Estudo de fluidização de biomassas lignocelulósicas visando a proposição de um modelo empírico físico

Abstract

Thermal energy conversion industries have been increasingly employing fluidized bed combustion technology, as it currently is one of the most efficient and feasible alternatives for a low carbon scenario. Fluid dynamics researches of fluidized beds provide valuable information for the design and operation of these systems. However, fluidization mathematical modeling, even for homogeneous particles, is a complex subject: it involves modeling gas flow, solids flow and the interaction between them. In the case of mixtures of particles with different physical properties, predict fluidization behavior is a great challenge. Usually empirical correlations are used to determine design parameters, such as characteristic fluidization velocities. The objectives of the present work were to compare characteristic fluidization velocities obtained experimentally with predictions of correlations from literature, and to verify if other physical and physicochemical parameters could be more appropriate for prediction of these velocities. Biomasses evaluated were sugarcane straw and bagasse, eucalyptus wood, rice husk and apple tree branches, in mixtures with sand in the following proportions: 20, 40, 60 and 80 percent of biomass volume fraction. Materials were characterized by their mean particle size (Sauter and Feret methods), true density, loose and tapped bulk densities, chemical, proximate and elemental compositions, higher heating value, repose angle, sphericity and aspect ratio. A bench scale reactor, with 94 mm internal diameter, was used to fluidize biomass and sand mixtures. Measured variables were air flow rate and pressure drop along the bed. Multiple linear regression was performed on SPSS 22 software. Materials characterization showed that biomasses present similar elemental composition, but differences in chemical and proximate compositions. True mass densities varied between 1002 kg/m³ for straw and 2587 kg/m³ for sand, and loose bulk densities varied between 98 kg/m³ for straw and 1499 kg/m³ for sand. Mean Sauter diameter of particles ranged from 363 μm for sand to 931 μm for straw, while Feret mean diameter ranged from 378 μm for sand to 2718 μm for straw. Fluidization tests showed minimum fluidization velocities Umf between 0.11 and 0.38 m/s, and complete fluidization velocities Ucf between 0.18 and 0.82 m/s for mixtures. Biomass and sand mixtures fluidized well in proportions up to 60% in volume of biomass. For proportions above this limit, there was intense formation of preferential channels and the occurrence of agglomerations in bed. Tested correlations were unable to predict Umf and Ucf in any situation, presenting great dispersion of results and relative errors higher than 60%. Multiple linear regression indicated mixtures bulk densities and repose angles as the most relevant parameters in the prediction of characteristic fluidization velocities. The two developed correlations from these properties obtained good results in prediction of these velocities: the adjusted R2 coefficient obtained at Umf prediction was 0.782, while at Ucf prediction it was 0.855. For Umf, on 90% of mixtures the relative error found was smaller than 24.3%, while for the Ucf on 90% of cases this error was smaller than 22.5%. Proposition of repose angle as a predictor for characteristic fluidization velocities are unprecedented, as well as the acknowledgment of bulk density as the most relevant parameter in the prediction of these velocities.