Abstract
Siloxanes are among the most technologically troublesome trace compounds present in biogas. As a result of their combustion, hard-to-remove sediments are formed, blocking biogas energy processing devices and reducing the efficiency of biogas plants. The purpose of this study was to help investors and designers to choose the optimal technology for the adsorptive removal of volatile methylsiloxanes (VMSs) from biogas and to identify adsorbents worth further development. This paper critically reviews and discusses the state-of-the-art technologies for the adsorption removal of siloxanes from biogas, indicating potentially beneficial directions in their development and deficiencies in the state of knowledge. The origin of VMSs in biogas, their selected physicochemical properties, technological problems that they can cause and their typical versus limit concentrations in biogases are presented. Both the already implemented methods of adsorptive VMSs removal from landfill and sewage gases and the ones being under development are verified and systematized. The parameters and effectiveness of adsorption processes are discussed, and individual adsorbents are compared. Possible ways of regenerating spent adsorbents are evaluated and prospects for their application are assessed. Finally, zeolite-based adsorbents—which can also be used for biogas desulfurization—and adsorbents based on polymer resins, as being particularly active against VMSs and most amenable to multiple regeneration, are identified.
Highlights
The most troublesome trace impurities present in biogas are volatile organic silicon compounds—siloxanes, which are oligomeric chemicals containing alternately connected silicon and oxygen atoms
Along with wastewater containing used cosmetics—especially shampoos, soaps, hair sprays, antiperspirants, etc.—volatile methylsiloxanes (VMSs) end up in wastewater treatment plants (WWTPs), where, together with sediments, they get into the fermentation chambers and as a result of the elevated temperature are released into biogas
VMSs inthey biogas are personal products, inand to unique characteristics such as: low surface tension, viscosity and chemical reactivity, high thermal together with sediments, they get into the fermentation chambers and as a result of the elevated temperature are released into biogas
Summary
The most troublesome trace impurities present in biogas are volatile organic silicon compounds—siloxanes, which are oligomeric chemicals containing alternately connected silicon and oxygen atoms. Silicon atoms are linked with hydrocarbon functional groups, mainly methyl (Table 1). In this case, considering their volatility under ambient conditions (Table 2), siloxanes are referred to as volatile methylsiloxanes (VMSs). The main source of VMSs in biogas are personal care products, in which VMSs, owing to their unique characteristics such as: low surface tension, viscosity and chemical reactivity, high thermal stability, compressibility, spreadability and hydrophobicity (Table 2), have been rapidly increasingly used in recent years. Nomenclature and formulas of main volatile methylsiloxanes (VMSs) detected in biogases. Formulas of main volatile methylsiloxanes detected in biogases.
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