Process and apparatus for recovery of boil-off gas from hydrogen liquefaction

Abstract

The hydrogen boil-off gas recovery method includes liquefying hydrogen (1) by cooling a hydrogen feed stream in a first cryogenic box (80 K CB), further cooling the hydrogen in a second cryogenic box (20 K CB), liquefying hydrogen in or downstream of the second cryogenic box, providing liquefied hydrogen to a loading bay housing at least one hydrogen transport truck, sending boil-off gas (BOG1) from the at least one hydrogen transport truck in the loading bay to which the liquefied hydrogen is provided to an indirect heat exchanger (BOG heater) external to the first and second cryogenic boxes to warm the boil-off gas, and mixing at least a portion of the warmed boil-off gas with hydrogen in a hydrogen refrigeration cycle (LP H2 COMP) used to provide cooling to the hydrogen feed stream upstream of the first cryogenic box or the second cryogenic box, depending on the pressure of the boil-off gas.

Description

[Technical field]

[0001] The present invention relates to a process and apparatus for recovery of boil-off gas from the liquefaction of hydrogen. The liquefied hydrogen is typically transported through a loading bay to supply one or more hydrogen transport trucks for transportation of the liquid hydrogen. [Background technology]

[0002] Hydrogen transport truck fill time (or boil off gas (BOG) recovery capacity) is limited today by the flexibility of the entire hydrogen liquefaction system (including the hydrogen generation system), which requires that the flow of BOG sent back to any BOG recovery system must match the ramp-up and ramp-down rates of the entire system to maximize recovery, thus limiting the rate at which the hydrogen transport truck can be depressurized and filled (or the amount of BOG that can be recovered).

[0003] From US 2013 / 291585 it is known to recycle BOG in a helium liquefier, the point to which the BOG is recirculated being selected purely depending on the temperature and purity of the BOG.

[0004] It has been found that the decision to recycle hydrogen liquefier BOG to either the feedstream or the hydrogen refrigeration cycle is more reliable based on BOG pressure measurements, in particular the phenomenon of liquid stratification within the trucks can mean that there is no direct relationship between the temperature and pressure of the BOG from the trucks.

[0005] According to the present invention, BOG recovery can be maximized by using these H2 molecules in one of three ways: heating the boil-off and then sending it to a liquefaction unit (multiple liquefaction unit trains can be implemented depending on the overall plant configuration) for compression and re-liquefaction; Sending boil-off upstream of the liquefier, especially at the inlet of the compression train; -Low temperature BOG is directly converted into a resource by utilizing its cooling power in the liquefaction equipment.

[0006] In such a configuration, it is important to separate the BOG according to its pressure level and to design the pressure train capacity to match the operation of the hydrogen transport truck. The process control principle of the whole system is based on the adaptability of the H2 feedstock load "according to" the BOG production.

[0007] According to the present invention, the duration of a hydrogen delivery truck filling sequence can be reduced (or the amount of BOG recovered can be maximized) by controlling the feed flow rate (at the inlet of the hydrogen liquefaction system) with the BOG flow rate (by simultaneously reducing the production capacity (or feed flow rate) of the hydrogen production unit with the recovery of BOG). Summary of the Invention [Means for solving the problem]

[0008] In accordance with the object of the present invention, a method for hydrogen boil-off gas recovery is provided, comprising: liquefying hydrogen by cooling a hydrogen feed stream at a first pressure in a first cryogenic box; further cooling the hydrogen in a second cryogenic box; cooling the second cryogenic box using a hydrogen refrigeration cycle; liquefying hydrogen in or downstream of the second cryogenic box; providing liquefied hydrogen to a storage or a loading bay housing at least one hydrogen transport truck; directing boil-off gas from the at least one hydrogen transport truck in the loading bay where the liquefied hydrogen is provided to an indirect heat exchanger external to the first and second cryogenic boxes to liquefy the boil-off gas. and warming the boil-off gas and measuring a pressure of the boil-off gas, where if the boil-off gas stream from the at least one hydrogen transport truck is below a predetermined pressure, at least a portion of the boil-off gas stream is warmed in a first heater and mixed with a hydrogen cooling cycle stream, and if the boil-off gas stream from the at least one hydrogen transport truck is above a predetermined pressure, at least a portion of the boil-off gas stream from the at least one hydrogen transport truck is warmed in the first heater or the second heater and mixed with a hydrogen feed stream upstream of the first cryogenic box.

[0009] According to another optional feature, When the boil-off gas stream from the at least one hydrogen transport truck is above a predetermined pressure, at least a portion of the boil-off gas stream from the at least one hydrogen transport truck is warmed in the second heater and mixed with the hydrogen feed stream upstream of the first cryogenic box, regardless of the temperature of the boil-off gas stream. At least a portion of the warmed boil-off gas is compressed in a dedicated compressor before being mixed with the hydrogen feed stream. At least a portion of the warmed boil-off gas is mixed with the hydrogen feed stream and then compressed to a first pressure. A hydrogen feedstream is produced by the hydrogen production unit and sent to the first cryogenic box in a first amount when the warmed boil-off gas is not mixed with the hydrogen feedstream, and the amount of the hydrogen feedstream sent from the hydrogen production unit to the first cryogenic box is reduced to a second amount less than the first amount when the warmed boil-off gas is mixed with the hydrogen feedstream. The predetermined pressure is between 1.5 and 3 bar, preferably equal to 2 bar. The boil-off gas stream is analyzed to detect the value of one of its components, and if its hydrogen content is below a predetermined value and / or its impurity content is above a predetermined value, the boil-off gas is not sent to the hydrogen feed stream or to the hydrogen cooling cycle. When the boil-off gas stream from the at least one hydrogen transport truck is below a predetermined pressure and below a predetermined temperature, at least a portion of the boil-off gas stream is not warmed in the first heater but is mixed with the hydrogen cooling cycle stream. The predetermined temperature is between -180°C and -210°C, preferably equal to -200°C. When the boil-off gas stream from the at least one hydrogen transport truck is below a predetermined pressure but above a predetermined temperature, the boil-off stream is heated in a first heater and returned to the hydrogen refrigeration cycle. If the boil-off gas stream from the at least one hydrogen transport truck is below a predetermined pressure and below a predetermined temperature, the boil-off stream is not heated in the first or second heaters and is heated in the heat exchangers of the first and / or second cryogenic boxes to cool the hydrogen feed stream (to be liquefied). The pre-determined pressure has a value selected between 2 and 10 bar, for example 2 bar.

[0010] According to another aspect of the present invention, a hydrogen liquefaction system and boil-off gas recovery system is provided, comprising a liquefaction system for liquefying hydrogen, the liquefaction system including a first cryogenic box for cooling a hydrogen feed stream at a first pressure, and a second cryogenic box for further cooling the hydrogen to liquefy the hydrogen in or downstream of the second cryogenic box, a loading bay housing at least one hydrogen transport truck, means for providing liquefied hydrogen to storage or to the loading bay housing the at least one hydrogen transport truck, a hydrogen refrigeration cycle used to provide cooling to the second cryogenic box, and means for detecting a pressure of the boil-off gas stream from the at least one hydrogen transport truck. the stage, first and second heaters external to the first and second cryogenic boxes; means for sending a boil-off gas stream from at least one hydrogen transport truck to the first heater if, and preferably only if, the detected pressure is below a predetermined pressure; means for sending the boil-off stream warmed in the first heater to be mixed with the hydrogen refrigeration cycle; means for sending the boil-off gas stream from the at least one hydrogen transport truck to the second heater if, and preferably only if, the detected pressure is above the predetermined pressure; and means for sending the boil-off stream warmed in the first heater to be mixed with the hydrogen feed stream upstream of the first cryogenic box.

[0011] Preferably, the liquefaction apparatus includes means for detecting a temperature of the boil-off gas stream from the at least one hydrogen transport truck, and means for directing the boil-off gas stream from the at least one hydrogen transport truck to the hydrogen refrigeration cycle without heating if the detected temperature is below a predetermined temperature and the detected pressure is also below a predetermined pressure.

[0012] The liquefaction system may include an additional first cryogenic box for cooling the hydrogen feed stream at the first pressure, an additional second cryogenic box (20K CB) for further cooling the hydrogen to liquefy it in or downstream of the additional second cryogenic box, means for providing liquefied hydrogen from the second additional cryogenic box or downstream thereof to a storage or loading bay housing at least one hydrogen transport truck, an additional hydrogen refrigeration cycle used to provide cooling to the second cryogenic box, means for sending a boil-off stream warmed in the first heater to be mixed with the additional hydrogen refrigeration cycle, and means for sending a boil-off stream warmed in the first heater to be mixed with the hydrogen feed stream upstream of the first additional cryogenic box.

[0013] This process also allows sharing the flexibility of all compressors and hydrogen generation systems by separating the BOG according to its pressure level and sending it to different coupling points with different inlet pressures, maximizing the flexibility of the integrated liquefaction system to speed up the filling sequence of hydrogen transport trucks or increase the recovery of BOG. ​​Temperature conditions can be considered, but preferably only if the BOG is cold enough to be sent to the hydrogen cooling cycle without any heating step.

[0014] Hydrogen liquefaction is a high cost process and BOG recovery of vaporized hydrogen molecules is very valuable, especially when gaseous hydrogen is produced at high cost, for example by electrolysis. BOG generation typically occurs through the cold end flash of the liquefier, the LH2 piping, the LH2 storage, and during the loading process of hydrogen transport trucks and subsequent delivery of liquid hydrogen.

[0015] The invention will now be explained in more detail with reference to the figures. [Brief description of the drawings]

[0016] [Figure 1]FIG. 1 shows a boil-off recovery process in which the boil-off gas is recycled to the hydrogen refrigeration cycle. [Diagram 2] FIG. 2 shows a boil-off recovery process in which the boil-off gas is recycled to the hydrogen feed compressor. [Diagram 3] FIG. 3 shows a single train boil-off recovery process in which the boil-off gas is recycled to the hydrogen feed compressor and hydrogen refrigeration cycle. [Figure 4] FIG. 4 shows a double train boil-off recovery process in which the boil-off gas is recycled to the hydrogen feed compressor and hydrogen refrigeration cycle. [Diagram 5] FIG. 5 shows a single-train boil-off recovery process in which the boil-off gas is recycled to the hydrogen feed compressor and then to the hydrogen refrigeration cycle via an additional compression step. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] There are various methods for recovering BOG from different areas of the plant (liquefaction unit, storage and loading). Figure 1 below shows the BOG recovery methods.

[0018] Hydrogen (1) can be produced by a steam methane reformer (SMR), shown in the diagram as H2 production, or by electrolysis or any other known means. Hydrogen (1) can also be obtained from industrial facility off-gas or from a pipeline. It enters the liquefaction unit after any components that may freeze are removed in a downstream cryobox.

[0019] In FIG. 1, the liquefier uses a pre-cooling cycle (typically nitrogen). The liquefier includes a first cryogenic box, an 80K cryogenic box (80K CB), cooled using a N2 freezing cycle with N2 compression (N2 COMP), and a second cryogenic box, a 20K cryogenic box (20K CB), using another hydrogen liquefaction cycle. The hydrogen refrigeration cycle compresses the cycle hydrogen using a LP compressor (LP H2 COMP) and a HP compressor (HP H2 COMP). In addition to or instead of the above cycles, it is understood that other types of refrigeration cycles can also be used, such as a mixed refrigeration cycle using a mixture of nitrogen and hydrocarbons.

[0020] Once liquefied in or downstream of the 20K cryobox, the liquid hydrogen is typically stored in a vacuum insulated storage burette tank LH2 storage or in a sphere depending on the volume. Downstream of the storage LH2 storage is the loading area, the loading bay, which includes multiple loading bays and sometimes a transfer pump. At least one truck is located in the loading bay, in which the liquefied hydrogen is being or has been filled. The term "loading bay" is therefore a shorthand way of referring to at least one hydrogen transport truck located therein. Figure 1 also shows the main BOG paths. For example, the boil-off gas BOG2 generated between the 20K cryobox and through the storage tank LH2 storage is generally recycled to the cold end of the liquefaction device. Downstream of the storage, in the loading bay, both the liquid and gas phases from the hydrogen transport truck (or trucks) are connected to the plant. Thus, gas phase BOG1 from the hydrogen transport truck is collected through the BOG network, typically warmed by an indirect heat exchanger through a BOG heater, and recycled to the LP compressor (LP H2 COMP) that is used to compress the hydrogen of the refrigeration cycle for the 20K cryobox. The BOG1 stream is then mixed with the H2 cycle stream. This BOG heater typically uses an external fluid such as steam or water to warm the BOG. ​​So, in this case, gaseous BOG1 is warmed outside the liquefier and sent to the liquefier at a suitable temperature to mix with the cycle gas.

[0021] In Figure 2, H2 production is performed at relatively low pressure (e.g. in the case of electrolysis) and a feed gas compressor (FEED COMP) is required upstream of the liquefaction unit to compress the hydrogen 1 produced in the hydrogen production unit. In this case, the BOG 2 produced during the loading process is warmed using an external fluid through a BOG heater similar to the one in Figure (1) and recirculated to the feed gas compressor (FEED COMP). The main difference with Figure 1 is that in Figure 1 the BOG is recirculated to the H2 cooling loop, while the boil-off gas BOG 2 of the hydrogen transport truck is recirculated to the feed line.

[0022] This has the advantage of recirculating the boil-off BOG1 without affecting the operation of the liquefier. Another advantage is that the BOG flow also does not affect the rated flow rate of the feed gas compressor train capacity, since the H2 generation unit can be adjusted less by the same amount to ensure that a constant flow rate is always supplied to the feed gas compressor FEED COMP. However, in general the feed gas compressor FEED COMP can be located much further from the loading bay where the BOG is generated than the LP compressor LP H2 COMP. The distance between the loading area and the feed compressor can be an issue in terms of maximum allowable pressure drop, since the refluxing BOG can be at an extremely low pressure (1-2 bar absolute).

[0023] The present invention focuses on BOG generation and recovery from the loading area. BOG generated in the loading area presents many challenges.

[0024] The amount of BOG that a hydrogen transport truck has when it returns to the loading bay will vary depending on the logistics chain, delivery method, distance to customer, number of customers to deliver to, billing method, etc.

[0025] BOG production can be varied and includes: Hydrogen transport truck depressurization: When a hydrogen transport truck arrives on-site under high pressure with remaining stored liquid H2 on board. - Refueling of hydrogen transport trucks. This action creates a reflux of low-pressure gaseous hydrogen. "Natural heat dissipation" around the storage system

[0026] Recycling any kind of BOG in a liquefier using the process according to FIG. 1 by offloading the H2 generation system leads to the following drawbacks: Liquefier compression trains may be over-designed, making it difficult to enforce wide ramp-up / ramp-down ranges on the liquefier machinery. · Processes are disrupted in the liquefaction unit to manage this additional peak.

[0027] Managing BOG solely through the solution in Figure 2 will also be a challenge as it will result in: Resourcing BOG above a certain minimum pressure (e.g., leading to BOG losses at low pressures due to natural heat ingress in the reservoir) Forcing a wide range of ramp-up / ramp-down on the feed gas compression train. Note that adding buffers to address this ramp-up issue is expensive.

[0028] BOG recovery in such systems is typically limited to about 70%.

[0029] It is important to note that when BOG is recycled to the liquefier, the feed gas flow rate is reduced by the same amount, therefore BOG recovery becomes much more valuable when H2 production reaches high OPEX costs, as is the case with electrolysis.

[0030] One embodiment of the invention shown in Figures 3A and 3B typically applies to liquefaction trains of about 5 to about 80 tpd. In this embodiment, three different BOG networks are created, with one or more high pressure BOG recovery networks separated from the low pressure BOG recovery network: · BOG generated during truck filling (e.g., pressure < 2 bar absolute) may generally be sent to the warm end of the LP cycle compressor as in Figure 1. Due to limited BOG recovery pressure in the network, the temperature of the BOG leaving the trucks is better controlled. It is therefore much easier to recover LP BOG at the cold end of the liquefaction unit in the same way as it is done for stored boil-off gas BOG2. · The BOG produced during depressurization is at high pressure (approximately 2-10 bar absolute) and can be sent to the feed gas compressor. Each BOG net frame can be connected to each loading bay.

[0031] Thus, FIG. 3A shows that low pressure boil-off gas BOG (LP) below a given pressure (here 2 bar) is heated in a first external heater BOG heater and sent to the inlet of the compressor of the hydrogen refrigeration cycle (if the temperature at reflux is above a given temperature, here -200°C) or sent directly to the cold end of the liquefier to be recovered through the hydrogen refrigeration cycle without passing through the first or second heater and heated in the heat exchangers of the first and / or second cryogenic boxes to cool the hydrogen feed stream to be liquefied (if the temperature at reflux is below a given temperature, here -200°C), and high pressure boil-off gas BOG (MP) above a given pressure (here 2 bar, e.g. 2-10 bar) is heated in a second heater BOG heater and sent to the inlet of the feed compressor upstream of the liquefier.

[0032] As shown in FIG. 3B, the loading bay housing at least one hydrogen transport truck produces boil-off gas, the pressure and temperature of which are detected using pressure detection means PI and temperature detection means TI. The pressure detector PI can open all three conduits, namely the conduit leading to the 20K cryobox, the conduit connected to the LP compressor of the hydrogen cooling cycle and the conduit connected to the MP compressor, i.e. the feed hydrogen compressor, and the temperature detector is connected only to the conduit leading to the 20K cryobox. In other words, the decision to send the BOG to the MP compressor or the LP compressor is made based only on the pressure value and does not depend on the temperature. So, any BOG with a pressure higher than a certain pressure is sent to the feed compressor regardless of its temperature. If the pressure is below a certain value, at least one inlet valve to at least one low pressure circuit is opened and the inlet valve to the high pressure circuit is closed. So, for example, low pressure boil-off gas below 2 bar is sent via the low pressure circuit to the low pressure heater and then to the hydrogen cooling circuit. If the pressure is above a predetermined value, at least one inlet valve to at least one low pressure circuit is closed and an inlet valve to the high pressure circuit is opened, so that the boil-off gas at a higher pressure, for example 2-10 bar, is sent to the hydrogen feed stream via the higher pressure circuit and compressed, preferably to a first pressure, in a feed compressor FEED COMP.

[0033] Only when the temperature is below a certain value and the pressure is also below a certain value, e.g. when the pressure is below 2 bar and the temperature is about -200°C, the low pressure boil-off gas is sent to the 20K cold box.

[0034] The advantages of this process are: The operating range of the compressor LP H2 COMP in the hydrogen cooling cycle is significantly reduced, simplifying the compressor design. - The compressor LP H2 COMP is not only easier to operate, given that there is no BOG returning to the liquefier, but can also function effectively when filling several hydrogen transport trucks simultaneously. Thus, the ramp-up and down times between operations with and without BOG recovery are reduced, improving the responsiveness of the cycle compressor LP H2 CMP. Liquefaction unit production capacity is independent of BOG recirculation to the hydrogen truck LP machine, since the LP BOG flow is much smaller and much more stable. · As mentioned above, the design of the feed gas compressor system can accommodate the depressurized BOG by reducing the production of the upstream H2 generation unit. The BOG flow is typically much smaller compared to the nominal capacity of the feed compressor, thus increasing the depressurized BOG flow and reducing the depressurization time. In this way, the overall loading process time can be reduced to the extent that one less loading bay can be considered for the plant design. Thus, ramping up and down of the built-in system to recover the depressurized BOG takes advantage of the shared flexibility of the hydrogen generation system and from the feed compressor. BOG recovery is maximized (up to over 90% recovery) and the size of the plant is largely independent of the recirculated BOG flow rate.

[0035] It will be appreciated that the liquefaction process produces liquid hydrogen that is sent to storage tanks or hydrogen transport trucks, while BOG is received from the storage tanks and / or hydrogen transport trucks being filled and / or from other hydrogen transport trucks, e.g., trucks returning to a facility after delivering the liquid hydrogen with BOG still remaining.

[0036] Purity detection can be used on the BOG from the loading bay, but any BOG below a certain purity level is discarded upstream of the detectors TI, PI, and the choice of destination for the BOG does not depend on its purity.

[0037] Figure 4 is similar to Figure 3 except that it includes two liquefaction trains, each of which has the same elements as the liquefaction unit of Figure 3. Water H2O is fed to an electrolysis unit ELY to produce a feed stream of hydrogen, which is compressed in multiple parallel compressors GH2 COMP upstream of a common purification unit PURIF to remove any impurities that may freeze in the cryobox. If required, gaseous hydrogen can be stored in hydrogen buffers GH2 BUFFERS.

[0038] The BOG systems for both trains are connected. The advantage of this invention when having multiple liquefaction trains is that BOG recovery is nearly independent of the shutdown of one liquefaction train. One loading bay is fed by two liquefaction trains.

[0039] When a loading bay housing at least one truck produces boil-off gas, its pressure and / or temperature are detected, and if the pressure and / or temperature are lower than a predetermined value, at least one inlet valve to at least one low pressure circuit is opened and an inlet valve to the high pressure circuit is closed.

[0040] Thus, low pressure boil-off gas, e.g. below 2 bar, is sent via a low pressure circuit to a low pressure heater and then to the hydrogen cooling circuits of the two liquefaction trains. If the pressure is higher than a predefined value, at least one inlet valve to at least one low pressure circuit is closed and an inlet valve to the high pressure circuit is opened. Thus, higher pressure boil-off gas, e.g. between 2 and 10 bar, is sent via a higher pressure circuit to the hydrogen feed streams that feed both liquefaction trains and is compressed in preferably five parallel compressors GH2 COMP.

[0041] Figure 5 is an alternative solution to Figure 3 above. In Figure 5, the LP and HP BOG streams are combined similar to Figure 1 or Figure 2, whereby one boil-off gas stream is heated in the boil-off heater BOG heater by heat exchange with an external stream. It is important that BOG is preferentially sent to the hydrogen refrigerated cycle, e.g., the inlet of the LP compressor LP H2 COMP of the hydrogen refrigerated cycle, since the average BOG flow of the truck throughout the entire loading sequence is very small and decompression BOG peaks are less likely to occur.

[0042] In the case of peak BOG flows, e.g. during truck depressurization, a BOG compressor is added to recycle the excess BOG stream to the hydrogen liquefaction system by increasing the pressure of the excess BOG2 stream to the feed process suction pressure (typically up to the suction of the feed compressor FEED COMP in the scheme described above). For this purpose a dedicated boil-off compressor BOG COMP is used.

Claims

1. A method for recovering hydrogen boil-off gas, comprising: liquefying hydrogen (1) by cooling a hydrogen feed stream at a first pressure in a first cryogenic box (80K CB); further cooling the hydrogen in a second cryogenic box (20K CB); cooling the second cryogenic box (20K CB) using a hydrogen refrigeration cycle; liquefying the hydrogen in or downstream of the second cryogenic box; providing liquefied hydrogen to a storage or loading bay housing at least one hydrogen transport truck; directing boil-off gas (BOG1) from at least one hydrogen transport truck in the loading bay to which liquefied hydrogen is provided to an indirect heat exchanger (BOG heater) external to the first and second cryogenic boxes to warm the boil-off gas; measuring the pressure of the boil-off gas; and when the boil-off gas stream (BOG(MP)) from at least one hydrogen transport truck is above a predetermined pressure, at least a portion of the boil-off gas stream from at least one hydrogen transport truck is warmed in the first heater or a second heater and mixed with a hydrogen feed stream upstream of the first cryogenic box.

2. 2. The method of claim 1, wherein when the boil-off gas stream (BOG(MP)) from at least one hydrogen transport truck is above a predetermined pressure, at least a portion of the boil-off gas stream from at least one hydrogen transport truck is warmed in a second heater and mixed with the hydrogen feed stream upstream of the first cryogenic box, regardless of the temperature of the boil-off gas stream.

3. 3. The method according to claim 1 or 2, wherein at least a portion of the warmed boil-off gas (BOG1) is compressed in a dedicated compressor before being mixed with the hydrogen feed stream (1).

4. 3. The method according to claim 1 or 2, wherein at least a portion of the warmed boil-off gas (BOG1) is compressed to the first pressure after being mixed with the hydrogen feed stream (1).

5. 3. The method of claim 1 or 2, wherein the hydrogen feed stream (1) is produced by a hydrogen generation unit (H2 generation, ELY) and sent to the first cryogenic box (80K CB) in a first amount when warmed boil-off gas (BOG1) is not mixed with the hydrogen feed stream, and when warmed boil-off gas is mixed with the hydrogen feed stream, the amount of hydrogen feed stream sent from the hydrogen generation unit to the first cryogenic box is reduced to a second amount less than the first amount.

6. The method according to claim 1 or 2, wherein said predetermined pressure is between 1.5 and 3 bar, preferably equal to 2 bar.

7. 3. The method of claim 1 or 2, wherein the boil-off gas stream is analyzed to detect the value of one of its components, and if its hydrogen content is below a predetermined value and / or its impurity content is above a predetermined value, the boil-off gas is not sent to the hydrogen feed stream or to the hydrogen cooling cycle.

8. 3. The method according to claim 1 or 2, wherein when the boil-off gas stream (BOG(LP)) from at least one hydrogen transport truck is below a predetermined pressure and below a predetermined temperature, at least a portion of the boil-off gas stream is not warmed in a first heater but is mixed with a hydrogen cooling cycle stream.

9. The method according to claim 8, wherein said predetermined temperature is between -180°C and -210°C, preferably equal to -200°C.

10. 7. The method of claim 6, wherein when the boil-off gas (BOG) stream from at least one hydrogen transport truck is below the predetermined pressure but above the predetermined temperature, the boil-off stream is heated in the first heater and returned to the hydrogen refrigeration cycle.

11. 7. The method of claim 6, wherein when the boil-off gas stream (BOG1) from at least one hydrogen transport truck is below the predetermined pressure and below the predetermined temperature, the boil-off stream is not heated in the first or second heater and is heated in a heat exchanger of the first and / or second cryogenic box (80K CB, 20K CB) to cool the hydrogen feed stream (1) to be liquefied.

12. The method according to claim 1 or 2, wherein the predetermined pressure has a value selected between 2 and 10 bar, for example 2 bar.

13. A hydrogen liquefaction system and boil-off gas recovery system, comprising: a liquefaction system for liquefying hydrogen (1), the liquefaction system comprising a first cryogenic box (80K CB) for cooling a hydrogen feed stream at a first pressure, a second cryogenic box (20K CB) for further cooling the hydrogen and liquefying the hydrogen in or downstream of the second cryogenic box, a loading bay housing at least one hydrogen transport truck, means for providing liquefied hydrogen to storage or to the loading bay housing at least one hydrogen transport truck, and a hydrogen refrigeration cycle (LP H2) used to provide cooling to the second cryogenic box. a means (PI) for detecting a pressure of a boil-off gas stream (BOG(LP)) from at least one hydrogen transport truck; first and second heaters external to the first and second cryogenic boxes; a means for sending the boil-off gas stream (BOG(LP)) from at least one hydrogen transport truck to the first heater if, and preferably only if, the detected pressure is below a predetermined pressure; a means for sending the boil-off stream warmed in the first heater to be mixed with the hydrogen refrigeration cycle; a means for sending the boil-off gas stream (BOG(LP)) from at least one hydrogen transport truck to the second heater if, and preferably only if, the detected pressure is above a predetermined pressure; and a means for sending the boil-off stream warmed in the first heater to be mixed with a hydrogen feed stream upstream of the first cryogenic box.

14. 14. The liquefaction apparatus according to claim 13, comprising means (TI) for detecting a temperature of a boil-off gas stream (BOG(LP)) from at least one hydrogen transport truck, and means for sending said boil-off gas stream (BOG(LP)) from at least one hydrogen transport truck to said hydrogen refrigeration cycle without heating if said detected temperature is below a predetermined temperature and said detected pressure is also below a predetermined pressure.

15. 15. The liquefaction apparatus of claim 13 or 14, comprising: an additional first cryogenic box (80K CB) for cooling a hydrogen feed stream at a first pressure; an additional second cryogenic box (20K CB) for further cooling the hydrogen and liquefying the hydrogen in or downstream of the second additional cryogenic box; means for providing liquefied hydrogen from the second additional cryogenic box or downstream thereof to the storage or to the loading bay housing at least one hydrogen transport truck; an additional hydrogen refrigeration cycle (LP H2 COMP) used to provide cooling to the second cryogenic box; means for sending the boil-off stream warmed in the first heater to be mixed with the additional hydrogen refrigeration cycle; and means for sending the boil-off stream warmed in the first heater to be mixed with the hydrogen feed stream upstream of the first additional cryogenic box.

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