Environmental Engineering Reference
In-Depth Information
introduced relatively few years ago (the industrial application started in the 1970s)
and results highly reliable and flexible.
Starting exclusively from Eqs.
2.1
and
2.3
, considering a stoichiometric
mixture of CH
4
and H
2
O completely converted to H
2
and CO
2
, and taking into
account the heat of reaction supplied by combustion of CH
4
, it is possible to
calculate the theoretical energy associated with lower heating value (LHV) of
methane to produce H
2
. The minimum energy consumption which can be reached
by this process corresponds to 2.59 Gcal/1000 Nm
3
H
2
when starting from water
vapour, and 2.81 Gcal/1000 Nm
3
H
2
when starting from liquid water, as the real
process [
6
].
Hydrogen plants designed with conventional technology utilize reforming
temperatures below 900C and high steam to carbon ratios ([2.5), to limit coke
formation problems. These plants are characterized by quite poor energy effi-
ciency, as significant amounts of process steam have to be condensed by large air
and water coolers. Moreover, investment costs are high, as large volumetric pro-
cess flows have to be handled [
6
].
Modern hydrogen plants utilize the new developments in SR and shift tech-
nology, allowing apparatus to be designed with reforming temperatures above
900C and steam to carbon ratios even lower than 2.0 [
6
]. These advanced SR
plants have improved energy efficiency and reduced hydrogen production costs.
Currently, the processes require about 2.98 Gcal/1000 Nm
3
H
2
implying that an
advanced reforming technology consumes about 6% more energy than the theo-
retical minimum.
In recent years, new concepts to produce hydrogen by methane SR have been
proposed to improve the performance in terms of capital costs reducing with
respect to the conventional process. In particular, different forms of in situ
hydrogen separation, coupled to reaction system, have been studied to improve
reactant conversion and/or product selectivity by shifting of thermodynamic
positions of reversible reactions towards a more favourable equilibrium of the
overall reaction under conventional conditions, even at lower temperatures. Sev-
eral membrane reactors have been investigated for methane SR in particular based
on thin palladium membranes [
14
]. More recently, the sorption-enhanced steam
methane reforming (Se-SMR) has been proposed as innovative method able to
separate CO
2
in situ by addition of selective sorbents and simultaneously enhance
the reforming reaction [
15
].
2.1.1.2 Hydrocarbon Partial Oxidation
An alternative route to produce synthesis gas starting from hydrocarbon feedstock
is the partial oxidation reaction (POX) [
16
]. This reaction utilizes the oxygen in the
air as oxidant and results moderately exothermic. The oxygen to carbon ratio is
lower than that required by stoichiometric complete combustion.
The stoichiometric equation for methane conversion is:
