Turow enters third phase

Phase III of the circulating fluidised bed combustion repowering of the Turow lignite burning power plant complex in Poland is now underway. After receiving a notice to proceed on 31March 2001, unit 4 of the former 10 x 200 MWe conventional generating plant is now being demolished ready to be replaced by the most advanced, efficient, and environmentally benign CFBC systems. These units will produce 261 MWe on a footprint no bigger than the original 200 MWe units.

Work was already underway on the damaged unit 5, which has been out of action since a major explosion in December 1998. Unit 5 is due to be commissioned in June 2002. Unit 4 is scheduled to follow after a 15 month gap, and unit six should follow no more than 11 months after that. The project, which aims to minimise the outage of all the units during the refurbishment programme, starting with the earliest units first, is due to be completed in the course of 2004.

The Turow power plant, located near the small town of Bogatynia in southern Poland, is right in the middle of the infamous Black Triangle on the borders of southern Poland, northern Czech Republic, and eastern Germany. The area is notorious for its dire pollution problems, most particularly its high levels of harmful particulate emissions. The earliest records of coal mining in the vicinity of Bogatynia date back to the 1770s.

Analyses of the Polish lignite fuel are shown in Table 1.

Phases I and II of the project, which commenced in June 1995 (see MPS, July 1995, involved replacing units 1 and 2 (Phase I) and unit 3 (Phase II), with Ahlstrom Pyroflow circulating fluidised bed boilers and ABB steam cycles producing 235 MWe apiece. Essentially the same consortium led by ABB is now handling Phase III, the repowering of units 4, 5 and 6, but the two parties have since evolved to become Alstom and Foster Wheeler.

In total, once all three phases are complete, six of the ten units at the site will have been repowered. In addition, three of the remaining four conventional units, the most recent to be built, will be modernised and uprated. Unit 7 was planned to be shut down permanently on commissioning of Phase III, but the ultimate aggregate output will still be well in excess of the original 2000 MWe, which made it Poland’s second largest generating station, supplying some 15 per cent of Poland’s electric power.

Emissions reduced

The repowered units 1 and 2 have now been operating for nearly 21/2 years since provisional acceptance in December 1998, with very high availability following the minimum of normal performance optimisation during the early months of commissioning.

On 1 June 2000, the PAC (provisional acceptance certificate) for unit 3 was signed and the 18 month guarantee run was initiated after a construction period of under 32 months compared with the schedule of 33 months.

Emissions from Phase I and II units, as listed in Table 2, represent a 92 per cent reduction in SO2, a 19 per cent reduction in NOx, and a 91 per cent reduction in particulates relative to the old plants.

New waste water treatment installed in 1998 reduces suspended matter in the return flow to the river from which the station takes its cooling water to an average level of 14.2 mg/m3 – a level substantially lower than that in the intake flow from the river. The overall efficiency of the entire plant is enhanced by the delivery of some 2200 MWt of district heating to consumers in Bogatynia.

Rehabilitating and repowering the old lignite burning plants was seen by Elektrownia Turow as an attractive means of accomplishing the goals of increasing efficiency and reducing pollution in a way which would meet anticipated new European legislative requirements.

Costs were minimised by retaining key items of plant that were still serviceable such as fuel feed systems and electrical interties. The major objectives of the rehabilitation include:

• meeting the prevailing environmental standards;

• increasing unit availability to higher than 80 per cent;

• reducing operating cost;

• increasing lifetime of the units by 30 years; and

• securing long term employment in the south-western area of Poland.

Project finance

Deregulation has been an ongoing battle in Polish political circles for years and privatisation still seems less than imminent. Arranging finance for Phase III of the Turow repowering was little if any less problematic than for the first two phases, although it may have been less protracted.

Phase 1 finance was based on a long term PPA with the Polish Power Grid Co, signed in September 1994. Final approval for a ministry of finance guarantee was granted in May 1995. Domestic and international debt finance was closed in June 1995. This amounted to some $215 million from a national Bank Handlowy syndicate, $55 million in soft loans from the National Fund for the Environment and Water Management, $105 million with Ministry of Finance cover from international lenders, $40 million over 20 years from the Nordic Investment bank, $54 million from Citibank with partial cover from Eximbank and FGB, and a substantial Swiss Franc loan from Swiss Bank with partial cover from ERG.

Phase II was also financed by Polish banks, with rather more competitive rates than for Phase I, as well as environmental funds and further credit from the Nordic Investment Bank and loans covered by foreign export agencies, but again there was no equity financing.

In the end there was no equity investment in Phase III, and again the project was financed largely on the basis of power sales contracts with PPGC. Local and foreign bond issues, as well as equity investment from outside countries had been considered for financing Phase II, but in the end the project was largely funded by more or less conventional national and international debt funding backed by export credit agencies.

In 1999 the completely new technical concept for the rehabilitation of units 4, 5 and 6 based on the most modern process parameters was developed. Notice to proceed (NTP) for unit 5 was given on 31 December 1999 and NTP for unit 4 followed on 31 March 2001 after financial closing of all credit agreements. NTP for unit 6 is planned to be issued in December 2001.

The Alstom/FWC contract for Phase III is worth some $667 million. Foster Wheeler will supply the three compact CFBC boiler islands, and Alstom will supply the turbine island, electrical and control systems and other auxiliary equipment. The total execution time for Phase III is 58 months.

Phase III boilers

The original conventional technology generating units are of Russian design and started operation in 1962. The lignite fuel, for which characteristics are listed in Table 1, is supplied from the open cast strip mine directly to the boilers by means of coal conveyors.

The new 261 MWe CFBCs for Phase III are described as the biggest Compact CFBCs in the world, but Foster Wheeler are known to be working on designs of units of up to 600 MWe with once-through supercritical pressure boiler technology.

The low and relatively uniform furnace heat transfer rates obtained in CFBCs facilitates a simple supercritical once through boiler design which is said to bring together the fuel flexibility and low emissions of CFBC with the higher thermal efficiency of supercritical steam pressures.

Design studies done by FWC for a 350 MWe version of such a boiler indicated that the design was technically feasible and that the increased cost associated with supercritical pressure in this case would be minimal.

The design parameters for Turow repowering Phases I and II were relatively modest, see Table 3. The enhanced figures for the Phase III technology are shown in Table 4. A distinguishing feature of a CFB boiler is the separator device at the furnace gas outlet, which collects bed material entrained in the flue gas for recycle back to the furnace. The bed material contains fuel ash, unburned fuel, spent limestone and unutilised limestone. Collection and recirculation of this material back to the furnace results in a very high level of fuel burnout and limestone utilisation.

The cyclone has become the most popular device for separation in fluidised bed systems, usually in the form of a steel shell lined with around 300 mm thickness of refractory. For this design capital cost is relatively low but operating costs are high due to refractory maintenance and heat loss from the shell to the environment. To reduce maintenance, Foster Wheeler has developed a steam-cooled design in which the cyclone is formed from steam-cooled tubing. This design minimises operating costs but has higher capital cost.

To combine the advantages of both cyclone types, a new type of separator was developed described as a “compact separator”. In the compact separator, which has flat walls rather than curved walls, gas and entrained solids leaving the furnace enter the separator through a tall, narrow opening in the centre and exit through two outlets in the separator roof.

The position of the separator gas inlet relative to the gas outlet imparts a swirl to the flow, causing solids separation just as if the separator walls were curved. To counter potential erosion, the interior walls of the separator are covered with a thin layer of refractory held in place by a dense pattern of metal studs, the same design used successfully in the lower furnace area and in steam cooled round cyclones.

The exterior walls are covered with insulation and lagging, like the furnace walls, to minimise heat losses. The Compact separator thus combines the low operating costs of the steam-cooled cyclone but with a reduced capital cost due to the use of flat rather than curved walls. The cooled construction minimises differential expansions between furnace and separator, minimising both the number and the motions of expansion joints.

A related innovation which enhances the basic Compact design is the new Intrex (Integrated Recycle Heat Exchanger) concept, which is a bubbling bed heat exchanger containing one or more tube bundles which are used to cool the circulating solids. Solids enter from the furnace via slots in the common wall (called internal solids circulation) or from the separator (called external solids circulation). The solids return to the furnace either via the solids return channels or through slots in the common wall. The immersed tube bundles, which can serve superheat or reheat duty, become highly efficient heating surface because of the high heat transfer rates. Also, by controlling the rate of fluidising airflow in the chamber and/or the solids return channels, the heat absorbed in the immersed bundle can be varied, which in turn can be used to control furnace temperature or steam temperature.

As boiler size increases, the furnace surface/volume ratio decreases and the furnace walls alone can no longer absorb sufficient heat to maintain furnace temperature at the desired level. The designer can then add either internal surface such as wingwalls or external surface via a heat exchanger.

The heat exchanger is fluidised with air, and so provides a location for heat transfer surface which avoids corrosive components in the flue gas. Locating only low temperature superheat surface in the backpass minimises backpass corrosion as well.

The potential for internal solids recirculation allows superheater tube bundle heat absorption even at low loads when the external solids circulation is low. Non-mechanical control of solids flow via fluidising overflow, rather then the control valve as used by some suppliers, minimises maintenance cost.

The heat exchanger enclosure is cooled by water from the drum and is integrated with the furnace and solids separator return leg, avoiding expansion joints and the associated maintenance.

An earlier application of the FWC compact CFBC boiler, two 150 MWe units at the Tha Toom complex in Thailand, which also supplies process heat to a paper mill and other local industries, uses the solid separator in place of the circular cyclone, but does not have the Intrex bubbling bed heat exchanger. Here the principle of the rectangular cross section unit being joined to the furnace without the need for expansion joints has already been proven.

Turbine-generator

Alstom’s scope of supply includes:

• project management and leading of the consortium;

• development of the technical concept for the rehabilitated units;

• design, delivery, construction/erection and commissioning of the turbine island, electrical supply system and the control and protection system.

The company’s factories in Poland are supplying the turbine set and the electrical equipment. Construction and erection works are also being handled by Polish contractors.

The entirely new 260 MW turbines consist of three cylinders (see diagram below): one single flow high pressure turbine, one single flow intermediate pressure turbine and one double flow low pressure turbine of the Alstom RT type.

The generator stator core and rotor windings are cooled by hydrogen.

Prospects

As was the case for Phase II, Phase III benefits from having a long term PPA already established with the local power grid company. The experience gained from negotiating the Phase I and Phase II projects will have educated the indigenous developers in dealing with the relatively tortuous and highly specified contract documentation required for dealing with Western financial and engineering participants. This should place Elektrownia Turow in good standing for further developments.

The fuel flexibility of the new boilers is such that virtually any kind of carboniferous fuel could be used if such resources were to become more economic in the region. Due to the low combustion temperature of the fluidised bed, oxides of nitrogen are produced at much lover levels than would be achievable in typical pulverised coal systems, and reductions in sulphur exceeding 90 per cent can be attained in the circulating fluidised bed.

Operating experience with Phase III at Turow will be anticipated with great interest.
Tables

Table 1. Fuel analyses for Polish lignite
Table 2. Guaranteed emission levels from Phase I and II units
Table 3. Phase I and II design conditions
Table 4. Phase III design conditions