IAQVEC 2016, 9th International Conference on Indoor Air Quality Ventilation & Energy Conservation In Buildings
Evaluation of a new thermal-regenerative air purifier for indoor
formaldehyde removal
Ru XIAO1,2, Jinhan MO1,2,*, Lin LIN1, Yuchen SHI1, Jiaqi SUN1
1
2
Department of Building Science, Tsinghua University, Beijing, China
Beijing Key Laboratory of Indoor Air Quality Evaluation and Control, Beijing, China
*
Corresponding email: [email protected]
ABSTRACT
Formaldehyde is a common indoor pollutant which has been named as carcinogenic to humans
by International Agency for Research on Cancer. This work aims to develop a new thermalregenerated air purifier to remove indoor formaldehyde for long-term applications. There are
four working modes in the air purifier: cleaning mode, thermal-regeneration mode, exhaust
mode and fresh air in-take mode. A small scale prototype purifier with commercial activated
carbon as adsorbent was developed and the modes were controlled by switching five valves
through a programmable logic controller (PLC). Experimental results indicate that the
formaldehyde removal efficiency of the prototype remained stable after 3 cycles of adsorption
and desorption. Compared with traditional purifiers without thermal regeneration, the proposed
purifier has longer lifespan for indoor formaldehyde removal. Moreover, the energy
consumption for thermal regeneration is small and acceptable.
KEYWORDS
Indoor air quality; Formaldehyde; Air cleaning; Adsorption; Desorption
INTRODUCTION
More than 65% of formaldehyde produced is used to synthesize resins used in wood-based
construction materials (Tang et al., 2009), which is therefore a main source of indoor
formaldehyde pollution (Bohm et al., 2012). In past decades, China has been experiencing rapid
urbanization and modernization. The urban residential building area grew from 4 to 21 billion
m2 from 1990 to 2010 (National Bureau of Statistics of China, 2010) and the production of
synthetic wood-based decorating materials was more than 150000 km2 in 2010 (Zhang et al.,
2013) , which leads to severe formaldehyde pollution in Chinese homes, especially in newly
built homes. Yao et al. (2005) had measured the formaldehyde concentrations in newly-built
homes in six Chinese cities in summer and winter. The average formaldehyde concentrations
in the six cities were between 200 to 500 μg/m3, which were 3 to 6 times greater than China
indoor formaldehyde standard level, 80 μg/m3 (GB/T 18801, 2002). Furthermore, formaldehyde
has adverse health impact on people and has been named as carcinogenic to humans by
International Agency for Research on Cancer (IARC, 2016). Du et al. reported that
formaldehyde was the top one cancer risk contributors among various indoor volatile organic
compounds inhaled for both males and females in urban China(Du et al., 2014). Therefore,
effective technologies for formaldehyde removal are in great need in urban China homes.
Common technologies for indoor air purification are adsorption, catalytic oxidation,
photocatalytic oxidation (PCO), and non-thermal-plasma (NTP). Catalytic oxidation, PCO and
NTP have problems of material poisoning and may produce potentially harmful intermediates
and by-products (Zhang et al., 2011). Besides, little is known about the practical PCO and NTP
air cleaning performance in real situations (Gallego et al., 2013). In contrast, adsorption
technology using activated carbon (AC) is cheap, stable and won’t produce secondary
pollutions, which is the most widely used to remove gas-phase contaminants in buildings
(VanOsdell et al., 1996). Some researchers had measured the adsorption performance of
activated carbon for formaldehyde (Chen et al., 2005a; Jing et al., 2008; Rengga et al., 2013;
Lee et al., 2013). However, most of the studies were finished in laboratory and using much
higher formaldehyde concentrations than those in real dwellings. Moreover, due to the low
adsorption capacity, the life span of activated carbon for formaldehyde is short, which limits
the application of activated carbon in indoor formaldehyde removal.
In this study, a new thermal-regenerated air purifier was developed to remove indoor
formaldehyde for long-term applications. The performance of the new air purifier was evaluated
for sub-ppm formaldehyde removal.
METHODS
Principal of the new thermal-regenerative air purifier
(a) Cleaning mode
(b) Regeneration mode
(c) Exhaust mode
(d) Fresh air in-take mode
Figure 1. Schematic of the thermal-regenerative air purifier
Figure 1 shows the configuration of the new thermal-regenerative air purifier, which is installed
on the inner wall of homes and can achieve four modes by switching five valves, K1 - K5. The
arrows with different colors in Figure 1 represent the air flow paths. (a) Cleaning mode, valves
K1 and K2 are opened and valves K3, K4 and K5 are closed. Indoor polluted air is driven into
the purifier and passes through the adsorption filter and fan, respectively. The adsorption filter
with activated carbon removes the indoor formaldehyde; (b) Thermal-regeneration mode,
valves K3 and K4 are opened and valves K1, K2 and K5 are closed. The inlet and outlet of the
air purifier are closed and the air inside the thermal-insulated purifier will continuously cycle
and is heated up to 80 °C by an electrical heater, which will results in the desorption of adsorbed
formaldehyde from the adsorption filter; (c) Exhaust mode, valves K2, K3 and K5 are opened
and valves K1 and K4 are closed. The air inside the purifier will be exhausted out after
completely desorption; (d) Fresh air mode, valve K1, K4 and K5 are opened and valve K2 and
K3 are closed. Outdoor fresh air is introduced indoor to dilute other indoor pollutants such as
carbon dioxide, and improve indoor air quality. All the four working modes are controlled by a
programmable logic controller (PLC).
Prototype set-up
A small-scale prototype purifier was produced with the basic parameters listed in Table 1.
Table 1. Basic parameters of the prototype.
Inner volume
0.036 m3
Inner size
Sealing material
0.2 m × 0.3 m × 0.6 m
Adsorbent material
1.5 kg activated carbon
Airflow rate
20 m³/h
Face velocity through filter
0.19 m/s
Power of the heater
100 W
Polystyrene
Performance evaluation of the prototype
The formaldehyde removal performance of the prototype purifier was measured in a 3m3 airtight glass chamber (1.28 m ×1.28m × 1.88 m). A fan was placed inside the chamber to mix the
air inside the chamber. The temperature and relative humidity inside the chamber were
controlled at 20 ºC and 20%, respectively. A micro-syringe-pump based generator using
formalin solution with 37% formaldehyde by mass was located in the chamber to generate
initial formaldehyde concentrations or constant emission rates. The concentration of
formaldehyde in the chamber was real-time monitored by a gas analyzer (INNOVA 1312).
INNOVA 1312 has been calibrated using MBTH (3-methyl-2-benzothiazolinonehydrazone
hydrochloride) spectrophotometry method with UV spectrophotometer (Unico, WFJ7200).
The performance evaluation experiment was carried out in four repeated cycles, each cycle is
consisted of 4 stages.
Firstly, the purifier was switched to “cleaning mode”. The initial CADR (Clean Air Delivery
Rate) and single-pass efficiency at each cycle were measured by a “Pull-down” test procedure
(Chen et al., 2005b). The micro-syringe-pump based generator rapidly evaporated formalin into
the chamber with the rate of 2 µL/min for 2 min. After the concentration of formaldehyde in
the chamber kept stable (roughly 0.5 mg/m3), the prototype purifier was turned on to purify the
air in the chamber. The concentration of formaldehyde in the chamber was continuously
measured for 60 min.
Secondly, the formaldehyde removal performance of the purifier was aged by continuously
absorbing formaldehyde. In this period, the purifier was kept at cleaning mode, while formalin
was evaporated with a constant rate of 0.8 µL/min for 12 hours. Another “pull-down” test was
performed to obtain the CADR and single-pass efficiency of the purifier after aging period.
Thirdly, the purifier was switched to “thermal-regeneration mode”. The air inside the purifier
was heated up to 80 ºC and then passed through activated carbon module, which resulted in the
desorption of formaldehyde from activated carbon. The desorption period lasted for 8 hours
with a total heating energy consumption of 0.4 kWh.
Fourthly, the purifier was switched to “exhaust mode”. The hot and high-polluted air inside the
purifier was exhausted out for 30 min. The regenerated activated carbon was cooled down
during the exhaust period.
These 4 stages took place in order during each cycle. After the exhaust period in the i# cycle,
the initial “Pull-down” test in the i+1# cycle begin. Thus, the performance in the first stage of
the i# cycle could represent the adsorption performance of the adsorbent after i-1 times of
adsorption-desorption cycles.
Mass conservation of formaldehyde in the chamber
Assuming that the air is well mixed in the chamber and other formaldehyde removal
mechanisms such as surface deposition and chamber leakage, can be characterized by a firstorder rate constant kn, the mass conservation of formaldehyde in each “pull-down” test can be
written as follows:
𝑉
d𝐶
= −(𝑘𝑛 𝑉 + 𝐶𝐴𝐷𝑅)𝐶 = −(𝑘𝑛 𝑉 + 𝑄𝜂)𝐶
𝑑𝑡
(1)
Where, V is the volume of the chamber [m3]; C is the average formaldehyde concentration in
the test chamber [mg/m3]; kn is the formaldehyde concentration decay rate without air purifier
operating [h-1], which was measured as 0.001 h-1 in this study; Q is the volumetric flow rate
through the purifier [m3/h]; η is the single-pass efficiency of purifier [%].
Assuming that CADR does not change during the test period, an analytical solution can be
obtained from Equation (1) as:
𝐶 = 𝐶0 exp [− (𝑘𝑛 +
𝑄𝜂
) t] = 𝐶0 exp[−(𝑘𝑛 + 𝑘𝑒 )t]
𝑉
(2)
Where, C0 is the initial formaldehyde concentration in the chamber [mg/m3]; ke is the total
formaldehyde concentration decay rate with air purifier operating [h-1].
CADR can be determined by linear regression of ln(C/C0) vs. t from measured concentration
decay curve, and the single pass efficiency can be calculated as:
𝜂=
𝑉(𝑘𝑒 − 𝑘𝑛 )
𝑄
(3)
RESULTS AND DISCUSSIONS
Figure 2 shows the adsorption performance in different periods of the first cycle and the first
stage of the second cycle. The initial removal efficiency for formaldehyde was 34.92%, and
CADR was 6.99 m³/h, which was relatively low due to the low air flow rate in the prototype.
After the aging period, the concentration of formaldehyde in the chamber continuously grew
up, indicating that the activated carbon in the purifier had reached saturation and the adsorbed
formaldehyde during aging period were released out in the “pull-down” test. However, after
the desorption period, it is found that the decay curve of formaldehyde was almost the same
with the initial test, which meant a full recovery of the adsorption capacity. The results prove
that the desorption process can effectively and totally remove adsorbed formaldehyde from
activated carbon.
Figure 3 shows the adsorption performance of the “pull-down test” for 4 cycles in the
experiment. It was found that the CADR and the removal efficiency remained stable after 3
adsorption and desorption cycles. From Figure 2 and Figure 3, we can conclude that the
purifier can effectively achieve self-regeneration by the adsorption and desorption cycle. The
thermal-regenerative process will greatly extend the life span of the purifier.
Concentration( mg/m3)
1.00
0.80
cycle 1#, adsorption period
热再生前
0.60
cycle 1#, after aging period
已饱和
cycle 2#, adsorption period
脱附完全后
0.40
0.20
0.00
0
10
20
30
40
50
60
time (min)
-0.20
Figure 2. “Pull-down test” performance in different periods of cycle 1# and cycle 2#
9
50
8
7
40
6
5
30
4
20
3
CADR (m³/h)
removal efficiency (%)
10
60
2
10
1
0
0
cycle1
cycle2
CADR
cycle3
cycle4
EFFICIENCY
Figure 3. Comparison of the adsorption performance in the first stage of different cycles
Considering the energy consumption in the desorption period, we calculated the extra electricity
consumption compared with purifiers without the function of thermal-regeneration. Common
commercial activated carbon module for formaldehyde removal is recommended to replace
every three months. We assume that the desorption process of this thermal-regenerative purifier
repeats once a month. Thus the extra electricity consumption for the desorption period will be
4.8 kWh per year. We can expect that the energy consumption is also small even for large scale
air purifier (with high CADR value, more activated carbon).
ACKNOWLEDGEMENT
The research was supported by the Natural Science Foundation of China (Nos. 51478235 and
51521005) and Beijing Municipal Science & Technology Commission (No.
Z151100001515007).
CONCLUSIONS
In this study, we develop a new thermal-regenerated air purifier with indoor formaldehyde
cleaning and self-regeneration function. We choose activated carbon as the adsorbent for
formaldehyde and the performance of the prototype is proved to be stable after 3 cycles of
adsorption-desorption process. The preliminary results indicate that the new purifier has a good
application prospect for indoor formaldehyde removal. Further evaluation for the real-scale
prototype is ongoing.
REFERENCES
Bohm M., Salem M.Z.M. and Srba J. 2012. Formaldehyde emission monitoring from a variety
of solid wood, plywood, blockboard and flooring products manufactured for building
and furnishing materials, Journal of Hazardous Materials, 221, 68-79.
Chen W., Zhang J.S. and Zhang Z. 2005a. Performance of Air Cleaners for Removing Multiple
Volatile Organic Compounds in Indoor Air, ASHRAE transactions, 111.
Chen W., Zhang J.S. and Zhang Z. 2005b. Performance of air cleaners for removing multiple
volatile organic compounds in indoor air, ASHRAE Transactions, 111, 1101-1114.
Du Z.J., Mo J.H. and Zhang Y. 2014. Risk assessment of population inhalation exposure to
volatile organic compounds and carbonyls in urban China, Environment international,
73, 33-45.
Gallego E., Roca F.J., Perales J.F. and Guardino X. 2013. Experimental evaluation of VOC
removal efficiency of a coconut shell activated carbon filter for indoor air quality
enhancement, Building And Environment, 67, 14-25.
Gb/T 18801 ( 2002) Indoor air cleaner
Iarc. 2016. Agents classified by the IARC monographs, Volumes 1-115, World Health
Organization,
International
Agency
for
Research
on
Cancer.
http://monographs.iarc.fr/ENG/Classification/latest_classif.php, 6.
Jing L., Zhong L., Bing L., Qibin X. and Hongxia X. 2008. Effect of relative humidity on
adsorption of formaldehyde on modified activated carbons, Chinese Journal of
Chemical Engineering, 16, 871-875.
Lee K.J., Miyawaki J., Shiratori N., Yoon S.-H. and Jang J. 2013. Toward an effective adsorbent
for polar pollutants: Formaldehyde adsorption by activated carbon, Journal of
hazardous materials, 260, 82-88.
National Bureau of Statistics of China. 2010. China Statistical Yearbook 2010, National Bureau
of Statistics of China: Beijing, China.
Rengga W., Sudibandriyo M. and Nasikin M. 2013. Adsorption of Low-Concentration
Formaldehyde from Air by Silver and Copper Nano-Particles Attached on BambooBased Activated Carbon, International Journal of Chemical Engineering and
Applications, 4, 332.
Tang X., Bai Y., Duong A., Smith M.T., Li L. and Zhang L. 2009. Formaldehyde in China:
Production, consumption, exposure levels, and health effects, Environment
International, 35, 1210-1224.
Vanosdell D.W., Owen M.K., Jaffe L.B. and Sparks L.E. 1996. VOC removal at low
contaminant concentrations using granular activated carbon, Journal Of the Air & Waste
Management Association, 46, 883-890.
Yao X.-Y., Wang W., Chen Y., Zhang W., Song Y. and Liu S. 2005. Seasonal change of
formaldehyde concentration in the air of newly decorated houses in some cities of China,
Journal of Environment and Health, 22, 353-355.
Zhang Y.P., Mo J.H., Li Y.G., Sundell J., Wargocki P., Zhang J., Little J.C., Corsi R., Deng Q.
and Leung M.H. 2011. Can commonly-used fan-driven air cleaning technologies
improve indoor air quality? A literature review, Atmospheric Environment, 45, 43294343.
Zhang Y.P., Mo J.H. and Weschler C.J. 2013. Reducing health risks from indoor exposures in
rapidly developing urban China, Environmental Health Perspectives (Online), 121, 751.