UPON EXTRACTION OF SYNGAS

TECHNICAL FIELD

The present application relates to a method for elimination of formation of dioxins and furans upon extraction of syngas according to claim 1 and system for elimination of formation of dioxins and furans upon extraction of syngas according to claim 10.

BACKGROUND ART

At all kind of incineration or gasification of organic material like municipal solid waste (MSW) or discharged plastics, etc. dioxins and furans are formed in the exhaust fumes. Dioxins and furans are extremely toxic to nature and humans. To clean the fume from waste incineration plants from the toxic dioxins and furans is costly and complicated and the result is questionable. Waste incineration plants also pollutes the atmosphere with carbon dioxide and a row of other unpleasant gases such as NO2.

SUMMARY OF THE INVENTION

An object of the present disclosure is to provide a method and system for elimination of formation of dioxins and furans upon extraction of syngas where the problems with prior art technologies are mitigated or at least alleviated.

The disclosure proposes a method for elimination of formation of dioxins and furans upon extraction of syngas, which comprises the steps of: providing a feedstock comprising organic materials, gasification of the feedstock, thereby forming a process gas flow of gaseous elements comprising carbon monoxide and hydrogen. The method further comprises cooling of the gaseous elements in at least two cooling steps. In the first cooling step, the gaseous elements are cooled from a starting temperature to a first predetermined temperature. In the second cooling step, the gaseous elements are cooled to a second predetermined temperature that is lower than the first predetermined temperature. In the first cooling step, water is provided as cooling medium. In the second cooling step, liquid carbon dioxide and/or water is provided as cooling medium, wherein the liquid carbon dioxide and/or water being used as cooling medium is fed directly into the process gas flow.

The method has a number of advantages.

The method provides for elimination of formation of dioxins and furans upon extraction of syngas in an environmentally friendly way since by means of the cooling being performed in at least two cooling steps, formation of dioxins and furans are eliminated or at least greatly reduced.

The method also provides for elimination of formation of dioxins and furans upon extraction of syngas from a feedstock comprising organic materials, such as any kind of organic waste, in a cost-efficient way since there is no need for sorting and separation of the organic materials of the feedstock prior to running the method.

By using carbon dioxide as cooling medium in the second cooling step, there is less need for handling and treatment of wastewater as compared to using water as cooling medium. By using water as cooling medium in the second cooling step, the same advantages are provided as described above, but with the need for wastewater treatment.

By the proposed method, a waste material, such as plastics waste and car tires, is recycled, instead of ending up as landfill, or even worse ending up in incineration plants where dioxins and furans are formed during incineration.

By the proposed method, syngas is formed in a cost efficient and environmentally friendly way.

According to a further development, the method further comprises the following steps after the second cooling step: cleaning the gaseous elements and extracting syngas from the gaseous elements, allowing carbon monoxide of the syngas to react with water according to the following formula: CO + H2O => H2 + CO2, separating hydrogen and carbon dioxide into two streams of hydrogen and carbon dioxide respectively, liquefying the carbon dioxide, and collecting the liquid carbon dioxide and/or the hydrogen in sealed containers.

Thereby emission of carbon dioxide gas to the atmosphere is prevented in contrast to conventional types of incineration processes. By providing the carbon dioxide in liquid phase, the carbon dioxide becomes easier to handle and to store in a sealed and container.

According to a further development, the method further comprises a third cooling step being performed directly after the second cooling step, wherein in the third cooling step, the gaseous elements are cooled to a third predetermined temperature that is lower than the second predetermined temperature.

The third cooling step provides for a simplified handling of the gaseous elements.

According to a further development, the collected liquid carbon dioxide is provided as cooling medium in the second and/or third cooling step(s) and thus a recirculation of carbon dioxide is achieved, and the carbon dioxide thus, being repeatedly separated from the hydrogen and provided in the second and/or third cooling step(s).

Thus, the collected liquid carbon dioxide is provided in the second cooling step as cooling medium. Thereby the carbon dioxide formed by the method is recirculated and used for cooling in a very efficient way. By using carbon dioxide as cooling medium, there is less need for handling and treatment of wastewater as compared to using water as cooling medium.

According to a further development, in the in the second cooling step, the cooling is performed rapidly during a time period of less than 10 seconds, preferably less than 5 seconds, most preferably less than 3 seconds in order to eliminate or at least greatly reduce formation of dioxins and furans.

By performing the second cooling step rapidly, the formation of dioxins and furans and other toxic elements which typically are formed at temperatures in the range of 500 to 200 °C are eliminated or at least greatly reduced.

According to a further development, the first predetermined temperature being in the range of 650 to 510 °C, preferably being a temperature in the range of 600 to 520°C.

The proposed first predetermined temperature is near, but not within, the temperature range where dioxins and/or furans are formed. Thus, by cooling to a first predetermined temperature being in the range of 650 to 510 °C, preferably to a temperature in the range of 600 to 520°C, typically no dioxins or furans are formed during the first cooling step.

According to a further development, the second predetermined temperature being below 200 °C, preferably below 180 °C, most preferably below 150 °C. At the second predetermined temperature of below 200 °C, furans or dioxins are typically not formed.

According to a further development, the third predetermined temperature being below 100 °C, preferably being in the range of 90 to 20 °C.

The reason for cooling to a temperature of below 100 °C is that water is condensed at temperatures below 100 °C and water soluble substances of the gases elements then are dissolved and separated from the gaseous elements.

According to a further development, the starting temperature being in the range of 1500 °C to 800 °C, preferably the starting temperature is about 1300 °C.

The starting temperature does not provide for any specific advantage, but is the typical temperature of the gaseous elements after gasification.

The disclosure further proposes a system for elimination of formation of dioxins and furans upon extraction of syngas, wherein said system is arranged for elimination of formation of dioxins and furans upon extraction of syngas according to the method.

The system is able to extract syngas according to the proposed method. Thus, the system provides for elimination of formation of dioxins and furans upon extraction of syngas having all of the above-mentioned advantages. In addition, the system may be modular, comprising a number of units, thus the system may be designed and redesigned depending on the system user’s needs.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically illustrates an example of a system for elimination of formation of dioxins and furans upon extraction of syngas according to the method of the present disclosure.

Fig. 2 schematically illustrates the method according to the present disclosure.

DETAILED DESCRIPTION

The present disclosure relates to a method for elimination of formation of dioxins and furans upon extraction of syngas. The method provides for elimination, or at least greatly reduced formation of dioxins and furans. By dioxins, polychlorinated dibenzodioxins (PCDDs), are meant a broad group of organic compounds, that are persistent organic pollutants in the environment. Persistent organic pollutants are also known as “forever chemicals” and are organic compounds that are resistant to environmental degradation through chemical, biological and photolytic processes.

Some of the dioxins are highly toxic. The different toxicity depends on the number of, and of the positions of the chlorine atoms.

Furans are heterocyclic organic compounds comprising a five-membered aromatic ring with four carbon atoms and one oxygen atom. Furans may further comprise fluorine atoms.

Both dioxins and furans are commonly formed as by-products upon burning or in various industrial processes.

Fig. 1 schematically illustrates an example of a system 100 for elimination of formation of dioxins and furans upon extraction of syngas according to the method of the present disclosure.

The system 100 may be modular and comprises a number of different units which will be described more in detail below. The modularity of the system 100 may facilitate design and redesign of the system 100. The system 100 may be tailor-made for example depending on type and size of a feedstock being fed into the system. In one example, the system may be arranged near or as a replacement for a waste incineration plant. Standard conduits for liquids and gases may be used for connecting the different units of the system 100 to each other.

As illustrated in Fig. 1 , the system 100 comprises a gasification unit 2 and a cooling unit 4.

The system 100 may comprise a chopper/shredder unit 1 to which a feedstock, i.e. a raw material, is provided. In one example, the chopper/shredder unit 1 is arranged for chopping/shredding the feedstock into a desired size. The chopper/shredder unit 1 may be used for mixing different types of materials, such as material with lower and material with a higher energy content. The feedstock comprises organic materials.

The feedstock may comprise a mix of material with different energy content, such as MSW, auto shredder residue, ASR, and/or plastics. The reason for mixing material with lower and higher energy content is to provide an efficient gasification process of all material. The system 100 may comprise a plurality of chopper/shredder units. The size and type of the chopper/shredder unit 1 may depend on the size and type of feedstock the system is arranged to process. In a further example, the feedstock is provided in a desired size and/or shape prior to entering the system 100 and a chopper/shredder unit 1 is not needed. In such case, the feedstock may be directly fed into a gasification unit 2.

The system 100 may further comprise a feeding unit (not shown) for feeding the feedstock into a gasification unit 2. The feeding unit may for example be a screw arrangement.

The system comprises a gasification unit 2. The gasification unit 2 may comprise a pyrolysis zone (not shown) and a plasma zone (not shown). The pyrolysis zone may be arranged for gasification of the feedstock in presence of a predetermined amount of oxygen. The oxygen may be provided from an oxygen generator 12. By providing pure oxygen instead of air, it is avoided that the resulting syngas comprises unnecessarily high amounts nitrogen or nitrogen compounds.

The amount of oxygen being supplied depends on the type of feedstock, i.e. on the energy content of the feedstock. In one example, the amount of oxygen provided in the method may be in the range of 0.1 -0.5 kg oxygen per kg feedstock when the feedstock has an energy content of about 17-26 MJ/kg. The lower energy content of the feedstock, the more oxygen needs to be supplied in order to obtain an efficient gasification.

The amount of oxygen provided may be controlled manually or automatically by means of a valve (not shown). The amount of oxygen provided and the temperature upon gasification may depend on the desired degree of gasification of the feedstock. The provision of oxygen is based on the temperature in the gasification unit 2. For example, if the temperature is too low in order to obtain a desired degree of gasification, a higher amount of oxygen is provided in order to maintain a desired temperature in the pyrolysis zone.

The plasma zone may comprise a light arc in which the formation into plasma of the partly gasified feedstock takes place. The light arc itself may have a temperature of about 8000 °C. Due to the very high temperature, the chemical bonds of the compounds of the feedstock are broken and the compounds may be decomposed into plasma. Oxygen and water steam may be provided by the oxygen generator to the plasma zone in order to form carbon monoxide, CO, of carbon being released upon the formation of plasma. The plasma formed in the gasification unit 4 comprises gaseous elements. These gaseous elements comprise a mix of elements, called syngas, also known as synthesis gas.

Syngas is a gas mixture comprising primarily of H2 and CO. Depending on the process in which the syngas is formed, such as the material of the feedstock and temperatures in the pyrolysis zone and the plasma zones of the gasification unit 2, the syngas may comprise small amounts of other gases, such as H2O, CO2, N2 and CH4. As an example, cooled syngas may comprise about 2 % Isl and about 10-15% CO2. Before being cooled, as will be described more in detail below, the syngas may also comprise up to 25 % of H2O. When the syngas is rapidly cooled to temperature of below 200 °C (which will be discussed more in detail below), the formation of dioxins and furans is eliminated and then cooled to a temperature below 100°C the main portion of H2O is condensed and thereby separated from the syngas.

The syngas being extracted by the proposed method may be used as fuel gas as it is. Alternatively, as will be discussed more in detail below, the syngas may be further processed to increase the ratio of hydrogen of the syngas and convert the carbon monoxide into carbon dioxide.

The temperature upon gasification of the feedstock may be in the range of 600-1200 °C, preferably 725-1125 °C, most preferably 850-1050 °C in a first zone and in the range of 2000-3000 °C in the plasma gasification zone.

Upon gasification, there may also be some residual materials formed in the gasification unit, such as inorganic materials, which are condensed before the gas is leaving the plasma zone or are not fully decomposed into plasma. These materials may be collected in a slag collection unit 3. The materials collected in the slag collection unit 3 are typically materials having a relatively high boiling point. Examples of such materials are metals, such as Fe, Cu and Al, and other metallic compounds. The materials collected in the slag collection unit 3 may be recycled and for example be re-used in manufacturing industry.

The system 100 may comprise more than one gasification unit 2 in order to ensure a high utilization of the material of the feedstock.

The system 100 further comprises a cooling unit 4. The cooling unit 4 is arranged to cool the gaseous elements being formed in the gasification unit 2. The cooling unit 4 is arranged to cool the gaseous elements in at least two cooling steps. The cooling unit 4 may comprise a radiation or convection cooler (not shown) and/or a water spray unit 19. In the first cooling step, the gaseous elements are cooled from a starting temperature to a first predetermined temperature, and in the second cooling step, the gaseous elements are cooled to a second predetermined temperature that is lower than the first predetermined temperature. In the first cooling step, water is provided as cooling medium. The water being used as cooling medium in the first cooling step is not fed directly into the process flow and does not come into directly contact with the gaseous elements. As will be discussed below, heat generated by the cooling unit 4 by means of the water in the first cooling step being vaporized may be used as an oxidant substance in the gasification unit 2 and/or for heating other arrangements of the system 100 by means of a closed loop 17. In the second cooling step, collected liquid carbon dioxide may be provided as cooling medium, wherein the liquid carbon dioxide being used as cooling medium is fed directly into the process gas flow (i.e. into the cooling unit 4). Preferably, the collected liquid carbon dioxide is provided as cooling medium and thus a recirculation of carbon dioxide is achieved, and the carbon dioxide thus, being repeatedly separated from the hydrogen and provided in the second cooling step. Alternatively, or in combination with liquid carbon dioxide, in the second cooling step, water is provided as cooling medium, wherein the water being used as cooling medium is fed directly into the process gas flow.

In the second cooling step, the cooling may be performed rapidly during a time period of less than 10 seconds, preferably less than 5 seconds, most preferably less than 3 seconds in order to eliminate or at least greatly reduce formation of dioxins and furans. As will be discussed with reference to the method below, the time periods during which the first and third cooling steps are performed are not critical.

The system 100 may further comprise at least one vessel 5 in which water-soluble elements, such as acid oxides like phosphorus oxide, being dissolved in the water may be collected. The water is provided to the cooling unit 4 by means of the water spray unit 19 being arranged in connection with the cooling unit 4. Water is provided to the system 100 regardless of if water or carbon dioxide is provided as cooling medium in the second and/or third cooling step(s) since the water is needed for dissolving water-soluble elements from the gaseous elements. The water spray unit 19 may further be arranged for providing water as cooling medium to the system 100.

In addition, further water-soluble elements may be dissolved and separated from the process flow by means of a cleaning unit 6 which is discussed more in detail below. In the latter case, the system 100 may comprise a vessel 21 being arranged in connection with the cleaning unit 6 and being arranged for collecting elements being dissolved and separated from the process flow. In one example, the first, second and/or third cooling step(s) are performed in the cooling unit 4. In yet an example, the first and second cooling steps are performed in the cooling unit 4, whereas the optional third cooling step is performed in the cleaning unit 6. In the latter case, the third cooling step is preferably performed by means of water.

The system 100 further comprises a filtering unit 9. The filtering unit 9 may comprise at least one filter, preferably, the filtering unit 9 comprises a plurality of filters. The at least one filter may be arranged to collect particulate compounds. The at least one filter may be a mechanical filter, such as a baghouse filter. The at least one filter may be cleaned by pressurized recirculated syngas at a predetermined time interval, or when needed due to a large amount of particles being collected by the at least one filter (not shown). Particulate compounds being collected by the at least one filter may be collected in at least one vessel 13. For example, poisonous metallic materials such as cadmium, may be collected into one or more slag collection units 13 being connected to the filtering unit 9. The compounds being collected in the at least one vessel 13 depends on the type of feedstock being provided into the system 100.

The system 100 may further comprise a cleaning unit 6. The cleaning unit 6 is arranged for cleaning and purifying of the remaining gaseous elements. In one example, the remaining gaseous elements may be led from the filter unit 9 to the cleaning unit 6 by means of a fan device 14.

The cleaning unit 6 may comprise filters and/or a cleaning liquid which is circulated in the cleaning unit 6 by means of a pump system (not shown). In one example, the gaseous elements are firstly led through one or more filters in order to purify the syngas from undesired compounds. Secondly, the gaseous elements are “showered” by the cleaning liquid. The cleaning liquid may typically be water, H2O. Water and water-soluble compounds may be separated from the gaseous elements. The extracted syngas may be repeatedly circulated within the cleaning unit 6 until the syngas has reached a desired purity.

The system 100 may comprise a plurality of cleaning units 6 arranged after one another. In one example, the plurality of cleaning units 6 may comprise different types of filters and/or cleaning liquids. The syngas resulting from the cleaning unit 6 typically comprises hydrogen gas (H2) and carbon monoxide (CO) in the ratio of approximately 1 :1. In addition, the syngas may comprise small amounts of carbon dioxide gas (CO2), nitrogen gas (N2) and traces of methane gas (CH4). The syngas resulting from the cleaning unit 6 may, but need not, be compressed by a compressor (not shown). The syngas resulting from the cleaning unit 6 may be used as it is for example as fuel gas or it may be exposed to further treatment as discussed below.

The system 100 may further comprise a water-gas-shift unit 7, to which the syngas is fed. In the water-gas-shift unit 7, a controlled amount of water steam is provided, wherein CO of the syngas reacts with water, thereby forming CO2 and H2 according to the following formula:

CO + H ₂ O => H ₂ + CO ₂

The water-gas-shift unit 7 may be a commercially available water-gas-shift unit.

The system 100 may further comprise a pressure swing adsorption, PSA, unit 8. Pressure swing adsorption is a well-known technique used to separate some gas species from a mixture of gases. In the system 100, the pressure swing adsorption unit 8 is arranged for separating CO2 and H2. The resulting H2 being separated by the swing adsorption unit 8 has a high purity, typically in the range of 99.99 %.

The H2 being separated in the PSA unit may be collected in a sealed container 10 and be used in for example industrial processes. The system can for example be arranged at, and connected to, an industrial plant requiring hydrogen in their processes and thereby transport of the hydrogen can be eliminated or reduced.

The CO2 being separated in the PSA unit 8 may preferably be liquefied and be collected in a sealed container 11 . Preferably, the sealed container 11 is thermally insulated in order to keep the carbon dioxide in liquid form at a temperature of about - 79 °C. The carbon dioxide may be liquefied by means of a compressor 20.

As will be discussed with reference to the method below, the collected CO2 may be provided as cooling medium in the second and/or cooling step(s). The liquid carbon dioxide being used as cooling medium is then fed directly into the process gas flow (i.e. the cooling unit 4), and thus a recirculation of carbon dioxide is achieved, and the carbon dioxide thus, being repeatedly separated from the hydrogen and provided in the second cooling step.

The collected CO2 may be subject to carbon capture and utilization, CCU. CCU is a process where carbon dioxide is captured and is recycled for further usage. The aim of CCU is to convert the captured carbon dioxide into e.g. plastics or biofuel in a controlled way, thereby preventing the CO2 from reaching the atmosphere. Alternatively, the CO2 may be subject to carbon capture and storage, CCS, wherein the CO2 may be permanently stored in an underground geological formation, thereby preventing the CO2 from reaching the atmosphere.

In one example about 1/3 of the carbon dioxide being separated in the PSA unit 8 and being collected in the sealed container 11 is used for the above mentioned second and/or third cooling step(s). By providing the collected liquid carbon dioxide as cooling medium in the second and/or third cooling step(s), a recirculation of carbon dioxide is achieved, and the carbon dioxide thus, being repeatedly separated from the hydrogen and provided in the second cooling step. Thus, 100% of the of the carbon dioxide being separated in the PSA unit 8 and being collected in the sealed container 11 may be subject to CCU and/or CCS.

In one example, as shown in Fig. 1 , a closed system 17 may be arranged to circulate a working fluid, such as water, by means of a pump 15 from the cooling unit 4 to a water-gas-shift unit 7 and back to the cooling unit 4. The working fluid may be circulated to different parts of the system, thereby heating and/or cooling different units of the system 100. In yet an example, the working fluid may be used for heating other arrangements, for example arrangements within the building in which the system 100 is arranged Typically, heat is generated in the cooling unit 4 and in the water-gas- shift unit 7.

Vaporized water or steam originating from water being provided to the cooling unit 4 by means of the water spray unit 19, may be used as an oxidant substance in the gasification unit 2.

Heat generated by the system may be transferred from the system to a heat consumer, such as via district heating. In yet an example, the system 100 may comprise a turbine generator 16, preferably being arranged in connection to the closed system 17 in which a working fluid is arranged to circulate. Electricity generated by the turbine generator 16 may be used for operation of the electricity of the system 100, and optionally also for operation of other arrangements of the building within which the system 100 is arranged.

Fig. 2 schematically illustrates the method according to the present disclosure.

The method 200 for elimination of formation of dioxins and furans upon extraction of syngas comprises the step of: providing a feedstock 201 comprising organic materials. By organic materials are meant materials comprising carbon-based compounds. The carbon-based compounds either may be naturally occurring carbon-based compounds, such as sewage sludge, or artificially produced, such as plastics.

The organic materials may comprise discharged plastics and tires, sewage sludge, municipal solid waste (MSW). As mentioned above, the feedstock may further comprise a mix of material having a both higher and lower energy content such as MSW, auto shredder residue, ASR, and/or plastics. ASR comprises for example glass fiber, rubber, automobile liquid and plastics. The reason that the feedstock further may comprise a material having a higher energy content compared to organic material like MSW is to obtain an efficient gasification process. The material, such as sewage sludge, and the material having a higher energy content compared to sludge may, but need not, be mixed in order to form a homogenous mixture, for example in a chopper/shredder unit before gasification of the feedstock.

The method 200 further comprises a step of gasification of the feedstock 202, thereby forming a process gas flow of gaseous elements comprising carbon monoxide and hydrogen. The gasification of the feedstock is described more in detail with reference to the system above.

The method further comprises a step of cooling of the gaseous elements in at least two cooling steps, wherein in the first cooling step 203, the gaseous elements are cooled from a starting temperature to a first predetermined temperature, and in the second cooling step 204, the gaseous elements are cooled to a second predetermined temperature that is lower than the first predetermined temperature.

The starting temperature may be in the range of 1500 °C to 800 °C, preferably the starting temperature is about 1300 °C. The starting temperature is the temperature of the process gas flow of gaseous elements comprising carbon monoxide and hydrogen after the gasification of the feedstock, prior to the cooling of the gaseous elements.

The first predetermined temperature may be in the range of 650 to 510 °C, preferably in the range of 600 to 520°C. Typically, dioxins and furans are not formed at temperatures above 500 °C.

The second predetermined temperature may be below 200 °C, preferably of below 180 °C, most preferably below 150 °C. The reason for cooling to a temperature of below 200 °C is that furans and dioxins typically are not formed below 200 °C.

As noted below, the method may further comprise a third cooling step being performed directly after the second cooling step, wherein in the third cooling step, the gaseous elements are cooled to a third predetermined temperature that is lower than the second predetermined temperature. The third predetermined temperature may be below 100 °C, preferably being in the range of 90 to 20 °C. It should be noted that the gaseous elements may be cooled to an even lower temperature, although no particular advantage is associated with cooling to an even lower temperature.

The method further comprises a step of cleaning 206 the gaseous elements and extracting of syngas from the gaseous elements. As discussed with reference to the system above, syngas, also known as synthesis gas, is a gas mixture comprising primarily of H2 and CO. By the cleaning, the syngas is purified from undesired compounds, especially from undesired water-soluble compounds. The cleaning may be performed as discussed above with reference to the system.

The method may further comprises a step of allowing carbon monoxide of the syngas to react with water 207 according to the following formula: CO + H2O => H2 + CO2.

The method may further comprise a step of separating hydrogen and carbon dioxide into two streams 208 of hydrogen and carbon dioxide respectively.

The method may further comprise a step of liquefying the carbon dioxide 209. This is preferably made by means of a compressor being arranged for compressing the carbon dioxide followed by cooling of the carbon dioxide into liquid carbon dioxide.

The method may further comprise a step of collecting the liquid carbon dioxide and/or the hydrogen in sealed containers. Preferably, the liquid carbon dioxide and hydrogen are collected in separate containers, i.e. the liquid carbon dioxide is collected in at least one sealed container and the hydrogen is collected in another, at least one sealed container.

In the first cooling step, water is provided as cooling medium. In one example, a predetermined amount of water is provided into a heat exchanger being arranged within the cooling unit for cooling of the gaseous elements. Thus, in the first cooling step, water does not come into direct contact with the process flow. By a predetermined amount of water is meant an amount of water needed for cooling the gaseous elements from a starting temperature to a first predetermined temperature.

In the second cooling step, liquid carbon dioxide and/or water is provided as cooling medium, wherein the liquid carbon and/or water being used as cooling medium is fed directly into the process gas flow. For example, the cooling medium may be sprayed into the process flow. The amount of carbon dioxide and/or water being provided is the amount of carbon dioxide and/or water being needed for cooling the gaseous elements to a second predetermined temperature that is lower than the first predetermined temperature.

In the third cooling step, liquid carbon dioxide and/or water may be provided as cooling medium, wherein the liquid carbon and/or dioxide being used as cooling medium is fed directly into the process gas flow. The amount of carbon dioxide and/or water being provided is the amount of carbon dioxide or water being needed for cooling the gaseous elements to the third predetermined temperature that is lower than the second predetermined temperature.

In one example, collected liquid carbon dioxide is provided as cooling medium and thus a recirculation of carbon dioxide is achieved, and the carbon dioxide thus, being repeatedly separated from the hydrogen and provided in the second and/or third cooling step(s). Alternatively, or in combination with liquid carbon dioxide, in the second cooling step, water is provided as cooling medium, wherein the water being used as cooling medium is fed directly into the process gas flow.

In the second cooling step, the cooling may be performed rapidly during a time period of less than 10 seconds, preferably less than 5 seconds, most preferably less than 3 seconds in order to eliminate or at least greatly reduce formation of dioxins and furans. By rapid cooling the time period in the temperature interval of about 500 to 200 °C is kept as short as possible which thereby results in an elimination of, or at least greatly reduced formation of dioxins or furans.

The time period for performing the first cooling step is not critical since there is typically no formation of dioxins and/or furans in the temperature range from the starting temperature, i.e. 1500 °C to 800 °C, preferably about 1300 °C, to the first predetermined temperature, i.e. in the temperature range of 650 to 510 °C, preferably the range of 600 to 520 °C. Preferably, the time period for performing the first cooling step is adapted such that a desired process flow is obtained. Thus, the time period for the first cooling step may be in the order of seconds, minutes, or even hours.

The time period for performing the third cooling step is not critical since there is typically no formation of dioxins and/or furans below 200 °C. Thus, the time period for the third cooling step may be in the order of seconds, minutes, or even hours. Preferably, the time period for performing the third cooling step is adapted such that a desired process flow is obtained.