Microwave enhanced gasification of carbon.

摘要

The gasification of carbonaceous material is a long standing industrial process which produces valuable feed stocks of H2 and CO gas (Syngas) that can be reacted further resulting in extended hydrocarbons for use in numerous areas of industry. The process of gasification is energy intensive, requiring temperatures >700 °C, which at the industrial scale requires large amounts of excess energy in order to achieve optimal reaction temperatures for ideal compositions and product yields. Conventional convective heating methods of current industrial processes thereby limits the overall efficiency of the gasification process even before the desired product is achieved.;Microwave radiation of dipolar or conducting materials is known to produce heat in a significantly different way than conventional heating. Microwaves can specifically heat materials which in turn heats microwave active material to a higher temperature than by conventional methods because less heat is lost to surroundings with limited loss of excess energy to the complete system. It is understood that microwave radiation also leads to enhanced rates of reaction, due to its ability to selectively heat materials and the mechanism by which it heats. An industrially relevant process such as carbon gasification is therefore an ideal candidate for investigation.;The disproportionation of CO2 over carbon (Boudouard reaction) as a fundamental reaction in carbon gasification provided a clean and clear starting point in which to study the effects of microwave heating. It was found that the use of microwave radiation to selectively heat the carbon resulted in a profound change in the fundamental thermodynamics of the reaction. From kinetic studies of the reaction under conditions of flowing CO2, it was found that the apparent activation energy decreased from conventional convective heating to the modified system under microwave irradiation. From measurement of the equilibrium constants as a function of temperature, the enthalpy of the reaction also dropped under microwave irradiation. The change in enthalpy affected the position of the equilibrium so that the temperature at which CO becomes the major product dropped significantly from the conventional thermal reaction to the microwave. The changes in the fundamental thermodynamics of the reaction are attributed to the enhanced reactivity of the CO2 with the steady-state concentration of electron--hole pairs that are present at the surface of the carbon as a result of the space-charge mechanism, which is understood to be heating the carbon.;Such a mechanism is unique to microwave-induced heating, and, given the effect it has on the thermodynamics of the Boudouard reaction, suggests that its use may yield large energy savings in driving the general class of gas--carbon reactions. To further elucidate the significant increase in efficiency, the carbon-steam process was also examined under microwave irradiation. From equilibrium measurements of the carbon-steam, as well as the various equilibria of comprising secondary reactions, it was observed that the same type of thermodynamic enhancement occurred for this more complicated system.