Field of the invention

The invention relates to a system and a method for producing synthesis gas by pyrolyzing biomass.

Background of the invention

Due to the increased focus on reducing the carbon dioxide emissions from the combustion of fossil fuels for producing power for e.g. households, biomass has become an increasing source of power for generating clean gas to be used in e.g. turbines and engines and thereby produce power. This is usually achieved by pyrolysis of biomass material and afterwards gasification. Biomass material comprises organic material (e.g. cellulose, hemicellulose and lignin) and inorganic material (e.g. ash) and, depending on the content of these, the pyrolysis temperature is different. However, for optimal pyrolysis of all the biomass, high temperatures are usually used in the process which in turn results in a considerable production of tar. Tar is most often considered an undesirable bi-product of the process as it may clog or break down the subsequent engines, turbines, filters or other. Therefore, it is important to produce synthesis gas with a minimum of tar content.

Thus, from US 9,518,227 B2 it is known to perform a multistage pyrolysis wherein fractions of the biomass are pyrolyzed at different temperatures for optimal pyrolysis of each fraction of the biomass material and each volatile product from the pyrolysis is removed before the pyrolysis of another fraction takes place such that tar production is minimized.

However, producing clean gas according to the prior art is relatively complex and not particularly efficient. It is therefore an objective of the present invention to provide for a more efficient process of producing clean synthesis gas. The invention

The invention provides for a method for producing synthesis gas. The method comprises the steps of: feeding biomass with an ash content above 3% by mass to a pyrolyzer arranged to pyrolyze biomass with an ash content above 3%, wherein said ash content is determined according to ASTM El 755 -01 “Standard method for the determination of ash in biomass” of 2003 Annual Book of ASTM Standards, vol. 11.05, (Philadelphia, PA), feeding pyrolysis gas from said pyrolyzer arranged to pyrolyze biomass with an ash content above 3% to a partial oxidation reactor to produce partially oxidized pyrolysis gas, feeding biomass with an ash content below 2% by mass to a pyrolyzer arranged to pyrolyze biomass with an ash content below 2% by mass, and wherein pyrolysis gas from said pyrolyzer arranged to pyrolyze biomass with an ash content below 2% is also fed to said partial oxidation reactor to produce partially oxidized pyrolysis gas, wherein the ash content is determined according to ASTM E1755-01 “Standard method for the determination of ash in biomass” of 2003 Annual Book of ASTM Standards, vol. 11.05, (Philadelphia, PA), feeding coke produced from said biomass with an ash content below 2% by mass from said pyrolyzer arranged to pyrolyze biomass with an ash content below 2% by mass to a gasifier arranged to gasify coke produced from biomass with an ash content less than 2% by mass, feeding said partially oxidized pyrolysis gas through said coke being produced from biomass with an ash content below 2% in said gasifier arranged to gasify coke produced from biomass with an ash content less than 2% by mass such that tar is removed from said partially oxidized gas and whereby synthesis gas is produced. Coke produced from biomass with ash content above 3% by mass can usually not withstand the high temperature necessary to run an efficient gasification process without melting and forming solid material (also called sintering) which may clog the gasifier system and/or severely reduce the efficiency of the gasifier. However, coke produced from biomass with ash content less than 2% is capable of withstand higher temperatures without sintering and it is therefore advantageous to use this type of coke for cleaning gas.

It is therefore advantageous to produce pyrolysis gas from biomass with ash content above 3% by mass and feed the pyrolysis gas to a partial oxidation reactor and afterwards through a gasifier arranged to gasify coke produced from biomass with ash content less than 2% by mass to bring the partially oxidized gas into contact with this type of coke during gasification, in that the synthesis gas is produced in a more efficient way.

Furthermore, this is advantageous in that coke produced from biomass with ash content less than 2% may withstand high gas temperatures without resulting in sintering problems as would usually be a problem if coke produced by biomass with ash content above 3% by mass were used at this temperature range (this type of coke is typically ideal for temperatures in the range of between 500 and 800 degrees Celsius). Furthermore, coke produced from biomass with ash content less than 2% is more efficient in cleaning pyrolysis gas than coke produced from biomass with ash content above 3% and this type of coke may clean more pyrolysis gas than what is generated in the production of the coke produced from biomass with ash content less than 2%.

It is advantageous to provide the step of feeding biomass with ash content less than 2% by mass to a pyrolyzer arranged to pyrolyze biomass with ash content less than 2% and wherein the pyrolysis gas from the this pyrolyzer is fed to the partial oxidation reactor in that the range of available biomass is increased whereby more synthesis gas may be produced. It is advantageous to provide the coke from biomass with ash content less than 2% from the this pyrolyzer in that the the coke generate herein is not wasted and also because the supply of coke produced from biomass with ash content less than 2% to the coke gasifier versus the operating speed of the pyrolyzer may be precisely linked such that the system is more efficient.

Also, it is advantageous in that supply of coke produced from biomass with ash content less than 2% from an external source is not necessary.

It is also advantageous that the method comprises pyrolyzing biomass with an ash content of below 2% by mass to produce the coke produced from biomass with an ash content of below 2% by mass feed the coke produced from biomass with ash content below 2% by mass to the gasifier which is arranged for gasifying coke produced from biomass with ash content less than 2% by mass in that the method hereby is self- sustained as regards the supply of coke produced from biomas with ash content less than 2% and because this type of coke may be produced in-situ. Furthermore, pyrolyzing the biomass with ash content less than 2% and feeding the coke produced by biomass with ash content less than 2% is advantageous in that the production of this type of coke may be scaled up or down in order to accommodate the coke demand in the gasifier which gasifies coke produced by biomass with ash content less than 2%.

It should be noted that the term “partial oxidation” should be understood such that some oxygen is added to the pyrolysis gas but not enough to fully combust the pyrolysis gas completely. I.e. according to this aspect enough oxygen is added such that the pyrolysis gas is partly combusted. Furthermore, it should be noted that the oxygen may be added in the form of pure liquid or gaseous oxygen, an oxygen containing compound - such as air or other, a mixture of oxygen and water vapor, a mixture of oxygen and C02 and/or in another form and/or mixed with another gas or vapor. Examples of “biomass with ash content above 3% by mass” could be straw (wheat, barley, rye, oats, grass and rape), rice hulls, nut shells, cotton trash, bamboo, corn, sugarcane and animal waste such as slaughterhouse waste, dairy waste, manure or other type of biomass with ash content above 3% by mass.

Examples of “biomass with ash content less than 2% by mass” could be waste wood, sawdust, wood pellets, wood chips and other type of woody biomass or biomass with ash content less than 2% by mass.

In this context, the term “pyrolyzer” should be understood as any kind of unit capable of running a pyrolysis process, which is a thermochemical decomposition of organic material or fossil fuel at elevated temperatures in the absence of oxygen (or any halogen). It should also be noted that in this context the term “pyrolysis” or “pyrolyzed” also covers torrefaction which is a mild form of pyrolysis at temperatures typically between 200 and 320 °C depending on the specific material being pyrolyzed.

It should be emphasized that the term “gasifier” is to be understood as any kind of device suitable for running a gasification process in which organic or carbonaceous based materials is converted into mainly carbon monoxide, hydrogen and carbon dioxide. This is achieved in the gasifier by reacting the material at high temperatures (typically higher than 700°C). The resulting gas mixture is in this text called synthesis gas but could also be referred to as syngas, product gas, producer gas or other and is itself a fuel.

The pyrolysis reactor and/or gasifier may be a fluid bed reactor (counter-current or co current). It should be emphasized that the term “fluid bed” is to be understood as a device or process where gas, air, steam or another fluid is passed up through the material at high enough velocities to suspend the material and cause it to behave as though it was a fluid. This process is also known as fluidization. Furthermore, the term “gas leading means” should be understood as pipes, ducts, tubes or other kind of gas leader for conducting a gas such as the pyrolysis gas, partially oxidized gas and/or synthesis gas.

The term “coke feeding means” should be understood as different types of conveyors such as belt conveyor, bucket conveyors and screw conveyors. The coke may also be conveyed by gravity, pneumatic force, augers, or other kind of coke feeders. The determination of ash content in biomass is performed according to the method described in ASTM E1755-01 “Standard method for the determination of ash in biomass” of 2003 Annual Book of ASTM Standards, vol. 11.05, (Philadelphia, PA) but may as well be determined according to the method described in “Technical Report NREL/TP-510-42622 January 2008, issue date 17 July 2005, A. Sluiter et al” since these methods are the same and will lead to the same result.

In an aspect of the invention, the method further comprises the step of extracting at least a portion of coke being produced from biomass with an ash content above 3% by mass from said pyrolyzer arranged to pyrolyze biomass with ash content above 3% by mass and feeding said coke being produced from biomass with ash content above 3% by mass to a coke gasifier arranged to gasify said coke being produced from biomass with ash content above 3% by mass.

It is advantageous to extract at least a portion of coke produced from biomass with ash content above 3% from the pyrolyzer for pyrolyzing biomass with ash content above 3% and feeding it to a separate gasifier in that this separate gasifier can better be arranged to handle the difficult coke. In an aspect of the invention, the coke gasifier arranged to gasify said coke being produced from biomass with ash content above 3% by mass is arranged in continuation of said gasifier arranged to gasify coke being produced from biomass with an ash content less than 2% by mass.

Feeding this coke to a coke gasifier for gasifying coke produced from biomass with ash content above 3% in that this coke may contribute to further clean the, now cooler, gas coming from the other gasifier which is arranged for gasifying coke produced from biomass with ash content less than 2%.

As previously mentioned, coke produced from biomass with ash content above 3% may not be suitable for cleaning high temperature gas (around 800-1100 degrees Celsius). However, at temperatures around 500-800 degrees Celsius, coke from biomass with ash content above 3% may be used for cleaning the gas without considerable sintering problems. Thus, it is advantageous to provide the system with a coke gasifier arranged to gasify coke from biomass with ash content above 3% arranged in continuation of the coke gasifier arranged to gasify coke from biomass with ash content less than 2% in that the coke from biomass with ash content above 3% is thereby utilized for further cleaning the (at this stage cooler) gas coming from the coke gasifier for gasifying coke from biomass with ash content less than 2%.

In an aspect of the invention, the partial oxidation reactor is arranged to partially oxidize said pyrolysis gas from said pyrolyzer arranged to pyrolyze biomass with an ash content above 3% and said pyrolysis gas from said pyrolyzer arranged to pyrolyze biomass with an ash content below 2% to produce said partially oxidized pyrolysis gas.

Partially oxidizing pyrolysis gas from both pyrolyzers in the same partial oxidation reactor is advantageous in that it provides for a inexpensive and efficient method. In an aspect of the invention, the pyrolyzer arranged to pyrolyze biomass with an ash content below 2% by mass and said pyrolyzer arranged to pyrolyze biomass with an ash content above 3% by mass are arranged in parallel.

This is advantageous in that the production of coke may thereby run simultaneously with the pyrolysis of the biomass with ash content above 3% by mass.

It should be noted that the “parallel” is to be understood such that the pyrolyzers are operatively arranged in parallel, i.e. they are arranged such that they may operate simultaneously, and the pyrolysis gas is led to the same partial oxidation reactor without intermixing of the pyrolysis gas from the pyrolyzers (i.e. the pyrolyzers arranged for pyrolyzing biomass with ash content less than 2% and above 3%, respectively).

In an aspect of the invention, the partially oxidized gas is cooled by a cooling unit before being feed through said coke in said gasifier.

It is advantageous to provide a cooling unit for cooling the partially oxidized gas in that the temperature of the partially oxidized gas may be precisely controlled prior to entering the gasifier such that the rate of gasification may be controlled. Furthermore, cooling the partially oxidized gas may protect mechanical components and equipment (pipes, grates, valves or other) from damage due to high thermal exposure.

In an aspect of the invention, the partially oxidized gas is cooled at least 100 degrees Celsius.

It is advantageous to cool the partially oxidized gas at least 100 degrees Celsius, in that the rate of gasification in the subsequent gasifier is thereby slowed down sufficiently and such that mechanical components and equipment may be protected. In an aspect of the invention, the partially oxidized gas is cooled at least 150 degrees Celsius.

It is advantageous to cool the partially oxidized gas at least 150 degrees Celsius in that the rate of gasification in the subsequent gasifier is thereby slowed down sufficiently and such that mechanical components and equipment may be protected.

In an aspect of the invention, the partially oxidized gas is cooled by injection of water or steam.

Cooling the partially oxidized gas by injection of water or steam provides an efficient and inexpensive method for cooling the partially oxidized gas.

In an aspect of the invention, a part of said pyrolysis gas from the pyrolyzer arranged to pyrolyze biomass with an ash content above 3% by mass is recirculate back to said pyrolyzer arranged to pyrolyze biomass with an ash content above 3% by mass.

It is advantageous to recirculate a part of the pyrolysis gas from the pyrolyzer arranged for pyrolyzing biomass with ash content above 3% by mass in that this gas may be used to provide the pyrolysis reactor with heat for sustaining the pyrolysis reaction.

It should be noted that the term “recirculation means” should be understood as components arranged to recirculate a part of the pyrolysis gas back to the pyrolysis reactor such as pipes, tubes, ducts in connection with fans, blowers, ejectors for driving the fluid. The ejector medium should be an inert gas (such as steam), which does not react with the product gas.

In an aspect of the invention, the method further comprises the step of feeding said pyrolysis gas from the pyrolyzer arranged to pyrolyze biomass with ash content above 3% to said pyrolyzer arranged to pyrolyze biomass with ash content less than 2%. Feeding the pyrolysis gas from the pyrolyzer arranged to pyrolyze biomass with ash content above 3% to the pyrolyzer arranged to pyrolyze biomass with ash content less than 2% is advantageous in that this pyrolysis gas will aid in running the pyrolysis process in the pyrolyzer arranged to pyrolyze biomass with ash content less than 2% and thereby reduce cost and provide for a more efficient method.

In an aspect of the invention, the method comprises the step of determining the ash content of said biomass according to ASTM E1755-01 “Standard method for the determination of ash in biomass” of 2003 Annual Book of ASTM Standards, vol. 11.05, (Philadelphia, PA), before feeding said biomass to said pyrolyzers.

Determining the ash content of the biomass with an ash content above 3% by mass before feeding it to the pyrolyzer arranged to pyrolyze biomass with an ash content above 3% and determining the ash content of the biomass with an ash content below 2% by mass before feeding it to the pyrolyzer arranged to pyrolyze biomass with an ash content below 2% is advantageous in that this ensures a more efficient method.

In an aspect of the invention, the biomass with an ash content above 3% by mass is straw material, non-wood waste or non-wood biomass slurry.

It is advantageous if the biomass with ash content above 3% by mass is straw material, non-wood waste or non-wood biomass slurry in that this type of biomass is abundant and therefore suitable as a source of energy. Also, this type of biomass usually has an ash content above 3% by mass.

The invention also relates to a system for producing synthesis gas by means of a method according to any of the previously discussed methods. It is advantageous to use such a synthesis gas producing system for producing synthesis gas in that coke produced from biomass with ash content less than 2% may clean the partially oxidized pyrolysis gas more efficiently than e.g. coke from biomass with ash content above 3% and thereby avoid sintering problems and tar in the product gas.

In an aspect of the invention, the system further comprises pyrolysis gas leading means for leading pyrolysis gas from a pyrolyzer arranged to pyrolyze biomass with an ash content below 2% by mass to the partial oxidation reactor.

It is advantageous if the system further comprises pyrolysis gas leading means for leading the pyrolysis gas from the pyrolyzer arranged to pyrolyze biomass with ash content less than 2% by mass to the same partial oxidation reactor in that the pyrolysis gas from the this pyrolyzer may also be used for producing synthesis gas.

Figures

The invention will be described in the following with reference to the figures in which fig. 1. illustrates the concept of the invention, fig. 2 illustrates an embodiment with a co-current coke gasifier arranged to gasify coke produced from biomass with ash content less than 2%, fig. 3 illustrates an embodiment with pyrolyzer arranged to pyrolyze biomass with ash content above 3% and pyrolyzer arranged to pyrolyze biomass with ash content less than 2% in series connection, fig. 4 illustrates an embodiment with a counter-current coke gasifier arranged to gasify coke produced from biomass with ash content less than 2%, fig. 5 illustrates an embodiment with a coke gasifier for gasifying coke produced from biomass with ash content above 3% provided in connection with the coke gasifier arranged to gasify coke produced from biomass with ash content less than 2%, and fig. 6 illustrates an embodiment with recirculation means.

Detailed description

Fig. 1 illustrates the concept of the invention. The concept of the invention is to feed biomass 3 with ash content above 3% by mass to a pyrolyzer 2 for producing pyrolysis gas from biomass with ash content above 3%. This is usually performed under application of e.g. hot steam from a heat source 20 for sustaining the pyrolysis reaction. The heat may be provided by e.g. excess heat generated from combustion of the synthesis gas in a combustion engine. The pyrolysis gas from the pyrolyzer 2 which pyrolyzes biomass with ash content above 3% is fed to a partial oxidation reactor 5 by means of pyrolysis gas leading means 4 for partially oxidizing the pyrolysis gas. The partial oxidation may take place by the addition of e.g. air or steam. In a preferred embodiment, the pyrolysis gas from a pyrolyzer 8 arranged to pyrolyze biomass 17 with ash content less than 2% may be fed to the partial oxidation reactor 5 but in another embodiment, the system does not necessarily comprise a pyrolyzer 8 arranged to pyrolyze biomass with ash content less than 2%. Also, in an embodiment, the pyrolysis gas from the pyrolyzer 8 for pyrolyzing biomass with ash content less than 2% may be fed to a separate cooling unit (not shown) for partially oxidizing only this pyrolysis gas. The partially oxidized gas from the partial oxidation reactor 5 may then be led, via the partially oxidized gas leading means 6, to a cooling unit 11 for cooling the gas before entering the coke gasifier 7 arranged to gasify biomass with ash content less than 2% which is via the coke feeding means 10. The cooling unit 11 may cool the partially oxidized gas by injection of water, air, recirculated synthesis gas or by use of a heat exchanger. The partially oxidized gas, now slightly cooler, is then fed to a coke gasifier 7 arranged for gasifying coke produced from biomass with ash content less than 2% for bringing the partially oxidized gas into contact with this type of coke 9 produced from biomass with ash content less than 2% such that the gasification reaction takes place. The coke 9 contributes to the removal of tar from the gas such that substantially clean synthesis gas may be extracted from the synthesis gas outlet 19. The synthesis gas may then be used in e.g. turbines for generating power.

In this embodiment, the coke 9 produced from biomass with ash content less than 2% is supplied by the pyrolyzer 8 arranged to pyrolyze biomass with ash content less than 2%. However, in another embodiment, the coke 9 produced from biomass with ash content less than 2% may be supplied by an external source such as e.g. an external stock which has coke 9 produced from biomass with ash content less than 2%.

The coke 16 produced by the pyrolysis of biomass with ash content above 3% is then extracted via coke outlet 24. This coke 16 may then be used for treatment of soil or for other purposes.

In this embodiment, the cooling unit 11 is a separate unit. However, in another embodiment, the cooling unit 11 may be integrated in the system e.g. by injecting cooling medium directly into the partially oxidized gas flow via nozzles.

Fig. 2 illustrates an embodiment with a co-current gasifier 7 arranged to gasify coke 9 produced from biomass 17 with ash content less than 2%. In this embodiment the partial oxidation unit 5, cooling unit 11 and coke gasifier 7 arranged to gasify coke produced from biomass 17 with ash content less than 2% are arranged in a single unit. The pyrolyzer 8 arranged to pyrolyzing biomass 17 with ash content less than 2% is directly connected to the upper end of the unit such that the pyrolysis gas may flow directly into it while screw conveyors 23 convey the coke 9 produced from biomass with ash content less than 2% such that it may fall by gravity to the coke gasifier 7 arranged to gasify coke 9 produced from biomass 17 with ash content less than 2%. At the same time, pyrolysis gas from pyrolysis of biomass 3 with ash content above 3% (typically around 600 degrees Celsius at this stage) is conducted from the pyrolyzer 2 which pyrolyzes biomass 3 with ash content above 3% by mass. The coke 16 produced from biomass 3 with ash content above 3% is conveyed by screw conveyors 23 and extracted by means of coke feeding means 15 such that only pyrolysis gas produced from pyrolysis of biomass with ash content above 3% is fed to the partial oxidation reactor 5. The partial oxidation reactor 5 partially oxidizes the pyrolysis gas such that its temperature increases to around 1100 degrees Celsius. Before entering the coke gasifier 7 arranged to gasify coke produced from biomass with ash content less than 2%, the partially oxidized gas is cooled in the cooling unit 11 to around 850-1100 degrees Celsius. In this embodiment, the coke gasifier 7 arranged to gasify coke 9 produced from biomass 17 with ash content less than 2% comprises a grate 21 for suspending the coke 9 and for allowing ashes to fall through the apertures of the grate 21 for extraction via the solid residue outlet 18. The grate 21 may also comprise nozzles for blowing gas (e.g. air) upwards such that the coke bed is fluidized.

It is advantageous to provide the pyrolyzers with screw conveyors 22 in that they may aid the pyrolysis reaction by kneading and treading the biomass as well as convey it. The screw conveyors 23 may also be electrically heated such as to provide heat to the biomass. Furthermore, using screw conveyors 23 is advantageous in that the residence time of the biomass in the pyrolysis reactor may be controlled by e.g. the pitch and/or the rotation speed of the screw conveyor 23. Fig. 3 illustrates an embodiment with pyrolyzer 2 arranged for pyrolyzing biomass with ash content above 3% and pyrolyzer 8 arranged for pyrolyzing biomass with ash content less than 2% in series connection.

In this embodiment, the pyrolysis gas from the pyrolyzer 2 arranged to pyrolyze biomass with ash content above 3% is fed to the pyrolyzer 8 arranged to pyrolyze biomass with ash content less than 2% via the pyrolysis gas feeding means 4 such that this pyrolysis gas may contribute to heating the pyrolyzer 8 and also for producing synthesis gas.

In this embodiment, the partially oxidized gas from the partial oxidation reactor 5 is cooled by injection of e.g. water or steam (as indicated by the arrow). However, in another embodiment, the cooling unit 11 could be a separate unit such as a heat exchanger.

Fig. 4 illustrates an embodiment with a counter-current coke gasifier 7 arranged to gasify coke produced from biomass with ash content less than 2%. In this embodiment, the coke gasifier 7 is arranged as a counter-flow gasifier, i.e. the partially oxidized gas from the partial oxidation reactor 5 is cooled in the cooling unit 11 and fed into the coke gasifier 7 arranged to gasify coke 9 produced from biomass 17 with ash content less than 2% from below the grate 21 such that the coke is fluidized. The synthesis gas is then extracted from the synthesis gas outlet 19 at the upper portion of the coke gasifier 7.

Fig. 5 illustrates an embodiment with a coke gasifier 14 for gasifying coke 16 produced from biomass 3 with ash content above 3% provided in connection with the coke gasifier 7 arranged to gasify coke 9 produced from biomass 17 with ash content less than 2%. The pyrolysis gas from the pyrolysis reactor 2 arranged to pyrolyze biomass 3 with ash content above 3% and pyrolysis gas from the pyrolyzer 8 arranged to pyrolyze biomass 17 with ash content less than 2% is fed to the same common partial oxidation reactor where the gas is partially oxidized resulting in an increase of temperature to around 1100 degrees Celsius. Screw conveyors 23 feed the coke 9 produced from biomass with ash content less than 2% and coke 16 produced from biomass with ash content above 3% to the coke gasifier 7 arranged to gasify coke 9 produced from biomass 17 with ash content less than 2% and the coke gasifier 14 arranged to gasify coke 16 produced from biomass 3 with ash content above 3%, respectively. The partially oxidized gas from the reactor 5 is cooled at the cooling unit 11 such that the temperature is lowered to around 900 degrees Celsius before entering the counter-flow gasifier 7 arranged to gasify coke 9 produced from biomass 17 with ash content less than 2%. At this stage, the gas from the gasifier 7 has further cooled to around 800 degrees Celsius which enables the use of coke 16 produced from biomass 3 with ash content above 3% for further removing tar from the gas. Therefore, in this embodiment, the gas from the coke gasifier 7 is brought into contact with coke 16 in the coke gasifier 14 (also arranged as a counter-flow gasifier) such that the gas is further cleaned from tar. The final synthesis gas is then extracted at the synthesis gas outlet 19.

Fig. 6 illustrates an embodiment with recirculation means 13.

In this embodiment, the pyrolizer 8 arranged to pyrolyze biomass 17 with ash content less than 2% and pyrolyzer 2 arranged to pyrolyze biomass 3 with ash content above 3% as well as the coke gasifier 7 arranged to gasify coke produced from biomass 17 with ash content less than 2% comprise recirculation means 13 for recirculating a part of the pyrolysis gas and product gas, respectively. A part of the pyrolysis gas from the pyrolyzer 8 and pyrolyzer 3 may be extracted and fed through a heat exchanger 22 before being recirculated back into the pyrolyzers 3,8. Heat to the heat exchangers 22 for heating the pyrolysis gas could originate from the combustion of synthesis gas in a combustion engine which would thereby produce heat.

The recirculated synthesis gas from the coke gasifier 7 arranged to pyrolyze biomass 17 with ash content less than 2% may be recirculated back into the partial oxidized gas leading means 6 for cooling the partially oxidized gas originating from the partial oxidation reactor 5.

In this embodiment, the recirculation means 13 comprises an ejector medium in the form of steam. However, the fluid (anywhere in the system) may also be driven via a pump, blower or fan. Also, in another embodiment, ejector mediums may be used for driving the flow of the gas (pyrolysis gas, synthesis gas, partially oxidized gas)

The invention has been exemplified above with reference to specific examples of gasifiers 9, cooling unit 11, gas leading means 6, recirculation means 13 and other. However, it should be understood that the invention is not limited to the particular examples described above but may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims.

List

1. System for producing synthesis gas

2. Pyrolyzer arranged to pyrolyze biomass with ash content above 3% by mass

3. Biomass with ash content above 3% by mass 4. Pyrolysis gas leading means

5. Partial oxidation reactor

6. Partial oxidized gas leading means

7. Gasifier arranged to gasify coke produced from biomass with ash content less than 2% 8. Pyrolyzer arranged to pyrolyze biomass with ash content less than 2% by mass

9. Coke produced from biomass with ash content less than 2% by mass

10. Coke feeding means

11. Cooling unit

12. Pyrolysis gas leading means 13. Recirculation means

14. Coke gasifier arranged to gasify coke produced from biomass with ash content above 3% by mass

15. Coke feeding means

16. Coke produced from biomass with ash content above 3% by mass 17. Biomass with ash content less than 2% by mass

18. Solid residue outlet

19. Synthesis gas outlet

20. Heat source

21. Grate 22. Heat exchanger

23. Screw conveyor

24. Coke outlet