A study of electrical efficiency improvements in Waste CHP plants

Examensarbete för masterexamen
Master's Thesis
Sustainable energy systems (MPSES), MSc
Tapia Molander, Max
By 2045, Sweden aims to achieve climate neutrality and to achieve such a goal work in each sector is of importance. One way to achieve such a goal is through the strategy of waste and biomass incineration to phase out fossile fuels, and in particular through combined heat and power, CHP, systems where the fuel can be used as efficiently as possible. For this thesis, the work is aimed towards that of waste fired CHP plants, and the ways of increasing the electrical efficiency, such that the generation of electricity to the grid is increased without increasing carbon emissions. Due to the nature of municipal solid waste, MSW, being combusted in such plants, being high in chlorine and alkali metals such as Na and K, the steam data is typically lowered to reduce problems with deposition and high temperature corrosion, HTC. This work examines the potential of increasing the electrical efficiency in existing plants through experimental work at a reference plant, and through alterations or future plant designs using modelling. In the modelling work, two primary models representing the reference plant were constructed for the two seasonal extremes: winter and summer, during which the operation of the plant differs the most from one another. Through these primary models, the effects of increased temperature and pressure could be examined. From there, larger changes were modelled such as the addition of a reheat cycle, the effects of changing the MSW to biomass, and in a final case also the impact of co-firing, both indirectly and in parallel. These models showed that there exists great potential to increase the electrical efficiency in a variety of ways. For the cases where biomass was used in modelling, a method was developed to roughly estimate which steam temperature would be applicable for each fuel through comparison of the fuel and ash composition and simulations of ash melt fractions. Through examining the alkali content and if there exists margins in the S/Cl ratio, one could determine if the steam data is set to cautiously and thus the possibility for increasing said values. Such margins were looked at both on a day-to-day basis through on site data collection, and through long term evaluation of data from the reference plant Lillesjöverket. For the existing plant, it was determined that both on a day-to-day basis and in the long term, there were no peaks large enough to support such an increase at present conditions without better precision in alkali measurements.
Process modelling , Simulations , Ebsilon , Ash , HTC , Sulphation , Electrical efficiency
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