![Production of hydrogen after regeneration: (a) 900 °C and 4 hours, and (b) 925 °C and 2 hours [95].](https://www.researchgate.net/profile/Mahdi-Yousefi-7/publication/356397172/figure/fig3/AS:11431281079648522@1660801604397/Production-of-hydrogen-after-regeneration-a-900-C-and-4-hours-and-b-925-C-and-2_Q320.jpg)
![Hydrogen mole fractions attained during the thermodynamic equilibrium of the CH4 cracking at different temperatures and atmospheric pressure [104].](https://www.researchgate.net/profile/Mahdi-Yousefi-7/publication/356397172/figure/fig4/AS:11431281079619080@1660801604446/Hydrogen-mole-fractions-attained-during-the-thermodynamic-equilibrium-of-the-CH4-cracking_Q320.jpg)
![Non-catalytic and catalytic types are employed for the thermal cracking of methane and their temperature range [104].](https://www.researchgate.net/profile/Mahdi-Yousefi-7/publication/356397172/figure/fig5/AS:11431281079619081@1660801604503/Non-catalytic-and-catalytic-types-are-employed-for-the-thermal-cracking-of-methane-and_Q320.jpg)
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Technical challenges for developing thermal methane cracking in small or medium scales to
produce pure hydrogen - A review
Mahdi Yousefi, Scott Donne
PRC for Frontier Energy Technologies and Utilization, University of Newcastle, Callaghan, NSW 2308, Australia
Abstract
Hydrogen is an important chemical commodity and plays a key role in the clean, secure and affordable
energy scenarios of the future. There is a significant interest in the development of small plants for
hydrogen generation besides other plants where hydrogen has been consumed as raw material and it
is because of the very high cost of compression and transportation of hydrogen.
Thermal methane cracking (TMC) is an alternative process for high purity hydrogen manufacturing
along with the traditional commercial processes such as steam reforming, coal gasification, partial
oxidation, and water electrolysis. Employing the TMC process for very high purity hydrogen
production on a small or medium-scale plant with the minimum requirement of separation units is the
main incentive of this review.
Given the results of the review, using catalysts for TMC can decrease the working temperature to
below 800 °C but it could create some significant issues, especially catalyst deactivation (a low
catalyst life due to carbon deposition), so it is not yet a viable method to employ on the production
plants. On the other hand, supplying the heat of reaction and reactor blockages are two basic
challenges for a non-catalytic reaction way.
1. Introduction
Hydrogen can potentially play a key role in the clean, secure and affordable energy scenarios of
the future as it offers ways to decarbonise a range of established industry sectors where it is proving
difficult to reduce greenhouse gas (GHG) emissions (e.g. transport, chemicals, and iron and steel).
Hydrogen can also open up new opportunities in other economic sectors, chief among them being
decentralised residential power generation.
The utilisation of the traditional energy carriers (e.g., oil, natural gas and coal) leads to emissions of
NOx and SO2 into the environment. NOx and SO2 are known as toxic gases and are also a source of
acidic rains, making them damaging to the environment. Hydrogen combustion only produces water
and heat and, as a fuel, can compete with conventional energy carriers such as natural gas, gasoline
and coal. Therefore, hydrogen could be a viable substitute if its production did not produce as much
toxic and greenhouse gases or emissions such as CO2, CO, SO2. Likewise, the direct combustion
2
applications of hydrogen in electricity production is another feature of H2 as an energy carrier[1, 2].
Hydrogen can be turned directly into electrical energy in a fuel cell in a reverse water electrolysis
process. Cold combustion of H2 with O2 as an electrochemical reaction can generate electricity [3].
Fuel cell technology applications have had a fast growth in recent years, based on them being able to
produce electricity ‘just in time’ and for being environmentally more efficient [4], especially in
vehicles, residential and power plant applications. An automobile’s driving force can be provided by
electricity which is generated from an onboard fuel cell, and it can be an alternative to conventional
combustion engines[5].
Due to the expensive transportation costs and the limitations of hydrogen storage, there is a
significant interest in developing a small-scale platform for hydrogen generation. Indeed, in some
areas, hydrogen storage and transportation costs have exceeded their selling prices [6]. Hydrogen
production in a small plant is always desirable in its downstream fields. Furthermore, the production
of very pure hydrogen can open a new window in the development of the fuel cell industry. Therefore,
the main motivation for this review is to assess a viable and inexpensive method for the production
of pure hydrogen for small or medium-scale applications with a view to eliminating the compression,
liquefaction, transportation and storage energy-related costs.
Figure 1: The hydrogen economy and supply chain
1
.
The major commercial processes specific to hydrogen production are steam reforming, coal
gasification and partial oxidation [1, 2]. In most of the processes, hydrogen is generated along with
other species such as CO, CO2, H2O and CH4. Therefore, separation and purification units are required,
1
https://ihsmarkit.com/research-analysis/the-role-of-hydrogen-in-a-deeply-decarbonized-future.html
3
which mostly are bulky, costly unit operations and do not indicate a feasible path for employing in
small or medium-sized hydrogen production plants.
Thermal methane cracking is a promising and feasible process for hydrogen production in small and
medium-scale applications.
CH4 →2H2+C Δ= 74.85 kJ Equation 1
There is a potential to generate hydrogen as the only gaseous product in the thermal methane cracking
process. Therefore, it is much less complicated than conventional processes such as steam reforming,
where the hydrogen has been manufactured by using CO2 or CO and there is a need for bulky and
complicated separation unit operations. The hydrogen which is produced by methane thermal
cracking is very suitable for direct consumption in fuel cells which require a high purity in order to
avoid significant deactivation in the fuel cells [4].
The methane cracking processes often work at atmospheric pressure and are less endothermic than
some other conventional processes like steam reforming. For example, a steam reforming process
needs about two times more energy than the catalytic TMC process (at the same capacity). The lower
energy demand makes the possibility of using solar energy or even hydrogen to supply the required
reaction energy, making it a greener hydrogen production process. As noted, TMC has not been
widely used for hydrogen production, despite having superior characteristics such as its high purity,
cost-effectiveness and environmental benefits. The vision pursued in this research is to survey an
advanced yet simple and inexpensive TMC technology platform for the production of high purity
hydrogen for small to medium-sized applications.
2. Thermal methane cracking process
Generally, methane can be cracked to hydrogen and carbon using two main methods, catalytic and
non-catalytic. In the past, this reaction has been investigated mostly as one of the side reactions in the
steam reforming plants, where catalyst deactivation and the elimination of carbon formation were the
main goals of the research[6]. However, most of the recent scientific investigations have been focused
on catalytic and non-catalytic TMC for the purposes of high purity hydrogen and carbon production
[5, 7].
2.1. Catalytic thermal methane cracking
Methane is the most stable hydrocarbon because of its structural symmetry and high C-H bond
energy
2
. Thus, it needs a high level of temperature (more than 1200 °C) for complete decomposition.
Using a catalyst can reduce the H-C energy bonds at moderate working temperatures (below 750°C).
Generally, the catalytic thermal cracking of methane mainly involves five steps [8]:
2
440 kJ/mol
4
The CH4 adsorption on the catalyst’s faces.
- The craking of the C–H bonds of methane:
) Equation 2
Equation 3
- Converting the H2 into molecules, as follows:
Equation 4
Aggregating the atomic carbon leading to encapsulated carbon and progressive catalyst deactivation.
The nucleating of the carbon and crystalline growth nanofibres (CNFs) or carbon nanotubes on the
catalyst’s faces.
Equation 5
The kinetics and rates for each of the reaction steps are not completely clear so far and they can be
unique for every type of catalyst [9]. A wide range of catalyst types have been tried for catalytic TMC
and they can be classified into two general categories, metal-based catalysts and carbon-based
catalysts (see Figure 2).
Figure 2: Catalyst types that can be used in thermal methane cracking.
TMC
Catalytic
Metal catalyst
Metal oxide
supported
catalyst
Metal supported
catalyst
Carbon based
catalyst
Carbon based
metal doped
Non-
Catalytic
5
2.1.1 Metal Catalysts
A wide range of metal-based catalysts have been tested for catalytic thermal methane cracking and
they can be categorized into three subgroups: metal catalysts, metal oxide catalysts and metal-
supported catalysts [1, 2].
Generally, the mechanisms of the methane cracking reaction have the same concepts as the metal-
based catalysts. However, they can have different preparations or chemical composition types (in
non-supported or metal-based catalysts) so that they can specify the morphology and structure of
the formed carbon on the catalyst’s faces [3, 7, 10].
Table 1 provides a summary of the properties of various kinds of metal-based catalysts which were
collected by Ashik et al. [7], which has been adapted based on some other recent research.
Table 1: Catalytic stability and activity of different metal-based catalysts[7].
Catalyst
Stability and time
Max. CH4
conversion
Max. H2
produced
Flow rate
Ref.
T
t
For ( t, T, and F)
Ni
500
37
9
--
60b
[4]
Fe
900
>75
98
--
20b
[11]
80% Fe/Al2O3
700
3
74
74
30b
[12]
40% Fe/Al2O3
700
3.3
57.8
74
30b
[12]
15% Fe/ Al2O3
700
0
2.1
2.1
30b
[12]
Ni–Cu
900
5
96
--
110c
[13]
Fe–Cu
600
5
51
--
110c
[14]
Ni–Cu–Al
800
2.75
--
75
120,000a
[8]
Ni/Ce–MCM-41
580
>23
75
--
75b
[15]
Ni–Cu–Zn/MCM-22
800
>50
85
--
10b
[16]
Ni/SiO2
600
>10
22
--
--
[17]
Ni/SiO2
650
4
42
--
15b
[18]
Ni/TiO2
700
8
--
73
20b
[19]
Ni/Al2O3
700
3
--
73
12a
[20]
Ni/La2O3
600
5
75
--
110c
[21]
Fe/Al2O3
800
3
--
91
1.5a
[20]
6
Fe/Al2O3
900
6
68
--
6000a
[22]
Fe/MgO
800
3
--
55
12,000a
[23]
NiCu/Al2O3
750
>7
--
80
12,000a
[24]
Ni–Ca/SiO2
580
3
39
--
100b
[25]
Ni–K/SiO2
580
3
40
--
100b
[25]
Ni–Ce/SiO2
580
3
90
--
100b
[25]
Ni–Fe/SiO2
650
>4
46
--
15b
[18]
Ni–Cu/SiO2
750
45
88
86
1800a
[26]
Ni–Cu–TiO2
700
8
--
80
20b
[19]
Ni–Cu/MgO
700
3
--
79
12a
[20]
Ni/MgAl2O4
700
5
37
--
80b
[27]
Ni–Cu/La2O3
900
>26
97
--
110c
[21]
Ni/Ce–SiO2
600
2
50
--
100b
[28]
Fe–Mo/MgO
800
3
--
92
1.5a
[20]
FeMo/MgO
950
3
--
96
1000a
[23]
FeMo/Al2O3
800
3
--
88
12000a
[23]
Co/Ce–TiO2
500
2
5
--
100b
[28]
Co/Al2O3/SiO2
700
30
90
--
1900 h−1
[29]
CoO–MoO/Al2O3
700
2
78.9
--
250b
[30]
Pt–Ni/MgAl2O4
700
4
45
--
80b
[31]
MgO/SiO2
900
200
--
45
60–65b
[32]
K/MgO/SiO2
900
200
--
77
60–65b
[32]
Ni/K/MgO/SiO2
900
200
--
61
60–65b
[32]
LaNiO3perovskite
700
4
81
--
15b
[33]
LaNiO3perovskite
800
5
91
--
20b
[34]
NiO/La2O3
800
5
93
--
20b
[34]
50% Ni/Al2O3
675
0
64
64
12a
[35]
50% Ni-5%Fe/Al2O3
675
8.3
60
14
12a
[35]
50% Ni-15%Fe/Al2O3
675
5
69.9
68.7
12a
[35]
50%Ni-10% Fe/Al2O3
700
0
85
30
12a
[36]
25%Fe-25%Co/MgO
700
9.5
69.8
6.2
50c
[37]
1Ni−2Fe−1A
700
0.4
58
12
70b
[38]
40%CoO-Mo/Al2O3
700
2
79
16
250b
[30]
7
(Mass (g), T = temperature (°C); t = time (h); flow rate (a: mL/(gcat.h), b: mL/min, c: NmL/min, unless other
units are stated); conversion (%); not mentioned in the original paper).
As seen in Table 1, a wide range of metal catalysts has been tried for thermal methane cracking.
Some non-metal supported catalysts, such as Co, Ni, Fe, Cu, Ru, Pt, Ir, Mo and Ti, have excellent
activity for a catalytic TMC reaction. The working temperatures of the metal-based catalysts vary
between 500 and 950 °C. The activity order for the non-supported metal catalysts is Ni, Co, Ru, Rh,
Pt, Re, Ir, Pd, Cu, W, Fe and Mo, respectively [7].
The advantage of a non-supported catalyst, compared to other catalysts, is its magnetic properties,
which makes for easier separation of the catalyst after the reaction [39]. Nickel is the most active
amongst all of the catalysts which have been used for TMC, which is mostly because of its crystal
size.
A quick encapsulation of a catalyst’s active sites by the deposited carbon is the main issue of the
catalytic TMC process, and catalysts with a higher activity have a quicker encapsulation. Carbon
encapsulation because of rapid aggregation will deactivate non-supported nickel-catalyst quickly,
particularly at a high temperature (more than 600 °C) where the reaction is faster [25].
Iron-based catalysts have also received considerable attention for catalytic TMC due to their cost-
effectiveness and greater stability at a high-temperature range (700 to 950 °C) [50]. Nonetheless,
the iron-based catalysts deliver less invaluable carbon (nano-carbon) as a by-product [40]. Cobalt
and copper activities are less than nickel and have much quicker deactivation properties [41, 42].
Noble metals, such as Ru, Rh, Re, Ir, Pd and Pt, have no cost-effectiveness due to having less
activity compared with nickel and also their quicker deactivation properties [42]
The catalytic performances of the metals are changed when located on supports such as Al2O3, SiO2
or other metal-based supports. The metals are polarized by the effects of the support [43], which
changes their properties. The main factors which influence catalytic activity are the dispersion of
the metal particles, crystallite size, electronic state of the metal particles, catalyst preparation
method, and the pore geometry of the support [44, 45].
Many studies have been conducted on improving the properties of metal catalysts by employing
different types of support in the catalytic TMC process. For example, Qian et al. [46] surveyed the
8
Ni-Cu and Co-Mo catalysts on an Al2O3 base, Saraswat and Pant [16] used a Ni–Cu–Zn catalyst on
an MCM-22 base, and Hornes et al. [47] employed a Cu–Ni catalyst on a CeO2 base.
Some of the researches have shown that a high improvement can be achieved by employing the
proper support for the metal catalysts. Cunha et al. [13, 14] used Ni and Fe catalysts on a spongy
type of Cu support which showed better stability in the decrease in carbon encapsulation. Punnoose
et al. [48] used M/Fe catalysts on an Al2O3 base (M = Ni, Mo and Pd) which resulted in a lower
deactivation by cracking because the nanoparticles grow away from the catalyst.
Oxide supported catalysts can also promote the surface properties and chemisorption of metal
particles [43]. As shown in Table 1, the metal-oxide supported catalysts, such as SiO2, TiO2, MnO,
MoO, MgO and CoO, have been studied by many researchers [23]. Saraswat et al. [26] used a Ni-
Cu catalyst which was supported with silica (50%Ni–10%Cu/SiO2) in a fixed bed reactor which
resulted in a maximum of 88 % methane conversion. Pinilla et al. [24] tested the same experimental
test with a Ni-Cu catalyst which was supported with alumina and the results revealed a maximum
80 % hydrogen yield. Lee et al. [30] used a CoO-MoO catalyst on alumina support and observed a
maximum 80 % conversion of methane but with a very quick deactivation. Otsuka et al. [49, 50]
investigated the influences of some supported catalysts like Ni-TiO2, Ni-SiO2, Pd-Ni-SiO2 and Ni-Al2O3
on catalytic TMC and the performances activity of the catalysts were shown to depend on their
supports based on the sequence: SiO2<TiO2< Al2O3.
Muradov and Veziroglu tested Fe2O3 catalysts and observed the formation of a massive amount of
CO2 and CO in the first two hours, because of the iron-oxide reduction, which resulted in poor H2
yield, and after these high activities were revealed at above 600 °C [51, 52]. Shah et al. [53] used
Al2O3 supported Fe catalysts between 400 and 1200 °C, with about a 68 % conversion, but the
catalyst was highly deactivated by sintering at more than 800 °C[53].
Pinilla et al. [23] used Fe-based catalysts at optimized operating conditions and found that adding
Mo will promote the performance of the Fe catalyst which was supported by MgO and that it
showed similar or slightly poorer results for the Fe catalyst which was supported by Al2O3[23].
It has been shown that nickel catalysts have more activity while the iron catalysts reveal better
stability, and nickel-iron based catalysts establish better activity and stability properties compared
to when they are used alone. Wang et al. [18] tested a Fe-Ni-SiO2 catalyst for methane de-
carbonation at 550 °C and 650 °C and found that the catalyst’s life is extended and activity improved
while producing a better carbon filament structure [18]. Saraswat et al. [16] also found that adding
9
Cu and Zn to a nickel/MCM-22 catalyst can promote the carbon filament structure and produce
multi-wall carbon nanotubes [16]. Furthermore, they studied the influence of Cu on the
performance of nickel/silica catalysts separately and a maximum of 88 % methane conversion was
observed [26]. Guevara et al. [15] investigated the effects of adding Ce to a nickel/MCM-41 catalyst
structure and observed no major catalytic deactivation after 1400 min (with 60 % to 75 % conversion)
due to its large surface area and the ordered pore system of the Ce-MCM-41 structure [15]. Parada
et al. [28] tested nickel/Ce–SiO2 catalysts and observed a 50 % increase in methane conversion for
nickel/Ce–SiO2 catalyst compared to a Ni/SiO2 catalyst (500 °C) due to the presence of CeO, which
creates small nickel particles (≈5 nm) highly dispersed on the catalyst’s surface. Cobalt/Ce–TiO2
catalysts were also tested by them but revealed a lower activity and stability [28]. Adding Ce to an
iron-based catalyst can also improve the catalyst’s performance. Tang et al. [54] showed that adding
Ce to an iron catalyst structure (Fe–CeO2) helped to maintain the iron’s active surface. They
discovered that a continuous production of a trace amount of CO through the reaction can reduce
the catalyst’s deactivation [54].
Zein and Mohamed [67] used Mn/Ni/TiO2 catalysts which resulted in a 48% to 58% methane
conversion for 180 min continuously, and the catalyst was stable for 6 times regeneration. They
found that the lifetimes of catalysts are closely associated with the type of filamentous carbon
formed [55]. Chen et al. [44] also showed that nickel’s crystal size has a major effect on carbon
nanotubes growth.
Only a few surveys have been conducted for hydrogen production using cobalt in the active phase.
Generally, cobalt has a lesser usage for methane cracking, with higher costs and toxicity. However,
Takenaka et al. [2] tested some cobalt catalysts supported on Al2O3, MgO, TiO2 and SiO2 and found
that the catalyst’s performance (activity and lifetime) was firmly dependent on the support type.
They showed that Cobalt/Al2O3 and Cobalt/MgO have better catalytic properties than Cobalt/MgO
and Co/TiO2 [2].
2.1.2. Carbon-based catalysts
Carbon-based catalysts are another type of catalyst that has been widely tested in thermal methane
cracking reactions. Generally, the carbon-based catalysts are less active than the metal-based
catalysts and need a higher working temperature. However, carbon-based catalysts are almost
10
cheaper and are more resistant than metal-based catalysts,
3
and they produce more valuable
carbons, e.g., nanotube or nanofibre carbons as a by-product [56, 57].
Table 2 was collected by Ashik et al. [23], and it has been adapted based on some other recent
research references. It demonstrates a wide range of carbon-based catalysts which have been
investigated for thermal methane cracking, including coal chars, activated carbon, glassy carbon,
carbon black, diamond powder, fullerenes, acetylene black, graphite, soot, carbon monolithic
honeycomb design carbon and nanotubes.
Table 2: The effects of experimental parameters for carbonaceous catalysts.
Catalyst
Operating conditions
Max conversion
Time
Ref.
Temp.
Flow
CH4
H2
DCC-N103(P) (CB)
851
15,000a
2
--
2
[58]
870
4
--
2
900
5
--
2
925
5
--
2
950
13
--
2
CG Norit (CB)
850
20b
--
72
4
[59]
50b
--
62
4
100b
--
48
4
Fluka 05120
850
20b
--
60
4
50b
--
51
4
NORIT CG (commercial AC)
951
601a
--
95
4
[60]
900
--
85
4
850
--
68
4
Xiaolongtan char
1000
200b
96
90
2
850
69
48
2
700
29
20
2
600
10
9
2
3
no metal carbides are formed
11
[61]
ACPS
850
1764 h−1
50
48
--
[62]
850
882 h−1
35
62.51
--
850
441 h−1
21.2
77
--
850
295 h−1
16.7
81.6
--
850.1
294 h−1
13.2
83.1
--
850.1
441 h−1
22
78.3
--
850.1
882 h−1
34.9
62.5
--
850.1
1764 h−1
41.5
57.9
--
850.1
882 h−1
34.9
62.5
--
824
882 h−1
37.6
60.9
--
801
882 h−1
48.7
50.9
--
774
882 h−1
59.5
39.1
--
Nickel–Activated Carbone
550
50b
4
--
1
[63]
650
6
--
2
750
13
--
2
850
27
--
1
BP1300 (CB)
850
36 h−1
--
92
6.5
[64]
72 h−1
--
70
6.5
144 h−1
--
41
6.5
BP2000 (CB)
850
36 h−1
--
59
6.5
72 h−1
--
35
6.5
144 h−1
--
18
6.5
BP2000 (CB)
950
144 h−1
--
78
6.5
900
--
59
6.5
850
--
18
6.5
AC/Pd-5
850
54b
58
--
4
[65]
AC/Pd-10
850
52
--
4
[65]
AC/Ni-10
850
15.1
--
4
[66]
AC/Ni-20
850
52
--
4
[66]
AC/Ni-30
850
54b
61
--
4
[66]
AC/Ni-40
850
54b
75
--
4
[66]
12
Ni/SHCC
850
--
80
--
5.5
[67]
Ni/SLCC
850
50b
76
---
5.1
[67]
Ni/RC
850
50b
59
--
9
[67]
Al/RC
850
50b
60
--
5
[68]
Si/RC
850
50b
11
--
5
[68]
AC from coconut
850
--
--
13
1.5
[68]
(AC)R/SiO2
850
10b
9
--
2
[69]
(AC)R/SBA-15
850
8
--
2
[69]
(T = temperature (°C); F = flow rate (
/(gcat.h)
/min); W = catalyst mass (g); time (h); conversion
(%); tmax = time at which maximum methane conversion or hydrogen production occurs (h); t not mentioned
in the original paper).
The active-carbon and carbon-black catalysts have been investigated more than the other carbon-
based catalysts due to their high activity and greater stability [70]. Moliner et al. [60] conducted a
study on activated carbons with different surface chemistries and textural shapes. The results
showed that the mesoporous material has, on average, more activity due to having fewer diffusion
restrictions in carbon-based catalysts and therefore provides more sustainable hydrogen production.
On the other hand, microporous carbons have a highly active surface and initial activity but they are
deactivated sharply. Furthermore, the catalyst’s porosity size and surface chemisorption play main
roles in the initial rate and the long-term sustainability of the carbon-based catalyst [60]. Lee et al.
[71] employed some carbon-black catalysts for TMC processing with maximum conversion rates of
37 to 70 % at 1100 °C which were achieved for different types of carbon-black catalysts. They
concluded that at high temperatures,
4
a high range of non-catalytic methane de-carbonation will
occur which can play a catalytic role [71]. Muradov [72] researched over 30 different elemental
carbon catalysts such as carbon-blacks, activated carbons, nanostructured carbons, graphite,
synthetic diamond powders and glassy carbon. The study demonstrated that the activated carbons
catalysts have more initial activity than carbon-blacks. Catalysts with a disordered structure of
carbon (e.g., AC, CB, acetylene black) also have higher activity than the ones with an ordered
structure (e.g., diamond powder and graphite) [72]. Lopez et al. [73] used 8 types of metal-based
catalysts (nickel- and iron-based catalysts were the best metal-based catalyst types) and six types of
carbon-based catalysts to compare their performance in catalytic TMC. The carbon-black catalyst
(Vulcan type) had the best stability performance and produced four times more hydrogen than the
4
More than 1100°C
13
other metal catalysts. However, the nickel catalyst (ex LDH-II) was the most active catalyst below
750°C [73].
Gatica et al. [74] used a carbon-based monolithic honeycomb for TMC and found that there is no
relation between a monolithic structure and catalytic performance. Dufour et al. [75] employed a
wood char carbon as a catalyst and found that it can reach up to a 70% conversion at 1000 °C (in
120 minutes).
A low methane space velocity and high temperatures favour hydrogen production. Muradov et al.
[56] showed that the methane cracking mechanism of carbon-based catalysts starts with
dissociative adsorption of the CH4 inactive-sites or radicals and it leads to the generation of C and
H2 [56]. The carbon often grows at the edge of existing crystals by two processes: carbon crystal
growth and carbon nucleation [56].
Carbon catalysts that are boosted by metal doping are the area of some investigations. Some
researchers have tried to improve the activity of carbon-based catalysts by doping with a trace
number of metals. Lijun et al. [76] used an activated carbon catalyst which was supported by iron-
alumina. Adding iron and alumina reduces the pore and surface areas but creates a mesoporous
structure (about 4.5 nm) and results in higher initial conversion and stability [76]. It has also been
found that the higher activity introduces extreme loading and makes the pores blocked and covered
so that it did not perform properly for use in catalytic TMC [76].
Zongqing et al. [63] used an activated carbon that was loaded with nickel. Their experimental tests
showed higher activity compared with an AC catalyst itself and resulted in a higher conversion rate.
However, the nickel carbide formation during the methane cracking introduced a permanent
catalyst deactivation [63].
Jianbo et al. [77] used an AC catalyst with added SiO2 or SBA-15 for thermal methane cracking. It was
found that the silicate and silica enhanced the micro-porosity of carbon-based catalysts and resulted
in the formation of ordered porous carbon. The pore structures and surface areas of AC catalysts
were improved by introducing SiO4 or SBA-1 and showed a higher catalyst activity and quicker
deactivation compared with the AC on its own [77].
Sarada et al. [66] conducted some tests to find the effects of the metal’s quantity on the AC activity.
The results showed that the AC supported nickel catalysts (nickel-10, nickel-20, nickel-30 and nickel-
14
40) had more activity and that nickel-30 had the best activity and nickel-40 had the least initial
conversion among all of the AC-nickel catalysts [66]. The initial conversion of methane for the
activated carbon/lead-10 was higher than that of the activated carbon/lead-5. The higher activity of
the activated carbon/lead-10 was due to its larger surface area (245.1 /g) compared to the
activated carbon/lead-5 (44.2 /g) [65].
Hierarchical porous carbon, with Al2O3 as an additive, exhibits a significant conversion of methane
from about 28 % to 60 % (after 10 hours at 850 C°).
The temperature and the space velocity
5
are the main parameters that influence the rate of
methane de-carbonation for the carbon-based and metal-based catalysts. The methane conversion
will be enhanced if the reaction temperature is increased, however, there is a limit on increasing
the temperature for each catalyst as it can damage the catalyst’s structure and composition. It has
been found that the methane conversion is increased as the space velocity of the methane feeding
decreases [78].
The results have also shown that a higher partial pressure of methane causes an increase in the
conversion rate [78]. At higher partial pressures of methane, the catalyst produces a greater amount
of carbon deposit for a smaller period [79], which is because of the higher diffusion rate of the CH4
into the inner part of the particles.
Meanwhile, the distribution of pore sizes affects the activity of carbon-based catalysts. For example,
a microporous (<2 nm) activated carbon deactivated very quickly due to the small diameter of the
micro-pores which are blocking with carbon first [80]. The studies on AC have shown that methane
conversion increased mainly within the AC microspores [62]. It has also been found that the
mesoporous activated carbons (2–50 nm) have larger surface areas which create better hydrogen
production and stability [60]. Botas et al. [80] employed ordered mesoporous carbons and showed
that the activity of catalysts such as CMK-3 and CMK-5 are more active compared with some other
carbon-based catalysts like CB or AC. Zhang et al. [81] also used a mesoporous carbon-based catalyst
which was prepared from direct coal liquefaction residue. The results showed a higher initial
conversion compared to commercial carbon-based catalysts for thermal methane cracking. An acid
washing was used to prepare the mesoporous catalysts. The catalyst washing caused the elimination
5
Volume of gas feed per hour per volume of catalyst
15
of the mineral and it improved the porosity structure.
6
The washing operation makes it mesoporous
on the surface of the catalyst by eliminating the soluble salt [81].
2.1.3 Catalytic deactivation
Very fast catalyst deactivation is a basic issue for catalytic methane cracking. Chocking,
encapsulation, poisoning, sintering and chemical and mechanical degradation are the main reasons
for the deactivation [82-84]. There have been several studies that have focussed on catalytic
deactivation and the related parameters for the catalytic decomposition of methane. The
mechanisms and reasons for a catalyst’s deactivation, for both the metal and carbon-based
catalysts, will be discussed below.
➢ The deactivation of metal catalysts
Generally, many parameters can play a role in the deactivation of metal catalysts however, the main
reason is the coking or encapsulation of active sites by carbon [85]. Shah et al. [53] found that
surface poisoning or re-crystallization are not the main reasons for a catalyst’s deactivation, and
mostly it is because of follows reasons:
- Catalyst oxidation
- Catalyst reduction
- Catalyst carburization
The operating conditions like the flow rate or space velocity, the temperature of the methane and
the hydrogen partial pressures, together with the catalyst’s structure also have strong impacts on
the catalytic deactivation [86, 87]. There is wide research on the reaction flow rate and temperature
impacts on catalytic deactivation, and the results show that both have a high influence on the
deactivation. It has also been found that the volume of the deposited carbon and the morphology
are affected by the reaction temperature [88].
Ermakova et al. [86] researched to study the effects of the structures of catalysts for catalytic
methane cracking. The results showed that the pores of the oxide matrix of a catalyst must be
nearby to molecules of the catalyst’s textural promoter to maximize the catalyst’s stability [86].
Villacampa et al. [87] also found that H2 competes with CH4 in the catalyst’s active sites to prevent
the encapsulation and formation of carbon filaments, which deactivates the catalyst. Thus, an
6
Total pore volumes (Vt) increased by 956, 1597 m2 g−1 and 0.59, 0.91 cm3 g−1, respectively.
16
increase in the concentration of the methane in the reactor feed leads to higher conversion and
catalyst deactivation [87].
The carbon encapsulating at low temperatures is slow, and there is enough time for full access of
the carbon atoms to the whole of the catalyst support by diffusion, and to prepare an opportunity
for new filament nucleation.
Moreover, a higher feed space velocity or flow can change the balance of both the nucleation and
diffusion of the carbon, therefore the carbon nucleation rate is faster at higher flow rates (or space
velocities). The most active site could be filled with the higher nucleated carbon and creates a
quicker catalyst deactivation. Thus, a lower flow or space velocity leads to a higher conversion rate
[84].
Suelves et al. showed that the sintering of nickel catalyst doesn’t include a considerable impact on
catalytic deactivation. Thus, the catalyst deactivation could not be the result of sintering during the
catalytic de-carbonation of the methane which was generated in the catalytic bed [8].
➢ Deactivation of carbon-based catalysts
The encapsulation of the carbon-based catalyst’s active sites is the main reason for their
deactivation as well. The carbon which is generated by the thermal catalytic decomposition of the
methane can have a structural order between that of graphite and amorphous. An amorphous
structure possesses higher activity and graphite shows a weak activity [62]. A carbon-based
catalyst’s deactivation involves the conversion of an amorphous state to a non-amorphous state
(such as graphite).
Lázaro et al. [64] investigated the mechanisms of carbon black deactivation. The results revealed
that non-porous carbon sediment grows on the mouth of the holes (pores) so that the holes are
gradually plugged with the generated carbon, making a non-porous material. Furthermore, the
experimental tests showed that at high temperatures and low space velocities the catalyst’s activity
will be increased. The authors found a linear relationship between the quantity of carbon
sedimented and the total porosity [64].
Moliner et al. [60] surveyed the influence of surface chemistry and textural properties on carbon
catalyst deactivation. The results revealed that the pore size distribution and the surface chemistry
had influenced the initial conversion rate and the catalyst deactivation phenomenon, respectively.
Catalysts with a microporous structure and a high amount of oxygenated surface groups were found
17
to have higher initial conversion rates but were deactivated very quickly. On the other hand,
catalysts with mesoporous structures, with more surface area, leads to a higher hydrogen
generation [60].
The molecular sieve impact in carbon catalysts is related to pore mouth blocking. Due to carbon
sedimentation, the pore mouths of catalysts are decreased, and the catalyst’s interface becomes
unreachable for CH4 adsorption. The diffusion effect can also be explained by the methane diffusion
phenomenon on the insides of tiny sized pores. The diffusion rate increases with temperature which
causes an increase in the deposition inside the pores. Hence, the thermal catalytic decomposition
of methane mostly takes place inside the pores at high-temperature ranges, where most of the high-
energy activated carbon is located [71].
2.1.4. The effects of co-feeding on catalyst stability
One of the biggest issues for thermal catalytic CH4 de-carbonation, for both the carbon-based and
metal-based catalysts, is the deactivation of catalysts because of the encapsulation of their active
sites by carbon. As previously discussed, the deactivation of a catalyst is because of three separate
mechanisms. Firstly, the dilution of the active sites on a catalyst’s surfaces by the formation of coke
which leads to a low-level deactivation. Secondly, the active phase, where the covering creates a
high-level deactivation and, finally, a full-level deactivation is caused by the formation of coke which
grows on the catalyst’s particle [29]. It has been found that carbon-based catalysts have higher
stability against the deactivation phenomenon compared to metal-based catalysts.
Methane co-feeding (with other organic materials) has been examined by some researchers as a
solution to inhibit catalyst deactivation. Methane co-feeding with other organic materials have been
used to analyse the reactions of the generating carbon-based sediments which have a higher activity
(compared with methane cracking sediment). It has been found that co-feeding into the reaction
system can further increase CH4 de-carbonation by inhibiting catalytic deactivation [52]. The
activities of the carbon deposits produced by different hydrocarbons are as follows [89, 90]: C
benzene > C acetylene > C ethylene > C propane > C methane.
Muradov tested ethylene and other hydrocarbons to survey the effects of methane co-feeding on
catalytic TMC [52, 75]. Pinilla et al. [91] used undiluted propane and ethane for thermal de-
carbonation at temperatures of higher than 850 °C by using a CB as a catalyst and the results showed
that the ethane and propane can produce an enriched stream of 40 % hydrogen. It was found that
18
the co-feeding of methane with ethane and propane leads to more hydrogen being produced in the
reactor and helps catalyst stability or lessens catalyst deactivation [91].
Malaika et al. [89] employed ethylene as a methane co-feed for the catalytic TMC process. The
results showed a combination of ethylene co-feeding can improve the catalyst’s deactivation
properties.
Rechnia et al. [92, 93] used ethanol as a methane co-feed for catalytic TMC. The results revealed a
better catalyst efficiency and lower deactivation. However, the disadvantage of using ethanol in
methane co-feeding is the production of carbon oxides in the reactor as an impurity within the
hydrogen. Pinilla et al. [94] also employed a mixture of CH4-CO2 as a feed in a reactor with a nickel-
alumina catalyst (600 to 700 °C). The presence of carbon dioxide in a reactor can turn the graphite
layer on the catalyst into carbon monoxide (gasification process) so that it will delay a catalyst’s
deactivation. However, its disadvantage is the production of carbon oxides in the reactor as an
impurity within the hydrogen as well.
The co-feeding of methane has been employed on activated carbon, carbon black and metal
catalysts in the thermal catalytic decomposition process. Among these, the activated carbon catalyst
has had more attention because of its considerable activity, while the carbon black and metal-based
catalysts have revealed low enhancement in catalytic stability compared to activated carbon [7].
Methane co-feeding can improve a catalyst’s deactivation properties however it is not effective
enough to prevent deactivation in an acceptable period. As discussed, co-feeding can bring some
impurities into the hydrogen stream which then need another unit operation for hydrogen
purification. Therefore, methane co-feeding is not an acceptable solution to catalyst deactivation in
the TMC process.
2.1.5. Catalyst regeneration
Practically, the catalysts which are employed for the thermal cracking of methane do not have
permanent activity as carbon deposition, carbon encapsulation, sintering and poisoning make a
catalyst’s deactivation quick. As discussed, the co-feeding methods were not a successful way to
prevent a catalyst’s complete deactivation. Finally, carbon sediment is an unavoidable by-product
of the thermal cracking of methane which encapsulates a catalyst’s active sites and causes a
deactivation. Therefore, the regeneration of deactivated catalysts and/or the replacement of spent
19
catalysts are two solutions to continue cracking methane. The gasification and combustion of the
deposited carbon are two processes that have been employed for catalyst regeneration. Air
(oxygen) is used for the burning of carbon [87] and carbon gasification can use carbon dioxide [95]
or steam [96], as per the following equations:
C+O2→CO2, ΔH=−394.7kJ/mol Equation 6
C+H2O→CO+H2, ΔH=135.9kJ/mol Equation 7
C+CO2→2CO, ΔH=174.5kJ/mol Equation 8
During the regeneration process, the carbon will be eliminated from the catalyst’s surface and the
surface area will be restored.
A gasification method can be employed for the regeneration of both metal-based and carbon-based
catalysts. The key advantage of the gasification method is that it avoids thermal shock or catalyst
oxidation. Furthermore, gasification by steam produces a higher range of hydrogen compared to
the oxidation method. In this method, about 1.4 moles
7
of H2 will be produced due to the reaction
of the H2O with C, leading to the generation of an overall 3.4 moles of H2 for each mole of CH4 [97].
In steam gasification, the process can be defined as a steam reforming process that has two steps.
Only pure H2 is generated in the first step, while in the second the H2 is mixed with CO and CO2.
Amendola et al. [121] considered three strategies for catalyst regeneration, including CO2
gasification, steam regeneration and carbon oxidation. They compared the results based on the
regeneration period, coke removal efficiency and catalyst performance. The results showed that the
gasification process required a longer regeneration period and that they could also not eliminate all
of the deposited carbon on the catalyst (by steam or CO2). Therefore, it was not possible to produce
non-stop and stable hydrogen by employing the gasification regeneration method. While the
combustion method was found to be faster and was able to remove all of the deposited carbon on
the catalysts, it has a catalyst re-oxidation problem which makes permanent catalyst deactivation
take a while [121].
7
Two moles of hydrogen were obtained from each mole of methane during the cracking step and an additional 1.4
moles were produced during the subsequent steam gasification of the deposited carbon, leading to an overall
hydrogen yield of 3.4 moles for each mole of CH4.
20
Pinilla et al. [95] used CO2 gasification for carbon-based catalyst regeneration. The results revealed
that temperature can play an important role in the efficiency of regeneration. As Figure 3 illustrates,
the efficiency of the regeneration can be increased by more than 20 % with an increase of 25 °C in
the regeneration temperature, while the regeneration time was halved [95].
Generally, a medium temperature (about 750 °C) is better for methane cracking due to less carbon
being generated inside of the catalyst’s structure, and a higher level of temperature (950 to 1000 °C)
is better for the regeneration process as it can remove most of the carbon deposited over the
catalyst [82, 98].
Figure 3: Production of hydrogen after regeneration: (a) 900 °C and 4 hours, and (b) 925 °C and
2 hours [95].
The carbon combustion regeneration eliminates all of the carbon produced on the catalyst with
higher performance. The merit of combustion regeneration is the released heat which provides heat
for the cracking reaction and it is faster than the gasification method. Nevertheless, the
disadvantages which come from a higher regeneration temperature are catalyst degradation,
oxidation and sintering. Lower CH4 concentrations are better for the combustion regeneration since
they produce less dense carbon, which could avoid heat being released over the catalyst. On the
other hand, a lesser concentration of oxygen is better for the combustion method due to the
combustion with more oxygen density would raise the temperature, which could lead to catalytic
21
deactivation (oxidation of active sites). It has also been found that a catalyst with a lower density of
metal (wt.%) is favourable for the carbon burning (combustion) method [14].
Zhang et al. [120] tested a Ni-SO4 catalyst for the de-carbonation of methane and also employed
steam for catalyst regeneration. The results showed that at 923K a full regeneration can be
obtained. However, a small amount of carbon remained in the gasification process in small pockets
but with no high-level decrease in the CH4 conversion being observed after 10 regeneration cycles.
Air combustion was also used by them to regenerate a catalyst at 550°C. The catalyst was fully
regenerated, but during the oxidation process, the high temperature affected the catalyst through
degradation and the production of a fine powder [120].
Otsuka et al. [111] employed a combustion method for the regeneration of lead-nickel/silica,
nickel/titanium oxide and nickel/alumina catalysts and observed a full catalyst recovery at higher
than 773 K for five regeneration cycles. Partial regeneration can be a preferred way to overcome
the problems of catalyst degradation or sintering and oxidation during the regeneration process,
but in the partial regeneration process, it was found that the carbon filament is always difficult to
oxidize and the encapsulated carbon only is removed (oxidized) [99].
Rahman et al. [60] used a partial regeneration method for catalyst regeneration. A stream with five
% oxygen was used and the results showed that the efficiency of the carbon removal decreased
after each regeneration cycle, so that the total catalyst mass dropped a maximum of 30 % in the
first cycle, 28 % during the second cycle, and a 23 % drop for third regeneration cycle [60].
2.1.6 Industrial plants by using catalytic TMC process
The HYPRO project
8
introduced the first commercial process for methane catalytic thermal de-
carbonation [100]. A fluidized bed reactor was employed in the HYPRO process which used a nickel-
alumina catalyst (at atmospheric pressure and 980 °C temperature). The HYPRO process could
convert light hydrocarbons (pure methane or mixed) to 90 % H2 with 10 % unreacted CH4. Carbon
mono-oxidation was used for catalyst regenerating in another fluidized bed reactor and the heat of
the reaction was used to supply the energy requirements of the process [62]. The capital costs of
this plant were less than the methane steam reforming methods [62, 100], but the operating costs
were higher [101]. In the process, the biggest problem for continuous production was the
regeneration of the spent catalyst, which was essential from a feasible economics aspect [101].
8
by Universal Oil Products in the 1960s
22
The Hazer Group is an Australian company that developed a pilot plant for the catalytic TMC process.
They used an iron-based catalyst for its low price and magnetic properties. The distinguishing
features of the Hazer process, compared to existing hydrogen production technologies, including
the use of low-cost iron ore fines as a catalyst for the process and the co-production of high purity
graphite. The graphite produced by the Hazer process has highly crystalline 'synthetic' graphite and
high purity (>90%wt). The technology utilizes unprocessed Fe ore as a low-cost catalyst for the
thermal methane decomposition which produces hydrogen and graphite. Using a low-cost,
disposable catalyst with excellent magnetic properties eliminates the need for re-use and re-
activation, and enables the recovery of the produced graphite. The process uses moderate
temperatures (800 to 900 °C) inside a single reactor. The feeding, separating and disposing of a large
amount of catalyst can be a significant issue for small or medium-scale plants. Meanwhile, with this
technology, the conversion cannot be completed due to the presence of some oxide components in
the iron ore, which can produce COx or other impurities which needs purification units for the
production of pure hydrogen.
2.1.7. Overall conclusions on catalytic thermal decomposition of methane
The thermal catalytic cracking of methane could be a most promising and beneficial process for the
production of very pure hydrogen in small or medium-sized hydrogen plants and use in fuel cell
applications. This section of the review has provided a general assessment of the different types of
catalysts, with or without co-feeding, and of the regeneration of deactivated catalysts. Metal,
carbon and metal-doped carbonaceous catalysts have been studied. It has been found that the
carbon-based catalysts and the metal-doped in carbon-based catalysts have higher stabilities and
lesser deactivation rates compared to the metal-based catalysts, but they have lower conversion
rates. The nickel-based metal catalysts and nickel-doped carbon catalysts also have better activity
than others. The results of most of the studies showed that a low flow rate (space velocity) and high
temperature have better results for reaching a high purity of hydrogen. It showed that the initial
catalytic activity depends on the catalyst’s long-term activity and the chemical structure
corresponds to the catalysts physical characteristics. Many types of research have been conducted
on deactivation mechanisms, but most of them have concluded that carbon encapsulation is the
main reason for deactivation. The co-feeding of CH4 with other hydrocarbons to produce more
active carbon (in carbon-based catalysts) and the regeneration of spent catalysts are two practical
23
methods to overcome the deactivation obstacles. An addition of 40 % ethanol (C2H5OH) to CH4
increases hydrogen production 10 times more than without it. In addition, the merits and demerits
of the regeneration processes of combustion with air (oxygen), carbon dioxide and steam have been
surveyed. The review has shown that steam regeneration (gasification) increases hydrogen content
in the post-reaction stream by its contribution of 1.4 moles per mole CH4. However, oxygen is
twenty times more efficient than carbon dioxide in the regeneration process [11]. Oxygen fully
regenerates some catalysts such as CuAl2O4 by removing most of the deposited carbon. However,
catalyst sintering and the oxidation of active metal sites can be some of the side effects of
combustion regeneration. Despite the regeneration process restoring most of the primary activity
of the catalyst, the regenerated catalyst can have a quick or permanent deactivation in the following
de-carbonation cycles. Eventually, a separation unit or purification process is unavoidable due to
the presence of un-reacted CH4 in the out stream or other co-components, including CO and CO2,
in the case of co-feeding and regeneration. The purification process and the separation units
necessary in thermal catalytic cracking eliminate the advantages of this process for hydrogen
production in small or medium-sized plants compared to other processes like methane steam
reforming. The Hazer Group has recently developed a new process for catalytic methane cracking
by employing the low price and magnetic properties of some iron-based catalysts. The Hazer process
utilizes unprocessed iron ore as a low-cost catalyst for the decomposition reaction which eliminates
regeneration and re-uses the catalyst while enabling the collection of the graphite produced. On the
other hand, the feeding, separating and disposing of a large amount of catalyst can be a significant
issue for small or medium-sized plants. However, with this technology, the conversion will not be
completed and due to the presence of some oxide components in the iron ore, it is most likely to
produce CO and CO2 or other impurities which need purification units for the production of pure
hydrogen.
In light of the results of the latest studies and experimental tests, despite the catalytic cracking of
methane being a promising and beneficial method, it has not a viable process for the production of
pure hydrogen in small or medium-sized plants yet, and there are still some knowledge gaps for
industrial development.
2.2. Non-Catalytic thermal methane cracking
Using a catalyst reduces the working temperature, but it also creates other problems, especially
catalyst deactivation (because of the carbon sedimentation), and particularly a decline in catalyst
24
activity after several regeneration cycles which leads to permanent deactivation. On the other hand,
methane conversion is not completed in most of the catalysts which oblige the employment of
purification and separation units for the production of pure hydrogen. However, in non-catalytic
thermal methane cracking, as Figure 4 shows, the conversion rate is more than 99 % at above 1200
°C. Meanwhile, Figure 5 shows that the working temperature of some of the catalysts are so high
that they are close to the non-catalytic working temperature range. Therefore, a non-catalytic
reaction can be a sound alternative to CH4 de-carburation without the catalyst limitations and
difficulties [102, 103].
Figure 4: Hydrogen mole fractions attained during the thermodynamic equilibrium of the CH4 cracking at
different temperatures and atmospheric pressure [104].
Figure 5: Non-catalytic and catalytic types are employed for the thermal cracking of methane and their
temperature range [104].
25
Generally, methane thermal de-carbonation is an endothermic reaction in which the (minimum)
starting temperature is about 300 °C at atmospheric pressure. A full methane conversion takes place
at higher than 1200 °C in the case of kinetic and thermodynamic conditions.
2.2.1 Non-catalytic thermal methane cracking mechanisms and kinetics
Although CH4 is the simplest hydrocarbon, the mechanisms of its cracking reaction are unclear and,
despite many kinds of research conducted in this area, an accurate characterization of the
mechanisms has not yet been reported. Because the C-H bonds of CH4 are significantly greater than
the C-C bonds of the products, other sub-reactions play an important role in the first stages of the
reaction and make the details unclear [105].
During the cracking process, the carbon produced as a by-product can play a catalytic role in the
reactions [106]. As discussed in the last section of this study, the presence of carbon black results in
an increase in the conversion of CH4 and hydrogen yield. This is because of the reactor particles’
surface areas which serve as reaction sites and strongly affect the reaction rates [107, 108].
However, regardless of the catalytic effect of carbon in the reactions, the mechanisms of the
reactions in higher temperatures (more than 1000 °C) can be different than with lower
temperatures (less than 1000 °C). The following tries to show the mechanisms of the reactions at
low and high temperatures which have been proposed by several references.
❖ Methane thermal cracking reaction mechanisms at lower temperatures:
The following mechanisms are primary reactions and produce ethane and hydrogen [108].
Equation 10
Equation 11
Equation 12
……………………………………………………………
Equation 10 is the rate controller and after that Equation 11 takes place faster. In the early steps
of the decomposition, Equation 10 is the only radical source. Secondary reactions of ethane:
26
Equation 13
Equation 14
Equation 11
…………………………………………………………………
Equation 13 is the rate controller and its reverse reaction is mostly small [108].
Secondary reactions of ethylene:
Equation 15
Equation 16
Equation 11
……………………………………………………………
Radical chain methylation:
Equation 17
Equation 18
Equation 11
……………………………………………………………
Secondary reactions of acetylene:
Equation 19
Equation 20
27
Radical chain methylation:
Equation 21
Equation 22
Equation 11
……………………………………………………………
Secondary reactions of propylene:
Equation 23
Equation 24
Equation 11
……………………………………………………………
Radical chain methylation:
Equation 25
Equation 26
Equation 11
……………………………………………………………
28
❖ Thermal methane cracking reactions mechanism at a higher temperature:
The thermal methane cracking reactions at higher temperatures (more than 1200 °C) takes place
very quickly. The H2 and carbon formation are explained by a free radical mechanism. however, the
details of the next steps (which make a higher conversion rate) and the carbon formation are not
yet completely clear [109].
The initial formation of the C2H6 and H2 are illustrated as follows:
Primary formation of ethane and H2:
Equation 10
Equation 11
Equation 12
……………………………………………………………
Equation 10 8 is the rate-controlling stage and the source for free radicals in the first stages. The
uni-molecular de-carbonation shifts to methyl radicals (Equation 11) [109]. The secondary reactions
of the ethane are suggested by the uni-molecular cracking, the reverse of Equation 12, and by a
free radical chain mechanism as the following reactions:
Secondary reactions of ethane:
Equation 27
Equation 13
Equation 28
The ethyne formation could be shown by the following equations [110]:
Secondary reactions of ethylene:
Equation 29
Equation 30
29
Equation 31
Propene production can be shown by the equations below, starting from the ethane methylation
[109]:
Equation 32
Equation 33
The primary gasses produced during CH4 decomposition are ethane, hydrogen, ethyne and ethene.
Furthermore, the formation of propene and small amounts of C3H4 (allene and propyne) can be
defined by the secondary reactions of ethyne and propene. Equation 34 has been considered to be
the most important source for the formation of C4 hydrocarbons [109]:
Equation 34
Benzene could be the main intermediate product based on the reaction conditions [109].
Equation 35
Holmen et al. described a model for the production and consumption of the main hydrocarbons at
both high and low conversion rates of CH4 in the temperature range of 1196-1450 °C (as shown in
Table 3) and the results of the experimental tests and the model predictions were almost close.
There was a minor deviation between the experimental data and the simulated results at low
conversion rates and low temperatures which could have been because of the faults in the
monitoring of the gas temperature or gas residence time or the influence of deposited carbon in a
catalyst role[109].
30
Table 3: Mechanisms and rate constants for the thermal cracking of methane at 1473 K
9
[109].
Reaction
A
E
3.5lE + 15
0.0
104000
2.25E + 04
0.3
8768
1.01E + 15
_ 0.64
0
5.54E + 02
3.5
5174
0.55
4.0
8296
2.00E + 13
0.0
39700
1.00E + 16
0.0
32000
6.62
3.7
9512
2.00E + 11
0.0
7170
1.32E + 06
2.53
12258
1.93E + 28
- 4.783
51123
l.00E + 13
0.0
0
1.58E + 16
0.0
38000
l.00E + 15
0.0
88000
1.16E + 10
0.0
43200
5.00 E + 13
0.0
35000
l.00E + 13
0.0
0
5.00 E + 12
0.0
1500
9
Modified Arrhenius equation: k = A.T exp( - E/RT).
31
1.26E + 13
0.0
0
5.00E + 11
0.0
7315
6.02E + 13
0.0
22300
1.81E + 11
0.0
17300
l.00E + 14
0.0
15000
l.00E + 14
0.0
41400
1.81E + 14
0.0
0
l.l0E + 12
0.0
4000
1.80E + 12
0.0
10400
6.02E + 12
0.0
9000
l.00E + 16
0.0
108000
2.71E + 11
0.0
23400
3.07E + 13
0.0
0
7.94E + 12
0.44
88760
9.64E + 13
0.0
0
6.20E + 11
0.0
20000
8.00E + 12
0.44
81150
1.58E + 12
0.0
8800
At higher temperatures ranges (1250°C to 1600°C), the CH4 and other hydrocarbons cracked to yield
and solid carbon. Holmen et al. [109] showed that a higher temperature and more residence
time produce the reactions to form and carbon as a final product. However, lower
temperatures and short reaction times lead to higher yields of aromatics and olefins. A low pressure
favours a higher conversion rate but it also leads to the generation of more olefins which will
32
decompose to carbon and with more reaction time in the reactor. Figure 6 presents a
conceptual graph of the CH4 conversion for thermal methane cracking as a function of temperature,
contact time and pressure [111].
Figure 6: Conceptual graph for methane conversion in reactor conditions [111].
Wullenkord et al. [107] investigated the effects of the reaction times on the conversion of methane at
different temperatures and in atmospheric pressures, as shown in Figure 7 below.
Figure 7: Methane conversion and yield of hydrogen as a function of the residence time [107].
33
2.2.2 Using tubular reactors for TMC
In 2011, a research project at CIEMAT
10
under the supervision of Professor Carlo Rubbia, the winner
of the 1984 Nobel Prize in Physics, was conducting experimental tests on tubular reactors regarding
the thermal decomposition of methane[112]. In this study, two models of tubular furnaces were
employed for the CH4 de-carbonation. The first furnace supplied heat to a maximum of 1100 °C
which was used to study methane conversion at various temperatures. Hydrogen and argon as
dilution gases were used as sweeping gases to prevent blockages and also to adjust the residence
times. Another tubular furnace that supplied heat up to 1700 °C was employed in a second step to
reach full conversion of the methane (100% methane de-carbonation) at a very high temperature
(1100 to 1700 °C). Figure 8 illustrates the experimental schematic, including the inert gas system
and a two-layer tube structure so that an alumina tube is an outer tube and a porous graphite tube
is located inside of it. The inert gas (He, Ar or H2) was injected into the CH4 line via the porosity of
the inner tube which was proposed to avoid a tube blockage by carbon deposition.
Figure 8: Schematic of the experimental setup for CH4 de-carbonation [112].
The first experimental results for the first reactor are presented in Figure 9 which shows the amount
of H2 in the reactor outlet, with three residence times and three flow rates in the atmospheric
pressure.
10
“Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas”, Madrid, Spain.
34
Figure 9: Hydrogen conversion rate as a function of the flow, residence time and temperature [112].
The results revealed that the production of hydrogen increases with the temperature rising. The
hydrogen conversion rate reaches a maximum of 55% at below 1000 °C and at a 96 second residence
time. Meanwhile, the reaction’s kinetics are faster in higher temperatures so that the residence
time is negligible. In temperatures higher than 1450 °C there was almost a full conversion of
methane (residence times above 0.2 seconds). At higher than 1350 °C, the cracking reaction did not
have a minor dependence on the residence time and the methane converted just to hydrogen
without any other intermediate products.
In this study, different tube porosities were employed to test whether a plug of deposited carbon
could be prevented or not and the results indicated that it is not an effective method to avoid the
formation of carbon plugs. There was a carbon plug formed in every one of these experimental tests
after a certain time (30 minutes to 4 hours due to the reactor temperature and hydrogen flow rates).
At higher temperatures, which leads to higher H2 and carbon generation, a carbon blockage
occurred sooner (see Figure 10). Some carbon dust came out from the reactor via the gas flow. A
carbon layer formation was also observed on the external face of the tube due to the penetration
of carbon into the tube’s pores, as shown in Figure 11.
35
Figure 10: Carbon plug phenomenon obtained from Gr-4 experiment cross-section [112].
.
Figure 11: Carbon sediment on the external surface of the porous tube reactor [112].
Carbon blockages create operational problems. Therefore, a silicon carbide reactor (SiC) was tested
to assess whether the carbon blockages could be prevented by changing the tube material but the
tests proved that it had only a minor success (Figure 11). The experiments showed that at a
temperature of 1200 °C a soft carbon formed along the tube reactor and that it can be easily
removed (see Figure 12), while the carbon which is deposited at a higher temperature (above 1350
°C), or in the hottest zone of the tube reactor, was very hard [112].
36
Figure 12: Soft carbon plug formation on the reactor at 1200 °C [112].
2.2.3 Using liquid metals based reactors for TMC
Carbon blocking is the major issue for pipe type reactors for thermal methane cracking. On the other
hand, providing the heat for the endotrophic reaction (de-carbonation of methane) is another
problem with pipe reactors due to the very high temperature and the difficulty of selecting a
material with a high range of conductivity.
A solution for steady-state methane cracking without reactor clogging involves the use of molten
metals as a heat transfer media in the reactor. Additionally, liquid metals can play a role as a catalyst
in the de-carbonation of methane.
In 1930, Tyrer et al. [113] conducted a process for thermal catalytic cracking of methane by using
molten Fe (at a temperature range of 1250–1300 °C). Martynov et al. [114] used a reactor that was
filled with molten Bi-Pb to de-carbonate methane and more than an 85 % conversion rate at 700 C°
was achieved in their reactor ( Figure 13).
37
Figure 13: Molten Bi-Pb reactor de-carbonation of methane[114].
Paxman et al. [115] conducted some experimental tests for CH4 de-carbonation with a molten
metal reactor. Their motivation was to employ a prototype solar reactor by using molten metal for
the thermal methane decomposition (see Figure 14). A 69% conversion rate using molten Sn was
achieved at 1100 C° [115].
Figure 14: Schematic of Liquid Metals Based Reactors for methane decarbonization [115].
An investigation into methane thermal cracking by employing molten tin through a capillary reactor
in a slug flow was conducted by a Dortmund University researcher [116]. The test was conducted in
a quartz-glass reactor with 2 mm ID. A special advantage of capillary reactors over the bubble
38
column reactors is the possibility of adjusting the residence time based on the de-carbonation
reaction demands. However, a low-level conversion rate (average 32 %) was reached at 1100 °C.
The reactor was working for more than 5 hours continuously and there was no carbon deposit
observed in the reaction zone [116].
The density difference between the molten metals and the deposited carbon is the main mechanism
for the separation of the carbon. Generally, the employing of a molten face in the thermal de-
carbonation of methane is promising, although more experimental testing is needed to confirm its
reliability for industrial scales.
Plevan et al. [104] conducted a study of thermal methane cracking by using a molten-metal bubble
column reactor. The study investigated the effects of the flow rate at a variety of three different
nominal temperatures for CH4 conversion. The results showed that molten tin had no catalytic effect
in the melting point temperature range, but that it can reduce the formation of intermediate
products. The results also showed that the majority of the CH4 conversion occurred in the gas phase
above the molten phase.
Shimotake et al. [117] conducted a study into the dynamic corrosion of tin in a stainless steel bubble
column reactor. Corrosion is the main issue when molten metals are employed for the fabricating
of reactor bodies (see Figure 15). The corrosion rates for the Sn at a molten phase were very high
when using stainless steels reactor tubing, and were of the order of 30 μm/h at 700 °C [117].
Figure 15: Stainless steel reactor tube cross-sections at different heights: (a) from a low-temperature lower
section, and (b) from a high-temperature upper section showing reduced wall thickness due to corrosion by
molten Sn [118].
39
In 2015, Geibler et al.[118] conducted some experiments in a molten metal-packed bed reactor
which employed a combination of stainless steel and quartz glass at working temperatures between
825°C and 1000 °C. The photograph and a detailed schematic of the reactor are shown in Figures 16
and 17.
Figure 16: Photograph of the methane cracking experimental facility [104].
40
Figure 17: Schematic of the reactor with the Sn filling system (left) and dimensions (right) [118].
In the Geibler et al. results, the hydrogen production increased as the CH4 flow rate declined which
was because of an increase in the gas contact time which creates a higher conversion rate. A
maximum hydrogen concentration of 45 % and a 30 % methane conversion rate were reached at
1000 °C (see Figure 18).
Figure 18: H2 generation as a function of inlet CH4 flow rate and temperature in the experimental reactor:
(a) measured H2 mole fraction in the product gas, and (b) estimated H2 flow rates [118].
Based on Geibler et al.’s modelling [118], the hydrogen conversion as a function of the bubble
reactor residence time for a range of molten temperatures with steady kinetic parameters and a
steady bubble diameter is shown in Figure 19. Based on the results, with an increase in the
temperature of the molten metal the H2 yield (as the methane conversion) will be increased. Due
to the methane temperature being lower than at a molten phase, the heat transfer between the
41
molten phase and the bubble’s surface is through convection and conduction phenomena inside of
the bubble. A heat transfer is essential for reaching the required temperatures for the reaction to
take place. Figures 19 and 20 show the bubble temperatures at the centre as a function of the
bubble contact time for various bubble diameters with a constant bubble surface temperature [118].
Figure 19: Hydrogen conversion as a function of the bubble residence time for different molten phase
temperatures (db = 3 mm, Tb,0 = 24.85 °C) [118].
Figure 20: Bubble centre temperature as a function of the bubble residence time for different bubble
diameters (TLM = 900 °C, Tb,0 = 24 °C) [118].
42
For a full conversion rate, it is necessary for the temperature of the cracking reaction to being more
than 1100 ° C. At higher temperatures, the endothermic reaction is quicker than the heat transfers
inside of the bubbles, therefore the endothermic reaction will consume the energy at the bubble’s
boundary at the inner face. Thus, the heat transfer into the centre of the bubble is a limited step for
a full conversion [118].
The operating pressure is another reaction parameter that can impact the CH4 conversion rate.
There is a hydrostatic pressure change due to the reactor’s height which makes an operating
pressure inside the reactor. Theoretically, an increase in pressure has an adverse impact on the
conversion rate [118]. Based on Geibler et al.’s results (Figure 21), the conversion is highly
dependent on the bubble contact time and molten temperature and the reactor pressure has no
significant influence on the conversion rate (around 200 KPa) [118].
Figure 21: Conversion (H2 yield) as a function of the bubble residence time for different initial pressures,
bubble diameter = 3 mm (TLM = 1100 °C, Tb,0 = 24 °C) [118].
43
3. Solar reactors
Many researchers believe that industrial heating for commercial thermal methane cracking
processes can come from solar energy in the future [119]. Solar reactors usually have a cavity-
receiver type configuration which is body insulated with refractory materials so that they just have
a small opening for contact with concentrated solar radiation. Abanades et al. [120] recently tested
an aerosol tubular quartz reactor system that employing fine carbon-black particles suspended in a
feed stream in a solar furnace [120]. Feeding the carbon particles into the feed stream increased
the heat transfer by direct absorption of concentrated solar radiance.
To reach temperatures of more than 1200 °C, a very high sunlight concentration factor (about 3000)
is required [121]. Thus, using a solar reactor needs a massive facility with mirrors and structures,
and the location of the plant plays a significant role in its performance. Therefore, for an application
in a small or medium-sized plant for hydrogen production, using a solar reactor cannot be a proper
option.
4. Theoretical and numerical studies for modelling TMC reaction and reactors
This part of the review evaluates the results and suggestions obtained from some of the theoretical
and numerical researches to model the thermal methane cracking reaction and reactors. The results
obtained in this section can reveal the techniques and practical solutions that can be employed to
overcome the current knowledge gaps for developing the thermal methane cracking reactors on
small and medium scales. Most studies and simulations have been in the use of solar reactors for
non-catalytic methane cracking at high temperatures. Increasing the performance and efficiency of
the reactors along with preventing the reactor plugging have been the main objectives of the studies.
Abanades et al. [119] modelled non-catalytic thermal methane cracking in a solar reactor. A
graphite nozzle-type receiver was considered to absorb the solar heat and handovers to the
methane flow for cracking to hydrogen and carbon black. The operating conditions were obtained
in the experimental tests and then the reactor was modelled by a Computational Fluid Dynamic
Modelling (CFD). The CFD was employed to forecast the mapping of reactor temperature and
concentration of species (Figure 22). Most of the reaction has taken place at the highest heat
exchange zone located on the walls that directly receive the concentrated solar flux and the
temperature had a great influence on the reaction progress. The results revealed that the nozzle
44
geometry and gas flow rate, concentration and temperature have a strong outcome on the general
conversion rate. It also showed that using a simple tubular receiver is not sufficient to heat the bulk
gas in the middle zone of the reactor and results in a lower conversion rate and the reaction will be
just concentrated on a thin layer close to the reactor wall (Figure 23). The study showed a 98%
conversion rate will be achieved by an improved feed nozzle [119].
Figure 22: Profile of gas temperature in the nozzle receiver [104].
Figure 23: Temperature profiles (gas and graphite walls) and axial gas velocity [104].
In a similar research Valde´s et al. [122] simulated another tubular solar reactor for thermal methane
cracking by a numerical study of momentum, heat and mass transferring and considered indirect
45
heating and an argon-methane feed mixture. The simulations showed that the methane conversion
is about 100% at higher than 2000K temperature. furthermore, it reveals that most of the reaction
takes place at the zone with the highest temperature.
The influence of reactor geometry on temperature distributions in a solar reactor was investigated
by Costandy et al. [123] as well. The CFD study results show that reactor geometry has a great impact
to get a uniform temperature distribution and it improves the conversion yield of methane inside
the reactor (Figure 24). it reveals the spherical reactors has a better performance compared to
cylindrical reactors.
Figure 24: (a) Spherical and (b) cylindrical reactor temperature contours for 100 kW/m2 solar radiation[123].
In terms of carbon separation inside the reactor Jaya et al. [124], conducted a numerical study on
carbon deposition of a cyclone solar reactor type (Figure 25). The aim was to find out a feasible way
to prevent carbon blockage in the reactor and an ‘‘aero-shielded solar cyclone reactor” was
suggested and tried to have a prediction of carbon accumulation at various locations inside the
reactor by using CFD modelling. The gas flow rate was from 10 to 20 L/min and particle mass flow
rate was considered from 7 to 1.75 kg/s. The study showed the influence of
mass flow rate of particles, particle diameter, feed flow rate, wall screening gas and type of window,
all have great influence to decrease the carbon blockage of the reactor. It is believed a possible way
to overcome the obstacle of carbon deposition is an appropriate selection of reactor parameters.
46
Figure 25: Meshed geometry of aero-shielded solar cyclone reactor [124].
Ozalp et al. [124] investigated Kinetics and heat transfer in a solar reactor in presence of a carbon
catalyst for thermal methane cracking. Results showed that the reactor temperature is uniformed
by the carbon seeding and it noticeably improves the reactor performance. Furthermore, the carbon
particles can absorb the radiants and decrease the reactor wall temperature and also be a catalytic
reaction site for the thermal decomposition of the methane.
In another study Rodat et al. [125], conducted a kinetic simulation of thermal methane cracking
reactions in a solar tubular reactor by Dsmoke software. Separate reaction zones including the pre-
heating zone, isothermal zone, and gas cooling zone of the reactor were considered in a graphite
cavity receiver and kinetic details of alkane transformation were studied. The sequence of methane
decomposition and temperature sensitivity analysis (1500–2300K) were investigated by using
kinetic analysis of the chemical system. the results reveal a stepwise dehydrogenation with the serial
Intermediates from C2H6 to C2H4, and C2H2 (Figure 26).
47
Figure 26: Methane conversion versus residence time at different temperatures obtained from Dsm
oke simulations and a plug-flow reactor model [125].
Parolin et al.[126] employed numerical methods to study a semi-batch annular fixed-bed membrane
reactor for thermal methane cracking in a fixed-bed catalytic membrane reactor. the results showed
that using a membrane creates a possibility to decrease the reactor temperature due to overcoming
equilibrium limitations. The impact of the pressure on the reactor performance was studied. The
model showed an un-uniform behaviour of the reactor efficiency in various catalyst activities and
pressures.
Paxman et al. [115] conducted a theoretical and experimental study on thermal methane
decomposition in a Tin molten media. A preliminary mathematical model was developed by Matlab
and some important parameters such as diffusion, heat exchange and residence time of bubbles in
the molten media were studied. The results demonstrated the molten media would allow efficient
heat transfer to the gas and traps the generated carbon. About 70% conversion rate was reached at
1373K with feed gas flow rates of both nitrogen and CH4 set to 15 mL/min.
Gautier, et al. [127] investigated the impact of pressure and temperature on the size of the carbon
black during thermal methane cracking. A computational fluid dynamics modelling was carried out
and different parameters such as convection, conduction and radiation heat transfer, carbon
nucleation and growing the carbon particles were considered. The study showed that an increase in
temperature and pressure leads to a higher hydrogen yield and then a tighter particle size
distribution so that particles population inclines to be mono-dispersed.
48
5. Conclusion
As observed in the first section of the review, using catalysts can decrease the working temperature
to below 800 °C but they create some significant issues, especially catalyst deactivation (a low
catalyst life due to carbon deposition), so it is yet to use as a viable method on the small or
medium-scale.
At higher temperatures (of more than 1200 °C), methane cracking takes place quickly in a non-
catalytic way. Abandes [112] conducted some experimental tests for the direct thermal cracking
of methane in tubular reactors. As the results showed, a high conversion rate was observed when
the temperature exceeded 1170 °C (a more than 99% conversion rate). Some dilution gases like Ar
or H2 were also used as inert gas to avoid reactor blockage due to carbon deposition, but the results
showed that it was not effective. Even a porous tube reactor for injecting the inert gas was
considered to avoid the formation of blockages in the hot zone of the reactor, but it also did not
work out. On the other hand, the endothermic reaction needs to be able to continually supply
energy which can be another issue in this regard. From another point of view, for laboratory
research, electric heating could be advisable (to provide the heat of the endothermic reaction), and
temperature control is very easy and can be employed for any temperature range if needed.
However, it does not fit well with the economies of H2 production on an industrial scale. Indeed, it
is worth noting that we use electricity to produce hydrogen, and one of the major applications for
hydrogen is in electricity production.
Generally, a more important problem associated with the current research can arise during the
scale-up of the process. There is a need to find a solution to providing the needed heat for the
endothermic reaction. On an industrial scale, it could not be a simple multiplication of tubes to
attain a reasonable amount of hydrogen production. The economic viability aspect of the system
may require that the reactor is designed with a higher volume that needs to be heated by an external
heat source in order to reach the operating temperature without any reactor clogging. A device
based on maintaining a gaseous volume is limited by the thermophysical properties of the gas itself
if it is to be required to maintain a stable high temperature. For example, the thermal diffusivity and
thermal conductivity will limit the volume of a temperature-controlled process in which an
endothermic reaction is taking place. In this regard, melted metal reactors could be a good choice
but, as previously observed in the review, the methane conversion rate was not acceptable to
49
prevent the need for a purification unit. It was also noted that increasing the temperature for a
complete conversion led to other problems like corrosion, carbon separation and melted metal
feeding and recycling. It is believed that the melted metal reactors could be useful for the long-term
target of producing hydrogen, however, it is not mature yet to be employed for pure hydrogen
production in residential, small or medium-sized scales.
Given the results of the literature review and the research objective, it is clear that the solution can
come from a non-catalytic reaction but that there are two basic knowledge gaps:
➢ How to supply the heat for the reaction (methane decomposition), and even for heating the
methane to high temperatures.
➢ Finding a solution to the carbon deposition inside the reactors which it creates blockages.
6 Acknowledgment
The authors acknowledge the financial support provided by the Priority Research Centre for Frontier
Energy Technologies and Utilization, the University of Newcastle Australia, for the work presented
in this paper.
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