With the increase in siloxane use in consumer products, concentrations of siloxanes are increasing in the waste streams. As a result, levels of siloxanes in biogas from anaerobic digesters and landfills are increasing. When biogas is combusted, the siloxanes are oxidized leaving white deposits on heated metal surfaces (engine components, pistons, cylinder heads, turbine blades, and spark plugs), reducing engine performance.
The elemental compositions of deposits formed during combustion of biogas or landfill gas were investigated using scanning electron microscopy (SEM). The comparison of the types and levels of siloxanes in biogas shows that volatile methyl siloxanes (VMS) present in landfill gas are primarily trimethylsilanol (TMSOH), Octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) as well as small concentrations of hexamethyldisiloxane (L2) and octamethyltrisiloxane (L3). The VMS in biogas from anaerobic digesters are mainly D5, D4 and hexamethylcyclotrisiloxane (D3). It is interesting to note that the landfill conditions promote formation of TMSOH and D4 is present at higher concentrations than D5. On the other hand, in the biogas forming in anaerobic digesters, D5 levels are significantly higher than those for D4.
Relatively high levels of carbon were present in the crystalline deposits forming during the combustion of biogas. Analysis of the deposits showed that carbon present is not in carbonate form but in bound form to the crystal structure. C:O:Si ratios were about 5:7:1 in the deposits obtained from the engines operated with biogas from the anaerobic digesters and 2:3.5:1 in the deposits from the engines operated with landfill gas. In general, the silicon content in the deposits in the facilities operated with biogas from the anaerobic digesters was lower than those operated with biogas from landfills.
Partial funding for this research was provided by Hinkley Center for Solid and Hazardous Waste Management of University of Florida (grant no FOER00010329).
Department of Civil and Environmental Engineering, Florida International University, College of Engineering and Computing, 10155 West Flagler St, Miami, FL 33174, USA
The objectives of this study were to compare composition and morphological characteristics of the deposits forming in engines operated with biogas from anaerobic digesters and from landfills. Engine deposit samples were collected from biogas to energy facilities in Florida, USA. The deposit samples were analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Similarities and differences between the deposit samples collected from the engines operated with biogas from anaerobic digesters and biogas from landfills were analyzed in terms of chemical composition and morphology. The sample obtained from the anaerobic digester gas did not contain potassium while calcium and zinc were present at a higher ratio. Relatively high levels of carbon were present in the crystalline deposits forming during the combustion of biogas. C:O:Si ratios were about 5:7:1 in the deposits obtained from the engines operated with biogas from the anaerobic digesters and 2:3.5:1 in the deposits from the engines operated with landfill gas. In general, the silicon content from deposits found in facilities utilizing biogas from anaerobic digesters was lower than those operated with landfills gas. The difference was made up with higher phosphorus, sulfur and calcium content in the deposits from the engines operated with digester gas.Go To Renewable Energy