MODELING OF SOLIDS HOLDUP IN CIRCULATING FLUIDIZED BED RISERS Q. Miao, J. Zhu, S. Barghi, C. Wang*, X. L. Yin*, C. Z. Wu* Particle Technology Research Centre, University of Western Ontario, London, Canada N6A 5B9 *CAS Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China Abstract: Hydrodynamics plays a crucial role in defining the performance of gas-solid circulating fluidized beds (CFB). A two dimensional model was developed considering the hydrodynamic behavior of CFB gasifiers. In the modeling, the CFB riser was divided into two regions: a dense region at the bottom and a dilute region at the top of the riser. Radial distributions of bed voidage were taken into account in the upper zone by using Zhang et al. (1991)’s correlation. For model validation purposes, a cold model CFB was employed, in which sawdust was transported with air as fluidizing agent. The column was 10 m in height and 280 mm in diameter, and was equipped with pressure transducers to measure axial pressure profile and with a reflective optical fiber probe to measure local solids holdup. A satisfactory agreement between the model predictions and experimental data was found. Keywords: Hydrodynamics, Modeling, Circulating Fluidized Beds, Biomass, Gasification 1. INTRODUCTION With continuously growing energy demand for global economic development and increasing environmental issues caused by fossil fuels, considerable attention is being given to the use of biomass for energy generation, particularly for fuel gas/product gas production by gasification as the produced fuel gas can be flexibly applied in boilers, engines, gas turbines or fuel cells. CFB gasification is now undergoing rapid commercialization for biomass (Yin et al. (2002); Kersten et al. (2003)). The flow structure of gas-solid flows in a CFB is inherently very complex. Up to now, considerable work has been done on modeling of CFB reactors (Grace et al. (1997)). Many modeling efforts with various assumptions and different mathematical formulations have been reported in literature. Harris and Davidson (1994) have classified these into three categories: (I) those that predict axial variation in solids suspension density, but not radial variations; (II) those that predict both axial and radial variations by assuming two or more regions, for example core-annulus flow structure or clustering annular flow models; (III) those that employ the fundamental equations of fluid dynamics to predict two-phase gas-solid flow. Type I models are oversimplified to provide rough predictions. The radial variations are completely neglected, which seriously limited the ability of the models to represent CFB reactors. Type III models are the most rigorous, but the required simplifying assumptions combined with their mathematical complexity limit their use to studies of specific flow structures within the riser. Type II models provide more details concerning the radial distribution of solids and allow one to investigate the flow structure of CFB risers in a two-dimensional way. Although the development of hydrodynamic modeling is prosperous, very limited work is focused on biomass particles. Whereas there has been considerable effort to develop new biomass gasification, combustion, pyrolysis and bio-conversion processes, relatively few authors have characterized the relevant flow characteristics of biomass particles in circulating fluidized beds or investigated measures that could assist in resolving flow issues. Cui and Grace (2007) reviewed recent researches on the hydrodynamics and mixing of biomass particles in fluidized beds.