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New technologies and clean energy
Article taken from "Automazione Oggi", issue 277 - February 2005

Immagine 1
Super-Flash is Automa's development system for making supervision and human-machine interface applications, used at the Ostiglia thermoelectric power station

Automa's Super-Flash has been used by Sis.El. to develop the supervision and control software of the DEMI plant at the Ostiglia thermoelectric power station.

By Massimiliano Gandellini, Elena Bertillo

The use of production processes operating with respect for the environment is one of the fundamental points of Endesa Italia's policy - the company which acquired Elettrogen from the Enel group in 2001 and which is now at the head of seven power generation sites spread out over Italy (four thermoelectric, two hydroelectric and one simple gas turbine centres). The company's strategy follows two main lines: the first regards improvement of efficiency of the power stations, thanks to implementing new technologies such as combined cycles, and the second is relative to attention paid to the environment. In fact, all the power stations are or will be certified UNI EN ISO 14001 by the end of 2005 and recorded in the EMAS register, because the aim of the company is to manage environmental aspects appropriately and to set up a path for improvement, especially where environmental impact is most significant.
With this in view, Endesa Italia has seen to converting some production units of the thermoelectric power stations in Ostiglia (Mantua) and Tavazzano - Montanaso (Lodi) into combined cycles.

Immagine 2
The network structure used at the Ostiglia thermoelectric power station

The power station in Ostiglia, which is strongly committed to completion of the conversion process from the traditional steam cycle into a combined one, consists of four thermoelectric units, two of which have already been reconverted and a third which will be by the end of 2005. The traditional cycle, still present in the fourth unit, provides that a heat generator supplies steam, by means of combustion, to a turbine which activates the alternator. Combustion takes place in a boiler burning natural gas (80%) and fuel oil (20%), whose reaction produces the thermal power needed to convert the water running inside the generator pipes (boiler) into steam. By means of pipes, the steam is delivered to the turbine which converts the thermal energy into mechanical energy. Finally, the alternator, connected to the turbine, converts the mechanical energy into electricity to a total of 330 MW.
The new power station in operation
The combination of two technological cycles is defined by the term 'combined cycle': one consists of a gas turbine (Brayton cycle) and the other of a steam turbine, which exploits the steam produced in a recovery generator thanks to using the exhaust gases of the gas turbine. The two turbines are then connected to their respective power generators. The Brayton or gas turbine cycle consists of a compressor which, through a battery of filters and a silencer, draws in 1,700,000 Nm³/h of air from the external surroundings, taking it up to a pressure of about 18 bar. The compressed air is put into the combustion chamber of the turbines together with the fuel, consisting exclusively of natural gas (methane). The gases produced by high temperature combustion expand in a gas turbine which, by rotating, drives an alternator which generates electricity. The turbine exhaust gases are still very hot (about 600°C) and, if these were released directly into the atmosphere, there would be a waste of energy which the plant would pay for in terms of efficiency. To prevent this waste and increase efficiency of the power station, the turbine exhaust gases are sent to a heat recovery steam generator - a large heat exchanger where the gases give up heat to the water, vaporising it. At this point the gases can be sent to the stack, as by this time they are at a very low temperature (about 80-90 °C) and therefore have a low energy content. The steam produced is delivered to the steam turbine and, after having given up most of its energy, is discharged into a condenser where it first condenses, cooled by the water taken up from the Po river by means of pumps, and is then pumped back into the heat recovery steam generator to restart the cycle. Finally, the electricity produced by the two alternators has its voltage stepped up (to 380 kV) by means of transformers connected to the national grid through long distance lines.

Immagine 3
The DEMI plant for demineralising water has been completely overhauled

The advantages of combined cycle units are obvious, both with regard to the technical performance of the plant - since the amount of energy produced goes from 330 MW to 380 MW for each unit and efficiency increases to 56% for the combined cycle from 40% for the traditional one - and with regard to emissions into the atmosphere. In fact, natural gas, mostly consisting of methane (CH4), is a fossil fuel with limited environmental impact. During combustion, by reacting with oxygen, it produces mainly CO2 and H2O and, unlike fuel oil which contains sulphur, does not give rise to those sulphurous emissions responsible for acid rain. The quantities of solid emissions (dusts) are very small - practically negligible - when compared with those of other fossil fuels. Furthermore, since the gas has a composition richer in hydrogen and poorer in carbon than fuel oil, it also produces lower emissions of carbon dioxide (CO2), in accordance with the trend lines drawn up at the Kyoto Conference for reducing greenhouse effect gas emissions.
Conversion from traditional to combined cycle of the first two units took place successfully and the third unit is now in the final conversion stage. At technical level, the major difficulties met with were of logistic type during the demolition stage of the old plants and stacks in order to make space for the new works, whereas at organisational level, the personnel underwent intensive training to acquire the information needed to manage the converted power generation units.
The importance of water
The power station in Ostiglia is situated near the Po river from where it takes up water: some of this is used to cool the four condensers (one for each unit) and the rest, following suitable demineralisation, is a fundamental element for making the four units operate - both those with combined cycle and those with traditional cycle. The demineralised water is also needed for the demineralisation plant itself during the so-called regeneration stage.
The DEMI plant, which has the function of making the Po river water as pure as possible until a conductivity of 0.05 µS/cm is reached, consists of two distinct sections. In the first - the clarifier-flocculator - a preliminary water purification treatment is carried out to lower the temporary hardness. The second section consists of 4 production lines and some storage tanks. The lines operate on the ionic exchange principle, thanks to cationic and anionic filters which the water has to pass through to give up all the substances (metals and hydrates) which increase conductivity, thereby becoming sufficiently pure to be able to enter the steam cycle, where the components would otherwise be subject to greater wear and a high possibility of breakdowns.

Immagine 4
The whole plant has been managed by means of supervision software developed by Sis.El., a company specialised in plant automation in various sectors

The filters consist of resins which must be regenerated every 1000 m³ of demineralised water produced (about once a day) with an acid solution and with soda, to be restored to their original condition and therefore always operate correctly. Approximately 20 hours are needed for a complete regeneration process.
Each line produces about 50 m³/h of water. To fulfil the power station requirements for demineralised water, at least 2 lines must always be in operation. Each unit does, in fact, need about 10 m³/h of fresh water for a total of 40 m³/h. A further 40 m³/h approximately of water are needed for the regeneration process and part of the water must be kept in reserve for emergencies. Two production lines, in rotation, are therefore always in operation, the third is subjected to regeneration and the fourth is kept as a spare or to be used at the same time as the first two in cases where a large amount of water is needed, such as during the start-up stage of the units.
A new logic for the DEMI plant
The DEMI plant for water demineralisation has been completely renewed. In fact, in 2003 the automation system based on an electromechanical/analogue logic was replaced with a new one based on a digital logic.
The whole plant has been managed by means of supervision software developed by Sis.El, a company specialised in plant automation in various sectors.
To upgrade the supervision and control system, Sis.El. has made a redundant architecture (CPU of the PLC, field bus and HMI) to guarantee its safety and reliability. Thanks to the use of a field bus and to GE Fanuc 90/30 series PLCs and Versamax, the company has managed to limit the use of cables, thereby obtaining savings in terms of times and costs, not only initial ones, but also those regard maintenance. About the supervision software, Sis.El has once again confirmed its preference for Super-Flash, Automa's development system with which it has created a standard application. It manages 1,130 alarms and about 300 parameters, mainly used to control and manage the numerous valves in the production lines (more than 200), as well as the delicate regeneration process which makes use of hazardous substances, such as acids, and which, unless correctly carried out, does not allow efficient production of demineralised water. With MicroC - the C language compiler allowing the potential of Automa's development system to be extended - the plant parameter management, event management (changing the parameters and operations carried out on the users), user hour meters, free management of trends, and management of PID adjustment functions have also been implemented.

Immagine 5
The Ostiglia thermoelectric power station

Thanks to the flexibility of Super-Flash, Sis.El was able to easily adapt the graphics of its SCADA to the requirements of the power station where a single interface for all supervisors was wanted: the DEMI plant one and the one made using ABB's Tenore, which controls 3 of the 4 units.
There are three Super-Flash stations: two near the DEMI plant and the other one in the operating room. The plant can therefore be managed both locally and remotely.
Finally, thanks to an Applicom OPC, Super-Flash is able to exchange information with the other SCADA thereby creating a single integrated supervision system.
BOX: For Free Supervision
Super-Flash is Automa's development system for making supervision and human-machine interface applications. These can simply monitor the trend of a plant or machine, or can go further to include supervision and control functions, with the aim of "managing" the production by means of a PC. Thanks to the Wrunfile and Runfile engines, the Super-Flash applications can count on a high level of operating continuity. Apart from being extremely reliable, the development system is also flexible, i.e. suitable for developing applications for most industrial sectors and for more specific spheres, such as Home Automation and Building Automation. Super-Flash is also user-friendly, even for users who do not know any programming language, and allow effective graphic interfaces to be made, to organise information in the most appropriate way. The benchmark product of Supervisione Libera (a range of products to facilitate making SCADA/HMI applications and their integration both towards the field and towards the information systems), Automa's development system is available with some specific services, among which are technical assistance and either basic or advanced training courses, held by the developers of the products themselves.
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