Measurement solution for siloxane measurement in land-fill gas via FTIR
What are siloxanes?
The generation of electricity from municipal digesters and land fill is of increasing interest, as the biogas generated is a form of renewable energy. Organosilicon compounds are often present in products such as shampoo and cosmetics, as well as industrial products and lubricants, and they move into land-fill. These volatile compounds find their way into land-fill or digester gases as siloxanes.
Problems caused by siloxanes
Siloxanes are low-level hazards to the atmosphere in terms of their emissions, however when they are combusted in gas engines the hard silica that is produced is very harsh to the moving parts of the gas engine. Ultimately this increase maintenance cost and gives a lower energy output, making the generation of power less efficient. With this ever-growing market, the need for analysis of the siloxane content of the biogas pre-generator is important. A land-fill gas plant operator can determine the amount of siloxane removal of the pre-combustion feed gas. This enables a more cost-effective cleaning system to be employed. They can also determine whether an existing clean-up system is operating effectively.
There is currently no standard method for gaseous siloxane measurement and to date no on-line gas process monitoring system. The usual method of gas analysis has involved the extraction of a sample gas, via bag or cylinder, and analysis off-plant. Sampling losses of siloxanes inevitably occur, as the sample can be adsorbed on to the sample container wall. The common method of analysis is absorbing into a liquid and analysis via GC-MS or GC-FID, but this is not a direct reading of the gaseous sample.
Protea has developed an analytical model for our atmosFIR FTIR gas analyser that allows speciation of siloxanes on-line. The full spectrum technique of FTIR also allows for the measurement of the main gas components, CH4, CO2, NH3 etc. A range of common siloxane and other species found in land-fill gases has been identified, see Table 1. The siloxanes present can vary from different land-fill sites, different sewage plants and other sources, but Protea has found this list to cover most straight and cyclic siloxane compounds present. Trimethylsilanol, while not a siloxane, can lead to siloxane formation and is so included in the analytical model.
Although absorption features of the siloxane groups are similar, there are small changes in response due to the underlying vibrational frequency difference of the molecules. This enables us to “fingerprint” each species of siloxane and generate an accurate analytical model of the biogas process.
The measurement of siloxanes in the Mid-IR spectrum is in the absorption region 850 – 1250cm-1. Here we find the IR absorptions due to the Si-O stretch, with interference from CH4 at a minimum
Protea prides itself on the generation of accurate calibration information, and on incorporating this into an application specific chemometric model for the application. For siloxane analysis we use PLS modelling techniques, which are able to find the subtle differences in our calibration spectra, and then use this information to predict concentrations. Thus allowing speciation and quantification of each siloxane.
In addition we also model the interfering responses due to CH4 and CO2, to allow us to accurately account for the overlapping absorbances of background gases.
Protea’s atmosFIR FTIR analyser can be supplied with the suitable performance specifications for each application. The detection limits for this standard siloxane analysis model are summarised in Table 2. These values are calculated using our standard product. For customers with specific needs these can be improved upon by making modifications to the hardware and data collection parameters. atmosFIR FTIR analyser use DTGS detectors for siloxane measurements. Other FTIR systems employ the temperature cooled MCT variety of detectors. These MCT detectors have a limited IR detection range and cannot detect all the absorption peaks due to siloxanes. In order to analyse spectral absorptions below 950cm-1, MCT detector require cryogenic cooling – not ideal for fixed, hasslefree continuous monitoring.
Resulting data from online runs on land-fill gives the predominant siloxane species in this case to be D4, Octamethylcyclotetrasiloxane, with traces of other siloxanes.
Protea’s continuous monitoring systems can be set-up to read speciated siloxanes or Total Siloxanes, whichever is most required. As well as the siloxane measurement, the quantification of background gases of CH4 and CO2 helps to control and improve the overall biogas process.
With the power of atmosFIR, we can also identify other species present in biogas, such as NH3 and sulphurous compounds such as H2S and Mercaptans. This provides the complete analytical tool for the biogas plant.