Process for producing a synthesis gas

Abstract

Process for manufacturing a hydrogen-containing synthesis gas from a natural gas feedstock, comprising the conversion of said natural gas into a raw product gas and purification of said product gas, the process having a heat input provided by combustion of a fuel; said process comprises a step of conversion of a carbonaceous feedstock, and at least a portion of said fuel is a gaseous fuel obtained by said step of conversion of said carbonaceous feedstock.

Claims

1. A method for revamping a plant for the production of a hydrogen-containing synthesis gas, said plant comprising a conversion section, at least one fired device producing heat for said conversion section and a fuel line being directed to said fired device; said plant being fed with a natural gas feedstock and said natural gas feedstock being split into a first fraction used as process gas in the conversion section and a second fraction used as fuel and directed to said fired-device; wherein: a gasification section including a gasifier fed with a carbonaceous feedstock is added to said plant, wherein said carbonaceous feedstock is solid or liquid; said gasification section being arranged to produce at least part of said fuel directed to said fired device, replacing a corresponding part of said second fraction of natural gas, wherein the conversion section includes a reforming section, and the effluent of said gasifier is used as fuel, not as a process gas, and provides at least part of the total amount of fuel directed to said reforming section.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is an illustrative scheme of the process for the production of hydrogen-containing synthesis gas according to an embodiment of the invention.

(2) FIG. 2 is a scheme of the front-end section of an ammonia plant according to a first embodiment.

(3) FIG. 3 is a scheme of the front-end section of an ammonia plant according to a second embodiment.

(4) FIG. 4 is a scheme of an embodiment of the invention for ammonia-urea process.

DETAILED DESCRIPTION

(5) FIG. 1 illustrates a block scheme of a process for producing a hydrogen-containing synthesis gas according to an embodiment of the invention.

(6) Block 300 denotes a reforming section, preferably of an ammonia plant, where a natural gas feedstock 301 is converted into a gas mixture 302, which is purified in a purification section 500 to obtain a product gas 303. The purification section 500 preferably comprises a shift section, a CO2 removal section (502, shown in FIG. 4) and optionally a methanation section.

(7) Block 400 denotes a coal gasification section, where a coal feedstock 401 is converted into a gaseous fuel 402 by a gasification process with an oxidant such as air or oxygen 403 and water or steam 404.

(8) The gaseous fuel 402 provides at least part of the total amount of fuel directed to the reforming section 300. Accordingly, the total amount of the feedstock 301 required for a particular production rate of ammonia is reduced. Alternatively, a larger amount of the feedstock 301 is available for the process, namely for generation of the product gas 303, hence the production of ammonia may be increased. Optionally, a portion of said fuel may be still taken from the natural gas feed 301. Said portion (also called fuel fraction) is represented with a dotted line 304 in the figure.

(9) FIG. 2 shows a subunit 100 of the front-end section of an ammonia plant. Said subunit 100 comprises two sections: a first section 101 for the production of a reformed gas 8 from a natural gas feedstock 1, and a second section 102 for the gasification of a coal feedstock 21 and its conversion into a gaseous fuel 35.

(10) Said first portion 101 comprises a primary reformer 103, which is in turn divided into a radiant section 104 and a convective section 105; a pre-heater 106 and a desulphurizer 107 which are positioned upstream said primary reformer 103.

(11) The natural gas 1 enters said pre-heater 106, where it is heated to a first temperature, e.g. around 350° C., and subsequently is directed to said desulphurizer 107, resulting in a stream 4 of desuplhurized natural gas. Said outlet stream 4 is mixed with superheated steam 5 generating a stream 6 of process gas.

(12) Said stream 6 is fed to the convective section 105 of the primary reformer 103 and it is further heated to a higher temperature, e.g. around 500° C., in a heat exchange coil 108.

(13) The heated stream 7 is subsequently fed to the radiant section 104 of the primary reformer 103, containing an array of tubes filled with catalyst where the conversion into a hydrogen-containing synthesis gas is carried out. The radiant section 104 is provided with a series of burners 201 generating the reforming heat for the aforementioned conversion.

(14) The convective section 105 of the primary reformer 103 substantially recovers heat from the flue gas generated by said burners, which leaves the reformer 103 at line 210. In particular, due to the high temperatures of said flue gas, the convective section 105 is mainly used to superheat the steam and to heat the process air feed to the secondary reformer (not shown in the figure). For these reasons, the convective section 105 is typically provided, besides the aforementioned heat exchange coil 108 for the feeding stream 7, with at least one steam superheater coil 109 and a heat exchange coil 110 for the process air.

(15) FIG. 2 also shows an auxiliary boiler 111 separated from the reforming section 103 and producing additional steam. It should be noted that this setup is purely illustrative and several variants are possible.

(16) As already said above, stream 35 of gaseous fuel is generated in a second section 102 where the gasification of a coal feedstock 21 takes place.

(17) Said second section 102 comprises a gasifier 112 and a series of purification equipment for removing undesirable impurities, e.g. cyclone or gas filter 114 and hydrogen sulphide adsorber 117.

(18) Said coal feedstock 21, an oxidant stream 22 and steam or water 23 are fed to said gasifier 112, where they react at a high temperature (typically around 1000° C. or higher) to produce a gaseous fuel 25 containing, besides H.sub.2 and carbon monoxide, impurities like sulphur, nitrogen and mineral matter.

(19) A continuous stream 24 of ash and unconverted carbon is provided from the bottom of said gasifier 112 to prevent the accumulation of solids in the gasifier 112 itself.

(20) Said gaseous fuel 25 free of most solid particles leaves the gasifier 112 from the top and is passed through a heat recovery unit 113. Said heat recovery unit 113 typically comprises a high pressure steam waste boiler and/or a high pressure steam superheater. In some lower cost embodiments, the gasifier effluent can be cooled by water quench.

(21) After waste heat recovery, the resulting cooled synthesis gas 26 flows through said cyclone or gas filter 114, which removes fine particulate matter 27 still present in the synthesis gas 26. Removing fine entrained solids 27 is an important step as fine particles in the synthesis gas may foul or corrode downstream equipment, reducing performance.

(22) The resulting clean synthesis gas 28 leaves the cyclone 114 and flows to an arrangement of heat exchangers 115, where it is cooled with an optional heat recovery to near ambient temperature and condensed unreacted steam 30 is removed in a separator 116.

(23) Subsequently, the cooled gas 31 leaving the separator 116 enters said absorber 117, in which it is scrubbed with a solvent 32 in order to remove hydrogen sulphide. The lean solvent 32 is typically an amine solution. Elemental sulphur may be recovered from this hydrogen sulphide by a suitable catalytic sulphur removal process (not shown in the figure). The loaded solvent is removed as stream 33 for external regeneration.

(24) Said removal of hydrogen sulphide in the absorber 117 may optionally be carried out by means of a biological process.

(25) The scrubbed gas 34 mainly containing CO and H2, leaving the top of the absorber 117, is optionally reheated in a heat exchanger 118 resulting in a heated stream 35.

(26) Said stream 35 represents the fuel gas which provides the fired heating for the operation of the plant.

(27) More in detail, referring to FIG. 2, said stream 35 fuels the burners 201 of the radiant section 104 and, if present, the burners 200 of the desulphurizer preheater 106, the burners 202, 203 of the convective section 105 and the burners 204 of the auxiliary steam generator 111.

(28) According to FIG. 2, the fuel 35 is split into portions from 10 to 14, each supplying one of the aforementioned burners. In particular: portion 10 fuels the burner 200 of the desulphurizer preheater 106; portion 11 fuels the burner 201 of the radiant section 104; portion 12 fuels the burner 202, provided to control the temperature of the stream 6 fed to the convective section 105; portion 13 fuels the burner 203, provided to control the temperature of the superheated steam generated in the coil 110 of the convective section 105; portion 14 fuels the burner 204 of the auxiliary steam generator 111.

(29) FIG. 3 shows another embodiment of the present invention, and the components are indicated by the same reference numbers.

(30) The gasifier 112 is additionally supplied with a stream 36 of sulphur sorbent, typically limestone, in order to remove most of the sulphur present in the coal feedstock 21.

(31) The spent sorbent is discharged from the bottom of the gasifier 112 together with ash and unconverted carbon in stream 24.

(32) After passing through a heat recovery unit 113, a cyclone 114, the synthesis gas stream 28 substantially free of sulphur and solid particles is used as fuel and supplied to the burners.

(33) FIG. 4 discloses another embodiment of the invention for implementation in an ammonia-urea plant. The syngas 303 is a make-up gas for synthesis of ammonia which is converted into ammonia 601 in an ammonia synthesis section 600. At least some or all of the ammonia 601 is used in a urea section 602 for the synthesis of urea 603 with a carbon dioxide feed 604.

(34) A first portion 605 of the total CO2 requirement 604 for conversion of the ammonia into urea comes from the CO2 removal unit 502, typically comprising an MDEA or potassium carbonate washing unit, forming part of the purification section 500 of the reformed gas 302.

(35) A second portion 606 of carbon dioxide is obtained from a portion of the fuel 402, i.e. from the gasification of coal. Said second portion 606 is a more substantial part of the total CO2 requirement 604 when the reforming section 300 only comprises a primary steam reformer and most or all the ammonia is converted to urea.

(36) More in detail, said portion 402 is directed to a shift reactor 608 to convert the carbon monoxide contained therein into carbon dioxide. The so recovered carbon dioxide is separated, for example in a washing unit 609, and mixed with said first portion 605 to form the above mentioned feed 604. Desulphurization of 402 is not shown.

(37) The remaining portion 411 of the fuel 402 is sent to the reforming section 300.

Example 1

(38) An integrated ammonia/urea plant based entirely on natural gas as feed and fuel produces 2200 tonnes/day of ammonia of which approximately 85% is converted into urea, of which the production is accordingly 3300 tonnes/day. Total energy requirement for the integrated plant, which is completely supplied in the form of natural gas, amounts to 5.2 Gcal LHV basis per tonne of urea product, amounting in total to 715 Gcal/h. Of this total natural gas import, 3.1 Gcal/tonne (426 Gcal/h) is required as process feed for the steam reforming process, with the balance of 2.1 Gcal/t (289 Gcal/h) used as fuel in the steam reformer and for the generation and superheating of high pressure steam.

(39) The whole of this natural gas consumption as fuel can in principle be replaced with a fuel gas generated from coal in a gasification facility as described herein. However it is assumed that due to miscellaneous losses the total LHV heating value required would be 10% higher (318 Gcal/h) after conversion from all natural gas firing to all coal-derived fuel gas. A fuel gas stream having a total LHV heating value of 318 Gcal/h can typically be produced by gasification of approximated 75 tonnes/h of bituminous coal (dry ash-free basis) at approximately 10 bar/1000° C. in a fluidized bed gasifier requiring around 45 tonnes/h of 95% purity oxygen.

(40) By contrast a revamp of the 2200 tonnes/day ammonia plant forming part of an integrated ammonia/urea plant so as to use coal as process feedstock would require gasification of approximately 110 tonnes/h of bituminous coal (dry ash-free basis), typically at 50 bar with around 95 tonnes/h of oxygen at around 60 bar—requiring a much larger capital investment than the coal gasification scheme above. Moreover import of a material amount of high pressure steam from an external boiler plant (assumed to be coal fired) would be necessary to ensure sufficient steam and mechanical power for the ammonia plant and the downstream urea plant.

Example 2

(41) In a plant for the methanol synthesis, whereby the gas production process is based on a pure steam reformer, 93.2% of the natural gas feed is required as process feed, with the balance of 6.8% used as fuel. The total gas consumption for methanol production according to this process route is around 7.4 Gcal/MT, based on the natural gas LHV.

(42) Application of a first embodiment of the invention allows replacing the fuel fraction, which is 6.8%. Hence, it allows reducing the natural gas consumption to 93.2% of the original value, i.e. 6.9 Gcal/MT based on the gas LHV.

(43) The amount of natural gas used as process feed can be drastically reduced by application of another embodiment of the invention, i.e. adding CO2 recovered from the gasifier effluent to the primary steam reforming. Accordingly, only 74.3% of the total original amount of natural gas is needed as process feed, or 5.5 Gcal/MT. The fuel fraction is produced by the gasifier. Hence, the gas consumption is reduced by more than 25%, compared to the original value of 7.4 Gcal/MT of methanol.

(44) It is worth considering that in a methanol synthesis plant according to the art, whereby the syngas generation is based on a primary steam reformer followed by an oxygen auto-thermal reformer (i.e. based on combined reforming), the total natural gas consumption is 7.0 Gcal/MT. This value is still 20% higher than the consumption value achieved by the embodiment described above.

(45) The invention can be applied also to a methanol plant based on combined reforming.