Biofuels- A Journey Through the Generations

Summary

In response to the world’s demand for sustainable energy sources, biofuels have arisen as a promising alternative to fossil fuels. The journey begins with the first-generation biofuels derived from food-based crops such as sugarcane and corn. These biofuels have advantages over conventional fuels but face challenges due to their competition with food and limited greenhouse gas emissions. To address these challenges, second-generation biofuels have been developed. These biofuels use nonfood-based biomass, including cottonseed oil or agricultural wastes, and produce cellulosic-based biofuels. Second-generation biofuels emit lower greenhouse gases and address feed security problems. However, due to high production costs and the need for pretreatment technologies, these biofuels require additional research for greater efficiency. The next focus shifted toward third-generation biofuels, which introduced oleaginous feedstocks, including algae or cyanobacteria. These biofuels have high oil content and energy density, but commercialization is difficult due to high cultivation costs and the need for advanced technology. This resulted in the introduction of fourth-generation biofuels based on lignocellulosic biomass genetic modification. These biofuels minimize land competition and water use and are carbon negative. However, they require sustainable development to be commercialized. This article gives a concise overview of the different generations of biofuels. The problems and prospects of these generations have also been covered at the end of the article.

Introduction

Biofuels are produced from organic materials known as biomass, including plants and microbial or animal waste. They may be used as substitutes for fossil fuels in transportation, electricity generation, and heating to minimize carbon dioxide emissions by producing a closed-loop system in the carbon cycle.

Carbon dioxide is used by biomass feedstock for growth, which is then converted into fuel, burned, and the cycle continues. Currently, there are two common types of biofuel: ethanol and biodiesel. Bioethanol is an alcohol produced by the fermentation of carbohydrates in sugar or starch crops.

Ethanol can be used as a fuel for vehicles in its pure form, but it is usually used as a gasoline additive to increase octane ratings and improve emissions. Biodiesel is produced from oils and fats via transesterification. It can be used as a fuel for vehicles in its form, but it is usually used as a diesel additive to reduce particulates, carbon monoxide, and hydrocarbons in diesel-powered vehicles. Other biofuels include biogas, syngas, methanol, butanol, straight vegetable oil, bio-jet oils, bioether, etc.

History or Background

Since the discovery of fire, humans have been utilizing biofuels as a source of energy. Biofuels during that period included wood, crop residues, and animal manure. Since the early 1700s, vegetable oil has been used in lamps for lighting, and the first biofuel-based engine appeared in 1824.

However, their use was undermined by the discovery of fossil fuels during the Industrial Revolution. Rudolf Diesel, a German scientist, invented a diesel engine that could run on vegetable oil in the late 1800s. Likewise, Henry Ford used crop-based ethanol in his Model T automobile in 1908 [1]. Due to the scarcity of fuel during World War I, biofuels gained interest in replacing fossil fuels.

Biofuels can be classified into generations based on feedstock and production technologies (Figure 1). During World War I, first-generation biofuels were derived from edible crops (corn, sugar beet, wheat, or rice barn). These biofuels were used instead of petroleum-based fuels, either pure or blended with other fuels. In World War II, biofuels were again in great demand due to the shortage of conventional fuels [2].

Due to global warming and economic concerns in the twentieth century, biofuels have become an essential alternative to fossil fuels because they are cost-effective, ecologically beneficial, and may help reduce dependence on fossil fuels. However, owing to the concerns relating to first-generation biofuels, such as food security and land usage, researchers have begun focusing on other biomass sources, such as nonfood parts of plants and grasses.

Even though second-generation biofuels have addressed the problems associated with first-generation biofuels, they still face challenges, including inefficient conversion due to the complex cellulosic structure and the need for land. Advancements in biotechnology, process technologies, and genetic engineering have driven continuous research and development of biofuels to overcome the limitations of second-generation biofuels. This resulted in exploring third-generation biofuels derived from algae and other nonfood biomass and fourth-generation biofuels created using synthetic biology and carbon capture technologies [3].

Figure 1: Different generations of biofuels [4]

Biofuel Production Processes

Biofuels are available in the form of solids, liquids, or gases. Some solid biofuels (wood) can be used directly for heat production. However, all liquids (such as ethanol and biodiesel) and gaseous biofuels (such as methane or biogas) require specific treatments for optimum burning. Several processes convert biomass into fuel depending on the type of biomass and the application of fuel, including direct combustion, bacterial decomposition, and conversion into liquid or gas [5]. Figure 2 shows the several routes involved in biomass conversion into biofuels.

Figure 2: Different routes for biomass conversion into biofuels [5]

Both direct combustion and bacterial decomposition processes generate energy that may be used to produce heat. The conversion of biomass into liquid or gas is commonly done using fermentation. Microorganisms (yeast or bacteria) are used in this approach to ferment sugar and produce biofuel.

The advanced processes for converting biomass into biofuels involve multiple steps, including the deconstruction step and upgrading. Depending on the feedstock type, deconstruction breaks down the complex structure of biomass into essential components at high or low temperatures. Pyrolysis, hydrothermal liquefaction, and gasification are examples of high-temperature deconstruction, while hydrolysis is an example of low-temperature deconstruction.

Figure 3: Steps involved in biomass conversion into liquid or gaseous fuel [6]

In pyrolysis, biomass is broken down in the absence of oxygen to produce a bio-oil intermediate, whereas, in hydrothermal liquefaction, wet biomass is converted into bio-oils under elevated pressure and temperature. Gasification is the process of heating biomass at high temperatures in the presence of oxygen to produce hydrogen gas, followed by cleaning and conditioning to convert it into a functional form.

In hydrolysis, biomass is chemically or mechanically processed to open its structure before being broken down with other chemicals or enzymes to produce fuel intermediate or other valuable molecules. Intermediate produced during the deconstruction step are then upgraded to produce valuable biofuels, which may be accomplished using chemical or biological processing depending on the product required [6].

First Generation Biofuels

First-generation biofuels have emerged as an effective renewable energy option, utilizing food crops as the primary feedstock for biofuel production. They can be produced by converting the sugar, starch, or oil content of food crops into bioethanol or biodiesel.

Bioethanol is produced through fermentation, while biodiesel is derived from transesterification, a process that involves the transformation of vegetable oils, recycled cooking oils, or animal fats. These biofuels have garnered attention for their potential to reduce greenhouse gas (GHG) emissions compared to fossil fuels, thus contributing to climate change mitigation.

By displacing conventional fuels, first-generation biofuels can enhance energy security by decreasing dependence on finite and geopolitically sensitive fuel sources. They offer a renewable and sustainable alternative that has gained momentum worldwide [7].

In the production process of these biofuels, glycerin is obtained as a by-product, and the biodiesel is refined in a distillation column to remove impurities. First-generation bioethanol is often produced using corn, potato, wheat, and sugarcane crops. These crops are fermented using yeast to convert sugar or starch into ethanol [8]. The various amounts of bioethanol produced by different feedstocks are given in the following table,

Table 1. The amounts of bioethanol produced by different feedstocks

Feedstock	Amount of Bioethanol Produced (L/kg of biomass)	References
Rice straw	117	[9]
Corn meal	10 – 11	[10]
Corn stover	5 – 6	[11]
Barley straw	0.05	[12]
Palmwood	0.023	[13]
Eucalyptus	0.6	[14]
Olive tree pruning	0.02	[15]
Paddy straw	0.001	[16]

However, the utilization of first-generation biofuels is not without its challenges and limitations. Some of them are discussed as follows [17-21],

One of the most pressing concerns is the potential impact on food security. The use of food crops for biofuel production competes directly with their utilization for human consumption. This competition can increase food prices, exacerbate food scarcity, and negatively affect vulnerable populations. Striking a balance between biofuel and food production is crucial to mitigate these concerns
The cultivation of crops for biofuel production can have significant environmental repercussions. Expanding agricultural land to meet biofuel demands may lead to deforestation, habitat destruction, and biodiversity loss
Sustainable land management practices and stringent regulations are necessary to prevent these adverse consequences and ensure the environmental sustainability of biofuel production
Another consideration is the energy balance and carbon emissions associated with first-generation biofuels. While they are generally considered renewable and have a lower carbon footprint than fossil fuels, some studies have raised concerns about their overall net energy gain
The production processes involved, such as crop cultivation, harvesting, and conversion, may consume significant energy and release more carbon emissions than the feedstock captures during its growth. Achieving a positive net energy gain and minimizing carbon emissions are crucial aspects that need to be addressed in developing and implementing biofuel technologies.

Researchers and scientists have been actively exploring alternative biofuel sources to overcome the limitations of first-generation biofuels. Second-generation biofuels, for example, utilize nonfood biomass, including agricultural residues (such as corn stover and wheat straw), energy crops (like switchgrass and miscanthus), and forest residues.

These feedstocks reduce competition with food products because they are derived from agricultural and forestry waste materials. Moreover, advancements in technology and research have led to the emergence of third-generation biofuels, such as algae-based biofuels. Algae offer higher oil yields and can be cultivated in non-arable land or wastewater, minimizing the environmental impact and enhancing overall sustainability.

First-generation biofuels have demonstrated their potential as renewable energy sources with the capacity to reduce GHG emissions and enhance energy security. However, challenges related to food security, environmental sustainability, and net energy gain have prompted the exploration of alternative biofuel sources [22].

Second Generation Biofuels

The second generation utilizes nonfood crops and agricultural or municipal waste materials as feedstock to solve the issues associated with first-generation biofuels. Second-generation feedstocks, known as lignocellulosic materials, contain lignin, hemicellulose, and cellulose that are difficult to convert into fermentable sugars. Therefore, physical, thermochemical, and biochemical treatments have been proposed to synthesize biofuels from second-generation feedstocks [23].

Physical treatments include the following,

Briquetting: It involves the conversion of biomass into highly dense solid blocks
Fiber extraction: In this process, the biomass is processed to make fibers
Pelletizing: This process involves the formation of pellets from biomass under high pressure.

In thermochemical treatments, pyrolysis, liquefaction, gasification, and direct combustion are used for the production of biofuels. On the other hand, biochemical treatments involve biomass fermentation under different conditions to produce alcohol, carbon dioxide, and water.

Second-generation biodiesel is produced from non-edible oils such as jatropha, jojoba, Karanja, castor, and cottonseed oil, whereas bioethanol sources include grasses, agricultural wastes, and wood chips. The feedstocks of these biofuels are sustainable because of diversified feedstocks, including nonfood crops and waste materials, which reduces food security concerns.

According to the life cycle assessment of second-generation biofuels, they can produce positive energy gains and emit lower GHGs than first-generation feedstocks, lowering their negative environmental impact. Despite their advantages, second-generation feedstocks have several challenges, including the need for advanced technologies for feedstock pretreatment, inefficient conversion owing to their complex structures, and the requirement for land and water for feedstock cultivation [17].

The logistics of providing a competitive and year-round supply of second-generation biomass feedstock is a significant challenge. Likewise, only limited improvements are possible in the performance and costs associated with the commercial-scale conversion process [24]. Biofuel production from lignocellulose is not cost-effective, although microbial fermentation is an eco-friendly way to convert lignocellulose into biofuel.

The impact of second-generation biofuels on food security depends on whether the feedstock competes with traditional crops or is a co-product in their production. Dedicated biomass, like warm season grasses, likely competes at least somewhat with food crop production, potentially increasing food prices. Biofuel from crop residues, such as corn stover and wheat straw, can lead to more land in these uses, potentially reducing food and feed prices [18, 25].

Third Generation Biofuels

Third-generation biofuels use oleaginous feedstocks (derived from microalgae or cyanobacteria) with unique characteristics. These feedstocks have high lipids and carbohydrate content, photosynthetic conversion efficiency, high energy density, rapid growth rate, and reduced water and land requirements compared to land-based crops [23].

The carbohydrate content of the oleaginous feedstock is utilized in bioethanol production, and the oil content is used to make biodiesel. Third-generation biofuels are carbon-neutral; they absorb and emit the same amount of carbon from the atmosphere. Owing to these properties, these are considered promising alternatives for producing fuels and chemicals.

The oleaginous feedstock does not require pretreatment, and processes used to transform it into different biofuels include [26],

Thermochemical methods (pyrolysis, gasification, and liquefaction)
Biochemical methods (fermentation and anaerobic digestion)
Chemical reactions (transesterification)
Direct combustion

Third-generation biofuels encompass a range of renewable fuels, including biodiesel, biogas, bioethanol, and biomethane derived from algae. These advanced biofuels offer higher efficiency and lower greenhouse gas emissions than first- and second-generation alternatives. However, the production costs associated with third-generation biofuels remain relatively high due to the energy-intensive nature of algae cultivation.

Algae, as a primary source for third-generation biofuels, require specific growth conditions, including access to nutrients, sunlight, and fresh water. These requirements contribute to the overall energy consumption and costs associated with their cultivation. Researchers have explored wastewater as a nutrient source for algae production to reduce expenses.

However, there are concerns regarding the potential algae contamination when using wastewater. Wastewater may contain hazardous chemicals, which pose a risk to cultivated algae and subsequent biofuel production. Proper wastewater treatment and monitoring protocols are necessary to mitigate these risks to ensure the absence of harmful substances in the cultivated algae [27].

Further research is essential to develop cost-effective biofuel production methods. Current studies focus on optimizing cultivation techniques, improving nutrient recycling systems, and enhancing overall process efficiency.

Additionally, integrating innovative biorefinery concepts can enhance the economic viability of third-generation biofuel production. Biorefineries aim to extract various components from algae biomass, including biofuels, high-value chemicals, proteins, and nutraceuticals. This integrated approach maximizes the value derived from algae cultivation, contributing to the overall cost-effectiveness of the biofuel industry [28].

Third-generation biofuels hold significant promise as sustainable alternatives to fossil fuels. While their production costs remain challenging, ongoing research on optimizing cultivation techniques, utilizing wastewater as a nutrient source, and integrating biorefinery concepts is expected to lead to cost-effective and sustainable biofuel production [17].

Fourth Generation Biofuels

Fourth-generation biofuels are currently under development and focus on advanced technologies and feedstock to further enhance sustainability and efficiency. Fourth-generation biofuels use genetic engineering to modify microalgae, yeast, fungi, cyanobacteria, or nonfood biomass [23].

Genetic modifications involve designing and developing new biological parts or redesigning a natural feedstock. The aim is to increase its oil content while reducing carbon dioxide gas in the environment. In addition to genetic modification, some fourth-generation technologies involve pyrolysis, gasification, upgrading, and solar-to-fuel pathways.

Fourth-generation biofuels represent an advanced category of renewable fuels that surpass the limitations of traditional biofuels. They encompass diverse energy sources such as butanol, biodiesel, hydrogen, and solar fuels.

These innovative biofuels are produced through various methods. One of the prominent is the utilization of photosynthetic microorganisms. This method combines photovoltaic and microbial fuel cells or synthetic cell components engineered explicitly for producing desired fuels [29]. The bio-oil contents produced by different species of algae are given in the following table,

Species	Oil composition (%)	Reference(s)
Stichococcus	30 – 40	17
Dunaliellatertiolecta	32
Botryococcusbraunii	35 – 70
Chlorella	45 – 50
Ankistrodesmus	30 – 40	107
Chlorella protothecoides	40 – 50
Nannochloropsis	35 – 45
Chlorella vulgaris	35 – 40	108

Table 2. Bio-oil contents produced by different species of algae

One of the notable characteristics of fourth-generation biofuels is their ability to consume carbon dioxide during their growth phase and employ carbon dioxide capture technology during combustion. This carbon dioxide capture technology involves techniques that capture the carbon dioxide produced during fuel combustion and subsequently store it underground. By sequestering carbon dioxide underground, fourth-generation biofuels contribute to reducing greenhouse gas emissions and mitigating climate change [30].

The production of fourth-generation biofuels also involve some new technologies that have garnered significant attention in recent years. Photosynthetic microorganisms, such as certain algae and cyanobacteria, are crucial in producing these biofuels. These microorganisms can convert sunlight, water, and carbon dioxide into energy-rich compounds through photosynthesis.

In addition to photosynthetic microorganisms, integrating photovoltaic and microbial fuel cells is another promising approach for producing fourth-generation biofuels. Photovoltaic cells capture sunlight and convert it into electrical energy, which can then be utilized to power microbial fuel cells. Microbial fuel cells employ certain microorganisms to catalyze the conversion of organic matter into electricity. By combining these technologies, researchers can effectively harness solar energy to drive the microbial production of biofuels [31, 32].

Furthermore, developing synthetic cell components for fuel production represents a significant advancement in fourth-generation biofuels. These synthetic components are engineered to mimic natural metabolic pathways, allowing for the production of desired fuels with high efficiency. By constructing artificial cell systems, researchers can optimize the biochemical processes involved in fuel synthesis and achieve enhanced yields and specific fuel compositions [33].

The carbon-negative nature of fourth-generation biofuels holds great promise for sustainable energy production. The ability to capture and store carbon dioxide underground not only helps reduce greenhouse gas emissions but also promotes the utilization of carbon dioxide as a valuable resource. Moreover, using renewable energy sources, such as solar energy, in producing these biofuels ensures a clean and sustainable energy supply.

Fourth-generation biofuels exhibit carbon-negative characteristics by utilizing carbon dioxide during their growth and employing carbon dioxide capture technology during combustion. By storing captured carbon dioxide underground, fourth-generation biofuels contribute to mitigating climate change and offer a promising pathway toward sustainable and environmentally friendly energy production [17].

Path To A Sustainable Biofuel Industry

Sustainability refers to meeting the needs of the present generation without compromising the ability of future generations to meet their own needs. One area of focus in achieving sustainability is promoting economic growth through biofuels. At present, they account for 56% of the overall transportation fuel.

However, it is vital to consider the advantages and disadvantages associated with different generations of biofuels. A number of factors should be considered in this regard,

Production methods
Net greenhouse gas emissions
Energy efficiency

First-generation biofuels have the highest biofuel output and energy efficiency. However, they are less successful in effectively reducing greenhouse gas emissions. While these biofuels offer significant production benefits, their impact on environmental sustainability is limited.

On the other hand, third-generation biofuels demonstrate the potential to reduce net greenhouse gas emissions significantly. They can effectively contribute to the overall reduction of environmental pollution. However, the production process for these biofuels typically requires a higher energy input, often relying on fossil fuels for power generation. The energy consumed during their production often counteracts the emission reductions achieved. This raises concerns about the ecological benefits of third-generation biofuels.

In order to secure a sustainable future for biofuels, it is vital to explore the potential of fourth-generation feedstocks. This generation holds promise as a sustainable source of biofuel. However, further research is needed to identify cost-effective production methods while improving energy efficiency [34, 35].

A precise comparison of the different generations of biofuels is given as follows,

Table 3. A comparison among the different generations of biofuels.

Generation	Feedstock	Conversion Process	Greenhouse Gas Emissions	Land Use Impact	Water Use Impact
First	Food Based Crops (Corn, sugarcane, vegetable oils)	Fermentation or Transesterification	Moderate to High	Significant land conversion	Moderate water consumption
Second	Nonfood Biomass (Castor oil, cottonseed oil, agricultural wastes)	Biochemical or thermochemical	Moderate to Low	Reduced land competition	Moderate water consumption
Third	Oleaginous Biomass (Algae, macroalgae, cyanobacteria)	Biochemical, Thermochemical, or Hydrothermal	Low to Moderate	Efficient land utilization	Low water consumption
Fourth	Genetically modified Lignocellulosic Biomass	Biochemical, Thermochemical	Low to Moderate	Reduced land usage	Low water consumption

An analysis of the literature on biofuels reveals that sustainability requires balancing the current generation’s needs with the well-being of future generations. Biofuels play a significant role in promoting economic growth, but different generations have varying advantages and disadvantages.

While first-generation biofuels offer high output and energy efficiency, they struggle to reduce greenhouse gas emissions effectively. Third-generation biofuels show the potential to reduce emissions but have a higher energy requirement during production, which may limit their ecological benefits. By investing in research and development, we can identify low-cost options and improve energy efficiency, thus paving the way for sustainable biofuel sources in the future. [22].

Future Perspective and Challenges

The future of biofuels should aim to achieve a clean and renewable energy source that reduces the environmental impact and the cost of production. First-generation biofuels have been used over the last many years but are unsustainable due to their reliance on food commodities. Therefore, it is necessary to develop alternative biomass for biofuel production to mitigate global hunger.

Second-generation biofuels use nonfood biomass, thus lowering food competition. They may offer a more sustainable alternative to first-generation biofuels, but their competitiveness with first-generation biofuels and electric mobility is still uncertain. Nonetheless, first- and second-generation biofuels violate the sixteenth Sustainable Development Goal (SDG) goal of preventing land degradation and preserving ecosystems.

As a result, alternative biomaterials like algae, cyanobacteria, lignocellulose biomass, or waste material (third and fourth-generation biofuel) hold the potential to reduce land usage and ensure biodiversity conservation. The transition to third and fourth-generation biofuels is driven by the need to integrate biomass-derived fuels more seamlessly into the existing petroleum-based infrastructure.

Microalgae farming demands a considerable amount of freshwater, which may conflict with the human need for a clean water supply. Nonetheless, microalgae may be grown in wastewater and operate as a natural wastewater remediator. The growth of microalgae in wastewater businesses for biofuel and bioproduct development should be prioritized.

Due to the high cost of processing technologies (e.g., lipid extraction and drying procedure), third-generation biofuels have not yet been completely commercialized. As a result, alternative and cost-effective processing methods must be thoroughly examined and investigated.

Fourth-generation biofuels can address high costs, greenhouse gas emissions, and land and water resource needs. Algae-to-biofuels technology is a crucial aspect of fourth-generation biofuel production. It allows for quick growth of non-toxic and biodegradable materials that can be grown without competing with food or feed crops.

Future research is expected to focus on finding novel biofuel-production species, optimizing culture conditions, genetic engineering of biofuel-producing species, and practical techniques for the mass cultivation of microorganisms.

One of the constraints for fourth-generation biofuel is a lack of gene and biological information on microorganism species, yet the technology is still in its early phases. Future research may be conducted to determine the most suited microalgae species for biofuel production while also evaluating the unknown dangers and repercussions that may result from the discharge of any genetically engineered microorganisms.

The effects (for example, on the environment, economy, and society) of utilizing genetically engineered microalgae as a biofuel feedstock are yet unknown, and they have unknown properties that may be detrimental if not adequately regulated. As a result, more research on fourth-generation biofuels would be beneficial in understanding the biofuels’ sustainability and practicality [36-39].

Conclusions

From first to fourth generations utilizing diverse feedstocks, biofuels have contributed to the evolution of fuels. They have enormous potential to control global energy and environmental problems. First-generation biofuels cleared the path for biofuel production and infrastructure development. Although they have advanced and well-established technologies resulting in the highest biofuel production and energy efficiency, they also highlight the importance of sustainability and minimizing food security problems. Second-generation biofuels introduce the use of nonfood biomass, decreasing food and competition by allowing the use of nonfood crops or biomass wastes to overcome challenges associated with first-generation biofuels. However, lignocellulosic high production costs and technological limits reduce its efficiency. With the advent of third-generation biofuels, algae and macroalgae were introduced as efficient feedstocks with increased lipid content and reduced land needs. However, Lipid extraction from microalgae consumes approximately 50% of the total energy input, reducing energy efficiency. Although technological challenges persisted, they opened the door to better yields and improved sustainability. Fourth-generation biofuels, an emerging generation of biofuels, include genetically modified feedstocks and have great potential for decreased land usage and carbon capture. Genetically modified algae or cyanobacteria could produce up to 35% more lipids and be cultivated in harsh environments. However, more studies are needed to find sustainable, low-cost biofuel production methods while increasing energy efficiency.

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