Influence of particle residence time distribution on the biomass pyrolysis in a rotary kiln

Rotary kilns are widely used in industry to perform several different operations. When numerically modeling such systems, a common simplification is to neglect the residence time distribution of particles. The present study numerically investigates the influence of particle residence time distribution on the pyrolysis of biomass in a cocurrent heated rotary kiln. First, the model (balance equations, reaction scheme, flow model, and heat transfer mechanisms) is developed. The applied particle flow model (Seaman's model) is then validated by comparing the calculated mean residence time tau and bed height to the results of a discrete element method (DEM) simulation of an example case. The mean residence time (tau = 552 s) and the bed height (H(z=0) = 52 mm) of the Seaman's model was found to be in good agreement with the results of the DEM simulation (tau = 542 s, H(z=0) = 50 mm). A detailed discussion of the residence time distribution's influence on the simulated conversion for the example case is then presented. We performed 54 simulations with various Pe ' clet numbers between 1 and 100 and showed that the product yield significantly changes for Pe ' clet numbers below 10, that is, neglecting residence time distribution (RTD) may not be a true assumption in all operating conditions. Finally, performing a comprehensive parameter study, the RTD's influence under various operating conditions and kiln designs is investigated. The varied parameters are the kiln diameter (D = 0.1-1 m) the ratio of particle to kiln diameter (d/D = 5 x 10-3-40 x 10-3), the ratio of kiln length to kiln diameter (L/D = 1-10), the kiln's inclination angle (beta = 0.1-8 circle), the Froude number (Fr = 10-3-10-2), the rotational Reynolds number (Re = 102-16 x 103), and the Pe ' clet number (Pe = 5-100). Using more than 13000 simulations, we show that in one third of the investigated cases the change of product yield is higher than 2% when changing from plug flow behavior to high axial dispersion (Pe = 5). For cases where the influence of the RTD on the final product yield is small, conditions along the kiln might still change significantly. The biggest change of the product yield from plug flow behavior to high axial dispersion was found to be 5.5 wt% for the following conditions: D = 1 m, d/D =5 x 10-3, L/D = 1, beta = 0.1, Fr = 10-2 and Re omega = 16,000.