Regenerable Sorbents for High-Temperature Desulfurization of Syngas from Biomass Gasification

Single-metal high-temperature solid sorbents for syngas cleaning using Mn, Ca, Fe, Cu, or Mo supported on g -Al 2 O 3 were synthesized, characterized, and tested in a fixed-bed reactor. H 2 S and SO 2 concentrations in the gas after treatment at T = 400 to 700 (cid:2) C were compared with thermodynamic calculations. The Mn-based sorbent showed the best ability to achieve a low sulfur residual in the gas, especially at temperatures above 600 (cid:2) C. Sorbents with Fe, Cu, and Mo gave SO 2 formation in the initial phase, but this could be avoided by a pre-reduction treatment of the sorbent material.


Introduction
Biomass is a sustainable and renewable source of energy. There are several pathways to convert biomass to more valuable energy forms, including thermochemical methods, biochemical methods, or physical conversion methods [1]. Among these conversion methods, biomass gasification has received considerable attention recently due to its higher overall flexibility and efficiency as well as the mature technologies available for further conversion of the produced syngas [2]. The syngas from biomass gasification is an important energy source and raw material for chemicals synthesis. However, the raw product gas from biomass gasification usually contains several contaminants, including particulate matters, tar, sulfur compounds, nitrogen compounds, etc., which are detrimental to downstream equipment and especially catalysts [3]. Sulfur, which is mainly present in the raw syngas in forms of hydrogen sulfide (H 2 S) and sulfur carbonyl (COS), is regarded as the key contaminant, because it is heavily corrosive, toxic to the catalysts, and polluting to the environment [4,5]. The tolerance of transition metal-catalyzed processes towards sulfur compounds is generally extremely low, hence, detrimental effects of sulfur have been intensively studied. For the cobalt-based Fischer-Tropsch process, the sulfur concentration in the feed gas needs to be lower than 0.1 ppm [6]. It is also reported that the sulfur content should preferably be lower than 0.1 ppm to protect the Cu catalysts involved in the methanol synthesis process [7]. Therefore, sulfur must be thoroughly removed from the syngas prior to being fed into downstream processes. Conventionally, low-tempera-ture sulfur removal methods, which often involve the absorption in solvents, are applied to chemically or physically capture sulfur compounds together with some other impurities [8]. However, the operating temperatures of these methods are usually lower than 100°C, which induces a significant investment in heat exchange equipment and energy loss for the raw biogas coming from a gasifier in the temperature range of 800 to 900°C [8,9], especially when the process layout requires reheating of the gas for further conditioning or processing. Therefore, high-temperature desulfurization with metal oxide-based solid sorbents has received considerable interest because it can efficiently prevent energy loss and reduce investments [10].
High-temperature desulfurization is developed based on the work by Westmoreland and Harrison [11]. The authors conducted a thermodynamic calculation to evaluate the feasibility of metal candidates for high-temperature sulfur removal. They proposed eleven transition metals, i.e., zinc, iron, manganese, molybdenum, vanadium, calcium, strontium, barium, cobalt, copper, and tungsten, as candidates for high-temperature solid sorbents (HTSS). The reactions involved in high-temperature desulfurization with metal oxides are listed as [5]: where M stands for metals. Metal oxides can extract H 2 S from the gaseous phase through the reaction in Eq. (1) and produce metal sulfide in the solid phase. The produced sulfide can later be regenerated back to oxide states through the reactions involved in Eqs.
(2)-(4), depending on the selected regeneration agent. All reactions can occur at medium to higher temperatures, i.e., 400 to 900°C [4]. Among all the metals proposed by Westmoreland et al. [11], the Ca-, Zn-, Fe-, Cu-, and Mn-based solid sorbents were found to be promising candidates for the desulfurization process [4,5]. Many efforts have been involved to improve the stability and performance of these solid sorbents, including using different support materials, addition of different promoters, and applying various synthesis methods. Accordingly, sorbents based on these metals show excellent sulfur capture capacity and stability. However, there has been less focus on the residual sulfur levels in the syngas after the desulfurization process, which is crucial for syngas applications. Thermodynamic calculations suggested that copper, zinc, and manganese oxides are favorable for H 2 S removal at temperatures lower than 650°C and can reduce the H 2 S concentration to sub-ppm level [12]. However, the reported experimental data for the residual H 2 S concentration were divergent, varying from 1 ppm [13][14][15] up to several hundred ppm [16]. Furthermore, some measurements were restricted by the instrument limitations so that the sulfur residual could not be precisely detected. Therefore, further studies are necessary to investigate the sulfur residue level of the relevant metal oxides and, thereby, their suitability as desulfurization sorbents.
In our previous work, we developed a method to investigate the capacity and stability of sorbent materials and have investigated supported Mn-based sorbents [17]. Furthermore, we have developed and studied a Mo-promoted material with improved properties [18]. Recently, we have introduced a sensitive sulfur analyzer to monitor residual H 2 S and SO 2 in the off-gas from the desulfurization reactor at very low concentrations [19]. By measuring the effluent concentrations of H 2 S and SO 2 from the sorbent bed, we have also examined the role of the oxide material in forming SO 2 during the initial phases of the sorption cycle, and how prereduction of the material can prevent this from occurring.
In this work, we have investigated five different solid sorbents for HTSS (Mn, Ca, Fe, Cu, and Mo metal oxides, all supported on g-alumina) and compared their properties and ability to reduce the gas-phase sulfur residue level down to the necessary gas purity over a range of relevant temperatures, i.e., 400-700°C. In addition, we have compared the experimental results with theoretical values from thermodynamic calculations and the formation of SO 2 over calcined sorbent samples.

Chemicals and Materials
Gamma-alumina purchased from Strem Chemicals was used as a support material in the study.

Sample Preparation
The desulfurization sorbents used here consist of metal oxides as active phase, supported on g-alumina as carrier. The support was pre-calcined with air for 10 h at 500°C and then impregnated with the precursor to synthesize the corresponding metal-based sorbent. After being aged overnight and dried for 24 h under 90°C, the samples were calcined in air for 5 h in a muffle oven at 700°C. The calcined samples were crushed and sieved to 150 and 250 μm. The nominal loading of the metal is 15 wt % for all single-metal sorbents and named as 15M, where M represents the metal applied.

Characterization of Sorbents
The specific surface area of the sorbents according to the BET isotherm as well as the pore diameter and pore volumes were measured with N 2 adsorption at 77 K using a Micromeritics TriStar 3020 instrument. Further details can be obtained in our previous work [17]. We also investigated the crystallinity states of all samples with X-ray powder diffraction on a Bruker AXS D8 Focus diffractometer using CuKa radiation (l = 1.54 Å). The crystallite size of the active phase was calculated by the Scherrer equation from selected reflections with the shape factor, K, applied as 0.9. The temperature-programmed reduction (TPR) measurements were conducted with a laboratory setup. About 100 mg sample were dried at 130°C for 30 min under inert gas flow and then cooled down to 100°C, followed by heating up to 750°C with a ramp rate of 5°C min -1 under a gas mixture consisting of 7 vol % H 2 in Ar with a gas flow of 50 mL min -1 [17].

Residual Sulfur Concentration Tests
The sorbents, 15Fe, 15Ca, 15Cu, and 15Mo, were first tested for their breakthrough capacities as done previously for the Mn-based sorbent [17,18]. The tests were performed with a laboratory setup consisting of a gas feeding and metering system, a quartz fixed-bed reactor heated by a furnace, and an analytical section using a quadrupole mass spectrometer (ThermoStar GSD 320 T1 C, MS) to monitor the gas composition. More details can be found in our previous work [17]. 0.2 g of a sorbent was loaded into the reactor and heated up to 600°C under inert gas (N 2 ) with a flow rate of 50 mL min -1 . Then, the gas mixture (100 mL min -1 ), containing 40 vol % H 2 , 20 vol % N 2 , 39.6 vol % Ar, and 0.4 vol % (4000ppm) H 2 S, was introduced into the bypass line for 25 min to stabilize the signal in the MS. The sorption was initiated by switching the gas mixture from the bypass line to the reactor line and the composition change of the desulfurized gas exiting the reactor was detected by the MS. The whole desulfurization process was monitored quantitatively based on a regular calibration of the MS signals. The desulfurization capacity of the sorbent before the breakthrough was calculated by the following formula: where Q is the flow rate of the model gas, i.e., 100 mL min -1 ; C in and C out represent the inlet and outlet concentrations of H 2 S (ppm), respectively; V m is the molar volume of gas at 1 atm and 20°C (24.04 L mol -1 ), m s is the mass of the loaded solid sorbent, and M H2S is the molecular weight of H 2 S. For analyzing the residual sulfur concentration, a specific sulfur analyzer was utilized, Thermo Fisher 450i. This analyzer is able to measure the H 2 S and SO 2 concentrations in the gaseous phase online with a typical time resolution of 1 min. The upper concentration limitation is 10 ppm for H 2 S and 100 ppm for SO 2 , which is quite low, and hence, it is critical to know the time passing before the breakthrough, in order to adapt the testing time and protect the analyzer. 0.2 g sorbent was loaded in the reactor and heated up to the desired temperature, i.e., 400, 500, 600, or 700°C, under inert gas (N 2 ) with a flow rate of 50 mL min -1 . The sorbent 15Mn went through a slightly different heating process, which was the same as that used for the desulfurization capacity and stability test, i.e., heating to 600°C under 50 mL min -1 50 vol % H 2 /N 2 gas flow and keeping for 1 h, then changing the temperature to the desired sorption temperature. Then, the model gas was introduced into the reactor to start the sulfur residue test. A lower feeding H 2 S concentration (compared to the capacity measurements) was applied to prolong the time before the breakthrough and ensure the collection of enough data points. A gas mixture (100 mL min -1 ) of 40 vol % H 2 , 20 vol % N 2 , 19.8 vol % Ar, and 0.2 vol % (2000 ppm) H 2 S was fed to the reactor with the solid sorbent, and the off-gas was analyzed by the sulfur analyzer. To avoid exposing the instrument to sulfur concentrations above the upper limit, the measurement was terminated once a sudden increase of the sulfur concentration was observed or when the experiment had lasted 40 min. With this setup, the sulfur content can be measured precisely down to approximately 100 ppb. Depending on the capacity of the sorbent used, the measurements usually lasted for 15-40 min. The gas was diluted with synthetic air by a volume factor of 5 before entering the analyzer, and the concentrations of SO 2 or H 2 S are calculated by the following equation: where RC s is the concentration in the off-gas and RC D is the observed value from the analyzer. The lowest attainable sulfur levels were obtained by establishing steady state in the sulfur residue state and calculating the average values. The steady state is defined according to the H 2 S concentration curves and defined as 10 continuous minutes with the lowest average H 2 S and SO 2 concentrations.

Thermodynamic Calculations
The theoretical residual sulfur concentration in the gas phase at equilibrium was also calculated for the tested metal oxides using FactSage. Westmoreland et al. [20] proposed that the maximum temperature applied for zinc oxide should be below 600°C, due to the vaporization issue. The thermodynamic calculation results also suggest a high content of Zn vapor when the temperature is higher than 600°C. Normally, iron or titanium is added as a promoter to inhibit zinc vaporization by forming zinc ferrite or zinc titanate [13,21]. In this study, the desulfurization performance of the single metalbased sorbents is compared at temperatures in the range of 400 to 700°C. Hence, zinc oxide was excluded from the experimental study.

N 2 Adsorption Measurement
The textural properties of all fresh sorbents are shown in Tab. 1. The surface area of the alumina support was measured to be 169 m 2 g -1 with an average pore width of 12.7 nm and a pore volume of 0.61 cm 3 g -1 . The surface areas of the samples decreased to different degrees, however, most of them are similar, in the range from 121 to 140 m 2 g -1 . The 15Ca and 15Mo solid sorbents had even lower surface areas, 72 and 104 m 2 g -1 , respectively. These two samples also had slightly larger average pore widths (18.0-18.3 nm), compared to 12.8-14.1 nm for the other samples. The pore volume was similar for all samples, in the range of 0.44-0.66 cm 3 g -1 .

X-ray Diffraction Measurement
The XRD patterns of all fresh (calcined) samples are shown in Fig. 2    the sharp peaks at 2q values of 21, 22.5, and 25.5°are evidence of high crystallinity of the binary phase. However, aluminum molybdate (Al 2 (MoO 4 ) 3 ) can be regarded as a combination of Al 2 O 3 and MoO 3 . In this study, more attention will be paid to the phenomena related to the chemical composition of the sorbent not the bonding relationship between the active phase and the support. Hence, in the latter part, the active phase of 15Mo will be considered to be MoO 3 .
The crystallite sizes were obtained by using the Scherrer equation. The calculated crystallite sizes of all species on the sorbents and the selected positions of the corresponding characteristic peaks are listed in Tab. 2. The particle sizes of Mn 2 O 3 , Fe 2 O 3 , and CuO are between 20 and 30 nm. The size of CaO is larger (38 nm), and the Al 2 (MoO 4 ) 3 particle is the largest (64 nm).

Temperature-Programmed Reduction
The reducibility of the samples was investigated by temperature-programmed reduction (TPR) in the temperature range of 100 to 750°C using diluted hydrogen (7 vol % in Ar). The resulting TPR profiles are shown in Fig. 3. The TPR profile for the 15Ca-Al 2 O 3 sorbent (curve A) illustrates very low reducibility. Two very low-intensity reduction peaks at 500 and 600°C are most probably due to impurities in the Ca precursor or in the support material since the CaO is not expected to be reduced at these conditions. The XRD patterns in Fig. 2 (cure A) revealed that the dominant Mn phase in the sorbent 15Mn is Mn 2 O 3 , and therefore, a stepwise reduction is expected: Mn 2 O 3 Mn 3 O 4 MnO [22]. The TPR profile of 15Mn (curve B) confirms this, as two peaks were observed, one at approximately 336°C, corresponding to the reduction of Mn 2 O 3 to Mn 3 O 4 , and the other peak at 420°C, corresponding to the reduction of Mn 3 O 4 to MnO. The sharp peak at 255°C in the TPR profile of 15Cu (curve C) indicates that CuO is much easier to reduce and that metallic Cu is formed. Although these results have not been quantified, the sample size and scale of the profiles are comparable, confirming the high reducibility of supported Cu. The observed profile is in good agreement with other studies [23,24]. It has also been reported that the reduction behavior of Cu-based catalysts may be different at higher Cu loadings (higher than 10 wt %) with high dispersion, and TPR profiles with two reduction peaks have been reported [25]. The single peak observed here could indicate a rather poor dispersion of Cu on the support. The TPR profile for the 15Mo sorbent is rather more complex, with two broad peaks at 430 and 571°C and a small shoulder at approx. 540°C (curve D). This indicates the range of oxidation states possible in this metal oxide [26,27]. Combining the TPR results with XRD, it can be proposed that the main Mo-containing compound on the sorbent is Al 2 (MoO 4 ) 3 , representing Mo 6+ . The two main reduction peaks in the TPR curve suggest that the reduction happens through two steps, i.e., Mo 6+ Mo 5+ Mo 4+ , possibly with some further reduction to Mo 2+ . Hence, it can be deduced that Mo is present as MoO 2 following a reduction in the sorption experiment (600°C in H 2 ). The 15Fe sample (curve E) exhibits one main broad reduction peak starting at 230°C and peaking at 438°C, probably linked with the reduction of Fe 2 O 3 to Fe 3 O 4 . There are also broad and weak peaks at higher temperatures, around 550 and 650°C, indicating that further reduction to Fe(II) and metallic Fe is possible but incomplete also in the TPR experiment.

Evaluation of Desulfurization Capacities of the Sorbents
Desulfurization performance tests were carried out on all sorbents to evaluate the breakthrough capacities and also to predict when the breakthrough will happen in order to protect the analytical sulfur analyzer. All materials behave in a similar fashion: first, the H 2 S signal in the mass spectrometer declines sharply from the highest level (input gas level) during the stabilization step to the lowest level and then stabilizes for some time, depending on the sorbent's capacity. The capacities of these samples are listed in Tab. 2. It is worth mentioning that the sorption processes also produced more water, which was detected but not quantified in the MS for sorbents 15Mo, 15Fe, and 15Cu. The amount of  water initially increased and was stable for several minutes (approx. 4 min for 15Fe and 15Cu, 8 min for 15Mo sorbent), and subsequently decreased to a lower, stable level. This confirms the reducibility of the active phases of these sorbents, as also observed in the TPR experiments. The capacities of these single metal-based sorbents at 600°C at these conditions are reported in Tab. 3 and decline in the following order: 15Mn > 15Fe > 15Mo > 15Cu > 15Ca.

Residual Sulfur Concentrations on Mn-based Sorbent
The 15Mn sorbent was pre-reduced as described in Sect. 2.4, and as discussed in the TPR section, it is assumed that Mn 2 O 3 was reduced to MnO during pre-reduction. The measured residual H 2 S concentration for the 15Mn solid sorbent at different temperatures are presented in Fig. 4a. There are some fluctuations at the beginning of the measurements, but after a few minutes, the detected levels are low and relatively stable. The H 2 S residual concentrations at the steady state measured at 400, 500, and 600°C were similar (0.16 to 0.22 ppm) and lower than the observed residual level at 700°C (0.92 ppm) (Fig. 4c). The breakthrough occurred after 27 min at 400°C, and after 37 min at 500°C, respectively, at higher temperatures, the experiment was stopped before the breakthrough occurred. The duration before the breakthrough reflects the capacity of the sorbent, and these findings are in agreement with previous studies indicating that the capacity of the Mnbased sorbents increases with increasing temperature [28][29][30]. As mentioned before, there are also traces of SO 2 in the gas phase. Over 15Mn, the concentration is very low, during the stable phase of the cycle, the concentration stayed below 0.08 ppm. We proposed two mechanisms for SO 2 formation during the high-temperature desulfurization process in our previous study [19], one is through sulfate decomposition and the other is through H 2 S oxidation by metal oxides with higher oxidation states. A very low SO 2 concentration from the pre-reduced 15Mn solid sorbent, especially at the beginning of the sorption, confirms that in this case the second mechanism is important. However, it is notable that the overall measured SO 2 residual concentrations at all temperatures are higher than their corresponding theoretical calculated values (Fig. 1b). This might be due to the reaction between H 2 S and traces of Mn 2 O 3 due to insufficient reduction. The average effluent SO 2 levels were in the range of 0.01-0.06 ppm for all temperatures (Fig. 4c).

Residual Sulfur Concentrations on Ca-based Sorbent
The Ca-based solid sorbent had the shortest pre-breakthrough time of all tested samples, which corresponds to the lowest capacity (Tab. 1), especially at 400°C where only four valid data points were captured (Fig. 5a). Similar to 15Mn, the capacities of 15Ca increased with increasing temperature, but in addition, the profiles were less stable. The measured residual H 2 S at 700°C was in the range of 2.5 to 5.4 ppm, higher than at lower temperatures, here, the concentration exhibits a clear maximum after 8 min and a minimum after 16 min. The H 2 S level at 600°C was lower than that at 500°C, which is again unexpected in light of the   thermodynamic relations (Fig. 1a). SO 2 formation was very low over 15Ca. This is in agreement with the TPR of this sample, showing a very limited reducibility (Fig. 3).
The average minimum values shown in Fig. 4c are very low, but it is worth taking into account that the capacity is low and that it was difficult to obtain a steady-state situation where good measurements could be performed. Calciumbased sorbents have been reported to be promising for high-temperature desulfurization, for either in situ or downstream use [5]. However, the most applied Ca-based materials are minerals like dolomite or limestone, which are unsupported and, thus, contain more active material per total mass unit [30][31][32][33].

Residual Sulfur Concentrations on Fe-based Sorbent
The measured residual H 2 S concentration results for the Fe-based sorbent are presented in Fig. 6a. Similar to the Mn-based sorbent, there are some fluctuations at the beginning of the measurements, which were stabilized after 10 min. The H 2 S level was higher at 700°C than at lower temperatures during the stable period after 10 min. Significant SO 2 formation was observed at the beginning of the sorptions at all temperatures, but this decreased rapidly to stable low levels after around 5 min. The main compound on the support of the sorbent 15Fe, according to the XRD patterns, was Fe 2 O 3 , which can be reduced to FeO under the reducing environment [16].
The lowest attainable values of residual H 2 S and SO 2 of all temperatures are given in Fig. 6c. The stable concentrations of SO 2 decrease with increasing temperatures in the range of 400-600°C, but the peak level during the initial perturbation is higher for higher temperatures, indicating a limited amount of reducible oxide and possibly a kinetic influence on the shape of the curve. This is confirmed by the experiment shown in Fig. 6d,    (pre-reduced at 600°C under a gas flow of 40 vol % H 2 in N 2 for 10 min), underwent a sulfur residue test at 600°C. Following reduction, virtually no SO 2 was detected, indicating the role of Fe 2 O 3 in SO 2 formation. However, the stable value for residual H 2 S was 0.62 ppm, which is higher than in the case without prereduction.

Residual Sulfur Concentrations on Cu-based Sorbent
The behavior of the 15Cu sorbent is shown in Fig. 7. The period before the breakthrough for the 15Cu sorbent is much shorter compared to the Mn-and Fe-based sorbents, under all tested temperatures. This might be due to the reduction of copper oxide to elementary copper at higher temperatures, combined with sintering of the active phase [34].
After the initial peak, the H 2 S curves reached steady state after around 10 min (Fig. 7a), and the expected temperature response on the stable level was observed. The stable low level was below 1 ppm up to 700°C (Fig. 7c). SO 2 formation was apparent at the beginning of the measurement, which demonstrates the conversion of H 2 S to SO 2 by copper oxides. Theoretical calculations confirm that copper oxides are more reducible than the other oxides studied here, and thus, have a stronger ability to convert H 2 S to SO 2 . The average SO 2 residue levels under Cu-based solid sorbents are shown in Fig. 7c, passing through a maximum at 600°C. Fig. 7d shows the sorption experiment at 600°C following the same pre-reduction protocol as described for 15Fe. Prereduction virtually eliminated SO 2 formation, but also led to a slightly higher H 2 S level at steady state (0.60 ppm vs 0.34 ppm). The reducibility of the Cu-based sorbent is considered less ideal for this application concerning sulfur capture capacity [5], and studies have been conducted to avoid CuO reduction by adding promoters [35,36] or using suitable support materials [37,38] to increase their efficiency. Reduction by H 2 S can be considered to occur according to Eqs. (8) and (9) [34], and metallic Cu adsorbs H 2 S according to Eq. (10) [16].

Residual Sulfur Concentrations on Mo-based Sorbent
The results of residual sulfur tests on sorbent 15Mo are shown in Fig. 8. The period before breakthrough at 400°C was shorter than at the other temperatures, however, with a very low H 2 S residual level. The H 2 S residual levels increased with increasing temperature, staying below 1 ppm up to 600°C (Fig. 8a). SO 2 formation was only detectable at temperatures of 500°C and above (Fig. 8b), and the values were very low at steady state. The absence of SO 2 formation at the lowest temperature is surprising since the material was partly reducible in TPR at temperatures below 400°C. This could indicate that the proposed Al-Mo compound (Al 2 (MoO 4 ) 3 ) is less reactive towards H 2 S. The H 2 S average residual values at steady state increased with increasing temperature but were below 1 ppm up to 600°C (Fig. 8c). The maximum SO 2 concentration (both in terms of peak value and steady-state level) was observed at 500°C, confirming a more complex behavior of this material. Also, for 15Mo pre-reduction of the material at 600°C eliminated SO 2 formation (Fig. 8d). But in this case, the H 2 S residual was also lower for the reduced sample.

Summary and Discussion
To summarize and compare the lowest attainable residual H 2 S and SO 2 concentrations with the theoretical values, all single-metal sorbents exhibited lower residual H 2 S levels at all tested temperatures comparing with corresponding values from thermodynamic calculations. This is probably because the thermodynamic calculations are done for a batch situation with a set number of moles of the species and cannot reflect the experimental system where the fixed bed provides several cleaning stages from the entry to the exit of the bed. The H 2 S levels at 600°C decreased in the order 15Mo > 15Ca > 15Cu > 15Fe > 15Mn, i.e., they do not follow the order shown in Fig. 1a. This could be due to slightly different ratios (calculations were done for 1 mol of metal oxide, experimentally the weight-loadings were the same), and in some cases, the sorption stoichiometry is different. The sorbents also have different physical properties, such as specific area and porosity, and the active phases have different particle sizes, all of which may kinetically influence the sorption reaction. Hence, deviations from the predicted values are not surprising. The formation of SO 2 is a practical issue that will influence the design and operation of a cleaning stage based on these sorbents. But this phenomenon also provides some insight into the behavior of the material during early stages of the sorption process. It can be noted that for the sorbents 15Fe, 15Cu, and 15Mo, pre-reduction suppressed the initial SO 2 formation, demonstrating the importance of the reducibility of the metal oxide and providing a practical solution to the issue of SO 2 formation if the sorbents are applied in an industrial process. But the effect of the pre-reduction on the stable residual level (the ability of the material to provide a sufficiently clean gas) should also be noted, the effect was negative on the sorbents 15Fe and 15Cu, while it was positive on 15Mo.

Conclusion
In this study, five single-metal sorbents, namely, 15Mn, 15Ca, 15Fe, 15Cu, and 15Mo, all supported on gamma-alumina were prepared and the lowest residual gas-phase sulfur concentrations (H 2 S and SO 2 ) in a model syngas after being exposed to single-metal sorbents in a fixed-bed, continuous reactor at different temperatures, were recorded and compared to thermodynamical calculations. In terms of the residual sulfur level during steady state, Fe-, Mn-and Cu-based sorbents showed better performance than 15Ca and 15Mo, and H 2 S residual concentration was found to be below 1 ppm over the whole temperature range of 400 to 700°C at the investigated conditions. The Ca-and Mo-based sorbent can also eliminate H 2 S to lower than 1 ppm, but only at temperatures up to 600°C. The formation of SO 2 during the initial stages of sorption was also investigated. When it comes to SO 2 formation, this is strongly linked with the reducibility of the sorbents and is not an important issue for Ca-based sorbents. For the other sorbents, a pre-reduction step significantly reduced or eliminated this issue, confirming that the main route is through oxidation of sulfur by the metal oxides and simultaneous reduction of the metal oxide to a lower oxidation state.