PYROLYSIS AND GASIFICATION OF LIGNOCELLULOSIC SOLID WASTES FOR ACTIVATED-CARBON PRODUCTION

This work was conceived as a preliminary experimental evaluation of the technical feasibility of converting solid-waste materials into powdered activated-carbon adsorbents suitable for wastewater treatment. Lignocellulosic materials (i.e. natural or modified plant tissues) were selected as the class of solid wastes to be studied. The production process under consideration was pyrolysis of the raw material (precursor) to form a carbonaceous char followed by "activation," or expansion of the pore system of the char by gasification with carbon dioxide at 900(DEGREES)C. The primary focus of the research was the influence of the conditions during pyrolysis (heating rate, final temperature) on the yield and porosity of the chars and final activated-carbon products. The pyrolysis conditions investigated were heating rates of 1, 15 and greater than 100(DEGREES)C/min and final temperatures of 500(DEGREES), 700(DEGREES) and 900(DEGREES)C.;Selected chars prepared by pyrolysis at the lower heating rates (1 and 15(DEGREES)C/min) to 500(DEGREES), 700(DEGREES), and 900(DEGREES)C were gasified for various lengths of time in a CO(,2) atmosphere at 900(DEGREES)C. Pore-volume analysis of the final products was performed by nitrogen adsorption 77K and mercury porosimetry. The rate of mass loss and development of the pore system during gasification was found to vary with prior pyrolysis conditions: e.g., low heating rate or longer exposure to high temperature during pyrolysis led to lower rate of gasification. However, the pore volume developed for a given mass loss due to gasification reactions was independent of prior pyrolysis conditions. The latter result was apparently due to the similarity of the pore systems present in the chars immediately prior to the onset of the gasification reactions, i.e. after the char had heated to the gasification temperature. Because the heating rate during pyrolysis below 500(DEGREES)C was the critical factor controlling char yield at 900(DEGREES)C, the final yield of activated carbon (i.e. gasified char) of a specified pore volume was also influenced mainly by the pyrolysis heating rate below 500(DEGREES)C.;Char yield was found to be determined by precursor composition (percent lignin, holocellulose, etc.) and pyrolysis heating rate. Both factors were found to exert their main influence on char yield during pyrolysis below 500(DEGREES)C. Neither factor was found to have a significant effect on the open micropore volume in the char (i.e., that measurable by CO(,2) adsorption at 298K). The fact that the absolute volume of open micropores did not change significantly as temperatures rose from 500(DEGREES)C to 900(DEGREES)C suggested that the char microstructure was effectively established at or below 500(DEGREES)C.