Water Online

July 2016

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.

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sparger is used to introduce moisture at 100 percent relative humidity in the biogas. After mixing in a manifold, the feed gas mixture is then directed into the reactor. A valve system allows the gases to bypass the reactor and flow directly to the analytical system for accurate measurement of the feed gas composition as needed. The sorbent reactor consists of a 1.5" outside-diameter spring- loaded stainless reactor. One hundred grams of sorbent particles in the 8-20 mesh size are loaded into the reactor for testing. The reactor is spring-loaded and has a length/diameter (L/D) ratio of 8 with a bed volume of 100 milliliters (ml). The reactor has three thermocouple ports to monitor the sorbent bed temperature. A back pressure regulator is used to control the adsorption pressure. After exiting the reactor, the CO 2 and CH 4 content of the stream are monitored by an on-line NOVA multi-gas analyzer and Vaisala CO 2 and humidity probes. Continuous analysis of CO 2 allows the monitoring of breakthrough gas concentrations and measurement of total CO 2 adsorption capacity. The desorption line is equipped with a BOC Edwards scroll (oil-free) vacuum pump. The pump can easily reach vacuums of less than 1 pound per square inch absolute (psia). The apparatus is fully automated using a control system from Opto 22 Corporation and can run without an operator for long periods of time, including overnight. The control system controls the test conditions, logs the analytical data, and also safely shuts down the apparatus in case of a malfunction. A simulated biogas composition of 60 percent CH 4 and 40 percent CO 2 on a dry basis were used for the bench-scale evaluations (water content was 3 percent by volume). In these bench-scale tests, the life of the sorbent was demonstrated for over 2,900 cycles without any loss in performance, and the sorbent beds produced high-purity methane above 99 percent. Figure 3 shows the results from these bench-scale tests. Pilot-Scale Tests A pilot-scale, fully automated, VSA-based carbon dioxide and moisture removal system for biogas was designed and fabricated. This system is part of the biogas purification subsystem and is installed downstream of the SulfaTrap desulfurization system and a biogas storage sphere. The storage sphere was used to store biogas and feed the carbon dioxide and moisture removal system. It can achieve greater than 95 percent methane (CH 4 ) purity in the product gas with greater than 90 percent methane recovery, reducing the inerts to less than 3 percent (i.e., combined nitrogen [N 2 ] and CO 2 ) in the product gas and a moisture content lower than 7 lbs/millions of standard cubic feet (MMscf ). The system was designed and fabricated for operation in a Class 1 Division 1 environment and is skid-mounted inside a NEMA 4 enclosure equipped with a purge system and is rated for installation in an outdoor environment. Figure 4 shows a picture of the system after fabrication. The purification system was demonstrated in conjunction with a food waste anaerobic digestion study conducted at the USAFA. This particular test site was selected due to the plentiful supply of food and grease trap waste. The pilot-scale biogas purification system was installed and tested with biogas generated via anaerobic digestion of a variety of food wastes, including pre- and wateronline.com n Water Innovations 29 BIOSOLIDS&RESIDUALS Figure 4. Picture of the pilot scale VSA system for CO 2 and moisture removal from biogas Figure 2. Bench-scale, two-bed VSA system Figure 3. Bench-scale tests in a two-bed vacuum swing cycling system. CH 4 = 60 percent, CO 2 = 40 percent, (dry basis), H 2 O = saturation at 22°C, space velocity = 125 hour -1 ; T = ambient, parallel and distributed simulation (P ads ) = 19.0 psia, parallel discrete event simulation (P des ) = 0.2 psia, L/D = 8. Table 1. Typical composition of raw, sweetened biogas and bio-methane from food wastes during field tests Sample Date: Raw biogas 7/16/2014 Sweetened biogas 7/16/2014 Bio-methane 7/16/2014 CH 4 64.40 61.70 96.35 CO 2 34.80 36.00 2.03 N 2 0.60 1.66 1.11 O 2 /Ar 0.23 0.67 0.52

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