Upload
mkpq
View
48
Download
2
Embed Size (px)
DESCRIPTION
Mod 5
Citation preview
Objectives :
• Significance of pyroprocessing.
• Burnability of Rawmix
•Effect of Raw mix in burning process.
•Formation of clinker and Granulation
• Effect of S, Cl, Na, K in burning process
“Heating of the raw meal to the required temperature so as to produce the desired clinker compounds in an economic way at higher productivity in the preheater & kiln.”
Pyro processing
Production of Good quality clinker from the kiln depends on :
Good raw mix design which in turn depends on
1. Desired clinker minerals
2. Allowable free lime
3. Nature of the raw material
Liquid ratio & Residence time in the kiln
Fuel quality & Combustion
Fineness of the raw meal to the desired level
Reactions involved in Pyro processing
1250Formation of liquid phase5
1400Completion of reaction & Equilibrium6
1200Formation of C2S, C3A 4
805Evolution of CO2 from CaCO3 and MgCO3
3
550Evolution of chemically bonded water2
100Evaporation of free water from the feed1
Required minimum temperature, Deg C
Reactions taking placeSl no
BURNABILITY OF RAWMIX
• The readiness with which a rawmix is transformed into clinker minerals in the course of high temperature treatment
• Determined by their chemical, mineralogical and granular composition
• Deciding factor for the temperature necessary for a good clinker product
• Different rawmix with the same chemical composition and equal fineness may differ in their burnability due to their different mineralogical composition
• Poor burnability of rawmix may be caused by coarse grains of Calcite (rich in CaCO3) or Quartz (rich in SiO2)
COARSE GRAINS• Clinker consists of alite, belite, liquid phase, free
lime and pores • The effect of coarse grains is studied with optical
microscope• Coarse quartz and calcite results in poor burnability,
high free lime and too little C3S in the clinker• Critical size of particle for residual free lime after
30 minutes Quartz - 45 micronsCalcite - 125 microns
• 1% increase of Quartz +45mic ----> 0.93% Free CaOCalcite +125mic ----> 0.93% Free CaO
Free CaO1400 = 0.33 (LSF –95)
(30minutes) + 1.8 (SM-2)
+ 0.93 SiO2 +45 micron
+ 0.56 CaCO3 +125 micron
FREE LIME
Kuehl’s Burnability Index
BI = C3S / (C4AF+ C3A)
Example :
BI = 45 / (14.5 + 7.5 )
= 2.045
• For the desired free lime, the required fineness can be found out to improve the burnability.
• The burnability can also be improved by reducing the LSF and Silica Modulus
• A mineraliser like flouride or sulfate (gypsum) can also be added to reduce the temperature at which liquid phase is formed
• mineraliser modifies the viscosity and surface tension of the clinker liquid to promote the formation of clinker minerals.
Chemical Composition
Parameter Variation Burnability
LSF
AR
SR
Parameter Variation Burnability
C3S
C4AF + C3A
Flux & Mineraliser
Desired Clinker QualityMineralogy of Raw meal
Chemical CompositionFineness
% Liquid
Effect of Raw mix in burning
FLUX & MINERALISER
Flux : Reduce the temperature of liquid formation.
Examples : Fe2O3, Al2O3
Mineraliser : Reduce the temperature for the Alite formation
Examples :TiO2, P2O5
LIQUID CONTENT
• Raw meal melts at more than 1200 deg C depending on the amount of fluxing material
•Liquid formation is important for
Effective Granulation
Stable coating
Protecting the refractory
% Liquid = 2.92 Al2O3
+ 2.25 Fe2O3
+ Mgo
+ Alkali
CLINKER FORMATIONPoor granulometry and Dusty Clinker leads to.• More wear rate in cooler and more maintenance
• Formation of unstable, porous coating instead of dense, stable coating
• Poor grindability of clinker
• Problematic clinker handling and dust nuisance
• Unstable kiln operation
NODULISATION• Particles are held together by capillary forces of the
liquid• Nodulisation depends on the amount of liquid,
particle size and the speed of the kiln• In the kiln, the liquid is formed in a narrow temp
interval at aprox 1300 degC• Formed C3S crystals sinter together to form coarse
C3S particles and slow down the nodulisation process• Nodulisation is enhanced by liquid phase and
counteracted by large C3S particles.
• At higher BZ temp, the formation of C3S particles is faster and hence smaller will be the nodule size
• Reducing the Alumina / Iron ratio (to 1.6) will improve the nodulisation as the formation of liquid phase starts at lower temperature
• Lowering of the silica modulus increases the amount of liquid and thereby improves the nodulisation
• Reducing the LSF reduce the potential C3S and thus increase the nodulisation
CHANGES IN CHEMISTRY & CLINKER NODULISATION
Nodulisation
N
Amount of C3S
% Liquid
Length of burning
zone
Particle size
Temperature Residence time
Common Morphological features of Clinker Phases
SlNo
ClinkerPhases Colour Shape Microstructures
1 Alite – C3S(3CaO.SiO2)
Straw yellow,brown,yellowishbrown,brownish yellow
Hexagonal,Pseudo-hexagonal, Lath,Subhedral,Anhedral etc.
Fused grains,Stretched, Twinned,Granulated, Brokenout line grain
2 Belite – C2S(2CaO.SiO2)
Blue, Bluishyellow,yellowish blue,greenish yellow,yellowish green
Rounded, Sub-rounded,Elliptical,Subhedral,Anhedral etc.
Clusters of varioussize, Fused grains,Twinned grains,Corroded grainmargins, Striationson belite grainsurface asinclusions
3 Free lime–CaO
Multiple highorderinterferencecolours of pink,green, yellow,blue etc.
Rounded,Sub-rounded,Subhedral,Anhedrtal etc.
Clusters, Striationson the grain surfaceas inclusions.
Characteristic Optical Properties of Clinker Phases
Major Clinker mineralsBouge’s Formulae:
when A.R > 0.64
C3S = 4.071 C - 7.6 S - 6.718 A- 1.43 F - 2.852 SO3
C2S = 2.867 S - 0.754 C3S
C3A = 2.650 A - 1.692 F
C4AF = 3.04F
VOLATILE MATTER
Volatile Matters present
SO3
Cl
K2O
F
OH
Na2O
Circulation of volatile matter• A fraction of the volatile components evaporates in
the kiln burning zone and condense in the back end or rawmeal and re-enter the burning zone
• The repeated evaporation and condensation results in an Internal circulation where the concentration can go up to fifty times the input concentration
• At equilibrium state, the output of volatiles along with clinker is equal to the total input from rawmeal and fuel
• Higher degree of volatiles concentration exists either due to more input or due to a high degree of volatility
External circulation• Volatile matter in rawmeal like sulfur, is burnt to
SO2 gas in the preheater upper cyclones at around 400 - 600 degC and expelled out from preheater but effectively precipitated in ESP
Condensation in Preheater• Volatile matter with low melting point condenses in
preheater walls and rawmeal particles causing build ups on cyclone
• SO2 gas combines with calcined rawmeal and condenses as CaSO4
CaO + SO2 + 1/2 O2 --------> CaSO4
Raiser Duct &
4 CycloneKiln
S in Raw meal a3
S
Kiln firing
a2
SO3 (b)
Through Raw meal
SO3 thro’ Clinker (c)
INTERNAL CIRCULATION
Recycled SO3 (e)
S
PC firing
a1
PREHEATER FIRST STAGE ESP
S in Raw meal
SO3
LOWER STAGES
SO3
IN HOTGAS
SO3 thro’ stack
EXTERNAL CIRCULATION
Recycled Raw meal
kiln
1
23
4
ESP
Raw meal
PC
Kiln Coal
Types of Circulation
Internal Circulation
External Circulation
Melting Pt & Boiling Pt of Volatile Matter
CompoundK Na Ca
MPt BPt MPt BPt MPt BPt
Oxide dec 350 sub 1275 2850 -
Carbonate 894 dec 850 dec dec 825
Sulphate 1074 1689 884 - 1190 -Chloride 768 1411 801 1440 772 1600
Hydroxide 360 1320 328 1390 580 -
MPt : Melting Point, BPt : Boiling Pt,dec : Decompose, Sub:Sublimate
Affinity between the Volatile Components
• Chloride reacts first with alkalis to form NaCl and KCL• Excess chloride reacts with Calcium to form CaCl2
• Alkalis in excess of chloride combine with sulphur to form Na2SO4 and K2SO4
• Alkalis not combined with chloride or sulphur will be present as Na2O or K2O
• Sulfur in excess of alkali combines with CaO to form CaSO4
VOLATILITY• Volatility is expressed in terms of evaporation factor
% within clinkerEvap. Factor E = 1 - -------------------------
% at kiln inlet LOI free basis• Chloride compounds KCl, NaCl and CaCl2 are having
an E of 0.99 to 0.996. At 800 degC they are melted and at 1200 - 1300 degC they are almost entirely evaporated
• Sulphate compounds with alkalis K2SO4, Na2SO4 are more stable ( E = 0.30 - 0.90), which is very desirable
Molecular Ratio of Sulfur and Alkali• Sulfur and alkali overall concentration and their
proportion is very important• Individually they are more damaging than their
sulphate compounds(K2SO4, Na2SO4) as K2SO4 has such a very high evaporation temp and they come out along with clinker
• Only half of Na2SO4 comes out as it begins to split into Na2O and SO3 at around 900 degC
• Sulfur in excess of alkali will form a more volatile CaSO4, which has a high evaporation factor
• An optimum molecular ratio is a must to avoid the presence of the damaging individual components
Optimum (SO3 / Alkali) Ratio
SO3 SO3 / 80
------ = ----------------------- = 1.1
Alk K2O + 0.5 * Na2O------ -------- 94 62
Operational Aspects of VC• Formation of build-ups in preheater riser
pipes and cyclones reduces the air volume • Reduced kiln production and increased
circulation of sulfur compounds due to less availability of excess oxygen
• Higher heat consumption• Dusty clinker formation
Nor mal Limits
Max Limits
K 2O eq=K 2O+1 .5Na2O 3.70% 6%
Chlor ine as Cl - 0.80% 2.00%
Sulf ur as SO3 2.50% 5%
Limits On Volatile Components In Bottom Cyclone Stage in a SP kiln system on LOI free Basis
CONTROL LIMITS
Nor mal Limits
Max Limits
K 2O+0.65*Na2O 1 .00% 1 .5%
Chlor ine as Cl - 0.02% 0.02%
Sul f ur as SO3 1 .00% 1 .6%
Max Allowable Input of Volatile Components for a SP kiln system Without bypass on LOI free Basis
CONTROL LIMITS
Nor mal Limits
Max Limits
K 2O+0.65*Na2O 1 .00% 1 .5%
Chlor ine as Cl - 0.01 5% 0.01 5%
Sul f ur as SO3 0.80% 1 .2%
Max Allowable Input of VC for a CALCINER Kiln system Without bypass on LOI free Basis
CONTROL LIMITS
Raw mix control
SO3 / Alkali ratioKiln by pass
Excess airFlame adjustments
Reducing Evop factor
ControlMethods
Discard the filter dust
Kiln Inlet Coating PROBLEMS• CO formation in kiln inlet• reduced atmosphere • increased kiln inlet draught• reduction in kiln inlet Nox content• reduced kiln feed• blockage of meal chute and riser duct• premature refractory failure• dusty clinker formation and poor clinker quality• less intake of secondary air • kiln hood positive
Kiln Inlet Coating Remedial Measures• increased oxygen content in kiln inlet ( to be
maintained around 2 %)• higher fineness of fine coal• smooth surface of kiln inlet lining• maintaining the alkali sulfur ratio near unity• using air blasters in kiln inlet and riser duct• using coating repellant refractory castable at kiln
inlet like silicon carbide castable• periodical cleaning of kiln inlet using compressed
air lance during plant running condition
Solving the Problem with Sulfur• Reducing the volatility- especially of CaSO4, by
lowering the BZ temp• By making the rawmeal easy to burn by means of
higher fineness / lower silica ratio • Maintaining optimum molecular ratio of sulphur to
alkali to avoid excess sulfur circulation• Reducing thermal load in the kiln• Increasing the availability of oxygen in kiln to shift
the equilibrium towards Alkali Sulphate• In the Case of reducing atmosphere, free carbon in
fuel reacts with alkali sulphates to liberate alkali oxides which increases the VC circulation
Kiln burning zone SO2 Formation :
Fuel S + 02 SO2
Alk SO4 <=> Alk-O + SO2 + ½O2
SO2 absorption :
Na2O + SO2 + ½O2 <=> Na2S04
K2O + SO2 + ½O2 <=> K2SO4
CaO + SO2 + ½O2 <=> CaSO4
Ca SO4 + C => CaO + SO2 + CO
Alk2 SO4 + C => Alk2O + SO2 + O
During Reducing Atmosphere in Kiln
Calcining zone SO2 Formation :
Fuel S + 02 S02
CaSO4+ C <=> CaO + SO2 + CO
SO2 absorption :
CaO + SO2 <=> CaSo3
CaCO3 + SO2 <=> CaS03 + CO2
CaSO3 + ½O2 <=> CaSo4
PREHEATER SO2 Formation :
Sulphides(FeS) + O2 Oxides + SO2
Pyrite
Organic S + O2 SO2
SO2 absorption :
CaCO3 + SO2 <=> CaS03 + CO2
CaSO3 + ½O2 <=> CaSo4
Solving the Problems with Chloride• As the evaporation factor of chloride is very high,
it is very difficult to reduce volatility.• Avoiding the chlorine rich raw material• Avoiding the peaks of chlorine content in rawmeal
by pre homogenization of raw material with respect to chlorine
• Discarding the ESP dust• Installation of alkali by pass system
Solving the Problems with Alkali• Maintaining the optimum the alkali sulfur ratio
• Addition of sulfur in the system in the form of gypsum