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Nearly all digester and landfill gas contains one or more species of siloxanes, a chemical used extensively in industrial products such as lubricants and in personal care products like cosmetics, hair spray and deodorants. These siloxanes make their way into municipal wastewater and ultimately into the methane liberated in municipal digesters. Discarded cosmetics and cosmetic containers introduce siloxanes into the solid refuse that decomposes in landfills, producing methane that is contaminated. The combustion of methane containing siloxanes causes damage that can be quite severe in generator engines. Deposits of solid silica (silicon dioxide) on the engine internals can increase maintenance requirements by a factor of 5 to 10 times. Normally designed to run 20,000 to 40,000 hours between teardowns, many engines are requiring re-builds in as short as 14,000 hours and, in severe cases, after only 2,000 to 4,000 hours. While some engine manufacturers are specifying that the siloxane level in fuel gases fed to their engines should be below 200 ppb (parts per billion), moderate damage has also been experienced at concentrations as low as 50 ppb. SAGTM technology reduces the siloxane level in fuel gases to below 20 ppb. Though there are hundreds of different siloxanes, the most common siloxanes found in landfill and digester biogas are "MM", "MDM", "D3", "D4", "D5", and "D6". Siloxanes MM and MDM are linear molecules and are less volatile than D3, D4, D5, and D6, which are cyclical molecules. MM, D4 and D5 are the most commonly found siloxanes in biogas. Siloxane D3 is difficult to detect and quantify. It is believed to be a transitional molecule and to condense or recombine to form heavier molecular weight siloxanes.
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