Optimal compaction pressure, particle size and binder ratio for quality briquettes made from maize cobs

The quality of briquettes varies greatly among small scale producers in Uganda due to the different methods of production, absence of standardization, lack of technical knowledge and quality control procedures. The study aimed at investigating the effect of cassava binder ratio, compaction pressure, and particle size on thermo-physical properties of briquettes made from maize cobs and cost benefit analysis of briquette production at the optimal condition. A 3×4×5 factorial experiment with four replicates was used. Three particle size levels (small, medium and large), four compaction pressures (P1=2MPa, P2=4MPa, P3=6MPa and P4=8MPa), and five cassava binder ratios (B1=5%, B2=7.5%, B3=10%, B4=12.5%, B5=15%) were used. Briquettes were made from carbonized maize cobs using a manually operated hydraulic briquette press. Proximate analysis (moisture content, volatile matter content, fixed carbon content, ash content), calorific value and density of the briquettes were determined. Linear regression models were used to investigate the effect of compaction pressure, binder ratio and particle size on each of the dependent variables. All the independent variables had significant effect on briquette quality. Particle size and binder ratio had significant effect on heating value (p-value: 1.421×10-08) at 5% level of significance. There was no significant effect on moisture content at 5% level of significance (p-value: 0.1248). Unlike compaction pressure and particle size, binder ratio had significant effect on volatile matter content ( p-value: 3.54×10-11) and fixed carbon content (p-value: 3.37×10-08) at 5% level of significance. Much as binder ratio and compaction pressure did not have significant effect on ash content at 5% level of significance (p-values > 0.05), particle size had significant effect (p-value: 0.01107) . Compaction pressure, particle size and binder ratio had significant effect on density (p-value: < 2.2×10-16) at a significance level of 5%. Despite all briquettes meeting the minimum quality requirements, medium sized particles (4mm to < 6mm), 5% binder ratio and 8MPa compaction pressure produced superior quality briquettes (8.421% ash content, 12.923% volatile matter content, 65.38% fixed carbon content, 13.358% moisture content, 25247.5 J/g heating value, and 409.8824 Kg/m3 relaxed density). In addition to the high quality briquettes, 5% binder ratio is low enough to minimize amount of cassava used for briquetting, hence helping in the fight against food insecurity. The volatile matter content of the briquettes was low (12.923%) which implies that they burn without smoke and therefore shall greatly reduce the high death toll caused by indoor air pollution. Cost benefit analysis of briquette production at optimal condition (medium sized particles, 5% binder ratio and 8MPa hydraulic cylinder pressure) had 69,809,400 Ugs ($20,532.2) net present worth, 1.15471 benefit cost ratio, 36% internal rate of return, and a discounted payback period of 3.603 years which shows economic viability. It is recommended that studies be conducted to enhance the pasting properties of cassava binder for briquette production, assess the strength of briquettes at optimal condition to ascertain whether they can withstand crumbling during handling and transportation, and investigate other raw materials since briquette quality is greatly influenced by its composition.