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Review

New Strategies for Sustainable Biofuel Production: Pyrolytic Poly-Generation of Biomass

by
Fanbin Meng
1,* and
Donghai Wang
2
1
College of Resource and Environment, Anhui Science and Technology University, Fengyang 233100, China
2
Department of Biological and Agricultural Engineering, Kansas State University, Manhattan, KS 66506, USA
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(5), 1945; https://doi.org/10.3390/su17051945
Submission received: 20 January 2025 / Revised: 19 February 2025 / Accepted: 21 February 2025 / Published: 25 February 2025
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Abstract

:
Biomass serves as a promising renewable and sustainable feedstock for energy production through thermochemical conversion. It can be transformed into sustainable biofuels by means of pyrolysis. Among these methods, the pyrolytic poly-generation of biomass, a novel biomass thermal conversion technology, can concurrently produce three valuable products, namely biochar, bio-oil, and combustible gas, without generating any byproducts. In contrast, conventional thermal conversion processes, such as carbonization for biochar, liquefaction for bio-oil, gasification for syngas, and combustion for heat, only yield single products, have limited efficiency, and give rise to byproducts. Clearly, pyrolytic poly-generation holds significant advantages over conventional thermal conversion processes. Nevertheless, the pyrolytic poly-generation process and its products are remarkably influenced by numerous factors, including the raw biomass properties, pretreatment methods, operating parameters, and catalysts. This article reviews the processing parameters and technology for biomass pyrolytic poly-generation, and also explores future research areas, with the aim of identifying research gaps and promoting its industrial implementation.

1. Introduction

Biomass refers to all kinds of organisms formed through photosynthesis using readily available carbon dioxide, water, and sunlight in the atmosphere, including all animals, plants, and microorganisms [1], and is the fourth major fuel after coal, oil, and natural gas. It has the characteristics of being renewable, sustainable, high yielding, and carbon neutral [2]. However, a large amount of biomass is discarded as waste, and its industrial application rate is relatively low [3]. According to the International Energy Agency (IEA), bioenergy accounts for approximately one-tenth of the world’s total primary energy supply, and has enormous potential for further development [4]. Considering the renewable and sustainable characteristics of biomass [5], increasing the utilization ratio of biomass in the primary energy supply will be beneficial for energy security and environmental protection [6]. Therefore, the high-efficiency conversion and high-value utilization of biomass have become the main focuses at present [7].
Pyrolysis is one of the most promising methods for converting biomass into biofuels [8,9]. Typical biomass pyrolysis can be classified into three categories based on their products: (1) slow pyrolysis for producing biochar [10], (2) fast/flash pyrolysis for producing bio-oil [11,12], and (3) oxygen-limited pyrolysis (or gasification) for producing syngas [13]. However, all these methods yield a single product and discard high-value byproducts, which cannot avoid the drawbacks of energy waste, environmental pollution, and high production costs [14]. To overcome the disadvantages of conventional pyrolysis, a new state-of-the-art approach called pyrolytic poly-generation has been designed [15,16]. This method can be profitable without government subsidies [17]. Biomass pyrolysis for poly-generation can be summarized as producing three useful products (biochar, bio-oil, and combustible gas) [18], or two useful products with the third product being utilized in the process [19,20].
From the Web of Science, for the topic “pyrolysis of biomass”, there are 22,997 (from 1950 to 31 December 2023) and 12,495 (in the last five years) search results. However, for the topic “pyrolytic poly-generation of biomass”, there are only 24 (from 1950 to 31 December 2023) and 5 (in the last five years) search results. Although there are some review articles focusing on biomass pyrolysis [21,22], there is currently no review concentrating on biomass pyrolysis for poly-generation. Therefore, this paper begins by summarizing the pathways of biomass pyrolysis for poly-generation. Then, the catalytic technologies for biomass pyrolysis for poly-generation are introduced. Additionally, the effects of the processing parameters on biomass pyrolysis for poly-generation are discussed in detail. Finally, the future development directions of biomass pyrolysis for poly-generation are prospected. The goal of this review is to identify the research gaps and promote the industrial implementation of biomass pyrolysis for poly-generation as soon as possible. The schematic diagrams of biomass pyrolysis for a single product and poly-generation are presented in Figure 1.

2. Pathways of Pyrolytic Poly-Generation of Biomass

The intermediate pyrolysis of biomass is commonly utilized to generate multiple useful products at a relatively low temperature, ranging from 300 to 700 °C [17]. In this section, the pathways of biomass pyrolysis for producing three useful products or two useful products with the third product being utilized in the poly-generation process will be discussed in detail (Figure 2).

2.1. Three Useful Products

Biomass pyrolytic poly-generation was developed to simultaneously produce biochar, bio-oil, and non-condensable gas in a single process [23]. The advantage of biomass pyrolytic poly-generation is that it improves the conversion efficiency by collecting all the products and avoiding by-products. However, the disadvantage is mainly reflected in the complex reaction system [24]. Table 1 presents the product distribution of biomass pyrolytic poly-generation for producing three useful products. It can be seen that biomass pyrolytic poly-generation is heated by an external heater, such as an electric furnace or hot flue gas, and is carried out in a fixed-bed reactor under a slow heating rate or in a fluidized bed reactor under a fast heating rate, respectively [25]. In addition, the product distribution varies with different processing parameters [26]. Therefore, there is not a unified pathway for biomass pyrolytic poly-generation for biofuels [27].
In Table 1, the types of biomass pyrolytic poly-generation for simultaneously producing biochar, bio-oil, and combustible gas are summarized, according to actual needs [37]. Some new technologies based on these samples will be further researched to gain new knowledge [38]. At present, studies on biomass pyrolytic poly-generation for three useful products are lacking, and there are research gaps to be addressed. They can be summarized as follows:
(1)
The relationship between raw materials and multiple products.
(2)
Stable equipment for pyrolytic poly-generation of three useful products.
(3)
Product distribution of pyrolytic poly-generation under a standard process.

2.2. Two Useful Products

The processes for producing two useful products with the third product utilized in the pyrolysis process can be classified into three types: (1) That producing non-condensable combustible gas and bio-oil, while the biochar is used as the catalyst to adjust the operating parameters of the system. (2) That producing biochar and non-condensable combustible gas, while the bio-oil is burned to supply heat for the system. (3) That producing biochar and bio-oil, while the non-condensable combustible gas is burned to supply heat for the system.

2.2.1. Non-Condensable Combustible Gas and Bio-Oil

Non-condensable combustible gas and bio-oil are two main products of biomass pyrolytic poly-generation. Cheng et al. [39] optimized the pyrolysis conditions for Crofton weed for bio-oil and combustible gas production in a resistance furnace. The results demonstrated that the bio-oil mainly consisted of aldehyde, phenol, and ketone, with a yield of 29.35%, and the combustible gas accounted for 82.51% of the biogas. If the main products of biomass pyrolytic poly-generation, including biochar, bio-oil, and combustible gas, can be collected or partially used for system heating or as a catalyst, the process can be considered as poly-generation. Dong et al. [40] obtained high-quality syngas and bio-oil through microwave pyrolysis of moso bamboo over a bamboo-based biochar catalyst. Norouzi et al. [41] produced hydrogen-rich gas and phenolic-rich bio-oil from the pyrolysis of green macroalgae Cladophora glomerata using homologous biochar as the catalyst.

2.2.2. Biochar and Non-Condensable Combustible Gas

There are few reports on the production of biochar and non-condensable combustible gas during biomass pyrolytic poly-generation [42], since bio-oil cannot be avoided during biomass pyrolysis [43]. Additionally, bio-oil is composed of multi-component compounds, making it difficult to utilize for specific goals [44]. Therefore, it is important to explore the effects of the pyrolysis temperature on the physicochemical properties of the gas and biochar obtained from the pyrolysis of crop residues [45].

2.2.3. Biochar and Bio-Oil

In recent years, many studies on the production of biochar and bio-oil have been reported [46]. Biochar and bio-oil were set as the desired products, while the combustible gas was used as the fuel gas for the biomass pyrolytic poly-generation process [47]. Yoder et al. [48] found that the potential revenue of biomass pyrolytic poly-generation is determined by the quantity and quality of the biochar and bio-oil. Hasan et al. [49] developed a novel technology called grinding pyrolysis to improve the bio-oil and biochar yield from Mallee wood. The continuous feeding system consisted of two hoppers connected in series and a shaft-less screw. The reactor was a horizontal cylindrical drum heated by electrical heating mats. The results indicated that grinding has a significant effect on the bio-oil yield and has little effect on the biochar yield. Wang et al. [36] studied the fast pyrolysis of microalgae remnants for bio-oil and biochar production in a fluidized bed reactor. A lab-scale reactor was made from a 316 stainless steel tube with a length of 310 mm and a diameter of 38.1 mm. Two gas cyclones and two condensers were used to collect the biochar and bio-oil, respectively. The yields of biochar and bio-oil produced in this system were 53% and 31% at 500 °C. Nhuchhen et al. [50] used a microwave reactor to carry out wood pellet pyrolysis to produce biochar and bio-oil. The microwave reactor was made in Germany, with a capacity of 3000 W and a frequency of 2.45 GHz. This system could produce biochar with a good higher heating value (HHV) and fine pore size, as well as bio-oil with an HHV in the range of 12–14 MJ/kg. Yue et al. [51] investigated the characterization of biochar and bio-oil obtained from the carbonization of various biomass materials in a Jenkner-type retort reactor. The reactor was made of cylindrical stainless steel with a length of 270 mm and an inner diameter of 130 mm, which was heated externally by an insulated electrical furnace. The maximum biochar yield and bio-oil yield obtained from apricot stone, hazelnut shell, grapeseed, and chestnut shell were 34.65%, 51.53%, 52.98%, and 58.80%, and 35.82%, 41.95%, 49.11%, and 36.11%, respectively.

3. Effect of Factors on Pyrolytic Poly-Generation of Biomass

Pyrolysis is a typical thermal treatment process that is significantly influenced by many factors, including the raw materials, pretreatment methods, and operating conditions [52]. Therefore, understanding the main factors affecting pyrolysis is crucial for optimizing the poly-generation products [53].

3.1. Effects of Raw Materials

Biomass refers to any organic matter produced by living organisms and their metabolites [54]. In general, it consists of cellulose, hemicellulose, and lignin as its three main components [55]. These three components have different thermal stabilities and can be converted into biochar, bio-oil, and combustible gas under different pyrolysis conditions. In comparison, lignin and cellulose are both highly thermally stable due to their heavily cross-linked structures and the long polymers in their glucose units, respectively. On the other hand, hemicellulose is less thermally stable because it is composed of polysaccharides with random amorphous structures and many branches [56]. According to the difference in the volatile content of cellulose (95.5%), hemicellulose (76.8%), and lignin (58.9%), the yield and quality of bio-oil, biochar, and combustible gas are different. Among them, lignin contributes to the main mass of biochar generated from the decomposition of biomass, while cellulose degradation leads to the formation of gases and a small mass of char with high porosity [15]. Chen et al. [57] reported that the pyrolysis mechanism is associated with these three components. Yang et al. [58] studied the pyrolysis characteristics of cellulose, hemicellulose, and lignin and concluded that they have different decomposing temperatures and yield different thermal decomposition products as well. The main thermal decomposition products of cellulose, hemicellulose, and lignin are presented in Figure 3 [59,60,61].
The type of biomass has a significant effect on the pyrolysis products due to the varying proportions of its three main components [62]. Chen et al. [63] found that rapeseed stalk and rice husk are favorable for producing high-quality gas fuel and good carbon-based adsorbent biochar, while cotton stalk and tobacco stem are beneficial for bio-oil production. In addition, the ash in the solid product is composed of some inert soil and active alkali/alkaline earth metals, which also have an impact on the pyrolytic poly-generation process [64]. Among these, the inert soil in biomass is mainly derived from the growth environment during biomass collection and cannot be fully removed even after pretreatment methods, such as washing, acid treatment, or alkaline treatment [4]. As of now, some raw biomass has been used as feedstock in pyrolytic poly-generation. However, there are still some potential raw materials available that have not been studied yet. When designing a pyrolytic poly-generation system, the effect of the raw biomass on product yield and distribution should be fully considered. The typical distribution of the yield products, as affected by the type of biomass and biomass composition at a constant pyrolysis temperature of 550 °C, is presented in Table 2.

3.2. Effects of Pretreatment Methods

Pretreatment is typically employed to alter the physical and chemical properties of the biomass in order to enhance the quality or yield of the syngas, bio-oil, or biochar obtained from pyrolysis [69]. Various pretreatment methods have been reported, including milling pretreatment, pelleting pretreatment, drying pretreatment, microwave pretreatment, ultrasound pretreatment, torrefaction pretreatment, acid pretreatment, and alkaline pretreatment [70,71]. Each of these methods has its own advantages and disadvantages. The main purpose of pretreatment for biomass pyrolytic poly-generation is to change the surface area, remove moisture, or adjust the ratio of the three major components in the biomass [72]. The major pretreatment methods used for biomass pyrolytic poly-generation are presented in Figure 4.
Milling is an essential method for increasing the surface area of raw biomass. Zhang and coworkers [73] reported that ball milling can break cell walls by disrupting the structure of lignin, hemicellulose, and cellulose. It can also change the microstructure of the biomass by adjusting the pretreatment parameters and increasing the surface area through severe destruction. Bai and coworkers [74] studied the effect of milling on wheat straw pyrolysis and found that the particle size, cellulose crystallinity, and morphological structure are the main factors affecting the pyrolysis processes. In addition, the particle size is a key factor affecting the heat flow and mass transfer during biomass pyrolytic poly-generation [75]. Varma and Mondal [76] reported that the particle size has no significant effect on bio-oil, and larger-sized particles favor the biochar yield but decrease syngas production. Similar conclusions were reported by Demirbas [77]. By contrast, pelleting is an effective pretreatment method to densify biomass into pellets with a regular shape for easy transportation and storage. However, pelleting also has the function of changing the biomass particle size through inertial forces, elastic and viscoelastic deformations, and van der Waals forces during the pelleting process [78]. Yang and coworkers [32] compared the effect of particles and briquettes (cotton stalk and rice husk) on pyrolytic poly-generation and found that briquettes favor non-condensable gas and biochar production. For the same raw material, the larger the particle size, the smaller the surface area [79], and vice versa. Therefore, a milling or pelleting pretreatment should be considered according to the processing requirements of the biomass pyrolytic poly-generation.
Drying, microwave, ultrasound, and torrefaction pretreatments are used to remove moisture content from biomass [80]. Among these, drying pretreatment is the most common method for improving the grindability of biomass before pyrolysis [81]. Additionally, vacuum freeze-drying pretreatment is beneficial for the production of high-quality porous biochar by enhancing the pore structure through volume expansion (water–ice transformation) [80]. Microwave pretreatment is a relatively new drying method, with the advantages of being rapid, highly efficient, and economical [82]. However, it also has disadvantages, such as lowering the heating value, energy yield, and elemental carbon content [83]. Ultrasound pretreatment is usually used in fruit processing [84]. It can increase the mass transfer rate and reduce the drying time through high-frequency vibration [85]. Torrefaction is a reliable pretreatment method for feedstock deoxygenation and densification [86]. This approach can partially decompose cellulose and hemicellulose and increase the lignin content in the treated biomass [87]. Wang and coworkers [66] reported that when using wet-torrefied corn stover for pyrolytic poly-generation, the yield of H2 and bio-oil increased, with an increase in the sugar content in the latter, while the yield of biochar decreased, and the porosity of the biochar was enriched. By contrast, Chen and coworkers [88] reported that when using dry-torrefied cotton stalk and corn stalk for pyrolytic poly-generation, the yield of biochar increased, while it improved the HHV of the combustible gas and bio-oil, but the yield decreased. These results indicate that wet and dry torrefaction have different effects on biomass pyrolytic poly-generation. In addition, Zhu and coworkers [89] found that combining a torrefaction pretreatment and co-pyrolysis was a promising way to improve the porous structure of biochar derived from bio-oil distillation residue and walnut shell.
Acid and alkaline pretreatments are employed to adjust the ratio of the three major components of the biomass [90]. Acid pretreatment is used to change the ratio of cellulose, hemicellulose, and lignin by disrupting the lignocellulosic matrix through solubilizing most of the hemicellulosic portion and part of the lignin [91]. Sulfuric acid, phosphoric acid, inorganic acids, and organic acids have been studied for biomass pretreatment. Alkaline pretreatment is used to adjust the three components by solubilizing the lignin and part of the hemicellulose, as well as reducing the cellulose crystallinity [92]. Alkali lignin and native lignin can be obtained by different pulping processes [93]. To some extent, a combined acidic–alkaline pretreatment has a better effect on lignin and hemicellulose removal [94].
Different pretreatment methods have different effects on biomass samples [95]. The pretreatment method should be selected based on the product distribution or quality requirement of the biomass pyrolytic poly-generation [96]. In addition, a detailed comparison of the different pretreatment techniques’ efficiency and the environmental impact of the biomass pyrolytic poly-generation should be conducted to clarify their effects on the overall sustainability of the process as the next step. Finally, the development of new and advanced pretreatment methods for biomass pyrolytic poly-generation is highly needed [97].

3.3. Effects of Operating Conditions

The operating conditions are the key factors that influence the process and product distribution of biomass pyrolytic poly-generation [98]. Zhang and coworkers [15] reported that when the pyrolysis temperature and residence time were 350 °C and 30 min, respectively, using a moving-bed reactor and bamboo chip as the raw material, the yields were approximately 33.3% biochar, 34.1% liquid product, and 34.5% combustible gas. Chen and coworkers [99] reported that when the pyrolysis temperature increased from 250 to 950 °C and cotton stalk was used as the raw material, the biochar yield decreased from 66.5 to 25.2 wt.%, the combustible gas yield increased from 11.8 to 35.71 wt.%, and the bio-oil yield varied with different trends. Similar results were reported by Xia and coworkers [100]. The appropriate operating conditions should be obtained through a systematic optimization of the processing conditions. For biomass pyrolytic poly-generation, the reactor style and pyrolysis temperature/residence time are the most important parameters [101]. The typical optimal operating conditions for pyrolytic poly-generation are presented in Table 3.
An optimized pyrolysis temperature and residence time are associated with the reactor style and raw biomass properties [102]. Generally, bench-scale reactors are designed using external heat, which has the advantage of enabling reaction temperature control. Chen and coworkers [103] made an externally heated, fixed-bed reactor for biomass pyrolytic poly-generation using a vertical stainless steel tube. Then, Chen and coworkers [68] used this set-up to study the effect of the heating rate (10, 30, and 50 °C/min) and pyrolysis temperature (400, 450, 500, 550, and 600 °C) on poplar pyrolytic poly-generation under a nitrogen atmosphere, and found that the optimized conditions were 550 °C and 50 °C/min, 600 °C and 50 °C/min, and 400 °C and 10 °C min for bio-oil, non-combustible gas, and biochar, respectively. In addition, Chen and coworkers [57] studied pine nutshell pyrolytic poly-generation for a temperature range from 300 to 700 °C with a 100 °C interval and a residence time of 10 min, and reported that 500 to 600 °C was the optimum temperature to produce high-quality non-condensable gas, bio-oil, and biochar. Gao and coworkers [101] developed a bench-scale moving-bed reactor by coupling a vertical stainless steel tube with a moving silica sample carrier, as well as a demonstration-scale moving-bed reactor by linking a torrefaction furnace to a pyrolyzing furnace. They studied the influence of temperature (250 °C to 950 °C with a 100 °C gradient) on rapeseed stalk pyrolytic poly-generation and a residence time of 30 min, and concluded that the optimum operating temperature of the moving-bed reactor for balanced yields of biochar, bio-oil, and gas was 650 °C. Chen and coworkers [30] employed a bench-scale fixed-bed reactor (inner diameter of 38 mm and height of 600 mm) to study tobacco waste pyrolytic poly-generation and found that the yield and quality of biochar, bio-oil, and combustible gas varied in different ways when the operating temperature increased from 250 to 950 °C, with a residence time of 30 min. According to the above-mentioned experimental results, the optimum temperature for high-quality biochar without condensable gas and bio-oil was 500–600 °C when pine nutshell was used for pyrolytic poly-generation, and the optimum temperature for high-quality charcoal, gas, and liquid oil was 550–750 °C when cotton stalk was used [99]. According to an economic analysis, the optimum operating temperatures were 450 °C for Chinese chestnut shells and 350 °C for Jatropha curcas shells [100]. To conclude, the optimum temperature for pyrolytic poly-generation varies significantly with the type of biomass. Compared to the heating rate, the pyrolysis temperature is the dominant factor.

4. Catalysts for Pyrolytic Poly-Generation of Biomass

Catalysts are used to lower the activation energy for a reaction and can improve the product quality and distribution from biomass pyrolysis [104,105], and they can potentially be adopted in the pyrolytic poly-generation process [106,107,108]. Eschenbacher and coworkers [109] studied wheat straw gasification in combination with catalytic tar upgrading to co-produce high-quality bio-oil, nutrient-rich char, and producer gas. The results indicated that both HZSM-5/γ-Al2O3 and γ-Al2O3 were effective catalysts, as the tar treatment improved the bio-oil quality. The heating value and revalorization efficiency of the bio-oil increased significantly through the reduction in the oxygen content, total acid number, and basic nitrogen content by the catalysts. Wang and coworkers [110] studied the effect of different catalysts on larch sawdust pyrolysis for phenol-rich bio-oil production and found that HZSM-5 promoted the formation of phenols and enhanced the total phenolic compounds in bio-oils. Maisano and coworkers [111] investigated the catalytic pyrolysis of Mediterranean sea plants for bio-oil production and found that CeO2, NiCe/HZSM-5, and Ni/CeO2 were all effective catalysts for increasing bio-oil yield by promoting deoxygenation reactions. Ozbay and coworkers [112] studied the catalytic pyrolysis of banana peel and found that Al-SBA-15 increased oxygen removal from the bio-oil and also aided the production of desirable products. In addition, Chai and coworkers [113] studied the effect of a catalyst on hydrogen production from the co-pyrolysis of waste plastics and biomass and found that Ni-CaO-C favored H2 production and lowered the CO2 concentration in the gaseous products. Liu and coworkers [114] studied synthesis gas production from microalgae catalytic gasification and found that Fe2O3 could improve the synthesis gas and CaO could improve the CO and H2 production. As far as we know, there is currently no study on catalytic pyrolysis focusing on boosting biochar production. Therefore, catalysts act mainly on the active components of biomass, including cellulose and hemicellulose, which can change the yield and composition of the bio-oil and combustible gas during the pyrolysis process [115].
Chen and coworkers [116] studied the effect of a catalyst on the pyrolytic poly-generation of pinewood and found that the HZSM-5 catalyst had a significant effect on the pyrolysis products, especially bio-oil. This is the only literature on the catalytic pyrolytic poly-generation of biomass. Further studies on catalytic pyrolytic poly-generation are needed to explore the catalytic mechanism for multiple-product distribution. In addition, biomass catalytic pyrolytic poly-generation to produce two useful products has been reported. Mohamed and coworkers [107] used solid additives as microwave absorbers and catalysts during switchgrass pyrolysis and found that K3PO4 and bentonite were beneficial for increasing the BET surface area of the biochar and that SiC favored a decrease in the water content of the bio-oil. Dai and coworkers [105] studied soapstock microwave-assisted co-catalytic fast pyrolysis to produce bio-oil and biochar and showed that the addition of bentonite not only increased the bio-oil yield but also enhanced the BET surface area of the biochar. Shen and coworkers [104] investigated the effects of alkaline-earth-metal additives on biomass pyrolysis for the co-production of bio-oil and biochar and found that MgCO3 was an appropriate additive to enhance the bio-oil and biochar yield. Norouzi and coworkers [41] also found that biochar with a porous structure and an alkaline earth metal was suitable for utilization as a catalyst to promote hydrogen-rich gas and phenolic-rich bio-oil production. Dong and coworkers [40] concluded that bamboo-based biochar favored bio-oil upgrading and high-quality syngas production during moso bamboo microwave pyrolysis. However, the effect of alkali- and alkaline-earth-metal adherents in the biomass and other catalysts on pyrolytic poly-generation of three useful products has not been reported, and it may be an interesting research topic in the field [117]. The effect of different catalysts on the various feedstocks for pyrolytic poly-generation is shown in Table 4.

5. Areas for Future Research

With the depletion of fossil fuels, biomass, as the fourth-largest renewable energy source, will soon become a major energy source. To realize the industrial application of biomass pyrolytic poly-generation, future research should focus on the following aspects:
(1)
Fundamental research on the mechanism of biomass pyrolytic poly-generation: Understanding the detailed chemical and physical processes that occur during biomass pyrolytic poly-generation is crucial. This includes studying the decomposition pathways of cellulose, hemicellulose, and lignin under different pyrolysis conditions and how they contribute to the formation of biochar, bio-oil, and combustible gas. Investigating the interactions between the different components of biomass and the effects of various operating parameters on the pyrolysis mechanism could help to optimize the process and improve product quality.
(2)
Development of advanced biomass pretreatment methods to increase pyrolytic poly-generation efficiency: Continue to explore new pretreatment methods that could enhance the properties of biomass for pyrolysis, For example, developing more efficient drying techniques that could remove moisture while minimizing energy consumption and maintaining the quality of the biomass; investigating the combination of different pretreatment methods to achieve synergistic effects and improve the overall efficiency of pyrolytic poly-generation; and studying the impact of pretreatment on the environmental sustainability of the process, such as reducing waste and minimizing the use of chemicals.
(3)
Ways to obtain desirable ratios of pyrolysis products: Developing strategies to control the ratio of biochar, bio-oil, and combustible gas produced during pyrolysis could involve adjusting the operating conditions, using catalysts, or modifying the biomass composition through pretreatment. An understanding of the market demand for different pyrolysis products and tailoring the process to meet those demands is needed. For example, if there is a high demand for bio-oil for use as a transportation fuel, research could focus on optimizing the process to increase the bio-oil yield while maintaining acceptable yields of biochar and combustible gas. Investigating the potential for the use of the by-products or waste streams from the pyrolysis process to produce additional valuable products could thereby increase the overall economic viability of the process.
(4)
Development of continuous processing systems: The designing and optimizing of continuous pyrolysis reactors that could handle large amounts of biomass and operate efficiently over long periods includes addressing issues such as heat transfer, mass transfer, and reactor stability. Developing integrated systems that combined pyrolysis with other processes, such as gasification or combustion, could maximize energy recovery and minimize waste. Implementing advanced control systems to monitor and adjust the process parameters in real time could ensure consistent product quality and process efficiency.
(5)
Further studies on catalytic pyrolytic poly-generation: Conducting in-depth research on the catalytic mechanisms involved in biomass pyrolytic poly-generation includes identifying the most effective catalysts for different biomass feedstocks and pyrolysis products, as well as understanding how catalysts affect the reaction pathways and product distributions. New catalysts that could selectively promote the formation of desired products while minimizing the formation of unwanted by-products could be developed. For example, catalysts that could enhance biochar production or improve the quality of bio-oil and combustible gas. An investigation of the stability and recyclability of catalysts to reduce costs and environmental impacts could also be performed.
(6)
Reactor scale-up and commercialization: The scaling up of laboratory-scale pyrolysis reactors to industrial sizes while maintaining process efficiency and product quality would require addressing engineering challenges, such as heat transfer, material handling, and process control. Economic analyses could be conducted to determine the viability of biomass pyrolytic poly-generation at different scales. This would include considering factors such as capital investment, operating costs, and revenue from product sales. Collaborating with industry partners to develop commercial-scale pyrolysis plants and bring the technology to market could involve demonstration projects and pilot plants to showcase the feasibility and benefits of biomass pyrolytic poly-generation.
(7)
The application and upgrading of pyrolysis products: Research exploring new applications for biochar, bio-oil, and combustible gas, for example, could include using biochar for soil improvement, carbon sequestration, or as a precursor for advanced materials. The development of methods to upgrade bio-oil to higher-quality fuels or chemicals could involve catalytic cracking, hydrotreating, or other upgrading processes. Investigating the potential for using combustible gas in combined heat and power systems or as a feedstock for chemical synthesis should consider the environmental impacts of pyrolysis products and develop strategies to minimize any negative effects, for example, by ensuring that biochar is used in a sustainable manner and that emissions from combustible gas combustion are properly controlled.

6. Conclusions

Pyrolytic poly-generation is a promising thermochemical process that can simultaneously convert solid biomass into two or more useful products at a relatively low temperature (300–700 °C) and air pressure. The distribution of multiple products (biochar, bio-oil, and combustible gas) can be controlled by altering the biomass types, employing different pretreatment methods, adjusting the processing conditions, and the selection of catalysts. Based on this review, the following conclusions can be drawn:
(1)
Biomass types and their effects on product distribution: ① The ratio of cellulose, hemicellulose, and lignin in the biomass significantly influences the product distribution of the pyrolytic poly-generation. ② A higher cellulose and hemicellulose content favors bio-oil production. ③ Higher amounts of anhydro-saccharides and light oxygenates are beneficial for gas generation. ④ A higher lignin content favors biochar production.
(2)
Impact of pretreatment on biomass: ① The pretreatment of the biomass can change its surface area, reduce moisture content, and adjust the ratio of the three components (cellulose, hemicellulose, and lignin). ② This helps to control the product distribution of pyrolytic poly-generation.
(3)
Processing conditions and their effects: ① The optimized processing conditions for pyrolytic poly-generation are related to the reactor style and raw biomass properties. ② A higher pyrolysis temperature and longer residence time favor combustible gas production. ③ A higher heating rate and shorter residence time favor bio-oil production. ④ A lower pyrolysis temperature and lower heating rate are beneficial for biochar production.
(4)
Flow diagrams for the biomass pyrolytic poly-generation process to produce two or more useful products have been proposed.
(5)
The effects of different catalysts on various feedstocks for pyrolytic poly-generation have been presented.

Author Contributions

Conceptualization, F.M.; methodology, F.M.; software, F.M.; investigation, F.M.; resources, F.M.; data curation, F.M.; writing—original draft preparation, F.M.; writing—review and editing, D.W.; project administration, F.M.; funding acquisition, F.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Educational Department of Anhui Province [grant number 2024AH050323, 2023AH010062], Anhui Science and Technology University [grant number 200341, XK-XJGY001], and Anhui Hengyu Environmental Protection Equipment Manufacturing Co. Ltd. [grant number 881417].

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic diagram of biomass pyrolysis for single product and poly-generation.
Figure 1. Schematic diagram of biomass pyrolysis for single product and poly-generation.
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Figure 2. Flow diagram of biomass pyrolytic poly-generation for three and two useful products.
Figure 2. Flow diagram of biomass pyrolytic poly-generation for three and two useful products.
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Figure 3. The main thermal decomposition products of cellulose, hemicellulose, and lignin.
Figure 3. The main thermal decomposition products of cellulose, hemicellulose, and lignin.
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Figure 4. Major pretreatment methods used for biomass pyrolytic poly-generation.
Figure 4. Major pretreatment methods used for biomass pyrolytic poly-generation.
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Table 1. Product distribution of biomass pyrolytic poly-generation.
Table 1. Product distribution of biomass pyrolytic poly-generation.
Raw MaterialSet-UpHeating EquipmentHeating RateBiocharBio-OilCombustible GasRef.
Rapeseed cakeFixed-bed reactor Electrical furnaceSlow27.40%59.70%12.80%[28]
Corn strawFixed-bed reactorElectrical ovenSlow25%34%41%[29]
Tobacco wasteFixed-bed reactorElectrical furnaceSlow33.30%33.30%33.30%[30]
Corn stoverHorizontal screw conveyer reactorFive-segment electrical furnaceSlow47.88%15.95%36.17%[31]
Poplar and bambooBench-scale vertical reactorFlue gasSlow30%33%37%[15]
Briquetted cotton stalk and rice huskCommercial biomass pyrolytic poly-generation systemHot gasSlow33.30%40.70%26%[32]
Straw and DDGSPilot-scale rotary kiln reactor Flue gas Slow12.7–27.27%41.8–72.73%14.57–30.93%[33]
Palm kernel shellsFluidized bed reactorElectrical furnaceFast24.5–28.9%21.1–23.1%50.0–53.4%[34]
Corn stover and cobsFree-fall fast pyrolysis reactorWatlow ceramic heatersFast25–37%45–55%11–17%[35]
Chlorella vulgaris remnantsFluidized bed reactorClamshell heatersFast31%53%10%[36]
Table 2. Typical product yield distribution with various biomass compositions.
Table 2. Typical product yield distribution with various biomass compositions.
CompositionsProductsRef.
CelluloseHemicelluloseLigninSyngasBio-OilBiochar
Cotton stalk21.98 35.50 29.87 32.40 40.10 27.50 [63]
Rapeseed stalk19.92 37.12 22.10 24.40 48.50 27.10 [63]
Tobacco stem11.78 26.43 18.63 22.10 31.70 46.20 [63]
Rice husk19.00 21.90 27.80 21.70 39.90 38.40 [63]
Bamboo26.10 40.10 30.90 32.20 48.40 19.40 [63]
Corn stalk31.5325.8613.9932.3049.6019.70[65,66]
Pine nut shells41.42 6.90 48.97 27.60 42.80 29.60 [57,67]
Poplar wood49.3122.6724.4926.2241.8331.95[68]
Cellulose100.00 0.00 0.00 45.30 46.10 8.60 [62]
Hemicellulose0.00 100.00 0.00 33.70 40.50 25.80 [62]
Lignin0.00 0.00 100.00 19.60 17.30 63.10 [62]
Table 3. Typical optimal operating conditions for pyrolytic poly-generation.
Table 3. Typical optimal operating conditions for pyrolytic poly-generation.
FeedstockReactor StyleTemperature (°C)Residence Time (min)ProductsRef.
Pine nutshellFixed bed500–60010Non-condensable gas with higher HHV a, bio-oil with lower water content and elevated heating value, biochar with substantial fixed-carbon content and greater specific surface area.[57]
Poplar woodFixed bed550–60010Biochar with surface area of 411.06 m2/g, non-condensable gas with HHV of 14.56 MJ/m3, bio-oil with HHV of 14.39 MJ/kg.[68]
Chinese chestnut shellFixed bed45030The maximum NPV b reached USD 7.28 million/yr.[100]
Jatropha curcas shellFixed bed35030The maximum NPV reached USD 25.315 million/yr. [100]
Tobacco waste Fixed bed65030Non-condensable gas is 33.3%, bio-oil is 33.3%, and biochar is 33.3%.[30]
Bamboo chipFixed bed550–75030Non-condensable gas, bio-oil, and biochar with stable product yield and high quality.[15]
Cotton stalkFixed bed550–75030Non-condensable gas with HHV of 8–9 MJ/m3, bio-oil can be used as platform product in biorefinery, and biochar with HHV of 28 MJ/kg and surface area > 200 m2/g.[99]
Rapeseed stalkMoving bed65030The maximum profit could reach CNY 25 million per year.[101]
Poplar chipMoving bed550–700-Non-condensable gas, bio-oil, and biochar with stable product yield and high quality.[15]
a HHV = higher heating value. b NPV = net present value.
Table 4. Effect of different catalysts on various feedstocks for pyrolytic poly-generation.
Table 4. Effect of different catalysts on various feedstocks for pyrolytic poly-generation.
FeedstockObjectiveCatalystFunctionRef.
Wheat strawGasification tarHZSM-5/γ-Al2O3, γ-Al2O3Upgrading bio-oil via reducing oxygen content, total acid number, and basic nitrogen content.[109]
Larch sawdustPyrolysis vaporHZSM-5Enhancing total phenolic compounds in bio-oils via promoting the formation of phenols.[110]
Sea plantPyrolysis vaporCeO2, NiCe/HZSM-5, Ni/CeO2 Increasing bio-oil yield by promoting deoxygenation.[110]
Banana peelPyrolysis vaporAl-SBA-15Increasing bio-oil and desired products through removing oxygen.[112]
Waste plastics and biomassCo-pyrolysis vaporNi-CaO-CFavoring H2 production and lowered CO2 concentration in the gaseous products.[113]
MicroalgaePyrolysis vaporFe2O3Improving synthesis gas. [114]
MicroalgaePyrolysis vaporCaOImproving CO and H2 production.[114]
PinewoodPyrolysis vaporHZSM-5Improving pyrolysis products, especially bio-oil.[116]
SwitchgrassPyrolysis vaporK3PO4, bentoniteIncreasing BET surface area of biochar.[107]
SwitchgrassPyrolysis vaporSiC Favors decreased water content of bio-oil.[107]
SoapstockPyrolysis vaporBentoniteIncreasing the bio-oil yield and enhancing the BET surface area of biochar.[105]
Rice huskPyrolysis vaporMgCO3Enhancing bio-oil and biochar yield.[104]
MacroalgaePyrolysis vaporAlkaline earth metalPromoting hydrogen-rich gas and phenolic-rich bio-oil production.[41]
Moso bambooPyrolysis vaporBamboo-based biocharFavors bio-oil upgrading and high-quality syngas production.[40]
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Meng, F.; Wang, D. New Strategies for Sustainable Biofuel Production: Pyrolytic Poly-Generation of Biomass. Sustainability 2025, 17, 1945. https://doi.org/10.3390/su17051945

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Meng F, Wang D. New Strategies for Sustainable Biofuel Production: Pyrolytic Poly-Generation of Biomass. Sustainability. 2025; 17(5):1945. https://doi.org/10.3390/su17051945

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Meng, Fanbin, and Donghai Wang. 2025. "New Strategies for Sustainable Biofuel Production: Pyrolytic Poly-Generation of Biomass" Sustainability 17, no. 5: 1945. https://doi.org/10.3390/su17051945

APA Style

Meng, F., & Wang, D. (2025). New Strategies for Sustainable Biofuel Production: Pyrolytic Poly-Generation of Biomass. Sustainability, 17(5), 1945. https://doi.org/10.3390/su17051945

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