Conventional and Intensified Steam Reforming of Bio-Oil for Renewable Hydrogen Production: Challenges and Future Perspectives

Abstract

The increasing demand for clean and sustainable energy has driven significant research into hydrogen production from biomass-derived feedstocks. Unlike the gasification route, the pyrolysis of biomass followed by steam reforming of bio-oil (SRBO) offers several advantages, including the liquid nature of bio-oil and the operation at lower temperatures, which facilitate easier transportation and storage compared to raw biomass. The conventional SRBO process faces several limitations, mainly catalyst deactivation due to significant coke formation and metallic sintering, as well as low hydrogen yield and purity. Hence, the intensified sorption-enhanced steam reforming of bio-oil (SESRBO) is a promising strategy to overcome these drawbacks, to simultaneously produce high-purity hydrogen and capture carbon dioxide in situ from the reaction media. This critical review presents an in-depth comparative analysis of conventional and intensified steam reforming of bio-oil, with a focus on associated challenges. Special attention is given to recent developments in the design of bifunctional materials (BFMs), which integrate both catalyst and sorbent into a single particle, along with process optimization focusing on key parameters, i.e., reforming temperature and steam presence. Finally, the review highlights key research gaps and future directions to overcome existing challenges in achieving cost-effective and scalable hydrogen production.

1. Introduction

According to the recommendation of the UN Climate Change Conference COP-27 (held in Sharm El-Sheikh, Egypt, 2022) and the goal commitment to achieve climate neutrality by 2050, new challenges are emerging for the energy sector. All countries are urged to make an extra effort to combat the climate crisis. Serious concerns have been expressed about greenhouse gas (GHG) emissions. In this context, hydrogen has attracted considerable interest as a carbon-free energy carrier that features a high energy density (120 MJ/kg) [1]. It also has numerous applications as a chemical feedstock (e.g., the production of ammonia, alcohols, dimethyl ether (DME), formic acid, etc.), as a reducing agent in metallurgy, and in fuel cell applications. Currently, the demand for hydrogen is around 45 million tons per year, according to the International Energy Agency (IEA), and it is predicted to increase by 4–5% annually over the next five years [2]. Until now, H₂ production has relied predominantly on steam reforming (SR) of fossil fuels, particularly natural gas as a common feedstock [3]. However, this approach is becoming less and less favorable due to (i) the excessive production of CO₂ (environmental considerations; large amounts of CO₂ (>9 kg CO₂ per only 1 kg H₂) are generated during steam reforming of natural gas [4]) and (ii) the non-renewable nature of fossil fuel feedstock (sustainability considerations).

Significant effort is being made to produce renewable hydrogen to reduce reliance on fossil fuels and their environmental impact (Figure 1a). By 2050, renewable energy sources are anticipated to account for approximately 75% of total energy consumption, with hydrogen energy contributing around 34% [5]. To produce hydrogen from biomass, two main thermochemical pathways can be adopted, namely (i) gasification and (ii) fast pyrolysis followed by the steam reforming of the resulting liquid phase, commonly referred to bio-oil (BO) [6]. Basically, bio-oil is a complex mixture of oxygenated organic compounds. It is composed of light and heavy fractions, including acids, alcohols, phenols, and aldehydes. Due to its liquid nature, bio-oil is more convenient to store and transport than raw biomass, making it a promising feedstock for hydrogen production via steam reforming [7]. Compared to gasification, the pyrolysis-based route offers several advantages, such as operating at lower temperatures (typically 500–700 °C instead of 800–1000 °C) and minimizing tar formation. These advantages together lead to lower energy consumption, reduced operating costs, and fewer technical complications, making this approach more efficient and cost-effective [8].

Over the past decade, significant attention has been given to hydrogen production via conventional steam reforming of bio-oil (SRBO). However, key challenges such as low hydrogen purity and catalyst deactivation caused by metal sintering and excessive coke deposition, hinder its efficiency [9,10]. In energy sector applications (such as fuel cell applications), highly pure hydrogen (>99.9%) is imperative. However, during conventional SR processes, the hydrogen concentration in the gas product is typically limited to around 70% due to thermodynamic constraints, while CO₂ concentrations can exceed more than 20% [11]. To achieve a high level of hydrogen purity, it is essential to implement purification and separation procedures in the downstream process, which increases total capital costs and complexity and reduces overall process efficiency. There are several techniques for separating CO₂ from gas mixtures, based on solid sorbents, liquid solvents, membranes, cryogenic distillation, pressure swing adsorption (PSA), etc. [12]. Given the efficiency of solid sorbents at high temperatures, sorption-enhanced steam reforming (SESR) has been proposed as a promising intensified technology that integrates CO₂ capture into the steam reforming process [13]. The primary aim of SESR is not merely to separate CO₂ from a gas mixture but rather to shift the reaction equilibrium toward higher H₂ yield and purity according to Le Chatelier’s principle. For this purpose, various high-temperature solid sorbents can be used to capture CO₂ generated in the reaction environment. To this end, SESR relies on hybrid materials, either physical (mechanical) mixtures of catalyst and sorbent or bifunctional materials (BFMs) that integrate a reforming catalyst and a CO₂ sorbent into a single particle. As a result, this intensified process enables the production of high-purity hydrogen in a single step (in situ) without additional purification.

Interest of the Review Within Current Literature

A bibliometric analysis was conducted using data from the past decade (2015–2025), sourced from the Scopus database, widely recognized for its reliability in scientific research. As illustrated in Figure 1b, the number of publications on hydrogen production through both conventional and intensified processes has shown a significant upward (exponential) trend, particularly in the last 3 years. Since keyword analysis plays a crucial role in indexing publications, it helps researchers identify hot topics and emerging trends in a specific field. For the purpose of this review, a keyword map-view generated using VOSviewer (version 1.6.20) is presented in Figure 2. In this map, the size of the nodes reflects the frequency of keyword occurrence, while the thickness of the connecting lines indicates the strength of co-occurrence between keywords. The results highlight that “hydrogen production” and “steam reforming” form a strong cluster with a thick link. In contrast, “sorption-enhanced steam reforming” and “BFMs” are associated with smaller nodes and much thinner links, confirming that this approach remains relatively underrepresented. Furthermore, although various feedstocks can be utilized for hydrogen production, the analysis demonstrated that bio-oil has gained considerable research attention in recent years.

Given the rapid advancement and growing interest in hydrogen production from bio-oil, several reviews have been published on various aspects of the SRBO process. These works include: synthesis and performance of different catalysts [14,15], optimization of operating conditions [1,16], reaction mechanism and kinetics considerations [7,17,18], as well as techno-economic analysis [19]. However, reviews dedicated specifically to SESR of bio-oil (SESRBO) remain extremely scarce. To the best of our knowledge, only one recent review has attempted to address this topic [20], but it provided only a general overview of SESRBO fundamentals without offering an in-depth analysis of the challenges associated with bio-oil feedstocks within this complex process.

For the first time, this review fills an important gap in the current literature by benchmarking SESRBO vs. SRBO in terms of benefits and challenges. This critical review presents a systematic understanding of the fundamental aspects of the cyclic carbonation-calcination behavior of hybrid materials (i.e., BFMs and mechanical mixtures of both catalyst and sorbent particles). Particular attention is devoted to assessing the effects of the main operating parameters, such as temperature, steam-to-carbon (S/C) ratio, and feed flow rate, on both catalytic activity and carbonation process. Furthermore, it provides a detailed discussion of the strategies developed to improve the sorption capacity and cyclic stability of BFMs. While SESR is well known for its effectiveness in limiting coke formation, recent studies on SESRBO have frequently reported this unwanted phenomenon stemming from the high complexity of bio-oil. Given the considerable impact of coke deposition on the catalytic activity and sorption capacity of BFMs, this review also devotes an in-depth analysis of coke deposition during the SESRBO process. Overall, the present work intends to provide a timely and valuable contribution to the field of sustainable hydrogen production.

2. Challenges of H₂ Production from Bio-Oil

2.1. Complexity of Bio-Oil as Feedstock

Despite its visual similarity to crude oil (a dark brown color), bio-oil is characterized by a high content of oxygenated compounds. Typically, it is derived from biomass and contains a complex mixture of more than 300 oxygenated compounds belonging to different chemical families [21]. These compounds range from highly volatile to non-volatile fractions, making their separation and upgrading more difficult. Except for water, the majority of oxygenates are acids, alcohols, phenols and their derivatives, and cyclic aldehyde compounds. The constituents of bio-oil can also be categorized as: (i) light components like methanol, ethanol, acetic acid, and acetone, and (ii) heavy components like furan, phenol, catechol, and m-methyl phenol. Most of them are frequently studied as representative model compounds. The compositional complexity of bio-oil poses several challenges, including high acidity, corrosiveness, and thermal instability. Moreover, bio-oil properties can vary depending on the feedstock and pyrolysis conditions, as well as storage conditions and duration. The key properties of bio-oil are summarized in

Table 1. Main features of raw bio-oil.

Parameter	Properties
Viscosity	A dense complex mixture of oxygenated organic compounds, consequently, it becomes difficult to pump (operational challenge).
pH	Acidic nature, with a pH commonly between 2 and 4. The acidity of bio-oil can cause corrosion and stability issues.
Heating value	Relatively low, typically lower than traditional fossil fuels such as gasoline and diesel.
Stability	Highly unstable and incompatible with conventional liquid fuels, primarily due to its high-water content. Over time, bio-oil degrades, resulting in the formation of sludge and solid residues. This degradation can be further accelerated by factors such as elevated temperatures, exposure to oxygen, and UV light.
Composition	The composition of bio-oil varies depending mainly on the applied feedstock and the pyrolysis conditions. It also may contain some impurities, especially sulfur, which can deactivate the catalyst.

. These complex characteristics also introduce operational challenges, such as reactor clogging. Studies have reported that different simulated bio-oils composed mainly of lighter fractions tend to achieve higher hydrogen yields at lower temperatures and S/C ratios, whereas heavier fractions require more severe reaction conditions to achieve comparable performance [22]. This behavior is attributed to the increased aromatic content, which promotes coke formation due to the low solubility and high polymerization tendency of aromatic compounds.

2.2. Catalyst Deactivation

During the SRBO process, the main catalyst deactivation typically progresses through five stages, as presented in Figure 3.

(1)

Stable equilibrium: The reaction operates under thermodynamic equilibrium. During this stage, excess catalyst compensates for initial deactivation.

(2)

Initial deactivation: The catalyst activity slightly declines owing to morphological changes in the catalyst due to catalyst sintering phenomena.

(3)

Pseudostable state: Despite some deactivation, the catalyst retains high activity.

(4)

Major deactivation: A sharp drop in H₂ and CO₂ production, with increased hydrocarbon formation. The reaction indices change rapidly but slow down as time-on-stream (TOS) increases. This stage is highly linked to excessive coke deposition over the catalytic sites.

(5)

Slow deactivation: The catalyst loses reforming activity but retains residual function, particularly for cracking oxygenates and facilitating the water-gas shift (WGS) reaction. This remaining activity is linked to the support interactions with catalytic active sites.

2.2.1. Coke Formation

Coke or fouling is the physical deposition of unwanted carbonaceous species from the fluid phase onto the catalyst surface, leading to activity loss due to blockage of active sites and/or pores. The coke properties and their effect on catalytic activity are usually explained in terms of four main features: content, location on the catalyst surface, morphology, and chemical nature [25]. These features determine not only the rate of deactivation but also the possibility of catalyst regeneration. SRBO is always associated with low H₂ yield and excessive coke formation, causing catalyst deactivation. In this context, Ochoa et al. [15] presented a comprehensive review of coke formation and deactivation during the catalytic reforming of biomass, highlighting the mechanisms of coke formation and the features of coke deposits. Figure 4 shows the possible deactivation pathways for metal-supported catalysts during SRBO: (i) strong chemisorption in the form of monolayers, leading to carbide formation or physisorption in multilayers, which hinders access to active sites; (ii) complete encapsulation of the active site, consequently becoming inaccessible to (C_nH_mO_k oxygenates and steam) reactants; (iii) obstruction of micropores and/or mesopores in the catalyst; (iv) blocking access to active sites within inner pores; (v) changes and/or disintegration of the catalyst structure and reactor blockage at advanced stages of coke growth.

The formation of various oxygenated hydrocarbons, C_nH_m groups, and aromatic polymers under high-temperature SRBO conditions can readily block catalyst pores, resulting in accelerated deactivation [26,27]. Coke deposition is considered to be primarily attributed to the polymerization of aromatic and unsaturated components present in the lignin-derived fraction of bio-oil, particularly phenolic compounds [28]. These compounds are highly reactive and tend to condense on the catalyst surface, forming dense carbon layers. In addition, poly-substituted hydroxy- and methoxy-phenols derived from lignin pyrolysis are considered undesired components in raw bio-oil, as they undergo condensation and repolymerization reactions that generate carbonaceous material, leading not only to severe catalyst deactivation but also to reactor fouling and operational instability [29]. In an investigation of the effect of some common impurities (acetic acid, oxalic acid, and methanol) on the performance of UpGraded Slag Oxides (UGSO)-supported Ni catalyst during the steam reforming of glycerol (SRG) under varying operational conditions (temperature range 580–730 °C, S/C ratio of 3 and 6), the authors reported that the presence of these acidic compounds significantly enhances coke formation by generating reactive intermediate species, which in turn reduces hydrogen yield [30].

Coke deposition is influenced by several factors, including operating conditions, bio-oil composition, and catalyst type and morphology, which determine the type and position of the carbon deposits. For example, low temperatures (<450 °C) favor coke deposition, as secondary reactions are promoted over reforming and coke gasification [10]. Other works [31,32] highlighted the effect of temperature on the nature of the formed coke, e.g., the formation of amorphous carbon at 550 °C and a filamentous structure at 700 °C, during the SR of raw bio-oil (derived from pine sawdust). Conversely, a higher space time, which corresponds to a higher proportion of catalyst mass relative to the fed reactants flow, is reported to reduce the amount of coke deposited, since secondary reactions are minimized. In this regard, Valle et al. [32] studied the effect of space time on the nature of coke deposited during the steam reforming of raw bio-oil over the Ni/La₂O₃-αAl₂O₃ catalyst. The authors claimed that low values of space time increase the content of encapsulating and filamentous fractions, and high values decrease the formation of encapsulating coke. It has been concluded that a S/C molar feed ratio greater than 4 is suitable at high temperature (700 °C) and high space time (0.38 g_catalyst·h/g_bio-oil), presenting an initial H₂ yield of 87%, which decreased to 70% after TOS of 5 h. Furthermore, an increase in the presence of steam and oxygen favors the gasification and/or combustion of coke precursors and coke itself, if formed. On the other hand, an increase in the aromatic nature of the feed subjected to reforming is reported to promote coke deposition due to polymerization reactions of aromatics [33].

To reduce this phenomenon, numerous efforts have been made to improve the resistance of Ni-based catalysts to coke formation by incorporating noble metals (e.g., Pt, Ru, Rh, Pd, Ir) or redox promoters/supports such as CeO₂, ZrO₂, and La₂O₃. It is reported that adding CeO₂ to Ni-based catalysts favors coke gasification, thereby reducing catalyst deactivation [34]. Meanwhile, incorporation of CeO₂ to ZrO₂ support improves the capacity of H₂O sorption and dissociation, keeping the Ni surface free of carbon due to their high redox ability [35]. In this framework, Rioche et al. [36] conducted SRBO experiments using Pt, Pd, and Rh catalysts supported on Al₂O₃ and CeO₂-ZrO₂ materials. Their findings indicated that the catalyst supported on CeO₂-ZrO₂ exhibited superior activity compared to Al₂O₃-based catalyst. In another study, Shokrollahi Yancheshmeh et al. [37] reported that the incorporation of CeO₂ into NiAl₂O₄ spinel catalyst significantly reduced filamentous carbon formation on the nickel surface and promoted coke gasification by providing an oxidative environment around nickel active sites, thanks to the formation of well-dispersed CeAlO₃ (Figure 5).

Another promising trend focuses on improving the catalyst-support framework to achieve strong metal-support interaction, i.e., spinel or perovskite [10], mesoporous framework [38], and nanocomposites [39]. These highly stabilized structures reduce the rate of carbon formation. For example, Xu et al. [40] investigated the impact of the Ce/Zr ratio and mesopore structure on promoting catalytic performances and found that the catalyst supported on Ce/Zr (molar ratio 1:1) showed the highest catalytic activity due to the redox property of CeO₂-ZrO₂ mesoporous nanocrystalline solid solution. Other studies [41,42] explored SRBO on Ni/CeO₂-ZrO₂ composite catalysts and demonstrated that CeO₂-ZrO₂ plays a key role in activating H₂O molecules, promoting the WGS reaction, and mitigating coke formation.

2.2.2. Sintering of Active Sites

Sintering is a thermally activated physical phenomenon that occurs in both supported and unsupported catalysts. It involves the growth of surface metal particles, thereby remarkably reducing the active surface area. This loss of surface area results in fewer accessible active sites, thereby diminishing the catalytic activity per unit mass of catalyst. Besides particle growth, sintering is frequently accompanied by the agglomeration of fine particles (Ostwald ripening phenomenon) [43]. Thus, the combined effects of surface area loss and pore blockage hinder reactant diffusion, ultimately leading to a substantial decline in catalytic performance over reaction time. It has been reported that elevated temperatures (>700 °C), high pressures (>40 bar), and the presence of steam and certain impurities containing poisoning elements (S and Cl) in the reaction atmosphere significantly accelerate the sintering process [15]. In contrast, filamentous carbon formed on the spent catalyst surface may help reduce sintering by physically separating Ni particles.

There are two main approaches to minimizing the sintering of active sites. The first involves the use of noble metals, i.e., Ir and Rh, and bimetallic catalysts such as Ni-Cu or Ni-Co alloys [44,45]. The second approach focuses on improving the metal dispersion and strengthening the metal-support interaction to eliminate particle migration. Sintering typically occurs above the Tamman temperature (TT), at which metal atoms become mobile. Several studies have therefore focused on developing the morphological characteristics of the support to favor metal dispersion [46,47,48]. For instance, Mulewa et al. [48] reported that the use of nanocomposite catalysts (Ni/TiO₂ nanoparticles supported on montmorillonite (MMT) clay) significantly improved Ni resistance to sintering during the steam reforming of ethanol (ESR). As shown in Figure 6, the Ni/MMT-TiO₂ nanocomposite, synthesized via a sol-gel-assisted impregnation method, demonstrated prolonged catalytic stability and a hydrogen yield 1.5 times higher than that of its microparticle counterpart over a 20-h reaction period.

3. Conventional Steam Reforming of Bio-Oil

Bio-oil, derived from the fast pyrolysis of biomass, contains a considerable amount of water, typically ranging from 20 to 50 wt.% depending on the feedstock characteristics and pyrolysis operating conditions [49]. Water removal is not only technically challenging due to its strong miscibility with polar oxygenated compounds in bio-oil but also economically unfeasible with current technologies because of the high energy demand and operational costs associated with separation methods [16]. However, the inherent water content in bio-oil is beneficial, as it acts as a reactant in the SRBO process [50]. During SRBO, various reactions can occur simultaneously, affecting the hydrogen yield and purity (

Table 2. Main chemical reactions during steam reforming of oxygenates.

Process	Reaction	$\mathsf{\Delta }{\mathit{H}}_{298}^{\mathit{o}}$(kJ/mol)	Equation
SR of oxygenates	$C_n{H}_m{O}_k+\left(n-k\right)H_{2}O\to nCO+\left(n+{\displaystyle \frac{m}{2}}-k\right)H_2$	>0	(1)
Water gas shift (WGS)	$nCO+n{H}_{2}O\leftrightarrow nC{O}_2+{nH}_2$	−41.2	(2)
Overall reaction (SR (1) + WGS (2))	$C_n{H}_m{O}_k+\left(2n-k\right)H_{2}O\to nC{O}_2+\left(2n+{\displaystyle \frac{m}{2}}-k\right)H_2$	>0	(3)
SR of methane	$C{H_4+H}_{2}O\leftrightarrow CO+3H_2$	206	(4)
Boudouard reaction	$2CO\leftrightarrow C{O}_2+C\left(coke\right)$	172.4	(5)
Cracking of oxygenates	$C_n{H}_m{O}_k\to C_x{H}_y{O}_z+C_r{H}_s+C{H_4+H}_{2}O+CO+C{O}_2+H_2+C\left(coke\right)$	>0	(6)
Methane decomposition	$C{H}_4\leftrightarrow 2H_2+C\left(coke\right)$	74.9	(7)
Coke gasification	$C\left(coke\right)+H_{2}O\to CO+H_2$	131.3	(8)

). Furthermore, coke generation (Equations (5)–(7)) and gasification (Equation (8)) lead to the reduction of catalyst activity and H₂ yield.

3.1. Catalyst

Selecting an appropriate catalyst is crucial for maximizing hydrogen yield, as catalyst performance is influenced by several factors such as active metal type, particle size, support material, and promoter addition. A high-performance catalyst can achieve hydrogen yields approaching or exceeding 80% from bio-oil via steam reforming, while the resulting gas mixture typically contains less than 70% H₂ and approximately 20% or more CO₂ [25]. The ideal catalyst for SRBO should possess the following characteristics: (i) high catalytic activity for reforming reactions, (ii) strong selectivity toward hydrogen generation, and (iii) resistance to deactivation caused by carbon deposition and metal particle sintering.

Catalysts are generally classified into two main categories: transition metal oxides and noble metal-based catalysts [7]. Among the transition metal oxide catalysts, nickel (Ni) has significant advantages due to its remarkable intrinsic activity at moderate temperatures, favorable effect on the WGS reaction, high availability, and cost-effectiveness [36]. Whereas Fe is often used as a metal catalyst due to its ability to break the C-C bond, it is known for its poor reactivity and low reducibility [51]. On the other hand, Cu-based catalysts can break the C-H bond and are effective for SRBO, but they lack the ability to break the C-C bond in compounds such as acetic acid or acetone [52]. Hu et al. [53] conducted a comparative study to evaluate the performance of different catalysts (Ni, Co, Fe, and Cu) in the steam reforming of acetic acid as a bio-oil model compound. Among them, Ni and Co catalysts were the most effective owing to their ability to activate both C-C and C-H bonds. In terms of activity, the catalysts ranked in the order of decreasing activity as Ni > Co > Fe > Cu.

Table 3. Transition metal oxide catalyst used for hydrogen production from model compounds and bio-oil (simulated and raw).

Feedstock	Operating Conditions			Reactor	Catalyst		Catalytic Activity		Ref. & Year
	T (°C)	S/C	Feeding Rate		Type	Prep. Method	Conv. (%)	YH2 (%)
Model compound
Acetic acid	650	5.58	GHSV = 13,000 h−1	FDB	Ni/Al2O3	CP	46	-	[54] 2005
	700	7.5:1	LHSV = 5.1 h−1	FB	Ni/La2O3 Co/La2O3 Ni-Co/La2O3	WI	96.9–100	-	[55] 2016
	800	3	WHSV = 5 h−1	FB	Ni/CeO2-ZnO	CP & WI	-	57.8:69.4	[56] 2020
Ethanol	500	3	GHSV = 51,700 h−1	FB	Co/α-Al2O3	WI & MM	86	64	[57] 2019
	500	1.5	WHSV = 2773 h−1	FB	Ni/Al2O3-TiO2	WI	93	88	[58] 2017
	450	5	WHSV = 0.19–2.88 h−1	FB	Co/CeO2	-	97.1	93.3	[59] 2017
	600	6	GHSV = 10,432 mL/g·h	FB	Ni/CeO2-MgO	Dip-coating technique	100	70	[60] 2017
Acetone	700	9	LHSV = 850 h−1	FB	Ce-Ni/Ce	WI	-	72	[61] 2015
Phenol	600	5	GHSV = 4968 h−1	FB	Ni/Al2O3	WI	94.7	80.8	[62] 2022
Glycerol	575	3	WHSV = 0.85 h−1	FB	20Ni-20Co HTl c	CP	-	97	[63] 2012
Bio-oil
Ethanol and phenol (sBO a)	600	2.65	GHSV = 54,000 h−1	FB	NiO/Al2O3	WI	81	66	[64] 2015
Acetic acid -hydroxy acetone, furfural and phenol a	600	4 2.67 13.2 11	-	FB	Ni/SBA-15 Ni-Cu/SBA-15 Ni-Co/SBA-15 Ni-Cr/SBA-15	WI	91.5–98.4	>95	[65] 2019
Acetic acid, Cresol and Benzyl Ether a	650–810	3 and 6	GHSV = 500:11,790 h−1	FB	Commercial Ni-based catalyst d	-	-	70:90	[66] 1999
Aqueous fraction a	800	4.9	-	FB	Ni/CeO2-ZrO2	WI	-	69.7	[41] 2010
Acetic acid, Acetone and Glycerol a	600–800	4	-	FB	Ni/CeO2-ZrO2 & La-Promoted Ni/CeO2-ZrO2	CP & WI	60–85	42–70	[42] 2016
Ethanol, acetone, acetic acid, and phenol a	700	9	LHSV = 850 h−1	FB	Ni/Al2O3 modified by Mg, Ce and Co	WI	92.3	-	[61] 2015
Fresh aqueous fraction of bio-oil a	700	2	ST = 0.22 gcatalyst/h·gbio-oil	FDB	Ni/α-Al2O3 & Ni/La2O3-Al2O3	WI	100	96	[67] 2012
Raw bio-oil (rBO b)	550–700	6	ST = 0.10 gcatalyst·h/gbio-oil.	FDB	Ni/La2O3-αAl2O3	WI	100	88	[32] 2018
	800	2	WHSV = 0.5 h−1	FB	Fe/olivine	WI	97.2	79.3	[68] 2016
	450	17	GHSV = 6000 h−1	FB	Ni/HZSM-5 Zeolite	WI	≈100	90	[69] 2011
	800	-	WHSV = 1.7 h−1	FB	Ni/UGSO	SSI	≈100	94	[70] 2018

Conv.: bio-oil conversion, Y_H2: hydrogen yield, FDB: fluidized-bed reactor, FB: fixed-bed reactor, SBA-15: mesostructured silica, CP: co-precipitation, WI: wet impregnation, MM: mechanical mixing, SSI: solid-state impregnation, ST: space time, WHSV: weight hourly space velocity, ^a simulated bio-oil, ^b raw bio-oil, ^c hydrotalcite-like material, and ^d UC G-90C catalyst manufactured by United Catalyst Inc. (Fountain Inn, SC, USA).

lists several examples of transition metal oxide catalyst formulations, along with preparation methods, operational conditions, and performance in the SRBO process. It is evident that Ni remains the most widely utilized catalyst.

Regarding the noble metal-based catalysts, palladium (Pd), platinum (Pt), ruthenium (Ru), and rhodium (Rh) have been considered due to their (i) high activity in converting bio-oils into gaseous products and (ii) superior resistance to coke (carbon) formation and sulfur poisoning, which prevents catalyst deactivation [36]. In a study by Basagiannis et al. [71], the performances of Pd, Pt, Ru, and Rh catalysts supported on Al₂O₃ were compared for SR of acetic acid. The results ranked them according to their activity as follows: Rh > Ru > Pd > Pt. The main limitation of using this category in industry is the high cost associated with producing them on a large scale. Therefore, combining Ni with a second metal M (noble or non-noble) to form a Ni-M alloy can improve catalyst properties by enhancing metal distribution, reducing metal particle size, and strengthening metal-support interactions. Bimetallic systems, such as Ni-Ru and Ni-Co, effectively combine the advantages of different active metals to enhance catalyst activity. In addition, the incorporation of Ru or Rh as promoters in Ni-based systems positively affects activity by improving the reducibility of Ni species. On the other hand, Ni-Co bimetallic catalysts with varied Ni and Co loadings exhibit synergistic interactions between Ni and Co, resulting in high reforming activity that promotes the SR reaction and does not favor methanation [65].

3.2. Catalyst Support

In catalyst design, the support also plays a vital role in enhancing performance, durability, and overall efficiency of the catalytic system. It significantly influences the dispersion and reducibility of the active phase and helps prevent sintering by stabilizing metal particles through strong metal-support interactions. A highly porous support promotes better dispersion of metal or metal oxide phases, thereby increasing the number of accessible active sites and improving catalytic performance compared to bulk metals. Moreover, the support contributes to mechanical strength and thermal stability, allowing the catalyst to sustain the harsh conditions of SR.

Various support materials are used in the SR process, including common oxides, alkaline earth oxides, redox oxides, and mineral-type materials, each type presenting its own advantages and disadvantages. Although Al₂O₃ is the most widely used support for Ni-based catalysts (Table 3), thanks to its high specific surface area and excellent chemical and mechanical stability under reaction conditions, it is prone to deactivation via carbon deposition and sintering due to its acidic sites. In contrast, basic oxide supports such as MgO and ZrO₂ have been shown to enhance metal dispersion and reduce carbon buildup. Therefore, a balance must be maintained between catalytic activity and resistance to deactivation to develop catalysts with optimal acidic properties.

As a redox oxide, CeO₂ has attracted significant interest in the SRBO process due to the oxygen vacancies on its surface, which readily form during the reduction process. CeO₂ also has redox cycles between oxidation states of +4 (oxidized) and +3 (reduced) and effective acid-base surface properties. Moreover, the incorporation of CeO₂ into ZrO₂ significantly increases the oxygen storage capacity and promotes the mobility of lattice oxygen atoms, thereby improving the coke-resistance of the catalyst. In this regard, Yan et al. [41] evaluated the performance of Ni/CeO₂-ZrO₂ catalyst with varying Ni and Ce loading in the production of hydrogen from raw bio-oil in comparison with the commercial catalyst Z417 (Ni/Al₂O₃-K₂O), in a fixed-bed reactor. The reaction temperature ranged from 400 °C to 850 °C, and the steam-to-bio-oil (S/BO) ratio ranged from 3.2 to 5.8. The results showed that at 800 °C, Ni/CeO₂-ZrO₂ catalyst with Ni and Ce loadings of 12 wt.% and 7.5 wt.%, respectively, and a S/BO ratio of 4.9 led to the highest H₂ yield of 69.7%, which significantly surpasses the H₂ yield obtained over the commercial Z417 catalyst (≈62%). Similarly, Gallegos-Suarez et al. [72] studied the application of a Ni/CeO₂ composite catalyst and found that Ni was highly dispersed and exhibited a small crystallite size in contact with ceria, resulting in high hydrogen selectivity (98.8%) and low carbon deposition (0.04 g_carbon/g_catalyst).

Another interesting category involves industrial waste-driven supports, such as fly ash (FA) and UGSO. For example, Wang et al. [73] evaluated the performance of the Ni/FA catalyst using acetic acid and phenol as model compounds and compared it with that obtained with FA. The study found that the Ni/FA catalyst exhibited some activity and stability at a reaction temperature of 700 °C and an S/C ratio of 9.2. The conversion of acetic acid was found to be 98.4%, which was almost double compared to FA (57.5%). The corresponding hydrogen yields were 85.6% and 50.2% for Ni/FA and FA, respectively. The excellent performance of the Ni/FA catalyst can be mainly attributed to the formation of highly dispersed Ni active sites, which strongly interact with the Al₂O₃-rich FA support, thereby promoting both reforming and WGS reactions. Likewise, the steam reforming of two raw bio-oils over Ni/UGSO catalyst was evaluated at various temperatures and weight hourly space velocities [70]. The catalyst remained stable throughout the tests, even when some coke deposition occurred. The highest yield (94%) and carbon conversion (X_c = 100%) were observed at temperatures above 800 °C and WHSV = 1.7 h⁻¹. Beyond their effectiveness, the use of waste-derived materials offers notable environmental and economic benefits by valorizing industrial residues.

4. Intensified SESRBO for High-Purity H₂ Production

Sorption-enhanced steam reforming (SESR) is an intensified process with notable advantages over conventional SR. A major benefit is the ability to achieve significantly higher hydrogen purity and yield at lower reforming temperatures within the range of 400 to 500 °C [74]. Additionally, SESR integrates hydrogen production and CO₂ separation in a single step, thereby eliminating, in most hydrogen-based applications, the need for downstream purification (Figure 7). From an economic perspective, SESR enhances energy efficiency by utilizing the heat released during the carbonization reaction and the WGS process to offset the endothermic reforming reaction. Moreover, the use of lower reaction temperatures in SESR contributes to retarding catalyst deactivation, ultimately extending the effective lifetime of the catalyst and sustainability of the overall process [75].

The selection of appropriate solid CO₂ sorbent materials is the key factor for a successful SESR process. An effective sorbent must combine adequate sorption capacity with rapid sorption kinetics, as well as adequate thermal stability during cyclic carbonation-calcination at elevated temperatures [76]. Common high-performance sorbents for this process include CaO-based (synthetic or natural ones like limestone and dolomite) and alkaline ceramics (e.g., Li₄SiO₄, Li₂ZrO₃, and Na₂ZrO₃) [77]. Among them, CaO-based sorbents are appealing due to their cost-effectiveness, high CO₂ sorption capacity, and favorable reaction kinetics [78].

To understand the mechanism of SESR as an intensified process, it is important to pay attention to the calcium looping mechanism, which is based on the reversible gas-solid reaction between calcium oxide (CaO) and carbon dioxide (CO₂) to form calcium carbonate (CaCO₃) (Equation (9)). The sorbent is generally present in a high mass percentage (>70%), making it the dominant compound in hybrid catalyst-sorbent materials (physical mixture and BFMs) [79]. As displayed in Figure 8a, the typical curve of CaO conversion vs. carbonation time is characterized as the knee-bended shape (initial steep straight line and final quasi-horizontal plateau joined by a rounded knee-bend). The carbonation reaction occurs according to two subsequent regimes: (i) the rapid kinetically controlled stage and (ii) the CO₂ diffusionally controlled stage [80]. In the rapid stage, carbonation occurs on the CaO surface through nucleation and growth of CaCO₃. According to CO₂-based chemistry, the adsorption and transformation of CO₂ into negatively charged intermediates govern subsequent reaction pathways. The rapid termination of the fast stage occurs when the formed CaCO₃ creates a continuous layer over the unreacted sorbent. As a result, the first rapid stage is mainly controlled by kinetics, while the subsequent, much slower, is governed by CO₂ diffusion through the CaCO₃ layer. During the carbonation reaction, the ratio of reaction rates between the initial and subsequent stages is usually very high, approximately 100 [81].

$$CaO\left(s\right)+C{O}_2\left(g\right)\leftrightarrow CaC{O}_3\left(s\right)$$

(9)

As depicted in Figure 8b, SESR is generally characterized by three stages. The pre-breakthrough stage is linked to the fast-kinetic step of the carbonation reaction, which is controlled by the surface reaction. At this duration, the fresh sorbent could exclusively capture all the generated CO₂; as a result, CO₂ concentration is retained at an extremely low level. In contrast, H₂ purity is maximized and sustained at a high level. In the breakthrough stage, the CO₂ concentration in the output gas stream gradually increases, resulting in a continuous decrease in H₂ purity and, to some extent, an increase in CO and CH₄ concentrations. This transition stage is characterized by the slow kinetics of the carbonation reaction, in which CO₂ diffuses through a thick CaCO₃ product layer to access the active CaO particles. In the post-breakthrough stage, the sorbent becomes fully saturated. As a consequence, the corresponding process represents the traditional SR without any intensification effect.

Table 4. Investigations related to SESR of bio-oil (simulated and raw) and its model compounds.

Feedstock	BFM	Prep. Method	Reactor	Operating Conditions			No. of Cycles (-)	H2 Purity (%) c	H2 Yield (%)	Ref.
				T (°C)	S/C	WHSV (h−1)
Model compound
Acetic acid	Ni/CaO-La2O3	Sol–gel	FB	650	3	0.63	9	92.2/85	81	[82] 2017
	Ni/CaO-Ca12Al14O33	WI	FB	650	3	1.18	20	96/83	78	[83] 2019
	Ni/CeO2-ZrO2-CaO	WI	FB	550	4	0.48	15	95/90	80	[84] 2020
	Ni/CexZr1−x O2-CaO	Sol–gel	FB	550	4	-	15	98/88	-	[85] 2017
Ethanol	Ni/Al2O3-CaO	CP	FB	500	4	-	10	96/90	73.5	[86] 2020
	NiO/CaO-Ca12Al14O33	CP	FB	600	2	0.6	10	87/85	70	[87] 2019
	Ce-Ni/ MCM41/CaO	WI	FB	600	3	1.99	1	90	94	[88] 2012
Phenol	Ni/CaO-Ca12Al14O33	CP	FB	500–650	11	-	50	98.8/96	78	[89] 2020
	Ni-M/CaO-Ca12Al14O33 (M = Cu, Co, and Ce)	WI	FB	650	3	-	5	69.7/-	62.3	[11] 2020
Glycerol	Co-Cu/CaO	CP	FB	525	4	3.41	10	99.2/97.5	81	[90] 2017
	NiO/CaO-Al2O3	CP	FB	550	3	-	5	90/80	-	[91] 2015
	NiO/CaO-Ca9Al6O18	CP	FB	550	9	1.55	5	98/98	91	[75] 2017
	Ni/CaO-FA	WI	FB	550	3	1.72	20	97/96	90	[92] 2020
	Ni/CaO-UGSO	WI	FB	550	3	1.55	2	95/95	90	[93] 2020
Bio-oil
Acetic acid and acetone (sBO a)	Pd-Ni/Co-Dolomite	WI	FDB	475–725	3.33–6.67	0.6757–1.3158	1	99.2– 99.4	83.3– 88.6	[94] 2016
Acetic acid, acetone, phenol, furfural and 1-butanol a	Ni-Co/Olivine-Dolomite	WI	FB	575	7	0.8–3.8	3	99/97	70	[95] 2020
Ethanol, acetic acid, acetone and phenol a	Ce-Ni-Co/ Al2O3-CaO	WI	FB	700	9	0.23	1	93.3	83.8	[96] 2015
Acetic acid, acetone, ethanol, and phenol a	Ni/CaO-UGSO	WI	FB	550	3	1.408	1	96.2	90	[97] 2024
Acetic acid, acetone, ethanol, and phenol a	Ni/CaO-CeO2-ZrO2	WI	FB	600	3	1.408	10	94/90	80	[98] 2025
(rBO b)	Ni/CeO2-ZrO2-CaO	WI	FB	550	4	0.48	15	90/89	77	[84] 2020
	NiAl2O4 spinel/Dolomite	CP	FB & FDB	600	3.4	6.67	10	99/95	69	[99] 2023
	Ce-Ni-Co/Al2O3-CaO	WI	FB	750	12	0.15	1	90	85	[100] 2023

^a simulated bio-oil, ^b raw bio-oil, ^c 1st cycle/last cycle

summarizes some published papers on the SESR of bio-oil using model compounds, simulated bio-oil, and raw bio-oil, and summarizes the main characteristics and operating conditions. The effectiveness of the SESRBO process in producing highly pure H₂ is clearly evident.

4.1. Effect of Operating Parameters

4.1.1. Effect of Temperature

Temperature is a critical parameter that influences not only the catalytic performance but also the CO₂ sorption capacity of BFMs. Furthermore, it directly affects the overall reliability of the process and the long-term stability of BFMs by governing key deactivation mechanisms, i.e., coke deposition and sintering. Basically, the carbonation reaction occurs when the CO₂ partial pressure (${\mathrm{P}}_{\mathrm{C}{\mathrm{O}}_2}$) in the flow stream exceeds the equilibrium partial pressure of CO₂ (${\mathrm{P}}_{\mathrm{C}{\mathrm{O}}_{2,\mathrm{e}\mathrm{q}}}$) at a specific temperature. The difference between these terms represents the driving force (${\mathrm{P}}_{\mathrm{C}{\mathrm{O}}_2}$ − ${\mathrm{P}}_{\mathrm{C}{\mathrm{O}}_{2,\mathrm{e}\mathrm{q}}}$) of the carbonation process [96]. When ${\mathrm{P}}_{\mathrm{C}{\mathrm{O}}_2}$ increases, the system shifts towards CaCO₃ formation [101]. However, it is important to recognize that ${\mathrm{P}}_{\mathrm{C}{\mathrm{O}}_{2,\mathrm{e}\mathrm{q}}}$ also rises with the temperature, as described by Equation (10), thereby reducing the driving force and the carbonation reaction rate. Thus, a moderate increase in temperature can enhance reaction kinetics and CO₂ diffusion only to a certain extent, beyond which the thermal degradation of CaCO₃ occurs (usually around 700 °C) [102]. Wang et al. [103] further optimized the temperature in both steam reforming of glycerol (SRG) and sorption-enhanced steam reforming of glycerol (SESRG). From a thermodynamic perspective, SESR is inherently more complex than conventional SR, primarily because it simultaneously involves both the carbonation and WGS reactions, which are exothermic. These reactions provide additional heat to the integrated system; as a result, the optimal reaction temperature for SESR was found to be approximately 100 °C lower than that for conventional SR. In support of this, Dang et al. [89] experimentally evaluated the effect of reaction temperature on the performance of SESR of phenol in the range of 500–650 °C at a S/C ratio of 11 over a 5Ni/CaO-Ca₁₂Al₁₄O₃₃ BFM with Ca:Al = 2.8. Their results showed that above 650 °C, the purity of H₂ exhibited a gradual decline, whereas the concentrations of CO and CO₂ followed an opposite trend (Figure 9a) because WGS and carbonation reactions are not favored at such high temperatures.

$$P_{{CO}_2,eq}(atm)=4.137\times {10}^7{exp}^{\left(-{\displaystyle \frac{20474}{T(K)}}\right)}$$

(10)

In their study on SESR of raw bio-oil over a Ni/Ce_1.2Zr₁Ca₅ BFM, Li et al. [84] reported a noticeable decrease in H₂ yield from 77.8% to 71.8% when the temperature increased from 550 °C to 650 °C, respectively (Figure 9b). Although steam reforming is an endothermic process that typically benefits from higher temperatures, excessive heating (>650 °C) can promote coke formation and favor undesired thermal cracking reactions of organic compounds. However, operating at temperatures lower than 500 °C is also detrimental, as it promotes the formation of unstable by-products from the decomposition and dehydration of bio-oil constituents. In line with this, Esteban-Díez et al. [94] concluded that 575 °C was optimal for the SESR of acetic acid and acetone, model compounds of bio-oil. A remarkably high H₂ purity (99.3–99.4%) and high H₂ yields (90.2–95.9%), together with low concentrations of CH₄, CO, and CO₂ were achieved using S/C ratios of 3 for acetic acid and 5 for acetone. Additionally, the SESR of tested blends of acetic acid and acetone (with molar ratio of 1:3, 1:1 and 3:1, respectively) led to lower H₂ yield values, ranging from 83.3% to 88.6%, when compared to the SESR of individual model compounds, whose yields were 90.2% (acetic acid) and 95.9% (acetone). This decrease in H₂ yield becomes more pronounced as the proportion of acetone in the blend increases, due to the relative stability of its carbonyl group. It therefore becomes difficult to convert it completely under SESR conditions.

The energy requirements associated with elevated temperatures significantly influence the overall energy efficiency of the SESR system. Therefore, a blind increase in reaction temperature is not an effective strategy. Instead, an optimal temperature must be carefully determined to balance the kinetics and thermodynamics of SR, WGS, and carbonation. At relatively low temperatures (<500 °C), the reforming reaction is kinetically limited, resulting in incomplete hydrocarbon conversion and lower hydrogen yield. In contrast, at higher temperatures (>650 °C), faster kinetics enhance the overall conversion, but the exothermic WGS and carbonation reactions become less favorable, consequently reducing CO₂ capture efficiency and increasing the risk of catalyst sintering and coke formation. To sum up, operating temperatures between 550 °C and 600 °C seem to be a reasonable regime for SESRBO.

4.1.2. Effect of Steam

Another key factor is the presence of steam, which directly affects the sorption and reforming processes. In the intensified process that integrates steam reforming with in situ CO₂ capture, steam is a predominant component of the reaction environment (>40% of the reaction gas). Steam plays a dual role as a reactant and a medium for heat transfer. Numerous studies have reached a consensus that the presence of steam improves the overall sorbent capacity. For instance, Linden et al. [104] observed that raising the steam pressure to 0.3 atm at a carbonation temperature of 550 °C resulted in a 33% improvement in CaO conversion. The precise mechanism by which steam impacts the carbonation reaction remains, however, under debate. Thus, several hypotheses have been proposed in the literature, which can be broadly categorized and summarized as follows:

(i)

Chemical and kinetic points of view: The enhancement of carbonation conversion in the fast reaction-controlled stage is attributed to the formation of Ca(OH)₂ as a transient intermediate (Equation (11)). This hypothesis found support in experiments conducted by Shokrollahi Yancheshmeh et al. [75], who associated the accelerated carbonation rate during the initial stage with the formation of bicarbonates, which enable the sorption of two CO₂ molecules per active site, thereby enhancing the CO₂ capture efficiency of CaO-based sorbents (Equation (12)). Consequently, it could be concluded that the carbonation reaction in CaO-based BFMs is kinetically more favorable at wet carbonation conditions.

$$\mathrm{C}\mathrm{a}\mathrm{O}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)\leftrightarrow \mathrm{C}\mathrm{a}\left({\mathrm{O}\mathrm{H})}_2\right(\mathrm{s}),\mathrm{\Delta }{\mathrm{H}}_{298}^{\mathrm{o}}=-69.1\mathrm{kJ}/\mathrm{mol}$$

(11)

$$\mathrm{C}\mathrm{a}\left({\mathrm{O}\mathrm{H})}_2\right(\mathrm{s})+{2\mathrm{C}\mathrm{O}}_2(\mathrm{g})\leftrightarrow {\mathrm{C}\mathrm{a}\left(\mathrm{H}\mathrm{C}{\mathrm{O}}_3\right)}_2,\mathrm{\Delta }{\mathrm{H}}_{298}^{\mathrm{o}}=-158\mathrm{kJ}/\mathrm{mol}$$

(12)

(ii)

Morphological point of view: The dynamic formation and decomposition of Ca(OH)₂ leads to an increase in the surface area and an expansion in pore volume. This improvement in the textural properties under wet conditions promotes CO₂ diffusion. Increasing the steam concentration during the carbonation process serves to alleviate the decline in CO₂ capture capacity by developing a more stable pore structure. As a result, it supports a higher carbonation rate, particularly during the second carbonation step, which is predominantly controlled by CO₂ diffusion. Wang et al. [105] proposed that OH formation is crucial for enhancing CO₂ diffusion in the product layer. Therefore, the carbonation reaction is governed by the diffusion of CO₃²⁻ and O²⁻ ions between reaction interfaces.

Although several studies reported that the role of steam during carbonation was limited to promoting the sorption capacity [106,107], Elsaka et al. [97] recently demonstrated that the presence of steam improved both sorption capacity and cyclic stability of UGSO-stabilized BFM (Ni-CaO-UGSO) under wet conditions (5 and 9.5 vol.% steam), with nearly zero reduction in CO₂ capture capacity over 15 carbonation-calcination cycles (Figure 10). Nevertheless, while moderate steam improves the sorption performance, an excessive increase in its concentration during the carbonation process can negatively impact performance by reducing ${\mathrm{P}}_{\mathrm{C}{\mathrm{O}}_2}$ in the reaction environment. This reduction in ${\mathrm{P}}_{\mathrm{C}{\mathrm{O}}_2}$ lowers the driving force for carbonation, thereby potentially diminishing CO₂ uptake. Also, during the calcination stage, steam plays a beneficial role. The introduction of steam into the calcination atmosphere lowers the decomposition temperature of CaCO₃, thereby reducing the risk of CaO sintering [75].

Regarding the effect of steam on the reforming process, the S/C (or (S/BO)) ratio directly influences energy consumption, gas product quality, and H₂ yield. For both SR and SESR processes, increasing the S/C molar ratio consistently improves hydrogen yield and purity. This enhancement is accompanied by a reduction in CH₄ and CO concentrations. A noteworthy advantage of a high S/C is the reduction of coke formation by promoting steam dissociation and enhancing coke gasification (Equation (8)) [1]. For example, in SRBO over a Ni/La₂O₃–αAl₂O₃ catalyst, the S/C ratio (1.5–6) was systematically studied [108]. At 700 °C and S/C of 6, H₂ purity (66%) and yield (93%) were achieved. Although the hydrogen yield decreased to 70% after 7 h due to catalyst deactivation, increasing the S/C ratio effectively minimized coke deposition and enhanced catalyst durability. In comparison, lower S/C ratios (1.5 and 3) resulted in more severe deactivation, with hydrogen yields dropping to 40% and 63%, respectively. In a related work, Ghungrud and Vaidya [109] studied the effect of the S/C ratio (3–12) in the sorption-enhanced steam reforming of ethanol (SESRE) over BFMs composed of cobalt (10 wt.%) and calcium-based sorbents stabilized with CeO₂, ZrO₂, and MgO. The highest H₂ concentration (92.1%) was achieved with Co-CaO-CeO₂ for a S/C ratio of 10 (Figure 11a). Notably, an extremely high S/C ratio led to the dilution of H₂ concentration and limited CO₂ sorption. As a result, from an economic perspective, it has been recommended that a S/C value within 3–6 would be reasonable for reforming reactions on an industrial scale. This suggestion underscores the importance of balancing the S/C ratio to achieve optimal performance in industrial processes and economic considerations.

4.1.3. Effect of Other Operating Factors

Selection of an appropriate space velocity in the SESR process is crucial for achieving high H₂ production and minimizing the reactor size. It has been reported that H₂ yield decreases with the space velocity. The shorter contact time at higher weight hourly space velocity (WHSV) values results not only in lower fuel conversion but also in increased coke deposits.

It is important to note that the feeding flow rate in SESR is more critical than in traditional SR because reforming is a faster reaction than carbonation. Therefore, achieving complete CO₂ separation from the product stream resulting from reforming and WGS reactions is more effective at lower feeding rates, thereby enhancing the sorption process. In their work, Liu et al. [86] conducted a set of experiments using Ni-CaO-Al₂O₃ BFM to explore the effect of feeding rate ranging from 0.03 to 0.08 mL/min on the SESR of bio-ethanol. The findings illustrated in Figure 11b indicate a clear trend: increasing the feeding rate significantly shortened the duration of the pre-breakthrough period.

The effectiveness of the SESR process largely depends on the reactor configuration, especially when processing complex feedstocks such as raw bio-oil. Landa et al. [99] investigated the effect of reactor configuration (specifically, fixed-bed (FB) and fluidized-bed (FDB) reactors) on coke deposition during SESR of raw bio-oil, using a mixture of NiAl₂O₄ catalyst and dolomite. Under optimized conditions (600 °C and S/C ratio of 3.4), the initial hydrogen yields during the CO₂ capture period reached 99% in the PBR and 92% in the FBR. Despite the slightly lower H₂ yield observed in the FBR, this configuration provided better resistance to catalyst deactivation, mainly due to enhanced coke gasification enabled by continuous particle movement.

4.2. Progress in the SESRBO

4.2.1. Development and Application of Hybrid Catalyst-Sorbent Materials

Hybrid materials in the SESR process can be broadly categorized into two main patterns: (i) bifunctional materials (BFMs), in which both the catalyst and sorbent are integrated within the same particles, and (ii) mechanically mixed systems, where the catalyst and sorbent are physically blended but remain as separate entities. Among them, BFMs are considered more advantageous due to the shorter distance between the catalyst, sorbent, and stabilizer components [26]. This intimate contact promotes more efficient heat and mass transfer, thereby accelerating the carbonation reaction and improving the overall sorption kinetics [41].

Still, sintering of the sorbent is the main cause of BFM deactivation, leading to two main issues: (i) the agglomeration of ultrafine particles resulting in the destruction of the porous structure [83], and (ii) a reduction in the specific surface area and pore volume of CaO-based sorbents [110]. It is widely recognized that the sintering of sorbents is significant, especially in the initial cycles of carbonation-calcination [111]. Silaban et al. [112] noticed a reduction of the sorption capacity of CaO-derived limestone from 61% to 35% of its theoretical value only after 6 carbonation-calcination cycles. Similarly, other studies reported a reduction of more than 45% in the sorption capacity of CaO sorbent derived from limestone after only 5 carbonation-calcination cycles [113]. This degradation is due to the fact that the carbonation reaction of CaO occurs at temperatures higher than TT of CaCO₃ (533 °C), which leads to a dramatic deterioration in specific surface area due to the agglomeration of small CaO particles [114].

It is important to note that the regeneration step, particularly with CaO-based sorbents, typically requires temperatures above 700 °C; consequently, it increases energy consumption and accelerates sintering. Therefore, optimizing regeneration conditions, particularly calcination temperature and duration, is crucial for preserving the structural integrity and reactivity of CaO [13]. Excessively high calcination temperatures (>850 °C) or prolonged regeneration times accelerate pore collapse, resulting in a significant loss of surface area and CO₂ uptake capacity. Conversely, incomplete regeneration at too low temperatures (<650 °C) can result in residual CaCO₃, lowering sorption efficiency in subsequent cycles [99]. Two approaches have been proposed to improve the calcination process: the first involves developing materials with high resistance to sintering, while the second focuses on lowering ${\mathrm{P}}_{\mathrm{C}{\mathrm{O}}_2}$ by injecting steam, thereby reducing the driving force for CaCO₃ decomposition [115,116]. Alternatively, Dang et al. [117] introduced an innovative approach by developing an integrated process combining SESRG with methane reforming of carbonates over a 1% palladium (Pd) incorporated into Ni-Ca-Al bifunctional material. Through this strategy, several advantages are achieved: (i) the sintering of CaO particles was eliminated by lowering the calcination temperature from 800 °C to a more practical 650 °C. Such a reduction in calcination temperature can significantly decrease thermal energy demand; for example, Nimmas [118] reported that even a 50 °C reduction in calcination temperature during SESRE resulted in a 14% decrease in thermal energy consumption. (ii) Validation of the sustainable carbon cycle was achieved through valorization of the released CO₂ during the calcination process. The developed material demonstrated about 80% CH₄ conversion, and 98.5% H₂ concentration was also generated.

Improvement of Sorption Capacity

The growth of CaO grain sizes and the generation of CaCO₃ on the sorbent surface led to a dramatic reduction in sorbent porosity and, consequently, the CO₂ sorption capacity over multiple cycles. To enhance the sorption capacity of CaO-based sorbents, various strategies, including additional pretreatments such as using organic CaO precursors, hydration reactivation, and the template-assisted approach, have been explored. Furthermore, researchers have investigated the effects of different calcium precursors and the doping of elements on improving texture properties [92].

Basically, synthetic CaO-based sorbents have better performance than natural sorbents due to the enhanced porosity and the presence of nanosized particles [84]. This superiority is linked to the decomposition of organic precursors (e.g., citric acid) during calcination, which releases gases and volatile organic compounds such as acetone, thereby fostering a more porous structure. For example, Nimmas et al. [87] studied the effect of different precursors on the properties of CaCO₃. Four distinct sorbent precursors, including calcium chloride (CaCl₂) and calcium acetate (CaAc₂) as calcium precursors, and sodium carbonate (Na₂CO₃) and urea (CO(NH₂)₂) were used as carbonate precursors. The results revealed that the sorbent derived from CaAc₂-Urea showed the highest CaO conversion of 80% at 700 °C due to its textural properties (S.A: 9.83 m²/g, P.V: 0.063 cm³/g, and P.S: 27 nm, see Figure 12). Additionally, whole synthetic CaO-based sorbents were employed in (Ni/Ca₁₂Al₁₄O₃₃-stabilized CaO) BFMs for H₂ production via SESRE. Among them, NiO/CaO_Ac2-Urea-Ca₁₂Al₁₄O₃₃ achieved reasonable H₂ purity (87%) at pre-breakthrough duration for 60 min with good cyclic stability over 10 SESR cycles. A similar investigation was conducted by Yang et al. [119] to produce four types of CaO-based sorbents from different calcium precursors (calcium acetate monohydrate, calcium carbonate, calcium hydroxide, and calcium oxide) through a combination of calcination and hydration reactions. In cyclic CO₂ capture experiments, it was observed that the CaO-based sorbents obtained from calcium acetate and calcium carbonate exhibited better CO₂ sorption capacities (0.299 and 0.284 g_CO2/g_sorbent at 650 °C, respectively) compared to 0.253 g_CO2/g_sorbent for CaO. These developed sorbents showed excellent cyclic stability, maintaining CO₂ capture capacities of 0.303 and 0.285 g_CO2/g_sorbent, respectively, after 9 carbonation-calcination cycles.

During the carbonation process, the transition to the 2nd carbonation (diffusion-controlled) stage (Figure 8a) occurs when the CaCO₃ layer reaches a thickness of approximately 50 nm [120]. Therefore, nano-scaled CaO helps accommodate the significant volumetric expansion during the carbonation process, because CaCO₃ has nearly twice the molar volume of CaO (36.9 vs. 16.7 cm³/mol, respectively). In light of this concept, Luo et al. [121] successfully synthesized nano-sized CaO-based sorbent using a sol-gel method with a calcium nitrate tetrahydrate precursor. Compared to micro-sized sorbent, the resulting sorbent displayed well-dispersed, uniform particles of 200 nm. This innovative sorbent demonstrated a remarkable CO₂ sorption capacity of 0.51 g_CO2/g_sorbent and good stability over 20 carbonation-calcination cycles.

Hydration is a pivotal strategy for enhancing the efficiency of CO₂ capture in CaO-based sorbents; the associated reaction is represented by Equation (11). It is well known that Ca(OH)₂ has a higher CO₂ sorption capacity compared to CaO [122]. The morphological characteristics of sorbents produced from hydrated and untreated dolomite were compared by Rong et al. [123], who found that hydrated dolomite produced sorbents with a more porous structure, a higher surface area, and a higher pore volume than those produced from untreated dolomite.

The template-assisted approach is a reliable route for producing hollow and porous CaO-based materials, offering benefits such as enhanced stability, precise control, and predictable properties. This process involves three key steps: (i) creating the template, (ii) covering it with CaO-based precursors, and subsequently (iii) burning the template. The final materials typically retain the structure of the template, resulting in a highly porous framework with a large surface area. In their study, Akgsornpeak et al. [124] utilized the sol-gel method to synthesize CaO from Ca(NO₃)₂ precursor and explored the influence of Ca(NO₃)₂ concentration and the addition of cetyltrimethylammonium ammonium bromide (CTAB), a widely used cationic surfactant, on CaO properties. The results indicated that a higher Ca(NO₃)₂ concentration led to a reduction in particle size. The addition of CTAB, however, had mixed effects: it resulted in larger agglomerated particles (negative effect) but also increased porosity and reduced particle size (positive effect). This led to the tailoring of morphology, indicative of an optimal porous structure that offered a higher active area for CO₂ capture.

Dang et al. [125] proposed a hollow porous Ni-Ca-Al-O BFM synthesized from an HTl precursor using a two-step hydrothermal method (Figure 13a) to enhance the performance of SESRG. The hollow microsphere structure formed by layered metal oxides effectively alleviated the sintering of Ni and CaO. As a result, the developed BFM demonstrated good performance, maintaining 99% hydrogen purity for 25 min (pre-breakthrough period). However, after 20 consecutive SESRG-regeneration cycles, the duration of this intensified process decreased to less than 15 min, corresponding to approximately 60% of its initial value. In a recent study, Olivier et al. [126] synthesized Ca₁₂Al₁₄O₃₃-stabilized CaO sorbent using a microsphere-assisted method to create a hollow sphere structure (Figure 13b). This advanced approach significantly improved both sorption capacity and cyclic stability of the developed materials compared to traditional methods. After 15 carbonation-calcination cycles at 600 °C, the material exhibited minimal sorption capacity loss (<1%), whereas the material produced without carbon microspheres underwent a 34% decline.

Other treatments have also demonstrated significant improvements in sorbent effectiveness. For example, Radfarnia et al. [127] proposed a novel two-step calcination technique, involving an initial treatment in argon (inert) atmosphere followed by calcination in air, which significantly improved sorption activity compared to one-step calcination. In another study, Shokrollahi Yancheshmeh et al. [34] developed a novel synthesis approach for Ni-CaO-based BFMs, namely Ca₃Al₂O₆-CaO/NiO-CeO₂. They first prepared three pure CaO sorbents from natural limestone using three different treatment methods (i.e., thermal decomposition, acid treatment, and ethanol/water treatment). The results revealed that the ethanol/water treatment method yielded the highest CaO molar conversion, owing to the formation of a well-developed porous structure. The cyclic experiments demonstrated higher activity and stability over 10 SESRG cycles, with a constant H₂ purity of around 96%. Even simple treatments, such as washing natural limestone, have proven essential for removing impurities (e.g., NaCl and sulfur-containing species) that could negatively affect the sorption capacity of CaO. By investigating the influence of ball milling treatment of UGSO on the CO₂ capture performance of CaO-10 wt.% UGSO sorbent, Aissaoui et al. [93] demonstrated that reducing the UGSO particle size by ball-milling significantly enhanced the cyclic stability of the developed material during repeated carbonation-calcination cycles. After 18 cycles, the ball-milled sorbent exhibited more than a 7% higher CaO conversion compared to the untreated sample. This enhancement was attributed to the improved textural properties (30.5 vs. 13.5 m²/g), which mitigated sintering and maintained higher sorption activity over repeated operation.

Enhancement of Pore Structure Stability

During SESR cyclic operation, the tendency of CaO to sinter and agglomerate into larger particles becomes pronounced. Therefore, incorporation of a stabilizer into the BFM matrix may prove to be essential due to (i) the large gap between the desorption temperature of CaO (>800 °C) and the TT of CaCO₃ (~533 °C), and (ii) the weak interaction between Ni and CaO [128]. For optimal stabilization, the employed synthesis methods must ensure uniform distribution of the inert phase between CaO particles to effectively reduce particle agglomeration [129] (Figure 14). Additionally, the ratio between sorbent and stabilizer must be optimized. A higher ratio of stabilizers improves the cyclic stability. However, it reduces the portion of active CaO, resulting in a lower sorption capacity. It is therefore imperative to develop and synthesize well-designed BFMs in order to significantly improve the efficiency of SESR processes.

Hashemi [131] used seven distinct metals (aluminum, cerium, lanthanum, magnesium, neodymium, yttrium, and zirconium) with an established molar ratio of metal to sorbent (M:Ca = 1:14) to investigate their performance as stabilizers within a Ca-based sorbent. The findings revealed that Mg-stabilized CaO demonstrated the highest cyclic stability, its superiority being attributed to the elevated TT of MgO, which exceeds 1250 °C. In another study, Rahmanzadeh and Taghizadeh [132] reported an enhancement in the CO₂ sorption capacity and cyclic stability of several developed CaO-based sorbents. As shown in Figure 15a, Zr-CaO and Na-Zr-CaO demonstrated more stable sorption capacity, slightly shifting from 0.337 and 0.348 in the 1st cycle to 0.329 and 0.336 at the 5th cycle, respectively. In contrast, the unmodified CaO sorbent displayed significant instability, with more than a 50% reduction in the sorption capacity at the last carbonation cycle. Ca₃Al₂O₆ has also demonstrated a significant stabilizer effect in the Ru/Ca₃Al₂O₆-CaO BFM [133]. As it has yielded excellent results during methane SESR (Figure 15b), this BFM could find potential applications in other SESR processes, including SESRBO.

Using stabilizers with high redox properties is of great interest. Since it can facilitate the oxidation of carbonaceous species and the activation of water, which ultimately increases both the activity and stability of BFM. In particular, CeO₂ is an appealing choice for this purpose [134]. In this regard, Ghungrud et al. [109] studied the effect of CeO₂, ZrO₂, and MgO as inert stabilizers for Co-CaO BFMs over 20 SESRE cycles. The study emphasized that CeO₂ and ZrO₂-stabilized BFMs displayed superior sorption capabilities and an extended pre-breakthrough period compared to MgO-stabilized BFM. Specifically, the CeO₂-stabilized BFM achieved the longest pre-breakthrough duration, lasting up to 45 min. In another study, Hu et al. [85] synthesized a series of Ni/Ce_xZr_1-xO₂-CaO (Ni/CZC) BFMs and reported that Ni/CZC-2.5 displayed excellent CO₂ sorption capacity over 15 consecutive carbonation-calcination cycles, even at a high calcination temperature of 900 °C. During SESR of acetic acid at 550 °C and S/C = 4, Ni/CZC-2.5 showed a high H₂ purity (98%) during the pre-breakthrough (maximum intensified) period of 18 min. This performance was attributed to the presence of CaZrO₃ phase, which acted as a barrier against CaO sintering at high temperature.

Developed BFMs stabilized on porous materials, e.g., meso- and microporous, spinels, perovskites, and nanocomposite materials, offer enhanced metal dispersion and reduced sintering for the effective SESR process [15]. In this context, Elsaka et al. [98] successfully synthesized a well-designated CeO₂-ZrO₂ nanocomposite (CZ) via a hydrothermal method and investigated it, for the first time, as a stabilizer in Ni/(CaO-20CZ) BFM for the SESRBO. As shown in Figure 16, the developed BFM demonstrated good stability over 10 consecutive SESRBO cycles in terms of hydrogen purity, yield, and pre-breakthrough duration. Up to the 10th cycle, the catalyst sustained stable performance, achieving an average hydrogen purity of 92% for approximately 27 min. The slight decline observed during the initial cycles, particularly the reduction of the pre-breakthrough period, was ascribed to incomplete decarbonation and/or structural rearrangements occurring at the regeneration temperature (750 °C) [135]. In another study, Aihara et al. [136] developed CaO/CaTiO₃ composites, which demonstrated a CaO conversion rate of 0.65 (2.6 times higher than that recorded for pure CaO without CaTiO₃ incorporation) after 10 carbonation-calcination cycles. By using hydrothermal synthesis, Wu et al. [137] synthesized a CaO-based metal-organic framework (MOF) co-promoted with MgO/Al₂O₃, which exhibited excellent CO₂ capacity (up to 0.72 g_CO2/g_sorbent) and maintained a stable performance level of 77% over a series of 100 cyclic tests.

Although a direct comparison among the various BFMs is very difficult due to wide variations in operating and regeneration conditions,

Table 5. Performance of BFMs prepared by different synthesis methods during cyclic SESR operation.

Hybrid Material (SESR Feedstock)	Prep. Method	Conditions			CO2 Capture Capacity (gCO2/gsorbent)		SESR Conditions			H2 Purity (%)		Ref.
		Carbonation	Calcination	No. of Cycles	1st Cycle	Last Cycle	Reaction	Regeneration	No. of Cycles	1st Cycle	Last Cycle
Co/CaO-Ca12Al14O33 (Glycerol)	WI	550 °C, 20% CO2/N2, 40 min	700 °C, 100% N2, 60 min	10	0.22	0.17	525 °C, S/C: 4, FR:0.02 mL/min	700 °C, 100% Ar, 60 min	50	96	94	[138]
Ni/CeO2-ZrO2 + CaO (Mechanical mixture) (sBO a)	WI	600 °C, 15% CO2 + 9.5% H2O/N2, 25 min	750 °C, 100% N2, 40 min	15	0.37	0.22	-	-	-	-	-	[98]
Ni/CaO-CeO2-ZrO2 (sBO a)	WI + HT	600 °C, 15 vol.% CO2 + 9.5% H2O/N2, 25 min	750 °C, 100% N2, 40 min	15	0.42	0.38	600 °C, S/C: 3, WHSV: 1.408 h−1	750 °C, 100% Ar, 20 min	10	94	90	[98]
Ni/CaO-CeO2-ZrO2 (Acetic acid)	SG	600 °C,10% CO2/Ar, 40 min	900 °C, 100% Ar	15	0.31	0.27	550 °C, S/C: 4, FR: 0.016 mL/min	700 °C, 100% N2, 60 min	15	98	88	[85]
Ni-Al2O3/CaO pellet b (Ethanol)	SG + CT	850 °C, 70% CO2/N2, 5 min	850 °C, 100% N2, 5 min	100	0.57	0.21	650 °C, S/E = 4, FR: 0.08 mL/min	900 °C, 100% N2, 15 min	10	95	90	[139]
Ni/CaO-Al2O3 (Ethanol)	CP	600 °C, 15% CO2/N2, 30 min	900 °C, 100% N2, 30 min	20	0.61	0.48	600 °C, S/E: 4, FR: 0.05 mL/min	900 °C, 10% H2/N2, 15 min	10	96	90	[86]
Ni/CaO-Ca12Al14O33 (Phenol)	CP + CAT	-	-	-	-	-	575 °C, FR: 0.04 mL/min	800 °C, 100% N2, 30 min	50	98	98	[140]
Ni/CaO-Ca12Al14O33 (Glycerol)	HT	-	-	-	-	-	550 °C, S/C = 4, FR: 0.02 mL/min	800 °C, 100% N2, 30 min	20	99	99	[125]
Ni/CaO-Ca12Al14O33 (Glycerol)	WI + CM	650 °C, 20% CO2/N2, 30 min	750 °C, 100% N2, 30 min	15	0.45	0.37	600 °C, S/C:3, WHSV: 2.59 h−1	700 °C, 100% Ar	9	93	91	[126]

^a simulated bio-oil composed of 35% acetic acid, 35% acetone, 20% ethanol, and 10% phenol. ^b CaO pellets were manufactured by an extrusion-spherization technique. S/E: steam-to-ethanol; SG: Sol–gel; CT: Cellulose template; CAT: carbon template; CM: carbon microsphere.

provides an overview of the cyclic stability of BFMs prepared through different synthesis routes. Obviously, BFMs developed via hydrothermal and carbon-template methods (in particular, Ni/CaO-Ca₁₂Al₁₄O₃₃) demonstrate superior stability, maintaining nearly constant H₂ purity and exhibiting minimal decay in CO₂ uptake during multiple cycles. In addition to the presence of stable mixed oxide phases such as Ca₁₂Al₁₄O₃₃, this improved stability primarily stems from the structural and morphological advantages of these synthesis techniques. In the hydrothermal route, controlled crystallization promotes the integration of Ni and CaO phases, thereby generating highly dispersed particles that remarkably reduce sorbent agglomeration [123]. Similarly, the carbon-template approach typically produces mesoporous structures that enhance CO₂ diffusion and provide sufficient space to accommodate the volume expansion during carbonation [137]. Therefore, controlled synthesis strategies and well-optimized regeneration conditions are crucial for mitigating sorbent deactivation and ensuring the sustained performance of BFMs during cyclic SESR processes.

From a sustainable perspective, valorization of waste materials derived from different industrial processes related to coal-fired thermal power plants, mines, metallurgy, etc., is an extremely attractive approach [141]. This strategy offers cost-effectiveness and a positive environmental impact [142]. Given that this topic is relatively recent, and the majority of studies have emerged in literature within the last ten years, there is ample space for innovative ideas in several key areas: (i) the exploration of waste sources, (ii) their suitable pre-treatment/activation routes, and (iii) advanced synthesis methods for waste-derived-material-supported and -stabilized catalysts and BFM.

Table 6. Performance of different waste material-stabilized CaO sorbents.

Sorbent	Waste	Prep. Method	Carbonation Conditions	Calcination Conditions	Cycles	Sorption Capacity at Last Cycle (gCO2/gsorbent)	Specific Surface Area (m2/g)	Ref.
CaO-24% SiO2	Husk ash	DM	700 °C, 15% CO2, 25 min	950 °C, 100% N2, 10 min	10	0.44	-	[143] 2012
CaO-SiO2	Husk ash	WI	700 °C, 15% CO2, 15 min	850 °C, 100% N2, 20 min	20	0.39	25	[144] 2012
CaO-10% UGSO	UGSO	WI	650 °C, 15% CO2, 9.5% H2O, 20 min	750 °C, 100% Ar, 40 min	18	0.54	30.5	[93] 2020
CaO-50% FA	Fly ash	CP	700 °C, 15% CO2, 20 min	850 °C, 100% N2, 10 min	20	0.37	16.41	[145] 2017
CaO–CS	Carbide slag (CS)	-	750 °C, 100% CO2, 60 min	900 °C, 100% N2, 90 min	20	0.42	20.8	[146] 2016
CaO–Nano SiO2	Photovoltaic waste (SiCl4)	DM	700 °C, 100% CO2, 5 min	900 °C, 100% N2, 3 min	30	0.32	7.46	[147] 2016
CaO-10% FA	Fly ash	WI	650 °C, 15% CO2, 9.5% H2O, 30 min	750 °C, 100% N2, 40 min	20	0.45	28.95	[92] 2020
Ni-CaO-10% FA	Fly ash	WI	550 °C, 15% CO2, 9.5% H2O, 30 min	800 °C, 100% N2, 20 min	20	0.57	23.83	[92] 2020
Ni-CaO-UGSO	UGSO	WI	650 °C, 15% CO2, 9.5% H2O, 25 min	750 °C, 100% N2, 40 min	18	0.45	10.5	[97] 2024

DM: dry mixing.

shows the potential of industrial wastes to provide a practical and effective approach to mitigating material sintering. It demonstrates the ability of industrial waste materials to produce highly reactive sorbents with suitable thermal stability during cyclic performance.

In Iliuta’s research group, many efforts are being made to valorize complex wastes (such as UGSO and FA) in order to develop catalytic and BFMs [148]. For example, Aissaoui et al. [93] studied the effect of UGSO as a stabilizer for increasing the cyclic stability of CaO-based sorbents. A UGSO-stabilized CaO material was developed via a wet mixing method involving limestone acidification followed by sonication and two-step calcination. During SESRG at T = 550 °C and S/C = 3, H₂ purity of about 95% was achieved during the pre-breakthrough period and lasted for ≈ 30 min. In another study, Gao et al. [92] investigated the use of various FA samples (from different sources around the world, referred to as FA1-FA12) as stabilizers for synthesizing CaO-FA sorbents and Ni-CaO-FA BFM. CaO-FA5 BFM containing 10 wt.% FA5 displayed the best sorption capacity and cyclic stability during 20 sorption-calcination cycles, attributed to a higher specific surface area of this BFM and the presence of relatively high amounts of inert components (Al₆Si₂O₁₃ and SiO₂) in FA5. Ni-CaO-FA5 consistently achieved high and stable hydrogen purity of approximately 97% and a yield of about 90% in SESRG. Similarly, the application of Ni/CaO-based BFMs stabilized by FA or UGSO in the SESRBO [97] led to very high H₂ purity (>96%) at 550 °C. The remarkable cyclic stability of BFM-UGSO was mainly due to the uniform distribution of mixed oxides generated by Ni and CaO interaction with UGSO-containing metal oxides.

Improvement of Coke Deposition Resistance

Whereas the SESR process has widely demonstrated the advantage in reducing the coke deposits due to (i) the basic nature of the CO₂ sorbent (e.g., CaO) and (ii) the relatively low operating temperature compared to SR [89,92], SESRBO still faces this issue, albeit to a lesser extent [31,99]. In this context, Landa et al. [31] highlighted that the type of reforming process influences both the content and the nature of coke deposits. As illustrated in Figure 17, the total coke deposition under SESRBO conditions was significantly lower than that of conventional SRBO. In the absence of sorbent (sorbent to catalyst ratio (SC_MR) = 0), SRBO led to a coke content exceeding 18 wt.%, predominantly consisting of high-temperature (HT) filamentous carbon, whereas SESRBO has reduced coke formation by nearly half, with a larger fraction of low-temperature (LT) amorphous carbon. Further increasing SC_MR to 10–20 effectively suppressed coke accumulation.

In general, coke deposition during the SESR reaction is affected by three main factors:

(i)

The chemical composition of the feeding stock [31,99]. For example, Dang et al. [90] and Gao et al. [92] observed low carbon deposition during SESR of glycerol. However, for crude glycerol, the tendency of coke formation (both amorphous and graphitic) becomes much more significant due to the presence of some constituents, especially acetic and oleic acids [30]. An increased formation of coke was also noted in another study on SESR of crude glycerol, mainly due to the presence of fatty acid methyl esters [149]. According to TPO results reported by Li et al. [84] (Figure 18), the amount of coke in the case of raw bio-oil (resulting from sawdust pyrolysis) was much higher than that of acetic acid after SESR over Ni/Ce_1.2Zr₁Ca₅ BFMs. The authors argued that the presence of high C/H compounds, such as phenols and their derivatives, in raw bio-oil accelerates coke formation, as these compounds polymerize into complex carbonaceous structures (coke precursors), leading to catalyst deactivation [150]. Similarly, Esteban-Diez et al. [94] achieved high H₂ purities (99.2–99.4%) and H₂ yields of 90.2–95.9% during the SESR of individual model compounds of BO (acetic acid and acetone) at 575 °C (optimized temperature) and a S/C of 3, using a mechanical mixture of Pd/Co-Ni + dolomite with an SC_MR ratio of 5. However, when processing the blend of these compounds in the same conditions, H₂ yield was reduced to 83.3–88.6%. This significant decline was attributed to incomplete reactant conversion caused by excessive coke deposition.

(ii)

Operating temperature. Valle et al. [32] emphasized that temperature has a significant influence on the nature of the coke formed. After SR of raw bio-oil (from pine sawdust) at 550 °C, amorphous carbon was detected on the spent catalyst (Ni/La₂O₃-αAl₂O₃), compared to the filamentous one at 700 °C. A similar conclusion was reached by Landa [31]. The reactor type also influences the coke deposition. Unlike fixed-bed reactors, fluidized-bed reactors are less prone to coke formation. For instance, after 30 min of time on stream (TOS) during SESRBO runs operated by [99], the coke deposited on the developed hybrid material (mechanical mixture of Ni/Al₂O₃ (catalyst) + dolomite (sorbent)) was reported as 3.7% and 2.9% in the fixed- and fluidized-bed reactors, respectively.

(iii)

BFM composition. Supporting materials and catalyst promoters play a crucial role in enhancing the removal of carbon deposits from the catalyst/BFM surface. In particular, rare-earth oxides such as CeO₂, ZrO₂, and La₂O₃ are widely recognized for their effectiveness in enhancing coke resistance in catalytic processes by facilitating redox reactions, enhancing oxygen mobility, and promoting carbon gasification. The presence of basic oxide supports like alkaline earth oxides (e.g., MgO) and transition metal oxides (e.g., Fe₂O₃) improves the neutral property of acidic supports, such as alumina, by mitigating coke formation on strong acidic sites [35,151].

4.2.2. Process Simulation and Kinetics Insights

To obtain a reliable intensification suitable for industrial application, achieving high H₂ purity is not the sole objective; a prolonged pre-breakthrough period is also imperative in the process design and operation to minimize the reactor switching, thereby enhancing the process reliability [78]. This conclusion was also highlighted by Iliuta et al. [152], who developed a mathematical model for simulating the SESRBO process. The results showed that the extended pre-breakthrough stage associated with high H₂ production can be achieved through increasing the sorbent loading and S/C ratio, as well as reducing the inlet gas velocity. While raising the inlet temperature increases the pre-breakthrough duration, it decreases the H₂ purity. The same group evaluated for the first time the kinetics of CO₂ sorption and steam reforming of bio-oil over Ni-CaO-UGSO and CeNi-CaO-UGSO BFMs, and further developed a comprehensive mathematical reactor model to evaluate the performance of the SESR process [153]. The good agreement between the simulation and the experimental data of different operation temperatures (600–650 °C), WHSVs (1.408–2.816 h⁻¹), and SESRBO cycles (1–5 cycles) (Figure 19) confirms the reliability of the proposed models in facilitating the design of larger-scale SESRBO operations.

5. Conclusions and Recommendations

This review highlights the recent advancements, challenges, and opportunities in steam reforming and sorption-enhanced steam reforming of bio-oil for hydrogen production. Based on the key findings discussed, several limitations and future trends are summarized as follows:

(1)

Bio-oil (derived from biomass) has a high energy density. While it represents a complex mixture of very interesting compounds, such as ethanol, acetic acid, and phenol, it also contains considerable amounts of water. Therefore, the steam reforming of bio-oil enables its direct utilization and eliminates the need for an expensive water separation step before use (an economic and practical advantage). This process displays the feasibility of reliable H₂ production.

(2)

Due to the intrinsic complexity of bio-oil, most works considered single-model compounds to study the steam reforming of bio-oil. However, to truly understand the complexity of this process, significant research on a more realistic bio-oil composition is highly needed.

(3)

Combining the valorization of (i) bio-oil (liquid waste from residual biomass) as reforming feedstock and (ii) solid residues as supporting material for SRBO and/or stabilizers for SESRBO represents an effective approach for sustainable and renewable hydrogen production, maximizing the benefit of the available resources, while also reducing the environmental impact.

(4)

Metal sintering and coke deposition are key challenges for commonly used catalysts. Further in-depth research is still required to better understand the mechanisms underlying these phenomena.

(5)

Achieving high H₂ purity alone is not sufficient to ensure the practicality of the intensification process in industry. Extending the pre-breakthrough period through proper system design and operation is equally vital to limit switching between reforming/carbonation and sorbent regeneration periods, thereby optimizing efficiency and economic viability.

(6)

In the context of the future development of SESRBO, particular attention should be paid to catalyst activity loss caused by coke deposition, which is particularly pronounced for feedstocks with a high C/H ratio, such as simulated and raw bio-oils.

(7)

The successful industrial implementation of SESRBO requires sustained development and optimization of cost-effective BFMs that exhibit high activity, long-term stability, and strong regenerative capacity. Integrating solid industrial waste into the formulation of BFMs can also represent a promising approach, offering strong synergy between economic benefits and environmental considerations.

(8)

The combination of SESRBO with other advanced technologies, such as dry reforming of methane, could offer significant potential in terms of reducing thermal energy requirements and improving overall system efficiency. In such a configuration, the CO₂ captured by the sorbent during the SESR process can then be used as a reactant in dry reforming of methane, enabling the production of high-purity hydrogen.

(9)

Converting the in situ-captured CO₂ into value-added chemicals (e.g., CO/syngas, methanol, formic acid, etc.) could enhance the process economics and establish integrated biorefinery concepts.

(10)

Further research is still needed to fully understand the reaction kinetics and mechanisms involved in sorption-enhanced processes, as well as to optimize the treatment of raw or aqueous bio-oil mixtures for practical applications. In this context, reactor simulation can be combined with quantum mechanical modeling, such as Density Functional Theory (DFT), which provides a powerful tool for investigating reaction mechanisms with high accuracy and microscopic resolution, particularly in CO₂ capture and related catalytic processes.

(11)

To assess the economic feasibility of the SESRBO process, a comprehensive techno-economic analysis of the integrated system would be of interest, considering factors such as high-temperature regeneration and reactor switching requirements for cyclic operation.