Tackling industrial waste Cement kilns versus Incinerators - An environmental comparison

2. What types of industrial wastes are considered?

2.1 Which five waste streams are the focus of this study?

This assessment only focuses on major waste streams that meet three criteria, namely they:

Among the waste currently used in cement kilns that meet the above criteria, five waste streams stand out, because they alone represent 91% of this total (see figure).

  1. Solvents and waste oils. Solvents mainly come from the chemical industry, where they have been used for chemical processes. They become waste once they can no longer be reused as a solvent. The waste oils partly originate from industry, but largely from oil changes of cars.
  2. Sludge comes from municipal and industrial wastewater treatment processes, for instance from the chemical and paper industry. The sludge from biological wastewater treatment must be dealt with in a safe and effective manner especially if it contains hazardous material. This sludge still contains a lot of water, the rest being mainly organic residue.
  3. Filter cake comes from wastewater filtration in industry and in waste water treatment plants. Filter cake grows in the course of filtration and becomes thicker as particulate matter is being retained. After being removed from the filter and put through a press, the filter cake looks relatively solid, even though it still contains 50% of water.
  4. Paint and ink residues are leftovers and mixtures of paints or inks used in industrial processes, for instance in paint spray machines. These residues also contain volatile organic compounds (VOCs). In this assessment, this waste stream is mainly referred to as impregnated sawdust, because paints and inks are mixed with sawdust when used as fuel in cement kilns.
  5. Fluff comprises a mixture of shredded plastics, textiles, and other non reusable or recyclable materials. It includes for instance material left over after an automobile has been dismantled and the ferrous metal and other marketable materials have been separated. Fluff can only be used in cement kilns if it is shredded fine enough and if it contains no more than a small amount of chlorine and water.

The wastes to be treated mainly come from industrial facilities and wastewater treatment plants located in various parts of Belgium, northern France, western Germany (near the Belgian border) and southern Netherlands. More...


2.2 How much waste and fuel is used in cement kilns?

The five types of waste accounted for about 500 kilotonnes of waste used in the Belgian cement industry in 2006.

Another major waste stream intervening in the Belgian cement industry is animal meal, including bone meal and slaughterhouse waste. Though this waste stream represented about 100 kilotonnes in 2006, it was excluded from this assessment as it is not subject to taxation on secondary fuels.

Excluding animal meal, the five types of waste cover about 91% of the total mass of all waste streams used in the Belgian cement industry as secondary fuels in 2006. The remaining 9%, jointly referred to as “other wastes”, includes seeds, tyres and rubber. These streams are largely similar to the five selected waste streams in terms of energy content and composition.

Besides waste, the Belgian cement industry also used about 480 kilotonnes of fossil fuel to meet its energy requirements, mainly petcoke – a solid fuel produced as a by product of the oil-refining process – but also coal.

The total amount of waste used (about 660 kilotonnes) outweighs the amount of fossil fuel (about 480 kilotonnes) in terms of mass.

However, fossil fuels, which generally burn better than waste, provided 56% of the energy needed to produce cement (for the six cement kilns considered). The five waste streams considered in this assessment cover about 30% of the overall energy requirements. More...

Figure: Average characteristics of wastes compared to fossil fuels 


2.3 How well does waste burn?

Each type of waste differs in the amount of energy released, moisture contained, and ashes left behind.

On average, the amount of energy released by a tonne of waste ranges from nearly 5000 MJ in the case of filter cake (that contain more than 50% of water) to nearly 30 000 MJ in the case of waste oil/solvents (that burn well and contain less than 15% of water). A tonne of waste oil/solvents releases about the same amount of energy as a tonne of coal or petcoke.

Whereas solvents produce little or no ash, filter cake leaves behind nearly a quarter of its weight in ashes. More...

Figure: Average characteristics of wastes compared to fossil fuels 


2.4 How and why is waste pretreated in some cases?

Paint & ink residues are mixed with sawdust before
                                        use in cement kilns
Paint & ink residues are mixed with sawdust before use in cement kilns

Apart from filter cake (from wastewater treatment plants), all industrial waste streams considered in this assessment require pre-treatment before they can be used in cement kilns and one of them, fluff, also requires pre-treatment prior to incineration.

In the case of cement kilns the fossil fuels and raw materials used must also be pre-treated.

Transport of waste streams to specific cement kilns and waste incinerators must also be taken into account to compare the environmental impact of the two systems. More...

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