Ocorrência de siloxanos no biogás de reatores UASB e avaliação de procedimentos e parâmetros de amostragem

Abstract

The biogas generated in Sewage Treatment Plants (STPs) contains, among other components, siloxanes, compounds present in various products, such as cosmetics and personal hygiene items. During the combustion of biogas, siloxanes are transformed into silicon dioxides, which cause abrasion of the moving parts of the power generation system. In this sense, the energy use of biogas depends on meeting the siloxane limits required by manufacturers of energy conversion equipment and/or by the Brazilian Oil Agency Resolution no. 685/2017. However, the removal of siloxanes from biogas is a recent requirement in Brazil, and there are very few studies that have evaluated its concentration in biogas generated in STPs, mainly in UASB reactors. At the same time, the application of the sampling methodology with impingers (gas bubblers), is not yet fully consolidated in terms of analytical robustness in Brazil. Therefore, the present research aimed to verify the occurrence of eight cyclic and linear siloxanes in biogas from UASB reactors and evaluate procedures and sampling parameters using impingers. For this, the concentration of siloxanes in the biogas from the UASB reactor was monitored, evaluating the variation in the concentration of these compounds throughout the day and between days of the week and the effect of the sampling point (in a reservoir or in the gas line) on the representativeness of the sample in terms of the average daily concentration of siloxanes. Additionally, analytical instrumentation for solvent partition sampling using crimp-type flasks was developed and evaluated. Finally, sampling with impingers using three solvents (methanol, acetone and acetonitrile) was tested, seeking to increase the recovery of the compounds in the gaseous matrices. The results indicated that, in terms of qualitative composition of siloxane, the biogas from UASB reactors and from sludge digesters are similar, with D5 being the most abundant siloxane, followed by D4. However, in quantitative terms, the median concentration of total siloxanes in the biogas from the UASB reactor was more than 2.5 times lower than the medians found in the international literature for sludge digesters. A significant difference was verified for the results of the biogas samples collected at different times of the day, while the results of the samples collected during weekends and weekdays were not different in terms of average concentration of siloxanes. The representativeness of the sample, in terms of the average daily concentration of siloxanes, was not affected by the sampling point when considering the experimental conditions used in this study. Sampling siloxanes directly in the crimp-type flasks presented a performance equal to or superior to that of sampling with impingers, thus allowing the increase of safety, simplicity and economy of the procedure. Additionally, it was verified that acetonitrile can be an alternative for sampling siloxanes, since it presented moderate to high absorption of siloxanes in relation to methanol and low volatility in relation to acetone. The results obtained contribute to the adequate monitoring of the concentration of siloxanes in biogas and biomethane, which is essential to guarantee their safe energy use.