Influence of coal co-firing on the particulate matter formation during pulverized biomass combustion

Biomass is regarded as CO₂-neutral, while the high contents of potassium and chlorine in biomass induce severe particulate matter emission, ash deposition, and corrosion in combustion facilities. Co-firing biomass with coal in pulverized-combustion (PC) furnaces is able to solve these problems, as well as achieve a much higher generating efficiency than grate furnaces. In this work, the particulate matter (PM) emission from biomass co-firing with coal was studied in an entrained flow reactor at a temperature of 1623 K simulating PC furnace condition. PMs were sampled through a 13-stage impactor, and their morphology and elemental composition were characterized by scanning electron microscopy and electron dispersive X-ray spectroscopy. SO₂ emissions were measured to interpret the possibility of potassium sulfation during co-firing. Results show that PMs from the separated combustion of both biomass and coal present a bimodal particle size distribution (PSD). The concentration and size of fine-mode submicron particles (PM_1.0) from biomass combustion are much higher than those from coal combustion because of the high potassium content in biomass. For the co-firing cases, with the coal ratio increasing from 0% to 50%, the PM_1.0 yield is reduced by more than half and the PM_1.0 size becomes smaller, in contrast, the concentration of coarse-mode particles with the size of 1.0–10 μm (PM_1.0-10) increases. The measured PM_1.0 yields of co-firing are lower than the theoretically weight-averaged ones, which proves that during the biomass and coal co-firing in PC furnaces, the vaporized potassium from biomass can be efficiently captured by these silicon-aluminate oxides in coal ash. In the studied range of coal co-firing ratio (≤50 wt.%), the chlorides and sulfates of alkali metals from biomass burning are the dominating components in PM_1.0, and a certain amount of silicon is observed in PM_0.1-1. The analysis of chemical composition in PM_1.0, together with that of SO₂ emission, indicates a marginal sulfation of alkali metal chloride occurring at high temperatures in PC furnaces.