Release to the Gas Phase of Inorganic Elements during Wood Combustion. Part 1: Development and Evaluation of Quantification Methods

Abstract

During wood combustion, inorganic elements such as alkali metals, sulfur, chlorine, and some heavy metals are partly released to the gas phase, which may cause problems in combustion facilities because of deposit formation and corrosion. Furthermore, it may cause harmful emissions of gases and particulate matter. The aim of this study is to obtain quantitative data on the release of inorganic elements during wood combustion, which will serve as input data for models aiming to address ash-related problems. Three quantification methods were developed. In all three methods, the release of inorganic elements was quantified by a mass balance based on the weights and inorganic compositions of the fuel and the ash residues obtained by high-temperature (500−1150 °C) treatment in a laboratory-scale tube reactor. However, method A involved the pyrolysis and combustion of a small fuel sample (∼30 g) in this reactor, whereas methods B and C involved initial pyrolysis and combustion, respectively, of a large fuel sample (∼5 kg) in a bench-scale fixed-bed reactor at 500 °C. The methods were evaluated by comparing the data on the release of Cl, S, K, Na, Zn, and Pb from fiber board obtained by the three methods. The release data were interpreted by use of literature information, equilibrium calculations, and scanning electron microscopy analysis of the ash samples. Large differences in the release trends (especially for S, Na, and Zn) were observed for the three methods because of the differences in sample size, oxidizing/reducing conditions, and the ash formation process. The combined results of the three methods provide a good understanding of the ash transformations and release of inorganic elements during wood combustion on a grate. Method A gives information on the local (or primary) release, whereas methods B and C provide insight into the influence of secondary reactions taking place in larger fuel beds.