Methane recovery from solid digestate

Abstract

A method for producing biogas from digestate from a digester, said method comprising: A step of recovering the digestate at the outlet of a digester, a post-digester or a storage tank; A step of separating the digestate into a solid digestate and a liquid digestate; A step of introducing the solid digestate into at least one closed tank; A step of anaerobic digestion in the tank without heating or mixing, and A step of recovering the biogas at the outlet of the tank.

Description

Title of the invention: Recovery of methane from solid digestates

[0001] The present invention relates to the production of biogas from digestate from a digester.

[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, the conditions of which are controlled, called a methanizer or digester, then possibly 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 sludge, manure / slurry, agricultural residues, energy crops, food waste, etc. In Europe, agricultural residues and energy crops (dedicated or intermediate) represent a large proportion of the biomass processed, particularly in Germany and Austria, then in France. In Germany, 15% of agricultural digesters are operated in mono-digestion, i.e., with 100% crops. The majority of digesters mix crops with manure / slurry (co-digestion).

[0004] Digestate is the residue of biomass after anaerobic digestion. Digestate is generally pasty, rich in carbon, nitrogen, potassium, phosphorus and trace elements. Its properties for fertilizing and / or improving soils are recognized, while being of biological origin and generating less odor than conventionally spread slurry or manure. The digestate is spread according to a strictly regulated procedure.

[0005] The digester, that is to say 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 sky), where it is collected. Post-digesters are similar to digesters.

[0006] 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.

[0007] Biogas mainly contains methane (CH4) and carbon dioxide (CO2) in variable 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, trace levels, including H2S, between 10 and 50,000 ppmv.

[0008] Depending on the degraded organic matter and the techniques used, the proportions of the components differ, but on average the biogas contains, on dry gas, 30 to 75% methane, 15 to 60% CO2, 0 to 15% nitrogen, 0 to 5% oxygen and trace compounds.

[0009] Biogas is used in different ways. After light treatment, it can be used near the production site to provide heat, electricity or a mixture of the two (cogeneration).

[0010] From this, a problem that arises is to improve the production of biogas.

[0011] A solution of the present invention is a method for producing biogas from digestate, said method comprising:

[0012] a. A step of recovering the digestate at the outlet of a digester, a post-digester or a storage tank; b. A step of separating the digestate into a solid digestate and a liquid digestate; c. A step of introducing the solid digestate into at least one closed tank; d. An anaerobic digestion step in the tank without heating or mixing, and e. A stage of recovery of the biogas at the outlet of the tank.

[0013] In other words, the solution according to the invention makes it possible to recover the residual biogas generated by the solid digestate.

[0014] Depending on the case, the method according to the invention may have one or more of the following characteristics:

[0015] - the anaerobic digestion step is carried out with inerting of the tank with CO2 or nitrogen, - the anaerobic digestion stage lasts between 1 and 6 months. - the process is carried out during the summer months and the digestion stage lasts between 1 and 4 months. - the process is carried out during the winter months and the digestion stage lasts between 3 and 6 months. - the tank is a concrete tank closed by a rigid or flexible cover allowing the tank to remain watertight after it has been closed, - steps c) and d) implement at least two tanks in parallel, - at each moment of step d) the quantity of biogas produced from the solid digestate is measured, this quantity is compared to a target value and the anaerobic digestion of the solid digestate is stopped when the quantity of biogas produced is greater than this target value. This makes it possible to increase the quantity of biogas generated while retaining the agronomic properties of the solid digestate (in particular sufficient carbon content to make it a good amendment). - the target value is between 25 and 400 Nm3 of biogas per tonne of volatile matter, preferably between 100 and 300 Nm3 of biogas per tonne of volatile matter.

[0016] The present invention also relates to a process for producing biogas from biomass, comprising:

[0017] a. A step of introducing the biomass into a digester and / or a post-digester, b. An anaerobic digestion step in the digester and / or post-digester, c. A stage of production of a first flow of biogas and digestate, d. A step of separating the digestate into a solid digestate and a digestate liquid e. A step of introducing the solid digestate into at least one closed tank, f. A step of anaerobic digestion in the tank without heating or mixing, g. A step of recovering a second flow of biogas at the outlet of the tank, And h. A step of mixing the first biogas stream and the second biogas stream so as to form a stream M.

[0018] The process of producing biogas from biomass makes it possible to improve the biogas yields of digesters because the residual biogas is thus recovered. Furthermore, this improves the environmental impact of the digester, because there are no greenhouse gas emissions into the atmosphere from the solid digestate.

[0019] Depending on the case, the process for producing biogas from biomass has one or more of the characteristics below:

[0020] - the flow M is purified so as to recover biomethane and CO2 and step f) is carried out with inerting of the tank with the CO2 resulting from the purification of said mixed flow. - step f) is carried out with inerting of the tank with nitrogen. - The process of producing biogas from biomass includes a step introduction of liquid digestate into a storage tank, an anaerobic digestion step in this storage tank and a step of recovery of a third biogas flow at the outlet of this storage tank. - the third biogas stream is mixed with stream M.

[0021] In the context of the invention, the solid digestate is introduced into a tank, preferably made of concrete, with a size between 100 and 1000 m3. The tank will be closed by a rigid or flexible cover allowing the tank to be sealed after its closure. No heating or agitation system is incorporated. The loading of the solid digestate into the tank could be carried out manually by tractors already present on site if the tank is located at a distance from the separator of the digestate into liquid digestate and solid digestate. If, on the other hand, the tank is at the base of the phase separator, loading will simply take place by gravity. Then, anaerobic digestion will occur:

[0022] i. either slowly with natural inerting over time, ii. either with inerting by CO2, preferably CO2 from biogas purification, iii. either with nitrogen inerting, preferably nitrogen from an external supply.

[0023] For anaerobic digestion, the solid digestate will be left in the closed tank for 1 to 6 months: 1 to 4 months during the summer months and 3 to 6 months during the winter months because the kinetics will be slower at low temperature. Note that several tanks can be used in parallel to treat a larger fraction of the produced solid digestate.

[0024] After anaerobic digestion, the second biogas stream generated by the solid digestate will be recovered at the top of the tank and mixed with the first biogas stream generated by the digester. Said mixture will then be purified / cogenerated.

[0025] The tank will then be emptied, for example, using a tractor.

[0026] The solution according to the invention therefore makes it possible, at lower cost, to recover the residual biogas from the solid digestate, without impacting the operation of the methanizer. In particular, this solution is interesting when the digester is operating at maximum capacity, and therefore the digestate cannot be recirculated. Also, this makes it possible to improve the carbon footprint of a methanization site, by limiting the release of greenhouse gases and pollutants into the atmosphere (nitrogen dioxide and carbon dioxide, ammonia).

Claims

Claims

1. A method for producing biogas from digestate from a digester, said method comprising: a. A step of recovering the digestate at the outlet of a digester, a post-digester or a storage tank; b. A step of separating the digestate into a solid digestate and a liquid digestate; c. A step of introducing the solid digestate into at least one closed tank; d. A step of anaerobic digestion in the tank without heating or mixing, and e. A step of recovering the biogas at the outlet of the tank.

2. A process for producing biogas according to claim 1, characterized in that the anaerobic digestion step is carried out with inerting of the tank with CO2 or nitrogen.

3. Production method according to one of claims 1 or 2, characterized in that the anaerobic digestion step has a duration of between 1 and 6 months.

4. A process for producing biogas according to claim 3, characterized in that the process is carried out during the summer months and the digestion step has a duration of between 1 and 4 months.

5. A process for producing biogas according to claim 3, characterized in that the process is carried out during the winter months and the digestion step has a duration of between 3 and 6 months.

6. Method for producing biogas according to one of claims 1 to 5, characterized in that the tank is a concrete tank closed by a rigid or flexible cover allowing the tank to be sealed after it has been closed.

7. Method for producing biogas according to one of claims 1 to 6, characterized in that steps c) and d) use at least two tanks in parallel.

8. A method for producing biogas according to one of claims 1 to 7, characterized in that at each instant of step d) the quantity of biogas produced by the digestate is measured, this quantity is compared to a target value and the anaerobic digestion of the solid digestate is stopped when the quantity of biogas produced by the digestate is higher than this target value.

9. A method for producing biogas according to claim 8, characterized in that the target value is between 50 and 400 Nm3 of biogas per tonne of volatile matter, preferably between 100 and 300 Nm3 of biogas per tonne of volatile matter.

10. A method of producing biogas from biomass, comprising: a. A step of introducing the biomass into a digester and / or a post-digester, b. An anaerobic digestion step in the digester and / or a post-digester, c. A stage of production of a first flow of biogas and digestate, d. A step of separating the digestate into a solid digestate and a liquid digestate e. A step of introducing the solid digestate into at least one closed tank, f. An anaerobic digestion step in the tank without heating or mixing, g. A step of recovering a second flow of biogas at the outlet of the tank, and h. A step of mixing the first biogas stream and the second biogas stream so as to form a stream M.

11. Process for producing biogas according to claim 10, characterized in that the flow M is purified so as to recover biomethane and CO2 and step f) is carried out with inerting of the tank with the CO2 resulting from the purification of said mixed flow.

12. Process for producing biogas according to claim 10, characterized in that step f) is carried out with inerting of the tank with nitrogen.

13. Method for producing biogas according to one of claims 10 to 12, characterized in that it comprises a step of introducing the liquid digestate into a storage tank, a step of anaerobic digestion in this storage tank and a step of recovering a third flow of biogas at the outlet of this storage tank.

14. A method of producing biogas according to claim 13, characterized in that the third biogas stream is mixed with stream M.