Rates of NOx absorption in calcined limestones and dolomites


Nigel J. James1 and Ronald Hughes*. Department of Chemical Engineering, University of Salford, Salford M5 4WT, England. The rate of sorption of NO was...
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Rates of NO, Absorption in Calcined Limestones and Dolomites Nigel J. James' and Ronald Hughes* Department of Chemical Engineering, University of Salford, Salford

H T h e rate of sorption of NO was measured on five samples of calcined limestone and dolomite. Measurements were made in a differential reactor a t temperatures from 250 to 420 "C and a t NO concentrations from 500 to 10 000 ppm. Rates were measured as a function of gas velocity, particle size, temperature, concentration of NO, and stone type. Particular care was taken to prevent the intrusion of mass transfer resistances so that true chemical kinetics were measured. The sorption rate was first order in NO concentration, and the activation energy was 22.6 kcal/gmol. The retarding effect of solid nitrate was correlated by an empirical power expression.

M5 4WT, England

Table 1. Petrographic Description of Stones Sample no.

1

2 3 4

5

The use of calcined limestones and dolomites as sorbents for removal of SO2 in pollution control studies has attracted a great deal of attention over the past few years. Processes such as stone injection into boiler furnaces and the use of fluidized bed combustors and slurry absorbers have all depended upon the use of these materials in one form or another. This led to kinetic studies of SO2 absorption such as those of Borgwardt ( I ) , Pigford and Sliger ( 2 ) ,the work a t Battelle by Coutant et al. ( 3 ) ,and work published by the present authors (4).

In principle, calcined limestones and dolomites .may also be used as sorbents for the oxides of nitrogen, NO,. Thermodynamics indicates that two reactions are of importance here:

+ 0 2 + 2N02 CaO + 2N02 + f / 2 0 2 + Ca(N03)2 2N0

(1) (2)

Calculations show that CaO absorbs NO a t lower temperatures than for SO*, with decomposition of the nitrate back to lime and NO becoming appreciable a t temperatures above 500-600 "C. Proposals have been made, however, for the use of limestones either by injection into flue gases as for desulfurization, addition to fluidized bed combustors (5,6),or by scrubbing with limestone slurries (7). Despite this interest, there appears to be little or no work published on the kinetics of absorption of NO by calcined limestones or dolomites. Kinetic data are a prerequisite to any design, and it is the object of the present paper to report kinetic results for this reaction.

Experimental The five stone samples used in this work are described in Table I; all were fully calcined before use. Nitric oxide was obtained from cylinders as pure gas (99.9%) or as mixtures with nitrogen containing 227 or loo0 ppm NO. The laboratory air used was purified by passing through soda lime to remove COP and through silica gel to remove water vapor. T h e sorption of oxides of nitrogen from low percentage mixtures in air by calcined limestones and dolomites was determined in the temperature range 250-420 "C using a thin bed of solid a t high gas velocities. The equipment consisted of a quartz tubular flow reactor together with a flow metering/mixing section. The input and output gas compositions were monitored with a modified Hersch meter, and in all cases

Present address, CIBA GEIGY (UK) Ltd., Duxford, Cambs, England.

Type

Composition and impurities

Lime- +- 2 % consisting stone magnesite, quartz Lime- + 5 % consisting stone silicates, quartz Dolo- 41 % MgO with +3% silicate mite Dolo- 53% MgO with +-2% clay mite Chalk

Disllngulshlng characlerlslics

Source

Coarse grains Wells, Somerset Buxton, Hard, finegrained stone Derbyshire Powdery soft Coleford, Glos stone Soft powdery Breedon, reddish brown Derbyshire color Bishopston, Very soft, whitish stone Glamorgan

the operation was differential with very low gas conversions per pass. The quartz reactor tube was 18 mm in diameter with a long preheat section to ensure adequate heating of the entering gas mixture. Three Pt-13% R h P t thermocouples were positioned in the bed of calcined material, one at the center and the other two symmetrically positioned near the edge of the bed. The bed (100 mg) was supported on a fine mesh stainless steel gauze. Blank tests without any calcined material present showed no measurable absorption of NO even at very low flow rates. The complete reactor was heated by Nichrome resistance wire, and temperature control to fl "C was achieved using a controllerlautotransformer system. Flow rates were measured using calibrated rotameters prior to the mixer. The nitrate in the stone after NO, absorption was determined by a chemical method (8).The sample was dissolved in water by soaking with Amberlite IR-120, a strong acid cation exchanger, and made up to 300 mL with distilled water and 1.5 g of Devarda's alloy added. A 42% solution of sodium hydroxide was then added, and the mixture was distilled, the distillate being collected in a measure quantity of 0.1 N HC1. This was titrated against standard sodium hydroxide, and the nitrate calculated from the relation: 1mL N-HCl = 0.08501 g NaN03

0.6201 g NO,

The stone sample was heated in a dry atmosphere of nitrogen to the required temperature. The gas mixture was then passed over the sample for a given period a t the selected flow rate. The sample was then cooled in flowing nitrogen, and the sample analyzed for nitrate as above.

Results and Discussion Preliminary Experiments. These were made to determine the boundaries within which true chemical kinetics could be measured. The most likely physical restrictions on the chemical rate are caused by gas film diffusion resistance external to the particles and by internal diffusional resistance in the particles. T o determine the effect of the external mass transfer resistance on the sorption rates, the gas flow rate was varied from 350 g/m2-sto 712 g/m2.s (equivalent to 5-10 L/min a t room temperature and 1atm). The experiments were made on 100 mg of Type 1 limestone (Table I) a t 400 "C and a NO concentration of 3000 ppm, and the observed conversion varied randomly over these flow rates within a range predicted by the extent of experimental error. This indicates that Volume 11, Number 13, December 1977

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sorption of NO under these conditions is unaffected by film mass transfer for this range of flow rates. Furthermore, since these experiments were made a t almost the highest temperature used, film mass transfer would be absent a t all temperatures lower than 400 "C. This conclusion was confirmed by calculating the drop in concentration of NO across the gas film using the correlation of Ruthven (9).The calculation was made for a gas flow rate of 456 g/m2-s a t 400 "C and a particle size of 96 pm. The drop in concentration of NO was only 0.6%. For all subsequent experiments, a gas flow rate of 356 g/m% was adopted as being sufficient to neglect external mass transfer. The effect of particle diameter on conversion was determined experimentally by varying the particle size from 49 to 96 pm. These experiments were also made using 100-mg beds of 100%calcined limestone (Type 1)a t a bed temperature of 400 "C and an NO concentration of 3000 ppm. Figure 1shows a plot of stone capacity against particle diameter for three different reaction times; for particles less than about 80 pm, no intraparticle diffusion limitation occurred. T o check this, a calculation was made using the criterion of Weisz and Hicks ( I O ) for the absence of diffusion-limited reaction. On this basis, particles would have to be less than 140 pm, which is in broad agreement with the experimental values. Therefore, particles of 63-74 pm were used in the main experiments described below. Effect of Stone Type. The absorption of NO was measured on all five types of stone listed in Table I. Measurements were made a t 400 "C on 100-mg samples with an initial concentration of NO of 3000 ppm. The results are shown in the form of stone capacity against time in Figure 2. The chalk-type limestone gave the best sorption characteristics; this was

01

I

I

I

PARTCLE

I

I

80

60

iC

DIAMETER,

I

100

p

Figure 1. Effect of particle diameter on sorption rate for three different reaction times

TYPE

2w

190

603

800

low

TIME SECS

Figure 2. Influence of stone type on reaction rate 1192

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Table II. Surface Area and Porosity of Fully Calcined Stones All stones 100% calcined by heating in air at 980 "C for 120 min BET surface area, m2/g

Pore vol

Av pore dlam,

Stone type and sample no.

Sire of particles, !J

Limestone

96

8.53

0.28X

0.16

68

5.81

0.50X

0.43

68

8.22

0.44X

58

6.78

0.26X

0.19

68

10.14

0.22X

0.11

(
p),

m3/g

!J

(1) Limestone

(2) Dolomite

lop6

0.27

(3) Dolomite (4)

Chalk

(5)

followed by the coarse-grained limestone (Type 1) and then by the two dolomites, with the hard fine-grained limestone proving to be the least effective sorbent. To determine whether the physical properties of the various stones played an important role in the rates of sorption, the surface areas and pore volumes were determined by the BET method (nitrogen absorption a t 77 K ) and mercury porosimetry, respectively. The results are given in Table I1 which also lists the average pore diameter. All samples were fully calcined. A comparison of this table with Figure 2 shows that the sorption rate correlates with the average pore diameter, a small average pore size giving a high rate of sorption. Since a small pore size corresponds to a large internal area, this observation agrees with the conclusion reached by other workers, that the most important consideration for efficient sorption is a high area calcine (3,4 ) . Figure 2 shows a wide activity range for various calcines. A comprehensive investigation of all of these was beyond the scope of this investigation so that all work described below was confined to what was regarded as a typical calcine, Le., the coarse-grained limestone designated as Type 1in Tables I and 11. Order of Reaction. The effect of NO concentration on the reaction order is illustrated in Figure 3, where the adsorption of NO is plotted against time for NO concentrations ranging from 500 to 10 000 ppm. Since the temperature variation in the bed during an adsorption run varied by only one or two degrees from the initial temperature, the bed could be assumed to be isothermal. Furthermore, the bed operated as a differential reactor with a very low conversion per pass; therefore, the order of reaction with respect to the concentration of NO may be obtained by a log-log plot of reaction rate against NO concentration. A plot of this type for three levels of conversion (fractional conversion, X = 0.05,0.10,and 0.20) with the same range of NO cqncentration is given in Figure 4. Straight lines of slope equal to unity are obtained, demonstrating a first order dependence of sorption rate on NO concentration which is retained throughout the whole concentration range investigated. A similar first order dependence has been reported for SOz sorption on calcined limestones (I, 3,4). Effect of Temperature. Sorption rates were determined a t temperatures from 250 to 420 "C. As before, 100-mg samples of fully calcined stones were used, and the NO concentration was kept constant a t 3000 ppm. The results obtained are summarized in the Arrhenius plot of Figure 5 for two levels of conversion (X= 0.1 and 0.21, assuming that the sorption rate with respect to the solid is of zero order. The latter is a common assumption in noncatalytic gas solid processes of this type. Good straight lines are obtained, the effect of conversion

6

5 L

3 2

LW

23c

600 TIME

8oC

lOC0

SECS

Figure 3. Variation of absorption with NO concentration 15

16

17 3

1 x 1 0 .

18

19

-1

K

Figure 5. Arrhenius plot for two conversion levels

t 0

x

=005

0

x

[email protected]

x

x

= 3.20

'o'@r--lo 5,000 PPM NO

I

I

3'

1c CCNC'N

NO,

I A S YO,

I

0

10'

NITRATE LOADING i g m i 1 9

caoi .lo3

Figure 4. Order of reaction in NO concentration

Figure 6. Effect of nitrate loading in solid on reaction rate

being to move the line away or toward the reciprocal temperature axis while keeping the same slope. This confirms that the sorption was in the chemically controlled region. The reliability of the temperature measurements was good since the bed temperature was measured on three thermocouples, and no significant temperature gradients were noted during sorption of NO. Any small temperature fluctuations were taken into account by taking a simple mean of the temperatures observed. The fluctuations were never greater than 2 "C. The activation energy and preexponential factor were determined by the method of least squares. Since the assumption of zero order with respect to solid would only be expected to hold a t low conversions where the retarding effect of solid nitrate formation on the surface was minimal, the data used in determining the temperature dependent parameters were taken a t conversions up to 10%. The expressions obtained for the rate and rate constant for a reaction first order with respect to gas and zero order in solid are:

and reaffirms that reaction was occurring free from physical transport effects. Effect of Nitrate Formation on Reaction Rate. Figures 2 and 3 show that the rate falls off with time as the conversion of CaO to CaN03 increases. At high conversion levels this effect becomes important. Although no attempt is made in this work to provide a theoretical interpretation of this effect since this will be incorporated in modeling studies already in progress, an attempt has been made to quantify this effect on an empirical basis. Thus, recognizing that the rates of sorption are obtained under differential conditions where the concentration of NO in the gas stream does not diminish appreciably in passing through the very shallow bed, a plot of log [sorption rate] against log [nitrate loading] may be made as in Figure 6. Here, plots are made in this manner for three concentrations of NO, namely 5000,3000, and 1000 ppm. Corresponding slopes were -1.52, -1.58, and -1.72, indicating that the retarding effect may be considered approximately proportional to the - 3 / 2 power of the nitrate loading. This effect is obtained on one stone only, and too much significance should not be placed on the order of -3/2 which is approximate and empirical. I t is probable, however, that similar effects (albeit with different magnitudes of the exponent) would be obtained for the other calcines. The important point is that the reaction rate is retarded as reaction proceeds

Rate = 7750 exp [-(I1 400 f 150)/T]gmol/g.s or k = 1.565 X 104[-(11 400 f 150)/T] m/s The corresponding activation energy of 22 600 cal/gmol is high enough to represent reaction in the chemical control region

Volume 11, Number 13, December 1977

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due to the accumulation of solid products, and this retardation may depend on the conversion raised to a power greater than unity.

Acknowledgment

Conclusions The rate of sorption of NO on various calcines of limestone and dolomite has been measured, the majority of the results referring to the more active of the two limestones employed. Chalk was the most active, but the activity was directly related to the surface areas of the calcines employed. On this basis, the two particular dolomites employed in this study were not the most reactive, and for practical purposes a coarse grained limestone might prove most effective. The order of reaction in NO was of first order, and a t low conversions a zero order dependence on the solid calcine may be assumed. However, a t high utilization of the calcines, the nitrate formed has a pronounced retarding effect on the sorption rate, but this can be accommodated by an empirical dependence on the nitrate loading. The activation energy and preexponential factor for the absorption have been measured. The value of the activation energy obtained, 22.6 kcal/gmol, suggests that the data were obtained in the chemical control region and confirms that the precautions taken to eliminate mass transfer resistances in the system were adequate.

Literature Cited

We acknowledge with gratitude the provision of stone samples by the British Quarrying and Slagging Federation. (1) Borgwardt, R. H., Enuiron. Sci. Technol., 4,59 (1970). (2) Pigford, R. L., Sliger, G., Ind. Eng. Chern. Process Des. Deu., 12, 85 (1973). (3) Coutant, R., Simon, R., Campbell, B., Barrett, R. E., Final Rep., “Investigation of Reactivity of Calcined Limestone and Dolomite for Capturing SO2 from Flue Gas”, to NAPCA, Contract CFA 70-111, Battelle Laboratories, Oct. 1, 1971. (4) James, N. J., Hughes, R., paper presented a t 2nd Int. Conf. on Control of Gaseous Sulphur and Nitrogen Compound Emission, Salford University, Salford, England, 1976. (5) Jonke, A. A., “Reduction of Atmospheric Pollution by Fluidized Bed Combustion”, Monthly Rep. No. 8, Argonne National Laboratory, Lemont, Ill., Mar. 1969. (6) Robinson, E. B., Bishop, J. W., Harvey, W. T., Jr., Ehrlich, S., Preprint 45C, 64th Nat. Mtg. AIChE, New Orleans, La., Mar. 1&20, 1969. (7) Crynes, B. L., Maddox, R. N., Chern. Technol., 502 (Aug. 1971). (8) Snell, F. D., Ettre, L. S., “Encyl. of Ind. Chem. Anal.”, Interscience, New York, N.Y., 1972. (9) Ruthven, D. M., Chern. Eng. Sci., 23,759 (1968). (10) Weisz, P. B., Hicks, J. S., ibid., 17, 265 (1962).

Received for review January 24, 1977. Accepted July 11,1977.

National Survey of Elements and Radioactivity in Fly Ashes Absorption of Elements by Cabbage Grown in Fly Ash-Soil Mixtures A. Keith Furr and Thomas: F. Parkinson Office of Occupational Health and Safety, and Nuclear Reactor Laboratory, Virginia Polytechnic Institute, Blacksburg, Va. 24061

Roger A. Hinrichs Physics Department, State University College, Oswego, N.Y. 13126

Darryl R. Van Campen US. Plant, Soil and Nutrition Laboratory, U S . Department of Agriculture, Cornell University, Ithaca, N.Y. 14853 Carl A. Bache, Walter H. Gutenmann, Leigh E. St. John, Jr., Irene S. Pakkala, and Donald J. Lisk’ Pesticide Residue Laboratory, Department of Food Science, Cornell University, Ithaca, N.Y. 14853

An analytical survey of 45 elements was conducted in fly ashes from 21 states by use of several instrumental methods. A high degree of correlation was found between the concentrations of pairs of chemically similar elements in the fly ashes and between the magnitude of gamma emission of the fly ashes and their respective concentrations of Th or U. Cabbage was grown to maturity in potted soil amended with 7% (w/w) of the various fly ashes. The concentrations of As, B, Mo, Se, and Sr in the cabbage showed a high degree of correlation with those in the respective fly ashes in which the plants were cultured.

present (2-5) such as selenium which can be absorbed from it by plants (2),foraging animals ( 6 ) ,and aquatic species (7). Furthermore, the presence of radionuclides in a limited number of fly ash samples has been reported (8). An analytical survey of the elemental content of fly ashes produced in this country has not been published. In the work reported, fly ashes from 2 1 states were analyzed for 45 elements and gamma emission by use of several instrumental methods. Cabbage grown on potted soil amended with the fly ashes was analyzed for these elements for possible correlation between the magnitude of element absorption by the crop and the element content of the respective fly ash.

Approximately 40 million tons of fly ash were collected in the United States in 1974, an increase of 15% over that produced in 1973 ( I ) . About 8.5% of this total was utilized in concrete and asphalt products and road stabilization ( I ) , the remainder being trucked to landfills or piped to settling ponds. Since fly ash typically contains nutrient elements, its use as a soil amendment in agriculture has been investigated to a limited extent (2). Numerous toxic elements may also be

Experimental In 1975 a description of our proposed study was sent to coal-burning power plants in 29 states with a request that they participate and return a representative sample of their fly ash to us for analysis. The power plants that participated and the data they provided pertaining to their fly ashes are listed in Table I. The fly ashes were mixed in a lucite twin shell blender and subsampled for analysis. Nondestructive neutron activation analysis for 36 elements

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