(554f) Recovery and Purification of Methane in Biogas Using One-Tower Vpsa-Methane with Integrated Pretreatment Adsorbent Layers

From the viewpoints of both energy conservation and alternative energy in the context of high-efficiency power generation, it is very useful to consume highly concentrated methane as a gas-engine fuel, purified from biogas. The authors of the present report designed and fabricated a biogas purifier which is compact and has a low energy power consumption rate. This biogas purifier features one-tower VPSA-methane loaded with pretreatment adsorbent layers (H₂S, Siloxane, and CO₂ from bottom to top). We experimented with bio-gas purification using this one-tower VPSA-methane (inlet gas rate of 400m³N/day, inlet methane concentration of 60vol%), and confirmed the performance thereof in this study as having a methane recovery ratio of 90% and a purified methane concentration of 90vol%. The flow sheet for one-tower VPSA-methane is indicated in Fig.1, while the operating conditions and performance are listed in Table 1.

[Adsorption Stage]

One-tower VPSA-methane consists of an adsorption tower (H₂S adsorbent, Siloxane adsorbent, CO₂ adsorption), a blower/vacuum pump combination rotary mechanism, valves, tubes and pipes, a pressure equalization tank, a product tank, and a raw material tank. The raw material (methane concentration of 60vol%, H₂S concentration of less than 200ppm, Siloxane consisting of 10ppm, and a bio gas rate of 16m³N/h). The raw material (16m³N/h) is supplied at an adsorption pressure of 140kPa-abs and an adsorption period of 60 seconds, such that H₂, Siloxane, and CO₂ are adsorbed. At the outlet, the purified methane (methane concentration of greater than 90vol%, H₂S of less than 1ppm, Siloxane undetected, and a purified 9m methane gas rate of 9m³N/h) is collected.

 

[Desorption Stage]

The adsorption tower saturated with H₂S, Siloxane, and CO₂ (60 second adsorption period) is connected to the blower and vacuum pump combination rotary mechanism and the internal pressure is reduced to the desorption pressure of 10 kPa-abs. The desorption period is set at 60 seconds, i.e., the same as the adsorption period. During the desorption period, the purified methane can be released from the auxiliary tank containing purified methane product to maintain outlet flow continuity. The above-mentioned one-tower VPSA-methane operation is characterized by a purified methane concentration level of 90vol%, a methane recovery ratio of 90%, and an electric power consumption rate of 0.3kWh/m³N-methane. With a 1m³ adsorption tower, purified methane of 173m³N/h can be recovered. We therefore concluded that the one-tower VPSA-methane in this study demonstrated strong economical performance.

Fig.1 Schematic diagram of VPSA-methane

oTable.1 VPSA-methane performance

Biogas composition

CH₄ 70vol%

CO₂ 30vol%

H₂O 4vol%

H₂S 100ppm

Siloxane 10ppm

Biogas flow volume

5~1500m³N/h

CO₂, H₂O, H₂S, CH₃SH,

Siloxane removal

>90%

Methane recovery

>80%

Electric power consumption

1~100kW

Methane recovered

10~100m³N/h

ï¼?Methane purity>95vol%ï¼?

Energy power consumption

0.16~0.2kWh/m³N-CH₄

Energy/Generation ratio

Approx. 4%