WGS Catalysts

Figure 17.6 illustrates a gasification process integrated with the calcium looping process. Once the water gas mixture is formed at the exit of the gasifier, calcium oxide fines are injected into the fuel gas stream. As the fuel gas flows past the WGS catalyst, the WGS reaction takes place and forms additional C02. The injected CaO sorbent particles react with C02 and H2S in the gas stream, thereby allowing further catalytic WGS reaction to occur. The reactions involved in the calcium looping scheme are... 

Today s low- and high-temperature WGS catalysts are sufficiently active and stable for use in stationary facilities for 2-10 years before requiring replacement. The incentive for companies to develop catalysts that last longer than 10 years is limited. Even so, it is reasonable to expect some advance to occur within the next several years just as it did when Cu was added to the iron-chromia high-temperature catalysts about 20 years ago. 

High- and low-temperature WGS catalysts based on Fe and Cu respectively, require slow and carefully controlled activation procedures. After reduction they are highly reactive toward air and can be a fire hazard to the consumer. 

For on-board fuel processing, there are two principal concerns (1) the feasibility of keeping the iron-chrome and copper oxide catalysts in the reduced state, especially during periods of shutdown and (2) the pyrophoric nature of the copper oxide catalyst in the reduced state [29], Because of these concerns, considerable research and development is being conducted to develop new WGS catalysts for on-board fuel processing. 

Catalyst systems for the WGS reaction that have recently received significant attention are the cerium oxides, mostly loaded with noble metals, especially platinum 42—46]. Jacobs et al. [44] even claim that it is probable that promoted ceria catalysts with the right development should realize higher CO conversions than the commercial Cu0-Zn0-Al203 catalysts. Ceria doped with transition metals such as Ni, Cu, Fe, and Co are also very interesting catalysts 37,43—471, especially the copper-ceria catalysts that have been found to perform excellently in the WGS reaction, as reported by Li et al. [37], They have found that the copper-ceria catalysts are more stable than other Cu-based LT WGS catalysts and at least as active as the precious metal-ceria catalysts. 

While the H20/CO ratio is crucial for the performance of LT WGS, it was particularly interesting to study the activity of catalysts at stoichiometric ratio and at H20/CO ratio of 3 1. Both are lower than those used in the commercial LT WGS processing of the gas exiting HT WGS. This was done deliberately for two reasons. The first is that there was no C02 present in the feed. Hence, the H20/CO ratio could be lower because there was no need to compensate the C02 influence on equilibrium with higher H20 concentration (due to reverse WGS reaction). The second reason was the intention to study the behavior of LT WGS catalysts at relatively low inlet CO concentration (0.5 vol%) with respect to the usual inlet CO concentrations used in the industrial process (1.5 to 3 vol%). The feed composition used here was similar to that reported in Refs. [45,46], except that the CO concentration and the H20/C0 ratio were lower. 

How can the use of modem quantitative theoretical approaches help in designing new and better WGS catalysts ... 

Figure 14.2 shows that the production of 99% pure hydrogen requires many catalytic processes. The desulfurization section is used to reduce the sulfur content of the natural gas to 0.01 ppm to protect the SMR and WGS catalysts downstream. A supported cobalt-molybdenum catalyst (CoMoS) converts the sulfur compounds into H2S, which is removed by a ZnO catalyst [5]. 

Natural gas feedstock is very dependent of the source location in some cases it has high levels of H2S, CO2 and hydrocarbons. Organic sulfur compounds must be removed because they will irreversibly deactivate both reforming and WGS catalysts. Hence a preliminary feed desulfurization step is necessary. This process consists in a medium-pressure hydrogenation (usually on a cobalt-molybdenum catalyst at 290-370 °C), which reduces sulfur compounds to H2S, followed by H2S separation through ZnO adsorption (at 340-390 °C) or amine absorption [9]. 

Commercial WGS catalysts have been optimised for more than 50 years for the massive H2 production in petrochemical plants. However, on-board production requires new catalytic properties such as short response in a dynamic regime, sulfur tolerance, low toxicity and safety (commercial catalysts generally contain Cr and are pyrophoric) and above all a higher efficiency to minimize the size of reactors. Imme-... 

This strategy will be illustrated below for the search of new WGS catalysts, being referred to as Strategy WGS 1 . 

For the present case study, a first attempt to predict quantitatively performances of WGS catalysts by ANN regression technique led to a rather poor correlation between predicted and experimental CO conversion values (Fig. 10.13). This suggests that, in addition to noisy data, the used descriptors, which were restricted to the single elemental composition of the catalysts, do not contain per se sufficient... 

Pros Converging conclusions were found between strategies applied for optimising WGS catalysts, such as the major role of some key elements or combination of elements. However, additional trends were revealed by the strictly ES... 

Andreev and co-workers [28] and Mellor [29] have prepared catalysts by leaching Cu-Zn-Al in aqueous sodium hydroxide solutions. Their studies have shown that skeletal Cu-Zn catalysts have significantly greater activities than commercial WGS catalysts when operated at temperatures below 573 K. 

An example of a homogeneous WGS catalyst is Rht(CO)i2 in aqueous pyridine.51 Analogous reactions with CO and alcohols instead of water are also known.52 Some important reactions of CO, H2, and methanol are shown in Fig. 22-5. 

The known WGS catalysts based on ceria or copper oxide both suffer greatly from the deactivation with time-on-stream and/or in shut-down/ restart operation. The mechanism of such deactivation was ascribed to the... 

The Ni-based catalyst typically used in the prereforming process can act as a sulfur trap to remove traces of sulfur in the feed. This can improve the lifetime of the catalyst in the main reformer and also the WGS catalyst used in the downstream. 

Significantly less mechanistic information is available for the CoMo WGS catalyst this is mainly due to the lack of stability of the active sulfided species under experimental conditions employed by the mechanistic study. Hakkarainen and Salmi10 and Hakkarainen and coworkers11 observed a consistent loss of sulfur upon establishing the operating conditions for transient response experiments. Nevertheless, the information provided by those studies, as summarized below, is probably capturing the most important mechanistic features relevant to a practical kinetic model. 

Despite the extensive list of potential drawbacks with existing commercial catalysts, few new WGS catalyst formulations have been commercialized in the past decades. Based on the data compiled by Armor,20,21 no new WGS catalysts were commercialized during the 1980s, while only two new catalysts were developed during the 1990s by the Siid-Chemie Group a ferrochrome catalyst promoted with Cu was developed for enhanced activity with suppressed Fischer-Tropsch by-product... 

There are some recent literature reports about new commercial catalysts designed to overcome some of the drawbacks associated with the traditional WGS catalysts. 

---