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Tri-reforming of methane: reactions, mechanism and catalysts
A novel concept of reforming process has been proposed for the first time by Song et al. [43,82], assuming the usage of the gas feed containing CH4, CO2, H2O, and O2.
The presence of the oxygen and water vapor improve energy efficiency and can positively affect coke mitigation . However, the biggest advantage of the process is the production of synthesis gas which may be further converted into methanol, di-methyl ether (oxo-synthesis) and liquid fuels (via Fischer-Tropsch synthesis). Another application of the tri-reforming of methane could be converting low-quality CO2-rich natural gas into industrially useful products . The syngas obtained in methane tri-reforming process has molar ratios in the range of 1.5-2.0 . The synthesis gas can be also further processed into either hydrogen or carbon monoxide alone, so that the obtained gas can be used in other processes, e.g. hydrogenation or carbonylation. It should be stressed, however, that for the future implementation of tri-reforming on industrial scale, an active, stable (free from coke formation), and selective catalyst is a key requirement.
Flue gases from natural gas power station for tri-reforming of methane
The scenario applies to fossil fuel-fired power plants, where carbon dioxide is one of the components of flue gases. The process does not require the pre-separation of carbon dioxide, and mixtures containing CO2, H2O and O2 (with methane added) may be directly used for the synthesis gas production (H2 and CO).
Fig. 1.6 shows the visualization of the concept where flue gases from a power station, with natural gas added, are directed to the tri-reformer. As a result, synthesis gas, which is a raw material for a number of chemical syntheses, is obtained. In this concept the direct CO2 usage, without its pre-separation, is assumed. This results in energy savings, since conventional methods, such as CO2-MEA absorption, require high energy for absorbent regeneration .
CO2 separation for tri-reforming of methane
Another approach to utilization of carbon dioxide flue gases is based on its separation and purification (via absorption, adsorption or membrane separation). Tri-reforming of methane is then a following processes applying pure CO2. In this case, different molar ratios between the reactants can be used, compared to the ones which are present in the real flue gas. The adjusted mixture should lead to the thermodynamically more favorable results, than in case of the gas mixture typical for flue gases.
Tri-reforming of methane can be carried out with various feed gas compositions as described by Song and Pan . Their thermodynamic calculations showed that high conversions of both CH4 and CO2 (respectively over 95, and ca. 80 % at 850 °C) may be obtained, assuming molar ratios of CO2/H2O
= 1.0, O2/CH4 = 0.1, and (CO2+H2O+O2)/CH4 = 1.05. Some experimental catalytic studies of TRM assumed the same and/or very close gas compositions to those proposed in literature calculations . Majewski et al.  and Garcia-Vargas et al.  studied TRM process with the feed of CH4/CO2/H2O/O2 = 1/0.5/0.5/0.1. Pino et al. , worked with feed gas composition of CH4/CO2/H2O/O2 = 1/0.46/0.46/0.1, while Si et al.  studied catalysts at CH4/CO2/H2O/O2 = 1/0.5/0.375/0.25., Sun et al.
 applied in their study CH4/CO2/H2O/O2 = 1/0.45/0.45/0.1, and Song and Pan [43,82,83] assumed CH4/CO2/H2O/O2= 1/0.475/0.475/0.1 and CH4/CO2/H2O/O2= 1/0.56/0.48/0.1. The presence of oxygen may improve catalytic performance of the reforming process. However, it has to be stressed that possible methane ignition should be prevented, and upper and lower flammability limits (UFL, LFL, respectively) have to be considered. Above the upper flammability limit the CH4 concentration is too high, thus in the presence of spark methane will not combust. The flammability limits are found through calculations, and from methane and oxygen mixture UFL is: 1.57, while for methane in air it is 0.94. On the other hand, when CH4 concentration is below the LFL its content is too low to start ignition. The lower flammability limits are: CH4/oxygen = 0.22, and for CH4/air: 0.05 .
Dry reforming of methane (DRM)
Dry methane reforming (DRM), also called CO2 reforming, is considered one of the promising processes for chemical CO2 utilization [44,67,95–100]. The process was introduced by Fischer and Tropsch for the first time in 1928 [101,102]. However, the extensive investigation on the DRM started when increasing concerns about greenhouse effects were raised by the international scientific community in the 1990s .
DRM leads to synthesis gas production, a mixture of hydrogen and carbon monoxide that is appropriate for products including Fischer-Tropsch fuels (process on Fe-based catalysts) and dimethyl ether (DME) . In comparison to conventional technologies, i.e., steam reforming and partial oxidation of methane, the DRM is also suitable for remote natural gas or crude oil fields, where water supplies are limited . Moreover, since natural gas deposits include large amounts of CO2, its emission to the atmosphere can be avoided if carbon dioxide were used in DRM. This would lower the purification costs . Due to the strong endothermicity of dry reforming of methane, the process can be environmentally valuable only if the required reaction heat comes from nuclear or renewable energy [96,102,103].
Similarly as TRM, the DRM reaction is accompanied by several side reactions, including previously mentioned direct methane decomposition (DMD) (Eq. 7), and Boudouard reaction (Eq. 8). Stoichiometric H2/CO ratio of dry reforming of methane is 1.0, but it is often lower due to the reverse water-gas shift (RWGS) reaction (Eq. 12). CO2 + H2= CO + H2O ΔH0 = 41 kJ/mol (Eq. 12).
Steam reforming of methane (SRM)
Steam reforming of methane (Eq. 2) is a chemical process assuming conversion of a hydrocarbon rich feedstock to hydrogen and syngas (hydrogen, carbon monoxide and carbon dioxide). Syngas, as previously mentioned, is an important industrial raw material, and it is used to produce a wide variety of commercially significant products, such as hydrogen (via WGS reaction) for further synthesis, e.g. ammonia [120,121]. Steam reforming of methane produces synthesis gas with high H2/CO molar ratio of ca. 3, that is higher than the one required for downstream methanol and hydrocarbon conversion processes. SRM was developed at the beginning of the 20th century to produce hydrogen . The main reason for expanding production of hydrogen was connected to the discovery of the Haber-Bosch process, important for the ammonia synthesis. The process was further developed in the 1970’s .
According to Rostrup-Nielsen , replacing carbon dioxide with steam in the reforming reaction has no drastic effect on the mechanism. In the steam reforming process CH4 is dissociated on the surface of a catalyst, typically Ni-based, molecular hydrogen is formed, and the remaining carbon reacts with water to form additional molecular hydrogen and carbon monoxide . The process takes place at high temperatures and, most economically, if the carbon-to-oxygen ratio in the feed gas is close to stoichiometric. Such conditions, however, lead to the graphitic carbon formation, which deactivates a used catalyst. The rate of carbon formation is known to be far lower on noble metals than on nickel-based catalysts, which is ascribed to a lower dissolution of carbon into these metals . Rostrup-Nielsen [119,125,126] reported a suppression of carbon deposition caused by sulfur poisoning in steam reforming of methane. This effect was explained by a partial blockage of a catalyst surface/sites. The process has been developed industrially by Haldor Topsøe AS, and known as the SPARG process .
Typically nickel-based catalysts are used for industrial steam reforming . The catalysts are heated to temperatures of up to 900 °C to obtain a satisfying conversion of methane [128,129]. The demand for high throughput, low pressure drop and high pressures in synthesis loops dictates the 20– 40 bar pressure in reforming units .
Partial oxidation of methane (POM)
In the 1940s, Prettre et al.  first reported the formation of synthesis gas by the catalytic partial oxidation of CH4. In contrast to dry and steam reforming of methane, methane partial oxidation is mildly exothermic, leading to savings in energy use. On the other hand, the partial oxidation requires pure oxygen, which is produced in expensive air separation units that are responsible for up to 40% of the cost of a synthesis gas plant. It should be additionally taken into account that the two reagents (CH4 and O2) are explosive if the reaction is not conducted under appropriate conditions. Therefore, the catalytic partial oxidation of methane did not attract much interest for nearly half a century, and steam reforming of methane remained the main commercial process to manufacture synthesis gas [131–133]. The H2/CO molar ratio of POM is close to 2, which is suitable for Fischer-Tropsch process (over cobalt-based catalysts) and methanol synthesis . Moreover, the POM can be performed under very high gas hourly space velocities (GHSV) of 100,000-500,000 h-1 . The application of POM in chemical technology up to date are gas-to-liquids (GTL) plants operated by the Royal Dutch Shell PLC .
Therm odynamic analysis of DRM, SRM, POM, TRM
In this chapter, thermodynamic equilibrium calculations are presented for dry reforming of methane (DRM). A comparison of DRM with thermodynamic results obtained for steam reforming of methane (SRM), partial oxidation of methane (POM), and tri-reforming of methane (TRM) was also carried out. Finally, for tri-reforming of methane, a feed composition was examined, which may be useful for a natural gas-based power plant.
The analyzes assumed the volumetric ratios of feed compositions considering two scenarios described in the Subchapters 126.96.36.199. and 188.8.131.52:
(i) CO2 can be separated from flue gases, and TRM reaction may be carried out according to the gas composition suggested in the literature [85,86]: (CH4/CO2/H2O/O2/Ar= 1/0.5/0.5/0.1/7.9).
(ii) direct usage of CO2 from flue gases from a power plant fired with natural gas, and the gas composition suggested for the first time in this thesis.
Table 3.1 presents the feed compositions which were proposed for the calculations. The following compositions refer to the volumetric ratios of the components: (1) DRM: CH4/CO2/H2O/O2/Ar = 1/1/0/0/8, (2) SRM: CH4/CO2/H2O/O2/Ar = 1/0/1/0/8, (3) POM: CH4/CO2/H2O/O2/Ar = 1/0/0/0.5/8.5, and (4) TRM: CH4/CO2/H2O/O2/Ar = 1/0.5/0.5/0.1/7.9.
Table of contents :
Goals of doctoral thesis
Chapter 1 – Current status and literature review on CO2 utilization
1.1. CO2 and climate
1.2. Reduction of CO2 emissions – solutions and technologies
1.2.1. Carbon dioxide capture and storage (CCS)
1.2.2. Current and emerging carbon dioxide utilization technologies
184.108.40.206. Chemicals production – mature technologies
220.127.116.11. Chemicals production – mature and emerging technologies, and future prospects
1.3. Tri-reforming of methane: reactions, mechanism and catalysts
1.3.1. Two concepts of tri-reforming of methane
18.104.22.168. Flue gases from natural gas power station for tri-reforming of methane
22.214.171.124. CO2 separation for tri-reforming of methane
1.3.2. Tri-reforming of methane (TRM): three main reactions
126.96.36.199. Dry reforming of methane (DRM)
188.8.131.52. Steam reforming of methane (SRM)
184.108.40.206. Partial oxidation of methane (POM)
220.127.116.11. Autothermal steam reforming (ATR)
18.104.22.168. Combined dry reforming and partial oxidation of methane (CRPOM)
1.3.3. Catalysts for tri-reforming of methane
22.214.171.124. Dry reforming of methane (DRM)
126.96.36.199. Steam reforming of methane (SRM)
188.8.131.52. Partial oxidation of methane (POM)
1.3.4. Double layered-hydroxides as potential catalysts for tri-reforming of methane
184.108.40.206. Properties, synthesis and application
220.127.116.11. Double layered-hydroxides in methane reforming processes
Chapter 2 – Experimental part
2.1. Catalyst preparation
2.1.1. Co-precipitation technique
2.1.2. Co-impregnation technique
2.1.3. Incipient wetness impregnation
2.2. Characterization methods
2.2.1. X-ray diffraction (XRD)
2.2.2. X-ray fluorescence (XRF)
2.2.3. Low temperature nitrogen sorption
2.2.4. Temperature programmed reduction (TPR-H2)
2.2.5. Temperature programmed desorption (TPD-CO2)
2.2.6. H2 chemisorption
2.2.7. Transmission Electron Microscopy (TEM)
2.2.8. High-Resolution Electron Microscopy (HRTEM)
2.2.9. Thermogravimetric analysis coupled by mass spectroscopy (TGA/MS)
2.2.10. Raman spectroscopy
2.3. Catalytic tests
2.4. Thermodynamic calculations
2.4.1 Minimization of Gibbs free energy
2.4.2. Calculation method
Chapter 3 – Thermodynamic analysis of DRM, SRM, POM, TRM
3.1. Thermodynamic equilibrium analysis of methane reforming processes
3.1.1. Dry reforming of methane calculations
3.1.2. Dry reforming of methane versus other reforming processes
3.1.3. The influence of feed gas composition on carbon deposition
3.2. Thermodynamic calculations for tri-reforming of methane assuming direct application of CO2 from flue gases
Chapter 4 – Dry reforming of methane (DRM)
4.1. Yttrium promotion of Ni-based double layered-hydroxides
4.1.1. Physicochemical properties
4.1.2. Reducibility, basicity, Ni dispersion and crystallite size
4.1.3. Catalytic activity and stability in DRM
4.1.4. Characterization of the spent catalysts
4.2. Co-impregnation with zirconium and yttrium of Ni-based double layered-hydroxides
4.2.1. Physicochemical properties
4.2.2. Reducibility, basicity, Ni dispersion and crystallite size
4.2.3. Catalytic activity and stability in DRM
4.2.4. Characterization of the spent catalysts after the TPSR test
4.2.5. Characterization of the spent catalysts after the isothermal tests
4.3. Co-precipitation with zirconium and impregnation with yttrium versus co-precipitation with zirconium and yttrium
4.3.1. Physicochemical properties
4.3.2. Reducibility, basicity, Ni dispersion and crystallite size
4.3.3. Catalytic activity and stability in DRM
4.3.4. Characterization of the spent catalysts after the isothermal tests
4.4. Co-precipitation with cerium and impregnation with yttrium of Ni-based double layeredhydroxides
4.4.1. Physicochemical properties
4.4.2. Reducibility, basicity, Ni dispersion and crystallite size
4.4.3. Catalytic activity and stability in DRM
4.4.4. Characterization of the spent catalysts after the isothermal tests
4.5. Overall conclusions on dry reforming of methane
Chapter 5 – Tri-reforming of methane and other reactions on selected catalysts
5.1. Partial oxidation of methane – one of main reactions in tri-reforming of methane
5.1.1. TPSR catalytic tests
5.1.2. Isothermal catalytic tests of partial oxidation of methane
5.2. Combined CO2 reforming and partial oxidation of methane as a part of the process of trireforming of methane
5.3. Tri-reforming of methane
5.3.1. Feed gas composition of (CH4/CO2/H2O/O2/Ar=1/0.5/0.5/0.1/7.9)
18.104.22.168. Catalytic tests
22.214.171.124. Characterization of the spent catalysts
5.3.2. Gas composition of flue gases from natural-gas-fired power station (CH4/CO2/H2O/O2/Ar = 3/1/2/0.3/3.7)
126.96.36.199. Catalytic tests
188.8.131.52. Characterization of the spent catalysts
5.4. The comparison of HTNi and HTNi-Y2.0 catalysts in DRM, POM, CRPOM and TRM