BUSCADOR RECOLECTA

The high-pressure oxidation of acetylene–dimethoxymethane (C2H2–DMM) mixtures in a tubular flow reactor has been analyzed from both experimental and modeling perspectives. In addition to pressure (20, 40, and 60 bar), the influence of the oxygen availability (by modifying the air excess ratio, λ) and the presence of DMM (two different concentrations have been tested, 70 and 280 ppm, for a given concentration of C2H2 of 700 ppm) have also been analyzed. The chemical kinetic mechanism, progressively built by our research group in the last years, has been updated with recent theoretical calculations for DMM and validated against the present results and literature data. Results indicate that, under fuel-lean conditions, adding DMM enhances C2H2 reactivity by increased radical production through DMM chain branching pathways, more evident for the higher concentration of DMM. H-abstraction reactions with OH radicals as the main abstracting species to form dimethoxymethyl (CH3OCHOCH3) and methoxymethoxymethyl (CH3OCH2OCH2) radicals are the main DMM consumption routes, with the first one being slightly favored. There is a competition between β-scission and O2-addition reactions in the consumption of both radicals that depends on the oxygen availability. As the O2 concentration in the reactant mixture is increased, the O2-addition reactions become more relevant. The effect of the addition of several oxygenates, such as ethanol, dimethyl ether (DME), or DMM, on C2H2 high-pressure oxidation has been compared. Results indicate that ethanol has almost no effect, whereas the addition of an ether, DME or DMM, shifts the conversion of C2H2 to lower temperatures.

The oxidation of acetylene (as soot precursor) and dimethyl ether (DME, as a promising fuel additive) mixtures has been analyzed in a tubular flow reactor, under high-pressure conditions (20, 40 and 60 bar), in the 450–1050 K temperature range. The effect of varying the air excess ratio (λ≈0.7, 1 and 20) and the percentage of DME with respect to acetylene (10 and 40%) has been analyzed from both experimental and modeling points of view. The addition of DME modifies the composition of the radical pool, increasing the production of OH radicals which cause a shift in the onset temperature for C2H2 conversion to lower temperatures; the higher the amount of DME, the lower the temperature. The presence of DME favors the oxidation of C2H2 towards products such as CO and CO2, eliminating carbon from the paths that lead to the formation of soot. On the other hand, in the presence of C2H2, DME begins to be consumed at temperatures higher than those required for the high-pressure oxidation of neat DME, around 175–200 K more. Consequently, the negative temperature coefficient (NTC) region characteristic of this compound at low temperatures is not observed under those conditions. However, an additional analysis of the influence of DME inlet concentration (at 20 bar and λ=1) indicates that, if the amount of DME in the mixture is increased to 500 ppm and more (700 or 1000 ppm), the reaction pathways responsible for this high DME reactivity at low temperatures become more relevant and the NTC region can now be observed.

The present work addresses the oxidation of ammonia and ammonia-nitric oxide mixtures in a CO2 atmosphere, characteristic of oxy-fuel processes and/or biogas combustion, from both experimental and kinetic modelling points of view. A parametric study of NH3 and NH3/NO mixtures oxidation is carried out, evaluating the influence of the temperature (700–1500 K), stoichiometry (from pyrolysis, λ = 0, to significantly oxidizing conditions, λ = 3.3), gas residence time (low values, 195/T(K) s and high values, 3100/T(K) s) and NH3/NO ratio (0.5–2.2), at atmospheric pressure under well-controlled laboratory conditions using two tubular flow reactor setups. Experimental results have been simulated with an updated literature reaction mechanism, which has been used to interpret the experimental observations.

An experimental and modeling study of the oxidation at high pressure of dimethyl carbonate (DMC) has been performed in a quartz tubular flow reactor. Experimental and simulated concentrations of DMC, CO, CO2 and H2 have been obtained for different temperatures (500–1073 K), pressures (20, 40, and 60 atm) and stoichiometries (λ = 0.7, 1, and 35). Both pressure and concentration of oxygen are important parameters for conversion of DMC. The simulations have been carried out using a detailed kinetic mechanism previously developed by the research group. In general, the model is able to reproduce the experimental trends of the different concentration profiles, although some discrepancies are observed between experimental and simulation results. The performance of the model was also evaluated through the simulation of literature data of the oxidation of DMC at atmospheric pressure in a flow reactor and of the DMC ignition delay times under low and high pressures. In this sense, this work contributes to the knowledge of the combustion process of DMC, by providing new experimental data on the conversion of DMC at high pressures and using a kinetic model for the interpretation of the results.

In the present work, experimental results from the literature on the effect of CO on the NH oxidation in the absence and presence of NO are supplemented with novel flow reactor results and interpreted in terms of a detailed chemical kinetic model. The kinetic model provides a satisfactory prediction over a wide range of conditions for oxidation in flow reactors and for flame speeds of CO/NH. With increasing levels of CO, the generation of chain carriers gradually shifts from being controlled by the amine reaction subset to being dominated by the oxidation chemistry of CO, facilitating reaction at lower temperatures. At elevated temperature, presence of CO causes a change in selectivity of NH oxidation from N to NO. The present work provides a thorough evaluation of the amine subset of the reaction mechanism for the investigated conditions and offers a kinetic model that reliably can be used for post-flame oxidation modeling in engines and gas turbines fueled by ammonia with a hydrocarbon or alcohol as co-fuel.

Alkyl nitrates thermally decompose by homolytic cleavage of the weak nitrate bond at very low temperatures (e.g., around 500 K at reaction times of a few seconds). This provides the opportunity to study the subsequent chemistry of the initially formed radical (or its subsequent pyrolysis products, if unstable) and nitrogen dioxide at such mild conditions. In this work this idea is applied to isopropyl nitrate (iPN) pyrolysis, which is studied in a tubular reactor at atmospheric pressure, temperatures ranging from 373 to 773 K, and residence times of around 2 s. At the experimental conditions, iPN decomposition starts at 473 K with O–N bond fission producing isopropoxy radical (i-C3H7O) and NO2. i-C3H7O is rapidly converted to acetaldehyde (CH3CHO), which is the most abundant product detected, and methyl radicals. Other major products detected are formaldehyde (CH2O), methanol (CH3OH), nitromethane (CH3NO2), NO, methane, formamide (CHONH2), and methyl nitrite (CH3ONO). Four literature nitrogen chemistry models─three of those augmented with iPN specific reactions─have been tested for their ability to predict the iPN decomposition and product profiles. The mechanism by the Curran group performs best, but it still underpredicts the observed high formaldehyde and methanol yields. A rate analysis indicates that the branching ratio of the reaction between methyl radicals and nitrogen dioxide is of significant importance. Based on recent theoretical and experimental data, new rate expressions for the two reactions CH3 + NO2 → CH3O + NO and CH3 + NO2 + He → CH3ONO2 + He are calculated and incorporated in the kinetic models. It is shown that this change clearly improves the predictions, although additional work is needed to achieve good agreement between calculated and measured species profiles.