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Summary This study was carried out to evaluate the physico‐mechanical and combustion indices of briquettes made from blends of cornhusk (CH) and cassava peel (CP). The CH and CP was torrefied at 200°C to 300°C and were mixed in the ratio 90/10 to 10/90 for briquette production using D‐optimal crossed design. The fuel application for co‐firing in coal engines was evaluated with fuel ratio, combustibility index and volatile ignitability. The findings revealed that the density, durability index and compressive strength of briquettes blend at 10/90 (CH/CP) torrefied at 300°C and water preconditioned, were respectively improved by 18.19, 2.98, and 32.25% compared with those for raw briquettes at 10/90 blend ratio. Furthermore, the fixed carbon and calorific value increased with the increase in the torrefaction temperature and time. The produced briquettes at the mixing ratios of 10/90 and 30/70 (CH/CP) torrefied at 300°C for at least 1 hour is sufficient for co‐firing in coal engines. The CH and CP wastes with low calorific value which are hitherto discarded could be used to produce briquettes with appropriate qualities attributes of alternative fuel source for heating applications.

Today, agricultural waste is one of the most common resources in Nigeria that could solve environmental, fuel and energy issues. However, it has some limitations such as low bulk densities, loose and irregular sizes, handling and storage problems, low energy density, reduced fixed carbon, low calorific value, high volatile matter and high moisture content etc. making it difficult to be utilized for fuel. One of the viable and promising technologies to upgrade the properties of raw biomass is through torrefaction technique which is capable of upgrading the combustion and fuel characteristics of biomass, demonstrated from behaviours that are similar to coal during combustion. During this process, about 70% of the initial biomass weight and about 90% of the original biomass energy is obtained as torrefied biomass while the remaining 30% biomass weight and 10% biomass energy is given off. In addition, the presence of moisture content in raw biomass that could aid biological degradation is reduced (< 3% w.b.) while combustion efficiency is being enhanced through upgraded fixed carbon and calorific value (15-25% wt) and reduced volatiles. These upgraded properties makes torrefied Nigerian biomass suitable to be used independently or co-fired in power plants and as an upgraded feedstock for domestic and industrial applications in a developing country like Nigeria. Unfortunately, there is scarce research materials on biomass torrefaction in Nigeria which could be attributed to the cost of acquiring torrefaction plant and other resources for torrefaction characterization which are on the high side. This paper therefore explores and reviews the property upgrades of raw biomass through torrefaction technique. The challenges of biomass energy in Nigeria, torrefaction effects on some Nigerian biomass, equipment used for the analysis of torrefied samples, alongside the torrefaction properties, combustibility indices and their products were examined. The review study concluded that torrefaction technology is a promising technique in Nigeria which is capable of improving and upgrading the quality, energy value and other properties of raw Nigerian biomass and could at the same time serve as an alternative source of energy asides hydropower energy if embraced by the concerned bodies.

Agricultural waste management through energy recovery is one of the critical options that could drive the clean energy production industry and properly replace the use of coal in Nigeria if done sustainably. The objective of this work therefore is to study the synergetic effect of waste feedstock mixture and torrefaction pre-treatment on some physical and performance characteristics of briquettes from cornhusk (CH) and sawdust (SD) wastes. In this work, CH and SD wastes were processed raw and torrefied at 200 to 300 °C and were mixed in the ratios between 90/10 and 10/90 for briquette production using D-optimal crossed design. Cassava starch of 20 g to 100 g of the feedstock (w/w) was used as binder. The performance characteristics of CH/SD briquettes were evaluated using standard procedures while the generated data were processed using ANOVA, regression and pareto analysis. The thermal efficiency of 29.94% and water boiling time of 12 min were obtained for CH/SD briquette blend of ratio 10/90 torrefied at 300 °C. The maximum density and water resistance index of the torrefied briquettes at different blends respectively increased between 5.78–9.77% and 75.70–85.45% over those of the raw briquettes due to torrefaction and water preconditioning. Furthermore, the lowest value of burning rate was obtained for briquettes torrefied at 300 °C at 50/50 (CH/SD) ratio. ANOVA revealed that torrefaction and feedstock blending significantly influenced the characteristics of CH/SD briquette at p<0.05. This study showed the potential use of torrefied briquettes from cornhusk and sawdust wastes as alternative for coal and forest wood and a new source of energy for heating applications.