Estimating the levels of process parameters for tar and char production via fast pyrolysis of Hypericum perforatum and characterization of the products

Abstract

The influence of pyrolysis temperature, pressure and heating rate was investigated on the tar and char yield from the fast pyrolysis of Hypericum perforatum (St. John's wort). The experiments were conducted using a fixed bed reactor. Response surface methodology (RSM), with a central composite design (CCD), was used for modeling and optimization of the process parameters. The optimum conditions were found to be at a temperature of 513.13 °C and 400 °C, pressure of 1 and 41 bar and heating rate of 489.39 °C/min and 150 °C/min for maximum tar and char yield, respectively. Maximum yields of tar and char were obtained as 32.98% and 41.23% under optimum process conditions, respectively. Scanning electron microscopy (SEM), ultimate analysis, and gas chromatography/mass spectrometry (GC/MS) were used to determine the physico-chemical properties of char, and the chemical properties of tar. The higher heating value of the chars produced at pressurized atmosphere were 29 MJ/kg. GC/MS analysis showed that the tar was rich in phenol, methyl and methoxy phenols, oxygenated benzenes and fatty acids. Furthermore, the formation of polycyclic aromatic hydrocarbons (PAHs) in the tar, obtained at pressurized pyrolysis in a nitrogen atmosphere, was investigated. Pressure increased the residence time of the volatile components in the reactor and this phenomenon led to the formation of PAHs. The derived tars were found to contain mainly 2-ring (naphthalene and its derivatives), 3-ring (acenaphthylene, Fluorene, Phenanthrene, 2-methyl anthracene), and 4-ring (Fluoranthene) PAHs.