Knowledge Brief

How can you save coal in your Fixed Chimney Bull’s Trench Kiln (FCBTK)?

About 25% of the coal fed in a BTK can be saved. If you are a BTK owner in North India, you can save about 100–150 tonnes of coal or about Rs 5 lakh to Rs 15 lakh in a year.

What are the methods to save coal in a BTK?

The methods to save coal are:

1. Feed coal continuously, in smaller quantities
2. Increase the coal feeding zone to 3–4 rows
3. Reduce/avoid leakage of air into the kiln
4. Reduce heat loss from the kiln roof and wicket gates
5. Ensure that the brick cooling zone is not too long
6. Avoid long kiln circuits.

Feed coal continuously, in smaller quantities

In traditional operation of a BTK, coal is fed intermittently in one or two rows of fuel feeding holes. Usually two firemen feed coal together for 5–10 minutes and there is a time gap of about 30–45 minutes before the next round of feeding takes place. The coal is fed using large spoons of 1.5–2.0 kg capacity, and typically 150–300 kg of coal is fed per feeding session of 5–10 minutes.

Due to feeding of such large quantities of coal in such short duration, a considerable amount of coal gets accumulated on the floor of the kiln. This stack of coal at the kiln floor does not get sufficient air for combustion and remains partially unburned. The energy of the unburned coal therefore remains unutilised.

You can save coal by adopting continuous feeding of coal in smaller quantities (using small spoons) manually or using a mechanised coal feeding system. If due to any reason you are not able to practise continuous feeding of coal, at least ensure that the time gap between two consecutive coal feedings does not exceed 10–15 minutes.

Increase the coal feeding zone to 3–4 rows

In typical operation of a BTK, coal is fed in a short fuel feeding zone, consisting of one or two rows of fuel feeding holes. The volatiles released from the fuel fed in this short fuel feeding zone do not get sufficient time for combustion and are carried along with the flue gases to the brick preheating zone. These unburned volatiles come out of the kiln in the form of black smoke and part of the energy of the coal remains unutilised.

You can save fuel by increasing the length of the fuel feeding zone to cover 3–4 rows. This allows the volatiles to get more time for combustion and burn completely, which results in fuel savings. Keep in mind that in the freshly opened row, where the temperature is lower, fuel having high volatile matter (e.g. saw dust) should be fed.

Reduce/avoid leakage of air into the kiln

The inside of a kiln is at a lower pressure compared to the atmospheric pressure, hence any crack or hole in the envelope of the kiln will result in air leaking in. Leakage of cool air takes place from the tarpaulin (tirpal), through walls (particularly walls to close wicket gates), through holes in the ash layer on the roof of the kiln, through cracks in the chimney, etc. Take steps to plug all leakages in the kiln.

Reduce heat loss from the kiln roof and wicket gates

Heat is lost to the ambient from the hot kiln surfaces. The temperature of uninsulated feed hole covers could range from 400 ^oC to 600 ^oC. The temperature of the walls to close the wicket gates could range from 100 ^oC to 200 ^oC. The temperature of the ash layer on the top of the kiln could range from 70 ^oC to 100 ^oC. You can reduce heat losses by using insulated feed holes covers, employing a double wall system to close the wicket gates, and using thicker ash layer of around 9 inches or using insulation material mats on the top of the kiln.

Ensure that the brick cooling zone is not too long

In a continuous kiln like BTK, the stored heat in the fired bricks should be transferred quickly and efficiently to the combustion zone. This transfer can happen only if there is sufficient air flow across the entire cross-section of the kiln.

It is often noticed that the brick cooling zone is kept too long, about 90–100 m (about 300 feet). This is done in the hope that the fired bricks at the unloading end will become cooler. The longer cooling zone does not help as (1) it reduces the amount of air flow through the kiln due to higher friction losses, and (2) the cold air entering the kiln, being heavy, flows mostly along the bottom of the setting. Therefore, while the heat from the bottom of the setting is transferred forward, the heat contained in the top part of the fired brick setting is not transferred. Usually, a brick cooling zone of 60 m or less should be employed.

Avoid long kiln circuits

In a BTK, the fire travels around the kiln circuit. Typically, the fire completes one kiln circuit in 30–40 days. When the fire is at one particular location , it uses part of the fuel to heat up the surrounding ground and kiln walls. When the fire returns to the same spot, after 30–40 days, the ground and kiln walls have got cooled and need to be heated again. If instead of 30–40 days, the fire returns in a shorter time period (say 20 days), the ground and the walls would still be warm and would require less fuel for reheating.