Electricity demand in South Africa is rising, and national utility Eskom is investigating ways of cost-effectively increasing generating capacity. Alstom’s EcoRam product has highlighted ways in which existing power plants can be optimized to increase output.
Willem A. Laenen, Martin C. Boettcher, Eskom, South Africa
Manfred Gietz, Hendrick Lammers, Alstom Power Service, Germany
Since 1999 the electrical peak demand has been increasing at 3-5 per cent per year for Eskom’s power stations. It is expected that the need for electrical energy in South Africa and its neighbouring countries will continue to grow in the next few years at the same pace. South Africa is expected to run out of peaking capacity by around 2007, and base load capacity by around 2010, depending on the assumed average growth rate.
New capacity is expensive and takes several years to get on-line. If existing power stations can be upgraded to increase their power output capacity at a competitive price, this postpones the need for the construction of new power stations at a significant cost benefit to Eskom. In addition, such increased capacity can generally be brought on-line with a much shorter lead-time than new capacity.
Figure 1. Peak demand for Eskom power plants
In 2003 a project was initiated by Eskom to look at the possibility of increasing the capacity of the Arnot power station. A project team was set up consisting of Eskom and Alstom staff to investigate this option by using the EcoRam concept.
The aim of the team was to:
- Identify the requirements to achieve an increased power output up to 400 MWe gross per unit
- List the necessary modifications
- Estimate the costs of the modifications
- Calculate the effect on the heat rate.
Investigating these factors would enable the team to determine the optimum power output. The team decided to investigate distinctive steps based on the possible capabilities and improvements of the main steam turbine and steam generator.
Arnot power station
The Arnot power station is located 50 km east of Middleburg in the Mpumalanga Province of South Africa at 1674 m above sea level. The power station consists of 6 x 350 MW coal fired units and is a base load/load following plant. The steam generator operates at fixed pressure mode. Arnot was Eskom’s first modern coal fired power station and became fully operational in June 1975. Three of its units were mothballed in 1992 due to the surplus generating capacity Eskom had at the time. These units were recommissioned between January 1997 and December 1998.
The first step of this study was a site assessment of operation and design parameters at Arnot between 20 and 27 January 2003. The second step was the evaluation of information received and the review of systems and equipment with respect to the load cases to be investigated. A first draft report had to be finalized by the end of February 2003 to enable Arnot to make first decisions on subsequent measures. At the end of April the final report was handed over summarising the two steps for the capacity increase measures and presenting in detail the cases investigated.
Figure 2. The Arnot power station
Contents of the study
The simplifying formulas:
Pnet = Pgross – Paux
net power = gross power – auxiliary power
Pgross = m•Δh with Δh = (h2 – h1)
gross power = mass flow • enthalpy difference
show that only limited possibilities exist to increase the net power output. These are:
- To reduce the auxiliary electrical power consumption
- To increase the (steam) mass flow
- To extend the enthalpy difference.
Of course, an optimization of the equipment efficiency would also lead to an increase in power output. In order to change the enthalpy difference the following main possibilities exist:
- Increase steam temperatures and/or pressures (h1)
- Decrease turbine exhaust temperature and pressure (h2).
However, increasing the steam parameters was not seriously taken into consideration, as the design limits of the equipment such as the steam piping would have been reached or even exceeded. The possibility to decrease exhaust steam temperature and thus condenser vacuum was not investigated, as there is little chance of optimizing the cold end of an existing plant. For these reasons this study concentrated on increasing the main steam mass flow by maintaining main steam pressure and temperature.
Concerning the equipment, the focus of the study was laid on the boiler island and the main steam turbine, as these are the two major parts of a power plant and thus the cost driving components, which determine mainly the steps of a capacity increase. Nevertheless, most of the other equipment and systems were reviewed as well. As the electrical equipment such as generator or transformers is being renewed, it was not topic of this study.
Steam generator: The boiler was reviewed in terms of heat release rates, which are basic design parameters, and were compared with modern design values based on Alstom Power experience. The coal particle residence time in the combustion chamber was also calculated and rated for the investigated cases.
Main steam turbine: The maximum load of the existing turbine was calculated as well as the mechanical strength of shafts and couplings. Based on a lifetime evaluation performed in the early 1990s an outlook on the residual lifetime was given. Furthermore, the opportunities were investigated to re-blade (upgrade) or to replace the complete inner casings (retrofit) of the main steam turbine, which will also increase the efficiency of the turbine.
The survey of the boiler and the results of the investigation for the main steam turbine showed that in the first step a continuous load of 365 MWe is permissible concerning turbine stresses and forces. The steam generator would be operated at its design point (boiler maximum continuous rating – BMCR), and the main steam turbine would operate with control valves fully open.
The further survey on the boiler resulted in the conclusion that according to the reviewed design values and the coal analysis, a five per cent increase of the maximum continuous steam flow would be most probably feasible. This would lead to an electrical power output of 386 MWe gross for the turbine steam path upgrade or 388 MWe for the turbine retrofit.
To reach the 400 MWe target a main steam flow of 108 per cent BMCR has to be achieved in connection with a turbine retrofit, while an upgrade would result in 397 MWe. Since some of the boiler characteristic values would exceed in this case the particular design values, the study recommended performing a more detailed thermodynamic model for the boiler.
Figure 3. Steps of load increase study for the Arnot power station
More detailed studies are currently in progress on the extraction pumps, ash and coal systems, boiler combustion, pipework and milling plant.
In February 2004 the detailed design evaluation work was started as part of the EcoRam phase 2 process with a major focus on the boiler island and its associated systems but also for the components identified in phase 1 of the study. An environmental impact assessment and financial evaluation of the project is in progress and these should be completed together with the EcoRam work.
By the end of August the second phase of the EcoRam work will be finalized. Current indications are that a two-phased approach will be most suitable, first increasing output to 370 MWe per unit starting in 2005, and then increasing it to 400 MWe per unit starting in 2008. This will however depend on the actual growth in electricity demand.
EcoRam – Economic Reliability Availability Maintainability
Market needs are forcing plant owners to find ways of adapting existing power plants to meet new requirements. Many existing steam power plants were designed according to individual boundary conditions which are no longer valid today.
EcoRam is a newly developed product with the target to optimize existing steam power plants by systematically analysing the total plant (boiler, turbine, generator, auxiliaries, balance of plant) and working out improvement measures. The focus is not on the individual components but on the plant as a whole. The result of an EcoRam project is an overview of improvement possibilities and detailed proposals for the specific plant in which the technical feasibility, the resulting performance improvement and increase in profitability is considered. Plant and market specific boundary conditions are considered in the EcoRam process. EcoRam projects are performed in a joint team (owner + Alstom). The close cooperation of the plant owner and Alstom creates synergies by combining the operation experience of the owner and the component/system knowledge of a supplier. EcoRam provides the assessment strategy, the benchmark data and the required tools to facilitate such a project in a short time period, with projects being done in a two-phase approach. The goal of phase 1 is to provide an overview of the improvement possibilities, while in phase 2 the improvement proposals are worked out in more detail.
Typical improvement measures are components or systems upgrade, changes in maintenance strategies for specific components or availability improvement. The result of an EcoRam project is an implementation plan which takes into account the specific boundary conditions of the customer.