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. As the main energy-consuming and
greenhouse-gas–emitting stage of cement manufacture, improvement of kiln
efficiency has been the central concern of cement manufacturing technology.
Early
history
Portland cement clinker was first
made (in 1825) in a modified form of the traditional static lime kiln.
The basic, egg-cup shaped lime kiln was provided with a conical or beehive
shaped extension to increase draught and thus obtain the higher temperature
needed to make cement clinker. For nearly half a century, this design, and
minor modifications, remained the only method of manufacture. The kiln was
restricted in size by the strength of the chunks of rawmix: if the charge in
the kiln collapsed under its own weight, the kiln would be extinguished. For
this reason, beehive kilns never made more than 30 tonnes of clinker per batch.
A batch took one week to turn around: a day to fill the kiln, three days to
burn off, two days to cool, and a day to unload. Thus, a kiln would produce
about 1500 tonnes per year.
A kiln is basically an industrial oven,
and although the term is generic, several quite distinctive designs have been
used over the years. Although perhaps more normally associated with pottery
making, both ‘Bottle’ and their very close relatives ‘Beehive’ kilns, were also
the central feature of any cement works. Early designs tended to be updraft kilns, which were
often built as a straight sided cone into which the flame was introduced at, or
below, floor level. Reaching heights of up to 70 ft, the dome or bottle
shape of the kiln, known as the ‘hovel’, would be quite a prominent landmark.
As well as protecting the inner kiln or ‘crown’, the opening at the top of the
hovel also acted as a flue, to remove the smoke and exhaust gases that were
produced during the production process. There was a three to four foot gap
between the outer wall of the hovel and inner shell of the crown. Due to the
fact that the 1-foot-thick (0.30 m) crown wall would expand and contract
during firing, it was strengthened with a number of iron bands, known as
‘bonts’. These were set twelve inches apart and ran right around the circular
oven. The development of downdraft kilns in the early 20th Century proved to be
much more fuel efficient and were designed to force the heated air to circulate
more around the kiln. The design incorporated a gentle curve at the 'shoulders'
of the kiln, which served to reflect the rising heat from the fire at the
bottom of the kiln, back down again over the material. The smoke and exhaust
was then sucked out through holes at the bottom of the kiln via a flue, which
was connected to a nearby chimney. The chimney would also serve a number of
neighbouring kilns as well. The kiln would be fired for several days to achieve
the high temperatures required to produce cement clinker,
and although the above methods were successful, the problem with any batch kiln
was that it was intermittent and once the product had been produced, the fire
had to be extinguished and the contents allowed to cool. This not only wasted a
lot of the heat, but also added to the expense of the finished product.
In order to save money on fuel, a
kiln was required that could run almost continuously, whilst the raw material
was somehow fed through it. It was this scenario that lead to the development
of the ‘Chamber’ kiln in the late 1850s. This particular kiln comprised a
number of individual chambers, which were arranged so that the hot flue gases
from one chamber, were drawn off and used to pre-heat the material in the
following chambers, before they were drawn up the chimney. Once the first
chamber had been filled with raw material, coal was added through the roof
holes of the chamber and was then set alight. At the same time, the second
chamber was being filled with raw material. The airflow from the first chamber
was then adjusted, using a number of dampers, to funnel the hot air through to
the second chamber to pre-heat the material. More coal was then poured into the
second chamber and ignited, as the third chamber was being filled and so on.
This process continued along the length of the kiln, so that by the time the
last chamber had been fired, the first chamber had already been cleared and
re-filled with more raw material so that the process could start again.
Although such chamber kilns were still being installed as late as 1900, the
development of the rotary kiln was already starting to have a major impact. The
rotary kiln was a major advancement for the industry as it provided the
continuous production of a much more uniform product in larger quantities.
Around 1885, experiments began on
design of continuous kilns. One design was the shaft kiln, similar in design to
a blast furnace. Rawmix in the form of lumps and fuel were continuously added
at the top, and clinker was continually withdrawn at the bottom. Air was blown
through under pressure from the base to combust the fuel. The shaft kiln had a
brief period of use before it was eclipsed by the rotary kiln, but it had a
limited renaissance from 1970 onward in China and elsewhere, when it was used
for small-scale, low-tech plants in rural areas away from transport routes.
Several thousand such kilns were constructed in China. A typical shaft kiln
produces 100-200 tonnes per day.
From 1885, trials began on the
development of the rotary kiln, which today accounts for more than 95% of world
production.
The
rotary kiln
The rotary kiln consists of a tube
made from steel plate, and lined with firebrick.
The tube slopes slightly (1–4°) and slowly rotates on its axis at between 30
and 250 revolutions per hour. Rawmix is fed in at the upper end, and the
rotation of the kiln causes it gradually to move downhill to the other end of
the kiln. At the other end fuel, in the form of gas, oil, or
pulverized solid fuel, is blown in through the "burner pipe",
producing a large concentric flame in the lower part of the kiln tube. As
material moves under the flame, it reaches its peak temperature, before
dropping out of the kiln tube into the cooler. Air is drawn first through the
cooler and then through the kiln for combustion of the fuel. In the cooler the
air is heated by the cooling clinker, so that it may be 400 to 800 °C
before it enters the kiln, thus causing intense and rapid combustion of the
fuel.
The earliest successful rotary kilns
were developed in Pennsylvania around 1890, and were about 1.5 m in diameter and 15 m in
length. Such a kiln made about 20 tonnes of clinker per day. The fuel,
initially, was oil, which was readily available in Pennsylvania at the time. It
was particularly easy to get a good flame with this fuel. Within the next 10
years, the technique of firing by blowing in pulverized coal was developed,
allowing the use of the cheapest available fuel. By 1905, the largest kilns
were 2.7 x 60 m in size, and made 190 tonnes per day. At that date, after only
15 years of development, rotary kilns accounted for half of world production.
Since then, the capacity of kilns has increased steadily, and the largest kilns
today produce around 10,000 tonnes per day. In contrast to static kilns, the
material passes through quickly: it takes from 3 hours (in some old wet process
kilns) to as little as 10 minutes (in short precalciner kilns). Rotary kilns
run 24 hours a day, and are typically stopped only for a few days once or twice
a year for essential maintenance. One of the main maintenance works on rotary
kilns is tyre and roller surface machining and grinding works which can be done
while the kiln works in full operation at speeds up to 3.5 rpm. This is an
important discipline, because heating up and cooling down are long, wasteful
and damaging processes. Uninterrupted runs as long as 18 months have been
achieved.
The
wet process and the dry process
From the earliest times, two
different methods of rawmix preparation were used: the mineral components were
either dry-ground to form a flour-like powder, or were wet-ground with added
water to produce a fine slurry with the consistency of paint, and with a typical water
content of 40–45%.
The wet process suffered the obvious
disadvantage that, when the slurry was introduced into the kiln, a large amount
of extra fuel was used in evaporating the water. Furthermore, a larger kiln was
needed for a given clinker output, because much of the kiln's length was used
up for the drying process. On the other hand, the wet process had a number of
advantages. Wet grinding of hard minerals is usually much more efficient than
dry grinding. When slurry is dried in the kiln, it forms a granular crumble
that is ideal for subsequent heating in the kiln. In the dry process, it is
very difficult to keep the fine powder rawmix in the kiln, because the
fast-flowing combustion gases tend to blow it back out again. It became a
practice to spray water into dry kilns in order to "damp down" the dry
mix, and thus, for many years there was little difference in efficiency between
the two processes, and the overwhelming majority of kilns used the wet process.
By 1950, a typical large, wet process kiln, fitted with drying-zone heat
exchangers, was 3.3 x 120 m in size, made 680 tonnes per day, and used about
0.25–0.30 tonnes of coal fuel for every tonne of clinker produced. Before the
energy crisis of the 1970s put an end to new wet-process installations, kilns
as large as 5.8 x 225 m in size were making 3000 tonnes per day.
An interesting footnote on the wet
process history is that some manufacturers have in fact made very old wet
process facilities profitable through the use of waste fuels. Plants that burn
waste fuels enjoy a negative fuel cost (they are paid by industries needing to
dispose of materials that have energy content and can be safely disposed of in
the cement kiln thanks to its high temperatures and longer retention times). As
a result the inefficiency of the wet process is an advantage—to the manufacturer.
By locating waste burning operations at older wet process locations, higher
fuel consumption actually equates to higher profits for the manufacturer,
although it produces correspondingly greater emission of CO2.
Manufacturers who think such emissions should be reduced are abandoning the use
of wet process.
Preheaters
In the 1930s, significantly, in
Germany, the first attempts were made to redesign the kiln system to minimize
waste of fuel.
This led to two significant developments:
- the grate preheater
- the gas-suspension preheater.
Grate
preheaters
Gas-suspension
preheaters
The key component of the
gas-suspension preheater is the cyclone. A cyclone is a conical vessel into which a dust-bearing
gas-stream is passed tangentially. This produces a vortex within the vessel.
The gas leaves the vessel through a co-axial "vortex-finder". The
solids are thrown to the outside edge of the vessel by centrifugal action, and
leave through a valve in the vertex of the cone. Cyclones were originally used
to clean up the dust-laden gases leaving simple dry process kilns. If, instead,
the entire feed of rawmix is encouraged to pass through the cyclone, it is
found that a very efficient heat exchange takes place: the gas is efficiently
cooled, hence producing less waste of heat to the atmosphere, and the rawmix is
efficiently heated. This efficiency is further increased if a number of
cyclones are connected in series.
The
number of cyclones stages used in practice varies from 1 to 6. Energy, in the form
of fan-power, is required to draw the gases through the string of cyclones, and
at a string of 6 cyclones, the cost of the added fan-power needed for an extra
cyclone exceeds the efficiency advantage gained. It is normal to use the warm
exhaust gas to dry the raw materials in the rawmill, and if
the raw materials are wet, hot gas from a less efficient preheater is
desirable. For this reason, the most commonly encountered suspension preheaters
have 4 cyclones. The hot feed that leaves the base of the preheater string is
typically 20% calcined, so the kiln has less subsequent processing to do, and
can therefore achieve a higher specific output. Typical large systems installed
in the early 1970s had cyclones 6 m in diameter, a rotary kiln of 5 x 75 m,
making 2500 tonnes per day, using about 0.11-0.12 tonnes of coal fuel for every
tonne of clinker produced.
A penalty paid for the efficiency of
suspension preheaters is their tendency to block up. Salts, such as the sulfate
and chloride of sodium and potassium, tend to evaporate in the burning zone of
the kiln. They are carried back in vapor form, and re-condense when a
sufficiently low temperature is encountered. Because these salts re-circulate back
into the rawmix and re-enter the burning zone, a recirculation cycle
establishes itself. A kiln with 0.1% chloride in the rawmix and clinker may
have 5% chloride in the mid-kiln material. Condensation usually occurs in the
preheater, and a sticky deposit of liquid salts glues dusty rawmix into a hard
deposit, typically on surfaces against which the gas-flow is impacting. This
can choke the preheater to the point that air-flow can no longer be maintained
in the kiln. It then becomes necessary to manually break the build-up away.
Modern installations often have automatic devices installed at vulnerable
points to knock out build-up regularly. An alternative approach is to
"bleed off" some of the kiln exhaust at the kiln inlet where the
salts are still in the vapor phase, and remove and discard the solids in this.
This is usually termed an "alkali bleed" and it breaks the
recirculation cycle. It can also be of advantage for cement quality reasons,
since it reduces the alkali content of the clinker. However, hot gas is run to
waste so the process is inefficient and increases kiln fuel consumption.
Precalciners
In the 1970s the precalciner was
pioneered in Japan, and has
subsequently become the equipment of choice for new large installations
worldwide.
The precalciner is a development of the suspension preheater. The philosophy is
this: the amount of fuel that can be burned in the kiln is directly related to
the size of the kiln. If part of the fuel necessary to burn the rawmix is burned outside the kiln, the output of the system can be
increased for a given kiln size. Users of suspension preheaters found that
output could be increased by injecting extra fuel into the base of the
preheater. The logical development was to install a specially designed
combustion chamber at the base of the preheater, into which pulverized
coal is injected. This is referred to as
an "air-through" precalciner, because the combustion air for both the
kiln fuel and the calciner fuel all passes through the kiln. This kind of
precalciner can burn up to 30% (typically 20%) of its fuel in the calciner. If
more fuel were injected in the calciner, the extra amount of air drawn through
the kiln would cool the kiln flame excessively. The feed is 40-60% calcined
before it enters the rotary kiln.
The ultimate development is the
"air-separate" precalciner, in which the hot combustion air for the
calciner arrives in a duct directly from the cooler, bypassing the kiln.
Typically, 60-75% of the fuel is burned in the precalciner. In these systems,
the feed entering the rotary kiln is 100% calcined. The kiln has only to raise
the feed to sintering temperature. In theory the maximum efficiency would be
achieved if all the fuel were burned in the preheater, but the sintering
operation involves partial melting and nodulization to make clinker, and the rolling action of the rotary kiln
remains the most efficient way of doing this. Large modern installations
typically have two parallel strings of 4 or 5 cyclones, with one attached to
the kiln and the other attached to the precalciner chamber. A rotary kiln of 6
x 100 m makes 8,000–10,000 tonnes per day, using about 0.10-0.11 tonnes of coal
fuel for every tonne of clinker produced. The kiln is dwarfed by the massive
preheater tower and cooler in these installations. Such a kiln produces 3
million tonnes of clinker per year, and consumes 300,000 tonnes of coal. A
diameter of 6 m appears to be the limit of size of rotary kilns, because the
flexibility of the steel shell becomes unmanageable at or above this size, and
the firebrick
lining tends to fail when the kiln flexes.
A particular advantage of the
air-separate precalciner is that a large proportion, or even 100%, of the
alkali-laden kiln exhaust gas can be taken off as alkali bleed (see above).
Because this accounts for only 40% of the system heat input, it can be done
with lower heat wastage than in a simple suspension preheater bleed. Because of
this, air-separate precalciners are now always prescribed when only high-alkali
raw materials are available at a cement plant.
The accompanying figures show the
movement towards the use of the more efficient processes in North America (for
which data is readily available). But the average output per kiln in, for example,
Thailand is twice
that in North America.
Ancillary
equipment
Essential equipment in addition to
the kiln tube and the preheater are:
- Cooler
- Fuel mills
- Fans
- Exhaust gas cleaning equipment.
Coolers
Early systems used rotary coolers,
which were rotating cylinders similar to the kiln, into which the hot clinker
dropped.
The combustion air was drawn up through the cooler as the clinker moved down,
cascading through the air stream. In the 1920s, satellite coolers became common
and remained in use until recently. These consist of a set (typically 7–9) of
tubes attached to the kiln tube. They have the advantage that they are sealed
to the kiln, and require no separate drive. From about 1930, the grate cooler
was developed. This consists of a perforated grate through which cold air is
blown, enclosed in a rectangular chamber. A bed of clinker up to 0.5 m deep
moves along the grate. These coolers have two main advantages: they cool the
clinker rapidly, which is desirable from a quality point of view (to avoid that
alite,
thermodynamically unstable below 1250 °C, revert to belite and free CaO on slow cooling), and, because they do not
rotate, hot air can be ducted out of them for use in fuel drying, or for use as
precalciner combustion air. The latter advantage means that they have become
the only type used in modern systems.
Fuel
mills
Fuel systems are divided into two
categories:
- Direct firing
- Indirect firing
In direct firing, the fuel is fed at
a controlled rate to the fuel mill, and the fine product is immediately blown
into the kiln. The advantage of this system is that it is not necessary to
store the hazardous ground fuel: it is used as soon as it is made. For this
reason it was the system of choice for older kilns. A disadvantage is that the
fuel mill has to run all the time: if it breaks down, the kiln has to stop if
no backup system is available.
In indirect firing, the fuel is
ground by an intermittently run mill, and the fine product is stored in a silo
of sufficient size to supply the kiln though fuel mill stoppage periods. The
fine fuel is metered out of the silo at a controlled rate and blown into the
kiln. This method is now favoured for precalciner systems, because both the
kiln and the precalciner can be fed with fuel from the same system. Special
techniques are required to store the fine fuel safely, and coals with high volatiles
are normally milled in an inert atmosphere (e.g. CO2).
Fans
A large volume of gases has to be
moved through the kiln system.
Particularly in suspension preheater systems, a high degree of suction has to
be developed at the exit of the system to drive this. Fans are also used to
force air through the cooler bed, and to propel the fuel into the kiln. Fans
account for most of the electric power consumed in the system, typically
amounting to 10–15 kW·h per tonne of clinker.
Gas
cleaning
The exhaust gases from a modern kiln
typically amount to 2 tonnes (or 1500 cubic metres at STP) per
tonne of clinker made.
The gases carry a large amount of dust—typically 30 grams per cubic metre.
Environmental regulations specific to different countries require that this be
reduced to (typically) 0.1 gram per cubic metre, so dust capture needs to
be at least 99.7% efficient. Methods of capture include electrostatic
precipitators and bag-filters. See also cement kiln emissions.
Kiln
fuels
Fuels that have been used for
primary firing include coal, petroleum coke,
heavy fuel oil,
natural gas,
landfill off-gas and oil refinery flare gas.
High carbon fuels such as coal are preferred for kiln firing, because they
yield a luminous flame. The clinker is brought to its peak temperature mainly
by radiant heat transfer, and a bright (i.e. high emissivity)
and hot flame is essential for this. In favorable circumstances, high-rank
bituminous coal can produce a flame at 2050 °C. Natural gas can only
produce a flame of, at best 1950 °C, and this is also less luminous, so it
tends to result in lower kiln output.
In addition to these primary fuels,
various combustible waste materials have been fed to kilns, notably used tires,
which are very difficult to dispose of by other means. In theory, cement kilns
are an attractive way of disposing of hazardous materials, because of:
- the temperatures in the kiln, which are much higher than in other combustion systems (e.g. incinerators),
- the alkaline conditions in the kiln, afforded by the high-calcium rawmix, which can absorb acidic combustion products,
- the ability of the clinker to absorb heavy metals into its structure.
Whole tires are commonly introduced
in the kiln, by rolling them into the upper end of a preheater kiln, or by
dropping them through a slot midway along a long wet kiln. In either case, the
high gas temperatures (1000–1200 °C) cause almost instantaneous, complete
and smokeless combustion of the tire. Alternatively, tires are chopped into
5–10 mm chips, in which form they can be injected into a precalciner
combustion chamber. The steel and zinc in the tires become chemically
incorporated into the clinker.
Other wastes have included solvents
and clinical wastes. A very high level of monitoring of both the fuel and its
combustion products is necessary to maintain safe operation.
For maximum kiln efficiency, high
quality conventional fuels are the best choice. When using waste materials, in
order to avoid prohibited emissions (e.g. of dioxins) it is necessary to control the kiln system in a manner
that is non-optimal for efficiency and output, and coarse combustibles such as
tires can cause major product quality problems.
Kiln
control
The objective of kiln operation is
to make clinker with the required chemical and physical properties, at the
maximum rate that the size of kiln will allow, while meeting environmental
standards, at the lowest possible operating cost.
The kiln is very sensitive to control strategies, and a poorly run kiln can
easily double cement plant operating costs.
Formation of the desired clinker
minerals involves heating the rawmix through the temperature stages mentioned
above. The finishing transformation that takes place in the hottest part of the
kiln, under the flame, is the reaction of belite (Ca2SiO4) with calcium oxide to form alite (Ca3O·SiO4):
Ca2SiO4 + CaO → Ca3SiO5
C2S + C → C3S
Tricalcium silicate is thermodynamically unstable below
1250 °C, but can be preserved in a metastable state at room temperature by
fast cooling: on slow cooling it tends to revert to belite (Ca2SiO4) and CaO.
If the reaction is incomplete,
excessive amounts of free calcium oxide
remain in the clinker. Regular measurement of the free CaO content is used as a
means of tracking the clinker quality. As a parameter in kiln control, free CaO
data is somewhat ineffective because, even with fast automated sampling and
analysis, the data, when it arrives, may be 10 minutes "out of date",
and more immediate data must be used for minute-to-minute control.
Conversion of belite to alite
requires partial melting, the resulting liquid being the solvent in which
the reaction takes place. The amount of liquid, and hence the speed of the
finishing reaction, is related to temperature. To meet the clinker quality
objective, the most obvious control is that the clinker should reach a peak
temperature such that the finishing reaction takes place to the required
degree. A further reason to maintain constant liquid formation in the hot end
of the kiln is that the sintering material forms a dam that prevents the cooler
upstream feed from flooding out of the kiln. The feed in the calcining zone,
because it is a powder evolving carbon dioxide, is extremely fluid. Cooling of
the burning zone, and loss of unburned material into the cooler, is called
"flushing", and in addition to causing lost production can cause
massive damage.
However, for efficient operation,
steady conditions need to be maintained throughout the whole kiln system. The
feed at each stage must be at a temperature such that it is "ready"
for processing in the next stage. To ensure this, the temperature of both feed
and gas must be optimized and maintained at every point. The external controls
available to achieve this are few:
- Feed rate: this defines the kiln output
- Rotary kiln speed: this controls the rate at which the feed moves through the kiln tube
- Fuel injection rate: this controls the rate at which the "hot end" of the system is heated
- Exhaust fan speed or power: this controls gas flow, and the rate at which heat is drawn from the "hot end" of the system to the "cold end"
In the case of precalciner kilns,
further controls are available:
- Independent control of fuel to kiln and calciner
- Independent fan controls where there are multiple preheater strings.
The independent use of fan speed and
fuel rate is constrained by the fact that there must always be sufficient
oxygen available to burn the fuel, and in particular, to burn carbon to carbon
dioxide. If carbon monoxide is formed, this represents a waste of fuel, and also
indicates reducing conditions within the kiln which must be avoided at all
costs since it causes destruction of the clinker mineral structure. For this
reason, the exhaust gas is continually analyzed for O2, CO, NO and SO2.
The assessment of the clinker peak
temperature has always been problematic. Contact temperature measurement is
impossible because of the chemically aggressive and abrasive nature of the hot
clinker, and optical methods such as infrared pyrometry
are difficult because of the dust and fume-laden atmosphere in the burning
zone. The traditional method of assessment was to view the bed of clinker and
deduce the amount of liquid formation by experience. As more liquid forms, the
clinker becomes stickier, and the bed of material climbs higher up the rising
side of the kiln. It is usually also possible to assess the length of the zone
of liquid formation, beyond which powdery "fresh" feed can be seen.
Cameras, with or without infrared measurement capability, are mounted on the
kiln hood to facilitate this. On many kilns, the same information can be
inferred from the kiln motor power drawn, since sticky feed riding high on the
kiln wall increases the eccentric turning load of the kiln. Further information
can be obtained from the exhaust gas analyzers. The formation of NO from
nitrogen and oxygen takes place only at high temperatures, and so the NO level
gives an indication of the combined feed and flame temperature. SO2
is formed by thermal decomposition of calcium sulfate
in the clinker, and so also gives in indication of clinker temperature. Modern
computer control systems usually make a "calculated" temperature,
using contributions from all these information sources, and then set about
controlling it.
As an exercise in process control,
kiln control is extremely challenging, because of multiple inter-related
variables, non-linear responses, and variable process lags. Computer control
systems were first tried in the early 1960s, initially with poor results due
mainly to poor process measurements. Since 1990, complex high-level supervisory
control systems have been standard on new installations. These operate using expert system
strategies, that maintain a "just sufficient" burning zone
temperature, below which the kiln's operating condition will deteriorate
catastrophically, thus requiring rapid-response, "knife-edge"
control.
Cement
kiln emissions
Emissions from cement works are
determined both by continuous and discontinuous measuring methods, which are
described in corresponding national guidelines and standards. Continuous
measurement is primarily used for dust, NOx and SO2,
while the remaining parameters relevant pursuant to ambient pollution
legislation are usually determined discontinuously by individual measurements.
The following descriptions of
emissions refer to modern kiln plants based on dry process technology.
Carbon
dioxide
During the clinker burning process CO2 is emitted. CO2
accounts for the main share of these gases. CO2 emissions are both
raw material-related and energy-related. Raw material-related emissions are
produced during limestone decarbonation (CaCO3) and account for about 60%
of total CO2 emissions.
Dust
To manufacture 1 t of Portland
cement, about 1.5 to 1.7 t raw materials, 0.1 t coal and 1 t clinker (besides
other cement constituents and sulfate agents)
must be ground to dust fineness during production. In this process, the steps
of raw material processing, fuel preparation, clinker burning and cement
grinding constitute major emission sources for particulate components. While particulate
emissions of up to 3,000 mg/m3
were measured leaving the stack of cement rotary kiln plants as recently as in
the 1950s, legal limits are typically 30 mg/m3 today, and much
lower levels are achievable.
Nitrogen
oxides (NOx)
The clinker burning process is a
high-temperature process resulting in the formation of nitrogen oxides
(NOx). The amount formed is directly related to the main flame
temperature (typically 1850–2000 °C). Nitrogen monoxide
(NO) accounts for about 95%, and nitrogen dioxide
(NO2) for about 5% of this compound present in the exhaust gas of rotary kiln
plants. As most of the NO is converted to NO2 in the atmosphere,
emissions are given as NO2 per cubic metre exhaust gas.
Without reduction measures,
process-related NOx contents in the exhaust gas of rotary kiln
plants would in most cases considerably exceed the specifications of e.g.
European legislation for waste burning plants (0.50 g/m3 for new
plants and 0.80 g/m3 for existing plants). Reduction measures are
aimed at smoothing and optimising plant operation. Technically, staged
combustion and Selective Non-Catalytic NO Reduction (SNCR) are applied to cope with the emission limit values.
High process temperatures are
required to convert the raw material mix to Portland cement clinker. Kiln
charge temperatures in the sintering zone of rotary kilns range at around
1450 °C. To reach these, flame temperatures of about 2000 °C are
necessary. For reasons of clinker quality the burning process takes place under
oxidising conditions, under which the partial oxidation of the molecular nitrogen in the
combustion air resulting in the formation of nitrogen monoxide
(NO) dominates. This reaction is also called thermal NO formation. At the lower
temperatures prevailing in a precalciner, however, thermal NO formation is
negligible: here, the nitrogen bound in the fuel can result in the formation of
what is known as fuel-related NO. Staged combustion is used to reduce NO:
calciner fuel is added with insufficient combustion air. This causes CO to
form.
The CO then reduces the NO into molecular nitrogen:
The CO then reduces the NO into molecular nitrogen:
2 CO + 2 NO → 2 CO2 + N2.
Hot tertiary air is then added to
oxidize the remaining CO.
Sulfur
dioxide (SO2)
Sulfur is input
into the clinker burning process via raw materials and fuels. Depending on
their origin, the raw materials may contain sulfur bound as sulfide or sulfate.
Higher SO2 emissions by rotary kiln systems in the cement industry are
often attributable to the sulfides contained in the raw material, which become
oxidised to form SO2 at the temperatures between 370 °C and
420 °C prevailing in the kiln preheater. Most of the sulfides are pyrite or marcasite contained in the raw materials. Given the sulfide
concentrations found e.g. in German raw material deposits, SO2
emission concentrations can total up to 1.2 g/m3 depending on the
site location. In some cases, injected calcium hydroxide
is used to lower SO2 emissions.
The sulfur input with the fuels is
completely converted to SO2 during combustion in the rotary kiln. In
the preheater and the kiln, this SO2 reacts to form alkali sulfates, which are bound in the clinker, provided that
oxidizing conditions are maintained in the kiln.
Carbon
monoxide (CO) and total carbon
The exhaust gas concentrations of CO and organically bound carbon are a
yardstick for the burn-out rate of the fuels utilised in energy conversion
plants, such as power stations. By contrast, the clinker burning process is a material
conversion process that must always be operated with excess air for reasons of
clinker quality. In concert with long residence times in the high-temperature range,
this leads to complete fuel burn-up.
The emissions of CO and organically
bound carbon during the clinker burning process
are caused by the small quantities of organic constituents input via the
natural raw materials (remnants of organisms and plants incorporated in the
rock in the course of geological history). These are converted during kiln feed
preheating and become oxidized to form CO and CO2. In this process,
small portions of organic trace gases (total organic carbon) are formed as
well. In case of the clinker burning process, the content of CO and organic
trace gases in the clean gas therefore may not be directly related to
combustion conditions.
Dioxins
and furans (PCDD/F)
Rotary kilns of the cement industry
and classic incineration plants mainly differ in terms of the combustion
conditions prevailing during clinker burning. Kiln feed and rotary kiln exhaust
gases are conveyed in counter-flow and mixed thoroughly. Thus, temperature
distribution and residence time in rotary kilns afford particularly favourable
conditions for organic compounds, introduced either via fuels or derived from
them, to be completely destroyed. For that reason, only very low concentrations
of polychlorinated dibenzo-p-dioxins and dibenzofurans (colloquially "dioxins and furans") can be found in the exhaust gas from cement rotary
kilns.
Polychlorinated
biphenyls (PCB)
The emission behaviour of PCB is comparable to that of dioxins and furans. PCB may be
introduced into the process via alternative raw materials and fuels. The rotary
kiln systems of the cement industry destroy these trace components virtually
completely.[citation
needed]
Polycyclic
aromatic hydrocarbons (PAH)
PAHs (according to EPA 610) in the exhaust gas of rotary kilns
usually appear at a distribution dominated by naphthalene,
which accounts for a share of more than 90% by mass. The rotary kiln systems of
the cement industry destroy virtually completely the PAHs input via fuels.
Emissions are generated from organic constituents in the raw material.
Benzene,
toluene, ethylbenzene, xylene (BTEX)
As a rule benzene, toluene, ethylbenzene
and xylene are
present in the exhaust gas of rotary kilns in a characteristic ratio. BTEX is formed during the thermal decomposition of organic raw
material constituents in the preheater.
Gaseous
inorganic chlorine compounds (HCl)
Chlorides are minor additional constituents contained in the raw
materials and fuels of the clinker burning process. They are released when the
fuels are burnt or the kiln feed is heated, and primarily react with the
alkalis from the kiln feed to form alkali chlorides. These compounds, which are
initially vaporous, condense on the kiln feed or the kiln dust, at temperatures
between 700 °C and 900 °C, subsequently re-enter the rotary kiln
system and evaporate again. This cycle in the area between the rotary kiln and
the preheater can result in coating formation. A bypass at the kiln inlet
allows effective reduction of alkali chloride cycles and to diminish coating
build-up problems. During the clinker burning process, gaseous inorganic
chlorine compounds are either not emitted at all or in very small quantities
only.
Gaseous
inorganic fluorine compounds (HF)
Of the fluorine present in rotary
kilns, 90 to 95% is bound in the clinker, and the remainder is bound with dust
in the form of calcium fluoride stable under the conditions of the burning process.
Ultra-fine dust fractions that pass through the measuring gas filter may give
the impression of low contents of gaseous fluorine compounds in rotary kiln
systems of the cement industry.
Trace
elements
The emission behaviour of the
individual elements in the clinker burning process is determined by the input
scenario, the behaviour in the plant and the precipitation efficiency of the
dust collection device. The trace elements introduced into the burning process
via the raw materials and fuels may evaporate completely or partially in the hot
zones of the preheater and/or rotary kiln depending on their volatility, react
with the constituents present in the gas phase, and condense on the kiln feed
in the cooler sections of the kiln system. Depending on the volatility and the
operating conditions, this may result in the formation of cycles that are
either restricted to the kiln and the preheater or include the combined drying
and grinding plant as well. Trace elements from the fuels initially enter the
combustion gases, but are emitted to an extremely small extent only owing to
the retention capacity of the kiln and the preheater.
Under the conditions prevailing in
the clinker burning process, non-volatile elements (e.g. arsenic, vanadium, nickel) are completely bound in the clinker.
Elements such as lead and cadmium preferentially react with the excess chlorides and sulfates
in the section between the rotary kiln and the preheater, forming volatile
compounds. Owing to the large surface area available, these compounds condense
on the kiln feed particles at temperatures between 700 °C and 900 °C.
In this way, the volatile elements accumulated in the kiln-preheater system are
precipitated again in the cyclone preheater, remaining almost completely in the
clinker.
Thallium (as the chloride) condenses in the upper zone of the
cyclone preheater at temperatures between 450 °C and 500 °C. As a
consequence, a cycle can be formed between preheater, raw material drying and
exhaust gas purification.
Mercury and its compounds are not precipitated in the kiln and the
preheater. They condense on the exhaust gas route due to the cooling of the gas
and are partially adsorbed by the raw material particles. This portion is
precipitated in the kiln exhaust gas filter.
Owing to trace element behaviour
during the clinker burning process and the high precipitation efficiency of the
dust collection devices, trace element emission concentrations are on a low overall
level
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