Síntese e avaliação do catalisador Ni/maienita para o craqueamento do alcatrão produzido no processo de gaseificação de capim elefante

Abstract

The growing global energy demand and concerns regarding the availability of fossil fuels are among the most discussed issues in the 21st century. These issues encourage research into the development of renewable energy sources as an alternative to fossil energy sources. As a renewable energy, biomass sources have gained more interest in recent years due to the concerns about global warming and atmospheric pollution. Gasification is an attractive route for the conversion of biomass into a combustible gas, which can be used for power generation. The objective of this work was to evaluate the efficiency of the Ni/mayenite catalyst for reforming of the tar generated in the CO2 gasification of elephant grass in a fixed bed tubular reactor. The mayenite support was synthesized from calcium and aluminum hydroxides and subsequently impregnated with nickel (Ni) by the wet impregnation method. Firstly, a preliminary study was conducted with the purpose of comparing different methods for mayenite production. The ultrasound method was studied in detail from a 2k experimental design, where the variables studied were ultrasound time (10 - 50 min) and calcination temperature (900 - 1200 °C). The influence of the application of ultrasound was also studied in the impregnation of the mayenite support with Ni. The catalyst activity was evaluated in the elephant grass gasification experiments with carbon dioxide at 900 °C. Gasification was carried out in a fixed bed tubular reactor and catalytic cracking was performed in a second reactor. Experiments with the second empty reactor were also carried out to evaluate the thermal cracking at the used temperature. The results showed that the mayenite can be synthesized from several methods, the ultrasound mixing method being the most promising due to the lower energy consumption and its high selectivity for mayenite formation. In the detailed study of the method, only the temperature had a significant effect. High temperatures increased the yield of mayenite and also its sintering, resulting in a decrease of its surface area. Ultrasound treatment had a positive effect on Ni impregnation, increasing the dispersion of the metal in the support. Thermal cracking of the tar at a temperature of 900 °C provided a reduction of the gravimetric tar to about one-third of the initial value. However, an increase in gas production was not observed. The Ni/mayenite catalyst proved to be efficient for reforming of tar from CO2 gasification of elephant grass. Using the Ni/mayenite catalyst a reduction of the tar concentration from 1.77 to 0.13 g·Nm-3 was observed, accompanied by a considerable increase in H2 and CO yield of 168 and 71%, respectively. After the catalytic tests, the catalyst presented a carbon deposition of 0.7%.