IRSST - Institut de recherche Robert-Sauvé en santé et en sécurité du travail

Removal of toxic vapors by oxidation: Development of laboratory test procedures for in-duct air cleaning systems


Exposure to volatile organic compounds (VOC) in workplaces can cause acute effects such as irritation of the skin, the eyes, the mouth, and the nose. Some products may also cause chronic effects including asthma and cancer. Preventing these adverse effects on the health of workers through the adoption and implementation of measures to eliminate or to at least reduce the risk is important. Among the approaches developed so far to reduce the exposure to indoor air pollutants (e.g. VOCs), ventilation is the most conventional and applied technique. During ventilation, the outdoor air is brought into the indoor environment and mixed with the indoor air to lower the concentration of VOCs. In an approach to optimize energy costs for heating and cooling, part of contaminated return air is sucked into the ventilation system and is recirculated in the premises. Ventilation systems are then provided with a system of air purification to remove contaminants in the recirculated air. For removing gaseous contaminants, the use of traditional adsorption-based air cleaning systems such as activated carbon requires quality maintenance and regular media changes. New oxidation-based purification technologies, such as photocatalytic oxidation and non-thermal plasma, are now available for general ventilation systems. Such technologies can be more energy efficient and may require less maintenance because these systems work continuously in ideal conditions without the accumulation of pollutants affecting their performance. However, there is no standardized protocol to evaluate the effectiveness of these new technologies.

This project aimed at the development of a laboratory evaluation protocol for air purification systems that are directly applied in ventilation systems, using an oxidation process as the main removal mechanism of pollutants in gaseous or vapor form. Dynamic single pass tests were conducted in a test rig which consisted of four identical test ducts with individual flow control. This setup allowed for the simultaneous evaluation of four different air purification systems under identical conditions. Three oxidation-based air cleaning technologies were considered: photocatalytic oxidation (PCO), non-thermal plasma (NTP), and ozonation (O3). The first step was to test the installation to ensure to obtain reproducible results. The second step was to analyze the three technologies simultaneously under various conditions to understand their capacity and to develop a protocol for testing the systems’ air purification capability. The protocol developed in this study was then examined using a full-scale setup complying with the ANSI/ASHRAE Standard 145.2-2016.

Once the test method was examined through repeatability tests, 18 different configurations of oxidation-based air cleaning units were tested in the 4-duct test rig. These include 12 different commercial PCO units, one in-house pilot PCO, 3 plasma and 2 ozonation units (i.e., 430 ppb and 1300 ppb of ozone). Sixteen of them were tested for the removal of 0.1 ppm methyl ethyl ketone (MEK) and their single pass removal efficiency varied from 0 to 37%. Ozonation and PCO using ozone generating vacuum UV lamps generally showed higher efficiency than PCO with non-ozone generating UVC lamps or plasma units. Formaldehyde, acetaldehyde, and acetone were the oxidation by-products detected in MEK testing. PCO-based systems tended to generate more by-products. Based on this test outcome, four units using different technologies were selected: PCO-A for pure PCO technology, PCO-A1 for a combination of PCO and ozonation technologies, O3-A for ozonation, and NTP-A (or NTP-C) for plasma.

The selected four units were then tested for toluene removal at different air velocities. Increasing air velocity reduces the residence time required for oxidation reactions between air contaminants and oxidizing agents generated by the air cleaning systems. Therefore, increasing air velocity showed a decrease in removal efficiency. However, no clear trends were observed in by-product generation rates. It was also found that the performance of PCO-A1 and O3-A technologies was more sensitive to air velocity changes.

The selected units were tested for 6 different VOCs: n-hexane, n-octane, toluene, o-xylene, styrene, and iso-butanol. The test results showed that the removal efficiency and by-product generation patterns of an air-cleaning unit differed substantially depending on the challenge VOC. Styrene and iso-butanol removal efficiency were higher in all devices because they have higher rate constants for the reaction with hydroxyl radical, which is the main oxidizing agent in the considered air cleaning technologies. In comparison to the other VOCs, the removal efficiency of styrene and iso-butanol was more sensitive to the employed air cleaning unit. Therefore, in order to pinpoint which air cleaning system is more effective in removing VOCs, styrene, and iso-butanol could be more ideal as challenge air pollutants. On the other hand, styrene showed a clear interference in ozone monitoring, so it was ruled out.

The effects of challenge gas concentrations in the range of 0.05 to 2 ppm and relative humidity levels between 20 and 60% were also studied for iso-butanol, toluene, and o-xylene. The rapid decrease of removal efficiency in sub-ppm levels indicates that applying high challenge concentration to accelerate air cleaner testing is not suitable. It was also observed that the performances of oxidation-based air cleaning systems tend to be more sensitive to humidity under 40% RH; hence, low humidity conditions should be avoided as the standard test condition. In addition, some selected PCO systems were tested for ozone removal, and commercial PCO systems showed poor ozone removal efficiency.

Based on the test results, a recommended test protocol was developed by aiming at multifaceted performance evaluations considering two different challenge levels (for indoor air quality application and lightly polluted industrial setup), by-product generation, and the effects of residue on air quality. The developed test protocol for full-scale testing has then been applied in 14 commercial in-duct air cleaning units under the recommended test conditions: 7 PCO units, 3 units combining PCO and adsorption media, 2 NTP units, and 2 ozonation units. The test results indicate that the proposed test protocol can capture different characteristics in the performance of the different systems except for the NTP units, which showed a poor performance throughout the study.

Considering large variations in the efficiency of oxidation-based air cleaning systems and their toxic by-product generation, developing a proper standard test method is urgent. The developed test protocol can serve as a starting point or as an interim test method. Commercial non-thermal plasma units tested in this study showed poor performance, which needs to be further investigated. The test results indicate that the efficacy of by-product scrubbers needs to be studied more for optimum design of air cleaning systems.

Additional Information

Category: Research Report
Research Project: 2013-0062
Online since: October 05, 2022
Format: Text