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Show OVERVIEW OF MODEL The outputs of the model used in this investigation are: (1) quantitative predictions of the elemental composition of ash deposits formed in pulverized coal combustors as a function of combustor operating conditions, coal (ash) type and amount, and location within the boiler, and (2) quantitative estimates of the rates of several deposition mechanisms. From these outputs, qualitative indications of deposit strength, morphology, removability, and emissivity can be derived. Required input includes: (1) a description of the boiler in terms of both dimensions and operating conditions, and (2) a description of the coal and mineral matter, including chemical species infonnation about selected mineral components. The model is reviewed in detail elsewhere (.11). A conceptual overview is presented here. ADL VIC is distinguished from traditional industrial indices for fouling and slagging and from other deposition models described in the literature in the following ways: (1) explicit prediction of the affects of boiler operating conditions on deposition behavior; (2) explicit prediction of variations of deposit properties with location within a boiler, (3) explicit dependence on the total amount of inorganic material flowing through the boiler (essentially all industrial indices are based on the elemental composition, independent of the total amount); and (4) a predictive framework based on the specific mineralogy of the inorganic material in the coal, rather than on its elemental composition. It is significant that a mineralogical description of the coal is required, as opposed to an ASTM ash analysis. ASTM procedures can be used to generate much of the required information. For example, pyritic sulfur can be used to quantitatively estimate the fraction of pyritic iron in the coal and free silicon (silicon in the fonn of silica) can be estimated from the ratio of silicon to aluminium in the ash. Other coal mineralogies cannot be easily estimated from ASTM procedures. Principal among these are calcitic calcium, atomically dispersed species of any type, and the precise composition of silicates. Given these inputs, the model predicts the path of the particle clouds through the boiler and the response of the particles to the changing environment encountered along this path. These are cast in the fonn of a series of coupled, ordinary differential equations, the solutions to which indicate particle temperature, velocity, and position as a function of particle residence time. Also included in these equations is the rate of accumulation of ash on heat transfer surfaces. The solutions to the differential equations are used to predict deposit elemental composition and other properties. The four deposition processes that are treated by ADL VIC are: (1) inertial impaction (and particle capture), (2) thennophoresis, (3) condensation, and (4) heterogeneous reaction. All heat transfer surfaces within the boiler are involved in these processes. ADLVIC currently uses flat walls in turbulent flows as an approximation to waterwalls and single tubes in cross flow as an approximation to convective pass tubes. The four processes of ash deposition are assumed to have additive influences on deposit composition. That is, the rate of deposition of ash at particle residence time t is given by Cdmif· = I; (t, t) G,{ t, t) + T; (t, t) + C; (r, t) + R; (t, t) (1 ) In this equation, mj represents the mass of component i in the deposit. The factor Ii represents the rate of inertial impaction, Gj the particle capture efficiency, Tj the rate of thennophoretic deposition, Cj the rate of condensation, and Rj the rate of chemical reaction. The subscript i refers to each of the mineral components in the coal. These include pyritic iron, other forms of 3 |
