Jean-Pierre Goffin,
Tractebel Energy Engineering, Brussels, Belgium
Electrabel’s Verbrande Brug power plant, near Brussels, was originally built as a coal and fuel-oil fired steam power plant, consisting of three units (2×125 MWe and 1×140 MWe), see Table 1. The first unit entered commercial operation in 1959. The steam turbine of the 140 MWe unit 3 was destroyed in 1982 following a grid accident and replaced in 1986 with a new one. The repowered plant retains this “new” steam turbine. But the original coal boiler has been replaced with a Siemens V94.3A gas turbine and associated heat recovery steam generator. The remaining two units are being decommissioned as part of the repowering project.
The net power output of the repowered unit will increase from 140 MW to 386 MW, while net efficiency is increased from 38 per cent to 56 per cent.
The repowered unit also provides further advantages such as:
* low investment costs for high power output, thanks to extensive use of existing plant components and site infrastructure;
* reduced environmental impact, thanks to low dust, CO2 and NOx emission levels and no SO2 emissions, particularly when compared with the existing power plant.
The new power plant buildings have been built near the turbine hall of unit 3 on a free area originally intended for a possible extension of the old coal-fired power plant. This option has allowed work on the new plant to be carried out prior to dismantling of the unit 1 and 2 turbine halls.
Basic design approach
The basic design of the new power plant was determined during the preliminary engineering phase, whose main objectives were:
* to optimise the steam cycle taking into account the characteristics of the existing steam turbine;
* to check the functional adequacy of the existing equipment: mainly the cold source (condenser and main cooling tower) and the steam turbine.
A key task was integrating the new equipment, such as the gas turbine and heat recovery steam generator, with existing components (steam turbine, condenser, cooling tower) into an optimised electricity generation process.
The base option of the design was to maintain the steam turbine and the condenser, and operate them in new conditions taking into account their respective limitations.
The steam turbine is a three-part machine (HP, IP, double flow LP) with reheat.
In the early phase of the project it appeared that the most suitable cycle would use two pressure levels with HP and LP steam production, with reheat but without steam extraction for feedwater heating. In such conditions the mass flow through the HP and IP turbines decreases while it increases significantly through the LP turbine (see Table 2). This requires a careful check on whether the new conditions are within the allowable operating range for all parameters, as defined in co-operation with the original manufacturer.
A key issue was the volume flow in the condenser because it determines the velocity field around the tube bundle, hence the vibration level and the induced mechanical stresses. To ensure safe operation of the existing condenser, it was decided to limit the volume flow to 107 per cent of design value. This resulted in a minimum condenser pressure of 60 mbar, which is significantly higher than the original value of 42 mbar.
Economic analysis revealed that condenser replacement could not be paid back by the gain in electricity production so it was decided to maintain the existing equipment.
To comply with environmental regulations at the higher condenser thermal load (+ 45 per cent) the cooling tower has been equipped with new packing.
Despite the numerous constraints mentioned, it was possible to develop an optimised cycle with a net efficiency of 56 per cent, by taking advantage of the high exhaust temperature of the gas turbine (580°C) and of the high design inlet temperatures of the steam turbine (550°C/560°C).
Gas turbine
After a cost/performance analysis of several possibilities, the Siemens V94.3A gas turbine was selected (see Table 3).
Considering the high costs for a small benefit, a flue gas bypass of the boiler has not been included. The flue gases produced by the gas turbine during start up are directly used by the boiler to produce steam which is bypassed to the condenser until the conditions to start the steam turbine are met.
Heat recovery steam generator
The heat recovery steam generator, supplied by CMI of Belgium, is based on a classical vertical design, with two pressure levels, reheat and condensate preheater.
The boiler is equipped with drums and is designed for sliding-pressure operation. When considering the operational parameters of the HRSG (see Table 4), one has to bear in mind that the choice of the steam parameters has been made on the basis of both efficiency and compatibility with the existing steam turbine configuration.
Water–steam cycle
The water–steam cycle, apart from the boiler itself, is located in the existing turbine hall building; it consists of the condensate system, the feedwater system with the feedwater and deaerator tank and the steam system.
As much of the original equipment as possible has been retained, depending on its state and compliance with the new operating conditions:
* the condenser is unchanged;
* the two 50 per cent condensate pumps are equipped with new impellers to meet the new operating conditions;
* two new low-pressure feedwater pumps have been installed;
* the turbine driven high-pressure feedwater pump has been replaced by a new variable speed motor driven feedwater pump;
* the standby high pressure motor driven feedwater pumps have been equipped with new impellers and keep the same function;
* the feedwater tank is unchanged.
The steam connections between the several exchangers of the boiler and the steam turbine are completed by two intermediate and low pressure bypass valves to the condenser, to be able to dump steam to the condenser when the steam turbine is not available during start-up and transients.
A steam inlet section has also been added for the low pressure steam. This consists of shut-off and control valves leading to the pipe connection between IP outlet and LP inlet points.
Other features of the steam and water cycle are:
* circulation loop for low load operation;
* circulation of preheated condensate to raise the inlet temperature of the condensate to the required temperature level to prevent the flue gas temperature dropping below the water dew point.
Instrumentation and control
The basic design of the instrumentation & control system takes into account the fact that the steam turbine and most of the auxiliaries are kept unchanged; interfacing relays with this part of the plant are kept as far as possible (the I&C was renovated seven years ago).
After a comparison between several possibilities the same digital control system (DCS) was chosen for the balance of plant as for the gas turbine. This achieves a homogeneous man–machine interface.
Making use of the advantages of the new digital control systems, Electrabel asked Tractebel in November 1999, five months before the start of commissioning:
* to implement a new control room in the administrative building of the Vilvoorde site;
* to centralise in this room the control of the operation of the Drogenbos combined cycle power plant (460 MW) and of the Schaerbeek power-from-waste plant 3 x 17 MW);
* to upgrade the DCS of Drogenbos to allow it to be remotely operated from Vilvoorde and to adopt the same approach to the representation of systems.
This “project inside the project”, as it has been called, was successfully completed in April 2000, allowing the commissioning of the new plant from the new control room.
Environmental impact
The environmental benefits of the Vilvoorde repowering stem from:
* the high efficiency of the new cycle when compared with the old one;
* the use of natural gas as fuel compared with coal or heavy fuel-oil; and
* low NOx emissions, through use of a gas turbine equipped with dry low NOx burners.
Particular attention has also been paid to noise abatement since the closest residential area is only 500 m away.
An optimisation study was performed to identify the noise sources, to impose contractual limits at the specification stage, and to choose the most cost-effective mitigating techniques.
The result will be a noise level as low as 40 dB(A) at the most critical reference point, which is consistent with the operation permit given by the authorities.
Although the improvements due to the repowering are obvious compared with the previous situation, careful analysis of the impact of the plant on the ambient air has been performed. This demonstrates a reduction of more then 70 per cent in the amount of NOx emitted by the site, in addition to the elimination of SO2 emissions and significant decrease in dust and CO emissions.
Organisation and timetable
Electrabel has entrusted Tractebel Energy Engineering with all the tasks relating to the preliminary design and execution phases of the project.
Electrabel, with the help of specialised teams from Tractebel, is in charge of dismantling the existing plant and the unwanted auxiliaries, along with maintaining the equipment retained from unit 3, principally the steam turbine, and the main and auxiliary cooling systems.
The following work sequence was adopted to meet the tight time schedule and to optimise the interfaces:
* dismantling of the auxiliaries for coal and heavy fuel-oil handling, which are no longer needed;
* demolition of the three existing boiler chimneys;
* dismantling of the existing boiler of unit number 3 to permit installation of some auxiliary mechanical equipment.
This first phase was implemented in parallel with the civil engineering phase for the new plant. A wall was built between the turbine halls of units 1 and 2 and the turbine hall of unit 3. TEE is presently preparing to dismantle the Verbrande Brug 1 and 2 boilers and turbine halls. This will take place in 2001-2002, as originally scheduled.
The milestone dates were:
* issuing of the building and operating permit by the authorities – 15/02/1999;
* start of driving of foundation piles – 22/02/1999;
* final notice to proceed issued to the main equipment contractors – 15/03/1999;
* first firing of the gas turbine – 15/07/2000;
* expected commercial operation -February 2001, which is less than 24 months after the start of pile driving for the foundations.
Engineering tools
To manage such a complex project TEE has made extensive use of computerised engineering tools:
* The process and instrumentation diagrams are drawn by computer, with automatic extraction of engineering data.
* The layout is drawn using 3D computer aided design models including civil engineering, mechanical and electrical layout, permitting simulations reality and automatic drawing of piping isometrics.
* Isometrics for piping are automatically established considering the functional data of the PIDs, the standardised piping categories and the drawing data of the 3D models, with automatic extraction of bills of materials.
* An original system developed by TEE is used to maintain coherence between all technical computer applications. This system is called TIS (“Technical Information System”) and ensures flow of information between all technical disciplines. Decentralised databases, each corresponding to a different engineering speciality, are organised around a central database; during the design process, upstream activities (PIDs, system design, layout) inject design data into the database. Subsequently, the mechanical engineering, electrical engineering and equipment contractors use and update these data. The resulting engineering data allow technical follow-up during the construction.
* To improve communication within the project team, a site dedicated to the project has been implemented on the company’s intranet.
