Modeling the co-combustion of coal and biocoal from the novel process of frictional pyrolysis for reducing the emissions of coal plants

Coal industry is a significant pillar of the European economy and is responsible for a quarter of the energy production. The gradual decarbonization of the energy sector must take into consideration the regions that require a transition period. In this framework, this study presents the case of co-combusting conventional fuels and biocoals from agrowaste in order to reduce the carbon footprint of existing coal plants during this transition period. Along with the more common case of torrefied biomass, two novel biocoals that are produced by means of frictional pyrolytic conversion are also considered in this analysis and the co-combustion is modeled thermodynamically. The scope of the study has been the calculation of CO2 and NOx emissions for the different potential fuel blends. Of biocoals with conventional coal fuels, i.e., anthracite bituminous, subbituminous, and lignite coal. The heating values of the fuel blends varied significantly from 12.5-30 Mj/kg. The CO2 emissions per kg of input ranged from per kg of input 1.4-3 kg of CO2, with subbituminous coal projected the best performance in this metric. Coals of higher heating values, i.e., anthracite and bituminous coal, performed also well but the metric of CO2 emissions per MJ of input was not comparatively as good and exceeded the level of 95 g of CO2 per MJ of input. On the contrary, lignite CO2 emissions per MJ of input was closer to 90 g of CO2 per MJ of input and this could be explained from the fact that the higher carbon content of anthracite and bituminous coal did not translate to analogically higher heating values. The comparative performance of lignite coal becomes the optimal choice in respect to CO2 emissions per MJ of input for fuel blends that use biocoal ratios higher than 40%. This is an interesting outcome that provides an insight of an alternative pathway for reducing the carbon footprint of low rank coal energy production facilities and can be a helpful tool during the transition of European energy sector towards renewable energy systems.