Fundamental studies of biomass fluidization

Abstract

The world is presently at a point of making important decisions regarding how to minimize disasters such as super storm, super earthquakes, and landslides that are caused by global warming due to fossil fuel exploration and consumption. One route is to use varieties of renewable energy resources such as solar, wind, biomass etc. to replace some of the fossil fuels consumed. Biomass such as wood, energy plants (switchgrass, poplar, and willow) is the only renewable energy that can be converted into liquid fuel. However, in the process of converting biomass to fuel, there are numerous technical challenges that are encountered primarily due to assumptions made in measuring values of physical properties of biomass grinds (e.g. size, particle geometry, and voidage). These properties are utilized in the design and sizing of biomass conversion and processing equipment and facilities. For example, the use of average (mean) particle diameter in fluidization models and equation seems to imply that biomass ground particles are uniformly sized, thus neglecting the size distribution of biomass. In addition, as non-spherical particles, the use of a mean diameter is problematic because axis of measurement significantly influences the value of diameter. Some mean diameter that can be obtained from non-spherical particle include, the diameter of a sphere that has the same surface area to volume ratio as a particle, Martins, Ferret and Sauter mean diameter. At present comprehensive investigation on contribution of physical properties vis-a-vis particle size measurement scheme on predictability of loblolly pine wood grind minimum fluidization velocity and other parameters that are used in designing fluidized bed system is lacking in literature. Hence, this study investigated how the physical properties of loblolly pine wood influence the ability to predict important design parameters and the behavior of biomass grinds during fluidization. The result showed that loblolly pine wood grinds have mean particle density of 1460.6±7 kg/m3, bulk density of 311± 37 kg/m3 and porosity of 0.787 ± 0.003. With regards to particle size, the Ferret diameter was found to be higher than surface-volume diameter, Martin’s diameter and chord diameter by 18.3, 23.6, and 7.03% respectively. Also, the shape characteristic based on the sphericity value of biomass grinds ranges between 0.235 and 0.603, thus indicating that biomass grinds have flat shape. The minimum fluidization velocity of ground loblolly pine wood (unfractionated samples) was found to be 0.25 ±0.04 m/s. For fractionated samples, the minimum fluidization velocity increased from 0.29 to 0.81 m/s as fractionating screen size increased from 0.15 to 1.70 mm. Predictions of minimum fluidization velocity obtained from selected equations were significantly different from the measured values. Moreover, increase in moisture content increased the bulk density, particle density, and porosity but the particle size coefficient of variation reduced from 90 at 8.46% MC to 42 at 27.02 % MC. Increase in moisture content also increased the minimum fluidization velocity of unfractionated grinds from 0.20 to 0.32 m/s as moisture was increased from 8.45 to 27.02 % wet basis respectively. In addition, the correlation developed predicted the experimental data for minimum fluidization velocity with mean deviation less than 10%. For the modified Ergun equation, the coefficients K1 and K2 were estimated to be 201 and 2.7 respectively. The overall mean relative deviation obtained between the predicted and experimental pressure drop using loblolly pine wood grind and the equation developed in this study and the Ergun equation were -17.48 and 63.5 % respectively. Finally, an evaluation of the interface exchange drag law equations (Gidaspow, Syamlal-Obrien, Wen & Yu and non-spherical) showed that non-spherical drag law equation predicted the minimum fluidization velocity with mean relative deviation of 4.5%. It was also observed that bed entrainment occurred for all the drag law equation at 2 sec simulation time. The drag law equations under-predicted the bed pressure drop, and mass entrainment above 0.2 m/s superficial airflow velocity after bed material entrainment has begun. The incorporation of body force improved the predictability of the non-spherical drag law equation but exhibited little impact on the other drag law equations. Finally, this study has demonstrated the possibility of improving prediction of important parameters of fluidize bed system when particles with non-uniform and irregular geometry is used.