The role of H2O(g) in the formation of ultrafine PMs under O2/N2/H2O atmosphere

Abstract

To explore the effect of water vapor (H2O(g)) on the formation characteristics of ultrafine particulate matter (PMs), Huangling coal char reacted in a high-temperature drop tube furnace (DTF) at 1800 K under various O2/N2/H2O atmospheres, with the content of H2O(g) varied at 0, 5%, 10%, 20% and 30% respectively. The results showed that both the number-based and mass-based particle size distributions (PSDs), the bimodal distribution of ultrafine PMs remained almost unchanged with increasing H2O(g) content, while the peak of ultrafine PMs shifted and the magnitude of the main peak fluctuated. H2O(g) elevated the mass fraction of ultrafine PMs with a larger particle size (D50 = 0.2134 μm) and the number fraction of ultrafine PMs with a smaller particle size (D50 = 0.0093 μm and D50 = 0.0167 μm) significantly. With the increase of H2O(g) content, both the number and mass of ultrafine PMs decreased first and then increased with the inflection point at 5% H2O(g) content, indicating that lower H2O(g) content inhibited the formation of ultrafine PMs while higher H2O(g) content promoted its formation. Meanwhile, with the increase of H2O(g) content, the average mass of single ultrafine particle, being higher in wet combustion atmospheres than that in dry combustion atmosphere, increased first and then decreased with the inflection point at the content of 5%. In addition, when the H2O(g) content was 5%, the average particle size of ultrafine particle was the largest, which increased by 9.07% compared with that under dry combustion atmosphere. Finally, the influence mechanism of H2O(g) on the formation of ultrafine PMs during char combustion was clarified.