Greenhouse Gas Reforming

Marco Castaldi (Assistant Professor, EEE)
Noah Whitmore (EEE)

Greenhouse Gas Reforming
Here the research focuses on catalytic reactor behavior and species evolution in light of multiple reaction pathways, residence time effect, catalyst effectiveness, Theile analysis, etc. In addition catalyst surface science will be explored in terms of alloy, promoter and stabilizer effects on reaction systems as well as turnover frequency analysis coupled with dual site or competing site mechanisms. While there has been a significant amount of work for CO2 reduction on PGM and transition metal complexes the explanations for the mechanistic pathways remain unsolved. In addition, the application of such catalysis has not been done and the performance at throughputs of substantial levels or very high space velocities to affect the CO2 production cycle by combustion has not been investigated. The primary goal to produce syn-gas or other valuable products.

Results to Date
To ensure that the reactions observed were catalyzed by the precious metal, a test was done where only the un-catalyzed support was evaluated. Figure 1 shows the results of the testing for hydrogen generation for the un-catalyzed support compared to the catalyzed samples for the same temperature profiles up to 900oC. It is clear from the data that the un-catalyzed support had significantly less hydrogen production. In addition, not shown, there was no weight gain associated with the un-catalyzed support test. CO production is not shown for clarity but resulted in a similar comparison as that for hydrogen generation.

Figure 1. Comparison of activity (hydrogen production) between as-prepared, pre-reduced and uncatalyzed support

The effects of varying temperature on the dry reforming reaction have also been explored. While using pre-reduced Pt/g-Al2O3 catalysts, carbon deposition and synthesis gas conversion was explored at temperatures of 500, 700 and 900°C. Figure 2 shows the results of the various temperatures on mass.

Figure 2. The effects of varying temperatures for mass percent growth during dry reforming runs.Runs were performed on pre-reduced 0.5% Pt/g-Al2O3 samples with inlet stream of 7ml CH4, 7ml CO2, and 90ml N2.

There is an initial decrease in mass, likely due to any moisture and adsorbed gases prior to the experiments followed by a slight increase for 500oC, no gain for 700oC and a significant increase in mass for the 900oC tests. In addition, the lower temperatures do not exhibit the rapid mass loss (near-step decrease of 2-3%) near 45 minutes that is seen for the high temperature experiment and discussed above. It is clear that temperature has an effect on whether the catalyst experiences that sharp mass loss followed by a rapid increase in mass gain.While the 900°C trial showed significant carbon deposition, it also exhibited the greatest synthesis gas conversion. At 700°C and 500°C, carbon build up was 56% and 83% less than the 22% carbon deposition by weight on the catalyst exposed to 900°C. However, for the 900°C sample, though it showed high synthesis gas production, carbon was still increasing at the end of a two hour isothermal run. The greater carbon formation at higher temperatures is suspected to be due to thermal decomposition of the methane molecule, leaving solid carbon on the catalyst surface and hydrogen gas in the stream.

Figure 3 plots the concentration of hydrogen produced versus time on stream in the TGA for the three temperatures tested with pre-reduced samples.

Figure 3. H2 conversion as a function of time for varying temperatures.Runs were performed on pre-reduced 0.5% Pt/g-Al2O3 samples with inlet stream of 7ml CH4, 7ml CO2, and 90ml N2.

For the first 29 minutes all three samples behave similarly as the temperatures are continuously increased. After approximately 29 minutes, which is near the 700oC isothermal portion and already into the 500oC isothermal portion, there is a decrease in activity which persists for the remaining test time. Yet, the test where 900oC is the maximum isothermal temperature, there is a continued increase in activity followed by a constant performance during the isothermal portion of the test.
The effect of varying concentrations of CO2 has also been studied preliminarily at temperatures of 700°C and 900°C and at CO2:CH4 ratios of 1, 2, and 4. At 900°C, carbon deposition was greatest when the CO2:CH4 ratio was 4, next greatest at a ratio of 1, and lowest at a ratio of 2.The order of hydrogen production followed the same trend. At 700°C, more carbon deposition occurred when the CO2:CH4 ratio was 2 than at a one-to-one ratio.

The above results and additional evidence provided by surface measurements discussed below suggest that while carbon formation is always favored thermodynamically, there is a competing kinetic effect.Since carbon deposition is present at all conditions it is possible that the carbon gasification reaction (Boudouard reaction) is responsible for the stable activity at high temperatures. As carbon is formed at the surface it can combine with reactant CO2 to form CO, thus limiting the fouling effect the carbon buildup causes. Since the reaction is endothermic, at lower temperatures there is not enough energy to both initiate reforming and carbon gasification, however at high temperature there is sufficient energy.

 

References

Mohan, M., May, N., Assaf-Anid, N., Castaldi, M.J., (2005) “Biomass as an Alternative Energy Source: An Illustration of Chemical Engineering Thermodynamic Concepts” Accepted in Chemical Engineering Education, in press

Marco J. Castaldi and John P Dooher (2005) “Investigation into a Catalytically Controlled Reaction Gasifier (CCRG) for Coal to Hydrogen”International Journal of Hydrogen Energy, in review

Castaldi M.J., Jackson,T.L., and Whitmore, N.W., (2006), “The role of carbon deposition on precious metal catalyst activity during dry reforming of biogas”, Catalysis Today, in review


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