etproduction
R. Soltani*, M.A. Rosen, I. Din
Faculty of Engineering and Applied Science, U
North, Oshawa, ON L1H 7K4, Canada
a r t i c l e i n f o
ive with two other lo-
presence of CO2 in the
the ratio of hydrogen
.
Copyright 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
In order to keep global warming to less than 2 C, some statethat the atmospheric CO2 concentration should not exceed
easures have been
proposed to mitigate global warming, including carbon diox-
ide capture and sequestration (CCS), which some propose as
an effective way to stabilise atmospheric carbon dioxide
concentrations [2,3]. Fig. 1 presents a breakdown of industrial
* Corresponding author.E-mail addresses: [email protected], [email protected] (R. Soltani), [email protected] (M.A. Rosen), [email protected]
(I. Dincer).M, Dhahran 31261, Saudi Arabia.
Available online at www.sciencedirect.com
ScienceDirect
w.
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 01 Associated with Department of Mechanical Engineering, KFUPIntroduction450 ppmV CO2-equivalent [1]. Many man S/C ratio of 2.5, CO2 capture from a flue gas stream is competit
cations provided higher weighting factors are considered for the full
flue gases stream. Considering carbon removal from flue gases,
production rate and Ncc increases with rising reformer temperaturethe effects of oxygen enrichment of the furnace feed are investigated, and it is found that
this measure improves the CO2 capture conditions for lower S/C ratios. Consequently, forArticle history:
Received 11 July 2014
Received in revised form
24 September 2014
Accepted 29 September 2014
Available online xxx
Keywords:
Steam methane reforming
Hydrogen production
CO2 emission
CO2 capture
Oxygen enrichmentPlease cite this article in press as: Soltanreforming for hydrogen production,j.ijhydene.2014.09.161
http://dx.doi.org/10.1016/j.ijhydene.2014.09.10360-3199/Copyright 2014, Hydrogen Enercer 1
niversity of Ontario Institute of Technology (UOIT), 2000 Simcoe St.
a b s t r a c t
Steam methane reforming (SMR) is currently the main hydrogen production process in
industry, but it has high emissions of CO2, at almost 7 kg CO2/kg H2 on average, and is
responsible for about 3% of global industrial sector CO2 emissions. Here, the results are
reported of an investigation of the effect of steam-to-carbon ratio (S/C) on CO2 capture
criteria from various locations in the process, i.e. synthesis gas stream (location 1), pres-
sure swing adsorber (PSA) tail gas (location 2), and furnace flue gases (location 3). The CO2capture criteria considered in this study are CO2 partial pressure, CO2 concentration, and
CO2 mass ratio compared to the final exhaust stream, which is furnace flue gases. The CO2capture number (Ncc) is proposed as measure of capture favourability, defined as the
product of the three above capture criteria. A weighting of unity is used for each criterion.
The best S/C ratio, in terms of providing better capture option, is determined. CO2 removal
from synthesis gas after the shift unit is found to be the best location for CO2 capture due to
its high partial pressure of CO2. However, furnace flue gases, containing almost 50% of the
CO2 in produced in the process, are of great significance environmentally. Consequently,points in steam m hane reforming for hydrogen
Assessment of CO2 capture options from variousjournal homepage: wwi R, et al., Assessment oInternational Journal
61gy Publications, LLC. Publelsevier .com/locate/hef CO2 capture options from various points in steam methaneof Hydrogen Energy (2014), http://dx.doi.org/10.1016/
ished by Elsevier Ltd. All rights reserved.
world now use amine-based systems [12,13]. Nonetheless,
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e1 02CO2 emissions based on a 2008 IEA report [3]. There, it can be
seen that the (petro) chemical sector in 2005 was responsible
for about 16% of industrial CO2 emissions, and that steam
methane reforming (SMR) accounts for a large share. Also,
from more recent IEA publications [4,5], the petrochemical
sector, after the iron and cement sector, is still the major
source of carbon dioxide emissions.
Steam reforming is the commonly used and mature tech-
nology for industrial hydrogen production. According to a life
cycle assessment of global hydrogen production [6], about 75%
of world's total hydrogen is produced by steam methanereforming. Also, a 2008 IEA report estimated the global annual
hydrogen production in 2005 at 65 Mt, with 48% from SMR [3].
Other data on the share of hydrogen production from SMR
confirm SMR to be the main process for hydrogen production.
Although SMR might be replaced in the future by other effi-
cient hydrogen production techniques, e.g. such as thermo-
catalytic decomposition [7], it is still expected to be important
in the future.
As pointed out earlier, SMR facilities emit on average 7 kg
CO2/kg H2 [3], which was equivalent to 220 Mt CO2 globally in
2005 [3]. Compared to total global CO2 emissions, the share
contributed by SMR facilities is small, at around 3% [3]. This
share is expected to increase through decarbonising the
transportation industry, partly by using fuel cell systems and
hydrogen as an energy carrier. The IEA estimates that if fuel
cell technology is applied in the transportation sector suc-
cessfully by 2050, the global hydrogen demand will be as high
Fig. 1 e Direct global CO2 emissions of the industrial sector
in Gt/yr for 2005 [adapted from 3].as 275 Mt per year [8], resulting in emissions of about 2 Gt CO2per year if SMR facilities are used. CCS would then be more
attractive for decreasing these emissions. One advantage of
introducing hydrogen to the transportation sector is that
emission control is easier at a central SMR plant than in all
cars on the road. When purified, CO2 has many uses in the
chemical industry, in solid form (dry ice), liquid form (e.g.
refrigeration equipment) and gaseous form (beverage
carbonation). It also is widely used as a reactant in chemical
processes and as an inert blanketing gas to prevent oxidation
(e.g., for food products) [9]. Recently, it has received attention
to help increase oil recovery fromdepleted or high viscosity oil
fields [10].
Numerous studies have been carried out on CO2 removal
from SMR, usually focussing on in-process capture and end-
point capture, i.e. capturing furnace flue gases or post com-
bustion capture. The former involves the capture from
Please cite this article in press as: Soltani R, et al., Assessment oreforming for hydrogen production, International Journaj.ijhydene.2014.09.161amine-based technology have been identified is not suitable
for low molar fractions of CO2 (flue gases) [14], and other
shortcomings of amine-based technology for flue gases have
been described [15], e.g. extensive solvent corrosion issues
and energy intensiveness. Removal of CO2 from flue gases by
pressure swing adsorption (PSA) has recently been investi-
gated as an alternative technology for CO2 removal by Kikki-
nides et al. [9], Park et al. [16], and Ko et al. [17]. Voss [18] has
studied the feasibility of CO2 removal from different process
locations by different technologies and compared them based
on various criteria. He concludes that PSA technology is highly
competitive to mature amine-based methods due to its
comparatively simpler procedure. Also, he states that PSA is
favourable for pressurized feed streams, namely synthesis gas
and PSA off-gas streams. Economic studies have also been
reported, e.g. assessing techno-economically CO2 capture
systems [1,19]. Overall, it is expected that CO2 avoidance costs
at SMR facilities decrease by having high pressure sources of
CO2 with high concentrations. Two main factors dominate
capture costs: CO2 partial pressure and molar concentrations
of streams. In this study, mass flows of CO2 are also consid-
ered, as these can motivate investors who seek to avoid po-
tential high CO2 emission penalties in the future.
Oxygen enrichment can enhance the performance of
steam methane reforming and other industrial processes,
especially those involving combustion. In case of SMR, oxygen
enrichment has been shown to reduce the requirement for
natural gas feed [20], mainly due to better heat transfer from
the combustion gases to the reformer. This is a consequence
of lower nitrogen concentrations in the combustion gases,
which decreases the amount of sensible heat lost with flue
gases. The other main benefit of oxygen enrichment is,
because of the relatively lower nitrogen supply, higher carbon
dioxide concentrations are attained in the combustion prod-
ucts. In this study, a fixed hydrogen production rate is
considered when analysing the impact of parameter varia-
tions and the effect of oxygen enrichment only on the com-
bustion process is examined.
In this research, we analyse an SMR process to determine
how the S/C ratio affects the criteria that play major roles in
CO2 capture: partial pressure, concentration, and overall mass
flow rate of CO2. Also, focussing on post combustion capture
from furnace flue gases, oxygen enrichment of furnace com-
bustion is investigated to see if it makes CO2 capture from flue
gases reasonable compared to synthesis gases and PSA off
gases. Finally, hydrogen production and carbon removal are
studied together and compared for varying S/C ratios and
reforming temperatures.
Methods considered
General descriptions are provided of the threemain processesprocess streams and is discussed in following sections. Sim-
beck [11] indicates that carbon capture from the flue gas
stream of an SMR furnace is possible using amine-based
capture and, in fact, most pilot plants established in theconsidered in this study: SMR, CO2 capture, and oxygen
enrichment.
f CO2 capture options from various points in steam methanel of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
Steam methane reforming
A typical SMR system consists of four main sequential units:
desulfurizer, reformer, shift reactors and separation units. In
the desulfurizer, sulfur is removed from natural gas to avoid
the production of sulfur oxides and contamination of catalysts
in the reformer. To simplify the analysis, the natural gas feed
is assumed to be puremethane in this study. In the reformer, a
syngas containing H2 and CO is produced by reacting between
methane and steam (Equation (1)). There are two shift re-
actors: high temperature (HT) and low temperature (LT), both
of which convert CO produced in the reformer to CO2 and H2(Equation (2)). The main reactions involved in an SMR process
are as follows:
CH4 H2Og/CO 3H2 DHr 251 MJ=kmolCH4 (1)
COH2Og/CO2 H2 DHr 41:2 MJ=kmolCH4 (2)
Carbon dioxide capture
In externally fired steam methane reforming process, as
stated earlier, three CO2 containing streams can be identified:
1) Shifted synthesis gas upstream of the hydrogen purifica-
tion unit.
2) PSA tail gas from hydrogen purification.
3) Flue gas from steam reformer furnace system.
The three stream locations are shown in Fig. 2. Each stream
provides the potential for removing CO2. Each stream has
different specifications. The shifted synthetic gas has low CO2concentration and elevated pressure. The PSA tail gas has
high CO2 concentration and low pressure. The flue gas has low
CO2 concentration and low pressure, but it has the advantage
of containing the full mass flow of produced CO2, which
makes it of interest environmentally.
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 0 3The net overall reaction is endothermic and the required
heat is normally supplied to the reformer by a furnace. Some
typical reformer operating conditions are listed as a temper-
ature of 700e1000 C, a pressure of 15e50 bar and an S/C ratio
between 2 and 5 [21]. The produced syngas is cooled before
entering the shift reactor to remove the heat of the exothermic
shift reaction (Equation (2)). The gas stream exiting the shift
reactors consists of H2, CO, CO2, H2O and the remaining
methane. After separation and removal of the water using a
condenser, the dry shifted stream enters the hydrogen puri-
fication unit from which the final product H2 exits. There are
two main technologies for hydrogen purification: PSA and
membrane separation [22]. Due to complicated nature of pu-
rification process, all separation and purification units are
assumed in this study to be simple separation steps. This
simplification is invoked because the focus of this study is on
substance concentrations, pressures and conversion ratios.
After purification of the H2 stream, the remaining gas stream
(PSA tail gas) leaves the unit at near atmospheric and with a
high concentration of CO2. This tail gas is sent to furnace as a
secondary feed stream (in addition to the main fuel),
decreasing the fuel consumption in the furnace.Fig. 2 e Schematic of SMR proce
Please cite this article in press as: Soltani R, et al., Assessment oreforming for hydrogen production, International Journalj.ijhydene.2014.09.161Two main technologies exist for CO2 removal from SMR
streams: absorption and adsorption. The former refers to
amine-based capturing method known as the monoethanol-
amine (MEA) process and the latter refers to PSA techniques.
Some other technologies for CO2 removal also can be found
[24], including vacuum pressure swing adsorption, which was
recently been introduced for this purpose [18]. MEA is a
mature technology for CO2 removal from syngas, but it has not
yet been commercialized for post-combustion capture,
mainly because the low molar fractions of CO2 are not
considered favourable [14] and the process is highly energy
intensive. This application also has solvent corrosion issues,
raising its costs significantly. The presence of contaminants
like O2 and NOx significantly increase solvent deterioration,
further increasing operational costs [25]. Consequently, the
process conditions of stream 1 allow the application of MEA
and PSA, while only PSA systems are applicable for stream 2
due to the high CO2 concentration and amine based systems
are preferred for stream 3 [18]. However MEA is not commer-
cialized due to corrosion issues.
Although many detailed specifications exist for each cap-
ture technology, in this study the focus is on the investigationss without heat integration.
f CO2 capture options from various points in steam methaneof Hydrogen Energy (2014), http://dx.doi.org/10.1016/
Second, oxygen enrichment improves heat transfer in the
inadequately accurate).
The separation of water in the condenser is complete.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e1 04reformer because of the higher flame temperature and the
higher emissivity of combustion gases, associated with the
increase in CO2 and H2O concentrations (as a consequence of
decreased nitrogen) [20]. The improvement of heat transfer
through the tubes of a steam reforming furnace increases the
amount of heat received by the methane and steam mixture
and hence its temperature, so that the reforming equilibrium
is displaced toward higher hydrogen production [20]. This
phenomenon leads to higher conversion ratios and, therefore,
a lower natural gas inlet for the same hydrogen production.
For simplicity, these effects on conversion ratio are not
considered in this study. Only the effect of reducing the
furnace fuel consumption for the provision of the heat
required for steam production and reformer is of importance.
In other words, the decrease in the steam to methane ratio
due to oxygen enrichment is not studied. Overall, two positive
effects on steam methane reforming result from oxygen
enrichment in the furnace: a higher conversion ratio modified
flue gas and furnace fuel consumption. The former is not
investigated here. Different enriching technologies are
appropriate for various volumes of enriched gas, including
PSA, cryogenic refrigeration [26] andmembrane air separation
(MAS). The simplicity and modular nature of MAS makes it
well suited for retrofitting and upgrading current processes
and cycles. The usual limits in industry for enrichment are
between 25 and 35% [20]. Nonetheless, all levels of enrichment
are considered in this study, to better characterize the impact
on the CO2 partial pressure in flue gases through enrichment.
Model description
A schematic for the process investigated here is depicted in
Fig. 2. The model developed for this investigation is mainly
based on the flow diagram and industrial plant data provided
by the U.S. Department of Energy (DOE) in a project on SMR, as
reported in Ref. [23]. The processmodel is developed using the
Aspen HYSYS V7.3 process modelling and simulation soft-
ware, to permit the case study to be analyzed for various
operating conditions. Aspen HYSYS is a comprehensive pro-
cess modelling system used by many oil and gas producers,
refineries, and engineering companies around the world to
optimize process design and operations [27]. All pressures andof two dominant factors for CO2 removal: CO2 partial pressure
and molar concentration. Also, an environmental perspective
suggests that the presence of CO2 in process streams, as a
fraction of the full CO2 present in the flue gases, be considered
as another capture criterion.
Oxygen enrichment
The benefits of oxygen enrichment have been demonstrated
for many combustion and other processes industry. For
combustion, for instance, oxygen enrichment has two main
benefits. First, it decreases the concentration of parasitic ni-
trogen in the combustion gases, increasing the heat available
to the process because less heat is lost via nitrogen emissions.temperatures are based on this industrial case study, but the
hydrogen production rate is set to 1 kg/h. Then, the required
Please cite this article in press as: Soltani R, et al., Assessment oreforming for hydrogen production, International Journaj.ijhydene.2014.09.161 No excess steam is generated (even though excess steam isnormally a by-product of SMR plants for export).
The thermodynamic integrity of the simplified model is
assured by setting appropriate reactor temperatures and flow
stream temperatures exogenously. The developed model is
depicted in Fig. 3 and the pressures, temperatures and pres-
sure drops are presented in Table 1. Comparing Figs. 2 and 3, it
can be seen that there is no difference; one may ask why flue
the gases stream from the combustor is not connected to the
reformer reactor? The reason is that heat input to the
reformer is via heat, not a material stream. Therefore, the
value used as the energy input to the reformer is based on the
energy of the flue gases.
Approach
The favourability of carbon dioxide capture is assessed at
three locations using two steps. The criteria for this study are
carbon dioxide partial pressure, molar concentration and
mass flow ratio. First, the condition for typical air (21% O2) as a
furnace feed is investigated. Second, the enhancement of
capturing feasibility by oxygen enrichment is assessed and
the impact of oxygen enrichment is compared for the three
locations. Note that both the optimal and ideal (100%) levels of
oxygen enrichment are studied. The assessment is made
quantitatively in terms of a proposed CO2 capture number
(Ncc), thereby allowing the capture favourability to be evalu-
ated and compared for different conditions, as follows:
Ncc Imass Ipressure Iconcentration (3)where Imass denotes the CO2mass ratio, Ipressure the CO2 partial
pressure (in bar), and Iconcentration the CO2 molar concentra-
tion. Note that, considering environmental aspects, the car-natural gas feed rate and, by setting steam-to-methane ratio
(a design parameter), the required steam supply rate to the
process are both calculated. Several assumptions aremade for
design and analysis:
The heat interaction (transfer) among the heat exchangersis not considered, mainly because an energy analysis is not
the goal of the study.
The natural gas feed is sulfur-free, so a desulphurizationunit is not implemented.
The inlet water to the process is adequate for the process,so no treatment unit is added.
The hydrogen separation in the purifier (PSA) removes 90%of the hydrogen.
The product H2 stream is 100% pure with no othercontaminants.
The reformer reactor and the two shift reactors are equi-librium reactors.
The furnace is a Gibbs reactor (the presence of CO, H2, CO2makes a stoichiometric reactor model complicated andbon dioxide mass flow ratio is an attractive factor because
almost half of the carbon dioxide emissions is from the
f CO2 capture options from various points in steam methanel of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
Fig. 3 e Schematic of Aspen HYSYS mod
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 0 5furnace for various S/C ratios (see Table 2). Thus, a higher Imassis of environmental importance in this study. In many SMR
plants, due to the following results, the preference is not to
remove CO2 fromflue gases. Moreover, in this study, a value of
unity for Ncc implies the availability of pure CO2 at 1 atm
pressure. However, the weights of all criteria are assumed the
same here (at unity).
As pointed out earlier, the hydrogen production rate is set
to 1 kg/h, so natural gas feed flow rate is adjusted. Then by
setting the S/C ratio to different values, the water flow rate is
determined. Heat integration is not incorporated into the
process modelling because an energy analysis is excludedfrom this study. In addition the fuel inlet to the furnace is
required to satisfy the heat flow required by the reformer and
Table 1 e Operating parameters considered for modelingSMR process.
Flow stream Baseline parameter value
Temperature (C) Pressure (bar)
Steam feed 510 30.0
NG feed 510 28.5
Mixed feed to reformer 649 27.0
Reformed gas 815 19.5
Cooled gas shift to HT shift 350 19.0
Cooled feed to LT shift 204 18.0
Shifted gas to purification 213 17.0
Dry syngas 38.0 16.6
Pure H2 38.0 1.60
PSA tail gas 38.0 1.00
Furnace fuel 25.0 1.00
Air inlet to furnace 25.0 1.00
Device Outlet temperature (C) Pressure drop (bar)
Reformer 815 1.72
HT shift 428 1.03
LT shift 213 1.03
Condenser 38.0 0.34
Please cite this article in press as: Soltani R, et al., Assessment oreforming for hydrogen production, International Journalj.ijhydene.2014.09.161the steam generator, and the air flow rate is summed to the
observation of 4% excess O2 in the combustion products.
So far, the effects have been studied of oxygen enrichment
and S/C ratio on carbon removal parameters. In this step,
which concentrates on carbon removal from flue gases, the
target is to analyse the hydrogen production rate compared
with carbon capture availability (Ncc) at the process end point
(flue gases) for various S/C ratios. For studying the effect on
hydrogen production of varying Ncc, another parameter needs
to be modified. For this purpose, the reformer temperature is
varied from 700 C to 1000 C for each S/C ratio, using thesimulated process. Note that, unlike for previous steps, the
el of SMR without heat integration.hydrogen production rate is fixed to permit determination of
the effect of only S/C ratio. In this parametric study, therefore,
the natural gas feed rate to the reformer is set to 1 kg/h and
consequently the hydrogen production rate changes. where
the hydrogen production rate was fixed, in this step of our
parametric study the natural gas feed rate to the reformer is
set to 1 kg/h and consequently the hydrogen production rate
changes. Other process settings are as mentioned in the cor-
responding sections. Further, for each S/C ratio, the resulting
optimum oxygen enrichment level from previous steps is
considered for the furnace air.
Table 2 e Natural gas consumption and CO2 emissionresults at selected ratios S/C.
Parameter S/C ratio
2.5 3.0 3.5 4.0 4.5
Reformer NG feed (kg/h) 2.89 2.87 2.84 2.73 2.65
Furnace fuel consumption (kg/h),
for air with 21% O2 by volume
0.00 1.50 2.30 3.05 3.50
Reactor conversion ratio (%) 73.0 77.9 81.7 84.8 87.38
Process CO2 emissiona (kg/h) 5.88 6.29 6.30 6.32 6.32
Overall CO2 emission (kg/h) 8.18 12.3 13.5 15.84 16.84
a Locations 1 & 2. Emissions from furnace are not considered.
f CO2 capture options from various points in steam methaneof Hydrogen Energy (2014), http://dx.doi.org/10.1016/
Results and discussion
In first step the modelled process is run for S/C ratios from 2.5
to 4.5 (commonly used S/C ratios) and corresponding effects
are determined of conversion ratios in the reformer reactor
and CO2 partial pressures, concentrations and partial mass
flows. It is observed in Table 2 that, by increasing S/C, the
conversion ratio increases. This is due to an increase in the
availability of steam for reforming, which requires less natu-
ral gas feed to be used in the reactor for the same hydrogen
production and, consequently, less carbon dioxide emission
from the reactor. However, increasing the ratio S/C also raises
the energy requirement for steam generation and reforming.
Table 3 lists data for the three capture criteria considered in
typical air with 21% oxygen.
It is thus observed that the greater the ratio S/C, the lowerpartial pressure and higher molar concentration are of major
s in the furnace feed for air with 21% oxygen.
ration (%) Total pressure (bar) CO2 partial pressure (bar)
0 16.6 3.04
9 1.03 0.53
1 1.03 0.18
0 16.6 3.10
4 1.03 0.56
5 1.03 0.13
9 16.6 3.15
8 1.03 0.58
0 1.03 0.12
2 16.6 3.18
1 1.03 0.60
9 1.03 0.11
1 16.6 3.20
Fig. 4 e Effects of S/C ratio on two CO2 capture criteria at
location 1.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e1 06is the natural gas usage. But this is not complete because, in
the furnace, the fuel consumption increases with S/C ratio.
Table 2 shows that for a ratio of 2.5, because of the PSA tail gas
contaminants (hydrogen, carbon monoxide and unreacted
methane), no additional external fuel is needed. For this ratio,
the PSA tail gas contains molar concentrations of around 20%
H2, 20% CO and the remainder water vapour and carbon di-
oxide. As S/C rises, the fuel injection to the furnace rises more
quickly than the rate of feed inlet to reformer decreases.
Therefore, higher steam-to-carbon ratios lead to higher car-
bon emissions to the atmosphere. Moreover, in cases where
more hydrogen production is needed, higher steam-to-carbon
ratios are advantageous. So far, it is observed that the relation
between S/C ratio and overall carbon dioxide emission is
linear. Now, we examine the effect of S/C ratio on overall
capture criteria which, for this study, are as follows:
1) CO2 partial pressure
2) CO2 molar concentration
3) CO2 mass ratio
Table 3 presents data for these parameters to identify for
which locations andwhich S/C ratios the carbon dioxide is in a
better condition to be captured. The conditions with higher
Table 3 e Characteristics of selected locations and S/C ratio
S/C ratio Location CO2 mass ratio (%) CO2 concent
2.5 1 73.0 17.9
2 73.0 51.4
3 100 17.8
3.0 1 57.0 18.1
2 57.0 52.8
3 100 12.8
3.5 1 51.0 18.1
2 51.0 53.4
3 100 11.8
4.0 1 46.0 18.2
2 46.0 53.6
3 100 11.0
4.5 1 44.0 18.22 44.0 53.39
3 100 10.78
Please cite this article in press as: Soltani R, et al., Assessment oreforming for hydrogen production, International Journaj.ijhydene.2014.09.161interests. Considering environmental aspects, higher ranges
of this criterion are better.
For better clarity, Figs. 4e7 illustrate the effect of increasing
S/C on several CO2 capture criteria. It is observed that the CO2concentration increases for process streams at locations 1 and
2 as S/C increases, yet it decreases at location 3 (flue gases).
This is due to fact that raising S/C increases the amount of
natural gas converted to hydrogen, which also produces more
carbon dioxide and raises its molar concentration at locations
1 and 2. An opposite result is observed in the furnace: the
addition of excess steam reduces the concentration of carbon
dioxide and, as a result of the higher fuel consumption and of
more parasitic contaminant nitrogen in the air inlet, more
inlet air is needed and the carbon dioxidemolar concentration
declines.
In Table 4, it can be seen that location 1 provides the best
condition for carbon dioxide removal for all five S/C ratios
tested. Based on the results, location 1 has better condition for
CO2 removal, almost twice as advantageous as location 2.
Compared to locations 1 and 2, location 3 is not appropriate for
CO2 capture, since it has the lowest value of Ncc. Another
result is that a lower S/C ratio implies better CO2 capture
possibilities. It may be asked why higher S/C ratios with lower1.03 0.62
1.03 0.11
f CO2 capture options from various points in steam methanel of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
the least likely capturing location. In this step, the effect
of oxygen enrichment of furnace feed is examined to deter-
mine its impact on Ncc of the flue gas stream. Oxygen
enrichment varies from 21% to 100% ideally for each of the S/C
ratios. Fig. 8 shows that for all S/C ratios the partial pressure of
CO2 increases but at a decreasing rate. From this behaviour
and the assumption that increasing enrichment raises costs,
the optimum oxygen enrichment level can be determined by
drawing tangent lines and perpendicular product to on the
curves. The optimum point selection is shown for one S/C
ratio (4.0) as an example (See Fig. 9). In Table 5, the oxygen
enrichment optimum levels for all S/C ratios are presented.
Again, an S/C ratio of 2.5 represents the best level due to
Fig. 5 e Effects of S/C ratio on two CO2 capture criteria at
Table 4 e Values of Ncc at various S/C ratios.
S/C ratio Location
1 2 3
2.5 0.40 0.20 0.03
3.0 0.32 0.17 0.02
3.5 0.29 0.16 0.01
4.0 0.27 0.15 0.01
4.5 0.26 0.15 0.01
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 0 7location 2.reformer natural gas feed and higher CO2 concentration and
partial pressures, leads to lower Ncc values? The answer is
that, if the environmental importance of CO2 mass ratio is
neglected, the higher S/C ratios lead to a higher Ncc because
both CO2 concentration and partial pressure increase. But,
higher S/C values, as stated earlier, concentrate CO2 emissions
in the furnace flue gases so, environmentally speaking, Imassshould be taken into consideration.
Based on results of previous section, CO2 removal
from location 3, being the source of 100% of emissions, is
permittingmore enrichment in a relatively economicmanner.
It can be seen from Table 6 that with enrichment of the
furnace feed, at optimum levels, a value of S/C of 2.5 is the best
condition for CO2 capture. This condition has the highest ef-
fect on Ncc, i.e. the optimum enrichment of the furnace feed
Fig. 6 e Effects of S/C ratio on two CO2 capture criteria at
location 3.
Fig. 7 e Effects of S/C ratio on CO2 mass ratio at locations 1
and 2.
Please cite this article in press as: Soltani R, et al., Assessment oreforming for hydrogen production, International Journalj.ijhydene.2014.09.161Fig. 8 e Effects of oxygen enrichment of furnace feed on
CO2 partial pressure for various S/C ratios.Fig. 9 e Optimum oxygen enrichment selection.
f CO2 capture options from various points in steam methaneof Hydrogen Energy (2014), http://dx.doi.org/10.1016/
increases Ncc from 0.03 to 0.10 (almost triple), while the effect
of enrichment on Ncc at location 3 for other S/C ratios is not
that significant compared to an S/C of 2.5. Overall, an S/C ratio
of 2.5 exhibits the best carbon capture condition and,
considering a higher weighting factor for the CO2 ratio of
location 3 (i.e., 2, 3, etc.) other than state 1, capturing would be
competitive with other locations. This higher weighting could
be a consequence of higher future carbon costs, incenting
industry to invest more on capturing carbon dioxide from flue
hydrogen in the PSA tail gases increases, which leads to higher
Fig. 10 e Effects of reformer temperature on mass flow rate
of produced H2.
Table 5 e Optimum oxygen enrichment for selected S/Cratios.
S/C ratio Optimum oxygen enrichment (%)
2.5 46
3.0 41
3.5 40
4.0 37
4.5 36
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e1 08gases. Furthermore, it can be seen from Table 6 that the ideal
enrichment has a greater effect on Ncc at locations 1 and 2 at
higher S/C ratios (4.0 and 4.5). For instance, ideal enrichment
increases Ncc at location 1 at S/C 4.0 from 0.27 to 0.44, whichis significant, while it raises the carbon capture number at
location 1 for an S/C ratio of 3.0 from 0.34 to 0.47. Also, the
effect of ideal enrichment on Ncc of the flue gas stream is
almost negligible for high S/C ratios.
Fig. 10 shows that higher reformer temperatures result in
higher hydrogen production rates, directly due to the higher
corresponding conversion ratios in the reformer reactor as
result of the higher temperatures. However, at temperatures
above around 850 C, the rate of increase decreases. It can beseen that the S/C ratio has a direct effect on hydrogen pro-
duction, in that high ratios facilitate high production rates,
primarily as a consequence of the high availability of steam
for natural gas feed in the reactor. Note that the natural gas
feed rate is constant for all conditions. So far, it is observed
that high reforming temperatures and high S/C ratios are
Table 6eNcc for selected S/C ratios for optimum and idealoxygen enrichment.
S/C ratio Location Carbon capture number, Ncc
Optimumenrichment
Ideal enrichment(100%)
2.5 1 0.42 0.482 0.21 0.24
3 0.10 0.17
3.0 1 0.34 0.47
2 0.18 0.25
3 0.04 0.07
3.5 1 0.30 0.46
2 0.16 0.25
3 0.03 0.05
4.0 1 0.27 0.44
2 0.15 0.24
3 0.03 0.04
4.5 1 0.25 0.42
2 0.14 0.24
3 0.03 0.03
Please cite this article in press as: Soltani R, et al., Assessment oreforming for hydrogen production, International Journaj.ijhydene.2014.09.161more appropriate for hydrogen production. Now, the effects of
S/C ratio and reforming temperature are examined as they
relate to carbon removal, via determination ofNcc (see Fig. 11).
Fig. 11 shows that, for all S/C ratios, as reactor temperature
rises, Ncc increases, peaks at some level and thereafter de-
creases. The peak temperature as seen in Fig. 11, is at higher
temperatures for lower S/C ratios. For an S/C ratio of 2.5, for
instance, the peak occurs at 900 C, while it occurs at 800 C forS/C 4.5. It is observed in the previous sections that lower S/Cratios provide better carbon capture conditions, and similar
results are observed here by varying reactor temperatures.
Nonetheless, at an S/C ratio of 3.0, unexpected behaviour is
observed for a temperature of 850 C. Unlike product hydrogenrate, which benefits from a high S/C ratio and a high reform
temperature, lower S/C ratios are more favourable regarding
carbon capture criteria and high reforming temperatures, over
a peak level, have negative effects on carbon capture. Also, the
following observed factor is important: when temperature
increases in the reformer andmore hydrogen is produced, PSA
tail gases channelled to the furnace carry higher amounts of
H2 because the hydrogen purifier is less than 100% effective.
This effect can partially offset the higher required combustion
energy for the provision of higher reforming temperatures.
The remaining required energy can be met by adding backup
fuel to the furnace. The reason for the peak in Fig. 11 is that,
above a certain level, the ratio of additional required energy in
the reformer (more combustion needed) and additionalFig. 11 e Effects of reformer temperature on Ncc.
f CO2 capture options from various points in steam methanel of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
C ratios result in better carbon removal conditions, the ratio of
hydrogen production rate to Ncc is less for those S/C ratios.
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 0 9Also, at high S/C ratiosmore hydrogen is produced, raising the
ratio of hydrogen production rate to Ncc.
Conclusions
The analysis and assessment performed in this comparative
study lead to the following main conclusions:
Higher S/C ratios provide higher CO2 partial pressures andconcentrations for synthesis gas and PSA tail gas streams.
Considering the CO2 presence at different locations, higherS/C ratios decrease the CO2 removal possibility byfuel and air intakes and consequently more nitrogen as
parasitic substance. Overall, it results in a lower value of Ncc.
One other important factor is examined: the comparative
behaviour of hydrogen production and carbon removal. The
aim of this task is to determine at what condition the process
is more appropriate in terms of hydrogen production and
what conditions result in better carbon removal. Fig. 12 shows
that reforming temperature continues to have a positive effect
on hydrogen production, except for an S/C ratio of 2.5 at
800 C, for which a slight drop is observed. Also, since lower S/
Fig. 12 e Effects of varying reformer temperature on H2/Nccratio.decreasing the value of criterion Ncc at location 3, due to
the concentration of CO2.
If less pollution is desired (i.e., less overall natural gasconsumption in the reformer and the furnace), then an S/C
ratio of 2.5 is best.
If the hydrogen production rate is of more importance thanenvironmental pollution, then an S/C ratio of 4.5 is best.
Oxygen enrichment is found to have highest positiveimpact on Ncc for an S/C ratio of 2.5, for location 3.
If a weighting system is considered for the three studiedCO2 capture criteria changes, the results change notably.
For instance, a weighting factor of more than unity for
complete CO2 mass ratio at location 3 (which depends on
the pollution regulations of a region) raises the CO2 capture
from this location relative to the other two locations.
High S/C ratios increase hydrogen production rates, as aconsequence of the high availability of steam for natural
gas feed in the reactor.
1e10; 2005 [Costa Verde, Brazil].[16] Park JH, BeumHT, Kim JN, Cho SH. Numerical analysis on the
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[17] Ko D, Siriwardane R, Biegler LT. Optimization of a pressure-swing adsorption process using zeolite 13X for CO2sequestration. Ind Eng Chem Res 2003;42(2):339e48.
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conversion ratios in the reformer reactor as a result of the
higher temperature.
The highest value of Ncc results for an S/C ratio of 2.5 fortemperatures between 850 C and 950 C.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e1 010Please cite this article in press as: Soltani R, et al., Assessment oreforming for hydrogen production, International Journaj.ijhydene.2014.09.161f CO2 capture options from various points in steam methanel of Hydrogen Energy (2014), http://dx.doi.org/10.1016/
Assessment of CO2 capture options from various points in steam methane reforming for hydrogen productionIntroductionMethods consideredSteam methane reformingCarbon dioxide captureOxygen enrichment
Model descriptionApproachResults and discussionConclusionsReferences