Infrared Quantification of Siloxanes in Synthetic Biogas at Parts-per-Billion Levels in the Presence of Spectral Interferents

Cost effective utilization of renewable biogas requires that the siloxane content be maintained at low parts-per-billion (ppb v ) levels. Infrared (IR) spectrometric methods offer the potential for near-real-time siloxane quantification but are difficult to implement in the presence of certain spectral interferents such as oxygenated volatile organic compounds (VOCs). In this report, a novel two-step gas-stream modification process is implemented in the quantification of siloxanes in biogas by Fourier transform IR spectrometry. The method is demonstrated for synthetic biogas comprising a mixture of methane in nitrogen, with linear (L3) and cyclic (D4) siloxanes present at 100 ppb v levels, and ethanol, acetone, and acetic acid added as model VOCs at 100 parts-per-million (ppm v ) levels. The first step in the process involves sparging the biogas through water to significantly reduce VOC concentrations. The second step removes residual VOCs and the siloxanes by passage of the gas stream over a low temperature metal oxide catalyst. IR spectra acquired after sparging and after passage over the catalyst serve as near-real-time biogas sample and biogas blank spectra, respectively. The biogas blank spectrum and standard siloxane spectra are next used in a simple least squares reconstruction of the biogas sample spectrum for siloxane quantification. Limits of detection for L3 and D4 are determined to be 9.3 and 6.3 ppb v , respectively, while limits of quantification are 31 and 21 ppb v . This method will facilitate the development of simple, inexpensive IR-based devices for on-line and near-real-time monitoring of siloxanes in industrial biogas streams.