Principle of
the VSBK firing

VSBK – how it works

For initial firing, the shaft is loaded from the bottom with green bricks and coal. Initially a higher amount of coal is used at the bottom most batches, which then gradually reduces in subsequent batches to the top.

While ignition, the wood placed is lightened in the firebox with the help of kerosene at the bottom of the brick setting. Firewood is continuously stoked till the fire moves along the stacked bricks and starts to ignite the coal.

At this time the firebox is sealed with bricks and mud mortar plastering from both side to control excess air passage. After around 9hours of firing, the sealing is opened for natural draught to proceed. When the maximum temperature reaches above the central part of the shaft, a first batch is unloaded.

  Standard Operation

During continuous operation, one batch of dried green bricks is loaded in a predetermined pattern at the top at a time and a sized and weighed quantity of coal is spread uniformly. At the bottom of the shaft, batches of fired bricks are continuously removed using an unloading trolley, which moves on rails along the length of the unloading tunnel. Lifting and lowering of the trolley is done using a single screw unloading mechanism. For unloading, the trolley is lifted so that the whole stack of bricks in the shaft rests on it.

The I-bars are taken out, when released. The whole stack isthen lowered till the gap in the layer appears through which the I-bars are then reinserted. On further lowering the trolley, the load of the stack is taken by the I-bars except for the batch being unloaded, which comes down along with the trolley, which is later pulled out along the rails. Bricks are subsequently unloaded and sorted for stacking. The next batch is loaded at the top with the green bricks lifted up to the loading platform. The frequency of unloading and loading depends upon the productivity required, hence the quality of fired bricks. Ideally, the unloading time interval is every 1.5 to 2 hours. Hence, the firing interval for a batch is about 18 to 24 hours.

Process occurring inside VSBK

As the procession of batches gradually passes through the shaft, green bricks encounter the pre-heating, firing, and cooling zone before they reach the shaft exit. The bricks in the preheating zone absorb heat from the hot flue gases coming through the firing zone, which helps in evaporation of moisture in the green bricks. The temperature in the preheating zone is about 100?c-500?c. After passing through the pre-heating zone, the bricks enter the firing zone with temperatures increasing up to 950?c and then move down to the cooling zone. The unloaded bricks come out of the shaft in a temperature range of 100?c-150?c.

 

Kiln efficiency

The energy efficiency of a Vertical Shaft Brick Kiln is based on the principle of convection current of the air, hot flue gases go up and pass through the fresh green bricks and utilize the heat in brick drying. This recovery of sensible heat accounts for the high energy–efficiency of the kiln.

The shaft design and firing technique generates just enough draught to ensure complete and efficient combustion external fuel added. Firing temperature, soaking time at the peak temperature and movement of firing zone can easily be controlled through upper and lower damper operation provided in the two chimney systems, and efficient use of the lid cover arrangements. The draught in the shaft is controlled by variation in the spacing density of the bricks during loading. The cooling of bricks below the firing zone in turn, heats up the air in the combustion process. Minimizing heat losses increases energy efficiency of the kiln. A filling of broken brickbats, rubbish and burnt coal ash insulates the whole kiln. Once the kiln is stable at operating efficiency, most of the heat released from the fuel goes into firing of the bricks. No appreciable heat is lost through the exhaust gases nor in steady state though the kiln body.

Brick quality

The quality of the fired product mostly depends on the quality of green bricks being loaded. Compared to Hoffman, Fixed Chimney Bull’s trench kiln and clamps, VSBK has a typically narrow range of product quality. Except breakage, all the fired products can be classified into a single saleable class. Breakage of the fired bricks mostly depends upon the quality of the loaded green bricks and the unloading mechanism. Also, the drawbacks of the firemaster heavily accounts on handling breakage. In areas with plastic clays, there are less than 1% breakages, while in the case of sandy soils with low plasticity, the breakage rate can be around 5–6%.

Maintenance

Kiln maintenance is appreciably low, with the only expenditure being on seasonal minor maintenance of the unloading trolleys and the screw jack system. The only other expenditure is on painting roof truss and minor patching up of the chimney and inner refractory lining respectively.

Advantages of the vertical shaft brick kiln

  • It is a highly energy efficient means of brick firing, consuming around 40-50% less amount of coal than traditional BTKs.
  • Pollution is reduced by 90% compared with BTKs, making it an environmentally safe, sound and sustainable means of brick firing.
  • The bricks are better and of consistently uniform quality, under stable operating conditions.
  • The kiln is extremely compact occupying very little space.
  • It has a high flexibility in operation. The production rate can be adjusted based on the market demand.
  • Kiln roofing arrangements makes it suitable for operation throughout the year, even in monsoon season (subject to availability of green bricks).
  • The kiln is simple and easy to construct. Maintenance costs are nominal. It doesn't require an external power source, except for electric lighting during night operation.
  • Requires less working capital and has a quick return on investment. A stock of 50,000 green bricks is sufficient for starting a 2-shaft kiln with a daily requirement of 10-12 thousands of green bricks. Saleable bricks are produced within 72 hours of firing.

Disadvantages of the vertical shaft brick kiln

  • VSBK technology is ideally suited in areas with good soil quality. Being an up draught kiln, every brick in the pattern has to with stand upcoming load from the top. Also, the bottom most layers have to withstand the total load of the stack. So, particular attention has to be paid to dimensional tolerances, strength and moisture content of green bricks.
  • VSBK is a continuous firing kiln with every loading and unloading process occurring within 1.5 - 2 hrs interval. Thus, rigorous consistency in night operation of loading and unloading is considered to be an added disadvantage.
  • VSBK firing is extremely sensitive to minor changes in coal amount, setting density and unloading frequency. Unintentional changes in the above parameters will result in deterioration of quality and will affect productivity.