Wet torrefaction of empty fruit bunches (EFB) and oil palm trunks (OPT): Effects of process parameters on their physicochemical and structural properties

In this work, the effect of wet torrefaction using conventional acid digestion vessel at three different temperatures (i.e., 180, 210, and 240 °C for 3.0 hours) and three different residence times (i.e., 2.0, 2.5, and 3.0 hours at 240 °C) onto the physicochemical, phase, and fuel properties as well as minerals content of EFB and OPT are evaluated. In both EFB and OPT cases, increasing temperature from 180 to 240 °C at 3 hours residence time and increasing residence time from 2 to 3 hours at 240 °C led to the reduction in the mass yield and an increase in the energy density. Generally, OPT hydrochar has lower mass yield relative to that of EFB. Van Krevelen diagram analysis reveals that EFB and OPT experienced dehydration and decarboxylation reactions. Such reactions reflect the occurrence of hemicellulose and partial cellulose decomposition during wet torrefaction as indicated by the changes in the intensities of several FTIR bands as a function of temperature and residence time and also by the changes in the intensities of the three XRD peaks at 2θ of 15.5, 22.7, and 34.6o representing crystalline cellulose phase when subjected to wet torrefaction at 240 °C for 3 hours. XRD peak analysis also indicates the presence of Si, Mg, and CaCO3 in the original and wet torrefied EFB and OPT. XRF results show that wet torrefaction at up to 240 °C for 3 hours can effectively remove K, Ca, and Cl from EFB and OPT and maintain S content at below 3 wt.%, which may partially resolve the slagging and agglomeration problems related to the EFB and OPT use in boiler as well as their SOx emission issue upon combustion.