Charcoal Fines Recovery: Minimum Binder Fractions, Heat per Kilogram, and the Clay Limit for Briquettes

Every kiln produces charcoal fines — fragments too small to sell as lump — taken here as a central fraction of 0.12 of total charcoal solids, with a stated range from 0.08 to 0.18. This study, which is simulated, not measured, asks what can be recovered from those fines by pressing them into briquettes with clay or with starch, under a handling test defined in the paper: three drops from 1 m onto concrete, with at least 0.90 of the mass retained. Under a stated strength model in which an unbound compact retains 0.55 of its mass and binder raises retention linearly, the minimum binder fraction that passes is 0.1591 for clay and 0.0921 for starch. At those minima the heat per kilogram of product is 24.386 MJ/kg for the clay briquette and 27.895 MJ/kg for the starch briquette, against 29.0 MJ/kg for lump charcoal, and the burn time per kilogram at a stated stove power of 2.0 kW is 3.387 hours and 3.874 hours against 4.028 hours for lump. Clay contributes no energy, so its cost is exact: the product heat falls to 20.0 MJ/kg at a clay fraction of 0.3103 and below the 19.0 MJ/kg of dry wood itself at a clay fraction of 0.3448. Past roughly three tenths clay, the briquette is no longer worth its transport weight in place of lump charcoal, and past about 0.3448 it is worse, kilogram for kilogram, than carrying dry wood.