Making Sense of Siloxanes
Whether or not you have ever heard of a siloxane, there is no doubt that you have encountered many. They play an important behind-the-scenes role in our daily lives; they have washed our hands, brushed our teeth, cleaned our clothes, driven us to work, helped us lose weight, and even print our newspapers.
Siloxanes are most widely used in the cosmetics industry, adding beneficial qualities such as spreadability, enhanced skin feel, reduction in greasiness, increased absorption, that silky shiny look, and more. Their influence does not stop there though; siloxanes are a popular additive to plastic products since they provide numerous desirable qualities including flexibility, abrasion resistance, and heat resistance. Siloxanes are even used by the food industry as an oil substitute to create low-calorie alternative food products such as potato chips, salad dressings, and mayonnaise.
Though siloxanes provide these numerous desirable qualities, they are a nuisance when it comes to conditioning biogas for use as an alternative energy source. The siloxane name is derived from its components, which consist of a silicon, oxygen, and alkane compound. When siloxane-containing products are disposed of into landfills and anaerobic treatment systems, the lower molecular weight components volatilize into the biogas. The reason this constituent has gained a great deal of attention is that upon combustion, which is required to utilize the biogas, the siloxanes are converted to silicon dioxide, or sand. The sand will deposit within the combustion or post-combustion stages of the equipment, eventually causing reduced efficiency, increased operating costs, and failure. Determining the amount of siloxanes present in the biogas can also be a troubling task, as there are currently no standard methods for the analysis of siloxanes in biogas. As such, there are multiple methods for sampling, analyzing, and reporting siloxanes.
Sampling methods can be divided into two categories: the canister method and adsorption method. The canister method consists of collecting a sample of biogas in a cleaned vessel. It requires relatively cheap materials, has low shipping costs, and sample collection requires only a period of seconds. The volume of sample collected also provides the lab with an adequate sample to repeat the test if necessary. However, the canister method is currently considered inferior to the adsorption method once in the analytical category.
Comparison of Collection Techniques1
|Ease of sampling||Excellent||Poor|
|D4/D5 siloxane recovery||Fair||Excellent|
The adsorption method employs a specific media, either liquid or solid, that collects the siloxanes as the biogas is drawn through the media. The most widely used adsorption media is liquid methanol. Although this method has been successful, it typically produces low results due to the two-stage adsorption required for testing and then analysis, while the methanol is directly used in the analytical stage.
The methanol, impinger method involves a series of two chilled impingers containing 6 ml of methanol each. Biogas is drawn across the impingers at a specific flow rate (recommended at 112 mL/min) for three hours. As the biogas flows through the media, the siloxanes are captured in the media. The reporting limits vary depending on media volume, biogas flow rate, and siloxane concentration in the biogas. The major drawback to this method is the obvious sampling time, but it leads to increased holding time and accuracy as long as the methanol remains chilled.
There are numerous techniques applied to the analysis of siloxanes, all producing varying levels of accuracy and limits. Most techniques utilize a gas chromatograph (GC) with a specific detector. The three most common detectors include the flame ionization detector (FID), atomic emissions detector (AED), and the mass spectrometry (MS), all of which are compared on the chart below. The major problem with any analytical technique is that siloxanes require a very low detection limit, which leads to the requirement of sophisticated methods.
Siloxane Analysis Techniques
|Specific compound detection||Poor||Fair||Excellent|
|Detection of total siloxanes in biogas||Poor||Excellent||Fair|
The siloxane issue becomes even more complicated when it comes to the results. Depending on the sampling and analytical methods used, the accuracy and detection of the results can vary. Not only can the limits vary, but the units in which the lab expresses the results as well. For instance, if the canister method is utilized, it is more common for the results to be presented at parts per billion by volume (ppbv). If the adsorption method is utilized, the results will probably be presented as “mass of silicon per volume of media” (mg/L), which will then have to be converted to the concentration within the gas. The end user, whether it is an IC engine or a power boiler, will present its limits as mass per volume (mg/m3) or even mass per unit energy (ug/Btu). The chart below lists the siloxane limits required by the manufacturers of several commonly used engines.
Manufacturer Siloxane Limits2
There is a fourth alternative for siloxane detection, which involves after-the-fact analysis. Although this method is not necessarily recommended, it can be informative. Data such as this was recently obtained from a municipal facility that operated an IC engine on biogas for one year and only employed moisture removal. The resulting oil analysis indicated that the siloxane levels exceeded the recommended concentrations at certain points in time and also allowed us to identify other constituents within the biogas. The results also showed that even though these specific time periods indicated that the siloxane levels exceeded operating limits, they had caused no damage to the engine itself.
When biogas utilization projects are being evaluated, it is imperative to evaluate the constituents in the biogas, especially siloxanes. Siloxane use in our daily lives is not likely to decrease and, therefore, their impacts should not be ignored. Regardless of the complex testing procedures and results, extensive data is essential to ensure optimal operation and maintenance strategies for successful biogas utilization. Although standard methods for siloxane analysis have not been established, there are a plethora of analytical options available that make preliminary and continuous detection available to all operations.
1. Pierce, Jeffrey and Wheless, Ed. “Siloxanes in Landfill and Digester Gas Update.” Presented at the SWANA 27th LFG Conference, March 2004.
2. Hayes, Heidi et al. “A Summary of Available Analytical Methods for the Determination of Siloxanes in Biogas.” Presented at the 2003 SWANA Conference.
3. Tower, Paul and Wetzel, Jeff. “Making Power Generation Make Sense by Removing Siloxanes from Digester Gas.” Presented at the CWEA Conference, April 2006.
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