METHOD FOR INERTIZING ACTIVATED CARBON IN BIOGAS PURIFICATION EQUIPMENT

Abstract

Several embodiments of the present invention are directed to methods for preparing a biogas purification system. These embodiments include rinsing an adsorbent with a gas stream that is separable by the downstream biogas purification process. Other embodiments include using a separated gas stream to rinse a saturated adsorbent. Additional embodiments include using a gas stream comprising CO2.

Description

METHOD FOR INERTIZING ACTIVATED CARBON IN BIOGAS PURIFICATION EQUIPMENT BRIEF DESCRIPTION OF THE INVENTION Several embodiments of the present invention are directed to methods for preparing a biogas purification system. These embodiments include rinsing an adsorbent with a gas stream that is separable by the downstream biogas purification process. Other embodiments include using the separable gas stream to rinse saturated adsorbent. Additional embodiments include using a gas stream comprising CO2. BRIEF DESCRIPTION OF THE FIGURES Figure 1 illustrates an exemplary configuration of the present invention. In this configuration, biogas is admitted to adsorbent A for the removal of organic compounds and H2S. The valve to adsorbent B remains closed. The effluent flow from adsorbent A is passed to a separator for the separation of methane and CO2, such as a pressure swing adsorption (PSA) unit. Once sufficiently pure, the biomethane can then be admitted to the network. When the new adsorbent, adsorbent B, is installed, the CO2 flow can be diverted from the purge valve to purge adsorbent B until all the air is removed. Figure 2 illustrates an exemplary configuration of the present invention. In this configuration, the valves to adsorbent A, which now contain spent carbon, are closed; the biogas flow is diverted to adsorbent B, the gaseous byproducts of which then pass to a separator as a PSA unit. The CO2 separated from the separator is passed to adsorbent A, and the methane and volatile organic compounds displaced by it are burned until the flame is naturally extinguished by the incorporation of CO2 purge gas. Once this is complete, the contents of adsorbent A are no longer flammable, and the adsorbent is ready for removal and transport, followed by replacement with a new adsorbent B. Figure 3 illustrates the oxygen concentration during CO2 purging at a gas velocity of 6.3 cm / second through a 250 mL test facility. Figure 4 illustrates the oxygen concentration during varying CO2 purge rates with AP3-60 carbon. Figure 5 illustrates the methane concentration during CO2 purging at a gas velocity of 3.2 cm / second through a 250 mL test facility. DETAILED DESCRIPTION OF THE INVENTION This description is not limited to the specific systems, devices, and methods described, as these may vary. The terminology used in the description is for the purpose of describing the specific versions or modalities only and is not intended to limit its scope. The following terms shall have, for the purposes of this application, the respective meanings stated below. Unless otherwise defined, all technical and scientific terms used herein have the same meanings commonly understood by a person skilled in the art. Nothing in this description shall be construed as an admission that the embodiments described herein are not entitled to precede that description under the prior invention. As used in this document, the singular forms “a”, “an”, and “the” include references to the plural unless the context clearly dictates otherwise. “Optional” or “optionally” means that the event or circumstance described below may or may not occur, and that the description includes cases where the event occurs and cases where it does not. “Biogas” refers to a renewable, environmentally friendly fuel that contributes to lower global warming than traditional fossil fuels. Biogas is produced by the aerobic fermentation of biomass from various sources, including, but not limited to, domestic waste, manure, sewage, sludge, and municipal solid waste. Raw biogas consists primarily of a mixture of methane and carbon dioxide gases and may contain small amounts of nitrogen, hydrogen sulfide, moisture, and siloxanes. The amounts of methane and carbon dioxide components in biogas are variable and depend somewhat on the organic matter precursor. CH4 concentrations of 50%–70% and CO2 concentrations of 25%–38% are considered typical. When purified to fossil natural gas standards, biogas is referred to as “biomethane.” This purification process involves removing carbon dioxide, hydrogen sulfide, water, and other organic contaminants using one or more separation methods. When the adsorbent media in a biogas purification plant are changed, a large amount of air can be added to the biogas during startup. The downstream process is not capable of handling such a large amount of N2 and O2, the addition of which could degrade the quality of the resulting biomethane. Consequently, a method is required to reduce the air concentration in the biogas stream entering a separator such as a PSA. Such methods and systems are particularly important when using mobile filters, since the mobile filter is filled remotely from the consumer site, where the filter media can then be rinsed to remove air that would otherwise be considered a contaminant. The embodiments of the invention relate to methods for purifying biogas and purification systems configured to carry out those methods. The methods in these embodiments may include the step of rinsing a sorbent with a rinse gas before contacting the sorbent with the biogas. Although the embodiments are not limited to the particular rinse gas, in certain embodiments, the rinse gas may be separable from the gases collected during the purification process by subsequent downstream processing. For example, in some embodiments, the gas may be CO2. The gas used to rinse the sorbent may be obtained from an external source or, in particular embodiments, the gas may be obtained by recycling gases from the purification process and using the recycled gas to rinse the sorbent.Other methods include rinsing the used saturated sorbent, or using it to purge volatile organic contaminants bound to the sorbent during the purification process. Once rinsed, the sorbent is suitable for accepting coal and meets transportation regulations regarding flammable materials. Figures 1 and 2 illustrate an example of a system used to implement the methods described above, in which the rinse gas is recycled from a downstream portion of the system that includes two adsorbent-containing units: Adsorbent A and Adsorbent B. In Figure 1, the biogas enters the system and flows through valve 1 to Adsorbent A, where organic compounds and H2S are removed. The effluent is conveyed through valve 3 to a separator, such as a pressure swing adsorption (PSA) unit, where the biomethane is separated from the carbon dioxide (CO2). The biomethane can be further purified and discharged into the network. The removed CO2 is typically removed from the system through purge valve 4.In embodiments of the invention, the CO2 can be redirected back through the system to the rinse Adsorbent B, which can contain fresh sorbent, before the biogas is introduced into Adsorbent B. As illustrated in Figure 1, in some embodiments, the CO2 separated in the separator can be routed through valve 5 to Adsorbent B and can be purged from the system through purge valve 6. The use of two adsorbent vessels (Adsorbent A and Adsorbent B) allows for the almost continuous production of the desired purified gas flow. It also allows for pressure equalization, where the gas leaving the depressurized vessel is used to partially pressurize the second vessel. Figure 2 illustrates the system of Figure 1 arranged for biogas purification through Adsorbent B. Once the sorbent in Adsorbent A is saturated and Adsorbent B has been rinsed, the biogas flow can be diverted to Adsorbent B for the removal of organic compounds and H2S by closing valve 1 and opening valve 9. The organic compounds and H2S can be removed from the biogas in Adsorbent B, and the effluent can be passed through valve 7 and to the separator unit, where the biomethane is separated from the CO2. The CO2 can be sent through Adsorbent B and through Adsorbent A by closing valve 5 and opening valve 8. The CO2 can be used to purge volatile organic compounds from Adsorbent A through purge valve 2, where the volatile organic compounds can be burned.After purging, valve 8 can be closed and purge valve 4 can be opened, allowing the sorbent to be removed from Adsorbent A and disposed of. Fresh sorbent can then be introduced into Adsorbent A, and this sorbent can be rinsed by closing purge valve 4 and opening valve 8. The system described in Figures 1 and 2 thus provides rinsing and purging of the sorbent while allowing the continuous flow of biogas through the system. The sorbent used in Adsorbent A and B may be any type of adsorbent known in the prior art, including, but not limited to, carbonaceous material, activated carbon, reactivated carbon, carbon black, graphite, zeolite, silica, silica gel, alumina clay, metal oxides, or a combination thereof. In some embodiments, the sorbent may be a catalyst, or the sorbent may be impregnated with one or more additives that aid in the adsorption of organic impurities and / or hydrogen sulfide. The catalytic sorbent material can be any sorbent material capable of catalyzing a known reaction, for example, the oxidation of hydrogen sulfide (H₂S) to SO₂. Carbonaceous materials act as true catalysts in their capacity because only the rate of a given reaction is affected, and the carbonaceous materials themselves are not significantly altered by the reaction. Thus, in some embodiments, the catalytic adsorbent material can be a carbonaceous material. In others In modalities, the catalytic adsorbent material can be a carbonaceous material that has been subjected to processing to improve catalytic activity. In certain embodiments, carbonaceous material can be prepared from nitrogen-rich starting materials. Carbonaceous materials prepared from nitrogen-rich starting materials have been shown to catalyze reactions such as the decomposition of hydrogen peroxide. In other embodiments, the catalytic properties of a carbonaceous material that exhibits no catalytic activity or weak catalytic activity can be enhanced by exposing these materials to nitrogen-containing compounds such as urea, ammonia, polyamide, or polyacrylonitrile. In some embodiments, the exposure of the carbonaceous material to nitrogen-containing compounds can be carried out at a high temperature, such as above 700°C, and the carbonaceous material can be heated before, during, or both before and during exposure to the nitrogen-containing compound.In other methods, the exposure of the carbonaceous material to a nitrogen-containing compound can be carried out at temperatures below 700°C, or low temperatures. In still other methods, the carbonaceous material can be oxidized at high temperatures before being exposed to a nitrogen-containing compound. The carbonaceous material or catalytic activated carbon can be calcined. Calcination can be carried out between any steps in the process. For example, in some embodiments, the oxidized carbonaceous material can be calcined before being exposed to a nitrogen-containing compound, and in other embodiments, the oxidized carbonaceous material can be calcined after exposure to a nitrogen-containing compound or after activation. In still other embodiments, the carbonaceous material can be calcined between more than one step in the process. For example, the carbonaceous material can be calcined after exposure to a nitrogen-containing compound and after activation. Calcination is generally carried out by heating the carbonaceous material or catalytic activated carbon to a temperature sufficient to reduce the presence of surface oxides on the carbonaceous material.The temperature at which the surface oxides are removed can be from approximately 400°C to approximately 1000°C or any temperature in between, and in some forms, the calcination can be carried out in an oxygen-free or otherwise inert environment. Although the embodiments of the invention include any means for separating biomethane from CO2, in some embodiments, such as those described in Figure 1 and Figure 2, the means for separating biomethane from CO2 may be a pressure swing adsorption (PSA) unit. A PSA unit typically includes a plurality of vessels, each containing a bed of adsorbent material that adsorbs a different gas, such as water vapor, CO2, N2, and O2. In some embodiments, the PSA includes vessels containing at least two different adsorbent materials, and in certain embodiments, at least one adsorbent material is selective for methane, and at least one adsorbent material is selective for hydrogen. In some embodiments, the adsorbent materials may be layered within the adsorption vessel.For example, the container may include one or more layers of methane-selective adsorbent material interleaved with one or more layers of hydrogen-selective adsorbent material. Each of the adsorbent materials may be activated carbon, carbon molecular sieves, zeolite, silica gel, alumina clays, and similar materials and combinations thereof. rnarnn / zznz / E / YiAi Gas flow can be passed through the PSA under pressure. At higher pressures, the likelihood of adsorption of the target gas component increases, and when the pressure is reduced, the adsorbed gas components can be released or desorbed. Different target gases tend to adsorb at different pressures. Therefore, PSA processes can be used to separate one or more gas species from a mixture by adsorbing and releasing each species at a different pressure. Although PSA is typically operated at near-ambient temperatures, heat can be used to enhance the desorption of adsorbed species.For example, when pressurized air is passed through a vessel containing a bed of nitrogen-selective adsorbent material at a pressure that favors nitrogen adsorption, substantially all of the nitrogen will be adsorbed onto the material, and the gas stream leaving the vessel will be oxygen-enriched and nitrogen-depleted. When the bed reaches the end of its nitrogen-adsorbing capacity, it is regenerated by reducing the pressure, applying heat, or both, releasing the adsorbed nitrogen. It is then ready for another cycle of producing an oxygen-enriched stream. In some configurations, the biogas processing system may also include a biogas compression system, a hydrogen sulfide scrubbing system, a moisture modification vessel, one or more compressors, a biogas disposal system, a water supply system, and a final analysis and processing system. The various components of these systems may be connected by any suitable means, such as piping, hoses, conduits, and similar means, or combinations thereof. These connecting means can convey the material handled by the particular system component between components at the temperatures and pressures required for proper operation. In this way, pressure and temperature can be maintained between components moving through the connecting means. These systems may also include one or more valves to control the flow of biogas between system components. In some embodiments, the system may include one or more digesters that remove hydrogen sulfide from the biogas. These digesters may be placed upstream or downstream of Adsorbents A and B, as illustrated in Figures 1 and 2, and may operate in concert with Adsorbents A and B to remove hydrogen sulfide. In certain embodiments, the digesters may be placed upstream of Adsorbents A and B. Hydrogen sulfide degrades metal equipment and detectors and is therefore typically removed initially in the process and methods described herein. Additionally, hydrogen sulfide is toxic, even at very low concentrations, requiring its removal from the gas stream. In certain configurations, the system may include a moisture separator to reduce the moisture content of the biogas. The moisture separator may be located upstream and downstream of Adsorbents A and B, and in some configurations, it may be upstream of Adsorbents A and B and downstream of the digesters. In some configurations, the moisture separator may be positioned to remove the moisture present as the biogas exits the digester, reducing the moisture content to less than approximately 1.4%. Downstream condensation can create problems for system control, as it interferes with gas flow and pressure measurements. If not removed, condensation can cause failure of the compressor's lubricating oil filter and internal lubricated parts. Multi-mode systems may include one or more compressors that pressurize the biogas as it is conveyed through the system. Compressors can be incorporated at almost any position within the system. For example, in certain configurations, a compressor may be incorporated near the beginning of the system, such as upstream of the digesters and moisture separator, but downstream of Adsorbents A and B. In other configurations, the compressor may be placed downstream of Adsorbents A and B, but before a PSA (Pressure System Appliance). In still other configurations, compressors may be placed both upstream and downstream of Adsorbents A and B. In additional configurations, a compressor may be placed downstream of the PSA to pump biomethane to a storage vessel, a device configured to operate on the biomethane, or combinations thereof.The compressors of various types can be driven by an electric motor, a biogas-operated engine, or a crude methane-operated engine, and in some types, generators can provide the electricity needed to drive the compressors. These generators may include a biogas-operated engine or a crude methane-operated engine, and in some types, the engines can be powered by biogas or methane to allow the systems to operate autonomously. In some configurations, the systems may include an accumulator that combines biogas flows with recycled gas flows from other parts of the system, such as a scrubbing tank or a gas dryer. The combined biogas flow can then be directed from the accumulator to a system upstream of, for example, Adsorbents A and B or the PSA. In other configurations, the combined biogas can be introduced into the system upstream of the moisture separator. In certain configurations, the system may include a cooler that can reduce the temperature of the biogas flow to below approximately 21°C (approximately 70°C). The cooler can be placed anywhere within the system, and in certain configurations, it can be placed immediately upstream of the PSA or an eliminator, which is described later. Some embodiments of the systems of the invention may include a biogas eliminator. These eliminators typically remove carbon dioxide from the biogas through water absorption. Carbon dioxide is more soluble in water under pressure than at atmospheric pressure, while methane is primarily insoluble in water, even at high pressures. Pressurizing a methane / carbon dioxide biogas mixture in the presence of water brings the carbon dioxide into solution, but little methane dissolves in the solution. The gas flows counterflow or crossflow with the water. The resulting processed biogas has an enriched methane content because some or all of the carbon dioxide has been processed out of the gas and into the aqueous solution. The compressed operating pressure is a function of the temperature, the mole fraction of carbon dioxide in the gas, and the desired methane purity.These eliminators can be located downstream of a chiller, and are typically downstream of Adsorbents A and B and downstream of the PSA. In certain configurations, the eliminator system can be connected to a water supply system that pumps water into the eliminator or scrubber system. rnarnn / zznz / E / YiAi EXAMPLES Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. Therefore, the spirit and scope of the appended claims shall not be limited to the description and preferred embodiments contained herein. Several aspects of the present invention shall be illustrated with reference to the following non-limiting examples. The following examples are for illustrative purposes only and should not be construed as limiting the invention in any way. EXAMPLE 1 The dynamics of oxygen displacement by carbon dioxide rinsing through an adsorbent loaded with activated carbon were studied for design engineering purposes. The number of bed volumes required to inert a carbon-filled column was determined as a function of the CO2 gas velocity. Similarly, the inerting of a methane-loaded activated carbon column by CO2 rinsing was studied to determine the degree of purging required to comply with transportation and safety regulations. In particular, laboratory studies were required to provide experimental data to evaluate the effectiveness of a CO2 gas flushing procedure for safely inerting activated carbon in service filter equipment used for biogas purification. Biogas consists primarily of a mixture of methane and carbon dioxide gases and may contain small amounts of hydrogen sulfide, moisture, and siloxanes. The amounts of methane and carbon dioxide components in biogas are variable and depend somewhat on the organic matter precursor. CH4 concentrations of 50%–70% and CO2 concentrations of 25%–38% are considered typical. Laboratory-scale “inerting” tests were conducted under ambient temperature and pressure conditions. The tests simulated the flushing of a biogas adsorption system (when filled with activated carbon) with 99.5% CO2 gas. The laboratory “inerting” studies simulated two separate, distinct stages of filter equipment use: (a) oxygen removal from a “freshly filled” adsorption system prior to its “on-stream” application for biogas purification; and (b) methane removal from a “used” carbon filter system to make transport and decommissioning operations (i.e., carbon dumping) “safe.” The studies used a vertical, cylindrical test setup (internal volume of 250 mL) and evaluated various CO2 downflow rates, thus providing data for variations in contact time and gas velocity through the carbon bed. The tests evaluated the separate CO2 gas scrubbing of four grades of selected Chemviron granulated carbon. For comparison purposes, the extruded test carbons comprised two impregnated grades, which are generally intended for biogas purification, and two grades of base (i.e., unimpregnated) carbon granules that served as precursor granules for the two impregnated test carbons. The test carbons included: (i) SOLCARB®KS3, a high-performance impregnated 3 mm granular product specially developed for the rapid vapor-phase removal of hydrogen sulfide, organic mercaptans, and certain organic sulfides; (ii) ENVIROCARB® STIX®, an impregnated extracted carbon (4 mm granules) designed for the removal of hydrogen sulfide, acid gases, and other odorous compounds from air; (iii) ENVIROCARB® AP3-60, an extruded base carbon (3 mm granules), which served as the precursor base for the manufacture of SOLCARB®KS3; and (v) ENVIROCARB® AP4-50, an extruded base carbon (4 mm granules), which served as the precursor carbon base for the manufacture of ENVIROCARB® STIX®. A rigid plastic tube test setup was oriented vertically. The tube dimensions are shown below in TABLE 1. rnarnn / zznz / E / YiAi TABLE 1 Length of test installation tube 47 cm Internal diameter 2.6 cm Calculated cross-sectional area 5.31 cm2 Calculated internal volume 249.6 cm3 The CO2 gas flow (99.5% purity) was obtained from a regulated, high-pressure CO2 cylinder. The CO2 flow rate was controlled and measured using a rotameter-type flow meter recalibrated for CO2 gas. The 250 mL test unit was first filled with a freshly obtained sample of granulated (not pre-dried) test carbon, and the weight of the test sample was recorded. Under these carbon filling conditions, the test unit also contained a certain air / oxygen ratio, present both in the intergranule spaces and adsorbed onto the carbon pore structure. The purpose of inerting was to remove <0.1% oxygen from the adsorbent column system by "rinsing" with a controlled downstream flow of 99.5% pure CO2 gas at ambient temperature and pressure. The initial assay used a sample of SOLCARB®KS3 granules. The weight of the sample carbon used to fill the 250 mL test setup was recorded. A CO2 flow rate of 2 liters per minute was connected to the top of the carbon-filled test setup column, and a digital timer was started. The CO2 gas flow rate of 2 liters / minute corresponded to a calculated gas velocity of 6.3 cm per second through the test setup. At intervals during the CO2 gas flow and washout, the effluent gas from the bottom of the adsorbent was sampled using a calibrated syringe and analyzed for its oxygen component using the laboratory GC / MS (Agilent 7890A gas chromatograph and 5975C mass spectrometer) with the appropriate GC column, pre-calibrated for the O2 concentration. Due to the oxygen residence time of 2 minutes in the GC column, it was possible to sample the effluent gas from the test column every 2 to 3 minutes. The CO2 gas flow continued through the test adsorbent until the O2 concentration was < 0.1%. After the adsorbent test was completed, the rinsed carbon sample was removed from the test setup, and its “rinsed” weight was recorded. Samples of the other three test carbons were then tested similarly at a CO2 gas flow rate of 6.3 m / s, and the % oxygen results were recorded at various flow times. A summary of the % oxygen test values ​​for rinsing at a CO2 flow rate of 2 liters / minute for the four carbons is shown in Table 2 below and in Figure 3. τηατηη / ζζηζ / Β / γίΛΐ TABLE 2: Four test carbons - O2 concentration during CO2 purge at a gas velocity of 6.3 cm / second through the 250 mL test setup. CO2 flow rate: 2 liters per minute. CO2 flow rate: 8 empty bed volumes per minute. CO2 velocity through the carbon: 6.3 cm per second. Solcarb KS3 STIX 4 mm. AP3-60 base. AP4-50 base. Bed volumes % of oxygen Bed volumes % of oxygen Bed volumes % of oxygen Bed volumes % of oxygen 0 20.93 0 20.93 0 20.93 0 20.93 8 15.91 8 0.35 8 18.42 8 18.00 16 0.13 16 0.12 16 0.94 24 0.46 40 0.13 32 0.07 32 0.10 48 0.12 80 0.09 48 0.13 48 0.10 80 0.08 - - 80 0.12 80 0.10 - - Test values ​​at a CO2 velocity of 6.3 cm / second indicated very rapid oxygen purging from the test facility. For all test carbons, the % O2 in the effluent was reduced to <0.1% by a flow of approximately 30 to 40 bed volumes (i.e., representing a CO2 flow of 4 to 5 minutes). The two base carbons appeared to take slightly longer for oxygen flushing than the two impregnated grades. For all test samples, no discernible exothermic reaction was observed during CO2 purging. After examining the test values ​​obtained with a CO2 purge velocity of 6.3 cm per second, additional purge gas velocities were similarly evaluated for their oxygen reduction effect. Purge flow rates of 0.5, 1.0, and 6.0 liters of CO2 per minute were assessed. The ENVIROCARB® AP3-60 base coal was selected as the adsorbent for the variable-rate purge test and was thought to represent the typical purge properties of the other test coals. After each purge test, the ENVIROCARB® AP3-60 coal was discarded, and the test facility was refilled with a fresh quantity of base coal. The variable CO2 purge gas conditions tested for the ENVIROCARB® AP3-60 coal are shown in Table 3 below. τηατηη / ζζηζ / Β / γίΛΐ TABLE 3: Variable purge gas conditions for ENVIROCARB® AP3-60 test carbon CO2 purge flow rate (liters per minute) CO2 purge flow rate (bed volumes per minute) CO2 purge rate (cm per second) 0.5 2 1.6 1.0 4 3.2 2.0 8 6.3 6.0 24 18.8 The % O2 test values ​​obtained in the gaseous effluent under different purge gas velocity conditions are shown in TABLE 4 and Figure 4. Table 4: AP3-60 Carbon Oxygen Removal as a Function of Variable Gas Purge Conditions Oxygen purge of AP3-60 base carbon. CO2 flow rate (liters per minute): 0.5, 1.0, 2.0, 6.0. Bed volumes per minute: 2, 4, 8, 24. Velocity (cm per second): 1.6, 3.2, 6.3, 18.8. Bed volume (%O2): 0, 20.93; 0, 20.93; 0, 20.93; 0, 20.93; 2, 20.80; 4, 16.53; 8, 18.42; 8, 19.80; 6, 4.17; 8, 4.39; 16, 0.94; 16, 1.14; 10, 0.59; 16, 0.008; 32, 0.10; 24, 0.14; 16, 0.23. 48 0.13 20 0.22 - - - - - - Γηαρηη / ζζηζ / Β / γίΛΐ The test values ​​indicated an inverse relationship between the CO2 purge rate and the oxygen concentration reduction rate. In other words, the slower the CO2 purge rate, the faster the oxygen removal rate. The values ​​indicate that the effectiveness of the purge depended primarily on the adsorption (and diffusion) kinetics within the carbon / air system (i.e., the adsorption of CO2 gas and the removal of adsorbed air from the carbon pore structure). The initial air removal and the continued removal of desorbed air / oxygen from the pore structure into the intergranule voids would be relatively rapid. Regardless of the effects of adsorption kinetics, all CO2 purge rates removed oxygen from the test facility very quickly and indicated that approximately 16 to 20 purge bed volumes were effective. EXAMPLE 2 The purpose of this study was to evaluate the effectiveness of a CO2 purge flow for the safe removal of methane from a “used” biogas carbon filter system, in order to “make it safe” before any transport or decommissioning operation. The 250 mL laboratory test setup described above was used again, along with the two grades of impregnated granulated carbon and their respective base carbon precursors. Before each carbon was tested, it was filled into the test setup and then allowed to saturate and equilibrate in a flow of 1 liter per minute of 99.9% methane gas for 30 minutes. The test charcoal saturated with methane was then similarly rinsed with a CO2 gas purge flow of one liter per minute. At timed intervals, an effluent gas sample was analyzed for % CH4 using the Agilent GC / MS precalibrated for methane concentration. A summary of the % methane test values ​​for rinsing at a CO2 flow of 11 liters / minute for the four carbons is shown in Table 5 and Figure 5. TABLE 5: Four test carbons - CH4 concentration during CO2 purge at a gas velocity of 6.3 cm / second through the 250 mL test setup. CO2 flow rate: 1 liter per minute. CO2 flow rate: 4 bed volumes emptied per minute. CO2 velocity through the carbon: 3.2 cm per second. Solcarb KS3 STIX 4mm AP3-60 Base AP4-60 Base CO2 Bed Volumes % of Methane CO2 Bed Volumes % of Methane CO2 Bed Volumes % of Methane CO2 Bed Volumes % of Oxygen 0 100 0 100 0 100 0 100 4 98.93 4 96.71 4 97.27 4 95.62 12 1.06 12 0.15 12 0.18 12 0.15 24 0.10 24 0.15 20 0.13 24 0.14 Methane gas was easily flushed from all test coal using the CO2 purge gas flow. There was very little difference among the four grades of coal tested. The methane concentration in the effluent gas flow was approximately 0.1% in approximately 12 bed volumes purged with 100% CO2 gas flow. No heat of adsorption was observed during the methane purge tests. The examples above demonstrate that removing oxygen from the carbon-loaded adsorbent vessel, prior to its use for biogas purification, using a purge gas flow with 99.5% CO2 was effective. A slower rate purge gas flow of CO2 was more effective for oxygen removal from the adsorbent system than a relatively faster rate purge gas, due to the kinetics of the air desorption rate of the carbon adsorbent. The removal of methane from a “used” biogas purification vessel (before safe decommissioning and emptying) by flushing with a 99.5% CO2 gas flow was very quick and uncomplicated, with no noticeable heat evolution.

Claims

1. A biogas purification system comprising: at least two adsorbent units; a separator configured and arranged to separate biomethane from carbon dioxide operatively connected to each of at least two adsorbents by conduits configured to supply carbon dioxide to each of at least two adsorbents; and valves operatively connected to each of the conduits and arranged to supply carbon dioxide individually to each of at least two adsorbents.

2. The biogas purification system of claim 1, wherein each of at least two adsorbent units is operatively connected to the separator by conduits configured to supply biogas from each adsorbent unit to the separator.

3. The biogas purification system of claim 1, wherein each of at least two adsorbents further comprises a purge valve.

4. The biogas purification system of claim 1, further comprising a biogas source operatively connected to at least two adsorbent units by conduits configured to supply biogas to each adsorbent unit.

5. The biogas purification system of claim 1, wherein the separator is a pressure swing adsorption unit.

6. The biogas purification system of claim 1, further comprising a biogas compression system, hydrogen sulfide cleaning system, a moisture modification vessel, one or more compressors, a biogas disposal system, a water supply system, an accumulator, an analysis and processing system, or combinations thereof.