Supercritical combined cycle power plants incorporating a gas turbine are a recent development, but supercritical combined heat and power plants for district heating use are not new. Well over 200 supercritical units have been built in the former Soviet Union over the last 25 years, mainly with steam conditions of 240 bar/565°C.
In Germany, the Schwarze Pumpe 2 x 800 MWe power plant recently commissioned near Dresden in the Lausitz lignite mining area is a good example of current technology. These are thought to be the largest lignite boilers ever built, with physical dimensions of 161 m in height, 24 m x 24 m in cross section. These are once through single pass boilers design by EVT as 900 MWe units but with a high process steam output of up to 800 t/h to various process plants in the region. They have steam parameters of 268 bar/547°C/565°C for the maximum steam generating capacity of 2420 t/h.
This is said to be the first time that P91/T91 steel has been used in the manufacture of a power plant steam generator of this size. Used for the live steam and hot reheat piping, this material was selected as having higher creep rupture stress than the X20CrMoV121 normally used, which leads to lower tube wall thicknesses and hence reduction in thermal stresses. The weight reduction in turn reduces stress levels at the boiler connections and at the turbine connections as well as on the structural steelwork.
A seven-stage regenerative feed heating system driven by a turbine driven feed pump delivers feedwater at a final temperature of 270°C. The supercritical steam output supplies a single reheat turbine system in sliding pressure mode with main steam at 638 kg/s, 253 bar, 544°C and reheat at 52 bar, 562°C.
Feedwater temperature has become an increasingly critical parameter in recent years, with the added emphasis on emissions control. There are economic benefits in raising steam pressure to 300 bar arising from further increases in generating efficiency.
Since with the high process heat output at Schwarze Pumpe a fuel utilisation efficiency of some 55 per cent is claimed, it may be considered that cogeneration of heat and steam could be a more economic approach to beating Carnot, but there are not that many such instances of adequately large heat loads existing close to the mine mouth sites adjacent to the coal resources.
State of the art must be the Avedøre II district heating power plant under construction for SK Power near Copenhagen in Denmark. The plant has a total electrical generation capacity of about 626 MWe with steam conditions of 300 bar/ 580°C/600°C and district heating capacity up to about 618 MJ/s. This is the third of a series of “Convoj” ultra-supercritical multifuel power plants which are unique to this environmentally hypersensitive country. Final feedwater temperature is some 320°C, and the high overall thermal efficiency of 47.2 per cent is only partly due to the low cooling water temperature of 10°C.
By law, all generating utilities in Denmark have to include a specific quanitity of biomass burn in their fuel mix. This tends to be done in separate fluidised bed boilers, much of the output of which is used for feedwater heating in a larger power plant. The Skærbækværket and Nordjyllandsværket ultra-supercritical power plant units which are good examples of this have already been described in detail in Modern Power Systems. The second unit at Avedøre will be described briefly later in this review, but first we will summarise current progress in supercritical generating plant technology.
Two drivers
Technically viable and economically successful ultra-supercritical coal fired power plants are being built in places as far apart as Denmark and Japan. Worldwide anxieties about global warming and international agreements between industrialized nations to reduce carbon dioxide and other greenhouse gase emissions has put renewed emphasis on improving the thermal efficiency of fossil fired power plants.
Some improvements will be achieved by increased use of more efficient combined cycle units, but steam turbine technology for ultrasupercritical steam conditions has matured over the last 5-10 years to the stage where it now presents a viable and attractive option for new power plant construction within favourable energy economies.
Historically, it was widely foreseen that from the traditional 165 bar/538°C single-reheat cycle, dramatic improvements in power plant performance could be achieved by raising inlet steam conditions to levels up to 310 bar and temperatures to levels in excess of 600°C. Any supercritical condition for which the throttle and/or reheat steam temperatures exceed 566°C, ie well above the traditional 538°C limit of P22, is now known as “ultra-supercritical”. The graph above illustrates the relative heat rate gain for a variety of main steam and reheat steam conditions for single-reheat units compared to the base 167 bar/538°C/538°C cycle.
The first 100 MWe supercritical power plant in Russia, which commenced operation at Kashira in 1966, had steam conditions of 306 bar/650°C/565°C.
In Germany, Siemens delivered their first supercritical steam turbines with steam conditions as high as 300 bar/625°C in 1954. Main steam temperatures as high as 600°C were in use in the late 1940s.
In the USA also, straight electricity generating supercritical plants proliferated in the early 1960s. In the early days of supercritical power plant design, one plant was built with what would now be termed as ultra-supercritical steam conditions, of 300 bar/650°C/650°C before the days of low creep high chromium steels, but the results were not economically rewarding
A number of steam generator and turbine manufacturers around the world now claim that steam temperatures up to 700°C are possible, using expensive nickel based alloys, which might raise plant efficiencies to over 50 per cent, but still below the efficiency of the gas turbine combined cycle competition.
There are continuing doubts about the economic benefits of taking steam temperature above 635°C, particularly when the costs of nickel based alloys are taken into account. The extra costs of using nickel based alloys can probably be at least fully compensated by reductions in the amount of material required, ie thinner tube walls, and smaller overall dimensions of both plant and site requirements.
Greater hesitation in raising steam pressures much above 300 bar has been evident in some countries. Increased live steam pressure offers lower potential for improved performance due to the implications of the auxiliary power consumption.
In addition, increased pressure leads to a loss of thermal flexibility and this represents the highest cost factor. It has been stated that in Germany, the economics of the plant are reduced if the additional investments exceed DM 30 million per percent improvement in heat consumption.
One way round the economic burden is to integrate a large industrial gas turbine into the cycle to replace the forced draft air fan of the Benson boiler system. This yields thermal efficiencies of well over 50 per cent and also introduces substantial fuel flexibility advantages. Described by ABB Kraftwerke as the high-temperature concept, the configuration was described by Dr Heinrich Kotschenreuter in Modern Power Systems, October 1994.
Material improvements
Now that P92 type materials are becoming commercially available, and more suppliers are developing the required expertise in manufacturing in these materials, adventurous utilities are becoming increasingly confident about the predictability of the actual costs and the reliability performance of supercritical plants for steam temperatures of up to 635°C.
The technology is currently moving through the use of P91 to P92 type steels, 18-8 or 18-25 austenitic steels like Super304H and Esshete 1250 and towards the use of high nickel alloys such as Inconel 718 of the kind increasingly used in advanced gas turbines in order to raise steam pressures and temperatures to ever higher performance with a view to increasing thermal efficiency and lowering lifetime costs.
Research and development programmes in the area of improved materials for ultra-supercritical steam turbines and steam generators are becoming increasingly international. Long term creep, tube thinning and corrosion testing of full scale components are essential, in association with all available operating experience, before the new materials can economically be used in practical applications. Classic programmes are continuing within:
The EU Brite Euram, COST and Thermie programmes;
The Convoj double reheat projects in Denmark;
KEMA in the Netherlands;
EPDC and CRIEPI in Japan;
EPRI 1403 – 50 and later programmes in the USA.
The key to securing the required steam conditions is the development of high temperature materials, available at an acceptable price, particularly for thick section components in the boiler and turbine.
These considerations led to the establishment of a four year EPRI project with partners from Japan, UK, USA and Denmark whose objective was to establish strong 9Cr and 12Cr steels as practical, validated materials for thick section boiler components such as headers and main steam lines. This successful project has developed three strong steels for thick section components for plant operating in the temperature range 565-620°C and two of them, P92 (NF616) and P122 (HCM12A), received ASME Code approval during 1996.
This project has now entered its second phase with the fabrication of full sized headers which have been installed in the Nordjyllandsværket power plant in Denmark. In addition, there is further work on the long term microstructural stability of the steels, and a full sized pressure vessel test under accelerated conditions is under way.
The programme was well reported at the I Mech E international conference on advanced steam plant, 21 to 22 May 1997 in London. Also presented at the London conference was a valuable overview of advanced steam plant development in Japan, by K Miyashita of EPDC. EPDC, he reported, is presently undertaking Phase 2 of its USC (ultra-supercritical) steam cycle development, which started in 1994 and will continue until 2000.
Not only are the Japanese looking at new candidate materials for high temperature use but also materials for intermediate temperatures because of their possible contribution to reduction of boiler weight due to their higher strength. Specimens have been inserted into the Takehara unit 1 power plant with full scale models of coils, water wall panels, headers and main piping for testing in collaboration with Hitachi, MHI, and IHI.
Miyashita observes that, in addition to traditional solid solution strengthening (addition of Mo) and precipitation strengthening (addition of V, Nb, and Ti), the following features are incorporated: the control of Cr nitride and Cr carbide, the addition of minor elements such as B and N, the replacement of Mo by W, the addition of Cu to austenitic steel and overall organisation of alloying elements. The creep rupture strength has been increased by 50°C in general at the temperature which gives allowable strength of 49 MPa. Creep rupture strength alone is not sufficient for the selection of material. Other properties such as fracture toughness, adaptability to fabrication and manufacturing process, endurance in the service environment, etc. has to meet the requirement.
USA – ‘bin there, done that’
The improvement in overall performance of steam turbines for thermal power plants has been brought about largely through two kinds of advancements:
improvement in mechanical efficiency by reducing aerodynamic and leakage losses as the steam expands through the turbine;
improvement in thermodynamic efficiency by increasing the temperature and pressure at which heat is added to the power cycle.
Efforts to increase the efficiency of the Rankine cycle by raising pressure and temperature go back to the 1950s in a period of rapid growth in average power plant size. During that period, the reheat cycle became well established commercially and the maximum steam conditions were raised from 100 bar/538°C/538°C to 165 bar/538°C/538°C and to 310 bar, for an experimental double reheat unit with 621°C/565°C/538°C steam conditions at the Philo 6 unit, followed by the Eddystone 1 plant with steam conditions of 345 bar/649°C/566°C /566°C in 1959.
These projects led to the commissioning of several large capacity cross-compound units in 1960 with steam conditions of 241 bar, 566°C/566°C. One 325 MW 165 bar, 593°C/566°C/538°C double reheat unit was also installed in this time frame.
By 1969, a simpler tandem-compound double reheat design had been placed into service that combined 241 bar/538°C high pressure and 552°C first reheat turbine sections in a single opposed flow casing. The second reheat section with 566°C inlet steam was designed in a double flow configuration to provide adequate volume flow capability and to confine the highest temperature conditions to the middle of the casing. This design, which has experienced exceptionally good reliability while exceeding performance expectations.
In addition to units with double reheat, during the 1960s and 1970s numerous single reheat supercritical units with steam conditions of 241 bar, 538°C/538°C were commissioned including tandem-compound units of outputs between 350 MWe and 884 MWe.
Experience from these single and double reheat units, together with the knowledge gained on the advanced steam condition designs of the 1950s, served as the basis for several Electrical Power Research Institute (EPRI) studies conducted during the 1980s of double-reheat turbines designed for operation at the advanced steam conditions of 310 bar, 593°C/593°C/593°C.
GE points out that it has been using Inconel-718, particularly for HP inlet flange and horizontal joint bolting, which must seal the joints for a minimum of 30 000 hours, or until the first scheduled outage, without any intermediate tightening. This material has been used by GE on a trial basis for steam turbine applications of up to 566°C, as well as in advanced gas turbines and aircraft engines in applications of up to 649°C.
Thre is little demand for supercritical steam systems in North America in these days of deregulated power, but an example of an advanced steam turbine generator recently designed and manufactured by GE in association with Toshiba is a single-reheat cross-compound unit rated at 1050 MW with steam conditions of 248 bar/600°C/610°C for the Tachibanawan 1 power plant on Funshu Island in Japan.
Economic neccessity in Germany
The economic pressures to exploit the indigenous resources of Saar coal and Rhineland lignite, and, more recently, of East German lignite, have resulted in a large scale investment in technology development for the greater optimisation of generating power plant efficiency and maintainability.
Smaller supercritical units with steam conditions of up to 300 bar/650°C were installed during the mid 1950s to early 1960s in about 14 power units, mostly in chemical process plants such as Hüls (85 MWe, 294 bar/600°C /560°C /560°C), Hatigen, and Leverkusen.
The development of highly supercritical power plants in Germany, however, was substantially set back since the period of peak interest in 1994/95 with the shelving of the Hässler and Lübeck projects and later the postponement of RWE’s latest Frimmersdorf power plant unit (250 bar/580°C /600°C).
But if the EU’s aspirations towards cost transparency ever materialise, operating experience from recently built supercritical coal-fired plants like Bexbach (250 bar/575°C /600°C) and four big VEAG lignite stations in former eastern Germany – Boxberg, Lippendorf, Schwarze Pumpe (268 bar/547°C /565°C) and Jänschwälde – and also the Schkopau IPP, none of which exceed 40 per cent efficiency by much, will do a good deal to prove the outstanding issues.
Construction has now started on Unit K of RWE Energie’s 1000 MWe Nideraußem lignite burning power station which will boast steam conditions of 265 bar/576°C /599°C. Described as the most advanced lignite fired power plant in the world, with the initial phase of BoA (brown coal power generation with optimised plant technology) the plant is designed to achieve 43 per cent thermal efficiency. At a later date, when the new dry lignite technology for the BoA-Plus concept is introduced to the plant, a further increase in efficiency of from 3 to 5 per cent may be expected.
Construction of the new unit, Niederaußem K, has now begun with a view to commencing commercial operation in 2002. Some steam will be extracted from the turbines for district heating in public premises such as the kindergarten, schools, halls and swimming baths of the Bergheimer district of the town of Niederaußem.
Pioneers in Japan
Perhaps the most significant reference for Siemens KWU will be their latest supercritical plant contract, the 600 MWe Isogo unit being built for Tokyo Electric Power for commissioning in 2001. This is rated at 250 bar/600°C /610°C, with a high pressure extraction stage and a feedwater temperature of 300°C. Since nickel-based alloys such as Inconel 718 are generally thought to be necessary for reheat temperatures of 610°C and higher, there will be great interest in operational experience on this plant.
This represents a substantial increase in steam temperature over Chubu Electric’s 700 MWe Kawagoe double reheat systems which run on steam conditions of 311 bar/566°C /566°C /566°C. The first two of these units have been running since June 1989 with an efficiency gain of 5 per cent over that of previous conventional plants of comparable size at 45 per cent. These make strategic use of 12 per cent chrome cast steel in the Toshiba turbines and blades and 9 per cent chrome in the pipework.
This was the first use of P91 type alloys in Japan since the standard single reheat steam conditions of 242 bar/538°C /566°C were established for large fossil-fuelled steam turbines. Since the Kawagoe units demand the very high levels of availability that have been an essential requirement for large fossil-fired middle-load power plants in recent years, the effective operating experience gained from this station is of immense importance to the development of the advanced supercritical system.
Chinese challenge
Supercritical power plants are now being tendered in China. Mitsui Babcock Energy (MBEL) has a wealth of metallurgical and manufacturing expertise in this field and proposes to actively follow-up this market.
MBEL regional director John Prosser recounts that, according to a report by the Chief Engineer of State Power and Director of Planning & Investment Department in October 1998, “China will adopt 600 and 1000 MW class supercritical units vigorously within this century and from next century, under the condition that advanced technologies must be introduced into China”.
The introduction of ultra-supercritical technology is also in planning, but this will not be very fast. Supercritical technology has definitely been recognised as the advanced, high-efficiency clean technology, and the market for it will certainly increase. For the short-term future, however, the main steam generating technology for coal-fired plant in China is likely to remain subcritical units, of the 300 and 600 MWe class, with the priority being given to 600 MWe.
Tendering has recently been finalised on Waigaoqiao (Shanghai), a 2 x 900 MWe supercritical unit, which was awarded to Alstom EVT/Siemens, the steam conditions being 250 bar/538°C/566°C. The next supercritical project will be Wangqu (Shanxi), a 2 x 600 MWe plant for which the enquiry is expected soon, with steam conditions anticipated to be 242 bar/538°C/566°C.
The only Western supplied, ie non-Russian, existing supercritical plant in China is the 2 x 600 MWe Shidongkou 11 (Shanghai) plant, which was put into operation in 1992. CE/Sulzer supplied the boiler island and the steam conditions are 253 bar/541°C/566°C.
The major structural reform that is currently being undertaken within the power sector in China is that the provincial power companies and even local power companies will become more and more economically independent. So, as a result more attention will be paid to the economics of future plants, including operational cost, efficiency etc.
State Power plans to use a bidding price system for selling power to this grid by 2010. Trials of the system are to be undertaken firstly in Shanghai, Shandong and Zhejiang Provinces. Economics is the main driver for this development..
In 1995, the average coal consumption in Chinese thermal power plants was 412 g/kWh. The target set by State Power is to reduce that figure to 380 g/kWh by 2000.
Five years ago, it would have been very difficult indeed for anyone to contemplate the manufacture of high chromium P91/T91 in China, with P22 being the predominant material. Since then, however, the main Chinese boilermakers have developed their capabilities in order to be able to include the manufacture of P91/T91. Provided the end-user clients can be persuaded to accept Chinese manufacture, then the capability exists. The 12Cr1MoV Chinese material is substantially used in the domestic boiler market, and all domestic boilermakers have gained recognised experience of this material. The more exotic alloys and inconels remain unproven at this moment in time, but the Chinese will not be slow in coming forward once a commercial opportunity presents itself.
Interest in supercritical technology has been expressed throughout South-East Asia, but only South Korea, Australia and Taiwan have moved it forward to the point of considering it seriously for named projects. Ultra-supercritical technology is probably a step too far at present for the region.
Pure Danish power
Strong governmental demands for environmentally acceptable electricity production led Danish electricity generators to reduce NOx, SOx and CO2 emissions. Public pressure led to the installation of many groups of large capacity wind turbines, and a great many advanced small distributed combined heat and power plants burning natural gas, refuse, and agricultural biomass.
Low production flexibility of these distributed generation facilities has since resulted in a reversion to larger, very high efficiency, multi-fuel central generating plants. Anxiety about the cost and adequacy of future natural gas supplies turned attention towards coal based fossil fired plants.
Three technologies were studied – IGCC, PFBC and USC with an added analysis of cofiring with biomass. The overall conclusions from the study projects were reported to be that the most efficient and technically feasible option was the USC concept. Neither the IGCC nor the PFBC were found to show better efficiencies or potential, and the investment costs were calculated to be much lower.
Avedøre II
The “Multi-fuel Concept” devised by Henrik Noppenau of SK Power integrates the output from three separate combustion units into one common highly supercritical steam turbine system to produce electricity and district heating.
The plant is the result of a combination and integration of three different plants. The heart of the system is an ultra-supercritical steam plant, where a Benson boiler built by Burmeister and Wain, using P91 and P92 tubes supplied by French nuclear steam generator manufacturer Stein Industrie. This produces superheated and reheated steam burning oil and natural gas to feed a condensing and controlled extraction steam turbine-generator to produce electric power and district heating, in a regenerative water-steam cycle.
In parallel to the steam system there is an aeroderivative gas turbine plant from which the exhaust gas is used to preheat the feedwater for the steam system.
The third component of the system is a biomass incinerator, burning straw and producing steam at the same conditions as the main boiler (as described in the May 1996 Modern Power Systems Scandinavian Supplement).
In a parallel combined cycle arrangement with a coal/natural gas/oil fired conventional boiler, and a biomass combustion unit burning wood chips in a fluidized bed burner and straw in a specially designed combustor, a 100 MWe ICAD (intercooled aeroderivative) version of the Rolls Royce industrial Trent gas turbine with an unfired waste heat recovery boiler is planned to run in an innovative integrated cycle.
In the cold winter months when the natural gas becomes the premium fuel, the gas turbine will only be used for peak load use. During the spring, summer and autumn when natural gas fuel is more readily and economically available the ICAD module will be run more continuously.
The main unit can burn coal, oil or gas in any mixture. The biomass unit is thought to be considerably larger than any straw-fired unit built so far. The multi-fuel concept results in efficiencies of 48 per cent for the main unit, 41 to 47 per cent for the biomass unit, and between 56 and 60 per cent for the ICAD gas turbine unit as in Table 1. The very high efficiencies are a result of the ultra-supercritical steam conditions and the synergetic effects obtained by connected the units in a new, advanced cycle.
The steam plant is able to produce 534 MWe in the most powerful condensing mode and 618 MJ/sec plus 424 MWe in maximum district heating configuration. The better efficiency which can be met in the configuration steam plant plus gas turbine plant is more than 51.5 per cent net.
The steam plant alone, in pure condensation configuration, burning natural gas, can generate 380 MWe net with an overall thermal efficiency of 47.2 per cent. This extremely high figure is only partially due to the low temperature of the cooling water, 10°C design. The bulk of the advanced performance is due to the design of the steam turbine, to the steam parameters selected and to the very sophisticated system configuration. Steam parameters are in the ultra supercritical range, meaning that the steam temperature is well above the traditional 1000°F limit, dictated by the P22/T22 alloy capability. Also the steam pressure is in excess of 300 bars.
The resulting gross heat rate in pure condensing mode, without district heating, biomass boiler and gas turbine, is 6680 kJ/kWh, corresponding to a gross efficiency of 53.9 per cent. The most important objective raised by the Avedøre 2 project is the extremely high efficiency target which has to be guaranteed not for one or two operating points but for as many as 26 points.
The thermal cycle configuration is very sophisticated, consisting of a regenerative train of 10 feedwater heaters, plus a final desuperheater, all low pressure drain being pumped forward and with an extensive use of variable frequency pump drives.
The steam turbine design for Avedøre 2 is a departure from the Alstom double reheat designs for Skærbækværket and Nordjyllandsværket which have a single ultra high pressure stage, a single HP stage, double flow IP and three double flow LP stages which could be reconfigured for power only ouput (412 MWe) and combined heat and power (320 MWe and 450 MJ/s). It is a single reheat design has more in common with latest Siemens design.
The Avedøre 2 steam turbine consists of five bodies that are connected in tandem to house one single-flow high-pressure (HP) turbine, one single-flow intermediate-pressure (IP1) turbine, one double-flow intermediate-pressure (IP2) turbine, and two double-flow low-pressure turbines (LP1 and LP2). The machine is designed for full throttling admission and sliding pressure operation.
Both the HP and IP1 turbines are of standard reheat turbine module design. Apart from some minor modifications that were necessary to take into account the increased differential expansions that take place during transient conditions, the relevant modification is the application of COST 501 materials to the key components exposed to the increased steam conditions:
Forged HP and IP rotors (high temperature areas only): X 12CrMoWNiVNbN10 11.
HP& IP inner casings, main and RH valves bodies: G-X 12CrMoWNiVNbN1011.
Comparing the creep strengths of the selected alloys as function of temperature in comparison with that of a conventional 12 per cent Cr previously used up to 565°C, the chosen materials have, at 600°C and 100 000 hours, a creep strength higher by 50 MPa than the conventional 12Cr, or the same strength of 100 MPa at a temperature at least 33°C higher.
For the IP section operating at 600°C, the new material has the same creep strength as that of a conventional 12Cr at the 65°C temperature for which the standard module has been designed, while for the HP section the lower inlet temperature (580°C) offsets the higher inlet pressure of 300 bar (20 per cent more than that of the standard module).
The intermediate-pressure turbine (IP2) receives the steam from the IP1 section through cross-over piping with asymmetric double flow. Each flow discharges steam,through the exhaust section located in the lower casing, into a district heating exchanger. The two flows discharge steam at different pressures into the district heating exchangers reflecting the need to produce different temperatures for the two-stage water heating system.
IP turbine discharge is connected via crossover piping to the exhaust of the IP2 flow at the lower pressure. This requires a larger LP inlet area and only 4 stages resulting in a special design of this section. Low-pressure turbine 2 (LP2) can utilise a standard six stage LP module without any modification, since it is connected to the IP2 exhaust having a pressure in the standard design limit of the module.
The two LP turbines, which feature four 41 inch last stage blades, is designed to be able to handle the maximum condensing steam flow at the condenser pressure of 0.023 bar (given by the very low circulating water temperature) with the minimum of exhaust losses. This is one of the major contributions that is made to the unit heat rate in full condensing operation.
The feed water heating train, which consists of ten stages of heaters plus the top desuperheater, are divided into six low pressure stages and four high pressure stages. The sixth low pressure stage is the deaerator, combined with the feed water tank. The other six heat exchangers are of the shell-and-tube type, with tubes made of low carbon stainless steel.
Three heaters in the high pressure train are of the header type, in order to avoid operational limitations resulting from the excessive thickness of a tube sheet designed for a pressure in excess of 400 bar. A final top desuperhater (the 11th), fed with the first IP extraction, raises the feedwater temperature up to 320°C.
Biomass burning
The biomass boiler is fed by a dedicated feedwater pumping system. The steam condenser has titanium tubes and is rigidly connected to the LP turbine exhaust hood. District heating output derives from condensing steam extracted from the district heating LP turbine into surface heat exchangers.
Coal, oil or gas can be burned in the boiler. The new generations of boiler steel have made it possible to increase steam pressure and temperature to 300 bar and 580/600°C. The boiler is fired with gas until the boiler load is 20 per cent, then coal is added. When the load is above 30 per cent the boiler can then be fired with an arbitrary mixture of gas and coal. Combustion of coal and gas are together is done with burners at two levels – gas being combusted at the lower level and coal at the higher level to ensure that both fuels reach optimal burn-out.
The Avedøre 2 project is being partly funded by the European Union. It is the aim of this Thermie project to demonstrate the multi-fuel concept in which a biomass unit of 40 MWe is connected to a main unit of 380 MWe in parallel combined cycle with a gas turbine of 100 MWe output.
The ICAD gas turbine is being jointly developed by SK Power (ELKRAFT) together with American, Canadian, British, French and Dutch electricity companies. The first phase of the project, which was carried out with the participation of General Electric, Rolls Royce, Turbo Power and Marine Systems has now been concluded. A gas turbine based on the new concept, with an output of about 100 MWe, has been undergoing trials since late 1997.
In the new concept the gas turbine is connected to the main unit in Parallel Powered Combined Cycle, which means that the flue gas from the gas turbine plant is used to preheat the feed water in the main unit instead of using steam from the turbines. This results in a level of synergy between the two plants. Some operating experience with a power plant using a gas turbine in parallel combined cycle with the exhaust gas heat being used for feedwater preheat has been gained in the Suomenoja power plant in Finland.
The main USC plant can be operated without the gas turbine plant and the biomass unit, and the gas turbine plant can operate independently also. Where there is an ample supply of natural gas at a competitive price, all three plants can be in 24 hour operation. When the supply of natural gas is limited the following pattern can be followed:
The main USC plant is operated on coal and the biomass plant is on 24 hour operation.
On cold winter days when consumption of natural gas by other users rises, the gas turbine plant is started up only during the power generating system’s peak loads.
With this pattern, although only a small quantity of natural gas is used, the generated power is of high value to the supply system.
In spring and autumn when the load on the natural gas network is lower, the gas turbine plant can be operated on a 24 hour basis and thus replace older condensing steam power plant. During the summer time the main USC plant could also be operated on gas if this became cost effective. Power generated from the natural gas part of the plant with an approximate yearly rate of utilization of only 3000 hours can be produce at the same costs per kWh as electricity produced from a coal fired base load unit.
Trends
Irrespective of Kyoto and the dash for gas, the combustion of coal to generate electric power around the world is actually increasing rather than decreasing. Environmental and economic pressures to reach ever higher levels of supercritical steam conditions are driving both boiler and steam turbine manufactures to the use of specially developed high performance alloys more closely akin to the material used in the highest performance gas turbines and in the nuclear power industry.
Integration of gas turbines and multiple fuel conversion facilities into ultra-supercritical steam power plant cycles with high district heating and process heat outputs with effective thermal efficiencies of over 55 per cent are now bringing coal fired power plants into direct competition with gas turbine combined cycle units, with the added benefit of significant fuel flexibility.
TablesTable 1 Efficiencies of the multifuel concept and of its units for condensing and chp operation (lhv) Table 2 Advanced steam conditions in the Avedøre 2 power plant
