Method for producing biomethane by purifying biogas from non-hazardous waste storage facilities and facility for implementing the method

Abstract

A method for producing biomethane by purifying biogas from non-hazardous waste storage facilities involves compressing the initial gas flow, introducing the gas flow to be purified into at least one adsorber loaded with adsorbents capable of reversibly adsorbing the VOCs, and subjecting the VOC-depleted gas flow to at least one membrane separation step in order to partially separate the CO.sub.2 and O.sub.2 from the gas flow. The method also involves introducing the retentate from the membrane separation step into at least one adsorber loaded with adsorbents capable of reversibly adsorbing the major portion of the remaining CO.sub.2, subjecting the CO.sub.2-depleted gas flow exiting the adsorber loaded with adsorbents capable of reversibly adsorbing the major portion of the remaining CO.sub.2 to a cryogenic separation step in a distillation column in order to separate the O.sub.2 and N.sub.2 from the gas flow, and recovering the CH.sub.4-rich flow from the cryogenic separation step.

Claims

1. A facility for producing gaseous biomethane by purifying biogas from non-hazardous waste storage facilities (NHWSF), comprising: a compression unit for compressing an initial gas flow of the biogas to be purified; a volatile organic compound (VOC) purification unit arranged downstream of the compression unit to receive the compressed initial flow of the biogas and comprising at least one adsorber loaded with adsorbents capable of reversibly adsorbing VOCs to thereby produce a VOC-depleted gas flow; a membrane separation unit arranged downstream of the VOC purification unit to receive the VOC-depleted gas flow and subject the VOC-depleted gas flow to at least one membrane separation, wherein the membrane separation unit has 1 to 4 membrane stages of which the permeability enables more than 90% of CO.sub.2 and at least 30% of O.sub.2 to be separated from the VOC-depleted gas flow; a CO.sub.2 purification unit arranged downstream of the membrane separation unit to receive a retentate from the membrane separation unit, wherein the CO.sub.2 purification unit comprises at least one adsorber loaded with adsorbents capable of reversibly adsorbing the majority of remaining CO.sub.2 from the retentate to produce a CO.sub.2-depleted gas flow; a cryodistillation unit comprising a heat exchanger and a distillation column, the cryodistillation unit arranged downstream of the CO.sub.2 purification unit to receive the CO.sub.2-depleted gas flow and subject the CO.sub.2-depleted gas flow to a cryogenic separation to separate O.sub.2 and N.sub.2 from the CO.sub.2-depleted gas flow, wherein: the heat exchanger is arranged to receive the CO.sub.2-depleted gas flow from the CO.sub.2 purification unit to cool the CO.sub.2-depleted gas flow, the distillation column is arranged to receive the cooled CO.sub.2-depleted gas flow from the heat exchanger, wherein the distillation column separates the CO.sub.2-depleted gas flow into a liquid CH.sub.4 and a gas distillate, a first pipe connects the heat exchanger and the distillation column for flow of the cooled CO.sub.2-depleted gas flow from the heat exchanger to the distillation column, a second pipe connects the heat exchanger and the distillation column for flow of the liquid CH.sub.4 from the distillation column to the heat exchanger, a third pipe connects the heat exchanger and the distillation column for flow of the gas distillate from the distillation column to the heat exchanger, and the flow the liquid CH.sub.4 and the gas distillate into the heat exchangers cools the CO.sub.2-depleted gas flow by thermal exchange and as a result vaporizes the liquid CH.sub.4 to form the gaseous biomethane.

2. The facility for producing gaseous biomethane by purifying biogas from non-hazardous waste storage facilities (NHWSF) according to claim 1, wherein: a biogas source supplies the initial gas flow of the biogas, the compression unit is capable of compressing the initial gas flow of the biogas to a pressure of from 0.8 MPa to 2.4 MPa, the VOC purification unit comprises two adsorbers loaded with adsorbents capable of reversibly adsorbing the VOCs, the membrane separation unit comprises two separating membrane stages capable of partially separating the CO.sub.2 and the O.sub.2 from the VOC-depleted gas flow, and the CO.sub.2 purification unit comprises two adsorbers loaded with adsorbents capable of reversibly adsorbing the majority of the remaining CO.sub.2 in the retentate.

3. The facility for producing gaseous biomethane by purifying biogas from non-hazardous waste storage facilities (NHWSF) according to claim 1, wherein the adsorber loaded with adsorbents capable of reversibly adsorbing the VOCs is a pressure swing adsorber (PSA) and wherein the adsorber loaded with adsorbents capable of reversibly adsorbing the majority of the remaining CO.sub.2 is a pressure and temperature swing adsorber (PTSA).

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The contemplated embodiments and resulting benefits will become clear from the following example supported by the attached FIGURE.

(2) The FIGURE is a schematic representation of a facility according to a particular embodiment.

DETAILED DESCRIPTION

(3) According to this particular embodiment, the method aims to produce gaseous biomethane while optimizing the energy expenditure as much as possible.

(4) The facility comprises a source of biogas to be treated (1), a drying unit (2), a desulfurization unit (3), a compression unit (4), a VOC purification unit (5), a first CO.sub.2 purification unit (6), a second CO.sub.2 purification unit (7), a cryodistillation unit (8), a liquid nitrogen storage unit (9), an oxidation unit (10) and finally a methane gas recovery unit (11). All the apparatus are connected to each other by pipes.

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

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

(7) The compression unit (4) is in the form of a lubricated screw compressor (18). This compressor compresses the gas flow (17) to a pressure of between 0.8 and 2.4 megapascals (between 8 and 24 bars). The unit further comprises a module (19) for recovering the heat generated by the oil cooling circuit. The flow leaving is shown on the FIGURE by reference (20).

(8) The VOC purification unit (5) comprises 2 PSAs (21, 22). They are loaded with adsorbents specifically selected to allow adsorption of the VOCs, and the later desorption during regeneration. The PSAs function in production and regeneration mode alternately.

(9) In production mode, the PSAs (21, 22) are supplied with gas flow at their lower part. The pipe in which the gas flow (20) circulates splits into two pipes (23, 24), each equipped with a valve (25, 26) and supplying the lower part of the first PSA (21) and the second PSA (22) respectively. The valves (25, 26) will be alternately closed depending on the saturation level of the PSAs. In practice, when the first PSA is saturated with VOCs, valve (25) is closed and valve (26) is opened to start loading the second PSA (22). From the upper part of each of the PSAs leads a pipe (27 and 28) respectively. Each of them splits into 2 pipes (29, 30) and (30, 31) respectively. The VOC-purified flow coming from the first PSA circulates in pipe (29) while the VOC-purified flow coming from the second PSA circulates in pipe (31). The two pipes are joined so as to form a single pipe (51) supplying the CO.sub.2 purification unit (6).

(10) In regeneration mode, the regenerating gas circulates in the pipes (30, 32). It emerges at the lower part of the PSA. Thus, a pipe (33) equipped with a valve (35) leads from the first PSA (21). A pipe (34) equipped with a valve (36) leads from the second PSA (22). Pipes (33, 34) are joined upstream of the valves (35, 36) to form a common pipe (37). This pipe is connected to the oxidation unit (10).

(11) The first CO.sub.2 purification unit (6) combines two membrane separation stages (38, 39). The membranes are selected to enable the separation of around 90% of the CO.sub.2 and around 50% of the O.sub.2.

(12) The permeate loaded with CO.sub.2, O.sub.2 and a very small proportion of CH.sub.4 coming from the first membrane separation is used to regenerate the PSAs (21, 22). It circulates in pipe (40) then alternately in pipes (30, 32) depending on the operating mode of the PSAs. The retentate from the first separation is then directed towards the second membrane separation (39). The permeate from the second membrane separation is recycled by means of a pipe connected to the main circuit upstream of the compressor (18). This step enables a gas (42) with less than 3% CO.sub.2 and with a CH.sub.4 yield greater than 90% to be produced.

(13) The second CO.sub.2 purification unit (7) combines 2 PTSAs (43, 44). They are loaded with zeolite-type adsorbents. They are each connected to pipes according to a model identical to that described previously for the PSAs. They also function according to a production mode or a regeneration mode. In production mode, the gas flow (42) alternately supplies the PTSAs (43, 44) by means of pipes (45, 46) each equipped with a valve (47, 48). The CO.sub.2-purified gas flow from the PTSA (43) then circulates in pipe (49). The CO.sub.2-purified gas flow from the PTSA (44) then circulates in pipe (50). The two pipes (49, 50) are connected to a single pipe (52) connected to the next unit.

(14) In regeneration mode, the regenerating gas circulates in the pipes (53, 54). It emerges in the lower part of the PTSAs. Thus, a pipe (55) equipped with a valve (56) leads from the first PTSA (43). A pipe (57) equipped with a valve (58) leads from the second PTSA (44). Pipes (55, 57) are joined upstream of the valves (56, 58) to form a common pipe (59). This pipe is connected to the methane gas recovery unit (11).

(15) The cryodistillation unit (8) is supplied by the pipe (52) in which the gas to be purified circulates. It contains 3 elements: a heat exchanger (60), a reboiler (61), a distillation column (62).

(16) The exchanger (60) is an aluminum or stainless steel brazed-plate exchanger. It cools the gas flow (52) by thermal exchange with the flow of liquid methane (69) drawn off the distillation column (62). The gas flow (52) is partially liquefied (63). The 2-phase flow (63) ensures reboiling of the vessel reboiler (61) of the column (62) and the heat produced (64) is transferred to the column vessel (62). The flow (63) cools in the reboiler (61) and partially condenses (65). The partially condensed fluid (65) is held by means of a valve (66) at a pressure of between 0.11 and 0.5 megapascals (between 1.1 and 5 bars) absolute. The fluid then in the liquid state (67) is sent to the head of the column (62). The temperature must be greater than 90.7 K to avoid the methane solidifying.

(17) The liquid (67) then separates in the column (62) to form a gas (68) by means of the condenser (71). The cooling of the condenser (71) is ensured by charging with liquid nitrogen coming from an external source (9). The liquid nitrogen is transformed into vaporized nitrogen (72). The gas (68) yields its cold energy in the exchanger (60) on contact with the gas flow (52) coming from the PTSAs (43, 44). The gas flow obtained (70) loaded with CO.sub.2 and O.sub.2 is sent to the oxidation unit (10). In the illustrated embodiment, the gas flow (70) is oxidized in a common oxidation unit (10) with the flow (37) resulting from the regeneration of the PSAs, loaded with CO.sub.2, O.sub.2 and VOCs. Alternatively, oxidation is carried out in separate units.

(18) In another embodiment not shown, the N.sub.2-rich distillate (68) from the cryogenic separation is mixed with the vaporized nitrogen (72) used to cool the head of the column (62) to regenerate the PTSAs.

(19) The liquid (69) from the distillation column vessel (62) is sent to the reboiler (61) where it is partially vaporized. The gas formed (64) is sent to the column vessel (62). The remaining liquid (69) is vaporized in the exchanger (60) to form a pure methane gas (73).

(20) In the embodiment shown, the gas flow (73) is used to regenerate the PTSAs (43, 44). The flow (73) is further preheated thanks to the heat generated by the oil cooling circuit of the compressor (18), which is transferred from the module (19) by means of a pipe (74).

(21) According to the method illustrated, the gaseous methane is collected after regenerating the PTSAs.

(22) Other alternatives of the method may be envisaged, notably that aiming to collect liquid methane directly from the distillation column.