E| No. 19 THE WORLD BANK April 1999 Supercritical Coal Fired Power Plants 19666 A Technology Being Successfully Deployed in Developing Countries Ingo Paul Supercritical coal fired power plants with efficiencies Why High Performance Coal Fired of 45% have much lower emissions than subcritical Power Plants Matter plants for a given power output. The paper reviews the major technical and performance aspects of a coal fired plant using this technology. These include the Power Markets Today Many regions of the world turbine-generator set, the once-through boiler and are experiencing fast growing electricity demand. operational issues such as load change, fuelflexibility Permitted emissions from power plants have been and water Early experience with supercritical plants reduced so as to meet air quality standards. Power in the US indicated that they had poor availability i e. forced outages were greater than with subcritical plants are also a source of CO2. one of the greenhouse plants. However experience that takes account ofplant gasses that the Kyoto Protocol proposes should be performance in Japan and Europe as well as in China subject to legally binding emissions reductions or lim- and South Africa (where these once through boilers plants are common) shows that these plants are just a reliable as subcritical plants. Figure 1 Low Emission Levels Worldwide, more than 400 supercritical plants are are achieved by High Steam Conditions and Flue Gas Cleaning now in operation. Steam parameters Flue gas dleaning Reviewing the possibilities for the design and manu- 100- pr s 1 facture of components for supercritical-fired plants in developing countries, the paper notes that the differ- 95- 98- '" '' ences between subcritical and supercritical power 90 . .. - _ ____ plants are limited to a relatively small number of com- ponents; primarily the feedwater pumps and the high- 185 5 pressure feedwater train equipment. All the remaining E components that are common to subcritical and super- _ g-- 80 critical coal-fired power plants can be manufactured 1, ° 7 - in developing countries. The paper concludes with a g I X review of Schwarze Pumpe-the world's largest super- critical lignite fired steam power plant. C02, NOX, SOX, and particulates Nox SOX The World Bank Group * Energy, Mining & Telecommunications * Finance, Private Sector and Infrastructure Network Ingo Paul is Head of Product Management for fossil-fueled steam power plants at Siemens Power Generation (KWU). This paper has been prepared under the aegis of a Siemens/World Bank partnership program (c.f note on page 8). The paper was reviewed and edited by Masaki Takahashi, Stratos Tavoulareas and Kvran O 'Sullivan "1t7 of the Energy, Mining and Telecommunications Department of the World Bank. 2 Supercritical Coal Fired Power Plants itations. Capital scarcity and competition are maintain- What is Critical about Supercritical? ing downward pressure on prices of new plant. Meanwhile electricity generated from coal currently There's nothing "critical" about supercritical. "super- accounts for about 40 percent of total worldwide pro- critical" is a thermodynamic expression describing the duction. Coal is an abundant fuel resource in many of state of a substance where there is no clear distinction the worlds' developing regions and forecasts show that between the liquid and the gaseous phase (i.e. they are it is likely to remain a dominant fuel for electricity a homogenous fluid). Water reaches this state at a pres- generation in many countries for some years to come, sure above 22,1 megapascals (MPa) (Figure 2). It is against this backdrop that power plant suppliers The "efficiency" of the thermodynamic process of a have invested heavily in generation technologies that coal fired power describes how much of the energy fed produce power more efficiently. Enhanced plant into the cycle is converted into electrical energy. The reduces emissions of CO2 and all other pollutants by greater the output of electrical energy for a given using less fuel per unit of electricity generated. While amount of energy input, the higher the efficiency. If the efficiencies of older power plants in developing the energy input to the cycle is kept constant, the out- countries like China and India are still around 30% put can be increased by selecting elevated pressures lower heating value (LHV), modern subcritical cycles and temperatures for the water-steam cycle. have attained efficiencies close to 40% (LHV). Further improvement in efficiency can be achieved by using Up to an operating pressure of around 19 MPa in the supercritical steam conditions. Current supercritical evaporator part of the boiler, the cycle is subcritical. coal fired power plants have efficiencies above 45% This means, that there is a non-homogeneous mixture (LHV). One percent increase in efficiency reduces by of water and steam in the evaporator part of the boiler. two percent. specific emissions such as C02, NOx, In this case a drum-type boiler is used because the SOX and particulates (Figure 1). steam needs to be separated from water in the drum of the boiler before it is superheated and led into the tur- bine. Above an operating pressure of 22,1 MPa in the Figure 2 Density Changes (schematic) evaporator part of the boiler, the cycle is supercritical. of Water and Steam The cycle medium is a single phase fluid with homo- geneous properties and there is no need to separate Pressure (bor) steam from water in a drum. Once-through boilers are 400 ,2. * lr *o * therefore used in supercritical cycles. 300 illi gZI ; ^ .-- ~~~~~~superdarlcr 221.2 N X l > 't . f *rX* Advanced Steels Currently, for once-through boilers, 2200 1.2 1|1 i P t'- f ', - X ^ .operating pressures up to 30 MPa represent the state of the art. However, advanced steel types must be used * E , w * - * *subtial for components such as the boiler and the live steam and hot reheat steam piping that are in direct contact with steam under elevated conditions. Therefore, a 100I 200 300 400 500 600techno-economic evaluation is the basis for the selec- Temperature () tion of the appropriate cycle parameters. Figure 3 Energy Issues 3 to 31,5 MPa/620°C/620°C is achieved using Austenite, Figure 3 Supercritical Steam Conditions which is a proven, but expensive, material. Nickel- are the Key to Economical Operation based alloys, e. g. Inconel, would permit 35 MPa/700°C /720°C, yielding efficiencies up to 48%. Manufactur- HP bypass ers and operators are cooperating in publically spon- , _ , LP sored R&D projects with the aim of constructing a H I I I 1!1 1111 I turbine demonstration power plant of this type. HP x 1l Other improvments in the steam cycle and components Condenser can yield a further 3 percentage points rise in efficien- cy. Most of these technologies, like the double reheat Steam [P bypass concept where the steam expanding through the steam -n generator turbine is fed back to the boiler and reheated for a sec- HP leod Feedwr ond time as well as heat extraction from flue gases heater (2x) tank have already been demonstrated. However, these tech- nologies are not in widespread use due to their cost. Boiler P feed feed pump herter (4x) The Turbine Generator Set Design Features There are several turbine designs available for use in Super-critcal Steam Condifions - 250 MPa/5400C/5400C supercritical power plants. These designs need not fun- Condensate Polishing damentally differ from designs used in subcritical power plants. However, due to the fact that the steam Advantages pressure and temperature are more elevated in super- Higher Thermal Efficiency critical plants, the wall-thickness and the materials Proven Design, Components and Materials selected for the high-pressure turbine section need reconsideration. Furthermore, the design of the turbine generator set must allow flexibility in operation. While depicts a supercritical cycle arrangement with steam subcritical power plants using drum-type boilers are parameters that yield high efficiency while allowing limited in their load change rate due to the boiler drum the use of well-proven materials. (a component requiring a very high wall thickness), supercritical power plants using once-through boilers Steam Conditions Today's state of the art in supercrit- can achieve quick load changes when the turbine is of ical coal fired power plants permits efficiencies that suitable design. exceed 45%, depending on cooling conditions. Options to increase the efficiency above 50% in ultra-supercrit- High Pressure (HP) burbine In the HP turbine sec- ical power plants rely on elevated steam conditions as tion, the steam is expanded from the live steam pres- well as on improved process and component quality. sure to the pressure of the reheat system. which is usu- ally in the order of 4 to 6 Mpa. In order to cater for the Steam conditions up to 30 MPa/600°C/620°C are higher steam parameters in supercritical cycles, mate- achieved using steels with 12% chromium content. Up rials with an elevated chromium content giving greater 4 Supercritical Coal Fired Power Plants material strength are selected. The wall thickness of The Boiler the HP turbine section should be as low as possible and should avoid massive material accumulation (e.g. of Apart from the turbine generator set, the boiler is a key oxides) in order to increase the thermal flexibility and component in modem, coal fired power plants. Its con- accomodate fast load changes. cept, design and integration into the overall plant con- siderably influence costs, operating behavior and Intermediate Pressure (IP) lurbine Section The availability of the power plant. steamflow is further expanded in the IP turbine sec- tion. In supercritical cycles there is a trend to increase Once-through boilers have been favored in many the temperature of the reheat steam that enters the IP countries, for more than 30 years. They can be used up turbine section in order to raise the cycle efficiency. As to a pressure of more than 30 MPa without any change long as the reheat temperature is kept at a moderate in the process engineering. Wall thicknesses of the level (approximately 560°C) there is no significant dif- tubes and headers however need to be designed to ference between the IfP turbine section of a supercriti- match the planned pressure level. At the same time, the cal plant and that of a subcritical plant. drum of the drum-type boiler which is very heavy and located on the top of the boiler can be eliminated. Low Pressure (LP) lurbine Section In the LP turbine Since once-through boilers can be operated at any section the steam is expanded down to the condenser steam pressure, variable pressure operation was intro- pressure. The LP turbine sections in supercritical plants duced into power plants at the start of the 1930s to are not different from those in subcritical plants. make the operation of plants easier. Once-through boilers have been designed in both two- pass and tower type design, depending on the fuel Figure 4 Various Boiler Types requirements and the manufacturers' general practice. For the past 30 years, large once-through boilers have Once-through boiler Once-through boiler Drum tje boiler been built with a spiral shaped arrangement of the (vertical tubing) (spiral/vertical tubing) (vertkc tubing) tubes in the evaporator zone. The latest designs of once-through boilers use a vertical tube arrangement Other Cycle Components A comparison of the water- steam cycle equipment in subcritical and supercritical coal fired power plants shows that the differences are limited to a relatively small number of components i.e. to the feedwater pumps and the equipment in the high pressure feedwater train i.e. downstream of the feed- water pumps. These components represent less than 6% of the total value of a coal fired power plant. Energy Issues 5 Supercritical Plants Have High Fuel flexibility is not compromised in once-through Efficiency and Reliability boilers. All the various types of firing systems (front, opposed, tangential, corner, four wall, arch firing with Operational Issues More than 400 supercritical slag tap or dry ash removal, fluidized bed) used to fire power plants are operating in the US, in Europe, a wide variety of fuels have already been implemented Russia and in Japan. Due to different approaches in for once-through boilers. All types of coal as well as their design and operation performance results are not oil and gas have been used. The pressure in the feed- uniform. While the rapid introduction of very large water system does not have any influence on the slag- plants in the US in the early 70s created problems in ging behaviour as long as steam temperatures are the availability, due to forced outage, of these plants, kept at a similar level to that of conventional drum feedback from other operators is very positive. type boilers. Availability of supercritical plants are equal or even higher than those of comparable subcritical plants. Water chemistry has been perceived to be more com- plicated in supercritical power plants. Problems experi- A number of power plants operate with once-through enced in the past were largely due to the use of deoxy- boilers and supercritical steam conditions in develop- genated all-volatile (AVT) cycle chemnistry. The solu- ing countries today. The South African utility ESKOM tion to these problems was the combination of a con- has been operating a number of once-through boilers densate polishing plant with oxygenated treatment for several years and local industry has participated in (OT) which is a well proven procedure. No additional the design and manufacture of these plants. The 2 x 600 installations for supercritical power plants compared to MW supercritical coal fired power plant Shidongkou the standard in subcritical power plants are required. in the Shanghai area of China was put into operation in the early 90s. In addition, once-through boilers do not have a boiler blowdown. This has a positive effect on the water bal- There are no operational limitations due to once- ance of the plant with less condensate needing to be through boilers compared to drum type boilers. In fact fed into the water-steam cycle and less waste water to once-through boilers are better suited to frequent load be disposed of. variations than drum type boilers, since the drum is a component with a high wall thickness, requiring con- Design and Manufacture of trolled heating. This limits the load change rate to 3% Components for Supercritical Coal per minute, while once-through boilers can step-up the Fired Plants in Developing Countries load by 5% per minute. This makes once-through boil- ers more suitable for fast startup as well as for transient There is a misconception, that the components of conditions. One of the largest coal fired power plants supercritical coal fired power plants can only be equipped with a once-through boiler in Germany, the designed and manufactured in developed countries due 900 MW Heyden power plant, is even operating in two to the complexity of the technology. As discussed, the shift operation as is the 3 x 660 MW power plant in differences in the technology between subcritical and Majuba, South Africa. supercritical coal fired power plants are limited to small 6 Supercritical Coal Fired Power Plants number of components. All developing countries using Current designs of supercritical plants have installa- coal in base load (e.g. China and India) have already tion costs that are only 2% higher than those of sub- large manufacturing capacity in the components com- critical plants. Fuel costs are considerably lower due to mon to subcritical and supercritical plants and are now the increased efficiency and operating costs are at the building up capacity in those components that are spe- same level as subcritical plants. Specific installation cific to supercritical. For example manufacture of the cost i.e. the cost per megawatt (MW) decreases with turbine generator set and boiler for the 2 x 900 MW increased plant size. For countries like India and Waigaoqiao supercritical plant is being done in China. China, unit ratings from 500 MW up to 900 MW are possible due to their large electrical grids. In countries Life Cycle Costs of Supercritical Coal Fired Power with smaller grids, unit sizes of 300 MW are more Plants The life cycle costs of supercritical coal fired appropriate and the specific installation cost will be power plants are lower than those of subcritical plants. higher than that of larger plants. The Schwarze Pumpe Power Plant - A Nilestone The world's largest lignite-fired steam power Boiler feedwater is raised to a pressure of 320 bar plant Schwarze Pumpe in Germany (Figure 5) is by one single pump driven by a steam turbine equipped with two 800 MW steam turbine genera- before being fed through a multi-stage preheating tors designed for super critical steam conditions of zone and into the boiler at a temperature of about 25 MPa /544 C/562 C. The net efficiency of this 270°C. In the boiler the feedwater is further heat- plant is about 41%, a very high value for a plant ed and then superheated to 5470C. For each of using lignite. the 800 MW units one boiler feed pump turbine to drive the boiler feed booster and main pumps is The Schwarze Pumpe concept is based on a study used. The boiler feed pump turbine is supplied with conducted in 1991 for VEAG by an association steam from an extraction point at the intermediate formed by RWE Energie AG and VEBA Kraftwerke pressure turbine and a second supply from the Ruhr AG cold reheat line The design features: The following requirements were specified for the design of the turbine: * A twin unit plant (2 x 800 MW) * Utilization of flue gas heat for condensate * Super critical steam conditions, single reheat, heating operation in sliding-pressure mode with main * Dual train flue gas discharge (without bypass steam conditions at 638 kg/s, 25,3 MPa, around the flue gas desulphurization system) via 5440C and reheat steam conditions at 5,2 MPa, the cooling tower. 5620C. * Seven-stage regenerative feed heating with a final temperature of 2700C and turbine-driven boiler feed pump Energy Issues 7 'Figur,Fe 5 - ~~~~~~~~~- ~~~~~The Schwarze Pumpe 2 x 800 MW Steam __________________________ ~~~~~~~~~~~~~~~~~~~~~~Power Plant. 8 Supercritical Coal Fired Power Plants This paper is one of a series on fossil fuel generation tech- nologies. 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