On the significance of potassium and chlorine content of lignocellulose during torrefaction

Abstract

In this work, the effect of inorganic content during the torrefaction of black locust wood, wheat straw, and rape straw was studied. The torrefaction experiments were carried out at five temperatures: 200, 225, 250, 275 and 300 °C using 1 h isothermal period. In order to study the influence of inorganic content, the raw samples were washed with hot water to remove the majority of the water soluble inorganic components. The inorganic ion content of the original and washed raw materials was measured by inductively coupled plasma optical emission spectrometry (ICP-OES) method. The thermal stability and the effect of torrefaction on the chlorine content of lignocelluloses were studied by the thermogravimetry/mass spectrometry method. The degree of hemicellulose and cellulose decomposition of the original and washed samples during torrefaction at different temperatures was characterized by compositional analysis of the samples. The cellulose, hemicellulose, and Klason lignin contents of the samples were determined by acidic hydrolysis and subsequent high-performance liquid chromatography analysis. The thermal degradation properties of original and washed samples have been compared. Principal component analyses (PCA) have been applied to identify the similarities and differences between the untreated and various torrefied black locust wood, rape straw, and wheat straw samples. We found that contrary to cellulose the impact of the alkaline ions on the thermal decomposition of hemicelluloses is marginal. As a result of washing the hemicellulose content of the black locust and rape straw samples became thermally slightly more stable in the 200–300 °C temperature range, while in case of wheat straw this effect is not significant. The formation of methyl chloride was detected during thermal decomposition. We found that most of the methoxy groups of straw lignins were cleaved during torrefaction at 275 and 300 °C, hence the severely torrefied straw samples produced only small amount of methyl chloride during thermal decomposition.