Design of a rain-shower based cleaning system for simultaneous PM_2.5 removal and CO₂ capture of ambient air

The first solar assisted large scale cleaning system (SALSCS, 1st generation), consisting of a large flat-plate solar collector, a tower (chimney) and filter banks, was built in Xi'an, China for urban PM_2.5 mitigation. Experimental results showed that the Xi'an SALSCS maintained a strong nature convective flow created by thermal buoyancy, drawing a large volume of PM_2.5-laden ambient air towards the filter banks and the center of the tower. The polluted air was filtered before discharging from the top of the tower. While filtration method works well, it requires changing and disposing these filter wastes on a maintenance schedule. A modified 2nd generation SALSCS with 18 m × 18 m × 16 m was designed, in which the PM_2.5 removal applies an analogous method to the natural rain scavenging. With this concept, a rain-shower based cleaning system utilizing falling water drops to remove the ambient PM_2.5 was built at Yancheng Environmental Science Park, Jiangsu, China. In this system, there were a large array of spraying nozzles used to provide a sufficient residence time and high rainfall intensity to increase the collision frequency between incoming particles and falling droplets. Theoretical calculations showed that under flow speed of 0.2 m s⁻¹ and using only 752 nozzles, the created sprays can remove PM_2.5 by 35% in mass. Applying two layers of mesh screens to form water films to enhance the particle capture, experimental data showed an improved efficiency of 50%. Further increasing the number of nozzles and deployment area, the wet scavenging method is feasible to remove fine particles with much higher efficiency. In addition to the removal of PM_2.5, an additional benefit of using the rain-shower technology is that if appropriate alkaline is dissolved in the water, the rain shower can also remove the precursor gases, e.g., adding NaOH for capturing CO₂ to reduce the greenhouse gas in the atmosphere.