Limestone Kilns with Preheaters
Metso is the worlds leading supplier of pre-heater / rotary kiln lime calcining systems.
Metso's pre-heater / rotary kiln systems produce the highest quality lime, with the widest range of stone feed size.
Pre-heater / rotary kiln systems can use gas, oil or coal for firing. Metso Minerals has designs for single line plants up to 1,800 tpd.
Main benefits of preheaters are:
- High recovery of heat from kiln exit gas.
- Lower SOx and NOx emissions.
- Operates on a variety of fuels.
- Reduced fuel consumption.
- Wide range of stone feed sizes.
- High production capacities.
- Reduced kiln length.
- Reduced capital equipment cost.
- Control system for entire facility.
- Capable of installing anywhere in the world.
- In house engineering for custom fit applications.
Modern plants are composed of 3 key components to acheive high effiency: Cooler, Kiln, and Preheater.
Coal Fired, Preheater Kiln System with Pulse Jet Baghouse
A lime preheater is a piece of equipment used in conjunction with a rotary lime kiln that allows the length of the kiln to be reduced while increasing the fuel efficiency of the system. The connection of the preheater to the kiln is accomplished with a transfer chute, which is both a duct to convey hot gasses from the kiln to the preheater and a chute to convey stone from the preheater to the kiln feed end.
Metso preheaters are available in rectangular or polygon designs, depending on the capacity of the system desired. The preheater is composed of a refractory lined floor, roof and side walls with hydraulically powered discharge rams. Limestone of ambient temperature enters the top of the preheater by gravity flow through a series of stone chutes and builds up on the floor until a vertical bed of stone is established. Hot gasses from the kiln are drawn up through the bed of stone by the systems induced draft fan, giving an efficient transfer of heat from the gasses to the stone. Hydraulic cylinders push the rams which slide the lower part of the stone bed across the floor to the preheater discharge opening where the stone falls through the transfer chute and into the kiln.
The rams are sequenced and timed to give an appropriate feed rate to the kiln. The purpose of the preheater is accomplished by transferring vast quantities of heat from the kiln exit gas into the feed stone, thereby increasing calcining efficiency and reducing fuel consumption.
Limestone enters the calcining system by being fed into the stone bin by the stone conveying equipment. The stone exits the bin by gravity through the stone feed chutes and into the preheater. The internal geometry of the preheater holds the accumulated stone into a vertical bed until it is heated and ready for discharge. The hot kiln exit gases are pulled from the kiln, through the bed of stone, before exiting the preheater through the exhaust gas duct. As the hot gases pass through the stone bed, sufficient heat is transferred from the gas to the stone to start the calcination reaction. This heat recovery is one of the key elements in producing high quality lime with reduced fuel consumption.
After heating and partial calcining in the preheater, the stone is discharged by having the bottom of the bed of stone pushed by a hydraulic ram to the preheater discharge opening. As the ram is retracted additional stone flows from the stone bid to replace the material that was discharged. The discharged stone flows by gravity through the transfer chute and into the kiln. The high temperatures in the kiln complete the calcining process.
The limestone now calcined, has been converted to burnt lime. The burnt lime gravity flows from the kiln into the cooler where a vertical bed of lime is formed. Ambient air is blown through a series of louvers at the bottom of the bed, up through the bed of hot lime. The direct contact of the cooling air with the lime provides good transfer of heat from the lime into the air. The heated air leaves the cooler and is used by the kiln firing system as primary and secondary combustion air. This recycling of heat from the product stream back into the process stream is another key element in producing quality lime with reduced fuel usage.
Stone feed size to a preheater rotary kiln can range from 1/4” to 2-1/2” with the preferable ratio of top to bottom size of 2 : 1. In a modern, well designed preheater rotary kiln calcining system, fuel efficiency can be as low as 4 MBtu/Ton.
A preheater can be added to an existing kiln of any length. By retrofitting a preheater onto an existing kiln the system efficiency can be greatly improved. In some cases the preheater is added at the feed end of the existing kiln. In other cases the preheater is installed over the kiln, as the kiln continues to operate. After the preheater is completely erected, the kiln is shortened and a transfer chute is installed, connecting the preheater to the kiln. Downtime is reduced and operations are enhanced.
Feed size to a preheater rotary kiln can range from 1/2" to 3" with the preferable ratio of top to bottom size of 2:1. In a modern, well designed kiln the fuel efficiency can be as low as 4 MBtu/Ton.
Rotary kiln technologies/preheater
Rotary kiln technologies/cooler
A preheater can be added to any kiln of any length. By adding a preheater efficiency can be greatly improved. In some cases the preheater is added at the kiln end at feed hood. The latest upgrades the preheater is built above the kiln while the system stays in operation. After the preheater is completely erected, the kiln is shortened and a transfer chute is placed below the preheater. Downtime is reduced and operations are enhanced.