Birleşik ısı-güç üretim santralları

Abstract

Enerji gereksiniminin sürekli olarak artması ve buna karşılık, ham enerji kaynaklarının da hızla tükenmesi, günümüzde, yüksek verimli enerji dönüşüm çevrimlerinin geliştirilmesinin önemini artırmıştır. Kullanılabilir elektrik ve ısı enerjisinin, birlikte, eş zamanlı olarak üretimi biçiminde tanımlanabilen Birleşik Isı-Güç Üretimi (B.I.G.Ü.) sayesinde, % 85-90 düzeyinde bir toplam verime ulaşılabilmekte ve böylece ham enerji kaynaklarının kullanımında büyük bir tasarruf sağlanmaktadır. Bu tezde, bu önemi nedeni ile, endüstri tesislerinin ve toplu yerleşimlerin elektrik ve ısı enerjisi geretamimlerinin karşılanmasında yaygın olarak kullanılan B.I.G.Ü. Sistemleri incelenmiştir. Değişik tip B.I.G.Ü. çevrimleri, ayrıntılı olarak ele alınmış ve birbirlerine olan üstünlükleri, nerede hangi tip çevrimin kullanılması gerektiği maliyetler de dikkate alınarak saptanmıştır. Bu amaçla, Microsoft Excel paket programından faydalanılarak ele alman örnek bir tesis üzerinde, ekonomik incelemeler yapılmıştır. Ayrıca, bir B.I.G.Ü. Sistemi'nin planlanmasında kullanılacak teknik ve ekonomik kriterler verildikten sonra, optimum çevrim kapasitesi ve türünün saptanmasına ilişkin tesis ve üretim maliyet denklemleri oluşturulmuş ve bir çevrim örneği için, optimum güç, Mathcad paket programı yardımıyla, grafik yöntem kullanılarak hesaplanmıştır.

Over the last century, the demand for energy in all over the world has increased considerably, while the amount of primary energy sources have decrased dramatically. Therefore, the world will have a big difficulty for meeting its own energy demand in the future. This matter forces the engineers to develop highly efficient energy convertion cycles. Combined Heat and Power Generation (CHPG) or Cogeneration, which could be defined as the coincident production of both mechanical or electrical power and other forms of useful energy, like heat or process steam, from the same facility, is a power producing cycle contributing to this purpose with its fuel saving aspect. Altough industrial cogeneration has been used in large industrial facilities like pulp and paper, textile, petroleum, chemical and food industries from the beginning of the last century, with the rapid changes in energy prices in the 1970s, the fuel-conserving aspect of cogeneration became a major driver for the increased interest in this technology. Today, industrial power, produced by cogeneration, competes with utility power generation, especially in developed countries. While this technology has been used in industry since 1970s in our country, it has spread rapidly with new industrial and district heating applications over the last years. In a classical system, an industrial company, which needs both electricity and process heat, provides its electricity demand from the electricity utility and steam demand from a steam boiler found in its own plant area. On the other hand, electricity is usually generated in utility thermal power plants which have approximately 35 % efficiency. About 50 % of the thermal energy of fuel is rejected to the atmosphere by condenser in steam-turbine power plants and by exhaust gasses in gas-turbine power plants. This loss is the result of the technology used today to generate electricity in thermal power plants. Whereas, the exhaust energy that remains after the production of shaft power is of sufficiently high quality to be useful in thermal heat processes. When the exhaust energy is used in a process, overall efficiency could reach up to 85-90 % and significant amount of fuel savings could be obtained. Moreover, since less fuel is used to produce same amount of energy by this method of generation called CHPG, the environment is less damaged compared to the classical or separate generation method. From an energy source point of view, cogeneration is benefical only if it saves primary energy when compared with separate generation of electricity and steam (or heat). The cogeneration plant efficiency r^ is given by E + AHS where E = electric energy generated AHS = heat energy, or heat energy in process steam = (enthalpy of steam entering the process) - (enthalpy of process condensate returning to plant) Qa = heat added to plant (in coal, natural gas, etc.) For separate generation of electricity and steam, the heat added per unit total energy output is e t(l-e) "He % E where e = electrical fraction of total energy output = - (E + AHS) Tje = electric plant efficiency rjh = steam (or heat) generator efficiency The combined efficiency rjc for separate generation is therefore given by 1 Tlc = (e/rje)+(l-e)/r,h and cogeneration is benefical if the efficiency of the cogeneration plant (r}co), exceeds that of separate generation (tjc). In evaluating and comparing alternative cogeneration cycles, two concepts, Net Heat to Process (NHP) and Fuel Chargeable to Power (FCP), are key. Neat Heat to Process is defined as the net energy supplied by the cogeneration system to the process load, and it is necessary to maintain a constant NHP for all systems being considered and compared. Fuel Chargeable to Power is defined as the incremental fuel for the cogeneration sysem relative to the fuel needs of a heat only system divided by the net incremental power produced by the cogeneration system. These concepts have been discussed in detail in Chapter 2. Furthermore, a brief review of thermodynamics as applied to power-turbine cycles has been included in this chapter. VI Two types of cogeneration facilities are distinguished according to the types of cycles. These are topping cogeneration cycle and bottoming cogeneration cycle. The topping cycle is one where the primary focus is on the electrical energy demand to be satisfied. The fuel is primarily used to generate electricity while any excess heat, available from the electric production process, is used in any industrial plant process which also requires heat. On the other hand, the bottoming cycle is one where the primary focus is on the thermal energy demand to be satisfied. The fuel is primarily used to generate heat for an industrial plant process. Any excess heat, available from the industrial plant process, is used to generate electricity. Since the topping cycle is more commanly used in industry, the focus of this thesis will be on this type of cycle. However, there are four broad categories of cogeneration cycle in types of power generator equipment. These are steam-turbine cycle, gas-turbine cycle, steam and gas-turbine combined cycle and gas-Diesel engine cycle that each has its own merits and demerits. For instance, steam-turbine cogeneration cycles can be applied when a low-cost fuel is available such as coal, wood chips, refuse and so on, but, power -to- heat ratio, which is defined as the ratio of power produced to NHP, can not usually exceed about 80,6 kW/109 J in steam-turbine cycles. This is generally less power than that required to satisfy most industrial plant electrical energy needs. Thus, a purchased power tie or additional condensing steam-turbine capacity for power generation is necessary to provide the balance of the industrial plant power needs. However, the use of condensing steam-turbine cycle results in much higher FCP costs, which opposes to the aim of cogeneration. Although gas-turbine cycles provide higher power -to- heat ratios than do steam-turbine cycles at favorable fuel chargeable to power, and also have a lower capital cost than do steam-turbine cycles on a $/kW basis they use high quality fuels like natural gas and oil. Therefore, gas turbine cycles are typically applied when the power -to- heat ratio is greater than 80,6, and when natural gas or oil is an available and economical fuel. Combined cycles, consisting of gas-turbine and steam turbine cycles, are used in industrial plants having relatively high electricity demand and in utility plant applications. This type of cycle is also used when selling the electricity to utility is possible and economical. Gas and Diesel engine type cogeneration is used generally in hospital, hotel and commercial sector applications which have less electricity and heat demands compared to industrial applications. However, this type of cogeneration requires high quality and expensive fuels. The advantages and drawbacks of the cycles mentioned above, and which type of cycle should be used in an application have been set forth in Chaper 3. In addition, for a numerical example, performance calculations of a vıı steam-turbine cogeneration cycle have been done, key criterians like NHP, FCP and power -to- heat ratio have been found, and power -to- heat ratios versus turbine throttle conditions and process steam conditions have been plotted in graphs. The costs of cycles, except for gas and Diesel engine cycle, have also been presented in tables in Chapter 3. The economic evaluation techniques used to evaluate a cogeneration investment have been discussed briefly, and adventages and disadventages of each technique have been given in Chapter 4. Economics of cogeneration have also been included in detail in this chapter, and technical and economical data of an industrial plant have been used for this study. This industrial facility has several options for providing both steam and electricity. It can: 1. Install a gas-fired ambient-air boiler for the steam needs of the plant and purchase 40 MW of electricity from the utility. 2. Install a coal-fired cogeneration system sized to provide steam needs with any supplementary electricity purchased from the utility. 3. Install a simple-cycle gas turbine and Heat Recovery Steam Generator (HRSG) to supply steam needs with any supplementary electricity purchased from the utility. 4. Install a combined-cycle gas turbine, HRSG, and steam turbine sized to provide all of the steam needs and more electricity than the facilities demand, and to sell exceeding amount of electricity to the utility. Here, the economics studiy has been made in Microsoft Excel by using costs of cycles given in Chapter 3. By taking the first case as a referance, it has revealed that the third alternative is the most economical. Same study has been done for the case that the industrial facility already has a steam boiler and equipment, and the third alternative has been found most economical also in this case. Moreover, the graphics showing the change in investment pay-back years versus electric rate, electric buyback rate and natural gas fuel cost have been ptotted. Technical and economical criterions used in planning a cogeneration system have been discussed in Chapter 7. Some sort of preventations should be taken to provide the cogeneration system to produce reliable electricity and steam. In addition to this, long-term steam, electricity and fuel price contracts should be signed to decrease the risk of investment. The details of criterions and preventations have been included in this chapter. The study for determining the optimum capacity and type of cogeneration for an industrial facility has been done in Chapter 6. The equations of capital and production costs have been formed for different five alternatives, which are classical systems having a steam boiler and buying electricity from the utility, steam-turbine cycle, gas-turbine simple cycle, gas V111 and steam-turbine combined cycle, and gas-Diesel engine cycle. While preparing these equations, it was assumed that the industrial facilities electricity and heat demands change in time in a day. The period of a day studied has been divided into little periods to show the case mentioned above. Thus, the continuous change has been converted to the step by step change by assuming that the demands are constant in a little period and change from a period to another one. However, equations have been simplified by assuming that the demands of a day are same for all days in a year. It has been thought that the incremental capital costs of each cycles to the classical system are paid back by incremental production cost adventages obtained by these cycles. When the equations prepared according to this assumption are solved by suitable mathematical methods, the capacity values and operating schedules of cycles which give the minimum pay-back years will be obtained. After that, the pay-back years values of the cycle having the minimum pay-back years is compared, with market conditions, and then, the decision for investment is made according to this comparison. In this thesis, only the solution for gas-turbine simple cycle has been made, since sufficient technical and economical data of other cycles are not in hand. In the solution, for simplicity, it was assumed that the cycle produces power constantly in related value. The optimum power value giving the minimum pay-bak years has been determined by graphic method using Mathcad program. What's more, how the optimum power and the minimum pay-back years values change with utilities electric rate and buyback rate, and natural gas price, have been presented in graphics. When sufficient data are obtained, same study can be made for other cycles. The recommendations about the conditions in which a cogeneration cycle can be installed, and general conclusions of the thesis have been given in Chapter 7.