Biogas upgrading system and method of operating same

Abstract

A method for upgrading a biogas feed stream to produce a product stream that complies with a prescribed RNG specification that specifies a limit for total inert gas constituents and less than 2.0 Mol% CO2. The method comprises reducing CO2 concentration to between 0.2 Mol % and 0.005 Mol%, and removing N2 from the biogas only to a level necessary to comply with the limit for total inerts. The disclosed system comprises sensors for determining concentrations of total inerts and CO2 in said biogas, a controller programable to select a predetermined target CO2 concentration for said gaseous RNG between 0.2 Mol% and 0.005 Mol% as a function of the concentrations of total inerts and CO2, and a plurality of membrane-based gas separation stages to which said biogas feed stream can delivered and be processed into a process stream with the target CO2 concentration.

Description

BIOGAS UPGRADING SYSTEM AND METHOD OF OPERATING SAMETechnical Field

[0001] A biogas upgrading system and a method of operating same produce a gaseous renewable natural gas product stream with improved methane recovery rates. In an exemplary embodiment, a multi-stage membranebased system upgrades a landfill gas.Background

[0002] Biogas comprises a mixture of gases with methane being one its main constituents. Biogas upgrading systems upgrade biogas by increasing the percentage of methane, and this is done by separating the methane from the other gases present in the source biogas mixture and by producing a product gas that has a methane concentration of at least 95% on a molar basis, enabling the upgraded biogas, also known as biomethane or renewable natural gas (“RNG”) to be used as a combustible fuel that burns in a similar manner to fossil based natural gas. In this disclosure, when “percent” or “concentration” is used to describe a fractional constituent of a gas mixture, this percentage means the molar fraction of the individual constituent. By capturing and upgrading biogas, methane is prevented from escaping into the atmosphere where it would otherwise contribute to overall greenhouse gas emissions. Accordingly, there is an environmental benefit associated with capturing biogas and using it to displace fossil fuels.

[0003] The composition of raw biogas can vary depending upon the source, how the source is managed, and how the biogas is captured. Accordingly, the type of pretreatment that is needed depends on the composition of the raw biogas and the amount and nature of volatile organic compounds (“VOCs”), which is also a characteristic of the source of the biogas. VOCs and corrosive contaminants such as hydrogen sulfide are typically stripped from the raw biogas in one or more pre-treatment steps, collectively referred to herein as a pre-treatment stage, before being delivered to a biogas upgrading system, to increase the fraction of methane to a level where it can be used as RNG. Compared to the residual trace amounts of VOC’s and corrosive contaminants still remaining after the pre-treatment stage, the concentrations of carbon dioxide, nitrogen and oxygen in biogas are relatively large. With the objective of biogas upgrading systems being to remove non-methane constituents to increase the methane concentration to what is required by an RNG specification, after the pre-treatment stage, the focus of designers for biogas upgrading systems is on separating methane from these larger non-methane constituents.

[0004] This disclosure is directed to the part of a biogas upgrading system that receives the feed biogas from the pre-treatment stage and upgrades it to a composition that meets or exceeds the composition required by the RNG product specification. The RNG specification typically specifies a minimum methane concentration, so that the RNG has an energy content that allows RNG to be combusted as a substitute for fossil-based natural gas. The RNG specification typically also specifies maximum concentrations for individual non-methane constituents, such as carbon dioxide, oxygen, and nitrogen. By way of example, in Canada, Enbridge Gas Ontario is an energy company that operates natural gas pipelines and it publishes pipeline gas quality specifications that must be met for RNG to be admitted into its pipelines. This specification includes many of the smaller fractions that can be corrosive to apipeline, and Table 1 only sets out the specification for the non-methane constituents that together make up the largest non-methane constituents in biogas:Table 1, RNG Specification for Main Non-Methane Constituents by Concentration

[0005] In Table 1 , according to this RNG specification, nitrogen (N2) is not specifically named, but as an inert gas it does not add to the heating value of the RNG and it must be included in the measurement of “total inerts”. While some of these non-methane constituents are not inert and would contribute to the overall heating value, RNG specifications that require a maximum molar concentration of 4 % for total inerts is common, and roughly translates to a minimum methane molar concentration of 96% if none of the non-methane constituents in the biogas contribute to the biogas’ heating value. In the United States, the RNG specifications vary slightly from one company to another but most are very similar to the RNG specifications in Table 1 and most require a total inert gas concentration of no more than between about 4% and 5%, a carbon dioxide concentration that is no more than 2%, oxygen concentration below 0.2%, and nitrogen concentration that is no more than 4%.

[0006] Because landfill gas collection systems draw out the landfill gas through a network of pipes, this method of collection often uses a negative pressure to draw out the biogas. Because landfills are a poorly sealed and controlled environment, air is drawn into the landfill gas when it is collected. This is why landfill gas typically has higher amounts of nitrogen and oxygen, compared to other biogases, which are produced in more controlled environments, such as anaerobic digesters.

[0007] Polymeric gas-separation membranes are one of the preferred technologies for separating methane from carbon dioxide and water. This is enabled by the development of membranes that are selectively permeable for carbon dioxide and water compared to methane. This selectivity is based on the physical size and shape of carbon dioxide and water molecules compared to the size and symmetrical shape of methane molecules. That is, carbon dioxide, oxygen, and water molecules all have an axis of orientation with a smaller cross-sectional area compared to the smallest cross-sectional area of methane molecules. The portion of the fluid stream that passes through the membrane is called the “permeate stream” and the portion that exits a membrane-based gas separation module without passing through the membrane is called the “retentate stream”. Because of the selectivity of the membrane, the retentate gas has a relatively high percentage of methane, compared to the stream fed into a gas separation module (the “feed stream”), and the permeate stream has a higher percentage of non-methane constituents compared to the feed stream. However, membrane selectivity is not perfect and because the differences in molecular size and shape are small, membranes that are selective of carbon dioxide can still allow some methane to pass through, even though it may be in much smaller amounts compared to carbon dioxide. This is why prior artmembrane-based gas separation systems typically use a plurality of membrane gas separation modules arranged in series with each successive stage achieving a higher purity of methane. To improve methane recovery rates, some of the permeate streams are recycled and returned to the feed stream to capture methane that passed through the membranes. Recovery rates can also be improved by the choice of membrane material, the membrane surface area, the pressure of the feed stream, and the number of membrane fluid separation modules. Methane and nitrogen have similar molecular sizes so some membrane-based separation units are not good at separating methane from nitrogen while separating carbon dioxide at the same time. Compared to landfill gas, for most biogases separating methane from nitrogen is not a significant problem, because nitrogen is not normally a large fraction of the non-methane constituents. For raw landfill gas, the concentration of nitrogen can be between 2 and 20%, or even more, depending upon how a landfill site is designed and managed, the degree of vacuum applied to draw the landfill gas from the ground, and the composition of the landfill contents. For landfill gas, when nitrogen is a large fraction of the gas composition, conventional membrane-based gas separation systems need equipment specific to nitrogen removal to reduce the total inert gases to 4% or less. There are various types of nitrogen removal units that use different mechanism for separating nitrogen from methane, and when needed, a nitrogen removal unit is usually added to treat the gas exiting from the treatment stage that separates most of the nitrogen from the methane. Because carbon dioxide is normally the largest non-methane fraction for biogas, this means that after the carbon dioxide is separated, there is less gas to be treated to reduce the nitrogen concentration. Depending upon the type of nitrogen removal unit, adding a nitrogen removal unit can add to the capital cost, the operational costs, maintenance costs, and can also result in reduced methane recovery rates because of the inverse relationship between methane recovery and nitrogen removal. When the percentage of nitrogen is higher than 4- 5% in the feed gas, after most of the carbon dioxide is removed, the nitrogen concentration rises higher than the nitrogen concentration in the feed gas and becomes a larger fraction of the total inerts. With conventional systems, when the RNG specification for total inerts is no more than 4%, it is generally accepted that for upgrading landfill gas, because of the relatively high nitrogen concentration compared to other biogases, a nitrogen removal unit is unavoidable to comply with most RNG specifications. “Nitrogen rejection unit” or “NRU” are other names that can be used for the nitrogen removal unit described in this disclosure.

[0008] In addition to the choice of membrane material, the number of stages, and the permeable area of the membrane material, it is also known that flowing a gas on the permeate side of a membrane in a membrane fluid separation module can increase the performance of a membrane by decreasing the partial pressure of the fluids permeating through the membrane. In this disclosure, a gas that is introduced into a membrane fluid separation module in this manner is referred to as a “sweep gas”. If the carbon dioxide, nitrogen and oxygen constituents of the sweep gas have respective percentages that are lower than that in the feed stream, then the introduction of the sweep gas results a lower partial pressure on the permeate side of the membrane for these gases and increases the driving force that promotes a higher permeation rate for these gases. Similarly, if the sweep gas has a higher percentage of methane, compared to the feed stream, this increases a partial pressure for methane on the permeate side of the membrane, reducing the driving force for methane, causing less methane to permeate through the membrane. United States Patent No. 10,254,041 B2 (the ’041 Patent), which is incorporated by reference, discloses a system and method for processing a hydrocarbon-comprising fluid and using a sweep gas. The '041 Patentteaches combining a multi-stage membrane-based fluid separation system with a system for liquefying gas, namely a 3-phase separator, and using flash gas from the 3-phase separator as a sweep gas for at least one of the membrane fluid separation modules. However, because a multi-stage membrane-based fluid separation system on its own is capable of producing RNG that meets or exceeds RNG specifications for admission into pipeline distribution networks or for immediate use as a fuel, there is no reason to add a 3-phase separator system, making the extra cost and complexity unnecessary. The '041 Patent discloses that adding the sweep gas improves the separation of carbon dioxide and water from the feed stream, and because this reduces the amount of solid carbon dioxide and ice that forms in the liquefaction process, this reduces maintenance and clogging that would occur if more carbon dioxide and water was left in the process stream delivered to the liquefaction system. As a result, there is an additional benefit when a sweep gas is implemented because this increases the process efficiency in the 3- phase separator. The motivation associated with this additional benefit, is absent when a membrane-based biogas upgrading system is not combined with a liquefaction system. The '041 Patent discloses using a by-product of the liquefaction system without drawing any of the sweep gas from the upgrading system itself.

[0009] United States Patent No. 11,731,076 B1 (the '076 Patent), which is incorporated by reference, discloses a membrane process and system for high recovery of a nonpermeating gas utilizing a sweep gas that is taken from another unit process in the system, such as a temperature swing adsorption unit or liquefaction unit. Like the other prior art, the '076 Patent teaches using the by-product from another system as the source of the sweep gas. In Figures 2 and 6 of the '076 Patent show sweep gas being introduced into the first membrane fluid separation module. According to Table 2 in column 12 of the '076 Patent, by using a sweep gas the same product composition can be achieved with 75% of the membrane area, compared to a system that does not use a sweep gas. Figure 3 of the '076 Patent illustrates an arrangement for a membrane-based gas separation module with a sweep gas introduced through a sweep gas inlet at an end of the separation module that is opposite to the end associated with the feed gas inlet, with the permeate outlet closer to the end associated with the feed gas inlet. This shows a preferred arrangement that allows the sweep gas to flow in “counterflow” with the direction that the feed gas flows through the membrane separation module towards the product outlet. United States Patent Application Publication No. US 2025 / 0229220 A1 (the “220 Application”), which is incorporated by reference, discloses a 4-stage membrane process with sweep for biogas upgrading. The '220 Application teaches using the retentate stream exiting a fourth membrane stage as a sweep gas in the second membrane stage. The disclosed purpose for the sweep gas is to improve the efficiency of the gas separation achieved by the second membrane stage and to separate more carbon dioxide from the product gas, thereby increasing the purity of the product gas. United States Patent Application Publication No. US 2025 / 02136884 A1 (the “884 Application”), which is incorporated by reference, discloses methods and systems for producing a liquefied methane product from a feed stream containing methane and carbon dioxide. The '884 Application identifies anaerobic digester lagoons and landfill sites as examples of sources for biogas, but nothing is disclosed about how to treat landfill gas differently from other biogases, and discloses a method and system that is directed to treating biogas with only small amounts of non-methane constituents other than carbon dioxide. Because the '884 Application discloses a system and method for upgrading biogas for liquefaction and not for complying with standard RNG specifications for a gaseous product stream, which typically prescribe a maximum carbon dioxide concentration of about 2%, the '884 Application is directed to a system andmethod that reduces carbon dioxide to a concentration of about 0.002% so that there is no risk of generating any frozen carbon dioxide solids during liquefaction. The '884 Application discloses that this level of carbon dioxide can be achieved by using a membrane-based gas separation system that uses a second membrane stage that comprises a gas separation membrane that is more permeable to carbon dioxide than methane, and using a series of gas separation modules with the permeate stream from each module being used as a sweep gas for the preceding module. The permeate stream from the first module is the series is recycled to the feed stream for the first membrane stage. The disclosed purpose for the sweep gas is to improve the efficiency of the gas separation achieved by the second membrane stage and to separate more carbon dioxide from the product gas, thereby increasing the purity of the product gas. Like the '041 Patent, the '884 application discloses a system in which the purpose for using a sweep gas is to remove carbon dioxide so that the product gas can be liquefied without carbon dioxide freezing and clogging the liquefaction apparatus. The purpose is not to produce a product stream that complies with an RNG specification.

[0010] Because landfill gas has a different composition from other biogases, and because nitrogen is difficult to separate from methane, biogas upgrading methods and systems are needed that are adaptable to upgrade landfill gas to comply with RNG specifications for gaseous product streams. Many components in a biogas upgrading system are supplied by different specialty companies, each with its own expertise associated with the components specific to each supplier. Instead of maximizing the efficiency and performance of each component, it may be possible to improve overall system efficiency, performance and costs by taking a holistic approach to system design and operating methods.

[0011] Because there is an inverse relationship between nitrogen removal and methane recovery rates, meaning that increasing the amount of nitrogen that is removed, usually results in a decrease in the methane recovery rate, there is a benefit to reducing the amount of nitrogen that is removed in order to comply with RNG specifications.Summary

[0012] A method is disclosed of upgrading a biogas feed stream to produce a product stream that complies with a prescribed RNG specification that specifies a limit for Mol % for total inert gas constituents and less than 2.0 Mol % carbon dioxide. The disclosed method comprises reducing carbon dioxide concentration to between 0.2 Mol % and 0.005 Mol %; and removing nitrogen from the biogas only to a level necessary to comply with the limit for Mol % for total inert gas constituents. The prescribed RNG specification is set by the purchaser of the RNG or the operator of the pipeline that receives it. By way of example, in typical cases the limit for Mol % for total inert gas constituents is no more than 4%, and in other cases it can be no more than 5%. In some embodiments the method further comprises reducing carbon dioxide concentration by even more, to less than 0.1 Mol %. Prior art biogas upgrading systems that produce a gaseous RNG do not target a carbon dioxide concentration at these levels because most RNG specifications only specify that the carbon dioxide concentration not exceed about 2 Mol %, and once this specification is met, attention is directed to other requirements of the RNG specification. With conventional systems and methods, exceeding the limits set by specifications is not an objective because this is normally associated with adding unnecessary costs which could make a system unable to compete with competitors.

[0013] In some embodiments the biogas feed stream is collected form a landfill. The composition of landfill gas can comprise higher proportions of nitrogen compared to other biogases, and because of the inverse relationship between methane recovery and nitrogen removal, the disclosed method is especially suited for upgrading of landfill gas. That is, the disclosed method can be applied to increase the amount of carbon dioxide that is removed to reduce the carbon dioxide concentration by about an order of magnitude more than what is required to comply with the RNG specification. Contrary to conventional thinking, and this can result in higher methane recovery rates and lower capital and operating costs because less nitrogen must be removed to comply with the total inert concentration prescribed RNG specification.

[0014] The method can further comprise separating carbon dioxide from the biogas feed stream with a multi-stage membrane-based gas separation unit. In such embodiments, the amount of carbon dioxide can be increased by flowing a sweep gas through a permeate side of a membrane, such as the permeate side of a membrane associated with a second stage of the multi-stage membrane-based gas separation unit. The sweep gas has a carbon dioxide concentration that is less than that of gas flowing on a retentate side of the membrane being swept.

[0015] The method can further comprise selecting a target carbon dioxide concentration between 0.2 Mol% and 0.005 Mol% as a function of the composition of the biogas feed stream. When the method is applied to a system that has a biogas feed stream with a consistent composition without much variability then the method can employ a fixed target carbon dioxide concentration. When the method is applied to systems that receive a biogas feed stream that has a composition that is more variable, the method can comprise monitoring the concentration of total inerts and the concentration of carbon dioxide for the biogas feed stream, and adjusting the target carbon dioxide concentration responsive to changes in these concentrations. The function for determining the target carbon dioxide concentration can further comprise reducing overall operational costs as one of the factors, including the cost for removing carbon dioxide and the cost for removing nitrogen. That is, the method need not reduce carbon dioxide concentration to 0.005 Mol % if it is possible to comply with prescribed RNG specification with a target carbon dioxide concentration lower than an upper limit (e.g. 0.2 Mol % or 0.1 Mol %) but higher than 0.005 Mol %. For example, based on the composition of the biogas feed stream, the method can select a target carbon dioxide higher than 0.005 Mol % which results in a product RNG stream with less than the limit for Mol % for total inert gas constituents without the need for a nitrogen removal step. In other embodiments, the method can select a target carbon dioxide higher than 0.005 Mol % and include a nitrogen removal step if the overall operating costs are lower, or, if the methane recovery rate is higher, compared to complying with the prescribed RNG specification by reducing carbon dioxide to 2 Mol%, and then achieving compliance with the RNG specification for total inerts by removing nitrogen.

[0016] In one aspect, the method is a method of upgrading a landfill gas feed stream for producing a gaseous RNG product stream that complies with a prescribed RNG specification that specifies a limit for Mol% for total inert gas constituents and less than 2.0 Mol% carbon dioxide. This aspect of the method comprises determining a target carbon dioxide concentration between 0.2 Mol% and 0.005 Mol% as a function of the composition of the landfill gas feed stream and reducing carbon dioxide concentration to the target carbon dioxide concentration by: (a) compressing the landfill gas feed stream and delivering a compressed landfill gas feed stream to a first stage inlet of a first gas separation module, and in the first gas separation module, using a first stage membrane that is selectiveof carbon dioxide over methane to separate the landfill gas feed stream into a first retentate stream that does not pass through the first stage membrane, and a first permeate stream that passes through the first stage membrane;(b) in a second gas separation module, using a second stage membrane that is selective of non-methane fluids over methane, to separate the first retentate stream into a second retentate stream that does not pass through the second stage membrane, and a second permeate stream that passes through the second stage membrane, and recycling the second permeate stream to the landfill gas feed stream; and (c) monitoring the total inert concentration in the second retentate stream and if it is less than the limit for Mol% for total inert gas constituents, delivering the second retentate stream to the gaseous RNG product stream. If the total inert concentration in the second retentate stream is higher than the limit for Mol% for total inert gas constituents, then the method comprises treating the second retentate stream to reduce nitrogen concentration until the total inert concentration complies with the prescribed RNG specification. In this way, the method produces the gaseous RNG product stream that complies with the prescribed RNG specification, while also improving methane recovery rate, or reducing overall capital and operational costs.

[0017] This aspect of the method can further comprise delivering a sweep gas to a permeate side of the second stage membrane to help to reduce the carbon dioxide concentration in the second retentate stream. The method can comprise controlling the flowrate of the sweep gas to reduce the flow of sweep gas if the total inert concentration of the gaseous RNG is less than what is required by the prescribed RNG specification by more than a predetermined margin. There can be fluctuations in the composition of the landfill feed gas that cause fluctuations in the composition of the product RNG stream, and in this disclosure when the method is described as complying with the prescribed RNG specification, it is understood that the method controls the upgrading system to comply with the prescribed limits with a predetermined margin so that fluctuations in composition do not result in a product gas stream that does not consistently comply with the prescribed RNG specification.

[0018] In some embodiments, the sweep gas is a portion of the gaseous RNG product stream. The method can further comprise adjusting the flowrate of the sweep gas as a function of factors comprising at least one of, reducing operational costs by removing more carbon dioxide instead of removing nitrogen, and increasing methane recovery rate.

[0019] To improve methane recovery rates, the method can further comprise using a third stage membrane that is selective of non-methane fluids over methane in a third gas separation module, to separate the first permeate stream into a third retentate stream that does not pass through the third stage membrane, and a third permeate stream that passes through the third stage membrane, wherein the third retentate stream is recycled to the landfill gas feed stream. To remove more carbon dioxide the method can further comprise using a fourth stage membrane that is selective of non-methane fluids over methane in a fourth gas separation module, to separate the second retentate stream into a fourth retentate stream that does not pass through the fourth stage membrane, and a fourth permeate stream that passes through the fourth membrane. That is, an additional membrane stage can be added in series to remove more carbon dioxide from the product stream to achieve the target carbon dioxide concentration.

[0020] In a particular aspect, the disclosed method of upgrading a landfill gas feed stream for producing a gaseous RNG product stream that complies with a prescribed RNG specification that specifies a limit for Mol% for total inert gas constituents and less than 2.0 Mol% carbon dioxide, the method comprises: in step (a) determining a targetcarbon dioxide concentration between 0.2 Mol% and 0.005 Mol% as a function of the composition of the landfill gas stream and reducing carbon dioxide concentration to the target carbon dioxide concentration by: (i) compressing the landfill gas feed stream and delivering a compressed landfill gas feed stream to a first stage inlet of a first gas separation module, and in the first gas separation module, using a first stage membrane that is selective of carbon dioxide over methane to separate the landfill gas feed stream into a first retentate stream that does not pass through the first stage membrane, and a first permeate stream that passes through the first stage membrane; and (ii) in a second gas separation module, using a second stage membrane that is selective of non-methane fluids over methane, to separate the first retentate stream into a second retentate stream that does not pass through the second stage membrane, and a second permeate stream that passes through the second stage membrane, and recycling the second permeate stream to the landfill gas feed stream; and, in step (b) monitoring the total inert concentration in the second retentate stream and if it is less than the limit for Mol% for total inert gas constituents, delivering the second retentate stream to the gaseous RNG product stream; and if the total inert concentration in the second retentate stream is higher than the limit for Mol% for total inert gas constituents, treating the second retentate stream to reduce nitrogen concentration until the total inert concentration complies with the prescribed RNG specification, thereby producing the gaseous RNG product stream that is compliant with the prescribed RNG specification. Like other aspects, this particular aspect can also further comprise delivering a sweep gas to a permeate side of the second stage membrane to help to reduce the carbon dioxide concentration in the second retentate stream. The method can also further comprise controlling the flowrate of the sweep gas and reducing sweep gas flowrate if the total inert concentration of the gaseous RNG is less than what is required by the prescribed RNG specification by more than a predetermined margin. That is, the method manages the upgrading of the landfill gas to produce a RNG product stream that complies with the prescribed specification without unnecessarily exceeding what is required. In some embodiments the sweep gas can be a portion of the gaseous RNG product stream, and in these embodiments, it is especially beneficial to adjust the flow rate of the sweep gas, to reduce the sweep gas flow rate when it is possible to do so while still complying with the prescribed specification, because this directly affects the methane recovery rate.

[0021] In yet another aspect, disclosed is a method of upgrading a landfill gas feed stream for producing a gaseous RNG product stream that complies with a prescribed RNG specification that specifies a limit for Mol% for total inert gas constituents and less than 2.0 Mol% carbon dioxide, with this aspect of the method comprising: (a) compressing the landfill gas feed stream and delivering a compressed landfill gas feed stream to a first stage inlet of a first gas separation module, and in the first gas separation module, using a first stage membrane that is selective of carbon dioxide over methane to separate the landfill gas feed stream into a first retentate stream that does not pass through the first stage membrane, and a first permeate stream that passes through the first stage membrane; (b) in a second gas separation module, using a second stage membrane that is selective of non-methane fluids over methane, to separate the first retentate stream into a second retentate stream that does not pass through the second stage membrane, and a second permeate stream that passes through the second stage membrane, and recycling the second permeate stream to the landfill gas feed stream; (c) in a third gas separation module, using a third stage membrane that is selective of non-methane fluids over methane, to separate the first permeate stream into a third retentate stream that does not pass through the third stage membrane, and a third permeate stream thatpasses through the third stage membrane, wherein the third retentate stream is recycled to the landfill gas feed stream; (d) in a fourth gas separation module, using a fourth stage membrane that is selective of non -methane fluids over methane, to separate the second retentate stream into a fourth retentate stream that does not pass through the fourth stage membrane and a fourth permeate stream that passes through the fourth stage membrane, wherein the fourth permeate stream is recycled to the landfill gas feed stream; and (e) monitoring the total inert concentration in the fourth retentate stream and if it is less than the limit for Mol% for total inert gas constituents, delivering the fourth retentate stream to the gaseous RNG product stream; and if the total inert concentration in the second retentate stream is higher than the limit for Mol% for total inert gas constituents, treating the fourth retentate stream to reduce nitrogen concentration until the total inert concentration complies with the prescribed RNG specification, thereby producing the gaseous RNG product stream that is compliant with the prescribed RNG specification.

[0022] For implementing the disclosed method, also disclosed is a biogas upgrading system for processing a biogas feed stream and producing a gaseous RNG that complies with a predetermined RNG specification. This system comprises: (a) sensors for determining concentrations of total inerts and carbon dioxide in the biogas feed stream; (b) a controller programed to select a predetermined target carbon dioxide concentration for the gaseous RNG between 0.2 Mol% and 0.005 Mol% as a function of the concentrations of total inerts and carbon dioxide in the biogas feed stream; and, (c) a plurality of membrane-based gas separation stages to which the biogas feed stream can delivered and be processed into a process stream with the target carbon dioxide concentration. Some aspects of the system can further comprise sensors for determining concentrations of total inerts and carbon dioxide in the gaseous renewable natural gas that is delivered from the system. In some aspects the disclosed system can further comprise a nitrogen removal unit for removing nitrogen from the process stream that exits the plurality of membranebased gas separation stages, the nitrogen removal unit being controllable to remove only the amount of nitrogen that is needed to enable the RNG to comply with the RNG specification for total inerts. In some embodiments the plurality of membrane-based gas separation stages each employ a membrane that is selectively more permeable to carbon dioxide compared to methane, and the membrane-based gas separation stages comprise: (a) a primary stage in fluid communication to receive the biogas and wherein the membrane separates the biogas into a first retentate stream and a first permeate stream; (b) a secondary stage in fluid communication to receive the first retentate stream and wherein the membrane separates the first retentate stream into a second retentate stream and a second permeate stream; and wherein the second retentate stream is delivered to a gaseous RNG product stream; and the second permeate stream is recycled to the biogas feed stream and re-introduced into the primary stage. The disclosed system can also comprise a third stage that also employs a membrane that is selectively more permeable to carbon dioxide compared to methane, the third stage in fluid communication to receive the first permeate stream and the membrane separates the first permeate stream into a third retentate stream and a third permeate stream, wherein the third retentate stream is recycled to the biogas feed stream and re-introduced into the primary stage. In other aspects the system can further comprise a fourth stage that also employs a membrane that is selectively more permeable to carbon dioxide compared to methane, the fourth stage in fluid communication to receive the second retentate stream before it is delivered to the gaseous RNG product stream, and wherein the membrane separates the second retentate stream into a fourth retentate stream that is, and a fourth permeate stream, wherein the fourth retentate stream is the process stream with the target carbon dioxide concentration, andis delivered to the gaseous RNG product stream. As with the method, the disclosed biogas upgrading system can be advantageously used for upgrading a biogas feed stream sources from a landfill.

[0023] Some embodiments of the disclosed biogas upgrading system can further comprise a flow controller and a conduit for delivering a sweep gas a permeate side of a membrane associated with at least one of the plurality of membrane-based gas separation stages. In some embodiments the sweep gas can be sourced from the gaseous RNG product stream.

[0024] There are many variations to the systems described herein and how they are operated, for example, the choice of membrane materials, controlling fluid pressure to influence permeation rates through the membranes, and the number of membrane stages, that can be used to practice the disclosed method. Accordingly, it is understood that while the disclosed systems are enabling embodiments, the disclosed method is not limited to these systems.Brief Description of the Drawings

[0025] Figure 1 is a schematic diagram illustrating a two-stage membrane-based biogas upgrading system combined with a pretreatment stage, including conduits for delivering a sweep gas.

[0026] Figure 2 is a schematic diagram illustrating a three-stage membrane-based biogas upgrading system including conduits for delivering a sweep gas.

[0027] Figure 3 is a schematic diagram illustrating a two-stage membrane-based biogas upgrading system combined with a nitrogen removal unit.

[0028] Figure 4 is a schematic diagram illustrating a four-stage membrane-based biogas upgrading system combined with a pretreatment stage.

[0029] Figure 5 is a process flow diagram illustrating the disclosed method for upgrading a biogas.Detailed Description

[0030] This detailed description provides exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the disclosed system and method. Rather, the ensuing detailed description of the exemplary embodiments will provide those skilled in the art with an enabling description for implementing these exemplary embodiments and the disclosed method. Various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention.

[0031] Reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification in order to provide context for other features.

[0032] Unless otherwise indicated, the articles “a” and “an” as used herein mean one or more when applied to any feature in embodiments of the disclosed landfill gas upgrading system described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used.

[0033] When biogas is upgraded to RNG it must meet a product fuel gas specification usually based on a heating value and Wobble index. As such, the product gas has a minimum required methane content with the residual gasbeing a mixture of inert gases. There are specific limits to the concentrations of these residual inert gases and there is also a total sum limit because the RNG specification includes a minimum methane content so that the RNG has the requisite energy content for use as a fuel.

[0034] Some of the disclosed systems and methods introduce a sweep gas on the permeate side of the membranes. When a sweep gas has a lower concentration of inert gases than the gas entering the gas separator on the retentate side of the membrane, the lower partial pressure on the permeate side of the membrane drives more inert gases through the membrane. Typically, this means more carbon dioxide is separated from the process gas, and this improves the methane recovery rate because of the inverse relationship between methane recovery and nitrogen removal. That is, removing more carbon dioxide means that more nitrogen may remain in the RNG while staying below the total sum limit for inert constituents and this improves the overall methane recovery because less nitrogen must be removed. In some embodiments enough carbon dioxide is removed to enable a gaseous product stream to comply with an RNG specification without the need for an extra nitrogen removal unit. Even when an upgrading system is combined with a nitrogen removal unit, it can be beneficial to reduce the utilization of the nitrogen removal unit by reducing the concentration of non-methane constituents other than nitrogen, for example, if it is less costly to reduce the concentration of carbon dioxide, even though it is already below the maximum concentration specified by the RNG specification.

[0035] With reference to Figure 1 , a schematic diagram illustrates biogas upgrading system 100 comprising a multi-stage membrane-based gas separation unit. In some embodiments, the biogas can be sourced from a landfill. The raw biogas is collected and delivered to pretreatment stage 110. Whereas, as previously disclosed above, the raw biogas constituents can comprise of 40-70% methane, 30-60% carbon dioxide, up to 20% nitrogen (5-12% nitrogen in most cases), up to 3% oxygen, 0 to 5000 ppm hydrogen sulfide, and trace levels (less than thousands of ppm) of siloxanes, and VOCs, pretreatment removes most, if not substantially all, of the hydrogen sulphide, VOCs and siloxanes. Pretreatment stage 110 is optional in some embodiments if the biogas fed into biogas upgrading system 100 is of a quality that does not require the biogas upgrading system to be combined with a pretreatment stage. For example, the biogas feed stream can come from a source that does not product the type of contaminants removed by pretreatment or the biogas feed stream can have already undergone pretreated at the source. Pretreatment apparatuses are well known and while the specific apparatus that is chosen is based on the apparatus that is most suited to treating the characteristic constituents of the raw biogas, the installation and use of a pretreatment apparatus is not novel in itself, apart from being used in conjunction with the combination of elements and method steps associated with the disclosed system. Figure 1 shows that system 100 can include sensors 104 for monitoring the composition of the biogas feed stream, and sensors 106 for monitoring the composition after pretreatement stage 110, and after any recycled permeate streams have been returned and combined with the biogas feed stream. In this disclosure, the biogas feed stream combined with the recycled permeate stream(s) is referred to herein as the “combined feed stream”. The sensors 104 and 106 represent one or a plurality of sensors that monitor the concentration of some or all of the constituents for the mixed gas, with carbon dioxide concentration being one of the particular constituents that is monitored to enable exemplary embodiments of the disclosed method. System 100 can also include sensor 162 for monitoring the composition of the RNG product stream to confirm that RNG product stream 165 complies with the parameters set by the RNG specification. Controller 170 is programableto select a predetermined target carbon dioxide concentration for the gaseous RNG between 0.2 Mol% and 0.005 Mol% as a function of the concentrations of total inerts and carbon dioxide in the biogas feed stream. Controller 170 can also receive data inputs from sensors for measuring the concentration of total inerts and carbon dioxide in the gaseous RNG delivered from the system.

[0036] After pretreatment stage 110, the biogas feed stream is combined with any recycled permeate streams with the combined feed stream being delivered to an inlet for compressor 120. Compressor 120 operates to increase the pressure of the combined feed stream before it is discharged through a compressor outlet and delivered to multistage membrane separator 130 (comprising the modules shown within the dashed outline). In this embodiment multi-stage membrane-base gas separation unit 130 has two gas separation modules, arranged in series. The first stage inlet of first gas separation module 132 receives the combined feed stream, which typically comprises between about 40-70% methane, 30-60% carbon dioxide. First gas separation module 132 has a first stage membrane that is selective of carbon dioxide, whereby the portion of the biogas that passes through the membrane, called the first permeate stream, has a higher percentage of carbon dioxide, compared to the feed stream, because the portion of the feed stream that does not pass through the membrane, called the first retentate stream, has a higher percentage of methane, compared to the feed stream. The first retentate stream exiting first gas separation module 132 is delivered to a second stage inlet of second gas separation module 134. The second stage membrane installed in second gas separation module 134 need not be the same as the first stage membrane. For example, the second stage membrane can be made from a different material or made with a different selectivity, or it can have a different membrane area. Nevertheless, like the first stage membrane, the second stage membrane is ordinarily also selective for carbon dioxide over methane.

[0037] Second gas separation module 134 processes the first retentate stream, which is delivered to a second stage inlet. The first retentate stream flows next to the second stage membrane and the fluid that does not pass through the second stage membrane exits second gas separation module through a second stage retentate outlet. The fluids that pass through the second stage membrane form the second permeate stream and exit biogas upgrading system 100. The second permeate stream is rich in carbon dioxide, and can be further purified in a separate system and sold commercially as a compressed gas or liquefied and sold as liquid carbon dioxide.

[0038] In the embodiment illustrated by Figure 1, second gas separation module also comprises an inlet for accepting a sweep gas that is supplied from second retentate stream 135 or the RNG product stream in delivery conduit 165. The sweep gas enters the second gas separation module on the permeate side of the second stage membrane and preferably flows in a direction opposite to the direction of flow for the gas flowing from the second stage inlet to the second stage retentate outlet. Because sweep gas stream 139 has a high percentage of methane and a very low percentage of carbon dioxide, this reduces the partial pressure for the carbon dioxide on the permeate side of the second stage membrane and drives more carbon dioxide through the second stage membrane. By driving more carbon dioxide through the second stage membrane, this reduces the amount of residual inert gases in second retentate stream 135, and in some embodiments, this reduces the amount of residual inert gases to a level below the total sum limit allowed by the desired RNG specification. Using a sweep gas can help to reduce the carbon dioxide concentration to a target carbon dioxide concentration between 0.2 Mol % and 0.005 Mol%

[0039] With reference to the process flow diagram shown in Figure 2, biogas upgrading system 200 is like biogas upgrading system 100, with pretreatment multi-stage membrane separator 230 further comprising additional third gas separation module 236. In this embodiment sweep gas stream 139 is delivered to the permeate side of the membrane of at least one gas separation module. In exemplary embodiments, as shown in Figures 1 and 2, the modules that receive sweep gas include the last gas separation module in the series of modules that produces the RNG product stream. This is because in this final stage, most the carbon dioxide has already been removed, so it is harder to remove the remaining amount of carbon dioxide, and the extra driving force provided by the introduction of sweep gas stream 139 has a greater effect compared to when sweep gas stream 139 is introduced into an earlier stage in the series. However, in exemplary embodiments, sweep gas would not be delivered to gas separation modules that generate permeate streams that exit the biogas upgrading system because this would result in the loss of the methane included in sweep gas stream 139. In Figure 2, a system is show that delivers sweep gas stream 139 to first gas separation module 132 and second gas separation module 134 and the methane present in this sweep gas is mostly recovered with the gas that is recycled back to the feed stream via recycle conduit 137 as part of the normal methane recovery process.

[0040] The biogas upgrading systems shown in Figures 1 and 2 are illustrative of systems that are capable of reducing the methane concentration to less than the 2% level required to meet the RNG Specification. More specifically, the disclosed systems are capable of reducing carbon dioxide to a target concentration that is between and upper limit of at least about 0.2% and more preferably at least about 0.1% and a lower limit of about 0.005 Mol%. Reducing carbon dioxide concentration to these levels is not required to comply with the RNG specification, but it can help with compliance with the specified total inert concentration and enable reducing the amount of nitrogen that must be removed to comply with the specification for total inert gas concentration. In addition to the systems shown in Figures 1 and 2, other configurations can be used for membrane-based biogas upgrading systems.

[0041] In all embodiments, it is only necessary to remove non-methane constituents from the product gaseous RNG to levels that meet the RNG specification, and exceeding the specification can result in higher operational costs and higher capital cost if the system is over-designed. Because RNG specifications typically set a limit for carbon dioxide that is around 2%, this may explain why known systems do not teach reducing the carbon dioxide concentration in RNG to levels near 0.2% or lower unless there is a reason for doing so, such as when the product gas is to be liquefied, and for liquefaction, a carbon dioxide concentration lower than 0.005% is needed. The disclosed systems and method take a more holistic approach, and targets a composition for the inert gases that is the least expensive to produce. That is, if the RNG specification allows a concentration of inert gases up to 4%, then the system is controlled to keep this concentration at less than 4% to include a safety margin, but close to 4% to avoid unnecessary costs associated with excessively exceeding the RNG specification. What is different in the present disclosure is the composition of the inert gases that make up the total inert concentration. Compared to previously known methods and systems for producing a gaseous RNG product stream, the disclosed method and system is designed to achieve a carbon dioxide concentration that is at least an order of magnitude lower, enabling a higher concentration of nitrogen.

[0042] With reference to Figure 3, biogas upgrading system 300 is similar to biogas upgrading system 100 but with the addition of nitrogen removal unit 360. When it is not possible to remove enough carbon dioxide to obviate the need for a nitrogen removal unit, the disclosed system and method can still be an improvement over the prior art because reducing carbon dioxide to a target concentration between 0.2 Mol % and 0.005 Mol % removes more carbon dioxide compared to prior art biogas upgrading systems employed for producing a gaseous RNG product stream, and this means that less nitrogen must be removed by nitrogen removal unit 360, and this results in higher methane recovery rates because of the inverse relationship between methane recovery and nitrogen removal.

[0043] With reference to Figure 4, biogas upgrading system 400 comprises multi-stage membrane separator 430 that has four gas separation modules 132, 134, 236 and 440. Fourth gas separation module 440 delivers a fourth retentate stream to RNG product stream 165 and fourth permeate stream 447 is recycled back to the feed stream together with second permeate stream 137. Reference numbers that are the same as the reference numbers in Figures 1 and 2 indicate the same system components. Fourth stage 440 helps to reduce the carbon dioxide concentration to between 0.2 Mol % and 0.005 Mol % and can avoid the need for using a sweep gas, demonstrating that the method can be implemented by a variety of system configurations.

[0044] Nitrogen is an inert gas so many RNG specifications do not specify a limit for nitrogen, but it is included in the limit for total inerts. Nitrogen removal unit 360 is operable to remove nitrogen until the concentration of total inerts in the RNG is lower than the maximum concentration specified by the RNG specification. As a result, less nitrogen is removed by reducing the other non-methane constituents have been reduced to the lowest levels that can be attained without costing more than it would cost to remove the same amount of nitrogen using a nitrogen removal unit. By controlling the upgrading of the landfill gas according to the total inert concentration, instead of considering the concentration of each individual non-methane constituent in isolation from the other non-methane constituents, the RNG specification can be met while managing the operational costs efficiently.

[0045] The disclosed method is described with reference to the process flow diagram shown in Figure 5. Depending upon the quality of the raw untreated biogas and the specification for gaseous RNG product, the disclosed biogas upgrading method for producing an RNG product stream can be combined with known pretreatment stages that remove, for example, hydrogen sulfide, volatile organic compounds and siloxanes. The disclosed biogas upgrading method processes the feed stream to produce a process stream that complies with the limits set by the RNG specification with respect to the concentration of total inert gas constituents and the concentration of carbon dioxide. Some embodiments of the disclosed method will not need to use all of the illustrated steps, and different embodiments can use different set points for the carbon dioxide concentration to upgrade the process stream to produce a gaseous RNG product stream. The disclosed method starts at step “A”, with receiving a pretreated biogas feed stream. Step “B” is a decision point. At step “B” the composition of the feed gas is considered. It would be unusual for the feed gas to already comply with the RNG specification with regard to the concentration of carbon dioxide and total inerts, but if it did, the method allows the feed gas to be delivered to an RNG product stream in step “C”, ending the disclosed method for upgrading biogas until the composition of the feed gas requires the method to upgrade it. At step “B” the normal process for the method is to choose a predetermined target carbon dioxide concentration based on the feed gas composition with the target concentration being between 0.2 Mol% and 0.005 Mol%. In some cases, this range is narrower, for example, between 0.1 Mol% and 0.01 Mol%.The target carbon dioxide concentration can be chosen by a controller that is programable to choose the target carbon dioxide concentration as a function of the concentrations of the total inerts and carbon dioxide in the feed gas. The controller could also consider the concentration of nitrogen as a factor in determining the target carbon dioxide concentration. The controller can also make adjustments to the target carbon dioxide concentration as a function of the concentration of total inerts and carbon dioxide in the gaseous RNG product stream that is produced by the biogas upgrading system. The controller can be programmed to choose a target carbon dioxide concentration that reduces overall costs and that increases the methane recovery rate. In step “D” the method processes the feed gas to reduce the carbon dioxide concentration to the target concentration. Step “E” is another decision point. In step “E”, if the concentration of total inerts is less than the concentration required by the RNG specification, then the process stream can be delivered to the RNG product stream in step “C”, ending the biogas upgrading process. At step “E” if it is determined that the concentration of total inerts is higher than what is required by the RNG specification, then the method employs step “F” to reduce the concentration of total inerts by removing nitrogen. RNG specification typically specify that the concentration of total inerts be no more than 5 Mol% and some specifications specify no more than 4 Mol%. Step “F” is optional because, depending upon the composition of the feed gas and the concentration of total inerts necessary to comply with the RNG specification, in some cases, step “F” is not needed. When the composition of the feed gas enables the method to produce a gaseous RNG product stream without step “F” by reducing carbon dioxide concentration to as low as 0.005 Mol% this is advantageous because this simplifies the system and avoids the capital and operational costs associated with a nitrogen removal unit. However, if the composition of the feed gas is such that use of a nitrogen removal unit is unavoidable, then in some cases it may be more cost effective to set a target carbon dioxide concentration that is below 0.2 Mol% but not as low as 0.005 Mol%. Even with step “F” included, compared to known biogas upgrading methods, because the disclosed method reduces carbon dioxide concentration by at least an order of magnitude more than what is normally done for producing a gaseous RNG product stream, while some nitrogen is still removed, more nitrogen can be left in the RNG product stream. Accordingly, the disclosed method enables higher methane recovery rates because of the inverse relationship between methane recovery and nitrogen removal.

[0046] Part of the novelty of the disclosed method for producing a gaseous RNG product stream is that it discloses reducing the carbon dioxide concentration by at least an order of magnitude more than what is needed. By reducing the carbon dioxide concentration to between 0.2 Mol% and 0.005 Mol% this enables significantly higher concentrations of other inert gases to remain if the RNG product stream, and a benefit of that is enabling more nitrogen to remain in the RNG product stream. This is especially relevant when the source of the biogas feed stream is a landfill because landfills normally have more nitrogen compared to other biogas sources. While it is possible to reduce the carbon dioxide concentration to levels lower than 0.005 Mol% the cost caused by the extra treatment to achieve this can outweigh the benefit of allowing higher concentrations of other inerts. Accordingly, this is the reason for choosing a predetermined target concentration for carbon dioxide that is between 0.2 Mol % and 0.005 Mol%. In some embodiments a target concentration of 0.1 Mol% can be enough to eliminate step “F”. Depending upon the composition of the feed gas, reducing the concentration of carbon dioxide to 0.005 Mol% will still result in the concentration of total inerts being higher than what is required by the RNG specification, so the predetermined target concentration for carbon dioxide can be higher than 0.005 Mol% because step “F’ isunavoidable. That is, while different embodiments may reduce carbon dioxide concentration to different predetermined levels, all embodiments reduce the amount of nitrogen that is removed to comply with the RNG specification, and in some embodiments, there is no need for treatment specifically for nitrogen removal, and in these embodiments this can result in significant cost savings, and reduced complexity in the method and the system needed to enable it. Because there can be variability in the composition of a biogas feed stream, in step “B” in exemplary embodiments, the composition of the feed stream is continuously monitored and adjustments are made to the target carbon dioxide concentration

[0047] In the following claims, letters are used in method claims to identify claimed steps (e.g. (a), (b), and (c)). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims.

Claims

Claims1 . A method of upgrading a biogas feed stream to produce a product stream that complies with a prescribed renewable natural gas specification that specifies a limit for Mol % for total inert gas constituents and less than 2.0 Mol % carbon dioxide, said method comprising: reducing carbon dioxide concentration to between 0.2 Mol % and 0.005 Mol %; and removing nitrogen from the biogas only to a level necessary to comply with the limit for Mol % for total inert gas constituents.

2. The method of claim 1 wherein the limit for Mol % for total inert gas constituents is no more than 4%.

3. The method of claim 1 wherein the limit for Mol % for total inert gas constituents is no more than 5%.

4. The method of claim 1 further comprising reducing carbon dioxide concentration to less than 0.1 Mol %.

5. The method of claim 1 wherein said biogas feed stream is collected from a landfill.

6. The method of claim 1 further comprising separating carbon dioxide from said biogas feed stream with a multi-stage membrane-based gas separation unit.

7. The method of claim 6 further comprising flowing a sweep gas through a permeate side of a membrane of a second stage of said multi-stage membrane-based gas separation unit.

8. The method of claim 7 wherein said sweep gas has a carbon dioxide concentration that is less than that of gas flowing on a retentate side of the membrane of said second stage of said multi-stage membrane-based gas separation unit.

9. The method of claim 7 wherein said sweep gas is supplied from said product stream.

10. The method of claim 6 wherein nitrogen is removed from a retentate stream that exits said multi-stage membrane-based gas separation unit.

11. The method of claim 1 further comprising selecting a target carbon dioxide concentration between 0.2 Mol% and 0.005 Mol% as a function of the composition of said biogas feed stream.

12. The method of claim 11 wherein the function for determining said target carbon dioxide concentration includes as a factor reducing overall costs, including the cost for removing carbon dioxide and the cost for removing nitrogen.

13. The method of claim 11 further comprising monitoring the concentrations of total inerts and carbon dioxide for said biogas feed stream and adjusting said target carbon dioxide concentration responsive to changes in the concentrations of total inerts and carbon dioxide for said biogas feed stream.

14. A method of upgrading a landfill gas feed stream for producing a gaseous renewable natural gas product stream that complies with a prescribed renewable natural gas specification that specifies a limit for Mol% for total inert gas constituents and less than 2.0 Mol% carbon dioxide, said method comprises determining a target carbon dioxide concentration between 0.2 Mol% and 0.005 Mol% as a function of the composition of said landfill gas feed stream and reducing carbon dioxide concentration to said target carbon dioxide concentration by: a. compressing said landfill gas feed stream and delivering a compressed landfill gas feed stream to a first stage inlet of a first gas separation module, and in said first gas separation module, using a first stage membrane that is selective of carbon dioxide over methane to separate said landfill gas feed stream into a first retentate stream that does not pass through said first stage membrane, and a first permeate stream that passes through said first stage membrane; b. in a second gas separation module, using a second stage membrane that is selective of nonmethane fluids over methane, to separate said first retentate stream into a second retentate stream that does not pass through said second stage membrane, and a second permeate stream that passes through said second stage membrane, and recycling said second permeate stream to said landfill gas feed stream; and c. monitoring the total inert concentration in said second retentate stream and if it is less than said limit for Mol% for total inert gas constituents, delivering said second retentate stream to said gaseous renewable natural gas product stream; and if the total inert concentration in said second retentate stream is higher than said limit for Mol% for total inert gas constituents, treating said second retentate stream to reduce nitrogen concentration until the total inert concentration complies with said prescribed renewable natural gas specification, thereby producing said gaseous renewable natural gas product stream that is compliant with said prescribed renewable natural gas specification.

15. The method of claim 14 further comprising delivering a sweep gas to a permeate side of said second stage membrane to help to reduce the carbon dioxide concentration in said second retentate stream.

16. The method of claim 15 further comprising controlling the flowrate of said sweep gas to reduce flow if the total inert concentration of said gaseous renewable natural gas is less than what is required by said prescribed renewable natural gas specification by more than a predetermined margin.

17. The method of claim 15 wherein said sweep gas is a portion of said gaseous renewable natural gas product stream.

18. The method of claim 14 further comprising, in a third gas separation module, using a third stage membrane that is selective of non-methane fluids over methane to separate said first permeate stream into a third retentate stream that does not pass through said third stage membrane, and a third permeate stream that passes through said third stage membrane, wherein said third retentate stream is recycled to said landfill gas feed stream.

19. The method of claim 15 further comprising adjusting the flowrate of said sweep gas as a function of factors comprising reducing operational costs by removing more carbon dioxide instead of removing nitrogen, and increasing methane recovery rate.

20. The method of claim 14 wherein the function for determining said target carbon dioxide concentration includes reducing overall costs, including the cost for removing carbon dioxide and the cost for removing nitrogen.

21. A method of upgrading a landfill gas feed stream for producing a gaseous renewable natural gas product stream that complies with a prescribed renewable natural gas specification that specifies a limit for Mol% for total inert gas constituents and less than 2.0 Mol% carbon dioxide, said method comprises: a. determining a target carbon dioxide concentration between 0.2 Mol% and 0.005 Mol% as a function of the composition of said landfill gas stream and reducing carbon dioxide concentration to said target carbon dioxide concentration by: i. compressing said landfill gas feed stream and delivering a compressed landfill gas feed stream to a first stage inlet of a first gas separation module, and in said first gas separation module, using a first stage membrane that is selective of carbon dioxide over methane to separate said landfill gas feed stream into a first retentate stream that does not pass through said first stage membrane, and a first permeate stream that passes through said first stage membrane; and ii. in a second gas separation module, using a second stage membrane that is selective of non-methane fluids over methane, to separate said first retentate stream into a second retentate stream that does not pass through said second stage membrane, and a second permeate stream that passes through said second stage membrane, and recycling said second permeate stream to said landfill gas feed stream; and b. in a third gas separation module, using a third stage membrane that is selective of non-methane fluids over methane, to separate said first permeate stream into a third retentate stream that does not pass through said third stage membrane, and a third permeate stream that passes through said third stage membrane, wherein said third retentate stream is recycled to said landfill gas feed stream;c. in a fourth gas separation module, using a fourth stage membrane that is selective of non-methane fluids over methane, to separate said second retentate stream into a fourth retentate stream that does not pass through said fourth stage membrane and a fourth permeate stream that passes through said fourth stage membrane, wherein said fourth permeate stream is recycled to said landfill gas feed stream; d. monitoring the total inert concentration in said fourth retentate stream and if it is less than said limit for Mol% for total inert gas constituents, delivering said fourth retentate stream to said gaseous renewable natural gas product stream; and if the total inert concentration in said second retentate stream is higher than said limit for Mol% for total inert gas constituents, treating said fourth retentate stream to reduce nitrogen concentration until the total inert concentration complies with said prescribed renewable natural gas specification, thereby producing said gaseous renewable natural gas product stream that is compliant with said prescribed renewable natural gas specification.

22. The method of claim 21 wherein the function for determining said target carbon dioxide concentration includes reducing overall costs, including the cost for removing carbon dioxide and the cost for removing nitrogen.

23. A biogas upgrading system for processing a biogas feed stream and producing a gaseous renewable natural gas that complies with a predetermined renewable natural gas specification, said system comprises: a. sensors for determining concentrations of total inerts and carbon dioxide in said biogas feed stream; b. a controller programable to select a predetermined target carbon dioxide concentration for said gaseous renewable natural gas between 0.2 Mol% and 0.005 Mol% as a function of the concentrations of total inerts and carbon dioxide in said biogas feed stream; and c. a plurality of membrane-based gas separation stages to which said biogas feed stream can delivered and be processed into a process stream with the target carbon dioxide concentration.

24. The biogas upgrading system of claim 23 further comprising a nitrogen removal unit for removing nitrogen from the process stream that exits said plurality of membrane-based gas separation stages, said nitrogen removal unit being controllable to remove only the amount of nitrogen that is needed to enable said renewable natural gas to comply with the renewable natural gas specification for total inerts.

25. The biogas upgrading system of claim 23 wherein said plurality of membrane-based gas separation stages each employ a membrane that is selectively more permeable to carbon dioxide compared to methane, and said membrane-based gas separation stages comprise: a. a primary stage in fluid communication to receive said biogas and wherein said membrane separates said biogas into a first retentate stream and a first permeate stream;b. a secondary stage in fluid communication to receive said first retentate stream and wherein said membrane separates said first retentate stream into a second retentate stream and a second permeate stream; and wherein said second retentate stream is delivered to a gaseous renewable natural gas product stream; and said second permeate stream is recycled to said biogas feed stream and re-introduced into said primary stage.

26. The biogas upgrading system of claim 25 further comprising a third stage that also employs a membrane that is selectively more permeable to carbon dioxide compared to methane, said third stage in fluid communication to receive said first permeate stream and said membrane separates said first permeate stream into a third retentate stream and a third permeate stream, wherein said third retentate stream is recycled to said biogas feed stream and re-introduced into said primary stage.

27. The biogas upgrading system of claim 26 further comprising a fourth stage that also employs a membrane that is selectively more permeable to carbon dioxide compared to methane, said fourth stage in fluid communication to receive said second retentate stream before it is delivered to said gaseous renewable natural gas product stream, and wherein said membrane separates said second retentate stream into a fourth retentate stream that is, and a fourth permeate stream, wherein said fourth retentate stream is said process stream with the target carbon dioxide concentration, and is delivered to said gaseous renewable natural gas product stream.

28. The biogas upgrading system of claim 23 further comprising a flow controller and a conduit for delivering a sweep gas to a permeate side of a membrane associated with at least one of said plurality of membranebased separation stages29. The biogas upgrading system of claim 28 wherein said sweep gas is sourced from said gaseous renewable natural gas product stream.

30. The biogas upgrading system of claim 23 wherein said biogas feed stream is sourced from a landfill.

31. The biogas upgrading system of claim 23 further comprising sensors for determining concentrations of total inerts and carbon dioxide in said gaseous renewable natural gas.

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