Emission characteristics of particulate matter during combustion of cotton stalk

Abstract: In this paper, DEKATI low pressure impactor and drop tube furnace combustion system were combined to study the emission characteristics of particulate matter during the combustion. The formation mechanism of particulate matter was investigated by scanning electron microscope (SEM) and X-ray fluorescence (XRF). Proximate and ultimate analysis of cotton stalk sample showed that the ash content and the fixed carbon content of the cotton were lower, the volatile content was higher and the overall calorific value was lower compared with coal. It was found that the yield of PM1 during the combustion experiments decreased significantly with the increase of the combustion temperature by mass distribution analysis. The combustion temperature increased from 1173 to 1473 K. The yield of PM1 decreased from 68.90 to 14.02 mg/m3. At the same time, the total yield of particulate matter also decreased from 75.41 to 16.30 mg/m3. And the peak of particle distribution moved from around 0.609 to around 0.261 μm. The smaller the particle size of the raw material, the lower the yield of PM1. The yield of PM1 decreased from 39.84 mg/m3 to 23.06 mg/m3 with the decrease of the particle size of the raw material. By contrast, the yield of PM1-10 increased from 5.18 to 8.38 mg/m3. The yield of particle under PM0.1 was very low, only 0.38 mg/m3, so the particle size of the raw material had no obvious effect on it. The mass-based particle size distributions (PSD) of particulate matter exhibited a bimodal distribution. The analysis of elements showed that the fine particle (0.028-0.261 μm) mainly consisted of K, Cl and a little amount of S. The main forming pathway of the fine particle was vaporization- condensation of the alkali compound such as KCl, KOH and K2SO4. However, heterogeneous condensation, agglomeration and coalescence contributed to PM1 formation at a certain degree during the combustion. The microstructure of the fine particle was regular and the main component was KCl. The elements of coarse particle (1.590-9.860 μm) were mainly composed of Ca, Mg and a small amount of Si and P. The main forming pathway of the coarse particle was the fracture of large particle or the transformation of Ca-rich, Mg-rich and Si-rich particles with heterogeneous condensation. In addition, the formation of silicates and phosphates influenced the formation of coarse particle. The surface shape of the coarse particle was mostly spherical. Intermediate particle (0.261-1.590 μm) belonged to the transition section and had the characteristics of both fine particle and coarse particle in complex microstructure.