Evaluation of Sorbents for High Temperature In Situ Desulfurization of

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Evaluation of Sorbents for High Temperature In Situ Desulfurization of Biomass-Derived Syngas Moritz Husmann* and Christoph Hochenauer Graz University of Technology Institute of Thermal Engineering Inffeldgasse, 25b 8010 Graz, Austria

Xiangmei Meng and Wiebren de Jong Delft University of Technology Department of Process & Energy Leeghwaterstraat, 44 2628 CA Delft, The Netherlands

Thomas Kienberger Agnion Highterm Research GesmbH Conrad v. Hötzendorfstrasse, 103a 8010 Graz, Austria ABSTRACT: In a preparative study, the use of different sorbent materials for in situ desulfurization of biomass derived synthesis gas is evaluated. Results of phase equilibrium calculations using FactSage version 6.4 for the H2S equilibrium concentration with a variation of different parameters are compared to values for the residual H2S content found in literature. The focus is strictly set on a high temperature, high steam application for biomass-derived syngas conditions. Possible synergistic effects between different sorbent components and deviations from simulated results are discussed subsequently. The use of copper-based sorbent material turns out to be promising due to a predicted positive impact of high steam conditions for sorption equilibria. Results for the implementation of a CaO−BaO mixed phase sorbent are confirmed, qualifying this material combination as a promising candidate for in situ application. Equilibrium calculations predict a desulfurization of syngas to a sulfur level of 2.1 ppmv H2S at a steam content of about 40 vol % and 820 °C which is sufficient for further catalytic gas processing applications.

■

INTRODUCTION The thermal conversion of woody or herbaceous biomass into a syngas of high heating value increases the flexibility of biomass use for heat and power generation and opens the future perspective for substitute natural gas (SNG) and biofuel production from economically less valuable sources. The use of wood chips in fluidized bed gasifiers is state of the art,1 whereby the generation of power from wood cannot be profitable without substantial funding. Possible means to reduce the costs of the produced gas can be a simplification of the process equipment design, an increase of the overall efficiency of the process and most effectively the use of cheaper fuel sources such as green waste, felling residue, or sewage sludge. Those fuels usually contain a higher amount of sulfur, chlorine, and ash forming elements.2,3 In order to obtain a high quality product gas with no detrimental effects on downstream equipment, the raw product gas has to be cleaned from sulfur and chlorine components which are corrosive as well as poison catalysts used in subsequent gas conditioning. Tars and alkali components have to be removed as well in order to lower the dew point of the gas and thus prevent pipe blocking or other severe failures due to fouling. The most common procedure for syngas cleaning in biomass gasification applications is the wet scrubbing process4 in which the gas is cooled and organic impurities are transferred to a liquid organic sorbent. In order to significantly improve the overall efficiency and to simplify the process equipment, the implementation of catalytic hot gas cleaning is a technological © 2014 American Chemical Society

development that has been pursued since the 1970s for coal gas cleaning4,5 and later for biomass derived syngas as a renewable energy carrier. With the objective of further developing catalytic syngas treatment, as described by Kienberger et al.,6 a setup has been established at TU Graz where syngas is produced from a bubbling fluidized bed gasifier (BFBG) operated with steam at 820 °C under atmospheric pressure. The syngas is filtered at 350 °C and desulfurized using ZnO sorbent in a fixed bed reactor at 300 °C. Tars contained in the gas stream are then converted to permanent gas components over a nickel catalyst.6 Deep desulfurization prior to tar conversion is necessary as sulfur contained in the gas forms stable nickel sulfide by reaction of sulfurous compounds with NiO. The consumption of nickel catalyst is a major cost factor in catalytic gas treatment, thus the formation of catalytically inactive NiS should be prevented by the employment of economically less valuable sorbent materials such as zinc oxide.6 With a rise in sulfur content of the fuel, the consumption of ZnO sorbent is also increased. Considering the deterioration during sorbent regeneration the necessity of a more frequent sorbent replacement is counteracting the benefit of using economically feasible fuel sources with a high degree of impurity and inhomogeneity. Received: November 18, 2013 Revised: February 6, 2014 Published: March 19, 2014 2523

dx.doi.org/10.1021/ef402254x | Energy Fuels 2014, 28, 2523−2534

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temperature of 300 °C. The consumption costs of in situ sorbent together with the necessary investment for process equipment design modifications have to be lower than the consumption of ZnO in the mentioned fixed bed reactor. Therefore, the investigations of sorbents exclude expensive rare earth-materials and focus on ZnO, CuO, MnO, and calciumbased materials. Also considered is a combined CaO-BaO sorbent described by Stemmler et al.7 Iron based sorbent is excluded, as it is considered to have a low capture efficiency at the intended process conditions.8

Therefore, a concept is pursued in which the combination of different bed materials buffers the high content of sulfur via in situ desulfurization to a level that is acceptable for further increased desulfurization and enhanced catalytic cleaning. Depending on the achievable level of H2S by in situ desulfurization a subsequent coarse desulfurization via fixed bed sorption could be necessary. As organic sulfur compounds might rise to a critical content level with a rise in total sulfur amount a hydrodesulfurization (HDS) conversion step could be employed to convert these organic compounds to H2S which is readily captured as metal sulfide in the downstream deep cleaning unit. Possible gas processing methods for a product gas with high sulfur content are displayed in Figure 1. The number of necessary cleaning steps depends on the effectiveness of in situ desulfurization.

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Figure 1. Concept for the desulfurization of a product gas with high sulfur content. The displayed values for sulfur content represent the outlet margins of each process step.

Gas desulfurization by means of desulfurization on metal oxide sorbents proceeds according to the following general equation (eq 1), with H2S being the main sulfur compound in the untreated gas stream. MeOx + x H 2S ↔ MeSx + x H 2O

EXPERIMENTAL SECTION

In order to validate suitable sorbents that comply with the aforementioned requirements, evaluation has been done by a thorough analysis of results obtained by other researchers as well as by simulative assessment corresponding to experimental data gained by former research activity at TU Graz described in Kienberger et al.6 The review of literature presented within this work is strictly focusing on high temperature and high steam conditions. Many of the published experimental results for H2S sorption equilibria are destined for coal derived syngas composition. Such syngas contains less steam as coal is usually gasified in oxygen with little or no steam added.9 Thus, the results are not directly transferable to the syngas obtained from gasification of biomass in BFBG-units. In order to generate a better prediction of achievable H2S concentrations in the syngas after in situ desulfurization, gasification conditions of the BFBG at TU Graz were simulated using the FactSage 6.4 Gibbs energy minimization tool.10 Calculations were made using the Fact Pure Substances database and the Factmisc database as the contained thermodynamic data for the System S−Fe−Mn-Cu is more specific within this extended set.11 The investigated gasifier is fluidized by steam in allothermal application at a steam-excess-ratio of 4 at atmospheric pressure. The steam-excess-ratio σ is defined as shown in eq 2: x wt,H2O σ= x wt,H2O,min (2) with xwt,H2O being the mass fraction of steam in the reactor and xwt,H2O,min the minimum amount for stoichiometric gasification, respectively. It can be determined, as shown in eq 4, as follows from the gasification reaction (eq 3).

(1)

The approach of using a substance as in situ sorbent is beneficial concerning the simplicity of process equipment design as a coarse desulfurization can be implemented without additional reactor design modifications. On the other hand, it narrows the possibilities of adjusting the conditions of desulfurization as the parameters for a stable gasification process are not supposed to be changed. Therefore, a profound research for potentially suitable sorbents at the conditions within the studied BFBG at TU Graz is inevitable. For successful in-bed application, the equilibrium constant of the sorbent concerning desulfurization reaction (eq 1) has to be high enough to ensure a low equilibrium H2S concentration even under high temperature and high steam conditions. Additionally, the sorbent has to be stable in the chemically active phase against reduction under the highly reductive hot syngas atmosphere. Furthermore, the vapor pressure of all present phases of the sorbent has to be low enough to prevent significant loss by evaporation at the gasifier operation temperature of 820 °C. Therefore, the choice of potentially suitable in-bed materials for sulfur sorption is limited, especially considering economic aspects, which require the sorbent material to be either cheap enough for single use without relevant increase of the gas price or regenerable for multicycle use. The benchmark for an in situ sorbent in the considered application is the cost of desulfurization with a fixed bed zinc oxide sorbent at a medium

CHnOm + (1 − m)H 2O ↔ CO +

x wt,H2O,min =

M H2O MCHnOm

⎛n ⎞ ⎜ + 1 − m⎟H 2 ⎝2 ⎠

(3)

(1 − m)(1 − x wt,H2O,Fuel)

(4)

According to the elemental composition of the fuel, a molar mass MCHnOm is determined to convert molar to mass fractions and calculate xwt,H2O,min under consideration of xwt,H2O,Fuel the water contained in the fuel. The gasification temperature is approximately 820 °C with a fuel ratio of 300 g/h equaling 1.5 kW. For gasification of wood pellets this results in a gas composition corresponding to Table 1 according to gas analysis measurements. The N2 content is a result of feedstock inert flushing. A detailed explanation of the BFBG unit is given by Kienberger et al.6

Table 1. Composition of Syngas Derived from Wood Pellet Gasification with a Steam-Excess-Ratio of 4 vol %

2524

H2

CO2

CO

CH4

N2

H2O

tars g/Nm3

H2S ppmv,dry

29

16

8

4

4

38

7−9

15


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With the objective of reproducing the permanent gas composition of the experimentally obtained syngas, the input of substances for simulation was chosen according to an elemental analysis of wood pellets as the standard fuel for gasification experiments performed in the BFBG at TU Graz. The elemental composition is 50.7 wt % C, 6.0 wt % H, 42.2 wt % O, 0.36 wt % N, 915 °C) or a reduced oxygen content.31 The reforming at moderate temperature prevents sintering of the sorbent particles and as reported, reforming at 700 °C in an oxygen depleted atmosphere (6 vol % O2 in N2) still leads to moderate sulfate formation, but no significant loss in sorbent activity is observed.33 The suitability of manganese sorbents for longterm use was also proved by a study conducted by Bakker at al.34 who investigated the changes in capacity over duration of 110 cycles of loading and regeneration with no significant loss of capacity. The regeneration conditions were compared between steam regeneration, SO2 regeneration and regeneration in diluted air. They also investigated the influence of temperature during sorbent loading, pointing out that the increase of temperature up to about 900 °C increases the sorbent capacity due to improved diffusion in the solid phase. This might especially be true for alumina supported particles used by Bakker et al. as a bulk phase of MnAl2O3 is formed.35 Another series of investigation examining the potential synergies of a combined Mn−Fe−Zn sorbent on Al2O3 support indicates a decline of sorbent reactivity with increasing number of loading/regeneration cycles and an increase of capacity with increased temperature.36 As the sulfidation takes place at 500− 650 °C diffusion limitations in the bulk phase and further incorporation of the active species into the support lattice might be accountable for this phenomenon.35 The use of natural manganese ore investigated by Yoon et al.37 confirms the above-mentioned temperature dependency due to diffusion resistance. In order to lower the achievable equilibrium H2S level, Alonso and Palacios38 investigated the combination of manganese based sorbents with either 10 mol % CuO or ZnO in a quartz tube reactor at 700 °C with gas composed of 1 vol % H2S, 10 vol % H2, 5 vol CO2, 15 vol % CO, 15 vol % H2O and N2. They confirm the above-mentioned result that Mn does not stabilize CuO in its oxidation states resulting in no measurable positive effect by CuO addition. The addition of ZnO reveals its effect by X-ray diffraction (XRD) analysis showingwith 77% compared to 22%a significantly higher share of tetragonal Mn3O4 crystallite phase in the sorbent structure. This change in crystallite structure results in a higher sorbent performance with Mn 3 O 4 having the highest equilibrium constant among Manganese oxides. Even after 70 cycles of sulfidation and regeneration the H2S concentration in the outlet gas is about 5 ppm showing that ZnO is still present in the crystallite lattice.

component. H2S concentrations of less than 10 ppmv were measured for a combined sorbent.24 Abbasian and Slimane17 as well as Li and FlytzaniStephanopoulus22 investigated the use of Cu with Cr2O3 and Cu with CeO2 as sorbent combinations. Such stabilized Cu sorbents were tested in the temperature range from 650 to 850 °C in a fixed bed reactor with a gas consisting of 20 vol % H2 and 10 vol % H2O in N2. Desulfurization levels from 5 to 10 ppm H2S were reached, being far below the equilibrium level for elemental Cu thus proving the occurrence of synergetic effects of mixed metal sorbents. For Cu/Cr2O3 sorbent, the formation of a stable CuCr2O4 phase was confirmed, whereby for CuO/CeO2 the reduction of Cerium, and thus its participation in sulfidation, is attributed to the good sorbent performance. Further investigations on the interaction between ceria and copper have been undertaken, proving this effect.27 Stemmler et al.7 investigated an in-bed desulfurization concept for the Güssing gasifier in Austria.28 The gasifier is destined for biomass gasification and based on the combination of two fluidized beds; one serving for heat generation by combustion of remaining coke and the other for the gasification of fresh biomass. They achieved a desulfurization from 350 to 250 ppmv with Güssing-gas-composition (36 vol % H2, 11 vol % CO2, 25 vol % CO, 17 vol % H2O, 4 vol % N2, 7 vol % CH4) using CuO−olivine as sorbent at 800−900 °C under fixed bed conditions. These results are close to the equilibrium H2S levels for metallic copper indicating no stabilization effect from olivine. For a less reducing gas environment as resulted from the oxygen blown gasification plant located in Värnamo, Sweden (11.8 vol % H2, 11.9 vol % CO 37.7 vol % H2O, 27.9 vol % CO2, 8.2 vol % CH4, 1 vol % N2), the same researchers calculated an equilibrium of about 100 ppmv at 950 °C.29 Considering economic aspects, CuO-sorbents are more expensive than ZnO-based sorbents. CuO supported on Cr2O3 and doped with other metals is commercially available as the so-called Adkins-catalyst, used in the conversion of fatty acids or other oleochemical applications. The use of such Cubased material might be beneficial compared to ZnO because of superior behavior concerning multicycle use. Abbasian et al.17 investigated the reforming properties of CuO/Cr2O3 sorbents in a simulated coal derived syngas (10 vol % H2, 20 vol % CO, 10 vol % CO2, 10 vol % H2O, 2 vol % H2S in N2). After cyclic sulfidation and regeneration at a temperature of 600 and 750 °C, respectively, they stated an increase in desulfurization capacity over the first 14 cycles. After 20 loading cycles no significant deterioration of the sorbent capacity was observed. Manganese-based. Being a cheap sorbent with good regeneration and thermal stability properties, manganese oxide exhibits some benefits as sorbent material.4 It is proposed that manganese sorbents are especially suitable for the use in high steam and high carbon syngas environment as occurring in biomass gasification.25 Intense analyses of MnO as sulfur sorbent for coal derived syngas have been carried out by Slimane et al.30−32 pointing out the stability of Manganese sorbents against reduction or volatilization even under changing process conditions as a major favorable feature of this sorption material. The high melting point of all formed manganese species enables the longterm use at temperatures in excess of 800 °C without significant volatilization of the sorbent.30 Fixed bed testing of the developed sorbent material revealed a residual sulfur concentration of about 222.5 ppmv after fixed bed testing with a simulated coal gas (13 vol % H2, 24 vol % CO, 5 vol % CO2, 5% 2526


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Cheah et al.25,39 conducted intense investigations on the high steam content in biomass derived syngas toward sorption performance and mechanisms on manganese sorbents. They compared the results obtained from a simulated syngas composition (10.5 vol % H2, 8.3 vol % CO2, 10.5 vol % CO, balance inert) with different steam contents in a fixed bed reactor. For a steam content of 8 vol % the outlet H2S content was at 5 ppmv passing through a bed of manganese supported on alumina at 700 °C. At a higher steam content of 45 vol % H2O a reduction of the gas-hourly space-velocity (GHSV) from 15000 to 3300 h−1 was necessary in order to observe a desulfurization effect. With the reduced GHSV a significant reduction of the outlet H2S content from 392 to about 5 ppmv was detectable over a short period of time. Therefore, an interaction between sorbent material and H2S took place. As this value is below the prediction of thermodynamic equilibrium calculations Cheah et al. conclude that mainly surface reactions determine the sorption mechanisms in high steam environment. This assumption is supported by the fact that there was no crystalline MnS phase detectable by XRD measurements.25 Many recent investigations 40−43 are focused on the combination of manganese sorbents with cerium. This approach could be promising as the combination of the two oxides yields a synergy between the low outlet H 2S concentration of rare earth oxides combined with the high sulfur retaining capacity of manganese. A prebreakthrough H2S content of 4.1 ppm has been detected for a Mn−Ce−La sorbent on alumina at a temperature between 627 and 750 °C.43 Calcium-based. Concerning sorbent cost, availability and toxicity Ca-based sorbents show superior properties. Naturally occurring as limestone or dolomite the composition of different minerals and content of trace elements varies according to the origin of the sorbent material. Yrias et al.44 have compared different dolomite and limestone compounds according to composition and desulfurization performance. As in-bed sorbents Ca based materials show favorable temperature dependence of reactivity for the use above 800 °C. Superimposed to the equilibrium of sulfidation (eq 1) the carbonate equilibrium determines the present form of Casorbent according to eq 5. CaCO3 ↔ CaO + CO2

Still Stemmler et al.7 tested different Ca-based sorbents with Güssing Gas composition (see above) and achieved a desulfurization down to 190 ppm for dolomite and 50 ppm for slag lime which is far below the calculated equilibrium. Therefore, the contents of trace elements in these sorbents and interactions thereof have a scope of impact that is beyond simulative assessment. In this context, Zevenhoven et al.45 concluded that a general statement about different Ca-based sorbent performance is not advisable as the observed initial reaction rates and product layer diffusion could not be attributed to porosity but rather to influences of other catalytic metals. Other comparative studies46,47 report a better performance of dolomite compared to pure lime, which is attributed to sintering of a CaS layer on the surface of lime particles. This layer increases the diffusion barrier and reduces the reactivity of the sorbent. In the case of dolomite this effect is less pronounced as the mixed crystal structure of CaO and MgCO3 for partly calcined dolomite prevents the formation of a homogeneous CaS layer on the particle surface.44,47 Corella and colleagues48−50 as well as Xie et al.51 conducted detailed investigations on the impact of different bed materials on the gas composition whereby the use of dolomite might result in a positive synergy between in situ desulfurization and tar content of the gas. Besides the limitation to residual H2S levels of 50 ppmv in case of positive synergies or above 100 ppmv for pure lime the regeneration properties of CaO based sorbents limit their economic feasibility. Due to attrition52 and formation of stable sulfate from CaS53 the sorbent performance decreases drastically upon multicycle use. In the case of Ca-based sorbents, the investment is low enough for single use, but landfill of the used sorbent is likely to pose problems as it is expected that the conversion to CaCO3 and H2S does not comply with legal limitations.54 Therefore, a prior conversion step to stable CaSO4 would be necessary.47 In a recent work, Stemmler et al.7 developed a Ca-based sorbent that might circumvent the limitations of sulfate formation and unsatisfactory equilibrium as they have synthetized a CaO-BaO mixed oxide that is capable of desulfurizing biomass derived syngas (Güssing composition, see above) to a level below 1 ppmv at 850 °C. This is to be accounted for a solid solution of CaO and BaO that forms due to a slight miscibility of both oxides at elevated temperature. The incorporation of BaO in a CaO matrix prevents the barium from converting into carbonate, which would normally occur under gasification conditions. Further explanation about the synthesis of this mixed sorbent is given by Stemmler et al.7 The stabilized BaO has an equilibrium-constant high enough to desulfurize the syngas to such low levels if the temperature level is kept above the calcination temperature of the embedding Caphase. According to experimental results, even a dechlorination down to below 0.5 ppm HCl is possible at a temperature level of 800−900 °C. The regeneration of this sorbent has to be experimentally confirmed but according to Stemmler et al. a regeneration could be possible by cooling down the sorbent under non oxidizing atmosphere, thus preventing the formation of bariumsulfate.7 Still attrition of the sorbent might prevent ongoing multicycle use. Reviewing experimental results shows that through the use of stabilized sorbents in a fixed bed set up residual H2S contents of about 10 ppmv have been achieved for various sorbent

(5)

However, the calcined form shows a significantly higher reactivity toward CaS formation resulting in lower equilibrium desulfurization limits for calcined lime. Another common Ca based sorbent is dolomite, which consists of a mixture of CaCO3 and MgCO3 with MgCO3 content of commonly 20−45 wt %.44 For dolomite three different phases exist in dependence of CO2-partial pressure and temperature as the equilbria for calcination of MgCO3 and CaCO3 are different. Therefore, calcination proceeds as shown in eqs 6 and 7 with increasing temperature and decreasing CO2-partial pressure. MgCa(CO3)2 ↔ CaCO3 + MgO + CO2

(6)

CaCO3 + MgO ↔ CaO + MgO + CO2

(7)

According to literature H2S equilibrium values using calcined CaO as sorbents range between 100 and 500 ppmv depending on the gas composition and steam content at around 800−900 °C.8,44 2527


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combinations under high temperature conditions. This accounts for ZnTiO4 mixed with traces of Mn and Cu,15 CuO-sorbent on Cr2O3,22 Mn3O4 sorbent with addition of ZnO38 and BaO sorbent stabilized in a CaO solid solution.7 Still the steam content being essential for sulfidation equilibrium is usually lower as most of the former research is dedicated to coal gas desulfurization. High steam conditions have been investigated by Cheah et al. resulting in somewhat unsatisfactory results for the tested CeCuO and Mn-sorbents.25 Still the results by Stemmler et al.7 demonstrate that achieving a residual H2S content of less than 1 ppmv can be possible under high temperature and high steam conditions. They managed this by applying a stabilized BaO-sorbent in a CaO-BaO solid solution which prevents the BaO from forming its carbonate as long as temperatures remain above the calcination temperature of the surrounding CaO-phase.7 Simulated Results. In order to evaluate influencing factors that are not available in literature for in situ application of biomass derived syngas desulfurization a simulative approach thereof is presented within this work. Variation of Steam Content. Being on the product side of the desulfurization reaction (eq 1) the steam content of the gas has a major impact on the H2S equilibrium. Therefore, sorbent specific behavior with varying steam content is an essential criterion for the evaluation of suitable sorbents for in situ application. To validate the influence of varying steam content in the syngas, a variation thereof has been examined depending on potential sorbents. The steam content present in the syngas originates partly from the fuel with a water content of about 6%. Mostly it originates from the steam that is used to fluidize the bed material and fuel in the BFBG. According to studies on coke formation on downstream catalytic gas cleaning units and validation of stable process conditions conducted by Kienberger et al.,6 a steam-excess-ratio σ of 4 has been established as standard process condition for the investigated gasifier. The steam content in the syngas is also influenced by gas phase reactions such as the water−gas shift reaction (eq 8) and methane reforming (eq 9) whereby the formation of methane is suppressed in the simulative approach. CO + H 2O ↔ CO2 + H 2

CH4 + H 2O ↔ CO + 3H 2

[ΔHR = − 42 kJ/mol]

Table 2. Simulated Steam Content with Corresponding Syngas Composition at 820 °C steam-excess-ratio

4

5

6

7

8

steam content mol %

29.0

34.7

39.6

43.9

47.6

35.4 11.7 14.0 9.8

33.1 11.9 11.1 9.0

31.1 11.8 9.0 8.2

29.2 11.6 7.7 7.7

27.5 11.3 6.4 7.1

mol %

H2 CO2 CO N2

steam-excess-ratio of 6 most closely represents the measured steam content in real process conditions (Table 1) it has been used as a standard input for further validation of influence factors. According to the simulated results, the dependence between the content of steam in the syngas and the residual level of H2S varies for different sorbents. Therefore, the influencing effects on the changes in the H2S equilibrium values with varying steam-excess-ratios have to be considered separately for each sorbent. Figures 2 and 4−6 show the change of equilibrium H2Svalues for different sorbents where the bold dotted lines

Figure 2. Deviation of H2S equilibrium for MnO in dependence of temperature for different steam-excess-ratios. TBFBG indicates the intended temperature of application.

(8)

represent the actual process conditions with a steam-content in the gas of approximately 40 vol %. The dashed lines show deviations representing σ = 4 and 8 with the corresponding steam contents displayed in Table 2. For MnO the change of equilibrium is consistent as for rising temperature and rising steam content the equilibrium values for H2S content increase as can be seen in Figure 2. This is a consequence of the exergonic sulfidation reaction under formation of H2O (eq 10).

[ΔHR = + 206 kJ/mol] (9)

Simulation of the gas composition with data obtained from fuel analysis and a steam-excess-ratio of 4 resulted in a higher hydrogen and lower steam content compared to formerly obtained experimental data from gas analysis (Table 1). The authors explain this as being due to a nonequilibrium state in real process conditions which becomes obvious by the presence of methane that is not predicted by equilibrium calculations. Within the temperature range from 500 to 1000 °C, the simulated steam content in the gas changes about 16% according to the temperature dependent shifts in the equilibrium of gas phase reactions mainly eq 8. As the water−gas shift reaction is slightly exergonic its equilibrium is shifted to the reactant side with rise in temperature, therefore, the water content of the gas rises with increased temperature. The total steam content in the gas with respect to its dependence on the steam-excess-ratio is listed in Table 2 predicted for equilibrium conditions at a temperature of 820 °C. These results were determined using FactSage 6.4. As a

MnO + H 2S ↔ MnS + H 2O

(10)

At the intended in situ condition of 820 °C with a steam content equaling the values for σ = 6, no sorption is predicted by thermodynamics. Even for lower steam content as represented by the σ = 4 equilibrium-curve, the use of single metal sorbents based on manganese seems not to be a suitable option as the equilibrium values at 820 °C are generally high. As reported by Alonso and Palacios,12 the addition of 10 mol % ZnO can stabilize manganese sorbent as Mn3O4, its most affine oxidation state toward sulfidation. A comparison of equilibrium constants is given in studies conducted by Bakker et al.8 Equilibrium calculation for Mn3O4 under the exclusion of MnO formation predicts a desulfurization down to 42 ppm H2S 2528


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at 820 °C and a steam excess ratio of 4. To evaluate the possibility of stabilizing manganese as Mn3O4 under the intended process conditions the simulated oxygen partial pressure in the equilibrium gas composition with 40 vol % steam content is shown in Figure 3 superimposed to a property

even though the equilibrium of the competitive calcination reaction (eq 5) is still on the carbonate side at temperatures below 760 °C for the simulated gas composition. CaO + H 2S ↔ CaS + H 2O

(11)

The minimum of equilibrium curves is equal for the different steam contents and indicates the temperature of calcination at about 760 °C where all the CaCO3 is transformed to CaO at the specific gas composition (Table 2). With further rise in temperature, the behavior of CaO is comparable to MnO except for the higher equilibrium constant of CaO, which leads to lower H2S contents even at elevated temperature. The rise in the H2S content for the σ = 4 equilibrium-curve at temperatures below 580 °C occurs due to coke formation and thus a reduced molar gas fraction with a relatively increased H2S content. In accordance with research conducted by Stemmler et al.,1 the FactSage 6.4 simulation predicts the achievement of equilibrium values below 5 ppmv for residual H2S content in the syngas when using a stabilized BaO−sorbent in a CaO− BaO solid solution. In this mixed sorbent, BaO is the active phase responsible for the low equilibrium content of H2S in the gas phase. The CaO matrix only stabilizes the BaO against formation of BaCO3. Therefore, a prediction of the achievable values for residual H2S level is valid under consideration of BaO only. As shown in Figure 5 a variation of the steam content

Figure 3. Property diagram for manganese species in dependence of oxygen partial pressure and temperature. TBFBG indicates the intended temperature of application.

diagram of Mn species. Rising temperatures and decreasing oxygen partial pressures promote the reduction of MnO2 to MnO. At a temperature of 820 °C, a pO2 of 10−3 to 10−8 bar would be required to sustain Mn in Mn3O4 oxidation state. Clearly the equilibrium state of manganese is the reduced MnO-form in nonstabilized sorbents for the intended process conditions. The effect of ZnO addition on lattice stabilization against the prediction for single phase behavior needs experimental assessment. The equilibrium curves for CaO sorbent, displayed in Figure 4, show a minimum due to the calcination reaction (eq 5).

Figure 5. Deviation of H2S equilibrium for BaO in dependence of temperature for different steam-excess-ratios. TBFBG indicates the intended temperature of application.

from 29 to 48 vol % leads to an increase of the equilibrium values from 0.93 to 3.49 ppmv H2S for TBFBG, the intended temperature of operation. Simulated values for temperatures below the predicted temperature of calcination of CaO (Figure 4) have to be considered as theoretical only, as the transformation of the CaO matrix to CaCO3 prevents the sulfidation of BaO to BaS. This has been determined experimentally7 therefore an application of this sorbent with the low simulated values below calcination temperature cannot be possible under real process conditions. Figure 6 shows the simulated sorption equilibrium values for CuO sorbent whereby copper oxide is readily reduced to elemental copper at the considered process conditions, over the entire temperature range. As shown in Figure 6 the variation of the steam content of the syngas in the range between 29 and 48 vol % barely shows any influence on the predicted equilibria for H2S sorption. This is in clear contrast to the simulated results for other sorbent substances. The most pronounced deviation

Figure 4. Deviation of H2S equilibrium for CaO in dependence of temperature for different steam-excess-ratios. TBFBG indicates the intended temperature of application.

Until a temperature level of 660 °C, the calcium is present as carbonate. With the progress of conversion from CaCO3 to CaO with rising temperature, sorption of H2S takes place. Looking at the σ = 4 equilibrium-curve the equilibrium calculations predict the formation of CaS starting from 680 °C whereas for a higher water content corresponding to σ = 8 the onset of CaS formation starts at about 720 °C. The temperature of beginning H2S-sorption lowers as the difference in chemical potential of the desulfurization reaction (eq 11) increases with lower steam content. Therefore the overall minimization of Gibbs enthalpy yields in the formation of CaS 2529


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assessment with equilibrium calculations does not produce the same results for a steam-excess-ratio of 4 with especially the hydrogen content being higher in the simulated gas (Table 2). In order to validate the influence of differences within the simulated syngas and the produced syngas under real process conditions, a variation of the C/H ratio of the fuel is studied. The values are varied from C/H = 0.7, which equals the analysis data for wood pellet-fuel, to C/H = 1.9. The corresponding gas composition for 820 °C, the intended operation temperature, is shown in Figure 7.

Figure 6. Deviation of H2S equilibrium for CuO in dependence of temperature for different steam-excess-ratios. TBFBG indicates the intended temperature of application.

at 1000 °C is only at 5% between the value for highest and lowest steam content. This effect is explained by application of law of mass action on eqs 12, 13, and 14, which are shifted to the reactant side by a higher steam content in the gas.20 CuO + H 2 ↔ Cu + H 2O

(12)

2CuO + H 2 ↔ Cu 2O + H 2O

(13)

Cu 2O + H 2 ↔ 2Cu + H 2O

(14)

Figure 7. Differences in syngas composition with varied C/H ratio at a temperature of 820 °C.

The unconventional behavior of CuO sorbent with varied steam content is explained further by a rising O2-partial pressure and lower partial pressures of H2 and CO (Table 2) with rising steam content of the syngas. H2 as well as CO are strong reducing agents under formation of H2O (eqs 12−14) and CO2. At the intended process conditions and a temperature of 820 °C the O2-partial pressure is in the range of 10−17 bar under equilibrium conditions. It rises with temperature as exergonic oxygen consuming reactions are shifted toward the reactant side and with a lower ratio of C/O in the gas as is valid for a higher steam-excess-ratio. Therefore, the atmosphere for sulfidation reaction is less reducing at a higher steam content and the influence of CuO on sorption equilibrium is more pronounced. The simulated results for ZnO−sorbent show no unexpected behavior with variation of the steam content. The H2S equilibrium values for 820 °C range from 123 ppmv at a steam content of 29 vol % to 274 ppmv at a steam content of 47 vol %. As an expected continuous volatilization of the ZnO due to significant vapor pressure of elementary zinc is not considered in the simulation, simulated data for ZnO desulfurization at 820 °C is not regarded as a realistic preview for experimental results. Excluding ZnO because of volatilization, according to FactSage simulation CuO shows the best sorption properties for the investigated process conditions among the conventional single oxide sorbents. For the implementation of CuO-sorbent equilibrium calculations predict a residual H2S content of 555 ppmv. Still the predicted results for a stabilized BaO-sorbent promise far superior performance with equilibrium values of 2.1 ppmv H2S at a steam content of about 40 vol % and 820 °C Variation of C/H Content. In gasification experiments conducted by Kienberger et al.,6 a steam-excess-ratio of 4 yields in the gas composition shown in Table 1. Simulative

With increased C/H ratio the H2 content in the gas decreases. The changes in the gas composition are comparatively small due to equilibrium conditions which causes the replacement of H2 by water−gas shift reaction (eq 8). This forces the rise in CO2 content as the water-gas shift reaction is driven to the product side. For the variation of C/H ratio the steam-excess-ratio was set at 4 according to the real process conditions. The effects on H2S sorption for different sorbents are displayed in Figure 8 where the bold symbols represent a C/H-ratio of 0.7 as given by elemental analysis. Deviations thereof are displayed by the smaller dashed symbols.

Figure 8. Influence of varied C/H-ratio of the fuel on H2S sorption. TBFBG indicates the intended temperature of application. 2530


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Figure 8 shows different H2S contents for the variation of C/ H ratio. Due to the reduction of the hydrogen content of the fuel, the total amount of formed gas is reduced which results in a higher initial H2S content with increased C/H ratio. For reasons of lucidity, only the initial H2S values for C/H = 0.7 are displayed in Figure 8. As the steam-excess-ratio of the gas is calculated depending on the carbon content of the fuel the amount of steam is also relatively increased (Figure 7). These effects explain the deviation in manganese sorption with changed C/H ratio. As the sorption of H2S formally is a function of temperature depending equilibrium constant and steam content only, manganese sorbent displays a typical sorption behavior, as no oxidation state transition or other side reactions occur. For CaO the influence of a rising CO2 content in the gas leads to a raised calcination temperature and thus a higher minimum of sorption equilibrium. Above the calcination temperature the increased steam content with increased C/H ratio leads to slightly elevated equilibrium curves. Cu sorbent shows an opposite behavior as sorption equilibria are lowered at increased C/H ratio. This occurs due to a less reducing atmosphere resulting from a lower hydrogen content in the gas. Lowering the potential of reduction to elemental copper, the influence of the high equilibrium constant of CuO toward desulfurization is more pronounced. Therefore, the superimposed oxidation state transition counteracts the inhibiting effect of raised steam content in the gas. This effect is most distinct at high temperatures as the partial oxygen pressure rises at elevated temperatures. The variation in simulated results for equilibria of other sorbent material is insignificant. For BaO, the equilibrium values rise from 0.93 to 1.12 ppm H2S due to elevated steam content with increased C/H ratio. Influence of Pressure. The water−gas shift reaction (eq 8) as the main gas phase reaction is equimolar. As the formation of methane is suppressed, in the simulative approach, a pressure dependence is not expected concerning the composition of the gas phase. Considering the gasification itself as well as the reduction of sorbent in nonequimolar reactions, the rise in pressure leads to increased soot formation at lower temperatures. The influence on conversion of oxidation state of sorbents is negligible. This excludes the conversion of CaCO3 to CaO with the calcination reaction being strongly dependent on the CO2 partial pressure. Therefore, the pressure influence on CaO sorbent performance is relevant for possible process applications with elevated pressure. This is displayed in Figure 9 with the corresponding CO2 partial pressures. The values are simulated for a gas containing 39.6 vol % steam thus equaling a steam-excess-ratio of 6. The CO2 content is approximately 12 vol % (Table 2). In Figure 9 the displayed curve for 1 bar pressure equals the dotted line in Figure 4 for CaO sorption at a steam-excess-ratio of 6 as it represents the standard conditions. The calcination temperature marks the point where the equilibrium of eq 5 is shifted to the product side depending on CO2 partial pressure and temperature. At a process pressure of 3 bar the calcination temperature is raised to 820 °C at a CO2 partial pressure of 335 mbar. This also marks the intended operational temperature of gasification. At a pressure of 5 bar calcination temperature rises to 860 °C with a minimum equilibrium H2S content of 958 ppmv in the gas phase. The elevated calcination temperature limits the minimum residual H2S level in the gas phase. Even for 3 bar, where the equilibrium calcination temperature falls

Figure 9. Dotted lines show the change of sorption equilbria of CaO sorbent for different calcination temperatures. TBFBG indicates the intended temperature of application. The dashed lines for px,CO2 display the partial pressure of CO2 in dependence of pressure and temperature.

together with the operational temperature, a higher residual H2S content in the gas phase might occur compared to atmospheric pressure. Gas phase desulfurization reactions in the colder freeboard zone of the gasifier would be limited as calcination (eq 5) and sulfidation (eq 11) would be occurring at the same time thus inhibiting deeper desulfurization. Therefore, the raising of reactor pressure should be minimized if calcium based sorbents are implemented for desulfurization. For other sorbent substances no change in reaction equilibrium occurs with the change of the reactor pressure. Influence of Sulfur and Sorbent Content. As the EquilibTool within FactSage calculates the overall equilibrium of all the contained substances and the possible formation products, it is only the elemental ratio of the initial compounds determining the product composition. Therefore, the differentiated simulative investigation of sorption equilibria of diverse sulfurous compounds with the investigated sorbents is not possible as products will always be formed according to the overall minimization of Gibbs enthalpy. Thus, the variation of sulfurous input compound does not change the result of minimum Gibbs enthalpy calculations when the elemental ratio of overall input remains unchanged. The same is true for different sulfur contents in the reaction mixture where variations show little effect on the result of Gibbs enthalpy calculation as long as the sorbent is added in abundance and sulfur content is small compared to the permanent gas fractions in the syngas. In this case, the formation of H2O during sulfidation of the sorbent has a negligible influence on the gas composition. The equilibria do not shift with variation of the sulfur content of a simulated gasification fuel. Some of the sorbents simulated, especially CaO, contribute in the ash forming process and therefore influence the formation of volatile ash components such as KCl or NaOH. However, as the content of other ash forming constituents is low for simulation of wood gasification, exceeding the stoichiometry for desulfurization leads to deposition of sorbent in the solid fraction. Therefore, a variation of sorbent content only varies the solid to gas ratio in the product but the equilibria remain unchanged. 2531


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DISCUSSION Evaluating the simulated results, the influence of the steam content shows for MnO and CaO sorbent a behavior according to the law of mass action with H2O as the equilibrium determining component. Due to a superimposed transition of oxidation state, which is delayed by a higher steam content, the variation of the H2O fraction between 29 and 47 vol % does not significantly influence the H2S equilibrium value for CuO sorbent. This qualifies stabilized CuO sorbents for potential use under high steam content conditions. The variation of C/H ratio within the fuel does not show any significant change in sorption equilibria, whereas the postulated influence of CO2 content observed by Li et al.43 seems to be surface related and beyond the scope of equilibrium simulation. Considering the benchmark of intermediately high temperature ZnO desulfurization according to simulated results, no single metal sorbent seems to be suited for in situ application. Simulation for ZnO-H2S-equilibrium at the intended process conditions resulted in 198 ppmv as the equilibrium value. Though the volatilization of ZnO is not regarded in a simulative approach, therefore, if at all only stabilized ZnO sorbents as presented by Bu et al.15 could be employed for in situ desulfurization. Considering the higher cost of stabilized ZnO sorbent and increased equilibrium H2S levels with increased temperatures, the benefit compared to the employment of untreated ZnO at lower temperatures is doubtable. For MnO sorbent the calculated equilibrium values confirm a high influence of the steam content and result in values that do not justify the use of manganese sorbents for in situ application. Unless a stabilization in Mn3O4 oxidation state can be achieved or further promising results on the combination of manganese and rare earth sorbents under high steam content conditions are published, manganese seems not to be a suitable in situ sorbent for the investigated process conditions. For the in situ application of CaO-based sorbent at 820 °C a residual H2S content of 740 ppmv is predicted (Figure 4). This value is still high and does not justify its use as an in situ sorbent. Solely the simulated results for BaO sorbent confirm low equilibrium values for the investigated process conditions thus qualifying a combined CaO-BaO sorbent as highly promising material for in situ desulfurization (Figure 5). Comparing the simulated results with data from literature shows that there is a tendency for the measured values for residual H2S content found in literature to be below the thermodynamically predicted equilibria for single metal components. This can be mostly attributed to differences in process conditions concerning the steam content and the reaction temperature but also the subsequently described factors limit the accuracy of the predicted desulfurization results. Differences occur by stabilizing effects between different metal components, which prevent the reduction of metal oxides in the case of stable mixed phases. Examples thereof are the formation of Zn2TiO4 in zinc-titanate sorbents,15 CuCr2O4 in a combined CuO/Cr2O3 sorbent17,22 or the effect on lattice structure from the addition of 5% Zn to MnO, stabilizing the manganese as Mn3O4.38 As reported by Li et al.,43 combinations of metal sorbents might even enhance the positive properties of each metal component such as the capacity of manganese and the desulfurization potential of CeO beyond the simple combination thereof.

Another effect is the influence of traces of other elements such as those present in many Ca-based sorbents. The predicted equilibrium values are based only on the CaO-CaS equilibrium at the specific conditions not considering further interactions of the mineral composition or the content of impurities. This includes the neglection of the negative effect of sintering of a CaS layer in case of a pure limestone as well as unpredicted positive results for a slag lime as used by Stemmler et al.7 The adsorption of H2S on the particle surface seems to be another reason for lower experimental H2S values than predicted by simulated results. By measuring H2S sorption on manganese and CeO + CuO particles, Cheah et al.25 observed that by XRD measurements, in accordance with thermodynamic predictions for manganese sorbent, no crystalline phase of MnS was detectable. Yet a desulfurization effect could be measured which indicates surface sorption instead of sulfide formation. For the ceria sorbent the measured sulfur content after desulfurization was higher than the Cu-content of the sorbent, whereby thermodynamics would not predict any formation of Ce2O2S- phase under high steam conditions. Thus Cheah et al. concluded25 that for the prediction of the achievable desulfurization under surface determined conditions, thermodynamic modeling by minimization of Gibbs enthalpy in the bulk phase of the sorbent is not a sufficient tool anymore. Other influencing factors which need experimental assessment are sorbent specific properties such as catalytic activity toward the conversion of different sulfur compounds. These are not considered in Gibbs enthalpy minimization simulation. The latter can effectively determine the minimum achievable sulfur content in the cleaned gas. This accounts for the example of ZnO-sorbent at lower sorption temperatures where the residual sulfur content is relevantly determined by organic sulfur compounds such as thiophenes and COS in the gas.6 Experimental results also show a variation of kinetics with variation of the gas composition. Cheah et al.25 observed a major influence of GHSV with increasing steam content whereby necessary residence times increased with elevated steam content of the gas. It is especially true for in-bed applications with statistic residence time distribution due to sorbent attrition that such influence factors are important for dosage ratio and sorbent content but are clearly beyond the scope of equilibrium calculation.

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CONCLUSION The evaluation of thermodynamic calculations on H2S sorption at 820 °C and the conditions present in the BFBG unit at TU Graz indicate that an in situ, deep desulfurization cannot be implemented to a useful extent using single metal sorbents under equilibrium conditions. According to simulated results, the residual H2S content cannot be reduced to values below 400 ppmv under in situ conditions by use of single metal oxides. Even for a coarse desulfurization depending on sorbent cost and other positive side effects a benchmark of about 50 ppmv H2S should be reached. For a solid solution sorbent consisting of BaO and CaO, equilibrium, calculations predict a desulfurization of syngas to a sulfur level of 2.1 ppmv H2S at a steam content of about 40 vol % and 820 °C. This is sufficient for further catalytic gas processing applications. Contradictory to the unsatisfactory simulated results for single metal sorbents, experimental results of other researchers show that also for other sorbent substances the combination of different sorbent and support materials can lead to positive 2532


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(9) Zheng, L.; Furinsky, E. Comparison of Shell, Texaco, BGL, and KRW gasifiers as part of IGCC plant computer simulations. Energy Convers. Manage. 2005, 46 (11−12), 1767−1779. (10) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Mahfoud, R. B.; Melancon, J.; Pelton, A. D.; Petersen, S. FactSage thermochemical software and databases. Calphad 2002, 2 (26), 189− 228. (11) Bale, C.; Bélisle, E.; Chartrand, P.; Decterov, S.; Eriksson, G.; Hack, K.; Jung, I.-H.; Kang, Y.-B.; Melançon, J.; Pelton, A.; Robelin, C.; Petersen, S. FactSage thermochemical software and databases Recent developments. Calphad 2009, 33 (2), 295−311. (12) Meng, X.; de Jong, W.; Pal, R.; Verkooijen, A. H.M. In bed and downstream hot gas desulphurization during solid fuel gasification: A review. Fuel Process. Technol. 2010, 91 (8), 964−981. (13) Torres, W.; Pansare, S. S.; Goodwin, J. G. Hot gas removal of tars, ammonia, and hydrogen sulfide from biomass gasification gas. Catal. Rev. 2007, 49 (4), 407−456. (14) Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos, M. Hightemperature H2S removal from fuel gases by regenerable zinc oxidetitanium dioxide sorbents. Ind. Eng. Chem. Res. 1989, 28, 535−541. (15) Bu, X.; Ying, Y.; Zhang, C.; Peng, W. Research improvement in Zn-based sorbent for hot gas desulfurization. Powder Technol. 2008, 180 (1−2), 253−258. (16) Mojtahedi, W.; Abbasian, J. H2S removal from coal gas at elevated temperature and pressure in fluidized bed with zinc titanate sorbents. 1. Cyclic tests. Energy Fuels 1995, 9 (3), 429−434. (17) Abbasian, J.; Slimane, R. B. A regenerable copper-based sorbent for H2S removal from coal gases. Ind. Eng. Chem. Res. 1998, 37, 2775− 2782. (18) Jalan, V. Studies Involving High-Temperature Desulfurization/ Regeneration Reactions of Metal Oxides for Fuel Cell Development;Giner Inc.: Waltham, MA, Oct. 1983. (19) Sick, G.; Schwerdtfeger, K. Hot desulfurization of coal gas with copper. Metall. Mater. Trans. B 1987, 18 (3), 603−609. (20) Kyotani, T. K. H.; Tomita, A.; Palmer, A.; Furimsky, E. Removal of H2S from hot gas in the presence of Cu-containing sorbents. Fuel 1989, 68, 74−79. (21) Patrick, V.; Gavalas, G. R.; Flytzani-Stephanopoulos, M.; Jothimurugesan, K. High-temperature sulfidation-regeneration of CuO−Al2O3 sorbents. Ind. Eng. Chem. Process Des. Dev. 1989, 28, 931−940. (22) Li, Z.; Flytzani-Stephanopoulos, M. Cu−Cr−O and Cu−Ce−O regenerable oxide sorbents for hot gas desulfurization. Ind. Eng. Chem. Res. 1997, 1 (36), 187−196. (23) Yasyerli, S.; Dogu, G.; Ar, I.; Dogu, T. Activities of copper oxide and Cu−V and Cu−Mo mixed oxides for H2S removal in the presence and absence of hydrogen and predictions of a deactivation model. Ind. Eng. Chem. Res. 2001, 40 (23), 5206−5214. (24) Gasper-Galvin, L. D.; Atimay, A. T.; Gupta, R. P. Zeolitesupported metal oxide sorbents for hot-gas desulfurization. Ind. Eng. Chem. Res. 1998, 37, 4157−4166. (25) Cheah, S.; Parent, Y. O.; Jablonski, W. S.; Vinzant, T.; Olstad, J. L. Manganese and ceria sorbents for high temperature sulfur removal from biomass-derived syngasThe impact of steam on capacity and sorption mode. Fuel 2012, 97, 612−620. (26) Desai, M.; Brown, F.; Chamberland, B.; Jalan, V. Copper-based sorbents for hot gas cleanup. Prepr. Pap. - Am. Chem. Soc., Div. Fuel Chem. 1990, 35 (No. 1), 87−94. (27) Kobayashi, M.; Flytzani-Stephanopoulos, M. Reduction and sulfidation kinetics of cerium oxide and Cu-modified cerium oxide. Ind. Eng. Chem. Res. 2002, 41 (13), 3115−3123. (28) Proell, T.; Rauch, R.; Aichernig, C.; Hofbauer, H. Fluidized bed steam gasification of solid biomassPerformance characteristics of an 8 MWth combined heat and power plant. Int. J. Chem. React. Eng. 2007, 1 (5), 1−21. (29) Stemmler, M.; Müller, M. Chemical hot gas cleaning concept for the “CHRISGAS” process. Biomass Bioenergy 2011, 35, S105−S115.

synergies that yield in residual H2S contents of about at 10 ppm at conditions comparable to the ones in the BFBG at TU Graz. This accounts for ZnTiO4 mixed with traces of Mn and Cu,15 CuO-sorbent on Cr2O317,22 and Mn3O4 sorbent with addition of ZnO.38 The use of copper based sorbent material supported on Cr2O3 turns out to be promising due to a predicted positive impact of high steam conditions on sorption equilibria. As many of the influencing factors are not exactly predicted by equilibrium calculations out of the reviewed sorbents for high temperature desulfurization, some mixed metal combinations and especially the solid solution sorbent consisting of BaO and CaO, bare a high potential to be suited for in situ application. Real process conditions with nonequilibrium atmosphere will have to show the limits and performance of these sorbents and then be evaluated according to the overall benefit compared to midtemperature ZnO desulfurization. Establishing an initial coarse desulfurization with reduced sorbent related process costs is an important precondition for the use of cheap fuel sources for gasification processes. This is a major part in bridging the conflict between high value fuels with low content of impurities and economically feasible fuels that raise the expenditure for gas cleaning. Future experiments within the BFBG at TU Graz will help to evaluate the feasibility of in situ desulfurization as process improvement.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +43 316 873 7805. Fax: +43 316 873 7305. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Austrian Research Promotion Agency (FFG) for the funding of the project.

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REFERENCES

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Evaluation of Sorbents for High Temperature In Situ Desulfurization of

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