• Vol 10, No 6 (2019)
  • Mechanical Engineering

Thermogravimetric Analysis on Combustion Behavior of Marine Microalgae Spirulina Platensis Induced by MgCO3 and Al2O3 Additives

Sukarni Sukarni, Sumarli Sumarli, Imam Muda Nauri, Ardianto Prasetiyo, Poppy Puspitasari

Corresponding email: sukarni.ft@um.ac.id


Cite this article as:
Sukarni, S., Sumarli, S., Nauri, I.M., Prasetiyo, A., Puspitasari, P., 2019. Thermogravimetric Analysis on Combustion Behavior of Marine Microalgae Spirulina Platensis Induced by MgCO3 and Al2O3 Additives. International Journal of Technology. Volume 10(6), pp. 1174-1183
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Sukarni Sukarni -Department of Mechanical Engineering, State University of Malang, Semarang Street 5, Malang 65145, Indonesia -Centre of Advanced Materials for Renewable Energy, State University of Malang, Semarang
Sumarli Sumarli Department of Mechanical Engineering, State University of Malang, Semarang Street 5, Malang 65145, Indonesia
Imam Muda Nauri Department of Mechanical Engineering, State University of Malang, Semarang Street 5, Malang 65145, Indonesia
Ardianto Prasetiyo Master Program of Mechanical Engineering, Graduate School, State University of Malang, Semarang Street 5, Malang 65145, Indonesia
Poppy Puspitasari -Department of Mechanical Engineering, State University of Malang, Semarang Street 5, Malang 65145, Indonesia -Centre of Advanced Materials for Renewable Energy, State University of Malang, Semarang
Email to Corresponding Author

Abstract
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The impact of MgCO3 and Al2O3 additives on the thermal behavior of Spirulina platensis (SP) biomass during combustion in a thermal analyzer was evaluated to understand their catalytic effect in the decomposition process. The samples were pure SP and a mixture of SP and additives at mass fractions of 3, 6, and 9 (wt,%). Each sample of around 8.5 mg was mounted in a thermobalance and subjected to a furnace on a heating program of 10oC/min. The 100 ml/min air atmosphere was kept continuously flowing during the combustion process from 30–1200oC. The thermal behavior of the sample was then characterized from the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves, those were recorded by a computer during the experiment. The Horowitz–Metzger method was used to evaluate the impact of additives on the kinetic parameters of the samples. The results indicated that the presence of additives shifted the main decomposition stage toward a lesser temperature. The rate of mass loss (ML) in the main decomposition zone decreased in the 1st peak and increased in the 2nd, in accordance with the increase in the fraction of additives. This indicates that additives play different roles during the decomposition process. The mass mean activation energy (Em) increased at the additive fraction of 3% for both MgCO3 and Al2O3, as well as at 6% MgCO3 compared to combustion with no additives. Conversely, the presence of greater additives promoted a shift in Em toward smaller values. These results confirm that both additives significantly influenced the thermal behavior and kinetics of the SP combustion.

Additives; Combustion; Microalgae; Spirulina platensis; Thermogravimetry

Introduction

Global energy demand is estimated to show a continuous increase of around a third by 2040 (BP Energy Outlook, 2018); conversely, there is a rapid depletion of fossil-based reserves (Sukarni et al., 2017). This situation has prompted the exploration of alternative energy sources, and biomass has received increasing interest (Purwanto et al., 2015; Ghatak & Mahanta, 2017; Wahyudi et al., 2018). Among the various biomass-based fuels, aquatic microorganisms have attracted attention due to their extensive spread across the globe, high efficiency during the photosynthetic process, ability to rapidly multiply their biomass,  non-dependence on land, thus  creating no interference with food production, automatic reproduction process, high capacity for transforming CO2 during the growth phase to become biomass, and their low sulfur and nitrogen content (Sukarni et al., 2014; Gai et al., 2015).

Various techniques for converting microalgae to energy have previously been performed, including biochemical conversions such as digestion (Passos et al., 2014) and fermentation (Hossain et al., 2015), as well as thermochemical conversions such as pyrolysis, gasification, liquefaction, and combustion (Sanchez-Silva et al., 2013; Sukarni et al., 2015, 2018a, 2018b; Viju et al., 2018). Combustion is the most important choice owing to the ease of transferring heat to energy using currently available technological devices. The combustion method accounts for over 97% of the world’s bioenergy production (Demirbas, 2004).

Several studies on microalgae direct combustion have been performed previously. Combustion of Nannochloropsis oculata (Sukarni et al., 2015), Nannochloropsis gaditana (Sanchez-Silva et al., 2013), Chlorella vulgaris (Chen et al., 2011), and Scenedesmus almeriensis (López et al., 2013) revealed the diverse range of thermal behavior among the species. These differences in their thermal behavior were strongly thought to derive from variances in their organic matter and the mineral compounds that made up the compartment of each species. Differences in the major components of algal biomass, i.e., protein, carbohydrate, and lipid, affect thermal behavior because each component has a different thermal degradation. From the different literature, it was also understood that the mineral content of algal biomass highly determined its ash content (Chen et al., 2014) and that this would significantly affect its decomposition behavior due to its catalytic effect during thermal degradation (Gai et al., 2015). However, the catalytic effect of each mineral compound on microalgal combustion has yet to be reported by scholars.

This paper presents the catalytic effect of MgCO3 and Al2O3 additives on the thermal behavior of Spirulina platensis (SP) biomass during oxidative-thermal degradation under thermal analyzer equipment. The characteristic temperatures were determined according to the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves. The Horowitz–Metzger fitting method was applied to evaluate the kinetic parameters in terms of activation energies, frequency factors, and reaction orders.


Conclusion

The catalytic effects of MgCO3 and Al2O3 in the decomposition process of Spirulina platensis during combustion have been studied using a thermal analyzer on a heating program of 10oC/min. In the main decomposition and combustion zone, it was found that the presence of MgCO3 and Al2O3 inhibited the thermal degradation of carbohydrates and protein, and slowed down their rate of thermal degradation. Increasing the mass ratio of MgCO3 and Al2O3 up to 6% induced severe thermal degradation of lipids. The addition of MgCO3 and Al2O3 up to a 6% mass ratio had a positive impact on microalgal thermal conversion mainly at temperatures above 200 oC and below 340oC, whereas the addition of 9% of both additives was not recommended as they made little contribution to enhancing the thermal conversion process. The presence of additives led to an increase in activation energies of up to 4.83 kJ/mol, thus indicating that the presence of additives might increase the entire reaction time. Similarly, the additives influenced the increasing frequency factors. The change in both kinetic parameters (activation energies and frequency factors) confirmed that the thermal conversion dynamics of the material had also been changed due to the presence of additives.

Acknowledgement

This research is supported by the Directorate of Research and Community Services, Ministry of Research, Technology, and Higher Education, Government of Indonesia under the scheme of “Penelitian Dasar Unggulan Perguruan Tinggi” 2019, with grant number 19.3.82/UN32.14.1/LT/2019.

Supplementary Material
FilenameDescription
R1-ME-3611-20191023063115.jpg 04. Figure 1 TG-DTG curves of the SP and its mixture
R1-ME-3611-20191023063203.jpg 05. Figure 2 Magnification of TG-DTG curves
R1-ME-3611-20191023063500.jpg 06. Figure 3 The incremental mass loss after addition of additives into the SP
R1-ME-3611-20191023063820.jpg 07. Figure 4 The Horowitz–Metzger parameters
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