Cement Plant Realizes Solid Energy Savings with Variable Frequency Drives
As the internal binder used to make concrete, which goes into roads, skyscrapers and bridges that are the visible monuments of modern life, the production of cement requires the use of high-end technology. At its Ste. Genevieve cement manufacturing facility near Bloomsdale, Mo., about 50 miles south of St. Louis, Holcim (US) engaged Danfoss for advanced variable frequency drive technology to assist in making its manufacturing site one of the most energy-efficient cement plants in the world.
The Holcim, Ste. Genevieve facility is the largest single kiln line cement plant in North America. Testing and land acquisition for the 3,900-acre site began in 1996. Ground was finally broken in 2005, with first production in 2009.
"It took four years and 7.5 million man-hours to develop this site," says Kendall Walden, electrical and process controls manager for the facility. "Holcim chose the site for its high-quality limestone deposit and proximity to the Mississippi River distribution network. As one of the largest cement manufacturing facilities in the world, we are capable of producing over four million metric tons of cement per year and 12,000 metric tons of clinker per day."
Clinker is the critical mineral compound used in making cement. It is produced by mixing and grinding limestone, alumina, iron and silica raw materials into a "raw meal" mixture that is then heated in a rotary kiln furnace. Finally, the clinker is ground with gypsum and limestone to produce fine cement.
“The kiln must be run with very tight tolerances on speed, temperature and air flow to optimize the clinker manufacturing process,” explains Walden. “Otherwise, energy and operating inefficiencies increase plant operating costs."
To make quality clinker, pre-heated raw meal is conveyed into a single rotary kiln — a large steel cylinder lined with refractory that spans 305 feet in length with a diameter of 21.7 feet. Positioned on a slight angle, the kiln rotates at approximately 210 revolutions per hour, as the raw meal travels its length. The raw meal is in contact with hot gases traveling counter current to the flow of material. These gases reach temperatures as high as 2,462° to 2,642°F, which ultimately transforms the mixture into the intermediate product, clinker. At the discharge end of the kiln, the hot clinker falls into a cooler, where the temperature is rapidly lowered to approximately 212°F by air quenching through the use of large fans.
Several fan motors, ranging up to 350 hp, are used to create proper air flow in the cooler. When initially installed, the motors used across-the-line starters, with dampers that applied full-voltage to the motor terminals at startup. As a result, the motors experienced high start-up inrush currents that were five to six times the full-load amps. Additionally the dampers were operating at less than 100 percent open for a majority of the year. This combination presented an opportunity to increase overall efficiency — plus the opportunity to avoid utility demand charges for excessive amperage draw at motor startup.
Walden assigned the engineering of the project to Michael Ifurung, an electrical engineer on his team. Looking for a more energy-efficient solution, Ifurung liked the idea of using Danfoss VLT®
AutomationDrives supplied by Decatur Industrial Electric, Decatur, Ill.
"It’s well known that variable frequency drives (VFDs) can dramatically cut energy costs in fan applications,” explains Ifurung. “For example, if speed can be reduced 20 percent, then kW consumption can be cut up to 50 percent due to the physics of motor Affinity Laws. The critical engineering decision is to select and configure the right kind of VFD for the application.”
After careful consideration of various VFD suppliers and their equipment capabilities, Walden and Ifurung selected the Danfoss VLT®
AutomationDrives for this application.
According to Ifurung, “Danfoss VLT®
Drives are variable frequency drives that are specially designed to handle industrial applications. In this case, the job called for 15 VLT®
Drives for several 460-volt AC motors —five 300 hp, eight 250 hp, and two 150 hp.