Volume 4, Number 3
A Gossman Consulting, Inc. Publication
March 1998

Minimum Calorific Value for Energy Recovery in Cement Kilns

Jerry Forgey - ERAtech Environmental, Ltd.
David Gossman - Gossman Consulting, Inc.

The establishment of a minimum calorific content for waste derived fuel (WDF) must be based on the ability of the waste to contribute heat to the cement manufacturing process during combustion. Considering the above, energy recovery from WDF occurs under the following condition: The WDF has an autogenic heating value necessary to make a net positive input to the thermal process that supports calcination.

Cement kilns use counter-current flow for exchange of heat energy from the process gases to the raw material. There are four important stages in the process of manufacturing cement. The first stage is the heating of the raw material to about 900C. At this temperature the second stage begins which is a chemical reaction called calcination. The third stage is further heating of the material to about 1300C. The final stage is a chemical reaction resulting in the formation of the clinker.

The clinkering reactions take place only after the calcination has occurred and the materials have been brought to high enough temperatures. Once this has been done, these reactions are exothermic and sufficient heat is released to bring the materials to the maximum temperature. The clinkerization process is not the one requiring large amounts of energy; it just requires high temperatures. The majority of the heat energy is used to preheat the feed solids and convert the limestone to lime (calcination).

The use of WDF often lowers the temperature of the combustion gases in the hottest part of the kiln. This does not result in the use of more traditional fuel if the energy produced by combustion of the WDF provides energy to the entire system. The maintenance of the raw material temperature at about 1400C in the clinkering zone is accomplished by the exothermic reaction. Lowering the combustion gas temperature actually has the benefit of reducing NOX emissions. Some kiln operators have done this with the use of "low NOX" burners but it can also be accomplished with WDF.

From an engineering standpoint, energy recovery occurs when useful energy is transferred from an outgoing process flow to an incoming flow so that the energy efficiency of the system is increased. For the cement kiln, the outgoing flow is the exhaust gas and the incoming flow is the raw material feed. In a preheater/precalciner kiln the outgoing gas stream contacts incoming feed solids and provides counter-current heat exchange until the gas exits the preheater, typically at 370C. For long-wet kilns the exit gas temperature is even lower.

It is ability of the waste, when combusted, to transfer heat energy to the raw materials in the manufacturing process not the temperature at the hottest end of the kiln that is important for determining energy recovery. In fact, not all WDF is input at the hottest portion of the kiln. Many kiln operators add WDF in the form of whole tires or containerized chemical wastes in the middle portion of the kiln where the temperature is closer to 1000C.

It is also important to note that wastes with low calorific content can be beneficial to cement manufacturing for other reasons. Kilns operate more efficiently when fuel has a narrow calorific content range. Fuel with calorific content significantly higher or lower than the average makes operation of a kiln more difficult. Blending of wastes with low and high calorific content allows for a more consistent fuel to be produced. The stable operation of a cement kiln has certain environmental benefits that include:

  1. Less off-specification product is produced
  2. Undesirable emissions are reduced
  3. More efficient operation of the kiln resulting in higher clinker production/ton of fuel

In addition, the establishment of a minimum calorific content in WDF should not preclude the use of wastes that contain significant quantities of secondary raw materials or the use of waste waters for quench water.