Gas cleaning with hot char beds studied by stable isotopes

• The use of isotope ratio mass spectrometry has demonstrated increasing irreversibly binding of tar compounds as a function of temperature.
• The use of stable isotope dilution mass spectrometry has demonstrated a trend: benzene < naphthalene < phenanthrene in binding efficiency.
• The use of GC–MS has demonstrated a pronounced hydrogen/deuterium exchange coinciding with the irreversible binding of tar compounds.
• Decarbonylation of phenol was demonstrated to be the initial thermally induced reaction leading to formation of naphthalene.
• Water-assisted gasification resulted in CO/CO2 equally labelled with 13C.

The chemistry taking place in a high temperature char bed used for binding aromatic tar compounds has been studied in detail. 13C labelled tar compounds were used to trace the incorporation into the char bed using isotope ratio mass spectrometry (IRMS) and GC–MS. Furthermore, compounds labelled with 2H (deuterium) were used to support proposed reaction mechanisms.It was found that at 700–800 °C the aromatic tar compounds bind irreversibly to the char by a radical reaction and, hence, become an integrated part of the char. Thermally induced reactions may occur prior to the binding reaction. The findings explain the tar reducing properties of the char bed in two-stage gasifiers.A living char bed for hot gas cleaning is discussed.