Full Length ArticleBioethanol production from acid pretreated microalgal hydrolysate using microwave-assisted heating wet torrefaction
Graphical abstract
Introduction
Recent research towards the development of alternative energy is receiving more attention due to the increase in worldwide population and industrialization [1], [2]. Besides, further issues such as global warming, increasing fuel demand, and the depletion of fossil fuel sources are promoting the invention of a renewable and sustainable green alternative fuel with minimal greenhouse gas emissions [3], [4], [5], [6]. With renewable characteristics, biomass is one of the alternative sources in the production of bioenergy and biofuel to meet the worldwide energy demand [7], [8], [9], [10], [11], [12], [13]. Microalgae have been considered as one of the significant third-generation biomass resources in the production of solid and liquid biofuels such as biochar, biodiesel, bio-oil, and bioethanol for energy application [14], [15], [16], [17]. The characteristics of microalgae with high carbon fixing efficiency which is beneficial towards the mitigation of greenhouse gas emissions and the high biomass productivity with simple cultivation showed its provision as a renewable feedstock for biofuel production [18], [19], [20].
Bioethanol is expected to be the most widely used biofuel around the world based on the utilization of renewable energy resources and environmental benefits from the conversion of starch-rich biomass sources such as maize and sugarcane [1], [21], [22]. Bioethanol shows its importance as an alternative fuel to blend with the current petroleum liquid fuel for the application in transportation and industry for engine combustion [23], [24], [25]. Other than that, the application of bioethanol can be seen in power co-generation systems, fuel cells, electric power generation, and in the chemical industry as the raw chemical and enhancers [26], [27]. With the recognition of bioethanol as a sustainable green fuel [28], [29], global production is in rapid growth and expected to achieve breakthrough production by the year 2020 [30]. Hence, the advancement of the pretreatment methods and technologies can be looked into to meet the rapid growth in bioethanol production [31], [32], [33]. When one is concerned with the feedstock for bioethanol, microalgal biomass with no food conflict issues is a potentially suitable candidate for the production of bioethanol [31], [34], [35].
Thermochemical conversion by torrefaction is one of the recent promising techniques in bioenergy production [36], [37], [38]. Torrefaction is a pretreatment thermochemical process that occurs at a low-temperature range from 150 to 300 °C under the controlled pressure in an inert environment, where the process can be divided into two categories, namely, dry torrefaction and wet torrefaction, based on the operating parameters and media conditions [15], [39], [40]. Wet torrefaction with its high product yield using lower energy can be employed to perform acid hydrolysis pretreatment in the production of bioethanol fuel as the sustainable green energy [41], [42]. The previous study on microwave-assisted acid hydrolysis using sago pith waste showed some good energy saving in the production of bioethanol [43]. The study of microwave-assisted acid hydrolysis on bode (Styrax tonkinensis) woody biomass also presented the glucose production for bioethanol production [44]. There was also a recent study that elucidated the potential of isoflavone aglycones, α-glucosidase and α-amylase inhibitory activities from soybean to study the nutraceutical values using microwave-assisted acid hydrolysis [45]. Also, microwave-assisted acid hydrolysis has been employed in the production of sugar from Jabon kraft pulp [46]. However, up to date, there is limited literature on the study of pretreatment using dilute acid on microalgal biomass via microwave-assisted heating wet torrefaction towards the production of bioethanol.
Due to the reason described above, this study focused on microalgal bioethanol production from the co-production of solid biochar and liquid hydrolysate from microwave-assisted heating wet torrefaction using dilute acids towards a sustainable green technology. This study also aimed to determine the effects of the pretreatment with acids on microalgal biomass through wet torrefaction towards the production of reducing sugars and the formation of reducing sugar by-product in the liquid hydrolysate. The fermentation process utilizing the microalgal hydrolysates produced was carried out to determine bioethanol production. In addition, the total reducing sugar content in hydrolysates was determined throughout the fermentation process to study the ethanol yield and efficiency for future optimization and application. Attention was also paid to the acid pretreatment using dilute acids for feasible bioethanol production with good reducing sugar content in the microalgal hydrolysate. It was concluded that microwave-assisted heating wet torrefaction for acid pretreatment could be one of the green technologies towards the co-production of bioethanol and biochar for future applications in alternative energy production.
Section snippets
Experimental materials
The dried microalgal biomass of Chlorella sp. GD and Chlorella vulgaris ESP-31 species were provided as described in the previous study [47], [48]. The diluted acids used for the pretreatment wet torrefaction were from the dilution of concentrated sulfuric acid (H2SO4, CAS: 7664-93-9, Merck) to 0.1 and 0.2 M. The microorganism species used for the fermentation process was Saccharomyces cerevisiae (S. cerevisiae) Type II yeast from Sigma-Aldrich. The yeast extract (DifcoTM YM Broth), bacterial
Components of microalgal biomass
The raw Chlorella vulgaris ESP-31 consisted of a large portion of carbohydrate content (57.50%) with components of protein (18.30%), lipid (15.38%), and other remaining components (8.82%). However, a large portion of protein content (59.75%) with components of carbohydrate (8.64%), lipid (7.86%), and other remaining components (23.75%) were found in the raw Chlorella sp. GD. Compared to other studies [60], [61], microalga C. vulgaris ESP-31 showed a slightly high carbohydrate component where
Conclusions
This study has demonstrated the feasible bioethanol production from microalgal hydrolysates on microwave-assisted heating wet torrefaction using dilute acids. The carbohydrate-rich microalga Chlorella vulgaris ESP-31 showed a better performance in comparison with Chlorella sp. GD where higher carbohydrates content produced higher reducing sugar and this aids in the fermentation for bioethanol production. Microalgal hydrolysate obtained after the pretreatment consisted of a total reducing sugar
CRediT authorship contribution statement
Kai Ling Yu: Data curation, Formal analysis, Writing - original draft. Wei-Hsin Chen: Funding acquisition, Conceptualization, Project administration, Supervision, Resources, Validation, Writing - review & editing. Herng-Kuang Sheen: Resources, Validation. Jo-Shu Chang: Resources, Validation. Chih-Sheng Lin: Resources, Validation. Hwai Chyuan Ong: Funding acquisition, Conceptualization, Project administration, Supervision, Resources, Validation, Writing - review & editing. Pau Loke Show:
Acknowledgments
This study is supported by the University of Malaya, Malaysia under the SATU Joint Research Scheme with the grant number of ST023-2019 and partnership grant with National Cheng Kung University (NCKU), Taiwan, under the grant number of RK002-2019. This research is also supported by the Higher Education Sprout Project, Ministry of Education to the Headquarters of University Advancement, Taiwan at NCKU. The financial support from the Ministry of Science and Technology, Taiwan, R.O.C., under grant
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