Sustainable Biofuels: Opportunities and Challenges

By Ramesh C. Ray

Sustainable Biofuels: Opportunities and challenges, a volume in the “Applied Biotechnology Reviews series, explores the state-of-the-art in research and applied technology for the conversion of all types of biofuels. Its chapters span a broad spectrum of knowledge, from fundamentals and technical aspects to optimization, combinations, economics, and environmental aspects. They cover various facets of research, production, and commercialization of bioethanol, biodiesel, biomethane, biohydrogen, biobutanol, and biojet fuel. This book discusses biochemical, thermochemical, and hydrothermal conversion of unconventional feedstocks, including the role of biotechnology applications to achieve efficiency and competitiveness. Through case studies, techno-economic analysis and sustainability assessment, including life cycle assessment, it goes beyond technical aspects to provides actual resources for better decision-making during the development of commercially viable technology by researchers, PhD students, and practitioners in the field of bioenergy. It is also a useful resource for those in adjacent areas, such as biotechnology, industrial microbiology, chemical engineering, environmental engineering, and sustainability science, who are working on solutions for the bioeconomy. The ability to compare different technologies and their outcome that this book provides is also beneficial for energy analysts, consultants, planners, and policy-makers.

The “Applied Biotechnology Reviews series highlights current development and research in biotechnology-related fields, combining in single-volume works the theoretical aspects and real-world applications for better decision-making.

• Covers current technologies and advancements in biochemical, thermochemical, and hydrothermal conversion methods for production of various types of biofuels from conventional and nonconventional feedstock
• Examines biotechnology processes, including genetic engineering of microorganisms and substrates, applied to biofuel production
• Bridges the gap between technology development and prospects of commercialization of bioprocesses, including policy and economics of biofuel production, biofuel value chains, and how to accomplish cost-competitive results and sustainable development

• Language: English
• Publisher: Academic Press
• Release date: Apr 8, 2021
• ISBN: 9780128223925

Book preview

1

Sustainable biofuels: opportunities and challenges

Preshanthan Moodley,    NRF SARChI Research Chair in Waste and Climate Change, School of Engineering, University of KwaZulu-Natal, Durban, South Africa

Abstract

Global energy demand is expected to increase by 48% in the next 20 years owing to the precipitous increase in the global population. Currently, 80% of the energy demand is met by fossil fuels. However, rapidly depleting fossil fuel reserves coupled with the negative environmental impacts from its combustion has prompted significant interest in sustainable biofuels. This will aid in the transition toward a carbon-neutral bio-economy. Several feedstocks have been identified as possible substrates for biofuel production. Agricultural residues have shown significant potential since they are environmentally benign, abundant, and low cost. Nevertheless due to its structural complexity, an appropriate pretreatment is required to enhance enzymatic and microbial conversion. Currently, first-generation biofuels such as bioethanol do not require intensive pretreatments; however, the major drawback is the utilization of food crops, thus contributing to the food versus fuel debate. Additionally, greenhouse gas emissions associated with first-generation biofuels are another obstacle. Second-generation biofuels such as bioethanol, biohydrogen, and biomethane appear to be most promising owing to its bioconversion from waste material. A major bottleneck in this process is the requirement of costly pretreatments and subsequent effluent treatment. Third-generation biofuels such as bioethanol from microalgae also show potential since process optimization could significantly enhance yields. Fourth-generation biofuels aim to utilize genetically optimized feedstocks that are designed to enhance capture of carbon dioxide; however, carbon capture and sequestration technology has limited the commercialization of this process. Integrated biorefineries have the potential to produce several generations of biofuels in one process, thereby completing valorizing the feedstock and enhancing the life cycle and techno-economic assessment of the bioprocess.

Keywords

Agricultural residues; biofuels; biobutanol; biohydrogen; biorefineries; algal biofuel; first-generation biofuels; second-generation biofuels; third-generation biofuels; fourth-generation biofuels

1.1 Introduction

1.1.1 Fossil fuels and energy crisis

The global population is estimated to exceed 9.7 billion by 2050, thus directly affecting the global energy consumption from 549 quadrillion British thermal units (Btu) in 2012 to 815 quadrillion Btu in 2040 (International Energy Outlook, 2016). Energy demands are primarily met by fossil fuels, accounting for 80% of the total energy market. The transport sector alone accounts for 60% of this usage (Zabed et al., 2016). Coal, a common fossil fuel, is formed through the slow decomposition of plant material and is mainly used for electricity generation and as an important source of energy for industry. Coal can be ranked in different categories and is determined by the degree of plant decomposition that dictates the pyrolytic devolatilization rates. In addition to this, aromatic content, calorific value, moisture and oxygen content play a key role in the quality of coal (Yang et al., 2019).

On the other hand, oil can be differentiated into two types, namely oil shale and oil sand. Shale is organic-rich sedimentary rock that contains high quantities of kerogen mineral pores. This type of fuel is gaining significant interest owing to the decline in petroleum and natural gas. It is usually composed of between 3.5% and 20% oil and is utilized via retorting and combustion. However, this method is plagued by low utilization efficiency that hinders industrialization (Hu et al., 2014). By contrast, oil sand is mainly composed of water, sand, and bitumen and is characterized by high viscosity and higher oil content (10%–30%) compared to shale (Zhang et al., 2017).

Current extraction rates indicate the complete depletion of oil and coal reserves in the next few decades (Day and Day, 2017). Additionally, greenhouse gas (GHG) emissions have increased exponentially from the combustion of these conventional fuels. This has led to increases in global temperatures, thus creating significant environmental concerns (Aditiya et al., 2016). The International Energy Agency estimates that fossil fuel usage, market flow, and GHG emissions would follow their current unsustainable trajectory until 2030 (Milano et al., 2016). For this reason, sustainable and greener alternative fuels are highly sought.

1.1.2 Renewable energy

The utilization of renewable energy sources has several benefits including energy security, sustained economic growth, and significant reduction in GHG emissions. Despite these benefits, renewable energy alternatives account for less than 17% of the total energy demand worldwide (Sener et al., 2018). Solar and wind power are affected by variations in solar radiation and wind speed, respectively, thus restricting the stable operation of these systems further causing output fluctuation. On the contrary, biofuels are gaining significant interest and have emerged as the potential replacement to traditional fuels owing to their highly renewable and sustainable nature. In addition, biofuels: (1) can be easily extracted from biomass and other waste material, (2) biodegradable, (3) environmentally friendly, and (4) are carbon neutral.

Globally, almost 50% of all renewable energy consumption in 2017 was derived from biofuels. This trend is expected to continue over the next few years and biofuels are anticipated to be the largest renewable energy source by 2023 (Mandley et al., 2020). Across the EU, biofuels have been shown to be the most flexible and commonly used renewable energy source, accounting for 64% of total usage.

1.2 Feedstocks for biofuels

There are several feedstocks that have been extensively investigated for biofuel production. Some of these include energy crops, agricultural residues, wastewater, and microalgae. These feedstocks have been shown to be effective in the production of various fuels such as bioethanol, biohydrogen, biomethane, and biodiesel. Some examples are listed in Table 1.1. Additionally, many of these substrates have been studied in a biorefinery-type system where more than one bioproduct is produced through the process. Bioenergy can be obtained from biomass through five main channels: (1) production of crops that yield sugar and starch, (2) burning of solid waste, (3) production of biogas through anaerobic digestion to further produce heat and electricity, (4) methane production using landfills, and (5) biofuel production such as ethanol, methanol, and biodiesel (Demirbas, 2001).

Table 1.1

1.2.1 Energy crops

Energy crops, such as corn and sugarcane, are one of the main sources of biomass-based feedstocks that have been examined for biofuel production. This includes biogas, biodiesel, and bioethanol with the potential to supply 435 EJ/year (Yamamoto et al., 2005; Hijazi et al., 2016). Moreover, energy crops are highly ranked as suitable feedstocks for biofuels owing to two main factors, land availability and yield of energy crop (Chao et al., 2019). However, there are also several drawbacks that have stagnated the industrial progress of these crops, and some of these include: (1) debate on social and ecological effects on food security and (2) negative impacts on farmers’ livelihoods and biodiversity preservation owing to large-scale land conversion (Ostwald et al., 2013). However, research is currently driven toward bridging the gap between these two divides. Table 1.2 outlines several investigations into biofuels from energy crops.

Table 1.2

Sugarcane and corn have been extensively studied for this purpose. Theoretically, sugarcane can yield 7400 MJ of energy, taking into account the straw, bagasse, and sugar (Leal, 2007). Dhaliwal et al. (2011) explored the production of bioethanol from sugarcane. These authors reported an ethanol concentration of 71.9 g/L employing a galactose-adapted thermotolerant strain of Pichia kudriavzevii. A significantly lower concentration of butanol was produced from a combination of sugarcane and sweet sorghum, resulting in 8.0 g/L from Clostridium beijerinckii DSM 6423 (Rochon et al., 2019). In contrast, several studies have demonstrated the efficiency of corn to produce higher yields of ethanol. For instance, the combination of distillers’ grain and corn produced 121.52 g/L ethanol employing Saccharomyces cerevisiae (Li et al., 2019). In another study, 137.50 g/L ethanol was generated from corn with a high solid loading of 33% using S. cerevisiae (Li et al., 2018).

1.2.2 Agricultural residues

The agricultural sector is one of the most important industries for the global economy. Globally, agriculture generates 23.7 million tons of food per day with annual waste estimations projected to reach 4 billion tons by 2050 (Duque-Acevedo et al., 2020). Sugarcane is an important agricultural crop globally, with annual waste yields (trash) amounting to approximately 131.2 Tg. This waste is disposed of either via landfilling or field burning, both of which have severe negative impacts on the environment. In the same line, this waste represents a reservoir of potential energy owing to its high sugar content, albeit requiring pretreatment (Moodley and Gueguim Kana, 2017a). Corn waste, another important agricultural crop, yields 1.03 billion metric tons annually with 50% of this considered waste. It also has a high energy density, exceeding 5210 MJ/kg, and owing to its relatively low lignin content, corn waste is a good candidate for biofuel production (Sewsynker-Sukai et al., 2018). In addition to these, other commonly examined residues include wheat straw, rice straw, water hyacinth, Napier grass, sorghum leaves, bamboo, rapeseed straw, and rice hulls, among others (Saha and Cotta, 2006; Lopez-Linares et al., 2013; Liong et al., 2012; Dagnino et al., 2013; Tawfik and Salem, 2012).

Sugarcane leaf waste was shown to yield 28.81 g/L bioethanol in a batch system employing S. cerevisiae BY4743 (Moodley and Gueguim Kana, 2019) (Table 1.3). Comparative results of 36.92 g/L bioethanol were reported from corncobs in a prehydrolysis simultaneous saccharification system also using S. cerevisiae BY4743 (Sewsynker-Sukai and Gueguim Kana, 2018). In a separate study, wheat straw was found to yield 38.10 g/L bioethanol employing S. cerevisiae TMB3400 (Olofsson et al., 2008).

Table 1.3

Biohydrogen has also been well reported from agricultural waste. For instance, sugarcane bagasse yielded 100 mL H2/g volatile solids (VS) from anaerobic sludge (Jafari and Zilouei, 2016), whereas corn stalks exhibited a higher yield of 163.1 mL/g from heat-shocked cow dung (Yang et al., 2015). Similarly, rice straw was found to yield 155 mL H2/g VS using boiled anaerobic activated sludge (Cheng et al., 2011).

All of these studies have demonstrated the feedstock potential of agricultural residues. In addition, the uses of this waste in bioprocessing or biorefinery systems mitigate some harmful environmental impacts while simultaneously adding value to waste.

1.2.3 Wastewater

Anthropogenic activity necessitates high quantities of water and as such, produces large amount of wastewater. Wastewater is often considered the effluent of many domestic and industrial processes, and it has been shown to be toxic when released into the environment owing to high concentrations of toxins, pollutants, and chemicals. Globally, almost 80% of all wastewater is released into the environment without treatment (Sharmila et al., 2020). For example, the United States, China, Japan, and India each produced 60.40, 37.98, 16.93, and 15.44 km³ of municipal wastewater (Mateo-Sagasta et al., 2015). In addition, 220,000 m³ of brewery wastewater is generated for every 100,000 m³ of beer produced (Mohammedawi et al., 2019). Other wastewater streams include dairy, juice, olive mill, domestic and paper mill, among others. However, wastewater also contains sufficient nutrients (C, N, P, etc.) to support the growth and bioprocess of certain types of microorganisms. This can lead to the production of valuable chemicals such as biofuels while simultaneously treating and reducing the contaminants in the wastewater.

Table 1.4 outlines the biofuel yield using different wastewater streams. Rice mill wastewater has been shown to be a good feedstock for biofuels, owing to a hydrogen yield of 200 mL/mL (Haridoss, 2016). Lower yields of 31.50 and 9.62 mL/mL biohydrogen have been reported from olive mill wastewater and paper mill wastewater, respectively (Eroglu et al., 2008; Hay et al., 2016). In another study, photosynthetic microalgae Nannochloropsis gaditana produced 70.3 mg bioethanol/g biomass from municipal wastewater (Onay, 2016). S. cerevisiae was capable of producing 52.4 g/L bioethanol from olive mill wastewater under nonaseptic conditions (Sarris et al., 2014).

Table 1.4

1.2.4 Microalgae

Microalgae are photosynthetic unicellular microorganisms that are usually found in ocean and freshwater environments. These organisms have shown promise as alternative feedstocks to biofuels owing to their high growth rates, high photosynthetic efficiency ease of cultivation in large quantities, and since they do not require arable land (Phwan et al., 2018). A major advantage of microalgae is their ability to grow in fresh, salt, and wastewater. Microalgae, similar to lignocellulosic biomass in this regard, require a pretreatment stage to release the enclose sugars to enhance fermentation. Biofuels generated from microalgal biomass are referred to as third generation.

Chlorella is a well-studied microalgal species for both bioethanol and biohydrogen. For instance, Chlorella vulgaris yielded 0.40 g/g of bioethanol subsequent to acidic pretreatment, whereas Chlorococcum infusionum was found to produce a significantly lower yield of 0.26 g/g after alkali pretreatment (Lee et al., 2011; Harun et al., 2011). Chlorella pyrenoidosa was shown to yield 93.86 mL H2/L, a noteworthy yield compared to that of Chlorella lewinii KU201 which demonstrated a yield of 11.50 mL H2/L (Dalatony et al., 2016; Pongpadung et al., 2015) (Table 1.5).

Table 1.5

1.2.5 Pretreatment for enhancing feedstock digestibility

Lignocellulosic-based feedstocks are mainly composed of cellulose and hemicellulose bound in a recalcitrant matrix with lignin. This matrix severely hampers accessibility of the sugars by both microorganisms and enzymes. For this reason, a pretreatment regime is necessary, to delignify the material, thereby enhancing enzymatic hydrolysis and further sugar release for bioprocessing (Moodley and Gueguim Kana, 2017b). There are three main categories of pretreatment: (1) physical, (2) chemical, and (3) biological. Physical pretreatment usually comprises milling, extrusion, or radiation. Chemical pretreatment usually encompasses the use of acid, alkali, ionic liquids, organosolvents, and inorganic salts. Lastly, biological pretreatment involves the use of fungal organisms that are capable of degrading the cellulose-rich structure of biomass. For an effective pretreatment, two or more of the above strategies are combined for enhanced activity (Kumar et al., 2020). Some methodologies, such as acid and alkali, have some drawbacks such as high cost, high toxicity, corrosiveness, and production of inhibitors by-products through the pretreatment process, whereas others such as inorganic salt and microwave radiation are relatively low cost, nontoxic, and highly efficient (Moodley et al., 2020). Table 1.6 outlines recent pretreatment studies with the corresponding sugar yields.

Table 1.6

BMIMCl, 1-Butyl-3-methylimidazolium chloride; PEG, Polyethylene glycol.

1.3 Sustainable biofuels

1.3.1 First generation

First-generation biofuels are those that are produced from edible energy crops such as sugar-based crops (sugarcane, sugar beet, and sorghum), starch-based crops (corn, wheat, and barley) or oil-based crops (rapeseed, sunflower, and canola). Initially, these biofuels showed promise in minimizing reliance of conventional fossil fuels and lowering the emission of GHG associated with its combustion (Rodionova et al., 2017). However, the production of first-generation biofuels has raised serious concerns on food supply, food security, and arable land requirements (Phwan et al., 2018). Nonetheless, research has continued to pursue first-generation biofuels; some studies are shown in Table 1.7. Most notable, biodiesel and bioethanol are first generation–associated fuels, commonly produced from corn, sugarcane, sorghum, and sunflower.

Table 1.7

1.3.2 Second-generation biofuels

Second-generation biofuels are considered the more sustainable option since the feedstock is lignocellulosic-based biomass that is abundant, inexpensive, and usually consists of nonedible plants (Alawan et al., 2019). This generation of fuel has also attracted significant interest owing to the use of waste plant biomass. This biomass could be used to generate heat or electricity by burning, feedstock for wastewater treatment, or as a cost-effective source for biofuel production (Alawan et al., 2019). Most research has been focused in this line, with significant output relating to pretreatment of biomass to enhance sugar release and enhancing biofuel yield. Lignocellulosic biorefineries have also recently come into the spotlight, since it underscores a potential circular process that aims to completely valorize the waste feedstock by producing multiple products or fuels (Moodley et al., 2020). However, one of the major bottlenecks in this stage is the development of an appropriate pretreatment strategy to efficiently release maximum sugar while being low cost. For this reason, much research has jointly examined pretreatment together with biofuel production. Some examples of second-generation biofuels are biodiesel, bioethanol, biohydrogen, and biomethane.

1.3.2.1 Bioethanol

Bioethanol is a versatile biofuel since it can be produced from a number of feedstocks using a variety of fermentative microorganisms. This process can be broken down into four main stages: (1) pretreatment, (2) hydrolysis, (3) fermentation, and (4) product recovery (Jonsson and Martin, 2016). As described earlier, the hydrolysis step is important for sugar release, and its efficiency is dictated by the effectiveness of the pretreatment stage. Additionally, the production of inhibitors during the pretreatment and hydrolysis stage can significantly hamper the fermentation process. Several studies have examined bioethanol production from lignocellulosic-based feedstocks (Table 1.8).

Table 1.8

1.3.2.2 Biohydrogen

Biohydrogen is considered a promising alternative to conventional fossil fuels, owing to its clean burning nature, high energy content, and low-cost methods of production. This fuel can be produced from almost any type of organic waste material using a variety of microbial consortia, via two main channels: dark fermentation and photofermentation (Soares et al., 2020). In order to maximize hydrogen yields, optimization of key factors that influence the fermentation process are essential, and these can include: (1) hydraulic retention time, (2) temperature, (3) inoculum pretreatment, (4) pH, (5) agitation, and (6) aeration, among others (Moodley and Gueguim Kana, 2015b). Inoculum pretreatment is often required to deactivate potential hydrogen consumers, particularly when employing mixed cultures such as anaerobic sludge or manure. Table 1.9 outlines various studies that have examined biohydrogen production from different waste and microbial sources.

Table 1.9

1.3.3 Third-generation biofuels

Third-generation biofuels are those that are derived from aquatic feedstock, either macro- or microalgae (IEA, 2008). Generally, the biomass is cultivated in the first step, with the second step including harvesting and production of the biofuel. Third-generations biofuels from microalgae have significant advantages that include high growth rates with low cultivation times and high-quality arid land is not required (Saladini et al., 2016). Several studies have explored bioethanol and biohydrogen production from microalgae, and some of these are also shown in Table 1.5. One of the major demerits of microalgal growth is the requirement for glucose, which is costly and can account for up to 80% of total cost of growth media. Consequently, mixotrophic cultivation of microalgae is currently being explored since carbon sources can include waste organic material (Alawan et al., 2019).

1.3.4 Fourth-generation biofuels

Biofuels produced in this generation are usually from genetically engineered microorganisms such as fungi, yeast, microalgae, and cyanobacteria. These modified organisms often have increased carbon entrapment capabilities with overall increases in cultivation, harvesting, and fermentation lipid yield (Dutta et al., 2014). In addition to this, the genetic modification also aims to enhance light penetration by reducing size of the chlorophyll antennae, to reduce photoinhibition by pigment manipulation, and to improve photosynthetic efficiency. This cultivation can occur either via a noncontained or contained system with the latter being more efficient albeit incurring high cost. While uncontained systems are lower cost, the potential for these genetically modified organisms to be released from these systems is high since they are prone to leakage (Abdullah et al., 2019). Owing to this, much of fourth-generation biofuel production processes have been limited to research laboratories because of the associated risks.

1.3.5 Integrated biorefinery approach

Biorefineries are now widely regarded as a cost-effective approach to decreasing GHG emissions, lowering reliance on fossil fuels, and moving toward a bio-economy. In a biorefinery system, either a process or multiple processes are employed in the bioconversion of biomass into a wide spectrum of products such as biochemicals, biomaterials, or biofuels (Moshkelani et al., 2013). A simple schematic for a biorefinery is shown in Fig. 1.1. The main stages in the biorefinery include the processing and pretreatment of the raw waste material and usually, these steps significantly affect the process economics. Lab research data are usually required to perform simulated processes for a biorefinery followed by a cost analysis to determine process feasibility prior to scale up (Ajao et al., 2018). Several studies have examined lab-scale biorefinery type studies. For instance, Moodley and Gueguim Kana (2019) examined the production of bioethanol from sugarcane leaf waste while subsequently directing the fermentation effluent toward animal feed production. In another study, municipal organic waste was used to generate five different products, namely lactic acid, biosuccinic acid, single-cell protein, biomethane, and biohydrogen (Khoshnevisan et al., 2020). Similarly, Papadaskalopoulou et al. (2019) examined the production of ethanol and biogas from kitchen waste.

Figure 1.1 Lignocellulosic processing in a biorefinery system. Source: Adapted from Kurian, J.K., Nair, G.R., Hussain, A., Raghavan, G.S.V., 2013. Feedstocks, logistics and pre-treatment processes for sustainable lignocellulosic biorefineries: a comprehensive review. Renew. Sustain. Energy Rev. 25, 205–219.

1.4 Life cycle assessment and techno-economic analysis of biofuel production

Life cycle assessment (LCA) refers to an environmental assessment methodology that is employed as a key indicator for the environmental sustainability of a process. In particular, LCA looks at assessing the environmental impacts of a specific product in a chain and subsequently examined factors such as effects on global warming, acidification, human toxicity, and impacts on biodiversity (Lecksiwilai and Gheewala, 2020). Information on these factors provides insight into multiple environmental impact categories and thus avoids the transfer of such factors into the next stage of the life cycle. Several studies have examined the LCA of specific processes at a lab scale. For instance, LCA analysis for the production of biodiesel and biogas from microalgae showed a significantly high emission of GHG from the biodiesel stage but zero emission for the biogas stage (Sun et al., 2019). In this case, since the emissions had no ripple effect, focus could be placed on solely addressing this in the first stage. Another study examining the LCA of the production of bioethanol from cow manure reported that 41.6 m³ of wastewater was generated from the process with the stage of phase separation accounting for the highest fraction of 38.7 m³ (Azevedo et al., 2017).

Techno-economic assessment is an exercise that simulates a process at industrial scale, often using software such as Aspen Plus and SuperPro Designer. It is essentially a blueprint of the entire process and provides important economic and environmental data. Usually, a flow sheet is designed first with different unit operations to perform a mass and energy balance, which ultimately evaluates and optimizes the efficiency and economic performance of various processes (Rajendran and Murthy, 2019). The ketone ammoximation process was optimized by evaluating energy and energy requirements, which resulted in an optimally energy-efficient process based on the lowest total annual cost (Zhu et al., 2019). A preliminary analysis of biodiesel production over solid biochar revealed that the break-even price of the product (biodiesel) was computed to be $1.55/kg, which further provided insight toward reaching a selling price of $1.70/kg (Lee et al., 2020). Another study looking at a safflower-based biorefinery producing bioethanol and biodiesel found that employing Zymomonas mobilis instead of S. cerevisiae as the fermenting organism enhanced profitability of the process by decreasing ethanol production costs from $0.12 to $0.09/L and thus reduced minimum selling price of ethanol from $0.67 to $0.43/L. In addition, biodiesel production was found to boost overall sale values by between 39% and 55% (Khounani et al., 2019).

1.5 Current advances in sustainable biofuels

In recent years, there have been significant advances in the development of biofuels and its related technologies. On the pretreatment front, inorganic salt was recently shown to be a low cost, environmentally friendly, and efficient alternative to commonly employed acid and alkali strategies (Moodley and Gueguim Kana, 2017a,b,c). There has also been an interest in exploring green liquor dregs (GLD), a waste product from the paper mill industry, as a low-cost alkali alternative (Sewsynker-Sukai et al., 2020). A recent study concluded that GLD was highly effective in enhancing sugar yield from corncobs and thus comparable to yields employing NaOH (David et al., 2020). Keeping with pretreatment, microwave irradiation has exhibited excellent heating efficiency with significantly lower treatment times and lower energy requirements (Moodley and Gueguim Kana, 2017b; Liang et al., 2019; Milkulski and Klosowski, 2020). These advances in pretreatment have the potential to mitigate high costs associated with feedstock pretreatment, thereby enhancing process feasibility. Recent efforts have also been challenged toward reducing process cost of fermentation. Simultaneous saccharification and fermentation systems have recently shown to produce competitive bioethanol yields while trimming costs since the entire process occurs in a single vessel (Seifollahi and Amiri, 2020; Prato et al., 2020). Magnetic nanoparticles have also been shown to enhance the yield of bioethanol when employed in the fermentation media (Sanusi et al., 2020). All of these developments have the potential to enhance process economics, thereby allowing scale-up processes to occur.

1.6 Current challenges in sustainable biofuels

Biofuel production processes still face many challenges that hamper its commercialization. At the forefront of the process is pretreatment, and this process has the potential to generate toxic by-products that require treatment prior to release. Similarly, lignin, phenolics, and other volatiles can form in the pretreatment waste hydrolysate that requires efficient treatment prior to disposal (Moodley et al., 2020). Another major challenge is the high water consumption required for biofuel and overall biorefinery systems. Water is a key requirement is pretreatment, fermentation, and downstream processes and its absence would significantly hinder the bioprocess. An alternative to this would be the exploration of wastewater as a water source, albeit some treatment may be necessary prior to use (Sewsynker-Sukai et al., 2020). Coupled with water use is the high energy requirement. Bioprocesses usually consist of several large unit operations that require significant energy input. These energy costs should be equated against product yield to determine process feasibility. Additionally, many biofuel processes are hampered by low yields. Lower yields impact on process economics and therefore research should also focus on the improvement of biofuel yields. Process optimization may be looked into with a specific focus on key process parameters.

1.7 Concluding remarks and future prospects

The biofuel production landscape has made significant strides in recent years through the advancement in new technologies and the identification of novel feedstocks. One of the key areas that will impact the future prospects of sustainable biofuels is efficient conversion technologies. Since most research is shifting toward waste, lignocellulosic-based feedstocks, a low-cost, environmentally friendly, and efficient pretreatment is required in order to maximize sugar yield and consequent product yield. With this mind, lignocellulosic biorefineries should be consistently examined and exploited. Several LCA studies have shown that using waste feedstock generally has a net-zero impact on GHG emissions. These studies have also indicated that a major processing cost lies in feedstock pretreatment. Techno-economic assessments can provide crucial insights into process costs. By including more than one revenue stream, as is the case in an integrated biorefinery, there would be less economic strain on the process, thus enhancing economic outlook. Processes should also be conceptualized with being low energy with a low carbon footprint. Investigation into waste feedstocks such as organic municipal waste, agricultural waste, and wastewater are encouraged to promote sustainable biofuel production processes.

Acknowledgments

The financial assistance of the National Research Foundation (NRF) of South Africa toward this research is hereby acknowledged. Opinions expressed and conclusions arrived at are those of the author and are not necessarily attributed to the NRF.

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