Supercritical steam generators


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Supercritical steam generators

  1. 1. INTRODUCTION1.1 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 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 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
  2. 2. adiabatic expansion of the vapor/steam on the turbine to obtain mechanicalwork. It is an isentropic process. The temperature of the steam is reduced andit falls below the saturated vapor line. The dryness fraction is reduced to lessthan one and a mixed liquid vapor phase is present. Process 4-1 is thecondensation process. This mixture is condensed in a condenser isothermallyand brought to the liquid phase back to the pump. FIGURE 1.2 Temperature vs. Entropy diagram for Rankine cycleThe 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 overalleffect is an increase in the thermal efficiency since the average temperature atwhich the heat is added increases. Moisture content at the exit of the steam isdecreased as seen in the figure 1.3.Superheating is usually limited to 620°C owing to metallurgicalconsiderations. 2
  3. 3. Figure1.3 Rankine cycle with superheating1.2 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 3
  4. 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/kgThus, 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. 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. 4
  5. 5. 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. 5
  6. 6. SUPERCRITICAL RANKINE CYCYLE2.1 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 Phase diagram of water 6
  7. 7. 2.2 Efficiency: The Rankine cycle can be greatly improved by operating in the supercritical region of the coolant. Most modern fossil fuel plants employ the 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)2.3 Definition: Figure 2.2 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. 7
  8. 8. 2.4 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. 8
  9. 9. CHAPTER 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 Predicted Recirculation inside the combustion chamberTable 3.1 lists the peak temperatures and burnout for designs A, B and C. the tablealso lists the standard deviations of the predicted temperature and oxygen fields. The 9
  10. 10. lowest values for C indicate the higher degree of homogeneity. Thus the symmetricalboiler seems to be the most suitable design. A B CPeak Temperature (K) 2618 2437 2106Burnout % 97 99 100Standard deviation of 375 238 90temperature (K)Standard deviation of 3 5 1O2 concentration % Table 3.1 Results of the boiler shape determination 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. Upfired DownfiredHeat transfer rate, kW/m2 220 261Outlet temperature, K 1722 1568 Table 3.2 Results of the burner location determination 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 10
  11. 11. enthalpy cannot be efficiently used. On the contrary, in the large boiler although the heat fluxes are uniform, 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 boilerFiring Density 774 238 89 3kW/mOutlet temperature, 1805 1558 1299K Table3.3 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
  12. 12. MATERIAL SELECTION4.1 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
  13. 13. Heat surface Tube material Header material Economiser SA-210 C SA-106 C Furnace Walls SA-213 T12 SA-106 C Super SA-213 T12 SA-335 P12 heater/Reheater SA-213 T23 SA-335 P91 SA-213 TP 304H SA-335 P911 SA-213 TP347HFG SUPER 304H Steam Piping SA 335 P91 Table 4.1 Materials for different boiler parts The materials for the other parts of the power plant (like turbine) also must besustainable for the super critically heated steam. The following table gives a detailidea on the turbine materials of a plant operating on a supercritical cycle. (Table 4.3) 13
  14. 14. Component 1,050° F 1,150 °F 1,300° F 1,400 °F Casings CrMoV (cast) 9–10% Cr (W) CF8C-Plus CCA617 (shells, valves, 10CrMoVMb 12CrW (Co) CCA617 Inconel 740 steam chests, nozzles) CrMoWVNbN Inconel 625 CF8C-Plus Nimonic 263 Bolting 422 9–12% CrMoV Nimonic Nimonic 105 105 9–12% CrMoWVNbN CrMoV Nimonic Nimonic 115 115 Nimonic 80A Waspaloy U700 Rotors/Discs 1CrMoV 9–12 % CrWCo CCA617 CCA617 12CrMoVNbN 12CrMoWVNbN Inconel 625 Inconel 740 Nozzles/Blades 422 9–12% CrWCo Wrought Wrought Ni-based Ni-based 10CrMoVNbN 10CrMoVCbN Table 4.2 Materials for other partsThe 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 14
  15. 15. SUPERCRITICAL BOILERS5.1 A typical Supercritical Boilers: Largest CFB and first supercritical CFB sold to date is the Lagisza 460 MWeunit Ordered by Poludniowy Koncern Energetyczny SA (PKE) in Poland. The designis Essentially complete with financial closing expected in the first quarter of 2006 atwhich time Fabrication and construction will commence. The largest capacity units inoperation today are the two (2) 300 MWe JEA repowered units which were designedto fire any Combination of petroleum coke and bituminous coals. The physicallylargest Foster Wheeler boilers in operation are the 262 MWe Turow Units 4, 5, and 6which were designed to fire a high moisture brown coal. The design and configurationof these units with Compact solids separators and INTREX™ heat exchangers wereused as the basis for the Lagisza design as well as for this study. The Lagisza designwas adjusted to accommodate a typical bituminous coal and the steam cycle. Figure 5.1 The Lagisza 300 MWe plant in Poland 15
  16. 16. The following table gives a detail report on the performance of the Lagisza 800MWePower plant.Parameters ValuesSteam ConditionsMain Steam Pressure(barg) 315Main steam Temperature(°C) 604Reheat Steam temperature(°C) 615Feed Water temperature(°C) 289EmissionsSO2 mg/NM3 111NOx mg/NM3 100Power generationGross power MWe 805Net power MWe 739Net Plant Efficiency % 40.7 Table 5.1 Performance Parameters5.2 Super and Sub Critical Boilers (comparative study): There are many advantages of super critical boilers over normalsubcritical boilers, the prime advantage being the increased efficiency and reducedemissions. There are many more advantages like no need of steam dryers, higheroperating pressures leading to more work output etc. It is thus very important to have a comparative study of both theboilers. 16
  17. 17. Technology Efficiency (%) Steam Typical emissions pressure/temperatureUltra Supercritical >242 bar and SO2-0.408 593.33°C kg/MHh 33–35 NOx-0.286 kg/MWh CO2-0.96 T/MWhSupercritical >221.2 bar and SO2-0.431 537°C kg/MHh 36–40 NOx-0.304 kg/MWh CO2-1.02 T/MWhSubritical 165 bar 537°C SO2-0.445 kg/MHh 42–45 NOx-0.31 kg/MWh CO2-1.02 T/MWh Table 5.2 Comparison of sub and supercritical boilers 17
  18. 18. ADVANCE IN SC TECHNOLGY AND FUTURE IN INDIA6.1 Natural Gas Production with a Supercritical Geothermal Power Application By-product: In many cases, the source for Natural Gas production is geothermally heatedbrine. In these cases, the brine temperatures range from 240 °F (390 K) up to 360 °F(455 K) depending linearly upon well depth. In this temperature range, the brinepasses through a heat exchanger to feed a supercritical Rankine cycle for Propane,which has a critical temperature of 206 °F (370 K) and a critical pressure of 616 psia.Theoretically, the brine may be able to run the cycle directly but there are too manycontaminants and compositional variations for this to be feasible. If a power cycle likethis were employed, the sites producing Natural Gas could potentially generate powerfor Grid use or, at a minimum, generate the majority of the plant’s electricalRequirements. In the United States, there are several areas along the Texas and theLouisiana Gulf Coast where this type of power cycle is feasible. The benefit to thiscycle is that it is extremely simple in terms of system components. The systemRequires only a single phase heat exchanger, a turbine, an air cooled condenser, and apump. The nominal operating pressure of this system is approximately 1000 psia,which suggests that all of the support piping and equipment is commerciallyavailable. Given a 15 Million BTU/hr brine source, this system could generateapproximately 400 kW net powers with a thermal efficiency of 9%. Additionally, thesystem can be built to be self regulating by using the power grid as a dynamic brakefor the turbine-generator set. In effect, this acts as a speed control for the turbineduring slight variations in system demand under normal operating conditions.Additional controls can be implemented to automate the system based on brinetemperature and flow rates, all of which minimize the need to have a full-timeoperator, thus reducing operational costs. 18
  19. 19. 6.2 Supercritical Boilers in India: There haven’t been any supercritical boilers in use in India so far. TheEuropean countries, USA, Japan have been using supercritical technology since thelast two decades. However, there are upcoming projects to build power plantsworking under the supercritical technology in India. The National Thermal Power Corporation (NTPC) had entrusted a technoeconomic study to M/s EPDC for super-critical Vs Sub-critical Boilers for theirproposed 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 mostapplicable steam conditions in India shall be 246 kg/cm2, 538° C/566° C. With thesesteam parameters, M/s EPDC has estimated that the capital cost for a supercriticalpower station (4x500 MW) shall be about 2% higher than that of sub-critical powerplant 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 byincrease 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 19
  20. 20. CONCLUSION The supercritical Rankine cycle, in general, offers an additional 30% relativeimprovement in the thermal efficiency as compared to the same system operating inthe subcritical region. The cycle has been successfully utilized in fossil fuel plantsbut the current available materials prohibit reliable application of the supercriticalcycle to nuclear applications. There is much work to be done in order to advancematerials to the point where they will be able to reliably withstand the stresses of asupercritical environment inside a nuclear reactor for a designed life span of 60 years. Supercritical boiler technology has matured, through advancements in designand materials. Coal-fired supercritical units supplied around the world over the pastseveral years have been operating with high efficiency performance and highavailability. 20
  21. 21. REFERENCES1. “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. “” 21