Trace element contents of bubbling fluidized bed

Sinks a Vital Element of Modern Waste Management 2nd International Conference on Final Sinks 16 – 18 May 2013 Espoo, Finland

Trace element contents of bubbling fluidized bed boiler sand material as a function of particle size M. Mäkelä (1), O. Dahl (1), G. Watkins (1), R. Pöykiö (2) and H. Nurmesniemi (3) (1) Aalto University, School of Chemical Tchnology, Finland ([email protected], [email protected], [email protected]), (2) City of Kemi, Finland ([email protected]), (3) Stora Enso Oyj, Finland ([email protected])

Abstract When assessing the effects of ash or other combustion residues, the physico-chemical properties, such as particle size distribution and the possible occurrence of toxic minerals and/or metals/metalloids and their concentrations are important factors for assessing the possible health effects of residue handling. Metal/metalloid trace elements that are associated in fine particles can penetrate into lungs much easier than coarse particles and thus the determination of the size distribution of trace element concentrations in combustion residues are essential. This work was aimed at characterizing and fractionating fluidized bed combustion bed sand material and analysing respective element concentrations the size-distributed sand fractions. Element concentrations in the bed the bed sand material were enriched at a particle size below 0.045 mm, due to the highest concentrations for Cd (3.6 mg kg-1, d.w.), Pb (210 mg kg-1, d.w.), Cr (276 mg kg-1, d.w.), Zn (571 mg kg-1, d.w.), As (30.6 mg kg-1, d.w.), V (41.5 mg kg-1, d.w.), Ni (68.0 mg kg-1, d.w.), Ba (1990 mg kg-1, d.w.), Mo (8.6 mg kg-1, d.w.), Mn (22,400 mg kg-1, d.w.) and Ti (910 mg kg-1, d.w.) found in this fraction. The smallest particles are of the greatest concern when combustion residues are handled at landfill disposal or utilization sites, particularly in windy weather, with potential risks associated with prolonged exposure to these residues. We conclude that, in terms of human health risk assessment, careless handling of this residue may pose a human health risk, e.g., if particles with notable trace element contents enter the human gastrointestinal tract through inadvertent wiping the mouth with dirty hands or inhalation of ash particles. Keywords: bed sand, fluidization, municipal district heating plant, sieving, trace element


1

Introduction

Fluidization refers to combustion where a granular material behaves like a fluid. In fluidized bed combustion, fuel is burned in a suspension of gas and solid material (usually sand), which is called a bed layer or bed sand material (Goncalves et al. 2011, Kouvo & Backman 2003, Sippula 2010). The combustion temperature in the bed layer is generally kept below the ash fusion temperature. When using low gas velocities which are below the minimum fluidisation velocity, large gas bubbles can be seen in the bed layer, hence the name bubbling fluidized bed (Raiko 2009). Fluidized bed boiler technology dates back to 1925 (Bowman et al. 2010), and it is now regarded as an efficient and environmentally benign combustion technique for a wide range of fuels, especially heterogeneous fuels. The bed material functions as a thermal flywheel, which ensures rapid ignition and a stable temperature profile. Efficient heat and mass transfer allow operation at low temperatures typically in the range of 850–1 000 °C. In this temperature range thermal NOx is not formed, while the temperature is still high enough to enable the use of SNCR (i.e., selective non-catalytic reduction) for NOx reduction. Bed additives can also be used to limit the SOx-content in flue gas (Wilén et al. 2004). Fluidized bed combustion has therefore been increasingly used by industries and utilities as an effective and environmentally acceptable means for producing steam for process heat and electricity generation (Tangsathitkulchai & Tangsathitkulchai 2001). However, in fluidized bed combustion, a large surface area is provided by the bed sand particles and they may act as absorbents for gaseous ashforming compounds, including trace elements (Lind et al. 1999). When assessing the effects of ash or other combustion residue (i.e., bed sand material) handling, the physicochemical properties, such as particle size distribution and the possible occurrence of toxic minerals and/or metals/metalloids and their concentrations, are important factors for hygienists and analytical chemists in characterizing the possible health effects of ash handling at workplaces, such as incineration plants and disposal sites (Hlaway et al. 1992, Osán et al. 2002). Metal/metalloid trace elements that are associated in fine particles can penetrate into lungs much easier than coarse particles. Therefore, the examination of physical and chemical properties and determination of the size distribution of trace element concentrations in combustion residues such as ash fractions and bed sand material are essential (Chen et al. 2008, Jumppanen et al. 2012). In addition, knowledge of the size distribution of combustion residues may be used to evaluate the possibilities for recycling and utilization of these residues (Chimenos et al. 1999, Ganesh et al. 2011). Sieving is a simple, portable and widely used method of classifying ashes according to their physical size alone, independent of other physical and chemical properties (Horwell 2007). Sieve analysis could be a cheaper and a more common method of determining the particle size of many non-sticky particulate materials than other techniques like particle size analysis. This investigation reports the characterization and size fractionation of fluidized bed combustion bed sand material with respective element concentrations the size-distributed sand fractions. This information is useful for assessing the potential risks associated with, e.g., bed sand handling and especially the differences in variable particle size fractions.

2

Material and Methods 2.1

Sampling and specification of the bed sand material

The bed sand material investigated in this study was obtained from a municipal district heating plant located in Northern Finland (Watkins et al. 2011). The plant has a 32 MW bubbling fluidized bed boiler (BFB), and prior to the sampling period the temperature varied between 810 and 830 °C, with approximately 70.0% of the energy produced by the BFB originated from the incineration of peat, and ca. 30% from the incinerating of clean forest residues (i.e., 10% of stump, 10% of sawdust and 10% of recycled wood). The sampling was carried out over a period of five days and five individual samples (1 kg per sampling day) were obtained during maintenance of the plant when the boiler was shut down and allowed to cool in order to enable a technician to enter the combustion chamber and take samples directly from the bed material.


2.2

Determination of the mineralogy and physical and chemical properties of the bed sand material

For the determination of mineralogical composition of the bed sand material, an X-ray diffractogram of a powdered sample was obtained with a Siemens D 5000 diffractometer (Siemens AG, Karlsruhe, Germany) using CuK radiation. The scan was run from 2 to 80° (2-theta-scale), with increments of 0.02° and a counting time of 1.5 second per step. Operating conditions were 40 kV and 40 mA. Peak identification was carried out with the DIFFRACplus BASIC Evaluation Package PDFMaint 12 (Bruker axs, Germany) and ICDD PDF-2 Release 2006 package software package (Pennsylvania, USA). The pH and electrical conductivity (EC) of the bed sand material were determined by a combination pH/EC analyser equipped with a Thermo Orion Sure Flow pH electrode (Turnhout, Belgium) and a Phoenix conductivity electrode (Phoenix Electrode Cor., Texas, USA) with a cell constant of 1.0. pH and EC were determined according to the European standard SFS-EN 13037 at a solid to liquid (S/L) ratio of 1:5. Determination of the dry matter content of the bed sand was carried out according to the European standard SFS-EN 12880, in which a sample is dried overnight to a constant mass in an oven at 105 °C. The total organic carbon (TOC) content was determined according to the European standard SFS-EN 13137 using a Leco CHN-600 analyser (Leco Inc, USA), in which a sample is combusted and the evolved carbon dioxide is measured by infrared spectrometry. A comprehensive review of the standards, analytical methods and instrumentation is given in our previous study (Watkins et al. 2011). For the determination of trace element distributions in different particle size fractions, the bed sand material was dry sieved on an automatic sieve shaker (Retsch Virbo, Haan, Germany) through stainless-steel sieves using a stack of nested sieves (DIN 4188, Retsch 5657, Haan, Germany) with the following particle size fractions: 31.516, 16-8, 8-4, 4-2, 2-1, 1-0.5, 0.5-0.25, 0.25-0.125, 0.125-0.075, 0.075-0.045 and