Semi-dry process of making clinker
Before we start discussing about clinker manufacturing in VSK technology — there it is important to know about kiln geometry and its overall relation with kiln diameter.
Here D (effective dia after brick insulation) gives an idea about kiln capacity in terms of Mt/day as all other dimensions are proportionately related. The top conical portion is inclined to an angle between 9 –11 degrees, considering volume shrinkage of 25 to 30%.
Once kiln effective volume (m³) is known and its utilization depends on material descending rate (cm/min.).
Length of three imaginary zones varies with kiln effective dia (D) to hold the material in respective zones for equal length of time — similar to rotary kiln.
Therefore, D is directly proportional to L (L1,L2 and L3) to maintain Retention Time (T1,T2 and T3) unchanged in respective zones. Recommended mat retention in all three zones believe to be significant for clinker formation and clinker quality.
With increase in kiln effect. dia — thermo processing zones are proportionately increases.
As it mentioned, mat retention time in respective zones remains unchanged for different dia kiln for a specific meal type. However a small reduction in retention time is possible with easy mix character to achieve higher volume utilization factor.
To avoid dissimilarities in mat residence time for bigger dia kiln having extended thermal zones, there faster clinker bed movement (descending rate in cm/min) is normally maintained.
Therefore, its important to maintain different zone length and material bed movements to attain trouble free kiln operation. Here we get an idea of kiln production capacity (T.P.D.) as entire kiln vol gets discharge in 7 to 8 hours. Volume utilization factor for VSK kiln is normally found between 3.5 -3.6 TPD/m³ .
Accordingly, max grate rotation (circumferential speed cm/min) for clinker discharge is calculated as: D*π*100/(3.05*60).
Material balancing to produce 1 kg clinker involves 1.63 Kg kiln feed (1.55 kg mix + 0.08 kg solid fuel). Fuel is premixed with raw mix, based on average heat requirement between 750–775 K.Cal/Kg clinker. Accordingly combustion air is needed 1.10 Kg with an excess air factor 1.15.
Black-mix is made to nodules of size 6 — 8 mm before feeding to kiln with 12 to 13 % water or 0.13 x 1.63 = 0.212 kg water/kg clinker and this is the reason for manufacturing process is termed as semi-dry process.
Kiln exhaust gas quantity remains as 2.29 kg with an average density 1.825 Nm³/kg.
Computing heat balance for VSK, this is found heat losses with flue gases is very less compare to dry-process rotary kiln as gas temp remains less and there is no additional air is used for clinker cooling. Evaporation loss is high also the radiation losses due to its smaller volume.
Combustion air is calculated based on heat input of 800 K.Cal/Kg clinker for fuel type petroleum coke. Accordingly air requirement is shown less as inherent O2 content in P.Coke is more. For any other coal/coke use, combustion air and flue gases quantity may come more. Therefore, a minimum 20% additional vol is considered for blower(s) and flue gas fan capacity calculation.
Kiln pressure as shown in above chart depends on kiln volume and its utilization factor (clinker rate in tph) and heat use.
= 10*tph*heat use (K.Cal/Kg)*SQRT(kiln effct. height in m)/(effct. dia in m*785)²
High kiln pressure is an indication of kiln up-set condition of having more uncalcined dust, inclined or weak center fire and/or under sized feeding.
A single nodule is itself a kiln within the kiln. When periphery of nodule changed to clinker- preliminary reactions are only started at inner core.
Therefore, length of high temp zone, descending rate of nodules or retention time, equally nodule size and meal fineness — all are very important to achieve quality clinker.
Kiln is normally operated in two methods, (i) Open-fire and (ii) Dark-fire.
In open-fire, side fire remains visible and burning zone depth is maintained less. Easy to operate as there is less chances of center fire missing but demand more fuel and heat losses through exhaust gas used to be more. Cause high particulate emission due to feed nodule bursting as it dry up faster and flue gas velocity increases with the reduction in gas density, able to carry out more fine particles.
Choice of operation sometime depends on heat content in meal, which is premixed and method of operation is being adjusted accordingly.
The above figure also illustrate the true position of zones which is more inclined towards center where availability of air remain deficient.
Counter current flow of air and material helps in recovering heat from descending hot clinker — thus air is preheated first. While exit out, heat is again transferred to cold and moist nodules.
Similarity in process is observed in Rotary kiln also. The only difference is, in Dry-process kiln — additional air is used for clinker cooling but in VSK , required combustion air is only used for cooling.
Therefore, very fast cooling of clinker or quenching process is being avoided in VSK process and it may unable to prevent phase transformation due to slow cooling of clinker.
Another disadvantage in this process commonly faced is homogeneity of heat distribution and maintaining center fire of clinker bed due to uneven air distribution.
A delayed combustion activities is observed at center of the kiln. Deficiency of combustion air availability towards kiln center is due to air flow characteristics — unless air is not forced. Uneven distribution of air (more at periphery and less at center) is the reason for weak center fire and more CO presence.
And problem may get escalated with increase in kiln dia and in such condition, a part of combustion air may be introduced just at the binning of the shell conical portion. It may help to keep center fire strong and preventing center discharge.
Bigger dia kiln may demand more perfection in mix quality and homogeneity, balanced fuel to air ratio, right water addition to get better nodule porosity and size, maintaining required temp in various processing zones.
All such information shall be made available to kiln operator to achieve optimum clinkerization — both in quality and quantity.
Kiln flue gases which contains the reaction products of fuel and carbonates and residual substance such as particulate matter are treated using pollution control devices.
VSK, as semi-dry process, release water vapour and don’t allow dust particles to rise due to high moisture present in flue gases. It help reducing particulate emission by 30–40% in compare to any dry gases.
Recent use of more Pet Coke as solid fuel has increased sulfur intake but presence of its product during combustion (SOx) in flue gases still found negligible. As gases runs through the clinker bed — which is rich in CaO and helps in desulfurization, where sulfur is left behind and get absorbed.
VSK is probably the first type of kiln that was used for making cement.
Vertical Shaft Kiln is having the advantage of good heat preservation, long refractory life, small space occupation and low investment cost.