Method and system for producing a synthetic fuel from biomass
Patent Information
- Application Number
- AU2025240709
- Authority / Receiving Office
- AU · AU
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-03-21
- Filing Date
- 2025-02-19
- Publication Date
- 2026-09-17
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Abstract
Description
Method and system for producing a synthetic fuel from biomass The present invention relates to a process for producing synthetic fuel from biomass and to a plant for producing synthetic fuel. An example of a known means of thermal conversion of biomass for production of raw materials or synthetic fuels from renewable energies is first to react carbon dioxide or carbon monoxide with hydrogen in a methanol synthesis to give methanol in accordance with the following two reaction equations: (I) CO2 + 3H2 CH3OH + H2O (II) CO + 2H2 CH3OH The carbon dioxide or carbon monoxide required in the case of a methanol synthesis is provided, for example, in a synthesis gas which is produced by means of an upstream combustion or gasification of biomass, which is also referred to as a combustion route or gasification route. The combustion of biomass is typically effected with an excess of oxygen, such that the carbon present in the biomass is converted predominantly to carbon dioxide, as illustrated in the following reaction equation (3): (III) biomass + O2 ^ CO2 + H2O This reaction is highly exothermic. The combustion offgas consequently consists essentially of carbon dioxide and water. In order to provide a synthesis gas for the methanol synthesis according to the above reaction equation (I), hydrogen is finally added to the combustion offgas, preferably after water has been condensed out. Owing to the high carbon dioxide content resulting from the combustion - see reaction equation (III) - the water content produced in the methanol synthesis according to the above reaction equation (I) has to be reduced considerably in complex steps in a downstream distillation stage in order to provide methanol with a certain degree of purity for the production of fuels. The providing and operating of the distillation stage is complex in terms of process engineering and costly. As an alternative to the combustion route, it is possible to choose the gasification route, which is of increasing interest. The methanol synthesis is preceded here by a gasification of biomass with a deficiency of oxygen or an oxygen deficit. The gasification affords a gasification offgas or a synthesis gas composed of carbon monoxide, carbon dioxide and hydrogen according to the following reaction equation (IV): (IV) biomass + O2 + H2O ^ CO + CO2 + H2 The partial O2 reaction is exothermic here. The carbon monoxide and the carbon dioxide are converted in the methanol synthesis according to the ratios of the above reaction equations (I) and (II) to methanol, or what is called crude methanol in the form of a mixture of methanol and water. Owing to the carbon dioxide present in the gasification offgas or in the synthesis gas, there is a certain proportion of water in the crude methanol that has to be reduced in a complex manner by distillation. What can be considered to be advantageous about the gasification route is that, by comparison with the combustion route, less water that has to be separated off by distillation is formed. On the other hand, combustion is a process which is much more robust in terms of process technology than gasification. For example, biomass of low or heterogeneous quality can be converted in a simple manner by combustion, whereas the gasification route requires a biomass reactant of comparatively high or constant quality. Aside from the established gasification or combustion routes via methanol synthesis, there is increasing awareness of the production of synthetic fuels in which biomass-to-liquid (BtL) concepts with alternative process routes are employed, in which hydrogen generated from renewable energy sources in particular is to be used. Against the background of advancing climate change, processes for biomass combustion or gasification are moving increasingly into the focus of current development, since synthetic fuels that can be produced from the resulting offgases (CO2, CO, H2), owing to their biogenic carbon content, do not lead to aggravation of the greenhouse effect and are certified as green. These "biomass-to-liquid" (BtL) concepts are usually based on gasification of the biomass with oxygen and steam at high temperatures, forming a synthesis gas composed of CO2, CO and H2. In order to adjust the stoichiometric H2 / CO ratio (e.g. 2:1) required for the synthesis, in what is called a water-gas shift (WGS) reaction, a portion of the carbon monoxide is reacted with steam to give hydrogen H2 and CO2, i.e. the H2 content of the synthesis gas is increased. CO2 can then be separated off in order to obtain a pure synthesis gas composed of CO and H2, which is converted in a chemical synthesis, especially a Fischer-Tropsch synthesis, to a series of liquid hydrocarbons, for instance diesel. The economic viability of these green fuels produced in this way is still poor at present, in particular in comparison with conventional fuels produced from fossil raw materials. Further developments in process technology with the aim of increasing the degree of conversion of carbon present in the carbon dioxide CO2 rather than separating off the carbon dioxide content CO2 are very energetically demanding and complex in terms of process engineering. For example, a reverse water-gas shift reaction (RWGS) that follows the process of gasification of the biomass has been proposed. In the RWGS reaction, the carbon dioxide CO2 present in the gas mixture of synthesis gas and carbon dioxide that is produced by the biomass gasification is reacted with hydrogen H2 and reduced to carbon monoxide CO. (V) CO2 + H2 ^ CO + H2O A disadvantage is that the RWGS reaction is strongly endothermic and has to be conducted at a very high process temperature of 800°C-900°C and requires corresponding heat sources. In addition, because of the high reaction temperature, this process variant requires considerable adaptations in terms of apparatus and process technology in order ultimately to be able to implement the desired processes of conversion of the synthesis gas to synthetic fuels, for example in a downstream Fischer-Tropsch synthesis. This has not yet been developed and optimized industrially to the extent that an economic mode of operation for a synthetic fuel from biomass gasification with a simultaneously high degree of conversion of the carbon dioxide can already be envisaged. Proceeding from the known concepts described above, it is an object of the present invention to specify an improved method of producing synthetic fuel from biomass that has a high degree of utilization of the carbon and at the same time is economically viable. Furthermore, a corresponding plant for producing synthetic fuel from biomass is to be specified. The object directed to a method is achieved in accordance with the invention by a method of producing a synthetic fuel that comprises the following method steps: (S1) performing a first reaction process, wherein the first reaction process is supplied with biomass and oxygen and produces a gas mixture of synthesis gas and carbon dioxide, wherein the synthesis gas contains carbon monoxide and hydrogen, (S2) separating carbon dioxide from the gas mixture and feeding hydrogen into separated carbon dioxide for a second reaction process, (S3) performing a second reaction process, wherein, in the second reaction process, a methanation is performed using the reactants carbon dioxide and hydrogen, producing methane and water as an intermediate product, (S4) feeding methane and water obtained from the second reaction process back into the first reaction process, generating a gas mixture comprising synthesis gas, (S5) discharging synthesis gas and converting synthesis gas to a synthetic fuel. The invention proceeds from the original finding that, in the processes established in the prior art, the carbon dioxide formed in the biomass gasification either has to be separated from the gas mixture, such that the carbon yield of the biomass used falls, or separated off in an energy-intensive RWGS reaction and a subsequent complex condensation of the water formed according to reaction equation (V). By contrast, the process regime of the invention enables virtually complete carbon utilization of the carbon bound in the biomass and efficient conversion to fuel from the synthesis gas obtained. This invention is a move away from a very complex reverse-water-gas shift reaction (RWGS) that has been pursued to date and the associated complex conditioning processes. The process pathway proposed in the present context is a methanation of carbon dioxide CO2 separated from the gas mixture of the gasification, wherein the carbon is fully recycled into the first reaction process via the methanation. (VI) CO2 + 4H2 ^ CH4 + 2H2O The process, which is also referred to as the Sabatier reaction (VI), can very advantageously be performed cyclically and continuously and can be connected integrally downstream of the removal of carbon dioxide. It is thus possible to increase the degree of utilization of carbon present in the biomass by incorporation into methane. The Sabatier reaction is exothermic and at a distinctly lower temperature level that is better adapted to the first reaction processes than, for example, a RWGS reaction. Therefore, the recycling of the methane CH4 and water H2O intermediates obtained from the methanation is particularly efficient and performable in coordination with the first reaction process. Continuous cyclical application is advantageously envisaged here. A further reactant supplied and fed to the Sabatier reaction is renewably obtained hydrogen H2, for instance from the electrolysis of water. The first reaction process, which is preferably performed as biomass gasification according to the above reaction equation (IV), and the second reaction process of the methanation are operated in a mutually coordinated manner. The specific feeding-back of methane CH4 and water H2O from the Sabatier reaction makes it possible in particular to completely dispense with addition, as used to be customary, of external water or steam together with the biomass in the first reaction process, or to distinctly reduce the feed with respect to the conversion of matter. Considerable advantages of the invention arise here firstly over use of the water-gas shift reaction (WGS), since the WGS, given rising prices for biomass, is accompanied by a large loss of carbon, which subsequently has to be separated off as CO2. In this concept, only some of the biomass can be used for the production of fuels. In addition, the discharge of CO2 to the atmosphere results in emission of greenhouse gases, and so this process regime - in spite of use of biomass - can only be qualified as climate-neutral to a limited extent. In any case, the potential for climate neutrality is exploited only to a small degree owing to the low degree of utilization of the WGS route. Secondly, however, the invention is also superior to the use of the reverse water-gas shift reaction (RWGS), since the invention proposes a technically more efficient route with complete utilization. The RWGS reaction according to reaction equation (V) is likely to lead to a higher degree of utilization of the carbon in the biomass and to the provision thereof for downstream synthesis processes, for instance Fischer-Tropsch. In the case of this process variant via RWGS for very substantial utilization of the carbon present in the biomass, which is currently the subject of vigorous discussion in research and development, there are further disadvantages which the invention has recognized and which have had the effect that this process variant is not being implemented significantly on an industrial scale to date: For instance, the RWGS reaction takes place at very high temperatures of 800°C-900°C and is an endothermic reaction, meaning that heat has to be supplied from the outside in order to reach and to maintain the required reaction temperature in the reactor. Therefore, in current concepts, the RWGS is often heated electrically or by combustion of valuable hydrogen H2, which has an adverse effect on the economic viability of the whole biomass-to-liquid process (BtL). In addition, the reaction temperature of the endothermic RWGS reaction is well above the reaction temperature of a downstream exothermic Fischer-Tropsch synthesis. Therefore, sufficiently efficient heat transfer — for example by means of complex heat pump systems — from the Fischer-Tropsch synthesis back to the RWGS reaction is impossible. In the RWGS reaction, the water formed also has to be removed from the synthesis gas, i.e. condensed out, for a subsequent Fischer- Tropsch synthesis. In addition, a low technology readiness level (TRL) is currently still being attested for RWGS owing to technical and economic challenges resulting from the high process temperature. Questions arise here as to the lifetime of the catalyst and the demanding heating of the process gases for the RWGS reaction. Carbon dioxide CO2 is separated from the gas mixture obtained from the biomass in the first reaction process by integration of an established separation method based on adsorption or absorption of the carbon dioxide. CO2 separation and storage, also called CO2 sequestration and CCS, generally refers to a process in which carbon dioxide CO2, either from the environment or directly at the sources of fossil CO2 emissions of an industrial or energy-related nature, is separated, processed, compressed and transported to a storage site and hence removed from the natural CO2 cycle in the atmosphere in a very long-lasting manner. In the present context, for example, a separation of carbon dioxide CO2 from the gas mixture downstream of the first reaction process, i.e. the biomass gasification in particular, may be employed, analogously to a CO2 separation in a post-combustion process. In this case, what is called a CO2 scrub may be installed as a cleaning step for the gas mixture from the first reaction process. Various scrubbing methods for a CO2 gas scrub are available. For example, amine scrubbing is known on an industrial scale from natural gas treatment. In the amine scrub, the CO2 is adsorbed onto the support at 40°C by means of finely divided amine droplets. In a further step, the amines go into a separator (stripper), where they release the CO2 in concentrated form again in a controlled manner at 150°C. Thus, the carbon dioxide CO2 separated from the gas mixture is provided again and sent to use in the second reaction process of the methanation. In general, a specific selection of a separation process integrated in the process of the invention for carbon dioxide CO2 is not restricted, and so it is possible in principle to employ established and available CO2 separation methods that are based on selective adsorption or selective absorption of carbon dioxide CO2 in a suitable adsorbent material or absorbent material. In a particularly preferred execution of the method, the second reaction process is performed at a reaction temperature between 300°C and 400°C and a pressure of 18 to 22 bar. This achieves a particularly advantageous and efficient process regime for the methanation, wherein the integration of a Sabatier reaction results in a moderate reaction temperature and reaction pressure that are adapted both to the first reaction process and to the subsequent synthesis of the fuel. In the Sabatier reaction according to equation (VI) integrated here into the process, carbon dioxide CO2 separated from the gas mixture reacts at temperatures of 300 to 700°C, preferably between 300°C and 400°C, with hydrogen H2 to give methane CH4 and water H2O. This reaction is exothermic and releases an enthalpy of reaction of AH0 = 165.0 kJ / mol. However, the reaction is accelerated by a catalyst. This purpose is usually served by nickel catalysts that are improved with various promoters and stabilizers such as aluminum oxide and zirconium dioxide, but the catalytic action of ruthenium has also been examined. An alternative pathway of a Sabatier reaction can be implemented with carbon monoxide CO as reactant: CO + 3H2 ^ CH4 + H2O. In the present context, the pathway according to equation (VI) with carbon dioxide CO2 as reactant is followed in order to recycle the carbon bound as methane CH4 into the first reaction process. In a preferred configuration of the method, steam is additionally fed into the first reaction process. The steam can be removed from the product of the Sabatier reaction, since not only methane CH4 but also water H2O is formed at the process temperature. This is recycled and fed to the first reaction process. In this way, the need to feed water in the form of steam from the outside to the first reaction process is reduced or can even be eliminated entirely, since it is generally the case that covering of the demand for water H2O can already be achieved by the second reaction process, the Sabatier reaction. The addition of external process steam in parallel with the addition of oxygen O2 for the first reaction process is therefore possible, but not absolutely necessary. The first reaction process — preferably a biomass gasification — can alternatively be operated solely with supply of oxygen O2 and biomass without addition of externally provided process steam. In the inflow from the Sabatier process there are significant amounts of water that are available and advantageously utilizable for biomass gasification. In a particularly preferred configuration of the method, hydrogen H2 is obtained from an electrolysis of water H2O and fed to the second reaction process as reactant. With regard to climate-neutral production of the synthetic fuel, it is advantageously possible here to obtain the hydrogen H2 from the electrolysis from renewable power sources, for example from wind power or photovoltaics. The method here is notable for a considerable gain in efficiency. Comparing the processes according to the prior art that are based on combustion of biomass, the same amount of end product can be produced in the present context with a much smaller amount of costly hydrogen H2 from the electrolysis. The requirement for hydrogen H2 supplied can be reduced by nearly 50% by the process regime of the invention. This is because, in the gasification process, the hydrogen atoms present in the biomass and introduced via added steam are converted to the constituents of the synthesis gas and chemically bound therein. In contrast, these hydrogen atoms are converted in combustion processes to water H2O, which is laboriously condensed out and is lost to the process overall. However, the invention achieves particularly efficient and virtually loss-free hydrogen utilization in order to produce synthetic fuels. In a further-preferred embodiment of the method, oxygen O2 is obtained from the electrolysis of water H2O and fed to the first reaction process. In this way, the oxygen O2 from the electrolysis is also utilizable in method step S1 and can be reacted together with the biomass so as to produce a gas mixture of synthesis gas and carbon dioxide CO2, wherein the synthesis gas contains carbon monoxide CO and hydrogen H2. A biomass gasification is preferably conducted here in the first reaction process. In a particularly preferred configuration of the method, the amount of oxygen O2 and water H2O supplied to the first reaction process is adjusted, with coordination with respect to the amount of methane CH4 fed back into the first reaction process. The recycling of methane and water from the Sabatier reaction into the first reaction process lowers the requirement for externally supplied water or steam. It is thus possible to considerably reduce the supply of external process steam and match it to the demand for the first reaction process with regard to complete conversion of matter. For this purpose, the amount of external oxygen and water or process steam supplied is matched as required to the recycled amount of methane and water correspondingly formed in the Sabatier reaction and recycled. Thus, in addition to the requirement for externally supplied hydrogen H2, the water demand is also reduced compared to the conventional process and a high degree of utilization of the reactants is achieved. This is done in a controlled manner depending on the consistency and composition of the biomass supplied with regard to its water content. Thus, in the case of biomass gasification with the mass flow of methane CH4 and water H2O recycled to the reaction, advantageous matching of the metered addition can be performed. This is done via appropriate open-loop or closed-loop control or matching of the external addition of water H2O or oxygen O2 to the first reaction process, which in particular comprises biomass gasification. In a preferred configuration of the method, a substream of the carbon dioxide CO2 separated from the gas mixture in step S2 is fed to the first reaction process, wherein a barrier effect is brought about. Controlled branching-off of a substream of carbon dioxide CO2 provides, in a simple manner from the process itself, an inert barrier gas which is metered into the gasification process in a controlled manner. In addition to the safety-relevant barrier effect against gas transfer, this brings about the establishment of a particularly homogeneous gasification bed of biomass in the first reaction process, since the barrier gas favors the establishing of a very substantially uniform conversion temperature over the entire volume. This is enabled by a corresponding distribution topology for the barrier gas with corresponding nozzle arrangements within the reaction space of the gasification reactor. A further advantage of the utilization of the carbon dioxide CO2 is that carbon dioxide CO2 is noncombustible and the use thereof as barrier gas does not cause any gas extrinsic to the process, such as nitrogen, to get into the process. In a particularly advantageous configuration of the method, the thermal energy obtained in the methanation in the second reaction process is utilized in the execution of the first reaction process. The Sabatier reaction is highly exothermic, and so a high thermal energy is obtainable from the enthalpy of reaction of AH0 = 165.0 kJ / mol and is utilizable in a controlled manner for the maintaining of the first reaction process. Heat integration is also particularly simple to accomplish and is already largely adapted with regard to the temperature level. The hot product gases from the Sabatier reaction, which is run at about 300°C to 400°C, are fed to the gasification reactor for the biomass, where the thermal energy thereof assists the gasification reaction at a set gasification temperature. This depends on the gasification process chosen and on the composition of the biomass. The gasification of biomass sets in after drying even at temperatures of 150°C, with escape of steam and oxygen at first. At higher temperatures, the solid constituents of the biomass, in particular the lignin and the cellulose, are gasified. This gas ignites as soon as secondary air is supplied; the ignition temperature is 230°C to 280°C. Industrial biomass gasification is a partial combustion with the aid of a gasifying or oxidizing agent, usually air, oxygen, carbon dioxide or steam, without ignition at temperatures of 700°C to 900°C, in which these are oxidized not to carbon dioxide CO2 as in the case of combustion but essentially to carbon monoxide CO. Further components of the gas mixture formed are hydrogen H2, carbon dioxide CO2, methane CH4, steam H2O and, depending on the biomass used and the gasification process, a number of organic substances in different concentrations. The solid residue that remains is ash and residues of biochar. When the temperature of the process gas is lowered, the steam, mixed with organic constituents, condenses to form a tar or to form an organically contaminated wood gas condensate which is separated off. The combustible product gas can be further oxidized in a subsequent process by a combustion (fuel gas) or a chemical synthesis (synthesis gas) with release of energy (exothermic process). In a further preferred configuration of the method, the thermal energy obtained in the methanation in the second reaction process is converted to electrical energy. Alternatively or additionally to the above-described advantageous heat integration of the process heat obtained from the exothermic methanation, it is advantageously possible to partly also energetically utilize the thermal energy obtained and to transform it correspondingly. It is thus possible to generate process steam, which in turn is used to generate power in a steam turbine set. The heat that arises in the Sabatier reaction is therefore at a temperature level of particularly good suitability for utilization by heat integration into the method or, alternatively or additionally, for generation of electrical energy. In contrast to the RWGS reaction, the Sabatier reaction is an exothermic reaction that takes place with strong evolution of heat. In contrast to an RWGS reaction, external heating is unnecessary — correspondingly lower apparatus complexity and energy expenditure leads to an increase in economic viability. Owing to the net heat generation of the overall process, heat export is therefore advantageously possible. Other processes at the installation site of a plant can be supplied with heat. In a particularly advantageous configuration of the method, the discharged synthesis gas is supplied to a Fischer-Tropsch synthesis, wherein carbon monoxide CO and hydrogen H2 are converted to a synthetic fuel comprising hydrocarbons. It is particularly appropriate to feed the synthesis gas obtained and processed in this way to a Fischer-Tropsch synthesis, in which case a catalyst is additionally used for the synthesis. The obtained synthesis gas composed of carbon monoxide CO and hydrogen H2 is already well matched in terms of temperature and pressure level to a subsequent performance of a Fischer-Tropsch synthesis. Fischer-Tropsch synthesis, also referred to as "Fischer-Tropsch process" or "FT synthesis" for short, is an industrial scale, heterogeneously catalytic process for preparing hydrocarbons. In this process, carbon monoxide adsorbed on cobalt- or iron-containing catalyst surfaces is hydrogenated with hydrogen. The reactions take place at temperatures of about 150 to 350°C and pressures of 1 to about 25 bar. FT synthesis requires the production of synthesis gas as reactant and the conversion thereof to Fischer-Tropsch products and further processing thereof. In the present context, biomass gasification is the method used for the production of the synthesis gas, and FT synthesis is integrated into the overall process. Carbon-containing starting materials that are generally available for a Fischer-Tropsch synthesis are coal, natural gas or organic waste as possible raw material sources. Owing to the multitude of possible feedstocks, Fischer-Tropsch synthesis occupies a central position in the search for alternatives to crude oil for obtaining of liquid hydrocarbons as fuel. The overall equation of the Fischer-Tropsch synthesis can be represented by the following reaction equation: (VII) nCO + 2nH2 ^ (CH2)n + nH2O The provision of cleaned synthesis gas from a biomass gasification is particularly efficient by the method of the invention comprising methanation and recycling of methane. A good degree of cleaning of the synthesis gas can already be achieved here, and so very demanding and complex gas cleaning and gas processing steps are dispensed with. Additional cleaning and adjustment of the state of the synthesis gas is nevertheless preferable, but is less complex by virtue of the purity already achievable originally and the pressure and temperature level achieved. By contrast, in conventional applications, the process component of providing purified synthesis gas for FT synthesis is very complex or less efficient in terms of the degree of utilization, as described further up in relation to the RWGS reaction or the WGS reaction. In particular, when coal is used - in the case of coal gasification - water-gas is first produced, followed by the preparation of the hydrogen additionally required. The last step is the cleaning of the gas to free it of unwanted sulfur- and nitrogen-containing constituents which, because of the sensitivity of most catalysts to these substances, have to be carefully removed. Gasification processes are frequently run with pure oxygen, although capital and operating costs are higher owing to the air fractionation required. However, the yield in the synthesis stages is higher since, when air is used, the synthesis gas formed is greatly diluted by the nitrogen and the subsequent processes are less efficient. In a preferred configuration of the method, biomass is gasified in the first reaction process, wherein an excess air ratio L between 0 < L < 1 is established, which is controlled with regard to the amount of methane CH4 and water H2O fed back in step S4. Thus, gasification of biomass is envisaged in the first reaction process, preference being given to an excess air ratio L of greater than 0.5; in particular, it may be preferable to perform the gasification reaction in a working range with an excess air ratio of 0.6 < L < 0.8. External water H2O or process steam is merely optionally fed in, since water H2O or steam is already available via the recycling of the products from the methanation. In addition, an external supply of pure oxygen O2 together with the biomass into the gasification process is preferred, so as to avoid introduction of gases extrinsic to the process, such as nitrogen, from the air. The gasification reactor for the biomass can be run under various conditions, which results in high flexibility. In practice, depending on the fuel and gasifier type, an adapted and optimized mode of operation will be established in a closed-loop controller or process control technology system. Methane CH4 is selectively fed into the gasifier, preferably at a point in the gasification reactor that promotes the conversion process to the desired synthesis gas constituents carbon monoxide CO and hydrogen H2. For the biomass gasification itself, it is possible to use different technical gasification reactors which differ primarily in the nature of the contact between biomass and gasification agent (air, oxygen or steam). In principle, three basic reactor types are usable here in a plant: the fixed bed gasifier, the fluidized bed gasifier and the entrained flow gasifier. The object directed to a corresponding plant for producing synthetic fuel from biomass is achieved in accordance with the invention by a plant for producing a synthetic fuel, comprising: - a gasification reactor for producing a gas mixture comprising carbon dioxide and synthesis gas, wherein the gasification reactor is set up to gasify biomass and has an offgas conduit for discharging the synthesis gas produced, - a separation device which is connected to the offgas conduit and is set up to separate carbon dioxide from the gas mixture and to provide it for a methanation via a first reactant conduit, - a Sabatier reactor set up to perform a methanation using carbon dioxide from the separation apparatus and hydrogen which is feedable to the Sabatier reactor via a second reactant conduit, - a recycle conduit which is connected to the Sabatier reactor and connected to the gasification reactor and set up to feed methane and water into the gasification reactor, and - a Fischer-Tropsch reactor which is connected to the offgas conduit downstream of the separation apparatus and which is set up for synthesis of fuel from the synthesis gas. The technical effects and advantages elucidated above for the proposed method are accordingly equally applicable to the proposed plant. It is advantageously possible via the recycle conduit in particular to recycle products from the Sabatier reactor, methane CH4 and water H2O, specifically to the gasification reactor, such that virtually complete conversion of matter is achievable using the carbon from the biomass, and particularly economically viable operation of the plant. The combination of a CO2 separation apparatus with the Sabatier reactor in terms of plant engineering implements extensive utilization. Continuous recycling enables virtually complete conversion of carbon dioxide CO2 to synthesis gas, such that the plant is capable of achieving an enrichment and cleaning effect with which virtually pure carbon monoxide CO and hydrogen H2 is providable in the offgas conduit as synthesis gas. This enables easier utilization and transfer of the synthesis gas that has already been largely formulated into the downstream Fischer-Tropsch reactor. However, an additional cleaning device may also be provided in the offgas conduit in order to free the synthesis gas of residual impurities and possible catalyst-damaging substances, for instance sulfur constituents or nitrogen constituents. However, cleaning intensity is much lower than when an RWGS reactor is used. A bypass conduit is preferably provided in the plant, which is led out of the separation device and is connected to the gasification reactor while bypassing the Sabatier reactor, such that carbon dioxide is feedable to the gasification reactor as barrier gas. The barrier gas serves to prevent introduction of external gas extrinsic to the process into the gasification reactor, but also as a barrier to escape and release of constituents of the gas mixture, for instance carbon monoxide, from the gasification reactor. At the same time, the barrier gas serves to compensate for a temperature gradient in the reactor space, such that a very substantially homogeneous gasification temperature is established in operation for the biomass to be gasified. In addition, as a particularly preferred configuration, a metering device comprising a gas distributor is connected to the recycle conduit, such that methane is introducible selectively and locally into the gasification reactor. For a better outcome in terms of yield and quality (purity) of the synthesis gas produced in the gasification reactor, it has been found that controlled feeding of methane from the Sabatier reactor into the gasification reactor is appropriate. Therefore, a gas distributor topology at the exit from the recycle conduit is advantageous. This enables particularly uniform and precise spatial contacting of the gasification material with methane obtained from the Sabatier reactor. The metering device with the gas distributor is designed and arranged in the gasification reactor such that metering of a respective volume flow can be established spatially and locally via a nozzle arrangement. The metered addition may be adjusted via a number of controllable metering valves, where one metering valve supplies a nozzle or a group of nozzles of the gas distributor. The invention is elucidated in detail hereinafter by way of example by preferred embodiments with reference to the appended figures, and the features described below may represent an aspect of the invention, either on their own or in different combinations with one another. The figures show: FIG 1 a schematic diagram of a plant for producing a synthetic fuel from biomass; FIG 2 a further plant with an improved degree of utilization of the carbon compared to the plant shown in FIG 1; FIG 3 a plant for producing a synthetic fuel from biomass according to the invention; FIG 4 a flow diagram of a method of producing a synthetic fuel from biomass. FIG 1 shows a possible plant 1A for producing a synthetic fuel F from biomass BM in a first execution. This implements a biomass-to-liquid (BtL) concept in order to obtain liquid fuel F from the gasification of biomass BM as starting material. As can be seen in FIG 1, the plant 1A comprises a first reactor in the form of a gasification reactor 3 for biomass BM, having an offgas conduit 5 in which a water-gas shift reactor 21 is disposed or integrated. The WGS reactor 21 integrated into the offgas conduit 5 thus assumes the function of the second reactor. Connected to the offgas conduit 5 in downstream succession are a separation device 7 for carbon dioxide CO2 and a Fischer-Tropsch reactor 15. The plant 1A further comprises a feed conduit 25 for feeding in water H2O, wherein the feed conduit 25 is connected to the WGS reactor 21 according to the working example shown in FIG 1. The flow direction in the plant 1A shown in FIG 1 here - as in the further figures - is aligned essentially from left to right and is indicated by the arrow directions of the synthesis gas SG. Biomass BM is fed to the gasification reactor 3 together with oxygen O2 and water H2O, and the biomass is gasified at a temperature of about 750°C. The gas mixture thus obtained contains carbon monoxide CO, hydrogen H2 and carbon dioxide CO2. The carbon monoxide CO and hydrogen H2 constituents form the synthesis gas SG of particular economic interest. In order to obtain the ratio of hydrogen H2 to carbon monoxide CO necessary for a subsequent Fischer-Tropsch synthesis FTS of fuel F in the Fischer-Tropsch reactor 15, for example 2:1, in what is called a water-gas shift reaction WGS, some of the carbon monoxide is reacted with steam D to give hydrogen H2 and CO2 according to the following reaction equation (VIII): (VIII) CO + H2O ^ CO2 + H2 This increases the proportion of hydrogen H2 in the synthesis gas SG. The carbon dioxide CO2 is then separated off in the separation device 7 in order to obtain synthesis gas SG of maximum purity with the carbon monoxide CO and hydrogen H2 constituents. The synthesis gas SG thus obtained is processed further and fed to a chemical synthesis, in the present context to an FT reaction in the Fischer-Tropsch reactor. In this way, long-chain hydrocarbons (CH2)n can be synthesized and a synthetic fuel F, for example liquid fuels such as diesel, can be produced accordingly. The separation of carbon dioxide CO2 in the separation device 7 is accompanied by a large loss of carbon that is not converted to fuel F, and therefore the degree of utilization of the carbon provided in the biomass BM is low. The economic viability of a green fuel F produced in this way is inadequate in comparison with conventional fossil fuels. FIG 2 shows a further plant 1B with a distinctly improved degree of utilization of the carbon bound in the biomass BM compared to the plant 1A shown in FIG 1. The plant 1B according to FIG 2 is therefore further equipped and modified particularly advantageously compared to the plant according to FIG 1A in order to perform a distinctly improved process for producing synthetic fuel F from the gasification of biomass BM. As can be seen in FIG 2, the plant 1B shown therein comprises a first reactor in the form of a gasification reactor 3, having an offgas conduit 5 in which a reverse water-gas shift reactor 23 is now disposed or integrated. The RWGS reactor 23 integrated into the offgas conduit 5 thus assumes the function of the second reactor. This makes it possible already to be able to largely convert carbon dioxide CO2 present in the gas mixture by running an RWGS reaction in the RWGS reactor 23 according to the abovedescribed reaction equation (V): (V) CO2 + H2 ^ CO + H2O The plant 1B further comprises a feed conduit 25 for feeding in hydrogen H2, wherein the feed conduit 25 according to the working example shown in FIG 2 may be upstream or else alternatively -not shown in FIG 2 - downstream of the RWGS reactor 23 in flow direction. The RWGS reactor 23 is followed by a condensation device 27 and a Fischer-Tropsch reactor 15 as further plant elements in the plant 1B. As can be seen in FIG 2, a gasification is first conducted at a process temperature in the gasification reactor 3. This first reaction process uses biomass BM and oxygen O2 to produce an offgas containing carbon dioxide CO2 in a gas mixture. The gas mixture further comprises carbon monoxide CO and hydrogen H2 and is also referred to in the present context as the synthesis gas SG produced. The first reaction process may be an industrial biomass gasification at process temperature of greater than 750°C. Alternatively, the first reaction process may be any other form of exothermic biomass utilization that produces a gas mixture comprising carbon dioxide CO2 and synthesis gas SG. Lower process temperatures are also possible here. This is followed by a further reaction process in the form of a reverse water-gas shift reaction at a second temperature using the gas mixture formed in the gasification reactor 3. The person skilled in the art will be able to select a suitable RWGS catalyst such that the RWGS reaction according to the above reaction equation (V) increases the proportion of carbon monoxide CO in the synthesis gas SG. The RWGS reaction is supplied externally with an excess of hydrogen H2 via the feed conduit 25, where the size of the surplus amount of hydrogen H2 is guided by the proportion of carbon dioxide CO2 in the synthesis gas SG produced in the first reaction, taking account of the above RWGS reaction equation (V). This RWGS equilibrium reaction is a reverse reaction of the WGS reaction shown in FIG 1 and takes place at very high temperatures of about 800°C-900°C and at moderate pressures of 1-20 bar. If sufficient hydrogen H2 is added to the synthesis gas SG and a sufficiently high conversion of CO2 is achieved in the RWGS, the great majority of the synthesis gas SG can thus be converted in the FTS reaction to hydrocarbons (CH2)n. Water H2O is removed from the synthesis gas by condensation prior to the FTS reaction. The synthesis gas SG modified by the RWGS reaction can be cooled, i.e. the synthesis gas SG having an elevated carbon monoxide content and a reduced carbon dioxide content compared to the synthesis gas SG that emerged directly from the gasification reactor 3 from the first reaction process. The cooling involves controlled condensing of water H2O out of the synthesis gas SG. For this purpose, a condensation device 27 is provided, which is connected into the offgas conduit 5. The condensed water H2O is collected and discharged via a drain. The synthesis gas SG that has been largely freed of water H2O and carbon dioxide CO2 and cooled is introduced into the Fischer-Tropsch reactor 15 and converted to synthetic fuel F in an FTS reaction. Although the process route via a RWGS reaction has a high degree of utilization of the carbon bound in the biomass BM, it is very energy-intensive and complex in terms of process engineering and will still be at an early technology stage for the foreseeable future, and so there is no sign of an early commercial application on an industrial scale. In addition, the water H2O is condensed out and discharged from the process, and therefore the level of utilization is low with respect to hydrogen H2. By contrast, FIG 3 shows a plant 1 for producing a synthetic fuel from biomass that overcomes these disadvantages of the methods described in FIG 1 and FIG 2. The plant concept of FIG 3 and the process regime makes virtually complete carbon utilization of the carbon bound in the biomass BM possible in a particularly efficient process regime, and conversion to fuel F from the synthesis gas SG obtained. For this purpose, FIG 3 shows a plant 1 for producing a synthetic fuel F from biomass BM. The plant 1 has a gasification reactor 3 for producing a gas mixture of carbon dioxide CO2 and synthesis gas SG. The gasification reactor 3 is set up to gasify biomass BM and has an offgas conduit 5 for discharging the synthesis gas SG produced. The synthesis gas SG comprises carbon monoxide CO and hydrogen H2. Also provided is a separation device 7 connected to the offgas conduit 5. The separation device 7 is set up to selectively separate carbon dioxide CO2 from the gas mixture with the synthesis gas SG and to provide carbon dioxide CO2 thus obtained via a first reactant conduit 9A for a methanation. For this purpose, a Sabatier reactor 11 is provided, which is set up to perform a methanation using carbon dioxide CO2 from the separation apparatus 7 and for an addition of external hydrogen H2. The external hydrogen H2 is advantageously obtained by electrolysis from an electrolyzer 29 from a renewable power source and is suppliable to the Sabatier reactor 11 via a second reactant conduit 9B. Connected to the Sabatier reactor 11 is a recycle conduit 13 which opens into the gasification reactor 3 and is set up to feed methane CH4 and water H2O from the Sabatier reaction SAR according to the reaction equation (VI) CO2 + 4H2 ^ CH4 + 2H2O into the gasification reactor 3. Downstream of the separation apparatus 7, a Fischer-Tropsch reactor 15 is connected to the offgas conduit 5 and is set up to synthesize fuel F from the purified synthesis gas SG, carbon monoxide CO and hydrogen H2. The plant 1 is additionally equipped with a bypass conduit 17 which is led out of the separation device 7 and is connected to the gasification reactor 3 while bypassing the Sabatier reactor 11, such that carbon dioxide CO2 is feedable to the gasification reactor 3 as barrier gas. The recycle conduit 13 opens into a metering device 19 to which a gas distributor is connected, such that methane CH4 is introducible selectively and locally into the gasification reactor 3 and distributable in a controlled manner. This achieves a particularly uniform distribution and homogeneous reaction in the gasification reactor. The operation of the plant 1 can be illustrated by the simplified flow diagram shown in FIG 4, which illustrates method steps for producing synthetic fuel F from biomass BM, with reference to some of the elements of FIG 3: In a first method step S1, a first reaction process is conducted, wherein the first reaction process is supplied with biomass BM and oxygen O2 and produces a gas mixture of synthesis gas SG and carbon dioxide CO2, wherein the synthesis gas SG contains carbon monoxide CO and hydrogen H2. The gasification of the biomass BM in the first reaction process is conducted in such a way that an excess air ratio L between 0 < L < 1 is established, where the gasification reactor 3 is controlled to an operating point at 0.6 ^ L ^ 0.8. There is likewise closed-loop control of the amount of methane CH4 and water H2O fed back in method step S4. Additional external feeding of water H2O or steam D into the gasification reactor 3 is thereby avoided or at least distinctly reduced. In a second method step S2, there is a controlled separation of carbon dioxide CO2 from the gas mixture and feeding of hydrogen H2 into separated carbon dioxide CO2 for a second reaction process. In a third method step S3, the second reaction process is performed. In the second reaction process, a methanation is performed using the reactants carbon dioxide CO2 and hydrogen H2, producing methane CH4 and water H2O as an intermediate product. In a method step S4, the methane CH4 and water H2O obtained from the second reaction process is returned to the first reaction process, producing a gas mixture comprising synthesis gas SG. In method step S5, synthesis gas SG is discharged and, in an FTS reaction, a conversion of synthesis gas SG to a synthetic liquid fuel F is brought about. In the second reaction process in step S3, the thermal energy Q obtained in the exothermic methanation is at least partly utilized in the execution of the first reaction process. Consequently, in addition to the products from the Sabatier reaction SAR, heat Q is also recycled and introduced into the gasification process. In method step S3, the Sabatier reaction SAR is performed at a reaction temperature between 300°C and 400°C and a pressure of 18 to 22 bar. However, it is also conceivable to utilize the heat of reaction Q released for power generation. Electricity can be generated, for instance, by steam generation from heat Q and expansion in a steam turbine set. Optionally, if necessary, water H2O in the form of steam D can additionally be supplied in the first reaction process in method step S1. The water input from the Sabatier reaction SAR via the recycle conduit 13 generally already sufficiently covers the water demand for biomass gasification. The demand for hydrogen H2 as reactant for the Sabatier reaction SAR is met from an electrolysis of water H2O, which is fed into the second reaction process in method step S3 as reactant in addition to the carbon dioxide CO2. Moreover, closed-loop control is performed in that the amount of oxygen O2 and water H2O supplied to the first reaction process is precisely adjusted, with coordination with respect to the amount of methane CH4 fed back into the first reaction process. In addition, a substream of the carbon dioxide CO2 separated from the gas mixture in step S2 is fed to the first reaction process, wherein a barrier effect is brought about in the gasification reactor 3. The synthesis gas SG discharged and optionally purified via cleaning steps that are not shown in detail is fed to a Fischer-Tropsch synthesis FTS. This converts carbon monoxide CO and hydrogen H2 to a synthetic fuel F including liquid hydrocarbons. These are long-chain (CH2)n molecules that can be used as liquid fuels F. This process regime of FIG 4 and the plant 1 of FIG 3 according to the invention is superior to the use of the reverse watergas shift reaction (RWGS) since a technically significantly more efficient route is proposed with complete utilization of the carbon. The disadvantages associated with the RWGS reaction on account of its complexity and the high reaction temperatures or heat balance can be overcome by the invention, and so an economically interesting and industrially implementable alternative to known BtL processes is provided. The temperature level of the Sabatier reaction SAR alone, at 300°C-400°C, is much lower than that of the RWGS reaction. In addition, the reaction takes place under moderate pressure (~e.g. 20 bar). Compared to an RWGS reaction, fewer challenges with regard to material resistance might therefore be expected in the industrial implementation of the plant concept. Moreover, with regard to the process pressure in the invention, it is very advantageous that all parts of the process - depending on the method chosen for the CO2 separation - can be performed virtually at a single pressure level as the nominal working pressure of the plant 1. Process pressures in the region of about 10 bar have been found to be favorable here. Energy-intensive repeated compressions that would otherwise be necessary after intermediate decompression are thus dispensed with, as for instance in the previously known approaches. Comparing gasification or BtL processes from the prior art that are based on combustion of biomass BM, the same amount of end product can be produced with a much smaller amount of hydrogen H2 from an electrolysis plant. Estimates have shown that the method proposed here can achieve a reduction in the H2 demand by nearly 50%. This is because, in the gasification process, the hydrogen atoms present in the biomass BM and introduced via added steam D are converted to the components of the synthesis gas SG. In contrast, these hydrogen atoms are converted to water in combustion processes, which is condensed out and is lost to the process of conversion to a biofuel.
Claims
1. A method of producing a synthetic fuel (F), comprising the steps of:(S1) performing a first reaction process, wherein the first reaction process is supplied with biomass (BM) and oxygen (O2) and produces a gas mixture of synthesis gas (SG) and carbon dioxide (CO2), wherein the synthesis gas (SG) contains carbon monoxide (CO) and hydrogen (H2),(S2) separating carbon dioxide (CO2) from the gas mixture and feeding hydrogen (H2) into separated carbon dioxide (CO2) for a second reaction process,(S3) performing a second reaction process, wherein, in the second reaction process, a methanation is performed using the reactants carbon dioxide (CO2) and hydrogen (H2), producing methane (CH4) and water (H2O) as an intermediate product,(S4) feeding methane (CH4) and water (H2O) obtained from the second reaction process back into the first reaction process, generating a gas mixture comprising synthesis gas (SG),(S5) discharging synthesis gas (SG) and converting synthesis gas to a synthetic fuel (F).
2. The method as claimed in claim 1, in which the second reaction process is performed at a reaction temperature between 300°C and 400°C and a pressure of 18 to 22 bar.
3. The method as claimed in claim 1 or 2, in which water (H2O) in the form of steam (D) is additionally fed into the first reaction process.
4. The method as claimed in any of the preceding claims, in which hydrogen (H2) is obtained from an electrolysis of water(H2O) and fed to the second reaction process as reactant.
5. The method as claimed in any of the preceding claims, in which oxygen (O2) is obtained from an electrolysis of water (H2O) and fed to the first reaction process as reactant.
6. The method as claimed in any of the preceding claims, wherein the amount of oxygen (O2) and water (H2O) supplied to the first reaction process is adjusted, with coordination withrespect to the amount of methane (CH4) fed back into the first reaction process.
7. The method as claimed in any of the preceding claims, inwhich a substream of the carbon dioxide (CO2) separated from thegas mixture in step (S2) is fed to the first reaction process, wherein a barrier effect is brought about.
8. The method as claimed in any of the preceding claims, inwhich thermal energy (Q) obtained in the methanation in the second reaction process is utilized in the performance of the first reaction process.
9. The method as claimed in any of the preceding claims, inwhich thermal energy (Q) obtained in the methanation in the second reaction process is converted to electrical energy.
10. The method as claimed in any of the preceding claims, inwhich discharged synthesis gas (SG) is supplied to a Fischer-Tropsch synthesis, wherein carbon monoxide (CO) and hydrogen (H2) are converted to a synthetic fuel (F) comprising liquid hydrocarbons.
11. The method as claimed in any of the preceding claims, inwhich biomass (BM) is gasified in the first reaction process, wherein an excess air ratio L between 0 < L < 1 is established, which is controlled with regard to the amount of methane (CH4) and water (H2O) fed back in step (S4).
12. A plant (1) for producing a synthetic fuel (F), comprising- a gasification reactor (3) for producing a gas mixturecomprising carbon dioxide (CO2) and synthesis gas (SG), wherein the gasification reactor (3) is set up to gasify biomass (BM) and has an offgas conduit (5) for discharging the synthesis gas (SG) produced,- a separation device (7) which is connected to the offgasconduit (5) and is set up to separate carbon dioxide (CO2) fromthe gas mixture and to provide it for a methanation via a firstreactant conduit (9A),- a Sabatier reactor (11) set up to perform a methanationusing carbon dioxide (CO2) from the separation apparatus (7) andhydrogen (H2) which is feedable to the Sabatier reactor (11) via a second reactant conduit (9B),- a recycle conduit (13) which is connected to the Sabatier reactor (11) and connected to the gasification reactor (3) and set up to feed methane (CH4) and water (H2O) into the gasification reactor (3), and- a Fischer-Tropsch reactor (15) which is connected to the offgas conduit (5) downstream of the separation apparatus (7) and which is set up for synthesis of fuel (F) from the synthesis gas (SG).
13. The plant (1) as claimed in claim 12, in which a bypass conduit (17) is provided, which is led out of the separation device (7) and is connected to the gasification reactor (3) while bypassing the Sabatier reactor (11), such that carbon dioxide (CO2) is feedable to the gasification reactor (3) as barrier gas.
14. The plant (1) as claimed in claim 12 or 13, in which a metering device (19) comprising a gas distributor is connected to the recycle conduit (13), such that methane (CH4) is introducible selectively and locally into the gasification reactor (3).