John Bromly

Kinetic and Thermodynamic Sensitivity Analysis of the NO-Sensitised Oxidation of Methane

Abstract Thermodynamic parameters such as species heats of formation and entropies may have a significant impact on the output of detailed kinetic models, but attention is generally only given to considering kinetic parameters in model development and optimisation. In this paper, a method for comparing the impact of uncertainties in both kinetic and thermodynamic parameters on the predictions of detailed kinetic models is described. This method employs kinetic and thermodynamic sensitivity analysis to define an impact factor for all parameters under consideration as the product of the sensitivity of a particular model output to the parameter and the uncertainty in the value of that parameter. Kinetic and thermodynamic impact factor analysis has been applied to the results of an experimental study of the interaction between NOx (ca. 0-200 ppm)and methane (ca. 40-1300 ppm) in the presence of oxygen (ca. 0-1%) in an atmospheric pressure flow reactor, for temperatures ranging from 500 to 700 C. A detailed che...

An experimental investigation of the mutually sensitised oxidation of nitric oxide and n-butane

The interaction between NO (0.01 to 200 ppm) and n-butane (50 to 600 ppm) in air has been investigated in a flow reactor at atmospheric pressure and temperatures from 330° C to 450° C (600 K to 720 K). Low concentrations of NO in n-butane/air systems promote the oxidation of the n-butane; conversely, low concentrations of n-butane in air promote the oxidation of NO to NO2. For given [NO]/[n-butane] ratio and reaction time, there is a critical sharply-defined ‘crossover’ temperature at which the system goes from being unreactive to reactive, with 100% conversion NO→NO2 occurring at T>Tcrossover. The crossover temperature increases, and becomes less sharply defined, with increasing [NO]/[n-butane] ratio. The conversion NO→NO2 is accompanied by the consumption of n-butane and the formation of CH3CHO, CO, HCHO, (CH3)2CO, various butenes, and propene. The extent of n-butane consumption depends in a complex manner on the experimental conditions especially on the relationship of the experiment temperature to the characteristic turnover temperature which marks the onset of the region of negative temperature coefficient (NTC) for n-butane/air reaction (≈380° C or 650 K). Trace quantities (as little as 0.02 ppm) of NO have a profound promoting effect on n-butane consumption in the vicinity of the turnover temperature by virtue of the ability of NO to convert unreactive HO2 radicals into reactive OH: HO2+NO→NO2+OH Other reactions of NO believed to be important in this system are RO2+NO→RO+NO2 and NO+OH+M→HONO+M The mutual sensitisation of the oxidation of n-butane and NO has implications for emissions of NO2 from combustion appliances and for hydrocarbon ignition phenomena in the presence of NO, such as occurs in engines.

NONCATALYTIC PARTIAL OXIDATION OF METHANE INTO SYNGAS OVER A WIDE TEMPERATURE RANGE

Noncatalytic partial oxidation of methane has been studied over a wide temperature range from 823 to 1531 K using two flow reactors. Highly diluted fuel-rich CH4/O2/N2 mixtures were reacted in uncoated tubular flow reactors at 1.2 atm. Residence time was varied from 1 to 164 s. Major and minor products of the partial oxidation were measured using a gas chromatograph. Kinetic modeling was performed to simulate experiments and key rate-controlling reactions were revealed by sensitivity analysis. It was found that chain-branching reaction H + O2 = OH + O, recombination CH3 + CH3 (+M) = C2H6 (+M), and methyl radical oxidation CH3 + O2 = CH2O + OH govern the overall rate of the process at short residence times. Recent measurements of these rate constants were analyzed and appropriate modifications in the detailed reaction mechanism were proposed. The model was adjusted to reproduce the measurements accurately at short residence times. At longer residence times, a significant impact of the heterogeneous reactions leading to inhibition of the overall process was observed. The model developed in the present study correctly reproduced temporal profiles and final compositions of the products over the entire range of temperatures and the initial mixture compositions.

The Comparative and Combined Effects of Nitric Oxide and Higher Alkanes in Sensitizing Methane Oxidation

The comparative and combined effects of nitric oxide (NO) and higher alkanes on methane oxidation were examined by experimentation and kinetic modeling. Experiments were conducted using fuel-lean, lower-alkanes mixtures with NO (0–400 ppm v/v) in an atmospheric flow reactor at residence time of 2 s over the temperature range of 820–950 K. NO was found to greatly promote methane conversion, and its sensitizing effect strengthened with the increasing concentration that was added to the system. The promoting effect of higher alkanes on methane conversion was also evident, particularly at zero or low NO concentrations. A strong dependency of the sensitizing effect on the concentration of higher alkanes present was also observed. The kinetic mechanism from Le Cong et al., performed reasonably well in reproducing the experimental trends. However, the sensitizing propensity of higher alkanes in the presence of NO could not be unambiguously ranked under all conditions. Modification to the kinetic mechanism of Le Cong et al., was attempted. Specifically, the submechanism of C3 peroxy radicals was added, but the modeling results indicated a lower impact on methane conversion than was initially expected. The most sensitive reactions were revealed, and the generalized reaction pathways for methane oxidation sensitized by higher alkanes, with or without the presence of NO, were also proposed, following detailed sensitivity analysis.