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449Three dimensional mathematical models for sim ulating furnace processes with clear 2D and 3D repre sentation of results, which have found wide use in recent years, serves as an essential aid for analyzing technical solutions used in designing equipment and experimental data obtained on boilers. Undoubtedly, applications of software systems implementing the methods of computational hydrodynamics for simu lating furnace processes will find growing use as these packages become more advanced and as solution of new problems is imposed on them. At the same time, one should not fully rely on trustworthiness of the results obtained from using complex scientifically sub stantiated models. This relates primarily to calcula tions carried out for arbitrary kinds of coal and boiler design, including determination of the total heat transfer in the furnace (the amount of absorbed heat and temperature of gas flow at the furnace outlet ).The results obtained from calculations of the total heat transfer in a furnace using both empirical meth ods and mathematical models depend to a determining extent on selection of the quantity characterizing the thermal resistance of waterwall tubes with deposits. In the standard method [1], the thermal efficiency coef ficient ψ 98 and ζ 98 = ψ 98 /x ww , where x ww is the water wall view factor, are used as indicators that take into account degraded absorption of heat by a waterwall covered with deposits (here and henceforth, the super script "98" means that the indicators correspond to the values specified in the third edition of the standard method of thermal design [1]). In the zone wise cal culation method [2] and in the mathematical models f '' ϑ of a furnace process, thermal resistance of waterwalls Ro (m 2 °C/kW) or a heat conductance coefficient or heat flux (quantities functionally connected with Ro) are specified.We point out once again that the normative thermal efficiency coefficient of waterwalls ψ 98 is not a suffi cient physical characteristic of external deposits (in particular, the same value of ψ 98 may correspond to real state of a waterwall ranging from its operationally clean to slagged state) [3][4][5]. Its values were selected as empirical coefficients intended for bringing the results obtained using this method in consistency with experimental data on the total heat transfer. It should be pointed out that the values of ψ 98 are underesti mated as compared with its true values if the waterwall thermal efficiency coefficient is understood to mean the experimentally determined and physically sub stantiated ratio between the absorbed q abs and incident q inc heat fluxes: ψ = q abs /q inc = (1 -q rev )/q inc , where q rev is the reverse heat flux. Figure 1 shows the values of the coefficient ψ 98 calculated using the standard method taking into account experimental data and ψ av = Σ(q abs /q inc ) determined from the results of zone wise calculation (summation by elements). It should be noted that the values of normative coefficients could be taken closer to their real valu...

The combustion of coal mixtures is regarded both as a necessary step and as a technology which offers noticeable technical, economic, and ecological advantages. The choice of an optimal, or even acceptable, mixture requires knowledge of the characteristics and properties of the coals being blended and of how their behavior changes in a mixture. With mixing, all the indicators characterizing a coal undergo changes and an improvement in one may be accompanied by the deterioration of others. Based on experimental and published data, we have analyzed the major characteristics of mixtures that control the operation of coal-dust thermal power plants, first the ones which are not additive, and then those whose additivity is not evident without experimental study.The preparation and combustion of mixtures of coals with similar characteristics has long been done in worldwide practice, primarily to maintain the quality of the fuel in the market. In recent years European thermal power plants have customarily burnt mixtures of coals with different characteristics and properties that have been mined on different continents. The blends are prepared at coal depots in ports and at power plants. The permissible quality of the mixture is determined by the primary application intended for the mixture and by how much the characteristics of the coals differ from one another.Coal traditionally arrives at many Russian thermal power plants from several suppliers and is alternately fed "by rota" to the boiler bins in the form of a controlled mixture of the different coals from the depot. With rare exceptions, this does not yield an adequately uniform mixture, primarily because there is no operational control of the quality of the arriving coal or modern equipment for blending it at the coal depots. However, when there is a system for operational monitoring at the intake and for batching of the storage pile, or for separative storage of coals from different suppliers, high quality coal blending can be achieved in some cases even with available equipment.The combustion of coal mixtures is regarded both as a necessary step and as a technology which offers noticeable technical, economic, and ecological advantages. The use of coal blends for new technologies and thermal power plants makes it possible to standardize equipment, mass produce tested new designs, and provide high power thermal power plants with stable fuel.The advantages of using coal blends at operating power plants include the following: extending the range of acceptable coals and thereby reducing the dependence on a single (or monopoly) fuel supply; and, the possibility of burning coals which could not be burnt in their "pure" form at a given power plant, in particular coals that were not incorporated in the plant design, without substantial modification of equipment. The use of blends may be an optimum solution of the problem of reducing harmful emissions, such as sulfur oxides -by adding coal with a low sulfur or solid particulate content -through burning of a low-ash coal mi...