Numerical modeling of the co-firing process of an in situ steam-torrefied biomass with coal in a 230 MW industrial-scale boiler

This paper presents the CFD modeling results of the torrefied maize straw co-firing with sub-bituminous coal in various mass ratios in the industrial scale boiler, to recognize possible application issues of the coal substitution with upgraded biomass. The steam torrefaction of biomass took place in a pilot in a counter-flow torrefaction reactor fed with superheated steam from the OP-230 (Rafako, Poland) boiler. Using a TGA, it was possible to analyze the combustion indexes and synergy effects after burning the torrefied biomass-coal mixtures. Additionally, a kinetic model of pyrolysis devolatilization was established and used in the modeling along with Ansys Fluent kinetics of the coal and gas-phase combustion models. Due to the numerical modeling, it was possible to determine the temperature distribution in the boiler's furnace chamber, the heat flux densities, the simulated distribution of carbon monoxides and carbon dioxide concentration, and the decomposition of nitrogen oxides resulting from co-combustion. Steam torrefied biomass indicates higher combustion activity compared to coal, ignites easier, and burns more intensely with better combustion stability. A synergistic effect between the coal-torrefied blend was observed. According to numerical analysis, it was found that with the increase of the share of torrefaction in the fuel mixture, the share of unburned fuel in fly ash increases. Additionally, an increased share of the torrefied biomass in the fuel blend from 30% to 40% results in a slight increase in the molar NO concentration in the furnace chamber. The authors strongly recommend the continuation of work on further investigation of the co-firing of the coal with torrefied biomass in the pre-mixed blends injected through all burners.