Energy efficiency of lime brining
Practical Action
Table 3: Molecular weights of components
calcite, CaCO3
dolomite, CaCO3. MgCO3
calcium oxide, CaO
100.09
184.42
56.08
magnesium oxide, MgO
40.32
A more precise calculation of the fraction of calcite limestone CaCO3, and any dolomite
CaCO3.MgCO3, converted to CaO and CaO + MgO, is complex and involves using these
molecular weights.
Table 4: Indicative calorific values of some common fuels
Fuel Calorific value (MJ/kg)
Commercial Butane
58
Diesel fuel
44
Heavy fuel oil
42
Charcoal (2% moisture)
29
Anthracite coal
33
General purpose coal (non-coking)
23
Wood (15% moisture)
15
Table 5: Approximate conversion of units for calorific value
MJ/kg
cals/gram
Btu/lb
1 239 430
kWh/tonne
278
Example of efficiency calculation
Having obtained values for all the terms in the equation we can now calculate efficiency:
E = Hc x Ls
Cf x Mf
E = 3078 x 0.60
23 x 200
E = 0.4015
E = 0.40 or 40%
It is obvious from the formula, and indeed from the application of common sense, that for
maximum efficiency lime-burners should aim for maximum conversion of calcium carbonate
to calcium oxide while using the minimum quantity of fuel. However, minimising fuel use
without evaluating the quality of the lime produced can give a very false impression of
efficiency.
An indication of a target efficiency using, for example, a forced draught vertical shaft kiln, is
50%. In terms of monitoring the performance of any lime burning process, it is evident from
the above example that the essential information to collect and calculate is: a chemical
analysis of the limestone, a value for theoretical heat of calcination (Hc) based on molecular
weights, a value for the available lime content (Ls) obtained from the Rapid Sugar Test, the
calorific value of fuel (Cf) and the mass of fuel per tonne of quicklime produced (Mf).
Conclusion
In developing this methodology Practical Action seeks to ensure that various kiln designs and
operational practices can be optimised in terms of energy usage and ultimately production
costs. This approach is by no means offered as definitive or untouchable; indeed it is
designed to encourage debate.
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