Lime kilns switch process control systems, without shutdown

Facing an estimated four day shutdown to install a new process control system, operators, technicians, and engineers from National Lime & Stone Co. (Carey, O.) worked with engineers from A. E. Ehrke & Company (Cleveland, O.) to avoid shutting down lime kiln production.

Kilns at the Carey mill operate 24 hours a day, seven days a week to annually process 20 million tons of limestone used in asphalt, carpet backing, ready-mix concrete, and many more nonorganic substances used in everyday life. Because it takes at least 12 hours for a cold kiln to reach operating temperature, shutting down is highly undesirable.

Each kiln has a horizontally rotating hearth that moves limestone through combustion gases well above 2,0008F. Over about one-hour, hot gases drive off the carbon dioxide. The goal is reduce the residue carbon dioxide to 1-2% in the shortest time. If too hot, limestone sinters into lumps and fuel is wasted. If too cool, cycle times increase and profits decrease.

Challenged to increase production and reduce variability, the old control system was replaced with a Fisher-Rosemount Systems Inc. (Austin, Tex.) Delta V scalable process automation system.

Beginning on a Monday in early February, 1998, the old system was removed and surprises immediately surfaced. With none of the original signal wires tagged, each pair of wires were tested and labeled, and a lot of unused wires were removed.

By Wednesday afternoon, all wires had been identified, application software configuration was complete, remote devices were working, and the system was powered up. With the controller and operator station on-line, a sensor problem affecting fan-damper control appeared. Having the kiln hot made troubleshooting easier and allowed system tuning. Customer acceptance testing was completed on Thursday. Only completion of operator training remained. The kilns had never stopped producing.

By mid-day on Saturday, three shifts of kiln operators were trained to use a mouse to navigate the graphical kiln displays and make operational changes.

In addition to the original 7 temperature, 5 flow, 10 pressure, 8 amperage, and 2 oxygen transmitters, 15 temperature transmitters were added to provide a profile of kiln performance and lime quality.

Control logic includes a master kiln temperature loop with adjustable air/fuel ratios to trim combustion air and natural gas demands. Cross limiting of fuel and air ensures safe combustion control during load changes.

System logic monitors and remembers modes and setpoints and restarts control logic following power outages; a fully manual task in the old system.

Fitted with a modem, the new system supports remote diagnostics and engineering help within minutes, 24 hours a day.

Online data collection and graphing of temperature and speed variables is proving beneficial in conducting laboratory analysis and meeting customer specifications.

Coming in on budget and on time, the new system has sparked ideas to improve the process. Improvements being considered include:

Installation of a fuzzy logic controller to stabilize kiln temperatures caused by raw material variations;

New, robust oxygen analyzers capable of performing self diagnostics;

Development of fully automatic kiln control to reduce process variability caused by manual interventions; and

Optimizing production control by identifying process constraints and manipulating other production equation factors to compensate for constraints.

Installation of a new process control system without a complete shutdown is a challenge, but with the help of a good technical team, we accomplished more than we thought possible.

For more information, visit www.controleng.com/freeinfo to read “New Heights in Process Improvement”

Author Information

Dave Harrold, senior editor dharrold@cahners.com

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