Recent Advances in the Removal of Volatile Methylsiloxanes from
|
|
99-330
Peter H. Glus Kit Y. Liang, P.E. Ramon Li, P.E. Richard J. Pope. P.E. ABSTRACT The use of biogas from sewage treatment plants (STPs) and landfills to generate electricity has become an established alternative to conventional fuels. However, trace compounds contained in the biogas, such as sulfides and other organic substances, have been identified as causing deleterious effects to combustion devices and their add-on air pollution control equipment. One class of compounds that has recently been identified as a significant contributor to these deleterious effects is called volatile methylsiloxanes (VMSs). The deleterious effects to combustion devices, such as accelerated engine and turbine wear and fouling of add-on air pollution control devices, have resulted in millions of dollars of financial damage to private industry and municipalities. Various technologies are available to remove VMSs from biogas, such as activated carbon, refrigeration systems, synthetic resins, liquid adsorbants, and membranes. Some of these technologies have seen significant but isolated developmental work. Only the activated carbon, refrigeration, and liquid adsorbent technologies have been used in full scale systems for more than two years. This paper will focus on VMSs, and discuss their origin, how they become entrained in biogas, and the specific problems caused by VMSs when they are oxidized to silica ash. The paper will then review recent developments to remove VMSs from biogas using activated carbon, refrigeration, synthetic resins, liquid adsorbants, and membranes. INTRODUCTION Biogas from STPs and landfills is often used on-site as an alternative to conventional fuels when generating electricity. The predictably high methane component of biogas plus the on-site generation has provided significant cost savings at large municipalities for a number of years. In addition to methane and carbon dioxide, it is also well established that biogas contains trace amounts of sulfides and other organic substances. Some of these trace compounds contained in the biogas have caused deleterious effects to combustion devices that use biogas as fuel. This paper focuses on a class of compounds called volatile methylsiloxanes (VMSs), which at elevated concentrations in the biogas have been identified as causing deleterious effects to IC engines and turbines. This paper will discuss the origin of VMSs, the problems they cause, and a review of recent efforts to remove them from biogas. ORIGINS OF VOLATILE METHYLSILOXANES Volatile methylsiloxanes are low-viscosity silicone fluids used for commercial and personal applications. One typical use for VMSs is as carriers in applications like skin cream and stick deodorants. Two specific VMSs, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) have been identified in the biogas at a number of STP and landfills. Table 1 presents the physico-chemical properties of D4 and D5. D4 and D5 have vapor pressures of 1.7 and 0.3 mmHg at 90oF, the typical temperature of biogas in anaerobic digesters. Because they have relatively high vapor pressures and low water solubilities, they also have high Henry?s Law constants (H). D4 has an H value between 3 and 17, very high for a chemical with a molecular weight of 296.6, which suggests D4 has a strong tendency to move from water to air.1 Due to their widespread commercial and personal use, VMSs become deposited in the environment and a fraction ends up in the influent at STPs. Recent work was performed to trace the distribution of VMSs in a typical STP.2 A fate model was developed and calibrated using two pilot plants and two sewage treatment plants. The model predicted that STPs remove about 82 percent of VMSs from the wastewater, and leave approximately 18 percent in the treated effluent. The majority of the VMS mass removed by the STP was removed in the sewage sludge (approximately 46 percent), with the remaining balance being volatilized. The model predictions suggest the majority of the VMSs being removed in a typical STP become entrained with the sewage sludge. One would expect, then, that when the sewage sludge is heated in anaerobic digesters to approximately 100oF, the VMSs partition to the gas phase and become entrained in the biogas. Test data confirms this, since VMSs have been identified in the biogas of 21 different STP across the country.3 PROBLEMS CAUSED BY VMSs Based on the STP model, the identification of VMSs in the biogas at many STPs across the US, and similar findings in Europe4 it is understood that a fraction of VMSs used in commercial and personal applications ultimately find their way into STP biogas. The problem for most STP operators begins when the biogas containing trace amounts of VMSs is combusted in IC engines and gas turbines. When VMSs become exposed to temperatures higher than 750oF during combustion, they become oxidized and form amorphous silica ash.5 This silica ash generally creates two problems, accelerated wearing of the engine?s physical components, and fouling of add-on air pollution control equipment. Accelerated wearing of engine and turbine components has been observed in at least five different STPs. Typically, on IC engines the slip liners become scratched and the piston crowns become worn. The valves become clogged due to ash buildup and do not seal properly. If the engines have pre-combustion chambers they can also foul. Elevated silicon levels are also observed in the lubrication oils due to the silica being created in the combustion cylinders. The normal maintenance and overhaul intervals of these engines can be seriously shortened. On gas turbines, the silica ash becomes deposited on the turbine blades and clogs the cooling action. This increases the temperature of the blades and sometimes results in premature breakage. Fouling of add-on air pollution control equipment such as oxidative and SCR catalysts has been observed in at least 15 different STPs. Typically, the silica ash formed during combustion is swept with the exhaust into the catalytic unit. The silica appears to attach to the active sites on the catalyst, slowly plating the modules. In some cases noticeable fouling was observed in a matter of hours, with full deactivation taking place in less than 100 hours. The economic consequences of these problems cannot be understated. Based on the authors? estimate, approximately 30 million dollars worth of catalytic units have been fouled by silica and rendered inactive. The engine damage caused by silica and the increased need for overhaul has also certainly cost the industry many more millions of dollars. SILOXANE REMOVAL IN BIOGAS In the following sections, five different technologies have been examined to identify recent developments in the removal of VMSs from biogas. The information presented for each technology was provided by specific vendors and was, in most cases, verified independently by the authors. The inclusion of a specific vendor into this paper is not meant to be an endorsement by the authors nor by Malcolm Pirnie, Inc. Activated Carbon Activated carbon is widely used to remove organic substances from gases and liquids because it has excellent adsorbent properties and a large surface area. In the late 80?s and early 90?s, there were four known pilot scale attempts using activated carbon to remove VMSs from biogas in STPs and landfills. However, none of the earlier attempts were scaled up to a full sized system. One recent project which involved a full sized system was installed at the Carson Ice Plant in Sacramento, CA. The Carson Ice Plant uses digester gas from the nearby Sacramento Regional WWTP. They installed a gas filtration process because the catalytic oxidizers used on the 50 MW gas turbines were degrading rapidly from silica fouling. The gas filtration process was designed by Filtration and Media Group (FMG) of Union Gap, WA (Please Note FMG is now Applied Filter Technology, AFT). The FMG process consists of a series of adsorber vessels containing polymorphous porous graphite (PPG), a specific type of activated carbon. Each bed contains 380 cu. ft. of media, and can process between 175 to 250 MMscf of gas before requiring regeneration, which takes place off-site. The process uses SAGTM technology (segmented activity gradient) developed by Applied Sorbent Technologies (AST) of Bellevue, WA (Please Note AST is now Applied Filter Technology, AFT) for the removal of poorly adsorbed species in the presence of high concentrations of competing organic contaminants in air and gas streams. The FMG installation at the Carson Ice Plant has been operating for approximately 2 years, and the turbines and their catalytic oxidizers have shown no fouling typical of digester gas installations. In addition, FMG has regenerated the PPG beds six times already, and has not reported significant reductions in performance. Overall removal efficiencies of VMS, identified through periodic sampling, are consistently greater than 99 percent. Refrigeration/Condensation Condensation can be used to remove selected compounds by dropping the temperature or pressure of the gas and letting the compound condense to a liquid form so it can settle out. Condensing can be achieved by either refrigeration or through depressurization of a pressurized system. One known condenser system is being operated at a STP in the US; however, it was not designed to remove VMSs and appears to achieve only modest removals. In 1993, the City of Dayton WWTP was experiencing accelerated engine deterioration on each of their three Waukesha 720 KW engine generator sets when they burn digester gas. The engines displayed significant deterioration of cylinder slip liners and heads, and required complete overhauls every three years. Lubricating oils also had a tendency to collect the silica ash, and had to be changed every 500 hours of operation. To mitigate the problem, the City of Dayton WWTP installed a three stage condenser to remove siloxanes and various other deleterious substances. The condenser was designed by Black and Veatch, and cools the digester gas from 130oF to approximately 35oF. According to Mr. Lalit Gupta, Plant Engineer, the installation of the condenser has tripled the time period between oil changes from 500 engine hours to 1500 engine hours and significantly reduced the frequency of engine overhaul. The condenser has been operating since 1995, and the manufacturer estimates that 80 to 90 percent of the siloxanes are removed. However, this estimate has not yet been substantiated through periodic testing. Synthetic Resins Synthetic resins, like activated carbon, can remove VMSs through adsorption onto their surface. They have surface area densities (i.e., surface area to mass ratio) equivalent to activated carbon, and can be specially formulated to remove specific classes of compounds. Synthetic resins have been recently used in a pilot scale project to remove VMSs from biogas at the City of Sunnyvale Water Pollution Control Plant. Sunnyvale was interested in purifying their biogas because they were experiencing accelerated engine wear in their two Caterpillar engine generators and degradation of their catalytic oxidizers used on the exhaust. The pilot scale project, designed by American Purification, Inc., of Newport Beach, CA, was a regenerative vapor recovery system using a polymeric adsorbent. The pilot unit processed 50 cfm of gas, and reported removal of siloxane compounds to non-detectable levels. According to American Purification, regeneration of a full scale unit is accomplished using microwave technology, which provides a uniform heating to approximately 350oF. The regeneration is completed in a vacuum under a nitrogen blanket and into a condensation system to simplify disposal. Liquid Absorbants Liquid absorbants have been used in at least seven installations in Europe to remove organics and siloxane from landfill gas. In the US, liquid absorption is used by a handful of landfill operators to treat biogas prior to use in combustion devices such as gas turbines. One popular liquid absorbent, called SELEXOLҬ is manufactured by Union Carbide. SELEXOLҠis designed to provide CO2, H2S, COS, mercaptan, and BTEX removal from biogasses.6 It is used by a number of landfill operators to remove deleterious substances, including VMSs, from the biogas prior to combustion. One specific company which reportedly has succeeded in removing siloxane is Ecogas, Inc. a division of Getty Synthetic Fuels. Their system uses SELEXOLҠin conjunction with a condenser and activated carbon. Reported removal efficiencies are high, and the processed gas in one case is used at a local university?s cogeneration turbine which has catalytic oxidizers for carbon monoxide and hydrocarbon removal. Membranes Membrane technology has been advancing in recent years, and a number of pilot scale installations have been operated to upgrade biogas to pipeline quality. Notably, a project was undertaken in upstate New York to remove CO2 and organics from digester gas. Although siloxanes were identified in the digester gas they were not tested for removal. Other notable efforts are being taken at the University of Southern California. Based on discussions with W.R. Grace and Monsanto, it is believed that a membrane could be developed which specifically removes siloxane from biogas. However, membranes can be very susceptible to acid deterioration that is sometimes present in biogas. CONCLUSIONS The presence of VMSs in biogas at STPs and landfills is well documented. In addition, the accelerated engine and turbine wear and the fouling of add-on air pollution control equipment caused by VMSs are beginning to be more fully understood. However, the search to find technically and commercially proven solutions to remove VMSs from biogas is still beginning. There are a number of promising technologies which may be viable for VMS removal; however, more closely monitored, problem-free operating hours must be accrued before they could be considered commercially proven. Lastly, it cannot be stressed enough how important it is to share information on problems such as VMSs in biogas. Many STP operators and engineering firms did not find out about the potential problems caused by VMSs in the biogas because the knowledge and data from past problems was being guarded due to contractual and litigation issues. A clearinghouse of information, set up on a password protected Internet site, should be established by interested parties to facilitate this goal. ACKNOWLEDGEMENTS The authors would like to acknowledge the assistance of the following individuals who have greatly facilitated the collection of information used for this paper: Mr. Jeff Wetzel, FMG, Inc.; Mr. Jeff White, Carson Energy; Mr. Lalit Gupta, City of Dayton WWTP; Mr. Tom Mohr, City of Sunnyvale WPCP; Mr. Philip Hodge, American Purification, Inc.; Mr. David Burns, Union Carbide; and Dr. Alex Stern, Syracuse University. REFERENCES
|
