Desiccant and Waste Management: Removing siloxanes to extend the life and efficiency of LFG processing equipment.
The Environmental Protection Agency (EPA) reports that in 2014, 6,870 million metric tons of CO2 equivalent greenhouse gases (GHG), or roughly 15,114 billion pounds, were emitted into the atmosphere in the U.S. According to the same report, 11% of these emissions were methane gas. Unfortunately, one pound of methane gas traps roughly 25 times more heat than one pound of CO2. However, methane can be used as a renewable fuel source. In the U.S., a large portion of methane gas emissions are emitted as part of Landfill Gas (LFG) (a mix of carbon dioxide, methane gas, and small amounts of other compounds). There are challenges, however, to effectively converting LFG into clean energy. One such challenge is removing siloxanes from LFG.
First, what are siloxanes?
Siloxanes are non-toxic silicon based compounds frequently used in a wide range of items such as personal care products, plastics, as well as food packaging. As these items breakdown in landfills, siloxanes and many other Volatile Organic Compounds (VOC’s) migrate into the LFG. Though many VOC’s are destroyed when processed through a facility’s Gas Collection and Control System (GCCS), siloxane is not. When combusted in GCCS engines siloxane transforms into crystalline silica (commonly known as quartz, or if amorphous, glass) and adheres to components inside of the engine. Once adhered, the crystalline silica creates an abrasive insulating surface causing excessive wear and heat buildup. Unfortunately, siloxane deposits can affect many parts of an engine, including: spark plugs, heads, valves, pistons, liners, rings, exhaust manifolds, sensors, turbo chargers, waste gate valves, heat recovery units, as well as exhaust stacks. A buildup of deposits in any of these areas leads to unscheduled and shortened maintenance intervals and costs.
What’s the solution to siloxanes?
AGM offers Bulk white (non-indicating) silica gel desiccant that is commonly used for removal of siloxanes from LFG. Silica gel is an amorphous form of silicon dioxide (SiO2), which is found as a naturally occurring substance, or may be manmade as most commercially available silica gels are today. This material may be processed into beaded or granulated form. Much like a sponge, silica gel is highly porous. This characteristic makes silica gel a good mechanical solution to capturing siloxanes from LFG being filtered through it. However, silica gel can become saturated and operate less effectively over time.
Landfill Gas-to-Energy (LFGE) facilities are encouraged to take regular gas samples to monitor siloxane levels both before entering a silica gel vessel as well as after. Effective silica gel may lower a siloxane count from X% to 0.1-0.3%; however, once the reduction of siloxanes between the two samples falls below 40% new desiccant should be considered. Bear in mind that these numbers are not universal and allowable siloxane levels should be decided on by the operator of each separate facility. Refer to an engine manufacturer’s recommended VOC tolerances in order to set a siloxane concentration limit for a GCCS.
Though it is possible to recharge silica gel after saturation, the economic feasibility of doing so is dependent upon the size and purpose of each individual facility. As an alternative, it may be possible to landfill the desiccant after saturation. However, check with local and state authorities before landfilling used bulk white (non-indicating) silica gel or any other desiccant, as there may be other contaminants in the LFG that the desiccant has collected that are controlled by the local or state regulations of your area.
Landfills generate methane gas due to the decomposing of buried refuse. This methane can be used to generate electricity and heat, or sold as a renewable energy source replacing fossil fuels. In order to use this methane, LFGE facilities capture the gas for conversion to energy. LFGE facilities “tap” the landfill, drilling wells into the decomposing mass in order to vent the methane into manifolds and header pipes. The gas then feeds into several filters and a condensate trap before flowing through a “tower” filled with silica gel where it is scrubbed, or filtered, to remove potentially damaging and harmful components, such as siloxanes. After the gas has been scrubbed it continues processing until it is eventually fed into an engine.
Why is it important?
Siloxane removal is essential to the efficient conversion of LFG and methane into a productive and positive renewable resource. Removing siloxanes from LFG before it enters a facility’s micro turbines, combustion turbines, or reciprocating engines extends the life of this equipment and increases the efficiency of landfill gas-to-energy processing facilities. As a result, siloxane removal yields cleaner, more efficient gas for energy conversion, limits reliability on fossil fuels, and reduces GHG emissions.