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Capstone Turbine Corporation • 21211 Nordhoff Street • Chatsworth • CA 91311 • USA Application Guide: Landfill/Digester Gas Use with the Capstone MicroTurbine Siloxane (D4) (Octamethylcyclotetrasiloxane) CH3 O Si O CH Si CH CH3 CH Si CH 3333 CH3 O Si O CH3 Figure 3. Chemical Structure for a Siloxane (D4) Molecule Siloxanes have limited water solubility, and they agglomerate in the solids (sludge) fed to digesters at wastewater treatment plants. In the hot environment of digesters, concentrations of volatile siloxanes increase due to the decomposition of silicones and other polymers composed of siloxanes. As a result, the concentration of siloxanes in digester gas is in the measurable ppb (parts per billion) or ppm (parts per million) range. As biogas that contains siloxanes is combusted, the silicon reacts with oxygen to form silicon dioxide (SiO2), a solid white powder commonly known as silica. Sand (quartz) is nearly pure silica. Silica particles are abrasive and have a very high melting temperature. When siloxanes are present in the fuel to a Capstone MicroTurbine, tiny particles of silica form in the combustion section. The silica particles travel with the exhaust gases at very high speeds through the nozzle vanes into the turbine wheel, and then exit through the recuperator and heat exchanger (if installed). Over time, these abrasive particles can cause erosion of some of the metal surfaces they contact, as well as fouling and plugging of heat exchanger surfaces - leading to a gradual increase in fuel consumption and exhaust temperature and a decrease in system efficiency. Additionally, silica may deposit all throughout the combustion section of the MicroTurbine and behind the turbine wheel, which could lead to seizing of the turbine shaft. In other power generating equipment (internal combustion engines and gas turbines) used for landfill gas and digester gas, troublesome silica deposits and erosion have also been found. These deposits are often found on the cylinder heads and rings of internal combustion engines, and on the heat recovery steam generator tubes of gas turbines. Maintenance and rebuild requirements tend to be very high, as shown by unit availability data. It is not uncommon for internal combustion engines at waste water treatment plants to have top-end rebuilds twice a year. 480002 Rev D (October 2011) Page 12 of 14 Capstone reserves the right to change or modify, without notice, the design, specifications, and/or contents of this document without incurring any obligation either with respect to equipment previously sold or in the process of construction.PDF Image | Application Guide Landfill/Digester Gas Use with the Capstone MicroTurbine
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