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By Patrick J. Evans with Gokhan Alptekin, Ambal Jayaraman, and Michael Stevens F ood is the largest component of municipal solid waste (21 percent), and currently innovative processes are being developed that divert food waste from landfills to recover this valuable resource. Anaerobic digestion is an effective process where food wastes including 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. Biogas is frequently produced by anaerobic digestion at municipal wastewater treatment facilities and at wastewater treatment plants for the food and beverage industry. Biogas is the result of decomposition of organic wastes, but the methane is diluted with large amounts of carbon dioxide (greater than 30 percent), and it therefore possesses less energy per unit volume than pipeline methane (natural gas). In addition to carbon dioxide (CO 2 ) and methane (CH 4 ), the biogas generated in the digesters and fermentation units also contains moisture at saturation and various trace contaminants such as sulfur compounds (e.g., hydrogen sulfide) and siloxanes. These contaminants must be removed and CO 2 and other inerts reduced to produce a higher-quality fuel that contains more than 90 percent methane (bio-methane). Biogas Purification Challenges Although various adsorbents or solvent systems are available to remove hydrogen sulfide (H 2 S), the most common form of sulfur in the biogas, the biogas also contains a wide range of organic sulfur compounds, from mercaptans to higher-molecular-weight disulfides. Unfortunately, the conventional desulfurization systems do very little to remove the organic sulfur compounds, particularly the disulfides. The conventional sorption systems such as iron sponge also have disadvantages with respect to safety and material handling. Another class of compounds present in biogas are siloxanes. Siloxanes are generated during anaerobic digestion of waste-activated sludge that concentrates silicone-based personal hygiene, healthcare, and industrial products. Siloxanes must be removed from biogas prior to use as an energy source. Biogas Purification System Description A low-cost, two-stage, complete biogas purification system has been developed by TDA Research in Wheat Ridge, Colorado, that removes various contaminants, such as inorganic sulfur, organic sulfur, siloxanes, CO 2 , and moisture to produce greater than 95 percent bio-methane. The purification system was recently demonstrated in biogas derived from anaerobic digestion of food wastes at the U.S. Air Force Academy (USAFA) in Colorado Springs, Colorado, to demonstrate mono-digestion of food waste; fats, oil, and grease (FOG); solids reduction; and stable bio- methane production. Two replicate digesters were operated for nearly one year, and a mixture of food waste and canola oil was fed to the digesters at various organic loading rates. The first stage is for sulfur removal and is based on a low-cost, high-capacity, and expendable sorbent called SulfaTrap™ that simultaneously removed sulfur and siloxane down to ppb levels. The second stage is a vacuum swing adsorption (VSA) system based on a regenerable mesoporous carbon media modified with surface functional groups to reduce the CO 2 and H 2 O concentration in the biogas to pipeline specifications. Figure 1 shows the two-stage biogas purification process to bio-methane. The second stage is for CO 2 and moisture rejection and is based on a VSA system that uses TDA Research's proprietary CO 2 adsorbent to reduce the CO 2 and other inerts in the biogas to less than 5 percent. The approach is similar to the pressure swing adsorption (PSA) and VSA systems that have been successfully used for years in small- to medium-scale air separation processes to produce very high-purity oxygen. A simple vacuum swing cycle consists of three steps. The adsorption of CO 2 from the biogas stream is carried out at the biogas delivery pressure (about 1.3 absolute atmospheric pressure [atm]), while the sorbent is regenerated and CO 2 recovered under vacuum (at about 0.2 absolute atm). The bed is subsequently pressurized with the feed (biogas) gas. The methane loss from the system is reduced by using intermediate pressure equalization steps between the main adsorption and regeneration portions of the cycle. The methane loss with the full vacuum swing cycle is minimal (i.e., less than 10 percent). Bench-Scale Tests The CO 2 sorbent's performance was demonstrated in a bench- scale, two-bed vacuum swing system (Figure 2). This system is capable of counter-current adsorption and desorption operation simulating the VSA operation expected in the full-scale system. In this system, the desired gas mixtures (CH 4 and CO 2 ) are directed into a bench-scale reactor that contains the sorbent. All gas flows are controlled with electronic mass flow controllers. An in-line 28 wateronline.com n Water Innovations Energy Boost: From Biogas To Bio-Methane A pilot project demonstrates how biogas purification yields better fuel in the form of high-purity bio-methane. Figure 1. Two-stage biogas purification process to bio-methane