(122e) The Effect of Pressure On Bubble Parameters in Jetting Fluidized Bed At High Fluidizing Gas Velocity

The effect of pressure on bubble parameters in jetting
fluidized bed at high fluidizing gas velocity

Junguo Li, Zhonghu Cheng, Yitian Fang

 State Key
Laboratory of Coal Conversion, Institute
of Coal Chemistry, Chinese Academy
of Sciences, Taiyuan,
030001, P. R. China

Introduction

       Lignite
is abundant and makes up approximately 40% of coal reserves in the world.
Commercial use of lignite is forecast to grow substantially owing to the
occurrence of deposits in such countries as India,
China, and Southeast Asia,
which are experiencing the strongest growth in electricity demand.¹ Also inferior anthracite and non-coking
bituminous coal is abundant in China.
What is worse, more than 50 percent of coal has a high ash melting point. Commercial use of the
above mentioned coal is forecast to grow substantially owing to the lack of oil
in China, which is experiencing the strongest growth in the development of coal
chemical industry.² However, inferior
anthracite usually has high ash and sulfur content and high ash fusion
temperature. In addition, lignite has high water and oxygen content, low energy
density, being prone to weathering and spontaneous combustion. All these
characteristics limit their high efficient utilization. Fortunately, fluidized
bed is considered as a promising gas producing technology due to its better
adaptability to the poor coal and effectiveness in the control of pollutant
emissions. However, it has
the disadvantages of difficult scale-up, and relative lower capacity. Considering
the developed large-scale gasification technology is all on the base of
increasing of pressure, the behaviour of gas-fluidized beds at high pressures
is of interest from theoretical and practical points of view.

       Jetting
fluidized bed (JFB), another type of spouted bed, where the jetting phenomena
is an important considerations, is the spouted fluid bed, especially when the
jet does not penetrate through the bed like that in a spouted bed.³ One of the important applications of
the JFB is the ash-agglomerating fluidized-bed coal gasifier (U-gas, KRW, and ICC).
Several demonstration projects of this coal gasification process have been
established in China
and USA.⁴ Many researchers pay
attention to the jet penetration depth; however, too few papers report on the
bubble dynamic of the JFB. After all, bubble characteristics have been well
recognized as one of the most important parameters of conventional gas-solid
fluidized bed.

       The
present work aimed to perform experimental investigations on the bubble dynamic
in a two dimensional JFB. It focused on systematically examining the effects of
pressure, fluidizing gas velocity, jetting gas velocity and static bed height
on the bubble parameters and its new correlation.

Experimental
Setup

Apparatus and materials

Experiments
were conducted utilizing a high-pressure fluidization cold model facility.
Briefly the facility comprises a pressure vessel, a
nitrogen supply system, CCD video cameras, a video imaging system,
an optic fiber system and peripheral equipment. The pressure vessel, capable of
operating at pressures up to 3.3MPa, is 1.6 m
high with an inside diameter of 1.2 m.
The interior of the vessel is lighted by internal LED lamps. The nitrogen
supply system uses a high-pressure reciprocating pump. No nitrogen
recirculation system is used. Supply pressure in the reciprocating pump is maintained
by a central liquid nitrogen system. Flow control is accomplished by use of a
parallel mass flow meter setup. The control valve is used for fluidized bed
distributor as the fluidized gas, and the control valve is used for jetting
tube as the jetting gas. An upstream pressure control valve and a back pressure
control valve are used to maintain the system pressure. Experiments were
carried out in a 2D Plexiglas jetting fluidized bed which has a cross section
of a = 300 mm, b = 30 mm and total height of 1450 mm (see Fig. 1). The area of jetting
nozzle is c = 50 mm, b = 30 mm and height l₃ = 400 mm. A V type gas distributor which has a b
= 90 ° conical angle and height l₂ = 100 mm was located at the bottom of the bed
and the total area of all orifices is 1.2%.

The
bed material was polystyrene with the sphericity close to 1. The mean particle
diameter was 1.32 mm and
the particle density was 1020 Kg/m³,
which is close to the group B/group D boundary using Geldart's definition. The
minimum fluidization velocity, u_mf, was then determined from
the plot of pressure drop vs. velocity of nitrogen in the traditional way at
0.7, 1.1, 1.6 and 2.1 MPa. The terminal velocity, u_t, was then
determined from the terminal velocity of a falling particle under different
pressure. A summary of experimental conditions and the particles fluidization
properties studied in this work were listed in Tables 1 and 2, respectively.

Video imaging system and image
processing

The
low-cost CCD cameras which are able to work under pressure are customized and
the image is clear under lower pressure. However, as the pressure
increases, the density and refractive index of the gas are constantly larger
and camera image becomes increasingly blurred. Fortunately, changing the focal
length appropriately, the image becomes clear again under higher pressure. In
the experiment three groups of CCD cameras which have different focal length
are installed to observe bubbles and solids motion form in the high-pressure
fluidization cold model facility. The image of the first group cameras is clear
only under lower pressure (0.1 - 0.7 MPa), the second is clear under middle
pressure (0.8 - 1.4 MPa), and the last is clear only under higher pressure (1.5
- 2.1 MPa). The cable, sealed in the flange, is capable of transmitting imaging
signal to an external video imaging system. And then the films of digital video
image from the video camera are transferred to PC computer. With the image
processing toolbox of Matlab, the software is developed to automate the
procedures for picture acquisition, data processing and analysis for frame by
frame. Then the data about bubble diameter in the bed is acquired.

Table 1
Experimental conditions

Table 2 Minimum
and terminal velocities in fluidization of polystyrene under pressure

Fig. 1. Scheme of
the experimental set-up. 1-computer; 2-data processing system; 3-mass flow meter/controller;
4-pressure vessel;
5-optical fiber
probe; 6-two dimensional
jetting fluidized bed; 7- LED lamps; 8-CCD camera.

Results and
Discussions

       Experimental
data about bubble equivalent diameter distribution versus height above the top
of air distributor at 2.1 MPa are presented in Fig. 2 for an identical static
height of H₀/D = 0.87, jetting gas velocity u_j = 1.25 u_t and fluidizing gas velocity u_f - u_mf = 0.20 and 0.3m/s.
It is shown that with the increase of bubble height or fluidizing gas velocity,
bubble diameters almost keep constant. It may be due to the effect of jetting
gas velocity. In the jetting fluidized bed, the jetting gas is very high and
almost occupies half of the total gas.

       To
highlight the trend of bubble growth along bed height, the arithmetic average
bubble diameters are also plotted in Fig. 2. It is reflected that the average
diameters remain stable and then decrease at the top of jetting fluidized bed. The
reason may that the equilibrium between the coalescence and splitting of
bubbles is broken up. The bed is in the turbulent regime.

Fig .2. Bubble diameter distributions as a function of
height above the distributer at 2.1 MPa: (a) u_f - u_mf = 0.20 m/s,
(b) u_f - u_mf = 0.30 m/s.

The systematically effects
of pressure, fluidizing gas velocity, jetting gas velocity and static bed
height on the bubble parameters and its new correlation will be presented in the
congress.

Notation

D                 
Width of 2-dimensional jetting fluidized bed, mm

H₀                
Static bed height, mm

u_f                  
Fluidizing gas velocity, m/s

u_j                Jetting gas velocity, m/s

u_mf              Minimum fluidization
velocity, m/s

u_t                  
Terminal velocity of a free-fall particle, m/s

Literature Cited

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BL. Development and application of an on-line fouling gauge for brown coal
power stations. Industrial &
Engineering Chemistry Research. Mar 1999;38(3):1159-1162.

2.             Li
FH, Huang JJ, Fang YT, Wang Y. Formation Mechanism of Slag during Fluid-bed
Gasification of Lignite. Energ Fuel. Jan
2011;25:273-280.

3.             WC
Y. Fluidization, Solids Handling, and Processing: Industrial Application. Noyes Publication. 1998.

4.             Zhang
K, Zhang J, Zhang B. Experimental and numerical study of fluid dynamic
parameters in a jetting fluidized bed of a binary mixture. Powder Technology. 2003;132(1):30-38.