FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1997

Project Title: DIRECT REVEGETATION OF COAL SLURRY AFTER AMENDMENT WITH FBC RESIDUES

ICCI Project Number: 96-1/3.4A-1

Principal Investigator: William R. Roy, Illinois State Geological Survey (ISGS)

Other Investigators: Gary B. Dreher, John D. Steele, ISGS; Robert G. Darmody, David Tungate, University of Illinois; William E. Giles, Steven C. Phifer, Freeman United Coal Mining Company

Project Manager: Daniel D. Banerjee, ICCI

ABSTRACT

The overall goal of this research program is to develop a method for the environmentally acceptable reclamation of coal slurry impoundments by the co-disposal of residues from the fluidized bed combustion (FBC) of coal with coal slurry solids (CSS). An active coal slurry basin was prepared for planting. Three randomized test blocks, each with 18 equally sized plots, were designed. Unweathered FBC residue was incorporated at three different dosages into the top 15 cm of the slurry, and was allowed to weather for about five months to allow for the conversion of calcium oxide and hydroxide to calcium carbonate. Three agronomic crops were planted in the fall on the test plots; birdsfoot trefoil, Kentucky-31 tall fescue, and alfalfa. An excelsior blanket was cut and placed as a mulch on randomly selected plots. During the spring, the mulch was removed, and the plots were re-seeded, then re-mulched. Sweet clover was substituted for the birdsfoot trefoil.

Based on short-term (one growing season) observations, mixtures of FBC residue and CSS will support plant growth under field conditions. An increase in application rate of FBC residue, however, resulted in reduced growth. The addition of FBC residue resulted in a decrease in the total concentrations of Si, Al, and K, while the total amount of Ca, Mg, S, and Zn in the mixtures increased. The concentrations of As, B, Cr, Fe, Pb, Na, P, and Se were not significantly changed. The large Ca concentrations in the mixtures may lead to deficiencies of B, Fe, Mn, P, and Zn for optimal plant growth. These plant nutrients may need to be added.

Based on short-term observations, the addition of FBC residue to the coal slurry basin did not result in groundwater contamination. The concentration of sulfate in the shallow groundwater beneath the experimental plots cannot be used as an indicator of pyrite oxidation because the water at the site is supersaturated with respect to gypsum. Because of the long-term nature of pyrite oxidation, there is a need to extend this field study into additional growing seasons. The ability to predict the extent of pyrite oxidation was advanced by revisions in the model PYROXID.

EXECUTIVE SUMMARY

The overall goal of the research program is to develop a method for the environmentally acceptable reclamation of coal slurry impoundments by the co-disposal of residues from the fluidized-bed combustion (FBC) of coal with coal slurry solids (CCS). At capacity, slurry impoundments presently must be reclaimed to prevent the oxidation of pyrite contained in the CSS, and the resulting generation of acidic leachate. In Illinois, the reclamation of slurry impoundments is usually accomplished by covering the de-watered CSS with a minimum of 1.2 m (4 feet) of soil. Fresh FBC residues contain strongly alkaline components which could neutralize the acidity created by the oxidation of pyrite. The objective of the field project is to test the ability of mixtures of FBC residue and CSS to support the germination and continued growth of three species of plants. The plants being tested, sweet clover, tall fescue, and alfalfa, are commonly used in reclamation activities in Illinois.

This research project is essentially the second year of a three-year effort. During the first year, an active coal slurry basin near Farmersville, IL was prepared. Three randomized test blocks, each with 18 equally sized plots, were designed. Unweathered FBC residue was incorporated at three different dosages into the top 15 cm of the slurry, and seeds were planted during the first quarter of the project.

During the third quarter of the project, the mulch was removed, and the plots were re-seeded, then re-mulched. Sweet clover was substituted for birdsfoot trefoil. An early frost in 1996 killed many of the young plants. The alfalfa and tall fescue survived the winter, but the extent of germination was not evenly distributed throughout the plots, and the plants appeared to be weak.

The addition of FBC residues to coal slurry solids reduced the amount of Si, Al, K, and coal in the mixtures. Not surprisingly, the addition of FBC increased the amount of Ca and Mg. The amount of Ca increased by a factor of 2.6 in the 1x plots, and by 4.3 in the 2x plots. The amount of Fe, Na, and P appeared to remain unchanged by the amendments. Most of the S in the coal slurry occurred as either organic forms or as pyrite. Sulfur as sulfate-containing phases accounted for about 20% of the sulfur present. The 2x application of FBC residue increased the total amount of S by less than about 1%, but the proportion of sulfate-S increased to about 50% of the S present. The amount of S as pyrite did not appear to change significantly with application rate.

The addition of FBC residue to the slurry solids did not significantly increase the concentrations of As, B, Cr, Hg, Pb, and Se which could potentially leach into the shallow groundwater. Although there was considerable variation in concentration, the amount of Zn in the mixtures was about 2.4x greater than that in the untreated coal slurry, suggesting that FBC residue contributed Zn to the experimental plots.

Standard soil fertility analyses were performed on samples collected 74 days after FBC residue addition. Aluminum, B, Ca, Mg, Zn, and soluble salts tended to increase with increasing amounts of added FBC residue, while the amount of Cu, Fe, K, extractable P, and soluble S remained relatively unchanged. Sodium and Mn tended to decrease in concentration, possibly because of dilution effects.

The increased levels of S were the result of the S in the both the CSS and FBC residue, and it should not limit plant growth. The extractable S content should decrease as the mixtures age because of the leaching of soluble S, and formation of gypsum as a product of the neutralization of oxidized pyrite. The extractable P content of the mixtures may be insufficient for optimal plant growth. Phosphorous deficiencies are likely to become a problem with direct revegetation unless sufficient P fertilizer is added.

Decreased solubility and possible deficiencies of B, Fe, Mn, P, and Zn may occur because the CSS/FBC mixtures have an excess of Ca. These nutrients might need to be added as fertilizer for optimal plant growth. Sodium should not pose a problem for plant growth if the Na leaches from the root zone before the seeds are sown.

Since the spring planting, the plants grew. After about two months of growth, the greatest yields were associated with the control plots. It should be noted, however, that as pyrite oxidation and neutralization occurs during longer time intervals, the distribution of plant yield may change. It was apparent, however, that mulch is essential for plant growth regardless of plant type and treatment. Of the three plants, the mean yield of the sweet clover exceeded those of alfalfa and fescue.

The concentrations of various constituents in plant tissues appeared to indicate stress in the productive plants. An increase in application rate of FBC residue appeared to be associated with smaller amounts of N, S, P, K, and Fe in the tissues. When compared with alfalfa and fescue, larger amounts of N, S, and P were associated with the clover.

Based on pH, electrical conductivity, B, and sulfate, the chemical composition of the shallow groundwater beneath the plots was not significantly different from that in the control well. A pond sample, however, suggested that the pond is more alkaline and dilute when compared with the shallow groundwater. The concentration of sulfate in the shallow groundwater beneath the plots was not significantly different from that in the control well. All of the shallow groundwater samples collected were supersaturated with respect to gypsum (CaSO4.2H2O). Hence, the concentrations of sulfate in either the experimental plots or the control well are limited by the solubility of gypsum. Therefore, the concentration of sulfate cannot be used as an indicator of pyrite oxidization or neutralization in the experimental plots.

A new subroutine, SATIND was added to the pyrite oxidation model PYROXID. The subroutine corrects for mineral precipitation errors that have been a long-term problem in development of PYROXID. Output data from PYROXID compared favorably with that from the published code MINTEQA2.