Reforming of tars and organic sulphur compounds in a plasma-assisted process for waste gasification

• An advanced two-stage thermal process for waste gasification is examined.
• A 20 hours waste gasification run was performed on a pilot plant.
• The plasma effect promotes complete conversion of aromatic species.
• No detectable organic sulphur is observed in the product gas.

Waste gasification is considered a valuable and sustainable solution to the production of clean energy (via gas turbines or gas engines) and bio-fuels, such as synthetic natural gas and bio-hydrogen, provided that the syngas produced in the gasifier is free of condensable tars and organic sulphur contaminants that cause equipment fouling and deactivation of catalytic stages downstream. In particular, catalytic reaction stages are highly sensitive to specific trace contaminants (e.g. PAHs, thiophenes, etc.), necessitating the use of additional cleaning operations to remove these residues to levels where the catalyst degradation is acceptable. In this work, the use of thermal plasma (coupled with primary waste treatment) to completely reform tars and organic sulphur compounds to simple gaseous products (predominantly H2 and CO) is assessed. To this end, a 20-hour waste gasification run was performed on a two-stage fluid bed-plasma demonstration plant to investigate the tar evolution in the syngas, with special attention on the chemistry of generic and sulphur-substituted aromatics within the plasma stage. The organic fraction in the gas phase was found to be completely reformed under plasma conditions, leaving essentially CO, H2 and H2S as ultimate products. In particular, reduction efficiencies typically exceeded 96%v/v for complex organics (e.g. PAH) and thiophenes were observed. The syngas, after a tertiary simplified gas cleaning process, is suitable for high efficiency power generation, or conversion to a fuel gas capable of injection into national or industrial supply grids.