A wide range of coals are used in cement plants.
As is the case with many coal applications, the plant can be
designed in part to suit the coal. Although coal the quality
criteria are not very exacting, it is important that variability in
quality be small, as the composition of the raw feed must normally
be tailored to the coal ash chemistry. As the rotary kiln is by the
most common cement plant in operation the following discussion
focuses on this type of plant.
In direct firing systems the coal is milled
on-line with the pulverized coal being directly fed to the kiln
burner. The quantity of primary air used with direct firing systems
is normally that required to dry the coal and sweep the mill, and
is more than required for transporting the pulverised coal or
producing a satisfactory flame.
Direct firing systems are not particularly suited
to high moisture coals because:
The moisture which is evaporated during milling
is fed into the kiln with the primary air. This has the effect of
lowering the flame temperature, and process efficiency.
Additional mill air may be required so that the
coal can be effectively dried without having excessive air preheat.
This additional air also enters the kiln as primary air. Excessive
primary air also has a similar effect to moisture.
In indirect firing systems the coal is milled
off-line and is stored in a bin from which it is fed to the kiln
burner. High moisture in coals can be associated with low rank and
the propensity for spontaneous combustion. This could mean problems
when storing the PF in badly designed bins of an indirect fired
system. If coals which are subject to spontaneous combustion are to
be used successfully, they may need to be sold selectively to
plants with a proven track record in this area.
Milling behaviour of coals for cement kilns has
special significance because kiln operators generally place some
emphasis on tailoring the coal fineness to suit its reactivity. The
accepted measure of coal particle size in the cement industry is
the percentage greater than 90ìm. It is widely held that low
volatile matter in the coal can be compensated by finer grinding.
In order for this option to be open the mills must have the spare
capacity to achieve finer grinding, or the coals must have a high
Hardgrove Grindability Index (HGI). The trade-off between HGI and
VM is therefore important. Fortunately, low VM bituminous coals
tend to have high HGI values.
The main consideration for coal storage is the
propensity for spontaneous combustion and explosion of stored PF.
If fresh PF remains in one spot, particularly in the presence of
hot moist air, self heating and ignition can occur rapidly. This
causes any dispersion of coal dust in air, within certain
concentration limits, to ignite and explode.
In order to heat the clinker to the required
temperature of around 1500°C, it is necessary to have a
flame temperature of around 1700°C. This is achieved by
providing preheat to the secondary air and limiting the quantity of
primary air. As noted above, for high moisture coals in direct
fired systems there may be an excess of moist primary air and the
required temperature may not be obtained. However for indirect
firing systems using dry primary air, a satisfactory flame
temperature can be achieved even with low energy coals.
The correct distribution of temperature along the
kiln requires that the flame front be located close to the
discharge end of the kiln, close to the burner. Coals which are
unreactive and slow to ignite may therefore cause problems in some
kilns, and in these cases medium to high volatile coals would
normally be specified. The use of swirl burners with low volatile
coals is not always successful, as the diverging jet may impinge on
the kiln walls just as the coal is igniting. The reducing
conditions that this causes in the clinker affects product quality,
and damages the refractory lining of the kiln.
While it is normal to have some build up of
deposits in the clinkering zone of a cement kiln, excessive
deposits in this area, called "clinker ring", can hinder the
movement of solids through the kiln and in extreme cases require
plant stoppages. A second problem is the formation of deposits in
the suspension preheaters. These problems are commonly associated
with the presence of chlorine, sodium, potassium, and sulphur. When
the levels of these elements are considered, the combined effects
of coal ash and raw feed composition must be considered. The major
source of sodium and potassium would normally be the raw feed,
however coals with unusually high sulphur or chlorine could
contribute to deposit formation and would not be favoured as cement