The manufacture of cement is particularly energy-intensive. To reduce the use of valuable primary fuels like coal, Heidelberg Cement is opting for alternative fuels, such as waste tires.
The calorific value of rubber is comparable to that of hard coal. And because the iron from the armouring can be incorporated into the cement, it reduces the need to add ferrous corrective substances.
“With every ton of waste tires that we’re using, we’re replacing the same amount of valuable hard coal,” explains Michael Becker.
He is the director of the Heidelberg Cement plant in Lengford, a small town in Germany. The company, headquartered in Heidelberg, is one of the largest cement manufacturers worldwide, with eight cement and three grinding plants.
“Waste tires have a high heat content, meaning that they are the ideal fuel for our production,” says Becker.
About 20,000 tons of hard coal are used in the plant annually to produce about 20 percent of the overall heat required, equal to 20 million tires.
“This is how we can substitute the primary fuel with the tires as secondary fuel.”
A roller conveyor guides tires of different sizes through the system.
Replacing two systems with one
The tires used are production waste coming from manufacturers as well as waste tires from trucks and cars. This means they vary greatly: their diameters vary between 30 and 160 cm, their widths between 10 and 40 cm. Weights average between eight and 60 kilograms.
To feed the different tire materials to the rotary kiln inlet, Heidelberg Cement previously operated two systems: one line transported the smaller and lighter car tires, the other one the larger and heavier truck tires.
“We didn’t think that this solution was efficient enough,” Becker says.
“Our employees hung the tires individually by hand into the hook lift and then they were transported into the kiln inlet. The heavy truck tires were handled by an excavator before they were fed into the transport stream. With the new fully automatic system we were able to not only improve this process with regard to occupational safety, but also optimize the working conditions for our employees in combination with an increased performance.”
The new system was supplied and installed by Beumer Group, a manufacturer of intralogistics systems based in Beckum, Germany. It sorts, separates and organizes the tires of different sizes and weights and feeds them to the rotary kiln inlet.
“We delivered a feeding and dosing box to feed the wheel loader, hook separators, separating lines for the tires, a flat belt conveyor with corrugated side walls including a conveyor bridge, a tire transport system for the pre-heater tower and a tire sluice,” notes Ralf Lehmkühler, senior sales manager at Beumer Group.
Wheel loaders slowly take waste tires from the collection point and feed the feeding and sorting box. With a volume of 140 cubic metres, it’s big enough to provide enough material for one shift.
Tires are initially dumped into a feed box equipped with a moving floor.
The sorting box is equipped with a moving floor: plate fins moved by a hydraulic drive transport the waste tires to the outlet side of the feeding and dosing box.
Photocells, mounted at different heights, measure the fill level. When a tire arrives at the discharge side, it is taken over by the hook separator. It then turns upwards and changes direction at the drive station. The tire then falls on a roller conveyor.
“The hook separator starts operating only when the roller conveyor sends a release signal and the photocell at the hook separator is free,” explains Becker. This prevents the tires from falling on top of each other. Additionally, they lose any water that might have collected inside the tire due to the impact when falling on the roller conveyor, and any dirt is removed.
Tires on rims are rejected from the system.
A checking device recognizes if the tires are damaged or if the rims are still on. Those tires are discharged from the system.
The “good” material is transported from a timing roller conveyor to a flat belt conveyor with corrugated side walls. It provides a separate compartment for each tire.
The conveyor transports the combustible material into the pre-heater tower. A scale determines the weight of the tire on the conveyor. The result is registered by the control system and evaluated for the feed regulation of the tires.
Tires are fed individually to the sluice.
The tires enter the sluice through an inlet chute one at a time. An arched chute and guide plate bring the tires from horizontal to vertical.
At the rotary kiln inlet, only one of the flap valves is open at a time to avoid heat losses and flashbacks. “First the upper one opens. Then it closes and the lower flap valve opens,” Lehmkühler notes. A compressed-air tank also ensures that all flaps are closed in case of failure.
The conveying capacity can reach a maximum of three tons per hour, or approximately 700 tires. Thanks to the new system, Heidelberg Cement can now feed its kiln with a continuous stream of tires, and faster than before.