Page 46 - Industrial Plant 2015

reactor/s to increase the hydrogen yield and natural gas, up to approx. 60% of the total CO2
syngas cooling section to recover heat from the
produced syngas;
• raw hydrogen purification, which, in modern
plants, is accomplished via Pressure Swing
Adsorption (PSA) unit and allows to achieve
hydrogen product purity above 99.99% mol.
In addition to the core process sections above
described, compression is often needed to raise the
feedstock and product hydrogen pressures.
The reforming reaction between steam and
hydrocarbons is highly endothermic and takes place
across specially formulated nickel catalyst contained in
vertical tubes situated in the radiant section of the
reformer. The simplified chemical reactions are:
CnH(2n+2) + nH2O = nCO + (2n+1) H2
(for saturated hydrocarbons)
CH4 + H2O = CO + 3H2 delta H = + 206 kJ/mol
(for methane)
In the adiabatic CO shift reactor vessel, the moderately
exothermic water gas shift reaction converts carbon
monoxide and steam to carbon dioxide and hydrogen:
CO + H2O = CO2 + H2 delta H = - 41 kJ/mol
The PSA purification unit removes from the hydrogen,
by adsorption, CO, CO2 and CH4 gases.
SMR is a mature technology and is now less likely to
yield any large step changes in economic benefit from
technological developments. Improvements and
optimization opportunities are constantly investigated
with reference to plant efficiency and plant profitability.
2. The CO2 balance in the
hydrogen plant
In a modern steam reforming hydrogen plant fed by
Fig. 1 – Simplified
H2 block flow
diagram
(pressures are in
bar absolute)
Industrial Plants - May 2015
44
produced is contained in the shifted gas (and then
in the PSA tail gas), while the remaining 40% is the
product of the combustion of the additional fuel
gas required by the steam reformer. At last, all the
CO2 ends up in the flue gas of the SR heater
(figure 1).
Typical CO2 flow rates and partial pressures for a
100,000 Nm3/h hydrogen plant are reported in table 1.
It is clear from this example that one ton of hydrogen
produced will also produce about 9 tons of carbon
dioxide.
The CO2 could be captured from any of these three
streams (figure 2), with removal efficiency of about
90% (from PSA tails gas and from SR flue gas) and up
to more than 99% (from raw H2 at higher pressure). The
total CO2 potentially removed (ηCO2) from the three
locations, calculated with the formula
ηCO2 (%) = 100 × (1 – CO2 in flue gas after rem. / CO2 in flue gas without CO2 rem.)
is reported in table 2.
It is also possible, in principle, to combine CO2 removal
1 or 2, with 3 (from flue gas), and obtain an overall ηCO2
of about 96% and 94% respectively.
3. CO2 removal technologies
There are several available technologies for CO2
removal, at different stage of development and
commercialisation. The most important de-carbonization
technologies that can be employed for capturing CO2 in
SMR hydrogen plants include:
• pre-combustion processes (i.e. from syngas);
• post-combustion processes (i.e. from flue gas).
The next paragraphs will briefly review such technologies
and their application. For sake of simplicity, the review
is here limited to the CO2 removal cases 1 and 3, yet
recognising the potential carbon capture also from PSA
tail gas.