Experimental Study on the Circulating Characteristics of the Return Particles in CFB and the Validity of the Particles to Desulphurization
The cold-model experimental system that particles flow from the top of CFB reactor is set up to study the circulating characteristics of the particles in the reactor. In order to get further information about the particles distribution and interaction with air flow, the two-phase gas-particle flow and particle concentration in the tower are studied with the use of phase-Doppler particle dynamic analyzer (PDA) system. The effects of the operating parameters including the mass flow rate and the superficial gas velocity on the process of the particles return to the main flow were investigated. The definition of valid internal circulation is put forward in this paper to describe the contribution of the particle to the SO 2 removal reaction. The results showed that the concentration distribution tends to be right high and left low in the radial direction. In axial, more particles concentrated in the upper of the reactor, especially in the case of high superficial gas velocity. With the decrease of the superficial gas velocity, the maximum fallback length of particle and the average particle concentration increase quickly. While the valid particle concentration increases slowly. The quantity of valid particles increases a little. Sending the particle from the top of the reactor has little contribution to the SO 2 removal reaction.
Flow patterns in the square cross-section riser of a circulating fluidised bed and the effect of riser exit design
Abstract Solids flow patterns were observed and measured in the square cross-section riser of a laboratory circulating fluidised bed (CFB). Measurements in the 0.14 m square riser used a sampling probe, of internal diameter 3.4 mm, to measure upwards and downwards solids fluxes throughout the cross-section. Interpolation gave complete solids flux profiles over the entire 0.14×0.14 m cross-section. Integration of the solids flux profile gave the external solids circulation rate, in reasonable agreement with measurements external to the riser. Special features of the solids flux profiles are (i) much higher down flux in the corners of the square section and (ii) slightly higher down flux half-way between the corners, near the walls. These features may be due to secondary flow in the square section duct and to enhanced sheltering provided by the two side walls. There is qualitative similarity between these solids flux profiles in the 0.14 m square riser and the profiles measured by Werdermann ((1992, Feststoffbewegung und Warmeubergang in zirkulierenden Wirbelschichten von Kohlekraftwerken. Dr-Ingenieur dissertation, Technical University Hamburg-Harburg) for large industrial combustors. But it is clear that there is a scale-up effect: the film, i.e. the cross-sectional area of duct containing the downflow (adjacent to the walls), is a much smaller proportion of the total area in the large units as compared with the laboratory units. The detail design of the riser exit or outlet bend at the top of the riser has a profound effect upon solids flux profiles in the riser and in particular on the reflux ratio defined as k m =(solids downward flow)/(external circulation rate). Measured values of k m varied by a factor of about 20 according to outlet bend design. These measurements were mostly near the top of the riser, but indications are that k m is affected, throughout the riser, by the design of outlet bend, a phenomenon previously observed in a CFB of circular cross-section by Brereton and Grace ((1994). In A. A. Avidan, Circulating Fluidized Bed Technology IV (pp. 137–144). New York: AIChE).
Hydrodynamics of circulating fluidized bed risers: A review
Any vessel in which solids are transported upward by a gas stream and then recycled to the bottom may be classified as a Circulating Fluidized Bed (CFB). We describe possible CFB operating regimes in the context of this broad classification and highlight commercial processes that employ CFB technology and potential applications. Process design and development require a fundamental understanding of gas and solids hydrodynamics — solids hold-up, mixing and velocity distribution. We discuss techniques used to measure solids mass flux, which is a critical parameter for both design and control. In the last decade, significant research efforts have been devoted to new experimental techniques to measure both gas and solids spatial and temporal distribution. We list these techniques and detail the different modelling approaches that have emerged based on the new data. Characterization of the data is still incomplete and the available models require further refinement to reliably predict the effect of scale, operating conditions and particle characteristics on hydrodynamics.