Fundamental Studies of Organic Silicon Combustion Chemistry and Characterizing the Presence of Silicon Species in Landfill and Sludge Gas

Abstract

Organic silicon species play important roles as reactants in waste-to-energy systems and in material synthesis processes. This research addresses some of the critical challenges posed by silicon species in biogas, which hinder energy recovery due, in part, to silica production during combustion. Despite over a decade of recognizing the impact of silicon compounds on biogas utilization, comprehensive characterization of their presence in biogas is lacking. In addition, understanding combustion chemistry of silicon compounds is essential to evaluate and mitigate their impacts on biogas combustion and improve material synthesis, but detailed reaction mechanisms and thermochemistry data of these chemicals remain scarce and have high uncertainties and known discrepancies with experimental observations. This study aims to fill key knowledge gaps by advancing the understanding of silicon species including their presence in biogas and fundamental combustion chemistry of some canonical compounds. The technical approach for characterizing waste-to-energy concerns included statistical analysis of biogas data reported in the literature, on-site analysis at landfill gas energy facilities, and qualitative interviews with waste-to-energy stakeholders. For advancing the reaction chemistry, flat flame burners and ignition studies were used. The fundamental laboratory combustion experiments focused on trimethylsilanol (TMSO) and hexamethyldisiloxane (HMDSO) due to the canonical structure of these silicon compounds. Burner studies leveraged well-established initial and boundary conditions and novel application of recent diagnostic advances to measure in situ temperature at unprecedented spatial resolution. The ignition studies leveraged the simplified geometry and well-established data interpretation methods to consider chemistry effects on flame speeds. Key outcomes of this work include a new database on silicon species concentrations in biogas. The longitudinal study showed widespread presence of silicon species in global and U.S. biogas systems. TMSO, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) were identified as sentinel species, where their concentrations were identified correlated with total silicon species concentrations. The database established in this work provide an important quantitative foundation for technology development to recover silicon species from biogas and to develop methods for silica abatement and mitigation. In addition, the new correlations discovered between specific and total silicon species concentrations provide new opportunities for developing methods to monitor silicon species and potentially improve waste-to-energy facilities, e.g., by reducing maintenance costs associated with mitigating the presences of these species in biogas. In laboratory burner studies, x-ray fluorescence (XRF) spectroscopy was applied to measure the in-situ temperature fields for the first time in flames with silicon reactants. Findings provide direct insight into the reaction pathways consuming TMSO and HMDSO. Specifically, TMSO and HMDSO reactions were initiated in low-temperature low-oxygen regions. The results indicate radicals from the methane flame system initiate reactions with TMSO and HMDSO through H-abstraction pathways. Additionally, gas sampling measurements identified TMSO as an intermediate of HMDSO reactions, contrary to recently proposed reaction mechanisms in the literature. Surprisingly, the flame speed measurements showed no significant impact from HMDSO on methane flame speeds at the conditions studied, suggesting transport effects may dominate the HMDSO chemistry. The results are the first attempt to measure siloxane flame speeds and provide important knowledge on how to conduct further measurements in the future with improved accuracy. The original findings of the burner and ignition studies provide new insights into the combustion chemistry of silicon species (namely the radical interaction and production pathways), and provide quantitative data to further advance reaction theory and kinetics.