Even for a leading producer like San Antonio, Texas-based Capitol Cement, a division of Capitol Aggregates, Ltd., meeting today's unprecedented demand for cement — driven by solid growth and reconstruction efforts — presents a formidable challenge. Operating for more than 40 years, Capitol produces cement powders and clinker for use in the construction and oil well service industries. Its arsenal includes a wet process kiln, a dry process kiln, and two finish mills to grind clinker and gypsum to cement powder. Part of the Zachry Group headquartered in Texas, Capitol ranks among industry pacesetters in adopting the latest technologies to produce approximately 900,000 tons of cement each year.

THE CHALLENGE

In Texas, day-to-day exigencies of cement manufacturing — like varying feedstock composition, fuel changes, process upsets, grade transitions, and high energy costs — are intensified by market demand outstripping supply. Although Capitol upgraded its dry process kiln in 1999 from 1,500 to 2,000 tons per day, providing a 33 percent increase in clinker output, its finish mills were not equal to the expanded operations. Accordingly, as each incremental production gain translates into increased profit under current market conditions, the producer endeavored to alleviate the finish-mill constraint.

THE APC SOLUTION

After researching several options, Capitol Cement turned to Pavilion Technologies for an advanced process control (APC) and optimization solution to eliminate the mill bottleneck and help achieve production goals. Initially, the producer received an estimate of value and a satisfaction guarantee prior to starting the project as part of Pavilion's ValueFirst program. Exceeding predicted results, production increased 10 percent; product variability decreased 30 percent; and, energy consumption dropped 4 percent in just weeks. A return on investment within months prompted Capitol to deploy Pavilion's control and optimization solution on the dry kiln line, including a precalciner and grate cooler, to further increase production and reduce energy use. Today, the operation provides a singular model of an optimized, performance-driven, integrated mill and kiln APC solution.

“We tried fuzzy logic advanced control tools,” explains Plant Manager Tom Spaits, “but we found these solutions required more ongoing attention than we were able to provide. Talking with our process manager, Tom Giuliani, we agreed that an advanced process control solution that didn't require a lot of upkeep might help us to reduce the milling bottleneck.”

PERFECTING THE PROCESS

Pavilion's cement solution combines the capabilities of its advanced process control and environmental compliance software. By means of the automated system, cement manufacturers like Capitol implement raw-mill, kiln, precalciner/preheater, clinker-cooler, finish-mill and alternative-fuel control; environmental monitoring and reporting; and, plant-wide performance management. The Pavilion8 integrated software suite combines modeling, control, monitoring, analysis, visualization and integrated functionality to deliver return on investment for cement customers within three to 12 months, system developers note.

Similar to the way an experienced operator makes decisions, the Pavilion8 control solution continually improves performance by evaluating historical trends and predicting future performance. Providing an advantage over less sophisticated technologies that rely merely on past and current conditions without evaluating interrelationships between process parameters, Pavilion's software closely monitors performance and considers a greater number of interactions to develop a long-term projection of outcome. The operator is thus permitted to focus on mission-critical process issues, thereby improving production efficiency to capitalize on market opportunities while reducing costs.

“Because our operators could see what the controller was anticipating as its next move, they quickly became comfortable, knowing that the [Pavilion] multivariable predictive control [MPC] solution was operating in a safe and responsible manner,” observes Process Manager Tom Guiliani. “Almost from the first day, operators felt confident that the MPC wasn't going to fly the plane into the ground.”

HARD AND FAST RESULTS

Pavilion's cement solution was initially installed on Capitol's finish mill #6 to maximize mill new feed, subject to the constraint of elevator amps while maintaining the desired cement Blaine. Exceeding all criteria for a successful application, the system generated a 10 percent increase in plant production within a few weeks,

Spurred by that success, Capitol proceeded with the Pavilion8 Finish Mill application on a second unit. Again, rapid results included increased production rate, reduced specific power consumption, improved product consistency and quality, plus lower overall operating and maintenance costs. Capitol Cement thereby eliminated its finish-mill bottleneck and increased the value of its kiln investment without purchasing additional costly equipment.

Finish Mill	Cement Type	Increased Production
Finish mill #6	Type I	6.8%
	Type III	5.1%
Finish mill #5	Type I	7.9%
	Type III	9.9%

After realizing significant benefits from the Pavilion8 Finish Mill Application, Capitol adopted Pavilion's APC: Dry Kiln to enhance the productivity and efficiency of its dry kiln line, including a precalciner and grate cooler. Local market conditions presented the opportunity to capitalize on excess clinker capacity by selling clinker and shifting production from Capitol's older and less efficient wet kiln to the modern, dry precalciner kiln.

To maximize production, minimize fuel consumption, reduce NOx emissions and improve clinker consistency, a kiln-cooler control application implemented the following processes:

• Controlling tower stage-four temperature to stabilize the calcining operation
• Controlling kiln burning-zone temperature by monitoring kiln exit NOx as a surrogate for temperature
• Controlling tower-exit excess oxygen to minimize heat lost to the stack
• Predicting free-lime changes on the basis of temperature
• Controlling free lime to maintain clinker quality
• Controlling kiln production rate in order to stabilize overall line operation.

The Pavilion8 Kiln Control Application used staged modular implementation to coordinate multivariate and interactive kiln-cooler operating conditions. Replacing a less-than-effective PID loop used previously, a single-variable controller on the cooler served to regulate cooler bed depth (CFG fan manifold pressure) by manipulating grate speed. Control of the kiln and precalciner tower, including tower stage-four temperature control previously effected in a PLC PID loop, was achieved by means of a multi-variable controller. Supporting the controller are three calculation block applications, providing external functionalities and data such as analyzer signal validation, free-lime prediction, and free-lime lab biasing.

Using Pavilion's Cement Solution enabled Capitol Cement to increase throughput, reduce excess oxygen, reduce NOx and increase tertiary air temperature — all of which contribute directly to energy efficiency. Increased stability was another benefit. Besides increasing production, furthermore, the system has dramatically eased operations through the myriad changes caused by variable feed chemistry, coal quality, and operational factors such as roller press starts/stops. These dynamic situations are handled automatically — no operator intervention is needed. Pushing the kiln towards more efficient operation by applying Pavilion's Kiln Control solution has allowed Capitol to maximize its existing infrastructure to meet market demand. The project took approximately three months to execute, yielding a return on investment for the dry kiln application in less than six months.

Affirms Capitol Cement Vice President Max Frailey, “In just a few short weeks, Pavilion's solution allowed us to increase our production by more than 10 percent. The financial benefits we expect to realize encouraged us to extend the application throughout our facility.”

KEY BENEFITS
PAVILION APC: FINISH MILL

• Increased production rate year over year: 5% to 10%
• Decrease in variability of product: 5% to 50%
• Reduced specific power consumption by 4%
• Increased product consistency by 5% to 50%
• Decreased overall maintenance costs

PAVILION APC: DRY KILN - KEY BENEFITS

• Increased average feed by 2.5%
• Reduced energy use by approximately 3%
• Reduced exit oxygen by 0.58% and reduced oxygen standard deviation by 41%
• Reduced NOx lb. by 8% and reduced standard deviation by 42%
• Increased average tertiary air temperature by 7% and reduced standard deviation by 33%
• Increased average free lime by 14% (closer to limit) and reduced standard deviation by 36%
• Realized controller uptime of 89%