Avaliação de Modelos Cinéticos para Combustão de Etanol e Butanol e Implicações em Química Atmosférica

Abstract

Despite the economic advantages related to the production of ethanol in Brazil, this biofuel shows some disadvantages as hygroscopicity, low flash point, high vapor pressure and corrosivity. Its substitution by n-butanol is, therefore, under discussion. In contrast, a comparative analysis of the combustion profiles of ethanol and butanol has not been reported in the literature yet. In this work, an analysis of the chemical mechanism for the butanol combustion is performed through rate analysis and sensitivity analysis. Ignition delay times have been evaluated, as well as the emission some pollutants (aldehydes, carbon monoxide, hydrocarbons and NOx), in simulations of both ethanol and butanol combustion in Otto cycle spark-ignition engines. The software kintecus® has been used to solve the set of differential equations related to the kinetic model. The kinetic model has been revalidated, through comparison between simulated and experimental ignition time values, being found a good agreement. Rate and sensitivity analysis have been performed for the combustion of a mixture butanol and oxygen, in stoichiometric relation, diluted in nitrogen and initial temperature and pressure values of 1199 K and 3.18 atm. Results of these analysis indicate that the mechanism is initiated by CC dissociation reactions. During the propagation, hydrogen atoms and hydroxyl radicals play fundamental role in hydrogen abstraction reactions. The termination is governed by stable specie formation reactions (H2, H2O and CO2). Moreover, the H + O2  OH + O reaction is shown to be of great importance for the ignition. Simulations for butanol and ethanol combustion in ideal Otto cycle engines were performed for six compression rates, varying from 7:1 to 12:1. Ignition time values between 3.1410-5 and 7.0410-5 second were observed for butanol, whereas for ethanol, ignition delay times varying from 5.5910-2 to 8.5910-5 second were obtained. In respect to atmospheric pollutants emissions, the carbon monoxide percentage in the final butanol combustion gas mixture varied from 4.21% to 7.16%, whereas for ethanol combustion, values between 0.69% and 2.73% were observed. Aldehydes emissions within butanol combustion gases varied between 1.55104 and 4.84104 ppbv, whereas from 3.44102 to 8.00103 ppbv in ethanol. NOx concentrations were found in the range from 3.46103 to 5.25103 ppm in butanol combustion final gas mixture and from 2.25103 to 9.01103 ppm in ethanol combustion final gas mixture. The smallest values obtained for the pollutant emissions from ethanol combustion are related to the compression rate in which the initial temperature does not rise abruptly. In conclusion, both fuels show similar combustion profiles and, according to the chemical mechanism and the spark-ignition model, the ignition of butanol yields NOx in smaller concentration, whereas the ignition of ethanol yields less aldehydes and carbon monoxide