摘要:

Another possible mechanism of porous medium combustion describes waves which propagate by consuming the oxidizer initially stored in the pores of the sample. This occurs for abnormally high pressure and gas density. In this case, uniformly propagating planar waves, which are kinetically controlled, can propagate. Diffusion of oxidizer decreases the wave velocity. In addition to the reaction and diffusion layers, the uniformly propagating wave structure includes a layer with a pressure gradient, where the gas motion is induced by the production or consumption of the gas in the reaction as well as by thermal expansion of the gas. The width of this zone determines the scale of the combustion wave in the porous medium.

ISSN:0010-2202    

DOI:10.1080/00102200008947304

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Combustion of chars obtained from the pyrolysis of lignocellulosic materials at 1200 K has been studied using a TGA at different temperatures (583-670 K) and oxygen partial pressures (10-101 kPa). The kinetic parameters vary significantly depending on burn-off when calculated on the basis of multiple and independent combustion runs. The apparent reaction order increases from 0.64 to 0.95 and apparent activation energy increases from 116 to 125 kJmol−1, Experiments carried out with gas and temperature shifts suggest that the dependency of the kinetic parameters on bum-off are spurious consequences of changes in the internal area. The shift technique yields an oxygen reaction order of 0.5, which decreases slightly with bum-off, and an activation energy of 120 KJmol−1, which increases slightly with burn-off. These values are found for the combustion of nine chars irrespective of their panicle sizes, ash content or parent material. The data obtained are attributed to a strong pore diffusion control on a common intrinsic kinetics characterised by zero reaction order and the activation energy of 240 Umol−1.

ISSN:0010-2202    

DOI:10.1080/00102200008947305

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A study is presented on the effect of convective conditions on the endothermic gasification and subsequent ignition of a slab of solid material heated by a radiant flux. A simple solid-phase thermal model is numerically integrated to approximately reproduce results of a previous experimental study of piloted ignition. The model is formulated in terms of the transient energy equation including endothermic Arrhenius-type pyrolysis and Beer-law in-depth absorption of the radiant flux. Temperature evolution curves are obtained for different values of the heat flux and convective coefficient, and are shown to accurately predict experimental surface temperature measurements prior to ignition. Material gasification rates are calculated and used in conjunction with the experimental ignition results to show that the piloted ignition delay closely corresponds to the time required by the solid to reach a critical value of the gasification rate. The critical gasification rate is nearly constant and independent of the heat flux for given convective conditions, and increases linearly with the convective coefficient. It is shown that the graphs of piloted ignition delay vs. heat flux may be accurately determined from this critical gasification criterion. Finally, a non-dimensional parameter that represents the relative amount of gasified fuel transported by the oxidizer stream is presented as a suitable measure of the flammability of the material.

ISSN:0010-2202    

DOI:10.1080/00102200008947306

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The problem of a flame generated by three coaxial flows is solved by extending the Burke-Schumann methodology to include a third stream. The solution is particularly relevant to flame synthesis wherein multiple tubes are often employed either to introduce inert as a diffusion barrier or to introduce more than two reactants. The general problem is solved where the inner and outer tubes contain reactants and the middle tube contains either an inert or a third reactant. Relevant examples are considered and the results show that the triaxial Burke-Schumann flame can be substantially more complicated than the traditional Burke-Schumann flame. When the middle flow is inert the flame temperature is no longer constant but increases axially, reaching a maximum at the flame centerline. At the exit the flame does not sit on the tube exit but instead resides between the inner and outer tubes, resulting in an effective barrier for particle build-up on the burner rim. For the case of a third reactant in the middle flow, synthesis chemistry where the inner reaction is endothermic and the outer reaction is exothermic is considered. In addition to showing the flame temperature and flame shape, the results identify conditions wherein reaction is not possible due to insufficient heat transfer from the outer flame to support the inner flame reaction.

ISSN:0010-2202    

DOI:10.1080/00102200008947307

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor