Heterogeneous Catalytic Gasification of Biomass to Biofuels and Bioproducts: A Review

Abstract

Biomass gasification is one of the most viable approaches for exploiting biomass. It systemically employs several agents in stimulating the desired reactions that lead to the conversion of biomass feed stocks to fuels/other products. Owing to the biodiversity of the products of biomass gasification, they are fast becoming substitutes for fossil-based products/fuels. In order to ensure optimal production of potential bioproducts, the process conditions of a gasification process have to be essentially controlled within the ambience of high throughput. Based on existing literature, biomass gasification to fuels is not essentially new; however, till date, none of the existing works seeks to unveil the role of heterogeneous catalysis in biomass gasification toward ensuring high bioproducts yield. Heterogeneous catalysis seems to play a crucial role in biomass gasification owing to the fact that catalyst involvement in gasification processes helps to lower the activation energy of specific reactions; hence, under thermal influence, the catalysts tend to hasten such reactions for the desired conversions. Furthermore, it is also pertinent to take into consideration viable approaches for extending or maintaining the service life of the catalyst employed by deploying apt modalities that abate catalyst poisoning (aging, deactivation and fouling); this paper seeks to cover such challenges.

Introduction

Since the era of industrial revolution, in lieu of the fact that fossil fuels have helped to power the world's economy, they have also contributed significantly to climate change. The main reasons for the desire for renewable energy as viable substitutes for fossils include environmental pollution resulting from the release of greenhouse gases (GHG) alongside climate change caused by fossil fuels [1]. The energy market is dominated by fossil fuels, which account for about 85% of the total global energy supply with coal being the primary source [2]. In another report [3], it was asserted that the consumption of fossil fuels has significantly contributed to carbon emissions. It emphasizes the importance of transitioning from non-renewable to renewable energy resources such as biomass energy. As a result, there is a strong push for human existence to depend heavily on renewable energy sources for sustainable development [3, 4]. Prior to the use of fossil fuels, lignocellulosic materials such as wood pellets/chips were widely used for heating and cooking [5, 6]. Biomass is a plant- and animal-based product that is used to generate biogas, biopower, liquid biofuels and bioheat [1]. Between 2000 and 2019, the bioenergy sector has gained expansion at a 3% annual rate [3]. Wood fuel with a capacity of about 2 billion cubic meter and wood pellets with a production capacity of about 40 million tonnes were generated worldwide in 2019 [3]. Thermal, chemical, thermochemical and biochemical conversion are some of the techniques used to convert biomass into viable and useful products. Thermochemical conversion (gasification, pyrolysis or liquefaction) is the breakdown of biomass in an anoxygenic or oxygenic environment at temperatures ranging from 523 to 1673 K [7, 8]. It includes various types of technologies, as illustrated in Fig. 1. The biomass gasification plant enroutes biomass conversion via a two-stage gasifier to obtain, char, tar, biogas and syngas.

Fig. 1

A biomass gasification plant for syngas and hydrogen production [9]

Amongst the aforementioned thermochemical conversion processes for biomass, gasification seems to be the most self-sufficient autothermic energy-intensive route for converting biomass to bioproducts/biofuels [9]. The energy recovery and heat capacity of biomass gasification are greater than those of pyrolysis and combustion, which contribute to the most efficient use of the available biomass feed stock for power and heat generation, because the inherent H₂ and carbon contribute significantly to its calorific value [10]. In addition, pyrolysis and liquefaction are both complex processes that are strongly reliant on operational conditions and the occurrence of a secondary reaction between volatiles and hot solid particles. As a result, the H₂ and CO conversions in liquefaction and pyrolysis processes are usually poor [11]. For instance, conversion processes, such as separation techniques and catalytic hydrogeneration, are not cost-effective, thus making the conversion of bio-oils obtained from these processes to other useful chemicals uneconomical. On the other hand, syngas produced by gasification is easily converted to synthetic natural gas via catalytic methanation of both CO₂ and CO [12,13,14]. As a result, biomass gasification has been regarded as the desired viable alternative for converting a wide range of biomass feed stocks to bioenergy, ranging from plant residues to kitchen waste, food waste, agro-waste, farm waste, and industrial waste [8, 15]; air, O₂ or supercritical water/steam may be employed as the gasifying medium. It is also worthy to note that tar may ensue as a product which appears tough to purify and reduces the H₂ yield, which in turn poses serious challenge to the process efficiency [16].

This review paper focuses on heterogeneous catalytic valorization of biomass to fuels and bioproducts via gasification (a component of thermochemical conversion), which is thought to be the most appealing method for biomass conversion due to its ability to treat a wide range of biomass and waste-derived feedstocks, including wood, sludge, crop and agricultural residues [4, 10]. According to Biomass Gasification Market Report [17], the resources generated from biomass gasification were worth a total of $98.2 billion in 2020, and it is expected to increase moderately between 2021 and 2026. As a result, the future demands of biomass-derived products cannot be overemphasized. According to a study, 50% drop in CO₂ emission can be attained by 2050, thus bringing biomass use to the tune of 26%.

This study emphasizes on biomass valorization via gasification using various catalysts, alongside the identification of different types of gasifiers, gasification agents, reaction conditions and kinetics of biomass gasification to give bioproducts and fuels as imposed by varying SBR, ER, CGE. It also sheds some lights on the recent advances in biomass gasification technologies with a clear perspective on the potential challenges and future considerations for improved heterogeneous catalytic gasification of biomass. Also, considering the findings of previous literature, no study seems to have bothered on the essentiality of combining the effects of new catalysts (dolomite, nickel and olivine) for heterogeneous gasification processes with a view to highlighting key considerations for optimum throughput/gas yield. Issues bothering on challenges such as catalyst deactivation, low carbon to hydrogen ratio, tar formation and conversion, thermal effect/sintering, low hydrogen/syngas yield, etc., have not been addressed by previous literature on the subject; hence, this work seems to identify conditions leading to such challenges as well as charts the way forward on how to abate/control such situations.

Biomass Gasification Technologies

Biomass gasifiers differ primarily in the following ways: (a) by mode of contact between the biomass and the gasifying agent, (b) nature of heat transfer, and (c) the residence time of the biomass in the reaction zone [18,19,20,21].

Various innovative solutions can be employed to accomplish the required plant configurations; for instance, the mode of contact between the gasification agent and the biomass material may be by cross, counter-current, or co-current flow and heat can be transferred from the outside wall of the gasifier or instantly within the reactor as aided by a combustion agent; the residence time may be in hours, minutes or seconds depending on the reactor type and the process conditions [22, 23]. Biomass gasification reactors/gasifiers may appear in the form of plasma, fixed bed, entrained flow, rotary kiln or fluidized bed reactors [24].

Parameters Influencing Gasifier Operation

Since gasification entails various complicated physiochemical reactions that are hard to monitor extrinsically, some investigators have towed the path of unraveling the underlying ideologies since its inception [21, 24, 25]. The biochemical reactions in gasification have many unique characteristics that greatly affect the effectiveness of the process as well as the potential end user applications [26]. Depending on the operational parameters, gasification involves several stages including oxidation, biomass drying, reduction, and pyrolysis, and each process must be conducted under optimal conditions to attain the desired product quality [6, 20, 27]. Table 1 displays a number of process parameters to be considered during biomass gasification.

Table 1 Process parameters for effective gasifier operation

Fixed, moving, and fluidized beds, as well as entrained flow gasifiers are the most frequent types of biomass gasifiers. Entrained flow and fluidized bed gasifiers provide efficient and effective interactions between the solid biomass and gas, which give rise to higher conversion efficiencies and reaction rates. Fixed bed gasifiers (FBGs) are usually beguiled with lower heat and mass transfer rates with high tendencies for char and tar-deposition. The FBG, on the other hand, is easier to operate and design and is best used in small sizes. Table 2 contains the performance data for the aforementioned gasifier types.

Table 2 Performance of most common gasifier types

Biomass Gasification

Gasification converts solid/liquid organic materials into a gas and/or vapor phase; however, at the initiation step, the solid phase is entirely the biomass [23]. After progressive gasification, the resulting gas is commonly referred to as "syngas," which can be used to generate electricity or biofuel. The solid phase, known as "char," contains the organic unreacted fraction as well as inert material from the biomass [29]. This conversion ensues from the partial oxidation of carbon in the feed, and it is typically performed in the presence of gasifying agents, such as CO₂, oxygen, steam and air [30]. Biomass gasification is thought to be a method of increasing the use of biomass for energy production, thus providing a variety of products for use [16, 22]. The potential consequences of fossil fuels on the earth’s climate, as well as the consistent rise in oil prices, are essential drivers of the recent advances recorded in biomass gasification processes [10, 31]. The syngas produced is a mixture of hydrogen, carbon dioxide, methane, oxygen, light hydrocarbons (C₂H₆ and C₃H₈) and heavier hydrocarbons including tar which condense at temperatures ranging from 260 to 290 °C [25, 27, 30]. Unwanted pollutants such as hydrogen sulfide and hydrogen chloride, as well as inert gases like nitrogen, can be found in syngas [31]. Their occurrence is determined by the type of biomass used and the operating parameters of the gasification process [32]. Depending on the nature of the biomass, the operating conditions and the gasification technology employed, the LHV of the produced syngas is usually in the range of 4–12 MJ/Nm³ [23]. Char is a mixture of unreacted organic fractions, mostly ash and carbon. The quantity of unreacted organic fractions is primarily determined by the operating conditions and the gasification technology adopted [12, 19, 32]. On the contrary, the quantity of ash is determined by the type of biomass pre-treatment technique employed. Depending on the quantity of unreacted organic fractions, the LHV of the char falls between 25 and 30 MJ/kg [33]. The primary reactions of gasification are endothermic, and the energy required for their occurrence is usually supported by the oxidation of a portion of the biomass via an autothermal or allothermal process [34].

The gasifier is internally heated through partial combustion in the autothermal process, whereas the energy required for gasification is supplied externally in the allothermal process [18, 26]. Gasification can be viewed as a series of stages in the auto-thermal system. Figure 2 is a schematic illustration of the elementary steps involved in pyrolysis and biomass gasification processes. The following are the main stages involved in a gasification process:

Fig. 2

a Elementary steps/mechanisms in the conversion process from pyrolysis to gasification [1], b pyrolysis and biomass gasification [15]

Oxidation (exothermic-stage) $\to$ Drying (endothermic-stage) $\to$ Pyrolysis (endothermic-stage) $\to$ Reduction (endothermic-stage).

To ascertain the occurrence of light hydrocarbons caused by tar decomposition, an additional step can be included.

Biomass Feed Stock and its Influence on Gasification

Biomass, which includes agricultural, woody, forest, and energy crops, is primarily inputted as feed into the gasifier [24]. As illustrated in Table 3, there are several variations of biomass based on their proximate, chemical and ultimate compositions, which in turn affect the composition of syngas produced from the processed feed [10, 15, 27]. The major components of biomass are lignin, cellulose and hemicellulose, which vary based on the biomass type [18]. When compared to herbaceous crops, woody biomass contains higher lignin than hemicellulose and cellulose [11], and their reactions during gasification differ due to the differences in their inherent functional groups and molecular structures [20]. Hemicellulose gives the highest gas yield relative to lignin and cellulose under the same gasification conditions, whereas, lignin gives the least calorific value and gas yield [31]. Considering data from thermogravimetric analysis, cellulose and hemicellulose release more volatiles compared to lignin [22, 27]. The nature of biomass polymer and chemical composition significantly affect the formation of tar [29]; high amount of lignin increases tar yield relative to cellulose and hemicellulose. Hemicellulose in the biomass accounts for least tar formation [34]. Higher moisture content, than what is required for gasification, causes a decrease in gasification temperature as a result of the endothermic nature of the steam gasification reaction [24], because it slows down the pyrolysis process which may require more energy during drying, thus affecting both the quality of gas and operation; this in turn reduces the higher heating value and cold gas efficiency of the syngas [35]. For biomass particle sizes, compared to bigger particles, smaller/finer particles encounter rapid heating rates, resulting in char gasification and faster devolatilization, thus enhancing gas yield with a greater proportion of hydrogen and carbon monoxide concentration [32]. As revealed for the gasification of chicken manure [35], higher ash content in biomass may hinder the gasification process. Biomass having high ash or mineral content agglomerates due to the minerals with low melting points, such as Na and K [19]; O/C and H/C ratios also have an impact on gasification efficiency [36]. Biomass feed with high surface area and porosity possesses a high diffusion and temperature uniformity, thus resulting in high composition of syngas [37]. According to James et al. [38], the nature of biomass influences tar generation, with grass (prairie hay), agricultural residue and wood chips producing different tar contents of 8.0 g/m³, 2.5 g/m³, and,1.95 g/m³, respectively. Woody biomass having a high content of lignin (20%) gives higher tar than sorghum and grass, which contain 4.48 and 2% lignin [15, 21, 39]. Lignin, as opposed to cellulose and hemicellulose, produces more tar [13]. To help enhance the quality of raw biomass, pre-treatment methods are provided, such as torrefaction (in which T = 300 °C), which improves the quality of biomass by high amount of hemicellulose, reduced moisture, and light volatiles. Compared to raw/untreated biomass, the H/C and O/C ratios are reduced, which increases the grand-ability and energy density [15]. Torrefied biomass is said to significantly boost the concentration of CO in syngas; nevertheless, torrefied biomass char has lower reactivity with steam during the process of gasification [39].

Table 3 Ultimate and proximate analysis of different types of biomass that can be used in biomass gasification reactions

Conceptual/Theoretical Framework

Biomass is a biological matter that stores energy in the form of sunlight through photosynthesis. It is primarily derived from living organisms such as animal, plants, and crop residues [40, 47, 48]. Because biomass gasification involves a series of complex thermochemical reactions, it is impossible to divide the gasifier into various zones that carry out multiple gasification reactions at the same time [49]. The gases obtained from a gasification process are used to generate a variety of outputs, including H₂ production and electricity. Contingent on temperature, biomass gasification can be categorized into high-temperature gasification (HTG) and low-temperature gasification (LTG). Hydrogen and gaseous products can be produced from biomass of low calorific value using LTG process. Its main advantages are its ease of use, avoidance of ash-related problems and efficiency of operation protocols [48]. Furthermore, with the use of LTG, tars are formed from lighter hydrocarbons than in HTG [49]. During gasification, a series of interconnected reactions occur. Quick drying is used first, and then fast pyrolysis (i.e., thermal conversion to gaseous products and char) and then gasification (partial oxidation reaction between the oxygen-source and pyrolysis products) [28, 50]. Pure CO₂, O₂, steam, air, or a combination of these materials are among the most commonly used oxidants [37]. CH₄, CO, H₂, H₂O and CO₂, H₂ are among the products of gasification as well as ash, char and tar. The conversion and reduction of char (carbonaceous materials with a polycrystalline structure) and tar are the most important factors influencing biomass gasification as well as syngas yield and process efficiency [44, 48, 51, 52]. The obtained syngas quality is influenced by a number of factors, including temperature, gasifier type, catalyst, heating rate, water content, pyrolysis-product oxidation and biomass composition [49]. As a result, predicting the exact components of the gaseous products is difficult and complex. The water–gas equilibrium at specific thermal conditions is one way of identifying the product-gas composition [48,49,50,51].

Direct or indirect heat can be used to meet the criteria of endothermic reactions. An innovative approach for various products made from renewable resources is formed through the conversion of biomass materials to gas with a high H₂ content. Furthermore, the generation of hydrogen is a vital aspect, which can effectively minimize the dependence on traditional fuels and reduce emissions associated with these fuels [44]. Multiple reactions are required to produce hydrogen from biomass. First, the fuels are reformed, then a two-step WGS reaction is applied, followed by the purification of CO and finally, complete removal of CO₂. From the reaction steps, the WGS removes tar and converts the CO [46, 51]. To achieve hydrogen from the resulting gas, steam gasification is used since it is more economical, dependable, and efficient in facilitating biomass conversion into enriched H₂ syngas containing 55% hydrogen on a dry basis [53]. Steam (which reacts with carbon monoxide to produce hydrogen and carbon dioxide) can be obtained from dehydration reactions in the steam gasification process [54].

Reactions in Biomass Gasification

The gasification process (Fig. 2) is primarily composed of reactions such as oxidation, pyrolysis, partial oxidation, and several other reactions that occur between carbon, moisture, the products of partial oxidation and pyrolysis as given in Table 4. An endothermic pyrolysis reaction converts fuel into various products such as volatile gases, tar, and char [54, 55]. However, because pyrolysis is endothermic at 560 °C but exothermic at higher temperatures, there are differences in the heat effect during pyrolysis [50]. Partial oxidation or oxidation reactions cause pyrolysis products to react with oxygen, thus generating more gaseous products and releasing heat to aid the gasification process [29].

Table 4 Chemical reactions that result in biomass conversion

Role of Catalyst in Biomass Gasification

Research on catalysts for use in gasification is often carried out specifically in relation to gasifier design or biomass feed type [38]. In biomass gasification, a catalyst plays a crucial role in promoting and enhancing the gasification process. Biomass gasification is the thermochemical conversion of biomass (organic materials such as wood, agricultural residues, and other organic waste) into a mixture of gases known as syngas (synthesis gas), which primarily contains CO, H₂, CO₂, CH₄, and trace amounts of other gases [40]. The presence of catalysts can significantly influence the efficiency and selectivity of the gasification reactions. However, the criteria for the catalysts are fundamentally the same and may be summarized as follows: (a) the catalysts must be effective in the removal of tars; one of the main challenges in biomass gasification is the production of tar, a complex mixture of organic compounds that can condense and cause operational issues in downstream processes and equipment [46]. Catalysts can aid in cracking and reforming tar into simpler, more valuable gases, thus reducing the tar content in the syngas [33]; (b) if the desired product is syngas, the catalysts must be capable of reforming methane [28]; (c) the catalysts should provide a suitable syngas ratio for the intended process; (d) the catalysts should be resistant to deactivation as a result of carbon fouling and sintering [40]; (e) the catalysts should be easily regenerated; some catalysts can be regenerated or reactivated after deactivation, allowing for longer catalyst lifespans and reduced operating costs [46]; (f) the catalysts should be durable and stable [22]; (g) the catalysts should be inexpensive [29].

Biomass gasification typically requires high temperatures to drive the gasification reactions. A catalyst can lower the required operating temperature, which can lead to energy savings and extend the lifetime of reactors and equipment [33, 46]. Steam gasification is a common biomass gasification method. Catalysts can improve the reactivity of biomass with steam, thus increasing the gasification rate and improving the overall process efficiency [49]. Catalysts can also be used downstream in syngas cleanup processes to remove impurities like sulfur compounds, ammonia, and particulates, which in turn makes the syngas suitable for various applications, including power generation, chemical synthesis, and fuel production [53]. Energy recovery from biomass via catalytic gasification is of great importance for sustainable and renewable energy development owing to the fact that the process renders the efficient and effective transformation of solid biomass into gas/liquid fuels and value-added materials by lowering the actual activated energy of the reaction as compared with the situation without the involvement of any catalyst. Besides providing an alternative reaction pathway for clean energy production alongside improved energy efficiency relative to other transformation techniques, catalysts provide improved reaction efficiency and product yields. In lieu of the aforementioned, a robust catalyst for efficient biomass conversion to gaseous fuels requires close examination.

Several gasification reactions are aided by catalyst addition during biomass gasification. In the presence of a catalyst, dry reforming, tar cracking, steam dealkylation, steam reforming, and thermal cracking occur [54, 55]. The decomposition of tar produces gaseous products such as CO, CO₂, H₂, and CH₄ [56]. Biomass has the potential to become a critical resource for H₂ production [16]. Compared to traditional gasification methods, steam gasification of biomass provides the most efficient way to produce an enhanced product gas [57]. When producing syngas with hydrogen enrichment as the goal for biomass gasification, there are two possibilities. The first approach is steam gasification, which entails introducing steam into the WGS reaction to give hydrogen [58, 59], and the second is catalytic steam gasification [60]. Catalytic tar removal is intended for energy-efficient operation at temperatures ranging from 400 to 750 °C [61]. To improve tar conversion into useful gases, a catalyst is used to enhance reaction rates at a lower temperature. As a result, catalysts increase gas yield by improving the conversion of char and tar and thus catalyze the reaction for the intended gaseous product formation [48, 62]. Tar can be efficiently removed/reduced by using the catalytic steam reforming method, which also converts tar into more useful gases [63,64,65]. The CO generated during the reforming process then participates in the WGS reaction to produce the required gases [66]. The effect of catalyst during steam reforming occurs during the dehydrogenation of hydrocarbons in the tar. This results in the formation of carbon on the catalysts' active sites, which can then react with H₂O to form CO [67]. By adjusting the temperature and employing catalysts, the kinetic drawbacks of tar removal can be overcome [42].

Factors Influencing Gasification

A number of factors influence the outcome of syngas production during a gasification process. To design the ideal gasification system, the gasifier type and operating conditions (for example temperature, catalyst, bed-material type, biomass particle size, and gasification agents) must be carefully examined. These elements will be discussed more extensively in the sections that follow.

Effect of the Gasifier Type

Fixed bed, entrained flow, moving bed, and fluidized bed gasifiers are the various types of gasifiers that can be employed in a gasification process [68]. Gasifiers are classified according to how the biomass is embraced in the reaction chamber, the direction of flow of both the biomass/catalyst and oxidant, as well as the heat source into the reactor [41, 44, 48]. The biomass bed is stationary in a fixed bed gasifier type, while the reaction front passes through it; however, the bed can undergo mechanical displacement [69,70,71. Fluidized beds are classified based on their heat transfer modes and fluid dynamics. The bubbling and the circulating fluidized beds are the two most common types of fluidized bed gasifiers [72]. According to a survey of current commercial or near-commercial biomass gasification technologies, directly heated bubbling fluidized bed (BFB) gasifiers are the most commonly used gasifiers that operate with a wide range of parameters such as temperature, pressure, and throughput [50, 51, 73, 74]. Several studies have been conducted to compare the advantages and disadvantages of fixed beds and fluidized bed gasifiers (Table 5).

Table 5 The pros and cons of gasifier types

Different gasifier (fixed or moving bed, fluidized bed, gas entrained and plasma) designs and configurations can affect gasifier efficiency, operating conditions, and syngas composition. Here are some key effects of gasifier type on gasification:

• (1) Gasification Efficiency

Gasifier type can influence the overall efficiency of the gasification process. Some gasifiers are designed for high efficiency and can convert a larger portion of the biomass feedstock into syngas. Higher efficiency gasifiers result in less wasted energy and better utilization of biomass [74].

• (2) Gasification Temperature

Gasifiers can operate at different temperatures depending on their designs. High-temperature gasifiers, such as entrained flow and fluidized bed gasifiers, can reach temperatures above 1000 °C, while low-temperature gasifiers, like fixed-bed gasifiers, operate at temperatures around 700 °C or lower. Temperature affects the gasification reactions, the composition of the syngas, and tar formation [50].

• (3) Syngas Composition

The gasifier type can impact on the composition of the syngas. Some gasifiers such as the entrained flow gasifier and bubbling/dual fluidized bed gasifiers are better suited for producing hydrogen-rich syngas, while others might produce syngas with a higher content of carbon monoxide or methane. The choice of gasifier should align with the intended application of the syngas [51].

• (4) Tar Formation

Tar is a common byproduct of biomass gasification and can cause operational issues and reduce syngas quality. Different gasifier types have varying abilities to handle tar formation. Some gasifiers, like fluidized bed gasifiers, are known for their tar-cracking capabilities, while others might require additional tar reforming stages [54].

• (5) Feed Stock Flexibility

Gasifiers can have different capabilities regarding the type and size of biomass feedstock they can handle. Some gasifiers can process a wide range of feedstock, including wood, agricultural residues, and energy crops, while others might be more limited in their feedstock flexibility [23, 72].

• (6) Gasifier scale

Gasifiers come in various sizes, from small-scale systems suitable for distributed energy production to large-scale systems for industrial applications. The gasifier type can influence the optimal scale and deployment of the gasification process [10].

Effect of Temperature

When temperatures higher than 1250–1350 °C were adopted, the outcomes were as follows: little to no amount of CH₄/tar was formed and significantly increased amounts of hydrogen and CO production were recorded without the requirement for additional conversion steps [65]. Rapagnà and Latif [75] investigated the steam gasification of almond shells in a fluidized bed and found that the highest yields of the gaseous products (hydrogen, carbon monoxide, methane, and carbon dioxide) obtained were 1.56, 0.8, 0.32, 0.1, and 0.3 m³ /kg biomass. The analysis revealed that the temperature increase from 650 to 850 °C, reduced the tars and char concentration, whereas the H₂ and CO yields increased. Similar to the works of Ref. [76], gasification was conducted in a fluidized bed using empty fruit bunch from pineapple as the raw material and air as the gasifying agent at temperatures ranging from 750 to 1050 °C. The concentration of hydrogen increased from 11.17 to 37.99 vol. % as the temperature increased; the concentration of CO₂ decreased, whereas that of CO increased. Furthermore, the concentration of methane increased from 6.04 to 13.72 vol%. At 1050 °C, the vol% of tar and char decreased, and the overall gas yield increased, thus reaching about 93 wt%. In another study, Lv et al. [77] used sawdust as the biomass resource in a fluidized bed and varied the temperature from 750 to 950 °C. The findings demonstrated that higher temperatures improved the H₂, yield with decreased amount of CO₂, CO, and methane concentrations. At increased temperatures, the quantity of H₂ increased from 1.4 Nm³/kg biomass to 2.50 Nm³/kg biomass and from 22 to 38 vol%, respectively; these findings were also corroborated by the results in ref. [78]. The carbon conversion efficiency (CCE) increased with increase in temperature. By increasing the temperature from 750 to 800 °C, the LHV decreased initially and then increased to 8.6 MJ/Nm³. Li et al. [30] investigated the steam gasification of empty fruit bunch (EFB) using a tri-metallic catalyst in a fixed bed reactor at temperatures ranging from 700 to 950 °C, and the findings showed that at elevated temperatures, the production of H₂ increased as well as the total gas yields. At 950 °C, the optimum value of H₂ and total gas yields were 1.47 m³/kg and 2.47 m³/kg, respectively. The hydrogen and carbon dioxide concentrations increased as the temperature increased, whereas, the concentrations of methane and carbon monoxide concentrations decreased. As the temperature increased from 700 to 950 °C, the product gas LHV decreased from 10.98 to 9.05 MJ/Nm³. Furthermore, in a research published by Ref. [79], the gasification of bamboo in a fluidized bed using air produced a different trend in terms of temperature effect. The concentrations of CO and H₂ decreased as the temperature increased, with a resultant increase in CO₂. The overall gas yield remained unchanged as the temperature increased. When the temperature was 500 °C, the maximum CCE and LHV were measured. Table 6 summarizes the results of several studies on temperature effects on the gasification of various biomasses. According to these research findings, higher temperatures resulted in higher H₂ concentrations and low char and heavy tar concentrations, as well as an increase in gas yield, thus releasing more volatiles. The increase in hydrogen production was caused by tar degradation, which also reduced the concentration of tar [50]. From Le Chatelier’s principle, an increase in temperature favors the backward reaction/formation of exothermic reaction of reactants or the forward reaction/formation of endothermic reaction products. As a result, the endothermic hydrocarbon reforming reaction was enhanced by an increase in temperature (Boudouard reaction), steam-methane-reforming (SMR) and water–gas shift reaction. An increase in the concentration of hydrogen resulted from the prominent SMR and water–gas shift reactions [80]. An increase in SMR resulted in a decrease in methane concentration as well as an increase in CO and hydrogen concentrations [32]. An increase in temperature favored the production of CO due to the increased rate of SMR, Boudouard, and WGS reactions [50]. In the meantime, the exothermic reaction of the partially combustible char influences the composition of CO in the producer gas, because higher temperatures were unfavorable for the production of CO. More so, the CCE increased as a result of increased steam and carbon conversion via WGS and endothermic Boudouard reaction [81]. Higher temperatures favored the production of H₂ and other associated gases [32].

Table 6 Effect of gasification temperature and biomass type on total syngas yield, LHV, H₂, CO, CH₄ and CO₂

Effect of Biomass Particle Size

In Ref. [37], the particle size of the biomass used ranges from 0.3 to 1.0 mm; however, reports have shown that biomass particle sizes outside of this range can lead to feeder blockage. With an increase in particle size, the overall gas yield decreases, while the tar and char yields increase. Studies have shown that the highest gas yield was 74.79 wt% for particles less than 0.3 mm, whereas, for particles ranging from 0.5 to 1.0 mm the lowest gas yield (72.74 wt%) was obtained [39, 48, 54]. The investigation also showed that smaller particle sizes produced more CH₄ and CO with lesser amount of carbon dioxide than was recorded for larger particles; H₂ yields remained nearly constant (i.e., ~ 31.89 and 34.93 vol%) for particles sizes of 0.3 mm and those of 0.3–0.5 mm, respectively, before decreasing to 22.07 vol%. Compared to particle sizes ranging from 0.5 to 1.0 mm, when the particle size of the raw material was in the range of 0.3–0.5 mm, the product gas had the maximum LHV and the best product gas composition. Similarly, Dassey et al. [70] found that particle sizes ranging from 0.15 to 5 mm reduced the hydrogen and total gas yields. Smaller particle sizes also produced more hydrogen and carbon dioxide with less methane, carbon monoxide, and C₂H₄. On the other hand, the LHV increased with an increase in particle size and approached the maximum value (10.28 MJ/Nm³) for the largest particle size. The value of the LHV is a measure of the amount of gases or gas composition of the gaseous product obtained and can be expressed as: LHV = 10.78 H₂(%) + 12.63 CO(%) + 35.88 CH₄(%) + 56.5 C₂H₂(%) + 64.5 C₂H₆(%). This then implies that for smaller particle sizes which favor high CH₄ and CO yields, the LHV is bound to increase if the quantities of the gases are quite appreciable as well as greatly influence the resulting LHVs, whereas, for larger particle sizes, the reverse may suffice and there is a shift in paradigm with respect to CH₄ and CO, which in turn lower or affect the LHV of the product gas; it should also be noted that this suffices owing to the fact that C₂H₂ and C₂H₆ are usually obtained in relatively smaller amounts. According to another study, smaller particles produced more methane, carbon monoxide, and less carbon dioxide [32]. The smallest particle size also gave corresponding CCE, gas yield, and LHV of 95.10%, 1.53–2.57 Nm³/kg biomass, and 8.7 MJ/Nm³, respectively. In the study carried out in Ref. [31], biomass particle sizes ranging from 0.075 to 1.2 mm were used in the steam gasification of pine wood chips at 900 °C. During the investigation, as the size of the particles increased, H₂, CCE, and total gas yield decreased, while the concentration of char and tar increased. For the smallest particle used, the overall gas yield and hydrogen gas yield were 1.42 and 0.8 m³ /kg biomass. They also found that smaller particles produced lesser CO and more CO₂ than those of larger particles. Nevertheless, a different trend was noticed in the work of Ref. [63], where biomass of particle size ranging from 0.15 to 3 mm was used. At 900 °C, the gas yield steadily increased from 1.71 to 1.83 Nm³/kg, but later reduced slightly to 1.79 Nm³/kg as the particle size increased from 0.12 mm to 0.4–0.8 mm. For particle sizes of 0.4–0.8 mm, a maximum gas yield of 1.83 Nm³/kg was achieved. Table 7 summarizes a few works on the effect of biomass particle size on gas yield from gasification.

Table 7 Effect of biomass particle size on biomass gasification products (CH₄, CO₂, CO and H₂)

Effect of the Gasification Agent

There are presently numerous studies on biomass gasification using various agents such as O₂, air, steam, or mixtures of these materials. Table 8 summarizes a few works on the effect of gasification agents on the throughput from a gasification process. Chhiti and Kemiha [74] compared steam and air as gasification agents and found that steam gasification was more effective in maximizing H₂ yield. By gasifying bamboo in a fixed-bed reactor using steam/air, 0.45/0.33 m³ of H₂/kg of biomass was produced. Furthermore, in a study involving air gasification of wood biomass with 20% moisture, the percentage composition of the product gas includes H₂: 21.66, CO: 20.55,CO₂: 12.36, CH₄: 1.011, N₂: 44.42, NO: 0 and NO₂: 0%, while that of air gasification of municipal solid waste with 16% moisture, gave producer gas composition of H₂: 19.09, CO:19.44,CO₂: 10.24,CH₄: 0.648, N₂: 50.58, NO: 0, NO₂: 0 and SO₂: 0% [61]. They discovered that when air/steam were adopted as gasifying agents, the amount of hydrogen and carbon monoxide in the product gas was also higher. According to Daorattanachai et al. [18], air gasification produced a gas yield of 1.39–2.39 m³ on dry basis/kg biomass, which was considerably higher than the yield from steam gasification whose values ranged from 0.77 to 1.01 m³ on dry basis/kg biomass. The gasification output was also influenced by the steam to biomass ratio (S/B). According to Ref. [27], increasing S/B from 0 to 1.32, increased the total gas and hydrogen yields to their highest values (i.e., 2.39 m³ /kg and 1.374 m³ /kg, respectively). An increase in steam to biomass ratio from 1.41 to 2.72, reduced both the overall gas and hydrogen yields. The composition of the hydrogen in the product gas was about the same measure. The percentage of hydrogen gas composition increased substantially when the steam-to-biomass ratio was increased from 0 to 1.31, but slightly decreased when the S/B ratio increased further from 1.31 to 2.72. The findings were in agreement with those of ref. [10], who investigated the effects of $\alpha -$ cellulose gasification at 850 °C when the ER was 0.27 with an increase in the S/Bs between 0 and 1.5 at an interval of 0.5. Upon increasing the S/B from 0 to 1, the percentage volume of H₂ increased from 12.90 to 19.06 vol%. Nevertheless, it began to fall when the S/B ratio was raised from 1 to 1.5. The S/B value of 1 also produced the highest total gas yield. The ER of an air gasifier can influence the composition of the produced syngas. According to ref. [37], increasing the ER from 0.12 to 0.30 reduced the tar and char yields from 13.12 to 3.92 wt% and 7.73 to 2.01 wt%, respectively. According to ref. [50], for an increase in ER from 0.12 to 0.3, the tar and char concentration decreased slightly. The total gas yield increased to a maximum of 0.7 wt% at an ER of 0.22 before decreasing slightly to 0.77 wt% when the ER = 0.4.

Table 8 Effect of gasification agent on biomass gasification products (syngas/total gas yield, CO, CO₂, H₂ and CH₄)

By increasing the ratio of SB, the reaction is influenced in a direction that lowers the steam concentration. Thus, the high steam rate accelerates methane-reforming reaction and gasification with high hydrogen production, which also accelerates the WGS reaction. Notwithstanding, too much steam in the gasifier may cause the system to lose a lot of energy in order to heat up the biomass, which is not good for energy production [10], and also, it lowers the reaction temperature, thus resulting in poor gas quality [27]. As a result, higher ERs were attributed to lower concentration of CO, lower tar and char yields, higher CO₂ concentration and lower LHVs. A high ER can lead to lower concentrations of carbon monoxide and hydrogen in the output gas while increasing the concentration of CO₂ [37]. Higher ERs, on the other hand, cause exothermic–oxidation reactions, which provide much more heat to the gasification process and in turn optimize the product quality [82]. However, at low ERs, the temperature in the gasifier drops, thus making further reactions of the biomass in the gasifier unfavorable [39]. ER is a key factor that determines the quality of gas produced during biomass gasification [50, 53].

Influence of Catalyst on Syngas Composition

Generally, a catalyst is a substance that alters the rate of a physical/chemical reaction. The use of a catalyst in biomass gasification improves heat and mass transfer between particles, which enhances gasification reactions such as partial combustion, Boudard, SRM and the WGS reactions [83,84,85,86]. Air gasification in the presence of a catalyst increases the hydrogen and carbon monoxide content of syngas, thus resulting in an increase in the HHV from the cracking and conversion of tar into a gaseous product (CO and H₂) [87]. Catalytic steam gasification increases the H₂-enriched syngas yield by favoring the WGS and SRM reactions in the presence of the catalyst with a higher H₂ yield [88]. On the contrary, the carbon monoxide content decreases as the WGS reaction consumes a portion of the CO, thus boosting the overall H₂/CO ratio [89]. To a certain extent, the methane content also drops [65]. Tar reduction contributes to the growth of H₂ and improves the total gasification yield and efficiency [90]. The CO₂ produced during gasification can be adsorbed by solid-based catalysts such as CaO, thus increasing the concentration of H₂ to a higher percentage [73]. When Ni-catalyst is employed during steam gasification, the resulting tar decomposes into CO, H₂, CO₂ and CH₄, thereby increasing the gas yield [91].

• (i) Selection criteria for catalysts

A catalyst that is meant to be used in a biomass gasification process must be well-thought out in terms of the biomass feed that will be processed, gasifier configuration, the operating conditions, cost, stability (mechanical, hydro and chemical stability), reusability as well as the intended product yield in order to ascertain the most appropriate synthetic route/method of production for the catalyst. Catalyst-reusability, deactivation resistance, stability, mechanical strength, activity, and cost are all factors to consider when choosing a suitable catalyst [10, 92]. Berrueco et al. [93] discovered that the same catalyst performed completely different on lignin, cellulose, and hemicellulose; thus, biomass with varying chemical properties encounter different catalytic effects from the same catalyst; hence, biomass properties should be taken into account when selecting a suitable catalyst [29]. The choice of a catalyst is also influenced by whether it is used in a post-gasification process or used in in situ tar cracking, as this affects process' economics, effectiveness, and stimulates catalyst deactivation [9, 43]. A primary catalyst is occasionally used as a guard bed/catalyst support for commercial catalysts to prevent deactivation. Several authors [46, 50, 60, 75, 90, 93,94,95,96], have summarized the characteristics of catalysts to be considered for use in gasification processes; for example, a catalyst: (a) should be efficient in the removal of tar at temperatures ranging from 650 to 950 °C; (b) should have the capacity of reforming CH₄ if syngas is the intended product; (c) should be capable of reforming higher hydrocarbons/aromatic compounds; (d) must be compatible with a minimum hydrogen/carbon monoxide ratio; (e) must be resistant to deactivation due to sintering and coking; (f) should be effective in preventing fouling by impurities; (g) should be resistant to sulfur poisoning; (h) must possess a good measure of reusability and stability; (i) should be easily regenerated; (j) should have a wide measure of availability and must be cheap; (k) should be nontoxic to the environment; and (l) must be resistant to attrition. Each group of catalyst that has been used in gasification have had different catalytic activities, as well as offered varying advantages and disadvantages, which are summarized in Table 9.

• (ii) Catalyst preparation for biomass gasification

Table 9 Advantages and disadvantages of different catalysts employed in gasification processes

Catalyst-reusability, deactivation resistance, stability, mechanical strength, catalytic activity, and cost are all factors to consider when deciding on a catalyst’s method of production [10, 92]. The first approach involves mixing the choice-catalyst with biomass so as to speed up the gasification process; this in turn converts the residual tar inside the reactor [1, 56, 85]. The second method employs a separate catalytic reactor located downstream of the gasifier to convert the residual tar [97]. Although the latter has a high efficacy in converting tar, it is either too expensive or too complex for small- and medium-scale systems [18, 98], whereas, in situ tar cracking is the most cost-effective and most promising approach [37, 68]. Christodoulou et al. [94] differentiated catalysts into the following categories: (I) Ni-catalyst, (ii) alkali and other metal catalysts, and (iii) dolomite catalyst, whereas Meng et al. [92] gave the following catalyst categories: (i) synthetic catalyst and (ii) mineral catalysts, with further sub-categorization. As illustrated in Fig. 3, three types of catalyst for biomass gasification suffice: (i) alkali-based catalyst (K, Cs, Li, Fr, Na, K, and Rb), (ii) natural catalysts (CaO, Olivine and dolomite), (iii) metal-based catalysts such as Ni [54, 92]. Natural catalysts are less expensive [89]; however, synthetic catalysts, such as noble metal-based catalysts, are synthesized via chemical treatments but involve higher costs [99]; nickel-based or iron-based catalysts possess high catalytic activity [85, 88]. As shown in Fig. 3, the catalytic activity of these catalysts varies depending on their characteristics and the gasification requirements.

Fig. 3

Catalyst for biomass gasification

A catalyst aids in the reduction of tar as well as tar reforming into useful product gases [71]. The catalyst is added directly to the feed stock (biomass) by dry mixing or wet impregnation prior gasification to catalyze the reactions, or are used as bed materials in fluidized bed gasifiers. Both gas clean-up and gasification occur in the same reactor, thus eliminating the need for a separate catalytic reactor downstream or the need for additional heating, which may result in low plant capital and operating costs [63, 79, 100]. When a secondary catalytic reactor is used downstream of the gasifier, its operating conditions differ from those of the gasifier unit. At elevated temperatures, these catalysts have a little effect on reforming hydrocarbons and CH₄ [95, 101]. Secondary catalysts have higher durability in downstream applications because coke formation on the surface of the catalysts is reduced [64, 73]. Natural and alkali catalysts can be used as primary catalysts; nevertheless, if used in separate catalytic reactors, they can function as secondary catalysts. Because of its affinity for coking deactivation, Ni-based catalyst is most suitable as secondary catalyst in downstream reactors [97, 102].

• (iii) Natural catalysts

Dolomite, shell and olivine, can be used as catalysts upon pretreating and calcination. Olivine and dolomite are widely used in fluidized bed gasifiers, while CaO is most preferred as carbon dioxide absorbent.

• (a) Dolomite.

Dolomite comprises mainly of Mg and Ca and has attracted significant research interests for use as catalyst, especially as a catalytic bed [103,104,105,106] since it is effective in the reduction of tar up to 95% and can increase the gaseous product yield [104]; this is attributed to its abundance, low cost, and effectiveness in tar conversion and reduction.

Dolomite increases the hydrogen and carbon dioxide concentration in syngas by enhancing the WGS reaction [106]; it can also increase the gasification efficiency and gas yield [107, 108]. The catalytic performance of CaO and MgO in dolomite increases significantly at 800 °C, thus resulting in more gas formation [84]. Dolomite may be useful as a bed material, sometimes in combination with other materials [109], or as a secondary catalyst in the downstream stage of a gasifier [100].

Dolomite becomes more chemically stable when it is calcined at temperatures above 900 °C. Because of the larger internal surface area provided by the increased pore size and oxides present in calcined dolomite, it is more active [93] and exhibits better catalytic effects [90] than raw dolomite. Calcined dolomite gives high H₂/CO ratio between reaction temperatures ranging from 700 to 800 °C [103]. The CaO/MgO mixture in calcined dolomite reacts with contaminants such as HCl, SO₂ and PAHs in the producer gas in order to convert them into fuel gas, thereby lowering their concentrations [104]. A few investigations on dolomite as a catalyst are evident in confirming the usefulness of dolomite as a viable catalyst. Due to the availability of Fe₂O₃ in dolomite, which promotes steam reforming and WGS reaction, the gasification of coconut shells resulted in 85.06% tar conversion efficiency. Using raw untreated dolomite with a sand bed, researchers discovered that as the amount of dolomite increased from 10 to 40%, solid particulate matter and tar decreased sequentially as the active site of the catalyst surface cracked more tar at lower temperatures [85, 109]. Zhou et al. [110] discovered a significant effect on gasification using calcined dolomite, with gaseous products increasing by more than 14% and tar yield decreasing by more than 95.4% when an air–steam mix was adopted as the gasifying agent. Asadullah et al. [111] revealed that for air gasification of rice husk, the amount of dolomite accelerated from 10 to 40% when mixed with a sand bed, the hydrogen content increased from 2.75 to 3.39%, the CO and CO₂ concentrations increased from 14.7 to 16.93%, and 9.3 to 11.9% respectively. According to ref [112], steam gasification of wood chips revealed that dolomite gave a high concentration of H₂ compared to that obtained for quartz (22.93%) and olivine (23.72%), thus implying that dolomite had enhanced catalytic activity. During steam gasification with dolomite as catalyst, the H₂ concentration increased significantly to 51.18 vol% and the concentration of CO increased to 25.09 vol%. In terms of mechanical strength, crystalline dolomite is more stable than amorphous dolomite because of its larger pore structure, which allows for faster release of gas [113]. Table 10 depicts the gaseous composition of the product gas using dolomite as catalyst, and Table 11 shows the tar and gas yields when dolomite was used as catalyst. Although dolomite proved to be an excellent choice as a tar reduction catalyst, its fragility poses a problem during operation.

• (b) Olivine

Table 10 Feed type, catalyst, gasification agent, temperature and syngas composition for dolomite-gasified biomasses

Table 11 Data comprising of different reactor-types, feed, bed-material, operating conditions, tar and gas yields

Olivine is primarily composed of silicate minerals in which Fe and Mg are integrated in the silicate tetrahedral [114]. It is an excellent in-bed additive for biomass gasification in fluidized bed gasifiers due to its hardness and high abrasion resistance [115]. The major benefit of olivine over silica sand is that it is more efficient on light tar than heavy tar [116]. Olivine is an appealing and preferred bed material for commercial, pilot and demonstration plant applications [117] due to its anti-wear/tar reduction properties and ease of availability [99]. Due to the migration of Fe (-Fe₂O₃ and -Fe₂O₃ phases) to the surface of olivine after pre-treatment, calcined olivine displays higher catalytic activity relative to raw olivine. Iron tends to leave the structure of olivine during calcination to form FeO₂ or FeO₃, and the amount of free iron oxide is dependent on the calcination temperature [102, 106, 118, 119]. Barisano et al. [120] observed that while both olivine and dolomite display similar catalytic behaviors in terms of tar degradation and production of gaseous products, olivine outperforms dolomite in terms of minimal solid fine particle production. When the amount of olivine in the sand bed was increased from 10 to 30% in the gasification of rice husk with air as gasifying agent, the solid particulate matter (SPM) and tar were reduced by 40–45% [121]. Proll et al. [122] discovered that, when compared to quartz sand which gave tar reduction of 51.11 g/Nm³, a tar reduction of 37.6 g/Nm³ was recorded for olivine at T = 750 °C, S/B = 0.8, thus confirming the better performance of olivine. The gas yield at 800 °C was approximately twice that achieved at 700 °C, while the tar formation was 10 times reduced using olivine as catalyst. Calcination of olivine improves its tar cracking ability [123]. The tar conversion potential of olivine was elevated by 51.5 and 56.2% when calcined at 950 °C and 1100 °C, respectively [124]. Xu et al. [125] observed a 35% tar reduction during day-2 of their operation and an increase in H₂ gas yield with the highest hydrogen/carbon monoxide ratio of 1.6. Olivine has similar tar-cracking characteristics with quartz and can further be improved by adding active metals such as, Co, Fe, Ce and Ni. Several authors have described the use of olivine as a metal support for catalysts, such as Ni/olivine, [126], Fe/Olivine [115], Ni-Ce-Mg/olivine [127] and Ni–Fe/MgO/olivine [128], which were synthesized using the sol–gel/wet impregnation method. Metals such as Ni and Fe in olivine typically provide an active site during catalysis, thus resulting in higher activity of the catalyst [129]. Table 12 depicts the gas composition of syngas when olivine was used as catalyst, while Table 13 gives the tar and gas yields when olivine was the catalyst adopted in the gasifier.

• (c) Calcium oxide

Table 12 Composition of syngas when olivine was used as catalyst in the gasifier

Table 13 Tar and gas yield with olivine as catalyst

CaO functions as a sorbent for carbon dioxide during biomass pyrolysis and gasification and in turn generates hydrogen-rich gas [130, 131]; hence, CaO is fast gaining attention due to its low cost and widespread availability [34, 54]. CaO contains minerals such as dolomite and limestone, which have high tar-cracking qualities [26]. Steam gasification aided by the addition of CaO sorbent into the gasifier can serve as a tar reducer, CO₂ adsorber and a heat carrier during endothermic gasification [127]. CaO can serve as a tar-reforming catalyst [132, 133]. CaO enhances CO₂ adsorption at lower temperatures (500–600 °C) to yield more hydrogen within lower CO and CO₂ concentrations, although at higher temperatures (750–850 °C), the catalytic influence of CaO becomes noticeable in cracking reactions, steam gasification of volatiles and char, WGS reaction, and pyrolytic conversion of volatiles, thus increasing the hydrogen yield [48]. The CaO/biomass ratio helps in tar decomposition [42] and increased contact between the CaO sorbent and tar species to break down more tar and increase the hydrogen yield [134, 135]. According to Srinivasakannan and Balasubramanian [85], an increase in the CaO to biomass ratio from 0 to 2, increased the hydrogen concentration from 22.17 to 53.44%, whereas, the H₂ yield increased by 2.7 times [45]. They also observed that during steam gasification of sawdust, where the concentration of hydrogen increased from 33.7 to 58.9%, the yield of H₂ increased from 40 to 59 g/kg biomass by increasing the CaO to biomass ratio from 0 to 2 [126, 128]. Table 14 shows the syngas composition when CaO or its derivatives were used as catalysts in gasifying different biomasses.

• (d) Alkali catalysts

Table 14 The composition of syngas for calcium oxide/calcium carbonate-based absorbent for CO₂ separation

The most frequently utilized catalysts are alkali (Na and K), alkaline earth (Mg and Ca), and transition metals such as Fe [132]. Tar reduction capabilities via steam reforming and improved product gas quality are among the benefits that alkali catalysts such as Li, Na, and K provide [106, 117, 128]; they also offer good resistances to carbon deposition [133, 136]. These catalysts are usually mixed with/impregnated in biomass fed into the reactor during a gasification process [135, 136]. Report has shown that alkali metals catalyze gas-phase reactions such as the WGS reactions [99] as well as tar cracking [78, 99], but they primarily promote the WGS reaction rather than steam reforming [90]. Since alkali carbonates constitute alkali metals, they are used in steam gasification of biomass as catalysts [111]. Due to their agglomeration susceptibility and cost involvement during gasification, they pose some operational challenges [49], although, evaporation of the catalysts during gasification poses challenges in its recovery/regeneration step [81]. The use of primary alkali catalyst in a fluidized bed increases the rate of char conversion, catalyzes the carbon steam reaction, and modifies the reaction pathway during pyrolysis to reduce tar and CH₄ yields, while significantly increasing the yield of char and other gases with the exception of CH₄ [60]. Steam gasification of tobacco stalk blended with K₂CO₃ and CH₃COOK demonstrated that it stimulates hydrogen production in addition to carbon conversion at increased temperatures in the range of 650–750 °C. Carbon conversion and hydrogen yield decreased with KCl loading due to KCl inhibition on gasification [137]. At 950 °C, alkali-feldspar tested in a fluidized catalytic bed with raw producer gas generated by another gasifier unit (Chalmers 2–4 MW) demonstrated that the catalyst largely supported WGS reaction with total reduction of ethyne, propyne, and other tar species. The catalyst was mechanically strong against attrition and did not agglomerate or deactivate after several hours of operation [133]. For the first four days of operation, it was observed that using alkali-feldspar in a 2-MW Chalmers gasifier is a suitable replacement for olivine and sand, with a tar conversion efficiency of about 48% [134]. According to some researchers, the alkali-feldspar mechanical properties prevent attrition and it has a high melting point despite the inherent high K and Na levels. Because feldspar contains alkali metals, additional alkali metals released from biomass may not cause further catalyst activation or agglomeration [134,135,136,137,138]. Chen et al. [83] investigated the gasification of sewage sludge in supercritical water using alkali catalysts Na₂CO₃, NaOH, K₂CO₃, and KOH, and they discovered that K₂CO₃ improved the gasification efficiency. The catalytic activity was in the order of KOH > K₂CO₃ > NaOH > Na₂CO₃ based on H₂ production [139]. The char and tar concentrations decreased with an increase in the gasification temperature from 950 to 1150 °C for pine sawdust biomass mixed with K₂CO₃ due to potassium's catalytic activity in inhibiting PAH growth and thus demonstrating its ability in decomposing light tars [95].

• (iv) Transition metal catalysts (TMCs)

• (a) Ni‑based catalyst

TMC, particularly Ni, has higher catalytic activity for tar conversion via reforming and cracking reactions/gasification of biomass, thus significantly improving the quality of gas [77, 92, 137], and hence, TMCs are regarded as commercial catalysts, which are effective in steam reforming at temperatures ranging from 550 to 1000 °C [106]. Ni-based catalysts exhibit a strong catalytic activity in the breakdown of tar and aids in CH₄ reforming and WGS reactions, thus making it an attractive option for biomass gasification [139]. Due to secondary cracking of vapors over Ni-based catalysts, the high tar conversion efficiency results in increased gas yield [140, 141]. As secondary catalysts, Ni-based catalysts are recommended for use after gasification in a distinctive catalytic reactor. Han et al. [138] provided a comprehensive overview of a nickel catalyst used during tar catalytic reforming, alongside its preparatory work, methodologies and performance based on its stability and catalytic activity/selectivity towards low molecular weight compounds. Ni-based catalysts are typically used as supplementary catalysts; nevertheless, only few works have demonstrated their use as primary catalysts. Fuchs et al. [143] utilized 3 different catalysts (ICI-46-1, BASF G1-50, and TOPSE R-63) in a secondary catalytic reactor and found that the tar conversion efficiency ranged from 96 to 99.2%. The product gas yield obtained based on the 3-catalysts was greater than 2.4 Nm³/kg of biomass. Notwithstanding, nickel-based catalysts suffer deactivation, sintering, carbon deposition on the surface of the catalyst, poisoning [74, 144], and are exposed to in-bed high attrition in fluidized bed gasifiers [145]. In addition, loss of catalyst activity might be caused by coke formation and dust, which are mainly influenced by tar generation [126], which in turn leads to tar being deposited on the catalyst-active site, thus reducing the catalyst’s reactivity and fouling of some pores [108]. An appropriate support material is required to resolve the problems arising when using NBCs. Numerous support materials, such as olivine, zeolite, dolomite, and metal oxides such as CeO₂, La₂O₃, MgO, Fe₂O₃, Al₂O₃, ZnO, SiO₂, ZrO₂, Fe, TiO₂, MgO, and La can be employed with Ni catalysts [138, 146], such that the Ni catalyst is modified by the addition of promoters (i.e., Fe, Cr, Na, Cu, La, K, Li, Mo, Co, and Mg) [109]. Acharya et al. [147] used the impregnation method to create Ni/SiO₂-Al₂O₃ and Ni/Al₂O₃, whereas precipitation and subsequent deposition were used to create Ni/dolomite using an aqueous solution of Ni(NO₃)₂·6H₂O. The efficiency in coke production was in the order of nickel/dolomite > nickel/Al₂O₃ > Nickel/SiO₂-Al₂O₃, which is believed to be due to the presence of CaO and MgO in dolomite which has affinity for toluene and tar. [39, 69, 91]. Guoxin and Hao [148] produced a nano-nickel/-Al₂O₃ using a deposition precipitation methodology for use in corncob air–steam gasification. When compared to non-catalytic gasification reaction, the nickel/-Al₂O₃ significantly increased the gas yield from 1.79 to 2.44 Nm³/kg. The yield of tar dropped from 33.1 to 0.12 g/Nm³, thus suggesting that the conversion of tar was as high as 99.5% [149]. Higher temperatures/catalyst loading (i.e., Nickel/Al₂O₃ and Nickel/CeO/Al₂O₃) were discovered to be more beneficial for the cracking of tar and H₂ production [102]. Olivine, as a catalyst support for Ni, experiences lower carbon deposition, greater tar-cracking activity, and higher H₂ yield, thus making it ideal for circulating catalytic beds [105, 113, 114]. Harrison [149] found that at 900 °C, a NiO/Olivine bed performed extremely well in producing hydrogen at 55.2 vol % compared to the quartz bed type, which gave 31.4 vol%, and olivine bed, which gave 41.1 vol% of the gas. The H₂/CO ratio continued to improve from 2.4 to 2.9 for olivine and NiO/olivine, compared to those of the quartz bed. A new catalyst, Ni-loaded brown coal char (Ni/BCC), demonstrated comparable catalytic activity relative to Nickel/Al₂O₃, with the additional benefit such as resistance to carbon deposition due to the actions of steam during coke gasification and the reforming of tar, which prevent deactivation, and excellent resistance to coke formation [18]. Figure 4 shows the tar conversion efficiencies of some notable catalysts (dolomite-based, olivine-based, alkaline-based and nickel-based catalyst). Table 15 contains some synthetic Ni-based catalysts, their synthetic routes and conditions of synthesis.

• (b) Non‑nickel-based transition metal catalysts

Fig. 4

*Tar conversion efficiencies (%) for different catalysts [35]. *D-CS: Dolomite with coconut shell as biomass, S/B = 2, T = 850⁰C; DS: Dolomite with sawdust as biomass, ER = 0.25,T = 800 °C; O-MTN: Olivine with model tar-naphthalene,T = 900 °C; O-S: Olivine with sawdust, ER = 0.25,T = 800 °C; 10Fe-O-MTT- 10-iron-olivine-model tar with toluene, T = 850 °C; ɤ-Al₂O₃-NiO-S: gama-bauxite-nickel oxide with sawdust, ER = 0.25,T = 800 °C; Ni–Fe-char-RH: Nickel–iron-char with rice husk, ER = 0.25,T = 800 °C; Ni-D-MTN: Nickel-dolomite with model tar-naphthalene, T = 750 °C; NiO/O-PS: Nickel oxide/olivine with pine sawdust, S/B = 0.65, T = 850 °C; Nano-NiO-ɤ-Al₂O₃-RH: nano-nickel oxide-gama bauxite with rice husk ash, ER = 0.22, SB = 1.33, T = 800 °C; MgO-Al₂O₃-NiO-MTN: Magnesium oxide-bauxite-nickel oxide with model tar naphthalene, T = 850 °C

Table 15 Methods of synthesis of Ni-based catalysts (NBCs), their precursor solutions, process conditions and nickel loading

For the steam reforming of biomass tar, non-Ni-based metal catalysts/other transition metal-based catalysts such as Pt, Co, Fe, Pd, Rh, and Ru have been developed. Nobel metal catalysts such as Ru, Pt, and Rh have extremely good tar reforming ability; nevertheless, their high costs make them unsuitable for use commercially [150,150,152,153,154]. Metal catalysts such as Cu, Co, Fe, Zn, and Cu, in addition to noble catalysts, have been used in most studies for tar reforming and were found to have demonstrated better catalytic activity compared to nickel [79, 89, 108]. Rh/CeO₂/SiO₂ catalyst performed better compared to NBCs due to the evidential reduced sulfur adsorption it exhibited when used in a fluidized bed reactor where partial oxidation of tar takes place [155]. The catalytic effect of the supported Rh catalyst was similar to that of the unsupported Rh catalyst but with much lower deposition of carbon [156]; however, introducing basic metal oxides to the support has demonstrated reduced coke deposition [157]. When compared to commercial NBCs, the Rh catalyst that is supported with 0.5 wt.% loading (Rh/Ce–Zr–Mg–O) at 350–550 °C, performed better in terms of CO/CO₂ production, hydrogen yield (88%) and phenol conversion (90%) [142, 149, 153]. Lopez et al. [158] investigated the effect of different supports on the performance of Ni and Pt-based catalysts for steam reforming of naphthalene/benzene as a model tar; the results showed improved and more stable catalytic activity for the Pt-Al₂O₃ catalyst during hydrogen production at 800 °C. The Ni/Ru-Mn/Al₂O₃ demonstrated excellent toluene reforming as well as high coking resistance and high stability at temperatures > 600 °C, which could be attributed to the influence of Mn in promoting the activity of the catalyst (Ni/Ru-Mn/Al₂O₃) [111]. Table 16 also shows the syngas production from biomass gasification schemes when Ni-based catalysts were employed.

Table 16 Syngas composition for Ni-based catalyst used in gasification experiments

Implications of Selecting a Wrong Catalyst for Biomass Gasification

In order to critically examine the implication of selecting a wrong catalyst for a gasification process, the situation of yellow horn biomass gasification, as investigated by Granados-Fernández et al. [159], was re-examined, in which a nonisothermal thermogravimetric mass spectroscopic (TG-MS) analysis of YH- biomass gasification was carried out using catalytic and non-catalytic isothermal gasification systems. Catalytic as well as non-catalytic yellow horn (YH) residual biomass gasification was conducted in a SDT Q600 thermobalance-TA Instrument integrated in a Pfeiffer vacuum Prisma™ QMS 200 mass spectrometer for off-gas analysis. Samples consisting of 10 mg biomass + silicon carbide/biomass + catalyst (9:1 wt. ratio) were loaded onto a crucible made of alumina within a bed configuration and heated between 30 and 800 °C at an interval of 10°C min⁻¹ using 20% gasifying agent comprising of 1% H₂O + 1% CO₂ (v/v)) balanced in He. The m/z signals of hydrogen, methane, water, ethene, ethyne, carbon monoxide, ethane, carbon dioxide, hydrogen sulfide and sulfur iv oxide were recorded/observed. Prior to running the experiments, the signals of the aforementioned gases were calibrated to avoid any form of interference between compounds. The signals were also normalized using the maximum intensity of each signal as reference.

Figure 5a is an illustration of the observed TG profiles within the nonisothermal regime for a feed flow rate of 100 mLmin⁻¹ comprising of 1% CO₂ + 1% H₂O (v/v) (as gasifying agent in helium, in contact with biomass of particle size ranging from 0.125 to 0.150 mm, ramp of 10°Cmin⁻¹, for initial sample mass of 10 mg and catalyst of 10 wt%. It is worthwhile to mention here that the TG profiles of the non-catalytic and catalytic processes appear superimposed, with the biomass received for both cases having comparable weight loss within the specified temperature range. Different mechanisms/elementary steps were seen to have stimulated the yellow horn biomass reaction/conversion in the presence of H₂O and CO₂. The first reaction took place below 200 °C with about 8% loss in weight which corresponded to the expulsion of moisture (≤ 100 °C) as well as structural water at approximately ≈200 °C, as the process was associated with some measure of heat gain. At the second stage, i.e., between 200 and 500 °C, the loss in weight was more significant as the recorded weight losses were in the region of 35–40%, as induced by pyrolytic decomposition of the biomass via thermal cracked proteins and unrecoverable residual lipids from the biomass. The third stage, involved heating the biomass sample within 500–800 °C, thus giving a weight loss of about 15% within the high temperature region of the YH-biomass gasification; the weight loss recorded at this stage was in the range of 45–60%. Figure 5b shows the variation of the activation energy for the catalytic and non-catalytic gasification processes of the yellow horn biomass. The energy barrier in both cases seem the same and this is a rare case of gasification where the catalytic and non-catalytic processes gave similar performance. In this situation, other gasifying agents can be tried alongside new catalysts (single/hybrid) for improved performance over the non-catalytic process.

Fig. 5

Catalytic and non-catalytic thermogravimetric analysis of YH biomass with H₂O + CO₂ as gasifying agent: a weight loss vs temperature profile, b activation energy profile

Homogeneous vs Heterogeneous Catalysis in Relation to Biomass Gasification Reactions

Homogeneous catalysts have been explored for biomass gasification, but their use in this process is not as common as heterogeneous catalysts. Homogeneous catalysts are soluble in the reaction medium (e.g., liquid or gas phase) and are typically used in homogeneous reactions, where the catalyst and reactants are in the same phase [77]. In biomass gasification, the gasification reactions involve solid biomass and gaseous reactants (e.g., steam or oxygen) [17]. As a result, heterogeneous catalysts, which are typically solid materials, are more commonly used. These heterogeneous catalysts are often supported on solid surfaces, like metal oxides or zeolites, and they facilitate gasification reactions at the catalyst’s surface [21]. However, there have been some research efforts to explore the use of homogeneous catalysts in specific gasification scenarios as discussed in the following subsections.

Gasification in Supercritical Water

Supercritical water gasification is a process where biomass is gasified in water above its critical temperature and pressure. Under these conditions, water becomes a unique medium that exhibits both liquid-like and gas-like properties [12]. In such conditions, some homogeneous catalysts have been investigated to promote gasification reactions in the supercritical water medium [83].

Alkaline Catalysts

Homogeneous alkaline catalysts, such as potassium hydroxide (KOH) or sodium hydroxide (NaOH), have been used in some biomass gasification processes, especially for the gasification of certain types of biomass feedstocks, like rice straw or herbaceous materials. These catalysts can aid in reducing tar formation and enhancing the gasification rate [61, 70].

Plasma gasification

In plasma gasification, a high-energy plasma is used to disintegrate and gasify the biomass [44]. Plasma itself acts as a homogeneous catalyst by providing an environment of high-energy particles that facilitate the gasification reactions [160]. Despite these efforts, homogeneous catalysts still face certain challenges and limitations in biomass gasification applications. The use of homogeneous catalysts can be more difficult to control compared to heterogeneous catalysts, and catalyst separation from the reaction products can be complex and costly. Additionally, the harsh conditions and heterogeneous nature of biomass gasification make the process largely heterogeneous. This then implies that what makes a gasification process homogeneous or heterogeneous is largely the phase transformations or dominance of the reactants, catalysts and products. It is largely impossible to liquify biomass prior use such that all the ensuing products are in liquid phase, just as it is essentially difficult to ensure all products are in gas phase; there is also no way to ensure that the catalysts and products remain in solid phase before, during and after reaction, and hence, the limitation of homogeneous catalysts essentially makes them become heterogeneous in nature if compatible with the biomass gasification process since the products, catalyst and reactants all exist in different phases. This further implies that biomass gasification can be homogenized in relation to ensuring non-phase transition behavior. As a result, the basic catalysts that have attained successful biomass gasification are heterogeneously oriented since there has not been any successful attempt to control the reaction pathways of gasification processes to limit or confine the product spec in one phase without the occurrence of phase transition and the existence of reactants, products and the catalyst in different phases. Furthermore, heterogeneous catalysis allows for the use of catalyst supports and promoters which may be unevenly hybridized in order to ensure optimal yield of products while ensuring an improved service life for the catalyst, catalyst stability and efficiency, unlike the homogeneous catalysts which are often disproportionated, and hence, the use of catalyst supports is rarely encouraged. Furthermore, liquid-/gas-phase homogeneous reactions are reactions occurring in gas/liquid or aqueous states whose reactants, products and catalysts are in a single/the same phase and such reactions include condensation and hydrolysis (i.e., acid-catalyzed decomposition of methyl acetate to acetic acid and methanol) reactions, which can be catalyzed by strong acids (i.e., H + ions). Table 17 consists of some generic differences between homogeneous and heterogeneous catalysts, which inform their peculiarities for different reactions.

Table 17 Differences between homogeneous and heterogeneous catalysts

Factors Affecting the Roles of Catalysts in Heterogeneous Gasification Reactions

Some factors that influence gasification reactions as well as have significant effects on catalyst-performance and gasification efficiency include:

Superficial Velocity (SV)

Low SVs can impede catalytic activities involving gasification reactions, thus yielding unburnt tar and high char yield. However, higher SVs decrease char yield owing to increased reaction rates of fast pyrolysis which drastically reduces the gas residence time, thus giving rise to low tar catalytic cracking efficiency. Literature has shown that SVs ranging from 0.4 to 0.6 m/s are apt for effective catalytic gasification reactions [27].

Gasifier Temperature

High gasifier temperatures are suitable for good catalyst activity and improved biomass carbon conversion to combustible gases but lowers the tar yield; this happens as a result of improved gasification rates since the material is volatilized and hence reduces the catalyst poisoning potential of the substrate/feed material. However, occasions may arise where trade-off relationships have to be established since some instances may result in high H₂ concentration at first with subsequent decline at increased temperatures [28, 161, 162].

Equivalence Ratio (ER)

ER has a significant impact on the effect of catalysts during gasification, especially in terms of the resulting product gases that ensue as well as tar formation reduction at elevated temperatures. High ER reduces the heating value of syngas. As a result of the inherent water gas shift (WGS) reaction which occurs during gasification, a high steam/biomass ratio increases the catalyst’s tendency for tar cracking and boosts the potential for H₂ yield. Thus, for efficient catalytic biomass gasification, the ER must be in the range of 0.2–0.35 for efficient gasification [9, 29].

Residence Time (RT)

RT significantly impacts on the tar formation tendency. An increase in RT may cause a corresponding reduction in the existence of oxygenated/oxygen-containing compounds with the production of aromatics at the catalyst’s site, such that 3–4 ring compounds become pronounced [30, 163]. Based on literature, tar reduction may be induced by increased space/residence time when gasification is carried out on a dolomite bed used as catalyst support [26, 31].

Moisture content (MC) of Biomass

MC is a measure of the relative humidity of the environment from which the biomass was harvested or kept, but a weak function of air temperature of the environment [12]. High amount of moisture affects or slows down the catalytic activity of the catalyst, which in turn imparts on the rate of consumption of biomass during pyrolysis prior to gasification. A high percentage of moisture of biomass has an impact on the performance of the catalyst and gasifier efficiency, which also mares the quality of gaseous products received from the gasifier [32]. In a gasification reaction where a downdraft gasifier was employed, high percentage of moisture (i.e., 40)% in the biomass posed detrimental consequences to the catalyst’s activity which in turn resulted in poor gasifier operation and efficiency [2].

Gasifying Agents and Steam to Biomass Ratio

Usually employed agents of gasification include air, steam, oxygen and CO₂. Air, when used as a gasifying agent, produces syngas with low heating value, whereas a combination of steam and oxygen as gasifying agent, produces syngas with medium heating value [22]; however, when a combination of air and steam (i.e., Air–steam) is employed, the H₂ yield increases [24]. The catalytic activity is modified by the different gasifying agents which in turn causes change in the reaction pathways such that the reaction path is adjusted in the direction of the preferred product; hence, there is change in the catalyst’s structure in terms of product selectivity.

Surface Area

Smaller biomass particulates possess a larger surface area-to-volume ratio than larger particles; hence, the former helps to speed up the rate of a chemical reaction owing to the availability/provision of more active sties for reaction progression as well as completion [164].

Selectivity

The selectivity of a catalyst is a measure of its preferential affinity for a synthetic product. It is a property of a catalyst borne as a result of its atomic constituents; the affinity of one molecule or atom for another results in the emergence of an attractive force between the target species/component and the catalyst. Thus, the selectivity of a catalyst helps to make room for the existence or creation of alternative routes/paths that will allow for the synthesis of a particular product relative to another. High catalyst selectivity for a product is a tilt towards increased yield, desorption or precipitation of that particular product [165].

Method of Preparation of Catalyst

The method of preparation has a way of influencing catalytic activity. For instance, catalysts synthesized via the one pot synthesis method and those prepared by impregnation method have been found to exhibit varying characteristics when used to catalyze reactions involving similar substrates. For instance, in a study, the one-pot synthesis method was seen to give better results over the impregnation method in a nanocatalyzed reaction. The preferred catalyst preparation method improved catalytic performance by enhancing the catalyst’s physicochemical properties whilst altering the metal–support interaction, thus changing the kinetics of the catalyst towards enhanced productivity, lower costs and optimized energy requirements [166]. In another investigation, the sol–gel and impregnation methods were used to synthesize iron oxide and molybdenum oxide catalysts supported on alumina for the production of carbon nanotubes. The results proved that catalysts prepared via impregnation method were stickier, flatter, had less impurities, were dispersed as well as easier to dip onto the probe/substrate, which is somewhat beneficial for large-scale synthesis of carbon nanotubes. Alumina was reported to have a larger surface area for a more even dispersion of iron and molybdenum oxide onto its surface. The received TEM images of the tubes showed that the average diameter of the carbon tubes synthesized by impregnation decreased by 23.3%. Hence, the impregnation method improved the quality of the carbon nanotubes relative to those obtained via sol–gel method [167].

Catalyst Geometry, Process Conditions and Particle Size

The activity of a catalyst can be influenced by the catalyst’s particle-shape and process conditions. For instance, in a study conducted by Miloš et al. [168], where slab, spherical and hollow cylinder-shaped particles of similar and different diffusion lengths/sizes, catalyst distribution (i.e., uniform and eggshell type distribution for spherical particle type), and the process conditions (temperature, pressure, syngas composition and degree of conversion) were investigated in relation to how they influence the catalyst’s effectiveness factor and CH₄ selectivity within the catalyst pellet, numerical simulations were conducted using kinetic parameters in order to determine the rates of CO conversion and CH₄ formation in the presence of Co/Re/γ-Al₂O₃ catalyst, and it was observed that particles of 0.2–0.5 mm or eggshell distribution of larger particles of layered thickness of 0.13 mm helped to avoid a negative impact on diffusion limitations for CH₄ selectivity under typical Fischer Tropsch conditions. However, they observed that for monolith reactors with wash-coated catalysts, diffusion limitations can be abated by employing catalysts of less than 0.11 mm layered thickness at a temperature of 473 K, a pressure of 25 bar alongside a molar ratio of H₂:CO of 2.

Mass Transfer

The rate of diffusion of molecules onto the surface of a catalyst is a measure of how fast or slow a reaction occurs. When the mass transfer resistance is high, it can slow down the rate of diffusion of reactant molecules through catalyst pores thus, reducing the degree of conversion and vice-versa. The reacting gas flow situations for heterogeneous reactions (deep oxidation and methane steam reforming reactions) involving a porous catalyst volume of honeycomb occurring in a monolith reactor with triangular channels was investigated [169]. Spatial distributions of the gas flow profile, the local mass transfer rate between the monolith reactor-channel walls, as well as the interaction at the mass transfer interface were examined using computational fluid dynamics (CFD). Under the reaction conditions, a non-stable reacting flow over the entire reactor-channel length was observed. However, there was an intensive change in the distribution pattern of the flow of gas close to the channel inlet, which resulted in high local rates of inter-phase exchange processes of up to two orders of magnitude. They asserted that the higher reaction rate existed at the initial part of the flow due to penetration of feed components through the catalyst pores to the catalyst’s surface, which in turn increased the effectiveness factor, such that it was > 1 close to the channel’s inlet. Furthermore, over time, due to the high volume of transported molecules/reactant build-up which tend to fill up the pores of the catalyst located within the monolith length, the reaction rate dropped as a result of high mass transfer resistance.

Type and Quantity of Feed Stock

The nature of a raw material has a way of determining the results obtained from a biomass gasification process. For instance, biomasses vary in their syngas compositions when processed; hence, the reason for which they exhibit variations in their syngas yields under similar reaction conditions [170].

Catalyst Selection for Heterogeneous Gasification Reactions and Reaction Mechanisms for a Typical CO₂ gasification Reaction

Catalyst Selection

The essence of involving a catalyst in gasification reactions is essential, because without the involvement of catalysts, the reactions will take a longer time to reach completion or some may never even commence at all owing to the fact that the energy required to initiate the reaction step is lacking. A catalysts helps to alter the rate of gasification i.e., it speeds up the gasification reaction by first altering the activation energy of the reaction. By lowering the energy requirement for the reaction initiation step, the activation energy of the reaction is easily attained; thus, the breaking of bonds in the biomass sets in by the addition of heat and inducement of molecular bombardment by the gasification agents, which in turn brings about product formation. However, these catalysts are not involved in the reactions themselves. The selection of a catalyst for a gasification process depends on the following, the nature of feed/biomass, the operating conditions as some catalysts experience sintering at harsh reaction conditions, suitability for tar reduction/conversion, high product/gas yield, sustainable active life/durability, stability, recyclability/regeneration potential and cost, all of which have to be carefully taken into consideration with a view to establish trade-offs since some catalysts have good tar reduction/elimination potentials while others are tilted towards high product yield [161]. In the char gasification process carried out by Jiao et al. [162] on sawdust char using CO₂, the effect of gasification temperature, CO₂ adsorption, and influence of K-modified transition metal composite catalysts was examined and it was discovered that temperature was one of the most influential operating variables in the gasification process, and carbon conversion was seen to increase about 2.55 times within 40 min when the temperature was adjusted from 750 to 800 °C. Also, the composite catalysts were seen to have effectively improved sawdust-char conversion at low temperatures, in the following order of catalytic performance: KCo > KNi > KFe > KCe. They asserted that the catalytic CO₂ gasification activity depended not only on the amount/quantity of CO₂ adsorbed, but also on the CO₂ decomposition activity that took place on the catalysts’ surfaces. The CO₂ reaction mechanism over Co, Ni, and Fe surfaces was projected to tow the following similar reaction steps: M + CO₂ → M-Oads + CO, M-Oads + C* → M + CO. Whereas, over the cerium oxide (CeO₂) surface, the reaction mechanism was likened to the following: CeO₂ + C* → CeO₂vacO + CO, CeO₂vacO + CO₂ → CeO₂ + CO; where “ads” referred to adsorption sites while “vac” referred to vacant sites which may be adjacent to the active sites. This goes further to explain that the mechanism of a catalytic gasification process somewhat depends on the type of catalyst selected for the anticipated reaction. In addition, the catalyst orientation or configuration in terms of particle size, geometry and structure also have a way of influencing the resulting products from a gasification reaction. Smaller particle sizes/increased particle concentration of feed and catalyst help secure a high surface area to volume ratio, which boost reaction rates, and according to the law of mass action, increased particle collisions are favored by increased concentration of catalysts/reactants. Also, it should be noted that while catalysts are reaction rate boosters, there is need to also consider their threshold limits which is indicative of the optimum conditions for best catalytic activity; this serves as a critical point beyond which diminishing returns set in in terms of catalyst performance or loss of activity, which are sometimes caused by imposing harsh reaction conditions or straining the catalyst beyond its activity limit. More so, catalyst selectivity or preferential selection or affinity for reacting species is also one significant attribute of a catalyst that determines the extent of reaction or how far a heterogeneous reaction progresses, and at what instance it comes to completion. In addition, catalysts used in gasification reactions should possess efficient tar elimination abilities within 650–950 °C; must possess good methane-reforming potential, especially when syngas is the desired product, must be capable of converting higher hydrocarbons/aromatics to viable products, must possess a good measure of compatibility with a minimum hydrogen/carbon monoxide ratio for optimal performance, should be resistant to deactivation due to sintering and coking while being effective in safeguarding against fouling by impurities or unwanted products. A reliable catalyst for gasification reactions needs to be resistant to sulfur poisoning amid having good stability and high reusability-potential with a non-complex regeneration procedure; also, it must be nontoxic, cheap, resistant to attrition, commercially viable and environmentally friendly.

Mechanisms for Heterogeneous Catalytic Gasification Processes

Largely, heterogeneous reactions occur in seven steps, these include: (i) External diffusion from the bulk fluid through the fluid–solid interface to the catalyst’s surface, (ii) inward penetration/internal diffusion through the pore-mouth of the catalyst to the internal wall of the catalyst, (iii) attachment to the internal surface of the catalyst, (iv) surface reaction between/among the reactants to form products, (v) desorption of the products, (vi) internal diffusion through the catalyst pore/channel to the pore mouth of catalyst, and (vii) external diffusion of the products from the catalyst pore-mouth. However, these steps occur via three mechanisms: the single site, dual site and Eidel Riley Mechanisms [163]. Thus, assuming that steps, i, ii, vi and vii are very fast steps and steps iii, iv and v are the slowest steps, it then implies that the reaction rate will be defined by any of the steps iii, iv and v, therefore, it then suffices to do the following:

The Single Site Mechanism (Involving One Reactant and a Catalyst)

Supposing an irreversible reaction occurs between a fluid (gas) and solid on a catalyst’s surface, the adsorption surface reaction and desorption steps (schemes 1-3) can be expressed as:

$${\text{A }} + {\text{ S}} \to {\text{A}}.{\text{S }}\left( {\text{adsorption step}} \right)$$

(1)

$${\text{A}}.{\text{S}} \to {\text{B}}.{\text{S }}\left( {\text{surface reaction step}} \right)$$

(2)

$${\text{B}}.{\text{S}} \to {\text{B }} + {\text{ S }}\left( {\text{desorption step}} \right)$$

(3)

where A = the molecule diffusing onto the catalyst surface, S = the vacant site of catalyst, A.S = Adsorbed molecule or A attached or adsorbed onto the catalyst’s site, B.S is the product of the conversion or reaction of A.S on the catalyst’s surface, B.S is the product B attached or formed on the site of catalyst, B = desorbed product obtained from the reaction of A on the catalyst’s surface.

The Dual Site Mechanism (Involving Two Reactants and Two Sites)

$${\text{A }} + {\text{ B }} + {\text{ 2S}} \to {\text{A}}.{\text{S }} + {\text{ B}}.{\text{S }}\left( {\text{adsorption step}} \right)$$

(4)

$${\text{A}}.{\text{S }} + {\text{ B}}.{\text{S}} \to {\text{C}}.{\text{S }} + {\text{ D}}.{\text{S }}\left( {\text{surface reaction step}} \right)$$

(5)

$${\text{C}}.{\text{S }} + {\text{ D}}.{\text{S}} \to {\text{C }} + {\text{ D }} + {\text{ 2S }}\left( {\text{Desorption step}} \right)$$

(6)

Again, in schemes 4, 5 and 6, A and B = reactants, 2S = two vacant sites of catalyst, A.S = Adsorbed molecule of A onto S, B.S = adsorbed molecule of B onto S, C.S = first product formed on one of the catalyst sites from the reaction of A and B, D.S = second product formed on the other catalyst site from the reaction of A and B, and, C and D are the desorbed products from the catalyst’s sites and surface.

Another variant of the dual site mechanism is:

$${\text{A}}.{\text{S }} + {\text{ S}}^{\prime} \to {\text{A}}.{\text{S}}^{\prime} \, + {\text{ S }}\left( {\text{adsorption step}} \right)$$

(7)

$${\text{A}}.{\text{S}}^{\prime} \to {\text{B}}.{\text{S}}^{\prime} \, \left( {\text{surface reaction step}} \right)$$

(8)

$${\text{B}}.{\text{S}} \to {\text{B }} + {\text{ S}}^{\prime}$$

(9)

In the above reaction schemes 7, 8 and 9, reactant A is already attached to a catalyst which comprises mixed metal atoms (such as Co and Ni) or a hybrid catalyst, where a promoter or catalyst support may be involved. In this case, the site S = one specie site on the main catalyst say Co, while site S’ = another specie site on the support or promoter, say Ni.

For the adsorption step, there is a plausible displacement of the molecule A from S to S’, thus causing A to be adsorbed onto site S’.

In the surface reaction step, the adsorbed A onto site S’ (i.e., A.S’) reacts to give product B attached onto site S’ (i.e., B.S), while the desorption step entails the detachment or separation of B from site S’ to give B and S’, respectively.

The third case or variant of the dual site mechanism is:

$${\text{AB}} + {\text{ 2S}} \to {\text{A}}.{\text{S }} + {\text{ B}}.{\text{S }}\left( {\text{dissociative adsorption}} \right)$$

(10)

$${\text{A}}.{\text{S }} + {\text{ B}}.{\text{S}} \to {\text{C}}.{\text{S }} + {\text{ D}}.{\text{S }}\left( {\text{surface reaction step}} \right)$$

(11)

$${\text{C}}.{\text{S }} + {\text{ D}}.{\text{S}} \to {\text{C }} + {\text{ D }} + {\text{ 2S }}\left( {\text{desorption step}} \right)$$

(12)

The above schemes 10, 11 and 12 can be seen for the case of a molecule such as carbon monoxide separating or splitting into its constituent elements (carbon and oxygen) owing to the difference in affinities/proximity of the vacant sites. The split elements are adsorbed onto each of the vacant sites giving A.S and B.S.

The Eidel Riley Mechanism

Eidel Riley proposed an exception that may likely occur beyond what have been discussed in the previous mechanisms) [164]. According to the concept, supposing reactant A already adsorbed to a surface combines with a gas and there is just an active site available for adsorption of the non-gaseous species, the steps are as follows:

$${\text{A }} + {\text{ B }}\left( {\text{g}} \right) \, + {\text{ S}} \to {\text{A}}.{\text{S }} + {\text{ B}}\left( {\text{g}} \right) \left( {\text{adsorption step}} \right)$$

(13)

$${\text{A}}.{\text{S }} + {\text{ B }}\left( {\text{g}} \right) \to {\text{C}}.{\text{S }}\left( {\text{surface reaction step}} \right)$$

(14)

$${\text{C}}.{\text{S}} \to {\text{C }} + {\text{ S }}\left( {\text{desorption step}} \right)$$

(15)

Considering schemes 13 to 15, the adsorption step simply suggests that the solid molecule becomes attached to the available site leaving the gas. The next step is the surface reaction step where the solid molecule is then provided the best ambience for contacting or reacting with the gas so that substantial contact is made between the gas and the solid reactant with the help of the catalyst as a holder for the solid species; this leads to the formation of product C attached to site S (i.e., C.S). Thereafter, the desorption or detachment of C takes place where the product is separated from the catalyst’s site.

Challenges and Future Prospects

Some challenges associated with high biomass conversion include low carbon conversion efficiency caused by induced coking, the presence of syngas impurities, low H₂/CO ratio, low CO₂ conversion in the presence of CO, the poor influence on/interference of gasifying agents with the biomass, the nature of biomass such as those bearing large particles, choice of catalyst and reactor-type; all of these have a way of affecting the syngas yield, tar reformation and volume of hydrogen produced during gasification. However, efforts should be made in selecting the ideal reactor type, catalyst/promoter/support, apt process conditions, ideal particle size range, S/B ratio, ER, gasifying agents, which may be combined if necessary in order to ensure high tar conversion or reformation of volatiles to give higher gas yield. Furthermore, since transition metals have proven to be very useful catalysts, which help in ensuring high tar conversion and high syngas/H₂ yield, efforts should be put in place to explore/exploit new/newly discovered catalysts of the lanthanide and actinide series, including the outer and inner transition metals in order to ascertain their efficacy in ensuring biomass and residual gas conversion for improved gas yield during biomass gasification. Other products generated from gasification such as CO₂ can be converted to light olefins as a way of reducing the carbon footprint; literature has it that CO₂ can be used in generating electricity. The char obtained can be used as adsorbent for detoxifying contaminated waters. Hydrogen obtained from the process can be used in the hydrodechlorination of pesticide effluents entrained with harmful chemicals like 4-chlorophenol. The methane produced can serve as biogas for generating electricity.

Conclusion

Heterogeneous gasification of biomass can result in the production of different useful materials ranging from hydrogen to CH₄, CO, CO₂, SO₂, tar, oil, etc., which find useful applications in the production of essential chemicals and other industrial materials. Several types of catalysts have been employed in catalytic gasifiers in order to improve on syngas/H₂ yield. Heterogeneous catalysts have also shown better performance relative to their single counterparts, especially with respect to the dolomite and Ni-derived catalyst families. Despite the huge potentials evident in these catalysts, it is also pertinent to mention here that the operational parameters such as SB ratio, ER, CGE must approach optimum conditions in order to ensure maximum utilization of the catalyst’s capacity. Also, in selecting ideal catalysts for gasification processes, it is necessary to establish/test the measure of compatibility amongst catalyst support, catalyst and promoter alongside the nature of feed stock, all of which can impede or positively influence catalyst activity. However, three things can lead to catalyst deactivation and these include coke deposition, aging and catalyst poisoning; these in turn render the catalyst inactive and thus imply that efforts should be put in place to ensure high catalyst activity during gasification. Furthermore, any ideal catalyst for gasification of a choice-biomass, must have a good measure of reusability within its service life after being regenerated. In addition, a highly effective catalyst must be preferentially selective towards the desired product. Since catalysts are reaction-specific, it then follows to say that no particular catalyst is suitable for all heterogeneous gasification processes. This then informs that an apt selection criteria need be thoroughly observed in order to pick the ideal catalysts for specific gasification processes in order to ensure high product throughput. Furthermore, a good understanding of the underlying mechanisms for heterogeneous catalytic gasification reactions should be well understood in terms of the induced physical and chemical interactions that precede the final product synthesis stages; this will not only serve as a viable approach for selecting an apt/most reliable catalyst for a specific gasification process but will also help to guarantee high selectivity and optimum product yield.

Data and material availability

All the data and materials required for a proper understanding of this research as well as to ensure data reproducibility have been included and are available in the manuscript.