Kinetics of the low-temperature pyrolysis of chromated copper arsenate-treated wood

Abstract

In this paper, a kinetic evaluation of non-isothermal thermogravimetric (TG) data is carried out by the method of least squares for both untreated and chromated copper arsenate (CCA)-treated wood. The type of wood used is Pinus sylvestris and the slow-heating decomposition in an inert (nitrogen) atmosphere (low-temperature pyrolysis) is studied. In the case of untreated wood, best fit parameters are determined for the three independent parallel reactions scheme, proposed by several other researchers. For CCA treated wood a comparative study is performed, in which five different kinetic models are considered. These are based on the assumption of: (1) a single reaction; (2) independent parallel reactions; (3) competitive reactions; (4) a chain of successive reactions and (5) a combination of two independent parallel reactions and one subsequent reaction. The choice between which of these models offer the best fit, is based partly on chemical considerations and partly on the quality of the fit. Altogether, the DTG curve of CCA treated wood is well described by assuming two independent parallel reactions at low temperature, which may be catalyzed by the metals, followed by the successive decomposition of the residue. A comparison is made with the kinetic parameters derived for untreated wood, resulting in the following observations and conclusions drawn therefrom: hemicellulose decomposition is catalyzed by the metals (lower activation energy, pre-exponential factor and peak temperature and higher contribution to volatiles production); cellulose decomposition starts once the major part of chromium(III) arsenate is decomposed and volatilised (higher activation energy) and may then even be catalyzed by the metals (higher pre-exponential factor) and lignin decomposes very slowly at higher temperatures together with intermediate products from hemicellulose and cellulose decomposition (low pre-exponential factor and activation energy). During this slow charring process the metals act no more as catalysts. This hypothesis is confirmed by a TG analysis, performed on a chromium(III) arsenate precipitate, which is the major arsenic compound present in CCA treated wood, and by a study concerning the microdistribution of copper, chromium and arsenic in the pyrolysis residue of CCA treated wood.