Biofuel and Biorefinery Technologies

Abstract

The global demand for energy is expected to rise up to 59% by the year 2035. This is due to the increasing technology developments and contemporary industrialization. Continues trends of these simultaneously will affects the crude fossil oil reserves progressively. Therefore, biofuels that are predominantly produced from the biomass based feedstocks such as plant, algae material and animal waste. Liquid or gaseous biofuels are the most simple to ship, deliver, and burn since they are easier to transport, deliver, and burn cleanly. The key contributor to the elevated green house gaseous concentration is carbon dioxide (CO2). Two-thirds of global anthropogenic CO2 emissions are due to fossil fuel combustion, with the remaining third attributed to land-use changes. Interestingly, recent literature has announced that the utilization of liquid biofuels capable of reducing the CO and CO2 emissions. Other positive impacts of the liquid biofuels are; (1) reduce the external energy dependence, (2) promote the regional engineering, (3) increase the Research & Development activities, (4) reduce the environmental effects of electricity generation and transformation, (5) improve the quality of services for rural residents and (6) provide job opportunities.

1. Introduction to biofuel

The global demand for energy is expected to rise up to 59% by the year 2035 [1, 2, 3, 4, 5, 6, 7]. This is due to the increasing technology developments and contemporary industrialization. Continues trends of these simultaneously will affects the crude fossil oil reserves progressively. Therefore, biofuels that are predominantly produced from the biomass based feedstocks such as plant, algae material and animal waste [2, 3]. Liquid or gaseous biofuels are the most simple to ship, deliver, and burn since they are easier to transport, deliver, and burn cleanly [4]. The key contributor to the elevated green house gaseous concentration is carbon dioxide (CO₂). Two-thirds of global anthropogenic CO₂ emissions are due to fossil fuel combustion, with the remaining third attributed to land-use changes. Interestingly, recent literature has announced that the utilization of liquid biofuels capable of reducing the CO and CO₂ emissions [5, 6]. Other positive impacts of the liquid biofuels are; (1) reduce the external energy dependence, (2) promote the regional engineering, (3) increase the Research & Development activities, (4) reduce the environmental effects of electricity generation and transformation, (5) improve the quality of services for rural residents and (6) provide job opportunities [7].

1.1 Type of biofuels

The oxygen content is the most important difference between biofuels and petroleum based fuels [8]. The biofuels produced from different renewable resources are typically non-toxic, accessible and abundant. Biogas, syngas, biobuthanol, bioethanol, biodiesel, bio-ether, and green fuel are various forms of biofuels. Gaseous biofuels are commonly used for heat and energy production purposes.

Biogas, is a gas fuel that burns much like fossil fuels, and for this reason, it gradually gains its position. Biogas consists mainly of methane gas, although it is produced from the degradation of anaerobic biomass. Many agricultural businesses use biogas, and the fuel is currently being packaged for domestic use in gas cylinders. The fuel is extracted from a combination of flora and fauna since each provides a specific ingredient (animals and plants). Plants have significant carbon and hydrogen in them, while they have nitrogen in them for animals. The components above are necessary and required for the production of biofuels.

Liquid biofuels, on the other hand, are widely used in the automotive industry. Biogas, obtained by anaerobic fermentation from organic materials, has a 40–70% CH₄ composition, 30–60% CO₂ and other gases such as H₂S, H₂, N₂ and CO. Biobutanol is capable of replacing both petrol and diesel. Using a bacterium to ferment biomass appears to be the most promising approach for processing biobutanol at the moment. The acetone-butanol-ethanol process is the name of the method. Acetone (propanone), butanol and ethanol are produced here. Clostridium species such as Clostridium beijerinckii, Clostridium acetobutylicum, Clostridium saccharoperbutylacetonicum and Clostridium saccharobutylicum, are used for the fermentation process of acetone-butanol-ethanol to produce biobased butanol [9]. Algae, sugar beet, sugar cane, maize, sorghum and cassava are the feedstocks used successfully so far. To make pure butanol, the materials are fractionally distilled. Bioethanol is a transparent liquid that is biodegradable, non-toxic, and environmentally safe. It’s chemically known as ethyl alcohol, and it’s made from the plant’s fermentable sugars (such as glucose, sucrose, and other sugars) through microorganisms. It is possible to mix bioethanol with gasoline as well. Fermentation of bioethanol is a biological process in which microorganisms convert sugars to produce bioethanol and CO₂. Yeasts are the most widely used microorganism in the fermentation process and Saccharomyces cerevisiae is the preferred option for bioethanol fermentation among yeasts [10]. For example, fermentation bacteria used are Clostridium acetobutylicum, Lactobacillus fermentum etc. The basic schematic fermentation process is as follow (Figure 1a).

Figure 1.

(a) Fermentation process, (b) Transesterifcation reaction of triacylglycerol using methanol and (c) Transesterification of triacylglycerol using ethanol.

Green fuels (green-diesel and green-gasoline) are an oxygen-free hydrocarbon comprised of short chain and long chain carbon fractions within a range of C₈–C₁₂ and C₁₃–C₂₀, respectively. The green fuels also free from sulfur and aromatic compounds. They contain n-alkanes and n-alkenes, which similar to those found in the petroleum-based gasoline and diesel. As the green fuel is entirely compatible as the petroleum-derived fuels, their fuel properties are vastly similar to each other. However, green fuel is completely different in chemical structure as compared to the well-established commercialized biodiesel [11, 12, 13]. To produce green diesel, biological oil feedstocks such as algae, vegetable or plant oils, and animal fats are thermally catalytically hydrocracked. Hydrocracking is a refinery process that uses elevated temperatures and pressure to break down these larger molecules (natural oils) into a shorter mixture of hydrocarbon chains in the presence of strong chemical heterogeneous catalysts. Green diesel also known as renewable diesel [14]. One of the most well-established liquid biofuels is biodiesel or fatty acid methyl ester (FAME), not only because of its lower environmental effect, but also because it provides the benefits of being renewable, biodegradable and non-toxic. By transesterification reaction with alcohol, biodiesel will be fabricated from either vegetable oils or animal fats using required catalyst [15]. It can be used as a diesel engine fuel in its pure form (B100) or in mixtures (B10, B15), but it is typically used as a diesel additive to reduce the levels of diesel-powered particulates, CO and hydrocarbons. The following is a diagram of the transesterification process (Figure 1b). Biodiesel fuel used in car and lorry engines typically comprised of higher boiling FAME fraction and its alkanes contain 14–22 carbon atoms [16]. Biodiesel also can be a mixture between their FAME and fatty acid ethyl ester (FAEE) and FAEE is prepared as following reaction (Figure 1c).

Table 1 depicts the most recent global developments on modern transportation fuels in 2019, published by NS Energy [17]. There are three liquid biofuels – bioethanol, biogasoline and biodiesel account for the vast majority of global biofuel production and use today. The United States is on the first world rank of biofuel manufacturer followed by Brazil. In Europe, Germany is the largest producer. Argentina produced almost similar production amount with the Germany. As been expected, China is the leading country for biofuel production in Asia. The majority of biofuels were produced from soybean, sugarcane, rapeseed, corn and waste cooking oil (WCO). In the case of China, bioethanol and ethanol-blended gasoline are primary products. In the United States and Brazil, soybean oil is widely used. Many European countries, primarily Germany, use rapeseed oil for biofuel production. Note that Germany also utilized the WCO as a biofuel feedstock. It should be noted that the WCO derived biofuels initiative is also used in many nations, including Australia, China, Italy, Portugal, the United Kingdom, the United States, Austria and Spain. Brazil also produced WCO-based biodiesel, but it only accounted for 0.5% of total biodiesel production. In Brazil, only 2.5% of the WCO produced in Brazil is estimated to be reused for biofuel production, while the rest is improperly discarded [18]. Sugarcane is widely used in Brazil and Argentina. Brazil is the global leader in producing bioethanol from sugarcane.

Country	Production (barrel/day)	Type of biofuel	Feedstock
US	1190.2	Bio-gasoline; Bioethanol	Soybean
Brazil	693.2	Bio-gasoline; Biodiesel; Bioethanol	Sugarcane; Soybean
Indonesia (Indexmundi2020)	>63.0	Biodiesel	Palm oil
Germany	75.8	Biodiesel	Rapeseed; WCO
Argentina	70.6	Bioethanol; Biodiesel	Sugarcane; Corn
China	68.0	Ethanol; Ethanol-gasoline blend	Import

Table 1.

Top countries for biofuel production across the globe in 2019 [17].

Overall, all liquid biofuels are made mainly from agricultural commodities, such as grain and sugar (bioethanol) and vegetable oil, based on the above results (biodiesel, bio-gasoline). It can be observed that each nation concentrated on distinct feedstock. Lowest oil price, high oil content, required fatty acid composition (saturated or unsaturated acid), low cultivation maintenance and expense, controllable growth and harvesting season, consistent seed maturity rates, and potential demand for agricultural by-products are all desirable characteristics when choosing the best biofuel feedstock [19].

1.2 Biofuel uses

There are applications of biofuel other than an alternative to diesel fuel. Most claim that the material is used for transportation only. But hydrogen, cleaning oil, cooking oil, and more can be provided by biofuel. As an alternative to substitute energy needs from vehicle fuel to core home heating, biofuels can work.

Here are the top ten biofuel applications.

1.2.1 Transportation services

In the United States, about 30% of the energy consumed is used for moving cars. Transport accounts for 24% of electricity and more than 60% of the absorbed oil worldwide. This means that more than a third of the oil is used for vehicle operations.

The key issue with alternatives is that it is not feasible for transport to use solar, wind, and other renewable energies. Experts think that successful breakthroughs in developments in practical technology are still decades away.

In short, biofuel can be converted into steam of hydrogen that is intended to be used in the fuel cell adjacent to it. More important automotive brands have already invested in biodiesel vehicle stations.

1.2.2 Generating electricity

Fuel cells provide a power-generating application that is used for electricity and providing fuel for transport. In backup systems where pollutants matter the most, biofuels could be used to generate electricity. This involves facilities located in suburban areas, such as schools, hospitals, and other styles. In reality, the greatest biofuel market in the United Kingdom will turn over 350,000 homes from landfill gas into power generation.

1.2.3 Provide heat

Over the last few years, bioheat has developed. The heat coming from hydraulic fracking would contribute to natural gas development as the primary use of natural gas that comes from fossil fuels. Although there is no need for natural gas to come from fossil materials, it can also derive from newly grown materials.

There is a large amount of biofuel that is used for heating. Since wood is the most practical heating process, houses that use wood-burning stoves instead of gas or electricity are used. A biodiesel blend would reduce the production of both nitrogen and sulfur dioxide.

1.2.4 Electronics charging

According to scientists from Saint Luis University, a fuel cell was built with cooking oil and sugar to produce electricity; customers would be able to use these cells instead of generating electricity. Instead of batteries, customers will be able to use fuel cells to charge everything from laptops to mobile phones. Cells have the ability to become a ready source of power when they are still in the process of growth.

1.2.5 Spills and grease from clean oil

Biofuel is considered to be environmentally friendly and can also help clean up oil and grease spills. For areas where crude oil polluted the waters, it was checked to act as a possible cleaning agent.

It has also been found that the results improve the areas of recovery and allow it to be extracted from the water. Biofuel can also be used for metal cleaning as an industrial solvent, which is also useful because of its lack of harmful effects.

1.2.6 Cooking

Although the most common ingredient to be used for stoves and non-wick lanterns is kerosene, biodiesel works equally well.

1.2.7 As a lubricant

In order to decrease the Sulphur concentration, diesel fuel is needed as Sulphur offers the most fuel lubricity. When it comes to maintaining the engine running correctly and preventing infection’s premature failure, this is critical.

1.2.8 Remove paint and adhesive

Biofuels can replace toxic materials in order to eliminate paint and adhesives. The best approach for eliminating non-critical applications is often known to be biofuel.

1.2.9 Create energy when fossil fuel runs out

As the supply of oil is beginning to run out. This has led us to ask how, without damaging the ecosystem, fuel can be extracted. Biofuel would assist the government in forming a sustainable, cost-effective method of generating energy.

1.2.10 Reduce cost and need for imported oil

In the United States, over 84% of the world’s petroleum is used. The U.S. has recently begun to reduce the need since 2006, despite the rise in fuel requirements. This makes it possible for biofuels to become the strongest emission reduction factor.

Analysts claim that when oil is disrupted, substituting imported oil with biofuel would help to balance the Economy. It does not matter how much the Americans Spend on oil imports, but how to balance the overall Economy (Top 10 Uses for Biofuel, 2016).

1.3 Biofuel feedstock

Biomass feedstocks for energy production can be produced from plants directly grown for energy use or parts of plants, waste, residues, and materials extracted from humans and animals’ activities. In 2005, the U.S. Department of Energy evaluated these feedstocks’ results and found that it was possible to sustainably harvest and deliver more than 1 billion tons of agricultural and forestry-related biofuels to biorefineries. Feedstocks may be defined by plant or residue types, the energy products they make, or any other way. The following categories of feedstocks will be used for discussion purposes.

1.3.1 Sugar and starch crops

Many of the sugar and starch crops that are contenders for biofuel production are already being used for agricultural and food grains or sweetener sugars. Root and tuber starches are usually used across the globe as food staples. Via conventional fermentation methods, these crops and their particular products can easily be transferred to ethanol and related alcohols for transport and other uses.

Competition for resources and the need for genetic, development, and manufacturing modifications to increase energy production sustainably would be the unique challenges facing most of these crops. Some examples are:

• Corn Grain

• Sweet Sorghum

1.3.2 Fibre and grass cellulosic crops

Many of the grass and related crops that have been cultivated for decades as pasture and grazing for feeding livestock or for soil conservation can be used as an energy resource. In general, these crops are higher in fiber (cellulose, hemicellulose, lignin) and poorer in carbohydrates, proteins, and oils. A variety of methods may turn these crops into energy, including direct heat and/or power combustion, cellulosic conversion into ethanol, thermochemical processing for fuel supplements, or anaerobic methane digestion. Some examples are:

• Miscanthus

• Energy cane

1.3.3 Oil crops

While several crops generate at least a small amount of vegetable oil, 15–50% of oil is provided by various crops. By grinding the seed and squeezing the oil out, oil can be extracted. To produce biodiesel, the oil is transesterified. Oil crops may also be transformed as alternatives to fossil fuel materials into high-value biochemicals and biomaterials, thus reducing the use of fossil fuels in turn. Some examples are:

• Soybean

• Canola/Rapeseed

• Mustard

• Camelina

• Warm Climate Feedstocks for Biodiesel

• Hazelnut/Filbert

1.3.4 Crop residues, manures, and organic wastes

Critical biomass residues remain after corn, sugar, starch, or oil plants are harvested for feed and food components. Abundant crop residues that can be transformed into renewable fuels are corn stover, corn cobs, wheat, and small grain straw. Sustainable maintenance of the agricultural production system is a crucial obstacle when removing crop residues. To increase soil organic matter quality as well as soil and water conservation objectives, crop residues are usually incorporated into the soil. In order to evaluate the influence of stover removal on the sustainability of crop production, a great deal of research is performed to study the effects on ecosystem services and the diversity of insects, vertebrates, and microbes. Some examples are:

• Corn Stover

• Corn Cobs

Manures are a result of livestock’s digestion of plants. Anaerobic digestion techniques have been used for years to transform these and other organic waste to methane and related gases in addition to their usual land application for nutrient content. In exchange, methane can be used explicitly for combustion heat, fueling diesel generators, or supplementing natural gas with further processing and cleaning.

In metropolitan areas, food production and industrial waste, including restaurant grease, leaves, grass cuttings, and other garden waste, are contained in large quantities and can be processed and converted to electricity through a number of methods.

1.3.5 Wood products

Trees and their associated products were used as a direct source of ignition and combustion for heating and cooking for decades. The thermal efficiency of these wood products, when dry, is about two-thirds that of coal and about 10% greater than that of deciduous plant biomass. As the fuel source for gasification and cellulosic conversion to ethanol, wood and its derivatives have also been used. Although it is generally possible to use any wood supply, different research projects have been ongoing to create so-called “energy forests” or “wood energy farms.” Woody Crops [20]. Some examples are:

• Hybrid Poplar

• Willow Shrub

1.4 Advantages of biofuels

Biofuels offer a wide range of benefits.

1.4.1 Renewable energy sources

Globally, there is strong energy demand. Nonetheless, most power sources are non-renewable, lead to the greenhouse effect, or, as is the case with nuclear energy, may lead to major ecological problems. Biofuels, which are renewable fuel sources and environmentally friendly, are derived from plant and animal manure.

The majority of fossil fuels will expire and one day wind up in flames. Since most sources, such as manure, maize, switchgrass, soybeans, crop, and plant waste, are renewable and are not likely to be running out any time soon, the use of biofuels in nature is effective. These crops can also be replanted again and again, as well.

1.4.2 Sovereignty

Unlike fossil fuels, whose deposits aren’t really found in all countries, any country can undertake biofuels’ development without interference with all other countries’ energy sources. By impacting or determining the world’s fuel prices and petroleum-based goods, countries with fossil fuel reserves have always taken full advantage of their economic resources. If a nation can manufacture its own biofuel, it can easily set its own prices for goods without many regional and global constraints.

Although local crops have decreased the nation’s reliance on fossil fuels, many experts agree that addressing our energy needs will take a very long time. We need more renewable energy options to reduce our dependence on fossil fuels as crude oil prices are hitting sky high.

1.4.3 Ensure economy’s sustainability

The sustainable quality of biofuels has contributed to states worldwide adopting them and supporting a decrease in fossil fuel use. Instead of high-cost imports of fossil fuels from Middle Eastern countries, policymakers should reduce this reliance and instead fund biofuel plants that are cheaper in the long term.

Locally generated biofuels can minimize reliance on other fuels and thus increase the security of energy and economic prosperity. Fewer imports imply more exports and, therefore, greater self-dependence.

1.4.4 Low expenses

The majority of biofuels are easy to manufacture and cheaper than fossil fuels. Therefore, their use will make life easier for ordinary citizens and help boost people’s living standards by reducing the increasing cost of living globally due to reliance on fossil fuels. As of now, as gasoline does, biofuels cost the same as the market. However, the net cost-benefit of using them is much more significant. They are safer fuels, which implies that they generate lower burning pollutants. They also have the ability to become cheaper in the future with the growing demands for biofuels.

1.4.5 Clean fuel

A lot of carbon is emitted by fossil fuels, which results in large levels of air pollution. This carbon also mixes with other greenhouse gases, such as methane, which contributes to unfavorable weather conditions. On the other hand, since they are clean fuels, biofuels do not release this amount of carbon into the environment.

1.4.6 Efficient fuel

Biofuel is made from renewable resources and, compared to fossil diesel, is relatively less combustible. It has considerably stronger hydrating characteristics. Compared to standard diesel, this produces less toxic carbon emissions. It is possible to manufacture biofuels from an extensive range of materials. The net cost-benefit of someone using them is considerably greater.

1.4.7 Extensive durability of vehicles’ engine

In most conditions, biofuels are able to adapt to existing engine designs and perform very well. It has higher levels of cetane and more robust lubricating properties. The longevity of the engine improves when biodiesel is being used as a flammable fuel.

Engine conversion is not required. This allows the engine to run for longer, needs less maintenance, and reduces the cost of pollution control overall. Engines intended to run on biofuels generate fewer emissions than other diesel engines.

1.4.8 Less smoke generation

Automobiles and factories using fossil fuels such as petroleum and diesel commonly create a lot of atmospheric smoke. As biodiesels have oxygen atoms in their chemical structure, they burn better and contain less carbon deposits. Biodiesels emit less smoke as a byproduct and are more environmentally friendly.

1.4.9 Minimize monopoly

Fossil fuels are more likely to be favored by biofuels due to their widespread use. This has created a monopoly over the years, contributing to price increases and the ever-increasing standard of living. Since biofuels are equivalent replacements for fossil fuels, they can be used to help minimize the fossil-fuel monopoly.

Biogas could be used in the same way as fossil fuels, for instance. Consequently, people have the option of converting to Biogas when natural gas prices go up. And vehicles can opt for ethanol or butanol when fossil diesel rates increase.

1.4.10 Less toxic

As a result of combustion, all forms of fuels, fossil fuels, and biofuels form carbon compounds. In the atmosphere, fossil fuels emit toxic carbon dioxide, especially in the presence of water vapor and methane gas. On the other side, biofuels’ carbon occurs in nature and is used for photosynthesis by plants, serving as an energy source for plants.

1.4.11 Employment source for locals

Most of the bio plants are geographically set up, and human capital is required in the process, such as construction engineers, farmers, project managers, fuel distributors, and logisticians. This helps to generate new work opportunities for locals.

1.4.12 Lower levels of pollution

Using fossil fuels such as coal, sulfur, and lead can be produced along with acid rain. Unlike biofuels, sulfur is not contained in biofuels. Biofuels are renewable resources that emit fewer emissions into the environment. Nevertheless, this is only one reason why biofuels are promoted.

They emit lower levels of various contaminants, such as carbon dioxide than conventional diesel. It plays a part in lowering air pollution. Furthermore, biofuels are biodegradable, reducing the risk of soil degradation during transport, storage, or use.

Social and environmental studies reveal that biofuels minimize greenhouse gas emissions by up to 65%. When fossil fuels are burnt, they release huge amounts of greenhouse gases into the atmosphere, affecting the environment. The greenhouse gases absorb the sunlight, which causes the earth to be hot. Besides, burning coal and oil is a source of climate change. Various countries are opting to use biofuels as a way to reduce greenhouse gases.

1.4.13 Agricultural promotion

Increased demand for the production of biofuels will lead to further farming of the appropriate crops. Crops with high carbon and cellulose composition can be planted on a massive scale, and after harvesting the edibles, the rest of the plant components can be used for the production of biodiesel.

1.5 Disadvantages of biofuels

1.5.1 High production cost

Biofuels are very costly to manufacture in the current market, even with all the advantages associated with biofuels. The interest and capital investment put into the biofuels production are relatively low as of now, but it can balance demand.

If demand rises, then it will be a long-term process to raise the supply, which will be very costly. Such a downside also prevents the use of biofuels from becoming popular globally.

1.5.2 Monoculture

Monoculture is the method of growing the same crops year after year in a single field, rather than generating different crops in multiple fields. Although this may be lucrative for farmers, growing the same crop every year would deprive the soil of nutrients returned by cover crops and farming overused areas. The reasons for planting a single crop over large tracts of land are discussed. First of all, the environment changes when only one crop is grown, and pests can ruin the entire crop.

Besides, complete pest control can be accomplished with pesticides. Even certain pest insects would inevitably develop resistance to the chemicals we use to fight them, and they would be able to live in a single crop area.

As we intend to encourage insect resistance to our pest, the next obstacle comes with genetically modified species. The change is not likely to impact any species, and the related problem remains.

Biodiversity, which requires various varieties of plants and animals, is thus the key to healthy agricultural fields.

1.5.3 Application of fertilizers

Biofuels are derived from crops, and to grow better, these crops need fertilizers. The drawback to using fertilizers is that they can cause water contamination and have adverse effects on the surrounding environment. Nitrogen and phosphorus are found in fertilizers. It is possible to wash them away from the soil into surrounding lakes, rivers, or ponds.

1.5.4 Food scarcity

Biofuels are obtained from plants and crops which have high sugar levels in them. Many of these crops are also used as food crops. Even though plants’ waste material may be used as raw resources, there will still be a need for such food crops. Other crops can take up farm space, which can cause several problems.

The use of existing biofuel land may not lead to acute food shortages, but it will undoubtedly pressure current plant growth. One big problem that people face is that the rising use of biofuels could also increase food prices.

Algae, which grows in rather inhospitable regions and has a small impact on land use, is favored by some people. The issue with algae, however, is water use.

1.5.5 Pollution in the industry

When burned, the carbon footprint of biofuels is smaller than the conventional sources of fuel. The method in which they are made makes up for that. Production depends to a large degree on lots of water and oil.

It is understood that large-scale industries intended for biofuel production produce large quantities of emissions and also cause small-scale water pollution. The total carbon pollution would not have a very significant dent in it unless more effective production means are placed.

1.5.6 Extensive use of water

In order to irrigate biofuel crops, large quantities of water are needed and can, if not handled wisely, place a strain on local and regional water supplies. Vast amounts of water that could place unnecessary pressure on local water supplies have been used in order to manufacture maize-based ethanol to satisfy consumer demands for biofuels.

1.5.7 Future price hike

The existing technology used for biofuel production is not as effective as it should be. Scientists are interested in the creation of better measurements that enable us to extract this fuel. However, testing and potential installation expense means that a significant increase will be seen in biofuels’ price.

As of now, gasoline prices are equivalent and are still practicable. The use of biofuels can be as tough on the economy as the rising gas prices are doing right now.

1.5.8 Land use changes

Land must be cleared of natural vegetation if it is used to produce a biofuel feedstock, contributing to ecological harm done in three ways.

First, the harm is caused by community habitat loss, animal dwellings, micro-ecosystems, and the general wellbeing of the region’s resources will be diminished.

In extracting CO₂ from the atmosphere, the native forest is almost always better than a biofuel feedstock, partially because the CO₂ stays trapped and is never extracted by burning as with the fuel stock.

Secondly, the damage of the generated carbon debt is significant. This contributes to the production of greenhouse gases as it is necessary to deforest an area and prepare it for agriculture as well as to grow a crop, and puts the region at a net positive development of GHG even before the production of a specific biofuel. Estimates have shown that a carbon debt that can take up to 500 years to repay can actually be created by deforestation of native land.

Finally, almost always converting land to an agricultural status means that fertilizers can be used to get the most yields per area. Runoff and other agricultural emissions are a problem.

1.5.9 Global warming

The biofuels, which mainly burn hydrogen and carbon, create carbon dioxide that causes global warming. Biofuels generate less GHG emissions than fossil fuels, but that can only help slow down global warming and not avoid or reverse it.

Biofuels could therefore be able to help alleviate our energy requirements, but they will not solve all of our issues. In the short term, it can only act as a replacement as we invest in other technologies.

1.5.10 Weather issues

For use at low temperatures, biofuel is less satisfactory. It is more likely than fossil diesel to draw moisture, which in winter conditions causes problems. The engine that coats the engine filters also enhances microbial growth (Various Advantages and Disadvantages of Biofuels, 2020

2. Biorefinery technologies

2.1 What is biorefinery?

A biorefinery is a specialized facility that uses tools and materials for processing biomass into fuels, electricity, and useful chemicals. The biorefinery is situated next to the petrochemical industry, making various petroleum products, including gasoline and plastic. A biorefinery benefits from using multiple biomasses and intermediate items, thereby maximizing the value of biomass feedstocks. One potential example for biorefinery product is low-volume yet high-value chemical products and high volume but low-value fluid or liquid transportation fuel such as biodiesel or bioenergy. Because of its high efficiencies, energy efficient technology that generates electricity and captures the heat (CHP) technology can generate electricity for its own use and sell surplus electricity to the community. High-value products boost profitability, high-volume fuels help meet energy needs, and power generation helps lower energy costs and mitigate greenhouse gas emissions from traditional power plant installations.

Nevertheless, the production of fuel and chemicals in biorefinery is limited, which may become harder to do. Interdependent societies and trading firms demand vast quantities of fossil fuels to supply the bulk of their energy and chemical supplies. The manufacture and use of fossil fuels cause environmental degradation, resulting in toxins, greenhouse gases, and dangerous materials. The increasing demand for energy and chemicals makes the environment more dependent. The amount of waste produced is also continuously rising alongside the growth in our global population. Our country generates roughly 250 million tons of municipal solid waste per year (MSW). Overall, 35% of MSW is recycled and composted, 13% of this waste is used to generate power, and 53% of MSW is buried in landfills.

It is important to find alternative resources to make energy and chemicals due to limited fossil fuel resources and the increasing demand for energy and chemicals. In this process, biomass has been recognized as a possible future source of chemicals and energy and addresses the environmental risks of burning fossil fuels. Various biomass sources are available but can also be collected from various waste sources such as agricultural waste, municipal solid waste, and industrial contaminants such as paper factories and pulp factories. In a holistic waste management plan, biomass waste valorization also plays a prominent role in waste recycling. A biorefinery that uses renewable biomass as a feedstock for the production of chemicals is more sustainable as opposed to using fossil fuels. A biorefinery may contribute to economic expansion while also reducing air pollution in the environment [21].

2.2 Biorefinery technology, product, and application

To create an integrated biorefinery that aims to build on the biomass conversion process in such a way that the maximum added value can be derived from the sustainable biomass feedstock, it integrates a range of different technologies. New methods of bio-refining are being explored in hopes of saving the world. Biorefineries combine/integrate different technologies for the conversion of biomass into a variety of products (i.e., food, feed, chemicals, materials, petroleum, coal, heat, and/or electricity) and are defined as ‘sustainable processing of biomass into a marketable commodity and energy spectrum’ by IEA Bioenergy Task 42. The concept focuses on the method of how various petroleum products are refined and made into usable fuels.

As it is subject to unpredictable circumstances, such as when farmers use various farming methods, and climate changes, a biorefinery’s precise technical specification can vary from case to case. Environmental and social factors decide which feedstock is available for processing. Numerous varieties of switchgrass, sugarcane, wheat, corn, wood, crop waste, sugar cane, surplus food, straw, freshwater biomass, and the biomass component of municipal and other sources of waste (MSW) can be used in a bio-refinery. Chemicals, biofuels, energy and heat, materials, food and feed, minerals and CO₂, are the main product groups in a biorefinery (Biorefinery, ctc).

Biorefineries can be classified based on the number of main characteristics they have. Various feedstocks for biofuel production include perennial grasses, starch crops (e.g., wheat and maize), sugar crops (e.g., beet and cane), lignocellulose crops (e.g., controlled forest, short growing coppice, switchgrass), lignocellulose residues (e.g., stover and straw), oil crops (e.g., palm and oilseed rape), and inorganic matter (e.g., stover and straw) (e.g., industrial, commercial and post-consumer waste).

Feedstocks can be treated on a number of platforms used by biorefineries. These platforms include biogas, consisting of single carbon molecules such as methane and carbon dioxide, starch, sucrose, or cellulose carbon carbohydrates; a mixed stream of 5 and 6 hemicellulose-derived carbon carbohydrates, lignin, oils (plant-based or algal), organic grass solutions, pyrolytic liquids. By integrating biological, thermal, and chemical processes, these main platforms can be changed to produce different products. Awareness of the feedstock, platform, and product that a biorefinery uses allows the business to be represented critically. Biorefinery practice creation helps compare biorefinery systems, improves understanding of global biorefinery growth, and allows technology differences to be developed.

The biorefinery classification examples include:

• C6 biorefinery sugar generating ethanol and animal feed from staple crops

• Syngas biorefinery from cellulosic biomass yielding FT-diesel and naptha

• Biorefinery of C6 and C6/C5 sugar and syngas containing ethanol, FT-diesel, and furfural from cellulose and hemicellulose crops

2.3 Biorefinery pathways

Biomass can be used as food, heating fuel, or converted into a liquid or gaseous form that can then be used for energy resources. There are different methods to transform biomass into biofuels. A distinction is made between biochemical conversion and thermochemical conversion. Anaerobic digestion, saccharification, and hydrolysis are common conversion technologies used throughout industries. 5 distinct sub-categories of thermochemical conversion include gasification, pyrolysis, liquefaction, gasification, and combustion.

As oxygen is completely removed, the aerobic decomposition of organic carbon diminishes organic nonwoody content. It sells easier, less volatile chemicals, including methane and carbon dioxide. However, the biochemical conversion process takes a lot of time and uses just a biomass portion. Thermochemical conversion technologies are more generally regarded as being superior for their flexibility and efficacy.

Most second-generation biofuels are generated by concentrating lignocellulosic biomass into different products. Containing three main constituents: cellulose, hemicellulose, and lignin. Under conditions between 200 and 380°C, hemicellulose is the easiest to break down, and cellulose decomposes between 320 and 400°C. The most stable substituent that breaks down when heated to 400°C is cellulose.

Temperature, heating rate, and residence time are three important moving parts in chemical-reaction thermochemistry. Combustion is currently the leading source of energy (approximately 80%) in the worldwide supply. Many alternative methods for pollution control, such as gasification and pyrolysis, are still in the research and development stage due to their high cost and low performance [22].

Biomass can be converted into an extended range of chemicals in two main ways:

1. Thermochemical pathway

2. Biochemical pathway

2.3.1 Thermochemical pathway

Thermochemical processing is used for two major routes:

One method that produces biomass is heating biomass with regulated oxygen quantities at high temperatures and pressure (a mixture of carbon monoxide and hydrogen). The process is called gasification. The gasification of solid biomass produces many industrial compounds. The effect of gasification on various liquid fuels will be addressed in the biofuels unit.

The second technical approach involves high-temperature heating of the biomass, but it works without relying on the atmosphere. As a method, pyrolysis is well-known. Glue must be used easily, so the reaction time must be short. If not, the top sector will be the carbon industry (char). This process is known as rapid pyrolysis, and the main product produced is organic oil [23].

The thermochemical conversion aims to minimize the entire biomass used in the chemical-making phase to steam. The Fischer–Tropsch process is a thermochemical conversion, which is why it is an example. Thermochemical biomass conversion is not a key application of chemical transformation. The main driver of this five-way conversion route is the output of thermal energy:

1. The Combustion

2. Torrefaction/Carbonization

3. Pyrolysis

4. Gasification

5. Liquefaction

The biomass is first transformed into syngas in the thermochemical pathway, converted by synthesis or some other method into ethanol.

2.3.1.1 Combustion

Considering that humans started with fire discovery, combustion was the first method of using living materials for energy production. Wood-burning forestry has taught people how to survive, heat and cook. Chemically, biomass combustion is an oxygen-based exothermic reaction. Here the biomass is oxidized by two major stable compounds, hydrogen and carbon dioxide. The heat generated by the reaction currently accounts for over 90% of energy consumption.

Bio-mass derived energy is mainly generated from heat and electricity. Biomass also provides renewable cooking fuel and heat in rural communities. Combustion of biomass also includes industrial heating and regional heating. Pellet stoves and wood-burning fireplaces are widely used in areas with cold climates. The use of biomass for electricity is important for modern-day environmental practices. Combustion of biomass in boilers and the power-producing steam turbine are the most common activities. Biomass is used as an alternative to fossil fuel in a boiler, typically for heating. The latter approach is more effective in lowering carbon dioxide emissions from a high-emission fossil-fuel plant than the prior solution.

2.3.1.2 Carbonization

Like torrefaction, carbonization is recommended for the productivity of biomass as a safe and efficient solid fuel. The biomass is continuously heated up to 200–300°C with little or no oxygen contact in torrefaction. This process changes the biomass hydrocarbon’s chemical composition to increase its carbon content while reducing its oxygen content. Torrefaction also raises the density of biomass and increases hygroscopic biomass. These qualities thus increase the commercial value of the timber for electricity manufacturing and transport. The general goal for other carbonization processes is to form carbon-rich reliable products under different conditions.

2.3.1.3 Pyrolysis

Pyrolysis occurs in an oxygen-free environment, unlike combustion, even when using partial combustion to heat the reaction. It can be used to quickly and effectively transform biomass into gases, liquids, and solids.

Parts of biomass are broken down in the pyrolysis process. Slow pyrolysis end products include reliable charcoal and gas, while rapid pyrolysis produces only bio-oil. In order to turn biomass into liquid fuels, pyrolysis is suitable. It is not an endothermic reaction, while combustion releases heat.

2.3.1.4 Gasification

Solid, liquid, and gaseous fossil fuels are converted into usable gases. One needs a medium for gasification reactions, water, or steam. An air, oxygen, or both make up the gaseous environment.

Natural gas production through fossil fuel emissions is more common than bio-gasification to produce biogasoline. Gasification moves the fuel from one type of fuel to another. There are numerous reasons for the change from one form of language to another.

• The fuel’s heating value is enhanced by eliminating non-combustible organic content such as nitrogen and water.

• To extract sulfur in the fuel gas so that it does not contribute to global warming.

• To lower the overall carbon to hydrogen (C/H) ratio in the gasoline.

• To optimize the fuel’s hydrofluorocarbon (HFC) and perfluorocarbon (PFC) emissions

The higher the hydrogen content in fuel, the more likely the fuel will be in its gaseous state. The relative hydrogen content in the substance is produced by adding air to the material by using gasification or pyrolysis.

1. At high temperatures, exposure to hydrogen.

2. Indirect exposure to high temperature and pressure resulted in a hydrogen-rich product. The method also includes steam reforming.

Compared to the amount of oxygen in cellulosic biomass, the oxygen concentration in natural gas is decreased. Gasification decreases the overall carbon footprint and creates a more usable commodity.

Some natural gas is gasified in order to use as a source of energy and as a means of ammonia production. Nature gas reforming helps in steam output (a mixture of H₂ and CO).CO, which is present in the biogas, is indirectly hydrogenated through the smog to produce methanol. However, these systems use natural gas, causing more carbon dioxide to be produced than other systems. Biomass can be used as a replacement for fossil hydrocarbons in various manufacturing processes.

Changing liquid transportation fuels generates biomass gasification, providing a strong basis for carbon dioxide and hydrogen. The process may also generate methane, useful as a source of energy.

2.3.1.5 Liquefaction

There are several methods of bringing solid biomass into liquid fuels, including pyrolysis, gasification, and hydrothermal processes. The conversion of biomass into an oily liquid can be achieved by heating the biomass at higher temperatures (300–350°C) and under high pressure [24].

2.3.2 Biochemical pathway

Biochemical conversion requires the breakdown of biomass to make the carbohydrates usable for refining into sugars, which can then be transformed using microbes and catalysts into biofuels and bioproducts. The following are possible stocks of fuel mixtures and other bioproducts:

• Renewable gasoline

• Ethanol and other alcohols

• Renewable chemical products

• Renewable diesel

The significant challenges of breaking down the complex structures of cellulosic biomass include key challenges for biomass’s biochemical conversion. To have access to these beneficial sugars, the Bioenergy Technologies Office explores more effective and affordable means of processing the sugars.

The critical challenge is to turn sugars into biofuels more efficiently and effectively. To achieve our target, the Bureau has developed new directions and technologies.

2.3.2.1 Step by step chemical conversion

In addition to heat and other chemicals, the biochemical conversion uses biocatalysts to convert the hemicellulose and cellulose into an intermediate stream of sugar. Such sugars are an intermediate stage in the manufacture of advanced biofuels and other biochemicals or are catalyzed chemically to generate useful substances. The whole method is made up of the following necessary steps.

1. Feedstocks are chosen because they contain the properties required for biochemical reactions. For fast, efficient plant operations, efficient feedstock handling systems are required.

2. Pretreatment: The biomass is heated to break down the fibrous cell walls and make it easier to hydrolyze cellulose and hemicellulose (see next step).

3. After hydrolysis, the sugars are separated from cellulose and hemicellulose in the pretreated content over the course of days.

4. D1. Bacteria are added to produce new chemicals, including fuels or building blocks for other chemicals, from sugars.

5. D2. Rather than chemical conversion, sugars can be converted using chemical catalysts to produce fuels and other useful items.

6. Recycling: When oils, solids, and residual impurities are separated from products.

7. Distribution: Fuels and other goods are shipped to refineries. Other products and intermediary products may be delivered to manufacturing plants for use in a wide range of consumer products.

8. The organic matter left is mostly comprised of lignin that can be combusted as natural gas and fuel [25].

2.4 Integration of biorefining in the processing industry

Due to the wide variety of biorefinery systems and their component selection, there are major energy properties variations. The process will be affected by the form of feedstock, crude oil metabolism, and end product. The freedom to select and change these parameters is a complex task that takes a great deal of thinking. The biorefinery portion should be considered to optimize the biorefinery’s energy characteristics to better conform the overall process integration.

The model is dynamic and implemented several degrees of freedom. The optimization is difficult to do due to the substantial uncertainty in potential energy markets, investment costs, and, especially, the cost of CO₂. Optimization research on the different levels of these key parameters should also be carried out in order to find “robust” solutions, i.e., perfect ones for technical, environmental, and economic performance at multiple levels of these variables. The incorporation of a biorefinery model into a process industry is very comparable, in theory, to the foreground/background approach or, often, the Complete Site o ne for a given biorefinery design.

• It will combine two broad process components. It is possible to do this on many levels:

• Independent integration of processes within a biorefinery

• Independent process incorporation of the host process

• The complete study of the site or foreground/background of the two components

• The study of process integration considers all streams as components of one large process.

One experience from integrated biorefinery process integration studies is that a very non-integrated host process may be more appropriate for integration with the biorefinery than an integrated biorefinery process. Therefore, if a biorefinery in an organization is considered shortly, any planned energy-saving measures should be postponed or carefully evaluated. This will jeopardize the possibility of effective overall integration. It also proves that the above fourth alternative that combines all the streams must always be done as a first step.

The standard theoretical integration technique is used in the article. The third alternative will then be checked by deciding whether the flows could be integrated into structures of various kinds. The distance between the gas measurements indicates the ability to save electricity. It is the complicated approach that suits the problem and the straightforward solution that is easier to manage. Functional limitations can often make the most powerful solutions impractical, but an efficient targeting method is essential.

In certain cases, the integration possibilities often depend on the availability of energy or excess heat in the refinery and part of the biorefinery. Since heat is not extracted by convection in the original form, it is only cooled by the cheapest possible means of a cooling system. Therefore, it is important that the possible amounts of excess heat from the process can be analyzed when reasonable heat exchange is applied to increase these temperatures and provide a targeted protocol.

2.4.1 Biorefinery concepts in different types of process industry

In the process industry, there is a vast variety of proposed and researched ideas for biorefineries. Only some important examples of concepts are presented below. While the integration of processes would support all forms of biorefineries, refineries processing bulk goods will be of greater importance than chemical products. The examples are, therefore, all bulk goods.

Examples of Process Integration Results Studies

2.4.2 The industry for pulp and paper

• Biomass gasification and electricity processing, methanol or diesel Fischer-Tropsch

• Black liquor gasification with DME generation or green electricity generation

• Processing of ethanol using partially existing pulping machinery or docked to a pulp mill

• Precipitation and upgrading of lignin

• Hemicellulose precipitation and upgrading with water extraction Refineries for Oil

• Biomass gasification and hydrogen production, or diesel Fischer-Tropsch Industry of Petroleum

• Biomass gasification and methanol or SNG processing

The overall outcomes of these studies are:

• In almost all situations, energy-saving steps can be found by process integration between the biorefinery definition and the process industry.

• Energy-saving opportunities are usually strong for the overall system, up to 25%.

• The reduction of CO₂ emissions by integrating biorefineries with process industries depends on society’s marginal energy production technology, particularly electricity. As long as coal convection plants are the marginal power generation technology, the global reduction in CO₂ emissions largely depends on the possibilities for biomass-based power generation as a by-product in integrated and non-integrated biorefinery networks.

• In almost all of the cases examined, the economic benefits of integrating biorefineries are fair or strong but depend entirely on future conditions about policy instruments, i.e., future levels of CO₂ charges (Berntsson).

3. Production of solid fuel biochar from waste biomass

Due to the possibility of exhausting fossil energy and the increase in climate change resulting from the excessive use of fossil fuels, there is a growing need to use renewable energy sources in future in place of fossil energy. Biomass energy is becoming an increasingly popular type of renewable energy, primarily due to its worldwide availability [1]. At present, biomass combustion alone (or co-combustion with coal) to produce heat and power in current coal-fired systems is widely accepted as a low-risk process and one of the least expensive methods of reducing CO₂ levels in the atmosphere [2]. However, raw biomass is not an effective energy carrier and there are significant barriers hindering the direct use of biomass due to its innate properties, including high moisture content, poor grindability and low energy density [3]. For instance, biomass has a fibrous structure that generates poor grindability, and this causes substantial increases in energy consumption and impairs the processes of fuel preparation and feeding. Moreover, in terms of high moisture content, it can reduce the maximum combustion temperature, which in turn reduces thermal efficiency and increases toxic emissions [3]. To address these issues, a pre-treatment process can be carried out to enhance the fuel quality of raw biomass before combustion.

To convert biomass feedstock into biofuels with a high-energy density, pyrolysis has been carried out. This process generates three primary product streams, namely liquid (bio-oil), solid (biochar) and gas products. The pyrolysis conditions largely determine the distributions and properties of these product streams. At present, a majority of research attention is focused on the liquid and gaseous products, and a number of different processes have been developed to generate increased yields and enhance the quality of the two target products [4, 5, 6, 7]. Nonetheless, given the high instability and complexity of its composition, it is not possible to use bio-oil. Rather, more upgrading is necessary [8]. With regard to gaseous products, there is a low yield, whilst the separation and purification processes are largely complex. This ultimately limits the large-scale application of such products in practice [7]. In such cases, it can be beneficial to optimise the use of biochar, and this can ultimately enhance biomass utilization efficiency.

In comparison to liquid and gaseous products, very few studies have investigated the production of solid fuel biochars from waste biomass. In fact, a majority of these studies have focused on examining the improved physicochemical properties of woody biomass [9, 10, 11, 12, 13, 14, 15, 16]. On the other hand, a few studies have examined the biochars produced from abundant agricultural wastes, although studies comparing the fuel quality of biochars obtained from woody biomass and agricultural residue are lacking. Moreover, biomass combustion has low thermal efficiency and produces extremely pollutant emissions. It also generates serious ash-related problems (i.e., fouling and slagging). Nonetheless, even though they are key issues when applying biomass as a solid fuel, very little research has examined the combustion qualities and ash issues relating to pyrolytic biochars in relevant literature [17, 18].

3.1 Biochar production

Several different biomass materials can be used to make biochar, including agriculture waste, animal waste, sewage sludge waste and algal waste. To produce biochar from biomass materials, a number of methods have been developed, such as pyrolysis, hydrothermal carbonisation and gasification.

3.1.1 Pyrolysis

Pyrolysis is a process carried out to thermally decompose biomass without oxygen. There are two stages involved in this decomposition process, namely the primary and secondary stages. Dehydration, dehydrogenation, and decarboxylation take place during the primary stage [19], after which a secondary reaction starts to take place, in which larger molecules are cracked and the solids are converted into gases and biochar. Moreover, there are two key types of pyrolysis, namely slow and fast pyrolysis and this is determined by the operating conditions. During slow pyrolysis, heat is set at a lower rate (0.1–1°C s⁻¹) and the reaction takes place over a long period of time (hours to days) at a temperature (300–900°C). This provides a favourable environment that facilitates the secondary processes and increases the production of biochar. By contrast, the biomass in fast pyrolysis is heated at a higher temperature (300–1000°C) and with a high heating rate (10–1000°C s⁻¹) for a short period of (0.5–2 s) [22], and this process results in the production of a solid (biochar), liquid (bio-oil) and gas (syngas) [24]. The solid carbonaceous substance biochar can be employed as either a catalyst, adsorbent or fuel. On the other hand, syngas (which is made up of CH₄, CO₂, H₂, CO, and other low molecular gases) is typically used in gas engines. Bio-oil is made up of water, phenolic compounds, alcohol, nitrogenous compounds (pyrazine, pyridine, and amines) and aliphatic and aromatic hydrocarbons. Thus, bio-oils are often used in boilers to produce heat [19]. The key properties of biochar (i.e., porosity, surface area and functional groups) are determined by the temperatures used during the pyrolysis process. At higher temperatures, the biochar’s surface area and porosity increase. This is because the aliphatic alkyl and esters groups in the organic compounds break and this facilitates the removal of pore-blocking substances [25]. When lower temperatures are used during pyrolysis, the resultant biochar is hydrophilic and has a graphene structure with fewer functional groups on the surface. On the other hand, biochar created at higher temperatures are hydrophobic and with functional groups being reshuffled and new groups (carboxyl, lactone, phenol, pyridine) being introduced that serve as electron donors and acceptors [26]. In an experiment, Akinfalabi et al. [27] carried out pyrolysis at 400 °C for two hours to produce biochar from sugarcane bagasse biomass. During the process, the latter was sulphonated with ClSO₃H, which increased the surface area from 98 to 298 m² g⁻¹. When used as a catalyst to produce biodiesel production, a yield of 98.6% fatty acids methyl esters (FAMEs) was generated [28]. It is important to note that the yield and quality of the resultant biochar are determined by the types of biomass feedstock used. Conducting pyrolysis sing forestry plants generates a 30% biochar yield, whilst using lignin produces a slightly higher yield of 45.69%. Thus, this suggests that the biochar yield is largely determined by the lignin content [29]. Moreover, Zhang et al. [30] pyrolyzed a lotus stem at 800°C and found that it produced biochar with 55% greater surface area (1610 m² g⁻¹) than porous carbon made from leaves. This is because there is a greater number of metal ions in the stem.

3.1.2 Gasification

Gasification refers to the processes of converting biomass into gaseous fuel through decomposition (H₂, CO, CH₄, etc.). To do this, higher temperatures (500–1400 °C) and oxygen-deficient conditions are required. To enhance the production of the gaseous product, different gasification agents (e.g., steam, CO₂ and some gas mixtures) can be used. Approximately >50% of the biomass converted into gaseous fuel and biochar during this process was smaller in size, and resistant to chemical oxidation [28]. Temperature plays a critical role in the gasification process and results in the increased production of hydrogen and carbon monoxide. At higher temperatures, however, the levels of carbon dioxide, methane, and hydrocarbon are reduced [31]. In general, the surface area of biochar created during the gasification process is smaller and possesses fewer functional groups (i.e., hydroxyl, carbonyl and carboxyl groups) than that yielded during the pyrolysis process [32]. It is important to note that the equivalence ratio (ER) also impacts the yield and quality of biochar. A higher ER indicates that a high quantity of oxygen has been added to the gasifier and it can positively or negatively impact the properties of the resultant biochar. Yao et al. [33]conducted a study and found that there was a decrease in biochar yield from 0.22 to 0.14 kg g⁻¹ and a reduction in carbon content from 88.17% to 71.6%. Additionally, the ER increased from 0.1 to 0.6 [34]. In general, if oxygen molecules are present in the compound, more ash content will be produced, whilst the yield and mechanical strength of the biochar will be reduced. A further study carried out by James et al. [35] investigated the impacts that airflow has on the properties of Pine woodchip biochar [22]. Their findings indicated that airflow of 8 to 20 L min⁻¹ produced basic biochar (pH > 7.0) and that there were no acidic functional groups at a high airflow rate. The content of alkalis and alkaline earth metals is higher when biochars are created through gasification, although the exact content varies based on the type of biomass used [36]. There are several different gasifiers that can be used in such processes, including fixed bed, fluidized bed and circulating fluidized reactors. These will be discussed in more detail at a later stage [20].

3.1.3 Hydrothermal technology

During this process, wet biomass is thermochemical converted into hydrochar. Hydrochar can be produced in the same ways as biochar (i.e., using the methods discussed above). Moreover, the process is very much like the process of forming natural coal. Moderate temperatures (150–350°C) and conditions under 10–15 bar are established to perform the hydrothermal treatment process [25]. Interestingly, the properties of water change significantly at higher temperatures and pressure to become more of an organic solvent. In such cases, reactions involving acid-based catalysts are favourable in promoting biomass decomposition [37]. At present, the exact mechanism facilitating the hydrothermal treatment process is unknown. However, it likely involves dehydration, hydrolysis, decarboxylation, aromatization and recondensation. Biomass breaks down to form saccharides and lignin throughout the hydrolysis process. Moreover, during the dehydration process, the hydroxyl group is eliminated, which ultimately removes water from the biomass. Meanwhile, during decarboxylation, all CO₂ is removed and this facilitates subsequent aromatization. A number of different compounds are produced during these processes, including phenols, aldehydes and acids and all such compounds are subjected to recondensation with aromatic polymers, which results in the production of hydrochar [38]. The key benefit of using hydrothermal technology is it can transform wet biomass into carbonaceous solids with no extensive drying required. Additionally, this produces a high yield. Other benefits include adaptable surface functionalities, conductive behaviour, the production of natural binders and high calorific value [39]. However, the hydrochar yield is reduced at higher temperatures (~350°C) (29%), as is the yield of bio-oil (31%). However, there is a substantial increase in gas fractions (67%) [40]. The O/C and H/C rations also decline with increased temperatures. Wang et al. [41] used a combination of thermal carbonization and activation to convert sunflower stalks into hydrochar [42]. The resultant hydrochar had a large surface area (1505 m² g⁻¹), as well as a 35.7 Wh kg⁻¹ energy density. Several different chemical and physical methods can be used to produce hydrochar. For instance, KOH is a chemical activating agent that facilitates the production of hydrocar with a higher surface area than other chemicals (ZnCl₂, HCl, NaCl, and MgCO₃). This is because KOH can easily reach the outer surface layer of carbon material [21, 23, 25, 43, 44, 45, 46, 47, 48, 49].

4. Conclusion

Solar and wind energy have the potential to supplement the existing energy resources in order to meet the growing global demand for electricity. It is necessary to make renewable energy more economically and efficiently efficient in order to make it more accessible. Biochar has been investigated for use in the fabrication of electrode materials and catalysts for use in the catalysis of processes involved in the generation of biodiesel and biohydrogen. It is necessary to conduct research into the synthesis of biochar with the needed qualities in order to increase the efficiency of the biochar-mediated process. Recent advancements in biochar-based material research in the field of renewable energy indicate that it has the potential to be a source of future energy in the future. The scarcity of fossil fuel, environmental pollution and the rise of energy demand have triggered many researchers to find alternatives fossil fuels feedstock. Various sources of biomass have been studied and this present study has provided the most recent promising feedstock for the alternative liquid fuel production. Criteria of excellent biomass-derived biofuel were discussed. Price, availability, the total content of oil in seed and quality are critical factors. Edible palm oil and soybean oil were proven highly promising feedstock for biofuel production; nevertheless, due to the “food versus fuel” issue make the use of these edible oil as biofuel feedstock not viable. Non-edible oil derived from jatropha oil and WCO presented promising feedstock for biofuel production. Based on the chemical composition and physicochemical properties of raw vegetable feedstocks, all feedstocks have failed to meet diesel standards (ASTM D6751 and EN 14214) specification, which in turn strong affirmed that raw vegetable is unacceptable to be used in the diesel engines.