EETD Scientists Develop Effective Approaches for Removing VOCs from Indoor Air

November 29th 2011
SEM images of synthesized catalyst LBNL-100 and of commercial MnO2

Researchers estimate that those of us in developed countries spend 90 percent of our time indoors, which means that most of the time we are breathing air polluted by emissions from indoor sources. Providing more outdoor air ventilation can improve indoor air quality; however, energy is needed to heat, cool, humidify or dehumidify, and sometimes filter the ventilation air brought indoors from outdoors. Studies have shown that about 10 percent of the energy consumed in the U.S. commercial buildings is used to thermally condition ventilation air. To improve a building's energy efficiency, we would like to reduce ventilation rates while maintaining good air quality—or better yet, to do so while improving indoor air quality.

Through their work at the Indoor Environment Department of Lawrence Berkeley National Laboratory's Environmental Energy Technologies Division (EETD), William Fisk, Hugo Destaillats, and Meera Sidheswaran are devising solutions to this challenge. Recently, they have been evaluating two ways to reduce indoor air pollutants without increasing ventilation rates: by developing a synthetic catalyst to reduce indoor formaldehyde concentrations, and by evaluating the effectiveness of activated carbon fiber filters in reducing other volatile organic compound (VOC) concentrations.

Tackling Formaldehyde with a Manganese Oxide Catalyst

Formaldehyde is a common indoor pollutant that the World Health Organization and the U.S. Department of Health and Human Services lists as a human carcinogen. Formaldehyde concentrations in indoor air are routinely above the maximum recommended indoor level, so efforts to improve indoor air quality often target formaldehyde.

"Mean formaldehyde concentrations in a typical U.S. building are about 17 parts per billion," says Destaillats, "although 20 to 50 parts per billion are fairly common." The California Environmental Protection Agency guideline for the maximum recommended long-term-average formaldehyde concentration is 9 parts per billion.

To reduce these formaldehyde concentrations, Fisk, Destaillats, and Sidheswaran developed a catalyst that could be applied to the filters routinely used to remove particles from airstreams. They formed the catalyst samples by co-precipitation of manganese-containing precursors, and cured them at different temperatures to compare their effectiveness. The synthesis resulted in a black powder containing agglomerates of particles smaller than 50 nanometers (nm) in diameter, giving the formaldehyde plenty of surface area with which to react. The research team used porosimetry and surface area analysis; X-ray diffractometry; SEM imaging analysis; and ICP-MS analysis to characterize the catalyst.

"Surface area and porosity are important for good reactivity with the formaldehyde," explains Fisk, "but the manganese oxide can also have a variety of crystal structures and chemical compositions that influence their effectiveness as a catalyst. "

The team applied the catalyst to particle filters, passed air containing formaldehyde and other contaminants through the filters, and measured the formaldehyde removal rates by the treated filter over time. Experiments were performed with various air speeds and with variable humidity. For reference, they also tested a commercially available manganese oxide under the same conditions.

The results have been encouraging. With air velocities typical of those in particle filtration systems, the initial formaldehyde removal efficiency was 80 percent, and even after 2,300 hours of continuous operation, the formaldehyde removal efficiency was approximately 60 percent.

"Typically, particle filters are replaced approximately every 1,500 hours of ventilation system operation, so the catalyst remains effective over the necessary time frame," says Destaillats. "The catalyst is inexpensive enough to be able to deploy it on particle filters and not have to recover the material when the filter is changed out every three or four months," says Fisk.

The synthesized sample that was conditioned at 100°C (LBNL-100) performed significantly better than the commercial sample. In fact, even when the amount of the commercial catalyst used was three times that of the LBNL-100 catalyst, the LBNL-100 catalyst still performed better, showing consistent single-pass formaldehyde removal of > 80 percent—far outperforming the commercial catalyst, which removed only 5 to 10 percent of formaldehyde over four days. Tests currently under way have shown that the LBNL catalyst also removes other VOCs.

Destaillats and Fisk attribute the effectiveness of the LBNL-100 to its higher available surface area, and its intrinsic redox properties seem to contribute to its much-longer effective lifetime, compared to the commercial version. "The synthesized catalyst has a much higher surface area, and a different particle size and chemical composition than the commercial product, which results in superior performance," says Fisk.

In a separate set of experiments, measurement of upstream and downstream formaldehyde and CO2 concentrations showed that mineralization (breakdown of the formaldehyde into CO2 and water) in both experiments reached 100 percent, without formation of formic acid, a potential by-product of an incomplete reaction. "This indicates to us that the catalyst should be able to achieve complete mineralization in buildings with the typical level—tens of parts per billion—of formaldehyde," says Destaillats. Preliminary tests at high velocities indicate that particles from the catalyst are not being entrained into the airstream.

Relative humidity has minimal effect on the effectiveness of the LBNL-100 catalyst. High relative humidity (90 percent) slightly reduced its formaldehyde removal efficiency, but its efficiency returned once relative humidity levels were reduced.

The 60 to 80 percent formaldehyde removal efficiencies are more than adequate. In fact, even a 20 percent formaldehyde removal efficiency in the supply airstream in a commercial building ventilation system could counteract the expected indoor formaldehyde increases associated with a 50 percent reduction in minimum outdoor air supply.

In some commercial buildings, this formaldehyde control could, by itself, enable energy-saving reductions in ventilation rates. However, in many situations the catalyst-treated filters would need to be supplemented by air cleaning systems or by pollutant source control measures for other pollutants. That approach could result in improved indoor air quality and simultaneous, significant ventilation energy savings. One very promising approach is to use activated carbon fiber air cleaners to remove other air pollutants.

Activated Carbon Fiber (ACF) Filters Prove Effective in Removing VOCs

While formaldehyde is a key indoor air pollutant, high concentrations of other VOCs in indoor spaces also pose health risks and inhibit the ability to decrease ventilation rates and energy use. In a separate study focusing on reducing ventilation and energy consumption while maintaining or improving indoor air quality, Fisk, Destaillats, and Sidheswaran evaluated the use of a commercial activated carbon fiber (ACF) media as a filter for cleaning air in heating, ventilating, and air conditioning (HVAC) systems.

Volatile organic compounds in the air flow adsorb to the ACF filter, removing them from the indoor air. To create space on the filter for more VOCs to adsorb, the VOCs must be desorbed from the filter periodically and exhausted outdoors—a process known as "regeneration." The research team studied three difference regeneration methods for the filters, using outdoor air under ambient conditions, with humidified air, and with the filter or regeneration air heated. The best performance occurred when the ACF filter was regenerated for 15 minutes once every 12 hours using air heated to 150°C. The air flow during regeneration is only 1 percent of the airflow during the 12 hour period of air cleaning, so only a very small amount of air must be heated, and the amount of energy required for regeneration is small.

The research team studied ACF system performance with mixtures of VOCs, with VOC properties ranging from those of formaldehyde (with a molecular weight of 30 and a boiling point of -21°C) to undecane (with a molecular weight of 156 and a boiling point of 196°C). For all VOCs other than formaldehyde, the time averaged VOC removal efficiency was above 70 percent. The efficiency of formaldehyde removal was approximately 20 percent. However, using a double layer of the ACF cloth, the efficiency of formaldehyde removal jumped to 40 percent, and the efficiency for other VOCs exceeded 90 percent. The ACF system imposed a low airflow resistance, so the system will have only a minor impact on fan energy use.

Modeling indicates that the combination of ACF air cleaning and a 50 percent reduction in ventilation can decrease indoor concentrations of VOCs by 60 to 80 percent and reduce formaldehyde concentrations by 12 to 40 percent. Thus, the system reduces exposures to VOCs and formaldehyde, while allowing the ventilation rate to be cut in half to save energy

"Energy modeling indicated the potential to reduce the energy required for heating and cooling of ventilation air by 35 percent to almost 50 percent," says Sidheswaran.

Ongoing Work Looks at Associated Issues

Fisk, Destaillats, and Sidheswaran continue to evaluate both solutions; looking at the catalyst's lifespan, the effect of other VOCs on its effectiveness, and its ability to remove other VOCs, as well as a combined approach that uses both the LBNL catalyst and the ACF filter.

"We're really pleased with the results on both products so far," says Fisk. "Often when you work on something like this, things go wrong, but overall, our results here have been very satisfying. "

This research was funded by the Department of Energy's Office of Energy Efficiency and Renewable Energy.

Mark Wilson