Performance of Power Plants

Transcription

1 CHAPTER Performance of Power Plants 1.1 INTRODUCTION The demand for commercial energy in the form of process heat and electricity is increasing exponentially with industrial growth. This is depleting the limited resources of fossil fuels (coal, oil, natural gas), resulting in higher fuel prices and enhanced environmental pollution by thermal and material discharges from inefficient power systems. The oil crisis of 1973/74 has finally proved that the mankind for its survival has to go in for development of efficient energy conversion technologies to improve the overall yield of fossil fuels and to reduce the thermal discharges into the atmosphere. The overall yield of the fuels may be improved in the conventional power systems along the following lines (Singal, 1988a): (i) by improving the internal efficiency of the thermal machinery and auxiliary equipment by decreasing the mechanical, aerodynamic and radiation losses; (ii) by extensive CARNOTIZATION OF THE POWER CYCLES by raising the source temperature and reducing the sink temperature in simple cycles or by hybridization (combined-cycle systems); (iii) by cogeneration of heat and electricity. A large number of researchers and practising engineers are involved in the design improvements and analysis of conventional power plants, combined cycle systems and cogeneration or combined heat and power plants in order to improve their thermo-economic performance. Some of their findings and recommendations published in the recent literature have been reviewed in what follows. 1

2 2 DESIGN OF COMBINED CYCLE POWER PLANTS 1.2 PERFORMANCE LIMITATIONS OF CONVENTIONAL POWER PLANTS Some 69 percent of electricity produced in India by 31 st March, 1966 will be generated in coal-fired central power stations. Coal-based thermal power plants will constitute 59,850 MW out of a combined generating capacity of 83,000 MW consuming 195 million tonnes of coal (Sumit Mitra, 1995). With uncertain programme of nuclear power expansion and the heavy project costs and long construction periods of hydro power plants, coal is projected to provide more than 70 percent of Indian electric power by the year 2000 (Sharad Pawar, 1995a, 1995b). Modified Rankine cycle can attain high thermal efficiencies of steam power plants (El-Wakil,1985) by raising the mean temperature of heat supply, by increasing boiler pressure, reheating between turbine cylinders and feedwater heating. But the inlet steam conditions to turbine are restricted by the metallurgical limitations. The most common steam parameters at turbine throttle are 140 bar and 540 C (Brown Boveri Review, 1988f). A typical Sankey diagram as shown in Fig. 1.1 (Fraas, 1982) gives the heat loss distribution in a steam power plant with a thermal F. d. fan 0.5 Fuel to boiler 301 Radiation loss 3 Stack loss 18 Outside losses 5 Available to boiler Available to turbine Charged to turbine Bled steam for feed heating 23 F. d. fan 0.5 Alternator output 120 MW Rejected to condenser 158 MW Fig. 1.1 : Sankey Diagram of Conventional Steam Cycle 120 MW e for 301 MW th Input (39.9% Efficiency)

3 PERFORMANCE OF POWER PLANTS 3 efficiency of 39.9 percent. For an electrical output of 120 MW, 158 MW equivalent of heat has to be rejected to the environment through the condenser cooling water. For a design water temperature rise of 10 C, 1200 m 3 /min of cooling water is required for a 500 MW thermal power plant. The energy production cost from a conventional power plant is high due to low thermal efficiency, large capital investment, long construction periods and high cost of infrastructure. Although National Thermal Power Corporation is selling electricity at ` 1. 50/kWh, the rates being quoted by USA companies for Cogentrix project in Karnataka and Enron project in Maharastra are higher, i.e., ` 2.30 to ` 2.59/kWh (Ranjit Mathrani,1995). 1.3 ATTAINMENT OF HIGH PERFORMANCE OF CONVENTIONAL POWER PLANTS There are two main reasons for low thermal and operational performance of thermal power plants in India. (i) Low quality coal with high ash content of more than 30 percent. (ii) High redundancy in the plant equipment and improper maintenance policy and practice Combustion Technologies The incompetency of pulverised coal-fired boilers to burn low-quality, highash coals has attracted the attention of engineers and scientists to explore the application of fluidised-bed coal combustion and coal gasification technologies to thermal power plants. The atmospheric and pressurised fluidised-bed coal combustion and coal gasification plants can produce electricity at lower cost than conventional thermal power plants. Further these technologies may discharge reduced pollution. Some technical and economic details are available as projected for Lurgi Pressure Gasification Process for fuel gas production (Lurgi, 1975) which can be used in place of pulverised coal-fired boilers. Gasification can be used with a gas turbine plant also. It may be possible to achieve a thermal efficiency of as much as 45 percent by applying gasification to advanced power cycles (combined cycles). Indian Institute of Chemical Technology, Hyderabad is seeking ` 400 crores to put up a 100 MW pilot project to demonstrate the effectiveness of the green technology called coal gasification (IICT, 1995). Both atmospheric as well as pressurised fluidised-bed combustion boilers have been developed (Combustion Engineering Review,1988) to burn the solid and dirty coal for clean and efficient combustion. A wide range of fuels such as high-ash coal, coke breeze, lignite, high moisture content bio-mass and other refuses can be used as fuel in fluidised-bed combustion boilers (Ignifluid Boilers Review, 1986). A number of demonstration and commercial plants with 100 MW to 200 MW capacity Atmospheric Fluidised Bed Combustion Boilers exist (Regan, et al, 1989). There is a special interest in the combustion of bio-mass such as agricultural residues, etc., in the fluidised-bed combustion boilers to design small, decentralised power plants for rural areas (Singal,1986, 1988b, 1995) to disconnect rural

4 4 DESIGN OF COMBINED CYCLE POWER PLANTS consumers from the national grid in order to improve the load factor of central power plants and to reduce transmission losses. Some work in the design of fluidised bed combustors for burning biomass has been done in other countries as well (Cook, 1984), but the development of fluidised-bed technology is based on experimentation for individual applications. Pilot plant study and indigenous efforts are needed for each case Load Factor Improvements There is always more demand of electricity than its supply as the current shortfall is estimated as 20,000 MW. However, the national average plant load factor of percent is rather on lower side. This can be attributed to faulty plant design, improper operation and maintenance policies and practices (Narayana, et al, 1988). Some computer models based on interference theory have been developed (Dutta, et al, 1987) as guidance for design of new thermal power plants. Computer aided reliability study of power plant equipment (Guchait, et al, 1980) have pinpointed the boiler as the weakest link in a thermal power plant. By-pass stations have also been proposed for flexible operation of steam power plants (Martin, et al, 1973) both for base load as well as peak load duties to improve the plant load factor Performance Analysis The complexity of power generating units has increased considerably to achieve improved utilization of energy sources. The owners of the plants demand strict guaranteed performance. This requires comprehensive thermoeconomic calculations of a large number of configurations during design and optimization stage and hence quick and precise computer-aided methods are needed for thermal power calculations (Pertz, 1991). Although thermal power plants are designed on the basis of first law of thermodynamics, but the traditional thermodynamic analysis methods are not satisfactory. Therefore Exergy method of thermodynamic analysis is being recommended to be applied to thermal power plants to improve their performance (Kotas, 1985). The Rational efficiencies of components based on Second law can be effectively used to calculate the Rational efficiency of the power plants (Horlock, 1992a) New Cycles Development The new Kalina Cycle based power plants can be installed in India from 1997 onwards which results in 20 percent increase in efficiency and 20 percent reduction in capital investment (Technorama, 1994). The higher efficiency is brought about by using varying ammonia-water mix as the operating fluid. The tall claims are still to be validated by field experience. In spite of the above efforts being made to improve the performance of steam power plants, experts are of the opinion that the state-of-the-art technology cannot cross a thermal efficiency of 44 percent. In search for clean and efficient power plant

5 PERFORMANCE OF POWER PLANTS 5 technologies, the combined cycle power plant has attracted the maximum attention of the research workers and practising engineers. This technology is well validated where clean fuel is available in plenty for combustion in a gas turbine plant. It has the promise to produce electricity at a cheaper rate than coal-fired steam power plants. It bids fair to become the dominant technology for power production in the twenty-first century (Nag, 1995). 1.4 COMBINED CYCLE POWER PLANTS The term combined cycle refers to any power plant system in which a highertemperature thermodynamic cycle rejects its heat to a lower temperature thermodynamic cycle, ordinarily employing a different working fluid. However, the term combined-cycle plant has now become the accepted shortened form of the term combined-cycle, gas turbine-steam turbine (Fraas, 1982). An example of a combined cycle plant is one operating on mercury and steam vapour cycles. However, technical problems associated with mercury have limited use of this cycle. Other examples of combined power plants may include: (Horlock, 1992b): (i) a magneto-hydrodynamic/steam turbine plant in which the exhaust gases from an open-circuit MHD generator operating at high temperature, are utilized to raise steam for a closed-circuit steam plant; (ii) a gas-turbine/steam-turbine binary cycle plant in which heat is supplied to the gas turbine from a nuclear reactor MHD-Steam Turbine Plants MHD generation has great potential for generating power in excess of 1000 MW (Singal, 1984). It can be used as topper for coal-fired thermal power plant, thereby improving the efficiency and lowering the capital cost and environmental pollution of the plant. India had set up a pilot plant at Tiruchirapalli for further studies. Similarly, Russia, Japan and other countries had set up MHD pilot plants. Some theoretical modelling has been proposed by a number of research workers (Aspenes, et al, 1978; Dicks, et al, 1974; Pudlik, 1975; Nakamura, 1975) but there is no commercialisation of MHD technology as yet. Recently, the development work on MHD technology has been shelved in India and other countries due to very high auxiliary power consumption by the magnets and very low plant efficiency as a consequence Gas Turbine-Steam Turbine Plants Combined power plants based on gas turbine as topper and steam turbine as bottoming plant are the most promising mode of energy recovery and conservation and is an economically interesting proposition where inexpensive low-ash fuel is available for the gas turbine. HBJ pipeline has provided an excellent opportunity for the installation of combined-cycle plants where low cost natural gas is available for driving the gas turbines. The plants being installed along this pipeline, i.e., Kawas, Gujarat (560 MW), Anta, Rajasthan (560 MW), Auraiya, Uttar Pradesh (380 MW) will be operating on the principle of combined-cycle system (Singal,1987b).

6 6 DESIGN OF COMBINED CYCLE POWER PLANTS There are very many possible arrangements of combined cycle gas turbinesteam turbine power plants and of the attainable outputs and efficiencies (Wunsch, 1988a). There is a possibility to use a variety of fuels, waste heat boilers with and without supplementary firing. Steam cycle may use single-pressure boiler, dualpressure boiler or triple-pressure boiler with feedwater heating by steam from boiler or bleed from turbine. The size of the plant may be from merely 50 MW to more than 500 MW with cycle efficiency of percent or more. The electricity production cost, taking into account the annuity factor of plant cost, fuel cost and operation and maintenance expenses, can be worked out for simple gas turbine plant, steam turbine plant and combined-cycle plants. The decision diagrams will then show at a glance the economic superiority of combined power plants (Wunch,1988b). An existing gas turbine plant can also be converted into combined-cycle plant for achieving higher efficiencies. The combined cycle power plants offer very favourable thermo-economic performance data as compared to conventional steam power plants. There may be reduction in the total plant costs by 10 percent, increase in net efficiency by 6 percent; reduction in construction time by 25 percent, SO 2 removal ratio is reduced from 90 percent to 95 percent and NO X in flue gases from kg/ GJ to kg/gj (Kehlhofer, 1988d). The cooling water requirement may be reduced to half Coal-fired Combined Cycle Plants At present there is a desire to introduce natural gas combined cycles to achieve high cycle efficiencies while meeting environmental standards. But there are relatively limited supplies of gas available. In contrast coal is the most abundant fossil fuel and on a world-wide basis remains the natural choice of the fuel for power generation. There are many advanced coal-fired power generation systems being developed world-wide. A closed-cycle gas turbine plant fired with pulverised coal was installed at Coburg, Germany in 1968 and has been operating successfully (Bammert, 1983). Due to dry working medium (air) the turbomachinery is not subjected to corrosion and erosion. Fouling of heat exchangers is not expected. US Department of Energy has initiated a multi-purpose High Temperature Turbine Technology Programme to develop gas turbines for high temperature duty in dirty gas streams (Wolf, et al, 1983). Fluidised-bed processes represent a significant advance over conventional coal combustion technologies. Coal is burned in a (bubbling) fluidised bed combustor which is enclosed in a pressure shell. Steam is raised to drive a steam turbine while the hot flue gases, after suitable cleaning, can be expanded through a gas turbine. The expected efficiency is in the range of per cent. The gas turbine inlet temperature of C and a standard sub-critical steam bar and 540 C can be used (Minchener, 1994). The combined cycle plant concept with integral coal gasification, as developed by Brown Boveri and Lurgi, demonstrates that it is possible today to generate

7 PERFORMANCE OF POWER PLANTS 7 power from coal economically and with minimum environmental impact (Kehlhofer, 1988d). Since the plant at Cool Water and Dow Chemical, Plaquemine, USA verified the suitability of integrated coal gasification combined cycle (IGCC) several years ago, several large European utilities are now showing interest in this new technology (Becker, et al, 1992) Biomass Fuel for Combined Power Plants Small, decentralised power systems based on locally available biomass fuel can very effectively meet the need for cheaper source of power; need for new capacity addition at low cost and a need for higher and stable load factor for central power plants (Singal, 1986). The biomass-gasifier/gas turbine (BIG/GT) is a progressive technology for biomass electricity generation because of expected high efficiencies and low capital costs at the small scales appropriate for biomass applications. Some biomass gasifiers are undergoing commercial development (Larson,1992). Some successful experimentation work was conducted in Livermore Laboratory, USA for Solar gasification of a mixture of coal and biomass with steam (Gress, et al,1980). In recent years, considerable attention has been given to the development of fluidised bed combustion of biomass in U.K. (Cook, 1984). The pressurised fluidised-bed combustion system gives high combustion efficiency coupled with greater heat transfer coefficients (Reddy, et al, 1991). All the power needs of Indian villages can be met from locally available surplus biomass by using high technology of fluidised bed combustion and combined cycle power plants {Singal, 1988b and 1986) Solar and Nuclear Combined Cycle Power Plants A conceptual design of a combined-cycle with a two-shaft gas turbine and a solar centering receiver system receives air from atmosphere and is compressed and heated to 815 C. It expands through power turbine to generate electricity and exhaust is sent to a steam Rankine cycle via a thermal storage system (El-Wakil, 1985). A combined cycle for nuclear plants presupposes that a high temperature gascooled nuclear reactor (HTGR) is the heat source for the gas turbine cycle. Such a reactor-turbine combination uses helium gas as the reactor coolant and gas turbine cycle working fluid in a closed cycle (El-Wakil, 1985). This seems to be a very attractive proposition but not much published literature is available in this area Thermo-economic Analysis of Combined Power Plants The combination of first law and second law approaches provide a good tool for the analysis of combined-cycle power plants for all loss-sources to be located and quantified. The new generation gas turbines together with multi-pressure steam cycle configuration will make it possible to reach net cycle efficiencies in the range of percent for large combined cycles (Bolland, 1991). For tropical conditions where ambient temperatures may be 40 C, it has been proposed to cool the air to 5 C by installation of an air conditioner. After deducting

8 8 DESIGN OF COMBINED CYCLE POWER PLANTS the power spent in the air condition plant there is a net increase of 20 percent in power output of gas turbine (Jericha, et al, 1991). Unfired combined cycles achieve superior efficiencies at low emission levels. The introduction of supplementary firing permits, within certain limits, an increase in unit output without a reduction in overall plant efficiency. In spite of supplementary firing, there is only negligible change in CO 2. and NO X emissions per kwh of electricity generated, as compared with an unfired combined cycle (Finch, et al, 1992). For medium power and large power outputs, the combined cycle is superior to all other gas turbine cycles. Only for small size plants, a Dual-Recuperated- Intercooled-After-cooled Steam Injected Gas Turbine cycle is able to compete with combined cycle (Bolland, et al, 1995). Combined cycles have demonstrated the highest power generation efficiencies and the lowest cost, in sizes above 50 MW. With decreasing size, the steam turbine represents an increasing portion of the overall cost. Therefore, in smaller sizes/steam injected gas turbines, which do not need a steam turbine, become attractive (Tuzon, 1992). The Kalina cycle based on using a binary working fluid, often ammonia and water, is more efficient than even triple-pressure Rankine cycle as bottoming plant for the gas turbine combined cycle. Optimised three stage Kalina cycle results in almost a two percent increase in efficiency (Marston, et al, 1995). Dynamic simulation of combined cycle plant during design stage can be used in selecting a control philosophy and evaluating safety for various planned and upset operating scenarios (Ahluwalia, et al, 1990). This can be a good design tool for evaluating system response to all anticipated steady-state and transient conditions during operation of the power plant. The combined pinch and exergy approach (Dhole, et al, 1995) and Rational Efficiency of components based on Second-law (Horlock, 1992a) can be used effectively for design improvements in combined cycle power plants. 1.5 COGENERATION PLANTS Cogeneration presents an efficient way of utilizing our limited energy sources because the same fuel is used simultaneously to produce two forms of useful energy, including electricity and heat. Cogeneration also presents a good investment opportunity for the prices of both electricity and heat have been escalating manyfold during the past ten years (David Hu, 1985). There are a lot of publications on cogeneration in recent years and some important papers have been included in the references. But these plants have limited size and application as cogeneration plants as such are not used for base load power generation. Of course, it is a very powerful tool for energy conservation. These can be used very effectively for sea-water desalination plants, district heating plants and for supply of electricity and process heat/steam to various types of industries (Singal, 1988a).

9 PERFORMANCE OF POWER PLANTS 9 Cogeneration systems have a very large potential application in Indian industries. There are more than 200 industries in India consuming more than 25 tonnes per hour of steam where cogeneration plants can be installed (Singal, 1987a). There can be many configurations which can be used for cogeneration plants. The combined cycle plants have an edge over other cycles/plants to supply heat/ steam and electricity to industries at cheaper rates. The research findings of cogeneration combined cycle plants can be used for the design and analysis of combined cycle power plants for electricity generation. 1.6 CONCLUSIONS It is clear from the above literature review that combined cycle power plants have very favourable thermoeconomic performance potential. There can be a very large configurations of combined cycle power plants and there is a need for their proper documentation and nomenclature system. There is a lot of research work on combined cycle plants operating on ash-free liquid and gas fuels. For Indian situation we need to take up a special development programme for combined cycle power plants which can be run on low-quality high-ash lignite and abundantly available thorium. A large amount of work has been reported on cycle and parametric analysis of combined cycles as applicable to small cogeneration plants and medium capacity peak load power plants. There is but scanty information on design and operating experience with large base load utility plants. Similarly, little information and research work have been reported on economic aspects of these types of plants. It is strange to note that combined cycle power plants coupled to nuclear reactors have very high promise but no work is published in this area. Therefore, there is a need for research and development in the above areas. qqq