The fuel bound of a small lime kiln, and a weighing test for completeness

Pure calcium carbonate requires 1.78 MJ per kilogram of stone, or 3.178 MJ per kilogram of quicklime. With air-dry wood at 15.0 MJ/kg and stated efficiencies of 0.15 for a clamp or pit kiln and 0.30 for a small shaft kiln, the fuel bound is 1.412 kg of wood per kilogram of quicklime in the clamp and 0.706 kg in the shaft kiln. A 100 kg charge of pure stone yields 56.03 kg of quicklime and needs 79.13 kg of wood in a clamp or 39.56 kg in a shaft kiln. Because pure CaCO3 loses 43.97% of its mass as CO2, weighing stone in and product out bounds the uncalcined fraction through x = (1 − m_out/m_in)/(p × 0.4397), where p is purity — a completeness check needing only a scale. The claim that any pale rock that looks like limestone makes good lime does not survive a numerical test: a stone with 80% CaCO3, 10% silica and 10% inert material gives only 0.355 kg of free CaO per kilogram of stone against 0.560 kg for pure limestone, and pure dolomite gives only 0.304 kg of CaO per kilogram of stone while raising wood per kilogram of CaO by 68.4%. **Design status: SIMULATED, NOT MEASURED.** The kiln sizing and fuel figures are a design study from stated inputs. Nothing here is described as built or as measured in a firing.