September 1, 1995, through August 31, 1996

Project Title: CODISPOSAL OF FBC RESIDUE AND COAL SLURRY SOLID--PREPARATION OF FIELD SITE

ICCI Project Number: 95-1/3.4A-2

Principal Investigator: Gary B. Dreher, Illinois State Geological Survey (ISGS)

Other Investigators: Robert G. Darmody, Robert E. Dunker, University of Illinois; William E. Giles, Steven C. Phifer, Freeman United Coal Mining Co.; John D. Steele, ISGS

Project Manager: Daniel D. Banerjee, ICCI

The objective of this research project is to prepare a field site for testing the hypothesis that plants suitable for use in reclamation of coal slurry impoundments can be grown in mixtures of residue from the fluidized bed combustion (FBC) of coal and coal slurry solid (CSS). CSS is the waste material removed during coal preparation. CSS typically contains principally pyritic, but also other forms of sulfur, which can produce acidic solutions during oxidation, the reaction of these sulfides with oxygen and water. The FBC residue is a highly alkaline material, which, when mixed with coal slurry solid, can neutralize the acidic solution produced by sulfide oxidation, and can slow the oxidation reaction.

A site for the field study was identified in an area of an active coal slurry impoundment at Freeman United Coal Mining Company's Crown III mine. At this impoundment, the discharge point is located at the distal end of the pond during the summer relative to the preparation plant, and at the proximal end during the winter. The study area is located at the distal end of the impoundment. Six grab samples of CSS were collected to screen the area for pyritic sulfur and carbonate carbon content. The area was found to have an excess acid generating potential, which made it a suitable site for the planned field test.

Three rectangular blocks, each containing 18 plots, were designed on the surface of the CSS. A composite sample for chemical characterization was collected from each of the 54 plots. A randomized treatment plan, which defined the set of treatments to be received by each plot, was designed. The amount of FBC residue to be added to each plot was calculated on the basis of the pyritic sulfur and inorganic (carbonate) carbon content in the plot's representative sample and the calcium carbonate equivalent of the FBC residue.

The FBC residue was added to each plot according to a pre-designed treatment schedule, and all plots were tilled to mix the FBC residue into the CSS. Water sampling wells of two- and six-foot depths and tensiometers were installed in randomly selected plots. Water samples were collected and analyzed. Paste pH values were determined for solids from each of the plots that contain wells.

In this research program we are investigating the use of residues from the fluidized bed combustion (FBC) of coal to neutralize acid generated during the oxidation of pyrite in coal slurry solids (CSS), and the ability of mixtures of these two solid wastes to serve as a growth medium for plants used in reclamation. The purpose of this procedure is to provide a method for the reclamation of coal slurry solids that is less expensive than the currently required method of covering the coal slurry solid with a minimum four-foot-thick layer of soil, followed by vegetation of the area. The development of such a reclamation method will provide a means of utilizing FBC residues at coal mines, several of which in Illinois accept FBC residues from coal users as a term of the contract for sale of the coal.

The objective of the present research project is to prepare a site for a field test of the ability of mixtures of FBC residue and CSS to support the germination and growth of three varieties of plants commonly used in reclamation in Illinois. The original test site was to be constructed within and on an impoundment of weathered coal slurry at Freeman United Coal Mining Company's Crown II mine near Farmersville, IL. A berm of coarse coal refuse was to be constructed to enclose an area approximately 150 feet by 200 feet. Within the area, fresh coal slurry was to be pumped and allowed to dewater by gravity drainage and evaporation. The procedure was to be repeated until a sufficiently thick layer of fresh coal slurry solid is deposited within the enclosed area. Following the deposition of the fresh coal slurry solid, fresh FBC residue was to be mixed into the upper six inches. The mixture was then to be allowed to weather for a period to allow time for the calcium oxide in the FBC residue to react with atmospheric carbon dioxide. This reaction would cause the formation of calcium carbonate, a less alkaline substance than calcium oxide. To sow seeds into the calcium oxide would possibly kill the seeds or the new plants.

The original test site was wet through the summer of 1995 and did not dry sufficiently to be used in the planned manner by September. Alternate study sites were sought at the same mining complex, and a new potential site was identified in December. The selected site is located in an actively used coal slurry impoundment at another mine in the complex. During previous summers the outfall of the coal slurry discharge was located to drain into the area selected for the field study, at the distal end of the impoundment, relative to the preparation plant. The level of the sediment in this area is higher than in the immediately surrounding area. During the winter months mine personnel located the discharge pipe closer to the preparation plant. In order to use the selected site, the summer location of the discharge pipe was placed sufficiently far from the study site to prevent disturbance by the discharge for the duration of the study.

A test site consisting of 3 blocks, each containing 18 equally sized rectangular plots, was marked off in the study area on the surface of the coal slurry solid. Samples of material from the top six inches of the coal slurry solid were collected from each corner and the center of each plot. All five portions from a plot were mixed in a plastic bucket, then a portion of each composite sample was saved in a plastic zipper-lock bag for analysis.

Each sample was analyzed for pyritic sulfur and carbonate carbon content. The amount of FBC residue to be added to the coal slurry solid in each plot was calculated from the pyritic sulfur and carbonate contents of the coal slurry solid and the calcium carbonate equivalent of the FBC residue.

Each sample of coal slurry solid was also analyzed by instrumental neutron activation analysis, optical emission spectrography, energy-dispersive spectrometry, and atomic absorption spectrometry for trace element composition and by x-ray fluorescence spectrometry for major and minor element composition.

A randomized treatment schedule was designed in which the treatment received by each plot was defined. The factors included in the treatment schedule are mulch, plant variety, fertilizer, FBC residue treatment, well and tensiometer installation, and temperature probe installation.

The FBC residue was added to each plot and all plots were tilled to mix the FBC residue into the top six inches of the CSS. After addition of the FBC residue, wells were installed in six plots of each block, according to the treatment schedule, at depths of two feet and six feet, to enable sampling of groundwater. Tensiometers were installed in the same plots. Two wells, which serve as background wells, were installed at a location remote from the study area.

Samples of solid for the determination of paste pH were collected from one-inch and six-inch depths at the plots that contain wells. The paste pH values ranged from approximately 6.2 to approximately 12.5, depending on whether FBC residue had been added to the plot.

Water samples were collected immediately after installation of the wells and approximately one week after well installation. The samples were analyzed and found to contain elevated concentrations of calcium (480 mg/L), sulfate (2400 mg/L), and sodium (1500 mg/L). The concentrations of these solutes in the two background wells were similar to those from the wells in the plots.

We plan to allow the study area to weather until October 1996 so that the calcium oxide in the FBC residue can react with atmospheric carbon dioxide and be converted to calcium carbonate. This reaction should result in paste pH values of approximately 8, and should be more favorable to seed germination than the current pH values of 12.