Conversion of Biogas to Syngas via Catalytic Carbon Dioxide Reforming Reactions: An Overview of Thermodynamic Aspects, Catalytic Design, and Reaction Kinetics

Abstract

Biogas production has continuously increased worldwide during the last decades. Nowadays, heat, electricity, and biomethane production are the main utilization of biogas at large-scale industrial processes. The research and development on biogas valorization is currently related to synthesis gas production via reforming process, since syngas allows obtaining various chemicals and fuels of high-added value. However, biogas reforming is a complex process, which implies various reactions in parallel, and needs high temperature (>800 °C) to obtain high methane conversion. The development of a highly-performing catalyst, which must be active, selective, thermally stable, and resistant to solid carbon formation on its surface, is crucial. This chapter is devoted to an update of the thermodynamic aspect of biogas reforming under different conditions. This chapter also reviews recent significant works related to catalyst design as well as kinetic and mechanistic studies of biogas reforming processes.

18.1 Introduction

The green alternatives to fossil fuel have been constantly developed in order to reduce the greenhouse gas emissions. One of the possible solutions is related to biogas, which is considered as a renewable energy source to replace conventional fuels to produce heat, electricity, and chemicals. Biogas is obtained from the decomposition of organic matter under anaerobic conditions (absence of oxygen). The main feedstocks for biogas production include energy crops (e.g., sunflower, wheat grain, sugar beet, etc.), agricultural residues, animal manure, municipal solid waste (MSW), industrial wastes, wastewater, and sewage sludge. These feedstocks contain mostly complex organic compounds, which are degraded by microorganisms under the absence of air through four main steps, such as hydrolysis, acidogenesis, acetogenesis or dehydrogenation and methanation (Jaffrin et al. 2003; Weiland 2010; Mudhoo 2012; Pullen 2015). The hydrolysis of complex polymers into oligomers and monomers (e.g., sugars, amino acids, fatty acids) takes place in the first step. These oligomers and monomers are then degraded into volatile fatty acids (>C₂), acetate, and hydrogen. The volatile fatty acids are later converted into acetate, H₂, and CO₂ by acetogenic bacteria. Finally, acetate, H₂, and CO₂ produce CH₄ through methanation. This multi-step transformation needs specific conditions including the absence of air (anaerobic condition), uniform temperature, optimum nutrient supply, optimum and uniform pH (Kaparaju and Rintala 2013).

From the technical point of view, large-scale biogas production can be done in an anaerobic digestion plant or in a landfill site, leading to the formation of digested gas and landfill gas, respectively. The size of these bioreactors varies from some cubic meters to several thousand cubic meters (Persson et al. 2009; Mudhoo 2012; Pullen 2015; Di Maio et al. 2018). Depending on the reactor design, feedstock used, and operational conditions, the composition of biogas can vary. However, biogas typically contains mostly CH₄, CO₂, H₂, N₂, O₂, light hydrocarbons, water vapor, and some pollutants (hydrogen sulfide, ammoniac, siloxanes, and organometallic complexes) (Persson et al. 2009; Pullen 2015; Mudhoo 2012; Kvist and Aryal 2019; Hoo et al. 2018; Eklund et al. 1998; Biogas Renewable Energy 2019). The digested gas generally contains more CH₄ (50–75 vol%) and less CO₂ (30–40 vol%) than landfill gas (CH₄: 35–65 vol% and CO₂: 15–50 vol%). Landfill gas contains varying amounts of N₂ (5–40 vol%) and O₂ (0–5 vol%) while the digested gas generally contains <2 vol% of these gases. This is due to the difficulty of sealing control in landfill sites during landfill gas recovery. Thus, the lower heating value (LHV) of the digested gas (5.5–7.5 kwh/Nm³) is higher than that of landfill gas (around 4.4 kwh/Nm³) (Okonkwo et al. 2018; Persson et al. 2009).

Biogas production worldwide continuously increases during the last decades. According to IRENA (2017), the worldwide installed capacity for biogas production increased from 6323 MW in 2007 to 15,752 MW in 2016. Europe represents the highest biogas production capacity with 10,533 MW in 2016 or 66.9% followed by North America with 2639 MW or 16.8% in the same year. In Europe, the top-five biogas producers are, in descending order, Germany, UK, Italy, France, and Netherlands (Achinas et al. 2017). In addition, Scarlat et al. (2018) predicted that biogas production in Europe would still increase with the similar rate during the next years.

Among different feedstocks, MSW is particularly interesting because of its specific characteristics. MSW is generated by households and includes various types of organic and inorganic wastes, which are mostly food waste, waste paper, cardboard, wood, plastic, rubber, and glass. However, it should be noted that the following wastes are not considered as MSW sources, e.g., municipal wastewater sludge, industrial wastes, automobile bodies, combustion ash, or construction and demolition debris. According to Kaza et al. (2018), the worldwide MSW production is predicted to increase from 2.01 billion tons in 2016 to 3.04 billion tons in 2030, mostly due to population explosion and urbanization. MSW composition largely depends on the income of each country. In high-income countries, MSW contains less food waste but more paper and cardboard wastes (from packaging), more rubber and leather (from used tires, clothes, and accessories) compared to low-income, lower-middle, and upper-middle income countries (Kaza et al. 2018). An example of the typical MSW composition in the United States is presented in Table 18.1. The total organic biomass content represents 69.5 wt% illustrating the high potential of biogas generation from MSW.

Table 18.1 MSW composition in the United States (Themelis and Ulloa 2007)

Up-to-date, the MSW collection is different from each country. In high-income countries, most of the MSW is collected (~100% for both urban and rural zones). This collection rate is lower in other countries, around 85%, 71%, and 45% from urban municipalities in upper-middle, lower-middle, and low-income countries, respectively. In contrast, 45%, 33%, and 26% of MSW are obtained from the rural municipalities in the upper-middle, lower-middle, and low-income countries, respectively. Thus, the effort is still needed for improving MSW collection in these countries. The MSW management by landfill technique is also different from each country. In several developing countries of low and lower-middle income, and particularly in rural zone, MSW is still burnt in open atmosphere without considering the pollution impacts, technical specification of waste treatment. MSW in some developing countries is treated in open dumping sites without leachate treatment and biogas recovery. In developed countries, sanitary landfill sites are used, which are equipped with lining systems for leachate and biogas recovery, geo-membrane for securing underground water, covering and drainage systems (Youcai and Ziyang 2017). More research is invited to improve the valorization of biogas generated by landfill sites in the world.

18.2 Biogas Utilization and Valorization

18.2.1 Heat and Electricity Production

Biogas mainly contains CH₄ and CO₂ together with other compounds (e.g., water vapor, N₂, O₂, H₂S, NH₃, and siloxanes). One of the primary valorization techniques is the production of heat and electricity. Biogas production in rural areas can be directly used for cooking, household heating, and street lightening without purification or upgrading. This is largely applied in developing countries with the financial incentives by local governments to boost cost-effective waste recycling, local income, and simple utilization (Yasmin and Grundmann 2019; Clemens et al. 2018; Pradhan and Limmeechokchai 2017; Aziz et al. 2019; Khalil et al. 2019; Mittal et al. 2018).

In large-scale urban regions, biogas can be used as a fuel for producing heat and electricity. Biogas utilization is found in the following aspects (Kaparaju and Rintala 2013):

• 1. Production of heat and/or steam using a boiler.

• 2. Production of electricity combined with waste heat recovery (combined heat power).

• 3. Production of heat, steam and/or electricity and cooling using a micro-turbine.

• 4. Production of electricity using a fuel cell.

It is worth mentioning that raw biogas must be purified before its utilization in an engine to removal pollutants like sulfur-, nitrogen- and chlorine-containing compounds, siloxanes, etc. (Kaparaju and Rintala 2013). In general, the requirement for biogas purification can be classified as follows: fuel cell > gas engine, gas turbine and micro-turbine > stirling engine > boiler (Kaparaju and Rintala 2013).

18.2.2 Biomethane Production

Biogas can also be upgraded into biomethane having the quality of natural gas. This biomethane can be then injected to the gas grid. This valorization way is nowadays commercialized in several countries. The pollutants and CO₂ must be removed from biogas to increase CH₄ content to at least 80 vol% (Elfattah et al. 2016). Different methods are available for biogas upgrading including water scrubbing, chemical scrubbing, biological absorption, pressure swing adsorption, membrane separation, and cryogenic technology, although each technology has its own advantages and disadvantages (Khan et al. 2017; Sahota et al. 2018; Angelidaki et al. 2018).

Water scrubbing is a simple method with low operation cost and allows CH₄ recovering of more than 80%. However, it requires high energy and water consumption, and has high corrosion risk because of the dissolution of acid gas like hydrogen sulfide. Chemical scrubbing is faster and more efficient, but also more expensive than water scrubbing. Adsorption needs specific operational conditions of pressure, temperature, and adsorbent to reach high efficiency (Pertiwiningrum et al. 2018). According to Angelidaki et al. (2018), the applied commercial technologies for biogas upgrading are as follows: water scrubbing > chemical scrubbing ~ membrane separation > pressure swing adsorption > other technologies. As previously presented, France is in the top-five biogas producers in Europe. The number of biomethane injection stations to the gas grid in France was 37 in June 2017, which has increased to 87 in May 2019 with a total production of 1382 GWh/year (GRDF 2019).

18.2.3 Syngas Production

Synthetic gas (or syngas—a gas mixture rich in CO and H₂) is well known as a versatile platform gas to produce various chemicals and fuels, e.g., methanol, light hydrocarbons, gasoline, diesel, waxes, and alcohols (Brown 2011; Puigjaner 2011; Abdoulmoumine et al. 2015; Liu et al. 2010; Shah 2017; Asimakopoulos et al. 2018; Abdulrasheed et al. 2019). The commercial syngas is produced from fossil resources, e.g., coal, natural gas and heavy hydrocarbons by catalytic steam reforming process (Schildhauer and Biollaz 2016; Liu et al. 2010; Shah 2017). Biogas, mainly containing CH₄ and CO₂, can be used as a renewable feedstock for syngas production.

Syngas production by catalytic steam reforming of biogas has been deployed at large-scale facilities. In the case of the VABHYOGAZ project in France, purified landfill gas was reformed with a molar ratio of steam/water around 4:1 to produce syngas, which is similar to the conditions of natural gas reforming, according to Eq. (18.1). This syngas was later transformed into hydrogen by the water-gas shift process (Grouset and Ridart 2018; Nanda et al. 2017). However, the large excess of steam makes the process highly intensive in energy because the reforming takes place around 900 °C. Current research on biogas reforming into syngas is focused on catalytic dry reforming and tri-reforming processes.

As previously mentioned, biogas mainly contains CH₄ and CO₂. This gas mixture can be transformed into syngas according to Eq. (18.2) which is the chemical reaction of dry reforming of methane (DRM). In this process, CO₂ is the principal oxidizing agent of the reforming of the CH₄ molecule. However, biogas generally contains more CH₄ than CO₂. In addition, water vapor and oxygen are generally present in biogas and can play the role of oxidizing agents in the steam reforming (Eq. 18.1) and partial oxidation (Eq. 18.3) of methane. The combination of three reactions, steam reforming, dry reforming, and partial oxidation of methane constitutes the process of tri-reforming of methane (TRM) (Eq. 18.4) (Table 18.2).

Table 18.2 Methane reforming reactions and side reactions

Biogas reforming into syngas is accompanied by several side reactions (Eqs. 18.4–18.7), which have an important impact on methane conversion and syngas selectivity as well as catalyst stability. This chapter is focused on the thermodynamic aspects, catalytic design, and reaction kinetics of dry- and tri-reforming of methane. The study of the thermodynamic equilibrium allows determining the limit of a given chemical process under defined conditions of temperature, pressure, and initial composition. The common method of the thermodynamic equilibrium calculation is based on the minimization of Gibbs free energy (Faungnawakij et al. 2007; Nikoo and Amin 2011; Néron et al. 2012).

Nikoo and Amin (2011) have detailed the basis of this method for the dry reforming of methane. Chemical compounds at both gas and solid phases have been considered. The solid considered in this process is related to solid carbon formed during the reaction. The gaseous molecules considered include CH₄, CO₂, CO, H₂, water vapor, methanol, ethylene, ethane, and dimethyl ether (Nikoo and Amin 2011). In the literature, work has been devoted to the thermodynamic study of dry- and tri-reforming of methane (Song and Pan 2004; Faungnawakij et al. 2007; Nikoo and Amin 2011; Jafarbegloo et al. 2015; Abdullah et al. 2017; Pham Minh et al. 2018a, b; Cao et al. 2018; Chein et al. 2017; Chein and Hsu (2018); Rahnama et al. 2014; Diez-Ramirez et al. 2016; Rego de Vasconcelos (2016)). Similar conclusions have been obtained by these studies. This section summarizes the main characteristics of each process.

18.3 Dry Reforming of Methane

For dry reforming of the methane process, Fig. 18.1 shows the amounts of different species in equilibrium from an equimolar mixture of CH₄ and CO₂ (1 mole for each molecule) at 1 and 16 atm. These results were obtained by ASPEN Plus software on the principle of Gibbs free energy minimization described elsewhere (Nikoo and Amin 2011). When the gas mixture is set at 1 atm, solid carbon (C_(s)) and water vapor are strongly favored below 700 °C. On the other hand, H₂ and CO are predominant above 700 °C. The conversion of CH₄ and CO₂ as well as the selectivity of H₂ and CO are significantly favored above 900 °C. However, catalyst thermal sintering can occur at this high temperature, which is considered as irreversible catalyst deactivation. Therefore, the design of a thermally stable catalyst is a main challenge of dry reforming of methane. Another characteristic of the dry reforming process is related to solid carbon formation (Eqs. 18.6 and 18.7).

Fig. 18.1

Thermodynamic equilibrium of a gas mixture initially composed of 1 mol of CH₄ and 1 mol of CO₂ at (a) 1 atm and (b) 16 atm

Wang and Lu (1996) have shown that the upper limit of the Boudouard reaction is equal to 700 °C while the lower limit of the methane cracking reaction is equal to 557 °C. Theoretically, there is no temperature range wherein solid carbon formation could be absent. Therefore, dry reforming catalyst must be not only thermally stable but also resistant to solid carbon formation. The formation of solid carbon can be reduced by increasing the molar ratio of CO₂/CH₄. By keeping the temperature at 700 °C and the total pressure at 1 atm, Rego de Vasconcelos (2016) showed that no solid carbon is formed when the molar ratio of CO₂/CH₄ is equal to 2.5/1. However, increasing this ratio also favors the formation of water vapor and slightly decrease the H₂ amount.

Dry reforming of methane produces more gaseous molecules than the initial gaseous reactants (Eq. 18.2). Thus, increasing pressure does not favor this reaction. This decreases CH₄ and CO₂ conversion as well as H₂ and CO selectivity while increases the selectivity in water vapor and solid carbon (Nikoo and Amin 2011; Rego de Vasconcelos 2016; Abdullah et al. 2017; Jafarbegloo et al. 2015). Figure 18.1b illustrates the equilibrium amount of an equimolar mixture of CH₄ and CO₂ when the total pressure is set at 16 atm. At 1000 °C and 16 atm, the unreacted CH₄ and CO₂ amounts are 11.1 and 8.8%, respectively, which are much higher than those at 1000 °C and 1 atm (<1%).

18.4 Tri-reforming of Methane

Tri-reforming process combines steam, dry reforming, and partial oxidation of methane. According to Eq. (18.4), CH₄ can be reformed by varying amounts of water vapor, CO₂, and O₂. Song and Pan (2004) have proposed a concept of tri-reforming of methane starting from natural gas (CH₄ and CO₂ source) and flue gas (CO₂, O₂, and water vapor) wherein various mixtures of CH₄, CO₂, O₂, and water vapor were considered. Pham Minh et al. (2018a, b) have also simulated various mixtures of landfill gas with water vapor. From the energetic point of view, partial oxidation of methane is exothermal (Eq. 18.3), thus heat from this reaction reduces the energy consumption of the global tri-reforming process (Chein and Hsu 2018). Furthermore, by varying the feeding composition, the molar ratio of H₂/CO can be varied, which is favorable for the downstream application of syngas (Song and Pan 2004). Above 750 °C and at 1 atm, the tri-reforming process generally has higher CH₄ conversion, higher molar ratio of H₂/CO and lower solid carbon amount compared to simple dry or steam reforming (Song and Pan 2004).

Figure 18.2 shows the thermodynamic equilibrium of a mixture initially composed of 2 mol of CH₄, 1 mol of CO₂, 1 mol of water vapor, and 0.1 mol of O₂ at 1 and 16 atm. The sum of oxidizing agents (O₂, CO₂, and water) is in excess for completely converting methane into syngas according to Eq. (18.1–18.3). The composition of this mixture is obtained from the addition of water vapor to a purified landfill gas having the molar ratio of CH₄/CO₂/O₂ of 2/1/0.1. At the total pressure of 1 atm, high temperature of at least 800 °C is required to selectively convert this mixture to syngas. Under these conditions, solid carbon could be prevented. In addition, increasing the pressure to 16 bar has negative impacts on reaction conversion and selectivity. At this pressure and 1000 °C, CH₄ conversion only reaches 86.3%.

Fig. 18.2

Thermodynamic equilibrium of a gas mixture initially composed of 2 mol of CH₄, 1 mol of CO₂, 1 mol of water vapor, and 0.1 mol of O₂ at (a) 1 atm and (b) 16 atm

From the thermodynamic point of view, both dry- and tri-reforming of methane are strongly influenced by the initial feeding composition, temperature, and total pressure. The gas mixture containing more oxidizing agents (e.g., water, CO₂, and O₂) than the stoichiometric amounts favors CH₄ conversion. High temperature also favors CH₄ conversion and syngas selectivity because of the endothermicity of steam and dry reforming of methane. On the other hand, high pressure is unfavorable considering the stoichiometric coefficients of methane reforming reactions.

18.5 Catalysts Development

The development of an active and stable catalyst is one of the key points necessary for the implantation of biogas reforming technologies at large-scale operations. Catalyst deactivation related to carbon deposition, sulfur poisoning, and catalyst sintering is one of the biggest issues related to catalytic performance. Therefore, researches have focused on the development of more resistant catalysts using, for example, combination of different active metals, new supports, and promoters. Studies developed through the years on this area have shown that the performance of the catalysts mainly depends on the active metal used on the nature of the support, on the metal-support interaction as well as on other features, such as metal particle size and catalyst surface area. The following topics will discuss the recent advances in catalyst design and application in dry- and tri-reforming of biogas.

18.5.1 Catalysts in Dry Reforming of Methane

Noble metal-based catalysts have been reported as highly active and stable catalysts for dry reforming of biogas with only low amounts of coke formation (Papadopoulou et al. 2012). The main reasons related to the coke resistance of these catalysts are the small equilibrium constants for the decomposition of CH₄ as well as the carbon dissolution into their lattices (Papadopoulou et al. 2012). These findings were reported by Rostrup-Nielsen and Hansen (1993) when they compared the performance of different noble metals catalysts (Ru, Ir, Pd, Pt, and Rh) on dry reforming of methane at temperatures varying between 500 °C and 650 °C. Their results showed that ruthenium (Ru) and rhodium (Rd) had the best catalytic performances with high selectivity for carbon-free operation, thus proving the high coking resistance. Similarly, Hou et al. (2006) evaluated the performance of several noble metals (Ru, Pd, Pt, and Ir) doped on alumina (α-Al₂O₃) catalysts in the dry reforming of methane reaction at 950 °C. The results proved the coking resistance and the stability of these noble metal catalysts with CH₄ and CO₂ conversions higher than 84%. The following order of performance was observed: Rh > Ru > Ir > Pd > Pt. The less efficient performance of the Pd- and Pt-based catalysts was related to the metal particles sintering at high temperature.

Despite the high activity and stability of noble metal-based catalysts, their high cost and low availability render them unattractive for industrial-scale uses. Transition metals have become a suitable alternative due to their low cost, high availability, and good activity in the dry reforming reaction (Pakhare and Spivey 2014). Nickel-based catalysts are often studied since they are already used for advanced reforming process at industrial scale, such as steam reforming (Eq. 18.1). Goula et al. (2015) investigated the performance of Ni/Al₂O₃ catalysts with nickel (Ni) loadings varying between 8 and 16 wt% on the biogas reforming reaction. The authors reported that the catalysts with lower nickel loading exhibited higher specific surface area and low nickel particles than the catalysts with high nickel loadings, which was related to the coverage of the internal surface area by the nickel species adsorbed on the alumina active sites. However, the authors reported that the nickel loading had no direct effect on the catalytic performance. In this case, the preparation method showed to be the most influencing parameter since it had a direct impact on the reducibility of the nickel species and thus on the overall performance of the catalysts. The 16 wt% Ni/Al₂O₃ catalyst prepared by incipient wetness impregnation method showed the best catalytic performance at 750 °C with CH₄ and CO₂ conversions equaled to 69.1% and 88.3%, respectively.

Despite their low cost and proven catalytic activity, transition metal-based catalysts have been repeatedly reported as more prone to deactivation due to carbon deposition than noble metal-based catalysts (Pakhare and Spivey 2014). Kroll et al. (1997) showed Ni/SiO₂ as a better performing catalyst in the dry reforming reaction at 700 °C showing CH₄ conversion of 93% during the first hours of reaction. However, the catalyst quickly deactivated after less than 10 h of reaction due to carbon deposition. Hence, recent studies have focused on improving the catalytic properties of transition metal-based catalysts.

Bi-metallic catalysts using a combination of two transition metals or between transition metals and noble metals have been tested to improve the catalytic properties of the catalysts. Among the various combinations of transition metals used, nickel (Ni) and cobalt (Co) are by far the most investigated combination due to the synergy between these two metals as well as to the property of cobalt of decreasing the rate of coke formation (Estephane et al. 2015). The influence of cobalt addition to the performance of Ni/ZSM5 catalyst on dry reforming reaction at temperatures ranging from 600 °C to 800 °C was investigated by Estephane et al. (2015). The authors suggested that the cobalt presence could help oxidizing the carbon species close to the catalytic sites, thus increasing the stability of the catalyst. The best catalytic result was obtained using a Ni/Co ratio equal to 1:2. This catalyst exhibited CH₄ and CO₂ conversion equal to 80% and 85%, respectively, at 800 °C, proving the good catalytic performance of this bi-metallic catalyst.

Similar results were achieved by Xu et al. (2009). The addition of cobalt significantly increased the catalytic performance of the Ni/La₂O₃-Al₂O₃ catalyst. The authors reported CH₄ and CO₂ conversions equal to 93.7% and 94%, respectively, in biogas reforming at 800 °C and 290 h when a bi-metallic catalyst with a Ni/Co molar ratio equal to 7:3 was used. Besides carbon deposition, sulfur poisoning also leads to catalyst deactivation during biogas reforming. Recent studies have shown that the use of bi-metallic catalysts could improve the sulfur resistance of the catalysts. Sapountzi et al. (2018) showed that doping Ni/GDC (gadolinia-doped ceria) catalyst with 2.3 wt% of gold (Au) hindered the formation of strongly bonded sulfur components by producing an Au-Ni alloy.

The textural and chemical promoters have also been used to improve the catalytic performance of the catalysts. According to Jang et al. (2019), promoters are usually used to enhance the textural properties of the catalysts, which helps preventing or delaying sintering of the active phase. Chemical promoters provide new active sites or enhance chemical properties, such as basicity and redox property, which prevent carbon formation (Jang et al. 2019). San José-Alonso et al. (2011) investigated the use of potassium (K) and strontium (Sr) on the performance of Co/Al₂O₃ on CO₂ reforming of CH₄ at 700 °C. The authors reported that the Co/Al₂O₃ catalyst showed CH₄ and CO₂ conversions of 73% and 82%, respectively. The percentage of deposited carbon was 0.53%. When this catalyst was doped with 1 wt% K or 1 wt% Sr, CH₄ and CO₂ conversions decreased to about 15%. The percentage of deposited carbon also decreased, achieving 0.08% when K was used. The main reason for the decrease in the CH₄ conversion was related to the partial coverage of the active sites for CH₄ decomposition and to the ability of the catalyst to gasify the carbon species.

Ceria (Ce) has also been reported to improve the coke resistance of the catalysts. Daza et al. (2010) investigated the effect of Ce addition and loading on Ni/Mg-Al catalyst on dry reforming at 700 °C. The addition of 1 wt% of Ce to the catalyst was responsible for increasing CH₄ conversion from 34.9% to 88.3%. The authors reported that Ce addition increased the reducibility and the total basicity of the catalyst, which was responsible for the increase in the catalytic activity. However, increasing Ce content from 1 to 10 wt% did not have an influence on the catalyst activity but it greatly hindered the formation of coke deposits. The catalyst doped with Ce was stable in the reaction conditions during 100 h of reaction.

18.5.2 Catalysts in Tri-reforming of Methane

As presented in the previous sections, tri-reforming is a synergetic combination between dry reforming, steam reforming, and partial oxidation. Hence, it is not surprising that the catalytic systems used in the tri-reforming process are similar to those used in the other advanced reforming process and face similar problems, such as low stability due to coke deposition and metal sintering. However, the tri-reforming process is a much more complex process due to the presence of three oxidizing agents (e.g., O₂, CO₂, and H₂O), requiring a catalyst capable of adsorbing and activating all three reactants efficiently as well as to withstand the highly oxidizing atmosphere. The following topics will briefly present the recent advances in catalyst development for this process as well as the efforts for scaling up the technology.

Nickel supported on conventional catalytic supports, such as Al₂O₃, ZrO₂, SiO₂, and CeO₂ is the most studied active metal. However, these catalysts may behave differently in tri-reforming conditions mainly due to their re-oxidation by the oxygen in the feed, which leads to their deactivation (Amin et al. 2015). Majewski and Wood (2014) investigated the performance of Ni/SiO₂ core shell catalyst in steam reforming of methane in tri-reforming conditions at 750 °C. The authors reported that changing the percentage of each oxidizing agent (O₂, CO₂, and H₂O) in the feed mixture had a different impact on the performance of the catalyst. Increasing the amount of oxygen in the feed mixture increased the CH₄ conversion to 90% but did not improve the H₂/CO ratio in the produced syngas. When steam was not present in the feed, CO₂ conversion increased to 90%, but CH₄ conversion decreased. When steam was added with a CH₄/H₂O ratio of 1:0.5, the H₂/CO ratio of the syngas decreased from 2.5 to 1.5 and coke deposition increased from 5 mg g_cat⁻¹ up to 99 mg g_cat⁻¹. These results show the potential of Ni/SiO₂ core-shell catalyst in the tri-reforming process. However, it also highlights the complexity to maintain high reactants’ conversion and good catalyst stability.

The role of the support in tri-reforming is also not yet defined and further studies to correlate the physicochemical properties of the catalysts to their performance are still needed. Different supports have been used to prepare catalysts for tri-reforming reaction, including CeO₂ (Pino et al. 2011), TiO₂ (Jiang et al. 2007), ZrO₂ (Si et al. 2012; Singha et al. 2016; Anchieta et al. 2019), MgAl₂O₄ (Lino et al. 2019), and SiC (García-Vargas et al. 2014a, 2014b, 2015b). In attempt to investigate the influence of different support materials on the performance of nickel-based catalysts, Kumar et al. (2019) prepared different catalysts using Al₂O₃, ZrO₂, MgO, CeO₂-ZrO₂, SBA-15, and TiO₂ as the supports and evaluated their performance in tri-reforming at 800 °C and with a feed composition of CH₄/CO₂/H₂O/O₂/N₂ of 1:0.23:0.46:0.07:0.28. The authors reported that Ni/Al₂O₃ had the best catalytic performance in the test conditions used with CH₄, H₂O, and CO₂ conversion rates equal to 8.72 × 10⁻¹ mmol g_cat⁻¹ s, 4.2 × 10⁻² mmol g_cat⁻¹ s, and 2.31 × 10⁻² mmol g_cat⁻¹ s, respectively. However, Ni/CeO₂-ZrO₂ and Ni/ZrO₂ showed better stability against oxidation, due to their redox and oxophilic properties, respectively. On the other hand, Ni/MgO and Ni/TiO₂ catalysts showed only very low CH₄ and CO₂ conversion rates since they presented a very low degree of reducibility. The authors concluded that small Ni particles with high dispersion, high degree of reducibility, strong metal-support interaction as well as high concentration of basic sites on the surface of the catalyst are crucial for the good catalytic performance of the catalysts.

García-Vargas et al. (2014b) investigated the performance of Ni/Al₂O₃ and Ni/CeO₂ catalysts in tri-reforming conditions at 800 °C. The best catalytic result was obtained using Ni/CeO₂ catalyst, showing CH₄ and CO₂ consumption rates around 8 and 3 mol s⁻¹ g_Ni⁻¹, respectively, which was explained by its basic properties. Ni/Al₂O₃ showed the lowest CH₄ and CO₂ conversion rates due to the formation of NiAl₂O₄ phase, thus decreasing the reducibility of the catalyst. Pino et al. (2011) have found that Ni/CeO₂ (5 wt%) catalysts prepared by flaming combustion at 350 °C were highly active and selective in tri-reforming of methane at 800 °C with methane conversion up to 93%. The addition of La slightly increased the catalytic performance of these catalysts.

Singha et al. (2016) investigated Ni/ZrO₂ catalysts synthesized by hydrothermal method in the reforming of different mixtures containing CH₄, CO₂, water vapor, and oxygen at 500–800 °C. In most of the case, CH₄ conversion likely linearly increased with the reaction temperature while the molar ratio of H₂/CO varied in narrow range of 1.7–2.0. The optimal Ni content was found at 4.8 wt%, explained by the dependency of Ni particle size on Ni content. At 800 °C, this catalyst was found very active and stable with reactants’ conversion close to 96–98% for 100 h of reaction time. This catalytic performance was mostly explained by the formation of metal nanoparticles strongly interacted with the support.

During the last decade, much work has been devoted to the design of a performing catalyst for biogas reforming, e.g., dry- and tri-reforming. Nickel has been found as the most appropriate active metal for these processes. Such a catalyst is generally constituted of (a) a thermally stable support at 700–1000 °C, (b) highly dispersed metal particles (<40 nm), (c) strong metal-support interaction, and (d) controlled basicity. The addition of promoters could also improve catalyst performance. Despite encouraging results, large pilot application of dry- and tri-reforming is still limited.

18.5.3 Catalyst Supports

Besides the active metal and promoters, the nature of the support also plays a significant role in the performance of the catalysts. Catalyst supports provide mechanical and thermal stability to the catalysts and confer high specific surface area, leading to high metal dispersion, which favors the CH₄ adsorption and dissociation (Papadopoulou et al. 2012). Moreover, strong metal-support interaction is responsible for the high stability of the metal particles during the reaction, which will prevent sintering and coking (Jang et al. 2019). Other support features, such as basic-acid characteristics and oxygen storage capacity also play an important role in the overall performance of the catalysts (Mohamedali et al. 2018).

Al₂O₃ is one of the support materials most used for reforming applications due to its low cost and high mechanical and thermal stability (Papadopoulou et al. 2012). Mo et al. (2015) recently investigated the application of Ni/Al₂O₃ porous catalysts in dry reforming of CH₄. The catalyst synthesized by hydrolysis-deposition method had high specific surface area of 477 m²/g and small nickel particles of about 7 nm, which ensured high CH₄ and CO₂ conversions of around 95% during 10 h of time on stream. However, depending on the synthesis method used, large metal particles (>10 nm) or even phases difficult to reduce, such as nickel aluminate can be formed, thus decreasing the performance of the catalyst. The acid sites present in this support rendered the alumina-based catalysts more prone to deactivation by carbon deposition.

Hydrotalcite-like materials containing aluminum (Al) and magnesium (Mg) have been intensively investigated for dry reforming applications mainly due to their basic character, which improve coke gasification to their high surface area and to the presence of di and trivalent cations that leads to a homogeneous distribution of the active phase in the matrix (Dębek et al. 2015). de Vasconcelos et al. (2018a) recently investigated the performance of MgAl hydrotalcite-based catalysts doped with Ni and with MgO loads varying from 0 wt% to 70 wt% in the dry reforming reaction at 700 °C. The authors reported that the catalyst with an MgO loading of 70 wt% presented the best catalytic performance with CH₄ and CO₂ conversions around 80% during 50 h of time on stream as well as the lowest deactivation rate of about 0.004% h⁻¹. The catalytic performance of this catalyst was related to the presence of strong basic sites that favored the carbon gasification during the reaction. Moreover, a NiO-MgO solid solution was formed, which stabilized the active phase and prevented its sintering.

Rare earth-based supports, specially ceria, have been also been studied due to its ability to enhance the availability of hydroxyl (OH_ads) and surface oxygen (O_ads) species, which controls the formation of carbon deposits (Papadopoulou et al. 2012). Wolfbeisser et al. (2016) studied the dry reforming reaction over ceria-zirconia-supported Ni catalysts at 600 °C. The authors reported that the presence of Ce did not improve the catalytic stability of the catalyst. However, it considerably increased its stability but hindering the formation of filamentous coke. The authors further reported that the influence of ceria strongly depends on the catalyst synthesis method, which can affect the presence of surface oxygen species and of oxygen vacancies. These factors are crucial for the carbon removal at the metal/support surface.

Ceria/zirconia-based supports have been investigated by many authors (Djinovic et al. 2012, 2015; Aw et al. 2014a, b, 2015; Djinovic and Pintar 2017). Solid solutions of ceria/zirconia with high specific surface area, high oxygen storage capacity, and high interaction with nanoparticles of nickel and cobalt as active metals have been found as the main factors responsible for high catalytic performance in dry reforming of methane. These catalysts strongly limited the formation of coke during dry reforming reaction. However, water has been usually found as byproduct with high selectivity (up to 20% in the gas mixture at the reactor outlet) (Aw et al. 2015), suggesting that the designed catalyst favors reversed water-gas shift reaction.

Finally, hydroxyapatite (HAP, chemical formula: Ca₁₀(PO₄)₆(OH)₂) has also been found as a new promising catalyst support for the dry reforming of methane (Boukha et al. 2007, 2019; de Vasconcelos 2016; de Vasconcelos et al. 2018a, c; Phan et al. 2018). Hydroxyapatite-supported metals catalysts (e.g., Pt, Ni, and Co) have been found active, selective, and stable in dry reforming reactions and are competitive compared to catalysts prepared from conventional supports like Al₂O₃ and MgAl₂O₄ (de Vasconcelos et al. 2018a). The performance of hydroxyapatite in this reaction could be explained by a high thermal stability, a controlled acido-basicity via the molar ratio of Ca/P, and a good cation exchange capacity allowing a strong support-metal interaction between metal nanoparticles and hydroxyapatite surface (de Vasconcelos 2016; Boukha et al. 2019).

18.6 Kinetic Models and Mechanism Aspects

In this section, an overview of reaction mechanisms and kinetics for dry- and tri-reforming of methane are systematically discussed. Understanding the inherently catalytic mechanisms and establishing comprehensive kinetic models for precisely predicting the consumption and formation rates of the corresponding reactants and products are crucial for catalyst design, process optimization, and reactor fabrication and scale-up.

18.6.1 Kinetics of Dry Reforming of Methane

Dry reforming of methane is widely known as a highly endothermic reaction, which is thermodynamically favored at high reaction temperature (Aramouni et al. 2018). However, at such high reaction temperature, dry reforming is also accompanied by several parallel side reactions such as CH₄ decomposition, which provoke carbonaceous deposits leading to catalytic deactivation (Pakhare and Spivey 2014; Kawi et al. 2015; Bian et al. 2017). Therefore, the understanding of fundamental dry reforming mechanism is crucial for kinetically resisting carbonaceous formation and simultaneously enhancing syngas yield as well as H₂ selectivity. Generally, the mechanistic pathways for syngas production from dry reforming of methane can be summarized into four major mechanisms, such as: (a) dissociative CH₄ adsorption, (b) dissociative CO₂ adsorption, (c) formation of hydroxyl groups, and (d) oxidation and desorption of intermediate species. The comprehensive review for these main dry reforming mechanisms is provided as follows:

• 1. Dissociative CH₄ adsorption: The dissociation energy for CH_x-H bonds in CH₄ reactant on the catalyst surface is mainly reliant on the intrinsic attributes of active phases and CH₄ dissociation step is widely recognized as the rate-determining step in dry reforming of methane (Tang et al. 2014; Horn and Schlögl 2015). In particular, CH₃ species are adsorbed on the top of active metal atoms, whereas CH₂ species are adsorbed in the middle of two active metal atoms also referred as bridged adsorption (Tang et al. 2014).

• 2. Dissociative CO₂ adsorption: CO₂ adsorption and dissociation are dependent on surface structure and structural defects of catalysts (Aramouni et al. 2018). In fact, this process can occur through three routes including C-only coordination, O-only coordination (in which an active metal surface is bonded with two O-atoms), as well as C and O coordination (wherein C and O atoms are adsorbed on catalyst surface meanwhile another O atom is left to expose) (Papadopoulou et al. 2012; Guharoy et al. 2018).

• 3. Formation of hydroxyl groups: Numerous dry reforming kinetic studies reveal that the water-gas shift (WGS) reaction nearly reaches equilibrium, indicating the rapid rate of associated surface reaction. The adsorbed H species from H₂ spillover on active metal surface reportedly migrate to support and subsequently react with adsorbed O-species to form hydroxyl groups. However, this process is unfavorable at high temperature beyond 800 °C (Papadopoulou et al. 2012; Bobadilla et al. 2017).

• 4. Oxidation and desorption of intermediates: The adsorbed CH_x species on active sites, denoted as S-CH_x, could react with surface oxygen leading to the formation of S-CH_xO and/or S-CO groups. Several proposed pathways for CO formation reportedly include the generation of intermediate S-CH_xO species and reduction of carbonates (formed by adsorbed CO₂) by surface carbon species. In general, the formation and/or decomposition of S-CH_xO species to H₂ and CO gases are recognized as the rate-determining step (Papadopoulou et al. 2012; Jiang et al. 2017; Das et al. 2018).

In general, the kinetic models typically employed in dry reforming of methane can be mainly categorized into empirical power law, Langmuir Hinshelwood-Hougen Watson (LHHW), and Eley Rideal (ER) models. Amongst these kinetic models, the practical and most simple power law model, as provided in Eq. (18.11), is commonly used since it is regarded as one of the easiest computational models for estimating the pertinent reaction parameters.

$$ -{r}_{C{H}_4}=k{P}_{C{H}_4}^m{P}_{C{O}_2}^n $$

(18.11)

where k and P_i (i: CO₂ or CH₄) are the corresponding apparent rate constant of CH₄ and partial pressure of reactants, while m and n represent the reaction orders for the corresponding CH₄ and CO₂ reactants, and –r_CH4 is the CH₄ reaction rate.

The simplicity of power law model, which does not require the full understanding of elementary dry reforming reaction steps since this model is not derived from mechanistic pathways has made it to be extensively employed in many studies for approximate kinetic parameters estimation. As the power law model is not based on the surface mechanism of dry reforming of methane, this model is slightly inaccurate and inappropriate for the investigation of wide-ranging feedstock compositions in comparison with other mechanism-based kinetic models. Nevertheless, this empirical model is advantageous for providing an initial guess of reactants’ power constants and apparent activation energy, E_a (kJ mol⁻¹). These useful parameters can be used for the justification of catalytic activity and comparison purpose among reported catalysts in literature.

Table 18.3 lists the computed reaction orders of CH₄ and CO₂ reactants and CH₄ activation energy obtained via power law model for numerous catalysts in recent dry reforming studies. Ayodele et al. (2016) found that CH₄ and CO₂ reaction orders were 3.66 and 0.35, respectively, with the apparent CH₄ activation energy of 96.4 kJ/mol on the La₂O₃-supported cobalt catalyst in dry reforming of methane. The higher CH₄ reaction order than that of CO₂ could indicate the larger dependence on CH₄ partial pressure for CH₄ dissociative adsorption. In addition, they found that the great oxygen storage capacity of La₂O₃ support supplied mobile lattice oxygen to enhance coke gasification from the catalyst surface. This evolution reasonably explains the low CH₄ activation energy (96.4 kJ/mol) on Co/La₂O₃ compared to other catalysts such as Pt/ZrO₂ (Wei and Iglesia 2004) and Ni/α-Al₂O₃ (Cui et al. 2007) with E_a of 100.56 and 106.84 kJ/mol, correspondingly.

Table 18.3 Kinetic parameters obtained via power law models for dry reforming on various catalysts

In the kinetic study of dry reforming of methane over SmCoO₃ perovskite catalyst, Osazuwa et al. (2017) found similar activation energy values for CH₄ and H₂ whereas CO₂ activation energy was reportedly close to that of CO. Thus, they suggested that H₂ and CO formation rates were mainly affected by the corresponding CH₄ and CO₂ consumption rates. Karam and Hassan (2018) used SBA-15 supported Ni catalysts for dry reforming of methane and found that the activation energies of reactants (e.g., CH₄ and CO₂) were smaller than those of products (e.g., H₂ and CO), indicating that the formations of H₂ and CO via surface reaction between adsorbed species are the rate-determining step.

As previously mentioned, the empirical power law models cannot elucidate the involvement of inherent elementary steps on surface reaction during dry reforming of methane since these models are mainly reliant on statistical criteria and could yield untrue kinetic parameter values. Hence, ER and LHHW models are broadly implemented to capture reactant consumption rates and product distribution in dry reforming as they are derived from the intrinsic mechanistic reaction steps. In the dry reforming study of Ru or Pt loaded TiO₂, Singh and Madras (2016) found that the typical peaks belonging to CO₂ adsorption were not detected but CO₂ in the gas phase was observed in in-situ Fourier transform infrared spectroscopy (FTIR) studies. In addition, adsorbed CH₄ species on catalyst surface were evidenced suggesting that dry reforming of methane over the abovementioned catalysts followed the ER mechanism. Thus, an ER kinetic model was proposed with the assumption of molecularly adsorbed CH₄ on active site and the subsequent CH₄ dissociation as the rate-determining step as shown in Eq. (18.12) of Table 18.4. The dissociated carbon species were further reacted with CO₂ in the gas phase to yield CO gas. This derived ER model demonstrated an excellent validation with experimental data.

Table 18.4 Representatives of LHHW and ER kinetic expressions for dry reforming of methane

Bao et al. (2017) examined NiCeMgAl catalyst for dry reforming of methane and developed an LHHW kinetic model (Eq. 18.13 in Table 18.4) based on the assumption of irreversibly adsorbed carbon gasification by surface –OH species as a rate-determining step and consideration of the significant contribution of the reverse water-gas shift reaction. Both CO₂ and CH₄ reactants were also assumed to adsorb dissociatively on the same active site of catalyst. The fitting of CH₄ consumption rate data to the derived kinetic model showed a high correlation coefficient of 0.97, indicative of satisfactory projection for dry reforming reaction rate.

In the kinetic and mechanistic studies of dry reforming of methane over La₂RhZr₂O₇ pyrochlores, Pakhare et al. (2014) conducted the deuterium kinetic isotope experiments with the employment of CD₄ during dry reforming and revealed that the rate-determining step was CH₄ activation or dissociation on active sites. Hence, an LHHW model involving dissociative CH₄ adsorption as a rate-determining step (Eq. 18.14 in Table 18.4) was proposed and in line with the obtained experimental data of kinetic and transient pulsing measurements.

18.6.2 Kinetics of Tri-reforming of Methane

Tri-reforming of methane is typically a synergistic combination of multiple reactions involving steam reforming of methane (SRM), dry reforming of methane, and partial oxidation of CH₄ (Song and Pan 2004). However, the kinetic and mechanistic studies of tri-reforming of methane are comparatively less than other reforming reactions owing to its complexity with the highest number of concomitant side reactions. Thus, mechanistic and kinetic investigation of tri-reforming of methane would turn out to be a crucial and alluring research area in the coming years.

The fundamental understanding of mechanism-derived kinetics is still vague and fewer studies have been carried out for tri-reforming of methane. Song and Pan (2004) conducted the first kinetic study of tri-reforming of methane over Ni-based catalysts. To identify the varied CO₂ conversion rate associated with reactant partial pressures, an empirical power rate law (Eq. 18.15) was simplified by retaining the CH₄ and O₂ partial pressures as constants. Therefore, the contribution of CH₄ and O₂ partial pressures would be reflected by other model parameters:

$$ -{\mathrm{r}}_{\mathrm{i}}={\mathrm{A}}_{\mathrm{exp}}\left(\frac{-{\mathrm{E}}_{\mathrm{app}}}{RT}\right){\mathrm{P}}_{C{O}_2}^{\alpha }{\mathrm{P}}_{{\mathrm{H}}_2\mathrm{O}}^{\beta } $$

(18.15)

where −r_i, E_app, and A are the corresponding CH₄ or CO₂ reaction rates, apparent activation energy, and pre-exponential factor. P_H2O and P_CO2 are the partial pressure of H₂O and CO₂ reactants, whereas α and β are reaction orders belonging to CO₂ and H₂O, respectively. The computed values of kinetic parameters obtained from Eq. (18.15) for tri-reforming of methane over different Ni-based catalysts are summarized in Table 18.5. From the experimental data and computational results, Song and Pan (2004) observed the negative reaction order values for H₂O reactant during the fitting of CO₂ consumption rates to the abovementioned power law model (Eq. 18.15) for all catalysts. This observation indicated the competitive adsorption between H₂O and CO₂ on the catalyst surface for reacting with CH₄ to yield H₂ and CO. As a result, an increase in P_H2O reasonably reduced the rate of CO₂ consumption. As seen in Table 18.5, reaction orders also varied considerably depending on catalysts employed.

Table 18.5 Kinetic parameters obtained from simplified power law model for tri-reforming of methane by Song and Pan (2004)

To intrinsically scrutinize the interaction between catalyst surface and CO₂ molecule with respect to mechanistic steps, they also employed a simplified LHHW model, which was derived from dry reforming side reaction for minimizing the intricacy of tri-reforming of methane. These mechanistic steps including the adsorption of CH₄ and CO₂ reactants as well as surface reaction are provided in Table 18.6. As seen in the simplified kinetic model, the interaction between catalyst surface and CO₂ molecules was the major factor accounting for the equilibrium CO₂ adsorption constant, i.e., K₂. The enhancing interaction of CO₂ with catalyst surface would provoke an increase in the number of adsorbed CO₂ species at high P_CO2. Hence, these adsorbed CO₂ species occupied most of active sites available for CH₄ adsorption, thereby inducing a drop in CH₄ consumption rate. This could be indicative of negative CO₂ reaction order.

Table 18.6 Tri-reforming of methane mechanistic steps for LHHW model proposed by Song and Pan (2004)

In the assessment of Ni-Mg/β-SiC at varied temperature and reactant feed ratios for tri-reforming of methane, García-Vargas et al. (2015b) observed that methane partial oxidation with the rapid achievement of full conversion was the predominant process among other concomitant main reactions since oxygen reactant was virtually absent in gaseous effluent from tri-reforming of methane. Based on this observation, a kinetic model merely in association with steam reforming, dry reforming, and water-gas shift reaction was proposed for capturing the individual rate of these reactions (see Table 18.7). The apparent activation energy of steam reforming, dry reforming, and water-gas shift reaction is 74.7, 77.8, and 54.3 kJ/mol, respectively, and the kinetic models show a relatively good fitness to experimental data with mean deviations from 10.8% to 19.4%.

Table 18.7 Kinetic expressions and estimated parameters for steam reforming, dry reforming, and water-gas shift reaction side reactions of tri-reforming of methane (García-Vargas et al. 2015a)

Unlike the kinetic expressions suggested by García-Vargas et al. (2015a), Chein et al. (2017) proposed that different kinetic rate models involved steam reforming, reverse CO₂ methanation (RCM), and water-gas shift reaction (Table 18.8) since they found that the presence of H₂O induced a great influence on tri-reforming reaction rate in their parametric investigation. They also reported that in the absence of water, dry reforming of methane became the predominant reaction inducing a low H₂/CO ratio of 1, while steam reforming of CH₄ was the main reaction with the absence of CO₂ reactant.

Table 18.8 Kinetic rate models for steam reforming, reverse CO₂ methanation, and water-gas shift reactions in tri-reforming of methane (Chein et al. 2017)

As partial conclusion, this section presents a comprehensive review on various kinetic models derived from the respective rate-determining step in association with different proposed mechanistic steps. Many kinetic models were widely reported into three forms, namely power law, ER, and LHHW models. The power law models could provide empirical reaction orders to reveal the dependence of reaction activity on reactant partial pressure. However, it could not reflect the essential influence factor of rate-determining step toward catalytic activity with respect to mechanistic pathway. Thus, numerous rate-determining steps presented in LHHW and ER models have been suggested for predicting the consumption rate of reactants. Generally, these rate-determining steps involve C–H bonds cleavage in CH₄ molecule on metal, dissociation of oxidizing agent (e.g., O₂, CO₂, or H₂O) on metal or support, formation of intermediate species and decomposition, oxidation of carbonaceous species, and several other parallel reactions. To implement the dry reforming and tri-reforming technologies for industrial application, the best-fit kinetic modeling associated with the intrinsic dry reforming and tri-reforming mechanism is indispensable for efficiently designing the catalyst system and optimizing the reactor design.

18.7 Industrial Applications

Despite the interest in scaling up this technology, problems related to catalyst activity and stability still need to be solved to advance the readiness level of this process. To the best of our knowledge, only very few reports are available on the literature regarding the scale-up of dry reforming of methane. Rego de Vasconcelos and Lavoie (2018) recently reported the scale-up of a dry reforming technology combined with renewable electricity. In this case, an electrified fixed-bed reactor is used in combination with steel wool as a catalyst. The tests performed showed CH₄ and CO₂ conversions equal to 100% over 200 h of time on stream. The advantages of this approach are the very low cost and the good catalytic performance of the catalyst as well as the use of renewable electricity to provide the energy to the reaction. The technology evolved from bench scale with capacity of treating less than 1 L/min of a mixture containing CH₄ and CO₂ to a pilot capable of treating 40 L/min of the same gaseous mixture. The technology developed is compatible for biogas, tail gas, and natural gas.

The Linde Group has also claimed the development of an innovative dry reforming process that convert large volumes of CO₂ into syngas using a nickel-based catalyst, which has been tested for more than 1000 h (Linde 2019). Besides CH₄ and CO₂, water is also used in this process to prevent catalyst deactivation by coke formation (Tullo 2016). The Linde Pilot Reformer research facility is located at Pullach (Germany) and is currently under scale up for commercial application.

Steam reforming of natural gas has been deployed at large industrial scale. This technology can be transposed to the reforming of landfill gas, as the case of the VABHYOGAZ3 project, financed by ADEME in France (ADEME 2016). This project aims to design a complete production chain of liquid hydrogen production from landfill gas containing up to 60% of CH₄. Landfill gas is firstly reformed into syngas at around 900 °C, with a large excess of steam (molar ratio of steam/methane of around 4:1). In fact, the landfill gas used in this project containing CH₄, CO₂, and O₂ is fed together with steam to the reforming catalytic reactor. The syngas is then converted into a mixture rich in H₂ and CO₂ by water-gas shift reaction. Pure hydrogen (99.99%) is obtained by the pressure swing adsorption process, which is then compressed to 350–700 bar into liquid hydrogen for injection in hydrogen-fueled vehicles. CO₂ obtained from the pressure swing adsorption process is valorized by producing sodium bicarbonate for flue gas treatment. The current production unit developed by the industrial partners of this project has a capacity of 10 kg of liquid hydrogen per day (ADEME 2016; Pham Minh et al. 2018a, b). The consortium of this project can deploy production units of 80–800 kg of liquid hydrogen per day from landfill gas with competitive cost of liquid hydrogen versus other production ways (e.g., electrolysis of water).

18.8 Conclusions

Biogas production from biological degradation of biomass and biowaste continuously increases worldwide. Heat and electricity production from biogas have been largely deployed. Current research and development on biogas valorization mostly consists in the production of value-added chemicals via biogas reforming. Both dry- and tri-reforming processes need high temperature (e.g., 800–1000 °C) and low pressure (e.g., atmospheric pressure) to reach high methane conversion. The utilization of a performing catalyst is indispensable to obtain exploitable reaction rate and good syngas selectivity. The main criteria of such a catalyst include high thermal stability, high coke resistance, high activity, and stability. Nickel is the most appropriate active metal for biogas reforming, while various supports have been found suitable. Significant efforts have also been devoted to kinetic and mechanism studies of dry- and tri-reforming processes. This is mandatory for reactor design for further deployment of these processes at large-scale.