Keeping the lime kiln on course

Mark Willliamson

Advanced process control (APC) can deal with lengthy time delays and stabilize the kiln operation from shift to shift. Pulp mill bottlenecks have been solved and significant energy savings have been achieved.

Chemical pulp mill processes often involve unexpected upsets and lengthy time delays, sometimes measured in hours. These disturbances that carry forward from shift to shift are often difficult for even experienced operators to deal with.

Modern pulping and recovery line advanced process controls (APCs) are now playing an important role in nipping many of these upsets in the bud, stabilizing the line operation and allowing operators to fine-tune their performance indicators. These model-based process controls regulate the mass flow, chemical reaction rates, combustion and heat transfer efficiency of pulping and recovery processes. In many cases the advanced controls emulate the best operating practices, as if the best operator were on duty twenty four hours per day. However, the controls can often do better than the best operator since the outcome is managed consistently based on quickly updated process measurements, automated online chemical analyses and predictive algorithms. As the controls work tirelessly and dependably, the operating conditions from shift to shift are steady and shift transfers are made without disturbances.

Solving a dilemma

The lime kiln process presents a particular challenge because of the unique and complicated process dynamics involved. Operators may be frustrated when trying to maintain a stable process which changes from shift to shift and can have understandably different opinions about how to run the kiln.

This control dilemma is created by the different and interrelated dynamics of the combustion and chemical reaction processes. Some control parameters, like flue gas temperature, respond within a few minutes to changes in fuel flow. Others, like the lime reaction process and residual carbonate level take 10 to 15 hours to change. Altogether, it takes a long time to achieve a stable lime kiln process, and there are often many instances of over-control or under-control. It's hard to keep it on course. This is somewhat like the frustrating task of trying to drive a car with severe under-steer or over-steer and sluggish brakes.

The effects of lime kiln instability on the entire recovery line can be profound. Changes in lime quality have a cascading effect on causticizing efficiency, white liquor production, and eventually the fiber line production.

Lime kiln APC can alleviate many of these problems and keep the lime conversion process on course. These supervisory systems oversee the operation of the lime mud washing, fuel combustion, lime conversion and kiln operation controls to stabilize the lime quality.

Stable temperature = even quality

The temperature profile from the inlet to the outlet is an important variable which, if properly controlled, will result in more even lime quality. The calcination process is stabilized by controlling the temperature change in the lime mud as it progresses through the kiln. The temperature of the lime mud cannot be measured practically so the temperature profile of the flue gases is controlled to regulate the lime mud temperature.

In a typical mill the flue gas temperature profile can be managed by:

• The fuel feed, which regulates the temperature level.

• The primary air flow, which affects the flame shape and provides effective heat transfer.

• The flue gas fan rpm, which affects the kiln temperature profile. Oxygen and carbon monoxide measurements modify the draft flow control to maintain a proper level of excess oxygen in the flue gas and low carbon monoxide emissions.

• The kiln rpm, which affects the lime mud depth (degree of filling).

Six percent energy savings

In one European kraft pulp mill the main control variable was determined to be feed end temperature which is regulated by natural gas flow as a ratio of production rate. Consequently, it is the most heavily weighted in the overall optimization control strategy. The flue gas oxygen level measurement is used to control the flue gas flow by regulating the flue fan speed.

By evening out the combustion conditions in the kiln some very significant energy savings were realized at this mill. It's a typical case of taking advantage of lower variations by shifting operating targets. This was made possible by keeping the temperature profile constant with respect to the kiln load. With the lower variation in the combustion level, the fuel flow could be managed to keep flue gas temperatures as low as possible. During a trial run the energy consumption was reduced by 6.16%. The payback time for this result alone was calculated to be less than one year.


Removing a bottleneck

With the improved stability this mill has run the kiln at a consistently higher lime production rate. The improved lime kiln control has also had a stabilizing effect on the complete recovery line. White liquor production has been constant and the causticizing plant has been used at full capacity more days per year. The improved recovery line performance and other factors have allowed this mill to achieve a sustainable fiber line production increase. Other mills equipped with lime kiln APC report lower and more stable residual carbonate levels, increased causticizing plant throughput with lower variability, and significant specific energy savings.

For different mills the control approach might be adapted or tweaked to suit the specific process dynamics and responses. It's worth a look to see if it can benefit your process.

The application and results of APCs on other fiber line and recovery line unit operations will be explored in future articles.