Extensive cooling of the lime product and significant preheating of secondary combustion air

Features

With heat recovery efficiencies in excess of 90%, this direct contact, counter-flow heat exchanger provides both extensive cooling of the lime product and significant
preheating of secondary combustion air to the kiln firing system.

  • Cooler assembly is integral with (kiln) Firing Hood, directly above.
  • Stainless steel internal cooling air distributors.
  • Efficient counter-flow heat transfer
  • Four cooling compartments, each with a dedicated discharge mechanism
  • Dampered cooling air intakes
  • Associated PLC-based controls for control of bed level and material discharge temperatures

Benefits

  • Cools pebble lime product with airflow equivalent to the secondary combustion air required for calcination, minimizing fan horsepowers..
  • High recovery of heat, which remains within the process.
  • Uniform cooling air distribution provides uniform cooling of the lime product and protects product handling equipment.
  • Quality materials of construction afford a long service life

Process overview

Pebble lime calcining is a process by which carbonates of calcium and magnesium (CaCO3 and MgCO3), or limestone, collectively, are heated to temperatures high enough to drive off CO2 and produce oxides of calcium and magnesium (CaO and MgO), lime product.  The process takes place within three major pieces of equipment—preheater, rotary kiln and stationary cooler.  The preheater, using hot process gases from the rotary kiln, preheats and begins to calcine the limestone feed material.  The material finishes calcining in the rotary kiln.  The kiln discharges into a stationary cooler where the lime product is cooled to temperatures low enough to be handled by typical rubber belt conveying equipment.

How it works

 Ambient air is blown into the four quadrants of the cooler’s lower chamber through a common cooling air duct.  The cooling air enters the bed of hot pebble lime product, uniformly, through a collection of fixed stainless steel louvers.  The level of material in the cooler is controlled by how often the four vibratory feeders beneath the cooler are cycled between maximum and minimum speeds.  Thermocouples mounted in each of the four cooler discharge spouts monitor product temperatures, and material flow through each spout is automatically adjusted to provide uniform cooling of the lime product, and at the same time, maximize heat recovery.


 

Technical info

Standard cooler sizes of IV, VI, VIII and X are available for pebble lime production rates from 300 to 1200 tons per day.  Because cooler size is influenced by a number of factors other than the desired rate of production—the elevation of the installation site, the friability of the stone/lime being processed, and to a degree, the type of fuel(s) being used, each size cooler is able to accommodate a range of production capacities.  In addition, due to the variability of the sizing criteria, these production capacity ranges overlap from one cooler size to the next.

Metso supplies all of the spare and replacement parts that are integral to the design of its “Niems-style” pebble lime coolers, such as cooler grate castings and cooler grate support tubes.  Metso can also provide trained field service personnel to assist in the installation and maintenance of its coolers, as well as help trouble shoot and resolve any operational challenges that may arise.

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