ANALYTICAL METHODS FOR THE DETERMINATION OF SILOXANES IN BIOGAS

The importance of measuring siloxanes has increased with the recent growth of the landfill-gas-to-energy (LFGTE) market. Siloxanes and other biogas impurities can form a residue upon combustion in engines that may increase maintenance and reduce engine lifetime. A siloxanes analysis can assist with either determining if a biogas purification system is needed, or if a system is already in place, determining if it is operating properly. To date, various methods have emerged to measure siloxanes involving different sampling techniques and detectors. This paper summarizes the available methods for the determination of siloxanes in air to enable members of the LFTGE industry to select the most appropriate method for their application. This paper presents information not only from method development by Air Toxics Ltd., but also from available technical literature by analytical laboratories, environmental consulting firms, and trade conferences. The specific sample collection techniques to be compared are: • Collecting a whole air sample in a Summa TM canister; and • Drawing sample air through methanol-filled impingers; • Drawing sample air through a charcoal sorbent tube. The specific analytical detectors to be compared are gas chromatography (GC) coupled with a: • Flame ionization detector (FID); • Atomic emission detector (AED); and • Mass spectrometer (MS). This paper specifically compares the performance of collection and analytical methods in terms of: • Ease of sampling; • Collection of a representative sample; • Siloxane recovery; • Detector selectivity and availability; • Reporting limit; and • Calibration techniques . INTRODUCTION Biogas is naturally produced from microbes degrading organic substances in landfills and wastewater treatment plants. One byproduct of this reaction is methane which typically comprises 50 to 60% of the biogas. As an alternative to releasing this greenhouse gas to the environment or simply flaring the gas, plant operators can utilize biogas to generate energy. The high methane content of the biogas provides a medium grade fuel with a heating value ranging from 550 to 680 British Thermal Units per cubic feet of gas (Liang et al., 2002). However, when operators begin to collect landfill and digester gas for energy conversion, trace impurities in the biogas often present an operational challenge. Siloxanes are one of the most difficult trace compounds to control in the biogas. Siloxane compounds are commonly found in commercial and consumer products, such as cosmetics, lotions and detergents. While some siloxanes quickly volatilize into the atmosphere, other less volatile compounds end up in the biogas. Table 1 lists some typical siloxane compounds and their properties. When the biogas is combusted, siloxanes oxidize to form silica. The formation of silica in the combustion engine is problematic since silica is abrasive and also acts as a thermal and electric insulator. As a result, combustion engine performance may be greatly reduced and post combustion catalysts may be less effective (Slezak et al., 2002, Liang et al., 2002). Siloxane analysis of the biogas can assist the plant operator in determining whether a siloxane removal system is TABLE 1. VARIOUS SILOXANES AND THEIR PROPERTIES Compound Abbreviation(s) Molecular Weight Boiling Point (? C) Pentamethyldisiloxane 148 86 Hexamethyldisiloxane L2, MM 162 101 Octamethyltrisiloxane L3, MDM 237 153 Decamethyltetrasiloxane L4 311 194 Dodecamethylpentasiloxane L5 385 230 Hexamethylcyclotrisiloxane D3 222 134 Octamethylcyclotetrasiloxane D4 297 175-176 Decamethylcyclopentasiloxane D5 371 210 Dodecamethylcyclohexasiloxane D6 445 245 needed or whether the removal system is operating correctly. Various analytical methods for the determination of siloxanes have been applied to the analysis of biogas. Sampling techniques typically include collecting a whole air sample using a Summa canister, drawing gas sample through solvent filled impingers, or drawing gas through a sorbent tube. The collected samples can then be analyzed using various instrumentation including gas chromatography coupled with a flame ionization detector (GC/FID), an atomic emission detector (GC/AED), and a mass spectrometer (GC/MS). The performance and applicability of each method is summarized. The differences between two specific methods, impinger GC/MS and canister GC/AED and GC/MS, are discussed by comparing the method results of a real digester gas. SAMPLE COLLECTION TECHNIQUES Canisters: Whole Air Sampling Using the canister collection method, siloxanes present in the gas phase of the biogas are collected as a whole air grab sample. Collection of a biogas in a canister utilizes a clean and evacuated Summa TM canister. The evacuated canister and a particulate filter are attached to the process feed line and the canister valve is opened to fill the canister with sample (Saeed et al., 2002b). After collection, the canister valve is closed and the sample is sent to the laboratory for analysis. Other whole air collection media are available and have been determined to be less suitable than canisters for the collection of siloxanes. Hagmann et al. (1998) concluded that the performance of bags and glass containers was not acceptable. Sample bags utilizing silicone septa and adhesives resulted in high blank levels of siloxanes, while glass containers demonstrated high adsorption of siloxanes