Development Status and Prospects of Biomass Energy in China

Abstract

With the increasingly serious problems of energy shortage and environmental degradation, countries around the world are actively developing safe, environmentally friendly, and renewable energy. Biomass energy has become an ideal substitute for fossil fuels due to its abundant reserves, good renewable performance, and zero carbon emissions. This paper discusses the importance and potential of biomass energy as a renewable energy source for China’s energy development, mainly including the three biomass conversion methods of physics, chemistry, and biology, seven utilization technologies, such as direct combustion, gasification, and pyrolysis, and five application approaches, such as biomass power generation, biomass gas fuel, biomass liquid fuel, and bio-based materials. This review systematically analyzes the challenges faced by China’s development of biomass energy and discusses the future development direction of biomass. The utilization of biomass resources should take a comprehensive and high-value path. China is actively looking for new energy utilization paths, and biomass energy has become a key measure to cope with carbon emission reduction, climate change, and ecological environment protection.

1. Introduction

With the continuous use of fossil energy sources such as coal, oil, and natural gas, the problem of resource shortage and environmental degradation is becoming more and more serious [1]. In this context, the development and utilization of renewable energy has become the focus of global attention. Among them, biomass energy with carbon capture and storage performance has become the world’s fourth-largest energy source after coal, oil, and natural gas (Global CCS Institute, 2019) [2], accounting for about 77% of the global renewable energy supply in 2019, thereby becoming an important alternative to fossil fuels in various countries, as shown in Figure 1.

Biomass energy is essentially a form of energy that stores solar energy in the form of chemical energy in biomass. It directly or indirectly comes from the photosynthesis of plants and can be converted into solid, liquid, or gaseous fuels through physical conversion, chemical conversion, and biological conversion [3]. The main sources of biomass energy include crop straw, forestry waste, agricultural products and residues, livestock and poultry manure, waste cooking oil, municipal solid waste, industrial wastewater/domestic sewage, and cottonseed oil. Biomass energy has the characteristics of clean and safe, low cost, abundant reserves, and sustainable development.

At present, China is facing severe energy challenges. Firstly, the demand for energy is increasing with rapid industrial development, and the dependence on imported energy is gradually increasing. Secondly, the energy structure of rich coal, poor oil, and less gas means that China’s energy utilization is always based on coal, which also leads to China becoming the world’s largest carbon emitter. According to the International Energy Agency [4], China’s future demand for fossil fuels will continue to increase. By 2040, China’s per capita energy consumption will exceed that of the EU, which will lead to a series of environmental and ecological risks related to climate change and biodiversity. Therefore, the Chinese government is actively looking for new energy utilization paths aimed at reducing carbon emissions and dependence on fossil fuels. This review mainly analyzes China’s biomass energy resources, utilization technology, and industrial status, and discusses the development potential and future development trend of each technology.

2. Assessment of Biomass Resources and Potential in China

China is a large agricultural country with abundant biomass reserves. During the period from 2004 to 2006, China had approximately 130 million hectares of farmland and produced more than 600 million tons of crop residues per year [5]. In 2020, China’s agricultural straw theoretical resource strength was about 829 million tons, and it is expected to reach 916 million tons in 2030; the total amount of livestock and poultry manure in China reached 1.868 billion tons, and it is expected to reach 1.983 billion tons in 2030; the total amount of forest residues available was 350 million tons, and is expected to reach 427 million tons by 2030; the amount of domestic waste was 310 million tons, and it is expected to reach 404 million tons in 2030; the annual production of waste oil was about 1055.1 million tons, and it is expected to reach 11.313 million tons in 2030; the annual output of sewage sludge was 14.47 million tons of dry weight, and it is expected to reach 30.9496 million tons in 2030. It can be seen that agricultural residues, led by crop straw, are one of the most important raw materials for the production of bioenergy. China’s agricultural straw resources are mainly distributed in major agricultural grain-producing provinces. The top five crop straw resources are in Heilongjiang, Henan, Jilin, Sichuan, and Hunan, accounting for about 60% of the country’s total [6]. In addition, another study [7] showed that the total area of marginal land suitable for the growth of energy plants in China is 130.34 million hectares. If one-thirtieth of this land is used to plant energy plants, the output of biofuel will reach 13.39 million tons. In 2020, researchers evaluated the sustainable potential of China’s agricultural waste for bioenergy, based on geographic information systems. The results show that the potential of agricultural residues in each province is huge, with a maximum of 27.8 TWH per year [8]. In this context, the development and utilization of biomass energy is an effective way to solve China’s energy problems. Therefore, in 2016, China’s National Energy Administration issued the 13th Five-Year Plan for Biomass Energy Development, pointing out that by 2020, the biomass energy industry would basically achieve commercialization and large-scale utilization, and the annual consumption would reach 58 million tons of standard coal [9]. In 2017, the Guiding Opinions on Promoting the Development of Biomass Heating [10], issued by the National Development and Reform Commission and the National Energy Administration, pointed out that by 2035, the installed capacity of biomass heat and power cogeneration will exceed 25 million kilowatts, the annual utilization of biomass briquette fuel will be about 50 million tons, and the annual utilization of biomass gas will be about 25 billion cubic meters. The total heating area of biomass heating is about 2 billion square meters, and the annual direct replacement of coal is about 60 million tons. In addition, the 14th Five-Year Plan for Bio-economic Development (2022) and the 14th Five-Year Plan for Renewable Energy Development (2022) both put forward the development requirements for the diversified development of biomass energy, with biomass power generation, cogeneration, biomass fuel, and so on as the key points of technology development and application. It can be seen that with the background of the carbon neutrality goal, the development of biomass energy will become a key measure for China in order to cope with carbon emission reduction, climate change, and ecological environment protection.

3. Biomass Conversion Technology

Biomass can be processed and utilized through physical transformation, chemical transformation, and biological transformation. Biomass utilization technologies mainly include direct combustion, liquefaction, gasification, pyrolysis, solidification, and fermentation. The available products mainly include briquette fuel, bio-oil, biogas, biodiesel, and bioethanol. The various types of biomass utilization technology are shown in Figure 2.

3.1. Physical Transformation

Solid Formed Fuel

The biomass preparation of solid fuel technology is the most mature biomass utilization technology, which has achieved industrial application. This technology solves the problems of high transportation costs and difficulties in the large-scale utilization of biomass due to uneven distribution, low energy density, inconvenient transportation, and storage. The main process is to crush the biomass to a certain particle size, and biomass density can reach 0.8–1.3 kg/m³ after high-pressure extrusion. According to the formed shape, the current forming technology can be divided into rod-forming technology, granular-forming technology, and cylindrical-block-forming technology. The process mainly includes drying, crushing, and forming, among which molding technology is the core link in the preparation technology of molding solid biomass fuel. At present, the main development trends include hot-pressing molding technology and carbonization molding technology [11,12]. These two technologies can be compressed and heated at the same time. The obtained fuel also has high calorific value and can also provide by-products. At present, the annual output of biomass briquette fuel in China has reached about 20 million tons [13].

3.2. Chemical Conversion

3.2.1. Direct Combustion

Direct biomass combustion technology is the earliest biomass utilization technology, which can be divided into suspension combustion, layer combustion, and fluidized combustion. For most biomass, this utilization technology can be used to complete the energy conversion process. The application scenario of direct combustion is single and the cost of tail-gas treatment is high. Different types of biomass have different properties, so it is necessary to develop various types of combustion furnaces to match them, which makes it difficult for direct combustion schemes to be widely used. At present, the main research direction is to improve the thermal combustion efficiency of biomass and reduce coking and ash deposition due to the high alkali content.

3.2.2. Biomass Gasification

Biomass gasification refers to the thermochemical process by which biomass reacts with a gasification agent to generate syngas (mainly composed of H₂, CO, and CH₄) and coke under high-temperature conditions. According to the different gasification agents, biomass gasification can be divided into air gasification, oxygen gasification, steam gasification, and hydrogen gasification. The most widely used process is air gasification, which has a low cost and a low calorific value of gas. The combustible gas obtained by biomass gasification technology can not only be used as a clean fuel but can also be used as a raw material for Fischer–Tropsch synthesis to prepare liquid fuel.

Gasifier is the core equipment for biomass gasification, which can be divided into fixed bed, fluidized bed, and entrained flow bed types [14]. The fixed bed gasifier mainly includes the updraft gasifier, downdraft gasifier, open-heart gasifier, and cross-suction gasifier [15]; the fluidized bed gasifier mainly includes a bubbling fluidized bed and circulating fluidized bed, and the latter can be divided into internal circulation, external circulation, and double circulation types [16,17,18]. Fixed bed and fluidized bed gasifiers are common furnace types. Usually, the fixed bed gasifier structure is simple, easy to operate, and is suitable for small-scale enterprises where gas quality requirements are not high, such as in household or rural gas supply; the fluidized bed, especially the circulating fluidized bed gasifier, is suitable for large-scale continuous production, such as power generation and synthesis gas preparation. At present, biomass gasification technology faces the main problems of unstable gas quality, low calorific value, high CO content and high tar content. The development of a low-cost high-calorific value gas preparation path, the development of efficient and stable catalysts, and the improvement of the tar removal process are key measures to improve the efficiency of biomass gasification.

3.2.3. Biomass Liquefaction

Biomass liquefaction is the process by which biomass is chemically converted into a liquid product, the main product of which is bio-oil. Under proper conditions, the maximum yield of bio-oil can be more than 70% [19]. Biomass liquefaction technologies can be divided into direct and indirect liquefaction. Indirect liquefaction mainly includes biomass gasification followed by synthesis and hydrolysis followed by fermentation, etc. Direct liquefaction refers to the direct reaction of biomass to produce bio-oil in a high-pressure environment and under the action of a catalyst. Biomass liquefaction can also be divided into the thermochemical method, biochemical method, esterification method, and chemical synthesis method. Thermochemical liquefaction can be divided into fast pyrolysis technology and high-pressure liquefaction technology (direct liquefaction). Biochemical methods mainly include converting starch to ethanol, cellulose to ethanol, and biomass fermentation to butanol. The chemical synthesis method includes indirect liquefaction to diesel and alcohol ether and esterification and hydrogenation to aviation fuel.

3.2.4. Biomass Pyrolysis

Biomass pyrolysis is the process of converting biomass into bio-oil under anoxic and high-temperature conditions (usually 400–800 °C), accompanied by the formation of a small amount of gas and coke [20]. Biomass pyrolysis is the only negative carbon energy conversion technology and is combined with biochar utilization and carbon materials [21]. Pyrolysis is generally divided into conventional pyrolysis, microwave pyrolysis, catalytic pyrolysis, hydrothermal treatment, and hydro-pyrolysis. Conventional pyrolysis includes slow pyrolysis (10 °C/min), medium pyrolysis (10–500 °C/min), fast pyrolysis (>10–200 °C/s), and flash pyrolysis (>200 °C/s) [22]. The main products of slow pyrolysis are coke and tar, and valuable chemicals such as acetone, methanol, and acetic acid can be obtained during the pyrolysis process [23]. The products of fast pyrolysis are mainly bio-oil and biogas, and the products are mainly bio-oil, which can reach 75–80 wt.% [24]. In addition to the above pyrolysis methods, two new pyrolysis technologies have emerged recently: solar pyrolysis and plasma pyrolysis.

Solar pyrolysis uses concentrated solar energy as a heat source to promote the pyrolysis reaction of biomass and realize the upgrading and utilization of biomass. In this process, solar energy is converted into the original chemical energy of biomass, which breaks the chemical bonds in biomass and releases more energy. Solar pyrolysis technology has the characteristics of high efficiency and environmental protection, which is of great significance in promoting sustainable energy utilization. Plasma is an ionized gas, which is composed of almost equal numbers of positive ions and negative electrons. Plasma is divided into hot plasma and cold plasma. Plasma pyrolysis can promote the rapid pyrolysis reaction, increase the production of gas in the pyrolysis process, and reduce the production of heavy tar [25].

Biomass pyrolysis is accompanied by complex chemical processes, and its complexity is mainly due to the diversity of biomass components and the complexity of the structure, as well as the heterogeneity of the pyrolysis process [26]. The process involves many influencing factors, such as the choice of raw material types, the level of pyrolysis temperature, the application of catalysts, the length of residence time, the speed of heating rate, and the choice of reactor types. For example, biomass with higher cellulose content tends to produce more bio-oil during pyrolysis, while biomass that is rich in hemicellulose is more likely to generate gas, and biomass with high lignin content mainly produces biochar [27]. At present, the research on biomass pyrolysis mainly focuses on technological improvement, high-efficiency catalyst development, and product diversification.

3.3. Biotransformation

3.3.1. Anaerobic Digestion

The anaerobic digestion of biomass refers to the digestion technology wherein, under anaerobic conditions, biomass is decomposed into CH₄, CO₂, H₂O, and H₂S under the action of facultative bacteria and anaerobic bacteria. The products of anaerobic digestion mainly include biogas rich in CO₂ and CH₄, wet sludge with high carbon content, and slurry rich in nutrients. Animal manure, domestic waste, sewage sludge, agricultural residues, forest residues, industrial waste, and other biomass can be used as raw materials for anaerobic digestion [28]. Different raw materials have different effects on the production efficiency of anaerobic digestion, and the materials that are most likely to be anaerobically digested into biogas are those with C/N ratios between 20:1 and 30:1 [29]. In addition, temperature also has a key impact on the anaerobic digestion process. Anaerobic digestion in the temperature range of 35–37 °C can maximize the concentration of CH₄ and H₂ in biogas. The anaerobic digestion of biomass is currently the most mature commercial biomass energy product [30] and the development of small household biogas engineering technology is the most mature. The development of medium and large biogas engineering technology is relatively mature in the United States and European Union countries. These countries have realized biogas purification as a vehicle fuel. China’s biogas technology is mainly based on small biogas projects for rural households. Compared with developed countries such as Europe and the United States, China’s large-scale industrial biogas engineering technology still displays a certain gap.

3.3.2. Pretreatment Hydrolysis Fermentation

Biomass hydrolysis fermentation is mainly divided into three stages: pretreatment, hydrolysis, and fermentation, which is commonly used to prepare bioethanol. Pretreatment will separate cellulose, hemicellulose, and lignin in biomass, cut off their hydrogen bonds, destroy the crystal structure, and reduce the degree of polymerization [31]. Hydrolysis refers to the process in which cellulose and hemicellulose are decomposed into monosaccharides under the action of a certain temperature and catalyst after pretreatment. The commonly used catalysts are inorganic acids and cellulases. Acid hydrolysis refers to the hydrolysis method of using acid as a catalyst, which is mainly divided into concentrated acid hydrolysis and dilute acid hydrolysis. Enzymatic hydrolysis is an enzyme-catalyzed hydrolysis method [32]. The fermentation process is carried out under the action of microorganisms. These microorganisms mainly include Saccharomyces cerevisiae, flocculating bacteria, Escherichia coli, tubular yeast, Pichia stipites, Candida shehatae, thermophilic anaerobic bacteria, etc. [33]. The fermentation process is affected by the quality of biomass raw materials, microbial activity, temperature (35–40 °C), and pH value (4–5). Generally speaking, the process takes 2–4 days to complete. Compared with other technologies, the biomass hydrolysis and fermentation process is simple and low-cost, with convenient transportation and a wide source of raw materials [34]. The fermentation methods of biomass hydrolysis and fermentation to produce bioethanol mainly include indirect fermentation, mixed strain fermentation, simultaneous saccharification and fermentation (SSF), non-isothermal simultaneous saccharification and fermentation (NSSF), immobilized cell fermentation, and so on. At present, there are still the following problems in the process of biomass hydrolysis and fermentation: (1) the price of biological enzymes for cellulase fermentation is too high, (2) there are few microorganisms that can ferment hemicellulose under anaerobic conditions, among others.

4. Application Status of Biomass Energy in China

In recent years, countries around the world have established clear development goals for biomass energy, and have formulated detailed development plans and policy frameworks accordingly, to steadily promote the sustainable development of renewable energy such as corn ethanol in the United States, sugarcane ethanol in Brazil, biomass power generation in Northern Europe, and biogas technology in Germany [35].

At present, the development status of biomass energy in China shows a trend of continuous improvement. By 2020, the total installed capacity of biomass power generation in China had reached 30 million kW, the annual utilization of biomass solid fuel had reached 50 million tons, the annual utilization of biogas had reached 44 billion m³, the annual utilization of biofuel ethanol had reached 10 million tons, and the annual utilization of biodiesel had reached 2 million tons. China’s biomass gasification and bio-liquid fuel production have become a large-scale industry and are the world’s second-largest producer of biomass energy. We have taken the new biomass utilization projects in the first half of 2024 in the three northeastern provinces of China (Heilongjiang, Jilin, and Liaoning) as an example, as shown in Figure 3. In the first half of 2024, a total of 26 green methanol projects were registered or signed in the 3 northeastern provinces, and 9 new biomass power generation projects were added. It can be seen that China’s biomass utilization has developed rapidly under government incentives and policy support.

4.1. Biomass Power Generation and Heating

At present, biomass power generation is the most mature technology and represents the largest application of biomass conversion and utilization technology. In renewable energy power generation, wind power and photovoltaics occupy a major position in the low-carbon transformation of power systems. However, the intermittent nature of wind and solar energy cannot guarantee the stability of the power supply. Therefore, thermal power generation is still the unshakable foundation of power systems. Traditional thermal power generation is mainly based on coal-fired power generation, which will emit a great deal of CO₂. Therefore, biomass power generation has become one of the most important paths for thermal power generation to achieve low-carbon transformation. Biomass power generation in China started late but developed rapidly. The cumulative installed capacity of biomass power generation increased from 4.5 GW in 2012 to 44.77 GW at the end of 2023 [36].

Biomass power generation is mainly divided into three types: biomass direct combustion, co-combustion, and gasification power generation [37]. Among them, biomass direct combustion power generation technology is the most mature, followed by biomass and coal co-combustion power generation, and both types have reached the stage of commercial application. At present, the most widely used biomass-burning boilers in engineering are circulating fluidized bed boilers and grate-burning boilers. The circulating fluidized bed boiler is widely adopted because of its strong adaptability to fuels, high controllability, low level of pollutant emissions, and low-temperature combustion. In 2006, China’s first large-scale biomass direct-fired power generation demonstration project with a 130 t/h grate-burning boiler was put into operation in Shanxian in China, with a power generation efficiency of up to 25 MW, which is equivalent to 100,000 tons of standard coal. In 2011, the Guangdong Zhanjiang 50.0 MW biomass circulating fluid bed (CFB) boiler was put into operation. In 2016, the world’s first 125.0 MW biomass CFB boiler to be designed and manufactured by China was put into operation in Thailand. As of April 2020, the number of direct-fired biomass units in China was nearly 440, including 336 CFB units. Although direct combustion is the most widely used method, its thermal efficiency is very low, and the rise and fall of loads are not flexible. It is necessary to ensure the stability of biomass supply [38]. Therefore, researchers have developed biomass–coal-fired power generation and gasification power generation and have tried to find out the differences between the different technologies to promote the development and transformation of biomass power generation. Yun et al. [39] found through modeling analysis that biomass-coupled coal-fired power generation is more low-carbon and profitable. Compared with coal power alone, co-combustion power generation has a lower cost and higher cost profit rate. Yan et al. [40] calculated the electricity cost of biomass–coal-fired indirect power generation technology by using the levelized electricity cost (LCOE) model; this was shown to be nearly 8% higher than that of pure coal-fired units. Yi et al. [41] found that the electricity cost of small-scale gasification distributed biomass straw gasification power generation is USD 0.056/kWh, which is lower than the local biomass feed-in tariff. It was found that the largest factors affecting the cost of power generation are plant size and straw cost. Therefore, biomass–coal-fired power generation represents a coal-fired power elimination scheme with economic and environmental benefits. Biomass–coal-fired power generation technology can use the existing large-capacity generating units, which have high flexibility. Besides, it can also effectively improve the utilization efficiency of biomass and avoid a series of problems that are caused by pure combustion. It is a more economical and feasible power generation scheme at this point in time.

The main methods of biomass heating are biomass cogeneration, biomass boiler central heating, household boilers, and so on. Biomass heating is mainly used in industrial parks, industrial enterprises, public service facilities, and heating areas such as in heating systems for rural residents. At present, China’s biomass heating capacity exceeds 300 million GJ.

4.2. Biomass Gas Fuel

4.2.1. Biogas Production from Biomass

Biogas is an atmospheric mixed gas that is produced by microorganisms through hydrolysis, acidification, acetic acid, and methanation under anaerobic or anoxic conditions. The main components are CH₄, CO₂, H₂O, H₂S, NH₃, and H₂. CO₂ and H₂S must be removed, because the combination of water vapor and hydrogen may cause H₂S to react, which will produce sulfuric acid and corrode the metal surface. The concentration of CO₂ in biogas is generally 25–50%, which is not flammable and will reduce the grade and combustion performance of biogas, resulting in a decrease in the utilization efficiency of the produced biogas [42]. At present, the most commonly used biogas preparation technology is anaerobic digestion. Compared with other technologies, anaerobic digestion technology is a more economical, environmentally friendly, and efficient method for producing biogas from waste biomass resources. CH₄ gas with a concentration of more than 95% can be obtained by purifying biogas, which is also called biomethane or bio-compressed natural gas. At present, the most commonly used biogas purification methods are membrane separation, water washing, pressure swing adsorption, low-temperature separation, chemical absorption, physical absorption, biological conversion, and in situ methane concentration [28], as shown in

Table 1. Comparison of biogas purification technologies [28,43,44,45].

Technical	Advantages	Disadvantages	Percentage of Application
membrane separation	CH4 content of 90.3%; low operating costs; environmentally friendly; simple equipment and operation; small footprint	membrane elements are fragile, short-lived, and costly	25%
water washing	CH4 content of 96.1%; good regeneration	high investment and operating costs; large footprint; difficult CO2 recovery	31%
variable pressure adsorption	CH4 content of 95.8%; low energy consumption; good regeneration; simultaneous removal of H2S	high CH4 loss rate; high cost; complex process; prone to fouling	14%
cryogenic separation	High CO2 separation; liquid CH4 can be produced	high energy consumption; high investment and operating costs; complex equipment	1%
chemical absorption	CH4 content of 94.6%; simple process; high efficiency; high product purity	high investment costs; high absorbent toxicity; difficult to dispose of waste	21%
biotransformation	CH4 content of 94%; low cost and low energy consumption; environmentally friendly	requires the addition of microbial nutrients; volatile acids tend to accumulate	8%

. Water washing technology, membrane separation technology, and chemical absorption technology are the three more frequently applied biogas purification technologies, accounting for 31%, 25%, and 21%, respectively [43]. Biogas is mostly used for direct combustion for heat, fuel cells or microturbines for electricity generation, cogeneration for power generation, or as a vehicle fuel. Overall, biogas production shows significant sustainability advantages and produces fewer greenhouse gases and hazardous-to-health compounds than fossil fuels. However, biogas production still has a certain degree of net CO₂ emissions, which may pose a potential threat to global warming. Currently, the main methods for reducing the negative impact of biogas production are the improvement of biogas purification technologies, the optimization of biogas residue treatment methods, and the effective control of total electricity consumption.

4.2.2. Hydrogen from Biomass

Hydrogen is a clean, efficient, safe, and flexible energy source that can be produced from a variety of sources, such as primary and secondary energy sources, which helps this renewable energy to achieve large-scale consumption, promotes the large-scale peaking of power grids, and cross-seasonal and cross-territory energy storage, and can realize the decarbonization of various industries. Hydrogen can be classified into grey, blue, and green hydrogen, according to the raw materials used for its preparation. Grey hydrogen refers to hydrogen produced from fossil energy sources, industrial by-products, etc., and is mainly divided into hydrogen produced from coal and that produced from natural gas, accounting for 96% of the hydrogen energy supply [46]. Blue hydrogen is produced by applying carbon-capture technology to grey hydrogen, avoiding direct CO₂ emissions into the atmosphere. Green hydrogen is made from renewable and nuclear energy. In terms of green hydrogen preparation, hydrogen production from biomass has great potential. As shown in

Table 2. Comparison of hydrogen production methods from biomass [47,48,49,51,52].

Primary Classification	Secondary Classification	Strengths and Weaknesses
thermochemical hydrogenation	gasification	high reaction temperature; catalyst required; tar difficult to control
	pyrolytic reforming	low reaction temperature; simple process; tar is difficult to control
	supercritical water conversion	no tar; high conversion rate; high-temperature and high-pressure environment; clogging and coke problems
	chemical chain	high hydrogen purity; high-temperature and high-pressure environment; high- performance oxygen carriers are expensive
biological method for producing hydrogen	photolytic water	O2 in the product; low light energy conversion
	optical fermentation	no O2 generation; high conversion rate; complex system
	dark fermentation	high economy; fast hydrogen production rate; good stability; volatile acids tend to accumulate
	light–dark fermentation	high energy conversion efficiency; low carbon dioxide emissions; high hydrogen production rate; complex technology and high cost

, hydrogen production from biomass can be divided into two main categories: thermochemical [47] and biological [48]. Thermochemical methods mainly include gasification, pyrolysis reforming, supercritical water conversion, and chemical chain hydrogen production. Biological hydrogen production mainly includes photolysis, photo-fermentation, dark fermentation, and light–dark fermentation. Biomass gasification is considered to be the most economical and efficient method of hydrogen production, and is usually carried out at 700–1200 °C. The process of hydrogen production from biomass gasification requires a gasification agent, which is generally air, water vapor, O₂, and their mixtures, and the products are mainly H₂, CO, CO₂, CH, and other hydrocarbon compounds. Studies have shown that when water vapor is used as a gasification agent, the hydrogen yield is three times higher than that from air, and the calorific value of the gas is high, but the energy cost is also higher [49,50].

4.3. Biomass Liquid Fuels

Biomass liquid fuels refer to biomass manufactured into liquid fuels, such as ethanol or oil, through fermentation and purification or biochemical synthesis. At present, the most common biomass liquid fuels on the market are mainly bioethanol, biodiesel, bio-methanol, and bio-aviation fuel.

In the forecast of the Blue Book 3060 Zero Carbon Biomass Development Potential, published in China [53], from 2021 to around 2030, the application of fuel ethanol and biodiesel will be one of the most important ways to reduce emissions in road transport, and bio-aviation fuels will gradually be applied in aviation. By 2030, it is expected that the use of biomass liquid fuels will exceed 25 million tons, which will reduce carbon emissions by about 180 million tons in the transport sector.

4.3.1. Bioethanol

Bioethanol is ethanol with less than 0.5% water, obtained by dehydrating biomass after it has been fermented and distilled to produce ethanol with a concentration of about 95%. According to the different raw materials, bioethanol can be divided into the first generation, the 1.5 generation, the second generation, and the third generation. First-generation bioethanol is mainly based on food crops such as corn and wheat, and this type of bioethanol production accounts for more than 96% of the total production [54], but the production process requires the use of arable land, which brings its production in conflict with food and feed production. Therefore, it cannot be considered a viable fuel in the long term. Generation 1.5 bioethanol is mainly based on non-food crops such as cassava, sweet potato, and sweet sorghum. The production process takes up a certain amount of land and feedstock resources are limited, so this type cannot be produced on a large scale. The feedstock for second-generation bioethanol comes mainly from agricultural and forestry waste (lignocellulosic ethanol), which accounts for less than 3% of total bioethanol production. The production process does not conflict with food crops, the raw materials are widely available, and the carbon emissions are lower than those of first-generation bioethanol, so it has good prospects for development. The feedstock for third-generation bioethanol is mainly algae, and this type of bioethanol production method is currently in the research and development stage. In addition, other researchers have proposed fourth-generation bioethanol with genetically modified organisms (GMOs) as the main feedstock, including algae, trees, plants, etc., the main disadvantage of which is that the genes and combinations on the host metabolic system are limited [55].

At present, achieving the industrialization of cellulosic ethanol has become the development consensus of countries around the world. The demonstration capacity scale of cellulosic ethanol in China has reached 50,000 t. Chu et al. [56] developed a process to convert cellulose from corn stover into highly concentrated bioethanol by the continuous hydrogenolysis of corn stover, yielding an ethanol concentration of up to 6.1%, which provides a competitive aqueous-phase conversion of stover to highly concentrated bioethanol. The current production cost of cellulosic ethanol is high due to multiple factors and has not yet been realized on a commercial production scale. This is because the pretreatment process of bioethanol raw materials necessitates high energy consumption and enzyme preparation cost, and the treatment of the waste liquid and residue is difficult. Therefore, researchers have investigated various enzyme preparations and biorefinery processes, with the aim of reducing the production cost of cellulosic ethanol. Hou et al. [57] prepared an adsorbent named AEPA₂₅₀ using rice straw enzymatic residue, which is able to detoxify ferulic acid (fermentation inhibitor); the removal efficiency of ferulic acid detoxification was as high as 94.393%, which was able to effectively improve the production of bioethanol. Liu et al. [58] proposed a novel biorefinery system for the production of bioethanol, xylose, electricity, and steam from corn cobs. The system was able to achieve a bioethanol yield of 194.2 L/t of dry corn kernel. Yang et al. [59] achieved bioethanol yield and xylose recovery of 189.75 and 150.95 g/kg, respectively, by the combined pretreatment of eucalyptus wood chips with NH₄Cl impregnation and refining. Zhang et al. [60] developed a GIS (geographical information system)-based analysis and optimization modeling approach that focused on locating bioethanol facilities; they also designed a bioethanol feedstock supply chain to minimize the total system costs.

4.3.2. Biodiesel

Biodiesel is a renewable energy source produced from biomass, such as waste oil and grease, which can be used as a direct replacement for fossil diesel as a fuel for existing engine systems. Compared with ordinary fossil diesel, biodiesel has the advantages of fuel performance, lubrication performance, and renewability. At the same time, it can also significantly reduce the emissions of toxic substances such as greenhouse gases, sulfur, and aromatic hydrocarbons. The main raw materials for biodiesel production are vegetable oils such as palm oil, soybean oil, corn oil, and rapeseed oil, followed by catering waste oil and animal fats. The main production methods are direct blending, transesterification, pyrolysis, and microemulsion [61,62], of which transesterification is the main production method for biodiesel [63,64]. Biodiesel can be classified into three generations according to different feedstocks, as shown in

Table 3. Classification of biodiesel feedstock and its advantages and disadvantages [61,62,63,64,65].

Generation	Material	Production Process	Vantage	Drawbacks
1	oilseeds, plant refined oil, and edible-crop refined oil (such as soybeans, rapeseed, peanuts, etc.)	ester exchange	mature technology; simple process; the most mature development	poor low-temperature fluidity; not suitable for long-term storage; increases food security
2	non-food crops, agricultural biomass waste, forestry waste biomass (such as wampee, mango, animal fats, rubber seeds, etc.)	hydrogenation of fats and oils	no food security issues; high carbon emission reduction effects	purification separation; purification is more difficult and costly
3	microalgae, microorganisms (such as autotrophic microalgae, heterotrophic microalgae, yeast, etc.)	photosynthesis	no pretreatment is required; high lipid production; the breeding method is simple and rapid; sewage purification	high cost; need a lot of water resources; may be toxic; poor fluidity at low temperatures

. Among them, the first generation of biodiesel mainly uses oilseeds, plant refining oil, and edible crop refining oil as raw materials, and the production process is the ester exchange method. The second generation of biodiesel mainly uses non-food crops, agricultural biomass waste, forestry waste biomass, and other non-food crops as raw materials, and the production process is mainly oil hydrogenation technology. In addition to exacerbating the food crisis, the above two generation forms of biodiesel also need to pre-treat those raw materials with larger particle sizes, resulting in a higher cost and higher energy consumption. Therefore, a third generation of biodiesel using microalgae and microorganisms as raw materials has emerged. This method does not raise food issues, can promote carbon emission reduction, and does not require the pretreatment of raw materials.

In recent years, biodiesel promotion has also received national attention. In 2001, China’s biodiesel market began to move forward into the industrialization process. In 2021, China’s biodiesel market size was about 4.324 billion RMB, and grew to 6.623 billion RMB in 2022, a year-on-year growth of 53.17%. At present, the main problems that still exist in China’s biodiesel industry are the serious shortage of oil production, the low recycling rate of waste oil and grease, high production costs, and difficulty in industrialization and scale development. In addition, biodiesel has the disadvantages of a high pour point and turbidity point, high viscosity, high NOx emission, low volatility, low energy content, and poor spray performance [65].

4.3.3. Bio-Methanol

Methanol is the simplest alcohol and has a wide range of industrial applications for the production of other chemicals such as formaldehyde, acetic acid, and various types of plastics [66]. The preparation of methanol is mainly divided into methanol from coal, methanol from natural gas, and methanol from CO₂, of which methanol from coal has the lowest cost and uses the most mature technology, but yields the most serious environmental pollution [67,68]. The common pathways for the preparation of bio-methanol can be divided into biomass gasification to methanol and biomass biogas to methanol, as shown in

Table 4. Comparison of the bio-methanol preparation pathways [67,68,69].

Production Route	Material	Technological Route	Advantages and Disadvantages
Methanol from biomass gasification	Biomass, H2	pretreatment → gasification → syngas treatment → methanol synthesis, distillation	higher thermochemical efficiency; difficult to scale up
Methanol from biomass biogas	Biomass, O2	biogas → pretreatment → methane reforming → syngas compression → methanol synthesis and distillation	slow process; low biomass energy density; difficult disposal of waste residues

. Methanol from biomass gasification refers to the use of oxygen/water vapor as the gasification agent; the syngas produced by gasification is purified, CO₂-converted, and CO₂/H₂ ratio-adjusted, and CO₂ and H₂S have been removed. Finally, the methanol synthesis reaction was carried out in the synthesizer. Biomass biogas to methanol refers to biomass through the biogas project to produce biogas. Biogas after purification can be used to prepare methanol as a new energy fuel. The main component of biogas is CH₄, and the current method for the large-scale production of methanol is to use CH₄ as the raw material.

Currently, China’s methanol production ranks first in the world, of which coal-based methanol is the main preparation method, with CO₂ emissions of 1.9–2.5 t/t methanol. Compared with coal-based methanol and natural gas-based methanol, bio-methanol has the advantages of high performance, low flammability, wide-ranging feedstock resources, low carbon, and environmental protection, as well as better economic benefits [70,71]. Bio-methanol is chemically indistinguishable from fossil-based methanol and is, therefore, fully miscible with water, conventional methanol, gasoline, and different organic compounds [72]. Methanol from biomass offers a solution to reducing CO₂ emissions at source and producing methanol sustainably. The CO₂ emissions in the biomass-to-methanol process are mainly in the syngas purification section of the biomass source, and the biogenic CO₂ emitted from this system is carbon-neutral and can be considered to have nearly zero CO₂ emissions. If the CO₂ capture process is introduced in the purification section, the CO₂ is captured and purified, liquefied, and sequestered; therefore, negative CO₂ emissions can be realized in the methanol production process. Wang et al. [73] evaluated the development prospects of the corn stover preparation of bio-methanol fuel based on life-cycle assessment, and the results show that the energy consumption of biomass-based methanol is 51.02 × 10 MJ/t, and the CO₂ emission is 667.53 kg/t. Compared with coal-based methanol, the full life-cycle economic cost increased by 24.46% and CO₂ emissions decreased by 59.39%. Bio-methanol can be used as a substitute for gasoline and diesel in internal combustion engines, both in methanol-fueled vehicles and in hybrids, as well as for power generation through gas turbines or fuel cells and as a household fuel source [74]. As the world’s largest market for methanol vehicles, China is exploring a “green methanol vehicle” route: the use of CO₂-methanol or biomass-methanol in vehicles with internal combustion engines. Li et al. [75] found that CO₂-methanol vehicles emit 24% less CO₂ over their life cycle compared to gasoline vehicles. Compared with coal and natural gas-based methanol, bio-methanol is more expensive and energy-intensive to produce, making it difficult to achieve large-scale production [76].

4.3.4. Bio-Based Aviation Fuels

Bio-based aviation fuels are aviation fuels that are made from biowaste or renewable resources and are certified as safe and sustainable. Bio-based aviation fuels are not only able to meet the performance requirements of aviation jet fuel but also have superior stability, better combustion properties, and good material compatibility. In addition, bio-based aviation fuels can reduce carbon emissions by 50% to 90% compared to fossil jet fuel [77]. The main raw materials for synthesizing bio-based aviation fuels include vegetable oil biomass, algal biomass, lignocellulosic biomass, sugars, and starch biomass. Different raw materials need different technological conversion routes. The current synthetic routes and technological methods for bio-based aviation fuels are shown in Table 5, which mainly include: the catalytic hydrogenation of vegetable oils, F-T synthesis, the use of vegetable oils for aviation fuels, the conversion of low-carbon alcohols for aviation fuels, and sugar for aviation fuels [78].

The catalytic hydrogenation of vegetable oils refers to the hydrogenation of vegetable oils or related raw materials into liquid hydrocarbon fuels for a range of aviation fuels under the action of catalysts, and the main raw materials are vegetable oils that are rich in triglycerides and fatty acids. Currently, catalysts used for the catalytic hydrogenation of vegetable oils to make aviation fuels can be divided into two main categories [79,80,81]: precious metal catalysts (Pt, Pd, Ru, Rh, etc.) and non-precious metal catalysts (Ni, Co, Fe, Mo, etc.). The application cost of precious metal catalysts is high, so the development of non-precious metal catalysts with high selectivity, high catalytic activity, and that are cheap and easy to obtain is an inevitable development trend. Zheng et al. [82] prepared Ni-Cu bifunctional catalysts and found that Ni-2Cu/MCM-41 catalysts effectively promoted the conversion of oleic acid; the conversion rate, as well as the amount of hydrocarbons, respectively, reached 100% and 85.68%, and the C8–C14 selectivity was 43.40%. Wang et al. [83] prepared MoC-loaded nitrogen-enriched carbon (MoC/CN) and mesoporous carbon (Mo₂C/MC) for the catalytic hydrogenation of oleic acid, and found that the MoC/CN catalyst exhibited remarkable activity at 310 °C, with a conversion and selectivity of 94.3% and 90.3%, respectively, which are significantly higher than those of the Mo₂C/MC catalyst (84.8% and 70.1%).

The F-T synthesis of bio-based aviation fuels involves the conversion of syngas (rich in CO and H₂) from biomass gasification to liquid aviation fuels after catalytic reforming. The most commonly used catalysts are mainly group-VIII metals, e.g., Ru, Fe, Co, Rh, and Ni, among which Fe and Co are widely used in industrial production. Li et al. [84] prepared a 10% Co-loaded ZrO₂-SiO₂ catalyst to catalyze the preparation of biobased aviation fuels from syngas, with a selectivity of 83.3% for C8–C16. Li et al. [85] successfully prepared F-T synthesis catalysts with a core-shell structure, using conventional F-T synthesis catalysts (Co/Al₂O₃ and Co/SiO₂) as the core and zeolite molecular sieves as the shell, and obtained biobased aviation fuels with up to 60–70% yield.

Sugar and its derivatives can also act as feedstocks for bio-based aviation fuels, which are mainly converted to bio-based aviation fuels through catalytic conversion and fermentation processes [86]. The catalytic conversion pathway mainly involves the deoxygenation of biomass and the C-C coupling reaction, which is a more complex process. Sugar can also be fermented to produce bioethanol and biobutanol, which can be converted to liquid hydrocarbon fuels, but the cost is 6–8 times higher than that of traditional petroleum-based aviation fuels.

Other methods of preparing bio-based aviation fuels include the fast pyrolysis of biomass and hydrothermal liquefaction. Among them, biomass-based fast pyrolysis requires precious metal catalysts and is more costly. At the same time, the product composition is complex and difficult to separate, and the oxygen and water content is also high. Hydrothermal liquefaction technology requires expensive special alloy reactors due to the presence of water, leading to a complex environment and increased corrosion in the reactor.

Table 5. Comparison of the synthetic pathways and technological approaches for bio-based aviation fuels [78,79,80,81,86].

Preparation Method	Catalyzer	Advantages and Disadvantages
Catalytic hydrogenation of vegetable oils	Pt, Pd, Ru, Rh, Ni, Co, Fe, and Mo	high fuel octane; low aromatic, oxygen, and sulfur content; expensive catalyst
Fischer–Tropsch synthesis	Ru, Fe, Co, Rh, and Ni	higher fuel thermal mass; lower energy density; lower aromatic content; higher production costs
Sugar conversion methods	basic catalyst; acid catalysts; acid-based bifunctional catalysts	complicated process; higher costs

Table 5. Comparison of the synthetic pathways and technological approaches for bio-based aviation fuels [78,79,80,81,86].

Preparation Method	Catalyzer	Advantages and Disadvantages
Catalytic hydrogenation of vegetable oils	Pt, Pd, Ru, Rh, Ni, Co, Fe, and Mo	high fuel octane; low aromatic, oxygen, and sulfur content; expensive catalyst
Fischer–Tropsch synthesis	Ru, Fe, Co, Rh, and Ni	higher fuel thermal mass; lower energy density; lower aromatic content; higher production costs
Sugar conversion methods	basic catalyst; acid catalysts; acid-based bifunctional catalysts	complicated process; higher costs

4.4. Bio-Based Material

Bio-based material can be obtained from biomass through biomanufacturing or chemical processing. According to the final use of the bio-based material, it can be divided into bio-based energy storage material, bio-based medical material, and bio-based agricultural material [87]. According to the application of the bio-based material, it can be classified into several categories such as bio-based chemicals, bio-based polymers, bio-based plastics, bio-based chemical fibers, bio-based rubbers, bio-based coatings, bio-based material auxiliaries, and bio-based composites [88], as shown in

Table 6. Classification of bio-based materials [87,88,89].

	First Classification	Secondary Classification
Bio-based material	Bio-based chemicals	ethanol, lactic acid, 1,3-propanediol, etc.
	Bio-based plastics	non-biodegradable bio-based plastics (polylactic acid, polycaprolactone, polybutylene terephthalate, starch-based plastics, etc.)
		biodegradable bio-based plastics (polyethylene, polyamide, polyethylene terephthalate and modified natural polymers, etc.)
	Bio-based chemical fibers	polylactic acid fiber
		bio-based polyamide fibers
		polyterephthalate (PTT) fibers
		chitin fiber
		seaweed fiber
		protein-modified fibers
	Bio-based elastomers	bio-based rubber
		other elastomers
	Bio-based material additives	bio-based plasticisers
		bio-based combustion agents
		bio-based adhesives
		bio-based lubricants
		bio-based cleaners
		bio-based surfactants
		bio-based other additives
	Bio-based coatings	bio-based anticorrosion coatings
		bio-based light-curing coatings
		other bio-based coatings
	Bio-based Composites	wood-based composite materials
		bamboo-based composite materials
		starch-based composite materials

.

In recent years, with the continuous progress of biorefinery technology and bio-catalysis technology, green, low-carbon, environmentally friendly, and resource-saving bio-based materials have received widespread attention. China’s bio-based materials have already reached a certain industrial scale, and some of the technologies are close to an internationally advanced level. Currently, China’s bio-based materials industry is growing at a rate of 20–30% per year, gradually moving toward the industrial-scale practical application and industrialization stage [32]. In 2021, the output of China’s bio-based materials was 7 million tons, with an output value of more than 150 billion RMB, accounting for 2.3% of the total output value of the chemical industry [90]. Among them, bio-based materials produced from lignocellulose, such as using lignin, cellulose, and hemicellulose as the raw materials, have developed rapidly [91]. In addition, biomass carbon material has recently received extensive attention from researchers. Bio-based carbon material refers to the conversion of biomass into high-value functional carbon materials, such as porous carbon materials and carbon nanotubes, through pyrolysis modification and other treatments. These materials have great application prospects in the fields of energy storage and conversion, catalysis, adsorption, separation, etc., because of their unique properties. Wu et al. [92] used biomass pyrolysis tar as a carbon source to prepare hierarchical porous bio-chars with macroporous–mesoporous–microporous interconnections and used them in supercapacitors with good application results. Zhu et al. [93] used bamboo charcoal as a raw material for biomass at 1200–1400 °C to obtain a large number of multi-walled carbon nanotubes, and proposed that the gas–liquid–solid process is the key mechanism for the formation of carbon nanotubes.

4.5. Biofuel Cells

The biofuel cell (BFC), as one kind of fuel cell, is a power generation device that uses enzymes or microorganisms as catalysts to convert the chemical energy of fuels into electricity through an electrochemical oxidation reaction. It is a renewable green battery with a simple structure, a wide source of raw materials, high power generation efficiency, mild operating conditions, low environmental pollution levels, and high carrying capacity. As shown in

Table 7. Classification and comparison of biofuel cells.

Classification	Type	Advantages and Disadvantages
Catalyst type	microbial fuel cell	Wide range of raw materials Mild operating environment No pollution and low cost Low output power density
	enzyme fuel cell	High catalyst selectivity Catalysts are costly, prone to deactivation and have poor stability
Electronic transfer method	direct biofuel cell	Low current and power density
	indirect biofuel cells	High efficiency of electronic transfer

, biofuel cells can be classified into microbial fuel cells and enzyme biofuel cells according to the type of catalyst, and into direct biofuel cells and indirect biofuel cells according to the method of electron transfer [94].

The microbial fuel cell (MFC) uses electricity-producing microorganisms to oxidize the fuel under anaerobic conditions, releasing electrons and protons, and converting the chemical energy of the fuel into electrical energy. The main features of the microbial fuel cell are a wide range of raw materials, a mild operating environment, no pollution, and low cost, but low output power density. The enzymatic biofuel cell (EBFC) is a kind of energy conversion device that uses enzymes as catalysts and renewable biomass as fuels (such as glucose, ethanol, hydrogen, etc.) and converts the chemical energy of the fuels into electrical energy by using the anode of the biofuel cell and the cathode of the biofuel cell. Its main feature is that the enzyme catalyst is highly selective and active, but the catalyst is costly and easy to deactivate, so it is not stable. A direct biofuel cell is one in which the fuel is oxidized at the electrodes and electrons are transferred directly from the fuel molecules to the electrodes. The role of the biocatalyst is to catalyze the reaction of the fuel on the electrode’s surface. An indirect biofuel cell is one in which the fuel does not react on the electrode but, instead, in the electrolyte or elsewhere, and electrons are transported to the electrode by a redox-active mediator. Compared to direct biofuel cells, the use of mediators in indirect biofuel cells improves the efficiency of electron transfer, and the output power of the biocatalytic electrode is increased. Currently, biofuel cells are widely used in the fields of implantable device power supply, wearable devices, self-powered sensors, and wastewater treatment [95,96,97]. However, biofuel cells are currently unable to achieve high-power power supply and can only be used for small power supply devices.

5. Progress in the Integrated Use of Biomass Energy in China

At present, various biomass conversion technologies are facing the development status of a single product and a poor economy. In order to make the use of biomass energy more efficient and economical, the diversified and comprehensive use of biomass has become essential for development. The comprehensive utilization of biomass focuses on the development of green manufacturing, the promotion of multi-energy complementarity, and the development of the integrated utilization of industry, agriculture, and commerce. As a result, a variety of integrated biomass utilization approaches have emerged, such as biomass-based cogeneration technologies, green chemical production technologies, coupled with renewable energy sources such as solar energy and carbon dioxide capture, storage, and utilization technologies (CCUS).

Sahoo et al. [98] proposed a solar-biomass poly-generation system for power generation, refrigeration, and desalination, with a primary energy saving of 50.5%. Baldelli et al. [99] investigated a biomass thermochemical conversion system for hydrogen production, coupled with the anaerobic fermentation of biomass, pyrolysis of biomass residues, and electrical reforming of syngas for hydrogen production. The energy inputs to the system are biomass and electricity. When the solar power system is not able to meet the demands of the power system, the power will be obtained from the grid. If there is excess solar power generation, the thermal energy demand of the plant can be met by converting the surplus power into thermal energy storage (TES). The system achieves a hydrogen production rate of 5.37% kgH₂/kg biomass, which is significantly higher than the typical value for a single process (about 3%). F. Calise et al. [100] demonstrated a liquid green methane production system consisting of an anaerobic digester, a biogas upgrading and biomethane liquefaction unit, photovoltaic panels, vacuum solar collectors, and electrical and thermal storage systems. In particular, the system used membrane separation technology for biogas upgrading, and the authors proposed a biomethane liquefaction process based on the Linde cycle. The primary energy savings of the system reached 50%. Ahmad et al. [101] evaluated the performance of three PV/biomass hybrid renewable energy systems (HRES), namely, a PV/bio/grid-connected hybrid system using a biomass gasification composition, a PV/bio/grid-connected hybrid system using a direct biomass combustion system, and a PV/bio/grid-connected hybrid system utilizing a biomass pyrolysis process, then used a multi-objective genetic algorithm (MOGA) for modeling.

At present, the comprehensive utilization of biomass in China has developed very rapidly and has achieved remarkable results. In biomass cogeneration, Liu et al. [102] proposed a novel hybrid energy system driven by thermal melting biomass gasification and solar energy, which combines biomass roasting, biomass gasification, a solar thermal collector, and absorption refrigeration. The hybrid energy system can save 1.05 × 10 MJ of heat per year, which is equivalent to about 358.26 tons of standard coal or 550.59 tons of CO₂ emission reduction. Zhu et al. [103] proposed a novel biomass-fired combined cooling, heating, and power (CCHP) system, incorporating a combination of an organic Rankine cycle (ORC), mono-efficiency lithium bromide absorption refrigeration, and mono-ethanolamine (MEA)-based CO₂ capture. The designed system uses pressurized hot water as a heat source for the ORC system and the refrigeration cycle. The results show that the maximum power of the system is 7.4%, the cooling efficiency is 14%, the thermal efficiency is 81.5%, and the primary energy saving is 13.3%. Zhu et al. [104] presented a geothermal-driven cogeneration system based on biomass digestion and a supercritical CO₂ cycle. The system can provide 500.8 kW of electrical energy, 900.2 kW of heating power, and 4.931 kW of cooling power. Meanwhile, the system can achieve a power generation efficiency of 23.08%, with a payback period of 6.87 years. Dou et al. [105] proposed a novel hybrid multigeneration system based on geothermal and biomass energy that can simultaneously provide electricity, cooling, heating, and hybrid freshwater production. The system can produce 776.3 kW of power, 237 kW of heating power, 15.5 kW of cooling power, and 20.35 kg/s of fresh water. Zhao et al. [106] proposed a novel biomass syngas and natural gas solid oxide fuel cell (SOFC)-combined cycle system utilizing liquefied natural gas (LNG) cold energy for carbon capture. The results show that the thermal, electrical, and energy efficiencies of the system are 60.28%, 66.20%, and 55.59%, respectively, under the design’s operating conditions. The carbon capture purity and mass flow rate of liquid CO₂ were 99.35% and 54.50 kg/h, respectively.

For the preparation of green chemicals from biomass, Xin et al. [107] proposed a hybrid biomass–solar gasification system for sustainable fuel production, as shown in Figure 4, which combines solar pyrolysis and PV-SOEC to facilitate sustainable fuel production. Compared with the conventional biomass gasification systems, this approach uses solar energy to provide heat for the biomass pyrolysis process, replacing a portion of the biomass combustion. When the system produces methanol, the total energy conversion efficiency can reach 73.06%, and the carbon utilization efficiency of the biomass can reach 66.81%. These values are improved by 10.45% and 54.25%, respectively, compared to a conventional system with the same methanol yield.

Guo et al. [108] proposed and analyzed a biomass and nuclear energy system for hydrogen production using high-temperature waste heat from a small fluoride salt-cooled high-temperature advanced reactor, which uses a nuclear energy method for hydrogen production from biomass derivatives. Zhang et al. [109] used Aspen Plus to develop a novel poly-generation system based on biomass gasification and syngas chemical cycling as shown in Figure 5. Electricity, dimethyl ether, and ammonia can be supplied. The system has an energy efficiency of 60.15% and an operating efficiency of 46.70%, and users can recover the initial investment cost within eight years. Wu et al. [110] proposed a new biomass gasification combined-cycle system for methanol synthesis, which consists of CO₂ hydrogenation to methanol, methanol separation, biomass gasification, and gas turbine combined-cycle power generation systems. The approach promotes the multi-level conversion and stepwise utilization of green hydrogen chemical energy and significantly improves the performance of the new cycle.

In the face of increasingly serious energy shortage and carbon emission problems, using biomass as a raw material, with solar energy and other renewable energy sources as an auxiliary, to enable green fuel technology has been widely studied by researchers, including green methane, green hydrogen, and green methanol. Although hydrogen as a fuel can achieve zero carbon emissions, its storage and transport problems are still difficult to solve, and biomass still emits a small amount of CO₂ during the conversion and utilization processes. Therefore, some researchers have begun to focus on the CO₂ hydrogenation to green fuel conversion pathway, as shown in Figure 6. Water electrolysis for hydrogen production is a mature technology using simple equipment, and the hydrogen produced is of high purity and is non-polluting. When the electricity used in the water electrolysis hydrogen production system comes from photovoltaic or wind power, it not only reduces CO₂ emissions but also provides more clean energy. Among the many green energy sources, green methanol, as a new type of clean energy, is considered one of the most important fuels of the future, which can promote the decarbonization transition in the fields of transportation and air transport. Therefore, CO₂ hydrogenation to methanol has received extensive attention from researchers for its great potential in the recycling of CO₂. Fournas et al. [111] investigated the production of renewable methanol by the gasification of forestry residues, using thermodynamic modeling and environmental–economic assessment. Kylee Harris et al. [112] proposed a commercial assessment system comparing two processing methods for biomass methanol and CO₂ methanol. This system comprised a new economic assessment method for renewable methanol technologies. The results showed that biomass gasification to methanol can meet the cost requirements of competitive markets. Zhang et al. [113] coupled a water electrolysis system, supercritical CO₂, and biomass gasification to a methanol system and used a supercritical CO₂ cycle to improve the performance of the system; the results showed that the system has an energy efficiency of 67.98%. In multi-system coupling and multi-energy complementary bio-methanol production systems, it is common to combine a water electrolysis system or a biomass gasification system with a methanol synthesis system [114]. In our latest research, a novel hybrid multi-system green methanol production system is proposed, as shown in Figure 7. This mainly includes a solar photovoltaic subsystem, a biomass anaerobic digestion subsystem, a biomass oxygen-enriched combustion subsystem, a water electrolysis subsystem, and a bio-methanol synthesis subsystem. The system utilizes biomass more efficiently and economically while producing green methanol.

6. Conclusions

Biomass is a renewable, green, and low-carbon energy source and is an ideal substitute for fossil fuels. The production of clean energy through biomass is an important way to achieve the sustainable development of energy, the environment, and the economy. It is also one of the most effective measures to achieve the goal of carbon neutrality.

China, as a large agricultural country, has abundant biomass energy stores with huge development potential, which can be used not only to produce biofuels but also to produce high-value-added food or industrial raw materials. Biomass energy products have huge development capacity due to their performance advantages. In recent years, China has paid more and more attention to the use of biomass energy, and biomass energy utilization technology has been rapidly developed. In general, biomass solid-forming fuels, food and non-food fuel ethanol, biodiesel, biomethane, power generation, and heating technologies have basically been industrialized. Cellulosic fuel ethanol, biomethane, and gasification power generation have entered the early stage of industrialization. Biomass pyrolysis oil, bio-jet fuel, and biosynthetic gas technologies have reached the demonstration stage. However, biological hydrogen production technology is still in the laboratory research stage.

At present, the main problems faced by the use of biomass energy are as follows: (1) the number and types of raw materials are abundant, but the distribution is relatively dispersed, which means that the cost of centralized utilization is relatively high. Besides, there may be a situation involving competition with food production. (2) In terms of the environment, in the process of converting biomass into high-grade energy, different technologies may produce different degrees of secondary pollution in the environment. (3) In terms of biomass solid-forming fuels, there are problems such as the dispersion of raw materials and unclear molding mechanism. (4) In terms of biomass power generation, direct combustion power generation has problems such as the deposition of slag in combustion devices and serious corrosion. There are some problems in gasification power generation technology, such as low efficiency, small scale, poor tar-disposal effect, large dispersion, and an unclear forming mechanism. (5) In terms of biomass liquid fuels, the development of China’s bio-ethanol conversion technology is rapid, but there is still a shortage in other fuel synthesis technologies. There are still problems, such as an unclear conversion reaction mechanism and a lack of efficient and long-life catalysts. (6) In terms of biomass gas, anaerobic digestion biogas technology is the most mature, but there are problems such as the high cost of the conversion process, difficulty in separating the products, the low efficiency of quality enhancement, and the instability of the products. (7) In terms of biomass-based materials and fuel cells, there are still big challenges in terms of product performance, manufacturing cost, key technology integration, industrialization scale, etc., and biomass technologies are still in the preliminary stages of development.

7. Future Development and Prospect

At present, the research and development of biomass energy application technology is mainly conducted from the perspective of the ecological environment and environmental protection. In the medium and long term, its usage can make up for the shortage of limited resources. Therefore, the development and utilization of biomass energy have far greater social benefits than economic benefits.

(1) Policy support: In the current stage of development, the development of biomass energy requires national policy support and financial support. It is essential to formulate relevant policies to encourage and support enterprises to invest in biomass energy development projects. In 2005, China promulgated the Renewable Energy Law, indicating that the country’s major policy was for the development and utilization of clean and efficient biomass energy. The biomass energy industry has now entered the initial stage. The period from 2007 to 2015 was important for the exploration and development of biomass energy production. The Chinese government has clearly defined the important position of biomass energy and has promulgated and implemented a number of plans, such as the Medium and long-term development plans for renewable energy, the 12th Five-Year Plan for Biomass Energy Development, the 13th Five-Year Plan for Biomass Energy Development, etc., which provide sufficient guarantees for the large-scale and commercial development of the biomass energy industry. Since 2021, China has seen a period of steady growth in biomass energy production. The National Development and Reform Commission issued the 14th Five-Year Plan for Bio-economic Development to promote the development and integration of new bio-manufacturing industries by technological innovation, and to continuously improve and realize the transformation and upgrading of the biomass energy industry.

(2) Development trend: The use of biomass resources should take the path of integration and high value. China’s biomass development trends are mainly as follows: (i) biomass power generation, from pure power generation to cogeneration; (ii) biomass energy utilization, from power generation to a comprehensive energy supply, such as transportation, electricity, heating, heating, etc.; (iii) biomass energy application technology presents diversification, such as bio-gas, fuel ethanol, cogeneration, biodiesel technology, etc.; (iv) make good use of biomass energy utilization in a hierarchical manner, and achieve an effective allocation of resources; (v) biomass energy use to promote scaling and commercialization.

(3) Industrialization transformation: on the one hand, it is necessary to focus on the development of flexible and controllable biomass energy technology, improve its participation in the power market and auxiliary services, and meet the peak and frequency regulation needs of the power system; on the other hand, it is important to continue to expand the scale of the non-electric utilization of biomass energy, conduct in-depth research, steadily promote pilot projects in the fields of bio-natural gas and bio-liquid fuel, and build a sustainable commercial and industrial development path.

At present, China is facing challenges such as population aging, food safety, energy shortage, and environmental degradation. In order to ensure population health and food security and to promote energy conservation and emission reduction, it is necessary to accelerate the development and application of new crop varieties, green planting technologies, biofuels and power generation, bio-environmental protection technologies, and bio-based products. It is worth noting that there are some problems in the biological industry, such as imperfect management mechanisms, imperfect market access regulations, the loose connection of scientific research and industry, and a lack of competitive enterprises. It is necessary to take measures to solve these problems and create favorable conditions to promote the development of the biological industry.