A multiplicity of sizes and geometries affords a “best fit” for a given pebble lime calcining application

Features

  • Two basic preheater shapes, a few different module geometries and a wide range of preheater sizes.
  • Efficient counter-flow heat transfer.
  • The largest hydraulically actuated discharge plungers in the industry.
  • Installed stand-by hydraulic system pump
  • Compartmentalized design includes a stone feed pin gate and an exhaust gas isolation damper at each module
  • Robust design, with refractory lining throughout, affords decades of service life

Benefits

  • Excellent heat recovery
  • Compartmentalized design accommodates on-line removal of any scaling or building within the module that may occur, without compromising production.
  • Large plunger workface allows for more work to be done with fewer moving parts.
  • Redundant hydraulic pump affords excellent system availability/reliability.

Metso supplies all of the spare and replacements parts that are integral to the design of its pebble lime preheaters, such as plunger castings, plunger rods, plunger rod seals, hydraulic cylinders, wall castings, etc. Metso can also provide trained field service personnel to assist in the installation and maintenance of its preheaters, as well as help trouble shoot and resolve any operational challenges that may arise.

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.

Metso plunger style, packed bed preheater for pebble lime rotary kilns - How it works

The preheater is made-up of a series of adjoining modules arranged in a polygon shape (in plan) with a common discharge hopper at the center, or arranged in two opposing rows of modules to form a rectangular shape, with a discharge “slot” in between, depending upon, among other things, the production capacity desired.
Each preheater module receives stone through a flat-sided, vertical chute that connects the module to a corresponding discharge opening in the stone bin directly above. Once inside the preheater, the stone comes to rest on a sloped floor, upon which it builds to form a packed bed. Hot process gases from the rotary kiln, below, are pulled through the packed bed of material by an induced draft fan. The heat content of the gases entering the preheater is such that a nominal 10 to 20 percent of the CO2 in the limestone feed is released in the preheater. Once through the material bed, the gases exit the preheater through each of two exhaust gas manifolds. Each of the two process gas collection manifolds is connected to onehalf of the preheater modules by means of a short exhaust gas duct from each module.
Partially calcined material is pushed from the sloped floor of each module into the center hopper of the preheater by an individual discharge plunger drive assembly.

Each of these assemblies includes a cast alloy steel plunger that is moved by plunger rods connected to a hydraulic cylinder drive mechanism. The flow of limestone through the preheater is controlled by the sequencing of the hydraulically actuated discharge plungers. An hydraulic power unit provides the pressure necessary to operate the hydraulic cylinders that move the plungers. The kiln system operator sets the desired production rate through his computer interface.
The control system PLC determines the number of strokes per hour needed to achieve the desired rate of production, and controls the frequency with which the discharge plungers are stroked, as well as the sequence in which they are stroked.



Specifications
Rectangular design
6 to 10 modules 300 tpd to 700 tpd
   
Polygon design
10 to 22 modules 600 tpd to 1200 tpd
   

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