Abstract
Introduction
Design of the fast fluidized BED
Performance of the CFBG
Conclusion
Xu Bingyan
Luo Zengfan
Wu Chungzhi
Huang Haitao
Zhou Xiguang
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences Guangzhou 510070, China
Paper No.9407
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This paper introduces a circulating fluidized bed gasifier (CFBG) with a diameter of 410 mm, a height of 4000 mm and a load of 250 kg/h for wood powders. The CFBG system, including gasifier, feeding and circulating are all operated pneumatically. The design of the CFBG is presented. The main parameters which affect the gasification processes are elucidated in the paper. The performance of the CFBG is expressed as follows: |
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Productivity: |
2000 kg/m²h |
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LHV of gas: |
7000 kJ/m3 |
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Heat efficiency of cold gas; |
75% |
Gasification has become more and more important in biomass utilization. A number of types of gasifiers has been developed to meet the different needs. The development of the fluidized bed gasifier (FBG) for small particle materials has made great progress for biomass gasification. The productivity of the FBG has increased by five times that of the fixed bed gasifier and the heating value of gas increases by about 20% [1] [2]. However, the flying char loss makes the FBG a kind of low heat efficiency gasifier (around 60%). The circulating fluidized bed gasifier has the following features: fast fluidization which enhances the heat and mass transfer so as to speed up the gasification process; and the circulation of the char which increases the residence time of char so as to satisfy the need of reduction reaction and decrease the char loss. The CFBG represents an attractive and promising gasifier.
A CFBG has been built and operated in the Zhanjiang Press Wood Products factory in China, utilizing the wastes produced during the wood processing over a period of three years. Four to six tons per day of wood powders are treated by the CFBG. Both purposes of recovering energy from wastes and preventing powders from pollution are achieved successfully. With the mature experience of the first CFBG in China, another CFBG of the same size has been built in the Sanya Timber Processing Complex and has been in operation for more than half a year.
The formation of a fast fluidized bed depends on the following conditions: (1) small particle materials; (2) high operating gas velocity; (3) continuous solid circulation. Small particles provide huge gas-solid contact surface, minimizing the transfer resistance inside the particles. High gas velocity with solids circulation means that there is an absence of bubbles forming a dilute continuum of discrete particles with solids clusters in it. The continual forming and dispersing of the solid clusters promote the contact of gas and solid and enhance heat and mass transfer. Continuous solid circulation maintains the density of the bed and uniform solids profile along the radical cross section. Consequently, the fast fluidized bed provides a high reaction rate for gasification and enough residence time of solids to complete the reactions.
The Size Profile of The Wood Powders
The size of the different samples is listed in table 1.
Table 1. Size of wood powders
|
Size profile (mm) |
Wt % |
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|
Sample No. |
(1) |
(2) |
(3) |
|
>0.6 (6-25mm in length) |
6.77 |
6.38 |
|
|
0.3-0.6 |
44.02 |
72.89 |
10.7 |
|
0.15-0.3 |
32.96 |
17.41 |
21.35 |
|
0.125-0.15 |
5.19 |
2.83 |
18.91 |
|
0.097-0.125 |
8.35 |
0.36 |
29.03 |
|
0.05-0.097 |
2.71 |
0.13 |
20.02 |
|
Average diameter (mm) |
0.329 |
0.363 |
0.157 |
Fast Fluidization Velocity
An appropriate fast fluidization velocity is a prerequisite for designing an optimum fast fluidized bed. Too low a velocity will result in a bubbling fluidized bed; too high will result in dilute phase pneumatic conveying. Based on the measurement of axial pressure drops tested in a plexiglass bed, a superficial velocity of 1.4 m/s is chosen to operate the CFBG, at which the average bed voidage is around 87%, and the voidage at the exit of the bed is around 96%. The dense phase bed occupies 75% of the whole bed. The fluidization characteristics of wood powders are listed in Table 2.
Table 2 shows that, the fast fluidization velocity (Uc) is much higher than the terminal velocity of the particles (Ut), i.e, Uc= (3-5) Ut. However, owing to the solids circulation, the bed is operated on a relatively dense phase.
Determination of The Bed Diameter and Height
The following parameters are requested to support the determination of the bed diameter and height:
Feed rate: 250 kg/h (requested) Air equivalence ratio: 20% (selected)
Fast fluidization velocity: 1.4 m/s (selected from experimental data)
Solid circulation rate: 25 times as high as the feed rate (calculated from experimental data)
Average voidage: 87% (calculated from experimental data)
Average residence time of solids: 8 s (designed and calculated)
After determining the fast fluidization velocity, at a certain designed feed rate (250 kg/h), air equivalence ratio (20%), and requested residence time of solid particles, the diameter and height of the bed are determined as follows:
Diameter: 410 mm
Height: 4000 mm
Table 2. Fluidization characteristics of wood powders
|
Average diameter mm |
Density kg/m3 |
Bulk Density kg/m3 |
Incipient Velocity Uf, m/s |
Terminal Velocity Ut, m/s |
Fast fluidization Velocity Uc, m/s |
Uc/Ut |
|
0.157 |
430 |
215 |
0.08 |
0.26 |
1.4 |
5.5 |
|
0.329 |
430 |
215 |
0.12 |
0.40 |
1.4 |
3.5 |
|
0.363 |
430 |
215 |
0.14 |
0.48 |
1.4 |
2.9 |
The characteristics of the feedstock (rubber wood powders) are shown in Table 3.
Table 3. Characteristics of feedstock
|
Moisture content |
% |
5 | |
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Elemental analysis |
% |
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|
|
C |
47.25 | |
|
|
H |
6.04 | |
|
|
O |
46.71 | |
|
Ash |
% |
2.46 | |
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Higher heating value |
kJ |
18320.5 | |
|
Particle size (average) |
mm |
0.329 | |
|
Density |
kg/m³ |
430 | |
|
Incipient fluidization 'velocity |
m/s |
0.12 | |
In order to investigate the performance of the CFBG, studies were conducted under different conditions, such as at different air equivalence ratio (ER). The ER is expressed as the ratio of the air consumed of a unit of dry wood for gasification to the air consumed of a unit of dry wood for complete combustion. It is found that the dominating factor which influences the other parameters is ER. Varying ER, several parameters, such as temperature and gas composition are varied correspondingly.
Fig. 1. Bed temperature versus ER
Reaction temperature is a very important parameter which controls the reaction rate, composition and amount of products of various processes such as pyrolysis, reduction, combustion or other secondary reactions. The reaction temperature in a CFB gasifier can be easily controlled by adjusting the equivalence ratio (ER). Fig.1 shows the relationship between bed temperature and equivalence ratio.
From Fig.1, it can be seen clearly that when ER varies from 0.13 to 0.285, the bed temperature raises from 500 to 1050 °C, which can be reached either by altering air flow rate or changing solids feed rate. The adjusting ability of operating temperature is one of the advantages of the CFBG.
Gas Composition
The composition of the produced gas at different equivalence ratio and bed temperatures from 8 runs are shown in Table 4
Table 4. Gas composition
|
No. |
ER |
Temp. |
Gas composition |
LHV kJ/m3 |
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|
°C |
CO2% |
CO% |
CH4% |
CnHm% |
H2% |
O2% |
N2% |
|||
|
1 |
0.171 |
630 |
14.9 |
16.8 |
9.43 |
1.3 |
5.5 |
1.1 |
47.71 |
7382.3 |
|
2 |
0.192 |
690 |
16.3 |
13.0 |
8.18 |
3.0 |
5.71 |
1.0 |
52.83 |
7424.5 |
|
3 |
0.192 |
685 |
18.9 |
14.4 |
8.35 |
2.1 |
5.34 |
0.9 |
50.02 |
7058.8 |
|
4 |
0.201 |
748 |
16.1 |
17.8 |
8.35 |
1.2 |
5.79 |
1.1 |
50.22 |
6972.7 |
|
5 |
0.203 |
760 |
20.4 |
12.2 |
7.42 |
2.1 |
10.78 |
3.9 |
46.26 |
6996.5 |
|
6 |
0.224 |
919 |
15.8 |
16.1 |
7.32 |
1.2 |
13.24 |
1.1 |
44.80 |
7190.4 |
|
7 |
0.257 |
974 |
15.6 |
16.7 |
6.90 |
1.0 |
16.32 |
0.7 |
43.25 |
7264.1 |
|
8 |
0.283 |
1042 |
13.1 |
17.6 |
5.34 |
1.3 |
16.57 |
0.6 |
45.42 |
6938.4 |
From Table 4, it can be seen that the gas composition is influenced obviously by the ER and temperature, the percentage of hydrogen increases but that of methane decreases with temperature increasing. This tendency is similar to the chemical equilibrium reaction. For the percentage of CO2, there is a peak value at ER around 0.2. Lower or higher ER make the CO2 content smaller. It is because that lower ER means less oxidant used resulting in less combustion and less CO2 producing. While higher ER means higher temperature which favors the reduction and shift reactions, so that the CO2 decreases. In spite of the change of gas composition at different conditions, the heating value of the gas remains the same range, around 7000 kJ/m³ . It is the second advantages of the CFBG. Meanwhile, the heating value of the gas from the CFBG is much higher than that from fixed bed air blown gasifier (around 5000 kJ/m³), and also higher than that from fluidized bed gasifier (around 6000 kJ/m³), It indicates that in the fast fluidized bed gasifier, the high heating rate and reaction rate of the particles enhance the gasification process, and the char circulation reduces the flying char loss and increases the residence time of char and then strengthens the reduction reaction.
Load of The Gasifier
The CFBG can adapt the change of load in a big range. When it changes in the range of 180-378 kg/h, the gasifier can be operated in steady condition and keeps the produced gas in good quality. The high load flexibility is another advantages of the CFBG.
Gasification Results of The CFBG
The typical results of the CFBG at different condition are shown in Table 5. and Fig.2.
Table 5. Gasification results of the CFBG
|
No. |
Total air m³/h |
Feed rate kg/h |
Operation temp. °C |
Gas productivity m³/kg |
LHV kJ/m3 |
C.Conv. % |
Heat Effi % |
Output kW |
|
1 |
190 |
260 |
686 |
1.66 |
7374 |
57.5 |
47.3 |
885 |
|
2 |
236 |
240 |
964 |
1.724 |
7190 |
85.0 |
70.0 |
776 |
|
3 |
230 |
210 |
974 |
1.927 |
7264 |
92.8 |
74.6 |
743 |
|
4 |
220 |
182 |
1042 |
2.101 |
6938 |
93.6 |
78.3 |
594 |
Fig. 2. Graph of cold gas efficiency and carbon conversion vs.ER
Table 5 and Fig.2. show that the carbon conversion and cold gas heat efficiency increase with the temperature increasing obviously. The main reason of that is the gas productivity greatly affected by the reaction temperature. Another reason is that the reduction and shift reaction almost does not occur at a temperature lower than 700 °C, which results in large amount of char remaining unreacted and accumulating in the bed or blown out of the gasifier.
The fast fluidization and continuous circulating of char feature the circulating fluidized bed gasifier, which benefit the gasification process dramatically. The features enhance heat and mass transfer, raise reaction rate, strengthen fast pyrolysis, reduction and shift as well as other gas-solid reactions etc. which make the productivity of the CFBG much higher and gas quality much better than other kinds of air blown gasifiers. They reach 2000 kg/m² .h and 7000 kJ/m³ respectively.
In operating the CFBG, reaction temperature can be easily adjusted by altering the air equivalence ratio; and the load is flexible in wide range under conditions of steady operation and the gas quality can be maintained in favorable range under different conditions. These are the advantages in operating the CFBG.
In order to get higher heat efficiency, the following parameters recommended will favor the fast pyrolysis, reduction, shift and secondary reactions. They are:
Equivalence ratio: 0.2-0.28
Reaction temperature: 800-1000 °C
References
1. Bingyan Xu, Flanigan V.J., W.E. Huang, and O.C, Sitton, "Design and Operation of A 6.0 Inch Fluidized Bed for Rice Hulls". Symposium Paper of "Energy from Biomass and Wastes IX". IGT, pp.595-614, 1985.
2. Xu Bingyan, Luo Zengfan et al: "Improved Up-draft Gasifier", Proceedings of Chinese Solar Energy Society Symposium (Book Biomass Energy) p.87-p.95, 1991
