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CemNet Training - Course TCM01
Module 4 - Session 2
Cement Manufacturing Technology 4. Pyroprocessing – raw mix to cement clinker. 4.2. Alkali cycles
Cement Technology Training ©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
In session 4.1 we discussed the importance of venting the exhaust gases from the kiln and the resistance and pressure drop to that venting caused by the preheater. Cyclone 1 Any changes in the process which restrict the preheater and increase the resistance to venting the exhaust gases from the kiln will be detrimental to kiln operations. Some minor components of the kiln feed will melt and evaporate in the burning zone of the cement kiln.
Feed Cyclone 2 Cyclone 3
Cyclone 4
Cyclone 5
Rotary Kiln
Cement Technology Training
©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
In session 4.1 we discussed the importance of venting the exhaust gases from the kiln and the resistance and pressure drop to that venting caused by the preheater. Cyclone 1 The minor components which behave in this way are the chloride and sulphate salts of the alkali metals. After evaporating in the burning zone they are carried back to the preheater in the exhaust gases where they recondense on the feed and are again carried back into the kiln. Cement Technology Training
©2005
Feed Cyclone 2 Cyclone 3
Cyclone 4
Cyclone 5
Rotary Kiln
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
1
CemNet Training - Course TCM01
Cement Technology
Module 4 - Session 2
4.2 Alkali Cycles
In session 4.1 we discussed the importance of venting the exhaust gases from the kiln and the resistance and pressure drop to that venting caused by the preheater. Cyclone 1 This causes them to concentrate in the kiln in a so-called “alkali cycle”. Some of the alkalis also condense on the walls of the ducts in the preheater, causing the feed to stick to the walls and creating build-up which restricts the gas flow through the preheater.
Feed Cyclone 2 Cyclone 3
Cyclone 4
Cyclone 5
Rotary Kiln
Cement Technology Training
©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
In session 4.1 we discussed the importance of venting the exhaust gases from the kiln and the resistance and pressure drop to that venting caused by the preheater. Cyclone 1 In the worst cases this “alkali cycle” can lead to clogging and blockage of the preheater. The kiln must then be stopped to clear the preheater. This is a dangerous task and also means that the thermal energy investment to raise the kiln temperature above 1400°C must be repeated. Cement Technology Training
©2005
Cement Technology
Feed Cyclone 2 Cyclone 3
Cyclone 4
Cyclone 5
Rotary Kiln
4. Pyroprocessing
4.2 Alkali Cycles
In session 4.1 we discussed the importance of venting the exhaust gases from the kiln and the resistance and pressure drop to that venting caused by the preheater. Cyclone 1 Control of this “alkali cycle” is therefore critical for stable cement kiln operations.
Feed Cyclone 2 Cyclone 3
We will explore the mechanism of this “alkali cycle” in this session, and also the means available to control the cycle.
Cyclone 4
Cyclone 5
Rotary Kiln
Cement Technology Training
©2005
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
2
CemNet Training - Course TCM01
Cement Technology
Module 4 - Session 2
4.2 Alkali Cycles
The mechanism of the alkali cycle is as follows: Volatile species in the kiln feed pass down the preheater and into the rotary kiln.
Cement Technology Training ©2005
4. Pyroprocessing
Cement Technology
4.2 Alkali Cycles
The mechanism of the alkali cycle is as follows: Volatile species in the kiln feed pass down the preheater and into the rotary kiln. Some of the volatiles pass out of the kiln in the clinker.
Cement Technology Training ©2005
4. Pyroprocessing
Cement Technology
4.2 Alkali Cycles
The mechanism of the alkali cycle is as follows: Volatile species in the kiln feed pass down the preheater and into the rotary kiln. Some of the volatiles pass out of the kiln in the clinker. Some evaporate in the burning zone of the kiln and are carried away with the combustion product gases.
Cement Technology Training ©2005
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
3
CemNet Training - Course TCM01
Cement Technology
Module 4 - Session 2
4.2 Alkali Cycles
The mechanism of the alkali cycle is as follows: Volatilised material from the feed is joined by any volatile material from the fuel.
Cement Technology Training ©2005
4. Pyroprocessing
Cement Technology
4.2 Alkali Cycles
The mechanism of the alkali cycle is as follows: Volatilised material from the feed is joined by any volatile material from the fuel. The combined volatilised material from the feed and fuel are carried to the preheater in the exhaust gases where they condense on the incoming feed and are carried back into the kiln. Cement Technology Training ©2005
4. Pyroprocessing
Cement Technology
4.2 Alkali Cycles
The mechanism of the alkali cycle is as follows: The cycle of volatile material builds up…….. ….until the amount of material exiting in the clinker equals the amount of material entering in the feed & fuel. Concentration of recirculating volatile material is significantly higher than in either the feed, fuel or clinker. Cement Technology Training ©2005
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
4
CemNet Training - Course TCM01
Cement Technology
Module 4 - Session 2
4.2 Alkali Cycles
The mechanism of the alkali cycle is as follows: Measurement of the input levels in feed and fuel……. …….output levels in the clinker…... …….and the levels in the hot meal. Allows the volatility of each species in the particular kiln to be estimated, and a model of the alkali cycle for the particular kiln to be constructed. Cement Technology Training ©2005
4. Pyroprocessing
Cement Technology
4.2 Alkali Cycles
The mechanism of the alkali cycle is as follows: The concentration of the volatile species in the hot meal at the bottom of the preheater increases the “stickiness” of the feed. The hot meal can stick to the walls of the preheater ducts causing restrictions and in the worst cases cause blockage of the preheater.
Cement Technology Training ©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. Part of the exhaust gases exiting the rotary kiln to the preheater are extracted before entering the preheater…... …….and quench cooled to precipitate the volatiles. The effectiveness of alkali bleeds or bypasses is closely connected with the cycle itself. Cement Technology Training ©2005
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
5
CemNet Training - Course TCM01
Cement Technology
Module 4 - Session 2
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. A bypass relies on the concentration of recirculating volatile material being significantly higher than in either the feed, fuel or clinker. This means that bleeding out a small proportion of the recirculating material………. ……………is sufficient to take out the equivalent amount that is entering in the feed and fuel. Cement Technology Training ©2005
4. Pyroprocessing
Cement Technology
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. A bypass relies on the concentration of recirculating volatile material being significantly higher than in either the feed, fuel or clinker. The concentration in the recirculating material reduces, and with it the tendency for the preheater to block.
Cement Technology Training ©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. The concentration factor of recirculating volatile material is the ratio of the quantity in the hot meal to the quantities entering the kiln in the feed and the fuel. Concentration factor is directly related to the volatility of the recirculating species in the burning zone. Alkali bypasses are effective for highly volatile materials which have high concentration factors. Cement Technology Training ©2005
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
6
CemNet Training - Course TCM01
Cement Technology
Module 4 - Session 2
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. Chlorides have a very high volatility in the burning zone (>97%), and therefore high concentration factors. Alkali bypasses are very effective in reducing the cycle of chlorides, and alleviating chloride based clogging of the preheater. If the raw materials and/or fuel have chloride content in excess of 0.03% on clinker an alkali bypass will be required to allow the kiln to operate without preheater clogging problems. Cement Technology Training ©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. Chlorides have a very high volatility in the burning zone (>97%), and therefore high concentration factors. Alkali bypasses are very effective in reducing the cycle of chlorides, and alleviating chloride based clogging of the preheater. When alternative fuels are burnt in a cement kiln, installation of an alkali bypass can be necessary to alleviate chloride based preheater clogging problems. Cement Technology Training ©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. When a bypass is installed on a cement kiln the volatility of the potassium, K+, and sodium, Na+, in the feed can be increased by the addition of chloride to the feed as calcium chloride, CaCl2. Potassium and sodium will preferentially combine with chloride in the hot meal of the kiln and their volatility will be boosted by the volatility of the chloride. Cement Technology Training ©2005
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
7
CemNet Training - Course TCM01
Cement Technology
Module 4 - Session 2
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. When a bypass is installed on a cement kiln the volatility of the potassium, K+, and sodium, Na+, in the feed can be increased by the addition of chloride to the feed as calcium chloride, CaCl2. This means that a bypass can be used to produce low alkali clinker from high alkali raw materials if calcium chloride is added to the kiln feed. Cement Technology Training ©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. However, alkali bypasses are not so effective as the solution for sulphate based recirculation problems and preheater clogging. Provided oxidising conditions prevail in the kiln, sulphates have a low volatility in the burning zone. This means the concentration in the recirculating material does not build up sufficiently for a bypass to be effective in bleeding out sulphates, unless very high bypass rates are used. Cement Technology Training ©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. However, alkali bypasses are not so effective as the solution for sulphate based recirculation problems and preheater clogging. Provided oxidising conditions prevail in the kiln, sulphates have a low volatility in the burning zone. Only if there are reducing conditions in the burning zone does the concentration of sulphate in the hot meal build up sufficiently for a bypass to be effective. Cement Technology Training ©2005
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
8
CemNet Training - Course TCM01
Cement Technology
Module 4 - Session 2
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. Reducing conditions and the presence of CO result in breakdown of sulphates in the burning zone. SO3 + CO Æ SO2 _ + CO2 _ The volatilised SO2 reacts with CaO and alkalis to form sulphates in the preheater. SO2 + CaO Æ CaSO3 + _O2 Æ CaSO4 Recirculating sulphates then build up sufficiently to make a bypass effective. Cement Technology Training ©2005
4. Pyroprocessing
Cement Technology
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. However, the reducing conditions create the problem of high sulphate recirculation and also the circumstances where a bypass can solve the problem. SO3 + CO Æ SO2 _ + CO2 _ If the reducing conditions are eliminated the problems of high sulphate recirculation usually disappear. The bypass is then not be needed. Cement Technology Training ©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
One solution to this alkali cycle problem can be the installation of an alkali bypass or bleed. The cause of reducing condition and attendant recirculating sulphate problem is usually the combustion in the main burner of the kiln. SO3 + CO Æ SO2 _ + CO2 _ Solving these combustion problems will solve the problems of recirculating sulphates. Efforts should be focused on the main burner of the kiln. Cement Technology Training ©2005
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
9
CemNet Training - Course TCM01
Cement Technology
Module 4 - Session 2
4.2 Alkali Cycles
Sulphides in the raw materials create an entirely different problem which require a different solution. Sulphides in the raw materials oxidise and volatilise in the top stages of the preheater and escape the preheater in the exhaust gases as SO2. S2- + O2 Æ SO2 _ SO2 in the exhaust gases is partially absorbed by the limestone in the raw mill to form calcium sulphite, CaSO3, which is then readily oxidised to calcium sulphate, CaSO4 . SO2 + CaCO3 Æ CaSO3 + CO2 CaSO3 + _O2 Æ CaSO4 Cement Technology Training ©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
Sulphides in the raw materials create an entirely different problem which require a different solution. Sulphides in the raw materials oxidise and volatilise in the top stages of the preheater and escape the preheater in the exhaust gases as SO2. S2- + O2 Æ SO2 _ However, the raw mill will only absorb 80% of the SO2 at best and the absorption rate falls off with decreasing moisture content in the raw materials .
Cement Technology Training ©2005
Cement Technology
4. Pyroprocessing
4.2 Alkali Cycles
Sulphides in the raw materials create an entirely different problem which require a different solution. Sulphides in the raw materials oxidise and volatilise in the top stages of the preheater and escape the preheater in the exhaust gases as SO2. S2- + O2 Æ SO2 _ When the raw mill is stopped virtually all the SO2 will be lost from the kiln as emissions. These, and other emissions from the cement kiln, and their mitigation, are the subject for the next session of the course.
Cement Technology Training ©2005
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
10
CemNet Training - Course TCM01
Module 4 - Session 2
Cement Manufacturing Technology 4. Pyroprocessing – raw mix to cement clinker. 4.2. Alkali cycles.
Cement Technology Training ©2005
4. Pyroprocessing
This document is for the sole use of students enrolled on course TCM01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © 2005 http://training.CemNet.com
11
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