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Gasification of Hemp to Produce Activated Carbon
World Renewable Energy Congress VI (WREC2000) © 2000 Elsevier Science Ltd. All rights reserved. Editor: A.A.M. Sayigh
2377
PYROLYSIS/GASIFICATION OF HEMP TO PRODUCE ACTIVATED CARBON
Anton R. Reed 1, David J. Johnson2 and Paul T. Williams 1 Department of Fuel and Energyland School of Textile Industries 2 The University of Leeds, Leeds LS2 9JT (TEL: UK (#44) 1132 332504 ; FAX : UK (#44) 1132 440572)
ABSTRACT Hemp derived as a by-product from the textile industry was pyrolysed in a fixed bed reactor at temperatures between 430 and 750 °C. The derived char was gasified between 700 and 800°C in a steam atmosphere to produce activated carbon. Char surface areas were increased from 3 mZg1 before gasification, to over 600 m 2 g-1 after activation. The process variables influencing the surface area of the activated carbons were identified as, the heating rate and final temperature of biomass pyrolysis to produce the initial char, and gasification temperature.
KEYWORDS Pyrolysis; gasification; surface area; bio-gas; bio-oil; activation; bumoff.
BIOMASS AS A TEXTILE WASTE Biomass is a raw material that has been utilised for a wide variety of tasks since the dawn of civilisation. Important as a supply of fuel in the third world, biomass was also the first raw material in the production of textiles. Hemp is a biomass textile and is a 'bast' fibre. Bast fibres come from the stem of the plant, lying in bundles beneath the outer covering or bark (Ansari et al., 1995). The textile industry can utilise only about 25% by weight of the stems, the remainder is waste (Ward et aL, 1954). Energy from waste biomass can be realised by direct use in combustion, or by its thermochemical conversion via pyrolysis to bio-oil, bio-gas or high calorific value carbon char (Bridgwater et al., 1991). Pyrolysis process conditions can be optimised to maximise the production of oils, gases or chars (Home et al., 1996). The aim of this project was to optimise pyrolysis conditions and the use of H20 gasification techniques to produce activated carbons from the textile waste material. Activated carbon is a material widely used for the adsorption of pollutants from both liquid and gaseous waste streams, for the recovery of solvents and as a catalyst or catalyst support (Satya Sai et al., 1997).
2378 Oxidants such as H20 eliminate carbon atoms from the char particle via CO and/or CO + H2 in a way which favours selective burning of the interior of the particle, with the subsequent creation of porosity (Rodriguez-Reinoso et al., 1995). The BET calculation can then be used to determine the surface area of activated chars, which is an indication of their usefulness as a pollutant adsorbent. EXPERIMENTAL A slow pyrolysis, stainless steel, static batch reactor, heated with an electric ring fumace was developed for the pyrolysis of the hemp. Purged continuously with nitrogen, the gases evolved were removed after a residence time of one minute (Williams, et al., 1996). 30g samples of hemp were heated at controlled rates of 1, 2, 5 and 10 °C min -1. Final temperatures of 430, 550, 650, 750 °C were used and then held at that temperature for one hour. The gasification process was carried out at 700 and 800 °C for the chars produced at 650 °C. Water was added to a preheater at 400 °C, via a syringe pump with a flow rate of 2.5 ml/hr. The steam was pumped into the inert nitrogen flowing over the char as the oxidising agent. Activation took place for 2, 4.5, 5,5 and 7.5 hrs. Figure 1. Shows the activation rig. The BET calculations for surface area determination were carried out on results from a Quantachrome Quantasorb.
Thennocouples~~
l Gas Sampling Point
I I um oo I
Bubble Trap
Char ] Steam Generating Fumace I
I I
I
I"
Syringe Water Pump I
I
Fig. 1. Schematic Diagram of the activation rig
RESULTS AND DISCUSSION It was found that at different heating rates, variations of product yields were achieved for the pyrolysis of hemp fibres. As the heating rate was increased there was a decrease in the char yield. As a result, the gas and the oil yield increased as the rate of heating increased. Table 1. shows that with the heating rate kept the same, but increasing the final temperature of pyrolysis, the percentage of gas produced decreases while the oil yield steadily increases. At the higher temperatures the yield of char is also decreased. This
2379 is demonstrated clearly in Table 1. Here the results for the heating rate of 2Kmin 1 are shown to different final temperatures of pyrolysis. Table. 1. Yields ofBio-gas, Bio-oil and Char at different final pyrolysis temperatures. Oil Yield (wt%)
Char Yield (wt%) Final Pyrolysis Temperature 430 550 650
28.27 27.21 25.38
Gas Yield (wt%)
41.56 43.8 50.72
30.27 28.99 23.9
The char has been shown (Teng., et al 1998) to consist of both carbon and partially pyrolysed material such as hydrocarbons of high molecular weight. With the increase in temperature these degrade further to increase the liquid yield of the pyrolysis process. High yields of char at the low pyrolysis temperature of 430°C may suggest that the biomass has undergone only partial as opposed to full pyrolysis. This may prove to be a key factor in the ability of a char to be created into a useful activated carbon. The gasification of the carbon char with steam can make a large difference to the surface area of the carbon. The initial surface area ofthe hemp char was 3 mZ/g. Once treated with steam at 700 and 800 °C a dramatic increase in surface area was found, for example an increase to a surface area of almost 700 m2g-1 was identified for 5.5hr burnoff with a gasification temperature of 800 °C. Table 2. demonstrates the increase achieved in surface area as the sample is gasified for an extended period of time. The corresponding steam gasification reactions are endothermic, and demonstrate how the steam reacts with the carbon char (Bacaoui et al., 1998).
,
>
c o (g) + H~O (g)
'
>
CO~ (g) + H~ (g)
(H)
CO2 (g) + Cx (s)
'
7:>
2 C 0 (g) + Cxq (s)
(Ill)
HzO (g) + Cx (s)
He (g) + CO (g) + Cxq (s)
(I)
Table. 2. Surface Areas of hemp char pyrolysed at 650°C at a heating rate of 2K/min and then gasified at 700 and 800°C with steam for different lengths of time. Gasification Temp. °C 700°C 800°C
2 hrs Burnoff 125 m2g-1 449 m2g-1
4.5 hrs Burnoff 381 mZg1 531 meg-1
5.5 hrs Bumoff 452 m2g-1 672 m/g 1
The surface areas achieved are high, but a char used from pyrolysis at 650 °C does not perform as well as the partially pyrolysed char from pyrolysis runs with a lower final temperature. The difference noted in the product yield in table 1. is considered a key factor in the final properties of the activated carbon. Table 3 demonstrates the differences in surface area achieved for the same gasification conditions, with the only variation being the final temperature the chars reached during pyrolysis of 700 °C for 2 hrs bumoff time.
2380
Table 3. Surface Areas of hemp char pyrolysed to different final temperatures but each gasified at 700°C for 2 hours.
Surface Area m2g-1
430 391
Pyrolysis Temperature (°C) 550 650 243 125
It can be seen from the results given that there are a number of factors, which affect the ability of a biomass char to be converted into a useful activated carbon. The slow heating rate in preference to a fast heating rate produces a greater carbon yield. The final pyrolysis temperature is also a key factor, with lower pyrolysis temperatures being preferred. The gasification temperature of 800°C and a long burnoff time seem to give preference to the high surface areas achieved from the char of hemp, creating a usable product from a waste material. ACKNOWLEDGEMENTS We would like to thank the U.K. Science and Engineering Research Council for support for this work under grant number GR/L35126. REFERENCES Ansari, I. A. M. I. (1995). Natural Cellulosic Fibres for Industrial Uses. PhD. Thesis, Department of
Textile Industries, University of Leeds. Bacaoui, A., A. Yaacoubi, A. Dahbi, C. Bennouna, J. Ayele and M. Mazet. (1998). Activated carbon production from Moroccan olive wastes-influence of some factors. Environmental Technology,19, 1203-
1212. Bridgewater, A. V., and Bridge, S. A. (1991). A review of Biomass pyrolysis and pyrolysis technologies. In: Biomass pyrolysis liquids upgrading and utilisation. A. V. Bridgwater and G. Grassi (Eds). Elesevier Applied Science, London. Home, P. A. and P. T. Williams (1996). Influence of temperature on the products from flash pyrolysis of Biomass. Fuel 75, 9, 1051-1059. Rodriguez-Reinoso, M. Molma-Sabio and M. T. Gonzalez (1995). The use of Steam and CO2 as activating agents in the preparation of activated carbons. Carbon, 33, 1, 15-23. Satya Sai, P. M., J. Ahmed. And K. Krishnaiah. (1997). Production of activated carbon from cocunut shell char in a fluidized bed reactor. Ind. Eng. Chem., 36, 3625-3630. Teng, H., and Tien-Sheng, Yeh. (1998). Preparation of activated carbons from bituminous coals with Zinc Chloride activation. Ind. Eng. Chem. Res., 37, 58-65. Ward, H. Jr. (1954) In'J-Iigh polymer cellulose, Eds, E. OHH. M. Spurlin., and M. W. Graffin, Interrscience publication Inc. 5, 1, 539-549. Williams, P. T., and S. Besler. (1996). The influence of temperature and heating rate on the slow pyrolysis ofbiomass. Renewable Energy, 7, 233-250.
2377
PYROLYSIS/GASIFICATION OF HEMP TO PRODUCE ACTIVATED CARBON
Anton R. Reed 1, David J. Johnson2 and Paul T. Williams 1 Department of Fuel and Energyland School of Textile Industries 2 The University of Leeds, Leeds LS2 9JT (TEL: UK (#44) 1132 332504 ; FAX : UK (#44) 1132 440572)
ABSTRACT Hemp derived as a by-product from the textile industry was pyrolysed in a fixed bed reactor at temperatures between 430 and 750 °C. The derived char was gasified between 700 and 800°C in a steam atmosphere to produce activated carbon. Char surface areas were increased from 3 mZg1 before gasification, to over 600 m 2 g-1 after activation. The process variables influencing the surface area of the activated carbons were identified as, the heating rate and final temperature of biomass pyrolysis to produce the initial char, and gasification temperature.
KEYWORDS Pyrolysis; gasification; surface area; bio-gas; bio-oil; activation; bumoff.
BIOMASS AS A TEXTILE WASTE Biomass is a raw material that has been utilised for a wide variety of tasks since the dawn of civilisation. Important as a supply of fuel in the third world, biomass was also the first raw material in the production of textiles. Hemp is a biomass textile and is a 'bast' fibre. Bast fibres come from the stem of the plant, lying in bundles beneath the outer covering or bark (Ansari et al., 1995). The textile industry can utilise only about 25% by weight of the stems, the remainder is waste (Ward et aL, 1954). Energy from waste biomass can be realised by direct use in combustion, or by its thermochemical conversion via pyrolysis to bio-oil, bio-gas or high calorific value carbon char (Bridgwater et al., 1991). Pyrolysis process conditions can be optimised to maximise the production of oils, gases or chars (Home et al., 1996). The aim of this project was to optimise pyrolysis conditions and the use of H20 gasification techniques to produce activated carbons from the textile waste material. Activated carbon is a material widely used for the adsorption of pollutants from both liquid and gaseous waste streams, for the recovery of solvents and as a catalyst or catalyst support (Satya Sai et al., 1997).
2378 Oxidants such as H20 eliminate carbon atoms from the char particle via CO and/or CO + H2 in a way which favours selective burning of the interior of the particle, with the subsequent creation of porosity (Rodriguez-Reinoso et al., 1995). The BET calculation can then be used to determine the surface area of activated chars, which is an indication of their usefulness as a pollutant adsorbent. EXPERIMENTAL A slow pyrolysis, stainless steel, static batch reactor, heated with an electric ring fumace was developed for the pyrolysis of the hemp. Purged continuously with nitrogen, the gases evolved were removed after a residence time of one minute (Williams, et al., 1996). 30g samples of hemp were heated at controlled rates of 1, 2, 5 and 10 °C min -1. Final temperatures of 430, 550, 650, 750 °C were used and then held at that temperature for one hour. The gasification process was carried out at 700 and 800 °C for the chars produced at 650 °C. Water was added to a preheater at 400 °C, via a syringe pump with a flow rate of 2.5 ml/hr. The steam was pumped into the inert nitrogen flowing over the char as the oxidising agent. Activation took place for 2, 4.5, 5,5 and 7.5 hrs. Figure 1. Shows the activation rig. The BET calculations for surface area determination were carried out on results from a Quantachrome Quantasorb.
Thennocouples~~
l Gas Sampling Point
I I um oo I
Bubble Trap
Char ] Steam Generating Fumace I
I I
I
I"
Syringe Water Pump I
I
Fig. 1. Schematic Diagram of the activation rig
RESULTS AND DISCUSSION It was found that at different heating rates, variations of product yields were achieved for the pyrolysis of hemp fibres. As the heating rate was increased there was a decrease in the char yield. As a result, the gas and the oil yield increased as the rate of heating increased. Table 1. shows that with the heating rate kept the same, but increasing the final temperature of pyrolysis, the percentage of gas produced decreases while the oil yield steadily increases. At the higher temperatures the yield of char is also decreased. This
2379 is demonstrated clearly in Table 1. Here the results for the heating rate of 2Kmin 1 are shown to different final temperatures of pyrolysis. Table. 1. Yields ofBio-gas, Bio-oil and Char at different final pyrolysis temperatures. Oil Yield (wt%)
Char Yield (wt%) Final Pyrolysis Temperature 430 550 650
28.27 27.21 25.38
Gas Yield (wt%)
41.56 43.8 50.72
30.27 28.99 23.9
The char has been shown (Teng., et al 1998) to consist of both carbon and partially pyrolysed material such as hydrocarbons of high molecular weight. With the increase in temperature these degrade further to increase the liquid yield of the pyrolysis process. High yields of char at the low pyrolysis temperature of 430°C may suggest that the biomass has undergone only partial as opposed to full pyrolysis. This may prove to be a key factor in the ability of a char to be created into a useful activated carbon. The gasification of the carbon char with steam can make a large difference to the surface area of the carbon. The initial surface area ofthe hemp char was 3 mZ/g. Once treated with steam at 700 and 800 °C a dramatic increase in surface area was found, for example an increase to a surface area of almost 700 m2g-1 was identified for 5.5hr burnoff with a gasification temperature of 800 °C. Table 2. demonstrates the increase achieved in surface area as the sample is gasified for an extended period of time. The corresponding steam gasification reactions are endothermic, and demonstrate how the steam reacts with the carbon char (Bacaoui et al., 1998).
,
>
c o (g) + H~O (g)
'
>
CO~ (g) + H~ (g)
(H)
CO2 (g) + Cx (s)
'
7:>
2 C 0 (g) + Cxq (s)
(Ill)
HzO (g) + Cx (s)
He (g) + CO (g) + Cxq (s)
(I)
Table. 2. Surface Areas of hemp char pyrolysed at 650°C at a heating rate of 2K/min and then gasified at 700 and 800°C with steam for different lengths of time. Gasification Temp. °C 700°C 800°C
2 hrs Burnoff 125 m2g-1 449 m2g-1
4.5 hrs Burnoff 381 mZg1 531 meg-1
5.5 hrs Bumoff 452 m2g-1 672 m/g 1
The surface areas achieved are high, but a char used from pyrolysis at 650 °C does not perform as well as the partially pyrolysed char from pyrolysis runs with a lower final temperature. The difference noted in the product yield in table 1. is considered a key factor in the final properties of the activated carbon. Table 3 demonstrates the differences in surface area achieved for the same gasification conditions, with the only variation being the final temperature the chars reached during pyrolysis of 700 °C for 2 hrs bumoff time.
2380
Table 3. Surface Areas of hemp char pyrolysed to different final temperatures but each gasified at 700°C for 2 hours.
Surface Area m2g-1
430 391
Pyrolysis Temperature (°C) 550 650 243 125
It can be seen from the results given that there are a number of factors, which affect the ability of a biomass char to be converted into a useful activated carbon. The slow heating rate in preference to a fast heating rate produces a greater carbon yield. The final pyrolysis temperature is also a key factor, with lower pyrolysis temperatures being preferred. The gasification temperature of 800°C and a long burnoff time seem to give preference to the high surface areas achieved from the char of hemp, creating a usable product from a waste material. ACKNOWLEDGEMENTS We would like to thank the U.K. Science and Engineering Research Council for support for this work under grant number GR/L35126. REFERENCES Ansari, I. A. M. I. (1995). Natural Cellulosic Fibres for Industrial Uses. PhD. Thesis, Department of
Textile Industries, University of Leeds. Bacaoui, A., A. Yaacoubi, A. Dahbi, C. Bennouna, J. Ayele and M. Mazet. (1998). Activated carbon production from Moroccan olive wastes-influence of some factors. Environmental Technology,19, 1203-
1212. Bridgewater, A. V., and Bridge, S. A. (1991). A review of Biomass pyrolysis and pyrolysis technologies. In: Biomass pyrolysis liquids upgrading and utilisation. A. V. Bridgwater and G. Grassi (Eds). Elesevier Applied Science, London. Home, P. A. and P. T. Williams (1996). Influence of temperature on the products from flash pyrolysis of Biomass. Fuel 75, 9, 1051-1059. Rodriguez-Reinoso, M. Molma-Sabio and M. T. Gonzalez (1995). The use of Steam and CO2 as activating agents in the preparation of activated carbons. Carbon, 33, 1, 15-23. Satya Sai, P. M., J. Ahmed. And K. Krishnaiah. (1997). Production of activated carbon from cocunut shell char in a fluidized bed reactor. Ind. Eng. Chem., 36, 3625-3630. Teng, H., and Tien-Sheng, Yeh. (1998). Preparation of activated carbons from bituminous coals with Zinc Chloride activation. Ind. Eng. Chem. Res., 37, 58-65. Ward, H. Jr. (1954) In'J-Iigh polymer cellulose, Eds, E. OHH. M. Spurlin., and M. W. Graffin, Interrscience publication Inc. 5, 1, 539-549. Williams, P. T., and S. Besler. (1996). The influence of temperature and heating rate on the slow pyrolysis ofbiomass. Renewable Energy, 7, 233-250.