Syngas Cleaning

Syngas Cleaning | Hot-Gas Filtration | Desulfurization, Tar Cracking, Ammonia Cracking
Trace Metal Capture | Preparation and Characterization of Catalysts and Sorbents
PSDF Transport Gasifier | Laboratory-Scale Gasifer

Trace Metal Capture

Beyond tars and acid gases, which are themselves a significant detriment to reforming catalysts and associated equipment, fuels cells, and gas turbine blades, semi-volatile metals can also damage cleanup systems, catalysts, turbine blades, fuel cells, and contaminate fungible chemical products. Metals are a difficult challenge to deal with whether using hot-gas filtration or low-temperature processing. Even though most of the metal tends to condense before the barrier filter of hot-gas cleanup systems, some small percentage of the metal (large enough to damage syngas-reforming catalysts, candle filters themselves, fuel cells, and gas turbine blades) does pass through these barrier filters along with the clean syngas. Low-temperature processing requires expensive measures to remove metals from the process stream. Significant costs are required to remove these metals.

One approach that Southern Research engineers use to solve the metals problem is to use high-temperature sorbents to capture all of the semi-volatile metals upstream of the barrier filter, which would prevent even small amounts of metal from passing through the filter with the clean syngas. High-temperature sorbents have already been developed that have been shown to be effective at capturing semi-volatile metals from vitiated combustion effluent, i.e., high-temperature flue gas. Southern Research develops designer sorbents and injection technologies to optimize the effectiveness of these sorbents at capturing metals from syngas, protecting the barrier filters from damage, and protecting the catalysts, fuel cells, or other downstream equipment from damage.

Figure 1. Insoluble metals found on the filter cake, as a function of Kaolinite/K2O ratio
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Figure 1 contains a plot of insoluble metals found in the filter cake following slipstream tests at the Power Systems Development Facility, plotted as a function of the sorbent/metal equivalence ratio measured in respective filter cakes.

As shown in Fig. 1, very little soluble silicon or aluminum was found in the filter cake for any of the tests, suggesting that few if any soluble metal/sorbent reaction products were formed. This is also evidence that the meta-kaolinite crystal structure remained in tact, and that little if any melting of the particles occurred. Furthermore, as little as 50% and as much as 85% of the potassium was insoluble for all filter cakes, suggesting that significant reactive capture of potassium occurred.

Nevertheless, the data in Fig. 1 indicates significant sorbent utilization. Sorbent/metal equivalence ratio metal-capture-limit lines in Fig. 1 indicate the maximum percentage of metal that can be captured based on four different equivalence ratios, = 1/1, 1/2, 1/4, and 1/8. Maximum in-flight capture of sodium and lead by kaolinite in vitiated post-flame air was previously published as being limited to an equivalence ratio of 1/1*. Subsequently, it was shown that the limit based on = 1/2 and even 1/4 could be reached for the same conditions, if sorbent melting occurred, where the aluminosilicate crystalline structure decomposed to allow the formation of separate silicates and aluminates**.

Figure 2. Scanning electron microscope image of filter cake - Southern Research

Even though the K2O product is a different metal oxide than the PbO and Na2O considered in the previous work, it is likely that the mechanisms involved will be similar and the maximum utilization limits should apply to all these types of metal. However, unlike the previous investigation, metal capture percentages were obtained in this work, as shown in Fig. 1, that reached equivalence ratios as high as 1/8, and this without any melting of the sorbent matrix.

The high capture percentages (i.e., insoluble metal %) observed (see Fig.1) at low equivalence ratios were encouraging. The sorbent was not allowed to build up on the filter before metal vaporization and injection began. Therefore, higher capture percentages of the metal may be possible in full-scale applications.

Figure 3. EDS spectra of Area 1 in Fig. 2.

Figures 2 and 4 show SEM images of potassium collected on a candle filter without any sorbent injection. As shown in the EDS spectra of Figure 3, the material was primarily aluminum and potassium, indicating that the potassium reacted with the candle filter element to form potassium aluminates. Subsequent tests with sorbent indicated that the sorbent reacted with the metal before it was able to react with the candle filter, thus protecting the candle filter. An SEM of a sorbent/metal filter cake is shown in Figures 4.

The low equivalence ratios observed and the high sorbent utilizations combined with a lack of sorbent melting in the experiments conducted in this work is an exciting result relative to high-temperature sorbents for capturing metals from hot syngas. It is desirable to avoid eutectic melting of the sorbents, which potentially could significantly contribute to melting on the candle filters and clogging of the filter elements.

Figure 4. Scanning electron microscope image of sorbent/metal filter cake - Southern Research

Even for eutectic melting, we have found that the syngas temperatures should be above 1500°F. The gas temperatures in the main reaction chamber were intentionally kept below this temperature, for this reason. In addition to remaining crystalline, clearly non-melted sorbent/metal reaction products will help eliminate sticky filter cakes, filter clogging, and bridging between candles. The observation that high sorbent utilizations can be obtained without melting is a result that was previously unknown. This result may be associated with the ability of kaolinite clay powders to capture metals more efficiently over long periods of time, while in a dust cake on a filter rather than while suspended in the gas phase for a few seconds**.

 

 

Syngas Cleaning | Hot-Gas Filtration | Desulfurization, Tar Cracking, Ammonia Cracking
Trace Metal Capture | Preparation and Characterization of Catalysts and Sorbents
PSDF Transport Gasifier | Laboratory-Scale Gasifer

Referenced Publications
*Mwabe, P. O. and Wendt, J. O. L., “Mechanisms Governing Trace Sodium Capture by Kaolinite in a Downflow Combustor” Proc. Combust. Inst., 26: 2447-2453 (1996).
*Davis, S. B. and Wendt, J. O. L., “Mechanism and Kinetics of Lead Capture by Kaolinite in a Downflow Combustor”, Proc. Combust. Inst., 28:2743-2749 (2000).
**Gale, T.K., and Wendt, J.O.L., "High Temperature Interactions between Multiple-Metals and Kaolinite” Combust. Flame, 131(3): 299-307 (2002). Printers Correction: Combust. Flame 133: 383 (2003).
**Gale, T.K., and Wendt, J.O.L., “Mechanisms and Models Describing Sodium and Lead Scavenging by a Kaolinite Aerosol at High Temperatures” Aerosol Sci. Technol. 37: 865-876 (2003).
**Gale, T.K. and Wendt, J.O.L., “In-Furnace Capture of Cadmium and Other Semi-Volatile Metals by Sorbents”, Proc. Combust. Inst., 30: 2999-3007 (2005).
**Gale, T.K., “Mechanisms Governing Multi-Species Metal Capture by Kaolinite, Hydrated Lime, and Novel Sorbents in High Temperature Combustion Environments” Ph.D. Dissertation, Chemical Engineering Dept., University of Arizona, Tucson, AZ (2001).