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.

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).

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.

Under pyrolytic conditions, algal biomass is converted to volatiles and biochar. The following differential rate equation can describe this conversion. dα dt = k ( T )  f ( α ) (1) where k ( T ) and f ( α ) are the rate constant and reaction mechanism, respectively. k ( T ) can be expanded to show its dependency on absolute temperature T , frequency factor A and activation energy E a , and the universal gas constant R as   dα dt = A exp ( E a RT ) f ( α ) (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.

The following correlations can describe thermodynamic parameters, such as changes in enthalpy, Gibbs free energy, and entropy. Δ H = E a − R T P (6) Δ G = E a + R T P ⁡ ln ( k B T P h A   ) (7) Δ S = Δ H − Δ 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 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.

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.

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).

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.

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.

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).

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.