Title : Development of cost-effective catalytic systems for total oxidation of VOCs by utilization of solid fuel combustion by products
Coal-based energy production still plays a leading role worldwide, growing in a number of fast-developing countries. The European Union's strategy for the next decade is to limit the burning of coal, but the number of incinerators and gasifiers for municipal waste continues to increase rapidly. The combustion of coal and household municipal waste generates a solid by-product from the thermal modification of the mineral components contained therein, the so-called ash that is disposed or utilized. The utilization of coal ash for the production of building materials is based on the self-cementing properties at high lime content and on the pozzolanic reaction at significant presence of aluminosilicates, and is well studied and widely applied in practice. National and international standards for the classification of coal ash as a construction raw material have been introduced while the development of technological solutions for the recovery of ash from municipal waste is still pending. A solution also awaits the fertilization of huge quantities of already deposited coal ash and the restoration of soils. The application of the concept of circular economy in production, including energy generation, requires a minimum consumption of raw materials and maximum utilization of by-products in terms of efficient energy consumption. This requires the development of efficient and smart approaches to waste recovery by converting them into products with high practical and environmental benefits.
The present study aims to utilize coal ash to develop cost-effective and highly active catalytic systems for the total oxidation of volatile organic compounds (VOCs). Coal ash usually contains more than 70 wt. % aluminosilicates and the other components are mainly oxides of alkali, alkaline earth and transition metals. The content of iron oxides in the form of hematite, magnetite or maghemite, as well as of iron ions incorporated in aluminosilicates, usually varies between 5 and 15 wt% and depending on the type of coal can reach even higher concentrations. This composition of coal ash allows it to be processed into zeolite-like materials by simple alkaline conversion, as aluminosilicates form the structural framework of the zeolite phase, and iron oxides and other metal particles from the source ash are evenly distributed in the zeolite matrix, forming active catalytic centres. These self-assembled catalytic systems of porous support and active centers show high efficiency toward total oxidation of VOCs comparable to that of platinum group metal catalysts (PGMs), which are extremely expensive and PGMs are in critical supply worldwide. Coal ash catalytic systems (CACats) were obtained by two-stage alkaline conversion and ultrasonic homogenization of the reaction mixtures. They are characterized in terms of phase composition of the zeolite matrix, morphology, surface properties and thermal stability, and the state and distribution of iron oxide components. An additional modification was applied by wet impregnation technique with copper or cobalt oxides to improve their catalytic properties. The thermal and dynamic functions of catalytic oxidation of CACats to model VOCs and competitive catalysis in VOCs mixtures were studied. Total oxidative destruction of VOCs was achieved with a sharp decrease in the process temperature below 500 C.
Acknowledgements: This work was financially supported by the National Science Funds, Ministry of Education and Science of R. Bulgaria under contract DN 17/18 (2017).