Environmental Engineering Reference
In-Depth Information
transportation by trucks, in both high-pressure gas and liquid, should become
competitive to move only small quantities of hydrogen.
On the other hand, a distributed manufacture, based on either on-site small
reforming plants or on small-size electrolyzers, would permit storage at the vehicle
facility and strongly reduce the specific impact of fuel distribution and transport
costs on a global techno-economic assessment.
Taking into account that hydrogen can be transported in pipelines similar to
natural gas, it is interesting to verify if the existing pipeline infrastructure could
be used, evaluating in particular the energetic limitations and the current
performance of materials of construction and of shut techniques. This analysis first
requires a comparison between the energy flows of hydrogen and methane.
The volumetric flow rate through a pipeline can be calculated according to the
following equation [
101
]:
s
p
1
p
2
Þ
2
ð
Q
¼
CD
2
:
5
k
ð
2
:
27
Þ
dZTLf
where Q is the volumetric flow rate of the gaseous fuel (Nm
3
/h), C is a propor-
tionality constant (about 0.00013), D is the inner diameter (mm), k is the pipeline
efficiency, p
1
and p
2
are the inlet and outlet pressure, respectively (kPa), d is
the relative density compared to air, Z is the compressibility factor, T is the
absolute temperature, L the length (km) and f is the friction factor. In Fig.
2.7
,a
scheme of a section of pipeline is shown with the indication of main parameters
affecting gas flow rate.
Considering the upper heating values of H
2
and CH
4
(12.8 and 40 MJ/Nm
3
,
respectively) as well as the wide difference in density values (0.09 for hydrogen
and 0.68 kg/Nm
3
for methane, respectively), similar pressure drops have to be
overcome to match the same energy demand. On the other hand, the accurate
evaluation of compressibility and friction factor indicates that hydrogen is able to
transport at least 80% of the energy carried by NG [
102
].
Also the aspects related to compression stage and pressure reduction stations
need to be accurately considered. The performance of the centrifugal compressor,
quite satisfactory for natural gas processing, becomes significantly limited when
hydrogen is used, because of the need of higher rotational velocity, not compatible
with material strength [
101
]. On the other hand, hydrogen fragileness [
103
] could
represent a further problem for a sufficient reliability of the pipings and fixings
Fig. 2.7 Section of a pipe-
line with indication of the
main parameters useful for
volumetric flow rate calcula-
tion (Q)
p
1
p
2
d,T
Q
D
L
