TG-FTIR Study of the Influence of Potassium Chloride on Wheat Straw Pyrolysis

The interest in utilizing biomass as a CO2 neutral fuel by combustion, gasification, or pyrolysis processes is increasing due to concern about the emission of greenhouse gases from fossil fuel combustion. In thermal fuel conversion, pyrolysis is an important step which determines the split of products into char, tar, and gas. In this work, a combination of thermogravimetry and evolved gas analysis by Fourier transform infrared analysis (TG-FTIR) has been applied to study the influence of potassium chloride (KCl) on wheat straw pyrolysis. Raw straw, washed straw, and washed straw impregnated with KCl have been investigated. To facilitate interpretation of the results, pyrolysis of biopolymers (cellulose, xylan, lignin) in the presence and absence of KCl was investigated as well. The raw straw decomposed in a single broad featureless peak. By washing, two peaks appeared in the derivative weight loss curve, corresponding to the decomposition of hemicellulose and cellulose components in the straw. Washing reduced the char yield from 23 wt % (daf) to 12 wt % (daf), reduced the yields of gases, and increased the tar yield from 32 wt % (daf) to 66 wt % (daf). Adding 2 wt % (daf) KCl to the washed straw resulted in a char yield which was close to that of the raw straw, and the yields of tar and gases were between those from the raw and washed straw. Furthermore, the peaks corresponding to hemicellulose and cellulose decomposition moved to lower temperatures, from 670 to 633 K for the cellulose peak, but did not collapse to a single peak as in the raw straw. The influence of KCl on the peak temperature of hemicellulose and cellulose decomposition was not observed with the single biopolymers. This indicates that minerals in straw influence the interaction between the biopolymers in whole biomass. Combustion of the char remaining after pyrolysis showed that char combustion is catalyzed by the minerals present in wheat straw. Char from the washed straw with KCl added burned with two peaks in the derivative weight loss curve corresponding to a catalyzed and noncatalyzed part, indicating that the added salt did not behave in the same way as the inherent minerals in the straw.