Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.
Issue link: http://wateronline.epubxp.com/i/694011
postconsumer food waste, waste cooking oil, and grease trap waste to produce pipeline-quality bio-methane. Typical composition of the raw biogas and the bio-methane produced in the field tests are provided in Table 1. The sulfur in the raw biogas was typically around 1,000 to 1,500 ppm H 2 S with trace amounts of organic sulfur compounds. SulfaTrap-R7 desulfurization sorbent removed the sulfur compounds to less than 0.25 parts per million by volume (ppmv). Initially, breakthrough tests were carried out with the CO 2 sorbent beds in the field using desulfurized food waste-derived biogas to measure the capacity of the saturated VSA adsorbent bed, which were above 4.4 weight percent (wt%) CO 2 . The VSA cycles were optimized in the field, and the optimized VSA cycle scheme was used to produce high-purity bio-methane with methane recovery greater than 90 percent. VSA cycle schemes with both feed-end and product-end pressurizations provided working capacities in excess of 2.8 wt% and the CO 2 concentration in the bio-methane product was reduced to less than 0.5 percent by volume. The dew point of the biogas was reduced from 10° to 15°C to less than -35°C, providing essentially a dry bio-methane product. Figure 5 shows the methane purity of the bio-methane as measured by an IR-based methane analyzer. The biogas purification system was operated for a total of 50 hours, purifying more than 4,000 standard cubic feet (scf ) of biogas to produce bio-methane with greater than 90 percent methane recovery. Economic Evaluation A VSA unit was designed that is sized to process 1,000 m 3 /day of biogas with a composition of 60 percent CH 4 , 40 percent CO 2 (on dry basis), and saturated amount of moisture at 24°C. The vacuum power requirement was estimated to be 7.3 kilowatt-electric (kWe), the sorbent bed size to be 336 L/bed, the operating power cost was $0.04 per m 3 CH 4 produced, and the total operating cost including the sorbent replacement cost was $0.07 per m 3 CH 4 produced with a methane purity and recovery of 99.5 percent and 80.3 percent, respectively. The methane recovery can be further increased to 90 percent or above by relaxing the methane purity to 96+ percent. Conclusions Anaerobic digestion of both pre- and postconsumer food waste, waste cooking oil, and grease trap waste can be converted to biogas. This biogas can be further purified and converted to bio-methane, which contains more than 95 percent methane. Bio-methane can then be used for transportation purposes or to generate combined heat and electricity using fuel cells. A major challenge is cost- effectively purifying biogas, while simultaneously minimizing energy requirements. Several contaminants must be removed, and CO 2 and others inerts reduced, to produce a higher-quality fuel that contains more than 90 percent methane (bio-methane). The piloting of an innovative biogas purification system at the USAFA has successfully demonstrated a very effective sorbent- based sulfur removal and VSA system for the purification of biogas streams. The pilot enabled the optimization of VSA system performance and demonstrated the sorbent performance in both bench-scale and pilot-scale vacuum swing systems operating on simulated and real biogas derived from food wastes. The pilot-scale unit processed more than 4,000 scf of actual food waste-derived biogas to produce bio-methane with greater than 90 percent methane recovery. The total operating cost for a 1,000 m 3 /day bio-methane production was estimated to be $0.07 per m 3 of bio- methane produced including the vacuum pump power and sorbent replacement cost. Acknowledgement This research was conducted with support from the Department of Defense Environmental Security Technology Certification Program (ESTCP) as Project ER-200933 under Contract W912HQ- 10-C-0001 from the U.S. Army Corps of Engineers Humphreys Engineer Center Support Activity. Additional funding was received from the Water Environment Research Foundation (WERF) under project number ENER14R14, which is gratefully acknowledged. Further information is available at https://www.serdp-estcp.org/ Program-Areas/Environmental-Restoration/ER-200933. n 30 wateronline.com n Water Innovations BIOSOLIDS&RESIDUALS; Patrick Evans, PhD, is a VP with CDM Smith and has 30 years of experience in environmental remediation, wastewater and drinking water treatment, and renewable energy. He specializes in research, development, and demonstration of innovative technologies in these areas with a focus on chemical engineering and environmen- tal microbiology. He received his PhD in chemical engineering from the University of Michigan and conducted a postdoctoral fellowship in environmental microbiology at the New York University Medical Center. He has served as the principal or co-principal Investigator on numerous research projects funded by the Department of Defense SERDP/ESTCP, the Air Force Civil Engineer Center, the Water Research Foundation, and the Advanced Research Projects Agency-Energy. About The Author Figure 5. Biogas purification system performance under actual biogas at USAFA (Colorado Springs, CO) showing the high-purity bio-methane production