FACILITY FOR PRODUCING GASEOUS METHANE BY PURIFYING BIOGAS FROM LANDFILL, COMBINING MEMBRANES AND CRYOGENIC DISTILLATION FOR LANDFILL BIOGAS UPGRADING

Abstract

A facility for producing gaseous biomethane by purifying biogas from landfill, comprising: a compression unit, a volatile organic compound (VOC) purification unit; a membrane separation unit, a CO.sub.2 polishing unit, a cryodistillation unit comprising a heat exchanger and a distillation column, an O.sub.2 depletion unit, a dryer arranged.

Claims

1/ A facility for producing gaseous biomethane (78) by purifying biogas from landfill (1), comprising: a compression unit (4) for compressing an initial gas flow of the biogas (1) to be purified, a volatile organic compound (VOC) purification unit (5) arranged downstream of the compression unit (4) to receive the compressed initial flow of the biogas (19) and comprising at least one adsorber (20, 21) loaded with adsorbents capable of reversibly adsorbing VOCs to thereby produce a VOC-depleted gas flow (50); a membrane separation unit (6) arranged downstream of the VOC purification unit (5) to receive the VOC-depleted gas flow and subject the VOC-depleted gas flow (50) to at least one membrane separation (37, 38) to partially separate the CO.sub.2 and O.sub.2 from the gas flow producing a methane rich retentate (41), a CO.sub.2 polishing unit (7) arranged downstream of the membrane separation unit (6) to receive the methane rich retentate (41) from the membrane (37, 38), wherein the CO.sub.2 polishing unit (7) comprises at least one adsorber loaded with adsorbents capable of reversibly adsorbing the majority of remaining CO.sub.2 from the methane rich retentate (41) to produce a CO.sub.2-depleted gas flow (51); a cryodistillation unit (8) comprising a heat exchanger (59) and a distillation column (61), arranged downstream of the CO.sub.2 polishing unit (7) to receive the CO.sub.2-depleted gas flow (51) and subject the CO.sub.2-depleted gas flow (51) to a cryogenic separation to separate O.sub.2 and N.sub.2 from the CO.sub.2-depleted gas flow and to produce a gas distillate (70), wherein a booster (9) is arranged downstream the membrane separation unit (6) and upstream the cryodistillation unit (8) and the cryodistillation unit (8) comprises further a subcooler, the said cryodistillation unit (8) being capable to produce two methane enriched flows respectively a low pressure (LP) and a medium pressure (MP) methane enriched flows, and wherein it further comprises a compressor capable to compress the low pressure (LP) methane enriched flow in order to mix it with the medium pressure methane enriched flow, to produce a medium pressure methane enriched flow.

2/ A facility for producing gaseous biomethane by purifying biogas from landfill according to claim 1, characterized in that the booster is arranged downstream the membrane unit and upstream the CO.sub.2 polishing unit.

3/ A facility for producing gaseous biomethane by purifying biogas from landfill according to claim 1, characterized in that the booster is arranged downstream the CO.sub.2 polishing unit and upstream the distillation unit.

4/ A facility for producing gaseous biomethane by purifying biogas from landfill according to claim 1, characterized in that it further comprises an O.sub.2 depletion unit arranged downstream the cryodistillation unit to receive the medium pressure methane enriched flow capable of converting the O.sub.2 present in medium pressure methane enriched flow into CO.sub.2 and H.sub.2O to produce an O.sub.2 depleted gas flow, and a dryer arranged downstream the O.sub.2 depletion unit capable of removing H.sub.2O from the O.sub.2 depleted gas flow.

5/ A facility for producing gaseous biomethane by purifying biogas from landfill according to claim 4, characterized in that the dryer (77) is a TSA (Temperature Swing Adsorption).

6/ A facility for producing gaseous biomethane by purifying biogas from landfill according to claim 4, characterized in that it further includes a booster arranged downstream the dryer.

7/ A facility for producing gaseous biomethane by purifying biogas from landfill according to claim 2, characterized it further includes an O.sub.2 depletion unit (76) arranged downstream the booster and upstream the CO.sub.2 polishing unit.

8/ A facility for producing gaseous biomethane by purifying biogas from landfill according to claim 7, characterized in that the CO.sub.2 polishing unit (7) comprises at least one adsorber loaded with adsorbents capable of reversibly adsorbing the majority of remaining H.sub.2O contained in the O.sub.2 depleted gas flow.

9/ A facility for producing gaseous biomethane by purifying biogas from landfill according to claim 1, characterized in that the volatile organic compound (VOC) purification unit is a pressure swing adsorber (PSA).

10/ A facility for producing gaseous biomethane by purifying biogas from landfill according to claim 1, characterized in that the CO.sub.2 polishing unit is a Pressure Temperature Swing Adsorption (PTSA).

Description

[0112] The invention and resulting advantages will become clear from the following example supported by the attached figures.

[0113] FIG. 1 is a graph showing vapor pressure curves of methane, nitrogen, and oxygen,

[0114] FIG. 2 is a schematic illustration of a facility in accordance with an embodiment of the disclosure showing a single-column NRU, with methane loop to act as a refrigerant,

[0115] FIG. 3 is a schematic representation of a facility of the invention according to a preferred embodiment.

[0116] The facility comprises a source of biogas to be treated (1), a drying unit (2), a desulfurization unit (3), a compression unit (4), a VOC purification unit (5), a first CO.sub.2 polishing unit (6), a second CO.sub.2polishing unit (7), a cryodistillation unit (8), an oxidation unit (10) and finally a methane gas recovery unit (11). All the apparatus are connected to each other by pipes.

[0117] The drying unit (2) comprises a pressurizer (12), a heat exchanger (13) and a gas liquid separation vessel (14). As already mentioned, this step enables the gas to be pressurized from 20 to a few hundred hectopascals (500 hPa (from 20 to a few hundred millibars (500 mbar) relative maximum). Cooling the gas to between 0.1 and 10 C. enables it to be dried. The gas flow exiting (15) therefore has a pressure of between 20 and 500 hPa (between 20 and 500 mbar) and a dew point of between 0.1 C. and 10 C. at the outlet pressure.

[0118] The desulfurization unit (3) is in the form of a tank (16) loaded with activated carbon or iron hydroxides. This unit enables the H.sub.2S to be captured and transformed into solid sulfur. The flow of gas exiting (17) contains in practice less than 5 mg/Nm.sup.3 of H.sub.2S.

[0119] The compression unit (4) is in the form of a lubricated screw compressor (18). This compressor compresses the gas flow (17) to a pressure of between 0.8 and 2.4 megapascals (between 8 and 24 bars). The flow leaving is shown on FIGS. 1-3 by reference (19)

[0120] The VOC purification unit (5) comprises 2 PSAs (20, 21). They are loaded with adsorbents specifically selected to allow adsorption of the VOCs, and the later desorption during regeneration. The PSAs function in production and regeneration mode alternately.

[0121] In production mode, the PSAs (20, 21) are supplied with gas flow at their lower part. The pipe in which the gas flow (19) circulates splits into two pipes (22, 23), each equipped with a valve (24, 25) and supplying the lower part of the first PSA (20) and the second PSA (21) respectively. The valves (24, 25) will be alternately closed depending on the saturation level of the PSAs. In practice, when the first PSA is saturated with VOCs, valve (24) is closed and valve (25) is opened to start loading the second PSA (20). From the upper part of each of the PSAs leads a pipe (26 and 27) respectively. Each of them splits into 2 pipes (28, 29) and (30, 31) respectively. The VOC-purified flow coming from the first PSA circulates in pipe (28) while the VOC-purified flow coming from the second PSA circulates in pipe (30). The two pipes are joined so as to form a single pipe (50) supplying the CO.sub.2 polishing unit (6).

[0122] In regeneration mode, the regenerating gas circulates in the pipes (29, 31). It emerges at the lower part of the PSA. Thus, a pipe (32) equipped with a valve (34) leads from the first PSA (20). A pipe (33) equipped with a valve (35) leads from the second PSA (21). Pipes (32, 33) are joined upstream of the valves (34, 35) to form a common pipe (36). This pipe is connected to the oxidation unit (10).

[0123] Optionally, the process comprises a further step of purification of the gas flow from VOCs by filtering the VOC-depleted gas flow in at least one filter loaded with activated carbon (non represented). Advantageously, there are 2 filters to be able to implement the process continuously. Indeed, when the first filter is saturated with VOCs, it is substituted by the second filter which has itself been previously regenerated.

[0124] The first CO.sub.2 polishing unit (6) combines two membrane separation stages (37, 38). The membranes are selected to enable the separation of around 90% of the CO.sub.2 and around 50% of the O.sub.2.

[0125] The permeate loaded with CO.sub.2, O.sub.2 and a very small proportion of CH.sub.4 coming from the first membrane separation is used to regenerate the PSAs (20, 21). It circulates in pipe (39) then alternately in pipes (29, 31) depending on the operating mode of the PSAs. The methane rich retentate from the first separation is then directed towards the second membrane separation (38). The permeate from the second membrane separation is recycled by means of a pipe (40) connected to the main circuit upstream of the compressor (18). This step enables a gas circulating in the conduit (41) with less than 3% CO.sub.2 and with a CH.sub.4 yield greater than 90% to be produced.

[0126] The second CO.sub.2 polishing unit (7) combines 2 PTSAs (42, 43). They are loaded with zeolite-type adsorbents. They are each connected to pipes according to a model identical to that described previously for the PSAs. They also function according to a production mode or a regeneration mode.

[0127] In production mode, the gas flow (41) alternately supplies the PTSAs (42, 43) by means of pipes (44, 45) each equipped with a valve (46, 47). The CO.sub.2 purified gas flow from the PTSA (42) then circulates in pipe (48). The CO.sub.2 purified gas flow from the PTSA (43) then circulates in pipe (49). The two pipes (48, 49) are connected to a single pipe (51) connected to the cryodistillation unit.

[0128] In regeneration mode, the regenerating gas circulates in the pipes (52, 53). It emerges in the lower part of the PTSAs. Thus, a pipe (54) equipped with a valve (55) leads from the first PTSA (42). A pipe (56) equipped with a valve (57) leads from the second PTSA (43). Pipes (54, 56) are joined upstream of the valves (55, 57) to form a common pipe (58). This pipe is connected to the oxydation unit (10).

[0129] In that example, the regeneration of the PTSA(s) is be made by the N.sub.2 rich distillate (74) from the cryogenic separation.

[0130] In the illustrated embodiment, the membrane separation unit (6) is separated from the CO.sub.2 polishing unit (7) by a booster (9) which is capable of increasing the pressure from between 110 psi to 230 psi to the feed pressure requested by distillation unit (300 psi to 600 psi).

[0131] In another embodiment which is not illustrated, the booster may be arranged downstream the PTSA as mentioned before.

[0132] The cryodistillation unit (10) is supplied by the pipe (51) in which the gas to be purified circulates. It contains 4 elements: a heat exchanger (59), a reboiler (60), a distillation column (61) and a subcooler (80).

[0133] The heat exchanger (59) is fed by the HP CO.sub.2 purified gas flow (51). The flow has a pressure of between 5 and 25 bar absolute, preferably a pressure of between 8 and 15 bar absolute, a temperature of between 273 and 313 K, typically 288 K, and comprises between 50 and 100% methane, up to 50% of N.sub.2 and up to 4% of O.sub.2.

[0134] The HP CO.sub.2 purified gas flow (51) is cooled and partially liquefied (62) to a temperature of between 100 and 200 K, in the heat exchanger (59) by exchange with a portion (63) of the distillate of the upcoming liquid enriched in CH.sub.4 (71) drawn off a bottom of the distillation column, and with a gas flow enriched in O.sub.2 and N.sub.2 (70) drawn off from the head of the distillation column.

[0135] The cooled HP CO.sub.2 depleted gas flow is then partially condensed. The cooled HP CO.sub.2 depleted gas flow (62) is sent to a reboiler (60) where it is further cooled and partially condensed by heat exchange with a portion (64) by exchange with a portion (63) of the distillate of the upcoming liquid enriched in CH.sub.4 (71) drawn off the bottom of the distillation column which is vaporized. The vaporized liquid enriched in CH.sub.4 (65) is introduced at a lower level of the distillation column to generate a gas enriched in CH.sub.4 for use in distillation.

[0136] The partially condensed cooled HP CO.sub.2 depleted gas flow (66) is then expanded in a valve (67) which produces a high cooling of the expanded fluid (68) to the operating MP pressure of the distillation column (62), between 1 and 5 bar absolute.

[0137] The MP partially condensed cooled CO.sub.2 depleted gas flow (68) contains a liquid fraction and a vapor fraction which are then separated in the head (69) of the column (62) to form a gas flow (70) enriched in O.sub.2 and N.sub.2 and a liquid flow (71) enriched in CH.sub.4. The cooling of the head of the column is ensured by charging a condenser (72) with a portion (81) of the liquid (71) enriched in CH.sub.4 drawn off the bottom of the distillation column and circulating in a the subcooler (80) before being decompressed from MP to LP in the valve (82) to produce a LP liquid (83) enriched in CH.sub.4.

[0138] The liquid fraction (71) is sent to a level of the distillation column above the level at which the vaporized liquid enriched in CH.sub.4 (65) is introduced and the vaporized bottom stream and the liquid fraction enter into contact for ensuring distillation.

[0139] The gas flow enriched in O.sub.2 and N.sub.2 (70) drawn off from the head of the distillation column is decompressed in the valve from MP to LP and transfer its cold energy in the exchanger (59) on contact with CO.sub.2 depleted gas flow (51). The LP gas flow obtained (74) serves for regenerating the PTSA (42, 43). The gas flow exiting from the bottom of the PTSA is loaded with CO.sub.2 and O.sub.2 and is sent to the oxidation unit (10). In the illustrated embodiment, the gas flow (58) is oxidized in a common oxidation unit (10) with the flow (37) resulting from the regeneration of the PSAs, loaded with CO.sub.2, O.sub.2 and VOCs.

[0140] As explained above, a portion (63) of the MP liquid enriched in CH.sub.4 (71) drawn off a bottom of the distillation column is sent to the heat exchanger (59), where it is vaporized by exchange with CO.sub.2purified gas flow (51) and form a first MP vaporized gas flow (75).

[0141] The LP liquid (83) enriched in CH.sub.4 is discharged in the condenser (72) and sent to the subcooler (80) and is vaporized in the heat exchanger (59) to produce a LP gas flow (84) enriched in CH.sub.4.

[0142] The LP gas flow (84) enriched in CH.sub.4 is compressed in a compressor (85) to produce a second MP gas flow (86) enriched in CH.sub.4.

[0143] The MP first and second gas flow (75, 85) enriched in CH.sub.4 are mixed (87) in the same pipe.

[0144] The MP vaporized gas flow (87) comprises between 97 and 100% of methane and less than 3% O.sub.2, preferably less than 1%. It is at a pressure of between 1 and 5 bars absolute, advantageously higher than bars absolute and at room temperature, typically between 273 and 313K, advantageously 288K.

[0145] When the MP single column distillation unit does not meet the oxygen pipeline specification, an additional treatment has to be done on the product.

[0146] The solution of the invention consists in adding an O.sub.2 depletion unit that will remove oxygen from the RNG.

[0147] According to FIG. 1, The MP vaporized gas flow is directed to a deoxo (76) in order to deplete O.sub.2 from said gas flow.

[0148] Practically, the deoxo comprises a bed containing a catalyst, especially a platinum-based catalyst. The bed is heated at a temperature below 500 C. advantageously between 130 and 300 C. by heating means which are included in the deoxo. The deoxo also comprises some air and/or liquid means for cooling the gas and advantageously a moisture separator.

[0149] The deoxo allows to obtain a gas containing less than 100 ppvm of O.sub.2.

[0150] The O.sub.2 depleted gas is then sent to a dryer, especially a TSA (77) comprising at least one adsorber loaded with adsorbents capable of reversibly adsorbing the majority of remaining H.sub.2O for example a zeolite or alumina based catalyst.

[0151] Advantageously, at least two TSAs are used so as to be able to implement the process continuously. Indeed, when the first TSA is saturated with H.sub.2O, it is substituted by the second TSA which has itself been previously regenerated. Preferably, the TSA(s) is/are heat regenerated by using natural external gas.

[0152] According to FIG. 1, the gas flow is finally compressed in a booster (78) at a pressure depending on the specification of the grid (79), typically between 10 and 15 bars for the gas supply network, between 80 and 100 bars for the gas transportation network.

[0153] As explained before, landfill gas upgrading is not easy, due to the presence of multiple impurities to remove in the raw biogas: CO.sub.2, air gases (nitrogen and oxygen), water, VOCs, H.sub.2S, siloxanes.

[0154] The applicant has introduced a technology that simplifies the landfill gas upgrading process into RNG. This patented technology (patent FR3046086, US2019/0001263) combines the benefit of the best process for CO.sub.2 removal on one hand (multiple stages of gas permeation membranes), with the best process for nitrogen and oxygen removal on the other hand (cryogenic distillation).

[0155] The instant invention highlights the potential of combining those two technologies for landfill gas upgrading, by adapting another process of cryogenic distillation. The choice between the cryogenic distillation (LP column versus single MP column) becomes a decision of economics (CAPEX and OPEX). Additionally, and most importantly, the choice should take into account the ease of operation and the up-time of the unit. The more equipment there is in a process, the lower total up-time of the unit; and as a consequence, the lower the annual incomes.