Emerging trends in hydrogen and synfuel generation: a state-of-the-art review

Abstract

The current work investigated emerging fields for generating and consuming hydrogen and synthetic Fischer-Tropsch (FT) fuels, especially from detrimental greenhouse gases, CO₂ and CH₄. Technologies for syngas generation ranging from partial oxidation, auto-thermal, dry, photothermal and wet or steam reforming of methane were adequately reviewed alongside biomass valorisation for hydrogen generation, water electrolysis and climate challenges due to methane flaring, production, storage, transportation, challenges and opportunities in CO₂ and CH₄ utilisation. Under the same conditions, dry reforming produces more coke than steam reforming. However, combining the two techniques produces syngas with a high H₂/CO ratio, which is suitable for producing long-chain hydrocarbons. Although the steam methane reforming (SMR) process has been industrialised, it is well known to consume significant energy. However, coke production via catalytic methane decomposition, the prime hindrance to large-scale implementation of these techniques for hydrogen production, could be addressed by coupling CO with CO₂ conversion to alter the H₂/CO ratio of syngas, increasing the reaction temperatures in dry reforming, or increasing the steam content fed in steam reforming. Optimised hydrogen production and generation of green fuels from CO₂ and CH₄ can be achieved by implementing these strategies.

Introduction

The world’s population continues to expand at a rapid rate, leading to a significant increase in energy demands. This demand is even higher than the population growth itself (Abdelkareem et al. 2022). The depletion of fossil fuels, which is influenced by factors such as geographical distribution and extraction accessibility, adds to the challenge (Sagar et al. 2024). Moreover, heavy reliance on fossil fuels contributes to the accumulation of greenhouse gases like carbon (IV) oxide (CO₂) in the atmosphere, which is the primary cause of global warming. Consequently, there is an urgent need to explore alternative, environmentally sustainable energy sources (Cao et al. 2024). Given this pressing need, there is a growing interest in alternative energy sources driven by both the rising global energy demand and concerns about the carbon footprint of fossil fuels (Lara Sandoval et al. 2024). Amongst the potential options, hydrogen stands out as a promising and environmentally friendly fuel source (Singh et al. 2017; Abdelkareem et al. 2022). Projections indicate that global demand for hydrogen energy across various industries will increase by approximately 400 Mt/year over the next five decades (Ighalo and Amama 2024).

Syngas (a mixture of hydrogen (H₂) and carbon monoxide (CO)) is an intermediate feedstock used to produce a variety of fuels and chemicals, including H₂, methanol, FT fuels, dimethyl ether (DME) and ethanol (Peña et al. 1996; Elvidge et al. 2015; Taherian et al. 2021; Jin et al. 2021). Its efficient commercialisation is gaining significant attention worldwide. H₂ is the perfect upcoming clean energy alternative (Puangpetch et al. 2009; Kong et al. 2019; Araiza et al. 2021; Li et al. 2022). Over the past few decades, emissions from burning fossil fuels like coal, oil, CO₂ and natural gas have contributed to a steady, rapid increase in global warming, resulting in severe environmental pollution (Wang et al. 2018b; Qingli et al. 2021; Nabgan et al. 2022; Sasidhar et al. 2022). A fossil fuel-based source of energy is non-renewable (Qingli et al. 2021); therefore, the search for alternative, cleaner and eco-friendlier forms of energy has gained significant attention in recent years (Makertiharta et al. 2017; Alhassan et al. 2022; Lee et al. 2022).

Biodiesel is an excellent choice for utilising the potential of biomass. Globally, more than 27 million metric tonnes of biodiesel are generated each year. Glycerine (10% of which is a by-product) may contain water, free fatty acids, manufacturing residues and trace amounts of heavy metals, impacting its purity and suitability for specific uses. Before being used, the crude glycerine must be refined. To produce hydrogen from biomass, two thermochemical processes are available. The first is biomass gasification, while the second is catalytic steam reforming of biomass pyrolysis oil (also known as bio-oil) (Wang et al. 2014). Biomass steam reforming (SR) via pyrolysis, the second method, which involves hydrogen production via biomass pyrolysis (SR of bio-oil), is a more cost-effective alternative due to the high bio-oil yield and mobility. Nonetheless, the challenge of purification associated with the techniques results in a significant price increase (Wang et al. 2014; Chen et al. 2017; Yi et al. 2023).

Most of the nearly 210 billion Nm³ of coke oven gas (COG) by-products from the metallurgical sector are either burned directly as fuel or discharged directly into the atmosphere, squandering energy and harming the environment. Due to its high hydrogen content (48–55 mol % H₂), it has been described as one of the raw materials most likely to attain large-scale commercial H₂ production in the short and medium terms. Physical separation technologies like pressure swing adsorption (PSA) are used to extract H₂ from COG. Still, this process also removes other components like CO, methane (CH₄), tar and hydrocarbons like polycyclic aromatic hydrocarbons (PAHs), xylene and toluene, whose further separation takes up large amounts of ammonia and contaminates water sources (Xie et al. 2017).

It has been proposed that chemical looping hydrogen (CLH) technology is an innovative and viable method of manufacturing H₂ incorporating CO₂ segregation, good product quality and high efficiency. The steam or fuel reactor (SRE/ FR) and the air reactor (AR) are the most common reactors in a CLH. In the steam reactor (SR), steam oxidises the reduced oxygen carrier (OC) to produce high-purity H₂. The challenges for the process include fuel conversion, steam conversion, heat duty and the optimum ratios of OC to COG, steam to OC and air to OC (Xiang and Zhao 2018). The splitting of water into H₂ and oxygen (O₂₎ using solar energy is another possibility being considered since H₂ gas is a powerful energy source because of its high gravitational energy density and is environmentally benign via near-zero greenhouse gas production (Tolod et al. 2016).

Typically, large-scale H₂ generation is mainly accomplished via fossil fuel reforming and water electrolysis, even though this latter technique accounts for just 5 % of total H₂ production (Meloni et al. 2020). Photocatalytic water splitting is amongst the most recent techniques for generating H₂. It allows the conversion of solar energy to chemical energy, enabling the use of solar energy and water. This is a viable approach for moving from a fossil fuel economy to a green, hydrogen-powered one (Puangpetch et al. 2009; Sayed et al. 2019). The SR of CH₄ (SRM), partial oxidation (POX), dry reforming (DRM) and auto-thermal reforming (ATR) are examples of fuel reforming processes. SRM is the oldest and most practical method for converting CH₄ to H₂ amongst the reforming processes. It is typically described as the consequence of (Eq. (1)) and (Eq. (2)) and has a high H₂/CO ratio of 3:1; working temperatures over 700 °C are needed for this reforming reaction, and steam-to-methane ratios of 2.5 to 3.0 are often used to minimise coke formation (Matas Güell et al. 2011; Yentekakis et al. 2021).

According to a recent report by Meloni and co-researchers (Meloni et al. 2020), the most used catalyst for SRM is Ni supported over ceramic oxides or oxides of metallic or metalloid elements. Other group VIII metals are active, although they have specific disadvantages; for instance, Fe oxidises rapidly, Co cannot survive steam partial pressures, and precious metals (Rh, Ru, Pt and Pd) are outrageously costly for practical use. Supports often employed include alumina (Zhang et al. 2017; Pirshahid et al. 2023), magnesia (Bian et al. 2016; Alabi et al. 2020), calcium aluminate (Batuecas et al. 2021; Zhang et al. 2024) and magnesium aluminate (Alabi et al. 2020).

The catalytic POX is more energy efficient due to its rapid kinetics and exothermic nature, eliminating the need for huge reactors and significant quantities of superheated steam. Furthermore, the stoichiometry of POX (6) produces a synthesis gas with an H₂/CO ratio of 2:1, allowing its direct use for methanol or FT synthesis without further adjustment. However, the need for pure O₂ and the risk of explosion are associated problems in addition to the seldom-required adjustment of the product ratio (Arku et al. 2018; Chen et al. 2020).

Because of its capacity to absorb two greenhouse gases, hydrogen generation from DRM has garnered a lot of interest in recent years. CH₄ and CO₂ (Eq. (1)) generate lucrative feedstocks (syngas) with a better H₂/CO ratio, which is required as a highly valued feedstock for FT synthesis and methanol production (Afzal et al. 2020; Li et al. 2021; Ibrahim et al. 2022). Again, nickel catalysts supported by various metal oxides like ZrO₂, Al₂O₃, MgO, CeO₂ or La₂O₃ have been widely used in DRM due to their relatively high catalytic activity and low cost (Charisiou et al. 2016; Goula et al. 2017; Chaudhary et al. 2020; Ibrahim et al. 2022).

The ATR method for figuring out the value of methane is an important part of making syngas by combining adiabatic (SR) and non-catalytic (POX) processes. As a result of the sintering, production and deposition of coke, the activity of Ni catalysts, which are frequently used in this process, is diminished. Consequently, they require support from metal oxides such as CeO₂ (Song et al. 2016; Araiza et al. 2021; Zhang et al. 2023) and SiO₂ (Nath et al. 2022; Alhassan et al. 2024).

This review compares the various natural gas reforming processes, especially the potential for their implementation on an industrial scale for valuable feedstock, energy and hydrogen production; discusses the need for hydrogen-driven energy processes, utilisation, storage and transportation of CO₂; and the impacts of flared natural gas. Cutting the 300 to 400 million tonnes of CO₂ released by methane flaring is also addressed here. Novelties at each methane reforming method are well summarised, and a combination of wet and dry reforming perspectives, world energy supply and fuel-gasifier interaction from combustion, gasification, pyrolysis and drying zones was classified into oxidation, methanation, non-coke side reactions and coke side reactions.

Review novelty and objective

Considering the various challenges associated with CO₂ emissions, numerous research institutions have recently focused on valorising CO₂ into fuels and chemicals, commonly called carbon capture and utilisation (CCU). This approach is recognised as a multidimensional method. In addition to its environmental and health advantages, the process of CO₂ upgrading holds promise for addressing the challenges associated with depleting energy resources and uneven distribution. The main objectives of this study were to comprehensively assess and examine recent advancements in catalytic technologies employed for converting CH₄ and CO₂ into chemically useful compounds and energy resources. Hydrogen emissions are environmentally beneficial because, when combined with oxygen in fuel cells, they only produce water vapour. The production of hydrogen through electrolysis, which is powered by renewable energy, offers a carbon-free process. Hydrogen has a wide range of applications and can replace fossil fuels in transportation, manufacturing and energy storage. This provides a promising solution to the environmental problems caused by conventional fuels.

Sources types and technologies to produce hydrogen

Hydrogen is an efficient energy type and has now been identified as an energy carrier that can be obtained from both renewable and non-renewable sources. However, over 70% of existing technologies to produce H₂ are based on reforming natural gas. However, reports from (Taylor and Balat 2008; Howarth and Jacobson 2021) argue that, thus far, hydrocarbon reformation, specifically methane, produces more than 96 % of H₂ from fossil fuels like coal, natural gas and petroleum. Hydrogen can dramatically lessen the environmental damage caused by fossil fuels compared to other fuels (Avci and önsan 2018).

Figure 1 provides a comprehensive overview of energy production processes and technologies that are essential for sustainable energy transitions. Figure 1A to D provide existing data regarding fuel energy demand, biomass energy demand and total energy makeup, while section E is divided into three main parts that cover pathways involving fossil fuel resources, biomass/waste utilisation and water splitting for hydrogen production. The section (1E) focuses on the utilisation of fossil fuel resources. It showcases methods such as natural gas conversion and coal gasification with carbon capture, utilisation and storage (CCUS). These technologies highlight strategies for mitigating carbon emissions while making use of existing fossil fuel infrastructure. The next segment highlights biomass and waste utilisation. It illustrates pathways for converting organic materials into energy sources. Technologies such as biomass conversion and waste-to-energy processes demonstrate the potential to reduce dependency on fossil fuels while addressing waste management challenges.

Fig. 1

Final energy demand by scenario by industrial sub-sector (A); energy demand scenario by fuel type (B); biomass consumption in energy demand (C) and D total final energy demand per energy sector (EU27 + UK), reused with permission from (Johannsen et al. 2023) and E Main Techniques to produce hydrogen

Finally, the section explains various techniques for hydrogen production through water splitting. These techniques include direct water splitting, high-temperature electrolysis and low-temperature electrolysis, each offering unique advantages and contributing to the development of sustainable hydrogen economies. Production of H₂ has been reported via numerous routes, amongst those presented in Fig. 1E: (i) low-carbon pathways utilising a variety of domestic resources, including fossil fuels; biomass conversion and waste to energy technologies, which mostly require biological processes such as anaerobic digestion and the action of microorganisms; (ii) CCUS and (iii) photoelectrochemical splitting of water into hydrogen and oxygen using either nuclear energy or renewable sources such as wind, solar, geothermal and hydroelectric power (Dutta and Vaidyalingam 2003; Lu et al. 2017; Sittipunsakda et al. 2021).

Amongst the production technologies, biomass conversion in anaerobic digesters is less efficient as they consume more time, require extensive infrastructure and inevitably must be subjected to several cleaning processes. Even though producing green hydrogen requires a feedstock that is entirely renewable and has as little carbon footprint as feasible, there are evident records in the literature (Cuéllar-Franca and Azapagic 2015; Arku et al. 2018; Xu et al. 2020) that declare that the combined impact of the method for fuel production and the extent of waste generation are critical factors for its classification. Accordingly, based on the source of production and method of separation, H₂ may be classified as brown or black (produced via pyrolysis and gasification of carbonaceous materials like coal, etc.); green (produced majorly from wind, solar, tidal, or via electrolysis, water splitting, stored as an energy vector, transferrable in space and time); blue ( a low-carbon H₂ produced mainly via SR with an effective cost-benefit, product storage via CCS and minimisation of pollution); grey (in resemblance to the blue hydrogen, but where emissions from the process are released directly into the atmosphere) and turquoise in which case, CH₄ is cracked from a temperature above 600 °C to about 1200–1400 °C, generating solid coke and H₂ gas (Menon and Selvakumar 2017; Howarth and Jacobson 2021).

As reported by (Osman et al. 2022) and shown in Fig. 2, the production, safety, storage, utilisation and upgrading of H₂ were highlighted. Fuel production for transportation, power generation, the production of nitrogen fertiliser by the Odda process and NH₃ production, industrial metallurgical processes and the production of hydrocarbon fuels have been amongst the most significant uses of hydrogen. Importantly, hydrotreating processes get rid of the stubborn carcinogenic heteroatoms (N, O, S, F, etc.) in the oil pool. For example, denitrogenation gets rid of the extra nitrogen molecules that stop the acidic sites on conversion catalyst molecules from working, which makes the crude oil fractions undesirable. There are corrosive nitrogen compounds; hence, natural oils above 0.25 wt % are removed during refining. Hydro-deoxygenation and hydrodesulphurisation are other industrial processes that take up a lot of H₂. Hydrotreatment also ensures the removal of inherent carcinogenic and toxic compounds of sulphur, which cause the sourness of crude. Sulphur is removed from fuels during refining to comply with laws to decrease sulphur-related air pollution. Water, minerals and other pollutants are also found in crude oil. If not removed, these salts and heavy metals produce acids, corroding downstream process equipment when heated. Oxygen dispersion in petroleum fractions induces gum development in different reactors and pipelines in refinery operations, resulting in clogging and loss of equipment efficiency (Verstraete et al. 2007; Ahmad et al. 2011; Sbaaei and Ahmed 2018).

Fig. 2

Routes for hydrogen production, conversion and applications. Copied from Osman et al. (2022) with creative commons licence permission

Highlight of published data, author keywords and hydrogen production

The prevalence of techniques in the literature emphasizing the need for a switch to more green energy sources with less negative potential for the environment cannot be overemphasised (Aw et al. 2014; Papageridis et al. 2016; Alhassan et al. 2019; Ahmed et al. 2020). Unarguably, sufficient data has been published addressing the critical issues of production, safety, storage, utilisation, photoproduction, CH₄ pyrolysis, SR and other related terms, which are presented in Fig. 3. The search terms “hydrogen production” and “energy generation” were typed in the search box on the Web of Science (WoS) interface under all fields. The outcome was used to generate the plain text file from which the cluster shown in Fig. 3 was generated. The author keywords of 1000 publications were refined to 15 or more appearances, indicating that only the keywords that satisfy the threshold appear in the cluster. Interestingly, the closest terms to H₂ production are CH₄ conversion, thermal decomposition, energy generation, biomass gas systems, visible light and water gas from the top; storage, kinetics, electrolysis, catalysts and temperature from the left; and evolution, decomposition, optimisation, visible light irradiation and photo-fermentation from the bottom.

Fig. 3

Cluster displaying the relationship between the strongest author keywords. Generated using R-studio from plain text file extracted from the WoS

Hydrogen production for energy and natural gas conversion has been a serious and important aspect over the last 2 decades. Based on an explanation for similar clusters in the studies (Alhassan et al. 2022; Hatta et al. 2023), the figure emphasised that photo-fermentation has not been established in the literature as much as the methane conversion route. This is evident from the text size of the keyword and its closeness to the other keywords. Astonishingly, the generation of natural gas as an energy alternative from food waste, especially via the incorporation of the action of microorganisms, is also shown by the appearance of fermentation, food waste and renewable H₂ in the cluster.

Opportunities and challenges in CO₂ utilisation for green fuels

Increased CO₂ emissions are the principal cause of global warming, an unavoidable threat that has attracted international attention (Bruhn et al. 2016; He and Liu 2017). Carbon cycles exist in bulk between the continental environment and the ocean. Global CO₂ emissions surpassed 9.68 billion metric tonnes in 2014, 60% of which remained in the atmosphere, according to BP world energy data (British Petroleum 2021). CO₂ output must be reduced by 60–70% to ensure equilibrium as nature absorbs 3 billion metric tonnes of carbon every year. As of October 2021, the average CO₂ concentration had shot up to roughly 414 ppm, an increase of approximately 47.86% relative to pre-industrial revolution levels (280 ppm). This dramatic increase in CO₂ concentration led to further global climate change. According to the Goddard Centre for Space Studies, the land-ocean temperature index (°C) has grown from − 0.16 °C in 1880 to 1.02 °C in 2020 (Chai et al. 2022).

Supercritical CO₂ functions in polymerisation, refrigeration and as a working fluid. Subsequently, the absorption, utilisation and valorisation of CO₂ are needed to combat pollution and global warming caused by their expanding sources. By 2030, process advancements should cut post-combustion capture costs from 52 to around 25–30 USD/tonne (Valluri et al. 2022). Similarly, its application in iron and steel production includes iron ore extraction, usually improved by flotation or magnetic separation, pelletisation for transport into the blast boiler, reduction into pig iron and steel refining using a basic oxygen boiler or electric arc boiler. Figure 4 depicts CO₂ separation and capture technologies in a contemporary system. Modern CCU technologies abate CO₂ by absorbing air supply, isolating CO₂ (although in reduced concentration) and diluting it in N₂ and NOx molecules (post-combustion). The fundamental concept behind the pre-combustion approach is gasification, which occurs when carbonaceous biomass and coal are pyrolysed at high temperatures to form syngas, which results in the creation of H₂, CO and CO₂, particularly at high reaction temperatures (Valluri et al. 2022). The oxy-combustion technique burns fuel in recycled flue gas (RFG) with a proportion of oxygen rather than pure air (in conventional post-combustion). A considerable proportion of CO₂ gas and minute amounts of water vapour are produced; these gases are separated by adding a desiccant (such as SiO₂) that can absorb the moisture, yielding CO₂ with a purity of between 80 and 90% and O₂ with a purity of 95%, respectively. During the chemical looping process, an O₂ carrier transfers the necessary O₂ for combustion from the combustion air to the fuel, typically an oxidised metal. Alternating between the fluidised bed for air and the one for fuel is the oxygen carrier (metal oxide). After mixing with the fuel, the MeO produces water vapour and carbon monoxide as exhaust gas (Rajabloo et al. 2023).

Fig. 4

Existing technologies for CO₂ capture and utilisation

The carbon cycle process has a significant impact on environmental, climate and energy production. Energy efficiency and carbon efficiency for the FT synthesis have respectively increased by 18.4% and 86.9% compared to conventional coal-liquid and by 15% and 100% for methanol production systems, based on the report of Chen (Chen et al. 2016). Another prime contributor to CO₂ emissions is the heat from fossil fuels, which accounts for 70% of the global electricity supply, around 50% of which is obtained from the combustion of coal (Chen et al. 2016).

Methane flaring and climate change

The world’s energy consumption is thriving. The accomplishment of manufacturing civilisation, the expansion of the economy and the population’s standard of living depend on energy usage. Consequently, the global energy consumption of all fuel sources has increased substantially. The usage of fossil sources (petroleum, coal and natural gas) is still dominant. It will continue to be essential soon, despite the year-by-year development of renewable energy sources (Sujianto 2020; Hatta et al. 2021). Global energy consumption, 1971–2019, displayed in Fig. 5a, highlights the six primary world energy sources, ranging from coal, oil, biofuels, hydro, natural gas and nuclear (This data is subject to the IEA’s terms and conditions: https://www.iea.org/t_c/terms and conditions/. Natural gas is one of the most effective and considerable energy sources on the planet today. Due to its reliability and relatively high fuel economy, it remains an attractive option for the industrial and electric power sectors (Hatta et al. 2021). The worldwide usage of natural gas has increased during the previous two decades. According to (British Petroleum 2021, 2022), global natural gas consumption in 2021 was approximately 4.04 trillion cubic meters. The United States and Russian Federation are the most significant producers, accounting for almost 42% of the world’s total natural gas, while in Fig. 5b, the various stages of raw biogas treatment to obtain clean biomethane are presented.

Fig. 5

A World total energy supply (by source), plotted based on data adapted from https://www.iea.org/t_c/termsandconditions/ with creative common’s licence (“Key World Energy Statistics 2021”); B biogas cleaning/upgrading processes. Reused with permission from Rafiee et al. (2021)

Although natural gas is an excellent fossil fuel and strategies to utilise the excess are ongoing, the scientific community agrees that anthropogenic greenhouse gas generation has consequently affected the global climate and that drastic reductions in these emissions are necessary to mitigate its adverse effects on climate change. Fossil fuel combustion, widely used for electricity, heat and transportation, is the primary source of emissions globally. Natural gas flaring releases approximately 300–400 million metric tonnes of CO₂ annually (Saidi 2018). The most common fossil fuels are coal, petroleum and natural gas. However, one common disposal method is flaring when there is inadequate infrastructure to use the gas locally, store it for energy, or transfer it via pipelines to market (Farniaei et al. 2014).

Most flared gas is CH₄, with minor amounts of volatile organic chemicals and inorganic molecules, such as CO₂, N₂ and water. There are two different types of gas flaring; associated flaring, which takes place in oil and gas (associated) reservoirs during exploration processes, and non-associated flaring, in which accumulated gas from refineries and petrochemical plants is flared for safety reasons, especially during normal routine operation. The volume and content of these gases vary depending on their production regions, as well as the temperature and pressure of the underground reservoirs from which they are extracted. Although no clear experimental data regarding the composition of the flared gas is available within our reach, in approximation, 80% of related gas flares are due to economic and technical constraints. According to a recent study by (Elvidge et al. 2015), upstream exploration and extraction facilities produce 90 % of flared gas globally. In recent years, extraction companies, pushed by environmental regulations and financial indications, have studied several options to reduce the quantity of flared gas. Collecting and transporting the gas to the market, converting it to a liquid fuel similar to gasoline, using it for electricity and heat generation, using it as a fuel for onsite needs, reinjection into underground strata to improve oil and gas extraction and the production of syngas are all methods for reducing gas flaring in addition to dry reforming processes (Orosa and Zardoya 2022; Tahmasebzadehbaie and Sayyaadi 2022; Owgi et al. 2023b), as summarised in Fig. 6. In a report published by (Fisher and Wooster 2019), gas flaring was mentioned amongst environmental catastrophes, potential global environmental crises and problems requiring attention.

Fig. 6

Strategies for flare gas recovery and utilisation

Injection involves capturing flare gas emitted during flaring and reinjecting it into underground formations, such as depleted oil or gas reservoirs. This utilises the natural porous structure of the formations for storage. The technique allows for the storage of flare gas, which has the potential to reduce GHG emissions associated with flaring (Dinani et al. 2023). It also makes use of existing infrastructure, such as wells and pipelines. However, the effectiveness of injection depends on the geological characteristics of the storage formations. There may also be regulatory and operational challenges to overcome, such as obtaining permits and ensuring proper well integrity (Zayer Kabeh et al. 2023; Dinani et al. 2023). Despite these challenges, injection serves not only to mitigate emissions but also to enhance oil or gas recovery. It can be used in a variety of industries for emissions reduction and storage.

The gas to liquid/chemical (GTL/GTC) processes convert flare gas into liquid fuels or chemical products. This is achieved through methods like FT synthesis or steam reforming. The processes yield valuable products, including DME, methanol, ethylene, ammonia and hydrogen. These technologies provide a way to monetise flare gas by producing higher-value products. They also offer alternatives to traditional fossil fuels and help diversify the energy mix (Orisaremi et al. 2023). However, GTL and GTC processes often require significant capital investment and complex infrastructure. They also have high energy and resource requirements, which can impact efficiency and the environmental footprint. Despite these challenges, GTL and GTC technologies offer solutions for emissions reduction and provide valuable products for various industrial applications (Dinani et al. 2023). They contribute to the transition towards sustainable energy systems.

Gas-to-power technologies efficiently convert flare gas into electricity using various methods, such as gas turbines, engines, combined cycle systems and solid oxide fuel cells (SOFCs). These technologies allow for the immediate utilisation of flare gas for onsite or grid-connected power generation (Orisaremi et al. 2023). While gas-to-power technologies can reduce flaring-related emissions and enhance energy efficiency, their deployment may require upfront investment in equipment and infrastructure. Additionally, efficiency and emissions performance can vary depending on the selected technology and operational conditions. Despite these challenges, gas-to-power technologies offer versatile solutions for using flare gas in various applications. This includes industrial operations, grid-connected power generation and off-grid power supply. They contribute to energy security and emissions reduction objectives (Orisaremi et al. 2023).

Strategies for mitigating methane prevalence

Generally, the strategies to modify methane reforming as the potential for mitigating pollution and energy generation have been in-depth, especially in recent years (Gao et al. 2018; Bahaghighat et al. 2019; Gao et al. 2021; Hatta et al. 2021; Yentekakis et al. 2021; He et al. 2022). The primary source from which methane is obtained is the associated and non-associated reservoirs (Muraza and Galadima 2015). As presented in Fig. 7, the most prevalent techniques range from the generation of synthetic gas from reforming such as auto-thermal, POX, dry and SR to other CH₄ and CO₂ conversion, utilisation and storage processes.

Fig. 7

Methane reforming perspectives. Generated using R-studio from plaintext file extracted from WoS

Dry reforming of methane (DRM)

The catalytic DRM (Eq. (1)) has attracted a lot of interest lately. This reaction produces syngas from two harmful greenhouse gases, CH₄ and CO₂. In addition to this benefit, the syngas is suitable for a variety of industrial processes, such as the production of higher hydrocarbons and oxygenated derivatives (such as methanol), because the molar ratio of H₂ to CO in the syngas is approximately equal to 1 (Radlik et al. 2015; Ibrahim et al. 2019; Kim et al. 2019).

$${CH}_4+{CO}_2\to 2 CO+2{H}_2\Delta {H^0}_{298\ K}=247 kJ{mol}^{-1}$$

(1)

The prime problems associated with DRM are excessive coke formation associated with the catalysts, the high temperature for the reaction and side reactions such as RWGSR (2), CH₄ cracking (3) and Boudouard reaction (BR) or CO disproportionation (4) (Schulz et al. 2015; Das et al. 2017; Bahari et al. 2021).

$$CO+{H}_2O\kern0.5em \leftrightharpoons C{O}_2+{H}_2\kern2.5em \Delta {H^0}_{298\ K}=-41.1\ kJ{mol}^{-1}$$

(2)

$${CH}_4\leftrightharpoons C+2{H}_2\kern6.75em \Delta {H^0}_{298\ K}=75 kJ{mol}^{-1}$$

(3)

$$2 CO\leftrightharpoons C+C{O}_2\kern5.5em \Delta {H^0}_{298\ K}=-172 kJ{mol}^{-1}$$

(4)

Equations (1) and (3) can favour the forward reaction by applying Le Chatelier’s principle, increasing the reaction’s temperature, removing the products as they form and introducing more moles of the reactants. In contrast, the reverse processes apply to Eqs. (2) and (4). Therefore, the RWGS and CO disproportionation prevail at low reaction temperatures, thereby depositing coke and generating CO₂.

Auto-thermal reforming (ATR)

The stand-alone technology capable of converting the entire methane in one reactor in the presence of fusion between exothermic POX and endothermic SR mechanisms is known as auto-thermal reforming (ATR). Catalytic ATR technology requires three critical reagents; CH₄, H₂O (steam) and air (O₂) to generate value-added syngas, as expressed in Eq. (5). This technology usually performs at a lower pressure than the POX process and has a low methane slip. The findings in the literature reported that the value of H₂/CO generated via catalytic ATR CH₄ is typically within the range of 1–2. Generally, the aim of combining POX and SR routes is to conserve energy since it does not require an external heat source. Indeed, catalytic ATR may boost the thermal conversion efficiency of H₂ generation and save operating expenses. However, the drawback of ATR CH₄ is the higher explosion risks due to the employment of oxygen as one of the necessary reactants. Besides, since this technology requires pure O₂, an expensive and complex O₂ separation unit must be installed in the system (Chong et al. 2019; Alhassan et al. 2023).

$$C{H}_4+0.25\ {O}_2+0.5\ {H}_2O\to \kern0.5em 2.5\ {H}_2+ CO$$

(5)

Like other CH₄ reforming processes, ideal catalyst selection contributes significantly to guaranteeing conversion and limiting coke accumulation during ATR since each type of catalyst can stimulate the reforming reaction through distinct pathways. Various catalysts in the literature have been used in ATR CH₄, including noble metal-based, non-noble metal-based and bimetallic catalysts (as summarised in Table 1). Li et al. compared the temperature profiles and the ATR performance of several noble metals (Pt, Rh and Pd) supported by spherical Al₂O₃ (Li et al. 2004)_. Amongst the employed systems, the Rh/Al2O3 catalyst demonstrated superior methane conversion (~ 100%) with a lower feed temperature than Pt and Pd-based catalysts. It was claimed that the metal particle distribution influenced the performance since the result of catalyst characterisation proved that Rh metal particles have better distributions, followed by Pt and Pd, in line with reforming activity. Indeed, the lower feed temperature exhibited by Rh/Al₂O₃ proved that the combustion reaction zone coincides with the reforming zone, which led to the heat transfer enrichment towards the endothermic section from the exothermic section.

Table 1 Summary of various catalyst systems utilised in CH₄ ATR

In different studies, Ni et al. (2014) examined the impact of Ce-based oxides (Ce-LaO_x, Ce-GdO_x, Ce-SmO_x and Ce-ZrO_x) introduction over noble metal Rh supported with Al₂O₃ in CH₄ ATR. The authors also investigated the effectiveness of alkaline-earth metal oxides like K, Ca and Mg incorporation on the stability and coke resistance of Rh/Al₂O₃. The authors confirmed that the incorporation of Ce-ZrOx effectively lessens the CO generated during the reaction, resulting in a high H₂/CO ratio compared to other Ce-based oxides. Besides, keeping the atomic ratio between Ce and Zr nearly 1 to 1 was the ideal amount to improve the catalyst’s thermal stability and catalytic activity efficiently. Alkaline-earth metal oxides were added, but no coke was deposited; yet, adding MgO showed a more stable performance than Rh-based catalysts combined with K and Ca metals.

Since precious metals are so costly and challenging, researchers have started looking into using cheaper, more abundant metals like nickel instead. On the other hand, nickel is frequently associated with deactivation due to coke deposition and sintering difficulties. The effect of Ni particle size on Ni-based catalyst deactivation in CH₄ ATR was examined by Shi et al. (2021). The researchers found that, in CH₄ ATR, smaller Ni-based catalyst particles were more active and stable than bigger ones, particularly at space velocities below 54,000 h⁻¹. They further clarified that the slower rate of side reaction (CH₄ decomposition) is responsible for coke deactivation compared to the rate of oxidative removal of surface carbons, causing the incomplete conversion of O₂. This phenomenon triggers Ni oxidation and leads to Ni deactivation.

Concerning deactivation matter, considerable effort has been made in the literature in modifying Ni-based catalysts for auto-thermal reforming CH₄. Kim and co-researchers (Kim et al. 2013) incorporated cerium oxide as the promoter for nickel-supported γ-alumina to tackle the deactivation and unstable issue due to coke accumulation. It was found that adding cerium oxide effectively enhanced the performance of the Ni-supported γ-alumina by recording ~ 100% of CH₄ conversion with no coke accumulation for 100-h reforming activity. The authors claimed that the CeAlO₃ phase formation within this catalyst structure accelerated the oxidation of coke and CO, affecting the catalytic stability and lowering CO selectivity. Indeed, an H₂/CO molar ratio of about 1.9 was generated from this process, lower than the typical ratio generated via POX and SR of CH₄ due to the unfavourable water-gas shift. In a different approach, Lisboa et al. (2011) employed Ni supported over Ce-ZrO₂ for the ATR of CH₄. The superior reforming activity (CH₄ conversion = ~ 55.0%) and stability compared to 10% Ni/α-Al₂O₃ was achieved by Ni/Ce_0.75Zr_0.25O₂ within 25 h, assigned to the helpful between the metallic surface area and O₂ storage capacity and metallic surface area. It was noticed that O₂ generated during the dissociation of CO₂ assists in re-oxidise the support to stimulate a redox mechanism for continuous coke cleaning.

Besides developing monometallic catalysts, bimetallic catalysts have been successfully utilised in ATR CH4. The impact of monometallic and bimetallic catalyst configurations using noble metals for the ATR of CH4 was compared by (Karakaya et al. 2013). Their work reported that the bimetallic Rh-Pt catalyst generated via incipient-to-wetness impregnation depicted an excellent conversion of CH₄ (31.0–51.2%) compared to monometallic catalysts (12.8–46.6%) regardless of reaction temperature (773–923 K). This trend resulted from the effective Pt-Rh interaction, which generated a synergetic effect that triggers higher conversion and reaction rates. A similar excellent catalytic performance of bimetallic catalysts was experienced by Ismagilov et al. using the Ni-Pd combination supported with Ce_0.5Zr_0.5O₂/Al₂O₃ via sequential impregnation technique (Ismagilov et al. 2014). Within the temperature range of 1023–1223 K, the authors reported an acquired 95% CH₄ conversion and ∼ 75% H₂ yield attributed to the catalyst’s reducibility and dispersion enhancement.

Partial oxidation (POX) of methane

Catalytic POX CH₄ (Eq. (6)) is another promising alternative for generating syngas from CH₄ for downstream process requirements. This route typically operates at a high temperature (1473–1773 K) in the absence of a catalyst and a moderate temperature (1023–1173 K) with catalysts, along with utilising a non-stoichiometric ratio of CH₄/O₂ as necessary reactants (Zhan et al. 2010; Velasco et al. 2014). Since this route is considered a mildly exothermic reaction, it offers a more economically feasible process as it consumes less energy than highly endothermic SR. Additionally, the synthesis gas generated via this technology typically consists of an H₂/CO ratio of 2, appropriate for downstream processes like FT and methanol synthesis without further adjustment. Remarkably, the generated undesired gases along this route also contain extremely low undesirable CO₂ compared to others, which must be eliminated before employing syngas in downstream reactions. Although the POX route offers advantages in terms of energy efficiency, the requirement of O₂ separation and desulphurisation units involves expensive operational costs, limiting this technology’s extensive implementation for industrial applications (Elbadawi et al. 2021). Indeed, due to the rapid reaction steps, it is challenging to eliminate the heat generated within the system, which is risky and could even trigger explosions.

$$C{H}_4+{~}^{1}\!\left/ \!{~}_{2}\right.{O}_2\kern0.5em \to \kern0.5em CO+2{H}_{2\kern0.75em }\kern2em \Delta {H^0}_{298\ K}=36 kJ{mol}^{-1}$$

(6)

In 1929, Liander and co-researchers initiated the POXM for syngas production (Liander 1929). They claimed that high yields of syngas were only attained at temperatures around 1123 K, while non-equilibrium product distributions were acquired at a temperature lower than 1123 K. Since then, various catalysts, including supported noble and non-noble metal oxides as well as bimetallic catalysts, have been utilised for the catalytic methane POX. Table 2 summarises several common catalysts that have been used in the POXM.

Table 2 Summary of catalysts reported for the partial oxidation of methane

Noble metals are known for their better performance in the catalytic oxidation of methane in terms of activity and resistance to coke formation. The performance of a range of noble catalysts supported by Al-Mg (Pt, Ir, Pd, Rh and Ru) synthesised via impregnation was evaluated by Khajenoori et al. (2013) for catalytic POXM at 973 K and CH₄/O₂ ratio of 2. Amongst the tested catalysts, Rh and Ru were found to be the most active, with about ~ 73–74.0% CH₄ conversion, close to the thermodynamic equilibrium values (76.3%) for the catalytic oxidation of CH₄, followed by Ir (72.1%), Pt (68.4%) and Pd (59.0%). This trend corresponded to the excellent metal distribution with a small crystallite size (5 nm) on the Al-Mg support. Additionally, the H₂/CO ratios for all the catalysts were obtained close to equilibrium levels (2.11), at about 1.8–1.9. However, because of the reverse water gas shift, the more significant value of H₂/CO (2.4–5.8) was obtained at low temperatures (923 K). Amazingly, none of the tested noble catalysts showed any signs of deactivation after 50 h of stable functioning. Ahn and co-workers also reported a similar outstanding performance on noble metals during the comparative evaluation of CeO₂-supported metallic catalysts (Pt, Ir, Pd, Ru, Ni and Rh), accredited to smaller particle size, excellent metal distribution and intense metal support interaction (Ahn et al. 2011). Nevertheless, the expensive cost and minimal reserves of these types of noble metals shifted the attention of researchers towards non-noble metals like Ni and Co, which are more attractive and practical for commercialisation.

Swaan et al. (1997) compared Ni- and Co-based catalysts for the POXM to syngas at 873–1173 K with a feeding ratio of CH₄/O₂/He/N₂ about 10/5/80/5. They found that Ni-based catalysts were superior in activity and selectivity in the POXM, although Co-based catalysts were very reactive for the combustion of CH₄ to CO₂. Since then, massive efforts on Ni-based catalyst development for POXM have been reported in the literature. Liu et al. evaluated the performances of Ni-supported catalysts by distinct alumina species, including α-Al₂O₃, γ-Al₂O₃ and θ-Al₂O_3, in the POXM (Liu et al. 2002). It was evidenced that Ni supported on γ-Al₂O₃ exhibited superior CH₄ conversion ~ 89.0% and stable within 24 h compared to Ni/θ-Al₂O₃ (~ 86.2%) and Ni/α-Al₂O₃ (~ 78.3%), owing to the inferior size of Ni particles (7.8 nm) and vast surface area (191 m² g⁻¹). The high CH₄ conversion (90.5–94.7%) and CO selectivity (93.9–96.8%) were also experienced by Lu et al. during the employment of Ni/γ-Al₂O₃ catalysts, regardless of reduction temperature (873–973 K) and GHSV of (612–1152 L g⁻¹ h⁻¹) (Lu et al. 1998).

Although Ni-based catalysts demonstrated comparable activities with noble metals, this material is known for their sintering and coke issues which contribute to deactivation. Several approaches have been implemented to overcome these drawbacks, ranging from support selection and modification to incorporating second metals as promoters or forming bimetallic catalysts. Mesoporous alumina synthesised via the post-hydrolysis method, according to Kim et al., improved Ni activity and lowered the coke accumulation during POXM (Kim et al. 2004). The authors stated that the incorporation of mesoporous alumina with Ni (Ni/Al ratio 1:10) led to intense Ni-Al interaction, causing the excellent distribution of Ni particles. Indeed, the relatively huge surface area (282.4 m² g⁻¹) and pore volume (0.26 cm³ g⁻¹) with a narrow pore size distribution (3.3 nm) explained the superior activity (73.4%) and were more resilient to coke accumulation than the Ni catalyst impregnated on commercial alumina (Ni-IMP) during 20 h of reaction.

Lucrédio and colleagues tackled the coking nature of Ni-based catalysts in POXM by introducing lanthanum and cerium via the anion exchange technique (Lucrédio et al. 2007). It was noticed that interaction between Ni supports was considerably improved after both promoters’ incorporation, attributed to enhancements in Ni active species distribution on the support surface. Although there was no significant improvement in CH₄ conversion after incorporating lanthanum and cerium, the authors observed stable conversion activity with enrichment in CO yield within 6 h on the stream. The authors justified that adding those promoters favoured the adsorption and decomposition of O₂ on the catalyst’s surface, thus assisting the carbon gasification process. Similar findings were reported in the literature with the incorporation of various kinds of promoters, including Cu (Habimana et al. 2009; Shokoohi Shooli et al. 2018), Ca (Qiu et al. 2007; Habimana et al. 2009) and Mg (Ma et al. 2019).

Besides monometallic catalysts, several series of bimetallic commonly consisting of Ni-based have been widely utilised in the POXM since these combinations can improve catalytic activity, stability, selectivity and coke resistance. Cheephat et al. (2018) synthesised Re-Ni bimetallic catalyst via impregnation and tested it in POX within 673–973 K (Cheephat et al. 2018). The authors found that Re-Ni bimetallic catalyst with a Re:Ni ratio of 3:7 depicted superior CH₄ conversion (~ 100) and product yields (CO = 92.8 %, H₂ = 87.4%) at temperature 973 K compared to monometallic Ni/γ-Al₂O₃ and Re/γ-Al₂O₃. Bimetallic catalysts did exhibit remarkable catalytic stability with a very small deactivation of H₂ production (7.4%), whereas monometallic Ni/γ-Al₂O₃ and Re/γ-Al₂O₃ experienced enormous catalyst deactivation of about 63.9% and 19.1%, respectively. This more excellent outcome was attributed to a better distribution of Ni after combining with Re species. Fazlikeshteli et al. (2021) and Javed et al. (2021) also reported comparable findings after forming bimetallic catalysts between Co-Ni and Pd-Ni species. Both authors justified that the bimetallic combination approach effectively improved the dispersion metal and metal support interaction, causing the increment in the accessible active site and coke resistance, thus, leading to excellent catalytic activity and stability.

Steam reforming of methane (SRM)

The first attempt to investigate the reaction between CH₄ and steam in the industry was done and published by Neumann and Jacob in 1924 (Ighalo and Amama 2024). SRM is concerned with the thermo-catalytic transformation of natural gas (mainly CH₄) into syngas and pure H₂. This material is a valuable feedstock for a variety of high-value petrochemicals. However, due to its high heating value of roughly 140 kJ/g and the creation of carbon-free H₂, it appears to be an appealing energy option and hence a critical starting point in the emerging H₂ economy (Ali et al. 2023). The SRM involves three bidirectional reactions depicted above in Eqs. (2), (7) and (8), respectively. Two of the reactions (Eqs. (7) and (8) are endothermic, whereas the water gas shift process (Eq. (2)) is exothermic (Guo et al. 2012; Seelam 2013; Boretti and Dorrington 2013). Conditions of reactions, steam-to-methane ratio and important findings from related works in steam reforming (SR) are presented in Table 3.

$${CH}_4+{H}_2O\leftrightharpoons CO+3{H}_2\kern2.25em \Delta {H^0}_{298\ K}=+206 kJ{mol}^{-1}$$

(7)

$${CH}_4+2{H}_2O\leftrightharpoons C{O}_2+4{H}_2\kern2.25em \Delta {H^0}_{298\ K}=+165 kJ{mol}^{-1}$$

(8)

Table 3 Reported conditions for steam reforming of methane in recent literature

Catalyst deactivation occurs majorly by coke formation due to CH₄ cracking (Eq. (3)) and the BR, also known as the CO disproportionation (Eq. (4)).

Photothermal Reforming (PTRM)

Solar energy could replace fossil fuels well because it is clean, widespread and never runs out. The notion of photothermal catalysis and its use in DRM processes was recently published. In line with the literature, photothermal catalytic DRM may significantly increase syngas production compared to a single thermal condition by combining infrared light’s thermal action with ultraviolet light’s photoelectric impact. For the above reasons, photothermal catalysis is developing into a competitive and promising technology. Based on publicly available research studies, Ni-based photothermal catalysts have high DRM catalytic activity (Zhong et al. 2022). Solar energy can be used to electrify, fuel, heat and cool buildings using photovoltaics, concentrated solar heating/power, photo/thermal chemical conversion and so on (Chen et al. 2022; Zheng et al. 2022). Amongst the different solar-to-chemical conversion technologies, solar-to-chemical conversion is both challenging and intriguing since it can alleviate two contemporary human problems: the energy crisis and environmental pollution. Since CO₂ and CH₄ are both greenhouse gases and relatively stable (C––O bond energy is 750 kJ/mol, C–H bond energy is 430 kJ/mol), solar-powered DRM would yield syngas, which can be easily recycled, more energy-saving with reduced emissions and enhanced storage potential.

Zhang et al. (2019) and Araiza et al. (2021) conducted in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at various temperatures with and without light irradiation to gain a better knowledge of the thermal influence on energetic hot carriers (EHC) (Fig. 8). At the furnace temperature of 150 °C, no indication of DRM initiation can be seen in the absence of light irradiation. The presence of HCO₃ indicates that both CO₂ and CH₄ were activated in the presence of light (1685/1420 cm⁻¹), ${CO}_3^{2-}$ (1557 cm⁻¹), COOH (1653 cm⁻¹) and CHX (2824/1440 cm⁻¹) species. This demonstrates unequivocally that the EHC may surmount the DRM thermodynamic barrier. The EHC uses two routes in photocatalytic DRM: (i) converting CO₂ to CO right away with ${CO}_2^{-}$ as an intermediate; (ii) reducing ${CO}_3^{2-}$ to COOH as an intermediate and subsequently to CO. In the second process, water can be produced as a by-product. The majority of EHCs opt to choose the second route, which lowers the H₂/CO ratio because there are fewer EHCs at normal temperatures that have the energy to overcome the higher redox potentials of reactants. When the temperature is increased to 150 ⁻C in photothermal catalytic DRM, the CO₂ peak becomes stronger. In contrast, the peaks of other intermediates in the second pathway weaken, indicating that the two routes are augmented by increased photo-induced EHC and decreased CO₂/CO reduction potential, resulting in an elevated H₂/CO ratio.

Fig. 8

In situ DRIFTS spectra with and without illumination for a DRM and b CO₂ reduction with H₂. c Improved TOF with EHC and thermally assisted TOF at varying temperatures in DRM. d Increased electron transfer with EHC and thermally assisted TOF at different temperatures in the CO₂ reduction process. e Improved TOF with EHC at varying temperatures on P25-supported Pt NPs of varying sizes, and f a schematic of the thermal suppress effect on EHC (adapted with copyright permission from (Zhang et al. 2022a))

Heterogeneous dissociation of CH₄ in the presence of a photocatalyst might occur due to the presence of a local electric field. The catalyst’s ability to transport electrons is most likely a critical factor in the activation of CH₄ and the formation of methyl radicals (He et al. 2022). As a result, lowering the activation energy barrier effectively may be accomplished by CH₄ catalytic conversion. Therefore, developing more cost-effective and effective CH₄ utilisation methods with catalyst support is extremely important. During the light-off operation, the temperature decreased quickly, stabilising the heat release and dissipating the photothermal storage capacity of ATR@PCM and ATF@PCM, which first increased quickly before decreasing to a constant state. The thermal and energy storage efficiencies were computed using heat loss, which is minimal at low temperatures, and phase transition enthalpy, which increases storage efficiency when solar energy is absorbed (Peng et al. 2022). A summary of findings reported on the recent photothermal conversion of methane is presented in Table 4.

Table 4 Abridged novelties in photothermal catalysts for methane conversion

Dual methane reforming (bi-reforming)

The production of syngas through combined steam and dry methane reformation (CSDRM), known as bi-reforming, seems to be a highly promising CO₂ valorisation process, providing metgas (CO + 2H₂) with an H₂/CO molar ratio close to 2 (Jabbour et al. 2017). This intuitive way (Eq. (9)) achieves a syngas with a desired H₂/CO ratio and combines both DRM (Eq. (1)) and SRM (Eq. (7)) processes (Singh et al. 2017; Jabbour 2020). For example, specific FT procedures are designed to prepare long hydrocarbon chains, while they may be utilised directly in methanol (Hatta et al. 2023) or dimethyl ether (Owgi et al. 2023a; Nabgan et al. 2023) synthesis. Conventional DRM (Eq. (1)) and SRM (Eq. (7)) give an H₂/CO ratio of almost 1 (too low) or nearly 3 (too high), requiring additional cycles (often expensive) if the product ratio needs to be changed to be around 2 for the following stages of the process.

$$3{CH}_{4\kern0.75em }+{CO}_2+2{H}_2O\kern0.5em \to \kern0.5em 8{H}_2+4 CO\kern1.25em \Delta {H^0}_{298\ K}=+220\ kJ{mol}^{-1}$$

(9)

From a practical standpoint, the CSDRM process has the added benefit of utilising CH₄, CO₂, and water as its principal reactants. These gases can also be present in biogas, a fuel that does not derive from fossil fuels. As a result, the approach’s complex development provides a way for creating metgas from renewable energy sources without using additional separation and purifying processes. Recent progress in the field emphasises more on combined reforming because it consumes more CH₄ than conventional reforming processes in which water has been reported to ignite side reactions. When DRM and SRM are combined with water, as in (Eq. (9)) above, less carbon may accumulate on the catalyst surface and more H₂ will be produced, resulting in syngas with the ideal composition for synthetic fuels (Dan et al. 2021). Table 5 summarises performing conditions, methods and catalyst systems in combined reforming reactions.

Table 5 Catalysts systems utilised for combined reforming of methane

The fate of captured gaseous species (based on previous investigations)

Gasifier updraft stages include combustion, gasification, pyrolysis and drying zones. In the drying zone, the product gas is liberated and burns with the fuels; in the pyrolysis zone, dry fuel (char + volatiles) is produced; and the gasification zone involves the conversion of coke and CO based on the reactions:

$$C+C{O}_2=\kern0.5em 2 CO$$

(10)

$$C+{H}_2O=\kern0.5em CO+{H}_2$$

(11)

$$C+2{H}_2=\kern0.5em {CH}_4$$

(12)

$$CO+{H}_2O=\kern0.5em {CO}_2+{H}_2$$

(13)

In the combustion zone, CO and CO₂ are made when partial and complete coke oxidation happens. Table 6 shows the different reactions of gaseous species.

Table 6 Outcomes and confrontation of the various reforming equations

Future prospective

Finding more efficient ways to produce syngas has received a lot of attention recently since it is a necessary intermediary in producing several chemicals and fuels, including dimethyl ether, methanol, propylene, ethylene and FT fuels. Using waste CO₂, DRM transforms natural gas into syngas. For some years, the industry has used a mix of steam and dry methane reforming (SMR + DRM) instead of DRM, which has yet to be fully industrialised (Bahari et al. 2022; Owgi et al. 2023b).

Maximizing reaction temperatures is crucial for enhancing product conversions, especially in the context of DRM, a process essential for reducing GHG emissions. However, increased temperatures also result in enhanced conversion rates, highlighting the potential for improved efficiency in hydrogen and synfuel production. It is important to note the challenges associated with lower DRM temperatures, such as the production of high-water content streams and the promotion of endothermic processes like RWGS. While DRM and RWGS methods are important for hydrogen and synfuel production, there is a growing recognition of the significance of other reforming processes, such as SR and carbon gasification. These processes require significant amounts of water but provide alternative methods for synthesising products and enhance the overall adaptability of hydrogen and synfuel generation.

Despite the benefits of higher temperatures and diverse reforming procedures, achieving high selectivity for hydrogen production remains challenging, especially when the CO₂/CH₄ ratio exceeds 1. The predominance of the RWGS reaction in these circumstances leads to reduced specificity for hydrogen, highlighting the need for innovative approaches to enhance product distribution. The emergence of triple-methane reforming reactors represents significant progress in addressing the challenges associated with conventional reforming procedures. These reactors utilise steam, oxygen and photothermal DRM systems to overcome issues such as excessive coking, agglomeration and uneven distribution of reactants and products. Moreover, current research efforts focus on optimising reactor design to increase efficiency, reduce energy demands and improve overall process control.

The development and improvement of catalysts are also crucial areas of focus for future studies. Durable, efficient and cost-effective catalysts are necessary for the generation of valuable chemicals from CH₄ and CO₂. Through current characterisation and computational modelling techniques, researchers aim to understand catalyst performance better. Further research and innovation are needed to fully harness the potential of hydrogen and synfuel generation. By strategically incorporating emerging trends, employing advanced methodologies and fostering interdisciplinary collaboration, it is possible to effectively tackle existing challenges and establish a resilient and sustainable energy future. Figure 9 presents the challenges and potential paths for the development of reformer technologies, based on the progress and status of various reformer technologies reviewed in related studies.

Fig. 9

The challenges and potential paths for reformer development are based on the progress and status of various reformer technologies in related studies (reused with permission from the reference (Bolívar Caballero et al. 2022))

Conclusion

This article has covered all the many methods for valuing methane, including dry, auto-thermal, steam, photothermal and partial oxidation techniques. Carbon capture and utilisation (CCU) and carbon capture and storage (CCS) are two carbon management mechanisms that have recently received a lot of attention from researchers concerned about the effects of human-caused emissions of greenhouse gases on the environment.

Methane reforming via water electrolysis, a method that currently accounts for a mere 5% of the overall production of hydrogen, has been an area of considerable discourse. Although the process of converting CO₂ into fuels and compounds shows potential for a future without carbon emissions, the complete eradication of carbon-based products continues to present difficulties. As a result, it is critical to embrace a well-rounded strategy that combines the use of carbon-based resources with the conscientious management of atmospheric CO₂ through the establishment of a human carbon cycle. Chemical hydrogen looping, biomass pyrolysis and coke oven gas utilisation are just a few of the novel approaches that have recently emerged and show promise as a means of producing energy sustainably. However, it is critical to address the energy requirements associated with methane abatement, as well as problems related to storage, transportation and manufacturing.

Future studies should focus on making methane valorisation technologies more efficient and scalable. Motivating the adoption of carbon-neutral practices and promoting the integration of renewable energy sources into existing infrastructure should be the focus of collaborative initiatives including academia, industry and policymakers. To further drive innovation in this crucial sector, interdisciplinary techniques that utilise materials science, chemistry and engineering insights are necessary. Through the progression of knowledge regarding methane valorisation and carbon management, it is possible to establish a pathway towards an energy future that is more resilient and sustainable, all while alleviating the detrimental effects of greenhouse gas emissions on both the environment and human health.