Hoagland, D. R., Arnon, D. I., “The Water-Culture Method for Growing Plants Without Soil,” California Agr. Expt. Sta. Circ. 347 (revised) (1950). Jackson, M. L., “Soil Chemical Analysis,” Prentice Hall, New York, N.Y., 1958. John, M. K., “Computerized Acquisition and Interpretation of Soils Data,” Agron. Abstr., 1970a, p 138. John, M. K . , Soil Sci., 109, 214-20 (1970b). John, M. K., Enoiron. Lett., 2, 173-9 (1971). John, M. K., Chuah, H. H., VanLaerhoven, C. J., ENVIRON. SCI. TECHNOL., 6, 555-7 (1972). Kropf, R., Geldmacher-v. Mallinckrodt, M., Arch. Hyg., 152, 218-24 (1968). Lagerwerff, J. V., Specht, A. W., ENVIRON. Scr. TECHNOL., 4, 583-6 (1970). Lewis, G. P., Lyle, H., Miller, S., Lancet, 11, 1330-3 (1969). Nandi, M., Jick H., Slone, D., Shapiro, S., Lewis, G. P., Lancet, 11, 1329-30 (1969).
Pionke, H. B., Corey, R. B., Soil Sci. SOC. Amer. Proc., 31, 749-52 (1967). Ross, R. G., Stewart, D. K. R., Can. J. Plant Sci., 49, 49-52 ( 1969). Schroeder, H. A., J. Chronic Dis., 18, 647-56 (1965). Schroeder, H. A,, Balassa, J. J., Hogencamp, J. C., ibid., 14, 236-58 (1961). Schroeder, H. A., Nason, A. P., Tipton, I. H., Balassa, J. J., ibid., 20, 179-210 (1967). Stanford, G., DeMent, J. D., Soil Sci. SOC.Amer. Proc., 21, 612-17 (1957). Tabatabai, M. A., Bremner, J. M., Agron. J., 62, 805-6 (1970). Tsuchiya, K., Keio J. Med., 18, 181-94 (1969a). Tsuchiya, K., ibid., 196913, pp 195-211. Walkley, A., Black, C. A., Soil Sci., 37, 29-38 (1934). Receiced for review September 13,1971. AcceptedJuly 17,1972
Desulfurization of Fuels with Half-Calcined Dolomite: First Kinetic Data Lawrence A. Ruth, Arthur M. Squires, and Robert A. Graff’ Department of Chemical Engineering, The City College of The City University of New York, New York, N.Y. 10031
w Kinetic data are given for the first time for reaction of half-calcined dolomite with H2S (55O-80O0C, 0.005-0.2 atm of H2Sin gas at 1 atm). The reaction can be used to remove H2Sfrom a fuel gas made by gasifying coal with air and steam at high pressure. Gas at 20 atm from a dilute-phase slagging gasifier can be desulfurized in an operation at about 920°C to reduce the SO2emission from power generation to roughly that expected from coal containing 0.15 sulfur. Gas at 20 atm from an ash-agglomerating fluidized-bed gasifier can be desulfurized to an even greater extent in an operation at about 760°C. In light of data given here for 700” and 8OO”C, the reaction can be expected to proceed to completion under these conditions at a faster rate than that atforded by the reaction of fully calcined dolomite, reported earlier. Data at lower temperatures reveal kinetic curiosities which may prove important to understanding of the reverse reaction, useful in regenerating half-calcined dolomite and releasing H,S.
S
quires (1967, 1968) proposed half-calcined dolomite as an agent to capture sulfur from a fuel gas at high pressure and at temperatures greater than about 750°C. At that time, no data were available for the reaction. [CaCO,+MgO]
+ H,S
=
[CaS+MgO]
+ H 2 0 + CO,
(1)
but Squires reasoned that tiny crystallites of CaC0, embedded in a porous matrix along with tiny crystallites of MgO might display good reactivity toward H?S. The present work has confirmed this expectation. Reaction 1 is livelier at 800°C than the reaction of fully calcined dolomite [CaO+MgO]
+ H2S = [CaS+MgO] + H 2 0
To whom correspondence should be addressed.
(2)
reported on earlier (Squires et al., 1971 ; Pel1 et al., 1971). Reaction of pulverized limestone with H2S is very slow at 800°C. Equilibrium for Reaction 1 is a strong function of temperature, and the reverse reaction [CaS+MgO]
+ HSO + COS = [CaCO,+MgO] + H,S
(3)
can be used at about 600°C to regenerate half-calcined dolomite and to release H2Sfor conversion to elemental sulfur. At 550” and 600”C, the rate of Reaction 1 drops sharply after partial conversion. A high partial pressure of COz or H 2 0 cancels this effect and allows the reaction to proceed to completion at a good rate. A brief burst of oxygen restores reactivity. An understanding of these kinetic curiosities may prove important to the study of Reaction 3. The City College is studying the kinetics of Reactions 1-3 with the object of contributing toward development of clean power systems in which coal or oil would be either gasified or pyrolyzed at high pressure in fuel-treating processes which are satellite to power generation by gas and steam turbines. In such processes, sulfur in the fuel would be converted to H&, which would then enter either Reaction 1 or 2. A modification of the Du Pont 950 Thermogravimetric Analyzer has adapted this instrument for study of the reaction of a corrosive gas, HiS. The good reactivity of half-calcined dolomite makes it attractive for use in a panel bed filter (Squires et al., 1970) which removes both dust and sulfur from fuel gas produced by gasifying coal or oil at high pressure. Reaction Conditions
Squires et al. (1971) gave the equilibrium for Reaction 1: loglo [HzO] [CO,] P/[H&]
=
7.253 - 5280.5/T
where [ . . .] signifies mole fraction; P is the pressure, atm: and T i s temperature, OK. In a clean power process in which Reaction 1 is used to remove sulfur from a fuel gas at high pressure, the equilibrium is generally favorable for absorption of sulfur only at high temperatures, above about 750°C. Volume 6, Number 12, November 1972
1009
THERMOCOUPLE SAMPLE PAN
\
r
7
f
BALANCE HOUSING ,-QUARTZ
BEAM
BAFFLE1
_ _ _REACTION CAS PURGEGAS
Figure 1. Modification of Du Pont TGA to separate reaction zone from balance
Work is in hand to develop the capability of studying Reactions 1-3 at high pressure, but studies to date have been made at atmospheric pressure. Study of Reaction 1 necessitates imposing a sufficient partial pressure of COz to prevent dissociation of CaCO,, and for this reason Reaction 1 has not yet been studied at temperature greater than 800°C. Data were obtained for Reaction 1 at 700" and 800°C to provide insight into the capability of Reaction 1 for desulfurizing fuels at these and higher temperatures. Reaction 1 has been studied at lower temperatures to provide information useful for later study of Reaction 3, for which Reaction 1 is the back reaction. Equipment
Since Reaction 1 produces a weight change, a thermogravimetric analyzer can provide differential kinetics for reaction of a sample of solid spread as a fine powder in a thin layer on a balance pan, the gaseous reactants flowing past the solid at a rate sufficient to preclude any substantial change in a known gas composition. In the Du Pont TGA, which we used for this purpose, gases flow horizontally and past a sample pan which is suspended from a quartz beam at the center of a furnace. As supplied, the Du Pont TGA cannot handle corrosive H& which would damage the balance. Figure 1 depicts our modification of the unit. A nitrogen purge flows through the balance housing, and a modified quartz furnace tube and baffle provide a separation between balance and reaction zone. The modification has two purposes: to keep corrosive H2S from the balance, and to keep purge nitrogen away from the solid sample, so that the composition of gas in the reaction zone is not affected. In the modification, purge gas enters the end of the balance bell jar and flows through a slot in the balance housing for the beam. Reaction gas enters the inner tube of the concentric furnace tube seen in Figure 1, and passes over the solid sample. The reaction gas leaves the inner tube through slots in the circular platinum baffle held in place at the end of the inner tube by means of a sidearm (not shown in Figure 1) attached to the balance housing. The slots in the baffle provide passage of balance beam and thermocouple. Reaction gas and nitrogen purge pass together through the annular space between inner and outer tubes and are discharged together through an opening near the end of the outer tube. Consideration of flow and diffusion in the vicinity of the slot in the balance housing for the beam showed that a purge nitrogen flow of 750-800 ml (OOC and 1 atm) /min. would be adequate. After many experiments using flows in this range, there is no change in the appearance of shiny copper wire placed inside the balance housing to detect H2S. 1010 Environmental Science & Technology
Of greater concern was the possibility that purge gas might penetrate to the vicinity of the sample. Data were taken on the weight gain vs. time for finely powdered copper exposed at 600°C to mixtures of oxygen and nitrogen of known composition. These data permitted judgment of the penetration in runs in which oxygen was used as the purge gas, nitrogen as the reaction gas, and copper powder as the sample. The penetration was found to depend on both rate of flow of reaction gas and ratio of purge gas to reaction gas. For example, at a reaction gas flow of 42.5 ml/min and a purge-toreaction-gas ratio of 4: 1, there resulted a penetration of purge gas amounting to 1 % of the reaction gas flow. At a reaction gas flow of 100 ml/min, however, there was negligible penetration at purge gas flows as high as 1000 ml/min. With the baffle provided, the flow of reaction gas needed to be at least 75 ml/min to prevent purge gas penetration at the purge flow rates believed to be necessary. In kinetic work, flows of reaction gas and purge gas have been 100-300 ml/min and 750800 ml/min, respectively. Reaction gas mixtures were made up to known composition by metering individual components through capillary flowmeters having 600-mm scales. Low-pressure pan-cake regulators maintain constant pressure to the flowmeters. The flow system is similar to that described by Squires et al. (1971). Procedures
All runs were made with dolomite from the Greenfield formation in Ohio supplied by Davon Inc. from their Plum Run quarry. About 25 mg of powder, -250 $270 mesh in size, was spread flat over the platinum sample pan. The mass of the pan is about six times that of the sample; this helped minimize temperature gradients in the sample during reaction. Dolomite was calcined to yield [CaC03+MgO] by heating under COSat 20"C/min until 825°C was reached; the sample was held at 825°C for 5 min, and then cooled under CO, to the desired reaction temperature. The reaction gas mixture was turned on, CO, turned off, and Reaction 1 proceeded isothermally with conversion of [CaCO,+ MgO] to [Cas+ MgO] while the TGA produced a record of the sample weight vs. time. In runs where complete conversion of CaC0, to C a s was not carried out, the temperature was raised at the conclusion of the run to produce complete conversion. The total weight change was used to compute the degree of conversion of CaC0, to C a s during the actual run. Reaction gas contained H& COZ, Hs, and usually N1. To preserve CaC03, C 0 2in the reaction gas was above the equilibrium decomposition pressure of CaCO, at the run temperature. This pressure is less than 0.01 atm at 600"C, but rises to 1 atm at about 900°C. No runs were made under conditions conducive to the recarbonation of MgO, a slow reaction in any event. The reaction gas must contain Hz to prevent dissociation of H2S,the amount required increasing with temperature. Results at 700' and 800°C
Figures 2 and 3 present kinetic results for Reaction 1 at 800" and 700°C, respectively. Each figure gives plots of log (1 - x) vs. time, where x is the fractional conversion of CaC0, to Cas. The quantities of COSnoted in Figures 2 and 3,25 and 7 %, respectively, were the amounts needed to preserve CaC03 with just a little extra for safety. Pell et al. (1971) and Pell (1971) gave similar kinetic plots for Reaction 2. The present work has led to the important discovery that the reaction of half-calcined dolomite with
Table I. Process Applications for Desulfurizing Fuel Gas with Half-Calcined Dolomite Dilute-phase slagging Dilute-phase gasifier with Ash-agglomerating slagging carbonization fluidized-bed gasifier zone gasifier Pressure 15 atm 20 atm 21 atm Sulfur content of coal 3 . 8 wt 3 . 0 wt Z 3 . 8 wt % Fuel gas composition 0.5mol 3.3mol CH4 0.5mol % co 26.3 22.7 31.8 24.2 24.9 15.6 Hz 3.7 7.0 0.5 coz 9.1 10.9 0.5 HzO 35.6 30.7 50.4 Nz A H2S COS 0.6 0.5 0.7 Temperature of desulfurization step 86OOC 92OOC 760°C Approx. sulfur content of coal equivalent to emissions from 0.15 wt % 0.15 wt % 0.04 wt combustion of desulfurized coal
+
+
H2S can be faster than the reaction of fully calcined dolomite. For example, at 800°C and 57, H2S, the time for the halfcalcined material to achieve 507, conversion is only 15 sec, while it is 280 sec for fully calcined solid. Runs made with powdered calcite and an argillaceous limestone at 800°C and 5 % H2Sconfirmed the expectation that the reactivity of CaCO:, toward H2Sis far less than the reactivity of half-calcined dolomite. In runs with calcite and limestone, 30 % conversion to Cas was attained only after 7000 sec, and at this conversion the rate had become extremely slow. Halfcalcined dolomite achieved this conversion in less than 10 sec. A straight line in a plot of log (1 - x) vs. time indicates that the reaction rate is proportional to the amount of unreacted CaC03 in the sample. At both 800" and 700°C, the rate drops off faster than it should if it were proportional to unreacted CaCO,. The effect is greater at 700°C. If the reciprocal of the time for 50% conversion is taken as an arbitrary index of reaction rate, the kinetics are seen to be less than first order in H?Sat both 800" and 700°C. Pell (1971) observed similar behavior of Reaction 2 at temperatures below about 700°C. He explained the main features of his data by the hypothesis that activated adsorption of H2S by unreacted CaO hinders the rate of Reaction 2. His reaction
model required three kinetic constants, and good Arrhenius plots were obtained for the three constants (Pell et al., 1971). An attempt to fit Pell's model to the data for Reaction 1 has not yet been made, for reasons which will become evident in the discussion to follow of results for Reaction 1 at temperatures below 700°C. Pell obtained data for both spherical and powder samples of stone. Data obtained from spheres disclosed that Reaction 2 generally occurs homogeneously throughout the solid. Pell encountered diffusional resistance to Reaction 2 only in runs on 8-mm spheres at 950°C. The present work on Reaction 1 has been conducted under the hypothesis that this reaction also takes place homogeneously in finely powdered samples. No evidence has been obtained that this is not the case. Work is planned on spheres at 800°C to test this hypothesis. Process Applications
Table I gives data for three process applications in which a fuel gas produced by gasifying coal with air and steam is desulfurized by action of half-calcined dolomite. The first column of the table is for a gas produced at 15 atm from a coal containing 3.8% sulfur by a dilute-phase slagging
Figure 3. Log (1 - x) plots at 700''C, 7 %
Figure 2. Log (1 - x) plots at 800"C, 2 5 % CO?, and three H S levels
CO?, and levels
i CL
"0
001
002
003
004
005
TIME, 103SECONDS
006 007
i
three
HzS
l
0I o
k$ni+6r*3k--GT
57
TIME, I O ~ S E C O N D S
Volume 6, Number 12, November 1972 1011
gasifier (Robson et al., 1970). Before the gas is desulfurized by Reaction 1, the gas would be cooled to 860"C, just below the equilibrium decomposition temperature for CaC03 at the C 0 2partial pressure in the gas. At equilibrium for Reaction 1 and for water-gas shift, desulfurization would be about 96 complete. Emission of SO2,at no increase in power-generating efficiency, would be roughly equivalent to that from coal containing 0.15 % sulfur. The nature of the system is that emission of SO2will remain relatively constant over a wide range in the sulfur content of the coal gasified. The second column of the table is for a gasifier concept in which coal would be fed to a dilute-phase carbonization zone which receives hot gases from a dilute-phase slagging gasification zone; char would be transferred from the former to the latter zone (Diehl and Glenn, 1970). Dent (1961) reported that fluidized-bed gasification of carbon by steam can proceed at temperatures beyond about 1050"C to thermodynamic equilibrium for this reaction. It will be important to determine whether or not Dent's observation holds also for gasification in a fluidized bed with air and steam. If so, an ash-agglomerating fluidized-bed gasifier (Squires, 1971) operating at 21 atm and 1100°C on coal containing 3.8 % sulfur would yield the gas shown in the third column of Table I. Desulfurization of the gas at 760°C would reduce emission of SO2 to that from coal containing about 0.04% sulfur. The liveliness of Reaction 1, illustrated by Figures 2 and 3, suggests that half-calcined dolomite might advantageously be used in a panel bed filter (Squires and Pfeffer, 1970; Paretsky et al., 1971). The filter would remove both dust and sulfur ahead of a combustion supplying hot gas to a gas turbine (Graff et al., 1971). Gases from a coal gasification bed operating at 1100°C will contain volatilized alkali salts. If the sulfur-absorbing bed does not adequately remove the alkali fume which appears when the fuel gas is cooled, it will be necessary to provide a second auxiliary panel bed simply for removing such fume as zcn
u a
>
TIME. 103SECONDS
well as last traces of dust. This bed should probably operate at a temperature below 800°C. Recent data obtained at The City College suggest that a panel bed filter provided with a filter aid can remove alkali fume at an efficiency greater than 99.9%.
Curious Results at Lower Temperatures
Figures 2 and 3 give the kinetic results of the present research which bear on the usefulness of Reaction 1 in desulfurizing a fuel gas at high pressure. Work at lower temperatures was undertaken to contribute toward study of Reaction 3, which must in practice be conducted at such temperatures. Runs at 600°C with just sufficient COz present to prevent decomposition of CaC03 provided a surprise. Figure 4 gives the data. The striking feature is that runs at all H2Slevels from 1-20 % begin rapidly and break sharply when the conversion to Cas reaches about 15-20Z. Final rates appear linear in the log (1 - x ) plots, but are very slow indeed; complete conversions could not be practicably achieved at the conditions of Figure 4. The initial rate was highest for the 20% run and lowest for the 1 run, although this comparison cannot be seen at the scale of the plots in Figure 4. Figure 5 gives data for a run at 600°C made with a reaction gas containing 5 H2S, 10 Hz, 60 % C02,and 25 N2.The difference between this run and the run at 5 % H,S in Figure 4, made at 3 % Con, is spectacular. The initial rates are about the same, but the run at 3 % C 0 2 stops dead after reaching 15 % conversion of CaCO,, whereas the run at 60% COScontinues at a high rate to reach substantially complete conversion of the solid. Behavior at intermediate COZ pressures is also shown in Figure 5. Figure 6 shows the C 0 2effect at 700°C; it is less than 600°C. An interesting point is that for 5 % H2S and 60 % COz, more time is required to achieve complete conversion at 700" than at 6OO0C,although the initial rate at 700°C is faster. Figure 7 gives data at 550°C. The runs at 20 % H2S provide still another surprise. The rates at 0.5 and 1 % H2S are low; the rate at 5 % H2S is comparatively high and the run achieved complete conversion. The runs at 20% H2S begin at even higher rates, but slow sharply, well short of complete conversion. Runs at several Hz levels, with constant C 0 2level, showed little, if any, effect of H2on the reaction. Figure 8 shows the effect of varying temperature from 550" to 750°C in 50" increments for a series of runs at 0.5% HzS, 60 % Con,and 39.5 % H?. If an Arrhenius plot is made using the final slopes of the curves of Figure 8, the points lie on a
-
Figure 5. Log (1 - x ) plots at 600°C, 5 % HzS, and four COZ levels
1
' a
1012 Environmental Science & Technology
O '0 I
I
-
04
05
TIME, IO'SECONDS
$6
017
Figure 6. Lag (1 - x) plots at 7OO0C,5 % HzS, and two COz levels
Figure 9. Log (1 - x ) plots at 600°C and two H20 levels, 5 Z HIS, 3 Z COZ, 10 H2,remainder N t
IO
20%HrS 48%cot
05 04
550eC
I ‘
\
BALANCEOF GASxHz
1
1
Figure 7. Log (1 - x ) plots at 55OCC,high C 0 2 level, and four H2Slevels
n
10
1
1
Figure 10. Log (1 - x ) plots at 550 “C and two H 2 0 levels
l+ i*o
TIME, 103SECONDS
=I 3
0.1 - o
TIME, I O ~ S E C O N D S
U
O.S%HzS 60% co2 03
I‘ ,
,
O4iI\\
Figure 11. Differential kinetics for reaction of halfcalcined dolomite at 6OOOC with 5% HzS, 3 % COP, 10 % H2,and balance Nz
‘ZERO“OXYGEN
Figure 8. Log (1 - x ) plots at 0.5% HzS, 60% COP, and five temperature levels
07
PULSE OF
,
L
0.3 0,4[
I
I
0 OZo
IO
20
I
,
,
1
30
40
50
60
Dashed curve is for gas containing at most a few hundreths % 0 2 ; solid curve shows effect of two pulses of a gas containing approximately 0.3% 02,each pulse lasting about lOsec
TIME, MINUTES
TIME. 103SECONDS
straight line, and the apparent activation energy is 60 kcal/gmol. Steam provides reactivity in a manner similar to COS.Figure 9 shows the effect at 600”C, and Figure 10 shows the much more dramatic effect at 550°C. Comparison of Figures 7 and 10 shows that 60% H 2 0 is more effective in promoting the reaction than 60% COz at 550°C. Comparison of Figures 5 and 9 shows that the reverse is true at 600°C. Finally, bursts of oxygen have a startling effect on reactivity (Figure 11). An oxygen pulse apparently revives the solid, permitting another increment of conversion. On the other hand, oxygen supplied steadily from the outset of a run at 600°C and at levels up to about 0.5 merely alters the conversion, by only a few percent, at which the reaction “drops dead.” Prolonged exposure to oxygen also has the undesirable effectof converting C a s to CaS04. If the CO, level had been increased from 3 to 60% at any time after about 10 min into the run illustrated in Figure 11, there would have been no effect on the rate in absence of a pulse of oxygen. However, if the COS is increased simultaneously with an oxygen pulse, the reaction proceeds at the high rate characteristic of the high CO, level, as seen for example in the run at 60 % COSin Figure 5. Pel1 (1971) found that steam accelerates Reaction 2 at 550°C. There was no effect at 700°C. In light of his hypothesis that Reaction 2 is hindered by activated adsorption of HgS on unreacted CaO, one might postulate that steam adsorbs preferentially to H?S at 55O”C, and further that HSSfrom the gas
OXYGEN
\/*0.3%
phase can readily react with the CaO surface carrying adsorbed H20.
Perhaps the kinetic curiosities displayed by Reaction 1 at lower temperatures can be explained by an analogous elaboration of Pell’s adsorption model, but an alternative line of thought is appealing. The gas environment may affect the way in which C a s grows within the solid microstructure. It may be that the reaction “drops dead” when an impervious layer of “green” C a s forms on a crystallite of CaC08. The role of an oxygen pulse or presence of C 0 2 or HaO while the C a s is forming may be to promote curing of the CaS layer with formation of cracks opening the interior of the crystallite to action of H2S. Alternatively, their role may be to promote diffusion of a species in the solid. Kinetic data alone are not likely to produce sharp distinctions among the several possibilities, and further kinetic studies of Reaction 1 will be paralleled with inspections of the solids produced. Ackno wledgrnent Samuel Dobner and Basil Lewris contributed to the work, and John Bodnaruk and George DiIorio assisted with the experimental arrangements. Literature Cited Dent, F. J . , Trans. Znst. Chrm. Eng. (London), 39,22--3 (1961). Diehl, E. K., Glenn, R. A., “Desulfurized Fuel from Coal in Inplant Gasification,” paper presented at Second International Conference on Fluidized Bed Combustion, sponVolume 6, Number 12, November 1972 1013
sored by National Air Pollution Control Association, Oxford, Ohio, October 1970. Graff, R. A., Pfeffer, R., Squires, A. M., in Proc. Clean Air Congr., pp 764-71, Academic Press, New York, N.Y., 1971. Paretsky, L.,Theodore, L.,Pfeffer, R., Squires, A. M., J. Air Pollut. Contr. Ass., 21, 204-9 (1971). Pell, M., PhD thesis, The City University of New York, 1971. Pell, M., Graff, R. A., Squires, A. M., in “Sulfur & SOz Developments,” “Chem. Eng. Progr. Tech. Manual,” pp 151-7, 1971. Robson, F. L.,Giramonti, A. J., Lewis, G. P., Gruber, G., “Technological and Economic Feasibility of Advanced Power Cycles and Methods of Producing Nonpolluting Fuels for Utility Power Stations,” report from United Aircraft Research Laboratories to National Air Pollution Control Administration, December 1970.
Squires, A. M., Aduan. Chem. Ser., 69, 205-29 (1967). Squires, A. M., in “Power Generation and Environmental Change,” D. A. Berkowitz and A. M. Squires, Eds., pp 175-227, MIT Press, Cambridge, Mass., 1971. Squires, A. M., U S . Patent 3,402,998 (September 24, 1968). Squires, A. M., Graff, R. A., Pell, M., Chem. Eng. Progr. S y m n Ser.. 67. 23-34 (1971). Squires, A. M,, Pfeffer, R., J Air Pollut. Contr. Assoc., 20, 534-8 (1970). Received for review November 12, 1971. Accepted August 14, 1972. This work was supported by Research Grant No. AP-00945 from the Ofice of Air Programs of the Enoironmental Protection Agency. Presented at the Division of Industrial and Engineering Chemistry, 161st meeting, ACS, Los Angeles, Calif:,March 1971.
Effects of Ambient Levels of Ozone on Navel Oranges C. Ray Thompson,’ Gerrit Kats, and Earl Hensel Statewide Air Pollution Research Center, University of California, Riverside, Calif. 92502 Ambient air levels of nitrogen dioxide had little effect on yield Mature navel orange trees were enclosed in plastic-covered greenhouses and exposed to ambient air, carbon-filtered air, and carbon-filtered air containing either ambient or one-half ambient air levels of ozone for eight months, from blooming to picking time, to determine how much injury this pollutant is causing to citrus trees. No visible leaf injury was observed. Leaf drop, fruit drop, and total yield were determined. Fruit drop, as measured by the percent of total fruit-set which dropped, showed an increase with the amount of ozone. One half the ambient level of ozone had no statistical effect on yield of either number or weight of mature fruit, but a significant reduction was shown by the ambient level of ozone. However, ambient air containing the totsll photochemical smog complex reduced yields considerably, further showing that the deleterious effect of peroxyacyl nitrates, oxides of nitrogen, etc., add to the injury caused by Los Angeles-type smog.
A
ir pollutants in the Los Angeles Basin and in other major U.S. and foreign cities are produced by incomplete burning of fossil fuels. The principal source is usually the automobile. The pollutants consist of hydrocarbons, carbon monoxide, nitric oxide, lead derivatives, and many other minor compounds. Some of these emissions react rapidly in sunlight to form photochemical smog which contains ozone as the major component and much lower levels of peroxyacyl nitrates and nitrogen dioxide. Major losses to agricultural and forest plants are caused by this mixture of pollutants. Previous studies at this Center have shown photochemical smog to cause severe reductions in the yield of both citrus and grapes (Thompson and Taylor, 1969; Thompson and Kats, 1970).
To whom correspondence should be addressed. 1014 Environmental Science & Technology
of citrus in a subsequent study (Thompson et al., 1971). Thus, because ozone is the component of smog which occurs in the highest concentration in the atmosphere and the effects shown by fumigating greenhouse-grown vegetables with similar concentrations of this gas cause extensive injury, a study was carried out to find out how much injury ambient levels of ozone were causing to mature, producing navel orange trees in the field. Methods
Twenty-seven navel orange trees enclosed in ventilated plastic-covered greenhouses (Thompson and Taylor, 1969) located near Upland, Calif., were used for this study. The trees were 16 years old and in good condition. The area has climatic conditions and photochemical smog levels typical of the Los Angeles Basin which extends 30-40 airline miles inland from the Pacific Ocean. The trees were irrigated on a 14-day schedule, but when soil suction near the trees exceeded 50 cbars at a depth of 0.5 meter and at a point 1.5 meters from the tree trunk, supplementary water was provided. Fertilization followed commercial practice. Biological controls for pests were used where feasible and were successful in controlling aphids and scale insects. Red citrus mites required spraying with a mixture of chlorobenzilate and ovotran twice during the study. Experimental treatments were randomized to minimize effects of a previous study in which fumigations with NOz (Thompson et al., 1971; Cochran and Cox, 1957) at ambient levels had been carried out in 1968. This randomization was used to avoid the large expense involved in moving the structures and the attendant equipment to another location. The previous experiment had shown minimal effects on the trees and randomization was considered adequate to remove the influence of previous treatments. Cross contamination between greenhouses was not a problem because ozone escaping from any one house would never exceed ambient levels of ozone of the previous day and the dis-
