Environ. Sci. Technol. 1985, 79, 1059-1065
Wall Loss of Gaseous Pollutants in Outdoor Teflon Chambers Daniel Grosjean
Daniel Grosjean and Associates, Inc., Suite 645, 350 N. Lantana Street, Camarillo, California 93010
rn Loss rates have been measured for -20 gaseous pollutants in FEP Teflon chambers. Loss rates were 1(1-4) X min-’ for toluene, 0-cresol, benzaldehyde, biacetyl, pyruvic acid, SO2, and methyl nitrate. Higher loss rates, (10-20) X lo4 m i d , were measured for ammonia, benzoic acid, and nitric acid. Ozone loss rates in dry air were (0.5-3.0) X min-l in the dark and (2-8) X min-’ in sunlight and increased in humid air. These loss rates are consistent with collision yields of 1 2 X (number of molecules lost per collision on the Teflon surface). Loss by chemical reactions cannot be ruled out for the most reactive compounds studied. For oxides of nitrogen, the measured loss rates reflected the competing pathways of thermal oxidation of NO (dark), photolysis of NO2 (sunlight), loss of NO, and loss of products (e.g., HON02)to the walls, and heterogeneous hydrolysis of NO2. The latter pathway contributes to the production of free radicals, with k(N02 + wall HONO) = (1.5-4.0) X min-’ under our conditions.
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Introduction Environmental chambers (commonly referred to as smog chambers) have been extensively used in atmospheric chemistry studies (e.g., see ref 1-5). In fact, most of the experimental information concerning hydrocarbon photochemistry, ozone production, and aerosol formation in urban air has been derived from smog chamber studies of “model” systems such as single hydrocarbon-NO, mixtures in purified air. Experimental results obtained in smog chambers are also used extensively for the formulation, testing, and validation of increasingly detailed computer kinetic models (e.g., see ref 6-8) describing the gas-phase chemistry of atmospheric pollutants. As for any other laboratory instrument, smog chambers are not without shortcomings and limitations. Wall effects, including film contamination, heterogeneous reactions on the chamber walls, and release of reactive species including free radicals and their precursors have been the object of several reports (9-12). Wall-promoted processes are obviously related to the extent of gas-phase pollutant loss to the chamber walls. In a typical example, a net flux of free radicals may result from a sequence of processes involving diffusion of NO2to the walls, reaction of NO2with water on the wall to form nitrous and nitric acids, release of volatile nitrous acid from the wall to the gas phase, and subse uent photolysis of nitrous acid to form hydroxyl radica s. While the importance of these heterogeneous effects is now well recognized, there is a paucity of data regarding the loss rates of organic and inorganic pollutants in environmental chambers. For example, current computer kinetic models take only into account measured or estimated loss rates for ozone and, in some instances, oxides of nitrogen. A more comprehensive assessment of wall loss rates for gaseous and particulate pollutants is critical to, for example, the validation of computer kinetic models, the derivation of carbon, sulfur, and nitrogen mass balances in chemically reactive mixtures, and the determination of organic aerosol formation rates from precursor hydrocarbons. In the course of recent studies of organic aerosol formation from olefins (13),aromatic hydrocarbons (14))and
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organosulfur compounds (15),we have measured loss rates for a number of pollutants in outdoor environmental chambers constructed from fluorinated ethylene-propene copolymer film (FEP Teflon). While the chamber facility we employed no longer exists, our results are applicable to other Teflon chambers, a number of which are currently being employed for atmospheric chemistry and physics studies (e.g., see ref 16 and 17). While the emphasis of this report is on gas-phase pollutants, detailed studies of aerosol losses have also been carried out and are described elsewhere (18, 19).
Experimental Section The outdoor chamber facility and its dedicated instrumentation have been described in detail elsewhere (13-15), and only a summary of information relevant to this study is given below. Outdoor chambers were constructed from panels of FEP 200A Teflon film heat sealed together, with the seams externally reinforced with 5 cm wide strips of plastic tape (mylar). Large chambers were constructed from 10 panels, each 9.14 X 1.22 m, with a total wall area of 111.6 m2. For a typically initial volume of -80 m3 (fully inflated chamber), the initial surface to volume ratio was -1.4 m-l. Smaller chambers were constructed from four panels, each 3.05 X 1.22 m, with a wall area of 14.9 m2. For a typical initial volume of 3.9 m3, the initial S/V was 3.8 m-l. Newly constructed chambers were first “conditioned” by sunlight irradiation for 6-10 h of purified air containing 1-5 ppm of ozone followed by purging several times with purified air. This protocol has been shown to minimize, in subsequent runs, contamination due to organic impurities desorbing from the Teflon film (9). Following this initial “conditioning” protocol, the chambers were purged at least once with purified air after each experiment, and sunlight irradiations of purified air were carried out at regular intervals. Detailed studies for a number of pollutant mixtures (20)have shown that “memory”effects, i.e., the possible retention of a fraction of the reactants and products from one run to the next, were reduced to negligible levels by using this cleanup protocol. In a typical experiment, a test atmosphere was prepared by injecting the pollutant of interest into the chamber containing purified air provided by an Aadco 737-14 air purification system. Stability studies were conducted in the dark (chamber covered with opaque plastic film which removed 299% of the incident sunlight) and in sunlight (opaque cover removed after injection and mixing). Measurement methods and other pertinent information are summarized in Table I. Temperature (YSI-741-A-10 sensor), dew point (EG&G 880-C-1 hygrometer), and sunlight intensity (Eppley ultraviolet radiometer) were also monitored. While most experiments were conducted with dry matrix air (typically dew point = -20 to -16 OC at T = 18-26 “C), the stability of ozone, NO,, and ammonia was also studied in humid air. Compounds that photolyze rapidly in sunlight such as pyruvic acid (27) and other carbonyls were obviously tested only in the dark. While most runs were carried out for 4-6 h, shorter (- 2 h) and longer runs (-10-12 h, overnight) were also included. Control experiments were also carried out. The matrix air provided by the purified air generator contained low
0 1985 American Chemical Society
Environ. Sci. Technol., Vol. 19, No. 11, 1985
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Table I. Experimental Conditions compound ozone nitric oxide nitrogen dioxide sulfur dioxide ammonia
source/preparation Welsbach generator lecture bottle gas cylinder, 100 ppm in Nz gas cylinder, 50 ppm in Nz lecture bottle
nitric acid
air stream through diluted aqueous solution n-butane d n-pentane d styrene d P-methylstyrene d o-cresol d benzaldehyde d biacetyl d pyruvic acid d peroxyacetyl nitrate synthesized (23) (PAN) methyl nitrate synthesizedh benzoic acid d
injection diluted output b b b b direct
measurement method ultraviolet photometry, Dasibi 1003-AHa chemiluminescence, Teco 14B/E4 chemiluminescence, Teco 14B/En pulsed fluorescence, Monitor Labs 8550a collection on oxalic acid impregnated filters, ion chromatography chemiluminescence, Teco 14B/E,' and collection on nylon filters/ion chromatography (21) gas chromatography-flame ionization detection (GC-FID) (22) GC-FID (22) GC-PID' GC-PID' gas chromatography-photoionization detection (GC-PID) (14) GC-PID' gas chromatography-electron capture detection (GC-ECD)" collection in impingers, ion chromatography (23) GC-ECD (24),also chemiluminescencec
b b b b b b b b diluted output, or prepared in situg b GC-ECD" prepared in situ' collection in KOH impingers, ion chromatography with ultraviolet detection (25)
Calibrated according to U.S. EPA approved methods involving NBS-traceable standards. Diluted with pure nitrogen in 200-cm3 glass bulbs. Instrument response to nitric acid and to PAN is quantitative in NOz mode of instrument. Commercial source, purity ?98%, used without further purification. 'On Teflon, TCEP, or S P 1240 packed columns. fSame column and conditions as for PAN. #From irradiated chlorine-acetaldehyde-NO, (24), propene-NO,, or biacetyl-NO, mixtures. Synthesized according to ref 26. I From styrene-ozone or pmethylstyrene-ozone mixtures in the dark. (I
Table 11. Summary of Pollutant Loss Rates in Outdoor Teflon Chambers" no. of runs
range of initial concn, ppb
ozone
14 8
NOc NOz? NO,
11 11 11 3 7 2 3 1 4 2
10-1280 340-1450 6-590 4-460 10-710 200-500 100-420 125 25-100 130 300-1100 500-1000 500 900-1100 500-1000 25 50-400 200-400 50-500 15-400
compound
so; 3"
nitric acid n-butane n-pentane toluene styrene p-methylstyrene o-cresol benzaldehyde benzoic acid biacetyl pyruvic acid methyl nitrate PAN
1
4 3 1 3 4 3 8
chamber sizeb L S L, s
L, s
L, s S S
S
L, s
S
L, s S
S S
S S S S S
loss rate, x ~ O -min-' ~ dark sunlight 1.3 k 0.9 3.4 f 1.4 0.4 f 4.15 -1.6 f 4.1d 0.7 f 1.1 1.3 A 0.3 4-17' 15 f 1 12.d 12.6 52.51 52.d 12.9 13.d 3.4 f 1.7 10.8 52.d 52.d 10.8' 54.3h
5.1 f 2.8 6.7 f 2.2 0.5 f 1.1 3.9 f 3.6 2.6 f 1.7
12.d s2.@ 12.51 52.d NAf NAB NAg
3.9 f 1.6h L, s In dry air (dew point = -20 to -16 "C at T = 18-26 "C) unless otherwise indicated. S = small (V = 3.9 m3, initial S / V = 3.8 m-l); L = large (V = 80 m3, initial S / V = 1.4 m-l). ?In mixtures of NO and NOz; see Table IV for details. dNet production of NOz in some runs by thermal oxidation of NO; see text. eIncludes runs with humid air (-55-50% RH). f N o measurable loss was observed. The upper limit listed was calculated from the precision of the measurements and the length of the run. ENA = not applicable; compound photolyzes in sunlight. h I n the presence of NOz.
levels of oxides of nitrogen (typically 6-12 ppb) and of non-methane hydrocarbons [ 1200 "C) for SO2 control is not optimal, and (2) a significant increase in sorbent reactivity will be attained by calcination at temperatures below 1200 "C, the lowest value assessed in the present experiment.
Introduction The injection of pulverized limestones into the radiant section of a pulverized-coal-fired boiler has been proposed as a method for the control of sulfur dioxide (SO,) emissions through the EPA-defined process of limestone injection into multistaged burners (LIMB). Development efforts are in progress to elucidate the principal controlling parameters of SO2capture in a flame environment (1,2). The present paper reports on the first phase of this effort in which the activation and sulfation of pulverized limestones are assessed at the high temperatures (>1100 "C) representative of those found in the combustion zone of commercial coal-fired utility boilers. Limestones injected into a high-temperature, sulfurladen environment follow a series of p.rocesses: heat-up, calcination/activation, sulfation, deactivation, and/or regeneration. Calcination encompasses the process of COP evolution from the sorbent as a result of particle heating. The surface area increases, thereby increasing the number 0013-936X/85/0919-1065$01.50/0
of active sites for SO2 capture. Activation may consist merely of calcination (the escaping COzleaving the particle open for reaction). Sulfation, the active process of SO2 capture, is described by the following general reaction under oxidizing conditions: CaO + SOz + 1/202 CaS04 (1) Although the physical process of eq 1remains unknown, data from several laboratory experiments have been correctly predicted by a model incorporating zero-order intrinsic kinetics (3). Due to diffusional resistances, however, the apparent, or global, reaction order with respect to SO2 has been observed as zero (41, one-half ( 5 ) ,and one (6). Several limestone-sorbent properties-specific surface area, pore size, total porosity, and crystallite size-influence the sulfation reaction rate (7,8). However, the latter three of these properties manifest themselves in the specific surface area. As a result, the specific surface area is a physical property of special interest in evaluating the ability of sorbents to uptake SO2. Deactivation and regeneration of the sorbent also are important. Deactivation is associated with sintering, a process that reduces the active surface area of the material. This may occur thermally or be augmented by interaction with other minerals (e.g., coal ash) that lower the melting point (hence, the Tammann temperature) of the solids. Regeneration is the desorption of sulfur species. This may leave the particle active for sorption but does so at the expense of sulfur release. Regeneration is mainly a concern for fuel-rich capture systems (e.g., low NO, burners) where initial capture occurs by the mechanism CaO + H2S CaS HzO (2) followed by the regeneration reaction CaS + 3/202 CaO + SO2 (3) Early failures of the LIMB process resulted from overburning the sorbents upon high-temperature injection. In addition, the thermodynamics of sulfation (eq 1)becomes unfavorable above about 1200 "C. Many subsequent studies of calcination and sulfation have therefore been limited to nonflame environments and relatively low tem-
0 1985 American Chemical Society
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Environ. Sci. Technol., Vol. 19, No. 11, 1985
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