Speciation of copper in ashes from municipal solid waste combustion

Sammanfattning: Copper is one of the most important trace elements in municipal solid waste (MSW) combustion. Knowledge of the speciation of copper is fundamental for the understanding of the effects of copper compounds on the combustion chemistry as well as of the environmental impact of the ash. An increased understanding of the chemistry of copper in combustion and in the ashes could support the development of management and recycling techniques, not only for the copper metal but also for the bulk of the ash. It could also increase the understanding of how it might be possible to reduce the amount of dioxins formed and thereby reducing the toxicity of the ashes as well as the flue gases. In this work the speciation of copper in four ash flows from a bubbling fluidized bed (BFB) boiler and one fly ash from a grate fired mass burn (MB) combustor has been investigated using synchrotron based X-ray absorption spectrometry. Additionally the copper speciation of leaching residues from the BFB filter ash and MB fly ash, leached in ammonium nitrate and nitric acid, have also been investigated. The results from the BFB ash flows showed that copper occurred mainly as copper metal, copper oxides and mixed oxides in the ashes from and close to the combustion bed. The concentrations of copper sulphate, hydroxides and chlorides increased further down the BFB flue gas system, closer to the filter. Copper in oxidation states 0, +I and +II was found in all ash flows, except in the BFB filter ash where mainly copper(II) was found. The MB fly ash showed significantly different copper speciation than in the BFB fly ash, with mainly phosphate or silicate together with a mix of copper metal, copper(II) oxide and copper(I) chloride. The residues from leaching with ammonium nitrate showed that the copper speciation was similar in both residues, containing a mix of mainly phosphate or silicate together with a mix of copper(II) oxide and copper(I) chloride. The results showed that the chemical speciation may be an important factor affecting the release of copper. This work also included the collection of XAS-data for a large number of copper compounds that could possibly be present in ashes. This collection of XAS-data will be useful in future work.