Integrated microalgal biorefinery – Routes, energy, economic and environmental perspectives

Highlights

• Technological progress in microalgal fuels is yet to attain commercial feasibility.

• Integration of different biofuel productions is inevitable for cost effectiveness.

• Increased energy outputs (MJ kg⁻¹) 15.4 and 18.8 were attained on integration.

• Integration resulted in cost reduction by 60% in biomass production for biodiesel.

• Integrated production of fuel and bioproduct seems attractive and lucrative.

Abstract

Commercialization of microalgal biofuels is not yet attained even after plentiful of research and extensive scientific projects. Expending the cost and energy for producing single microalgal product proven to be unviable, and hence integrating multiple product generation from single batch of biomass was considered as effective. This review focusses on delineating the challenges associated with individual fuel production pathways and merits of integrating different fuel production pathways. The advantages of integrated microalgal biorefinery have been summarized along with energy output and economic impact. Integrated production of different biofuels enhances the energy output significantly (biodiesel + biocrude oil - 18.8 MJ kg⁻¹, biodiesel + bioethanol - 15.4 MJ kg⁻¹, biodiesel + biogas - 13.4 MJ kg⁻¹) and reduced the cost of biomass production by about 60% and 40% for biodiesel and other fuel productions, respectively. In addition, the integrated production of bioproducts with fuels also emphasized in the present study.

Introduction

Ceaseless rise of global energy demand and severe environmental degradation are the two inexorable issues associated with conventional fuel usage which dictate the urge for finding alternative energy resources (Branco-Vieira et al., 2020). For almost three decades, biofuel research receives paramount importance along with eventual growth and development (Mathimani et al., 2019). Microalgae offer a significant space for the promising production of biofuels and other value-added products (Deviram et al., 2020; Raheem et al., 2018). The prominence of microalgae-based fuel production heavily relies on three aspects which include theoretical possibility for profitable production, environmental sustainability with reduced carbon footprints and value rich quality for complete utilization (Lakshmikandan et al., 2020; Sun et al., 2020). On considering the first aspect, microalgae hold the supremacy in areal lipid productivity than any other terrestrial crops and enhancing the tendency of lipid accumulation is an added attractive feature (Remmers et al., 2018). The second aspect deals with mitigating the adverse environmental effects such as CO₂ emission control, CO₂ capture and wastewater treatment (Barati et al., 2021; de Assis et al., 2019). Generating microalgal biomass using wastewater as nutrient medium or flue gas as source of CO₂ are comprehensively advantageous in terms of mitigating wastes and emissions leading to sustainable environment. Life cycle assessment (LCA) study stating that microalgal biomass production under flue gas could contribute to positive impact on greenhouse gas (GHG) emission reduction (75%) (Yadav et al., 2020). The third aspect of microalgae-based fuel production is charging towards complete utilization of microalgal biomass to achieve zero wastes approach. For instance, it was suggested that isolating particular fraction for different bioenergy product seems costlier, and inevitable requirements for cost-effective technologies are needed especially for extracting the biocomponents (protein, lipids and carbohydrates) (Vermuë et al., 2018). These three promising aspects of microalgae show the unleashing potential of microalgae which supports its continual exploration for fuel and other applications. Also, utilizing wastes (wastewater and CO₂ from flue gas) for biomass production and biorefinery potential with many value-added integrated productions and zero waste concept might lead to prominent pavement of microalgae towards cleaner and sustainable production in every dimension. The potential bottlenecks in microalgal based productions are i) costlier process and ii) infancy level of understanding about integrated microalgal biorefinery. For the first case, the economic and energy demerits associated with each step such as cultivation, optimization towards hyper accumulating targeted product, harvesting, product extraction and fuel conversion were separately studied and potential optimization techniques for universal applications are scattered (Arcigni et al., 2019). Hence, a consolidated approach which involves the sequential process optimization is required to achieve finite improvement in terms of economy and energy perspective. El-Dalatony et al. (2019) demonstrated an integrated method in which successive fermentation of microalgal biomass was involved in the order of carbohydrates, proteins followed by transesterification of lipids in Chlamydomonas mexicana. Following to which, the leftover lipids and glycerol from transesterification can be further fermented. Such high throughput fermentation resulted in sectioned production of bioethanol (from carbohydrates), higher alcohols (from proteins), biobutanol (from the leftover lipids and glycerol) along with biodiesel from transesterification of lipids resulting in about 89% conversion efficiency and about 46% of overall energy recovery were achieved. González-González et al. (2018) suggested that integration of biodiesel production with biogas production in a closed loop can substantially improve the energy, economy and environmental safety (carbon balance). In another study, Gluconobacter oxydans was used for producing dihydroxyacetone from glycerol byproduct obtained from biodiesel produced from Scenedesmus dimorphus (Carbajal et al., 2020). In the last decade, the focus on microalgal biorefineries are intensified and byproducts of biodiesel conversions are potentially used (Silva et al., 2016).

Fig. 1 displays the published literatures with the term “microalgal biorefinery” in last decade which shows an increase in emerging interests over the concept. Such increase in interest towards integrated microalgal biorefinery is needed to attain further detailed insights about tailoring microalgal biochemical composition for multiple products. In such context, the review mainly focused on summarizing the challenges (technological and economic) associated with individual product (fuels) production from microalgae and suggesting the merits of different integrated microalgal biorefinery. To date, no other review articles encompass such depiction of integrated biofuel production pathways along with energy outputs and economic vitalities. To present with comprehensiveness, this review also analyzed the associated challenges in energy, economic and environmental aspects.