Article

Renewable Hydrogen Production Processes

for the Off-Gas Valorization in Integrated Steelworks
through Hydrogen Intensified Methane
and Methanol Syntheses
Antonella Zaccara 1, Alice Petrucciani 1, Ismael Matino 1, Teresa Annunziata Branca 1,
Stefano Dettori 1, Vincenzo Iannino 1, Valentina Colla 1,*, Michael Bampaou 2
and Kyriakos Panopoulos 2
1 Scuola Superiore Sant’Anna, TeCIP Institute, Via Moruzzi 1, 56124 Pisa, Italy;
a.zaccara@santannapisa.it (A.Z.); a.petrucciani@santannapisa.it (A.P.); i.matino@santannapisa.it (I.M.);
t.branca@santannapisa.it (T.A.B.); s.dettori@santannapisa.it (S.D.); v.iannino@santannapisa.it (V.I.)
2 Chemical Process and Energy Resources Institute (CPERI), Centre for Research and Technology Hellas

(CERTH), 6th km. Charilaou-Thermi, 57001 Thessaloniki, Greece; bampaou@certh.gr (M.B.);

panopoulos@certh.gr (K.P.)
* Correspondence: valentina.colla@santannapisa.it; Tel.: +39-050-882-328

Received: 3 November 2020; Accepted: 16 November 2020; Published: 18 November 2020

Abstract: Within integrated steelmaking industries significant research efforts are devoted to the
efficient use of resources and the reduction of CO2 emissions. Integrated steelworks consume a
considerable quantity of raw materials and produce a high amount of by-products, such as off-
gases, currently used for the internal production of heat, steam or electricity. These off-gases can be
further valorized as feedstock for methane and methanol syntheses, but their hydrogen content is
often inadequate to reach high conversions in synthesis processes. The addition of hydrogen is
fundamental and a suitable hydrogen production process must be selected to obtain advantages in
process economy and sustainability. This paper presents a comparative analysis of different
hydrogen production processes from renewable energy, namely polymer electrolyte membrane
electrolysis, solid oxide electrolyze cell electrolysis, and biomass gasification. Aspen Plus® V11-
based models were developed, and simulations were conducted for sensitivity analyses to acquire
useful information related to the process behavior. Advantages and disadvantages for each
considered process were highlighted. In addition, the integration of the analyzed hydrogen
production methods with methane and methanol syntheses is analyzed through further Aspen
Plus®-based simulations. The pros and cons of the different hydrogen production options coupled
with methane and methanol syntheses included in steelmaking industries are analyzed.

Keywords: renewable hydrogen production; electrolysis; biomass gasification; off-gas recovery;

methane production; methanol production; steelworks sustainability; process simulation

1. Introduction
The steel industry is energy intensive, being the second-largest industrial energy consumer and
one of the most relevant CO2 emission sources [1,2]. Steel production is mainly based on fossil fuels
for energy supply and accounts for about 4–5% of total world CO2 emissions [3,4]. In particular, for
every ton of steel produced, 1.9 tons of CO2 are emitted [4].

Metals 2020, 10, 1535; doi:10.3390/met10111535 www.mdpi.com/journal/metals

Metals 2020, 10, 1535 2 of 26

The primary steel production in integrated steelworks needs more energy than the steel
manufacturing in electric arc furnace (EAF), where scrap is used and no chemical energy to reduce
iron ore is necessary [5]. The blast furnace (BF) and the basic oxygen furnace (BOF) exploit an amount
of energy in the range of 13–14 GJ/t of produced steel, while the scrap/EAF route needs 4–6 GJ per
ton of produced steel [6]. Generally, in the steelworks energy costs account for about 20% of the total
operation costs [7,8].
The ever increasing international attention toward environmental issues, landscape and health
protection, as well as tightening of environmental regulations, exert a further pressure on steel
companies in order to increase the sustainability of their production cycles [9]. During the last United
Nations Climate Change Conference in Paris (COP 25), governments around the world approved that
international climate policy should limit the increase of global average temperature to less than 2 °C
with respect to pre-industrial period [10]. To reach this target a substantial reduction of
anthropogenic green house gases (GHG) emissions is required in all sectors. European process
industries are therefore focusing many actions and research activities toward the optimization of
resource management and the reduction of climate-changing emissions. In this scenario, it is
undeniable that the European steel industry is committed to achieve important objectives in terms of
production costs, environmental impact, and sustainability [4]. This can be observed, for instance, in
the work described in [11] where different simulation techniques have been exploited for analyzing
future scenarios dedicated to improving the management of water or solid by-products streams in
integrated steelmaking. The importance of improving the sustainability of steelworks, including the
electric one, is also evident in the work presented in [12], where Aspen Plus® simulation model is
applied in order to study the correlations among electric energy consumption, steel grade, slag
quantity, and composition. The objectives of sustainability and reduction of costs are usually linked,
as an efficient use of energy and resources allows reducing production costs, GHG emissions and
environmental impact [13]. Therefore, energy saving is essential to guarantee the competitiveness of
steel companies, as it also entails significant reductions of operative costs [2,5]. Over the last few
decades, the steel industry has reduced its energy consumption by 50%. Currently, significant further
reductions of the fossil fuel derived energy consumption and carbon emissions cannot be achieved
unless introducing breakthrough steelmaking technologies that can use renewable energy most likely
in the form of hydrogen [4].
Integrated steelworks are relevant resource consumers, but they also produce an important
volume of by-products, among which are the off-gases. A correct and efficient management of all by-
products [14], including process gases, can play a decisive role in increasing the economic and
environmental sustainability of the integrated steel production route [15].
Process off-gases are generated in the three main steps of steel production in an integrated steel
plant: Coke oven gas (COG), blast furnace gas (BFG), and basic oxygen furnace gas (BOFG). These
by-products are a very precious source of energy and a valid alternative to natural gas (NG) in several
operations: They can be exploited to meet the energy demand of production processes, to produce
steam and as energy sources for power plants, which can both satisfy internal electricity demand and
offer part of their production to the external energy market [16]. The formation of off-gases is
obviously related to production steps and it cannot be avoided, but a reliable prediction of its
production or demand can improve their management and reuse, as shown in detail in [17], where
the description of two models based on Echo State Neural Networks (ESN) is reported. These models
aim to forecast the amount and energy content of BFG and its demand by main users, in order to
obtain useful information for the optimization of the off-gases management by a decision support
system. The strong interaction between production scheduling and off-gases network makes the
management of these by-products very complex, and their distribution is often difficult to optimize,
as discussed by Maddaloni et al. [18], who presented a quadratic programming for the optimization
of off-gas management. In particular, when an overproduction of off-gases occurs, the excess gas
needs to be flared with consequent waste of energy and CO2 emissions, while, when their production
is not sufficient to meet the demand, NG is used leading to economic and environmental costs. An
optimal exploitation of those off-gases plays a crucial role in integrated steelworks sustainability.
Metals 2020, 10, 1535 3 of 26

Therefore, in the last few years, companies and research institutions have developed several projects,
aimed at investigating the best use of this important resource. For instance, a decision support system
based on flowsheeting static models forecasting gas consumption and demands was developed by
Porzio et al. in [19]. Multi-objective optimization techniques have been intensively applied to find
optimal distribution of process gases, such as Genetic Algorithms (GA) [20] and Mixed Integer Linear
Programming (MILP) [21,22], which are suitable to face the multiple and often complex constraints
which characterize this optimization problem and allow considering also the cost of gas network
structural modifications [20]. Further studies compared such approaches [23], while a more recent
study introduced multi-period optimization to this purpose [24].
A step ahead in the internal off-gas optimized management was carried out in the project co-
funded by the European Union through the Research Fund for Coal and Steel (RFCS), which is
entitled “Optimization of the management of the process gases network within the integrated
steelworks—GASNET” [17,18,25]. Within this project a Decision Support System (DSS) was
developed helping plant managers to optimally exploit process off-gases by minimizing energy
wastes and flaring and considering environmental and economic constraints as well as synergies
among producers and consumers of gas, heat, electricity, and steam.
As far as the utilization of integrated steelworks off-gases within different chemical processes is
concerned, Maruoka and Akiyama in [26] investigated the topics of methanol production by first
steps of methane production through steam reforming and ad-hoc energy recovery process. Other
works have also aimed at investigating the use of exhaust gases for the production of chemical
substances, as analyzed in [27]. Some recent projects and researches, such as VALORCO [28],
Carbon2Chem [29], Renewable-Steel-Gases [30], and the works of Kim and Han [31], of Gao et al.
[32], and of Shin et al. [33], were dedicated to the production of methane, methanol, ammonia and
other chemicals through adequate exploitation of off-gases. However, often the composition of these
gases is not suitable to reach high reaction yield in the production of methane and methanol. Indeed,
the content of hydrogen (H2) in off-gases is often insufficient to reach the required stoichiometric ratio
for the reactions involved in the methane and methanol productions. To avoid this problem, the
addition of further H2 is a key element in methane and methanol syntheses when steelworks off-gases
are used as feed. Therefore, a suitable hydrogen production process must be selected in order to
obtain advantages in terms of process economy and sustainability.
The work presented in this paper is part of the project entitled “Integrated and intelligent
upgrade of carbon sources through hydrogen addition for the steel industry (i3upgrade)” co-founded
by the European Union through the RFCS, which aims at valorizing off-gases for the production of
methane and methanol through the improvement of steelworks off-gases by adding hydrogen. In
this way, besides economic advantages, carbon dioxide emissions can be reduced, if added H2 is
produced through “green” technologies. The CO2 emission reduction is obviously not only obtained
through a suitable technology choice for H2 production, but it is also due to the adopted system
operating conditions and to the exploitation of renewable material and energy sources.
In particular, the paper presents a comparative analysis of different H2 production processes
exploiting renewable sources for identifying advantages and limitations for their integration in
methane and methanol syntheses, whose investigation is also introduced in the paper. The selected
and investigated processes are polymer electrolyte membrane (PEM) electrolysis, solid oxide
electrolyzer cell (SOEC) electrolysis and biomass gasification. The comparison was carried out
through Aspen Plus® V11-based simulations exploiting ad-hoc developed models. Sensitivity
analyses have been carried out in order to analyze the modelled processes’ behavior at different
operating conditions.
The main novelties of the proposed work lie in the modelling approach adopted for the three
considered technologies for green hydrogen production that allow both a comprehensive analysis of
the considered technologies and especially the integration of the developed models with further
models related to methanol and methane syntheses, as well as in the carried out simulation analyses.
In order to consider a credible inclusion in the steel industrial context, some realistic scenarios are
Metals 2020, 10, 1535 4 of 26

analyzed related to a medium size integrated steelmaking plant regarding different utilization
options of the off-gases.
The paper is organized as follows: Section 2 presents a state-of-the-art related to technologies of
H2 production with low environmental impact and suitable for integration in steelworks. It also
contains a brief summary of methane and methanol syntheses. In Section 3 the Aspen Plus®-based
simulations of the selected hydrogen production methods are reported and their integration in
methane and methanol synthesis of steelworks off-gases is introduced. Section 4 discusses the main
results and Section 5 reports the main concluding remarks of this work.

2. Sustainable Hydrogen Production

Hydrogen is a clean gas and a source of environmentally friendly energy, but it is not easily
obtainable in nature. There are different processes for hydrogen production from conventional and
alternative energy sources, as schematically summarized in Figure 1 [34]. Presently, the global
hydrogen production mainly derives from fossil fuels and the most developed technologies are
pyrolysis [34] and hydrocarbon reforming, such as steam methane reforming (SMR) that is the most
common and cheapest method used to produce hydrogen in large scale [35,36].

Renewable
Fossil Fuels
Sources

Hydrocarbon Hydrocarbon Biomass Water

Reforming Pyrolysis Process Splitting

Steam
Biological Eletrolysis
Reforming

Partial Thermochemi
Thermolysis
Oxidation cal

Autothermal
Photolysis
Reforming

Figure 1. Hydrogen production methods [34].

In the present work, an accurate literature analysis [34,37–45] has been conducted in order to
study and compare different techniques for the hydrogen production. According to the state-of-the-
art investigation, water electrolysis and biomass gasification are the most promising sustainable
technologies with low environmental impact and with an appropriate development degree for the
integration in steelworks. The following two subparagraphs provide a description of both
approaches.

2.1. Electrolysis
Water electrolysis is an electrochemical process: The water is separated in hydrogen and oxygen
by exploiting electrical and thermal energy [46]. The anode and the cathode are immersed in the
electrolytes and the separation of hydrogen takes place when electrical current is applied [34]: At the
cathode the reduction occurs and the hydrogen is produced, while at the anode an oxidation reaction
allows the production of oxygen [47].
This technology produces hydrogen with very high purity without carbon and sulfur
contamination [36,46]. The main issues are Joule effect and parasite reactions within solution that
cause high-power dissipation with a consequent costs increase [48].
The electrolysis technique is generally indicated for producing hydrogen close to the users,
especially due to the compactness of the required plant, and for small-scale applications [36]. This
process is energy intensive, but can be coupled to renewable energy sources in order to achieve a
completely “green” and sustainable hydrogen production [49]. Considering these aspects, the
Metals 2020, 10, 1535 5 of 26

renewable hydrogen system can be one of the solutions to be followed to obtain hydrogen without
impact on carbon footprint. In addition, the use of solar and wind energy with hydrogen storage
system can result in using renewable and clean resources as well as in environmental protection [39].
There are different technologies for electrolysis process, the main are: Low temperature (70–90
°C) alkaline electrolysis (AEL), proton exchange membrane (PEM), and high temperature (650–850
°C) solid oxyde electrolyzers cells (SOEC). In particular, the last two methods are more suitable for
the purpose of the present investigation, as AEL is sensitive to the effects of fluctuations in power
supply (i.e., in the case of renewables): Its efficiency is compromised by presenting large inertia in
transporting ions [50]. A further pioneering technology is anion exchange membrane (AEM)
electrolysis. The AEM combines the low cost of alkaline electrolysis and the high power of PEM, but
this technology is still under development [51].

2.1.1. PEM Electrolysis

The PEM electrolysis is characterized by the use of thin splitting polymer membrane of Nafion®
instead of the electrolytic liquid in the AEL process. The thickness of the membrane is usually 0.2 mm
to ensure a close proximity of the electrodes [52,53]. A simplified scheme of PEM is depicted in Figure
2a.

(a) (b)

Figure 2. (a) Schematic representation of polymer electrolyte membrane (PEM) electrolysis; (b)
Schematic representation of solid oxide electrolyzer cell (SOEC) electrolysis.

At the anode the oxidation of water occurs, as follows:

H2O  O2 + 2H+ + 2e−.

At the cathode, the hydrogen ions are reduced, according to the following reaction, and pass
through the membrane [52,53]:
2H+ + 2e−  H2.
In general, the following operating conditions hold for PEM:
 Operation temperature To = 40 ÷ 80 °C [52,54,55];
 pressure of the cathode section Pcat < 35 bar [52,56];
 pressure of the anode section Pan < 3.5 bar [52,56];
 fresh water consumption of 0.9 ÷ 1 L of H2O/Nm3 of H2 (10.1 ÷ 11.2 kg of H2O/kg of H2) [57–60];
and
 energy demand of 54 ÷ 80 kWh/kg of H2 [55,57–59,61].
PEM is not sensitive to the effects of fluctuations in power supply thanks to the rapid response
of the proton transport across the polymeric membrane. For this reason, it is suitable to the use when
Metals 2020, 10, 1535 6 of 26

renewable energy sources are available. On the other hand, the investment costs are high, as the
membranes and the electrodes are composed of noble metals [50].

2.1.2. SOEC Electrolysis

The SOEC electrolysis is composed by three-layer solid structure (porous cathode, electrolyte,
and porous anode) and an interconnect plate as shown in Figure 2b [45,62].
The steam at high temperature is introduced at the cathode side and reduced into hydrogen,
producing oxide anions, according to the following reaction:
2H2O + 4e−  2H2 + 2O2−.
At the anode, the oxide anions, that have migrated through the electrolyte, are combined to form
oxygen molecules, releasing electrons, according to the following reaction [45,62]:
2O2−  O2 + 4e−.
The SOEC electrolysis operating conditions are:
 To = 600 ÷ 1000 °C [52,62,63];
 Pstack = 1 ÷ 30 bar [64];
 fresh water consumption of 9.1 kg of H2O/kg of H2 [55]; and
 energy demand of about 50.3 kWh/kg of H2 (i.e., it requires less electric power than other water
electrolysis technologies) [55].
This technology allows achieving a higher energy conversion efficiency with respect to other
water electrolysis techniques [62]. On the other hand, SOEC technologies need high purity of the
water as feedstock to avoid corrosion and early degradation of components [64]. The high
temperature operating mode leads to high maintenance costs and generally the SOEC is convenient
when a high-temperature heat source is available [52,64].

2.1.3. Hydrogen Production from Biomass

Presently, in the case of hydrogen production from biomass, the most important thermochemical
conversion technologies are pyrolysis and gasification [43,65]. Among these technologies, steam
gasification can provide the highest yields of hydrogen that can reach 52% (thermal-to-hydrogen)
[34].
The biomass gasification process converts biomass into a gaseous fuel (syngas) at high
temperature and without combustion with a controlled amount of air, oxygen and/or steam [34,66].
Bio-syngas contains mostly CO and H2, and the other elements are CO2, CH4, light hydrocarbons
(HC), coal, and tar [43,67]. Absorbers and special membranes are generally exploited to separate the
hydrogen from the gas stream [66].
The gasification processes of biomass are divided into three main groups (which are
summarized in Table 1), depending on the type of gasifying agent [67].
Among the different biomass gasification processes, the steam gasification is the best
compromised in terms of hydrogen efficiency and costs for the purpose of this work.

Table 1. Comparison between biomass gasification processes [67].

Oxygen
Air Gasification Steam Gasification
Gasification
Product heating
Low, 4–6 Medium, 10–15 High, 15–20
value (MJ/Nm3)
CO, H2, Water, CO2, HC, H2, CO, CO2, CH4, light
Products CO, H2, HCs, CO2
Tar, N2 HCs, tar
Average product gas H2 15%, CO 20%, CH4 H2 40%, CO 40%, H2 40%, CO 25%, CH4
composition 2%, CO2 15%, N2 48% CO2 20% 8%, CO2 25%, N2 2%
Metals 2020, 10, 1535 7 of 26

Reactor temperature
900–1100 1000–1400 700–1200
(°C)
Cost Low High Medium

2.2. Integration of Hydrogen Production with Methane and Methanol Synthesis

The integration of the sustainable hydrogen production with methane and methanol synthesis
based on steelwork off-gases as feedstock is necessary in order to achieve the required stoichiometric
ratio because the compositions of steelworks gases are often not suitable without hydrogen addition.
The typical volume compositions of steelworks process gases are:
 BFG: CO 23%, H2 4.5%, CO2 21%, N2 51%;
 BOFG: CO 60.9%, H2 4.3%, CO2 17.2%, N2 15.5%, CH4 0.1%; and
 COG: CO 4.6%, H2 48.9%, CO2 3.2%, N2 3.75%, CH4 21.35%, CxHy 1.9% [16].
The methanation is a method to obtain methane from carbon monoxide and carbon dioxide with
the addition of the required stoichiometric hydrogen. In the last few years this process has gained the
attention of researchers as it is considered a promising solution to store energy in future energy
systems. Therefore, many recent studies and projects are studying the possibility of improving this
technology or simplifying the process itself to make it competitive. For instance, Neubert et al. in [68]
proposes a new catalytic methanation reactor with heat pipe integration into a structured reactor,
that can be considered suitable alternatives to classical reactors for CO2 methanation due its efficient
temperature control; Leimert et al. in [69] suggests the combination of heat pipe reformer biomass
gasification process with hydrogen intensified methanation. Further studies on the production of
methane from biomass are reported in [70–72], as well as from COG in [73]. In this last work the use
of COG for methanol (MeOH) synthesis is also described. The catalytic methanol synthesis is
exothermic and thermodynamically favored by lower temperatures and higher pressures. Today
most of the world methanol production is covered with natural gas derived synthesis gas that after
H2/CO ratio adjustment is catalytically processed at 50–100 bar and temperatures between 200–300
°C (temperatures required for the activation of the employed catalyst). The stoichiometric number
( ) characterizes the hydrogen input related to CO and CO2 components for MeOH synthesis:
[ ]−[ ]
= (1)
[ ]−[ ]
where [ ], [ ] and [ ] denote the molar flows of the feed components. If SN = 2, the reaction
is stoichiometric, although the most favorable case is slightly over-stoichiometric [74]. Methanol
synthesis normally results in conversion close to what the thermodynamic equilibrium regulates.
Thus, any additional input hydrogen is not consumed in the process and remains unexploited in the
product gases [75]. Table 2 shows the reaction equations for the methane and methanol syntheses
from CO and CO2 [69,76].

Table 2. Reaction equations for the methane and methanol synthesis of CO and CO2 [69,76].

Reaction Equation Reaction Enthalpy ΔΗ0 (kJ/mol)

CO + 3H2  CH4 + H2O −206
Methane synthesis CO2 + 4H2  CH4 + 2H2O −165
CO2 + H2  CO + H2O +41
CO + 2H2  CH3OH −90
Methanol synthesis CO2 + 3H2  CH3OH + H2O −49
CO2 + H2  CO + H2O +41

3. Materials and Methods

The software Aspen Plus® was adopted for the simulation of hydrogen production with the
selected renewable technologies. In the following paragraphs each model is described.
Metals 2020, 10, 1535 8 of 26

3.1. PEM Electrolysis

The model has been developed, tuned and validated by considering both literature data [52–
56,64] and producers’ information [57–61,77]; some assumptions and simplifications have been
carried out. The model is stationary and the Aspen Plus® flowsheet is depicted in Figure 3.
The input stream of the model is fresh water, which is assumed to be already deionized, at a
temperature of Tin = 25 °C and a pressure Pin = 1 bar. The needed heat is provided in the heater block
“HEATER” in order to reach the temperature of 80 °C. Moreover, the values Pcat = 15 bar and Pan = 1.5
bar (10% of cathode section pressure) are considered [52,54–56].
The stack is the crucial component in the PEM Electrolysis model; it is represented in the orange
square in the Figure 3 and it is where water split reaction occurs. In the reactor “REAC” energy is
supplied until the outlet temperature reaches the maximum value of 90 °C (maximum difference
between inlet and outlet is set to 10 °C in order to avoid the overheating of the stack) [64].
The hydrogen and oxygen are separated between the cathode and the anode sections. In
particular, the permeation of the produced gases through the membrane, which are indicated as
and , respectively, is represented with the combination of separators units and ad-hoc
written calculator blocks (i.e., “CH2PERM” and “CO2PERM”) according to the following relations
[57,64]:
.
= (0.0009 ∙ )∙∆ ∙ (2)
where is the temperature, ∆ is the pressure difference between cathode and anode, is the active
area (estimated) [54,56].
= 0.5 ∙ (3)
In addition, in order to consider the water dragging by hydrogen ions (protons) from anode to
cathode, it is assumed that two moles of water are dragged per mole of hydrogen [56,64,78].
All the steps required for the purification of the hydrogen are included in the cathode section in
the fuchsia square in Figure 3. In particular:
 The flash unit “SEPH2OCT” removes the dragged water, that is recovered and recycled at the
inlet of the PEM;
 the block “H2PUR” represents the last separation step that purifies the hydrogen stream from
the remaining water and from the oxygen traces that is permeated from the anode (the molar
fraction composition of stream “H2CAT” before the purification is: H2 = 0.951, H2O = 0.048, O2 =
0.001).
On the other hand, the anode section contains the flash unit “SEPH2OAN,” which represents
the separation step of water from the produced oxygen (the molar fraction composition of stream
“O2PROD” before the separation step is: O2 = 0.867, H2O = 0.133, H2 trace); separated water is
recycled.
The final step is the compression of hydrogen to achieve a storage pressure of 30 bar; the related
compressor “STORAGE” provides the required power.
A further block is the thermodynamic efficiency calculator “THEREFF,” which calculates the
thermodynamic efficiency of the electrolyzer through the following equation [52]:
∙
= (4)

where is volumetric flow of produced H2 [Nm3/s], is the low calorific value (LCV) of H2
[MJ/Nm ] and
3 is the Total consumed energy [MW].
Metals 2020, 10, 1535 9 of 26

Figure 3. Aspen Plus® flowsheet of PEM electrolysis model.

Metals 2020, 10, 1535 10 of 26

3.2. SOEC Electrolysis

The SOEC model has been built considering the following main assumptions and
simplifications:
 The simulation is developed under the assumption of thermo-neutral conditions [54,64,79];
 at the cathode outlet of the stack, there is always a mixture of steam and hydrogen because the
steam conversion in the stack is lower than 100%, therefore the yield of conversion of steam into
hydrogen is fixed at 75% [54,64];
 SOEC is supposed as an isothermal steady-state system [54,62,80];
 a fraction of the cathode outlet flow is recirculated to the cathode inlet in order to exploit the
hydrogen capacity of counteracting undesired oxidation of the electrodes by the action of pure
steam and to preheat the fresh inlet stream [45,54,62,81,82];
 at the cathode, the flash unit is fed with steam and recycled hydrogen. At the anode, the
formation of oxygen is ensured by the recombination of the oxide anions, produced on the
cathode and transported through the solid electrolyte [50,83]; and
 perfect thermal insulation on the stack is considered and heat losses toward the environment are
neglected [54].
The Aspen Plus® flowsheet of SOEC model is depicted in Figure 4.
The fresh water already deionized (as high water purity is required [64]), is the input stream of
the model with Tin = 20 °C and Pin = 1 bar [55].
The first step of the model is the increasing of the fresh water pressure up to 30 bar [64]. The
temperature of the water rises to 749 °C in the heat recovery section (blue frame in Figure 4) and high
temperature steam is obtained.
In the stack (orange square in Figure 4), at the operative conditions of 877 °C and 30 bar, the
water split occurs [64] and the produced hydrogen and oxygen are separated in the cathode and
anode sections of the model. The cathode outlet flow is characterized by a mixture of steam and
hydrogen. In order to supply the hydrogen required in the gas feeding mixture (10% mol of H2 and
90% mol of H2O), preventing the oxidation of the electrode material and preheating the feed, a
fraction of the cathode outlet flow is sent to the cathode inlet [45,63,83].
In the last step, the pure hydrogen is achieved by condensing the steam in “HE-1” block [54] and
separating it from water in the flash unit “FLASH-H2” (stream H2: Mole fraction H2 95%, H2O 5%)
followed by the separator block “SEP-H2”. The condensed water is reused in the process [27].
The simulation is carried out considering several literature data with particular attention to the
data related to energy and water consumption reported in [55], that are mainly employed for the
validation of the model.

3.3. Biomass Gasification

The biomass gasification model has been developed considering the following main
assumptions and simplifications:
 The process is considered steady state and isothermal;
 the operations occur at atmospheric pressure (~1 bar);
 the pressure and temperature are considered uniform inside the gasifier;
 the heat losses occurring in the gasifier are neglected;
 the ash and tar formations are neglected: Char has been simplified as pure carbon;
 sulfur (S) is converted to H2S; and
 pressure drops are ignored [84–87].
Metals 2020, 10, 1535 11 of 26

Figure 4. Aspen Plus® flowsheet of SOEC electrolysis model.

Metals 2020, 10, 1535 12 of 26

The biomass input of the model is wood residue with a low content of water, therefore, the
drying step is not required. The biomass and the ash are specified as non-conventional components.
The proximate analysis and ultimate analysis inserted in Aspen Plus® are illustrated in Table 3 [85].

Table 3. Proximate analysis and ultimate analysis inserted in the model [85].

Wood Residue  Moisture 5.01 wt.%

Proximate Analysis (wt.%, dry) Ultimate Analysis (wt.%, dry)
Volatile matter 81.81 C 50.08
Fixed carbon 17.83 H 6.70
Ash 0.36 O 42.51
N 0.16
S 0.20
Cl -
Ash 0.36

The Aspen Plus® flowsheet of the model is illustrated in Figure 5. The decomposition of biomass
in its chemical compounds (i.e., hydrogen, carbon monoxide, carbon dioxide, oxygen, nitrogen, and
sulfur) with pyrolysis process is the first step and it is carried out in the reactor “PYRO”. In the
separator block “CHAR-SEP” the separation of solid (char) from volatile part takes place. The steam,
which is necessary for the gasification, enters the block “GASI” at 150 °C and 1 atm. The gasification
occurs in the reactor “GASI” based on Gibbs free energy minimization by using as calculation option
the “Restrict chemical equilibrium”. The reactions are showed in Table 4.

Table 4. Restricted chemical equilibrium in the GASI reactor.

Reaction Heat of Reaction

Reaction Scheme Reaction Name
Number ΔH (kJ/mol)
1 C + O2  CO2 Carbon combustion −393.0
2 C + 0.5 O2  CO Carbon partial oxidation −112.0
3 C + CO2  2 CO Boudouard reaction +172.0
4 C + H2O  CO + H2 Water gas shift reaction +131.0
5 CO + H2O  CO2 + H2 Water gas shift reaction −41.0
6 C + 2 H2  CH4 Methanation of carbon −74.0
7 H2 + 0.5 O2  H2O Hydrogen partial combustion −242.0
8 CH4 + H2O  CO + 3 H2 Steam reforming of methane +206.0
9 H2 + S  H 2 S H2S formation −20.2

The char gasification occurs in plug flow reactor “CHAR-GAS”, where the kinetic reaction is
inserted. The power law reaction kinetic has been used with the parameters reported in [88].
The cleaning of the produced syngas is the last stage of biomass gasification; this stage allows
removing water, ash and H2S and obtaining a syngas with high content of hydrogen. It is carried out
in the simulation by different separation units in series.
The validation of the model has been conducted using literature data, especially the ones
provided by [89].
Metals 2020, 10, 1535 13 of 26

Figure 5. Aspen Plus® flowsheet of biomass gasification model.

Metals 2020, 10, 1535 14 of 26

3.4. Integration of Hydrogen in Methanol and Methane Synthesis

The integration of the three types of renewable hydrogen production, described in previous
paragraphs, with methanol and methane syntheses by using steelworks off-gases as feedstock is
shown in the simplified overall process flowsheet depicted in Figure 6. In a further part of the work
carried out in i3upgrade project, methane and methanol syntheses from steelworks off-gases utilizing
renewable hydrogen have been modelled by Aspen Plus®. However, the description of these models
is out of the scope of the present paper and only a brief overview is provided here.
The first step for methanol synthesis is the compression to about 70 bar of the feed gas in three
stages with intermediate cooling and free water removal to reduce compression costs and avoid
unwanted side-effects of liquid water in the machinery and the reactor. Subsequently, H2 is added in
the required stoichiometric ratio and the inlet mixture is preheated before entering the synthesis
reactor. After the reactor there is a separation step in which the liquid (mainly methanol and water)
and gaseous (unreacted hydrogen and the rest of the carbonaceous feedstock) products are flashed.
The last step is the purification of methanol in a distillation column, which removes the contained
product water.
The first step for methanation only requires a moderate compression of the gaseous feed at <10
bar, with single stage compression applied. After the compression, renewable H2 is added and the
mixture is preheated and directed to the reactor. The products contain water that is separated via a
flash separator as liquid.

Figure 6. Integration of hydrogen production with methanol and methane synthesis: Stream 1—feed,
stream 2—H2 feed, stream 3—reactor inlet, stream 4—reactor outlet.

4. Results and Discussion

4.1. Hydrogen Production Model Results

As mentioned before, the hydrogen produced is necessary to the enrichment of off-gases from
integrated steelworks. From preliminary analyses about different possible case studies related to
partial or complete use of steelworks off-gases produced in a medium size integrated steelworks
(with about 6 Mton of annual steel production capacity) for methane and/or methanol production,
on an average base, the amount of needed hydrogen is about 10 kg/s. Although this H2 demand is
significant, it serves as the calculation basis hereafter; deeper details about the problems related to
the required quantity of hydrogen are provided in Section 4.2. The developed hydrogen production
Metals 2020, 10, 1535 15 of 26

models have been used in order to simulate the generation of this amount of hydrogen and the results
reported in Tables 5 and 6 have been obtained. They are in line with literature and producers’ data
as can be seen by comparing them with the information reported in Sections 2.1 and 2.2.

Table 5. Hydrogen production results based on PEM and SOEC models.

Required Required Thermodynamic

H2O Feed H2O Feed
Power Energy Electrolyser Efficiency
(kg/s) (kg/kg of H2)
(MW) (kWh/kg of H2) (%)
PEM 97.8 1988.9 54.8 9.8 62.60
SOEC 89.2 1494.5 41.6 8.9 83.50

Table 6. Hydrogen production results based on biomass gasification model.

Biomass Feed Efficiency Syngas Molar Composition

(kg/s) (%) (%)
CO2 = 18.4
CH4 = 0.4
77.39 12.90
H2 = 61.7
CO = 19.4

As expected, the PEM electrolysis requires more water and power than the SOEC as shown in
Table 5 but as described in Section 2.1, it is particularly suitable to address power supply fluctuations
of green energy with respect to SOEC. In addition, in both cases the required power is composed of
different contributions:
 In the case of PEM, 72.20% of the power is required for electrolysis, 26.21% are dissipations,
0.75% is needed for water heating, 0.03% for pumping and 0.81% for H2 compression;
 for SOEC, 84.98% of the power is required from the stack, 15.00% is needed for steam generation
and the remaining 0.02% for pumping.
Obviously, SOEC requires a considerable energy amount for steam generation and, for this
reason, it is convenient if a high-temperature heat source is available, such as in the case of steelworks.
On the other hand, although a renewable source is used, the biomass process produces a
considerable amount of carbon dioxide as shown in Table 6, that negatively affects both the purity of
hydrogen, which thus requires highly efficient separation steps, and carbon footprint of the process.
A series of sensitivity analyses have been conducted by varying different operating conditions,
in order to better understand the behavior of the three processes and finding useful indications.

4.1.1. PEM Electrolysis

In the PEM electrolysis model, the operating pressure and temperature have been changed and
the following variables have been monitored:
 Required power (before and after the final compression to obtain H2 stored at 30 bar);
 water feed; and
 composition of gas stream produced in the cathode section before the final purification step.
The cathode pressure is varied between 12 bar and 30 bar (anode section is always considered
10% of cathode ones) and the results are shown in Figure 7.
Metals 2020, 10, 1535 16 of 26

(a) (b)

(c)

Figure 7. Results related to cathode operating pressure changes for 10 kg/s H2 production: (a) Total
required power; (b) water feed rate; (c) molar fraction in gas flow obtained in cathode section.

The increasing of pressure leads to a decrease of the required power and water feed. In
particular, if there is an increase of the operating pressure, energy consumption for compressing the
produced hydrogen can be avoided. In addition, Figure 7c shows that the purity of hydrogen stream
in the cathode section, before the last step of separation, slightly increases, despite the higher pressure
difference between the two electrolyzer sections tends to rise the permeation of undesired products
from cathode to anode sections and vice versa. Therefore, the increase in the efficiency of the
electrolysis reaction has a preponderant influence over the undesired permeation and the last step of
purification can be less efficient (and thus less expensive). Although the advantages of operating at
higher pressures, it is necessary considering the materials and the units suitable for high pressure.
Therefore, a compromise is required.
The effects of increasing the reactor inlet temperature between 71 °C and 80 °C, by keeping the
maximum difference between inlet and outlet temperature at 10 °C, are reported in Figure 8. The
increase of temperature leads to an almost linear and slight rise of the amount of required power
(especially related to the water heating) and water feed. Obviously, the final compression of
hydrogen increases the amount of total required power. However, the amount of power required for
compression is negligibly affected by the reactor inlet temperature, such as shown by the almost
constant difference between the two curves in Figure 8a. On the other hand, the composition of
hydrogen stream before the last step of purification appears not considerably affected by the
temperature change. It could be that the increase of the operating temperature allows a faster
electrolysis but it cannot be evaluated by exploiting the developed and used stationary model.
Metals 2020, 10, 1535 17 of 26

(a) (b)

(c)

Figure 8. Results related to reactor inlet temperature changes for 10 kg/s H2 production: (a) Total
required power; (b) water feed rate; and (c) molar fraction in gas flow obtained in cathode section.

4.1.2. SOEC Electrolysis

In SOEC Electrolysis the temperature and the pressure are the key parameters in energy
requirement of the system. For this reason, the sensitivity analyses by changing the temperature
between 600–1000 °C and the pressure between 1–30 bar have been carried out.
The Figure 9 shows the variation of required thermal power of stack and required thermal power
of evaporator. The power of pump required for increasing the fresh water pressure up to 30 bar is
constant and equal to 325.9 kW.

Figure 9. Effect of temperature change in the power requirement of stack and evaporator for 10 kg/s
H2 production.
Metals 2020, 10, 1535 18 of 26

The increase of temperature leads to a linear increase of stack thermal power; on the other hand,
the power required by the evaporator decreases because more thermal energy is recovered in the heat
exchangers. The total required power (including stack, evaporator and water pump) increases from
1482 MW up to 1499 MW, with an increment of 1%. From these results, it emerges that, in terms of
required power, it seems better to work at lower temperatures to reduce the required power, but
according to literature this leads to a decrease in the efficiency of the stack due to the decrease of
electrode activity, the increase of overpotential and polarization losses [90]; however, the developed
model type cannot allow the monitoring of this aspect. Furthermore, if higher operative temperatures
are reached by increasing steam temperature, the electrical energy demand in the stack is reduced in
favor of the thermal energy demand that is required to reach the desired steam temperature and that
replaces part of the electric energy required for the reactions to occur. This is advantageous because
it offers more opportunities to recover industrial waste heat or to use alternative heat sources, such
as geothermal source [91]. Therefore, it is necessary to find a compromise between energy resources,
efficiency and costs (high temperatures increase the material costs). Obviously, if a heat source is
available, such as geothermal source or industrial waste heat, its temperature will be a dominant
factor in the choice of operative temperature of the stack and will provide some constraints for the
entire system.
Figure 10 represents the effect of water feed pressure on the required power of the SOEC.

(a) (b)

Figure 10. (a) Effect of pressure on required power for the operation of the stack and the evaporator
for 10 kg/s H2 production; (b) Effect of pressure on required power of pump and compressor for 10
kg/s H2 production.

The increase of inlet water pressure has advantages in terms of required power, in fact, although
the power needed for the pump increases, the benefits in the reduction of required power for
hydrogen compression up to 30 bar and steam generation (evaporator) are greater. The total required
power decrease from 1598 MW to 1489 MW with a reduction of 6.9%.
Another important parameter in SOEC electrolysis is the amount of hydrogen recirculated in the
cathode inlet, essential to prevent the creation of oxidizing environment by pure steam at elevated
temperature and to preheat the fresh inlet stream. The molar fraction of H2 at the cathode inlet has
been varied between 0.1 (reference case: 10% mol of H2 and 90% mol of H2O) and 0.5 by changing the
amount of recirculated hydrogen. Figure 11 shows the effect of the molar fraction of hydrogen on the
required power of the SOEC and on the consumed water.
The increasing of molar fraction of H2 in the gas feeding mixture leads to an increase of required
power and a reduction of the produced H2 by the electrolysis reaction with consequent slight increase
of required water feed to reach the desired amount of 10 kg/s of H2. In addition, if a high amount of
hydrogen is present at the cathode inlet, the reaction will not occur normally because the quantity of
water for electrolysis is too little. The results of this analysis confirm the choice of operating with a
low content of hydrogen in the mixture at the cathode inlet: A fraction of 10–20% mol of H2 is
sufficient to avoid the oxidation of the electrode and to guarantee a sufficient preheating of the feed
and an effective electrolysis reaction [63].
Metals 2020, 10, 1535 19 of 26

(a) (b)

Figure 11. (a) Effect of H2 recirculation at the cathode inlet on the power requirements; (b) effect of H2
recirculation at the cathode inlet on the water feed rate (for 10 kg/s H2 production).

4.1.3. Biomass Gasification

In the biomass gasification the steam/biomass ratio and the temperature of gasifier are the main
parameters that impact the composition of syngas. Figure 12a depicts the effect of steam/biomass
ratio on composition in mole fractions of dry syngas, while Figure 12b shows how this composition
is affected by the gasifier temperature.

(a) (b)

Figure 12. (a) Effect of steam/biomass ratio and gasifier temperature on the water free composition of
the produced syngas; (b) effect of gasifier temperature on the water free composition of the produced
syngas.

The increase of steam/biomass ratio leads to an increase of the H2 and CO2 yields in the syngas
at the expense of CO and CH4. Instead, the temperature does not significantly affect the H2 amount
in the produced syngas but increases the amount of carbon monoxide with a linear behavior.
Therefore, a good compromise should be achieved, according to the availability of steam, on the cost
related to its production and to the separation of produced hydrogen and the reduction of carbon
dioxide.

4.2. Integration of Hydrogen in Methanol and Methane Synthesis

Hydrogen intensified methane and methanol syntheses in the steel industry has been studied
by coupling the hydrogen models with methane and methanol production ones by considering two
Metals 2020, 10, 1535 20 of 26

base scenarios. A medium size integrated steelmaking facility with an annual steel production
capacity of about 6 Mt of steel was considered.
In particular, the CH4 scenario and the MeOH scenario are defined as follows:
 CH4 scenario: 100% exploitation of by-product gases produced in an integrated steelmaking
plant for generation of methane: In this scenario the entire available amount of off-gases is used
to produce methane (assuming that the vast natural gas market and transporting infrastructure
can uptake easily such production). The renewable hydrogen produced either by PEM or SOEC
is inserted in basic stoichiometric ratio to produce methane.
 MeOH scenario: Methanol synthesis of a fraction of the by-product gases (~65%) due to the fact
that MeOH is a product that has a limited market compared to the natural gas market and
infrastructure. Again, the production of H2 is assumed to be coming from either PEM or SOEC.
Table 7 presents the results of the AspenPlus® simulations of the two scenarios focused on the
hydrogen requirements and consumption, electrolysis demands, product yields, and carbon
conversion.

Table 7. Main results of the analyzed scenarios.

CH4
MeOH Scenario
Scenario
Off-gas feed (kg/s) 190.3 125.3
Feed Compression (MW) 38.3 85.4
H2 Feed (kg/s) 20.9 8.4
H2 Consumption (%) 98.4 37.4
CH4 Product (kg/s) 50.4 -
CH3OH Product (kg/s) - 25.1
Carbon Conversion (%) 98.6 38.8
CO2 Utilization (%) 96.9 5.8

Considering the high amount of required hydrogen for enriching the steelworks off-gases, the
study shows that the use of electrolyses-based production process is preferred to the biomass-based
ones. Apart from the purity of the obtainable H2, the electrolysis processes do not affect the steelworks
carbon footprint if they use renewable energies than the biomass process. However, the energy
content and cost of the required hydrogen used exceed the energy off-set of the steelwork plants.
In Figure 13 the PEM and SOEC electrolysis requirements are represented in the range of GWs.
It is evident that both the cases are restrictive for employment in full-scale, considering also that the
capacities of current available biggest commercial electrolyzers are about two order of magnitude
lower.
Starting from the results of the two simulated scenarios and converting each material streams in
energy content, Figure 14 reports Sankey diagrams which allow comparing the energy content of the
feedstock and the electrolysis requirements (in case of PEM use) for the two scenarios. In the CH4
scenario, 540% of the energy content of the feedstock is required for PEM electrolysis, while for the
MeOH scenario 339%.
Metals 2020, 10, 1535 21 of 26

Electrolysis Requirements (GW)

4

PEM SOEC
3

0
CH4 Scenario MeOH Scenario

Figure 13. PEM and SOEC electrolysis requirements (MeOH: S.N. = 1.7, CH4: Stoichiometric).

Figure 14. Sankey diagram—energy analysis of CH4 scenario and MeOH scenario.

5. Conclusions
Different selected renewable hydrogen production processes have been analyzed by means of
AspenPlus® flowsheet simulations in order to obtain indications on the most suitable technologies
for hydrogen-based enrichment of integrated steelworks off-gas to be used as feedstock in methane
and methanol syntheses. In particular, PEM electrolysis, SOEC electrolysis and biomass gasification
have been examined. Simulations and sensitivity analyses have been carried out in order to examine
the advantages and drawbacks of the considered technologies and operating conditions, especially
in terms of purity of produced hydrogen, consumed energy and water and thermodynamic
efficiency, by taking into account that their application requires the coupling with renewable energy
sources (in the case of electrolysis) and with the synthesis processes of methane and methanol starting
from steelworks off-gases. The study shows that the use of electrolyses-based production process is
preferred to the biomass-based ones: Biomass gasification appears less suitable for enrichment of
steelworks gases, as less pure hydrogen is obtained with considerable production of CO2.
The study highlighted that the change in operating condition can lead to improvement of the
performances: For instance, higher operating pressure leads to decrease of global amount of required
energy in PEM and in SOEC systems and, in the case of PEM electrolysis, it leads to a less water
consumption and to a purer hydrogen stream before the final purification stage that thus can be less
efficient and with less economic impact. The sensitivity analysis also demonstrates that temperature
is a key parameter in SOEC design: Its increase leads to an increase of total required power, but
working at lower temperature leads to a decrease in the efficiency of the stack. Furthermore, higher
operative temperature reduces the electrical energy demand in favor of the thermal energy demand.
In terms of energy and water consumption SOEC technology seems to be more efficient than PEM.
Metals 2020, 10, 1535 22 of 26

However, PEM shows the advantage of being more stable in case of fluctuations of power supply (as
due to green power) and to be carried out with lower temperatures (the pressure is similar) and,
consequently, less issues related to the equipment or corrosion issues related to the use of hot steam.
On the other hand, SOEC electrolysis appears attractive if a high temperature heat source is available
and if a considerable amount of industrial waste heat can be recovered.
The two analyzed scenarios for methane and methanol syntheses from off-gases of medium
capacity steelworks show the importance of hydrogen and how it represents the greater energy costs
in the overall process. Therefore, future works should focus on the recycling and reuse of any residual
hydrogen from the synthesis processes in order to have a significant reduction in the electrolysis
requirements, by making the process sustainable within the integrated steelworks in terms of both
environmental and economic impacts. In addition, considering that the capacity of currently available
commercial electrolyzers are not suitable to meet the required hydrogen demand, the design of novel
electrolyzers or the assembly of multiple electrolyzers should be investigated in order to obtain a
suitable hydrogen production by also exploiting the advantages of the scale economy.
For these reasons, an accurate economic analysis is under development in order to evaluate the
full scale feasibility considering the already analyzed scenarios and furthermore specific ones (e.g.,
with reuse of residual hydrogen) for the implementation in real industrial contest. Concrete business
case studies will be analyzed taking into account the volatility of energy green markets and the
application of a dispatch controller. Indeed, the results of the simulations represent the basis for
further work about the implementation of innovative advanced control system, namely dispatch
controller, that will manage and will optimize the use and distribution of steelworks off-gases and
H2 from volatile power sources in the methane and methanol syntheses. Also, the optimization of
OPEX related to H2 intensified methane and methanol syntheses by using steelworks off-gases will
start by considering the results of this work.

Author Contributions: Conceptualization, I.M., K.P., M.B., and V.C.; methodology, I.M., K.P., M.B., and V.C.;
software, I.M., A.Z., A.P., S.D., and V.I.; validation I.M., V.C., and T.A.B.; formal analysis, I.M. and V.C.;
investigation, A.Z., A.P., I.M., T.A.B., S.D., V.I., and V.C.; resources, V.C.; data curation, A.Z., A.P.; writing—
original draft preparation, A.Z. and A.P.; writing—review and editing, V.C., I.M., T.A.B., K.P., M.B., S.D., and
V.I.; visualization, A.Z. and A.P.; supervision, V.C. and I.M.; project administration, V.C.; funding acquisition,
V.C. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by the European Union through the Research Fund for Coal and Steel
(RFCS), Grant Agreement No 800659.

Acknowledgments: The work described in this paper was developed within the project entitled “Integrated and
intelligent upgrade of carbon sources through hydrogen addition for the steel industry”, i3upgrade) (i3upgrade,
GA No. 800659), which has received funding from the Research Fund for Coal and Steel of the European Union.
The sole responsibility of the issues treated in this paper lies with the authors; the Commission is not responsible
for any use that may be made of the information contained therein.

Conflicts of Interest: The authors declare no conflict of interest.

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