Method for recovering compressed natural gas stored under pressure in a tank

Abstract

The recovery method comprises the following steps: - providing at least one recovery container (28) for compressed natural gas that is at least partly empty and has an internal pressure lower than the pressure of the compressed natural gas in a mobile test tank (6), - placing the mobile test tank (6) in fluid communication with the recovery container (28), - transferring at least a portion of the compressed natural gas from the mobile test tank (6) to the recovery container (28) by equalizing the internal pressure between the mobile test tank (6) and the recovery container (28) during a pressure equalization transfer step.

Description

[0001] The present invention relates to a method for recovering compressed natural gas stored under pressure in a mobile test tank.

[0002] The invention also relates to a method for metrological control of a device for distributing compressed natural gas to a motor vehicle implementing such a recovery method.

[0003] It is planned to subject a compressed natural gas (CNG) supply station to motor vehicles to regular metrological checks in order to ensure the proper functioning of the various elements of this station. In particular, each CNG distribution device to a motor vehicle is regularly checked to ensure that the transfer of CNG from a CNG source to a motor vehicle is carried out under good conditions.

[0004] To achieve this, it is intended to use each dispensing device to transfer CNG from the CNG source to one or more mobile test tanks, the operation of the dispensing device and certain characteristics of the dispensed CNG being monitored or measured during this transfer operation.

[0005] Once filled, the mobile test tank(s) must be emptied of the CNG they contain in order to be reused. This is usually done by flaring the CNG contained in the mobile test tank(s), i.e., burning the natural gas.

[0006] However, such a flaring process is highly polluting as it releases combustion gases directly into the atmosphere. In addition, the CNG is "wasted," meaning it is only used to conduct metrological inspections and is not used to fuel a motor vehicle. Given that a CNG supply station typically includes several dispensing devices, each of which must be inspected, the amount of wasted gas quickly becomes very significant.

[0007] One of the aims of the invention is to overcome these drawbacks by proposing a method for recovering compressed natural gas which has been previously transferred into a mobile test tank allowing this gas to be recovered so that it can be reused and not flared.

[0008] To this end, the invention relates to a method for recovering compressed natural gas stored under pressure in a mobile test tank, comprising the following steps: providing at least one compressed natural gas recovery container that is at least partially empty and has an internal pressure lower than the pressure of the compressed natural gas in the mobile test tank, placing the mobile test tank in fluid communication with the recovery container, transferring at least a portion of the compressed natural gas from the mobile test tank to the recovery container by equalizing the internal pressure between the mobile test tank and the recovery container during a pressure equalization transfer step.

[0009] The invention allows the mobile test tank to be emptied so that it can be reused for another metrological check, the compressed natural gas being transferred into a recovery container which can be stored for the purpose of later reuse of the compressed natural gas. Thus, the compressed natural gas is recovered and not flared and can be reused, for example, to power a motor vehicle or to replenish the source of compressed natural gas.

[0010] The recovery method according to the invention may further comprise one or more of the following characteristics, considered alone or in any technically conceivable combination: the recovery method further comprises the following step if a volume remains available in the recovery container and if a volume of natural gas remains present in the mobile test tank: placing a compressor in fluid communication with the mobile test tank on the one hand and with the recovery container on the other hand, transferring at least a portion of the natural gas remaining in the mobile test tank into the recovery container by means of said compressor;the recovery method further comprises the following step if the recovery container is filled with the natural gas transferred from the mobile test tank and if a volume of natural gas remains present in the mobile test tank: breaking the fluid communication between the mobile test tank and the recovery container, placing the mobile test tank in fluid communication with another at least partly empty recovery container, transferring at least a part of the natural gas remaining in the mobile test tank into the other recovery container; the compressed natural gas is transferred from the mobile test tank to one or more recovery containers until the pressure in the mobile test tank is close to atmospheric pressure, more particularly less than or equal to 1 bar;the recovery method further comprises at least one step of re-injecting the compressed natural gas present in at least one recovery container by placing said recovery container in fluid communication with the compressed natural gas tank of a motor vehicle or with a compressed natural gas injection point of a station for supplying compressed natural gas to motor vehicles and by transferring said compressed natural gas from the recovery container to said motor vehicle tank or said injection point; said re-injection step is carried out by means of a compressor arranged in the fluid communication between the recovery container and the motor vehicle tank or the injection point; the pressure of compressed natural gas in the mobile test tank (6) is substantially between 200 and 250 bars before the pressure-balancing transfer step. ;

[0011] According to another aspect, the invention also relates to a method for metrological control of a device for distributing compressed natural gas to a motor vehicle, comprising the following steps: placing the dispensing device in fluid communication with a source of compressed natural gas on the one hand and with at least one mobile test tank on the other hand, actuating the dispensing device to fill the mobile test tank from the source of natural gas, measuring at least the quantity and the flow rate of the gas dispensed by the dispensing device upon its actuation, after filling the mobile test tank, implementing a recovery method as described above to transfer the natural gas from the mobile test tank to at least one recovery container.

[0012] The control method according to the invention may further comprise one or more of the following characteristics, considered alone or in any technically conceivable combination: the compressed natural gas source comprises a plurality of storage containers distributed in at least two transfer levels to the mobile test tank, the distribution device being arranged: to transfer the compressed natural gas stored in at least one storage container of one level by pressure balancing to the mobile test tank and when the pressure differential between said storage container and the mobile test tank is less than a predetermined value, to transfer the compressed natural gas stored in at least one storage container of the other level by pressure balancing to said mobile test tank; the compressed natural gas injection point of a natural gas supply station is in fluid communication with the natural gas source, the compressed natural gas reinjected into said injection point being used to resupply said compressed natural gas source with compressed natural gas.

[0013] Other aspects and advantages of the invention will appear on reading the following description, given by way of example and with reference to the appended drawings, in which: there Fig. 1 is a schematic representation of a device for distributing compressed natural gas, and the Fig. 2 is a schematic representation of the different stages of a process for recovering compressed natural gas stored in a mobile test tank.

[0014] In reference to the Fig. 1 , a distribution device 1 for dispensing compressed natural gas (CNG) from a CNG source 2 is described for transferring the CNG from the CNG source 2 to a vehicle tank 4 or to a mobile test tank 6. Such a distribution device 1 is intended to be part of a station for supplying CNG to motor vehicles using CNG as fuel.

[0015] The distribution device 1 comprises in particular a device 8 for injecting CNG into a vehicle tank 4 or into the mobile test tank 6, the injection device 8 being in fluid communication with the CNG source 2, possibly via a control device 10 and a valve device 12.

[0016] The injection device 8 is for example formed by a conventional gun for this type of application which can be placed in fluid communication with the internal volume of the vehicle tank 4 or the mobile test tank 6, as shown schematically in the Fig. 1 .

[0017] The CNG source 2 is for example formed by a plurality of storage containers 14 each containing CNG at a pressure substantially between 250 and 300 bars. The CNG contained in the storage containers is gas having been compressed from the national gas distribution network. The storage containers 14 are for example distributed between at least two, preferably three, transfer levels to the injection device 8, the gas pressure being different in the storage containers 14 of each transfer level. More particularly, the storage containers 14 are distributed between at least one lower bank 16 and one upper bank 18, the CNG in the lower bank being supplied to a tank before the CNG in the upper bank when the CNG is supplied to a vehicle tank.Preferably, the storage containers 14 are equally distributed in an intermediate bank 20, with the CNG in the intermediate bank 20 being supplied to a tank before the CNG in the upper bank and after the CNG in the lower bank when the CNG is supplied to a vehicle tank, as will be described in more detail below.

[0018] The valve device 12 makes it possible to open and close the fluid communication between the CNG source 2 and the injection device 8. When the storage containers 14 are distributed in several transfer levels, the valve device 12 makes it possible to select the bank from which the CNG is transferred from the CNG source 2 to the injection device 8, by opening the fluid communication between one of the banks and the injection device 8 and by closing the fluid communication between the other banks and the injection device 8. Thus, the valve device 12 comprises for example three valves 22 arranged respectively in the fluid communication between the lower bank 16 and the injection device 8, the intermediate bank 20 and the injection device 8 and the upper bank 18 and the injection device 8.

[0019] The control device 10 makes it possible to control the valve device 12 to open and close the fluid communication between the CNG source 2 and the injection device 8 when a user actuates the injection device 8. The control device 10 is in particular arranged to select the valve 22 to be opened in the valve device in order to open the fluid communication between one of the transfer levels and the injection device 8 as a function of the pressure at which the gas can be injected into a vehicle tank 4 during normal operation of the distribution device 1. Thus, the control device 10 is arranged to measure the flow rate of the gas upstream of the injection device 8 by means of a flow sensor 24 and to control the valve device 12 in order to guarantee an optimum filling flow rate as a function of the pressure differential between a storage tank 14 and a vehicle tank 4, by means of a computer 26.

[0020] More particularly, when a user places the injection device 8 in fluid communication with a vehicle tank 4, the control device 10 is arranged to begin the transfer of CNG between the lower bank 16 of the CNG source 2 and the vehicle tank 4 by pressure balancing when the pressure differential between the lower bank 16 and the vehicle tank 4 allows a satisfactory filling flow rate to be obtained. When the pressure differential does not allow such a filling flow rate to be obtained, the control device 10 closes the fluid communication between the lower bank 16 and the injection device 8 and opens the fluid communication between the intermediate bank 20 and the injection device 8.Likewise, when the pressure differential between the intermediate bank 20 and the vehicle tank 4 does not allow a satisfactory filling flow rate to be obtained, the control device 10 closes the fluid communication between the intermediate bank 20 and the injection device 8 and opens the fluid communication between the upper bank 18 and the injection device 8. The filling of the vehicle tank 4 continues by pressure balancing until a pressure of approximately 200 bar is obtained in the vehicle tank 4.

[0021] When the pressure in the storage containers 14 of the different transfer levels becomes lower than a predetermined threshold, which is different for each transfer level, the storage containers 14 are replenished with CNG from the distribution network.

[0022] In order to ensure that the dispensing device 1 described above is functioning correctly, it is regularly subjected to a metrological control during which the dispensing device 1 is used as described above to fill the mobile test tank 6 instead of a vehicle tank 4. This filling makes it possible to measure at least the quantity and the flow rate of the gas dispensed by the dispensing device 1 when the injection device 8 is actuated. It is understood that other parameters may also be measured during the metrological control. When the results of the control are not satisfactory, the dispensing device 1 is made unavailable until the anomalies are overcome.

[0023] The mobile test tank 6 is a container, for example of the steel cylinder type, which can contain compressed gas. By mobile, it is meant that the mobile test tank 6 can be transported, for example by being placed in a vehicle, between different distribution devices and between different CNG supply stations so that the mobile test tank 6 can be used for different metrological control sessions. The mobile test tank 6 has for example a volume of 70 liters and a diameter of 30 cm. During the test, the mobile test tank 6 is filled by placing it in fluid communication with the injection device 8 and actuating the latter until a filling rate of approximately 9 to 10 kg / min and a pressure of approximately 220 bar is reached. Such a filling corresponds to approximately 12 kg of CNG in the mobile test tank 6.After filling the mobile test tank 6, the pressure in the mobile test tank 6 is substantially between 200 bar and 250 bar. The operation of filling the mobile test tank 6 is carried out several times in order to validate the control. Thus, it is for example planned to completely fill the mobile test tank 6 three times and to half fill it twice. Consequently, a metrological control session of a CNG distribution device 1 consumes almost 50 kg of CNG. Knowing that a distribution station generally comprises three to five distribution devices 1, it is understood that the consumption of CNG to carry out the metrological controls of the distribution devices is very significant.

[0024] The invention makes it possible to recover the CNG used during these checks so that this significant quantity of CNG can be reused and not flared, as is currently done. This recovery method will now be described with reference to the Fig. 2 .

[0025] When the mobile test tank 6 has been filled during a metrological control phase as described above, the mobile test tank 6 is placed in fluid communication with an at least partially empty CNG recovery container 28, as shown at the top of the Fig. 2 . The recovery container 28 has at this stage an internal pressure lower than the CNG pressure in the mobile test tank 6. The recovery container 28 is for example of the same type as the mobile test tank 6 and has for example a capacity similar to or greater than that of the mobile test tank 6 so that the CNG contained in the mobile test tank 6 can be completely emptied into the recovery container 28 if the latter is empty at the start of the implementation of the recovery method.

[0026] During this step, the recovery container 28 is in direct fluid communication with the mobile test tank 6, i.e., the recovery container 28 and the mobile test tank 6 are directly connected by a gas line so that the CNG contained in the mobile test tank 6 is transferred to the recovery container 28 without its characteristics being modified. Thus, during this step, at least a portion of the CNG contained in the mobile test container 6 is transferred to the recovery container 28 by balancing the internal pressure between the mobile test tank 6 and the recovery container 28.

[0027] At the end of this step, if the mobile test tank 6 still contains CNG and if the recovery container 28 still contains an empty volume, the direct fluid communication between the mobile test tank 6 and the recovery container 28 is undone and a compressor 30 is placed between the mobile test tank 6 and the recovery container 28. In other words, the compressor 30 is placed in fluid communication with the mobile test tank 6 on the one hand and with the recovery container 28 on the other hand, as shown in the middle of the Fig. 2 . The compressor 30 allows the mobile test tank 6 to be completely emptied into the recovery container 28 despite the zero or negative pressure difference between the pressure in the mobile test tank 28 and the pressure in the recovery container 28 at the end of the pressure-balancing filling step. The compressor 30 is, for example, a mobile CNG compressor that can be transported with the mobile test tank container 6 from one dispensing device 1 to another and from one dispensing station to another.

[0028] When the mobile test tank 6 still contains CNG and when the recovery container 28 is full following the transfer steps described above, the CNG remaining in the mobile test tank 6 is transferred to another recovery container. To do this, the fluid communication between the mobile test tank 6 and the recovery container 28 is released and the mobile test tank 6 is placed in fluid communication with another at least partially empty recovery container, either directly or via the compressor 30. The CNG remaining in the mobile test tank 6 is then transferred to the other recovery container either by pressure balancing or by means of the compressor 30 depending on the pressure difference between the mobile test tank 6 and the other recovery container.

[0029] The CNG contained in the mobile test tank 6 is transferred into one or more recovery containers 28 until the pressure inside the mobile test tank 6 is close to atmospheric pressure. Atmospheric pressure means that the pressure inside the mobile test tank 6 is less than or equal to 1 bar. The mobile test tank 6 can then be reused for a new metrological control.

[0030] Thus, the recovery method according to the invention makes it possible to recover the CNG transferred into a mobile test tank 6 during a metrological inspection of a CNG distribution device 1 without having to flare this CNG to empty the mobile test tank 6. This recovery avoids a significant release of CNG into the atmosphere and avoids polluting and costly waste.

[0031] According to one embodiment, the CNG contained in the recovery container 28 can be reused, for example by being transferred into one or more vehicle tanks 4 or by being transferred into a CNG injection point 32 of a natural gas supply station in fluid communication with the CNG source 2, as shown at the bottom of the Fig. 2 The injection point 32 thus makes it possible to replenish the CNG source 2, by filling one or more of the storage containers 14. The transfer of the CNG contained in the recovery container 28 to a vehicle tank 4 or to the injection point 32 is done for example by means of the compressor 30 described previously.

[0032] The CNG contained in the recovery container(s) 28, used to carry out one or more metrological checks, can thus be recovered to be consumed as motor vehicle fuel after these metrological checks.

Claims

1. Method for recovering compressed natural gas stored under pressure in a mobile test tank (6), comprising the following steps: - providing at least one recovery container (28) for the compressed natural gas which is at least partly empty and has an internal pressure lower than the pressure of the compressed natural gas in the mobile test tank (6), - placing the mobile test tank (6) in fluid communication with the recovery container (28), - transferring at least a portion of the compressed natural gas from the mobile test tank (6) to the recovery container (28) by equalizing the internal pressure between the mobile test tank (6) and the recovery container (28) during a pressure equalization transfer step.

2. A recovery method according to claim 1, further comprising the following step if a volume remains available in the recovery container (28) and if a volume of natural gas remains present in the mobile test tank (6): - placing a compressor (30) in fluid communication with the mobile test tank (6) on the one hand and with the recovery container (28) on the other hand, - transferring at least a portion of the natural gas remaining in the mobile test tank (6) into the recovery container (28) by means of said compressor (30).

3. A recovery method according to claim 1 or 2, further comprising the following step if the recovery container (28) is filled with natural gas transferred from the mobile test tank (6) and if a volume of natural gas remains present in the mobile test tank (6): - breaking the fluid communication between the mobile test tank (6) and the recovery container (28), - placing the mobile test tank (6) in fluid communication with another at least partly empty recovery container, - transferring at least a portion of the natural gas remaining in the mobile test tank (6) into the other recovery container (28).

4. A recovery method according to any one of claims 1 to 3, wherein the compressed natural gas is transferred from the mobile test tank (6) to one or more recovery vessels (28) until the pressure in the mobile test tank (6) is close to atmospheric pressure, more particularly less than or equal to 1 bar.

5. A recovery method according to any one of claims 1 to 4, further comprising at least one step of re-injecting the compressed natural gas present in at least one recovery container (28) by placing said recovery container (28) in fluid communication with the compressed natural gas tank (4) of a motor vehicle or with an injection point (32) of compressed natural gas of a station for supplying compressed natural gas to motor vehicles and by transferring said compressed natural gas from the recovery container (28) to said motor vehicle tank (4) or to said injection point (32).

6. Recovery method according to claim 5, wherein said reinjection step is carried out by means of a compressor (30) arranged in the fluid communication between the recovery container (28) and the motor vehicle tank (4) or the injection point (30).

7. A recovery method according to any one of claims 1 to 6, wherein the pressure of compressed natural gas in the mobile test tank (6) is substantially between 200 and 250 bars before the pressure balancing transfer step.

8. A method for metrologically controlling a distribution device (1) for supplying compressed natural gas to a motor vehicle, comprising the following steps: - placing the distribution device (1) in fluid communication with a source (2) of compressed natural gas on the one hand and with at least one mobile test tank (6) on the other hand, - actuating the distribution device (1) to fill the mobile test tank (6) from the natural gas source (2), - measuring at least the quantity and the flow rate of the gas distributed by the distribution device (1) during its actuation, - after filling the mobile test tank (6), implementing a recovery method according to any one of claims 1 to 7 to transfer the natural gas from the mobile test tank (6) to at least one recovery container (28).

9. Metrological control method according to claim 8, wherein the source (2) of compressed natural gas comprises a plurality of storage containers (14) distributed in at least two transfer levels towards the mobile test tank (6), the distribution device (1) being arranged: - to transfer the compressed natural gas stored in at least one storage container (14) of one level by pressure balancing to the mobile test tank (6) and - when the pressure differential between said storage container (14) and the mobile test tank (6) is less than a predetermined value, to transfer the compressed natural gas stored in at least one storage container (14) of the other level by pressure balancing to said mobile test tank (6).

10. Metrological control method according to claim 8 or 9, implementing a recovery method according to claim 5, in which the compressed natural gas injection point (32) of a natural gas supply station is in fluid communication with the source (2) of natural gas, the compressed natural gas reinjected into said injection point (32) being used to resupply said source (2) of compressed natural gas with compressed natural gas.

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