DESCRIPTION

Field of the invention

The present invention relates to a zero-emission process for producing syngas. Specifically, the present invention deals with a production of high-quality syngas production using natural gas (methane) without any greenhouse flue gas released in the atmosphere.

State of the art

It is known to produce syngas in different ways. One of them is the reforming of natural gas according to the overall reaction of steam methane reforming (Quirino et al., 2021, 2020) according to following reaction:

CH ₄ + H ₂ O = CO + 3 H ₂ or by means of the autothermal path according to following reaction:

CH4 + O.5 O ₂ = CO + 2 H ₂

Another way to generate syngas is the coal (or solid fuel) gasification as in the following simplified lumped scheme: c + H ₂ O = CO + H ₂

There are also other paths and processes to generate syngas (and hydrogen), such as the reforming of methanol (Palo et al., 2007), the ammonia splitting (Cha et al., 2021), the acid gas to syngas conversion or other sources such as biomasses, biogas, municipal solid wastes, plastic wastes and so on, but no one of them are so widespread and adopted for bulk productions.

In addition, the integration of renewable energy sources is not yet relevant in the production of syngas and, hence, its derivatives (ammonia, methanol, dimethyl ether to quote a few).

The syngas is, in fact, a starting source for generating hydrogen. All the bio and fossil refineries are reforming natural gas as in the below reaction

CH ₄ + H ₂ O = CO + 3 H ₂ to obtain syngas and then, shifting the syngas to hydrogen according to the water-gas shift reaction: CO + H2O = CO2 + H2

If any carbon capture technology is adopted after the shifting, the hydrogen produced assumes the “blue” labelling.

However, the CO2 to be sequestrated is not only the one generated at the Water Gas Shift Reactor, but also the one (the majority) released as flue gas at the firebox of the steam reformer, caused by the exothermic combustion reaction of methane with oxygen present in air,

CH ₄ +2O2 = CO2+2H2O heating the tubes, of the steam reformer, wherein the steam endothermic reforming reaction CH ₄ + H2O = CO + 3 H ₂ takes place.

Therefore, the above-described known process present two main disadvantages the reactor duty generation necessary in the steam reformer for carrying out the endothermal steam reforming reaction and the release of flue gases.

Summary of the invention

To overcome the aforementioned problems, a process and a relative plant to produce syngas reducing and preferably avoiding any emission starting from methane (or natural gas), has been conceived.

It is therefore an object of the present invention a process for producing syngas comprising the steps of: a) burning methane or natural gas with oxygen and water steam for producing flue gas comprising CO2 and H2O according to the following reaction:

[1] CH ₄ +2O ₂ -^ CO2+2H2O b) mixing the flue gas coming from the previous step with steam and methane; c) carrying out reforming reactions to the stream coming from step b) according to the following schemes producing a hot stream of low-grade syngas:

[2] CH ₄ + CO ₂ 2 CO + 2 H ₂

[3] CH ₄ + H2O CO + 3 H ₂ d) cooling the hot stream of syngas coming from the step c) by heat exchange with a water stream which is thereby vapourised, e) separating water from wet syngas coming from step d); f) carrying out an electrolysis of a steam stream in a solid oxide electrolytic cell (SOEC), thereby obtaining oxygen gas and wet hydrogen gas according to the following reaction scheme:

[4] H ₂ O(g)^ H2+I/2O2 g) separating wet hydrogen from oxygen, h) drying wet hydrogen by water condensation, i) mixing syngas coming from the step e) with hydrogen of step h) for producing a high-grade stream of syngas.

Advantageously, the process allows to produce syngas without any emissions starting from methane (or natural gas). Furtherly, it is possible to produce high quality syngas using a combination of hydrogen produced by Solid Oxide Electrolysis Cell (SOEC) and a low quality syngas coming from a bi-reforming unit. In this way using the CO2 produced during combustion to produce syngas, the process allows to reduce the emissions. The coupling of SOEC and burner units present several benefits:

(i) consuming lower amounts of fossil fuels,

(ii) exploiting green hydrogen for CO2 hydrogenation, and

(iii) reducing to zero greenhouse gases (GHG) emissions.

DESCRIPTION OF DRAWINGS

Figure 1 : is a schematic representation of a preferred embodiment of the plant according to the present invention;

Figure 2 reports in table 1 the steam spreadsheet for the plant of figure 1 obtained by conducting Aspen Hysys vl 1 simulation.

Figure 3 represents a schematic representation of the plant according to a further preferred embodiment;

Figure 4 reports a graphic for the selection of the optimal operating condition for SOEC (Hillestad, 2018). DETAILED DESCRIPTION

For the purposes of the present invention the wording “comprising” does not exclude the possibility that further stages/ elements not explicitly listed after said wording are contemplated.

On the contrary the wording “consisting of’ excludes the above possibility.

For the purposes of the present invention the wording “plant” means an assembly comprising one or more reactive and/or operating units in fluid and/or thermal communication among each other.

For the purposes of the invention with the wording “reactive units” we mean reactors.

For the purposes of the invention for operating units we mean heat exchanger, pumps, separators, condenser etc.

The present invention further relates to a plant for carrying out the process according to the present invention, comprising:

- a burner unit (OXY-STEAM COMB);

- a first mixing unit (BIREF MIXER) in fluid communication with the burner unit (OXY-STEAM COMB) and methane and optionally water steam sources;

- a reforming unit (BIREF) in fluid communication with the mixing unit (BIREF MIXER) and associated to a waste heat boiler (BOILER)

- a waste heat boiler (BOILER) comprising a plurality of tubes surrounded by a shell

- a first water condensation unit (DeWatl) in fluid communication with the waste heat boiler (BOILER) and a second mixer (STEAM SPLITTER);

- a solid oxide electrolytic cell (SOEC) in fluid communication with the waste heat boiler (BOILER), with the burner unit (OXY-STEAM COMB) and a separator unit (SEP) of wet hydrogen from oxygen;

- a second water condensation unit (DeWat2) in fluid communication with said separator unit (SEP) of wet hydrogen from oxygen and a second mixing unit (SYNGAS MIXER) for obtaining high grade syngas

- said second mixing unit (SYNGAS MIXER) in fluid communication with said first (DeWatl) and second condensation unit (DeWat2). Advantageously, the plant then allows to produce high-quality syngas to be used in other processes reducing, preferably annulling the emission of flue gas.

It is to be noted that the process of the present invention and the relative plant can be applied to all the processes aimed at producing bulk organic chemicals, syngas and derivates, hydrogen for example but not limiting in the field of application of methanol, ammonia, dimethyl ether, acetic acid, hydrogen, polymers, methanol to olefins, hydrogenation, hydrotreatments, formylation, carbonylation.

Advantageously, the process and the plant of the present invention allow to produce syngas with zero flue gases emissions.

Advantageously, the process and the plant of the present invention allow through the electrification production of syngas zero environmental impact and high-quality syngas.

Advantageously, the process and the plant of the present invention allow to produce conditioned syngas (ready for synthesis) without any CO2 capture plant, amine washing, sweetening or other technologies.

Advantageously, the plant of the present invention can be chemically and thermally integrated with existing plants.

For the purposes of the present invention, the definition of “chemically integrated” means that the streams leaving one unit of the plant of the invention are partially/completely used as reactants in the existing plant and viceversa. For the purposes of the present invention, the definition of “thermally integrated” means that the either the enthalpy or the thermal energy produced in one of the units of the plant of the invention is used for the operation of another unit of the existing plant and viceversa.

Preferably in the process of the invention step a) is carried out in the presence of steam for mitigating the exothermic reaction [1],

In this way it is possible to mitigate the temperature of the flue gas produced at a temperature range of from 1000 to 1400°C, preferably 1100°C with a mass ratio of methane, O2 and water steam ranging from 1 :4:7 to 1 :4:7.5 preferably is 1 :4:7.1. at a pressure comprised between 20 and 40 bar, preferably 30 bar. Preferably the molar ratios of methane, O2 and water steam respect the optimal values reported in Seepana and Jayanti (Seepana and Jayanti, 2010). It is to be noticed that the oxygen is provided to the reaction [1] in under-stoichiometric condition not leaving any residual unreacted oxygen. In some alternative embodiments, O2 is fed in step a) in an excess with respect to the stoichiometric molar amount required to carry out reaction [1] of from 2 to 5% preferably of from 2 to 4% more preferably of 3% regulation the oxygen residues during the process.

According to a preferred embodiment, step a) is a steam-moderated oxy-fuel combustion carried out at a temperature ranging from 1050 to 1500°C.

Preferably the oxygen is fed to the step a) from an oxygen external source such as an oxygen tank or an air separation unit and/or the step f).

For example, in the preferred embodiment reported in Figure 1 or 3 the oxygen fed at the step a) comes in part from the one produced in the step f).

Preferably, the heat of the flue gas allows to sustain the pre-heating (superheating steam) and the highly endothermic bi-reforming as well.

According to one embodiment, step b) is carried out in a mixer together with fresh methane and optionally steam. Specifically, the hot flue gas (around 1100°C) is then quenched injecting fresh methane (around 25°C) or preheated methane (about 250°C) and optionally steam (preferably at 850°C), preferably in the distribution holed plate at the top of the reformer. Preferably, the step b) is carried out at a temperature range comprised between 1000 and 1100°C preferably at 1050°C, at a pressure comprised between 20 and 40 bar, preferably 30 bar.

It is to be noted that in the step b) the mass ratio of flue gas, methane and optionally water steam ranges from 1 :4:7 to 1 :4:7.5. The oxygen could be slightly understoichiometric, preferably in the methane-to-oxygen ratio 1 :4 to 1 :3.97.

The step c) is carried out in relative bi-reforming unit producing the hot stream of low-grade syngas. The step c) operates in autothermal regime according to the reactions [2] and [3], The hot stream coming from step c) allows to carry out the endothermic reactions [2] and [3], According to one embodiment, step c) takes place in a catalytic tube bundle at a temperature comprised between 900 and 1000°C preferably at 950°C, at a pressure comprised between 20 and 40 bar, preferably 30 bar.

It is to be noticed that low-grade stream of syngas obtained in step c) has a molar ratio H2/CO ranging from 0.5 to 3, preferably from 1 to 3 and more preferably from 2 to 3.

The step d) is carried out in a waste heat boiler wherein the hot low-grade stream of syngas coming from step c) enters the tube side causing fed water stream at the shell side to boil generating medium pressure (MP) steam of from 20 to 40 bar, preferably 30 bar. Specifically, the heat of hot low-grade stream of syngas is recovered to heat the water up to the vaporization and cooling at the same time the hot low-grade stream of syngas for the following steps. It is to be noted that, the hot low-grade stream of syngas coming from step c) enters the tube side in a temperature range 900°C and 1000°C, preferably 950°C causing fed water stream at the shell side to boil up to the vaporization. Preferably, in step d) the hot low-grade stream of syngas is cooled to a temperature ranging between 30 to 40°C preferably 35°C favouring the separation of water in step e).

Specifically, the medium pressure steam leaving the waste heat boiler is split into streams whereby a first stream is sent to step a). Such first stream is used to support the burning reaction preferably for the thermal buffer as above described. The medium pressure steam is further split in a second stream heated up to a temperature comprised between 800 and 900°C, preferably 850° C. In other words, the second stream of steam is super-heated by means of a heat exchanger or a combustion chamber. It is to be noted that, the second stream of steam is heated according to the graphic of figure 4 (Hillestad et al.2018) to be supplied at least in part to the step f). Furthermore, the medium pressure steam is split also in a third stream to be purged. This purge stream is strictly necessary since in the step f) of electrolysis of a steam stream in a solid oxide electrolytic cell (SOEC) the available energy is not able to superheat the entire fraction of steam which is not recirculated to the burner of the step a).

According to a preferred embodiment, the second stream is furtherly divided into two streams in a further splitter. The first one is conveyed to solid oxide electrolytic cell SOEC of the step f) and the second one can be recycled to step b) as shown figure 1. In such configuration, the superheated steam is directly injected to the bi-reformer through the mixer. Alternatively, the second stream can be expanded in a turbine as shown in figure 3. It is to be noticed that the expansion up to 1.2 bar allows to release mechanical work which ideally is fully converted into electrical energy.

The process comprises the step e) separating water from syngas coming from step d) which is cooled down. Specifically, the step e) is configured to condense and remove water from the syngas, coming from step d), thereby obtaining a stream consisting essentially dry low-grade syngas. Preferably, during step e) the low-grade syngas coming from step d) cooled before removing water is exploited in a dewatering unit. Accordingly, the separated water can be acid comprising CO2 and the there is a need to treat it to remove the entrapped CO2 in a known in the art way.

The process comprises the step f) wherein an electrolysis of a steam stream is carried out in a solid oxide electrolytic cell (SOEC), whereby steam is split into oxygen gas and hydrogen gas according to the reaction scheme [4], Namely, a portion of the medium pressure steam is used to generate oxygen and hydrogen, as above described. According to alternative embodiments, the electrolysis can be carried out with known device other than solid oxide electrolytic cell to produce oxygen and hydrogen such as for example: Polymer Electrolyte Membrane (PEM), Alkaline Electrolyser (AE)

As anticipated the oxygen generated in the step f) after separation from the hydrogen can be sent to the step a) for reducing the oxygen demand form the reservoir to obtain a self-sustainable burning process. The hydrogen instead can be used for the following steps preferably after step g).

Preferably the step f) is carried out at a temperature comprised between 800 and 900°C preferably at 850°C, at a pressure comprised between 20 and 40 bar, preferably 30 bar.

According to one embodiment, the step f) comprises a pre-cooling sub-step of cooling hydrogen stream coming from step f) by heat exchange with a water stream which is thereby vapourised and optionally purged with the third stream of step d), to be used in the process of invention and/or in an integrated plant to produce energy or other products. In this way, the process allows to recover the wet syngas enthalpy by generating high- pressure steam at 30 bars by means of the heat exchanger

The process comprises the step g) of separating. Specifically, the step g) allows to separate oxygen and hydrogen generated in the step f). The oxygen stream is sent to the step a) for the burning step while the hydrogen stream is sent to the step i) for producing syngas

The process comprises the step h) of drying wet hydrogen by water condensation coming from the step g). In detail, the hydrogen stream is treated, preferably after the precooling sub-step, to reduce the content of water by means of a dewatering unit.

Preferably, the hydrogen stream generated at the step h) and g) to be sent to the step i) is almost pure hydrogen over 99,5%.

The process comprises the step i) of mixing syngas coming from the step e) with hydrogen of step h) for producing a high-grade stream of syngas. In this way it is possible to reach ratio H2/C0 ranging from 3.0 to 5, defining the high-grade stream of syngas.

It is a further object of the present invention a plant for producing syngas configured to carry out the process of the invention. It is to be noted that it is clear from the context in which units the steps of the process are carried out.

The plant comprises a burner unit OXY-STEAM COMB configured to carry on the step a) preferably the burner unit OXY-STEAM COMB comprises a furnace. The burner unit OXY-STEAM COMB is configured to receive methane or natural gas, oxygen and steam. Namely, the burner unit OXY-STEAM COMB can be put in fluid communication with a methene or natural gas, such a production plant, with oxygen source such as a reservoir and/or oxygen production unit and with steam source such as other unit which product steam.

The plant comprises a first mixing unit BIREF MIXER in fluid communication with the burner unit OXY-STEAM COMB and methane and optionally water steam sources. Such first mixing unit BIREF MIXER is configured to carry on the step b) mixing the flue gas with methene preferably from reforming. According to one embodiment, the first mixing unit BIREF MIXER is configured to receive and mix steam with the flue gas and methene.

The plant comprises a reforming unit BIREF in fluid communication with the first mixing unit BIREF MIXER and associated to a waste heat boiler BOILER. The reforming unit BIREF is configured to carry on the step c) generation hot low grade syngas stream.

The plant comprises a waste heat boiler BOILER comprising a plurality of tubes surrounded by a shell. The waste heat boiler BOILER is configured to carry out step d). Specifically, the waste heat boiler BOILER is in fluid communication with the burner unit OXY-STEAM COMB through the first mixing unit BIREF MIXER to receive the low- grade syngas in the tube side and in fluid communication with a source water unit to receive the water stream in the shell side. In this way, the waste heat boiler BOILER allows the heat exchange.

The plant comprises a first water condensation unit DeWatl in fluid communication with the waste heat boiler BOILER and a second mixer STEAM SPLITTER. The first water condensation unit DeWatl is configured to carry our step e) to produce a dry low grade syngas stream. Preferably, the first water condensation unit DeWatl can be associated to a treatment unit of the separated water from the syngas to reduce the CO2 content.

The plant comprises a solid oxide electrolytic cell SOEC in fluid communication with the waste heat boiler BOILER and with the burner unit OXY-STEAM COMB and a separator unit SEP of wet hydrogen from oxygen. The solid oxide electrolytic cell SOEC is configured to carry out step f) producing oxygen and hydrogen streams. Specifically, the solid oxide electrolytic cell SOEC upon production of the oxygen and hydrogen streams is configured to send oxygen stream to the burner unit OXY-STEAM COMB and the hydrogen stream to downstream solid oxide electrolytic cell SOEC preferably to a second mixing unit SYNGAS MIXER.

It is to be noted that, separator unit SEP of wet hydrogen from oxygen to fed respectively step a) with the oxygen and step i) with hydrongen is configured to carry on the step g).

The plant comprises a second water condensation unit DeWatl in fluid communication with said separator unit SEP of wet hydrogen from oxygen and a second mixing unit SYNGAS MIXER for obtaining high grade syngas. The second water condensation unit DeWatl is configured to carry on the step h) removing water from the produced hydrogen.

The plant comprises a second mixing unit SYNGAS MIXER in fluid communication with said first DeWatl and second condensation unit DeWatl. Specifically, the second mixing unit SYNGAS MIXER is configured to carry on the step i) for producing high grade syngas.

It is to be noted that the plant can comprise a heat exchange unit cooling humid hydrogen coming from the separation unit SEP and before entering the dewatering unit DeWatl wherein pure hydrogen is obtained. Also, this heat exchanger uses as cooling fluid a water stream that in the heat exchanger is transformed into medium pressure steam cooling the hydrogen stream.

Preferably the plant comprises a first splitting unit STREAM SPLITTER configured to split the medium pressure steam produced from the waste heat boiler BOILER in a first stream to be sent to the burner unit OXY-STEAM COMB, in a second stream to be further heated in as described above, and in a third stream to be purged. The latter can be purged together with the medium pressure steam produced downward from the solid oxide electrolytic cell SOEC. More preferably, the plant comprises a second splitting unit SH SPLITTER configured to further split the second heated stream of steam into to a first stream to be sent to the solid oxide electrolytic cell SOEC and to a second stream that can be sent to the first mixer BIREF MIXER figure 1 or to turbine unit Figure 3.

It is to be noted that the high-pressure steam produced in the plant and the energy from the turbine can be used in the plant itself for the self-consistency and/or to reduce the used resources and/or to support other process and plant associated to the ones of the present inventions.

Example 1 (recycle configuration)

A preferred embodiment of the plant according to the present invention is depicted in detail in Figure 1. Apart from energy/process integration, the layout includes all the relevant unit operations and reactions. Figure 1 depicts the detailed flowsheet of the invention. In the NZE BIREF, the steam-moderated oxycombustion (SMOEC) takes place in the combustion chamber (OXY-STEAM COMB) and the methane (CEL fuel) is consumed generating carbon dioxide and heat (Qcomb) which is distributed within the chamber. The released heat sustains the pre-heating (superheating steam, QSH heat stream) and the highly endothermic bi-reforming as well (QBiRef). The hot flue gas (1100°C) are then quenched injecting fresh methane (CEL quenching) in the distribution holed plate at the top of the reformer (BIREF mixer). The resulting mixture (to BIREF) is reformed in the reforming section. The exiting syngas (Wet syngas) is then cooled in the boiler. The outlet temperature from the BIREF reactor is set to 950°C which is the industrial typical value. The syngas (Cold syngas) is cooled at 35°C and then dewatered to get the dry syngas (Crude syngas). The acid water leaving the dewatering process should be post-treated to remove the entrapped CO2. Thanks to this assumption (i.e., Cold syngas temperature fixed at 35°C), it is possible to estimate the maximum amount of saturated steam that the boiler can generate. The removed sensible heat is exploited to generate saturated steam at 30 bar. The produced steam is split into three streams: (1) a first stream (Steam recycle) (7.1 kg/s) is delivered back to the burner as thermal buffer, (2) a second stream (Steam to SH) is recirculated in a combustion chamber/heater (Super) to heat up the steam up to 850°C (Steam to SH) which is the required temperature to supply the steam to the SOEC (Figure 4, Hillestad et al.2018) and finally (3) the third stream is purged (Steam purge). This purge stream is strictly necessary since the SMOEC is not able to superheat the entire fraction of steam which is not recirculated to the burner. The superheated steam after the heater (Super Heater SH) is furtherly divided into two streams in the splitter (SH splitter). A fraction is conveyed to the SOEC (to SOEC) to produce the hydrogen required at the syngas conditioning, while the complementary fraction (SH steam recycle and Steam to BiRef which are the same streams, but Aspen Hysys v. l l require a recycle tool to aid the numerical convergence) is directly injected to the bi-reformer. The SOEC produces hydrogen and oxygen. The pure oxygen (O2 SOEC) can be recycled back to the burner to reduce the ASU energy requirement, while the hydrogen (Wet H2) undergoes cooling and dewatering is sequence. The dry syngas (Crude syngas) and the produced hydrogen (H2 SOEC) are mixed to get the optimal composition in terms of hydrogen content as H2/CO and SN (Conditioned syngas). This is the syngas condition that consists of a hydrogen content refinement. Finally, the recovery boiler allows to save the enthalpy release when cooling the wet hydrogen. This sensible/latent heat is exploited to furtherly produce saturated steam at 30 bar (Steam rec). The recovered and purged steam are collected into a single stream (Steam excess). The stream spreadsheet of the plant of Figure 1 resulting from Aspen Hysys vl 1 simulation is given in Table 1 reported in Figure 2.

Example 2 (co-generation configuration)

A further preferred embodiment is reported of the plant according to the present invention is represented in figure 3 differing from that reported in figure 1 in the splitting of the second superheated stream. In fact, the pressurized superheated steam is expanded in a turbine (up to 1.2 bar) to release mechanical work which ideally is fully converted into electrical energy.

Bibliography

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