Non-premixed fluidized bed combustion of C1-C4n-alkanes

The non-premixed combustion of C₁-C₄n-alkanes with air was investigated inside a bubbling fluidized bed of inert sand particles at intermediate temperatures: 923 K ? T_B ? 1123 K. For ethane, propane and n-butane, combustion occurred mainly in the freeboard region at bed temperatures below T₁ = 923 K. On the other hand, complete conversion occurred within 0.2 m of the injector at: T₂ = 1073 K. For methane, the measured values of T₁ and T₂ were significantly higher at 1023 K and above 1123 K, respectively. The fluidized bed combustion was accurately modeled with first-order global kinetics and one PFR model to represent the main fluidized bed body. The measured global reaction rates for C₂-C₄n-alkanes were characterized by a uniform Arrhenius expression, while the global reaction rate for methane was significantly slower. Reactions in the injector region either led to significant conversion in that zone or an autoignition delay inside the main fluidized bed body. The conversion in the injector region increased with rising fluidized bed temperature and decreased with increasing jet velocity. To account for the promoting and inhibiting effects, an analogy was made with the concept of induction time: the PFR length (b_i) of the injector region was correlated to the fluidized bed temperature and jet velocity using an Arrhenius expression. These results show that the conversion of C₂-C₄n-alkanes can be estimated with one set of critical bed temperatures and modeled with one Arrhenius kinetics expression.