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Show INTRODUCTION A central problem in the modeling of atmospheric pressure fluid-ized bed combustion (AFBC) of coal is the quantitative description of the reaction between oxygen and char particles resulting from devola-tilization, swelling, and/or fragmentation of the coal feed. This problem is usually treated by dividing the bed gas flow into bubbles and emulsion, with mass transfer between the two streams. The reactive (char) particles are usually assumed to be present only in the emulsion phase (Davidson and Harrison, 1963). Modifications of this description include consideration of bubble clouds (Kunii and Levenspiel, 1969t Chapter 8), allowance for axial dispersion in the bubble and emulsion phases (May, 1959; van Deemter, 1961), and distributed gas-particle contact times (Orcutt, Davidson, and Pigford, 1962). Gilliland and Knudsen (1970) allowed for a variable void fraction by assuming that particle concentration is proportional to a power of the (distributed) gas residence time. A probe for the measurement of time-resolved oxygen concentrations in AFBC was developed by the FluiDyne Engineering Corporation, Minneapolis, MN for the Tennessee Valley Authority and the U.S. Department of Energy, Morgantown Energy Technology Center (Saari and Davini, 1983). The probe is based on a sensor manufactured by Robert Bosch, GmbH, for automotive application. The sensing element is a Pt,0?;ZrO?;0?,Pt electrochemical cell with a shield and coating which protect the outer platinum electrode from erosion by particles. The response time of this sensor in a fluidized bed is about 30 ms (Saari and Davini, 1983). The apparent randomness of the signal from this probe during fluidized bed combustion at realistic values of u /u _ o mf complicates the identification of characteristic bubble and emulsion 0p concentrations. In the present paper some properties of the distribution of the sensor output are examined. The treatment presented is but one of several alternative interpretations of the fluctuating 0 measurement. EXPERIMENTAL Measurements of the oxygen concentration were made in the 700 kW AFBC of the MIT Combustion Research Facility. The equipment has been 2 |
