Refinement of indicators characterizing fouling and slagging for coals with low slagging propensities as applied to the standard and zone-wise methods for thermal design of furnace chambers and to mathematical models

Abstract

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...

A mathematical model of slagging of the furnace of the pulverized-coal-firing boiler

“…For practical purposes (determining the char acteristic distribution of fouling and the localized dis tribution of the heat transfer coefficient) this model can be used in the form presented here. The calcula tion comes to a close as soon as a weighted mean value of the thermal resistance of the walls that corresponds to the operating conditions (the operating tempera ture at the furnace outlet) or has been evaluated by other methods, for example [18], is attained. Thus, when calculating, the iterations can be carried out within the interval of weighted mean values of the thermal resistance from the condition that occurs immediately after cleaning the heating surfaces to the typical operating condition.…”