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Technical Progress Report -'@2 '3 A62 4-0 49 9. Y6n qfi c)

DOE Grant #DE-FG22-93PC93213 $$, * Reporting Period: 7/1/95 - 9/30/95 ' / ""8 %/ g$.

c;ilj., \/

"Rate Inhibition of Steam Gasification by Adsorbed Hydrogen'' '$7

Dennis J. Miller, Principal Investigator

"US-DOE Patent Clearance is not required prior to the publication of this document"

Work during the eighth quarter of the grant period has focused on expanding the core ma& of gasification experiments with pre-annealed Saran char to include reactions to very low extents of carbon conversion, to relatively high extents of conversion, and using merent gas coqositions than originally planned. In addition, several experiments have been conducted in which gas composition is changed during experiment and the resultant rates measured. Together these experiments help to better explain the role of adsorbed hydrogen in jnhiiithg gasification rate.

Results and Discussion

Several experiments have been conducted to very low extents of Saran char conversion (<0.5%) and the adsorbed hydrogen concentration following reaction has been measured. These reactions were conducted to develop a better understanding of gasification behavior during the early stages of char gasification, where rate is initially high and rapidly decreases. Gasification was carried out at 850°C in a mixture of 40% H20 / 60% H2 at 1.0 Mpa total pressure. Typical rate data for these conditions is given as Figure 1, where it is seen that both CO and CH, formation rates decline sharply at conversions of less than one percent.

Results of adsorbed hydrogen measurements for experiments to low conversion are given in Figure 2. Initial measurements of adsorbed hydrogen concentration at low conversion, shown by the solid circles ('aged" char) in Figure 2, gave quantities of adsorbed hydrogen higher than expected. In order to determine ifthese results were valid, we did several checks of the experiments including measuring adsorbed hydrogen concentration of unreacted Saran char. We found hydrogen adsorbed onto unreacted Saran char in the amount of 10-15 cm3 (STP)/gram. This was surprising, as the Saran char had been pre-annealed to 1500°C to drive off any hydrogen present and then stored in bottles under nitrogen in a dessicator. We then repeated the experiments at low conversion, but before gasification each char sample was re-annealed at . 1500°C to drive off any hydrogen present. The quantity of hydrogen adsorbed following these gasification experiments is given as the solid squares ('Yiesh" char) in Figure 2, and is on average 10-15 cm3/g less than in the original set of experiments. Thus, it is clear that the stored char does pick up hydrogen during storage, perhaps when the bottles are opened to remove a sample for gasification. We are currently re-evaluating other experiments with this "aged" char to determine ifhydrogen uptake during storage gave artijicially high values of adsorbed hydrogen concentration following gasification.

The results of these low conversion experiments suggest that hydrogen adsorbing and blocking reactive sites is the dominant mode of hydrogen jnhiiition during the initial stages of reaction.

Only at very low conversion do we observe both a sharp decline in gasification rate with conversion and a sharp increase in adsorbed hydrogen concentration. At higher extents of conversion, gasification rate levels out or even increases, while hydrogen adsorption also slowly increases. We are continuing to analyze our gasification results at these low conversions to further solidi@ this claim.

Experiments have also been performed to carbon conversions as high as 75%. A typical rate curve for steam gasification (40% H20 / 60% Argon) to low conversion is given in Figure 3; rate increases slowly beyond the conversion range shown in Figure 3 out to 75%. (We did not conti,nuously measure rate over the course of these very long experiments, which took as long as 48 hr of gasification the . ) The corresponding N2 BET Surface areas given in Figure 4 show that an extensive micropore structure develops as conversion proceeds. The quantity of hydrogen adsorbed as a h c t i o n of carbon conversion and gas composition is given in Figure 5. These results clearly show that the quantity of hydrogen adsorbed is at most weakly dependent on composition ofreactant gas and heavily dependent on extent of char conversion. Noting in Figures 1 and 3 that gasification rate in H20/Ar is about three-fold higher than rate in H20/H2, it is clear that adsorbed hydrogen cannot account for the differences in rate between the two gas compositions. On a per unit d c e area basis, both gasification rate and adsorbed hydrogen concentration increase moderately with conversion following the initial rapid changes at low conversion. The fact that hydrogen adsorbs on newly formed d a c e area and that gasification takes place on the same area suggests that, at high conversion, the sites upon which hydrogen adsorb are different than the sites at which gasification takes place.

To further investigate the effects of inhibition, we conducted several experiments in which reactant gas was periodically switched during the course of gasification. Results ofthese experiments show that the gasification rate returns to essentially the same value following a switch fiom H20/Ar to H20/H2 and then back to H20/Ar. This verifies that hydrogen inhibition at high conversions is primarily by "reverse oxygen exchange," which is essentially reduction of the Surface oxygen complexes that. form CO.

Conclusion .

As we continue our gasificationladsorbed hydrogen concentration measurements, an overall picture of the role of hydrogen inhibition in €40/H2 gasification is beginning to emerge. . Gasification rate initially declines very rapidly out to about 1% carbon conversion; adsorbed hydrogen concentration increases rapidly over the same conversion range. We believe that hydrogen adsorbs on active sites initially present on the char, blocking gasification to some extent.

Once a conversion of about 1% is reached, gasification rate levels out and the quantity of hydrogen adsorbed increases much more slowly. Both gasification rate and quantity of adsorbed hydrogen increase slowly (on a unit d c e area basis) out to high conversion (>70%); this suggests that adsorbing hydrogen no longer is able to block gadication sites. One of two possiiilities may be occuring: either hydrogen adsorbs on sites which are not active gasification sites, or an equilibrium between steam and hydrogen is established in the competition for active sites.

In any case, for extents of reaction above 1%, gasification rates at different gas compositions and in experiments in which gas composition is periodically switched clearly show that the presence of gas phase hydrogen has much more of an effect on gasification rate than hydrogen adsorbed on the surface.

Plans for Continued Research

We have expanded the core gasification experiments beyond those originally proposed to better understand the unexpected effect of adsorbed hydrogen on gasification rate. We have now essentially completed those expanded results, and will in the next quarter move on to other primary issues in the grant. These include comparing the results of coal char and demineralized coal char to those obtained for Saran char, studying the effects of treatment with molecular oxygen on the resultant gasification rate and adsorbed hydrogen concentration, and studying hydrogen adsorption in the presence of a gasification catalyst. Our focus in the next quarter will be on gasification of the coal char materials.

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed. or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recorn- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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Figure 1: H20/H2 Gasification Rate of Annealed Saran Char at 8500C and 1.0 MPa Reactant Gas Comp.= 40% H20 / 60% @

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