FR3050656A1 - PROCESS FOR THE PRODUCTION OF LIQUID BIOMETHANE BY CRYOGENIC SEPARATION - Google Patents

Abstract

Process for the production of liquid biomethane by cryogenic separation from a biogas stream (1) comprising methane and carbon dioxide, comprising: - a first stage of production of biomethane (4) by cryogenic separation of the biogas stream (1) ) in a cryogenic regenerator (3), - a second biomethane liquefaction step (4) in an exchanger, with the first and the second stage using a single nitrogen circuit.

Description

The present invention relates to a process for producing liquid biomethane by cryogenic separation.

As part of its valuation, biomethane - as a renewable substitute for natural gas with the same characteristics as this one - can be injected into a natural gas distribution or transmission network that connects gas producers and consumers. .

Biomethane is obtained from biogas.

Biogas is a gas produced by the natural or artificial fermentation of vegetable or animal organic matter (methanisation). It contains mainly methane (CH4) carbon dioxide (CO2), but also - to a lesser extent - water, nitrogen, hydrogen sulphide, oxygen, and other organic compounds , in the form of traces. Depending on the organic matter degraded and the techniques used, the proportions of the components differ, but on average the biogas comprises, on dry gas, 30 to 75% of methane, 15 to 60% of CO2, 0 to 15% of carbon dioxide. nitrogen, 0-5% oxygen and trace compounds. Biogas is valued in different ways. It may, after a light treatment, be upgraded near the production site to provide heat, electricity or a mixture of both (cogeneration); the high content of carbon dioxide reduces its calorific value, increases the compression and transport costs and limits the economic interest of its valuation to this use of proximity.

Further purification of the biogas allows its wider use, in particular, a thorough purification of the biogas makes it possible to obtain a biogas purified to the specifications of the natural gas and which can be substituted for it; the biogas thus purified is "biomethane". Biomethane thus completes the natural gas resources with a renewable part produced in the heart of the territories; it is usable for exactly the same uses as natural gas of fossil origin. It can feed a natural gas network, a filling station for vehicles, it can also be liquefied to be stored in the form of liquid natural gas (LNG) ... The modes of valorization of the biomethane are determined according to the local contexts: local energy needs, possibilities of valorization as biomethane fuel. existence close to natural gas distribution or transmission networks, in particular. Creating synergies between the different actors working on a territory (farmers, industrialists, public authorities), the production of biomethane helps the territories to acquire a greater energy autonomy.

The present invention proposes to provide a simplified process for the production of biomethane. A solution of the present invention is a process for producing liquid biomethane by cryogenic separation from a biogas stream 1 comprising methane and carbon dioxide, comprising: a first step of producing biomethane 4 by cryogenic separation of the stream of biogas 1 in a cryogenic regenerator 3, - a second step of liquefying the biomethane 4 in an exchanger, with the first and the second step implementing a single nitrogen circuit.

By "single nitrogen circuit" is meant a process in which the nitrogen used in the first stage comes exclusively from the vaporized nitrogen in the second stage.

By "cryogenic regenerator" is meant a regenerator type exchanger operating at cryogenic temperature.

Depending on the case, the process according to the invention has one or more of the following characteristics: the first biomethane production stage comprises the following successive stages: i) introduction of the biogas 1 at a pressure of between 3 and 15 bar in a cryogenic regenerator, ii) cooling of the biogas 1 in contact with the matrix contained in the cryogenic regenerator and previously cooled by the flow of nitrogen 2 until solidification of the carbon dioxide, (the matrix has been previously cooled so as to present a temperature profile ranging from room temperature to the end where the biogas is injected to a temperature below -90 ° C at the end where the biogas comes out) and iii) recovery of a gaseous biomethane stream 4, and the second biomethane liquefaction step comprises the following successive steps: iv) introduction of the gaseous biomethane stream 4 recovered in step i ii) and the flow of liquid nitrogen (7) in an exchanger 6, v) liquefaction of the gaseous biomethane stream 4 in the exchanger 6 by heat exchange with the liquid nitrogen 7, and vi) recovery of a flow of liquid biomethane 8. - the liquid nitrogen circuit comprises a series passage of the flow of liquid nitrogen in the exchanger 6 and then in the cryogenic regenerator 3. - the temperature of the liquid nitrogen 7 in step iv) is between -196 ° C and -190 ° C and the temperature of nitrogen 2 in step i) is between -196 ° C and -120 ° C. the cryogenic regenerator comprises a first and a second bottle 3a and 3b, said method comprises a third regeneration step, and the first bottle 3a undergoes the first production step while the second bottle 3b undergoes the third regeneration step before to be reversed. - The third regeneration step comprises a sweep of the second bottle 3b by a nitrogen stream 2 from the exchanger at a pressure between 1 bar and 3 bar until sublimation of the solidified CO 2. exchanger 6 operates continuously. the liquid biomethane 8 is sent to a storage.

The present invention will now be described in more detail with the aid of FIG. 1. The installation represented in FIG. 1 comprises a cryogenic regenerator 3 with two bottles, a first bottle 3a and a second bottle 3b. The second bottle 3b undergoes the third regeneration step. In other words, the second bottle 3b is swept by the flow of nitrogen 2 and the low pressure of the nitrogen stream of between 1 and 3 bar allows the sublimation and evacuation of carbon dioxide solidified and trapped in the matrix. This nitrogen sweep also makes it possible to cool the matrix comprised in the second bottle 3b and to obtain a temperature profile comprised between ambient temperature and a temperature below -90 ° C. After this cooling of the matrix included in the second bottle, the first bottle and the second bottle are inverted; that is, the second bottle 3b becomes the first bottle 3a and will undergo the biomethane production step and the first bottle 3a becomes the second bottle 3b and will in turn undergo the regeneration step. This inversion is made possible by a set of valves. The biogas at a pressure of between 3 and 15 bar is introduced into the first bottle 3a and is cooled to a temperature below -90 ° C. in contact with the cooled matrix during the regeneration step. This cooling leads to the solidification of the carbon dioxide which then remains trapped in the matrix. The output of the first bottle of biomethane gas 4 is recovered, which is then liquefied in a heat exchanger 6 by heat exchange with a stream of liquid nitrogen 7. Note that the flow of liquid nitrogen corresponds to the upstream flow of the nitrogen flow used. in the regeneration step. A stream of liquid biomethane is recovered at the outlet of the exchanger.

Note that the liquid nitrogen 7 can come from a nitrogen storage tank. It takes 3 kg of nitrogen to liquefy 1 kg of biomethane.

Claims (8)

claims A process for producing liquid biomethane by cryogenic separation from a biogas stream (1) comprising methane and carbon dioxide, comprising: a first step of producing biomethane (4) by cryogenic separation of the biogas stream (1) in a cryogenic regenerator (3), - a second biomethane liquefaction step (4) in an exchanger, with the first and the second step using a single nitrogen circuit. 2. Method according to claim 1, characterized in that: the first step of producing biomethane comprises the following successive steps: i) introduction of the biogas (1) at a pressure of between 3 and 15 bar in a cryogenic regenerator (3) ii) cooling of the biogas (1) in contact with the matrix contained in the cryogenic regenerator and previously cooled by the flow of nitrogen (2) until solidification of the carbon dioxide, and iii) recovery of a flow of carbon dioxide; gaseous biomethane (4), and the second step of biomethane liquefaction comprises the following successive steps: iv) introduction of the flow of gaseous biomethane (4) recovered in step iii) and flow of liquid nitrogen (7) in an exchanger (6), v) liquefying the gaseous biomethane stream (4) in the exchanger (6) by heat exchange with the liquid nitrogen (7), and vi) recovering a flow of liquid biomethane (8) . 3. Method according to claim 2, characterized in that the liquid nitrogen circuit comprises a series passage of the liquid nitrogen stream in the exchanger (6) and then in the cryogenic regenerator (3). 4. Method according to one of claims 2 or 3, characterized in that the temperature of the liquid nitrogen (7) in step iv) is between -196 ° C and -190 ° C and the temperature of the nitrogen (2) in step i) is between -196 ° C and -120 ° C. 5. Method according to one of claims 2 to 4, characterized in that: - the cryogenic regenerator (3) comprises a first and a second bottle (3a) and (3b), - said method comprises a third regeneration step, and - the first bottle (3a) undergoes the first production step while the second bottle (3b) undergoes the third regeneration step before being inverted. 6. Method according to claim 5, characterized in that the third regeneration step comprises a sweep of the second bottle by a stream of nitrogen (2) from the exchanger (6) at a pressure between 1 bar and 3 bar until sublimation of the solidified CO 2. 7. Method according to one of claims 1 to 6, characterized in that the exchanger (6) operates continuously. 8. Method according to one of claims 1 to 7, characterized in that the liquid biomethane (8) is sent into a storage.