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The rapid increase in fossil fuel depletion, environmental degradations, and industrialization have encouraged the need and production of sustainable fuel alternatives. This has led to the increase in interest in biofuels, especially third-generation biofuels produced from microalgae since they do not compete with food and land supplies. However, the global share for these biofuels has been inadequate recently, especially due to the ongoing global pandemic. Therefore, this paper offers a review of the state-of-the-art study of the production field of third-generation biofuel from microalgae. The current review aims to focus on the different aspects of algal biofuel production that requires further attention to produce it at a large scale. It was found that several strategies during the life cycle of algal biofuel production can significantly increase its quality and yield while reducing cost, energy, and other related attributes. This paper also focuses on the challenges for large-scale production of third-generation biofuels pre and post COVID-19 to better understand the barriers. The high cost of this fuel’s production and sale tends to be the major reason behind the lack of large-scale production, hence, inadequacy to meet the global need. Third-generation biofuel has so much to offer including many integrated applications and advanced uses in the future fuel industry. Therefore, it is important to cope with the ongoing circumstances and emphasize the future of algal biofuel as a sustainable source.

Restoring native grassland vegetation can substantially improve ecosystem service outcomes from agricultural watersheds, but profitable pathways are needed to incentivize conversion from conventional crops. Given growing demand for renewable energy, using grassy biomass to produce biofuels provides a potential solution. We assessed the techno‐economic feasibility and life cycle outcomes of a “grass‐to‐gas” pathway that includes harvesting grassy (lignocellulosic) biomass for renewable natural gas (RNG) production through anaerobic digestion (AD), expanding on previous research that quantified ecosystem service and landowner financial outcomes of simulated grassland restoration in the Grand River Basin of Iowa and Missouri, United States. We found that the amount of RNG produced through AD of grassy biomass ranged 0.12–45.04 million gigajoules (GJ), and the net present value (NPV) of the RNG ranged − $97 to $ 422 million, depending on the combination of land use, productivity, and environmental credit scenarios. Positive NPVs are achieved with environmental credits for replacement of synthetic agricultural inputs with digestate and clean fuel production (e.g., USEPA D3 Renewable Identification Number, California Low Carbon Fuel Standard). Producing RNG from grassy biomass emits 15.1 g CO2‐eq/MJ, which compares favorably to the fossil natural gas value of 61.1 g CO2‐eq/MJ and exceeds the US Environmental Protection Agency's requirement for cellulosic biofuel. Overall, this study demonstrates opportunities and limitations to using grassy biomass from restored grasslands for sustainable RNG production. Restoring native grasslands in agricultural watersheds can enhance ecosystem services and offer profitable routes for biomass use in renewable natural gas (RNG) production. Our study assessed the techno‐economic and life cycle impacts of converting grassy biomass to RNG via anaerobic digestion, observing RNG outputs between 0.12 and 45.04 million gigajoules. Economic viability, based on a net present value ranging from − $97 to $ 422 million, relies on environmental credits. RNG production from this pathway emits significantly lower CO2 compared to fossil fuels, meeting stringent EPA standards for cellulosic biofuels, and showcasing the sustainable potential and challenges of this approach.

Many countries are immersed in several strategies to reduce the carbon dioxide (CO2) emissions of internal combustion engines. One option is the substitution of these engines by electric and/or hydrogen engines. However, apart from the strategic and logistical difficulties associated with this change, the application of electric or hydrogen engines in heavy transport, e.g., trucks, shipping, and aircrafts, also presents technological difficulties in the short-medium term. In addition, the replacement of the current car fleet will take decades. This is why the use of biofuels is presented as the only viable alternative to diminishing CO2 emissions in the very near future. Nowadays, it is assumed that vegetable oils will be the main raw material for replacing fossil fuels in diesel engines. In this context, it has also been assumed that the reduction in the viscosity of straight vegetable oils (SVO) must be performed through a transesterification reaction with methanol in order to obtain the mixture of fatty acid methyl esters (FAMEs) that constitute biodiesel. Nevertheless, the complexity in the industrial production of this biofuel, mainly due to the costs of eliminating the glycerol produced, has caused a significant delay in the energy transition. For this reason, several advanced biofuels that avoid the glycerol production and exhibit similar properties to fossil diesel have been developed. In this way, “green diesels” have emerged as products of different processes, such as the cracking or pyrolysis of vegetable oil, as well as catalytic (hydro)cracking. In addition, some biodiesel-like biofuels, such as Gliperol (DMC-Biod) or Ecodiesel, as well as straight vegetable oils, in blends with plant-based sources with low viscosity have been described as renewable biofuels capable of performing in combustion ignition engines. After evaluating the research carried out in the last decades, it can be concluded that green diesel and biodiesel-like biofuels could constitute the main alternative to addressing the energy transition, although green diesel will be the principal option in aviation fuel.

Only trace amount of isobutanol is produced by the native Saccharomyces cerevisiae via degradation of amino acids. Despite several attempts using engineered yeast strains expressing exogenous genes, catabolite repression of glucose must be maintained together with high activity of downstream enzymes, involving iron–sulfur assimilation and isobutanol production. Here, we examined novel roles of nonfermentable carbon transcription factor Znf1 in isobutanol production during xylose utilization. RNA-seq analysis showed that Znf1 activates genes in valine biosynthesis, Ehrlich pathway and iron–sulfur assimilation while coupled deletion or downregulated expression of BUD21 further increased isobutanol biosynthesis from xylose. Overexpression of ZNF1 and xylose-reductase/dehydrogenase (XR-XDH) variants, a xylose-specific sugar transporter, xylulokinase, and enzymes of isobutanol pathway in the engineered S. cerevisiae pho13gre3Δ strain resulted in the superb ZNXISO strain, capable of producing high levels of isobutanol from xylose. The isobutanol titer of 14.809 ± 0.400 g/L was achieved, following addition of 0.05 g/L FeSO4.7H2O in 5 L bioreactor. It corresponded to 155.88 mg/g xylose consumed and + 264.75% improvement in isobutanol yield. This work highlights a new regulatory control of alternative carbon sources by Znf1 on various metabolic pathways. Importantly, we provide a foundational step toward more sustainable production of advanced biofuels from the second most abundant carbon source xylose. The engineered yeast strain could be used as a cell factory to produce high levels of isobutanol from the second most abundant sugar xylose due to activation of Znf1 target genes in the the valine biosynthesis, Ehrlich pathway and iron–sulfur assimilation.

The production of microalgal biomass on a commercial scale remains a significant challenge. Despite the positive results obtained in the laboratory, there are difficulties in obtaining similar results in industrial photobioreactors. Changing the cultivation conditions can affect not only the growth of microalgae but also their metabolism. This is of particular importance for the use of biomass for bioenergy production, including biofuel production. The aim of this study was to determine the biomass production efficiency of selected microalgal strains, depending on the capacity of the photobioreactor. The lipid and ash content of the biomass were also taken into account. It was found that as the scale of production increased, the amount of biomass decreased, irrespective of the type of strain. The change in scale also affected the lipid content of the biomass. The highest values were found in 2.5 L photobioreactors (ranging from 26.3 ± 2.2% for Monoraphidium to 13.9 ± 0.3% for Chlorella vulgaris). The least favourable conditions were found with industrial photobioreactors, where the lipid content of the microalgal biomass ranged from 7.1 ± 0.6% for Oocycstis submarina to 10.2 ± 1.2% for Chlorella fusca. The increase in photobioreactor capacity had a negative effect on the ash content.

The contribution of castor oil for reaching the targets set by RED1 and RED2 in Europe can be tangible if the problem related to the mechanical harvesting is overcome. Dwarf hybrids suitable for mechanical harvesting are already available on the market but the residual moisture of plants and capsules has to be lowered in order to allow mechanization. In the present case of study, three common terminating products (Glyphosate GLY, Diquat DIQ and Spotlight DEF) were tested on Kaiima C1012 hybrid in a complete randomized block design to assess the effectiveness of using chemical products to decrease residual moisture in castor plants. Plants were harvested via combine harvester equipped with cereal header to evaluate seed loss (due to dehiscence, impact and cleaning shoe) and the dehulling capacity of the combine harvester’s cleaning shoe. DIQ decreased significantly moisture content of capsules (7.32%) in comparison to the other treatments, while the lowest plant moisture was recorded in DIQ (62.38%) and GLY (59.12%). The use of DIQ triggered the highest impact seed loss (61.75%) in comparison with GLY (46.50%) and DEF (29.02%). Control plants could not be harvested mechanically due to the high residual moisture content and high density of weeds. The present case of study provides highlights regarding the need to further investigate the best practice to terminate castor plants and to develop a specific combine header to reduce seed loss from impact.

This paper focuses on developing countries that are striving to understand the requirements for the sustainable, commercial development of algae for the production of biofuels. The paper will review the sustainable development of biofuel production, including the major issues that must be addressed before embarking on the path to sustainable biofuel production. The sustainable production of biofuel should be implemented with an ecologically friendly perspective to ensure that future generations will enjoy prosperity of the planet that we share. We can find more than one path for the development of biofuel production from algae but sustainable development must be stressed to ensure prosperity for future generations.

In the current investigation, fuel blend combination (FBC) of spirulina microalgae biodiesel (SMAB), diesel, ethanol, and methanol is prepared and assessed. Engine performance and spray attributes of FBC were investigated through Diesel-RK Software (DRS) at different engine loads using FBC0, FBC1, FBC2, FBC3, FBC4, FBC5, and FBC6. The viscosity of SMAB decreased with the addition of alcohol. Increasing percentage of SMAB from 0 to 40% in the FBC increases the spray characteristics by 11.9%. Performance of engine was enhanced by adding ethanol and menthol to SMAB and diesel combinations. Brake thermal efficiency (BTE) and brake specific fuel consumption (BSFC) were 1.7% higher and 6.6% lower for FBC6 blend compared to FBC0 (diesel fuel). Increase in percentage of SMAB, ethanol, and methanol in hybrid fuels has shown a decrease in smoke emissions, NOX emissions by 41.3 and 43% at various engine loads.

One of the more promising technologies for future renewable fuel production from biomass is hydrothermal liquefaction (HTL). Although enormous progress in the context of continuous experiments on demonstration plants has been made in the last years, still many research questions concerning the understanding of the HTL reaction network remain unanswered. In this study, a unique process model of an HTL process chain has been developed in Aspen Plus® for three feedstock, microalgae, sewage sludge and wheat straw. A process chain consisting of HTL, hydrotreatment (HT) and catalytic hydrothermal gasification (cHTG) build the core process steps of the model, which uses 51 model compounds representing the hydrolysis products of the different biochemical groups lipids, proteins, carbohydrates, lignin, extractives and ash for modeling the biomass. Two extensive reaction networks of 272 and 290 reactions for the HTL and HT process step, respectively, lead to the intermediate biocrude (~200 model compounds) and the final upgraded biocrude product (~130 model compounds). The model can reproduce important characteristics, such as yields, elemental analyses, boiling point distribution, product fractions, density and higher heating values of experimental results from continuous experiments as well as literature values. The model can be applied as basis for techno-economic and environmental assessments of HTL fuel production, and may be further developed into a predictive yield modeling tool.

In this modern era due to multidimensional problems associated with petrochemical fuels, the scientific community is showing a burgeoning interest in microalgae due to their potential applications which are indispensable for economic amelioration. Microalgae are a fundamental source of oils and various other biomolecules that can be used in the production of biofuels and various other value-added bioproducts. However, implication of microalgae-based biofuels is not economically viable due to various factors. One of these prime reasons is the cost associated with its harvesting. This review focuses on various harvesting techniques applied to microalgae in the last 2/3 decades, presenting the main benefits and drawbacks of each method to allow the selection of appropriate method(s) for economically harvesting microalgal biomass. According to this review, use of any single technique is not viable for harvesting microalgal biomass. However, keeping in view the morphological characteristics of the microalgae, growth density, utility purpose of the harvested biomass, harvesting scale and physico-chemical characteristics of the production medium, these techniques should be applied in suitable combinations to obtain fruitful results.