Bull's Trench Brick Kiln
|Introduction|| Advantages of the Bull´s trench kiln
The use of fired ceramic bricks goes back more than three thousand years, and bricks are still the preferred house construction material in most countries around the world. In many countries, the brick industry is the largest single user of energy and also employs a large number of workers, due to the labour intensive manual brickmaking process. Suitable clays for manufacturing bricks exist almost everywhere, and the brick-making process can be done with simple manual methods, or in highly automated plants, depending on the prevailing labour cost and the demand of the market.
Firing the bricks, also termed baking, gives them strength and turns the plastic clay irreversibly into a permanent hard material that can no longer be slaked in water. Originally, bricks were fired in clamps or scove kilns. These are not permanent structures, but simply a pile of green bricks covered with a sealing layer of mud, with the fuel placed under the bricks. Later, permanent kilns were used for firing bricks. Both types of kiln are loaded with green bricks, which are heated up to the desired temperature and then cooled again before the bricks can be drawn from the kiln. All the heat energy used for firing is lost during cooling, and such so-called periodic kilns waste energy.
In 1857, in Germany a continuous brick kiln was invented by F. E. Hoffmann. The first kiln had a circular, arched tunnel surrounding the chimney. This reduced the fuel consumption by more than 50% compared to the periodic kilns. Thirty years later, a British engineer, W. Bull, designed an archless version of the Hoffmann kiln, which is now called a Bull' s trench kiln. It is widely used in Pakistan, India, Bangladesh and Myanmar, but is little known elsewhere. Its greatest advantage is its low cost of construction and comparatively low energy consumption.
Bull´s trench kiln seen from the loading end (Bangladesh)
The kiln can be made circular or elliptical in shape. It is constructed on dry land, by digging a trench, 6 - 9 m wide, 2 - 2.5 m deep, and 100 - 150 m long. An alternative method is to build up the sides of the kiln with bricks, especially where drainage is a problem. Gaps are left in the outer wall for easy assess to the trench during setting and drawing of bricks.
The green bricks to be fired are set in rows, two to three bricks wide, with holes in between that allow feeding of coal and a sufficient flow of air through the setting (Figure 2). A linking layer of bricks is made across the width of the kiln and half way up, to stabilize the setting. On top of the bricks, two layers of bricks, covered with ash or brick dust, seal the setting. A large piece of canvas, paper or metal sheet is placed vertically across the brick setting to block air from entering from the wrong side of the chimneys (Figure 3). The trench contains 200 - 300,000 bricks at a time.
Chimneys, 6 - 10 m high, made of sheet metal, are placed on top of the brick setting. They are moved around as the firing progresses and they have to be light, so that the firing crew can carry them. Wires attached to the top of the steel chimneys support them. The need for lightness and the cost of replacing the chimneys often have the effect that the height of the chimneys becomes too low. That means the exhaust temperature has to be higher in order to maintain sufficient draught and the chimneys are placed closer to the firing zone. Thereby, less heat of the exhaust gases can be reutilized. Small circular Bull's trench kilns use only one chimney, whereas the larger elliptical kilns need two chimneys.
The firing in a Bull's trench kiln is continuous, day and night. Green bricks are loaded and finished bricks are drawn all the time. The fuel saving is achieved by reusing part of the energy that is otherwise lost in periodic kilns. As shown in Figure 4, the air for combustion is drawn through the already fired but still hot bricks. The cooling bricks transfer their heat to the combustion air, pre-heating it before it enters the firing zone. After combustion, the hot exhaust gases pass through the yet unfired bricks on their way to the chimneys. This pre-heats the bricks, so less fuel is needed to bring the bricks to the maximum temperature. Once every 24 hours the chimneys are moved forward 5 to 7 m. Daily output is 15 - 25,000 bricks.
Design of a Bull´s trench kiln
Normally, the firing crew consists of six men organized in two teams, who take turns stoking the kiln. When the chimneys are moved, all six men are needed. The whole operation of shifting the chimneys forward takes about one hour.
The firemen stoke the fire through removable cast iron holes at the top of the brick setting. Ideally, stoking should be done 3 - 4 times per hour, but especially at night, the workers tend to stoke large amounts of fuel at long intervals, causing an increase in fuel consumption. The firing of the kiln demands great skill, which may take years to master well. The fuel can be any combustible material or a combination of them; coal, lignite, peat, firewood, saw dust, agricultural waste, such as rice husk, brand or coffee shells. Natural gas or oil can also be used, but such fuel is normally too expensive. Old tires cut into pieces are more commonly used, but the combustion gases are very toxic, and in many countries the use of tires has been banned.
Fired bricks being drawn from the kiln (Pakistan)
A canvas prevents air from entering the kiln from the wrong end (Pakistan)
There are two main drawbacks with moveable chimneys;
This loss of temperature increases, fuel consumption by about 5 - 10%. In the dryer areas of the Indian subcontinent, a Bull's trench kiln with a fixed central chimney is now widely useed (Figure 5). Compared to the moveable chimneys the initial cost is higher, but the saving the cost of replacing the metal chimneys and the slightly better fuel economy soon pays back the initial investment. A large central flue channel is constructed in the centre of the elliptical kiln, and through this, the exhaust gases flow to a brickwork chimney. Where electricity is, available and in stable supply, an exhaust fan can be used instead of a chimney.
A Bull´s trench kiln with a fixed chimney (Pakistan)
The cost of establishing and operating a Bull's trench kiln depends on the local cost of labour and fuel. The figures given here are average of data collected on the Indian subcontinent, and therefore the cost figures may not be correct in other regions, with other costs of fuel and labour, and with a different culture and infrastructure.
|Text, drawing and photographs
German Appropriate Technology