An experimental study of the reactivity of cellulosic-based chars from wastes

Abstract

The objectives of this work were to determine the reactivity of the chars of different cellulosic-based wastes. In energy recovery from waste, the burnout of chars is a part of the overall complex combustion process. Being able to establish simple models for the char burnout process is therefore important. In this study, chars made from wood chips, newspaper and glossy paper were chosen as being representative of the cellulosic fractions of municipal solid waste. The ash content of these parent materials ranged from 0.4% to 24.8% by weight, on a dry basis. After devolatilization, the chars had ash contents in the range 2.2–61.8%.The reactivities of the chars examined were determined from weight loss and surface area measurements. Weight loss was measured via thermogravimetry and surface areas were measured via gas adsorption techniques using CO2 as the adsorbing gas.The reactivity of each of the cellulosic-based chars was found to be a function of conversion. The glossy paper char was found to be much more reactive than the wood-chip char, which in turn was found to be slightly more reactive than the newspaper char. In order to obtain the reactivity for the glossy paper char from the gravimetric measurements, it was crucial to account for calcium carbonate in the sample, a clay used in the paper-making process to give the paper its glossy appearance. Calcium carbonate (CaCO3) desorbs CO2 at the high temperatures used during char preparation and adsorbs CO2 at the low temperatures used in the reactivity tests to ensure kinetics-limited oxidation.The average intrinsic reactivities for the three chars, during reaction in 10vol.% oxygen at 500°C and atmospheric pressure, were found to be 0.46×10-6,0.21×10-6 and 1.36×10-6gm-2s-1, respectively, for the wood-chip, newspaper and glossy-paper chars and the corresponding peak reactivities were 1.43×10-6,1.06×10-6and 5.80×10-6gm-2s-1. The reactivities for the chars differ as a function of conversion and ash content, but no trend in terms of either increasing or decreasing reactivity with increasing ash content was found. Moderate heat treatment, where the char is exposed to 1277°C for 47ms, had no significant influence on char reactivity.