The Binder Bound for Residue Briquettes: Minimum Binder Fractions for Sawdust and Charcoal Fines and Their Cost in Heat

Hand-formed briquettes made from sawdust or charcoal fines need a binder, and every kilogram of binder displaces fuel or adds inert mass. This paper computes, from stated assumptions, the smallest binder mass fraction at which a briquette meets an explicit handling criterion, and what that fraction costs in heat per kilogram. The criterion is a compressive strength of 0.30 MPa, chosen to represent surviving a 1 m drop onto earth and 24 h of stacked storage. Under a linear strength law calibrated so that its predictions fall in the ranges reported in Food and Agriculture Organization briquetting guides, the minimum fractions are: sawdust with starch 5.0%, with molasses 7.5%, with clay 15.0%; charcoal fines with starch 6.0%, with molasses 10.0%, with clay 20.0%. The corresponding heat penalties are 0.075, 0.45 and 2.85 MJ per kg for sawdust, and 0.75, 1.70 and 6.00 MJ per kg for charcoal fines. Starch is therefore the cheapest binder in heat on both feedstocks, and clay on charcoal fines destroys one fifth of the fuel value to buy handling strength. The study is simulated, not measured; all numbers come from the stated model and must be checked against pressed samples before production use.