Ultrastructural development of the softwood cell wall during pyrolysis

The pyrolytic conversion of pine wood at mild temperatures between 200 degrees C and 300 degrees C was investigated by transmission electron microscopy (TEM). Based on TEM imaging and image analysis, a novel method was developed for determining the local orientation of the cellulose microfibrils in the secondary wall S2 which gives a measure for the progression of pyrolytic conversion of the cell wall. Elemental composition of pyrolysed specimens was determined up to 600 degrees C. TEM imaging together with the evaluation of the elemental composition shows that first the polyoses are degraded, while the cellulose microfibril orientation is still visible up to 225 degrees C. The cellulose microfibrils could not be observed at temperatures higher than 250 degrees C, while lignin containing compound middle lamella (CML) was still visible. After a gradual decrease of the CML up to 275 degrees C, the cell wall became entirely isotropic beginning at 300 degrees C. Based on the presented results, we propose an early degradation of the supramolecular structure of the cell wall.