Residue Briquettes Priced in Delivered Heat: Rice Husk, Coffee Husk and Groundnut Shell

A fuel should be priced by the heat it delivers to the pot, not by the heat it contains in a laboratory. This paper computes delivered heat for briquettes made from three agricultural residues, using stated energy contents, ash fractions and stove efficiencies that penalise high-ash fuels. Rice husk, with a higher heating value of 13.5 MJ per kg, an ash fraction of 0.22 and a stove efficiency of 0.16, delivers 2.16 MJ per kg and 1.512 MJ per hour (0.42 kW), needs 2.315 kg per family meal requiring 5.0 MJ delivered, and leaves 0.509 kg of ash per meal. Coffee husk delivers 3.696 MJ per kg and 2.218 MJ per hour, needs 1.353 kg per meal and leaves 0.095 kg of ash. Groundnut shell delivers 4.392 MJ per kg and 2.416 MJ per hour, needs 1.138 kg per meal and leaves 0.046 kg of ash. Against stated workability criteria of at least 3.5 MJ delivered per kg and at most 0.15 kg of ash per meal, groundnut shell and coffee husk are workable cooking fuels in briquetted form and rice husk is not: it delivers less than half the heat per kilogram of groundnut shell (ratio 0.492) while producing more than eleven times the ash per meal (ratio 11.07). The study is simulated, not measured.