Secondary utilization of the spent fly ashes-derived sorbents

Abstract

In the years 2017-20, the research on the use of fly ashes and bottom ashes from power plants for the preparation of adsorbents for carbon dioxide capture from flue gases was carried out at the involved institutes. As part of the research, adsorbents were prepared by alkali fusion and hydrothermal processes. The obtained products were subjected to cyclic adsorption and desorption tests simulating their industrial use. One of the partial tasks was to evaluate how the worn adsorbents can be subsequently utilized. In the study presented here, the materials collected after the adsorption testing were subjected to several procedures, which aim was to verify the applicability of these materials as substrates for the reclamation of excavated brown coal and lignite quarries. The basic property assessed was the ability to retain humidity and thus contribute to the water balance in the reclaimed landscape. A hydration test followed by slow drying of the sample in a thermogravimetric analyzer was proposed for this purpose. With regard to the raw materials from which the adsorbents were prepared, attention was also paid to the risk of undesired leaching of toxic substances. The last measured parameter was the course of CO2 desorption from the pores of the used adsorbent. It answers the question of whether the adsorbent is usable for terrain reclamations in the state after the last adsorption (i.e. CO2 saturated), or whether it requires thermal desorption as the final step. Based on the results of hydration tests of the spent adsorbents, it can be concluded that they could be applied in the reclamation of closed lignite quarries, and these materials would allow a more sophisticated application than just as a stabilizer. In the tested samples, the high retention of water and its slow release was confirmed by the TGA method, which (in the case of the mentioned use) could improve the management of soil humidity and its distribution to woody plants used in the biotechnical reclamation phase. Another, but as yet unproven, possibility is to use the porous structure of the materials as a suitable substrate for colonization by nitrifying bacteria. These bacteria would subsequently improve the self-cleaning ability of the water tank created in the case of the hydraulic reclamation method. Due to the results of measurements of carbon dioxide desorption from saturated adsorbents, it is necessary to recommend that thermal desorption should be included in their preparation before the use in reclamation. Tests of toxic elements leaching have not identified any potential risk from their mobilization into groundwater or surface water.