CFD analysis of fast pyrolysis process in a pilot-scale auger reactor

In this work, an auger pilot-scale fast pyrolysis process computational fluid dynamic (CFD) model was developed for use as a design tool for scale-up. Multiphase flow dynamics and chemical kinetics were included in the multi-fluid model (MFM). Rotating reference frame (RRF) was adopted to simulate the effect of rotation of the auger in the reactor. The model predictions were validated with experimental data at three temperatures (450, 475, and 500°C) and four biomass feed rates (1, 1.5, 2.5, 3.5 kg/h). Good agreement was observed between the simulations and the experiment. A parametric study of the process was carried out to study the impact of operating factors including biomass feed rate (1-4 kg/h), operating temperature (400-600°C), and vacuum pressure (0-500 mbar). Other parameters studied included using nitrogen as a carrier gas (1-10 kg/h) and varying the angular velocity of the screw (45-95 rpm). The results illustrate that the predicted optimum temperature for maximising bio-oil production is 500°C. Bio-oil yield increased as the biomass feed flow rate increased due to shorter vapour residence time, minimising further reaction of the non-condensable fraction in the vapour phase. Introducing nitrogen shows the same effect, increased yield due to decreased vapour residence time. Increasing the angular velocity of the screw enhances the flow of vapours in the reactor; however, the rotational speed must be balanced against the increase in unreacted biomass. The simulation gave an optimum of 70 rpm for the angular velocity of the screw.