Development of Bioenergy from Chromium-Copper-Arsenate (CCA) Treated Wood Wastes

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Disposal of woods treated with chromated copper arsenate (CCA) preservatives has become a serious problem in the world. Environmentally benign disposal technologies have to be developed. We have tried to find pyrolysis conditions for CCA treated wood in order to maximize the retention of arsenic in wood charcoal along with a high oil yield. In 2002, a CCA treated sample given an extra heat treatment at 450℃ for 10 min was thoroughly investigated in order to establish the details of the reaction in which arsenic is captured in the pyrolysis residue prior to volatilization. Composition and structure of the metal compounds in the pyrolysis residue were examined by transmission electron microscopy. Alarge number of particles were found of variable diameter between 10 and 100 nm. The smaller ones were mostly spherical, sometimes faceted. The larger ones were lumpy. CCA compounds and their reaction products like Cr_2As_4O_<12> and As_2O_3 were identified in conventional TEM by Selected Area Electron Diffraction. In high resolution, the nanoparticles exhibited lattice fringes as indication of their mono crystalline character, fitting, e.g. the d_<210> = 0.204 nm of Cr. The volatility of arsenic during pyrolysis of CCA treated wood was measured by XRF (X-Ray Fluorescence) analysis at temperatures up to 500℃ and at times up to one hour. Weight change and arsenic content of the pyrolysis residue were measured after dissolution in HMO_3. More than 20 % of arsenic was already lost at 300℃, which may have been caused mainly by the volanlization of the unreacted arsenic compound after reduction of As(V) in As_2O_5 to As(III) in As_2O_3. Arsenic is probably released as As_4O_6, which is very difficult to capture and toxic. By an additional heat treatment this reduction can be prevented and the dry separation of the metals may be allowed