnamese and Chinese anthracites were shipped to the UK for test firing in a 160 kW single burner facility at Mitsui Babcock’s Technology Centre. Burnout and NOx production were examined for each coal as a function of excess air and burner primary zone stoichiometry, and a blend of the Vietnamese coals was also tested. A model of the Yue Yang boiler was built up, enabling Mitsui Babcock to develop a full-scale plant understanding of the combustion behaviour of the Vietnamese coals.

The tests confirmed the difficult combustion characteristics of the Vietnamese coals, but indicated no unusual blend characteristics. The results also confirmed Mitsui Babcock’s initial assessments of combustion efficiency. “It was another challenge,” said Picken, “but the test work gave us comfort that we could actually operate the boilers at the required load conditions”. Further testing will be carried out during commissioning to validate the test rig runs.

Modelling of the furnace was carried out using advanced computational fluid dynamics (CFD) techniques. With downshot firing, it is imperative that the furnace and flame are wholly compatible in order to optimise combustion efficiency, minimise NOx production and predict the thermal performance of the furnace. CFD techniques enabled Mitsui Babcock to more thoroughly understand flame development and furnace utilization at Yue Yang and to make forward projections for Pha Lai II. The CFD technique was validated over the load range using test data from Yue Yang before use with the Pha Lai II furnace and fuels. The particle track, gas temperature contours, velocity contours, char burnout rate, oxygen utilisation and surface heat flux were all examined and a favourable comparison with field observations was obtained.

Optimising design

The understanding of the coals provided by the burnout model enabled the optimisation of the Pha Lai II furnace and the combustion system in order to counteract the more difficult burnout potential of the Vietnamese coal. The Pha Lai II boilers will use a greater than usual coal fineness as well as a larger furnace in order to increase residence time to more than twice that usually required for the modern wall firing of bituminous coals. Despite their lower output, the Pha Lai II boilers will be almost the same size as the Yue Yang boilers, and will deliver a similar combustion efficiency on a much more difficult coal.

The Yue Yang boilers have demonstrated the best record of availability for units in China in recent years and the Pha Lai units will incorporate some of their design features. These include an arrangement of the headers and interconnecting pipework that enhances the fatigue life of the boiler pressure parts, and gas velocities selected to minimise erosion.

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The furnaces will also include a number of design features for the control of slag formation, in particular the unique membraning of secondary and final superheaters to prevent the distortion of the elements and inhibit slag build up, and the provision of a curtain of air injected down the front and rear walls to provide a relatively cool, oxidising environment local to the walls. The economiser will be the high-efficiency plate-gilled type as used at Yue Yang, and the use of high quality materials throughout will further enhance availability and station life.

In addition, CFD modelling of the Pha Lai II furnace prompted Mitsui Babcock to incorporate alternative burner slot positions to assist with the recirculation of rising hot gases into the root of the flame and so improve ignition. This will also create an additional air stream adjacent to the front and rear walls, offering greater protection against slagging, corrosion and refractory tile wastage.

Reaching goals

The CFD observations were also instrumental in the understanding of the NOx potential of Pha Lai II. The extent to which fuel-derived NOx from anthracite can be efficiently controlled in the flame is limited, and thus the control of NOx production in a downshot arrangement is achieved by understanding the thermal NOx mechanisms.

In a downshot arrangement with a refractory lined furnace, highly stabilised flames are short and hot, and therefore produce high levels of NOx. However, the grouped burner slot arrangement in the boiler allows flame stability to be imparted via the external recirculation of hot gases to the periphery of each group, and penetration to be achieved through delayed combustion within the centre of the group. This allows NOx control without sacrificing flame stability or combustion efficiency. Mitsui Babcock estimates that NOx can be reduced by 15-30 per cent using these techniques, without penalty on combustion efficiency.

According to the project partners, work on the Pha Lai site is advanced and on schedule for first unit commercial operation on 16 January 2001. The second unit will start commercial operation six months later. The civil works are nearing completion, and commissioning work is due to start in mid-2000.

When operational, Pha Lai II will help Vietnam attain its economic goals through the efficient and environmentally acceptable use of a domestic resource. For the project partners, it is a benchmark project that demonstrates their ability to overcome the challenges presented by a fast-track schedule and a new customer. For Mitsui Babcock, working in Vietnam was also a first, and it hopes to participate more in this growing power sector. “We see the country as a market for us,” said Picken. “We want to build on Pha Lai.”

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