Transporte pneumático horizontal de biomassas: experimentos em unidade piloto e simulações numéricas em cfd

Abstract

Biomass pneumatic conveying has been widely used in energy and chemical industries. The characteristics and stability of the gas-solid flow are very important for the final product quality, design, and operation of the industrial plants. Due to its different physical properties, operational problems are frequently verified during the transport of biomass, generally associated with the feeding process control. Bearing this in mind, one of the aims of this work was to evaluate the rotary valve feeding efficiency in a positive pressure pneumatic conveying line, using biomass with different physical properties and varying operating conditions such as: air velocity, valve rotational speed, and height of material in the feeding silo. The biomasses used were: milled corn, corn cob, corn straw, rice husk, and wheat bran. They were characterized as to their density, size, shape, angle of repose, and moisture content. It was observed that the height of material in the silo influenced the feeding efficiency. Materials of low bulk density obtained greater feeding efficiencies for the conditions of lower material height in the silo due to easier breaking of the mechanical/cohesive arches formed. Moreover, it was proposed a modification in an empirical correlation presented by Lourenço (2019) to predict the pressure drop in the feeding Tee, from the insertion of a form factor in its equation, due to the irregularity of the biomass particles. The results obtained by the modified correlation showed good agreement with the experimental data, with a maximum deviation of less than 10%. In addition, a study of the pressure drop was carried out for the pneumatic conveying of biomass in dilute phase in a horizontal stretch of 9 m, in which it was possible to observe that the increase in the solids' mass flow rate, as well as the air velocity, provided the increase of the pressure drop in the line. Through static pressure profiles along the length, the regions of acceleration and fully developed flow were identified and, then, the acceleration length was determined for each material and operational condition evaluated. It was concluded that the acceleration length increases with increasing air velocity, solids' mass flow rate, and, consequently, solids loading. Numerical simulations in CFD (Computational Fluid Dynamics) were performed to predict the pressure drop in the horizontal length to describe the dilute phase pneumatic conveying of corn cob, using the Granular Eulerian approach and applying the RGN turbulence model (k-ε) – Dispersed. The numerical study was able to satisfactorily predict the static pressure profile along its length, showing an average deviation of 0.71% in relation to the experimental data for the operational condition of air velocity of 16 m/s and solids' mass flow rate of 437 kg/h. In addition, the model was able to describe qualitatively the distribution of particles along the line, making it possible to identify the formation of rope in the curve and the establishment of the fully developed flow region according to the similarity of the profiles of solids' volumetric fractions in the cross-sections of the straight stretch.