Graphite-assisted microwave carbon dioxide gasification of wet stalks

Abstract

In order to realize the low-carbon gasification of the typical wet stalks, the graphite was used as the microwave-absorbing agent and the heat transfer medium in the microwave field, and CO2 was used as the gasification agent to explore the characteristics of microwave CO2 gasification. After mixing the corn stalks, the rice stalks or the wheat stalks with different water contents and the graphite in a mass ratio of 4:1, CO2 was introduced for gasification in a microwave field. Experiments showed that with the increase in the water content, the carbon conversions, the gas yields and the volume fractions of the effective components all increased slowly in the front and rear sections, and increased rapidly in the middle section. When the CO2 flow rate was 0.6 Nm3·kg−1·min−1, the carbon conversions, the gas yields and the volume fractions of the effective components all reached their peaks. It was found that the structure of the three stalks changed from smooth to porous during the intermediate process of the graphite-assisted microwave gasification. The deashed stalks with water content of 60 % was gasified in a microwave field of 700 W for 1.25 min, and the BET specific surface areas of the generated porous carbon reached 2041–2054 m2 g−1, and the volume fractions of the effective components of the gasification gas were more than 95 %. With the prolongation of the gasification time, the carbon conversions increased slowly in the early and late stages, but increased rapidly in the middle stage. The reason was that the large amount of porous carbon generated in the middle stage was the gasifiable strong microwave-absorbing agent, which greatly increased the gasification temperature and accelerated the reaction rates. When the graphite was reused 10 times, the carbon conversions could still reach more than 80 %. The XRD tests were carried out on the graphite used many times to explore its deactivation mechanism. This paper has practical significance for the low-carbon CO2 recycling of stalks biomass, and provides a new way for the preparation of biomass-based porous carbon.