The dry-tower based cooling system at the Mátra power plant near Budapest has undergone a major upgrade, involving addition of a wet cooling tower to create a “hybrid” system. It is one of the largest such projects ever carried out, and the first involving a dry-tower cooling system of the Heller type. The result is increased competitiveness and efficiency, with a 250 000 t reduction in annual carbon dioxide emissions.
The lignite fuelled Mátra plant, 70 km east of Budapest, majority-owned and operated by RWE, is the largest coal-fired plant in Hungary (see Figures 1a and 1b). Units 4 and 5, originally rated at 200 MW each, were initially commissioned in the early 1970s, and retrofitted in the late 1990s to provide 230 MW nominal capacity each.
Láng Engineering Works (today part of Alstom) delivered the steam turbines under licence from Brown Boveri Company (BBC). The generators are hydrogen cooled units manufactured by Ganz, while the Heller type dry-tower cooling systems were supplied by EGI (today owned by GEA). In the Heller system the dry cooling tower is connected to the turbine condenser via a water filled intermediate closed circuit. The condenser is of the direct contact type, in which the steam is condensed by bringing it into direct contact with cooling water films, rather than a conventional surface condenser (editor’s note: see pp 26-30 for more on the direct contact jet condenser concept).
In 2004 the Mátra plant management initiated a 60 million euro investment programme to increase the generating capacity of units 4 and 5 and to give the plant greater operating flexibility to meet changing market demands. This has been achieved by adding a 30 MW natural-gas-fired topping gas turbine to each of the units, for commissioning by mid 2007.
This power uprate had significant implications for the overall water–steam cycle of the units, since in the modified units the heat for feedwater preheating is in part provided by the residual thermal energy of the gas turbine exhaust flue gas flows, via new heat recuperators (see Figure 2). Consequently, the amount of steam extracted from the turbine to serve the original feedwater heaters is substantially reduced, overloading the steam turbine, especially its cold end. This meant that the capacity of the cooling system would no longer be sufficient.
Furthermore, there was the inherited problem of a 10% capacity deficiency in summer (relative to original nominal capacity of 200 MW) due to the dry-tower cooling system.
Wet and dry cooling
There are a variety of approaches to dissipating the residual heat of fossil fired power plants but they can be divided into two main types: those employing a wet cooling tower; and those using a dry cooling tower. In the dry cooling tower systems, the sole cooling medium is the ambient air, whereas in wet systems you need a water source, which can be sea, river, lake or underground.
The dry cooling tower system offers the advantage of very low water consumption, which is obviously a factor if there are limited water resources. The main drawback, at most geographical locations, is lower plant efficiency compared with use of wet-tower systems.
A hybrid cooling system merges the dry-tower and wet-tower cooling technologies into one combined system. These systems are rare, most of them were built recently, and very few are conversions from fully wet-tower or fully dry-tower systems (not counting systems where water is sprayed onto the surface of the dry air heat exchangers).
“Conversion-to-hybrid” upgrades can be classified into two groups:
• Dry-to-hybrid. In this case, a wet tower is added to the existing air-cooled plant. This can restore performance hampered by ambient air conditions during peak temperature periods, or even enhance it on a permanent basis. The critical issue, though, is the increased water consumption.
• Wet-to-hybrid. In this case the water consumption of an existing wet cooling system can be significantly reduced by adding a dry air-cooled tower. The potentially lower efficiency of the dry system could be partly or fully offset by measures such as a steam turbine blading upgrade or complete turbine retrofit.
A well designed conversion-to-hybrid cooling upgrade is a cost-effective intermediate technology combining the advantages of air- and water-cooled systems while offsetting their disadvantages.
The Mátra solution
Water is available but limited at the Mátra site, which is why units 1, 2, 4 and 5 were originally designed with dry cooling towers (one per unit); only unit 3 had a wet cooling tower. The conversion-to-hybrid upgrade solution was viable for units 4 and 5, but the constraints were tight. Engineers at the steam turbine’s OEM, Alstom Power Hungary, therefore proposed an optimised conversion-to-hybrid cooling upgrade (see Figure 2). They committed themselves to 230 MW nominal capacity in summer at 30ºC ambient air temperature and 246 MW in winter at 2ºC.
The approach adopted for Mátra 4 and 5 is to have two different types of condenser (of different capacities) operating in parallel: the original direct condenser, cooled by the existing dry tower, and a newly installed surface condenser cooled by a new wet cooling tower added to each unit.
Consequently, the exhaust steam flow leaving the low pressure steam turbine casing has to be divided into two flows. At the increased nominal load about two-thirds of the total steam flow is still condensed in the direct contact condenser, but the remaining third is channelled, via a window created on the condenser neck, to the new surface condenser (see Figure 3).
The new configuration does not require any further structural modifications to the original dry cooling system.
The hybrid cooling scheme combines the typically lower water consumption of the dry cooling towers with the better condenser vacuum associated with wet cooling systems, thus providing improved unit capacity. The concept delivers significantly better performance than any of the other options for boosting cooling capacity proposed by other suppliers bidding for the project in 2005.
The predicted capacity increase is 24 MW (annual average) for each unit, without additional fuel. In addition, the forecast minimum block efficiency increase is more than 1 percentage point.
But there were some critical prerequisites for the proper execution of the chosen solution. These included extensive design and site knowledge of the installed steam turbine and turbine hall equipment, and the ability not only to design tailor-made surface condensers, but also to model the direct contact condensers and to optimise the whole water–steam cycle.
The project is not only the biggest conversion-to-hybrid upgrade ever executed anywhere in the world, it is also the first one applied to a Heller-type dry-tower cooling system.
Mátra management signed the project contract with Alstom Power Hungary in February 2006, along with two further contracts for the major overhaul, upgrade and life extension of the two steam turbines – which meant that Alstom could develop the optimal technical solution, while limiting the technical and logistical risks involved in site co-ordination. The total value of the three contracts exceeded 20 million euro out of the total investment programme of 60 million euro.
The entire project, from signature of the contract to completion, took 14 months, and just nine months from contracting to commissioning of the first unit – which significantly reduced the payback period on the investment. For the most part, it was possible to execute the project without disturbing the normal day-to-day operation of the plant, and remaining work could be done during the major power plant overhaul period.
For unit 5 this outage started in August 2006 and the unit was re-commissioned by the end of October 2006, two days ahead of schedule.
Initial winter performance tests confirmed that unit 5 was achieving the winter load performance that had been promised. On 2 February 2007, at an ambient temperature of 6ºC, the unit was running at 240 MW with the original dry air-cooling configuration. When the wet-tower system was put into operation, the unit ran at more than 247 MW. At this load the calculated efficiency increase was more than 2% – exceeding the contractual commitment.
The unit 4 outage started in early March 2007 and the unit was back in operation by the end of May, a few days ahead of schedule. For this unit, Alstom was also responsible for the major overhaul of the 257 MVA Ganz generator.
The potential environmental benefits of the cooling upgrade are substantial. According to Mátra’s own calculations, it will save annually more than 250 000 t of CO2 (and 150 t of SO2). So it represents a significant contribution to reducing Hungary’s carbon footprint.
In recognition of these achievements, Alstom Power Hungary has already won two environmental awards. First the Hungarian Association of Environmental Service and Equipment Manufacturing Companies’ Pro Environment 2007 Award, which encourages manufacturing and service enterprises that make outstanding contributions to the environment, together with individuals who are dedicated to environmental protection. The company also received the Innovation Award 2006 from the Ministry of Environment and Water Affairs, for the most successful product development in the field of environment protection in Hungary in 2006.