Optimisation of lignite beater wheel mills using modern gas flow measurements


A new digital time-of-flight-based cross-correlation technique, employing pairs of robust infrared sensors, enables hot recirculation gas flow into lignite beater wheel mills to be accurately measured, with significant potential benefits for mill control

Modern beater wheel mills are still lacking sufficient measurement in order to be controlled by their total instantaneous mass energy balance. Besides unreliable measurement of air flows into the mill the main obstacle is the unknown amount of hot recirculation gas from the boiler which is used to dry the lignite in the mill and at the same time keep the mill inert so that no explosion hazard can occur.

Cross correlation technique

The control state space in which a beater wheel mill is operated and which is defined mainly by the enthalpy flows in and out of the mill can change rather quickly if individual parameters such as the water content of the lignite is changing rapidly. In this instance the mill gas in- and output flows can undergo changes which are hard to control as the amount of heat into and out of the mill is not known.