Experimental Research on The Co-firing of Mixtures of Lignite, Waste Woody Biomass and Miscanthus in The Direction of Energy Sector Transition

Decarbonization of the energy sector is a necessary but long-term process. It includes at least partial substitution of fossil fuels using renewable and alternative fuels. However, this substitution, apart from not always being possible, is often accompanied by limitations and unknowns. Those unknowns are dominantly related to the possibility of establishing stable, highly efficient and low-waste combustion as a fundamental process of primary energy conversion from fuel. Such situations are very common considering the necessity of decarbonization in a very large number of fossil fuel-based energy plants, especially coal-based ones. The possibility of establishing an energetically, economically and environmentally acceptable combustion process is a function of a number of variables, of which the collective properties of the fuel, including the ash from that fuel, are dominant in this sense. With the motive of a scientific contribution to the energy transition, and with the aim of obtaining new knowledge about the characteristics of the combustion of lignite coals with different types of biomass, a set of laboratory research was carried out. Various mixtures of lignite, waste woody biomass and Miscanthus as a fast-growing energy crop were subjected to combustion. With a change in the composition of the fuel mixture, the tests were performed at a process temperature of 1250 °C and with a staged supply of combustion air.  With these combustion conditions, the emission of undesirable components into the environment, the efficiency of combustion and the ash estimated tendency of fouling of the boiler heating surfaces were determined. It has been shown that even with co-firing at a temperature of 1250 °C, it is possible to establish a efficient process with an acceptable content of unburnt carbon in the slag (unburnt carbon content, UBC<4%) as well as CO emissions (e_CO≤340 mg/m_n³), a relatively low emission of nitrogen oxides e_NOx≤670 mg/m_n³. Also, the process proved to be well controlled from the aspect of possible ash slagging in the furnace. The net emission of CO₂ decreases in proportion to the share of biomass in the mixture, while the emission of SO₂ is high, at the level of e_SO2≤2500 mg/m_n³.