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PRODUCTIVITY IMPROVEMENTS AT CRICKET MOUNTAIN

World Cement August 1999

Gino Bittante and Greg Hassell, Universal Dynamics Technologies Inc., USA, and Joe Brokke, Plant Manager, Continental Lime Inc., USA, describe the recent KilnMax™ installation project on the No. 3 kiln at the Cricket Mountain lime plant; looking at the optimisation process, the results of the project, and opportunities for further improvement.

Background

Continental Lime's Cricket Mountain plant, located 35 miles south of Delta, Utah, started production from its first 500 tpd kiln in July 1980. A second kiln was added in 1987, a third in 1992, and a fourth, 1200 tpd kiln, started production in February 1998. As a result, this plant is now one of the largest, most modern and efficient lime plants in the Western US, with a total capacity of over 900 000 tpa. High quality limestone is quarried in the Cricket Mountains, six and a half miles west of the plant. The limestone is crushed and sized at the quarry and delivered to the plant by truck. The stone is then processed in four, modern, coalfired, preheater rotary kilns. The plant manufactures a full range of bulk quicklime and crushed limestone products. Storage and shipping facilities provide a reliable supply to customers by both truck and rail.

During the construction of kiln No. 4, an upgrade of the controls on kilns 2 and 3 was initiated to standardise the features and characteristics of kilns 2-4. A Windows NT based control system and operator interface was installed, and the Programmable Logic Controllers upgraded to the same models as those installed for No. 4 kiln. The process sensors, analysers and actuators for all kilns were already all up to date and performing reliably.

The Cricket Mountain plant ran three kilns with just one operator per shift. The operator's duties included taking regular product samples and performing quality tests for all three kilns, as well as controlling the kilns themselves. It became clear that some type of smart 'cruise control' was needed on at least one of the kilns to enable the same operator to handle the increased workload of a fourth kiln while maintaining the same level of equipment monitoring and quality testing.

No. 3 kiln was selected as the first kiln on which to implement the KilnMax* optimisation system, supplied by Universal Dynamics. The reason for selecting Kiln 3 over the new kiln under construction was that it was the newest and best instrumented of the existing kilns, and had an established performance baseline. Kiln 3 was also considered the best performing kiln in Continental Lime's operations. If Universal Dynamics could improve the operation of this kiln, then bigger payoffs could be expected on the company's other kilns.

After the start-up of No. 4 kiln in February 1998, work on the optimisation of kiln No. 3, the KilnMax control system, began. The optimisation goals were to improve the kiln's ability to make lime of several different customer specifications, improve fuel efficiency, and automate some of the kiln operator's duties.

In June, the initial KilnMax setup on No. 3 kiln was completed and left in control for over a month. No. 3 kiln was purposely run at reduced capacity while No. 4 kiln was pushed hard to verify its production capabilities. During the first full month of the new control, No. 3 kiln produced a lime with very tight specifications and with very little operator intervention. This smooth initial start-up and operation with almost no operator intervention at lower production rates had a 'good news/bad news' effect on the overall project. The 'good news' was that operators immediately accepted and liked the system because it worked very well and allowed them to almost completely forget about No. 3 kiln. The 'bad news' was that therefore the need for in-depth operator and supervisor training was overlooked.

Figure 2. KilnMax's BrainWave kiln temperature control. Top trend panel: the yellow line is the desired kiln temperature setpoint. The red trend shows the actual kiln temperature. The light blue line shows changes to fuel rate and the green line shows the changes to stone feed rate to maintain the target temperature. Bottom trend panel: graphical representation of model for one of the process variables.

The requirement for more operator training became evident when market conditions in late July required that No. 3 kiln be pushed to maximum capacity. When the operators switched KilnMax on at very high production rates for the first time, it did not function as the operators expected. They did not have the training, practice, or confidence to make the necessary expert system setting changes on their own and turned the system off. This problem was overcome by additional operator and supervisor training away from the control room and by providing better documentation of the features and characteristics of the control system. In addition, Universal Dynamics' engineers in Vancouver, Canada connected to the plant in real-time control mode via modem and had telephone discussions with each operator on a one-on-one basis over a period of approximately two months.

The combination of formal training and remote support ensured that each operator experienced a wide range of operating conditions. This type of training and support is essential to the long-term success of a kiln optimisation project as operators need to fully understand the behaviour of the kiln and feel comfortable with the compromises that the control system makes when the physical limits of one or more pieces of kiln equipment is ultimately reached.

KilnMax was switched on again in late August and was used off and on for several periods until the end of November 1998. This allowed a good comparison of kiln performance with and without the system to be made. During this time, the kiln performance was monitored by Universal Dynamics through a modem link to their Vancouver, Canada office. Additional opportunities for improvement were identified and control program changes were made online without interfering with kiln operation.

The changes made from September to early November significantly improved kiln performance over the range of kiln operation. Here the use of Universal Dynamics' patented multivariable BrainWave controller within KilnMax's rule-based expert system showed its advantage. It allowed KilnMax to operate from 60% to over 100% of the kiln production range at higher than previously possible quality levels. Results from this period confirmed that kiln performance increased as time went on. In late September and again in early November, the aggressiveness of the stone feed control loop was increased to make it respond more quickly to temperature disturbances at all production levels. In mid October, a more robust method of calculating ram output was implemented to improve kiln bed depth control.

Results

No. 3 kiln performance data for the September to November period includes laboratory lime quality test data and the control system trender data. This time span includes significant periods of both manual and KilnMax control. It provides a good comparison between kiln performance with and without the control system.

KilnMax controls four areas of the kiln, as follows:

• Heat input (through coal feed rate).
• Excess oxygen (through cooler air flow and kiln draft).
• Kiln temperature (through stone feed rate).
• Kiln bed depth (through kiln speed and ram stroke rate).

Along with these measurable items, the plant laboratory test data recorded the quicklime product characteristics.

Heat input

The system controls the fuel feed rate: it pushes heat input to the kiln, trying to efficiently burn as much coal as possible. The coal feed rate is automatically increased to the maximum amount set by the operator and the oxygen/combustibles levels on the kiln flue gas. Coal feed rate automatically adjusts to changing operating conditions and drops when there is a need to protect the kiln.

Excess oxygen

The system trims the cooler air to maintain excess oxygen values. Under manual control the average variation from set point was 0.47%. With KilnMax, the variation was 0.22%. Tighter excess oxygen control results in a more efficient kiln.

Kiln temperature

Analysis of kiln operating data and lime laboratory data shows that lime quality characteristics are related to the kiln temperature profile and bed depth for any particular limestone feed. The kiln temperature profile is the most significant variable affecting lime quality. The limestone feed rate is adjusted by the system to maintain a temperature index derived from the kiln temperature profile. The temperature index is the major determinant of lime quality.

Comparison of the kiln

The temperature index shows that the kiln temperature is much steadier with KilnMax than without it, and the control reduced the average temperature deviation from over 4% to less than 1%.

Bed depth

Prior to the installation of the control system, optimum bed depth control was next to impossible. Trying to match kiln speed with the preheater ram stroke rate was very difficult because the amount of stone moved by each ram stroke varies by up to 50% over time.

The preheater ram lb/stroke is calculated by the system, which adjusts the kiln speed and ram stroke rate to maintain a constant bed depth in the kiln. With this feature, the kiln produces good quality quicklime under most conditions with bed depths of up to 1700 lb/rev (higher values have not yet been tried). Therefore the kiln is able to operate with bed depths that are roughly 100 lb/rev higher than previously. The higher bed depth increases fuel efficiency by 2 - 4%.

Lime quality

Tight control of bed depth and the kiln temperature profile produces lime with less variability (LOI and reactivity). Laboratory data for the period from 14 September to 16 November 1998 showed that KilnMax operation increased the quicklime quality by over 50% compared to operation without it. During this time, the control system operation was still being improved and operators were still learning how to use it.

Other findings

Cooler control

As part of the project an improved control scheme for the contact cooler was installed. In addition to balancing the four cooler discharge temperatures, the cooler control system automatically detects and takes corrective actions to prevent lime from free flowing or bridging within each discharge leg. Test data shows that a 10% change in cooler level causes about a 100° change in firing hood temperature. This demonstrates the importance of keeping the cooler level as high as possible (for efficiency) and as steady as possible (for quality).

Coal quality

The kiln experiences frequent temperature disturbances from such things as changes in stone size from segregation in the preheater, changes in the amount of stone per preheater ram stroke, and changes to the heat value of the coal. Sometimes the kiln temperature will suddenly start to increase or decrease rapidly. The stone feed rate is adjusted by the control system to compensate, eventually bringing temperature back into range. In many cases, the cause of the temperature disturbance is change in coal quality (either moisture content or heat value).

If the kiln is fed drier coal, the weigh feeder ensures that the mass flow of coal is kept constant, but as the coal is now drier, the kiln receives more coal and less water. This causes a drop in excess oxygen and an increase in combustibles. KilnMax compensates by adding cooler air to rebalance the fuel air mix. The effect is the same as a simple coal feed rate increase. This increase in 'effective coal' results in the kiln getting hotter. The opposite happens if the system starts burning wetter coal. From the operating data it was determined that the coal heat value fluctuates by S - 10%.

Because temperature deviations make it more difficult to achieve consistent product quality, it is important to keep coal as consistent as possible. When this is not possible, a control scheme that senses the change in coal quality and automatically compensates for this (by adjusting cooler airflow) will achieve even tighter control over lime quality.

Conclusion

The trend data and laboratory results show that the KilnMax system significantly improved kiln performance and reduced product variability by over 50%. Kiln fuel efficiency increased by over 3% and production of one tightly specified product increased by 15%. Acceptance of the system by operators was very good, as they had more time to concentrate on other duties and improve the overall operating efficiency of the plant.

In hindsight, additional training for production, maintenance and operations staff in the use and features of the system earlier in the optimisation program would have accelerated understanding and acceptance by all.

Occasionally, at the end of a production campaign, the ID fan capacity on kiln No. 3 is limited from plugging of the preheater. This has caused some degradation of KilnMax performance under this difficult operating condition. Further productivity and quality improvements will be possible even under these restricted conditions when on-line quicklime quality test measurement equipment is installed. Lime quality measurements will be automatically logged and correlated to optimise kiln control settings to meet the various customer quicklime specifications.

Enquiry no: 7

* KilnMax is a trademark of Universal Dynamics Limited