Sirkülasyonlu akışkan yataklı yakıcıların dizaynı; kömürün yüksek bir verimle yakılması ve baca gazı emisyonlarının kabul edilebilir seviyelerde tutulması için oldukça önemlidir. Bununla beraber, sirkülasyonlu akışkan yataklı yakıcıların optimum tasarım ve işletme parametrelerinin tespitinde, gerçek boyuttaki yakıcılarla çalışmanın gerek zaman ve ekonomiklik, gerekse uygulama açısından pratik olmayacağı açıktır. Bu nedenle, geliştirilen bir model ile sirkülasyonlu akışkan yataklı kömür yakıcılarında etkin olan tasarım ve işletme parametrelerinin tespiti ve sistemin optimum çalışmasını sağlayacak gerekli parametrelerin belirlenmesi mümkün olacaktır. Bu amaçla, bu çalışmada kömür yakan sirkülasyonlu akışkan yataklı yakıcılar için dinamik iki boyutlu bir model geliştirilmiştir. Model sonuçları Gazi Üniversitesi Isıl Güç Labaratuvarı’ndaki 50 kW’lık pilot sirkülasyonlu akışkan yatak yakıcı ve 18 Mart Çan Termik Santrali’nde bulunan 160 MW’lık bir sirkülasyonlu akışkan yatak yakıcının test sonuçları ile karşılaştırılmıştır. Model sonuçlarının küçük ve büyük ölçekli yatak test sonuçları ile göstermiş olduğu uyum modelin geçerliliğini kanıtlamıştır. Modelde ayrıca sirkülasyonlu akışkan yataklı yakıcılar için yakıcı verimi tanımlanarak, küçük ve büyük ölçekli yataklar için hava fazlalık katsayısının yakıcı verimi üzerindeki etkileri; farklı yatak basınç değerleri için incelenmiştir. Genel olarak hava fazlalık katsayısının pilot ve endüstriyel ölçekli yatakta verimi olumsuz etkilediği görülmekle beraber hava fazlalık katsayısı sabit tutulurken artan basıncın yatak içerisindeki yanmayı iyileştirdiği ve pilot ölçekli yatakta yakıcı verimi üzerinde belirgin bir değişime sebep olmazken endüstriyel ölçekli yatakta basıncın etkisinin daha fazla olduğu görülmüştür.

Fluidized bed combustion allows clean and efficient combustion of coal. A well-designed circulating fluidized bed combustor can burn coal with high efficiency and within acceptable levels of gaseous emission. It is also important to determine the effects of operational parameters in circulating fluidized bed combustors via a simulation study instead of expensive and time consuming experimental studies. A very good appreciation of the combustion process is needed for a reliable performance prediction through modeling. The main goal of the modeling of circulating fluidized bed combustors is to constitute a system that maximizes combustion efficiency, and minimizes operating and investment costs and air pollutant emissions. From this point of view, in the present study a dynamic two dimensional model for a circulating fluidized bed combustor integrates and simultaneously predicts the hydrodynamics, heat transfer and combustion aspects, which can be employed to simulate under a wide range of operating conditions, has been developed. In the model, the circulating fluidized bed riser is divided in two regions: bottom zone as a fast bubbling fluidized bed and upper zone as a core-annulus solids flow structure. In the model, pressure drop due to solids acceleration is considered. Char particles are divided into n size groups in the model burn and undergo attrition. In the model, volatiles are released at a rate proportional to the solid mixing rate. Volatile nitrogen and sulphur increases as a function of bed temperature. Heat transfer model is based on cluster renewal process. In the model, return cycle of solids has been considered. Inputs for the model are combustor dimensions and construction specifications (insulation thickness and materials, etc.), primary and secondary air flow rates; coal feed rate and particle size distribution, coal properties, calcium to sulfur ratio, limestone particle size distribution, inlet air pressure and temperature, ambient temperature and the superficial velocity. Simulation model calculates the axial and radial distribution of voidage, velocity, particle size, pressure drop, gas emissions and temperature at each time interval for gas and solid phases. Developed model’s hydrodynamic structure is given and validated with cold beds test results obtained from various CFB test rigs of different size in the literature in previous studies (Eskin and Güngör, 2005a-b). The model should be flexible enough in order to be used in different applications of circulating fluidized beds. The computer code should be modular to allow users to update component modules easily as new findings become available. The set of differential equations governing mass, momentum and energy are solved using the Gauss-Seidel iteration and combined Relaxation Newton-Raphson methods. Time step is 10-11 seconds using in the model. The calculation domain is divided intogrid nodes, in the radial and the axial directions respectively. With the cylindrical system of coordinates, a symmetry boundary condition is assumed at the column axis. At the walls, a partial slip condition is assumed for the solid and the gas phases. The simulation results are compared with test results obtained from 50 kW Gazi University Heat Power Laboratory pilot scale unit of 12.5 cm internal diameter. In this comparison, oxygen molar ratio and carbon monoxide emissions along the bed axis and oxygen, carbon dioxide mole ratios and sulphur dioxide emissions response are obtained for the pilot circulating fluidized bed unit using the same input variables in the tests as the simulation program input. The simulation results are in good agreement with experimental ones. Simulation results are also compared with the data obtained during the commissioning period from 160 MW Çan Power Plant circulating fluidized bed unit of 8.55 m internal diameter and 37 m in height. Simulation and test results at the riser exit were compared at different coal feed rates and the results are in good agreement with large-scale circulating fluidized bed unit data. In this study, combustor efficiency has been defined and combustor efficiency for pilot scale and industrial scale has been investigated. It is generally observed that air to fuel ratio has a negative effect on combustor efficiency in both pilot and industrial scale circulating fluidized bed combustors. As pressure increases for constant air to fuel ratio, combustion in the bed becomes more effective, causing little distinctive change on pilot scale bed combustor efficiency, whereas an explicit change is observed in industrial scale bed due to pressure effect.