Environmental Engineering Reference
In-Depth Information
power size. The potentialities of small-scale plants suggest a deeper discussion
about hydrogen distribution network scenarios that is carried out in
Sect. 2.2
.
The scientific community interest has been focused in recent years especially on
H
2
catalytic production by partial oxidation of methane, due to the large diffusion
of natural gas as primary feedstock. Coke formation and its deposition on catalyst
active sites represent, as well as for SR process, the main barrier to be abated for a
practical utilization of CPO in hydrogen production plants.
Methane CPO has been intensively studied to select new advanced catalysts
able to maximize hydrocarbon conversion, hydrogen yield and especially to
control catalytic deactivation phenomena, strictly connected to coke deposition
problem, similar to SR process [
18
-
23
]. The role of transition metal-based cata-
lysts in methane CPO reaction mechanism has been detailed [
24
], evidencing that
fuel dissociation step is crucial for a viable overall process rate at reasonable
temperatures, as expected taking into account the stability of methane molecule.
LPG could be another favourable feedstock for distributed hydrogen production
since it is easy to store and transport. Furthermore, LPG and NG appear attractive
because hydrocarbon mixtures with short aliphatic chains (C
1
-C
4
) and no-sulphur
or other electronegative atoms (Cl, P) could limit carbon deposition and catalyst
poisoning. Commercial Ni catalysts used for SR plants have resulted very active
also for CPO of methane and propane, but deactivation resistance due to coke is
not yet acceptable [
25
-
28
]. Ni-based catalyst modification with rare-earth metal
oxide La
2
O
3
can reduce the Lewis acidity of the catalyst surface and enhance its
ability to suppress carbon deposition [
29
], while among the various noble metal
catalysts Rh has been reported as active and stable [
30
]. Bi-metallic Ni-Pt cata-
lysts supported on Al
2
O
3
result very promising if compared with monometallic
catalytic solids [
31
]. Mixed oxides containing Ce seem useful to formulate a
catalyst suitable for a durable hydrogen production, in particular CeO
2
is known to
be an oxy-transporter, i.e. it is capable to oxidize deposited carbon particles and to
actively participate in mechanism of redox catalytic reactions [
32
]. On the other
hand, the incorporation of ZrO
2
into CeO
2
lattice promotes the CeO
2
redox
properties, increasing the oxygen mobility within the solid solution formed [
33
].
If the water quantity added as feed increases up to a value corresponding to
neutral energetic balance between exothermic and endothermic reaction steps, the
overall process is denominated autothermal reformer (ATR). This approach
combines both SR and POX catalytic processes and it has been recently proposed
to optimize the performance in terms of compactness and efficiency of small-
medium production plants. This technology could permit a compromise between
the good efficiency of SR and the fast start up of POX. However, it needs a careful
control of in going mass stream [
6
,
7
].
2.1.1.3 Hydrocarbon Decomposition
The direct thermal decomposition of methane or higher hydrocarbons represents
the unique approach for a theoretical direct decarbonization strategy [
34
,
35
].
