On the mechanism of bubbling fluidized bed combustion of gas oil

A study has been carried out on atmospheric fluidized-bed combustion (FBC) of conventional liquid fuels at temperatures lower than the value classically adopted for FBC of solid fuels (i.e., 850 °C). The study comprised an experimental program of steady-state tests for characterization of the combustion mechanism, mainly through proper acquisition of gas compositions and passive measurements of pressure signals. To this end, a bench-scale, bubbling-bed facility was used, and gas oil (diesel fuel) was chosen as a reference fuel and fed in a nonpremixed, submerged setup. Extending the bed operating temperature down to a value as low as 550 °C allowed the relevant features of the combustion mechanism to become more evident and separately investigable. Operability of the bench-scale unit and feasibility of the combustion process were always ensured in the temperature range 550-800 °C. However, tests carried out at T < 750 °C always exhibited an irregular combustion behavior, such as the continuous random occurrence of uncontrollable micro-explosions. Despite an operation globally at steady state, the outlet gas composition and bed pressure were actual dynamic variables. The paper proposes an interpretation of the above fancy features in relation to the mechanism of liquid fuel combustion in a fluidized bed, the main steps of which are fuel atomization and vaporization, formation of a rising fuel vapor bubble, coalescence with air bubbles, and onset of ignition. Further, the paper introduces two characteristic variables, i.e., the average frequency of micro-explosion events (fe) and the average overpressure (Pmax), which are easily computable by the investigator and allow discrimination of the occurrence of a “regime with micro-explosions”. Furthermore, another output of the study is a model for a first-approximation estimation of the frequency of microexplosion. The model development is simply based on the prediction of the coalescence between fuel and air bubbles, and the resulting ignition of the combustible mixture.