Tunnel Kiln

Tunnel kiln is a continuous moving ware kiln in which the clay products to be fired are passed on cars through a long horizontal tunnel. The firing of products occurs at the central part of the tunnel. The tunnel kiln is considered to be the most advanced brick making technology. The main advantages of tunnel kiln technology lie its ability to fire a wide variety of clay products, better control over the firing process and high quality of the the products.

1).In a tunnel kiln, a continuous moving ware kiln,the clay products/bricks to be fired are passed on cars

   (1.1) through a long horizontal tunnel

   (1.2) The firing of bricks occurs at the central part of the tunnel.The length of tunnel can vary from 60 m to 150 m;
2).Generally green bricks are produced by mixing powdered fuel with clay. Green bricks are then moved in the tunnel or chamber dryers on cars for drying. Heat from the hot flue gases coming out of the kiln is utilized for the drying of bricks;
3).The cars loaded with dried green bricks are pushed in the kiln.  The cars are moved inside the kiln intermittently at fixed time intervals. The duration of the firing cycle can range from 30 to 72 hours;
4).Three distinct zones appear in an operating tunnel kiln:
4.1 Brick firing zone where the fuel is fed and combustion is happening;
4.2 Brick preheating zone (before the firing zone) where the green bricks are being pre-heated by the hot flue gases coming from the firing zone;
4.3 Brick cooling zone (ahead of the firing zone) where fired bricks are cooled by the cold air flowing into the kiln.
5).Fuel(granulated/pulverised coal) is fed into the firing zone of the kiln through feed holes provided in the kiln roof.The firing zone usually extends up to 8 cars.The temperature in the firing zone is maintained at 900–10500C.
6).There is counter current heat transfer between the bricks and the air. Cold air enters the kiln from the car exit end and gets heated while cooling the fired bricks. After combustion, the hot flue gases travel towards the car entrance end losing a part of the heat to the green bricks entering the kiln.
7).Hot air/gases are extracted from the tunnel kiln at several points along the length of the kiln and are supplied to the drying tunnel/chamber. In some of the kilns, there is also provision of a hot air generator to supplement the requirement of hot air for drying.
8).The flue gases from the drying tunnel are released in the atmosphere through a Chimney or air blower.