Novel sour water gas shift catalyst (SWGS) for lean steam to gas ratio applications

Fuel Processing Technology 134 (2015) 65–72

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Novel sour water gas shift catalyst (SWGS) for lean steam to gas ratio applications Bonan Liu a, Qiuyun Zong b,⁎, Xian Du a, Zhaoxi Zhang a, Tiancun Xiao a,⁎, Hamid AlMegren c a b c

Inorganic Chemistry Laboratory, Oxford University, South Parks Road, OX1 3QR, UK Qindao Lianxin Chemical, Qingdao, Shandong Province, PR China Petrochemical Research Institute, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 14 July 2014 Received in revised form 19 December 2014 Accepted 13 January 2015 Available online 17 February 2015 Keywords: Sour water gas shift Catalyst Lean H2O/CO ratio Syngas Industrial application Coal gasification

a b s t r a c t Dry powder coal gasification is emerging as one of the most energy efficient methods for coal conversion. However, the low steam content, high temperature and high content of CO in the raw syngas make it difficult for a conventional sour water gas shift catalyst to be directly used for syngas conditioning. Conventional sour water gas shift takes place at very high steam to carbon monoxide ratio (H2O/CO), often 2 or above, but the H2O/CO ratio from a dry powder coal gasifier is often less than 0.8. To develop a sour water gas shift catalyst suitable for the lean steam raw syngas, we have prepared a series of MgAl2O4 spinel modified alumina supported CoMoOx catalysts by changing the content of K2O in the promoter, and tested them under lean steam to carbon monoxide (H2O/CO) ratio conditions for sour water gas shift process. Our results show that the addition of potassium into the catalyst increases the catalyst water gas shift activity at a lean steam to gas ratio, and that catalyst activity increases with the K2O content increase in 0–10 wt.% range; the potassium additive helps to increase the dispersion of MoO3 and improves catalyst strength and surface area. The increase of K2O content leads to higher catalyst activity for the CO shift reaction with little methane yield, reducing the hot spot formation in the catalyst bed. This may be due to the high K2CO3 content in the catalyst enhancing the surface affinity to steam in the syngas, and the basicity of K2CO3 depresses methane formation over the MoS2 active site. The 10.0 wt.% of K2O-containing SWGS catalyst showed the highest stability even in the absence of H2S in the feed gas for up to 90 min, and little H2S is released from the catalyst (reverse sulfurization) under such conditions. The optimized K2O-containing SWGS catalyst, e.g., QDB-5-10 has been used in an industrial plant for 2 years in a coal to methanol plant and showed stable and unique performance under the lean steam conditions, allowing the 1st stage SWGS reactor to be well within control. The potassium carbonate included in the catalyst is stable and little leachate occurred even after 2-years of use time on stream. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Coal is one of the main energy and chemical resources in China; currently 60% of the energy comes from coal, and over 70% of the power supply is generated by burning coal [1,2]. However, the direct combustion for power generation has caused serious environmental pollution and also emitted large amounts of CO2 into the atmosphere, increasing the threat of global warming [1,3,4]. How to utilize coal resources in a cleaner manner is a challenge worldwide when facing the current and future problems in relation to use of the resources. Gasification is considered to be a relatively energy efficient technology for coal utilization, and the results syngas can be used to produce various products and chemicals. In addition, CO2 can be separated and captured from the gasified stream [5–9]. Coal gasification technologies have been developed for several decades and have been commercially used in many industries [10–14]. The currently available gasifiers can ⁎ Corresponding authors. E-mail addresses: [email protected] (Q. Zong), [email protected] (T. Xiao).

http://dx.doi.org/10.1016/j.fuproc.2015.01.015 0378-3820/© 2015 Elsevier B.V. All rights reserved.

be divided into three types; entrained-bed gasifiers, fixed bed gasifiers, and fluidized-bed gasifiers. The entrained-bed gasifier has the feature of fast residence time (typically less than 5 s) in the gasifier as well as high pressure. Fluidized-bed gasifiers have been developed primarily for the application to low-grade coal and wastes that contain lots of impurities. Fixed-bed gasifiers have a long history in industrial application, such as Lurgi slurry gasifier, which is still used extensively in China. Its feature is reliable, but has relatively low efficiency and is not suitable for the single large scale gasifier [10,15–17]. The slurry gasifier has been used in industry for many years and the raw syngas from the slurry gasifier often contains high steam content. This gives a high ratio of H2O/CO in the feed for water gas shift reaction [18,19]. The gas conditioning for this raw syngas can be easily carried out using typical sour water gas shift catalyst like K8-11 [10, 16,20–23]. But the raw syngas from the dry powder coal gasifier (such as Shell GSP) contains up to 70% of CO but very low steam content. Therefore the conventional sour water gas shift catalysts are not suitable for direction application, so often a huge amount of steam has to be added to the raw syngas to increase steam to carbon ratio. This is

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B. Liu et al. / Fuel Processing Technology 134 (2015) 65–72

because most sour water gas shift catalysts operate at a H2O/CO ratio above 1, in fact above 2 or 3 [24–26] so as to depress carbon formation and increase the catalyst stability. However, the exothermicity of the water gas shift reaction makes it difficult to control the reactor temperature for the high CO-containing syngas shift process. Thus in industrial operation, to control the temperature under the operating conditions for the 1st stage shift reactor, sometimes high temperature SWGS catalysts were employed with extra steam input into the raw syngas from dry powder gasifier. The adoption of high temperature operation is to limit CO conversion, as predicted by the thermodynamics [27,28], so as to generate less heat. This requires another cooling stage before the follow-up water gas shift process. To make the process simple and allow easy control of the shift reaction stage, a catalyst that can operate at lean steam conditions and especially can work at H2O/CO (mol. ratio, the same in the following) below 0.5, which is often the composition of the raw dry powder gasified products is highly demanded. There have been many studies in the development of the SWGS catalysts, mostly based on alumina supported MoS2 catalysts promoted with Co or Ni [29–31]. As we pointed out before, these developments targeted the slurry gasifier, and often operate at a H2O/CO ratio above 1 [25,32–34]. A Pt/CeO2 catalyst has been reported suitable for lean steam condition [27]. In this work we have prepared a series of MgAl2O4 modified Al2O3 supported Co–Mo–S catalysts using potassium carbonate as the promoter. The catalysts showed robust performance for sour water gas shift reaction at low H2O/CO ratio conditions. The catalyst can even maintain its performance in H2S-free feed stream for a short period. The catalyst has been used in an industrial plant for 2 years, which reduced steam requirements and energy input significantly compared to the use of the benchmark commercial catalyst. 2. Experimental The Mg modified alumina support material was prepared by impregnating γ-AI2O3 support (China Aluminum Co., Zibo China, chemical grade, surface area, 213 m2/g) with a proper amount of aqueous solution of magnesium nitrate hexahydrate, which was followed by drying for 4 h at 120 °C in air and calcining for 2 h at 500 °C in air. The content of MgO is 10.5 wt.% and presents in the form of MgAl2O4, confirmed by X-ray diffraction measurement. This support has been designed and made for the QDB-4 SWGS catalyst, which contains 3.5 wt.% of Co and 8.1 wt.% of MoO3 in the oxide form catalysts. The proprietary preparation details of this catalyst have been reported in the literature [35–37]. The QDB-04 catalyst has been used for relatively high H2O/CO conditions for many years, and the industrial operation results have been reported in literature [35, 38]. Here the spinel modified γ-Al2O3 is marked as Al2O3–MgAl2O4. The new K containing SWGS catalysts were prepared by impregnating Al2O3–MgAl2O4 support (extrudate, 3.5 mm in diameter, 20 mm length) with the aqueous solution of Co nitrate, ammonium molybdate and K2CO3 solution using our proprietary method, which can effectively bond the potassium carbonate to the support [43,44]. The solubility of the ammonium molybdate was adjusted through the addition of ammonia. The loading of the Co, Mo and K was carried out through incipient impregnation method, which means that the support can absorb all the solutions onto it; the total content of the CoO and MoO3 in the catalyst oxide form is 3.5 wt.% and 8.1 wt.% respectively. The K2O content in the catalysts was controlled by K2CO3 concentration in the impregnation solution. This novel catalyst series are named as QDB-5-x series catalysts, where x represents the K2CO3 content in the catalyst. The QDB-5 series catalysts containing nominal K2O content of 0 wt.%, 2.0 wt.%, 4.0 wt.%, 6.0 wt.%, 8.0 wt.%, 10.0 wt.%, 12.0 wt.%, and 14.0 wt.% are remarked as QDB-5-0 (no K2O), QDB-5-2 (2 wt.% K2O), QDB-5-4 (4 wt.% K2O), QDB-5-6 (6 wt.% K2O), QDB-5-8 (8 wt.% K2O), QDB-5-10 (10 wt.% K2O), QDB-5-12 (12 wt.% K2O), and QDB-5-14 (14 wt.% K2O). The ICP analysis of the K2O content in the QDB-5 series catalyst showed that the K2O contents in the oxide catalysts are 0.001 wt.%, 1.96 wt.%,

3.92 wt.%, 5.91 wt.%, 7.92 wt.%, 9.88 wt.%, 11.78 wt.% and 13.65 wt.%, which are very close to the nominal value, due to the adoption of the incipient impregnation method. The slightly lower content of the real K2O may result from the some metal salts contacting the impregnation tank. 2.1. Catalyst test Catalyst testing was carried out in a fixed bed micro-reactor system, to mimic industrial conditions (3.7 Mpa, 423 °C) in the first conversion reactor of a Shell gasifier [39]. Each time, 10.0 g of the sieved catalyst (20–40 mesh) was loaded into the reactor with a gaseous hourly space velocity (GHSV) of 3000 h− 1. The gas feed stock composition is adjusted to be close to the industrial outlet of a Shell gasifier (dry feed coal gasification derived syngas composition, CO 64.9%, CO2 8.1%, H2 25.1%, and N2-1.9% for complement, inlet H2S concentration 3000 ppm). The inlet temperature is between 200 and 300 °C, with a reactor pressure of 3.7 Mpa. Water is injected into the system using a HPLC pump, passing through a pre-heater operating at 150 °C to feed the steam. The outlet gas is directly transferred into online GC system for analysis (Agilent 7890, with TCD, FID and FPD). Before the reaction, the gas line was switched to a parallel gas line (3 mol.% H2S, 47 mol.%H2 and 50 mol.% N2), and the catalyst was presulfurized at 250 °C for 3 h, until the gas outlet sulfur composition is constant. The 3 hour-holding time was to completely sulfurize the active phase of Co–Mo–Ox. Afterwards, the catalyst bed was heated to 400 °C at 1 °C/min in the sulfurizing gas to stabilize the active phases. Then the sour syngas was conducted into the reactor and the catalysts were tested under the specific conditions to mimic the lean-steam water gas shift process. The gas analysis was carried out after 4-hour time on stream when the reaction reached steady state. The catalyst activities have been expressed by the CO conversion or the change of CO content in the outlet gas under the same test conditions, and the side reaction is expressed by the CH4 content in the outlet gas [27,30,45]. The industrial operation results were obtained from Henan Zhongyuan Dahua Fertilizer where a Shell Dry Coal Powder gasifier was used. There were two trains of the gasifiers each with 3 stages SWGS reactor after the gasification. For comparison across the parallel streams, the water gas shift catalyst was initially replaced by QDB-5-10 catalysts in one train, only in the 1st stage SWGS reactor, then gradually replacing the 2nd stage SWGS stage reactor with the QDB-5-10 catalysts. The utility consumption and the exit CH4 were recorded for comparison in parallel. When unloading the industrial catalyst after use, the catalyst was purged with flowing N2 at the reaction temperature under atmosphere pressure and cooled down to room temperature naturally. The reactor containing the SWGS catalyst was placed in static air to passivate the industrial catalysts before collection. 2.2. Catalyst characterization The phase of the components in the catalyst crystal size and crystalline structure of the prepared CoMo/Al2O3–MgAl2O4 samples was determined using X-ray diffraction (XRD) with an X' PeRT Pro Alpha 1 diffractometer with Cu Kα radiation (λ = 1.5406 Å), operated at a tube current of 40 kV and a voltage of 40 Ma. Line trace was collected over 2θ values from 20° to 70°, and scanned at a speed of 1°/min. Fourier transform infra-red (FT-IR) spectra were obtained on Bruker Vertex-70 by diffused reflectance accessory technique. Laser Raman spectra were obtained using a Perkin-Elmer Raman station 400F Raman spectrometer. 3. Results and discussion 3.1. Effect of K2O content on the SWGS catalyst performance The K2O containing sour water gas shift catalyst has been tested in a micro-reactor system to study the effect of K2O content on the CO

B. Liu et al. / Fuel Processing Technology 134 (2015) 65–72

conversion over QDB-5-x catalyst performance, and the results are shown in Fig. 1. The catalyst has been tested at various temperatures and it is shown that at the lower temperature (260 °C), the catalyst activity changes less with the K2O loading. However, CO conversion is always higher than the catalyst containing no K2O. The catalyst with K2O content of 8–10 wt.% has the highest CO conversion, about 12%, while no methane or any other side products were detected. When the catalysts were tested at higher temperature such as 350 °C and 450 °C, the CO conversions increased by 5% and 13% for the QDB-5-0 (no K2O) compared to the catalysts at 260 °C, and the conversion increased significantly with the increase of K2O content. The QDB-5-8 catalyst has the highest CO conversion for the SWGS reaction at 350 °C and 450 °C; conversion then tends to decrease with further increasing K2O. This suggests that the optimum content of K2O in the QDB-5 catalyst is about 8.0–10 wt.%. Therefore in the next stage, we would use the QDB-5-10 as the optimized formulation for industrial application. The reason for the low activity of QDB-5 with K2O content more than 8 wt.% may be due to the coverage of the activity site with the K2CO3 in the catalyst surface. This is in agreement with the literature results [27,33], which also shows that too high or low K content cannot give high CO conversion even under high steam to CO ratio condition. These results showed that the K2O content has a significant effect on the catalyst activity, and higher temperature gave higher conversion, which is not favorable from a thermodynamic view, as the water gas shift reaction is exothermic. This result demonstrates that the water gas shift reaction is kinetically controlled, so the reaction conditions and catalyst property have a significant impact on the conversion.

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Fig. 2. Dependence of the out CO on the H2O/CO ratio in the feed gas. Reaction condition: dry gas composition CO 64.9%, CO2 8.1%, H2 25.1%, N2-1.9%, H2S: 3000 ppm, inlet temperature of feed: 250 °C, the temperature reading in the reactor: 423 °C, pressure: 3.7 MPa.

Based on the optimized catalyst preparation, the effect of H2O/CO ratio on the outlet CH4 content and CO conversion has been tested for QDB-5 catalysts containing 0, 6 and 10 wt.% of K2O; the results are shown in Figs. 2 and 3. The outlet contents of CO after the shift reaction over the various catalysts at different H2O/CO ratios are shown in Fig. 2. A low content of outlet CO means the higher catalyst activity of the catalyst. The general trend over the various catalysts is that the outlet CO concentration drops with the increase of H2O/CO ratio, which can be easily understood from the equilibrium of the water shift reaction, as the increase of H2O as reactant would lead to the more CO being consumed by the higher H2O feeding. However, the QDB-5-x catalysts with higher K2O content give lower CO outlet content under all the H2O/CO ratios conditions,

although the difference between the catalysts with various K2O contents tends to decrease with the increase of H2O/CO ratio. This means that the H2O addition increases the catalyst activity more significantly at lower H2O/CO ratio, and intends to have less effect when the H2O/ CO ratio increases. This may be due to the competitive absorption of H2O and CO over the catalyst under different H2O/CO ratio conditions. Compared to the concentration of outlet CO which reflected the catalyst activity, the side product e.g., CH4 content is more important for the 1st stage SWGS process, as an increase of 1% of CH4 in the product would lead to as high as 70 °C temperature rise in the reactor [43,46, 47], which in return would accelerate the methanation reaction and lead to the out of control of the reactor. The change of outlet CH4 in the products with the H2O/CO ratio over various catalysts is given in Fig. 3. Generally, the methane content in the outlet gas is less than 2.5 vol.% in all the QDB-5 catalysts, and the highest CH4 content in the product occurs over the QDB-5-0, where no K2O is included. Under the same H2O/CO condition, the catalyst with 6 wt.% K2O generates less than 1 vol.% CH4 and with 10 wt.% K2O gives 0.4 vol.% of methane under the lowest H2O/CO condition. The CH4 content decreases with the increase of H2O/CO ratio, but in all the cases, the QDB-5-10 catalyst with the highest K2O has the lowest yield of methane, which tends to produce no CH4 at H2O/CO of 1, almost the same over the catalyst.

Fig. 1. The effect of K2O content on the QDB-5 catalyst performance for SWGS reaction at different temperatures. Reaction conditions: H2O/CO = 0.3, GHSV of the syngas: 3000 h−1, pressure: 3.7 Mpa, the outlet pressure of the Shell Gasifier. Dry syngas composition of CO 64.9%, CO2 8.1%, H2 25.1%, N2 1.9%, H2S: 3000 ppm, water flow was controlled to give H2O/CO = 0.5 (mol. ratio).

Fig. 3. Dependence of the out CH4 on the H2O/CO ratio in the feedstock over various catalysts. Reaction conditions: dry gas composition: CO 64.9%, CO2 8.1%, H2 25.1%, N2-1.9%, H2S: 3000 ppm, inlet temperature of feed: 250 °C, the temperature reading in the reactor: 423 °C, pressure: 3.7 MPa.

3.2. Effect of H2O/CO ratio on the CH4 side product content in the exit gas

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This showed that the low H2O/CO ratio feed condition favors the CH4 formation in the SWGS process, but the addition of K2O can significantly depress the methane generation under the condition. 3.3. Effect of H2S content in the feed on the catalyst performance It is generally accepted that the active phase for the sour water gas shift is MoS2, promoted with cobalt oxide [31,34,48], as MoS2 could absorb and activate CO. H2S in raw syngas from the gasifier often varies significantly due to the variations of the feedstock, gasification of coal with high sulfur content produces syngas with high H2S content while coal with low sulfur content gives syngas with little or no H2S. The active component of SWGS, e.g., MoS2 may experience reverse sulfurization to become MoSxOy, and may lose its catalytic activity when low or no H2S is present in the syngas. To ensure a stable industrial operation, the SWGS catalyst is required to be active under all possible industrial conditions. The change of CO conversion over QDB-5 catalysts containing various K2O contents with the H2S content in the feed is shown in Fig. 4. The data were obtained after the catalysts were tested under the gas conditions for 60 min, which appears to be the maximum period of gas variation according to our discussion with the Shell gasifier operators. The catalyst with 10% K2O has the highest CO conversion at 47.5%, and tends to increase with the increase of H2S in the syngas feed. The highest CO conversion reached 50.3% when H2S rises to 600 ppm or above. The QDB-5-8 catalyst has CO conversion of 40.1% at H2S of 0, but increases with the H2S content. The catalyst with 6% K2O has the lowest CO conversion when H2S is absent in the raw syngas, but increases more significantly with the H2S content rising. CO conversion over the 3 catalysts reached the same when H2S increased to 600 ppm or above. These results showed that higher K2O loading stabilized the QDB-5-x catalyst, which still keeps the catalytic activity for CO conversion even with no H2S presence in the feed. The K2O additive may stabilize the MoS2 as the consequence of desulfurization by the steam in the reactants. The QDB-5-10 catalyst is more suitable for industrial operation as it had a stable CO conversion in a very broad range of H2S content. To further explore the changes of the QDB-5 catalyst under lean H2S condition and provide a basis for industrial application, the QDB-5-6 and QDB-5-10 catalyst have been tested in syngas containing 0.02 vol.% H2S. The results are shown in Fig. 5. The catalyst with 10 wt.% K2O loading has higher CO conversion (48.1%) than the one with 6 wt.% (36.2%), and both catalysts showed stable CO conversion in the initial 20 min when H2S was kept at 0.02% vol. After 20 minutes operation, the H2S was stopped while the rest

Fig. 4. The change of the CO conversions over QDB-5-x catalysts with the H2S content in the syngas stream. Reaction condition: dry gas composition: CO 64.9%, CO2 8.1%, H2 25.1%, N2-1.9%, reactor temperature of feed: 350 °C, pressure: 3.7 MPa.

Fig. 5. Dependence of the QDB-5 catalyst performance and exit H2S on the time on stream. Test conditions: Dry syngas composition of CO 64.9%, CO2 8.1%, H2 25.1%, N2-1.9%, initial H2S: 200 ppm, water flow was controlled to give H2O/CO = 0.5 (mol ratio), temperature: 450 °C, pressure: 3.7 MPa.

test conditions remained unchanged; the CO conversion was almost unchanged over the QDB-5-10 catalyst but QDB-5-6 catalyst activity dropped gradually. Meanwhile there was trace amount of H2S detected in the exit gases of the both catalysts probably due to the contamination of H2S in the gas pipeline. However, the outlet H2S over QDB-5-6 catalyst is much higher than the one over QDB-5-10, suggesting that more H2S may be released from the QDB-5-6 after H2S is stopped, e.g., the reverse sulfurization process occurred, which releases H2S into the reactor system. Therefore its H2S content in the exit gas stream decreased much more slowly than the QDB-5-10 catalyst. The loss of sulfur of QDB-5-6 catalyst leads to the active components, MoS2 to change to MoOSx, which in return deactivated the SWGS catalyst gradually. These results suggest that the increase of K2O not only helps to improve the catalyst activity, but also prevents the reverse sulfurization under lean H2S conditions. 3.4. The catalyst performance in industrial operation Based on the above catalyst screening results, the QDB-5-10 catalyst was scaled up and manufactured in large quantities and used in several coal conversion plants where Shell dry coal powder gasifier was adopted. Here we only present the results from Liuzhou Coal to Chemical Plant, Guangxi Province China since Dec 2007. Table 1 shows the SWGS catalysts used in the same coal conversion plant, where Shell dry coal powder gasifier was introduced. The typical raw syngas coming out from the gasifier composed of 58 vol.% of CO, steam content 16–18 vol.%, 7–7.5 vol.% CO2, 15–17 vol.% H2 with H2S ranging from 100 to 2000 ppm, depending on the coal feedstock. Normally there are 3 SWGS reactors for the raw syngas conditioning. Before the QDB-5-10 catalyst was introduced into industry, an industrial bench mark catalyst was used, which required H2O/CO above 2 [24,25], so a steam generator is needed to increase the steam content in the gas. The 3-stage SWGS reactors normally have input temperature of from 240 °C to 286 °C; sometimes, the hotspot temperature generated in the SWGS reactor is as high as 492 °C, which consequently led to the formation of CH4 in the outlet gas at 1.23 vol.% in the 1st SWGS reactor and accumulated to 2.2% after the 2nd SWGS reactor and to 2.8% after the 3rd stage WGS reactor. In addition, the steam input for the plant is 35–60 ton/h for the SWGS reactor to keep the high H2O/CO ratio, so as to depress the carbon formation and promote the SWGS reaction. However, much more hot spots and higher methane were generated in the catalyst. After the 1st SWGS catalyst bed, the reaction reached the equilibrium, and the excess steam was then used for next steam water gas

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Table 1 Comparison of plant indexes with the high and lean H2O/CO water gas shift catalyst adopted. Shift technology

Shell gasifier Bench mark SWGS high H2O/CO QDB-5-10 catalyst

Reactor no.

Steam/gas ratio

Temperature °C

Inlet

Outlet

Inlet

Hot spot

inlet

Methane % Outlet accumul

1st 2nd 3rd 1st 2nd 3rd

1.82 0.79 0.49 0.26 0.23 0.26

0.79 0.49 0.35 0.03 0.04 0.03

286 260 240 210 210 200

492 373 336 417 371 317

0 0 0 0 0 0

1.23 2.2 2.8 0.02 0.03 0.05

Steam, ton/h

Condensed liquid ton/h

35–60

30–55

0

0–5

Pressure: 3.7 Mpa, and GHSV of the dry syngas: 3000 h−1.

shift reaction. After 3-stage SWGS process, the CO was reduced to the desirable level, to about 20–23% in the gas stream, which is ready for the methanol synthesis process. However, when QDB-5-10 catalyst was used, the H2O/CO ratio in the crude syngas directly from the Shell dry powder gasifier was about 0.21, with CO inlet 66.1% (dry base). At 235 °C, no extra steam was added with the raw syngas shifted over QDB-5-10 directly. The CO content was reduced to 38–43% after the first SWGS bed and the hot spot temperature in the SWGS catalyst bed rose to 417 °C, which was much lower than the benchmark catalyst, making the system easier to be controlled. The shifted products were then added with a small amount of water to decrease the syngas gas temperature from the 1st stage SWGS reactor, and the added water amount is decided by the inlet temperature requirement. Typically the water added made the H2O/CO ratio just about 0.25 which can decrease the inlet temperature to 220 °C, the syngas product had CO content of 20–23% after the 2nd SWGS process, and the temperature only increased to 370 °C. To cool down this temperature to the desirable SWGS condition, more water is dripped to the gas stream. On one hand, this can decrease the reactor temperature; on the other hand, it also provides the steam for the 3rd stage SWGS process. The H2O/CO ratio was 0.20 when the added water cooled the 2nd stage syngas to 220 °C, which then passed to 3rd stage QDB-5-10 SWGS catalyst; the CO was decreased to 9–11 vol.%, with the hot spot temperature only at 317 °C. This showed that the introduction of QDB-5-10 catalyst worked effectively at H2O/CO ratio and the resultant syngas did not require any further external cooling, but just through adding liquid water into the syngas to generate steam and also cooled down for next stage water gas shift process. This simplified the process significantly and also made it easy to operate and control the whole plant. It is known that the industrial operation is not always constant, and the outlet syngas from the gasifier changes with the gasifier conditions and the coal feedstock in an acceptable range. The QDB-5-10 catalyst has been used for a year in the Shell Dry Coal Powder gasification plant, and the operation record showed (as shown in Table 2) that the inlet temperature of the raw syngas changes from 210 °C to 243 °C, and the steam to gas ratio in the raw syngas from the gasifier ranged from 0.26 to 0.29. The water gas shift catalyst results in the 2-year operation showed that the hotspot in the first SWGS reactor changes from 417 °C to 451 °C, and the outlet CO from the reactor was very stable, decreasing from about 65 vol.% to about 36 vol.%, with almost no methane formation over the SWGS stage process.

3.5. Characterization of the industrial catalyst The analysis of the spent QDB-5-10 and the benchmark catalysts has been carried out for the carefully collected samples and the results are shown in Table 3. The QDB-5-10 has much higher strength which decreased to 121 N/cm−1 from 128 N/cm−1 after 24 month usage. The surface area only decreased by about 7.0 m2/g while the average pore size dropped to 0.3264 cm3/g after 24 month use. In contrast, for the industrial benchmark catalyst, the strength decreased from 65 N/cm−1 to 46 N/cm−1, while the surface area changed from 60.3 m2/g to 38.6 m2/g. This may be due to the benchmark catalyst itself has the weak strength, and also used under the high H2O/CO condition, which in return accelerates the surface shrinking. The XRD patterns of the fresh and spent QDB-5-10 and the benchmark catalysts are shown in Fig. 6. Both the fresh catalysts show the main diffraction peaks of γ-Al2O3 at 2θ of 38, 44.2 and 65°, but the γ-Al2O3 for the benchmark catalyst's peaks are sharper than the QDB5-10. Also there are MgAl2O4 presenting in both catalyst, and K2CO3 is present in the QDB-5-10. No Mo or Co oxides are detected in the catalyst samples, which are due to the highly dispersion. However, in the spent catalyst, sharp peaks corresponding to MoS2 are detected which are the active phase for the SWGS reaction. The γ-Al2O3 support peaks become even sharper in the benchmark spent catalyst, which is in agreement with the surface area measurement. The spent catalysts also contain K2CO3, which become sharper, suggesting that no or little K loss occurred over the QDB-5-10 catalyst. The spinel MgA2O4 is present in both the catalysts, which is not changed by the industrial operation. Fig. 7 shows the FTIR spectra of the QDB-5-10 and QDB-5-0 catalyst before and after industrial applications. As pointed by Nikolova et al. [33,34,49], potassium loaded over the QDB-5-10 catalyst exists in the catalyst as a stable bicarbonate species coordinated to the K+′, which could be attributed to the decomposition of the CO2− 3 ions in the precursor. The characteristic band at 600–900 cm−1 is due to the strong IR absorption of Al2O3, and no KAlO2 or other form of aluminate was detected with FTIR in the fresh QDB-5-10 catalyst, suggesting that the addition of K2CO3 does not lead to aluminate formation, which does not damage the support structure. After 24 month industrial application, the absorption peaks corresponding to the K2CO3 in the QDB-5-

Table 3 Comparison of the physi-chemical properties of QDB-5-10 SWGS catalyst before and after industrial use. Catalysts

Table 2 CO conversion and CH4 content in the outlet gas of the first SWGS reactor in Liuzhou Coal to Methanol Plant at different operation periods. H2O/CO

Temp, °C Inlet

Hot spot

210.5 231.0 239.6 243.1

417 448.9 451.1 446.7

0.26 0.28 0.29 0.27

Gas composition in the first reactor % Inlet CO

Outlet CO

Inlet CH4

Outlet CH4

65.1 64.71 64.09 64.93

35.5 35.01 33.20 36.10

0.003 0.016 0.026 0.00

0.003 0.02 0.03 0.00

Pressure: 3.7 Mpa, and GHSV of the dry syngas: 3000 h−1.

Strength (N cm−1) Specific surface (m2 g−1) Pore size (cm3 g−1) Average pore radius (nm) Pore distribution b25 nm 25–50 nm 50–100 nm N150 nm

QDB-5-10

Benchmark industrial A

Fresh sample

After test

Fresh sample

After test

128 113.9 0.332 89.7 53.6 5.5 14.3 27.7

121 106.8 0.326 78.9 48.4 3.7 8.8 39.2

65 60.3 0.239 97.1 45.0 3.8 9.4 42.0

46 38.6 0.283 77.6 40.7 4.2 18.5 36.6

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Fig. 6. XRD patterns of the QDB-5 catalyst with and without K modifiers used in industrial reactor for 2 years.

10 become weaker compared to the absorbed water at 1630 cm− 1, the O–H oscillation vibration in of the OH group in the absorbed water on support. However, for the fresh QDB-5-0 catalyst, the IR band at 1630 cm− 1 and 600–900 cm− 1 is much stronger, and very weak peaks at 1385 cm−1, which may be due to trace amount of carbonate or sulfate species. After one year industrial application, the IR band at 1630 cm−1 becomes weaker and even weaker with the 2 years of time on stream. Compared to the QDB-5-0, the absorbed water IR band at 1630 cm−1 becomes less significant, while the IR band corresponding to K2CO3 decreases, which suggests that there might be some K2CO3 leak, but the catalyst surface is still more hydrophilic than the catalyst without K2CO3. It is known that water absorption over the catalyst is important for the SWGS process. The newly formed bicarbonate species coordinated to the K cations over the QDB-5 catalysts might be related to the anti-methanation

Fig. 8. Raman spectra of the fresh QDB-5-10 catalyst (with K) and QDB-5-10 2 years served; fresh QDB-5-0 catalyst (K free) and QDB-5-0 2 year industrial application.

property of the catalyst. The formed carbonate species somehow inhibited the formation of C\H bond on the catalyst surface thus depressed methanation. Also it has shown that the acidic properties of MoS2 favor the formation of hydrocarbon, which can be depressed with the promotion of alkali additives, especially by potassium [1,2,50, 51]. This may be another reason for the low methane yield over the QDB-5-10 catalyst. In terms of the operation, because the lean H2O/CO ratio was adopted, there would be less H2 generated from the equilibrium of the reaction: CO + H2O = H2 + CO; thus low concentration H2 was produced in the lean H2O/CO system, which in turn leads to less methanation from the reaction of CO + 3H2==CH4 + H2O. The higher activity for the K2CO3 modified QDB-5 catalysts may also be due to the strong affinity of the potassium to water, as reflected in the IR which can improve the steam reactant to be absorbed onto the catalyst and thus have more frequency to react with the CO absorbed onto the active site. Also K2CO3 in the catalyst may adjust the interaction between the molybdenum sulfide and the support, and increase the active site exposed to the catalyst surface. The laser Raman spectra of the various QDB-5 catalysts before and after industrial application are shown in Fig. 8. The strong Raman bands at 910 cm−1 and 1050 cm−1 show that the addition of K2CO3 leads to more MoO3 clusters existing over the catalyst surface. After 2 years time on stream, the catalyst surface still had strong Raman bands of MoO3. For the catalyst without K2CO3, the Raman bands of MoO3 become weaker after 2 years' industrial application. This suggests that the inclusion of K2CO3 in the SWGS catalysts helps to stabilize the active phase of MoO3, and relieves the interaction of MoO3 with Al2O3 support. The almost non-change of the MoO3 Raman band in all the catalysts suggests that the cluster size and dispersion of MoO3 may tend to be the same. 4. Conclusion

Fig. 7. FT-IR spectra of fresh QDB-5-10, fresh QDB-5-0 (K free), post-run QDB-5-10 cat. (1–2 years) and post run QDB-5-0 cat. (2 years).

Al2O3–MgAl2O4 supported CoMo catalysts with various K2CO3 loadings have been prepared using our proprietary method and the effect of K2CO3 on the sour water gas shift catalyst for Shell dry coal powder gasifier has been studied in both lab and industrial reactors. The addition of K2CO3 increased the catalyst SWGS activity under H2O/CO

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gas condition significantly and the maximum activity reached with 10.1 wt.% of K2CO3 loading. In addition, the inclusion of K2CO3 to the SWGS catalyst helps depress the methane formation, which therefore helps to avoid the hot spot formation in the water gas shift catalyst bed, and lower methane content in the shifted products. The catalyst containing higher K2CO3 has higher activity for CO conversion, and little reverse sulfurization occurred in 90 min in the absence of H2S in the feed syngas. The higher K2CO3 loading enhances SWGS activity at lower H2S concentration in the containing gas, but does not have this effect when the concentration of H2S is increased under the same conditions. Industrial application results after 2 years' operation showed that the catalyst has high activity for CO conversion and high stability under the lean H2O/CO conditions, with little or no methane formation in the water gas shift reaction. Also the low steam content in the raw syngas from the Shell coal dry powder gasifier is enough to be used directly for the 1st stage SWGS reactor, which removes the need for a steam injection system thus reducing steam consumption and also ensuring smooth and stable operation. The lean steam SWGS performance of the QDB-5-10 catalyst may result from the stabilization effect of the K2CO3 on the active phase, e.g., MoS2, and also the increase of the basicity of the catalyst which depresses the carbon formation and retards the methane hydrogenation of CO with H2. In addition, the K2CO3 has higher affinity to steam, which improves the steam absorption over the catalyst surface, and thus promoting the reaction.

Acknowledgments We would like to thank Dr. Thomas P Hickey for his constructive discussion and Mr. Terry Pollard for his reading and correction. Also thanks are due to Guangzhou Boxenergytech Ltd China for their help in the measurement of the catalyst physical properties.

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Further Reading [1] D. Nikolova, T. Grozeva, R. Edreva-Kardjieva, Low content (K2O)NiO/Al2O3 sulphurresistant catalysts for water-gas shift reaction, Journal of Environmental Protection and Ecology 2 (2001) 747–752.