WO2011141635A1

WO2011141635A1 - Method of producing a hydrocarbon composition

Info

Publication number: WO2011141635A1
Application number: PCT/FI2011/050430
Authority: WO
Inventors: Jukka Koskinen; Jan Wahlström; Isto Eilos; Sebastian Johansson; Sami Toppinen; Marianne Pettersson
Original assignee: Neste Oil Oyj
Priority date: 2010-05-10
Filing date: 2011-05-10
Publication date: 2011-11-17
Also published as: FI20105503A; US20130072583A1; EP2569397A1; EA201291188A1; BR112012028677A2; CA2797878A1; CN102918136A; FI20105503A0

Abstract

Method of producing a hydrocarbon composition. The method comprises providing a biomass raw-material; gasifying the raw-material in the presence of oxygen to produce a gas containing carbon monoxide, carbon dioxide, hydrogen and hydrocarbons possibly together with inert components; separately increasing the hydrogen-to-carbon monoxide ratio of the gas to a value of about 2; feeding the gas to a Fischer-Tropsch reactor; converting in the Fischer-Tropsch reactor at least a significant part of the carbon monoxide and hydrogen contained in the gas into a hydrocarbon composition containing C₄-C₉₀ hydrocarbons; and recovering the hydrocarbon composition. According to the invention, fresh external hydrogen is introduced into the gas before feeding into the Fischer-Tropsch reactor. By using external hydrogen feed, the capacity of a biomass gasification process can be increased and the need for a conventional Water Gas Shift for producing hydrogen from carbon monoxide and steam can be eliminated.

Description

Method of producing a hydrocarbon composition

The present invention relates to hydrocarbon compositions. In particular, the present invention concerns a method according to the preamble of claim 1 of producing a hydrocarbon composition which can be used as such or as an intermediate for the production of various hydrocarbon products.

In a Fischer-Tropsch reactor (in the following also abbreviated FT reactor), hydrogen and carbon monoxide are reacted in the presence of a transition metal catalyst, such as cobalt or iron, to form a composition containing a broad range of linear alkanes. This hydrocarbon composition is useful as an intermediate in the production of many products in the chemical and refining industry and business.

A number of carbonaceous sources have been used as raw-materials for producing a hydrogen and carbon monoxide containing gas (also known as a "syngas") which can be fed into the FT process. Originally, coal was used as the primary raw-material, but lately also natural gas has been taken into use in commercial processes. Even more recently various processes have been developed in which biological materials, such as plant oils, plant waxes and other plant products and plant parts or even oils and waxes of animal origin, are gasified and processed to produce a suitable feed. In a further alternative approach, viz. in the BTL process (biomass to liquid process), a biomass comprising whole plants is used as a raw-material. The BTL process allows for the utilization of forestry residues. Typically, gasification of biomass for producing a syngas takes place in the presence of oxygen. For fuel production by the FT process, it is preferred to use an oxygen-containing gas, particularly oxygen gas, for the gasification in order to attain reasonably high temperatures and to reduce the formation of nitrogenous by-product. A typical temperature in the gasification is about 750 to 1200 °C. At these conditions, biomass, such as lignocellulosic materials, will produce a gas containing carbon monoxide, carbon dioxide, hydrogen and water gas. Further it usually contains some hydrocarbons and impurities, such as sulphur, nitrogen compound and trace metals. In case of gasification in the lower temperature range of about 750 - 950 °C, the product of the gasification will still contain some unreacted hydrocarbons. In order to convert all hydrocarbons to syngas components, the effluent of a gasifier is typically fed into a reformer, either a thermal reformer or catalytic reformer, wherein the gas is subjected to further thermal reactions which give a syngas product mix containing less by-products.

The gaseous effluent of the reformer has to be freed from carbon dioxide, water, sulphur and any other catalyst poisons before it can be used as a syngas for a FT reaction.

Furthermore, the hydrogen-to-carbon monoxide ratio needs to be increased. In particular, whereas a gasifier produces a gas having a molar ratio of hydrogen to carbon monoxide of about 0.5 to 1.5, and reforming only marginally increases the ratio, the Fischer-Tropsch reaction requires that the reactants are present in a higher molar ratio of about 2: 1.

Therefore, it is necessary to increase said ratio in the gas produced in a conventional gasifier.

It is possible to achieve the latter goal by subjecting the gas to a water gas shift (WGS) reaction, in which hydrogen is produced by the reacting carbon monoxide with water to produce carbon dioxide and hydrogen. However, this reaction is disadvantageous in the sense that it will also increase the concentration of carbon dioxide by sacrificing some of the desired carbon monoxide. Before the FT reactor, the concentration of carbon dioxide has to be reduced to a level which is rather low, typically below 3 mole-%. The amount of carbon dioxide which has to be removed from the syngas is therefore significant, and it corresponds to about 50 % of the total gasification capacity of the whole process. In other words, approximately half of the carbon content in the gas produced in the gasification of the biomass can be utilized for production of hydrocarbons by the FT process.

It is an aim of the present invention to eliminate at least a part of the problems related to the art and to provide a novel method of producing hydrocarbon compositions by gasification of biomass and Fischer-Tropsch processing of hydrogen and carbon monoxide.

The invention is based on the concept of introducing fresh hydrogen into the gas produced by gasification of biomass before the gas is fed into the Fischer-Tropsch reactor. By using fresh (externally produced) hydrogen for increasing the hydrogen to carbon monoxide ratio, the capacity of the process can be improved and the volumes of carbon dioxide exiting the process can be reduced.

More specifically, the present invention is characterized by what is stated in the characterizing part of claim 1.

Considerable advantages are obtained by means of the invention. By using external hydrogen feed, the capacity (i.e. the production rate) of a biomass gasification process can be increased with at least about 40 % (cf. Figure 1). In such an embodiment, there is no need for a conventional Water Gas Shift (WGS) reactor for producing hydrogen from carbon monoxide and steam.

In addition to using external hydrogen to compensate for or replace hydrogen production by WGS, additional external hydrogen can be used to convert all or part of C0₂ present in the synthesis gas to CO in a reverse WGS reactor. In that embodiment C0₂ is separated from synthesis gas and recycled to a reverse WGS reactor where C0₂ is reacted with external hydrogen to produce CO and water. In that embodiment, the capacity of the process is increased at maximum by 160 %. Both embodiments will naturally reduce investment costs by either eliminating the need for a separate Water Gas Shift reactor or greatly reducing the size of the equipment. Thus, the use of external hydrogen will enhance the chemical bounding of green carbon into the product instead of forming C0₂ which is exhausted into atmosphere. This decreases C0₂ emissions of the whole process compared to a conventional FT process for the production of hydrocarbon from syngas obtained by gasification of biomas. The reduction of carbon dioxide emission can be on the order of 5 to 90 %, in particular about 10 to 80 % based on volume.

On the other hand, at a fixed production rate of the FT reactor ("fixed capacity"), the invention allows for a reduction of gasifier capacity for example depending on the availability of biomass raw-material.

Next the invention will be examined more closely with the aid of a detailed description with reference to the attached drawings, in which

Figure 1 shows in graphical form the amount of feed of external hydrogen vs. FT production capacity increase; Figure 2 shows the process scheme of a first embodiment of the invention; and

Figure 3 shows the process scheme of a second embodiment of the invention.

As discussed above, the present invention concerns a method of producing hydrocarbon compositions by a Fischer-Tropsch reaction from a synthesis gas produced by gasification of biomass. The hydrocarbon compositions are suitable as raw-materials for various hydrocarbon compositions used in the chemical and petrochemical industry. They can be used, for example, as fuels or lubricants. A particularly interesting alternative is to use the hydrocarbons in the production of fuels for combustion engines or jet engines.

The hydrocarbon compositions typically contain linear hydrocarbons having 4 to 90 carbon atoms. There can be some branched hydrocarbons in the product. Primarily the

hydrocarbons are saturated (alkanes) although unsaturated compounds can be included in minor amount of less than 10 mol-%, in particular less than about 5 mol-%. Depending on the catalyst used, some oxygenated hydrocarbons can be formed as impurities in the FT reaction.

In the first step of the process, an organic raw-material is gasified in the presence of oxygen to produce a gas containing carbon monoxide, carbon dioxide, hydrogen and hydrocarbons possibly together with inert components, and tarry compounds and some inorganic impurities, including metal particles. The organic raw-material or feedstock of the process is preferably a material composed of biological matter, i.e. of a matter of vegetable or animal origin. In the present context, the term "biomass" will be used for designating any such raw-material.

A typical feature of the feedstock materials of the present process is that they contain carbon, in particular in excess of about 20 %, preferably in excess of about 30 %, advantageously in excess of about 40 % by dry matter. The biomass feedstock is preferably selected from annual or perennial plants and parts and residues thereof, such as wood, wood chips and particles (saw dust etc), forestry residues and thinnings; agricultural residues, such as straw, olive thinnings; energy crops, such as willow, energy hay, Miscanthous; and peat. However, it is also possible to use various waste materials, such as refuse derived fuel (RDF); wastes from sawmills, plywood, furniture and other mechanical forestry wastes; and waste slurries (including industrial and municipal wastes). Also microorganism residues and wastes are available as biomass feedstock. In addition to said materials of vegetable origin, various animal products such as fats and waxes can be used.

The biomass is generally gasified in a fluidized bed reactor or a circulating fluidized bed reactor (CFB) gasifier in the presence of oxygen at a temperature in the range of about 700 to 1200 °C, preferably gasification is carried out in medium- high temperature range of about 750 to 950 °C or 750 to 900 °C. The circulating bed is formed by a granular or particulate bed material, such as aluminosilicate (e.g. sand) or a similar inorganic material. CaO, which can be obtained by introducing Ca carbonate into the gasification reactor, is used as a catalyst for the decomposition of tars in the gasification. The biomass can be in the form of particles, granules or chips or similar coarse or finely divided parts. According to one embodiment, the biomass can be used roughly as such as harvested. According to another embodiment, the biomass is milled or grinded to an average particle or granule size of less than about 50 mm, preferably less than about 40 mm, in particular about 25 to not more than 1 mm before gasification. The biomass can also be fed into the gasifier in the form of a liquid stream, e.g. a liquid stream obtained by pyro lysis of biomass. Such pyrolysis products include charcoal and tars.

In the case of solid biomass, it is typically fed into the reactor with a moisture content of less than 30 % by weight, preferably less than 25 % by weight, for example about 5 to 20 % by weight.

Gasification can be promoted by feeding steam, air or oxygen into the reactor, particularly advantageous results being obtained with oxygen and oxygen in combination with steam.

Depending on the biomass and the temperature and on the concentration of oxygen, the "carbon conversion", i.e. conversion of elemental carbon contained in the raw-material into light compounds, hydrocarbons and tar, is higher than 70 %, preferably higher than 75 %, in particular in excess of 80 % by weight of the carbon in the raw-material.

Based on the above, by gasification, a gas containing carbon monoxide, hydrogen and carbon dioxide as main components along with some water or steam is produced. The gas is recovered. It can be used in the Fischer-Tropsch process for producing hydrocarbons by reacting carbon monoxide with hydrogen in the presence of a catalyst for converting at least a significant part of the carbon monoxide and hydrogen contained in the gas into a hydrocarbon composition containing C4-C90 hydrocarbons.

According to an embodiment of the invention, the hydrocarbon composition thus obtained is recovered and subjected to further processing for use, for example, as a fuel or lubricant for combustion engines, or even for jet engines. The fuel may be, for example, LPG (liquefied petroleum gas), gasoline, diesel or any jet fuel.

In case of waxes and similar hydrocarbons which are solid or semi-solid at ambient temperature and, generally, and also in case of any high-molecular weight hydrocarbons, the FT hydrocarbon composition is preferably further processed by hydrogenation with hydrogen gas at an increased temperature in the presence of a catalyst in order to produce a hydrocarbon composition suitable as a diesel class hydrocarbon or as composition from which such a hydrocarbon can be produced. Typically, hydrogenation with hydrogen gas is performed at a temperature of about 220-270 °C in a fixed bed reactor. The catalyst is typically a supported or unsupported metal catalyst, e.g. nickel on carbon.

After the hydrogenation, preferably, an isomerization step is performed to produce paraffinic hydrocarbons and similar composition for use as fuels.

Hydrocarbon compositions suitable for fuel applications have distillation cut points in the range of about 150 to 300 °C, in particular 180 to 240 °C. The carbon numbers of such compositions are in the range of 10 to 25. Lubricant compositions can be obtained from the FT product of the instant invention. Typically such compositions have carbon numbers in the range of 30 to 40.

In a conventional gasification reactor, a product gas exhibiting a molar ratio of hydrogen to carbon monoxide of 0.5 to 1.5 is produced. In particular, gasification of a wood, annual plant or peat raw-material will upon gasification in the presence of oxygen gas yield a product gas in which the molar ratio of hydrogen to carbon monoxide is about 0.8 to 1.1. In practice, the molar ratio of hydrogen-to-carbon monoxide needs to be raised to about 2 before the FT reaction. For this reason, there is a need for a separate step in which the ratio is increased, said step being carried out at the latest immediately before the Fischer- Tropsch reaction.

In a preferred embodiment, the molar ratio of hydrogen-to carbon monoxide is increased by introducing fresh hydrogen into the gas before the gas is fed into the Fischer-Tropsch reactor.

According to a particularly preferred embodiment, fresh hydrogen is introduced at a point immediately before the Fischer-Tropsch reactor in order to raise the hydrogen-to carbon monoxide ratio of the gas to about 2.

The fresh hydrogen is derived from an external source of hydrogen. By the term "external source" is meant a source which is not an integral part of the conventional processing steps of the process. Conventionally, hydrogen can produced from the gasifier gas by a water gas shift reaction (WGS) in which some of the carbon monoxide is sacrificed for producing hydrogen by reducing water (steam) with carbon monoxide to liberate hydrogen from the water which oxidizing the carbon monoxide into carbon dioxide. This process step will increase the proportion of carbon dioxide which has to be withdrawn from the gas stream which eventually will be fed into the FT reactor. In the present invention, at least a part of the hydrogen is obtained from another source than a WGS reactor. As will be discussed more in detail below, in one embodiment, the present invention allows for the elimination of a WGS reactor totally, and in other embodiments, the required hydrogen production capacity thereof can be greatly reduced. Thus, less than 20 mole-%, preferably less than 10 mole-%, in particular less than 5 mole- % of the carbon monoxide produced the biomass raw-material is used for producing hydrogen gas for use in the Fischer-Tropsch reactor.

Preferably, the external hydrogen fed (mol/h) in relation to the carbon dioxide from the gasifier (mol/h) is as follows (Figure 1): Generally the ratio is 0.5: 1 to 6: 1, in particular 0.9: 1 to 4:1. At a value of 0.5:1 (a capacity increase of 40 %) no WGS is used and C0₂ is being converted to CO. At a value of 4: 1 (a capacity increase of 160 %), the hydrogen is fed in a volume sufficient for converting all C0₂ to CO. One particularly interesting source of hydrogen is formed by natural gas, although other sources of light hydrocarbons, in particular methane, such as landfill gas, biogas, hydrogen produced by bioelectricity (i.e. electricity produced by use of renewable energy resources) and methane hydrate are also possible.

Generally, any external hydrogen source which comprises hydrogen produced by electricity in particular without emission of carbon dioxide and other greenhouse gases is particularly interesting. It should be pointed out that during electrolytic production of hydrogen, considerable volumes of oxygen gas of high purity are obtained. This oxygen can be employed in the gasification of the biomass and in reformation of the gasification effluent.

Depending on the desired composition of the hydrogen feed gas, the source of methane and other light hydrocarbons can also be subjected to reformation optionally in combination with a shift reaction.

Natural gas is a very clean source of methane. It typically contains up to 98 vol-% methane or even more, the balance being formed by ethylene and C₃ and C₄ alkanes. As a feed for clean gas catalytic reformation (i.e. catalytic reformation essentially in the absence of catalyst poisons such as particles and sulphide and amine compounds) natural gas is highly suitable.

A typical reactor set up includes at least one reformer and to at least one shift reactor, said reactor units being placed in the indicated order in a cascade.

In the reformer, reaction 1 takes place, and in the shift reactor, reaction 2 takes place:

CH₄ + H₂0 ^ CO + 3H₂ (1)

CO + H₂0 ^ C0₂ + H₂ (2)

By the reforming and/or shift reactions, methane and other light hydrocarbons are therefore first converted to hydrogen and carbon monoxide by reaction of methane with steam (reaction 1), and then more hydrogen is produced from the carbon monoxide by reacting it with steam to yield carbon dioxide and hydrogen (reaction 2). Reformation can also be carried out in the presence of oxygen.

As will be evident, by merely subjecting the source of methane and other light

hydrocarbons to reformation, a product mixture is obtained having a hydrogen-to-carbon monoxide molar ratio of 3 : 1. This may be sufficient for raising the hydrogen-to-carbon monoxide ratio flow of the syngas produced by biomass gasification to a value in the range of 2. The product mixture of hydrogen and carbon monoxide is interesting also because the carbon monoxide is one of the essential components of the FT feed.

By subjecting the reformation effluent to a shift reaction, more hydrogen is obtained along with carbon dioxide. Although the concentration of carbon dioxide in the FT feed should be restricted, a carbon dioxide can be used for producing carbon monoxide by a revered shift reaction as disclosed herein. Therefore the effluent of a reaction cascade comprising a reforming unit and a shift unit can be fed into a reversed water gas shift reactor.

Generally, the catalytic reformation for converting methane into hydrogen can be used for replacing a hydrogen unit, as explained in connection with the embodiment of Figure 3. However, a reformer can also be incorporated into the process either as a part of the reformation carried out for raw syngas or as a separate reformation of purified, i.e. clean syngas. The latter is typically obtained by removing impurities, such as gaseous compounds selected from hydrosulphide (H₂S), ammonia (NH₃), hydrochloride (HC1), hydrogen cyanide (HCN) and particles all of which may act as catalyst poisons for the FT process.

In one embodiment, the external hydrogen is fed directly into a reformer or into a reversed water gas shift reactor or into both. In one embodiment, the present invention comprises a combination of low- to moderate- temperature gasification (750 to 950 °C) followed by catalytic reforming of the raw syngas produced by the gasification. Such an embodiment may comprise the following steps: - gasification of the raw-material in the presence of oxygen to produce a gas containing carbon monoxide, carbon dioxide, hydrogen and hydrocarbons possibly together with inert components;

- conducting the gas obtained by gasification of the raw-material into a reformer;

- reforming the gas in the presence of oxygen in order to increase the molar ratio of hydrogen to carbon monoxide in a gaseous effluent of the reformer to a value in the range of 0.5 to 1.5;

- withdrawing the gaseous effluent from an outlet of the reformer; and

- further increasing the hydrogen-to-carbon monoxide ratio of the gaseous effluent to a value of about 2 by introducing fresh hydrogen therein.

In an alternative of the above embodiment, gasification is carried out at a first temperature and reforming at a second temperature, which is higher than the first temperature. For example, the high-temperature reforming can be carried out at a temperature in excess of 1000 °C, preferably about 1050 - 1250 °C, typically without a catalyst. Autothermal reformation, on the other hand, is a catalytic process, not thermal, and is carried out at temperatures of 900 - 1300 °C, typically at 1200 °C.

In another alternative of the above embodiment, reforming is carried out in the presence of a catalyst, at excess temperatures of about 500 - 900 °C. This is possible since the temperature of the reformation can be kept lower or equal to the temperature of the gasification when catalytic reformation is used.

Still a further embodiment comprises high-temperature gasification. For example such an embodiment may comprise the steps of

- gasification of the raw-material in the presence of oxygen at a temperature in

excess of 1000 °C to produce a gas containing carbon monoxide, carbon dioxide, hydrogen and hydrocarbons possibly together with inert components; and

- further increasing the hydrogen-to-carbon monoxide ratio of the gas effluent to a value of about 2 by introducing fresh hydrogen into the gas.

In any of the above embodiments, carbon dioxide needs to withdrawn from the gas before it is fed into the Fischer-Tropsch reactor. The carbon dioxide concentration of the syngas fed into a FT reactor is 1 to 10 %, and typically no higher than about 3 %, as mentioned above. Instead of purging the carbon dioxide into the ambient it is preferred, for the purpose of an embodiment of the present invention, to recover the carbon dioxide.

Carbon dioxide can be withdrawn from the gas at any point from or downstream any gas treatment process arranged before the Fischer-Tropsch reactor. Many gas treatment processes include hydrolysation reactors, washing units, units for removing water and hydrogen sulphide and purging beds for other impurities, such as HC1 and carbonylic compounds. Carbon dioxide can be recovered even from a high-temperature outlet stream of the gasifier or any reformer by, e.g., a metal membrane (a hydrogen cell).

The temperature of the gas subjected to carbon dioxide removal in the above units (except for hydrolysation reactors) is generally less than 100 °C, typically about 20 to 80 °C.

The pressure of the gas effluent of the gasifier and any optional reformer is suitably about 1 to 20 bar (absolute pressure), e.g. about 3 to 10 bar, and it has to be raised about 30 bar before the FT reactor. In certain cases even underpressure, s.o. pressure below air pressure (absolute pressure less than 1 bar) can be used.

There are various means available for separating and washing away of carbon dioxide from gas streams containing carbon dioxide. Thus, carbon dioxide can be separated from the gas for example by membrane, by pressure swing absorption (PSA) or by washing with a liquid, e.g. methanol or amine, capable of absorbing carbon dioxide.

The particular advantage of using a methanol or amine washing unit for recovering carbon dioxide is that the carbon dioxide thus separated from the gas flow is pure and there is no particular need further to purify it, unless the sulphur content is too high. In this case, absorber bed reactors are needed to decrease the sulphur content, e.g. from a level of 100 - 200 ppb of sulphur to a level of 10 - 20 ppb. The carbon dioxide can be recycled completely or partially to a reverse WGS reactor (to be discussed in more detail below), or part of it can be emitted to the ambient.

Conventionally, methanol or amine washing units are expensive and they are in an embodiment of the invention replaced by at least one membrane unit or by at least one pressure swing absorption unit for partial or total removal or recovery of carbon dioxide. There are various PSA masses which are selective for C0₂, hydrogen and water. Molecular sieves for absorption of C0₂ comprises for example aluminosilicates and alkaline earth metals. For adsorbing water, various alumina compounds are commonly used (cf. for example US Patent No. 5 604 047). The feed gas of the PSA unit contains hydrogen, carbon dioxide and carbon monoxide. A temperature and a pressure on the levels indicated above (for example temperature of about 40 °C and pressure higher than 10 bar and up to about 30 bar) are suitable for the PSA absorbers. Another option is to separate carbon dioxide with a selective membrane from the gaseous effluent of the previous unit. Selective membranes of polymeric type based on polyamines and polyimide are commercially available for selective carbon dioxide separation from synthesis gas. For a membrane unit, the temperature and pressure can be on the same level as indicated above for the PSA unit.

The recovered carbon dioxide can be used for forming carbon monoxide. According to one embodiment, at least a part of the carbon dioxide is used for forming carbon monoxide by a reversed water gas shift reaction by use of external hydrogen.

The reversed water gas shift reaction can be carried out at a temperature generally in the range of about 500 to 1000 °C. In particular the reversed water gas shift reaction can be carried out at a temperature of about 700 to 900 °C. The pressure range is preferably about 1 to 10 bar, a pressure range of about 4 to 8 bar being preferred. Such conditions will favour the reaction of carbon dioxide and hydrogen to yield carbon monoxide and water. The reactions are endothermic which means that the temperature of an adiabatic reactor will drop with about 130 °C during operation. Suitable catalysts are optionally supported iron and nickel metal catalysts.

In another embodiment, external hydrogen is being fed into the gas both in order to increase the hydrogen-to-carbon monoxide ratio of the gas and for forming carbon monoxide by the reversed water gas shift reaction. The molar ratio between the hydrogen fed into the gas related to carbon dioxide from gasifier and that used for forming carbon monoxide and satisfying the desired hydrogen to carbon monoxide ratio of 2, respectively, is in the range 0.5: 1 to 6: 1. in particular 0.9: 1 to 4: 1.

Thus, based on the above, an embodiment of the invention comprises the steps of

- feeding the carbon dioxide together with fresh hydrogen into the gaseous effluent of a reformer or a high-temperature gasifier in order to produce a modified gaseous flow; and

- feeding the modified gaseous effluent into a reaction zone for a reversed water gas shift reaction.

Before the FT reactor there are preferably still at least some guard beds for removing metals and hydrogen sulphide.

One embodiment of the present invention comprises the concept of essentially not using any of the carbon monoxide produced by gasification of the biomass raw-material for producing hydrogen gas for use in the Fischer-Tropsch reactor. Instead a corresponding volume of external, fresh hydrogen gas is introduced into the process. The recovered carbon dioxide can be used for forming carbon monoxide. According to one embodiment, at least a part of the carbon dioxide is used for forming carbon monoxide by a reversed water gas shift reaction by use of external hydrogen.

Therefore, the capacity of the process will be greatly increased. In that embodiment, the capacity of the process is increased at maximum by 160 % (cf. the right-hand side of the graph in Figure 1).

The molar ratio of the fresh hydrogen fed into the gaseous effluent to the carbon monoxide produced by gasification of the biomass raw-material amounts to 2 to 3 in particular 2,4.

Next, the invention will be elucidated with the aid of the attached drawings. The following reference numerals are used: Hydrogen unit 1

Reformer 2; 12

Reversed shift reactor 3; 13

Scrubber 4; 14

C0₂ membrane 5; 15

Gas line 6; 16

CO₂ removal 9; 19

Washer 7; 17

Fischer-Tropsch reactor 8; 18

Clean gas reformer 10

Clean gas shift reactor 11

Turning first to Figure 2, it depicts an external hydrogen unit 1. The hydrogen unit is separated from the actual process as indicated by the hashed line. It can be formed by any source of hydrogen readily available for example, such as feed line for hydrogen produced from natural gas.

Reference numeral 2 signifies a reformer which typically is a catalytic reformer which can be operated at temperature up to about 1000 °C. As explained above, the reformer is for instance a catalytic reactor with solid catalyst beds and provided with feed for oxygen or other gases for enhancing the reformation reactions.

The task of the reformer is to free gas fed into the reformer from tarry compounds and to convert hydrocarbons to synthesis gas components. The feed for gas from a gasifier into a reformer 2 is indicated with an arrow pointing at reformer 2.

The gasifier can be of any conventional type, typically a circulating bed reactor wherein biomass is combusted at increased temperature in the presence of oxygen.

There is a line 6 for release of gases from the Fischer-Tropsch reactor unit 8.

The effluent from the reformer 2 typically contains a product mixture of carbon dioxide, carbon monoxide, water and hydrogen as main components. Depending on the biomass combusted, there are also some sulphuric gases and nitrogen compounds as well as hydrocarbons. The effluent of the reformer 2 is, in the embodiment of the figure, fed into a reversed water gas shift reactor 3. Further, the feed of the shift reactor includes a stream of hydrogen gas from the hydrogen unit 1 along with some recycled gases separated from the gas mixture conducted to the Fischer-Tropsch reactor 8. Although the two or three gas flows can be separately fed into the reversed shift reactor 3 as indicated in Figure 2, it is equally possible to combine the gas flows before the reversed shift reactor 3.

In the reversed shift reactor 3, carbon dioxide and hydrogen, primarily external, fresh hydrogen from hydrogen unit 1 , are used for producing carbon dioxide by a reversed water gas shift reaction (reaction 3)

C0₂ + H₂ <→ CO + H₂0 (3)

The reaction is an equilibrium reaction and by increasing the temperature and proportion of hydrogen and the proportion of recycled carbon dioxide, the production of carbon monoxide will be increased.

The reaction is typically carried out at about 700 to 900 °C. Generally, the reaction can be performed by using a nickel catalyst or another catalyst which is capable of withstanding the reaction conditions prevailing in this "dirty reversed shift" reaction. This expression refers to the fact that the gas fed into the reversed shift reactor can contain considerably high amounts of hydrogen sulphide and other sulphuric compounds (in concentrations from about 10 to several hundred ppms). Typically, steam can be introduced into the reactor to mitigate the risk of catalyst deactivation. The pressure of the reaction is generally 1 to 15 bar, preferably about 5 to 10 bar. The space velocity, GHSV is preferably in the range of about 3,000 to 5,000 1/h, although a broader range of about 1,000 to 10,000 is equally possible.

The gaseous effluent of the reversed shift reactor is withdrawn and conducted through a series of washing units 4 and units for specific removal of carbon dioxide 5. The washing unit 4 of the drawing is for example an amine washer wherein the gas is conducted with an organic amine to bind gaseous impurities, typically sulphuric compounds and other catalyst poisons. As will be understood to a person skilled in the art, the washing unit 4 is optional. The unit for selective carbon dioxide removal 5 can be a methanol washing unit or a membrane unit or a PSA unit, as explained above. The carbon dioxide is circulated via recycle line, but a part thereof can be emitted to the ambient, for example after a further purification step in unit 9.

After unit 5 there can be an optional washer unit 7, for example an amine washer, as shown in the drawing. Finally, the syngas is fed into a Fischer-Tropsch reactor 8 wherein hydrocarbons are synthesized by reacting carbon monoxide and hydrogen. The conditions in reactor are typically: a pressure of about 30 bar and a temperature of about 200 to 250 °C. There is a catalyst comprising an iron or cobalt catalyst. The reactor type can be a slurry-type reactor or a fixed bed reactor, wherein the strongly exothermic FT reaction is controlled by the use of efficient cooling means.

Finally, a FT product (of the above explained kind) comprising a hot, viscous liquid stream of hydrocarbons is recovered. Light volatile gases are separately removed and recycled, as explained above. Figure 3 shows an alternative embodiment, in which the hydrogen unit 1 of Figure 2 has been replaced by a reformer 10 and a shift reactor 11. The reformer 10 can be fed with a source of methane and other light hydrocarbons. Typically such a source is formed by natural gas, which in one embodiment contains at least 98 vol-% CH₄, up to 1 vol-% C₂H6 and up to 0.5 vol-%> C₃ and C₄ alkanes.

In the reformer 10, methane is partially oxidized by an exothermic reaction. The temperature of the reaction is about 800 to 950 °C and the pressure about 5 to 100 bar (abs). A metal catalyst, such as a transition or nobel metal catalyst is typically used. The shift reaction 11 is typically carried out at a temperature generally in the range of about 150 to 400 °C and at a pressure of about 1.5 to 10 bar (abs.). In particular the shift reaction can be carried out in two stages, comprising a first, high temperature shift reaction at about 350 °C and a second, low temperature shift reaction at a temperature of about 180 to 220 °C. Suitable catalysts are various metal oxide catalysts, such as transition metal oxides and mixtures thereof on supports, including iron oxide, chromium oxide and zinc oxide.

As shown in Figure 3, the hydrogen flow containing either carbon monoxide (reforming only) or carbon dioxide (reforming and shift) is preferably fed to the reversed gas shift reactor 13, The operation of units 14, 15, 17, 18 and 19 is the same as in the embodiment of Figure 2.

Finally, some words about Figure 1. Said figure shows the feed of external hydrogen vs. FT production capacity increase. In the figure, the various points stand for the following amounts of added hydrogen (H₂), mol/h, vs. carbon monoxide (CO), mol/h, coming from the gasifier: No WGS point: 0.9: 1, No WGS + full reversed shift: 4: 1.

Claims

Claims:

1. Method of producing a hydrocarbon composition, comprising the steps of

- providing a biomass raw-material;

- gasifying the raw-material in the presence of oxygen to produce a gas containing carbon monoxide, carbon dioxide, hydrogen and hydrocarbons possibly together with inert components;

- separately increasing the hydrogen-to-carbon monoxide ratio of the gas to a value of about 2;

- feeding the gas to a Fischer-Tropsch reactor;

- converting in the Fischer-Tropsch reactor at least a significant part of the carbon monoxide and hydrogen contained in the gas into a hydrocarbon composition containing C4-C90 hydrocarbons; and

- recovering the hydrocarbon composition,

c h a r a c t e r i z e d by

- introducing fresh hydrogen into the gas before the gas is fed into the Fischer- Tropsch reactor.

2. The method according to claim 1, wherein fresh hydrogen is introduced at a point immediately before the Fischer-Tropsch reactor in order to raise the hydrogen-to carbon monoxide ratio of the gas to about 2.

3. The method according to claim 1 or 2, wherein the fresh hydrogen is derived from an external source of hydrogen.

4. The method according to any of preceding claims, wherein the hydrogen is derived from a source selected from natural gas, methane, hydrogen gas produced by bioelectricity and methane hydrate.

5. The method according to claim 4, wherein hydrogen is produced from natural gas or another source of methane and other light hydrocarbons by catalytic reforming.

6. The method according to claim 5, wherein hydrogen is produced from natural gas or another source of methane and other light hydrocarbons in a cascade formed by at least one unit for catalytic reforming and one unit for a water gas shift reaction.

7. The method according to any of claims 1 to 6, comprising the steps of

- feeding the gas obtained by gasification of the raw-material into a reformer;

- reforming the gas in the presence of oxygen in order to increase the ratio of

hydrogen to carbon monoxide in a gaseous effluent of the reformer to a value in the range of 0.5 to 1.5;

- withdrawing the gaseous effluent from an outlet of the reformer; and

8. The method according to claim 7, wherein gasification is carried out at a first temperature and reforming at a second temperature, said second temperature being essentially higher than the first temperature.

9. The method according to claim 7 or 8, wherein reforming is carried out in catalyst bed reformer at a temperature in excess of 850 °C, preferably about 900 - 1200 °C.

10. The method according to any of claims 1 to 6, comprising the steps of

- gasifying the raw-material in the presence of oxygen at a temperature in excess of

1000 °C to produce a gas containing carbon monoxide, carbon dioxide, hydrogen and hydrocarbons possibly together with inert components; and

11. The method according to any of claims 1 to 10, wherein carbon dioxide is withdrawn from the gas before it is fed into the Fischer-Tropsch reactor and used for forming carbon monoxide by a reversed water gas shift.

12. The method according to claim 11, wherein carbon dioxide is withdrawn from the gas downstream any gas washing process arranged before the Fischer-Tropsch reactor.

13. The method according to claim 11 or 12, wherein carbon dioxide is separated from the gas by membrane filtration, by pressure swing absorption or by washing with a liquid capable of absorbing carbon dioxide .

14. The method according to any of claims 11 to 13, wherein significantly all of the carbon dioxide contained in the gas is removed before it is fed into the Fischer-Tropsch reactor and used for forming carbon monoxide by a reversed water gas shift reaction by use of external hydrogen.

15. The method according to any of claims 11 to 13, wherein only a part of the carbon dioxide contained in the gas is removed and used for forming carbon monoxide by a reversed water gas shift reaction by use of external hydrogen.

16. The method according to claim 15, wherein external hydrogen is both fed into the gas in order to increase hydrogen-to-carbon monoxide ratio and used for forming carbon monoxide by reversed water gas shift reaction.

17. The method according to claim 16, wherein the molar ratio between hydrogen and C0₂ fed into the gas and used for forming carbon monoxide, respectively, is in the range of 0.5: 1-6: 1 in particular 0.9: 1- 4: 1.

18. The method according to any of claims 11 to 17, comprising the steps of

- feeding the carbon dioxide together with fresh hydrogen into the gaseous effluent of a reformer or a high-temperature gasifier in order to produce a modified gaseous effluent; and

19. The method according to any of claims 14 to 18, wherein the reversed water gas shift reaction is carried out at a temperature in the range of about 500 to 1000 °C, in particular about 700 to 850 °C.

20. The method according to any of the preceding claims, wherein less than 20 mole-%, preferably less than 10 mole-%, in particular less than 5 mole-% of the carbon monoxide produced the biomass raw-material is used for producing hydrogen gas for use in the Fischer-Tropsch reactor.

21. The method according to claim 20, wherein essentially none of the carbon monoxide produced the biomass raw-material is used for producing hydrogen gas for use in the Fischer-Tropsch reactor.

22. The method according to any of the preceding claims, wherein the molar ratio of the fresh hydrogen fed into the gaseous effluent to the carbon monoxide produced by gasification of the biomass raw-material amounts from 0,55 to 2,4.

23. The method according to any of the preceding claims, wherein the recovered hydrocarbon composition is further treated to produce a fuel or lubricant for combustion engines.

24. The method according to claim 23, comprising producing hydrocarbon compositions suitable for fuel applications having distillation cut points in the range of about 150 to 300

°C, in particular 180 to 240 °C.

25. The method according to claim 23, comprising producing hydrocarbon compositions suitable for lubricant applications, said compositions having carbon numbers in the range of 30 to 40.

26. The method according to any of the preceding claims, wherein the external hydrogen is fed directly into a reformer or into a reversed water gas shift reactor or into both.

27. The use of a method according to any of the preceding claims for decreasing C0₂ emissions of a Fischer-Tropsch process wherein hydrocarbons are produced from syngas obtained by gasification of a biomass raw-material.