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 93 meters in length with a diameter of 6.6 meters. 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 1350 to 1450 degrees C, 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 100 degrees C 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 startup 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® Automation VT Drives 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 Automation VT Drives for this application.
According to Ifurung, “VLT drives are variable frequency drives that are specially designed to handle industrial applications. In this case, the job called for 15 VLT drives, specifically the FC 322 model, for several 460-volt AC motors — five 300 HP, eight 250 HP and two 150 HP.
“These are variable-torque type drives,” Ifurung continues. “At faster speeds, a fan encounters more pressure. That requires more torque to spin the fan. Because torque reductions vary as the cube of speed, every time you can reduce torque you can reduce RPMs. Thanks to Affinity Law physics, that means you can cut kW exponentially. That’s why the VLT drives slashed the plant’s utility bill by enough to pay for the project in less than two years. Plus, they reduced utility demand charges by reducing inrush current at start up.”
Danfoss VLT drives are soft-start devices that gradually ramp voltage up and down when starting and stopping, which eliminates sudden fluctuations in amperage that may result in demand charges.
In operation, the VLT FC 322 drive can vary the current frequency from 0 to 32 kHz to deliver a variety of AC motor speeds. Motor speed can then be matched to the required fan RPM to maintain process quality and reduce electric consumption whenever full torque is not required.
To tap those qualitative and quantitative benefits, the drives use DeviceNet™ protocol to communicate with programmable logic controllers (PLCs). Proprietary process-control software crunches the pressure and temperature data, then instructs the PLCs to tell the VLT drives to change fan RPMs and sequencing as needed. The PLCs also receive 4-20 ma signals from pressure and temperature transducers as a backup. The VLT drives and PLCs are managed through a human machine interface (HMI) panel in the control room.
“The VLT drives operate in an open loop mode with a speed accuracy of +8 RPM — which makes it easy to fine tune air flow,” says Walden. “That kind of accuracy gives us reliable fan control to modulate gas flow and the rate at which ambient air is inputted into the cooler for proper clinker temperature.”
The VLT drives also provide positive benefits for the plant’s electrical system.
“We were able to use the Danfoss Harmonic Distortion calculator to determine the transformers’ K-factor needed in the project,” says Ifurung. “K-factor is a metric of how the harmonic electric load currents affect heat buildup in the transformer. Knowing the K-factor saved us some money on the transformer.”
Electrically, the VLT drives practically eliminate harmonic distortion by incorporating a DC Link reactor. The result is a high power correction (over 0.90) that helps to avoid utility penalties.
Since the plant was going to buy transformers, Decatur was also able to implement a three-contactor bypass with the electronically controlled bypass (EMB2) option. This option provides several modes of operation, including programmable automatic bypass operation with adjustable delay, common motor start/stop command and a firefighter’s override mode. The drives and bypass can be operated by serial or HMI control to provide complete operational control with no loss of command or communication.
To protect against dust and heat, the drives are housed in NEMA 12 cabinets. Unshielded cable lengths up to 1,000 feet and shielded cable up to 500 feet allow the drives to be separated from the motors – a major benefit in the environment surrounding the kiln.
But the big bottom-line benefit of the VLT implementation was the substantial energy savings. “Everyone knows that anytime you can reduce fan motor RPMs you can get exponential energy savings,” says Walden. “In our case, the results are significant. This technology has aided our overall plant electrical efficiency. As a result of those savings, our payback for the project is less than two years. And for the second year in a row, our plant has received EPA’s ENERGY STAR® for Super Energy Efficiency. Danfoss VLT drives are helping our Ste. Genevieve plant build a solid reputation for saving energy.”