ARTICLE pubs.acs.org/EF
Occurrence of Zinc and Lead in Aerosols and Deposits in the Fluidized-Bed Combustion of Recovered Waste Wood. Part 1: Samples from Boilers Sonja Enestam,*,†,‡ Christoffer Boman,§ Jere Niemi,† Dan Bostr€om,§ Rainer Backman,§ Kari M€akel€a,† and Mikko Hupa‡ †
Metso Power Oy, Post Office Box 109, FIN-33101 Tampere, Finland Åbo Akademi University, Biskopsgatan 8, 20500 Åbo, Finland § Energy Technology and Thermal Process Chemistry, Umea University, SE-901 87 Umea, Sweden ‡
ABSTRACT: Combustion of recovered waste wood (RWW) has led to increased fouling and corrosion of furnace walls, superheaters, and economizers. These problems have been associated mainly with chlorine, zinc, and lead in the deposits but also with sodium and titanium. The presence of lead and zinc compounds, especially lead and zinc chlorides, has been shown to increase the corrosivity of the deposits even at relatively low metal temperatures (230-450 °C). The present work determined experimentally the distribution and speciation of zinc and lead compounds in aerosol particles and deposits in the fluidized-bed combustion of RWW. Measurements were conducted in both a full-scale (20 MWth) plant with as-received RWW and in a pilotscale (2 MWth) setup with as-received RWW and RWW doped with zinc and lead. The results show that the amount and speciation of zinc and lead in the deposits vary depending upon the fuel composition, flue gas temperature, and metal temperature. Both lead and zinc chlorides are found in temperature ranges typical for the primary superheater area. A caracolite-type compound [Na3Pb2(SO4)3Cl] was identified in deposits from the economizer area and K2ZnCl4 in the sub-micrometer aerosol particle fraction.
1. INTRODUCTION 1.1. Recovered Waste Wood (RWW) as a Fuel. In recent years, concerns about the environment, depletion of fossil fuel resources, and economic considerations have increased interest in the use of biomass and waste-derived fuels for power production. A potential renewable CO2-neutral fuel is RWW, comprising packaging materials, demolition wood, timber from building sites, and used wood from residential, industrial, and commercial activities.1 The total amount of RWW generated in the 20 European countries participating in the COST Action E31 is approximately 29.6 megatons/year, which corresponds to 0.7% of the primary energy consumption in these countries.1 The current management of recovered wood, under which 34% is used for energy generation, 38% is recycled, and 28% is put to other uses (landfill, reuse, and compositing), contributes to fossil fuel savings of 150 PJ/year and a CO2 reduction of 11 megatons/ year. Because of its relatively low price, there is a market demand for RWW as a fuel for biofuel boilers. The price per megawatt of waste wood in Sweden during the period of 2005-2009 was 4060% lower than that, for example, of forest residue (Figure 12). Waste wood is, however, often contaminated with paint, plastic, and metal components, which result in concentrations of, e.g., zinc, lead, chlorine, sodium, and sometimes sulfur that are elevated compared to those found in virgin wood.3-5 Zinc and lead originate to a large extent from the surface treatment of the wood, which accounts for 80% of the lead and 70% of the zinc according to Krook et al.,6 who also determined that 10% of the lead in waste wood originates from plastic and approximately 14% of the zinc originates from galvanized metal. r 2011 American Chemical Society
Boilers burning waste wood have in several cases experienced increased fouling and corrosion of furnace walls, superheaters, and economizers. These problems have been associated mainly with chlorine, zinc, and lead in the deposits but also with sodium and titanium.4,5,7,8 Typically, corrosion related to the combustion of waste wood occurs at metal temperatures of 350-450 °C, which is lower than the corrosion range for boilers burning virgin biomass. The location of the deposits/corrosion and the composition of the deposits seem to vary with the fuel composition, boiler design, combustion parameters, flue gas temperature, and metal temperature.4,5 Another problem associated with combustion of waste wood is the formation of molten metal at the bottom of the boiler.5 Zinc and lead introduced into the boiler might be reduced to metallic form in the lower part of a boiler with staged air supply [e.g., a bubbling fluidized-bed (BFB) boiler], where the air/fuel ratio λ is
