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Show II. Backeround It has been proven that gas bubbles breaking at the bed sUlface maintain their entity in the freeboard(I-4). these entities are move in a coherent form and slowly disperse into the bulk gas flow. The result is fast decay of turbulence and poor mixing of the gas and constituents in the freeboard. The bubbles project particles when they erupt at the surface of the bed. on a weight basis, the proportion of particles returning to the bed from the splash zone is much greater than the proportion carried out The particles returning to the bed will convey sensible hea~ limiting the freeboard temperature rise. Also, some of the unburnt volatiles will be carned into the freeboard by these isolated bubbles. The rate of reaction in the freeboard will then be limited by the temperature and the rate at which the bubbles entrain oxygen (5). On the other hand, it has been . shown that further reactions of NOx reduction; NO-CHAR and NO-VOLATILES are possible. They may take place if (6) : a) the entrained char burns in the freeboard, thus, increasing the freeboard temperature, b) the freeboard turbulence level is sufficiently high for improved mixing. III. Cold Flow Model Experiments The experimental set up consisted of two parts. The fluidized bed unit with an appropriate air handling system,the measurement system, and the laser Doppler velocimeter unit. A transparent model of the FBC columns was designed and built to carry out the first stage of the experimental program. The column (O.3m dia.) was constructed such that sections could be added to vary the height of the unit as shown in Figure 1. The total height above the distributor plate was 1.2m. The column was made of plexiglass. First-quality crown glass was used for one column section to ensure good optical access; this section was interchanged with other sections when measurements were made in other freeboard locations. The entire column was mounted on a threedirectional traversing table to enable measurements at various locations with the freeboard. The fluidizing (primary) air was supplied, at atmospheric pressure and room temperature, by a variable speed centrifugal blower. The secondary air was supplied by a separate blower for independent control of the air flow rate. Both air flow rates 3 |