Abstract: The present study presents a three-dimensional numerical model for co-firing lignite and biomass in large-scale utility boilers. The model takes into account the non-spherical form of the biomass particle, which influences the drag coefficient and its devolatilization and combustion mechanisms. Simulations under different co-firing rations and biomass particle sizes are performed for a 300 MWe pulverized-fuel, tangentially fired boiler located in Northern Greece, operating with low quality lignite. Validation of the simulations is performed using plant data for the reference case of lignite combustion. The results obtained, which include temperature and species fields, particle tracks and burnout per burner level and NOx calculations, provide useful conclusions regarding the maximum allowable biomass particle size and substitution ratio for an efficient boiler operation. In particular, the combustion behavior of particles from different burner levels suggests the optimum positions for dedicated biomass burners.
Abstract: In the current work the co-combustion of Solid Recovered Fuels’ (SRFs’) with brown coal in large scale pulverised coal boilers under different operational conditions is numerically investigated. In order to overcome the difficulty of the complex, inhomogeneous nature of waste recovered fuels, SRF is modelled as a mixture of two different fractions, the biogenic and the plastic one. For each fraction different combustion mechanisms are presented, whilst for the first time the proposed combustion mechanism of the plastic fraction is incorporated in a commercial CFD code and validated against available experimental data. A 600 MWe brown coal boiler is simulated as a reference and its operational characteristics are compared with parameterised scenarios of SRF co-firing conditions. Based on the numerical results, the optimum co-firing concepts regarding the more efficient operation of the boiler (hot spots and fuel’s burnout) are identified, decreasing the environmental impact of the boiler’s emissions.