PROCESS FOR INJECTING BIOMETHANE INTO A NATURAL GAS NETWORK

Abstract

A process for injecting biomethane into a network which has a gross calorific value of value X between X1 and X2, comprising the injection of nitrogen into the biomethane network before the injection of the biomethane into the network which has a gross calorific value of value X so as to reduce the calorific value of the biomethane network to a value between X1 and X2, with the nitrogen derived from the retentate of at least one membrane stage.

Claims

1-12. (canceled)

13. A process for injecting biomethane into a biomethane network that has a gross calorific value of value X between X1 and X2, comprising the steps of: injecting biomethane having a gross calorific value greater than X2 into a biomethane network; and injecting nitrogen into the biomethane network in an amount sufficient to achieve an overall calorific value of the injected biomethane and nitrogen of between X1 and X2, wherein the nitrogen being injected is obtained from a retentate of at least one membrane stage.

14. The process for injecting biomethane of claim 13, wherein X1=9.5 kWh/Nm.sup.3 and X2=10.5 kWh/Nm.sup.3.

15. The process of claim 13, further comprising the steps of: feeding, to the at least one membrane stage, air from an internal network of the process or from an air compressor; separating the fed air into an impure oxygen permeate and an impure nitrogen retentate, the impure nitrogen retentate being the nitrogen that is injected into the biomethane network; and controlling the amount of nitrogen injected into the biomethane network via a control valve located on a feed of the at least one membrane stage or via adjustment of a production capacity of the air compressor.

16. The process of claim 15, wherein upstream of the compressor the air is dried and de-oiled and is at a pressure greater than or equal to a pressure of the biomethane network.

17. The process of claim 15, wherein a purity of the nitrogen injected into the biomethane network is controlled based upon a concentration of oxygen in the impure nitrogen retentate or upon a pressure of the impure nitrogen retentate.

18. The process of claim 13, wherein the membrane from which the nitrogen-enriched retentate is obtained also produces an oxygen-enriched stream.

19. The process of claim 18, wherein the oxygen-enriched stream is injected into a digester that produces biogas.

20. The process of claim 18, wherein the oxygen-enriched stream is injected upstream of an activated carbon filter of a biogas purification unit to facilitate abatement of H2S by the activated carbon filter from biogas fed to the biogas purification unit.

21. A plant for injecting biomethane into a network having a gross calorific value of value X, comprising: a biomethane production unit; a biomethane network in fluid communication with the biomethane production unit, the biomethane network having a gross calorific value of value X; a nitrogen-selective membrane that is adapted and configured to produce a nitrogen-enriched retentate from an air stream, the biomethane network being in fluid communication with the nitrogen-selective membrane and receiving the nitrogen-enriched retentate therefrom; a system for producing compressed air at a pressure greater than or equal to a pressure of the biomethane network, the system for producing compressed air being in upstream flow communication with the nitrogen-selective membrane.

22. The plant of claim 21, wherein said plant further comprises: an oxygen concentration analyzer located on the retentate of the membrane upstream of the biomethane network, the analyzer being adapted and configured to measure an oxygen concentration of the membrane retentate; a pressure sensor located on the retentate of the membrane upstream of the biomethane network, the pressure sensor being adapted and configured to measure a pressure of the membrane retentate, and a control valve located on the retentate of the membrane downstream of the analyzer and upstream of the biomethane network, the control valve controlling the air stream fed to the membrane.

23. The plant of claim 21, wherein the system for producing compressed air is fed with air and comprises, in a flow direction of the air: an air inlet, an air compressor, a compressed gas cooling system, a condensate separator, an activated carbon filter adapted and configured to remove residual oil particles from the fed air, a particle filter adapted and configured to remove activated carbon particles from the fed air, a dryer, and a compressed air storage tank.

Description

[0037] The invention will be described in greater detail using FIGS. 1, 2 and 3.

[0038] FIG. 1 represents a plant according to the invention when the air used for producing the nitrogen is taken from an instrument air network.

[0039] FIG. 2 represents a plant according to the invention when the air used for producing the nitrogen is produced by a dedicated compressor.

[0040] In both scenarios, the air stream 1 supplies a membrane stage consisting of one or more membranes in parallel 2 and enabling the production of pressurized nitrogen. A nitrogen-enriched retentate 3 is recovered from the membrane. Depending on the amount of oxygen tolerated in the biomethane network, a more or less pure nitrogen is produced. In order to control this purity of the nitrogen, the retentate passes into an analyzer 4 that measures the oxygen concentration and the purity of the nitrogen injected into the biomethane network 6 is controlled via a control valve 5. The stream of nitrogen produced is controlled 15 by adjusting the flow rate of air entering the membrane stage, either via a control valve 16 (FIG. 1), or by adjusting the production capacity of the air compressor 17 (FIG. 2); a flowmeter and also an analyzer of CH.sub.4 from the biomethane make it possible to check that the GCV complies with the injection specification.

[0041] FIG. 3 depicts what the air production system may be: the air may be compressed to a pressure greater than 5 bar in an air compressor 7, then cooled 8. The air stream thus compressed and cooled is introduced into a condensate separator 9, before passing successively through an adsorber 10 comprising activated carbon so as to eliminate the residual oil particles and through a particle filter 11 so as to eliminate the activated carbon particles. A compressed and purified air stream is then recovered which may be stored 12 before supplying the membrane 2.

[0042] Tables 1 and 2 below illustrate the need for injection of nitrogen in order to comply with the biomethane injection specification from the point of view of the GCV and the Wobbe index in L gas networks:

TABLE-US-00001 TABLE 1 Without With Biomethane composition N.sub.2 N.sub.2 N.sub.2 % mol. 0.0% 6.0% O.sub.2 % mol. 0.0% 0.0% CO.sub.2 % mol. 2.5% 2.5% CH.sub.4 % mol. 97.5% 91.5% Total 100.0% 100.0% GCV kWh/Nm.sup.3 10.81 10.15 Wobbe index kWh/Nm.sup.3 14.22 13.06

TABLE-US-00002 TABLE 2 L gas GRT specification max. GCV kWh/Nm.sup.3 10.5 max. Wobbe index kWh/Nm.sup.3 13.06