1. Introduction
Algae biofuel is known to be a good substitute for fossil fuels leading to the research and development of many aspects of the topic to investigate its viability and sustainability. Exploration of algae as a biofuel source is also associated with the limitation of lignocellulosic biomass as a second-generation biomass, e.g., with a complex structure as well as intensive and expensive pretreatments [
1]. Algal biofuel is manufactured from microalgae biomass such as green algae species
Chorella sp.,
Dunaliella salina,
Scenedesmus sp., [
2], and
Chamydomonas reinhardtii, since they are capable of accumulating a high amount of lipids—up to 60%—within a short period of time due to their rapid reproduction rate [
3]. They can be converted into renewable biofuel oil or gas which is more sustainable than fossil fuels [
4]. Microalgae are unicellular photosynthetic simple plants that convert sunlight, water, and carbon dioxide into chemical energy contained in the microalgal biomass [
3,
5].
Nannochloropsis sp. produces 20 tons of oil per hectare, which is 3.5 times greater than palm oil and 20 times that of sunflower and rapeseed [
6]. Microalgae can fix CO
2 10 to 50 times better than terrestrial plants [
7], and they consume nutrients from runoff water from nearby land areas or by channeling sewage or wastewater treatment plants since they can be cultivated in fresh-, salt-, brackish-, and wastewater [
4]. They can also be cultivated in open ponds or closed photobioreactors [
8].
Algae biofuel is manufactured in the following three stages: (1) cultivation of microalgae, (2) biomass harvesting, and (3) manufacturing of the desired products [
8,
9]. Algal oil is extracted from the microalgal biomass after harvesting through physical, chemical, and biochemical approaches prior to transesterification of crude algal oil to crude biofuel. Microalgae also can be fermented for direct ethanol synthesis or undergo thermochemical conversion to produce synthesis gas (syngas) that are fuel gases [
8,
10].
Nannochloropsis gaditana lipid extracted algae (LEA) used in this study was obtained from the subcritical water extraction of algal oil under certain experimental conditions.
The thermochemical method of conversion of LEA offers simpler pathways from feedstock to product and is more advantageous than biochemical conversion [
11]. Raheem et al. [
12] reported that gasification is the most efficient method of conversion of biomass to gaseous products, since it is suitable for application with a wide range of biomasses, for instance, algae, palm oil waste, wood, peat, and solid waste. Thermogravimetric analysis (TGA) is used for biomass lignocellulosic composition determination, such as lignin, cellulose, and hemicelluloses [
13,
14], pyrolysis of biomass [
15,
16,
17,
18], combustion of palm oil biomass [
19], reaction kinetics analysis [
20], gasification of biomass [
21,
22], and torrefaction of wood [
23]. They were performed by varying process parameters to mimic the actual process that aims to predict the behavior of the respective biomasses under certain thermal treatment processes. TGA is also performed for proximate analysis determination [
21,
24] since the thermogravimetric (TG) curve and first-order derivative of the thermogravimetric curve (DTG) is used to analyze the decomposition rate of the feedstock.
Different temperatures of the TGA process indicate different material losses. For instance, moisture (M) is released at 120 °C [
13], below 150 °C [
15,
19], in the range of 100–150 °C [
16], at 110 °C [
17], and below 220 °C [
14]. The difference in the temperature of moisture removal depends on the atmosphere, or the carrier gas used [
21]. Volatile matter (VM) evaporation occurred at a temperature range from moisture removal, at up to 550 °C [
13], up to 580 °C [
15], 485–600 °C [
25], up to 490 °C and 628 °C under N
2 and air atmosphere, respectively [
21]. The next peak on the DTG curve above 500 °C denotes fixed carbon (FC) content, and the final composition of the feedstock indicates ash content in the feedstock.
In this study, the LEA was from subcritical water extraction (SWE) of algal oil. To the best of our knowledge, no studies are available in the literature reporting the thermal behavior of LEA from the subcritical water extraction (SWE) of algal oil. The TGA process of LEA was conducted using N2 at different heating rates (5, 10, and 15 °C/min) from room temperature to 1000 °C, and the thermal behavior of the samples during TGA was studied.
2. Materials and Methods
2.1. Materials
The material used in this study was lipid-extracted algae (LEA) from Nannochloropsis gaditana obtained from the subcritical water extraction (SWE) of algal oil. The LEA was in a solid form and crushed in a mortar and pestle and sieved using a sieve mesh to achieve 90 microns size.
2.2. Proximate Analysis
Determination of moisture (M), volatile matter (VM), fixed carbon (FC), and ash was carried out in a thermogravimetric analyzer (TGAA/SDTA851, Mettler Toledo, OH, USA). A total of 20 mg of each algae residue sample was put in an alumina crucible and was heated in a furnace continuously from room temperature to 1000 °C under different heating rates (5, 10, and 15 °C/min), using N2 as the carrier gas at 50.00 mL/min.
2.3. Ultimate Analysis
The elementary compositions of LEA were performed in a CHNS analyzer (LECO True Spec CHNS628, USA). This analysis revealed the organic elements in LEA in wt% of carbon (C), hydrogen (H), and oxygen (O) as well as sulfur (S) and nitrogen (N). Dried samples of about 3 mg were put in the analyzer and the analysis was run at 1000 °C in the presence of oxygen, nitrogen, and helium as the carrier gases, and the elemental compositions were automatically calculated by the analyzer. The ultimate analysis data are useful in the computation of a higher heating value (HHV) by using Dulong’s formula in Equation (1):
where C, H, O, and S are mass fractions of carbon, hydrogen, oxygen, and sulfur in wt% of the samples [
26]. This method is applicable for samples that are less than 1 g and are not enough for the bomb calorimetry test for calorific value.
2.4. Thermogravimetric Analysis (TGA)
A thermogravimetric analyzer (TGAA/SDTA851, Mettler Toledo, OH, USA) was used for thermal degradation. The gasification experiments were carried out by varying the heating rate from 5–25 °C/min with 5 °C/min increment using N2 as a carrier gas under dynamic conditions with a flow rate of 50.00 mL/min. The gasification temperatures were raised from room temperature to 1000 °C. The duration of each experiment depends on the heating rate of TGA. The mass and heat transfer problem was overcome by fixing the mass of samples to 20 mg. After weighing the sample, it was directly placed into a 150 µL ceramic crucible and the temperature was kept isothermal for about one minute until a steady condition was obtained before increasing to 1000 °C. LEA weight loss and rate of weight loss were measured and recorded continuously as a function of temperature and time. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of each sample were obtained as an output. These curves were used to analyze the thermal characteristics of LEA.
2.5. Gasification Experiments
LEA was gasified in a vertical fixed bed Temperature Programmed Gasifier (TPG) (model: VSTF50/150-1100, 1.2 kW/6 A) equipped with a type K thermocouple and connected to a digital gas flowmeter (model: FMA5506A; Omega Engineering, Inc., Selangor, Malaysia). There were two parameters studied at five levels, namely the reaction temperature (600, 700, 800, 900, and 1000 °C) and biomass loading (03, 0.4, 0.5, 0.6, and 0.7 g). ER, biomass loading, and airflow are correlated to each other in the ER formula, given in Equation (2):
The gasifying medium used was compressed air, with a volumetric flow rate depending on the parameters being tested. Syngas from the reaction was passed through 8” molecular sieves moisture trap, cotton wool, and ice bath to remove moisture and particles from the syngas. The syngas was then collected in a 3 L Tedlar gas storage bag and the inlet was sealed with plastic paraffin film (Parafilm) to minimize gas loss before further analysis of the syngas.
2.6. Syngas Analysis
The produced syngas was analyzed in a Gas Chromatograph with a Flame Ionization Detector (GC-FID). A Gas Chromatography with Flame Ionization Detector (GC-FID) (Agilent Technologies, 6890 N) was used in the syngas analysis to study the mole compositions of desired gases, namely hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4) in the syngas. Analysis of the syngas was carried out according to ASTM Method D3612-96. The sample gas was injected into a 30 m × 0.53 mm Carboxen-1010 PLOT column. The oven was set at 350 °C and argon gas was used as carrier gas under a 3.0 mL/min flow. The presence and composition of H2, CO, CO2, and CH4 were analyzed from the chromatogram generated from GC-FID in parts per million (ppm) unit.
2.7. Kinetic Study of Gasification
TGA provides the measurement of weight loss as a function of temperature and time that is useful as a tool for comparing kinetic data of various reaction parameters [
16]. Two iso-conversional models were used to calculate the activation energy of the gasification processes, namely the Kissinger–Akahira–Sunose (KAS) model and the Flynn–Wall–Ozawa (FWO) model. These kinetic models require TGA from any type of biomass, where TGA provides the data for weight loss as a function of temperature [
16]. There were five different heating rates used (5, 10, and 15 °C/min).
The isoconversional method is based on the assumption that the conversion, α is expressed as a product of two functions that are independent of each other; k(T), which is solely dependent on the temperature, and f(α) which is dependent on the conversion of the decomposition process. In the non-isothermal decomposition of solid biomass, the sample mass was recorded as a function of temperature, where the general rate of thermal decomposition is expressed as:
where
wo is the initial weight of the sample, w is the weight of the sample at the corresponding time (min) or temperature (K), and the w
f is the final weight of the sample after the reaction [
27,
28]. The rate of the reaction can be defined by the Arrhenius equation,
where A (min
−1) is the pre-exponential factor of the Arrhenius equation, E (J mol
−1) is the activation energy, and R (8.314 J mol
−1 K
−1) is the universal gas constant. Algae biomass is a multi-component mixture, hence, the complexity of the reaction increases with conversion (α) [
28]. This resulted in an extremely low possibility that different components have the same activation energy (Ea). Consequently, Ea and A are the functions of α. At a constant heating rate of the sample (β = dT/dt), the substitution of Equation (4) into Equation (3) resulted in Equation (5).
The natural log of Equation (5) is expressed as Equation (6):
This equation is the basis for the Friedman method. Temperature is a function of time and increases with a constant heating rate,
β, as shown in Equation (8):
Rearranging Equation (7) gives:
Substituting Equation (8) into Equation (5) gives:
Equation (10) is the integrated form of Equation (9). Different kinetics methods were developed from this equation,
where g(a) = integrated form of conversion dependence f(α), x = E/RT, A = constant, R = universal gas constant, 8.314 kJ mol K
−1, α = conversion value, and T = temperature in unit K. Different kinetic methods are applied in the study of thermal degradation of biomass from this equation [
29,
30].
The activation energy and pre-exponential factor were determined using two kinetics methods The first is (1) the Kissinger–Akahira–Sunose (KAS) method, adapted from Kissinger, 1957,
and the second is the (2) Flynn–Wall–Ozawa (FWO) method, adapted from Flynn and Wall., 1966,
The activation energy for the specified value of conversion, α, of the thermal decomposition reaction is obtained from the slope of the curves,
versus
(−1.052E/R) and lnβ versus 1/T (−E/R) for the KAS and FWO models, respectively [
21,
30,
31]. Repeating the activation energy determination for the whole range of α from 0 to 1 will result in activation energy of progressing values of α for both models.
4. Conclusions
Thermogravimetric analysis of Nannochloropsis gaditana LEA was carried out in a thermogravimetric analyzer (TGA) at 30–1000 °C under N2 flow as the carrier gas. As observed from the analysis, the following three stages of decomposition occurred: moisture release as observed at the first peak of DTG curves, pyrolysis of organic materials at the second peak, solid residue decomposition, or fixed carbon release at the third peak. From these stages, maximum degradations were observed at the second stage at all heating rates. An increase in the heating rate increased the devolatilization of volatiles rate, which resulted in a lower ash content as well as maximum degradation temperature that was also ascribed by thermal equilibrium within the samples’ particles. The highest activation energy was found at α = 0.3, which fell under the devolatilization stage, proven by the highest AR value found at this stage. Gasification of LEA also was carried out at two different parameter variables, where the highest H2 yield was attained at 800 °C and 0.7 g. Since gasification of LEA showed a considerably high H2 yield and requires low activation energy, LEA was found to be suitable to be used in gasification for syngas production.
In the future, a kinetics analysis of other species of LEA could be performed to study the suitability of samples for gasification. This could, in turn, maximize the utilization of microalgae for producing value-added products.