(587y) Biogas Upgrading Technology With Gas Separating Membrane

Introduction

 In Japan, major gas companies are legally required to utilize 80 % or more of excess biogas from swage treatment plants and so on which can be obtained at fair costs by 2015. Thus, major gas companies are expected to make systematic effort to use biogas both at on-site and off-site.

 In general, biogas from methane fermentation is composed of around 60 vol % methane, 40 vol % carbon dioxide and other trace components. In order to utilize biogas effectively, it is preferable to be upgraded to city gas quality by removing carbon dioxide. Moreover, impurities such as hydrogen sulfide, water vapor and siloxane in biogas are required to be eliminated before gas consumption at various appliances or transport grid injection. Therefore, upgrading biogas technology is very important. 

 Currently, the most popular biogas upgrading technologies are pressurized water scrubbing (PWS), pressure swing adsorption (PSA), organic physical scrubbing and chemical scrubbing. In Japan, PWS and PSA have been commercially adopted. However PWS is limited to large-scale adoption and PSA method has low methane recovery. Membrane separation technology which has been put to practical use in Europe shows high performance in biogas upgrading and great advantage in compactness, which are favorable to smaller scale plants [1] [2]. In addition, capital and operational costs of membrane separators are usually lower than those of other separators. However there are few cases of demonstrating membrane separation for biogas upgrading in Japan It is required to evaluate the possible application capability of membrane separation.

 There are various types of separation membrane, some of which are organic and the others are inorganic. The materials of membrane determine the separation principal. For example, polyimide membrane, which is organic type, uses the difference of gas permeation rate through membrane for gas separation. The permeation rate of carbon dioxide is higher than that of methane permeation rate. Therefore carbon dioxide is exported out of membrane by preferential permeation. Also zeolite membrane, which is inorganic type, has molecular sieving effect and it uses the difference of molecular size for gas separation.

 The objectives of this study are to evaluate the capability of an organic membrane for biogas upgrading.

Experimental

 Polyimide hollow fiber membrane modules were set to a small-scale gas separation test unit. Biogas from methane fermentation of food wastes was desulfurized and collected in a gasholder before being supplied to the test unit as feed gas. The biogas was consisted of 55 to 60 vol % methane and 40 to 45 vol % carbon dioxide and it was supplied to the membrane modules under a condition of the feed gas pressure of 0.6－0.9 MPaG. The composition of the upgraded gas and the permeate gas were analyzed with mass spectrometer to evaluate the methane recovery rate of the membrane module.

 Also simulated gas of about 60% methane, 40% carbon dioxide and 600 ppm hydrogen sulfide was supplied to investigate the effect of impurities on the upgrading performance.

Results and discussion

 The concentration of methane in upgraded gas was 98 % and methane recovery rate was 80% under a condition of the feed gas pressure of 0.9 MPaG and the upgraded gas flow rate of 40 liter per minute. When the permeate gas was recirculated to the feed gas and upgraded again, the methane recovery rate increased to 94 %. It was confirmed that polyimide membrane has high capability for updating biogas by recirculating permeate gas to feed gas.  The operational conditions and the upgraded gas composition were stable during two hours test.

 The dew point of the upgraded gas was about −40 ºC when the pretreated biogas of −19 ºC dew point was fed to the test unit. This corresponds to 800−850 ppm water vapor removal.

 In the test with simulated biogas of 600 ppm hydrogen sulfide, the removal rate of hydrogen sulfide with the membrane was 31 % under a condition of the feed gas pressure of 0.9 MPaG and the upgraded gas flow rate of 21 liter per minute, which gave longer gas retention time in the hollow fiber than that of the previous test with biogas previously described. Despite the longer gas retention time, the removal rate of hydrogen sulfide was 31 %. It was found to be hard to remove hydrogen sulfide sufficiently only with polyimide membrane.

 These results showed high capability of polyimide hollow membrane for upgrading biogas, though desulufration has to be well considered for process design.

Conclusion

・A small-scale biogas upgrading test with polyimide hollow fiber membrane was performed.

・Polyimide hollow fiber membrane showed high capability for biogas upgrading. The methane concentration in the upgraded gas was 98 % and the methane recovery rate was 94% when the permeate gas was recirculated to the feed gas.

・800−850 ppm water vapor was removed from the feed gas of −19 ºC dew point with polyimide membrane. The dew point of the upgraded gas was −40 ºC.

・Hydrogen sulfide was not removed sufficiently from the feed gas with polyimide membrane. Desulfuration should be well considered for biogas upgrading process design.

・A long-term test will be performed to evaluate the stability of membrane separation and the durability against impurities such as water vapor, hydrogen sulfide and so on.

References

[1] A. Makaruk, M. Miltner, M. Harasek, “Membrane biogas upgrading processes for the production of natural gas substitute”, Separation and Purification Technology, Vol. 74, Issue 1, P. 83-92 (2010).

[2]A. Petersson, A. Wellinger, “Biogad upgrading technologies developments and innovations”, IEA Bioenergy.