PLANT FOR THE PRODUCTION OF LIQUID CO2 AND BIOMETHANE WITH AN AGENT TO PREVENT THE FORMATION OF HYDROGEN AND OXYGEN

Description

[0001] The present invention relates to an installation and a method for producing biomethane and liquid CO2 from a biogas stream.

[0002] Biogas is the gas produced during the degradation of organic matter in the absence of oxygen (anaerobic fermentation), also called methanization. It can be a natural degradation - it is observed in marshes or household waste dumps - but the production of biogas can also result from the methanization of waste in a dedicated reactor, with controlled conditions, called a methanizer or digester, then in a post-digester, similar to the digester and allowing the methanization reaction to be pushed further.

[0003] Biomass is any group of organic matter that can be transformed into energy through this methanization process, e.g., sewage treatment plant sludge, manure / slurry, agricultural residues, food waste, etc.

[0004] The digester, i.e. the reactor dedicated to the methanization of biomass, is a closed tank, heated or not (operation at a fixed temperature, between ambient temperature and 55°C) and whose contents consisting of biomass are mixed, continuously or sequentially. The conditions in the digester are anaerobic and the biogas generated ends up in the head space of the digester (gaseous headspace), where it is collected. Post-digesters are similar to digesters.

[0005] Due to its main constituents - methane and carbon dioxide - biogas is a powerful greenhouse gas; it also constitutes, at the same time, a significant source of renewable energy in a context of increasing scarcity of fossil fuels.

[0006] Biogas mainly contains methane (CH 4 ) and carbon dioxide (CO 2 ) in varying proportions depending on the method of production and the substrate, but can also contain, in smaller proportions, water, nitrogen, hydrogen sulfide (H2S), oxygen, as well as other organic compounds, in trace amounts, including H 2 S, between 10 and 50,000 ppmv.

[0007] Depending on the degraded organic matter and the techniques used, the proportions of the components differ, but on average biogas contains, on dry gas, 30 to 75% methane, 15 to 60% CO2, 0 to 15% nitrogen, 0 to 5% oxygen and trace compounds.

[0008] Biogas is used in various ways. After light treatment, it can be used close to the production site to provide heat, electricity or a mixture of the two (cogeneration); the high carbon dioxide content reduces its calorific value, increases compression and transport costs and limits the economic interest of its use to this local use.

[0009] US2021 / 094894A1 describes a plant usable for the production of biomethane and liquid CO2 as well as a process using this unit.

[0010] Further purification of biogas allows its wider use, in particular, advanced purification of biogas makes it possible to obtain a purified biogas to the specifications of natural gas and which can be substituted for it; the biogas thus purified is "biomethane". Biomethane thus supplements natural gas resources with a renewable part produced in the heart of the territories; it can be used for exactly the same purposes as natural gas of fossil origin. It can supply a natural gas network, a vehicle filling station, it can also be liquefied to be stored in the form of liquid natural gas (bioLNG)...

[0011] On the other hand, biogas also consists largely of carbon dioxide, which can be used as a product for various markets such as carbonated drinks, greenhouses, and cleaning. This carbon dioxide needs to be liquefied to be transported to its intended use. This liquefaction also allows the required gas quality to be achieved. The remaining gaseous compounds, loaded with methane, are recycled into the biogas purification process.

[0012] This recycled gas mainly contains carbon dioxide, between 45 and 75%, methane, between 25 and 55%, nitrogen, oxygen and hydrogen if present in the biogas. Oxygen and hydrogen will tend to accumulate between the biogas purification process and the CO 2 liquefaction process until reaching explosive compositions.

[0013] Indeed, hydrogen is a very small molecule which permeates through the membranes and therefore ends up in the vents of the purification unit which are directed towards the CO2 liquefaction unit.

[0014] Hydrogen is non-condensable and will be fully recycled to the purifier after treatment of the vents by the liquefier. If the hydrogen is recycled indefinitely between the two units, its concentration will increase over time and may reach a level that poses a risk of explosion.

[0015] Oxygen partially permeates through the membranes and therefore ends up, in part, in the vent. Oxygen is also non-condensable and will end up in the recycle to the scrubber. A high oxygen concentration in the return gas to the scrubber presents an explosion hazard.

[0016] From this point on, a problem arises: to provide a biomethane and liquid CO2 production facility that includes a means of avoiding the accumulation of hydrogen and oxygen in the recycle.

[0017] A solution of the present invention is a plant for producing biomethane and liquid CO2, comprising: A biogas production unit, At least one biogas membrane separation unit for producing biomethane and a gas mixture M1 comprising mainly carbon dioxide, A cryogenic distillation unit for the gas mixture M1 for producing liquid CO2 and a gas mixture M2 comprising carbon dioxide, methane, oxygen and hydrogen, One or more membranes located on the flow of the gas mixture M2 and for separating the methane from the oxygen-hydrogen mixture included in the gas mixture M2, A means for recycling the methane from the gas mixture M2 to the membrane separation unit, and A vent for evacuating the oxygen-hydrogen mixture from the gas mixture M2.

[0018] An example of an installation according to the invention is shown [ Fig. 1 ].

[0019] Depending on the case, the installation according to the invention may have one or more of the characteristics below:The installation comprises between the biogas production unit and the membrane separation unit a means for compressing the biogas; the compression means is a lubricated screw compressor; The installation comprises between the biogas production unit and the membrane separation unit a unit for removing at least part of the hydrogen sulfide and the volatile organic compounds; the unit for removing at least part of the hydrogen sulfide and the volatile organic compounds is a pressure-modulated adsorption unit. the membrane(s) located on the gas mixture flow M2 is more permeable to oxygen and hydrogen than to methane; the membrane separation unit comprises three or four membrane stages. the membrane separation unit comprises: ▪ A first membrane separation stage provided with a first membrane capable of receiving the biogas flow and providing a first permeate and a first retentate,said first membrane being more permeable to carbon dioxide than to methane, ▪ A second membrane separation stage provided with a second membrane adapted to receive a second feed gas and to provide a second permeate and a second retentate, said second membrane being more permeable to carbon dioxide than to methane, and said second membrane separation stage being connected in series with the first membrane separation stage such that the first retentate constitutes the second feed gas, ▪ A third membrane separation stage provided with a third membrane adapted to receive a third feed gas and to provide a third permeate and a third retentate, said third membrane being more permeable to carbon dioxide than to methane,and said third membrane separation stage being connected in series with the first membrane separation stage such that the first permeate constitutes the third feed gas. the membrane separation unit may comprise a fourth membrane separation stage provided with a fourth membrane adapted to receive a feed gas and to provide a permeate and a retentate, said fourth membrane being more permeable to carbon dioxide than to methane, and said fourth membrane separation stage being connected in series with the third membrane separation stage such that the third retentate constitutes the fourth feed gas. ,

[0020] The present invention also relates to a process for producing liquid methane and CO2, implementing the installation according to the invention and comprising: a) A biogas production step, b) A first step of membrane separation of the biogas so as to produce biomethane and a gas mixture M1 comprising mainly carbon dioxide, c) A step of cryogenic distillation of the gas mixture M1 so as to produce liquid CO2 and a gas mixture M2 comprising carbon dioxide, methane, oxygen and hydrogen, d) A second step of membrane separation of the gas mixture M2 so as to separate the methane from the oxygen-hydrogen mixture included in the gas mixture M2, e) A step of recycling the methane from the gas mixture M2 to the membrane separation unit, and f) A step of evacuating the oxygen-hydrogen mixture from the gas mixture M2.

[0021] Depending on the case, the method according to the invention may have one or more of the characteristics below: The process comprises between steps a) and b) a biogas compression step. The process comprises between steps a) and b) a pre-purification step for removing at least part of the hydrogen sulfide and the volatile organic compounds. The pre-purification step is an adsorption purification step; step f) is carried out continuously.

[0022] The solution according to the invention proposes to use one or more membranes to continuously vent a flow of this recycled gas from the CO2 liquefier to the biogas purifier. The membrane(s) used preferentially separate oxygen and hydrogen from methane and carbon dioxide.

[0023] The membrane is a multitude of micrometric tubes, called fibers, made of a porous material through which gaseous compounds are likely to pass. The separation of the different gaseous compounds through a membrane is done by pressure difference between the flow inside the membrane fibers and the flow outside the membrane fibers and by difference in affinity of the gaseous compounds with the membrane fiber. The gas flow that passes through the pores of the membrane fiber is called permeate. The gas flow that does not pass through the pores of the membrane fiber is called retentate. Hydrogen and oxygen diluted in carbon dioxide are found in the membrane permeate while methane diluted in carbon dioxide is found in the membrane retentate. The membrane retentate is recycled to the biogas purifier while the membrane permeate is vented.

[0024] The present invention will be described in more detail with the help of the example below. EXAMPLE

[0025] Biogas contains 100 ppm hydrogen and 0.2% oxygen. The hydrogen must be completely purged through the membrane.

[0026] This solution, which impacts biomethane yield, is necessary to ensure the safety of the installation. It is illustrated in this table, given for information purposes only. This table takes the example of treatment of a maximum biogas flow rate (1100 Nm 3 < / h of raw wet biogas) at nominal composition. Biogas LCO 2 Bimethane Purge Dry flow 1043 Nm3 / h 374 Nm3 / h 667 Nm3 / h 1.7 Nm3 / h N2 0.4% 0.0% 0.7% 0.6% O2 0.2% 0.0% 0.3% 8.3% Methane 62.4% 0.0% 97.5% 8.0% CO2 37.0% 100.0% 1.6% 76.7% H2 0.01% 0.0% 0.0% 6.4% BioCH4 yield 99.98%

[0027] In other words, the solution according to the invention not only ensures the safety of the operation by eliminating the compounds responsible for explosion risks, but also increases the methane yield.

Claims

1. Installation (1) for producing biomethane (4) and liquid CO2 (6), comprising : - A unit (3) for producing biogas (2), - At least one biogas membrane separation unit (5) making it possible to produce biomethane (4) and a gaseous mixture M1 comprising mainly carbon dioxide, - A unit (7) for cryogenic distillation of the gas mixture M1 to produce liquid CO2 (6) and a gas mixture M2 comprising carbon dioxide, methane, oxygen and hydrogen, - One or more membranes (9) located on the flow of the gaseous mixture M2 and used to separate the methane (8) from the oxygen-hydrogen mixture (10) contained in the gaseous mixture M2, - A means (11) for recycling the methane (8) from the gas mixture M2 to the membrane separation unit (5), and - a vent (13) for discharging the oxygen-hydrogen mixture (10) from the gas mixture M2.

2. Installation according to claim 1, characterised in that it comprises a biogas compression means between the biogas production unit (3) and the membrane separation unit (5).

3. Installation according to claim 2, characterised in that the compression means is a lubricated screw compressor.

4. Installation according to one of Claims 1 to 3, characterised in that it comprises, between the biogas production unit (3) and the membrane separation unit (5), a unit for at least partially eliminating hydrogen sulphide and volatile organic compounds.

5. Installation according to claim 4, characterized in that the unit for at least partial removal of hydrogen sulphide and volatile organic compounds is a pressure swing adsorption unit.

6. Installation according to one of claims 1 to 5, characterized in that the membrane (9) located on the flow of gaseous mixture M2 is more permeable to oxygen and hydrogen than to methane.

7. Installation according to one of claims 1 to 6, characterized in that the membrane separation unit (5) comprises three or four membrane stages.

8. Installation according to claim 7, characterised in that the membrane separation unit (5) comprises : - A first membrane separation stage provided with a first membrane capable of receiving the biogas flow (2) and providing a first permeate and a first retentate, said first membrane being more permeable to carbon dioxide than to methane, - A second membrane separation stage provided with a second membrane adapted to receive a second feed gas and to provide a second permeate and a second retentate, said second membrane being more permeable to carbon dioxide than to methane, and said second membrane separation stage being connected in series with the first membrane separation stage such that the first retentate constitutes the second feed gas, - A third membrane separation stage provided with a third membrane adapted to receive a third feed gas and to provide a third permeate and a third retentate, said third membrane being more permeable to carbon dioxide than to methane, and said third membrane separation stage being connected in series with the first membrane separation stage such that the first permeate constitutes the third feed gas.

9. Installation according to claim 8, characterized in that the membrane separation unit (5) may comprise a fourth membrane separation stage provided with a fourth membrane adapted to receive a feed gas and to supply a permeate and a retentate, said fourth membrane being more permeable to carbon dioxide than to methane, and said fourth membrane separation stage being connected in series with the third membrane separation stage such that the third retentate constitutes the fourth feed gas.

10. A process for producing methane and liquid CO2, using the Installation (1) as defined in any of claims 1 to 9 and comprising: a) A biogas (2) production stage, b) A first stage of membrane separation of the biogas so as to produce biomethane (4) and a gaseous mixture M1 comprising mainly carbon dioxide, c) A cryogenic distillation stage for the gaseous mixture M1 to produce liquid CO2 (6) and a gaseous mixture M2 comprising carbon dioxide, methane, oxygen and hydrogen, d) A second stage of membrane separation of the gaseous mixture M2 so as to separate the methane (8) from the oxygen-hydrogen mixture (10) included in the gaseous mixture M2, e) A step for recycling the methane (8) from the gas mixture M2 to the membrane separation unit (5), and f) a step for discharging the oxygen-hydrogen mixture (10) from the gaseous mixture M2.

11. Process according to claim 10, characterized in that it comprises between steps a) and b) a biogas compression step.

12. Process according to one of claims 10 or 11, characterized in that it comprises, between steps a) and b), a pre-purification step making it possible to remove at least some of the hydrogen sulphide and the volatile organic compounds.

13. Process according to claim 12, characterized in that the pre-purification step is a purification step by adsorption.

14. Process according to one of claims 10 to 13, characterized in that step f) is carried out continuously.