Process and equipment for producing bioethanol without CO2 emissions by conversion of synthesis gas obtained from the thermal conversion of waste at high temperatures.

Abstract

The present invention is a process and apparatus for producing bioethanol with no CO2 emissions by anaerobic fermentation of syngas produced by thermal conversion at high temperatures of feedstock consisting of municipal solid waste (MSW), agricultural waste or its derivatives such as refuse derived fuels (RFD), or even industrial waste such as non-recyclable plastic waste, or a combination thereof, with extra hydrogen being added by electrolysis to balance the H2 / CO ratio, thus maximizing the conversion of the organic components in the fermentation step and preventing the emission of CO2 to the atmosphere.

Description

[Technical field]

[0001] The present invention relates to the field of processes and equipment suitable for converting RDF (Refuse Derived Fuel), MSW (Municipal Solid Waste) and similar materials such as plastic residues into products with higher added value by ensuring environmental compatibility in terms of the absence of greenhouse gas emissions, and in particular consists of a process and equipment for producing bioethanol without CO2 emissions by converting synthesis gas obtained from the thermal conversion of waste at high temperatures. [Background technology]

[0002] Global warming is forcing the European Union to implement a strategy to reduce greenhouse gas (GHG) emissions in Europe by 2050 by finding alternatives to the use of natural gas or other hydrocarbons.

[0003] The traditional method of disposing of municipal, agricultural and industrial waste by incineration is also one of the major sources of greenhouse gas emissions, mainly CO2, CH4 and nitrogen oxides. In contrast, waste can be used as a source of carbon and hydrogen by converting it into synthesis gas, which is a mixture of CO, CO2 and H2. Processes for such conversion are known in the art.

[0004] For example, US Pat. No. 5,399,633 describes a gasification process by use of pure oxygen for the conversion of waste to synthesis gas, the temperature of which is controlled along the vertical axis of the reactor.

[0005] US Pat. No. 5,399,433 describes a high temperature waste conversion process to produce synthesis gas, followed by purification of the synthesis gas and adjustment of the H2 / CO ratio for the purpose of producing methanol.

[0006] In both of the cited examples, there is an excess of carbon in the feed matrix relative to the final product, which leaves the process mainly in the form of CO2.

[0007] Further conversion of the synthesis gas to desired products, in particular ethanol, can be carried out by different methods.

[0008] US Pat. No. 5,399,633 describes a process for converting synthesis gas by anaerobic bioconversion into oxygenated hydrocarbon products; the hydrocarbon products are, inter alia, ethanol, acetic acid, n-propanol and n-butanol.

[0009] US Pat. No. 5,399,433 describes a different method for improving the CO / H2 composition of a gas stream available to supply the microbial production of oxygenated compounds.

[0010] Finally, in US Pat. No. 5,233,993, the production of ethanol is obtained by anaerobic fermentation from CO-rich synthesis gas, which means that part of the carbon is lost as CO, whereas the production of CO can only be reduced or avoided as the amount of available H increases.

[0011] Therefore, there remains a need for zero-emission processes and equipment for producing chemicals, such as ethanol, or biofuels from carbon contained in municipal solid waste (MSW), agricultural waste, or its derivatives, such as refuse derived fuels (RDF), and industrial waste, such as non-recyclable plastic residues.

[0012] Refuse-derived fuel (RDF) is a type of fuel produced after separation of the organic fraction, metals, paper and plastics. Additionally, there is a need for processes and equipment that have zero emissions, especially with regard to CO2 emissions. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] International Publication No. 2018 / 066013 [Patent Document 2] International Publication No. 2018 / 134853 [Patent Document 3] U.S. Patent No. 9,518,237 [Patent Document 4] US Patent No. 2019 / 0078121 [Patent Document 5] US Patent Application Publication No. 2019 / 0323042 Summary of the Invention [Problem to be solved by the invention]

[0014] The object of the present invention is to overcome the limitations of the prior art by providing a process and an apparatus suitable for converting municipal waste and other industrial wastes, such as non-recyclable plastics, into bioethanol, in compliance with the regulatory guidelines aimed at reducing greenhouse gas emissions in the environment to zero. [Means for solving the problem]

[0015] The solution provided is a process and associated equipment in which CO2 emissions are avoided due to the synergistic effect of the fermentation and methanation steps, also taking into account that gasification of such feedstocks is well known to be accompanied by a greater production of CO and CO2, producing a synthesis gas enriched with extra hydrogen from the electrolysis process, thus maximizing the biological fermentation process to produce ethanol. [Brief description of the drawings]

[0016] A better understanding of the invention can be obtained by reference to the following detailed description and the accompanying drawings which show preferred embodiments by way of non-limiting examples.

[0017] [Figure 1] 1 shows a general conceptual diagram of the present invention. [Diagram 2]FIG. 1 shows a block diagram of a preferred embodiment of the present invention with units for introducing hydrogen upstream and downstream of the fermenter, refining raw ethanol, and purifying recovered water. [Diagram 3] FIG. 1 shows a block diagram of an alternative embodiment of the invention in which hydrogen is introduced completely upstream of the fermentor. [Figure 4] FIG. 1 shows a detailed diagram of a preferred embodiment with hydrogen introduction upstream and downstream of the fermenter. [Diagram 5] 5 is an alternative embodiment to FIG. 4 in which hydrogen is introduced only upstream of the fermenter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Detailed Description of the Invention The present invention relates to a process for producing raw bioethanol by fermentation of synthesis gas produced by thermal conversion of a feedstock consisting of one or more types of waste materials.

[0019] The present invention further describes an apparatus for carrying out the aforementioned process.

[0020] The described processes and apparatus allow for the conversion of waste into biofuels, such as ethanol; the waste includes municipal solid waste (MSW), agricultural waste or its derivatives, such as refuse-derived fuels (RFD), or even industrial waste, such as non-recyclable plastic waste.

[0021] According to a first characteristic of the invention, the process makes use of the availability of hydrogen produced by electrolysis to enable the conversion of carbonaceous elements contained in the gasification waste into ethanol.

[0022] The availability of hydrogen produced by electrolysis further makes it possible to avoid the emission of CO2 into the atmosphere, since the carbonaceous elements not converted during the fermentation step are converted into methane by methanation reactions simply by adding further hydrogen. The methane thus produced can be sent for distribution or recycled to the gasifier to control the waste conversion temperature.

[0023] The developed process stems from the consideration that synthesis gas produced by high temperature conversion of municipal solid waste, whether treated or not, or by thermal conversion of agricultural or industrial waste, has a H2 / CO ratio by volume equal to about 1.

[0024] The optimal H2 / CO ratio required to produce ethanol by fermentation is about 2.0÷2.2 by volume, and it is therefore clear that thermal conversion of waste materials as described above to produce synthesis gas having a ratio value lower than the optimal one will result in excess carbonaceous components coming out of the process, mainly in the form of carbon dioxide.

[0025] In accordance with another aspect of the present invention, a process is provided that includes a plurality of high temperature waste converters capable of producing synthesis gas and a plurality of electrolysis cells for producing hydrogen and oxygen.

[0026] By integrating the synthesis gas produced by the converter with hydrogen (H2) produced by electrolysis, it is possible to use almost the entire carbonaceous components in the form of CO / CO2 of the synthesis gas to produce ethanol. Furthermore, in order to maximize the CO2 conversion and thus avoid emissions into the atmosphere, it is provided that the residual components still present in the downstream stream of the fermenter are converted into synthetic methane by the so-called methanation reaction. CO2+4H2←→2H2O+CH4

[0027] Advantageously, the methane can be used in high temperature conversion of waste materials for better control of the process and can be sent at battery limits for use outside of this process.

[0028] Block diagrams of the described embodiments can be seen in Figures 4 and 5. However, alternative embodiments of the invention are possible without changing the inventive concept underlying the invention.

[0029] Rather, the embodiments described herein are intended to provide a thorough and complete disclosure that will fully convey the scope of the invention to those skilled in the art.

[0030] The invention therefore relates to an environmentally compatible process for the production of ethanol from waste through a fermentation step of syngas, which fermentation is optimized by adding extra hydrogen obtained by electrolysis of water, while the oxygen of the same electrolysis is used as combustion in a thermal conversion step, making it possible to avoid the emission of CO2 into the atmosphere.

[0031] In a preferred but non-limiting embodiment, such a process comprises the following main steps: High-temperature thermal conversion of waste with production of raw syngas (100); · Production of H2 and O2 by electrolysis (102); · Purification of raw synthetic gas (101); Fermentation of purified syngas to produce raw bioethanol after addition of all or part of the hydrogen produced by electrolysis to achieve the optimal H2 / CO ratio required for the fermentation reaction (103); Partial or total conversion of residual carbon dioxide present in the purge gas from fermentation to methane (105); Separation of the produced methane from the unconverted carbon dioxide (106) and recycling of all or a portion of the recovered methane and all of the separated CO2 to the high temperature thermal conversion step (100); · Purification of raw bioethanol produced by fermentation (104); The main step may also include several further sub-steps such as: Purification of water obtained from the production of raw bioethanol and recycling it into the same water electrolysis unit (107).

[0032] The secondary steps mentioned above contribute to optimizing the process in terms of utility consumption.

[0033] In the preferred embodiment described, the high temperature waste to syngas conversion step (100) includes multiple conversion trains or reactors. More specifically, it has been chosen to use at least two, and preferably three, thermal conversion trains to convert the feedstock to syngas.

[0034] Such a solution makes it possible to precisely manage maintenance periods and guarantee a certain continuity of use, thereby allowing operation at a greater capacity even if one train is stopped to maintain the other two, thus ensuring a minimal turndown (75-80%) and a continuous, almost constant operation for the downstream steps of the thermal conversion.

[0035] Furthermore, the operational selection to carry out thermal conversion for several trains makes it possible to equalize the composition of the various streams of syngas leaving each gasifier before the subsequent purification steps, which means the possibility to adjust the feedstock supplied to each gasifier in order to obtain a raw syngas suitable for processing in the subsequent purification steps, increasing the flexibility of the plant.

[0036] For example, a waste flow rate of 8-10 t / h was chosen, which is fed to each individual thermal conversion line for a total of 24-30 t / h of convertible waste.

[0037] The typical composition of the waste is shown in Table 1. [Table 1]

[0038] Each thermal converter is supplied with pure oxygen produced by the electrolysis process (102) as gasification agent. In addition, the introduction of a specific aliquot of natural gas (CH4) recycled from the methanation step (105) and subsequent separation (106) is provided for the purpose of controlling the temperature profile within the reactor.

[0039] The CO2 stream can also be advantageously used to inert the waste feed system, thus preventing any syngas loss and any air ingress. In the described embodiment, it was chosen to use CO2 as an inerting agent, which is likely available in the process generated streams.

[0040] However, other alternative inert gases to CO2, such as a nitrogen-rich stream, can be used to achieve the same goal of preventing any syngas losses and any air ingress into the waste feed system.

[0041] For that purpose, it was chosen to add an additional amount of hydrogen so as to allow a certain amount of CO to be present downstream of the methanation step, to separate it from the methane produced and to recycle it completely as a deactivating agent to the thermal conversion step (100).

[0042] This allows the unconverted amount of carbon dioxide to continue circulating within the plant, by carrying out the fermentation step (103) and the subsequent methanation step (105) in such a way that the CO2 is not completely converted, thus achieving an optimization and stabilization of all processing steps.

[0043] Each conversion train consists of a conversion reactor operating at near atmospheric pressure (up to 500mbarg) and distributing synthesis gas at a temperature of 1100-1200°C. The synthesis gas is rapidly cooled to 90°C by an evaporation quench that fixes the composition of the synthesis gas obtained at high temperatures in the reactor, avoiding the occurrence of collateral reactions responsible for the formation of pollutants such as dioxins and furans.

[0044] The invention comprises, downstream of the thermal conversion unit (100), a purification step (101) operating at dual pressure, including several units depending on the capacity of the plant, the purpose of which is to remove particulates, metals, chlorides, ammonia, COS and H2S.

[0045] In the low pressure region downstream of the quench, the syngas leaving all gasifiers is at a pressure close to atmospheric, up to about 100-500 mbarg. The syngas is sent to an acid wash column operating at a pH of 1-3; these conditions allow for the removal of any collected particles and metals from the syngas stream.

[0046] The synthesis gas coming out of the three conversion trains at the outlet of the acid column is collected together and sent to a common alkaline washing column, which neutralizes it by raising the pH above 7 and reducing any corrosion phenomena of downstream equipment.

[0047] A further purification step is performed by a wet electrostatic precipitator (WEP), the purpose of which is to remove the collected dust, optionally followed by a subcooled water wash in a column with the aim of further reducing dust and particulates in the syngas.

[0048] At the outlet of the low temperature washing section, a gas storage tank makes it possible to handle any flow rate and pressure fluctuations of the synthesis gas. According to the invention, the pressure at the gas meter is set at about 40mbarg.

[0049] Before entering the high pressure purification section, the syngas is compressed up to 12 barg via a dedicated compression unit. The pressurized syngas is then sent to an adsorption bed for the purpose of removing residual dust, particulates and heavy metals.

[0050] This operation is followed by a catalyst / adsorbent bed allowing the removal of HCl and a hydrolysis reactor allowing the conversion of COS and HCN to H2S and NH3, respectively.

[0051] The synthesis gas exiting the hydrolysis reactor is sent to a Hg removal bed and an H2S removal system according to known techniques.

[0052] In this sense, removal of H2S can be achieved by systems that allow conversion of H2S to elemental sulfur and remove it as sulfur sludge, or by amine washing.

[0053] A typical composition of the refined synthesis gas is shown in Table 2.

[0054] The purified synthesis gas can be fed to an ethanol fermentation section (103).

[0055] The data shown in Table 2 confirm what has already been stated regarding the lower than optimal H2 / CO ratio for the fermentation step. [Table 2]

[0056] According to the invention, the additional hydrogen required to obtain an optimal H2 / CO ratio is provided by an electrolysis step (102) based on several cells, depending on the capacity of the plant, suitable for the production of hydrogen and oxygen.

[0057] In a preferred, but non-limiting embodiment of the invention, a portion of the hydrogen produced by electrolysis is added to the syngas upstream of the fermentation step (103) and the remaining portion is added downstream of the fermentation unit (103) and before the methanation step (105), with the aim of optimizing both steps with regard to the associated equipment volumes and operating conditions. In an alternative embodiment, the electrolytically produced hydrogen stream is added completely before the fermentation step (103), facilitating the maximization of the conversion of CO and CO2 to ethanol.

[0058] In both of the described solutions, oxygen is sent to a conversion train to perform waste conversion (100).

[0059] According to the invention, the conversion of CO and CO to ethanol is carried out in a fermentation step (103), which takes place in one or more bioreactors containing bacterial cultures dispersed in a liquid nutrient medium.

[0060] As is known, organic streams consisting of a single carbon atom (C1) and therefore containing one or more of the elements CO and CO2, are converted into ethanol in combination with H2 at low temperatures of 30-40°C and low pressures of 3-5 barg.

[0061] Thus, according to the invention, synthesis gas containing CO, CO2, with the addition of H2 coming from electrolysis is bubbled inside the liquid nutrient medium. The conversion results in the formation of ethanol dispersed in the nutrient medium in the aqueous phase, and a residual gas stream containing unreacted components and the CO2 produced by the biological conversion process during fermentation.

[0062] The ethanol concentration in the stream leaving the fermentation step is comprised in the range of 3-6% by weight and therefore a specific step is required to separate the ethanol from the aqueous phase in order to obtain anhydrous ethanol.

[0063] In a preferred embodiment, the synthesis gas, in addition to a portion of the hydrogen produced by electrolysis and thus having a H2 / CO ratio of 2.0-2.2% by volume, is converted to raw ethanol and purge gas.

[0064] In an alternative embodiment, the synthesis gas has an H2 / CO ratio between 5 and 5.2% by volume, in addition to all the hydrogen produced by electrolysis, thus converting CO and CO2 to ethanol. According to this mode of operation, the CO2 initially contained in the synthesis gas is converted during the fermentation process, which results in an increased ethanol yield and minimizes the purge gas flow.

[0065] The purge gas produced by the fermentation step is further processed in a methanation step (105) to convert any CO2 still present in the gas into methane to prevent its emission into the environment.

[0066] In the preferred embodiment described, the remaining part of the hydrogen produced by electrolysis (102) is added to the purge gas stream downstream of the fermentation step (103) in order to optimize H2 / C1 to carry out methanation of the amount of CO2 still present in the purge gas and to produce synthetic methane. In the alternative embodiment described, in which the hydrogen produced by electrolysis is added completely before the fermentation step, thus producing a minimal amount of purge gas, the remaining amount of residual H2 in the purge gas outlet stream at the outlet of the fermenter is sufficient to carry out methanation of the residual CO and CO2 without further addition of fresh H2.

[0067] According to the present invention, the methane produced in step 105 is separated from unreacted CO2 in step 106 with the aim of avoiding emission of CO2 into the atmosphere.

[0068] As previously mentioned, a portion of the methane is recycled to the conversion reactor (100) for purposes of controlling the temperature profile within the reactor, and the remaining portion is sent for other uses or sold externally.

[0069] The CO2 is recycled as an inerting agent in the waste feed system, partly to the methanation step (105) and partly to the thermal conversion step (100). In particular, in the described alternative embodiment in which hydrogen from electrolysis (102) is added entirely upstream of the fermenter (103), the amount of CO2 recovered in the separation step (106) is small and not sufficient to inert the waste feed system.

[0070] Therefore, in the described alternative embodiment, in addition to the recovered CO2, a nitrogen-rich stream derived from a known CO2 separation process is used, which corresponds to a by-product of the same block 106 as shown in Figure 5.

[0071] The liquid phase coming out of the fermentor containing the diluted ethanol can be sent to fermentation to recover the product, the raw bioethanol produced by purification step (104). Generally, this step typically involves recovery of the biocatalyst, which is recycled back to the fermentor, and an ethanol recovery and purification step, generally performed by several distillation columns, followed by dewatering by molecular sieve filtration to meet the residual water requirement.

[0072] Thus, from the purification unit (104) is obtained a stream of anhydrous ethanol and a stream of water containing traces of alcohol produced as by-products during the fermentation process. Typically, such by-products consist of butanediol, ethyl acetate and other superior alcohols.

[0073] The aqueous stream produced during the distillation step may be sent to a wastewater treatment step (107) to allow recycling of the water purified in the electrolysis step (102).

[0074] Furthermore, according to the invention, the stream comprising the by-products consisting of butanediol, ethyl acetate and higher alcohols, as well as the excess of bacteria introduced into the fermenter (103), can advantageously be sent to a thermal conversion step (100), thereby obtaining the double advantage of not dispersing organic components in the environment and of further contributing to the control of the thermal conversion with respect to the composition of the raw synthesis gas produced.

[0075] Advantageously, the described process makes it possible to produce bioethanol from waste materials, such as RDF, municipal solid waste, plastic residues, etc., without introducing CO2 into the atmosphere. Indeed, the synergy between the fermentation step and the subsequent methanation step of the purge gas produced during fermentation makes it possible to reduce or completely decompose the amount of CO2 in the outlet stream, thus avoiding its introduction into the atmosphere.

[0076] As stated in the described embodiment, the methanation step does not completely remove the CO2, but preserves a certain amount of it separated downstream of the methanation, thus preventing its introduction into the atmosphere. This recovered CO2 stream is advantageously used to inert the waste feed system, thus preventing any synthesis gas loss and any air ingress that would result in a reduction in the conversion yield of the reactor and a subsequent imbalance in the fermentation step.

Claims

1. A process for producing ethanol by anaerobic fermentation of synthesis gas, wherein - the synthesis gas is produced by thermal conversion of a feedstock containing municipal solid waste (MSW), agricultural waste or derivatives thereof, such as refuse-derived fuel (RDF), or even industrial waste, such as non-recyclable plastic waste, or combinations thereof, at a temperature above 1000 °C, ・ H ２ Hydrogen is further added to the synthesis gas produced by electrolysis so that the H / CO ratio balances at a value equal to at least 2.0÷2.2 by volume, and thus the CO ２ conversion of the organic components in the fermentation step is maximized to avoid the emission of CO into the atmosphere wherein the process comprises the following steps: - high-temperature conversion of waste (100) by producing raw synthesis gas; ・ Generation of H ２ and O ２ by electrolysis (102); ・ Purification (101) of raw synthesis gas by a purification unit acting at two pressure levels, for the purpose of removing particulate matter, metals, chlorides, ammonia, COS, and H ２ S from the raw synthesis gas; ・ Optimal H required for the fermentation reaction ２ Fermentation (103) of the purified synthesis gas to produce raw bioethanol after addition of all or part of the hydrogen produced by electrolysis to achieve the / CO ratio; - partial or total conversion of residual carbon dioxide present in the purge gas emerging from the fermentation to methane (105); ・ Separation of methane produced by unconverted carbon dioxide (106), and recycling of all or part of the recovered methane and all of the separated CO ２ to all of the thermal conversion steps (100), a process characterized by comprising.

2. The process according to claim 1, further comprising a step (104) of purifying the raw ethanol produced by fermentation, from which a stream of anhydrous ethanol and an aqueous stream containing alcohol and other organic products emerge.

3. The process according to claim 2, further comprising a step (107) of purifying the water obtained in the purification step of the raw ethanol in order to recycle the recovered water to an electrolysis unit.

4. The process according to claim 1, characterized in that the high-temperature waste conversion step (100) for producing raw synthesis gas consists of a plurality of converters providing at least two trains, preferably three conversion trains.

5. The process according to claim 1, characterized in that the step (101) of purifying the produced synthesis gas consists of a plurality of trains depending on the capacity of the plant.

6. The process according to claim 1, characterized in that in the water electrolysis step, several cells are used depending on the capacity of the plant.

7. Hydrogen generated in the electrolysis step (102) is partially added upstream of the fermentation step, thus shifting the H ２ / CO ratio to a value of 2.0 to 2.2% by volume, and the remaining part is added downstream of the fermentation step, thereby enabling the occurrence of the methanation reaction (105), and thus the process is optimized with respect to the volume and operating conditions of the fermentation and methanation steps. The process according to claim 1, characterized in that.

8. The hydrogen produced in the electrolysis step (102) is completely added upstream of the fermentation step, and thus the H ２ / CO ratio is shifted to a value of 5 to 5.2% by volume, maximizing the conversion process of CO and CO ２ to bioethanol. The process according to claim 1, characterized in that.

9. The process according to claim 1, characterized in that the oxygen produced by electrolysis is used as a combustion agent in the conversion step for converting waste into synthesis gas.

10. The process according to claim 1, characterized in that the fermentation step (103) is carried out in one or more bioreactors containing a bacterial culture dispersed in a liquid nutrient medium, depending on the capacity of the plant.

11. The process according to claim 1, characterized in that the amount of methane stream generated during methanation (105) and exiting the separation step (106) is recycled to the thermal waste conversion step (100) in order to control the conversion temperature of the feedstock.

12. The process according to claim 1, characterized in that the excess bacteria exiting the fermentation step (103) and the by-products consisting of butanediol, ethyl acetate, and higher alcohols exiting the water purification unit (107) are sent to the converter in order to control the conversion of waste with respect to the composition of the synthesis gas produced.

13. The CO emerging from the separation process (106) ２ is used either individually or mixed with other streams consisting of inert substances and forming by-products of the same separation step (106) in order to deactivate the waste supply system and thus prevent any syngas loss and any air ingress and enable equalization of the conversion yield, the process according to claim 1, characterized in that.

14. An apparatus for producing ethanol by anaerobic fermentation of synthesis gas, including the conversion of the synthesis gas, produced by thermal conversion of municipal solid waste (MSW), agricultural waste or derivatives thereof, such as refuse-derived fuel (RDF) or even industrial waste, such as non-recyclable plastic waste, into said ethanol, said process using electrolysis for the production of additional hydrogen added to said synthesis gas to balance the H ２ / CO ratio, thus without CO ２ emissions to the atmosphere and thus maximizing the conversion of the organic components in the fermentation step, Here, the apparatus comprises the following steps: - at least one thermal high-temperature waste conversion unit (100) involving the production of raw synthesis gas consisting of a plurality of conversion reactors; ・At least one unit (102) for generating H ２ and O ２ by electrolysis consisting of a plurality of electrolytic cells; ・At least one raw syngas purification unit comprising a plurality of purification trains (101) acting at two-stage pressures, low pressure and high pressure, wherein the purpose of the purification train (101) is to remove particulates, metals, chlorides, ammonia, COS, and H ２ S, at least one raw syngas purification unit; ・ The optimal H ２ At least one unit (103) for the fermentation of purified synthesis gas consisting of a plurality of bioreactors for producing raw bioethanol when all or part of the hydrogen produced by electrolysis is added to achieve the optimal H / CO ratio; - at least one unit (105) for the partial or total conversion of residual carbon dioxide present in the purge gas from the fermentation to methane; - at least one separation unit (106) for separating the methane produced from unconverted carbon dioxide and a line for recycling both streams to the converter; - at least one recycle line exiting the separation step (106) and sent to the thermal conversion step (100) for recycling all or part of the methane exiting the separation step as a temperature control vector in the thermal conversion step; ・ All of the CO recovered in the separation step ２ at least one recycle line exiting the separation step (106) and sent to the thermal conversion step (100) for recycling all of the CO as an inactivating agent in the thermal conversion step, the apparatus comprising.

15. The apparatus according to claim 14, further comprising at least one purification unit (104) for purifying the raw ethanol produced by fermentation, from which a stream of anhydrous ethanol and an aqueous stream containing alcohol and other organic products exits.

16. The apparatus according to claim 14 or 15, further comprising at least one water purification unit (107) for the aqueous stream produced in the raw ethanol purification step, in order to recycle the recovered water to the electrolysis unit.

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