Numerical Study for the Design of a Fluidized Bed Reactor for Biomass Gasification

Abstract

In biomass waste gasification, raw atmospheric air is commonly used as the gasification agent due to its affordability and availability. This leads to syngas with a low heating value and high tar content, limiting its industrial application. While fluidized bed gasifiers are the prefered gasification technology due to their enhanced heat and mass transfer rates, experimenally investigating their hydrodynamic parameters and optimizing the gasification process is challenging. The high temperatures involved, reaching of up to 1000 oC, complicate fluidized bed experiments, making numerical simulations valuable for such investigations. Existing simulation models are tailored for room temperature, rendering them inadequate for high-temperature applications. This study aimed to numerically investigate the fluidized bed hydrodynamics at high temperatures and optimize the gasification process to maximize syngas quality while reducing the amount of tar. The first part of the study investigated fluidized bed hydrodynamics at high temperatures. Computational fluid dynamics simulations were conducted using OpenFOAM. A 3D model based on the Eulerian-Eulerian approach was developed. Inert sand particles of three sizes (233, 335, and 500 μm) were used as the bed material, with temperatures ranging from 25 to 400 oC. The simulation results were validated using experiments conducted in a laboratory-scale fluidized bed unit under the same temperature and particle size range. It was found that the temperature of the bed materials significantly affected the fluidized bed hydrodynamics. Increasing the temperature from 25 to 400 °C led to a 40 % decrease in the minimum fluidization velocity, while increasing the particle size from 233 to 500 μm resulted in a 61 % increase in the the minimum fluidization velocity. Additionally, the bed porosity at the minimum fluidization point increased by 4.6 % while the bed height increased by 17 % over the same temperature range. The simulation results were in good agreement with the experimental results, with a percentage mean absolute error of 6.1 %. The second part of the study was a sensitivity analysis on the effect of gasification input variables on the gasifier outputs using Aspen Plus. The optimal ranges for the equivalence ratio, air preheating temperature, and gasifier pressure were 0.15-0.33, 25-625 oC, and 1-4 atm, respectively. A multi-objective optimization using the Box-Behnken design response surface methodology was then conducted within these optimal ranges to maximize syngas composition and higher heating value while minimizing tar. The analysis of variance revealed that the equivalence ratio had the most impact on hydrogen and methane production, higher heating value, and tar content, while pressure had the least influence. Conversely, air temperature was the most influential factor in carbon dioxide production, whereas the equivalence ratio had the least effect. Two optimal solutions were obtained for gasifiers. The first solution is a pressurized gasifier, operating at 4 atm.The optimal values for hydrogen, carbon monoxide, methane, higher heating value, and tar were 11.41 %, 14.41 %, 2.19 %, 4.30 MJ/Nm³, and 23.68 g/Nm³, respectively. The most optimized points for the equivalence ratio and air temperature were 0.16 and 575 °C, respectively. The second solution is an atmospheric gasifier, operating at 1 atm. The optimal values for hydrogen, carbon monoxide, methane, higher heating value, and tar were 10.07 %, 14.52 %, 2.21 %, 4.14 MJ/Nm³, and 29.17 g/Nm³, respectively. The most optimized points for the equivalence ratio and air temperature were 0.15 and 445 °C, respectively. This study provides valuable insights into fluidized bed hydrodynamics and the optimization of gasification processes parameters. The developed models offer practical guidelines for selecting optimal gasifier conditions to maximize syngas energy density and minimize tar content.