Comprehensive Review of Biomass Pyrolysis: Conventional and Advanced Technologies, Reactor Designs, Product Compositions and Yields, and Techno-Economic Analysis

Abstract

Pyrolysis is an environmentally friendly and efficient method for converting biomass into a wide range of products, including fuels, chemicals, fertilizers, catalysts, and sorption materials. This review confirms that scientific research on biomass pyrolysis has remained strong over the past 10 years. The authors examine the operating conditions of different types of pyrolysis, including slow, intermediate, fast, and flash, highlighting the distinct heating rates for each. Furthermore, biomass pyrolysis reactors are categorized into four groups, pneumatic bed reactors, gravity reactors, stationary bed reactors, and mechanical reactors, with a discussion on each type. The review then focuses on recent advancements in pyrolysis technologies that have improved efficiency, yield, and product quality, which, in turn, support sustainable energy production and effective waste management. The composition and yields of products from the different types of pyrolysis have been also reviewed. Finally, a techno-economic analysis has been conducted for both the pyrolysis of biomass alone and the co-pyrolysis of biomass with other raw materials.

1. Introduction

The increasing consumption of fossil fuels has caused significant issues in energy security and environmental pollution. As the global population grows, energy demand continues to increase. Traditional fossil fuels, which currently account for around 80% of the world’s major energy supply [1], are not only finite but also major contributors to greenhouse gas emissions. In 2023 alone, CO₂ emissions derived from fossil fuels reached a staggering 37.4 billion tons [2].The urgent need for sustainable and renewable energy sources is, thus, unavoidable.

Biomass, with its abundant supply and near-zero carbon neutrality, stands out as one of the most promising alternatives to fossil fuels. Lignocellulosic biomass, consisting primarily of cellulose from 15.2% (pistachio shell) to 67.0% (cotton stalk), hemicellulose from 5.2% (sunflower) to 38.2% (pistachio shell), and lignin from 6.5% (rye) to 53.5% (walnut shell) [3], can be a significant contributor to the production of renewable energy. Aquatic biomass, rich in proteins (14.0–65.2%), carbohydrates (3.3–30.2%), and lipids (1.1–51.0%) [4], also offers considerable potential. Biomass can be processed through different ways, such as combustion, physical processes, biochemical processes, and thermochemical processes (pyrolysis, gasification, supercritical fluid extraction, and direct liquefaction). Biomass combustion applications can decrease life-cycle greenhouse gas (GHG) emissions by up to 90% compared to their fossil fuel counterparts in both solid and gaseous bioenergy pathways [5]. At the same time, biomass ash-related problems, such as slagging, fouling, bed agglomeration, and corrosion, are significant issues in the biomass energy sector, primarily due to the formation of low-melting-point compounds such as potassium chloride and potassium sulphate during combustion [6]. Understanding the mechanisms of ash deposition and prevention techniques is crucial to alleviating these issues [7]. Co-combustion of biomass with fossil fuels is an effective technology to reduce greenhouse gas emissions from the heat and power generation sector, especially using local biomass and waste, with existing fossil fuel boilers being retrofitted at low cost to accommodate up to 20% biomass [8]. While co-combustion can significantly reduce nitrogen oxides without increasing carbon monoxide emissions, challenges such as feed type, moisture content, and combustor design need continuous optimization, and the eventual phase-out of coal power plants may limit its long-term potential. In the physical method of conversion, biomass is compressed into solid briquettes, increasing its energy density and making it easier to store and transport [9]. Biochemical conversion methods, including anaerobic digestion and syngas fermentation, break down non-woody organic materials without oxygen, producing stable compounds such as methane and carbon dioxide [10]. These processes primarily utilize cellulose and hemicellulose and face challenges such as the high cost and the difficulty of breaking down cellulosic biomass and efficiently converting sugars into biofuels. Among thermochemical conversion methods, biomass pyrolysis technology could produce different fuels, chemicals, and carbon materials by controlling the pyrolysis conditions. The word pyrolysis is derived from the Greek words “pyro” (fire) and “lysis” (separation), referring to the chemical decomposition of materials at elevated temperatures in the absence of oxygen [11]. Nitrogen is the most frequently chosen carrier gas because of its price compared to that of argon and helium. Unlike photolysis, which involves the breakdown of substances using light, pyrolysis uses heat to break down chemical compounds. During pyrolysis, the organic matrix of biomass decomposes, producing solid (char), liquid (aqueous phase, organic phase, and tars), and non-condensable gas products. Pyrolysis offers rapid conversion of biomass and is especially effective in producing liquid (so-called bio-oil) and char [12]. However, it also presents challenges, including the need to address the high oxygen content and the thermal instability of bio-oils through further processing, such as catalytic pyrolysis [13].

Pyrolysis of biomass continues to attract significant research interest, as evidenced by the annual increase in the number of related publications indexed in the Scopus database, as shown in Figure 1.

In 2014, there were 1072 publications, and this number has increased consistently each year, reaching 2625 publications in 2023. This significant rise suggests that biomass and pyrolysis have become increasingly important topics in scientific research, likely because of their relevance in renewable energy and environmental sustainability. Recently published reviews focused on the main research topics are summarized in

Table 1. Main research topics discussed in the latest published reviews on pyrolysis of biomass.

Main Research Topics	References
Biomass pyrolysis for energy recovery, recent advances, feedstock compositions, techno-economic analyses, and future research paths.	[14]
Pyrolysis reaction mechanisms, process modelling, and challenges (e.g., aerosol and tar formation), enhancing process control and performance.	[15]
Hydrogen production via pyrolysis, polygeneration systems, impact of reaction conditions on hydrogen and chemical formation, and two-stage pyrolysis.	[16]
Role of catalysts in pyrolysis and benefits and drawbacks of catalytic pyrolysis by combining different catalysts for enhanced performance.	[17]
Catalytic fast pyrolysis (CFP), optimization of bio-oil production, challenges in CFP, advanced upgrading methods, and applications of CFP products.	[18]
Production and properties of activated carbon (AC) from biomass pyrolysis, and applications of AC in the adsorption of pollutants and gases.	[19]
Production and applications of char from waste biomass, environmental remediation, engineering aspects, and water treatment.	[20]
Directional pyrolysis based on the element economy (carbon, hydrogen, oxygen, and nitrogen), the production of high-quality fuels, chemicals, carbon materials, and the enviro-economic assessment.	[21]
Advanced technologies for biomass conversion to biofuels, thermochemical and biochemical methods, efficiency improvements, and environmental impact.	[22]

.

Table 2. A comparison of the present review with the previously published reviews on biomass pyrolysis.

Research Topic	[14]	[15]	[16]	[17]	[18]	[19]	[20]	[21]	[22]	This Review
Conventional Pyrolysis	✓	✓	✗	✓	✓	✗	✓	✗	✓	✓
Reactors Design	✗	✗	✗	✗	✗	✗	✗	✗	✗	✓
Advanced Technologies	✓	✓	✓	✓	✓	✗	✗	✓	✓	✓
Product Compositions and Yields	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓
Techno-economic Analysis	✓	✓	✓	✗	✗	✗	✗	✓	✗	✓
Future Outlooks	✓	✓	✓	✓	✓	✓	✓	✓	✓	✓

✓: Topic is covered in the review article. ✗: Topic is not covered in the review article.

provides a comparison of the research topics discussed in this review with the previously published reviews on biomass pyrolysis. As indicated in Table 2, this review is focused on six main research topics. The research gap regarding the designs of reactors dedicated to biomass pyrolysis has been filled in this work.

While numerous reviews on the pyrolysis process have been published in recent years, this paper focuses specifically on biomass pyrolysis. To underscore the unique contributions of our work in relation to previous reviews [14,15,16,17,18,19,20,21,22], we provide a comparative summary in Table 2. A major distinguishing feature of our review is its detailed analysis of reactor designs. Additionally, a significant novel aspect of our study is the proposed classification of pyrolysis types based on defined heating rate ranges. Given the variation in reported heating rate ranges for different pyrolysis types, we propose specific ranges for slow, intermediate, fast, and flash pyrolysis, grounded in the existing literature. Moreover, this paper uniquely addresses the relationship between heating rate and pyrolysis product yields and compositions, underscoring the critical influence of heating rate in optimizing the pyrolysis process.

2. Conventional Pyrolysis

Conventional pyrolysis in the review literature is classified into different types, which can vary depending on the authors criteria: (i) slow, intermediate, and fast [10,14]; (ii) slow, fast, and flash [4,18]; and (iii) slow, intermediate, fast, and flash [17,23,24]. The classification is most often based on commonly used operational conditions such as heating rate (HR), temperature (T), hot vapor residence time (HVRT), and solid residence time (SRT), which refers to the residence time of the sample in the reactor. The operational parameters characterizing individual types of pyrolysis given in the review articles are clearly different, depending on the types of raw materials, types of reactors, and their scale. For example, the HRs reported in °C/s are as follows:

(i)

Slow pyrolysis < 0.17 [10,25]; 0.1–1 [14,26]; 0.08–0.34 [19]; 0.17–0.5 [20].

(ii)

Intermediate pyrolysis 1–10 [14]; 3–5 [27]; 1–100 [28].

(iii)

Fast pyrolysis 10–100 [29]; 10–200 [14,18,26]; 100–1000 [25]; 1000 [20].

(iv)

Flash pyrolysis > 1000 [23,26]; ≤2500 [30]; 1000–10,000 [31].

Figure 2 presents a classification of the four types of pyrolysis, organized based on the HR values listed in

Table 3. Operating parameters of different types of pyrolysis.

Pyrolysis Type	HR, °C/s	T, °C	HVRT, s	SRT, min	PS, mm	CG, -	References
Slow	0.07	400, 500	7200 a	120	<0.5	N2 ᵇ	[34]
	0.07	500	1800 a	30	2	N2 ᵇ	[35]
	0.08–0.33	700	—	71–310	0.075–0.25	N2:H2 (1:1)	[36]
	0.083	600	—	120	0.075–0.15	N2	[37]
	0.083	400	—	740	~2	N2	[38]
	0.083	900	—	180	19	N2	[39]
	0.17	300–500	3600 a	60	<2	N2ᵇ	[40]
	0.17	400, 600, 800	—	30–60	0.7–0.8	N2	[41]
	0.25	300–600	—	1.7	1–2	Ar	[42]
	0.33	500, 700	—	55–65	—	N2	[43]
	–	450–650	1–60	1–20	0.25–1.4	N2	[44]
Intermediate	0.42	500	17 c	—	0.5–1	N2	[45]
	0.5	500	—	—	—	N2	[46]
	0.5, 1, 5	300–550	—	—	0.18–0.25	N2	[47]
	0.55	600	—	78	1.18	N2, CO2, and N2:CO2 (3:1)	[48]
	0.83, 1.33, 1.67	350–600	—	10	0.1–0.2	N2	[49]
	0.83, 2.5, 4.17	400–700	—	—	<1.0	Ar	[50]
	1.2	500	20	7	0.3–0.75	N2	[51]
	1.67	500	<20	1.2	0.5–1.0	N2	[52]
	1.67	400–700	—	9–12	0.4–0.92	N2	[53]
	1.67, 5	400–700	—	10	0.85–1.25	N2	[54]
	3.33	350–500	—	2.5–40	—	flue gas	[55]
	—	450	2	10	—	N2	[56]
	—	350–500	20–40	10	2–4.5	N2	[57]
Intermediate and Fast	1.5–48	500	0.5–0.8	—	2–3	N2	[58]
	5, 8.33, 11.6	400–700	—	10	~0.85	N2	[59]
	5–100	550–800	—	—	0.25–0.43	N2	[60]
	6.67	500	—	1	<0.5	N2	[61]
	10	450–550		3.6	0.2–0.5	N2	[62]
Fast	48	480–790	—	—	<1	He	[63]
	110	500	—	0.072	<2	N2	[64]
	166.7	500	<1	—	0.5–1	N2	[52]
	180	765	0.1	—	—	flue gas	[46]
	200	300–600	0.5–1	0.025	0.175	He	[65]
	300	300–750	0.5–1	—	<1	N2	[66]
	400	400–600	1.2–12	—	0.2	N2	[67]
	1000	400–900	—	—	5	N2	[68]
	—	500	3.3	10	0.09–0.180	N2:air (2:3)	[69]
Fast and Flash	10–3000	350–1400	—	0.017–0.067	0.05–2	N2	[70]
	100, 1000, 5000, 10,000	600	—	0.17	0.4	He	[71]
	600–2200	250–610	0.2–0.32	—	0.84–1	N2	[72]
Flash	2500, 7000	500	0.015–0.025	0.016	0.074	N2	[73]
	3826–4578	439–521	—	—	<2	N2	[74]
	5000	485, 515	0.02	0.083	<0.15	N2	[75]
	5000	485	1	—	1–2	N2	[75]
	10,000	500	—	0.34	0.15	N2	[76]
	11,875	400–500	0.05–2	—	<0.07	N2	[77]
	13,000–21,000	477–627	0.115–0.240	—	0.05–0.07	Ar	[78]
	20,000	520	—	0.25	0.15–0.25	He	[79]
	20,000	300–500	—	—	0.5–1.5	He	[80]

HR—heating rate, T—temperature, HVRT—hot vapor residence time, SRT—solid residence time, PS—particle size, CG—carrier gas. ^a For reactors without removal of the carrier gas during pyrolysis, the value of the HVRT is equal to the SRT. ^b Nitrogen was used to remove air from the reactor before pyrolysis. ^c Calculated as the ratio of the inert gas flow rate to the reactor volume.

, to better systematize the knowledge about pyrolysis types. The data collected in Table 3 do not include experiments using thermogravimetric analyzers, which are usually used for slow, intermediate, and fast pyrolysis [32]. Additionally, certain reactors, such as circulating fluidized bed reactors [33], are commonly used for large-scale biomass pyrolysis. However, it is challenging to estimate the HR of a biomass particle in these reactors. Therefore, most of the results in Table 3 include the HR. The results without specified HRs are still important, as they provide insights into other pyrolysis parameters, such as HVRT and SRT. Standardization of experimental parameters and product yield measurements remains a challenge, necessitating further lab-scale studies to optimize and compare results across different research efforts.

2.1. Slow Pyrolysis

Slow pyrolysis (carbonization) is primarily utilized to generate char as the main product (yield from 30 to 40 wt.%) [24], with bio-oil and non-condensable gases as by-products. It is carried out at temperatures between 300 °C and 900 °C, with a HR < 0.4 °C/s and prolonged solid residence time (SRT) even reaching 12 h [34,35,36,37,38,39,40,41,42,43,44]. The hot vapor residence time (HVRT) value is rarely reported in the literature. The reason for that is that it is not easy to determine the HVRT (even though it is critical in the reaction mechanism occurring in the vapor, which finally determines the amount and composition of produced liquid and gas fraction). However, the HVRT can range from 1 s for reactors with N₂ flow during pyrolysis to 7200 s when N₂ is used only to remove air from the reactor before the pyrolysis process. The particle size (PS) of biomass used in slow pyrolysis can range from 0.075 to 19 mm. Since this pyrolysis has the longest SRT, the large PS is not a problem. Furthermore, all types of biomasses (regardless of moisture content) can be employed in slow pyrolysis.

2.2. Intermediate Pyrolysis

Intermediate pyrolysis has recently gained recognition as a distinct thermochemical technique characterized by a moderate HR (0.4–10 °C/s), short HVRTs (0.5 to 40 s), reaction temperatures ranging from 350 to 700 °C [45,46,47,48,49,50,51,52,53,54,55,56,57], and SRT ranges from 1.2 to 78 min. This process typically produces a liquid yield of 40–50 wt.% [24], non-condensable gases, and solid char, achieving values between those of the products of slow and fast pyrolysis. The bio-oil generated through intermediate pyrolysis stands out due to its reduced viscosity and low tar content, making it suitable for direct thermal applications. Furthermore, the char produced is dry and brittle, which makes it ideal for uses such as biofertilization and gasification. This method also supports a wide variety of feedstocks, including woody biomass, straws, grasses, and agricultural residues, without the need for extensive grinding. The flexibility of intermediate pyrolysis allows larger feedstock sizes and contaminated biomass to be handled, enhancing its applicability.

2.3. Fast Pyrolysis

Fast pyrolysis is an advanced thermochemical process designed to rapidly convert biomass into bio-oil, char, and gaseous products. This process involves heating the biomass to temperatures ranging from 300 to 1400 °C at high HRs of 10 to 1000 °C/s, with a very short HVRT of 0.1 to 12 s and a SRT of 0.017 to 10 min [46,52,58,59,60,61,62,63,64,65,66,67,68,69]. The primary aim of fast pyrolysis is to maximize the yield of bio-oil, which typically constitutes 50–75% in weight [24] of the output, significantly outweighing the yields of char and gases. These yields are achieved by heating the biomass to a temperature at which thermal cracking occurs while minimizing the exposure time that promotes char formation. Key parameters critical for optimizing fast pyrolysis include high heating rates, controlled temperatures around 500 °C, effective removal of char, and dry biomass feed with less than 10 wt.% water content [81]. If the bio-oil has a low pH, it must be upgraded before use. Fast pyrolysis is not only significant for bio-oil production but also has applications in producing food flavors and certain chemicals [82].

2.4. Flash Pyrolysis

Flash pyrolysis represents a highly advanced thermal conversion method capable of swiftly transforming solid organic matter into liquid or gaseous products [83]. Operating at temperatures ranging from 300 to 1400 °C and with an astonishingly rapid heating rate of 1000–21,000 °C/s, biomass particles are exposed to a brief heat pulse lasting merely 0.015 to 2 s [68,70,71,72,73,74,75,76,77,78,79]. This brief exposure induces the rapid decomposition of organic macromolecules, such as lignin and cellulose, releasing volatile compounds that are quickly extracted from the high-temperature zone and rapidly cooled to prevent further secondary reactions. The solid residence time for particles (with sizes 0.05 to 2 mm) is from 0.016 to 0.34 min. Challenges in scaling flash pyrolysis include reactor design to accommodate ultra-short residence times under extreme heating rates, with concerns cantered on bio-oil stability and quality due to catalytic effects of char and ash residues. Despite these challenges, flash pyrolysis holds promise for efficient energy conversion, albeit necessitating advanced technologies for product refinement and pollutant removal, particularly to mitigate the corrosive and unstable nature of the bio-oil produced.

3. Reactors Type and Design

Generally, biomass pyrolysis reactors can be divided based on the (i) mode of operation (batch and continuous) [84]; (ii) source of heat supplied to the biomass (flue gases, inert media, electric heater, microwaves, or solar energy) [30]; (iii) pressure conditions (vacuum, atmospheric, and gas overpressure) [85]; (iv) implemented type of pyrolysis [26,28]; and (v) method of supplying biomass to the reaction zone [86]. In this study (see Figure 3), pyrolysis reactors are categorized, based on how the biomass are moved into the reaction zone, into the following types: pneumatic bed reactors, gravity, stationary bed, and mechanical reactors.

3.1. Pneumatic Bed Reactors

In pneumatic reactors, the movement of biomass within the reaction zone is driven by preheated and pressurized nitrogen or recirculated gas. Pneumatic reactors are particularly suitable for the fast pyrolysis of pre-treated biomass, with previous processes such as grinding and drying. This category of reactors includes bubbling fluidized bed reactors [69], conical spouted-bed reactors [87], circulating fluidized bed reactors [88], and entrained flow reactors [89], as illustrated in Figure 4a–d.

Bubbling fluidized bed (BFB) pyrolizers are simple in construction and operation (Figure 4a), offering efficient heat transfer and accurate temperature control, making them suitable for both laboratory and commercial-scale applications. Typically, an inert gas such as nitrogen, along with sand, is used to fluidize and achieve good heat transfer to biomass particles, which are often smaller than 2–3 mm in size to ensure high heating rates [81]. These reactors maintain a semi-suspended state of particles by using gas velocities above the minimum fluidization velocity (which defines the lower velocity threshold required for the fluidizing medium to achieve fluidization), creating a fluidized bed with uniform mixing and temperature distribution. On the other hand, the maximum effective velocity (MEV) represents the upper gas velocity limit at which bio-oil production is optimized [90]. The fluidization and mixing of sand and biomass particles significantly influence product yields in BFB reactors. The process yields high bio-oil outputs, ranging from 50 to 70 wt.% (based on dry wood) [91], but can result in fine char contamination in the liquid product, necessitating the use of cyclone separators for char removal. BFB reactors are efficient in continuous operation, providing consistent performance and high bio-oil yields, although they require careful management of heat transfer, particularly at larger scales. They are also sensitive to the biomass type, size, and gas velocity for fluidization.

The conical spouted bed (CSB) pyrolizer offers a robust alternative to fluidized bed reactors for biomass fast and flash pyrolysis. As shown in Figure 4b, biomass is introduced into the reactor through a top-mounted opening and nitrogen, regulated by a mass flow controller, serves as the spouting agent. The volatiles that exit the reactor are directed through a cyclone and filter, aided by a forced convection oven, to remove fine char particles. The conical design of the reactor supports continuous solid feeding and removal of char, facilitated by the density difference between the pyrolyzed biomass and the sand [92]. This design allows for efficient handling of larger and irregular biomass particles, enhancing heat and mass transfer rates. By reducing gas residence times, the CSB reactor minimizes secondary cracking reactions, thus optimizing bio-oil yields. Amutio et al. [93] reported that bio-oil yield can be as high as 80 wt.% from biomass pyrolysis. This reactor was successfully scaled up to a pilot plant with a capacity of 25 kg/h [94]. The reactor’s ability to handle a wide particle size distribution and different densities, along with its suitability for continuous operation, underscores its potential for large-scale biomass pyrolysis applications.

Circulating fluidized bed (CFB) pyrolizers, as shown in Figure 4c, are advanced systems used for biomass pyrolysis, distinguished by their operational similarity to fluidized bed reactors, but with notable enhancements. Unlike traditional fluidized bed reactors, which use a single heated reactor, CFB reactors employ two interconnected units: the first for the pyrolysis reaction and the second for combusting char to generate heat. This design allows for the continuous circulation of an inorganic heat carrier, typically sand, which is heated in the second unit and returned to the first to sustain the pyrolysis process. Operating at high gas velocities, CFB reactors achieve short biomass residence times, promoting efficient gas-solid contact and superior mixing, thereby enhancing temperature control and reaction uniformity. Despite their complexity and higher energy requirements, CFB reactors offer significant advantages, including high biomass throughput, effective use of byproduct char and non-condensable gases for energy recovery, and improved bio-oil yields. CFB reactors, widely used in the oil industry, demonstrate potential for high-capacity biomass processing with pyrolysis oil yields of up to 78 wt.% [33].

In an entrained flow reactor (EFR) used for biomass pyrolysis, dried biomass is typically utilized. Biomass is introduced into the reactor through a feeder (Figure 4d), which regulates the feeding rate of the particles. The primary gas (also called transporting gas), is preheated in a preheater, transports the biomass particles into the reactor. The velocity of the biomass particles is governed by the flow rate of this transporting gas. Upon entering the reactor, the biomass particles pass through a heated zone where the pyrolysis reaction occurs. To prevent premature contact between the biomass particles and the hot gases within the reactor, a secondary gas stream (typically nitrogen) stream is introduced through the hopper and screw feeder. The EFR system is also equipped with a pressure control mechanism to maintain optimal operating conditions [95]. Unlike fluidized bed reactors, EFRs are designed to handle processes at temperatures as high as 1500 °C [96]. They also achieve exceptionally high heating rates, up to 10,000 °C/s [97], making them particularly effective in laboratory-scale applications. Typically, the HVRT in these reactors is around 5 s [89].

3.2. Gravity Reactors

A cost-effective and straightforward approach to achieve the continuous movement of substrates and products within a pyrolysis reactor is to harness the force of gravity. In a continuous pyrolizer, gravity facilitates the downward flow of shredded biomass particles from the feeder, through the reactor, and into the char collection chamber. The rate of settling of these particles within the reactor is influenced by their size and density. Low-density biomass particles tend to settle more slowly, particularly under high-temperature pyrolysis conditions, where convective heat currents oppose gravitational forces.

The group of gravity reactors includes free-fall reactors [98] and drop-tube reactors [99]. The differences between the designs of these reactors are insignificant and Figure 5 shows an example diagram of a free-fall reactor.

Gravity reactors are suitable for fast and intermediate pyrolysis. Biomass is supplied to these reactors by means of a screw feeder [100] or a synergic pump [101]. To prevent the biomass feeder from overheating, the upper part of the reactor is equipped with a water cooler. Typically, two streams of carrier gas are used: a primary (cold) stream flowing through the feeder and a secondary (heated) stream at a set temperature. Biomass particles descend through the reactor by gravity. The lower part of these reactors may be equipped with a char catch [98] or impactors [101]. In these types of reactors, up to 72 wt.% [102] are bio-oil. However, there are some challenges, including temperature gradients and limited reactor capacity due to their relatively low volume. These reactors are mainly laboratory-scale installations with biomass feed rates of up to 2 kg/h [100,102]. Additionally, it should be noted that the drop-tube reactor can be integrated with other systems, such as a fixed bed reactor [103], to improve the efficiency of biomass conversion.

3.3. Stationary Bed Reactors

Stationary bed pyrolizers are widely used in laboratory-scale studies because of their simplicity and effectiveness. In these reactors, a slow flow of inert gas is employed, typically 50 to 300 mL/min, to ensure that the biomass material remains stationary within the reactor. Depending on the specific design, the biomass sample can be placed directly in the reactor (as shown in Figure 6a–c), on a ceramic or metal boat, or in a layer of mineral wool (as shown in Figure 6d).

The stationary bed reactors are employed for slow [35], intermediate [51], and fast pyrolysis [104]. Batch reactors (BR) operate according to the principle of a closed system, where there is no inflow or outflow of materials during the pyrolysis process. This means that the biomass is loaded into the BR and the reactor is sealed before pyrolysis begins. After the pyrolysis process, the char produced is cooled together with the reactor [105]. As a result, condensing gases may settle on the char. To ensure a uniform temperature distribution, these reactors are usually relatively small, e.g., 1 dm³ [105]. One of the key benefits of a BR is its ability to achieve high conversion rates by allowing materials to remain in the reactor for extended periods (up to several hours), ensuring complete pyrolysis. However, BRs also have several challenges: (i) the volume of biomass that can be processed at a time is limited, reducing overall productivity; (ii) manual loading, unloading, and cleaning of the reactor between batches requires significant labor; (iii) operational costs and scaling up of BRs for industrial purposes.

Compared to BRs, semi-batch reactors (SBRs) (Figure 6b) are frequently used for biomass pyrolysis because of their enhanced process control and product separation capabilities. In SBRs, solid biomass feedstock remains within the reactor throughout the pyrolysis process [106], or is continuously removed [40,107]. The gases produced by the pyrolysis are also continuously removed. This continuous removal of gases allows for the effective separation of bio-oil from the non-condensable gases, optimizing the yield and purity of the desired products. These reactors are effective for performing slow [40] and intermediate [106] pyrolysis.

A variation of a SBR is the continuous reactor shown in Figure 6c. Its main advantage is the continuous feeding of biomass and removal of char [107]. These reactors perform well on a pilot scale [108]. At the bottom of the bed, the char is manually removed, while the gases pass through a heated pipe to a post-combustion chamber. At the reactor outlet, gaseous products travel through an electrically heated line to a post-combustion unit, passing through a cyclone that separates fine particles.

The design and construction of fixed bed reactors (FBRs) are relatively straightforward, and they can efficiently process feedstock of different sizes. As shown in Figure 6d, FBRs are available in two configurations, horizontal [109] and vertical [110]. The FBR is a commonly used laboratory-scale pyrolizer for biomass, which operates cyclically. The reactor’s operating cycle is influenced by several factors, including the reactor and sample size, biomass particle size, temperature, and the method used for char removal and cooling. For example, a typical pyrolysis cycle in a horizontal FBR with a 1.5 g biomass sample at 500 °C lasts approximately 30 min [51]. This includes 5 min of nitrogen purging, 7 min of pyrolysis at 500 °C, and approximately 17 min of sample cooling. A two-stage FBR is an advanced version of the traditional FBR [111]. Investigations using two-stage reactor include a wider range of studies, such as those examining co-pyrolysis of biomass with another material in separate beds and those on catalytic pyrolysis, where the catalyst and biomass are not mixed together [112]. In addition, designs with multiple heating zones enable the selection of different temperatures for each bed.

3.4. Mechanical Reactors

Mechanically driven pyrolysis reactors represent a distinct category in which the contact between biomass particles and the heating agent is facilitated by mechanical means. Various mechanisms are employed to move biomass within these reactors, including rotating cones (Figure 7a), stirrers (Figure 7b), screws (Figure 7c), rotating disks (Figure 7d), and rotary motion of the entire reactor (Figure 7e).

The rotating cone reactor is a device designed for efficient biomass pyrolysis through intense mechanical mixing of biomass and a heating material, typically sand (Figure 7a). The process begins when the biomass and sand are fed separately into the reactor, where they first encounter an impeller at the base of the heated reactor cone [91]. The biomass and sand mixture then spirals upwards along the cone wall due to the rotary action, ensuring thorough mixing and uniform heat transfer. When the mixture reaches the top of the reactor, the mixture of char (produced during the pyrolysis) and sand exits the reactor. The char is then combusted, and the sand is reheated and recirculated back into the reactor. This design eliminates the need for a carrier gas, and obtains tar yields of even 70 wt.% on a dry biomass basis [113]. Additionally, the reactor supports a high throughput capacity and rapid heating, which minimizes cracking reactions and enhances bio-oil yield. In the Hengelo plant in the Netherlands, 3200 kg of pyrolysis oil is produced per hour through fast pyrolysis [114].

Figure 7b shows a stirred bed reactor (SBR) in vertical operation mode. In an SBR, the biomass is continuously introduced into the reactor at a controlled feed rate. The reactor operates at a temperature of, typically, around 500 °C with an inert gas, such as nitrogen, flowing through the system to ensure an oxygen-free environment. For non-catalytic experiments, the SBR contains pure sand as the bed material, while for catalytic experiments, a mixture of sand and catalyst is used [115]. During the process, the biomass is rapidly heated, and the volatile components are released and collected as pyrolysis oil. The reactor operates continuously, with a typical run time of 60 min, to ensure complete devolatilization of the biomass and to produce sufficient pyrolysis liquid for analysis [115]. In turn, horizontal SBRs are significant because of their adaptability to a wide range of particle diameters, effective sealing, and precise control over temperature and residence time [116]. In this typology of pyrolizer, biomass particles are introduced at one end of the reactor and gently agitated by a horizontal agitator as they gradually progress towards the reactor out.

The auger reactor (AR), also known as the screw (see Figure 7c), is a prominent choice both for lab-scale (<1.0 kg of biomass per hour [117]) and large-scale pyrolysis plants (50 tons per day [118]). In this reactor, biomass is continuously fed into a single- or twin-screw system, where the rotating auger conveys the biomass along the heated axis of the reactor [119]. As biomass moves through the heating zone, it is decomposed into gases, organic volatiles, and char, with gases exiting the AR and char being collected at the bottom. The residence time of the biomass within the reactor can be precisely controlled by adjusting the screw rotation speed or flight pitch, allowing for optimized pyrolysis conditions [120]. The highest yield of bio-oil reported by Brown and Brown [121] was 73.6 wt.%. The ARs have longer vapor residence times compared to FBRs, which favor secondary reactions and char formation. In order to mitigate this, some designs incorporate gas exits along the reactor wall to reduce vapor residence time and use small particulate heat carriers such as hot sand or steel shot to enhance heat transfer [121,122]. ARs remain attractive for their effective particle mixing, controlled thermal exposure, and suitability for various pyrolysis processes, including slow, intermediate, and fast pyrolysis. Twin-screw configurations further enhance mixing and prevent material buildup, making the technology robust and efficient for biomass pyrolysis [123].

The ablative pyrolysis reactor (Figure 7d) facilitates pyrolysis of biomass by pressing biomass particles against a hot surface under high pressure and centrifugal force, allowing effective heat transfer. Unlike traditional pyrolysis reactors, which rely on indirect heat transfer, the ablative method involves direct contact between the biomass and the heated reactor disc, creating a mechanism similar to pressing frozen butter against a hot pan [124]. This direct contact leads to rapid surface renewal and high heat transfer rates, enabling the pyrolysis of larger biomass particles (35 × 200 mm) [125]. The process is characterized by high efficiency and control, producing bio-oil yields up to 81 wt.% [126]. However, it is limited by the thermal conductivity of biomass, which can be mitigated by using larger biomass particles and optimizing the temperature and heating rates [127]. Although mechanically complex and potentially costly at larger scales, ablative pyrolysis offers improved controllability and reduced equipment size, making it a promising method for bio-oil production from diverse biomass sources [128].

Rotary kiln reactors (RKRs) are pivotal in biomass pyrolysis due to their versatility and efficiency in handling diverse feedstocks. The diagram of an RKR is shown in Figure 7e. Primary types of rotary kilns, both internally and externally heated, offer various heating methods, including steam, gas, and electrical systems, tailored for specific pyrolysis needs [129,130]. For example, the Haloclean reactor operates at temperatures between 320 and 500 °C, providing short residence times for solid residues and variable times for gases and pyrolysis liquids, facilitated by an internally heated, nitrogen purged screw and metal spheres for enhanced heat transfer [131]. RKRs typically feature a slight downward angle, ensuring uniform axial temperature distribution and continuous operation, making them ideal for industrial applications [132]. Their design eliminates the need for feedstock particle reduction, accommodating a wide range of biomass sizes and shapes. Rotary kilns also simplify operational control and improve energy efficiency, as highlighted by recent advancements that aim to integrate capacity and energy consumption for optimal performance. Studies have demonstrated their efficacy in various contexts, from slow pyrolysis of olive stones [133] to torrefaction of wood [134], underscoring their gentle mixing capabilities that improve the flowability of the char and reduce the oxidation. This makes rotary kilns a preferred choice for both standalone pyrolysis and as an intermediary stage in multistep gasification, significantly contributing to char production and thermal treatment of particulate solids.

4. Advanced Technologies

Recent advancements in pyrolysis technologies aim to improve the efficiency, yield, and quality of pyrolysis products, contributing to sustainable energy production and waste management. This section provides a comprehensive analysis of the most advanced biomass pyrolysis technologies, including co-pyrolysis, catalytic pyrolysis, microwave pyrolysis, hydrothermal pyrolysis, and plasma pyrolysis. Each technology is examined in terms of its mechanisms, advantages, and potential applications.

4.1. Co-Pyrolysis

Co-pyrolysis involves the simultaneous pyrolysis of biomass with another material, such as plastics or coal [135]. This method enhances the quality and yield of the pyrolysis products, without the addition of a catalyst, solvent, or large amount of hydrogen, which can be expensive. For instance, co-pyrolysis with plastics can increase the production of liquid hydrocarbons, making the process more economically viable. Co-pyrolysis would allow the production of stable pyrolysis bio-oils from several feeds, which, in many cases, cannot be achieved separately. For example, oils derived from biomass pyrolysis cannot be fully blended with those produced from plastic fractions, which, after a while, end up generating several phases. The synergistic effects observed in co-pyrolysis can lead to improved thermal degradation and product distribution. Studies have shown that co-pyrolysis can significantly enhance the calorific value of the resulting bio-oil and reduce the oxygen content, thereby improving its stability and energy density. The high oxygen content in bio-oil poses significant challenges for its use as a substitute for fossil fuels, as it leads to a lower calorific value, increased corrosion issues, and instability [136].

The mechanism of co-pyrolysis involves complex interactions between the biomass and the co-reactant. The presence of plastics, for example, can provide additional hydrogen, which helps in the deoxygenation of bio-oil, leading to higher yields of hydrocarbons. The process parameters, such as temperature, heating rate, and the ratio of biomass to co-reactant, play crucial roles in determining the efficiency and product distribution of co-pyrolysis. Additionally, co-pyrolysis can be easily applied to existing pyrolysis plants with minimal modifications, making it a cost-effective option for up-grading biomass pyrolysis systems.

4.2. Catalytic Pyrolysis

Catalytic pyrolysis employs catalysts to improve the quality and yield of produced bio-oil and gas. Metallic minerals present in biomass can function as natural catalysts, influencing the pyrolysis products. Specific catalysts such as zeolites, metal oxides, and mesoporous materials can be employed to reduce the oxygen content in bio-oil, resulting in higher-quality fuel. This method also allows for the selective production of valuable chemicals. The use of catalysts can enhance the deoxygenation, cracking, and reforming reactions during pyrolysis, leading to the production of hydrocarbons and other valuable chemicals [137].

The catalytic treatment can be done ex situ or in situ. Taking into account only the in situ option, there are also several ways of implementation. The catalyst can be directly introduced in the reactor where the biomass is pyrolyzed. This option simplifies the catalytic pyrolysis process, and it could reduce significantly the decomposition temperature of the biomass components [137]. The disadvantage is that it makes more complicated the separation of the catalyst from the remaining pyrolysis solid, and, therefore, its reuse. In addition, the operating parameters used in the pyrolysis reactor have to be somewhere between those best suited for pyrolysis and those best suited for catalysis.

In case the catalytic treatment is done online in a second stage [138], only the volatiles generated from the pyrolysis would be treated. This will allow a longer use of the catalyst. Different batches of biomass can be pyrolyzed while maintaining the same catalytic bed. Furthermore, the operating conditions can be optimized separately for both pyrolysis and volatile treatment, depending on the type of catalyst used, the type of solid desired, the type of liquid, and the target gas for the process. In fact, the catalyst used and the operating parameters employed are key aspects for the yields and quality of pyrolysis products obtained [139].

Recent advancements in catalytic pyrolysis have focused on developing novel catalysts with high activity, selectivity, and stability. For instance, modified natural zeolites have shown promising results in enhancing the yield and quality of bio-oil. The catalytic activity of these materials can be further improved through thermal and acid activation processes. The catalytic activities of the acid catalysts are highly dependent on acid properties, particularly, the acid strength, acid type, and number of acid sites [135]. ZSM-5 zeolite has been widely employed in as-received or in modified way to create a bio-oil with more olefins and more aromatic compounds [140,141].

The integration of catalytic pyrolysis with other processes, such as hydrodeoxygenation (HDO), can further enhance the quality of the bio-oil, making it suitable for use as a transportation fuel. However, challenges such as high coke production, stability, catalyst deactivation, and regeneration remain significant hurdles. Coke generation is the major deactivation mechanism of zeolites, reducing their lifetime. This coke could also increase the pressure drop across the bed in case a fixed bed reactor is used, which is the simplest and more versatile catalytic reactor type [142]. There are, nowadays, technological solutions to the rapid deactivation by coke, such as the continuous regeneration of catalysts in the Fluidized Catalytic Cracking (FCC) process of oil refining. This regeneration is an essential step due to the formation of coke, which rapidly deactivates the catalyst. However, these solutions are complex in design, operation, and maintenance as well as costly.

4.3. Microwave Pyrolysis

Microwave pyrolysis utilizes microwave radiation to heat the biomass. This method offers several advantages, including rapid heating, uniform temperature distribution, and higher energy efficiency. Microwave pyrolysis can produce high-quality biochar and bio-oil with lower energy consumption compared to conventional methods. The microwave heating mechanism involves the direct interaction of microwave energy with the biomass, leading to efficient and uniform heating. Microwave dielectric heating can have two types of effects: thermal and non-thermal. Thermal effects are due to different temperature regimes, created due to microwave heating. Non-thermal effects (e.g., changes in reactivity and selectivity or acceleration) refer to circumstances where the result of a synthesis performed under microwave conditions differs from the result with conventional heating at the same temperature. However, the existence of specific (non-thermal) effects of microwaves is a rather controversial scientific issue at present [143,144,145].

The operating conditions used are critical in the case of using microwaves as a heating medium for the biomass to be pyrolyzed. Particle size will be key in the energy transfer achieved, as well as in the composition of the biomass itself. Water is often the main constituent of the biomass that absorbs microwaves, so a minimum moisture content may be necessary. This lack of moisture can be overcome by adding heating aids (microwave adsorbers) [146]. The different heating characteristics of the lignocellulosic biomass components mean that the components themselves show different decomposition under microwave-assisted heating and conventional electrical heating [147]. For instance, if microalgae biomass were used instead of cellulosic biomass, microwave pyrolysis would be limited due to poor absorption of microwave irradiation [148]. To overcome this limitation, studies have mixed algae with sorbents such as metal oxides, activated carbons, and coals to achieve high temperatures during the process [149,150]. This technology has shown potential for scaling up and integrating with other processes for enhanced biomass conversion.

4.4. Hydrothermal Pyrolysis

Hydrothermal pyrolysis involves the thermal decomposition of biomass in the presence of water at high temperatures and pressures [151,152]. This method is particularly effective for wet biomass and can produce bio-oil with a high energy density. It also allows for the recovery of valuable nutrients from the biomass. The hydrothermal conditions facilitate the breakdown of complex biomass structures, leading to the formation of bio-oil, biochar, and aqueous phase products. This technology is advantageous for processing high-moisture feedstocks and can be integrated with nutrient recovery systems.

4.5. Plasma Pyrolysis

Plasma pyrolysis employs plasma arcs to generate extremely high temperatures, which can efficiently break down biomass into syngas and biochar [153]. This method is highly efficient and can handle a wide range of biomass feedstocks, including those with high moisture content. The plasma arc generates reactive species that facilitate the rapid and complete decomposition of biomass. Plasma pyrolysis has shown promise for waste-to-energy applications and the production of high-purity syngas.

The mechanism of plasma pyrolysis involves the generation of a plasma arc, which creates a highly reactive environment with temperatures exceeding 10,000 °C. This extreme temperature facilitates the complete breakdown of biomass into its constituent elements, primarily carbon monoxide, hydrogen, and carbon dioxide [154,155]. The high energy density of the plasma arc ensures efficient conversion of biomass, even with high moisture content. Plasma pyrolysis systems are typically compact and can be integrated with other waste treatment processes, making them suitable for decentralized waste management.

5. Yield and Composition of Products

The composition and yield of pyrolysis products depend on several factors, including the type of biomass, heating rate (HR), hot vapor residence time (HVRT), solid residence time (SRT), particle size (PS), temperature (T), and the type of reactor used. This section presents the compositions and yields of bio-oil, char, and non-condensable gases from the studies listed in Table 3. The pyrolysis products (Figure 8a–c, Figure 9a,b and Figure 10a,b) were analyzed based on the HR and were obtained exclusively at a temperature of 500 °C. To ensure uniform results for condensed products (char and bio-oil), the compositions are given as ash-free, and in the case of pyrolysis gases, as nitrogen-free. It is important to note that although many experimental studies on biomass pyrolysis are available, they often do not report the HR. This is because it is difficult to measure or estimate this crucial pyrolysis parameter in reactors such as FBR [44], AR [57], BFB [69,156], SBR [106], RKR [134], and CSBR [157].

5.1. Bio-Oil

Bio-oil (also called biocrude) is one of the biomass pyrolysis products. Its dark-brown liquid inherently contains a significant aqueous phase, with a water content ranging typically from 20% to 40% of the feed for slow pyrolysis [158]. This water fraction is influenced by the initial moisture content of the feedstock, which is generally air-dried and commonly limited to a maximum of 10% to optimize the fast pyrolysis yield [159]. During the pyrolysis process, both the moisture presented in the biomass and the water generated as a byproduct contribute to the final bio-oil composition. Notably, while pyrolysis liquids can tolerate some water, there is a critical threshold beyond which phase separation occurs, resulting in a liquid that is no longer miscible with water. One of the ways to reduce the water content in bio-oil and reduce its oxygen content is co-pyrolysis of biomass with plastic waste [160] or end-of-life tires [112]. As a result of co-pyrolysis with plastics, the water and oxygen content in bio-oils can be approximately 2% and 4%, respectively [160].

Figure 8a illustrates the bio-oil yield in mass percentage (included water) as a function of the HR obtained at 500 °C.

The results indicate that for slow pyrolysis, the bio-oil yields range from 17.5 wt.% [42] to 45 wt.% [43]. For intermediate pyrolysis, operating at HRs between 0.4 °C/s and 10 °C/s, the bio-oil yield varies from 29 wt.% [59,61] to 75% [58]. The highest yield of 86% was achieved during flash pyrolysis at a heating rate of 2500 °C/s in a wire-mesh reactor [73]. In general, Figure 8a shows a positive correlation between heating rate and bio-oil yield, suggesting that increasing HR promotes bio-oil production when the same type and fraction of biomass are used.

In addition to yield, the composition of bio-oil is crucial because it determines its potential applications. Therefore, Figure 8b presents the molar ratios of hydrogen to carbon (H:C) and oxygen to carbon (O:C) in bio-oil. The H:C ratio ranged from 1.34 [40] to 1.89 [54], while the O:C ratio ranged from 0.08 [54] to 0.58 [49]. A higher H:C ratio generally indicates a higher hydrogen content, which can be beneficial for certain applications. Bio-oils with a higher H:C ratio tend to have better fuel properties because they are more similar to conventional hydrocarbons found in petroleum fuels [161]. As reported by Annamalai et al. [162], the H:C and O:C ratios are 2.25 and 0 in gasoline, 1.92 and 0 in diesel fuel and 1.9 and 0.1 in biodiesel, respectively. The molar ratios of H:C and O:C in pyrolysis bio-oils [50,54] are consistent with the restrictive requirements for biodiesels.

A lower O:C ratio indicates a lower oxygen content, which is desirable because a high oxygen content in bio-oil leads to poor stability, high acidity, and low energy density. Therefore, minimizing the O:C ratio is essential to produce high-quality bio-oil with improved properties and a greater applicability [163].

The HHVs of the bio-oils highly depend on the biomass feedstocks used. For instance, in [54], with safflower seed used for bio-oil production, its HHV could achieve 40.9 MJ/kg, while in the case of using residues from cassava farming, the HHV was of less than 20 MJ/kg [62].

Bio-oil contains various organic compounds such as acids, alcohols, aldehydes, alkanes, alkenes, aromatics, ketones, nitriles, and phenols [35,40]. Of the functional groups mentioned, the relative share of phenols, organic acids, ketones, and aromatics (mainly benzene, toluene, and xylene) is shown in Figure 8c. Bio-oil can contain up to 54% of phenols [49]. Phenols and other aromatic compounds are mainly produced by breaking down lignin in biomass and by reforming and polymerizing light compounds [164]. The highest aromatic content for noncatalytic pyrolysis at 500 °C was 20% [35]. Enhancing the production of aromatics requires the use of catalysts [165]. Ketones and organic acids in bio-oils are produced through complex reactions involving monomer sugars that form when cellulose and hemicellulose are broken down by hydrolysis [166]. As can be seen in Figure 8c, the percentage of acids was always higher than that of ketones for bio-oils from fast and flash pyrolysis. The presence of acids determines the acidity of bio-oils, 2.5 ≤ pH ≤ 5.3 [35,45,61].

5.2. Char

The determination of the char yield is a fundamental aspect of the characterization of the pyrolysis process. It is well established that the char yield decreases as both the process temperature and the heating rate increase [167]. Figure 9a illustrates the char yields for all types of pyrolysis. For slow pyrolysis, the yield was typically below 50%, ranging from 29% (with HR of 0.25 °C/s, PS ≤ 2 mm, and SRT of 1.7 min) [42] to 45% (with HR of 0.07 °C/s, PS of 2 mm and SRT of 30 min) [35]. This indicates that, in addition to the type of biomass and reactor used, SRT significantly influences the char yield. In intermediate pyrolysis, the char yields varied from 13% (with HR of 4.16 °C/s, PS < 1 mm and unspecified SRT) [50] to 47.5% (with HR of 6.67 °C/s, PS < 0.5 mm and SRT of 1 min) [61]. The most divergent char yield values were observed for fast and flash pyrolysis. The lowest char yield found was 5% (with HR of 7000 °C/s, PS < 0.074 mm and SRT of 0.016 min) [73], while the highest was 44.5% (with HR of 48 °C/s, PS < 1 mm and unspecified SRT) [63].

Based on the analyzed literature [34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79], the carbon (C) content in char varies significantly depending on the type of biomass. A high ash-content biomass would certainly decrease the carbon content in the char. The char contains, approximately, from 37.06% C [74] up to 93.9% C [43]. According to the classification by the International Biochar Initiative [168], these correspond to Class 2 chars (which have more than 30% carbon) and Class 1 chars (which have more than 90% carbon). The molar H/C ratio can be used to predict the degree of aromatization and the stability of char [169]. Ratios above 0.7 indicate the presence of non-pyrolytic carbon [170]. If the H/C ratio is below 0.4, it suggests that the biochar was produced at temperatures over 500 °C for more than 10 min, making it safe for animal consumption. The H/C ratio is also a key indicator of how long biochar remains stable [171]. The stability of biochar carbon in soil is crucial for it to be effective in preventing further climate change. Essentially, assessing biochar stability involves studying what happens to biochar once it is added to soil, specifically measuring how much carbon is eventually released back into the atmosphere as carbon dioxide. The reported higher heating value (HHV) of char ranges from 19.9 MJ/kg [74] to 32.5 MJ/kg [68].

5.3. Non-Condensable Gas

When biomass is subjected to pyrolysis, volatile compounds are released from the biomass particles, leading to the formation of non-condensable gases such as carbon dioxide (CO₂), carbon monoxide (CO), hydrogen (H₂), methane (CH₄), ethene (C₂H₄), ethane (C₂H₆), propene (C₃H₆), and propane (C₃H₈). The composition and yield of non-condensable gases are significantly influenced by various parameters, including T, HR, PS, VRT, SRT, pre-treatment methods (physical, chemical, and biological), vapor condensing temperature, and the reaction environment. Typically, the gas yield is calculated by subtracting the combined mass of bio-oil and char from the initial biomass mass. Higher pyrolysis temperatures generally increase both the mass yield and energy yield of non-condensable gases [172]. As the pyrolysis temperature increases, the cleavage of glycosidic bonds accelerates, enhancing the evaporation of the products and favoring the formation of gaseous products [173]. Lower pyrolysis temperatures tend to produce higher amounts of CO and CO₂, while higher temperatures result in increased CH₄ and H₂ content [17]. Additionally, the cellulose, hemicellulose, and lignin content in biomass plays a crucial role in the production of non-condensable gases. Cellulose primarily promotes CO formation, hemicellulose leads to increased CO₂ production, and lignin generates the highest levels of CH₄ [174].

The results of the literature review presented in Figure 10a show the yield of non-condensable gases from various types of biomass at 500 °C. Yields varied, ranging from 13% [35] to 52% [42] in slow pyrolysis, from 12% [46,50] to 36% [52] in intermediate pyrolysis, and from 8% [73] to 55% [66] in fast and flash pyrolysis. The absence of a clear trend in the yield of non-condensable gases with varying HRs indicates that other factors play a significant role in pyrolysis. These factors include the type of reactor, particle size, moisture content, biomass type and composition (including cellulose, hemicellulose, and lignin content), and the carrier gas used. For future literature reviews, it is essential to consider not only the constant pyrolysis temperature but also the type of pyrolizer used, the biomass PS, VRT, and SRT.

Figure 10b shows the four most identified gases in the composition of non-condensable gases as a function of the heating rate. Across the range of HRs evaluated, CH₄ typically has the lowest content in the non-condensed gases, while CO₂ or CO generally have the highest contents. The HHV of the obtained gases ranges from 5.7 MJ/m³ [74] to 14.6 MJ/m³ [66].

Using the gas yield data (Figure 10a) and the gas composition percentages (Figure 10b), the production of CO, CH₄, and H₂ was calculated in cubic centimeters per gram of raw material. The highest CO production, CH₄ production, and H₂ production was 272 cm³/g, 77 cm³/g, and 47 cm³/g, respectively [66]. To increase the H₂ production, catalysts and steam reforming are required, which can increase the hydrogen output of biomass to up to 634 cm³/g [175].

6. Techno-Economic Analysis

Techno-economic analyses have become increasingly common in recent years, particularly in the study of biomass pyrolysis, both as a standalone process and when combined with other feedstocks.

Table 4. Techno-economic analysis of biomass pyrolysis and co-pyrolysis with other raw materials.

Pyrolysis Type	Biomass or Raw Material	Feed Rate	Reactor Type	Payback Period in Years	Economic Analysis Findings	Cost of Bio-Char/Bio-Oil	Refs.
Slow	Cassava branches (CB) and peels (CP)	2.9 t/h (CB) 0.2 t/h (CP)	Muffle	5 (CB) 8 (CP)	Economic feasibility is achieved with processing scales above 3.4 t/h. Competitive selling price if feedstock cost reduced >40%.	Biochar: USD 1.6/kg (target USD 1.25/kg for CB)	[176]
Slow	Coffee pulp	30 L, 100 L and 200 L batch	BR	2	Highest NPV with 200 L batch at USD 9781. Profit in 10 years: USD 15,399	Biochar: USD 0.7/kg Bio-oil: USD 1.03/kg	[177]
Slow	Cellulose, hemicellulose, lignin	100 t/h	CSTR	—	The revenues from char production show a positive net profit for all pyrolysis cases, with profits exceeding USD 90/t for lignin, cellulose, and hemicellulose.	Biochar: USD 110/t (lignin), USD 285/t (cellulose), USD 296/t (hemicellulose)	[178]
Slow	Madhuca indica tree	—	CSTR	3.14	At the targeted interest rate of 20%, the total capital cost was found to be USD 10,753,500 per year.	Bio-oil: USD 1.11/kg	[179]
Slow	Swine manure	1 t/h	—	4.6	If the selling price of biochar decreases by 20%, the investment payback period extends beyond 8 years.	Biochar: USD 116/t Bio-oil: USD 154/t	[180]
Intermediate	Forest residues	338–2549 t/day	RK	—	The MSP (10% IRR) of upgraded bio-oil was more than double that of crude bio-oil. Economy-of-scale benefits are evident.	Bio-oil: USD 0.71/L (crude), USD 1.25/L (upgraded)	[181]
Fast	Date palm waste	10 t/day	BFB	2.57	Net savings: USD 556.8/t of waste. Potential earnings of USD 44.8 million annually with 50% waste processed	—	[182]
Fast	Miscanthus	2000 t/day	BFB	7.6–8.9	The best economic performance with IRR: 11.3%, ROI: 13.1%.	Bio-oil: CNY 6.89/L	[183]
Fast	Corncob	96.8 t/h	CFB	13	Pyrolysis shows a positive NPV, provided the biomass cost is below USD 75.5/t.	Bio-oil: USD 1.47/gasoline gallon equivalent	[184]
Fast	Sugarcane bagasse	10 t/h	SBR	—	The cost to build the pyrolysis plant was USD 52 million. The NPV was negative with USD 65.7 million.	Bio-oil: USD 1.19/L	[185]
Fast	Eucalyptus	2000 t/day	BFB	10	MSPs for bio-oil: USD 1.04/L (single facility), USD 0.58/L (distributed). Single facility more economically favorable.	Bio-oil: USD 1.04/L (single), USD 0.58/L (distributed)	[186]
Fast	Rice husk	1000 t/day	BFB	—	Double the profit results in 57% higher of the selling price. 75% of operating cost is on utilities.	Bio-oil: USD 0.55/L	[187]
Fast	Forestry and agricultural residues	100 kg/h (mobile) 4000 kg/h (fixed)	BFB	6 years (mobile)	Total capital investment: CNY 0.86 million (mobile).	Biochar: CNY 1.2/kg, Bio-oil: CNY 1.25 /kg.	[188]
Fast	Rice straw (RS), waste tire (WT)	20 t/h	BFB	6.23	Plant (20% RS, 80% WT) most economical: NPV, USD 5.63 million.	Biochar (20% RS, 80% WT): USD 0.07/kg, Bio-oil: USD 0.36/kg.	[189]
Fast	Sewage sludge (SS), wheat straw (WS), sawdust (SD)	1.2–4.0 t/campaign	—	—	Net present worth (40% SD, 60% SS): CAD 8.71 million. Single pyrolysis of SS not profitable.	SS Biochar: (CAD 1.33/kg) and from WS (CAD 4.99/kg).	[190]

T—ton, CSTR—Continuous stirred-tank reactor, NPV—net present value, IRR—internal rate of return, ROI—return on investment, MSP—minimum selling price, CAD—Canada dollar.

provides a summary of the key findings of these analyses, covering slow [176,177,178,179,180], intermediate [181], and fast [182,183,184,185,186,187,188,189,190] pyrolysis methods.

A few studies have examined the techno-economic aspects of slow pyrolysis using different biomass feedstocks. Alonso-Gómez et al. [176] studied biochar production from cassava waste biomass, focusing on cassava branches (CB) and peels (CP) as feedstock. They found both CB and CP promising, yielding large amounts of biochar. Economic feasibility was achieved at processing scales greater than 3.4 tons per hour. Capital expenditure (CapEx) for a CB pyrolysis plant was estimated at USD 1,870,305 with a payback period of 5 years, while the CapEx for CP was lower at USD 374,503, but had a longer payback period of 8 years. A key insight was that reducing feedstock costs by more than 40% would significantly improve the market competitiveness of cassava biochar. Setiawan and co-researchers [177] analyzed the slow pyrolysis of coffee pulp in a 200 L batch reactor. Their economic evaluation showed that the highest net present value (NPV), USD 9781, was achieved with this batch size. Over a 10-year period, the process was projected to generate a profit of USD 15,399, suggesting that batch pyrolysis of coffee pulp can be economically viable, especially when operated at the optimal batch size. Muhammad Shahbaz et al. [178] explored how different biomass components, cellulose, hemicellulose, and lignin, affect slow pyrolysis results. Their results revealed that lignin produced significantly more biochar and was the most cost-effective feedstock, with a processing cost of USD 110 per ton, compared to USD 285 per ton for cellulose. Furthermore, lignin pyrolysis led to lower carbon emissions, making it a more environmentally sustainable option. As found by Rahul et al. [179], the payback period for the production of bio-oil and biochar from Madhuca indica through the biorefinery process was estimated at 3.14 years. Techno-economic studies show that this biorefinery process is both highly profitable and feasible. This analysis confirmed that biodiesel production is technically and financially viable. A techno-economic assessment of large-scale swine manure biochar production in China, conducted by Hu et al. [180], found that the project is sensitive to both the selling price of biochar and the operating costs. The results show that when the biochar price exceeds USD 116 per ton, the net present value (NPV) is positive, making USD 116 per ton the minimum selling price (MSP) for swine manure biochar. However, it is important to note that if the biochar price drops by 20%, the payback period extends beyond 8 years, suggesting that the project has relatively weak resistance to financial risk.

The intermediate pyrolysis of forest residues using a rotary kiln reactor was discussed in [181]. The economic analysis revealed that the minimum selling price (MSP) for crude bio-oil was USD 0.71 per liter, while upgraded bio-oil was priced at USD 1.25 per liter. The study demonstrated economy-of-scale benefits, indicating that larger-scale operations could enhance economic feasibility by reducing per-unit costs and improving profitability.

Several studies have explored the techno-economic feasibility of fast pyrolysis using various feedstocks, highlighting the process’s potential for biofuel production and waste management. For example, the fast pyrolysis of date palm waste in a 10-ton-per-day fluidized bed reactor was analyzed in [182], demonstrating net savings of USD 556.8 per ton of waste processed, with potential annual earnings of USD 44.8 million if 50% of the waste is utilized. This underscores the economic benefits of converting agricultural waste into bio-oil, particularly for large-scale waste management and biofuel production. Similarly, the study in [183] investigated the conversion of miscanthus to hydrocarbon biofuel through fast pyrolysis followed by catalytic hydrotreatment. The soil amendment scenario showed the best economic performance, with an internal rate of return (IRR) of 11.3% and a payback period of 7.6 years, indicating that optimizing product use can enhance profitability. On the contrary, the fast pyrolysis of corncob in a bubbling circulating fluidized bed reactor with a capacity of 96.8 kg per hour, as studied in [184], presented economic challenges. The long payback period of 13 years and a bio-oil selling price of USD 1.47 per gasoline gallon equivalent (gge) suggest difficulties in achieving economic viability without favorable market conditions or cost reductions. Further challenges were noted in the study [185] of sugarcane bagasse in a large-scale semi-batch reactor with a capacity of 10,000 kg per hour. The economic evaluation revealed a negative net present value (NPV) of USD −65.7 million and a minimum selling price (MSP) of USD 1.19 per liter, indicating that significant process improvements or market adjustments are needed for financial viability. On the other hand, the economic feasibility of eucalyptus wood for fast pyrolysis electricity generation was evaluated in [186], concluding that a single large facility was more competitive than multiple smaller ones, with a minimum bio-oil selling price of USD 1.04 per liter. This finding suggests that scale and centralized processing can improve economic outcomes. Wang and Jan [187] found that fast pyrolysis of rice husk in a 1000-ton-per-day facility could achieve a minimum bio-oil selling price of USD 0.55 per liter, with utility costs accounting for 75% of total operating costs. This highlights the importance of managing operational expenses to improve profitability. The potential for mobile pyrolysis systems was explored by Xing Chen et al. [188], who studied forestry and agricultural residues in an internally interconnected fluidized bed reactor. The mobile system, with a capacity of 100 kg per hour, became profitable in the 6th year, demonstrating the economic viability and flexibility of smaller mobile units.

Co-pyrolysis studies also provided insight into the improvement of economic outcomes. Khan et al. [189] evaluated the co-pyrolysis of rice straw and waste tires, finding that a mixture of 20% rice straw and 80% waste tires yielded the best economic performance, with a net present value of USD 5.63 million and a payback time of 6.23 years. This configuration reduced operating costs compared to using 100% waste tires. Finally, O’Boyle et al. [190] assessed the co-pyrolysis of sewage sludge with wheat straw and sawdust, finding that co-pyrolysis with sawdust was the most profitable, with a net present worth of 8.71 million Canada dollar. Although sewage sludge pyrolysis alone was not profitable, it was still cost-effective compared to conventional sludge treatment methods, highlighting the economic advantage of coprocessing diverse feedstocks.

These studies collectively highlight the diverse approaches and economic considerations in biomass pyrolysis and co-pyrolysis. Key factors influencing economic viability include feedstock costs, reactor design, plant capacity, and product pricing. Co-pyrolysis often offers improved economic outcomes by optimizing feedstock blends and enhancing product yields. However, challenges such as high capital investments, sensitivity to market prices, and operational costs must be carefully managed to achieve financial sustainability. Scaling up operations and reducing feedstock and utility costs are critical strategies for enhancing the economic feasibility of pyrolysis-based biofuel production.

7. Future Outlooks

The field of biomass pyrolysis is poised for significant advancements that could dramatically improve both the efficiency and scalability of the technology. One of the key areas for future research lies in optimizing reactor designs. Hybrid reactors that integrate multiple pyrolysis techniques could be developed to enhance product yields while reducing operational costs. These new designs would not only improve the energy efficiency of the process but would also make it more adaptable to different types of biomass feedstocks.

Another promising area is the integration of catalytic and co-pyrolysis methods. Future research could focus on discovering novel, active, stable and cost-effective catalysts that are derived from sustainable sources. This would enhance the quality of bio-oils and chars produced, making them more viable for commercial applications. The exploration of co-pyrolysis with various waste materials could also lead to higher yields of valuable chemicals and fuels, presenting a sustainable solution for waste management.

Scaling up pyrolysis technologies from laboratory to industrial scales will be a crucial step forward. However, this comes with challenges such as ensuring economic feasibility and navigating regulatory landscapes. Future efforts should focus on comprehensive techno-economic analyses that evaluate the long-term economic benefits and potential risks associated with large-scale operations. Addressing these challenges is essential for the successful commercialization of pyrolysis products.

Environmental sustainability remains a critical consideration for the future of biomass pyrolysis. Research should continue to focus on reducing the carbon footprint of pyrolysis processes and improving the overall sustainability of bioproduct chains. Moreover, innovations like plasma pyrolysis and microwave-assisted pyrolysis, which offer more efficient conversion processes and higher energy densities in bioproducts, should be prioritized in future studies.

The role of government policies and regulations cannot be overlooked. Future perspectives must consider how policy frameworks can incentivize the research, development, and adoption of pyrolysis technologies. Supportive policies could accelerate the transition from conventional energy sources to biobased alternatives, contributing to global sustainability goals.

Finally, global collaboration in research and development will be vital. By standardizing practices and sharing knowledge across regions, the global scientific community can accelerate the advancement of pyrolysis technologies and ensure their successful implementation worldwide. The future of biomass pyrolysis is bright and has the potential to play a significant role in sustainable energy production and environmental protection.