Method and system for producing a synthetic fuel from biomass

Abstract

The invention relates to a method for producing a synthetic fuel (F), comprising the steps (S1): carrying out a first reaction process, wherein the first reaction process creates a gas mixture of synthesis gas (SG) and carbon dioxide (CO2) with the addition of biomass (BM), oxygen (O2), wherein the synthesis gas (SG) contains carbon monoxide (CO) and hydrogen (H2); (S2): separating carbon dioxide (CO2) from the gas mixture and supplying hydrogen (H2) to separated carbon dioxide (CO2) for a second reaction process; (S3): carrying out a second reaction process, wherein in the second reaction process methanation is carried out using the reactants carbon dioxide (CO2) and hydrogen (H2), wherein methane (CH4) and water (H2O) are produced as an intermediate product; (S4): feeding back methane (CH4) and water (H2O) obtained from the second reaction process into the first reaction process, wherein a gas mixture containing synthesis gas (SG) is produced; and (S5): discharging synthesis gas (SG) and converting synthesis gas into a synthetic fuel (F). The invention further relates to a system (1) for producing a synthetic fuel (F), which is designed in particular to carry out the method.

Description

[0001] Description

[0002] Process and plant for producing a synthetic fuel from biomass

[0003] The present invention relates to a method for producing synthetic fuel from biomass and to a plant for producing synthetic fuel.

[0004] For the thermal conversion of biomass to produce raw materials or synthetic fuels from renewable energies, it is known, for example, to first convert carbon dioxide or carbon monoxide with hydrogen to methanol in a methanol synthesis according to the following two reaction equations:

[0005] ( I ) CO2 + 3H2CH3OH + H20

[0006] ( II ) CO + 2H2CH3OH

[0007] The carbon dioxide or carbon monoxide required in the case of methanol synthesis is provided, for example, in a synthesis gas that is produced by means of upstream combustion or gasification of biomass, which is also referred to as the combustion route or gasification route.

[0008] The combustion of biomass typically takes place under an excess of oxygen, so that the carbon contained in the biomass is mainly converted into carbon dioxide, as illustrated in the following reaction equation ( 3 ):

[0009] ( III ) Biomass + 02CO2 + H20

[0010] This reaction is highly exothermic. The combustion exhaust gas therefore consists essentially of carbon dioxide and water. To provide a synthesis gas for methanol synthesis according to the above reaction equation (I), hydrogen is finally added to the combustion exhaust gas, preferably after the water has been condensed out.

[0011] Due to the high carbon dioxide content resulting from combustion – see reaction equation (III) – the water content produced in the methanol synthesis according to the above reaction equation (I) must be significantly reduced in a downstream distillation stage in complex steps in order to provide methanol with a certain degree of purity for the production of fuels. The provision and operation of the distillation stage is technically complex and cost-intensive.

[0012] As an alternative to the combustion route, the gasification route, which is attracting increasing interest, can be chosen. In this process, methanol synthesis is preceded by the gasification of biomass under an oxygen deficiency or oxygen deficit. The gasification produces a gasification exhaust gas or a synthesis gas consisting of carbon monoxide, carbon dioxide, and hydrogen according to the following reaction equation (IV):

[0013] (IV) Biomass + 02+ H20 CO + CO2+ H2

[0014] The O2 partial reaction is exothermic. In methanol synthesis, the carbon monoxide and carbon dioxide are converted into methanol or crude methanol in the form of a mixture of methanol and water according to the ratios of the above reaction equations (I) and (II). Due to the carbon dioxide contained in the gasification exhaust gas or in the synthesis gas, the crude methanol contains a certain amount of water, which must be laboriously reduced by distillation.

[0015] The advantage of the gasification route is that, compared to the combustion route, less water is produced, which must be separated by distillation. On the other hand, combustion is a much more robust process than gasification. For example, biomass of low or heterogeneous quality can be easily converted by combustion, whereas the gasification route requires a biomass feedstock of comparatively high or constant quality.

[0016] Aside from the established gasification or combustion routes via methanol synthesis, the production of synthetic fuels has come into focus, in which biomass-to-liquid (BtL) concepts with alternative process routes are applied, with particular emphasis on the use of hydrogen produced from renewable energy sources.

[0017] Against the backdrop of advancing climate change, biomass combustion or gasification processes are increasingly becoming the focus of current developments, as synthetic fuels can be produced from the resulting exhaust gases (CO2, CO, H2). These fuels, due to their biogenic carbon content, do not exacerbate the greenhouse effect and are certified as green. These so-called "Biomass-to-Liquid" (BtL) concepts are usually based on the gasification of biomass with oxygen and water vapor at high temperatures, forming a synthesis gas from CO2, CO, and H2. In order to achieve the stoichiometric H2 / CO ratio required for the synthesis (e.g., 2:1), a portion of the carbon monoxide is combined with water vapor in a so-called water-gas shift (WGS) reaction to form

[0018] Hydrogen H2 and CO2 are converted, i.e. the H2 content of the synthesis gas is increased. CO2 can then be separated to obtain a pure synthesis gas from CO and H2, which is converted into a range of liquid hydrocarbons, such as diesel, in a chemical synthesis, in particular a Fischer-Tropsch synthesis. The economic viability of these green fuels produced in this way is currently still poor, especially in comparison to conventional fuels made from fossil raw materials. Further process developments to increase the conversion rate of the carbon contained in the carbon dioxide CO2 instead of separating the carbon dioxide content CO2 are proving to be very complex in terms of energy and plant technology. For example, a reverse water gas shift reaction (RWGS) has been proposed, which follows the biomass gasification process.In the RWGS reaction, the carbon dioxide CO2 contained in the gas mixture of synthesis gas and carbon dioxide produced by biomass gasification is reacted with hydrogen H2 and reduced to carbon monoxide CO.

[0019] (V) CO2 + H2CO + H20

[0020] A disadvantage is that the RWGS reaction is highly endothermic and must be carried out at a very high process temperature of 800°C - 900°C, requiring corresponding heat sources. Furthermore, due to the high reaction temperature, this process variant requires considerable equipment and process engineering adaptations in order to ultimately carry out the desired conversion processes from the synthesis gas to synthetic fuels, for example in a downstream Fischer-Tropsch synthesis. Technically, this is not yet sufficiently developed and optimized to foresee an economical operation for a synthetic fuel from biomass gasification with a simultaneous high degree of carbon dioxide conversion.

[0021] Based on the known concepts described above, the object of the present invention is to provide an improved process for producing synthetic fuel from biomass that has a high carbon utilization rate and is simultaneously economical. Furthermore, a corresponding plant for producing synthetic fuel from biomass is to be provided.

[0022] The object directed to a method is achieved according to the invention by a method for producing a synthetic fuel , which comprises the following process steps : ( 51 ) Carrying out a first reaction process , wherein the first reaction process produces a gas mixture of synthesis gas and carbon dioxide with the supply of biomass and oxygen , wherein the synthesis gas contains carbon monoxide and hydrogen ,

[0023] ( 52 ) separating carbon dioxide from the gas mixture and adding hydrogen to separated carbon dioxide for a second reaction process ,

[0024] ( 53 ) Carrying out a second reaction process, wherein in the second reaction process a methanation is carried out using the reactants carbon dioxide and hydrogen, wherein methane and water are produced as intermediate products,

[0025] ( 54 ) feeding back methane and water obtained from the second reaction process into the first reaction process, whereby a gas mixture containing synthesis gas is produced,

[0026] ( 55 ) Discharge of synthesis gas and conversion of synthesis gas to a synthetic fuel.

[0027] The invention is based on the knowledge that in the processes established in the prior art, the carbon dioxide produced during biomass gasification must either be separated from the gas mixture, so that the carbon yield of the biomass used decreases, or must be separated in an energy-intensive RWGS reaction and a subsequent complex condensation of the water produced according to reaction equation (V).

[0028] In contrast, the process of the invention enables virtually complete utilization of the carbon bound in the biomass and efficient conversion of the obtained synthesis gas into fuel. The invention thereby represents a departure from the previously pursued and very complex reverse water gas shift reaction (RWGS) and the associated complex conditioning processes. The proposed process path is the methanation of carbon dioxide CO2 separated from the gasification gas mixture, with the carbon being fully recycled back into the first reaction process via the methanation.

[0029] (VI ) CO2+ 4H2CH4+ 2H2O

[0030] The process, which is also known as the Sabatier reaction (VI), can be carried out very advantageously cyclically and continuously and can be integrated downstream of the separation of carbon dioxide. This means that the utilization rate of the carbon contained in the biomass can be increased by incorporating it into methane. The Sabatier reaction is exothermic and takes place at a significantly lower temperature level that is better adapted to the initial reaction processes than, for example, an RWGS reaction. This makes recycling the intermediate products methane CH4 and water H2O from the methanation particularly efficient and can be carried out in a way that is adapted to the initial reaction process. Continuous cyclic application is advantageous. Renewably produced hydrogen H2, for example from the electrolysis of water, is provided as an additional reactant and fed to the Sabatier reaction.The first reaction process, which is preferably carried out as biomass gasification according to the above-mentioned reaction equation (IV), and the second reaction process of methanation are operated in a coordinated manner. Through the targeted recycling of methane (CH4) and water (H20) from the Sabatier reaction, the previously common addition of external water or steam along with the biomass in the first reaction process can be completely eliminated, or the feed can be set at a significantly lower level with regard to the material conversion.

[0031] On the one hand, the invention offers significant advantages over the use of the water-gas shift reaction (WGS), since WGS, with rising biomass prices, is associated with a large loss of carbon, which must subsequently be separated as CO2. With this concept, only a portion of the biomass can be used to produce fuels. In addition, the release of CO2 into the atmosphere leads to the emission of greenhouse gases, so that this process - despite the use of biomass - can only be qualified as climate-neutral to a limited extent. In any case, the potential for climate neutrality is only exploited to a limited extent due to the low utilization rate of the WGS route.

[0032] On the other hand, the invention is also superior to the application of the reverse watergas shift reaction (RWGS), as it proposes a technically more efficient route with full utilization. The RWGS reaction according to reaction equation (V) does lead to a higher utilization of the carbon in the biomass and its availability for subsequent synthesis processes such as Fischer-Tropsch. This process variant using RWGS, which is currently being hotly debated in research and development for the extensive utilization of the carbon contained in the biomass, has further disadvantages that the invention has recognized and which have led to this process variant not being implemented on a significant industrial scale to date:

[0033] The RWGS reaction takes place at very high temperatures of 800 °C - 900 °C and is an endothermic reaction, i.e., to reach and maintain the required reaction temperature in the reactor, heat must be supplied from outside. Therefore, in current concepts, the RWGS is often heated electrically or by burning valuable hydrogen H2, which has a negative impact on the economic viability of the entire biomass-to-liquid (BtL) process. In addition, the reaction temperature of the endothermic RWGS reaction is significantly higher than the reaction temperature of a downstream exothermic Fischer-Tropsch synthesis. Therefore, sufficiently efficient heat transport - for example, through complex heat pump systems - from the Fischer-Tropsch synthesis back to the RWGS reaction is not possible. In the RWGS reaction, the water formed must also be removed from the synthesis gas, i.e. condensed, for subsequent Fischer-Tropsch synthesis.In addition, RWGS is currently still considered to have a low technology readiness level (TRL) due to technical and economic challenges posed by the high process temperature. This raises questions about the catalyst's lifetime and the complex heating of the process gases required for the RWGS reaction.

[0034] The separation of carbon dioxide (CO2) from the gas mixture obtained from the biomass in the first reaction process is achieved by integrating an established capture process based on adsorption or absorption of carbon dioxide. CCt capture and storage, also known as CCp sequestration and CCS, generally refers to a process in which carbon dioxide (CO2) is captured, processed, compressed, and transported to a storage site, either from the environment or directly at the sources of fossil CO2 emissions of an industrial or energy-related nature, and thus removed from the natural CO2 cycle in the atmosphere for as long as possible.

[0035] In this case, for example, a separation of carbon dioxide CO2 from the gas mixture downstream of the first reaction process, i.e. in particular biomass gasification, can be used, analogous to CO2 separation in a post-combustion process. In this case, a so-called CO2 scrubbing process can be installed as a cleaning step for the gas mixture from the first reaction process. Various scrubbing processes for CO2 gas scrubbing are available. Amine scrubbing, for example, is well known on an industrial scale from natural gas processing. In amine scrubbing, the CO2 is deposited on the carrier at 40 °C by means of finely distributed amine droplets. In a further step, the amines reach a separator (stripper) where they release the CO2 again in concentrated form at 150 °C. The carbon dioxide CO2 separated from the gas mixture is thus made available again and used in the second reaction process of methanation.In general, a specific selection of a separation process for carbon dioxide CO2 integrated in the process of the invention is not restricted, so that in principle established and available CCp separation processes can be used which are based on selective adsorption or selective absorption of carbon dioxide CO2 in a suitable adsorbent material or absorbent material.

[0036] In a particularly preferred embodiment of the process, the second reaction process is carried out at a reaction temperature between 300 ° C and 400 ° C and a pressure of 18 to 22 bar.

[0037] This results in a particularly advantageous and efficient process for methanation, whereby the integration of a Sabatier reaction results in a moderate reaction temperature and reaction pressure that are adapted to both the first reaction process and the subsequent synthesis of the fuel.

[0038] In the Sabatier reaction according to equation (VI) integrated into the process here, carbon dioxide CO2 separated from the gas mixture reacts with hydrogen H2 at temperatures of 300 to 700 °C, preferably between 300 °C and 400 °C, to form methane CH4 and water H2O. This reaction is exothermic and releases a reaction enthalpy of AH° = 165.0 kJ / mol. However, the reaction is accelerated by a catalyst. Nickel catalysts are usually used for this purpose, which are improved with various promoters and stabilizers such as aluminum oxide and zirconium dioxide, but the catalytic effect of ruthenium has also been investigated. An alternative route of a Sabatier reaction can be carried out using carbon monoxide CO as the reactant according to CO + 3H2— CH4 + H2O. In the present case, the path according to equation (VI ) is followed with carbon dioxide CO2 as the reactant in order to recycle the carbon bound as methane CH4 into the first reaction process.

[0039] In a preferred embodiment of the process, additional steam is added to the first reaction process.

[0040] The steam can be extracted from the product of the Sabatier reaction, since in addition to methane CH4, water H2O is also formed at the process temperature. This is recycled and fed back into the first reaction process. This reduces or even eliminates the need to add water in the form of steam from outside to the first reaction process, since the demand for water H2O can usually be met by the second reaction process, the Sabatier reaction.

[0041] 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. Alternatively, the first reaction process—preferably biomass gasification—can be run with only the addition of oxygen (O2) and biomass, without the addition of externally supplied process steam. The feed from the Sabatier process contains significant amounts of water that are available for biomass gasification and can be used advantageously.

[0042] In a particularly preferred embodiment of the process, hydrogen H2 is obtained from an electrolysis of water H2O and fed to the second reaction process as a reactant.

[0043] With a view to climate-neutral production of the synthetic fuel, it is advantageous to obtain the hydrogen H2 from electrolysis using renewable electricity sources, such as wind power or photovoltaics. The process is characterized by a considerable increase in efficiency. If one compares the state of the art processes based on the combustion of biomass, the same amount of end product can be produced from electrolysis using significantly less expensive hydrogen H2. The demand for supplied hydrogen H2 can be reduced by almost 50% using the process according to the invention. This is because during the gasification process the hydrogen atoms present in the biomass and introduced via added steam are transferred into the components of the synthesis gas and chemically bound there.In contrast, these hydrogen atoms are converted into water (H2O) during combustion processes, which is then condensed at great expense and lost from the process altogether. In contrast, the invention achieves a particularly efficient and virtually loss-free use of hydrogen to produce synthetic fuels.

[0044] In a further preferred embodiment of the process, oxygen O2 is obtained from the electrolysis of water H2O and fed to the first reaction process.

[0045] In this way, the oxygen O2 from the electrolysis can also be utilized in process step S1 and can be reacted with the biomass to produce a gas mixture of synthesis gas and carbon dioxide CO2, which contains carbon monoxide CO and hydrogen H2. Biomass gasification is preferably carried out in the first reaction process.

[0046] In a particularly preferred embodiment of the process, the amount of oxygen O2 and water H2O fed to the first reaction process is adjusted, with an adjustment being made with regard to the amount of methane CH4 fed back into the first reaction process.

[0047] By recycling methane and water from the Sabatier reaction into the first reaction process, the need for externally supplied water or steam is reduced. This means that the supply of external process steam can be significantly reduced and adapted to the requirements of the first reaction process with regard to complete material conversion. To this end, the supplied amount of external oxygen and water or process steam is adjusted as required to the returned amount of methane and the corresponding water formed and returned in the Sabatier reaction. This means that not only the need for externally supplied hydrogen H2 but also the water requirement is reduced compared to the conventional process and a high degree of utilisation of the reactants is achieved. This takes place in a controlled manner depending on the consistency and composition of the supplied biomass in terms of its water content.Thus, in the case of biomass gasification, the mass flow of methane (CH4) and water (H20) recycled to the reaction allows for advantageous adjustment of the dosage. This is achieved by appropriate control, regulation, or adjustment of the external addition of water (H20) or oxygen (O2) to the first reaction process, which in particular comprises biomass gasification.

[0048] In a preferred embodiment of the process, a partial stream of the carbon dioxide C02 separated from the gas mixture in step S2 is fed to the first reaction process, whereby a blocking effect is brought about.

[0049] By deliberately diverting a partial flow of carbon dioxide CO2 from the process itself, an inert barrier gas is easily provided and dosed into the gasification process. In addition to the safety-relevant barrier effect against gas transfer, this also results in the creation of a particularly homogeneous gasification bed of biomass in the first reaction process, as the barrier gas promotes the creation of the most uniform conversion temperature possible across the entire volume. This is made possible by an appropriately designed distribution topology for the barrier gas with corresponding nozzle arrangements within the reaction chamber of the gasification reactor. A further advantage of using carbon dioxide CO2 is that it is not flammable and when it is used as a barrier gas no foreign gas, such as nitrogen, enters the process.

[0050] In a particularly advantageous embodiment of the process, the thermal energy obtained in the second reaction process during methanation is used in the execution of the first reaction process.

[0051] The Sabatier reaction is highly exothermic, so that a large amount of heat energy can be extracted from the reaction enthalpy of AH° = 165.0 kJ / mol and used specifically to maintain the initial reaction process. Heat integration is also particularly easy to accomplish and the temperature level is already largely adjusted. The hot product gases from the Sabatier reaction, which operates at approximately 300 °C to 400 °C, are fed to the biomass gasification reactor, where their heat energy supports the gasification reaction at a set gasification temperature. This depends on the selected gasification process and the composition of the biomass.

[0052] The gasification of biomass begins after drying at temperatures as low as 150 °C, with water vapor and oxygen initially escaping. At higher temperatures, the solid components of the biomass, primarily lignin and cellulose, are gasified. This gas ignites as soon as secondary air is introduced; the ignition temperature is between 230 °C and 280 °C.

[0053] Technical biomass gasification involves partial combustion with the aid of a gasification or oxidizing agent, usually air, oxygen, carbon dioxide or water vapor without ignition, at temperatures of 700 °C to 900 °C. During this process, the fuel is not oxidized to carbon dioxide CO2 as in combustion, but mainly to carbon monoxide CO. Other components of the resulting gas mixture are hydrogen H2, carbon dioxide CO2, methane CH4, water vapor H2O and, depending on the biomass used and the gasification process, a number of organic substances in varying concentrations. Ash and residues of biochar remain as solid residues. When the temperature of the process gas is reduced, the water vapor, mixed with organic components, condenses to form tar or an organically contaminated wood gas condensate, which is separated out.The combustible product gas can be further oxidized in a subsequent process by combustion (fuel gas) or chemical synthesis (synthesis gas) with the release of energy (exothermic process).

[0054] In a further preferred embodiment of the process, the thermal energy obtained in the second reaction process during methanation is converted into electrical energy.

[0055] Alternatively or in addition to the advantageous heat integration of the process heat obtained from the exothermic methanation described above, it is advantageous to also partially use the heat energy obtained for energy purposes and convert it accordingly. In this way, process steam can be generated, which in turn is used to generate electricity in a steam turbogenerator. The heat generated in the Sabatier reaction is therefore at a temperature level that is particularly well suited to being used through heat integration into the process or, alternatively or additionally, for generating electrical energy. In contrast to the RWGS reaction, the Sabatier reaction is an exothermic reaction that takes place with strong heat evolution. In contrast to an RWGS reaction, external heating is not necessary - correspondingly lower equipment and energy expenditure leads to increased economic efficiency.Due to the net heat generation of the entire process, heat export is advantageous. Other processes at the plant's installation site can be supplied with heat.

[0056] In a particularly advantageous embodiment of the process, the discharged synthesis gas is fed to a Fischer-Tropsch synthesis, whereby carbon monoxide CO and hydrogen H2 are converted into a synthetic fuel containing hydrocarbons.

[0057] It is particularly expedient to feed the synthesis gas obtained and processed in this way to a Fischer-Tropsch synthesis, additionally using a catalyst for the synthesis. The synthesis gas obtained from carbon monoxide CO and hydrogen H2 is already well adapted in terms of temperature and pressure levels to subsequent Fischer-Tropsch synthesis. The Fischer-Tropsch synthesis, also known as the "Fischer-Tropsch process" or "FT synthesis" for short, is a large-scale, heterogeneous catalytic process for the production of hydrocarbons. Carbon monoxide adsorbed on cobalt- or iron-containing catalyst surfaces is hydrogenated with hydrogen. The reactions take place at temperatures of approximately 150 to 350 °C and pressures of 1 to approximately 25 bar. The FT synthesis requires the production of synthesis gas as a starting material and its conversion to Fischer-Tropsch products and their further processing.Biomass gasification is used to produce the synthesis gas, and FT synthesis is integrated into the overall process. Generally, coal, natural gas, or organic waste are also available as carbon-containing starting materials for Fischer-Tropsch synthesis. Due to the variety of possible feedstocks, Fischer-Tropsch synthesis plays a central role in the search for alternatives to petroleum for the production of liquid hydrocarbons as fuel. The overall equation for Fischer-Tropsch synthesis can be represented by the following reaction equation:

[0058] (VI I ) nCO + 2nH2( CH2) n + nH2O

[0059] The provision of purified synthesis gas from biomass gasification is particularly efficient with the process of the invention, which comprises methanation and recycling of methane. A good degree of purification of the synthesis gas can be achieved, thus eliminating the need for very costly and complex gas purification and gas conditioning steps. Additional purification and conditioning of the synthesis gas is nevertheless preferred, but is less complex due to the purity already achievable initially and the achieved pressure and temperature levels.

[0060] In contrast, in conventional applications the sub-process of providing purified synthesis gas for an FT synthesis is very complex or less efficient in terms of utilization, as described above for the RWGS reaction and the WGS reaction. In particular, when coal is used - in coal gasification - the first step is to produce water gas, followed by the production of the additionally required hydrogen. The final step is to clean the gas of unwanted sulfur and nitrogen components, which must be carefully removed because most catalysts are sensitive to these substances. Gasification processes are often operated with pure oxygen, although the investment and operating costs are higher due to the required air separation.However, the yield in the synthesis stages is higher because when air is used, the resulting synthesis gas is strongly diluted by the nitrogen and the subsequent processes are less efficient.

[0061] In a preferred embodiment of the method, biomass is gasified in the first reaction process, wherein an excess air number L is set between 0 < L < 1, which is controlled with respect to the amount of methane CH4 and water H20 fed back in step S4.

[0062] Thus, the first reaction process involves the gasification of biomass, with an excess air ratio L of greater than 0.5 being preferred. In particular, carrying out the gasification reaction in a working range with an excess air ratio of 0.6 < L < 0.8 may be preferred. External water H2O or process steam is only fed in optionally, since water H2O or steam is already available from the recycling of the products from methanation. Furthermore, an external supply of pure oxygen O2 together with the biomass into the gasification process is preferred, so that the introduction of gases foreign to the process, such as nitrogen from the air, is avoided. The gasification reactor for the biomass can be operated under various conditions, resulting in a high level of flexibility.In practice, an adapted and optimized operating mode is set in a control or process control system depending on the fuel and gasifier type. Methane CH4 is fed selectively into the gasifier, preferably at a point in the gasification reactor that favors the conversion process into the desired synthesis gas components carbon monoxide CO and hydrogen H2. Different technical gasification reactors can be used for biomass gasification itself, which differ primarily in the type of contact between biomass and gasification agent (air, oxygen or steam). As a rule, three basic reactor types can be used in a plant: the fixed bed gasifier, the fluidized bed gasifier and the entrained flow gasifier.

[0063] The object directed to a corresponding plant for producing synthetic fuel from biomass is achieved according to the invention by a plant for producing a synthetic fuel comprising: - a gasification reactor for producing a gas mixture containing carbon dioxide and synthesis gas, wherein the gasification reactor is designed for gasifying biomass and has an exhaust line for discharging the synthesis gas produced,

[0064] - a separation device connected to the exhaust gas line, which is designed to separate carbon dioxide from the gas mixture and to make it available for methanation via a first reactant line,

[0065] - a Sabatier reactor set up to carry out methanation using carbon dioxide from the separation device and hydrogen, which can be fed to the Sabatier reactor via a second reactant line,

[0066] - a return line connected to the Sabatier reactor, connected to the gasification reactor and designed to feed methane and water into the gasification reactor, and

[0067] - a Fischer-Tropsch reactor connected to the exhaust gas line downstream of the separation device and designed to synthesize fuel from the synthesis gas.

[0068] The technical effects and advantages explained above for the proposed process apply equally to the proposed plant. In particular, the return line makes it advantageous to specifically return products from the Sabatier reactor, methane CH4 and water H2O, to the gasification reactor, so that almost complete material conversion can be achieved using the carbon from the biomass and particularly economical operation of the plant. The technical combination of a CO2 separation device with the Sabatier reactor implements extensive utilization. Continuous recycling enables almost complete conversion of carbon dioxide CO2 into synthesis gas, so that the plant can achieve an enrichment and purification effect, with which almost pure carbon monoxide CO and hydrogen H2 can be made available in the exhaust line as synthesis gas.This allows for easier use and transfer of the already largely prepared synthesis gas to the downstream Fischer-Tropsch reactor. However, an additional purification device can still be provided in the exhaust line to remove residual impurities and potentially catalyst-damaging substances, such as sulfur or nitrogen components, from the synthesis gas. However, the purification effort is significantly lower than when using an RWGS reactor.

[0069] Preferably, a bypass line is provided in the plant, which leads out of the separation device and is connected to the gasification reactor, bypassing the Sabatier reactor, so that carbon dioxide can be supplied to the gasification reactor as a barrier gas.

[0070] The seal gas serves to prevent the entry of external gases from the process into the gasification reactor, but also to prevent the escape and release of components of the gas mixture, such as carbon monoxide, from the gasification reactor. At the same time, the seal gas serves to compensate for a temperature gradient in the reactor chamber, ensuring the most homogeneous gasification temperature possible for the biomass to be gasified during operation.

[0071] In addition, as a particularly preferred embodiment, a metering device is connected to the return line, which comprises a gas distributor, so that methane can be introduced selectively and locally into the gasification reactor. For a better result in terms of yield and quality (purity) of the synthesis gas produced in the gasification reactor, it has been shown that a targeted feeding of methane from the Sabatier reactor into the gasification reactor is expedient. A gas distributor topology at the outlet of the return line is therefore advantageous. This enables the gasification material to be particularly uniformly and precisely spatially exposed to methane obtained from the Sabatier reactor. The metering device with the gas distributor is designed and arranged in the gasification reactor in such a way that the metering of a respective volume flow can be spatially and locally adjusted via a nozzle arrangement.The dosage can be adjusted via a number of controllable dosing valves, with one dosing valve supplying one nozzle or a group of nozzles of the gas distributor.

[0072] In the following, the invention is explained by way of example with reference to the attached figures on the basis of preferred embodiments, wherein the features presented below can represent an aspect of the invention both individually and in various combinations with one another. They show:

[0073] FIG 1 shows a schematic representation of a plant for producing a synthetic fuel from biomass;

[0074] FIG 2 shows another plant with improved carbon utilization rate compared to the plant shown in FIG 1;

[0075] FIG 3 shows a plant for producing a synthetic fuel from biomass according to the invention;

[0076] FIG 4 is a flow diagram of a process for 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 embodiment. A biomass-to-liquid (BtL) concept is implemented here to obtain liquid fuel F from the gasification of biomass BM as a 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 exhaust gas line 5 in which a water-gas shift reactor 21 is arranged or integrated. The WGS reactor 21 integrated into the exhaust gas line 5 thus takes over the function of the second reactor. Connected downstream of the exhaust gas line 5 are a separation device 7 for carbon dioxide CO2 and a Fischer-Tropsch reactor 15.The plant 1A further comprises a feed line 25 for feeding water H2O, the feed line 25 being connected to the WGS reactor 21 according to the embodiment shown in FIG 1. The flow direction in the plant 1A shown in FIG 1 is here - as in the other figures - essentially from left to right and based on 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 approximately 750 °C. The gas mixture thus obtained contains carbon monoxide CO, hydrogen H2 and carbon dioxide CO2. The components carbon monoxide CO and hydrogen H2 form the economically particularly interesting synthesis gas SG.In order to obtain the ratio of hydrogen H2 to carbon monoxide CO required for a subsequent Fischer-Tropsch synthesis FTS of fuel F in the Fischer-Tropsch reactor 15, for example 2:1, a portion of the carbon monoxide is converted with water vapor D to hydrogen H2 and CO2 in a so-called water-gas shift reaction WGS according to the following reaction equation (VI II):.

[0077] (VI II) CO + H2O CO2 + H2 This increases the proportion of hydrogen H2 in the synthesis gas SG. The carbon dioxide CO2 is then separated in the separation device 7 in order to obtain the purest possible synthesis gas SG with the components carbon monoxide CO and hydrogen H2. The synthesis gas SG thus obtained is further processed and fed to a chemical synthesis, in this case an FT reaction in the Fischer-Tropsch reactor. In this way, long-chain hydrocarbons (CH2) nsynthesized and a synthetic fuel F can be produced accordingly, for example, liquid fuels such as diesel. The separation of carbon dioxide C02 in the separation device 7 is accompanied by a large loss of carbon, which is not converted into fuel F, which is why the utilization rate of the carbon provided in the biomass BM is low. The economic viability of a green fuel F produced in this way is inadequate compared to conventional fossil fuels.

[0078] FIG. 2 shows a further plant 1B with a significantly improved utilization rate 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 upgraded and modified particularly advantageously compared to the plant according to FIG. 1A in order to carry out a significantly improved process for producing synthetic fuel F from the gasification of biomass BM.

[0079] As can be seen in FIG. 2, the system 1B shown there comprises a first reactor in the form of a gasification reactor 3, having an exhaust line 5, in which a reverse water-gas shift reactor 23 is arranged or integrated. The RWGS reactor 23 integrated into the exhaust line 5 thus takes over the function of the second reactor. This makes it possible to largely convert the carbon dioxide CO2 contained in the gas mixture by operating an RWGS reaction in the RWGS reactor 23, according to the previously described reaction equation (V): (V) CO2 + H2CO + H2O

[0080] Plant 1B further comprises a supply line 25 for supplying hydrogen H2. According to the exemplary embodiment shown in FIG. 2, the supply line 25 can be arranged upstream of the RWGS reactor 23 in the flow direction or, alternatively—not shown in FIG. 2—downstream of the RWGS reactor 23. Further plant elements in plant 1B include a condensation device 27 and a Fischer-Tropsch reactor 15.

[0081] As can be seen in FIG 2, gasification is initially carried out in the gasification reactor 3 at a process temperature. This first reaction process produces an exhaust gas containing carbon dioxide CO2 in a gas mixture using biomass BM and oxygen O2. The gas mixture also contains carbon monoxide CO and hydrogen H2, which is also referred to herein as the generated synthesis gas SG. The first reaction process can be a technical biomass gasification at a process temperature of greater than 750 °C. Alternatively, the first reaction process can be any other form of exothermic biomass utilization that produces a gas mixture containing carbon dioxide CO2 and synthesis gas SG. Lower process temperatures are also possible.

[0082] A further reaction process then takes place 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 can select a suitable RWGS catalyst so that the RWGS reaction increases the proportion of carbon monoxide CO in the synthesis gas SG according to the above reaction equation (V). An excess of hydrogen H2 is fed externally to the RWGS reaction via the feed line 25, the amount of excess hydrogen H2 being determined based on the proportion of carbon dioxide CO2 in the synthesis gas SG produced in the first reaction, taking into account the above RWGS reaction equation (V).

[0083] This RWGS equilibrium reaction is a reverse reaction of the WGS reaction shown in FIG. 1 and takes place at very high temperatures of approximately 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 CO2 conversion is achieved in the RWGS, the vast majority of the synthesis gas SG can be converted to hydrocarbons (CH2) in the FTS reaction. nbe converted. Water H2O is removed from the synthesis gas by condensation before the FTS reaction. This can result in cooling of the synthesis gas SG modified by the RWGS reaction, i.e. the synthesis gas SG with an increased carbon monoxide and a reduced carbon dioxide content compared to the synthesis gas SG directly resulting from the first reaction process in the gasification reactor 3. The cooling involves a targeted condensation of water H2O from the synthesis gas SG. For this purpose, a condensation device 27 is provided which is connected to the exhaust gas line 5. The condensed water H2O is collected and drained off via a drainage system. The synthesis gas SG, which has been largely freed of water H2O and carbon dioxide CO2 and cooled, is introduced into the Fischer-Tropsch reactor 15 and converted into synthetic fuel F in an FTS reaction.Although the process route via an RWGS reaction demonstrates a high degree of utilization of the carbon bound in the biomass BM, it is very energy- and process-technically complex and is still at an early stage of technology, so that an imminent commercial application on an industrial scale is not in sight. Furthermore, the water H2O is condensed out and discharged from the process, so that a low degree of utilization is recorded with regard to hydrogen H2. In contrast, FIG 3 shows a plant 1 for producing a synthetic fuel from biomass, which overcomes the disadvantages of the processes described in FIG 1 and FIG 2.With the plant concept of FIG 3 and the process control, a practically complete carbon utilization of the carbon bound in the biomass BM is possible with a particularly efficient process control and conversion to fuel F from the obtained synthesis gas SG.

[0084] 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 generating a gas mixture of carbon dioxide CO 2 and synthesis gas SG . The gasification reactor 3 is set up to gasify biomass BM and has an exhaust gas line 5 for discharging the produced synthesis gas SG . The synthesis gas SG comprises carbon monoxide CO and hydrogen H 2. Furthermore, a separation device 7 connected to the exhaust gas line 5 is provided. The separation device 7 is set up to selectively separate carbon dioxide CO 2 from the gas mixture with the synthesis gas SG and to provide the carbon dioxide CO 2 thus obtained via a first reactant line 9A for methanation. For this purpose, a Sabatier reactor 11 is provided, which is designed to carry out methanation using carbon dioxide C02 from the separation device 7 and for the addition of external hydrogen H2.The external hydrogen H2 is advantageously obtained from an electrolyzer 29 from a renewable power source by means of electrolysis and can be fed to the Sabatier reactor 11 via a second reactant line 9B. A return line 13 is connected to the Sabatier reactor 11, which flows into the gasification reactor 3 and is designed to convert methane CH4 and water H20 from the Sabatier reaction SAR according to the reaction equation.

[0085] (VI) C02 + 4H2CH4 + 2H2O is fed into the gasification reactor 3. Downstream of the separation device 7, a Fischer-Tropsch reactor 15 is connected to the exhaust gas line 5. This reactor is designed 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 line 17, which leads from the separation device 7 and is connected to the gasification reactor 3, bypassing the Sabatier reactor 11, so that carbon dioxide CO2 can be fed to the gasification reactor 3 as a seal gas. The return line 13 opens into a metering device 19, to which a gas distributor is connected, so that methane CH4 can be selectively and locally introduced into the gasification reactor 3 and distributed in a targeted manner. This achieves a particularly uniform distribution and homogeneous reaction in the gasification reactor.

[0086] The operation of plant 1 can be illustrated by the simplified flow diagram shown in FIG 4, which illustrates process steps for producing synthetic fuel F from biomass BM, with reference in part to elements of FIG 3:

[0087] In a first process step S1, a first reaction process is carried out, wherein the first reaction process produces a gas mixture of synthesis gas SG and carbon dioxide CO2 with the supply of biomass BM, oxygen O2, wherein the synthesis gas SG contains carbon monoxide CO and hydrogen H2. The gasification of the biomass BM in the first reaction process is carried out in such a way that an excess air number L is set between 0 < L < 1, wherein the gasification reactor 3 is controlled to an operating point at 0.6 < L < 0.8. The amount of methane CH4 and water H2O fed back in process step S4 is also controlled. Additional external feeding of water H2O or water vapor D into the gasification reactor 3 is thereby avoided or at least significantly reduced.In a second process step S2, carbon dioxide CO2 is specifically separated from the gas mixture, and hydrogen H2 is added to the separated carbon dioxide CO2 for a second reaction process. In a third process step S3, the second reaction process is carried out. In the second reaction process, methanation is carried out using the reactants carbon dioxide CO2 and hydrogen H2, producing methane CH4 and water H2O as intermediate products. In a process step S4, the methane CH4 and water H2O obtained in the second reaction process are returned to the first reaction process, producing a gas mixture containing synthesis gas SG. In process step S5, synthesis gas SG is discharged, and in an FTS reaction, synthesis gas SG is converted into a synthetic liquid fuel F.

[0088] In the second reaction process in step S3, the heat energy Q obtained in the exothermic methanation is at least partially used in the first reaction process. Therefore, in addition to the products from the Sabatier reaction SAR, heat Q is also recycled and introduced into the gasification process. In process step S3, the Sabatier reaction SAR is carried out at a reaction temperature between 300 °C and 400 °C and a pressure of 18 to 22 bar. However, it is also conceivable to use the released reaction heat Q to generate electricity. Electricity can be generated, for example, by generating steam from the heat Q and expansion in a steam turbogenerator. If necessary, additional water H2O in the form of steam D can be added to the first reaction process in process step S1.The water input from the Sabatier reaction SAR via the return line 13 generally already sufficiently covers the water requirement for biomass gasification. The need for hydrogen H2 as a reactant for the Sabatier reaction SAR is covered by electrolysis of water H2O, which is fed into the second reaction process in process step S3 alongside the carbon dioxide CO2 as a reactant. In addition, control is carried out such that the amount of oxygen O2 and water H2O fed to the first reaction process is precisely set, with an adjustment being made with regard to the amount of methane CH4 fed back into the first reaction process. In addition, a partial stream of the carbon dioxide CO2 separated from the gas mixture in step S2 is fed to the first reaction process, creating a barrier effect in the gasification reactor 3.The discharged synthesis gas SG, which may be purified through unspecified purification steps, is fed into a Fischer-Tropsch synthesis FTS. This process converts carbon monoxide CO and hydrogen H2 into a synthetic fuel F containing liquid hydrocarbons. These are long-chain (CH2) hydrocarbons. n Molecules that can be used as liquid fuels F .

[0089] This process control according to FIG. 4 and plant 1 according to FIG. 3 according to the invention is superior to the application of the reverse water-gas shift reaction (RWGS), as it proposes a technically significantly more efficient route with full carbon utilization. The disadvantages associated with the RWGS reaction due to its complexity and the high reaction temperatures and heat balance can be overcome with the invention, thus providing an economically attractive and technically feasible alternative to known BtL processes.

[0090] The temperature level of the Sabatier reaction SAR alone, at 300 °C - 400 °C, is significantly 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 can therefore be expected during the technical implementation of the plant concept. Furthermore, with regard to the process pressure, it is a great advantage of the invention that all sub-processes - depending on the process selected for CO2 capture - can be carried out at practically a single pressure level as the nominal working pressure of plant 1. Process pressures in the range of around 10 bar have proven to be advantageous here. This eliminates the need for energy-intensive repeated compressions, which would otherwise be necessary after intermediate expansions, as in previously known approaches. If one compares gasification orIn state-of-the-art BtL processes based on the combustion of biomass BM, the same amount of end product can be produced with significantly less hydrogen H2 from an electrolysis plant. Estimates have shown that the process proposed here can achieve a reduction in H2 demand of almost 50%. This is because during the gasification process, the hydrogen atoms present in the biomass BM and introduced via added steam D are converted into the components of the synthesis gas SG. In contrast, in combustion processes these hydrogen atoms are converted into water, which is condensed and lost in the conversion process to a biofuel.

Claims

Patent claims 1. A process for producing a synthetic fuel (F), comprising the steps: (51) Carrying out a first reaction process, wherein the first reaction process produces a gas mixture of synthesis gas (SG) and carbon dioxide (CO2) by supplying biomass (BM) and oxygen (O2), wherein the synthesis gas (SG) contains carbon monoxide (CO) and hydrogen (H2), (52) Separating carbon dioxide (CO2) from the gas mixture and adding hydrogen (H2) to separated carbon dioxide (CO2) for a second reaction process, (53) Carrying out a second reaction process, wherein in the second reaction process a methanation is carried out using the reactants carbon dioxide (CO2) and hydrogen (H2), whereby methane (CH4) and water (H2O) are produced as intermediate products, (54) feeding back methane (CH4) and water (H2O) obtained from the second reaction process into the first reaction process, thereby producing a gas mixture containing synthesis gas (SG), (55) Discharge of synthesis gas (SG) and conversion of synthesis gas to a synthetic fuel (F) .

2. The process according to claim 1, wherein the second reaction process is carried out at a reaction temperature between 300 °C and 400 °C and a pressure of 18 to 22 bar.

3. Process according to claim 1 or 2, wherein water (H2O) in the form of steam (D) is additionally added to the first reaction process.

4. A process according to any one of the preceding claims, wherein hydrogen (H2) is obtained from an electrolysis of water (H2O) obtained and fed into the second reaction process as a reactant.

5. Process according to one of the preceding claims, in which oxygen (O2) is obtained from an electrolysis of water (H20) and is fed to the first reaction process as a reactant.

6. A method according to any one of the preceding claims, wherein the amount of oxygen (02) and water (H20) supplied to the first reaction process is adjusted, with an adjustment being made with respect to the amount of methane (CH4) fed back into the first reaction process.

7. Process according to one of the preceding claims, in which a partial stream of the carbon dioxide (C02) separated from the gas mixture in step (S2) is fed to the first reaction process, whereby a barrier effect is brought about.

8. Process according to one of the preceding claims, in which heat energy (Q) obtained in the second reaction process during methanation is used in carrying out the first reaction process.

9. Process according to one of the preceding claims, in which heat energy (Q) obtained in the second reaction process during methanation is converted into electrical energy.

10. Process according to one of the preceding claims, in which discharged synthesis gas (SG) is fed to a Fischer-Tropsch synthesis, whereby carbon monoxide (CO) and hydrogen (H2) are converted into a synthetic fuel (F) comprising liquid hydrocarbons.

11. A process according to any one of the preceding claims, wherein biomass (BM) is gasified in the first reaction process, wherein an excess air number L is between 0 < L < 1 which is controlled with respect to the amount of methane (CH4) and water (H20) fed back in step (S4).

12. Plant (1) for producing a synthetic fuel (F) comprising - a gasification reactor (3) for producing a gas mixture containing carbon dioxide (C02) and synthesis gas (SG), wherein the gasification reactor (3) is designed for gasifying biomass (BM) and has an exhaust gas line (5) for discharging the synthesis gas (SG) produced, - a separation device (7) connected to the exhaust gas line (5) which is designed to separate carbon dioxide (C02) from the gas mixture and to make it available for methanation via a first reactant line (9A), - a Sabatier reactor (11) arranged to carry out methanation using carbon dioxide (C02) from the separation device (7) and hydrogen (H2) which can be fed to the Sabatier reactor (11) via a second reactant line (9B), - a return line (13) connected to the Sabatier reactor (11), which is connected to the gasification reactor (3) and is designed to feed methane (CH4) and water (H20) into the gasification reactor (3), and - a Fischer-Tropsch reactor connected to the exhaust gas line (5) downstream of the separation device (7) (15) , which is designed to synthesize fuel (F) from the synthesis gas (SG).

13. Plant (1) according to claim 12, in which a bypass line (17) is provided which leads out of the separation device (7) and is connected to the gasification reactor (3) bypassing the Sabatier reactor (11), so that Carbon dioxide (CO2) can be fed to the gasification reactor (3) as a barrier gas.

14. Plant (1) according to claim 12 or 13, wherein a metering device (19) comprising a gas distributor is connected to the return line (13), so that methane (CH4) can be introduced selectively and locally into the gasification reactor (3).

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