Knowledge Brief

How can you improve the performance of your Fixed Chimney Bull’s Trench Kiln (FCBTK)?

The main problems that you face in a Bull’s Trench Kiln (BTK) are:

1. High consumption of fuel
2. Low percentage of good quality bricks
3. High air pollution.

Some of these are inherent problems of a BTK and, to a large extent, they will remain so. The better way would be to shift from BTK to Zigzag Kiln technology or to other efficient kiln technologies. However, there are several ways to improve the performance of your existing BTK.

What are the different ways to improve the performance of your BTK?

Some of the ways to improve the performance of your BTK are listed below.

1. Improve fuel-feeding practices
2. Reduce air leakages into the kiln
3. Reduce heat losses from the kiln
4. Introduce good housekeeping and operational practices.

How do improved fuel-feeding practices enhance the performance of your BTK?

The current practice in a BTK is that the fuel is fed for 5–10 minutes continuously every 30–45 minutes. During this 5–10 minutes session, about 150–300 kg of coal is fed by two firemen using large spoons, with each spoon having a capacity of 1.5–2.0 kg of coal.

Such heavy intermittent feeding of coal results in large quantities of coal getting accumulated at the base of the kiln. This pile of fuel does not get sufficient air for combustion and thus does not burn completely, leading to wastage of coal and high air pollution.

The combustion of coal can be improved by improving fuel-feeding practices as suggested below.

1. Feed coal continuously, in small quantities, without any gap in time.
2. Use smaller spoons to feed coal and use only one fireman for feeding.
3. Ensure that the coal is crushed to a size of less than half inch.
4. Alternatively, a mechanised fuel-feeding system can be used to ensure continuous and uniform feeding of fuel at a desired feeding rate.

Improvement in fuel combustion will result in reduction in fuel consumption and air pollution.

How can reduced air leakages into the kiln improve its performance?

Air is required in the kiln for the combustion of fuel to produce heat and for heat recovery. Heat recovery means the transfer of heat from the brick-cooling zone to the firing zone and from the firing zone to brick preheating zone. Ideally, all the air should enter the kiln from the brick unloading end and should flow through the cooling zone, the firing zone, the preheating zone, and the flue gas ducts before coming out through the chimney.

The inside of the kiln is at a lower pressure compared to the atmospheric pressure. Therefore, wherever there is an opening, air from the surroundings will enter the kiln. Hence, only the brick unloading end is kept open for entry of air. However, there is a possibility of cold air from the surroundings leaking into the kiln through wicket gates, kiln roof, flue gas ducts, kiln walls, and tarpaulin (tirpal).

Any leakage of air into the kiln will reduce the amount of air entering the kiln from the brick unloading end, and hence will reduce the extent of heat recovery.  Also, combustion efficiency is reduced due to lesser amount of air being available for the combustion of fuel.  Leakage of cold air near to the firing zone will also create difficulty in maintaining high temperatures and will eventually increase the fuel intake.

Preventing air leakages will reduce fuel consumption and improve the product quality.

How can reduced heat losses from the kiln improve its performance?

In a BTK, heat losses take place through the kiln roof, side walls, wicket gates, and covers of the fuel feed holes. You can reduce these heat losses by:

1. Closing the wicket gates by double wall with cavity filled with ash
2. Using double-wall insulated feed-hole covers
3. Putting a thicker layer of ash on the kiln top to increase the thermal insulation.

Reducing heat losses in the kiln will result in lesser fuel consumption.

How can good practices of housekeeping and operation improve the performance of your kiln?

Good housekeeping and operational practices can help a long way in improving the performance of your BTK. A few such best practices are listed below.

1. Keep the kiln floor levelled and paved. It will reduce the breakages of bricks.
2. During the setting of bricks, ensure that the brick column is straight and aligned. Also locate the first ledge on which fuel is fed centrally below the feed holes. It will result in proper distribution of fuel in the kiln, better air flow in the kiln, and less breakages of bricks.
3. The draught from the chimney should be maintained by preventing air leakages through the chimney walls and preventing choking of ducts by cleaning the ducts periodically.
4. Accumulation of fuel on ledges or at the bottom of the kiln should be prevented. It will reduce the chances of over-firing of bricks and wastage of fuel.
5. Properly dried green bricks should be loaded in the kiln. It will reduce the breakages of bricks and fuel consumption.
6. Ensure that the brick cooling zone is not too long. Longer cooling zone reduces the amount of air flow through the kiln. If bricks are not getting cooled and the cooling zone is required to be kept long, then check for the possibility of any air leakages or obstruction to air flow in the cooling zone and rectify it.