On the reactivity of carbonate esters examined by weak flames in a micro flow reactor with a controlled temperature profile
Session chaired by Pr. Christine Rousselle
Oxygenated fuels produced from renewable biological resources can be effective solution to the environmental issues caused by the combustion of fossil fuels. Recently, carbonate esters have attracted attention as a combustion additive. Some studies reported that DEC (DiEthyl Carbonate) is easily miscible with diesel without phase separation and is excellent in terms of PM emissions in practical engines. Fundamental ignition and combustion experiments, and kinetic modeling of DEC and DMC (DiMethyl Carbonate) have been conducted. Carbonate esters are extensively used also as a solvent for lithium ion batteries. Unwanted accidents of the lithium ion batteries are an international concern. Fundamental understandings on ignition and combustion characteristics of carbonates esters are indispensable to safety for lithium ion batteries. In this study, the ignition and combustion characteristics of carbonate esters were investigated using a micro flow reactor with a controlled temperature profile (MFR). DMC, EMC (Ethyl Methyl Carbonate) and DEC which are typical solvents for electrolyte of lithium ion batteries were chosen in the present study. They have methyl or ethyl groups with the structure of a carbonate ester (RO-C(=O)-OR’).
In MFR, a cylindrical quartz tube is employed as a reactor channel and the inner diameter of the tube is smaller than the ordinary quenching diameter. The tube is heated by an external heat source and a stationary temperature profile along the inner surface of the tube wall is formed in the flow direction. Weak flames were observed at low flow velocities of several cm/s. Theoretical analysis showed that weak flames represent ignition characteristics of test fuels. The reactivity of the mixture can be evaluated by observing the temperature region where flame was formed: higher/lower reactivity fuels make weak flames at lower/higher temperature region. In the present study, a quartz tube with an inner diameter of 2 mm was employed as a reactor channel. H2/air premixed flat-flame burner was used as an external heat source. The maximum wall temperature was set to 1300 K. A stoichiometric mixture with air was introduced to the reactor from the lower temperature side at a flow velocity of 2.0 cm/s to observe weak flame with a digital still camera. The position of the maximum chemiluminescence in the flame image was defined as the weak flame position. Under low flow velocity conditions, flow in MFR can be modeled as a one-dimensional reactive diffusive laminar flow without a boundary layer. Therefore, computations were performed using the steady-state one-dimensional flame code (PREMIX code in the ANSYS Chemkin-pro ver. 19.3) with an additional term of a heat transfer between the reactor wall and the gas in the energy equation. The same wall temperature profile as in the experiments were used in the computations. As detailed chemical kinetic models, DMC mechanism developed by LLNL group (257 species, 1563 reactions) and DEC mechanism developed by Tsinghua group (341 species, 1980 reactions) were employed. As no mechanism for EMC has been developed yet, we developed a trial mechanism using rate parameters of EMC reactions modeled by analogy with those of DEC and DMC reactions. The computational conditions were same as the experimental conditions. The maximum peak position of the heat release rate (HRR) was defined as the weak flame position in computations. The computational weak flame positions were compared with the experimental ones.
The wall temperatures at the weak flame positions obtained in the experiments were 1149–1183 K for DMC, 1128–1162 K for EMC, and 1142–1154 K for DEC: in the computations, 1177 K for DMC, 1140 K for both EMC and DEC. In both experiments and computations, a weak flame of DMC was stabilized at higher temperature compared to those of the other samples, which means lower reactivity of DMC. EMC and DEC exhibited almost the same reactivity with each other. To investigate the difference in the reactivity, the weak flame structures of DMC with methyl groups and DEC with ethyl groups were compared. DEC had a negative heat release rate around 970 K where in the upstream of the main heat release took place and it was found that the decomposition of DEC is a major pathway of DEC consumption. On the other hand, most of the initial DMC were consumed by H-abstraction reactions. This is probably because the molecular structure of DMC is stronger than that of DEC. It is considered that the DEC decomposed at low temperature because of the low bond energy of ethyl group on DEC. Focusing on the profiles of fuel mole fractions, DEC was completely consumed on 1050 K, while DMC remained up to 1165 K. It is considered that the difference in the initial reaction between DMC and DEC leads to the difference in the reactivity. EMC also had a similar tendency to DEC, suggesting that the presence of the ethyl group would control the reactivity.
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