Patent Application
20240343567
The present invention concerns a method for hydrogen production coupled with CO2 capture, which is also configured to ensure zero export steam.
As it is known, carbon dioxide is a by-product of many industrial processes and a final combustion product of carbon containing fuels. As such, it is generated in large quantities and emitted in the gaseous effluents of industrial and energy production sites, and in smaller and distributed amounts in building heating, transportation, etc.
In all cases, since process feedstocks and fuels are almost all of fossil origin, the CO2 emitted contributes the to anthropogenic carbon emission, causing an increase of the CO2 concentration in the atmosphere and contributing to climatic changes.
CO2 is a primary greenhouse gas and it is estimated that stationary CO2 emissions contribute for about 60% of the overall global CO2 emissions [https://www.ipcc.ch/pdf/special-reports/srccs/srccs_chapter2.pdf [Accessed April 2018]].
Main industrial sectors contributing to the stationary CO2 emissions are represented by power plants, and energy intensive industries. In particular, the refinery sector contributes to around 6% of the total stationary CO2 emissions [Jiri van Straelen, Frank Geuzebroek, Nicholas Goodchild, Georgios Protopapas, Liam Mahony, CO2 capture for refineries, a practical approach, Energy Procedia 1 (2009) 179-185].
On the other hand, demand of hydrogen has reached almost 75 million of tons in 2018 and increases by approximately 6% per year [IEA.org/reports/the-future-of-hydrogen, June 2019], with more than 50% used for refinery applications as hydrotreating and hydrocracking and the remaining fraction mainly for ammonia and methanol production. According to IEA, roughly 0.83 bn tons of CO2 are associated today to yearly hydrogen production. The importance of this carbon flow is such that in the coming years the CO2 emissions associated to each single chemical process are going to become the first and major parameter to assess for technology selection including the one to make hydrogen.
Steam Reforming is currently the most cost-effective technology to produce hydrogen, particularly in refineries, where natural gas or off gases are used as feedstock. Steam Reforming of natural gas and light naphtha is the workhorse for such production being a quite efficient process, with the highest H2/CO ratio and the lowest Cost of Production (CoP). Such process accounts for at least up to 20% of the CO2 emissions in refinery [J. van Straelen, F. Geuzebroek, N. Goodchild, G. Protopapas, L. Mahony, International Journal of greenhouse control, 4 (2010) 316].
In steam reforming process, a part of CO2 (typically ˜ 50-60% of the total amount) is generated inside the process syngas in the steam reforming (CH4+H2O=CO+3H2) and water gas shift (CO+H2O=CO2+H2) reactor and downstream stages, while another part (40-50%) is additionally generated in the steam reformer furnace where heat provided by external fuel combustion supplies the necessary thermal input to the endothermic reaction [G. Collodi, Chemical Engineering Transactions 19 (2010) 37]. It is estimated that around 0.9 kg CO2 are produced per Nm3 of H2.
In addition, the presence of a furnace with burners installed inside makes the steam reformer configuration quite complex and the overall plant is characterized by quite large footprint.
As a matter of fact, the key challenge in achieving a carbon-neutral energy and production systems is to decarbonize the sectors that are currently heavily dependent on fossil fuel resources, such as oil and natural gas. The most promising option for future decarbonization of final energy and feedstock use in the chemical industry is to convert the relatively abundant potential of wind and solar energy, produced in the form of electricity into heat, chemicals and fuels. In fact, electrification has the potential to release major progress on sustainability and reduction in fossil energy and feedstock use. Electrification of conventionally fired chemical reactors has the potential not only to reduce CO2 emissions but also to provide more flexible and compact solutions for heat generation.
Under the assumption that the cost of electricity is going down in the future and more and more renewable electricity will be available, it makes sense to replace the combustion step which today supplies the heat of reaction in the steam reformer with an electric device, with the possibility to eliminate the CO2 emissions contribution from furnace burners.
In addition, the possibility to maximize the efficiency of thermal exchange may also help to reduce the feed consumption, thereby limiting CO2 emissions from the process side as well.
Under such basis a reconfiguration of process architecture is required.
According to the prior art, the process architecture of a conventional Hydrogen Production Unit (HPU) with steam reforming of natural gas feedstock includes the following conventional process steps:
Optionally in between steps (ii) and (iii) a pre-reforming step may be added, depending on the hydrocarbon feedstock used.
Making reference to
The required hydrogen (˜3 mol %, typical value with natural gas feedstock) is recycled from the H2 Product stream and/or taken from an available hydrogen source from battery limit. The produced hydrogenated compounds are then sent to the desulfurization step, in which they react typically with zinc oxide beds for H2S adsorption, optionally equipped with a material for hydrogen chloride adsorption.
The treated feedstock is then mixed with a controlled quantity of steam according to the selected value for the steam/carbon molar ratio (S/C=3 mol/mol, typical value) and preheated at 550° C. (typical value) in the convection section of the reformer furnace 2.
The heart of the process is the endothermic reaction of methane with steam over Ni catalyst (reaction 1).
CH4+H2O⇔CO+3H2 ΔH0=+206 kJ/mol (1)
The reaction is conducted in a tubular catalytic reactor heated up by external fuel combustion in the radiant section of a furnace. In series to the main steam reforming reaction, the water gas shift reaction (reaction 2) converts part of the CO produced by the first reaction into additional H2 and CO2.
CO+H2O⇔CO2+H2 ΔH0=−41 kJ/mol (2)
The process steam added to the feed is in excess of the stoichiometric quantity so as to improve the hydrocarbons conversion and prevent any carbon deposition over the catalyst. Reforming temperatures are selected in a high range (typically 850÷920° C.) in order to obtain high hydrogen yields. The operation of the steam reforming reaction inside a fired heater causes an excess heat generation related to the low thermal efficiency of the radiant section. The excess heat is normally recovered in the convection section through high pressure steam generation.
Besides this, additional steam is generated by the process gas boiler employed to cool down the process syngas at the outlet of the steam reformer. The total steam produced is more than necessary for the process itself, therefore, a quantity of export steam is made available at battery limits as a by-product. The cooled process gas is then fed to a high temperature shift conversion stage (HTS) at an inlet temperature of about 320° C.
The HTS shift reactor 3 is a fixed bed adiabatic reactor using an iron/chromium/copper oxide catalyst which converts the carbon monoxide and steam present in the syngas into additional hydrogen and carbon dioxide according to the water gas shift reaction (reaction 2). In some cases, an additional stage of shift conversion at lower temperature (LTS) is installed downstream and operated.
The process syngas at the outlet of the shift conversion stage is cooled down to about 40° C. through a heat recovery section and a final cooler. Downstream, the equipment for water condensate removal is installed, from which the syngas is sent to a PSA unit 4 to perform the raw hydrogen purification.
The PSA unit operates through short adsorption/desorption cycles conducted over selected adsorbent materials and operated in parallel vessels at different time stages.
The hydrogen is released from the PSA unit 4 at the set pressure (typically about 20 barg for refinery applications, for example). The hydrogen recovery factor of PSA can achieve values up to 90%, while the hydrogen balance, together with the impurities present in the raw hydrogen stream, is released in an off-gas stream and leaves the PSA unit 4 at low pressure (˜0.3 barg). PSA unit 4 can reach hydrogen purity up to 99, 9999% vol. Typical hydrogen purity specification in refinery is >99.9%.
The off-gas from PSA recovered at near atmospheric pressure and containing the produced CO2 and residual hydrogen (an exemplary composition of this stream being CH4 18% mol, CO 10.24% mol, CO2 45.10% mol, H2 26% mol, H2O 0.55% mol) is recycled back (recycle stream) to the reformer furnace 2, where residual hydrogen and CO are burned with make-up fuel and the generated flue gas is sent to the stack.
It is also known that carbon capture and storage (CCS) is the process of removing or reducing the CO2 content of streams normally released to the atmosphere and transporting captured CO2 to a location for permanent storage. CCS can be applied to a wide range of large single-point sources, such as process streams, heater and boiler exhausts, and vents from a range of high CO2 footprint industries, including power generation, refining, natural gas treating, chemicals, cement production and steel production.
There are three main CO2 capture systems associated with different combustion processes, namely, post-combustion, pre-combustion and oxyfuel combustion.
In Post combustion capture processes the removal of CO2 is performed after combustion has taken place. The flue gases exiting combustion plants are typically treated using chemical or physical sorbents to selectively remove CO2 from the gas mixture. It is an end-of-pipe solution, where CO2 is removed from the flue gas before the flue gas is emitted to atmosphere via the stack.
The advantage of the post-combustion process is that it is suited not only to new installations but may also be retrofitted to existing plants [SUZANNE FERGUSON and MIKE STOCKLE, Carbon capture options for refiners, PTQ Q2 2012 77].
The main challenge is that the CO2 level in combustion flue gas is normally quite low, from 5% vol to 20% vol depending on off-gas content in mixed fuel gas.
In Pre-combustion capture processes the fuel (normally coal or natural gas) is pretreated before combustion. In particular, it is generally gasified or reformed to a syngas stream, which is then subject to water-gas shift reaction and subsequent gas clean up to separate the produced hydrogen from the CO2. The gas cleaning step is usually achieved using similar methods employed as described for post-combustion processes, although there are advantages to removing the CO2 from the syngas mainly associated with the pressure of the gas which reduces compression energy requirements. The hydrogen is used as the input fuel to the combustion process, whilst the CO2 is available in a concentrated form for compression, transport and storage. The high concentration (>20%) of CO2 in the H2/CO2 fuel gas mixture facilitates the CO2 separation [S. T. Wismann, J. S. Engbaek, S. B. Vendelbo, F. B. Bendixen, W. L. Eriksen, K. Aasberg-Petersen, C. Frandsen, I. Chorkendorff, P. M. Mortesen, Electrified methane reforming: A compact approach to greener industrial hydrogen production, Science, 2019, 364, 756-759].
In Oxyfuel combustion, oxygen, instead of air, is used for combustion. This reduces the amount of nitrogen present in the exhaust gas that affects the subsequent separation process. The major components of the flue gases are CO2, water, particulates and SO2. After the removal of particulates, SO2 and water, the remaining gases contain a high concentration of CO2, about 80-98% (depending on the fuel used).
Both pre combustion and post combustion technologies for CO2 capture can be applied to a steam reforming plant.
The pre combustion technologies would apply to the syngas stream exiting water gas shift reactors, whereas the post combustion technologies would apply to flue gas from furnace.
However, in the first case, only the CO2 coming from the process would be captured, in the second case all CO2 can be captured, however the cost of such option would be higher taking into consideration the low CO2 partial pressure in the flue gas, when compared to CO2 partial pressure in the syngas. Additionally, since the flue gas is available at about atmospheric pressure, large size CO2 capture systems would be needed, making the cost of such option still higher.
Electrical steam reforming has been studied a lot in the last years. In this context still recently, Haldor Topsoe has published about the joint research work they are carrying out in collaboration with the Technical University of Denmark based on the application of electric power to internally catalytic coated tubes for steam reforming reaction [S. T. Wismann, J. S. Engbaek, S. B. Vendelbo, F. B. Bendixen, W. L. Eriksen, K. Aasberg-Petersen, C. Frandsen, I. Chorkendorff, P. M. Mortesen, Electrified methane reforming: A compact approach to greener industrial hydrogen production, Science, 2019, 364, 756-759]. An experimental and model investigation is under progress to check the performance of the proposed solution. First results showed that the intimate contact between the electric heat source and the reaction site may drive the reaction close to thermal equilibrium, increase the catalyst utilization and limit unwanted byproduct formation, as well as potentially reduce in size the current reformer platforms by an order of magnitude.
In view of all the above, it is evident the need for reducing or removing CO2 emissions resulting from the production of hydrogen starting from hydrocarbons, in particular natural gas.
In this context and in consideration that government regulations are more and more forcing the industry to reduce CO2 emissions, it is proposed the solution according to the present invention, with the aim of providing for replacing the combustion step, which supplies the heat of reaction in the steam reformer according to the prior art, with a step wherein heat is provided by an electric device, by maximizing the efficiency of thermal exchange and improving conversion of natural gas to H2, to reduce the natural gas feed consumption, thereby limiting CO2 emissions from the process side as well.
In this context it is proposed the solution according to the present invention, providing for a method for hydrogen production coupled with CO2 capture based on the coupling of an electrical steam reformer with CO2 capture technology, in order to enable the simultaneous production of a hydrogen stream with a CO2 stream at minimum CO2 emissions.
Moreover, according to the present invention, the produced CO2 stream can be valorized for downstream use.
In particular, the method for hydrogen production coupled with CO2 capture according to the present invention involves the use of an electrical steam reformer instead of a fired heated steam reformer, in order to maximize the efficiency of thermal exchange, improve conversion of natural gas to H2 and eliminate the CO2 contribution coming from fuel combustion.
The method for hydrogen production coupled with CO2 capture according to the present invention is based on the innovative proposal that by replacing a conventional fired heated steam reformer with an electrically heated steam reformer, the stream resulting after separation of the hydrogen product can be recycled to the feed. Only a small amount needs to be purged to avoid accumulation of inert components in the system, if the amount in the feed is not compatible with the required purity of produced hydrogen.
It is therefore an aim of the present invention to provide a method for hydrogen production coupled with CO2 capture allowing for overcoming the limits of the solutions according to the prior art and achieving the previously described technical results.
A further aim of the invention is that said method for hydrogen production coupled with CO2 capture can be implemented with substantially limited costs.
Not last aim of the invention is that of proposing a method for hydrogen production coupled with CO2 capture being substantially simple, reliable and in particular less risky in terms of explosion related to combustion.
It is therefore a first specific object of the present invention a method for hydrogen production comprising the following steps:
In particular, according to the present invention, the syngas production system comprises a desulfurization unit upstream the electrical steam reformer, sulfur, chlorides and olefins being removed from the hydrocarbon feed in said desulfurization unit.
In particular, according to the invention, at least part or all of the recycle stream is fed to the desulfurization unit.
Alternatively, according to the present invention, CO2 is removed from said hydrogen enriched syngas comprising hydrogen and CO2 or from said compressed recycle stream, or both.
Moreover, always according to the invention, part of the recycle stream is intermittently or continuously purged, in particular 7 vol % or less of the recycle stream is purged, preferably between 0.1 and 5 vol % of the recycle stream is purged and most preferably about 2 vol % of the recycle stream is purged.
Additionally, according to the present invention, the steam-to-carbon ratio in said step of reacting said hydrocarbon feed added with said compressed recycle stream with steam is comprised between 2.8 and 3.
In particular, according to the invention, electricity fed to said electrical steam reforming is derived from renewable sources, for example solar, wind or hydro.
Finally, it is an additional specific object of the present invention a plant for hydrogen production starting from a hydrocarbon feed, comprising an electrically powered steam reformer and at least one CO2 capture system, arranged downstream said electrically powered steam reformer.
The invention will be disclosed herein below for illustrative, but non limitative purposes, according to preferred embodiments, with reference in particular to the figures of the enclosed drawing, wherein:
The method for hydrogen production coupled with CO2 capture proposed according to the present invention can be implemented according to three different configurations.
Making reference to
In particular, the method for hydrogen production coupled with CO2 capture according to this embodiment is composed of the following steps: a natural gas (NG) feed is mixed with a recycled stream coming from a pressure swing absorption (PSA) unit 14, heated up to 380° C. and conveyed to a pre-treatment unit 10, where sulfur, chloride and olefins are removed. The purified process stream is then mixed with steam, in a preferred steam-to-carbon ratio of 2.8-3. The ratio 2.8 is optimized to have zero export steam. It is possible to further lower the steam to carbon ratio depending on the possibility to have a catalyst able to operate at low steam-to-carbon ratio without deactivation.
The stream is subsequently pre-heated through a heat exchanger r (not shown) up to 550° C. The heat exchanger preferably uses the reformed stream as heat exchanging fluid, the temperature of the reformed stream being 850-900° C., to heat up the process stream. The heat of the reformed stream can also be used in a different heat exchanger to produce the steam required for the reforming reaction. Alternatively, a stand-alone heater can be used, such as an electric heater or a gas heater. In particular, the use of an electric heater would be preferable in order to eliminate the CO2 contribution coming from fuel combustion. Alternatively, in order to reduce electric power consumption, it is possible to replace the electrical heater with a gas heating step, carried out by burning a portion of natural gas with air. The flue gas generated is sent to stack. However, this solution is less compact and would be more impacting in terms of CO2 emissions, but would make the system a slightly bit more independent from availability of power produced from renewable feedstock.
The pre-heated stream is then sent to an electrical steam reformer 11, at a temperature of the pre-heated stream that allows both to protect the catalyst of the steam reformer 11 and to enter on the catalytic bed of the steam reformer 11 already over the threshold catalyst temperature.
Along the catalytic bed a temperature profile is established that is dependent on both endothermicity of the reaction and the heat provided by the electrical heat source, to achieve at the outlet of catalytic bed a temperature in the range of 850-900° C.
In some plant configurations, mainly when heavy feedstocks are used, a pre-reforming step can be installed upstream to the steam reformer 11. In this additional step a preliminary reforming occurs at lower temperature.
The reformed stream is subsequently cooled down, the heat of the reformed stream being preferably at least partly recovered to produce steam or to pre-heat the process stream upstream the reformer 11 as discussed above, the outlet stream, at a high or medium temperature, depending on available heat, preferably at 340° C., is then sent to a water gas shift reactor 12, in which a certain CO conversion to CO2 is reached.
Water gas shift conversion may be carried out at high (water gas shift inlet temperature about 320° C.-350° C.) or medium temperature (water gas shift inlet temperature about 250° C.-280° C.), depending on the heat recovery in the overall process.
The process stream from the water gas shift reactor 12, which is a stream that is rich of H2 and CO2, is cooled by a series of exchangers (not shown) to recover heat and then is sent to a CO2 capture unit 13 (namely amine, membrane separation, cryogenic, adsorption systems and combination of them) where the CO2 is separated; a pure CO2 stream is collected and, eventually, valorized.
Finally, the gas rich in H2 is sent to a PSA system 14 to be purified, meanwhile the off-gas or recycle stream is compressed in a compressor 15 and recycled to the front end of the plant. A split of purge gas (in the order of 2-3% of total) is separated from the main recycle stream to manage the amount of N2 or other inerts which are present in the natural gas to prevent accumulation thereof in the recycle stream.
With reference to
According to this alternative configuration, the process stream from the water gas shift reactor 12, after heat recovery is sent to the PSA system 14 to be purified, meanwhile the off-gas or recycle stream is compressed in a compressor 15 and then is sent to a CO2 capture unit 13′ where the CO2 is separated (compression and CO2 separation can also be integrated a single unit); the pure CO2 stream is collected and the remaining stream is recycled to the front end of the plant, after an optional purging of around 2-3 wt %.
Finally, in a third embodiment of the method of the present invention, a plant for hydrogen production coupled with CO2 capture is shown in
The method for hydrogen production coupled with CO2 capture according to the present invention represents an overcoming of disadvantages of the state of the art technology based on conventional fired heated reformer coupled with CO2 capture due to the fact that:
The method for hydrogen production coupled with CO2 capture according to the present invention is based on the assumption that electricity needed in order to operate the electric steam reformer is produced from renewable sources, thus allowing for no CO2 production. However, in a near term scenario of energy transition, the availability of renewables may not completely satisfy the hydrogen market based on such technology. Since the method for hydrogen production coupled with CO2 capture according to the present invention enables for a reduction of CO2 emission, a breakeven point has been calculated, making the assumption that to make equal the CO2 emissions of a fired heated reformer and an electrical one, up to 30-40% of power from fossil/coal may be accepted, the remaining share coming from renewables.
The method for hydrogen production coupled with CO2 capture according to the present invention was used to treat a natural gas feed with the composition shown in Table 1:
The main technical results achieved by the method for hydrogen production coupled with CO2 capture according to the present invention were evaluated, in comparison with a traditional fired heated steam reformer, by treating the natural gas according to the embodiment disclosed with reference to
(*)without taking into consideration optional utility consumption of CO2 removal step (in case the CO2 capture step should need electric power or natural gas consumption to be operated)
(**)Residual part in purge gas
(***)CO2 captured (%) being higher than indicated capture efficiency (70%) due to CO2 recycling back at the front end of the plant
Efficiency to hydrogen has been calculated as Feed (LHV)+Fuel (LHV)/Hydrogen production (Nm3), LHV standing for Lower Heating Value.
The technical results achieved by the method for hydrogen production coupled with CO2 capture according to the present invention were also evaluated, in comparison with a traditional fired heated steam reformer, by treating the same natural gas of the previous example according to the embodiment disclosed with reference to
(Table 3 follows)
(*)without taking into consideration optional utility consumption of CO2 removal step (in case the CO2 capture step should need electric power or natural gas consumption to be operated)
(***)CO2 captured (%) being higher than indicated capture efficiency (70%) due to CO2 recycling back at the front end of the plant
Efficiency to hydrogen has been calculated as Feed (LHV)+Fuel (LHV)/Hydrogen production (Nm3), LHV standing for Lower Heating Value.
The technical results achieved by the method for hydrogen production coupled with CO2 capture according were also evaluated, in to the present invention comparison with a traditional fired heated steam reformer, by treating the same natural gas of the previous examples according to the embodiment disclosed with reference to
(*)without taking into consideration optional utility consumption of CO2 removal step (in case the CO2 capture step should need electric power or natural gas consumption to be operated)
(***)CO2 captured (%) being higher than indicated capture efficiency (70%) due to CO2 recycling back at the front end of the plant
Efficiency to hydrogen has been calculated as Feed (LHV)+Fuel (LHV)/Hydrogen production (Nm3), LHV standing for Lower Heating Value.
Independently on the type of installation of the CO2 capture unit, the method for hydrogen production coupled with CO2 capture according to the present invention allows to reduce the CO2 emissions of up to 45%.
An important technical highlight has to be made with reference to inert components (such as nitrogen) possibly contained in natural gas.
One of the peculiar aspects of the method for hydrogen production coupled with CO2 capture according to the present invention, independent of the embodiment, is the possibility to recycle back the off-gas from PSA directly to the feed section, thereby reducing the amount of needed make up feed, provided that a compression step is applied. This solution is not applied in conventional fired heated reformer, since in the latter case, due to the presence of at least one burner, it is more convenient, from a technical point of view, to recycle back the off-gas from PSA to the fuel section, thereby reducing the amount of needed make up fuel and simultaneously avoiding the step of off-gas from PSA recompression.
The main benefits of the method for hydrogen production coupled with CO2 capture according to the present invention are highlighted herein below:
Moreover, since electricity becomes cheaper and is increasingly coming from renewable sources, the benefits of the method for hydrogen production coupled with CO2 capture according to the present invention will increase over time.
Additionally, according to the first embodiment disclosed above, wherein also the process stream upstream the reformer 11 is pre-heated in an electrical heater, the method for hydrogen production coupled with CO2 capture according to the present invention also involves the following advantages:
With reference to the embodiments shown in
Still another advantage of the method for hydrogen production coupled with CO2 capture according to the present invention is the possibility to avoid to recycle back a portion of hydrogen from battery limits to carry out the desulfurization step. In fact, the amount of hydrogen required for such a step is already included in the recycled stream from PSA. However, the proposed solution can work even with a recycled stream from PSA routed directly to the electrical steam reformer. In this last case, there is the need of hydrogen from battery limits to enable the hydrodesulfurization of the hydrocarbon feedstock.
Another advantage of the method for hydrogen production coupled with CO2 capture according to the present invention is the possibility that no pollutant is released into the atmosphere. This last benefit is dependent on the end use for the split purge gas coming from the recycle stream (PSA off-gas). If it is sent to flare, it contributes to pollutants emissions. Otherwise, if it can be valorized, the environmental impact can be reduced.
Even if the purge gas is sent to flare, the overall CO2 emissions are significantly lower (about 90%) than CO2 emissions of a conventional fired heated reformer coupled with CO2 capture.
Still another advantage, depending on plant capacity and type of used shift reactor, is that all required heat to make steam production and additional services, like feed preheating can be produced by heat recovery from process stream.
The present invention was disclosed for illustrative, non-limiting purposes, according to a preferred embodiment thereof, but it has to be understood that any variations and/or modification can be made by the persons skilled in the art without for this reason escaping from the relative scope of protection, as defined in the enclosed claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IT2021/000039 | 8/4/2021 | WO |
