As more operators convert landfill gas into electricity or transportation fuel, siloxanes—chemicals common in cosmetics, cleaning products and industrial applications—are becoming more of a headache.
Siloxanes that are landfilled can volatize into biogas. And this can wreak havoc on power-generating equipment.
“When the gas is combusted to make power, siloxanes are converted to silica, which can deposit on the combustion [equipment] surfaces,” says Kenneth Kruszynski, principal at Civil & Environmental Consultants in Lombard, Ill. “These deposits affect the combustion efficiency, emissions and heat transfer capacity of the equipment. When deposits are considerable, abrasion problems can cause severe damage.”
As a result, some operators are looking at how best to deal with siloxanes in order to prevent their problematic effects. As it stands, Jeffrey Pierce, a senior project director with SCS Energy/SCS Engineers, estimates that fewer than 20 percent of power plants have siloxane treatment systems.
“Siloxane treatment systems are absolutely required only if a power plant has post combustion, catalytic air pollution control equipment or employs a turbine with a recuperator,” Pierce says. “And only a few have these appurtences.”
Additionally, turbine and engine manufacturers set maximum limits on landfill gas siloxane concentration. Exceeding a manufacturer’s maximum siloxane limit runs the risk of voiding a warranty or service agreement, says Pierce.
For a boiler, siloxane removal is not generally required, as the powdery silica can be cleaned off the tubes relatively easily. But for reciprocating engines and turbines, silica deposits are harder to remove, driving up equipment maintenance costs.
The Los Angeles County sanitation district runs a Solar Mercury 50 turbine system at its Calabasas landfill gas-to-energy facility in Agoura, Calif. It provides power to the sanitation district’s facilities and to the grid.
“We went with this power-generating system because it has low emissions,” says Mark McDannel, engineer for the L.A. County sanitation district’s solid waste department. “We could burn a lower quality of landfill gas, and get higher efficiency as far as the amount of electricity generated per unit of fuel burned.”
The district has invested in siloxane control.
“In this case, the alternative would have been to destroy the recuperator. We have seen no deposits in the seven years [since installing siloxane control technology],” says McDannel.
Willexa Energy, headquartered in Charlotte, N.C., developed regenerative siloxane reduction technology, which is considered the industry standard. The media to adsorb siloxanes can be reused for about two years.
“Operating costs are a fraction of non-regenerative systems. You pay a little more up front, but the return in investment is short, so you save significantly over the life of the project,” says Brad Huxter, sales agent for Willexa Energy.
The payback is greatest when burning gas in reciprocating engines that drive generators to produce electricity, says Huxter.
“[Operators] never again have to change their oil, change their spark plugs, or do a top-end overhaul because of siloxanes,” he says. “So in terms of siloxane-related operating costs, downtime, and loss of efficiency, we would eliminate it entirely.”
With regenerative technology, heat is driven into the media inside of vessels to release siloxanes and other organic compounds that adsorb to the media.
“After regeneration is complete, the media is cooled with ambient air, and once the vessel gets under 100 degrees Fahrenheit media can adsorb contaminants once more," explains Dave Moniot, president and CEO of Venture Engineering & Construction, a Pittsburgh-based engineering firm and biogas equipment manufacturer. "Contaminants during the regen are flared. But a nonregenerative system can serve its purpose for smaller projects, mainly with gas flows under 1,000 standard cubic feet per minute. In these cases there is often not excess landfill gas available to operate the regen flare and the plant."
While investing in siloxane removal will often eliminate downtime, and lower engine maintenance costs associated with decoking activities, if and how much impact it has often depends on several factors.
Moniot says many plants running internal combustion systems that leverage Venture’s siloxane removal technology save, on average, $695,000 per year, with greatest savings in coastal regions with high siloxane concentrations.
Still, many project operators and developers chose to absorb the higher maintenance costs, except when it is critical due to high siloxane levels, high gas flow rates, and requirements of energy-converting machinery.