This SBIR Phase I project will develop a method that utilizes a carbon based adsorptive surface flow membrane to perform continuous concentration of methane from raw bio-gas from large animal feed operations. The membrane can upgrade biogas at 40 to 65% CH4 to 90%+ CH4 with <10% CO2 at minimal/no pre-compression. Then, the upgraded methane can be used to generate pipeline quality methane, to produce peak time electrical power from on-site gas storage tanks for resale to the grid and/or to generate power or heat for in-house use. The resulting methane can be used on site, supplied via pipeline after being pressurized, or be feed to an on site electrical generating plant.
The broader/commercial impact of the project will be first to reduce the release of the greenhouse gas methane, second to produce a stream of high quality natural gas at a theoretically lower cost than competing technologies.
The primary objective of this project was to demonstrate the feasibility of using carbon molecular sieve (CMS) membranes for the upgradingof biogas generated methane via the removal of CO2. During the Phase I program, we successfully demonstrated that: CMS membranes are suitable for upgrading methane in biogas generated via anaerobic digestion. The membranes show good CO2 permeance, moderate selectivity, and good permeance stability in the presence of contaminants, particularly water. Further, it was possible to achieve >90% methane purity from a synthetic mixture of CO2/CH4 (40/60%) at low pressures (<35psig) and high water relative humidity using a permeate air sweep to enhance the permeation driving force and operating at elevated temperature of ca. 100 to 110?C. The CMS membrane is particularly suited for the moderate temperature operation required to yield stable operational performance. Because of the presence of significant water in the biogas, yielding relative humidities above 85% at 70?C, it is necessary to operate the membrane at temperatures in excess of 100?C to prevent pore condensation of water and hence membrane flux loss. During the Phase I program, we demonstrated that the CMS membrane is stable under these conditions. Purging of the permeate side of the membrane was demonstrated to improve both the recovery and purity of the methane. Due to the relatively low feed side pressure of ca. 30 to 40 psig and the moderate CO2 content (ca. 40 to 50%), it is difficult to deliver high CH4 recovery due to the lack of CO2 driving force. By purging air on the permeate side, the CO2 partial pressure is reduced yielding adequate driving force for CO2 permeation. Further, bypass of CH4 into the permeate purge stream could be recovered via burning for energy recovery, so that in general no methane losses are experienced. Field testing of the CMS membrane in an actual high temperature anaerobic digester was conducted. Preliminary field testing of the CMS membrane for CO2 removal from biogas was conducted during the Phase I program. A three tube CMS bundle was made available for this purpose. The results were consistent with the laboratory synthetic gas mixture testing, although there was a modest step change decay in overall CO2 permeance on initial exposure to the gas stream. It is suspected that this reduction was due to fouling of pores in a distinct size range that are accessible to organic vapor components in the biogas (various carboxylic acids, aldehydes, etc.) Pore condensation of CO2 was demonstrated using the CMS membrane, effectively producing a membrane with potential infinite selectivity capability. Pore condensation of CO2 in the CMS membrane points to the potential to develop a membrane with essentially infinite selectivity by simply blocking all pores to permeation by other gases. Phase I test results show that CO2 pore condensation is occurring at pressures of about 70 to 80 psig at ~1?C and slightly over 100 psig at room temperature. However, several problems were encountered during the Phase I program, including: The CO2/CH4 selectivity, although suitable to deliver CH4 at ca. 90% from anaerobic digestion generated biogas, was relatively low at ca. 10 and was just barely adequate for commercial implementation. At the moment, we are investigating improvements in the membrane performance, via modifying the membrane pore size distribution and better defect control, to deliver pore condensation of CO2 at temperatures above ambient and pressures of ca. 30 to 40 psig. Pore condensation of CO2 would prevent permeation of CH4 and hence is expected to deliver very high and perhaps infinite selectivity. In this case, biogas CH4 purities over 99% would be achievable and practical. Complications at the pilot testing facility. The pilot testing activities conducted at the dairy farm were cut short due to a problem with the farm equipment. A failure in one of the plumbing lines resulted in considerable contamination of our pilot testing equipment and a necessary recall of the equipment to our facility for cleanup and overhaul. Due to the length of time to bring the anaerobic digesters back on-line to steady state gas production, it was deemed infeasible to complete the full battery of pilot testing activities planned for the Phase I program.
