Torrefaction of spent coffee grounds for solid fuel production: A review

Lignocellulosic solid fuels derived from biomass are emerging as a promising alternative to fossil fuels. However, due to a high moisture content, low energy density, and bulky volume, biomass is less favorable in terms of transport, storage, handling, and conversion. Pretreatment of biomass has shown to address these limitations, improving the efficiency of converting biomass into bioenergy. Among various methods, torrefaction is particularly promising, as it produces solid biogenic fuels that show a relatively higher conversion rate into bioenergy (on an equal mass basis) compared to the original biomass from which they are derived. This paper provides a comprehensive review of the state-of-the-art in torrefaction and its role in thermochemical conversion processes. Particularly, the characteristics of spent coffee grounds (SCGs) are examined, together with the potential of torrefaction for solid biofuel production from SCGs. Reactor technologies that may be used for this purpose are explored and the impact of process parameters and operating conditions is discussed. To gain deeper understanding of the reactions and mass & heat transfer phenomena involved in torrefaction, commonly applied reaction kinetics and previously established Computational Fluid Dynamics (CFD) models are reviewed. Challenges associated with scaling up methods for industrial applications and future perspectives of torrefaction in bioenergy production are also discussed. Finally, conclusions are drawn and research needs for further improvement of the torrefaction process are highlighted.