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Co-combustion of waste and coal modelling in cement industry

The present cement production is facing two main problems. The first one is the production of large amount of greenhouse gases, around 5 % of world’s anthropogenic CO2 emissions, and second one is the high fuel prices, mainly coal. The cement producers are therefore under increasing pressure to reduce their fossil fuel consumption and associated greenhouse gases emissions. It was found that partial substitution of coal by alternative fuels like waste derived fuels and biomass may play a major role in the reduction of CO2 emissions. As waste disposal at landfills is the last option in the waste management strategy, energy recovery of waste derived fuels, commonly known as solid recovered fuels – SRF, in the cement industry has a high potential. Incineration of high share of SRF in cement calciners still faces significant challenges, mainly because it is well known that the use of alternative fuels in existing pulverized burners alters the flame shape, the temperature profile inside the furnace, and the burnout of the fuels used. A possibility for the ex- ante control and investigation of the incineration process are computational fluid dynamics - CFD simulations. CFD simulations have shown to be a powerful tool during the development and optimisation of chemical engineering processes and involved apparatuses. They can show some important flow characteristics and mixing phenomena, which cannot be experimentally investigated, and because of that CFD together with experiments and theory, has become an integral component of combustion research. The main focus of this work is to present the numerically efficient method for simulation of thermo-chemical processes occurring inside a cement calciner. This implies good knowledge of state-of- the-art solid fuel combustion models, such as coal, biomass and solid recovered fuel combustion model, as well as proper definition of the endothermic calcination process. In order to correctly numerically study the role and interaction of solid fuel combustion and limestone calcination within cement calciner, new and improved physical and chemical models were introduced. To verify the accuracy of the modelling approach, the new models were extensively analysed, and the numerical predictions of each new model was compared with experimental data. To represent the applicability of the modelling approach, two different three dimensional geometries of real industrial cement calciners were used for the numerical simulations. The result of this research is the better understanding of co-firing of waste derived fuels with coal inside the cement calciners, and its influence on the calcination process. The results gained by this simulation can be used for the optimization of cement calciner’s operating conditions, and for the reducing of its pollutant emissions depending on coal-waste derived fuel ratio and composition of both fuels. Furthermore, the obtained results can be used by manufacturers of combustion equipment to produce more efficient combustion units.

Co-combustion; waste; coal; numerical modelling; cement industry

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