Buhar çevrimli kojenerasyon sistemlerinin termoekonomik optimizasyonu
Thermoeconomic optimization of steam cycle cogeneration systems
Yazar: SEHER METE
Danışman: PROF. DR. TANER DERBENTLİ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2012
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Bileşik ısı güç santrali = Combined heat power plant ; Ekserji = Exergy ; Kömür yanması = Coal combustion ; Termoekonomi = Thermoeconomy

Özet:
Son yıllarda artan enerji talebi fosil kaynaklı yakıtların tükenme hızında artışa sebep olmuştur. Tüketimle beraber miktarları giderek azalan yakıtların fiyatları artmaya başlamıştır. Bu nedenle mevcut enerji kaynaklarının verimli kullanımına yönelik yapılan çalışmalar ağırlık kazanmıştır. Öte yandan CO2 başta olmak üzere yakıt emisyonlarını önlemek için çeşitli teknolojiler geliştirilmiş ve uygulanmaya başlanmıştır. Fosil yakıtlardan kaynaklanan bu emisyonların azaltılmasına yönelik çalışmalar günümüzde de yoğun ilgi çeken bir çalışma alanıdır. Bu çalışmalarla enerjiye duyulan sürekli ihtiyacın olumsuz sonuçlarının önüne geçilmeye çalışılmaktadır. İşletmelerin veya bireylerin günlük hayatta en çok ihtiyaç duydukları enerji çeşitleri ısı ve elektriktir. Bu iki enerji çeşidini tek bir kaynağı kullanarak eş zamanlı olarak üreten bileşik ısı güç üretim sistemlerinin (kojenerasyon sistemleri) kullanımı yaygın hale gelmiştir. Bu sistemlerin amacına uygun olarak seçilmesi ve kurulması yakıt ve para tasarrufunun miktarını belirleyecektir. Bu durumda işletme esnasında ortaya çıkabilecek yakıtın aşırı değer kazanması, kapasite faktörünün değişmesi gibi durumlara sistemin nasıl karşılık vereceğini bir başka deyişle maliyetlerin bu tür değişimlerden nasıl etkileneceğini bilmek gerekir. Bu bakımdan termoekonomik incelemeler ve ekserji analizleri önemlidir. Bu tezde sistemin her bir akışı için ekserjiler farklı türbin ve kazan verimleri için hesaplanmış, sistem için maliyet denklemleri oluşturulup çözülmüştür. Ayrıca termoekonomik inceleme ile sistem ekserji verimliliği açısından değerlendirilmiştir. Kojenerasyon santralinde, ekserji kırımının diğer elemanlara göre daha fazla olduğu yerler bulunmuştur. Sistemin tüm elemanları için eksergoekonomik faktör bulunmuş ve toplam eleman maliyetlerinde ekserji kırımlarının ve yatırım maliyetlerinin payları incelenmiştir. Yapılan eksergoekonomik incelemeler sonucunda elektriğin maliyeti 0,12 TL/kWh olarak bulunmuştur. Bölgesel ısıtmaya gidecek olan sıcak suyun maliyeti ise 0,49 TL/ton olarak hesaplanmıştır. Kazanın ve türbinin maliyetlerinin bu elemanların verimlerine göre değişimi bulunmuş ve elektrik maliyetini en düşük seviyeye getirecek kazan ve türbin verimleri elde edilmiştir. Buna göre kazan verimi % 86 değerindeyken elektrik maliyeti minimum olurken türbin için bu değer %84 olmaktadır.Çalışmada ayrıca bir değere getirilmiş maliyet yöntemiyle birim enerji maliyeti bulunmuştur. Bu yöntem kullanılarak yapılan çözümlemeler ile de elektrik maliyeti 0,12 TL/kWh olarak bulunmuştur. Bu yöntemde maliyetler yatırım maliyeti, yakıt maliyeti ve bakım onarım maliyeti olarak 3 bileşene ayrılmış ve birim enerji maliyeti üzerindeki etkisi en fazla olan bileşenin yakıt maliyeti olduğu gözlemlenmiştir.Çalışmada geri ödeme süresi yöntemi de uygulanmış buna göre sistemin kendini 1,6 yılda geri ödediği bulunmuştur. Bu süreye bakıldığında da sistemin kurulmasının avantajlı olacağına karar verilmiştir.

Abstract:
For centuries mankind used different forms of energies. This requirement is growing rapidly with the technological developments. Electricity is one form of energy which is highly demanded and power plants are operating all over the world for meeting this demand. These power plants use fossil fuels to a great extent, however engineers are also trying to develop new power generation techniques which utilize renewable sources.In the last few decades, increasing energy demand caused a rise in the consumption rates of fossil fuels. In turn the prices of fossil fuels increased. As a result research and development on the efficient use of energy sources gained importance.On the other hand, global warming is an important current issue which must be considered. International aggreements have been signed to limit the CO2 emisssions which is seen as the main cause of global warming. Therefore the effective use of fossil fuels is important from this perspective also.Heat and electricity are two forms of energy which are used concurrently in human dwellings and industrial plants. Combined heat and power generation systems, in other words cogeneration systems provide thermal and electrical energies simultaneously, with less fuel use.The thermodynamic and economic gain that can be obtained from a cogeneration system is dependent on choosing and sizing the system correctly. Therefore the thermal and electrical energy requirements of the plant or district must be calculated properly and a cogeneration system that satisfies the demand must be selected. The response of the system to fuel costs and changes in demand must be calculated. The thermodynamic and exergetic analysis gain importance in these stages of analysis.In this study, a cogeneration system which uses lignite coal as fuel and steam turbine as the prime mover for generating electricity has been considered. Heat obtained from the condensing steam is utilized for district heating. As is known a significant part of the Turkish lignites have low calorific values and also they contain high percentages of nitrogen and sulphur which will cause production of NOx and SOx during combustion. Furthermore CO2 emissions present in all fossil fuel burning systems is also a problem. Therefore as a part of this study, clean power generation systems and carbon capture and storage technologies were investigated.Fluidized bed combustion is one of the methods of burning high sulphur lignites effectively. Not only is the combustion process is enhanced by better mixing and turbulence but also the limestone fed into the combustion chamber helps to reduce the SOx emissions by forming sulfates.Thermoeconomic analysis of a cogeneration system forms the backbone of this study. Therefore a review of equations and terminology related to exergy and thermoeconomics was made in Chapter 4 of the thesis. Exergy concept is related to the maximum of work that a system can do in a given environment. In contrast to energy, exergy may be destroyed during a process. This provides a measure that can be used to improve a process. Exergy may also be used to determine the costs of thermal and mechanical flows in a process. Costing on an exergy basis is gaining more acceptance among the engineers compared to energy costing.The steam cycle cogeneration system which is studied in this work has nine main components. These are a boiler, a back pressure steam turbine, a district heating heat exchanger, an economizer for heating the inlet air with the exhaust gases of the the boiler , a feed water heater, low and high pressure condensate pumps, a pressure reducing valve and a generator for converting mechanical energy to electricity. Superheated steam at 6 MPa and 4500 C is produced in a boiler by burning lignite coal. After leaving the boiler, steam enters a back pressure turbine where it expands to 150 kPa, producing work. In the district heating heat exchanger steam condenses, giving its energy to water which is circulating in the district heating cycle. Feedwater leaves the heat exchanger as saturated liquid. It is then pumped to the feedwater heater and subsequently to the boiler. Steam extracted from the turbine at 500 kPa is sent to the feedwater heater.The first law of thermodynamics was applied to each component of the system to determine the mass flow rates, properties of the streams, the power produced and the heat transferred to the district heating loop. The second law of thermodynamics was applied in the form of entropy balance equation to calculate the entropies and exergies of the streams and the exergy destruction in the components. Cost balance equation was applied to all components of the system and in combination with the boundary conditions and the auxiliary equations a system of linear equations was formed. Solution of this set of linear equations yielded the cost flow rates of the streams and the costs of electricity and the district heating water which are the two main products of the cogeneration system.A computer program was prepared for making the calculations stated above. The program was used to investigate the changes in the cost of the products according to changes in various system parameters such as the boiler and turbine efficiencies. The results were presented in graphical form. For a boiler efficiency of %87 and turbine efficiency of %90, the cost of generating electricity for this system was found as 0,12 TL/kWh. The cost of thermal energy that goes to district heating was found 0,49 TL/tone.The exergy destructions in the components and costs per unit exergy of all the internal streams were also determined. Thus it was able to make a comparison between various components with respect to irreversibilities. Exergoeconomic factor values were used to pinpoint the sources of irreversibilities and where to start the improvements in the system. For the cogeneration system considered, the boiler exergy destruction is higher than the other components. This is mainly due to combustion which is a highly irreversible process and heat transfer at finite temperature differences ocuurring in the boiler.Parametric studies were done by changing the boiler and turbine efficiencies. The costs of tubine and boiler were found according to these system element?s efficiencies. Unit cost of electricity was found according to changing turbine and boiler efficiencies. Cost of electricity has been minimum at the %86 boiler efficiency and % 84 turbine efficiency.In addition to calculation of costs of electricity and heat by the exergoeconomic approach, a purely economic method was used to find the levelized life cycle costs of electricity. In this method the cost of the commodity is divided into three parts. These are the cost resulting from capital investment, cost resulting from fuel expenditure and cost accruing from the operation and maintenance. The cost of electricity by this approach was also found as 0,12 TL/kWh. This computation showed that the contribution of capital, fuel and operation and maintenance to the total cost of electricity was %37, %49 and %14 respectively.Another approximate but widely used parameter for determining the feasibility of power generation systems is the pay back period. The pay back period for the system considered in this study was found as 1,6 years, which is an acceptable value.The CO2 emissions of the cogeneration system considered is 27537030 kg per year. This is a %36 reduction as to seperate production of heat and electricity.It should be again emphasized that the literature review conducted and the conclusions based on the results of this study show that the heat and electricity demands must be evaluated carefully before choosing the cogeneration system. It can be concluded that coal burning cogeneration systems are feasible alternatives for providing the thermal and electrical energy needs of group of apartment buildings, housing areas and small cities.