Evolved Gas Analysis and Kinetics of Catalytic and Non-Catalytic Pyrolysis of Microalgae Chlorella sp. Biomass With Ni/θ-Al2O3 Catalyst via Thermogravimetric Analysis

This study investigates the efficacy of a prepared Ni/θ-Al₂O₃ catalyst during the pyrolytic conversion of Parachlorella kessleri HY-6 and compares the results with non-catalytic conversion. The catalyst was characterized by techniques such as Brunauer–Emmett–Teller (BET) for surface area, acidity, and X-ray powder diffraction (XRD). Isoconversional and combined kinetic methods were used to study the pyrolytic kinetics of the process. Ni/θ-Al₂O₃ was used at 10, 20, and 30% of the algal biomass. The addition of Ni/θ-Al₂O₃ facilitated the conversion by lowering the mean activation energy during pyrolysis. The catalytic effect was more pronounced at lower and higher conversions. The presence of the catalyst facilitated the pyrolysis as indicated by the lower value of activation energy and ∆H, and ∆G. Gases evolved during pyrolysis were qualitatively analyzed by FTIR to see the effect of catalyst on evolved gas composition during the pyrolysis process.

Introduction

The search for an appropriate alternative energy solution to fossil fuel is still in the research and development stage (Xu et al., 2020). Biomass, being a sustainable energy resource, has been under consideration for the past few decades. To date, the use of 1^st and 2^nd generation biomass (lignocellulosic and edible materials) is limited due to more land requirements, food vs. fuel concerns, and seasonal variations (Sekar et al., 2021). Microalgae is a potential third-generation biofuel as it can proliferate on unarable land using wastewater. The growth of microalgae offers the advantage of wastewater treatment and CO₂ fixation. It has a higher per unit area yield of lipids for biodiesel than terrestrial plants (Lee et al., 2020). Microalgae is a unicellular microorganism composed of lipids, proteins, and carbohydrates found in the aquatic environment and proliferate (Gong et al., 2020).

Microalgae can be converted into multiple biofuels such as biodiesel and jet oil through three main thermochemical conversion routes such as gasification, pyrolysis, and hydrothermal liquefaction (Fan et al., 2020). Pyrolysis converts biomass to an intermediate liquid product which is later refined for drop-in hydrocarbon biofuels, oxygenated fuel additives, and petrochemical alternatives (Zhao et al., 2013). Pyrolysis is a widely used thermochemical conversion process where organic material is chemically dissociated by heating in the lack of oxygen (Naqvi et al., 2018; Chen et al., 2021). Pyrolysis occurs in the absence of oxygen at a particular temperature (300–700°C). The final product is obtained mainly as bio-oil and biochar, along with some non-condensable gaseous components. Pyrolysis is generally classified as slow, fast, and flash pyrolysis based on heating rates. The prediction of product type and its properties depends on the kind of microalgae, operating conditions, reactor types, and conversion mechanism (Chen et al., 2021). Usually, the bio-oil obtained from slow and fast pyrolysis has a noticeable amount of oxygen which can adversely affect the stability and quantity of the product (Azizi et al., 2018). The solution to this problem is the use of catalysts. The use of catalysts also helps to modify the reaction process for the production of upgraded fuels. The catalysts can be used along with biomass feed as well as in catalytic bed form. The utilization of catalysts in the pyrolysis process improves the quality of biofuels by reducing their acidity and viscosity (Babich et al., 2011). Catalysts influence the yield and composition of pyrolysis products, i.e. bio-oil, gas, and char. Various materials have been used as effective catalysts, such as zeolite, nickel, platinum, magnesium, and cobalt (Babich et al., 2011).

HZSM-5 is widely investigated because of its ability to improve bio-oil quality through deoxygenation. However, HZSM-5 has little effect on the nitrogen content of bio-oil (Mustapha et al., 2021). Bio-oil yield and its quality depend on the type of catalyst and temperature used during catalytic pyrolysis (Guo et al., 2012). Among these catalysts, Ni-based catalysts perform better during the pyrolysis due to their activity for decarboxylation or decarbonylation reactions during the hydride oxygenation process (Sekar et al., 2021). Alumina-supported Ni catalyst enhanced the cracking and reforming of volatile and tars during the gasification of biomass (Díaz-Rey et al., 2015). Silica-supported nickel phosphide catalyst improved the boil-oil yielded with low content of oxygen content (Zeng et al., 2013). Like many non-noble metals, Ni-based catalysts are ideal for pyrolysis catalysts because of their low cost and availability instead of noble metals (Rahemi et al., 2013). Nickle base catalysts were investigated for the deoxygenation process of triglycerides (Zhao et al., 2013).

However, many studies are available on the catalytic degradation of microalgae via pyrolysis processes to optimize biofuel production (Zhao et al., 2021). But still, it is difficult to establish its industrial-scale employment due to the lack of deep understanding of pyrolysis mechanism, conversion rate, the extent of conversion, and kinetic behavior (Ali et al., 2021). Kinetic modeling is a mathematical interpretation of how the reaction mechanism proceeds and determines the kinetic parameters. Typically, two types of kinetic approaches are available to measure the kinetic and thermodynamic parameters to evaluate the reaction path and behavior. One is a model-fitting approach that is based on the reaction mechanism. While the model-free approach is based on different heating rates, it is also called the isoconversional method (Naqvi et al., 2018). Knowing the kinetic and thermodynamic parameters of these reactions during thermal degradation will help design, optimize, and establish process conditions at the commercial level as kinetics help understand the reaction rate. At the same time, thermodynamics gives information about the equilibrium state of the reaction.

Thus, this work investigated the impact of Ni/θ-Al₂O₃ catalyst on the pyrolysis of Parachlorella kessleri HY-6 algae and focused on evolved gas analysis during pyrolysis. Results of catalytic conversion were compared with non-catalytic conversion. Isoconversional and combined kinetic methods are used to study the pyrolytic kinetics of the process. Ni/θ-Al₂O₃ is used at 10, 20, and 30% of the algal biomass. Knowing kinetic and thermodynamic parameters from these reaction models during thermal degradation will help design, optimize, and establish process conditions at the commercial level. The gases that evolved during the process were analyzed to assess the impact of the catalyst on the pyrolysis process.

Materials and Methods

Microalgae Biomass

Microalgae Parachlorella kessleri HY-6 was used in this research. Microalgae biomass was produced using modified Bold’s Basal Medium (BBM) as reported (Wadood et al., 2020)and briefly described here. Composition of BBM follows; 3.386 NaNO₃, 0.170 CaCl₂·2H₂O, 0.304 MgSO₄·7H₂O, 0.054 KH₂PO₄, 0.049 K₂HPO₄, 0.428 NaCl, 0.185 H₃BO₃, EDTA solution (in mM) which contained 0.171 Na₂EDTA·2H₂O, 0.554 potassium hydroxide, ferric solution (in μM) having 17.9 FeSO₄·7H₂O. Trace metals solution (in μM) was composed of 7.28 MnCl₂·4H₂O, 30.7 ZnSO₄·7H₂O, 1.680 Co(NO₃)₂·6H₂O, and 6.290 CuSO₄·5H₂O. The photobioreactor having a capacity of 4.0 L was used to conduct the growth at 25 ± 1°C under the continuous illumination of 60 μmol m⁻² s⁻¹. The aeration flow rate was 2.0 vvm with 3.5% CO₂ in a batch series. The harvest period for microalgal cells was 16 days per batch via centrifugation process under the condition of 7,000 rpm for 4 min. After centrifugation, the sample was rinsed using distilled water and vacuum dried at 105°C. Biomass was ground in a kitchen grinder before its thermogravimetric analysis. Ultimate analysis of biomass was done using the elemental analyzer model Euro Vector EA 3000. Proximate and ultimate analysis of biomass is given in Table 1. ASTM standard methods E1756-08, E1755-01, and D3174-12 were used to obtain moisture, volatile matter, and ash contents, respectively.

TABLE 1

TABLE 1. Ultimate and proximate analyses of Parachlorella kessleri HY-6.

Synthesis of Catalyst

The catalyst (Ni/θ-Al₂O₃) was synthesized using a certain amount of metal precursors (nickel nitrate hexahydrate Ni(NO₃)₂·6H₂O) onto commercial θ-alumina support through the incipient wetness impregnation method. Synthesis was done by adding 5 wt% of nickel precursor at each run to attain the anticipated 20 wt% Ni loading using syringe under vacuum and at room temperature. The paste was dried for 25 h, at laboratory conditions. The catalyst paste was reduced in a gaseous stream of 10% hydrogen in helium at 750°C in a vertical quartz glass tube, placed in a Thermcraft furnace. The detailed preparation is described in our preceding study (Adamu et al., 2018).

Thermogravimetric Analysis and Evolved Gas Analysis of Biomass Under Catalytic and Non-catalytic Conditions

The TGA of microalgae biomass with and without catalyst was carried out under 100 ml/min of nitrogen flow rate and at the temperature range of 50–700°C. Thermogravimetric experiments for kinetics analysis were conducted under three different heating rates, i.e., 5, 10, and 20°C/min using SDT Q600 equipment. During the non-catalytic process, about 7.0 mg of microalgae biomass was positioned in an alumina crucible and heated to 700°C at each heating rate. Under the catalytic process, biomass and catalyst samples were tested at 10% catalyst loadings using the same experimental conditions as non-catalytic processes. The percentage of weight loss and the differential weight loss with respect to temperature data were verified and evaluated in the results and discussion segment. Evolved gases were analyzed by TGA-FTIR as described (Abdul Jameel et al., 2017). Briefly, the evolved gases from TGA passed to IR cell (maintained at 200°C) via a heated line kept at 190°C with a built-in Swagelock 15 μm filter. The evolved gases spectra were recorded by FTIR equipment (Thermo Scientific Nicolet iS10) connected to the iZ10 interface. Pyrolysis reaction for evolved gas analysis was conducted from 25 to 950°C.

Catalyst Characterizations

Surface Properties of the Nickel Catalyst on an Alumina Support

The Micromeritics ASAP 2020 instrument measured surface properties via N₂ as the probe molecule at 77 K. The catalyst sample was vented for 2.5 h at 300°C to eradicate contamination or bound moisture. The N₂ adsorption/desorption experiments were performed to obtain the relevant data to compute the BET pore volume and surface area.

X-Ray Diffraction and SEM Analysis

A Rigaku Miniflex diffractometer functioning at 40 kV and 15 mA was used to inspect the crystalline properties of the Nickel/alumina catalyst. The scanned rate was 5°/min every 0.08° for a 2θ angle of 10°–90° for each run. The Joint Committee on Powder Diffraction Standards (JCPDS) data was applied in identifying the peaks at which the elements and stages exist. SEM image for the catalyst prepared was obtained using MIRA3 TESCAN high-resolution scanning electron microscope. The sample was coated with gold to 5 nm thickness using Ion Sputter Q 150 R S, from Quorum Technologies.

Temperature Program Desorption

NH₃-TPD was done in a Micromeritics’ Autochem II 2920 analyzer to estimate the catalyst’s acidic value and examine the NH₃ desorption kinetics of the catalyst as reported previously (Adamu et al., 2018). Briefly, about 0.10 g of sample was sited in a U-shaped quartz tube reactor. The catalyst was heated to 500°C in steps under reducing conditions (i.e., a gaseous mixture of H₂ and helium in the ratio of 1:9) tracked by a 2 h isothermal degassing under the argon atmosphere. Catalyst samples were then flooded for 1 h with a gaseous mixture of 94.48% helium and 5.52% NH₃ at 50 ml/min to soak the catalyst at 120°C. The reactor was heated to 750°C at the rate of 10 C/min. Ammonium desorption from the surface of the catalyst was watched by a thermal conductivity detector (TCD).

Kinetic Analysis

Under pyrolytic conditions, algal biomass is converted to volatiles and biochar. The following differential rate equation can describe this conversion.

$\frac{\text{dα}}{\text{dt}}=\text{k}\left(\text{T}\right)\text{ f}\left(\text{α}\right)(1)$

where $k\left(T\right)$ and $f\left(\alpha \right)$ are the rate constant and reaction mechanism, respectively. $k\left(T\right)$ can be expanded to show its dependency on absolute temperature $\text{T}$, frequency factor $A$ and activation energy $E_a,$ and the universal gas constant $\mathrm{R}$ as

$\frac{\text{dα}}{\text{dt}}=\text{A}\exp \left(\frac{{\text{E}}_{\text{a}}}{\text{RT}}\right)\text{f}\left(\text{α}\right)(2)$

where $E_a$ is the minimum energy to form an activated complex, whereas the frequency factor $A$ accounts for a minimum number of collisions in a specific direction to cause a reaction.

Isoconversional Methods

Various analytical solutions of Eq. 2 in differential and integral forms are used to describe the kinetics of the conversion. Friedman and Kissinger-Akahira-Sunose (KAS) are commonly used differential and integral isoconversional solutions of the rate equation.

$\text{Friedman:}\ln \left(\frac{\text{dα}}{\text{dt}}\right)=\text{const}-\frac{{\text{E}}_{\text{a}}}{\text{RT}}(3)$

$\text{KAS:}\ln \left(\frac{\text{β}}{{\text{T}}^2}\right)=\text{const}-\frac{{\text{E}}_{\text{a}}}{\text{RT}}(4)$

Estimation of the minimum energy for a conversion ${\text{E}}_{\text{a}}$ is obtained from the linear regression.

Combined Kinetics

Combined kinetics offers a linearized rate equation for a single-step reaction. The reaction mechanism is described by a generalized form $\text{f}\left(\text{α}\right)=c{\left(1-\text{α}\right)}^{\text{n}}{\text{α}}^{\text{m}}$ where c, n, and m are parameters obtained by maximizing R² for the linear equation.

$\ln \left[\frac{\text{dα}/\text{dt}}{{\left(1-\text{α}\right)}^{\text{n}}{\text{α}}^{\text{m}}}\right]=\ln \left(\text{cA}\right)-\frac{{\text{E}}_{\text{a}}}{\text{RT}}(5)$

Thermodynamic of Activation

The following correlations can describe thermodynamic parameters, such as changes in enthalpy, Gibbs free energy, and entropy.

$\Delta H=E_a-R{T}_P(6)$

$\Delta G=E_a+R{T}_P\ln \left(\frac{k_B{T}_P}{hA }\right)(7)$

$\Delta S=\frac{\Delta H-\Delta G}{T_P}(8)$

where $T_P$ is the temperature corresponding to maximum conversion rate whereas $k_B$ and $h$ are the Boltzmann and Planck constants, respectively. Activation energy in Eqs 6, 7 are estimated from the combined (net) reaction kinetics.

Results and Discussion

Biomass Characterization

Results on ultimate and proximate analysis of biomass are summarized in Table 1.

Sufficient hydrocarbons and volatile matters coupled with lower moisture content indicate better quality and high yield of biofuels. Similarly, lower sulfur content is desirable from the environmental point of view. Nitrogen is an element of protein that is a major component of microalgal biomass. Besides these major components of microalgae biomass, a sufficiently higher amount of metals (K, Na, Mg, Ca) is also present due to growth media. The presence of high metal contents makes the microalgae biomass unique from lignocellulosic biomass. These metals can act as a catalyst as well during the pyrolysis process (López-González et al., 2014). Pyrolysis of microalgae is a complex process than lignocellulosic biomass because of the presence of various proteins, carbohydrates, lipids, and pigments along with metals.

Catalyst Characterization

Figures 1A,B shows the N₂–adsorption desperation isotherms and the corresponding pore size distribution of the catalysts. The BET surface area, total pore volume, and average pore diameter are given in Table 2. The BET surface area is 65.63 m²/g. According to the IUPAC classification, the nitrogen adsorption-desorption isotherms for the catalyst show a typical type IV isotherm with H₃ hysteresis loop (observed in P/Po 0.83–0.95), which indicated that the sample has a mesoporous texture with slit-like spores. The pore volume of the prepared catalyst is 0.52 cm³/g. It is believed that larger pore volumes tend to help store more reactants, and interconnected porous networks facilitate the transportation of reactant molecules and products, resulting in higher reaction rates (Khan et al., 2020).

FIGURE 1

FIGURE 1. N₂ adsorption-desorption (A) BJH desorption branch pore size distribution (B).

TABLE 2

TABLE 2. Physical properties and surface chemical composition of Ni/θ -Al₂O₃ pristine catalyst.

Figure 2 shows the XRD patterns of θ-Al₂O₃ support and Ni/θ-Al₂O₃ supported catalysts. The peaks at theta values of 20.2, 32.8, 36.7 38.9, 46.2, 66.9 correspond to (201), (400), (202), (111), (311), (512) planes of pure θ-Al₂O₃ depending on the intensities of planes to corresponding 2θ values (JCPDS Card No. 35–0121). The peaks at 24.2, 33.8 37.7 45.0, and 59.6 assigned to the (005), (101), (015), (018), and (110) planes are in good agreement with the planes of α-nickel hydroxide (JCPDS 380715) (Sánchez-De la Torre et al., 2013). The 2θ values of 44.09°, 51.70°, and 76.09° are associated with the (111), (200), and (220) lattice planes of nickel (PCPDF-04–0850). Unidentified peaks could be due to the presence of nickel-aluminate (Jiménez-González et al., 2013).

FIGURE 2

FIGURE 2. XRD of θ-Al₂O₃ and Ni/θ-Al₂O₃.

One the main challenges with the application of nickel-based catalyst in thermo-catalytic conversions of biomass is their fast deactivation due to sintering, coking and morphological changes especially during catalyst preparation or during reaction above 600°C (Adamu et al., 2018). Therefore we conducted SEM to ascertain the morphology of the of the nickel particles on the alumina support after preparation. Supplementary Figure S1 shows the SEM image of the catalysts, with highly dispersed nickel species on the surface of the catalyst. Since the catalyst was calcined at 750°C, it will be very stable against sintering during the biomass pyrolysis at the present reaction conditions. In particular, two types of nickel particles were observed on the theta-alumina surface. One type consist of approximately spherical particles with slight deformation and relaxed contours, connected by successive metal impregnation and annealing protocol (Adamu et al., 2019). These grains belong to NiO particles, while the much smaller sized particles represent the Ni nanoparticles (Sánchez-De la Torre et al., 2013). The observed stable (post-calcination) morphology suggest that sufficiently strong metal-support interaction was achieved due to the synthesis protocol adopted for the Ni/Al₂O₃ catalysts which was aimed to tackle sintering, as well as carbon deposition (Goula et al., 2015; Adamu et al., 2017).

Thermogravimetric Analysis

The pyrolytic conversion of Parachlorella kessleri HY-6 is described by three stages in Figure 3. Differential thermogravimetric (DTG) and conversion rate at three different heating rates (5,10,20 C/min) and three different catalyst loadings (10, 20, 30 wt%) are tested. The higher heating rate increased the conversion rate. However, due to heat and mass transfer hindrance, the peaks shifted to higher temperatures with increasing heating rates without affecting the shape or reaction mechanism.

FIGURE 3

FIGURE 3. (D) TG curves of Parachlorella kessleri HY-6, (A) without catalyst, (B) with 10% catalyst, (C) with 20% catalyst, and (D) with 30% catalyst.

Most of the moisture was removed below 150°C during non-catalytic and catalytic pyrolysis. The primary degradation occurred in the temperature range of 150–500°C, resulting in a significant mass loss which is also shown by the high content of volatile matters. In this temperature range, at least three peaks are convoluted in the form of overlapping conversions, indicating complex interactions and degradations. Weight loss due to the decomposition of these structural microalgae components occurs between 200–500°C (Vuppaladadiyam et al., 2019). Carbohydrates and part of proteins, and lipids decomposed through devolatilization during the main pyrolysis step (Xu et al., 2020). Carbohydrates, proteins, lipids, and pigments undergo severe cracking in the active pyrolysis zone. As a result, various hydrocarbons, oxygen-rich, and nitrogen-containing organic compounds are formed alongside simpler non-condensable gases such as CH₄, CO, CO₂, H_2, etc.

The last stage is at > 500°C where mineral matter and carbon material are decomposed further. The effect of catalytic activity is evident from an additional peak between 200–300°C. Weight loss was almost stable after 600°C during non-catalytic pyrolysis of biomass. However, weight loss around 800°C was observed during catalytic pyrolysis. The residual solid mass remained at 15.9 and 19.24% at the end of non-catalytic and catalytic pyrolysis, respectively. Different decomposition patterns during catalytic pyrolysis might be due to heat and mass transfer limitations in the presence of the catalyst and its catalytic activity. At a higher catalyst dose, the left and right shoulders around the main peak almost disappeared. The corresponding increased conversion rate can be seen at a catalyst dose of 30%, reaching a peak maximum conversion rate of 0.00369 s⁻¹ at 20°C/min.

Pyrolysis of microalgae is a complex process due to various types of constituent lipids, proteins, and carbohydrates. Each constituent of microalgae has a different pyrolysis mechanism. The presence of all three in one system makes the process very complicated. Lipids are made of triglycerides (TAG), phosphor- and glycolipids (Farooq et al., 2013), and their relative amount depends on the growth conditions (Farooq, 2021). They decomposed through decarboxylation, decarbonylation, and fragmentation process. Lipids decomposed at 200–450°C. Proteins are made of amino acids and decomposed via dehydration, deamination, and decarboxylation reaction. Carbohydrates are decomposed by dehydration, glycosidic bond cleavage, and rearrangement (Wang et al., 2017). Generally, microalgae decomposed between the temperature range of 200–450°C. The temperature of the maximum mass loss peaks followed the order: lipid > protein > carbohydrate (Wang et al., 2017).

Isoconversional Kinetics for Activation Energy Measurement

Friedman and KAS methods were used to estimate the activation energy profiles of Parachlorella kessleri HY-6 during pyrolytic conversion. Average activation energy without catalyst was recorded at 253.54 (±58.81)kJ/mol from the Friedman method and 241.91 (±53.05) kJ/mol from the KAS method. Below 0.2 and above 0.8 are considered uncertain conversion ranges because of heterogeneity and therefore excluded from the kinetic analysis. Table 3 summarizes the activation energies of various microalgae during pyrolysis estimated from different isoconversional methods without any catalyst.

TABLE 3

TABLE 3. Isoconversional activation energies of different microalgal species during pyrolysis without catalyst published during 2018–2020.

With the addition of 10% catalyst, the mean activation energy dropped to 202.88 (±30.95) kJ/mol as estimated from the Friedman method and 186.40 (±43.46) kJ/mol from the KAS method. At 20% of the catalyst dosage, the average activation energy remained at 214.75 (±30.95) kJ/mol as determined by the Friedman method and 190.81 (±49.75) kJ/mol from the KAS method. With 30% of catalyst dose, the activation energy decreased further to 186.29 (±16.42) kJ/mol for the Friedman method and 166.81 (±30.54) kJ/mol for the KAS method. The recorded average activation energy was least at 30% less with catalyst dosage.

Figure 4 indicates the evolution of activation energy along the conversion path of Parachlorella kessleri HY-6 pyrolysis. Significant changes can be observed at lower and higher conversions. They were showing that the catalyst is effective at the lower as well as at the elevated temperature. The middle section seemed barely affected by the catalyst.

FIGURE 4

FIGURE 4. (A) Friedman, and (B) KAS activation energy profiles of Parachlorella kessleri HY-6 pyrolytic conversion with and without catalyst.

Combined Kinetics

Linearized rate law uses a generalized model function to retrieve the kinetic parameters through the maximization of R² under the assumption of a single reaction. Figure 5 compares the kinetic parameters of the pyrolytic conversion of Parachlorella kessleri HY-6 with and without catalyst determined via combined kinetics. Kinetic parameters obtained from the combined kinetic analysis elaborated the thermodynamic characteristics of the conversion as given in Table 4.

FIGURE 5

FIGURE 5. Combined kinetics of Parachlorella kessleri HY-6, (A) without catalyst, (B) with 10% catalyst, (C) with 20% catalyst, and (D) with 30% catalyst.

TABLE 4

TABLE 4. Kinetic and thermodynamic parameters of pyrolytic conversion of Parachlorella kessleri HY-6.

Positive ∆H and ∆G values indicate that the conversion of Parachlorella kessleri HY-6 under pyrolytic conditions with or without a catalyst is endothermic and non-spontaneous. However, the difference in E_a and ∆H is below 5 kJ/mol, which shows that a small amount of energy in excess is needed to form an activated complex. Hence, the conversion can be considered favorable.

Evolved Gas Analysis by TGA-FTIR

The relative amount of pyrolysis products (solid, liquid and gas) is affected by the composition of raw material, pyrolysis process and its experimental conditions such as temperature, heating rate, catalyst, type of catalyst, and gas flow rate. TGA coupled with FTIR is a helpful technique for analyzing the composition of evolved gases. Microalgae biomass decomposed via dehydration and depolymerization of carbohydrate fraction and produced various low molecular weight products such as CO, CO₂, aldehyde, ketones, and alcohols at the temperature range of 150–300°C (López-González et al., 2014). Their characteristic bands from FTIR help to identify the primary compounds. The maximum concentration of evolved gases given by the normalized Gram-Schmidt (GS) curve coincides with the DTG curve. Normalization was carried out by dividing the Gram-Schmidt intensity with the mass of the sample without catalyst. The delay in the peak of the GS curve compared to the DTG is due to the travel time of gases from the pyrolysis reactor to the analyzer (Abdul Jameel et al., 2017). The GS shows the different decomposition mechanisms during catalytic pyrolysis compared to non-catalytic. However, a detailed investigation of liquid and solid fractions is required to elucidate the pyrolysis mechanism, especially for catalytic pyrolysis.

The decomposition of microalgae biomass (MB) without a catalyst is shown in Figure 7 for three different decomposition steps during pyrolysis.

Figure 7A shows the IR spectra for the dehydration step, which is dominant at a temperature <200°C. The bands around 1,500 cm⁻¹ and 3,500–4,000 cm⁻¹ showed bending and plane stretching of -OH groups. The results are consistent with reported results for non-catalytic pyrolysis of microalgae (Marcilla et al., 2009). The IR spectra of the evolved gases during the second and most important decomposition step are given as three overlapping peaks at 291, 326, and 461°C in Figure 6B. It showed a complex degradation process. This figure showed different chemical bonds or functional groups via absorbance of vibrational modes of each functional group at the corresponding temperature vs wavelength (Marcilla et al., 2009). The majority of vibrational modes contributed to the bands at 1800–1,200 cm⁻¹. Water is still being produced at 3,500–4,000 cm⁻¹. Strong stretching on the C=O band appeared at 1870–1,540 cm⁻¹ could be the carbonylic compounds like aldehyde etc. The band at 1,700–1,800 cm⁻¹ represents the esters. Bands at 2,900–2,800 cm⁻¹ described vibration of C-H bands as methyl group and methylene groups.

FIGURE 6

FIGURE 6. Thermogravimetric curves for decomposition of microalgae during non-catalytic (A) and catalytic (B) pyrolysis.

Higher CO₂ and water formed during the last stage of pyrolysis of microalgae. Microalgae decomposition in the presence of Ni/θ-Al₂O₃ catalyst is given in Figure 7 (a',b' and c'). The IR scan of evolved gases during catalytic pyrolysis significantly changed during the second and main decomposition stage. The evolution of water is minor during the first and second stages compared to the non-catalytic process. The intensity of all gases during the last and second stages is also reduced during the catalytic process. These patterns showed a different reaction mechanism than non-catalytic pyrolysis. It can be seen from the spectra that the release of the gases takes place at all temperatures. Release rate of CO₂ was intermittently in the temperature ranges of 300–450°C and 650–950°C. Absorption peaks in the range of 4,000–3,500 cm⁻¹ and 1,600–1,300 cm⁻¹ were due to the release of H₂O from the decomposition of oxygen-containing compounds (Liu et al., 2008; Wang et al., 2013). The absorption peak between 3,100 and 2,675 cm⁻¹ indicated the emission of CH₄.

FIGURE 7

FIGURE 7. IR spectra of evolved gases from catalytic and non-catalytic pyrolysis of microalgae. Dehydration step (A) (a and a’), main decomposition (B) (b and b’), solid residue decomposition (C) (c and c’).

The release of methane was mainly from the cracking of the methoxy (–O–CH₃) group-containing compounds (Yang et al., 2007). The peaks around 2,400–2,240 cm⁻¹ and 680–660 cm⁻¹ are the characteristic peaks of CO₂, which are mainly produced from the degradation of carbonyl (C=O) and carboxyl (COOH) containing compounds. The peaks between 2,240 and 2060 cm⁻¹ were because of the CO emission, resulting from the cracking of organic compounds containing ether (C–O–C) and carbonyl (C=O) functional groups (Yang et al., 2007). The absorption band in the wavenumber range of 1,900–1,600 cm⁻¹ can be associated with the release of aldehydes (Gong et al., 2020). The evolved gases from the pyrolysis of MB were water, CH₄, CO₂, CO, and -HCO. According to the peak intensity, since the intensity of the peaks corresponds to the concentration of the gases (Fan et al., 2020), the concentration of evolved gases is in the descending order as H₂O > CH₄ > CO₂>-CHO > CO. Gas yield increased with temperature due to secondary cracking of char and pyrolysis vapors (Hu et al., 2013). The presence of Ni in the catalyst was reported to decrease the yield of water and tar (Lu et al., 2020). Gas yield during the pyrolysis of biomass depends on temperature and nature of catalyst as well (Durak, 2016).

Figure 8 shows the intensity of evolved gases during the pyrolysis of microalgae biomass at 10 wt% Ni/θ-Al₂O₃ catalyst. The intensity of peaks in Figure 8B at wave no. 1,000–1,500 and 3,000–3,600 cm⁻¹ are reduced compared to non-catalytic pyrolysis, as shown in Figure 8A. Relative contents of evolved gases are different for both non-catalytic and catalytic pyrolysis, as shown in Figures 8C,D, and the significant portion of evolved gas composed of CO₂. The increase in CO₂ content could be primarily from the cracking of oxygenated organic compounds at higher temperatures (Abhijeet et al., 2020). The comparison of Figure 8D with Figure 8C clearly showed the variation in the composition of evolved gases as evidence of the catalytic activity of the catalyst. The intensity of all gases was lower during catalytic pyrolysis at all temperatures. CO₂ yield decreased during catalytic pyrolysis along with the composition of all the other gases. The increase in CO₂ content at higher temperatures could be primarily from the cracking of oxygenated organic compounds. Gas formation increased in the presence of a catalyst at a higher temperature, as reported earlier (Aysu et al., 2016). The likely reason for increase in CO₂ and CH₄ might be char gasification and reforming reactions (Vuppaladadiyam et al., 2019).

FIGURE 8

FIGURE 8. Evolved gases intensity during pyrolysis of microalgae biomass (MB) during non-catalytic (A–C) and catalytic pyrolysis (B, D) using TGA-FTIR (A, B) 3D FTIR spectra, (C, D) Gases evolved at different temperatures.

CO₂ decreased first and then increased at a temperature higher than 750°C compared to the non-catalytic process (Chattopadhyay et al., 2011). Secondary decomposition reactions become dominant at temperature >600°C leading to higher gas yield. Moreover, the presence of inorganic (K, Na, Mg and Ca, etc.) facilitated the carboxylation reaction and enhanced the CO₂ production during the decomposition of carbohydrate (Aysu and Sanna, 2015). Figure 8 shows a clear difference between catalytic and non-catalytic pyrolysis. However, the mechanism of pyrolysis cannot be described by variation in evolved gas analysis. Variation in solid residue and the liquid fraction must also be monitored along with gases. During pyrolysis, solid, liquid, and gas yield depends on temperature, heating rate, active metal, and nature of catalytic support (Kar et al., 2019). Alumina tends to form more char due to its acidity. At higher temperature, the bio-oil and char products are converted to gas product because of secondary cracking reactions (Durak et al., 2019). However, at this point, the unavailability of data on solid and, mainly, liquid fraction is the limitation of this work that will be addressed in future studies.

Conclusion

This study reports the application of Ni/θ-Al₂O₃ catalyst during the pyrolysis of Parachlorella kessleri HY-6. Kinetic analysis shows the efficacy of Ni/θ-Al₂O₃ in lowering the mean activation energy of Parachlorella kessleri HY-6 during pyrolysis. The influence of the catalyst at lower and higher conversion was significant. Thermodynamic analysis indicates the process’s non-spontaneity and endothermicity; the conversion was still favorable with less than 5 kJ/mol of energy required to form activated complexes. FTIR analysis of evolved gases along with TGA profile showed that microalgae degrade in three important steps. The catalyst was active in accelerating the decomposition process by lowering the activation energy. The composition of the evolved gases differed with temperature and the presence of a catalyst during the pyrolysis process.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

WF and SR write and conceptualize the work. IA obtained the funding and did kinetics analysis. MS and HA did TGA and gas analysis. SA synthesizes the catalyst. All authors reviewed the manuscript.

Funding

The authors extend their appreciation to the Deputyship for Research and Innovation, Ministry of Education, Saudi Arabia for funding this research work through the project number (IFPIP:778-829-1442) and King Abdul Aziz University, DSR, Jeddah, Saudi Arabia.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenrg.2021.775037/full#supplementary-material

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