A method and system for upgrading biogas to biomethane

Abstract

A method for purifying biogas to biomethane, comprising the following successive steps: (a) passing raw biogas through a first buffer tank (Z1) equipped with a demister and an activated‑carbon filter (F1) and compressing the pre‑purified gas in a compressor (C1) to 6-12 bar; (b) raising the temperature of the compressed biogas to 200–300 °C; (c) removing sulphur compounds in one or more desulphurization reactors (R1, R2) filled with a zinc‑oxide (ZnO) sorbent bed at 200–300 °C; (d) cooling the desulphurized gas to 30–40 °C while condensing and removing liquid; (e) upgrading the cooled gas in a two-stage pressure-swing adsorption (PSA) unit (A1–A8) containing sorbents for CO₂, N₂ and O₂, wherein in the first stage (A1–A4) adsorption is carried out at line pressure, producing biomethane with ≥ 98 vol % CH₄, which is forwarded to a gas grid or to a condensate tank (S1), and in the second stage (A5–A8), vacuum pressure‑swing adsorption (vPSA) is performed at (−1) to 25 kPa, the methane‑rich product being recycled to the suction side of the compressor (C1); and (f) catalytically oxidizing the residual methane contained in the PSA off‑gases in a methane-oxidation reactor (R3) using air, oxygen‑enriched air or pure oxygen.

Description

[0001] A METHOD AND SYSTEM FOR UPGRADING BIOGAS TO BIOMETHANE

[0002] TECHNICAL FIELD

[0003] The present disclosure concerns a method for upgrading biogas to biomethane and a system for implementing that method. It is particularly suitable for treating various biogas categories and for producing biomethane that meets the quality levels required by law.

[0004] BACKGROUND

[0005] The EU directive 2009 / 28 / EC on the promotion of the use of energy from renewable sources, as well as national laws such as the Polish renewable energy sources act of 20 February 2015, define biogas as gas obtained from biomass, especially from installations that process animal- or plant-based feedstocks, and also from sewage-treatment-plant and landfill gases. Biomethane is defined as gas derived from biogas, agricultural biogas or renewable hydrogen that has been purified and is either injected into the gas grid, transported in compressed or liquefied form by means other than the gas grid, or used directly as vehicle fuel without transport.

[0006] Numerous techniques for raising the methane content of biogas above 95 % are known from the scientific literature and from patents. Industrially, the following upgrading approaches are widely applied: physical absorption, high-pressure water scrubbing; chemical absorption; amine scrubbing; pressure-swing adsorption (PSA); membrane separation; cryogenic separation.

[0007] CN104910988A discloses a two-stage membrane-separation process that delivers high-purity, high-methane gas suitable for automotive use. The patent also proposes oxidizing the residual methane in the off gas over a catalyst to convert it to CO2 and water. Because membrane units are employed, the process operates at a high pressure of around 2.5 MPa.

[0008] EP4001381A1 teaches producing biomethane with a CH4 purity above 95 % by directly methanating the CO2 present in biogas, after an intermediate water-removal step. This eliminates any need to dispose of CO2 and increases the overall biomethane yield.

[0009] KR101207532B1 discloses methods to upgrade biogas to >95 % CH4 by first washing out CO2 and ILS with water, then drying the gas and removing the remaining impurities by pressure-swing or temperature-swing adsorption. The adsorbent bed comprises roughly 70-80 % molecular sieves and 20-30 % activated alumina.

[0010] KR101598818B1 teaches a method that combines PSA and membrane separation in series. PSA separates certain components, while membrane separation further splits CPU from CO2. Operating the two stages in tandem boosts purification efficiency, improves biomethane quality and enables CO2 recovery and utilization.

[0011] CN108530251A, likewise, integrates PSA and membrane separation methods. It removes hydrogen sulphide and volatile organic compounds via PSA and activated-carbon capture, while membrane separation upgrades the biogas to high-quality biomethane.

[0012] SUMMARY OF THE INVENTION

[0013] The object of the invention is to upgrade biogas, whose composition varies with its source (see Table 1), to high-methane gas containing more than 98 % CH4, independent of initial CO2, z, O2 content.

[0014] Table 1

[0015] In one aspect, the present invention relates to a method for purifying biogas to biomethane, comprising the following successive steps:

[0016] (a) passing raw biogas through a first buffer tank equipped with a demister and an activated-carbon filter and compressing the pre-purified gas in a compressor to 6- 12 bar;

[0017] (b) raising the temperature of the compressed biogas to 200-300 °C; (c) removing sulphur compounds in one or more desulphurization reactors filled with a zinc-oxide (ZnO) sorbent bed at 200-300 °C;

[0018] (d) cooling the desulphurized gas to 30-40 °C while condensing and removing liquid;

[0019] (e) upgrading the cooled gas in a two-stage pressure-swing adsorption (PSA) unit containing sorbents for CO2, N2 and O2, wherein in the first stage adsorption is carried out at line pressure, producing biomethane with > 98 vol % CH4, which is forwarded to a gas grid or to a condensate tank (SI), and in the second stage vacuum pressure-swing adsorption (vPSA) is performed at (-1) to 25 kPa, the methane-rich product being recycled to the suction side of the compressor; and

[0020] (f) catalytically oxidizing the residual methane contained in the PSA off-gases in a methane-oxidation reactor using air, oxygen-enriched air or pure oxygen.

[0021] Preferably, step (b) is carried out in two heat exchangers arranged in series, the first heat exchanger being heated by the desulphurized-gas stream and the second heat exchanger by the exhaust gases from the methane-oxidation reactor.

[0022] In another aspect, the invention relates to a system for purifying biogas to biomethane, comprising, in succession downstream of a raw biogas inlet:

[0023] (a) a pretreatment unit with a separator and demister and an activated-carbon filter;

[0024] (b) a compressor;

[0025] (c) a desulphurization unit with at least one desulphurization reactor;

[0026] (d) a two-stage pressure-swing adsorption (PSA) unit comprising of two sub-units each containing at least four adsorbers, the first sub-unit operating under pressure and the second sub-unit under vacuum;

[0027] (e) a twin adsorption filter located downstream of the first PSA sub-unit and upstream of a second buffer tank for polishing methane to > 99 vol % CH4;

[0028] (f) a methane-oxidation reactor for oxidizing methane present in the off-gas from the vacuum PSA sub-unit;

[0029] (g) two heat exchangers arranged between the compressor and the desulphurization unit for heating the compressed biogas with (i) the desulphurized-gas stream and (ii) the exhaust gases from the methane-oxidation reactor, respectively; (h) a fourth heat exchanger in the desulphurized-gas line downstream of the first heat exchanger for reheating the vPSA return gas, followed by a fifth heat exchanger for cooling the desulphurized gas before PSA; and

[0030] (i) a separator with a demister downstream of the fifth heat exchanger for condensing and removing liquid from the desulphurized gas prior to PSA.

[0031] A third heat exchanger can be installed in the raw-biogas line downstream of the two heat exchangers and upstream of the desulphurization unit.

[0032] A sixth heat exchanger can be arranged upstream of the methane-oxidation reactor for heating the combined stream of air, oxygen- enriched air or oxygen and vPSA return gas.

[0033] The system includes all necessary pipelines, valves, instrumentation and analyzers, selected in accordance with standard engineering practice.

[0034] Therefore, the invention employs two PSA stages. Gas leaving the second (vacuum) PSA stage is recirculated to the compressor inlet, enriching the feed and allowing the first PSA stage to recover biomethane at > 99.3 % CPU regardless of the CO2, N2 and O2 levels in the raw gas.

[0035] The second PSA stage also captures methane from desorption (regeneration) gases generated during pressure relief and purge. After methane recovery these residual gases, now containing < 2 % CPU, are sent to a catalytic methane-oxidation reactor, where their methane content is reduced to < 5 ppm.

[0036] The invention offers the following technical advantages: it brings biogas to the pressure and composition required by gas-pipeline operators, making it fully equivalent to natural gas for transmission, LNG production and engine fuel; it eliminates methane emissions: the utilization system lowers exhaust-gas methane to < 5 ppm; it re-uses waste heat within the process, reducing auxiliary-energy demand and improving overall energy efficiency; it operates reliably even when raw biogas contains more than 230 ppm H2S.

[0037] These and other features, aspects and advantages of the invention will become better understood with reference to the following drawings, descriptions and claims. BRIEF DESCRIPTION OF DRAWINGS

[0038] The invention is shown by means of an example embodiment on a drawing, wherein Fig. 1 shows a flow diagram of the system.

[0039] DETAILED DESCRIPTION

[0040] The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention.

[0041] Figure 1 schematically illustrates one preferred embodiment of the purification system.

[0042] A continuous stream of landfill biogas (170 Nm3h ') containing 46.28 vol % CFL, 50.63 vol % CO2, 2.07 vol % N2 and 1.70 vol % O2 at 15 °C is first routed through the activated-carbon filter Fl for coarse removal of organic vapors and aerosols. The pre-purified gas is collected in the first buffer tank Zl, which is fitted with a demister. Condensed liquid is discharged to condensate tank SI. The gas from the fist buffer tank Zl is compressed by a compressor Cl from 0.056 mbar(g) to 12 bar(g).

[0043] The hot (~ 80 °C) compressed gas is then sent to an integrated heat-recovery section. In a first heat exchanger El, such as a plate heat exchanger (e.g. a diaphragm heat exchanger) it is heated to 180 °C by the desulphurized gas exiting a second desulphurization reactor R2. Downstream, the partially heated stream passes through a second heat exchanger E2 such as a plate heat exchanger (e.g. a diaphragm heat exchanger), where it absorbs heat from the exhaust of a catalytic methane-oxidation reactor R3 and reaches 200 °C. During plant start-up, when no hot process streams are yet available, a third heat exchanger (E3), such as electric heater, supplies the necessary energy to raise the gas temperature from 80 °C to 200 °C.

[0044] At 200 °C the gas enters the desulphurization train consisting of two zinc-oxide desulphurization reactors R1 and R2. The desulphurization reactors Rl, R2 can be operated singly, in parallel or in series, so that the outlet hydrogen-sulphide and organic-sulphur concentration does not exceed 7 mg m3.

[0045] Leaving the desulphurization section at about 200 °C, the gas is cooled in two stages. In a fourth heat exchanger E4, such as a plate heat exchanger (e.g. a diaphragm heat exchanger), it transfers heat to the return gas from the vacuum PSA (vPS A) stage and cools to 80 °C. It is then further cooled to 40 °C in a fifth heat exchanger E5, preferably an air cooler. A downstream separator S2, equipped with a demister, removes the condensate generated during cooling.

[0046] The cooled, desulphurized biogas (169.4 Nm3h ', 10.4 bar(g), 40 °C) is fed to the first PSA module (adsorbers A1-A4). The adsorber beds, arranged in either parallel or series configuration according to the raw-gas composition, simultaneously dehydrate the gas and adsorb CO2, N2 and O2. Purified biomethane (71.3 Nm3h 99 vol % CH4) is withdrawn from the top of the beds, polished in one of the twin adsorption filters F2A / F2B to > 99 vol % CH4, collected in a second buffer tank Z2 and delivered either to the transmission grid, to downstream CNG / LNG facilities or directly to local consumers.

[0047] Regeneration of adsorbers A1-A4 produces a mixture of return gases (98.1 Nm3h 7.99 vol % CH4, 86.73 vol % CO2, 3.46 vol % N2, 1.73 vol % O2) at about 2 bar(g). These gases enter the second, vacuum PSA stage (adsorbers A5-A8). Here additional methane is recovered on suitably selected sorbents. The vPSA delivers a 7.1 Nm3h1methane-rich stream, which is routed via a third buffer tank Z3 back to the suction of the compressor Cl .

[0048] The residual off-gas from the vPSA contains only 0.86 vol % CH4 (balance mainly CO2) and, after vacuum regeneration at 40 °C, is drawn off by vacuum pump Pl. Before oxidation it is pre-heated to 80 °C in the fourth heat exchanger E4. At the inlet of the methane-oxidation reactor R3 the pre-heated stream is mixed with atmospheric air supplied by fan VI and pre-heated in a seventh heat exchanger E7, preferably a shell-and-tube exchanger, using hot reactor effluent. In the methane-oxidation reactor R3 the trace methane is quantitatively oxidized over a palladium catalyst at 350 °C, reducing its concentration to < 5 ppm.

[0049] The reactor effluent (92.5 Nm3h ', 350 °C) transfers its heat successively to heat exchangers E2 and E7, thereby cooling to ambient temperature before being exhausted to atmosphere.

[0050] The described configuration thus produces pipeline-quality biomethane, maximizes methane recovery and reduces residual methane emissions to negligible levels while efficiently re-using internal heat streams. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. Therefore, the claimed invention as recited in the claims that follow is not limited to the embodiments described herein.

Claims

AMENDED CLAIMS received by the International Bureau on 19 November 2025 (19.11.2025)1. A method for purifying biogas to biomethane, comprising the following successive steps:(a) passing raw biogas through a first buffer tank (Zl) equipped with a demister and an activated-carbon filter (Fl) and compressing the pre-purified gas in a compressor (Cl) to 6-12 bar;(b) raising the temperature of the compressed biogas to 200-300 °C in two heat exchangers (El, E2) arranged in series, wherein the first heat exchanger (El) is heated by a desulphurized gas and the second heat exchanger (E2) is heated by exhaust gases from a methane-oxidation reactor (R3);(c) removing hydrogen sulphide and organic sulphur compounds in one or more desulphurization reactors (Rl, R2) filled with a zinc-oxide (ZnO) sorbent bed at 200- 300 °C to obtain the desulphurized gas;(d) cooling the desulphurized gas downstream of the first heat eachanger (El) in two stages by first passing it through a fourth heat exchanger (E4) to transfer heat to a return gas from a vacuum PSA stage, and then cooling the desulphurized gas to 30- 40 °C in a fifth heat exchanger (E5), while condensing and removing liquid from the desulphurized gas in a separator with a demister (S2) downstream of the fifth heat exchanger (E5);(e) upgrading the cooled gas in a two-stage pressure-swing adsorption (PSA) unit (Al- A8) containing sorbents for CO2, N2 and O2, wherein in the first stage (A1-A4) adsorption is carried out at line pressure, producing biomethane with > 98 vol % CFE, which is then passed through two adsorption filters (F2A, F2B) to polish the methane content to > 99 vol % CH4and collected in a second buffer tank (Z2) for delivery to a gas grid or to a condensate tank (SI), and in the second stage (A5-A8), vacuum pressure-swing adsorption (vPSA) is performed at (-1) to 25 kPa, the methane-rich product being recycled to the suction side of the compressor (Cl); and(f) catalytically oxidizing the residual methane contained in the PSA off-gases in a methane-oxidation reactor (R3) using air, oxygen- enriched air or pure oxygen.

2. A system for purifying biogas to biomethane, comprising, in succession downstream of a raw biogas inlet:(a) a pretreatment unit with a separator and demister (Zl) and an activated-carbon filter (Fi);(b) a compressor (Cl);(c) a desulphurization unit with at least one desulphurization reactor (Rl, R2) filled with a zinc-oxide (ZnO) sorbent bed for removing hydrogen sulphide and organic sulphur compounds to obtain desulphurized gas;(d) a two-stage pressure-swing adsorption (PSA) unit containing sorbents for CO2, N2 and O2, comprising two sub-units each containing at least four adsorbers, the first sub-unit (A1-A4) operating under pressure and the second sub-unit (A5-A8) operating under vacuum, and having an outlet connected to the suction side of the compressor (Cl) for recycling a methane-rich stream;(e) two adsorption filters (F2A, F2B) arranged in parallel downstream of the first PSA sub-unit and upstream of a second buffer tank (Z2) for polishing the biomethane to > 99 vol % CH4;(f) a catalytic methane-oxidation reactor (R3) for oxidizing methane present in the off-gas from the vacuum PSA sub-unit, using air, oxygen-enriched air or pure oxygen;(g) two heat exchangers (El, E2) arranged between the compressor and the desulphurization unit for heating the compressed biogas with (i) the desulphurized gas and (ii) the exhaust gases from the methane-oxidation reactor (R3), respectively;(h) a fourth heat exchanger (E4) in the desulphurized-gas line downstream of the first heat exchanger (El) for reheating the vPSA return gas, followed by a fifth heat exchanger (E5) for cooling the desulphurized gas before the PSA; and(i) a separator with a demister (S2) downstream of the fifth heat exchanger (E5) for condensing and removing liquid from the desulphurized gas prior to the PSA.

3. The system according to claim 2, wherein a third heat exchanger (E3) is installed in the raw-biogas line downstream of the two heat exchangers (El, E2) and upstream of the desulphurization unit.

4. The system according to claim 2, wherein a sixth heat exchanger (E6) is arranged upstream of the methane-oxidation reactor (R3) for heating the combined stream of air, oxygen-enriched air or oxygen and vPSA return gas.[0001][0002]Statement under article 19(1)[0003]Replacement pages which replace the original pages 8-9 of the International application as filed, contain amended claims under Article 19 PCT.[0004]The original claims 1 and 3 are amended.[0005]The original claim 2 is deleted.[0006]Original claims 3, 4, 5 are renumbered accordingly.[0007]Claim 1 has been amended to include:[0008]- two heat exchangers in step (b), to correspond with feature (g) of claim 3[0009]- two-stage cooling and liquid removal in step (d), to correspond with feature (h) of claim 3; - two adsorption filters and second buffer tank in step (e), to correspond with feature (e) of claim 3;[0010]- sulphur compounds defined in step (c): the claim now specifies that the sulphur compounds removed in the ZnO reactor are hydrogen sulphide and organic sulphur compounds. This clarification is supported by the description, which indicates that the ZnO desulfurization reactors reduce “the outlet hydrogen sulphide and organic sulphur concentration” to a very low level.[0011]Claim 3 has been amended to include: - ZnO sorbent bed and sulphur compounds defined in step (c), in line with feature (c) of claim 1 ;[0012]- PSA sorbents in step (d), to correspond with feature (e) of claim 1 ;[0013]- details of the reactor R3 in step (f) to correspond with feature (f) of claim 1. In both claims, the deemed-unclear term “twin filter” has been changed to “two filters”.