Chlorella sorokiniana thermogravimetric analysis and combustion characteristic indexes estimation

1 Chlorella sorokiniana thermogravimetric analysis and combustion characteristic indexes estimation. Authors: Sergio Paniagua1, Luis Fernando Calvo1*, Carla Escapa1, Ricardo N. Coimbra1, Marta Otero2, Ana I. García1. 1Universidad de León, Departamento de Química y Física Aplicadas, IMARENABIO, Avda. Portugal 41 (24009), León, Spain 2University of Aveiro, CESAM-Department of Environment and Planning, Campus Universitário de Santiago 3810-193, Aveiro, Portugal * Corresponding author email: [email protected]; Telephone: +34987291828 Keywords: Chlorella sorokiniana; combustion characteristic indexes; heating rate; microalgae culture; thermogravimetric analysis. Abstract. This work aimed to investigate thermal decomposition of microalgae throughout the different culture stages. For this purpose, Chlorella sorokiniana was cultured in photobioreactors (PBRs), and microalgae biomass was sampled at different days throughout the development of the culture. The aim was to analyse the energetic value of this biomass by thermogravimetric analysis (TGA) as well as to calculate combustion characteristic indexes during the different culture stages. In all cases, thermal decomposition of microalgae biomass during combustion denoted two stages. The first one encompassed the carbohydrates and proteins decomposition and the breakdown of hydrocarbon chains of fatty acids, whereas the second step was closely related with the combustion of the formed char. Fuel composition analysis denoted a microalgae HHV value quite similar to that of Poplar (considered as an energy crop) biomass and slightly higher than published values for herbaceous biomass. In relation with the culture stages, it was found that a better combustion performance (higher thermal indexes as well as 2 higher DTGmax values) for microalgae biomass sampled at days 19 and 21. These results point to the importance of the culture stage for the thermal valorization of microalgae biomass. 1.- Introduction. The depletion of fossil fuel reserves and the environmental pollution associated to their burning have prompted scientist and engineers to develop new technologies and find alternative energy sources [1]. According to the 2013 Survey of the World Energy Resources (WER), 223 million tonnes of crude oil and 209 million cubic meters of natural gas remain in our planet. These global reserves of crude oil and gas are estimated to last for 56 and 55 years, respectively. This report also estimates that there are 869 million tonnes of coal reserves, which based on current production rates should last for around 115 years [2]. Consequently, biomass fuels are gaining particular attention as an alternative, clean and renewable energy. Among these biofuels, photosynthetic organisms have the advantage to fix CO2 in the atmosphere and for this reason are considered as CO2-neutral fuels when being combusted [3]. In this sense, microalgae have been proposed as a thirdgeneration biofuel source due to their rapid growth rate and high oil contents, high yield per area, no competition with crops for arable land or freshwater and their capacity to use of CO2 as feedstock. In addition, microalgae are able to use nutrients from most wastewaters, providing an alternative method for wastewater treatment [4]. Thermochemical conversion of biomass is considered as one of the most promising routes for biomass utilization [5]. Nowadays, combustion is the most simple and direct technology available for biomass utilization, which is responsible for over 97% of the world´s bio-energy production [6]. For the prediction of combustion performance in 3 boilers and obtaining relative combustion characteristics of fuels, thermogravimetric analysis (TGA) is known to be most useful [7]. Nevertheless, only very recently were published studies on the TGA of microalgae combustion. Chen et al. [8] reported the combustion behaviour of Chlorella vulgaris under different oxygen concentrations. Gai et al. [9] carried out a kinetic analysis of thermal decomposition characteristics of Chlorella pyrenoidosa and Spirulina platensis under non-isothermal conditions. López et al. [5] investigated thermal behavior of Nannochloropsis gaditana, Scenedesmus almeriensis and Chlorella vulgaris under oxidizing atmosphere by TGA coupled to mass spectrometry. Liu et al. [10] used TGA for the determination of the ash content and the qualitative or semi-quantitative analysis of the carbohydrates, proteins and lipids in four microalgae species based on TG analysis ash content. These authors [10] pointed to the possible utilization of TGA for microalgae screening and cultivation monitoring. However, to our best knowledge, such an assessment has not been published yet. During cultivation of microalgae, their growth affects the nutrients removal rates, and a nutrients limitation improves lipids accumulation in microalgal cells [11]. Consequently, cell composition of microalgae varies along the different stages of culture. This feature may affect the behaviour of thermal decomposition [9]. For this reason, it is important to study thermal properties evolution of the microalgae throughout the different culture stages. In this work, Chlorella sorokiniana was cultured and microalgae biomass withdrawn throughout the development of the culture and analyzed by TGA. The aim of this research was to assess the evolution of thermal characteristics of the combustion of microalgae Chlorella sorokiniana throughout the different stages of the culture. 4 2.- Material and methods. 2.1. - Microalgae culture. The microalgae strain used in this study was Chlorella sorokiniana CCAP 211/8 K (UTEX Culture Collection). Inoculum for the experiments was cultivated in 250 ml Erlenmeyer flasks in the standard culture medium Mann and Myers [12], which was autoclaved for 20 min at 1 atm pressure to ensure aseptic ambient. Firstly, the inoculum was cultivated in 250 mL Erlenmeyer flasks and, in a second stage, microalgae were cultured in tubular bubbling photobioreactors (PBRs) at pilot scale (0.0875 m diameter and 0.3 m height with 1.2 l capacity). PBRs were inoculated with the same volume of pre-cultured microalgae to ensure the same initial concentration (0.1 g l-1.) Growth conditions were maintained constant, under controlled temperature (25±1 ºC), irradiance (µE m-2s-1) and photoperiod (12:12), inside a vegetal culture chamber. In the same way, pH was controlled (7.2-7.5) by the injection of 7% CO2 enriched air, filtered through 0.2 µm sterile air-venting filter (Millex-FG50, Millipore). The PBRs surface was illuminated with 8 fluorescents lamps (33W, 2150 lumen, Philips, France) providing 650 µE m-2s-1 light intensity under 12:12 light/dark cycle. PBRs were operated under two conditions, batch (or discontinuous) or semi-continuous. In the period of time in which the PBRs are in discontinuous mode, point dilutions were made every 24 hours of 0.5 g l-1. These dilutions were not performed when culture was under discontinuous mode. Daily dilution of the culture volume was carried out to guarantee a biomass concentration of 0.5 g l-1. PBRs were then operated in semicontinuous mode until the volume of culture medium exchanged was equal to 2.5 times the volume. Then, the culture was kept in batch mode for three days, after which 5 the culture was ended. Fig. 1 represents the growth of microalgae biomass in PBRs throughout the culture times. Biomass samples of the culture were taken four times, as indicated in Fig. 1. The first sample was taken after 14 days (t14). The above sample was related with the last day of the cultivation in batch and the first day at which a dilution was effected. The second sample point was done after 17 days (t17) of the start of the trial; this point denoted a middle point during the semi-continuous stage. The third point, at 19 day (t19), was associated with at time at which the semi-continuous stage ended as well as the last dilution was done. The last sampled point, at 21 days (t21) after the start of the experiment was associated with the end of the culture. The above point was at a Batch stage. Culture growth was monitored by the determination of microalgae biomass concentration. For this purpose, optical density at 680 nm (OD680) was daily measured by spectrophotometric (UV/Vis spectrophotometer BECKMAN DU640). The dry mass biomass concentration was measured by filtering 25 ml of culture thought 0.45 µm filter and drying it in an oven at 105 ºC during 24 h. Biomass harvested after 14, 17, 19 and 21 under cultivation was centrifuged at 6461 g during 5 min (SIGMA 2-16P). The pellet was washed twice with distilled water, dried in an oven at 105ºC during 24 h and homogenized for subsequent analysis. 2.2.- Chlorella sorokiniana elemental, proximate and calorific value analysis. The fuel properties of this microalgae strain were assessed by the determination of the elemental and proximate analysis as well as by the determination of the calorific value. Moisture content was determined gravimetrically by the oven drying method. Higher heating value (HHV) at a constant volume was measured by means of an adiabatic oxygen bomb calorimeter. A set of standardized rules were used to calculate the 6 Chlorella sorokiniana values shown in Table 1. Moisture (ASTM 3320), volatiles (UNE 32019), ash content (UNE 32004), carbon, hydrogen and nitrogen (ASTM 5373), sulphur (ASTM 4239). Also, heating value (HHV) was determined according to UNE 32006 rule. For comparison purposes, this Table 1 also shows both the elemental and proximate analysis as well as the heating value for certain biomass crops (poplar and rice) and a fossil fuel (carbon). With this data, authors want to give the readers a preliminary idea about the possible thermal valorization of Chlorella sorokiniana biomass. 2.3.- Thermogravimetric analysis (TGA). Dried and homogenized microalgae biomass samples were preheated according methodology followed by [5] to further reduce the moisture content. Then, samples were milled on a Fritsch mill Model P-19 to a 1 mm particle size. Afterwards, by using a Retch ball mill model MM200, particle sizes around 125 µm were obtained. After such conditioning of microalgae biomass samples, TGA was carried out using a TA Instruments SDT2960, which is able to supply a continuous measurement of sample mass as a function of time or temperature. Milled samples weighing 4–6 mg were placed in a pottery crucible and heated at three heating rates (10, 20 and 40 K min-1) from ambient to 1150 K. This heating was carried out under an air flow of 100 ml min-1 (at a gauge pressure of 1 atmosphere) to carry out the combustion process. Then, derivative thermogravimetric curves (burning profiles) were obtained for the combustion of microalgae biomass samples collected throughout the culture. 2.4. – Combustion characteristic indexes. 7 In agreement with the opinion of several authors, thermal indexes can give fast and truthful information on the kinetics of the combustion of the biomass extracted from the microalgae [5, 13]. The following indexes were calculated in this work: 2.4.1. - Determination of ignition temperature (Te) and ignition index (Di). Considering a typical DTG curve for the combustion of microalgae biomass (Fig. 2), the ignition temperature (Te) as defined as follows [14, 15]: firstly, through the DTG peak point a, a vertical line was made upward to meet the TG oblique line at point b; secondly, a tangent line to TG curve was made at point b, which met the extended TG initial level line at point c; thirdly, another vertical line was made downwards through point c, which met the cross axle at point d. The corresponding temperature of point d was defined as Te [16]. The ignition index (Di) represents the ignition capacity of a fuel so that, the higher Di, the easier the fuel ignition occurs. This index was determined by the following equation [17]: 𝐷𝐷𝑖𝑖=(𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑 )𝑚𝑚𝑚𝑚𝑚𝑚 𝑡𝑡𝑝𝑝·𝑡𝑡𝑒𝑒 where (dw/dt)max is the maximum combusiton rate (% min-1) , tp is the time (min) at which the largest peak (at a temperature above 293 K) occurs and te is the ignition time (min). 2.4.2. - Determination of burnout index (Df). The burnout index Df denotes the combustion capacity of a fuel and was here determined to evaluate the burnout performance of microalgae biomass. This index values were estimated according to Eq.2 [17]: Eq. (1) 8 𝐷𝐷𝑓𝑓=(𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑 )𝑚𝑚𝑚𝑚𝑚𝑚 ∆𝑡𝑡1/2 ·𝑡𝑡𝑝𝑝·𝑡𝑡𝑓𝑓 Δt1/2 is the time (min), in the first half of the DTG for the particular stage, since the half of the maximum DTG value is reached until achieve this DTGmax value (min), tf is the time at which the end of the peak takes place (starting counting time zero to 293 Kelvin degrees and considering the final moment as that in which it reaches the 2% of DTGmax). This index is very similar to the Di but gives greater importance to the end of the peak and does not consider the ignition temperature. 2.4.3. - Devolatilization index (D). This parameter, which is related to the release of volatiles during combustion, was estimated as depicted in Eq. 3: 𝐷𝐷=(𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑 )𝑚𝑚𝑚𝑚𝑚𝑚 𝑇𝑇𝑚𝑚𝑚𝑚𝑚𝑚·∆𝑇𝑇 Tmax is the temperature at which DTGmax is achieved (K), ΔT is the difference between Tmax and Te (K). 2.4.4. - Combustion characteristic index (S). This index can be used for a preliminary assessment of the microalgae combustion performance [18] and represents the energy requiered to burn a fuel. The S was calculated according Eq. 4: 𝑆𝑆=(𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑 )𝑚𝑚𝑚𝑚𝑚𝑚 · (𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑 )𝑚𝑚𝑒𝑒𝑚𝑚𝑚𝑚 𝑇𝑇𝑒𝑒2·𝑇𝑇𝑓𝑓 where (dw/dt)mean is the average combustion rate considering the 1% of the DTGmax as the start and the end of the process (% min-1), Te is the igntion temperature (K) and Tf is the temperature value at which the end of the peak is achieved (1% of the DTGmax ). Eq. (2) Eq. (3) Eq. (4) 9 3.- Results and discussion. 3.1.- Fuel properties of Chlorella sorokiniana. Carried out the characterization of Chlorella sorokiniana, results obtained are shown in Table 1. Percentages obtained for the C, H, N and S elements are in concordance with the literature related to this microalgae genus (50% for C, 6 –8% in the case of H, 7– 10% for N and <1 % talking about S content (w/w)) [19, 20, 5, 21]. In the case of the calorific value, results here obtained are also similar to those obtained by other authors for this particular genus (values about 18-20 MJ kg-1 of HHV) [22, 19]. For comparison purposes regarding the fuel quality, besides the data for Chlorella sorokiniana, the elemental and proximate analysis as well as the calorific value of an herbaceous crop (rice straw), a lignocellulosic energy crop (poplar) and a bituminous coal [23] exploited in thermal stations) have been depicted in Table 1. Compared with rice straw [24], microalgae showed higher values for all the parameters included in the elemental analysis with a major difference in the case of nitrogen. Regarding the proximate analysis, it is to highlight the relative lower ash content of microalgae, which is an important feature for fuel. Moreover, the HHV of microalgae is higher than that of rice straw. Compared with a lignocellulosic crop (poplar) [25], microalgae have similar H and C contents, but lower N and higher S contents. Meanwhile, the ash content of microalgae is slightly higher and the volatiles content lower than for poplars. In any case, microalgae and poplars have quite similar HHV (around 20 MJ kg-1). In the last term, if we make a comparison between the microalgae and a conventional coal, data showed for the Chlorella elemental analysis, H, N and S values are higher for the microalgae (not so in the case of carbon). Following with the comparison between coal and algae, HHV of the coal was higher than the same parameter for the microalgae, 16 parameter defining the semicontinuous, which is the dilution with a frequency of 24 hours. Therefore, no biomass growth occurred during the semi-continuous culture. Differently, exponential growth was observed under batch culture. 4.- Conclusions. Chlorella sorokiniana thermal decomposition denoted two stages. 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Thermogravimetric studies of the behavior of wheat straw with added coal during combustion. Biomass and Bioenergy. 2009;33(1):50-6. doi:http://dx.doi.org/10.1016/j.biombioe.2008.04.013. 33. López-González D, Fernandez-Lopez M, Valverde JL, Sanchez-Silva L. Thermogravimetric-mass spectrometric analysis on combustion of lignocellulosic biomass. Bioresource technology. 2013;143:562-74. doi:http://dx.doi.org/10.1016/j.biortech.2013.06.052. 0 0,5 1 1,5 2 2,5 12 13 14 15 16 17 18 19 20 21 Microalgae culture biomass/g l -1 Time/days Batch I Semicontinuous 1st dilution Last dilution Fig. 2 Thermogravimetric curves for the combustion process of Chlorella sorokiniana microalgae. Points a, b, c, and d are employed to calculate ignition temperature (Te) index. Fig. 1 Concentration of microalgae biomass throughout the culture of Chlorella sorokiniana . In circles, days to which samples were taken (14,17,19 and 21). Batch II Fig. 3 Thermogravimetric combustion curves at the different heating rates for the times at which Chlorella sorokiniana biomass was sampled: (A) 14 days, (B) 17 days, (C) 19 days and (D) 21 days. Table 1 – Analysis (elemental and proximate) and calorific value of Chlorella sorokiniana and three other biomasses for comparison purposes. a In percentage. All values are in dry basis except moisture. b HHV: high heating value Elemental analysis Proximate analysis Calorific value Ca Ha Na Sa Moisturea Asha Volatilesa HHVb/MJ kg-1 Chlorella sorokiniana (this work) 47.9 6.40 8.74 0.78 9.6 7.83 76.1 18.72 Rice Straw [23] 37.87 4.61 0.63 0.14 7.43 19.07 67.95 14.71 Poplar* [24] 49.50 5.80 0.56 0.11 7.90 3.28 79.90 19.78 Coal [22 ] 62.07 2.3 1.16 2.21 11.2 30.33 8.01 24.38 Table 2 – DTG peaks and characteristic temperatures and maximum values for the temperature programmed combustion under different heating rates of Chlorella sorokiniana biomass sampled at different culture times. Heating rate/K min-1) 10 20 40 Time Peak Toa/K Tfb/K TDTGmaxc/K DTGmaxd/% min-1 To/K Tf/K TDTGmax/K DTGmaxd/% min-1 To/K Tf/K TDTGmax/K DTGmax/% min-1 14 1 443.25 588.52 556.24 3.324 447.28 605.67 573.39 7.024 470.49 600.63 580.45 15.430 2 594.57 614.75 603.65 2.965 608.77 645.02 616.77 7.636 611.73 661.16 625.85 14.380 3 737.83 907.32 827.62 3.033 748.93 918.42 854.86 4.767 774.15 949.69 867.97 7.184 17 1 453.34 589.53 551.19 2.926 462.42 607.69 563.3 6.648 462.42 617.78 578.43 13.570 2 593.57 652.08 604.66 2.469 609.71 659.14 621.81 5.421 619.8 670.24 631.90 10.750 3 669.23 858.59 759.02 1.888 673.27 798.36 726.73 6.933 674.27 808.45 727.74 7.919 19 1 467.46 589.53 563.31 3.309 470.49 602.65 575.41 6.885 474.52 603.65 584.49 15.600 2 594.57 620.81 605.67 3.356 608.72 636.95 616.77 8.082 606.68 677.30 624.81 16.910 3 735.81 912.36 813.53 2.404 638.96 773.14 749.94 4.453 703.53 812.49 765.07 8.733 21 1 456.36 596.59 554.22 4.658 470.49 604.66 568.34 9.831 462.42 610.72 574.40 21.960 2 598.61 619.87 602.65 2.436 608.74 624.84 616.77 7.184 616.77 679.32 626.86 13.780 3 715.64 896.22 779.19 2.761 768.13 921.44 804.42 3.592 739.85 903.28 817.53 6.012 a T o : temperature at which a certain peak starts b Tf: temperature at which a certain peak ends c T DTGmax: temperature associated to DTGmax d DTGmax: largest value of DTG in the considered process Table 3 - Combustion characteristic indexes for the temperature programmed combustion under different heating rates of Chlorella sorokiniana biomass sampled at different culture times – Heating Rate/K min -1 10 20 40 Peak 1 Peak 2 Peak 3 Peak 1 Peak 2 Peak 3 Peak 1 Peak 2 Peak 3 a T e /K Time 14 525 595 775 525 600 775 530 600 770 Time 17 520 610 775 520 600 740 530 600 770 Time 19 525 595 730 550 610 820 535 610 780 Time 21 525 582 725 545 610 770 550 605 765 b D i /% min -3 · 10 -4 Time 14 54 41 12 420 280 70 3400 2100 410 Time 17 49 33 5 420 200 37 3000 1600 180 Time 19 52 35 10 360 310 39 3300 2500 230 Time 21 75 25 13 550 300 59 4500 2000 380 c D f /% min-4 · 10-4 Time 14 11 402 3.30 173 1000 32 3060 5430 277 Time 17 11 43 0.80 187 253 19 2750 3360 256 Time 19 13 55 1.45 159 511 23 3620 5020 395 Time 21 21 264 1.94 352 1450 39 5340 4400 185 d D/% min-1 K-2 · 10-4 Time 14 1.98 8.52 0.68 2.59 5.33 0.69 5.39 9.30 0.83 Time 17 1.75 7.15 0.59 2.71 3.86 0.26 5.32 5.51 0.38 Time 19 1.58 6.45 1.32 4.36 19.35 0.16 5.39 25.24 0.18 Time 21 2.97 2.18 1.80 7.78 22.39 0.23 14.35 10.05 0.35 e S/% min -2 K -3 · 10 -7 Time 14 0.54 0.58 0.14 2.37 2.03 0.35 5.47 5.88 0.58 Time 17 0.43 0.50 0.04 2.10 1.10 0.14 8.50 4.27 0.27 Time 19 0.57 0.51 0.14 2.13 2.53 0.25 11.44 10.52 0.47 Time 21 1.02 0.31 0.21 4.20 2.40 0.39 19.80 6.51 0.91 a T e : ignition temperature b Di: ignition index c Df: burnout index d D: devolatilization index e D: combustion characteristic index