Abstract

Danish power companies now operate six supercritical (SC) and two ultra supercritical (USC) pulverised coal-fired (PF) power plants featuring 250-400 MWe, this paper be present some of the most interesting operating experiences with these plants. Supercritical steam conditions are primarily attached to lower electricity costs through improved fuel economy, and with a modest increase in investments and high availability a sound economy of the concept is guaranteed. Sea water cooling further improves net efficiencies up to 45%.

Since the first SC plants were commissioned high temperature steel has improved remarkably. The P91 steel has been used for headers, steam piping, casings and rotors, which allow for operation at USC steam conditions (290 bar/580°C) and further improvements of efficiency. However, better steels are already available and a steel named P92 has now been bought for the next Danish power project.

However, steel cannot continue to improve forever and crossing the "steel barrier" will need nickel-based "Super Alloys" for the hottest areas of the water steam cycle. A demonstration programme on this issue was launched in 1998 by a group of 40 European utilities, research laboratories and equipment manufacturers with economic support from the European Commission's THERMIE program. Commissioning of the advanced plant is foreseen for 2010 and maximum steam temperatures will be in the range of 700-720°C and net efficiences 52-55%, depending on site and fuel conditions. An important part of the project is also to demonstrate ways of lowering investment costs.


1    Introduction

Since the energy crisis in 1973 and 1979 a lot of effort has been made to reduce the energy consumption in Denmark and increase the use of indigenous fuels and renewables. In recent years also the need of reduced CO2 emission from power generation has been a prime driver in Danish energy policy. However, experience from the introduction of new technologies for power generation has shown that in particular renewables are difficult to introduce in Denmark where the major sources are wind and straw.

Table 1 gives an indication of how power was generated in the ELSAM area in 1996 and 1997. It is seen that the major part of the electricity production is still coal based but its share of the total electricity generation is highly dependent on export of electricity. Comparison of the figures for 1996 and 1997 of Table 1 offer an excellent demonstration of the volatility of the open markets for power generation and the need for operational elasticity of the fossil fueled installations.

Renewables like wind contributes with some 5% of total generation. The target of the government to burn 1.2 m tonnes of straw by the utilities has now been delayed by four years from Year 2000 to 2004.

Combined heat and power production has found widespread application and a large number of small de-centralised and mainly gas-fired cogeneration installations are now working with a share of some 20% of total generation.

Table 1. ELSAM. Generating Capacity and Electricity Production in 1996.

All coal-fired power plants commissioned in the 1980s and 1990s have been designed for super critical operation with high efficiency ratings. Before the introduction of the super critical power plant concept, the generally applied concept was a drum-type boiler, which was replaced by the once-through type boiler to increase efficiency by at least 3% (relative) for practically the same investment.

It is the experience of ELSAMPROJEKT that the positive Danish experiences concerning the economy of the super critical technology concept versus drum boiler technology has now been reckognized world wide and the number of super critical power plants is increasing. In Europe all new power plant and projects are based on super critical stream parameters.

However, there will still be a number of power projects - typically with small outputs below some 300 MW - based on drum boilers because the local manufacturers in the newly industrialised parts of the world can build these boilers easily due to their simple design and at low costs.

The first Danish super critical power plant started operation in 1984 at the Studstrup power station and today a total of eight super critical units are in operation whereof six belong to the ELSAM power generators. Main design parameters for the six coal-fired ELSAM plants are shown in table 2.

Steam parameters of the first four super critical units are rather conventional and well proven but during the 1989 VGB Congress "Power Stations 1989" in Hamburg, ELSAM presented their ideas of new generating capacity based on further improvements of conventional super critical PF technology. These ideas have now been realised in the new units with USC steam parameters at the Skaerbaek and Nordjylland power stations and please note that unit 3 at Skaerbaekvaerket is a gas-fired twin unit of unit 3 at Nordjyllandsvaerket.

Table 2. Thermodynamic Main Parameters of Super Critical Plants in the ELSAM Area.

Basic to ELSAM's ideas was to use a new 9% Cr steel named P91 which was emerging at that time. P 91 had 30% better creep strength than well proven X20 CrMoV 121 and made it possible to design thick walled sections like headers and rotors for steam temperatures in the range of 580 C. Further, despite advanced steam parameters, all international regulations on operating behaviour and characteristics (start time, gradients etc) can be met.

ELSAM's idea of improving the coal to wire conversion efficiency by introducing better steel to improve the efficiency of conventional PF technology and lower the electricity prices and CO2 is now accepted in other areas of Europe. This is best illustrated by the new lignite-fired 800-900 MW power plants owned by German VEAG which all have introduced P91 to increase steam temperatures and improve net efficiency.

Figure 1 indicates the development of steel for thick walled boiler sections and steam piping which represent one of the bottlenecks for further improvement of conventional coal-fired technology. It is seen that X20 CrMoV 121 was in use for a very long period of more than 30 years before P 91 took over the predominating role. However, the leading role of P 91 only lasted for 10 years before P 92 took over but further improvements - named Super Alloys and to be mentioned later - after another 10 years are also indicated in figure 1.

Figure 1. Development of high temperature materials.


2    Experience with Operation of Super Critical Power Plants.

The super critical plants have all been built according to the "split package/multi contract principle" in which a project is divided into several packages put out to tender individually. Applying this principle has been very satisfactory, as it enables the owner to utilise his experience from plants already in operation and combine the experience of different suppliers.

The general experience with the operation of super critical plants has been good: the time-based availabilities of the seven units are all within the range of 90-94%. It is important to note that no impact on availability has been observed due to the super critical steam parameters and the time between planned overhauls has now been enhanced from one to two years.

Information concerning the operation of unit 3 at Esbjergv rket, in particular the no. of operating hours and net efficiency, is shown in table 3 for the period 1994 to 1997. Also, table 3 demonstrates the tendency to extend time intervals for planned overhauls from one to two years. It may be concluuded that the figures show a very reliable plant, featuring actual annual efficiencies in the range of 45% corresponding to the design values.

Table 3. Operating data for Esbjerg Power Plant.

Operation of the boiler is the most troublesome part of plant operation and some of the major problems will be addressed shortly. All boilers have demonstrated good operating characteristics and the guaranteed boiler efficiencies, in the range of 93-95% and more, have not shown any tendency to decrease with boiler age. The most common reason for forced outage time is still tube failures caused by fly ash erosion.

The operating experience and behaviour of the economiser and water walls has been good and no major problems have arisen. Despite the higher water/steam temperatures in the water walls of super critical boilers, tube failures have not occurred in these sections. Super critical steam pressure has not increased the number of tube failures in superheaters and reheaters and high temperature corrosion has not been observed at Esbjerg 3 which was the first plant to operate at steam temperatures above 540 C considered as the ultimate steam temperature for many years.

Typical for super critical boilers with single reheat is the very low temperature of the cold reheater, which increases the amount of energy absorbed in the reheater compared to a subcritical unit. As cooling of the big reheater bundles is crucial to a super critical power plant in all modes of operation, a 100% HP bypass replaces the safety valves on the high-pressure part of the boiler.

In one of the plants, the HP bypass valves started to vibrate heavily at partial load and much effort was put into solving the problem. Due to good co-operation with the valve manufacturer and a new arrangement of the steam lines around the valves, the vibration problem was eventually solved.

A relatively large share of ELSAMs coal is imported from Poland but also from most other major coal exporting countries world-wide. Blending of coals is only used in extreme cases to overcome operating problems.

Coal is pulverised in vertical tube mills at Esbjerg Power Plant and in roller mills at the rest of the plants. These tube mills have exhibited very satisfactory operating behaviour with low maintenance costs and are capable of operating for some 30,000 hours between overhauls and it seems difficult for the roller mills to reach this long time between overhauls. However, one of the shortcomings of the tube mills is their high power consumption, up to twice as much as the roller mills.

The fine milling also leads to slightly reduced tendency to slagging and less fouling problems as all ash particles cool well before entering the first superheaters. Typically, a tendency of reduced soot blowing of the furnace walls is seen and the amount of bottom slag is reduced, while more fly ash is being generated.


3. The Skaerbaek and Nordjylland Power Plants.

3.1 USC Water / Steam Cycle

In 1993 ELSAM started construction of two 400 MW twin units with USC steam conditions. The design was based on new steel P 91 for headers, steam piping and turbine, the water/steam cycle of the two double reheat plants is shown in figure 2, which also indicates the low condenser pressure due to seawater cooling and the 10 stage regenerative heating of main condensate and feed water. A detailed presentation of these plants were given during the VGB Conference "Fossil-fired Power Plants with Advanced Design Parameters", in June 1993 in Kolding, Denmark.

The idea of double reheat is to lower generating cost and reduce severe erosion of the last stage buckets in the LP turbines by water droplets as seen in some of ELSAM's super critical units. The water content in the exhaust steam of seawater cooled single reheat plants may increase to 15% during winter time operation of single reheat cycles and it will be reduced by some 5% through double reheat.



Figure 2. USC Water/Steam Cycle (1997)

The steam turbines for both plants have been supplied by ALSTOM and new 9-10% Cr steel has been foreseen for valve bodies, turbine casings and rotors in direct contact with high steam temperatures. However, also the cooling of the hottest sections by colder steam has been foreseen to ensure the continuous use of older more proven steel. The turbo set is split as follows:

• A single-flow very-high-pressure turbine, without control stage.
• A combined high and intermediate pressure turbine of asymmetric double-flow construction.
• A further IP turbine of asymmetric double-flow construction which allows for two steam extractions at very low steam pressures and a two stage heating of the water circulating in the district heating system. This design guarantees the optimum thermodynamical advantages of the low temperatures of the circulating water of the district heating systems.
• Two double-flow low-pressure turbines.

Both steam generators are identically designed by Danish Burmeister & Wain Energi and built in a consortium with Aalborg Industries A/S and V lund Energy A/S. The different main fuels, of course, mean differences in the combustion and flue gas systems. These differences, however, are allowed for in such a way that conversion from gas to coal-firing is possible without major modifications.

Figure 3 shows a schematic view of the super critical once-through single-pass steam generator where the top of the boiler house is 90 m above ground level. The furnace has spirally-wound furnace walls and the cross-section is a square with a side length of 12.25 m. 16 low NOx burners with staged combustion are arranged in the corners at four levels and the main steam flow is controlled by sliding pressure operation at fully open turbine valves.

All well-known ferritic and ferritic/martensitic steels are used but the advanced steam parameters forced a general change to improved steels in most areas of the boiler, steam piping and turbine.

In particular, the high main steam pressure and double reheat mean that at rated load water/steam temperatures in the furnace walls will increase and in the cyclones, temperatures will be close to 480 C. However, it is important to note that 13CrMo44 is still applicable for the furnace walls but to guarantee the safe operation of the furnace walls very thorough stress analyses were performed of the tubes in the burner zone at the transitions from spiral wound tubes to vertical tubes and those parts of the upper pass walls most exposed to heat.

For HP 1 and the final superheater, finely grained austenitic steel HTP 347 FG was supplied by Sumitomo Steel and for thick-walled headers, main steam and hot reheat piping new 9% Cr-steel P 91 were supplied by DB Kraftwerksrohrleitungsbau. The procurement of steam piping indicated no increases in price due to new P 91 and erection demonstrated that welding of P 91 is easier than for older 12% Cr steel like X20 CrMoV 121.

Figure 3. USC Steam Generator (1997).


3.2    Experience from Commissioning and early Operation

The flue gas cleaning path of gas-fired Sk rb k 3 is much simpler than for a coal-fired plant and, therefore, it might be expected that the commissioning would be easier. However, the commissioning of Skaerbaek 3, in particular, suffered from problems with HP heaters and turbine valves.

During acid cleaning of the furnace walls and super heaters of Skaerbaek 3 it was discovered that the conductivity of the feed water remained high and after many investigations it was concluded that a layer of phosphate created during production of HP heater tubes still remained on tube surfaces. The removal of the phosphate was very time consuming and the HP heater tubes were acid cleaned on the feed water side until conductivity was approved. On the steam side it was approved to let the condensate wash the tubes during normal plant operation and watch the conductivity fall. At Nordjylland 3 both sides of the tubes were acid cleaned and conductivity could be approved after a short time.

Pressure areas and spring forces of the intercept valves of the IP 0 turbine had to be modified to ensure the safe closing of the valves. Also tolerances between moving parts of main and pilot valves were too small and had to be increased but problems concerning rubbing between stationary and moving parts still need to be solved. Working fluid for the hydraulics of the turbine valves has been changed from turbine oil to phosphate esther with good results. In other major valves and bypass stations in the boiler area the working fluid will also be changed from oil to phosphate esther.

During commissioning the generator had to be balanced - can be done in air - before and after over speed tests. However, the vibration level was still too high and during plant overhaul in August, new possibilities for balancing bolts were arranged which have reduced vibrations to half of the guaranteed figures.

Skaerbaek 3 started commercial operation on October 1, 1997 and generator output has suffered from an oversized swallowing capacity in the VHP turbine. This also meant that the first cold reheat temperature was too high causing too much spray and adding to problems concerning the overall heat balance of the boiler. During plant overhaul, the nozzle boxes of the VHP turbine have been modified so the swallowing capacity now is correct.

The correct combustion of natural gas was more difficult than expected and in March 1998 burners were modified. During plant overhaul in August 1998 further burner modifications were done and optimisation is now ongoing to ensure full reheat temperatures at all loads between 60 and 100% load.

At Skaerbaek 3 the flue gas path is based on singular 100% components and this principle has demonstrated excellent operating behaviour so far.

The commissioning of Nordjylland 3 suffered from the same problems as Skaerbaek 3 but also benefitted from the experiences at Skaerbaek 3 and all in all the commissioning of Nordjylland 3 proceeded without any major problems. Acid cleaning and steam blowing was done in February and April 1998 followed by:

Steam to turbine:	June 1, 1998.
Start coal-firing:	June 10, 1998.
Synchronisation:	June 15, 1998.
Start commercial operation:	September 15, 1998.

Typically for the second of the twin plants, Nordjylland 3 started commercial operation ahead of plan, in this case two weeks earlier than planned.

Early plant operation of Nordjylland 3 has suffered from the same problems concerning swallowing capacity of the VHP turbine and boiler combustion as Skaerbaek 3, and during this year's plant overhaul a number of modifications as effected at Skaerbaek 3 will be repeated.


4    The Joint European Demonstration Project.

The presentation of ELSAM during the 1989 VGB Congress in Hamburg also brought a graph which forecasted that net efficiency of a seawater cooled, pulverised coal-fired power plant could be raised into the range of 51% after year 2000 but with main steam temperatures in the range of 650 C. Today we feel very happy to note that this target seems possible to reach based on a double reheat cycle with main steam pressure in the range of 330 bar, main steam temperatures in the range of 610 C and reheat temperatures some 20K higher. Similar design parameters are now being used by German utilities for the planning of new generating capacity.

However, improvements of the coal-fired power plant technology do not stop at the level mentioned above and, in January 1998 a large group of major suppliers to the power industry and some of the leading utilities in Europe started a demonstration project named "Advanced ("700 C") PF Power Plant". The project is being financially supported by the European Commission's THERMIE programme. The aim of the project is to break the "steel barrier" and introduce new nickel-based super alloys for the highest temperatures in the steam cycle which will be in the range of 700 C, thus boosting the coal-to-wire efficiency into the range of 52-55% depending on site and fuel conditions. One of the cycles to be investigated is indicated in figure 4.

The project is technologically very advanced but even if it may succeed technically, the economy of the project is challenged by other fuels and technologies and to ensure the success of the whole project the complete plant structure will be reviewed to find other more cost effective ways of arranging the major components, i.e. boiler and turbine. This kind of advanced overall plant architecture has been called compact design.



Figure 4. Advanced Steam/Water Cycle of 2010.

The original idea of this project was generated during ELSAM's R&D programme of the mid 1990's and further promoted through a joint European programme named COST. Now, 40 partners of which 26 are industrial companies, counting all leading boiler and turbine manufacturers, material and steel manufacturers join the project and the remaining partners are utilities and material test laboratories. The organisation of the project has called for the formation of three main groups: a turbine group, a boiler group and a process group. The turbine group - headed by ALSTOM, Rugby, UK - is responsible for the materials and design of the turbine; the boiler group - headed by Babcock Kraftwerkstechnik, Oberhausen, Germany - is subdivided into three subgroups responsible for boiler design, boiler materials and combustion; and the process group - headed by ELSAMPROJEKT - is responsible for the cycle optimization, balance-of-plant, overall economy, etc. Overall project coordination is in the hands of ELSAMPROJEKT.

Following a preliminary design and feasibility study in parallel with initial materials testing, design and testing of large critical components will have to be performed as well as qualification of all optimized materials. An overall time schedule for the complete project, culminating with the construction of the plant somewhere in Europe in about 8 years, is found in figure 5.

The project proposal covers the first six years of a planned, total project duration of 17 years. The budget for the activities within materials testing, preliminary design and feasibility totals approx. ECU 20 m of which the Commission has been asked for a grant of 40% under the DGXVII THERMIE programme.


Figure 5. Phases of the USC-700^oC Project

Figure 5 indicates that a second phase of demonstration programmes will start in year 2001, which is three years after project start. This is a very important part of the whole project requiring many hosts and sponsorship and, hopefully, many other European utilities and manufacturers than those already joining will show their interest and join the project.