Kinetic analysis and pyrolysis mechanism of raw and impregnated almond shells

Abstract

Nowadays, lignocellulosic agro-wastes, as rice husks, walnut shells, almond shells, woody wastes, etc., are considered on the one hand as a serious environmental concern and on the other hand as a source of highly concentrated carbon, which can be transformed into energy and value-added products through a suitable thermochemical process. Respectively, pyrolysis is described as an appropriate thermochemical process of agro-waste conversion due to a significant reduction in the volume/mass ratio of agro-wastes, thus limiting the problem associated with their disposal or waste storage, in addition to producing value-added products. Various approaches and modifications of “classical” pyrolysis are sought as well in order to direct biomass pyrolysis process to a greater extent to one or another product or to obtain a product with certain characteristics. In this regard, thermal decomposition reactions studies are important, firstly to fundamentally clarify the mechanisms, i.e. the mechanism of reactions, the parameters that affect them and respectively the consequences on the yield and characteristics of the final products, and secondly to implement industrially the developed thermochemical technologies, which require in depth understanding of the kinetics of thermal decomposition. In the current study the thermal stability and kinetic parameters of pyrolysis of virgin and impregnated with ZnCl2 almond shells were investigated and compared. Unequivocally the treatment with ZnCl2 solution of initial almond shells leads to changes in the matrix of almond shells, which affects both, i.e. the process of pyrolysis and the final characteristics of the obtained pyrolytic products. The impregnation alters the thermal decomposition mechanism and it appears that the activation energy of pyrolysis does not depend on the conversion degree and is lower for preliminary untreated almond shells. Based on kinetic parameters, the thermodynamic functions describing the formation of activated complex were determined as well. It appears that preliminary treatment of almonds shells with ZnCl2 solution leads to higher speed proceeding pyrolysis and yields of pyrolytic solid product (char) with well-developed porosity. However pyrolysis of ZnCl2 solution treated shells requires higher energy consumption.