Kabarcıklı akışkan yatakta biyokütle gazlaştırma işleminin incelenmesi
Investigation of biomass gasification process in a bubbling fluidized bed
Yazar: MAHMUT CANER ACAR
Danışman: PROF. DR. YAKUP ERHAN BÖKE
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Makine Mühendisliği Bilim Dalı
Derece: Doktora, 2019
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Bu doktora tezinde kabarcıklı bir akışkan yatakta biyokütle gazlaştırma işlemi sırasında gerçekleşen hidrodinamik ve ısıl olaylar için hesaplamalı akışkanlar dinamiği yöntemi kullanılarak üç boyutlu ve zamana bağlı modelleme çalışmaları yapılmıştır. Tezin bir bölümünde de kimyasal reaksiyonları içeren simülasyon çalışmaları gerçekleştirilmiştir. Modelleme çalışmalarında Ansys Fluent programı, simülasyon çalışmalarında ise Aspen Plus programı kullanılmıştır. Modelleme çalışmaları için TÜBİTAK Marmara Araştırma Merkezi'nde bulunan bir kabarcıklı akışkan yatak reaktörü esas alınmış ve bu reaktörün tasarım ölçüleri dikkate alınarak model geometrisi oluşturulmuştur. Simülasyon çalışması kapsamında üç farklı simülasyon modeli geliştirilmiştir. Geliştirilen her bir simülasyon öncelikli olarak deneysel çalışmalarla doğrulanmıştır. Daha sonra, doğruluğu yapılmış simülasyonlar kullanılarak gazlaştırma performansına etki eden parametreler kapsamlı bir şekilde incelenmiştir.
Hidrodinamik ve ısıl model çalışmalarında Euler-Euler iki akışkan modeli kullanılmıştır. Bu modelde, gazlar birincil faz, katı tanecikleri ise ikincil faz olarak ele alınmaktadır. Gaz-katı arasındaki momentum değişimi ve ısı transferi için sırasıyla, Gidaspow ve Gunn modelleri kullanılmıştır. Ayrıca, ışınımla olan ısı transferi için de P1 ışınım modeli tercih edilmiştir. Türbülans etkileri için ise k-ε ayrık (dispersed) modeli seçilmiştir.
Modelleme çalışmaları kapsamında ilk olarak akışkan yatak gazlaştırıcı içindeki hidrodinamik olaylar incelenmiştir. Bu kapsamda sırasıyla, model geometrisi oluşturma, ağ atama ve model kurulumu ve çözümleme işlemleri yapılmıştır. Model geometrisi oluşturulduktan sonra ağ atama işlemi gerçekleştirilmiş ve farklı eleman sayılarından oluşan üç farklı ağ (kaba, orta ve ince ağ) oluşturulmuştur. Modelin ağa bağımlılığını ortadan kaldırmak amacıyla, aynı başlangıç ve sınır koşulları için bu üç farklı ağ ile çözümlemeler gerçekleştirilmiş ve model çalışmaları için orta ağın kullanılmasına karar verilmiştir. Daha sonra, zaman tasarrufu açısından orta ağın eleman sayısını daha da düşürmeye yönelik ek bir çalışma yürütülmüş ve eleman sayısı yaklaşık %71 oranında azaltılmıştır.
Hidrodinamik model çalışmasına kabarcıklı akışkan yatak için en uygun bir akışkanlaşma hızının belirlenmesi ile başlanmıştır. Bu hız aynı zamanda yatağın boş durumundaki hızını (superficial gas velocity) ifade etmektedir. Katı taneciklerinin yatak içinde yer değiştirebilmesi ve kabarcıkların oluşabilmesi için boş yatak gaz hızının minimum akışkanlaşma hızından (Umf) büyük olması gerekmektedir. Bu nedenle hidrodinamik model çalışmalarında öncelik olarak, uygun bir akışkanlaşma hızının belirlenmesi için bir çalışma yürütülmüş ve yatak içindeki en uygun akışkanlaşma, boş yatak gaz hızının minimum akışkanlaşma hızından 3 kat daha büyük (3Umf) olduğu durum için elde edilmiştir. Uygun akışkanlaşma hızının belirlenmesinin ardından bu hız değeri için reaktör içerisindeki zamana bağlı hidrodinamik değişimler incelenmiştir.
Isıl model çalışmalarında kullanılmak amacıyla, akışkana ait yoğunluk, ısı iletim katsayısı, özgül ısı ve dinamik viskozite gibi termo-fiziksel özellikler için sıcaklığa bağlı denklemler türetilmiş ve bu denklemler Fluent programına tanıtılarak akışkan özelliklerinin sıcaklığa bağlı değişimleri de göz önüne alınmıştır. Isıl model için kurulum işleminin yapılmasının ardından çözümlemelere başlanmış ve bu kapsamda reaktör içindeki sıcaklığın zamana ve konuma göre değişimi ve sıcaklığın yatak hidrodinamiği üzerindeki etkileri incelenmiştir. Isıl model çalışmaları iki bölümde ele alınmıştır. Birinci bölümde, akışkanlaşma hızı olarak hidrodinamik çalışmalar kapsamında belirlenen boş yatak gaz hızı (3Umf) kullanılmıştır. İkinci bölümde ise model geometrisi oluşturulurken esas alınan deneysel çalışmadaki hava ve buhar için kullanılan hızlar dikkate alınmıştır.
Simülasyon çalışmasında, literatürde gazlaştırma işlemleri için sıklıkla kullanılan Aspen (Advanced System for Process Engineering) Plus paket programı tercih edilmiş ve Gibbs serbest enerjisinin minimizasyonuna dayanan kimyasal (termodinamik) denge modeli ve sınırlı kimyasal denge yöntemi göz önüne alınmıştır. Burada, bir KAYG'de gerçekleşen biyokütle gazlaştırma işlemi için üç farklı simülasyon geliştirilmiştir. Geliştirilen simülasyonlardan elde edilen sonuçlar ilk önce deneysel verilerle karşılaştırılarak model doğrulama işlemi yapılmıştır. Model doğrulama çalışmasının ardından gazlaştırma sıcaklığı, buhar/biyokütle oranı, eşdeğerlik oranı (equivalence ratio) ve biyokütle nem içeriği gibi çalışma parametrelerinin ürün gazı kompozisyonu, H2/CO oranı, ürün gazı alt ısıl değeri ve soğuk gaz verimi üzerindeki etkileri incelenmiştir. Parametrik çalışmalar ile elde edilen sonuçlardan yola çıkılarak, bir KAYG için en uygun çalışma koşulları belirlenmiştir.

Abstract:
In this Ph.D. thesis, a modeling study was carried out to investigate the hydrodynamic and thermal behavior of the bubbling fluidized bed gasifiers. Moreover, a simulation study was also performed for gasification reactions taking place in the gasifier. Ansys Fluent was used in the modeling studies, while Aspen Plus was preferred in the simulation studies. A bubbling fluidized bed reactor belongs to TÜBİTAK (The Scientific and Technological Research Council of Turkey) was used for modeling studies and model geometry was created based on this reactor. Three different simulation models were developed within the simulation study. The developed each simulation model initially was validated with relevant experimental study. Then, the effects of gasification temperature, equivalence ratio, steam to biomass ratio and biomass moisture content on syngas composition, H2/CO ratio, syngas heating value and cold gas efficiency were investigated.
The meaning and importance and also, the aim and content of this thesis were given in the first chapter. Then, general information about the gasification process was indicated in the second chapter. For this purpose, biomass and biomass gasification phenomena were firstly expressed. The importance of biomass and gasification technology and the reactions occurring during the gasification process were mentioned. Then, some basic information related to the most common gasifier types; fixed bed gasifier, fluidized bed gasifier and entrained flow gasifier was defined. The advantageous and disadvantageous of fluidized bed gasifiers in comparison with other gasifier types were emphasized. An extensive literature research has also carried out in this chapter. In this context, experimental studies performed with the lab scale and pilot scale gasifiers were searched. Parameters affecting the gasifier performance such as gasification temperature, equivalence ratio (ER), steam to biomass ratio (S/B) and biomass moisture content were analyzed. Modeling and simulation studies were also reviewed under the literature studies. Especially, modeling studies carried out with Ansys Fluent and simulation studies carried out with Aspen Plus were investigated.
Third chapter deals with the methods and equations used in modeling and simulation studies. Multi-phase flows can be modeled with two different approaches in the computational fluid dynamics. These are Euler-Lagrange (E-L) and Euler-Euler (E-E) methods. In E-L method, the fluid phase is treated as a continuum by solving the Navier-Stokes equations, while discrete phase is solved by tracking a large number of particles through the calculated flow field. The dispersed phase can exchange momentum, mass, and energy with the fluid phase. This method is more useful where the interactions between solid particles are negligible and the solid volume fraction is smaller than 10%. Since each particle is tracked and calculated separately, E-L is seen as time-consuming method in the modeling of fluidized bed gasifiers. Navier-Stokes equations are solved for both discrete phase and fluid phase and it is not required to track any particle in E-E method. Thus, E-E modeling approach is considered as a time-saving method for fluidized bed gasifiers where there are millions of particles. Therefore, this modeling approach was preferred during the hydrodynamic and thermal modeling studies carried out in this thesis.
Model geometry was created at the onset of modeling studies, in the fourth chapter. Then, a meshing process was performed for the created geometry. In this context, three different grid structures called coarse, medium and fine grids were formed and using these grid structures a grid independent test was performed. Coarse, medium and fine grids consist of 1,367,126, 1,761,958 and 2,142,026 elements, respectively. The results obtained with coarse grid were differs from the medium and fine grids. Grid independent test showed that the results obtained by using medium and fine grids were almost the same. Since the computational time for the fine grid is more, it was decided to use the medium grid in the modeling study. Even if the element number of medium grid is lesser than fine grid the required time for running process is still high due to the three-dimensional and time-dependent structure of the model. Therefore, an additional study has been carried out to decrease the element number of medium grid and thus, the element number was decreased by about 71%. This final grid has been used for hydrodynamic and thermal modeling studies. Fluidization velocity, which is defined as superficial gas velocity, is one of the most important parameters in terms of bubble formation and gas-solid homogeneity in the bed. The value of this velocity was calculated using an empirical correlation for air and steam as 0.178 m/s and 0.195 m/s, respectively. The value of superficial gas velocity should be greater than minimum fluidization velocity (Umf) to obtain a suitable fluidized bed structure. Thus, a preliminary parametric study was performed to determine optimal superficial gas velocity. For this purpose, five different gas velocities (Umf, 2Umf, 3Umf, 4Umf, 5Umf) were compared with each other and the optimal fluidized bed structure was obtained at the velocity of 3Umf. This velocity value was preferred in the modeling studies. The contour of solid volume fraction with respect to time was obtained for 3Umf in the x-y, x-z and y-z planes. According to the results, the bed height reached a dynamic steady state about 1 second for both air and steam conditions. The pressure drop between inlet and outlet of the reactor decreased significantly at the onset of fluidization and then oscillates around steady state value after 1 second. The fluctuation of the pressure drop is linked with bubble formation and rupture. The variation of time-averaged gas velocity and solid volume fraction with respect to distance from the reactor center were analyzed for both air and steam. Time-averaged air velocity decreases while time-averaged solid volume fraction increases towards the center. Therefore, there is an inverse relationship between gas velocity and solid volume fraction. Time-averaged particle volume fraction along the reactor height was also plotted within the context of hydrodynamic modeling study. The solid volume fraction dropped to zero where the height of the bed reached its maximum value (285 mm). Same solid fraction profile was obtained even though two different fluids (air and steam) were used. This is due to the considering same superficial gas velocity (3Umf) in case of air and steam. Thermal modeling study is divided into two parts. In the first part, the superficial gas velocity (3Umf) determined within the context of hydrodynamic studies was used as the fluidization velocity. In the second part, the velocities that were used for air and steam in the experimental study were taken into consideration as superficial gas velocity.
In the fifth chapter, simulation studies were carried out by taking into consideration the gasification reactions. Three different simulation models were developed. Both the chemical equilibrium model (CEM) and restricted chemical equilibrium method (RCEM) based on Gibbs free energy minimization were used in the first simulation. Almond shell and steam were used as a biomass and gasification agent, respectively. At the outset, simulation predictions were compared with the experimental data for syngas composition and gas yield. The predictions obtained from the RCEM were more consistent with the experimental data. Therefore, RCEM presents a promising approach for the simulation of almond shell gasification in a bubbling fluidized bed gasifier. After validation of the simulation a sensitivity analysis study was performed by using RCEM to investigate the effect of gasification temperature, S/B ratio and biomass moisture content on syngas composition, H2/CO ratio, low heating value (LHV) and cold gas efficiency (CGE). Rising temperature improved the hydrogen production. While the increasing temperature increased CGE, it decreased LHV until 850 °C. However, there was no remarkable change was seen in CGE and LHV beyond this temperature. Thus, it may be concluded that the optimal operating temperature should be between 850 and 900 C. Due to the partial conversion of incoming steam into H2 of syngas, the CGE exceeded 1 at the temperature of 1100 C. CGE was affected negatively at higher S/B ratios. Therefore, incoming steam increment may be preferred in the case of H2-rich syngas requirement. Increasing biomass moisture content degraded the gasifier performance. This is due to the decrease of LHV and CGE with S/B. Therefore, biomass should be dried before feeding to the gasifier. Second simulation study was performed for a bubbling fluidized bed by using RCEM. Almond shell was chosen as the feedstock and steam/oxygen was chosen as the gasifying agent. The developed simulation results were initially compared with the experimental data. The predictions were in good agreement with actual values. After validation process, a parametric study was conducted and effect of varying steam to biomass ratio and equivalence ratio on gasifier performance was studied. S/B ratio mostly affected the hydrogen concentration of product gas. In this study, an increase of %60 in hydrogen concentration was observed. However, rising steam amount causes an increase in tar formation due to the decreasing of gasifier temperature. Thus, S/B ratio should be increased in the case of a hydrogen rich syngas is required. Hydrogen and methane concentrations decreased with increasing equivalence ratio while carbon dioxide increased. A small change was seen in carbon monoxide concentration with ER. Since LHV and CGE decrease with increasing ER, it can be said that ER has a negative effect on gasifier performance. Last simulation study was performed for walnut shell gasification in an auto-thermal BFBG. Steam and air were separately used as the gasifying agents. Simulation estimations for syngas composition were compared with the considered experiment that was carried out both air and steam gasification process. Then, the effects of temperature and biomass moisture content on gasifier performance were investigated. In case of air gasification, increasing temperature increases the hydrogen and carbon monoxide concentrations while it decreases the carbon dioxide and methane concentrations. Because LHV and CGE are positively affected by temperature, the gasifier performance improves with increasing temperature in the case of air gasification process. Rising temperature increases the hydrogen and carbon monoxide and decreases the carbon monoxide and methane concentrations when steam is considered as gasifying agent. LHV and CGE decreases and increases, respectively, until 850 C and no further changes are seen for both parameters above this temperature point and they remain all most the same up to 1000 C. Moisture content has positive effect on hydrogen and carbon dioxide concentrations and negative effect on carbon monoxide and methane concentrations for both steam and air conditions. In addition, gasifier performance is degraded with increasing moisture content in both cases.