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Advanced controls using process running models and fuzzy logic have a stabilizing effect on lime quality and energy use
THE LIME KILN OPERATION in a kraft pulp mill is one of the most challenging to control. Firstly, it is a complicated mixed-phase chemical reaction process. Quickly moving combustion gases provide the heat input to chemically change the slowly moving solid-phase lime mud into lime for use in the recausticizing process.
Secondly, the many different types and degrees of process delays are very significant and difficult Io manage manually. hot flue gases travel through a kiln in less than a minute but the lime mud stays in a kiln about two to three hours. The hot end temperature stabilizes in about two to four hours after a change in the feed end. A change in the kiln rotation speed, to adjust production rate, results first in a feed end temperature upset and then, after about a half hour, the hot end temperature changes. It can take from three to four hours before the process temperature profile is stabilized again. The total mass of the kiln stabilizes in eight to 12 hours after a change in the operating temperature.
It sounds complicated and it is. Adequate, but not optimum, manual control takes a well developed feel for the process and a light hand on the individual control loop targets. In a process with such long time delays, it is common for operators to overreact to changes, resulting in long periods of instability and variable lime quality. Often, the operators in the next eight-hour shift are the first to see the effect of a change in operation. And any attempt at optimizing energy consumption is futile if the process is not stable.
ESTABLISHING STABILITY
To solve these lime kiln process stability problems Metso Automation has, to date, installed 13 DNAlime optimization control systems in kraft pulp mills. These high level supervisory systems oversee the coordination of the basic lime mud washing, fuel combustion, lime burning and kiln operation controls to stabilize the process and, most importantly, to attain even lime quality for a more stable causticizing process. These optimizing controls achieve a steadiness of operation that even the most experienced operators would find difficult or impossible...