ACTIVATED CHARS FOR TAR CONVERSION AT HIGH TEMPERATURE

The presence of tar compounds in the syngas is the main bottleneck for a wider commercial adoption of the gasification process. Tar is a mixture of heavy hydrocarbons that condense when syngas is cooled below 350°C, leading to clogging of pipes and downstream apparatus. In the last two decades, the scientific community has shown a growing interest on innovative solutions for syngas cleaning. One of the most promising options for tar conversion in medium-scale gasifiers is the use of biomass char as catalyst. However, the mechanism of tar conversion over char surfaces is still not well known. A deeper understanding of this conversion process could be useful to define the best pre-treatments for chars, able to improve their performances. This work aims at acquiring reliable data to study the influence of char internal structure and operating conditions on tar conversion kinetics. Three different chars have been obtained starting from the same parent coal. The activation protocol includes the coal devolatilisation at 900ºC and the char activation by steam at the same temperature, for different times. The specific surface area increases from 1 m2/g for the char without activation (raw char) up to 627 m2/g for the char activated for a time of 150 minutes. The activity of the different chars produced has been tested in a tubular reactor. The tests utilised a nitrogen stream doped with naphthalene. The tested reactor temperatures were 800°C and 900°C, by keeping the gas residence time and naphthalene concentration constant. More tests were carried out at 900°C under different residence times in the reactor. Preliminary results show a high conversion of naphthalene, especially with chars having higher specific surface areas. Reactor temperature and residence time as well as the char internal structure appear to have a significant influence on naphthalene conversion.