Article pubs.acs.org/EF
Assessment of Methods for the Measurement of Wood Fuel Compositions Mohammad Arifur Rahman and Philip K. Hopke* Center for Air Resources Engineering and Science, Clarkson University, Post Office Box 5708, Potsdam, New York 13699, United States ABSTRACT: The composition of processed wood fuels, such as pellets, is important, since large quantities of pellets are used to heat buildings and generate electricity. Prior work identified heavy metal contaminants in pellets as a result of waste wood being part of the input materials. Preparation methods for the analysis of the elemental composition of wood fuels with ICP-MS were evaluated using standard reference materials (SRM) and real wood pellet samples. The effect of different degrees of grinding (10−20 μm and 60−100 μm) on the homogeneity of the resulting samples was investigated. The finer-ground samples (particle size 10−20 μm) demonstrated good homogeneity. The amount of mass required for digestion was evaluated based on the relative mean error (%) for the elemental analyses for both direct HNO3-H2O2 and Ash-HNO3-H2O2 digestion methods. Well ground, larger pellet samples (10−20 μm and ∼1.0 g) demonstrated good agreement between two digestion methods. The precision of the analysis with ICP-MS was evaluated by repeatability standard deviation (Sr) measurements. The ratio of Sr(Dir)/ Sr(Ash) was between 0.29 and 2.78, implying very good agreement between the two digestion methods. The elemental compositions of 30-pellet samples were analyzed, and the relative standard deviation (RSD) was within 10% for both methods. Since the direct digestion method requires fewer steps, it was chosen as the preferred method. Thus, this work suggests the need for a reevaluation of the current standard method such that the sample size would increase and better grinding to finer particle sizes would be employed to ensure reliable measurements of the wood pellets’ elemental contents.
I. INTRODUCTION Wood pellets are a biofuel commodity used extensively throughout the world to produce energy.1 The market for pellets has been booming in many countries, driven mainly by the low cost per unit of energy to achieve the 2020 targets for energy from biomass.2 As a result, global pellet production has substantially increased over the past decade. In 2013, global production increased to approximately 22.1 million tonnes, with Europe and North America being responsible for the majority of global wood pellet production and consumption.3 However, despite its economic and climate benefits, residential biomass fuel combustion in rural areas often represents the major anthropogenic source of air pollution,4 particularly because of the metal content.5 The potential health effects of combustion-related trace element pollution have been discussed by several authors.6−8 Combustion of contaminated wood, such as lead painted wood or pressure treated wood, poses potential environmental problems because of the elemental content of the emitted particulate matter as well as problems of hazardous ash disposal. During combustion, the incombustible fraction of the fuels will result in ash. Inorganic elements, such Ca, K, Mg, Na, Cu, Zn, V, Ni, Zn, Cu, Cd, Pb, Cr, and Hg, are distributed between bottom ash, coarse fly ash particles (>1 μm), fine particles (
