Tunçbilek termik santrali'nin 5. ünite kazanındaki alev oluşumunun sayısal modellemesi

Abstract

Bu çalışmada, Tunçbilek Termik Santrali'nin 5. ünitesinde pülverize kömür yakan 150 MW ısıl kapasiteye sahip bir kazanın sayısal modellemesi yapılmıştır. Kazan 69 m uzunluktadır ve 12 m x 12 m kesit ölçülerine sahiptir. Kazanda dört adet köşelerde, iki adet de kenarlarda olmak üzere toplam altı adet brülör grubu bulunmaktadır. Her grupta ise alt, orta ve üst bölgede olmak üzere üçer adet brülör bulunmaktadır. Kazan geometrisi CATIA yazılımı kullanılarak kazanın ölçülerine göre oluşturulmuştur. Daha sonra GAMBIT yazılımı kullanılarak mevcut geometride ağ yapısı oluşturulmuştur. Ağ yapısında 4,2 milyon hücre kullanılmıştır. Sayısal model hazırlanırken iki farklı durum göz önüne alınmıştır. Bu durumlar arasındaki farklılık toplam hava ve yakıt miktarlarının brülörlere dağılma oranlarındaki farklılıktan kaynaklanmaktadır. Dağılma oranlarının santral verilerine göre hazırlandığı model Durum 1, varsayıma dayalı olarak hazırlandığı model Durum 2 olarak adlandırılmıştır. Kazanın sayısal modellemesi ANSYS yazılımı içindeki FLUENT modülü kullanılarak yapılmıştır. Kazan gerçek çalışma koşullarına uygun şekilde modellenmiştir. Oluşturulan modelde sınır şartları olarak kazan üzerinden ölçülen sıcaklık değerleri ve santral verilerinden elde edilen hava ve kömür debileri kullanılmıştır. Kazanın üst bölgesindeki borular karmaşık geometriye sahip olduğundan bunlara geometride yer verilmemiş, bunların neden olacağı basınç kayıpları bu bölgenin gözenekli ortam olarak tanımlanmasıyla hesaba katılmıştır. Modeldeki denklemler zamandan bağımsız olarak çözdürülmüştür. Modelde türbülans modeli olarak standart k-ε türbülans modeli, yanma modeli olarak ön karışımsız yanma modeli ve ışınım modeli olarak P1 ışınım modeli kullanılmıştır. Ön karışımsız yanma modelindeki PDF modülü sayesinde santralde kullanılan kömürün içeriği tanımlanmıştır. Ayrık faz modeli kullanılarak gaz fazındaki türbülansın katı parçacıkların yörüngelerine olan etkisi göz önünde bulundurulmuştur. Kömür giriş yüzeylerinde enjeksiyon yüzeyleri oluşturulmuş, deneysel verilerden elde edilen kömür çapları tanımlanarak kazan içerisine kömür püskürtülmüştür. Beşinci bölümde ise Tunçbilek Termik Santrali'ndeki çalışma koşulları daha önce Seyitömer Termik Santrali ile yapılan çalışmadaki aynı ısıl kapasiteye sahip kazana uygulanarak sonuçlar kıyaslanmıştır. Tunçbilek Termik Santrali için hazırlanan modellerden elde edilen sonuçlar incelendiğinde her iki modelin de kendi içinde tutarlı olduğu görülmektedir. Her iki durumda da kazan içindeki en yüksek sıcaklık olarak isimlendirilen alev bölgesi istenen şekilde kazanın merkezinde oluşmaktadır. Kesitlerdeki sıcaklık ve hız dağılımları beklendiği şekilde olmuştur. Bu çalışmada kazanın 3. grubuna ait brülörler kapatılarak modelleme yapılmıştır. Sonuçların karşılaştırılmasında kullanılan deneysel veriler ise uzun zaman aralığında çeşitli brülörlerin kapalı kalmasıyla elde edilmiş ortalama sıcaklık değerleridir. Bundan dolayı modelin çalışma şartlarına uygunluğu ile ilgili kesin bir değerlendirme yapılması söz konusu olmayıp ele alınan durumların kazan içerisindeki sıcaklık mertebelerine yakınlıkları karşılaştırılmıştır.

In this study, numerical modeling of the pulverized coal-fired boiler which has 150 MW thermal capacity in the 5th unit of Tunçbilek Thermal Power Plant was performed. There are six-burner groups in the boiler. Two of them are on the side and the rest are at the corners. In each group, there are three burner regions which are the lower, middle, and upper regions. Based on the dimensions of the boiler, geometry was generated by using CATIA. The boiler is approximately 69 m in length, 12 m in width, and 12 m in depth. Mesh was generated by using GAMBIT. At critical regions like air and coal inlets and boiler outlets, more mesh elements were used. In mesh generation, 4,2 million cells were used. When preparing numerical models, two different cases were taken into account. The difference between these cases was caused by differences in the distribution of the mass rate of total air and total coal to the burners. In Case 1 which was prepared based on the operating conditions of the power plant; the amounts of air that were sent to the upper, middle, and lower burners are 43%, 43%, and 14% of the total air respectively. The amounts of coal which were sent to the upper, middle, and lower burners are 43%, 35%, and 22% of the total coal respectively. In Case 2 which was prepared based on assumptions, total air and coal were distributed equally to each burner. In experimental studies, measurement of the combustion and heat transfer characteristics is expensive. Experimental studies require a long time, skill and they have geometric limitations. CFD codes provide to test these characteristics in a much shorter time at lower costs and they haven't geometric limitations. They enable to analysis of a system that involves combustion, heat transfer, turbulence, and chemical reactions. Therefore, in recent years CFD codes became popular to predict the performance of boilers in both scientific and industrial studies. In furnace studies, they are generally used to optimize combustion performance and reduce pollutant emissions. Therefore, numerical modeling of the boiler was performed by using Computational Fluid Dynamics codes. In this study, FLUENT was used as CFD codes. The numerical model was generated based on actual operating conditions. At air inlets, mass flow inlet was used as a boundary condition. At these regions, the mass flow rate and temperature of the air were defined. Some of the air was sent into the boiler from coal inlets. Thus, better dissemination of the coal particles was provided in the boiler. At coal inlets, coal particles were sent into the boiler by defining these regions as injection surfaces. Defining these regions as injection surfaces provides to define the size distribution of the coal particles. Using the Rosin Rammler method, the minimum and maximum diameters of the coal particles were defined. At the coal inlets, the mass flow rate and temperature of the coal were defined. At the outlet of the boiler, the pressure outlet was defined as a boundary condition. Pipes that are at the upper side of the boiler have very complex geometry. Therefore, these pipes weren't included in the geometry, but pressure losses caused by these pipes were taken into account by defining this region as a porous medium. Calculations were performed based on steady-state. In similar studies, k-ε models were used as turbulence models. There are three kinds of k-ε models which are Standard k-ε, Renormalization-group (RNG) k-ε and Realizable k-ε. The differences of these methods are due to the method of calculating turbulent viscosity, the turbulent Prandtl numbers governing the turbulent diffusion of k and ε, and the generation and destruction terms in the ε equation. In this study, Standard k-ε was used as a turbulence model. Because this model requires a shorter time and provides reasonable accuracy for a wide range of turbulent flows. Wherefore air and coal enter the boiler separately, the Non-premixed Combustion Model was used as a combustion model. In the Non-premixed Combustion Model, the content of the coal was defined using the PDF approach. There are five kinds of radiation models which are Discrete Transfer Radiation Model (DTRM), P-1, Rosseland, Surface-to-Surface (S2S), and Discrete Ordinates (DO). In this study, P-1 was used. Because; it provides reasonable accuracy in a shorter time and requires little CPU demand. To predict the trajectories of individual coal particles, Discrete Phase Model was used. This model is used to predict the effects of turbulence in the gas phase on the dispersion of particles. In our previous study, we had prepared a numerical model which was based on operating conditions for Seyitömer Power Plant. In the fifth section, the operating conditions of Tunçbilek Power Plant were applied to the Seyitömer Power Plant's boiler which has the same thermal capacity, and the results obtained from both studies were compared. According to the obtained results, the flame region occurs at a higher region for operating conditions of Tunçbilek Power Plant compared to operating conditions of Seyitömer Power Plant. It is understood that differences between coal contents and particle diameters cause this situation. When the results obtained from prepared models for Tunçbilek Power Plant are considered, it is seen that both cases are self-consistent. In both cases, the maximum temperature in the boiler known as the flame zone occurs in the center of the boiler as desired. In both cases, swirling combustion occurs at the center of the furnace. Temperature and velocity distributions in the cross-sections occur as expected. When the distribution of carbon dioxide is considered, it is seen that towards the center of the boiler from the burner surface carbon dioxide concentration increases. While the coal particles sprayed from the bottom burners move upward and downward, coal particles sprayed from the middle and top burners move upward. When coal particles enter the furnace, mass transfer from the coal particles starts immediately and reaches the maximum rate at the center of the furnace. It is seen that air and fuel mixing is intense near the burners and due to the intimate mixing of fuel and air and chemical reactions from this point towards the center of the furnace, concentrations of gas components also change rapidly. Experimental data were obtained 45 cm away from the surfaces where any burner does not take place. In this study, when the numerical model was prepared, only the burners belonging to the third group of the boiler were closed. Experimental data used for the comparison of the results were mean temperature values obtained by closing various burner groups in long time period. Therefore, it is not possible to decide if these results are suitable for the operating conditions of the power plant. On the other hand, it is possible to decide if obtained values close to the temperature levels in the boiler.