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
1191
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
Environmental Science & Technology
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
(
