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Ganesh final report Document Transcript

  • 1. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 1 1 INTRODUCTION 1.1 Basic Rankine Cycle: The Rankine cycle is the oldest functional heat cycle utilized by man. The Rankine cycle is the very a basic vapor power cycle which is adopted in all the thermal power plants. It is a four step process (Figure 1.1) which involves the heating of the working fluid to its saturation temperature and vaporizing it isothermally, expanding the vapor on a turbine (work cycle), condensing the steam isothermally to the liquid phase and pumping it back to the boiler. Figure 1.1.1 Basic Rankine Cycle Figure 2 represents the temperature-entropy diagram for the simplest version of the Rankine cycle. Although this simple version is rarely used it gives a very clear and simple picture on the working of the cycle. Process 1-2 is the pumping of the working fliud (water) into the boiler drum. The power required is derived from the overall power developed. Process 2-3 is the heating of the water upto its saturation temperature (100°C at 1 atm pressure for water) is reached and then isothermal heating of the water where the
  • 2. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 2 phase change from liquid to vapor occurs. Points 3 lie on the saturated vapor line. The steam here is completely dry. Process 3-4 is the adiabatic expansion of the vapor/steam on the turbine to obtain mechanical work. It is an isentropic process. The temperature of the steam is reduced and it falls below the saturated vapor line. The dryness fraction is reduced to less than one and a mixed liquid vapor phase is present. Process 4-1 is the condensation process. This mixture is condensed in a condenser isothermally and brought to the liquid phase back to the pump. FIGURE 1.1.2 Temperature vs. Entropy diagram for Rankine cycle The steam is however, usually, superheated so as to obtain more work output. Increasing the superheat to greater extent would lead to more work output. However the energy spent in superheating the fuel is also high. The overall effect is an increase in the thermal efficiency since the average temperature at which the heat is added increases. Moisture content at the exit of the steam is decreased as seen in the figure 1.3. Superheating is usually limited to 620°C owing to metallurgical considerations.
  • 3. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 3 Figure1.1.3 Rankine cycle with superheating 1.2 Energy Analysis of the Rankine Cycle: All four components in the Rankine Cycle (pump, boiler, turbine and condenser) are steady flow devices and thus can be analyzed under steady flow processes. K.E and P.E changes are small compared to work and heat transferred and is thereby neglected. Thus the steady flow equation (per unit mass) reduces to: Q+hini = W+hfinal Boiler and condenser do not involve any work and pump and turbine are assumed to be isentropic. The conservation of Energy relation for each device is expressed as follows:  Steam turbine: As the expansion is adiabatic (Q=0) and isentropic (S3=S4), then, W3-4=Wturbine= (h3-h4) kJ/kg
  • 4. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 4  Condenser: Heat rejected in the condenser, Q4-1+h4=h1+W4-1 Since W4-1=0, Q4-1=h1-h4 Thus, Q4-1=-(h4-h1) kJ/kg  Pump: Work required to pump water: Wpump=h1-h2 kJ/kg (-ve work)  Boiler: Heat added in boiler: Q2-3=h3-h2 kJ/kg=h3-h1-Wpump kJ/kg Thus, the Rankine Efficiency=Work done/Heat added = (h3-h4-Wp) / (h3-h1-Wp) Neglecting feed pump work as it is very small compared to other quantities, the efficiency reduces to: ηrankine= (h3-h4) / (h3-h1). 1.3 Factors increasing the Rankine Efficiency: i. Lowering the condenser pressure: Lowering the condenser pressure would lead to the lowering of temperature os steam. Thus for the same turbine inlet state, more work is obtained at lower temperatures.
  • 5. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 5 This method though cannot be extensively used as it reduces the dryness fraction x of the steam. This is highly undesirable as it decreases the turbine efficiency is reduced due to excessive erosion of the turbine blades. ii. Superheating the steam to high temperature: There is an increase in the work output if superheating of steam is done. It increases the thermal efficiency as the average temperature at which heat is added increases. There is also another benfit of superheating; the steam at the exit of the turbine is drier than in case of non superheated steam. iii. Increasing the boiler pressure: Increasing the boiler pressure raises the average temperature at which heat is added and thereby increases the theramal efficiency. However the dryness fraction decreases for the same exit temperature of the boiler. This problem can be solved by employing reheating procedure. If however the boiler pressure is raised to supercritical point greater efficiency is obtained as the latent heat absorbed during phase change is reduced to zero.
  • 6. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 6 2 SUPERCRITICAL RANKINE CYCYLE 2.1 Supercritical technology: When temperature and pressure of live steam are increased beyond the critical point of water, the properties of steam will change dramatically. The critical point of water is at 374 °C and 221.2 bar (218 atm), Figure 2.1, and it is defined to be the point where gaseous component cannot be liquefied by increasing the pressure applied to it. Beyond this critical point water does not experience a phase change to vapor, but it becomes a supercritical fluid. Supercritical fluid is not a gas or liquid. It is best described to be an intermediate between these two phases. It has similar solvent power as liquid, but its transport properties are similar to gases. Figure 2.1.1 Phase diagram of water 2.2Efficiency: The Rankine cycle can be greatly improved by operating in the supercritical region of the coolant. Most modern fossil fuel plants employ the
  • 7. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 7 supercritical Rankine Steam Cycle which pushes the thermal efficiency of the plant (see equation 4) into the low to mid 40% range. ηsupercritical = (h2-h1-h3+h4 )/( h2-h1) -(eqn 4) 2.2 Definition: Figure 2.2.1 T-S diagram for supercritical Rankine cycle For water, this cycle corresponds to pressures above 221.2 bar and temperatures above 374.15°C (647.3 K). The T-S diagram for a supercritical cycle can be seen in Figure 6. With the use of reheat and regeneration techniques, point 3 in Figure 2.1, which corresponds to the T-S vapor state of the coolant after it has expanded through a turbine, can be pushed to the right such that the coolant remains in the gas phase. This simplifies the system by eliminating the need for steam separators, dryers, and turbines specially designed for low quality steam.
  • 8. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 8 Material Concerns: The primary concern with this cycle, at least for water, is the material limits of the primary and support equipment. The materials in a boiler can be exposed to temperatures above their limit, within reason, so long as the rate of heat transfer to the coolant is sufficient to “cool” the material below its given limit. The same holds true for the turbine materials. With the advent of modern materials, i.e. super alloys and ceramics, not only are the physical limits of the materials being pushed to extremes, but the systems are functioning much closer to their limits. The current super alloys and coatings are allowing turbine inlet temperatures of up to 700°C (973 K). the fourth generation super alloys with ruthenium mono-crystal structures promise turbine inlet temperatures up to 1097°C (1370 K). Special alloys like Iconel 740, Haynes 230, CCA617, etc. are used. The metallurgical challenges faced and solutions:  Normal Stainless steel proves of absolutely no use in building SC and USC Boilers.  The high temperature and pressure in the boiler induce huge amount of stresses and fatigue in the materials. Also chances of oxidation are very high at such high temperature and pressure.  To resist these stress levels and oxidation different advanced materials and alloys should be introduced.  Also they should me machinable and weldable. This is a great metallurgical challenge.
  • 9. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 9 3 DESIGN AND WORKING 3.1 Boiler Design: The design of Super and Ultra supercritical boilers (also called as Benson Boiler) is very critical as the working pressures of these boilers are very high. The boiler shells, the economizer unit, super heaters, air preheaters are specially designed. Their location is also of great significance. i. Boiler shell: As shown in the figure 3.1 the geometry of the boilers and the configuration of the inlets determine the recirculation pattern inside boiler. The intensive recirculation created in the symmetric boiler results in a more uniform temperature field, lower temperature peaks, moderate oxygen concentration and complete burnout of the combustible gases and char Fig 3.1.1 Predicted Recirculation inside the combustion chamber Table 3.1 lists the peak temperatures and burnout for designs A, B and C. the table also lists the standard deviations of the predicted temperature and oxygen fields. The lowest values for C indicate the higher degree of homogeneity. Thus the symmetrical boiler seems to be the most suitable design.
  • 10. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 10 ii. Location of burners: The number of burners in the boiler shell is also of prime importance. Amongst all of them the downfired boilers are most suitable and advantageous. Table 3.2 gives a clear idea. iii. Boiler dimensions: One of the most important advantages of HTAC applications are high heat fluxes. Thus, compact combustion chambers can be built and the investment costs can be lowered. The fourth calculation series was carried out in order to find the combustion chamber dimensions which can, on one hand, ensure an efficient heat exchange between combustion gas and water/steam mixture and on the other hand, ensure high values of firing density. Three different sizes are tested and they are named in as the small boiler, the medium size boiler and the large boiler .It has been observed (see Table 3.3) that the small boiler is too short. At the top a region of high temperatures exists and its enthalpy cannot be efficiently used. On the contrary, in the large boiler although the heat fluxes areuniform, they are two times lower than in the medium size boiler. Therefore, the medium size boiler configuration is chosen for further investigations. Small boiler Medium size boiler Large boiler Firing Density kW/m3 774 238 89 Outlet temperature, K 1805 1558 1299 Table3.1.1 Results of the boiler size determination 3.2 Working: As already discussed, the working of Supercritical Boilers is similar to the working of sub-critical boilers. It works on the supercritical rankine cycle. Most supercritical boilers are being run at operating pressures above of 235 bars. The working of ultra supercritical boilers has operating pressures above 273 bars
  • 11. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 11 4 MATERIAL SELECTION 4.1 Metallurgical Problems: The available materials today like stainless steel which are usually used for boiler parts are not suitable for SC and USC boilers. They do not have the enough creep strength to resist the high pressure. Also there is high rate of oxidation at such high temperature and pressures which are beyond the capability of these materials to resist. Capable, qualified materials must be available to the industry to enable development of steam generators for SC steam conditions. Major components, such as infurnace tubing for the waterwalls, superheater/ reheater sections, headers, external piping, and other accessories require advancements in materials technology to allow outlet steam temperature increases to reach 760°C (1400F). Experiences with projects such as the pioneering Philo and Eddystone supercritical plants and the problems with the stainless steel steam piping and superheater fireside corrosion provided a valuable precautionary lesson for SC development. Industry organizations thus recognized that a thorough program was required to develop new and improved materials and protection methods necessary for these high temperature steam conditions. 4.2 Materials used: The materials used should be sustainable to the very high pressure being developed and should not get oxidized due to the very high temperature. Different high temperature materials are being used like 9 to 12% ferritic steels T91/P91, T92/P92, T112/P122 steel, Advanced Austenitic alloys TP347, HFG, Super 304, Nickel and chrome-nickel super alloys like Inconel 740. Table 4.2 gives a very brief idea about the boiler materials used for different parts of the boiler.
  • 12. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 12 Heat surface Tube material Header material Economiser SA-210 C SA-106 C Furnace Walls SA-213 T12 SA-106 C Super heater/Reheater SA-213 T12 SA-213 T23 SA-213 TP 304H SA-213 TP347HFG SUPER 304H SA-335 P12 SA-335 P91 SA-335 P911 Steam Piping SA 335 P91 Table 4.2.1 Materials for different boiler parts The materials for the other parts of the power plant (like turbine) also must be sustainable for the super critically heated steam. The following table gives a detail idea on the turbine materials of a plant operating on a supercritical cycle. (Table 4.3)
  • 13. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 13 Table 4.2.2 Materials for other parts The following figures show some of the materials used for SC and USC boilers. Iconel 740 is widely used for steam pipings in almost all of them. Figure 4.1 TP347HFG Figure 4.2 Iconel 740 Component 1,050° F 1,150 °F 1,300° F 1,400 °F Casings (shells, valves, steam chests, nozzles) CrMoV (cast) 10CrMoVMb 9–10% Cr (W) 12CrW (Co) CrMoWVNbN CF8C- Plus CCA617 Inconel 625 Nimonic 263 CCA617 Inconel 740 CF8C- Plus Bolting 422 9–12% CrMoV Nimonic 80A 9–12% CrMoV CrMoWVNbN Nimonic 105 Nimonic 115 Waspaloy Nimonic 105 Nimonic 115 U700 Rotors/Discs 1CrMoV 12CrMoVNbN 9–12 % CrWCo 12CrMoWVNbN CCA617 Inconel 625 CCA617 Inconel 740 Nozzles/Blades 422 10CrMoVNbN 9–12% CrWCo 10CrMoVCbN Wrought Ni-based Wrought Ni- based
  • 14. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 14 5 SUPERCRITICAL BOILERS 5.1 A typical Supercritical Boilers: Largest CFB and first supercritical CFB sold to date is the Lagisza 460 MWe unit Ordered by Poludniowy Koncern Energetyczny SA (PKE) in Poland. The design is Essentially complete with financial closing expected in the first quarter of 2006 at which time Fabrication and construction will commence. The largest capacity units in operation today are the two (2) 300 MWe JEA repowered units which were designed to fire any Combination of petroleum coke and bituminous coals. The physically largest Foster Wheeler boilers in operation are the 262 MWe Turow Units 4, 5, and 6 which were designed to fire a high moisture brown coal. The design and configuration of these units with Compact solids separators and INTREX™ heat exchangers were used as the basis for the Lagisza design as well as for this study. The Lagisza design was adjusted to accommodate a typical bituminous coal and the steam cycle. Figure 5.1.1 The Lagisza 300 MWe plant in Pola
  • 15. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 15 5.2 Super and Sub Critical Boilers (comparative study): There are many advantages of super critical boilers over normal subcritical boilers, the prime advantage being the increased efficiency and reduced emissions. There are many more advantages like no need of steam dryers, higher operating pressures leading to more work output etc. It is thus very important to have a comparative study of both the boilers. Table 5.2.1 Comparison of sub and supercritical boilers Technology Efficiency (%) Steam pressure/temperature Typical emissions Ultra Supercritical 33–35 >242 bar and 593.33°C SO2-0.408 kg/MHh NOx-0.286 kg/MWh CO2-0.96 T/MWh Supercritical 36–40 >221.2 bar and 537°C SO2-0.431 kg/MHh NOx-0.304 kg/MWh CO2-1.02 T/MWh Subritical 42–45 165 bar 537°C SO2-0.445 kg/MHh NOx-0.31 kg/MWh CO2-1.02 T/MWh
  • 16. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 16 6 ADVANCE IN SC TECHNOLGY AND FUTURE IN INDIA 6.2 Supercritical Boilers in India: There haven’t been any supercritical boilers in use in India so far. The European countries, USA, Japan have been using supercritical technology since the last two decades. However, there are upcoming projects to build power plants working under the supercritical technology in India. The National Thermal Power Corporation (NTPC) had entrusted a techno economic study to M/s EPDC for super-critical Vs Sub-critical Boilers for their proposed Sipat STPS (4x500 MW) in Madhya Pradesh. M/s EPDC has recommended that a first step to the introduction of super- critical technology, the most proven steam conditions may be chosen and the most applicable steam conditions in India shall be 246 kg/cm2 , 538° C/566° C. With these steam parameters, M/s EPDC has estimated that the capital cost for a supercritical power station (4x500 MW) shall be about 2% higher than that of sub-critical power plant but at the same time the plant efficiency shall improve from 38.64% to 39.6%. Being a pit head thermal power project, the saving in fuel charges is not justified by increase in fixed charges. Here are some upcoming projects in India:  North Karanpura, Jharkhand – 3x660 MW  Darlipali, Orissa – 4x800 MW  Lara, Chattisgarh – 5x800 MW  Marakanam, Tamilnadu – 4x800 MW  Tanda-II, Uttar Pradesh - 2x660 MW  Meja, Uttar Pradesh - 2x660 MW  Sholapur – 2x660 MW  New Nabinagar-3x660 MW  Many more projects including 800 MW ultra super critical units under consideration
  • 17. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 17 7 CONCLUSION The supercritical Rankine cycle, in general, offers an additional 30% relative improvement in the thermal efficiency as compared to the same system operating in the subcritical region. The cycle has been successfully utilized in fossil fuel plants but the current available materials prohibit reliable application of the supercritical cycle to nuclear applications. There is much work to be done in order to advance materials to the point where they will be able to reliably withstand the stresses of a supercritical environment inside a nuclear reactor for a designed life span of 60 years. Supercritical boiler technology has matured, through advancements in design and materials. Coal-fired supercritical units supplied around the world over the past several years have been operating with high efficiency performance and high availability.
  • 18. “SUPERCRITICAL TECHNOLOGY IN POWER PLANT” . 18 REFERENCES 1. “Design Aspects of the Ultra-Supercritical CFB Boiler”; Stephen J. Goidich, Song Wu, Zhen Fan; Foster Wheeler North America Corp. 2. “Novel conceptual design of a supercritical pulverized coal boiler utilizing high temperature air combustion (HTAC) technology”; Natalia Schaffel- Mancini, Marco Mancini, Andrzej Szlek, Roman Weber; Institute of Energy Process Engineering and Fuel Technology, Clausthal University of Technology, Agricolastr. 4, 38678 Clausthal-Zellerfeld, Germany; 6 February 2010. 3. “Supercritical (Once Through) Boiler Technology”; J.W. Smith, Babcock & Wilcox, Barberton, Ohio, U.S.A.; May 1998. 4. “Steam Generator for Advanced Ultra-Supercritical Power Plants 700 to 760°C”; P.S. Weitzel; ASME 2011 Power Conference, Denver, Colorado, U.S.A; July 12-14, 2011. 5. “Supercritical boiler technology for future market conditions”; Joachim Franke and Rudolf Kral; Siemens Power Generation; Parsons Conference; 2003. 6. “Steam Turbine Design Considerations for Supercritical Cycles”; Justin Zachary, Paul Kochis, Ram Narula; Coal Gen 2007 Conference;1-3 August 2007. 7. “Technology status of thermal power plants in India and opportunities in renovation and modernization”; TERI, D S Block, India Habitat Centre, Lodi Road, New Delhi – 110003. 8. “Applied Thermodynamics”; Dr. H.N Sawant; January 1992; revised July 2004. 9. “http://en.wikipedia.org/wiki/Boiler#Supercritical_steam_generator”