Research articleInvestigation of reducing ultrafine particulate matter formation by adding modified montmorillonite during coal combustion

- Raw and modified montmorillonite show great ability to reduce PM0.2 formation.
- Modification treatment promotes reaction between montmorillonite and alkali metal.
- SiO bonds are easier to break at high temperature due to the acid modification.
- New SiOAl bonds are formed after modified by polyhydroxy aluminum solution.

Up to now, nearly all sorbents used to reduce the formation of particulate matter (PM) during coal combustion are raw minerals in nature. In this work, a new idea is proposed to improve the control ability by modifying the natural minerals. First, hydrochloric acid (HCl) and hydroxyl polymeric aluminum were selected to modify raw montmorillonite, respectively. Then combustion experiment of pulverized coal alone and that mixed with the raw/modified montmorillonite was performed under O2/N2 atmosphere in a lab-scale drop tube furnace (DTF). The produced PM was collected via a low pressure impactor system (LPI). The results show that the raw montmorillonite reduced the PM0.2 yield by 17.99%. Compared with raw montmorillonite, PM0.2 yield further decreased by 29.00% and 13.11% when acid-treated montmorillonite and aluminum-treated aluminum were added. Further characterization show that the formation of ultrafine PM was reduced mainly because of the chemical reaction between the sorbent and the vaporized alkali metal. More importantly, the modification treatment optimized the surface characteristics of montmorillonite. Moreover, modification treatment increased the number of free oxygen active sites in montmorillonite due to the breakage or formation of certain metallic bonds. These changes promoted the process of capturing alkali metal and thereby the modified montmorillonite showed better performance in reducing PM0.2 formation.