Environ. Sci. Techno/. 1995, 29, 372-383
Suffihtion-RegeReration Cycles of ZnO- and CaO-ContainingSorbents S O P H I A C . CHRISTOFOROU, E V A N G E L O S A . EFTHIMIADIS,* A N D IACOVOS A. VASALOS Chemical Process Engineering Research Institute and Department of Chemical Engineering, Aristotelian Uniuersity of Thessaloniki, P.O. Box 151 7, 54006 University City, Thessaloniki, Greece
The desulfurization performance of regenerable metal oxides was tested in a fluidized bed reactor. A reductive regeneration step was applied between the oxidative regeneration and the subsequent sulfidation when the metal oxide was ZnO. The reaction of the sulfided CaO sorbents with H20 and CO2 produced CaC03 that decomposed under an inert atmosphere. The H2S removal ability of the regenerated CaO sorbents increased when H2 and CO were added to the regenerative gas mixture. Sorbents derived from the impregnation of a metal oxide on magnesia exhibited larger desulfurization capacity than those derived from the impregnation of the same oxide on alumina. The treatment of the CaO sorbents with methanol increased the surface area, the pore volume, and the desulfurization capacity of the sorbents. The pore structure analysis of sorbents a t different stages of the sulfidation-regeneration cycle showed insignificant changes in the physical properties of the sorbents.
Introduction The hot coal gas stream at the exit of a coal gas gasifier unit contains sulfur species,primarily HzS,that must be removed before its use in the gas turbines. Sorbents have been employed for this purpose. These are porous metal oxides (e.g.,ZnO, CaO, CuO, Fe2O3, MnO, MnzO3, and V203) that react with HzS and produce metal sulfidesand vapor water. The metal oxides can be converted to the initial metal oxide in the regenerator of the desulfurizationunit. Acommercial sorbent should be regenerable so that it can be used for multiple sulfidation-regeneration cycles. In the opposite case, the purchase of fresh sorbent and the disposal of the sulfided sorbent are expected to affect negativelythe overall effectivenessof the Integrated GasificationCombined Cycle (IGCC) system. The regeneration of a metal sulfide can be performed either by using 0 2 or by using water according to the following reactions:
and
The sulfidation and regeneration of zinc oxide and zinc titanate sorbents (mixturesofZnO and Ti03 were examined in a thermogravimetric reactor by Woods et al. (1). The rate of the regeneration reaction was favored by high oxygen concentrations and high (for a constant oxygen molar fraction) or low (for a constant oxygen concentration and above 1 atm) pressures. When the regeneration took place at high temperatures (above 720 "C),the temperature did not affect the observed oxidation rate. The reaction of a metal sulfide and oxygen can lead to the formation of a metal sulfate (MS04),a side reaction that leads to the loss of the sorbent desulfurizationcapacity. Depending on the reaction conditions, the sulfate decomposes to its metal oxide, to its metal sulfide, or remains unchanged. When the sorbent consists of ZnO, the zinc sulfate formed during the oxidative regeneration decomposes during the initial stages of the sulfidation releasing sulfur species 6 0 2 , HzS, and COS) into the gas stream (2). Grindley (2) examined the conditions under which ZnS04 decomposes. He proposed a reductive regeneration step (reaction of HZand CO with ZnSOd between the oxidative regeneration and the subsequent sulfidation to decompose the ZnS04. His results showed that the rate of the ZnS04 dissociation depends on the temperature and that a significant amount of the sulfate is converted to the sulfide. The oxidation of Cas under typical oxidative regeneration conditions produces CaS04. The reductive decomposition of CaS04was examined by Kamphuis et al. (3). Temperatures higher than 1300 K are required to form CaO while temperatures higher than 1100 K lead to Cas which, according to Kamphuis et al., undergoes a solid-solid reaction with CaS04 (in an inert atmosphere) producing CaO and SOz. However, the incomplete dissociation of the CaS04 is expected to form a solid mixture of CaS04-CaOa Author to whom correspondence should be addressed; e-mail address:
[email protected].
372 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL.
29, NO. 2,1995
0013-936W95/0929-0372$09.00/0
0 1995 American Chemical Society
Cas, and consequently, the regenerated solid will exhibit lower desulfurization capacity. An alternative regeneration route can be used to recover the initial metal oxide from Cas: Cas
+ CO, + H,O - CaCO, + H,S
(3)
The carbonate easily decomposes under an inert atmosphere producing CaO and COZ. Metal sulfates were formed during the regeneration of sorbents prepared using the impregnation of copper and manganese oxides on a zeolite by Atimtay et al. (4). No reductive regeneration step was needed for these sorbents; therefore, sulfate residuals were present in the regenerated sorbents. The H2Sconcentration at the exit of a fixed-bed reactor loaded with CuO-MnO sorbents was of the order of 200 ppmv. The high H2S concentrations before the breakthrough point could be attributed to the internal structure of the solid carrier that was characterized by micropores (zeolite surfacearea482 mz/gandporediameter 5.4 A before impregnation). The sulfidation and the regeneration of a metal oxide sorbent are gas-solid reactions with solid product formation, where the solid product occupies more (sulfidation) or less (regeneration) space than the solid reactant, respectively. The pore structure analysis of the sorbent at different stages of the sflidation-regeneration cycle gives information about the structural changes that are due to the chemical reactions. No significant differences were measured between the surface area and the total pore volume of the fresh and the regenerated zinc titanates by Woods et al. (1). This implies that the physical properties of these sorbents changed during sulfidation, but they were almost completely restored (the average pore diameter of the regenerated sample was lower than that of the fresh material) during the regeneration. Efthimiadis (5) compared the physical properties of unreacted and regenerated commercial zinc oxides. These measurements showed a significant surfacearealoss and almost complete restoration of the total pore volume after the regeneration. Beside that, the pore size distribution determined by the mercury intrusion data was shifted toward larger pore sizes relatively to that of the unreacted solid. The same structural change was also noticed in the porosimetric data of Grindley (2) and Efthimiadis and Sotirchos (6). Grindley noticed that the porosity of the oxidatively regenerated samples was lower than that of the reductively regenerated samples, and he attributed these measurements to the decomposition of zinc sulfate. The scope of this study is to prepare sorbents using the impregnation of ZnO and CaO on porous carriers and to examine their desulfurization performance during multiple sulfidation-regeneration experiments. Experimentswere performed in a fluidized-bed reactor since this reactor practically eliminates the temperature increase due to the exothermic regeneration reaction. The sulfidation of fresh sorbents was studied by Vasalos et al. ( 3 ,where a detailed investigation of the parameters that affect the HPSremoval capacity of a sorbent was performed. In this study, we changed the initial pore structure of a CaO sorbent by treating the porous solid with methanol, and we compared its desulfurization capacity with that of the untreated sorbent. Metal oxide sorbents that contain ZnO have been tested previously in thermogravimetric and fixed-bed
arrangements. On the contrary, the performance of CaO in multiple sulfidations has not been tested extensively, probably because CaS cannot be converted to CaO easily. An example of a mixed metal oxide with CaO is tricalcium silicate ((CaO)sSiOz),which was used for the removal of H2S by Yo0 and Steinberg (8). The sulfided product ((CaS13Si021 was oxidized to produce (CaS04)sSiOz. This sulfate was then reactedwith COZto produce the initial oxide. The above sulfidation-regeneration cycle was performed at temperatures higher (950-1000 "C) than those of the gas stream at the exit of the Shell (900 "C) and the Texaco (700 "C) gasifiers (9). Therefore, the objective of this work is to find out those reaction conditions that allow the use of the sorbents in multiple sulfidations and in the same time are applicable to the above commercial power plant units.
Themedynamic Studies Thermodynamic computations can be used to estimate the concentration of the chemical components involved in a gas-solid reaction when equilibrium is reached. The thermodynamic criteria that characterize an efficient desulfurization sorbent are low HzS concentration at the exit of the sulfidation reactor and complete conversion of its sulfide to the initial sorbent during regeneration. The thermodynamic desulfurization potential of sorbents was examined in previous studies (10, 111, where it was calculated that the extent of the H2S removal in the sulfidation reactor is a function of the reaction temperature and the inlet gas composition. Our computations agreed with those of previous studies, and efficient H2S adsorption-H2S concentration of the order of a few parts per million by volume-was predicted for metal oxides with high affinityfor HIS. In this section, we examine the effect of the reaction conditions during regeneration (composition of the gas mixture and temperature) on the conversion of the sulfides to the initial oxides. We limited our thermodynamic studies to the regeneration of ZnO and CaO, because these metal oxides were experimentally tested in our fluidized-bed facility. We applied CHEMQ, a program for the computation of chemical equilibrium, developed by Kirkpatrick and Pike (12)to predict the behavior of gas-solid systems at various reaction conditions. The program is based on the free energy minimization method and requires the initial composition of the species involved in the chemical reactions and two system parameters, such as temperature and pressure. The program uses a database of chemical species with their thermodynamic properties and predicts the formation of the new species due to the chemical reaction. We initially examined the reaction of ZnS with an oxygen in nitrogen mixture at different temperatures, considering that the moles of the oxygen were double or more than those of ZnS. We predicted that at temperatures higher than 1000 K all ZnS is converted to ZnO (reaction l), while at temperatures lower than 873 K almost all ZnS is converted to ZnSOr. At intermediate temperatures, our thermodynamic results showed the formation of a mixture of ZnO and ZnS04. When the same calculations were performed at pressures higher than 1 atm, the formation of ZnS04was favored because no gas product is produced during this reaction. Regeneration of ZnS at high temperatures is expected to cause sintering of the porous solid (loss of the initial surface area and pore volume) and, consequently, VOL. 29, NO. 2,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
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1 .oo
moles C02 =
moles COS
10 = 0.1
Temperature, K
FIGURE 1. (a) Effectof the HzOconcentration on the oxidation of Cas. (b) Conversionof the initial CaS to CaCOa or CaSOlfor the thermodynamic data of panel a.
degradation of its initial desulfurization ability. A reducing gas can be used to convert the sulfate to the oxide. Our thermodynamic computations showed that CaS reacts with oxygen to form CaS04 at temperatures lower than 1273 K. This sulfate can decompose to the cxide at high temperatures under a reducing atmosphere. A more attractive way to produce CaO from CaS is reaction 3 followed by decomposition of the CaC03. A parametric investigation was performed to examine the effect of the temperature, reactive gas (COZand HzO),and solid reactant (Cas) concentrations on the thermodynamic predictions. The reaction of CaS and the CO2-HZ0 mixture gives rise to the formation of CaS04 in addition to CaC03. If we consider that the reaction of COZor HzOwith CaS produces CaS04, then the following gas-solid reactions can describe the formation of the sulfate:
+ 4C0, - CaSO, + 4CO Cas + 4H,O - CaSO, + 4H,
Cas
(4) (5)
However, our computations showed that only reaction 5 is thermodynamically possible for a gas-solid mixture of stoichiometric composition. 374 = ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, KO. 2. 1995
In Figure la,b we present the thermodynamic results for a C02-HZO-Nz-CaS gas-solid system where the mole ratio of COZ and CaS is 100 and the HzO concentration varies between 10 and 60%vol. At high temperatures (8731173 K), an increase in the water concentration decreases the fraction of the unreacted CaS for a given temperature (Figure la). This fraction becomes zero for water concentrations higher or equal to 40%. The percentage of CaS that is converted to CaS04 depends on the temperature. According to our predictions shown in Figure lb, the maximum formation of CaS04-minimum formation of CaCOs-corresponds to 973 K. We also considered a constant water (40% mol) and CaS (0.2%mol) concentration, and we changed the COzcontent in the reactive gas mixture. Complete conversion of the CaS (similarlyto the results of Figure la) was predicted at temperatures lower than 873 K or COz concentrations higher than 40%. An increase in the CO2 concentration gave rise to higher CaC03 concentration. Formation of CaS04was predicted in almost all the reaction conditions examined. In general, an increase in the concentration of H20 or COZ (reactive gas species in reaction 3) shifts reaction 3 to the right.
1
.oo %
CO
%
C02
%
H2
moles H 2 0 = 10
500
800
900
1000
1100
1 30
Temperature, K
WCO m
n
J
WCO2
WH,
2
30 FIGURE 2. (a) Effect of the Hz and CO on the oxidation of Cas. (b) Conversion of the initial CaS to CaCOs or CaSO, for the thermodynamic data of panel a.
Given that CO and H2 are used to decompose the CaS04, addition of these gas species to the initial mixture is expected to inhibit the formation of the sulfate. Consistent with this remark are the results shown in Figure 2a,b where a small percentage (0.1% and 2%vol) of CO and H2 was included in the reactive gas. Notice that no CaS04was predicted in the temperature range of interest even when small amounts of CO and H2 were present in the initial gas mixture. It is interesting that higher concentrations of Hz and CO do not favor the conversion of CaS to CaC03. Our explanation for this result is that a part of the unreacted CaS reacts with CO:, producing CaO, S02, and CO. Obviously, this reaction is inhibited by the presence of the reducing gases.
Materials Synthesized sorbents and a natural limestone were employed in our experiments. Names were assigned to the synthesized sorbents according to the solid carrier, the impregnated oxide, and the number of the preparation experiment. For instance, for the preparation of sorbent Mg3-Ca5 the C3.MgO magnesia was used as the porous substrate where CaO was impregnated during the sorbent preparation experiment number 5. A list of the materials
tested in this study and the preparation techniques are given in Table 1. The raw materials for the preparation of the sorbents were alumina and magnesia porous carriers. The alumina was an almost pure (98.7% wt) a-alumina supplied by Condea with the commercial name Puralox. The magnesia sampleswere suppliedby Grecian Magnesite. The main chemical compounds of the magnesia samples were MgO, SOz,and CaO. Metal oxides were impregnated on the carriers using the dry and wet impregnation techniques. The preparation procedure, a detailed compositional analysis of the solid carriers, and a complete characterization of their pore structure can be found in the work of Vasalos et al. (7). Sample VKO-19 is a limestone that was provided to our laboratory by the University of Twente, The Netherlands. This limestone was calcined under nitrogen at 850 "C to release the COz from the carbonates (CaC03and MgC03). The inductively coupled plasma and atomic emission spectroscopy (ICPIAES) technique was employedto measure the percentage of CaO and ZnO in the unreacted samples shown in Table 1. A n improvement in the HIS removal capacity of metal oxide sorbents is possible if the pore volume and the surface area of the porous solid increase. In the CaO sorbents, this VOL. 29. NO. 2, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1
Preparation Technique, Solid Reactant Content, Physical Properties, and t i n of Sorbents sorbent
Mg3-Ca5
Mg3-Ca6
Mg3-Ca6MT
Mg3-2116 W
AI-Znll
carrier preparation technique CaO, %wt ZnO, %wt pore vol, rnUg s. area, rn2/g kin, rnin
C3.Mg0 dry irnpreg.
C3.Mg0 dry irnpreg.
C3.Mg0 wet impreg.
alumina (Puralox) dry irnpreg.
7.0
8.54
C3.MgO dry impreg. methanol treated 7.2
406
0.30 11 496
0.38 40.75 351
3.0 5.5 0.42 20.9 40 1
11.4 0.23 27.18 424
s
t
Mq3--06
VKO-19 calcined at 850 "C 40.5 0.38 14.22 47 1
MT
Mg3dn6W AI-7n 11 V K q F 19
Pore Diameter, microns FIGURE 3. Pore size distributions of the unreacted sorbents.
change in the intraparticle structure can be achieved with the treatment of the sorbent with methanol (13). Withum and Yoon reacted Ca(OH)2with methanol to produce Ca(OH)(OCH3),which can also react with methanol yielding Ca(OCH3)2.The methanol-treated Ca(OH12 had a higher pore volume and surface area than the untreated hydrated lime, followed by larger SO2 capture capacity. The latter was attributed to the physical property changes rather than the effects of the calcium methoxides. We treated our CaO sorbents with methanol in the same way as Withum and Yoon did to examine the effect of the pore network changes in the reaction of porous CaO sorbents with H2S. Preparation of Methanol-Treated Sorbents. CaO was impregnated on the porous magnesia carrier using the dry impregnation technique. The sorbent was then impregnatedwith distilled water of volume equal to the void space of the porous sample, the latter determined by the mercury porosimetry analysis. The soaked mass was dried at 100 "C for 1 h to convert the CaO to Ca(0H)Z. Following that, 100 mL. of high-purity ('99.9%) methanol was heated at 30 "C under dry nitrogen purge, and 40 g of the solid was progressivelyadded to the solution. The solid was carefully mixed with the methanol mixture. Methanol could evaporate and then condense in a condenser unit. The heat treatment of the metal oxide with methanol at 30 "C lasted 1 h. The mixture was then cooled to room temperature, and a rotary evaporator was used to remove the methanol. This sample was kept at room temperature for 24 h under continuous nitrogen flow. The methanol-treated samples were stored in air-tight bottles. 378 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 2,1995
Physical Properties of Sorbents. The physical properties of the samples used in our experiments are given in Table 1 and in Figure 3. The calcined limestone (sample VKO-19) exhibits a very narrow pore size distribution and small surface area relatively to the other sorbents of this study. The pore space of the Al-Znll sorbent was significantly lower than that of the other samples. The overall pore volume and the surface area of the methanol-treated material were higher than those of the untreated solid, implying that the methanol-treated sample is characterized by a larger number of small pores. The comparison between the pore structure of samples Mg3-Ca6 and Mg3-Ca6 MT showed that the treatment with methanol caused the expected changes in the physical properties of the sorbent.
Experimental Procedures and Conditions The sulfidation-regeneration experimentswere performed in a fluidized-bed reactor unit. The reactor was a quartz tube of 3.5 cm 0.d. (3.2 cm i.d.) equipped with a gas distributor and a K-type thermocouple. A three-zone furnace was used to maintain the reaction temperature constant. The reactor was loaded with 26-28 g of the sorbent that was in the particle form. The particle size of the VKO-19 sample was 100-210 pm, while that of all the other samples was 125-180pm. The stream at the exit of the reactor was fed in a stainless steel reactor where the unreacted H2S was oxidized to SOz. The reactors operated isothermally and at ambient pressure. AU sulfidation runs took place at 600 "C, while the oxidation of the HzS was at 700 "C. The effect of the regeneration temperature on the
E 500 0Q
400
t
.-0
300
L
-w
t Q)
200
U
t 0
0 100 cr) 0 4
I
O
Normalized Time, t/tmin FIGURE 4. Effect of the methanol treatment on the desulfurization capacity of the Mg3-Ca6 sorbent.
extent of the reaction was examined in the range of 550670 "C. In the sulfidation runs, the reactive gas was prepared from the mixture of a gas stream of 1%H S in NZwith pure N2. In the regeneration runs, the reactive gas was either a mixture of O2 and N2 or a mixture of COZ(or COp/CO), H20,and N2. HZwas occasionallyused in the regeneration experiments. The total flow rate in all of the experiments was 1000 mL/min (at ambient conditions). This flow rate corresponds to a superficial velocity of 2 cmls in the quartz reactor. The gas product of the sulfidation reaction was mixed with 1000 mL/min air, and this mixture entered the stainless steel reactor. Details about the reaction scheme and the schematic of the reactor arrangement can be found in the work ofVasalos et al. (7). The extent of the sulfidation and the regeneration was monitored by measuring the SO2 (gas product of the H2S oxidation and the regeneration reaction) concentration in a pulse fluorescent analyzer.The gas product of the CaC03 calcination, i.e., C02, was measured using a nondispersive infrared analyzer. In a typical sulfidation-regeneration run, the fresh sorbent was sulfided and regenerated more than once. The reactor was continuously flushed with NZwhen no reactive gas was used. About 0.2 g of the solid was collected from the reactor loading at the end of different sulfidationregeneration cycles. These samples were used to determine the composition, the solid phases, and the internal pore structure of the reacted sorbent. These are the steps that were followed in the sulfidation-regeneration cycles: ZnO Sorbents. (a) Sulfidation: 0.2% HzS in Nz, at 600 "C. (b) Oxidative regeneration: 2% 0 2 in Nz, at 700 "C. (c) Reducing regeneration: 2% H2 in N2, at 650 "C. CaO Sorbents. (a) Sulfidation: 0.2% H2S in N2, at 600 "C. (b) Regeneration: 12.4% COZ,75.2% H20, and 12.4% Nz, at 550-670 "C or 17.7%COz, 2% CO, 11.0%H20,2.3% Hz, and 67% N2, at 650 "C. (c) Calcination: N2, at 800 "C.
Results and Discussion The treatment of a sorbent with methanol not only changes the physical properties of the sample, but also its chemical composition. Withum and Yoon (13) detected calcium methoxides in the methanol-treated hydrated lime using Fourier transform infrared spectroscopy (FTIR),thermogravimetric analysis (TGA), and X-ray diffraction (XRD)
techniques. However, their reactivity data showed that the enhancement of the hydrated lime sulfation should be attributed to the structural changes due to the methanol treatment rather than the presence of calcium methoxides in the samples. The methanol-treated Mg3-Ca6 MT sorbent was employed in a TGA experiment. The sorbent lost about 14.6% of its initial weight in the temperature range 315-475 "C and about 2.2% in the temperature range 480-700 "C. The former weight change was attributed to the release ofwater from the sample and the latter to the decomposition of methoxides. We examined the effect of the heat treatment in the fluidized-bed reactor as follows: The reactor was loaded with 28 g of the Mg3-Ca6 MT sorbent and heated up to 600 "C under nitrogen. We left the sorbent at 600 "C for 15 min under nitrogen purge, we cooled the reactor down to room temperature, and then we measured the sorbent weight. The weight loss due to the above heat treatment was 16% wt. This result agrees with the TGA measurements and indicates that the chemicalcomposition of the heated Mg3-Ca6 MT sorbent is the same as that of the Mg3-Ca6 sorbent. The same result was also obtained from the comparison of the XRD data of the two samples. Effect of Methanol Treatment on Sulfidation. Two samples of sorbent Mg3-Ca6, onewith and the other without methanol treatment, were sulfidedunder identical reaction conditions. The experimental data shown in Figure 4 are presented as the evolution of the HzS concentration with the normalized time (t/tmiJ. The latter is defined as the ratio between the reaction time and the minimum required time for complete sulfidation of the solid reactant (CaO)in the sorbent under the reaction conditions of each experiment. The values of tmin for all samples of this study are listed in Table 1. The percentage of CaO in the Mg3-Ca6 MT sample is lower than that of the Mg3-Ca6 sample, although the untreated sorbentwas preparedwith the same procedure. However, the compositional analysis of the former sample was performed after the methanol treatment. If we account for 16%wt losses during the heat treatment in the reactor, the CaO percentage of the dry sample is 8.35%wt. The breakthrough point, or equivalentlythe CaO utilization, of sorbent Mg3-Ca6MT is more than four times that of sorbent Mg3-Ca6 (Figure4). The difference between VOL. 29, NO. 2. 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
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Real Time, min >
(b)
0
0
10
Time, min FIGURE 5. Sulfidation (a) and regeneration (b) curves for the AI-Znll sorbent.
the desulfurization performance of the two samples shown in Figure 4 clearly displays the effect of the initial pore structure on the desulfurization performance of a sorbent. Sulfldation-Regeneration Experiments. The reaction of metal oxides impregnated on inert porous carriers with H2Swas investigated in the work of Vasalos et al. (7). These experimental results showed that the preparation procedure, the porous carrier, the impregnated metal oxide, and the size of the solid reactant are parameters that affect the desulfurization performance of the sorbent. However, the desulfurization performance of a commercial sorbent does not depend only on its ability to remove H2S from a gas stream but also on its regenerability. A regenerable sorbent should maintain its desulfurization capacity for as many sulfidation-regeneration cycles as possible. Successive sulfidation-regeneration were performed in our fluidized-bed reactor in order to study the regeneration of the sulfided sorbents and the desulfurization performance of the regenerated solids. The experimental data of this study can be classified in two categories based on the metal oxide (ZnO or CaO) that was impregnated on the solid carrier. 378 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 2, 1995
ZnO Sorbents. Sorbent Al-Znll was sulfided and regenerated using the standard reaction conditions in our facilities. The experimental data for 2.5 sulfidationregeneration cycles are given in Figure 5a,b, respectively. The regeneration curves of Figure 5b have similar shape. However, this is not the case for the three successive sulfidation curves C.1, C.2, and C.3 that describe the evolution of the H2S concentration with the reaction time. The H2Sconcentration increased sharply at the beginning of the second and the third sulfidation runs (curves C.2 and C.3 of Figure 5a) and then dropped smoothly to about 80 ppmv. A second sharp increase in the breakthrough curves of these runs was also noticed, the latter corresponding to the breakthrough point. The method that we use to detect the sulfur gas species at the exit stream of the reactor cannot separate H2S and SO2 because H2S is oxidized to the latter gas species at the oxidation reactor. This is the reason that we refer to H2S and SO2 concentration at the y-axis of Figure 5a. We analyzed the regenerated Al-Znll sorbent, and we identified ZnSOl (about 0.060.1%wt) using the XRD technique, the 113GRACE method,
Time,
min
FIGURE 6. Effect of the reaction temperature during the reducing regeneration step of the AI-Znll sorbent.
and the ionic chromatography technique. As a result, we attributed the high sulfur-gas concentration at the beginning of the second and third sulfidation reaction to the release of SO2 derived from the zinc sulfate. Obviously, ZnSO4 is formed during the oxidative regeneration. The thermal decomposition of ZnS0, in an inert atmosphere is described by the reaction: ZnSO,
-
ZnO
+ SO, + 1/20,
(6)
Zinc sulfate is also reduced in the presence of hydrogen according to the following reactions (14):
+ 4H, - ZnS + 4H,O ZnSO, + H, - ZnO + SO, + HzO ZnSO,
(7) (8)
High HZconcentration is expected to lead to the formation of ZnS (reaction 71, while lower HZconcentration to the formation of ZnO (reaction 8). Therefore, the release of SO2 from the sulfate is favored when the solid is heated in a reducing atmosphere (the HzS-NZ mixture can be regarded as a reducing gas and thus favors the ZnS0, decomposition to ZnO). We tested experimentally the thermal decomposition of ZnSOl at 680-700 "C under inert atmosphere (N,),but the SO2 concentration measured in the chemiluminescence analyzer was very low. It was, thus, concluded that this sulfate does not decompose to the oxide form under the above conditions. Our next attempt to decompose the ZnSO, was to introduce a reduction step after the regeneration of the solid. We used low H2 concentration (1%vol H2 in NZin the experiment shown with the square markers in Figure 6 and 2%vol HZin N2 in all the other experiments of the same figure) and different reduction temperatures to experimentally find out the conditions that lead to complete decomposition of ZnS04. The experimental results of Figure 6 imply that SO2 is easily released from ZnSO, when the reaction temperature is higher than 600 "C and that a temperature increase leads to faster removal of SO2 from the solid. Use of a two-step regeneration (oxidative and reductive regeneration) affected the overall desulfurization performance of the sorbent as follows: During the sulfidation of the regenerated sorbent, no SO,
was detected at the beginning of the reaction, and the desulfurizationcapacity of the regenerated Al-Znl 1sorbent was equal to that of the unreacted sorbent. The formation of sulfates during the regeneration of ZnO-containing sorbents was also noticed by Gangwal et al. (15) and by Gupta et al. (16). In the former study, a reductive regeneration step was conducted at 550 "C using a clean coal gas stream to decompose the sulfates, while in the latter study regeneration took place at temperatures higher than 760 "C to prevent the sulfate formation. Our sorbents were initially calcined at 600 "C; it is, therefore, expected that sintering of the porous solids will occur if they were reacted at higher temperatures. ZnO-CaO Sorbents. Sorbent Mg3-Zn6Wwassulfided, regenerated with oxygen, and reduced with hydrogen for seven cycles. The H2S removal ability of the sorbent after various sulfidation-regeneration cycles was compared. The breakthrough points after the first sulfidation (sulfidations 2-7) were less than one-tenth that of the first sulfidation. We explained this behavior as follows: Sample Mg3-Zn6W is a metal oxide mixture with ZnO and CaO as the metal oxides that react with HzS to produce their sulfides (ZnS and Cas). The reaction of ZnS and CaS with oxygen-at the reaction conditions of this study-produces ZnO (andZnS04 that decomposes to ZnO under a reducing atmosphere) and CaSO,, according to our thermodynamic predictions. The CaS04 decomposes to CaO only at high temperatures (2900 "C)releasing SOz in a reducing atmosphere (3).The presence of CaS04 in the regenerated sorbent affects the desulfurization capacity of the sorbent as follows: the portion of the fresh CaO that is converted to CaS04 does not adsorb H2Sand the reactive gas diffuses through a CaS04 layer to react with the solid reactant. Given that the molar volume of CaS04is three times larger than that of CaO, the formation of the sulfate may also lead to pore closure and inaccessible pore space formation. Therefore, another regeneration route should be followed when the sorbent contains CaO. CaO Sorbents. Our thermodynamic computations showed that a C02-HzO-Nz mixture can be used to regenerate the CaO-containingsorbents. Experimentaldata for six successive sulfidations of sample Mg3-Ca6 are presented in Figure 7. The fist three sulfidationsof Figure VOL. 29, NO. 2, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
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Time,
min
FIGURE 7. Breakthrough curves of the Mg3-Ca6 sample during multiple sulfidation-regeneration experiments.
.-0 L
w
c a) U
c
0
0 v,
w
I
150
200
Time,
250
300
350
min
FIGURE 8. Breakthrough curves of the Mg3-Ca6 MT sample during multiple sulfidation-regeneration experiments.
7 have almost the same breakthrough point, while the desulfurization capacity of the sorbent decreased drastically at the subsequent three sulfidations. The analysis of the regenerated sample using the XRD technique showed that CaS04 and CaS existed in the sorbent in addition to the CaO. Therefore, we attributed the loss of the sorbent capacity to incomplete conversion of CaS to CaO and to formation of CaS04 during the regeneration. Notice that the solid carrier in the Mg3-Zn6W and the Mg3-Ca6 sorbents is the same. Therefore, differences in the experimental data for sorbents Mg3-Zn6W and Mg3-Ca6 should be attributed to the impregnated metal oxide. Specifically, when ZnO is impregnated on the porous carrier, the desulfurization capacity of the fresh sorbent is larger than when CaO is used. The comparison between the above two sorbents gives the reverse result when the criterion for the selection of the sorbent is the desulfurization capacity of the sorbent during successive sulfidation-regeneration cycles. In Figure 4,we compared the desulfurization performance of the Mg3-Ca6 MT sorbent with that of sorbent 380 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 2,1995
Mg3-Ca6to demonstrate the enhancement of the reactivity due to the methanol treatment. Sample Mg3-Ca6 MT was also used in three successive sulfidations. A decrease in the sorbent desulfurizationcapacity after each sulfidationregeneration cycle is noticed in Figure 8. The breakthrough point of the first sulfidation is more than double that of the second sulfidation, while an even smaller breakthrough point was measured in the third sulfidation. The desulfurization capacity of sorbent VKO-19 was tested as follows: 2.3 g of the sorbent was mixed with 25.7 g of a-alumina (Puralox). The solid mixture was loaded in the fluidized bed reactor, and it was then sulfided and regenerated five times. The breakthrough point of the first three sulfidations (curvesC1-C3 in Figure 9) was practically the same, while that of the fourth and fifth sulfidation (curves C4 and C5) was significantlylower. If we compare the sulfidation data of sorbent Mg3-Ca6 (Figure 7) with those of sorbent VKO-19,we notice that the former sample exhibits a progressive decrease in the breakthrough points of different cycles,while the latter sample exhibits a sudden change in the breakthrough points after the third cycle. A
>
E
400
Q Q
c
.-0
co 4
4
4
e
200
c
Q) U
D$
Time,
min
FIGURE 9. Breakthrough curves of the VKO-19 sample during multiple sulfidation-regeneration experiments.
500 400 300
200 nn 4
I 20
co 4 c. 5
60
40
80
100
1 !O
Time, min FIGURE IO. Beakthrough curves of the Mg3-Ca5 sample during multiple sulfidation-regeneration experiments. HZand CO were added to the regenerative gas mixture.
possible explanation for this difference is that the pore sue distribution of the -0-19 sample is narrow with respect to that of the Mg3-Ca6 sample. Therefore, the formation of CaS04 is expected to cause more significant structural changes in theVKO-19 sample. Unfortunately, the mercury porosimetry analysis of the sulfided VKO-19 sample was not possible since it was mixed with alumina in the reactor. Lyke (17) postulated that CaC03may react with H20 or COz and H2S producing CaS04 as follows: CaCO, CaCO,
+ 3H,O + H,S - CaSO, + CO, + 4H,
+ 3C0, + H,S - CaSO, + 4CO + H,O
(9) (10)
The gas products in the side reactions that produce CaS04 are Hz and CO. Consequently,when these gas species exist in the inlet gas stream, the formation of Cas04 should be inhibited. A new reactive gas mixture (COZ17.7%, H 2 0 11%, CO 2%, Hz 2.3%, and Nz 67%) was used in the regeneration experiments of sorbent Mg3-Ca5 shown in
Figure 10. Comparison between the experimental data of Figures 7 and 10 shows that in both experiments the three initial sulfidations gave about the same breakthroughpoint, but when HZand CO were added to the regeneration gas mixture, the HzSremoval capacity of the sorbent was maintained in the fourth, fifth and sixth cycles. The disadvantage of using fuel gases (HZ and CO) in the regeneration gas mixture is loss of the overall efficiency of the power generation integrated system, while the advantage is the use of the same sorbent for a larger number of sulfidation-regeneration cycles.
Physical Properties of Reacted Sorbents The characterization of the internal pore structure of sorbents prepared from the impregnation of ZnO showed that the surface area and the pore size distribution of the sorbent did not change signrficantly after sulfidation. When CaO is impregnated on the inert carrier, the initial metal oxide is converted to the following solids during the sulfidation-regeneration cycle: Ca0-Cas-CaC03-CaO. VOL. 29. NO. 2,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
381
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3 -
0
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z 3
0
10
Pore Diameter, microns FIGURE 11. Evolution of the pore structure of the Mg3-Ca6 sorbent during sulfidation-regeneration experiments.
Samples were collected from the reactor loading of the experiment shown in Figure 7, and they were analyzed using the mercury porosimetry and nitrogen adsorption techniques. The initial sorbent is characterized by the pore size distribution denoted with the curve “Fresh Mg3-Ca6” in Figure 11. The pore size distribution of the solid after the reaction of the sulfided sorbent with HzO and COZat the fourth cycle is described by the “CF4 Mg3-Ca6” curve (CaO is converted to CaC03),and that after the calcination of CaC03 is curve “R4 Mg3-Ca6”. The analysis of the sulfided for the fifth time sample gives rise to the pore size distribution “S5 Mg3-Ca6” in Figure 11. The mercury porosimetry data in this figure show that all curves are bunched closely together, although the molar volume of the involved solids varies as follows: ~ca0=16.89,vcas = 25.76, and VC~CO,= 36.92 cm3/gmol (physicalproperties of pure solids (18)). The surface areas ofthe Ca6-Mg3 samples of Figure 11 are the following: the Mg3-Ca6 Fresh sample has 11.0m2/g,the CF4 Mg3-Ca6 sample has 3.55 m2/g,the R4 Mg3-Ca6 sample has 8.30 m2/g, and the S5 Mg3-Ca6 sample has 8.27 mz/g. The surface area of the sorbent remains almost the same after five sulfidations, with the exception of the carbonated solid, probably due to the large difference in the molar volume of the involved solids. The pore structure analysis of sorbents impregnated with ZnO or CaO indicated that the sorbents maintain their initial internal structure after multiple sulfidation-regeneration cycles. Consistent with this remark were the SEM micrographs of the fresh and the regenerated four times Mg3Ca6 sorbent.
were significantly higher. The breakthrough points of the former sorbents decreased after the third successive sulfidation. This behavior was attributed to the formation of CaS04, a solid product of molar volume almost three times that of CaO. Thermodynamic computations using CHEMQ, a program that uses the free energy minimization method, showed that the temperature and the concentration of Cas, HzO, and COz affect the extent of the CaS conversion to CaC03 and CaS04. Our thermodynamic results showed that the addition of HZ and CO to the regenerative gas mixture eliminates the formation of the sulfate. In accordance to our predictions, we added Hz and CO to the H20-COZ-N2 mixture, and we noticed an improvement in the desulfurization performance of the regenerated sorbent. CaO sorbents were treated with methanol in order to increase their surface area and pore volume. The fresh samples exhibited enhanced desulfurization performance with respect to that of untreated sorbents, although their desulfurization capacity decreased at the subsequent sulfidations. Samples of the sorbents were collected at different stages of the sulfidation-regeneration cycles, and they were analyzed using the mercury porosimetry, nitrogen adsorption, and scanning electron microscopy techniques. No significant changes were measured in the pore structure of the sorbent, implying the absence of thermal or chemical sintering effects.
Conclusions
This work was funded by the Commission of the European Community, under Contract JOUF-0051-C.
The HzS removal ability of sorbents derived from the impregnation of alumina or magnesia porous carriers with ZnO or CaO was studied in a fluidized-bed reactor system. The regeneration of ZnO sorbents was carried out in two stages: the oxidative (2% O2 in N2 at 700 “C) and the regenerative (2%HZin N2 at 650 “C). A different regeneration route was followed when CaO sorbents were employed, Le., the sulfided sorbent was reacted with COZand HzO producing CaC03 that decomposed at 800 “C under nitrogen. The prebreakthrough HzSconcentrations for the CaO sorbents were 0 ppmv, while those for the ZnO sorbents 382 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 2 , 1 9 9 5
Acknowledgments
Literature Cited (1) Woods, M. C.; Gangwal, S.K.; Jothimurugesan, K.; Harrison, D. P. Ind. Eng. Chem. Res. 1990, 29, 1160. (2) Grindley, T. Sidestream Zinc Ferrite Regeneration Tests; Eighth Annual Gasification and Gas Stream Cleanup Systems Contractors Review Meeting; DOEIMETC-88/6092;U.S. Department of Energy: Morgantown, WV, 1988. (3) Kamphuis, B.; Potma, A. W.; Prins, W.; Van Swaaij, W. P. M. Chem. Eng. Sci. 1993, 48(1), 105. (4)Atimtay, A. T.; Gasper-Galvin, L. D.; Poston J. A. Environ. Sci. Technol. 1993, 27(7), 1295.
(5) Efthimiadis, E. A. Ph.D. Dissertation, University of Rochester, Rochester, NY,1991. (6) Efthimiadis, E. A.; Sotirchos S. V. Chem. Eng. Sci. 1993, 48(11), 1971. (7) Vasalos, I. A.; Efthimiadis, E. A.; Christoforou, S. C. High
Temperature Desulfurization of Coal Gases by Regenerative Sorption; Final Report to the EC research project JOUF-0051-C
0; CPERI: Thessaloniki, Greece, Jan 1994. (8) Yoo, H. J.; Steinberg M. Calcium Silicate Cement Sorbentfor HzS Removal and Improved Gasification Process; Final Report DOE/ CH/00016-1494,U.S. Department ofEnergy: Morgantown, WV, 1983. (9) Heesink, A. B. M., Ed. Development of Systems for Regenerative Desulfurization of CG Gases with a Separate FB or CFE Absorber, Progress Report 1to the EC research project JOUF-0051-C(TT); University of Twente: Enschede, The Netherlands, 1991. (10) Westmoreland, P. R.; Harrison D. P. Environ. Sci. Technol. 1976, 10(7), 659. (11) Furimsky, E.; Yumura M. Sci. Technol. 1986, 39, 163. (12) Kirkpatrick, M. 0.; Pike, R. W. CHEMQ Prediction of Chemical Equilibrium User’sManual; Department of Chemical Engineering, Louisiana State University: Baton Rouge, LA, 1992.
(13) Withum, J. A.; Yoon, H. Environ. Sci Techol. 1989, 23, 821. (14) Bissett, L. A.; Strickland, L. D. Ind. Eng. Chem. Res. 1991,30,170. (15) Gangwal, S. K.; Harkins, S. M.; Woods, M. C.; Jain, S. C.; Bossart, S. J. Environ. Prog. 1989, 8(4), 265. (16) Gupta, R.; Gangwal S. K.; Jain, S. C. Energy Fuels 1992, 6, 21. (171 Lyke S. E. Sulfide and Chloride Control with Solid Supported Molten Salt at High Temperature and Pressure; Proceedings of the Fourth Annual Contractors Meeting on Contaminant Control in Hot Coal-Derived Gas Streams, US. Department of Energy: Morgantown, WV, 1984. (18) Perry, H. P.; Chilton, C. H. ChemicaZEngineering’Handbook, 5th ed.; McGraw-Hilk Tokyo, 1973.
Received for review April 1 1 , 1994. Revised manuscript received July 25, 1994. Accepted October 24, 1994.@
ES9402258 @
Abstract published in AdvanceACSAbstrmts, December 1,1994.
VOL. 29, NO. 2, 1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY 1389
