Zhengyang Wang

Numerical Simulation of the Gas-Solid Flow in a Square Circulating Fluidized Bed with Secondary Air Injection

The dynamic behavior of gas-solid flow in an experimental square circulating fluidized bed setup (0.25 m × 0.25 m × 6.07 m) is predicted with numerical simulation based on the theory of Euler-Euler gas-solid two-phase flow and the kinetic theory of granular flows. The simulation includes the operation cases with secondary injection and without air-staging. The pressure drop profile, local solids concentration and particle velocity was compared with experimental results. Both simulation and experimental results show that solids concentration increases significantly below the secondary air injection ports when air-staging is adopted. Furthermore, the flow asymmetry in the solid entrance region of the bed was investigated based on the particle concentration/velocity profile. The simulation results are in agreement with the experimental results qualitatively.

Experimental Investigation on Secondary Air Jet Penetration Depth in the Dense Zone of a Circulating Fluidized Bed

The secondary air jet penetration depth of dense zone is measured in a 0.25m×0.25m×6.07m circulating fluidized bed. The influence of gas velocities (U0), nozzle diameters, jet velocities, nozzle angel and the solids inventory is studied. A modified empirical correlation for calculating the secondary air jet penetration depth in the dense zone is developed. SA jet penetration depth increases with the increase of jet velocity. With the same flow rate, jet penetration depth becomes smaller as the diameter increases. Horizontal jet has the maximum jet penetration depth. The increasing of gas velocities, nozzle angel and solids inventory will reduce the jet penetration depth.

Numerical Study on the Stereo-Staged Combustion Properties of a 600 MWe Tangentially Fired Boiler

Numerical study of the combustion process and NO emission of a 600MWe tangentially fired boiler with stereo- staged combustion system were performed by Fluent 6.2 CFD software. The stereo-staged combustion technology is consist of horizontal bias combustion (HBC) burners, separated over fire air (SOFA), horizontal offset secondary air and pulverized coal reburning arrangement. The temperature field and the distribution of combustion species concentration in the furnace were gained as well as the aerodynamic field. The simulation results shows that stereo-staged combustion system can keep good combustion stability and the NOx emission of the boiler reduces significantly when the HBC burners and horizontal offset secondary air are adopted.

Effects of Coarse Particle Addition on Solids Distribution of CFB with Air-Staging

To study the influence of coarse particle addition on solids distribution of circulating fluidized bed (CFB), some experiments has been done on a 0.25m¡Á0.25m¡Á6.07m square circulating fluidized bed in this paper. The axial pressure drop, local solids holdup and coarse particle fraction in the CFB system was measured and analyzed. Experimental results show that the pressure drop increases with the increasing of initial coarse particle fraction (Xc0) no matter with or without air staging. Comparatively, the pressure drop is larger for the case with air-staging. With same initial coarse particle fraction Xc0, the coarse particle fraction is larger in the bottom bed but smaller in the upper bed and external circulating system when air-staging is adopted.

Experimental Research on the Corner Secondary Air Injection of Square Circulating Fluidized Bed

Corner secondary air injection which four nozzles respectively located on four corners of square circulating fluidized bed (CFB) was investigated in this study. The cold CFB model has a square cross-section of 0.25m×0.25m and a height of 6.07m. The axial pressure drop profile along the riser has been got by differential pressure measurement to evaluate the axial solids concentration distribution in different operating conditions. Solids concentration increases significantly below the secondary air injection ports especially when high secondary air rate is adopted. Using CO2 as tracer, the dispersion of corner secondary air jet was investigated. It is found that if the solids concentration near nozzles’ outlet is lean, the secondary air injection with high velocity will make a swirling flow in the riser. However, higher solids concentration distribution especially in the corner region will limit the penetration of secondary air injection and no apparent swirling flow was found in these conditions. Keywordssquare circulating fluidized bed; corner secondary air injection; solids concentration; dispersion of secondary air.

Gas-Solids Flow Properties of a Square Circulating Fluidized Bed with Secondary Air Injection

Effects of secondary air (SA) injection or air-staging on the gas-solids flow properties in the riser of a circulating fluidized bed (CFB) were investigated. The experiments were carried out in a CFB cold model with a square cross-section of 0.25m×0.25m and a height of 6.07m. The axial pressure drop profile along the riser was reported. And the local solids holdup profile at the centerline and the diagonal line of some cross sections which influenced by the SA injection was measured with an optical fiber probe. Two SA arrangement modes, i.e. four SA nozzles located on four walls (Wall SA) and four corners (Corner SA) were conducted. Air-staging results in a denser bottom bed for both two SA modes. The Wall SA case has a higher solids holdup than the Corner SA case in most regions of lower bed except the corner region but was leaner in the vicinity of SA injection level. W-shaped solids concentration profile was found in the region immediately above the SA injection level for Wall SA case but not obvious for Corner SA case. Fractal dimension was analyzed for pressure drop fluctuations and local solids concentration signal. Air-staging led to smaller fractal dimension value than the case without SA injection. The SA jets affected the local gas-solids distribution and fluctuation in the region close to the SA injection. Radial and axial solids transfer should be considered for the fractal analysis.

Bottom Zone Flow Properties of a Square Circulating Fluidized Bed with Air Staging

The recycled solids fed to the circulating fluidized bed (CFB) riser on one wall have a limited lateral dispersion rate, which results in an uneven solids distribution in the bottom zone of the bed. Using a reflective optical fiber solids concentration measuring system and visual observation, the flow properties in the bottom zone of a fluidized bed was investigated. The cold model CFB riser has a square cross-section of 0.25m×0.25m and a height of 6.07m with quartz sand as bed material (dp=276μm). At the condition of u 0=4m/s, Gs=21kg/m2s, the back feeding particles can penetrate the gas-solid flow in the riser and get to the opposite wall. It makes a denser region near the front wall of the bed. When air-staging is adopted, solids holdup increases below the secondary air (SA) injection ports with the increase of the rate of the secondary air to the total air (SAR). The solids holdup increase also exists above the secondary air injection ports but the incremental amount decreases quickly along the riser. At SAR=0.3, a denser bottom zone limits the back feeding particles’ penetration and a relatively even solids distribution is formed in the bottom bed. A W-shaped solids concentration profile is formed just above the SA injection level. At SAR=0.5, uneven solids concentration profile exists in the bottom bed and no W-shaped solids concentration profile is formed for the deeper penetration of SA jets.