Understanding the Combustion Chemistry of Siloxanes: Reaction Kinetics and Fuel Interactions

This work explores the reaction kinetics of three complimentary organosilicon structures important to waste-to-energy and material synthesis applications. The chemical kinetics of the siloxane compounds were investigated using oxidation/auto-ignition and pyrolysis/thermal decomposition experiments. The twofold approach enabled a large range of state conditions and reaction chemistries to be studied, often for the first time. The effects of trimethylsilanol (TMSO) and hexamethyldisiloxane (HMDSO) on syngas (H2 and CO) auto-ignition behavior at temperatures of 1010 – 1070 K and pressures of 8 to 10.3 atm were quantified using a rapid compression facility (RCF). Trace concentrations of TMSO (100 ppm, mole basis) and HMDSO (100 ppm) were added to a surrogate syngas mixture of CO and H2 (with a molar ratio of 2.34:1), air levels of dilution, with molar equivalence ratio of ϕ = 0.1). The measured ignition delay times showed both siloxane species dramatically promoted reactivity of the H2 and CO reactants as indicated by reduced ignition delay times, with TMSO decreasing ignition delay times by approximately 37% and HMDSO decreasing ignition delay times by approximately 50% compared with the reference syngas mixture which contained no siloxanes. HMDSO also demonstrated a marked increase in energy release with an increase in pressure rise of approximately 20% compared with the reference syngas mixture. The thermal decomposition behavior of three organosilicon species, TMSO, HMDSO and hexamethylcyclotrisiloxane (HMCTSO), was investigated using two shocktube facilities: a diaphragmless shocktube (DFST) and a unique high-repetition rate shocktube (HRRST). This work provided first-of-their-kind laser schlieren densitometry results for understanding the thermal effects of the decomposition process. All three siloxane compounds demonstrated strongly endothermic behavior. Time-resolved speciation data were also obtained during the pyrolysis experiments using time-of-flight mass spectrometry (TOF-MS). The first-of-their-kind data provided vital new information at conditions not studied previously. Additionally, TOF-MS experiments using photo-ionization energy from a synchrotron facility provided further insights into the species relevant for thermal decomposition. The data showed the HMDSO, TMSO and HMCTSO do not decompose into smaller silicon based intermediates, as expected based on the limited information on these species available in the literature. Instead, small hydrocarbons were observed as were spectra attributable to larger stable siloxane species. The results of the oxidation and thermal decomposition experimental studies were used to propose and test hypotheses for siloxane reaction pathways important for this class of compounds. Importantly, the experimental data indicate significant reactivity at combustion conditions which may be attributed, in part, to increased production of the OH radical pool. However, the results also indicate direct reactions with the siloxane compounds or silicon-containing intermediates may be necessary to explain the observed behaviors.