FINAL TECHNICAL REPORT

September 1, 1996, through November 30, 1997

Project Title: ORGANIC SULFUR AND HAP REMOVAL FROM COAL WITH SUBCRITICAL WATER

ICCI Project Number: 96-1/1.1C-4

Principal Investigator: Mr. Chris M. Anderson, UNDEERC

Other Investigators: Mr. Ronald C. Timpe, UNDEERC

Project Manager: Dr. Ken Ho, ICCI

ABSTRACT

Economically feasible organic sulfur and hazardous air pollutant (HAP) precursor removal processes have not been developed, and an effective sulfur and selected HAP removal process is needed to enhance the utilization of high-sulfur coals and to comply with increasingly stringent regulations. Energy & Environmental Research Center (EERC) researchers have developed a process using subcritical water to remove organic sulfur and selected HAPs from Illinois Basin Coal Sample Bank coals. This technology differs from the previous methods, since it processes the coal at lower temperatures using environmentally friendly water solvent. Using a bench-scale system, the EERC reduced the sulfur level by 75% and mercury by 90% by hydrothermal treatment. Based on these results, the EERC has been working with ICCI for the past 3 years to integrate this technology with advanced physical cleaning and scaleup of the subcritical water extraction technology.

The third year of this project was spent integrating packed-column flotation with continuous slurry hydrothermal treatment. Several hundred pounds of product coal slurry from a 4-in. packed column was treated at conditions near the critical temperature (<374C) and subcritical pressure (<3200 psia) of water using a 3/4-in.-ID tubular reactor system. Results indicate that physical cleaning reduced the pyritic sulfur in Illinois No. 6 coal from 3.4% to 2.7% and the organic sulfur content from 2.2% to 2.0%. Hydrothermal processing at subcritical conditions in the tubular reactor reduced the organic sulfur by 25%, from 2.0% to 1.5%. Differences in process results between bench scale and pilot scale were attributed primarily to changes in operation mode from batch to continuous and reduced temperature conditions. The EERC completed limited testing with the tubular reactor configured in the batch mode to process the physically cleaned coal with water at higher temperatures. In addition, the EERC added an on-line catalyst module system to the process, which desulfurized the tars. In bench-scale testing, cobalt-molybdenum catalyst reduced the sulfur content of tar from 1.60 to 0.11 wt% in an aqueous environment, whereas the tar sulfur was reduced from 2.4% to 1.4% by the catalyst in the pilot scale-testing. The desulfurized tars were added back to the solids to increase the fuel recovery by 7 wt% (26% increase in volatile matter) to give a total fuel recovery of 85% with only 3.5 wt% ash on a moisture-free basis. Approximately one-third of the volatile matter was water-soluble or converted to gas. Mercury content was reduced from 0.319 g/g in the raw coal to 0.166 g/g in the deep-cleaned coal, while chlorine was reduced from 950 to < 50 ug/g.

Work was interrupted by the flood of the Red River in Grand Forks, North Dakota, which caused significant damage to the entire city, including the research facilities at the EERC. The EERC requested and was granted a 3-month no-cost project extension for the 1996-97 program to allow EERC researchers ample time to complete the project and put their private and professional lives back together.

EXECUTIVE SUMMARY

The primary objective of this project is to develop a continuous processing method for removing organic sulfur along with chlorine and selected hazardous air pollutants (HAPs) from physically cleaned coal with subcritical water techniques in combination with effective thermal and/or chemical treatment, if required. The goal is to produce a coal that will emit 1.2 lb of SO2/MMBtu or less upon combustion from coals supplied by the Illinois Basin Coal Sample Bank. The physical cleaning method selected for the Illinois Clean Coal Institute (ICCI) program development was packed-column flotation using a 4-in. packed-column cell.

Year 3 objectives were to treat 500 lb of physically cleaned Illinois coal samples using the integrated hydrothermal system at subcritical conditions. The test coals for the program were generated using a 4-in.-diameter packed-column flotation cell developed by West Virginia University and tested at Southern Illinois University (SIU). Flotation experiments were completed at the Energy & Environmental Research Center (EERC) using conditions previously determined by SIU and others for Illinois coals. On obtaining satisfactory operation of the column, more than 600 lb of physically cleaned coal was prepared for continuous testing on the hydrothermal process development unit.

Previous efforts by the EERC for deep cleaning physically cleaned Illinois bituminous coals have indicated that subcritical extraction is an effective technique to reduce sulfur as well as HAPs. Batch extractions were performed at pressure and temperatures up to supercritical water conditions under constant fluid flow (dynamic) conditions. Analysis performed on cleaned samples indicated that sulfur levels can be lowered to less than 0.8 wt% sulfur, with over 90% reduction in mercury content.

Using laboratory- and bench-scale systems, the EERC refined the technology to process physically cleaned coal on a continuous basis at the elevated temperature conditions. The EERC enhanced the heat exchangers and high-pressure slurry pump to maintain high residence times. The additional heat allows researchers to investigate, with continuous flow, conditions that are closer to the critical point of water (374C), where extraction properties reach an optimum point. The EERC also modified the pilot-scale system to enhance product quality and recoveries by cleaning the organic material present in the process water using a specially designed catalyst modular system. Several commercial catalysts were tested, with special emphasis on potential poisoning, efficiency, selectivity, and catalyst attrition.

Work at the EERC was interrupted by the spring flood of the Red River of the North in Grand Forks, North Dakota, which was of a magnitude never before witnessed in the region. The entire city, including the research facilities at the University of North Dakota EERC, was flooded. The EERC, along with the remainder of the University, was forced to close because of water in the bottom floors of the buildings causing loss of power, water and sewer, and equipment. Technical work at the EERC was not possible between April 17 and May 8, 1997. From May 8, 1997, forward, the premises were cleaned, electricity was restored, repairs were made, and equipment replacements were begun. As equipment was put back into service, project activity gradually increased. Fortunately, the subcritical extraction equipment used in the ICCI project was not seriously damaged and was prepared for testing as operators became available. As time progressed, personnel dealt with
flooded homes, limited office space, and lack of equipment at work and the mental stress brought on by both.

Following flood recovery, shakedown testing of the continuous hydrothermal system to determine operating parameters, including solids loading in the slurried feed, was carried out. Process residence times were increased to over 1 hour by using a Bran-Lubbe high-pressure pump system and adding a high-pressure reactor. Additional heat was applied with a superheater downstream from a series of three Dowtherm preheaters. Initial plugging problems caused by settling of the coal were corrected by increasing the solids loading from < 10% to >30%. The EERC treated over 200 lb of cleaned coal at near-critical temperature and pressure using the continuous slurry method.

Product coal slurry from a 4-in. packed column was treated at conditions near the critical temperature (<374C) and subcritical pressure (<3200 psia) of water using a 3/4-in.-ID tubular reactor system. Results indicate that physical cleaning reduced the sulfur in Illinois No. 6 coal from 3.4% to 2.6%, and the hydrothermal treatment in the continuous mode reduced the sulfur content from 2.6% to 2.3%. Hydrothermal processing of a second Illinois bituminous coal at subcritical conditions in the batch mode reduced the organic sulfur by 25%, from 2.0% to 1.5%, whereas bench-scale results had given sulfur values as low as 0.8% in the recovered coal. Differences in process results between batch bench and pilot scale and continuous pilot scale were attributed primarily to changes in operation mode from batch to continuous and reduced temperature conditions. The EERC completed limited testing with the tubular reactor configured in the batch mode to process the physically cleaned coal with water at higher temperatures. In addition, the EERC added an on-line catalyst module system to the process, which desulfurized the tars. In bench-scale testing, cobalt-molybdenum catalyst reduced the sulfur content of tar from 1.60 to 0.11 wt% in an aqueous environment, whereas the tar sulfur was reduced from 2.4% to 1.4% by the catalyst in the pilot-scale testing. The desulfurized tars were added back to the solids to increase fuel recovery by 7 wt% (26% increase in volatile matter) to give a total fuel recovery of 85% with only 3.5 wt% ash on a moisture-free basis. Approximately one-third of the volatile matter was water-soluble or converted to gas and, therefore, in these tests was not recovered in the condensed-phase fuel.

Additional testing at extended residence times and with additives had minimal effect on sulfur content. This indicates that temperature and mode of operation are primary factors in controlling the removal of sulfur from coal by extraction with water. Continuous operation includes excessive close contact between solids and sulfur during the processing, resulting in lack of sulfur removal. The batch process offers less long-term close contact and provides a tar-water stream that can be treated by on-line catalytic desulfurization. Near the end of the contract, the EERC completed limited testing with the tubular reactor configured in batch mode to process the physically cleaned coal with water at higher temperatures and reduced coal-sulfur extraction contact time. Although less than 50 lb was processed with the new system, early results indicated organic sulfur removal to be 33% better than the continuous mode. In addition, the EERC added to the process an on-line catalyst module system that desulfurized the aromatic tars. In bench-scale testing, a Co-Mo catalyst reduced the sulfur content of tar from 1.60 to 0.11 wt% in an aqueous environment. On-line pilot-scale testing reduced tar sulfur content from 2.4 wt% to 1.4 wt%, a 42% reduction. The desulfurized tars were added back to the solids to increase the fuel recovery by 7% and volatile matter by 26%. More fundamental and pilot testing is required to verify conditions and results from batch mode extraction. Mercury content was reduced from 0.319 g/g in the raw coal to 0.166 g/g. in the deep-cleaned coal, while chlorine was reduced from 950 to < 50 g/g. Previous tests with IBC-101 and IBC-102 have shown Hg reduction of 58% and 99%, respectively, indicating that Hg removal is effective by hydrothermal treatment and is dependent on the disposition of the Hg. Chlorine was reduced from 950 g/g in the feed, to 710 g/g in the physically cleaned coal, to <50 g/g in the hydrothermally treated product.

Accepting that the process still needs more fundamental and process engineering before being ready for demonstration at the commercial level, the EERC is confident in asserting that the technology of this project has led to several significant achievements. It was demonstrated that: