Sustainable Energy Sector Using Waste Woody Biomass and Miscanthus in the Energy Transition

- The thermal energy sector is very important at this time of global energy transition. From the aspect of stable energy independence, this is especially important in countries whose energy system is based on fossil fuels, especially coal. This is why decarbonisation of the energy sector is necessary, with a partial and gradual substitution of coal with renewable fuels. However, the use of these fuels is not always possible in existing plants. This refers to the establishment of energy, economic and environmentally acceptable proportion of the co-firing of these fuels, i.e. a mixture of coal and renewable fuels. The success of the establishment of this process is a function of several variables, the dominant of which are the aggregate properties of the fuel, basic and their mixtures, including the ash properties of those fuels. With the motive of contributing to a more successful implementation of the energy transition in the thermal energy sector, and with the aim of obtaining new scientific knowledge about the characteristics of the combustion of lignite and brown coal with different renewable fuels, laboratory research was carried out. In particular, different mixtures of lignite and brown coal, waste woody biomass and Miscanthus as a fast-growing energy crop were subjected to co-firing with variable process conditions. In addition to changing the composition of fuel mixtures, the test regimes included a significant change in process temperature (1250-1450 °C) and the primary measure of staged air supply to the furnace. In these combustion conditions, the emission of undesirable and harmful components into the environment, the efficiency of combustion, and the tendency of the ash from the fuel mixture to possible soiling of the heating surfaces in the furnace were evaluated - an analysis of the characteristics of the ash samples from the reaction zone and samples of the slag and ash to the furnace. Here, in both cases of co-firing, lignite and brown coal with different types of biomass, it was shown that in real conditions it is possible to establish a sustainable primary energy conversion process from fuel with a low unburnt carbon content in the slag (unburnt carbon content, UBC<1%) as well as low CO emission, below 350 mg/m n3 . At the same time, the NO x emission is below 320 mg/m n3 during the co-firing of lignite mixtures at 1250 °C, and in the case of brown coal mixtures below 740 mg/m n3 at 1450 °C. In both cases, the net CO 2 emission decreases in proportion to the proportion of biomass in the mixture, while the SO 2 emission is still high, at a level of up to 2500 mg/m n3 for lignite mixtures and up to 6400 mg/m n3 for brown coal mixtures. None of the treated types of biomass, up to the level of applicable content in the mixture with lignite and brown coal, does not worsen the progress of the process from the aspect of possible slagging/fouling heating surfaces in the boiler.