(443f) Convective Heat Supply Reactor Concepts for the High Temperature Pyrolysis of Methane

Contrary to the popular
belief, fossil energy consumption is not limited by its availability but is
constrained by environmental considerations. Over the past decades, the world’s
natural gas proven reserves have been increasing steadily, from 2592 trillion
cubic feet in 1980 to 6972 trillion cubic feet in 2014 [1], and if
non-conventional sources like methane hydrates or shale gas are included, this increase
is even steeper. Many scenarios for the future production of energy without CO₂-emissions
have been investigated. The consensus seems to be directed towards renewable
energy; however, a complete transition to renewable energy is not foreseeable
in the near future [2]. It is thus necessary to search for bridging
technologies. Among these, hydrogen presents itself as a very attractive
alternative energy carrier due to the possibility of using existing natural gas
infrastructure as well as for its facile integration in fuel cells and other clean
energy technologies.

Hydrogen can be generated
with low to zero CO₂-emissions by pyrolysing hydrocarbons at high
temperatures. The pyrolysis of methane can be described by the reaction:

CH4⇄Cs+2H2
      DHR,1000°C=91.52 kJmol-1   
 DGR,1000°C=-49.28 kJmol-1

This reaction is endothermic and becomes
thermodynamically spontaneous above 820 K and kinetically feasible at
temperatures around 1073 K. The reaction can be catalysed at lower temperatures,
but the carbon deposition on the catalyst and the limited equilibrium
conversions leading to a separation problem and recycle streams are still issues that have to be addressed. The high temperature
pyrolysis remains an alternative, but is not without challenges: carbon
deposition is still a major hurdle, which can lead to blockage of the reactor,
and the endothermicity of the reaction necessitates the introduction of heat at
higher temperatures.

Convective heat supply
reactor concepts offer advantages that can help circumvent both problems. In
this work, three concepts are presented:

The first two concepts are
based upon the use of liquid molten media, such as molten metals. On the one
hand, the liquid medium acts as a barrier preventing carbon deposition at the
heat transfer surface, as well as a heat transfer material due to its excellent
transport properties. The use of molten metal bubble columns was first proposed
by Steinberg [3]. A modified version was explored by Serban
et al. [4] exhibiting a relatively high conversion (57%) for low temperature
operation (750°C). In this publication, carbon deposition occurred at both the
reactor surface and in the sparger used to feed the methane. Currently a bubble
column reactor with a fixed-bed is being investigated by the group of Prof. Abanades in the IASS in Postdam in cooperation with
the Karlsruhe Institute of Technology amongst others [4]. Their results show a
hydrogen yield of 30% at 1000°C with little carbon deposition on the reactor
surface. However, the lack of control over the residence times in bubble
columns inherently leads to a decrease in conversion. To overcome this difficulty,
the use of a capillary reactor using molten metals in slug flow regime (Fig. 1)
was studied at temperatures up to 1100°C [5], showing promising results,
although only relatively low conversions (30% methane) have been achieved so
far.

Figure
1.  Molten Sn
capillary reactor in slug flow regime

To increase conversions,
working at still higher temperatures is the logical approach when the reactor
length is limited. However, initial experimental results (Fig. 2) indicate
increased carbon deposition, which can be explained by the increased rates of
carbon formation, coupled with insufficient wetting of the heat transfer area
by the molten metal as well as morphological differences in the carbon formed. To
resolve this problem, two techniques can be used: the first one is to tweak the
wetting properties of the molten metal, to avoid carbon deposition. A second
approach would be to modify the reactor design appropriately to guarantee film
formation while maintaining the advantages of slug flow operation.

Figure
2. Carbon deposition in a capillary
reactor at 1300°C

In this work, the
experimental results for the first approach are presented, where the wetting
properties of the melt were modified using a different alloy (Ga-In-Sn) rather
than just tin. For the second approach, a theoretical study on a countercurrent
falling-film reactor (Fig. 3) is presented. The falling-film reactor uses a
similar concept to the capillary reactor, employing the molten metal as both a
heat transfer medium and to protect the heat transfer surface, thus retaining
the advantages of the slug flow regime. In addition, countercurrent operation
inherently prevents the occurrence of the back reaction which could potentially
take place in the slug flow regime. Furthermore, the residence time of the gas
although not as well-defined as for the slug flow regime, can be still
considered narrow enough to assure plug flow. The mathematical modelling and the
simulation results are presented together with preliminary experimental results
regarding film stability.

Figure
3.  Falling molten Sn film reactor.

Finally,
the third concept uses gas as a convective heat carrier. Previous work at our
laboratory used a porous-wall reactor [6], where the carrier gas flowed through
a porous wall into the reaction zone, avoiding the carbon deposition on the
outer walls. However, methane diffusion through the membrane proved to be
higher than anticipated, and as a result carbon deposits were still found in
the outer region. To avoid this diffusion, a side-jet reactor (Fig. 4) concept
was developed, where the carrier gas flows through small holes in the inner
tube generating a greater gas velocity in comparison to the porous wall. This
minimizes the rate of diffusion of methane into the outer zone and provides a
better mixing with the methane and thus an improved heat transfer. In this
work, the modelling of the side-jet reactor is presented as are the preliminary
experimental results.

Figure
4.  Side-jet reactor concept.

Acknowledgements

Financial
support from the German Research Council DFG (Grant No. AG 26/15-1) is gratefully acknowledged.

References

[1] U.S. Energy
Information Administration. International Energy Statistics.
Retrieved from: http://www.eia.gov/cfapps/ipdbproject/iedindex3.cfm?tid=3&pid=3&aid=6&cid=regions&syid=1980&eyid=2015&unit=TCF
on October 2015.

[2]           G. Kreysa, DGMK Conference “The Future Role of Hydrogen in
Petrochemistry and Energy Supply”, Berlin, Germany, October, 2010.

[3]           M. Steinberg,
Int. J. Hydrogen Energy. 24(8) (1999) 771-777.

[4] M. Serban, M. A.
Lewis, C. L. Marshall, R. D. Doctor, Energy Fuels. 17 (3) (2003) 705-713

[5]           T.
Geißler , M. Plevan , A. Abanades, A. Heinzel, K. Mehravaran, R.K. Rathnam, C. Rubbia, D. Salmieri,
L. Stoppel, S. Stückrad, A. Weisenburger,
H. Wenninger, T. Wetzel, Int. J. Hydrogen Energy. 40 (41) (2015) 14134-14146.

[6]           I.
Schulz, D.W. Agar, Int. J. Hydrogen Energy. 40(21) (2015) 11422-11427

[7] Fabian Goebel,
Bachelor Thesis, Technische Universität Dortmund, 2015.