While not considered a human health hazard, siloxane residues can reduce LFGTE engine life and performance.

By Guy J. Graening

Few operators of wastewater treatment facilities and landfills have heard of siloxanes because they are not odiferous or hazardous to the environment (Dow Corning, 1997). When operators begin to collect the sewage digester gas or landfill gas for energy applications, however, siloxanes emerge as one of the most difficult contaminants to control in the biogas.

The problem generally appears when the biogas is used to fuel internal combustion engines or turbines that drive electrical generators. Although the biogas is primarily methane and carbon dioxide, it frequently contains trace impurities that form a residue upon combustion (Dow Corning, 1999). The combustion residues can contain calcium, sulfur, zinc, and silicon compounds and are a primary contributor to reduced engine lifetime (Niemann, 1997). The presence of silicon in the residue combined with the volatility of some siloxanes has prompted increased emphasis on improved techniques for sample collection and analytical detection of siloxanes (Niemann, 1997). This article introduces the reader to sampling and analysis of siloxanes by describing the specifications in Air Toxics Ltd. (ATL) Method @71 (Air Toxics Ltd., 2002; Saeed, 2002).

What Are Siloxanes?

Siloxanes are a family of organic compounds containing chains of silicon, oxygen, and methyl groups. Siloxanes are manufactured in a wide variety of forms, including low- to high-viscosity fluids, gums, elastomers, and resins (Dow Corning, 1997). Most analytical methods apply to the determination of selected volatile methylsiloxane (VMS) compounds of low viscosity and low molecular weight (i.e., less than 450). These VMS compounds can be referred to in shorthand notation consisting of M or D to represent their complex linear or cyclical structures of silicon, oxygen, and methyl groups attached to the silicon. As shown in Figure 1, the M indicates three methyl groups surrounding a silicon atom at the end of a linear siloxane chain. Similarly, a D indicates two methyl groups attached to a silicon atom in the middle of a linear (or cyclical) siloxane chain. Following this nomenclature, the linear structure of octamethyltrisiloxane can be represented as MDM (see Figure 2) and the cyclical structure of octamethylcyclotetrasiloxane can be represented as D₄ (see Figure 3).

Siloxanes are frequently found in such commercial and consumer products as detergents, shampoos, deodorants, and cosmetics. Most siloxanes volatize quickly into the atmosphere and eventually degrade into carbon dioxide, silica, and water (Dow Corning, 1997). Some siloxanes, however, end up in wastewater or landfills when consumers rinse or discard products.

How Do You Collect a Biogas Sample?

There are several techniques for collecting gas phase samples from the biogas collection and energy conversion system. It can be collected as a whole-air sample with a Summa canister that is evacuated prior to sampling or drawn through a sorbent tube or impinger so that the siloxanes are concentrated in the sorbent media or impinger solution. Note that these sample techniques differ from whole-air sampling in that the matrix (i.e., mostly methane and carbon dioxide) is not collected. ATL Method @71 specifies collecting the biogas sample with a series of two midget impingers containing methanol (see Table 1). Siloxanes present in the air phase dissolve in the chilled methanol solution and are subsequently capped and kept chilled until analysis. The suggested media hold time is 30 days, and the suggested sample hold time until analysis is 21 days.

Collect the sample by attaching inert, flexible tubing from the source air stream to the inlet of the first impinger (see Figure 4). Additional tubing connects the outlet of the first impinger to the inlet of the second impinger, and both impingers are chilled in an ice bath. If the source is not under pressure, a low-volume pump can supply the vacuum required to draw the sample though the impingers.

Figure 4. Impingers in Ice Bath and Rotameter With Built-In Needle Valve

A needle valve and rotameter can be used to adjust and measure the flow rate of sample through the impingers. The user must determine optimum sampling rate and volume to achieve the data quality objectives of the sampling program. Sampling rates from 100 to 1,000 mL/min. are appropriate as long as there is not a significant loss of impinger solution. The amount of sample air drawn through the impingers and the amount of methanol in the impinger determine the final reporting limit concentration. The more sample air drawn through the impingers equals more target constituent concentrated in the solution and thus lower reporting limits. Be careful not to oversample and saturate the solution. Less impinger solution means lower reporting limits but also less capacity to dissolve the target constituents. ATL Method @71 suggests filling each impinger with 6 mL of methanol and sampling at a flow rate of 112 mL/min. for 180 minutes. This arrangement results in a sampling volume of approximately 20 L.

How Do You Analyze Siloxanes?

There are several techniques for analyzing a siloxanes sample, including gas chromatography (GC) coupled with either a flame ionization detector, a mass spectrometry (MS) detector, or an atomic emission detector. ATL Method @71 specifies analysis of siloxanes dissolved in methanol using GC/MS instrumentation. Analytical details of the method, including instrument operating procedures and quality-control criteria, follow SW-846 Method 8000B protocol and are summarized in Table 2 (US Environmental Protection Agency, 1996).

ATL Method @71 involves injecting a 1 µL aliquot of sample from the impinger vial directly into the GC column and analyzing by a MS detector in the full scan mode. Five siloxanes are speciated in a calibration range from the reporting limit of 1 µG/mL (i.e., the low point on the curve) to an upper limit of 160 µG/mL. A method detection limit (MDL) study is conducted annually per the Code of Regulations Title 40. The initial calibration (ICAL) involves a minimum of five points, a second source check, and reporting-limit verification. Every 12-hour period of operation involves a tuning check of the GC/MS system, a continuing calibration verification (CCV), and a laboratory control spike (LCS). Laboratory blanks are run before samples are analyzed, and a duplicate sample analysis is performed on 10% of the samples. The addition of three internal standards and one surrogate complete the rigorous quality control for the ATL method.

Interferences to the method generally include high levels of water or hydrocarbons. Siloxanes can hydrolyze in the presence of water to form new compounds. High concentrations of hydrocarbons in samples might cause matrix interference and can result in sample dilution.

Compound List

 ATL Method @71 targets five siloxanes at reporting limits shown in Table 3. Siloxane compounds D₄, D₅, and MDM are included because they are commonly used in personal care products and have been detected in landfill gas streams (Niemann, 1997). The second column in Table 3 provides the method reporting limit expressed in µG/mL (i.e., mass of compound detected per unit volume of methanol). The third column provides an example of a final reporting limit concentration expressed in µG/L (i.e., mass of compound detected per volume of air sampled). This particular example involves drawing 20 L of air sample through two impingers, each filled with 6 mL of methanol. The resulting concentration of siloxanes that could be detected in each impinger separately would be 0.3 µG/L (or from 16 to 49 parts per billion by volume [ppbv]).

Where Has ATL Method @71 Been Applied?

ATL Method @71 is appropriate for analysis of common VMS compounds in methane gas sources such as landfill gas or digester gas. The method is especially useful for applications where the biogas is purified and subsequently converted to energy (e.g., internal combustion engines or turbines). Speciation of five siloxanes in the ppbv range assists with determining the efficiency of purification filters and processes. To date, the method has been used for more than 80 projects in 19 states (Arizona, California, Hawaii, Illinois, Indiana, Massachusetts, Michigan, Nevada, New Jersey, New York, Ohio, Oregon, Pennsylvania, Utah, Vermont, Virginia, Washington, Wisconsin, Wyoming) and in Canada, Italy, and Hong Kong. The list of clients spans several market segments, including waste management (wastewater treatment and landfill), utility (private and public), and manufacturing (engine and turbine).

References

Dow Corning, Environmental Information Updates. "An Overview of Volatile Methylsiloxane (VMS) Fluids in the Environment." May 1997.

Dow Corning, Environmental Information Updates. "Organosilicon Compounds in Biogas." November 1999.

Niemann, M. "Characterization of SI Compounds in Landfill Gas." 20^th Annual SWANA Landfill Gas Symposium, Monterey, CA. March 1997.

Air Toxics Ltd. "Siloxanes in Air by GC/MS Direct Inject Analysis." Standard Operating Procedures, SOP #71. Revision 0. March 2002.

Saeed, S. "Determination of Siloxanes in Air Using Methanol-Filled Impingers and Analyzed by Gas Chromatography/Mass Spectrometry (GC/MS)." 1^st Annual GTI Natural Gas Technologies Conference, Orlando, FL. September 2002.

US Environmental Protection Agency, Center for Environmental Research Information, Office of Research and Development. "Method 8000B Determinative Chromatographic Separations." Revision 2. December 1996.

Guy J. Graening, P.E., is business development manager with Air Toxics Ltd. in Folsom, CA.

MSW - March/April 2003