Study of cashew nut shells valorisation by gasification

Abstract

During these last three years the production of cashew nuts is expanding across West Africa, and according to the USAID the sub-region produces 85 % of the world harvest. Furthermore, after the tourism, cashew nuts are one of the pillars of Casamance economy, (In Senegal, cashew nuts yield 35 million CFA francs a year). Cashew nut shells are a residue obtained from cashew shelling. This biomass residue is abundant, cheap and generates high energy content. Currently, cashew nuts shells are rejected by artisanal traders without being valued. They were often burned in open air and cause several socio environmental problems. Therefore, the issue of energy recovery by thermochemical process arises to overcome this cashew nuts shells rejection and open air burning. Nevertheless, gasification technology is suitable for biomass residues conversion and remains an economical alternative for the valorisation of cashew nut shells in small scale industries. In order to evaluate the cashew net shells valorisation by gasification, several experiments were conducted on a char obtained from the pyrolysis of this biomass and using a fixed bed reactor. As gasifying agents, carbon dioxide, steam and the mixture of carbon dioxide and steam were used at different temperatures for the gasification of fine particles size (630 m) and gross particles size (3,000 m) CNS char. In order to compare the effect of each parameter studied, carbon conversion rate was calculated from gasification experiments results, by carbon mass balance, of the char carbon content and the produced gas carbon content. Kinetics parameters of CNS char gasification reaction were also determined, using a volume reaction model, in order to compare reactivity of the char, by the activation energy and pre-exponential factor comparison. From the results obtained, temperature has a positive effect on the kinetic of carbon conversion. The results obtained show clearly an improvement of CNS char reactivity with temperature increasing and using steam as gasifying agent. However, char particles size has no significant effect on the gasification reactivity.