Low-temperature pyrolysis of CCA-treated wood waste: Chemical determination and statistical analysis of metal input and output; mass balances

Abstract

Low-temperature pyrolysis is proposed as an alternative method to dispose of CCA-treated wood waste. In the frame of a study aiming at optimising the pyrolysis of CCA-treated wood, an experimental facility has been built to examine the influence of important process parameters (pyrolysis temperature, residence time, heating rate, particle size, …) on the release of metals and on the resultant mass reduction. In order to perform a mass balance calculation for the total system, a method for metal analysis was developed. Two leaching procedures and one dissolution procedure were tested and compared with each other, resulting in an optimal procedure to bring the metals into solution: “the BSI method” to determine the total amount of Cr, Cu and As in the dried wood and “the Reflux method” to determine the total amount of Cr, Cu and As in the pyrolysis residue. These results illustrate that Cr is more strongly bound in the pyrolysis residue compared to the CCA-treated wood. The analytical technique used was ICP-MS and the analytical problems like interferences and matrix effects were solved by using the appropriate isotope, an internal standard and mathematical corrections. The resulting optimal technique for CCA-treated wood (“the BSI method”) was applied to wood samples with different particle sizes. A statistical analysis of the Cr, Cu and As content in the CCA-treated wood shows the heterogeneous character of CCA-treated wood samples. Heterogeneity becomes less important when using samples with a small range of particle sizes. The smaller wood particles have significantly higher metal concentrations than the larger particles. Realistic mass balances for the metals were obtained and showed that most of the Cr, Cu and As remained in the pyrolysis residue.