Thermal decomposition of biomass wastes derived from palm oil production

Abstract The palm oil industry produces large amounts of biomass by-products, such as palm empty fruit bunch (EFB), mesocarp fiber (MF), and kernel shell (KS). The thermal behavior and decomposition of the oil palm biomass wastes have been studied by thermogravimetry/mass spectrometry (TG/MS) and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). The determination of biopolymer and inorganic composition facilitated the interpretation of the thermal decomposition results. It was established that the chemical compositions of KS and MF were similar, while EFB contained significantly higher amount of cellulose and potassium, and decomposed in a narrower temperature range. In addition to the thermogravimetric curves (TG and DTG), the product distribution of pyrolyzates also reflected the compositional differences. Py-GC/MS and TG/MS experiments showed that the increasing potassium content of the samples reduced the intensity of 4-hydroxy-5,6-dihydro-2H-pyran-2-one, which is formed via depolymerization and dehydration of xylan units. Consequently, these experiments proved that beside cellulose, the depolymerization of hemicellulose was also hindered, while the dehydration, fragmentation, and charring reactions were catalyzed by potassium. Additionally, the evolution of the characteristic cellulose and lignin products shifted to lower temperatures. The most dominant aromatic pyrolysis product was phenol from each sample, which formed to a large extent from the abundant 4-hydroxybenzoate (4Hb) lignin subunits by scission and decarboxylation. KS also produced 4-hydroxybenzoic acid in significant amounts during pyrolysis at 450 °C confirming the presence of 4Hb subunits in lignin. The evolution of phenol started at 220 °C from the 4Hb moieties, and ended at about 600 °C from the residual phenolic units, while the release of guaiacol, vinylguaiacol, and syringol occurred between 300 and 450 °C as monitored by TG/MS. Some characteristic carbohydrate products were attributed to either hemicellulose or cellulose degradation based on the evolution profiles of KS and MF with separated carbohydrate decomposition steps. The comparison of the oil palm biomass wastes showed that EFB is supposed to be treated separately from KS and MF during the utilization process due to the different thermal behavior and composition.