Timo Saarinen, Fortum Engineering, Helsinki, Finland
In December 1999, a contract was signed for the modernization of unit three at the Deva power plant in western Romania. The contract, between S.C. Termoelectrica S.A., formaly Conel, the Romanian National Electric Company, and a consortium consisting of Fortum Engineering of Finland, Alstom Energie of Germany and Itochu of Japan, is worth approximately a66 million ($63 million).
The project, which is being financed by the World Bank, is due to be completed in May 2002. The aim of the project is to install a modern turbine, control system and state of the art combustion technology that will enable the plant to decrease its nitrogen oxide emission levels well below the levels specified by the European Union.
The modernization project will increase the life of unit three by 100 000 hours as well as increase its overall availability. The contract will also ensure the achievement of at least the original design performance values of the unit, and provide a safe working environment for power plant personnel.
The Deva power plant consists of six units, each having a capacity of 210 MWe. Each unit consists of two boilers and one turbine. The power plant uses hard coal as the main fuel as well as natural gas and heavy fuel oil. Deva power plant’s annual production is about 1400 GWh of electricity and 11 TWh of district heat; it is one of the most important power producers within the Romanian power industry.
Romania’s electrical supply
In 1998, Romania had approximately 22.6 GW of electric generation capacity. Of this total, thermal power plants accounted for around 70 per cent of electricity supply, with hydroelectric plants producing most of the remainder. Approximately 60 per cent of the country’s power capacity is more than 20 years old, and about 10 GW will need to be rehabilitated or replaced by 2010. Rehabilitation is the cheaper option, and will cost an estimated $3 billion but should give Romania the opportunity to export electricity to its neighbours, and thus generating a valuable source of income for the government.
Since 1993, Romania’s electricity generation has surpassed the country’s consumption, but its old and inefficient power system, combined with difficulties in securing supplies of primary fuels, has resulted in periodic energy shortages and shutdowns in power generation. This has restricted state-owned Termoelectrica from exporting any significant amount of power.
The Romanian government’s privatization plans have promoted competition within the country’s power sector. The government has also signed a contract with Bulgaria to connect their power grids, allowing Bulgaria to serve as a transit centre for power exports from Romania to Turkey.
Under privatization, Termoelectrica, which owned 36 subsidiaries generating heat and electricity and 42 electric network subsidiaries, has retained ownership of the national grid, and has remained the parent company to supply and generating subsidiaries. Under the government’s restructuring plans, separate subsidiaries for thermal generation, hydropower generation, and distribution have been created in addition to a separate company for nuclear power generation.
The effective date of contract for the Deva refurbishment project was 1 June 2000 with site activities starting on 1 November 2000. Before site activities can begin on a refurbishment project, the Consortium conducts an intensive condition assessment of the existing units and its components. This assessment uses current material inspection data, plant evaluation, the experience of plant personnel, the operating and maintenance history as well as original design and construction data to carry out its analysis.
Power plant data can be supported by field analysis, including non destructive testing (NDT) and destructive testing (DT). The NDT can include magnetic partial, ultrasound, endoscopic, dye penetrant, eddy current and replica tests. The DT can include hardness, tensile strength and creep tests, and also chemical analysis.
Operational problems relating to the combustion system that have occurred in the past are also taken into account during the design and modification of the boiler and combustion system. Such operational problems can include unstable combustion, high non-combustible losses, corrosion, slagging, fouling, and over-heating in the furnace. Minimizing these problems is essential to the longevity of the refurbished unit.
Replace and refurbish
The Deva power plant was originally built in the 1970s. The turnkey modernization project to improve the overall output and increase the efficiency of the unit will include: new low-NOx combustion systems including low-NOx burners capable of firing coal; heavy fuel oil and gas; an over-fire air system; and anti-corrosion air systems. Replacement of the ash and slag slurry pumps and the coal mills, and partial replacement of the heating surfaces of the boiler are also included in the refurbishment plans for Deva.
The delivery of new soot blowing systems, a new automatic control system, and the new electrical systems, as well as the refurbishment of hundreds of valves, actuators and instruments will also increase the efficiency of the boiler.
The turbine refurbishment will include a high pressure cylinder with high pressure control valves and actuators, an intermediate pressure cylinder including intermediate pressure control valves and actuators, a low pressure cylinder without outer casing, a rotor, high pressure stop valves and actuators, intermediate pressure stop valves and actuators, replacement of the turbine control system, generator refurbishment and pre-rehabilitation tests.
The contract also includes the dismantling, installation, and commissioning of the unit. The consortium will also provide training for the staff and documentation for Termoelectrica.
The main issue in the design of the boiler modernization has been the determination of the existing performance of the boilers. The existing design has been verified by using the most modern process calculation and simulation tools. An important part has been to ensure the compatibility of the water/steam process design with the new combustion system and new furnace conditions. As a result of these checks, there will be only minor changes in the original design of the boiler which will mainly involve the replacement of the existing heating surfaces.
All the existing hammer type coal mills at Deva will be replaced with eight new bowl-type coal mills. The new coal mills, which will be delivered by Alstom Power Boiler, feature excellent grinding fineness enabling the achievement of low NOx emissions and a low rate of unburned carbon in fly ash and slag. As well as a low coal/primary air ratio enabling the achievement of low NOx emissions, the new coal mills will help to achieve high unit availability and an excellent tolerance against wear from abrasive coal.
The combustion system
The main fuel of the unit is hard coal but it will be also possible to reach the maximum capacity of the unit with natural gas, and 50 per cent capacity with heavy fuel oil.
The design of the combustion system is based on the intensive furnace simulation calculations. By utilising modern tools and analysis techniques, it has been possible to identify furnace conditions such as temperature fields, particle streams, and chemical conditions of the furnace gases in order to adjust the combustion system design.
The new low-NOx combustion system will be delivered by Fortum Engineering and it is based on the HT-NR (High Temperature NOx Reduction) low-NOx burners applied with an over-fire air (OFA) system. The fast ignition of the flame close to the burner tip is typical for the HT-NR technology. This fast ignition enables effective control of the fuel/air ratio in the flame and thus gives balanced temperature fields in the flame and the ability to control NOx.
The reduction of the NOx emissions will therefore be ensured by the use of the OFA system. About one third of the combustion air is inserted to the furnace via the over-fire air ports located above the burners. The burners operate under stoichiometric conditions with an air factor of approximately 0.85.
The HT-NR low-NOx burners have a long mechanical life, due to the application of special ceramic materials inside the burners which enable low maintenance costs and high availability. The simple construction enables simple and safe operation and the best available combustion performance. The excellent flame stability enables low minimum load without supporting fuel and thus improved operational economy.
There will be eight low-NOx burners for each boiler with one coal mill feeding two burners. The maximum capacity of a boiler is reached with six burners in operation.
The application of sub-stoichiometric conditions has some times increased the risk of corrosion in the furnace. In the new design, this risk has been reduced through the proper design of the air streams, and side air ports shielding the furnace walls will also be applied.
The main automatic control system will also be supplied by Fortum Engineering. The system will be implemented using the digital distributed control system (DCS) Damatic XDi. The system will provide control, monitoring and alarm annunciation combined with an interlock and sequence control system. There will be approximately 7000 I/O signals connected to the system. The unit will be operated and monitored via automation operator terminals in the central control room. The system will ensure the safe, reliable and efficient operation of the unit at Deva.
An additional ‘EXIS’ process information system will be provided. EXIS covers the procedures of power plant operation control including the acquisition of process measuring data, event and alarm data, process computing and reporting, and display functions for operation control. Additionally, EXIS covers advanced alarm functions, the processing of disturbance and failure data, and the long term storing of process data for later analysis.
The turbine and generator modernization will be implemented by Alstom Power Sp. z.o.o, which has excessive experience on the modernization of Russian LMZ-designed turbines. The turbine modernization includes almost completely new high pressure and medium pressure parts and modernization of the low pressure part as well as the modernization of the auxiliary equipment.
The advanced design of the low pressure part of the turbine will have a significant influence on the efficiency.
Shrouded rotor blades, three dimensional vane design, optimized transonic blading, jointly optimized last stage blades and diffuser design, and protection against last stage blade erosion are all important features to be applied to Deva’s unit three lower pressure modernization scheme, and will lead to excellent efficiency.
Once the project is completed, the customer will have a completely modernized unit with a life time extended by at least 100 000 operating hours. The modernized automatic control systems and instrumentation will ensure the safe operation of the unit with high availability. The production capacity of the plant will also be increased.
The refurbishment of the boilers and the turbine will increase the total efficiency of the unit. The increased efficiency with the new combustion system will enable the customer to achieve emission levels far below the national standards.
The refurbishment of Deva power plant, unit three, is a good example of how the existing overrating capacity can be upgraded for future operation in an environmentally and economically sound way. This approach requires a high level of professionalism from the feasibility study phase to the final implementation.
Close cooperation between the different parties, i.e. the power plant operators, designers and contractors is essential. A good understanding of the local conditions as well as the ability to combine modern technologies with existing technology is needed.
Erection and pre-commissioning of the refurbished unit is due to be completed on the 22 November 2001, with final commissioning of Deva’s unit three to be completed on the 23 May 2002.
Figure 1. HT NR Low NOx burners will help to keep emissions in check Figure 2. Deva power plant Unit 3 Damatic XDi control system provides control, monitoring and alarm annunciation and an interlock and sequence control system