In the black-meal process of the clinkerization based on VSK technology, the low-volatile fuel, i.e., coke-breeze or saleable low-volatile (SLV) coal, is uniformly mixed with raw materials during grinding and is fed into the vertical-shaft kiln from the top in the form of nodules of 5-10 mm diameter. The entire space of the kiln is filled with nodules. Air required for burning the clinker enters from the bottom of the kiln and moves upwards through the voids and gaps in the burnt clinker. The clinker is cooled rapidly and the air is preheated. Fuel particles of the raw-meal nodules bum, when the preheated air is in contact with them. The cooled clinker will be discharged out of the kiln from its bottom.
Good operation of the kiln aims at high productivity with improved clinker quality and low fuel and material composition, and fuel consumption, taking into consideration the structure of the kiln, and condition of air ventilation through the nodule/clinker bed.
Before lighting up the kiln, the following checks should be carried out:
(a) Rotary grate: The grate segments and crown with cutter teeth should be intact and in position. The grate should be allowed a full revolution to ensure that it does not touch the lining plates anywhere.
(b) Lining plates should not be loose.
(c) Refractory lining: The refractory lining must be checked and repaired wherever necessary. If the thickness of lining in the burning zone becomes reduced below 80 mm, it must be replaced.
(d) All starters/main switches of the electrical installation should be checked to ascertain whether they are in working condition.
(e) Roots blower: In accordance with the manufacturers recommendations, oil must be checked. Filters must be cleaned. The weights in the safety valve are to be adjusted to suit the anticipated kiln bed resistance.
(f) The chimneys damper, rotary feeder, kiln hood doors, and discharge gates must be checked for mechanical operation and lubrication.
(g) All the flanges of the kiln, bottom hood cup, chutes, discharge gates etc. should be properly sealed so that the combustion air does not leak through gaps between the joints.
After checking the equipment, the pre-selected old clinker or filling material (size 20 to 50 mm) is filled from the top of the kiln. The discharge gates/material block tube (MBT) should not be operated at this time. The filling is continued up to a level of 2.5 to 3 m from the rotary grate. Then the discharge gates/MBT must be operated for emptying the bottom cup. It has to be ensured that the rotary grate does not operate at this time. Next, the grate is operated at slow speed so that the clinker bed descends uniformly about 100 mm and then the grate is stopped. The filling is again started from the top until the kiln is 1.5 to 2.0 m empty form the top of the cone. The rotary grate is again started to ensure that the clinker bed descends uniformly by about 100 mm. About 100 kg of dry firewood (size 75 × 75 × 700 mm approx) is then stacked uniformly over the bed.
The chimney damper is kept partially open and the by-pass valve in the kiln air duct is kept open. The kiln can be lit. A few lighted torches of firewood are passed into the kiln through the kiln hood. The fire is allowed to spread uniformly. The draft is adjusted to suit the fire. After a uniform fire is achieved, coal is fed through the rotary feeder to make a uniform incandescent bed of fire. At a suitable stage the Roots blower is started with all by-pass valves open. The nodulizer is started and green nodules are fed over the fire bed. Initially it is preferable to have a slightly higher moisture content in the nodules. After about 10 minutes the rotary grate is started at minimum speed.
The bed level is built up slowly by operating the rotary grate at a lower speed. The quantity of air is increased in line with the feeding of nodules by closing the by-pass valves as per requirements. When the bed level reaches 40-50 cm from the top of the cone, the discharge rate is increased. Normally it takes 6 to 7 hours to complete these operations.
Stabilized kiln operation as obtained after the initial lighting up may be continued under the following conditions:
* A uniform discharge rate should be maintained as far as possible by adjusting the grate speed whenever necessary.
* The nodule size, feed rate and bed level must be maintained.
* The bed should always be covered with nodules by continuously feeding them in a circle some distance away from the refractory lining, so that the nodules themselves become distributed both over the periphery and towards the centre of bed. Continuous steam emerging from the centre of the bed indicates good fire and good combustion conditions. The temperature of various zones should be periodically recorded.
* It is necessary to ascertain frequently that the entire bed is descending uniformly. This is ensured by cleaning the sides frequently with the help of a rod with a flattened tip. While cleaning the sides, care must be taken not to damage the refractory lining.
* Once the kiln is stabilized, neither the process parameters nor the raw-mix composition should be subject to frequent change.
Strict controls should be observed in the preparation of feed from the raw material stage. The table feeders/weigh feeders should be monitored periodically so that the component ratios conform to the values for the raw-mix design. The blending operation is carried out until a uniform blending with the desired total carbonate (TC) content is obtained. The TC in raw mix should not vary beyond ± 0.2 per cent of the desired value.
The nodules should be checked for appropriate size porosity, moisture and strength on an hourly basis. The feed rate and discharge rate should be synchronized to maintain a constant bed level.
The combustion air can be regulated by adjusting the valve on the by-pass pipe. The measurement is done with the help of two instruments provided on the burners platform; one from the orifice plate to give combustion airflow and the other connected at the bottom for measuring the pressure at the inlet to the kiln which gives an idea of the bed resistance. If during a normal operation both the instrument readings are constant, the burner should understand that the kiln is filled with properly burning clinker. In the case where the inlet pressure varies, the burner receives an indication that the bed resistance is varying and can take necessary corrective action.
8.4.3 Discharge rate
Under stabilized operating conditions, the discharge and feed rates should be synchronized to maintain a constant bed level. If there are lumps that are too big or clinker that is too loose near the grate, the discharge rate will decrease or increase. Depending on the case, the grate speed is to be changed immediately, otherwise the bed level will rise or the burning zone will collapse destroying the entire kiln operation in the next few hours. Intermittent discharge is not desirable. The rotary-grate speed should not be abruptly changed; it should be done gradually step-by-step.
Normal kiln operation is also judged from the temperatures of flue gases, clinker, burning zone and cooling zone and also of the kiln shell from outside. The temperature of the kiln is judged from the thermocouple-based temperature indicators and cross-checked by touching the kiln shell around the periphery at various heights to feel the uniform temperature gradient. The desired temperature profile for a VSK is shown in figure 9.
If the operation of the kiln is not checked frequently or if the raw-meal composition changes, the kiln is liable to experience trouble. Some of the troubles are mentioned below.
Figure 9. Temperature profile for VSK
8.5.1 Lump formation
Sometimes a big lump may be noticed in the burning zone which is very tough and as big as the internal diameter of the kiln. This may not descend. This generally occurs due to melting of clinker on account of a very high temperature there. The raw-mix composition should be checked and corrected. Another reason for this trouble may be fuel with a low ash fusion or a higher quantity of fuel. The lump so formed should be broken so that it may pass through the grate.
When the clinker is so lumpy that it does not contain loose material and the entire kiln is filled with such material, there are chances that these lumps will form a bridge and rest over the sides of the kiln and not on the grate. There would be a gap just above the grate and clinker discharge would be stopped, resulting in a rise in the fire bed. In such cases the feed should be adjusted and air should be reduced. The bed should be poked from the burner platform. If necessary the Roots blower and feed should be stopped and poking through the grate should be continued until the bed starts to descend.
8.5.3 Side discharge
Sometimes, if the clinker produced becomes suddenly too loose, it will pass through the bigger lumps and be discharged quickly. Even red-hot clinker may be discharged indicating that over a certain portion there is no bed to support the burning zone. In such conditions, the grate speed should be reduced or it should be stopped, to reduce the discharge for some time, and to allow the burning zone to be built up slowly. The reduced rate of discharge should be continued until the trouble is overcome.