Environ. Sei. Technol. 1989, 23, 821-827
Treatment of Hydrated Lime with Methanol for In-Duct Desulfurization Sorbent Improvement Jeffrey A. Wlthum' and Heeyoung Yoon
Consolldation Coal Company, Research & Development, 4000 Brownsvllle Road, Library, Pennsylvania 15129 Laboratory results showed that methanol treatment of hydrated lime caused a chemical reaction forming calcium methoxide. The methoxide formation resulted in a drastic change in the surface of the lime particles, which form submicron platelike surface structures. This change increased the BET surface area of the lime to 10-100%. Pore volume and particle size were also increased. The SOz capture capacity of the sorbent was increased, in some cases by more than loo%, depending on the duration and the temperature of methanol treatment. The SO2capture capacity of the sorbent was well-correlated with the surface area and the calcium methoxide content. Calcium methoxide may be converted back to calcium hydroxide under wet conditions. If so, hydration with methanol/water mixtures may not produce calcium methoxide.
Background Several in-duct sorbent injection processes for electric utility SO2 control are under development. These processes share some common features. They use hydrated lime or slaked lime as the sorbent. The existing ductwork downstream of the air preheater and the particulate collection system serve as the SO2 capture zones. The processes generate a dry waste which can be removed and disposed of with the fly ash (1). These features minimize capital requirement and construction time, making the technology well suited for those retrofit applications for which available space or remaining boiler life is limited. One drawback of the in-duct processes is that the sorbent efficiency (utilization) is lower than for a conventional wet flue gas desulfurization (FGD) process. This makes the sorbent cost the major factor in the SOz control cost (2). Therefore, by increasing the sorbent utilization, the economics of in-duct sorbent injection processes can be significantly improved. We have studied various approaches to improve the sorbent utilization (3, 4). Methanol treatment of hydrated lime was one of the approaches studied. Literature information indicates that methanol can react with calcium hydroxide to produce calcium methoxide (5, 6);the effect of this chemical change on the reactivity was not reported. Others reported that hydration of calcined lime with methanol/water mixtures produced sorbents that had higher desulfurization activity, compared to the same lime hydrated with water only, in tests simulating boiler/furnace injection conditions (7-9). However, the desulfurization mechanism for boiler/furnace injection is different from that for in-duct systems because of the differences in reaction temperatures. At boiler/furnace injection temperatures (850-1200 "C), the calcium hydroxide thermally decomposes to the oxide before SOz is absorbed. A t in-duct injection temperatures (50-150 "C), no thermal decomposition takes place and the hydroxide is the material that reacts with and absorbs SOz. This paper describes the results of laboratory tests that were done to gain an understanding of the physical and chemical changes resulting from methanol treatment of hydrated lime and the effects of the treatment on SO2 capture performance. 0013-936X/89/0923-0821$01.50/0
Table I. Hydrate Analysis moisture (106 "C), w t % ash (925"C), wt % ash analysis, w t % NazO KZO CaO MgO
0.53 76.66
TiOz
0.12 0.21 93.94 3.18 0.29 0.02
PZ06
99.99% pure). Methanol Treatment Procedure. Methanol treatment was conducted in a 100-mL round-bottom flask suspended in a constant-temperature bath. A mechanically propelled stirrer was used to mix the methanol and the hydrated lime. For each treatment, 10 g of hydrated lime was stirred with 50 mL of methanol under dry nitrogen purge for a specified time at the specified temperature. Four different treatment temperatures (25,35,50, and 60 "C) and treatment times of 1 min-24 h were used. The temperatures chosen covered the range between room temperature and the boiling point of methanol (64.96 "C at 1atm). The mixture was cooled to room temperature, filtered in a nitrogen-purged glovebox and dried in a vacuum oven. The samples were stored in air-tight sample bottles. The treated samples easily passed through a 150-mesh screen without grinding. Five sets of duplicate samples were prepared to test the reproducibility of the treatment effects on sample desulfurization performance. One sample received a "triplet treatment" in which a hydrated lime sample was subjected to three consecutive treatment cycles, each cycle consisting of a 2-h treatment at 25 "C, followed by filtration and drying. Laboratory Desulfurization Test Procedure. Following preparation, the activity of each sample was evaluated with a laboratory reactor test unit. This unit consisted of a gas feed system, a saturating system, and a reactor with a fixed sorbent bed (Figure 1). The SOz content of the feed gas was set at lo00 ppm by mixing pure N2 with an S02/N2mixture. The pure nitrogen gas stream was saturated by passing it through a 350-mL-capacityPyrex glass bottle equipped with a fritted
0 1989 American Chemical Society
Environ. Sci. Technol., Vol. 23, No. 7, 1989
821
HEATING TAPE (165 OF)
,
TC
I ATURATOR BATH R"--
PLANT N 2
u I L REACTOR BATH 115OOF)
L5WO PPM SO2 IN N 2 PR = P R E S S U R E REGULATOR FI = FLOW INDICATOR PI = P R E S S U R E INDICATOR TC = THERMOCOUPLE PAS = P R E S S U R E R E L I E F SYSTEM R V = R E L I E F VALVE 115-20 PSIG)
Flgure 1. Schematic of laboratory desulfurization test unit.
disk and filled with deioinized water. This saturator was immersed in a constant-temperature bath. The reactor was a 9 mm i.d. X 15 cm long Pyrex tube. The sorbent (25 mg) was dispersed on a 10 cm X 10 cm quartz wool matrix, which was rolled up and inserted into the reactor and sandwiched between two glass wool plugs to prevent sorbent loss due to entrainment in the gas flow. The reactor was immersed in a constant-temperature bath for temperature control. The spent reactor contents were recovered by rinsing the quartz wool matrix with acetone through a 200-mesh screen and evaporating the acetone at 100 "C. Sulfur and calcium contents of the spent reactor samples were determined with a Leco analyzer and by atomic absorption spectrophotometry, respectively. The S/Ca molar ratio, or calcium utilization, was used to compare the desulfurization performance of the treated samples. The test procedure involved the following. The reactor, with sorbent charge, was lined out with a stream of nitrogen gas at controlled temperature and humidity conditions. After the line-out, the S02/N2mix was added for the specified test period (1-60 min), then the gas flow was stopped, and the reactor contents were removed and analyzed. For the tests presented here, the gas feed rate was 2.125 L/min, the saturated bath temperature was set to provide feed gas having 60% relative humidity at the reactor and gas feed temperature (65.6 "C). For each product sample, runs to measure calcium utilizations were conducted at contact times of 1,2,5,10,30, and 60 min with the S02/N2 mixture. Analysis Methods. The samples were analyzed for elemental carbon and hydrogen with a Leco Model CHN-600 analyzer. The carbonate contents were determined by Coulometric titration of COz liberated during acid digestion. Calcium contents were determined by atomic absorption spectroscopy. BET surface area measurements and thermogravimetric analyses were also performed on each sample. 822
Environ. Sci. Technol., Vol. 23, No. 7, 1989
Mercury porosimetry, X-ray powder diffaction, Coulter Counter particle sizing, SEM photos, and FTIR analyses were performed on the untreated hydrated lime and on selected samples representing 2-5-h treatment times and 25-50 "C treatment temperatures.
Results and Discussion Effect of Methanol Treatment on Physical and Chemical Properties of Sorbent. Chemical Changes. The changes in physical and chemical properties of hydrated lime after treatment with methanol are summarized in Table 11. The only noticeable change in the chemical composition of the sorbents was the appearance of calcium methoxides, Ca(OCH3)2and Ca(OH)(OCH,), by the following reactions: Ca(OH), + CH30H Ca(OH)(OCH3)+ HzO (1) Ca(OH)(OCH3)+ CH30H -+ Ca(OCH3)z+ H2O (2) The presence of these calcium methoxides was confirmed by Fourier transform infrared spectroscopy (FTIR), chemical analysis, thermogravimetric analysis (TGA), and X-ray powder diffraction. In general, the conversion of calcium hydroxide to the calcium methoxides was a slow reaction over the entire temperature range studied (25-60 "C), although the extent of conversion did increase with an increase in the treatment temperature. A comparison of the diffuse reflectance FTIR spectra for raw and treated hydrates gives the best proof that calcium methoxides, not physically adsorbed methanol, were present in the methanol-treated samples. Figure 2 shows the diffuse reflectance FTIR spectra for an untreated hydrated lime, a hydrated lime treated for 2 h at 50 "C, the difference between the treated and untreated spectra, and the absorption spectrum of pure methanol. The three peaks in the region 2800-2950 cm-l in the methanol-treated sample are C-H stretch peaks, and they roughly correspond to the C-H stretch peaks of the methyl group in methanol. A sharp peak at ca. 1060 cm-' is present in the spectrum of the methanol-treated sample, but not the untreated sample. This is a C-O stretch peak, +
Table 11. Results of Methanol Treatment: Physical and Chemical Changes
treatmt temp, OC untreated 25
time, h 0.017 0.017 0.5 1 2 2
24 24 35
50
1 5 5 6 1 n
L
60
2 5 1 2 5
surf. area, m2/g 21.5 23.4 19.4 27.7 37.3 33.6 34.0 37.2 35.5 37.2 37.7 37.5 37.7 44.1 39.1 39.9 38.8 38.7 43.2 40.4
chem anal. results, wt % CH30 OH Ca 1.0 0.5 2.0 7.0 7.2 7.8 13.5 15.8 9.1 13.5 17.4 15.7 7.8 13.6 12.6 20.5 15.0 16.9 19.2
42.8 43.2 42.3 25.6 30.7 38.8 37.8 34.0 30.7 29.3 33.0 28.6" 31.6 30.6 34.8 33.9 27.8 25.2" 24.6O 22.9O
52.4 51.1 54.4 53.7 48.3 50.9 49.0 48.4 52.7 52.6 49.5 46.7 41.7 52.8 51.3 48.5 47.3 50.1 44.9 48.8
three 2-h treatments at 25 "C
32.4
23.8
24.4
46.2
high-purity calcium methoxide
59.0
44.7
6.4
42.1
TGA results, wt 90
CH30
OH
13.8 12.3 14.5 10.4 15.0 14.5 19.9 13.7 15.8 15.4
45.1 41.6 41.3 39.9 35.4 37.0 32.4 34.7 36.8 33.9 34.0 40.8 33.1 34.7 32.6 32.6 28.2 34.2 32.6 34.3
21.0
29.1
2.6 4.3 3.8 9.1 8.6 10.9 12.7 11.0 11.1
pore vol (0.014.1 pm), mL/g 0.134 0.160
mean partcle size, pm
av cryst size, nm calcium calcium methoxide hydroxide 17.6 15.9
8.1
0.257
19.2
17.4
0.302
13.0
15.6
14.9 17.4
15.8 16.9
0.313 0.319
9.7
aAtom balances showed unusually high CaO content (>16 wt %), suggesting that the hydrogen analysis was incorrect for these samples.
which indicates the presence of methoxyl groups. This peak is shifted slightly compared to pure methanol (1034 cm-l), because the 0 atom is not bonded to an H atom in the treated sample as it is in methanol. The 0-H stretch peak for pure methanol (between 3100 and 3600 cm-l), which is very broad because of the strong hydrogen bonding that occurs, is not present in the spectfum of the treated sample. This indicates that the methoxyl groups in the sample do not exist as free or adsorbed methanol and, therefore, must be calcium methoxide. The very sharp peak a t 3650 cm-' is due to the 0-H stretch of Ca(OH)2. Close examination of the methanol-treated spectrum reveals a very small 3675 cm-' shoulder peak attached to this Ca(OH)2peak. This 3675 cm-' frequency corresponds to that of a very small 0-H stretch peak found in the spectrum of a high-purity calcium methoxide produced in our laboratory, which was attributed to Ca(0H)(OCH,), calcium hydroxyl methoxide. In general, the FTIR results agree well with those reported in a Russian paper on FTIR spectra of metal alkoxides (IO). Each sample was subjected to chemical analysis for carbon, carbonate, hydrogen, and calcium content. From this information, the hydroxide and methoxide weight fractions were calculated. This acted as additional confirmation of the presence of methoxyl groups. This information, listed in Table 11, shows that the methoxyl group content of the samples increased with both treatment time and treatment temperature. The data indicate that a long contact time would be necessary to attain equilibrium for the methoxyl-hydroxyl exchange. Arai et al. (6)report that the equilibrium methoxyl group content is 32 wt %; the temperature was not reported, but we presume it was room temperature. The effect of temperature on the equilibrium was not reported. Thermogravimetric analysis (TGA) was performed on each sample as a check of the chemical analysis results. Typical TGA curves are shown in Figure 3 for a methanol-treated and an untreated hydrated lime. The weight loss that occurred at ca. 470 "C was caused by decomposition of Ca(OHI2to CaO. The 23.3% weight loss for the
TREATED-UNTREATED
I UNTREATEDHYDRATE
4400 4000 3600 3200 2800 2400 2000 1600 1200 800
400
0
WAVENUMBERS
Figure 2. Diffuse reflectance FTIR spectra.
untreated hydrated corresponds to approximately 95% calcium hydroxide, in agreement with chemical analysis. The TGA curves of the methanol-treated samples show a weight loss occurring between 650 and 800 OC, probably due to loss of methoxyl groups from the calcium methoxides. The weight % CHBOin the original sample can be calculated from the weight lost in this temperature Environ. Sci. Technol., Vol. 23. No. 7, 1989
823
40
RATE: 150.0 OC/MIN
-\
V
TREATMENT TEMP. 44
UNTREATED HYDRATE
-e
40
E 111 5
36
N
323.3%
32
Y
a
U
TREATED HYDRATE l2HRS AT 5 0 O C )
I-
I
K
2
u
28
Y
z 24
20
it 0
0
I 0
I
I
I
100 200 300
I
I
I
I
I
I
I
I
I
I
I
1
2
3
4
5
6
A
I
24
NOL-TREATED HYDRATE MEAN: 9.7UM MEDIAN: 10.36 UM MODE: io.mum
Y
400 500 600 700 800 900 1000
TEMPERATURE (C)
DEVIATION: 2.23 UM
Flgure 3. TGA curves for untreated and methanol-treated samples showing weight loss as a function of temperature.
region. CH30 contents by TGA analyses were in agreement with chemical analyses as shown in Table 11, except for four samples. For the four samples, the hydrogen contents by the chemical analyses were not measured correctly, as indicated in the table. X-ray powder diffraction analysis also showed that the treated samples contained calcium methoxide by the detection of the 001 plane reflection, identified from the literature (6). Physical Changes. Methanol treatment significantly changed the physical structure of the sorbent. The methoxide formation altered the surface morphology from spherical agglomerates to flat, platelike structures and substantially increased the surface area, over 100% in some cases. In addition, the 0.01-0.1 pm pore volume and the average particle size both increased. The BET surface area of the methanol-treated hydrates was high (28-44 m2/g) compared to the original hydrate surface area of 21 m2/g. Treatment time and temperature affected the extent of surface area increase (Figure 4). The surface area increased with treatment time to a maximum after 1-2 h of treatment. However, continuing treatment caused the surface area to decrease. This behavior was also seen in a similar study (6). Porosity increase may be contributed to the surface area increase. Mercury intrusion porosimetry results show that methanol-treated hydrates had greater pore volumes (higher porosity) than the untreated hydrate. Table I1 shows that increases in both treatment time and temperature increased the pore volume. The pore volume was highly sensitive to treatment temperature since a run at 50 OC resulted in a greater pore volume (0.32 cm3/g) than at 25 OC (0.26 cm3/g) for the same 2-h treatment time. The effect of time was significant initially, with a 2-h run producing a greater pore volume (0.26 cm3/g) than a 1-min run (0.16 cm3/g), both done at 25 "C. As measured by Coulter Counter analysis, the average particle size increased from 8.1 to 9.7 pm during a 2-h, 50 "C treatment. Figure 5 shows a decrease in the total fraction of particles smaller than 4 pm and an increase in 824
Environ. Sci. Technol., Vol. 23, No. 7, 1989
.,
16
I
Y
2
2
0
12
>
UNTREATED HYDRATE 8.11 UM MEAN: MEDIAN: 9.3 UM MODE: 12.52 UM DEVIATION: 2.36 UM
Lu -1
u a
P d
2
2
4
U
0 8 0 0
1
2
4
8
16
32
PARTICLE SIZE (microns)
Figure 5. Particle size distribution for a methanoktreated and untreated hydrate.
the fraction of larger particles, indicating that particle swelling and/or agglomeration occurred. SEM photos of untreated (Figure 6, upper) and methanol-treated (Figure 6, lower) samples show the dramatic change in surface structure following treatment. The untreated sample consisted of 1-2-mm units agglomerated into 10-pm particles. The surace of the methanol/treated particles, however, was covered with flat plates -1 pm diameter by 0.1 pm thick. These could be crystals containing calcium methoxide, which was shown to have this structure (5, 6). This surface change was probably responsible for the large surface area increase caused by methanol treatment. X-ray powder diffraction was done for selected samples to determine the size of the Ca(OH), crystallites, which
Table 111. Results of Methanol Treatment: Sorbent Activity Changes
BET surf. area. treatmt temp, OC
time, h
m2/g
1 min
21.5 23.4 19.4 27.7 37.3 33.6 34.0 37.2 35.5 37.2 37.7 37.5 37.7 44.1 39.1 39.9 38.8 38.7 43.2 40.4
5.0 4.7 5.2 4.9
three 2-h treatments at 25 'C
32.4
high-purity calcium methoride
59.0
untreated 25
0.017 0.017 0.5 1 2 2
24 35
24 1 5 5
6 50
60
1 2 2 5 1 2 5
Ca utilization in lab desulfurization teats, 9b 2 min 5 min 10min 30min
8.2 11.9 9.1
16.0 16.8 18.5 21.0 14.2 20.6 15.2 19.8 20.2 18.6 15.9 17.1 17.2 16.7 13.7
15.8 18.5 17.5 19.3 24.6 30.7 25.0 25.6 19.5 31.3 21.7 24.4 24.1 21.5 19.8 28.6 27.1 25.6 23.2
19.4 24.4 21.3 23.5 33.6 28.5 34.0 41.8 33.6 32.8 44.7 40.2 42.0 34.4 38.0 42.1 44.7 35.1 34.5 40.4
23.7 25.8 23.9 25.9 33.5 34.4 38.1 45.9 42.7 37.4 47.6 46.6 43.2 37.2 47.9 44.6 49.9 46.4 46.7 55.7
1.2
5.3
14.6
31.8
41.5
51.1
3.5
9.2
19.8
26.6
30.8
31.3
0.6 4.9 7.5 5.7 4.1
6.0 1.3 5.6 4.8 7.0 4.2 8.6 6.7 6.1 6.2 6.3
13.1 10.5 9.6 14.1 16.0
60min
1.5 5.6 8.4 9.3 7.3 11.6 9.3 7.1 6.2 9.3 8.2 9.8 9.8 11.7 10.2
6.6
35
30
25
20
5MIN OESULFURIZATION RUN
5L 0
21
METHANOL TREATMENT TIME lhourtl
Figu.7. Effectotbeanantanemsaantacavnv. salnpka&ated at 25 "C.
e 8. (Upper)SEM photo of an unheated hydrate (1 cm = 2.58 I".(Lower) SEM photo of mthanoCtreated hydrate (1 cm = 1.42 W
w).
d d affect the reactivity. Table I1 shows that the average crystallite size did not significantly change as a result of methanol treatment. Effect of Methanol Treatment on Sorbent Activity. Treatment Time Effects. The changes in sorbent de-
sulfurization activity for methanol-treated samples are summarized in Table 111. Figure 7 shows the effect of treatment time on laboratory desulfurization performance of methanol-treated samples prepared a t 25 "C. The initial activity, as indicated by calcium utilization a t 5-min desulfurization run time, increased slightly with methanol treatment time, but the saturated calcium utilization a t 60-min desulfurization run time (or sorbent mpacity) was significantly increased with the treatment time. 2- and 24-h treatments at 25 O C increased the sorbent capacity by 50 and 75% (relative), respectively: 24.37, and 44% saturated calcium utilizations for untreated sample, and 2- and 24-h treated samples, respectively. The effect of Envirm. scl. Techmi.. Vol. 23. NO. 7. 1989 825
50
t
0
40
0
60 MIN DESULFURIZATION RUN
5 MIN DESULFURIZATION RUN
I 5 MIN DESULFURIZATION RUN
5 lo
0
1
0 20
1
30
40
50
60
METHANOL TREATMENT TEMPERATURE (OCI
Flgure 8. Effect of treatment temperature on sorbent activity. Samples treated for 2 h.
treatment time was even greater for sorbents prepared at higher temperatures. The sorbent capacity increased by 50 and over 100% (relative) for 1- and 5-h treatments at 50 "C. The saturated utilizations were 37 and 50% for the 1-and 5-h treatment samples (Table 111). Treatment Temperature Effects. The methanol treatment temperature also had a significant effect on the SO2 capacity of the sorbents over the 25-60 "C treatment temperature range studied. Figure 8 shows the temperature effect at a 2-h treatment time. The temperature effect on the sorbent initial activity, as indicated by the 5-min calcium utilization, was not significant, but the temperature effect on the sorbent capacity (60-min utilization) was significant. For 2-h treatment samples, the sorbent capacity increased by nearly 35% (relative) from 35 to 47%. Table I11 shows a similar effect of treatment temperature for the other treatment times. Multiple Treatment Effects. A number of short treatments increased the sorbent capacity more than one long treatment at the same temperature. The treated sample produced by three 2-h treatments at 25 OC had a 52 '70 saturated calcium utilization, whereas a sample subjected to one 24-h treatment had a 43-46% calcium utilization. However, the data are not conclusive as to whether this was due to short, multiple treatments or to multiple sample recovery steps. Methoxide Content Effects. The desulfurization performance was closely correlated with the methoxide content. However, since the surface area also increased with the methoxyl group content, the effect of methoxide content cannot be separated from the effect of surface area. Figure 9 shows the calcium utilization and the surface area as a function of methoxyl group content. The surface area increased rapidly from 20 to almost 40 m2/g as the methoxyl content increased from 0 to 15 wt %, but above 15 wt % methoxyl group the surface area levels off at -40 m2/g. The sorbent capacity behaved similarly, rapidly increasing from 24 to 45% calcium utilization over the range 0-15 wt 5% methoxyl group. The initial activity 826
Environ. Sci. Technol., Vol. 23, No. 7, 1989
0 0
I
I
I
I
L
5
10
15
20
25
WT % CH3O GROUP
Figure 0. Effect of methoxyl group content on surface area and sorbent activity.
(5-min desulfurization run) appeared to increase also with the methoxide content, although data scattering was considerable. To determine if calcium methoxide reacts with SO2, a high-purity calcium methoxide sample was prepared in our laboratory. This methoxide sample, made by condensing methanol vapor onto calcium metal (II), contained 44.7% (OCH3)- group, 6.4% (OH)- group, 5% CaO, and 4% CaC03 by weight (determined by chemical analysis; Table 11). The fact that the sample had significant CaO, CaC03, and (OH)- group indicates that the metallic calcium used may have reacted with air by exposure to the atmosphere. The sorbent capacity (31% calcium utilization; Table 111) for this material was lower than for all the methanoltreated hydrates, except those treated for less than 1 h, despite the fact that this methoxide had higher surface area (59 m2/g) and higher methoxyl group content (45 wt %) than all of the treated hydrates. Since the high-purity calcium methoxide was less reactive than the methanoltreated hydrates, the activity enhancement was probably due to physical effects rather than chemical effects. Pore closure has often been suggested as the reason for less than 100% calcium utilization in most calcium-based sorbents (12-15). This occurs because the difference in the molar volumes of the reactant, Ca(OH)2(0.055 nm3/ molecule), and products, CaS03/CaS0, (0.077-0.087 nm3/molecule), causes the reaction products to block the open pores and form a product shell layer around the particles. This blockage increases the pore diffusion limitation for SO2to reach unreacted Ca(OH), in the interior of the particle, and the extent of reaction would depend mainly on the porosity and surface area. If this mechanism is true, the physical changes resulting from methanol treatment would improve the sorbent performance by providing more active surface and pore volume. Relative Humidity Effect. Figure 10 shows desulfurization runs at 45 and 60% relative humidity for a methanol-treated (2 h, 50 "C)sample. The sorbent ca-
surface area and pore volume increases. 3. These physical and chemical changes, in general, became more pronounced with increasing treatment temperature and duration over the ranges of 25-60 "C and 1 min-24 h, respectively. 4. The calcium methoxide was not stable under wet conditions and converted back to calcium hydroxide.
45
40
35
E2
30
Acknowledgments
0
t
25
i I2
I
2
60% RELATIVE HUMIDITY
20
0
A u
a
v
15
We gratefully acknowledge G. A. Robbins of Consol R&D, who performed the FTIR work, and L. K. Bailey and A. E. Goodwin of Conoco Inc. R&D, who performed the X-ray diffraction and mercury intrusion porosimetry work, respectively. Registry No. CH30H, 67-56-1; SOz, 7446-09-5; Ca(OH)(OCHs), 119771-06-1; Ca(OCH&, 2556-53-8.
10
5
0
Literature Cited
tQ I
I
I
I
I
I
I
0
10
20
30
40
50
60
DESULFURIZATION RUN TIME (minuter)
Flgure 10. Effect of relative humidity. Open symbols: untreated hydrate. Closed symbols: methanol-treated hydrate prepared at 50 "C for 2 h.
pacity was lower at the low humidity as observed with untreated hydrated lime under similar conditions (3). However, unlike the untreated hydrate, the initial activity was about the same at both humidity conditions up to 10-min desulfurization run times. Stability of Methoxyl Group in Water. The calcium methoxide material in the methanol-treated hydrated lime was not stable in water. The stability was tested by stirring 1 g of a methanol-treated sample containing 15 w t % CHBOwith 10 g of water for 6 min and filtering. Analysis showed that 63% of the methoxide was recovered as methanol in the filtrate. Thus, many of the CH30 groups were exchanged by OH groups from the water. This may have implications for hydration with methanollwater mixtures. Lime hydration is usually performed with an excess of water (16). The excess water may suppress the calcium methoxide formation during hydration with a methanollwater mixture. Conclusions 1. Treatment of hydrated lime with methanol caused
the following chemical and physical changes: formation of calcium methoxide or calcium hydroxyl methoxide via reaction between calcium hydroxide and methanol, surface area increase of 10-100% caused by platelike structure formation on the surface of the particles, pore volume increase of up to 14070,and possibly some particle agglomeration and/or swelling. 2. Treated hydrated lime samples showed higher capacity for SOz sorption in laboratory desulfurization tests at 65.6 "C and 60% relative humidity; some samples showed twice the capacity for SO2 capture as untreated hydrates. Based on a low reactivity of a high-purity calcium methoxide sample produced from metallic calcium, the improved desulfurization performance of a treated sample may be primarily due to physical effects such as
(1) Statnick, R. M.; Burke, F. P.; Koch, B. J.; McCoy, D. C.; Yoon, H. Proceedings, Fourth Pittsburgh Coal Conference, Pittsburgh, PA, September 28 to October 2, 1987; pp 250-259. (2) Yoon, H.; Ring, P. A.; Burke, F. P. Proceedings, Coal Technology '85 Conference, Pittsburgh, PA, November 12-14, 1985; Vol. V, pp 129-151. (3) Yoon, H.; Stouffer, M. R.; Rosenhoover, W. A.; Statnick, R. M. Proceedings, Second Pittsburgh Coal Conference, Pittsburgh, PA, September 16-20, 1985; pp 223-236. (4) Yoon, H.; Withum, J. A.; Rosenhoover, W. A.; Burke, F. P. Proceedings of the Joint Symposium on Dry SOz Simultaneous S02/N0, Control Technology, Raleigh, NC, U.S. Environmental Protection Agency. US.Government Printing Office: Washington, DC, 1986; EPA-600/9-86029b. (5) Kubo, T.; Uchida, K.; Tsubosaki, K.; Hashimi, F. Kogyo Kagaku Zasshi 1970, 73 75-82. (6) Arai, Y.; Yasue, T.; Wakui, Y. Nippon Kagaku Kaishi 1981, NO.9, 1402-1408. (7) Gooch, J. P.; Dismukes, E. B.; Beittel, R.; Thompson, J. L.; Rakes, S. Proceedings of the Joint Symposium on Dry SO2 Simultaneous S02/N0, Control Technology, Raleigh, NC; US. Environmental Protection Agency. U.S.Government Printing Office: Washington, DC, 1986; EPA600/9-86-029b. (8) Muzio, L. J.; Offen, G. R.; Boni, A. A.; Beittel, R., Proceedings of the Joint Symposium on Dry SOz Simultaneous S02/N0, Control Technology, Raleigh, NC; U S . Environmental Protection Agency. US.Government Printing Office: Washington, DC, 1986; EPA-600/9-86-029b. (9) Moran, D. L.; Rostam-Abadi, M.; Harvey, R. D.; Frost, R. R.; Sresty, G. C. Prepr., Diu. Fuel Chem., Am. Chem. SOC. 1987, 32, 508-516. (10) Grigor'ev, A. I.; Turova, N. Ya. Dokl. Akad. Nauk SSSR 1965,162, 98-101. (11) Russell, R. W. US.Patent No. 3 009 964, 1961. (12) Ramachandran, P. A.; Smith, J. M. AIChE J. 1977, 23, 353-361. (13) Chrostowski, J. W.; Georgakis, C. ACS Symp. Ser. 1978, 65, 225-237. (14) Yortsos, Y. C.; Shankar,K. Ind. Eng. Chem.Fundam. 1984, 23, 132-134. (15) Prasannan, P. C.; Ramachandran, P. A.; Doraiswamy, L. K. Chem. Eng. Sci. 1985,40, 1251-1261. (16) Boynton, R. S. Chemistry and Technology of Lime and Limestone, second ed.; Wiley-Interscience Publishers: New York, 1980.
Received for review August 1,1988. Accepted February 16,1989.
Environ. Sci. Technol., Vol. 23, No. 7, 1989 827
