Sour Water Gas Shift Research Articles

Platinum Based Catalysts in the Water Gas Shift Reaction: Recent Advances

The water gas shift (WGS) is an equilibrium exothermic reaction, whose corresponding industrial process is normally carried out in two adiabatic stages, to overcome the thermodynamic and kinetic limitations. The high temperature stage makes use of iron/chromium-based catalysts, while the low temperature stage employs copper/zinc-based catalysts. Nevertheless, both these systems have several problems, mainly dealing with safety issues and process efficiency. Accordingly, in the last decade abundant researches have been focused on the study of alternative catalytic systems. The best performances have been obtained with noble metal-based catalysts, among which, platinum-based formulations showed a good compromise between performance and ease of preparation. These catalytic systems are extremely attractive, as they have numerous advantages, including the feasibility of intermediate temperature (250–400 °C) applications, the absence of pyrophoricity, and the high activity even at low loadings. The particle size plays a crucial role in determining their catalytic activity, enhancing the performance of the nanometric catalytic systems: the best activity and stability was reported for particle sizes < 1.7 nm. Moreover the optimal Pt loading seems to be located near 1 wt%, as well as the optimal Pt coverage was identified in 0.25 ML. Kinetics and mechanisms studies highlighted the low energy activation of Pt/Mo2C-based catalytic systems (Ea of 38 kJ·mol−1), the associative mechanism is the most encountered on the investigated studies. This review focuses on a selection of recent published articles, related to the preparation and use of unstructured platinum-based catalysts in water gas shift reaction, and is organized in five main sections: comparative studies, kinetics, reaction mechanisms, sour WGS and electrochemical promotion. Each section is divided in paragraphs, at the end of the section a summary and a summary table are provided.

Catalytic performance stability of Mo, W and Re-based sour water–gas shift catalysts

It has been shown in our previous findings that Mo-, W- and Re-based catalysts promoted with Ni, Co, and K are considered as a promising tri-component catalyst for sour WGS reaction. In order to evaluate the stability of the formed active sulfide and oxysulfidic species at sulfided Mo-, W- and Re-based catalysts at the same surface density of the main component, namely 2 atoms metal/nm2 γ-Al2O3 support, under influence of oxidizing agents as H2O steam and CO2, the WGS reaction was proceeded in absence of sulfur in gas feed. With the intent to enable a quick check, the applied approach was to perform the WGS reaction at high reaction temperature, 400 °C, in H2S-free syngas and varying the gas hourly space velocity (GHSV). This study gave information about the behaviour of sour molybdenum, tungsten and rhenium WGS catalysts during operation accident. It was found that the active sulfide and oxysulfidic species are more stable in absence of sulfur in feed at tri-component KCoMo and KCoRe catalysts. These catalysts possess lowest sulfur dependence of WGS activity and stability of the deactivation, which is an indication of the applicability of these two systems as optimal sour water–gas shift catalysts.

Study of sour water gas shift using hydrotalcite based sorbents

One of the most promising alternatives to the use of liquid amines and pressure swing adsorwption (PSA) for H2 production and CO2 capture in pre-combustion processes is the so called sorption-enhanced water gas shift reaction concept (SEWGS). This study presents the performance in SEWGS of two sorbents, a K-promoted hydrotalcite with Mg/Al ratio=2, which based on our previous work was selected as the most promising CO2 capture sorbent tested under sweet water gas shift (WGS) operating conditions, and a bi-functional material prepared by impregnation of the K-promoted hydrotalcite with Co and Mo in order to enhance the catalytic properties of the material.Short term sour CO2 capture tests and WGS tests at similar operating conditions (T=300–350°C; P=15bar) were carried out in order to study the effect of the presence of H2S in the feed gas stream in the CO2 capture capacity and catalytic activity of both sorbent materials as well as to compare their performance in developed tests. Finally, the feasibility of using the bifunctional material for sour SEWGS processes was also studied.Impregnation with Co-Mo did not adversely affect the adsorption capacity of the K-promoted hydrotalcite and enhanced catalytic activity, allowing total CO conversion even after adsorption capacity had been reached.

Sour water–gas shift reaction over Pt/CeZrO2 catalysts

The influence of H2S on the performance of platinum catalysts for the water–gas shift reaction was investigated. Platinum catalysts supported on different oxides (γ-Al2O3, ZrO2, CeO2, Ce0.75Zr0.25O2, and Ce0.25Zr0.75O2) were tested under industrial conditions in the absence and presence of H2S. Pt/CeO2 showed higher initial catalytic activity than Pt/Al2O3, but exhibited some deactivation and low recovery of catalytic activity when subjected to contaminated conditions, due to the formation of sulfur species on the ceria surface. Zirconia addition to CeO2 was beneficial to increase sulfur tolerance and preserve high activity. The deactivation ratios for Pt/CeZrO2 catalysts were very close and recovery ratios were 100%, indicating that for these catalysts the deactivation is reversible, which can be related to less interaction between H2S and these supports.

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

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 0–10wt.% 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 10.0wt.% of K2O-containing SWGS catalyst showed the highest stability even in the absence of H2S in the feed gas for up to 90min, 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 2years 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.

Data Reconciliation and Dynamic Modeling of a Sour Water Gas Shift Reactor

Integrated gasification combined cycle (IGCC) plants with CO2 capture have strong potential in the future carbon-constrained world. In these plants, the CO2 and COS content of the syngas at the inlet of the acid gas removal process should be within certain limits in order to satisfy the environmental limits on sulfur and CO2 emissions. To satisfy these constraints, the syngas from the gasification process is passed through water gas shift reactor(s). A premium is placed on sulfur-tolerant catalysts since the syngas may contain COS and H2S. In comparison to the sweet-shift process, the sour-shift process results in higher overall efficiency because of the higher temperature of the feed syngas and requirement of less additional steam for the shift reactions. The optimal operating conditions and the dimensions of the sour shift reactors can be obtained by considering the effect of a number of key variables. With this motivation, a 1-D mathematical model of a sour water gas shift (WGS) reactor has been developed by considering mass, momentum, and energy conservation equations. The experimental data available in the open literature are reconciled for measurement errors after gross errors are removed and then used to obtain the rate parameters. Subsequently, the developed model is used to study the performance of the WGS reactor as part of an IGCC plant with CO2 capture. The results presented in this paper are very useful in designing, analyzing, and operating the sour water gas shift reactors.

Plant-wide modeling of an indirect coal–biomass to liquids (CBTL) plant with CO2 capture and storage (CCS)

The net greenhouse gases (GHG) emissions of conversional Fischer Tropsch (FT) synthetic fuels derived from coal are about double of those from petroleum fuels. Adding moderate amounts of biomass to coal can substantially reduce the carbon footprint of the indirect fuel production plant. The indirect coal–biomass to liquids (CBTL) technology with CO2 capture and storage (CCS) is more environmental friendly than the conventional coal to liquids (CTL) processes. This paper focuses on the selection of CCS technologies and obtaining their optimal operating conditions for a CBTL plant. A detailed process model is developed in Aspen Plus V7.3.2 for this purpose. In this plant, syngas is produced in the biomass/coal-fed co-gasifier. Then, a sour water gas shift (WGS) reactor converts a portion of the CO in the syngas to CO2 to obtain the desired H2/CO ratio in the syngas feed to the FT unit. Substantial amount of CO2 is captured before the FT reactor by using a dual-stage, selective physical solvent-based process. In the FT unit, the Fe-based catalyst is used in the low temperature FT (LTFT) slurry reactor to convert syngas to hydrocarbons. For selection of the post-FT CO2 capture technology, three candidate technologies – Selexol, MEA and MDEA/PZ, are evaluated. The results show that the MDEA/PZ technology with intercooling has the lowest overall penalty. Impacts of two key design variables, H2/CO ratio at the inlet of the FT unit and biomass/coal ratio of the feedstock, on the product yield and utility consumptions are investigated.

A Step-Change Sour Shift Process for Improving the Efficiency of IGCC with CCS

A balanced portfolio of mitigating power generation technologies is required to meet global CO2 reduction targets, of which Integrated Gasification Combined Cycle with CO2 Capture and Storage (IGCC-CCS) from coal is one. The sour water gas shift (WGS) reaction is a key conversion step in the IGCC-CCS process and is responsible for about 44% of the efficiency penalty that CCS would confer on IGCC with current technology, due principally to the diversion of steam from steam turbines to the shift process. A multi-faceted development program has been undertaken to drive down this penalty, considering different reactor types, new catalyst formulations and process integration. Process evaluation has shown cost of electricity (CoE) reductions of 3-4% with the combination of significantly reduced steam addition and the utilisation of improved reactor designs with initial catalyst performance data. Higher reductions in CoE of up to 7% are obtainable, dependent upon the gasifier selection. This translates to reductions of up to US $8/MWh for power production on the most efficient flow-sheets.

Effect of Titania Addition on the Performance of CoMo/Al2O3 Sour Water Gas Shift Catalysts under Lean Steam to Gas Ratio Conditions

CoMo/Al2O3 sour water gas shift catalysts with and without TiO2 modification have been tested in parallel industrial reactors under lean steam/gas conditions for two years, and part of catalyst samples was taken out each year during the maintenance period. The catalyst samples have been characterized using temperature programmed sulfurization (TPS), X-ray diffraction (XRD), and laser Raman and BET surface area measurements. The results have shown that adding TiO2 to the catalyst makes the active components, e.g., Mo and Co, easier to be sulfurized with higher sulfur capacity in the catalyst itself. This may be the main reason why the TiO2 modified CoMo catalyst to be active even at low H2S gas stream. The results from industrial operation showed that adding TiO2 to the shift catalyst increases the catalyst activity and stability, presents the higher shift activity in a broader range of H2S content, depresses the aggregating of the molybdenum oxide, and reduces carbon deposition. In addition, the TiO2 addi...

Platinum based core–shell catalysts for sour water–gas shift reaction

Pt@CeO 2 and Pt@Ce 0.67 Zr 0.33 O 2 prepared by microemulsion method were studied for water–gas shift reaction in pure syngas and sour syngas from 573 K to 773 K. Both showed high catalytic activities in pure syngas with CO conversion (X CO ) nearly to equilibrium status at temperature above 723 K, and the side reaction of methanation was inhibited throughout the temperature range. Pt@Ce 0.67 Zr 0.33 O 2 showed lower X CO in pure syngas, but higher in sour syngas than Pt@CeO 2 , indicating that the former is more resistant to H 2 S than the latter. Although a decrease in X CO was observed in sour syngas relative to pure syngas, stability experiments carried out over Pt@Ce 0.67 Zr 0.33 O 2 indicated that the catalytic activity could be recovered after the removal of H 2 S. ► Platinum based core–shell catalysts were firstly studied for sour WGS. ► Core shell shows high activities in pure syngas but no methanation even at 773 K. ► The catalysts deactivation in sour syngas was partly irreversible. ► The reason is mainly due to the absence of surface platinum.

Sour water–gas shift reaction over Pt/CeO 2 catalysts

Ceria-supported catalysts with 0.38–2.0 wt.% Pt have been studied under high-temperature water–gas shift (WGS) conditions with and without 20 ppm H 2S present, which is a typical level in feed streams for WGS reactors in IGCC power plants. The effect of H 2S on CO conversion decreased with increasing temperature in the range 573–723 K and also with increasing Pt content. A 1 wt.% Pt catalyst exhibited excellent stability during a 300 h test at 673 K and GHSV = 7500 l kg −1 h −1 in an H 2S-charged feed with steam-to-CO ratio 2 as CO conversion remained stable around 73%. This was still close to the corresponding thermodynamic limit of 78.8%, which was reached under sulfur-free conditions. The Pt ionization degree in WGS-tested catalysts increased only slightly with the H 2S addition as the Pt 2+ fraction grew from ca. 40% to 50% and the Ce oxidation states remained similar too. The bimodal pore structure of the mesoporous CeO 2 was also retained after WGS experiments although the effective surface area had declined by nearly 20% following the 300 h test in sour syngas. This and the concomitant decline of surface OH groups were likely caused by adsorption of H 2S and subsequent formation of surface sulfates and thus responsible for lowering the WGS activity of the ceria surface in the presence of H 2S. We propose that Pt centers can mitigate this effect through hydrogen spillover in their vicinity thus maintaining a high WGS activity of these catalysts in sour syngas.

Carbon-Free Hydrogen and Electricity From Coal: Options for Syngas Cooling in Systems Using a Hydrogen Separation Membrane Reactor

Conversion of coal to carbon-free energy carriers, H2 and electricity, with CO2 capture and storage may have the potential to satisfy at a comparatively low cost much of the energy requirements in a carbon-constrained world. In a set of recent studies, we have assessed the thermodynamic and economic performance of numerous coal-to-H2 plants that employ O2-blown, entrained-flow gasification and sour water-gas shift (WGS) reactors, examining the effects of system pressure, syngas cooling via quench versus heat exchangers, “conventional” H2 separation via pressure swing adsorption versus novel membrane-based approaches, and various gas turbine technologies for generating coproduct electricity. This study focuses on the synergy between H2 separation membrane reactors (HSMRs) and syngas cooling with radiant and convective heat exchangers; such “syngas coolers” invariably boost system efficiency over that obtained with quench-cooled gasification. Conventional H2 separation requires a relatively high steam-to-carbon ratio (S/C) to achieve a high level of H2 production, and thus is well matched to relatively inefficient quench cooling. In contrast, HSMRs shift the WGS equilibrium by continuously extracting reaction product H2, thereby allowing a much lower S/C ratio and consequently a higher degree of heat recovery and (potentially) system efficiency. We first present a parametric analysis illuminating the interaction between the syngas coolers, high temperature WGS reactor, and HSMR. We then compare the performance and cost of six different plant configurations, highlighting (1) the relative merits of the two syngas cooling methods in membrane-based systems, and (2) the comparative performance of conventional versus HSMR-based H2 separation in plants with syngas coolers.

Co-production of hydrogen, electricity and CO 2 from coal with commercially ready technology. Part A: Performance and emissions

This two-part paper investigates performances, costs and prospects of using commercially ready technology to convert coal to H 2 and electricity, with CO 2 capture and storage. Part A focuses on plant configuration and the evaluation of performances and CO 2 emissions. Part B focuses on economics, establishing benchmarks for the assessment of novel technologies and guidelines for technological development. In the co-production plants considered in the paper, coal is gasified to synthesis gas in an entrained flow gasifier. The syngas is cooled, cleaned of particulate matter, and shifted (to primarily H 2 and CO 2) in sour water–gas shift reactors. After further cooling, H 2S is removed from the syngas using a physical solvent (Selexol); CO 2 is then removed from the syngas, again using Selexol; after being stripped from the solvent, the CO 2 is dried and compressed to 150 bar for pipeline transport and underground storage. High purity H 2 (99.999%) is extracted from the H 2-rich syngas via a pressure swing adsorption (PSA) unit and delivered at 60 bar. The PSA purge gas is compressed and burned in a conventional gas turbine combined cycle, generating co-product electricity. The H 2/electricity ratio can be varied by lowering the steam-to-carbon ratio in the syngas or by letting part of the de-carbonized syngas by-pass the PSA unit. Performances and emissions of H 2/electricity co-production with CO 2 capture are compared with those of a system that vents the CO 2. We examine different methods of syngas heat recovery (quench versus radiant cooling) and explore the effects of changing the electricity/H 2 ratio, gasifier pressure and hydrogen purity. Results show that state-of-the-art commercial technology allows transferring to de-carbonized hydrogen 57–58% of coal LHV, while exporting to the grid decarbonized electricity amounting to 2–6% of coal LHV. In contrast to decarbonizing coal IGCC electricity, which entails a loss of 6–8 percentage points of electricity conversion when capturing CO 2 as an alternative to venting it, CO 2 capture for H 2 production gives a minor energy penalty ( ∼ 2 percentage points of export electricity). For H 2 production, the efficiency gain achievable by hot syngas cooling vs. quench is a modest 2 percentage point increase in electricity for export, compared to 2–4 percentage points in the electricity case. Reducing H 2 purity or increasing gasification pressure has minor effects on performance.