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Poly- and Perfluorinated alkyl substances (PFAS) pose environmental and public health concerns. While incineration remains the most common PFAS remediation method, the complete combustion and pyrolysis mechanism of PFAS is unknown. This study aims to expand our understanding of the kinetics of gas-phase PFAS incineration by measuring the effect of difluoromethane (CHF) on propane ignition delay times (IDTs). The ignition delay times were measured by OH* emission and end-wall pressure time histories behind the reflected shock wave. Different concentrations of CH2F2 were mixed with fuel-lean propane-oxygen mixtures diluted in argon. Experiments were conducted at a nominal reflected shock pressure of P5 = 1 atm and reflected shock temperatures of 1200 < T5 < 1800 K. A new detailed chemical kinetic mechanism is presented. 135 new rate constants were computed using RRKM/ME theory, based upon stationary points computed using ANL0. The new mechanism is in excellent agreement with the measured ignition delay time. A novel sensitivity analysis helps to explain the elementary steps by which CH2F2 increases the ignition delay time. 

Previous generations of halocarbon refrigerants and flame suppressants cause intolerable levels of ozone depletion and global warming, motivating the search for environmentally-friendly alternatives, but the complex flammability of proposed halocarbon compounds has proven to be a challenge. Because the combustion chemistry of these greener halocarbon refrigerants and suppressants is very condition-dependent, the flammability of potential alternatives need to be screened under a variety of operational conditions prior to marketing and further product development. To facilitate this screening, kinetic models can be generated automatically in Reaction Mechanism Generator (RMG) to predict the flammability. RMG, a software package that automates the generation of detailed reaction mechanisms, has recently been extended to predict the chemical kinetics involved in halocarbon combustion. Full kinetic mechanisms with kinetic, thermodynamic, and transport properties generated from RMG are then evaluated with Cantera to predict laminar flame speeds under different reacting conditions. Recent work developed an RMG model of flame suppressant 2-BTP (CH2=CBrCF3) in methane flames. Current work has advanced RMG’s 2-BTP model to achieve improved quantitative agreement with experimental flame speeds. We also use RMG to generate models of binary halocarbon blends to determine the importance of “cross reactions” that are not accounted for through simple concatenation of individual combustion mechanisms. Flame speeds of RMG-generated blend models and blend models comprised of concatenated mechanisms are compared, along with experimental flame speeds found in literature. As demonstrated in current and previous work, automating the generation of full halocarbon kinetic models through RMG expedites screening for the next generation of environmentally-friendly refrigerants and suppressants, a task that would be both time- and cost-intensive if conducted without automation. 

Previous generations of halocarbon refrigerants and flame suppressants cause intolerable levels of ozone depletion and global warming, motivating the search for environmentally-friendly alternatives, but the complex flammability of proposed halocarbon compounds has proven to be a challenge. Because the combustion chemistry of these greener halocarbon refrigerants and suppressants is very condition-dependent, the flammability of potential alternatives need to be screened under a variety of operational conditions prior to marketing and further product development. To facilitate this screening, kinetic models can be generated automatically in Reaction Mechanism Generator (RMG) to predict the flammability. RMG, a software package that automates the generation of detailed reaction mechanisms, has recently been extended to predict the chemical kinetics involved in halocarbon combustion. Full kinetic mechanisms with kinetic, thermodynamic, and transport properties generated from RMG are then evaluated with Cantera to predict laminar flame speeds under different reacting conditions. Recent work developed an RMG model of flame suppressant 2-BTP (CH2=CBrCF3) in methane flames. Current work has advanced RMG’s 2-BTP model to achieve improved quantitative agreement with experimental flame speeds. We also use RMG to generate models of binary halocarbon blends to determine the importance of “cross reactions” that are not accounted for through simple concatenation of individual combustion mechanisms. Flame speeds of RMG-generated blend models and blend models comprised of concatenated mechanisms are compared, along with experimental flame speeds found in literature. As demonstrated in current and previous work, automating the generation of full halocarbon kinetic models through RMG expedites screening for the next generation of environmentally-friendly refrigerants and suppressants, a task that would be both time- and cost-intensive if conducted without automation. 

Standard enthalpies, entropies, and heat capacities are calculated for more than 14,000 halogenated species using a high-fidelity automated thermochemistry workflow. This workflow generates conformers at density functional tight binding (DFTB) level, optimizes geometries, calculates harmonic frequencies, and performs 1D hindered rotor scans at DFT level, and computes electronic energies at G4 level. The computed enthalpies of formation for 400 molecules show good agreement with literature references, but the majority of the calculated species have no reference in the literature. Thus, this work presents the most accurate thermochemistry for many halogenated hydrocarbons to date. This new dataset is used to train an extensive ensemble of group additivity values (GAV) and hydrogen bond increment groups (HBI) within the Reaction Mechanism Generator (RMG) framework. On average, the new group values estimate standard enthalpies for halogenated hydrocarbons within 3 kcal/mol of their G4 values. To demonstrate the significance of RMG’s improved halogen thermochemistry, a model for C3H2F3Br (2-BTP) is generated, and flame speeds are compared to a literature mechanism. A significant contribution towards the automation of detailed modeling of halogenated hydrocarbon combustion, this research provides thermochemical data for thousands of novel halogenated species and presents a comprehensive set of halogen group additivity values.