Paradigm Shift in Bioenergy: Addressing the System of Biomass Wastage and Environmental Pollution with Biomaterial Valorisation into Biochar

Abstract

The universal need for sustainable and renewable energy sources has accelerated the shift towards bioenergy as a valuable option to fossil fuels. However, a significant challenge remains in the underutilisation of biomass resources and the environmental pollution caused by improper biomass disposal methods. Biochar, a by-product of biomass pyrolysis rich with carbon, serves as a means to convert underused biomass into valuable energy and a tool for environmental remediation. Biochar can be integrated into a biorefinery for improved bioelectricity and biogas production, but there are challenges with regard to its production scalability, quality control, and standardisation. This article provides a comprehensive review of the prospective processes useful in the valorisation of biomass into biochar for bioenergy, co-firing potential with fossil fuels, and in waste biomass transformation. This article also provides insight into business development and policy-making by bioentrepreneurs, bioengineers, and the government, as it identifies grey opportunities for bioenergy production and improvement. The prospect of AI technology in improving the production, quality, and yield of biochar, by identifying the most efficient parameters and conditions, as well as optimising the application of biochar in various industries, is also highlighted. The transition to biofuels in aviation, a step towards a future in the industry that is more sustainable, is also suggested in this review.

1. Introduction

The demand for bioenergy has rapidly increased in recent years, with the International Energy Agency (IEA) reporting a fourfold rise and predicting that over 17% of the global energy will come from bioenergy by 2060 [1,2]. Among bountiful energy sources, bioenergy has garnered significant focus due to its sustainability capabilities compared to alternatives like wind and solar energy [3,4,5], thus necessitating this critical review of bioenergy.

Bioenergy is a renewable alternative energy stemming from the conversion of biomass—plant or photosynthetically generated organic material—into biofuel and bioelectricity [6]. Biomass can be valorised into functionalised ingredients for food that are utilised to produce healthy and finished products with more nutritious dietary fibre content [7]. Biomass feedstocks comprise diverse biological components, such as cellulose, starch, lignin, hemicellulose, and oils, as well as inorganic matter [8,9]. Annually, green plants produce about 170 billion biomass metric tons, of which 75% consists of carbohydrates [10], while about 1 billion tons become waste [11]. Currently, biomass waste and feedstock account for nearly 70% of the global renewable energy production [1]. Since about 70% of the global renewable energy is derived from the conversion of plant-based biomass, the choice of the biomass and feedstock is a critical step.

Typical biomass useful for bioenergy includes waste from timber industries (construction waste, sawmill residues, forest leftovers), sugar crops (sugar beet, sweet sorghum), starchy crops (maize, wheat), oil crops (rapeseed, sunflower), herbaceous lignocellulosic crops (miscanthus), and short rotation coppice (poplar, eucalyptus, willow) [8,12]. Biomass-to-bioenergy conversion occurs mainly through biochemical or thermochemical processes [13,14,15,16].

Feedstocks of second-generation biofuel—such as lignocellulosic biomass, non-edible oils, animal fats, cooking oils, pulp, and other non-edible plant materials—and wastes from industries, agriculture, sewage, and manure are converted into bioethanol, biodiesel, biohydrogen, and biogas within integrated biorefineries [6]. Additionally, third-generation feedstocks like oleaginous microorganisms (e.g., algae) are being explored for bio-jet fuel production, attracting growing investment [17,18,19,20]. The use of biomass for energy results in lower releases of harmful pollutants, like oxides of nitrogen and dioxide of sulfur dioxide, thereby reducing acid rain globally [12]. This, therefore, means that biomass from agricultural, industrial, and other organic waste sources, including lignocellulosic materials and algae, can be converted via bio- and/or thermochemical processes into cleaner biofuels and bioenergy with minimal global pollution.

Beyond bioenergy, biochar production has emerged as a powerful carbon reduction strategy, potentially more effective than direct bioenergy use [21]. Biochar addition is also being investigated for its ability to improve biomass-to-bioenergy conversion processes [22,23,24]. To fully understand biochar’s technological and environmental potential, it is essential to examine the diversity of its feedstocks since the origin and composition of biomass directly influence the properties and functionality of the resulting biochar.

Biochar feedstocks are categorised into five main groups, and they include agricultural wastes, such as residues from rice, corn, legumes, wheat, and sugar cane (Category 1); soft lignocellulosic biomass, like tree leaves, fruit peels, reeds, nut shells, and weeds (Category 2); livestock manure, like poultry cattle, or pig, manure (Category 3); sewage sludge (Category 4); and hard lignocellulosic biomass, like wood from pine trees, bamboo trees, oak, etc. (Category 5) [24,25,26]. The transformation of these diverse feedstocks into biochar relies on controlled thermochemical processes, where process parameters determine both yield and functional properties.

Biochar is produced by heating organic materials in oxygen-deficient or oxygen-free environments—a thermochemical conversion process [27,28]. The characteristics of biochar (like surface area, yield, carbon content, nutrient retention, and porosity) depend heavily on the production method [21].

The heating in an oxygen-deficient/independent environment, otherwise called thermochemical conversion, is a process that employs either slow pyrolysis, including thermal processing of biomaterial at a relatively reduced temperature (≥300 and ≤500 °C) and in an anoxic condition; fast pyrolysis, including quick biomaterial heating under deficient oxygen at a temperature between 500 and 1000 °C; or gasification, including pyrolysis of biomaterial in a regulated environment with limited oxygen supply at increased temperatures between 750 and 900 °C [21,28,29,30,31].

These processes also produce valuable by-products, like bio-oil and syngas, usable as biofuels [21,30]. However, biochar formed via thermochemical conversion may have limitations, including insufficient surface area, low porosity, and fewer water-soluble oxygen-containing functional groups, such as OH, C=O, and CO [32].

The market for biochar in 2023 was valued at approximately USD 541.8 million, and this market was projected to increase by 13.9% annually in 2024 [21]. The function of biochar in alleviating soil erosion, degradation, climate change, and pollution conforms to the United Nations Sustainable Development Goals [21].

While extensive work has been conducted on the production and characterisation of biochar from several biomass feedstocks, there remains a limited understanding of how feedstock diversity, pyrolysis parameters, and biochar functional properties interact to optimise carbon sequestration, soil fertility, and bioenergy integration.

Most existing studies have focused on either the chemical composition or the fuel potential of biochar, but lack a comprehensive comparative assessment linking feedstock type, pyrolysis conditions, and the resultant physicochemical and energy characteristics under standardised conditions. Furthermore, few studies integrate environmental performance (e.g., carbon stability, emission reduction) with bioenergy yield optimisation, leaving a gap in designing feedstock-specific biochar systems for sustainable bioenergy applications.

This study contributes to filling this gap by systematically comparing biochar yield and quality from different biomass categories (agricultural, lignocellulosic, and manure-based) under controlled thermochemical processes. The originality of this article is the fact that it provides insight into the diverse techniques of biochar production and optimisation, its applications and co-firing potential with waste biomass for increased bioelectricity and biogas production, and the scalability, quality control, and standardisation of biochar products for bioenergy production.

Also, we evaluate how feedstock composition influences biochar’s carbon content, porosity, and energy potential, which are critical for its dual role in bioenergy production and carbon sequestration.

Insight into business development and policy making by bioentrepreneurs, bioengineers, and the government are revealed in this study, including grey opportunities for bioenergy production and enhancement.

Recent advances in biochar applications in biofuel for jet planes and spaceships, as well as in carbon sequestration, energy storage and release, are noted here. The prospect of AI technology in improving the production, quality, and yield of biochar, by identifying the most efficient parameters and conditions, as well as optimising the application of biochar in various industries, is also highlighted.

This review, therefore, provides insight into how optimised biochar production can complement bioenergy systems, addressing sustainability goals and climate mitigation simultaneously.

2. Biochar Feedstocks and Production

The processes of biochar production involve the conversion of feedstocks to improve their desired properties. It is usually carried out by the pyrolysis of a respective organic-rich biomaterial in a low- or no-oxygen environment with a high amount of the by-product, achieved at temperature and time ranges of 450 to 500 °C and 45 to 60 min [33,34,35]. The finite property of the produced biochar is usually dependent on the process conditions and treatments selected based on the desired purpose of the product [23].

2.1. Feedstocks for Biochar Production

According to Adwani and Singh [36], agricultural and municipal wastes are the major feedstocks employed during biochar production. These wastes are usually organic and inorganic materials, such as municipal sludge, animal manure, wood wastes, and agricultural and kitchen residues, which yield a porous by-product [37]. The prospect of waste, such as feedstock, necessitates non-dependence on whole biomass, which serves other purposes since its unnecessary disposal poses a serious environmental danger. In some cases, whole woods are used in biochar production, although this presents a higher cost of production and depletion of plants that would serve other purposes [38]. Biochar generated from woody biomass has a higher calorific value and concentration, which further improves its cementing mortar strength [38,39]. Other biomaterials, such as bagasse, orange peels, sawdust, pine needles, and hardwoods, were also suggested as possible feedstocks for biochar generation [40]. The findings of Kumar et al. [41] suggested that various feedstock types showed different characteristics, reflected in their energy density, volatility, pH, and humidity. Therefore, they behave differently under various heating conditions, which directly influence biochar production and properties, such as ash content, carbon composition, and mineral concentration [38]. Biochar produced from woody feedstock possess a high specific surface area (SSA) in comparison to that produced from other feedstocks, yielding high-quality products but displaying lower cationic exchange capacities, as further suggested.

Biochar feedstocks can be grouped into woody and non-woody feedstocks. These materials primarily contain lignocellulosic and non-lignocellulosic biomaterials at different ratios, which are the structural constituent elements of biochar and crucial in determining its physical properties [38,42]. This provides biochar with high aromatisation and resistance to decomposition, resulting from thermal breakdown of this biomass, as reported by Rangabhashiyam and Balasubramanian [43]. Besides biochar’s porous nature, it also contains negative functional groups, and the lignocellulose component of biochar feedstocks provides multiple physicochemical properties [44,45,46,47]. However, non-lignocellulosic biomaterial, on the other hand, harms biological systems because it contains heteroatoms (nitrogen, phosphorus, and sulfur) as well as toxic heavy metals, which accumulate in the food chain or dissolve in water bodies, leading to their contamination [48,49]. Therefore, lignocellulosic feedstock is the preferred choice for biochar production because of the eco-friendly benefits it offers. Furthermore, the choice of feedstock and the temperature employed during biochar production influence the physicochemical characteristics and interactions of the final product [50]. Thus, it offers an innovative approach that valorises organic-rich wastes into valuable biochar products as a waste-to-resource solution to their bulk accumulation in the environment [51].

2.2. Pyro Processes and Other Techniques for Biochar Production

Biomaterials, as an alternative viable source of renewable energy, have been recognised as one of the potential viable substitutes for fossil-based fuels. However, because of its increased moisture level, low bulk density, poor grindability, and reduced energy density, using it directly as fuel seems less attractive. Therefore, pyrolysis is employed to transform biomass into biochar to overcome the aforementioned problems [52]. Pyrolysis, a thermochemical breakdown procedure that involves heating biomaterials in an anoxic condition under a moderate temperature (usually around 350 and 650 °C).

Cellulose, lignin, and hemicellulose, which make up most biomaterials, go through separate reaction pathways during thermochemical degradation, such as cross-linking, fragmentation, and depolymerisation at different temperatures, yielding gaseous, liquid, and solid products [53]. According to Al Arni [54] and Abou Rjeily et al. [55], biochar is the solid by-product of biomaterial pyrolysis, bio-oil is the liquid by-product, and biogas, also known as syngas, is the gaseous by-product, which contains methane, carbon monoxide, hydrogen, and carbon dioxide. Cha et al. [53] reported that the products accrued from pyrolysis are determined by the properties of raw residue materials and modified pyrolytic procedures.

Pyrolysis can be grouped into four processes, which include the following: a. slow, b. intermediate, c. fast, and d. flash pyrolysis. The pyrolysis process is categorised in accordance with the following parameters: temperature, residence time, and heating rate (

Table 1. Review of the literature on the types of pyrolysis, feedstocks, and products of biochar.

Pyrolysis Type	Temperature Range (°C)	Heating Rate (°C/min)	Residence Time (min)	Feedstock(s)	Main Products	References
Slow	400–600	10	NA	Vine shoot, corn stover, olive mill waste	Biochar, gas	[56]
	200–500	7	120	Vegetable waste, pinecone	Biochar	[57]
	300–700	60	NA	Metal (loid)-enriched plant biomass	Biochar, bio-oil	[58]
	400–600	13	60	Pigeon pea stalk, bamboo	Biochar	[59]
	400–800	10	30–60	Mixed wood waste, coconut husk waste	Biochar	[60]
	500–900	10	60	Sewage sludge	Biochar	[61]
Intermediate	773	10	7	Fibreboard, brewery spent grains, and soybean meal	Biochar	[62]
	500	10	60	Post-extraction rapeseed meal	Biochar, bio-oil, and gas	[63]
	110	33	60	Bambara groundnut shell	Biochar, bio-oil, gas	[64]
	750	NA	NA	Hemp biomass	Biochar, bio-oil, biogas	[65]
	600	10	5	Pine bark, wheat straw, and Tetra Pak waste	Biochar	[66]
	600	150	15.28	Maize cob waste	Biochar, bio-oil, and biogas	[67]
	500–550	NA	30	Date palm water	Bio-oil, biochar, and burnable gases	[68]
Fast	400–700	NA	3–9	Date palm stones	Bio-oil, biochar	[69]
	350–1400	600	<4	Pinewood, beechwood, straw of Danish wheat, alfalfa straw, and rice husk	Biochar	[70]
	400–700	NA	1	Rice straw, pine wood, and fruit bunch waste	Bio oil	[71]
	500–600	10	NA	Sweet lime fruit bunch waste	Bio-oil, biochar, and gas	[72]
	300–600	100–400		Waste pine sawdust	Phenols, ketones, aldehydes, sugars, etc.	[73]
Flash	400–700	NA	3	Date palm stones	Bio-oil, biochar, and gas	[69]
	450–600	10	<1	Sewage sludge	Gas, liquid, and biochar	[74]
	250–610	10	<1	Soybean flake	Bio-oil, Biochar	[75]
	900	7.5	NA	Wood pellet, polyethene, and polypropylene	Tar, gases	[76]
	500	10	NA	Pachira aquatica fruit shells	Bio-phenolics	[77]

NA: not available.

). However, other processes, like torrefaction, have also been shown to produce biochar of known quality.

2.2.1. Slow Pyrolysis

Slow pyrolysis is an ideal technique that uses lengthy residence times (10 to 30 min or 25 to 35 h) and slow heating rates (0.1 to 0.8 °C/s) under temperature ranges of 350 and 600 °C in an unoxygenated environment [78,79]. As stated by Laird et al. [80], slow pyrolysers can be categorised into two systems: batch systems, also known as “charcoal kilns”, or continuous systems, where a biomaterial is gradually heated up to above 400 °C in an oxygen-limited environment. Slow pyrolysis is widely chosen to yield biochar as a gas by-product. This technique is a rather easy and reliable process, mostly explored for small-scale and/or farm-based biochar synthesis [81]. Technologies for slow pyrolysis produce larger particles of biochar, up to several centimetres in size [82], and more biochar than fast pyrolysis [83]. Goyal et al. [84] stated that the final products of pyrolysis using this process are predominantly biochar (35%), bio-oil (30%), and syngas (35%).

Although the processes involved in slow pyrolysis are known to be easy, the yields and characteristics of its products can be mostly affected by the conditions of operation, such as particle size, peak, pressure, vapour phase residence time, and temperature.

Furthermore, the origin and source of the biomaterial significantly impact the production amount and properties of the biochar that is produced [56]. Mohamed Noor et al. [85] suggested that biochar synthesis via slow pyrolysis of a biomaterial is a potential carbon-negative practice since it eliminates the net carbon dioxide in the atmosphere and produces recalcitrant carbon appropriate for sequestration in soil. Sakhiya et al. [79] suggested that low pyrolysis-generated biochar can be utilised as a clean solid fuel, soil conditioner, coal-fired power plant fuel, and carbon alleviation method to reduce greenhouse gas emissions. While biochar is the primary product of slow pyrolysis, its features are favoured by the slow heating rate; however, it is inevitable that certain by-products, such as a mixture of gases and bio-oil, will occur. This bio-oil, valorised through gasification, supports the circular economy strategy and decreases wastage processes and energy losses [86].

Another intriguing route to improving the effectiveness of slow pyrolysis practice, particularly regarding economic savings, substitutes the utilisation of a comparatively high-cost inert gas, such as N₂, with CO₂ emanating from flue gas residues [87]. Manyà et al. [56] conducted slow pyrolysis research using three different biomass sources, including vine shoots, corn stover, and waste from a two-phase olive mill, to determine the various impacts of some factors, specifically maximum temperature, pressure, and pyrolysis atmosphere, on the stability prospects of biochar, including the yields of the major pyrolysis outputs. The findings showed that biomass transformation via high-pressure slow pyrolysis in a CO₂ environment is significant in concurrently achieving two value-added products, including biochar with suitable carbon seizure capabilities and a generated gas with a suitable formulation for energy resource recovery.

2.2.2. Intermediate Pyrolysis

This process yields a liquid with two phases, characterised by a low heating value and a high water content in the aqueous phase [88]. This process is primarily used to combine the advantages of both the slow and fast methods of pyrolysis. Fast pyrolysis generates high liquid output with comparatively few solid residues, whereas slow pyrolysis produces a high percentage of solid residues with low liquid output. Temperatures between 300 and 600 °C are commonly used for intermediate pyrolysis, with heating rates ranging between 0.1 and 10 °C per minute. It can process a wide variety of biomass types, as well as different particle sizes, such as pellets and chips, and tolerate moisture contents up to 40% [89]. These characteristics support the assertion by Yang et al. [90] that this type of pyrolysis serves as an effective alternative to both slow and fast pyrolysis.

Waste biomass is rich in organic components that can be transformed into high-calorific fuel through intermediate pyrolysis [62]. Jung and Kim [91] found that biochar produced at 500 °C and 800 °C had surface areas of 107 m²/g and 249 m²/g, respectively. Using a final activation temperature of 900 °C with a one-hour activation time under a N₂/CO₂ atmosphere (without cooling) yielded biochar with a surface area as high as 1126 m²/g. According to Luz et al. [92], biochar stemming from intermediate pyrolysis boosts microbial bioavailability and activity. Their study showed methane peak values of 62, 67, and 70 per cent at biochar production temperatures of 450, 500, and 550 °C, respectively. Typical yield ranges for intermediate pyrolysis are 15% to 25% for biochar, 40% to 60% for bio-oil, and 20% to 30% for gas.

The bio-oil that results from intermediate pyrolysis is distinguished by its low tar content and viscosity [89]. Intermediate pyrolysis can be used to generate high-quality biochar and bio-oil, as well as by-products for the chemical industry from lignocellulosic and high-protein biomass [62,63].

2.2.3. Fast Pyrolysis

In this process, biomass decomposes rapidly under a high heating rate (10–100 °C/s) in an oxygen-limited environment, with a short residence time of approximately 0.5 s to a few minutes depending on the biomass and equipment model (Table 1). In this type of pyrolysis, rapid vapour cooling is employed to maximise bio-oil yields [78,83,89,93]. The pyrolysis process decomposes biomass into vapours (oil and gases) and solid residue (biochar) through polymerisation and fragmentation reactions [79]. As a result, fast pyrolysis produces more pyrolysis gas and bio-oil than biochar. Biomass is first converted into liquid products rather than solid biochar. Fast pyrolysis typically provides 50–70% bio-oil, 10–30% biochar, and 15–20% syngas by mass, according to Laird et al. [80]. Woolf et al. [94] reported that fast pyrolysis produces more fuel products and less biochar compared to slow pyrolysis. The primary goal of fast pyrolysis is to achieve high liquid yields. Additionally, Bridgwater [95] highlighted that the bio-oil produced by fast pyrolysis offers significant advantages, including ease of transport and storage, along with the potential to generate a variety of valuable compounds. Unfortunately, it is disheartening to see that in the haste of starting new research, the lessons learned from earlier work are either ignored or lost. Some fundamental and necessary conditions must be met for fast pyrolysis to produce good bio-oil yields [95].

Additionally, bio-oils produced by fast pyrolysis are characterised by lower viscosity and lack of phase separation, making them well-suited for gasification [96]. The high bio-oil yield in fast pyrolysis largely results from the limited time available for side reactions, such as tar decomposition and repolymerisation. According to Frantzi and Zabaniotou [86], rapid heating rates promote the quick endothermic breakdown of biomass, increasing the production of volatile components while minimising mass and heat transfer limitations. During fast pyrolysis, biomass rapidly degrades, producing primarily aerosols, vapours, and condensable and non-condensable gases, with only trace amounts of gas and biochar. Upon cooling, the vapours and aerosols condense into a dark brown, single-phase liquid that has about half the calorific value of conventional fossil fuels [79,89]. Notably, Alcazar-Ruiz et al. [97] suggested that fast pyrolysis of agro-waste could serve as a promising alternative for producing improved biofuels.

Because of their exceptionally quick heat transfer rates, high gas velocity, large surface area contact, and exact control over vapour residence time during the pyrolysis reaction, fluidised bed reactors are generally considered the most appropriate and frequently used technology for fast pyrolysis [89]. Compared to other reactor types, they also provide comparatively easy scaling and simplified operation [98]. Using a fluidised bed reactor, Wang et al. [99] examined the fast pyrolysis of Chlorella vulgaris residues, demonstrating that energy and nutrients can be recovered from microalgal biomass after lipid extraction. Their results revealed that the bio-oil contained a diverse range of chemical compounds, including amides, aromatics, amines, phenols, carboxylic acids, and fatty acids. The high nitrogen content in the bio-oil was likely attributable to the protein-rich nature of the feedstock. Miscanthus rapid pyrolysis in a fluidised bed reactor was studied by Wang and Lee [100]. According to their research, the pyrolysis gas output rose with temperature, peaking at 550 °C, with a yield of over 60%. In a different study, Dong et al. [101] used fast pyrolysis in an auger reactor at 600 °C for one minute to create a cheap catalyst from charcoal obtained with pyrolysis, which was subsequently utilised for pre-esterification in the production of biodiesel.

2.2.4. Flash Pyrolysis

Flash pyrolysis is an enhanced and amended method of fast pyrolysis. This procedure entails igniting a flash fire in a bed of dense biomass under high pressure. Nunoura et al. [102] stated that instead of moving downhill through the bed, the fire goes upward, converting biomass into gas at high pressure and biochar with fixed-carbon yields that can get close to the thermochemical equilibrium limit. The particle diameters during flash pyrolysis range from 0.3 to 1.18 mm [103], and biomass breaks down at temperatures above 1000 °C in a brief period of time, usually less than a minute [104]. A high bio-oil yield is produced by the combination of quick heating, high temperatures, and brief vapour residence times. However, the production of biochar is decreased by this procedure [105]. The major aim of flash pyrolysis is to maximise the amount of bio-oil produced. According to Ighalo et al. [106], flash pyrolysis produces about 9.4–79.5 (%) bio-oil to 15.0–56.0 (%) biochar depending on the feedstock and biomass used. Compared to conventional kilns, flash pyrolysers are more likely to have heat-recovery equipment. Flash pyrolysis can also be carried out in fluidised bed and twin-screw mixing reactors; however, the need for extremely high temperatures and fast heating rates in reactors limits its industrial use [104]. Pardo et al. [107] investigated energy recovery from agri-food waste, specifically grape seeds and chestnut shells, using two pyrolysis methods: flash pyrolysis at 750 °C and 850 °C and conventional pyrolysis at 750 °C. Their results demonstrated that flash pyrolysis outperformed traditional pyrolysis in terms of product quality. Notably, the gas from flash pyrolysis had a high heating value that was about four times higher, which was explained by higher levels of hydrogen (H₂) and methane (CH₄). Over 90% polycyclic aromatic hydrocarbons were present in the grape seed bio-oil, which had a calorific value of up to 32 MJ/kg—roughly 7% more than the heating value of bioethanol. In a different investigation, Madhu et al. [108] examined how particle size, temperature, and sweep gas flow rate affected the flash pyrolysis of palmyra palm fruit in a fluidised bed reactor. At 500 °C, a particle size of 1 mm, and a sweep gas flow rate of 2 m³/h, they reported the maximum bio-oil production of 48.22%. According to Frantzi and Zabaniotou [86], flash pyrolysis has three main drawbacks: low thermal stability and high viscosity of bio-oils due to the catalytic activity of biochar and the formation of solid residues.

In summary, slow pyrolysis promotes extended residence durations and reduced heating rates, mostly yielding biochar beneficial for carbon sequestration and environmental remediation. Additionally, intermediate pyrolysis balances the production of biochar, bio-oil, and gas, with a focus on bioenergy and chemical applications. Finally, fast pyrolysis operates at high temperatures and heating rates with short residence times, optimising bio-oil and chemical production for fuels and industrial chemicals.

2.2.5. Torrefaction: Alternative Biochar Production Process

Torrefaction is a widely known thermochemical process that transforms biomaterials into energy-dense biochar with enhanced grindability and improved combustion properties [109]. As a promising technique, torrefaction plays a crucial role in upgrading biomass energy density and unlocking its full potential [110,111]. Raising bulk and energy densities, lowering water and oxygen content, and improving hydrophobicity are qualities that make transportation and storage easier. This method has been thoroughly explored to increase biomass. Furthermore, downstream thermochemical transformations, like gasification, benefit from the ease with which torrefied biomass may be pulverised into consistent particle sizes [112]. Torrefied biomass can improve soil fertility and carbon sequestration for sustainable agriculture, facilitate scalable decarbonisation within circular economy frameworks, and transform agricultural leftovers into useful industrial inputs [113]. It facilitates energy self-sufficiency, reduced carbon footprints, better economic returns, and sustainable supply chains. Torrefied biomass lowers greenhouse gas emissions and increases process efficiency when utilised to produce energy [114]. To identify the best uses for biochar, however, it is crucial to understand how changes in cellulose, hemicellulose, and lignin concentration impact its chemical and physical characteristics [115]. Ibitoye et al. [116] investigated biochar made from rice straw torrefaction as a viable substitute for coke and coal in the manufacturing of steel and iron. The study assessed how torrefaction yields were affected by heating rate (10–30 °C/min), temperature (200–300 °C), and residence duration (20–60 minutes). The yield ranges for solid, liquid, and syngas were 44.67–96.43%, 1.50–22.39%, and 2.07–36.79%, respectively. At 270 °C, ideal conditions produced about 64% solid biochar. The moisture, ash, volatile, and fixed carbon contents of the biochar varied from 7.43 to 8.80%, 5.76 to 6.87%, 21.75 to 28.26%, and 56.83 to 63.82%, respectively. Looking ahead, as technology advances and the demand for sustainable energy solutions grows, key future developments in torrefaction include improvements in reactor design, automation, and process optimisation. These advancements could enable commercial-scale torrefaction facilities, contributing to a cleaner and more efficient energy landscape [117]. The key trends are shown in Table 1 and are summarised as follows:

2.3. Temperature Effect on Biochar Yield

Temperature is one of the most important variables that influences biochar yield, composition, and functionality during the pyrolysis process. Generally, as the pyrolysis temperature increases, the biochar yield decreases, while the carbon content, surface area, and aromaticity of the resulting biochar increase [118,119]. This occurs because higher temperatures promote greater thermal degradation and volatilisation of organic components, such as cellulose, hemicellulose, and lignin—reducing the solid fraction and increasing the gaseous and liquid by-products [120,121]. At low pyrolysis temperatures (300–400 °C), the process is relatively incomplete, resulting in a higher yield (up to 50–60%) of biochar rich in oxygen-containing functional groups but lower in fixed carbon and surface area. Such biochars are often suitable for soil amendment and nutrient retention due to their high cation exchange capacity [122,123]. Conversely, high-temperature pyrolysis (500–700 °C) yields lower quantities (20–35%) of biochar with greater carbon stability, higher aromaticity, and enhanced porosity—attributes desirable for carbon sequestration and adsorption applications [105,124,125]. However, excessively high temperatures (>700 °C) can lead to the loss of nutrient elements (e.g., N, P, K) and excessive ash formation, thereby reducing the agronomic value of the biochar [126,127]. In summary, increasing pyrolysis temperature decreases overall biochar yield but enhances stability, surface area, and fixed carbon content (Figure 1). The optimal temperature depends on the desired application—lower temperatures for soil fertility enhancement and higher temperatures for carbon sequestration or pollutant adsorption.

3. Overview of Biochar Applications

Biochar has obviously garnered heightened awareness owing to its unique properties, including increased surface area, cation exchange capacity, high carbon content, nutrient retention capacity, stable structure, potential for reducing nutrient leaching, and climate change mitigation [79,128]. These rare features of biochar have helped to make it a practical, affordable, and environmentally friendly method of sequestering carbon from soil. Biochar’s adaptability to physicochemical characteristics offers a chance to maximise its effectiveness and achieve the intended results [129]. According to Neogi et al. [130], enhancing soil qualities, lowering greenhouse gas emissions, eliminating organic and heavy metal contaminants, creating biofuels, synthesising beneficial compounds, and producing cementitious materials are all benefits of biochar. Biochar is predominantly produced by utilising plant residues and other biomass wastes; however, its on-farm production and subsequent uses in a closed-loop agriculture system will avoid expensive transportation costs and promote a circular flow of materials and energy that preserves additional economic and environmental advantages [131].

Table 2. Summary table of biochar applications and effects.

Goals	Characteristics Application	Results	References
Carbon sequestration	Stable carbon source, suitable for long-term soil carbon sequestration	CO2 emission reduction and climate change mitigation	[132,133]
Soil quality enhancement	The unique structural porosity of biochar offers a conducive environment for microbes in soil, hastens the action of the soil biological chain, increases biodiversity, and promotes nutrient cycling	Nutrient retention, greenhouse gas abatement, enhanced soil fertility, reduced carbon footprint in agroecosystems, and soil bioremediation	[134,135,136]
Water treatment	Biochar has a porous structure that absorbs organic pollutants and heavy metals	Removal of organic and inorganic pollutants and maintenance of organoleptic properties	[137,138,139,140]
Energy storage and release	The structural porous properties of biochar and its capacitive characteristics as a composite material can store and release energy	Used in supercapacitors and battery technology, enhanced energy system sustainability, and an increase in energy densities	[48,141,142]

summarises biochar applications and their effects.

3.1. Mitigation of Climate Change and Carbon Sequestration

The process of absorbing and retaining carbon to prevent it from being released into the atmosphere is known as carbon sequestration [132]. The primary factor contributing to global warming is typically the temperature surge caused by various anthropogenic activities. As far back as the late nineteenth century, the average surface temperature of the Earth has increased by about 1.55 °C [143], obviously attributable to increased CO₂ via anthropogenic activities [129].

Biochar plays a pivotal role in the carbonaceous non-biodegradable solid product produced by pyrolysing biogenic organic wastes. Biochar is regarded to be among the most promising alternatives of negative emission technologies (NETs) due to its capacity to sequester carbon and its associated low cost and environmental footprint [144,145,146]. Since it is a stable carbon source, biochar remains in soil for a longer amount of time, allowing for long-term soil carbon sequestration. The sequestering impact of biochar has the advantage of alleviating global warming by decreasing greenhouse gas emissions (GHG) from soil [133]. The accessibility of biomass feedstocks ultimately determines the mitigating effects of biochar; therefore, the ability to provide biochar systems with biomass feedstocks for extended periods of time must be evaluated in relation to the CO₂ removal of biochar, which is pertinent to climate mitigation [144]. Kurniawan et al. [134] asserted that biochar can store CO₂ for an extended period of time and prevent its release back into the atmosphere following the decomposition process.

Research by Xu and his colleagues [147] revealed that the pyrolysis of switchgrass for biochar production on marginal land and subsequent application to soil brought about enormous environmental benefits by reducing greenhouse gas (GHG) emissions and sequestering carbon without crop land competition. A life cycle analysis of the widespread use of biochar in agriculture and associated possible advantages was presented by Yang et al. [90], who found that converting one tonne of crop waste into biochar may sequester almost 920 kg of CO₂.

3.2. Soil Fertility Enhancement

Innovative and viable solutions are essential for sustained agricultural outputs for the world’s growing population, mainly when climate change surges desertification and drought worldwide [148]. According to Kombat [149], agriculture accounts for about 40% of the climate crisis; therefore, the agricultural sector should also contribute to curbing the impacts of climate change purposefully. An environmental impact assessment can be carried out via soil cultivation, breeding of animals, and crops. Plants are definitely the source of the bulk of human food, which also contributes to enhancing nutritional health and improving food sufficiency.

In the quest to improve soil fertility for enhanced plant growth, chemical fertilisers have, however, long been utilised to boost the yield of cropland, which poses a threat because part leaches into the environment and results in the emission of greenhouse gases (GHG) [148]. According to the Sustainable Development Goals (SDGs) established by the United Nations, intentionally positive energies must be directed towards improving soil quality and viable green resource development [135]. Hence, Kombat [149] stated that there is a growing need to make agricultural soil more fertile via various viable means that encourage soil organic carbon sequestration and carbon dioxide emission. Therefore, applying biochar and microbial community biomass to the soil, for example, contributes to sustainable agriculture by sequestering carbon in the soil, increasing soil fertility, lowering the minimum amount of carbon released into the atmosphere, and improving the nutritional value of plants while also enhancing food sustainability.

Biochar, being an eco-friendly and affordable carbon-based material made from biomass residues, provides a natural solution for enhancing soil fertility and curbing energy use. It has numerous environmental advantages when expansively applied to soil for amendment, including soil bioremediation, greenhouse gas abatement, enhancing soil quality, reducing the carbon footprint of agroecosystems, and increasing microbial growth and activities [133,134,135,136].

The unique structural porosity of biochar offers a conducive environment for microbes in the soil, hastens the action of the soil biological chain, boosts biodiversity, and promotes nutrient cycling. The structural porosity of biochar enables the formation of a symbiotic microbe–plant relationship, which results in plant growth and diverse soil microbial groups [150]. The application of biochar as a soil enhancement mitigates 12% of the CO₂ emissions from land use yearly [134,151]. Woolf et al. [93] reported the high (12%) carbon dioxide sinking potential of biochar conversion from biomass.

3.3. Water Treatment and Purification

Significantly, about 70% of the surface of the Earth is covered by water, which is necessary for life. Currently, the growing human population has led to greater water demand, which has, in turn, raised the amount of waste produced by human activities [152]. Several scholars have demonstrated elevated levels of synthetic and emerging organic contaminants in aquatic environments in Nigeria and a few other selected countries within Africa [153,154,155,156,157]. A vast number of chemical contaminants are categorised as emerging organic contaminants (EOCs) since they are either new or recently discovered [158].

In developing nations, there is insufficient funding for the installation, maintenance, and operation of technology that provides clean drinking water [54,159].

Consuming impure water, which is common in developing nations, is the main method by which pollutants enter the human body from the environment. However, water quality issues have been neglected, and this poses threats to the supply of safe drinking water in most less developed countries, likely because of unskilled staff and exorbitant analytical equipment [138]. The progressively broad and enormous use of the most probable sources of emerging contaminants, like personal care products and pharmaceuticals in less-developed nations, makes their devastating environmental effects a true concern [153]. In Nigeria, however, there is little knowledge of the detrimental implications of emerging contaminants (ECs) in industrial-based wastewaters released into the environment, even though the existence and consequences of conventional pollutants linked to industrial wastewater discharge are well known [160]. According to Egbuna et al. [154], persistent exposure to these emerging pollutants may be linked to several health issues, including carcinogenesis, hepatotoxicity, neurotoxicity, and renal toxicity. To ameliorate the glitches associated with water contamination, various writers have suggested using existing cost-effective procedures, such as boiling, sand filtration, chemical disinfection, chlorination, and sedimentation [161,162], however, it has been reported that some of these existing methods, in turn, form cancer-causing agents and/or upsurged levels of chemical pollutants [138].

Biochar has become increasingly significant for a variety of environmental applications in recent years. It has demonstrated significant benefits such as favourable surface and structural characteristics, ease of preparation, and readily available feedstocks. These remarkable qualities portray it as a low-cost, efficient, and environmentally sustainable resource for diverse removal of impurities [137], making it suitable for low-income areas [163] and the removal of organic and inorganic pollutants [139]. It has also been reported that it maintains organoleptic properties [138].

García-Ávila et al. [140] assessed the efficiency of organic waste-derived biochars as filtration materials for the treatment of drinking water, and the results revealed that the biochars demonstrated improved removal of turbidity and colour. Likewise, Vikrant et al. [164] and Tan et al. [165] highlighted that a pyrolysis-derived filter is beneficial for lead removal from drinking water, which is toxic and causes persistent health problems in humans. According to Hersh et al. [166], biochar sorption properties and textural characteristics make it a unique method for removing a range of toxins from water, such as metals and toluene.

3.4. Energy Storage and Release

The utilisation of fossil-based fuels, including oil, coal, and natural gas, to energise a variety of actions has significantly supported the rising global need for energy [48]. For energy to be conserved and delivered to end users consistently and sustainably, energy storage is necessary. Cost-effective and sustainable energy storage materials are necessary for a viable supply of energy with minimal waste generation [141]. A variety of inexpensive and easily accessible biomaterials have been studied to yield carbonaceous catalysts since they are nature-based resources for carbon and other essential features [167].

There is a surge in attention among researchers on the production of activated carbon utilising biochar as a promising viable precursor, pyrolysed from biomass residues [168]. Biochar addresses environmental issues and encourages the sustainability of energy storage application development [48]. However, it can be tailored to improve its electrochemical properties and structural features by increasing porosity, enhancing surface functionalities by doping heteroatoms or increasing its surface area. These characteristics of biochar are extremely preferred for its effective use in energy storage (in batteries and supercapacitors) or hydrogen storage [142]. Research conducted by Husain et al. [141] revealed that porous biochar, derived from garden residues, demonstrated an increased energy density in an aqueous electrolytic solution and potential stable cycling with 88% capacitance retention. This could be linked to the graded structural porosity of biochar with heteroatom surface functionalities, which is a necessary characteristic for supercapacitor applications [142]. Also, according to Atinafu et al. [169], oilseed rape-derived biochar showed increased heating enthalpy. This could, however, be linked to its favourable structural properties, such as high mesopore proportions, physical structures of the supporting materials, and high specific surface area.

4. Biochar Integration in Energy Systems

In recent studies, researchers have investigated the possibilities of biochar as a sustainable electrochemical energy storage material because of its clear-cut abundance and physicochemical characteristics, such as surface functional groups, a substantive surface area, excellent electric conductivity, porosity, pore volume, and chemical stability [170,171]. Further modifications through activation, doping, and hybridisation enhanced its energy storage capacity, aimed at addressing the cost-effectiveness, efficiency, and environmental impact challenges of energy storage [170,171,172,173]. Porous biochar doping with heteroatoms, such as nitrogen (3.56%) and oxygen (6.4%), improves its capacitance, resulting in high density, cycling stability, and high surface area [141,174]. Thus, the doping process serves as a cost-effective means of biochar performance improvement. Some advancements in biochar energy storage research generated promising results, such as solid-state batteries, flow batteries, lithium-air batteries, thermal energy storage, supercapacitors, and hydrogen storage [175]. These supercapacitors, as suggested by Visser et al. [176], comprise pore structures capable of high conductivity and electrolyte absorbance of oxygen-rich functional groups, which improve their energy storage ability. Andrade et al. [177] reported a coffee ground-based biochar electrode with a large surface area and a high surface site oxygen content, which served as a supercapacitor to store electricity generated from a photocatalytic fuel cell (PFC). Biochar was also reported to possess high specific capacitance, and it is capable of maintaining 90% its capacitance after 5000 cycles of usage [174]. Poultry litter-based biochar, activated with potassium hydroxide (KOH), as suggested by Pontiroli et al. [178], generated highly activated carbon with increased electrical conductivity, high performance, and porosity, making the biochar sourced from this less expensive material an effective energy storage alternative that can be used as supercapacitors.

As reported by Liu et al. [179], biochar has shown potential as a sustainable energy source because of the abundance and availability of the feedstocks utilised to generate it. Biofuels, heat, and electricity were also reported to be generated from biochar, which reduces fossil fuel dependence and assists in the switch to these energy sources as an alternative to the former [180]. According to reports, biochar can be used as a heterogeneous catalyst in the process of producing biodiesel because of its expansive surface functionality, surface area, and resistance to both acidic and basic conditions [176]. The biodiesel generated from cooking oil wastes showed up to 90% conversion efficiency via biochar applications, as revealed by Chi et al. [181]. In a different study, 99.3% of the free fatty acids in Jatropha curcas oil were converted when biochar made from the plant was employed as a catalyst to esterify the oil into biodiesel [182]. Furthermore, biochar has also been applied as a catalyst and electrode for biodiesel and hydrogen generation in microbial fuel cells [183]. Biochar technology has shown many intriguing applications in energy systems and, therefore, has been ascertained to be an efficient sustainable development tool for energy generation and storage. Reports by Ahmed et al. [38] suggested that carbon extracts from biomass serve as raw materials for biofuel generation and an agent for water purification. This extract can be in the form of biochar with adsorptive and energy-producing capabilities, used to improve energy storage performance in various applications.

4.1. Co-Firing of Biochar and Fossil Fuels

More than one fuel source is said to undergo co-firing when combined and simultaneously combusted. Coal has been the major fuel source co-fired with other sources for efficient, environmentally friendly, and sustainable energy generation processes [15]. It has been in use over the years, showing 25–35% efficiency, but it emits bulk greenhouse gases [184]. The necessity for green and renewable energy has resulted in the process of co-firing coal with other fuel sources. Its advantages include burning new fuel in old plants without incurring the cost of constructing new ones, reducing accompanying greenhouse gas emissions, and reducing fossil fuel dependence. Research by Truong and colleagues [185] showed that co-firing coal with 20% rice straw biomass increased energy output, reduced CO₂ emissions by 4.68 metric tonnes, destroyed waste fractions, and improved low-energy fuel combustion rates. They further stated the advantage of co-firing, which utilises low-energy fuels, such as leftover coal pellets, to destroy waste fractions and increase energy output simultaneously. Coal and biomass co-firing also reduces the release of noxious polluting agents, such as carbon oxides, nitrogen, and sulphur [15,186]. The overall ability of the co-firing process to improve the combustion rate of low-energy fuels makes the process an ideal means of harnessing the energy potential of these biomasses.

Fossil fuel co-firing with biochar is an integrated process that provides the energy needed for the pyrolysis process to aid bio-oil and pyro-gas generation [187,188]. This boosts the temperature of the process to reach the required level of operation, and supplying the required amount of heat to the pyrolysing chamber generates the desired amount of product. Although different treatment conditions are required during pyrolysis processes, effective control and monitoring of these treatment methods are necessary since their impacts on the substrates determine the features of the final product. According to González et al. [187], these pyrolysis products act as a supplementary power source that can be utilised in cogeneration units to produce electricity. Anand et al. [189] also reported electricity generation from wheat straw and rice straw-derived biochar in the Haryana and Punjab states of India, leading to eco-friendly management by co-firing them in coal power plants via a slow pyrolysis process. This reduces the pollutants emitted when these wastes are burned by farmers, and they can serve as fuel replacements in power plants, as further reported. Electricity generation from palm kernel shell-based biochar in a direct carbon fuel cell (DCFC) has been reported, which depends on the transformation of chemical energy into electrical energy via chemical oxidation processes [184]. Biochar from biomass serves as a catalyst, anode, and cathode in microbial fuel cells (MFCs), with 4346 mW/m² maximum power density, as reported by Osman et al. [23]. Biochar, therefore, serves as an electricity generation source, an energy intermediate for electricity generation, and a conducting and storage device.

4.2. Biochar, Biofuels, and Bioenergy

4.2.1. Biofuels

The stresses posed by climate change have prompted researchers to search for alternatives to the generation and consumption of biofuels, such as biodiesel, bio-oil, bioethanol, and syngas, to mitigate these challenges [38]. Biochar utilisation as a catalyst helps to improve these biofuels and their various production strategies. It has been used to improve bio-oil quality and syngas yields during their respective production processes, and it is crucial in their hydrocarbon compositions [190]. Utilising biocatalysts in microbial bioconversion processes during biomass decomposition to generate biogas and biofuels, such as bioethanol, biodiesel, and biohydrogen, has been improved by the application of biochar [23,191]. For instance, improvement in bioethanol production by strains of S. cerevisiae and K. marxianu, when used as biocatalysts, by immobilisation on biochar with easy recyclability, yielded 7.2 and 7.3 g L⁻¹ h⁻¹ of the metabolite [192]. Also, enhanced ethanol production from lignocellulosic feedstock using a biochar-mediated fermentation process has been suggested as a promising strategy to overcome the inhibitory effect of acetic acid and furfural generated during lignocellulose pre-treatment [193].

The trans-esterification reaction between alcohol and fats or oils, employed in the production of biodiesel, is possible in the presence of homogeneous or heterogeneous catalysts [194]. Heterogeneous catalysts are usually preferred to homogenous ones and have been broadly utilised in producing biodiesel because of their easy separation from products, fast recycling, good stability, and long lifetime, which the latter lacks [195]. Biochar serves as a porous material that facilitates heterogeneous reactions between oil and alcohol to produce biodiesel [196]. Foroutan et al. [197] produced crystalline-structured biochar from eggshells, and brown alga was used to produce biodiesel with 98.8% efficiency. These catalysts are eco-friendly and can be recycled in several rounds of production before a drop in product yield. Chicken manure-based biochar produced 95.6% fatty acid methyl ester (FAME) yield at 350 °C from waste cooking oil, although the presence of calcium resulted in catalytic cracking of the oil residues [196]. Other materials, such as iron-impregnated palm kernel shells, were used to improve catalyst performance, achieving 90.2% biodiesel yield [198].

Bio-oil and syngas are generally produced from agricultural wastes, wood chips, or municipal wastes through a thermochemical process during biochar production. According to Frantzi and Zabaniotou [86], pyro-oils, which are plant by-products, serve as intermediate energy carriers for syngas production in gasification plants. The utilisation of bio-oil for advanced biofuel production is economically advantageous due to its simplicity of production through pyrolysis. In addition, bio-oil, as a liquid fuel, showed benefits such as economical storage and handling, a higher energy density than solid fuel, and improved fuel properties [199]. There has been increased interest in pyrolysis technology commercialisation resulting from bio-oil upgrades and applications in different processes, such as chemicals, transportation fuels, materials, and heat and energy generation, as Braimakis et al. [199] suggested. The reforming or steam gasification of bio-oil is primarily based on hydrogen production. During steam gasification, the gas mixture contains more hydrogen gas, which is proven to be profitable and promotes the circular economy by minimising energy losses and process wastes [86]. According to Osman et al. [23], biochar acts as a catalyst during dry methane reforming and methane decomposition, with a 13.4–97.75% hydrogen conversion rate and a 220.3% increase in hydrogen yield. Thus, biochar plays an important role in hydrogen production from pyrolysis and methane conversion processes.

Table 3. Pyrolysis methods, conditions, and percentage biofuel yields.

Pyrolysis Methods	Temperature (°C)	Residence Time (s)	Heating Rate (°C/s)	Particle Size (mm)	Yield (%)
					Oil	Char	Gas
Slow	300–950	330–550	0.1–1.0	5–50	30	35	10–30
Fast	850–1250	0.5–10	10–200	˂1	50	20	30
Flash	900–1200	˂1	˂1000	˂0.5	75	12	13–15

Adapted from [105,200,201].

lists biofuel yields with different biochar-producing pyrolysis types.

4.2.2. Bioenergy

Bioenergy is recognised as a feasible green energy resource and a promising alternative for reducing dependence on fossil fuels. Bioenergy is also broadly classified into two main types: biofuel (like liquid fuels, such as ethanol) and biopower (i.e., electricity) [202].

The transition from conventional fossil fuel energy to biochar-based bioenergy generation can aid in decreasing greenhouse gas emissions and their resultant deleterious impacts on the environment [203].

Pyrolysis-based bioenergy production relies heavily on a steady and suitable biomass supply [204]. Bioenergy plays a vital role in energy security because it is abundant and renewable, which gives it clear advantages over conventional fossil fuels. Energy is a fundamental necessity for the advancement of practically every societal component worldwide, as ecosystems, lives, and human civilisations all depend on it [205].

4.3. Biochar and Anaerobic Digestion

Anaerobic digestion (AD) technology is an effective and efficient means of organic biomass conversion into biogas. It has been reported as a practical approach for effective waste management and clean energy generation [206]. Its benefits include methanogenic lag phase reduction, toxic inhibition mitigation, functional bacteria immobilisation, and electron transfer enhancement between acetogens and methanogens, not without challenges, such as digester failure, lower productivity, and the presence of refractory substances [206,207,208,209]. Adding 5–50 g/L of biochar into anaerobic digesters during AD is effective in mitigating these challenges [207,208]. Parra-Orobio et al. [207] further reported improvements in methane yield of 70% and 450% when food waste and pig manure substrates were used for biogas production. Biochar increases AD’s buffering capacity due to its alkaline nature, maintaining a stable pH, with its pores serving as substrate for differently functioning microbial communities [210,211]. This will result in improved interspecies electron transfer and microbial activity to enhance volatile fatty acid degradation, resulting in an efficient and stable AD process that improves biogas generation [212,213,214]. Ebubechi et al. [215] suggested that during the anaerobic co-digestion of organic feedstocks, biochar acts as an efficient stabiliser to enhance biogas generation from these substrates.

During anaerobic digestion processes, biochar adsorbs and degrades inhibitory substances, adsorbents, and degraders, enhancing biomass utilisation to boost biogas generation from organic wastes [215]. These inhibitors, such as certain organic compounds, NH₃, nutrients, sulphides, and heavy metals, are removed by biochar supplementation via a sorption process, alleviating the risks posed by their accumulation and inhibitory effects during AD [206,216,217]. The AD process usually suffers ammonia (NH³) inhibition, leading to digester failure; however, biochar incorporation into the system effectively reduces the NH³ concentration in the digesters through NH₄+-N sorption, making the nitrogen content more available for mineralisation after soil application [211]. The amendment of AD digestate with carbon-rich biochar reduces carbon dioxide emission by 33%, with an increase in its organic content, highlighting its potential as a soil improver when employed in agricultural practices, as further reported by Viaene et al. [211]. According to Wang et al. [218] and Schmidt et al. [219], the biochar component in fertiliser serves as a stable carbon source with the ability to act as a long-term carbon sink, curbing climate change impacts and reducing CO₂ emissions. This makes biochar an efficient additive to both the AD system and the digestate, contributing to a bio-circular economy. AD and biochar supplementation strategies help to achieve recalcitrant compound bioconversion, with improved digestate physicochemical characteristics for agricultural applications [207].

5. Limitations and Future Directions

5.1. Limitations of Biochar Production and Scalability Costs

Although there are excellent instances of small-scale sustainable biochar production, using materials like Brazil nut shells [220], the development of global manufacturing capacity is necessary for the use of biochar to become feasible [221], especially as feedstocks or additives for bioenergy. One of the obvious challenges limiting the scalability and commercialisation of biochar is the technological challenge of large reactors underheating during pyrolysis. Seman-Varner et al. [222] and Hassebrook et al. [223] indicated that the rate at which biomass is processed in larger reactors is slower compared to smaller ones, as it takes more time for heat to be transferred into biomass in larger reactors. However, temperature-automated pyrolysis has been reported to overcome these heat transfer challenges by allowing a small amount of air into the reactor, which partially oxidises some of the pyrolysis products, thus creating thermal energy internally [222,224]. Nevertheless, biochar has diverse benefits and applications, including bioenergy; however, it is still underutilised because of technological and financial obstacles as well as a lack of collaboration between governments, industries, and academics [225,226]. The high cost of biochar production has also reduced its large-scale potential application [227]. Indeed, biochar has been largely employed in scientific studies and outdoor applications [228,229,230] due to the fact that it resonates with many SDG goals, making it an appropriate candidate for bioenergy transformation. To optimise its beneficial bioenergy impact, it is imperative to have cross-disciplinary collaboration, improve scientific innovation, and provide strong policy and regulation support. Convening participants and experts on biochar research and commercialisation, co-hosted by several organisations, highlighted the need for real-time, practical, coordinated, and large-scale research on the commercial production of biochar to solve critical economic, environmental, and agricultural issues [222,223].

5.2. Standardisation and Quality Control of Biochar Products

Due to biochar variability related to feedstock and biomass, there is a need for a sustainable and standardised biochar production protocol, focusing on not just addressing spatial soil property heterogeneity but also on bioenergy biochar requirements. Standardisation and certification of biochar are practical techniques for implementing sustainability policies [231]. These criteria help consumers make informed decisions by distinguishing products that meet sustainability criteria [232]. When standardising biochar production and application, especially in bioenergy, certain conditions and criteria are expected to be met. Köves and colleagues [231] highlighted the need for the establishment of biochar certification programs, certification scheme structuring, and approaches for proper biochar standardisation and certification. The certification scheme structure significantly impacts its performance, whether a standalone or integrated approach, like life cycle assessment (LCA), zero-waste plans, or contamination prevention strategies [233,234]. Also, to create resource-efficient biochar systems, a dual strategy of “sustainable production” and “sustainable application” is necessary [234,235]. Currently, there are three recognised and established biochar standardisation and certification programs: the British Biochar quality Mandate (BBF) [236], the European Biochar certificate (EBC) [235], and the International Biochar Initiative (IBI) [237]. The IBI is a non-profit organisation based in the United States that focuses on optimal industry procedures in biochar production, encouraging stakeholder engagement and adherence to strict environmental and ethical requirements. Its goals include creating biochar systems that are both financially feasible and environmentally friendly [237]. These established frameworks aim to guarantee the safety and quality of biochar products, as well as support the growth and commercialisation of the biochar industry [231]. One critical IB1-BS requirement in biochar production is that biochar products should have at least 10% organic carbon content. In addition, they must possess a hydrogen-to-organic carbon ratio of less than 0.7 (biochar stability indicator) and a declaration of product parameters, like total ash content, total nitrogen content, moisture content, pH value, electrical conductivity (salinity indicator), CaCO₃ concentration, and particle size range [231]. Also, EBC focuses on establishing a control mechanism based on scientific research, provides consumers with a reliable quality benchmark, validates producers’ adherence to rigorous quality criteria, disseminates current and relevant information to guide future regulatory frameworks, and addresses potential hazards associated with biochar utilisation [238]. Additionally, EBC is also concerned with electrical conductivity (EC), specific surface area, major elements, heavy metals, organic contaminants (polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated biphenyls (PCBs), ash content, elemental analysis (C, H, N, O, and S), and pH [231]. BQM sets Maximum Permissible Limits (MPLs) for toxicants and crucial biochar properties, including cation exchange and water retention capacity. It aims to create a two-tiered quality grading system (standard and high-grade) with specific standards for each [236]. According to the standards established by these frameworks and programs, biochar is divided into five groups depending on its anticipated benefits when applied to soils [239]. Although these programs fail to outline the specific application protocol for using biochar in bioenergy, it is critical to follow the requirements for feedstock, production procedures, and quality control established by these programs when applying biochar in bioenergy generation.

6. Research Gaps and Future Studies

Biochar has great prospects in many areas, ranging from energy industries to aviation industries, agriculture, environmental management, and policy development. The prospects of biochar in diverse areas are discussed in this section.

6.1. Prospect of Biochar as Electrodes

Among the many applications of biochar, its use in microbial fuel cells as electrodes stands out as a potential field of research demonstrating the increasing demand for sustainable sources of energy [24]. This is especially vital in the context of increasing concerns about climate change and renewable energy solutions. Aduba et al. [22] reported an improvement in bioelectricity and biogas production when the biochar dosage in a 4-litre anaerobic digester was increased. Although the application of biochar as electrodes in microbial fuel cells has been explored by various researchers in recent times, improved system performance and reduced contaminant proliferation [232,240,241] have been hampered by patents and novel research for industrial applications and scale-up. One benefit of using biochar as an electrode material is its re-usability without a significant drop in efficiency [240]. However, efficient and pocket-friendly electrode materials for microbial fuel cells are crucial for advancing this technology and meeting the global energy demands. These materials are essential for enhancing the performance and scalability of microbial fuel cells in order to make them a viable option for sustainable energy production on a large scale. Biochar and its syngas by-products obtained from biomass thermochemical conversion can be applied in electrochemical cells and energy generation [242,243]. Research and development in this area is ongoing to discover new materials with enhanced properties and efficiency. The goal is to create microbial fuel cells that can compete with conventional energy sources, ultimately reducing our fossil fuels. Zhao et al. [244] suggested that further investigation into biochar’s effect on MFC performance will highlight potential mechanisms to boost its production through cost-effective and efficient protocols. This will help to mitigate climate change impacts for a sustainable future through greenhouse gas emissions reduction. The successful development of these microbial fuel cells will revolutionise the energy industry. Microbial fuel cells are capable of transforming energy generation and storage. This innovation has significantly impacted global initiatives to switch to viable energy sources. The ability to harness electricity from bacteria opens up a whole new realm of possibilities for sustainable energy production. It could revolutionise the energy industry as we know it. By utilising bacteria to generate electricity, we could potentially reduce fossil fuel dependence and harmful greenhouse gases. The potential applications for this technology are vast and could lead to significant advancements in renewable energy sources.

6.2. Prospect of Biochar in Air and Water Pollution Control

Biochar has gained popularity because of its low cost, high porosity, large surface area, numerous surface functional groups, and high removal capability. Biochar has recently been explored in industrial air pollution control as a pollutant-removing biomaterial used before industrial waste is discharged into the environment [245]. Various research findings suggest that biochar eliminates gaseous pollutants found in industrial exhaust systems and micro-pollutants in water systems. According to Srinadh et al. [246], metal vapour, particularly mercury (Hg), acidic gases (SO₂, CO₂, and H₂S), fragrant organic pollutants, and oxides of nitrogen (NO_x) have all been demonstrated to be successfully eliminated by biochar. This elimination occurs mainly by precipitation, adsorption, and size exclusion mechanisms. Biochar’s potential application areas in air pollution control include gaseous emissions from remediation systems, incinerators, cremation, smelters, boilers/cookers, wastewater treatment plants, and agricultural production systems. Biochar’s commercial uses resulting from its different properties indicate potential for further improvement and scaling up in air pollution management [247]. The commercial biochar applications in air pollution control are limited mainly due to insufficient data regarding its catalytic performance and the complexities of industrial upscaling; however, its synthesis and activation methods can be manipulated to optimise surface characteristics and catalytic sites for diverse heterogeneous catalytic reactions [248]. Aside from the prospect of biochar in the abatement of air pollution, there are considerable prospects for biochar applications in GHG emission reduction [249] and contaminant removal from water systems. Petala et al. [240] reported the elimination of emerging contaminants from the aqueous phase in a hybrid electrochemical biochar-based water system.

6.3. Prospect of AI in Biochar Bioenergy

The prospect of using AI in biochar production and applications in bioenergy cannot be overlooked. The use of AI algorithms and technology can greatly enhance the efficacy and effectiveness of biochar production processes. Recently, there has been a surge in the development of AI-based methodologies across various disciplines. Because this subject area is quickly expanding, a wide variety of models are being described that employ deep learning (DL) and machine learning (ML) models. These models are often complex and fail to explain the decision-making process, hence the moniker “Black-Box” [250]. Wang et al. [251] identified machine learning and/or hybrid models (machine learning in combination with mechanism-based analysis) as one of the trending hotspots for future research, as the search for understanding machine learning Black Boxes persists. A possible solution to this complexity is the development of explainable artificial intelligence (XAI) systems. These systems aim to provide transparency into how machine learning models make decisions, ultimately increasing trust and usability in various applications. Although some biochar studies have used AI technology, most of these studies rely on data generated at laboratory-scale levels for training the models. However, more effort is required to construct AI models at industrial- or pilot-scale levels. AI can also help optimise biochar’s features and quality for precise applications. Artificial intelligence integration with biochar applications in bioenergy cannot be overlooked because it has the potential to revolutionise the industry. Recently, AI has shown promising results in predicting optimal conditions for biochar production [251,252,253], which resulted in increased sustainability and efficiency in the production of bioenergy. Coşgun et al. [252] confirmed the prospect of AI in biochar production and applications. They also inferred that AI technology can assist in accurately predicting a combination of variables that will yield biochar with desired properties that can be applied for biofuel production, resource allocation, and accurate decision-making. Additionally, AI technology can optimise the biochar production process by identifying the most efficient parameters and conditions, ultimately leading to improved biochar quality and yield, as well as optimising the application of biochar in various industries for maximum impact. Furthermore, AI technology can also help develop innovative uses for biochar beyond bioenergy production, such as agriculture, soil health improvement, and increased crop yields, and in continuous monitoring to improve biochar production processes. Ultimately, AI technology has the potential to create customised biochar blends tailored to specific needs as well as revolutionise the biochar industry and drive sustainable solutions for a variety of sectors.

6.4. Prospect of Biochar in Aviation Biofuel

Bioenergy enhancement, as well as the need for sustainable fuels in aviation and other industries, is a biochar market, which may drive technological growth and upscale [222]. Only four kinds of hydrocarbons, namely, aromatics, cycloalkanes (naphthenes), iso-alkanes, and n-alkanes, can be approved as substitutes for traditional jet fuel [254]. The necessary properties of jet fuels include high energy density, to decrease storage space; low freezing temperatures, to provide cold flow qualities at high altitudes; high flash points, to limit fire hazards; and good sealing, to prevent leaks [255]. The hydro-processing of lipids, Fischer–Tropsch (F-T) synthesis, alcohol conversion to jet fuel, biomass pyrolysis, hydrothermal liquefaction (HTL), and fatty acid methyl ester (FAME) blending are some of the approaches for converting biomass to jet fuel that have been investigated [256]. Of all the other pathways, fast pyrolysis (FP), a pyrolysis process that was already covered in this study, turned out to be the most promising choice, providing the highest fuel output (22.5%) [257,258]. The biochar generated by thermochemical conversion is not merely a by-product; it can be activated and altered to function as a catalyst for enhancing bio-oil applications [259]. Raw bio-oil is usually excessively acidic and has an elevated oxygen content [260,261], rendering it unsuitable for direct application in jet engines. Modifying biochar with metals or chemicals can facilitate reactions, such as hydrodeoxygenation (HDO), which eliminates oxygen, and catalytic cracking, which decomposes large molecules into smaller hydrocarbons [262] necessary for jet fuel. Consequently, fast pyrolysis, which transforms over 60% weight of lignocellulosic materials to bio-oil, subsequently undergoes hydro-processing for stabilisation, wherein carbonyl and carboxyl functional groups are converted into alcohols catalysed by noble metals, such as ruthenium (Ru), platinum (Pt), and palladium (Pd), with carbon and metal oxide support. Thereafter, stabilised oil undergoes cracking and hydrodeoxygenation, utilising ruthenium, nickel, or sulfide Co-Mo catalysts, with active transition metal carbides and phosphides [263]. Although the prospect of biochar research in the aviation sector is still in the early stages of expansion, progressive and consistent probing into the biocatalytic properties of biochar can offer new insights into the connection between biochar and aviation fuel. Kannapu et al. [264] reported 29% jet fuel production from MgO-modified or KOH-activated biochar catalysts (AC), unlike activated biochar alone, due to the strong carboxyl (–COOH) hydroxyl (–OH) groups it contains. Using biochar as a source of biofuel in the aviation industry is not just a prospect; it is also interesting. It significantly reduces carbon emissions and increases sustainability in the aviation sector [265,266]. This could revolutionise the way aircraft are powered, thus addressing environmental concerns. The transition to biofuels in aviation is sustainable for the industry, as it significantly reduces greenhouse gas emissions, thus combating climate change [203]. This transition could help lower the carbon footprint of air travel, making it a more eco-friendly industry. The transition to biofuels is not without its challenges, but it is an essential step in building a more sustainable future for industries by reducing the impact of aviation on the environment. It is necessary to continue investing in research and developing more affordable and widely available biofuels. This will ultimately lead to a significant decrease in greenhouse gas emissions from the aviation sector and mitigate climate change effects for more sustainable air travel. Also, it will be beneficial to the economy and environment by reducing fossil fuel reliance and promoting cleaner technologies. It will also improve air quality and public health for communities living near airports. Additionally, it will reduce the impact of noise pollution on these communities. Lastly, investing in sustainable aviation practices will lead to a more sustainable and efficient transportation system overall.

7. Conclusions

This article presents a comprehensive analysis of the pivotal role of biochar in the sustainable bioenergy transition, focusing on biomass waste valorisation to biochar. As environmental concerns and global energy demands intensify, biochar serves as an innovative solution addressing both energy generation and pollution mitigation. This review categorises pyrolysis into slow, intermediate, fast, and flash methods, as well as torrefaction as an alternative method for biochar production, each defined by specific temperature ranges due to rates of heating and residence times that impact biochar yield and quality. Its unique characteristics—such as increased surface area, porosity, nutrient retention, and chemical stability—inform its applications in sequestering carbon, improving soil fertility, purifying water, and storing energy. The energy applications of biochar are diverse and promising. Modified biochar doped with heteroatoms exhibits enhanced electrochemical properties, making it suitable for supercapacitors, batteries, and hydrogen storage. Biochar functions as a catalyst and electrode in biodiesel production and microbial fuel cells, contributing to cleaner biofuel generation and bioelectricity. Co-firing biochar with fossil fuels, notably coal, improves combustion efficiency and reduces harmful emissions, thus aiding in the transition towards cleaner energy. Biochar production faces scalability and economic challenges. Heat transfer limitations in large pyrolysis reactors, high production costs, and variability in biochar quality due to diverse feedstocks hinder widespread commercial adoption. Addressing these issues requires technological innovation, standardised production protocols, certification frameworks (such as EBC, IBI, and BQM), and interdisciplinary collaboration among academia, industry, and policymakers. Future research directions emphasise using biochar as an electrode in microbial fuel cells, enhancing air pollution control via biochar adsorption of gaseous pollutants, and integrating AI for optimizing biochar production and modifying its properties for tailored applications. AI-driven modelling can improve process efficiency, quality control, and predictive capabilities, facilitating industrial-scale production and innovative applications. In the aviation sector, biochar-derived biofuels offer a sustainable alternative to conventional jet fuels. Fast pyrolysis combined with catalytic upgrading of biochar enables the production of bio-oils with favourable fuel properties. Biochar-based catalysts enhance hydrodeoxygenation and cracking reactions, essential for producing high-quality bio-jet fuels. The adoption of biochar-derived fuels in aviation can substantially reduce carbon emissions, improve air quality near airports, and promote sustainable transportation. Thus, biochar represents a versatile, sustainable material with the capacity to transform bioenergy production, environmental management, and industrial applications. Its ability to sequester carbon, enhance soil fertility, purify water, store energy, and catalyse fuel production underscores its critical role in addressing climate change and advancing circular bio-economies. Overcoming production and scalability challenges through innovation, standardisation, and AI integration will unlock biochar’s full potential. Continued interdisciplinary research and policy support are vital to harness biochar’s benefits, fostering a sustainable, low-carbon energy future across diverse sectors, including agriculture, energy, environment, and aviation.