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��Pyrolysis Kinetics of Raw/Hydrothermally Carbonized Lignocellulosic Biomass Wei Yan,a Schinthia Islam,b Charles J. Coronella,b and Victor R. V�squezb a Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada 89512; wei.yan@dri.edu (for correspondence) b Department of Chemical and Metallurgical Engineering, University of Nevada, Reno, MS0170, Reno, Nevada 89557 Published online 21 March 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.11601 In the process we call hydrothermal carbonization (HTC), Pyrolysis of lignocellulosic biomass was performed in a also known as hydrothermal pretreatment and wet torrefac- thermogravimetric analyzer at temperature ranging 105 to 8008C at the heating rates of 5, 10, and 208C min21. Sam- tion, biomass is treated in hot compressed water, resulting in three products: gases, aqueous chemicals, and solid product. ples of raw loblolly pine and hydrothermally carbonized loblolly pine were used as feedstocks in pyrolysis study. Ther- The pretreatment temperature is in the range of 200 2608C, and the pressures are up to 700 psi. The solid product contains mogravimetric experiments showed that more significant decomposition occurred in the pyrolysis of raw biomass com- about 55 90% of the mass and 80 95% of the fuel value of the original feedstock. The gas product is about 10 20% by mass pared with hydrothermally carbonized biomass, so that the of the feedstock, and aqueous chemicals, primarily sugars and raw biomass displayed a much higher volatile content and furfurals, make up the balance [6, 7]. lower fixed carbon content. Derivative thermogravimetry also Hydrothermal carbonization produces a solid product showed that one major decomposition reaction took place at (biochar) with higher energy density than the starting bio- a specific heating rate for two biomass feedstocks. Assuming mass feedstock. Ultimate analysis proves that the solid prod- that the decomposition obeys first-order kinetics, kinetic uct has a greater carbon/oxygen ratio, making it similar to parameters of biomass pyrolysis were determined using two low-rank coal. In addition, the solid product is very friable methods proposed by Kissinger and Ozawa, respectively. Both and can be easily pelletized into desired forms, which greatly methods gave analogous values of activation energy for raw lower the handing and transportation cost. Equilibrium mois- and hydrothermally carbonized biomass. This study gave ture measurement indicates that the solid product absorbs further confirmation that the hydrothermal carbonization less moisture than raw biomass, which means excellent (HTC) process transforms lignocellulosic biomass into an intermediate feedstock with favorable properties for thermo- storage properties for a long period of time [6, 7]. Research has been conducted in the field of cellulose and chemical applications. � 2012 American Institute of Chemical biomass pyrolysis. For instance, Gronli et al. investigated cel- Engineers Environ Prog, 31: 200 204, 2012 lulose pyrolysis kinetics by thermogravimetry [8]. Teng et al. Keywords: pyrolysis kinetics, hydrothermal carbonization, described thermogravimetric analysis on global mass loss lignocellulosic biomass kinetics of rice hull pyrolysis [9]. Kim et al. reported pyrolysis kinetics and decomposition characteristics of pine trees [10]. However, there is no open literature of the pyrolysis of hydro- INTRODUCTION thermally carbonized biomass. Therefore, this study investi- In 2008 2009, 60% of oil consumed in the US was gated thermochemical performance of raw and hydrothermally imported from other countries [1]. The limited reserve of fos- carbonized biomass in pyrolysis. The thermogravimetric curves sil fuel and increasing concerns about climate change force (mass loss curves) were observed and the derivative thermog- us to focus on sustainable sources for green and renewable ravimetry (DTG) were obtained for three heating rates. Kinetic fuel. In the US, the Energy Independence and Security Act parameters of the volatile evolution were also determined (EISA) of 2007 clearly requires that at least 36 billion gallons using Kissinger s method and Ozawa s method. of renewable fuel must be produced and consumed in the U.S. by 2022 [2 4]. First-generation biofuels (ethanol pro- MATERIALS AND METHODS duced from starch and biodiesel from oil/fat) are not a viable option for achieving this goal based upon the availability and Biomass cost of feedstock. Because of the abundant supply of ligno- Loblolly pine (Alabama, USA) was used as the feedstock. cellulosic biomass in the US, research and development has On a mass basis, it consisted of 11.9% hemicelluloses, 54.0% been widely conducted to convert lignocellulosic biomass to cellulose, 25.0% lignin, 8.7% water-extractives, and 0.4% bio-based fuel, chemicals, and energy. In spite of low cost of ash [6]. Prior to the study, each pine sample was milled to lignocellulosic biomass, feedstock handling and transporta- the size of 0.5 1.0 mm and dried at 1058C for 24 h. tion are much more cost-intensive due to its diversity, widely spread distribution, and seasonal availability [5]. To deal with these difficulties, there is a need for a process to homogenize Hydrothermal Carbonization lignocellulosic biomass, while simultaneously producing a Hydrothermal carbonization of loblolly pine was per- stable, energy-dense, solid fuel. formed in a Parr Series 4560 bench-top reactor (100 mL) (Moline, IL). The temperature of the reactor was controlled � 2012 American Institute of Chemical Engineers at 2608C using a proportional integral derivative (PID) con- 200 July 2012 Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep Figure 2. Mass loss curves for hydrothermally carbonized Figure 1. Mass loss curves for loblolly pine at various heat- loblolly pine at various heating rates. ing rates. troller. The reactor pressure was not controlled and was carried out at the heating rates of 5, 10, 208C min21. approximately in accord with the water vapor pressure. For Raw loblolly pine started to decompose at 3008C and each run, a mixture of loblolly pine and water in a mass ratio significant decomposition continued until the temperature of 1:5 was loaded to the reaction vessel. The mixture was reached 4258C, where mass yield dropped to 30%. After that, stirred manually to ensure complete wetting. Nitrogen was pyrolysis progressed at slower rates as the temperature passed through the reactor for 10 min to purge oxygen from increased to 8008C, where the final yield reached 16% the reactor. When the reactor was heated to the desired (see Figure 1). Hydrothermally carbonized loblolly pine per- temperature, hydrothermal carbonization experiment was formed differently compared with raw pine in TGA experi- initiated. After 5 min, the reactor was rapidly cooled off by ments. Significant decomposition occurred in a narrower immersion in an ice-water bath. The gas product was temperature range (300 4008C) and the mass yield dropped released directly and the liquid and solid products were sep- less dramatically, only from 95 to 60%. Similarly, decomposi- arated by vacuum filtration. The solid product was dried at tion continued at slower rates until the ultimate mass yield 1058C for 24 h prior to further investigation. All experiments reached 36%. were performed at least three times. Figures 1 and 2 also indicated that there was nearly no change of mass yield in the temperature range of 700 8008C. The residue left was mostly fixed carbon and a trace amount Thermogravimetry of ash. The fixed carbon yields were 0.36 g/g hydrothermally Dynamic thermogravimetry analysis was performed using carbonized loblolly pine and 0.16 g/g raw loblolly pine. On a Perkin-Elmer TGA7 instrument (Waltham, MA). The sample the basis of the mass yield of hydrothermal carbonization ( 10 mg) was initially loaded on the platinum tray and posi- at 2608C [6, 7], fixed carbon yield increased by 28% per 1 g tioned in the center of the furnace. The measurements were biomass feedstock due to hydrothermal carbonization. On conducted under an inert atmosphere, where the flow rate of the other hand, volatile yields were 84% for raw loblolly nitrogen was set to be constant at 80 cm3 min21. To remove pine and 64% for hydrothermally carbonized loblolly pine. the moisture, the sample was first dried at 1058C for 5 min On the mass basis, hydrothermal carbonization reduced the prior to the heating period. Temperature programs for volatile yield by 58%. These could make hydrothermally dynamic TGA were from 105 to 8008C at a fixed heating rate. carbonized biomass much more favorable in thermochemical In this study, the heating rates of 5, 10, and 208C min21 were application. selected and investigated. The mass and temperature of the Figures 3 and 4 showed the derivative thermogravimetry samples were monitored and recorded using the Pyris soft- (DTG) of evolved volatiles corresponding to mass loss data ware associated with Perkin Elmer TGA 7. All experiments shown in Figures 1 and 2. For both raw and hydrothermally were performed at least three times. carbonized loblolly pine, there were common trends: (1) three peaks for three heating rates, (2) a larger peak at Morphological Analysis higher heating rates, (3) peak apex located in a narrow tem- perature range, (4) a higher peak temperature at higher heat- Scanning electron microscopy (SEM) was used to observe the microstructure and surface morphology of raw and pre- ing rates, (5) larger absolute DTG values (%/min) for raw loblolly pine than that for hydrothermally carbonized loblolly treated biomass. This analysis was carried out using a Hitachi pine. These thermogravimetric curves of pyrolysis of raw S-4300SE Field-Emission Scanning Electron Microscope loblolly pine were found comparable to similar pyrolysis (Toronto, Canada). Prior to the SEM analysis, the samples were dried at 1058C for 24 h, and coated with gold to pro- study of other pine trees reported in the literature [10]. vide about 200 � gold layer thickness using a vacuum sputter coater. Kinetic Models for Biomass Pyrolysis RESULTS AND DISSUSION Kissinger s Method First reported by Kissinger in 1957, this method has been Biomass Pyrolysis widely employed for determining the values of the kinetic Figures 1 and 2 show the mass loss curves of raw and parameters of pyrolysis [9, 11 14]. The evolution of volatiles hydrothermally carbonized loblolly pine at the fixed heating can be represented as a simple reaction with an assumed rates from the TGA experiments. TGA experiments were first-order rate (Eq. 1), with a rate constant, k (Eq. 2). Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep July 2012 201 Figure 3. Evolution of volatile products during pyrolysis of Figure 5. Plot of ln(B/T2) vs. 1/temperature for raw and loblolly pine at various heating rates. hydrothermally carbonized loblolly pine. d2Y dY dk ��k=B� ��Y Y �=B� � 0 2 dT dT dT �5� �at T � Tmax� Taking temperature derivatives on both sides of Eq. 2 gives dk 2 � A�E=RT � exp� E=RT ��6� dT Substitution of Eqs. 1 and 6 into Eq. 5 gives Eq. 7. 2 ln�B=Tmax� � �E=R� 1=Tmax � ln�AR=E��7� According to Eq. 7, the first order kinetic parameters E and A can be determined from the slopes and intercept of a linear plot of ln[B/(Tmax)2] versus 1/Tmax at various heating rates. Figure 5 showed the linear relationships for pyrolysis of raw Figure 4. Evolution of volatile products during pyrolysis of and hydrothermally carbonized loblolly pine, respectively. hydrothermally carbonized loblolly pine at various heating rates. Ozawa s Method The method describes the determination of the kinetic parameters, Arrhenius activation energy, and pre-exponential factor by thermogravimetry, based upon the assumption that the decomposition obeys first order kinetics [15 17]. It is nor- mally applicable to decomposition occurring in the range biomass ! volatiles from 100 to 10008C. Log(heating rate) were plotted against �1� dY 1/temperature for each specific conversion. This method is � k�Y Y � dt not applicable if the curve is nonlinear. For each of the thermogravimetric curves in Figure 1 and 2, k � A exp� E=RT ��2� the absolute temperature at the constant conversion was deter- where Y is the mass yield [%] of evolved volatiles up to time mined by plotting the logarithm of the heating rate versus the t [min], Y* is the ultimate mass yield of volatiles, t is time, T is reciprocal of the absolute temperature at which the conversion the absolute temperature, R is the gas constant [kJ (mol K)21], level was reached (see Figure 6). The activation energy and A is the pre-exponential factor (min21), and E is the activation pre-exponential factor were calculated in accordance with the energy (kJ mol21). If pyrolysis is performed at a fixed heating methods described in the original literature [15]. rate (Eq. 3), Eq. 1 can be expressed as Eq. 4. dT Kinetic Parameters of Biomass Pyrolysis � B �3� dt With the operative equation (Eq. 7) of Kissinger s method, activation energy and pre-exponential factor were calculated dY from the slope and intercept of the fitting linear line (Figure 5). � k�Y Y �=B �4� dT Table 1 showed that the activation energies were 149.3 and where B is the heating rate (8C min21). B is equal to 5, 10, and 203.6 kJ mol21 for raw and hydrothermally carbonized loblolly 208C min21in this study. The ultimate mass yield of volatiles pine, respectively. Using Ozawa s method, the activation was considered to be the mass yield of volatiles at 8008C since energy and pre-exponential factor were obtained from the mass yields of sample were stable at 700 8008C. It is clear that slope and intercept of the fitted linear line (Figure 6). Table 2 there is one major peak for each heating rate in Figure 3 and 4. summarized the activation energy and pre-exponential factors At peak temperature, the derivative of DTG should be equal to at specific mass yields for raw and hydrothermally carbonized zero, which is shown in Eq. 5. loblolly pine, respectively. 202 July 2012 Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep Figure 6. Plot of log (B) vs. 1/temperature at various conversion rates for raw and hydrothermally carbonized loblolly pine. Table 1. Activation energy and Arrhenius constants obtained using Kissinger s method. Biomass E (kJ mol21) A (min21) Raw loblolly 149.3 3.146 3 1011 HTC loblolly 203.6 2.039 3 1016 Table 2. Activation energy and Arrhenius constants using Ozawa s method at various mass yields. Biomass Mass yield E (kJ mol21) A (min21) Figure 7. SEM images of (a) raw loblolly pine, Raw loblolly 0.8 113.1 1.597 3 107 (b) hydrothermally carbonized loblolly pine. 0.6 165.2 6.165 3 1012 0.4 165.4 1.506 3 1013 HTC loblolly 0.8 181.8 3.127 3 1014 0.6 205.1 4.845 3 1015 This means that the evolution of volatiles occurs more easily 0.4 245.9 2.469 3 1016 during the pyrolysis of raw biomass, which may be inter- preted by the fact that the substances causing volatiles were partially converted into aqueous chemicals in hydrothermal carbonization. The SEM images (see Figure 7) also indicated Kinetic parameters were determined for the specific the fiber texture of raw loblolly pine was much smoother temperature range instead of the whole temperature range. than that of hydrothermally carbonized loblolly pine, show- For example, Kissinger s method uses the peak temperature ing substances (like hemicellulose, cellulose) reacted in the for each heating rate. Ozawa s method uses the temperature process of hydrothermal carbonization [6, 7]. corresponding to each conversion (or mass yield) for each heating rate. In this study, the temperature ranges of Kissing- er s method were 375 4008C and 360 3808C for raw and CONCLUSION hydrothermally carbonized loblolly pine, respectively. The Kinetic measurement and modeling showed different char- temperature range investigated using Ozawa s method was acteristics of hydrothermally carbonized lignocellulosic bio- very close to the temperature range using Kissinger s method mass. Within the temperature range of 360 4258C, pyrolysis of where mass yield was 0.6. Using both approaches, the hydrothermally carbonized biomass progressed less aggres- activation energies were very analogous (149.3 kJ mol21 vs. sively than that of raw biomass. After this initial significant 165.0 kJ mol21 for raw loblolly pine, 203.6 kJ mol21 vs. decomposition, the pyrolysis reactions continued at much 205.0 kJ mol21 for hydrothermally carbonized loblolly pine). slower rates with temperature increasing to 8008C. The result- The activation energy of pyrolysis of loblolly pine were ing derivative thermogravimetry also verified that raw biomass comparable with the reported values for the pyrolysis was more chemically reactive than hydrothermally carbonized of rice hull, pine tree, sugar cane, bagasse, and wheat straw biomass in the conditions investigated. Kinetic parameters of [9, 10, 16 18]. For instance, Kim et al. reported that two biomass pyrolysis were determined using two methods pro- simultaneous first order reactions were proposed in pyrolysis posed by Kissinger or Ozawa, respectively. Both methods of pine and the apparent activation energy increased from were applicable in this study, based upon the assumption that 145 to 302 kJ mol21 with increasing conversion [10]. the decomposition obeys first-order kinetics. The values for Activation energy of volatile evolution from raw biomass activation energy were quite comparable using the two meth- is lower than that from hydrothermally carbonized biomass. ods, and both methods showed the activation energy of vola- Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep July 2012 203 tile evolution of raw biomass was lower than that of hydro- torrefaction of lignocellulosic biomass, Energy Fuels, 24, thermally carbonized biomass. Kinetic results further con- 4738 4742. firmed that hydrothermal carbonization could turn lignocellu- 8. Gronli M., Antal M.J., & Varhegyi G. (1999). A round-robin losic biomass into a solid fuel, which may be a favorable feed- study of cellulose pyrolysis kinetics by thermogravimetry, stock for thermochemical application. Industrial Engineering and Chemistry Research, 38, 2238 2244. 9. Teng, H.S., Lin, H.C., & Ho, J.A. (1997). Thermogravimat- ACKNOWLEDGMENTS ric analysis on global mass loss kinetics of rice hull pyroly- This work was supported by the US Department of Energy, Award DE-FG36-01GO11082. The authors acknowledge mean- sis, Industrial Engineering and Chemistry Research, 36, 3974 3977. ingful discussions with Larry Felix from the Gas Technology 10. Kim S.S., Kim J., Park Y.H., & Park Y.K. (2010). Pyrolysis Institute [GTI] and Kent Hoekman from the Desert Research kinetics and decomposition characteristics of pine trees, Institute [DRI]. The authors also acknowledge the assistance of Bioresource Technology, 101, 9797 9802. Mojtaba Ahmadian-Tehrani from University of Nevada, Reno 11. Kissinger, H.E. (1957). Reaction kinetics in differential [UNR] in scanning electron microscopy. thermal analysis, Analytical Chemistry, 29, 1702 1706. 12. Reich, L. (1964). A rapid estimation of activation energy LITERATURE CITED from thermogravitric traces, Polymer Letter, 2, 621. 1. International Energy Statistics. (2008). EIA, US Energy 13. Sestak, J., Satava, V., & Wendlandt, W.W. (1973). The study Information Administration, US, Department of Energy. of heterogeneous processes by thermal analysis, Thermo- 2. Biomass Multiyear Program. (2008). Office of the Bio- chimica Acta, 7, 333 334. mass Program, Energy Efficiency and Renewable Energy, 14. Ozawa, T.J. (1970). Kinetic analysis of derivative curves U.S. Department of Energy, Washington, D.C. in thermal analysis, Journal of Thermal Analysis, 23, 3. Huber, G.W., & Dale B.E. (2009). Grassoline at the 3271 324. pump, Scientific American, July 52 59. 15. ASTM Standard Test Method for Decomposition Kinetics 4. Biofuels create green jobs: Growing transportation fuels by Thermogravimetry. E1641 07. and the nation s economy. (2008). Office of the Biomass 16. Antal. M.J., & Varhegyi, G. (1995). Cellulose pyrolysis Program, Energy Efficiency and Renewable Energy, U.S. kinetics: the current state of knowledge, Industrial Engi- Department of Energy, Washington, D.C. neering and Chemistry Research, 34, 703 717. 5. Tester, J.W. (2005). Sustainable energy, Cambridge, Mas- 17. Varhegyi, G, Antal, M.J., Szekely, T., & Szabo, P. (1989). sachusetts: MIT Press. Thermal degradation of hemicellulose, cellulose and 6. Yan, W., Acharjee, T.C., Coronella, C.J., & Vasquez, V.R. sugar cane bagasse, Energy Fuels, 3, 329 335. (2009). Thermal pretreatment of lignocellulosic biomass, 18. Ouajai, S., & Shanks, R.A. (2005). Composition, structure, Environmental Progress and Sustainable Energy, 28, and thermal degradation of hemp cellulose after chemi- 435 440. cal treatments, Polymer Degradation and Stability, 89, 7. Yan, W., Hastings, J.T., Acharjee, T.C., Coronella, C.J., & 327 335. Vasquez, V.R. (2010). Mass and energy balance of wet 204 July 2012 Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep

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