Biomass-Derived Tar Conversion via Catalytic Post-Gasification in Circulating Fluidized Beds: A Review

Abstract

Waste biomass gasification can contribute to the production of alternative and environmentally sustainable green fuels. Research at the CREC–UWO (Chemical Reactor Engineering Center–University of Western Ontario) considers an integrated gasification process where both electrical power, biochar, and tar-free syngas suitable for alcohol synthesis are produced. In particular, the present review addresses the issues concerning tar removal from the syngas produced in a waste biomass gasifier via a catalytic post-gasification (CPG) downer unit. Various questions concerning CPG, such as reaction conditions, thermodynamics, a Tar Conversion Catalyst (TCC), and tar surrogate chemical species that can be employed for catalyst performance evaluations are reported. Catalyst performance-reported results were obtained in a fluidizable CREC Riser Simulator invented at CREC–UWO. The present review shows the suitability of the developed fluidizable Ni–Ceria γ-alumina catalyst, given the high level of tar removal it provides, the minimum coke that is formed with its use, and the adequate reforming of the syngas exiting the biomass waste gasifier, suitable for alcohol synthesis.

1. Introduction

Biomass gasification is a process with high prospects for manufacturing alternative energy [1,2]. Biomass utilization offers, however, special challenges. Biomass is a low bulk density feedstock that is collected across large, distant, and frequently remote areas and is available on a seasonal basis, with transportation costs being a key for its utilization.

During gasification, biomass (solid or liquid) is converted into syngas, biochar, and tar [3,4]. The produced biomass-derived syngas (BMD-syngas) can be used as a chemical feedstock or, alternatively, as a fuel for heat and/or energy generation, while biochar can have valuable applications as a soil fertility enhancer. Biomass gasification can lead to the formation of tars, which consist of a liquid fraction, with molecules with carbon numbers exceeding six. Tar can be a nuisance for the syngas produced in view of its combustion in gas turbines [5].

Tars are considered to be a product of biomass pyrolytic decomposition during gasification [6]. When condensed in various gasifier downstream units, tars may cause operational problems and lower the quality of the produced syngas [7,8]. The proportions of biomass constituents, including cellulose, hemicellulose, and lignin, can affect the products obtained from the gasification process. For instance, when lignin is the main biomass constituent, the lowest gas yields of a low calorific value are obtained [6].

Table 1. Issues Related to the Presence of Tars in the Producer Gas [9].

Issues	Explanation
Pipeline blockage	Gaseous tars condense at low temperatures and obstruct pipelines.
Corrosion in downstream equipment	Tars have an acidic nature and can corrode downstream equipment if condensed.
Catalysts deactivation	Tars can deposit on the catalyst’s active sites.
Reduction in process efficiency	Formed tars can diminish the available energy in the produced syngas by up to 10–15%.
Production of phenolic wastewater	Phenolic compounds in tar can contaminate process wastewater. Contaminated process water has to be treated.
Human and environmental issues	Most tars are carcinogenic.

describes the different process issues that occur when tars are formed during biomass gasification. These unresolved issues may be classed as (a) pipeline blockage, (b) corrosion of downstream equipment, (c) catalyst deactivation, (d) reduction in gasification efficiency, (e) formation of phenolic species contaminating process water, and (f) environmental issues harmful to health.

Thus, a catalytic post-gasification (CPG) unit using a tar conversion catalyst (TCC) with formed tars being converted into valuable products such as H₂, CO, and CH₄ can play a significant role in biomass gasification viability. It is with these facts in mind that the present review assesses the possible formulation of new catalysts for the conversion of derived tars from biomass. Their evaluation is conducted using an advanced laboratory-scale fluidized reactor designated as the CREC Riser Simulator invented and developed at the Chemical Reactor Engineering Center (CREC), at the University of Western Ontario. It is also with this aim that this review considers a continuous CPG unit for tar conversion in the context of an integrated waste biomass gasification plant.

2. Biomass and Biomass Gasification Overview

2.1. Biomass Properties

Biomass is an organic matter, from either plant or animal origin, that is living, or was living, in the recent past. During the photosynthesis process, the energy required for biomass synthesis comes from the sun. Biomass is considered to be a renewable resource. It is a raw material that is CO₂ neutral, given that the same moles of carbon used for biomass photosynthesis are returned to the atmosphere when combusted [10]. When biomass C-H, C-O, and C-C bonds are broken, enthalpies of 337 kJ/mol, 1,076 kJ/mol, and 607 kJ/mol [11], respectively, are required [10]. Furthermore, when compared to fossil fuels, one can see that biomass conversion leads to lower gas emissions of chemicals such as SO_X and NO_X [10].

Biomass is mainly composed of cellulose, hemicellulose, and lignin components. The amount of cellulose, hemicellulose, and lignin present in biomass can vary from one source to another.

Table 2. Composition of different types of biomass [10].

Lignocellulosic Material	Cellulose (wt%)	Hemicellulose (wt%)	Lignin (wt%)
Hardwood stems	40–55	24–40	18–25
Corncobs	45	35	15
Grasses	25–40	35–50	10–30
Wheat straw	30	50	15
Leaves	15–32	80–85	0
Sugarcane bagasse	32–44	27–32	19–24

reports the cellulose, hemicellulose, and lignin of different types of biomass.

Additionally, biomass can be classified based on its origin, such as agricultural waste, forest waste, or industrial waste [9].

2.2. Biomass Gasification

Gasification is one thermochemical process that can be used to convert waste biomass into biofuels, gases, and chemicals [10,12]. When evaluating biomass in terms of its capacity to produce energy, one can envision it as a partial oxidation process [5], where a low-energy-density material is converted into gaseous products with a higher calorific value.

During the gasification process, a wide variety of heterogeneous and gas-phase reactions take place.

Table 3. Summary of Biomass Gasification Reactions [13]. Note: m and n subscripts represent the H and C element content in the ${\mathit{C}}_{\mathit{n}}{\mathit{H}}_{\mathit{m}}$. Formula.

Stoichiometry	Reaction
$\mathrm{B}\mathrm{i}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\to \mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}+\mathrm{t}\mathrm{a}\mathrm{r}+{\mathrm{H}}_2\mathrm{O}+\mathrm{g}\mathrm{a}\mathrm{s}\mathrm{e}\mathrm{s} (\mathrm{C}\mathrm{O},\mathrm{C}{\mathrm{O}}_2,\mathrm{C}{\mathrm{H}}_4\dots )$	Biomass volatilization	(1)
$\mathrm{C}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\to \mathrm{C}\mathrm{O}$	Partial combustion	(2)
$\mathrm{C}+{\mathrm{O}}_2\to \mathrm{C}{\mathrm{O}}_2$	Complete combustion	(3)
$\mathrm{C}+{\mathrm{C}\mathrm{O}}_2\to 2\mathrm{C}\mathrm{O}$	Boudouard reaction	(4)
$\mathrm{C}+{\mathrm{H}}_2\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$	Water–gas shift or carbon-steam reaction	(5)
$\mathrm{C}+2{\mathrm{H}}_2\to \mathrm{C}{\mathrm{H}}_4$	Methane formation	(6)
$\mathrm{C}\mathrm{O}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\to \mathrm{C}{\mathrm{O}}_2$	Carbon monoxide oxidation	(7)
${\mathrm{H}}_2+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}$	Hydrogen oxidation	(8)
${\mathrm{C}\mathrm{H}}_4+2{\mathrm{O}}_2\to {2\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$	Methane oxidation	(9)
${\mathrm{H}}_2\mathrm{O}+\mathrm{C}\mathrm{O}\to {\mathrm{H}}_2+\mathrm{C}{\mathrm{O}}_2$	Water–gas shift reaction	(10)
${\mathrm{H}}_2+\mathrm{S}\to {\mathrm{H}}_2\mathrm{S}$	Hydrogen sulfide formation	(11)
${\displaystyle \frac{1}{2}}{\mathrm{H}}_2+{\displaystyle \frac{1}{2}}{\mathrm{N}}_2\to \mathrm{N}{\mathrm{H}}_3$	Ammonia formation	(12)
${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}+{\displaystyle \frac{\mathrm{n}}{2}}{\mathrm{O}}_2\to \mathrm{n}\mathrm{C}\mathrm{O}+{\displaystyle \frac{\mathrm{m}}{2}}{\mathrm{H}}_2$	Partial oxidation	(13)
${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}+\mathrm{n}\mathrm{C}{\mathrm{O}}_2\to 2\mathrm{n}\mathrm{C}\mathrm{O}+{\displaystyle \frac{\mathrm{m}}{2}}{\mathrm{H}}_2$	Dry reforming	(14)
${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}+\mathrm{n}{\mathrm{H}}_2\mathrm{O}\to \mathrm{n}\mathrm{C}{\mathrm{O}}_2+\left({\displaystyle \frac{\mathrm{m}}{2}}+\mathrm{n}\right){\mathrm{H}}_2$	Steam reforming	(15)
${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}+{\displaystyle \frac{2\mathrm{n}-\mathrm{m}}{2}}{\mathrm{H}}_2\to \mathrm{n}\mathrm{C}{\mathrm{H}}_4$	Hydrogenation	(16)
${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}\to {\displaystyle \frac{\mathrm{m}}{4}}\mathrm{C}{\mathrm{H}}_4+{\displaystyle \frac{\mathrm{n}-\mathrm{m}}{4}}\mathrm{C}$	Thermal cracking	(17)

summarizes these possible gasification reactions, such as volatilization, partial combustion, water–gas shift, the Boudouard reaction, dry and steam reforming, and ammonia synthesis, among others. The various reactions of this complex reaction network determine the composition of the resulting gasification products.

The ensemble of these reactions together leads to a syngas blend consisting of CO, CO₂, H₂, CH₄, and H₂O, together with a solid residue and a liquid fraction designated as biochar and tar, respectively.

3. Tar Properties

Tars include a wide variety of chemical compounds. As a result, over the years, different tar definitions have been proposed, with tar classifications being based on diverse criteria. For example, in the Brussels Meeting of the IEA Bioenergy that took place in 1998, tars were defined as “hydrocarbons of higher molecular weight than benzene” [14]. In addition, tars are considered to be constituted of condensable organic compounds, mainly aromatic species, produced under thermal or partial-oxidation conditions [15]. However, it is found that in the above-described biomass-derived tars, one should also include oxygenated hydrocarbons, sulfur and chlorine species, alkali metals, and fuel-bound nitrogen [16].

One should mention as well, that the maximum amount of tars allowed in the producer gas depends on the specific syngas application. Valderrama et al. [17] state that tar content in syngas should be less than 100 mg/Nm³ in the case of internal combustion engines, while for gas turbines, it should be less than 5 mg/Nm³, and for fuel cell and methanol production, it should be less than 1 mg/Nm³.

Furthermore, tar composition can be affected by the gasifier’s operational conditions, such as temperature, total pressure, steam-to-biomass ratio, type of biomass feedstock, and gasifying agent. Given all this, tars can be classified based on (a) tar molecular weight, (b) tar molecular structure, (c) tar solubility, and (d) tar condensability.

An important physicochemical property of tar is its “dew point”. This is the temperature at which the tar partial pressure equals the tar saturation vapor pressure [18]. Thus, when the tar partial pressure is higher than the tar saturation vapor pressure, tar condenses. The tar dew point can be affected by tar composition. For example, in the case of polyaromatic hydrocarbons (e.g., naphthalene, anthracene), tar compositions have an important effect on tar dew point. On the other hand, for water-soluble tars (e.g., pyridine, phenol, cresols, toluene, xylene), tar composition modestly affects the tar dew point [16]. When these tars are condensed, they lead to equipment corrosion and affect the downstream gasification process.

Tars resulting from gasification can also be classified and studied on the basis of the extent of biomass conversion, as reported in

Table 4. Tar Classification Based on the Extent of Biomass Conversion During Gasification [6,15].

Class	Description	Examples of Constituent Chemical Species
Primary tars	Tars are composed of cellulose and hemicellulose-pyrolysis-derived products. Tars originate early during gasification.	levoglucosan, furfural
Secondary tars	Tars are the result of secondary gasification steps, during which primary tars are further converted with steam and oxygen into phenolic and olefin species, using a temperature in the 500 °C to 750 °C range.	phenolic, olefins
Tertiary tars	Tertiary tars originate from secondary tar species. Tars are composed of aromatic species with methyl substituent groups.	toluene, indene
Condensed Tertiary tars	Condensed tars originate from secondary tars, being composed of polyaromatic hydrocarbons without substituent groups.	benzene, naphthalene

[6,15].

Table 5. Typical Composition of Tars Obtained from Biomass Gasification [19].

Chemical Species	Composition (%)
Toluene	24
Other 1-ring aromatic hydrocarbons	22
Naphthalene	15
Other 2-ring aromatic hydrocarbons	13
3-ring aromatic hydrocarbons	6
4-ring aromatic hydrocarbons	1
Phenolic compounds	7
Heterocyclic compounds	10
Others	2

reports the typical percentages of aromatics, phenolic, heterocyclic, and other chemical constituents that make up a typical tar composition.

Considering these various issues regarding tar and the complexity of its composition, CREC–UWO researchers selected 4-methoxy-2-methylphenol (4M2MP) as a surrogate model compound [20] to study the efficiency of tar removal via CPG due to its similarity with lignin in terms of its oxygen, carbon, and hydrogen element contents. A 4M2MP displays a good balance between aromatic, methyl, methoxy, and hydroxyl groups. All these chemical groups are expected to be contained in lignin.

4. Catalytic Removal of Tars

An approach designated as “hot-gas tar cleaning” can be used to promote a series of tar conversion reactions during tar catalytic decomposition. Catalysts used in the “hot tar cleaning process” can be used both inside the sand gasifier unit itself [4,21,22,23,24,25] and in a separate fluidized unit following the sand fluidized gasifier [26]. Regarding these two possible options, the one where the CPG unit follows the sand gasifier is preferred by CREC researchers [27] to minimize issues with sand, ash, and catalyst separation in the industrial process.

In the two different unit configurations, catalysts may help favorably by simultaneously modifying the gasifier-produced syngas composition and by removing tar in a process where it decomposes into smaller molecules, such as permanent gases (H₂, CO, CO₂, CH₄) [6,28,29,30]. Thus, the function of these catalysts is to boost the tar removal rate and/or the selectivity of gasification reactions toward higher H₂/CO ratios [31].

The “hot-tar cleaning” process requires a fluidizable catalyst that addresses the following issues: (a) the metal crystallites and the catalyst support do not lead to the formation of unreactive solid phases (e.g., nickel aluminates) and (b) the metal crystallites themselves do not yield undesirable crystallite agglomeration, in a process where there may be a reduction in metal dispersion and an increase in the crystallite sizes [32].

Natural mineral materials such as olivine, clay minerals, calcined rocks, or ferrous metal oxides can be considered good catalysts for tar removal. Alternatively, one can employ synthetic materials such as char or activated carbon, catalytic cracking catalysts, alkali alumina, or transition metals (Ni, Pt, Zr, Ru) [33]. However, these various materials frequently display limits in their applications given the following: (a) they do not fluidize well, (b) they have limited resistance to attrition, and (c) they do not recover activity completely following catalyst regeneration.

Table 6. Advantages and Disadvantages of Using Different Catalytic Materials for the Conversion of Biomass-derived Tars. Adapted from [6,25,26,32,33,34,35,36,37].

Catalyst	Advantages	Disadvantages
Natural catalyst-Olivine	Suitable as a bed material; Better resistance to attrition than dolomite; Available at low cost.	Moderate or low activity for tar cracking; Poor performance in producing gaseous hydrocarbons; Limited specific surface area (olivine 0.42 m2g−1) Poor fluidization.
Alkali metal Catalyst	Good for syngas production and tar conversion; High tar-reforming efficiency; High hydrogen yields; Good for tar catalytic cracking; Good resistance to carbon deposition.	Not suitable as a secondary catalyst component in bimetallic formulations; Significant production of methane at >800 °C; Easy agglomeration, leading to pipe clogging at high temperatures; Produces huge amounts of ash; Costly and has poor economic viability.
Nickel-based catalyst	Adequate to be dispersed in metal-supported catalysts; High tar-reforming efficiency; High hydrogen yield; High tar conversion efficiency (>99%); Suitable bimetallic formulations; Compatible with the use of different supports and promoters.	Deactivates with tar-containing syngas; Susceptible to sulfur poisoning; Frequent coking; Drops catalytic activity due to sintering; Restricted to temperatures below 780 °C for better lifespan.

describes the advantages and disadvantages of using the above-described materials as catalysts for tar conversion.

Thus, for the “hot tar cleaning” process, catalysts involving the following characteristics are required [17,37]:

• Good activity and efficient tar conversion;
• Good activity for the water–gas shift reaction when aiming to achieve high H₂ concentrations;
• Good stability for coke deactivation and H₂S poisoning;
• Easy regeneration;
• Good resistance to attrition, with this being particularly important if the gasification involves a fluidization unit;
• Commercial availability and low cost;
• Free of negative environmental impact.

Typical TCCs employed in the “hot-tar cleaning process” have complex compositions and involve high surface area materials [38,39]. They contain three solid phases: (a) the active phase, (b) the promoter phase enhancing catalytic activity and/or stability, and (c) the support providing a high surface area that facilitates the dispersion of the active phase.

Transition metals can act as active phases given (a) their partially empty d-orbital, (b) their adequate crystallite surface for the adsorption of reactant molecules, (c) their ability to lower the activation energy involved in energy transition states, and (d) their property of exhibiting multiple oxidation states. Studies have shown the good performance of noble metals, such as Ru, Rh, Pd, Os, Ir, Pt, Au, and Ag. [40,41,42]. However, these catalysts are not attractive due to their high cost.

Consequently, Ni-based catalysts are considered a good option, as they display good performance at a lower cost [36]. Ni-based catalysts can contribute to the cracking of both C-H and C-C bonds. These catalysts have also shown good performance in biomass gasification, particularly given that they promote both steam and dry reforming to form hydrogen at adequately selected operating conditions [37]. Likewise, Ni-based catalysts can activate the H₂O and CO₂ species present, promoting both tar reforming and water–gas shift reactions [40].

Ni-catalysts can also be modified with other metals in order to improve Ni dispersion, oxygen capacity, reducibility, activity, and stability. In spite of Wulff’s rule, where the morphology of a crystalline substance determines the crystal planes exposed on the surface [43], catalysts usually exhibit an uneven and irregular morphology, with precursors and synthesis methods having an impact on this matter. This modification with other metals can also lead to changes in specific surface area and Ni crystallite particle size, with larger catalyst surface areas enhancing Ni particle dispersion.

TCC catalysts are manufactured using a catalyst support. The TCC catalyst support should have a porous and high specific surface area. Possible catalyst support materials can be perovskites, biochar, activated carbon, zeolites, silica gels, activated clays, and activated alumina. These support materials have a wide range of surface areas (1.5 to 1500 m²/g), pore volumes (from 0.4–1 cm³/g), and pore diameters (ranging from 0.4 to 2000 nm). One can consider Al₂O₃ as a valuable support choice, given its thermal stability and physicochemical properties [41]. Al₂O₃ can be found in different phases. However, the most popular ones for catalytic purposes are the α and the γ phases. Al₂O₃ displays advantages as a support for tar conversion catalysts, promoting strongly binding active phases that prevent metal leaching and the formation of acidic and basic sites and oxygen vacancies. Similarly, silica can be used as a support. Silica shares some of the advantages of Al₂O_3, such as having a high surface area, thermal stability, and preventing metal sintering due to the formation of metal silicates.

Given the various catalyst support alternatives available and the research focus on fluidized catalysts for CPG, a commercially available Sasol fluidizable γ-Al₂O₃ with good attrition resistance was selected by the CREC–UWO team. Furthermore, a cerium oxide alkaline earth metal was considered to reduce the acidity of the strongest acid sites of the γ-Al₂O₃ support [27]

5. Catalyst Deactivation by Coke

The mechanisms of coke formation have been extensively studied [44], and it is accepted that they follow three steps: (i) hydrogen transfer at acid sites, (ii) dehydrogenation of hydrocarbons, and (iii) polycondensation. Catalysts can deactivate in different ways, with the formation of coke causing catalyst deactivation (active site coverage) and/or pore blocking [45].

In particular, catalysts used in tar removal are susceptible to the formation of coke. This is due to the splitting of the C-H bonds [36], which leads to the formation of carbon-rich deposits [46]. Three types of coke have been identified: pyrolytic, encapsulated, and whisker carbon types. Pyrolytic and whisker coke are commonly formed at temperatures above 650 °C [36], while encapsulated coke is more likely to be formed at lower temperatures (600 °C) [47].

In the case of Ni-based catalysts, the whisker type of coke is the most common. This type of coke can be characterized as being filamentous and associated with high temperatures, low water content, and the presence of aromatics [36]. When filamentous coke is formed, the active surfaces of the catalysts are still available. However, as the catalysts are reused, the carbon accumulates, leading to an eventual significant drop in catalytic activity [46].

Thus, as a result, it is worth stating that the evaluation of coke formation under the 550–600 °C anticipated temperature conditions commonly employed with fluidized catalysts, as the one developed by CREC–UWO researchers [48] is of paramount importance, for the CPG process.

6. Ceria as a Promoter of Tar Removal Catalysts

A promoter can be a key component of a TCC [49,50,51]. In a CPG unit, a catalyst promoter can help to moderate the formation of carbon and to oxidize the carbonaceous species [52]. This can be achieved with the extra oxygen storage capacity and oxygen mobility induced by a promoter. Oxygen vacancies and oxygen transport to the active sites can help steam reforming and coke elimination, as well as assist the water–gas shift reaction [33]. In this manner, the inclusion of a promoter in a TCC can assist in delaying catalyst deactivation [27].

Ceria is a good candidate as a catalyst promoter, given its high oxygen storage capacity and the fact that it can absorb and release oxygen reversibly via oxygen vacancies [53,54]. CeO₂ has a fluorite cubic unit cell structure, as shown in Figure 1. In the fluorite, each Ce⁴⁺ is coordinated with eight oxygen atoms. Furthermore, each O^2- in the tetrahedral space is, in turn, coordinated with the four Ce⁴⁺ nearest atoms [55].

There is also evidence that ceria can improve the dispersion of the active species [56]. CeO₂ can delay the transition phase of alumina from gamma to alpha at temperatures above 730 °C [57]. Moreover, ceria can enhance the activity, stability, and resistance to coke formation on Ni/Al₂O₃ catalysts [56]. Altogether, when Al₂O₃ and CeO₂ are combined, the catalyst shows better stability and greater redox activity. This is the case given that fluorite displays vacant octahedral holes that can facilitate oxygen mobility through a defect structure [58].

It has been recognized that CeO₂ loadings ranging from 1 wt% to 5%wt play an important role in reducing the interaction between the Ni-based catalyst and the Al₂O₃ support. This leads to a stronger dispersion of the nickel particles. Other than the nickel dispersion, CeO₂ facilitates the electron transfer [27].

Table 7. CeO₂ Promoted Catalysts Used for Methane Dry Reforming.

Catalyst	Conditions	Results Obtained
Ni/CeO2-Al2O3 [53]	Aerogel and xerogel catalysts prepared via a sol–gel method, with 9 wt% Ni and 3 wt% ceria; Two different calcination temperatures used: 823 K and 1023 K; Good performance for methane dry reforming.	These Ni–ceria–aerogel catalysts were evaluated for methane dry reforming, showing better activity and stability due to their higher density of surface-active sites; The aerogel catalysts’ stability was attributed to their higher surface area and larger pore sizes; The addition of CeO2 activates reforming reactions at lower temperatures and improves nickel dispersion. Electronic interaction with CeO2 inhibits coke formation; The aerogel catalysts with the CeO2 displayed better Ni dispersion than the xerogel with CeO2, as shown via XRD; Both Ni/CeO2-Al2O3 aerogel and xerogel catalysts showed similar reforming after 10 h. After 10 h, the aerogel reduces activity abruptly; Coke formed was found in both encapsulated and whisker forms.
Nickel supported on ceria-alumina [59]	Fixed nickel loading: 8 wt%; Ceria loadings: 0 wt%, 10 wt% and 20 wt%; Catalysts prepared following the wet impregnation method; Reaction temperatures: 700–900 °C; CH4/CO2 ratio: 1.5.	This Ni–ceria–alumina catalyst exhibited good activity for both reforming and methane decomposition reactions; Ceria promotes favored coke gasification and water–gas shift in the 500–850 °C range; NiAl2O4 decreased as the CeO2 loading increased, as shown with XRD; Addition of CeO2 improved catalytic activity, suppressed coke formation, and enhanced nickel dispersion; CeO2 led to smaller specific surface areas attributed to partial pore blockage; Both ceria-promoted and unpromoted catalysts increased methane and CO2 conversion as temperature increased. At higher temperatures, methane conversion was closer to chemical equilibrium; Both ceria-promoted and unpromoted catalysts improved hydrogen yields as temperature augmented. Hydrogen yields were higher for the promoted catalyst.
Ni-CeO2/Al2O3 [54]	Catalyst prepared by co-impregnation; Nickel loading fixed to 15 wt%; Ceria loadings varied from 0 to 6 wt%; Catalytic performance studied for CO2 methanation reaction; Feed of CO2:H2 with a 1:4 ratio; Temperature range: 200 to 450 °C.	This Ni-CeO2/Al2O3 displayed a slightly reduced surface area and pore volume, attributed to micropore blockage; The addition of ceria enhanced CO2 conversion. The unpromoted catalysts showed a 45% CO2 conversion, while with 2 wt% CeO2, the CO2 conversion increased to 71%; The CO2 conversion showed maximum values at 350 °C, with a slight decrease at higher temperature reverse methanation; The 2 wt% ceria promoted catalyst displayed enhanced stability.
Ni/CeO2-Al2O3 [60]	Preparation methods: co-precipitation, excess solution, wet impregnation, sol–gel, and citric acid; Nickel loading: 10 wt%; Ceria loading: 20 wt%; Reaction temperature range: 873–1073 K; Feed of CH4:CO2:Ar in 40:40:20 ratio.	The preparation method of the Ni/CeO2-Al2O3 affected the catalyst’s textural properties and its thermal stability; The citric acid preparation method yielded the catalyst with the best surface area and pore size distribution; Citric acid and co-precipitation preparation yielded catalysts with low-carbon deposition. Co-precipitation catalyst produced filamentous coke; Evaluated catalysts showed better methane and carbon dioxide reforming conversions and higher H2/CO ratios as temperature increased; At 1073 K, the highest CH4 conversion of 75% and 70% for the co-precipitated catalyst and the citric acid-prepared catalyst, respectively, were obtained.
Ni-CeO2/Al2O3 and Ni-MgO/Al2O3 [61]	Catalysts prepared by the wet impregnation method; Fixed nickel loading: 10 wt%; Fixed promoter loading: 10 wt%; Feed: pine wood.	These Ni-CeO2/Al2O3 and Ni-MgO/Al2O3 catalysts allowed for establishing interactions between nickel and aluminum as follows: (a) when ceria was used, reduction temperatures decreased; thus, weaker interactions were conjectured, (b) when MgO was employed as a support, higher reduction temperatures were needed, thus, stronger promoter–Ni interactions were hypothesized; FTIR analysis of used ceria-promoted catalysts showed a 3500 cm−1 band that corresponded to water adsorption. This indicated that CeO2 may enhance the performance of the water–gas shift reaction; The MgO-promoted catalyst yielded the highest coke, followed by the unpromoted catalyst and the CeO2-promoted catalyst.
Ni-CeO2-Al2O3 [62]	Catalysts prepared using co-precipitation and wetness impregnation methods; Nickel loadings varied from 15% to 30%; CeO2 from 1.5 wt% to 3 wt%; Reaction temperature range: 600–700 °C; S/M ratio range: 1.5–3.5.	This Ni-CeO2-Al2O3 was prepared using co-precipitation and impregnation methods; The co-precipitated samples displayed a better dispersion of Ni particles; The unpromoted catalyst with 25% Ni showed an XRD peak attributable to NiAl2O4. The CeO2-promoted catalyst did not show this peak, suggesting that the CeO2 decreased the interactions between the Ni and Al2O3; For impregnated samples, the optimal Ni loading was 20%, while for co-precipitated catalysts, it was 25%; A total of 1.5% CeO2 was selected as optimal for both co-precipitated and impregnated catalysts; Co-precipitated catalysts showed better steam methane reforming conversion, while the impregnated catalysts exhibited better H2/CO ratio; After a stability test, promoted catalysts yielded 4% coke, while unpromoted catalysts yielded 9% coke.
CeO2-Ni/Al2O3 [63]	Catalysts prepared using the wetness impregnation technique; Fixed nickel loading: 10 wt%; CeO2 loadings: 5 wt%, 10 wt% and 15 wt%; Reaction temperature range: 600–800 °C; Feed: CH4 and CO2; Stability test for 36 h at 800 °C.	This CeO2-Ni/Al2O3 catalyst showed a reduced surface area, pore size, and pore volume with CeO2 addition; The calcined samples did not show any peaks associated with NiO (from XRD). This was attributed to a good nickel particle dispersion; The H2-TPR showed a predominant peak at around 800 °C, attributed to NiAl2O4 spinels; CH4 and CO2 dry reforming conversion increased with temperature. All catalysts showed good performance, but the 5 wt% CeO2 doped catalyst displayed best results; As temperature increased, chemical species distributions were closer to chemical equilibrium; CO2 conversion was higher than CH4 conversion. This was attributed to the reverse water–gas shift reaction; Based on TGA-DTA, the 5 wt% CeO2-modified catalyst had a lower weight loss than the unpromoted one. This showed that CeO2-enhanced catalyst had better stability, with less coke formed.

reports a summary of CeO₂-doped Ni Catalysts on Al₂O₃ supports used for methane dry reforming, as reported in the technical literature.

While these studies provide valuable references and precedents, one has to note that the performances of these Ni/CeO₂-Al₂O₃ catalysts were evaluated using the methane dry-reforming reaction in fixed bed laboratory scale units. Fluid dynamic and reaction conditions in these fixed units differ significantly with respect to the ones anticipated in circulating fluidized beds, as is the case in the CPG process of the present review.

7. The Catalytic Post-Gasification (CPG) Process

An integrated gasification process with CPG, as the one described in Figure 2, should involve two steps. In the first step, biomass is gasified in a fluidized gasifier unit. In the second step, tars are converted as follows: (a) biomass-derived tars are converted into lighter hydrocarbons consisting of CO, CO₂, and H₂; (b) the composition of the syngas exiting the gasifier is modified to an H₂/CO ratio larger than 2, as required for methanol synthesis. These two functions of the CPG unit are expected to be conducted via dominant tar steam cracking and water–gas shift reaction functionalities imparted to the TCC catalyst.

It is significant to observe that a successfully integrated gasification process, including tar CPG, should involve several steps with adequate thermal compatibility between the steps as follows:

(a)

T_gasifier = 800 °C > T_CPG = 550 °C;

(b)

T_regenerator = 615° C > T_{CPG downer} = 550 °C;

(c)

T_regenerator close to T_reducer,

with T_gasifier, T_CPG, T_regenerator, and T_reducer representing selected temperatures in the gasifier unit, the catalytic post-gasification unit, the regenerator, and the reducer units, respectively.

In this manner, the various units of the integrated gasification process, as shown in Figure 2, can operate within the desirable thermal ranges, maximizing enthalpy recovery for electrical power production and yielding syngas with a composition compatible with alcohol synthesis.

Thus, to accomplish this, a fluidizable TCC in a CPG unit is required to perform at 500–550 °C, 2.2 atm, and short contact times (e.g., 10 s). Furthermore, a high-temperature heat exchanger, as shown in Figure 2, is needed to recover a significant fraction of the available process thermal enthalpy for electrical power generation via a steam turbine. One should note that the temperature operating conditions at the syngas side of the heat exchanger are reduced from 800 °C to 550 °C to prevent tar condensation between the gasifier exit and the CPG downer unit entry.

8. Thermodynamics of Syngas Composition During Catalytic Post-Gasification Tar Conversion

The main anticipated reactions in catalytic post-gasification can be designated as being of the “primary type” or of the “secondary type”. The “primary type” reactions, such as surrogate tar steam cracking, are represented by Equation (18). The “secondary type” reactions (water–gas shift reaction, steam, and dry reforming) are described with Equations (19)–(21). The extent of these secondary reactions is strongly affected by the thermal level, the reaction environment (e.g., partial pressure of primary chemical product species and steam), and thermodynamics.

$${\mathrm{C}}_8{\mathrm{H}}_{10}{\mathrm{O}}_2+{\mathsf{\epsilon }\mathrm{H}}_2\mathrm{O}\to {\mathsf{\alpha }\mathrm{H}}_2+\mathsf{\beta }\mathrm{C}\mathrm{O}+\mathsf{\gamma }\mathrm{C}{\mathrm{O}}_2+\mathsf{\delta }{\mathrm{C}\mathrm{H}}_4+\mathsf{\sigma }{\mathrm{C}}_{\mathrm{m}}{\mathrm{H}}_{\mathrm{n}}{\mathrm{O}}_{\mathrm{o}}+\mathsf{\omega }{\mathrm{C}}_{\left(\mathrm{s}\right)}$$

(18)

where α, β, γ, ε, δ, σ, ω are stoichiometric coefficients; C_mH_nO_o is a generic hydrocarbon product resulting from the cracking and reforming of tar, and C_(s) is a solid carbonaceous deposit, designated as coke.

$$\begin{array}{cc}\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}_2+\mathrm{C}{\mathrm{O}}_2& \text{Water–Gas Shift (WGS)}\end{array}$$

(19)

$$\mathrm{C}{\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{C}\mathrm{O}+3{\mathrm{H}}_2\hspace{1em}\hspace{1em}\hspace{1em}& \text{Methane Steam Reforming (MSR)}$$

(20)

$$\mathrm{C}{\mathrm{H}}_4+{\mathrm{H}\mathrm{O}}_2\leftrightarrow 2\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2\hspace{1em}\hspace{1em}\hspace{1em}& \text{Methane Dry Reforming (MDR)}$$

(21)

While the overall tar conversion (R1) can be represented by Equation (18), the actual distribution of gaseous products in the tar catalytic conversion unit is affected by the relative reaction contribution of R2, R3, and R4, as shown in Equations (19)–(21).

Given the nature of the proposed Ni-based catalyst, which strongly promotes these three reactions, one can conclude that the extent of these R2, R3, and R4 reactions influence syngas composition. In order to clarify this matter, the K_y,eq equilibrium constants, at 550 °C and 800 °C and with total pressure at 2.2 atm are reported in

Table 8. Equilibrium Constants for WGS, MSR, and DMR. Note: K_y,eq is a chemical equilibrium constant defined on the basis of species molar fractions at given temperatures and at a given pressure.

Reactions	Designation	Ky,eq Defined at 800 °C and 2.2 atm Expected Gasifier Conditions	Ky,eq at 550 °C and 2.2 atm Expected CPG Conditions
R2	Water–Gas Shift (WGS)	0.55	3.74
R3	Methane Steam Reforming (MSR)	6.45	0.0058
R4	Methane Dry Reforming (MDR)	9.79	0.0021

, based on the chemical species molar fractions. These two temperatures are the anticipated typical thermal levels for the CPG and biomass gasifier units, respectively.

The thermodynamic equilibrium calculations in Table 8 show a dominant influence of R2, R3, and R4 reactions at gasification conditions (e.g., 800 °C and 2.2 atm), while only R2 rules at 550 °C in the CPG unit. In other words, one can expect that the composition of syngas coming from the gasifier is transformed in CPG, and this is given the significant influence of the water–gas shift reaction, with a substantial increase in the H₂/CO ratio.

Table 9. Typical syngas exiting biomass gasifier and chemical equilibrium calculated syngas composition at the outlet of the CPG unit at water–gas shift reaction chemical equilibrium (R2) at 550 °C and 2.2 atm. These hypothetical outlet gasifier conditions are selected to illustrate results. Note: Lights, Gases, and Molar ratios at the outlet of the CPG unit as follows: H₂/CO = 2.80; CO₂/CO = 2.26; H₂/CO₂ = 1.23.

Condition 1	Syngas Composition at the Gasifier Exit [64]	Syngas at Chemical Equilibrium via R2 at 550 °C
H2	0.072	0.1304
CO	0.112	0.0536
CO2	0.16	0.218
CH4	0.056	0.056
N2	0.40	0.4
H2O	0.20	0.1415

reports a syngas mixture, including CO, CO₂, H₂, H₂O, and N₂, as typically exiting a biomass gasifier [64], which is processed later in the tar CPG conversion reactor, assuming chemical equilibrium. One can see in Table 9 that a high H₂/CO ratio (e.g., 2.8 ratio) is anticipated at the outlet of the CPG unit at thermodynamic equilibrium.

Thus, on this basis, one can expect that by using a Ni-based catalyst, the R2 reaction is strongly promoted, achieving, in this manner, both the reduction in biomass-derived tar in syngas as well as increasing the H₂/CO ratio in the CPG unit. As a result, this makes the obtained syngas blend a suitable option for the synthesis of alcohols. As shown later in this review, the reported thermodynamic analysis described in this section is critical to establishing the syngas feed composition that can be used in tar catalytic conversion experiments.

9. Biomass Gasification Catalysts Evaluated in the CREC Riser Simulator

The CREC Riser Simulator, as shown in Figure 3, is a laboratory-scale fluidized reactor operated in the “batch mode” at conditions similar to the ones of industrial fluidized risers and fluidized downers [48], in terms of chemical species reactant partial pressures, reaction times, temperatures, and catalyst/reactant ratios.

By using the CREC Riser Simulator, several catalytic gasification biomass studies have been conducted under different operating conditions in Canada, Mexico, and Saudi Arabia university laboratories [48]. The value of this unit for establishing Ni- and Fe-based fluidizable catalyst performances by using various chemical tar surrogates has been reported in a significant number of publications, as reported in

Table 10. Catalysts Evaluated in the CREC Riser Simulator.

Catalyst	Conditions	Results Obtained
Ni/α-Al2O3 [65]	Glucose was used as cellulose surrogate; Steam-to-biomass ratios: 0.4–1.0; Catalyst loading: 1 g; Catalytic steam gasification at 700 °C and 5–30 s; Non-catalytic steam gasification at 700 °C and 30 s.	This Ni/α-Al2O3 catalyst increased the hydrogen formation and obtained higher S/B ratios; S/B increases reduced the CO; The water–gas shift reaction was enhanced, with CO2 and H2 increasing and a CO decreasing; Methane yields differed from equilibrium values, indicating non-equilibrium; At shorter contact times (<30 s), catalytic gasification became kinetically limited.
20% Ni/5% La2O3-γ-Al2O3 [66]	Glucose was used as a cellulose surrogate; 2-methoxy-4-methylphenol was used as a lignin surrogate; Temperatures: 600–700 °C; Steam-to-biomass ratios (S/B): 0.6–1.0 g/g; Reaction times: 5–30 s; Catalyst/biomass ratio was fixed at 12.5.	This 20% Ni/5% La2O3-γ-Al2O3 catalyst yielded 100% glucose, with non-detectable tars being formed; With 2-methoxy-4-methylphenol, at 700 °C, this catalyst gave a 90% carbon conversion to permanent gases, 5.7 wt% tar, and 3.3 wt% of coke; The H2/CO ratio was larger than 2.0.
Fe2O3/SiO-Al2O3 [67]	Catalysts were synthesized by the One-Pot method; Toluene was used as biomass surrogate; Fe loadings: 5–15 wt%; Temperature range: 400–600 °C; Steam-to-biomass ratios: 1, 1.5; Reaction time range: 10–25 s.	This Fe2O3/SiO-Al2O3 catalyst displayed good toluene conversion; At 500 °C and 20 s, as the Fe loading increased, toluene conversion decreased from 69% to 63%; Fe loading had an influence on producer gas yields. As it increased, the CO2 yield augmented, while CO decreased. On the other hand, CH4 yields were low, in all cases; At reaction times ranging from 15 s to 20 s, toluene conversion improved from 54% to 62%. At 25 s, there was only a 2% extra increase in toluene conversion.
20% Ni/5% CeO2-γ-Al2O3 [68]	2-methoxy-4-methylphenol was used as tar surrogate; Gasifying agents: steam–inert and steam–CO2; Reaction times: 10–30 s; Temperatures: 550 °C and 600 °C.	This 20% Ni/5% CeO2-γ-Al2O3 catalyst yielded a 98.5% tar conversion under a steam–10% CO2 atmosphere at 600 °C; Under inert steam, this catalyst gave 33% higher H2 than in thermal runs; Runs under a steam–CO2 atmosphere increased the CO conversion from 8% to 19%; Unconverted tar was 34% for thermal runs, 16% for catalytic runs under inert steam, and 3% for catalytic runs under steam–CO2.
FexOy/CaO-γ-Al2O3 [69]	2-methoxy-4-methylphenol was used as a lignin surrogate; Reaction times: 3–10 s; Temperatures: 500–550 °C; Steam-to-biomass ratios (S/B): 1.0 and 1.5.	This FexOy/CaO-γ-Al2O3 catalyst gave a 99.95% tar conversion at 500 °C, at an S/B ratio of 1.5 and 7.5 s, with 0.98% of coke being formed; This catalyst performed close to thermodynamic equilibrium, yielding a 97% selectivity to light fraction products; Up to 10% of CaO helped to reduce the thermal sintering and the Lewis acidity of the alumina support, improving basicity.
5% Ni-Ru/γ-Al2O3 [70]	2-methoxy-4-methylphenol was used as a surrogate for lignin; Temperatures: 550–650 °C; Steam-to-biomass ratio (S/B): 1.5 g/g; Catalyst/biomass ratios: 2.66–7.89 g/g; Ru loadings: 0.25–1.0%; Catalysts: 50 mg, 100 mg, and 150 g.	This 5% Ni-Ru/γ-Al2O3 catalyst displayed 80% 2-methoxy-4-methylphenol conversion, at 600 °C; Most runs were developed using the 0.25%-Ru promoted catalyst and the 5% Ni-Ru/γ-Al2O3 promoted catalyst; Coke selectivity was reduced when using the Ru-promoted catalyst; Coke selectivity augmented as the catalyst/biomass ratio increased; Coke decreased as the temperature increased.
NiO-Fe2O3/SiO2- γ-Al2O3 [71]	Toluene was used as a tar surrogate; Catalyst loading: 0.3 g; Reaction temperature: 600 °C; Reaction time: 10–20 s; NiO loading: 0 wt% and 10%wt; Thermal runs were performed for comparison.	These Fe-Co/Ce-ZrO2 catalysts yielded a 45% toluene conversion at 20 s, at an S/B of 1, using a catalyst/biomass ratio of 1.5; Ni addition promoted water–gas shift and methane CO2 reforming; At 10 s to 20 s reaction time, the toluene conversion increased from 4.8% to 46.3%, with permanent gases.
Fe-Co/Ce-ZrO2 [72]	Toluene was employed as a tar surrogate; Catalyst loading: 0.376 g; Temperatures: 600–700 °C; Reaction time: 20 s; Water-to-carbon ratio: 5.6 mol H2O/mol carbon; Thermal steam reforming was used to assess the effect of the thermal reaction; Cat A: 75 wt% Ce-25 wt% Zr; Cat B: 10 wt% Fe-10 wt%Co-60 wt%Ce-20 wt% Zr; Cat C: 5 wt% Fe-5 wt% Co-70 wt% Ce-20%wt Zr.	Addition of Fe and CO facilitated the water–gas shift reaction, leading to higher concentrations of H2 and lower concentrations of CO; The increase in temperature had a positive effect on the catalytic activity dominated by water–gas shift; Cat B slightly decreased the H2 and improved the CO conversion compared to Cat C; Cat B and C exhibited lower concentrations of CH4 and CO2 produced when compared to Cat A (without Fe-Co doping). This was attributed to the basic catalyst sites with added Fe-Co catalysts that promoted endothermic CO2 methane reforming.
Ni/Ce-Mesoporous Al2O3 [73]	Glucose was used as a surrogate of Cellulose; Ni loadings: 5 wt%, 10 wt%, 15 wt%, and 20 wt%); Temperatures: 550–750 °C; Catalyst loading: 0.20 g; Biomass feed: 0.2 mL (15 wt% glucose + water); Reaction time: 20 s.	This Ni/Ce-Al2O3 catalyst yielded 35% hydrogen, with the 10 wt% Ce catalyst providing the lowest yield of CO2 (38%); The interaction between the nickel and the ceria likely led to electronic effects that helped suppress the conversion of CO to CO2 and coke; Coke decreased as Ni loading augmented.
Ce-various Al2O3 [74]	A total of 2.5 wt% toluene, 12.5 wt% glucose, and 85 wt% water used as a tar surrogate; Temperatures: 500–700 °C; Catalyst loading: 0.20 g; Catalysts evaluated: 20%Ni/γ-Al2O3, 20%Ni/meso-Al2O3, 20%Ni/Ce-γ-Al2O3, and 20%Ni/Ce-meso-Al2O3.	This catalyst involved various alumina supports; The one using the meso-Al2O3 support provided a better dispersion of Ni particles when compared to the γ-Al2O3; The ceria promoter helped to reduce the formation of coke, COx, and hydrocarbons. Hydrogen yield improved with the addition of the ceria-promoter; At 700 °C, the Ni/meso-Al2O3 catalyst gave more undesirable hydrocarbons than the other catalysts.

. In this table, both the conditions and reaction results are described in detail. In this respect, significant results were obtained by using a diversity of chemical species to simulate tar produced from biomass gasification, such as glucose, 2-methoxy-4-methylphenol, toluene, and their blends. Ni and Fe catalysts supported on fluidizable alumina were the preferred TCC catalysts. Various dopants were added to these catalysts, such as La₂O₃, CeO₂, Ru, CaO, Co, and ZrO₂.

In spite of the well-documented progress in evaluating TCC catalysts in the CREC Riser Simulator, as documented in Table 10, more recently, the CREC–UWO team has established the concept of testing the TCC catalysts (fluidizable Ni/ceria on γ-Al₂O₃ catalyst) by using a specially selected syngas blend that emulates a “Water–Gas Shift (WGS) equilibrated syngas”. The composition of this “Water–Gas Shift (WGS) equilibrated syngas” can be calculated from chemical equilibrium thermodynamics at the CPG reaction conditions, as described in Section 8.

10. The TCC Catalyst Preparation

The TCC fluidizable catalyst proposed by CREC–UWO researchers was prepared by using the incipient wetness technique. A mesoporous γ-Al₂O₃ support was employed for this catalyst. Cerium oxide (IV) was used as a catalyst promoter, and nickel was employed as the catalyst active phase. The preparation of the catalyst via the incipient wetness technique was conducted as follows: First, the catalyst precursor salts were dissolved in distilled water. Then, the amount of precursor used was based on the desired precursor loading. The volume of the impregnating precursor solution was calculated based on the pore volume of the γ-Al₂O₃ support that was obtained from the N₂ adsorption analysis. Additional details regarding the preparation of the catalyst are provided in [20].

The prepared catalyst was characterized by using (a) nitrogen physisorption to obtain the specific surface area, the average pore diameter, and the pore volume; (b) ammonia-TPD to establish the acid site strengths of the catalyst γ-alumina support, and the influence on acidity of both cerium promoter and nickel; (c) CO₂-TPD to evaluate the cerium promoter and the nickel active phase effects on support basicity; (d) H₂-TPR to assess the reduction in the nickel active phase, including the active metal surface, the metal dispersion, and the average metal crystallite size; (e) XRD to establish the crystalline structures and phases present in the catalyst; (f) pyridine FTIR to evaluate the presence of Brønsted and Lewis acid sites.

In these respects, the most promising catalyst for tar reduction, identified as consisting of 15%Ni–5%CeO2 γ-Al₂O₃, has the following properties: (a) a 139 m²/g specific surface area, (b) 15 wt% nickel loading, (c) an 84% reducible active nickel phase, (d) a 0.97% nickel dispersion, (d) a 107 nm nickel crystallite size, and (e) a 6.8 cm 3 STP ammonia acidity and a 2.4 cm³ STP/g CO₂ basicity.

11. Evaluation of a Ni/Ce-γ-Al₂O₃ Catalyst Using a Close to “Equilibrium Syngas Blend”

The performance of the 15%Ni–5%CeO₂ on γ-Al₂O₃ catalyst was evaluated in the CREC Riser Simulator. The CREC Riser Simulator is a bench-scale batch mini-fluidized reactor unit. Figure 4 and Figure 5 show schematic diagrams of the reactor section and its auxiliary components, respectively. The reaction cell, as described in Figure 4, contains the solid sample (biomass or catalyst) in a 50 cm³ volume.

The catalyst is placed in the lower shell of the reactor inside a basket contained between two grids. An impeller in the upper shell fluidizes the catalyst sample inside the basket, with the reaction gas blend moving upwards inside the catalyst basket and downwards in the outer annulus located between the basket and the reactor body walls. A metal gasket is used to seal the upper- and lower-unit shells. Both shells of the reactor are equipped with rod heaters and thermocouples, allowing one to control the reactor temperature. There is an injection port located in the lower shell. This port allows one to inject the sample of feedstock to be processed almost instantaneously. After sealing the reactor shell and doing the leak test, a thermal insulation canister is set around the CREC Riser Simulator body. The Riser Simulator is also equipped with a water-cooling system, which protects the impeller graphite gaskets from thermal damage.

Figure 5 describes the CREC Riser Simulator and its auxiliary components. The CREC Riser Simulator has a vacuum box of approximately 1000 cm³ that connects to the reactor through a four-port valve (4PV). This valve allows for the entrance of the fluidizing agent (such as argon or helium) or the gases needed for the regeneration of the catalyst (air and hydrogen). Moreover, this valve isolates the reactor chamber from the vacuum box while the reaction is taking place.

Once the reaction time is completed, the four-port valve automatically opens and transfers the produced gas products to the vacuum box. The vacuum box is also equipped with a mixer that ensures that the product gas is well-mixed before an aliquot of the vacuum box contents is sent to the gas chromatograph.

The reactor and vacuum box are equipped with pressure transducers. Pressure transducers help monitor the total pressure changes throughout the different phases of a run: (a) at the feedstock injection, (b) during the reaction run, and (c) at reactor content evacuation. These pressure transducers are connected to analog/digital cards for data collection. Additionally, the reactor, the vacuum box, and the transfer lines are equipped with thermocouples connected to their respective temperature controllers that allow them to have the desired temperatures in these various CREC Riser Simulator components. The CREC Riser Simulator is also trained with a six-port valve (6PV) that allows the transfer of the produced gas samples to the gas chromatograph. To accomplish this, a 6PV loop is first filled with a produced gas sample when the 6PV is in the “load” position. The 6PV can be turned to the “injection” position to send the gas sample to the GC for sample analysis.

The configuration of the CREC Riser Simulator with all its auxiliary components has been demonstrated to be most valuable to study and evaluate fluidizable catalysts for several catalytic and heterogenous reactions: the catalytic cracking of hydrocarbons [48], the oxydehydrogenation of light paraffins [75,76], the chemical looping combustion of waste biomass [77], the catalytic gasification of biomass [66], under short contact times, providing on-line” product analysis using gas chromatography.

12. Typical Tar Conversion Runs in the CREC Riser Simulator

Biomass is mainly composed of cellulose, hemicellulose, and lignin, with the latter having the most complex chemical structure of the three. During the experiments in the CREC Riser Simulator, the tar model compound selected was the 2-methoxy-4-methlylphenol due to its chemical similarity with lignin [66,69,78].

The following procedure is followed for runs in the Riser Simulator: (a) 0.30 g of catalyst is loaded in the reactor; (b) prior to each reaction run, the catalyst is regenerated. This is accomplished by flowing air first through the CREC Riser Simulator unit and then by flowing pure nitrogen through it. Each of these steps takes place at 610–615 °C for 15 min. During the regeneration of the catalyst, the impeller is started to make sure that catalyst particles are in contact with either air or nitrogen; (c) after this preparatory step, the desired temperature is set, depending on the conditions for the experiment. As the system is heated up or cooled down, helium is flown through the catalyst sample; (d) once the reaction temperature is reached, the reactor pressure is lowered to 14.7 psi, and the vacuum box is set to 5 psi; (e) the impeller is started at 5000 rpm and the water/model compound, with a steam-to-biomass ratio of approximately 1, is injected; (f) once the reaction time is complete, the reactor is automatically opened, and the produced gas is transferred to the vacuum box; (g) then, a 1 mL loop representative aliquot is sampled via a 6PV and is sent to the gas chromatograph (GC) to be analyzed. Additional details regarding the analytical methods used can be found in [20].

Figure 6 displays the typical pressure profile obtained in the CREC-Riser Simulator during a run: (a) a first pressure jump at “A” at H₂/CO₂ gas feeding; (b) a second pressure jump at “B” at water–tar surrogate blend injection; (c) a third “B–C” progressive pressure increase during tar conversion, as well as syngas reforming; and (d) a fourth “C–D” sudden pressure drop showing the reactor content product evacuation, and the quick chemical species transport from the reactor toward the vacuum box.

13. Results of Catalytic Tar Conversion Runs

The selection of the syngas mixture to be used in the experiments in the CREC Riser Simulator presents special challenges. This is the case because, regardless of the composition of the syngas fed and the tar conversion catalyst loaded in the reactor unit, there is a significant influence of the catalytic water–gas shift reaction. As a result, CO and water react, favoring the formation of CO₂ and H₂ species (CO + H₂O = CO₂ + H₂), as described in Section 8 of this review. This water–gas shift activity was confirmed via various experiments, where high CO₂/CO ratios, in the range of 6, and H₂/CO ratios, in the range of 3–4, were observed.

As a result, it was decided to select a H₂/CO₂ = 1 syngas gas blend and to perform these runs under the assumption that the tar surrogate compound was being exposed to a syngas rich in CO₂, H₂, and H₂O, with a minor amount of CH₄ and CO contained, as is expected in the 500–550 °C range, under the presence of a water–gas shift active catalyst.

Figure 7a reports the 2M4MP surrogate tar conversions under steam and a H₂/CO₂ atmosphere when using a 15%Ni–5%CeO₂ γ-Al₂O₃ catalyst at 500 °C, 525 °C, and 550 °C and a 10 s reaction time. For further details regarding these conversion calculations refer to Appendix A. One can see that an 83% surrogate tar conversion is reached at 550 °C. Figure 7b shows the CO selectivity for these runs. At 550 °C, one can notice a higher 0.068–0.077 CO selectivity versus the 0.024–0.027 CO selectivity obtained under steam conditions only [27]. These selectivity results agree with those reported in [68], where it is shown that there is a similar increasing trend of CO selectivity when CO₂ is introduced in the CREC Riser Simulator feed. Figure 7c reports the CO₂ selectivity under the same conditions as in Figure 7a,b. At 550 °C, one can notice a higher stabilized 0.2–0.25 CO₂ selectivity under steam–H₂/CO₂ than the 0.11–0.13 CO₂ selectivity obtained using steam only [20].

On this basis, the CO₂/CO ratios under steam and H₂/CO₂ can be obtained as follows: (a) 7.4 at 500 °C, (b) 5.9 at 525 °C, (c) 3.0 at 550 °C. Thus, CO₂/CO ratios obtained show a decreasing trend with increasing temperature, with this being attributed to the influence of chemical equilibrium conditions for the exothermic water–gas shift reaction.

Furthermore, and for the same runs as reported in Figure 7a–c, Figure 8a shows the hydrocarbon product distribution as CH4, C₁⁺, and coke. One can notice in this respect, as reported in Figure 8a, that the CH₄ selectivity falls in the 0.045–0.058 range, augmenting with temperature. One can conclude on this basis that co-feeding steam and H₂/CO₂ favors CH₄ formation via the catalytic hydrogenation of the formed methyl radicals. This is the result of both the increased 2M4MP steam cracking and the expected amplified reverse dry reforming, as discussed in Section 8. In addition, Figure 8b displays the C₁⁺ species selectivity, with these selectivities falling in the 0.61–0.76 range. One can notice that at higher thermal levels, there is a beneficial and desirable reduction in C₁⁺ species selectivity. One should observe as well that C₁⁺ selectivity values reported in Figure 8b are comparable with the 0.82–0.87 C₁⁺ levels obtained under a steam atmosphere by Rojas Chaves et al. [27].

Finally, Figure 8c shows the coke yields for the runs described in Figure 7a–c and Figure 8a,b. One can notice in Figure 8c that the coke formed stabilizes at 550 °C, remaining at a low 0.0006–0.00067 gcokegcat-1 (0.06–0.067%) level. These low coke levels are encouraging, given that they show that only a small fraction of about 1/10 of the total catalyst stream is required to be recirculated in the CPG unit, as described in Figure 2, to be reactivated via combined coke combustion and active nickel species reduction.

One should note that in addition to the carbon-containing product fractions reported in Figure 7 and Figure 8, hydrogen was also monitored under steam–H₂/CO₂ blends at 550 °C during 5–10 s reaction times, as described in

Table 11. H₂/CO Ratios at 550 °C, Using 15%Ni–5%CeO₂ on γ-Al₂O₃ Catalyst. Notes: (a) Tar surrogate: 2M4MP, (b) Catalyst weight: 0.30 g, (c) Steam–H₂/CO₂ Atmosphere [27].

Catalyst	Reaction Time (s)	H2/CO Ratio
15%Ni–5%CeO2/γ-Al2O3	10	3.8
15%Ni–5%CeO2/γ-Al2O3	7	3.8
15%Ni–5%CeO2/γ-Al2O3	5	3.9

.

Table 11 shows that in all cases, the resulting H₂/CO ratios obtained at 550 °C when using the 15%Ni–5%CeO₂ on γ-Al₂O₃ catalyst stay at 3.8–3.9. These high H₂/CO ratios are anticipated from thermodynamics, as reported in Section 8. This demonstrates the value of a CPG unit for both tar conversions, as well as to provide syngas with an H₂/CO ratio larger than 2, which is the syngas-required ratio for alcohol production, such as in the case of methanol synthesis.

In summary, the experimental results obtained in the CREC Riser Simulator provide favorable indicators for a CPG process as follows: (a) very high tar 83% conversion; (b) a favorable H₂/CO ratio of 3.8, larger than 2; (c) reduced 0.7 C₁⁺ selectivity. Thus, the reported results set the stage for the full-scale implementation of an integrated waste biomass gasification process, as described in Figure 2.

14. Conclusions and Future Perspectives

Waste biomass gasification yields syngas with undesirable tar content and a relatively low H₂/CO ratio. Tar removal can be accomplished via different procedures available. However, there are still barriers to overcome for the commercialization of these technologies, with some of them being (a) low energy efficiency, (b) cost of implementation, (c) environmental impact, (d) health hazards, (e) syngas heating value, (f) process water treatment [14].

To implement waste biomass gasification, the following significant issues should be addressed, as described in Figure 2:

• Waste biomass gasification requires the integration of various process steps, accounting for thermal balances and thermal efficiency, as well as for the quality of the produced syngas and biochar. The produced syngas should be free of tars and have an H₂/CO > 2 quality to be suitable for alcohol synthesis;
• Waste biomass gasification needs a process configuration involving circulating fluidized beds. This is the case given the periodic catalyst regeneration and catalyst reduction in the continuous process. Thus, the catalyst used for CPG should have good fluidization properties and exhibit minimum attrition;
• Waste biomass gasification requires a CPG catalyst such as the 15%Ni–5%CeO₂ on γ-Al₂O₃ catalyst proposed by CREC–UWO researchers. This catalyst must achieve a high tar conversion while promoting steam hydrocarbon reforming, as well as the water–gas shift reaction, in order to enrich the syngas quality, yielding a hydrogen/CO ratio higher than 2, suitable for alcohol synthesis;
• The waste biomass gasification process with the CPG should be designed from reaction engineering principles and be developed with lab-scale data from devices suitable for the evaluation of fluidizable catalysts, such as in the case of the CREC Riser Simulator, which emulates the reaction conditions of a large-scale CPG gasification process.

These issues are addressed in the present review, which examines the encouraging results obtained by CREC–UWO researchers in the area of integrated waste biomass gasification. These positive indicators are obtained via experiments in the CREC Riser Simulator with 0.6 v% 15%Ni–5%CeO₂ on γ-Al₂O₃ catalyst volumetric concentrations. These volumetric concentrations are considered relatively low when compared with the ones expected both in industrial risers and downers, which are in the 2–5 v% range. Thus, it can be predicted that CPG could be significantly improved by using higher volumetric catalyst concentrations (e.g., 5 v%), yielding complete tar removal estimated at 99.7% or greater.