Energy efficient indoor VOC air cleaning with activated carbon fiber (ACF) filters
This study explores the potential environmental and energy benefits of using activated carbon fiber (ACF) filters for air cleaning in HVAC systems. The parallel aims for the air cleaning system were to enable reduced indoor exposures to volatile organic compounds (VOCs) and to simultaneously allow reduced rates and energy consumption for outdoor-air ventilation. We evaluated the use of ACF media to adsorb VOCs from indoor air during repeated simulated 12-hour to 24-hour periods of occupancy. In a cyclic regeneration process, VOCs were desorbed from the ACF media and vented outdoors to enable the next cycle of air cleaning. The VOC removal efficiency of the ACF media was measured using a 9.5-cm2 ACF specimen exposed to a mixture of VOCs that included toluene, benzene, o-xylene, 1-butanol, limonene, undecane and formaldehyde at 29 °C and 30% relative humidity. The concentrations of these model pollutants upstream of the ACF media were in the range 20 to 30 ppb, to simulate realistic conditions. Velocities through the ACF media were typical of those in normal particle filter systems (~0.5 m s-1). Initial tests were conducted to develop a modified multi-component Freundlich isotherm and estimate the maximum adsorption capacity of the media, which was determined to be 90 mg VOC per gram of ACF. Three different ACF regeneration methods were explored using relatively cleaner outdoor air under ambient conditions, with this air humidified, and with the filter heated. It was found that heating the ACF media to ~ 150 °C by circulation of a DC current through the fibers for a short period (15 minutes) yielded the best VOC removal results, allowing for subsequent consistent removal efficiencies of 70 – 80% for most VOCs. Regeneration with unheated outdoor air was also effective and used less energy (subsequent removal efficiency was 50 – 60% for most VOCs). ACF did not perform as well in eliminating formaldehyde, for which a maximum removal of 25-30% was achieved with heated regeneration. A mass balance model indicated that the combination of ACF air cleaning and a 50% reduction in ventilation will decrease indoor concentrations of VOCs by 60% to 80% and reduce formaldehyde concentrations by 12% to 40%. Energy modeling indicated the potential to reduce the energy required for heating and cooling of ventilation air by 35% to almost 50%.