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Physical and thermochemical characterization of rice husk char as a potential biomass energy source
Bioresource Technology 97 (2006) 2065–2070
Physical and thermochemical characterization of rice husk char as a potential biomass energy source S. Maiti, S. Dey, S. Purakayastha, B. Ghosh
*
Advanced Materials and Solar Photovoltaic Division, School of Energy Studies, Jadavpur University, Kolkata 700 032, India Received 12 April 2005; received in revised form 5 October 2005; accepted 6 October 2005 Available online 17 November 2005
Abstract The fixed bed pyrolysis of rice husk was studied under conventional conditions with the aim of determining the characteristics of the charcoal formed for its applicability as a solid fuel. Thermoanalytic methods were used to determine the kinetic parameters of its combustion. Palletisation using different binders and techniques to improve the time of sustained combustion of the char pallets were investigated. The optimum temperature for carbonization to obtain a char having moderately high heating value was found as 400 C. For the active char combustion zone, the order of reaction was nearly 1, the activation energy 73.403 kJ/mol and the pre-exponential factor 4.97 · 104 min1. Addition of starch as a binder and 10% ferrous sulphate heptahydrate or sodium hypophosphite as an additive enhanced the ignitibility of the char pallets. 2005 Elsevier Ltd. All rights reserved. Keywords: Rice husk char; Energy source; Pyrolysis; Fuel characterization; Thermoanalytic methods; Palletisation; Ignition temperature
1. Introduction Pyrolysis can be described as the direct thermal decomposition of the organic matrix in the absence or very limited quantity of oxygen to obtain an array of solid, liquid and gas products. The process has been practiced for centuries for production of charcoal from biomass and requires relatively slow reaction at very low temperatures to maximize solid yield. Researchers have extensively studied biomass pyrolysis at various reaction conditions. Some of the them are of rice husks (Islam and Ani, 2000; Teng et al., 1997; Koullas et al., 1998; Mansaray and Ghaly, 1998), sawdust (Aguado et al., 2000), sewage sludge (Ahuja et al., 1996), rapeseed stalk (C ¸ ulcuogˇlu and Karaosmanogˇlu, 2001), straw (Demirbasß and Sahin, 1998; Di Blassi et al., 2000; Ergu¨denler and Ghaly, 1992) and sugarcane bagasse (Bilba and Ouensanga, 1996; Gracia-Perez et al., 2001; Marshall et al., 2000). *
Corresponding author. Tel.: +91 33 2414 6823; fax: +91 33 2414 6853. E-mail address: [email protected] (B. Ghosh).
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.10.005
India produces around 78 million metric tons of rice against worlds production of 540 million metric tons, thus accounting for around 14% of world production. Due to the enormous rice production and processing activities, huge amount of residues are generated at the rice mill sites. A part of these is consumed in traditional uses such as fodder for livestock and industrial fuel for boilers. Until recently the majority of the rice mill generated wastes were not utilized and disposed off by burning in open fields. The aim of this work was to study on conventional pyrolysis of rice husk in a fixed bed pyrolytic reactor to produce rice husk char and process parameters were optimized in order to establish this carbonaceous residue as a potential solid fuel. 2. Methods 2.1. Rice husk Rice husk was collected from a rice mill locally. It was sun dried and the moisture content reduced to less than 10%.
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2.2. Experimental set up biomass char preparation The dried rice husk was taken in a stainless steel fixed bed reactor of dimensions 0.1 m · 0.1 m · 0.2 m. The feed was heated externally by an electric heater of 3 kw capacity. The temperature of the reactor was controlled by means of a temperature controller and measured by a thermocouple in the bed. The experiments were conducted at a fixed heating rate of 10 C min1. The temperature range of observation was 200–650 C at an interval of 50 C and the holding time was fixed at 60 min. The vapors and the gases that were evolved during the process were passed through a water-cooled condenser attached to a liquid receiver, which was in turn connected to a rotary vacuum pump. 2.3. Property determination 2.3.1. Yield The weight of the biomass was measured before and after the pyrolysis. The yield is given by Yield ð%Þ ¼ A=B 100; A is the weight of the material after pyrolysis and B is the weight of the material before pyrolysis respectively. 2.3.2. Volatile matter, fixed carbon and ash content The proximate analysis includes percentage of ash content, volatile matter and fixed carbon. These were determined by standard published procedures (ASTM 3174 and ASTM 3175). 2.3.3. pH The rice husk char sample was soaked with a specified volume of distilled water and boiled for around 10 min. The solution was allowed to cool down to room temperature, filtered using Whatman 40 filter paper and the pH was observed using a digital pH meter (Systronics 802). 2.3.4. Heating value Heating value is a major quality index for fuels. higher heating value, commonly known as HHV, defines the energy content of a fuel. Estimation of HHV from elemental composition of the fuel is one of the basic steps in performance modeling and calculations of thermal systems. Numerous correlation for calculation of HHV are available from literature, out of which the equation proposed by Cordero et al. (2001) compares very well with the others and has the advantage of its applicability to a wide range of carbonaceous materials. The equation relating HHV, fixed carbon and volatile matter has been derived from multiple linear regression analysis and can be given as HHV ðkJ=kgÞ ¼ 354:3FC þ 170:8VM ðVM þ FC þ Ash ¼ 100Þ For optimization of process parameters, the variation of the above properties of the char was examined in details
with the main parameter of pyrolysis, i.e. temperature of the bed. 2.3.5. Ultimate analysis The char samples were characterized for elemental analysis using energy dispersive X-ray fluorescence (ED-XRF) spectrometer (Jordan Valley EX-3600). For sample preparation technique similar to the one used by Aprilesi et al. (1984) was followed. Two hundred micromillilitres of 1000-ppm palladium as PdCl2, was added to 50 ml of the water sample. This served as internal standard and also as a co-precipitant. The solution was maintained at pH-9, by treating with liquid ammonia. Saturated NaDDTC (2.5 ml) (sodium diethyldithio carbamate, C5H10NaS2 Æ H2O) was added to precipitate the metal contents as their respective carbamates. The precipitate thus formed was collected on 25 mm diameter Nuclepore membranes of pore size 0.4 lm, by vacuum filtration using a Millipore vacuum filtration unit. A thin uniform layer (about 23 mm in diameter) was formed on the membrane that is used as sample for the ED-XRF measurements. The ED-XRF unit employs a Si(Li) semiconductor detector to detect X-rays emitted from the samples. Titanium and iron filters were used to detect some of the elements. 2.3.6. Thermal analysis The rice husk char samples were subjected to thermogravimetric (t.g.a.), derivative thermogravimetric (d.t.g.) and differential thermal analysis (d.t.a.) in air using heating rate of 10 C min1. A Shimadzu TG 41 Thermal Analyzer was used for the thermal analyses. In order to ensure the uniformity of temperature of the sample and good reproducibility, small size was taken. The sample (10 mg) was heated from ambient to 1000 C at a slow heating rate of 10 C min1. The data was analyzed to determine the t.g.a. and d.t.a. indices (thermal degradation rate, initial degradation temperature, peak temperature). The determination of kinetic parameters were based on the Arhennius equation and a technique proposed by Duvvuri et al. and applied by Ergu¨denler and Ghaly (1992). 2.3.7. Palletising the rice husk char Palletising experiments were performed using a hydraulic press. The maximum pressurising capacity of the hydraulic press was 1100 MPa between two plates with a speed of 45 mm min1. The cylindrical palletising die was made of hardened steel with an inner diameter of 25 mm and a height of 60 mm. Before palletising, the char samples were ground to pass through a sieve having an opening of 250 lm. For each experiment, 20 g of these samples mixed thoroughly with 2–4% binder, filled into the die and pallets were produced under pressures of 200–300 MPa. In order to determine the stability of the pallets due to the addition of binders, measurements of the length were taken immediately on removal from the die, after 1 week of exposure to the atmosphere and again after 3 weeks exposure. A series
2.3.8. Increase of ignition temperature and heat retentivity of the palletized rice husk char To ensure a sustained combustion of the rice-husk char, some experiments were done to enhance the ignition time and temperature of the palletised char sample. Several chemicals, each having different effects on the carbon at its unpiloted ignition temperature, were added to the char to the extent of 10–20%. The loose matter was then palletised using the most suitable binder. Experiments were done using some carbonates like mixture of sodium carbonate and bi-carbonate, magnesium carbonate and zinc carbonate. Alkaline pyrogallet solution (%), ferrous sulphate heptahydrate, boric acid, disodium tetraborate, sodium hypophosphite and zinc chloride were also used as additives. The time just at the start of visual and sustained combustion (smoldering or appearance of flame) was plotted against temperature. The rice-husk char alone in its palletised form was used as a reference. 3. Results and discussion 3.1. Variation of the yield, volatile matter, fixed carbon, ash, pH, bulk density and heating value of the rice-husk during pyrolysis with temperature of the bed of the reactor The effect of reactor temperature on the yield, volatile matter, fixed carbon, ash, pH and heating value of rice husk char is shown in Figs. 1 and 2. The drastic weight loss pattern observed between 350 and 400 C was due to the destruction of cellulose and hemicellulose in the original biomass. Pyrolysis apparently started at around 350 C where the volatile matter in the biomass began to vaporize. The yield became almost constant after this stage, which marked the maximum char yield. Volatile matter dropped rapidly between temperatures of 300 C and 400 C and remained constant from about 500 C. Similarities in the curves were noticed in case of generation and volatile matter. This can be explained by the fact that as the pyrolysis
pH
of tests were done to determine the density and heating value of the pallets formed.
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12
12
10
10
8
8
6 4
6 4
2
2
0
HHV (kJ/kg)
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0 250
300
350
400
450
500
550
600
650
700
Temperature (oC) HHV in kJ/kg
pH
Fig. 2. Variation of high heating value and pH of the resulting char with temperature.
proceeded, the gas generated reduces mass of the remaining char and also reduced the volatile matter of the biomass. The curve of variation of fixed carbon with temperature increased with increasing temperature and became constant from about 400 C. This was because the non-volatile flammable matter remained in the biomass after the volatile part was driven off. The ash content of rice husk was quite high and it increased with increasing pyrolysis temperature. Increased ash content was due to the reduction in the contents of other elements like nitrogen, carbon, hydrogen and oxygen. Sulphur was volatilized during heating but the inorganic salts in the ash did not. Hence as carbonization proceeded with increase in temperature, the ash content increased. Fig. 2 showed that a change in pH occurred in the temperature range of 300–550 C, the cause of which could be due to a separation of organic (carbon) and inorganic components (alkali metal salts) in the husk. After this temperature range, the pH remained almost constant, as the ash content remained relatively same. Pyrolysis at temperature of 600 C and above rendered all products alkaline. According to Abe (1988), the destruction of cellulose and hemicellulose around 300–350 C produced organic acids and phenolic substances, which lowered the pH of the product and it turned acidic. After 350 C, the alkali salts began to separate from the organic matrix and the pH was increased. Finally, when all the alkali salts were leached from the pyrolytic structure, the pH became constant at around 600 C. Higher ash content of rice husk decreases its heating value. Higher heating value indicates the biochars potential to be used as fuel. The optimum temperature for carbonization to obtain a char having moderately high heating value was 400 C. 3.2. Elemental analysis
Fig. 1. Variation of conversion of biomass to char, volatile matter, fixed carbon and ash content of the resulting char with temperature.
The elemental analyses of raw rice husk and the chars prepared at various heat treatment temperatures with a hold time of 30 min are shown in Table 1. As the temperature was increased, oxygen content decreased and ash content increased. The atomic ratio of H/C decreased from about 1.46 in the raw rice husk to 0.5 in the char prepared
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Table 1 Elemental analyses of rice husk heat treated under pyrolytic conditions at different temperatures
H
N
O
S
48.73 65.94 66.76 68.38 69.29 72.88
5.91 4.21 4.16 4.01 3.59 3.32
0.65 1.41 1.41 1.42 1.42 1.43
44.64 28.39 27.62 26.13 25.64 22.32
0.07 0.05 0.05 0.06 0.06 0.05
600 400 200
20.5 53.10 53.22 53.98 54.32 57.61
at 750 C. This implied that the char became progressively more aromatic with increasing temperature treatments. The determination of inorganic components showed high concentrations of metals like K (976 ppm), Ca (546 ppm), moderate levels of Fe (2027 ppb), Al (723 ppb) and high level of Si (2360 ppm) in rice hush char prepared at 350 C. Rb, V, S, Cu and Mn were also detected in trace amounts.
0 -200
w (mg)
C
800
dw/dt
% Dry ash
11000 10000 9000 8000 7000 6000 5000 4000 3000 200
˚
150 (raw rh) 350 450 550 650 750
% Dry ash free
1000
Temp diff ( C)
Temperature (C)
1200
150 100 50 0
0
3.3. Thermal analysis results Thermoanalytic techniques (d.t.a., t.g.a. and d.t.g.) have been widely utilized to study the thermal behaviour both of fossil fuels and biofuels. For coal, some correlations between d.t.g. data in air (burning profiles) and combustion efficiency in a pilot plant have been demonstrated (Alonso et al., 2001). Hence, it is possible that thermal analysis would make an important contribution to knowledge of the thermal behaviour of biomass. The results obtained by d.t.a., t.g.a. and d.t.g. of rice husk char are shown in Fig. 3. As previously showed by Ergu¨denler and Ghaly (1992), three steps of weight loss could be observed at the heating rate of 10 C min1, from the t.g.a. curve. From the start of the experiment until the temperature of around 150 C, the amount of weight loss was recorded due to loss of water present in the sample and the external water bound by surface tension. The second weight loss step occurred at 325–425 C and it was correlated with combustion of lignin in the fuel. The third step of combustion occurred at 425–490 C. The maximum rate of weight loss occurred at rapid decomposition of the sample and was observed between 360 and 490 C. This zone was referred to as the active zone. The d.t.a. curve showed the exothermic peak at 447.67 C, which coincided with the beginning of rapid decomposition on t.g.a. curve. This method of thermoanalytic study is attempted to characterize rice husk chars thermal behavior in an oxidizing atmosphere to improve its utilization as biomass fuel. Determining the kinetic parameters also provides information to design more effective conversion systems. In the temperature range of 360–490 C the maximum thermal degradation took place. Thermal degradation rate was 4.917% min1 and the percentage of total thermal degradation was 56.8. For this zone, the order of reaction was 0.71452 or nearly 1, the activation energy 73.403 kJ/mol
200
400
600
800
1000
Temperature (oC)
Fig. 3. d.t.g., t.g.a. and d.t.a. curves of rice husk char at 10 C min1 respectively.
and the pre-exponential factor 4.97 · 104 min1, with a regression coefficient 0.93601. 3.4. Characteristics of the rice husk char pallets Rice husk char is a material totally lacking plasticity and hence needs addition of a sticking or agglomerating material to enable a pallet or briquette to be formed. The binder should preferably be combustible, though a non-combustible binder effective at low concentrations can be suitable. For the purpose of palletizing the rice husk char, a number of binders were considered. The heating values of the char/ binder combinations were studied initially in order to select a suitable binder. Experiments were done using coal tar, polyvinyl alcohol, clay, starch, bentonite, sodium silicate and molasses. The results, as given in Fig. 4, showed that in terms of heating value coal tar was the best as a binder, but it produced a lot of smoke and residual ash on burning. Polyvinyl alcohol was second best to coal tar, but had to be
Fig. 4. Comparison of heating values of palletized rice husk char using different binders.
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Fig. 5. Variation of retention time of char ignition with temperature.
eliminated due to its high cost. Starch was chosen as the most effective one. About 4–8% of starch made into paste with hot water was adequate. The pallet had a fairly moderate energy content of 11.8 kJ/kg. Pallet density, which in this case was 0.52 kg m3, is an important parameter in briquetting or palletizing. The higher the density, the higher is the energy–volume ratio. Qualitative observations have shown that as the densities of the compacted biomass increased their ignitabilities decreased. It is very difficult to obtain optimum parameters for acceptable ignitability of pallets. The effect of moisture content of the pallets was managed through control over the initially ground char and binder mixture factor. The stability and pallet strength showed that the rice husk char pallets prepared using starch as binder were fairly stable.
for the same reason. Salts like ferrous sulphate heptahydrate, disodium tetraborate, sodium hypophosphite were added for their reducing properties. Boric acid is a common additive used as flame-retardants. The results as given in Fig. 5 showed best outcomes using ferrous sulphate and sodium hypophosphite as retention time before unpiloted ignition of the rice husk char at 375 C was 180 min. 4. Conclusion This study was an attempt to establish the rice husk char obtained from the rice husk gasification system as a potential solid fuel. Results showed that this waste could be a viable option of biomass energy source. Acknowledgements
3.5. Results of increase of ignition temperature and heat retentivity of the palletized rice husk char Unpiloted ignition is flaming ignition that occurs in absence of a pilot source. The external heat flux needed for unpiloted ignition is around 25 kW m2 depending on the apparatus. For radiant heating of biomass or any other cellulosic solids, unpiloted transient ignition has been reported at 600 C. With convective heating, unpiloted ignition has been reported as low as 270 C and as high as 470 C (White and Dietenberger, 2001). For rice husk char pallet using starch as binder, the unpiloted ignition temperature was found to be 375 C. A special type of unpiloted ignition is spontaneous ignition that occurs when heat generated from an internal source is not readily dissipated. Such ignition generally occurs for a prolonged period and involves smoldering. This process gives rise to a sustained combustion within the pyrolysing material. Carbonates of sodium and zinc were added to the char samples in order to cut off the oxygen supply by generation of carbon-di-oxide to sustain the combustion process. Alkaline pyrogallet solution being an absorber of oxygen was added
The authors thank CSIR, Govt. of India for the financial support provided for this study. They would also like to acknowledge the extended help from USIC, Jadavpur University, DAE-IUC, Kolkata centre and NML, Jamshedpur. References Abe, F., 1988. The thermochemical study of forest biomass. Forest Prod. Chem. 45, 1–95. Aguado, R., Olazar, M., Jose, M.J.S., Aguirre, G., Bilbao, J., 2000. Pyrolysis of saw dust in a conical spouted bed reactor. Yields and product composition. Ind. Eng. Chem. Res. 39, 1925–1933. Ahuja, P., Singh, P.C., Upadhaya, S.N., Kumar, S., 1996. Kinetics of biomass and sewage sludge pyrolysis: thermogravimertic and scaled reactor studies. Indian J. Chem. Technol. 3, 306–312. Alonso, M.J.G., Borrego, A.G., Alvarez, D., Kalkreuth, W., Mene´ndez, R., 2001. Physicochemical transformations of coal particles during pyrolysis and combustion. Fuel 80, 1857–1870. Aprilesi, G., Cecchi, R., Ghermandi, G., Magnoni, G., Santangelo, 1984. Calibration and errors in the detection of heavy metals in fresh and sea waters by PIXE in the ppb–ppm range. Nucl. Instr. Method B 3, 172–176. Bilba, K., Ouensanga, A., 1996. Fourier transform infrared spectroscopic study of thermal degradation of sugarcane bagasse. J. Anal. Appl. Pyrol. 38, 1–73.
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Cordero, T., Marquez, F., Rodriguez-Mirasol, J., Rodriguez, J.J., 2001. Predicting heating values of lignocellulosic and carbonaceous materials from proximate analysis. Fuel 80, 567–1571. C ¸ ulcuogˇlu, E., Karaosmanogˇlu, F., 2001. Thermogravimetric analysis of rapeseed cake. Energy Sources 23, 889–895. Demirbasß, A., Sahin, A., 1998. Evaluation of biomass residue, briquetting waste paper and wheat straw mixtures. Fuel Process. Technol. 55, 175– 183. Di Blassi, C., Branca, C., DErrico, G., 2000. Degradation characteristics of straw and washed straw. Thermochim. Acta 364, 133–142. Ergu¨denler, A., Ghaly, A.E., 1992. Determination of reaction kinetics of wheat straw using thermogravimetric analysis. App. Biochem. Biotechnol. 34, 75–91. Gracia-Perez, M., Chaala, A., Yang, J., Roy, C., 2001. Co-pyrolysis of sugarcane bagasse with petroleum residue. Fuel 80, 1245–1258. Islam, M.N., Ani, F.N., 2000. Techno-economics of rice husk pyrolysis, conversion with catalytic treatment to produce liquid fuel. Bio. Resour. Technol. 73, 67–75. Koullas, D.P., Nikolaou, N., Koukios, E.G., 1998. Modelling of nonisothermal kinetics of biomass prepyrolysis at low pressure. Bioresour. Technol. 63, 261–266. Mansaray, K.G., Ghaly, A.E., 1998. Thermal degradation of rice husks in nitrogen atmosphere. Bio. Resour. Technol. 65, 16–20. Marshall, W.E., Ahmedna, M., Rao, R.M., Johns, M.M., 2000. Granular activated carbons from sugarcane bagasse: production and uses. Int. Sugar J. 102, 1215.
Teng, H.S., Lin, H.C., Ho, J.A., 1997. Thermogravimetric analysis on global mass loss kinetics of rice hull pyrolysis. Ind. Eng. Chem. Res. 36, 3974–3977. White, R.H., Dietenberger, M.A., 2001. Encyclopedia of Materials: Science and Technology 9712–9716, ISBN: 0-08-0431526.
Further reading Blesa, M.J., Fierro, V., Miranda, J.L., Moliner, R., Palacios, J.M., 2001. Effect of the pyrolysis process on the physico-chemical and mechanical properties of smokeless fuel briquettes. Fuel Process. Technol. 74, 1–17. Boonmee, N., Quintiere, J.G., 2005. Glowing ignition of wood: the onset of surface combustion. Proc. Combust. Inst. 30, 2303–2310. Ghosh, B., 2004. First Handbook of Stakeholders, School of Energy Studies. Jadavpur University, Kolkata. Pindoria, R.V., Chatzakis, I.N., Lim, J.Y., Herod, A.A., Dugwell, D.R., Kandiyoti, R., 1999. Hydropyrolysis of sugar cane bagasse: effect of sample configuration on bio-oil yields and structures from two benchscale reactors. Fuel 78, 55–63. Ravindranath, N.H., Hall, D.O., 1995. Biomass, Energy and Environment—A Developing Country Perspective from India. Oxford Univ Press, Oxford, ISBN 0-19-856436-8. Yaman, S., 2004. Pyrolysis of biomass to produce fuels and chemical feedstocks. Energy Conversion Manage. 45, 651–671.
Physical and thermochemical characterization of rice husk char as a potential biomass energy source S. Maiti, S. Dey, S. Purakayastha, B. Ghosh
*
Advanced Materials and Solar Photovoltaic Division, School of Energy Studies, Jadavpur University, Kolkata 700 032, India Received 12 April 2005; received in revised form 5 October 2005; accepted 6 October 2005 Available online 17 November 2005
Abstract The fixed bed pyrolysis of rice husk was studied under conventional conditions with the aim of determining the characteristics of the charcoal formed for its applicability as a solid fuel. Thermoanalytic methods were used to determine the kinetic parameters of its combustion. Palletisation using different binders and techniques to improve the time of sustained combustion of the char pallets were investigated. The optimum temperature for carbonization to obtain a char having moderately high heating value was found as 400 C. For the active char combustion zone, the order of reaction was nearly 1, the activation energy 73.403 kJ/mol and the pre-exponential factor 4.97 · 104 min1. Addition of starch as a binder and 10% ferrous sulphate heptahydrate or sodium hypophosphite as an additive enhanced the ignitibility of the char pallets. 2005 Elsevier Ltd. All rights reserved. Keywords: Rice husk char; Energy source; Pyrolysis; Fuel characterization; Thermoanalytic methods; Palletisation; Ignition temperature
1. Introduction Pyrolysis can be described as the direct thermal decomposition of the organic matrix in the absence or very limited quantity of oxygen to obtain an array of solid, liquid and gas products. The process has been practiced for centuries for production of charcoal from biomass and requires relatively slow reaction at very low temperatures to maximize solid yield. Researchers have extensively studied biomass pyrolysis at various reaction conditions. Some of the them are of rice husks (Islam and Ani, 2000; Teng et al., 1997; Koullas et al., 1998; Mansaray and Ghaly, 1998), sawdust (Aguado et al., 2000), sewage sludge (Ahuja et al., 1996), rapeseed stalk (C ¸ ulcuogˇlu and Karaosmanogˇlu, 2001), straw (Demirbasß and Sahin, 1998; Di Blassi et al., 2000; Ergu¨denler and Ghaly, 1992) and sugarcane bagasse (Bilba and Ouensanga, 1996; Gracia-Perez et al., 2001; Marshall et al., 2000). *
Corresponding author. Tel.: +91 33 2414 6823; fax: +91 33 2414 6853. E-mail address: [email protected] (B. Ghosh).
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.10.005
India produces around 78 million metric tons of rice against worlds production of 540 million metric tons, thus accounting for around 14% of world production. Due to the enormous rice production and processing activities, huge amount of residues are generated at the rice mill sites. A part of these is consumed in traditional uses such as fodder for livestock and industrial fuel for boilers. Until recently the majority of the rice mill generated wastes were not utilized and disposed off by burning in open fields. The aim of this work was to study on conventional pyrolysis of rice husk in a fixed bed pyrolytic reactor to produce rice husk char and process parameters were optimized in order to establish this carbonaceous residue as a potential solid fuel. 2. Methods 2.1. Rice husk Rice husk was collected from a rice mill locally. It was sun dried and the moisture content reduced to less than 10%.
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2.2. Experimental set up biomass char preparation The dried rice husk was taken in a stainless steel fixed bed reactor of dimensions 0.1 m · 0.1 m · 0.2 m. The feed was heated externally by an electric heater of 3 kw capacity. The temperature of the reactor was controlled by means of a temperature controller and measured by a thermocouple in the bed. The experiments were conducted at a fixed heating rate of 10 C min1. The temperature range of observation was 200–650 C at an interval of 50 C and the holding time was fixed at 60 min. The vapors and the gases that were evolved during the process were passed through a water-cooled condenser attached to a liquid receiver, which was in turn connected to a rotary vacuum pump. 2.3. Property determination 2.3.1. Yield The weight of the biomass was measured before and after the pyrolysis. The yield is given by Yield ð%Þ ¼ A=B 100; A is the weight of the material after pyrolysis and B is the weight of the material before pyrolysis respectively. 2.3.2. Volatile matter, fixed carbon and ash content The proximate analysis includes percentage of ash content, volatile matter and fixed carbon. These were determined by standard published procedures (ASTM 3174 and ASTM 3175). 2.3.3. pH The rice husk char sample was soaked with a specified volume of distilled water and boiled for around 10 min. The solution was allowed to cool down to room temperature, filtered using Whatman 40 filter paper and the pH was observed using a digital pH meter (Systronics 802). 2.3.4. Heating value Heating value is a major quality index for fuels. higher heating value, commonly known as HHV, defines the energy content of a fuel. Estimation of HHV from elemental composition of the fuel is one of the basic steps in performance modeling and calculations of thermal systems. Numerous correlation for calculation of HHV are available from literature, out of which the equation proposed by Cordero et al. (2001) compares very well with the others and has the advantage of its applicability to a wide range of carbonaceous materials. The equation relating HHV, fixed carbon and volatile matter has been derived from multiple linear regression analysis and can be given as HHV ðkJ=kgÞ ¼ 354:3FC þ 170:8VM ðVM þ FC þ Ash ¼ 100Þ For optimization of process parameters, the variation of the above properties of the char was examined in details
with the main parameter of pyrolysis, i.e. temperature of the bed. 2.3.5. Ultimate analysis The char samples were characterized for elemental analysis using energy dispersive X-ray fluorescence (ED-XRF) spectrometer (Jordan Valley EX-3600). For sample preparation technique similar to the one used by Aprilesi et al. (1984) was followed. Two hundred micromillilitres of 1000-ppm palladium as PdCl2, was added to 50 ml of the water sample. This served as internal standard and also as a co-precipitant. The solution was maintained at pH-9, by treating with liquid ammonia. Saturated NaDDTC (2.5 ml) (sodium diethyldithio carbamate, C5H10NaS2 Æ H2O) was added to precipitate the metal contents as their respective carbamates. The precipitate thus formed was collected on 25 mm diameter Nuclepore membranes of pore size 0.4 lm, by vacuum filtration using a Millipore vacuum filtration unit. A thin uniform layer (about 23 mm in diameter) was formed on the membrane that is used as sample for the ED-XRF measurements. The ED-XRF unit employs a Si(Li) semiconductor detector to detect X-rays emitted from the samples. Titanium and iron filters were used to detect some of the elements. 2.3.6. Thermal analysis The rice husk char samples were subjected to thermogravimetric (t.g.a.), derivative thermogravimetric (d.t.g.) and differential thermal analysis (d.t.a.) in air using heating rate of 10 C min1. A Shimadzu TG 41 Thermal Analyzer was used for the thermal analyses. In order to ensure the uniformity of temperature of the sample and good reproducibility, small size was taken. The sample (10 mg) was heated from ambient to 1000 C at a slow heating rate of 10 C min1. The data was analyzed to determine the t.g.a. and d.t.a. indices (thermal degradation rate, initial degradation temperature, peak temperature). The determination of kinetic parameters were based on the Arhennius equation and a technique proposed by Duvvuri et al. and applied by Ergu¨denler and Ghaly (1992). 2.3.7. Palletising the rice husk char Palletising experiments were performed using a hydraulic press. The maximum pressurising capacity of the hydraulic press was 1100 MPa between two plates with a speed of 45 mm min1. The cylindrical palletising die was made of hardened steel with an inner diameter of 25 mm and a height of 60 mm. Before palletising, the char samples were ground to pass through a sieve having an opening of 250 lm. For each experiment, 20 g of these samples mixed thoroughly with 2–4% binder, filled into the die and pallets were produced under pressures of 200–300 MPa. In order to determine the stability of the pallets due to the addition of binders, measurements of the length were taken immediately on removal from the die, after 1 week of exposure to the atmosphere and again after 3 weeks exposure. A series
2.3.8. Increase of ignition temperature and heat retentivity of the palletized rice husk char To ensure a sustained combustion of the rice-husk char, some experiments were done to enhance the ignition time and temperature of the palletised char sample. Several chemicals, each having different effects on the carbon at its unpiloted ignition temperature, were added to the char to the extent of 10–20%. The loose matter was then palletised using the most suitable binder. Experiments were done using some carbonates like mixture of sodium carbonate and bi-carbonate, magnesium carbonate and zinc carbonate. Alkaline pyrogallet solution (%), ferrous sulphate heptahydrate, boric acid, disodium tetraborate, sodium hypophosphite and zinc chloride were also used as additives. The time just at the start of visual and sustained combustion (smoldering or appearance of flame) was plotted against temperature. The rice-husk char alone in its palletised form was used as a reference. 3. Results and discussion 3.1. Variation of the yield, volatile matter, fixed carbon, ash, pH, bulk density and heating value of the rice-husk during pyrolysis with temperature of the bed of the reactor The effect of reactor temperature on the yield, volatile matter, fixed carbon, ash, pH and heating value of rice husk char is shown in Figs. 1 and 2. The drastic weight loss pattern observed between 350 and 400 C was due to the destruction of cellulose and hemicellulose in the original biomass. Pyrolysis apparently started at around 350 C where the volatile matter in the biomass began to vaporize. The yield became almost constant after this stage, which marked the maximum char yield. Volatile matter dropped rapidly between temperatures of 300 C and 400 C and remained constant from about 500 C. Similarities in the curves were noticed in case of generation and volatile matter. This can be explained by the fact that as the pyrolysis
pH
of tests were done to determine the density and heating value of the pallets formed.
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12
12
10
10
8
8
6 4
6 4
2
2
0
HHV (kJ/kg)
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0 250
300
350
400
450
500
550
600
650
700
Temperature (oC) HHV in kJ/kg
pH
Fig. 2. Variation of high heating value and pH of the resulting char with temperature.
proceeded, the gas generated reduces mass of the remaining char and also reduced the volatile matter of the biomass. The curve of variation of fixed carbon with temperature increased with increasing temperature and became constant from about 400 C. This was because the non-volatile flammable matter remained in the biomass after the volatile part was driven off. The ash content of rice husk was quite high and it increased with increasing pyrolysis temperature. Increased ash content was due to the reduction in the contents of other elements like nitrogen, carbon, hydrogen and oxygen. Sulphur was volatilized during heating but the inorganic salts in the ash did not. Hence as carbonization proceeded with increase in temperature, the ash content increased. Fig. 2 showed that a change in pH occurred in the temperature range of 300–550 C, the cause of which could be due to a separation of organic (carbon) and inorganic components (alkali metal salts) in the husk. After this temperature range, the pH remained almost constant, as the ash content remained relatively same. Pyrolysis at temperature of 600 C and above rendered all products alkaline. According to Abe (1988), the destruction of cellulose and hemicellulose around 300–350 C produced organic acids and phenolic substances, which lowered the pH of the product and it turned acidic. After 350 C, the alkali salts began to separate from the organic matrix and the pH was increased. Finally, when all the alkali salts were leached from the pyrolytic structure, the pH became constant at around 600 C. Higher ash content of rice husk decreases its heating value. Higher heating value indicates the biochars potential to be used as fuel. The optimum temperature for carbonization to obtain a char having moderately high heating value was 400 C. 3.2. Elemental analysis
Fig. 1. Variation of conversion of biomass to char, volatile matter, fixed carbon and ash content of the resulting char with temperature.
The elemental analyses of raw rice husk and the chars prepared at various heat treatment temperatures with a hold time of 30 min are shown in Table 1. As the temperature was increased, oxygen content decreased and ash content increased. The atomic ratio of H/C decreased from about 1.46 in the raw rice husk to 0.5 in the char prepared
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Table 1 Elemental analyses of rice husk heat treated under pyrolytic conditions at different temperatures
H
N
O
S
48.73 65.94 66.76 68.38 69.29 72.88
5.91 4.21 4.16 4.01 3.59 3.32
0.65 1.41 1.41 1.42 1.42 1.43
44.64 28.39 27.62 26.13 25.64 22.32
0.07 0.05 0.05 0.06 0.06 0.05
600 400 200
20.5 53.10 53.22 53.98 54.32 57.61
at 750 C. This implied that the char became progressively more aromatic with increasing temperature treatments. The determination of inorganic components showed high concentrations of metals like K (976 ppm), Ca (546 ppm), moderate levels of Fe (2027 ppb), Al (723 ppb) and high level of Si (2360 ppm) in rice hush char prepared at 350 C. Rb, V, S, Cu and Mn were also detected in trace amounts.
0 -200
w (mg)
C
800
dw/dt
% Dry ash
11000 10000 9000 8000 7000 6000 5000 4000 3000 200
˚
150 (raw rh) 350 450 550 650 750
% Dry ash free
1000
Temp diff ( C)
Temperature (C)
1200
150 100 50 0
0
3.3. Thermal analysis results Thermoanalytic techniques (d.t.a., t.g.a. and d.t.g.) have been widely utilized to study the thermal behaviour both of fossil fuels and biofuels. For coal, some correlations between d.t.g. data in air (burning profiles) and combustion efficiency in a pilot plant have been demonstrated (Alonso et al., 2001). Hence, it is possible that thermal analysis would make an important contribution to knowledge of the thermal behaviour of biomass. The results obtained by d.t.a., t.g.a. and d.t.g. of rice husk char are shown in Fig. 3. As previously showed by Ergu¨denler and Ghaly (1992), three steps of weight loss could be observed at the heating rate of 10 C min1, from the t.g.a. curve. From the start of the experiment until the temperature of around 150 C, the amount of weight loss was recorded due to loss of water present in the sample and the external water bound by surface tension. The second weight loss step occurred at 325–425 C and it was correlated with combustion of lignin in the fuel. The third step of combustion occurred at 425–490 C. The maximum rate of weight loss occurred at rapid decomposition of the sample and was observed between 360 and 490 C. This zone was referred to as the active zone. The d.t.a. curve showed the exothermic peak at 447.67 C, which coincided with the beginning of rapid decomposition on t.g.a. curve. This method of thermoanalytic study is attempted to characterize rice husk chars thermal behavior in an oxidizing atmosphere to improve its utilization as biomass fuel. Determining the kinetic parameters also provides information to design more effective conversion systems. In the temperature range of 360–490 C the maximum thermal degradation took place. Thermal degradation rate was 4.917% min1 and the percentage of total thermal degradation was 56.8. For this zone, the order of reaction was 0.71452 or nearly 1, the activation energy 73.403 kJ/mol
200
400
600
800
1000
Temperature (oC)
Fig. 3. d.t.g., t.g.a. and d.t.a. curves of rice husk char at 10 C min1 respectively.
and the pre-exponential factor 4.97 · 104 min1, with a regression coefficient 0.93601. 3.4. Characteristics of the rice husk char pallets Rice husk char is a material totally lacking plasticity and hence needs addition of a sticking or agglomerating material to enable a pallet or briquette to be formed. The binder should preferably be combustible, though a non-combustible binder effective at low concentrations can be suitable. For the purpose of palletizing the rice husk char, a number of binders were considered. The heating values of the char/ binder combinations were studied initially in order to select a suitable binder. Experiments were done using coal tar, polyvinyl alcohol, clay, starch, bentonite, sodium silicate and molasses. The results, as given in Fig. 4, showed that in terms of heating value coal tar was the best as a binder, but it produced a lot of smoke and residual ash on burning. Polyvinyl alcohol was second best to coal tar, but had to be
Fig. 4. Comparison of heating values of palletized rice husk char using different binders.
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Fig. 5. Variation of retention time of char ignition with temperature.
eliminated due to its high cost. Starch was chosen as the most effective one. About 4–8% of starch made into paste with hot water was adequate. The pallet had a fairly moderate energy content of 11.8 kJ/kg. Pallet density, which in this case was 0.52 kg m3, is an important parameter in briquetting or palletizing. The higher the density, the higher is the energy–volume ratio. Qualitative observations have shown that as the densities of the compacted biomass increased their ignitabilities decreased. It is very difficult to obtain optimum parameters for acceptable ignitability of pallets. The effect of moisture content of the pallets was managed through control over the initially ground char and binder mixture factor. The stability and pallet strength showed that the rice husk char pallets prepared using starch as binder were fairly stable.
for the same reason. Salts like ferrous sulphate heptahydrate, disodium tetraborate, sodium hypophosphite were added for their reducing properties. Boric acid is a common additive used as flame-retardants. The results as given in Fig. 5 showed best outcomes using ferrous sulphate and sodium hypophosphite as retention time before unpiloted ignition of the rice husk char at 375 C was 180 min. 4. Conclusion This study was an attempt to establish the rice husk char obtained from the rice husk gasification system as a potential solid fuel. Results showed that this waste could be a viable option of biomass energy source. Acknowledgements
3.5. Results of increase of ignition temperature and heat retentivity of the palletized rice husk char Unpiloted ignition is flaming ignition that occurs in absence of a pilot source. The external heat flux needed for unpiloted ignition is around 25 kW m2 depending on the apparatus. For radiant heating of biomass or any other cellulosic solids, unpiloted transient ignition has been reported at 600 C. With convective heating, unpiloted ignition has been reported as low as 270 C and as high as 470 C (White and Dietenberger, 2001). For rice husk char pallet using starch as binder, the unpiloted ignition temperature was found to be 375 C. A special type of unpiloted ignition is spontaneous ignition that occurs when heat generated from an internal source is not readily dissipated. Such ignition generally occurs for a prolonged period and involves smoldering. This process gives rise to a sustained combustion within the pyrolysing material. Carbonates of sodium and zinc were added to the char samples in order to cut off the oxygen supply by generation of carbon-di-oxide to sustain the combustion process. Alkaline pyrogallet solution being an absorber of oxygen was added
The authors thank CSIR, Govt. of India for the financial support provided for this study. They would also like to acknowledge the extended help from USIC, Jadavpur University, DAE-IUC, Kolkata centre and NML, Jamshedpur. References Abe, F., 1988. The thermochemical study of forest biomass. Forest Prod. Chem. 45, 1–95. Aguado, R., Olazar, M., Jose, M.J.S., Aguirre, G., Bilbao, J., 2000. Pyrolysis of saw dust in a conical spouted bed reactor. Yields and product composition. Ind. Eng. Chem. Res. 39, 1925–1933. Ahuja, P., Singh, P.C., Upadhaya, S.N., Kumar, S., 1996. Kinetics of biomass and sewage sludge pyrolysis: thermogravimertic and scaled reactor studies. Indian J. Chem. Technol. 3, 306–312. Alonso, M.J.G., Borrego, A.G., Alvarez, D., Kalkreuth, W., Mene´ndez, R., 2001. Physicochemical transformations of coal particles during pyrolysis and combustion. Fuel 80, 1857–1870. Aprilesi, G., Cecchi, R., Ghermandi, G., Magnoni, G., Santangelo, 1984. Calibration and errors in the detection of heavy metals in fresh and sea waters by PIXE in the ppb–ppm range. Nucl. Instr. Method B 3, 172–176. Bilba, K., Ouensanga, A., 1996. Fourier transform infrared spectroscopic study of thermal degradation of sugarcane bagasse. J. Anal. Appl. Pyrol. 38, 1–73.
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Cordero, T., Marquez, F., Rodriguez-Mirasol, J., Rodriguez, J.J., 2001. Predicting heating values of lignocellulosic and carbonaceous materials from proximate analysis. Fuel 80, 567–1571. C ¸ ulcuogˇlu, E., Karaosmanogˇlu, F., 2001. Thermogravimetric analysis of rapeseed cake. Energy Sources 23, 889–895. Demirbasß, A., Sahin, A., 1998. Evaluation of biomass residue, briquetting waste paper and wheat straw mixtures. Fuel Process. Technol. 55, 175– 183. Di Blassi, C., Branca, C., DErrico, G., 2000. Degradation characteristics of straw and washed straw. Thermochim. Acta 364, 133–142. Ergu¨denler, A., Ghaly, A.E., 1992. Determination of reaction kinetics of wheat straw using thermogravimetric analysis. App. Biochem. Biotechnol. 34, 75–91. Gracia-Perez, M., Chaala, A., Yang, J., Roy, C., 2001. Co-pyrolysis of sugarcane bagasse with petroleum residue. Fuel 80, 1245–1258. Islam, M.N., Ani, F.N., 2000. Techno-economics of rice husk pyrolysis, conversion with catalytic treatment to produce liquid fuel. Bio. Resour. Technol. 73, 67–75. Koullas, D.P., Nikolaou, N., Koukios, E.G., 1998. Modelling of nonisothermal kinetics of biomass prepyrolysis at low pressure. Bioresour. Technol. 63, 261–266. Mansaray, K.G., Ghaly, A.E., 1998. Thermal degradation of rice husks in nitrogen atmosphere. Bio. Resour. Technol. 65, 16–20. Marshall, W.E., Ahmedna, M., Rao, R.M., Johns, M.M., 2000. Granular activated carbons from sugarcane bagasse: production and uses. Int. Sugar J. 102, 1215.
Teng, H.S., Lin, H.C., Ho, J.A., 1997. Thermogravimetric analysis on global mass loss kinetics of rice hull pyrolysis. Ind. Eng. Chem. Res. 36, 3974–3977. White, R.H., Dietenberger, M.A., 2001. Encyclopedia of Materials: Science and Technology 9712–9716, ISBN: 0-08-0431526.
Further reading Blesa, M.J., Fierro, V., Miranda, J.L., Moliner, R., Palacios, J.M., 2001. Effect of the pyrolysis process on the physico-chemical and mechanical properties of smokeless fuel briquettes. Fuel Process. Technol. 74, 1–17. Boonmee, N., Quintiere, J.G., 2005. Glowing ignition of wood: the onset of surface combustion. Proc. Combust. Inst. 30, 2303–2310. Ghosh, B., 2004. First Handbook of Stakeholders, School of Energy Studies. Jadavpur University, Kolkata. Pindoria, R.V., Chatzakis, I.N., Lim, J.Y., Herod, A.A., Dugwell, D.R., Kandiyoti, R., 1999. Hydropyrolysis of sugar cane bagasse: effect of sample configuration on bio-oil yields and structures from two benchscale reactors. Fuel 78, 55–63. Ravindranath, N.H., Hall, D.O., 1995. Biomass, Energy and Environment—A Developing Country Perspective from India. Oxford Univ Press, Oxford, ISBN 0-19-856436-8. Yaman, S., 2004. Pyrolysis of biomass to produce fuels and chemical feedstocks. Energy Conversion Manage. 45, 651–671.