Plant and process for the production of hydrogen by pyrolysis of methane with a liquid catalyst

Abstract

A plant for producing hydrogen by pyrolysis of natural gas using a liquid catalyst (3) comprises a reaction chamber (4) and a heat exchanger (1) designed to be positioned in the exhaust duct (2) of an electric arc furnace that produces flue gas at a temperature of at least 800°C, a feed line (5) and a return line (6) connecting the ends of the heat exchanger (1) to the reaction chamber (4) so as to form a circuit, a pump (7) circulating the liquid catalyst (3) in the circuit, heating means (8) for further heating the liquid catalyst (3) and keeping it in the liquid state even when the electric arc furnace producing the flue gas is switched off, means (12, 13) for removing from the reaction chamber (4) a layer (11) of solid carbon that forms on the surface of the liquid catalyst (3) as a result of pyrolysis, a gas separation device (10) arranged to receive a mixture of gases exiting the reaction chamber (4) and separate the unreacted natural gas from the hydrogen produced by pyrolysis, and a conduit for reintroducing the unreacted natural gas from said gas separation device (10) into the reaction chamber (4).

Description

[0001] PLANT AND PROCESS FOR THE PRODUCTION OF HYDROGEN BY

[0002] PYROLYSIS OF METHANE WITH A LIQUID CATALYST

[0003] The present invention relates to a plant and a process for producing hydrogen from methane pyrolysis using a liquid catalyst that is brought to its reaction temperature by exploiting the heat recovered from the flue gas of an electric arc furnace (EAF) in a steel plant. Specific reference will be made to this application below, but it is clear that the above can be applied to other plants where high-temperature flue gas is produced by other types of equipment. Furthermore, the term “methane” is used as a synonym for “natural gas” even though the latter contains small amounts of ethane, propane, butane, etc.

[0004] The steel industry is currently responsible for about 7% of global CO2 emissions, and most of these emissions come from the integrated production cycle based on the reduction of iron ore in blast furnaces, which accounts for 70% of the world's steel. With the implementation of ecological transition plans and CO2 quota systems, European steel companies are under pressure to significantly reduce their emissions. At present, steel companies are entitled to a free allocation of CO2 quotas for approximately 70-80% of their annual requirements and must purchase the remaining quantity on a special exchange, but the free allocation quotas will gradually decrease until they reach zero in 2030. In order to remain competitive, it is therefore essential for European companies to decarbonize as much as possible.

[0005] Electric arc furnaces (EAFs) that melt scrap iron offer a low-emission alternative to the integrated cycle for producing liquid steel, but around 25% of the energy supplied to an EAF is lost in the form of heat in the flue gas, which reaches peak temperatures of around 1300°C. Currently, this waste heat is only partially recovered through methods such as preheating the scrap to be melted or producing steam and electricity, for example via a Rankine cycle, which have limitations in terms of efficiency and operational flexibility. In fact, scrap preheating involves considerable operational complexities without the possibility of separating this process from furnace operation, while recovery through thermodynamic cycles is characterized by low overall efficiency.

[0006] On the other hand, the use of “green” hydrogen could further reduce emissions linked to the use of natural gas in various stages of the steelmaking process, particularly in furnaces for preheating semi-finished products (slabs, billets, blooms, etc.), in drying furnaces for ladles, and in burners in EAFs themselves. In addition, the production of high-quality steels, particularly flat products, requires the addition of up to 35% DRI (Direct Reduced Iron) or HBI (Hot Briquetted Iron) to replace scrap. In direct reduction processes of iron ore for the production of DRI and HBI, hydrogen can also be used to reduce emissions from the above-mentioned plants, which are also currently based on natural gas consumption. However, the high cost of hydrogen production, mainly due to high energy requirements, is a significant barrier to its widespread use.

[0007] Methane pyrolysis is a known technique for producing hydrogen without direct CO2 emissions, for example using a catalyst as described in US 2022 / 0348460, and is based on the spontaneous dissociation of gaseous hydrocarbon molecules in the presence of a catalyst (including nickel, iron, cobalt, or an alloy thereof). However, processes according to the prior art require a significant amount of energy (approximately 1 kWh / SMCm) to reach the temperatures necessary for the pyrolysis reaction.

[0008] The paper “Hydrogen production using methane: Techno-economics of decarbonizing fuels and chemicals ” by Brett Parkinson et al . , publi shed on 01.02.2018 in the International Journal of Hydrogen Energy, vol. 43, pp. 2540-2555, describes a methane pyrolysis plant with a pyrolysis reactor containing a Ni-Bi catalyst over which a layer of immiscible molten salt is placed, which is circulated outside the reactor for two purposes: a) to remove the carbon produced in pyrolysis, separating it from the salt in a special drum; b) to supply heat to the catalyst remaining in the reactor by passing the decarbonized molten salt through a heater before reintroducing it into the reactor. The paper states that the heater is powered by gas, hydrogen, or solar energy, and in particular, in the project illustrated, 22% of the recovered methane separated from hydrogen in a special gas separation device is used to produce the 110 MW required by the heater. Clearly, a fraction of the hydrogen produced could also be used, but in both cases, the problem of high energy consumption and CO2 production to obtain the required hydrogen remains.

[0009] In fact, in order to try to reduce consumption, this plant uses two heat exchangers in series through which the carbon-rich molten salt leaving the reactor is passed: the first exchanger is used to preheat the methane to 625°C and the second exchanger is used to preheat the decarbonized salt before passing it into the heater. In addition, there are two other heat exchangers in series through which the gas mixture exiting the reactor is passed: the first exchanger is used to preheat the methane to 800°C, and the second exchanger is used to produce high-pressure superheated steam used to generate electricity.

[0010] The paper “Methane pyrolysis as a potential game changer for hydrogen economy: Techno-economic assessment and GHG emissions” by Shokrollahi Marzieh et al., published on 13.04.2024 in the International Journal of Hydrogen Energy, vol. 66, pp. 337-353, describes a methane pyrolysis plant that is essentially identical to the one in the aforementioned paper, which is in fact explicitly recognized as the basis on which this paper was developed, differing only in that the heater is a gas, hydrogen, or electric heater and in the focus on the economic value of the carbon obtained as a by-product of pyrolysis.

[0011] The purpose of the present invention is to overcome the drawbacks of the prior art for producing hydrogen without CO2 emissions through the pyrolysis of natural gas. This purpose is achieved by means of a plant and a process that exploits the waste heat contained in the fumes from EAFs for steel production to heat a liquid catalyst, preferably consisting of an alloy of bismuth with nickel and molybdenum, which is circulated in a heat exchanger exposed to said fumes.

[0012] More specifically, such a liquid catalyst is described in the paper “Ternary NiMo- Bi liquid alloy catalyst for efficient hydrogen production from methane pyrolysis ” by L. Chen, Z. Song, S. Zhang et al. published in Science on 25.08.2023, and is a bismuth alloy containing 2.3% by weight nickel and 1.3% by weight molybdenum, which achieves maximum productivity at 800°C. It should be noted that although this is the preferred catalyst, the invention could also work with other types of liquid catalysts, albeit with lower efficiency, such as Fe-NaKCl, Mn-ChKCl, or Ni-Bi alloys.

[0013] The fundamental advantage of this plant and process is that it significantly reduces both the economic and environmental costs of hydrogen production, so that it can be used more widely as an alternative fuel to natural gas, contributing to the decarbonization of steel production. A second significant advantage stems from the fact that the heat exchange for heat recovery from the fumes is carried out by the liquid catalyst itself, thus maximizing the simplicity and efficiency of the plant without complicating the management of the furnace from which the waste heat is recovered.

[0014] Another advantage lies in the production of hydrogen at the same plant where it can be used as fuel, thus minimizing transport and storage problems. Furthermore, the need to preheat the reducing gas is drastically reduced when hydrogen is used in a direct reduction plant that requires an inlet temperature of the reducing gas of approximately 950°C, since the hydrogen produced by pyrolysis leaves the reaction chamber at approximately 800°C.

[0015] Further advantages and features of the plant and process according to the present invention will be apparent to those skilled in the art from the following detailed and nonlimiting description of some of its embodiments with reference to the accompanying drawings, in which:

[0016] Fig.l is a schematic view of the plant in its first basic embodiment, showing the flow of the liquid catalyst through a heat exchanger located in the flue gas duct of an EAF, and a reaction chamber for the pyrolysis of methane;

[0017] Fig, 2 is a partial schematic view similar to Fig. 1 of a second embodiment with additional elements inserted into the catalyst circuit;

[0018] Figs.3A-3B are enlarged details of the diagram in Fig.2 showing the flow of the catalyst in two different operating conditions of the plant; and

[0019] Fig, 4 is a partial schematic view similar to Figs.3 A-3B of a third embodiment with additional elements inserted into the catalyst circuit.

[0020] With reference to Fig. 1 , it can be seen that a plant according to the present invention in its simplest embodiment comprises a heat exchanger 1, inside the flue gas duct 2 of an EAF, in which a liquid catalyst 3 circulates directly and, after being heated, enters a reaction chamber 4, where the pyrolysis of methane takes place thanks to the supply by the catalyst itself of the heat necessary for the pyrolysis reaction. The circulation of the liquid catalyst 3 in the circuit, consisting of the heat exchanger 1 and the reaction chamber 4 connected by a feed line 5 and a return line 6, is ensured by a pump 7, preferably inserted in the feed line 5, and by an induction coil 8, preferably located on the return line 6, which further heats catalyst 3 to a temperature higher than that at the outlet of the heat exchanger 1 in order to increase the efficiency of the pyrolysis reaction. The coil 8 also serves to maintain catalyst 3 at a temperature such that it remains liquid even in the event of an EAF shutdown and, obviously, in order to load it into the plant, catalyst 3 must already be liquid, so it is preheated to a temperature of at least 420°C. Furthermore, it is clear that chamber 4 and lines 5 and 6 are insulated to minimize heat loss and maintain the temperature of catalyst 3.

[0021] Methane is preferably fed into the reaction chamber 4 through porous septa 9 located at the bottom of the reaction chamber 4, while the gas mixture exiting the top of the reaction chamber 4 is treated in a gas separation device 10 of a known type, which separates the hydrogen produced by pyrolysis from the unreacted methane that is recirculated to the supply line in order to use its heat to preheat the incoming methane. In fact, methane must be heated to the reaction start temperature of approximately 600°C, which occurs in the lower part 4A of chamber 4, before the pyrolysis reaction can take place, which occurs in the upper part 4B of chamber 4. In practice, the heat absorbed by catalyst 3 in the heat exchanger 1 is extracted in chamber 4 in two ways: in the upper part 4B through the endothermic pyrolysis reaction, and in the lower part 4A by heating the incoming methane to the reaction start temperature.

[0022] Note that the circulation of catalyst 3 in countercurrent with respect to the fumes in duct 2 and with respect to the methane in chamber 4, as shown in Fig.l, is preferable but not strictly essential, since the process could also work with catalyst 3 circulating in the opposite direction, albeit with less effectiveness. In particular, with heat exchanger 1 operating in countercurrent, a AT of 480°C can be obtained, from 950°C to 470°C, of the fumes across the heat exchanger 1, considering this relatively moderate fumes inlet temperature in order to minimize wear of the plant by placing the heat exchanger 1 in duct 2 downstream of the chamber for the removal of coarser dust by gravity, in order to limit the exposure of the heat exchanger 1 to dust that could damage and / or encrust it, reducing its efficiency. Obviously, the exact position of the heat exchanger 1 can be decided on the basis of construction and operational considerations depending on the specific device for which the heat from the flue gas is to be exploited, as it might be preferable to use fumes at a lower temperature (in any case, approximately at least 800°C) to increase the operating life of the heat exchanger 1, accepting an increase in the heat input of the additional heating device consisting of coil 8 or the like.

[0023] The heat exchanger 1 can be made of various nickel alloys, which are protected from catalyst 3 by creating a chemically inert barrier between the two by means of a graphite deposit made by catalyst 3 itself. In fact, in the methane pyrolysis process, for every kg of hydrogen produced, the pyrolysis reaction produces 3 kg of solid carbon, mainly in the form of graphite, a small part of which remains in the circulated catalyst 3 but is mainly separated from catalyst 3 by exploiting the difference in density of the carbon, which forms a surface layer 11 that floats above catalyst 3.

[0024] This layer 11 of solid carbon is periodically removed from chamber 4 through a door 12, located above the return line 6, by means of appropriate removal devices such as a skimmer 13 or similar. This by-product of natural gas pyrolysis can be used to produce electrodes for EAFs and ladle furnaces, or sold on the market, if this is more advantageous. Furthermore, it is theoretically possible to refine the pyrolysis process in order to maximize the yield of valuable by-products such as graphene, carbon nanotubes, and fullerenes.

[0025] With the plant and process according to the invention, the applicant has calculated that, considering a quantity of approximately 6000 t of the NiMo-Bi catalyst described above with an activation energy of 81.2 kJ / mol, for an EAF sized for 30 castings per day of 150 t, the system could produce approximately 33400 SMC of hydrogen and 9000 kg of solid carbon for each hour of EAF operation, considering a fumes flow of 125000 SMC / h undergoing a AT of 480°C as described above and a natural gas consumption of approximately 17650 SMC / h.

[0026] It should be noted that these are the maximum values obtainable by exploiting the maximum realistic AT of the fumes and the maximum realistic size of the plant, with specific reference to the NiMo-Bi catalyst, but the use of other catalysts and / or plants of different sizes would obviously give different results.

[0027] The process according to the invention can therefore be summarized in the following general steps: a) arranging a circuit consisting of a heat exchanger 1, positioned in the exhaust duct 2 of a device that produces flue gas at least at 800°C, and a reaction chamber 4 connected by a feed line 5 and a return line 6; b) circulating a liquid catalyst 3 in said circuit, which is equipped with heating means suitable for further heating the catalyst 3 coming from the heat exchanger 1 and for maintaining said catalyst 3 in the liquid state even when the device that produces the flue gas is turned off; c) introducing natural gas into the reaction chamber 4 so as to cause the pyrolysis of the natural gas using the heat supplied by catalyst 3, producing hydrogen gas and solid carbon 11; d) removing the gaseous mixture of hydrogen and unreacted natural gas from the reaction chamber 4 and separating said two gases, reintroducing the unreacted natural gas into the reaction chamber 4; e) periodically removing the solid carbon 11 from the reaction chamber 4.

[0028] As mentioned above, in the preferred embodiment illustrated above, these general steps are further specified as follows:

[0029] - in step a), the heat exchanger 1 is placed in the flue gas duct 2 of an EAF downstream of the coarse dust removal chamber, in a position where the incoming flue gas has a temperature of approximately 950°C;

[0030] - in step b), catalyst 3 is a bismuth alloy containing 2.3% by weight of nickel and 1.3% by weight of molybdenum, it is circulated in countercurrent to the fumes in duct 2, and the heating device is an induction coil 8 arranged on the return line 6;

[0031] - in step c), natural gas is introduced into the reaction chamber 4 in countercurrent to the flow of catalyst 3.

[0032] In a second embodiment illustrated in Fig.2, the system described above also includes a second pump 7' arranged in the return line and a storage tank 14, preferably equipped with a second induction coil 8', which extends vertically with two bidirectional openings between the feed line and the return line in a position such that it is upstream of the two pumps 7, 7' with respect to the direction of flow of catalyst 3. In addition, an auxiliary heat exchanger 15 may also be added, preferably located in the feed line downstream of the first pump 7, which is useful in case it is necessary to dissipate the heat recovered from the fumes, for example due to a shutdown of the reaction chamber 4 (which is not shown in the figure because that part of the plant remains unchanged).

[0033] More specifically, tank 14 divides the feed line 5 and return line 6 into two parts each 5A, 5B and 6A, 6B, where parts 5A and 6A are those between tank 14 and reaction chamber 4, while parts 5B and 6B are those between tank 14 and heat exchanger 1. It follows that the first pump 7 and the auxiliary exchanger 15 are in line 5B, while the second pump 7' and the first coil 8 are in line 6A. The two pumps 7, 7' operate in coordination with each other, so that their flow rates are adjusted to adapt the circulation of catalyst 3 towards the reaction chamber 4 or the storage tank 14 to the flow rates required by exchanger 1 to ensure the correct cooling of the EAF fumes and by the reaction chamber 4 for the desired hydrogen production.

[0034] As shown in Fig.3A, if the flow rate required by the reaction chamber 4 is lower than that circulating in exchanger 1, i.e., the flow rate of the second pump 7' is lower than the flow rate of the first pump 7, a portion of the catalyst 3 flow coming from line 6B is directed to chamber 4, while the remaining portion of catalyst 3 is directed to the storage tank 14. Conversely, as shown in Fig.3B, if the flow rate of catalyst 3 required by the reaction chamber 4 is greater than that coming from exchanger 1, i.e., the flow rate of the second pump 7' is greater than the flow rate of the first pump 7, the missing portion of catalyst 3 is automatically taken from the storage tank 14.

[0035] Based on the incoming or outgoing flows, the level of catalyst 3 inside storage tank 14 does not vary; what varies is the thermal energy stored in it, since it is a thermal stratification tank. When a portion of catalyst 3 from exchanger 1 is directed to tank 14, it enters the upper part and the same amount of catalyst 3 exits from the lower part of tank 14, maintaining the mass balance unchanged (Fig.3A). Conversely, when catalyst 3 is drawn by the second pump 7' also from the storage tank 14 to the reaction chamber 4, an equal amount of catalyst 3 coming from line 5A, instead of being directed back to exchanger 1, returns to the storage tank 14, maintaining the mass balance unchanged (Fig.3B).

[0036] In the third embodiment illustrated in Fig.4, a three-way valve 16 is further provided in line 6B so that the flow of catalyst 3 can be diverted to a bypass line 17, which extends to line 5B upstream of the first pump 7. In this way, if the temperature of catalyst 3 coming from the heat exchanger 1 does not exceed a minimum threshold required for pyrolysis in the reaction chamber 4, also taking into account the possible increase in temperature due to coil 8, then valve 16 closes line 6B and opens bypass line 17, preventing catalyst 3 from proceeding to the storage tank 14 and the reaction chamber 4. As a result, a portion of catalyst 3 can recirculate in the heat exchanger 1 until the minimum temperature required to be directed to the reaction chamber 4 and / or tank 14 is reached. During this transition, the entire amount of catalyst 3 circulating in the reaction chamber 4 is taken from the storage tank 14.

[0037] Conversely, if the temperature of catalyst 3 in line 6B is higher than the aforementioned minimum threshold, then valve 16 keeps bypass line 17 closed and the system operates as described with reference to the second embodiment of Figures 2, 3 A, and 3B. Obviously, the operation described above requires the presence of a plurality of temperature sensors and a control unit (not shown in the figures) that receives data from said sensors and controls valve 16. This control unit also serves to regulate the other operating parameters of the system, such as the flow rates of pumps 7, 7' and the quantity of methane introduced into the reaction chamber 4.

[0038] It is clear that the embodiments of the plant according to the invention described and illustrated above are only examples that are subject to numerous variations. In particular, the porous septa 9 could be replaced by nozzles or nebulizers to obtain smaller methane bubbles that react better with catalyst 3. Furthermore, the devices for heating catalyst 3 could also be positioned in other locations, for example inside the reaction chamber 4, and be made in a manner other than induction coils 8, 8', for example resistors.

[0039] Finally, other embodiments not illustrated can be easily derived from those described above by combining the various additional elements, for example, the auxiliary exchanger 15 could be present in the configuration of Fig.1 and absent in the configuration of Fig.2. Similarly, valve 16 and bypass line 17 could be present in the configurations of Fig.1 and Fig.2, even in the absence of the auxiliary exchanger 15.

Claims

CLAIMS1. A plant for the production of hydrogen by pyrolysis of natural gas, comprising a reaction chamber (4) suitable for containing a liquid catalyst (3), means for introducing natural gas into said reaction chamber (4), a gas separation device (10) arranged to receive a mixture of gases exiting the reaction chamber (4) and to separate natural gas that has not reacted from hydrogen produced by pyrolysis, and a conduit for reintroducing into the reaction chamber (4) the natural gas that has not reacted coming from said gas separation device (10), characterized in that the plant further comprises:- an electric arc furnace that produces flue gas at least at 800°C discharged through a gas exhaust duct (2),- a heat exchanger (1) suitable to be placed in said exhaust duct (2), preferably downstream of a coarser dust removal chamber, and yet more preferably in a position where the flue gas temperature is about 950°C ,- a feed line (5) and a return line (6) connecting the ends of said heat exchanger (1) to the reaction chamber (4) so as to form a circuit,- at least one pump (7), preferably arranged in said feed line (5), suitable for circulating said liquid catalyst (3) in said circuit,- heating means suitable for further heating said liquid catalyst (3) coming from the heat exchanger (1) and for maintaining said catalyst (3) in the liquid state even when said flue gas-producing electric arc furnace is turned off,- means for removing from the reaction chamber (4) a layer (11) of solid carbon formed on the surface of the liquid catalyst (3) by pyrolysis.

2. A plant according to claim 1, characterized in that the means for introducing natural gas into the reaction chamber (4) consists of porous septa (9) arranged at the bottom of the reaction chamber (4).

3. A plant according to claim 1 or 2, characterized in that the means for removal from the reaction chamber (4) of the layer (11) of solid carbon consist of a door (12) formed higher than the return line (6) and a skimmer (13).

4. A plant according to any of the preceding claims, characterized by further comprising a second pump (7') arranged in the return line (6) and a storage tank (14)extending vertically with two bidirectional openings between the feed line (5) and the return line (6) in a position such that it is upstream of the two pumps (7, 7') with respect to the flow direction of the liquid catalyst (3).

5. A plant according to any of the preceding claims, characterized by further comprising an auxiliary heat exchanger (15) arranged in the circulation circuit of the liquid catalyst (3), preferably arranged in the feed line (5) downstream of the first pump (7), suitable for dissipating the heat recovered from the flue gas.

6. A plant according to any of the preceding claims, characterized by further comprising a three-way valve (16) placed in the return line (6) so as to be able to divert the flow of the liquid catalyst (3) to a bypass line (17) extending to the feed line (5) at a location upstream of the first pump (7).

7. A plant according to any of the preceding claims, characterized by being configured to circulate the liquid catalyst (3) countercurrently with respect to the flue gas in the exhaust duct (2) and / or with respect to the natural gas in the reaction chamber (4).

8. A plant according to any of the preceding claims, characterized in that the heating means consist of induction coils, preferably a first coil (8) arranged on the return line (6) and a second coil (8') arranged on the storage tank (14) when present.

9. A process for the production of hydrogen by pyrolysis of natural gas, comprising the steps of:(a) arranging a circuit consisting of a heat exchanger (1), located in the exhaust duct (2) of an electric arc furnace producing flue gas at a temperature of at least 800°C, and a reaction chamber (4) connected by means of a feed line (5) and a return line (6);(b) circulating a liquid catalyst (3) in said circuit, which is provided with heating means suitable to further heat said liquid catalyst (3) coming from said heat exchanger (1) and to maintain the catalyst (3) in the liquid state even when said flue gas-producing electric arc furnace is turned off;(c) introducing natural gas into said reaction chamber (4) so that pyrolysis of the natural gas takes place using the heat supplied by the liquid catalyst (3), producing hydrogen gas and solid carbon (11) that floats on the surface of the liquid catalyst (3);(d) taking the gaseous mixture of hydrogen and unreacted natural gas from the reaction chamber (4) and carrying out the separation of said two gases, reintroducing theunreacted natural gas into the reaction chamber (4);(e) periodically withdrawing solid carbon (11) from the reaction chamber (4).

10. A process according to the preceding claim, characterized in that in step (a) the heat exchanger (1) is placed in the exhaust duct (2) of the electric arc furnace downstream of a coarser dust removal chamber, preferably in a position where the flue gas has a temperature of about 950°C.

11. A process according to claim 9 or 10, characterized in that in step (b) a liquid catalyst (3) is used which is an alloy of bismuth containing 2.3% nickel by weight and 1.3% molybdenum by weight, and said liquid catalyst (3) is preferably circulated countercurrently relative to the flue gas in the exhaust duct (2) and is preferably heated further in the return line (6) by an induction coil (8).

12. A process according to any of claims 9 to 11, characterized in that in step (c) the natural gas is introduced into the reaction chamber (4) countercurrently to the flow of the liquid catalyst (3).

13. A process according to any of claims 9 to 12, characterized in that in step (a) the circuit is also arranged with a three-way valve (16) placed in the return line (6) so that the flow of the liquid catalyst (3) can be diverted to a bypass line (17) extending to the feed line (5), and in that in step (b) the liquid catalyst (3) is circulated only between the heat exchanger (1) and said bypass line (17) until it reaches a minimum temperature threshold.

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