The XAS method has earlier been applied to the speciation of cadmium in fly ashes in a PhD thesis and showed promising results. The positive features of the method is that it is element specific, it can show the oxidation state of the metal and in favorable cases the compounds present can be identified and approximately quantified. The nearest neighbor atoms around the target atom (the investigated element) can be identified by theoretical modeling. Semi-quantitative determination of metal compounds is possible with linear combinations of XAS data from pure (reference) compounds obtained at the same beamline as the sample data. Thus, the quality of the quantitative result is dependent on the amount and quality of such standard data. A compound that is not included in the database cannot be found as a component in a sample. Therefore, an important part of this work has been the building up of a database of XAS data for copper compounds. In many cases this involved synthesis of reaction products that may occur in combustion residues. Our database now comprises 23 copper compounds. In addition, for a scientist to be able to do XAS data collection at a suitable synchrotron beam line the research project has to be approved by the scientific committee reviewing and choosing projects. The available beam time at synchrotron is very limited and therefore additional methods were investigated as well in this work.
Additional methods being used are:
- X-ray powder diffraction (XRD) gives information about the crystalline compounds present in amounts larger than 2% (w/w).
- Scanning electron microscopy with element detection by energy dispersive X-ray spectrometry (SEM-EDX) is used to investigate the spatial distribution of elements in the ash.
- Diffuse reflectance Fourier transform infrared spectroscopy (DR-FTIR) gives information about compounds that are not necessarily crystalline. In this project it mainly works as a complement to the XRD technique.
- Thermodynamic modeling of ash chemistry in furnaces, for predicting the ash composition in theoretical scenarios. Software used for this purpose is e.g. FactSage.
- High temperature reactions, as an investigation of what types of copper compounds could be expected from incineration. Some of the reaction products have also been used as reference compounds in the XAS study.
Ashes from three different boilers have, so far, been analyzed in this project. At a commercial bubbling fluidized bed (BFB) boiler, fired with sorted MSW, ash samples were collected at four different locations (the bottom bed, a hopper directly after the combustion chamber, a cyclone for separation of coarser particles in the flue gas and a textile filter for separation of finer particles). At a commercial grate fired (GF) boiler, fired with MSW, one sample of filter (electrostatic precipitator) ash was collected. At a research boiler with a circulating fluidized bed (CFB) ash samples were collected at four different locations (bottom bed, return leg, cyclone and filter) at seven different occasions (cases a-g). During the seven occasions the fuel varied somewhat. In cases a-d the fuel consisted of bark pellets, refuse derived fuel (RDF) and additives (kaolin, phosphate and sulfate). In cases e and f the fuel consisted of wood chips and RDF, with or without lime injection to the flue gas before the filter. In case g the fuel consisted of wood chips, shredder light fraction (SLF, also known as “carfluff”) and sewage sludge. Two filter ash samples (from the BFB and the GF) had previously been tested for leaching with acidic solution as well as ammonium nitrate solution. The solid residues from these leaching tests have also been investigated in this project.
In a parallel project, the BFB filter ash mentioned above, along with a filter ash from a fourth boiler, has been analyzed by sequential extraction. We have collected XAS data from the residues from each of the steps of the sequential leaching. These data has not yet been fully analyzed, but it looks promising for an increased knowledge in both the composition of the filter ashes and what actually happens during the sequential leaching.