Ten rules energy efficient cost effective brick firing

Practical Action

Example

Señora Jara, a Peruvian brickmaker, is using 0.40 tonnes of coal to fire 1,000 bricks,

230 x 110 x 70 mm. She fires 80,000 a month in clamps of 10,000. The coal has a

calorific value of 25,000 Mega Joules per tonne. A tonne costs $400. She estimates her

process is 50% efficient. That is, half the energy actually fires the bricks and the rest is

lost to the atmosphere. Of the 50% of energy lost, she thinks at least one-third is from

the sides of the clamps. How much money could she save by burning a whole month's

production in a single, large clamp?

Burning 0.40 tonnes of coal means 0.40 x 25,000 = 10,000 MJ per 1,000 bricks. So

eight clamps of 10,000 bricks uses 800,000 MJ. Half this, 400,000 MJ, is lost. A

third, 133,333 MJ, is lost from the walls at a cost of 133,333 MJ ÷ 25,000 MJ/tonne x

400 $/tonne = $2,133. Referring to the table, the surface area of a 10,000 brick clamp

is 27.46 m2. Eight clamps means a total cooling area of 219.68m2 - proportional to a

$2,133 loss. The surface area of an 80,000 brick kiln is 109.83m2, around half. So

this would correspond to a loss of $1,067.

What does this mean in practice? Almost 17% of waste energy is lost from clamp walls:

5.44 tonnes of coal per 80,000 bricks. If this loss is halved, the coal used can be

reduced to 366 kilograms per 1,000 bricks, saving $1,088 per month.

2 Square kilns are generally more efficient than rectangular ones

A kiln with equal sides has a smaller cooling area than a rectangular one of the same volume.

So efficiency is better because of reduced heat losses. From the table, a cubic kiln of 20,000

bricks has a surface area of 3.30 x 3.30 x 6 = 65.34 m2. A rectangular kiln with the same

number of bricks - the same volume - could be built by making it four times longer, halving

the height and the width. That is, it would measure 13.20m x 1.65m x 1.65m. However, the

surface area would be (4 x 13.20 x 1.65) + (2 x 1.65 x 1.65) = 92.57m2. Even considering

only the four 'cooling faces', the cubic kiln has a surface area of 3.30 x 3.30 x 4 = 43.56m2,

while the rectangular one is ((13.20 x 1.65 x 2) + (1.65 x 1.65 x 2)) = 48.21m2.

3 Increasing insulation reduces heat losses

Heat is lost through the top, the walls and, to a lesser degree, the bottom of the kiln.

Anything which reduces this heat loss increases efficiency. Thicker scoving, plastering with

mud, on clamps will help reduce losses as will using fired bricks in the outer layers. For kilns,

thicker walls or a wall with an air gap will help.

4 Placing fuel as close to the bricks as possible is most efficient

Obviously, if a brick is a long way from the heat, it will not 'burn'. So having the fuel closer to

the bricks is more efficient. Placing some or all the fuel in the clay mix can be very efficient.

Incorporating, for example, coal dust or saw dust into the body of bricks is an established

technique. In some brickworks all the fuel needed is inside the bricks. The fuel chosen

should be fine so as not to cause large voids in the bricks. Brickmakers should also try

distributing their fuel more evenly throughout the kiln rather than, say, burning it all at the

bottom in tunnels.

Example

How much fuel can be moulded into bricks? This varies with the clay type, the fuel type, and

the burning process. Some experts suggest 5% of fuel by weight as a maximum. Señora

Jara can buy coal dust of the same calorific value as her coal. It's cheaper because it's

regarded as waste, so she saves money. Using bigger clamps, she needs 366 kilograms per

1,000 bricks. Her bricks weigh about 3 kilograms, so this fuel requirement corresponds to

around 12% of the mass. That's probably too much to incorporate all of it into the brick, but

she could try using up to half her fuel as dust in the bricks and distributing the rest of the

coal, through the clamp.

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