(406i) Thermocatalytic Decomposition of Methane to Hydrogen and Carbon Nanotubes in Fluidized Bed Reactor Using Ni-Based, Cu-Zn-Promoted and Alumina-Supported Catalyst

Lately the term “hydrogen economy”
has gained an unprecedented degree of importance. Hydrogen is now being seen as
the alternative low-carbon energy source for today’s energy guzzling civilization.
The constantly declining availability of the conventional carbon-based sources
such as coal and petroleum and the rapid increase in the greenhouse gases in
the atmosphere leading to a fatal rise in the global temperatures which has
only fuelled a renewed interest in the hydrogen economy^[1]. Talking
about the current hydrogen production scenario, 23% accumulated growth in
hydrogen production is predicted from 2010 to 2030. Presently 80% of the demand
is concentrated in two sectors, namely the fertilizer industry (for ammonia
production) and the refineries. With the concept of hydrogen cars, FCVs (Fuel
Cell Vehicles) and “hydrogen mobility” coming in, the hydrogen mobility is
projected to become the major sector of hydrogen consumption. The trend will be helpful in battling with the
climate change if the hydrogen that is being produced is “green hydrogen.”

          There are two main viable
ways of hydrogen production: Steam Methane Reforming (SMR) process and Direct Thermo-catalytic
decarbonisation of hydrocarbons. Fuel cells are one efficient way of putting
hydrogen to good use. One limiting factor for use of fuel cell technology is
the production of hydrogen that meets desired quality standards. According to
ISO-FDIS rules, the maximum permissible limits for carbon dioxide and carbon
monoxide in hydrogen are 2 and 0.2 ppm respectively. It is difficult to
economically produce such high purity of hydrogen from the SMR method as
separation of CO_x from hydrogen progressively becomes more difficult
with increasing desired purity. The separation costs become too high to justify
the production. The inherent advantage of the TCD (Thermo-Catalytic
Decomposition) route is that it produces hydrogen from methane/natural gas
without any CO_xproduction. Thus, the process inherently
bypasses the need of CO_x separation/sequestration step. Thus the TCD
route is aptly suited for the production of CO_x-free hydrogen to be
used in fuel cells.

          The TCD reaction can be
carried out either in fixed bed or a fluidized bed reactor. Though the fixed
bed reactor has a higher conversion in comparison to the fluidized bed reactor,
it is difficult to operate it on an industrial scale due to the high rate of coking
and deactivation of the catalyst. Fluidized bed reactor provides easy regeneration
of the catalyst particles. In addition to the CO_X free hydrogen, the
CNT’s formed as a highly desirable by-product also help to make the process
more commercially feasible ^{[2, 3]}. The global market of CNT’s is
expected to grow from USD 2.26 Billion in 2015 to reach USD 5.64 Billion by
2020. CNTs can be used in nano-composites, metal composites, and ceramic
composites. One more proposed use is their application in super-capacitors for
use in electric vehicles. It is easy to show that TCD of methane route for
hydrogen (with CNT as a by-product and taking carbon credits into account) is
economically comparable to the SMR method (with sequestration).

          The methane conversion
observed in the process is greatly dependent on the catalyst used. A Ni-based,
Cu-Zn promoted and alumina-supported catalyst was used and more than 85%
methane conversion was achieved in fluidized bed reactor (Figure 1a). The
carbon by-product is also produced in the form of bamboo shaped carbon
nanotubes (Figure 1b). Apart from the catalyst, hydrodynamics of the fluidized
bed reactor system also greatly affects the observed conversion. Thus, it is of
paramount importance that the hydrodynamics is studied in detail to
industrially scale-up the process. It is interesting to note that the behaviour
of the catalyst particles is time-variable in the reactor. The hydrodynamic
behaviour of the particles evolves as the reaction proceeds. The freshly prepared
catalyst particles show Geldart-A type of behaviour. However, as the reaction
proceeds, carbon nanotubes begin to grow on the catalyst particles. The
accumulation of CNT’s over the catalyst particle changes the cohesive
properties and thus the fluidization behaviour (the behavior is more akin to
Geldart-C category particles). This changes the degree and type of fluidization
and thus inversely affects the methane conversion. So, regeneration of the
catalyst is required by separating the CNT’s. Thus, it is required to study the
time-variable fluidization behaviour of the particle to find out the time after
which the catalyst needs to be regenerated.

Figure 1: (a) Methane
conversion with time on stream, P_CH4 = 0.25, Avg. catalyst particle size
= 125 micron. (b) Transmission Electron Microscopy image of the produced bamboo
shaped carbon nanotube.

Acknowledgement

Authors acknowledge Hindustan
Petroleum Corporation Limited (HPCL), India for providing the financial support
for this research.

The author is a recipient of
Prime Minister’s Fellowship Scheme for Doctoral Research, a public-private
partnership between Science & Engineering Research Board (SERB), Department
of Science & Technology, Government of India and Confederation of Indian
Industry (CII). The author’s host institute for research is Indian Institute of
Technology Delhi, and the partner company is Hindustan Petroleum Corporation
Limited.

References

1. N. Z. Muradov, T. N.
Veziroglu, Int. J. Hydrogen Energy 2005, 30, 225.

2. H. F. Abbas, W.M.A. Wan Daud,
Int. J. Hydrogen Energy, 2010, 35, 1160.

3. S. K. Saraswat, K. K. Pant,
Int. J. Hydrogen Energy 2011, 36, 13352.