The combustion of single 0.5-4 mm black liquor particles and the formation of a localized flame around the particle were studied by means of numerical simulations using a detailed physical model. A critical Damkohler number for a localized CO flame was determined from pyrolysis experiments. These results were extrapolated to furnace conditions i.e. higher temperatures, higher slip velocities and lower O2 concentrations. The general trend found was that particles will act mainly as a source of combustible gases: the main gas oxidation takes place in the bulk gas. To verify this, experiments at temperatures and slip velocities corresponding to actual furnace conditions would be required. Black liquor particles are thermally large (i.e. non-isothermal). Drying and pyrolysing particles have a steep internal temperature gradient. This makes combustion stages simultaneous: char conversion may occur at the particle surface while the interior is still drying and pyrolysing. If no gases are oxidized around the particle, and no flame sheet is formed, O2 may reach the particle surface during pyrolysis. For smaller particles, O2 will penetrate more effectively into the particle and react with char. No significant CH4 oxidation takes place inside the particle. For larger particles, CO is effectively oxidized to CO2 inside the particle by the water CO shift reaction. During char combustion internal particle temperature is more uniform. The larger the particle the higher the internal mass transfer resistance and the thinner the relative thickness of the char conversion region. A reduced model that considers these mechanisms should be used for furnace CFD calculations, as was the objective of this work.
This paper consists of four main parts: 1) Description of the detailed physical combustion model, 2) Criterion definition for localized flame appearance, 3) Role of localized gas oxidation during in-flight furnace combustion and 4) Conversion mechanisms during pyrolysis and char combustion.