By pass system in the dry process
It is well known that cement is made mainly from limestone and clay. Both contain the essential oxides: silicon dioxide, calcium oxide, aluminum oxide, and iron oxide needed to form the clinker 4 major components in the rotary kiln. These four oxides exist in limestone and clay in different percentages. Limestone and clay have also other minerals that contain some elements that are not required in the process of forming clinker main components. Some of these minerals exist in different percentage. If these percentages exceeded certain limits then it creates trouble for the burning process especially in the preheater lower stages before and after the precalciner, in the kiln inlet area and sometimes in kiln tube itself.
These elements and components are called process adverse materials.
These harmful materials are sulfur, chlorine and alkali elements i.e. sodium and potassium. Their behaviour in the kiln and preheater atmosphere leads to build-up of layers of these components and trapping huge quantity of kiln dust. This build-up forces the kiln operator to shutdown the kiln system to clear this build-up that in most cases makes the operation next to the impossible. The kiln operation suffers because the build-up in the riser pipes and cyclones increases pressure drop in the system, so maintaining draft in the kiln becomes harder and more difficult.
This obliges the kiln operator to work with less excess air in the kiln, which makes things worse since the circulation of sulfur highly increases at low oxygen level and the problem becomes worse in the kiln and the build-up of accretion becomes very quick and its nature will be stronger and harder i.e. the build-up will be harder to clean. This leads directly to a decrease in kiln production and eventual shut down of the system.
The second result of working with higher levels of sulfur circulation is the formation of a very dusty clinker. This leads to deterioration of the heat profile of the kiln and preheater system because high amount of heat will be lost with the dust transported with the kiln exhaust gas.
This will shift the heat upward away from burning zone affecting the coating condition in the kiln-burning zone. The kiln then will have a long zone of unstable coating at the upper transition zone before the burning zone leading directly to brick in this area, since with every cycle part of the brick that is approximately 10 mm will be lost.
The only real and final remedy for this phenomena preventing it from troubling the burning process is to install a by-pass system.
The sole and major job of this installation i.e. by-pass installation is to control these elements to the degree that permits for smooth operation. Therefore, a universal fact should be noted that the by-pass system does not eliminate the circulation problem of volatile matters but just controlling it to the degree that makes the kiln operation smooth and economical. This section is devoted to study in detail the problem of the volatile matters in the raw materials and the by-pass system and how to make it work smoothly.
This term is used to represent the phenomena caused by the presence of the volatiles in the system i.e. alkali chlorides, Sulphates and other related components in the dry kiln system with preheater and precalciner.
Why this phenomenon is called circulation phenomenon? The circulating elements are sulfur and chlorine and the other two elements are sodium and potassium.
Fractions of these four elements first evaporate in the kiln burning zone depending on the degree of volatility of the component then carried away by kiln gas in the direction of the kiln inlet and condensate and precipitate on the kiln feed particles in the preheater lower stages and in the kiln inlet then return back to the kiln burning-zone again and volatile again.
So they start to circulate in the system between the kiln and preheater stages and will continue endlessly to evaporate in the burning zone of the kiln then precipitate in the preheater. They return again to kiln burning zone with the fresh material containing fresh supply of these elements in addition to what is precipitated on them in the preheater. However, we have to note here that part of these elements leaves the system with the clinker, the preheater chimney, preheater electrostatic precipitator dust, the by-pass’s chimney, and by-pass electrostatic precipitator dust.
These cycles continue as long as the kiln is in operation and the process is called Circulation Phenomenon.
These cycles increase the concentration of the alkali sulfates and chlorides in the system to a very high level as the operation of the kiln continue. Such concentration increases to fifty times its original concentration when the kiln is just started.
The INNER-CIRCULE of volatile matters or INTERNAL CIRCULATION PHENOMENON is between preheater tower lower stages and the kiln-burning zone.
As for the volatile matters that condensate on the material particles of the kiln feed and have the chance to leave the kiln system with the kiln gas leaving the upper most cyclones, they precipitate in the preheater electrostatic filter and eventually returning back to the kiln with the fresh feed but they just enrich the feed with these volatile matters to some degree. Their cycle from preheater to electrostatic precipitator to kiln feed and back again to the kiln system is called the EXTERNAL CIRCULE of volatile matters or EXTERNAL CIRCULATION PHENOMENON.
At a certain concentration, the system atmosphere becomes saturated to the degree that permits for the precipitation of the volatile matters in the melted phase condition that will lead to the formation of build-ups and rings in the lower part of the preheater system and the kiln inlet and in calcining zone in the kiln.
If the concentration of the volatile matters becomes dangerously high either because of high input of volatile matters to the system or due to high volatility of these matters, then the installation of a by-pass system is a must.
There are two limits, the normal limit which is applied when the kiln feed has moderate to normal silica modulus and normal value of free lime will be accepted in the clinker, while the high limit is applied when the kiln feed is easy to burn i.e. have low silica modulus accepting higher free lime in the clinker and/or the alkali/sulfur ratio in the kiln feed is ideal [this will be discussed later].
The high limits should not also be applied when the condition in the burning zone require very strong flame e.g. as in high silica ratio in kiln feed or the kiln feed has a relatively high volatiles content and we need to produce low alkali clinker cement from this feed with concentration of volatile matters in the kiln feed higher than the ideal concentration needed for low alkali clinker.
If these limits are surpassed and the total input of volatiles are higher, the precalciner kiln should be provided with a by-pass system by which a part of the kiln gas can by-pass the preheater altogether. This process decreases the concentration of volatile matters leaving the kiln to the preheater and precalciner.
The by-pass percentage of the kiln gas can be calculated in the following way.
% By-pass = % chlorine as Cl in the raw materials [raw meal basis] x 100
This procedure is applied to chlorine because it is nearly impossible to control the evaporation of chlorine in the kiln burning zone or control its concentration in the process by any means.
The raw materials contain chlorine [raw meal basis] =0.23 %. What is the bypass %?
The required bypass% = 0.25×100=25%
The circulation mechanism of circulating elements; potassium, sodium, chlorine, and sulfur can be explained in the following manner:
The circulating elements enter the kiln with the kiln feed that travels through the preheater to the kiln inlet.
Starting from lower most cyclones the temperature starts reaching 800ºC in the kiln system. From this temperature, part of these elements is volatilized i.e. dissociated and becomes part of the kiln atmosphere.
As the temperature starts to rise more and more the rate of volatilization increases.
In the kiln atmosphere the fuel will add to the kiln atmosphere additional sulfur after combustion. When the material reaches burning zone, all the chlorine will be evaporated with part from sulfate, sodium, and potassium [the harder the kiln feed to burn the higher will be the evaporation rate of the volatiles and this takes place also in case of a very strong flame in the main burner] and transported along with the kiln gas back to the areas of lower temperature. Because of this cyclic mechanism, the circulating elements increase in progression.
This enrichment continues until the total quantity of circulating elements brought into the kiln system corresponds to the total quantity carried out of it.
The dissociated components become then capable of reacting with other compounds in the kiln-gas, kiln-dust and burning material.
The main compounds made are:
- Alkali sulfate [K, Na]2 SO4
- Alkali chloride KCl, NaCl
- Alkali carbonate [K, Na]2 CO3
- Calcium sulfate Anhydrite CaSO4
- Sulfate spurrite 2C2 S CaSO4
- Sulfo-spurrite [K, Na] 2 SO4
Chloride reacts primarily with the alkalis, forming NaCl and KCl. Any excess of chlorides will react with calcium oxide available in the system to form CaCl2. A part of the alkalis in excess of chloride combine with sulfur to form Na2SO4, K2SO4 and double salts as Ca2K2(SO4)2. Alkalis not combined with chloride or sulfur are present as Na2O and K2O embedded in the clinker mineral.
Sulfur in excess of alkali combines with CaO to form CaSO4.
At temperatures between 800 and 1200ºC, these compounds condensate on the burning materials, kiln dust or on the walls of kiln and preheater system, some of them in a melted phase. The circulating elements, which condensate on the walls in the preheater and kiln tube and on the burning materials and some of these on the dust, reach areas of high temperature again, together with the burning material stream, where once again some are volatilized and transported back with the kiln gas.
As an example of how the concentration of a certain volatile element changes through the process:
If we introduced 1 kg of potassium each hour with the feed and when the reactions achieve a state of equilibrium of volatiles in the system, then we will have the following condition:
- 811 kg of potassium will leave with the clinker
- In the system the concentration of potassium will be 2.573 kg
- In the by-pass dust the concentration of potassium will be 0.221 kg.
- In the exhaust gas dust the concentration will be 0.042 kg of potassium.
The explanation is the following:
The first, 0.189 kg will remain from one kg, this will start making accumulation and continue on for hours with addition of new supply from the fresh material. When the concentration in the kiln atmosphere reaches the concentration of 2.573 kg/h then what is coming out from the by-pass system dust will be 0.221 kg and with E.P dust 0.042 kg/h
The second, the volatility of potassium permits for the remaining of 0.811 kg/h in the clinker coming out from the kiln while only 0.189 kg remained in the system but eventually it will return back to the system with the kiln feed enriching the system with potassium to reach higher concentration of that element and eventually will reach saturation level if the by pass system is not applied.
Third, the by-pass system permits only for controlling the volatile elements but not eliminating them from the system, since the concentration of the potassium coming out with kiln by-pass dust is only 0.042 kg /hour for each one kg added to the system through the fresh feed, while nearly 0.800 kg is going out with the clinker.
That indicates the less the volatility of the element in the raw mix in the kiln, the better will be the condition regarding the circulation phenomena.
Each of these elements will have its evaporation rate or volatility.
Volatility or evaporation factor defined as E of a volatile element or compound means that part of this element evaporates in the kiln burning zone instead of leaving the kiln with the clinker produced in the kiln burning zone where this element is evaporated
The concentration of the sulfate in the clinker in one sample is 1.05 %, SO3 concentration in cyclone 4 materials which is collected from the material pipe of cyclone 4 to the kiln inlet is 2 % and the loss on ignition of this same sample is 3.5%
What is the evaporation factor of sulfate [SO3] in this system?
% SO3 at the kiln inlet loss on ignition free basis=
= = 2.0725
Evaporation factor E = 1 – = 1 – 0.507 = 0.493
This calculation can be applied to any of the other volatile elements as potassium, sodium, or chorine.
The loss on ignition of the sample for chlorine is 3.8 %. Its concentration in the clinker is 0.03 % and its concentration in the hot meal [kiln feed] from cyclone 4 to the kiln inlet is 0.65 %. What is the evaporation factor of chlorine in this system?
% Chlorine at the kiln inlet loss on ignition free=
Evaporation factor E of chlorine = 1- = 1 – 0.04=0.96
The loss on ignition of the sample for potassium in kiln feed is 3.5% and the concentration of potassium in the clinker 0.29% and its concentration in the hot raw meal from cyclone 4 to the kiln inlet 0.39%. What is the evaporation factor of potassium in this system?
The % of potassium at the kiln inlet loss on ignition free =
Evaporation factor E of potassium=1 – = 1 – 0.725 = 0.275
But we have to indicate what is the meaning of the value obtained from the previous examples:
When E = 1 indicate that all volatile elements evaporate and none leave with the clinker
This is clearly indicated in the case of Example 2 of chlorine where the solution proved in a very unmistakable way this fact. [E in the example is nearly one].
When E = 0 indicate that none of the volatile elements evaporate and all leave with the Clinker.
This is clearly indicated in the case of Example 3 of potassium where the solution proved in a very unmistakable way this fact. [E in the example is very small].
Average evaporation factors
Average values for the evaporation factor of various volatiles element compounds are:
Evaporation factor E
K2O free from chlorine 0.1-0.4
Alkali SO3 0.2-0.9 [have a relatively high melting point of 1074ºC, boiling at 1689ºC]
Excess SO3 0.75
KCl 0.990-0.996 [have low melting point of 768ºC, boil at 1411ºC]
The volatile matters escape through the preheater dust [preheater valve].
K2O free from chlorine 0.15
Alkali SO3 0.05-0.25
Excess SO3 ———
The volatile matters escape through the preheater chimney [preheater filter valve]
K2O free from chlorine 0-0.10
Alkali SO3 0-0.20
Excess SO3 ———
Chloride compounds KCl, CaCl2 and NaCl are seen to have an evaporation factor of 0.990-0.996 in the kiln at 800ºC. These compounds melt and boil at 1400ºC.
But sulfate compounds with alkali as K2 SO4 [potassium sulfate] and Na2SO4 [sodium sulfate] will be more stable than CaSO4 [calcium sulfate]. The last component is formed when there is excess sulfur that cannot react with the alkali in the system.
Alkali sulfates have evaporation factors from 0.2 to 0.90 but they are mostly in the lower part of the range, while excess sulfur that cannot find alkali to react with has an evaporation factor of 0.75, therefore it is best that all sulfur react with alkalis to the highest extent.
The concentration of sulfur which may build up in the system through its circulation and its components causes huge trouble for kiln operation. The total quantity of sulfur or the deficiency of alkalis may be the cause. Sulfur that has not been combined with the alkalis present in the system is responsible for causing huge trouble to kiln operation than the other sulfate components such as potassium sulfate and sodium sulfate.
If the alkalis are in the right proportion with the sulfur in the system, both will combine together and become built in salts in the clinker minerals. But in the absence of alkalis i.e. if there is excess sulfur in the system, the more volatile calcium sulfate will be formed in the kiln system, and it has a higher evaporation factor.
The following equation is used for the estimation of optimum molecular ratio between sulfur and alkalis in the system:
If the sulfur and alkalis ratio exceeds 1.1 it means that the amount of sulfur present in the kiln feed material that react with the alkalis is in excess and the remaining excess sulfur will react to form CaSO4.
A kiln feed sample contains the following concentration
SO3=0.45 % K2O=0.37 % Na2O=0.38 %
What is the sulfur and alkalis molecular ratio in this system?
The result is indicating that there is no excess sulfur in the system.
A kiln feed sample contain the following concentration
SO3=0.57% K2O =0.21 % Na2O=0.15 %
What is the sulfur and alkalis molecular ratio in this system?
The result is indicating that there is excess sulfur in the system that will react to form CaSO4
The amount of excess sulfur is expressed in gram SO3 per 100kg clinker and calculated according to the equation.
E.S = 1000x SO3 –850x K2O – 650x Na2O [gram SO3/100kg clinker]
The limit on the excess sulfur is given to be in the range of 250-600g/100clinker
For easy burning kiln feed, the high value of 600gram SO3/100kg clinker will not cause any problem for the kiln operation, but for hard burning kiln feed, the lower value is the limit. Above these limits the coating formation starts in the preheater tower and kiln by-pass system.
A kiln feed sample contain the following concentration
SO3=0.57% K2O =0.21 % Na2O=0.15 %
E.S = 1000 x SO3 –850x K2O – 650 x Na2O [gram SO3/100kg clinker]
E.S = 1000x 0.57 – 850 x 0.21 –650 x 0.15
= 570 – 178.5 –97.5
= 294 gram SO3/100kg clinker
This kiln feed contains a relatively small amount of excess sulfur. But if the material of the kiln feed is hard to burn or the flame is very strong a coating problem may cause some trouble due to build-up in the preheater and the pressure loss may increase in the preheater.
Since the chlorine affinity for reaction with alkalis is higher than the sulfate therefore the following equation is applied to determine the optimum sulfate alkali ratio where the chlorine is subtracted from the alkalis
Therefore the optimum range is nearly 0.8 to 1.1.
By the condensation of the volatilized circulating elements, salt melts are formed when the temperature of the material in the system reaches 700ºC and more. The melt starts to trap particles of dust binding these materials together, and the build-up starts to exist in the system. Thus, the principal cause of build-up is the adhesion of the melt to the walls of the preheater and kiln.
In certain stage of build-up a new material starts to exist and causes more trouble in the system. The formation of spurrite [2 C2S . CaCO3] and sulfo spurrite [2 C2S . CaSO4] in case of the excess sulfur will exist in abundance. The formation of these compounds is promoted and made possible only by the presence of the alkalis, chlorine, and sulfur in the raw material of the kiln feed. The last mentioned build-up will not be due to the present melt but due to kiln mating together of the needle shaped spurrite crystals.
If there is a ring in the kiln related to the circulating elements it will exist in the calcining zone and its position will be 7 to 13 times kiln inner diameter from the kiln outlet (7:13 Di). These rings will contain enrichment of circulating elements, spurrite and sulfate-spurrite. In most cases the sulfo-spurrite will be the dominant component in this build-up.
However, these rings are not very dangerous to the kiln and can be detected from outside the kiln by the kiln hand pyrometer or by shell scanner. These rings always fall by themselves due to the rotation of the kiln and the fluctuation in the temperature inside it.
The build-up can occur in the connection area between the preheater and kiln riser duct.
Cyclone gas-duct i.e. riser-ducts and the cones of lowest cyclones. Coating in these areas can contain from 8 to 60%circulation elements compounds.
The diameters of the gas ducts decrease by the coating formation. In the area of the cyclones cones the build-up will end in closing the material pipes at the bottom of the cone and the eventual result will be the filling of the cyclone by materials and hours will be lost in clearing the material from the cyclone.
Also the crusts that may occur in the cyclones riser ducts may fell down closing the material pipe at the bottom of the cyclone.
It should be noted that the 4, 5, or 6 cyclone stages preheater with precalciner are very susceptible to the build-up caused by volatile materials. This can be attributed to the presence of secondary firing system in the precalciner and due to the temperature profile in such type of system which induce such type of build –up and the high circulation of dust in the lower part of the preheater.
The following features characterize the behaviour of such type of system regarding the circulating elements:
- Practically all of the amount of the volatile elements enter the system is trapped and caught within the kiln and preheater system which leads to considerable build-up of concentration of these elements in the system.
- The maximum concentration of these elements will be found in the area of kiln inlet and at the areas of lowest –cyclones. This phenomenon takes place because the temperature profile and the optimal condition for condensation in the area of the lower most cyclones.
Therefore it is a normal procedure to measure the concentration of circulating elements in the material coming out of the lowest-cyclones such as sulfur, chlorine, sodium and potassium. But still some part of these circulating elements can escape and reach the top most cyclones i.e. the dust enriched by these elements can pass through all four cyclones stages and eventually returns back to the kiln with fresh kiln after precipitating in the electrostatic precipitator.
The behavior of the volatile matters and their effect on operation can be greatly affected by the process used to burn that material to change it into clinker. In the long dry kiln the condition will not permit for high circulation in the system since the kiln dust from smoke chamber contains small concentration of volatile matters and the valve of the kiln exhaust fan is for all kiln gas, while in the 4 suspension-preheater system will be less sensitive to the volatile problem. The suspension preheater kiln with precalciner will be more sensitive to the volatile matters but both if the limit of the maximum allowable concentration of volatiles is exceeded the system should be supplied with by-pass.
12. What is the reason that makes suspension-preheater-kilns with precalciners more sensitive to the volatiles problem than the suspension –preheater kilns?
The reason for higher sensitivity is that in the precalciner kilns the kiln gas to clinker ratio [expressed as Nm³/kg clinker] in the precalciner kilns are lower than in the suspension-preheater kilns. In the precalciner kilns, the quantity of fuel burned in the kiln is 40% of the total fuel of the system i.e.320kcalorie/kg of clinker which results in higher concentration of volatile matters as gram/m³ of kiln gas in the kiln inlet smoke chamber reaching therefore critical values of saturation with less concentration of volatile matters in the kiln feed. While in the 4-suspension preheater kiln the fuel in the kiln is 100% required in the system and this means higher gas to clinker ratio in the kiln inlet smoke chamber i.e. kiln inlet connecting the kiln tube to the preheater. That dilutes the concentration of the volatile matters in this area, and therefore the system can work with higher concentration of volatile matters in the kiln feed.
The operation of suspension preheater kilns with precalciners explained before is very sensitive for circulation phenomena because if this problem is ignored or mismanaged the result will be :
- Frequent kiln stops due to cyclones blocking which need additional time for cooling and cleaning.
- Higher heat consumption due to this frequent stops, additional fuel used for reheating the system and higher kiln’s brick consumption.
- Reduced kiln production since the operator will try to continue work with less draft in the kiln and in most cases in reducing atmosphere with much CO in the system.
14.1 Interruption of the External Circulation
To reduce the level of circulating matter in the kiln system, the input of circulation matters can be reduced in some cases by discarding the fine fraction of the filter-dust; meanwhile, the effect of such measure will be in most cases very limited on the Internal Circulation but can be evaluated to determine its effectiveness by establishing a volatile mass balance.
14.2 Reducing the burning zone temperature
This means the reduction of the volatility of the alkalis, chloride and sulfate components. This can be done by reducing the burning zone temperature. The volatility of the sulfur compounds especially calcium sulfate is a function of the burning zone temperature. Calcium sulfate starts to decompose at 1220ºC and this thermal decomposition can be avoided by lowering burning zone temperature.
This can be done also by other means as :
14.3 Decreasing the silica ratio of the kiln feed and thus making the kiln feed easier to burn.
14.4 Finer grounding of coarser particles especially the free silicates if present in the kiln feed therefore easier to burn kiln feed. The result will be lower sintering temperature in the burning zone decreasing the volatility .
14.5 Accepting higher free-lime in the clinker. This requires less fuel in the burning zone, and there will be no overheating of the burning zone.
14.6 Controlling volatile content in the raw material used for grinding and used as kiln feed. That means observing the optimum molecular of sulfur to alkali and ensuring that the excess sulfur is minimized.
14.7 Controlling Oxidation condition in kiln atmosphere. When we have the oxygen level in the kiln in the higher side, the condition in the kiln will be oxidation condition. The dissociation of sulfate compounds achieves balance in the favor of forming alkali sulfate in the oxidation condition in the kiln. If we have reduction condition the alkali sulfate tends to dissociate to alkali oxide and oxygen. Therefore, the oxygen level in the kiln atmosphere should be maintained at approximately 2%, but increasing oxygen level beyond 2% will have limited effect on the volatility of the sulfate.
14.8 Controlling the reduction condition in the kiln atmosphere
Reducing condition in the kiln atmosphere due to incomplete combustion of the fuel [either due to lower fuel temperature or bad atomization of the fuel] either in the kiln or in the precalciner increases the calcium sulfate and alkali sulfate in the presence of free carbon. The reactions will be as follows:
Calcium sulfate + Carbon → Calcium oxide +SO2 + Carbon mono-oxide
Alkali sulfate + Carbon → Alkali oxide + SO2 + Carbon mono-oxide
These reactions increase sulfur circulation in the system since this reaction enhances the decomposition of the stable calcium sulfate as well as stable alkali sulfate.
Therefore it is of prime importance to maintain carbon mono-oxide % not more than 0.1% at the kiln inlet and oxygen level >1.5%. The incomplete combustion of the fuel should be prohibited in all conditions. This is very important since the high concentration of sulfur in the system leads to a very dusty clinker from the kiln in spite of the fuel in the kiln burner or increasing the kiln burning zone temperature. This ends in a very bad heat profile in the kiln itself and cyclic kiln operation.
14.9 Installation of a kiln By-pass system
If the total chloride input as indicated before exceeds 0.015% on raw material basis or 0.023% chlorine on clinker basis then the installation of a by-pass system is a must.
In such case the by pass system will be used mainly to control the chlorine and the removal of it from the kiln system. But the problem of chlorine in the system can’t be blamed alone as the cause of the problems caused by volatile matters in the suspension preheater kilns, since in this kiln the volatiles have no escape route in the kiln system.
This leads in the absence of by-system to formation of low-melting phases belonging to the CaSO4-Na SO4-Kcl system or the excessive entrapment of alkali/sulfur in the clinker.
This is partially overcome by the diverting part of the kiln exit gases with a bypass duct.
A modern by pass system consists of an air quench chamber, a shut-off valve, a water quench chamber and a dust collector. The air quench chamber is used to mix ambient air with the kiln gasses to quickly cool the harmful volatile compounds. The water quench chamber is used to cool the gases quickly to lower temperature for dust collection.
The by-pass systems installation will eventually lead to higher power and heat consumption in the kiln system. Also one of the major losses with this installation is the dust loss, since the dust-laden gas streams are thrown out of the system. At 30% bypass the fuel consumption increases by about 8-10% and material loss by about 3-6%.
The traditional by pass installation consists of
- The chimney
- The draft-fan
- The electrostatic-precipitator
- Dust handling from the electrostatic-precipitator collecting screw-conveyor to the collecting-pin and consists of: The bucket-elevator, the dust pin, and the granulator to change the dust into dust balls and the belt conveyor for the dust balls to the truck.
- Condition-tower for the hot gas from the quench chamber to decrease temperature of the gas from 450ºC to 150ºC by water spray system.
- The quenching chamber for mixing the hot gas with ambient air, laden with the volatile matters from the kiln riser-duct and decrease gas temperature from 1000ºC to 450ºC in a matter of seconds to freeze the volatile components in its solid state and prevent it from existing in the melting phase in the by-pass ducts system. This mixing chamber is always located in the duct taking the hot gas from the kiln riser duct and nearly 800mm away from the connecting point to the riser duct.