Alternative Fuels & Raw Materials (AFRs)
The term "Alternative Fuels" is generally a euphemism for waste. The waste that
is most often considered as fuel for cement kilns includes used tyres, rubber, paper waste,
waste oils, waste wood, paper sludge, sewage sludge, plastics and spent solvents and spent
Arguments that the cement industry use to justify the use of waste as a fuel are pretty standard. They generally include:
- Conservation of a non-renewable resource
The industry points out that substituting coal with unwanted materials that
need to be disposed of would save coal, a non-renewable resource.
This implies that waste is a renewable resource, which it certainly should not be. Any process which relies on a constant (and in the case of cement kilns, large) stream of waste is intrinsically unsustainable and can only encourage an increase in waste generation rather than any attempts to reduce it.
- Energy recovery from waste materials
As all the energy is used directly in the kiln for clinker
production, the use of alternative fuels maximises the recovery of energy
from waste. The recovery of the non-combustible parts of the waste is
also maximised as the inorganic part is a substitute for raw materials in
the cement and the need for disposal of slag or ash is eliminated.
If we accept that there is no option but to both generate and burn waste, then this argument holds true. The incorporation of heavy metals and other toxins in the cement should, however, be a cause for concern rather than celebration.
- Increased environmental performance
Because of high temperatures, long residence times, high
turbulence, a high PH environment, termal stability and the elimination of
ash residues, cement kilns are more efficient and have a lower environmental
impact than traditional incinerators.
Once again, the premise is that waste is a given, and that it must be burned is a given. It is true that if, indeed, there is no alternative to producing waste and burning it, it is better that it be burned at very high temperatures, leaving little residue. It must be remembered, however, that the waste burned contains heavy metals and toxins, and that these don't just simply disappear. Instead they either go up the stack or are incorporated into the cement.
Overall greenhouse gas emissions will be reduced
After considering the emissions from incinerators and landfills, there is an
overall decrease in greenhouse gas production if waste is co-processed in cement
kilns, because the kilns take these emissions over and no new emissions
Once again, this argument is based on the idea that waste is inevitable and must be burned, so it might as well be burned in cement kilns.
- There is a reduced risk of soil and groundwater contamination
By burning waste in cement kilns there is a reduced requirement
for landfills and potentially hazardous waste will be incinerated and incorporated
into the cement instead of being sent to landfills either as is or in the form of
ash from a traditional incinerator.
We are led to believe that there are no alternatives to waste or what could be done to it, and it is better to have the waste residues trapped in the cement and its products than elsewhere.
The use of AFRs will not significantly change emissions
If t-he basic rules of secondary and raw material usage are
followed (for example, feeding via the correct firing path, storing the waste
correctly, sourcing it from trustworthy sources and setting limits on the quality)
there should be no significant change in emissions from the cement kiln.
There is evidence to suggest that, under perfect operating conditions, emissions are largely limited. Unfortunately, however, perfect conditions rarely persist and during start-up and shut-down situations, as well as upsets during normal operation, emissions have been shown to be problematic.
An industry view can be found in Alternative Fuels in Cement Manufacture - Technical and environmental review.
Waste tyres are a popular fuel for cement kilns. They are a real problem in
waste terms, and are attractive to kilns because they have a high energy content.
In many instances tyres are the only "alternative" fuel that a kiln might use.
Some cement kilns can accept whole tyres, while others require that the tyres first be chipped.
A Friends of the Earth report, Gone to Blazes reports that tests of tyre burning at four California kilns showed the following emission increases when compared to coal:
|Emission||% Increase||Number of Tests|
|Dioxins||53% - 100%||4/4|
|PAHs*||296% - 2230%||3/4|
|Lead||59% - 475%||3/4|
A significant increase of zinc and lead input to the kiln, and between a two
to five times increase in dioxin emissions were found in a German study of a
Belgian kiln burning tyres.
There is plenty of tyre burning information here. A report, Options for the use and disposal of waste tyres, examines both the problems with burning tyres and alternative uses for waste tyres in the South African context.
Although about a test burn case at a paper mill facility, the note International paper and a boiler full of tyres has some interesting legal perspectives on test burns of tyres.
Although about a dedicated tyre incinerator, a letter to the Dixie County Advocate newspaper summarises very nicely a community's concerns about tyre burning.
Blended and Processed Fuels
Blending plants take waste a create fuel products from them. They are given
fancy names like Cemfuel, Climafuel and Profuel to make them sound green and
to disguise the fact that they are
actually just waste, often hazardous, in a different form. Manufacturers
of such fuels claim that they are helping to recycle waste that otherwise
cannot be recycled.
Some of these products are known as SLFs (Secondary Liquid Fuels). SLFs are a blend of organic and solvent wastes. Materials used in such fuels include Oils, Non Halogenated Solvents, Halogenated Solvents, Organic Acids. Glycols, Distillation Residues, Solvent Based Inks, Paints and Adhesives, Aqueous/Organic Mixtures, Viscous Organic Liquids, Toxic Solvents, Organic Sludges and Amines/Alkali. Such fuels can replace up to 40% of the traditional fuels used in the kilns. They are used by injecting them into the kiln burner.
One of the best known SLFs is Cemfuel, produced by a Castle Cement affiliate company in Britain. Castle had hoped that, once waste had been turned into Cemfuel, it would be classed as a fuel and would therefore no longer be seen to be waste. This would mean that it would not be subject to the trade restrictions imposed upon waste through mechanisms such as the Basel Convention. This hope was, however, for the moment at least, dashed in a court case where the judge deemed that Cemfuel remained waste until such time as it was burned and the energy recovered.
Fuels such as Climafuel and Profuel are made from cardboard, paper, plastics, textiles, carpet and other fibrous wastes that are expensive or difficult to recycle and would, says the industry, otherwise be disposed of in landfill sites. The materials are shredded and ground to pieces of about 20mm in size, and then mixed.
Meat and Bone Meal (MBM) products are produced at animal rendering plants through the high temperature processing of animal remains, largely waste from abattoirs. The fuel is the granular solid residue that is left after the tallow (fat) has been rendered out.
Processed Sewerage Pellets (PSP) are made from the sludge from sewerage works. The sludge is treated by drying and then the application of heat to produce a sterile, glassy, pelletised material.