Propagation of Reaction Between Bubbles with a Gas Burning in a Fluidised Bed

Abstract

A bubbling fluidised bed reactor has been used for investigating how combustion propagates between bubbles of premixed fuel + air, rising from the distributor towards the surface. Earlier work has shown that when the temperature of the sand gradually rises, the gases first burn in flames above the bed, regime A, then combustion moves under its surface, to occur in bubbles travelling up the bed, under regime B. Above a certain temperature, characteristic of the gas mixture composition, combustion descends towards the bottom of the bed. Ignition then occurs in small bubbles near the distributor, under the stable regime C. The kinetic model, used to calculate the delay for thermal ignition inside gas bubbles rising through the bed, gives correct predictions for regimes A and C but not for B. Under regime B, bubbles of the mixture begin to ignite under the bed's surface while their residence time in the bed is remains shorter than the delay times for thermal ignition derived from the kinetic model. As the temperature rises, the ignition delay rapidly decreases, and regime C is reached, in accordance with the model. In this work attention was focused on regime B. A laboratory reactor of quartz glass was used, with a bed of quartz sand. Fast video recording was employed to capture ignition phenomena as the bed's temperature was raised or lowered. Records of freeboard concentrations of O 2 , CO and of total hydrocarbons, VOCs, were obtained, confirming the specific aerodynamic and chemical character of regime B. It has been shown that combustion spreads from the surface to bubbles near the bed's surface and then to other bubbles close by. Such transfer of the reaction stabilizes combustion inside the bed, at temperatures appreciably lower than that for the thermal ignition of the mixture, given by the kinetic model. This is consistent with earlier findings, which have shown that the combustion of gaseous mixtures in bubbling fluidised beds is controlled by gas phase processes, as in flames.

Thermal decomposition of selected chlorinated hydrocarbons during gas combustion in fluidized bed

“…Red line shows location of dynamic bed surface. Taking into account previous considerations [13,14,22], the analysis of the stepped curve indicates two characteristic critical temperatures (T cr.1 and T cr.2 ) for each combusted mixture.…”

“…As studies [13] and [14] show the combustion process of gaseous fuels in a bubbling fluidized bed in the temperature range of 300–1000°C is carried out at a certain height above the distributor. It has also been shown that one can designate two characteristic bed temperatures (T cr.1 and T cr.2 ), which enable distinguishing of three temperature ranges (combustion regime A, B and C).…”

“…Between T cr.1 and T cr.2 , in regime B, combustion process in the bubbles is possible as a result of flame propagation between bubbles. During regime C - when the temperature of the bed is higher than the critical temperature T cr.2 - the burning occurs as a result of self-ignition of the fuel mixture in bubbles [13,14,22]. The composition of the flue gases strongly dependences on type of combustion regime.…”

“…The composition of the flue gases strongly dependences on type of combustion regime. It have been described earlier (for methane and ethane) [13,14] and will be briefly discussed later in this work (for LPG).…”

The influence of process parameters on combustion of plastic waste in the fluidized bed reactor

“…The experimental tests were conducted in the laboratory fl uidized bed reactor, previously used for solids or gaseous fuels combustion [13][14][15][16][17][18][19][20] . The experiments were carried out in a laboratory fl uidized bed reactor with a thermal power of about 5 kW.…”