Aspectos fundamentais da pirólise de biomassa em leito de jorro: fluidodinâmica e cinética do processo

Abstract

Pyrolysis is considered to be the most promising thermal conversion technology. The possibility of producing specialty chemicals and higher added value products makes the process very attractive when compared to other sources of green energy. Pyrolysis is characterized by thermal degradation of the solid fuel, what imply on the rupture of carbon-oxygen bond. The products formed during pyrolysis, as coal fines, gases, and volatiles (acid extract and bio-oil), have high calorific power and have been had many application in chemical industries and in energy generation. The chemical reactor is the main element of the applied chemical processes and for showing a good solid-gas contact, high mass, heat transfer and particle recirculation rates, the spouted and fluidized bed have become efficient options to the success of fast pyrolysis because they prone the physic-chemical processes that occur during pyrolysis. This work has as main objective, to study the fundamental aspects of the biomass pyrolysis in spouted bed that are highly important to the process optimization. Among these aspects, the pyrolysis kinetics and the fluid dynamic behavior of the fluid and the particles inside the equipment stand out. The pyrolysis kinetics of the sugarcane bagasse was analyzed, using experimental data of mass loss by thermogravimetry. The parameters of some of the main kinetic models available on literature were estimated. The results were compared and the model that best represents the data was chosen. The results of the investigation of the primary thermal degradation kinetics showed that the mass loss during thermogravimetry is best represented by parallel and independent reactions of its subcomponent. The calculated values of activation energy allow presuming the necessary amount of energy for the reaction to occur. Regarding the fluid dynamics of the process, the particle mixing experiments in spouted bed allowed a better comprehension of the segregation mechanisms in the equipment. Segregation occurs due to the different terminal velocities of the particles, which may occur due to size and density differences. Tests with glass spheres of different diameter showed that the bigger particles concentrate mainly on the superior annular region. Regarding particles with different densities and same size, represented by the mixing of glass sphere and polyethylene with same diameter, it was observed that the less dense particles, polyethylene, concentrate on the superior region of the bed. It may have occurred due to the low elasticity of the collision of the low density particles with the wall and its difficult on sliding into the wall. In this case, it was defined the minimal spout condition as being the air flow that both particles spout and an increase on the air flow did not cause changes on the pressure drop. Varying the particle size and density, experiments in spouted bed using bagasse and sand in different compositions were carried out. In this step, it was identified a range of composition in which the mixture shows good circulation in the bed, as it was evaluated the particle segregation index in bed. CFD simulations involving a multiphase dense system, with more than one particulate phase, were compared to the data obtained experimentally using glass spheres with different diameters, showing good prediction of the axial distribution data of the volumetric fraction of particles. Beyond that, data about the particle segregation in spouted bed, available on literature, were also reproduced by simulation using CFD. The knowledge of the kinetic and fluid dynamic of the process is the initial and preponderant step to implementation of pyrolysis process in pilot scale. As the determination of the ratio between sand mass and biomass is the critical point of the pyrolysis process, a good understanding of the segregation mechanisms of particle mixtures may help on the identification of the optimum conditions to operate the reactor.