Rules of Thumb for Reactor Design and Process Conditions for Thermochemical Treatment of Chromated Copper Arsenate (CCA) Treated Wood Waste

摘要

Lab-scale pyrolysis experiments have been performed to gain insight in the mechanisms influencing the metal behavior during carbonization of chromated copper arsenate (CCA) treated wood chips. To this end a fixed bed low temperature pyrolysis reactor and a thermogravimetric (TG) analyser have been used. Besides CCA treated wood chips, mixtures of metal and organic model compounds (As_2O_5, As_2O_3, CrAsO_4 + CrAsO_4 + Ca(OH)_2, As_2O_5 + glucose, As_2O_5 + lignin, As_2O_5 + activated carbon, As_2O_3 adsorbed on activated carbon, CrAsO_4 + glucose, CrAsO_4 + lignin, CrAsO_4 + Ca(OH)_2 + glucose, CrAsO_4 + Ca(OH)_2 + lignin) have been pyrolyzed in a controlled way. The aim of these model compound experiments is not to imitate the real CCA wood pyrolysis process, but to isolate some individual mechanisms (without being influenced by a variety of complex interactions) and to study the influence of different process parameters on these mechanisms. The parameters controlled during pyrolysis (and their corresponding range) are: temperature (330°C - 430°C), heating rate (5°C/min - 20°C/min), residence time (10 min -40 min), reactor pressure (0 bar gage - 5 bar gage), heater gas flow rate (1000 Nl/h - 1800 NI/h), oxygen concentration (1 vol% - 3 vol%) and steam concentration (5 wt% -15 wt%). Based on the experimental observations, reactor design guidelines are derived in order to identify an operating window that minimizes the volatilization of metals during thermochemical conversion of CCA treated wood, while ensuring a good quality of charcoal product.