Fixed Chimney Bull’s Trench Kiln (FCBTK) technology is the most popular technology for the production of bricks in India, Bangladesh, Nepal, and Pakistan.
The main reasons for its popularity are:
In an FCBTK, the bricks are stacked and fired/burnt in the space (called ‘trench’ or ‘dug’) between the central part of the kiln (called miyana) and the outer wall. FCBTK is a continuous moving-fire kiln in which the fire moves in a closed circular or oval circuit through the bricks to fire them.
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 600–750 feet (180–220 m). It usually takes about a month for the fire to complete one round of the kiln.
The bricks are stacked in the kiln in ‘column-blade’ brick setting in which the bricks are stacked in vertical columns in a row across the width of the trench. The rows of brick columns are arranged one ahead of the other in the direction of air flow. The air flows through the passages left in between the brick columns in a straight-line path.
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.
In an operational FCBTK, the bricks can be segregated into three distinct zones.
Usually, solid fuels such as coal, firewood, and agriculture residues are used in an FCBTK. 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 traditional operation of an FCBTK, fuel is fed intermittently. Usually two firemen feed fuel together for 5–10 minutes continuously and there is a time gap of about 30–45 minutes before the next round of fuel feeding takes place. Usually the fuel is fed in two to three rows of the brick setting simultaneously.
In an FCBTK, the fire moves along the direction of air flow. The air flow in the kiln is caused by the natural draught created by the chimney located at the centre of the kiln. The air flows in a straight-line 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. The central duct is connected to the kiln through the side ducts, which have one opening in the central duct and the other opening in the kiln. At a time, only one or two side ducts are in use to connect the kiln to the central duct for passage of flue gases. The openings in the kiln of the side ducts that are not in use are closed with temporary brick walls.
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, fuel feeding is stopped and fuel-feed holes in new rows in front of the firing zone are opened for fuel feeding. Usually, in every 24 hours, one or two new side flue ducts in the direction of fire travel are opened to sustain the fire movement.
The fire travels a distance of 20–30 feet (6–10 m) in 24 hours in the direction of air flow and fires 20,000 to 50,000 bricks daily.
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.