Conversion of rice husk into amorphous silica and combustible gas

Conversion of rice husk into amorphous silica and combustible gas

Energy Convers.Mgmt Vol. 32, No. 6, pp. 565-570, 1991 0196-8904/91 $3.00+ 0.00 Copyright © 1991 Pergamon Press plc Printed in Great Britain. All rig...

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Energy Convers.Mgmt Vol. 32, No. 6, pp. 565-570, 1991

0196-8904/91 $3.00+ 0.00 Copyright © 1991 Pergamon Press plc

Printed in Great Britain. All rights reserved

CONVERSION

OF RICE HUSK INTO AND COMBUSTIBLE

AMORPHOUS GAS

SILICA

A. CHAKRAVERTY and S. KALEEMULLAH Post Harvest Technology Centre, Indian Institute of Technology, Kharagpur-721 302, India

(Received 15June 1990; receivedfor publication 21 January 1991) Akitract--Results of the studies on thermogravimetric analysis (TGA) of raw and I(N) HCI acid treated rice husk (in still air) reveal that its thermal degradation takes place in three main stages of mass loss, namely (i) drying, (ii) devolatilization and (iii) slow oxidation of fixed carbon. Hydrochloric acid leaching of husk at 75°C for 1 h prior to combustion is necessary for production of amorphous silica of complete white colour. For production of low calorie-combustible gas along with amorphous silica from the rice husks containing 5.5-7% (wb) moisture, a furnace set temperature of 450°C appears to be optimal. Rice husk

Amorphous silica

Combustible gas

INTRODUCTION Husk is generally used for direct combustion, either in furnaces or in domestic stoves. As rice husk contains about 60-70% volatile matter [1], it produces a lot of smoke when subjected to burning. The environment is polluted and the efficiency of the furnace is significantly decreased by carrying away unburnt volatile matter with the smoke. Smoke is hazardous to life and property. It is necessary to reduce pollution and increase the efficiency of furnaces by converting the higher molecular carbon compounds to lower molecular carbon compounds (combustible gas). In order to understand the thermal degradation mechanism, thermogravimetric analysis (TGA) of rice husk has to be carried out. By burning the rice husk at a controlled temperature between 500 and 700°C, amorphous silica can be produced [2]. The metallic impurities present in the silica obtained from husk ash can be removed by pretreating the raw rice husk with some inorganic acids like HCI, HNO3, etc. [3]. Efforts are being made for manufacturing solar grade silicon materials from rice husk. The silica present in rice husk, being biogenic in origin, is inherently amorphous. This silica has been found to be an attractive source for manufacturing silicon of reasonably high purity (purer than 99.9%) [3]. It can be used for the production of photovoltaic cells. Ultrapure silicon is used for manufacturing semiconductors, microchips, transistors and integrated circuits. Keeping the above points in view, the present project has been undertaken with the following major objectives: (1) to study the thermal degradation of untreated and acid treated rice husk by thermogravimetric analysis (TGA); (2) to study the effects of acid leaching of rice husk on furnace temperature, husk bed thickness on the production of low calorie gas and amorphous white silica; (3) to study the structural nature of the above rice husk silica using X-ray diffractometry. M A T E R I A L S AND M E T H O D S

The husk was sieved to remove the dust and bran particles. The husk was thoroughly cleaned with water and then sun dried. The dried materials were leached with 1N HCI at 75°C for 1 h to remove metallic impurities. After this, the leached material was washed with tap water five times to remove the HCI. Finally, it was thoroughly washed with distilled water to remove all the impurities and the last traces of HC1. The wet material was dried in sunlight, so that its moisture content came down to 5.5-7% (wb). Moisture contents of the husk samples were calculated by determining the weights before and after drying them in an oven at a temperature of 100 + 2°C for 24 h. The TGA of rice husk was carried out using a thermogravimetric analyser (made by the Fertilizer Corp. of India Ltd). Untreated and acid treated samples were subjected to thermogravimetric analyses. 565

566

C H A K R A V E R T Y and K A L E E M U L L A H :

RICE H U S K CONVERSION

The sample was taken in a small platinum crucible. The initial weight of the sample was recorded. The sample inside the furnace was heated at a constant rate of heating (5 or 10°C/min) by controlling the input power with the help of an autotransformer. During the entire heating operation, the mass loss of the material under investigation was recorded at an interval of 5 rain. The sample was heated to 750°C. The experimental set up mainly consisted of a vertical electric furnace, temperature controller cum indicator and a temperature recorder. The energy meter was used to know the total energy consumed to heat the furnace and to maintain the desired temperature during the combustion. The furnace was specially designed and fabricated to conduct combustion studies of husk for the production of silica and gas. The furnace was getting the power from the three-phase main supply through an energy meter (three phase, 25 A, 220 V), a magnetic contactor and a temperature controller. The desired furnace temperature was controlled with the help of a temperature controller cum indicator. During combustion, the bed temperature of the husk was recorded with the help of a chromel-alumel thermocoupl¢ connected to a continuous strip chart recorder. The flame temperature was recorded using a chromel-alumel thermocouple connected to a digital multimeter. The energy meter reading and the time taken for combustion were noted. After cooling the furnace, the white ash was collected and weighed to know the percentage of white ash. The ash obtained from the raw and acid treated rice husk in the vertical furnace was analysed for residual carbon and structure. To determine the residual carbon present in the ash, a known quantity of ash was taken in a silica crucible and kept inside the muffle furnace set at 700°C. The sample was kept for 24 h and the loss in weight was taken as the residual carbon content of the ash. The structure of the silica in white ash was examined by the X-ray diffraction technique to determine the formation of any crystalline silica during thermal treatment of the husk at high temperatures with the help of an X-ray diffractometer at a chart speed of 2°/rain using Cu K~ radiation. RESULTS AND DISCUSSION

TGA of rice husk The TGA curves of rice husk representing the mass loss as a function of temperature are shown in Figs 1-4. When husk was heated, either at 5 or 10°C/min, the mass loss took place in four stages.

I

Time (mln)

Time (rain) 60

~ - 3 0 - ~ 1 0 I~ 25 -,++20 ::=

"~XI

MOSSof the ing rote:

1o

.I

I* 25 -,+,I 5,+,-- 33

420.9 mg 5 *C/min

sompte:

I

B

10

2O

2O

3O

3O

: ::17"t------55

::

Moss of the Iompte:423 mg Heotlng rote: 5 "C/mln

llI

_o 5o

~ 50 i

~ 6o

lIE

7O 8o 9o

+°t

90 I 100

I

I

I

I

I

I

200300400500000700800 Temperoture ('C)

Fig. 1. TGA curve o f ground and untreated rice husk.

100

o

I 1 100 200

I I 300 4 0 0 Temperature

I I 500 600 (*C)

I I 700 O00

Fig. 2. TGA curve o f ground and acid trvatext rice husk.

CHAKRAVERTY and KALEEMULLAH:

RICE HUSK CONVERSION

Time (rain} 2.5 =:-- 11 --;: h,-1T.5 ---H k,--- 19 A

Time (rain)

Moss of the sompte: 375.5 mg Heating rote: 10*C/min

l B C

0 10

567

22.5

,I

Moss of the sompto 415.4 n~ Heating rote 10"C/rain

10

20-

2O

30

3O

~ 5o

50

m

~

=E 6 0

60

70

!

80 : ~

\,v-,

~

80

90

E

F

9O

1OO

0

t

I00

[

200

I

I

I

1

300 4 0 0 500 600 Temperature ('C)

1

700

,,I

800

Fig. 3. TGA curve of ground and untreated rice husk.

100

0

I 100

I 200

I 1 I I 300 4 0 0 500 600 Temperature (eC)

I 700

I 800

Fig. 4. TGA curve of ground and acid treated rice husk.

T h e f o u r t h stage h a d two substages i n d i c a t e d as I V - a a n d IV-b. A l l these stages were m a r k e d in the figures as A - B , B - C , C - D , D - E a n d E - F , which were identified f r o m the t r e n d o f the T G A

curves. The temperature ranges for the various stages of mass loss and the rates of mass loss for the above two heating rates are given in Table 1.

TGA of rice husk at 5°C/min The TGA curves of untreated and acid treated husk at a heating rate of 5°C/min show that the temperature ranges o f the same stage are different. In the first stage, the temperature ranges were 25-175°C for the untreated sample and 25-150°C for the treated one. The rate of mass loss of moisture was 0.35%/min in the untreated sample and 0.42/min in the treated one [Fig. 5(a)]. Table 1. Details of thermogravimetric analysis of ground rice husk-both untreated and acid treated when heated at different heating rates

Fig. No.

Condition of husk

Heating rate (°C/min)

1

Untreated

5

2

1N HCI treated

5

3

Untreated

l0

4

1N HC1 treated

10

Decay stage I II III IV-a IV-b I II III IV-a IV-b I II III IV-a IV-b I II III IV-a IV-b

Temperature Mass range loss (°C) (% wb) 25-175 175-225 225-350 350-450 450-750 25-150 150-225 225-390 390-475 475-750 25-175 175-225 225-395 395-500 500-750 25-200 200-225 225-415 415-525 525-750

10.5 1.0 62.5 9.0 3.25 10.5 1.5 62.0 10.75 3.25 ll.0 1.0 62.5 12.0 1.5 10.0 0.25 61.50 12.25 1.75

Time (min) 30 10 25 20 60 25 15 33 17 55 15 5 17 10.5 25 17.5 2.5 19.0 11.0 22.5

Total time (min)

145

145

72.5

72.5

Rate of mass loss (%/rain) 0.35 0.10 2.50 0.45 0.054 0.42 0.10 1.88 0.63 0.06 0.73 0.2 3.7 1.14 0.06 0.57 0.1 3.24 1.1 l 0.08

Colour of ash Brownish white

White

Brownish white

White

568

C H A K R A V E R T Y and K A L E E M U L L A H :

RICE H U S K CONVERSION

(o) A t 5 ° C l m l n

C

Untreated husk

E ~, 4

. . . .

Treoted husk

Q3

E 3 1 0 I1: I

II

ITr

Different

IV-o

IV-b

stages

(b) At 10 ° C / m l n E ", 4 v

0 n-

I

II

n'r

IV-o

IV-l}

Different stages Fig. 5. Plot between different stages and rates of mass loss.

A mass loss of 10.5% occurred in this stage, corresponding to the removal of moisture from the material. In the second stage, the temperature ranges were 175-225°C for the untreated sample and 150-225°C for the treated one. There was only 1.5% mass loss in that stage, which was much less compared to any other stage. This stage was considered as a transition stage, at it might be due to the loss of the last trace of moisture (if any), CO2 gas, etc. The rate of mass loss in this stage was only 0.1%/min. The major part of the mass loss took place in the third stage. The sharp fall of the TGA curves, as shown in Figs 1 and 2, in the temperature ranges of 225-350°C in the untreated one and 225-390°C in the treated one were due to the removal of volatile matter. The mass loss of volatile matter was about 62.5% in the untreated sample and 62% in the treated one. The rates of mass loss of volatile matter were 2.55 and 1.88%/min in the untreated husk and treated husk, respectively [Fig. 5(a)]. It implies that the rate of removal of volatile matter was faster in the untreated sample than that in the acid treated sample. The fourth stage of the TGA curves, which showed a gradual decrease in the mass, was due to a slow combustion of solid (fixed) carbon. Initially, the combustion rate was high in this stage. But after some time, its rate drastically decreased. The temperature ranges of the substage IV-a were 350-450°C for the untreated one and 390-475°C for the treated one. The mass losses of the fixed carbon were 9 and 10.75% and the rates of mass loss of fixed carbon were 0.45 and 0.63%/min [Fig. 5(a)] for the untreated and acid treated husk, respectively. It reveals that the rate of removal of fixed carbon was faster in the case of treated husk than that in the untreated husk. The mass loss of carbon occurring in substage IV-b was only 3.25%, and it occurred in the temperature range 450-750°C in the untreated husk and 475-750°C in the acid treated husk, respectively. The rate of oxidation of fixed carbon in this substage was only 0.054-0.06%/min [Fig. 5(a)]. It shows that the rate of removal of fixed carbon was faster in the first substage and very slow in the second substage.

CHAKRAVERTY and KALEEMULLAH: RICE HUSK CONVERSION

569

TGA of rice husk at lO°C/min heating rate It may be seen from Figs 3 and 4 that the T G A curves of rice husk at 10°C/min heating rate also have the four stages. The only difference that occurred was in the temperature range of each stage. The temperature ranges of the different stages o f degradation in T G A at 10°C/min were higher than that at 5°C/rain. The temperature ranges of stages I, II, III, IV-a and IV-b were 25-175, 175-225, 225-395, 395-500 and 500-750°C for the untreated rice husk and 20-200, 200-225, 225-415, 415-525 and 525-750°C for the acid treated rice husk. The rates of mass loss in stages I, II, III, IV-a and IV-b were 0.73, 0.2, 3.7, 1.14 and 0.06%/min, respectively, in the untreated rice husk and 0.57, 0.1, 3.24, 1.11 and 0.08%/min, respectively, in the treated rice husk [Fig. 5(b)]. It was observed that the rate of mass loss in the husk at 10°C/min was higher than that at 5°C/rain heating rate due to the higher heating rate.

Effect of furnace set temperature and bed thickness on the production of low calorie-combustible gas The results of the effect of the furnace temperature and bed thickness on the production of low calorie-gas from the husk are given in Table 2. At a furnace set temperature of 450°C, the flame temperatures of the combustible gas which were produced during combustion of the husk were 690, 720 and 750°C for about 6, 11 and 18 rain at 20, 30 and 40 cm husk bed depths, respectively. When the furnace set temperature increased to 500°C, the flame temperatures of the gas were also increased to 700, 735 and 760°C for about 6, 12 and 20 min for husk bed thicknesses of 20, 30 and 40 cm, respectively. From the above, it is clear that, as the bed thickness increases, the flame temperatures and the duration of flame also increase. It also reveals that, as the furnace temperature increases, the flame temperature also increases.

Effect of furnace set temperature and bed thickness on husk bed temperature and the time required for the production of amorphous white ash with natural draught At 450°C furnace temperature, the maximum bed temperatures were 680, 680 and 690°C for 20, 30 and 4 0 c m bed depths, respectively, and it took 5, 6.5 and 9 h, respectively, for complete combustion of the husk. The corresponding residual carbon contents were 5.32, 4.8 and 3.85%(db), and the colour of the ash was brownish white. At the furnace temperature of 500°C, the maximum bed temperatures and the combustion times were 710, 710 and 715°C and 4, 5.75 and 7.5 h, respectively, for 20, 30 and 40cm bed depths. The corresponding residual carbon contents were 5.21, 4.24 and 2.2% (db), and the ash produced was brownish white in colour.

Effect of pretreatment of husk with acid (IN HCI) and the furnace temperature At 450°C furnace temperature, the maximum bed temperature was 660°C for the 20 cm bed depth, and it took 5 h for complete combustion of the acid treated husk. The ash was completely white in colour, and the residual carbon content was 2.17% (db). Table 2. Effect of furnace set temperature and husk bed depth on the production of low calorie-combustiblegas Moisture Furnace set Bed contentof Gas Flame temperature depth husk temperature time Mode of burning of husk S. No. (°C) (cm) (% wb) (°C) (min) Furnace temperature was raised and 450 20 5.74 690 6 set at the desired temperature from ambient temperature. Husk was charged into the furnace with natural air flow 450 30 5.82 720 11 450 40 5.72 750 18 500 20 5.68 700 6 500 30 6.02 735 12 500 40 5.93 760 20

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CHAKRAVERTY and KALEEMULLAH: RICE HUSK CONVERSION

1, Untreated rice husk osh 2. 1N HOt treated rice husk osh Radiation : CuKa

2

5

10

1,5

20

25

30

35

Brogg ongte (28 deg) Fig. 6. X-ray diffractograms of white ash obtained after heating rice husk in a vertical furnace of 500°C.

At the furnace temperature of 500°C, the maximum bed temperature and the combustion time were 720°C and 4 h respectively, for the 20 crn bed depth. The ash was milky white in colour, and the residual carbon content was 2.05% (db).

X-ray diffraction analysis of the white ash (silica)obtained from rice husk in a vertical furnace The white ash samples which were obtained by burning the untreated and acid treated husk with a bed depth of 20 cm at 500°C were examined for characterization of the silica using an X-ray diffractometer. As the X-ray diffractograms (Fig. 6) are not having any sharp peak, the above white ash samples were amorphous in nature. The maximum husk bed temperature recorded was 720°C for a depth of 20 cm at a furnace temperature of 500°C. It implies that amorphous ash was obtained even at a husk bed temperature of 720°C. CONCLUSIONS

(1) The thermal decomposition of the untreated and acid treated rice husk takes place in three distinct stages of mass loss, namely, (i) removal of moisture (drying), (ii) release of volatile matter (devolatilization) and (iii) oxidation of fixed carbon (slow combustion). (2) The temperature ranges of each of the three stages in TGA are different for the untreated and acid treated rice husk. (3) A furnace temperature of 450°C for the bed depths of 30 and 20 cm is recommended for the production of amorphous white ash from the untreated and acid treated rice husk. (4) For the production of low calorie combustible gas along with amorphous silica, the moisture contents of both the raw and acid treated husk ranging from 5.5 to 7% (wb) appear to be optimum at a furnace temperature of 450°C. Acknowledgement--The authors are grateful to Dr H. D. Banerjee of the Material Science Centre, I.I.T., Kharagpur for providing some instrumental facilities for a few experiments. REFERENCES 1. O. B. Juliano, Rice Chemistry and Technology. American Association of Cereal Chemists, St. Paul, Minn. (1985). 2. M. A. Hamad and I. A. Khattab, Thermochim. Acta 48, 343 (1981). 3. P. Mishra, A. Chakraverty and H. D. Banerjee, J. Mater. Sci. 20, 4387 (1985).