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Sulphur capture in fluidized bed boilers: The effect of reductive decomposition of CaSO4
The Chemical Engineering
Journal,
40 (1989) 59 - 69
59
Sulphur Capture in Fluidized Bed Boilers: the Effect of Reductive Decompositibn of CaS04 ANDERS LYNGFELT and BO LECKNER Chalmers Tekniska University of Technology,
Department
of Energy Conversion, S-41296
GGteborg (Sweden)
(Received November 26, 1987; in final form April 13, 1988)
ABSTRACT
Sulphur capture by lime was studied in a 16 MW stationary fluidized bed boiler (FBB). A marked fall-off in sulphur capture was noted at temperatures above about 880 “C. The proposed explanation is that the combustion produces reducing conditions in the particle phase, and thus allows for a reductive decomposition of CaSO,,. This explanation is supported by (i) thermodynamics showing the instability of CaS04 under reducing conditions; (ii) in-bed oxygen measurements indicating reducing conditions in the particle phase; (iii) the observed fall-off in sulphur capture with temperature, which is not seen in laboratory tests under oxidizing conditions or in a circulating FBB, where the sorbent particles experience oxidizing conditions to a greater extent; (iv) the observation that the temperature dependence of the sulphur emission is very strong even when the net sulphur capture is zero provided always that CaS04 is present and (v) literature data indicating the rate of the proposed reaction.
1. INTRODUCTION
The understanding of phenomena affecting sulphur capture in fluidized bed boilers (FBBs) is far from complete [ 11. A contributing factor to this is that experiments in fullscale FBBs are costly and time-consuming and often involve restrictions concerning variations in parameters. The complexity of the sy&em leads to problems in the interpretation of data. Comparisons of experiments and general conclusions are often difficult because of important differences in boiler design and operation, sorbent and fuel properties. Sulphur capture studies are also difficult owing 0300-9467/89/$3.50
to the length of time needed to obtain steady state. Furthermore, the self-absorption of most coals, caused by sulphur retention in mineral ash or unburnt combustible matter, introduces important uncertainties in the interpretation of the sulphur capture by added limestone. In the experiments reported here it was the intention to minimize parameter variations in order to allow for steady state conditions; therefore only temperature and lime addition were varied. A virtually ash-free fuel, petroleum coke, was used to eliminate the selfabsorption of fuel mineral ash. Ignaberga limestone was used as the sorbent because its sulphur capture properties have been studied previously in the stationary FBB used in this work [2, 31, as well as in a circulating FBB [ 3, 41, and in the laboratory [ 5 - 81. The important effect of temperature has been noted both with the present boiler and also with other FBBs. In spite of many proposed explanations a full understanding of the effect of temperature has not been reached. The effect of temperature was chosen as an object of study in the expectation that the discovery of the cause of this effect would increase the general understanding of sulphur capture in FBBs. Evidence in support of the theory that the temperature dependence is explained by the reductive decomposition of CaS04, is presented below in the form of thermodynamic, experimental and literature data.. 2. THEORY
A sorbent commonly used for sulphur capture in FBBs is limestone. The lime CaO, formed by the calcination of the limestone CaCOs reacts with sulphur dioxide in the presence of oxygen to form calcium sulphate 0 Elsevier Sequoia/Printed in The Netherlands
60
CaO + SO2 + %O, -----+ CaSO,
(1)
The capture of sulphur by lime in FBBs is strongly dependent on temperature. At low temperatures the limestone does not calcine, which results in the reduced rate of sulphur capture observed at temperatures below 800 “C. A marked fall-off has also been observed at increased temperatures [9, lo], but the reason for this has not, however, been finally resolved although many explanations for the temperature dependence of sulphur capture have been presented. These are usually concerned with reaction mechanisms and/ or the pore structure of the lime. Two of these explanations, however, associate the temperature dependence with combustion or rather with the reducing conditions caused by the combustion process. According to one of the theories [ 111, the particle phase is depleted in oxygen, which virtually stops the sulphur capture (see reaction (1)). According to the other [12], the captured sulphur is released from the calcium sulphate as a result of reaction with reducing compounds, such asCOorHz: CaSO, + CO/HP ----+ CaO + SO2 + COz/HzO
(2)
The effect of reducing conditions on sulphur capture has received little consideration in the literature, possibly for the following reasons: (i) Variations in excess air ratio have been found to have little effect on sulphur capture, as long as the ratio is greater than unity, which suggests that the availability of oxygen is of minor importance. However, the possible variations in the excess air ratio in an FBB may not be large enough to allow for a transition from reducing to oxidizing conditions in the particle phase. (ii) Under overall reducing conditions, i.e. gasification, lime has been shown to be very effective in sulphur capture. Sulphur is then captured in the form of H,S according to CaO + H2S --+
CaS + H20
(3)
In the few cases where reducing zones have been considered in sulphur capture [ 13, 141, it has been assumed that sulphur is retained according to reaction (3). A study of the thermodynamic conditions for reactions (1) and (3) shows that CaS is
-10
-15
-5
log(Pol) Fig. 1. Phase diagram for the system 02, SO*, HzS, HzO, CaS, CaO and CaS04: pressures are in bars; a partial pressure for Hz0 of 0.05 bar is assumed; see also Appendix B.
stable under reducing conditions and that CaSO, is stable under oxidizing conditions. There is also an intermediate region where CaO is stable, as shown by the phase diagram (Fig. 1). Measurements of the oxygen partial pressure have shown that the bed in the FBB used in the experiments reported below is in this intermediate concentration region during a large fraction of the time [ 15, 161. During combustion, oxygen is not in equilibrium with reducing compounds such as CO. Thus reaction (2) is possible even when CaS04 is stable with respect to the oxygen partial pressure. This is seen if CO is chosen instead of O2 for the horizontal axis in the phase diagram, as in Fig. 2. The diagram indicates that, at 900 “C, the reduction of &SO, with CO is a spontaneous reaction even at the low concentrations of CO prevailing in the flue gas (typically 200 ppm). The CO concentration in the reducing zones of the bed is obviously much higher. Since reaction (2) is spontaneous in an FBB, it follows that sulphur capture may be a competition between reactions (1) and (2).
CPSOI 1000 .G & so2
t
HzS 100 ppm
.G ,' QQ ,*' B #' I' #'
i
‘Ok 10
CaS
: ,' : I' : I' '\ ,' ,' '1 ,' : ,
'\ '. '*-__
CaO
100
1000
10000
CO, ppm Fig. 2. Phase diagram for the system CO, COz, SOz, HzS, HzO, CaS, CaO and CaSOd at a total pressure of 1 bar: partial pressures for Hz0 and CO2 of 0.05 and 0.12 bar are assumed.
61
Below, experimental and literature data will be presented which indicate that reaction (2) is fast enough to affect sulphur capture under the conditions prevailing in a stationary FBB.
3. EXPERIMENTAL
The tests were performed in the 16 MW fluidized bed boiler at Chalmers University of Technology. Some data on test conditions are given in Table 1. A full account of test conditions and results is presented in ref. 17. A summary is given below of those results which are relevant to the theory discussed above. The test was designed to give information on the temperature behaviour of sulphur capture in a fluidized bed. The investigation was performed as follows: (i) A zero-test giving the temperature dependence of SO, emission prior to lime addition. The fluidized bed in this condition consisted mostly of silica sand, The sulphur content of the bed material was low, indicating that little or no CaS04 was present. (ii) Temperature dependence tests showing the sulphur capture at various temperatures with a constant addition of limestone. The molar ratio of calcium to sulphur for the high temperature test reported below was 1.5. These tests continued for eleven days and involved a substitution of bed material for lime and calcium sulphate. (iii) A sulphur capture decay test showing the decrease in sulphur capture over a period of 58 h after limestone addition was stopped. The decay test followed the temperature dependence tests so that the bed was “loaded” with CaS04 and CaO. During this test the mass fractions of &SO4 and CaO in the bed material were approximately 30% and 20% respectively, indicating the presence of 3000 kg CaS04 in the bed.
4. RESULTS
4.1. Zero test The zero test showed no discernible correlation between temperature and sulphur emissions (Fig. 3). All measured data points for SOZ are corrected for excess air ratio by the simultaneously measured O2 value, so that all
TABLE 1 Test conditions Boiler Load Bed surface Bed height Fluidization velocity Excess air ratio Feed
10 MW 10 m* lm 1.4 m s-r 1.4 Above bed from fuel chute
Fuel Size Moisture Ash Volatiles, d.a.f. Sulphur, d.a.f. Nitrogen, d.a.f.
Petroleum coke 6-18mm 11% 0.6% 10% 2.6% 2.5%
Limestone Size CaC03
Ignaberga 0.2 - 2 mm 90%
d.a.f., dry and ash free.
so2 1000
ppm 500
1
0’
’
’
’
I
’
’
’
550
000
T,
’
’
’
’
900
ac
Fig. 3. SO2 emission (at 6% 02) us. bed temperature during the 19-h zero test: data points indicate 3-min averages.
SOZ values are given for 6% OZ. The average value for SOZ emissions during the zero test was 1504 ppm. This value corresponds very well with that calculated from the fuel sulphur content. 4.2. Temperature dependence tests The high temperature test showed a very clear temperature dependence (Fig. 4). When the temperature was increased from 850 to a little above 900 “C the sulphur retention fell from approximately 50% to zero and possibly even below zero. Similar results were obtained in additional high temperature tests. If the temperature was kept high the sulphur capture again increased, probably owing to the accumulation of unreacted lime. An overview of the whole temperature dependence test is given in Fig. 5 for a molar ratio of
62
,,: ;d, ,
1500 -
:!
(3)
4
SO2
ppmlooo
,’
8’
,_d”
_P*8’ _,-soo- ’ ’ ’ 150 MO 170
/
! /
(0
/’
’
/’
. A’(*)
.
880 890 1, *c
’
900
’
-
0.0
910
’ ’
’
700
’ ’
’ ’
’
800
’ ’ ’
Temperature,
Fig. 4. SO2 emission (at 6% 02) us. bed temperature: continuous lines indicate temperature increase and broken lines indicate temperature decrease, showing the difference between the initial increase (1) and the final decrease (2) in temperature 9 - 10 h later; when the temperature is increased to a high level and then kept approximately constant, the SO2 emissions decrease (3); the zero level of SO2 emissions is 1504 ppm according to the zero test (Fig. 3) and 1535 ppm according to fuel analysis (dotted line).
’
’
$00
’ ’ ’ ’
I
1000
OC
Fig. 6. Lime conversion us. temperature in laboratory tests [ 5 ] : 0, sulphation temperature (calcination temperature constant at 850 “C); A, calcination temperature (sulphation temperature constant at 850 “C); 0, sulphation temperature equal to calcination temperature; broken line, conversion according to Fig. 5 for comparison.
1400 SO2 1300
890
pm
,200 880 Tb.d
1100
OC 870
0.0 ’ 600 ’ ’
*
0
I
’ 650
’
’
’
’
’
900
T, "C
Fig. 5. Lime conversion for a calcium to sulphur ratio of 1.5 us. bed temperature: data points indicate 3-min averages.
calcium to sulphur of 1.5. Similar results were also achieved for a ratio of calcium to sulphur of 3. The temperature behaviour in the FBB is in striking contrast with that seen in laboratory experiments with the same limestone under oxidizing conditions (Fig. 6) [ 51. It is thus possible to associate the behaviour in the FBB with the presence of combustion. 4.3. Sulphur capture decay test The dependence of SO, emissions on temperature did not cease when the lime addition was stopped (Fig. 7). At the end of the sulphur decay test a very noticeable temperature dependence was still observed, even though the lime addition had been stopped for more than 50 h (Fig. 7). The net sulphur capture had then decreased to zero. In spite of this a very strong temperature dependence was observed, of approximately 15 ppm “C-l, or,
*60w 300
310 t,
320 hours
340
Fig. 7. SO2 emissions at 6% 02 (upper graph) and changing temperature (lower graph) us. time in the sulphur capture decay test: lime addition was stopped at 280.9 h.
since total emissions were 1500 ppm, 1% “C-‘. Within the resolution permitted by the data (3-min averages) the response of SO2 emissions to bed temperature is immediate. The temperature dependence of the sulphur capture decay test is striking in view of the absence of temperature dependence during the zero test. The only significant difference between the end of the sulphur decay test and the zero test is the presence of 3000 kg CaSO,. Thus the temperature dependence should be associated with the presence of CaSOd. 4.4. In-bed oxygen measurements In-bed oxygen measurements were made at 14 locations in a horizontal plane at approxi-
63
mately half bed height [15]. These showed an oxygen partial pressure below lo-” bar for 80% - 90% of the time. This value should not be seen as an average for the whole bed, since the measurements do not cover the whole bed area, and only refer to one height. However, the measurements clearly indicate that the bed material spends a considerable fraction of the time under reducing conditions. Previous measurements in the same boiler have given similar results [ 161 as have measurements in other fluidized beds [X3, 191. A discussion of what the oxygen probe actually measures in a system that is not at equilibrium is given by Minchener and Stringer [18]. After the addition of lime the NO emissions decreased by 60% - 80% depending on the temperature. This is shown in Fig. 8 where data for an excess air ratio of approximately 1.4 are shown. Similar plots were achieved for other excess air ratios [ 171. The significance of these measurements with respect to sulphur capture is that CaS04 and CaO are catalysts for the reduction of NO under reducing conditions [20, 211. The experience from circulating FBBs is that lime addition, especially at high molar ratios, gives a strong increase in NO emissions [ 31. This is explained by the catalytic oxidation of NH3 to NO under oxidizing conditions [20, 211. (In circulating FBBs the sorbent particles spend a large fraction of the time above the dense part of the bed and in the cyclone, i.e. where the particles are in better contact with oxygen.) The observations related to NO emissions indicate that the sorbent particles in the stationary FBB experience reducing conditions. These observations are thus in accordance with the in-bed oxygen measurements.
200
NO
Fig. 8. NO emissions (at 6% 02) as a function of bed temperature for an excess air ratio of 1.4: upper graph is prior to lime addition; lower graph is after lime addition; data points indicate 3-min averages.
900 “C. These studies include data on the reducing agents Hz, CO, coal/coke and natural gas. It was observed that with highly reducing conditions, i.e. pure Hz or CO, and at temperatures below 850 - 900 “C, CaS04 was reduced to CaS: CaS04 + 4Hz/4C0 + 4Hz0/4C02
The reduction of CaS04 has been studied as a possible way of producing sulphur compounds from natural gypsum or waste materials such as phosphogypsum. It has also been studied as a means of sorbent regeneration in sulphur capture processes. Although the reaction rates are slow for the purpose of industrial processes at temperatures below 900 “C, several studies of the reduction of CaS04 have been performed at temperatures of 800 -
(4)
At higher temperatures CaO is also formed [22,23]. It has often been assumed that the reduction of CaSO,, to CaO (reaction (2)) is in two steps [24 - 261, i.e. reaction (4) followed by the solid-solid reaction CaS + 3CaS04 -
4CaO + 4S02
(5)
which proceeds in the following consecutive stages [25 ] : 3CaS04 -
3Ca0 + 3S03
CaS + 3S03 +
CaO + 4S02
(6) (7)
Another possible reaction path [27 - 291 is a reduction of CaS04 to CaSO, which then decomposes: CaS03 +
5. REACTIONS UNDER REDUCING CONDITIONS
CaS
CaO + SO2
(8)
The two reaction paths proposed in the literature need not exclude each other if it is assumed that which reaction path is actually followed depends on the conditions. It is likely that a high concentration of reducing agent and of SOZ would promote reaction (4), whereas reaction (2), via reaction (8), is to be expected at lower concentrations of reducing agent and SOZ, i.e. when the conditions are as in a fluidized bed. Literature data indicate that whether the end product of the reduction of CaS04 is CaS or CaO depends on the
64
gas composition [22, 28,30,31]. This is in accordance with the thermodynamic calculations (Fig. 1). However, if CaS is formed in the first step, other reaction paths are possible in an FBB. For example water vapour may oxidize CaS : CaS + HZ0 ---+ CaO + H,S
(9)
provided that the subsequent oxidation of H2S to SO, is fast enough. CaS may also be oxidized according to CaS + 20,
---+ CaSO,
(19)
or CaS + 11/02 ----+ CaO + SO;!
(11)
depending on the oxygen partial pressure [ 321. It should be observed that the reactions in the reaction paths discussed above are overall and not elementary reactions.
6. REACTION RATES
On the basis of the literature data it is assumed that under the conditions prevailing in a fluidized bed, the reaction does not proceed via CaS but directly to CaO according to reaction (8). Within the temperature interval 800 - 900 “C no experiments have been performed with known gas concentrations and conditions where CaO is stable, for instance with a ratio of CO to CO, of 0.01 (Fig. 2). In the absence of applicable rate data for reaction (2), the rate can only be estimated from the rates of reactions (4) and (5). This probably gives an underestimate of the rate. Data for the reduction of CaS04 at 900 “C and below are shown in Table 2 and Fig. 9. These data indicate that the reduction is faster with H2 than with CO. The reduction with carbon is slower than with CO. This is expected
TABLE 2 Literature data on reduction of CaSO,; reaction product is CaS if not otherwise specified Reference
Reducer
Temperature WI
1331
Hz
885
[=I
Hz
880
Hz 20%
880
Rate
Symbol used in Fig. 9
32% in 6 min 80% in 9 min 60% in 15 min
D
0
V
V
(334
800 - 900 800 - 900
6% min-’ 16% - 22% min-’ 6.5% - 8% min-’ 2.3% - 3.7% min-’
co co 5%, 10% co CO+H2 CO+Hz CO+H2
840 - 900 900 700 800 850 900
162 exp(-8200/T)Pco 0.8%, 1.6% min-’ 96% in 45 min 8% in 60 min 26% in 30 min 91% in 25 min
co Coal 0.35h
800,850 800,850
36%, 92% in 60 min 9%, 67% in 60 min
0 +
[301
Coal 0.31h Coal 0.062h
900 900
6% min-’ 0.2% min-’
+ *
[401d [411e [311f 142 1g
Coke Coal Coke Gas mixture
800,900 900 800 800
25%, 60% in 10 min 20% + 20% in 12 min CaO + CaS 21% in 60 min CaO 80% - 90% in 60 min CaO
[341a 1351
[261b [361a [371C 1381
[391
Hz
850
Hz
800
co
- 900
- .. --min-’
CaO
aNot isothermal; approximate rate. bSulphated dolomite; 0.2 - 0.5 mm; P,o in atm. CWith Fez03 catalyst. dPhosphogypsum. Vresence of SiOz and AIzOs. fPresence of clay. gMixture of Hz, Hz 0, CO and CO2; phosphogypsum with 10% - 20% Nas S04. hAs weight of reductive agent divided by weight of CaSO4.
.... q
0 0 0
+ * * *
65
.
-
r_
,,.......
.E
-
,‘-‘.-“F”
E
-
0.100
2
s- 0.010
..,.-““’
,,,,,,,,.... “”
_
.?-
V+ -0
+_______.-------l
r -
q
:
l
i
*
l O.OOl-
1 ’
’ 800
4
a
’
’
850
’
Temperature,
’
k
’
’
so0
OC
Fig. 9. Rate for reduction reaction: for gaseous reducers rate is given for a partial pressure of 1 bar assuming first-order reaction; for symbol list see Table 2.
since the reaction proceeds with the gaseous intermediate CO [30]. The rate probably reflects the rate of gasification of the char [ 301. For the cases where the product is CaO, either the gas composition is not known, or there is an influence exerted by the presence of other compounds. In the FBB experiments carried out, approximately 3000 kg CaSOd was contained in the bed. Assuming a sulphur capture of 50% means that CaSO, was formed at a net rate of 1.1 kg min-‘. The effect of reduction is assumed to be important if the reduction rate is greater than this, i.e. greater than l.l/ 3000 = 0.0004 min-l . This rate will now be compared with that of reactions (4) and (5). According to Fig. 9 the rate of reaction (4) can be taken as approximately of the order of 0.1 min-’ bar’
for CO (and H,). Assuming a first-order reaction and a particle-phase concentration of CO (and H, ) of 5000 - 10 000 ppm (see Appendix A) and observing the stoichiometry of reaction (5) yields a reaction rate approximately one order of magnitude greater than the net sulphur capture rate calculated above. Although reaction (5) is slow at 900 “C and below (Table 3, Fig. lo), its rate is also approximately one order of magnitude greater than that of the sulphate formation rate. This applies to a stoichiometric ratio CaS:CaS04 of 1:3. Noting that the reaction is first order in CaS [26] (but not in CaS04), the sulphur release rate according to reaction (5) corresponds to the net sulphur capture rate if a few per cent of the CaS04 is converted to CaS. The comparison involves the following important uncertainties:
q
0.1000
Q
.I
0.0100
0
x
E
;
x
0.0010
..’
..’
,.’ .’
.’
A
:
/ ”
”
I
A
O.Ooo’ L 800
”
.,’ /
2
,
x
800
Temperature,
1000
I100
@C
Fig. 10. Rate for reaction (5): for symbol list see Table 3.
TABLE 3 Literature data on reaction (5); rate given as fractional conversion of CaS Reference
Temperature (“0
Rate (min-‘)
Symbol used in Fig. 10
[251
900 950 1000
0.009 0.039 0.15
0 0 0
WI
840 - 900 900 - 1100
2.11 X 10’ exp(-29100/T) 4.61 X 10” exp(-36916/T)
--- .. -
[241
800 900
X X X
I441
800 840 880
0.0007 0.0057 > 0.033 0.000094 0.00022 0.00074
900
0.0005
1431
1000
1391
n A A 0
66
(i) The reaction path and its rate are not clearly established. The presence of combustion intermediates may also be important. (ii) The actual concentrations of CO and H2 are not known at present. (iii) The sulphate present in the fluidized bed differs in important aspects from that mentioned in most literature data. The data of the greatest relevance are probably those which show the reactivity of sulphated sorbent particles (see ref. 26 and Table 2). The sorbent, however, is dolomite and the possible influence of MgO is not known. Notwithstanding these uncertainties the data clearly indicate that the reduction reaction is fast, in view of the large amount of CaSO, contained in the bed material, and should be expected to have an effect on sulphur capture behaviour. 7. DISCUSSION
Under oxidizing conditions CaS04 is stable, whereas CaS is stable under extreme reducing conditions. There is an intermediate region where CaO is stable. The size of this region increases rapidly with temperature. In-bed oxygen measurements indicate that the sorbent particles forming the bed material are under reducing conditions for a large fraction of the time. The observation that the sorbent particles experience reducing conditions is also supported by the response of NO emissions to the addition of lime. The occurrence of reducing conditions in the particle phase can be explained by means of the two-phase theory (see Appendix A). The picture inferred from in-bed oxygen measurements, the thermodynamic stabilities of calcium-sulphur compounds, and the literature data on reaction rates is that there is a competition between sulphur capture under oxidizing conditions and release of sulphur under reducing conditions. An increased reduction rate will produce active sorbent and increase the gross sulphur capture rate. The reduction is not “seen” as long as the net sulphur capture rate, i.e. gross sulphur capture less reduction, is not markedly affected. A decrease in the net sulphur capture rate is furthermore concealed by the long residence time of the sorbent particles not elutriated. This permits a complete sulphation even at a low net rate.
The reaction rates for both the release and the capture of SO2 increase with temperature. Two factors, however, greatly promote the release rate as the temperature is increased: (i) the shift in equilibrium by which the equilibrium pressure of SO* over CaS04 under intermediate conditions increases rapidly with temperature (Fig. l), and (ii) the increasingly reducing nature of the conditions in the particle phase caused by the combustion of volatiles [ll] and by increased char gasification and combustion rate. The proposed competition between capture and release implies that at a given temperature and with stationary conditions a balance in the amount of CaSOd and CaO arises. A temperature increase shifts the balance since the desorption rate increases faster with temperature than does the absorption rate. This is observed as an increase in the SOZ emission, which subsequently decreases as a new balance is approached. If the amounts of CaS04 and CaO in the bed are large this new balance is approached slowly and the SO2 emission will appear as a direct function of temperature. If lime is added continuously the balance is approached faster. The dependence of the sulphur capture behaviour on temperature predicted from the assumption of a reductive decomposition of CaSO& under the conditions in the particle phase corresponds to that observed in Fig. 4. The SO2 capture rate decreases to zero when the temperature increases above 900 “C, and then again increases if the temperature is held constant. This behaviour strongly indicates a desorption of SO, for the following reasons: (i) If the temperature dependence were explained solely by the arrest of the sulphur capture under reducing conditions, a level of zero would not be expected, since some of the sorbent particles are in oxidizing regions, for instance in the splash zone. (ii) If the decrease in sulphur capture at higher temperatures is related to impairment of pore structure the following behaviour would be expected: at a high temperature a large amount of the reactive CaO in the bed would become impaired and accordingly the sulphur capture at a given temperature should be lower following a temperature decrease compared with that following temperature increase. Figure 4 shows that the actual behaviour is the opposite, i.e. for a given tem-
67
perature the sulphur capture is higher after a decrease than after an increase. (iii) Other explanations for the temperature dependence found in the literature related to the pore structure or reaction rate cannot explain a sulphur capture decrease to zero. Laboratory tests on the temperature dependence for the actual limestone only show a slight influence of temperature within the interval 800 - 950 “C (Fig. 6). Thus, of a number of propositions made to explain the temperature dependence of sulphur capture [ll, 12,45 - 491, only one provides a good explanation for these observations, namely the reduction theory of Fieldes [12]. None of the other theories can explain why there is a strong temperature dependence during the sulphur decay test when the net absorption is zero. Since there was no temperature dependence during the zero test, the temperature dependence seen in the sulphur decay test should be associated with the presence of CaS04 in the bed. The strong temperature dependence during the sulphur decay test accordingly indicates a capture/release of soz. When considering the effects seen in these experiments it should be noted that the FBB used has a small recirculation of elutriated particles. A circulating FBB, or a stationary FBB with a larger recirculation, will have active sorbent particles in the freeboard that may conceal the effects of reducing conditions in the dense part of the bed. A comparison of the sulphur capture in a stationary and a circulating FBB, in which the same sorbent and the same coal were used, showed that the strong fall-off in SO2 capture with temperature observed in the stationary FBB is not seen in the circulating FBB [ 31. The effect of a higher volatile content would be both to create more strongly reducing conditions in the particle phase [ll] and to give higher concentrations of H2/HZ0. Thus a higher volatile content would be expected to increase the rate of reduction of CaS04. In the present experiments the fall-off in sulphur capture started at a temperature of 860 - 8’70 “C. The experiments were performed using petroleum coke with a low volatile content. However, in previous experiments in the same boiler and with the same sorbent but with a coal of ordinary volatile content the decrease started at a much lower temperature
and the sulphur capture rate was distinctly lower at 850 “c than at 800 “C [2,3].
8. CONCLUSIONS
Thermodynamics show that CaSO, is not stable under reducing conditions. The twophase theory explains why it is likely that an important part of the bed material may experience reducing conditions. In-bed oxygen measurements indicate the presence of reducing conditions in the bed during a large fraction of the time. Literature data indicate that the reductive decomposition of CaSO,, is fast compared with the net formation rate of CaSO1,. Evidence that the proposed reduction actually occurs is given by the following observations: (i) a decrease in SOZ capture from 50% to approximately zero with a temperature increase above 900 “C; (ii) a comparison of the zero test and the sulphur decay test showing that there is a strong dependence of SO2 emissions on temperature but only in the presence of CaSO,. Further support for the proposed influence of reducing conditions includes: (i) the absence of fall-off in sulphur capture at temperatures above 900 “C in laboratory tests with the same limestone but under oxidizing conditions; (ii) the much less pronounced temperature dependence of sulphur capture in a circulating FBB where oxygen-rich zones, i.e. those above the dense part of the bed and in the cyclone, have a high sorbent particle density. The observations presented here are circumstantial evidence that &SO4 is reduced with subsequent SO2 release and that this explains the strong temperature dependence of SO? capture. This explanation is attractive since it follows from basic thermodynamics. Although the thermodynamics are simple, it must be observed that the processes governing the capture/release of SO2 are complex and that at present knowledge of these is incomplete. Therefore experimental data from real FBBs are needed to determine under what conditions, i.e. of temperature, fuel volatile content, recirculation ratio, fluidization etc., the reductive decomposition of CaS04 affects the sulphur capture.
68
The increased understanding of sulphur capture in FBBs presented here could have important implications for the design and operation of fluidized bed boilers. ACKNOWLEDGMENT
This work was supported financially by the Swedish National Energy Administration. REFERENCES 1 D. L. Keairns, R. A. Newby and N. H. UIerich, Fluidized-bed combustor design, in P. Basu (ed.), Fluidized Bed Boilers: Design and Application, Pergamon, Toronto, 1983. 2 L.-E. hand, S. Johansson, M. Karlsson and B. Leckner, Emissions from a fluidized bed boiler, Rep. A86-156, 1986 (Department of Energy Conversion, Chalmers University of Technology, Goteborg, Sweden). 3 B. Leckner and L.-E. Amand, Proc. Znt. Confi Fluid. Bed Combust., 9 (1987) 891. 4 B. Leckner and S. Herstad, Emissions from a circulating fluidized bed boiler, Rep. A86-157, 1986 (Department of Energy Conversion, Chalmers University of Technology, Goteborg, Sweden). 5 S. Ghardashkhani and E. Ljungstrom, The effect of temperature on the sulphur dioxide uptake by Ignaberga limestone, Rep. OOK 87:4, 1987 (Department of Inorganic Chemistry, Chalmers University of Technology, Goteborg, Sweden). 6 S. Ghardashkhani and E. Ljungstrom, Thermogravimetric analysis of Ignaberga limestone. The effect of COz and SOz concentration on the sulphur dioxide absorption capacity, Report OOK 87:7, 1987 (Department of Inorganic Chemistry, ChaImers University of Technology, Goteborg, Sweden). pa t$r 7 K. Dam-Johansen, Svovldioxidbinding kalk, Ph.D. Thesis, Danmarks Tekniske Hdjskole, Copenhagen, 1987. 8 E. Andersson, C. Blid, 0. Lindqvist and B.-M. Nielsen, Svenska karbonatstenars kapacitet att absorbera svaveldioxid vid atmosfiirstryck, Rep. ABZ-01, 1982 (Department of Inorganic Chemistry, Chalmers University of Technology, GSteborg, Sweden). 9 G. Moss, Proc. Int. Conf Fluid. Bed Combust., 2 (1970) 11-6-l. 10 J. M. Castleman III, Proc. Znt. Conf. Fluid. Bed Combust., 8 (1985) 196. 11 J. S. Dennis and A. N. Hayhurst, 20th Int. Symp. on Combustion, The Combustion Institute, Pittsburgh, 1985, p. 1347. 12 R. B. Fieldes, Reaction of sulphur dioxide with limestone particles, Ph.D. Thesis, University of Cambridge, 1979. 13 D. Park and 0. Levenspiel, Fuel, 61 (1982) 578. 14 F. W. Cox, W. E. Genetti and Y. Y. Lee, AIChE Symp. Ser., 81 (245) (1985) 27.
15 D. Cooper and E. Ljungstr%m, The influence of bed temperature on the in-bed 02 partial pressures in a 16 MW AFBC fired with petroleum coke, Internal Rep. OOK A87 002, 1987 (Department of Inorganic Chemistry, Chalmers University of Technology, Goteborg, Sweden). 16 E. Ljungstrom, Proc. Int. Conf. Fluid. Bed Combust., 8 (1985) 853. 17 A. Lyngfelt, Sulphur capture in a fluidised bed combustor. The effect of temperature, Rep. A8 7167, 1987 (Department of Energy Conversion, Chalmers University of Technology, GSteborg, Sweden). 18 A. J. Minchener and J. Stringer, J. Inst. Energy, 57 (1984) 240 - 251. 19 D. P. Saari and R. J. Davini, Proc. Int. Conf. Fluid. Bed Combust., 7 (1981) 995. 20 T. Furusawa, M. Tsujimura, K. Yasunaga and T. Kojima, Proc. Int. Conf. Fluid. Bed Combust., 8 (1985) 1095. 21 Y. Y. Lee, S. M. S. Soares and A. Sekthira, Proc. Int. Conf. Fluid. Bed Combust., 9 (1987) 1184. 22 R. Kuusik and A. Trikkel, Eesti NSV Tead. Akad. Toim. Keem., 34 (1985) 58. 23 I. Colussi and V. Longo, Rass. Chim., 26 (1974) 14. 24 B. Neumann, Angew. Chem., 39 (1926) 1537. 25 J. Chen and R. Yang, Ind. Eng. Chem., Fundam., 18 (1979) 134. 26 E. Kikuchi, T. Iwasaki, T. Hatsuyama and Y. Morita, Nenryo Kyokai-Shi, 62 (1983) 204. 27 I. G. Kostylkov and V. N. Nosov, J. Appl. Chem. USSR (Engl. Transl.), 55 (1982) 1773. 28 I. G. Kostylkov and V. N. Nosov, J. Appl. Chem. USSR (Engl. Transl.), 54 (1982) 2236. 29 L. Robbins, Diss. Abstr. B, 27 (4) (1966) 1139. 30 E. T. Turkdogan and J. V. Vinters, Trans. Inst. Min. Metall., Sect. C, 85 (1976) 117 - 123. 31 W. Machu and K. M. Hanna, Egypt. J. Chem., 1 (1958) 153. 32 D. C. Lynch and J. F. Elliot, Metall. Trans. B, 11 (1980) B415. 33 W. D, Bond, Production of tritium by contained nuclear explosions in salt. II. Laboratory studies of the reduction of alkaline earth sulphates by hydrogen, Rep. ORNL-3334, 1964 (U.S. Atomic Energy Commission). 34 P. Longuet and B. Courtault, Silic. Ind., 28 (1963) 28. 35 D. J. Dijksman, Spec. Publ. Chem. Sot., 12 (1958) 333. 36 R. Kuusik, P. Saikkonen and L. Niinisto, J. Therm. Anal., 30 (1985) 187. 37 T. Zadick, R. ZavaIeta and F. P. McCandless, Ind. Eng. Chem., Process Des. Dev., 11 (1972) 283. 38 W. Gorselany, M. Mielczarek-Ziemska, J. Gasowski and E. Smietalska-Pietruszkiewicz, Ochr. Powietma, 10 (1976) 57. 39 H. 0. Hofman and W. Mostowitsch, Trans. Amer. Inst. Min. Eng., 40 (1910) 917. 40 Ye. P. Parfenov, V. M. Borisov and L. M. Razdorskikh, Sou. Chem. Znd. (Engl. Transl.), 7 (1975) 1452.
69
41 K. Leipner and B. Miiller, Chem. Tech. (Leipzig), 21 (1969) 770. 42 P. S. Vladimirov, J. Appl. Chem. USSR (En&. Transl.), 55 (1982) 659. 43 J. Gasowski, M. Mielczarek-Ziemska and T. Wasag, Pr. Nauk. Politech. Szczecin, 214 (1982) 47. 44 R. Matsuzaki, H. Masumizu, N. Murakami and Y. Saeki, Bull. Chem. Sot. Jpn., 51 (1978) 121. 45 M. Hartman and 0. Trnka, Chem. Eng. Sci., 35 (1980) 1189. 46 R. Yang, C. R. Krishna and M. Steinberg, Znd. Eng. Chem., Fundam., 16 (1977) 465. 47 N. A. Burdett, J. Inst. Energy, 56 (1983) 198. 48 M. Hartman and R. Coughhn, AZChE J., 22 (1976) 490. 49 G. Moss, Inst. Fuel, Symp. Ser. (London), 1, (1) (1975) D2-1. 50 M. M. Avedesian and J. F. Davidson, Trans. Inst. Chem. Eng., 51 (1973) 121. 51 I. Barin and 0. Knacke, Thermochemical Properties of Znorganic Substances, Springer-Verlag, Berlin, 1973. 52 I. Barin and 0. Knacke, Thermochemical Properties of Inorganic Substances. Supplement, Springer-Verlag, Berlin, 1977.
two-phase theory the following hypotheses can be made: (i) The fraction of time spent under reducing conditions found from the oxygen measurements largely represents the time that the probe spends in the dense phase. (ii) The combustion reaction is sufficiently fast compared with the interphase mass transport for the dense phase to be depleted in oxygen. This will lead to the formation of CO, H, and volatiles from pyrolysis and gasification reactions. The combustible gases formed are combusted in the bed, as a result of interphase mass transfer, and above the bed, where the gas from the dense phase is mixed with oxygen-rich gas. Because of the combustion in the splash-zone together with the dilution of the gases from the particle phase in the main gas flow, it can be assumed that in the dense phase the CO concentration is considerably higher than it is in the flue gas (200 - 400 ppm). Support for the two-phase theory and the importance of interphase mass transfer is given by Avedesian and Davidson [ 50 3.
APPENDIX A: THE TWO-PHASE MODEL
According to the two-phase model consists of a dense or particle phase void or bubble phase. The gas flow dense phase is given by the minimum tion velocity umf which is estimated 0.1 - 0.2 m s-r. The fraction of gas transported through the void is given u-u
mf
u
F=
the bed and a in the fluidizato be that is by
0.9
where u is the superficial gas velocity. 90% of the gas flow thus passes the bed in a bubble phase which, as determined by bed expansion, is approximately one-fifth of the expanded bed volume. Based on the
APPENDIX B: CONSTRUCTION DIAGRAM
OF PHASE
The phase limits are given by the equilibria for reactions (l), (ll), (10) and (9). The last of these is independent of oxygen, which explains the horizontal bending of the left leg in Fig. 1. For the low total SO2 and HzS pressures in question it can be shown that at equilibrium the concentrations of COS, Sz, Ss, S4 etc. are small compared with the sum of those of SO2 and H2S. The equilibrium for CO + 1/202 --f COz gives the transformation from Fig. 1 to Fig. 2. Thermodynamic data are taken from Barin and Knacke [51,52].
Journal,
40 (1989) 59 - 69
59
Sulphur Capture in Fluidized Bed Boilers: the Effect of Reductive Decompositibn of CaS04 ANDERS LYNGFELT and BO LECKNER Chalmers Tekniska University of Technology,
Department
of Energy Conversion, S-41296
GGteborg (Sweden)
(Received November 26, 1987; in final form April 13, 1988)
ABSTRACT
Sulphur capture by lime was studied in a 16 MW stationary fluidized bed boiler (FBB). A marked fall-off in sulphur capture was noted at temperatures above about 880 “C. The proposed explanation is that the combustion produces reducing conditions in the particle phase, and thus allows for a reductive decomposition of CaSO,,. This explanation is supported by (i) thermodynamics showing the instability of CaS04 under reducing conditions; (ii) in-bed oxygen measurements indicating reducing conditions in the particle phase; (iii) the observed fall-off in sulphur capture with temperature, which is not seen in laboratory tests under oxidizing conditions or in a circulating FBB, where the sorbent particles experience oxidizing conditions to a greater extent; (iv) the observation that the temperature dependence of the sulphur emission is very strong even when the net sulphur capture is zero provided always that CaS04 is present and (v) literature data indicating the rate of the proposed reaction.
1. INTRODUCTION
The understanding of phenomena affecting sulphur capture in fluidized bed boilers (FBBs) is far from complete [ 11. A contributing factor to this is that experiments in fullscale FBBs are costly and time-consuming and often involve restrictions concerning variations in parameters. The complexity of the sy&em leads to problems in the interpretation of data. Comparisons of experiments and general conclusions are often difficult because of important differences in boiler design and operation, sorbent and fuel properties. Sulphur capture studies are also difficult owing 0300-9467/89/$3.50
to the length of time needed to obtain steady state. Furthermore, the self-absorption of most coals, caused by sulphur retention in mineral ash or unburnt combustible matter, introduces important uncertainties in the interpretation of the sulphur capture by added limestone. In the experiments reported here it was the intention to minimize parameter variations in order to allow for steady state conditions; therefore only temperature and lime addition were varied. A virtually ash-free fuel, petroleum coke, was used to eliminate the selfabsorption of fuel mineral ash. Ignaberga limestone was used as the sorbent because its sulphur capture properties have been studied previously in the stationary FBB used in this work [2, 31, as well as in a circulating FBB [ 3, 41, and in the laboratory [ 5 - 81. The important effect of temperature has been noted both with the present boiler and also with other FBBs. In spite of many proposed explanations a full understanding of the effect of temperature has not been reached. The effect of temperature was chosen as an object of study in the expectation that the discovery of the cause of this effect would increase the general understanding of sulphur capture in FBBs. Evidence in support of the theory that the temperature dependence is explained by the reductive decomposition of CaS04, is presented below in the form of thermodynamic, experimental and literature data.. 2. THEORY
A sorbent commonly used for sulphur capture in FBBs is limestone. The lime CaO, formed by the calcination of the limestone CaCOs reacts with sulphur dioxide in the presence of oxygen to form calcium sulphate 0 Elsevier Sequoia/Printed in The Netherlands
60
CaO + SO2 + %O, -----+ CaSO,
(1)
The capture of sulphur by lime in FBBs is strongly dependent on temperature. At low temperatures the limestone does not calcine, which results in the reduced rate of sulphur capture observed at temperatures below 800 “C. A marked fall-off has also been observed at increased temperatures [9, lo], but the reason for this has not, however, been finally resolved although many explanations for the temperature dependence of sulphur capture have been presented. These are usually concerned with reaction mechanisms and/ or the pore structure of the lime. Two of these explanations, however, associate the temperature dependence with combustion or rather with the reducing conditions caused by the combustion process. According to one of the theories [ 111, the particle phase is depleted in oxygen, which virtually stops the sulphur capture (see reaction (1)). According to the other [12], the captured sulphur is released from the calcium sulphate as a result of reaction with reducing compounds, such asCOorHz: CaSO, + CO/HP ----+ CaO + SO2 + COz/HzO
(2)
The effect of reducing conditions on sulphur capture has received little consideration in the literature, possibly for the following reasons: (i) Variations in excess air ratio have been found to have little effect on sulphur capture, as long as the ratio is greater than unity, which suggests that the availability of oxygen is of minor importance. However, the possible variations in the excess air ratio in an FBB may not be large enough to allow for a transition from reducing to oxidizing conditions in the particle phase. (ii) Under overall reducing conditions, i.e. gasification, lime has been shown to be very effective in sulphur capture. Sulphur is then captured in the form of H,S according to CaO + H2S --+
CaS + H20
(3)
In the few cases where reducing zones have been considered in sulphur capture [ 13, 141, it has been assumed that sulphur is retained according to reaction (3). A study of the thermodynamic conditions for reactions (1) and (3) shows that CaS is
-10
-15
-5
log(Pol) Fig. 1. Phase diagram for the system 02, SO*, HzS, HzO, CaS, CaO and CaS04: pressures are in bars; a partial pressure for Hz0 of 0.05 bar is assumed; see also Appendix B.
stable under reducing conditions and that CaSO, is stable under oxidizing conditions. There is also an intermediate region where CaO is stable, as shown by the phase diagram (Fig. 1). Measurements of the oxygen partial pressure have shown that the bed in the FBB used in the experiments reported below is in this intermediate concentration region during a large fraction of the time [ 15, 161. During combustion, oxygen is not in equilibrium with reducing compounds such as CO. Thus reaction (2) is possible even when CaS04 is stable with respect to the oxygen partial pressure. This is seen if CO is chosen instead of O2 for the horizontal axis in the phase diagram, as in Fig. 2. The diagram indicates that, at 900 “C, the reduction of &SO, with CO is a spontaneous reaction even at the low concentrations of CO prevailing in the flue gas (typically 200 ppm). The CO concentration in the reducing zones of the bed is obviously much higher. Since reaction (2) is spontaneous in an FBB, it follows that sulphur capture may be a competition between reactions (1) and (2).
CPSOI 1000 .G & so2
t
HzS 100 ppm
.G ,' QQ ,*' B #' I' #'
i
‘Ok 10
CaS
: ,' : I' : I' '\ ,' ,' '1 ,' : ,
'\ '. '*-__
CaO
100
1000
10000
CO, ppm Fig. 2. Phase diagram for the system CO, COz, SOz, HzS, HzO, CaS, CaO and CaSOd at a total pressure of 1 bar: partial pressures for Hz0 and CO2 of 0.05 and 0.12 bar are assumed.
61
Below, experimental and literature data will be presented which indicate that reaction (2) is fast enough to affect sulphur capture under the conditions prevailing in a stationary FBB.
3. EXPERIMENTAL
The tests were performed in the 16 MW fluidized bed boiler at Chalmers University of Technology. Some data on test conditions are given in Table 1. A full account of test conditions and results is presented in ref. 17. A summary is given below of those results which are relevant to the theory discussed above. The test was designed to give information on the temperature behaviour of sulphur capture in a fluidized bed. The investigation was performed as follows: (i) A zero-test giving the temperature dependence of SO, emission prior to lime addition. The fluidized bed in this condition consisted mostly of silica sand, The sulphur content of the bed material was low, indicating that little or no CaS04 was present. (ii) Temperature dependence tests showing the sulphur capture at various temperatures with a constant addition of limestone. The molar ratio of calcium to sulphur for the high temperature test reported below was 1.5. These tests continued for eleven days and involved a substitution of bed material for lime and calcium sulphate. (iii) A sulphur capture decay test showing the decrease in sulphur capture over a period of 58 h after limestone addition was stopped. The decay test followed the temperature dependence tests so that the bed was “loaded” with CaS04 and CaO. During this test the mass fractions of &SO4 and CaO in the bed material were approximately 30% and 20% respectively, indicating the presence of 3000 kg CaS04 in the bed.
4. RESULTS
4.1. Zero test The zero test showed no discernible correlation between temperature and sulphur emissions (Fig. 3). All measured data points for SOZ are corrected for excess air ratio by the simultaneously measured O2 value, so that all
TABLE 1 Test conditions Boiler Load Bed surface Bed height Fluidization velocity Excess air ratio Feed
10 MW 10 m* lm 1.4 m s-r 1.4 Above bed from fuel chute
Fuel Size Moisture Ash Volatiles, d.a.f. Sulphur, d.a.f. Nitrogen, d.a.f.
Petroleum coke 6-18mm 11% 0.6% 10% 2.6% 2.5%
Limestone Size CaC03
Ignaberga 0.2 - 2 mm 90%
d.a.f., dry and ash free.
so2 1000
ppm 500
1
0’
’
’
’
I
’
’
’
550
000
T,
’
’
’
’
900
ac
Fig. 3. SO2 emission (at 6% 02) us. bed temperature during the 19-h zero test: data points indicate 3-min averages.
SOZ values are given for 6% OZ. The average value for SOZ emissions during the zero test was 1504 ppm. This value corresponds very well with that calculated from the fuel sulphur content. 4.2. Temperature dependence tests The high temperature test showed a very clear temperature dependence (Fig. 4). When the temperature was increased from 850 to a little above 900 “C the sulphur retention fell from approximately 50% to zero and possibly even below zero. Similar results were obtained in additional high temperature tests. If the temperature was kept high the sulphur capture again increased, probably owing to the accumulation of unreacted lime. An overview of the whole temperature dependence test is given in Fig. 5 for a molar ratio of
62
,,: ;d, ,
1500 -
:!
(3)
4
SO2
ppmlooo
,’
8’
,_d”
_P*8’ _,-soo- ’ ’ ’ 150 MO 170
/
! /
(0
/’
’
/’
. A’(*)
.
880 890 1, *c
’
900
’
-
0.0
910
’ ’
’
700
’ ’
’ ’
’
800
’ ’ ’
Temperature,
Fig. 4. SO2 emission (at 6% 02) us. bed temperature: continuous lines indicate temperature increase and broken lines indicate temperature decrease, showing the difference between the initial increase (1) and the final decrease (2) in temperature 9 - 10 h later; when the temperature is increased to a high level and then kept approximately constant, the SO2 emissions decrease (3); the zero level of SO2 emissions is 1504 ppm according to the zero test (Fig. 3) and 1535 ppm according to fuel analysis (dotted line).
’
’
$00
’ ’ ’ ’
I
1000
OC
Fig. 6. Lime conversion us. temperature in laboratory tests [ 5 ] : 0, sulphation temperature (calcination temperature constant at 850 “C); A, calcination temperature (sulphation temperature constant at 850 “C); 0, sulphation temperature equal to calcination temperature; broken line, conversion according to Fig. 5 for comparison.
1400 SO2 1300
890
pm
,200 880 Tb.d
1100
OC 870
0.0 ’ 600 ’ ’
*
0
I
’ 650
’
’
’
’
’
900
T, "C
Fig. 5. Lime conversion for a calcium to sulphur ratio of 1.5 us. bed temperature: data points indicate 3-min averages.
calcium to sulphur of 1.5. Similar results were also achieved for a ratio of calcium to sulphur of 3. The temperature behaviour in the FBB is in striking contrast with that seen in laboratory experiments with the same limestone under oxidizing conditions (Fig. 6) [ 51. It is thus possible to associate the behaviour in the FBB with the presence of combustion. 4.3. Sulphur capture decay test The dependence of SO, emissions on temperature did not cease when the lime addition was stopped (Fig. 7). At the end of the sulphur decay test a very noticeable temperature dependence was still observed, even though the lime addition had been stopped for more than 50 h (Fig. 7). The net sulphur capture had then decreased to zero. In spite of this a very strong temperature dependence was observed, of approximately 15 ppm “C-l, or,
*60w 300
310 t,
320 hours
340
Fig. 7. SO2 emissions at 6% 02 (upper graph) and changing temperature (lower graph) us. time in the sulphur capture decay test: lime addition was stopped at 280.9 h.
since total emissions were 1500 ppm, 1% “C-‘. Within the resolution permitted by the data (3-min averages) the response of SO2 emissions to bed temperature is immediate. The temperature dependence of the sulphur capture decay test is striking in view of the absence of temperature dependence during the zero test. The only significant difference between the end of the sulphur decay test and the zero test is the presence of 3000 kg CaSO,. Thus the temperature dependence should be associated with the presence of CaSOd. 4.4. In-bed oxygen measurements In-bed oxygen measurements were made at 14 locations in a horizontal plane at approxi-
63
mately half bed height [15]. These showed an oxygen partial pressure below lo-” bar for 80% - 90% of the time. This value should not be seen as an average for the whole bed, since the measurements do not cover the whole bed area, and only refer to one height. However, the measurements clearly indicate that the bed material spends a considerable fraction of the time under reducing conditions. Previous measurements in the same boiler have given similar results [ 161 as have measurements in other fluidized beds [X3, 191. A discussion of what the oxygen probe actually measures in a system that is not at equilibrium is given by Minchener and Stringer [18]. After the addition of lime the NO emissions decreased by 60% - 80% depending on the temperature. This is shown in Fig. 8 where data for an excess air ratio of approximately 1.4 are shown. Similar plots were achieved for other excess air ratios [ 171. The significance of these measurements with respect to sulphur capture is that CaS04 and CaO are catalysts for the reduction of NO under reducing conditions [20, 211. The experience from circulating FBBs is that lime addition, especially at high molar ratios, gives a strong increase in NO emissions [ 31. This is explained by the catalytic oxidation of NH3 to NO under oxidizing conditions [20, 211. (In circulating FBBs the sorbent particles spend a large fraction of the time above the dense part of the bed and in the cyclone, i.e. where the particles are in better contact with oxygen.) The observations related to NO emissions indicate that the sorbent particles in the stationary FBB experience reducing conditions. These observations are thus in accordance with the in-bed oxygen measurements.
200
NO
Fig. 8. NO emissions (at 6% 02) as a function of bed temperature for an excess air ratio of 1.4: upper graph is prior to lime addition; lower graph is after lime addition; data points indicate 3-min averages.
900 “C. These studies include data on the reducing agents Hz, CO, coal/coke and natural gas. It was observed that with highly reducing conditions, i.e. pure Hz or CO, and at temperatures below 850 - 900 “C, CaS04 was reduced to CaS: CaS04 + 4Hz/4C0 + 4Hz0/4C02
The reduction of CaS04 has been studied as a possible way of producing sulphur compounds from natural gypsum or waste materials such as phosphogypsum. It has also been studied as a means of sorbent regeneration in sulphur capture processes. Although the reaction rates are slow for the purpose of industrial processes at temperatures below 900 “C, several studies of the reduction of CaS04 have been performed at temperatures of 800 -
(4)
At higher temperatures CaO is also formed [22,23]. It has often been assumed that the reduction of CaSO,, to CaO (reaction (2)) is in two steps [24 - 261, i.e. reaction (4) followed by the solid-solid reaction CaS + 3CaS04 -
4CaO + 4S02
(5)
which proceeds in the following consecutive stages [25 ] : 3CaS04 -
3Ca0 + 3S03
CaS + 3S03 +
CaO + 4S02
(6) (7)
Another possible reaction path [27 - 291 is a reduction of CaS04 to CaSO, which then decomposes: CaS03 +
5. REACTIONS UNDER REDUCING CONDITIONS
CaS
CaO + SO2
(8)
The two reaction paths proposed in the literature need not exclude each other if it is assumed that which reaction path is actually followed depends on the conditions. It is likely that a high concentration of reducing agent and of SOZ would promote reaction (4), whereas reaction (2), via reaction (8), is to be expected at lower concentrations of reducing agent and SOZ, i.e. when the conditions are as in a fluidized bed. Literature data indicate that whether the end product of the reduction of CaS04 is CaS or CaO depends on the
64
gas composition [22, 28,30,31]. This is in accordance with the thermodynamic calculations (Fig. 1). However, if CaS is formed in the first step, other reaction paths are possible in an FBB. For example water vapour may oxidize CaS : CaS + HZ0 ---+ CaO + H,S
(9)
provided that the subsequent oxidation of H2S to SO, is fast enough. CaS may also be oxidized according to CaS + 20,
---+ CaSO,
(19)
or CaS + 11/02 ----+ CaO + SO;!
(11)
depending on the oxygen partial pressure [ 321. It should be observed that the reactions in the reaction paths discussed above are overall and not elementary reactions.
6. REACTION RATES
On the basis of the literature data it is assumed that under the conditions prevailing in a fluidized bed, the reaction does not proceed via CaS but directly to CaO according to reaction (8). Within the temperature interval 800 - 900 “C no experiments have been performed with known gas concentrations and conditions where CaO is stable, for instance with a ratio of CO to CO, of 0.01 (Fig. 2). In the absence of applicable rate data for reaction (2), the rate can only be estimated from the rates of reactions (4) and (5). This probably gives an underestimate of the rate. Data for the reduction of CaS04 at 900 “C and below are shown in Table 2 and Fig. 9. These data indicate that the reduction is faster with H2 than with CO. The reduction with carbon is slower than with CO. This is expected
TABLE 2 Literature data on reduction of CaSO,; reaction product is CaS if not otherwise specified Reference
Reducer
Temperature WI
1331
Hz
885
[=I
Hz
880
Hz 20%
880
Rate
Symbol used in Fig. 9
32% in 6 min 80% in 9 min 60% in 15 min
D
0
V
V
(334
800 - 900 800 - 900
6% min-’ 16% - 22% min-’ 6.5% - 8% min-’ 2.3% - 3.7% min-’
co co 5%, 10% co CO+H2 CO+Hz CO+H2
840 - 900 900 700 800 850 900
162 exp(-8200/T)Pco 0.8%, 1.6% min-’ 96% in 45 min 8% in 60 min 26% in 30 min 91% in 25 min
co Coal 0.35h
800,850 800,850
36%, 92% in 60 min 9%, 67% in 60 min
0 +
[301
Coal 0.31h Coal 0.062h
900 900
6% min-’ 0.2% min-’
+ *
[401d [411e [311f 142 1g
Coke Coal Coke Gas mixture
800,900 900 800 800
25%, 60% in 10 min 20% + 20% in 12 min CaO + CaS 21% in 60 min CaO 80% - 90% in 60 min CaO
[341a 1351
[261b [361a [371C 1381
[391
Hz
850
Hz
800
co
- 900
- .. --min-’
CaO
aNot isothermal; approximate rate. bSulphated dolomite; 0.2 - 0.5 mm; P,o in atm. CWith Fez03 catalyst. dPhosphogypsum. Vresence of SiOz and AIzOs. fPresence of clay. gMixture of Hz, Hz 0, CO and CO2; phosphogypsum with 10% - 20% Nas S04. hAs weight of reductive agent divided by weight of CaSO4.
.... q
0 0 0
+ * * *
65
.
-
r_
,,.......
.E
-
,‘-‘.-“F”
E
-
0.100
2
s- 0.010
..,.-““’
,,,,,,,,.... “”
_
.?-
V+ -0
+_______.-------l
r -
q
:
l
i
*
l O.OOl-
1 ’
’ 800
4
a
’
’
850
’
Temperature,
’
k
’
’
so0
OC
Fig. 9. Rate for reduction reaction: for gaseous reducers rate is given for a partial pressure of 1 bar assuming first-order reaction; for symbol list see Table 2.
since the reaction proceeds with the gaseous intermediate CO [30]. The rate probably reflects the rate of gasification of the char [ 301. For the cases where the product is CaO, either the gas composition is not known, or there is an influence exerted by the presence of other compounds. In the FBB experiments carried out, approximately 3000 kg CaSOd was contained in the bed. Assuming a sulphur capture of 50% means that CaSO, was formed at a net rate of 1.1 kg min-‘. The effect of reduction is assumed to be important if the reduction rate is greater than this, i.e. greater than l.l/ 3000 = 0.0004 min-l . This rate will now be compared with that of reactions (4) and (5). According to Fig. 9 the rate of reaction (4) can be taken as approximately of the order of 0.1 min-’ bar’
for CO (and H,). Assuming a first-order reaction and a particle-phase concentration of CO (and H, ) of 5000 - 10 000 ppm (see Appendix A) and observing the stoichiometry of reaction (5) yields a reaction rate approximately one order of magnitude greater than the net sulphur capture rate calculated above. Although reaction (5) is slow at 900 “C and below (Table 3, Fig. lo), its rate is also approximately one order of magnitude greater than that of the sulphate formation rate. This applies to a stoichiometric ratio CaS:CaS04 of 1:3. Noting that the reaction is first order in CaS [26] (but not in CaS04), the sulphur release rate according to reaction (5) corresponds to the net sulphur capture rate if a few per cent of the CaS04 is converted to CaS. The comparison involves the following important uncertainties:
q
0.1000
Q
.I
0.0100
0
x
E
;
x
0.0010
..’
..’
,.’ .’
.’
A
:
/ ”
”
I
A
O.Ooo’ L 800
”
.,’ /
2
,
x
800
Temperature,
1000
I100
@C
Fig. 10. Rate for reaction (5): for symbol list see Table 3.
TABLE 3 Literature data on reaction (5); rate given as fractional conversion of CaS Reference
Temperature (“0
Rate (min-‘)
Symbol used in Fig. 10
[251
900 950 1000
0.009 0.039 0.15
0 0 0
WI
840 - 900 900 - 1100
2.11 X 10’ exp(-29100/T) 4.61 X 10” exp(-36916/T)
--- .. -
[241
800 900
X X X
I441
800 840 880
0.0007 0.0057 > 0.033 0.000094 0.00022 0.00074
900
0.0005
1431
1000
1391
n A A 0
66
(i) The reaction path and its rate are not clearly established. The presence of combustion intermediates may also be important. (ii) The actual concentrations of CO and H2 are not known at present. (iii) The sulphate present in the fluidized bed differs in important aspects from that mentioned in most literature data. The data of the greatest relevance are probably those which show the reactivity of sulphated sorbent particles (see ref. 26 and Table 2). The sorbent, however, is dolomite and the possible influence of MgO is not known. Notwithstanding these uncertainties the data clearly indicate that the reduction reaction is fast, in view of the large amount of CaSO, contained in the bed material, and should be expected to have an effect on sulphur capture behaviour. 7. DISCUSSION
Under oxidizing conditions CaS04 is stable, whereas CaS is stable under extreme reducing conditions. There is an intermediate region where CaO is stable. The size of this region increases rapidly with temperature. In-bed oxygen measurements indicate that the sorbent particles forming the bed material are under reducing conditions for a large fraction of the time. The observation that the sorbent particles experience reducing conditions is also supported by the response of NO emissions to the addition of lime. The occurrence of reducing conditions in the particle phase can be explained by means of the two-phase theory (see Appendix A). The picture inferred from in-bed oxygen measurements, the thermodynamic stabilities of calcium-sulphur compounds, and the literature data on reaction rates is that there is a competition between sulphur capture under oxidizing conditions and release of sulphur under reducing conditions. An increased reduction rate will produce active sorbent and increase the gross sulphur capture rate. The reduction is not “seen” as long as the net sulphur capture rate, i.e. gross sulphur capture less reduction, is not markedly affected. A decrease in the net sulphur capture rate is furthermore concealed by the long residence time of the sorbent particles not elutriated. This permits a complete sulphation even at a low net rate.
The reaction rates for both the release and the capture of SO2 increase with temperature. Two factors, however, greatly promote the release rate as the temperature is increased: (i) the shift in equilibrium by which the equilibrium pressure of SO* over CaS04 under intermediate conditions increases rapidly with temperature (Fig. l), and (ii) the increasingly reducing nature of the conditions in the particle phase caused by the combustion of volatiles [ll] and by increased char gasification and combustion rate. The proposed competition between capture and release implies that at a given temperature and with stationary conditions a balance in the amount of CaSOd and CaO arises. A temperature increase shifts the balance since the desorption rate increases faster with temperature than does the absorption rate. This is observed as an increase in the SOZ emission, which subsequently decreases as a new balance is approached. If the amounts of CaS04 and CaO in the bed are large this new balance is approached slowly and the SO2 emission will appear as a direct function of temperature. If lime is added continuously the balance is approached faster. The dependence of the sulphur capture behaviour on temperature predicted from the assumption of a reductive decomposition of CaSO& under the conditions in the particle phase corresponds to that observed in Fig. 4. The SO2 capture rate decreases to zero when the temperature increases above 900 “C, and then again increases if the temperature is held constant. This behaviour strongly indicates a desorption of SO, for the following reasons: (i) If the temperature dependence were explained solely by the arrest of the sulphur capture under reducing conditions, a level of zero would not be expected, since some of the sorbent particles are in oxidizing regions, for instance in the splash zone. (ii) If the decrease in sulphur capture at higher temperatures is related to impairment of pore structure the following behaviour would be expected: at a high temperature a large amount of the reactive CaO in the bed would become impaired and accordingly the sulphur capture at a given temperature should be lower following a temperature decrease compared with that following temperature increase. Figure 4 shows that the actual behaviour is the opposite, i.e. for a given tem-
67
perature the sulphur capture is higher after a decrease than after an increase. (iii) Other explanations for the temperature dependence found in the literature related to the pore structure or reaction rate cannot explain a sulphur capture decrease to zero. Laboratory tests on the temperature dependence for the actual limestone only show a slight influence of temperature within the interval 800 - 950 “C (Fig. 6). Thus, of a number of propositions made to explain the temperature dependence of sulphur capture [ll, 12,45 - 491, only one provides a good explanation for these observations, namely the reduction theory of Fieldes [12]. None of the other theories can explain why there is a strong temperature dependence during the sulphur decay test when the net absorption is zero. Since there was no temperature dependence during the zero test, the temperature dependence seen in the sulphur decay test should be associated with the presence of CaS04 in the bed. The strong temperature dependence during the sulphur decay test accordingly indicates a capture/release of soz. When considering the effects seen in these experiments it should be noted that the FBB used has a small recirculation of elutriated particles. A circulating FBB, or a stationary FBB with a larger recirculation, will have active sorbent particles in the freeboard that may conceal the effects of reducing conditions in the dense part of the bed. A comparison of the sulphur capture in a stationary and a circulating FBB, in which the same sorbent and the same coal were used, showed that the strong fall-off in SO2 capture with temperature observed in the stationary FBB is not seen in the circulating FBB [ 31. The effect of a higher volatile content would be both to create more strongly reducing conditions in the particle phase [ll] and to give higher concentrations of H2/HZ0. Thus a higher volatile content would be expected to increase the rate of reduction of CaS04. In the present experiments the fall-off in sulphur capture started at a temperature of 860 - 8’70 “C. The experiments were performed using petroleum coke with a low volatile content. However, in previous experiments in the same boiler and with the same sorbent but with a coal of ordinary volatile content the decrease started at a much lower temperature
and the sulphur capture rate was distinctly lower at 850 “c than at 800 “C [2,3].
8. CONCLUSIONS
Thermodynamics show that CaSO, is not stable under reducing conditions. The twophase theory explains why it is likely that an important part of the bed material may experience reducing conditions. In-bed oxygen measurements indicate the presence of reducing conditions in the bed during a large fraction of the time. Literature data indicate that the reductive decomposition of CaSO,, is fast compared with the net formation rate of CaSO1,. Evidence that the proposed reduction actually occurs is given by the following observations: (i) a decrease in SOZ capture from 50% to approximately zero with a temperature increase above 900 “C; (ii) a comparison of the zero test and the sulphur decay test showing that there is a strong dependence of SO2 emissions on temperature but only in the presence of CaSO,. Further support for the proposed influence of reducing conditions includes: (i) the absence of fall-off in sulphur capture at temperatures above 900 “C in laboratory tests with the same limestone but under oxidizing conditions; (ii) the much less pronounced temperature dependence of sulphur capture in a circulating FBB where oxygen-rich zones, i.e. those above the dense part of the bed and in the cyclone, have a high sorbent particle density. The observations presented here are circumstantial evidence that &SO4 is reduced with subsequent SO2 release and that this explains the strong temperature dependence of SO? capture. This explanation is attractive since it follows from basic thermodynamics. Although the thermodynamics are simple, it must be observed that the processes governing the capture/release of SO2 are complex and that at present knowledge of these is incomplete. Therefore experimental data from real FBBs are needed to determine under what conditions, i.e. of temperature, fuel volatile content, recirculation ratio, fluidization etc., the reductive decomposition of CaS04 affects the sulphur capture.
68
The increased understanding of sulphur capture in FBBs presented here could have important implications for the design and operation of fluidized bed boilers. ACKNOWLEDGMENT
This work was supported financially by the Swedish National Energy Administration. REFERENCES 1 D. L. Keairns, R. A. Newby and N. H. UIerich, Fluidized-bed combustor design, in P. Basu (ed.), Fluidized Bed Boilers: Design and Application, Pergamon, Toronto, 1983. 2 L.-E. hand, S. Johansson, M. Karlsson and B. Leckner, Emissions from a fluidized bed boiler, Rep. A86-156, 1986 (Department of Energy Conversion, Chalmers University of Technology, Goteborg, Sweden). 3 B. Leckner and L.-E. Amand, Proc. Znt. Confi Fluid. Bed Combust., 9 (1987) 891. 4 B. Leckner and S. Herstad, Emissions from a circulating fluidized bed boiler, Rep. A86-157, 1986 (Department of Energy Conversion, Chalmers University of Technology, Goteborg, Sweden). 5 S. Ghardashkhani and E. Ljungstrom, The effect of temperature on the sulphur dioxide uptake by Ignaberga limestone, Rep. OOK 87:4, 1987 (Department of Inorganic Chemistry, Chalmers University of Technology, Goteborg, Sweden). 6 S. Ghardashkhani and E. Ljungstrom, Thermogravimetric analysis of Ignaberga limestone. The effect of COz and SOz concentration on the sulphur dioxide absorption capacity, Report OOK 87:7, 1987 (Department of Inorganic Chemistry, ChaImers University of Technology, Goteborg, Sweden). pa t$r 7 K. Dam-Johansen, Svovldioxidbinding kalk, Ph.D. Thesis, Danmarks Tekniske Hdjskole, Copenhagen, 1987. 8 E. Andersson, C. Blid, 0. Lindqvist and B.-M. Nielsen, Svenska karbonatstenars kapacitet att absorbera svaveldioxid vid atmosfiirstryck, Rep. ABZ-01, 1982 (Department of Inorganic Chemistry, Chalmers University of Technology, GSteborg, Sweden). 9 G. Moss, Proc. Int. Conf Fluid. Bed Combust., 2 (1970) 11-6-l. 10 J. M. Castleman III, Proc. Znt. Conf. Fluid. Bed Combust., 8 (1985) 196. 11 J. S. Dennis and A. N. Hayhurst, 20th Int. Symp. on Combustion, The Combustion Institute, Pittsburgh, 1985, p. 1347. 12 R. B. Fieldes, Reaction of sulphur dioxide with limestone particles, Ph.D. Thesis, University of Cambridge, 1979. 13 D. Park and 0. Levenspiel, Fuel, 61 (1982) 578. 14 F. W. Cox, W. E. Genetti and Y. Y. Lee, AIChE Symp. Ser., 81 (245) (1985) 27.
15 D. Cooper and E. Ljungstr%m, The influence of bed temperature on the in-bed 02 partial pressures in a 16 MW AFBC fired with petroleum coke, Internal Rep. OOK A87 002, 1987 (Department of Inorganic Chemistry, Chalmers University of Technology, Goteborg, Sweden). 16 E. Ljungstrom, Proc. Int. Conf. Fluid. Bed Combust., 8 (1985) 853. 17 A. Lyngfelt, Sulphur capture in a fluidised bed combustor. The effect of temperature, Rep. A8 7167, 1987 (Department of Energy Conversion, Chalmers University of Technology, GSteborg, Sweden). 18 A. J. Minchener and J. Stringer, J. Inst. Energy, 57 (1984) 240 - 251. 19 D. P. Saari and R. J. Davini, Proc. Int. Conf. Fluid. Bed Combust., 7 (1981) 995. 20 T. Furusawa, M. Tsujimura, K. Yasunaga and T. Kojima, Proc. Int. Conf. Fluid. Bed Combust., 8 (1985) 1095. 21 Y. Y. Lee, S. M. S. Soares and A. Sekthira, Proc. Int. Conf. Fluid. Bed Combust., 9 (1987) 1184. 22 R. Kuusik and A. Trikkel, Eesti NSV Tead. Akad. Toim. Keem., 34 (1985) 58. 23 I. Colussi and V. Longo, Rass. Chim., 26 (1974) 14. 24 B. Neumann, Angew. Chem., 39 (1926) 1537. 25 J. Chen and R. Yang, Ind. Eng. Chem., Fundam., 18 (1979) 134. 26 E. Kikuchi, T. Iwasaki, T. Hatsuyama and Y. Morita, Nenryo Kyokai-Shi, 62 (1983) 204. 27 I. G. Kostylkov and V. N. Nosov, J. Appl. Chem. USSR (Engl. Transl.), 55 (1982) 1773. 28 I. G. Kostylkov and V. N. Nosov, J. Appl. Chem. USSR (Engl. Transl.), 54 (1982) 2236. 29 L. Robbins, Diss. Abstr. B, 27 (4) (1966) 1139. 30 E. T. Turkdogan and J. V. Vinters, Trans. Inst. Min. Metall., Sect. C, 85 (1976) 117 - 123. 31 W. Machu and K. M. Hanna, Egypt. J. Chem., 1 (1958) 153. 32 D. C. Lynch and J. F. Elliot, Metall. Trans. B, 11 (1980) B415. 33 W. D, Bond, Production of tritium by contained nuclear explosions in salt. II. Laboratory studies of the reduction of alkaline earth sulphates by hydrogen, Rep. ORNL-3334, 1964 (U.S. Atomic Energy Commission). 34 P. Longuet and B. Courtault, Silic. Ind., 28 (1963) 28. 35 D. J. Dijksman, Spec. Publ. Chem. Sot., 12 (1958) 333. 36 R. Kuusik, P. Saikkonen and L. Niinisto, J. Therm. Anal., 30 (1985) 187. 37 T. Zadick, R. ZavaIeta and F. P. McCandless, Ind. Eng. Chem., Process Des. Dev., 11 (1972) 283. 38 W. Gorselany, M. Mielczarek-Ziemska, J. Gasowski and E. Smietalska-Pietruszkiewicz, Ochr. Powietma, 10 (1976) 57. 39 H. 0. Hofman and W. Mostowitsch, Trans. Amer. Inst. Min. Eng., 40 (1910) 917. 40 Ye. P. Parfenov, V. M. Borisov and L. M. Razdorskikh, Sou. Chem. Znd. (Engl. Transl.), 7 (1975) 1452.
69
41 K. Leipner and B. Miiller, Chem. Tech. (Leipzig), 21 (1969) 770. 42 P. S. Vladimirov, J. Appl. Chem. USSR (En&. Transl.), 55 (1982) 659. 43 J. Gasowski, M. Mielczarek-Ziemska and T. Wasag, Pr. Nauk. Politech. Szczecin, 214 (1982) 47. 44 R. Matsuzaki, H. Masumizu, N. Murakami and Y. Saeki, Bull. Chem. Sot. Jpn., 51 (1978) 121. 45 M. Hartman and 0. Trnka, Chem. Eng. Sci., 35 (1980) 1189. 46 R. Yang, C. R. Krishna and M. Steinberg, Znd. Eng. Chem., Fundam., 16 (1977) 465. 47 N. A. Burdett, J. Inst. Energy, 56 (1983) 198. 48 M. Hartman and R. Coughhn, AZChE J., 22 (1976) 490. 49 G. Moss, Inst. Fuel, Symp. Ser. (London), 1, (1) (1975) D2-1. 50 M. M. Avedesian and J. F. Davidson, Trans. Inst. Chem. Eng., 51 (1973) 121. 51 I. Barin and 0. Knacke, Thermochemical Properties of Znorganic Substances, Springer-Verlag, Berlin, 1973. 52 I. Barin and 0. Knacke, Thermochemical Properties of Inorganic Substances. Supplement, Springer-Verlag, Berlin, 1977.
two-phase theory the following hypotheses can be made: (i) The fraction of time spent under reducing conditions found from the oxygen measurements largely represents the time that the probe spends in the dense phase. (ii) The combustion reaction is sufficiently fast compared with the interphase mass transport for the dense phase to be depleted in oxygen. This will lead to the formation of CO, H, and volatiles from pyrolysis and gasification reactions. The combustible gases formed are combusted in the bed, as a result of interphase mass transfer, and above the bed, where the gas from the dense phase is mixed with oxygen-rich gas. Because of the combustion in the splash-zone together with the dilution of the gases from the particle phase in the main gas flow, it can be assumed that in the dense phase the CO concentration is considerably higher than it is in the flue gas (200 - 400 ppm). Support for the two-phase theory and the importance of interphase mass transfer is given by Avedesian and Davidson [ 50 3.
APPENDIX A: THE TWO-PHASE MODEL
According to the two-phase model consists of a dense or particle phase void or bubble phase. The gas flow dense phase is given by the minimum tion velocity umf which is estimated 0.1 - 0.2 m s-r. The fraction of gas transported through the void is given u-u
mf
u
F=
the bed and a in the fluidizato be that is by
0.9
where u is the superficial gas velocity. 90% of the gas flow thus passes the bed in a bubble phase which, as determined by bed expansion, is approximately one-fifth of the expanded bed volume. Based on the
APPENDIX B: CONSTRUCTION DIAGRAM
OF PHASE
The phase limits are given by the equilibria for reactions (l), (ll), (10) and (9). The last of these is independent of oxygen, which explains the horizontal bending of the left leg in Fig. 1. For the low total SO2 and HzS pressures in question it can be shown that at equilibrium the concentrations of COS, Sz, Ss, S4 etc. are small compared with the sum of those of SO2 and H2S. The equilibrium for CO + 1/202 --f COz gives the transformation from Fig. 1 to Fig. 2. Thermodynamic data are taken from Barin and Knacke [51,52].