WO2024036169A1

WO2024036169A1 - Nitrogen removal system for methane purification from landfill gas, and method thereof

Info

Publication number: WO2024036169A1
Application number: PCT/US2023/071867
Authority: WO
Inventors: Michael J. Mitariten
Original assignee: Archaea Energy, Inc.
Priority date: 2022-08-09
Filing date: 2023-08-08
Publication date: 2024-02-15

Abstract

The disclosure provides systems and methods that produce a product stream of methane from a mixture of methane and nitrogen. The systems and methods of the disclosure are capable of producing pipeline quality methane containing at least 95% by volume of methane from a landfill gas feed. The systems comprise two sequential nitrogen rejection units (NRUs), each of which contains an adsorbent selective for methane over nitrogen, wherein the adsorbed methane within the second NRU is desorbed and recycled back to the first NRU to enrich the product stream produced from the first NRU.

Description

NITROGEN REMOVAL SYSTEM FOR METHANE PURIFICATION FROM LANDFILL GAS, AND METHOD THEREOF

1. CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of a US application filed on August 9, 2022, entitled "NITROGEN REMOVAL SYSTEM FOR METHANE PURIFICATION FROM LANDFILL GAS, AND METHOD THEREOF", originally filed as a utility application and assigned US application no. 17/884,051 and for which a request to convert into a provisional application was filed on July 19, 2023. The provisional application, having an effective filing date of August 9, 2022, is assigned US provisional application no. . The contents of the application filed on August 9, 2022 are incorporated by reference herein in their entireties.

2. BACKGROUND

[0002] The removal of nitrogen from landfill gas is of considerable importance inasmuch as nitrogen is present in a significant extent. Nitrogen contamination lowers the heating value of the natural gas and increases the transportation cost based on unit heating value.

[0003] Applications that are aimed to remove nitrogen and other impurities from natural gas streams provide significant benefits to the U.S. economy. In 1993, the Gas Research Institute (GRI) estimated that 10-15% (~22 trillion cubic feet) of the natural gas reserves in the U.S. are defined as sub-quality due to contamination with nitrogen, carbon dioxide, sulfur. Most of these reserves, however, have discounted market potential, if they are marketable at all, due to the inability to cost effectively remove the nitrogen. Nitrogen and carbon dioxide are inert gases with no BTU value and must be removed to low levels (4% by volume typically) before the gas can be sold.

[0004] Unlike crude oil, it is not economical to bring imports of natural gas into North America, therefore pricing of natural gas could be expected to rise forcing end users to seek alternative fuels, such as oil and coal, that are not as clean burning as gas. While base consumption for natural gas in the U.S. is projected to grow, one segment may grow much more rapidly. Natural gas usage in electric power generation is expected to grow rapidly because natural gas is efficient and cleaner burning allowing utilities to reduce emissions.

[0005] Concurrently, due to the rising population, warfare conflicts, and/or delay of infrastructure upgrades to alternative heating sources, the worldwide demand for natural gas has increased. Further, capturing methane can reduce emissions of greenhouse gas, which is shorter-lived but far more potent than carbon dioxide.

[0006] There is a need for more efficient systems for capturing methane from biological sources.

3. SUMMARY

[0007] The present disclosure relates to processes and systems for the purification of feed gas containing methane, for example unmarketable sub-quality reserves and renewable feed gas, e.g., landfill gas. The present disclosure provides efficient processes and systems for separating methane from nitrogen to yield a highly purified methane that is suitable for transport in gas pipeline.

[0008] Methods and systems disclosed herein allow high recovery, such as up to 95% or higher recovery, of methane from feed gases.

[0009] Systems disclosed herein typically comprise at least two nitrogen rejection units (“NRUs”), referred to herein as a first NRU and a second NRU. In some embodiments, each NRU is configured for methane adsorption. Upon feeding gases comprising methane and nitrogen, the first NRU can adsorb methane and produce a product methane stream. The first NRU can also purge a nitrogen-enriched purge stream which comprises methane, e.g., methane not adsorbed in the first NRU. The second NRU, which is also configured to adsorb methane, can receive a methane-containing, nitrogen-enriched purge stream, such as produced from the first NRU. Methane adsorbed in the second NRU is then recycled through the first NRU, where it adsorbed for release into a product methane stream.

[0010] The use of a pressure swing adsorption (“PSA”) system in the second NRU advantageously allows for greater recovery of methane to be recycled into, and adsorbed in, the first NRU. Methane adsorption and recovery in the first NRU can also be achieved through a PSA system.

[0011] Examples of such systems include those described in Section 5, in Group A numbered embodiments 13 to 24, and in Group B numbered embodiments 58 to 84 and 98 to 127. [0012] Methods of enriching for methane, e.g., using the systems of the disclosure, can produce product methane streams comprising high percentages, such as up to 95% or higher, of the methane in feed gases. Examples of such methods include those described in Section 5, in Group A numbered embodiments 1 to 12, and in Group B numbered embodiments 1 to 57, 85 to 97, and 128 to 140.

[0013] In some aspects, the present disclosure provides a pressure swing adsorption method to produce a high purity methane product stream from a pretreated landfill feed, comprising:

1) Sending a pretreated landfill feed containing at least 80% by volume of methane to a first nitrogen rejection unit (“NRU”) containing a methane selective material to adsorb methane and to produce a purged stream;

2) Desorbing the methane within the first NRU in a PSA cycle to produce a product stream of methane;

3) Sending the purged stream to a second NRU containing a methane selective material to adsorb methane, and to produce a second purged stream;

4) Desorbing the methane from the second NRU in a PSA cycle to be recycled back to the pretreated landfill feed for the first NRU, to enrich the product stream to contain at least 95% by volume of methane, such as at least 97% by volume of methane.

[0014] In some aspects, the present disclosure provides a pressure swing adsorption system to produce a high purity methane product stream from a pretreated landfill feed comprising:

1) A first nitrogen rejection unit that contains a methane selective material and is in fluid communication with a pretreated landfill gas feed, such that the first NRU adsorbs methane and allows nitrogen to pass through to produce a purged stream, and that the adsorbed methane is then desorbed and recovered as a product stream to be removed in a separate outlet;

[0015] A second nitrogen rejection unit that contains a methane selective material and is in fluid communication with the first NRU to receive the first purged waste stream, such that the second NRU adsorbs methane, and allows nitrogen to pass through, to produce a second purged stream; wherein the second NRU is also in separated fluid communication with the pretreated landfill feed inlet to said first NRU, such that the adsorbed methane within the second NRU is desorbed, recovered and sent to the first NRU together with said pretreated landfill gas.

4. BRIEF DESCRIPTION OF THE FIGURES

[0016] FIG. 1 is a schematic view of an exemplary system and a method to produce a pipeline stream of methane from a landfill gas feed comprising two nitrogen rejection units.

5. DETAILED DESCRIPTION

[0017] The present disclosure relates to processes and systems for the purification of feed gas containing methane. Methods and systems disclosed herein allow high recovery, such as up to 95% or higher recovery, of methane from feed gases. This high recovery can increase the value of otherwise unmarketable gases and/or renewable gases, such as landfill gases. This high recovery can also benefit the environment by capturing methane (a highly potent greenhouse gas, with an average radiative forcing over a 100-year time frame estimated to be 20-fold to 80-fold higher than CO2) for renewable energy or other purposes.

[0018] In some aspects, the present disclosure relates to methods of enriching methane from a feed gas comprising nitrogen and methane, comprising adsorbing methane from a feed gas on a first adsorbent bed comprising a first adsorbent in a first NRU to produce a first nitrogen-enriched purged stream enriched in nitrogen and comprising methane; adsorbing methane from the first nitrogen-enriched purged stream on a second adsorbent bed comprising a second adsorbent in a second NRU to produce a second nitrogen- enriched purged stream; and recycling methane adsorbed in the second NRU to the first NRU. Methane adsorbed by the first NRU can be desorbed to yield a methane product stream, which can provide 95% or higher recovery of methane from the feed gas.

[0019] In some aspects, the present disclosure relates to systems for enriching methane, comprising two NRUs. In some embodiments, the systems can be pressure swing adsorption (PSA) systems comprising a first NRU that contains a first adsorbent bed comprising a first adsorbent and is in fluid communication with an inlet for a feed gas comprising nitrogen and methane, such that the first NRU adsorbs methane, allows nitrogen to pass through to produce a first nitrogen-enriched purged stream, and is configured to desorb adsorbed methane and remove desorbed methane as a product stream; and a second NRU that contains a second adsorbent bed comprising a second adsorbent and is in fluid communication with the first NRU to receive the first nitrogen-enriched purged stream, such that the second NRU adsorbs methane and allows nitrogen to pass through, to produce a second nitrogen-enriched purged stream; and the second NRU is configured to desorb adsorbed methane and recycle desorbed methane as a methane-enriched stream to the first NRU.

[0020] In some embodiments, the systems can comprise a first NRU that rejects nitrogen using adsorption, membrane separation, or cryogenic separation and is in fluid communication with an inlet for a feed gas comprising nitrogen and methane, such that the first NRU enriches nitrogen in a first rejected stream and enriches methane in a methane product stream; and a second NRU that contains an adsorbent bed comprising an adsorbent and is in fluid communication with the first NRU to receive the first rejected stream, such that the second NRU adsorbs methane and allows nitrogen to pass through, to produce a second rejected stream; wherein the second NRU is configured to desorb adsorbed methane and recycle desorbed methane as a methane-enriched stream to the first NRU.

[0021] In some aspects, the present disclosure relates to a pressure swing adsorption method to produce a high purity methane product stream from a pretreated landfill feed, comprising:

1) Sending a pretreated landfill feed containing at least 70% by volume of methane to a first nitrogen rejection unit (“NRU”) containing a methane selective material to adsorb methane and to produce a purged stream containing less than 40% by volume of methane;

3) Sending the purged stream to a second NRU containing a methane selective material to adsorb methane, and to produce a second purged stream of nitrogen that contains less than 20% by volume, such as less than 10% by volume of methane; 4) Desorbing the methane from the second NRU in a PSA cycle to be recycled back to the pretreated landfill feed for the first NRU, wherein the recycled methane stream contains no more than 70% by volume of methane, to enrich the product stream to contain at least 95% by volume of methane, such as at least 97% by volume of methane.

[0022] In some aspects, the present disclosure relates to a pressure swing adsorption system to produce a high purity methane product stream from a pretreated landfill feed comprising:

2) A second nitrogen rejection unit that contains a methane selective material and is in fluid communication with the first NRU to receive the first purged waste stream, such that the second NRU adsorbs methane, and allows nitrogen and other hydrocarbons to pass through, to produce a second purged stream; wherein the second NRU is also in separated fluid communication with the pretreated landfill feed inlet to said first NRU, such that the adsorbed methane within the second NRU is desorbed, recovered and sent to the first NRU together with said pretreated landfill gas.

[0023] Natural gas streams frequently contain components smaller than nitrogen, such as water vapor, carbon dioxide, and hydrogen sulfide. The gas stream to be treated in accordance with embodiments herein can have these contaminants removed prior to treatment of the feed gas stream in accordance with the process and systems of this disclosure. Methods and systems to pre-treat feed gas (e.g., landfill gas) to remove these contaminants are well known in the art. Specifically, in some embodiments, the feed gas (e.g., landfill gas) is first treated and produced at 100 psig, to contain about 70% by volume of methane; less than 2% by volume, such as less than 1 % by volume of water vapor; less than 10% by volume, such as less than 2% by volume of carbon dioxide; and less than below 1 ,000 ppm, such as less than 10 ppm by volume of H₂S.

[0024] In some embodiments, pretreatment of the feed gas can comprise compression (e.g., to 200 psig), removal of volatile organic compounds (VOCs) by adsorbent-based systems, removal of CO₂ by membrane separation, or any two or all three thereof.

[0025] The amount of nitrogen present in the feed gas stream is not critical in carrying out embodiments herein and can be as low as 5 mol percent to as high as about 35 mol percent. Typically, the nitrogen content is in the range of 5 to 20 mol percent.

[0026] The first NRU unit at first cycle achieves at least 85% by volume recovery of the methane from desorbing and recovering the methane within the methane selective adsorbent, and the % by volume will increase to at least a pipeline quality of 95% by volume of methane, after further enrichment of the feed gas mixed with the desorbed methane from the second NRU. The purged stream from the first NRU will produce a rejected nitrogen rich stream with about 15 to 50% by volume CH₄ and 20% to 50% by volume N₂. The concentration of the rejected nitrogen is dependent on the amount of nitrogen in the feed gas and the recovery rate of methane in the first stage NRU. A co-current depressurization step is introduced into the first NRU and the second NRU units to desorb methane.

[0027] The first nitrogen-enriched purged stream is optionally compressed and sent to the second NRU that contains a CH₄ selective adsorbent to treat the first nitrogen-enriched purged stream to adsorb the CH₄, while letting the nitrogen pass through the adsorbent bed to be purged. Unlike the first NRU unit, this second NRU, or High Recovery NRU Module (“HRM”) does not aim to produce pipeline quality gas, rather it enriches methane to an increased purity at less than pipeline quality (enriched as compared to the NRU reject stream). The adsorbed CH₄ in the second NRU is then regenerated in a PSA cycle, wherein the CH₄ is desorbed at about 80% by volume. The desorbed CH₄ is optionally compressed, and recycled back to the feed for the first NRU, to achieve a product stream of CH₄ of at least 95%, such as at least 97% by volume from the first NRU. The N₂ in the purged feed stream from the first NRU passes through the bed of the CH₄ selective adsorbent in the second NRU and becomes a final rejected or purged nitrogen stream, containing less than 10, such as less than 5% by volume of methane. [0028] Separation of nitrogen from methane can be performed using one or more techniques of combinations of techniques. In some embodiments, separation of nitrogen from methane involves fractional distillation at low temperature and (usually) high pressure, i.e. cryogenics. Since nitrogen has a lower boiling point than methane and the other hydrocarbons present in natural gas, it may be removed as a gas on liquefying the remaining constituents, which are then revaporized.

[0029] In some embodiments, separation of nitrogen from methane can comprise one or more of selective diffusion through a series of organic membranes, formation of lithium nitride by treatment with lithium amalgam, absorption of the nitrogen in liquid ammonia or in liquid sulfur dioxide.

[0030] In some embodiments, separation of nitrogen from methane can be achieved using membrane separation technology.

[0031] In some embodiments, separation of nitrogen from methane comprises pressure swing adsorption (“PSA”) systems that use nitrogen specific adsorbents to remove nitrogen.

[0032] In some embodiments, separation of nitrogen from methane comprises selective adsorption of the methane and higher hydrocarbons on an adsorbent such as activated charcoal. Adsorbents such as activated charcoal that more strongly adsorb methane than nitrogen {i.e., are selective for methane over nitrogen) are sometimes termed “equilibrium absorbents.” The adsorbed gases are then desorbed to yield a gas reduced in of nitrogen. Other molecules having smaller molecular dimensions than nitrogen, e.g., carbon dioxide, oxygen, water, small organic molecules and hydrocarbons other than methane, and/or carbon monoxide, will also tend to remain in the nitrogen stream and will tend not to be adsorbed with the methane and higher hydrocarbons.

[0033] Selective adsorption systems require a change in conditions to desorb an adsorbed material. Adsorption/desorption systems include, but are not necessarily limited to, pressure swing adsorption (PSA), thermal swing adsorption, displacement purge, and non-adsorbable purge/partial pressure reduction. Use of equilibrium absorbents in PSA is sometimes referred to herein as “equilibrium PSA.”

[0034] Various properties of first NRUs and second NRUs, such as dimensions, shape, and volume, number of adsorbent beds in each NRU, and number of adsorbent layers in each adsorbent bed, and bed and layer dimensions, shapes, and volumes can each independently be selected based on the feed gases, the desired throughputs, and other parameters.

[0035] As illustrated in FIG. 1 , a nitrogen removal system 100 according to some embodiments herein is operable to remove nitrogen from a methane-containing pretreated landfill feed 101 entering through a feed conduit 102. The feed from 102 enters a first NRU adsorption unit 104 that contains a methane selective bed (/.e., an adsorbent bed comprising an adsorbent that preferentially adsorbs methane over nitrogen, with such adsorbents described in more detail elsewhere herein). Methane is then desorbed, recovered, and sent out of unit 104 as a product stream 108 via a product conduit 106. Product stream 108 contains at least 90% by volume methane, such as over 95% by volume methane and less than 10%, such as less than 5% by volume of nitrogen and carbon dioxide.

[0036] A nitrogen rich, low methane purged stream is produced from unit 104, and this nitrogen-enriched purged stream is optionally compressed via compressor 110 and send to a second nitrogen rejection unit 112 via conduit 114, wherein the second nitrogen rejection module 112 contains a methane selective adsorbent bed that adsorbs methane, purges a purified N₂ waste stream 116 from the second NRU 112 unit via conduit 118. The final purged nitrogen waste stream contains at least 75% by volume, such as at least 90% by volume of nitrogen. A recycled methane stream is desorbed and recovered from the adsorbent bed of the second NRU, optionally compressed in a compressor 120, and recycled to feed conduit 102 via recycled conduit 122. The recycled methane stream contains no more than 70% by volume of methane.

[0037] The methane selective adsorbent bed employed in adsorption units 104 and 112 can comprise from the same or similar methane adsorbing material, such that the adsorbent beds have characteristics in which methane has an equilibrium adsorption amount greater than that of nitrogen. Thus, units 104 and 112 adsorb methane preferentially over nitrogen. Various types of adsorbent materials can be used: activated carbon, zeolite such as crystalline aluminosilicate zeolite (e.g., 13X) or a high aluminum X zeolite having a silicon-to- aluminum ratio of about one (/.e., 1 :1 ± 25% or 1 :1 ± 10%), or an amorphous adsorbent (e.g., silica gel or carbon), or the like having an average pore diameter of 4.5 - 15 A as determined by the MP method and an adsorbed quantity of methane at a temperature of 298 K of 20 Ncc/g or greater at atmospheric pressure can be used. Appiicable adsorbents include molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

[0038] In some embodiments, adsorbent beds in the adsorption units 104 and 112 can comprise multiple layers, such as from two to ten layers, such as of different materials described above. In some embodiments, adsorbent beds comprise two layers. In some embodiments, adsorbent beds comprise three layers. In some embodiments, adsorbent beds comprise 4-10 layers or subranges thereof.

[0039] The layers can be disposed in any orientation, e.g., vertically (stacked together) or horizontally. The layers can be disposed such that multiple layers are disposed between an inlet to the adsorption unit 104 or 112 and an outlet for a purge stream, product stream, or recycle stream. For example, if an inlet is at the bottom of a unit and an outlet is at the top of the unit, the layers can be vertically oriented. The layers can be discrete (/.e., one adsorbent material per layer), or can be gradients from a region of one substantially pure adsorbent material to another. A discrete layer may include some portion of an adsorbent material from an adjoining layer, arising from interactions between the layers during initial loading and/or during operation of the unit. The masses and/or volumes of each of multiple layers in adsorption units 104 and 112 can be the same or can vary.

[0040] In some embodiments, wherein an adsorbent bed comprises two layers, the mass and/or volume ratio of the layers can be in a range from 1 :9 to 9:1 . In a particular embodiment, the mass and/or volume ratio of the two layers is 1 :1 ± 20% or 1 :1 ± 10%. In some embodiments, wherein an adsorbent bed comprises three layers, the mass and/or volume ratio of the layers can be in a range from 1 :1 :18 to 1:18:1 to 18:1 :1. In a particular embodiment, the mass and/or volume ratio of the three layers is 1 :1 :1 ± 20% or 1 :1:1 ± 10%.

[0041] Independently of the number of layers, in embodiments wherein an adsorbent bed comprises two or more layers, the layers can be arranged so that each pair of adjacent layers comprises or consists of different materials, e.g., a first material in a first layer and a second material in a second, adjacent layer. In embodiments wherein a third layer is included and is adjacent to the second layer but not the first, the third layer can comprise or consist of a third material or the first material, and the like holds for a fourth layer, fifth layer, etc., if such are included.

[0042] Independently of the number of layers in any adsorbent bed, in some embodiments, the first NRU and/or the second NRU comprise(s) one, two, three or more (e.g., 4 to 10) adsorbent beds. Thus, in some embodiments, the first NRU and/or the second NRU comprise(s) one adsorbent bed. In some embodiments, the first NRU and/or the second NRU comprise(s) two adsorbent beds. In some embodiments, the first NRU and/or the second NRU comprise(s) three adsorbent beds. In some embodiments, the first NRU and/or the second NRU comprise(s) from 4 to 10 adsorbent beds. Multiple adsorbent beds in a single NRU can be the same or differ in number of adsorbent layers, adsorbent material(s) used, and/or other parameters. Multiple adsorbent beds within a single NRU can be used for a common subprocess, e.g., multiple adsorbent beds in the first NRU can be used for adsorbing methane, passing a nitrogen-enriched purge stream to the second NRU, and desorbing methane to a product stream. Adsorbent beds can perform different aspects of a subprocess at different times. Continuing the example, the first adsorbent bed of the first NRU can adsorb methane and pass a nitrogen-enriched purge stream, while an additional adsorbent bed of the first NRU can desorb methane to a product stream.

[0043] In some embodiments, adsorbents in the adsorption units 104 and 112 can be the same or can be different.

[0044] The adsorption performed in adsorption units 104 and 112 can be performed by any known adsorption process such as, for example, pressure swing adsorption (PSA), thermal swing, displacement purge, or nonadsorbable purge (/.e., partial pressure reduction). However, embodiments herein can be advantageously performed using a pressure swing cycle. Pressure swing cycles are well known in the art. In some embodiments, adsorption in units 104 and 112 can be “equilibrium” PSA that takes advantage of the higher adsorption capacity of methane over nitrogen.

[0045] During methane adsorption in units 104 and 112, the temperature within the units can be maintained in the range of from about 40°F to about 140°F, such as 70°F to 120T. The adsorption pressure in the NRU can be maintained in the range of from about one psia to about 200 psia, such as from five to 80 psia. [0046] In some embodiments, the adsorption pressure in NRUs, such as units 104 and 112, can be from 20 psia to 200 psia.

[0047] Desorption can be at any pressure less than the adsorption pressure, e.g., a pressure from the pressure of the feed gas down to vacuum or near-vacuum, e.g., 1 psia.

[0048] In some embodiments, the first nitrogen-enriched purged stream from the first NRU (e.g., unit 104) can be produced by co-current depressurization.

[0049] In some embodiments, methane can be desorbed in the first NRU (e.g., unit 104) by counter-current depressurization.

[0050] NRUs can adsorb or desorb methane or other materials independently of one another, e.g., a first NRU can adsorb methane at a time when a second NRU is desorbing methane.

[0051] The adsorption capacities of units 104 and 112 can vary depending on the nitrogen content of the feed gas. The adsorption capacities can be optimized by adjusting adsorption cycle times, pressure of feed gas and/or recycled methane streams, or other parameters of the process.

[0052] The units 104 and 112 can be skid-mounted, thus providing easy mobility to and between gas processing locations.

[0053] FIG. 1 shows two units 104 and 112. Systems comprising a series of three or more units are contemplated, wherein all but a final unit provide a nitrogen-enriched stream to subsequent units, a final unit purges a purified N₂ waste stream, and all but a first unit recycle methane to the first unit.

5.1. Numbered Embodiments

[0054] While various specific embodiments have been illustrated and described, it will be appreciated that various changes can be made without departing from the spirit and scope of the disclosure(s). The present disclosure is exemplified by two groups of numbered embodiments set forth below. Unless otherwise specified, features of any of the concepts, aspects and/or embodiments described in the detailed description above and one group of numbered embodiments below are applicable mutatis mutandis to both groups of numbered embodiments. [0055] The first group of numbered embodiments (the “Group A numbered embodiments”) is as follows:

1. A pressure swing adsorption method to produce a high purity methane product stream from a pretreated landfill feed, comprising: a) Sending a pretreated landfill feed containing at least 70% by volume of methane to a first nitrogen rejection unit (“NRU”) containing a methane selective material to adsorb methane and to produce a purged stream enriched in nitrogen; b) Desorbing said methane within said first NRU in a PSA cycle to produce a product stream of methane; c) Sending said purged stream to a second NRU containing a methane selective material to adsorb methane, and to produce a second purged stream; d) Desorbing said methane from the second NRU in a PSA cycle to be recycled back to said pretreated landfill feed for said first NRU, to enrich said product stream to contain at least 95% by volume of methane.

2. The method of embodiment 1 wherein said methane selective adsorbent of steps a) and c) is selected from the group consisting of a high aluminum X having a silicon-to- aluminum ratio of about 1, zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon and a combination of two or more thereof.

3. The method of embodiment 1 , wherein said landfill feed comprises about 80% by volume of methane, less than 2% by volume of water vapor, less than 10% by volume of carbon dioxide, and less than below 1 ,000 ppm by volume of H2S.

4. The method of embodiment 1 , wherein said pretreated landfill feed comprises about 5 to 20 mol percent of nitrogen.

5. The method of embodiment 1 , wherein said purged wasted stream from a) containing less than 40% by volume of methane. 6. The method of embodiment 1 , wherein said second waste stream of nitrogen in c) contains less than 20% by volume, preferably less than 10% by volume of methane.

7. The method of embodiment 1 where a co-current depressurization step is introduced into said first NRU unit to generate said product stream in b).

8. The method of embodiment 1 , wherein a co-current depressurization step is introduced into said second NRU unit to generate said recycled methane stream in d).

9. The method of embodiment 1 wherein said first and second NRU operate under the conditions of equilibrium selectivity.

10. The method of embodiment 1 , wherein said wherein said recycled methane stream in d) contains no more than about 80% by volume of methane.

11. The method of embodiment 1 , wherein said purged waste stream from said first NRU to said second NRU is compressed.

12. The method of embodiment 1 , wherein said recycled stream from said second NRU to said feed for said first NRU is compressed.

13. A pressure swing adsorption system to produce a high purity methane product stream from a pretreated landfill feed comprising: a) A first nitrogen rejection unit (“NRU”) that contains a methane selective material and is in fluid communication with a pretreated landfill gas feed, such that said first NRU adsorbs methane and allows nitrogen to pass through to produce a first purged stream, and that said adsorbed methane is then desorbed and recovered as a product stream to be removed in a separate outlet; b) A second nitrogen rejection unit that contains a methane selective material and is in fluid communication with said first NRU to receive said first purged waste stream, such that said second NRU adsorbs methane, and allows nitrogen and other hydrocarbons to pass through, to produce a second purged stream; wherein said second NRU is also in separated fluid communication with said pretreated landfill feed inlet to said first NRU, such that said adsorbed methane within said second NRU is desorbed, recovered and sent to said first NRU together with said pretreated landfill gas.

14. The system of embodiment 13 wherein said methane selective adsorbent of steps a) and b) is selected from the group consisting of a high aluminum X having a silicon-to- aluminum ratio of about 1, zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon and a combination of two or more thereof.

15. The system of embodiment 13, wherein said landfill feed comprises about 80% by volume of methane, less than 2% by volume of water vapor, less than 10% by volume of carbon dioxide, and less than below 1 ,000 ppm by volume of H2S.

16. The system of embodiment 13, wherein said landfill feed comprises about 5 to 20 mol percent of nitrogen.

17. The system of embodiment 13, wherein a compressor is located between said first NRU and said second NRU in b) to compress said purged stream from said first NRU.

18. The system of embodiment 13, wherein a compressor is located between said second NRU and said inlet that transports said landfill gas feed to said first NRU, to compress said recycled methane stream from said second NRU in b).

19. The system of embodiment 18, wherein said recycled methane stream contains no more than about 80% by volume of methane.

20. The system of embodiment 13, wherein said landfill gas feed comprises about 80% by volume of methane, less than 2% by volume of water vapor, less than 10% by volume of carbon dioxide, and less than below 1 ,000 ppm by volume of H2S.

21. The system of embodiment 13, wherein said landfill feed comprises about 5 to 20 mol percent of nitrogen.

22. The system of embodiment 13, wherein said purged wasted stream from a) containing less than 40% by volume of methane.

23. The system of embodiment 13, wherein said second waste stream of nitrogen in b) contains less than 20% by volume, preferably less than 10% by volume of methane. 24. A pressure swing adsorption system to produce a high purity methane product stream from a pretreated landfill feed comprising: a) A first nitrogen rejection unit (“NRU”) that rejects nitrogen using adsorption, membrane or cryogenic technology and is in fluid communication with a pretreated landfill gas feed, such that said first nitrogen rejection unit enriches nitrogen in a rejected stream and enriches methane in a methane product stream; b) A second nitrogen rejection unit that contains a methane selective material and is in fluid communication with said first NRU to receive said first waste stream, such that said second NRU adsorbs methane, and allows nitrogen and other hydrocarbons to pass through, to produce a second waste stream; wherein said second NRU is also in separated fluid communication with said pretreated landfill feed inlet to said first NRU, such that said adsorbed methane within said second NRU is desorbed, recovered and sent to said first NRU together with said pretreated landfill gas.

[0056] The second set of numbered embodiments (the “Group B numbered embodiments”) is as follows:

1. A method of enriching methane from a feed gas comprising nitrogen and methane, comprising:

(a) adsorbing methane from a feed gas on a first adsorbent bed comprising a first adsorbent in a first nitrogen rejection unit (“NRU”) to produce a first nitrogen-enriched purged stream enriched in nitrogen and comprising methane;

(b) adsorbing methane from the first nitrogen-enriched purged stream on a second adsorbent bed comprising a second adsorbent in a second NRU to produce a second nitrogen-enriched purged stream; and

(c) recycling methane adsorbed in the second NRU to the first NRU.

2. The method of embodiment 1, wherein recycling methane comprises:

(a) desorbing methane in the second NRU to produce a methane- enriched stream; and

(b) introducing the methane-enriched stream into the first NRU. 3. The method of embodiment 2, wherein the methane-enriched stream is combined with feed gas prior to its introduction into the first NRU.

4. The method of embodiment 2, wherein the methane-enriched stream is combined with feed gas in the first NRU.

5. The method of any one of embodiments 1 to 4, wherein the first nitrogen- enriched purged stream is produced by co-current depressurization.

6. The method of any one of embodiments 2 to 4, wherein adsorbing and desorbing the methane in the second NRU are performed by pressure swing adsorption (PSA).

7. The method of any one of embodiments 1 to 6, which comprises desorbing methane adsorbed in the first NRU to produce a product stream.

8. The method of embodiment 7, wherein adsorbing and desorbing in the first NRU are performed by a process comprising pressure swing adsorption (PSA).

9. The method of embodiment 7 or embodiment 8, wherein desorbing methane in the first NRU comprises counter-current depressurization.

10. The method of embodiment 9, wherein desorbing in the first NRU is performed at a pressure from below that of the feed gas down to 1 psia.

11 . The method of embodiment 8, wherein desorbing in the first NRU is performed in a vacuum.

12. The method of embodiment 6, wherein adsorbing and desorbing in the first NRU are performed by a process comprising thermal swing adsorption.

13. The method of embodiment 6, wherein adsorbing and desorbing in the first NRU are performed by a process comprising non-adsorbable purge or partial pressure reduction.

14. The method of any one of embodiments 6 to 13, wherein the product stream contains at least 95% by volume of methane.

15. The method of any one of embodiments 6 to 14, wherein the product stream contains at least 96% by volume of methane.

16. The method of any one of embodiments 6 to 15, wherein the product stream contains at least 97% by volume of methane. 17. The method of any one of embodiments 6 to 16, wherein the product stream methane content is improved by at least 2% by volume as compared to a process lacking step (c).

18. The method of any one of embodiments 6 to 17, wherein the product stream methane content is improved by at least 3% by volume as compared to a process lacking step (c).

19. The method of any one of embodiments 6 to 18, wherein the product stream methane content is improved by at least 4% by volume as compared to a process lacking step (c).

20. The method of any one of embodiments 6 to 19, wherein the product stream methane content is improved by at least 5% by volume as compared to a process lacking step (c).

21 . The method of any one of embodiments 1 to 20, wherein the first adsorbent and the second adsorbent are selective for methane over nitrogen.

22. The method of any one of embodiments 1 to 21 , wherein the first adsorbent and the second adsorbent are each independently selected from a high aluminum X zeolite, zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

23. The method of embodiment 22, wherein the high aluminum X zeolite has a silicon to aluminum ratio of 1 :1 ± 25% or 1 :1 ± 10%.

24. The method of any one of embodiments 1 to 23, wherein the first adsorbent and the second adsorbent are each independently selected from molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

25. The method of any one of embodiments 1 to 24, wherein the first adsorbent and/or the second adsorbent comprise or consist of molecular sieved activated carbon (“MSC”).

26. The method of any one of embodiments 1 to 24, wherein the first adsorbent and/or the second adsorbent comprise or consist of crushed activated carbon (“AC-1”).

27. The method of any one of embodiments 1 to 24, wherein the first adsorbent and/or the second adsorbent comprise or consist of granular activated carbon (“AC-2”). 28. The method of any one of embodiments 1 to 24, wherein the first adsorbent and/or the second adsorbent comprise or consist of pelleted activated carbon.

29. The method of any of embodiments 1 to 28, wherein the first adsorbent bed and/or the second adsorbent bed independently comprise more than one layer of materials.

30. The method of embodiment 29, wherein the first adsorbent bed and/or the second adsorbent bed independently comprise two layers of materials.

31 . The method of embodiment 30, wherein the two layers have a mass and/or volume ratio from 1:9 to 9:1.

32. The method of embodiment 30 or embodiment 31, wherein the two layers have a mass and/or volume ratio of 1 :1 ± 20% or 1 :1 ± 10%.

33. The method of embodiment 29, wherein the first adsorbent bed and/or the second adsorbent bed independently comprises three layers of materials.

34. The method of embodiment 33, wherein the three layers have a mass and/or volume ratio from 1:1:18 to 1 :18:1 to 18:1 :1.

35. The method of embodiment 33 or embodiment 34, wherein the two layers have a mass and/or volume ratio of 1 :1 :1 ± 20% or 1 :1 :1 ± 10%.

36. The method of embodiment 29, wherein the first adsorbent bed and/or the second adsorbent bed independently comprises from 4-10 layers of materials or subranges thereof.

37. The method of any one of embodiments 29 to 36, wherein the layers are arranged so that each pair of adjacent layers comprises or consists of different materials.

38. The method of any one of embodiments 29 to 37, wherein the first adsorbent bed and/or the second adsorbent bed independently comprises two or more layers each comprising or consisting of high aluminum X zeolite, zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), or pelleted activated carbon.

39. The method of any of embodiments 1 to 38, wherein the first adsorbent and the second adsorbent are the same.

40. The method of any one of embodiments 1 to 39, wherein the feed gas is introduced to the first NRU at a pressure from 20 psia to 200 psia.

41 . The method of any one of embodiments 1 to 40, wherein the feed gas is introduced to the first NRU at a pressure from 50 psia to 150 psia. 42. The method of any one of embodiments 1 to 41 , wherein the feed gas is introduced to the first NRU at a pressure from 75 psia to 125 psia.

43. The method of any one of embodiments 1 to 42, wherein the feed gas comprises at least 70% methane by volume.

44. The method of any one of embodiments 1 to 43, wherein the feed gas comprises at least 75% methane by volume.

45. The method of any one of embodiments 1 to 44, wherein the feed gas comprises at least 80% methane by volume.

46. The method of any one of embodiments 1 to 45, wherein the feed gas comprises less than 2% by volume of water vapor, less than 10% by volume of carbon dioxide, and less than 1 ,000 ppm by volume of H2S.

47. The method of any one of embodiments 1 to 46, wherein the feed gas comprises 5 to 20 mol percent of nitrogen.

48. The method of any one of embodiments 1 to 47, wherein the feed gas is a pretreated landfill gas.

49. The method of any one of embodiments 1 to 48, wherein the first nitrogen- enriched purged stream comprises less than 40% by volume of methane.

50. The method of any one of embodiments 1 to 49, wherein the second nitrogen-enriched purged stream contains less than 20% by volume of methane.

51 . The method of any one of embodiments 1 to 50, wherein the second nitrogen-enriched purged stream contains less than 10% by volume of methane.

52. The method of any one of embodiments 2 to 51 , wherein the methane- enriched stream contains no more than 80 ± 10% or 80 ± 5% by volume of methane.

53. The method of any one of embodiments 1 to 52, wherein the first nitrogen- enriched purged stream is compressed before being introduced to the second NRU.

54. The method of any one of embodiments 1 to 53, wherein the methane- enriched stream is compressed between the second NRU and the first NRU.

55. The method of any one of embodiments 1 to 54, wherein the first NRU and/or the second NRU comprise(s) two adsorbent beds.

56. The method of any one of embodiments 1 to 55, wherein the first NRU and/or the second NRU comprise(s) three adsorbent beds. 57. The method of any one of embodiments 1 to 56, wherein the first NRU and/or the second NRU comprise(s) from four to ten adsorbent beds.

58. A pressure swing adsorption (PSA) system for enriching methane, e.g., suitable for performing the method of any one of embodiments 1 to 57; comprising:

(a) a first nitrogen rejection unit (“NRU”) that contains a first adsorbent bed comprising a first adsorbent and is in fluid communication with an inlet for a feed gas comprising nitrogen and methane, such that the first NRU adsorbs methane, allows nitrogen to pass through to produce a first nitrogen-enriched purged stream, and is configured to desorb adsorbed methane and remove desorbed methane as a product stream; and

(b) a second NRU that contains a second adsorbent bed comprising a second adsorbent and is in fluid communication with the first NRU to receive the first nitrogen-enriched purged stream, such that the second NRU adsorbs methane and allows nitrogen to pass through, to produce a second nitrogen-enriched purged stream; and the second NRU is configured to desorb adsorbed methane and recycle desorbed methane as a methane-enriched stream to the first NRU.

59. The system of embodiment 58, wherein the first adsorbent and the second adsorbent are selective for methane over nitrogen.

60. The system of embodiment 58 or embodiment 59, wherein the first adsorbent and the second adsorbent each independently selected from high aluminum X zeolite, zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

61 . The system of embodiment 60, wherein the high aluminum X zeolite has a silicon to aluminum ratio of 1 :1 ± 25% or 1 :1 ± 10%.

62. The system of any one of embodiments 58 to 61 , wherein the first adsorbent and the second adsorbent are each independently selected from molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

63. The system of any one of embodiments 58 to 62, wherein the first adsorbent and/or the second adsorbent comprise or consist of molecular sieved activated carbon (“MSC”). 64. The system of any one of embodiments 58 to 63, wherein the first adsorbent and/or the second adsorbent comprise or consist of crushed activated carbon (“AC-1”).

65. The system of any one of embodiments 58 to 63, wherein the first adsorbent and/or the second adsorbent comprise or consist of granular activated carbon (“AC-2”).

66. The system of any one of embodiments 58 to 63, wherein the first adsorbent and/or the second adsorbent comprise or consist of pelleted activated carbon.

67. The system of any of embodiments 58 to 66, wherein the first adsorbent bed and/or the second adsorbent bed independently comprise more than one layer of materials.

68. The system of embodiment 67, wherein the first adsorbent bed and/or the second adsorbent bed independently comprise two layers of materials.

69. The system of embodiment 68, wherein the two layers have a mass and/or volume ratio from 1 :9 to 9:1.

70. The system of embodiment 68 or embodiment 69, wherein the two layers have a mass and/or volume ratio of 1 :1 ± 20% or 1 :1 ± 10%.

71 . The system of embodiment 67, wherein the first adsorbent bed and/or the second adsorbent bed independently comprises three layers of materials.

72. The system of embodiment 71 , wherein the three layers have a mass and/or volume ratio from 1 :1:18 to 1 :18:1 to 18:1 :1.

73. The system of embodiment 71 or embodiment 72, wherein the two layers have a mass and/or volume ratio of 1 :1 :1 ± 20% or 1 :1 :1 ± 10%.

74. The system of embodiment 67, wherein the first adsorbent bed and/or the second adsorbent bed independently comprises from 4-10 layers of materials or subranges thereof.

75. The system of any one of embodiments 67 to 74, wherein the layers are arranged so that each pair of adjacent layers comprises or consists of different materials.

76. The system of any one of embodiments 67 to 75, wherein the first adsorbent bed and/or the second adsorbent bed independently comprises two or more layers each comprising or consisting of high aluminum X zeolite, zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), or pelleted activated carbon.

77. The system of any one of embodiments 58 to 76, wherein the first adsorbent and the second adsorbent are the same. 78. The system of any one of embodiments 58 to 77, further comprising a compressor in fluid communication with the first NRU and the second NRU and configured to compress the first nitrogen-enriched purged stream before being introduced to the second NRU.

79. The system of any one of embodiments 58 to 78, further comprising a compressor in fluid communication with the second NRU and the first NRU and configured to compress the methane-enriched stream between the second NRU and the first NRU.

80. The system of any one of embodiments 58 to 79, wherein the second NRU further allows hydrocarbons other than methane to pass through to the second nitrogen- enriched purged stream.

81 . The system of any one of embodiments 58 to 80, wherein the methane- enriched stream is introduced to the first NRU through the inlet for the feed gas.

82. The system of any one of embodiments 58 to 81 , wherein the first NRU and/or the second NRU comprise(s) two adsorbent beds.

83. The system of any one of embodiments 58 to 82, wherein the first NRU and/or the second NRU comprise(s) three adsorbent beds.

84. The system of any one of embodiments 58 to 83, wherein the first NRU and/or the second NRU comprise(s) from four to ten adsorbent beds.

85. A method for enriching methane, comprising using the system of any one of embodiments 58 to 84 to enrich methane in a feed gas comprising nitrogen and methane.

86. The method of embodiment 85, which is as defined in any one of embodiments 1 to 57.

87. The method of embodiment 85 or embodiment 86, wherein the feed gas comprises at least 70% methane by volume.

88. The method of any one of embodiments 85 to 87, wherein the feed gas comprises less than 2% by volume of water vapor, less than 10% by volume of carbon dioxide, and less than 1 ,000 ppm by volume of H₂S.

89. The method of any one of embodiments 85 to 88, wherein the feed gas comprises 5 to 20 mol percent of nitrogen.

90. The method of any one of embodiments 85 to 89, wherein the feed gas is a pretreated landfill gas. 91 . The method of any one of embodiments 85 to 90, wherein the feed gas is introduced to the first NRU at a pressure from 20 psia to 200 psia.

92. The method of any one of embodiments 85 to 91 , wherein the first nitrogen- enriched purged stream comprises less than 40% by volume of methane.

93. The method of any one of embodiments 85 to 92, wherein the second nitrogen-enriched purged stream contains less than 20% by volume of methane.

94. The method of any one of embodiments 85 to 93, wherein the second nitrogen-enriched purged stream contains less than 10% by volume of methane.

95. The method of any one of embodiments 85 to 94, wherein the recycled methane stream contains no more than 80 ± 10% or 80 ± 5% by volume of methane.

96. The method of any one of embodiments 85 to 95, wherein the product stream contains at least 95% by volume of methane.

97. The method of any one of embodiments 85 to 96, wherein adsorbed methane is desorbed from the first NRU at a pressure from below that of the feed gas down to 1 psia.

98. A system for enriching methane, e.g., suitable for performing the method of any one of embodiments 1 to 57, comprising:

(a) a first nitrogen rejection unit (“NRU”) that rejects nitrogen using adsorption, membrane separation, or cryogenic separation and is in fluid communication with an inlet for a feed gas comprising nitrogen and methane, such that the first NRU enriches nitrogen in a first rejected stream and enriches methane in a methane product stream; and

(b) a second NRU that contains an adsorbent bed comprising an adsorbent and is in fluid communication with the first NRU to receive the first rejected stream, such that the second NRU adsorbs methane and allows nitrogen to pass through, to produce a second rejected stream; wherein the second NRU is configured to desorb adsorbed methane and recycle desorbed methane as a methane-enriched stream to the first NRU.

99. The system of embodiment 98, wherein the first NRU rejects nitrogen using cryogenic separation.

100. The system of embodiment 98, wherein the first NRU rejects nitrogen using membrane separation. 101. The system of embodiment 98, wherein the first NRU rejects nitrogen using an adsorbent bed comprising an adsorbent, optionally an adsorbent selective for methane over nitrogen.

102. The system of any one of embodiments 98 to 101 , wherein each adsorbent is selective for methane over nitrogen.

103. The system of any one of embodiments 98 to 102, wherein each adsorbent is independently selected from high aluminum X zeolite, zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

104. The system of embodiment 103, wherein the high aluminum X zeolite has a silicon to aluminum ratio of 1 :1 ± 25% or 1 :1 ± 10%.

105. The system of any one of embodiments 98 to 104, wherein each adsorbent is independently selected from molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

106. The system of any one of embodiments 98 to 105, wherein each adsorbent individually comprises or consists of molecular sieved activated carbon (“MSC”).

107. The system of any one of embodiments 98 to 105, wherein each adsorbent individually comprises or consists of crushed activated carbon (“AC-1”).

108. The system of any one of embodiments 98 to 105, wherein each adsorbent individually comprises or consists of granular activated carbon (“AC-2”).

109. The system of any one of embodiments 98 to 105, wherein each adsorbent individually comprises or consists of pelleted activated carbon.

110. The system of any of embodiments 98 to 109, wherein each adsorbent bed independently comprises more than one layer of materials.

111. The system of embodiment 110, wherein each adsorbent bed independently comprises two layers of materials.

112. The system of embodiment 111, wherein the two layers have a mass and/or volume ratio from 1:9 to 9:1.

113. The system of embodiment 111 or embodiment 112, wherein the two layers have a mass and/or volume ratio of 1 :1 ± 20% or 1 :1 ± 10%. 114. The system of embodiment 110, wherein each adsorbent bed independently comprises three layers of materials.

115. The system of embodiment 114, wherein the three layers have a mass and/or volume ratio from 1 : 1 : 18 to 1 : 18: 1 to 18:1 :1.

116. The system of embodiment 114 or embodiment 115, wherein the two layers have a mass and/or volume ratio of 1 :1 :1 ± 20% or 1 :1 :1 ± 10%.

117. The system of embodiment 110, wherein each adsorbent bed independently comprises from 4-10 layers of materials or subranges thereof.

118. The system of any one of embodiments 110 to 117, wherein the layers are arranged so that each pair of adjacent layers comprises or consists of different materials.

119. The system of any one of embodiments 110 to 118, wherein each adsorbent bed independently comprises two or more layers each comprising or consisting of high aluminum X zeolite, zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), or pelleted activated carbon.

120. The system of any one of embodiments 98 to 119, wherein the adsorbent of the second NRU and any adsorbent in the first NRU are the same.

121. The system of any one of embodiments 98 to 120, further comprising a compressor in fluid communication with the first NRU and the second NRU and configured to compress the first nitrogen-enriched purged stream before being introduced to the second NRU.

122. The system of any one of embodiments 98 to 121, further comprising a compressor in fluid communication with the second NRU and the first NRU and configured to compress the methane-enriched stream between the second NRU and the first NRU.

123. The system of any one of embodiments 98 to 122, wherein the second NRU further allows hydrocarbons other than methane to pass through to the second rejected stream.

124. The system of any one of embodiments 98 to 123, wherein the methane- enriched stream is introduced to the first NRU through the inlet for the feed gas.

125. The system of any one of embodiments 98 to 124, wherein the first NRU and/or the second NRU comprise(s) two adsorbent beds. 126. The system of any one of embodiments 98 to 125, wherein the first NRU and/or the second NRU comprise(s) three adsorbent beds.

127. The system of any one of embodiments 98 to 126, wherein the first NRU and/or the second NRU comprise(s) from four to ten adsorbent beds.

128. A method for enriching methane, comprising using the system of any one of embodiments 98 to 127 to enrich methane in a feed gas comprising nitrogen and methane.

129. The method of embodiment 128, which is as defined in any one of embodiments 1 to 57.

130. The method of embodiment 128 or embodiment 129, wherein the feed gas comprises at least 70% methane by volume.

131. The method of any one of embodiments 128 to 130, wherein the feed gas comprises less than 2% by volume of water vapor, less than 10% by volume of carbon dioxide, and less than 1 ,000 ppm by volume of H₂S.

132. The method of any one of embodiments 128 to 131 , wherein the feed gas comprises 5 to 20 mol percent of nitrogen.

133. The method of any one of embodiments 128 to 132, wherein the feed gas is a pretreated landfill gas.

134. The method of any one of embodiments 128 to 133, wherein the feed gas is introduced to the first NRU at a pressure from 20 psia to 200 psia.

135. The method of any one of embodiments 128 to 134, wherein the first nitrogen-enriched purged stream comprises less than 40% by volume of methane.

136. The method of any one of embodiments 128 to 135, wherein the second nitrogen-enriched purged stream contains less than 20% by volume of methane.

137. The method of any one of embodiments 128 to 136, wherein the second nitrogen-enriched purged stream contains less than 10% by volume of methane.

138. The method of any one of embodiments 128 to 137, wherein the methane- enriched stream contains no more than 80 ± 10% or 80 ± 5% by volume of methane.

139. The method of any one of embodiments 128 to 138, wherein the product stream contains at least 95% by volume of methane.

140. The method of any one of embodiments 128 to 139, wherein adsorbed methane is desorbed from the first NRU at a pressure from below that of the feed gas down to 1 psia. 6. EXAMPLES

[0057] Table 1 compares the Material Balances of a process using a system comprising only one NRU to the Material Balances of a process using a system comprising two NRUs, with the second NRU being a High Recovery Module (HRM), such as the system set forth in FIG. 1.

Table 1

[0058] In the table, both of the methane selective NRUs were running under equilibrium adsorption conditions. The activated carbon adsorbent used was equilibrium selective for methane and thus can be used under equilibrium conditions with similar recoveries as shown in Table 1. However, size selected adsorbent can also be used for the first NRU.

[0059] As can be seen from the above Table 1 , in FIG. 1 a pretreated NRU or landfill feed 101 that contained a mixture of about 80 % by volume methane and about 20% by volume nitrogen was introduced into an equilibrium PSA column together with a recycle stream from inlet 122, to form a feed mixture that contained about 80% by volume of methane. It can be seen from Table 1 in connection with FIG. 1 that the single pass recovery without HRM (high recovery NRU module) is 90% by volume. Single pass recovery is defined as methane mol fraction in the gas phase, multiplied by the mols of product per hour divided by the feed methane mol fraction gas phase times the mols of product per hour.

[0060] Thus, in connection with FIG. 1 , the feed gas to the first NRU is 1000 SCFM times 78.64% CH4 = 786.4 SCFM of contained CH4) and the product flow is 745 SCFM times 95% methane = 707.8 SCFM of contained CH4. Thus the CH4 recovery rate of the first NRU is the CH4 product flow divided by the feed CH4 flow or 707.8 SCFM divided by 786.4 SCFM or 90%. [0061] For the same feed flow rate and conditions (786.4 SCFM of contained CH4 in the feed) the use of the High Recovery Module increases the CH4 product flow to 801 SCFM times 95% = 761 SCFM. The added CH4 recovery by the use of the High Recovery Module is thus 761 SCFM divided by 786.4 SCFM or 96.8% CH4 recovery.

[0062] Theoretical considerations and the results of other studies (not shown) are generally in agreement with the results described above, that recapture of roughly two-thirds of methane that would otherwise be lost is typical for systems and methods of the present disclosure.

Claims

WHAT IS CLAIMED IS:

(c) recycling methane adsorbed in the second NRU to the first NRU.

2. The method of claim 1, wherein recycling methane comprises:

(b) introducing the methane-enriched stream into the first NRU.

3. The method of claim 2, wherein the methane-enriched stream is combined with feed gas prior to its introduction into the first NRU or in the first NRU.

4. The method of any one of claims 1 to 3, wherein the first nitrogen-enriched purged stream is produced by co-current depressurization.

5. The method of any one of claims 2 to 4, wherein adsorbing and desorbing the methane in the second NRU are performed by pressure swing adsorption (PSA).

6. The method of any one of claims 1 to 5, which comprises desorbing methane adsorbed in the first NRU to produce a product stream.

7. The method of claim 6, wherein adsorbing and desorbing in the first NRU are performed by a process comprising pressure swing adsorption (PSA).

8. The method of claim 6 or claim 7, wherein desorbing methane in the first NRU comprises counter-current depressurization.

9. The method of claim 8, wherein desorbing in the first NRU is performed at a pressure from below that of the feed gas down to 1 psia, or in a vacuum.

10. The method of claim 6, wherein adsorbing and desorbing in the first NRU are performed by a process comprising thermal swing adsorption or by a process comprising non-adsorbable purge or partial pressure reduction.

11 . The method of any one of claims 6 to 10, wherein the product stream contains at least 95% by volume of methane (e.g., at least 96% by volume of methane or at least 97% by volume of methane).

12. The method of any one of claims 6 to 11 , wherein the product stream methane content is improved by at least 2% by volume (e.g., by at least 3% by volume, at least 4% by volume, or at least 5% by volume) as compared to a process lacking step (c).

13. The method of any one of claims 1 to 12, wherein the first adsorbent and the second adsorbent are selective for methane over nitrogen.

14. The method of any one of claims 1 to 13, wherein the first adsorbent and the second adsorbent are each independently selected from a high aluminum X zeolite (e g., high aluminum X zeolite with a silicon to aluminum ratio of 1 :1 ± 25% or 1 :1 ± 10%), zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

15. The method of any one of claims 1 to 14, wherein the first adsorbent and the second adsorbent are each independently selected from molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

16. The method of any of claims 1 to 15, wherein the first adsorbent bed and/or the second adsorbent bed each independently comprises more than one layer of materials (e.g., two layers of materials, three layers of materials, or from 4-10 layers of materials or subranges thereof).

17. The method of any one of claims 1 to 16, wherein the layers are arranged so that each pair of adjacent layers comprises or consists of different materials.

18. The method of any one of claims 1 to 17, wherein the first adsorbent bed and/or the second adsorbent bed independently comprises two or more layers each comprising or consisting of high aluminum X zeolite, zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), or pelleted activated carbon.

19. The method of any of claims 1 to 18, wherein the first adsorbent and the second adsorbent are the same.

20. The method of any one of claims 1 to 19, wherein the feed gas is introduced to the first NRU at a pressure from 20 psia to 200 psia, such as from 50 psia to 150 psia or from 75 psia to 125 psia.

21 . The method of any one of claims 1 to 20, wherein the feed gas comprises at least 70% methane by volume, e.g., at least 75% methane by volume or at least 80% methane by volume.

22. The method of any one of claims 1 to 21, wherein the feed gas comprises less than 2% by volume of water vapor, less than 10% by volume of carbon dioxide, and less than 1 ,000 ppm by volume of H2S.

23. The method of any one of claims 1 to 22, wherein the feed gas comprises 5 to 20 mol percent of nitrogen.

24. The method of any one of claims 1 to 23, wherein the feed gas is a pretreated landfill gas.

25. The method of any one of claims 1 to 24, wherein the first nitrogen-enriched purged stream comprises less than 40% by volume of methane, optionally less than 20% by volume of methane or less than 10% by volume of methane.

26. The method of any one of claims 2 to 25, wherein the methane-enriched stream contains no more than 80 ± 10% or 80 ± 5% by volume of methane.

27. The method of any one of claims 1 to 26, wherein the first nitrogen-enriched purged stream is compressed before being introduced to the second NRU.

28. The method of any one of claims 1 to 27, wherein the methane-enriched stream is compressed between the second NRU and the first NRU.

29. A pressure swing adsorption (PSA) system for enriching methane, e.g., suitable for performing the method of any one of claims 1 to 28; comprising:

30. The system of claim 29, wherein the first adsorbent and the second adsorbent are selective for methane over nitrogen.

31 . The system of claim 29 or claim 30, wherein the first adsorbent and the second adsorbent are each independently selected from high aluminum X zeolite (optionally high aluminum X zeolite with a silicon to aluminum ratio of 1 :1 ± 25% or 1:1 ± 10%), zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

32. The system of any one of claims 29 to 31 , wherein the first adsorbent and the second adsorbent are each independently selected from molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

33. The system of any of claims 29 to 32, wherein the first adsorbent bed and/or the second adsorbent bed independently comprises more than one layer of materials, such as two layers of materials, three layers of materials, or from 4-10 layers of materials or subranges thereof.

34. The system of any one of claims 29 to 33, wherein the layers are arranged so that each pair of adjacent layers comprises or consists of different materials.

35. The system of any one of claims 29 to 34, wherein the first adsorbent and the second adsorbent are the same.

36. The system of any one of claims 29 to 35, further comprising a compressor in fluid communication with the first NRU and the second NRU and configured to compress the first nitrogen-enriched purged stream before being introduced to the second NRU.

37. The system of any one of claims 29 to 36, further comprising a compressor in fluid communication with the second NRU and the first NRU and configured to compress the methane-enriched stream between the second NRU and the first NRU.

38. The system of any one of claims 29 to 37, wherein the methane-enriched stream is introduced to the first NRU through the inlet for the feed gas.

39. A method for enriching methane, comprising using the system of any one of claims 29 to 38 to enrich methane in a feed gas comprising nitrogen and methane.

40. The method of claim 39, which is as defined in any one of claims 1 to 28.

41 . A system for enriching methane, e.g., suitable for performing the method of any one of claims 1 to 28, comprising:

42. The system of claim 41 , wherein the first NRU rejects nitrogen using cryogenic separation or membrane separation.

43. The system of claim 41 , wherein the first NRU rejects nitrogen using an adsorbent bed comprising an adsorbent, optionally an adsorbent selective for methane over nitrogen.

44. The system of any one of claims 41 to 43, wherein each adsorbent is selective for methane over nitrogen.

45. The system of any one of claims 41 to 44, wherein each adsorbent is independently selected from high aluminum X zeolite (e.g., high aluminum X zeolite with a silicon to aluminum ratio of 1 :1 ± 25% or 1 :1 ± 10%), zeolite 13X, carbon or silica gel, molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

46. The system of any one of claims 41 to 45, wherein each adsorbent is independently selected from molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or a combination of two or more thereof.

47. The system of any of claims 41 to 46, wherein each adsorbent bed independently comprises more than one layer of materials, such as two layers of materials, three layers of materials, or from 4-10 layers of materials or subranges thereof.

48. The system of any one of claims 41 to 47, wherein the layers are arranged so that each pair of adjacent layers comprises or consists of different materials.

49. The system of any one of claims 41 to 48, wherein the adsorbent of the second NRU and any adsorbent in the first NRU are the same.

50. The system of any one of claims 41 to 49, further comprising a compressor in fluid communication with the first NRU and the second NRU and configured to compress the first nitrogen-enriched purged stream before being introduced to the second NRU.

51 . The system of any one of claims 41 to 50, further comprising a compressor in fluid communication with the second NRU and the first NRU and configured to compress the methane-enriched stream between the second NRU and the first NRU.

52. The system of any one of claims 41 to 51 , wherein the methane-enriched stream is introduced to the first NRU through the inlet for the feed gas.

53. A method for enriching methane, comprising using the system of any one of claims 41 to 52 to enrich methane in a feed gas comprising nitrogen and methane.

54. The method of claim 53, which is as defined in any one of claims 1 to 28.