The application of zeolites in wastewater treatment is studied long ago but still many unexploited opportunities exist. Recent investigations on natural and synthetic zeolites in water decontamination are focused on the improvement of their adsorption and ion-exchange properties modifying them by functionalization, acidic treatment or by incorporation of metal nanoparticles. Another target is to obtain magnetic zeolites to facilitate their powders separation from decontaminated media. In the context of environmental protection and conservation of natural resources, zeolites are derived from waste aluminosilicates including also coal fly ash (FA). Fly ash zeolites (FAZ) with excellent surface characteristics are synthesized by double stage fusion-hydrothermal activation including high thermal stage prior to the hydrothermal. Drawback of this synthesis manner is that magnetite from FA undergoes thermal oxidation to hematite preventing the simultaneous presence of Fe2+/3+ ions into FAZ, which is otherwise important for their decontamination efficiency. Thus prepared FAZ also suffer from the disruption of their magnetic activity, as the magnetic zeolites adsorbents are removed from decontaminated media by a simple magnetic process. In this study, FAZ are loaded by magnetite nanoparticles between the two synthesis stages. The obtained nanocomposites and their parent FAZ are tested for decontamination of polluted waters. Acknowledgements This work was financially supported by National Science Funds, Ministries of Education and Science of Republic Bulgaria and Slovak Republic in the frame of bilateral research project under Grant DNTS Slovak Republic 01/6 (SK-BG-2013-0025).
The advantages of post-combustion capture of CO2 based on physical adsorption have been extensively studied in the last years regarding to the economy, environmental compatibility, and process’ efficiency. The industrialization of the process involves the construction of adsorption columns in which adsorption and desorption mass transfer processes occur. Each solid-gas system has particular mass and heat transfer properties. In this study the adsorption and desorption processes in the fly ash zeolite-CO2 system in dynamic conditions was investigated. The breakthrough curves were built in a series of experimental conditions aimed at establishing an optimal temperature of desorption. In the temperature range 50-200 oC, the adsorption capacity at the break point was obtained of 100-115 mg/g. The concentration limit of 5 vol.% CO2 in the exhaust gas stream was reached in 11.3-11.9 minutes. The small change in the adsorption capacities achieved for multiple adsorption-desorption cycles and the minor displacement in the break point determine the applicability of a low thermal swing adsorption process at 50 oC. This low temperature of desorption would increase the total energy efficiency of the process, as the required heat could be provided by waste streams from the electricity production process. Acknowledgements This work was financially supported by the NSF, Ministry of Education and Science of Bulgaria under contract DM17/6.
The global warming represents one of the main world problems in the recent decades as it is considered that electricity production by coal burning is responsible for 40 % of the total CO2 emissions. The post-combustion capture of CO2 based on physical adsorption has the highest potential for meeting the industrial needs. Development of new sorbents, especially including solid waste utilization such as zeolitization of coal ash, would improve the process’ parameters in terms of economic feasibility, safety, and environmental protection. The adsorption processes’ nature is complex and is described by many empirical constants which are experimentally determined for individual contact systems. In this study the heat of adsorption in the system fly ash zeolite (FAZ) – CO2 is experimentally investigated. The performed technique includes on-line measurements of temperature increase during the exothermic adsorption process in laboratory adsorption column filled with FAZ. The sample is under continuous flow of 30 ml/min pure CO2. The exothermic peak is registered at 95 s and the detected temperature increase is 32.88 oC. The calculated specific heat of adsorption amounts to Hads=36.83 kJ/kg. Acknowledgements This work was financially supported by the NSF, Ministry of Education and Science of Bulgaria under contract DM 17/6.
Coal-fired Thermal Power Plants (TPPs) are the main source of greenhouse gas emissions in the atmosphere, but they also generate huge amounts of solid by-products, including fly ash (FA). A modern strategy for sustainable energy production is to improve the ecology of the TPPs by developing technological solutions for zero-emission plants. However, this concept is still far from a practical realization and requires intensive research efforts. Thanks to its aluminosilicate nature, FA is investigated to be converted into zeolites for applications in gas cleaning systems. The development of technologies for CO2 capture by fly ash zeolites (FAZ) will provide a join solution for the two main ecological problems concerning coal supplied TPPs, namely utilization of solid residues and implementation of carbon capture technologies. In the present study, the comparative studies were performed on the adsorption of CO2 onto FAZ obtained from FA referred to classes F and C according to the standard specification of coal fly ash (ASTM 618). FA-derived zeolites of Na-X and Ca-Na-X types were studied with respect to their specific surface and porosity by model studies of their nitrogen adsorption/desorption isotherms at cryogenic temperatures. The adsorption isotherms of CO2 were measured in the relative pressure range p/p0=0.001-0.03 at 0 oC and mathematically analyzed. Acknowledgements: The financial support of Bulgarian National Science Fund (BNSF) under the project DN 17/18 (12.12.2017) is highly appreciated.
Deterioration of the air quality and the greenhouse effect requires replacement of the conventional fuels with renewable energy sources (RES). The main disadvantage of RES is the intermittent production of energy during the day and the different seasons. This requires efficient energy storage solutions. Affordable technologies applicable to household-scale heating are thermochemical energy storage systems (TES). TES are usually applied for concentration and storage of solar energy. TES based on zeolites as heat storage media use the exothermic effect of water adsorption. The preferred storage medium is commercial zeolite 13X, which is characterized by high porosity and a large specific surface area. In these systems, the charging is carried out at 130-180 °C and the energy storage efficiency is over 90 %. In the present study, storage media of zeolite X synthesized by the alkaline conversion of coal fly ash was studied in a laboratory assembled TES as a possible substitute of zeolite 13X. The results revealed a micro-mesoporous structure for coal fly ash zeolites (CFAZ) in contrast to the microporous 13X. The charge of TES with CFAZ can be carried out at lower temperatures, while its discharge is enough exothermic. The utilization of coal ash as raw materials for synthesis of heat storage media contribute to many environmental benefits.
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