MURDOCH RESEARCH REPOSITORY.

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1 MURDOCH RESEARCH REPOSITORY This is the author's inal version o the work, as accepted or publication ollowing peer review but without the publisher's layout or pagination. Yin, C-Y and Goh, B-M (2011) Thermal degradation o rice husks in air and nitrogen: Thermogravimetric and kinetic analyses. Energy Sources, Part A: Recovery, Utilization, and Environmental Eects, 34 (3). pp Copyright Taylor & Francis Group, LLC. It is posted here or your personal use. No urther distribution is permitted.

2 Thermal degradation o rice husks in air and nitrogen: thermogravimetric and kinetic analyses Chun-Yang Yin a,*, Bee-Min Goh b a School o Chemical and Mathematical Sciences, Murdoch University, Murdoch, 6150 Western Australia b Faculty o Applied Sciences, Universiti Teknologi MARA Perlis, 02600, Arau, Perlis, Malaysia Corresponding author: Chun-Yang Yin Address: School o Chemical and Mathematical Sciences, Murdoch University, Murdoch, 6150 Western Australia c.yin@murdoch.edu.au; yinyang@streamyx.com Abstract In this study, thermogravimetric and kinetic data o thermal degradation o Malaysian rice husks under air or nitrogen atmospheres are analyzed. Determination o elemental composition, ash content and gross caloriic value o MRH and their comparison with other biomass uel are also conducted. It is ound that MRH has a gross caloriic value o approximately 15 MJ/kg. Thermogravimetric analysis shows that signiicant thermal degradation occurs within temperature ranges o C (nitrogen) and C (air) which is primarily attributed to decomposition o hemicellulose and cellulose. Linear regression analysis using LINEST unction in Microsot Excel indicates that the nitrogen atmosphere data (order o reaction = 0.44) it the Arrhenius model better than the air data (order o reaction = 0.22). The data obtained rom our study can be used or preliminary evaluation o rice husks either as a combustible biomass or source or a small-scale thermochemical conversion system. Keywords rice husks, biomass uel, elemental analysis, thermogravimetric analysis, LINEST unction Running title (head) Thermal degradation rice husks in air and nitrogen. 1

3 Introduction Rice husks are the discarded external layers o rice grains which constitute the largest milling by-products o paddy, constituting more than 10 % o paddy by weight. It is estimated that 3.6 million tonnes o rice husks are produced annually in Malaysia (Lee et al., 2005) rendering complications or rice mill operators to dispose o the husks due to their abundance. However, since rice husks are essentially biomass which contains carbon source, one can surmise its application as an auxiliary uel. Rice husks are generally regarded as one o the major biomass uels used to generate power along with wood and oil palm residues (Mohamad-Yuso et al., 2008). The usage o Malaysian rice husks (MRH) as a combustion source or biomass or thermochemical conversion application represents a two-pronged approach in solving its disposal dilemma as well as providing an inexpensive renewable energy uel. Many pilot-scale initiatives on usage o MRH as a eedstock or energy recovery have taken place in Malaysia such as the 1.5 MWe Titi Serong power plant located in the northern state o Perak. The plant uses MRH as biomass uel rom the rice milling process. The plant is designed to generate 12 tonnes o superheated steam at 25 bar (g) at 300 C which are subsequently supplied to the 1.5 MW extraction turbine or electricity generation. As such, it can be said that MRH represents an important biomass eedstock in small-scale energy generation in Malaysia (Mokhtar, 2006). The main objectives o the study are to analyze and compare the thermogravimetric and kinetic data o thermal degradation o MRH under air or nitrogen atmosphere so that these data can aid in the uture design o MRH combustors or thermochemical converters. Previous related studies on analyses o thermogravimetric and kinetic data o thermal degradation o rice husks include research conducted by Mansaray and Ghaly (1998, 1999a, 1999b), Markovska and Lyubchev (2007) and Genieva et al. (2008) but to the best o the authors knowledge, there is no identiied study on direct comparison o kinetic data in air and nitrogen atmosphere as applied to the Malaysian context. Experimental Malaysian rice husks (MRH) were obtained rom a rice mill located in Tanjung Karang, Selangor, Malaysia. Elemental analysis o the RH was perormed using Flash EA 1112 ThermoFinnigan elemental analyzer. The RH were cut, ground and weighed on tin oil beore inserted into the instrument. The system was purged with helium gas at 140 ml/min prior to lash combustion process. Results were indicated as percentages o carbon, hydrogen and nitrogen while percentage o other elements was calculated by dierence. Thermogravimetric (TG) analyses (under air or nitrogen atmosphere) were conducted via Mettler-Toledo TGA/SDTA851 Thermogravimetric Analyzer. The instrument was set to increase temperature at a rate o 10 C/min (temperature range: 25 to 1000 C) under either pure nitrogen or air atmosphere with a low rate o 50 ml/min. The TG and derivative TG (DTG) curves were recorded concurrently with temperature increase. The gross caloriic value was determined using an adiabatic calorimeter bomb (IKA-WERKE C5003). All analyses were repeated three times to provide an average reading. 2

4 Results and Discussion Composition and Caloriic Value In general, MRH has a typical chemical composition that corresponds to the ollowing: cellulose (40-45%), lignin (25-30%), ash (15-20%) and moisture (8-15%) (Haji-Ali et al., 1992). To provide a more detailed investigation, elemental analysis (ultimate analysis) is thus conducted in this study. Table 1 shows the elemental composition, ash content and gross caloriic value o MRH and comparison with other biomass uel. The experimental values obtained or MRH in this study are consistent with those reported or other biomass uel. The experimental values o MRH (obtained rom the Northern state o Penang, Malaysia) reported by Mohamad-Yuso et al. (2008) seem to validate our indings, though MRH in our study appears to contain less silica judging by our lower percentage ash content. Other biomass uels contain appreciably lesser percentage ash content compared to MRH. The high silica content o MRH is due to the presence o opaline silica (plant-based hydrated amorphous orm o silica) ound predominantly in the outer epidermis o rice husks (Krishnarao and Godkhindi, 1992; Ryu et al., 1997). The gross caloriic value represents the experimental value o the higher heating value (HHV) concept whereby the latent heat o vaporization o water in the combustion products is considered. The HHV includes the heating value o the condensed steam given o and HHV at constant pressure measures the enthalpy change o combustion with water condensed (Demirbas and Demirbas, 2009). In our study, the gross caloriic value o MRH appears to be noteworthy considering its high ash content, albeit other biomass uels have higher values by approximately 20%. This implies that MRH may be suitable or small-scale energy recovery initiatives. Table 1 Elemental composition, ash content and gross caloriic value o MRH and comparison with other biomass uel MRH this study MRH Penang a Wheat straw b Almond shell c Sunlower shell c Elemental analysis Carbon (%) Hydrogen (%) Nitrogen (%) Sulur (%) Not detected Ash content (%) Gross caloriic value (MJ/kg) a From Mohamad-Yuso et al. (2008). b From Arvelakis and Koukios (2002). c From Demirbas (2002). 3

5 Thermogravimetric analysis Figure 1 shows the TG and DTG curves o MRH in air and nitrogen lows. For both lows, the MRH experiences a noticeable weight loss by approximately 5 wt % rom 25 to 130 C due to vaporization o physically adsorbed water. At 230 C, drastic MRH weight reduction begins to occur that ends at around 380 C (nitrogen) and 540 C (air). This constitutes wt % reduction o about 47 and 76 % or nitrogen and air, respectively. The thermal decomposition o rice husks can be described on the basis o the decomposition behaviors o its major constituents: cellulose, hemicellulose, lignin and ash (Mansaray and Ghaly, 1999a; Genieva et al., 2008). The irst three compounds are chemically active and decompose thermochemically in the temperature range o C (hemicellulose decomposes predominantly between 150 and 350 C, cellulose decomposes between 275 and 350 C and lignin undergoes gradual decomposition between 250 and 500 C) (Antal, 1983; Mansaray and Ghaly, 1998). In the case o nitrogen low, a signiicant degradation process takes place rom 230 to 380 C where there is generation o volatile compounds due to decomposition o hemicellulose and cellulose. From 380 to 640 C, it urther experiences gradual and marginal weight loss by approximately 9 wt %. This gradual weight loss is also reported by Mansaray and Ghaly (1998) in which they attribute this to lignin conversion to char. In the case o oxygen low, where CO 2 is likely produced, the degradation trend is dierent and rather unique since it constitutes a two-stage combustion process (230 to 390 C; 390 to 540 C). Mansaray and Ghaly (1998) coin this two-stage process as two separate reaction zones where the irst reaction zone is attributed to rapid evolution o volatile products while in the second zone, lignin, which has lower decomposition rates than cellulose and hemicellulose components o rice husk, is gradually reduced to char. 4

6 Air Nitrogen Weight (%) (a) Temperature ( o C) mg/min Air Nitrogen (b) Temperature ( o C) Figure 1. (a) TG and DTG (b) curves o MRH in air and nitrogen lows Kinetic Analysis Kinetic analysis or biomass degradation under air or nitrogen atmosphere is important to quantiy the relevant parameters necessary or design o combustor or energy recovery purposes as well as to aord a more in-depth explanation o the degradation process. Kinetic reaction parameters are determined using the established Arrhenius equation (Mansaray and Ghaly, 1999a; Yagmur and Durusoy, 2009; Jeguirim and Trouve, 2009): k = Ae E / RT (1) 5

7 where k = rate constant, A = requency actor, R = universal gas constant, T = absolute temperature, and E = activation energy o the reaction. Global kinetics o the revitalization reaction is expressed as (Jeguirim and Trouve, 2009): 1 w w o dw w w = k dt wo w n (2) where w o = initial mass at the start o thermal degradation, w = inal mass at the end o dw thermal degradation, w = mass at time t, = ratio o change in mass with respect to dt time, n = order o reaction. By integrating equations (1) and (2), the ollowing expression is obtained: Equation (3) can be written in the ollowing orm: where Ln 1 dw = Ln( A wo wt dt ) E RT y = B + Cx + Dz + w w nln wo w (3) (4) y = Ln w o 1 w dw ; dt 1 x = ; T w w z = Ln ; B = Ln(A) ; wo w E C = ; D = n (5) R Constants B, C and D are determined using linear regression analysis or temperature ranges rom 230 to 540 C (air) and 230 to 380 C (nitrogen). These temperature ranges are chosen because it is previously explained that signiicant weight loss (signiicant thermal degradation) occurs within these ranges. By using the LINEST unction in Microsot Excel, B, C and D are quantiied and subsequently, A, E and n values are determined. This LINEST unction essentially provides statistical inormation that describe a linear trend matching known data points, by itting a straight line using the least squares method. For this method, we speciy the sets o dependent variable (y) and independent variables (x and z) rom our thermogravimetric data. The third and ourth arguments in the LINEST unction are both set to be TRUE so that the constant B is not equal to nil and the statistical parameters used to evaluate the goodness o it o our data to the Arrhenius model can be shown. To the best o our knowledge, the ease o usage o the LINEST unction has not been ully exploited by previous researchers in determining thermal degradation kinetic parameters. Table 2 shows the determined kinetic and statistical parameters as a result o the regression analysis. Our analysis evidently indicates that the nitrogen atmosphere data it the Arrhenius model ar better than the air data since the ormer has correlation o determination, R 2 value which is almost at unity and lower σ res value. This indicates that or nitrogen, predicted data points are close to experimental data points while the 6

8 opposite is true or air. It is noteworthy to point out that the determined n value or air is consistent with previous reported n values or combustion o rice husks (Mansaray and Ghaly, 1999a) which are lower than 1. According to IUPAC, the requency actor, A expresses the empirical temperature dependence o the rate coeicient on temperature. The high A value or nitrogen data (at the order o 10 6 ) has also been previously reported or air (21 % oxygen and 79 % nitrogen) degradation o Lemont LG and ROK 14 rice husks varieties (Mansaray and Ghaly, 1999b). Table 2 Kinetic and statistical parameters Atmosphere Temperature range ( C) A (min -1 ) E (kj/mol) n R 2 Standard deviation o the residuals, σ res Air Nitrogen Conclusions The thermogravimetric and kinetic data o thermal degradation o MRH under air or nitrogen atmospheres have been analyzed. Our characterization results seem to indicate that MRH may be suitable or small-scale energy recovery initiatives judging by its gross caloriic value o approximately 15 MJ/kg. Thermogravimetric analysis shows that signiicant thermal degradation occurs within temperature ranges o C (nitrogen) and C (air) which is predominantly due to decomposition o hemicellulose and cellulose. Linear regression analysis using LINEST unction indicates that the nitrogen atmosphere data it the Arrhenius model ar better than the air data with orders o reaction less than 0.5 or both sets o data. The data obtained rom our study are important or preliminary evaluation o rice husks either as a combustible biomass or source or a small-scale thermochemical conversion system. Reerences Antal, M. J Biomass pyrolysis: a review o the literature. Part 1 - Carbohydrate pyrolysis. Advances in Solar Energy 11: Arvelakis, S. and Koukios, E. G Physicochemical upgrading o agroresidues as eedstocks or energy production via thermochemical conversion methods. Biomass and Bioenergy 22: Demirbas, A Fuel characteristics o olive husk and walnut, hazelnut, sunlower, and almond shells. Energy Sources 24: Demirbas, T., and Demirbas, C Fuel properties o wood species. Energy Sources, A 31: , Genieva, S. D., Turmanova, S. C., Dimitrova, A. S., and Vlaev, L. T Characterization o rice husks and the products o its thermal degradation in air or nitrogen atmosphere. Journal o Thermal Analysis and Calorimetry 93:

9 Haji-Ali, F., Adnan, A., and Chew, K. C Geotechnical properties o a chemically stabilized soil rom Malaysia with rice husk ash as an additive. Geotechnical and Geological Engineering 10: Jeguirim, M., and Trouve, G Pyrolysis characteristics and kinetics o Arundo donax using thermogravimetric analysis. Bioresource Technology 100: Krishnarao, R. V., and Godkhindi, M. M Distribution o silica in rice husks and its eect on the ormation o silicon carbide. Ceramics International 18: Lee, K. T., Mohtar, A. M., Zainudin, N. F., Bhatia, S., and Mohamed, A. R Optimum conditions or preparation o lue gas desulurization absorbent rom rice husk ash. Fuel 84: Mansaray, K. G., and Ghaly, A. E Thermal degradation o rice husks in nitrogen atmosphere. Bioresource Technology 65: Mansaray, K. G., and Ghaly, A. E. 1999a. Determination o kinetic parameters o rice husks in oxygen using thermogravimetric analysis. Biomass & Bioenergy 17: Mansaray, K. G., and Ghaly, A. E. 1999b. Determination o reaction kinetics o rice husks in air using thermogravimetric analysis. Energy Sources 21: Markovska, I. G., and Lyubchev, L. A A study on the thermal destruction o rice husk in air and nitrogen atmosphere. Journal o Thermal Analysis and Calorimetry 89: Mohamad-Yuso, I., Farid, N. A., Zainal, Z. A., and Azman, M Characterization o rice husk or cyclone gasiier. Journal o Applied Sciences 8: Mokhtar, H Biomass power plant ull scale demonstration project. Standard & Quality News SIRIM Malaysia 13: Ryu, S. -E., Kim, T. -N. and Kang, T. -K Pulverization o rice husks and the changes o husk densities. Journal o Materials Science 32: Yagmur, S., and Durusoy, T Oil shale combustion kinetics rom single thermogravimetric curve. Energy Sources, A 31: