Direct-fired inclined counterflow rotary kilns and use thereof

A method of operating a direct-fired inclined counterflow rotary kiln for the thermal treatment of material, the rotary kiln presenting a longitudinal axis around which the kiln rotates, said longitudinal axis being inclined with respect to the horizontal plane and extending longitudinally between an upper inlet end and a lower outlet end of the kiln, the kiln having an internal length Lint of from 20 m to 100 m and an internal diameter Dint and presenting a ratio Lint/Dint from 10 to 100, said method comprising the steps of:

• introducing material to be treated into the kiln at the inlet end and subjecting the introduced material to thermal treatment within the kiln, after which the treated material is evacuated from the kiln at the outlet end;
• introducing a main fuel and a main oxidant into the kiln at the outlet end so as to generate a main combustion of the main fuel with the main oxidant, said main combustion taking place in a main combustion zone inside the rotary kiln extending over a distance of ¼ to ⅓ of the internal length Lint of the kiln from the outlet end, wherein combustion gases generated by said main combustion travel through the kiln from the main combustion zone towards the inlet end through a heat exchange zone in which no combustion takes place and in which heat is transferred from the combustion gases to the material travelling in counterflow with the combustion gases, said heat exchange zone being located upstream of the main combustion zone, the combustion gases leaving the main combustion zone optionally containing combustible substances generated by an incomplete main combustion of the main fuel;
• allowing the combustion gases generated by said main combustion to be evacuated from the kiln via a flue gas outlet situated at the inlet end of the kiln;
• introducing supplementary oxidant into the kiln at the inlet end and also optionally supplementary fuel, wherein:
• the combustion gases from the main combustion zone contain combustible substances and both supplementary oxidant and supplementary fuel are introduced into the kiln;
• the supplementary oxidant introduced into the kiln has an oxygen content of from 50% vol to 100% vol;
• and
• combustion of the supplementary oxidant with the combustible substances or combustion of the supplementary oxidant with the optional supplementary fuel is carried out in a supplementary combustion zone inside the rotary kiln which extends over a distance from the inlet end of at most ¼ of the internal length Lint, so that the combustion gases generated by the supplementary combustion are evacuated together with the combustion gases generated by the main combustion via the flue gas outlet of the kiln.

An installation for the thermal treatment of material, comprising:

• a direct-fired counterflow kiln rotatable around a longitudinal axis, said longitudinal axis being inclined with respect to the horizontal plane and extending between an upper inlet end and a lower outlet end, the kiln having a longitudinal middle M halfway between the inlet end and the outlet end and having an internal length Lint of from 20 m to 100 m and an internal diameter Dint and presenting a ratio Lint/Dint from 10 to 100, the kiln comprising a longitudinal middle section presenting no oxidant or fuel inlets which extends over at least 50% of the internal length Lint of the kiln and which bridges the longitudinal middle M;
• a material supply system for supplying material to be treated to the kiln, the material supply system being in fluid connection with the inlet end of the kiln and adapted for introducing material to be treated at a controlled supply rate into the kiln at the inlet end;
• a material evacuation system in fluid connection with the outlet end and adapted to evacuate thermally treated material from the kiln;
• at least one main burner located at the outlet end of the kiln and adapted for introducing main fuel and main oxidant into a main combustion zone adjacent the outlet end of the kiln;
• at least one supplementary burner located at the inlet end of the kiln and adapted for introducing supplementary oxidant and supplementary fuel into a supplementary combustion zone adjacent the inlet end of the kiln;
• a flue gas outlet situated at the inlet end of the kiln and adapted for evacuating combustion gases from the kiln, wherein the at least one supplementary burner is in fluid connection with a source of an oxidant having an oxygen content of from 50% vol to 100% vol.