Dynamics of on-line ash monitors in monitoring and control systems

On-line ash monitors have been in use for many years in coal industry. Their operation is usually based on the scattering or the absorption of incident gamma radiation, and the derived ash value is the result of a time-averaged measurement. The natural radiation method is also used in on-line ash content measurements. A review of the ash monitors applied in Polish coal preparation plants is presented in this paper. Most of monitor installations are designed to monitor trends of ash content in coals, rather than to react to rapid changes of ash content. These data averaged over the period of tens of seconds (or several minutes) are often used by an operator to adjust the operation of a plant manually. In the case of closed loop control systems with on-line ash monitors, the dynamic properties of these instruments are important. The concept of an ash monitor with a time constant (or a time of measurement) adapting to variations of an input signal (ash content) have been analyzed. The fuzzy logic has been applied to establish the structure of the adaptive filter. Such a system allows us to speed-up the reaction of the instrument to rapid variations of ash content and at the same time to achieve better statistical accuracy over a longer period of time. This is particularly important in closed loop control systems or in splitting of a coal stream to different products. Simulation analysis of the coal blending control system applied in the "Sosnica" plant is presented in this article. Application of an on-line ash monitor with an adaptive time constant is discussed and some results of practical experiments are presented.