SLC-D Preheater System

With its high temperature pre-combustion chamber, the SLC-D offers unique flexibility in fuel use. Choose as new system, or quickly retrofit your existing ILC or SP preheater

FLSmidth SLC-D
  • The Basics
  • Design and Operation

Key Benefits

  • High material and gas retention times for effective operation
  • Suitable for many fuel types, especially low-volatile fuels
  • Low NOx operation
  • Enables cost-effective tower dimensions, as the calciner can be installed separately from the cyclone tower
  • Very well-suited to retrofits of existing SP or ILC preheaters due to short kiln downtime for tie-in

How it works

Separate line calciners are also known as “air-only” calciners, since the calcining chamber is at least partially offset from the kiln riser. Combustion air is drawn through a separate tertiary air duct.

Because kiln combustion gases don’t pass through the calciner, the calciner size can be reduced to attain the required gas velocity and retention time. This often results in a shorter, less costly preheater structure and a shorter erection time for retrofits.

The SLC-D configuration combines the advantages of the well-known separate-line calciners with the low NOx features of the in-line calciner when a portion of the calciner fuel is directed to the kiln riser duct. This effectively destroys thermal NOx formed in the kiln.

All Fuel Types

Because the combustion in the calciner takes place in hot atmospheric air and the combustion temperature in the calciner can be controlled independently of the temperature of the calcined material fed to the kiln, the SLC-D is very well-suited to many fuel types – especially low-volatile fuels.

Operating Characteristics

  • Combines separate, downdraft pre-combustion chamber prior to mixing with in-line calciner
  • High-temperature combustion in atmospheric air very well suited to burning low-volatile and waste-derived fuels and ensures low NOx generation
  • Larger downdraft calciners able to meet stringent residence time requirements at a given high temperature for hazardous wastes such as SWDF (Solid Waste Derived Fuel) or LWDF (Liquid Waste Derived Fuel).
  • Delivers high material and gas retention times in pre-combustion chamber whose dimensions are minimal since kiln gasses do not pass through it
  • Raw meal elevated to top of calciner by tertiary air to minimize preheater height
  • Incorporates DUOFLEX style burner to downdraft chamber to ensure flexible and effective combustion


In the SLC-D, raw meal entrained in hot tertiary air enters the downdraft calciner at the top through a 180° involute and exits through a truncated cone which forms a circular duct transition to the kiln riser duct.

The 180° involute ensures that the majority of the calcined meal is detained along the walls of the calciner. This protects the refractory lining while promoting hot core conditions for optimum combustion.

For the same production capacity, the SLC-D calciner has smaller dimensions than the ILC calciner, since no kiln exhaust air is led through the calciner. This makes the SLC-D system well suited for retrofit applications even when low reactive fuels are not planned.

Design Characteristics

  • Full capacity range of today’s plants, up to 12000 t/d or more
  • Built with single or multiple preheater strings
  • Ratio of firing in calciner: 55-65%
  • Normal bypass of kiln gas: 0-60%
  • Calcination at kiln inlet: 92-95%