National Cheng Kung University From the SelectedWorks of Wei-Hsin Chen Fall October, 2014 Thermal decomposition dynamics and severity of microalgae residues in torrefaction. Wei-Hsin Chen, National Cheng Kung University Available at: http://works.bepress.com/wei-hsin_chen/110/ Bioresource Technology 169 (2014) 258–264 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech Thermal decomposition dynamics and severity of microalgae residues in torrefaction Wei-Hsin Chen a,⇑, Ming-Yueh Huang a, Jo-Shu Chang b,c,d, Chun-Yen Chen c a Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan, ROC Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC c Center for Bioscience and Biotechnology, National Cheng Kung University, Tainan 701, Taiwan, ROC d Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan, ROC b h i g h l i g h t s g r a p h i c a l a b s t r a c t Thermal decomposition and severity a r t i c l e i n f o Article history: Received 9 May 2014 Received in revised form 21 June 2014 Accepted 24 June 2014 Available online 1 July 2014 Keywords: Torrefaction and pyrolysis Microalgae residues Torrefaction kinetics Thermogravimetric analysis Torrefaction severity index (TSI) Torrefaction severity index Torrefaction C. sp. JSC4 residue in N2 200-300 °C 0.8 High curvature Intermediate curvature 0.6 0.4 C. sorokiniana CY1 residue 0-60 min Curvature = 0 Low curvature 0.2 0 60 40 20 Torrefactio n time (min) 0 200 225 Tempe 250 rature 275 300 0 ( C) a b s t r a c t To figure out the torrefaction characteristics and weight loss dynamics of microalgae residues, the thermogravimetric analyses of two microalgae (Chlamydomonas sp. JSC4 and Chlorella sorokiniana CY1) residues are carried out. A parameter of torrefaction severity index (TSI) in the range of 0–1, in terms of weight loss ratio between a certain operation and a reference operation, is defined to indicate the degree of biomass thermal degradation due to torrefaction. The TSI profiles of the two residues are similar to each other; therefore, the parameter may be used to describe the torrefaction extents of various biomass materials. The curvature of TSI profile along light torrefaction is slight, elucidating its slight impact on biomass thermal degradation. The sharp curvature along severe torrefaction in the initial pretreatment period reveals that biomass upgraded with high temperature and short duration is more effective than using low temperature with long duration. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Microalgae have been considered as potential sources for producing biofuels; this arises from the fact that they are characterized by rapid growth, high carbon fixing efficiency, and avoiding ⇑ Corresponding author. Tel.: +886 6 2004456; fax: +886 6 2389940. E-mail address: weihsinchen@gmail.com (W.-H. Chen). http://dx.doi.org/10.1016/j.biortech.2014.06.086 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved. 1 Torrefaction severity index of microalgae residues in torrefaction are studied. Microalgae residues are thermally degraded in a thermogravimetric analyzer. Torrefaction severity index (TSI) is defined to indicate torrefaction degree. The curvature of TSI profile reveals the sensitivity of biomass thermal degradation. Biomass upgraded with high temperature and short duration is more effective. the problems of food shortage (Sayre, 2010; Chen et al., 2012), thereby achieving carbon capture and storage when they grow and are harvested. On account of high lipid content in some microalgae, they can be employed for biodiesel production (Li et al., 2008; Chisti, 2007). Microalgae can also be fermented to produce bioethanol (Harun et al., 2010) and biobutanol (Jang et al., 2012). For these reasons, microalgae are promising and crucial feedstocks for third generation biofuels. 259 W.-H. Chen et al. / Bioresource Technology 169 (2014) 258–264 Nomenclature A Ea k n R R2 T t X pre-exponential factor (min1) activation energy (kJ mol1) reaction rate constant (min1) order of reaction () universal gas constant (=8.314 J mol1 K1) coefficient of determination temperature (K) time (min) conversion of sample () After microalgae undergo lipid extraction, the residues are generally discarded or burned. The microalgae residues following oil extraction, in fact, still contain carbohydrates, proteins, and some lipids (Xu et al., 2011). For this reason, some studies, with interest in extending the utilization of microalgae residues, have been performed. For example, methane was recovered from the anaerobic digestion of microalgae residues to improve the renewability of biodiesel conversion process (Ehimen et al., 2011). Biogas was produced from the co-digestion of microalgae residues and carbon rich substrates, with emphasis on the higher methane yield of the co-digestion of the residues from amino extraction than from lipid extraction (Ramos-Suárez and Carreras, 2014). A carbon-based solid acid catalyst was synthesized from microalgae residue by in situ hydrothermal partially carbonization with H2SO4 (Fu et al., 2013); it was found that the catalyst had high catalytic activity and a well activity remained after the five-cycle regeneration. Low-molecular-weight maltodextrin was recovered in carbohydrate hydrolysis with acid, for the purpose of gaining valuable carbohydrates remained in microalgae residues (Lam et al., 2014). These evidences clearly suggest that the reuse of microalgae residues is of great potential for commercial applications. Pyrolysis, where biomass is thermally decomposed in an inert or oxygen-free environment at temperatures ranging from 350 to 750 °C (Elliott, 2007), is a noticeable route to obtain biofuels form microalgae and their residues (Miao et al., 2004). Unlike pyrolysis for bio-oil production, torrefaction is a thermal process mainly for solid fuel production. In torrefaction, biomass is also thermally degradated in inert or oxygen-free environments but the temperature is between 200 and 300 °C (Du et al., 2014). In recent years, biomass upgrade by torrefaction has received a great deal of attention, as a consequence of a number of advantages in improving the quality of solid biomass fuels (Rousset et al., 2011; Chen et al., 2012a; Lu et al., 2012, 2013). In reviewing literature relating microalgae and torrefaction, however, very few papers have been published. The study of Wu et al. (2012) revealed that the calorific values of a torrefied microalga and its residue, pretreated at 300 °C for 30 min, were increased by the factors of 26.70% and 17.16% when compared to the untreated counterparts. Chen et al. (2014) explored the isothermal and non-isothermal torrefaction characteristics and kinetics of a microalga, and pointed out that, under the same average temperature and torrefaction duration, non-isothermal torrefaction gave more severe pretreatment than the isothermal one. Mass production of biofuels from microalgae is a promising route for the applications of prospective alternative fuels, implying that a large amount of microalgae residues is likely to be produced. Despite numerous papers concerning biomass torrefaction reported, the understanding of the torrefaction characteristics of microalgae residues, especially in weight loss dynamics, remains insufficient. This study aims to figure out the thermal degradation dynamics of two microalgae residues in torrefaction, and a parameter of torrefaction severity index (TSI) will be introduced W DWI weight of sample (mg) weight loss increment (wt%) Subscript 0 i f ref beginning of torrefaction initial state (at 105 °C) final state (at 800 °C) reference operation (at 300 °C and 1 h) to evaluate the impact of torrefaction temperature and duration on biomass weight loss. The obtained results are able to provide a useful insight into the further utilization of third generation biofuels. 2. Methods 2.1. Material preparation and basic analysis In the present study, two microalgae residues served as the samples for torrefaction experiments; they were Chlamydomonas sp. JSC4 (C. sp. JSC4) and Chlorella sorokiniana CY1 (C. sorokiniana CY1) residues. The two biomass species were isolated from freshwater in southern Taiwan and pertained to oil-rich indigenous microalgae (Ho et al., 2010; Chen et al., 2013). The lipids in the two microalgae were extracted through the direct transesterification method (Su et al., 2007). After the oil-extraction of the two microalgae, the residual solids were dehydrated in an oven at 105 °C for 24 h to minimize the interference of surface water. Thereafter, the samples were ground into powders by a shedder followed by sieving using a screen to particle sizes less than 40 mesh (i.e., <0.42 mm). The powders were collected in sealed plastic bags and stored in a desiccator at room temperature until thermogravimetric analyses were carried out. The basic properties of the two microalgae residues, such as composition, proximate, elemental, and calorific (IKA C2000 Basic) analyses, are tabulated in Table 1. In the composition analysis, Table 1 A list of basic properties of two microalgae residues. Biomass C. sp. JSC4 C. sorokiniana CY1 Composition analysis (wt%) Crude protein Crude lipid Carbohydrate Others 12.18 6.85 35.70 45.27 18.81 9.90 35.67 35.62 Proximate analysis (wt%) Volatile matter (VM) Fixed carbon (FC) Moisture Ash 75.50 15.60 3.50 5.20 73.20 15.10 3.80 7.90 Photograph Elemental analysis (wt%, dry-ash-free) C 40.32 H 7.38 N 2.61 O (by difference) 44.50 Chemical formula CH2.2O0.83N0.06 HHV (MJ kg1, dry basis) 17.41 45.07 7.64 3.88 35.52 CH2.03O0.59N0.07 20.40 260 W.-H. Chen et al. / Bioresource Technology 169 (2014) 258–264 crude protein, crude lipid, and carbohydrate were determined by Kjeldehl method, Soxhelt-extract method, and phenol–sulfuric acid method, respectively (Peng et al., 2001a). The proximate analysis was performed in accordance with the standard procedure of American Society for Testing and Materials, as described elsewhere (Chen et al., 2011). The elemental analysis was carried out using an elemental analyzer (PerkinElmer 2400 Series II CHNS/O Elemental Analyzer) to measure the weight percentages of C, H, and N in biomass, whereas the weight percentage of O were obtained by difference (i.e., O = 100–C–H–N). The calorific values of the samples were detected by a bomb calorimeter (IKA C5000). where X0 is the conversion of the sample at the onset of torrefaction (t = t0). At the condition of n = 1, Eq. (3) reveals that the plot of ln (1 x)1 versus torrefaction time (t t0) gives a straight line with the slope equal to the rate constant k. If the order of reaction is not unity (i.e. n – 1), the plot of ð1 XÞ1n versus torrefaction time gives a straight line with the slope equal to (n 1)k. For a given material at a given torrefaction temperature, a set of reaction rate constant and torrefaction temperature can be obtained. The relationship between the reaction rate constant and reaction temperature is normally assumed to obey Arrhenius law, that is 2.2. Thermogravimetric analysis Ea k ¼ A exp RT The thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) analysis of microalgae residues were carried out by means of a thermogravimetric analyzer (TG, PerkinElmer Diamond TG/DTA). In TGA, around 5 mg of the sample was used for each experimental run, and the flow rate of carrier gas (i.e. N2) was fixed at 100 cc min1 (25 °C and 1 atm). Two different operating modes of TGA, including constant heating rate and isothermal torrefaction, were carried out. In the two modes, the heating rate in the TG was controlled at 20 °C min1 and the samples were heated from room temperature to 105 °C. Then, the samples were kept at 105 °C for 5 min to completely remove moisture in the samples and provide a basis for analysis. In the first mode, the samples were heated from 105 to 800 °C at a heating rate of 20 °C min1 to investigate the pyrolysis characteristics of microalgae residues. In the second mode, the heating process was made up of two dynamic heating periods and one isothermal heating period. The samples were heated from 105 °C to the desired torrefaction temperature at a heating rate of 40 °C min1 (i.e., the first dynamic heating period) followed by torrefaction for 60 min. At the end of torrefaction, the sample was heated at a heating rate of 20 °C min1 until 800 °C, namely, the second dynamic heating period. To figure out the torrefaction characteristics of microalgae residues, five different torrefaction temperatures of 200, 225, 250, 275, and 300 °C, which cover light, mild, and severe torrefaction, were taken into account in the study. To ascertain the measured quality of TGA, the TG was periodically calibrated and the experiment under any given condition was usually carried out more than twice. The relative error among the measurements of TGA was controlled within 3%. 2.3. Isothermal torrefaction kinetics The reaction rate of a sample is typically given by (Lu et al., 2013) dX ¼ kð1 XÞn dt ð1Þ where k is the reaction rate constant and n is the order of reaction. X is the conversion of the sample and defined by X¼ Wi W Wi Wf ð2Þ where W, Wi, and Wf designate the instantaneous sample weight, initial sample weight, and final sample weight, respectively. To provide a standard of analysis, the initial and final sample weights were identified at 105 and 800 °C, respectively. For isothermal torrefaction, the conversion of a sample is expressed as (Chen and Kuo, 2011) 8 > < > : ln 1X 0 1X ¼ kðt t 0 Þ if n ¼ 1 ð3Þ ð1 XÞ1n ð1 X 0 Þ1n ¼ kðn 1Þðt t0 Þ if n – 1 ð4Þ The logarithmic form of Eq. (4) gives ln k ¼ ln A Ea RT ð5Þ From the five sets of reaction rate constant and torrefaction temperature, the plot ln k versus 1/T gives a straight line with the slope equal to Ea/R and the intercept equal to ln A. Accordingly, the activation energy (Ea) and pre-exponential factor (A) in the kinetics of microalga residues torrefaction are obtained. 3. Results and discussion 3.1. Properties of microalgae residues The composition, proximate, elemental, and calorific (higher heating value, HHV) analyses of the two microalgae residues are given in Table 1. On account of undergoing oil extraction, the crude lipids in C. sp. JSC4 and C. sorokiniana CY1 residues are 6.85 and 9.90 wt%, respectively. In contrast, more carbohydrates are retained in the residues inasmuch as they are 35.70 and 35.67 wt% in C. sp. JSC4 and C. sorokiniana CY1 residues, respectively. Overall, the components in the residues are ranked as: carbohydrate > crude protein > crude lipid > ash > moisture, disregarding the content of others. The volatile matter contents in C. sp. JSC4 and C. sorokiniana CY1 residues are 75.50 and 73.20 wt%, respectively, accounting for the highest contents in the proximate analysis. This reveals that the reactivity of the residues is high, from the burning point of view. The ash contents in the residues range from 5.20 to 7.90 wt%. From the viewpoint of gasification, the slagging phenomenon in gasifiers could be reduced if these residues are blended with coals (Du et al., 2010). The carbon contents in C. sp. JSC4 and C. sorokiniana CY1 residues are 40.32 and 45.07 wt%, respectively. According to the elemental analysis, the chemical formulas of C. sp. JSC4 and C. sorokiniana CY1 residues are expressed as CH2.20O0.83N0.06 and CH2.03O0.59N0.07, which have a higher atomic H/C ratio when compared to lignocellulosic biomass (Chew and Doshi, 2011). Meanwhile, their atomic O/C ratios are in a comparable state. The HHVs of C. sp. JSC4 and C. sorokiniana CY1 residues are 17.41 and 20.40 MJ kg1, respectively. In view of lower carbon and higher oxygen contained in the C. sp. JSC4 residue, its HHV is lower than the other residue. These HHVs are close to those of lignocellulosic biomass materials (Parikh et al., 2005). Accordingly, torrefaction can be employed to upgrade the fuel properties of microalgae residues. 3.2. Pyrolysis of microalgae residues The pyrolytic characteristics of the two residues are examined in Fig. 1 where the TGA and DTG curves are plotted. As a whole, their thermal degradation processes are similar to each other and can be divided into four different regions. The weight loss curves 261 W.-H. Chen et al. / Bioresource Technology 169 (2014) 258–264 4th stage TGA (wt%) TGA 800 TGA 80 1.6 60 1.2 o 309 C o 304 C 40 (a) 100 2 JSC4 CY1 0.8 80 600 60 400 40 20 0.4 DTG 0 100 200 300 400 500 600 700 200 C 225 oC o 250 C 275 oC o 300 C Temperature 0 200 o 20 0 10 20 30 40 50 60 70 80 Temperature (oC) 3rd stage TGA (wt%) 2nd stage o 1st stage DTG (wt%/ C) 100 90 100 Heating time (min) 0 800 o Temperature ( C) (b) Fig. 1. Distributions of TGA and DTG of microalgae residues in N2 at a heating rate of 20 °C min1. 20 o 200 C o 225 C o 250 C 275 oC o 300 C 18 proceed from dehydration (the first stage, 25–200 °C), protein and carbohydrate decomposition (the second stage, 200–350 °C), lipid decomposition (the third stage, 350–550 °C), and then to the decomposition of other components (the fourth stage, 550– 800 °C). When the heating temperature is lower than 200 °C, the DTG curves depict that the decomposition rate is low. This reflects that the upgrade of microalgae residues should be carried out at temperatures no less than 200 °C. The rapid drop of the TGA curves in the second stage accounts for the main pyrolysis or devolatilization process of the residues due to the reactions of depolymerization, decarboxylation, and cracking (Peng et al., 2001a,b) of carbohydrate and protein (Rizzo et al., 2013). The maximum decomposition rates of C. sp. JSC4 and C. sorokiniana CY1 residues are 0.83 wt% °C1 (at 304 °C) and 1.02 wt%°C1 (at 309 °C), respectively. The maximum values are dominated by the thermal degradation of carbohydrates in that their contents in the residues are substantially higher than those of proteins (Table 1). Compared to the second stage, the lower DTG curves in the third stage can be explained by the lower lipid contents in the two residues. In the fourth stage, a low level of DTG curve is attributed to the continuous and slow weight loss of carbonaceous matters in the residues (Rizzo et al., 2013). 3.3. Weight loss dynamics of torrefaction The TGA and temperature curves of C. sp. JSC4 and C. sorokiniana CY1 residues in torrefaction are shown in Figs. 2a and 3a, respectively. The TGA curves depict that the thermal degradation of the two residues under light torrefaction (i.e., at 200 or 225 °C) are slight, implying that the role played by torrefaction time on the thermal decomposition of biomass is not significant. On the contrary, the weight loss under severe torrefaction (i.e., at 275 or 300 °C) is pronounced. This drastic weight drop is mainly exhibited in the initial torrefaction period, and impact of torrefaction time on weight loss is not crucial when it is long to a certain extent. Unlike light and severe torrefaction, the weight losses of the two residues under mild torrefaction (i.e., at 250 °C) progressively decline with time, and they are relatively sensitive to torrefaction duration. When the DTG curves of the two residues are examined, Figs. 2b and 3b depict that the amplitudes of the curves rise dramatically with increasing torrefaction temperature. This elucidates the prominent role played by torrefaction temperature on the thermal DTG (wt%/min) 16 14 12 3 10 2 8 1 6 0 4 9 12 15 2 0 0 10 20 30 40 50 60 70 Heating time (min) Fig. 2. Distributions of (a) TGA and temperature and (b) DTG of Chlamydomonas sp. JSC4. decomposition of microalgae residue. After the heating time is longer than 20 min, the DTG curves of mild torrefaction are higher than the others, implying that the thermal degradation of the residues under mild torrefaction is relatively subject to torrefaction duration when compared to light and severe torrefaction. The maximum decomposition rate during torrefaction and its torrefaction time, namely, the duration from the onset of torrefaction, corresponding to the peak of the DTG curve are tabulated in Table 2. The torrefaction time is located adjacent to the onset of torrefaction, regardless of the torrefaction temperature. This reveals that the maximum decomposition rate is mainly driven in the vicinity of the beginning of the pretreatment. 3.4. Torrefaction severity The weight loss increments of the two residues in the course of torrefaction are presented in Fig. 4, while their weight loss increments and solid yields at the end of torrefaction under the five different torrefaction extents are also given in Table 2. In light torrefaction, the growing tendency in the increment with time is not obvious. The increment of the samples from 1-h torrefaction is smaller than 15 wt%, and the solid yield is larger than 77% (Table 2). The weight loss increment in severe torrefaction is mainly achieved in the initial period and larger than 41 wt%, while the solid yield is lower than 48%. For mild torrefaction, torrefaction 262 W.-H. Chen et al. / Bioresource Technology 169 (2014) 258–264 (a) 100 50 TGA 80 60 400 40 200 C 225 oC o 250 C 275 oC o 300 C Temperature 0 200 o 20 0 10 20 30 40 50 60 70 80 Temperature (oC) TGA (wt%) 600 90 Weight loss increment (wt%) 800 30 20 200 oC o 225 C o 250 C 275 oC o 300 C C. sp. JSC4 10 0 0 10 20 30 40 50 60 Torrefaction time (min) 25 Fig. 4. Distributions of weight loss increments of Chlamydomonas sp. JSC4 (bold lines) and Chlorella sorokiniana CY1 (fine lines) in torrefaction. 200 oC 225 oC o 250 C o 275 C 300 oC 20 DTG (wt%/min) C. sorokiniana CY1 100 Heating time (min) (b) 40 residence time might be used to achieve the same degree of weight loss during torrefaction. Sabil et al. (2013) employed a nondimensionalized weight loss to measure the torrefaction degree of oil palm biomass. In the present study, a parameter of torrefaction severity index (TSI) is developed to identify torrefaction degree. The TSI is defined as 15 2 10 1 0 5 0 0 10 20 30 8 10 40 12 50 14 16 60 Torrefaction severity index ðTSIÞ ¼ 18 70 Heating time (min) Fig. 3. Distributions of (a) TGA and temperature and (b) DTG of Chlorella sorokiniana CY1. duration is an important factor affecting weight loss and solid yield over the entire upgrading process. After 1-h torrefaction, the weight loss increments of C. sp. JSC4 and C. sorokiniana CY1 residues are 31.08 and 34.09 wt%, respectively, whereas their solid yields are 59.57% and 55.67%, respectively. Li et al. (2012) and Peng et al. (2013) used the weight loss of biomass as an indicator to stand for torrefaction severity, and addressed that a different combination of temperature and DWI DWIref ð6Þ where DWI and DWIref stand for the weight loss increment at a certain torrefaction operation and a reference operation, respectively. Within the investigated ranges of torrefaction temperature and duration, the operation at 300 °C for 1 h gives the highest degree of torrefaction; hence this condition is adopted as the reference operation. Accordingly, the values of TSI at the onset of torrefaction and the conditions of 300 °C and 1 h are zero and unity, respectively. Overall, the three-dimensional profiles of TSI of the two residues shown in Fig. 5 are similar to each other. Accordingly, TSI is a feasible parameter to indicate the thermal degradation extent of biomass in torrefaction. Physically, the higher the curvature in the profile, the more sensitive the biomass weight loss to torrefaction operation is. The curvature along the torrefaction time of 0 is zero, and along the torrefaction temperature of 200 °C is low, regardless of torrefaction duration. The curvature along the torrefaction time of 60 min is intermediate, implying that the Table 2 A list of the decomposition characteristics of two microalgae residues in torrefaction. Temperature (°C) Maximum decomposition rate (wt% min1) Torrefaction time of maximum decomposition (min) Weight loss increment in torrefaction (wt%) Solid yield from torrefaction (%) C. sp. JSC4 200 225 250 275 300 1.02 1.44 2.30 6.76 16.80 0.87 1.30 2.42 2.14 1.60 7.04 14.68 31.08 41.71 46.20 86.87 77.31 59.57 47.07 40.57 C. sorokiniana CY1 200 0.73 225 1.12 250 2.29 275 7.27 300 22.90 0.40 1.53 2.52 3.16 1.58 5.23 12.84 34.09 44.11 49.21 85.76 77.22 55.67 45.65 38.33 263 W.-H. Chen et al. / Bioresource Technology 169 (2014) 258–264 0.8 High curvature 0.6 Intermediate curvature 0.4 Low curvature 0.2 0 Curvature = 0 250 275 300 40 20 225 (b) 0.4 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 High curvature Intermediate curvature Low curvature Curvature = 0 0.2 0 250 275 80 60 40 20 0 300 60 225 ( C) 40 ure t 0 200 a Torrefactio 20 r pe n time (min T em ) 0 Fig. 5. Three-dimensional profiles of torrefaction severity index of (a) Chlamydomonas sp. JSC4 and (b) Chlorella sorokiniana CY1 in torrefaction. torrefaction degree of biomass can be controlled in accordance with the temperature. In contrast, the curvature along the torrefaction temperature of 300 °C at short torrefaction time is high, elucidating the high sensitivity of weight loss to torrefaction operation. The obvious curvature along the operation of 300 °C indicates that biomass torrefaction at the temperature with short duration is a more energy-saving route to upgrade biomass when compared to the torrefaction at 60 min with lower torrefaction temperatures. 0 100 200 300 400 200 500 250 600 300 700 800 o Temperature ( C) (b) 2 o 200 C 225 oC o 250 C o 275 C 300 oC 1.5 o 1 0.6 100 1 0.5 Torrefaction severity index 0.8 o 200 C o 225 C 250 oC o 275 C o 300 C 1.5 0 ( C) ure t 0 200 a Torrefactio r pe n time (min T em ) 60 Torrefaction severity index 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 2 o 1 (a) DTG (wt%/ C) Torrefaction severity index DTG (wt%/ C) Torrefaction severity index (a) 100 80 1 60 40 20 0.5 0 0 100 200 300 400 200 500 600 250 300 700 800 Temperature (oC) Fig. 6. DTG distributions of (a) Chlamydomonas sp. JSC4 and (b) Chlorella sorokiniana CY1. 3.6. Torrefaction kinetics 3.5. Pyrolysis of torrefied residues As described earlier, the maximum decomposition rates of C. sp. JSC4 and C. sorokiniana CY1 residues in the non-isothermal pyrolysis processes (Fig. 1) are 0.83 wt% °C1 (at 304 °C) and 1.02 wt%°C1 (at 309 °C), respectively. When the two species undergo light torrefaction at 200 °C for 1 h followed by non-isothermal pyrolysis, Fig. 6 depicts that the maximum decomposition rates of C. sp. JSC4 and C. sorokiniana CY1 residues are 0.82 wt% °C1 (at 304 °C) and 0.91 wt%°C1 (at 310 °C), respectively, revealing that only small amounts of proteins and carbohydrates in the microalgae residues are consumed from the light torrefaction. After experiencing mild torrefaction, large amounts of proteins and carbohydrates are consumed, as a consequence of significant drop in the peaks; but the pyrolysis of lipids are slight. Of course, if the duration in mild torrefaction is reduced, the peak at around 300 °C will not drop so significant. Following severe torrefaction, lipids in the residues are also consumed apparently in that the pronounced decline in the DTG curves at the temperature range of 300–480 °C is exhibited. In isothermal torrefaction, five torrefaction temperatures are practiced; hence five reaction rate constants can be obtained from the plot of ln (1 x)1 or ð1 XÞ1n versus torrefaction time, depending on the value of n (i.e. Eq. (3)). In examining the values of the coefficient of determination (R2) from the regression lines of the plots of ln k versus 1/T, the best regression lines of C. sp. JSC4 and C. sorokiniana CY1 residues are obtained at n = 3 and 5, respectively, as listed in Table 3. From the slopes and intercepts of the two regression lines given in Table 4, the activation energy and the pre-exponential factor for the isothermal torrefaction of C. sp. JSC4 residue are 97.90 kJ mol1 and 7.96 107 min1, respectively, whereas they are 174.74 kJ mol1 and 1.76 1016 min1, respectively, for the isothermal torrefaction of C. sorokiniana CY1. According to Eqs. (3)–(5), it is known that the order of reaction (n) will affect the reaction rate constant (k), which consists of the activation energy (Ea) and pre-exponential factor (A). The thermal decomposition characteristics of the two microalgae residues are similar to each other (Fig. 1). However, as shown in Table 3, the optimum n values of C. sp. JSC4 and C. sorokiniana CY1 are 3 and 264 W.-H. Chen et al. / Bioresource Technology 169 (2014) 258–264 Table 3 Coefficient of determination (R2) of the isothermal kinetics of two microalgae residues. R2 n 1 2 3 4 5 6 7 8 9 C. sp. JSC4 C. sorokiniana CY1 0.9243 0.9921 0.9962 (Max.) 0.9864 0.9748 0.9645 0.9560 0.9491 0.9433 0.8430 0.9403 0.9735 0.9835 0.9857 (Max.) 0.9849 0.9832 0.9813 0.9793 Table 4 A list of torrefaction kinetics of two microalgae residues. Microalgae C. sp. JSC4 C. sorokiniana CY1 R2 n Regression line Ea (kJ mol1) A (min1) 0.9962 3 In k = 11,775/T + 18.912 97.90 7.96 107 0.9857 5 In k = 17,379/T + 29.817 174.74 1.76 1016 5, respectively. As a result, the activation energy (Ea) and preexponential factor (A) of C. sp. JSC4 are significantly different from those of C. sorokiniana CY1. Torrefaction temperature and duration are two important parameters affecting the torrefaction severity index (TSI), as shown in Fig. 5. In the present studies, only two microalgae residues are studied. The TSI profile may also depend on biomass species. For example, the TSI profile of lignocellulosic biomass may be different from that of microalgae residues. Furthermore, if the oxidative torrefaction of biomass is performed, the oxygen concentration in carrier gas and air superficial velocity may also influence TSI profile. The impacts of the three factors (i.e., biomass species, oxygen concentration, and air superficial velocity) on TSI profile deserve further investigation in the future. 4. Conclusions The thermogravimetric analyses of C. sp. JSC4 and C. sorokiniana CY1 residues have been performed to figure out their thermal degradation characteristics. A parameter of torrefaction severity index (TSI) is introduced to account for the sensitivity of weight loss to temperature and duration where the torrefaction temperature of 300 °C and duration of 1 h are adopted as the reference. The curvatures along the three-dimensional profiles clearly indicate that biomass torrefaction at 300 °C with short duration is a more energy-saving route to upgrade biomass when compared to the torrefaction at 60 min with lower torrefaction temperatures. 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