Biogas upgrading to methanol

Abstract

A method for upgrading biogas to methanol, including the steps of: providing a reformer feed stream comprising biogas; optionally, purifying the reformer feed stream in a gas purification unit; optionally, prereforming the reformer feed stream together with a steam feedstock in a prereforming unit; carrying out steam methane reforming in a reforming reactor heated by means of an electrical power source; providing the synthesis gas to a methanol synthesis unit to provide a product including methanol and an off-gas. Also, a system for upgrading biogas to methanol.

Claims

1. A method for upgrading biogas to methanol, comprising the steps of: a) providing a reformer feed stream comprising said biogas, b1)—optionally, purifying said reformer feed stream in a gas purification unit, b2)—optionally, prereforming said reformer feed stream together with a steam feedstock in a prereforming unit, c) carrying out steam methane reforming of said reformer feed stream in a reforming reactor comprising a pressure shell housing a structured catalyst arranged to catalyse steam reforming of said reformer feed stream, said structured catalyst comprising a macroscopic structure of an electrically conductive material, said macroscopic structure supporting a ceramic coating, where said ceramic coating supports a catalytically active material; said steam methane reforming comprising the following steps: c1) supplying said reformer feed stream to the reforming reactor, c2) allowing the reformer feed stream to undergo steam reforming reaction over the structured catalyst and outletting a synthesis gas from the reforming reactor, and c3) supplying electrical power via electrical conductors connecting an electrical power supply placed outside said pressure shell to said structured catalyst, allowing an electrical current to run through the electrically conductive material of said macroscopic structure, thereby heating at least part of the structured catalyst to a temperature of at least 500° C., and d) providing at least part of the synthesis gas of step c2) to a methanol synthesis unit to provide a product comprising methanol and an off-gas.

2. The method according to claim 1, wherein the electrical power supplied has been generated by means of renewable energy sources.

3. The method according to claim 1, wherein the reformer feed stream has a first H/C ratio and where a second hydrocarbon feed gas with second H/C ratio is mixed with the reformer feed stream upstream the reforming reactor, wherein the second H/C ratio is larger than the first H/C ratio

4. The method according to claim 1, wherein an electrolysis unit is used to generate a hydrogen rich stream from a water feedstock and where said hydrogen rich stream is added to the synthesis gas to balance the module of said synthesis gas to be in the range of 1.5 to 2.5.

5. The method according to claim 4, wherein said electrolysis unit is a solid oxide electrolysis cell unit and said water feedstock is in the form of steam produced from other processes of the method.

6. The method according to claim 1, wherein a membrane or PSA unit is included in the methanol synthesis unit to extract at least part of the hydrogen from said off-gas and return said at least part of the hydrogen from said off-gas to the synthesis gas to balance the module of said synthesis gas to be in the range of 1.5 to 2.5.

7. The method according to claim 1, wherein a combination of steam superheating and steam generation is integrated in waste heat recovery of said synthesis gas from said reforming reactor, and wherein the superheated steam is used as steam feedstock in step c) of the method for upgrading biogas to methanol.

8. The method according to claim 1, wherein the pressure of the gas inside said reforming reactor is between 20 and 100 bar.

9. The method according to claim 1, wherein the temperature of the gas exiting said reforming reactor is between 900 and 1150° C.

10. The method according to claim 1, wherein the space velocity evaluated as flow of gas relative to the geometric surface area of the structured catalyst is between 0.6 and 60 Nm.sup.3/m.sup.2/h and/or wherein the flow of gas relative to the occupied volume of the structured catalyst is between 700 Nm.sup.3/m.sup.3/h and 70000 Nm.sup.3/m.sup.3/h.

11. The method according to claim 1, wherein the plot area of said reforming reactor is between 0.4 m.sup.2 and 4 m.sup.2.

12. The method according to claim 1, wherein the production of methanol is regulated according to availability of renewable energy.

13. The method according to claim 1, wherein the method further comprises the step of upgrading the methanol to fuel grade methanol.

14. The method according to claim 1, wherein the method further comprises the step of upgrading the methanol to chemical grade methanol.

15. The method according to claim 1, wherein the method further comprises the step of using at least part of the methanol of step d) to a system for producing transportation fuel.

16. The method according to claim 1, wherein at least part of the off-gas is recycled to upstream said reforming reactor.

17. The method according to claim 1, wherein between 80% and 100% of the carbon of the biogas in said reformer feed stream is converted into methanol.

18. The method according to claim 1, wherein the biogas of said reformer feed stream amounts to 500 Nm.sup.3/h to 8000 Nm.sup.3/h.

19. The method according to claim 1, wherein a separation unit is used to remove part of the CO.sub.2 of the reformer feed stream subsequent to step a) and preceding step c).

20. The method according to claim 1, wherein part of the off-gas produced in step d) is recycled to a biogas production facility for producing the biogas to be upgraded.

21. A system for upgrading biogas to methanol, comprising: an optional gas purification unit, an optional prereforming unit, a reforming reactor comprising a pressure shell housing a structured catalyst arranged to catalyse steam reforming of a bio gas, said structured catalyst comprising a macroscopic structure of an electrically conductive material, said macroscopic structure supporting a ceramic coating, where said ceramic coating supports a catalytically active material; wherein said reforming reactor moreover comprises an electrical power supply placed outside said pressure shell and electrical conductors connecting said electrical power supply to said structured catalyst, allowing an electrical current to run through the electrically conductive material of said macroscopic structure to thereby heat at least part of the structured catalyst to a temperature of at least 500° C., a methanol synthesis unit arranged to receive a synthesis gas from said reforming reactor and produce a product comprising methanol and an off-gas.

22. The system according to claim 21, wherein catalyst pellets are loaded on top of, around, inside, or below the structured catalyst of the reforming reactor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] FIG. 1 is a schematic drawing of a system for biogas upgrading to methanol production;

[0065] FIGS. 2a-2c shows comparative cases for methanol plants based on a fired reformer versus an electric reformer versus alkaline electrolysis;

[0066] FIG. 3 shows CO.sub.2 equivalent emissions (CO.sub.2e) associated with production of MeOH as the combined contribution from: Plant emissions+Emissions from electricity generation; and

[0067] FIG. 4 is a graph of technologies with lowest operating expenses as a function of natural gas price and electricity price.

DETAILED DESCRIPTION OF THE DRAWINGS

[0068] FIG. 1 is a schematic drawing of a system 100 for biogas upgrading to methanol production. The system is a methanol plant comprising an electrically heated steam methane reformer (eSMR) 10.

[0069] The system 100 for biogas upgrading to methanol comprises a reforming section 10 and a methanol section 60. The reforming section 10 comprises a preheating section 20, a purification unit 30, e.g. a desulfurization unit, a prereformer 40 and an eSMR 50. The methanol section comprises a first separator 85, a compressor unit 70, a methanol synthesis unit 80, a second separator 90 as well as heat exchangers. The first and second separators 65 and 90 may e.g. be flash separators.

[0070] A reformer feed stream 1 comprising biogas is preheated in the preheating section 20 and becomes a preheated reformer feed stream 2, which is led to the purification unit 30. A purified preheated reformer feed stream 3 is sent from the purification unit 30 to the preheating section 20 for further heating. Moreover, steam 4 is added to the purified preheated reformer feed stream, resulting in feed gas 5 sent to a prereformer 40. Prereformed gas 6 exits the prereformer 40 and is heated in the preheating section 20, resulting in gas 7. In the embodiment of FIG. 1, hydrogen 14 is added to the gas 7, resulting in a feed gas 8 sent to the eSMR 50. The feed gas 8 undergoes steam methane reforming in the eSMR 50, resulting in a reformed gas 9 which is led from the eSMR 50 and from the reforming section 10 to the methanol section 60.

[0071] In the methanol section 60, the reformed gas 9 heats water 12 to steam 13 in a heat exchanger. In a first separator 85 water is separated from the synthesis gas 9 to provide a dry synthesis gas 11, which is sent to a compressor 70 arranged to compress the dry synthesis gas before it is mixed with recycle gas from a second separator 90 enters the methanol synthesis unit 80. Most of the produced methanol from the methanol synthesis unit 80 is condensed and separated in the second separator 90 and exits the methanol section as methanol 25. The gaseous component from the second separator 90 is split into a first part that is recycled to the methanol synthesis unit 80 and a second part that is recycled as an off-gas 17 to be used as fuel 18 to the preheating section 20 of the reforming section 10 and/or recycled as feed 16 to the eSMR 50. An additional compressor is typically used for recycling the first part of the gaseous component from the second separator 95 to the methanol synthesis unit 80. Water 12 is heated to steam within heat exchangers of the system 100 and in the given embodiment inside the cooling side of the methanol synthesis unit 80.

[0072] To achieve full carbon utilization, synergy can be obtained if an SOEC-based water electrolysis unit 110 is used. The SOEC unit 110 can utilize some of the steam production available from waste-heat management in the reforming and methanol sections, e.g. stream 13 and convert the steam to i.a. H.sub.2. The H.sub.2 can be used as a hydrogen source in the feed gas to the reforming reactor. It should be noted that a relatively small SOEC unit is needed to achieve this. Alternatively, any other appropriate hydrogen source may be utilized.

[0073] In the case, where a second hydrocarbon feed gas is added to or mixed with the reformer feed stream upstream the reforming reactor, the second hydrocarbon feed gas is typically added to the reformer feed stream upstream the prereforming unit and the purification unit. In FIG. 1, this would correspond to adding the second hydrocarbon feed gas to the preheated reformer feed stream 2. The second hydrocarbon feed gas may be a stream of natural gas having a higher H/C ratio than the H/C ratio of the reformer feed stream of stream 1.

[0074] In the case, where a separation unit is used to remove part of the CO.sub.2 in the biogas upstream the reforming unit, this separation units is advantageously upstream the preheating unit 20. When a major part of the reformer feed stream is biogas, by removing part of the CO.sub.2 in the reformer feed stream, it is possible to achieve a reformer feed stream with about 25% CO.sub.2, which is preferable for the downstream methanol production.

[0075] A system 100 according to the invention, comprising an electrically heated steam methane reformer and a methanol synthesis unit is also abbreviated eSMR-MeOH. Such an eSMR-MeOH system resembles a plant used in classical industrial process (SMR-MeOH) to a large extent, but deviates on some essential aspects. Firstly, use of the eSMR 10 removes the requirement for the intensive firing in the fired steam reformer of a classical SMR-MeOH system and thereby leaves only a small CO.sub.2 emission from the eSMR-MeOH layout associated with purge gas handling. Secondly, the use of biogas rather than natural gas as the reformer feed stream or as the main part thereof removes the requirement for oxygen addition to the synthesis gas as the natural high CO.sub.2 content of biogas allows for the module adjustment inherently, as described below:

[0076] From an overall plant stoichiometry where methane (as natural gas) is used as feedstock, the reaction scheme can be expressed as:

CH.sub.4+0.5O.sub.2.fwdarw.CO+2H.sub.2.fwdarw.CH.sub.3OH

[0077] Alternatively, if a CO.sub.2 feedstock is available, this can be used as oxygen source, giving an overall plant stoichiometry of:

0.75CH.sub.4+0.25CO.sub.2+0.5H.sub.2O.fwdarw.CO+2H.sub.2.fwdarw.CH.sub.3OH.

[0078] Higher temperatures can be reached in an eSMR compared with a fired reformer, which gives a better conversion of methane in this layout; in the end, this provides for less off-gas handling. It should be noted, that the CO.sub.2 content in biogas can vary, and therefore, an addition of hydrogen to the synthesis gas can be advantageous to increase the carbon utilization of the process. To achieve full carbon utilization, an excellent synergy can be obtained if SOEC based water electrolysis unit 110 is used, which can utilize some of the steam production available from waste-heat management in the reforming section 10 and the methanol section 60. This is illustrated as the parallel hydrogen source 14 in FIG. 1. Notice that a relatively small SOEC unit 110 is needed to achieve this, and the process can also run without it. The same methanol synthesis technology as in the classical approach can be used and the methanol reactor will in this layout have a CO/CO.sub.2 ratio corresponding to that of a typical methanol plant and therefore have a similar activity and stability. To some extent, at least part of the offgas from the methanol synthesis unit can be recycled to the reforming section as feedstock to increase the carbon efficiency and recover unconverted methane. In the same way, it is also possible to recover at least part of the off-gas from a potential methanol distillation and return this as feedstock, if this is compressed to operating pressure. At least to some extent, preheating can be done by the excess steam, because high preheating. Electrically heated reforming can e.g. use a monolithic-type catalyst heated directly by Joule heating to supply the heat for the reaction. In its essence, the eSMR 10 is envisioned as a pressure shell having a centrally placed catalytic monolith, which is connected to an externally placed power supply by a conductor threaded through a dielectric fitting in the shell. The shell of the eSMR is refractory lined to confine the high-temperature zone to the center of the eSMR.

[0079] From a reforming reactor point of view, the eSMR has several advantages over a conventional fired reformer. One of the most apparent is the ability to make a significantly more compact reactor design when using electrically heated technology, as the reforming reactor no longer is confined to a system of high external heat transfer area. A size reduction of two orders of magnitudes is conceivable. This translates into a significantly lower capital investment of this technology. The combined preheating and reforming section of an eSMR (including power supply) configuration was estimated to have a significant lower capital investment. As the synthesis gas preparation section of a methanol plant accounts for more than 60% of the capital investment in a classical fired reformer based methanol plant, a drastic saving on the reformer equipment will translate into a significant reduction in the cost of a methanol plant based on eSMR.

[0080] FIGS. 2a-2c show comparative cases for methanol plants based on a fired reformer (FIG. 2a) versus an electric reformer (FIG. 2b) versus alkaline electrolysis (FIG. 2c). A major advantage of the eSMR of FIG. 2b is that it does not require burning hydrocarbons to provide the heat for the reaction, and consequently direct CO.sub.2 emissions of this technology is significantly decreased. This is exemplified in FIGS. 2a-2c, showing how the consumables and CO.sub.2 emissions can be markedly changed when using the eSMR-MeOH technology compared with both the fired reformer approach and electrolysis. The consumption figures of the fired reformer layout (FIG. 2a) and the eSMR-MeOH layout (FIG. 2b) are both based on Haldor Topsoe developed flowsheets for chemical-grade methanol production (i.e., including product distillation), while electrolysis layout (FIG. 2c) is an overall best-case stoichiometric analysis coupled with published consumption figures for alkaline electrolysis (AEL) based H.sub.2 production and CO.sub.2 purification. It should be noticed that the consumables are, from a chemical standpoint, divided in substantially pure CH.sub.4 and CO.sub.2 to not disadvantage the SMR-MeOH layout by requiring firing with biogas, which would have increased the CO.sub.2 emissions from this plant considerably. In the given case, 30% reduction in methane consumption and 80% reduction in CO.sub.2 emissions are achieved by the eSMR-MeOH compared to the fired reformer (SMR-MeOH). It is emphasized that process improvement may be considered for all presented cases, and should therefore not be considered limiting. When no units are given, the presented figures represent relative molar flow of components in FIG. 2a-2c.

[0081] The overview of the consumables of FIGS. 2a-2c illustrates a markedly lower electricity use for methanol production when using eSMR-MeOH over electrolysis. By use of SOEC instead of AEL in the electrolysis layout, the electricity use could potentially decrease to 11-13 kWh/Nm.sup.3 MeOH (depending on availability of steam), which would be an improvement for this technology, but still markedly higher than eSMR-MeOH. Notice that the concept development still can be done on the electrolysis approach to improve the performance of this technology, but this is all at research stage and only established electrolysis technology, as AEL, combined with classical methanol synthesis technology can be considered ready for industrial application presently, why this is also the focus of the comparison.

[0082] Energy consumption of methanol production by AEL (“AEL-MeOH”) is calculated as: E.sub.total=E.sub.AEL+E.sub.CO.sub.2+E.sub.compress−E.sub.steam. Here, E.sub.AEL is energy use of alkaline electrolysis with an energy efficiency of 71%. E.sub.CO.sub.2 is the energy use of CO.sub.2 purification estimated as 2.6 MJ/Nm.sup.3 CO.sub.2 when using flue gas as feedstock. E.sub.compress is the compression power calculated at an efficiency of 75%, without including energy for cooling water, to be 0.7 kWh/Nm.sup.3 methanol. E.sub.steam is the potential energy recovery from steam production calculated as 75% recovery of the exothermic energy removed in the methanol synthesis estimated to be 0.7 kWh/Nm.sup.3 methanol. The calculation does not include any considerations on byproduct formation in the methanol synthesis unit or their integration in the plant layout.

[0083] FIG. 3 shows CO.sub.2 equivalent emissions (CO.sub.2e) associated with production of methanol for SMR, eSMR and AEM, respectively. For each of these production technologies, the black box represent overall equivalent emissions (CO.sub.2e) if the methanol was produced by renewable energy and the white box represent overall equivalent emissions (CO.sub.2e) if the methanol was produced with electricity from the Danish electricity network in 2019. When calculating the overall CO.sub.2 emissions from a chemical plant, the electricity consumption must be evaluated as well, as this could potentially also have a large CO.sub.2 emission footprint. The exact emissions will be dependent on the source of the electricity. Looking at the associated equivalent CO.sub.2 emissions (CO.sub.2e) when electricity is provided either by fully sustainable resources or as an example the Danish energy grid in 2019, in which more than 60% of the annual electricity use is covered by sustainable sources as solar cells, wind power, and biomass. The actual CO.sub.2e for production of methanol by the eSMR-MeOH technology was on this basis calculated as shown in FIG. 3 and benchmarked against the conventional fired technologies and AEL-MeOH. Irrespective of the source of electricity, eSMR-MeOH will markedly better the CO.sub.2 footprint of the methanol product over the conventional approach, viz. SMR-MeOH. While, based on the energy grid in Denmark in 2019, the electrolysis approach will not have a positive effect on the CO.sub.2e. Only when the electricity is fully renewable, the electrolysis approach will have an CO.sub.2e comparable to the eSMR-MeOH route, but AEL-MeOH will still be 35% higher

[0084] FIG. 4 is an overview of technologies with lowest operating expenses as a function of natural gas price and electricity price.

[0085] To make sustainable technology attractive, it must be cost-competitive compared to the established production routes. FIG. 4 shows an overview of which technology gives the lowest operating expenses as a function of gas and electricity price. It should be noted, that the overview only shows operating expenses. If expenses to plant depreciation are included in the production costs, the size of the area indicated “eSMR-MeOH” would markedly increase into the areas denoted “AEL-MeOH” and “SMR-MeOH”, because the eSMR-MeOH technology has a significantly lower capital investment compared with the two other technologies. From this overview it can be seen that the fired technology (SMR-MeOH) has been the cheapest production route for the last century because it is favored by the low gas prices. However, the decreasing electricity prices opens for an incentive toward the electrically driven technologies. An eSMR driven frontend is proposed as a next step for a cost-competitive route for methanol production. To exemplify the opportunity, competitive cases can be found when comparing with natural gas prices of ca. 6-8 $/MMBTU in Europe. The operating expenses of the eSMR-MeOH technology will be further favored in cases with CO.sub.2 taxation, which will increase the operating expenses of the fired reformer approach significantly. This is indicated by the dashed line in FIG. 4, with a representative CO.sub.2 tax of the Nordic countries today. It is emphasized that FIG. 4 is only indicative, as the development within the eSMR-MeOH layout is still in a relatively early phase. It is foreseen that development within eSMR-MeOH will improve the consumption figures further, and thereby the operating expenses.

[0086] While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant's general inventive concept.

EXAMPLE 1

[0087] Example 1 relates to an embodiment of the invention where a biogas is converted into methanol, cf. FIG. 1 for reference. A feed gas (1) is mixed with a recycle gas from a methanol loop to provide hydrogen for the subsequent desulfurization (30) and prereforming (40) steps. Using an electrically heated reformer (50), the gas is converted with steam (4) into a synthesis gas. This is cooled and separated into a condensate and dry synthesis gas (11), where the dry synthesis gas is compressed and fed to a methanol loop using a boiling water type methanol reactor (80). The compressed make-up synthesis gas is mixed with recycled gas (95) in the loop and sent to the methanol reactor (80) to produce methanol. By cooling and condensing this methanol is separated to produce the final product (25). Most of the off-gasses from this separation are recycled (95) directly to the methanol reactor, another fraction (16) is recycled to the feed, while the last fraction is exported as a fuel rich off-gas.

[0088] Overall, this embodiment of the process allows for converting 95.4% of the carbon feedstock (CO.sub.2+CH.sub.4) into methanol.

TABLE-US-00002 Example 1 Inlet Inlet pre- Inlet Feed Off-gas desulfurization reformer reformer Outlet (1) recycle (2) (5) (8) reformer T [° C.] 179 40 380 27 26.3 1050 P [barg] 30 85.5 29.5 293 343 25.3 Components [Nm.sup.3/h] CH.sub.3OH 0 3 3 3 0 0 CH.sub.4 1863 71 1933 1933 1997 113 CO 0 27 27 100 1 2208 CO.sub.2 627 24 651 580 617 294 H.sub.2 0 322 322 240 93 5421 N.sub.2 5 13 18 18 18 18 O.sub.2 5 0 5 1 0 0 H.sub.2O 0 0 0 2898 2926 1365 After Outlet Outlet recycle flash make-up- mixing and Outlet Outlet MeOH separator gas inlet MeOH MeOH recycle Product (11) compressor reactor reactor compressor (25) T [° C.] 40 123 220 260 46 40 P [barg] 23.9 90 90 87 90 90 Components [Nm.sup.3/h] CH.sub.3OH 0 0 92 2468 92 2376 CH.sub.4 113 113 2659 2659 2547 92 CO 2208 2203 3169 1005 966 39 CO.sub.2 293 293 1177 966 885 81 H.sub.2 5420 5409 17081 12116 11670 446 N.sub.2 18 18 471 471 453 18 O.sub.2 0 0 0 0 0 0 H.sub.2O 24 14 17 229 3 226 Off-gas recycle Off-gas T [° C.] 40 40 P [barg] 85.5 85.5 Components [Nm.sup.3/h] CH.sub.3OH 3 0 CH.sub.4 71 21 CO 27 8 CO.sub.2 24 8 H.sub.2 322 103 N.sub.2 13 3 O.sub.2 0 0 H.sub.2O 0 0