Syngas Purification by Plasma Tar Cracking

Gasification is a well-known technology able to convert any carbon-rich raw material into a producer gas containing mainly hydrogen, carbon monoxide, and methane, among others. Because of that, gasification has been revisited in the last decade as an option to avoid landfilling municipal solid waste (MSW) or even for recovering materials and energy from excavated MSW through the Enhanced Landfill Mining (ELFM) approach. However, the producer gas obtained consists of permanent gases and heavy organic molecules, called tar. Tar can quickly foul and clog pipes and auxiliary equipment in downstream applications like internal combustion engines, gas turbines, and fuel cells. Tar can also poison active sites in Fischer-Tropsch catalysts. Removing tar is, therefore, indispensable to add value to the producer gas by enabling its use in energy conversion and chemical synthesis. The use of cold plasma for this purpose has continuously gained attention because it can remove pollutants from syngas without being poisoned (like with catalysts) and without needing extremely high temperatures (like with thermal cracking). Despite these advantages, the use of cold plasma has been limited to the removal of model molecules at temperatures below 400°C, mostly under pure nitrogen. These conditions do not reflect the real situations found in gasification systems. The current temperatures studied would require syngas cooling, thereby reducing the energetic efficiency substantially. Real tar molecules and producer gas atmospheres are also needed to evaluate whether a cold plasma is a suitable alternative for tar cracking. In this thesis, a cold plasma system is experimentally explored in conditions closer to reality at the temperatures commonly found in gasifiers (600-1100°C), with model and real tar compounds, also exploring the effect of producer gas components in plasma cracking. To this end, an experimental set-up specially targeted to these higher temperatures was designed and built. The results suggest that a cold plasma environment and increasing temperatures have a synergistic effect on tar removal in both model tar molecules and real tar molecules while reducing the dew point of the remaining tar species due to the reduction of polyaromatic hydrocarbons (PAHs) formation. The characteristics observed show promising ways to integrate cold plasma at high temperatures with different catalysts to reduce even more syngas tar levels.

Publication year:2021

Accessibility:Open