Process for producing a hydrogen-containing synthesis gas

Abstract

Process including the production of a hydrogen-containing synthesis gas by conversion of a hydrocarbon feedstock, wherein said process has a heat input provided by combustion of a plurality of process fuel streams and said plurality of process fuel streams comprises at least one fuel stream of ammonia. Combustion of said at least one fuel stream of ammonia is performed non-catalytically in at least one fired equipment.

Claims

1. A process, comprising: producing a hydrogen-containing synthesis gas by conversion of a hydrocarbon feedstock; wherein said process having a heat input provided by combustion of a plurality of process fuel streams; and wherein said plurality of process fuel streams comprises at least one fuel stream of ammonia and combustion of said at least one fuel stream of ammonia is performed non-catalytically in at least one fired equipment.

2. The process of claim 1, wherein combustion of said at least one fuel stream of ammonia provides up to 50% of the total heat input of the process.

3. The process of claim 1, wherein combustion of said at least one fuel stream of ammonia provides up to 30% of the total heat input of the process.

4. The process of claim 1, wherein combustion of said at least one fuel stream of ammonia provides up to 15% of the total heat input of the process.

5. The process of claim 1, wherein said at least one fuel stream of ammonia is in a gaseous state or a vapour state.

6. The process of claim 1, wherein said plurality of process fuel streams comprises at least one fuel stream of ammonia which is supplied by a storage tank as trim fuel of the process, said trim fuel accounting for not more than 15% of the total combustion heat of the process.

7. The process of claim 1, wherein said plurality of process fuel streams comprises at least one fuel stream of ammonia which is supplied by a storage tank as trim fuel of the process, said trim fuel accounting for not more than 10% of the total combustion heat of the process.

8. The process of claim 1, wherein said plurality of fuel streams also includes a hydrogen-rich fuel stream.

9. The process of claim 8, wherein combustion of said hydrogen-rich fuel stream provides at least 50% of the total heat input of the process.

10. The process of claim 8, wherein combustion of said hydrogen-rich fuel stream provides at least 70% of the total heat input of the process.

11. The process of claim 8, wherein combustion of said hydrogen-rich fuel stream provides at least 85% of the total heat input of the process.

12. The process of claim 8, wherein said conversion of the hydrocarbon feedstock provides a raw product gas containing hydrogen, CO and CO.sub.2, said raw product gas is purified by at least a step of CO.sub.2 separation, after a shift reaction converting CO into CO.sub.2, thus obtaining a CO.sub.2-depleted synthesis gas, and wherein said hydrogen-rich fuel stream comprises, or consists of, a portion of said CO.sub.2-depleted synthesis gas.

13. The process of claim 1, wherein said hydrogen-containing synthesis gas comprises hydrogen (H.sub.2) and nitrogen (N.sub.2) and is catalytically reacted in a synthesis loop to produce ammonia, and wherein said plurality of process fuel streams comprises a portion of the ammonia so produced.

14. The process of claim 13, wherein said plurality of process fuel streams further comprises at least one fuel stream of ammonia which is supplied by a storage tank, said storage tank being preferably filled with ammonia synthesized by the process.

15. The process of claim 13, wherein said synthesis loop also releases a purge stream containing unreacted synthesis gas and said plurality of process fuel streams further including at least one of: said purge stream or a portion thereof; a hydrogen-rich gas obtained after hydrogen recovery from said purge stream; a tail gas obtained after hydrogen recovery from said purge stream.

16. The process of claim 1, wherein said hydrogen-containing synthesis gas includes a mixture of carbon oxides and hydrogen, and said hydrogen-containing synthesis gas is catalytically reacted in a synthesis loop to produce methanol.

17. The process of claim 16, wherein said synthesis loop also releases a purge stream containing unreacted synthesis gas and said plurality of process fuel streams also includes a hydrogen-rich fuel stream, said hydrogen-rich fuel stream comprising or consisting of at least a portion of said purge stream, said at least a portion of purge stream being subjected to removal of at least part of carbon-containing compounds.

18. The process of claim 1, wherein the conversion of said hydrocarbon feedstock includes a step of reforming or of partial oxidation, said step of reforming including at least one of the following: a fired primary reforming, a gas heated reforming (GHR), an air- or oxygen-fired secondary reforming, or an auto-thermal reforming (ATR).

19. The process of claim 1, wherein said at least one fired equipment, where ammonia fuel is combusted, includes at least one of the following: a primary reformer, a feedstock pre-heater, an auxiliary steam generator, a desulphurization pre-heater, a steam superheater, a heat recovery steam generator, or a gas turbine.

20. The process of claim 1, wherein a portion of said at least one fuel stream of ammonia is decomposed to nitrogen (N.sub.2) and hydrogen (H.sub.2), the hydrogen so obtained acting as combustion promoter for the combustion of ammonia.

21. The process of claim 1, wherein said portion decomposed to nitrogen and hydrogen is not greater than 50% of the overall fuel stream of ammonia.

22. The process of claim 1, wherein said portion decomposed to nitrogen and hydrogen is not greater than 30% of the overall fuel stream of ammonia.

23. A process according to claim 1, including a use of said least one fuel stream of ammonia, in said at least one fired equipment, during a transient or a start-up phase.

24. A method for revamping a plant for synthesis of ammonia from a hydrocarbon feedstock, the plant including: a front-end section including a reforming section for converting said hydrocarbon feedstock into a raw synthesis gas containing hydrogen, CO and CO.sub.2, and a purification section providing a purified synthesis gas comprising hydrogen and nitrogen in a suitable ratio of about 3:1; and a synthesis section for converting said purified synthesis gas into an ammonia product; the method comprising: separating a portion of the ammonia product and recycling said portion of the ammonia product to the front-end section for use of said ammonia as fuel in at least one fired equipment of said plant, wherein said ammonia fuel is combusted non-catalytically.

25. A method for start-up of a chemical plant for synthesis of ammonia, including non-catalytic combustion of ammonia in at least one fired equipment of the chemical plant during a start-up phase of the chemical plant.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a first embodiment of the invention.

(2) FIG. 2 shows a second embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(3) FIG. 1 illustrates a block scheme of a plant 100 for the synthesis of ammonia essentially comprising a reforming section 1, a purification section 2, a multi-stage compressor 3, a synthesis section 4 and a hydrogen-recovery unit 5. The reforming section 1 and the purification section 2 are part of a front-end section 6.

(4) A stream 10 of natural gas is supplied to the reforming section 1, wherein it is reformed in the presence of steam 11 and an oxidant 12 (e.g. air or enriched air) providing a raw synthesis gas 13 mostly composed of hydrogen and containing minor amounts of other components including e.g. carbon monoxide, carbon dioxide, water, methane.

(5) Said raw synthesis gas 13 is fed to the purification section 2, wherein carbon monoxide is converted into carbon dioxide to produce a shifted gas and said shifted gas is subjected to a carbon dioxide removal process, providing a purified gas 14 essentially containing hydrogen and a CO.sub.2-containing tail gas stream 15. For example, said carbon dioxide removal process is a pressure swing adsorption (PSA) process using molecular sieves.

(6) If appropriate, said purified gas 14 is mixed with nitrogen 16 from an air separation section (not shown) to provide an ammonia make-up gas 17. The make-up gas 17 is then compressed to the pressure of the synthesis section 4 within the multi-stage compressor 3.

(7) The gas 17 thus obtained, together with a flow of hydrogen 18 recovered from the hydrogen-recovery unit 5, feeds the synthesis section 4. Said synthesis section produces ammonia 19 and a flow 20 of purge gas treated in the unit 5 for recovery of the hydrogen contained inside it.

(8) The ammonia 19 splits into a first portion 21 and a second portion 22. Said first portion 21 is used, together with a tail gas 23 leaving the hydrogen-recovery unit 5, as fuel in one or more furnaces of the reforming section 1, for example in the burners of a primary reformer and/or in a charge pre-heater (not shown). The second portion 22 is exported. Optionally, a suitable small amount of natural gas 24 may also be used as fuel in the reforming section 1; however, it is preferably avoided to reduce CO.sub.2 emissions.

(9) FIG. 2 illustrates a block scheme of a plant 100 according to another embodiment of the invention. In this example, a portion 14a of the purified gas essentially containing hydrogen mixes with the tail gas 23 of the hydrogen-recovery unit 5 and is thus used as fuel in the reforming section 1.

EXAMPLE

(10) The following Table 1 refers to an ammonia production process carried out in a front-end section and a synthesis section. Inside the front-end section, natural gas is converted into a synthesis gas and CO.sub.2 is separated from said synthesis gas. The synthesis section produces an ammonia product and a purge stream, the latter being recovered for use as fuel in the front-end section. The front-end section requires further fuel, which comes from different sources according to the examples illustrated below.

(11) Table 1 compares the overall consumption of natural gas, the fuel consumption breakdown and the ammonia production rate in the following processes:

(12) 1.1 Fuel requirement essentially provided by natural gas (prior art);

(13) 1.2 Fuel requirement essentially provided by ammonia, produced in the ammonia production process (embodiment of FIG. 1 of the invention);

(14) 1.3 Fuel requirement essentially provided by ammonia and a CO.sub.2-depleted syngas, both produced in the ammonia production process (embodiment of FIG. 2 of the invention).

(15) TABLE-US-00001 TABLE 1 1.1 1.2 1.3 Overall NG consumption NG feed (as LHV energy) Gcal/h 460 610 590 NG fuel (as LHV energy) Gcal/h 90 0 0 Total NG consumption, feed + fuel Gcal/h 550 610 590 (as LHV energy) Fuel consumption breakdown NG fuel (as LHV energy) Gcal/h 90 0 0 Purge fuel (as LHV energy) Gcal/h 15 20 17 Ammonia fuel (as LHV energy) Gcal/h 0 120 45 CO.sub.2 depleted syngas as fuel Gcal/h 0 0 65 (as LHV energy) Ammonia production rate Ammonia end product t/h 83 83 83 Ammonia fuel consumption t/h 0 27 10 Ammonia total production t/h 83 110 93

(16) The processes according to embodiments of the invention (namely 1.2 and 1.3) eliminate the natural gas consumption as fuel. Accordingly, CO.sub.2 stack emissions from natural gas fuel combustion are eliminated.

(17) All processes 1.1, 1.2 and 1.3 have the same production rate of the ammonia end product, that is 83 t/h. However, the processes of the invention (namely 1.2 and 1.3) produce an excess of ammonia for use as fuel. In particular, process 1.2 makes about 32% more ammonia than process 1.1, while process 1.3 makes 12% more ammonia than process 1.1.

(18) Surprisingly, the processes of the invention (namely 1.2 and 1.3) have only moderately higher natural gas consumption than the prior art, despite the low CO2 stack emissions and oversized production capacity. In particular, process 1.2 has +11% consumption and process 1.3 has +7% consumption.

(19) Using ammonia fuel in combination with CO.sub.2 depleted syngas, according to process 1.3, is advantageous both as regards total gas consumption and as regards total ammonia required production. Accordingly, process 1.3 consumes less gas and requires a smaller plant (i.e. lower plant cost) than process 1.2.