boiler & steam turbine retrofit
Adding a much needed 300 MWe at South Africa’s Arnot coal fired power plant1 December 2008
As power stations built in the last thirty years approach the end of their design life, and the cost of new capacity continues to increase – along with demands for improved efficiency and lower emissions - an integrated approach to retrofit looks increasingly compelling. The ambitious upgrade project currently underway at the Arnot coal fired plant in South Africa – which will result in an uprate from 6 x 350 MWe to 6 x 400 MWe and a life extension of 20 years – illustrates the benefits.
Eskom, the South African state-owned energy giant, produces about 95% of the country’s electricity. At the turn of the century considerable changes in the South African electricity market occurred due to unprecedented economic growth resulting in a rapid increase in the country’s power demand. Periodic blackouts during peak hours started to occur frequently, and short-term delivery of extra capacity became essential to resolving the immediate shortfalls.
In 2003, the six-unit Arnot power station (6 x 350 MWe) had already passed its design life of 30 years, and various large components were in need of replacement. Combined with the increased demand for electricity, this presented an ideal opportunity to investigate the feasibility of increasing the capacity of the existing station, whilst at the same time extending the station’s lifetime.
Retrofitting existing equipment takes advantage of improved performance and mechanical features offered by modern designs. These features in turn provide tangible financial benefits, such as additional power revenues, reduced operational and maintenance costs, as well as lifetime extensions for key components. Each retrofit project is unique and presents its own set of challenges, but for any such opportunity to be successfully exploited and economically viable, it is imperative that an approach be taken that encompasses analysing all aspects of the generating plant as opposed to individual isolated systems or components.
As an initial step in facilitating any decisions regarding modification and investment by Eskom, Alstom was hired to conduct a comprehensive study to investigate the feasibility of a capacity increase. The target of the study was to assess the potential of achieving a gross output of 400 MWe per unit and to
include a preliminary estimate of investment requirements. As part of the integrated approach, the thermodynamics were analysed on a plant-wide basis – as opposed to impacts on individual components – thus creating synergies that optimised the entire unit thermodynamic cycle, the steam flow parameters, and ultimately yielding the targeted output increase. Alstom was subsequently awarded the retrofit contract.
Arnot is the first Eskom plant to undergo a major increase in capacity with substantial retrofit scope. Many parts of the existing power plant did not need modification or replacement, and those retained components identified as potentially being critically affected by the changes in plant parameters were assessed to ensure that they would be able to operate under the new conditions.
The first step in assessing the possibility of re-using equipment is a careful review of existing documentation and information. Next, a review of performance test data is carried out where applicable. This data is compared with actual operational data to allow for a plausibility check and to discover potentially weak points or defects. A further element of the study procedure is a site visit to collect updated data and feedback from plant staff. This data is then taken into account when performing cycle calculations to predict future projected operation and to form the basis of the decision to re-use or upgrade such assessed equipment.
The Arnot turbines were originally designed for a continuous load of 350 MWe, whereas the original boiler was based upon a 500 MWe design (down-sized for Arnot). Therefore, an initial assumption was that the turbines would be the limiting element.
The main components of the water steam cycle, cooling water systems, boiler and coal handling were reviewed with respect to the operating boundary conditions, the effects on heat rate, and the cost implications of any necessary modifications to achieve the respective increase in continuous output power. It was shown that the target output of 400 MWe could be achieved with a retrofit of the existing steam turbines, but the preliminary assessment revealed that the boiler would in fact also be a limiting element. Subject to confirmation, the study recommended increasing the steam flow from the boiler to 105% of existing BMCR (boiler maximum continuous rating).
The next objective thus became to ensure that the boiler was capable of producing the required steam to the turbine on an uninterrupted basis over a long period. The study identified a number of areas in which the existing equipment was deficient. Through focus on optimisation of the plant thermal cycle combined with consideration of suitability and serviceability of existing plant for the new conditions and maximum possible reuse of existing components, the scope required for retrofit and uprating was identified. Retrofitting the high pressure (HP) and the intermediate pressure (IP) turbines to increase performance and flow swallowing capacity was essential, along with retrofitting and upgrading of boiler pressure parts and coal feeding plant to provide increased steam flow. Replacement, retrofit and/or modifications to various retained boiler and turbine auxiliary components to ensure integration and capability to operate under new conditions were also identified.
As part of the capacity uprate process, all the mill gearboxes are being refurbished and upgraded, while the classifiers, the interconnecting pipework and the mill grinding materials are being replaced. Six new primary air (PA) fans are being incorporated in each unit together with their associated motors and oil systems. Modifications to the internals of the boiler drum without the need to weld to the main pressure vessel wall are also being carried out. The low pressure (LP) turbine is being dismantled and refurbished with minor modifications to increase LP turbine swallowing capacity, and complete retrofits of the HP and the IP turbines are being supplied. The turbine auxiliaries, including the valves and the oil systems, are all being stripped and refurbished. Modifications to increase the capacity of the electric feed pumps are being carried out, and each unit is being fitted with a new condensate extraction pump plus associated piping.
Boiler performance and circulation analysis
The first major task undertaken as part of the new boiler design was to conduct a boiler performance and circulation analysis. The Alstom proprietary Reheat Boiler Performance Program (RHBP) predicts overall boiler performance as well as the performance of selected boiler components. Heat balances are performed around the boiler envelope and individual components in order to generate the information that is required for detailed component evaluation. The software is structured in a modular fashion, and performs these calculations in a predetermined sequence.
The circulation analysis is performed using in-house software. Geometric data describing tubes, downcomers and risers is loaded into the software, and all flow links are established for pressure drop balance. The program then determines and distributes heat duties throughout the boiler furnace in line with the latest design standards. As well as the heat duties of other evaporative surfaces such as water wall screens, the program uses the total heat distribution to calculate the circuit pressure drops for a selected range of boiler circulating factors, using an HTFS separated flow model correlation for two-phase flow. This enables the flow in each of a number of typical circuits to be found, such that the net pressure drop from the steam drum, via the downcomers, circulation pumps, water walls and risers back to the steam drum is zero. The total flow in the boiler is then determined by adding all the flows entering or leaving the steam drum.
Optimising the steam water cycle
In retrofitting the HP turbine, the required flow can be accommodated at a wide range of turbine inlet pressures. The ability to redesign the turbine to accept increased flow at reduced pressures significantly increases the value of the HP retrofit beyond the gain from efficiency improvement alone, since it may allow previously untapped boiler reserves to be optimally exploited. Low pressure operation also generally improves the boiler performance with regard to both circulation and carry over.
The IP and LP turbines need to be considered with a view to being able to handle the increased steam flow and the associated increase in stage pressure. In this case, the IP cylinder steam path was replaced and the LP modified. The boiler feed pump turbine is supplied from extraction steam, having its own extractions. Exhausting to the feed heaters (rather than to the condenser) was analysed and considered suitable.
The safety valves on the steam drum of a natural circulation boiler, the superheater outlet header, and the reheater inlet and outlet headers must be checked for relieving capacity and operating pressure based on the design code. The boiler design and pressure vessel codes require that the boiler is provided with safety valves that are capable of discharging the maximum evaporation capability of the boiler. In addition, the superheater safety valves must be able to discharge a percentage of the boiler evaporation in order to provide adequate cooling flow through the superheater during upset conditions. The reheater system must also be provided with safety valves that are capable of discharging the maximum reheat steam flow. If the required relieving capacity for the upgrade is not available from the installed complement of safety valves, larger size valve internals may be required. The margin between the normal operating pressure and the pressure at which the safety valve operates must be sufficient to prevent unnecessary operation or simmering that can result in more frequent valve maintenance. The reseating pressure of the safety valve is just as important as the set pressure. On existing boiler installations, changes in operating conditions resulting from capacity upgrades can only be accommodated within the certified design pressure and the rules governing the selection, capability and operation of the safety valves.
The reheat temperature is a very important consideration. In retrofitting the HP turbine section, the increased efficiency results in a reduction in cold reheat temperature. As a consequence - assuming nothing else has changed - hot reheat temperature will be reduced with a detrimental effect on both power and heat rate.
Reheat temperature can be maintained by either operational adjustments (burner tilts, spray flows and similar) or by resurfacing the reheater. In some cases, the superheater may need to be resurfaced to maintain both superheat and reheat temperatures. In any case, a comprehensive boiler analysis is required in order to quantify the extent of changes, and in many situations the analysis highlights other areas of deficiency or suggests modifications to other areas that may help the reheater.
Equipment requiring retrofitting
The major retrofit required on the Arnot turbine plant is the complete replacement of the HP and IP turbines. However, the increased operational demands to achieve the guarantees offered and the project objectives had meant that much of the auxiliary equipment associated with the turbine plant was also affected, and therefore the scope was extended to include modifications to the existing LP turbines.
The HP turbine retrofit consists of one pre-assembled retrofit module per unit comprising a drum type HP rotor with integral coupling, reaction type blading, over-speed tested to 120% of nominal speed, and a new inner casing, with mounted blades designed for shrink ring closure. New steam seals and gland sealing elements are also being provided.
The existing coal mills, in their as-is configuration, were unable to meet the coal capacity throughput requirements of the 400 MWe load without modification to the mill classifier and gearbox. A larger, higher efficiency static classifier, providing higher coal throughput with lower overall mill differential pressure, replaced the existing classifier.
The existing hot primary air fans were operating close to their operational limit and were unable to meet the coal capacity throughput requirements of the 400 MWe load. New primary air fans were provided in order to achieve the milling plant potential. The new fans were supplied with new shafts and impellers, new casings, new electric motors, new couplings and new inlet louvre dampers. On boiler units, the primary air fan white metal bearings were refurbished or replaced so that after the boiler upgrade, all white metal bearings would be common.
Coal burner and secondary air nozzle changes are necessary to accommodate the increased fuel and airflow requirements at 400 MWe. A full change of burner nozzles (coal and air) is required in order to redistribute the secondary air within the windbox and to provide the correct relationship between primary and secondary air velocities.
The new ceramic coal nozzles, which were developed with local suppliers, are suitable for additional NOx-reduction measures. The Arnot power station is the first unit in the Eskom fleet fitted with low-NOx burners. The new nozzle design takes into account the manufacturing requirements of ceramic components. The newly developed ceramic inner coal nozzle incorporates flame attachment tips designed to stabilise and ignite the coal stream close to the nozzle to devolatilise the coal as quickly as possible. The outer secondary air nozzle is designed to maintain a constant airflow around the nozzle throughout the burner tilt range.
In order to reduce the flue gas temperature leaving the boiler and to improve the overall thermal efficiency, two additional rows of extended economiser surface were added to the top of the existing configuration. The economiser profile is similar to that already installed, and the support structure is suitable for the additional surface with just minor modifications.
Regenerative air preheater performance was enhanced by the application of additional measures to reduce air preheater air in-leakage. In order to meet the requirements of the specification and taking into account the service life of some of the airheater elements, a solution was provided to Eskom with assurances that the regenerative air preheaters would perform to expectations. The solution includes using new heater packs (provided by Eskom), and the refurbishment of existing seal systems on all boiler units. After refurbishment, air preheater air-in leakage was significantly reduced.
Boiler circulation system
The boilers at Arnot are of the controlled circulation type, with individual orifices located in the front and rear distributor headers that control the flow of boiler water to the furnace tubes. Correct selection of orifice sizes is paramount to ensuring that all furnace tube circuits have sufficient flow to protect the tube under all conditions of operation and that the circulation pumps have sufficient capability to meet those requirements. For the boiler upgrade, all distribution orifices and a number of orifice carriers were replaced.
The existing orifices are mounted in specially designed orifice carriers that are indexed to assure correct installation on any given circuit.
The indexing also simplifies any future removal/replacement efforts. In order not to compromise the circulation pump capability, the new orifices were carefully selected for the new specific operating conditions at 400 MWe.
The boiler design pressure cannot be compromised. Therefore, to accommodate the increased pressure drop through the superheater, the outlet header operating pressure was reduced. In order to reduce the pressure drop between the boiler and the turbine, the existing steam strainer previously located between the superheater outlet header and the boiler main steam stop valves was removed and a new pipe bend installed. The new piping is made of the same material and has the same dimensions as the piping previously installed.
Contracting, guarantees and commercial issues
Eskom awarded Alstom a single contract covering the entire capacity increase scope. The complexity of the project and the scope diversity required the involvement of a number of different divisions and called upon a range of expertise to provide an integrated project, in terms of both technology and management.
Considerations in terms of governmental policies for the development of local competences, industrial capability, and trading of high capital value equipment needed to be carefully and critically assessed. With extensive involvement of Alstom’s local companies, these issues were successfully resolved. Devolution of a significant portion of the scope to a local South African base was achieved, and all contracting criteria imposed were met.
Delivery times, particularly critical in the case of the hardware for the first unit to be retrofitted, as well as outage duration periods for the works required, were of utmost importance to Eskom in achieving their economic models. Through close co-operation between Eskom and Alstom, the contractually agreed project milestones were achieved.
The first major project milestone was achieved in 2008 when the unit 3 retrofit was successfully completed and the unit was restarted, exceeding project objectives and guarantees.
Equipment for the remainder of the units is currently at various stages of manufacture and supply, with the final unit due to be retrofitted and commissioned in 2012.
The Arnot project provides an excellent demonstration of how, through thorough analysis, consultation and planning, a capacity increase retrofit project can be developed on a plant-wide integrated basis to deliver optimal results. The resulting increase in capacity and lifetime extension has allowed Eskom to quickly address immediate and long-term concerns in an optimised and economic manner. The success of the project is seen as a forerunner to similar integrated approaches to existing assets owned by generating companies globally.