De nouvelles méthodes cryogéniques pour maîtriser la pollution de l'air intérieur (en anglais)

The journal Science of the Total Environment will publish in February 2019 an article exploring the capacity of cryogenic technologies to tackle indoor air pollution¹.

Recent studies show that chronic exposure to haze pollution can lead to health issues such as respiratory problems or cardiorespiratory dysfunctions. According to an article published by Michael Jerrett in Nature in 2015², exposure to fine particles in atmospheric pollution leads to 3.3 million premature deaths annually worldwide.

 

 

Some techniques of indoor air purification already exist, but their efficiency and their use is limited (limited target pollutants, need to frequently replace filters or adsorbents...). The team of researchers of the Nottingham Trent University who wrote the article tested cryogenic methods to remove indoor airborne particulate matter (PM) and typical associated gaseous pollutants NO₂. They relied on the assumption that phase transition of gaseous components by cryogenic methods may separate the haze particulates and gaseous pollutants from the air.

In order to conduct their experiment, the scientists used a condenser composed of two modules: a liquid nitrogen injection module and an intelligent temperature control module. The latter controlled the gas flow of liquid nitrogen to maintain the inner of the condenser at a temperature comprised between -60 and -25°C. The condenser was also equipped with a fan. The polluted air was introduced in a single-tube installed in the condenser. The pipeline was 15 metres long, with a diameter of 1.6 centimetres.

 

 

The study was conducted in a Chinese laboratory during heavy outdoor haze events in December 2014 and January 2015. Two different kinds of pollutants were analysed:

• Particle matters (PM), including PM₁₀, PM_2.5 and PM₁. The number in index indicates the diameter of the particle: PM₁₀ are particle matters whose diameter is less than 10 µm. PM_2.5 and PM₁ are considered as "fine particles" and "very fine particles". The smaller particles tend to penetrate into the gas exchange regions of the lung, and vert small particles (>100 nanometres) may pass through the lungs and affect other organs³. The results of the study focused more specifically on PM_2.5.
  
• NO₂, which is produced by internal combustion engines and thermal power plants, among others.

The results of the experiment showed that up to 99% of indoor PM_2.5 from ambient air were removed at -18°C. It also demonstrated that 98% of NO₂ is condensed and removed from the ambient air at -50°C. It implies that the method would be effective for multiple indoor pollutants with higher boiling points.

Two hypothesis can explain the enhanced particle removal inside the cooled pipelines: thermophoresis or cryogenic condensation.

• Thermophoresis is a phenomenon "observed in mixtures of mobile particles where the different particle types exhibit different responses to the force of a temperature gradient"⁴. In the experiment, it is assumed that aerosol particles may have deposited onto the wall of the pipeline when subjected to a temperature gradient.
• Condensation of water vapour is a pre-conditioning technique allowing fine particles enlargement.  At -10°C, a condensational growth of PM₁ was observed, explaining the reduction of exit mass concentration. The larger the particles are, the easier they are to remove, since they are prone to be separated from the polluted air and removed by gravitational or inertial settling within the pipe.
  

The results of the experiment may pave the way for the development of new generations of air conditioners that can not only control the temperature but also the quality of indoor air.

According to the authors of the article, "this study demonstrated the effectiveness of cryogenic circulation for removing both particulate and gaseous pollutants from indoor air, and cryogenic condensation was the principal removal mechanism. However, energy consumption, humidity control and other technical challenges should be further studied before practical application".

¹ TANG Y., BI L., MORTIMER R. J. G., et al. Cryogenic circulation for indoor air pollution control. Science of the Total Environment. 2019, vol.651, pp. 1451-1456. Available on Research Gate.

² JERRETT M. The death toll from air-pollution sources. Nature. 2015, vol. 525, pp. 330.331. Available on Nature website.

³ WIKIPEDIA. Particulates. Available following this link.

⁴ WIKIPEDIA. Thermophoresis. Available following this link.