Comparison of Primary Volatiles from Coal and Biomass Pyrolysis in N2 and CO2

Abstract

The present work compares the primary tars produced by fast pyrolysis in nitrogen (N2) and carbon dioxide (CO2) atmospheres of coal (bituminous colombian coal), raw biomasses (straw and walnut shells), and biomass macromolecules (cellulose, hemicellulose, and lignin). Primary tars have been obtained in a heated strip reactor (HSR). Particles are heated to temperatures of 1573 and 2073 K with a heating rate of 104K/s; meanwhile, the gas in the reactor is kept at near ambient temperature. Therefore, volatiles ejected from the hot particles are quenched immediately after volatilization, minimizing secondary reactions. Chemicophysical analyses, such as gas chromatography–mass spectrometry (GC–MS), size-exclusion chromatography (SEC), and ultraviolet–visible (UV–vis) spectroscopy, have been performed to detect the compounds present in primary tars. Anhydrosugars (levoglucosan) largely dominate the primary tars of cellulose and hemicellulose. Lignin primary tars are instead mainly composed of oxo-aromatics (vanillin and benzendiol). The chemical composition of tars from fast pyrolysis of two natural biomasses (walnut shells and straw) cannot be obtained as a linear combination of the chemical composition of the tars produced from pyrolysis of single lignocellulosic components. First of all, no anhydrosugars are present. Phenols and benzofurans prevail in straw tars, while benzendiols and some compounds related to the lignin structure (vanillin and cinnamaldehyde) prevail in tars of walnut shells. At the higher temperature, the results drastically change: oxygenated aromatic compounds are present in the form of naphtalenols rather than benzendiols, and larger amounts of heavy polycyclic aromatic hydrocarbons (PAHs, indenopyrene and benzofluorantene) appear. The primary tars of coal pyrolysis are very different from those of biomass: heavy aliphatics (C21–C30) are the most abundant class at 1573 K. At 2073 K, large amounts of heavy PAHs (benzophenanthrene and benzopyrene) are produced. A table is provided with the indication of the main components of primary tars, which can be of interest for the development and validation of pyrolysis models for coal and biomass.