Modelling of the turbulent gas–particle flow structure in a two-dimensional circulating fluidized bed riser

Laboratoire Matériaux, Procédés et Energie Solaire, CNRS-PROMES, 7 rue du Four Solaire, 66120 Font-Romeu Odeillo, France
Chemical Engineering Science 01/2007; DOI: 10.1016/j.ces.2006.08.066

ABSTRACT The gas–particle turbulent flow in a circulating fluidized bed (CFB) riser is investigated numerically by large eddy simulation (LES) coupled with Lagrangian approach. The gas phase model is based on locally averaged two-dimensional Navier–Stokes equations (N–S Eqs.) for two-phase flow with fluid turbulence calculated by LES, in which the effect of particles on subgrid-scale (SGS) gas flow is taken into account. The particles’ motion is treated by a Lagrangian approach, in which the particles are assumed to interact through binary, instantaneous, and non-elastic collisions. The model predicts the heterogeneous particle flow structure and the gas and particle mean velocities and turbulent intensities. The gas turbulence induces particle lateral dispersion. The instantaneous gas turbulent intensity is attenuated by the formation of clusters, whereas the intensity is enhanced by the increase of gas–particle interactions. Globally, the presence of clusters enhances the gas turbulent intensity. Finally, the turbulent model is noticeably affected by an empirically assigned gas constant Ck and it consistently predicts values of the gas and particle turbulent intensities higher than those predicted by Smagorinsky's method.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Dense gas-particle flows are frequently encountered in fluidized beds, riser and downer reactors, pneumatic transport and the near-wall zone of dilute gas-particle flows. Particle-particle collision plays an important role in the behavior of two-phase flows. In this paper a USM-Q two-phase turbulence model for dense gas-particle flows is proposed to account for both two-phase turbulence and inter-particle collision. For two-fluid large-eddy simulation of gas-particle flows, the author proposed a unified second-order moment (USM) two-phase SGS stress model and a two-phase k-k p SGS energy-equation stress model. The proposed models can fully account for the interaction between the gas and particle SGS stresses. Keywordsdensed gas-particle flows–second-order moment model–two-phase turbulence–large-eddy simulation
    Science China: Physics, Mechanics and Astronomy 07/2011; 54(7):1296-1303. · 1.17 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Carbon Capture and Storage (CCS) is the critical enabling technology that would reduce CO2 emissions significantly while also allowing fossil fuels to meet the world's pressing energy needs. The International Energy Agency analysis shows that although the developed world must lead the CCS effort in the next decade, there is an urgent need to spread CCS to the developing world. Given technologies for reducing GHG emissions originate mainly in developed countries, technology transfer, as an important feature emphasized by both the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol, therefore has a key role to play in bridging a gap between developed and developing countries. The main objective of this paper is to explore potential policies and schemes promoting the transfer of CCS technologies to developing countries. First, it reviews the global CCS status, analyzes the significant gap of CCS in developed and developing countries, and investigates stakeholder perceptions of diffusing CCS to China, which is a major developing country and a significant potential candidate for large-scale CCS deployment; then the authors make an attempt to understand technology transfer including its benefits, barriers, and definition. The UNFCCC explicitly commits the developed (Annex I) countries to provide financial and technical support to developing countries under favorable terms. The authors argue that the ultimate goal of technology transfer should not only be limited to apply CCS in developing countries, but also to enhance their endogenous capabilities, which will enable future innovation and ensure long-term adoption of low-carbon technologies. As a result, the authors propose a four-pronged approach to the transfer of CCS technologies, which involves physical transfer of explicit technologies, a financial mechanism, endogenous capacity building, and a monitoring mechanism. Concrete enhanced actions to promote CCS technology transfer are also proposed. The four-pronged approach and related enhanced actions proposed in this paper are also applicable to other low-carbon technology transfer.
    Energy Policy. 01/2011; 39(6):3106-3116.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The axial and lateral solids holdup profiles in a 2-D circulating fluidized bed (CFB) were measured with an optical fibre probe under a wide range of operating conditions. The CFB is 7.6m in height and has a 19×114mm2 narrow cross-section riser. The results showed that the operating conditions influence the flow structure significantly and control the flow in the same manner as that in cylindrical risers. The solids had lower concentrations at the riser centre than the near wall region. Compared with data from cylindrical columns, the axial and lateral profiles of solids holdup in 2-D riser had a similar pattern in shape, but were more uniform. The geometry of the riser was found to be an important factor that affects the solids distribution due to differences in terms of the perimeter per unit cross-sectional area and the wall-to-centre distance. To some extent, the two-dimensional and three-dimensional risers are more comparable under fast fluidization conditions. Generally, the solids distributions along the axial and the lateral directions in 2-D riser were dissimilar to those in cylindrical risers, while the main differences have been discussed in the current study.
    Chemical Engineering Science - CHEM ENG SCI. 01/2010; 65(20):5447-5454.