Market analysis and trends for products deriving from the biochemical and thermal treatment of multi-dispersed agricultural residues

Abstract

Rapid population growth, economic development, resource constraints and various geopolitical problems, as well as intensifying environmental issues are leading to a worsening energy crisis. Oil, natural gas and coal are still the main sources of primary energy. Biofuels, such as biochar, bio-oil, biogas, and hydrogen, are one of the most significant renewable and sustainable energy sources. Their use can reduce net greenhouse gas emissions as well as dependence on imported fossil fuels, in response to the above, while the production and exploitation of biomass mainly from agricultural residues can also create jobs in rural and depopulated areas. In this study, a market analysis for biochar, bio-oil, biogas, and hydrogen was conducted worldwide intending to provide detailed information for the economic viability of sustainable agricultural systems and to specify the prospects for an economically viable introduction of each of the bio-products into the energy market. The results revealed that indeed these biofuels are one of the most significant and energy sources offering sustainability by reducing gas emissions into the atmosphere but also by managing residual biomass, thus contributing to waste management. A rapid acceleration is expected in the next years followed by price reduction.

Graphical abstract

Keywords

biochar / biofuels / biogas / bio-oil / hydrogen / market analysis

1 Introduction

Rapid population growth and economic development are the major reasons for the ever-increasing demand for energy [1]. Energy is a vital commodity of our society, supporting and feeding the economy, industry, transport, and livelihoods. Over the years, various sources of energy are used, such as coal, oil, natural gas, but also more sustainable forms such as renewable energy sources (RES), e.g., wind, solar, hydroelectric, biomass, etc. But as the world’s population continues to increase, as well as to grow economically, the demand for energy has increased in turn, resulting in a strain on the ever-present energy crisis that now has far-reaching effects in the environment, the economy and society itself. More recently, the war between the countries in early 2022 abruptly ended a period of energy market stability after the pandemic-induced decline in 2020 and simultaneously fueled inflationary pressures and slowed economic growth. Market pressures had already been observed, but the war and geopolitical situations significantly exacerbated this trend. This led to a sharp rise in energy prices, especially for natural gas in the European markets, but highlighting other sources and other solutions on the other hand [2].

According to International Energy Agency (IEA) [3], global energy demand was expected to decrease by 5% in 2020, the relevant CO₂ emissions by 7%, and energy investment by 18% due, of course, to the COVID-19 pandemic. Despite the reductions observed in the consumption of fossil fuels and the relative rise of RES, fossil fuels remain the dominant source of primary energy. Because a drastic change in policies is not observed, a more rapid shift away from fossil fuel consumption cannot be predicted with certainty. But it is beyond question that such a change will bring about a stimulation of the economies, will offer new jobs, but also through a reduction of emissions will mitigate climate change [3].

The growth of global economies such as China, the United States of America, and Europe is connected with increase on energy consumption. The limitation of fossil fuels will give way to an increase in RES consumption, thus causing a drastic reduction in greenhouse gas emissions. However, it has been stated that among RES and for better climate protection and resource efficiency, the development of a sustainable bioenergy market based on bioresources is required [4]. But this newer approach to the need of increasing the use of biomass for energy production should be done not in competition with food, but through residual biomass [5].

Biofuels are produced from biological materials—most commonly from sugarcane, cereals, or residual agricultural and livestock biomass. They belong to RES because they are derived from plant organic matter which grows through photosynthesis and sequestration of CO₂ from the atmosphere. They are considered as a stable source of energy because there can be perennial cropping and biomass production against limited mineral resources. It has been shown, through extensive detailed research in two countries (Greece and China, respectively) that only biomass derived from agricultural and livestock residues has an energy potential that can cover almost all the energy needs of these countries [6]. On the other hand, they stand out among RES because they have the ability to bind CO₂ in order to grow and thus contribute to the mitigation of the greenhouse effect and subsequent climate change. In spite of this, the side emissions of CO₂ resulting from the processes of biomass production and its conversion into the final biofuels for use should be taken into account on a life cycle assessment (LCA) basis. This is a particularly important field and obviously depends on the type and yields of the crops but also on the biomass utilization methods [7].

Biofuels include biochar, bio-oil, biogas, and hydrogen and in their advanced types can meet technical and environmental constraints [8]. In addition, they are particularly useful for the transition to a zero-carbon footprint, as planned by the European Green Deal and the UN Sustainable Development Goals. The use of these advanced biofuels can reduce the consumption of fossil fuels by improving economic and environmental indicators, but also social ones due to the creation of jobs mainly in rural areas that may also face depopulation problems [9,10].

Before a new idea is materialized as a new product or new business, a market analysis is conducted as the first step to determine the consumers’ intentions. Thus, a market analysis is a documented first investigation of the market that is used to inform the project’s planning activities. More useful in the present case, of the utilization of biofuels and their further promotion, after the various technical issues of production and optimization have been successfully approached, is the knowledge of the market where they will be placed. The required market analysis gives useful information for the development of related businesses, for the comparison of technologies, and for customer requirements. In the end, it is the market analysis that plays a dominant role regarding the financial viability of the venture and the limits of the return of such an investment [11].

This study includes a detailed survey of the market analysis for biochar, bio-oil, biogas, and hydrogen, worldwide intending to provide detailed information for the economic viability of sustainable agriculture systems and to specify the prospects for an economically viable introduction of each of the bio-products into the energy market.

2 Biochar

Biochar is one of the typical pyrolysis products, along with bio-oil and gases [12]. It is a solid material rich in carbon that is produced by the thermal decomposition of organic material or biomass in the absence, or under limited supply of oxygen [13]. In theory, any material or feedstock with high carbon content can be employed to produce biochar. In the final biochar product, the feedstock’s compounds of carbon are converted into persistent and labile fractions. A wide variety of studies performed in the previous years predicted that the stable (persistent) carbon compounds are likely to remain in the soil for hundreds/or thousands of years. On the other hand, the labile carbon compounds are, most of the time, microbially degraded from weeks to years and this depends, among others, on climate [14].

2.1 Biochar production

Biochar can be produced by a broad range of feedstocks as well as from different pyrolysis technologies, and therefore it presents a high level of heterogeneity. Examples of feedstock for the production of biochar are peanut hulls, rice, sugarcane, leaves, cow manure, and sewage sludge. Biochar feedstocks can be converted into different type of products such as gases, solids, liquids, and heat via thermochemical decomposition processes [14]. After the establishment of the feedstock, different types of thermochemical processes can take place, such as slow pyrolysis, fast pyrolysis, flash pyrolysis, microwave pyrolysis, and so on [14−17]. Some thermochemical conversion processes, as well as the characteristics and reaction products of each process, are shown in Tab.1. The process type, the process condition, and the biomass type are parameters that biochar yield depends on [13].

Tab.1 Thermochemical conversion processes and products [15,16,18−20]

Thermochemical conversions	Slow pyrolysis	Fast pyrolysis	Flash pyrolysis	Micro-wave pyrolysis	Hydrothermal carbonization	Gasification	Torrefaction
Temperature/°C	300–800	400–600	750–1000	400–800	400–1000	800–1600	200–300
Heating rate/(°C·min−1)	< 10	> 200	> 1000	–	< 1	~1000	< 50
Aeration	Oxygen-free or limited	Oxygen free	Oxygen free	Oxygen-free or limited	–	Oxygen-free or limited	Oxygen-free
Residence time/min	> 60	~0.02	~0.04	–	5–240	0.2–0.4	15–60
Major product	Biochar	Bio-oil	Bio-oil	Synthesis gas (syngas)	Hydro-char	Syngas	Torrefied biomass
Biochar yield/%	30–60	10–26	~10	~27	50–79	14–25	69–80
Carbon content/%	95	74	85	–	65	–	51–55
Carbon yield/masscarbon	0.58	0.26–0.3	0.29	–	0.88	–	0.67–0.85
Byproduct	Bio-oil-combustible gas	Combustible gas	–	Biochar-combustible gas	–	Biochar-tars	None

2.2 Composition, structure, and physicochemical characteristics of biochar

Biochar consists mainly of carbon (> 60%), followed by hydrogen and oxygen at lower ratios [21]. It is characterized by high degree of carboxylate esterification and aromatization structure, high boiling point, low solubility as well as high stability. In addition, it is very resistant to any physical, chemical, and biological decomposition [22,23]. All the aforementioned, and under natural environmental conditions, allow biochar’s existence in the soil for a lot of, up to thousand, years [24].

Some advantages of biochar are the large surface area, the high porosity, and the alkaline properties [25]. Biochar is also characterized by good conductivity; high ion exchange capacity and it contains essential quantity of functional groups (such as carboxyl, carbonyl, and hydroxyl groups) [26]. The alkaline properties of biochar are related to inorganic minerals (e.g., carbonates and phosphates), and to the ash formed during the pyrolysis and the carbonization processes [25]. The physicochemical characteristics of biochar are influenced by various factors, including feedstock properties, pretreatment methods, pyrolysis temperature, heating rate, and residence time. However, feedstock properties and pyrolysis temperature play a critical role in the production of biochar [27−29]. The content of lignin, cellulose, and hemicellulose varies among different feedstocks, significantly affecting the yield of biochar. Feedstocks with higher lignin content result in higher biochar yield, whereas those with higher cellulose and hemicellulose content can produce a variety of oxygen-containing functional groups but yield less biochar [30]. The feedstock type also greatly influences the morphology, the aromatic carbon content, and the mineral content of biochar [28,31]. The decomposition of feedstocks (i.e., cellulose, hemicellulose, and lignin), the transformation of intermediates, the product distribution, and the ash content strongly correlate with pyrolysis temperature [28]. The content of surface functional groups on biochar is also affected by pyrolysis temperature, which is crucial for customizing the desired functional groups. Biochar’s specific area is in the range of 1.5–500 m²·g⁻¹, while it increases with the elevation of pyrolysis temperature [21,32]. Additionally, process pressure, residence time, and heating rate can more or less influence the yield, product distribution, and quality of pyrolysis products [33−35].

2.3 Biochar applications

Biochar presents some discreet physical and chemical properties, such as high surface area and stable carbon fraction. Therefore, a lot of research is conducted for a wide variety of biochar applications ranging from plant and animal agriculture to toxicant filtration for soil rehabilitation [14]. Lately, due to the increasing demand for food and environmental safety and reduction of greenhouse gas emissions, the applications of biochar are associated with sustainable agricultural development, soil management as well as carbon sequestration [21].

Biochar is used as fuel, but also as soil amendment to enhance plant growth and pH and for the mitigation of nitrous oxide emissions from soils. Certain amounts of inorganic mineral nutrients, such as nitrogen, phosphorus, potassium, magnesium, calcium, and so on are included in biochar. Adding biochar to soil can improve its nutrient content, especially when the biochar is prepared from livestock and poultry manure, which has a more obvious nutrient supplement effect on poor soil.

A very significant parameter for biochar’s adsorption process of heavy metals, microorganisms, and organic pollutants is its porous structure [36]. It cannot only reduce the content of pollutants but also reduce their activity and toxicity [20]. Therefore, biochar has a great application prospect in the prevention and control of water pollution [37], soil pollution [38], and air pollution [39].

Furthermore, biochar can be used for carbon sequestration to remove greenhouse gases from the atmosphere. The mitigation effect of biochar on climate change mainly comes from the inert characteristics of biochar itself, which can weaken the return process of fixed carbon produced by photosynthesis to the atmosphere, thus reducing the content of carbon elements in the atmosphere, and thus alleviating the greenhouse effect caused by CO₂ [40,41]. Biochar can also boost the growth of plants by improving soil fertility, which in turn causes more plants to photosynthesize and then consume more CO₂. In the process of preparing biochar, energy will be generated, or excess energy will be stored and exchanged as heat. As a supplement to fossil fuels, it will be used in other fields with high energy consumption, to decrease the use of fossil fuels and the CO₂ emissions. The addition of biochar to the soil also lessens the availability of nitrogen fertilizer in the soil, thus reducing NO_x emissions [42]. So, biochar plays a very essential role in the mitigation of greenhouse gas emissions [20].

Biochar also enhances animal growth performance via the improvement of digestion and nutrient metabolism in animals. For example, according to Chen et al. [21] with the application of 2% biochar in animals’ nutrition the specific growth rate of catfish rises, while the ammonia nitrogen emissions are reduced. In addition, dietary charcoal powder that includes wood vinegar, which is a biochar by-product, as a feed additive, improves the feed utilization efficiency of piglets, promotes piglet growth, and increases the average daily weight gain of ducks [21].

In addition, the raw materials for the preparation of biochar are usually the waste of agriculture, paper-making, and other industries. Biochar is prepared from these wastes through a series of methods, which can effectively reduce the discharge of solid wastes, enhance the utilization efficiency of resources, and conserve the environment [20,43].

2.4 Market analysis and future forecasts

Lately, the production and the application of biochar has been widely developed [44]. Biochar has taken center stage in agricultural and environmental sustainability discussions, owing to its diverse applications in boosting environmental restoration, mitigating environmental impact, storing energy, and addressing pollution in various areas [45]. Due to biochar’s ability to store CO₂ and thus contributing to greenhouse gas emissions mitigation, its potential in carbon sequestration has already been presented from an environmental point of view. Overall, the global demand for biochar is constantly increasing, and the biochar market has great potential.

Already from 2015, the essential agricultural use of biochar has been observed. A further acceleration of biochar application (e.g., agricultural and industrial) is awaited from 2020 onwards according to the European Biochar Certificate [46]. The agricultural applications of biochar vary from soil conditioners, composting additives, and carriers for fertilizers to manure treatment and stable bedding, silage additives, and feed additives. The industrial applications of biochar mainly concern the construction, plastics, paper, and textile industries. There are a lot of parameters that drive the European biochar market such as the increasing demand for organic farming and the stringent environmental regulations. Thus, the corresponding market awareness is increased over time. In 2022, the global biochar market size was valued at USD184.90 million while the same year the Asia Pacific biochar market size was valued at USD148.19 [47]. Asia Pacific ranks first in the global biochar market. Carbon sequestration and soil enhancement are the main drivers in biochar applications due to their significance in the region. Furthermore, the accelerated economic and agricultural development contributed significantly to the market expansion in this specific area. The main producer of char in the Asia Pacific region is China. The region faces the problems of quality degradation, soil pollution, uncontrollable crop residue disposal as well as greenhouse gas emissions. The use of agricultural waste for the production of biochar and the subsequent use of biochar as soil conditioner will help the region to overcome the aforementioned problems [47,48].

The market potential of biochar is closely related to the amount of biomass residues. The feedstocks for the production of biochar include raw materials such as crop and forestry waste, animal manure and municipal solid waste. Biochar’s potential for carbon sequestration around the world depends on parameters such as the population, land area, and agricultural production of each country. China, the United States of America, Brazil, and India present the highest biochar carbon potential, with a potential CO₂ removal of 468, 398, 303, and 225 MtCO₂e·year⁻¹, respectively [49]. China has the greatest potential for biochar carbon sequestration, with available crop waste production, forestry waste production, animal manure production, and municipal solid waste production of 75.68 × 10⁷, 78.92 × 10⁷, 11.32 × 10⁷, and 24.9 × 10⁷ t·a⁻¹, respectively. North and South America land mass includes countries that have high biochar potential (more than 25 MtCO₂e·year⁻¹). On the other hand, countries related to low biochar potential are present across North Africa, Middle East and in smaller regions of Europe and southern Africa [49].

The price of biochar varies significantly on a global scale (Tab.2 and Tab.3). Such a discrepancy can be attributed either to the great range of biochar products’ physical and chemical characteristics or to the diversity of potential uses and associated markets that range across agriculture, forestry, mining, horticulture, nursery, and industrial adsorbent sectors [50]. Thengane et al. [51] declare that the breakeven comes at 310 EUR·t⁻¹ of biochar in case only the units that are designed to produce biochar are taken into consideration. However, Keske et al. [52] proved that if biochar production is performed in mobile units the lowest possible cost of biochar is 423 EUR·t⁻¹. This large difference stems from the high sensitivity of biochar production in mobile devices to labor costs [53]. These discrepancies increase in global level taken into consideration that in East Asia the wholesale biochar price in the vicinity could be 170 EUR·t⁻¹ while in the United Kingdom could be more than 550 EUR·t⁻¹ [54−56]. The price of biochar is influenced to some extent by its raw material sources. Wood-derived biochar typically has a higher carbon content and stable pore structure, leading to a higher selling price. Compared to other sources, wood-derived biochar has higher raw material costs, and the main economic barriers it faces are the high costs of collection and transportation [57]. Biochar derived from agricultural waste, animal manure, and organic waste has a relatively simple carbonization process and lower production costs, but its carbon content and stability are not as high as wood-derived biochar. Therefore, it is mainly used for agricultural soil improvement and fertilizer, with a relatively lower selling price. Hamedani et al. [58] compared the environmental impacts and economic performance of wood-derived biochar and animal manure-derived biochar, finding that wood-derived biochar has advantages in all categories of environmental impact and cost.

Tab.2 Typical prices of biochar in different countries reported by companies in 2013 [14]

Country	Mean (pure)/(USD·kg−1)	Mean (blend)/(USD·kg−1)
Australia	3.44	–
Austria	0.68	–
Canada	3.47	–
Germany	3.40	1.85
Ghana	0.35	–
India	–	0.08
Ireland	–	3.40
Kenya	–	1.00
Philippines	0.09	0.10
South Africa	–	0.30
Spain	1.83	–
Sri Lanka	0.32	–
Switzerland	0.66	–
The United Kingdom	5.06	0.41
The United States of America	2.74	5.94

Tab.3 Typical prices of biochar in different countries

Country	Biochar price/(USD·t–1)	Feedstock	Ref.
Australia	800	City green waste and farm trash	[59,60]
Australia	> 400	Residues from eucalypt plantations	[61]
United Kingdom	5000	Woodcuttings	[62]
Philippines	< 100	–	[60]
The United States of America	> 1044	Forest residues	[63,64]
	5000	–	[62]
	448.78–1846.96	Orchard waste	[63]
	200–5000	Grape pomace	[65]
The United States of America	341–392	Tree limbs, wood log residue, and non-merchantable logs	[51]
Canada	1004.27	Forest biomass from clear-cut black spruce	[52]
Germany	1280	–	[66]
India	186–674	Rice and wheat straw	[67]
Ireland	3000	–	[66]
Switzerland	780	–	[66]
Indonesia	282–351	Palm empty fruit bunches	[68]
Spain	114–840	Olive tree pruning residues	[69−71]
Malaysia	116–197	Agricultural wastes	[71]
Vietnam	430	Rice husk	[72]
Nepal	144	Forest shrub	[73]
China	843–992	Rice straw and sugarcane bagasse	[74]
East Asia	182	Corn and wheat straw	[54]
Global	1834	Forest biomass	[50]

In recent years, the high demand for agricultural products and the need for higher soil fertility and enhanced crop yields resulted in a fluctuating growth trend of the global biochar market. According to relevant reports, in 2022 the global biochar market size worthed USD 184.90 million while in 2030 the market growth is expected to grow up to USD 450.58 million, exhibiting a compound annual growth rate (CAGR) of 11.9% during the forecast period [47]. According to market data forecast [75], in 2020 the European biochar market was worth USD 0.59 billion and it is evaluated to grow at a CAGR of 13.6% in order to reach USD 0.72 billion by 2025.

Recently, intense industrialization resulted to considerable rise of carbon emissions and thus to climate change. Globally, the governments try to mitigate climate change through the enhancement of renewable energy technologies. There are plenty of tax incentive programs that are offered in order to adopt these technologies. There are also numerous incentives available, in federal level, for adoption by nonprofit organizations, businesses, and municipal projects. Furthermore, there are government schemes, such as the Renewable Heat Incentive, that expect to lead the market due to their generation of significant interest in developing countries’ markets [47,48].

Enterprises mostly coming from Asia are responsible for high sales volumes of biochar (up to 7457 MT). In 2013 there were 175 MT and within a year have increased by 15%. Biochar prices ranged from 0.08 to 13.48 USD·kg⁻¹ (blended with other materials and unblended biochar products). Nowadays, biochar prices range from < 100 to 5000 USD·t⁻¹ (Tab.3).

North America ranks second in the market for char in the world. Next years, it is anticipated to grow due to the high demand for organic food as well as for the increasing meat consumption. Many local and large manufacturers support this expansion on national and global level. According to a survey conducted by US Biochar Initiative, domestic’s char production ranges from 35000 to 70000 t annually [47,48].

3 Bio-oil

The liquid fuel that is produced through biomass thermochemical conversion such as pyrolysis and hydrothermal liquefaction (HTL) is called bio-oil [76]. The bio-oil obtained from biomass pyrolysis is utilized as fuel and chemical product [1]. This liquid product is considered a promising candidate for the replacement of petroleum fuels [76]. The biomass composition as well as the parameters of the pyrolysis process affect the composition and the yield of bio-oil. By applying short residence times and fast cooling higher bio-oil yield can be achieved. As a pyrolysis product has a high content of water (15 wt%–30 wt%) and oxygen (35%–40%), low heating value, and has also a tendency to polymerize during storage [1].

3.1 Bio-oil production

Bio-oil can be produced through thermochemical methods, including slow pyrolysis, flash pyrolysis, vacuum pyrolysis, HTL, and gasification [77]. Among all these methods, HTL and pyrolysis are reported to be the most economically feasible and environmentally friendly biomass conversion pathways [78]. Flash pyrolysis and HTL have the highest bio-oil yields, reaching yields of 80% and 60%, respectively. In contrast, other thermochemical conversion methods, such as slow pyrolysis, vacuum pyrolysis, and gasification, have lower bio-oil yields, approximately below 45%. Therefore, flash pyrolysis and HTL are considered the main processes for bio-oil production [16,76−78]. Flash pyrolysis refers to the rapid thermal decomposition of organic compounds by heat in the absence of oxygen. The result of this process is the production of charcoal, bio-oil, and gaseous products. HTL which is also called direct liquefaction, refers to the hydrothermal upgrading/pyrolysis, depolymerization and solvolysis, which is fulfilled under high pressure and temperature—with or without the presence of a catalyst—in order to keep the water in liquid or supercritical state. HTL’s primary product is bio-oil or bio-crude, while the main byproducts are the solid residue, biochar, and water-containing soluble organic compounds. Flash pyrolysis requires dry biomass, but HTL is tolerant in high moisture and for this reason this process is ideal for biomass that comes from aquatic environments. Both processes are thermochemical technologies with organic compounds as feedstock and this confers the advantage of a relatively simple procedure with one reactor and low capital cost [76]. A detailed comparison of the two methods is presented in Tab.4.

Tab.4 Bio-oil production through flash pyrolysis and HTL [16,76,78]

Method	Treatment condition/requirement	Reaction mechanism/process description	Technique feasibility
			Pros	Cons
Flash/fast pyrolysis	Relatively high temperature (400–1000 °C); a short residence time (–2 s); atmosphere pressure; drying necessary	The light small molecules are converted to oily products through homogeneous reactions in the gas phase	High oil yield up to 80% on dry feed; lower capital cost	Poor fuel quality obtained
HTL/liquefaction/ hydrothermal pyrolysis	Lower temperature (250–400 °C); longer residence time (0.2–1.0 h); high pressure (4–22 MPa); drying unnecessary	Occurs in an aqueous medium which involves complex sequences of reactions	Commercialized already; better quality of bio-oil obtained (high heating value (HHV), low moisture content)	Relatively low oil yield (20%–60%); need high-pressure equipment, thus higher capital cost

3.2 Bio-oil characteristics

Bio-oil is a complex mixture of organic substances [1], some of which are water-soluble [79] including aromatic hydrocarbons, phenols, ketones, esters, ether, sugars, amines, alcohols, furans. In addition, bio-oil is highly oxygenated and chemically unstable [1]. Specifically, bio-oil has the following characteristics [79]. Lower heating value (LHV): 13–18 MJ·kg⁻¹; immiscible with mineral oils; pH: 2–3.7; ash content: 0.01 wt%–0.02 wt% (dry); density: 1110–1300 kg·m⁻³; moisture content: 20%–30%; kinematics viscosity: 10–80 cSt at 50 °C; at high moisture content presents tendency to separate into light and heavy phases; deterioration at high temperature storage conditions or at oxygen exposure.

All the above prove that bio-oil cannot be used in existing fuel oil applications without the modification of the system applied for its exploitation [79]. Thus, the proposed solution should have a new fuel system for the bio-oil and an auxiliary fuel system to bring up the working temperature of the bio-oil [1,79].

3.3 Bio-oil applications

Bio-oil can be stored and transported [76] and has numerous applications [1]. It can be used to produce fuels and chemical products or as a combustion fuel for energy production [12,76]. Some industrial uses of bio-oil include: as combustion fuel for heat production in boiler/burner/furnace systems; as a transportation fuel after upgrading; as combustion fuel for power generation in diesel engines/turbines; for the production of adhesives; for the production of anhydro-sugars; as a liquid smoke and wood flavor [12,76].

3.4 Market analysis and future forecasts

Under the net-zero target, countries are introducing relevant policies to accelerate renewable energy enhancement. As the most widely used renewable source, bio-liquid fuels have received unanimous encouragement internationally [80,81]. The most widely applied and developed bio-liquid fuels globally include fuel ethanol and biodiesel. In 2020, the total global production of biofuels (mainly fuel ethanol and biodiesel) reached 610 million barrels [82].

In 2006, the global biodiesel production was less than 10 million tons and in 2020 reached 42.9 million tons while the last decade the CAGR of global demand for biodiesel has reached 10%. The biodiesel global demand is expected to reach 80 million tons by 2030 [83]. The global biodiesel production was around 37.45 million tons in 2021. Europe, Indonesia, the United States of America, Brazil, Argentina, and China, are among the main production regions and countries with production volumes of 12.66, 8.4, 5.16, 5.1, 1.73, and 1.61 million tons, respectively [84]. Each country, based on its circumstances, employs different raw materials for biodiesel production. For example, the European Union (EU) primarily uses rapeseed oil, America relies mainly on soybean oil, South-East Asia focuses on palm oil while China uses waste cooking oil for biodiesel production.

Since 2015, according to statistical data from the Renewable Fuels Association of the United States of America, the global fuel ethanol production maintains slight growth. However, in 2020, under the effect of the COVID-19 pandemic, the global fuel ethanol production dropped to 26.059 billion gallons (approximately 99 billion liters). In general, the development and utilization of ethanol presents considerable asymmetry owed to varied factors such as the natural conditions, the technological investments, and the financial support. The largest ethanol producers are the United States of America and Brazil (> 80% of global ethanol production). The EU, China, India, and Canada are also major regions and countries that promote ethanol production [85].

Bio-liquid fuels, the earliest form of renewable energy to achieve large-scale industrialization and the most widely used, gradually leveled off after more than a decade of development fervor in the early 21st century. Recently, the growth rate and investment enthusiasm of bio-liquid fuels have lagged behind that of wind and solar power. Various organizations such as the Biofuture Platform, International Renewable Energy Agency, IEA, etc., have conducted investigations into factors influencing the development of biofuels in different scopes. It is widely believed that policy uncertainty, unstable raw material supply, and economic non-competitiveness are significant influencing factors [86].

The high cost of raw materials and transportation are significant reasons for the weak competitiveness of biofuels. For example, in the production of fuel ethanol, the conversion rate of corn and wheat is about 30% while the cost of raw materials accounts for about 80% of the total cost. Although straw is cheap, its conversion rate is less than 15%, and the transportation cost exceeds 35% of the total production cost [86]. In comparison with fossil fuels, there is a widespread problem of low economic competitiveness of biofuels worldwide. For example, in 2019, the production cost of biodiesel in major promotion countries and regions has exceeded 600 USD·t⁻¹ [87], while the export price of fossil diesel in China was 574.4 USD·t⁻¹ that year [78]. Zhu [88] analyzed the cost of corn, cassava, and straw fuel ethanol separately. Under the pricing model of fuel ethanol in China, cassava and corn fuel ethanol production enterprises can profit, while cellulose fuel ethanol is still not competitive compared to fossil fuels. Concerning biodiesel production, the raw material for food waste oil after deacidification and refining is about 5000 CNY·t⁻¹, with a conversion rate of 90%. The raw material cost in biodiesel production is more than 70% of the total cost [86]. At present, the cost of cellulose fuel ethanol is relatively high. The development of cellulose fuel ethanol requires more technological research and development efforts and needs to deal with bottleneck problems that concern raw material collection and storage, transportation as well as raw material pretreatment [88]. Tab.5 lists the plant size, feedstock price, and the estimated bio-oil cost until 2006 according to the National Renewable Laboratory of the United States of America. Department of Energy, while Tab.6 presents a summary of the techno-economic evaluation of various pyrolysis-based bio-refineries in more recent years. According to Tab.5, the values of the crude bio-oil until 2006 ranged from 0.09 to 0.54 USD·kg⁻¹ [89], while more recently the minimum fuel selling price, according to Tab.6, ranges from 1.04 to 5.10 USD·gal⁻¹.

Tab.5 Cost and selling price of bio-oil production [89]

Plant size/(t·d−1)	Feed cost/(USD·dry t−1)	Feed cost/(USD·GJ−1)a)	Bio-oil cost/(USD·kg−1)	Bio-oil cost/(USD·gal−1)a)	Bio-oil cost/(USD·GJ−1)a)
2.4	22	1.10	0.38	1.73	21.20
24	22	1.10	0.18	0.82	10.10
100	36	1.80	0.26	1.21	14.50
200	36	1.80	0.21	0.99	11.70
400	36	1.80	0.19	0.89	10.60
1000	46.50	2.33	0.09	0.41	5.00
1000	44	2.20	0.11	0.50	6.10
250	44	2.20	0.11	0.50	6.10
1000	20–42.50	1.00–2.13	0.13–0.54	0.59–2.46	7.30–30.00
250	11	0.55	0.10	0.46	5.60
1000	44	2.20	0.09	0.41	5.00

a) Assumes 20 MJ·kg−1 HHV for wood, 17.9 MJ·kg−1 HHV of bio-oil, and 4.55 kg·gal−1 density of bio-oil.

Tab.6 Brief summary of the techno-economic evaluation of various pyrolysis-based bio-refineries [90,91]

Feedstock	Plant capacity/(MT·d−1)	Conversion technology	Main product	Minimum fuel selling price/(USD·gal−1)
Wood	2000	Thermal deoxygenation (biofuel)	Biofuel	2.78
Forest residues	2549	Fluid catalytic cracking (FCC) of raw bio-oil (5%) with vacuum gas oil (VGO)	Upgraded oil	2.83 (without co-products), 2.68 (with co-products)
Forest residues	2549	Catalytic pyrolysis followed by its (10%) FCC cracking with VGO	Upgraded oil	5.10 (without co-products), 4.73 (with co-products)
Hybrid poplar	1000	Microwave assisted catalytic pyrolysis followed with catalytic hydrogenation	Jet fuel ranged cycloalkanes	3.78
Guayule bagasse	2000	Pyrolysis followed by hydrodeoxygenation	Gasoline ranked fuel	3.63
Red oak	2000	Hydroprocessing (transportation fuels)	Transportation fuel (gasoline and fuel ranged)	3.09 (transportation fuels)
Red oak	2000	Hydroprocessing (fuel), ketonization and alkylation (mixed alcohols)	Fuel (gasoline and diesel ranged)	2.85 (without co-product) 2.75 (with co-product)
Woody biomass	2000	Deoxygenation through hydroprocessing	Hydrocarbon ranked biofuels	3.69
Pine wood	1000	Torrefaction followed by pyrolysis	Bio-oil	1.04
Rice husk	1000	Fluidized bed fast pyrolysis	Bio-oil	2.08

4 Biogas

Biogas is a blend of gases (primarily CH₄ and CO₂) that is released when an organic waste is decomposed in the absence of oxygen. A few decades ago, biogas was mainly used as a fuel source by low-income individuals. However, in contemporary times, it has evolved into a major choice in the field of international energy planning [92].

4.1 Biogas production

Anaerobic digestion (AD) is a method that harnesses biomass to produce sustainable and cost-effective biofuels [93]. It is regarded as an environmentally conscious and economically viable solution for addressing the diverse biodegradable waste resulting from human activities [94]. The occurrence of AD is relatively common in nature. Biogas is produced in various locations, including sediments at the bottom of freshwater and marine environments, rice fields, wetlands, marshes, and the rumen of certain animals, among others. Along the way, organic matter (such as agricultural and forestry waste, livestock manure, urban organic waste) is decomposed by microbial communities in anaerobic environments, which not only effectively reduces the volume of waste, but also converts it into valuable energy [93,95]. The products of this process are biogas and digestate, where biogas can be utilized as fuel, and digestate is commonly used as a soil additive [94,95].

The AD process can be divided into four primary phases: hydrolysis, acidification, acetogenesis, and methanogenesis [96,97]. Initially, complex organic compounds are broken down by ectoenzymes into small molecules such as monosaccharides, fatty acids, and amino acids. Subsequently, in the acidification stage, small organic molecules enter microbial cells and are transformed into volatile fatty acids and hydrogen, among other substances. Following this, in the acetogenesis stage, the products from the previous stage are further converted into acetic acid, H₂, CO₂, and new cellular material. Finally, in the methanogenesis stage, the products from the preceding stages (i.e., acetic acid, H₂, CO₂) are transformed by methanogenic bacteria into CH₄ and CO₂ [93,98].

The configuration of different parameters can have varying effects on the efficiency of the AD process. These parameters include system configuration (one-stage or two-stage systems), pH, humidity (wet or solid AD, depending on the solids content), temperature (typically mesophilic or thermophilic range), reactor type (conventional or high-rate systems), organic loading rate, pretreatment of the substrates, microbial communities of the inoculum, hydraulic retention time, and physicochemical characteristics of the substrates and the co-digestion [96,99].

4.2 Biogas composition and characteristics

The composition of biogas is predominantly CH₄, accounting for 50%–75%, CO₂ making up 19%–34%, and minor quantities of H₂, NH₃, and H₂S [93,100]. The calorific value of biogas depends on the CH₄ content, so the higher the CH₄ content in biogas, the more energy it has. The sulfur content in biogas potentially needs to be removed in order to reduce the harm to equipment and the environment [101]. Theoretically, each cubic meter of biogas can be converted into approximately 6 kWh of energy, but due to conversion losses in biogas plants, only 2.2 kWh of energy is generated [100,101].

4.3 Biogas applications

Biogas has many uses (Fig.1), depending on the nature of the source and the local demand for a particular form of energy. In general, with biogas, heat through direct combustion can be produced, generating electricity via fuel cells or micro-turbines, and concurrently producing electricity and heat through cogeneration. It can also be used as a transport fuel, after the purification process, i.e., the removal of particles, H₂S, NH₃, H₂O, and its upgrade, i.e., the removal of CO₂ and the addition of propane.

Fig.1 The uses of biogas (CHP: cogeneration, combined heat and power).

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Generally, biogas serves as the fuel for internal combustion engines, propelling generators to produce electricity. Biogas power generation emerges as a promising approach for renewable energy utilization when the electricity and heat generated in the course of biogas generation can be efficiently put to use.

After purification and upgrading, biogas can be employed as a substitute for natural gas and can be utilized in diverse fields. Several biogas upgrading technologies have been developed globally. The use of biogas for cogeneration, replacing the use of traditional fossil fuels, has been widely practiced [102].

4.4 Market analysis and future forecasts

Biogas production diverges in developing and developed nations. In developing countries, biogas production primarily centers around small, household-scale digesters designed to supply fuel for cooking or lighting, contrasting with developed nations that concentrate on larger-scale, farm-based, and commercial biogas plants [103]. The United States of America and Europe lead in biogas production, and other regions are progressively embracing this technology. Europe is an advanced region in the global biogas industry, with over 40 years of industrialized biogas development experience. The technology and business models in this sector are mature and well-established.

In 2020, the biogas production in Europe amounted to 20 billion m³, equivalent to 0.71 EJ of energy and this production constituted over half of the global biogas output, with Asia following as the second-largest contributor, holding a 32% share. Germany, China, and the United States of America rank as the world’s top three biogas-producing nations [104].

In the European biogas sector, there are thousands of plants, with countries like Germany, Austria, Denmark, and Sweden leading the way in technology and boasting the highest count of modern biogas facilities. Taking Greece as an example, in the 80s, a few projects for the energy utilization of biogas were implemented in the country with raw material mainly livestock waste and waste from food processing industries, such as olive mill waste. Some of them were demonstration projects, which after the initial enthusiasm and the provision of scientific support stopped their operation. The main reasons for that were the lack of information, appropriate infrastructure, state of interest, and financial incentives [105].

Nowadays, the exploitation of biogas is a well-known technology in the cases of landfills and wastewater treatment plants. In Greece, an important effort is made for the production of biogas in order to be established as an alternative source of energy. Typically, in Greece, AD is employed as a waste management approach, without being linked with the production of biogas and energy. The approach that is followed is the disposal of the waste after some treatment rather than adopting a known and integrated technology such as AD for the parallel production of biogas and the use of the residue as a soil fertilizer [105].

In 2007, 15 biogas plants operated in Greece and in most cases the exploitation of biogas covered the thermal needs of the plants. A decade later, Greece boasts the operation of 37 biogas plants, with the majority (30) dedicated to the treatment of landfill or sewage gas. In 2018, the Hellenic Association of Biogas Producers was established. Up to date, there are 30 full members, with biogas plants of sizes from 100 to 5.250 kWh. Approximately 33.5 MWe represents the combined installed power capacity of the more active electricity-producing member plants (information given by the Greek Center for RES). Tab.7 shows the biogas production potential from livestock farms and industrial activities.

Tab.7 Biogas production potential from main livestock farms and and industrial activitiesa)

Primary source	Waste/(t·year−1)	Power/MWe
Cow farming	14450800	278
Pig farming	2268220	37
Slaughterhouses	204932	28
Cheese & milk industrial units	425647	7.21
Summary	17439599	350.21

a) Source: Greek Center for RES

The primary means of biogas production in China encompass household biogas digesters and biogas projects [106]. In 1999, there were only 7.8 million household biogas digesters in the country, but by 2022, that number had grown to 15.18 million (Fig.2). Similarly, in 1999, China had around 0.01 million projects, but by 2022, the country had established about 0.0751 million biogas projects (Fig.3) [107]. The rapid growth of China’s biogas industry is primarily attributed to substantial government support, including subsidies and investments. From 2000 to 2016, the Chinese government invested CNY 42 billion in biogas project construction, and from 2016 to 2020, this investment increased to CNY 50 billion [106].

Fig.2 Change trend of household biogas numbers.

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Fig.3 Change trend of biogas engineering numbers.

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China launched nationwide pilot projects for the upgrading and transformation of rural biogas from 2015 to 2017. As time progresses, local governments and enterprises are progressively directing their attention toward large-scale biogas projects and bio-natural gas pilot initiatives. Although the number of biogas projects has decreased overall, the total tank capacity and total gas production have increased, indicating that the monomer scale of biogas projects is gradually expanding, and biogas projects are gradually developing in the direction of scale and large-scale (Tab.8). However, the biogas project has a long industrial chain, and the commercial profitability operation model is not yet mature. Small and medium-sized biogas projects are always struggling, showing the same shrinking trend as household biogas [108].

Tab.8 Development of various types of biogas projects [107,108]

Year	Project quantity/piece				Total pool capacity/(1 × 104 m3)
	Large	Midsize	Small-sized	Total
2011	4998	9016	67027	81041	1192.74
2012	5597	9767	76588	91952	1433.43
2013	6160	10285	83512	99957	1573.04
2014	6713	10087	86236	103036	1690.81
2015	7077	10543	93355	110975	1892.52
2016	7523	10734	95183	113440	2013.41
2017	7875	10516	91585	109976	2068.19
2018	7966	10332	89761	108059	2197.81
2019	–	–	–	102650	–
2020	–	–	–	93481	–
2021	–	–	–	93427	–
2022	–	–	–	75115	–

The cost of biogas production varies significantly. It depends on parameters such as the substrate used and the possibility of distributing the resulting digestate in the surrounding agricultural area [109]. The cost of gene rating biogas in Sweden is 0.17–0.50 EUR·m⁻³. The pur chase price of the upgraded biogas and natural gas amounts to 0.70–0.90 EUR·m⁻³. Upgrading biogas in units of 200–300 m³·h⁻¹ incurs a cost of 0.01–0.015 EUR·kWh⁻¹ [105]. Lawson et al. [110] performed a techno-economic assessment on biogas plant in Denmark with an annual processing capacity of 100000 t, employing ex-situ bio-methanation as the upgrading method. Their findings indicate that biogas plants are economically viable only when the minimum biomethane sales exceed 0.66 EUR·Nm⁻³. Compared to alternative RES, such as wind and solar photovoltaics, biogas stands out for its ability to facilitate flexible power generation during periods of low wind and solar intensity [109].

Over the past few years, the number of biogas plants in Europe has steadily risen in tandem with the development of the biogas industry. Particularly noteworthy is the rapid increase in the number of plants engaged in bio-methanation, which demonstrated an addition of 95 in 2019. According to statistics from the European Biogas Association, the current total production of biogas and biomethane in Europe stands at 21 billion m³. Forecasts suggest a potential doubling by 2030, ranging from 35 to 45 billion m³. Looking to 2050, the production of biogas could increase at least 5-fold, reaching 167 billion m³, representing approximately 40% of the EU’s natural gas consumption in 2021. Under the assumption of a decline in natural gas demand, by 2050, biomethane is expected to have the capacity to fulfill up to 61% of the total natural gas demand [111].

Against the backdrop of achieving carbon peak, carbon neutrality and the rural revitalization strategy goals, China’s biogas industry has shown tremendous market potential. According to the 2021 annual report from the China Biogas Association, in the past 20 years, the central government has invested over CNY 48 billion in central budget funds and national debt funds to promote the development of the rural biogas industry [112]. At the same time, the rural biogas industry in China has successfully transitioned from household biogas development to various types of biogas projects and large-scale bio-natural gas projects. It has established and substantially improved the standardized system for biogas, and breakthroughs have been made in biogas technology. A valuable, scalable, and replicable set of mature technological models has been accumulated. It is predicted that by 2030, the biogas industry in China could have a production potential of approximately 169 billion m³, achieving a greenhouse gas emission reduction of 300 million tons of CO₂ equivalent. By 2060, the potential biogas production could reach 371 billion m³, leading to a significant reduction of 660 million tons of CO₂ equivalent in greenhouse gas emissions. This figure corresponds to replacing 68% of the national natural gas consumption in 2020 or more than 1.5 times the natural gas import volume in 2020 [112].

5 Hydrogen

Hydrogen, an element widely present in nature, can be discovered in various substances, including hydrogen sulfide, freshwater, biomass, and fossil fuels, among others. However, in nature, it typically forms chemical bonds with oxygen in water or carbon in hydrocarbons, making it difficult to find hydrogen as an isolated element. Hydrogen, forming chemical bonds with both water and organic compounds, constitutes more than 70% of the Earth’s surface [113].

Hydrogen energy finds widespread applications in various fields, including transportation, petroleum refining, NH₃ production, metal refining, and residential living. The potential of hydrogen gas as an energy carrier is significant for integrating various infrastructures, aiming to improve economic efficiency, reliability, and flexibility. The many uses of hydrogen will help reduce carbon emissions, and it is expected to play an important role in the future energy economy system [114]. Hydrogen energy is characterized by abundant resources, high quality, and energy density. Additionally, as hydrogen is free of carbon elements, it is regarded as a clean fuel and a prospective substitute for fossil fuels [115]. More and more countries realize the importance of hydrogen energy in the future energy landscape, leading to its elevation as a national strategic priority in many nations [116].

5.1 Hydrogen production

Hydrogen has a wide range of sources and diverse preparation technologies. Not only it can be produced through the reforming, thermal cracking, or microbial fermentation of fossil fuels such as coal, oil, and natural gas, but it can also come from industrial by-products such as coking, chlor-alkali, steel, metallurgy, and can also be prepared using electrolytic water. Large central factories, medium-sized semi central factories, or compact distributed equipment located very close to the point of use, such as gas stations or fixed power plants, can all be used for hydrogen production [117].

The production cost and process carbon emissions of hydrogen may vary greatly depending on the production technology and type of energy used. Hydrogen production technology is usually classified based on different colors, and different colors of hydrogen represent the cleanliness of the hydrogen production process [118−120]. (1) “Grey” hydrogen: hydrogen produced through the combustion of fossil fuels such as oil, natural gas, and coal, and this process results in CO₂ emissions. (2) “Blue” hydrogen: by combining advanced technologies such as carbon capture and storage with the production of hydrogen from fossil fuels, the greenhouse gas emissions from this process can be reduced. (3) “Turquoise” hydrogen: fossil fuel pyrolysis is utilized for hydrogen production, generating solid carbon as a byproduct. (4) “Green” hydrogen: the production of hydrogen uses renewable energy; It is produced without CO₂ emissions. (5) “Purple” hydrogen: hydrogen is generated from water using nuclear energy as the source. (6) “Yellow” hydrogen: hydrogen is obtained through the electrolysis of water using electricity from the grid.

5.2 Hydrogen main characteristics

One of hydrogen’s most prominent features is its minimal density, measuring 0.089886 kg·m⁻³ (at 0 °C and 101.325 kPa), only 1/14th that of air. It is the gas with the smallest known density in the world. Another essential characteristic of hydrogen is its diffusion properties, as it possesses the highest diffusion capacity, thermal conductivity, and diffusion rate among all gases at room temperature. Under normal conditions, hydrogen is a colorless, odorless gas that is highly flammable and poorly soluble in water. Although hydrogen has low solubility in water and fats, it has higher solubility in metals such as nickel, palladium, and molybdenum.

Hydrogen fuel, as a clean and pollution-free energy source, possesses numerous advantages when compared to traditional fossil fuels. First, it displays excellent combustion properties, creating a combustible mixture with air in the central combustion chamber characterized by a high ignition point, a broad flammable range, and a rapid combustion rate. Secondly, at an equivalent mass, hydrogen fuel possesses a high calorific value, releasing more energy than conventional fossil fuels. Furthermore, the final product of hydrogen fuel combustion in internal combustion engines is water, effectively minimizing the greenhouse effect. Lastly, hydrogen fuel results in lower noise generation and higher efficiency during engine combustion and work processes [121].

On the other hand, hydrogen also presents some disadvantages. First, the production cost of hydrogen is higher than the one of traditional fossil energy, making it challenging to produce hydrogen in large quantities at a low cost. In addition, transportation and storage of hydrogen can be dangerous because that hydrogen is combustible, and high-temperature collision or careless leakage of hydrogen during transportation and storage may lead to fire and explosion. Furthermore, there are potential safety hazards in use, including that hydrogen must be purified before ignition, hydrogen burns rapidly, which may lead to energy waste in the process of use [122].

5.3 Hydrogen applications

Hydrogen has a wide range of applications, functioning as fuel, raw material, energy carrier, and a means of energy storage. It has many potential applications in transport, building heat, industry and energy. In the field of transportation, hydrogen presents a highly favorable prospect. Research shows that hydrogen energy and fuel cell technology are very suitable for decarbonization in heavy or long-distance transportation applications. More than a third of the world’s energy demand comes from the heating and powering of buildings, representing a quarter of the global carbon emissions (8.67 billion tons of CO₂). It turns out that the industry is difficult to decarbonize, especially as regards heating, because only a few low-carbon alternatives can compete with natural gas, the most common heating fuel. Among these limited options, hydrogen solution is one of the most cost-effective and flexible ways to promote the energy transformation of the industry [123].

Hydrogen can also provide energy for industrial or off-grid applications. Hydrogen energy and electric energy are both secondary energy sources, so it is easier to couple electric energy, heat energy, fuel, and other energy sources, and establish an interconnected modern energy network with electric energy. More importantly, hydrogen energy can achieve discontinuous production and large-scale storage, which will significantly increase the flexibility of the power network [123]. In the future, hydrogen could potentially serve as a solution for decarbonizing aviation and shipping by producing liquid synthetic kerosene or other synthetic fuels. Fuel cells are considered to be the optimal energy conversion technology for the maximization of efficiency in power generation, using hydrogen as fuel. Nowadays, fuel cells achieve efficiencies of 40%–55% in terms of LHV regardless of their size.

5.4 Market analysis and future forecasts

Due to the extensive application scenarios and substantial value in the hydrogen energy industry chain, the development and utilization of hydrogen energy have become not only a crucial path for multiple economies to achieve energy transition but also a significant position in international competition. At present, hydrogen energy strategies have been implemented by 41 governments, representing close to 80% of the global CO₂ emissions associated with energy [124]. Presently, the United States of America, Europe, and China stand as the three major nations and regions globally leading in hydrogen production. Based on the United States Department of Energy, National Clean Hydrogen Energy Strategy and Roadmap, it is estimated that the domestic hydrogen demand in the United States of America will reach 10 million MT annually by 2030, will increase to 20 million MT by 2040, and further climb to 50 million MT by 2050 [125]. Additionally, the cost of hydrogen production is expected to decrease to 2 and 1 USD·kg⁻¹ by 2029 and 2035, respectively. The average cost of hydrogen is about 2.30 USD·kg⁻¹ [126].

Europe’s commitment to achieving net-zero emissions by 2050, combined with the recent announcement of a 55% decarbonization target by 2030, has driven a strong interest in hydrogen. Since 2015, the production cost of green hydrogen has already decreased by 40%, and it is anticipated to further reduce by another 40% by 2025. Factors contributing to this significant reduction include the decrease in the cost of renewable energy and the reduced cost of electrolysis equipment. It is projected that by 2030, the cost of green hydrogen could potentially drop to less than 2 USD·kg⁻¹, enabling it to compete with blue hydrogen [127].

In recent years, the EU has progressively established a clean energy strategic plan centered around hydrogen. Particularly, the war between countries in early 2022 has accelerated the pace of energy transition in EU countries, with security becoming a primary consideration for policymakers. The significance of renewable energy and hydrogen in guaranteeing energy security has become increasingly apparent. The Hydrogen Strategy for Climate-Neutral Europe outlines the goal of installing 6 gigawatts of electrolyzers within the EU by 2024, with the target of achieving an annual production of 1 million tons of green hydrogen. The aspiration for 2030 is to accomplish an annual production of 10 million tons of green hydrogen [128]. The recent “REpowerEU” plan from the EU reaffirms the 2030 goal of producing 10 million tons of green hydrogen [128]. Following the footsteps of Europe and the United States of America, the 2050 Carbon Neutrality and Green Growth Strategy from Japan aims to achieve domestic hydrogen production of 3 million tons per year by 2030 and by 2050 it plans to achieve a domestic annual hydrogen production of 20 million tons. The Promotion of the Hydrogen Economy and Hydrogen Safety Management Act from South Korea sets a goal to replace imported crude oil with imported hydrogen by 2050 [129].

Based on statistical data, China became the world’s leading producer of hydrogen in 2021, with a production of about 33 million tons, of which approximately 12 million tons adhered to industrial hydrogen quality standards. Additionally, China holds the top position globally in renewable energy installed capacity, indicating significant potential in providing clean and low-carbon hydrogen. Presently, while the hydrogen energy industry in China is in its early stages of development, it shows a positive growth trend. The Medium and Long-Term Plan for the Development of the Hydrogen Industry (2021–2035) issued by China in 2022 has outlined the development direction for the hydrogen industry. It clarifies the development direction of the hydrogen energy industry, and for the first time explicitly points out that hydrogen energy is an important component of the future national energy system. It sets development goals for each stage of the hydrogen energy industry, such as the achievement of an annual production of 100000 to 200000 t of renewable energy from hydrogen by 2025, the reduction of CO₂ emissions by 1 to 2 million tons annually and the establishment of a more complete hydrogen energy industry technology innovation system. In addition, the targets address the production of more clean energy and of a more efficient supply system by 2030, providing strong support for achieving carbon peak goals by 2035. The goal is to establish a diversified hydrogen energy application ecosystem by significantly increasing the share of renewable energy derived from hydrogen in the final energy consumption [130].

Currently, the consumption of hydrogen is mainly derived from fossil fuels. However, with the growing awareness of carbon emission reduction in various countries, the competitiveness of low-carbon hydrogen has been continuously enhanced, leading to a sustained decline in the demand for “grey hydrogen” [131]. The high production cost of low-carbon hydrogen is the main factor hindering its development, and reducing this cost will significantly promote the development of the hydrogen industry. In the process of “green hydrogen” production, due to the industrialization of electrolyzer manufacturing, the improvement of electrolysis efficiency, and the reduction of renewable energy electricity cost, cost that has significantly decreased over the past decade, from 10 to 15 per kg in 2010 to 4–6 per kg in 2020, and it is expected to further drop to 2.5 USD·kg⁻¹ by 2030 [123,132]. The improvement of electrolysis efficiency comes from advancements in production technology, such as the adoption of proton exchange membrane or alkaline water electrolysis technology. In the future, continued investment in technology, improved electrolysis efficiency, and increased deployment of electrolyzers will further reduce the economic barriers to the development of the hydrogen industry [123]. The development and construction of hydrogen storage, transportation, and related infrastructure are crucial links connecting upstream hydrogen production and terminal hydrogen utilization. Expanding the scale of infrastructure, improving transportation capacity, and increasing the number of hydrogen refueling stations are the keys to reducing costs [133,134].

The Hydrogen Council states that the accelerated deployment of hydrogen energy projects will expedite cost reductions in the hydrogen industry [123]. Wang et al. [135] predicted that the cost of green hydrogen in China is expected to decline to 20–30 CNY·kg⁻¹ by 2030. Moving toward 2060, it is anticipated that the cost of green hydrogen will further decrease to 12–18 CNY·kg⁻¹. Considering the cost of carbon emissions, green hydrogen is expected to become the most cost-effective method of hydrogen production. According to the Global Hydrogen Review 2023 report [124], the global consumption of hydrogen reached 95 million MT in 2022, marking an almost 3% rise compared to 2021. Governments have announced many support plans, but most have not yet been implemented or funded, which has hindered investment decisions for planned projects. Governments need to accelerate the implementation of these plans and provide funding to expand the scale of hydrogen production to achieve affordable hydrogen production.

6 Discussion

The increasing energy demand is primarily linked to both rapid population and economic growth. Nowadays, fossil fuels and specifically oil, gas, and coal are still the main sources of primary energy with oil to share the largest demand. The increasing use of these natural fuels will result in more CO₂ emissions in the atmosphere, contributing to climate change. For this reason, there is a big quest for the use of RES and the replacement of fossil fuels. Due to the high energy consumption especially from the biggest world economies such as China, the United States of America, and Europe which is combined with the recent higher electrical energy prices the countries should find alternative solutions to cover their energy needs. Biofuels, such as biochar, bio-oil, biogas, and hydrogen are very promising energy sources, and Fig.4 presents an example of biofuel production methods. The recent selling price of these biofuels is presented in Tab.9, which has been derived from the elements and data presented in the previous sections of this article where the bibliographic references are given. Here is a summary of that data.

Fig.4 Conversion of biodegradable and non-biodegradable agricultural residues into useful bio-products.

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Tab.9 Selling price of biochar, bio-oil, biogas, and hydrogen

Product	Selling price
Biochar	< 0.093–4.66 (EUR·kg−1)
Bio-oil	0.25–1.25 (EUR·L−1)
Biogas (upgraded)	0.70–0.90 (EUR·m−3)
Hydrogen	~2.10 (EUR·kg−1)

According to the findings of this study, biochar, which can be made from any material or feedstock with a high carbon concentration, can be turned into a variety of products that include gases, solids, liquids and heat via thermochemical decomposition processes. Usually, it is derived by waste coming from agriculture, papermaking and other industries. Biochar is prepared from these wastes through a series of methods, which can effectively reduce the discharge of solid wastes, improve the utilization efficiency, and protect the environment. In addition, it can be used as a soil conditioner to enhance plant growth, pH and to reduce nitrous oxide emissions from soils. Furthermore, biochar can be used for carbon sequestration in order to remove greenhouse gases from the atmosphere.

Biochar products are indeed sold at a wide range of prices (< 0.093 to 4.66 EUR·kg⁻¹). This is explained by the large variations in the values of their basic physicochemical characteristics, which determine the applications that biochar finds on the market, from simple to quite advanced, in agriculture, forestry and the adsorbent industry. If only units were designed to produce biochar the range would be from 170 to 550 EUR·t⁻¹. The global biochar market size was valued at EUR 169.96 million in 2022 and it is projected to grow from EUR 187.04 million in 2023 to EUR 411.72 million by 2030.

Bio-oil, which is the liquid product from the pyrolysis of biomass, is a renewable that can be stored and transported and has many applications. This product, because it usually has a relatively low calorific value and particular physicochemical characteristics, such as high viscosity, high water and ash content, volatility, and corrosiveness, can be upgraded through either flash pyrolysis or HTL, producing high-quality biofuel. The values of the crude bio-oil in the previous years ranged from 0.082 to 0.49 EUR·L⁻¹, while recently the prices range from 0.25 to 1.25 EUR·L⁻¹.

Globally, fuel ethanol and biodiesel are the most widely applied and developed bio-liquid fuels. In the production of fuel ethanol, the conversion rate of corn and wheat is about 30%, and the cost of raw materials contributes for about 80% of the total cost. Currently, the cost of cellulose fuel ethanol is relatively high. The production cost of biodiesel in major promotion countries or regions has exceeded 548.07 EUR·t⁻¹. Global biodiesel production has increased from about 10 million tons in 2006 to 42.9 million tons in 2020 and is projected to reach the level of 80 million tons by 2030.

Biogas is produced during AD of organic materials. Examples of these sources are sewage sludge, animal manure, organic fractions of household and industry waste, and energy crops. It is gas fuel with similar characteristics to natural gas. European countries such as Austria, Germany, Sweden and Denmark are among the technical leaders, presenting the largest number of modern biogas plants.

Since parameters such as the substrate used and the possibility of distributing the resulting digestate in the surrounding agricultural area can affect the cost of biogas production, the latter varies significantly. The purchase price of the upgraded biogas and natural gas amounts to 0.70–0.90 EUR·m⁻³ while the cost of upgrading biogas in units of approximately 250 m³·h⁻¹ is 0.012 EUR·kWh⁻¹ of upgraded biogas.

Hydrogen is an abundant element and can be found chemically combined in many substances in nature such as water and biomass. It is chemically linked to oxygen in water and to carbon in hydrocarbons, thus it is difficult to find it in nature as a separate element. Hydrogen energy is a clean (does not emit CO₂ and during its use, almost zero air pollutants are released), efficient and safe energy, which can be obtained through various methods and from different sources (fossil fuels, nuclear energy, RES, biomass, etc.) that grade its sustainability. The main hydrogen production technologies include coal gasification, reforming of natural gas with steam, biomass gasification, electrolysis using RES and nuclear processes. The source of hydrogen and its production technology determine not only the CO₂ emissions but also its production cost and the final sale price.

Nowadays, the United States of America, Europe, and China are the major hydrogen-producing countries and regions. The average cost of hydrogen is about 2.10 EUR·kg⁻¹, and the production cost is expected to decrease to 1.83 and 0.91 EUR·kg⁻¹ by 2029 and 2035, respectively. Concerning the demand for hydrogen, it is expected to grow from 327 TWh today to a level of 2500 TWh a year by 2050.

The environmental sustainability of services or processes is quantified through LCA. LCA is based on ISO 14040/44. The principles of ISO 14040/44 take into consideration all the emissions and the resource consumptions throughout the whole life of a product or a process [136]. In research conducted by Aravani [137], LCAs were applied to assess the environmental sustainability of a thermochemical conversion method and of a biochemical conversion method for managing common agricultural residues in Greece namely olive tree prunings and sheep and goat manure. The sustainable management scenarios were compared with the main management practices implemented in the country. More specifically, the LCA for olive tree prunings concerned their open-field burning and their use for energy production through gasification with main products of the latter the biochar and the synthesis gas (syngas). The alternative scenarios for sheep and goat manure were their composting to be used as fertilizer and their use for energy production through AD. The main products of the last-mentioned process were biogas and digestate. Gasification and AD demonstrated positive environmental impacts for the management of the aforementioned residues, and thus indicating that more environmentally friendly technologies should be used for the management of the residual biomass.

7 Conclusions

Under the net-zero target, countries are introducing relevant policies to accelerate the development of renewable energy. Biofuels, such as biochar, bio-oil, biogas, and hydrogen could become a viable alternative solution to the use of fossil fuels under the increasingly strengthened environmental restrictions. Before a new idea is materialized as a new product/or new business, a market analysis is conducted as the first step to determine the consumers’ intentions. In the present work, the market analysis of biochar, bio-oil, biogas, and hydrogen worldwide was conducted.

The technologies used to produce these products belong to the categories of thermochemical treatment (such as pyrolysis) and biochemical treatment (such as AD). However, despite research carried out worldwide and in Europe about these processes and their products, there are countries such as Greece, that there is still a lack of knowledge and awareness not only of farmers but also of industries and the general public about the energy recovery potential of agricultural residues, their end use and benefits. On the other hand, China is one of the largest and most important producers of biofuels. The proposal for a carbon neutrality goal requires China to accelerate the transition to clean energy and thereby successfully address the issues of environmental and energy security.

Before the energy crisis these biofuels produced by biomass had already started being involved in different applications and a rapid acceleration is expected in the next years. This study showed that indeed these biofuels are one of the most significant and sustainable energy sources by reducing gas emissions into the atmosphere but also by managing residual biomass, thus contributing to waste management. Due to the energy crisis, it is expected these biofuels to play a more significant role in energy production and climate change mitigation and their price through the years is expected to decline.

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Competing interests

The authors declare that they have no competing interests.

Acknowledgements

This research resulted from and was funded by the project “Integrated management and exploitation of multi dispersed agricultural residues—application to energy production”, a joint R&D project between Greece and China under the auspices of Ministry of Science and Technology (MOST) of the People’s Republic of China and the Ministry of Development & Investments/General Secretariat for Research and Innovation (GGEK) of the Hellenic Republic.

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