Performance of a CO2 sorbent for indoor air cleaning applications: Effects of environmental conditions, sorbent aging, and adsorption of co-occurring formaldehyde

Indoor air cleaning systems that incorporate CO2 sorbent materials enable HVAC
load shifting and efficiency improvements. This study developed a bench-scale experimental
system to evaluate the performance of a sorbent under controlled operation
conditions. A thermostatic holder containing 3.15 g sorbent was connected to a
manifold that delivered CO2-enriched air at a known temperature and relative humidity
(RH). The air stream was also enriched with 0.8-2.1 ppm formaldehyde. The CO2
concentration was monitored in real-time upstream and downstream of the sorbent,
and integrated formaldehyde samples were collected at different times using DNPHcoated
silica cartridges. Sorbent regeneration was carried out by circulating clean air
in countercurrent. Almost 200 loading/regeneration cycles were performed in the
span of 16 months, from which 104 were carried out at reference test conditions
defined by loading with air at 25°C, 38% RH, and 1000 ppm CO2 and regenerating
with air at 80°C, 3% RH and 400 ppm. The working capacity decreased slightly from
43-44 mg CO2 per g sorbent to 39-40 mg per g over the 17 months. The capacity increased
with lower loading temperature (in the range 15-35°C) and higher regeneration
temperature, between 40 and 80°C. The CO2 capacity was not sensitive to the
moisture content in the range 6-9 g/m3 and decreased slightly when dry air was used.
Loading isothermal breakthrough curves were fitted to three simple adsorption models,
verifying that pseudo-first-order kinetics appropriately describes the adsorption
process. The model predicted equilibrium capacities decreased with increasing temperature
from 15 to 35°C, while adsorption rate constants slightly increased. The
formaldehyde adsorption efficiency was 80%-99% in different cycles, corresponding
to an average capacity of 86 ± 36 μg/g. Formaldehyde was not quantitatively
released during regeneration, but its accumulation on the sorbent did not affect CO2
adsorption.