Tackling industrial waste Cement kilns versus Incinerators - An environmental comparison

3. How is waste used in cement production?

3.1 How is cement produced?

Cement kilns produce clinker
Cement kilns produce clinker

The Belgian cement industry mainly produces Portland cement, the most common type of cement in much of the world. A fine powder, it is a basic ingredient of concrete and mortar.

Portland cement is made of quarried limestone and other minerals containing calcium, silicon, aluminium and iron. Various production processes are available known as “wet process”, “dry process” or variations thereof.

They all share the following steps:

Raw materials, referred to as raw meal, are first:

They are then subjected to high temperatures (up to 1450 °C) in a cement kiln in order to convert them into cement clinker. The kiln consists of a long cylinder that is nearly horizontal and rotates slowly around its axis. The raw meal is fed into the slightly higher end and gradually moves down towards the lower end while being heated and mixed. Considerable quantities of fuel are needed to keep the kiln hot for the desired chemical reactions to take place.

The small lumps of solid material that come out of the cement kiln are referred to as clinker. They are cooled and some of the heat is recovered.

Subsequently, the clinker is ground and blended with other materials to produce the finished Portland cement. More...


3.2 How can waste replace part of the fuel and raw materials used in cement kilns?

In recent years, cement kilns have been used to treat various types of waste such as tyres, waste solvents and lubricants, hazardous waste (as hazardous organic compounds are completely destroyed by the process), and bone meal and slaughterhouse waste.

Cement kilns are generally heated by burning fossil fuel, but combustible material contained in the wastes can replace some or all of the fossil fuel. Residue (ash) from burning waste is incorporated into the cement, becoming part of the final product. This locks away some environmentally damaging waste in the cement and replaces some of the raw materials needed to produce the cement.

This study considers six cement kilns in Belgium that use two types of fossil fuel as primary fuel. The main fossil fuel is petcoke (short for petroleum coke) – a solid fuel that is a by-product of the oil-refining process. Though these cement factories also use coal as a primary fuel, adding one tonne of waste in practice would mainly replace petcoke.

The comparison considers the raw materials, wastes and fuels that go into the cement kiln, and the environmental impact of adding a tonne of waste to the process to replace some of the fuel and raw materials used. This would not change the overall amount of cement clinker produced since the fuel and raw material inputs would be adjusted accordingly.

In terms of energy content, adding 1 MJ of waste replaces 1MJ of fuel (mainly petcoke

In terms of mass, adding one tonne of a specific type of waste to the process replaces a certain amount of fuel (mainly petcoke) and a certain amount of raw materials (raw meal) as indicated in Table 2.

Table 2: Substitution scenarios for each of the five wastes in the cement kilns 

For example, one tonne of industrial sludge would on average replace 250 kg of petcoke (fossil fuel) and 270 kg of raw materials.

Fluff (mixture of plastics, textiles and other materials) burns better than sludge but still leaves a lot of ashes. Adding a tonne of fluff would on average replace 650 kg of petcoke and 120 kg of raw materials.

In the case of solvents and waste oils, because they burn even better and leave hardly any ashes, adding one tonne of waste replaces on average 880 kg of petcoke but requires a slight addition of raw materials. More...


3.3 How does the use of waste change the outputs of cement production?

To assess how the addition of a tonne of waste would change emissions to air and the composition of the clinker the different constituents of the waste and of the fossil fuel they replace are considered, including content in heavy metals, sulphur, chloride, carbon, etc.

For each substance that goes into the kiln it is determined how much would end up in the cement clinker or in emissions into the atmosphere. There are no emissions to water and soil. Matter, such as fine ash, that is not leaving the plant but is instead recycled or mixed with the final clinker, is not regarded as an emission to the environment. Theoretically this might lead to an increase in release to the environment, in case the cement comes into contact with rain or groundwater. However, previous studies have determined that this increase in release is so low that it cannot be detected by measurement devices. The environmental impact thereof is therefore considered to be negligible in the frame of this assessment.

For example, if we know that as a result of the change in the fuel mix the amount of sulphur that goes into the cement kiln is decreased, we can calculate the resulting decrease of sulphur emissions into the atmosphere. These calculations (referred to as transfer functions) are based on information obtained from the six Belgian cement kilns.

Table 1: Average properties of selected waste streams and fules 

It is assumed that no additional NOx is produced by using an extra tonne of waste to replace some of the fossil fuel used. From measurements in practical situations, it is known that the use of waste streams leads to a decrease in the formation of NOx. This means that in this study the assumption that the NOx emission will not increase, as a result of the use of more waste as a fuel, in fact is the worst case. More...

The Three-Level Structure used to communicate this Life Cycle Assessment is copyrighted by GreenFacts asbl/vzw