Gasification of Rice Husk in a Fluidized-Bed Gasifier without Inert

Oct 18, 2003 - A novel fluidized-bed gasifier was designed in which rice husk was ... of a counterflow fixed-bed reactor, cyclone separator, fan, scru...
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Ind. Eng. Chem. Res. 2003, 42, 5745-5750

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Gasification of Rice Husk in a Fluidized-Bed Gasifier without Inert Additives Hong Jiang, Xifeng Zhu, Qingxiang Guo,* and Qingshi Zhu Department of Chemistry, University of Science and Technology of China, Hefei 23006, China

A novel fluidized-bed gasifier was designed in which rice husk was fluidized and gasified without inert additives. The gas compositions of CO, CO2, H2, CH4, and O2 were determined by an online analyzer and gas chromatography. The results showed that low fluidized velocity and low gasification temperature (550-650 °C) were beneficial to the production of syngas with a high composition of CO. The composition of H2 was influenced slightly by the equivalent ratio. The average composition of H2 was almost unchangeable to different fluidized velocities. The minimal fluidized velocity vmf of rice husk without inert additives experimentally determined was 0.495 m/s, which was higher than that of the two-component system containing rice husk and sand (vmf < 0.3 m/s). The throughput of rice husk in this reactor was higher than that in a fixed bed with the same volume. This fluidized-bed reactor had a longer service term than a traditional fluidized bed for a two-component system. 1. Introduction Biomass including agriculture wastes, sawdusts, etc., was widely studied as a renewable energy. Crop production will continue to increase to feed the ever-increasing population of the world. The current world production of cereals is about 2.0 × 109 tons.1 There are about 7 × 107 tons of rice husk produced annually just in China.2 Furthermore, the development of world economics will be cumbered by the energy crisis. The renewable biomass energy will be a good substitution of fossil fuels. Biomass can be converted to gas, syngas, or chemicals such as methanol by gasification. The reactors for gasification include fixed and fluidized beds according to the movement of biomass in the reactors.3,4 The main advantages of fixed-bed reactors are the high carbon conversion efficiency and low tar content. Their disadvantages are low production capacity and bad heat transfer. Otherwise, fluidized beds have good heat and mass transfer between the phases of gas and solid, the best temperature distribution, fast heatup, and high specific capacity. They tolerate wide variations in fuel quality and have a wide particle size distribution. Their disadvantages are operational complication and high ash content in the producer gas.5 Many researchers have worked on biomass gasification. However, almost all kinds of gasification based on fluidized beds were carried out by adding inert additives to make the rice husk fluidize well and to keep a uniform temperature in the reactors. Some inert additives such as sand not only waste the limited space of the reactors but also wear out the inner surface of the reactors. In the present work, an attempt was made to investigate biomass gasification without inert additives. 2. Materials 2.1. Apparatus. Fixed- and fluidized-bed gasifiers were used in this research. The fixed-bed gasifier consists of a counterflow fixed-bed reactor, cyclone * To whom correspondence should be addressed. Tel.: +86551-3607466. Fax: +86-551-3606689. E-mail: qxguo@ ustc.edu.cn.

separator, fan, scrubber, and gas storage tank. The fixed-bed reactor is a 300 mm i.d. and 600 mm long steel vessel with a small air inlet, which is divided into two compartments with steel mesh: the upper one is for gasification and the lower one for ash removal. Rice husk is gasified in the reactor, and the producer gas is educed by the fan. The producer gas goes through the cyclone separator, in which the ash is separated, and goes through the scrubber, in which tar and fine particles are mostly removed. Then the nearly cleaned producer gas can be used as fuel or feedstock to produce syngas and chemicals. Wastewater containing high concentrations of tar can be treated and recycled to this system. A schematic diagram of the fixed-bed gasifier is shown in Figures 1 and 2. The fluidized-bed gasifier (Figures 3 and 4) is more complicated than the fixed-bed one. The entire system consists of the fluidized-bed reactor, rotameters, manometers, thermocouple thermometers, a screw feeder, air compressors, cyclone separators, and a purification system. The fluidized-bed reactor consists of two parts; the lower part is a 100 mm i.d. and 1300 mm long tube, and the upper part is a 165 mm i.d. and 2000 mm long tube. Both of them are made of a high-quality heatresistant stainless steel tube and cement. The rice husk is added into the fluidized bed by a screw feeder and fluidized by air or steam. Liquefied petroleum gas is added to fire the rice husk in the beginning of the operation. After a few minutes of heating, the rice husk is gasified to be gaseous products, which contain CO, H2, CH4, CO2, N2, etc. The raw gas flows into the cyclone separator, in which big ash particles are removed. The fine ash particles and tar in the gas are nearly completely removed by the scrubber. The wastewater is treated by another system. The biomass material used in this experiment was rice husk. The average particle size of the rice husk was 2 mm wide, 1 mm thick, and 8 mm long (Table 1). The gasification agent was air. All experiments were carried out at a pressure of 104 Pa and at ambient temperature. 2.2. Analysis Instruments. A gas chromatograph (GC; GC122, Shanghai Analytical Instruments Co. Ltd., China) with two detectors (thermal conductivity and

10.1021/ie0304659 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/18/2003

5746 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003

Figure 1. Schematic diagram of the fixed-bed gasifier.

Figure 2. Structure of the counterflow fixed-bed reactor.

Figure 4. Structure of the fluidized-bed reactor. Table 1. Analysis of the Rice Husk density (kg/m3) average size volatile (wt %) ash (wt %) moisture (wt %)

230 8× 2 × 1 mm 87.0 1.5 11.5

C (%) H (%) O (%) N (%) Si (%) K (%)

45.06 5.45 47.22 0.26 0.66 0.11

Table 2. Composition of the Producer Gas Produced by a Fixed-Bed Gasifier component composition (%)

CO 14.2

H2 7.4

CH4 2.5

CO2 16.9

O2 0.4

N2 56

analyzer consists of three infrared detectors for CO, CO2, and CH4 and two special detectors for H2 and O2. All data could be read on the analyzer and stored in a computer. Figure 3. Schematic diagram of the fluidized-bed gasifier: 1, biomass storage tank; 2, screw feeder; 3, fluidized bed; 4, cyclone separator; 5, air storage tank; 6, air compressor; 7, tar removal; 8, CO2 removal; 9, water storage tank; 10, producer gas storage tank; 11, for synthesis of methanol; 12, for fuel battery; 13, online gas analyzer; 14, thermocouple thermometer; 15, computer; 16, steam generator.

flame ionization) and an infrared online analyzer (JF2000 combined online gas analyzer, Beijing Junfang Analytical Instruments Co. Ltd., China) were used in this work. A column (Carbonxen 1000) with 1.5 m length and a diameter of 3 mm was used in the GC. The online

3. Results and Discussion 3.1. Experiment on a Fixed-Bed Gasifier. A gasification experiment was carried out in a counterflow fixed bed at around 600 °C at the center of the bed and 480 °C near the wall. Rice husk was fed into the bed at a rate of 10 kg/h and gasified with an equivalent ratio (ER) of 0.12. The average gas yield was 1.2 Nm3/kg of dry biomass. The fixed-bed gasifier was run more than 2 h during the experiment. The average compositions of the producer gas are listed in Table 2. 3.2. Experiment in a Fluidized Bed. The minimum fluidization velocity of rice husk without sand was

Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5747 Table 3. Compositions of Different Components (Air Flux, 14 m3/h)

Table 5. Compositions of Different Components (Air Flux, 22 m3/h)

composition (%)

composition (%)

temp (°C)

CO

CO2

H2

CH4

O2

temp (°C)

CO

CO2

H2

CH4

O2

540 550 560 570 580 590 600 610 620 650 680 700 720 750 770

14.0 13.1 12.4 11.2 11.7 14.0 11.7 13.3 13.2 7.8 8.2 7.9 7.8 8.0 10.3

14.0 13.9 13.8 14.4 14.0 13.5 14.0 14.0 14.3 15.3 15.2 15.1 15.0 15.1 14.6

2.2 2.3 2.3 2.5 2.3 3.0 2.5 3.0 3.3 2.2 2.2 2.3 2.3 2.4 2.6

4.74 3.94 3.38 3.07 3.08 3.75 2.87 3.96 3.90 1.80 1.85 1.90 2.00 2.06 2.89

1.67 1.64 1.64 1.60 1.63 1.57 1.60 1.63 1.63 1.89 1.88 1.92 1.95 1.98 2.05

550 560 570 580 590 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770

5.3 3.0 4.3 4.1 5.3 6.3 5.1 6.1 5.6 5.9 6.1 6.0 5.9 5.5 5.2 6.1 5.5 6.2 6.3 6.2 6.4 6.7 7.7

13.0 8.6 12.2 10.7 13.4 14.9 12.5 14.2 14.6 15.5 15.5 15.5 15.5 15.6 15.8 15.9 15.8 15.8 15.9 15.9 15.9 16.1 15.9

1.7 0.6 1.1 1.0 1.3 1.3 1.2 1.5 1.4 1.4 1.5 1.6 1.6 1.5 1.5 1.8 1.6 1.7 1.8 1.6 1.6 1.7 2.0

1.26 0.64 1.00 0.96 1.40 1.61 1.10 1.49 1.39 1.52 1.56 1.53 1.49 1.34 1.35 1.68 1.52 1.84 1.84 1.76 1.77 1.75 2.24

4.45 5.62 4.30 4.89 3.39 2.47 4.93 3.34 3.37 2.34 2.27 2.15 2.16 2.28 2.22 1.80 2.03 1.95 1.84 2.05 1.99 1.60 1.52

Table 4. Compositions of Different Components (Air Flux, 18 m3/h) composition (%) temp (°C)

CO

CO2

H2

CH4

O2

550 560 570 580 590 600 610 630 640 650 660 670 680 690 720 750 770

7.8 8.1 8.1 8.7 8.7 8.8 8.9 9.3 5.1 9.3 9.1 8.3 8.0 7.3 6.1 4.0 3.6

16.3 16.2 16.2 16.1 15.9 16.0 16.0 15.9 16.5 15.9 15.9 15.9 16.0 16.0 16.1 16.2 16.2

2.4 2.5 2.5 2.5 2.6 2.6 2.7 2.7 1.5 2.7 2.6 2.5 2.4 2.3 1.8 1.1 0.9

1.60 1.72 1.76 1.99 1.97 2.01 2.11 3.28 0.58 2.35 2.12 1.96 1.78 1.68 1.23 0.62 0.54

1.04 1.02 1.00 1.02 1.09 1.09 1.09 1.09 1.21 1.10 1.19 1.25 1.3 1.35 1.47 1.87 1.93

difficult to determine. The minimum fluidization velocity of a two-component system containing rice husk and sand was less than 0.3 m/s.6 Gasification experiments were carried out at temperatures of 550-770 °C (distraction II, Figure 4). Rice husk was fed into the system at a rate of 25 kg/h, and every experiment lasted for more than 2 h. Sampling of the producer gas was performed when gasification reached steady state, at which the temperature fluctuated in a small range for a few minutes. The compositions of CH4, H2, CO, CO2, and O2 in the producer gas were detected at different air fluxes and different gasification temperatures. The results are shown in Tables 3-6 and Figures 5-8. The superficial gas velocity vf could be calculated by eq 1, where Q stands for air flux, A for the square of

vf )

Q Q ) A π 2 d 4

(1)

the transverse section of the fluidized bed, and d for the diameter of the fluidized bed. The velocities for different air fluxes are listed in Table 7. When Q < 14 m3/h, the rice husk could not be fluidized well. Table 7 shows the operation time of the fluidized bed reactor before it was blocked up by rice husk, from which it could be known that the minimum fluidization velocity of this reactor was vmf ) 0.495 m/s, which is higher than that of a two-component system

Table 6. Compositions of Different Components (Air Flux, 25 m3/h) composition (%) temp (°C)

CO

CO2

H2

CH4

O2

560 570 580 590 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770

1.8 1.9 2.6 2.7 3.3 2.1 3.8 3.9 3.9 3.1 3.8 4.0 4.5 4.0 4.8 3.8 2.9 2.9 2.7 2.6 2.9 2.7

10.3 11.9 12.6 13.3 13.1 12.7 12.6 10.7 13.9 11.4 12.8 11.5 14.7 11.0 14.3 12.2 12.0 11.4 12.5 11.9 14.2 13.6

0.10 0.30 0.70 0.75 0.93 0.45 1.00 0.90 1.10 0.76 0.92 1.00 1.28 0.90 1.23 0.98 0.78 1.00 1.00 1.00 1.02 1.00

0.28 0.30 0.52 0.57 0.75 0.39 0.91 0.68 0.76 0.60 0.96 0.94 1.10 0.96 1.28 0.81 0.53 0.51 0.49 0.48 0.49 0.45

1.80 1.90 2.55 2.70 3.30 2.10 3.75 3.87 3.87 3.10 3.75 4.00 4.50 3.97 4.77 3.76 2.90 2.85 2.65 2.60 2.88 2.65

containing rice husk and sand (vmf < 0.3 m/s). In the table, ER stands for equivalent ratio, which is defined as the ratio of the actual oxygen (air) to fuel ratio divided by the stoichiometric oxygen (air) to fuel ratio required for complete combustion. According to eq 2, the stoichiometric oxygen for 25 kg of rice husk is about 115 m3 of air.7,8

CH1.5O0.7 + 1.025O2 ) CO2 + 0.75H2O

(2)

Figures 5-7 show that the composition of CO2 changes slightly with temperature. When vf ) 0.495 m/s, the compositions of CO and CH4 show different trends when the temperature is increased lower or higher than 620 °C; that is, the compositions of CO and CH4 when T < 620 °C are higher than those when T > 620 °C. The composition of H2 goes up slightly when the temperature is increased in the range of T < 600 °C and drops

5748 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003

Figure 5. Compositions of CO, CO2, H2, CH4, and O2 (ER ) 0.122, vf ) 0.495 m/s).

Figure 6. Compositions of CO, CO2, H2, CH4, and O2 (ER ) 0.157, vf ) 0.637 m/s).

Figure 7. Compositions of CO, CO2, H2, CH4, and O2 (ER ) 0.191, vf ) 0.778 m/s).

slightly in the range of T > 600 °C (Figure 5). When vf ) 0.637 m/s, the composition of CO increases slowly

Figure 8. Compositions of CO, CO2, H2, CH4, and O2 (ER ) 0.217, vf ) 0.884 m/s).

Figure 9. Composition of CO for different fluidized velocities.

when the temperature is increased until T > 650 °C and then decreases regularly (the coke increased). The compositions of the other components are almost constant (Figure 6). When vf ) 0.778 m/s, the composition of CO increases when the temperature is increased, whereas the composition of O2 drops slowly (Figure 7). When vf ) 0.884 m/s, the compositions of all components fluctuate in a range. The useful components for syngas are CO and H2 in the producer gas. Figures 9 and 10 compare the compositions of CO and H2 for all superficial velocities in the fluidized bed. This clearly indicated that low superficial velocity and low gasification temperature (550-650 °C) are beneficial to the production of syngas with a high composition of CO (Figures 9 and 10). From Figure 11, it can be seen that the average composition of H2 is almost unchangeable for different superficial velocities. Compared to the compositions of gas produced in two kinds of gasifiers, it was found that the compositions of gas produced in a fixed-bed gasifier were better than those produced in a fluidized-bed gasifier, but its gasification capacity was less than that of a fluidized-

Table 7. Maximum Operation Time before the Reactor Was Blocked Up air flux (m3/h) air velocity (m/s) ER operated time (min)

2 0.071 0.017 20

4 0.142 0.035 25

6 0.212 0.052 32

8 0.283 0.070 77

10 0.354 0.087 40

14 0.485 0.122 >120

18 0.637 0.157 >120

22 0.778 0.191 >120

25 0.884 0.217 >120

Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5749

fluidized bed uniform, they also made the efficient gasification capacity of gasifier decrease and the gasifier wall easier to wear out. No other inert materials were used in this work; rice husks could be fluidized when vf > 0.495 m/s. However, a longer fluidized-bed reactor was needed to acquire enough residence time for rice husk gasification. The heat value of the producer gas was about 4.0 MJ/m3. A method could be adopted to increase the heat value to 4.7 MJ/m3 by circulating the producer gas in the gasification system. The residence time also affects the gasification efficiency. To acquire a full residence time in rice husk gasification, a total 3.3 m long fluidized-bed reactor was used in our experiments. In this way, the residence time of rice husk gasification could be reached within at least 3.7 s when vf equaled 0.884 m/s. 4. Conclusions Figure 10. Composition of H2 for different fluidized velocities.

Figure 11. Average compositions of CO and H2 for different air fluxes.

bed gasifier. The effective gasifying volume inside the fixed-bed gasifier is equal to 0.04239 m3, and that inside the fluidized-bed gasifier is equal to 0.05295 m3. The fixed- bed gasifier has a gasification capacity of 236 kg of rice husk/m3, whereas the fluidized-bed gasifier has a gasification capacity of 472 kg of rice husk/m3. A high composition of H2 in the producer gas was not reached only with air as the gasification agent. The addition of steam and an increase in the gasification temperature would be beneficial to producing H2 because of reaction (3).

C + H2O ) CO + H2

(3)

The higher composition of the producer gas was obtained with a low ER value (ER ) 0.122, vf ) 0.495). ER could be decreased by increasing the feeding flux of rice husk and keeping the air flux constant. Thereby, the composition of CO and H2 would be increased. Rice husks are difficult to fluidize because of their peculiar shape, size, and density. Many researchers had reported that second solids, usually inert materials such as sand, calcite, and alumina, were added into the fluidized bed to facilitate fluidization. Although additional inert materials made the temperature in the

A novel fluidized-bed reactor was designed for gasification of rice husk. Because the length of the reactor is longer, it provides enough residence time for the rice husk. The results suggested that the inert additives, which would wear the reactor’s wall greatly and waste the effective reactor’s volume, were not needed for rice husk gasification. A low fluidization velocity and low gasification temperature (550-650 °C) were beneficial to producing syngas with a high composition of CO. The composition of H2 was affected slightly by ER. The average composition of H2 was almost unchanged for different fluidized velocities. The rice husks were fluidized well without additional inert materials when the fluidization velocity was greater than the minimum fluidization velocity (vmf ) 0.495 m/s) of the rice husks with a size of 2 mm wide, 1 mm thick, and 8 mm long. Compared with the gasification in a fixed-bed gasifier, the fluidized-bed gasifier had a higher gasification capacity. Furthermore, a longer working life would be gained than with traditional fluidized-bed gasifiers because no additives were used. Acknowledgment This research was supported by the CAS, MOST, NSFC, and the University of Science and Technology of China. Literature Cited (1) FAO. Yearbook production 1992. Food and Agriculture Organization; FAO Basic Data Unit Statistics Division: Rome, Italy, 1997; Vol. 46. (2) Hsu, H.-h.; Liu, G. Tradeoffs between Quantity and Quality of China’s Rice; Economic Research Service/USDA, Agriculture Trade Reports; USDA: Washington, DC, 2001; pp 26-29. (3) Williams, P. T.; Horne, P. A. Analysis of aromatic hydrocarbons in pyrolytic oil derived from biomass. J. Anal. Appl. Pyrolysis 1995, 31, 15-37. (4) Gil, J.; Corella, J.; Aznar, M. P.; Caballero, M. A. Biomass gasification in atmospheric and bubbling fluidized bed: Effect of the type of gasifying agent on the productdistribution. Biomass Bioenergy 1999, 17, 389-403. (5) Warnecke, R. Gasification of biomass: comparison of fixed bed and fluidized bed gasifier. Biomass Bioenergy 2000, 18, 489497. (6) Rao, T. R.; Bheemarasetti Ram, J. V. Minimum fluidized velocities of mixtures of biomass and sands. Energy 2001, 26, 633644.

5750 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 (7) Turn, S.; Kinoshita, C.; Zhang, Z.; Ishimura, D.; Zhou, J. An experimental investigation of hydrogen production from biomass gasification. Int. J. Hydrogen Energy 1998, 23, 641-648. (8) Corella, J.; Aznar, M. P.; Delgado, J.; Aldea, E. Steam gasification of cellulosic wastes in a fluidized bed with downstream vessels. Ind. Eng. Chem. Res. 1991, 30, 2252-2262.

Received for review June 2, 2003 Revised manuscript received September 15, 2003 Accepted September 17, 2003 IE0304659