Pirólise de madeira tratada com CCA em reator de leito fixo

Abstract

Wooden poles of the electricity distribution network are treated with preservatives to increase their shelf life. Among the various types of wood preservatives available on the market, the most used is the chromated copper arsenate (CCA), due to its high efficiency for preserving wood. The presence of metals in these preservatives requires proper disposal at the end of pole life. This study proposes the use of pyrolysis process (under controlled conditions) as an alternative to wooden poles disposal after removed from the electricity distribution network. The pyrolysis of CCA treated wood promotes the release of dangerous volatile compounds due to the presence of metals (Cr/Cu/As) in its composition. In order to minimize these emissions, low cost reagents (based on calcium) can be used during the pyrolytic process. In this work, the effect of different operating parameters (temperature, residence time, heating rate, wood/CaO ratio and superficial velocity) on the retention of metals in the char was evaluated, as well as the influence of calcium compounds on the calorific value of the fuel gas generated in the pyrolysis process. The characterization of CCA treated wood indicated differences in the chemical composition of the tested fractions (C1-sapwood, C2- outer core and C3-cord), especially in the metal content. The metals distribution analysis indicated that the concentration of metallic compounds tend to zero at the center of the pole, so the high concentration of metal is located on the outer portion. In the proximate analysis, a higher ash content on the outer portion was observed. Fourier Transform Infrared Spectroscopy (FTIR) showed bands very similar to a conventional biomass. In Thermogravimetric Analysis (TGA) peaks centered at 276 and 354 ºC were observed, relating to thermal degradation of hemicellulose and cellulose respectively. The activation energy determined by the Kissinger method was approximately 156 kJ·mol-1. The mean activation energy by the Flynn-Wall-Ozama method was 153 kJ·mol-1 for C1, 182 kJ·mol-1 for C2 and 170 kJ·mol-1 for C3. The degradation reaction mechanism determined by using the Criado is mostly controlled by diffusion processes. Under controlled operating conditions (temperature, residence time and air flow) calcium carbonate (CaCO3) was heated in order to obtain calcium oxide (CaO) with a higher surface area. Preliminary tests indicated that the calcination of CaO with greater surface area was obtained at 850 ºC with a residence time of 5 min. In the first series of pyrolysis tests, as CaO was added to the system, lesser metals emission and oil yield were observed. In the second set of pyrolysis tests, the relevant effects identified on the retention of metals was the high particle size of the wood, the addition of CaO and the heating rate. Due to the addition of CaO, the CO2 concentration was reduced and it was produced a combustible gas with a calorific value close to 25 MJ·Nm-3.