How to measure the energy used to fire clay bricks
Practical Action
MONITORING FORM: Energy consumption of brick firing processes
Name of producer
Fambidzanai Training Centre
(FTC)
Location/address
Dates and times of firing
4 Dovedale Rd, Stapleford, Start 1 Jan. 1998
Mount Hampden, Harare.
Finish 15 Jan. 1998
Type of clamp/kiln
Layered, scoved clamp
Type of fuel
Boiler waste
Mass of fuel used (kg)
4,680
Calorific value (kJ/kg)
Gross 20,170
Net 19,180
Moisture content (%) ?
No. of green bricks in kiln
14,817
Mass per brick (kg)
green 3.65
dry 3.25
fired 3.10
Brick moisture content (%)
= (3.65 - 3.25) x 100/3.65
= 10.96 (wet basis)
Method of forming
Weather conditions
Semi-dry pressing (Ceratec Intermittent heavy rain, but
press machine)
clamp sheltered by roof
Calculation of kiln efficiency
Mass of green brick (kg) 14,817 x 3.65 = 54,082
Mass of dry brick (kg) 14,817 x 3.25 = 48,155
Total moisture content (kg) 14,817 x (3.65 - 3.25) = 5,927
Drying energy (kJ) 2,591 x 5,987 = 15,356,857
Total energy (kJ) 4,680 x 19,180 = 89,762,400
Firing energy (kJ) 89,762,400 - 15,356,857 = 74,405,543
Mass of fired brick (kg) 3.10 x 14,817 = 45,933
Specific firing energy (kJ/kg) 74,405,543/45,933 =1,620
Qualifying information:
(i) Soil vitrification category
High temperature (>1000°C)
(ii) Bullers Bar no.
= 13 (≈ 890°C)‡
And/or
(iii) Avg. firing temp. (°C) ?
(iv) Firing time (hrs) ?
Notes: ‡Thermoscopes contained bars 11, 13, 15 & 17. Only six of eighteen were recovered.
Three from the sides of layers 4 and 15 and the centre of layer 26 were virtually unaffected.
One from the side of layer 8 showed Bar no. 13 just sagging. Two, from layer 14 (centre) and
layer 18 (side) had all the bars melted - turned totally to liquid in layer 14. With only six
samples it's not really valid to take an average.
Comments: Bricks are mostly underfired. Specific firing energy is thus quite low. The long firing
time indicates not enough air gaps - boiler waste packs 'tighter' than coal. Brick quality and
appearance is not good due to poor soil preparation, a worn press machine, and contamination
from to drying on dirty ground. Bricks are easily broken, revealing no unfired core, but a matrix
containing lumps of dry clay and stones - stones too big to make a strong brick. Bricks aren't
vitrified but merely quite well 'baked'. Some were 'hard-burned' - turning blue. These
corresponded to 'hot-spots' indicated by Thermoscopes. Clamp volume is 39 cubic metres while
the volume of bricks is around 27 cubic metres. Void volume is thus 12 cubic metres (about
30%) which includes the volume of coal initially in the clamp.
Recommendations: Green bricks should be thoroughly dry to save fuel. The firing process needs
modifying to produce well-burned bricks. The 'high temperature' soil is difficult to vitrify,
needing a temperature of more than 1000°C. Experience suggests fuel should be increased by
around 20%. More sensibly, this increase could be achieved by ensuring green bricks are dry.
The same fuel to brick mass ratio could then be retried. The other problem is air flow. The
clamp burned too slowly. Boiler waste could be distributed more evenly throughout, and air gaps
between bricks could be increased. Firing time could be reduced to around 7 days.
Signature, date, organisation & contact address
Kelvin Mason, IT-Zimbabwe, PO Box 1744, Harare. Tel: 490270, 496653, 496746/7 Fax:
496041
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