Hygroscopic property of biofuel obtained by torrefaction of wood in a quiescent layer of bentonite

Abstract Torrefaction is widely employed to improve the characteristics of wood for using it as a fuel. One of the possible ways of torrefaction is heating of biomass in a quiescent layer of mineral filler. In this paper the effect of the parameters of the torrefaction process in a quiescent bentonite layer on the hydrophobization of wood (pine, oak, poplar, and rowan tree) is experimentally studied. The hydrophobic properties of biofuels are evaluated by measuring the contact angle. However, the value of this parameter for biofuel depends not only on the hydrophobicity of the surface but also on its porosity. Therefore, the value of the contact angle for a smooth surface was estimated by using the method proposed by Liptakova and Kudela. It is found that torrefied pine has the largest hydrophobicity in comparison with oak, poplar, and rowan tree. The hydrophobicity of biofuel increases with the torrefaction temperature and holding time and decreases with the height of the bentonite layer in the reactor. The contact angle depends on the mass yield as [ c o n t a c t a n g l e ] o = 316 - 549 ∙ m a s s y i e l d + 289 ∙ m a s s y i e l d 2 . The hydrophobicity of biofuels increases as the content of oxidized compounds becomes higher. The surface free energy is estimated by using the Owens-Wendt method. It is demonstrated that the dispersive component of surface free energy rises, while a polar component decreases in the process of torrefaction. The critical surface energy is estimated via Zisman method. It is established that after torrefaction its value increases with the decrease in the height of mineral layer.