Evaluating thermochemical properties and optimising densification conditions of coffee husks under low pressure
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Over 90% of Uganda’s energy demand is supplied by biomass material mainly in the form of wood and charcoal. Currently, the country has surplus wood fuel reserves but it is projected that it will be in a deficit by the year 2020. Agricultural wastes could be used to substitute wood fuel and charcoal. However, crop residues such as coffee or rice husks have low density resulting in high cost for handling, storage and transportation, and poor combustion behaviour. Biomass densification technology can be used to overcome these problems. However, biomass densification is an intricate process because each biomass material has a unique set of physical conditions for producing a desired quality of briquettes. Furthermore, the combustion behaviour of biomass material is affected by their chemical compositions which also vary with the source of the material. Knowledge of densification conditions and thermochemical properties of coffee husks is currently lacking, therefore limiting its use as a densified fuel. This study was therefore carried out with the aim of assessing the feasibility of producing densified fuel from loose coffee husks. In the study, composite samples of loose coffee husks were taken and analysed through proximate and ultimate analyses. Results of proximate and ultimate analyses were used for evaluating the thermochemical properties of coffee husks. Determination of the optimum condition for densification of coffee husks was carried out following the response surface methodology (RSM). First, a 2k-1 fractional factorial experiment was executed as a screening tool to establish the statistically significant (p<0.05) factors that affect the quality of the briquettes. It was then followed by determination of the region of optimum response following the method of steepest-ascent. Finally, the central composite design (CCD) experiment combined with the desirability function technique was used to establish the optimum densification conditions. The validity of the results was checked through confirmation experiments by comparing values of responses predicted by the developed densification models with the experimental values. Results of proximate and ultimate analyses of coffee husks samples gave a volatile matter content of 89.57% db, Sulphur content of 0.18% db and Nitrogen content of 1.48% db. The higher heating value (HHV) of coffee husks was determined at 18.42 MJ/kg. Furthermore, the study showed that a die pressure of 14.91 MPa, moisture content of 8.00% wb, and binder content of 45.0% db were optimum for densification of coffee husks. With these settings, the briquettes produced had a particle density of 718.09 kg/m3, durability of 80.77% and stability of 14.98%. Based on the results of proximate and ultimate analyses, the Sulphur content of 0.18% suggests that combustion of coffee husks might pose corrosion problems due to SOx emissions. There is also a likely NOx pollution problem since the Nitrogen content of 1.48% in coffee husks exceeds levels for biomass fuels recommended by European Committee for Standardisation (CEN). The volatile matter content reported at 89.57% db suggests that coffee husks is easy to ignite and suitable for large-scale combustion and gasification applications. At 18.42 MJ/kg, the HHV of coffee husks is in the range of wood fuel and therefore it is competitive in terms of its heating values. However, the particle density of 718.09 kg/m3 and durability of 80.77% falls below CEN standards for wood fuel briquettes. In conclusion, coffee husk has competitive heating value, volatile matter and ash content for use as a fuel in large-scale combustion and gasification application. However, use of coffee husks briquettes produced at low die pressures could be hampered by its potential to produce excessive NOx and SOx pollutants. Another drawback is that the briquettes fall short of the density and durability requirements of woody biomass fuels under CEN Standards.