WO2015101717A1

WO2015101717A1 - System and method for producing synthetic fuel

Info

Publication number: WO2015101717A1
Application number: PCT/FI2014/051068
Authority: WO
Inventors: Ilkka HANNULA; Esa Kurkela
Original assignee: Teknologian Tutkimuskeskus Vtt Oy
Priority date: 2014-01-02
Filing date: 2014-12-31
Publication date: 2015-07-09
Also published as: FI20145001L

Abstract

A system (1) for producing synthetic fuel (2) comprises a synthesis gas supply system (4) configured to supply initial synthesis gas (5);a water-gas shift reactor (6) configured to receive a portion of the initial synthesis gas and to increase the hydrogen-to- carbon monoxide ratio thereof, thereby producing shifted synthesis gas (9);a bypass arrangement(8) configured to lead a portion of the initial synthesis gas past the water-gas shift reactor and to combine it with the shifted synthesis gas to form final synthesis gas (11); and a synthesis island (12) configured to receive the final synthesis gas, and to convert it to a synthetic fuel (2). According to the invention, the system further comprises an external hydrogen supply system (10) configured to supply external hydrogen to increase the hydrogen-to-carbon monoxide ratio of the final synthesis gas (11); and a bypass control arrangement (13, 14) configured to control, during the operation of the system (1), the portion of the by- passed initial synthesis gas (7) on the basis of the amount of the external hydrogen supplied.

Description

SYSTEM AND METHOD FOR PRODUCING SYNTHETIC FUEL

FIELD OF THE INVENTION

The present invention relates to the production of synthetic fuels from carbonaceous feedstock.

BACKGROUND OF THE INVENTION

Synthetic fuels can be produced via thermal gasifica^¬ tion from carbonaceous feedstock such as coal, bio- mass, and household or industrial waste. Gasification is a thermochemical conversion process that turns car^¬ bonaceous feedstock into a gas mixture rich in carbon monoxide and hydrogen, called product gas or synthesis gas (syngas) . Other major compounds include carbon di^¬ oxide, nitrogen, water, methane and a rich spectrum of hydrocarbons. The syngas is subsequently cleaned from impurities and finally used as a source material for a catalytic synthesis process, in which hydrogen and carbon dioxide are reacted in the presence of a cata^¬ lyst to produce hydrocarbons, alcohols or chemicals like methanol, paraffins or methane (also known as synthetic natural gas, SNG) .

The basic processes and many variations thereof for producing synthetic fuels are well known in the art. In the case of biomass as the source material, such processes can be called Biomass-to-Synfuels plants.

Depending on the feedstock, reactor design and process conditions, the produced gas will have a certain in- trinsic hydrogen-to-carbon monoxide (¾/CO) ratio that in some cases can be too low in relation to the stoi^¬ chiometric requirements of the downstream synthesis. Therefore, the hydrogen content shall be adjusted be^¬ fore the actual synthesis process to meet the required ratio for the end product at issue, which ratio typi^¬ cally varies from 1 to 3. Known solutions for the manipulation of the ¾/CO ra^¬ tio comprise the so called water-gas shift reac^¬ tor/process where the following reaction takes place in the presence of a catalyst: CO + H₂0 → CO₂ + ¾ . In this process, part of the initial carbon monoxide in the syngas is consumed and converted into carbon diox^¬ ide. The balance of this slightly exothermic reaction is more on the product side at lower temperatures. Un- der typical conditions, the water-gas shift process produces an excess amount of hydrogen in relation to what is required. However, the final adjustment of the H₂/CO ratio can be achieved via bypassing a certain amount of syngas around the shift reactor so that when the bypass and output stream from the water-gas shift process are once again combined, the resulting H₂/CO ratio of the final syngas matches the desired value.

As an alternative to the water-gas shift process, ad- ditional hydrogen can be supplied from an external source into the syngas. For example, electrolytic hy^¬ drogen from renewable energy sources can be used. By electrolytic hydrogen is meant here hydrogen produced from water by electrolysis, using electricity from a power grid supplied by renewable energy sources, like solar energy or wind power. When electricity produced by renewable energy sources is used, the hydrogen thereby produced can also be considered "renewable hy^¬ drogen". The hydrogen production can be adjusted ac- cording to the supply and demand variations of the grid; the less there are other loads (or the more there is supply) connected to the grid, the more elec^¬ tricity can be available for the electrolysis process. This way the electrolysis process can benefit from the resulting price variations of electricity, thus im^¬ proving the feasibility of the synthetic fuel produc^¬ tion process, and/or to balance the load of the grid. The latter option can also be seen as a way of storage of the renewable energy supplied to the grid.

An example of utilizing renewable hydrogen for the production of methanol is disclosed by Galindo and Badr in "Renewable hydrogen utilization for the production of methanol", Energy Conversion and Management, Volume 48, Issue 2, February 2007, Pages 519- 527. One problem associated with the utilization of renewable hydrogen in liquid fuel production is the non-constant hydrogen production adapted to the grid load variations, or to the low electricity price peri^¬ ods. The variations in the amount of hydrogen produced by electrolysis can be balanced by using an intermedi- ate storage of hydrogen as a buffer covering the hydrogen need during low hydrogen production periods. However, as known, hydrogen storage involves many challenges . Another solution is proposed by Mignard and Pritchard in "On the use of electrolytic hydrogen from variable renewable energies for the enhanced conversion of bio- mass to fuels", Chemical Engineering Research and De^¬ sign, Volume 86, Issue 5, May 2008, Pages 473-487. In the proposed approach, the power of the whole synthet^¬ ic fuel production process is adjusted according to the electrolytic hydrogen available. In other words, during periods of low hydrogen production, the synthesis is run at a reduced output, thus removing the need for intermediate storage of hydrogen. However, the synthetic fuel production plants are typically rather large and expensive systems which should be run at a nominal capacity as much as possible to keep the pro^¬ duction process optimal and economical.

PURPOSE OF THE INVENTION The purpose of the present invention is to provide novel solutions for efficient and flexible utilization of electrolytic renewable hydrogen in a synthetic fuel production process. On the other hand, it is also a purpose of the present invention to provide novel so^¬ lutions for efficient and flexible renewable electric^¬ ity grid load balancing via hydrogen electrolysis.

SUMMARY

The system and the method of the present invention are characterized by what is presented in claims 1 and 6, respectively .

According to a system aspect, the present invention is focused on a system for producing synthetic fuel. By synthetic fuel is meant here a liquid or gaseous fuel converted e.g. from biomass, waste, or coal by means of a conversion process. The system of the present invention comprises a syn^¬ thesis gas supply system configured to supply a gas mixture, which is here called "initial synthesis gas", containing hydrogen and carbon monoxide. Also other elements or compounds, e.g. carbon dioxide, methane, or nitrogen, may be contained in the initial synthesis gas. Hydrogen and carbon monoxide are required for the actual synthesis process finally converting, after various gas conditioning phases, the synthesis gas to a synthetic fuel. Thus, by synthesis gas is meant here a starting material for the later synthesis process.

In one preferred embodiment, the synthesis gas supply system comprises a gasification reactor configured to produce initial synthesis gas by gasifying a carbona- ceous source material. In one embodiment, the gasifi^¬ cation reactor is configured to gasify biomass serving as renewable feedstock for synthetic fuel production. The system also comprises a water-gas shift reactor configured to receive a portion of the initial synthe^¬ sis gas and to increase the hydrogen-to-carbon monox- ide ratio thereof by means of a water-gas shift reac^¬ tion, thereby producing so called shifted synthesis gas. The principles of water-gas shift reactors are well known in the art and apply also to the design and manufacture of the water-gas shift reactor of the pre- sent invention.

According to a principle as such known in the art, the system also comprises a bypass arrangement configured to lead a portion of the initial synthesis gas past the water-gas shift reactor and to combine it with the shifted synthesis gas to form final synthesis gas with a hydrogen-to-carbon monoxide ratio between those of the initial and the shifted synthesis gases. Under typical conditions, the hydrogen-to-carbon monoxide ratio of the shifted syngas exiting the water-gas shift reactor is higher than needed for the conversion of the syngas into a synthetic fuel in the actual syn^¬ thesis process. The bypass arrangement can be used to control the process so that the final synthesis gas, in particular the hydrogen-to-carbon monoxide ratio thereof, meets the specific stoichiometric conditions required for the synthesis process. In other words, the bypass arrangement enables adjustment of the level of the hydrogen-to-carbon monoxide of the final syn- gas.

Also the bypass arrangement can be designed and imple^¬ mented, in general, according to principles known in the art. For example, an initial syngas line can be just divided into two sub-lines, one leading to the water-gas shift reactor and another one bypassing the water-gas shift reactor and later joining later the output from the water-gas shift reactor.

The system of the present invention also comprises a synthesis island configured to receive the final syn^¬ thesis gas, and to convert it to a synthetic fuel. The "island" refers to the fact that the actual production of the synthetic fuel from the final syngas is typi^¬ cally carried out by using various different reactors and other processing equipment. For example, the syn^¬ thesis island can comprise, of course, an actual cata^¬ lytic reactor, but also equipment for product recov^¬ ery, recycle of unconverted syngas, and product up^¬ grading steps. The conversion of the syngas into syn- thetic fuels takes place in the catalytic reactor.

It is important to note that the elements of the sys^¬ tem, as described above, can be designed, manufac^¬ tured, and operated according to the principles as such known in the art. Therefore, no detailed descrip^¬ tion is required here.

On the other hand, as is clear for a skilled person, the system can also comprise equipment, sub-systems, and apparatuses configured to perform also other pro^¬ cessing steps than only those of gasification, water- gas shift reaction, and the actual catalytic synthe^¬ sis. For example, the syngas can be filtrated, re^¬ formed, and cleaned from impurities at various stages of the overall process. These possible further pro^¬ cessing steps can be carried out according to the principles known in the art.

According to the present invention, the system further comprises an external hydrogen supply system config^¬ ured to supply external hydrogen to increase the hy^¬ drogen-to-carbon monoxide ratio of the final synthesis gas; and a bypass control arrangement configured to allow controlling, during the operation of the system, the portion of the bypassed initial synthesis gas on the basis of the amount of the external hydrogen sup- plied so as to meet the stoichiometric requirements for the final syngas. By the "portion of the bypassed initial synthesis gas" is meant the portion of the in^¬ itial synthesis gas led past the water-gas shift reac^¬ tor. This can also be called unshifted synthesis gas to distinguish from the portion of the syngas pro^¬ cessed in the water-gas shift reactor.

The hydrogen supply system may comprise any devices and components suitable for supplying a flow of hydro- gen to the system and combining this flow with the flow of the bypassed initial synthesis gas, the output flow of shifted synthesis gas from the water-gas shift reactor, or the combined flow of those bypass and out^¬ put flows. Such devices and components can comprise e.g. various types of piping, valves, and flow regula^¬ tors. Given the basic principle of the present inven^¬ tion, a person skilled in the art is able to design and implement an appropriate hydrogen supply system according to known principles.

Correspondingly, said bypass control arrangement may comprise any suitable valve means and control equip^¬ ment for monitoring or receiving the amount/rate of the supplied external hydrogen, and controlling the valve means accordingly to adjust the bypassed portion of the initial synthesis gas. The control equipment can comprise e.g. a processor coupled to a memory to perform the actual control operations. In addition, or alternatively, the control arrangement can also com- prise program code which is configured to, when exe^¬ cuted by the processor, to perform the actual control^¬ ling of the bypass flow, e.g. by adjusting an adjusta- ble valve dividing the initial syngas into two sub- flows, one of which is led to the water-gas shift re^¬ actor and another one past it as a bypass flow. Thus, one important principle of the present invention is to combine both the water-gas shift reactor and the supply of external hydrogen, both serving to increase the hydrogen-to-carbon monoxide ratio in the final syngas, into a single system.

On the other hand, as an important feature, the system according to the present invention comprises the dy^¬ namic bypass control arrangement, i.e. a control ar^¬ rangement by which the portion of the bypassed initial syngas can be adjusted on-line, i.e. during the opera^¬ tion of the system, on the basis of the supply of the external hydrogen.

Said combination of the water-gas shift reactor and the external hydrogen supply system, together with the bypass control arrangement configured to allow dynamic control of the hydrogen supply, provide great ad^¬ vantages. First, the combination of two alternative hydrogen supplement systems into a single production system allows selecting and balancing between those two hydrogen addition possibilities. This can be used, for example, to adjust the production of synthetic fuel according to the available supplies of the carbo^¬ naceous source material and the external hydrogen. For example, by replacing part of the hydrogen supplement produced by the water-gas shift process with direct addition of external hydrogen, the total synthetic fuel production can be increased without increasing the carbonaceous source material consumption. This is due to the larger bypass flow, and consequently de^¬ creased consumption of carbon monoxide of the syngas in the water-gas shift reaction CO + ¾0 → CO₂ + ¾, resulting in an increase in the total amount of ¾ and CO in the final syngas.

Alternatively, in comparison with a case where only a water-gas shift reactor is used to increase the hydro^¬ gen content, the same production rate of final syn^¬ thetic fuel is achievable with less carbonaceous source material consumption. One further advantage resulting from combining an external hydrogen supply system into a synthetic fuel production system also having a water-gas shift reac^¬ tor is that when the portion of the syngas that is processed by means of the water-gas shift reactor is decreased, also the amount of undesired carbon dioxide produced in the water-gas shift reaction is reduced.

As a very advantageous feature of the present inven^¬ tion, such adjustment between the proportions of the hydrogen addition by means of the water-gas shift re^¬ actor and via the external hydrogen supply is not only possible to be made fixedly at the design phase of the system, but also dynamically, i.e. continuously during the operation of the system. This allows continuous adjustment and optimization of the synthetic fuel pro^¬ duction process on the basis of the desired process parameters and/or the available supplies of the carbo^¬ naceous source material and/or the external hydrogen. For example, when the external hydrogen is "renewable hydrogen", i.e. hydrogen produced from water by an electrolysis process driven by a variable supply of renewable electricity, the system can be used to con^¬ tinuously adjust the bypass flow of the initial syngas on the basis of the hydrogen production rate. In other words, the more there is available such renewable hy^¬ drogen, the greater can be the bypassed portion of the initial syngas not led to the water-gas shift reactor, and vice versa. As an essential difference to the known solutions utilizing variable supply of renewable hydrogen only to meet the stoichiometry requirements of the final syngas, no intermediate hydrogen storage is required to cover the hydrogen need during the low hydrogen production by the electrolysis process.

As yet another advantage, implementation of the pre^¬ sent invention into a synthetic fuel production plant is straightforward. In comparison with prior art, no modification of the actual synthesis gas supply sys^¬ tem, or the catalytic synthesis island, and the pro^¬ cesses thereof is required. In principle, in addition to combining both hydrogen content addition arrange- ments into a single system, all what is needed is some suitable equipment to enable said dynamic control of the bypass of the initial syngas on the basis of the supplied amount of the external hydrogen. Hence, the present invention is not only possible to be imple- mented in new synthetic fuel production systems in the design phase thereof, but it is also possible to modi^¬ fy an existing production plant so as to make it operate according to the present invention. The operation of the synthetic fuel production system according to the present invention can also be consid^¬ ered from the initial syngas supply point of view. In one embodiment, the synthesis gas supply system is configured to supply the initial synthesis gas at a constant output rate so that the production rate of the synthetic fuel varies along the supply of the ex^¬ ternal hydrogen, as explained above. This embodiment enables driving of the synthesis gas supply system, e.g. a gasification reactor for gasifying biomass or some other carbonaceous feedstock, with a constant output . In another embodiment, the synthesis gas supply system is configured to adjust the output rate of the initial synthesis gas, i.e. the rate at which the initial syn^¬ thesis gas is supplied, on the basis of the amount of the external hydrogen supplied so as to keep the pro^¬ duction rate of the synthetic fuel constant. In other words, when the amount of the available external hy^¬ drogen is known, the required supply of the initial synthesis gas, and the portion thereof for bypass, can be calculated and adjusted accordingly so as to meet the total amount and stoichiometric requirements for the final syngas for the given synthetic fuel type and production rate. What is stated above concerning the features, details, implementation into practice and advantages of the system aspect of the present invention applies, muta^¬ tis mutandis, also to the method aspect of the inven^¬ tion. The same applies vice versa, i.e. what is stated below in the context of the method applies, mutatis mutandis, also to the system aspect of the present in^¬ vention .

The method of the present invention for producing syn- thetic fuel comprises providing initial synthesis gas containing hydrogen and carbon monoxide; receiving a portion of the initial synthesis gas and processing it by means of a water-gas shift reaction to increase the hydrogen-to-carbon monoxide ratio thereof, thereby producing shifted synthesis gas; leading a portion of the initial synthesis gas without processing it by means of the water-gas shift reaction, and combining it with the shifted synthesis gas to form final syn^¬ thesis gas with a hydrogen-to-carbon monoxide ratio between those of the initial and the shifted synthesis gases; and converting the final synthesis gas to a synthetic fuel. Similarly to what was explained above in the context of the system aspect, the method steps above can be carried out by means of equipment and principles as such known in the art.

According to the present invention, the method comprises further supplying external hydrogen so as to increase the hydrogen-to-carbon monoxide ratio of the final synthesis gas; and controlling the portion of the bypassed initial synthesis gas on the basis of the amount of the external hydrogen supplied.

Thus, one key principle in the invention is to combine into a single process the two different hydrogen addi^¬ tion approaches, i.e. the water-gas shift reaction and the supply of external hydrogen, and to control the bypass flow on the basis of the external hydrogen sup^¬ ply.

Depending on the actual equipment construction of the synthetic fuel production plant at issue, the external hydrogen can be supplied to the bypass flow of the in^¬ itial synthesis gas or to the output flow of the shifted synthesis gas from the water-gas shift reac^¬ tor. However, most preferably, the external hydrogen is supplied to the combined flow of the bypass flow and the output flow from the water-gas shift reactor, said combined flow forming the final syngas. This sup- ply of the external hydrogen can take place wherever between said combination of those two sub-flows and the actual conversion of the final syngas into a syn^¬ thetic fuel in a synthesis island. For example, the final syngas formed by combining the bypass flow and the output flow from the water-gas shift reactor can be further processed or conditioned by various treat^¬ ments, after which the external hydrogen can be sup- plied to it as the very last step just before the ac^¬ tual conversion.

In a preferred embodiment, providing the initial syn- thesis gas comprises gasifying a carbonaceous source material. This source material can comprise e.g. bio- mass .

In one embodiment, the initial synthesis gas is pro- vided at a constant output rate so that the production rate of the synthetic fuel varies according to the supply of the external hydrogen.

In another approach, the output rate at which the ini- tial synthesis gas is provided is adjusted on the ba^¬ sis of the amount of the supplied external hydrogen so as to keep the production rate of the synthetic fuel constant . BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to pro^¬ vide further understanding of the invention and constitute a part of this specification, illustrate em^¬ bodiments of the invention and together with the de- scription help to explain the principles of the inven^¬ tion. In the drawings:

Figure 1 illustrates a schematic view of a system for producing synthetic fuel according to one embodiment of the present invention; and

Figure 2 illustrates, as a flow chart, a method according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The system illustrated in Figure 1 is configured to produce synthetic fuel, here exemplified by synthetic natural gas SNG, by using biomass 3 as an example of a carbonaceous source material. The biomass is gasified in a gasification reactor 4, producing at its output initial synthesis gas 5 containing hydrogen ¾, carbon monoxide CO, and some by-products.

A portion of the synthesis gas flow from the gasifica^¬ tion reactor is led to a conditioning station comprising a water-gas shift reactor 6.

In the water-gas shift reactor, the initial synthesis gas is conditioned in the presence of a catalyst and steam in such a way that part of the carbon monoxide is consumed and the hydrogen content of the gas is in- creased according to the following reaction: CO + ¾0 → C0₂ + ¾.

The portion of the synthesis gas flow from the gasifi^¬ cation reactor which is not led to the water-gas shift reactor 6 is led as a bypass flow 7 past the water-gas shift reactor. The system comprises adjustable valve equipment 8 to divide the initial synthesis gas flow from the gasification reactor to a flow to the water- gas shift reactor flow and to the bypass flow. After the water-gas shift reactor, the bypass flow of the unshifted syngas is again combined with the flow of the conditioned, i.e. shifted, syngas 9 coming from the output of the conditioning station/water-gas shift reactor 6.

The system further comprises an external hydrogen line 10 to supply a flow of external hydrogen to the system in order to increase the hydrogen-to-carbon monoxide ratio of the final synthesis gas. In the example of Figure 1, the external hydrogen is supplied to the combined flow of the shifted syngas from the output of the water-gas shift reactor and the bypassed portion of the unshifted syngas. Alternatively, it could be supplied e.g. to the bypass flow 7.

The combined flow of the bypassed initial syngas and the shifted syngas forms a flow of final syngas 11 which, after having been supplemented by the flow of external hydrogen, is led to an actual synthesis is^¬ land 12 where the syngas is converted into the desired synthetic fuel.

The system thus enables the increasing of the hydrogen content of the final synthesis gas via two processes: by conditioning the initial syngas by means of a wa^¬ ter-gas shift reactor, and by direct supply of exter- nal hydrogen. To balance the overall system between these two alternatives and to meet the stoichiometric requirements for the final syngas, the exemplary sys^¬ tem of Figure 1 comprises a control unit 13 configured to receive the supply rate of the external hydrogen, measured by a flow meter 14, and to control the ad^¬ justable valve equipment 8 so as to adjust the bypass flow 7 appropriately. This way, the system can be op^¬ erated at various hydrogen supply rates/amounts. Said adjustability is utilized in the example illus^¬ trated in Figure 1 so that the system 1 for producing synthetic fuel is connected to a plant 15 for produc^¬ ing hydrogen from water 16 by means of an electrolysis process. The hydrogen production plant is driven by electricity from a power grid 17 supplied by renewable energy produced by a wind farm 18. Electricity from such a grid is preferably used for hydrogen production during periods of ample electricity supply, low over^¬ all load connected to the grid, and/or low price of the electricity. Adjusting the hydrogen production this way means substantial variation in the hydrogen supply rate from the hydrogen production plant 15 to the synthetic fuel production system 1. The system of the present invention provides a way to utilize such variable hydrogen supply efficiently, without the need to store the excess hydrogen during periods of high production. On the other hand, during low hydrogen production, the synthetic fuel production system can be adjusted to replace part of the external hydrogen by simply increasing the portion of the initial syngas led to the water-gas shift reactor and decreasing the bypass flow accordingly.

When the hydrogen supply is increased, the portion of the initial syngas which is processed by means of the water-gas shift reactor is accordingly decreased. Then, if the supply of initial syngas is kept con^¬ stant, the overall amount of hydrogen and carbon mon^¬ oxide in the final syngas is increased. This is due to the decreased consumption of carbon monoxide in the water-gas shift reaction. Thereby, the total produc- tion rate of the synthetic fuel is increased. Alterna^¬ tively, if the synthetic fuel production rate is de^¬ sired to be kept constant, it is possible to reduce the supply of the initial syngas, e.g. by operating the gasification at a lower output. Naturally, when adjusting the initial syngas output from the gasifier, the bypass flow shall be adjusted differently than in the case of constant supply of initial syngas.

The flow chart of Figure 2 summarizes the steps of an exemplary method according to the present invention. The method begins with gasification of biomass or some other carbonaceous source material so as to produce initial syngas in the form of a gas mixture containing hydrogen and carbon monoxide. A portion of this ini- tial syngas is processed by means of a water-gas shift reaction to form shifted syngas having an increased hydrogen content. The other portion is not processed by means of the water-gas shift reaction, and it is combined with the shifted syngas to form final syngas with a hydrogen content between the initial syngas and the shifted syngas.

External hydrogen is supplied so as to increase the hydrogen-to-carbon monoxide ratio of the final syngas, wherein the bypass portion of the initial syngas, i.e. the portion not processed by means of the water-gas shift reaction, is controlled, i.e. adjusted, on the basis of the hydrogen supply rate so as to meet the stoichiometric requirements for the final synthesis gas . Finally, the final syngas is converted into a synthet^¬ ic fuel by a catalytic synthesis process.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The in^¬ vention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

Claims

1. A system (1) for producing synthetic fuel (2), the system comprising:

- a synthesis gas supply system (4) configured to sup- ply initial synthesis gas (5) containing hydrogen and carbon monoxide;

- a water-gas shift reactor (6) configured to receive a portion of the initial synthesis gas and to increase the hydrogen-to-carbon monoxide ratio thereof by means of a water-gas shift reaction, thereby producing shifted synthesis gas (9);

- a bypass arrangement (8) configured to lead a por^¬ tion of the initial synthesis gas past the water-gas shift reactor and to combine it with the shifted syn- thesis gas to form final synthesis gas (11) with a hy^¬ drogen-to-carbon monoxide ratio between those of the initial and the shifted synthesis gases; and

- a synthesis island (12) configured to receive the final synthesis gas, and to convert it to a synthetic fuel (2);

characteri zed in that the system further com^¬ prises :

- an external hydrogen supply system (10) configured to supply external hydrogen to increase the hydrogen- to-carbon monoxide ratio of the final synthesis gas (11) ; and

- a bypass control arrangement (13, 14) configured to control, during the operation of the system (1), the portion of the bypassed initial synthesis gas (7) on the basis of the amount of the external hydrogen sup^¬ plied so as to meet the stoichiometric requirements for the final syngas.

2. A system (1) as defined in claim 1, wherein the synthesis gas supply system comprises a gasification reactor (4) configured to produce initial synthesis gas by gasifying a carbonaceous source material (3) .

3. A system as defined in claim 2, wherein the gasification reactor (4) is configured to gasify biomass (3) .

4. A system (1) as defined in any of claims 1 to 3, wherein the synthesis gas supply system (4) is config^¬ ured to supply the initial synthesis gas (5) at a con^¬ stant output rate so that the production rate of the synthetic fuel varies along the supply of the external hydrogen .

5. A system (1) as defined in any of claims 1 to 3, wherein the synthesis gas supply system (4) is config- ured to adjust the output rate of the initial synthe^¬ sis gas (5) on the basis of the amount of the external hydrogen supplied so as to keep the production rate of the synthetic fuel constant.

6. A method for producing synthetic fuel (2), the method comprising:

- providing initial synthesis gas (5) containing hydrogen and carbon monoxide;

- receiving a portion of the initial synthesis gas and processing it by means of a water-gas shift reaction to increase the hydrogen-to-carbon monoxide ratio thereof, thereby producing shifted synthesis gas (9);

- leading a portion of the initial synthesis gas (7) without processing it by means of the water-gas shift reaction, and combining it with the shifted synthesis gas (9) to form final synthesis gas (11) with a hydro^¬ gen-to-carbon monoxide ratio between those of the initial and the shifted synthesis gases; and

- converting the final synthesis gas (11) to a syn- thetic fuel (2);

characteri zed in that the method further com^¬ prises : - supplying external hydrogen (10) so as to increase the hydrogen-to-carbon monoxide ratio of the final synthesis gas (11); and

- controlling the portion of the bypassed initial syn- thesis gas (7) on the basis of the amount of the ex^¬ ternal hydrogen supplied.

7. A method as defined in claim 6, wherein providing the initial synthesis gas (5) comprises gasifying a carbonaceous source material (3) .

8. A method as defined in claim 7, wherein the carbo^¬ naceous source material comprises biomass (3) .

9. A method as defined in any of claims 6 to 8, where^¬ in the initial synthesis gas (5) is provided at a con^¬ stant output rate so that the production rate of the synthetic fuel (2) varies along the supply of the ex^¬ ternal hydrogen (10) .

10. A method as defined in any of claims 6 to 8, wherein the output rate at which the initial synthesis gas (5) is provided is adjusted on the basis of the amount of the external hydrogen (10) supplied so as to keep the production rate of the synthetic fuel (2) constant .