Bioenergy Production

Abstract

Bioenergy, when compared to traditional fossil fuels, offers clear benefits due to its renewable nature and enormous supply, and so plays a critical role in ensuring energy stability while minimizing net greenhouse gas emission. However, the advancement of bioenergy can produce major environmental changes, the extent of which is unknown. This chapter highlights the overview of bioenergy, available technologies for bioenergy production, environmental implications, challenges, prospects and future work consideration for the successful transition to bioenergy economy. Consequently, a global bioenergy sector producing substantial amount of energy would be required for the transition to a low-carbon energy economy while meeting rising future energy demands.

1. Introduction

The world economy has developed within a concept that is heavily dependent on fossil fuel (coal, oil, and natural gas), which supply the vast large proportion of the substrate utilized in the synthesis of fuels and chemicals. The global energy utilization is increasing tremendously, and fossil fuels currently provide around 88% of the global energy. However, due to their finite reserves and non-renewable nature, the long-term exploitation of these limited resource is unreliable [1]. According to projections, the world’s energy requirement will rise by a factor of two or three throughout this century [2]. Similarly, the quantities of greenhouse gases (GHGs) in the environment are quickly increasing, with CO₂ releases from fossil fuels being the main significant contribution to this increase. It is necessary to cut greenhouse gas emissions to less than half of world emission rates of 1990 as to mitigate the consequences of global warming and climate change [3]. Another significant global concern is energy supply stability, which is complicated by the fact that the vast majority of known traditional oil and gas reserves are located in politically unstable countries.

Bioenergy is an alternative form of basic energy that offers an opportunity for greenhouse gas (GHG) reductions, provided that the feedstocks are exploited from a renewable source and that effective bioenergy technologies are utilized. It is possible that increasing the amount of electricity generated by this form of energy may help to achieve the Framework Convention on Climate Change (FCCC) goals of stabilizing atmospheric concentrations of greenhouse gases below toxic concentrations in the future. Biomass is an alternate provider of chemical feedstock and energy, and biorefining biomass is equivalent to petroleum processing [3, 4]. “A biorefinery,” according to the National Renewable Energy Laboratory (NREL), is described as “a system that incorporates biomass transformation operations and technology to synthesize fuels, electricity, and chemicals from biomass” according to National Renewable Energy Laboratory. Furthermore, bioenergy obtained from biological materials has historically been considered to be a significant form of energy that will help to lessen reliance on fossil fuels [5].

The notion of biorefineries is a sustainable strategy to the biomass transformation into useful products that may easily substitute fossil oil refineries, which are used to generate a number of fuels, chemicals, and other by-products from crude oil. Using biomass as a substrate, biorefining is the method of refining a variety of bio-based products such as chemicals, fuels, and power, all of which are utilized as end products. Biofuels are liquid or gaseous fuels that are predominantly derived from biomass. They can be employed to substitute or supplement diesel, gasoline, or other fossil fuels in a variety of uses, including transportation, stationary, portable, and other purposes. Biofuels, such as biodiesel, bioethanol, biogas, and bio-oil, are the most important products of the biorefining industry. When likened to conventional fossil fuels, biofuels have outstanding characteristics in aspects of renewability, relatively clean refining, locally distributed resources, biodegradability and non-hazardous, clean combustion, a favorable economic implication, improved fuel economy, reduced reliance on petroleum oil, and improved health advantages [6, 7]. The application of green technology-based biorefinery approach results in a crude oil non-reliance future, with a prosperous industry dependent on organic and environmentally friendly raw material including agricultural residues, cheese whey, household residues, forest residues, and algae. The advancement in technology makes it possible to produce biofuel from waste raw material in an efficient manner.

1.1 Classification of biofuels

1.1.1 Generations of biofuels

Historically, there are three generations of biofuels. First generation biofuels such as bioalcohols, biodiesels, biogas, bioethers, biosyngas and vegetable oil have been produced primarily from sugar, starch, and vegetable oil sugar, or animal fats, and they are produced through conventional techniques [7]. Advances based on various biomass possibilities have resulted in the development of 2nd and 3rd generation biofuels [8]. Biofuels derived from agroforestry residues lignocellulosic materials and waste biomass (wheat stalks, maize stalks, corn, and wood) as well as dedicated non-food based bioenergy materials (e.g. miscanthus, willow, and poplar), serve as the foundation for second generation biofuel production [9]. Advanced biofuels such as biohydrogen and bioethanol are examples of the second-generation biofuels. Algae-based biofuels such as biogas (biohydrogen and biomethane) are the third generation of biofuels [10].

1.1.2 Types of biofuels

Ethanol is the most popularly used alcoholic biofuel on the industry today. There are numerous motivations for its application as a sustainable energy, including: that it is made from renewable agricultural feedstock such as corn, sugar and molasses, rather than non-sustainable sources, and that ethanol and its byproducts are less hazardous than other alcoholic fuels [11]. Biodiesel is a liquid fuel made from animal fats, vegetable oils, and waste cooking oil that can be used as a substitute for diesel fuel and is regarded as a viable replacement to fossil diesel [12]. It is sustainable, non-hazardous, biodegradable, sulfur- and benzene-free, may be applied in standard diesel engines without adjustment, and can be blended with fossil diesel at any ratio [7, 13, 14]. Bio-oil is a combination of organic components, primarily acids, alcohols, aldehydes, esters, ketones, and phenols. This liquid is usually dark brown in color and free-flowing, with a smoky fragrance [15, 16]. Bio-oil can be considered an environmentally benign fuel when compared to fossil fuels because it emits less CO2 and produces reduced NOx emissions than diesel oil [16]. Biogas is a gas combination mostly made up of CH₄ and CO₂ that is generated from agricultural residue, manure, municipal trash, plant material, sewage, green waste, or food waste while biohydrogen is produced from microalgae and bacteria metabolism. It is a form of green energy. Biogas is a diverse sustainable energy source that may be employed to substitute fossil fuels in the generation of electricity and heat, as well as a gaseous automobile fuel.

2. Biomass conversion technologies for bioenergy production

Most techniques are appropriate for direct biomass conversion or intermediate conversion [17, 18]. Because the techniques are adequately mutable, gaseous, and liquid fuels that are undistinguishable to those derived from fossil feedstocks, or that are not matching but useful as fossil fuel alternatives, can be created. It’s worth noting that biomass feedstocks may be used to make practically all of the fuels and commodity chemicals that are made from fossil fuels. The techniques include a wide range of thermal [18] and thermochemical technologies [19] for the conversion of biomass via combustion, gasification, and liquefaction, as well as microbial transformation of biomass through fermentative methods to create gaseous and liquid fuels. There are numerous biomass conversion pathways for creating energy haulers from biomasses. Figure 1 depicts significant conversion pathways for producing heat, power, and transportation fuels that are now in use or under development. The accessible technologies for development in producing transportation fuels are categorized as combustion, gasification, and digestion, followed by the technologies available.

Figure 1.

Pathways for biomass conversion to finished products adapted from [19].

2.1 Physicochemical conversion processes

Physicochemical biomass transformation includes the generation of products employing physical and chemical conversion techniques at relatively close ambient temperatures and pressures. It is mostly linked with the conversion of fresh or used vegetable oils, animal fats, greases, tallow, and other apt feedstocks into beneficial liquid fuels and chemicals like biodiesel.

2.1.1 Extraction or separation method

There are varieties of procedures for the extraction of biomass including liquid–solid extraction, partitioning, acid–base extractions, liquid–liquid extraction, ultrasonic extraction (UE), and microwave assisted extraction (MAE) [20]. Several extraction procedures, such as enzyme assisted extraction and solvent extraction have also been examined in the past few decades [18]. However, there are certain disadvantages to these extraction processes. Liquid–liquid extraction and liquid–solid extraction are the two most used extraction methods. Two distinct solvents are typically used for liquid–liquid extraction, one of which is unvaryingly water. Cost, toxicity, and flammability are some of the downsides of this approach [21]. A solid-phase extraction (SPE) technique is also employed in separating analytes which are dissolved or suspended in a liquid mixture based on their physical and chemical properties from a wide range of matrices. Soxhlet extraction, percolation, sonication and steam distillation are examples of traditional procedures. Although these procedures are commonly used, they have numerous drawbacks: they are generally time-consuming, requiring massive quantities of polluting solvents which are susceptible to temperature, causing thermo labile metabolites to degrade (18). For extracting analytes from solid matrices, novel extraction techniques such as supercritical fluid extraction (SFE) and pressurized solvent extraction (PSE) have been developed [22]. SFE is a comparatively recent and an operative separation technology for extracting essential oils from various plant sources. Extracts could be applied as a viable substrate for pharmaceutical medications and additives in the perfume, cosmetics, and food industries. SFE has been shown to be active for essential oil separation and its derivatives for application in the food and pharmaceutical industries. This is found to yield high-quality essential oils which have more acceptable structures other than those obtained by orthodox hydro-distillation.

2.1.2 Trans-esterification

Both homogeneous and heterogeneous catalysis have been used to trans-esterify biomass such as microalgal oils for biodiesel synthesis. Because it catalyzes the reaction at low temperature and atmospheric pressure and can produce a significant conversion yield in a short period, homogeneous alkaline catalysis has been the most widely utilized method for biodiesel production. Alkaline catalysts including sodium hydroxide (NaOH) and potassium hydroxide (KOH) are extensively employed; however, because of the high free fatty acid concentration in microalgal oils, alkaline catalysts cause the free fatty acids in oils to generate soap and are not suited for microalgal biodiesel generation. As the content of free fatty acids is greater than 1%, acid catalysts are utilized to overcome the constraint of high free fatty acid content [23]. Sulfuric acid (H₂SO₄) and hydrochloric acid are the most used acid catalysts (HCl). In comparison to alkaline catalysts, they require longer response times and a higher temperature. Initially, an acid catalyst is utilized in some research to convert free fatty acids into esters by esterification. After the free fatty acid content in the oils has been decreased to less than 1%, the oils undergo a second transesterification phase employing an alkaline catalyst. Regardless of the excellent conversion yields achieved by homogeneous catalysts, catalyst loss occurs after the process. In this regard, heterogeneous catalysts are known to contribute significantly to the future for their advantages in terms of recovery and reuse [24].

2.2 Thermochemical conversion processes

This is a cost-effective technology. Dry (non-aqueous) and hydrothermal techniques are two types of dry (non-aqueous) procedures [20]. Biomass undergoes structural breakdown which degrades to condensable vapors, and eventually disintegrating to gaseous molecules in a dry thermochemical transformation method as the temperature rises. A better understanding of everything from the process of decomposition of a single component to the technoeconomic evaluation of the biofuel sector is needed to achieve commercial synthesis of biofuels via thermochemical transformation of biomass [25].

2.2.1 Conventional combustion

This is defined as the oxidative chemical reaction that produces light, heat, smoke, and gases in a flame front when combustible elements (hydrogen and carbon) are ignited in fuels. Nitrogen is relatively inert, though it burns endothermically with oxygen at high temperatures to generate the undesirable NOx pollutants [26]. Combustion techniques now provide a significant amount of biomass-based renewable energy [27]. Wood, dry leaves, hard vegetable husks, rice husks, and dried animal manure are all examples of biomass that can be burned in combustion plants. An exothermic chemical reaction occurs during the combustion process. When biomass is burn’t in the presence of oxygen, chemical energy is released. At about 800 to 1000°C, combustion occurs inside the combustion chambers. It’s worth noting that the biomass utilized to produce biofuels by combustion must have a moisture content of less than 50%. Traditional wood use is inefficient (sometimes as low as 10%) and causes pollution with dust and soot. The adoption of considerably improved heating systems, such as those that are automated, have catalytic gas cleaning, and use standardized fuel, has resulted from technological developments [25].

Effective biomass-to-electricity/heat conversion is achievable because of fluidized bed technologies and better gas purification. Biomass co-combustion, particularly in coal-fired power plants, is considered a single most rapidly developed biomass conversion route in numerous EU countries (including Spain, Germany, and the Netherlands). The benefits of co-firing are clear, with features such as improved total electrical efficiency (often about 40%) because of existing plant economies of scale, and little to non-existent investment costs when high-quality fuels such as pellets are utilized [26, 28]. Furthermore, direct avoided emissions are significant due to the direct substitution of coal. Since several coal-fired power plants are completely depreciated, co-firing is generally a very beneficial greenhouse gas (GHG) countermeasures alternative. Additionally, biomass combustion reduces sulfur and other emissions. Because many plants currently have some co-firing capability, there is a growing need for increased co-firing shares (up to 40%) [21].

2.2.2 Carbonization

Carbonization is the process of converting waste biomass into high-carbon, high-energy charcoal [10]. It redefines renewable energy and power producing principles. Char is made through a pyrolysis process in which biomass is burned to high temperatures in an inert atmosphere until the absorbed volatiles are released, hence increasing its heating value and energy content. Carbonization is an old process that is still employed today, but the increasing interest in it, particularly with biomass, stems from the fact that it opens new commercial and scientific opportunities. The carbon in the created char may be removed to make the valuable graphite and graphene. On a weight basis, the efficiency of these archaic systems is regarded to be quite low. For such operations, the wood to charcoal conversion rate is predicted to be between 6 and 12 tonnes of wood per tonne of charcoal [29]. Carbonization, also known as “dry wood distillation”, removes most the wood’s volatile components. Carbon accumulates mostly as the oxygen and hydrogen levels in the wood decline. The wood experiences a variety of physico-chemical changes as the temperature rises. The majority of water evaporates between 100 and 170°C, and gases, including condensable vapors like CO and CO2, between 170 and 270°C. Following that, condensable vapors (those with long carbon chain molecules) produce pyrolysis oil, which is used to generate chemicals or fuels. Exothermic reactions are defined as those that occur between 270 and 280°C and are characterized by the spontaneous creation of heat.

The advancement of industries such as the charcoal industry has resulted in significant improvements in production efficiency, with commercial synthesis, particularly in Brazil, currently with efficiency levels of >30%. The three main methods of generating charcoal are internally heated (by controlled burning of the raw material), externally heated (using fuelwood or fossil fuels), and hot circulating gas. Internally fired charcoal kilns are the prevalent type of kiln. It is estimated that these kilns waste 10–20% of the wood (w/w), with another 60% (w/w) lost in the transformation to, and emission of gases into the atmosphere [29]. Externally heated reactors fully eliminate oxygen, yielding higher-quality charcoal on commercial scale. They do, however, need the application of an external fuel source, which can be obtained from “producer gas” once pyrolysis has started.

2.2.3 Liquefaction

Thermochemical transformation of biomass to liquid fuels in a hot, pressurized water environ long enough to disintegrate the solid biopolymeric framework into predominantly liquid constituents is known as biomass hydrothermal liquefaction [30]. Hydrothermal processing temperatures range from 523 to 647 K, with working pressures ranging from 4 to 22 megapascal (MPa). The technique is meant to treat wet materials without the necessity for drying and provide access to ionic process parameters using a liquid water processing medium. The temperature is high enough to trigger pyrolytic process in biopolymers, and the pressure is high enough to control the liquid water processing phase. Hydrothermal method is classified into three distinct stages based on the severity of the working conditions. At temperatures <520 K, hydrothermal carbonization happens [17]. Hydrochar is the main product, and it resembles low-rank coal in qualities. The hydrochar from microalgae is largely made up of the carbohydrate and protein fractions, with the lipid fraction remaining intact, allowing the lipids to be recovered during hydrothermal carbonization.

At intermediate temperatures between 520 and 647 K, this process is called hydrothermal liquefaction (HTL), a promising thermochemical liquefaction technique and it produces a liquid fuel called bio-crude. Biocrude is like petroleum crude, and it may be used to make all the petroleum distillate fuel products. Gasification reactions take control at temperatures above 647 K, and the process is known as hydrothermal gasification, which creates synthetic fuel gas. One of the merits of hydrothermal gasification over liquefaction stems from the fact that the water phase that follows gasification contains less organic carbon, resulting in improved carbon efficiency [31]. In each case, the underlying goal is to remove oxygen to produce a final product which has a higher energy density. Unlike HTL, thermochemical liquefaction of biomass has received recognition in recent years as it provides a greater energy density and has a faster reaction time, and it can be used on a wider range of materials. HTL can efficiently treat wet and dry biomass without lipid content limitations, from lignocellulosic to organic waste. The product created in this process is known as bio-crude, which is the renewable analog to oil, because it is an energy-dense intermediate that may be refined to a fuel [22].

2.2.4 Pyrolysis

By adding heat to a feedstock in the absence of oxygen, long chain molecules are broken down into short chain molecules through pyrolysis [32]. Figure 2 depicts different bioenergy production routes of pyrolysis. Pyrolysis occurs at temperatures between 300°C and 700°C while the mild pyrolysis know as torrefaction (of wood chip) is evident at temperatures below 300°C [9]. The process is used in the manufacturing of syngas from biomass or waste as input (a mixture of hydrogen, volatile organic compounds, and carbon monoxide). By modifying the process settings, it is necessary to synthesize fluids similar to diesel and a variety of various products. Because of a greater understanding of the physical and chemical parameters that control pyrolytic reactions, the optimisation of reactor settings required for certain forms of pyrolysis has been made possible. More research is currently ongoing to produce hydrogen using high-pressure reactors and producing alcohol from pyrolytic oil using low-pressure catalytic techniques (which require zeolites) [20]. The advantages of pyrolysis and gasification is the conversion of their solid materials into vapor which are further burnt in turbines, providing fuel flexibility and security. The heat required to drive the chemical reactions that generate syngas is a key disadvantage of both technologies. As a result, some fuel must be used in the syngas production process.

Figure 2.

Pathways of pyrolysis processes for bioenergy production adapted from [32].

2.2.5 Gasification

Gasification is the process of partially oxidizing an organic feedstock to generate syngas (a mixture of hydrogen, volatile short chain organic compounds, and carbon monoxide) [33]. The fuel is typically biomass or waste, and the chemical proportions in the syngas can be controlled by changing the process conditions. The conversion of CO₂ from outside of a biomass into fuels such as the those in their synthetic forms are used in this technique for meet high carbon demands from renewable sources. The huge carbonate deposits on the planet and carbonates arising from the sea, containing about 360 parts per million (ppm) of CO₂ by volume, might all be used as renewable carbon resources [21]. As demonstrated in Figure 3, it can be produced using biomass gasification techniques and then converted into a variety of chemicals and fuels. For continuous water splitting, these can be subjected to electrochemical, biochemical, thermochemical, microbial, photolytic, and biophotolytic operations. Biomass represents about 10.5% of total energy utilization in most developed countries, according to estimates provided by the International Energy Agency (IEA) from a study of 133 countries in 2000.

Figure 3.

Sequence for derivation of syngas from biomass adapted from [20].

2.3 Biochemical conversion processes

Biochemical conversion mechanisms disintegrate biomass using enzymes produced by bacteria and other microbes. Microbes are employed to carry out the biomass transformation operation in most cases. Biochemical conversion is one of the few methods for extracting energy from biomass that is environmentally friendly.

2.3.1 Fermentation mechanism

Fermentation is a biochemical technique applied for bioethanol production after biomass pretreament (makes the cellulose accessible) and hydrolysis (breaks the polysaccharide in feedstock to free sugar molecules). Fermentation is a metabolic operation that uses enzymes to induce chemical reactions in organic feedstocks. There are three fermentation processes that are frequently employed in bioethanol synthesis: separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), and simultaneous saccharification and co-fermentation (SSCF). Separate hydrolysis and fermentation (SHF) is the most popular approach utilized in bioethanol synthesis. The hydrolysis of lignocellulosic biomass is excluded from the ethanol fermentation process in SHF. It is possible to deploy enzymes at elevated temperatures for improved efficiency while fermenting microbes can be utilized at mild temperatures for optimal sugar consumption. SSF and SSCF have a brief entire operation since the enzymatic hydrolysis and fermentation processes take place concurrently to keep the level of glucose as minimal as possible during the operation. In SSF, the fermentation of glucose is segregated from the fermentation of pentoses, but in SSCF, the fermentation of glucose and pentoses are carried out in the same facility [34]. SSF and SSCF are preferable over SHF because the procedure can be completed in the same vessel. The advantages of both procedures are cheaper costs, larger ethanol yields, and reduced operating times [35].

Fermentation of bioethanol can be done in a batch, fed-batch, repeated batch, or continuous mode, depending on the process. In a batch method, the feedstock is delivered at the start of the operation and the media is not added or removed throughout the operation [36]. This mode of fermentation is beneficial due to the absence of labour skills and ease of biomass management [37]. Continuous methodis accomplished by continuously introducing feedstock, culture medium, and nutrients to a bioreactor comprising functional microbes [38]. The merits of continuous systems over batch and fed-batch systems include increased yield, smaller bioreactor volumes, and lower capital and operating expense [37]. Fed-batch fermentationis an integration of batch and continuous modes of fermentation that involves the input of feedstock into the fermentor without withdrawing the medium from the fermentor. It has been successfully employed to mitigate the issue of biomass inhibition in batch operations. Comparing this procedure to other modes of fermentation, it accounts for an increased efficiency, produces more dissolved oxygen in the medium, requires less fermentation duration, and has a less harmful impact on the medium constituents [39].

2.3.2 Anaerobic digestion

Anaerobic digestion (AD) is a mechanism in which microbes disintegrate organic matter in the absence of oxygen, including animal dung, wastewater biosolids, and food residues. In order to produce biogas (biomethane), anaerobic digestion must occur in an airtight vessel known as a bioreactor, which can be built in a variety of forms and dimensions to accommodate the site’s and biomass requirements. These bioreactors include diverse microbial populations that decompose (or digest) the residue and generate biogas and digestate (the solid and liquid substance end-products of the AD operation), which are released from the digester once the waste has been broken down [40]. However, anaerobic co-digestionis the method of combining different organic substances in a single digester. Co-digested resources comprise manure, food wastes (including processing, distribution, and consumer generated materials), energy crops, crop wastes, and fats, oils, and greases (FOG) from restaurant grease traps. Co-digestion can raise the quantity of biogas produced from organic residue that is low yielding or challenging to digest.

The mechanism of anaerobic digestion is divided into four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In single-stage batch bioreactors, all residues are fed at the same time, and all four approaches as shown in Figure 4 are permitted to take place in the same reactor consecutively; the compost is then discharged after a specified retention interval or after the termination of biogas generation [39]. Hydrolysisis employed to break down organic macromolecules into their constituent parts, which can then be used by acidogenic bacteria [41]. During acidogenesis, acidogenic microbes are capable to manufacture intermediate volatile fatty acids (VFAs) and other compounds by accumulating the products of hydrolysis via their cell membranes and converting them into other products [42]. Acetogenesisis the mechanism by which these high VFAs and other intermediates are transformed into acetate, with H₂ and/or CO₂ being generated during the operation [42]. Methanogenesisis the ultimate step of anaerobic digestion, during which readily available intermediates are consumed by methanogenic microbes, resulting in the production of methane. The environmental requirements of methanogenesis are as follows: greater pH than earlier phases of anaerobic digestion, as well as lower redox potential [43].

Figure 4.

Four-step anaerobic digestion process adapted from [43] with modifications.

2.4 Biological process

Using microbes (microalgae and Cyanobacteria or blue-green algae), it is possible to produce biogas by a variety of processes, including biophotolysis, photo fermentation and dark fermentation.

2.4.1 Biophotolysis

Biological photolysis occurs when light or a microbiological species is present and leads to the dissociation of H2O into molecular H2 and O2. Biophotolysis is a metabolic mechanism that is reliant on light and can be classified into two types: direct photolysis and indirect photolysis [44, 45].

Direct Biophotolysis is a light-dependent route for hydrogen formation which occurs in two stages: first, the breakdown of H2O molecules in photosynthesis (Eq. 1), accompanied by the synthesis of hydrogen facilitated by hydrogenases (Eq. 2), which occurs in green algae and cyanobacteria and depends on light energy [46].

$${\text{2H}}_2\mathrm{O}\to {\text{4H}}^{+}+{\text{4e}}^{-}+{\mathrm{O}}_2$$E1

$${\text{4H}}^{+}+{\text{4e}}^{-}\to {\text{2H}}_2$$E2

Direct bio-photolysis comprises H2O oxidation as well as a light-dependent electron exchange to the [Fe] hydrogenase, which leads to H2 generation through photosynthesis [47]. Direct bio-photolysis was based on the photosynthetic ability of microalgae and cyanobacteria to quickly breakdown H2O into oxygen and hydrogen. Microalgae can employ solar energy via proton and electron obtained from the H2O—splitting process, but cyanobacteria receive their energy from photosynthetic activity to enhance H2 generation, which takes place by direct adsorption of light and electron transfer to two enzyme cateories—hydrogenase and nitrogenase [48]—responsible for the enhancement of the transformation of hydrogen ions to hydrogen gas [49]. These techniques showed tremendous potential, but they also had major limitations, such as the discrepancies of direct bio-photolysis to simultaneously generate H₂ and O₂, as well as the fact that the O₂ produced by bacteria throughout the procedure prevents considerable H2 production from being achieved.

During indirect biophototlysis, photosynthetic H2 can be formed by green algae amid sulfur deprivation conditions, as opposed to direct bio photolysis [48]. The restriction of sulfur—nutrients in the growth media of green algae prompted a reversible impediment in the O₂ photosynthetic operation of the green algae. Sulfur deprivation triggers a decrease in the activity of the photosystem II (PSII), which is responsible for enhancing electron extraction from water through photochemical oxidation, and the photosynthetic process decreases below the respiration activity, resulting in a decrease in oxygen discharge below the amount of oxygen expended by respiration [50]. The synergistic effect between photosynthesis and respiration attributed to sulfur deprivation leads to a net utilization of oxygen by cells, which enables the growth environment to become anoxic [51]. The potential to develop ways to reuse constituents of the photobioreactor and optimize the cost of chemical nutrients that aids algae development which account for around 80% of the overall operational costs are two of the setbacks of efficient commercial application of indirect bio-photolysis for biogas synthesis [52, 53].

2.4.2 Photo fermentation

Under anoxic environments with light, photosynthetic microbes are capable of converting the majority of organic acids or volatile fatty acids (VFA) into biohydrogen and carbon dioxide [54]. Nitrogenase is the enzyme responsible for the majority of the biohydrogen produced by photosynthetic bacteria. Luminous light has a significant effect on the synthesis of nitrogenase [55]. It is essential for biohydrogen synthesis that the feedstock have an appropriate ratio of carbon and nitrogen sources (C/N ratio). Nitrogen constraints have been shown to modify the metabolic activities of photosynthetic bacteria, directing it more towards the discharge of extra energy and reducing power in the form of biohydrogen. The process of photo fermentation is influenced by some variables, such as light intensity, inoculum age, nutrient type, and temperature. Temperature has a significant impact on the metabolic routes’ ability to shift to greater biohydrogen synthesis [56]. The biohydrogen metabolism of purple non-sulfur bacteria is primarily controlled by the activity of the enzymes; nitrogenase and hydrogenase [56]. As part of the process, the nitrogenase enzyme generates biohydrogen under nitrogen-deficient environments (Eq. 3), where the hydrogenase enzyme oxidizes the biohydrogen in order to reuse electrons, protons, and ATP for employ in energy metabolism [57, 58]. Because hydrogenase enzyme can operate in any direction, according to Eq. 4, some of them are physiologically dedicated to utilizing biohydrogen (in the presence of appropriate electron acceptors) while others are responsible for the synthesis of biohydrogen under stringent anaerobiosis [59].

Light

$${\text{2H}}^{+}+{\text{2e}}^{-}+\text{4ATP}\to \mathrm{H}2\mathrm{v}+\text{4ADP}+\text{4Pi}$$E3

Nitrogenase

$${\mathrm{H}}_2\to {\text{2H}}^{+}+{\text{2e}}^{-}$$E4

Nitrogenase

The overall metabolic route for the photo fermentation system is given as:

$$\text{Substrate}\to \text{TCA cycle}\to \text{NAD/NADH}\to \text{Ferredoxin}\to \text{Nitrogenase}\to {\text{H}}_2.$$

2.4.3 Dark fermentation

Anoxic and certain microalgae (green algae) perform heterotrophic fermentation on carbohydrate-based substrates in the absence of light energy, resulting in the synthesis of hydrogen [60]. When it comes to dark fermentation, the practicality of producing hydrogen is dependent on the fact that hydrogen can be generated by heterotopic bacteria satellites that are situated in the algae biomass slurries. The impediment of H₂-consuming microorganisms in a multi-microbial consortium that disintegrates algal biomass for the generation of H2 is a vital issue that presents a barrier to the effective use of dark fermentation technology. Dark fermentation is a mechanism in which organic feedstock are transformed by fermentative bacteria into biohydrogen, volatile fatty acids (VFA), and carbon dioxide in the absence of light. Carbohydrates (mostly glucose) are the primary energy sources for this mechanism, which results in the production of biohydrogen as well as volatile fatty acids (VFAs) such as acetic acid and butyric acid. Eqs. (5) and (6) demonstrate variation in product yield when acetic acid or butyric acid is the sole VFA product, the highest output of 4 and 2 mol H₂/mol glucose respectively can be obtained. A lesser output is frequently attained in reality, because glucose is not only utilized for biohydrogen generation, but also to nourish and sustain the development of the microbes [60]. Biohydrogen generation via this approach can be influenced by substrate, inoculum, bacteria growth conditions, and other operating parameters.

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+{\text{2H}}_2\mathrm{O}\to {\text{4H}}_2+{\text{2CH}}_3\text{COOH}+{\text{2CO}}_2$$E5

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+{\text{2H}}_2\mathrm{O}\to {\text{2H}}_2+{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\text{COOH}+{\text{2CO}}_2$$E6

2.4.4 Proposed multi-stage bioreactor for biogas production

A multi-stage bioreactor can be employed for the production of biohydrogen or hythane. A four-stage bioreactor produces significant amounts of hydrogen and recovers energy. In the first step which involves direct biophotolysis, blue-green algae employ visible light whereas photosynthetic microorganisms utilize unfiltered infrared rays in the second stage photo-fermentative reactor. The second phase photosynthetic reactor discharge is passed to a third stage dark fermentation for microbial transformation of substrates into H₂ and organic acids. The fourth stage involves converting organic acid (from dark fermentation) into biohydrogen via microbial cell electrolysis in the dark (ideally at night or in low light) [47]. The growing interest in hythane has led to substantial study into dark fermentation of biomass for hythane generation in two-stage processes. Hythane is a gaseous combination of H₂ (10–30%) and CH₄ (70–90%) used as a substitute to methane in the automotive sector. Hythane is now produced mostly from fossil fuels, however using sustainable sources will significantly minimize greenhouse gas emissions. The efficient biotechnology process of two-stage anaerobic digestion (AD) can generate biohythane in two-stages, dark fermentation, and methanogenic phases, for H2 and CH₄ synthesis respectively. Because H₂ is a sustainable energy source, its existence in hythane facilitates the reduction of CO₂ and NOx emissions. This product (hythane) is a clean-burning green energy that could be used as industrial biogas [61]. However, various issues need to be addressed before the multi-stage bioreactor technology may be efficiently utilized [62].

3. Economic and environmental implications, limitations and prospects

3.1 Economic feasibility of biofuel compared to fossil fuel

The rise of international bioenergy markets is critical to maximizing the utilization of global biomass resources and market potential [63]. The global biomass and biofuel markets, on the other hand, are still expanding and are subject to tariffs and non-tariff trade restrictions, resulting in substantial and often unexpected changes in the international trade flows [64, 65]. In contrast to fossil fuel markets, bioenergy markets have limited trade flows, which exacerbates these problems. Additionally, feedstock supply (easily accessible), offtake (easily secured contracts), capacity utilization (75%) and sustainability compliance are key factors required for bioenergy plant establishment [63, 65, 66]. According to Bloomberg New Energy Finance [65] the annual value of renewable energy capacity can rise from 395 USD billion in 2020 to 460 USD billion in 2030. This can result in bioenergy market been expanded by 7 USD trillion for the next two decades. The use of biofuel can be economically and environmentally advantageous to both developed and undeveloped countries [63, 65, 66]. Consequentially, biofuels have the potential to be a sustainable, renewable, and viable energy source, especially in the transportation sector. This makes the biofuels industry to have many potentials with ecological and economic benefits [67, 68].

However, when compared to the gasoline cost of production (from fossil fuel), which is about 0.3 USD – 0.4 USD/LGE (liter per gasoline equivalent) in 2020, sugarcane and corn ethanol production cost is approximately 0.40–0.50 USD/LGE, making ethanol less competitive commercially [69]. Likewise, sugar beet, maize, or wheat ethanol cost between 0.6 USD and 0.8 USD/LGE. The comparatively higher price and energy content of ethanol are significant drawback to its utilization as a viable sustainable biofuel and as a gasoline additive. The energy content of a gallon of ethanol is approximately one-third that of a gallon of gasoline. Consequently, ethanol has not been economically viable when likened to gasoline; however, with government incentives, the cost of producing ethanol will be significantly reduced [70]. In actual fact, when compared to fossil fuels, the use of biofuels will minimize the net cost of fuel through biofuel regulations which may reduce fossil fuel use by less than 2.5% at a cost of 67 USD billion plus a 6 USD billion gas tax [63, 65, 66]. The primary concern is that, in the near future, more biofuels will make overall fuel costs more expensive than fossil fuels. Notwithstanding, the long-term savings in fuel prices may offset the initial expenditures [71].

3.2 Environmental impacts and benefit

Replacing fossil fuels with biofuels (fuels made from renewable organic material) is possible to reduce conventional and greenhouse pollutant emissions. Additionally, producing energy from biomass has substantial distinctive environmental benefits. The abatement of acid rain, soil erosion, water pollution, and landfill pressure, while also providing habitat for wildlife and improving forest reserves through proper management are among some of the advantages [72, 73]. Although there are certain uncertainties about employing biomass indirect combustion, gasification, or pyrolysis processes can provide still significant environmental benefits. For instance, the production of SO2, CO2, and ash is often much lower in biomass power systems than in coal combustion and conversion systems [68, 72, 74]. The sources and side effects of coal combustion which makes biomass combustion more advantageous include reduce emission Hazardous air pollutants (HAP) and SO2 of the following [75]. Hence, sulfur and nitrogen content of biomass combustion are so low to be neglected.

Biomass, on the surface, appears to be an appealing renewable fuel for boilers, even though its composition is liable to change. For example, the ash composition of biomass differs significantly from the ash composition of coal. Also, many undesired processes in combustion furnaces and power boilers are caused by metals in ash when combined with other fuel constituents such as silica, sulfur, and chlorine [72, 73, 75, 77]. Conversely, in biomass combustors, elements such as Si, K, Na, S, Cl, P, Ca, Mg, and Fe are engaged in processes that can contribute to ash fouling and slagging [76, 77]. The effects of biomass content on combustion are non-hazardous and provide great environmental safety. The principal benefit of using biomass energy is the reduction of greenhouse gas pollution. Furthermore, reburning of biomass fly ash as a fuel-flexible material can provide well-burnt ashes for common fuels. Additionally, eliminating ash stabilization (chemical hardness) can significantly enhance ash potential. This can reduce NOX emissions by 20% while slightly increasing CO emissions. However, the rise of CO level is usually around 100–140 ppm, which are within the permissible average limit of 150 ppm CO [67, 75, 77]. Also, the ash produced can be returned into the forest, replenishing the nutrients loss by the soil. Therefore, the nutrient compounds in the ash can be recycled or repurposed as fertilizers for good sustainable energy practices based on biomass.

3.3 Limitations of bioenergy production

Improper burning of biomass releases CO2, N2O, CH4, and other hydrocarbons, all of which are detrimental to health. Human activity contributes 60% to global climate change [67, 73]. Activities such as using chemicals like chlorofluorocarbons (15%), agricultural biomass (12%), land-use alterations (9%) and other human activities (4%) also contribute to high levels of greenhouse gases in the atmosphere [68, 75]. Currently, global greenhouse gas emissions are increasing year on year. CO2 has been increasingly linked to global warming [78, 79]. The greenhouse effect caused by gases (with three or more atoms) with higher heat capacity than O2 and N2. The primary human-caused greenhouse gas is CO2 (CO2). CO2 emissions from fossil and biomass fuel combustion significantly contribute to the greenhouse effect and global warming. The reactivity of ash in biomass combustion can be detrimental. In the diverse activities of this sustainable feedstock, trace elements found in biomass play a significant role. Trace elements (usually metals) are biochemically important, as well as nutritionally and environmentally [76, 77, 78]. The amounts of trace element levels are related to biomass species, sample growing site, plant age, and distance from the pollution source. Metals such as Cd, and Hg ions are potentially detrimental to plants. As boilers flue gas undergoes chemical processes, phase transitions, and precipitation because of a wide temperature differential, high element concentrations in both biomass and boiler fly ash are essential [9, 13, 14, 75, 76, 77].

3.4 Potentials and future work considerations for effective bioenergy production

Since fossil fuels have caused havoc on the ecosystem, it is critical to explore solutions. Biofuels can provide energy requirements while limiting environmental impact by exploiting readily available biomass as feedstocks. According to life-cycle analyses, advanced biofuels and cellulosic biofuels have the potential to achieve baseline GHG reduction targets of 50% and 60%, respectively (including indirect land-use change). Although transportation currently contributes around 23% of all CO2 emissions caused by energy use. To achieve a 50% decrease in energy-related CO2 emissions by 2050, sustainably produced biofuels could account for 27% of total transportation fuel consumption [63, 66, 80]. In essence, biofuels derived from waste biomass could be the most sustainable energy alternative to fossil fuels in the transportation industry [81, 82, 83]. Nevertheless, concerns about the biomass supply chain, energy efficiency, and product yield persist. Different processing improvement techniques, either alone or in combination with nanomaterials, may be used to tackle these problems. Advancing biomass combustion technology can result in increased conversion efficiency at a low cost. Additionally, several research have reported on the use of nanomaterials in conjunction with microwave, mechanical vibration, pulsation, and ultrasonication to enhance biofuel production [19, 20]. Compared to other nanocatalysts, ferrofluids are easy to separate and move in oscillating magnetic fields [76, 77]. Therefore, they could be used with some of the technologies to improve the biomass-based energy economy.

Continuous biofuel synthesis using microchemical and Coiled Flow Inverters (CFI) are also possible. Heat transfer fluids (HTF) and ionic liquids (IL) could also be employed in biofuel production to save energy. In the future, the use of biomass in biofuel synthesis and utilization is very promising to be explored to further improve the overall process economy. According to the EU’s Renewable Energy Directive (RED), biofuels must meet certain sustainability standards before they may contribute to the binding national targets each member state [63, 65]. Several attempts to develop sustainability criteria and standards for biofuels are underway in this section. Other international initiatives include the Global Bioenergy Partnership, the Roundtable on Sustainable Biofuels (RSB), and ISO (International Organization for Standardization) standards aimed at increasing bioenergy production’s efficiency and lowering emissions [63, 64].

4. Conclusion

Bioenergy will be the most important sustainable energy source in the coming decades since it provides a cost-effective substitute to fossil fuels. The availability of low-cost biofuel and a wide range of viable forms of biofuel for the generation of heat, steam, electricity, and gas, as well as for use as a transportation fuel, will be critical to the growth of bioenergy. Many different sources, such as crops, grasses, leaves, manure, fruit and vegetable wastes, algae or other lignocellulose biomass can be used, and the procedure can be done on both small and large scale. This enables the production of biofuel everywhere in the entire globe. Significant advancements in process performance of existing technologies, as well as the establishment of novel techniques for biomass conversion, mixing, process monitoring, and process control, are required for further biofuel facility development. However, the major concern is lowering the cost of biofuel synthesis. Consequently, the biorefinery concept is required to more thoroughly exploit sustainable biomass and to produce additional value-added coproducts (e.g. bio-based products from lignin) that would lower the cost of biofuel synthesis. As a result, biofuel will be more cost efficient than fossil fuels to enhance effective transition to bioeconomy.