Steam Reforming of Ethanol/Gasoline Mixtures: Deactivation, Regeneration and Stable Performance

Steam reforming of ethanol/gasoline mixtures: deactivation, regeneration and stable performance

Introduction

Increasing concerns about environmental impacts of vehicles with gasoline powered combustion engines have increased efforts to find technologies to reduce emissions and fossil fuel consumption. Recently there has been an increased focus on using renewable sources to generate transportation fuels, such as utilizing agricultural waste to generate ethanol.  Ethanol has a lower vapor pressure relative to gasoline and thus ethanol/gasoline blends such as E85 (85% ethanol) are available rather than 100% ethanol within the current infrastructure.  Thus it is important to understand the reforming capabilities of these mixed transportation fuels.  There has been extensive research on the reforming of pure ethanol, however, literature on reforming ethanol-gasoline blends is limited.  For this study we look at steam reforming of ethanol at less energy intensive conditions: 650°C and steam/carbon ratio of 1.8 with a Rh/Pt catalyst (BASF RM-75ST) washcoated on a ceramic monolith.  At these less energy intensive conditions, however, catalyst deactivation due to either sulfur poisoning or carbon formation is more favored.  The objective of this study is to understand sulfur-induced catalyst deactivation and also potential catalyst regeneration methods in order to develop a process reforming E85 or other sulfur containing liquid fuels at less energy intensive conditions.   

Results and Discussion

The catalyst deactivated after exposure to fuel containing 5 ppm sulfur after 22 hours on stream whereas the catalyst did not deactivate reforming a sulfur-free fuel for at least 110 hours of continuous operation.  Although initially the catalyst was able to achieve equilibrium product distributions with 100% ethanol and gasoline conversion in both the sulfur-free and the 5 ppm sulfur test condition the catalyst operating in the presence of sulfur deactivated to an extent unsuitable for industrial hydrogen production, i.e. < 10 mole% hydrogen production and < 21% ethanol conversion after 100 hours of reforming.  After such significant levels of deactivation exposure to air for one hour was able to restore initial catalyst activity.  Although initial activity (100% ethanol and gasoline conversion and equilibrium hydrogen production) was restored catalyst deactivation was measured after less time on stream then with the fresh catalyst.  Successive air regenerations were also performed after intermediate levels of deactivation as shown in Figure 1. 

These results show that some deactivation was reversible and thus initial activity after air regeneration could be restored; however, an aspect of irreversible deactivation also must have existed which shortened the period of stable activity following the air treatment. Initial XPS results indicate that extended periods of time on stream contribute to irreversible changes in precious metal chemistry which may be attributed to the permanent loss in performance shown in Figure 1. Coupling reactor studies with characterization data it was found that these precious metal chemical changes may be avoided with preemptive regeneration.  Preemptive regenerations extended the period of stable performance during reactor studies.  These results imply that an industrial process could be developed utilizing preemptive regenerations to achieve stable activity while reforming a sulfur containing fuel.  

Significance

In industry, catalyst regenerations are performed more frequently as the catalyst ages, however, the irreversible aspect of the catalyst deactivation is often not understood.  The purpose of this study is to understand the difference in the irreversible and reversible aspects of deactivation in order to optimize an industrial process for reforming sulfur-containing fuels. 

References

1.            Swartz, S.L., Matter, P.H., Arkenberg, G.B., Holcomb, F.H., Josefik, N.M.,  J. Power Sources 188, 515 (2009)