Second law analysis of heat transfer surfaces in circulating fluidized beds
Introduction
Superior environmental performance of CFBs is one of the prime motivations of its extensive use in industry. CFBs are also characterized by their approximate isothermal nature and high rates of heat transfer between the fluidized medium and the heat transfer surfaces. Design or scale-up of heat transfer systems in CFBs require estimation of the effective heat transfer coefficient on heat transfer surfaces in contact with the fluidized medium. It is therefore essential to understand the mechanisms of heat transfer in CFBs, and to develop an appropriate model to predict the rate of heat transfer.
There have been many studies concerning the fundamental analysis of the heat transfer between the fluidized beds and heat transfer surfaces in both laboratory and industrial-scale units. Basu and Nag [1] and Glicksman [2] presented comprehensive reviews of CFB heat transfer. Recently, Grace et al. [2] experimentally and theoretically investigated the effects of various operating parameters on the heat transfer process. Xie et al. [3] proposed a 2D model which coupled radiation, conduction and convection from the hot core on the furnace side to conduction and convection into the coolant on the wall side. In a further study authors also developed an advanced model to accommodate the 3D membrane wall geometry Xie et al. [3]. Gupta and Reddy [4] proposed a mechanistic model to predict the bed-to-wall heat transfer coefficient in the top region of a CFB riser column for different riser exit configurations. Chen et al. [5] suggested that mechanistic models based on the surface renewal concept hold promise for design and scale-up of heat transfer systems for both bubbling dense beds and fast CFBs.
Although there have been many studies concerning the fundamental analysis of the heat transfer between the fluidized beds and their heat transfer surfaces as mentioned above, the second law analysis of the CFBs is very rare in the literature [6], [7], [8], [9], [10], [11], [12]. It is a well known fact that thermodynamics is concerned not only with the conservation of energy but also with quality of energy. The traditional first law analysis based upon component performance characteristics together with energy balance can always lead to the correct final result. But, this analysis cannot locate and measure the source of losses. This is mainly because the first law embodies no distinction between work and heat, no provision for quantifying the heat and no accounting for work lost. The first law efficiency does not take into account the quality of energy. It has long been understood that traditional first law analysis is needed for modeling the engine processes but a novel approach is also required to give the engineer the best insight into the engine’s operation. The heat transfer surfaces directly affect the hydrodynamic behavior and thus fluidized beds’ combustion performance. As for studies concerning fluidized beds’ heat transfer surfaces, there are only two investigations taking into account the exergy analysis of heat transfer surfaces. The first study conducted by Eskin and Kilic [8], investigates the estimation of cooling tube location in bubbling fluidized bed coal combustors through exergy analysis. Whereas the second study conducted by Gungor and Eskin [10] takes into account the thermodynamic analysis of heat transfer to the immersed surfaces in small-scale CFBs. It is clearly observed that the effect of heat transfer surfaces on the CFB combustion efficiency remains as a domain to be investigated in detail. This manuscript deals with the effect of the heat transfer surfaces on the dimensions, arrangement and types of surfaces in order to achieve maximum efficiency which is the main contribution of the manuscript to the literature. This study proves that both the location and the dimensions of the heat exchangers are extremely important design parameters of the CFB combustors for optimization of system efficiency.
As for the thermodynamics, exergy is defined as the maximum amount of work which can be produced by a system or a flow of matter or energy as it comes to equilibrium with a reference environment. Unlike energy, exergy is not subject to a conservation law (except for ideal, or reversible, processes). Rather exergy is consumed or destroyed, due to irreversibilities in any real process. Second law (exergy analysis) quantifies and locates the losses that help in optimizing the thermal systems. In order to evaluate the inefficiencies associated with the various processes – second law analysis must be applied [13]. For second law analysis, the key concept is ‘‘availability’” (or exergy). The availability of a material is its potential to do useful work. Unlike energy, availability can be destroyed with such phenomena as combustion, friction or mixing. The reduction of irreversibilities can lead to better engine performance through a more efficient exploitation of fuel, better environmental impact in general and higher potential savings. The use of exergy principles improves the engineers’ understanding of thermal and chemical processes and allows sources of inefficiency to be quantified. From this point of view, in this study, the thermodynamic second law analysis of heat transfer surfaces in CFBs is analyzed in order to define the parameters that affect the system efficiency. Using a previously developed 2D CFB model [14], [15] which uses the particle-based approach and integrates and simultaneously predicts the hydrodynamics and combustion aspects, second law efficiency and entropy generation values are obtained at different height and volume ratios of the heat transfer surfaces for CFBs. Besides that, the influences of the water flow rates and heat exchanger tube diameters on the second law efficiency are investigated. It must be noted that since the nonexistence of experimental data in the literature, in this study only theoretical analysis is given.
Section snippets
System analysis
The use of CFB modeling enables the analysis of a combustion system involving fluid flow, heat transfer, and combustion and pollutant emissions. The thermodynamic analysis of heat transfer in circulating fluidized beds is very important in defining the parameters that affect the system efficiency. For this purpose, thermodynamic analysis of the heat exchanger surfaces in the CFB combustor is evaluated through a model, which can be employed to simulate a wide range of operating conditions [14],
Simulation results and discussion
Using the presented heat transfer model, the second law efficiency of the CFB is obtained at various conditions. In performing the simulations, the volume ratio, γ, which is the ratio of the volume of heat exchanger in the control volume to the total heat exchanger volume (Fig. 2); and the height ratio, Φ, which is the ratio of the starting height of the heat transfer surfaces to the total height of the CFB are defined (Fig. 1). Simulation results are obtained from 300 kW CFB unit of 20 cm, i.d.,
Conclusions
The main contribution of this study to science domain and the designers lies in the fact that it proves that the heat exchanger surfaces have an irrefutable importance on the quality of energy to be derived from fluidized bed combustion which also involves many complicated factors such as chemical reactions and physical transport processes. In the real applications, especially in real fluidized bed applications where the heat derived from the bed is of utmost importance; the design and
Acknowledgement
The author would like to express his thanks to Prof. Dr. Nurdil Eskin for her valuable contribution to this work.
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