Cleaning Up Landfill Gas: Parker Hannifin’s GES Siloxane Removal System improves quality of landfill gas
by phil burnside
Landfill gas and the gas generated from wastewater treatment plants and other digester environments represent easily recoverable, abundant sources of methane gas. Recovering the energy locked in that gas makes sense economically and environmentally. When converted into useful electrical energy at the point of its release, such gas can serve as a useful and valuable resource rather than just another source of undesirable greenhouseforming vapor. But landfill gas and the biogas generated from wastewater digester systems emerge from deep within the bowels of some nasty stuff. It belches from its source full of contamination and curious chemical surprises in a range of concentrations and capable of undesirable behaviors.
One of the most challenging constituents commonly found in such a mix are siloxanes, a man-made group of compounds consisting of silicone, oxygen and some sort of alkane. In a landfill or biodigester environment, siloxanes volatilize and come off in the produced gas stream along with methane. During combustion, siloxanes break down. The silicon and oxygen molecules break away and recombine as silicon dioxide, which is just another name for common sand. It drops out as an abrasive white powder. When burned in a reciprocating engine or gas turbine, the result is the ongoing deposition of extremely hard silicon dioxide dust at the point of combustion and all points downstream.
Any reciprocating engine operating on biogas containing siloxanes tends to lose efficiency and demand extraordinary maintenance effort. Spark plugs tend to die young. The silicon dioxide contaminates lubricants and results in the need for frequent oil changes. Accumulated deposits result in the need for early and frequent top-end overhauls. Gas turbines fueled by such gas can also foul quickly, losing efficiency and in some cases requiring more frequent takedowns and expensive rebuilds.
Loss of efficiency and potential downtime increase with siloxane load. A treatment process introduced under the Green Energy Solutions (GES) product line by the Purification, Dehydration and Filtration (PDF) Division of Parker Hannifin Corp.’s Filtration Group received a warm welcome from engine and turbine operators in siloxane-laden environments, according to the company.
The PDF Division is headquartered in Lancaster, New York, U.S.A., and was formed from domnick hunter, LLC, which was purchased by Parker in 2006, and Airtek, which was acquired in January 2007. Siloxanes are an extremely useful family of chemicals that present no known health risks and are generally considered safe. They are commonly found in cosmetics, deoderants, defoamers, water repelling windshield coatings, driveway sealants, food additives and some soaps because they help such products smear well and resist cracking.
Siloxane contamination is limited to gas generated from landfills and wastewater digesters because of the siloxanes in the incoming wastewater or trash. Other biogas, such as that from most agricultural activity, does not contain appreciable amounts of siloxane because the incoming waste contains little or none to start with. Parker Hannifin’s GES Siloxane Removal System was introduced in 2007 after many years of research and development, according to Brad Huxter, market development manager for the GES product line. “Our company is regarded as being a market leader in the purification of gases,” said Huxter.
“The emerging biogas-to-energy market had a siloxane problem and there didn’t seem to be a good answer. We were asked if the adsorption technology we had used successfully in other applications could be adapted to remove siloxanes from biogas. The development team pursued development of a reliable and cost-effective siloxane removal system to eliminate siloxane accumulation as a cause of maintenance, downtime and loss of efficiency in biogas-to-energy projects.” The result was a twin tower temperature swing adsorption (TSA) system in which each tower contains a beaded media that has an affinity for siloxane molecules, Huxter said “One of the towers is online adsorbing siloxanes while the other undergoes regeneration then waits to come
Parker’s systems currently range from 8.5 m3/min at 345 mbar and up. “The only thing that limits us in terms of how big a system we can build is what we can ship,” Huxter said. “Our biggest single system is designed for 56.6 m3/min at 345 mbar. As one would expect, the system’s capacity goes up with pressure.” Capacity can also be increased by installing multiple tower pairs, he said. Some may have four or even eight media towers, depending on conditions and gas volume.
Treatment is based on the beaded media blend’s affinity for siloxane molecules. Inside the tower, the beads present an extremely large surface area. As the gas stream passes through and around it, siloxane molecules stick to the surface of the beads like iron filings stick to a magnet. Parker uses a proprietary blend of materials for its media. “What makes our product unique is the composition of our media and our ability to regenerate it automatically on-site so it can be used again and again,” Huxter said.
The regeneration process is straightforward. The system uses heated air to add energy to the exhausted media and that heat breaks the bonds between the beads and the siloxane molecules, which are then carried by the airflow to a flare where they are incinerated. “Our purpose is to provide a means for operators to invest a known amount up front to treat the gas and avoid greater, unpredictable maintenance expenses and loss of efficiency later, over the life of the project,” Huxter said.
“These systems offer a very rapid return on investment,” he said, “especially when you take into account the extended time between spark plug changes, oil changes and topend work. Based upon data gathered at one commercial installation, operators who carefully monitored their maintenance before and after they installed our siloxane removal system showed that they were able to more than triple the amount of time between such maintenance activities,” he said. He indicated that they also saw a marked increase in both engine availability and efficiency.
“What a lot of people don’t take into account is that at the same time we’re reducing maintenance expenses, we’re also increasing the average output of the engines over time,” Huxter said. “They’re producing more energy than they could have with untreated gas because the engines are offline less often for maintenance, and do not lose efficiency over time due to siloxane buildup. That increased production reduces the payback time even more.”
“After carefully researching the biogas industry, we decided that we would not go to market with any kind of siloxane removal system unless we could guarantee two things. Number one, we would guarantee the system would work. Number two, we would guarantee how much it would cost to keep it working.” As a result, Parker guarantees performance of their systems, and offers to maintain that guarantee long term at a fixed cost with an optional fiveyear performance package.
“We visit the customer’s site once a month to inspect the unit, calibrate its operation and pull gas samples, which we then have analyzed by a third-party laboratory to validate that it is working. We also change the media and the filter elements as necessary during the five years to maintain performance. That ability to determine operating costs in advance, combined with a clear performance guarantee, essentially eliminates all risk for the customer in terms of both performance and cost of ownership,” Huxter said.
A new media charge will typically and consistently perform well for about a year and half, Huxter said. The environmental economics of the media itself are very attractive. When the media does eventually need replacement, Parker will run it through an extended regeneration cycle. This normally cleans it enough that it can safely and inexpensively be disposed of in a landfill. As it removes siloxane, the media picks up other compounds, too. While difficult to quantify, the media is known to reduce volatile organic compounds (VOCs) in the gas stream and reduce its moisture as it passes through the tower, which is constructed of 304 stainless steel.
Huxter said that the media will also reduce hydrogen sulfide (H2S) levels to a limited degree, but that H2S removal, if needed, should be accomplished using a separate H2S removal system upstream from the siloxane removal system. Siloxane in gas turbine fuel presents a serious threat, too, according to Huxter. “There’s a misconception that turbines are more susceptible to siloxanerelated problems than engines, but history has shown that’s not really the case. Siloxanes do lower the efficiency of the turbine and in some cases have led to frequent rebuilds. The economics of siloxane removal in both turbine and engine applications are very clear,” he said.
To date, Parker has installed and commissioned over 20 GES Siloxane Removal Systems and is working with numerous global customers on additional systems to be in service this year and in the future. “Every unit that we’ve sold has had a performance guarantee that says we’ll deliver our promised result or we’ll give you your money back. We’ve never had to do that. As far as we are aware, nobody’s ever had a 100% success rate like that with siloxane removal before,” he said.
Source: Diesel & Gas Turbine Worldwide