Low-temperature pyrolysis of CCA-treated wood: thermogravimetric analysis

Abstract

A thermo-analytical study of untreated and chromated copper arsenate (CCA) treated wood samples is performed in order to obtain a better understanding of the low-temperature pyrolysis of CCA-treated wood waste in an inert atmosphere. The type of wood used in this study is Pinus sylvestris sapwood. The influence of the presence of CCA and the heating rate on the pyrolytic behaviour of wood samples is studied, as well as the release of volatile compounds and metals (Cr, Cu, As) during the pyrolysis process. This paper shows that CCA has a significant influence on the thermal behaviour of wood samples, which is more pronounced the higher the CCA concentration of the sample is. The temperature at the onset of pyrolysis, as well as the temperature where the maximum rate of decomposition occurs, are lowered by the CCA treatment. The final char yield (including the metals) is higher and the rate of weight loss is much more peaked for CCA-treated wood. It could be postulated that the CCA compounds act as promotors of the pyrolysis reactions favouring the formation of char. For higher heating rates, there is a shift of the DTG peak to higher temperatures for both untreated and CCA-treated wood samples. Within the accuracy of the evolved gas analysis (EGA) method applied, it is observed that the presence of CCA does not significantly influence the type and relative amount of measured volatiles. The volatilisation of metal compounds is shown to be strongly dependent on temperature and residence time of the wood sample at a given temperature. A critical point (10 min at 400°C) exists, below which the release of Cr and Cu is negligible and the release of As is below 10%. Above this critical point (longer times at 400°C), there is a dramatic increase in metal release for all three metals. The CCA concentration itself also has an influence in the sense that higher concentrations in the original sample give higher relative concentrations of metals in the resulting pyrolysis residue.