FINAL TECHNICAL REPORT

September 1, 1995, through August 31, 1996

Project Title: DESULFURIZATION OF COAL: ENHANCED SELECTIVITY USING PHASE TRANSFER CATALYSTS

DOE Cooperative Agreement Number: DE-FC22-92PC92521 (Year 4)

ICCI Project Number: 95-1/1.1D-3P

Principal Investigator: Dr. Stephen R. Palmer, SIUC

Other Investigators: Dr. Edwin J. Hippo, SIUC

Project Manager: Dr. Ken Ho, ICCI

ABSTRACT

Due to environmental problems related to the combustion of high sulfur Illinois coal, there continues to be interest in the development of viable pre-combustion desulfurization processes. Recent studies by the authors have obtained very good sulfur removals but the reagents that are used are too expensive. Use of cheaper reagents leads to a loss of desired coal properties.

This study investigated the application of phase transfer catalysts to the selective oxidation of sulfur in coal using air and oxygen as oxidants. The phase transfer catalyst was expected to function as a selectivity moderator by permitting the use of milder reaction conditions than otherwise necessary. This would enhance the sulfur selectivity and help retain the heating value of the coal.

The use of certain coal combustion wastes for desulfurization, and the application of cerium (IV) catalyzed air oxidations for selective sulfur oxidation were also studied. If successful this project would have lead to the rapid development of a commercially viable desulfurization process. This would have significantly improved the marketability of Illinois coal. However, the phase transfer catalysts, the cerium and the scrubber sledge did not catalize the sulfur removal significantly.

U.S. DOE Patent Clearance is NOT required prior to the publication of this document.

EXECUTIVE SUMMARY

Due to environmental problems related to the combustion of high sulfur Illinois coal, there continues to be interest in the development of commercially viable pre-combustion desulfurization processes. These processes would allow the continued use of high sulfur Illinois coal for electricity generation, without the need for post combustion scrubbers.

Recent studies by the authors use a combination of a selective oxidation pretreatment reaction with a subsequent base desulfurization reaction to obtain very good sulfur removals (up to 95%). Unfortunately the cost of the chemical reagents used in the oxidative pretreatment is too high for commercial development and the use of cheaper oxidants results in too much carbon oxidation and hence Btu loss.

Although these studies failed to provide a cost effective desulfurization process, they do point us towards the types of chemical reactions needed for successful desulfurization. It is believed that the key to the success of the two-step process lies in the ability of the oxidant to selectively oxidize the organic sulfur species to their sulfoxides, sulfones or sulfonic acids in the pretreatment step. This selectively weakens the C-S bonds and therefore makes the subsequent removal of sulfur much easier.

The challenge that we face at this time is finding an inexpensive oxidant system that retains the desired sulfur selectivity and preserves the desired properties of the coal such as Btu value. Identifying this inexpensive oxidant system is the primary goal of this study. Specifically the application of phase transfer catalysts to the selective oxidation of sulfur over that of carbon, using air and oxygen as oxidants, is being investigated.

Phase transfer catalysts are substances that have the ability to transport chemical reagents across phase boundaries. This ability promotes chemical reactions in heterogeneous systems that would not otherwise take place or which would take place only very slowly. In this sense they are true catalysts and as such permit the formation of desired products under much milder, and therefore more selective, reaction conditions. It is these features that make phase transfer catalysts so attractive for reactions involving coal. It is anticipated that phase transfer catalysts would act as selectivity moderators in the oxidation of sulfur in coal.

Although the moderation of oxidative selectivity using phase transfer catalysts is the primary area for research in this project, a number of related features are also being examined. These include the potential use of coal combustion residues for the desulfurization step and the application of cerium (IV) catalyzed air oxidation to the selective oxidation of sulfur in coal.

Specific objectives have been identified for the completion of this project. These are:

1. Use physically cleaned coals from the Illinois Basin, to prepare a variety of selectively oxidized coals using air and oxygen under phase transfer catalysis conditions.

2. Test these selectively oxidized coals for enhanced desulfurization under standard desulfurization conditions.

3. Compare these desulfurization results with those obtained for oxidized coals prepared without the phase transfer catalyst.

4. Investigate the applicability of cerium (IV) catalyzed air oxidation for selective oxidation of sulfur in coal, both with and without phase transfer catalysts.

5. Examine the ability of alkaline coal combustion wastes (scrubber wastes and fly ash) to desulfurize selectively oxidized and unoxidized coals.

6. Investigate the kinetics and mechanism of desulfurization under these various conditions by varying the time and temperature of the treatments on coals and model compounds.

7. Measure Btu contents after both oxidation and desulfurization reactions to quantify Btu recoveries after each process.

8. Test selected products for trace element content to determine if these elements can also be removed by the process.

9. Investigate the influence of the amount of phase transfer catalyst on the level of selective oxidation and subsequent desulfurization.

10. Examine the recovery of phase transfer catalysts from the reaction media to see if recycling of the catalyst would be possible.

As far as the authors have been able to determine the application of phase transfer catalysts to the selective oxidation of sulfur in coal had not been investigated before. Similarly, the authors could not find reference to the application of the cerium (IV) catalyzed selective air oxidation of organic sulfur to the study of coal desulfurization. In addition, the use of coal combustion wastes for the precombustion desulfurization of coals appears to be another unique feature of this project.

From the data received, we can see that recovery of the coal oxidation product is high with yields approaching 95-97% in many cases. Unfortunately, if we compare the sulfur contents of the oxidation products we see that there is very little difference between them and the sulfur content of the blank sample (ie. No oxidant, no catalyst). This indicates that no or very little sulfur has been removed during the oxidation process.

The sulfur reductions obtained for all of the oxidized samples after base desulfurization are around 60 to 65%. Thus the use of the phase transfer catalysts and the cerium catalyst, under the oxidation conditions employed, does not appear to have significantly impacted the level of desulfurization obtained. However, it should be noted that the introduction of the cerium catalyst, by itself and in combination with TBAC, did improve the level of desulfurization by around 5%.

Attempts to optimize the level of desulfurization by employing other variables such as increased reaction time (1hr to 24hrs), increased catalyst loading (5% to 20%), introduction a catalyst soak time (1hr to 24hrs), changing the solvent to include methanol and THF, increased oxidant flow rates (200ml/min to 600ml/min), and the introduction of ultrasonic reaction conditions, have not been successful at this time.