DECARBONISATION OF A CHEMICAL PLANT

Abstract

A chemical plant comprising an off-gas treatment unit arranged to accept a hydrocarbon-containing off-gas stream, the off-gas treatment unit comprising sequentially: (i) an autothermal reformer, arranged to accept a hydrocarbon-containing fuel stream comprising hydrocarbons and steam and produce a reformed gas stream; (ii) a water-gas shift section arranged to accept said reformed gas stream and produce a shifted gas stream; and (iii) a CO2 removal unit arranged to accept said shifted gas stream and produce a CO2 rich stream and a decarbonised fuel stream; wherein said hydrocarbon-containing fuel stream is derived from said hydrocarbon-containing off-gas stream, steam and any supplemental fuel; wherein the chemical plant is arranged such that the decarbonised fuel stream is combusted instead of said hydrocarbon-containing off-gas stream.

Claims

1. A method for retrofitting a chemical plant which is initially arranged such that a hydrocarbon-containing off-gas stream is combusted to provide at least some of the heating duty on the chemical plant, the method comprising the step of: installing an off-gas treatment unit arranged to accept said hydrocarbon-containing off-gas stream, the off-gas treatment unit comprising sequentially: (i) an autothermal reformer, arranged to accept a hydrocarbon-containing fuel stream comprising hydrocarbons and steam and produce a reformed gas stream; (ii) a water-gas shift section arranged to accept said reformed gas stream and produce a shifted gas stream; and (iii) a CO.sub.2 removal unit arranged to accept said shifted gas stream and produce a CO.sub.2 rich stream and a decarbonised fuel stream; wherein said hydrocarbon-containing fuel stream is derived from said hydrocarbon-containing off-gas stream, steam and any supplemental fuel; wherein said off-gas treatment unit is arranged such that the decarbonised fuel stream, optionally having first undergone further purification, is combusted instead of the hydrocarbon-containing off-gas stream; with the proviso that the hydrocarbon-containing off-gas stream is not a purge gas stream from a methanol loop; and with the proviso that if the hydrocarbon-containing off-gas stream is a tail-gas stream from the purification unit of a hydrocarbon reforming section comprising a fired steam reformer, water-gas shift and purification unit, then the decarbonised fuel stream from the off-gas treatment unit is not sent to the fired steam reformer of the hydrocarbon reforming section as a fuel.

2. The method according to claim 1, wherein the off-gas treatment unit is arranged to accept off-gases from two or more different hydrocarbon-containing off-gas streams from the chemical plant.

3. The method according to claim 1, wherein the hydrocarbon-containing off-gas stream is a purge stream.

4. The method according to claim 1, wherein the hydrocarbon-containing off-gas stream is a hydrocarbon-containing stream from a purification unit.

5. The method according to claim 4, wherein the purification unit is part of a steam reforming section.

6. The method according to claim 1, wherein the water-gas shift section comprises a high-temperature shift vessel and the off-gas treatment unit is arranged such that the hydrocarbon-containing feed to the autothermal reformer is pre-heated by heat exchange with a shifted gas stream generated by the high-temperature shift vessel.

7. A chemical plant comprising an off-gas treatment unit arranged to accept a hydrocarbon-containing off-gas stream, the off-gas treatment unit comprising sequentially: (i) an autothermal reformer, arranged to accept a hydrocarbon-containing fuel stream comprising hydrocarbons and steam and produce a reformed gas stream; (ii) a water-gas shift section arranged to accept said reformed gas stream and produce a shifted gas stream; and (iii) a CO.sub.2 removal unit arranged to accept said shifted gas stream and produce a CO.sub.2 rich stream and a decarbonised fuel stream; wherein said hydrocarbon-containing fuel stream is derived from said hydrocarbon-containing off-gas stream, steam and any supplemental fuel; wherein the chemical plant is arranged such that the decarbonised fuel stream, optionally having first undergone further purification, is combusted to provide at least some of the heating duty on the chemical plant; with the proviso that the hydrocarbon-containing off-gas stream is not a purge gas stream from a methanol loop; and with the proviso that if the hydrocarbon-containing off-gas stream is a tail-gas stream from the purification unit of a hydrocarbon reforming section comprising a fired steam reformer, water-gas shift and purification unit, then the decarbonised fuel stream from the off-gas treatment unit is not sent to the fired steam reformer of the hydrocarbon reforming section as a fuel.

8. The chemical plant according to claim 7, wherein the chemical plant is an oil refinery.

9. The chemical plant according to claim 7, wherein the off-gas treatment unit is arranged to accept off-gases from two or more different hydrocarbon-containing off-gas streams from the chemical plant.

10. The chemical plant according to claim 9, wherein the two or more different hydrocarbon-containing off-gas streams are combined into a single stream prior to entering the autothermal reformer.

11. The chemical plant according to claim 7, wherein the hydrocarbon-containing off-gas stream is a purge stream.

12. The chemical plant according to claim 7, wherein the hydrocarbon-containing off-gas stream is a hydrocarbon-containing stream from a purification unit.

13. The chemical plant according to claim 7, wherein the water-gas shift section comprises a high-temperature shift vessel and the off-gas treatment unit is arranged such that the hydrocarbon-containing feed to the autothermal reformer is pre-heated by heat exchange with a shifted gas stream generated by the high-temperature shift vessel.

14. A process of treating a hydrocarbon-containing off-gas stream from a chemical plant by converting said hydrocarbon-containing off-gas stream in an off-gas treatment unit to produce a decarbonised fuel stream and combusting said decarbonised fuel stream to provide at least some of the heating duty on said chemical plant, wherein the chemical plant is as defined in claim 7.

15. The process according to claim 14, wherein said decarbonised fuel stream comprises vol % 50-98 vol % H.sub.2.

16. The method according to claim 4, wherein the purification unit is part of a steam reforming section of a hydrogen, methanol or ammonia plant.

17. The method according to claim 1, wherein the off-gas treatment unit comprises a pre-reformer upstream of the autothermal reformer.

18. The chemical plant according to claim 12, wherein the purification unit is part of a steam reforming section of a hydrogen, methanol or ammonia plant.

19. The chemical plant according to claim 7, wherein the off-gas treatment unit comprises a pre-reformer upstream of the autothermal reformer.

Description

DESCRIPTION OF THE FIGURES

[0035] FIG. 1 is an illustration of an off-gas treatment unit according to the present invention. An off-gas stream (101) or a purified off-gas (103) optionally having first undergone treatment in a purification unit (102), is combined with steam (120) to generate a hydrocarbon-containing fuel stream (106) which is fed to an autothermal reformer (108). In the arrangement modelled in FIG. 1 the off-gas was purified by CO.sub.2 removal prior to combination with steam. An oxygen-containing stream (107) is also fed to the autothermal reformer. A reformed gas stream (109) is fed to a water-gas shift section (110) which includes one or more shift units. A shifted gas stream (111) is fed to a cooling and water recovery unit (112) where it is separated into a water stream (113) and a crude H.sub.2 stream (115). The crude H.sub.2 stream is fed to a CO.sub.2 separation unit (116) where it is separated in a CO.sub.2-rich stream (117) and a decarbonised fuel stream (118). The water stream (113), along with make-up water (119) is fed to a steam boiler (114) where it is used to generate steam (120) which is fed back to the process upstream of the autothermal reformer. In some embodiments supplemental fuel (121), typically having first undergone treatment in a purification unit (104), may also be fed to the autothermal reformer. The decarbonised fuel stream (118) is used as fuel in place of the off-gas (101). Combustion of the decarbonised fuel stream is not shown.

[0036] FIG. 2 is an illustration of another off-gas treatment unit according to the present invention. The reference numerals used in FIG. 2 are analogous (101).fwdarw.(201), (102).fwdarw.(202) etc., but details of the arrangement and new reference numerals are described below. An off-gas stream (201), having optionally undergone purification in a purification unit (not shown) to produce a purified off-gas (203) is fed downstream from the ATR and upstream from the WGS section (210). The arrangement is analogous to FIG. 1 but the decarbonised fuel stream (218) from the CO.sub.2 separation unit (216) is treated in a purification unit (222) to produce a H.sub.2-rich stream (224) and a H.sub.2-lean stream (223) which is fed back to the process upstream of the autothermal reformer in order to convert hydrocarbons present in the H.sub.2-lean stream. The H.sub.2-rich stream (224) is used as fuel in place of the off-gas (201) or the purified off-gas (203). Combustion of the H2-rich stream is not shown. This arrangement is particularly suitable where stream (201) or (203) has a high CO content.

[0037] For ease of understanding, the origin of the off-gas stream (101, 201) has not been shown in FIGS. 1 and 2. However, the claimed invention is subject always to the provisos set out in claim 1.

DETAILED DESCRIPTION

[0038] Any sub-headings are for convenience only and are not intended to limit the invention.

Chemical Plant

[0039] The present invention is applicable to a wide variety of chemical plants in which there is at least one hydrocarbon-containing off-gas stream which is combusted as fuel to provide at least some of the heating duty on the chemical plant.

[0040] A hydrocarbon-containing off-gas stream is fed to the OTU. In some embodiments the off-gas stream is a purge stream. The skilled person will be aware that many chemical processes operate as a loop; the product stream from a reactor is treated to separate out a stream containing the desired product and a stream containing unreacted materials which is recycled to the reactor. To avoid inerts building to unacceptably high levels in the loop a portion of the recycle stream is removed (a purge stream). For example, in some embodiments the off-gas stream may be a purge stream from a methanol plant or an ammonia plant.

[0041] In a preferred embodiment the off-gas stream is a hydrocarbon-containing stream from a purification unit. In some embodiments the purification unit is a component of a steam reforming section comprising a steam methane reformer (e.g. in a hydrogen, methanol or ammonia plant).

[0042] In a preferred embodiment the off-gas stream is a hydrocarbon-containing stream from an ethylene cracker.

Off-Gas Treatment Unit

[0043] The OTU comprises, sequentially, an autothermal reformer, a water-gas shift section, and a CO.sub.2 removal unit. Additional units (e.g. heat exchangers, steam removal etc. . . . ) may also be present in the OTU.

[0044] The OTU may be arranged to accept a single hydrocarbon-containing off-gas stream or two or more different hydrocarbon-containing off-gas streams. The latter option has the benefit that the OTU can be used to carry out steam reforming, shift and CO.sub.2 removal on multiple different streams from across the chemical plant. Where the OTU is fed by two or more hydrocarbon-containing off-gas streams, the off-gas streams are typically combined into a single stream prior to entering the ATR. For example, the various off-gas streams may be combined in a fuel gas header prior to the ATR.

[0045] In some embodiments the OTU is arranged to receive a supplementary fuel in addition to said off-gas stream(s). The energy available from burning the decarbonised fuel stream generated by the OTU is less than that available from burning the hydrocarbon-containing stream(s). This may not be problematic in all cases, but in some cases supplementary fuel is needed to compensated for the shortfall. The supplementary fuel is typically a natural gas stream. Where supplementary fuel is added, this is usually combined with the off-gas stream(s) prior to the ATR. If necessary, the supplementary fuel may be treated in a purification unit, e.g. to remove metal(s), sulfur compounds and/or halide compounds, prior to the ATR.

[0046] Depending on the composition of the off-gas stream(s), it may be necessary to carry out a purification step prior to introducing said off-gas stream(s) into the OTU. Purification may include for instance sulfur removal, CO.sub.2 removal and/or H.sub.2 removal. Removal of CO.sub.2 and/or H.sub.2 is preferred to avoid over-sizing the OTU.

[0047] Depending on the composition of the off-gas stream(s), and the composition of any supplemental fuel used, it may be necessary to carry out a pre-reforming step upstream of the ATR. Pre-reforming is well known to those skilled in the art.

[0048] Steam is added upstream of the ATR. The mixture at the inlet of the ATR, derived from said off-gases and any supplemental fuel (e.g. by combining the hydrocarbon-containing off gas stream(s), steam and any supplemental fuel), is referred to herein as the hydrocarbon-containing fuel stream. The composition of the hydrocarbon-containing fuel stream at the inlet to the ATR has a steam to carbon ratio, defined as the ratio of steam to carbon atoms present as hydrocarbon, which is typically around 2.6:1. For the avoidance of doubt, a feed containing 75 mol % H.sub.2O and 25 mol % CH.sub.4 has a steam to carbon ratio of 3.0:1, a feed containing 75 mol % H.sub.2O, 10 mol % CO and 15 mol % CH.sub.4 has a steam to carbon ratio of 5.0:1 and so on. A steam to carbon ratio of at least 2.0:1 is required to theoretically convert all of the carbon present in hydrocarbons into CO.sub.2 and H.sub.2. Steam to carbon ratios below 2.0:1 may be used if additional steam is added between the ATR and WGS section and/or if the OTU is operated with a recycle of hydrocarbon-containing gas to the ATR (e.g. generated by a H.sub.2 purification unit located downstream from the CO.sub.2 removal unit). Steam to carbon ratios above 2.0 to 1 may be beneficial to ensure that sufficient steam is present for complete conversion of hydrocarbons in the feed. Because of the energy cost of raising steam it is preferred that the steam to carbon ratio is not more than 3.5:1. It is preferred that the steam to carbon ratio at the inlet to the ATR is from 0.5:1 to 3.5:1, such as 1.0:1 to 3.0:1.

[0049] In embodiments where the off-gas stream has a high CO content it is preferred that the CO is removed prior to reforming in the OTU in order to reduce the size and throughput of the OTU. This may be achieved by treating the off-gas stream in a WGS unit located outside of the OTU. Alternatively, instead of introducing a separate WGS unit to treat the off-gas stream, the off-gas stream may instead be introduced to the OTU downstream of the ATR and upstream of WGS section. If the WGS section includes two or more WGS units (e.g. high temperature shift, medium temperature shift), the off-gas stream may be introduced between one or more of the WGS units. In this arrangement, the decarbonised fuel stream generated by the CO.sub.2 removal unit will also include hydrocarbons from the off-gas stream. It is therefore necessary in this embodiment to introduce a H.sub.2 purification unit downstream from the CO.sub.2 removal unit to separate the decarbonised fuel stream into a H.sub.2 rich stream and a H.sub.2 lean stream. The H.sub.2 lean stream which is rich in hydrocarbons is used to generate the hydrocarbon-containing fuel stream for the ATR. The H.sub.2 rich stream is combusted as fuel instead of the off-gas. This arrangement is shown in FIG. 2.

[0050] In embodiments where the off-gas stream has a high CO.sub.2 content it is preferred that the CO.sub.2 is removed prior to reforming in the OTU in order to reduce the size and throughput of the OTU. This may be achieved by treating the off-gas stream in a CO.sub.2 removal unit located outside of the OTU. Alternatively, instead of introducing a separate CO.sub.2 removal unit to treat the off-gas stream, the off-gas stream may instead be introduced to the OTU downstream of the WGS section and upstream of CO.sub.2 removal unit. In this arrangement, the decarbonised fuel stream generated by the CO.sub.2 removal unit will also include hydrocarbons from the off-gas stream. It is therefore necessary in this embodiment to introduce a H.sub.2 purification unit downstream from the CO.sub.2 removal unit to separate the decarbonised fuel stream into a H.sub.2 rich stream and a H.sub.2 lean stream. The H.sub.2 lean stream which is rich in hydrocarbons is used to generate the hydrocarbon-containing fuel stream for the ATR. The H.sub.2 rich stream is combusted as fuel instead of the off-gas.

[0051] In embodiments where the off-gas stream has a high H.sub.2 content it is preferred that the H.sub.2 is removed prior to reforming in the OTU in order to reduce the size and throughput of the OTU. This may be achieved by treating the off-gas stream in a H.sub.2 purification unit located outside of the OTU. Alternatively, instead of introducing a separate H.sub.2 purification unit to treat the off-gas stream, the off-gas stream may instead be introduced to the OTU downstream of the CO.sub.2 removal unit and upstream of a H.sub.2 purification unit. The H.sub.2 purification unit generates a H.sub.2 rich stream and a H.sub.2 lean stream. The H.sub.2 lean stream which is rich in hydrocarbons is used to generate the hydrocarbon-containing fuel stream for the ATR. The H.sub.2 rich stream is combusted as fuel instead of the off-gas.

[0052] In general, where the OTU includes a H.sub.2 purification unit downstream from the CO.sub.2 removal unit which separates the decarbonised fuel stream into a H.sub.2 rich stream and a H.sub.2 lean stream, the H.sub.2 rich stream is combusted as fuel instead of the decarbonised fuel stream.

Autothermal Reformer (ATR)

[0053] The skilled person will be familiar with the design of autothermal reformers and detail of their arrangement and suitable catalysts are summarised in WO2022/003312A1. An ATR generally comprises a burner disposed at the top of the reformer, to which the hydrocarbon-containing fuel stream and an oxygen-containing gas are fed, a combustion zone beneath the burner through which a flame extends, and a fixed bed of particulate steam reforming catalyst disposed below the combustion zone. In autothermal reforming, the heat for the endothermic steam reforming reactions is therefore provided by combustion of a portion of hydrocarbon in the hydrocarbon-containing fuel stream. The hydrocarbon-containing fuel stream is typically fed to the top of the reformer and the oxygen-containing gas fed to the burner, mixing and combustion occur downstream of the burner generating a heated gas mixture the composition of which is brought to equilibrium as it passes through the steam reforming catalyst.

[0054] The role of the ATR is to convert hydrocarbons in the hydrocarbon-containing fuel stream into hydrogen and carbon oxides through steam reforming reactions.

[0055] In a preferred embodiment the WGS section includes a high-temperature shift vessel and the off-gas treatment unit is arranged such that the hydrocarbon-containing feed to the gas-heated reformer is pre-heated by heat exchange with a shifted gas stream generated by the high-temperature shift vessel. High-temperature shift is operated adiabatically in a shift vessel with inlet temperature in the range 300-400 C., preferably 320-360 C., typically over a bed of a reduced iron catalyst, such as chromia-promoted magnetite. Alternatively, a promoted zinc-aluminate catalyst may be used. This arrangement reduces or eliminates the need for additional fuel to pre-heat the hydrocarbon-containing fuel stream. In a preferred embodiment the shifted gases from the high-temperature shift vessel provide all of the heating duty for the hydrocarbon-containing fuel stream.

[0056] The stream exiting the ATR is referred to herein as a reformed gas stream.

Water-Gas Shift (WGS) Section

[0057] The reformed gas stream is typically cooled before being fed to the WGS section. The WGS section includes one or more WGS shift stages and may include stages of high-temperature shift, medium-temperature shift, isothermal shift and low-temperature shift. These terms, as well as suitable catalysts therefor, are described in WO2022/003312A1.

[0058] The stream exiting the WGS unit is referred to as a shifted gas stream and ideally comprises CO.sub.2, H.sub.2, with the residual being steam and inert gases. A small amount of unreacted hydrocarbons may remain.

[0059] It is preferred that steam or water removal is carried out on the shifted stream before it is sent to the CO.sub.2 removal unit. Typically the shifted stream is cooled to a temperature below the dew point so that the steam condenses. Suitable techniques for steam removal are described in WO2022/003312A1.

CO.SUB.2 .Removal Unit

[0060] The role of the CO.sub.2 removal unit is to separate the shifted stream into a CO.sub.2 rich stream and a decarbonised fuel stream. Any suitable CO.sub.2 separation technology may be used and the skilled person will be aware of suitable technologies. Examples include physical wash systems, reactive wash systems (e.g. an amine wash system) and cryogenic systems.

[0061] The CO.sub.2 rich stream may be further purified if CO.sub.2 is a desired product or may be sent for carbon capture and storage or utilisation. An example of utilisation is in the manufacture of methanol.

[0062] The decarbonised fuel stream is used as fuel in the chemical plant. Whilst the decarbonised fuel stream should not contain significant amounts of hydrocarbons, CO or CO.sub.2 (to avoid emitting CO.sub.2 to atmosphere), because the decarbonised fuel stream is intended for use as fuel it does not need to be especially pure and may include inerts. Typically the decarbonised fuel stream contains 50-98 vol % H.sub.2, preferably 60-98 vol % H.sub.2, such as 85-98 vol % H.sub.2.

Retrofit Method

[0063] The above sections describe the arrangement of a chemical plant containing an OTU according to the invention. The retrofit method involves installing an OTU arranged as described above, such that a hydrocarbon-containing off-gas stream which was originally combusted to provide at least some of the heating duty on the chemical plant, is instead sent to the OTU and the decarbonised fuel stream generated by the OTU is combusted instead of the hydrocarbon-containing off-gas stream. For example, the decarbonised fuel stream may be used as fuel in a burner, e.g. to produce steam. It will be appreciated that the original burner may need to be replaced to be suitable for burning the decarbonised fuel stream having a high H.sub.2 content and the method may include installation of H.sub.2 fuel burners for the decarbonised fuel stream, and any other necessary modifications.

[0064] In some embodiments the off-gas stream is a hydrocarbon-containing stream from a purification unit which is a component of a steam reforming section comprising a steam methane reformer (e.g. in a hydrogen, methanol or ammonia plant). In these embodiments the decarbonised fuel stream generated by the OTU may be used as fuel for the steam methane reformer.

[0065] The invention includes the following embodiments. [0066] 1. A method for retrofitting a chemical plant which is initially arranged such that a hydrocarbon-containing off-gas stream is combusted to provide at least some of the heating duty on the chemical plant, the method comprising the step of: [0067] installing an off-gas treatment unit arranged to accept said hydrocarbon-containing off-gas stream, the off-gas treatment unit comprising sequentially: [0068] (i) an autothermal reformer, arranged to accept a hydrocarbon-containing fuel stream comprising hydrocarbons and steam and produce a reformed gas stream; [0069] (ii) a water-gas shift section arranged to accept said reformed gas stream and produce a shifted gas stream; and [0070] (iii) a CO.sub.2 removal unit arranged to accept said shifted gas stream and produce a CO.sub.2 rich stream and a decarbonised fuel stream; [0071] wherein said off-gas treatment unit is arranged such that the decarbonised fuel stream, optionally having first undergone further purification, is combusted instead of the hydrocarbon-containing off-gas stream; [0072] with the proviso that the hydrocarbon-containing off-gas stream is not a purge gas stream from a methanol loop; and [0073] with the proviso that if the hydrocarbon-containing off-gas stream is a tail-gas stream from the purification unit of a hydrocarbon reforming section comprising a fired steam reformer, water-gas shift and purification unit, then the decarbonised fuel stream from the off-gas treatment unit is not sent to the fired steam reformer of the hydrocarbon reforming section as a fuel. [0074] 2. A method according to embodiment 1, wherein the off-gas treatment unit is arranged to accept off-gases from two or more different hydrocarbon-containing off-gas streams from the chemical plant. [0075] 3. A method according to embodiment 2, wherein the two or more different hydrocarbon-containing off-gas streams are combined into a single stream prior to entering the autothermal reformer. [0076] 4. A method according to any of embodiments 1 to 3, wherein the off-gas treatment unit is arranged to receive a supplementary fuel in addition to said off-gas stream(s). [0077] 5. A method according to any of embodiments 1 to 4, wherein the hydrocarbon-containing off-gas stream is a purge stream. [0078] 6. A method according to any of embodiments 1 to 4, wherein the hydrocarbon-containing off-gas stream is a hydrocarbon-containing stream from a purification unit. [0079] 7. A method according to embodiment 6, wherein the purification unit is part of a steam reforming section. [0080] 8. A method according to embodiment 6, wherein the purification unit is part of a steam reforming section of a hydrogen, methanol or ammonia plant. [0081] 9. A method according to any of embodiments 1 to 8, wherein the method includes installation of H.sub.2 fuel burners for the decarbonised fuel stream. [0082] 10. A method according to any of embodiments 1 to 9, wherein the CO.sub.2 rich stream is arranged to be sent for carbon capture and storage or utilisation. [0083] 11. A method according to any of embodiments 1 to 9, wherein the water-gas shift section comprises a high-temperature shift vessel and the off-gas treatment unit is arranged such that the hydrocarbon-containing fuel stream to the autothermal reformer is pre-heated by heat exchange with a shifted gas stream generated by the high-temperature shift vessel. [0084] 12. A method according to any of embodiments 1 to 11, wherein the off-gas treatment unit includes a H.sub.2 purification unit downstream from the CO.sub.2 removal unit, arranged to separate the decarbonised fuel stream into a H.sub.2 rich stream and a H.sub.2 lean stream, wherein the H.sub.2 rich stream is combusted as fuel instead of the decarbonised fuel stream. [0085] 13. A chemical plant comprising an off-gas treatment unit arranged to accept a hydrocarbon-containing off-gas stream, the off-gas treatment unit comprising sequentially: [0086] (i) an autothermal reformer, arranged to accept a hydrocarbon-containing fuel stream comprising hydrocarbons and steam produce a reformed gas stream; [0087] (ii) a water-gas shift section arranged to accept said reformed gas stream and produce a shifted gas stream; and [0088] (iii) a CO.sub.2 removal unit arranged to accept said shifted gas stream and produce a CO.sub.2 rich stream and a decarbonised fuel stream; [0089] wherein the chemical plant is arranged such that the decarbonised fuel stream, optionally having first undergone further purification, is combusted instead of the hydrocarbon-containing off-gas stream; [0090] with the proviso that the hydrocarbon-containing off-gas stream is not a purge gas stream from a methanol loop; and [0091] with the proviso that if the hydrocarbon-containing off-gas stream is a tail-gas stream from the purification unit of a hydrocarbon reforming section comprising a fired steam reformer, water-gas shift and purification unit, then the decarbonised fuel stream from the off-gas treatment unit is not sent to the fired steam reformer of the hydrocarbon reforming section as a fuel. [0092] 14. A chemical plant according to embodiment 13, wherein the chemical plant is an oil refinery. [0093] 15. A chemical plant according to embodiment 13 or embodiment 14, wherein the off-gas treatment unit is arranged to accept off-gases from two or more different hydrocarbon-containing off-gas streams from the chemical plant. [0094] 16. A chemical plant according to embodiment 15, wherein the two or more different hydrocarbon-containing off-gas streams are combined into a single stream prior to entering the autothermal reformer. [0095] 17. A chemical plant according to any of embodiments 13 to 16, wherein the off-gas treatment unit is arranged to receive a supplementary fuel in addition to said off-gas stream(s). [0096] 18. A chemical plant according to any of embodiments 13 to 17, wherein the hydrocarbon-containing off-gas stream is a purge stream. [0097] 19. A chemical plant according to any of embodiments 13 to 17, wherein the hydrocarbon-containing off-gas stream is a hydrocarbon-containing stream from a purification unit. [0098] 20. A chemical plant according to embodiment 19, wherein the purification unit is part of a steam reforming section. [0099] 21. A chemical plant according to embodiment 19, wherein the purification unit is part of a steam reforming section of a hydrogen, methanol or ammonia plant. [0100] 22. A chemical plant according to any of embodiments 13 to 17, wherein the hydrocarbon-containing off-gas stream is a hydrocarbon-containing stream from an ethylene cracker. [0101] 23. A chemical plant according to any of embodiments 13 to 22, wherein the CO.sub.2 rich stream is arranged to be sent for carbon capture and storage or utilisation. [0102] 24. A chemical plant according to any of embodiments 13 to 23, wherein the water-gas shift section comprises a high-temperature shift vessel and the off-gas treatment unit is arranged such that the hydrocarbon-containing fuel stream to the autothermal reformer is pre-heated by heat exchange with a shifted gas stream generated by the high-temperature shift vessel. [0103] 25. A chemical plant according to any of embodiments 13 to 24, wherein the off-gas treatment unit includes a H.sub.2 purification unit downstream from the CO.sub.2 removal unit, arranged to separate the decarbonised fuel stream into a H.sub.2 rich stream and a H.sub.2 lean stream, wherein the H.sub.2 rich stream is combusted as fuel instead of the decarbonised fuel stream. [0104] 26. A process of treating hydrocarbon-containing off-gases from a chemical plant by converting said off-gases in an off-gas treatment unit to produce a decarbonised fuel stream and combusting said decarbonised fuel stream to provide at least some of the heating duty on said chemical plant, wherein the chemical plant is as defined in any of claims 13 to 25. [0105] 27. A process according to claim 26, wherein said decarbonised fuel stream comprises vol % 50-98 vol % H.sub.2.

Example

[0106] The process of FIG. 1 was modelled based on an off-gas feed, with supplemental natural gas feed, to illustrate the production of a decarbonised fuel. The results for FIG. 1 are as follows:

TABLE-US-00001 Stream in FIG. 1 101 121 106 107 109 115 117 118 Temperature ( C.) 33 20 420 363 342 65 45 40 Pressure (bara) 1.3 50.0 26.1 37.4 23.5 19.6 150 5.0 Mass flow (ton/h) 60.7 6.16 41.8 123.7 55.3 44.5 84.3 6.6 Composition (mol %) O.sub.2 95.0 H.sub.2O 0.75 47.5 23.7 1.37 0.01 0.39 H.sub.2 23.7 19.4 51.81 69.45 0.34 96.5 CO 14.5 11.8 7.11 0.71 0.04 0.99 CO.sub.2 51.0 2.00 0.45 16.12 27.0 99.6 N.sub.2 0.62 0.89 0.62 0.9 0.53 0.63 0.87 Ar 4.1 0.45 0.54 0.75 CH.sub.4 9.45 89.0 19.3 0.29 0.34 0.06 0.50 C.sub.2H.sub.6 7.00 0.92 C.sub.3H.sub.8 1.00 0.13 C.sub.4H.sub.10 0.10 0.01 C.sub.5H.sub.12 0.01 H.sub.2S (mg/Nm.sup.3) 5

[0107] The above example illustrates a situation whereby 69.5 te/h of CO.sub.2 equivalent in the off-gas are reduced to 1.5 te/h of CO.sub.2 equivalent in the decarbonised fuel product, via a process where 98% of the CO.sub.2 is captured for sequestration. Typical CO.sub.2 reduction via capture for sequestration is 95%, with ranges of 90 to 99%.