Practical Method of Gravimetric Tar Analysis That Takes into Account a Thermal Cracking Reaction Scheme

摘要

For biomass gasification to be effective, some difficult problems associated with tars must be overcome. One of these is the lack of a way to quantitatively analyze all of the gravimetric tar components with a single analytical device. Therefore, gravimetric tar components have been identified and their yields have been measured with a combination of two or more devices. However, a simple and practical analytical method is required for process development and pilot-scale testing. Here, we propose a practical method of gravimetric tar analysis that reflects the gravimetric tar reaction scheme and is suitable for industrial use. We applied this method to gravimetric tars produced by secondary biomass tar thermal cracking experiments. This method uses both ultimate analysis and ~1H NMR analysis. Ultimate analyses showed that the h/c molar ratio of the gravimetric tars decreased with increasing secondary thermal cracking temperature. ~1H NMR analyses showed that the hydrogen distribution depended on the thermal cracking temperature: as the temperature was increased, the number of aliphatic hydrogens decreased and the number of aromatic hydrogens increased. Analysis of the chemical shifts of ~1H NMR peaks of the main biomass tar components in a reference material showed that the components could be separated into monocyclic aromatics and polycyclic aromatics at a threshold chemical shift of 7.4 ± 0.1 ppm. From these results, we proposed a modified reaction scheme and converted the hydrogen distributions obtained by ~1H NMR analysis to carbon distributions. Even though gravimetric tar yields decreased with increasing thermal cracking temperature, the yields of polycyclic aromatics were almost constant and were independent of thermal cracking temperature. The yields of monocyclic aromatics decreased with increasing thermal cracking temperature, whereas the yields of monocyclic aromatics in the volatile organic compound fraction increased. The yields of monocyclic aromatics were almost constant at temperatures below 1073 K. Thus, the occurrence of ring-opening reactions was negligible below 1073 K. Decomposition of monocyclic aromatics started at temperatures above 1173 K. Dealkylation reactions were accelerated at temperatures above 1073 K.