The role of coal fired plants in power generation is changing, with an increasing number covering peak demand, rather than baseload. This, combined with the pressure to reduce carbon emissions, is encouraging operators to introduce significant plant upgrade programmes so they can operate their fleet in the most economic and environmental manner.
George N. Stamatelopoulos, EnBW Kraftwerke AG, Germany
The boundary conditions of coal fired power plants and their role in power generation have changed and continue to evolve. The fight against climate change and the greater contribution from renewable energy to Europe’s energy mix are forcing coal fired power plants to change the way they operate.
Instead of operating in a traditional baseload or medium load regime, coal fired power plants now frequently cover peak load demand. This is already taking place in Germany and is likely to be the case throughout Europe and elsewhere in the not too distant future.
|Figure 1: Opened LP machines at Unit 7 of the Karlsruhe power plant showing newly installed rotors
Source: EnBW Kraftwerke
Thus, plant operators are now focusing their efforts on operational flexibility, increased peak capacity, low-load operation, quick start-up times, maximum district heat extraction at low loads, reduced generation cost and back-up or reserve power stations for coal-based fleet. All of these contribute to the same development trend – namely the growing requirement for the more efficient and economic operation of coal fired power plants.
One operator facing these challenges is EnBW Energie Baden-Wuerttemberg AG, Germany’s third-largest utility, which owns and operates more than 2.5 GW of installed capacity based on bituminous coal with main plants in Altbach, Heilbronn and Karlsruhe. The company also holds significant shares in lignite fired power plants in Lippendorf and Buschhaus and in bituminous coal fired power plants in Bexbach, Mannheim and Rostock, representing a total installed capacity of more than 4 GW. EnBW is currently building a 910 MW unit in Karlsruhe, which through the use of state-of-the-art technology will have the highest steam parameters and efficiency in today’s market.
EnBW’s strong involvement in coal fired power generation has obliged the company to proactively optimize the operation and performance of these assets.1 Three projects that fulfil these targets of improving and modernizing coal-based generation assets are presented below. Specifically, the following will be discussed:
- a high (HP), intermediate (IP) and low-pressure (LP) turbine retrofit and a capacity increase at Heilbronn power plant’s Unit 7;
- a LP turbine retrofit and firing system optimization on Unit 7 at the Karlsruhe (RDK) power plant;
- an IP turbine retrofit and increase in capacity at Altbach power plant’s Unit 2.
TURBINE RETROFIT & CAPACITY INCREASE AT HEILBRONN
The bituminous coal fired Unit 7 of the Heilbronn power plant entered operation in 1985 with a capacity of about 750 MW and the capability to cogenerate heat for both industrial and domestic customers at about 300 MWth.
Its 2250 tonnes/hour forced-flow steam generation boiler, designed as a tower-type boiler, utilizes a tangential firing system and was supplied by EVT (Energie- und Verfahrenstechnik GmbH, now Alstom). The turbine consists of a single-flow HP part, one double-flow IP part and two double-flow LP parts, and was supplied by BBC (Brown, Boveri & Cie, now Alstom).
The rationale for the turbine retrofit and capacity increase project was two-fold. First, it could increase capacity and achieve a simultaneous rise in efficiency and a fall in specific carbon dioxide (CO2) emissions. Second, it could change the quality of bituminous coal supplied to the unit and enable co-firing of sewage sludge in the boiler. Prior to starting the project a feasibility study was conducted2, where the objectives of the retrofit were defined. Subsequently the scope of the project was specified and the project contract awarded to Alstom.
For the turbine retrofit, all three sections of the turbine – HP, IP and LP – were supplied with new blade technology to produce a better entropic efficiency in all three sections. The outer casings were kept in order to minimize erection work and outage time. In particular, in the LP section new hardening technology was applied to the top edge of the blades in the last row to protect against erosion, while not affecting the strength properties of the bottom edge. In addition to the upgrade of the blades, the inner casings, shafts and sealing system were also renewed and replaced.
Table 1 summarizes the results of the turbine retrofit. The power output increase is related to an efficiency increase of 1.1 percentage points and a corresponding 2.8 per cent decrease in the specific CO2 emissions; down to 830 g CO2/kWh. On the air/fuel side of the power plant, a series of modifications were conducted to accommodate the change in the properties of the bituminous coal and the co-combustion of sewage sludge:
- the unit’s four mills were replaced by ones with a higher capacity and a capability of burning a wider range of coal types and co-combusting sewage sludge;
- the number of each mill’s outlet ducts was increased from one to two, eliminating one splitter in the pulverized fuel ducting and reducing the pressure losses and the imbalance of coal/air distribution in the four corner burners;
- an additional primary air fan was installed to generate hot air after the air preheater at the inlet, raising the mills’ operational flexibility by increasing the energy input to them via the additional primary air;
- two additional steam air preheaters were integrated and a corresponding adjustment of the air ducting was made to prevent low-temperature corrosion in the flue gas rotary air preheater.
Figure 2 gives an overview of the modifications conducted on the four mills and compares their status before and after the retrofit. After the retrofit, Unit 7 went back into commercial operation in autumn 2009. In addition to the technical features upgrade and their successful implementation, it was also important for EnBW to achieve a lifetime extension for this unit and combine it with a reduction in the frequency of inspections.
|Table 1: Results of turbine retrofit at Heilbronn power plant’s Unit 7|
LP TURBINE RETROFIT & FIRING SYSTEM OPTIMIZATION AT RDK
Karlsruhe (RDK) power plant’s bituminous coal fired Unit 7 entered operation in 1985 with a capacity of about 535 MW and the capability to cogenerate heat for industry and households at about 220 MWth.
The 1534 tonnes/hour forced-flow steam generation boiler is designed as a tower-type boiler, having a boxer-type firing system with 32 burners at four levels and it was supplied by L&C Steinmüller.
The turbine consists of a single-flow HP part, one double-flow IP part and two double-flow LP parts and was supplied by BBC. The reasons for the LP turbine retrofit and the firing system optimization project were once again two-fold:
- after a successful retrofit of the HP and IP part of the steam turbine in 2005,3 the LP part represented the limiting factor in the lifetime extension of the whole power plant;
- the change in the quality of bituminous coal supplied to the unit combined with the potential to improve the firing system performance and reduce the ammonia consumption for NOX emissions control.
For the turbine retrofit, the LP sections of the turbine were supplied with new technology blades enabling a better isentropic efficiency of the LP sections. The contract was awarded to Alstom and was executed by a multinational team with project management from Germany, engineering and design from the UK, and manufacturing from Poland. As previously, the outer casings were kept in order to minimize erection work and thus the outage time, and the inner casings, the shafts and the sealing system were renewed and replaced. Figure 1 shows the opened LP machines with the new rotors installed.
The additional power output expected from the LP turbine retrofit, according to the design data of the retrofit project, was about 9 MWe. In order to evaluate the results, two performance tests were conducted – one before and one after the retrofit. As can be seen in Table 2, the additional power output was measured to be higher than 25 MWe.
This higher than expected performance is because of the lower power output measured before the retrofit and is linked to the ageing of the LP turbine, while the performance after the retrofit corresponds well with the design data. In this context, the power output increase is linked to an efficiency increase of about 1.0 percentage points and a corresponding decrease in the specific CO2 emissions of 2.5 per cent.
On the firing system side, the following modifications were carried out to cope with the change in the bituminous coal properties and to improve the performance of the firing system:
- modification of the boiler’s 32 burners to ensure improved mixing at sub-stoichiometric conditions;
- installation of side air and over-fire air openings to achieve a staged combustion and therefore reduce primary NOx formation;
- modification of the outlet ducts in two of the four mills, corresponding to the two upper burner levels, by installing a four-outlet systems instead of the existing single outlet. This further eliminates two stages of coal splitting dampers and ensures a homogeneous coal distribution to the different burners;
- in the same two mills as above, increasing the rotating velocity of the dynamic classifier to ensure that finer milled fuel reaches the two upper burner levels.
As a result of the firing system modification the excess air ratio was safely decreased to values below 1.2, compared to 1.25 prior to the modifications. This consequently reduced the self-consumption of the steam generator by cutting the loads of the fans. Furthermore, NOx formation was significantly reduced; for some coals by half of the value before the modification, being approximately 1000 mg/Nm3, on a 6 per cent O2 basis. This also reduced the NH3 consumption for the catalyst operation.
The RDK 7 Unit has two bunkers feeding one mill, a feature that allows the mixing of different coal qualities in one mill, providing wider fuel flexibility for the unit. For clarity the results highlighted here are therefore indicative and do not cover all possible coal combinations supplied to the power plant.
The Unit 7 of the Karlsruhe power plant was taken back into commercial operation in autumn 2010 after the retrofit and the firing system modifications. Both the LP turbine retrofit and the firing system modification took place within the planned overhaul of the unit.
|Table 2: Results of the LP turbine retrofit at RDK’s Unit 7|
IP TURBINE RETROFIT AND THE INCREASE OF CAPACITY AT ALTBACH
Unit 2 of Altbach, which is a bituminous coal fired plant power plant, entered operation in 1997 with a capacity of about 430 MW. The 1008 tonnes/hour forced-flow steam generation boiler is designed as a tower-type boiler, having a wall firing system with 12 burners at three levels, and was supplied by L&C Steinmüller.
The turbine consists of a single-flow HP part, one double-flow IP part and one double-flow LP part, and was supplied by Siemens. A characteristic feature of the unit is that it can also be operated as a compound block because it is linked on the water/steam side to a gas and steam turbine combined-cycle. The combined-cycle contributes up to about 95 MWe to the capacity of the unit.
During an inspection of the inner casing of the IP section of the steam turbine, lifetime limiting damages were discovered that resulted in the decision to perform an IP turbine retrofit. The contract was awarded to Siemens, and design work has already started. The manufacturing slot is also reserved, with the retrofit to be implemented in spring of 2012.
In the new configuration of the IP part the inner casing and its contents – including the shaft, blades and sealing devices – will be replaced by new components. The technical progress that has taken place since the original installation of the turbine in Unit 2 will be considered in the design of the new IP part. New blade geometry, optimized flow pattern and an advanced sealing concept in this section will result in an increase of the power output by 2 MWe, while keeping the same firing capacity. The outage time for this retrofit is expected not to exceed 37 days and the necessary outage for this is allocated for March 2012.
In the framework of a feasibility study, the possibility of increasing the unit’s capacity was also examined. Both the steam generator and the turbine were originally designed with high margins, enabling such a capacity increase. Therefore it was decided to make the necessary minor modifications to raise capacity by 21 MWe on a solo – not compound – operation basis. This project was awarded to a consortium led by Steinmüller Engineering and will be carried out in the same outage, as the IP turbine retrofit.
|Figure 2: Comparison of the mills at Heilbronn Unit 7 before and after modifications Source: Alstom|
The modifications that will be performed are: modifications to burners and mills; adaptation of the forced draft fan, comprising the installation of a frequency converter.
In all six mills the transmission speed of the gearbox will be increased, the rotation direction of the nozzle ring will be changed, the classifier will be modified and the sealing air fan will be upgraded. All those measures aim to increase the coal throughput through the mills, while maintaining fineness and an acceptable power consumption of the mill and classifier motors.
The 12 burners will be replaced by new ones, firstly to accommodate the capacity increase, while keeping a good firing performance and secondly to minimize the maintenance requirements of the burners. Currently, three different burner types are installed in the steam generator, as a result of past optimization and experimental work. Extensive CFD work on the design and performance of the burner was carried out during the design phase to ensure a good firing and environmental performance of the new burners with the wide range of coals expected to be utilized in Altbach in the future.
We have described the modifications of Unit 7 in Heilbronn, Unit 7 in RDK and Unit 2 in Altbach. The first two were performed in the last two years, while the latter is expected to be finished within a year. The results in these units are in line with EnBW’s strategy to continually upgrade its existing coal fired fleet, reduce its operational costs and prepare the units for the more demanding flexible operational mode in the years to come. Also the continuous improvement of the environmental performance of the coal fired units, in terms of emissions and in particular CO2 remains a major objective in the operation of these units.
1. EnBW Energie Baden-Württember AG: Die fossil befeuerten Kraftwerke der EnBW, Company Publication, December 2010.
2. Goll, B., Peter, T., Hestermann, R.: Sanierung wirtschaftlich optimiert – Leistungssteigerung von Block 7 des Heizkraftwerkes Heilbronn, BWK, Bd. 62 (2010), Nr. 7/8, S. 52-54.
3. Kirschning, F.-P.; Abröll, B.; Adamski, P.: Retrofitting of Steam Turbines in two hard coal-fired power stations of EnBW, POWER-GEN Europe, Cologne, Germany, May 2006.
This article was co-authored by Frank-Peter Kirschning, Rolf Seeger and Stefan Eberle of EnBW Kraftwerke AG, Germany.