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Energy & Fuels 2006, 20, 520-531
The Fate of Trace Elements during the Co-Combustion of Wood-Bark with Waste B. Miller, D. Dugwell,* and R. Kandiyoti Department of Chemical Engineering, Imperial College London, Prince Consort Road, London SW7 2AZ, U.K. ReceiVed April 13, 2005. ReVised Manuscript ReceiVed NoVember 9, 2005
An experimental study has been made into the behavior of trace elements during the combustion of spruce wood-bark with three auxiliary fuels, agricultural waste, pulp sludge, and plastic waste, under simulated fluidized bed combustion conditions. A novel suspension-firing reactor has been used to achieve these conditions, with operation at 800 and at 900 °C, with an air/fuel ratio of 1.2:1 throughout. Experimental results have been interpreted using a thermodynamic equilibrium model. The behavior of seven trace elements (Cd, Cr, Mn, Ni, Pb, Se, Zn) has been studied using inductively coupled plasma ionization-mass spectrometry. In addition, an atomic absorption-based technique has been used to study the behavior of Hg. The experimental data indicate significant emissions of all the elements except Ni and Se, which was present in the fuels at low concentration below the lower limit of quantification. Whereas the majority of fuel Hg is likely to be emitted from the combustor, irrespective of fuel blend employed, the level of emissions of Cd, Cr, and Mn may be affected significantly by the addition of auxiliary fuel. The use of wood-bark as a base fuel for boilers is pertinent to paper mills and timber producers where large quantities of bark are generated as a byproduct.
Introduction The controlled combustion of wastes of agricultural, industrial, and municipal origin promises to be an increasingly viable alternative to direct disposal to landfill. It allows useful energy to be recovered during thermal conversion, and the residual combustion ashes occupy only a small fraction of the initial waste volume. Furthermore, the European Union Directive 1999/ 31/EC specifically precludes the landfilling of combustible wastes, defined by maximum limits of 10% loss on ignition or 6% total organic carbon. The heat produced in the cocombustion process can be harnessed to generate electricity or provide district heating in the same way as heat from coal or natural gas-fired power stations. This practice will thus replace some fossil fuel-derived energy, thereby reducing dependence on expensive and often imported fossil fuels. Substitution of biomass-derived wastes for fossil fuels will also reduce the net carbon dioxide emissions to the atmosphere. The carbon in biomass is part of the natural vegetation growth-decomposition cycle. It has been sequestered recently from the atmosphere and will be reconsumed during the growth of new plant material. This is not the case for carbon in coal, oil, or gas, which has been trapped underground for millions of years. Combustion of these fossil fuels directly increases the carbon dioxide content in the atmosphere, which is approaching 380 ppm. Fluidized bed combustors (FBC) have already proved convenient for the disposal of wastes with subsequent energy production. Such combustors are tolerant of variable fuel quality (e.g., changes in moisture content, chemical composition, and calorific value) and type (due to seasonal and production variations). They are also useful for fuels that exhibit lowtemperature ash fusion properties since they normally operate in the range of 850-950 °C (in contrast to pulverized fuel * Corresponding author. Phone: +44 20 7594 5568. Fax: +44 20 7594 5604. E-mail:
[email protected].
combustors where maximum temperatures in excess of 1500 °C prevail). The variability in quality and availability of many waste fuels makes it attractive to burn such fuels in combination with a base fuel, coal being the prime choice. This approach has the added attraction of enabling waste fuels to be co-fired directly into existing coal-fired fluidized bed combustors with minimal modifications to plant. We have recently been involved in a major investigation into the potential for co-firing a range of fuels, including biomass, waste, and coal, in circulating fluidized bed boilers used by the paper industry. The investigation formed part of a multipartner European Union research project1 that included co-combustion tests at the bench scale, pilot scale, and full commercial scale. Our contribution has been at the bench scale, using a novel suspension-firing reactor to screen fuel blends for their suitability for testing on the larger-scale equipment operated by other partners in the project. This reactor mimics combustion in a fluidized bed reactor without interference from added bed solids on the measurement of trace (
