FINAL TECHNICAL REPORT

September 1, 1995, through August 31, 1996

Project Title: WASHABILITY OF AIR TOXICS IN ILLINOIS COALS AS APPLIED TO GRAVITY-BASED CLEANING

ICCI Project Number: 95-1/1.2C-1

Principal Investigator: Ilham Demir, Illinois State Geological Survey (ISGS)

Other Investigators: Rodney R. Ruch, John M. Lytle, John D. Steele, Chris C. Rohl, each at the ISGS

Project Manager: Ken K. Ho, ICCI

ABSTRACT

Advanced gravity-based cleaning can potentially reduce the contents of ash and air toxics, including sulfur (S) and trace and minor elements, in finely ground coals to levels predicted by laboratory float-sink (F-S) tests. Therefore, the goal of this project was to generate F-S washability data on ash, S, and 16 trace and minor elements (As, Be, Cd, Cl, Co, Cr, F, Hg, Mn, Ni, P, Pb, Sb, Se, Th, U - all identified as hazardous air pollutants (HAPs) by the Clean Air Act Amendments of 1990) in marketed Illinois coals. The data will assist in developing strategies to prevent or reduce the adverse effects of pending environmental regulations on the use of Illinois coals in power generation.

Samples of 29 marketed coal products from Illinois mines that were in operation during 1995 were ground to -100 mesh and separated into their specific gravity fractions (<1.3, 1.3-1.4, 1.4-1.6, >1.6) using a centrifugal F-S method. Also, representative splits of 10 samples selected among the 29 samples were ground to -200 mesh and subjected to similar F-S separation tests. Using the moisture and ash data on the F-S fractions, a composite sample of 80%-combustibles recovery was prepared for each F-S test.

For the twenty-nine -100 mesh samples, the ash content in the composite samples decreased (relative to parent coals) by 37% to 75%. Average decreases for As(63%), Cd(73%), Hg(70%), Mn(70%), and P(58%) exceeded the average decrease for ash (53%). Average decreases for other elements were: Co(33%), Cr(19%), F(35%), Ni(30%), Pb(45%), S(27%), Sb(18%), Th(28%), and U(12%). Only Be was enriched (on average, 118%) in composite samples relative to the original coals. However, the additional environmental risk from Be enrichment during fine-coal cleaning could be insignificant because this element has small initial concentration in the coals and relatively low atmospheric mobility during coal combustion. Grinding the samples to -200 mesh generally did not result in significant improvement in the F-S separation of ash, S, and HAPs, beyond the separations obtained at -100 mesh. Thus, grinding marketed Illinois coals to -100 mesh would be sufficient for significantly improving the quality of these coals through the use of advanced gravity-based cleaning techniques.

The F-S data of this study showed that the ultimate cleanability of Illinois coals is substantially greater than that indicated by release analysis data of a previous study.

EXECUTIVE SUMMARY

The Clean Air Act Amendments (CAAA) of 1990 require that emissions of 189 hazardous air pollutants (HAPs) and sulfur dioxide into the atmosphere from various human sources be reduced. Among the 189 HAPs, there are 16 elements (As, Be, Cd, Cl, Co, Cr, F, Hg, Mn, Ni, P, Pb, Sb, Se, Th, U ) that occur in coal and that can potentially be emitted into the atmosphere as a result of coal combustion. Electrical utilities are presently exempt from having to comply with the CAAA requirements on HAPs. However, this may change after the U.S. EPA completes its risk analyses and promulgates emission standards.

We previously determined contents of ash, sulfur (S), and HAPs in marketed coal samples from operating mines in Illinois and compiled existing data on channel samples¹. In another project, we investigated froth flotation cleanabilities of ash and HAPs in the same coals using a froth flotation / release analysis (FF/RA) procedure². The FF/RA data can be used to estimate the performance of the coals in standard column or froth flotation circuits. However, FF/RA data generally do not indicate maximum, theoretical cleanability of finely ground coal^3,4. Float-sink (F-S) washability tests appear to be more suitable than the FF/RA for determining the maximum cleanability of a coal sample at a given particle size. The purpose of this project, therefore, was to generate F-S washability data on ash, S, and HAPs in marketed coal products from all operating Illinois mines and to compare the results with the FF/RA data obtained previously.

New gravity-based cleaning technologies or technologies that combine gravity separation and column flotation potentially can achieve the same levels of washability efficiency predicted by results from F-S separation of finely ground coal in the laboratory. The F-S washability data produced by this project can then be used to evaluate the efficiency of physical fine-coal cleaning, especially advanced gravity-based cleaning, for reducing ash, S, and HAPs contents in marketed coal products from operating Illinois mines. The marketed coal products are currently ground to 70% -200 mesh size for utilization in PC boilers which comprise over 90% of the combustion boilers used in Illinois⁵. Therefore, because of no extra cost for grinding, an advanced cleaning strategy to produce compliance coal from Illinois coals could be economical.

For this project, samples of 29 marketed Illinois coals were ground to -100 mesh and then separated into their specific gravity fractions (<1.3, 1.3-1.4, 1.4-1.6, >1.6) using a centrifugal F-S method. In addition, representative splits of 10 samples selected among the 29 samples were subjected to similar F-S separation at -200 mesh size. For a given test, the first two and sometimes three lightest F-S fractions were combined in proportion to their moisture and ash contents to obtain a clean composite sample with the lowest possible ash content at an 80%-combustibles recovery.

The result of the F-S tests indicated that ash and S contents of marketed Illinois coals can potentially be reduced significantly, beyond conventional cleaning, through the use of advanced physical fine-coal cleaning. For the 29 samples ground to -100 mesh, the decrease in the ash content of the clean composite samples relative to the parent coals ranged from 37% to 75% with an average decrease of 53%. As a result, clean composite samples having ash contents of 2.8 to 3.9% were produced from 10 of the 29 samples. For the remaining composite samples, the ash content ranged from 4.0 to 5.3 % for 12 samples and 5.6% to 7.9% for 7 samples. The S content of the 29 composite samples varied between 0.6 and 3.5%, representing a decrease of a 8% to 47% with an average of 27%. The S remaining in the clean composite samples was overwhelmingly organic S.

Among the decrease values for HAPs, the average decreases for As(63%), Cd(73%), Hg(70%), Mn(70%), and P(58%) exceeded the average decrease for ash, indicating that these five elements were somewhat enriched in relatively coarse mineral grains that were effectively removed. Precombustion removal of As, Cd, and Hg from coal is important because of the relatively high atmospheric mobility of these elements during coal combustion. The average decreases for Pb(45%), Se(38%), and F(35%), three other elements with relatively significant atmospheric mobility, were smaller than the decrease for ash but still significant. The average decreases for other HAPs were: Co(33%), Cr(19%), Ni(30%), Sb(18%), Th (28%), and U(12%). Although Sb can be mobilized to a relatively significant degree by attachment to air-borne fly ash particles, small Sb concentrations (generally <1.5 mg/kg) in marketed Illinois coals could limit any environmental risk associated with Sb emission from combustion of these coals. Only Be was generally enriched in the -100 mesh washability products (on average, by 118%) when compared with the parent coals, suggesting that Be was associated with the coal macerals. Because some of the Be stays with bottom ash and slag during coal combustion, and because the Be content of marketed Illinois coals is usually <1.5 mg/kg, additional environmental risk due to the enrichment of this element could be very small or insignificant. Because a Cl-containing heavy medium was used in the F-S tests, the Cl analysis was compromised for this study. Further improvement in the F-S separation of ash, S, and HAPs made by grinding 10 of the samples to -200 mesh was generally insignificant. Thus, grinding the coals to -100 mesh would be sufficient for significantly improving their quality through the use of advanced gravity-based cleaning techniques.

The F-S process used in this study generally removed significantly more ash and HAPs than the previously used² FF/RA process. Therefore, the F-S tests can estimate the ultimate cleanabilities of marketed Illinois coals better than the FF/RA tests.

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¹Demir, I., Harvey R.D., Ruch, R.R., Damberger, H.H., Chaven, C., Steele, J.D., and Frankie, W.T., 1994, Characterization of Available (Marketed) Coals From Illinois Mines. Illinois State Geological Survey, Open File Series 1994-2, 26 p.

²Demir, I., Ruch, R.R., Harvey, R.D., Steele, J.D., and Khan, S., 1995, Washability of Trace Elements in Product Coals from Illinois Mines. Illinois State Geological Survey, Open File Series 1995-8, 22 p.

³Straszheim, W.E. and Markuszewski, R., 1989, Association of mineral matter with the organic coal matrix, Preprints ACS Div. Fuel Chem., v. 34(3), p. 648-655.

⁴Forrest, W.R., Adel, G.T., and Yoon, R.H., 1994, Characterizing coal flotation performance using release analysis. Coal Prep., v. 14(1-2), p. 13-27.

⁵Honaker, R.Q., Paul, B.C., and Wang, D., 1994, Advanced physical coal cleaning to comply with potential air toxic regulations. Quarterly Technical Report (9/1-11/30/1994) to the Illinois Clean Coal Institute.