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Transition metal oxides for hot gas desulphurisation
Fuel 78 (1999) 601–612
Transition metal oxides for hot gas desulphurisation W.F. Elseviers, H. Verelst* Department Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium Received 22 September 1997; received in revised form 20 July 1998; accepted 5 October 1998
Abstract Thermodynamic equilibrium simulations were made to determine suitable materials for intensive desulphurisation of fuel gases from an IGCC. Zinc-based materials are the most promising for high temperature intensive desulphurisation, but cause evaporation and regenerability problems. To overcome these problems, a new material was synthesised, containing zinc oxide supported on a titanium dioxide support. The material is able to reduce H2S levels from 3250 ppm to the thermodynamic equilibrium values in a synthetic fuel gas mixture at atmospheric pressure. At 600⬚C, the material can be regenerated completely. Sorbent sulphur capacity increases upon regeneration until 100% sorbent usage. Comparing experimental and simulation results, it is also concluded that Aspen Plus is a powerful tool in calculating high-temperature gas–solid equilibria. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: Simulation; H2S; Hot gas cleanup; Desulphurisation
1. Introduction Integrated Gasification Combined Cycles (IGCC) are an attractive alternative for future power production. The use of low rank fossil fuels leads to the formation of unwanted impurities like H2S, COS and NH3, which need to be eliminated prior to combustion in the gas turbine. Gasification fuel gas desulphurisation is traditionally carried out by means of low temperature absorption processes, capable of removing H2S to ppm levels. HCl and nitrogen compounds such as NH3 and HCN are removed simultaneously. Low temperature H2S absorption reduces the overall cycle thermal efficiency by a few percent as a result of the exergy losses in the cooling. The same residual H2S levels can potentially be obtained through high temperature gas cleaning, thus avoiding this efficiency loss. H2S removal at elevated temperatures can be carried out in a number of ways. Sorbent injection into the gasifier can serve as a bulk gas desulphurisation. Additional desulphurisation can be achieved through the use of (regenerable) transition metal oxides. In situ desulphurisation using limestone or dolomite can only reach H2S residual levels of about 100 ppm [1]. Although in most cases this degree of desulphurisation might be acceptable, other fields of application like Molten carbonate fuel cells require ppm or subppm residual H2S levels [2], but the major disadvantage of * Corresponding author. Tel.: ⫹32-2-629-32-50; fax: ⫹32-2-629-32-48. E-mail address: [email protected] (H. Verelst)
in situ desulphurisation is the production of large amounts of solid waste. Intensive desulphurisation processes using a regenerable sorbent will eliminate this problem. This paper will focus on the intensive desulphurisation as a stand-alone process. The goal of the research is two-fold: firstly, by comparing the experimental results and the data available from literature with simulation results, to show that process engineering computer simulations can be a useful tool in investigating this type of high-temperature gas–solid reaction; secondly, to come to a more thorough understanding of this type of reactions, so as to develop a more complex modelling approach.
2. Intensive desulphurisation 2.1. Sorbent requirements A good sorbent will allow for a deep desulphurisation to ppm levels and have good regeneration properties. This means the combination of a high affinity towards the reaction with H2S, as well as the formation of a sulphide which can be converted back to the oxide through oxidation with air or diluted air. Next to the residual H2S level, sorbent durability is the critical issue. For economical operation, a good sorbent has to maintain a large fraction of its desulphurisation properties for at least a hundred sulphidation-regeneration cycles, requiring excellent sorbent stability.
0016-2361/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(98)00185-9
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The regeneration reaction equilibrium strongly depends upon the oxygen partial pressure as can be seen from Eq. (4). An unwanted side-reaction is the formation of sulphates:
Table 1 Simulated coal gas composition Component
Concentration
CO2 CO H2 H2O H2S
12.5% mol 42.5% mol 32.175% mol 12.5% mol 3250 ppm mol
2.2. Reactions A number of transition metal oxides are known to have desulphurisation properties at elevated temperatures. A thermodynamic screening was performed by simulations with ASPEN PLUS. It is to be considered that this approach only makes use of the bulk properties of both gaseous and solid phases, and that at the interface between both phases where the reactions take place, different conditions may exist, also because of the use of supported metal oxide systems, containing smaller clusters of the active species. As it is difficult to obtain accurate values of the thermodynamic properties of the solid phases under these conditions, it is believed that a bulk approach can be a good first approximation of the actual situation in order to verify the thermodynamic feasibility of this type of reaction. Thus for bulk systems, ignoring the oxidation state of the metal, the desulphurisation reaction can be written as MeO
s
⫹ H2 S
g
$ MeS
s
⫹ H2 O
1
g
Kp H2 O=H2 S
2
while the regeneration is represented by MeS
s
⫹ 3 =2 O2
g
$ MeO
s
⫹ SO2
3
g
Kp p⫺1=2 × SO2 =O2 3=2
4
MeS s ⫹ 2O2 g $ MeSO4 s
5
Kp p⫺2 × O2 ⫺ 2
6
because the formed sulphate is inert with respect to the desulphurisation and thus there is a loss of active material. Consequently, it is important to know at which conditions this reaction (5) precedes the desired regeneration reaction (3). A second important process parameter is the SO2 concentration in the regenerator off-gas, which needs to be in excess of 5 vol% for a subsequent SO2/SO3 conversion for the production of sulphuric acid. The main composition of the fuel gas containing the H2S is determined by the CO-shift reaction: CO2 g ⫹ H2 g $ H2 O
g
⫹ CO g
7
In the absence of water vapour and hydrogen, soot formation can take place through the Boudouard reaction from CO and CO2: C s ⫹ CO2 g $ 2CO
g
8
2.3. Simulation input Gasification gas composition can vary over a wide range, depending on coal type and gasification operating conditions. Based upon previous calculations [1], the simulated coal gas with composition shown in Table 1 will be used. Regeneration gas consists of an O2/N2 mixture. Simulations were carried out for both sulphidation and regeneration. The components used in the desulphurisation and regeneration simulations are listed in Table 2 and Table
Table 2 Metal components considered in the desulphurization simulations Metal
Feed sorbent
Components in simulation
Al Ba Ca Co Cu Fe Mg Mn Mo Ni Sn Sr Ti W
Al2O3 BaCO3 CaCO3 Co3O4 CuO Fe3O4 MgCO3 MnO2 MoO2 NiO SnO2 SrCO3 TiO2 WO2
Zn Zr
ZnO or Zn2TiO4 ZrO2
Al, Al2O3, Al2S3 Ba, BaO, BaCO3, BaS Ca, CaO, CaCO3, CaS Co, CoO, Co3O4, Co9S8, CoS2, CoCO3 Cu, Cu2O, CuO, Cu2S, CuS Fe, FeO, Fe2O3, Fe3O4, FeS, FeS2, Fe3C, FeCO3, C Mg, MgO, MgCO3, MgS Mn, MnO, Mn3O4, Mn2O3, MnO2, MnS, MnS2, Mn3C, MnCO3 Mo, MoO2, MoO3, Mo2S3, MoS2, MoS3, Mo2C, MoC Ni, NiO, Ni2O3, NiS0.84, NiS, Ni3S2, Ni3S4, NiS2, Ni3C, NiCO3 Sn, SnO, SnO2, SnS, SnS2, Sn2S3, Sn3S4 Sr, SrO, SrCO3, SrS Ti, TiO, TiO2, Ti3O6, Ti4O7, TiS, TiS2, TiC W, WO2, WO2.72, WO2.90, WO2.96, WO3, W2O6, W3O8, W3O9, W4O12, WS2, W2C, WC Zn, ZnO, ZnS, Zn2TiO4, TiO2, TiS, TiS2 Zr, ZrO, ZrO2, ZrS, ZrS2
W.F. Elseviers, H. Verelst / Fuel 78 (1999) 601–612 Table 3 Components considered in the regeneration simulations Phase
Components in simulation
Gas Solid
O2, N2, SO2, Zn Zn, ZnO, Zn2TiO4, ZnSO4, TiO2, ZnS, TiS, TiS2
3, respectively. All phases are taken into account, including multiple solid phases as well as evaporation of the metal. Desulphurisation properties for vanadium were not examined because no thermodynamic parameters could be found in the ASPEN PLUS databanks for the sulphide, nor in literature [3–6]. The formation of carbonyl compounds from iron, nickel and cobalt could be completely neglected above 300⬚C. Sulphate formation is taken into consideration in the regeneration simulations only because desulphurisation takes place in a reducing atmosphere. For the desulphurisation simulations the influence of temperature from 300⬚C to 1500⬚C on residual H2S level at a fixed pressure of 15 MPa was studied for each case, using the simulated coal gas as input stream. All simulations used 100% excess oxide, i.e. the amount of metal fed was twice the amount required to capture all sulphur. For the regeneration simulations, spent desulphurisation sorbent is fed to an equilibrium reactor operating at 1 kPa. For each case the influence of temperature from 400⬚C to 1400⬚C on the composition of the sorbent after regeneration was studied as a function of the oxygen partial pressure. The amount of oxygen fed was twice the amount required to convert the sulphide back to the oxide. 2.4. Desulphurisation simulation results The oxides of aluminium, magnesium, titanium and zirconium are inert for reaction with H2S: no stable sulphides are formed. For the materials showing
603
desulphurisation capacities, Figs. 1 and 2 give the simulation results. Residual H2S levels at 400⬚C and 750⬚C are given in Table 4. For an IGCC-integrated high temperature desulphurisation process, only the temperature range above 300⬚C is of interest, as below 300⬚C kinetics become too slow. It is clear that SrCO3 and BaCO3 can be used for in situ desulphurisation in gasifiers operating at 1100⬚C–1500⬚C, where dolomite and limestone are no longer suited, resulting in even lower residual sulphur levels. At 400⬚C, nearly all transition metal oxides are suitable for desulphurisation. At higher temperatures residual H2S levels increase. If 100 ppm residual H2S is considered allowable, ZnO could theoretically be used up to the melting point of Zn metal, but as a result of Zn metal volatilization in a reducing atmosphere, its usage will be temperaturelimited, as will be discussed below. Zinc titanate Zn2TiO4 is able to reach the same residual H2S levels as pure ZnO. Tin metal volatilization will also limit the use of tin oxide SnO2 as desulphurisation sorbent to relatively low temperatures. For molybdenum and tungsten oxides, carbide formation must be considered. From a thermodynamic point of view, Mo2C or MoC respectively WC are the stable forms at the conditions studied. If carbide formation is neglected, Mo and W residual H2S level indicate a very good desulphurisation potential. However, if carbide formation is taken into account, MoO2 is converted to Mo2C below and to MoC above 900⬚C, resulting in a very poor desulphurisation with residual H2S levels of 900 ppm at 400⬚C. WO2 is completely converted to WC showing no desulphurisation capacity at all. As carbide formation only occurs at higher temperature [7], one could say that, if desulphurisation kinetics are fast enough at 400⬚C, carbide formation will have no noticeable effect on H2S removal. However, if the occurrence of higher temperatures is possible or if kinetics
Fig. 1. Residual sulphur level at 1.5 MPa as a function of temperature (part 1).
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Fig. 2. Residual sulphur level at 1.5 MPa as a function of temperature (part 2).
require higher temperatures, the use of tungsten and molybdenum oxide should be avoided. Cobalt and copper oxides are reduced to their corresponding metals (Table 4). As metal desulphurisation capacity is lower and desulphurisation kinetics slower, this will lead to a lower desulphurisation capacity if sorbent regeneration is incomplete. Supporting the oxide might improve reduction resistance and thus regenerability [8]. Nickel behaves quite well at low temperatures where excess oxide is converted into Ni3C, but desulphurisation is poor above 450⬚C, as reduction to Ni metal occurs. As the affinity for sulphur is very high, regenerability will be poor, making it unsuitable for regenerable desulphurisation processes [9]. Iron oxide desulphurisation potential is somewhat lower compared to the other materials studied. Furthermore, as iron catalyses the Boudouard reaction (Eq. 7), solid carbon deposition inside sorbent pores and on the surface will quickly limit sulphidation. This solid carbon formation is
possible until 850⬚C and has a negative effect on residual H2S level, as can be concluded from Fig. 3. Manganese is capable of maintaining ppm-range H2S levels up to 475⬚C with still only 50 ppm near 600⬚C. Above 400⬚C MnO is the stable form. No carbide formation nor reduction to Mn metal occurs. From a thermodynamic point of view, manganese is a most promising sorbent, which is also indicated by a number of experiments [10,11]. From these simulations, it can be concluded that for an IGCC integrated deep desulphurisation, zinc oxide is definitely the best sorbent, capable of reaching the ppm level. However, metal volatilisation can be a serious problem, especially at higher temperatures. Molybdenum oxide and tungsten oxide must be rejected because of the possibility of carbide. Manganese oxide also seems to be a most promising material as a result of its resistance to reduction, volatilisation and carbide formation. Sorbent activity, capacity and regenerability will decide on its applicability.
Table 4 Residual H2S levels and species formed at 15 bar Sorbent
ZnO Zn2TiO4 MoO2 MnO2 WO2 SnO2 Co3O4 CuO NiO Fe3O4
Residual H2S level (ppm)
Sulphide formed
Excess oxide present as
100 ppm
400⬚C
750⬚C
400⬚C
750⬚C
400⬚C
750⬚C
H2S at
0.03 0.6 0.6 2.5 3.4 19 24 39 41 91
0.06 27 143 216 429 3250 2464 401 1282 732
ZnS ZnS MoS2 MnS WS2 SnS Co9S8 Cu2S Ni3S2 FeS
ZnS ZnS Mo2S3 MnS WS2 SnS Co9S8 Cu2S Ni3S2 FeS
ZnO Zn2TiO4 MoO2 MnO WO2 Sn Co Cu Ni3C Fe3C
ZnO Zn2TiO4 Mo MnO WO2 Sn Co Cu Ni Fe3C
970⬚C 900⬚C 720⬚C 660⬚C 610⬚C 475⬚C 475⬚C 500⬚C 470⬚C 41⬚C
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605
Fig. 3. SO2 concentration in off-gas during ZnO regeneration at 0.1 MPa.
2.5. Regeneration simulation results Regeneration simulations were made only for zinc-based materials, namely zinc oxide ZnO and a zinc titanate, Zn2TiO4. Fig. 4 shows the ZnSO4 formation as a function of temperature and O2 partial pressure for the regeneration of pure ZnO. It can be seen that temperatures in excess of 700⬚C are required to avoid sulphate formation and that the minimum regeneration temperature strongly increases with the partial pressure of oxygen. From Fig. 4, showing the SO2 concentration in the regenerator off-gas versus the oxygen content of the regenerator feed gas, it is clear that in order to obtain SO2 concentrations suitable for H2SO4 production the
oxygen molar fraction in the feed gas should be higher than 10%. This implies that regeneration should be carried out at 800⬚C minimum, whereas desulphurisation is done at much lower temperatures. Implementation on an industrial scale will thus yield a more complex unit. Another fact which cannot be overlooked is the volatilisation of Zn at high temperatures. If desulphurisation and regeneration temperature are much different, temperature gradients from one reactor bed to another can be expected and zinc evaporation is likely to occur in the reducing atmosphere of the desulphurisation reactor. Fig. 5 gives the amount of Zn in the vapour phase as a function of temperature at sulphidation conditions. One can see that vapour losses for zinc
Fig. 4. ZnSO4 formation during ZnO regeneration at 0.1 MPa (O2 volume fraction in regenerator feed gas: (a) 100%; (b) 50%; (c) 20%; (d) 10%; (e) 1%).
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Fig. 5. Zn evaporation for ZnO and Zn2TiO4 during sulphidation at 1.5 Mpa as a function of temperature: (a) Zn2TiO4; (b) ZnO.
titanate are much smaller and thus that ZnO reduction resistance increases. Furthermore, sulphided Zn2TiO4 can be regenerated without sulphate formation above 350⬚C. From a thermodynamic point of view, zinc titanate definitely has major advantages over pure zinc oxide: the reduction resistance has increased and sulphate formation is suppressed, while H2S residual levels remain the same.
3. Other zinc-based sorbents These simulations indicate that the volatilisation and regeneration problems may be solved by supporting the oxides on a carrier material or also through the use of mixed metal oxides. A lot of research has already been done in this field and was mainly focussed on two types of zinc-based materials: zinc ferrites and zinc titanates. Zinc ferrites are mixed oxides of zinc and iron oxide and thus contain two oxides with desulphurising ability. Depending on the ratio of ZnO to Fe2O3, several crystalline phases can exist in the material: the mixed oxide ZnFe2O4 coexists with the excess of the other oxide [12]. The presence of iron oxide lowers the reduction rate of ZnO and the material is regenerable at 750⬚C. Kobayashi [13] reports a 25% capacity decrease over 20 sulphidation/regeneration cycles as a result of sintering and incomplete regeneration. At 10 MPa and 450⬚C, sub-ppm residual H2S levels were reached. Pineda et al. [14] supported zinc ferrites on alumina to further decrease the ZnO reduction rate. However, the material was not homogeneous and had a low sulphur capacity. Sasaoka [15] studied the soot formation over zinc ferrites in a CO-shift gas atmosphere and
reported the conversion of the zinc ferrite into cementite Fe3C and fibrous ZnO below 550⬚C. Soot formation was stronger in more reducing gas mixtures. Impregnation of zinc ferrites with small amounts of V2O5 increased the reduction resistance and sulphur loading. V2O5 was also reacted with H2S [16]. Recently, Jothimurugesan et al. [17] developed a zinc and iron oxide based sorbent, which reached sub-ppm H2S levels at 350⬚C and retained its full capacity over 100 cycles. Although zinc ferrites show lots of improvements over pure zinc oxide, ZnO reduction and regeneration still impose practical problems. The use of supported zinc oxides can improve reduction resistance and regenerability at the cost of lower sulphur capacity per unit mass of sorbent as a result of the inactivity of the support material towards the desulphurisation. In zinc titanates several crystalline phases can coexist, depending on the Zn/Ti ratio. ZnTiO3, Zn2TiO4 and Zn3TiO8 have been identified by Flytzani-Stephanopoulos et al. [18]. During sulphidation, partial or complete decomposition into ZnO and TiO2 occurred, but Zn vapour losses were not detected and regenerability was good in 2% O2. Sulphate formation was not observed, and 50% sulphur loading could be achieved over subsequent sulphidation cycles. The capacity decrease was attributed to a decrease in surface area and pore volume. Woods et al. [19] reported complete sulphidation and regeneration at 675⬚C and observed no ZnSO4 formation. However, Yrjas et al. [20] report sulphate formation at 20 MPa and below 700⬚C at higher oxygen partial pressures during regeneration. Regeneration was fast but much time was needed to obtain complete regeneration. Gupta et al. [21] carried out a 100 cycle test at 750⬚C on a pilot scale. Sulphur capacity dropped to about 40% of its initial value as a result of losses in surface area and pore volume.
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607
Fig. 6. XRD spectrum of zinc titanate sorbent.
Other additions to zinc titanate materials have been examined to improve sulphur capacity, durability and/or regenerability. Poston [22] added La2O3 to reduce sorbent spalling. Sorbent capacity increased slightly as a result of the reaction with La2O3. The addition of molybdenum caused migration of zinc towards the pellet surface but regenerability was poor [23]. Jothimurugesan et al. [24,25] synthesised a zinc titanate with Ni, W and Mo promoters. At 750⬚C, H2S levels below 70 ppm were measured and the material was also capable of decomposing ammonia according to 2 NH3 g $ N2 g ⫹ 3 H2 g
9
which is an important side-effect of this material, as in most gasification gases ammonia is present in about the same range of concentrations as H2S and will be converted into NOx upon combustion in the gas turbine. Other supported zinc oxides made use of alumina carriers, but these could not stop reduction and evaporation, although a new crystalline phase was formed [26,27]. Mixing ZnO with ZrO2 yielded somewhat better results but the stabilisation mechanism was not understood very well. A mixed oxide of CaO and ZnO showed no regenerability. Residual H2S concentrations are of the sub-ppm level, but sorbent capacity drops over repeated sulphidation/regeneration cycles as a result of significant material and strength losses [28,29].
4. Experimental It is clear that zinc-containing sorbents and more specific zinc titanates are among the most promising materials for hot gas cleaning purposes. A number of zinc-titanate sorbents were thus prepared to validate the simulation results through a series of experiments. Before carrying out experiments with the synthesised materials, the experimental setup was evaluated with ZnO and CuO. The obtained results corresponded well with the predictions from the simulations: ZnO is able to reach very low residual H2S levels, but lacks regenerability at temperatures lower than 850⬚C. CuO was converted into a mixture containing 50%mole CuS and 50%mole Cu2S and was not regenerable. 4.1. Sorbent preparation and characterisation A TiO2 (Merck) support was impregnated during 24 hours with a 4.2 M solution of Zn(NO3)2 (Merck) by wet impregnation. The slurry was dried for 24 hours at 140⬚C and calcined for 24 hours under oxygen at 600⬚C. The obtained sorbent was characterised with X-Ray Diffraction (XRD) to determine which crystalline phases were present (Fig. 6). Sorbent and support surface area were determined using N2 porosimetry, while the composition was determined through Energy Dispersive X-Ray Diffraction (EDX) (Table 5). The measured surface area values are rather low and the materials do not contain pores below 10 nm. The introduc-
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Table 5 Surface area from N2 porosimetry
Table 7 Experimental conditions for H2/H2S experiments
Material
SA (m 2/g)
Formula
Run
Sulphidation (⬚C)
Regeneration (⬚C)
TiO2 support (250–400 mm) Zinc titanate (100–250 mm) Zinc titanate (⬍100 mm)
7,9 4,6 3,1
TiO2 ZnO.(TiO2)2.6 ZnO.(TiO2)2.6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
500 500 500 500 500 600 600 600 600 600 600 600 550 500 500
750 700 650 600 600 600 600 600 600 600 600 550 800 800
tion of the zinc oxide onto the support decreased surface area by about 50%. No new crystalline phase could be identified from the XRD analysis, which revealed the sorbent to be consisting of finely dispersed ZnO on a TiO2 matrix. 4.2. Experimental setup The synthesised materials were tested on their desulphurisation abilities in a small, tubular packed-bed reactor. CO, CO2 and a mixture of 1 vol% H2S in H2 were mixed together through a set of calibrated rotameters to obtain the desired gas composition. H2O is produced in situ by the CO-shift reaction. For this reason, an inert packing, consisting of quartz sand, constituted the first 15 cm of the 40 cm reactor tube. The second part of the 7 mm inner diameter quartz tube reactor contained the active sorbent, mixed with inert quartz sand in order to reduce sorbent breakthrough time and increase bed residence time. Total sorbent bed length was about 10 cm, after which again a 15 cm inert bed was placed. The reactor tube was placed in a temperaturecontrolled oven. The reactor bed pressure drop was measured with an upstream electronic pressure indicator. The gas leaving the reactor tube was led through a zinc acetate buffer solution, allowing time-averaged H2S outlet concentrations to be measured accurately using the methylene blue method [30]. A qualitative monitoring of the total gas composition was done continuously by means of a quadrupole mass spectrometer, giving the intensities for mass 2 (H2), 18 (H2O), 28 (CO and CO2), 34 (H2S), 44 (CO2) and 60 (COS) as a function of time. Sorbent breakthrough time was determined as the time when the H2S outlet concentration starts increasing dramatically. 4.3. Results The first goal in the experiments with the synthesised materials was to check whether they were capable of capturing H2S from a gas. To eliminate the possible interference of the other fuel gas constituents, the experiments were done in Table 6 Experimental feed gas compositions (mol%) Component
Mixture 1
Mixture 2
Mixture 3
H2 H2S CO CO2 Ar
32,2 0,325 0 0 67,5
32,4 0,327 42,5 0 24,7
47,8 0,483 21,3 18,3 12,1
a H2/H2S mixture. Ar was added as an inert in order to work at the same hydrogen partial pressure as in the experiments with the simulated fuel gas. A second goal was the determination of the lowest possible regeneration temperature and the influence of the fuel gas constituents on the desulphurisation. A first series of desulphurisation experiments was made with the gas with composition ‘‘Mixture 1’’ from Table 6. Regeneration was carried out at different temperatures with argon-diluted air. The regeneration mixture contained 1 mol% O2, resulting in even lower SO2 concentrations, too low for H2SO4 production, but avoiding sulphate formation. In the subsequent sulphidation-regeneration cycles, desulphurisation temperature was kept at 500⬚C or 600⬚C, while regeneration temperature was gradually decreased from 900⬚C in 50⬚C steps. The minimum regeneration temperature will be defined as the lowest temperature causing no sorbent capacity losses in the subsequent sulphidation run. The temperature program is given in Table 7. A number of H2S breakthrough curves are given in Fig. 7, which gives the relative H2S concentration (0 corresponds with no H2S present, 1 with the inlet concentration) as a function of the reduced time, which is defined as the actual time divided by the estimated breakthrough time for 100% sorbent usage calculated from the sorbent composition. Table 8 Experimental conditions Run
Components
Mixture
1 2 3 4 5 6 7 8
Ar/H2/H2S Ar/H2/H2S Ar/H2/H2S/CO Ar/H2/H2S Ar/H2/H2S/CO/CO2/H2O Ar/H2/H2S/CO/CO2/H2O Ar/H2/H2S/CO/CO2/H2O Ar/H2/H2S/CO/CO2/H2O
1 1 2 1 3 3 3 3
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609
Fig. 7. H2S breakthrough curves for H2/H2S experiments.
The sorbent capacity increased upon successive regenerations up to about 75% of the total capacity predicted from sorbent overall composition. At 600⬚C the sorbent can thus be repeatedly sulphidated and regenerated over several cycles without large capacity losses. In the H2/H2S/Ar atmosphere the material was able to reach residual H2S levels of 0.15 ^ 0.05 ppm at 600⬚C, which corresponds with the thermodynamic equilibrium in the absence of the other fuel gas components. When regeneration temperature drops below 600⬚C, the sorbent capacity drastically decreased, probably because of sulphate formation. Regeneration at 800⬚C was not able to decompose the sulphate.
Consequently, the minimum regeneration temperature is taken as 600⬚C. A next series of experiments was carried out to determine the influence of the other fuel gas components: CO, CO2 and H2O. All experiments are carried out at 600⬚C. In subsequent sulphidation runs, the influence of the addition of each component to the reaction gas mixture is studied. An overview is given in Table 8, where gas compositions are taken from Table 6. The breakthrough profiles are given in Fig. 8. Between runs 3 and 4 also a mixture containing only Ar, CO and CO2 was fed to the reactor to check on the influence of the Boudouard reaction (8).
Fig. 8. H2S breakthrough curves for shift-gas experiments.
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Fig. 9. EDX element mapping of spent sorbent: (a) Visual; (b) Ti; (c) S; (d) Zn.
From these results we can conclude that the fuel gas composition does not have a large influence on the desulphurisation performance, except for the minimum residual H2S level which can be obtained. The thermodynamic equilibrium is slightly worse in case CO, CO2 and H2O are present; the corresponding levels of 2.2 ^ 0.1 ppm was reached. Sulphur capacity increased again upon regeneration (runs 1/2). The addition of CO did not influence the breakthrough time (runs 2/3) nor did the complete fuel gas
mixture (runs 5/6/7/8). It is also clear that the material is fully regenerable at 600⬚C in 1% O2. In a CO and CO2 containing atmosphere, soot formation takes place. This was observed at the outlets of the reactor tube where temperatures are lower, and also by comparing the breakthrough times for runs 3/4/5. Sulphur capacity decreases from run 3 to run 4 and is restored upon regeneration. This is as a result of the soot formation which took place during the time the CO/CO2/Ar mixture was passed over the
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reactor after regeneration 3 and before sulphidation 4. During regeneration 4 the soot was burned off and the sulphur capacity was restored from run 5 on. It can be concluded that either soot formation does not occur in the complete fuel gas mixture either that any formed carbon will be converted back to CO or CO2 when H2 or H2O are present. The conclusion is that the fuel gas atmosphere does not influence the desulphurisation capacity. From the sorbent breakthrough time and the H2S feed rate also the sorbent sulphur capacity could be calculated: 0.11 g sulphur/g sorbent, which corresponds with a nearly 100% sorbent usage. Sulphur, zinc and titanium maps were obtained by EDX for the spent sorbent (Fig. 9). The maps were taken over the cross section of a 150 mm sorbent grain. It is clear that Zn and S occur in the same regions on a Ti support and are located near the surface, where Ti is found more in the inner core of the material. As a result of the small amounts of material used in the experiments (0.1 to 0.5 g), changes in pore volume and surface area could not be measured. The increase in sulphur capacity upon regeneration is probably as a result of changes in pore structure and/or surface area. Further experiments with larger amounts of material will therefor be necessary to study these structural changes, also to obtain a better view on the sintering behaviour of these solid materials at high temperatures. However, a strong decrease in capacity over the total number of sulphidation-regeneration cycles was not measured and the particle size remained roughly the same and it is beleived that the material does not suffer too much from (macroscopic) sintering on this experimental scale. The comparison of Ti and Zn maps obtained by EDX for fresh and spent sorbent also showed a similar material distribution, indicating that on this scale the material is also stable enough under the studied conditions.
5. Conclusion Zinc-based materials are the most promising for high temperature intensive desulphurisation. To overcome the regenerability and reduction problems, a new material was made, containing zinc oxide supported on titanium dioxide. The obtained ‘zinc titanate’ did not contain any new solid phases, but is able to reduce H2S levels from 3250 ppm to the thermodynamic equilibrium values in a synthetic fuel gas atmosphere. At 600⬚C, the material can be regenerated completely. Sorbent sulphur capacity increases upon regeneration until 100% sorbent usage. Further experiments will be necessary to study the structural changes in the material and to determine regeneration conditions yielding SO2 concentrations sufficiently high for H2SO4 production. Comparing experimental and simulation results, we can also conclude that Aspen Plus is a powerful tool in calculating high-temperature gas–solid equilibria.
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Acknowledgements The authors would like to thank Oscar Steenhaut (VUB Department of Metallurgy) for the EDX measurements and Prof. Johan Martens (KU Leuven Center for Surface Chemistry and Catalysis) for the XRD measurements. This research is funded by the VUB-OZR.
References [1] Elseviers W, Van Mierlo T, Van de Voorde MJF, Verelst H. Thermodynamic Simulations of Lignite-Fired IGCC-Cycles with Desulphurisation and CO2 Capture - IGCC Cycle Simulations. Fuel 1996;75:1449–1456. [2] Gangwal SK, Harkins SM. Bench-Scale testing of Novel HighTemperature Desulfurization Sorbents, Proceedings of the 8th Annual gasification and Gas Stream Cleanup Systems Contractors Review Meeting, DE-AC21-86MC23126, DOE/METC, U.S. Department of Energy, Morgantown Energy Technology Center, Morgantown, WV, 1988, Vol. 1, 45-47. [3] Kubaschewski O, Evans E, Alcock CB. Metallurgical Thermochemistry. 4. London: Pergamon, 1967. [4] Mills KC. Thermodynamic Data for Inorganic Sulphides, Selenides and Tellurides. London: Butterworth, 1973. [5] Chase MW Jr, Davies CA, Downey JR Jr, Frurip DJ, McDonald RA, Syverud AN. JANAF Thermochemical Tables, 3rd Edition, Part II, Cr-Zr, Journal of Physical and Chemical Reference Data, Vol 14, Supplement 1, 1985. [6] Rao YK. Stoichiometry and Thermodynamics of Metallurgical Processes. Cambridge University Press, 1985. [7] Kofstad P. High Temperature Corrosion. London: Elsevier Applied Science, 1988 Chapter 15. [8] Kyotani T, Kawashima H, Tomoita A, Palmer A, Furimsky E. Removal of H2S from Hot Gas in the Presence of Cu-Containing Sorbents. Fuel 1989;68:74–79. [9] Bartholomew CH, Agrawal PK, Katzer JR. Sulfur Poisoning of Metals. Advances in Catalysis 1982;31:135–242. [10] Westmoreland PR, Gibson JB, Harrison DP. Comparative Kinetics of High Temperature Reaction between H2S and Selected Metal Oxides. Env. Sci. Tech. 1977;11:488–491. [11] Benslimane R, Hepworth MT. Desulfurization of Hot Coal-Derived Fuel Gases with Manganese-Based Regenerable Sorbents. 3. Fixed Bed Testing. Energy & Fuels 1992;9:372–378. [12] Lee Y-S, Kim H-T, Yoo K-O. Effect of ferric oxide on the hightemperature removal of hydrogen sulfide over ZnO-Fe2O3 mixed metal oxide sorbent. Ind. Eng. Chem. Res. 1995;34:1181–1188. [13] Kobayashi M, Nunokawa N, Shirai H. Development of regenerable desulfurization sorbent for coal gas sulfur removal below ppm level, In ‘High Temperature Gas Cleaning’ (Eds. Schmidt E, Ga¨ng P, Pilz T, Dittler A), Institut fu¨r Mechanische Verfahrenstechnik und Mechanik, Universita¨t Karlsruhe, Karlsruhe, 1996, pp. 618-629. [14] Pineda M, Fierrro JLG, Palacios JM, Cilleruelo C, Ibarra JV. Kinetic behaviour and reactivity of zinc ferrites for hot gas desulphurization. J. Materials Science 1995;30:6171–6178. [15] Sasaoka E, Iwamoto Y, Hirano S, Uddin MA, Sakata Y.* Soot formation over zinc ferrite high-temperature desulfurisation sorbent. Energy & Fuels 1995;9:344–353. [16] Akyurtlu JF, Akyurtlu A. Hot gas desulfurization with Vanadiumpromoted zinc ferrite sorbents. Gas Separation and Purification 1995;9:17–25. [17] Jothimurugesan K, Adeyiga AA, Gangwal SK. Hot Coal Gas Desulfurization with Zinc-Based Sorbents, In ‘High Temperature Gas Cleaning’ (Eds. Schmidt E, Ga¨ng P, Pilz T, Dittler A), Institut fu¨r
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W.F. Elseviers, H. Verelst / Fuel 78 (1999) 601–612 Mechanische Verfahrenstechnik und Mechanik, Universita¨t Karlsruhe, Karlsruhe, 1996, pp. 630-638. Flytzani-Stephanopoulos M, Jothimurugesan K, Lew S, Gavalas GR, Patrick V. Detailed studies of novel regenerable sorbents for hightemperature coal-gas desulfurization, Proc. 7th Ann. Gasif. and Gas Stream Cleanup Systems Contractors Review Meeting, DE-FC2185MC22193, DOE/METC/US Dept. Of Energy, Morgantown Energy Technolgy Center, Morgantown, WV, 1987, Vol 2, 726-740. Woods MC, Gangwal SK, Jothimurugesan K, Harrison DP. Reaction between H2S and Zinc Oxide-Titanium Oxide Sorbents. 1. SinglePellet Kinetic Studies. Ind. Eng. Chem. Res. 1990;29:1160–1167. Yrjas P, Huppa M, Konttinen J. Regeneration of sulfided Zn-titanate by oxidation under pressure, In ‘High Temperature Gas Cleaning’ (Eds. Schmidt E, Ga¨ng P, Pilz T, Dittler A), Institut fu¨r Mechanische Verfahrenstechnik und Mechanik, Universita¨t Karlsruhe, Karlsruhe, 1996, pp. 514-527. Gupta RP, Gangwal SK, Cicero DC, Henningsen GB, Katta S. Hot Coal Gas Desulfurization in Fluidized-Bed reactors Using Zinc Titanate Sorbents, In ‘High Temperature Gas Cleaning’ (Eds. Schmidt E, Ga¨ng P, Pilz T, Dittler A), Institut fu¨r Mechanische Verfahrenstechnik und Mechanik, Universita¨t Karlsruhe, Karlsruhe, 1996, pp. 543556. Poston JA. A Reduction in the spalling of zinc-titanate desulfurization sorbents through the addition of lanthanum oxide. Ind. Eng. Chem. Res. 1996;35:875–882. Siriwardane RV, Poston JA. Evans Jr, G. Spectroscopic Characterisa-
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tion of Molybdenum-Containing Zinc Titanate Desulfurization Sorbents. Ind. Eng. Chem. Res. 1994;33:2810–2818. Jothimurugesan K, Adeyiga AA, Gangwal SK. Simultaneous Removal of H2S and NH3 from hot coal gas, In Proc. AIChE Spring Meeting 1995. Jothimurugesan K, Gangwal SK. Catalytic Decomposition of Ammonia in Coal Gas, In ‘High Temperature Gas Cleaning’ (Eds. Schmidt E, Ga¨ng P, Pilz T, Dittler A), Institut fu¨r Mechanische Verfahrenstechnik und Mechanik, Universita¨t Karlsruhe, Karlsruhe, 1996, pp. 383-392. Sasaoka E, Hirano S, Kasaoka S, Sakata Y. Stability of Zinc Oxide high-temperatire desulfurisation for reduction. Energy & Fuels 1994;8:763–769. Christoforou SC, Efthimiadis EE, Vasalos IA. Sulfidation-Regeneration cycles of ZnO- and CaO-containing sorbents. Env. Sci. Tech. 1995;29:372–383. Ayala RE, Feitelberg AS, Furman AH. Development of a hightemperature moving bed coal gas desulfurization system, Proc. Pittsburgh Coal Conference 1995, 1053-1058. Khare GP, Delzer GA, Kubicek DH, Greenwood J. Hot Gas desulfurization using Novel Zinc-Based Sorbents, In ‘High Temperature Gas Cleaning’ (Eds. Schmidt E, Ga¨ng P, Pilz T, Dittler A), Institut fu¨r Mechanische Verfahrenstechnik und Mechanik, Universita¨t Karlsruhe, Karlsruhe, 1996, pp. 569-582. Cline JD. Spectrophotometric determination of hydrogen sulphide in natural waters. Limnology and Oceanography 1969;:454–458.
Transition metal oxides for hot gas desulphurisation W.F. Elseviers, H. Verelst* Department Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium Received 22 September 1997; received in revised form 20 July 1998; accepted 5 October 1998
Abstract Thermodynamic equilibrium simulations were made to determine suitable materials for intensive desulphurisation of fuel gases from an IGCC. Zinc-based materials are the most promising for high temperature intensive desulphurisation, but cause evaporation and regenerability problems. To overcome these problems, a new material was synthesised, containing zinc oxide supported on a titanium dioxide support. The material is able to reduce H2S levels from 3250 ppm to the thermodynamic equilibrium values in a synthetic fuel gas mixture at atmospheric pressure. At 600⬚C, the material can be regenerated completely. Sorbent sulphur capacity increases upon regeneration until 100% sorbent usage. Comparing experimental and simulation results, it is also concluded that Aspen Plus is a powerful tool in calculating high-temperature gas–solid equilibria. 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: Simulation; H2S; Hot gas cleanup; Desulphurisation
1. Introduction Integrated Gasification Combined Cycles (IGCC) are an attractive alternative for future power production. The use of low rank fossil fuels leads to the formation of unwanted impurities like H2S, COS and NH3, which need to be eliminated prior to combustion in the gas turbine. Gasification fuel gas desulphurisation is traditionally carried out by means of low temperature absorption processes, capable of removing H2S to ppm levels. HCl and nitrogen compounds such as NH3 and HCN are removed simultaneously. Low temperature H2S absorption reduces the overall cycle thermal efficiency by a few percent as a result of the exergy losses in the cooling. The same residual H2S levels can potentially be obtained through high temperature gas cleaning, thus avoiding this efficiency loss. H2S removal at elevated temperatures can be carried out in a number of ways. Sorbent injection into the gasifier can serve as a bulk gas desulphurisation. Additional desulphurisation can be achieved through the use of (regenerable) transition metal oxides. In situ desulphurisation using limestone or dolomite can only reach H2S residual levels of about 100 ppm [1]. Although in most cases this degree of desulphurisation might be acceptable, other fields of application like Molten carbonate fuel cells require ppm or subppm residual H2S levels [2], but the major disadvantage of * Corresponding author. Tel.: ⫹32-2-629-32-50; fax: ⫹32-2-629-32-48. E-mail address: [email protected] (H. Verelst)
in situ desulphurisation is the production of large amounts of solid waste. Intensive desulphurisation processes using a regenerable sorbent will eliminate this problem. This paper will focus on the intensive desulphurisation as a stand-alone process. The goal of the research is two-fold: firstly, by comparing the experimental results and the data available from literature with simulation results, to show that process engineering computer simulations can be a useful tool in investigating this type of high-temperature gas–solid reaction; secondly, to come to a more thorough understanding of this type of reactions, so as to develop a more complex modelling approach.
2. Intensive desulphurisation 2.1. Sorbent requirements A good sorbent will allow for a deep desulphurisation to ppm levels and have good regeneration properties. This means the combination of a high affinity towards the reaction with H2S, as well as the formation of a sulphide which can be converted back to the oxide through oxidation with air or diluted air. Next to the residual H2S level, sorbent durability is the critical issue. For economical operation, a good sorbent has to maintain a large fraction of its desulphurisation properties for at least a hundred sulphidation-regeneration cycles, requiring excellent sorbent stability.
0016-2361/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(98)00185-9
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The regeneration reaction equilibrium strongly depends upon the oxygen partial pressure as can be seen from Eq. (4). An unwanted side-reaction is the formation of sulphates:
Table 1 Simulated coal gas composition Component
Concentration
CO2 CO H2 H2O H2S
12.5% mol 42.5% mol 32.175% mol 12.5% mol 3250 ppm mol
2.2. Reactions A number of transition metal oxides are known to have desulphurisation properties at elevated temperatures. A thermodynamic screening was performed by simulations with ASPEN PLUS. It is to be considered that this approach only makes use of the bulk properties of both gaseous and solid phases, and that at the interface between both phases where the reactions take place, different conditions may exist, also because of the use of supported metal oxide systems, containing smaller clusters of the active species. As it is difficult to obtain accurate values of the thermodynamic properties of the solid phases under these conditions, it is believed that a bulk approach can be a good first approximation of the actual situation in order to verify the thermodynamic feasibility of this type of reaction. Thus for bulk systems, ignoring the oxidation state of the metal, the desulphurisation reaction can be written as MeO
s
⫹ H2 S
g
$ MeS
s
⫹ H2 O
1
g
Kp H2 O=H2 S
2
while the regeneration is represented by MeS
s
⫹ 3 =2 O2
g
$ MeO
s
⫹ SO2
3
g
Kp p⫺1=2 × SO2 =O2 3=2
4
MeS s ⫹ 2O2 g $ MeSO4 s
5
Kp p⫺2 × O2 ⫺ 2
6
because the formed sulphate is inert with respect to the desulphurisation and thus there is a loss of active material. Consequently, it is important to know at which conditions this reaction (5) precedes the desired regeneration reaction (3). A second important process parameter is the SO2 concentration in the regenerator off-gas, which needs to be in excess of 5 vol% for a subsequent SO2/SO3 conversion for the production of sulphuric acid. The main composition of the fuel gas containing the H2S is determined by the CO-shift reaction: CO2 g ⫹ H2 g $ H2 O
g
⫹ CO g
7
In the absence of water vapour and hydrogen, soot formation can take place through the Boudouard reaction from CO and CO2: C s ⫹ CO2 g $ 2CO
g
8
2.3. Simulation input Gasification gas composition can vary over a wide range, depending on coal type and gasification operating conditions. Based upon previous calculations [1], the simulated coal gas with composition shown in Table 1 will be used. Regeneration gas consists of an O2/N2 mixture. Simulations were carried out for both sulphidation and regeneration. The components used in the desulphurisation and regeneration simulations are listed in Table 2 and Table
Table 2 Metal components considered in the desulphurization simulations Metal
Feed sorbent
Components in simulation
Al Ba Ca Co Cu Fe Mg Mn Mo Ni Sn Sr Ti W
Al2O3 BaCO3 CaCO3 Co3O4 CuO Fe3O4 MgCO3 MnO2 MoO2 NiO SnO2 SrCO3 TiO2 WO2
Zn Zr
ZnO or Zn2TiO4 ZrO2
Al, Al2O3, Al2S3 Ba, BaO, BaCO3, BaS Ca, CaO, CaCO3, CaS Co, CoO, Co3O4, Co9S8, CoS2, CoCO3 Cu, Cu2O, CuO, Cu2S, CuS Fe, FeO, Fe2O3, Fe3O4, FeS, FeS2, Fe3C, FeCO3, C Mg, MgO, MgCO3, MgS Mn, MnO, Mn3O4, Mn2O3, MnO2, MnS, MnS2, Mn3C, MnCO3 Mo, MoO2, MoO3, Mo2S3, MoS2, MoS3, Mo2C, MoC Ni, NiO, Ni2O3, NiS0.84, NiS, Ni3S2, Ni3S4, NiS2, Ni3C, NiCO3 Sn, SnO, SnO2, SnS, SnS2, Sn2S3, Sn3S4 Sr, SrO, SrCO3, SrS Ti, TiO, TiO2, Ti3O6, Ti4O7, TiS, TiS2, TiC W, WO2, WO2.72, WO2.90, WO2.96, WO3, W2O6, W3O8, W3O9, W4O12, WS2, W2C, WC Zn, ZnO, ZnS, Zn2TiO4, TiO2, TiS, TiS2 Zr, ZrO, ZrO2, ZrS, ZrS2
W.F. Elseviers, H. Verelst / Fuel 78 (1999) 601–612 Table 3 Components considered in the regeneration simulations Phase
Components in simulation
Gas Solid
O2, N2, SO2, Zn Zn, ZnO, Zn2TiO4, ZnSO4, TiO2, ZnS, TiS, TiS2
3, respectively. All phases are taken into account, including multiple solid phases as well as evaporation of the metal. Desulphurisation properties for vanadium were not examined because no thermodynamic parameters could be found in the ASPEN PLUS databanks for the sulphide, nor in literature [3–6]. The formation of carbonyl compounds from iron, nickel and cobalt could be completely neglected above 300⬚C. Sulphate formation is taken into consideration in the regeneration simulations only because desulphurisation takes place in a reducing atmosphere. For the desulphurisation simulations the influence of temperature from 300⬚C to 1500⬚C on residual H2S level at a fixed pressure of 15 MPa was studied for each case, using the simulated coal gas as input stream. All simulations used 100% excess oxide, i.e. the amount of metal fed was twice the amount required to capture all sulphur. For the regeneration simulations, spent desulphurisation sorbent is fed to an equilibrium reactor operating at 1 kPa. For each case the influence of temperature from 400⬚C to 1400⬚C on the composition of the sorbent after regeneration was studied as a function of the oxygen partial pressure. The amount of oxygen fed was twice the amount required to convert the sulphide back to the oxide. 2.4. Desulphurisation simulation results The oxides of aluminium, magnesium, titanium and zirconium are inert for reaction with H2S: no stable sulphides are formed. For the materials showing
603
desulphurisation capacities, Figs. 1 and 2 give the simulation results. Residual H2S levels at 400⬚C and 750⬚C are given in Table 4. For an IGCC-integrated high temperature desulphurisation process, only the temperature range above 300⬚C is of interest, as below 300⬚C kinetics become too slow. It is clear that SrCO3 and BaCO3 can be used for in situ desulphurisation in gasifiers operating at 1100⬚C–1500⬚C, where dolomite and limestone are no longer suited, resulting in even lower residual sulphur levels. At 400⬚C, nearly all transition metal oxides are suitable for desulphurisation. At higher temperatures residual H2S levels increase. If 100 ppm residual H2S is considered allowable, ZnO could theoretically be used up to the melting point of Zn metal, but as a result of Zn metal volatilization in a reducing atmosphere, its usage will be temperaturelimited, as will be discussed below. Zinc titanate Zn2TiO4 is able to reach the same residual H2S levels as pure ZnO. Tin metal volatilization will also limit the use of tin oxide SnO2 as desulphurisation sorbent to relatively low temperatures. For molybdenum and tungsten oxides, carbide formation must be considered. From a thermodynamic point of view, Mo2C or MoC respectively WC are the stable forms at the conditions studied. If carbide formation is neglected, Mo and W residual H2S level indicate a very good desulphurisation potential. However, if carbide formation is taken into account, MoO2 is converted to Mo2C below and to MoC above 900⬚C, resulting in a very poor desulphurisation with residual H2S levels of 900 ppm at 400⬚C. WO2 is completely converted to WC showing no desulphurisation capacity at all. As carbide formation only occurs at higher temperature [7], one could say that, if desulphurisation kinetics are fast enough at 400⬚C, carbide formation will have no noticeable effect on H2S removal. However, if the occurrence of higher temperatures is possible or if kinetics
Fig. 1. Residual sulphur level at 1.5 MPa as a function of temperature (part 1).
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Fig. 2. Residual sulphur level at 1.5 MPa as a function of temperature (part 2).
require higher temperatures, the use of tungsten and molybdenum oxide should be avoided. Cobalt and copper oxides are reduced to their corresponding metals (Table 4). As metal desulphurisation capacity is lower and desulphurisation kinetics slower, this will lead to a lower desulphurisation capacity if sorbent regeneration is incomplete. Supporting the oxide might improve reduction resistance and thus regenerability [8]. Nickel behaves quite well at low temperatures where excess oxide is converted into Ni3C, but desulphurisation is poor above 450⬚C, as reduction to Ni metal occurs. As the affinity for sulphur is very high, regenerability will be poor, making it unsuitable for regenerable desulphurisation processes [9]. Iron oxide desulphurisation potential is somewhat lower compared to the other materials studied. Furthermore, as iron catalyses the Boudouard reaction (Eq. 7), solid carbon deposition inside sorbent pores and on the surface will quickly limit sulphidation. This solid carbon formation is
possible until 850⬚C and has a negative effect on residual H2S level, as can be concluded from Fig. 3. Manganese is capable of maintaining ppm-range H2S levels up to 475⬚C with still only 50 ppm near 600⬚C. Above 400⬚C MnO is the stable form. No carbide formation nor reduction to Mn metal occurs. From a thermodynamic point of view, manganese is a most promising sorbent, which is also indicated by a number of experiments [10,11]. From these simulations, it can be concluded that for an IGCC integrated deep desulphurisation, zinc oxide is definitely the best sorbent, capable of reaching the ppm level. However, metal volatilisation can be a serious problem, especially at higher temperatures. Molybdenum oxide and tungsten oxide must be rejected because of the possibility of carbide. Manganese oxide also seems to be a most promising material as a result of its resistance to reduction, volatilisation and carbide formation. Sorbent activity, capacity and regenerability will decide on its applicability.
Table 4 Residual H2S levels and species formed at 15 bar Sorbent
ZnO Zn2TiO4 MoO2 MnO2 WO2 SnO2 Co3O4 CuO NiO Fe3O4
Residual H2S level (ppm)
Sulphide formed
Excess oxide present as
100 ppm
400⬚C
750⬚C
400⬚C
750⬚C
400⬚C
750⬚C
H2S at
0.03 0.6 0.6 2.5 3.4 19 24 39 41 91
0.06 27 143 216 429 3250 2464 401 1282 732
ZnS ZnS MoS2 MnS WS2 SnS Co9S8 Cu2S Ni3S2 FeS
ZnS ZnS Mo2S3 MnS WS2 SnS Co9S8 Cu2S Ni3S2 FeS
ZnO Zn2TiO4 MoO2 MnO WO2 Sn Co Cu Ni3C Fe3C
ZnO Zn2TiO4 Mo MnO WO2 Sn Co Cu Ni Fe3C
970⬚C 900⬚C 720⬚C 660⬚C 610⬚C 475⬚C 475⬚C 500⬚C 470⬚C 41⬚C
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605
Fig. 3. SO2 concentration in off-gas during ZnO regeneration at 0.1 MPa.
2.5. Regeneration simulation results Regeneration simulations were made only for zinc-based materials, namely zinc oxide ZnO and a zinc titanate, Zn2TiO4. Fig. 4 shows the ZnSO4 formation as a function of temperature and O2 partial pressure for the regeneration of pure ZnO. It can be seen that temperatures in excess of 700⬚C are required to avoid sulphate formation and that the minimum regeneration temperature strongly increases with the partial pressure of oxygen. From Fig. 4, showing the SO2 concentration in the regenerator off-gas versus the oxygen content of the regenerator feed gas, it is clear that in order to obtain SO2 concentrations suitable for H2SO4 production the
oxygen molar fraction in the feed gas should be higher than 10%. This implies that regeneration should be carried out at 800⬚C minimum, whereas desulphurisation is done at much lower temperatures. Implementation on an industrial scale will thus yield a more complex unit. Another fact which cannot be overlooked is the volatilisation of Zn at high temperatures. If desulphurisation and regeneration temperature are much different, temperature gradients from one reactor bed to another can be expected and zinc evaporation is likely to occur in the reducing atmosphere of the desulphurisation reactor. Fig. 5 gives the amount of Zn in the vapour phase as a function of temperature at sulphidation conditions. One can see that vapour losses for zinc
Fig. 4. ZnSO4 formation during ZnO regeneration at 0.1 MPa (O2 volume fraction in regenerator feed gas: (a) 100%; (b) 50%; (c) 20%; (d) 10%; (e) 1%).
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Fig. 5. Zn evaporation for ZnO and Zn2TiO4 during sulphidation at 1.5 Mpa as a function of temperature: (a) Zn2TiO4; (b) ZnO.
titanate are much smaller and thus that ZnO reduction resistance increases. Furthermore, sulphided Zn2TiO4 can be regenerated without sulphate formation above 350⬚C. From a thermodynamic point of view, zinc titanate definitely has major advantages over pure zinc oxide: the reduction resistance has increased and sulphate formation is suppressed, while H2S residual levels remain the same.
3. Other zinc-based sorbents These simulations indicate that the volatilisation and regeneration problems may be solved by supporting the oxides on a carrier material or also through the use of mixed metal oxides. A lot of research has already been done in this field and was mainly focussed on two types of zinc-based materials: zinc ferrites and zinc titanates. Zinc ferrites are mixed oxides of zinc and iron oxide and thus contain two oxides with desulphurising ability. Depending on the ratio of ZnO to Fe2O3, several crystalline phases can exist in the material: the mixed oxide ZnFe2O4 coexists with the excess of the other oxide [12]. The presence of iron oxide lowers the reduction rate of ZnO and the material is regenerable at 750⬚C. Kobayashi [13] reports a 25% capacity decrease over 20 sulphidation/regeneration cycles as a result of sintering and incomplete regeneration. At 10 MPa and 450⬚C, sub-ppm residual H2S levels were reached. Pineda et al. [14] supported zinc ferrites on alumina to further decrease the ZnO reduction rate. However, the material was not homogeneous and had a low sulphur capacity. Sasaoka [15] studied the soot formation over zinc ferrites in a CO-shift gas atmosphere and
reported the conversion of the zinc ferrite into cementite Fe3C and fibrous ZnO below 550⬚C. Soot formation was stronger in more reducing gas mixtures. Impregnation of zinc ferrites with small amounts of V2O5 increased the reduction resistance and sulphur loading. V2O5 was also reacted with H2S [16]. Recently, Jothimurugesan et al. [17] developed a zinc and iron oxide based sorbent, which reached sub-ppm H2S levels at 350⬚C and retained its full capacity over 100 cycles. Although zinc ferrites show lots of improvements over pure zinc oxide, ZnO reduction and regeneration still impose practical problems. The use of supported zinc oxides can improve reduction resistance and regenerability at the cost of lower sulphur capacity per unit mass of sorbent as a result of the inactivity of the support material towards the desulphurisation. In zinc titanates several crystalline phases can coexist, depending on the Zn/Ti ratio. ZnTiO3, Zn2TiO4 and Zn3TiO8 have been identified by Flytzani-Stephanopoulos et al. [18]. During sulphidation, partial or complete decomposition into ZnO and TiO2 occurred, but Zn vapour losses were not detected and regenerability was good in 2% O2. Sulphate formation was not observed, and 50% sulphur loading could be achieved over subsequent sulphidation cycles. The capacity decrease was attributed to a decrease in surface area and pore volume. Woods et al. [19] reported complete sulphidation and regeneration at 675⬚C and observed no ZnSO4 formation. However, Yrjas et al. [20] report sulphate formation at 20 MPa and below 700⬚C at higher oxygen partial pressures during regeneration. Regeneration was fast but much time was needed to obtain complete regeneration. Gupta et al. [21] carried out a 100 cycle test at 750⬚C on a pilot scale. Sulphur capacity dropped to about 40% of its initial value as a result of losses in surface area and pore volume.
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607
Fig. 6. XRD spectrum of zinc titanate sorbent.
Other additions to zinc titanate materials have been examined to improve sulphur capacity, durability and/or regenerability. Poston [22] added La2O3 to reduce sorbent spalling. Sorbent capacity increased slightly as a result of the reaction with La2O3. The addition of molybdenum caused migration of zinc towards the pellet surface but regenerability was poor [23]. Jothimurugesan et al. [24,25] synthesised a zinc titanate with Ni, W and Mo promoters. At 750⬚C, H2S levels below 70 ppm were measured and the material was also capable of decomposing ammonia according to 2 NH3 g $ N2 g ⫹ 3 H2 g
9
which is an important side-effect of this material, as in most gasification gases ammonia is present in about the same range of concentrations as H2S and will be converted into NOx upon combustion in the gas turbine. Other supported zinc oxides made use of alumina carriers, but these could not stop reduction and evaporation, although a new crystalline phase was formed [26,27]. Mixing ZnO with ZrO2 yielded somewhat better results but the stabilisation mechanism was not understood very well. A mixed oxide of CaO and ZnO showed no regenerability. Residual H2S concentrations are of the sub-ppm level, but sorbent capacity drops over repeated sulphidation/regeneration cycles as a result of significant material and strength losses [28,29].
4. Experimental It is clear that zinc-containing sorbents and more specific zinc titanates are among the most promising materials for hot gas cleaning purposes. A number of zinc-titanate sorbents were thus prepared to validate the simulation results through a series of experiments. Before carrying out experiments with the synthesised materials, the experimental setup was evaluated with ZnO and CuO. The obtained results corresponded well with the predictions from the simulations: ZnO is able to reach very low residual H2S levels, but lacks regenerability at temperatures lower than 850⬚C. CuO was converted into a mixture containing 50%mole CuS and 50%mole Cu2S and was not regenerable. 4.1. Sorbent preparation and characterisation A TiO2 (Merck) support was impregnated during 24 hours with a 4.2 M solution of Zn(NO3)2 (Merck) by wet impregnation. The slurry was dried for 24 hours at 140⬚C and calcined for 24 hours under oxygen at 600⬚C. The obtained sorbent was characterised with X-Ray Diffraction (XRD) to determine which crystalline phases were present (Fig. 6). Sorbent and support surface area were determined using N2 porosimetry, while the composition was determined through Energy Dispersive X-Ray Diffraction (EDX) (Table 5). The measured surface area values are rather low and the materials do not contain pores below 10 nm. The introduc-
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Table 5 Surface area from N2 porosimetry
Table 7 Experimental conditions for H2/H2S experiments
Material
SA (m 2/g)
Formula
Run
Sulphidation (⬚C)
Regeneration (⬚C)
TiO2 support (250–400 mm) Zinc titanate (100–250 mm) Zinc titanate (⬍100 mm)
7,9 4,6 3,1
TiO2 ZnO.(TiO2)2.6 ZnO.(TiO2)2.6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
500 500 500 500 500 600 600 600 600 600 600 600 550 500 500
750 700 650 600 600 600 600 600 600 600 600 550 800 800
tion of the zinc oxide onto the support decreased surface area by about 50%. No new crystalline phase could be identified from the XRD analysis, which revealed the sorbent to be consisting of finely dispersed ZnO on a TiO2 matrix. 4.2. Experimental setup The synthesised materials were tested on their desulphurisation abilities in a small, tubular packed-bed reactor. CO, CO2 and a mixture of 1 vol% H2S in H2 were mixed together through a set of calibrated rotameters to obtain the desired gas composition. H2O is produced in situ by the CO-shift reaction. For this reason, an inert packing, consisting of quartz sand, constituted the first 15 cm of the 40 cm reactor tube. The second part of the 7 mm inner diameter quartz tube reactor contained the active sorbent, mixed with inert quartz sand in order to reduce sorbent breakthrough time and increase bed residence time. Total sorbent bed length was about 10 cm, after which again a 15 cm inert bed was placed. The reactor tube was placed in a temperaturecontrolled oven. The reactor bed pressure drop was measured with an upstream electronic pressure indicator. The gas leaving the reactor tube was led through a zinc acetate buffer solution, allowing time-averaged H2S outlet concentrations to be measured accurately using the methylene blue method [30]. A qualitative monitoring of the total gas composition was done continuously by means of a quadrupole mass spectrometer, giving the intensities for mass 2 (H2), 18 (H2O), 28 (CO and CO2), 34 (H2S), 44 (CO2) and 60 (COS) as a function of time. Sorbent breakthrough time was determined as the time when the H2S outlet concentration starts increasing dramatically. 4.3. Results The first goal in the experiments with the synthesised materials was to check whether they were capable of capturing H2S from a gas. To eliminate the possible interference of the other fuel gas constituents, the experiments were done in Table 6 Experimental feed gas compositions (mol%) Component
Mixture 1
Mixture 2
Mixture 3
H2 H2S CO CO2 Ar
32,2 0,325 0 0 67,5
32,4 0,327 42,5 0 24,7
47,8 0,483 21,3 18,3 12,1
a H2/H2S mixture. Ar was added as an inert in order to work at the same hydrogen partial pressure as in the experiments with the simulated fuel gas. A second goal was the determination of the lowest possible regeneration temperature and the influence of the fuel gas constituents on the desulphurisation. A first series of desulphurisation experiments was made with the gas with composition ‘‘Mixture 1’’ from Table 6. Regeneration was carried out at different temperatures with argon-diluted air. The regeneration mixture contained 1 mol% O2, resulting in even lower SO2 concentrations, too low for H2SO4 production, but avoiding sulphate formation. In the subsequent sulphidation-regeneration cycles, desulphurisation temperature was kept at 500⬚C or 600⬚C, while regeneration temperature was gradually decreased from 900⬚C in 50⬚C steps. The minimum regeneration temperature will be defined as the lowest temperature causing no sorbent capacity losses in the subsequent sulphidation run. The temperature program is given in Table 7. A number of H2S breakthrough curves are given in Fig. 7, which gives the relative H2S concentration (0 corresponds with no H2S present, 1 with the inlet concentration) as a function of the reduced time, which is defined as the actual time divided by the estimated breakthrough time for 100% sorbent usage calculated from the sorbent composition. Table 8 Experimental conditions Run
Components
Mixture
1 2 3 4 5 6 7 8
Ar/H2/H2S Ar/H2/H2S Ar/H2/H2S/CO Ar/H2/H2S Ar/H2/H2S/CO/CO2/H2O Ar/H2/H2S/CO/CO2/H2O Ar/H2/H2S/CO/CO2/H2O Ar/H2/H2S/CO/CO2/H2O
1 1 2 1 3 3 3 3
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Fig. 7. H2S breakthrough curves for H2/H2S experiments.
The sorbent capacity increased upon successive regenerations up to about 75% of the total capacity predicted from sorbent overall composition. At 600⬚C the sorbent can thus be repeatedly sulphidated and regenerated over several cycles without large capacity losses. In the H2/H2S/Ar atmosphere the material was able to reach residual H2S levels of 0.15 ^ 0.05 ppm at 600⬚C, which corresponds with the thermodynamic equilibrium in the absence of the other fuel gas components. When regeneration temperature drops below 600⬚C, the sorbent capacity drastically decreased, probably because of sulphate formation. Regeneration at 800⬚C was not able to decompose the sulphate.
Consequently, the minimum regeneration temperature is taken as 600⬚C. A next series of experiments was carried out to determine the influence of the other fuel gas components: CO, CO2 and H2O. All experiments are carried out at 600⬚C. In subsequent sulphidation runs, the influence of the addition of each component to the reaction gas mixture is studied. An overview is given in Table 8, where gas compositions are taken from Table 6. The breakthrough profiles are given in Fig. 8. Between runs 3 and 4 also a mixture containing only Ar, CO and CO2 was fed to the reactor to check on the influence of the Boudouard reaction (8).
Fig. 8. H2S breakthrough curves for shift-gas experiments.
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Fig. 9. EDX element mapping of spent sorbent: (a) Visual; (b) Ti; (c) S; (d) Zn.
From these results we can conclude that the fuel gas composition does not have a large influence on the desulphurisation performance, except for the minimum residual H2S level which can be obtained. The thermodynamic equilibrium is slightly worse in case CO, CO2 and H2O are present; the corresponding levels of 2.2 ^ 0.1 ppm was reached. Sulphur capacity increased again upon regeneration (runs 1/2). The addition of CO did not influence the breakthrough time (runs 2/3) nor did the complete fuel gas
mixture (runs 5/6/7/8). It is also clear that the material is fully regenerable at 600⬚C in 1% O2. In a CO and CO2 containing atmosphere, soot formation takes place. This was observed at the outlets of the reactor tube where temperatures are lower, and also by comparing the breakthrough times for runs 3/4/5. Sulphur capacity decreases from run 3 to run 4 and is restored upon regeneration. This is as a result of the soot formation which took place during the time the CO/CO2/Ar mixture was passed over the
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reactor after regeneration 3 and before sulphidation 4. During regeneration 4 the soot was burned off and the sulphur capacity was restored from run 5 on. It can be concluded that either soot formation does not occur in the complete fuel gas mixture either that any formed carbon will be converted back to CO or CO2 when H2 or H2O are present. The conclusion is that the fuel gas atmosphere does not influence the desulphurisation capacity. From the sorbent breakthrough time and the H2S feed rate also the sorbent sulphur capacity could be calculated: 0.11 g sulphur/g sorbent, which corresponds with a nearly 100% sorbent usage. Sulphur, zinc and titanium maps were obtained by EDX for the spent sorbent (Fig. 9). The maps were taken over the cross section of a 150 mm sorbent grain. It is clear that Zn and S occur in the same regions on a Ti support and are located near the surface, where Ti is found more in the inner core of the material. As a result of the small amounts of material used in the experiments (0.1 to 0.5 g), changes in pore volume and surface area could not be measured. The increase in sulphur capacity upon regeneration is probably as a result of changes in pore structure and/or surface area. Further experiments with larger amounts of material will therefor be necessary to study these structural changes, also to obtain a better view on the sintering behaviour of these solid materials at high temperatures. However, a strong decrease in capacity over the total number of sulphidation-regeneration cycles was not measured and the particle size remained roughly the same and it is beleived that the material does not suffer too much from (macroscopic) sintering on this experimental scale. The comparison of Ti and Zn maps obtained by EDX for fresh and spent sorbent also showed a similar material distribution, indicating that on this scale the material is also stable enough under the studied conditions.
5. Conclusion Zinc-based materials are the most promising for high temperature intensive desulphurisation. To overcome the regenerability and reduction problems, a new material was made, containing zinc oxide supported on titanium dioxide. The obtained ‘zinc titanate’ did not contain any new solid phases, but is able to reduce H2S levels from 3250 ppm to the thermodynamic equilibrium values in a synthetic fuel gas atmosphere. At 600⬚C, the material can be regenerated completely. Sorbent sulphur capacity increases upon regeneration until 100% sorbent usage. Further experiments will be necessary to study the structural changes in the material and to determine regeneration conditions yielding SO2 concentrations sufficiently high for H2SO4 production. Comparing experimental and simulation results, we can also conclude that Aspen Plus is a powerful tool in calculating high-temperature gas–solid equilibria.
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Acknowledgements The authors would like to thank Oscar Steenhaut (VUB Department of Metallurgy) for the EDX measurements and Prof. Johan Martens (KU Leuven Center for Surface Chemistry and Catalysis) for the XRD measurements. This research is funded by the VUB-OZR.
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