Burning premixed fuel-air mixtures in a bubbling fluidized bed is accompanied by some characteristic phenomena. The most striking one is the production of acoustic effects, indicating that combustion is not really continuous. A second, less obvious effect, is the NOx concentration in the flue gases falling with increasing bed temperature, observed above a certain critical mean bed temperature. To investigate the periodic burning of portions of methane-air mixture, photometric and acoustic signals were recorded simultaneously. Using a laboratory quartz reactor, explosions could be optically recorded in the bed, millimeters above distributor. With ethane fuel, the effective “combustion zone” in the reactor was also located by determining vertical temperature profiles, using eight thermocouples. When the bed temperature rises, maxima in the vertical temperature profiles associated with the “reaction zone” move from above the bubbling bed to the distributor. A mathematical model of unsteady combustion in a single bubble surrounded by bed material was used to simulate the process. Computed temperature maxima were compared with the experimental profiles. This meant finding the region where bubbles of premixed gases exploded, experimentally and from the model. A correlation between the NOx concentration and the location of the explosions (and diameter of the exploding bubbles) has also been found.

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