Method for producing biomethane incorporating the production of heat for the methaniser using membrane separation

Abstract

The present invention relates to a process for producing biomethane suitable for supplying a natural gas network that incorporates a process for providing heat for heating the biogas production step, the process comprising at least steps of producing biogas by anaerobic fermentation of organic matter, of pretreating and compressing the biogas and also of permeation in order to obtain, after a first separation by permeation, a stream of biomethane and a gaseous permeate having a reduced methane content; the process additionally provides the heat necessary for the anaerobic fermentation step via a boiler using the retentate from a second permeation step fed by the permeate from the first separation.

Claims

1. A process for producing biomethane suitable for supplying a natural gas network that incorporates a process for providing heat for heating the biogas production step, wherein the biomethane production process comprises at least: a step (a) of producing biogas by methanization of organic matter; a step (b) of pretreating the biogas produced during step (a); a step (c) of compressing the pretreated biogas with a compressor; a step (d) of using a first gas separation module to separate, by permeation, the pretreated and compressed biogas resulting from step (c) in order to produce, in comparison to the pretreated and compressed biogas, a first methane-enriched gaseous retentate having a methane content of greater than 89% and a first methane-depleted gaseous permeate having a methane content of between 10% and 25%; a step (e) of providing the gaseous retentate from step (d) as biomethane; a step (f) of heating water in a heating water circuit including a gas boiler having burners, heat being provided to step (a) via the heating water circuit; a step (g) of separating by permeation the first methane-depleted gaseous permeate to produce, in comparison to the first methane-depleted gaseous permeate, a second methane-enriched gaseous retentate and a second methane-depleted permeate, the methane content of the second methane-enriched gaseous retentate of step (g) being greater than or equal to 25%; a step (h) of supplying the burners of the boiler from step (f) with the second methane-enriched gaseous retentate from the separation step (g) as fuel for consumption by the burners; and a step (i) of discharging the permeate produced by the permeation separation step (g).

2. The process of claim 1, further comprising a step (j) of providing additional heat to the water circulating in said heating water circuit using heat recovered during steps (b) and (c).

3. The process of claim 1, wherein a variable fraction of the methane-enriched retentate from the separation step (g) is sent back to an inlet of the compressor of step (c) when the supply of heat to step (a) is greater than heat requirements of step (a).

4. The process of claim 1, wherein the gaseous retentate resulting from performance of step (d) has a methane content of greater than 96.5%.

5. The process of claim 1, wherein the gaseous retentate resulting from performance of step (g) has a methane content of between 30% and 40%.

6. A plant for carrying out the process of claim 1 for producing biomethane intended for supplying a natural gas network and the integrated provision of heat for heating the biogas production step, said plant comprising at least: a biomethane production unit comprising at least a source of organic matter, a digester for producing biogas by methanization of said organic matter, and a module for prepurifying the biogas produced; a compressor suitable for compressing the prepurified biogas; a first gas separation module for separating, by membrane permeation, the prepurified and compressed biogas, said first module being suitable for producing a gaseous retentate having a methane content of greater than 89% and a gaseous permeate having a methane content of between 10% and 25%; a system for heating the organic matter contained in the digester comprising at a heating water circuit suitable for heating the digester and a gas boiler being suitable for heating the water of the heating water circuit; and a second module for separating, by membrane permeation, the gaseous permeate resulting from the first permeation separation module, said second module being suitable for producing a methane-depleted permeate and a methane-enriched gaseous retentate having methane content greater than 25, wherein the second module provides the gaseous retentate retentate thereof as biogas, the second separation module supplies the retentate thereof to the burners of said gas boiler, and the second separation module discharges the permeate resulting therefrom.

7. The plant of claim 6, wherein the means suitable for cooperating in order to heat the organic matter contained in the digester comprise additional means suitable for recovering heat from the biogas prepurification module and the prepurified biogas compressor and also additional means suitable for providing the heat recovered to the water of the heating circuit of the digester.

8. The plant of claim 6, further comprising a line equipped with a flow control valve suitable for recycling a fraction of the retentate from the second membrane stage to the inlet of the compressor.

9. The plant of claim 6, wherein said first module is suitable for producing a gaseous retentate having a methane content of greater than 96.5%.

10. The plant of claim 6, wherein a second module is suitable for producing the methane-enriched gaseous retentate at a methane content of between 30% and 40%.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The FIGURE presents a schematic diagram illustrating various elements of the invention. For the sake of simplicity, only the elements of the plant useful for the understanding and implementation of the invention are referenced.

DETAILED DESCRIPTION OF THE INVENTION

(2) According to the diagram of the FIGURE, the biomethane production plant according to the invention operates in the following manner. The methanizer 1 delivers a biogas 2. The biogas 2 is sent to a pretreatment module 3 in which it is objected to 4 different treatments prior to a CO.sub.2/CH.sub.4 separation. The pretreated biogas 5 is compressed in the compressor 6 in order to provide the compressed pretreated biogas 7 at the inlet pressure to a first membrane stage 8 that carries out the CO.sub.2/CH.sub.4 separation. More specifically, the membrane stage 8 delivers a methane-enriched gaseous retentate 9, the methane content of which is sufficient to be substituted for natural gasat least 89%, if necessary 96.5% or more, depending on the specifications of the natural gas that must be substitutedand delivers a gaseous permeate 10 having a methane content of between 10% and 25%. In order to be able to use the methane contained in the permeate 10 as fuel in a simple boiler, the permeate 10 must firstly be enriched in methane, it is thus sent to the feed of a second membrane stage 11 which delivers a methane-depleted permeate 12 (less than 2% of the methane contained in the biogas produced is thus discharged into the permeate) and a retentate 13, the methane content of which is greater than 30%, preferably of the order of 35%. The gaseous stream 13the methane content of which is thus high enoughis sent to the burners of a boiler 14, which can therefore be a boiler of conventional technology. The permeate 12 is methane-depleted relative to the stream 11, its CH.sub.4 content is low enough to be able to be discharged to the atmosphere without expensive additional treatment of the vented gas. The boiler 14 provides heat to the water circuit 15 which heats the organic matter contained in the methanizer 1 in order to maintain therein the required temperature conditions for the correct operation of the anaerobic fermentation process that generates the biogas 2. Additional heat is provided to heat the water circulating in the heating circuit 15 using heat available from the pretreatment module 4 by recovery of heat from cold units and/or heat of compression from the compression module 7.

(3) When the heat requirements of the fermentation are lower than the available resources, a portion 13a of the stream 13 is drawn off from the stream supplying the boiler and is sent to the compressor 6 in order to be purified in addition to the stream of biogas 5. This flow 13a is controlled owing to a regulated FCV1 (flow control valve 1) valve installed on the line ensuring the recycling thereof to the compressor so as to adapt the flow as a function of the heat requirements of the methanizer. The table below presents the characteristics of the various streams resulting from the implementation of the invention applied to a pretreated biogas stream 5 containing 55% methane and 44.4% carbon dioxide. The percentages expressed are molar percentages, the contents of minor elements are not indicatednamely for example 0.6% for the biogas 5.

(4) TABLE-US-00001 Stream ref. Charact 5 7 9 10 12 13 % CH.sub.4 55 55 96.91 21.01 03.80 34.25 % CO.sub.2 44.4 44.4 02.29 78.24 95.54 64.92 P. bar 1.05 15.25 15 2.6 1.015 1.9 Nm.sup.3/h 51.32 51.05 22.54 28.52 12.40 16.12 Function: Biogas Memb.1 BioCH.sub.4 Memb.2 Vented Fuel feed feed gas

(5) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(6) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(7) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising, Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of, comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

(8) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(9) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(10) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(11) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.