Pyrolysis profiles and global kinetics of tropical biomass : coconut and cashew nut shells

Sammanfattning: Biomass has been recognised as renewable and environmentally friendly source of energy since decades. Tropical biomasses which are available in large quantities as residues in Latin America, Africa and Asia, have high energy potential for exploitation through thermochemical conversion process, as pyrolysis, gasification or combustion. Cashew nut and coconut shells are two of the most abundant biomass fuels in tropical countries, which are still not well studied as energy sources.This study aims at providing additional and unique data for the conversion of these specific tropical biomasses into energy. In fact, even though these shells are prolific in many tropical countries, they have so far been known principally as raw materials for non-energetic end-uses. Such uses are the production of activated carbon from coconut shells or cashew nut shells extraction from cashew nut shells, among others. Their use as energy sources is still traditional and unquestionably inefficient in most of these countries.In this study, cashew nut and coconut shells were submitted to a controlled pyrolysis process using thermogravimetry, in order to describe and characterise their devolatilisation patterns as well as their respective global pyrolysis kinetics. The study was carried out in an inert atmosphere (argon) from ambient temperature to 1000oC at different heating rates, explicitly 5, 10, 20, 40 and 50 ºC/min. For kinetic parameters determination, the parallel-independent decomposition reactions mechanism was used together with the Coats and Redfern Method. The devolatilisation kinetics parameters of these special shells were compared with that of wood pellets.This study is hitherto and, to the knowledge of the author, innovative and incomparable as no other studies in this perspective have been reported in the literature.The exclusive findings from the current study are two clear peculiar mass loss rate peaks, during cellulose and hemicelluloses thermal degradation, with different heights, found in both tropical biomass samples. These peaks differ from the ordinary woody biomass one-overlapping mass loss rate peak. Kinetics of the thermal decomposition process obtained by applying the Coats and Redfern method and parallel-independent reaction mechanism, showed that coconut shells components are the most reactive followed by cashew nut shells components. It is, Arrhenius constant is greater for coconut shells and lower for wood pellets. These differences become bigger at high temperatures than at lower temperatures.