Hello Calvin,

With respect to cement kilns. the term "reducing conditions" refers to the condition in which there is not enough oxygen available to completely burn all of the carbon in the fuel to CO2. The result is the presence of various amounts of carbon monoxide (CO), depending on the severity of the lack of oxygen. eg;-

Complete combustion of carbon:  C + O2 --> CO2

Incomplete combustion of carbon: C + 1/2 O2 --> CO

A low oxygen content (<1% O2) and the presence of CO (>200ppm) in kiln gases at the kiln inlet indicate reducing conditions.

Due to this lack of oxygen, SOx in the kiln inlet cannot form the less volatile alkali sulphates which would allow the sulphur to leave the kiln within the clinker. This results in a strong sulphur recirculation between burning zone and kiln inlet/lower preheater, causing buildups, blockages and rings.

Hope this helps...

Regards,

Ted.