The cement industry is highly affected by increased energy prices and CO2
penalties. The need for adaptation to a more environmentally friendly,
and cost effective, energy solution is urgent in the increasingly
competitive global market.
In cement plants, generic measures like preventive maintenance, use of
efficient motors, efficient and variable speed drives, proper
insulation, optimization of compressor and lighting systems can help
reduce energy consumption.
Cement Kilns
Cement kilns are used for the pyroprocessing stage of manufacture of Portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates.
Over a billion tonnes of cement are made per year, and cement kilns are the heart of this production process: their capacity usually defines the capacity of the cement plant.
Manufacturing of Cement
Cement is manufactured through a closely controlled chemical
combination of calcium, silicon, aluminum, iron and other ingredients.
Common materials used to manufacture cement include limestone,
shells, and chalk or marl combined with shale, clay, slate, blast
furnace slag, silica sand, and iron ore.
These ingredients, when heated
at high temperatures form a rock-like substance that is ground into the
fine powder that we commonly think of as cement.
The most common way to manufacture portland cement is through a dry
method.
Dry process
In the dry process, the raw materials are dried in a
combined drying and grinding plant to reduce the moisture content below
1%. The materials are ground in either a ball or a roller mill with
drying of the materials being achieved by sweeping the mill with the
exhaust gases from the kiln.
The drying capacity of the raw grinding
system may be supplemented by auxiliary hot air generating furnaces. The
dried, finely ground raw mix powder is then blended and homogenised in
silos with the mixing being provided by compressed air.
The blended and homogenised raw mix is then usually fed to a gas-suspension preheater ahead of a short rotary kiln.
These preheaters consist of successive heat exchanger and collection
cyclone stages arranged above one-another in series in a tower.
Partial
calcination of the raw mix takes place prior to the mix entering the
rotary section of the kiln.
Additionally a precalciner can be integrated
between the kiln and the suspension preheater which ensures complete
calcining of the raw mix before it enters the kiln. Precalciners
increase kiln capacity and reduce energy consumption.
Fuel consumption is lowest in this process and is
in the range of 2750-4000 kJ/kg of clinker.
The power consumption is in
the range of 120-125 kWh/tonne of cement (Chemical Universe, 2007).
However, the most modern, dry-process cement plants can have power
consumption in the range 80-100 kWh/tonne of cement by deployment of
compression grinding equipment for raw and finish milling rather than
ball mills.
Production of blended cements is also widely used to reduce
unit electrical energy consumption
Although the dry process is the most modern and popular way to
manufacture cement, some kilns in the United States use a wet process.
Wet Process
In the wet process the preparation of the raw mix
is achieved by grinding the dry raw materials with water in wash mills,
or ball mills with rubber linings. The resultant slurry is mixed in
slurry basins where compressed air is introduced and the slurry is
continuously stirred to produce a slurry of consistent composition.
In the conventional wet process this thick liquid
slurry, which may have water content of 30-40%, is fed directly into the
kiln where the water is evaporated in the drying zone at the kiln
inlet.
This drying zone is fitted with curtains of chains which promote
heat exchange between the slurry and the exhaust gases from the
combustion of fuel in the kiln passing counter-current with the slurry
feed up the kiln.
After passing through the drying zone the raw material
moves down the kiln through the preheating zone to be calcined and
finally burnt to clinker in the sintering zone.
Conventional wet kiln
technology has a high heat consumption and produces large volumes of
exhaust gases.
In more modern wet kiln systems water is driven off the
raw slurry prior to entering the kiln in a slurry drier.
Slurry
“thinning” chemicals may be added to allow the slurry to be pumped to
the kiln with lower water content.
These more modern wet kiln systems
have lower specific heat consumption compared to conventional wet kilns.
