18

220

Carbon Dioxide Reforming

of Coke Oven Gas Surplus
JOSÉ MIGUEL BERMÚDEZ, ANA ARENILLAS, J. ANGEL MENÉNDEZ

ABSTRACT
Currently 20–40 % of the coke oven gas (COG) produced is used as fuel in the
coke oven itself. However, there is a substantial surplus that is usually flared
or even directly emitted to the air. These practices give rise to environmental
problems, energy wastage and economic inefficiency. Surplus COG must be
better managed in order to solve these problems. The production of methanol
from the syngas generated from COG has attracted much attention, because
methanol is a valuable product, with a growing market and easy to handle.
If the technology used to produce the syngas is carbon dioxide (CO2)
reforming, several benefits are obtained: (i) it is possible to obtain a syngas
suitable for methanol production in just one step; (ii) it involves the partial
recycling of CO2; (iii) it entails lower energy requirements and provides higher
yields than conventional methanol production, which will help to improve
economic and energy efficiency.
Key words: Coke oven gas, CO 2 reforming, CO 2 emissions, Syngas,
Methanol, Energy efficiency

1. INTRODUCTION

Currently 20–40% of coke oven gas (COG) produced is fed back and used as
fuel to drive the coke oven itself. However, there is a substantial surplus
that is usually flared or directly emitted to the atmosphere, resulting in
environmental problems, energy wastage and economic inefficiency. Seeing
that the steelmaking industry is responsible for 5–7% of the total

Instituto Nacional del Carbón-CSIC, Francisco Pintado Fe, 26, 33007, Oviedo
Corresponding author: E-mail: Spainjmbermudez@incar.csic.es; aapuente@incar.csic.es;
angelmd@incar.csic.es
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anthropogenic carbon dioxide (CO2) emissions, increasing efforts are being

made to reduce such emissions and increase energy efficiency. A better
management of surplus COG is one of the proposed solutions. Several
technologies have been proposed for valorizing COG surplus (hydrogen
separation, syngas production, chemical looping combustion, polygeneration
of chemicals and energy). The most appealing alternative is syngas
production, since the COG surplus can be fully exploited. In addition, the
production of methanol from the syngas generated from COG is attracting
much attention because methanol is a valuable product, with a growing
market and it is easy to handle. CO2 reforming for the production of methanol
from COG offers important advantages, such as obtaining a syngas suitable
for methanol production in just one step, the partial recycling of the CO2
emitted when methanol is used, lower energy requirements and higher
yields than conventional methanol production.

2. THE COKE OVEN GAS

Coke oven gas can be considered as a by-product of the production of coke[1,2].

Coke is produced by the high-temperature carbonization (about 1000–1100ºC)
of bituminous coal in the absence of oxygen, giving rise to three different
fractions[2–5]:

1. Coke: A solid product with high carbon content (>90%) which plays a
key role in steel production, as it serves as a fuel, reducing agent and
most importantly, as a permeable support for the load in the blast
furnace[2–4]. Thousands of tons of coke are consumed daily in a steel
factory, which gives a general idea of the importance of this material
in this sector of industry.
2. Tar: A liquid product with a very high viscosity, the main components
of which are phenols, polyaromatic hydrocarbons and other heavy
organic compounds. This liquid fraction has many applications including
that of precursor of carbon materials, pavement seal coating or in the
production of oils, dyes and solvents[2].
3. Coke oven gas (COG): A high-energy mixture of gases which has a
complex composition when it leaves the coke oven [1,6,7,8]. After freezing
(i.e., when it is separated from the tars), COG is subjected to a number
of conditioning processes to eliminate benzene, toluene and xylene
(BTX) and reduce its H2S and NH3 contents so that it can be used in
the coke oven itself as fuel. After these conditioning processes, the
main components are H2, CH4, CO, N2, CO2 and other hydrocarbons
in low proportions. Table 1 shows the composition range of these
gases in COG after the conditioning processes[2,6,9,10].
Carbon Dioxide Reforming of Coke Oven Gas Surplus 5203

Table 1: Proportions of main COG components

Component Proportion (vol.%)

H2 55–60
CH4 23–27
CO 5–8
N2 3–6
CO2 <2
CnHm <2

2.1. Production and Conditioning

Coke became a key material in the steel and iron industry when Abraham
Darby first used it as fuel in a blast furnace in Coalbrookdale (England) in
1709[3]. This is a historical landmark because the successful use of coke in
the blast furnace was responsible for the subsequent development of the
steel and iron industry and the starting point of the Industrial Revolution.
At that time coal was heated in piles. Much has changed since then.
Nowadays, the production of coke takes place in ovens arranged in parallel
to form batteries, as shown in Fig. 1[4]. The ovens are filled with the
bituminous coal and heated up to the carbonization temperature[2,4]. Once
the carbonization is finished, the oven is opened and the coke is cooled
down using water.

Fig. 1: Battery of coke ovens[11]

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During the process COG and tars leave the oven through the risers
situated in the upper part of the batteries. These risers conduct these
fractions to the cooling and cleaning processes[6,10,12]. Fig. 2 shows a diagram
of this complex and thorough conditioning process[6,12]. The first stage is
the cooling process, where the raw coke oven gas temperature is lowered
to room temperature and the tar is separated. During the cooling stage,
aerosols are formed. These particles must be removed by means of
electrostatic separators to avoid fouling in the gas lines and nozzles. Once
the tar has been completely separated, several scrubbers are used to remove
different contaminants. First H2S is removed by means of a scrubber with
a solution of ammonia and then converted into elemental sulfur using the
Claus process. The COG is then scrubbed in water to remove NH3. The
solution that leaves this stage with the NH3 is distillated to obtain ammonia.
In the final stage BTX are removed by scrubbing COG in oil. Usually, in
this stage naphthalene is also removed. As a result of these cleaning stages,
sulfur, ammonia and BTX can be sold as by-products, in order to increase
the economic benefits of the process[6,12].

Fig. 2: COG cleaning process. Adapted from[6]

Apart from environmental reasons, this intensive treatment must be

carried out in order to prepare the COG itself for use[6,12]. Normally COG is
used as fuel for the coke oven itself or in other processes of the steelmaking
plants[2,6]. However, there is an important surplus that is normally burnt
off in gas flares or, in some cases, directly emitted to the air, causing serious
Carbon Dioxide Reforming of Coke Oven Gas Surplus 5225

env iron mental pr oblems, en ergy wastage and economic

inefficiency[1,10,13,14,15].

2.2. Energy Considerations in Relation to Coke Ovens and Coke

Oven Gas
Figure 3 shows the energy balance for coke ovens. As can be seen, the coal
fed to the oven and the coke obtained account for the greatest amounts of
energy (added to or removed) from the balance. The output, besides coke,
includes the energy from the other products obtained (COG, tars, BTX and
a negligible amount of sulfur) together with the energy losses. As coal is
not able to compensate for this energy output, some energy has to be
introduced into the system. This is achieved by means of electricity and
above all, coke oven gas, which is recycled to the oven to provide more than
7% of the energy input of the balance (which comprises approximately 95%
of the energy needed for the coking process)[6].

Fig. 3: Energy balance of a coke oven. Adapted from[6].

Since COG accounts for 17.9% of the total energy output of the coke
ovens and the energy from the COG recycled to the oven is 7.6% of the total
energy input, the energy of the COG produced exceeds the energy
requirements of the coke ovens. This gives rise to a considerable COG
surplus, which constitutes about 10% of the energy output from the oven,
representing about 1200 kWh/t of coke produced. This huge amount of
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energy cannot be ignored when making decisions about the use of COG
surplus, as has been the case in the last few decades.

2.3. Reasons for Exploiting Coke Oven Gas Surplus

Steelmaking is the largest energy consumer in the manufacturing
sector[16,17]. For this reason, the steelmaking industry is responsible for
about 5–7% of the total anthropogenic CO2 emissions. Given that steel
production has risen during the last few decades and is expected to increase
even more in the next few years, CO2 emissions can also be expected to
grow, giving rise to serious environmental problems[1,10,15,16,17].

Since the early 60s, the steel industry has responded positively to
movement in favor of for sustainability, especially in areas such as energy
efficiency and the reduction of greenhouse gases (GHG) emissions[16] .
However, the production processes are close to their physical limits
(especially in terms of carbon use) and this has incentivated the search for
alternative ways to improve the efficiency and sustainability in steelmaking
facilities[16]. One of these alternatives is better management and exploitation
of COG and in particular, surplus COG[6,10,16,17,18].

The most common practice for dealing with this surplus is gas flaring
and in some cases, where environmental polices and controls are more lax,
it is directly emitted to the air, giving rise to harmful environmental
problems[1,13,14]. Moreover, as has been mentioned, COG is a highly energetic
gas (its low calorific value varies in the range of 17 to 18 MJ/m3 depending
on its composition), due to its three main components, namely, H2, CH4
and CO as well as the other hydrocarbons present[6]. This energy content
therefore must be taken into account when making decisions about the use
and exploitation of COG surplus in order to avoid energy wastage and
economic inefficiency. In short, the need for a more efficient exploitation of
coke oven gas surplus is dictated by the negative environmental consequences
of non-exploitation, the need to ensure the energy efficiency of the process
and the economic balance of the coke making industry. These are all very
compelling reasons given the current socio-economic context and have
motivated a great deal of interest within the steelmaking industry in the
valorization of COG surplus. Examples of this recent surge of interest are
the COURSE50 program or the process modification carried out by the
U.S. Steel Corp[19,20]. The COURSE50 program (“CO2 Ultimate Reduction
in the Steelmaking Process by Innovative Technologies for Cool Earth 50”)
is an ambitious project that is currently being developed in Japan with the
aim of reducing CO2 emissions and improving energy efficiency in steel
industries by finding alternative uses for blast furnace gases and coke oven
gases[19]. In the case of the U.S. Steel Corp., this company has modified its
production process by using COG as fuel in its blast furnaces. This has
resulted in annual savings of more than 6 million dollars[20].
Carbon Dioxide Reforming of Coke Oven Gas Surplus
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3. TECHNOLOGIES FOR VALORIZING COKE OVEN GAS

SURPLUS

During the past few decades, various alternatives for valorizing COG surplus
have been proposed. These include using it for energy production, direct
utilization in the blast furnace to produce “pig iron” or treating it for the
production of chemicals and fuels. This last option, the synthesis of chemicals
and fuel appears to be the most promising, as well as the most challenging,
proposal[1,10]. The other possible alternatives can be grouped into four main
categories: hydrogen separation, synthesis gas production, chemical looping
combustion and polygeneration of chemicals and energy. As a result of these
technologies, many different products can be obtained from COG surplus,
as is illustrated in Fig. 4.

Fig. 4: Products obtained from COG and routes for obtaining them

3.1. Hydrogen Separation

Hydrogen is the principal and most valuable component in COG. Hydrogen
has been widely proposed as a future energy vector in the so-called Hydrogen
economy, an energetic model based on this gas, first proposed by General
Motors in 1970[21]. Hydrogen is seen as a clean alternative to conventional
fuel, since when it is consumed it produces water instead of CO2. However,
hydrogen is only as clean as its method of production. Actually, hydrogen is
produced mainly from oil or carbon, what finally gives rise to considerable
CO2 emissions. For this reason, scientific efforts are being directed to finding
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alternative sources of hydrogen, such as biomass and wastes or residual

streams from Industry like COG [21–23]. Of the different technologies studied
for producing hydrogen from COG, pressure swing adsorption (PSA) is
leading this field[15,24].

PSA is a low cost, low-energy and highly efficient gas separation

technology that employs several parallel units that operate in consecutive
steps. The first step of the process is adsorption, in which COG flows at the
highest operation pressure through a PSA unit filled with the adsorbent
material. In this step, the adsorbable substances are retained in the
adsorbent bed, while the rest of the gases leave the unit. When the adsorbent
is saturated the operation is stopped. At this point, the unit is depressurised
allowing the adsorbent to be regenerated. The depressurisation cycle is
terminated by means of a counter-flow expansion down to the lowest
pressure, called the dump step. The adsorbent is regenerated by a gas stream
that purges the adsorbed impurities remaining in the unit. Finally, the
unit is brought back to high-pressure conditions and adsorption is resumed.
These cycles do not require heating or cooling systems because the operation
occurs at constant temperature[25,26]. Various adsorbent materials are used
in the PSA units for hydrogen recovery. These include carbonaceous
materials, alumina oxides or zeolites[25,27].

As a result of the PSA process two different streams, a hydrogen-rich

stream and a highly concentrated methane gas are obtained. The methane-
rich stream can be employed as fuel in various plant processes in a similar
way to COG. Regarding to the purity of the hydrogen stream, it will also be
necessary to find the right balance between the amount of hydrogen
recovered and its purity, since as one of these parameters increases, the
other decreases. The inclusion of a final backfill step in the PSA process
could increase the purity of hydrogen at the expense of decreasing hydrogen
recovery. However, hydrogen purities higher than 99.99 % are very difficult
to achieve[1].

Most studies published in the field of PSA have been conducted using
mixtures with two or three components, whose behaviour differs from that
of COG. It has been demonstrated that a minor impurity like nitrogen plays
an important role in the process, since it modifies the breakthrough times
of the rest of the components fed into the PSA column[28]. Another key
parameter in the process is the composition of the adsorbent bed, since the
materials employed influence the concentration of the main impurity in
the final stream[29].

Further studies beyond fundamental research have been conducted to

explore the possibility of implementing this technology at industrial level.
Two interesting examples are the studies of Joseck et al.[7] and Hwang and
Chang[23]. Joseck et al. explored the possibility of H2/COG separation using
PSA technology to valorise the COG produced in the Rust Belt for use in
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Carbon Dioxide Reforming of Coke Oven Gas Surplus 9

fuel cell vehicles (FCVs). Although some of the conclusions of their study
may have lost weight due to the deep economic crisis affecting this sector,
the huge potential of COG as source of hydrogen has been confirmed. The
hydrogen obtained from the COG produced in this area could be enough to
fuel ca. 1.7 million FCVs. It should be clarified that considering only COG
surplus, this number would be reduced, but will be over the million of vehicles
in any case. Hwang and Chang[23] evaluated the possibility of using hydrogen
from different sources in fuel cell scooters in Taiwan by means of a Life-
cycle Analysis. Their study revealed that COG would be the most efficient
source, since it would lead to a remarkable reduction in GHG emissions
and an improvement in energy efficiency.

Besides PSA, membrane separation has also been proposed for the
recovery of hydrogen from COG[14]. Membrane gas separation is a pressure-
driven process that offers several advantages such as easy operation, low
capital and operating costs and low-energy requirements. In this process, a
mixture of gases at high pressure is forced to pass through the surface of a
membrane. This membrane is selectively permeable to one or more of the
gas components, giving rise to two different streams: the permeate (rich in
the components to which the membrane is permeable) and the retentate
(rich in the rest of the components)[30]. However, this technology has been
proposed mainly for application in membrane reactors for syngas production.

Other interesting technologies proposed for the recovery of hydrogen

from COG are cryogenic separation and hydrate formation, but these are at
an infant stage and certain drawbacks must be overcome before they can
be applied at industrial level[31,32].

3.2. Synthesis Gas Production

Synthesis gas, or simply syngas, is a mixture of H2 and CO that is widely
used in industry as a raw material for the production of many different
chemicals[33]. Although it is usually produced from natural gas, the concern
about fossil fuels depletion and GHG emissions have intensified research
efforts to find alternative processes of syngas production, including biomass
gasification[34], biogas reforming[35] and the thermal upgrading of COG[7,18,36–
44]
. COG thermal upgrading has been carried out using the conventional
technologies that are applied to natural gas: steam reforming (Reaction 1),
partial oxidation (Reaction 2) and CO2 reforming (Reaction 3). Turpeinen et
al. [45] reported an interesting thermodynamic study of the energy
requirements of these technologies and the CO2 emissions when they are
applied to convert COG to hydrogen compared to other potential hydrogen
sources (e.g. natural gas, biogas and refinery gas). The results of the study
highlighted the huge potential of COG as a source of syngas (and especially
hydrogen) and its advantages over the other sources. When steam or,
especially, dry reforming are used, COG has proved to be the best source so
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far in terms of energy consumption and CO2 emissions. In the case of partial
oxidation, COG gives rise to the lowest emissions, although natural and
refinery gas need less energy.

CH4 + H2O 3 H2 + CO (Reaction 1)

CH4 + ½ O2 2 H2 + CO (Reaction 2)

CH4 + CO2 2 H2 + 2 CO (Reaction 3)

Depending on the process of upgrading used, the H2/CO ratio of the

synthesis gas obtained varies a lot, allowing the production of a wide range
of chemicals (Fig. 5). This versatility is one of the reasons why the production
of synthesis gas appears to be the most appealing alternative for COG
upgrading.

In the case of steam reforming, one of the most critical factors to be

considered is the H2O/CH4 ratio. Normally this ratio is higher than one
because the steam is injected in excess over the stoichiometric value to
prevent deactivation due to carbon deposits[46]. When COG is used instead
of natural gas, it has been found that this ratio should be in the range of
1.1–1.3 if the process is carried out at temperatures of about 950ºC[44]. The
use of hot COG (as it leaves the coke oven, with no pre-treatment) has also
been studied with the aim of reforming methane as well as the tarry
components, taking advantage of the high temperatures of the gas to promote
the desired reactions[43]. This technology can produce 3–5 times more

Fig. 5: H2/CO ratios of the syngas obtained from COG using different syngas production
processes
Carbon Dioxide Reforming of Coke Oven Gas Surplus 528
11

hydrogen than the steam reforming of cold COG. However, this process
has a tremendous drawback: the low H/C ratio of the hot COG compared to
the treated COG, which leads to a huge production of carbonaceous deposits
and consequently the rapid deactivation of the catalysts[37]. Moreover, the
presence of H2S is also highly undesirable, since it acts as a poison for the
catalysts.

Partial oxidation of methane can be carried out in the presence of a

catalyst or in its absence[47]. However, in the case of partial oxidation of
COG, almost all the works published have focused on the catalytic process.
This process offers several benefits that will probably increase its importance
in the next few years. These benefits include high energy efficiency (since
partial oxidation is a mild exothermic reaction), high reaction rates and a
H2/CO ratio of 2 in the resulting syngas, which makes it suitable for the
production of methanol or for Fischer-Tropsch synthesis[47]. However, the
large amount of hydrogen present in the COG fed to the reactor neutralizes
this last advantage (Fig. 5). As in the case of steam reforming, where the
H2O/CH4 ratio plays a key role in the process, in partial oxidation the O2/
CH4 ratio is of outstanding importance[48,49]. When this ratio is increased
from 0.125 to 1.0, at temperatures of around 750ºC, the conversions increase
dramatically, whereas when the value is higher than 0.5, the selectivities
to H2 and CO decrease[48]. This is due to the consumption of excess O2 in
the complete oxidation of methane or in the oxidation of the H2 and CO
produced (reaction 4–6).

CH4 + O2 2 H2 + CO2 (Reaction 4)

H2 + ½ O 2 H2O (Reaction 5)

CO + ½ O2 CO2 (Reaction 6)

Besides the O2/CH4 ratio, space velocity also has a significant effect in
the resulting gas since as it increases the combustion of methane is favoured
to the detriment of partial oxidation. For these reasons it is necessary to
find a compromise solution between high syngas production (high space
velocity) and high syngas quality (low space velocity)[48].

One of the most important drawbacks for the industrial implementation

of partial oxidation is the high cost related to a pure oxygen supply, since
about the 40% of the expenses of the partial oxidation process come from
the production of oxygen[50]. Membrane reactors have emerged as a potential
solution and in the particular case of COG partial oxidation, membrane
reactors have been pretty much the only technology to be investigated in
recent years[13,51–54]. In this kind of reactors, air is fed in directly with no
previous separation of oxygen and inside the reactor, an oxygen permeable
membrane only allows oxygen to reach the catalyst, but not the rest of the
components present in the air. In the case of COG, with some types of
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membrane, the presence of large amounts of hydrogen influences the

performance of the membrane, since it acts as a “pseudo-catalyst” favouring
oxygen permeation[54].

Finally, the CO2 reforming of methane is the least studied of the COG
thermal upgrading processes, in spite of its several advantages[23,55]. In this
process, CO2 (the most important GHG) is used to transform methane and
obtain the synthesis gas, thereby consuming two of the main gases
responsible for global warming in a valuable raw material for the industry.
As in steam reforming, CO2 reforming requires the presence of a catalyst,
Ni being the metal that is most commonly used for this purpose. The studies
carried out on the catalyst are of critical importance for the industrial
implementation of this process, since the main drawback of the process is
the deactivation of the catalyst due to the intense formation of carbonaceous
deposits[23,55,56]. To date, only two processes based on methane dry reforming
have been industrially implemented: the SPARG process[57] and the CALCOR
process for CO production[58]. The SPARG process is especially attractive
for application in the CO2 reforming of COG, since it is based on the addition
of H2S to the process stream, leading to a partial poisoning of the catalyst
but preventing the formation of carbonaceous deposits. In this way, it is not
necessary to completely remove H2S from COG before using it in CO2
reforming, which reduces the costs associated with the scrubbing process.
The CO2 reforming of COG will be further discussed in Section 4.2. “The
process. CO2 reforming of coke oven gas”.

3.3. Chemical Looping Combustion

Chemical looping combustion is a growing technology that is used to improve
combustion efficiency and facilitate the capture of CO2[59,60]. The scheme
shown in Fig. 6 illustrates the idea upon which this technology is based. It
consists of two reactors, an air reactor and a fuel reactor and a circulating
metal oxide that works as oxygen carrier between both reactors. In the fuel
reactor, fuel is combusted using the oxygen present in the carrier as oxidizing
agent, thus working without the presence of air. As a result, the outlet
stream is only composed of CO2 and H2O, avoiding the presence of N2,
which dilutes these products. This absence of N2 facilitates the CO2 capture.
The oxygen carrier is reduced in this reactor and sent to the air reactor
where it is oxidised to its initial state and then sent back again to the fuel
reactor.

The chemical looping combustion of COG has been proposed by Wang et

al.[59]. In their work different oxygen carriers were studied, the best results
being obtained with that comprising 45% Fe2O3, 15% CuO and 40% MgAl2O4,
which allowed them to achieve a maximum fuel conversion of 92%. Moreover,
this oxygen carrier showed a high and stable activity over 15 reduction-
oxidation cycles. However, this has been the only study carried out in this
Carbon Dioxide Reforming of Coke Oven Gas Surplus 530
13

Fig. 6: Scheme of the process of chemical looping combustion

field and although the results are encouraging, this technology is still at its
infancy and still needs a lot of development.

3.4. Polygeneration of Energy and Chemicals

Polygeneration is a very attractive approach which is based on the combined
production of energy (electricity or heat) and valuable products (normally
hydrogen, methanol, dimethyl ether or other valued chemicals)[61]. Co-
generation is the simplest technology of polygeneration and the most
widespread. In the case of COG, co-generation has been put in practice by
Essar Steel Algoma Inc. in Ontario (Canada)[62]. In this plant COG and blast
furnace gas have been used in a 70 MW energy co-generation facility to
produce heat and electrical power that can be used in the steelworks since
2009. This facility produces half of the electrical power requirements of the
plant, thus being more independent of Ontario’s power grid. Moreover, direct
emissions of NOx have been reduced approximately in 400 tonnes annually
and indirect emissions (related to electricity generation) of 1259 tonnes of
NOx, 2391 tonnes of SO2 and 539616 tonnes of CO2 annually are avoided.

But co-generation is not the only type of polygeneration that could be

applied to COG. Two recent studies have proposed polygeneration of energy
and chemicals from COG combined with other sources[63]. Jin et al. proposed
a process in which COG and coal were used as raw materials for the
production of hydrogen and energy in the same system. This system would
give rise to high hydrogen recoveries and energy efficiencies at reduced
CO2 emissions. In another study, Li et al.[64] proposed a polygeneration
system in which methanol, dimethyl ether and dimethyl carbonate were
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produced from COG and coal gasification gas by means of an integrated
catalytic synthesis procedure. The study also included a simulation of the
proposed system to demonstrate the potential of the technology to efficiently
produce high added value chemicals. However, both works are theoretical
proposals that need to be further studied before being implemented in the
coke making industry.

4. CO 2 REFORMING OF COKE OVEN GAS FOR METHANOL

PRODUCTION

Among the different technologies for COG surplus valorization, syngas

production seems to be the most attractive option[1]. Several reasons support
this assertion. The components of COG are exploited to the maximum, the
technology is well known and is in widespread use and the products have a
high value and are in high demand. Of the wide range of products that can
be obtained (hydrogen, methanol, dimethyl ether or a wide variety of
hydrocarbons), methanol is the most appealing[1,65,66].

4.1. Methanol Production

Although research on the transformation of COG into syngas has been
mainly focused on the production of hydrogen as the final product, an
alternative to this line of research is gaining strength: the production of
methanol[1,65,66]. Methanol has the advantage of being liquid under ambient
conditions, which makes it easier to handle than hydrogen and syngas.
Moreover, it is a valuable product and the interest in it has increased in
recent years due to its applicability in many different industries, such as
the energy, chemical, food or pharmaceutical industries (see Table 2)[67].
However, the sectors that are mainly responsible for this increasing interest
are the energy and chemical industries. Methanol is expected to play a key
role in the future energetic model mainly due to its application as a raw
material for biofuel production and/or as a hydrogen carrier in the Hydrogen
Economy[68]. In the other sector, the chemical industry, the process of
methanol-to-olefins has experienced a “boom” that has multiplied the demand
for methanol in this process by more than 800 times in the last five years.

This increasing demand makes methanol market a growing opportunity

for novel processes of production, especially if the process is efficient and
environment friendly. Moreover, in the case of COG, there is a clear example
of the aroused interest in methanol production. China (the world leader in
coke and therefore, COG production) has built several industrial plants
that are expected to produce 1.2 million tons/year of methanol from COG[66].
Carbon Dioxide Reforming of Coke Oven Gas Surplus
53215

Table 2: Methanol supply and demand in millions of metric tonnes (period 2008–2013)[67]

2008 2009 2010 2011 2012 2013 Increase

(2008–
2013)
Production 40.3 42.1 48.9 54.8 60.6 64.6 + 60%
Demand 40.3 42.1 48.9 54.8 60.6 64.6 + 60%
Formaldehyde 15.2 14.2 16.3 17.6 18.4 19.3 + 27%
Acetic acid 4.3 4.2 5.0 5.2 5.3 5.7 + 33%
MTBE 7.0 6.7 7.3 7.7 8.2 8.5 + 21%
Methyl methacrylate 1.3 1.3 1.4 1.5 1.5 1.6 + 19%
DMT 0.5 0.5 0.5 0.5 0.5 0.5 + 4%
Methyl mercaptan 0.4 0.4 0.4 0.4 0.5 0.5 + 10%
Methylamines 1.2 1.1 1.3 1.4 1.4 1.4 + 23%
Methyl chloride 1.7 1.7 1.8 1.9 1.9 2.0 + 16%
Alternative fuels
Gasoline blending 3.1 4.9 6.2 7.1 8.3 9.2 + 198%
and combustion
Biodiesel 0.9 0.8 0.9 1.2 1.3 1.2 + 34%
DME 1.8 3.3 4.0 4.3 4.6 4.7 + 159%
Fuel cells 0.01 0.01 0.01 0.01 0.01 0.01 - 30%
Methanol-to-olefins 0.01 0.01 0.7 2.5 4.9 5.9 + 83896%
Others 3.0 2.8 3.3 3.6 3.9 4.0 + 32%

4.2. The Process of CO2 Reforming of Coke Oven Gas

The production of methanol from COG necessarily involves the prior

conversion of COG in syngas. Once the syngas is produced, it enters the
methanol synthesis reactor. Due to the low conversions of this reaction, it
is necessary to separate the products and the unreacted gases, which are
recycled to the methanol synthesis reactor. The methanol is then subjected
to different stages of purification until it reaches the level of purity required
for its use[68].

Methanol synthesis is a well-known process and research efforts are

being focused on the improvement of the reactor design more than on
changes to the process that could improve it. Moreover, in a conventional
methanol plant, syngas generation accounts for 55% of the financial outlay
required for the process units [68,69]. For these reasons research is focusing
on syngas production, which is the core of the process. The composition of
syngas needs to satisfy certain conditions: the H2/CO ratio should be around
2 and the R parameter (equation 1) should be in the range of 2.03–2.05.
These requirements are due to the reactions involved in the synthesis of
methanol (Reactions 7 and 8)[1,9,10,38,40,42,65,68–70].

R = (H2 – CO2) / (CO + CO2) (Equation 1)

2 H2 + CO CH3OH (Reaction 7)

3 H2 + CO2 CH3OH + H2O (Reaction 8)

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Using conditioning processes, the syngas produced by means of steam

reforming or partial oxidation can fulfil these requirements, but if the process
employed is CO2 reforming, no conditioning stages are required[9,38,40,42,65].
In fact, it is possible to produce a syngas suitable for methanol production
in just one step, provided that the dry reforming is conducted under
stoichiometric conditions of methane and CO2. Moreover, this process of
methanol production also involves the partial recycling of the CO2 (Fig. 7),
since theoretically, half of the CO2 produced upon methanol consumption is
recycled in the CO2 reforming of COG[10,65].

Fig. 7: Partial recycling of CO2 in the dry reforming of coke oven gas to produce
methanol

Nevertheless, beyond these positive considerations about CO2 reforming

of COG to produce methanol, there is a latent question that may raise
some scepticism about this technology. If there are almost no industrial
processes available for the CO2 reforming of methane, why should we trust
in this technology for the reforming of COG and the production of methanol?
Will it be possible to overcome the drawbacks of the CO2 reforming of
methane using COG? Research has been carried out with the aim of finding
an answer to this question and the results obtained suggest that the answer
may be affirmative.

The thermodynamics of the process show that it is of paramount

importance to work within a concrete range of operation conditions to obtain
the desired results[9]. It is necessary to work at temperatures higher than
800ºC and pressures as low as the economics of the process allow. Working
at lower temperatures will lead to poor conversions and the syngas thus
Carbon Dioxide Reforming of Coke Oven Gas Surplus 53417

obtained will not be suitable for methanol production, since its H2/CO ratio
and R parameter will be far from the desired values. Moreover, at these
temperatures the most important drawback of CO 2 reforming, the
deactivation of the catalysts due to carbon deposits, is also minimized, since
carbon formation under these conditions is extremely low. In the case of
pressure, the discussion is more complicated although its influence on the
process is not as great. Normally, methane reforming processes are carried
out at mild pressures (15–30 bar) for economic reasons, despite the decrease
in conversions. Usually, the subsequent processes in which syngas is used
are carried out at high pressures[71]. Working at high pressures before
reforming the natural gas makes it possible to use smaller reactors and
compress less volume of gas, since both steam and CO2 reforming double
the number of moles. This means that per litre of natural gas treated, two
litres of syngas are produced. However, in the case of the CO2 reforming of
COG, the increase in the number of moles is less than 1.4 times, which
means that the benefit obtained from pressurizing before the reforming
step is reduced. For this reason, the pressure at which the process should
be carried is the lowest pressure that will maximize the economy of the
process, i.e., a compromise solution between the cost of pressurizing and
the conversion achieved[9].

But not only the thermodynamics point to the potential viability of this
process, the theoretical results have also been confirmed experimentally.
High conversions and selectivities have been achieved in this range of
experimental conditions using three main types of catalyst (see Table 3):
carbonaceous materials, Ni supported catalysts and mixtures of both
catalysts.

From the studies that have produced the data in Table 3, two results in
particular are worthy of note: the reaction mechanism in the Ni/Al2O3
catalysts and the synergetic effect appeared when carbon materials are
mixed with Ni/Al2O3. The reaction mechanism of the CO2 reforming of COG
when Ni/Al2O3 catalysts are used is one of the most promising results pointing
out the possible viability of the process. In the CO2 reforming of methane,

Table 3: Conversions and selectivities of the catalysts studied in the dry reforming of
COG

Catalyst Temperature VHSV Conversions Selectivity Reference

(ºC) (L/g·h) (%) (%) (s)
CH4 CO2
[40]
Activated carbon 1000 0.75 82 95 90–100
[36]
Ni/SiO2 800 30 75 80 100
[42]
Ni/Al2O3 900 9 90 95 94
Ni/Al2O3 (67%)
[38]
Activated 800 3.75 85 93 85
Carbon (33%)
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the main drawback for its industrial implementation is the deactivation of

the catalyst due to the intense formation of carbon deposits. These carbon
deposits are a consequence of the reaction mechanism of the CO2 reforming
of methane[72] (Fig. 8, classic path). The classic reaction mechanism of the
CO2 reforming of methane is a concatenation of two reactions: methane
decomposition (reaction 9) and carbon gasification (reaction 10). Methane
molecules reach the active centres of the catalyst where they decompose
into H2 and solid carbon. CO2 then gasifies the solid carbon, giving rise to
CO. However, CO2 is not able to gasify all of the solid carbon, resulting in a
large number of unregenerated carbon deposits that block the active centres
and cause the deactivation of the catalyst.

CH4 2 H2 + C (Reaction 9)

CO2 + C 2 CO (Reaction 10)

However, when the gas fed to the reactor is COG instead of CH4, the
huge amount of H2 present in the reacting stream plays a key role, giving
rise to an alternative reaction mechanism[42] (Fig. 8, alternative path). The
H2 reacts with the CO2 through the reverse water gas shift reaction (reaction
11) to produce CO and water. The water produced then reacts with methane
via steam reforming (reaction 1), to produce H2 and CO.

H2 + CO2 H2O + CO (Reaction 11)

CH4 + H2O 3 H2 + CO (Reaction 1)

Fig. 8: Possible reaction paths of the CO2 reforming of COG

Carbon Dioxide Reforming of Coke Oven Gas Surplus 536
19

The deactivation rate of this alternative path is that of the steam

reforming of methane (a well-known and industrially widespread process),
which is considerably lower than that of the CO2 reforming of methane. It
is important to point out that the alternative reaction path is not the only
path that takes place in the process. Both reaction paths coexist, but the
studies suggest that the alternative path is the predominant mechanism.
Thus, the deactivation rate of the CO2 reforming of COG is closer to that of
the steam reforming of methane than to that of the CO2 reforming of
methane, but it is in between these two values.

The other striking feature of the CO2 reforming of COG is the synergetic
effect presented by carbon materials and conventional Ni/Al2O3 catalysts
when they are mixed and used together as catalysts[38]. The synergetic
effect between carbon materials and Ni/Al2O3, in terms of improvement of
conversions, has been previously reported for the CO2 reforming of
methane[73]. However, in the case of the CO2 reforming of COG, it has been
found that this synergetic effect influences not only the conversions of the
process, but also the selectivity and the resistance to deactivation of the
catalysts. Fig. 9(a) shows the results for the conversions achieved with
different mixtures of activated carbon and Ni/Al2O3, varying the amounts of
each material. As can be seen, when the mixtures are used, the conversions
achieved are higher than those predicted by the law of mixtures and in
some cases, higher than that achieved by each material separately.

In terms of selectivity, the mixtures are more selective than each material
separately. This can be checked from the production of water, which is the
main by-product of the process. Water production is lower when the mixtures
are used than when activated carbon and Ni/Al2O3 are used separately, as
is shown in Fig. 9(b). The last property affected by the synergetic effect is

Fig. 9: Synergetic effect of the combination of a carbon material (activated carbon)

and a conventional catalyst (Ni/Al2O3) in the CO2 reforming of COG. Results of
(a) conversions and (b) water production. Adapted from[38].
537
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the resistance to deactivation. It was found that the mixtures are more
resistant to the deactivation produced by carbon deposits. During the
reaction, the mixtures undergo a lower loss of BET surface area than that
experienced by the activated carbon and the Ni/Al2O3 when they are used
separately. All these results are of great interest due to the costs associated
with these catalysts. The partial replacement of an expensive material (Ni/
Al2O3) by a cheap material (activated carbon) can reduce the price of the
catalysts used in the process.

4.3. Comparison with Conventional Methanol Production

The thermodynamic equilibrium, the alternative reaction pathway and the
cheap catalysts are attractive features of methanol production from COG
by means of CO2 reforming. But there is a special advantage to be gained
from this process: the reduction of CO2 emitted per kg of methanol produced.
A comparison between this process (see block diagram shown in Fig. 10(a)
and a conventional process of methanol production from natural gas (see
block diagram shown in Fig 10(b), shows that obtaining methanol from
COG considerably reduces CO2 emissions[65].

Fig. 10: Block diagram of the (a) conventional methanol synthesis and (b) COG-based
methanol synthesis using CO2 reforming for syngas generation. Units labelled
as Cond are condensers.
Carbon Dioxide Reforming of Coke Oven Gas Surplus 538
21

But this is not the only advantage of this process. Table 4 shows the
results of the comparison of both processes[65]. It is clear that the COG-
based process is superior to the conventional one in terms of carbon and
hydrogen yields and the quality of the product obtained. This means that
the exploitation of the raw materials is more efficient in the COG-based
process. Moreover, the higher purity obtained in the COG-based process,
will decrease the costs of purification. Furthermore, the methanol obtained
in this process can be used directly as fuel since it fulfils the purity
requirements for this application. The question of energy consumption is
not so straightforward.

Table 4: Comparison between conventional methanol production and COG-based

methanol production in terms of energy consumption, yields and purity of the
final product [65]

Conventional COG-based
Energy consumption(KWh/Kg MeOH) Case 1 4.9 4.1
Case 2 2.6 2.6
Case 3 1.1 1.7
Yield (%) C 76 80
H 73 84
Purity (wt. %) 96.8 98.4

The comparison in terms of energy consumption has been carried out

on the basis of three different cases, to find if there is a gradual increase in
energy recovery:

1. Case 1: where only the process units that consume energy are taken
into account.
2. Case 2: besides the energy consumed by the units in case 1, the
energy that can be recovered from the purges in the methanol
synthesis loops is taken into consideration.
3. Case 3: additional energy that can be recovered from the units that
release energy is taken into consideration.

In Case 1, when only the energy consumption is considered, the energy

required by the COG-based process is lower than that needed for conventional
technology. However, as different energy recoveries are posed, the
conventional process has lower energy requirements. This means that the
energy consumption in the COG-based process is lower, even though
conventional technology allows greater energy recoveries. However, it is
worth pointing out that, these energy recoveries suffer from efficiencies
that would bring the final values closer to those of Case 1 than to those of
Case 3[65].

In the case of CO2 emissions, there is no need for discussion: the COG-
based process is superior to conventional production in all cases[65]. Fig. 11
539
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Fig. 11: CO2 emissions of the conventional and COG-based methanol production
processes. Adapted from[65]

shows the results of CO2 emissions for all the cases considered in the energy
consumption study. However, when discussing about CO2 emissions, the
indirect emissions associated to energy consumption also need to be taken
into consideration. These indirect emissions depend on the energy mix of
the geographical situation of the plant. In order to determine with the
maximum possible accuracy the CO2 emissions of each process, three
different geographical locations were considered: China, USA and the
European Union. As can be seen from the figure, in all of the cases the
emissions from conventional technologies are higher than those from the
COG-based process. The difference varies in the range of 6–27%, depending
on the energy integration and geographical situation of the plant. This
result will have a huge impact, as will be shown in section 4.5 “Impact of
the technology”.

4.4. Economic Considerations

It has been shown that this technology is technically feasible and has several
technological and environmental advantages over the conventional method
of methanol production from natural gas. Nevertheless, it is the economy
of the process that will ultimately decide in favour of or against its future
industrial implementation. Several projects of methanol production from
COG are already running in China, where the production of the synthesis
gas in these projects is carried out by means of partial oxidation[66]. Another
techno-economic study of methanol production from COG carried out by
Lundgren et al.[74] underscores the potential of the production of methanol
from COG by means of three other different processes: steam reforming,
autothermal reforming and mixing COG with synthesis gas obtained from
biomass gasification. Their results showed that these technologies could be
economically competitive with other methanol production processes.
Carbon Dioxide Reforming of Coke Oven Gas Surplus
54023

No economical analyses of the COG-based production of methanol by

means of CO2 reforming as the technology for the production of synthesis
gas have been carried out. Nevertheless, three factors together with the
successful experiences with partial oxidation and the positive economic
results of steam and autothermal reforming, suggest that such an economic
analysis would decide in favour of the industrial implementation of the
COG-based methanol production based on CO2 reforming for the generation
of syngas. These factors are:

• Two residual gases are valorised: COG and CO2. In this process a
valuable product is produced from these two residual gases. Moreover,
with this process it is possible to reduce CO2 emissions in comparison
with conventional technologies based on natural gas. This would yield
economic benefits in the emission markets.
• The possibility of using the catalysts composed of a mixture of activated
carbon and Ni/Al2O3 would reduce the cost of the reforming process,
since such mixtures are cheaper than conventional catalysts.
• The CO 2 reforming of COG takes place mainly via a reaction
mechanism different from the CO2 reforming of methane. For this
reason, the deactivation rate of the catalyst (which is the main obstacle
to the industrial development of CO2 reforming) would be lower than
in the case of the CO2 reforming of methane and would tend towards
that of steam reforming. If this drawback is avoided, the chances of
this technology for being implemented on an industrial scale would
be improved.

4.5. Impact of the Technology

The best way to show the potential of this technology for reducing CO2
emissions is to evaluate its impact with real cases. The Rust Belt (USA) and
the Shanxi region (China) are two geographical places of special interest
due to their important production of COG surplus[15,75,76]. The Rust Belt is a
region in the Midwest-Northeast of the USA with an important network of
coking and steel plants, which has been previously proposed for other
technologies of COG surplus exploitation such as H2 production[15]. The
Shanxi region is one of the most important carbon-producing regions in
China and therefore, one of the main producers of coke and COG[75,76]. Fig.
12 shows the COG surplus production for each zone and the amount of
methanol that can be produced with this COG surplus. In addition, the
emissions of CO2 avoided by replacing conventional methanol production
from COG-based methanol production are also shown. As can be seen, in
both cases the CO2 emissions avoided are more than considerable, especially
in the case of Shanxi. Moreover, this estimation does not take into account
the emissions avoided by preventing the flaring or direct emission of COG
that will increase the savings in CO2 emissions derived from substituting
conventional methanol production with COG-based methanol production.
541
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Fig. 12: COG surplus generation and possible methanol production and CO2 emissions
savings derived from the exploitation of this COG surplus in the Rust Belt
(USA) and the Shanxi region (China). Images from[11].

5. CONCLUSIONS

COG surplus non-exploitation is actually a big problem for the coke making
industry, giving rise to serious environmental problems, a considerable
waste of energy and economic inefficiency. The production of methanol is
an attractive solution to this problem, since it entails the production of a
valuable product with a growing market from a residual stream. If the
process is carried out using CO2 reforminfcg in the syngas generation step,
more benefits arise. This technology involves the partial recycling of CO2,
since half of the CO2 produced upon methanol consumption is recycled for
the CO 2 reforming of COG. Moreover, when compared with other
conventional technologies of methanol production from natural gas, the
COG-based methanol production through CO2 reforming, results in higher
yields, a higher quality of the methanol produced and lower energy
requirements. Moreover, this process would significantly reduce CO2
emissions, thereby contributing to the fight against global warming. All of
these features would improve the economic results of the coke making and
steelmaking industry.

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