Coal combustion heat transfer

Ga.s-to-Pa.rticle Heat Transfer. Heat transfer between gas and particles is rapid because of the enormous particle surface area available. A Group A particle in a fluidized bed can be considered to have a uniform internal temperature. For Group B particles, particle temperature gradients occur in processes where rapid heat transfer occurs, such as in coal combustion. 

The modeling of fluidized beds remains a difficult problem since the usual assumptions made for the heat and mass transfer processes in coal combustion in stagnant air are no longer vaUd. Furthermore, the prediction of bubble behavior, generation, growth, coalescence, stabiUty, and interaction with heat exchange tubes, as well as attrition and elutriation of particles, are not well understood and much more research needs to be done. Good reviews on various aspects of fluidized-bed combustion appear in References 121 and 122 (Table 2). 

A furnace bums a liquid coal tar fuel derived from coke-ovens. Calculate the heat transferred in the furnace if the combustion gases leave at 1500 K. The burners operate with 20 per cent excess air. 

It is also interesting to note that soft-sphere models have also been applied to other applications such as gas-particle heat transfer by Li and Mason (2000) and coal combustion by Zhou et al. (2003). Clearly, these methods open a new way to study difficult problems in fluidized bed reactors. 

Greenberg. J. B. and Goldman, Y. (1989). Volatilization and burning of Pulverized coal with radiation heat transfer effects on a counter flow combustor. Combustion Sci. Tech., 64 1-17. 

In practice, the gases exiting the fluidized bed reactor contain a certain amount of ash and have to be cleaned. Also, the combustion products of coal are sometimes corrosive, which means that in addition to air being fed into the reactor, various other chemicals are added to ensure "clean" combustion products that will not corrode turbine blades or violate environmental standards. Coal combustion is a very active field of research, and many exciting developments are occurring there. In this analysis, we make certain assumptions that illustrate the thermodynamic concepts as clearly as possible. Therefore, we do not examine the effect of hydrodynamics, heat, and mass transfer, which are very important in the combustion of the coal particle and the distribution of combustion products. We do not expect that this will have a significant impact on the analysis. 

Radiative heat transfer plays an important part in many fluidized bed processes operated at high temperatures, such as coal combustion and gasification. When treating a fluidized bed as a whole solid gray body, the radiative heat transfer coefficient ht between the fluidized bed at temperature 7), and a heating surface at temperature Ts is defined as... 

The release of alkaline compounds is also an important factor in coal combustion. Most high-temperature corrosion problems in fossil fuel boilers are the result of salt melts (54,55). Alkali metal compounds volatilize from coal at the high temperatures of conventional combustion and subsequently condense on heat transfer surfaces. The lower temperatures of FBC are expected to reduce salt volatility, a fact that has been confirmed by Vogel et al. (16). 

The properties of wood(7,14) were used to analyze time scales of physical and chemical processes during wood pyrolysis as done in Russel, et al (15) for coal. Even at combustion level heat fluxes, intraparticle heat transfer is one to two orders of magnitude slower than mass transfer (volatiles outflow) or chemical reaction. A mathematical model reflecting these facts is briefly presented here and detailed elsewhere(16). It predicts volatiles release rate and composition as a function of particle physical properties, and simulates the experiments described herein in order to determine adequate kinetic models for individual product formation rates. 

Intelligent design of circulating fluidized bed boilers depends on sufficient understanding of the physico-chemical hydrodynamics occurring in the combustors, such as chemical kinetics of coal combustion and pollutant formation, hydrodynamics of gas-solid two phase flow, mixing of gas and solids, distribution of heat released, and heat transfer between immersed... 

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