The concept of retrofitting boilers and steam turbines has now become well established as a way of improving the thermal performance and reliability of existing power plants, with integrated retrofit of both these major components offering considerable opportunities for innovation and maximisation of benefits.
As part of an expansion programme, the Belchatow power plant (PGE Elektrownia Belchatow) approved a 25 MWe uprating of its unit number 6 and awarded the contract to Alstom Power, the latter being well positioned to offer an optimised integrated retrofit as it has both boiler and turbine capabilities in-house. As part of the upgrade, most of the main equipment will be modernised, including boiler, steam turbine, feed heating system and generator. The project therefore provides a good case study for the benefits provided by the integrated retrofit process.
PGE Elektrownia Belchatow, in the Lodz province of Poland, approximately 170 km south west of Warsaw, is the largest power plant in Poland and the largest lignite-fired power station in Europe. It consists of 12 x 370/380 MW (originally 360 MW) lignite fired reheat units commissioned between 1982 and 1988 (initially 4320 MW installed capacity, now 4450 MW, following the various modernisation activities completed to date).
The plant accounts for 20% of the electricity produced in Poland. Upon completion of a new 858 MW supercritical unit (see Modern Power Systems, October 2007, p 12-17), currently under construction, and completion of modernisation programmes on ten of the existing units, the installed capacity will be around 5500 MW.
The Belchatow power plant was originally developed by the state-owned power utility, but today, as a result of the privatisation process is owned and operated by PGE (Polska Grupa Energetyczna).
PGE Elektrownia Belchatow is investing in extending the plant’s life, increasing output, enhancing its environmental performance and boosting availability. FGD systems have already been installed on the ten units being modernised and all low pressure turbines have been retrofitted. Meanwhile a modernisation programme for the boilers, and HP and IP turbines is in progress.
CO2 emissions have now become a major issue, with an investment programme initiated for the application of carbon capture. In collaboration with the Dow Chemical Company, Alstom will design and construct a pilot carbon capture plant at unit 12, with the aim of capturing about 100 000 tonnes per year of CO2 using Alstom’s advanced amine technology. The pilot will be jointly operated by Alstom and Elektrownia Belchatow and is expected to be in operation by mid 2011. In a second phase, Alstom and PGE plan to build a larger plant that will capture CO2 produced by the new 858 MW lignite-fired unit currently being built by Alstom. This capture plant is scheduled to be in operation by 2015.
Existing equipment
The Belchatow plant was built with 12 identical units, the main components being a boiler, type BB-1150, a turbine, type 18K360 and generator, type GTHW-360.
Rafako constructed the subcritical tower boilers, under licence from Sulzer and EVT. The boiler design was based on well-proven once-through technology with spirally wound furnace walls combined with a low load circulation system. The boiler is well suited for sliding pressure operation.
The furnace has a cross section of 15.7 m x 17.1 m, while the height of the furnace chamber, excluding ash hopper, is approximately 34 m.
Zamech supplied the condensing turbines under licence from BBC (both now in the Alstom Power group). Turbine design was based on proven BBC technology from the 1970s, with an impulse control first stage in the HP turbine with four control valves each feeding a separate segment of nozzles. All remaining stages are of the reaction type. Domel (now also part of Alstom Power) supplied the generators, built under licence from the former BBC and rated at 426 MVA at a power factor of 0.85. The stator is water-cooled and the rotor is hydrogen cooled.
The original turbine inlet conditions were rated at: live steam temperature, 535°C; reheat steam temperature, 535°C; and live steam pressure, 176.5 bar.
The current modernisation programme underway at the Belchatow plant was started during 2007 and has progressed as follows. The first unit to be modernised, unit 3, has been refurbished with PAC (provisional acceptance certificate) achieved during 2008. The refurbishment of unit 4 is underway, with erection work currently ongoing. Contracts for the modernisation of unit 5 have been awarded as separate component packages and are now being executed, with overall co-ordination being performed by the customer, PGE. The most recent modernisation contract to be awarded is that for unit 6 and is to be executed as an integrated project managed by Alstom Power. The major items will include the boiler, turbine, generator and rotary air preheater.
Modernisation objectives
Motivated by European environmental requirements and its own business growth plans, PGE Elektrownia Belchatow defined several plant modernisation objectives:
• Unit life to be extended up to a total of 320 000 operating hours.
• Thermal optimisation to increase generated output to between 390 and 400 MW, while maximising thermal efficiency.
• Reliability, availability, and maintainability (RAM) to be improved to achieve levels comparable with the best in the industry. The time between main overhauls also to be extended.
• Boiler firing system to be upgraded to meet emissions requirements of less than 200 mg/Nm3 for both CO and NOx, as well as achieving optimum coal burn parameters within the operating load range to ensure compliance with EU Directive 2001/80/WE (Large Combustion Plant Directive).
• Automated start-up, shutdown and operation of the unit throughout the load range, and operating load range extended from 60-100% MCR to 40-100% MCR plus compliance with grid code requirements.
Separate feasibility studies were carried out during 2002/3 for the boiler and turbine and a number of alternative turbine inlet steam conditions were considered.
PGE Elektrownia Belchatow initially decided to aim for a live steam temperature of 547°C, a reheat steam temperature of 568°C and live steam pressure of 170.2 bar, with the existing feedwater temperature, 255°C, for units 3 and 4.
Taking into account test results from the unit 3 modernisation and a review of available reserve capacity, it was decided to increase live steam temperature to 568°C, live steam pressure to 185 bar and feedwater temperature to 275°C for the subsequent modernisations (ie, unit 5 onwards).
The scope of the upgrade
PGE Elektrownia Belchatow decided as a consequence of the feasibility studies to modernise each of the Belchatow units based on the following scope:
• Turbine island modernisation to include HP and IP turbine retrofits, turbine auxiliaries, extraction pipework, generator retrofit, additional stage of feed heating, electro-hydraulic control system, I&C equipment.
• Boiler modernisation to include pressure parts, firing system, flue gas and air ducts, ID and FD fan retrofit and I&C equipment.
• Main steam pipes to be replaced.
• Air preheater refurbishment.
• Electrostatic precipitator refurbishment
• New distributed control system.
• New flue gas heat exchanger.
Boiler modernisation
To achieve the 33°C increase in SH and RH outlet temperatures (for units 5 onwards) improved materials are being used for the new boiler pressure parts, including, in the case of unit 6, T91 and VM12 steels to meet the new operating conditions and the requirements arising from the stress analysis.
To achieve the required SH outlet temperature with the selected range of coals and to improve the part load performance, a wall-SH is being implemented into the unit 6 furnace above the firing system.
The modernisation of the unit 6 firing system includes the installation of eight new pulverised coal (PC) burners (including PC ducts), and two over-fire air (OFA) levels, in order to achieve the required NOx/CO emissions reduction. A direct firing system is being supplied, without vapour separation.
New OFA Level 1 consists of twelve nozzles, and OFA Level 2 of ten lances. Each of the eight inclined jet burners is assigned to one coal mill and consists of the following equipment:
• two PC nozzles, each equipped with core air tubes;
• lower, intermediate and upper secondary air nozzles, each with three horizontally separated nozzles;
• refractory between the PC and secondary air nozzles, stabilised by air-cooled supporting tubes.
Hot air is provided to the secondary air nozzles and to the core air tubes as cooling air, and also to the OFA levels 1 and 2, via new hot air ducts, connected to the existing air distribution system. The design load condition is for operation with design coal at 100% MCR with six out of eight mills in operation.
The existing coal milling system, consisting of eight beater wheel mills (type N230.45S), is being modernised and adapted to the requirements of the new firing design.
The unit 6 combustion air system is also being adapted to support the newly introduced OFA levels 1 and 2. In addition the opportunity is being taken to replace worn out components, for example, expansion joints, dampers, ducts, as well as to improve the performance of FD and ID fans.
In addition the rotary air heater performance of unit 6 is being improved thanks to the introduction of high efficiency heating elements with reduced air leakage provided by a new sealing system.
These modifications to the firing system and pressure part replacements will require the retrofit or replacement of a number of boiler auxiliary systems including new water lances for cleaning the furnace, a new and extended soot blower system, a new afterburning grate and a modified steam coil air heater.
Turbine improvements
Over the last 30 years, the technology of coal-fired power plants has made tremendous progress. During this period the major focus has been on turbine aerodynamic efficiency and improved steam conditions. In order to maximise unit efficiency both aspects will be utilised in the Belchatow 6 retrofit project. As already noted, the original equipment was designed for inlet steam temperatures of 535°C, so no major improvement in unit performance would be possible without replacement of steam paths within the high pressure and intermediate pressure turbines.
Due to the changed steam parameters some of the auxiliary equipment, including extraction pipelines, drain system and gland system are also affected, and are included in the scope of the modernisation.
A further improvement in steam cycle efficiency derives from the increase in feedwater temperature to 275°C. This is achieved by the introduction of an additional stage in the high pressure feedwater system, in the form of two new last stage HP heaters, which operate in parallel. Steam to the heaters is taken from a new extraction from the new HP turbine. This requires a modification to the existing HP heater bypass system and modification to the feedwater pipework arrangement with installation of new extraction pipework.
The HP turbine retrofit consists of a pre-assembled retrofit module, one per unit comprising a drum type HP rotor with integral coupling and reaction type blading. The new inner casing has casing mounted reaction stationary blades and is designed with shrink ring closure. The HP turbine section is equipped with two stop valves and two control valves. Live steam is admitted to the inner casing through the extended diffusers. It then passes through two 180° scrolls and the radial stationary blade row to the axial reaction blading. The rotating reaction blading is pre-twisted by applying torsion to the shrouds.
The new IP turbine is also designed for the new thermal cycle conditions. After passing the stop and control valves, the steam flows through the extended diffusers to the inlet scrolls of the inner casing. These scrolls are designed to harmonise the steam flow for the first stage blading row. In addition, the first stationary radial blade-row optimises the steam flow for the most efficient expansion. After expansion through the turbine blading, the steam leaves the outer casing via the crossover pipe flange at the top of the turbine casing. A stepped balance piston in front of the blading is used to compensate for the axial thrust caused by the reaction rotor blading.
Additional systems pertaining to the turbine island are being modernised in order to meet the customer specification for unit operational flexibility and efficiency increase, as well as supporting the modernisation of the steam turbine.
The existing electro hydraulic turbine governing system will be replaced by a
new programmable microprocessor based control system. The system to be installed is the current version of the Alstom P320 TGC two-channel redundant control system, which is routinely installed as standard equipment on new Alstom steam turbines. The governor will be operated through the external plant DCS. The new electro-hydraulic control system and new DCS system also require some of the existing I&C equipment to be modernised.
After the unit modernisation the control
oil system will continue to operate with the existing oil pressure supply system. Therefore the standard valve actuators for the new
HP and IP valves will have to be adapted to accommodate the different supply pressure. Minor modifications of control oil pipework will also be required as part of this modernisation.
Generator upgrade
For the same amount of heat transferred from the boiler to the turbine, the retrofitted unit 6 will generate more electrical power, some 394 MW, necessitating modernisation of the existing generator.
For the generator upgrade and modernisation, a key requirement is to avoid changes to foundation design. The significant power output increase, from 370 to 394 MW, required the application of a new stator with modern stator windings with the new stator maintaining the existing foundation footprint.
The generator upgrade also includes modernisation of the existing rotor, new H2 coolers, HV bushings, excitation and voltage regulation system; new stator water skid and generator seal oil system upgrade.
Significant improvements
In summary, this integrated retrofit project will ensure that the plant will be able to operate for the next 25 years with increased efficiency and reliability.
The modernisation of Belchatow units 3 and 4 was based on improving steam conditions to 547°C/568°C. Based on experience from the first modernisation project, the results of the performance tests and the latest emissions requirements, PGE Elektrownia Belchatow decided to continue the modernisation programme of the remaining units based on revised steam conditions determined during a new feasibility study carried out during 2008/9, on the basis of which it was decided to increase the live steam temperature to 568°C and the feedwater temperature to 275°C. These are the new steam conditions being applied to units 5 and 6.
It is expected that both of these modernised units will enter operation during the second half of 2011.
Modernisation of units 3 and 4 has led to a significant improvement in unit efficiency providing a heat rate improvement of 1.35%, while the integrated approach adopted for unit 6 promises a further efficiency improvement of 0.85% on unit heat rate providing a total unit efficiency improvement of 2.2% compared with the original units.
The Belchatow upgrade can be seen as a textbook example of a customised integrated retrofit, encompassing both boiler and turbine.
