Slagg - en koldioxidsänka? En studie av karbonatisering i slagg från förbränning av hushålls- och industriavfall

Sammanfattning: From incineration of industrial- and household waste bottom ash is generated. A large volume of bottom ash is generated yearly in both Sweden and Europe. The natural carbonation process, which involves a reaction between carbon dioxide and calcium in the material, is initiated when the bottom ash reaches contact with air. The carbonation binds carbon dioxide to the bottom ash, decreases the pH and the solubility of metals and hence decreases risks of toxic leachate. Normally the bottom ash is stored during six months to lower the pH from circa 11 to a pH of 8.5-9. The purpose of the master thesis is to estimate the amount of carbon dioxide that can be stored in the material, in order to investigate whether bottom ash is a carbon sink. The usability of the material and the legal aspects of usage and storage of bottom ash is also examined. Previous studies present a wide spread of estimated amount of carbon dioxide uptake, between 12-251 kg of carbon dioxide per ton of bottom ash. In the case study of this essay the amount of stored carbon dioxide in bottom ash stored according to Sysav’s method in Malmö was analyzed. This material was stored for 5.5 months, the pH had dropped from 12.1 to 8.1 and it had absorbed 37 kg of carbon dioxide per ton of bottom ash. The carbonation process that occurs during this time is affected by how much air the bottom ash is exposed to, the chemical composition, the moisture content and the grain size of the material. An optimally estimated moisture content, received from studies conducted on laboratory level, is according to literature 15 percent. The case study at Sysav gave a moisture content of bottom ash of 10-20 percent that could not be linked to the rate of carbon uptake, due to the final sample design of the study. Life cycle analysis of bottom ash shows that use of the material in road construction reduces emissions of carbon dioxide, compared to landfill or use in cement production. In many European countries, the bottom ash is used as a construction material. In Sweden, however, the bottom ash is mainly used as cover material to closed landfills. It would be possible to facilitate the use of bottom ash through legislation. For example, a development in specification limits and guidelines could increase the usability which is also recommended in the Waste Framework Directive. If the laws regarding end-of-waste were expanded to include bottom ash, this could in addition facilitate the use. Storage, and the sequestration of carbon dioxide in the material, requires a use for it to be economically and practically viable. If there is no request or use of the material the alternative is to put it in landfills directly, why the possibilities for bottom ash to be a carbon sink is put to risk.