نبذة مختصرة : Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2012
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2012
Gazlaştırma temel olarak karbon içeren kömür, petrol, biyokütle ve katı atıklar gibi hidrokarbonlu yakıtların kısmi olarak oksidasyonu ile CO, H2, CO2 ve CH4 gibi gazlara dönüştürülmesi işlemidir. Gazlaştırma işlemi gazlaştrıcı adı verilen reaktörlerin içerisinde gerçekleştirilir ve ürün olarak oluşan gaz karışımına sentez gazı denilmektedir. Katı yakıt ve katı atık esaslı elektrik üretim teknolojileri arasında gazlaştırma prosesi, en düşük emisyona, katı ve sıvı atık değerlerine sahip temiz ve çevreci bir teknoloji olarak öne çıkmaktadır. Gazlaştırma işleminde yakma teknolojilerine göre daha az miktarda CO2, SO2 ve NOx oluşumu gerçekleşmektedir. Gazlaştırma teknolojileri dünya genelinde yaklaşık bir yüzyıldan beri ticari olarak hem sıvı yakıt üretimi hem de çeşitli kimyasalların üretilmesinde kullanılmaktadır. Gazlaştırma işlemi oksidant olarak hava, oksijen, su buharı ya da bunların çeşitli oranlarda karışımı kullanılarak; basınçlı veya atmosferik olarak sabit yatakta, kabarcıklı akışkan yatakta, dolaşımlı akışkan yatakta ya da sürüklemeli akışlı gazlaştırıcılarda gerçekleştirilebilmektedir. Dünya genelinde ticari ismiyle anılan bir çok gazlaştırıcı sistem bulunmaktadır. Gazlaştırma işleminde proses seçimi kullanılacak olan yakıt tipine, özelliklerine ve nihai elde edilecek sentez gazından hangi ürünün üretileceği göz önüne alınarak belirlenmektedir. Yakma sistemleri stokiyometrik hava miktarının üzerinde çalıştırılırken; gazlaştırma sistemleri stokiyometrik hava ihtiyacının altında çalıştırılmaktadır. Gazlaştırma proseslerinde gerekli oksijen miktarı genellikle teorik yanma için gerekli olan oksijen miktarının % 35’i ya da daha azı olarak kontrol edilmektedir. Bu tez çalışmasında gazlaştırma prosesi ve bu prosesin tarihsel gelişimi, uygulamaları hakkında bilgi verilmiş daha sonra ise sürüklemeli akışlı ve kabarcıklı akışkan yataklı gazlaştırma sistemlerinin Aspen HYSYS programı kullanarak benzeşimi yapılmıştır. Termodinamik denge prensibine göre çalışan benzeşim çalışmalarının geçerliliği araştırılmış ve düzeltilmiş denge sıcaklıklarında termodinamik denge modelinin deneysel çalışmalarla daha uyumlu sonuçlar verdiği görülmüştür. Türk kömür ve biyokütle örneklerinin gazlaştırma prosesi ile değerlendirilmesi ve işletme şartlarının elde edilecek gaz bileşimine etkisini araştırmak için termodinamik denge prensibine dayalı stokiyometrik olmayan doğrulaması literatürdeki verilerle karşılaştırılarak yapılmış olan düzeltilmiş model kullanılmıştır. Doğrulaması yapılmış modelde Soma kömürü ve Soma kömürüne çeşitli oranlarda ilave edilmiş fındık kabuğu ve fındık kabuğunun gazlaştırma prosesinde kullanılması durumunda elde edilebilecek sentez gazı bileşimi incelenmiştir. Termodinamik denge modelinin sürüklemeli akışlı gazlaştırıcılardan elde edilen gaz bileşimini tahminde büyük doğrulukta olduğu görülmüştür. Daha düşük sıcaklıklarda çalışan akışkan yataklı gazlaştırıcı sistemlerinin ise termodinamik denge koşullarından saptığı ve düzeltilmiş denge modellerinden elde edilen sonuçların daha doğru sonuçlar verdiği görülmüştür. Bu tip gazlaştırıcılarda nihai gaz bileşiminin tayininde karbon dönüşüm oranı ve düzeltilmiş denge sıcaklıklıklarının gaz bileşiminin doğru tahmininde etkili paremetreler olduğu belirlenmiştir. Kullanılan paket programından elde edilen sonuçlarda oksijen ortamında gerçekleştirilen gazlaştırma işleminde yüksek ısıl değerli sentez gazının elde edilebileceği görülmüş, ilave su buharının gaz bileşiminde etkili olan bir diğer paremetre olduğu ve hidrojen miktarını arttırıcı yönde etkide bulunduğu, artan hava yakıt oranın soğuk gaz veriminin düşmesine neden olduğu belirlenmiştir.
In the contex of climate change, efficiency and energy security, gasification is likely to play an important role. Gasification is a process of converting carbonecous materials such as coal, oil and biomass to combustible gases including CO, H2, CO2, and CH4 with partial oxidation. Gasification process takes place in reactors which are called gassifiers and the product realized in these gasifiers is called syngas. Among solid waste-based electricity generation process technologies, gasification process has becomes prominent as a clean and environmentally friendly technology for its low emissions, low solid and liquid waste values. Compared with combustion technologies, in gasification process, less CO2, SO2 and NOx is produced. Gasification technologies have been used in commercial plants to produce not only liquid fuels but also miscellaneous chemicals and combustible gaseous products for almost one century. In gasification process air, steam, pure oxygen, carbon dioxide or a mixture of these gases with diffrent ratios can be used as gasificating agent in athmospheric or pressurized; fixed beds, fluidized beds, or entrained flow gasifiers. Globally there are different type gasifiers which are known their commercial names. In contrast to combustion processes, which work with excess air, gasification processes operate at substoichiometric conditions. The oxygen supply controlled generally 35 percent of the amount of O2 theoretically required for complete combustion. Different kind of fuels can be used in gasifers to produce synthes gas. Biomass , fuel derived from organic matter on renewable basis, is among the largest sources of energy in the world, third only to coal and oil. Biomass which adsorbs CO2 from the athmosphere during photosynthesis, and the CO2 is then returned to atmosphere after combustion, is known as a CO2 neutral energy source. The widespread availability of biomass has been widely rocognised, as has its potential to supply much larger amounts of useful energy with fewer environmental impacts than fossil fuels. On the other hand the gas produced can be standardised in its quality and is easier and more versatile to use than the original biomass or coal. For example produced gas can be used to power gas engines and gas turbines or as achemical feedstock for production of liqued fuels. The performance of gasifiers could be characterised by several paremeters. Producer-gas composition and gasification efficiency are both of them. The composition of gas obtained from a gasifier depends on miscelleneous parameters, such as fuel composition, gasificating agent, operating pressure, temperature, moisture content of the fuels, fly ash recircletion, second air addition, gasifier design. Because of this complexity it is very difficult to predict the exact composition of gas from a gasifier. There are different type gasification models to predict gas composition from gasifiers. The main goals of these models are to study the thermochemical processes during the gasification of the biomass and evaluate the influence of the main input variables, such as moisture content, air/fuel ratio, temperature, producer - gas composition, pressure and the calorific value of the producer gas. Some studies only consider the final composition of chemical equilibrium, while others take into account the different processes along the gasifier, distinguishing zones. The models can be classified as kinetic rate models, thermodynamic eqilibrium models compotutional fluid dynamic models and neural network models. Kinetic models provide essential information on kinetic mechanisms to describe the conversion during coal and biomass gasification, which is crucial in designing and improving of gasifiers. Kinetic rate models describe the char reduction process using kinetic rate expressions obtained from experiments and permit better simulation of the of the experimental data where the residence time of gas and fuel is relatively short. Kinetic models always contain parameters that limit their applicability to different type plants. On the other hand thermodynamic equilibrium models are independent of gasifier design and they are more suitable for process studies. Equilibrium models have two general approaches; stoichiometric and non- stoichiometric models. The stoichiometric approach requieres a clearly defined reaction mechanism that incorparates all chemical reactions and species involved. On the contrary in the non-stoichiometric approach , no particular reaction mechanisms or species are involved in the numerical simulation. The only input needed to specify the feed is its elemental composition ,which can be obtained from elemental analyses. Neural network gasification modeling is quite new aproach and it requires experimental data to use. Aspen HYSYS progam makes model creation and updating easier since small sections of complex and integrated systems can be tasted modules by module. Program is a process modelling tool for concepttuel design, optimization, businnes planning and performance monitoring for oil and gas production, gas processing industries. The process simulator is equipped with a large property data bank which contains various stream properties requiered to model the material streams in a gasification plant with allowance for addition of in-house property data. In this thesis study gasification process and the historical backround of gasification processes and its applications are informed firstly. And then gasificatin process simulation models of an entrained fluidized, bubbling and circulating fluidized bed gasifiers are done with the Aspen HYSYS program which has a strong thermodynamic backround. The thermodynamic models are found to be a useful tool for preiminary comprasion and for process studies on the influence of the miscelleneous fuels and process parameters. The simulation program predict the gas product obtained from gasifier systems with non stoichiometric equilibrium model based on minimization of Gibbs free energy. The validation of the simulation process is done with the experimental and simulation results which obtained published data from in literature. At high temperatures gasification results obtained from simalatiıon program simulate the process at high accuracy. At lower tempratures process deviates from pure chemical equilibrum model. On the other hand when the carbon conversion rate is applied and quassi state tempretures used, results becomes acceptable accuracy . By this validation in last chapter validated simulation model is used for Soma coal and hazelnut shells’s gasification simulation case. A circulating fluidized bed system is simulated with Aspen HYSYS program. Effect of fuel charecteristic and operation parameters on gas product are investigated. The results shows that prediction capability of chemical equilibrium models depend on the hydrodynamic structure inside the gasifiers, operating conditions and fuel chracteristic. But simulation tool gives aproximate results to find the best operating point and the effect of the operation conditions of gasifying system. By this respect it can be seen from results that using O2 as gasification agent provokes the lower heeating value of syngas.The reason of it nitrogen in air dilutes the syngas and by this way heating value of produced gas decreases. İn addition to this adding water vapour increases the quality of product gas by increasing the H2 content in syngas. The optimum range to pruduce high content H2 is found between 0.2 -0.3 equivalance ratio. Pressure is also an positive effect on CH4 and H2O content in product gas. Cold gas efficiecy of soma lignite is higher than hazelnut shells cold gas efficiency. At the same time increasing equivalance ratio decreases the cold gas efficiency both of them.On the other hand gas obtained from hazelnut shells’ has lower H2S emissions. The equilibrium models are quite appliciable to see the capibility of different fuels in gasifying systems. Adding carbon conversion rate and applying quassi state temperatures increase the accuracy of model. To increase the sensitivity of model, tars properties can be added to system with the external codes and Aspen HYSYS program can be worked with that external code together.
Yüksek Lisans
M.Sc.
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