Knowledge Brief

How does a Natural Draught Zigzag Kiln (NDZK) operate?

Natural Draught Zigzag Kiln (NDZK) technology evolved in India during the 1990s and 2000s. The technology was developed by modifying the brick setting and operating practices of Induced Draught Zigzag Kilns (IDZKs). These modifications enabled the kiln to operate with air flow (in a zigzag path) caused by the natural draught created by the chimney, without the use of a fan.

How does an NDZK operate?

In an NDZK, the bricks are stacked and fired/burnt in the space (called ‘trench’ or ‘dug’) between the rectangular central part of the kiln (called miyana) and the rectangular outer wall of the kiln. NDZK is a continuous moving-fire kiln in which the fire burns continuously and moves in a closed rectangular circuit through the bricks stacked in the trench.

The flow of air into the kiln, which is required for combustion of the fuel and for the movement of fire in the forward direction, is caused by the draught created by the chimney. As the fire moves forward, the fired bricks behind the fire are taken out of the kiln after they cool down, while fresh green bricks are stacked ahead of the fire.

For a kiln of production capacity 20,000–50,000 bricks per day, the central perimeter of the kiln circuit is about 400–500 feet (125–150 m). It usually takes about 25 days for the fire to complete one round of the kiln.

The bricks are stacked in such a manner that distinct chambers of brick setting are formed in the kiln. Just like in the case of FCBTK, in Zigzag Kilns also, the bricks are stacked in vertical columns in a row across the width of the trench. However, unlike in FCBTK, all the brick columns are not of the same width. The rows of brick columns are arranged one ahead of the other in the forward direction of fire travel. In an NDZK, one chamber of brick setting consists of five such rows.

The air flow through the brick setting takes place through the gaps provided in between the brick rows. The openings for air flow in every fifth row (i.e., at the end of each chamber) are provided in such a way that they cause zigzag flow of air in the kiln. Usually double or triple zigzag brick settings are being practised in NDZKs.

The bricks stacked in the kiln are covered with a layer of ash and brick dust. This layer acts as a temporary roof of the kiln, which helps in preventing heat loss as well as air leakages into the kiln.

What are the three distinct zones in an operational NDZK?

In an operational NDZK, the bricks can be segregated into three distinct zones.

1. Brick firing zone where the fuel is being fed and combustion is taking place.
2. Brick preheating zone (ahead of the firing zone in the direction of air flow) where green bricks are stacked and are preheated by the hot flue gases coming from the firing zone.
3. Brick cooling zone (behind the firing zone) where the burnt bricks are cooled by the cold air flowing into the kiln.

What fuels are used in an NDZK and how is the fuel fed?

Usually, solid fuels such as coal, firewood, and agriculture residue are used in an NDZK. The fuel is fed in the kiln through the fuel-feed holes provided at the top of the kiln by the firemen standing on the top of the kiln. In each chamber, there are two rows of fuel-feed holes.

In an NDZK, the fuel is fed continuously by a single fireman. Usually two firemen are deployed in a shift who feed fuel alternately. The fuel is fed in six chambers of the brick setting simultaneously.

How does the air flow and fire movement take place in an NDZK?

In an NDZK, the fire moves along the forward direction of air flow. The air flow in the kiln is caused by the draught created by the chimney. The air flows in a zigzag path through the brick setting inside the kiln.

The back end of the brick cooling zone, where unloading of fired bricks from the kiln happens, is kept open to allow the entry of air from the surroundings into the kiln. The front end of the brick preheating zone is sealed with the help of polythene sheets or tarpaulin to guide the flue gases to the chimney through the flue gas duct system.

The flue gas duct system consists of a central duct and several side ducts. The central duct originates from the bottom of the chimney and extends along the length of the miyana in both directions till the end. The side ducts are L-shaped ducts, which are provided at regular spacings along the perimeter of the miyana. One end of each side duct opens in the kiln while the other end opens at the top surface of the miyana. Adjacent to the top openings of the side ducts, the central duct also has openings at the top surface of the miyana.

The side ducts are connected to the central duct with the help of a shunt. The shunt is an inverted U-shaped metallic duct that is used to connect the top opening of a side duct to the adjacent opening of the central duct located at the top surface of the miyana. At a time, only one or two side ducts are in use to connect the kiln to the central duct for the passage of flue gases. The top openings of the side ducts and the central duct that are not in use are closed with the help of concrete slabs.

Air from the surroundings enters the kiln at the brick unloading end and flows through the brick cooling zone into the brick firing zone where it is used for burning the fuel. The hot flue gases flow into the brick preheating zone, and then through the open side ducts and central duct before coming out through the chimney. In the process, the cold air gets heated by the burnt bricks and transfers heat from the cooling zone to the firing zone, and the hot flue gases get cooled by the green bricks and transfer heat from the firing zone to the preheating zone.

As burning of bricks gets completed in a chamber, fuel feeding is stopped and fuel-feed holes in a new chamber in front of the firing zone are opened for fuel feeding. Usually in every 24 hours, a new side flue duct in the direction of fire travel is opened to sustain the fire movement.

The fire travels a distance of 15–20 feet (5–6 m, 3 chambers) in 24 hours in the direction of air flow and fires 20,000 to 50,000 bricks daily.

How does loading and unloading of bricks take place in the kiln?

Fired bricks, after having cooled down, are taken out of the kiln daily from the back end of the brick cooling zone. An equivalent batch of green bricks is loaded ahead of the brick preheating zone.

Wicket gates are provided at regular spacings in the outer wall of the kiln to allow the movement of bricks and workers in and out of the kiln.