DK201700695A1

DK201700695A1 - System and process for synthesis gas production

Info

Publication number: DK201700695A1
Application number: DKPA201700695A
Authority: DK
Inventors: Mølgaard Mortensen Peter; Aasberg-Petersen Kim
Original assignee: Haldor Topsøe A/S
Priority date: 2017-12-08
Filing date: 2017-12-08
Publication date: 2019-06-25

Abstract

The invention relates to a system and a process for producing synthesis gas. The system comprises a partial oxidation reactor arranged to partially combust a first hydrocarbon feed together with sub-stoichiometric amount of oxygen to produce a first synthesis gas stream comprising hydrogen, carbon monoxide and carbon dioxide. The system also comprises a steam methane reforming reactor housing steam reforming catalyst, the steam methane reforming reactor comprising an inlet for inletting a second hydrocarbon feed and steam, and an outlet for outletting a second synthesis gas stream comprising reformed gas from the steam methane reforming reactor. The system furthermore comprises a third reactor comprising a third catalyst suitable for steam methane reforming/methanation and reverse water gas shift reaction. The system furthermore comprises conduits for leading the first and second synthesis gas streams into the third reactor, and the third reactor comprises an outlet for letting out a product synthesis stream.

Description

System and process for synthesis gas production

FIELD OF THE INVENTION

Embodiments of the invention generally relate to a system and a process for producing synthesis gas. In particular, the invention relates to a system and a process aimed at producing a reformed stream with a low H2/CO ratio.

BACKGROUND

Catalytic synthesis gas production by steam reforming of a hydrocarbon feed has been known for decades. The endothermic steam reforming reaction is typically carried out in reactors referred to as steam reformers or steam methane reformers. A steam reformer has a number of catalyst filled tubes placed in a furnace or fired heater to provide the heat for the endothermic reaction. The tubes are normally 10-14 meters in length and 7-15 cm in inner diameter. The heat for the endothermic reaction is supplied by combustion of fuels in burners in the furnace. The synthesis gas exit temperature from the steam reformer depends on the application of the synthesis gas but will normally be in the range from 650°C-980°C.

It is known that carbon formation on the catalyst used in catalytic synthesis gas production by steam reforming is a challenge, especially for production of synthesis gasses with a relatively low H2/CO ratio. Therefore, catalysts resistant to carbon formation are required for such synthesis gasses. Such carbon resistant catalysts are e.g. noble metal catalysts, partly passivated nickel catalysts, and promoted nickel catalysts. Moreover, industrial scale reforming of CO2 rich gas typically requires a co-feed of water to decrease the severity of the gas for carbon formation. From a thermodynamic viewpoint, it is advantageous to have a high concentration of CO2 and a low concentration of steam in the feed to proDK 2017 00695 A1 mote the production of synthesis gas with a low H2/CO ratio. However, operation at such conditions may not be feasible due to the possibility of carbon formation on the catalyst.

Alternative production of a synthesis gas with a low H2/CO ratio by steam reforming is a sulfur passivated reforming (SPARG) process which may be used for producing synthesis gas with a relatively low H2/CO ratio. This process requires desulfurization of the produced synthesis gas to produce a sulphur free synthesis gas.

More details of various processes for producing synthesis gas with low H2/COratio can be found in “Industrial scale experience on steam reforming of CO2rich gas”, P.M. Mortensen & I. Dybkjær, Applied Catalysis A: General, 495 (2015), 141-151.

The terms “reforming” and “methane reforming” are meant to denote a reforming reaction according to one or more of the following reactions:

CH4 + H2O θ CO + 3H2(i)

CH4 + 2H2O θ CO2 + 4H2(ii)

CH4 + CO2 θ 2CO + 2H2(iii)

Reactions (i) and (ii) are steam methane reforming reactions, whilst reaction (iii) is the dry methane reforming reaction.

For higher hydrocarbons, viz. CnHm, where n>2, m > 4, equation (i) is generalized as:

CnHm + n H2O nCO + (n + m/2)H2 (iv), where n>2, m > 4

Typically, reforming is accompanied by the water gas shift reaction (v):

CO + H2O θ CO2 + H2 (v)

DK 2017 00695 A1

Processes based on Autothermal Reforming (ATR) are an alternative route to production of synthesis gas, especially when a low ratio of hydrogen to carbon monoxide is required. The main elements of an ATR reactor are a burner, a combustion chamber, and a catalyst bed contained within a refractory lined pressure shell. In an ATR reactor, partial combustion of the hydrocarbon feed by sub-stoichiometric amounts of oxygen is followed by steam reforming of the partially combusted feedstock in a fixed bed of steam reforming catalyst. Steam reforming also takes place to some extent in the combustion chamber due to the high temperature. The steam reforming reaction is accompanied by the water gas shift reaction. Typically, the gas is at or close to equilibrium at the outlet of the reactor with respect to steam reforming and water gas shift reactions. The temperature of the exit gas is typically in the range between 850°C and 1100°C. More details of ATR and a full description can be found in the art, such as “Studies in Surface Science and Catalysis”, Vol. 152, ’’Synthesis gas production for FT synthesis”; Chapter 4, p.258-352, 2004.

ATR uses oxygen and steam, and optionally also carbon dioxide, in a reaction with a hydrocarbon feed stream to form synthesis gas. The ratio of hydrogen to carbon monoxide in the exit gas depends upon the selected operation conditions including the amount of steam and carbon dioxide added to the hydrocarbon feed stream and/or the ATR reactor. Increasing the amount of carbon dioxide will decrease the hydrogen to carbon monoxide ratio in the product gas, but will also increase the size of the reactor due to the higher flow.

Synthesis gas may also be produced by processes based on thermal partial oxidation (TPOX). In a TPOX reactor the hydrocarbon feed stream and an oxidant react thermally without catalyst in a refractory lined reactor at high temperature. The temperature of the synthesis gas leaving the TPOX will often be at about 1200-1300°C or even above. No catalyst is involved. Little or no steam or carbon dioxide is added to the hydrocarbon feed stream as this may promote the formation of soot. Catalytic partial oxidation is defined as a process where the

DK 2017 00695 A1 hydrocarbon gas and oxidant are mixed and the mixed feed is let into a catalyst bed.

SUMMARY OF THE INVENTION

In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention.

Furthermore, in various embodiments the invention provides numerous advantages over the prior art. However, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments, and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to the invention shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).

One embodiment of the invention provides a system for producing a synthesis gas stream. The system comprises a partial oxidation reactor arranged to partially combust a first hydrocarbon feed together with sub-stoichiometric amount of oxygen to produce a first synthesis gas stream comprising hydrogen, carbon monoxide and carbon dioxide; a steam methane reforming reactor comprising one or more heat sources and a number of reformer tubes housing steam reforming catalyst, the steam methane reforming reactor comprising one or more inlets for letting a second hydrocarbon feed and steam into the reformer tube(s), and an outlet for outletting a second synthesis gas stream comprising an at

DK 2017 00695 A1 least partly reformed gas from the steam methane reforming reactor; and a third reactor comprising a third catalyst active in catalysing steam methane reforming/methanation and reverse water gas shift reactions. The system moreover comprises conduits for leading the first and second synthesis gas streams into the third reactor and the third reactor comprises an outlet for letting out a third synthesis gas stream. This third synthesis gas stream may be the final synthesis gas stream or product gas stream; however, the third synthesis gas stream could also undergo further processing.

The term “partial oxidation reactor” is meant to cover a partial oxidation (CPOX) reactor, a thermal partial oxidation (TPOX) reactor or an autothermal reforming (ATR) reactor. In a partial oxidation reactor, the steam methane reforming and the water gas shift reactions take place.

When the first and second synthesis gas streams from the partial oxidation reactor and the steam methane reforming reactor, respectively, undergo steam methane reforming, methanation, and reverse water gas shift reactions in the third reactor, the resulting gas, viz. the synthesis gas stream from the system, is close to equilibrium with regard to these reactions.

The term “steam methane reforming” is meant to cover the reactions (i) and (ii), whilst the term “methanation” is meant to cover the reverse reaction of reaction (i) and/or (ii). Thus, the term “steam methane reforming/methanation reactions” is meant to denote the reactions (i) and (ii) running towards equilibrium. The term “reverse water gas shift” is meant to denote the reverse reaction of reaction (v). In most cases, all of these reactions are at or close to equilibrium at the outlet from the catalyst bed or catalyst zone of the reactor concerned.

The present invention enables production of synthesis gas compositions of CO,

H2, CO2, and H2O which is difficult and/or capital cost intensive utilizing standard reforming applications as the combination of the operation enables operating points inside an area which otherwise could be limited by carbon formation,

DK 2017 00695 A1 soot formation, or stoichiometry. This enables adjustment of a synthesis gas to a specific use.

The final gas, viz. the synthesis gas stream exiting from the third reactor, may undergo further processing, such as separation in a separation unit. The separation unit may e.g. be a pressure swing adsorption (PSA) unit, a temperature swing adsorption (TSA) unit, a unit with a membrane, CO2 wash or a combination of CO2 separation and a cold box. A cold box is defined as a cryogenic process for separation of a mixture of H2, CO, and other gasses into a somewhat pure stream of CO, a somewhat pure stream of H2, and a balancing stream of what remains from the feed stream.

In this way the gas stream outlet from the third reactor has a relative high concentration of carbon monoxide. The use of a third reformer comprising a third catalyst active in catalysing steam methane reforming, methanation and reverse water gas shift reactions may seem counterintuitive, since methane may be generated from the hydrogen and carbon monoxide of the first and second synthesis gas streams, by the reactions (i) and (iii) running towards the left side. However, the advantage of providing a synthesis gas with a high concentration of carbon monoxide whilst alleviating the risk of carbon formation on catalyst within the steam methane reforming reactor outweigh the potential disadvantage of a minor increase in the amount of methane in the synthesis gas stream. It should be noted that carbon dioxide and/or steam may also be fed into the partial oxidation reactor.

The first and second synthesis gas streams are mixed completely or partially prior to being fed into the third reactor or within the third reactor. Alternatively, part of either the first or the second synthesis gas stream bypasses the third reactor and the bypassed stream is optionally mixed with the third synthesis gas downstream the third reactor.

DK 2017 00695 A1

The term “hydrocarbon feed” is meant to denote a feedstock comprising one or more hydrocarbons and possibly other constituents, such as steam and possibly carbon dioxide. The term “a hydrocarbon gas stream” is meant to denote a gas stream comprising one or more hydrocarbons and possibly other constituents, such as hydrogen, carbon monoxide, carbon dioxide, nitrogen, argon and mixtures thereof. Examples of “a hydrocarbon gas” may be natural gas, town gas, or a mixture of methane and higher hydrocarbons. Typically, the first and second hydrocarbon feeds are hydrocarbon gas streams comprising minor amounts of hydrogen, carbon monoxide, carbon dioxide, nitrogen, or argon, or combinations thereof, in addition to the hydrocarbon gasses therein and in addition to steam and possibly carbon dioxide added thereto. The term “hydrocarbon” is meant to denote an organic compound consisting entirely of hydrogen and carbon; thus, the term “hydrocarbon” for example relates to methane or higher hydrocarbons, such as ethane or propane.

For reforming processes, an example of a “hydrocarbon gas and steam” is e.g. a mixture of methane, steam, and possibly other oxidizing gasses, such as carbon dioxide.

The “first hydrocarbon feed” and “second hydrocarbon feed” may be two parts of a hydrocarbon feed, such as two parts of a hydrocarbon gas stream, split up into two feeds or synthesis gas streams, but having substantially similar composition; or they may be hydrocarbon feeds, such as hydrocarbon gas streams, of different compositions. Steam reforming catalyst is also denoted steam methane reforming catalyst or methane reforming catalyst. Examples of steam reforming catalysts are Ni/MgAl2O4, Ni/Al2O3, Ni/CaAl2O4, Ru/MgAl2O4, Rh/MgAl2O4, Ir/MgAl2O4, Mo2C, Wo2C, CeO2, a noble metal on an Al2O3 carrier, but other catalysts suitable for reforming are also conceivable.

Typically, the hydrocarbon streams will have undergone desulfurization to remove any sulfur in the feed and thereby avoid deactivation of the catalysts in the process.

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Optionally, the hydrocarbon gas will together with steam also have undergone adiabatic prereforming according to reaction (iv) in a temperature range of ca. 350-550°C to convert higher hydrocarbons as an initial step in the process normally taking place downstream the desulfurization step. This removes the risk of carbon formation from higher hydrocarbons on catalyst in the subsequent process steps.

By the process of the invention, the first and second synthesis gas streams are mixed upstream or within the third reactor. This mixing allows for tailoring of the elemental H/C and O/C ratios of the gas made up of the first and second synthesis gas streams, and thus for tailoring of the H2/CO of the synthesis gas stream outlet from the third reactor.

Thereby it becomes possible to provide a synthesis gas stream with a low H2/CO ratio without the risk of carbon formation at the catalyst within the steam methane reforming reactor, even at severe operating conditions. The combination of the partial oxidation reactor, the steam methane reforming reactor and the third reactor thus renders it possible to change the H/C and O/C ratios of the product gas to a gas which would be considered critical with respect to carbon formation in a typical steam reformer configuration, without being critical in the concept of the invention.

Within this context, the term S/C or “S/C ratio” is an abbreviation for the steamto-carbon ratio. The steam-to-carbon ratio is the ratio of moles of steam to moles of carbon in hydrocarbons in a gas. Thus, S/C is the total number of moles of steam added divided by the total number of moles of carbon from the hydrocarbons in the gas. Moreover, the term “O/C” or “O/C ratio” is an abbreviation for the atomic oxygen-to-carbon ratio. The oxygen-to-carbon ratio is the ratio of moles of oxygen to moles of carbon in a gas. Furthermore, the term H/C or “H/C ratio” is an abbreviation for the atomic hydrogen-to-carbon ratio. The hydrogen-to-carbon ratio is the ratio of moles hydrogen to moles of carbon in a

DK 2017 00695 A1 gas. It should be noted that the term “C” in the ratio S/C thus is different from the “C” in the ratios H/C and O/C, since in S/C ”C” is from hydrocarbons only, whilst in O/C and H/C, “C” denotes all the carbon in the gas.

It should be understood that the term “an inlet” and “an outlet” is not intended to be limiting. Thus, these terms also cover the possibility where the units, e.g. the reformer tubes, have more than one inlet and/or outlet. For example, a reformer tube could have an inlet for hydrocarbon gas and another inlet for steam, so that the hydrocarbon gas and steam is mixed within the reformer tube.

In an embodiment, the partial oxidation reactor of the system is a catalytic partial oxidation (CPOX) reactor or a thermal partial oxidation (TPOX) reactor. However, as an alternative the partial oxidation reactor could be a gasification reactor and the hydrocarbon feed could be coal. It should be noted, that the terms “first hydrocarbon feed” and “second hydrocarbon feed” are not limited to gas streams, and thus the first hydrocarbon feed may be a feedstock in solid form, such as coal.

When the partial oxidation reactor is an ATR reactor, the first synthesis gas stream exiting the ATR reactor typically has a relatively high temperature, such as about 1000°C or even more, and a relatively high CO2 content. Here, a “relatively high CO2 content” is meant to denote that the first synthesis gas stream has a CO2 content of at least 5 dry mole%, such as at least 10 dry mole%, such as at least 20 dry mole%.

When the first synthesis gas stream is a gas with a relatively high CO2 content, as it is the case when the partial oxidation reactor is a catalytic partial oxidation (CPOX) reactor, a thermal partial oxidation (TPOX) reactor, or an autothermal reforming (ATR) reactor, a CO rich product gas is produced by the process of the invention, whilst alleviating problems of carbon formation on the catalyst material in any of the reactors. Within this context, the term “CO rich product gas” is meant to denote a product gas with a low H2/CO ratio, such as a product

DK 2017 00695 A1 gas with a H2/CO ratio below 2.5, preferably a product gas with a H2/CO ratio below 2.0, more preferably a product gas with a H2/CO ratio below 1.8, even more preferably a product gas with a H2/CO ratio below 1.6. Moreover, the term “CO2 rich gas” is meant to denote a gas stream with a CO2 content of at least 50 dry mole% CO2, such as at least 70 dry mole% CO2, such as at least 90 dry mole% CO2.

In an embodiment, the third reactor is an adiabatic reactor. Even though the reverse water gas shift reaction is endothermic, an adiabatic reactor is possible due to the relatively high temperatures of the first and second synthesis gas streams exiting from the partial oxidation reactor, e.g. an ATR reactor, and the steam methane reforming reactor. In one embodiment, the temperature of the synthesis gas stream exiting the adiabatic reactor is lower than the temperature of the first synthesis gas stream exiting the partial oxidation reactor and lower than the temperature of the second synthesis gas stream exiting the steam methane reforming reactor. In another embodiment, the temperature of the synthesis gas stream exiting the adiabatic reactor is lower than the temperature of the first synthesis gas stream exiting the partial oxidation reactor and higher than the temperature of the second synthesis gas stream exiting the steam methane reforming reactor.

Another aspect of the invention relates to a process for producing synthesis gas, comprising the steps of:

a) in a partial oxidation reactor, partially combusting a first hydrocarbon feed together with sub-stoichiometric amount of oxygen to produce a first synthesis gas stream comprising hydrogen, carbon monoxide and carbon dioxide,

b) in a steam methane reforming reactor comprising one or more heat sources and a number of reformer tubes housing steam reforming catalyst, letting a second hydrocarbon feed and steam into the reformer tube(s) and carrying out a reforming reaction of the second hydrocarbon feed and steam over the steam reforming catalyst to form a first reformed gas stream, and outletting a second

DK 2017 00695 A1 synthesis gas stream comprising reformed gas stream from the steam methane reforming reactor,

c) feeding the first and second synthesis gas streams into a third reactor and carrying out steam methane reforming/methanation reaction and reverse water gas shift reaction over a third catalyst active in catalyzing steam methane reforming/methanation and reverse water gas shift reactions, thereby producing the synthesis gas stream.

Even though not explicitly indicated above, the process also includes outletting the synthesis gas stream comprising synthesis gas from the third reactor, possibly for further processing, such as separation in a separation unit. The separation unit may e.g. be a pressure swing adsorption (PSA) unit, a temperature swing adsorption (TSA) unit, a unit with a membrane, CO2 wash or a combination of CO2 separation and a cold box. A cold box is defined as a cryogenic process for separation of a mixture of H2, CO, and other gasses into a somewhat pure stream of CO, a somewhat pure stream of H2, and a balancing stream of what remains from the feed stream.

In step a), optionally carbon dioxide and/or steam may also be fed into the partial oxidation reactor.

In an embodiment, the partial oxidation reactor is a catalytic partial oxidation (CPOX) reactor or a thermal partial oxidation (TPOX) reactor. In an alternative embodiment the partial oxidation reactor is an autothermal reforming (ATR) reactor.

In an embodiment, the third reactor is an adiabatic reactor. The catalyst within the third reactor is a catalyst active in catalyzing the steam methane/methanation and reverse water gas shift reactions.

In an embodiment, the molar ratio between carbon dioxide let into to the autothermal reforming reactor and hydrocarbons in the hydrocarbon feed is larger

DK 2017 00695 A1 than 0.5. The term “carbon dioxide let into the autothermal reforming reactor” is meant to comprise any carbon dioxide let into the ATR reactor directly as well as the carbon dioxide within the first hydrocarbon feed, if any.

In an embodiment, in step b), the S/C ratio of the second hydrocarbon feed is between about 0.7 and about 2.0. In production of a CO rich synthesis gas stream, the lower S/C ratio, the better. Having a low S/C ratio means that the amount of oxidant gas is low.

In an embodiment, the amount of steam, oxygen, and carbon dioxide let into the partial oxidation reactor and the amount of steam and carbon dioxide let into the steam methane reforming reactor is adjusted to ensure that the H2/CO ratio of the resultant synthesis gas stream is below 2.5. Preferably, the H2/CO ratio of the resultant synthesis gas stream is below 2.0, more preferably the H2/CO ratio of the resultant synthesis gas stream is below 1.8, and most preferably the H2/CO ratio of the resultant synthesis gas stream is below 1.6.

Typically, a heat source of the steam methane reforming reactor heats the steam reforming catalyst within the reformer tube to a temperature sufficient to ensure that the second synthesis gas stream exiting the steam methane reforming reactor has a temperature of between about 650°C and about 950°C.

Typically, the autothermal reforming reactor produces a gas stream with an exit temperature of between about 900°C and about 1100°C.

Typically, the catalyst material within the reformer tubes of the steam methane reforming reactor is a reforming catalyst. Advantageously, the catalyst material is arranged to catalyze steam methane reforming reactions (reactions (i) and (ii)), dry methane reforming (reaction iii) and water gas shift reactions (reaction v). In the case where the partial oxidation reactor is an ATR reactor, the ATR reactor may contain the same type of catalyst material. Moreover, the catalyst

DK 2017 00695 A1 material within the third reactor may be the same type of catalyst material. Examples of catalyst materials could be Ni/MgAbO4, Ni/Al2O3, Ni/CaAl2O4, Ru/MgAl2O4, Rh/MgAl2O4, Ir/MgAl2O4, Mo2C, Wo2C, CeO2, a noble metal on an Al2O3 carrier, but other catalysts suitable for reforming are also conceivable, such as catalysts having platinum or palladium as active metals. Moreover, it is possible to have a configuration with different types of catalyst materials (e.g. the ones mentioned above) in the partial oxidation reactor, the steam methane reforming reactor, and the third reactor.

The one or more heat sources of the steam methane reforming reactor is/are arranged to heat the catalyst material within the reformer tubes to temperatures of between about 650°C and about 950°C. It should be understood, that not all the catalyst material within the steam methane reforming reactor needs to be heated to a temperature between 650°C and about 950°C; instead at least some of the catalyst material is heated to a temperature between 650°C and about 950°C. Thus, in a part of the steam methane reforming reactor close to the inlet, the catalyst material may be heated to a temperature of e.g. 450°C or 500°C; and in a part of the steam methane reforming reactor close to the outlet, the catalyst material may be heated to a temperature of more than 950°C, such as e.g. 1000°C. The reformed gas exiting the steam methane reforming reactor has a temperature of up to 950°C. Typically, the pressure within the reformer tube is above 5 barg and below 35 barg, for example between 25 and 30 barg.

In an embodiment, the steam methane reforming reactor is a heat exchange reformer. In this embodiment, the heat source for the endothermic steam methane reforming reaction is at least part of the first synthesis gas stream from the partial oxidation reactor, optionally after the first synthesis gas has been mixed with the second synthesis gas within the steam methane reforming reactor. The at least part of the first synthesis gas stream, optionally together with the second synthesis gas, flowing outside the reformer tube(s) of the steam methane reformer is cooled by heat exchange with the gas within the reformer tube(s).

DK 2017 00695 A1

The use of a product stream of reformed gas as a source of heat in heat exchange reforming is known in the art. For example, EP-0033128 and EP0334540 deal with parallel arrangements, in which a hydrocarbon feed stream is introduced in parallel to a radiant furnace and heat exchange reformer. The partially reformed gas from the radiant furnace is then used as a heat source for the reforming reactions in the heat exchange reformer.

Other parallel arrangements combine heat exchange reforming and autothermal reforming. EP0983963, EP1106570 and EP0504471 deal with processes in which a hydrocarbon feed stream is introduced in parallel to a heat exchange reformer and an autothermal reformer. The hot product synthesis gas from the autothermal reformer is used as a heat exchanging medium for the reforming reactions occurring in the heat exchange reformer.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the present invention are explained, by way of example, and with reference to the accompanying drawing. It is to be noted that the appended drawing illustrates only an examples of an embodiment of this invention and is therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

Figure 1 is a schematic drawing of a system for producing synthesis gas according to the invention.

DETAILED DESCRIPTION

The following is a detailed description of an embodiment of the invention depicted in the accompanying drawing. The embodiment is an examples only and is in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiDK 2017 00695 A1 ments; but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

Figure 1 is a schematic drawing of a system 100 for producing synthesis gas according to the invention. The system 100 comprises a partial oxidation reactor 10 in the form of an ATR reactor, a steam methane reforming reactor 20 and a third reactor 30 in the form of an adiabatic reverse shift reforming reactor.

Feed streams to the system 100 comprise a stream of hydrocarbon gas 1, which is split up into a first hydrocarbon gas stream 2 and a second hydrocarbon gas stream 3. A CO2 rich stream 4, for example substantially pure CO2, and steam 5 are added to the first hydrocarbon gas stream 2, hereby forming a combined stream 7, viz. a first hydrocarbon feed, prior to inletting the combined stream 7 into the ATR reactor 10. An oxygen containing stream 8, such as air, a stream of steam and oxygen, an oxygen rich stream or substantially pure oxygen, is inlet into the ATR reactor 10 via an inlet. In the ATR reactor 10, partial combustion of the first hydrocarbon feed 7 by sub-stoichiometric amounts of oxygen from the oxygen containing stream 7 is followed by steam reforming of the partially combusted feed stream in a fixed bed 10a of steam reforming catalyst, thereby producing a first synthesis gas stream 11 comprising hydrogen, carbon monoxide and carbon dioxide. The first synthesis gas stream 11 exiting the ATR reactor 10 typically has a temperature of between about 900°C and about 1100°C, such as about 1000°C. Additional stream(s) of pure steam may also be added to the ATR reactor 10.

The steam methane reforming reactor 20 comprises one or more heat sources and a number of reformer tubes housing steam reforming catalyst. The steam methane reforming reactor 20 moreover comprises one or more inlets for letting a second hydrocarbon feed 3 in the form of a hydrocarbon gas stream, such as natural gas, and steam 9 into the reformer tubes, and an outlet for outletting a second synthesis gas stream 12 comprising at least partly reformed gas from

DK 2017 00695 A1 the steam methane reforming reactor 20. The second synthesis gas stream 12 exiting the steam methane reforming reactor 20 typically has a temperature of between about 650°C and about 950°C, such as about 800°C. In the embodiment shown in figure 1, the second hydrocarbon feed 3 and the steam 9 are combined to a single stream prior to being let into the steam methane reforming reactor 20.

The first and second synthesis gas streams 11, 12 are combined to a single stream 13 which is led into the third reactor 30 comprising a third catalyst active for catalyzing steam methane reforming/methanation and reverse water gas shift reactions. The third reactor 30 is an adiabatic reactor in this embodiment. The third reactor 30 comprises an outlet for letting out a third and final synthesis gas stream 15. Within the third reactor 30 the single stream 13 is equilibrated by the reverse water gas shift and steam methane reforming/methanation reactions. The steam methane reforming reaction is endothermic, the reverse water gas shift reaction is mildly endothermic, whilst the methanation reaction is exothermic. Therefore, the temperature of the synthesis gas stream 15 may be lower than the temperature of the single stream 13 or it may have the same temperature as the single stream 13. The synthesis gas stream 15 exiting the third reactor 30 may be further processed downstream the third reactor.

In the embodiment of the system of Figure 1, no further gas streams are added; in particular no further gas streams are added in upstream the third reactor 30 and downstream the ATR reactor 10 and/or the steam methane reforming reactor 20.

Carbon formation in the tubes of the steam methane reforming reactor is dictated by thermodynamics and in a typical design of a reformer it is a requirement that the reformer does not have affinity for carbon formation of the equilibrated gas anywhere in the catalyst material. This means that the feed stream 3 (figure 1) to the steam methane reforming reactor will have to be balanced with water 9 in order to circumvent the carbon formation area. Typically, the process

DK 2017 00695 A1 gas enters the steam methane reforming reactor at 400-600°C, and leaves it at a temperature of about 950°C (not experiencing temperatures above 1000°C). Thus, when designing a steam methane reforming reactor, there must not be an affinity for carbon formation of the equilibrated gas anywhere in the temperature range from 400°C to 1000°C. This criterion can be used to evaluate the carbon limit of the steam methane reforming reactor.

If potential for carbon formation exists, it will only be a matter of time before a shutdown of the steam methane reforming reactor is necessary due to too high pressure drop. In an industrial context, this will be expensive due to lost time on stream and loading of a new batch of catalyst material into the reformer tubes. Carbon formation at reforming conditions is often as whisker carbon. This is destructive in nature toward the catalyst material and regeneration of the catalyst material is therefore not an option. Thus, the possible operating range for a tubular steam methane reforming reactor will be defined by the conditions which will not have a potential for carbon formation.

The H2/CO ratio of the product gas can be controlled by adjusting the addition of H2O and CO2, where more H2O will increase the product towards hydrogen rich gas and more CO2 will increase the product towards CO rich gas. However, when producing a synthesis gas with a very low H2/CO ratio, in a steam methane reforming reactor, an accompanied high S/C ratio will be necessary to balance the severity of the gas to avoid carbon formation on a nickel catalyst.

EXAMPLE:

An example calculation of the process is given in Table 1 below. A hydrocarbon feed stream 7 comprising a hydrocarbon gas 1, a CO2 rich stream 4 and steam and having a S/C ratio of 0.6 is fed to the ATR reactor 10 of the invention as shown in Fig. 1. The hydrocarbon feed stream 7 is heated to 650°C prior to being let into the ATR reactor 10. The ATR reactor 10 produces a first synthesis gas stream 11. An oxidant gas stream 5 is added to the ATR reactor 10 and the

DK 2017 00695 A1 amount thereof is adjusted such that the temperature of the first synthesis gas stream 11 is 1050^OC.

In parallel, another hydrocarbon feed stream comprising a hydrocarbon gas 3 and steam 9 and having a S/C ratio of 1 is fed to the SMR reactor 20 of the invention as shown in Fig. 1 to produce a second synthesis gas stream 12.

When the first and the second synthesis gas streams are combined and undergo steam methane reforming/methanation and reverse water gas shift in the third reactor, a third synthesis gas is produced with a different composition. Such a synthesis gas would result in carbon formation if it had been produced in a single steam methane reforming unit. However, in the given example the exit temperature of the product gas stream exiting the adiabatic reforming reactor 30 is 941°C, which is well below the methane decomposition equilibrium temperature for the gas of 1069°C and above the Boudouard temperature for the gas of 880°C. Consequently, the product gas stream does not have potential for carbon formation. Thus, by the invention, it is possible to provide a product gas with other S/C and CO2/CH4 ratios, and with a relatively higher CO concentration, than those product gasses possible with a steam methane reforming reactor alone.

In this context, the methane decomposition temperature (T(MDC)) is calculated as the temperature where the equilibrium constant of the methane decomposition into graphite (CH4 θ C + 2H2) equals the reaction quotient of the gas. Formation of graphitic carbon can take place when the temperature is higher than this temperature. The reaction quotient QC is defined as the ratio of the square of the partial pressure of hydrogen to the partial pressure of methane, i.e. QC = P²H2/PcH4.

The Boudouard equilibrium temperature (T(BOU)) is calculated in a similar way, but from the Boudouard reaction (2CO θ C + CO2) and in this case formation of

DK 2017 00695 A1 graphitic carbon can take place when the temperature is lower than this Boudouard equilibrium temperature.

DK 2017 00695 A1

ATR 10 SMR 20 RWGS 30

Inlet T [°C]	650	650	1009
Outlet T [°C]	1050	950	941
Inlet P [kg/cm²g]	35.5	30	29.5
Outlet P [kg/cm2g]	35	29.5	29
Outlet T(MDC) [°C]	-	999	1069
Outlet T(BOU) [°C]	859	925	880
Inlet:
N2 [Nm³/h]	51	51	580
CO2 [Nm3/h]	41965	1965	29399
CH4 [Nm3/h]	37200	37200	14029
H2 [Nm3/h]	784	784	121325
H2O [Nm3/h]	22546	37200	60731
CO [Nm3/h]	0	0	74901
Oxygen feed:
O2 [Nm3/h]	23416
N2 [Nm3/h]	478
Oxygen feed T [°C]	371
Outlet:
N2 [Nm3/h]	529	51	580
CO2 [Nm3/h]	26651	2748	26400
CH4 [Nm3/h]	247	13783	10905
H2 [Nm3/h]	49505	71820	127698
H2O [Nm3/h]	47732	12999	60606
CO [Nm3/h]	52267	22634	81025
O2 [Nm3/h]	0	0	0
Outlet flow [Nm3/h]	176931	124035	307214

Table 1

As a comparative example, production of a similar synthesis gas of H2/CO = 1.6 at similar capacity is illustrated in Table 2 and here the corresponding product synthesis gas flow is 318709 Nm³/h, compared to 307214 Nm³/h in the example of the invention. This higher synthesis gas flow reflects the higher content of water and carbon dioxide in this process scheme. Consequently, the concept of the invention enables operation at more severe conditions than in a single SMR.

DK 2017 00695 A1

This benefit is obtained because the carbon limits are circumvented by using the CO2 already present in the first synthesis gas stream exiting the ATR reactor. Obtaining the given synthesis gas by ATR alone is additionally limited by minimum oxygen requirements and limitations in soot formation, which prevent 5 operation in this regime.

Standalone SMR

Inlet T [°C]	650
Outlet T [°C]	950
Inlet P [kg/cm²g]	29.5
Outlet P [kg/cm2g]	29
Outlet T(MDC) [°C]	1170
Outlet T(BOU) [°C]	856
Inlet:
N2 [Nm³/h]	78
CO2 [Nm3/h]	61361
CH4 [Nm3/h]	56589
H2 [Nm3/h]	1193
H2O [Nm3/h]	96201
CO [Nm3/h]	0
O2 [Nm3/h]	0
Outlet:
N2 [Nm3/h]	78
CO2 [Nm3/h]	31849
CH4 [Nm3/h]	4946
H2 [Nm3/h]	126612
H2O [Nm3/h]	74068
CO [Nm3/h]	81156
O2 [Nm3/h]	0
Outlet flow [Nm3/h]	318709

Table 2

To summarize, the invention relates to a system and a process for producing synthesis gas. The system comprises a partial oxidation reactor arranged to partially combust a first hydrocarbon feed together with sub-stoichiometric amounts of oxygen to produce a first synthesis gas stream comprising hydrogen, carbon monoxide, and carbon dioxide. The system also comprises a steam

DK 2017 00695 A1 methane reforming reactor housing steam reforming catalyst, the steam methane reforming reactor comprising an inlet for inletting a second hydrocarbon feed and steam, and an outlet for outletting a second synthesis gas stream comprising reformed gas from the steam methane reforming reactor. The sys5 tem furthermore comprises a third reactor comprising a third catalyst active in catalyzing steam methane reforming/methanation and reverse water gas shift reactions. The system furthermore comprises conduits for leading the first and second synthesis gas streams into the third reactor, and the third reactor comprises an outlet for letting out a synthesis gas stream.

Claims

CLAIMS:

1. A system for producing a synthesis gas stream, said system comprising:

- a partial oxidation reactor arranged to at l combust a first hydrocarbon feed together with sub-stoichiometric amount of oxygen to produce a first synthesis gas stream comprising hydrogen, carbon monoxide and carbon dioxide,

- a steam methane reforming reactor comprising one or more heat sources and a number of reformer tubes housing steam reforming catalyst, said steam methane reforming reactor comprising one or more inlets for letting a second hydrocarbon feed and steam into said reformer tube(s), and an outlet for outletting a second synthesis gas stream comprising at least partly reformed gas from said steam methane reforming reactor,

- a third reactor comprising a third catalyst active in catalyzing steam methane reforming/methanation and reverse water gas shift reactions, wherein said system comprises conduits for leading said first and second synthesis gas streams into said third reactor and said third reactor comprises an outlet for letting out said synthesis gas stream.

2. A system according to claim 1, wherein said partial oxidation reactor is a catalytic partial oxidation (CPOX) reactor or a thermal partial oxidation (TPOX) reactor.

3. A system according to claim 1, wherein said partial oxidation reactor is an autothermal reforming (ATR) reactor.

4. A system according to any of the claims 1 to 3, wherein the third reactor is an adiabatic reactor.

5 steam, oxygen and carbon dioxide let into the partial oxidation reactor and the amount of steam and carbon dioxide let into the steam methane reforming reactor is adjusted to ensure that the H2/CO ratio of said synthesis gas stream is below 2.5.

5. A system according to any of the claims 1 to 4, wherein the steam methane reforming reactor is a heat exchange reformer and the one or more heat sources is at least part of the first synthesis gas stream from the partial oxidation reactor.

DK 2017 00695 A1

6. A process for producing synthesis gas, said process comprising the steps of:

b) in a steam methane reforming reactor comprising one or more heat sources and a number of reformer tubes housing steam reforming catalyst, letting a second hydrocarbon feed stream and steam into said reformer tube(s) and carrying out a reforming reaction of said second hydrocarbon feed stream and steam over said steam reforming catalyst to form a first reformed gas stream, and outletting a second synthesis gas stream comprising reformed gas stream from said steam methane reforming reactor,

c) feeding said first and second synthesis gas streams into a third reactor and carrying out steam methane reforming reaction or reverse water gas shift reaction over a third catalyst suitable for steam methane reforming reaction and/or reverse water gas shift reaction, thereby producing the synthesis gas stream.

7. A process according to claim 6, wherein said partial oxidation reactor is a catalytic partial oxidation (CPOX) reactor or a thermal partial oxidation (TPOX) reactor.

8. A process according to claim 6, wherein said partial oxidation reactor is an autothermal reforming (ATR) reactor.

9. A process according to any of the claims 6 to 8, wherein said third reactor is an adiabatic reactor.

10. A process according to any of the claims 6 to 9, wherein the molar ratio between carbon dioxide let into to the autothermal reforming reactor (ATR) and hydrocarbons in the first hydrocarbon feed is larger than 0.5.

DK 2017 00695 A1

11. A process according to any of the claims 6 to 10, wherein, in step b), the S/C ratio in the second hydrocarbon feed is between about 0.7 and about 2.0.

12. A process according to any of the claims 6 to 11, wherein the amount of

10 13. A process according to any of the claims, wherein the steam methane reforming reactor is a heat exchange reformer and the one or more heat sources is at least part of the first synthesis gas stream from the partial oxidation reactor.