How to measure the energy used to fire clay bricks
Practical Action
Background
The factors which affect energy use in brickmaking are:
• The nature of the clay: refractoriness - resistance to change by heat; the presence of
fluxes - substances which aid vitrification; intrinsic calorific value.
• The moisture content of 'green' bricks going into the kiln.
• The type of fuel used, its calorific value, moisture content, and distribution.
• The design of kiln.
• The skill of the brickmakers controlling the burning process
• The climatic conditions: ambient temperature, wind direction and strength...
Normally, it's the effect of kiln design which is being investigated. Either that or varying the
operating conditions of similar kilns. Here, the way we gather data takes account of the
nature of the clay and its effect on energy consumption. We also consider only the energy
used in firing, ruling out drying energy, so we can compare processes directly. Then we
propose a way of measuring how much heat-work has actually been done - this is so we can
take account of how well bricks are fired. (It would be easy to think a process was efficient
because it used very little energy and then find the bricks were only dried mud - not vitrified
at all!)
Data collection
We categorise clay as high temperature (vitrifying above 1,000°C), medium temperature
(vitrifying between 950 and 1,000°C), or low temperature (vitrifying below 950°C). This
means access to a laboratory kiln which can reach such temperatures. The procedure is to
mould cones approximately 70 mm high with a base diameter around 15 mm. Cones are
placed in the kiln leaning slightly so that when vitrification temperature is reached the top
bends over. This temperature is noted and the clay categorised accordingly.
The exact number of bricks being fired is needed. This is easier to determine if bricks are
stacked in thousands prior to firing. Alternatively, the bricks in each layer can be counted as
the kiln is built. Then we need to know the mass of the bricks. A convenient number of green
bricks (perhaps 1 per 1,000, but not less than 24) should be selected at random and
weighed. Thence an average 'wet mass' can be calculated. Now we need to know the moisture
content of the green bricks to calculate the energy needed simply to dry them. The selected
bricks can be dried in a conventional oven or a simple field oven. The important thing is to
keep bricks between 80 and 100°C and weigh them periodically until no further weight loss is
noted. The average 'dry mass' can then be found. With this data it's possible to calculate the
mass of water and the drying energy:
Moisture content
(wet basis)
= wet mass - dry mass
wet mass
Mass of green bricks =
avg. mass of green brick x no. of bricks
Total moisture content
= moisture content of brick x mass of bricks
Drying energy
= specific drying energy x total moisture content
(Where specific drying energy' = 2,591 kJ/kg of moisture)
All fuel used should be weighed. If a fuel such as sawdust or pulverised fly ash is mixed into
the body of bricks, its mass must also be determined. A sample of each fuel used should be
taken promptly to test for calorific value and moisture content. If the clay has a natural fuel
content, it too should be sent for testing.
If such test facilities are unavailable or unaffordable, then the best that can be done is to use
the most recent local data. If a value is found in this way, it should not be recorded as the
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