INTERIM FINAL TECHNICAL REPORT

November 1, 1998, through October 31, 1999

Project Title: VALUE-ADDED PRODUCTS FROM FGD SCRUBBER SLUDGE

ICCI Project Number: 98-1/3.2A-3

Principal Investigator: Vivak M. Malhotra, Southern Illinois University at Carbondale

Other Investigators: Y. Paul Chugh, Southern Illinois University at Carbondale

Project Manager: Dr. Ronald H. Carty, ICCI

ABSTRACT

The overall goal of our project is to develop an economically feasible technology, which will convert flue gas desulfurization (FGD) scrubber sludge, both sulfate-rich and sulfite-rich, into value-added decorative composites which can be sawed, routed, drilled, and glued. The goal is to be accomplished by a three-phase approach. In the first phase of the project, the first year's focus is to develop decorative materials from sulfate-rich scrubber sludge. In our efforts to form materials from sulfate-rich scrubber sludge, we studied how water-to-scrubber sludge (w/s) ratio and the fabrication temperature affected the crystal growth behavior of the sludge under pressure. This was accomplished by conducting scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), and thermomechanical analyzer (TMA) measurements. Our results suggest that the hydration of hemihydrate, generated from the sludge, resulted in the formation of two phases of calcium sulfate in our materials formed at four different temperatures using two different w/s ratio settings. At T < 130^oC, parallelogram-shaped gypsum and rectangular-shaped b-hemihydrate crystals dominated our materials for w/s = 0.2. However, for w/s = 0.6, needle-shaped but orientated crystallites formed in the samples. At T > 130^oC, the samples mainly exhibited hemihydrate phase in the form of fibrous growths formed by the splitting of gypsum crystallites. The samples fabricated at 90^oC exhibited a higher degree of interlocking of the crystallites which imparted better mechanical property to our materials as depicted by the fracture strength. We also conducted systematic fabrication experiments in which we explored the effects of (1) type of polymer impregnation, (2) concentration of the polymer, (3) type of fiber, and (4) length of cure time on the mechanical strength and elastic properties of the composites formed using the procedures developed from pressure-temperature-water content experiments. Once we had optimized various aforementioned variable, we established that it is feasible to form larger sized composites, i.e., 4" X 4" X 0.25", 6" X 6" X 0.25", and 8" X 2" X 1", from CWLP (City Water Light and Power, Springfield, Illinois) scrubber sludge. We believe the technology developed during the first year of the project is conducive for the further scale up of our countertop and decorative materials from sludges.

Pages 1 to 24 contain proprietary information

EXECUTIVE SUMMARY

Objectives: The overall goal of our project is to develop an economically feasible technology, which will convert flue gas desulfurization (FGD) scrubber sludge, both sulfate-rich and sulfite-rich, into value-added decorative composites which can be sawed, routed, drilled, and glued. The goal is to be accomplished by a three-phase approach. In the first phase of the project, the current focus, we are to develop decorative materials from sulfate-rich scrubber sludge. In order to achieve this, the following tasks are planned: (a) design and assemble a high pressure molding facility, (b) develop protocols for the fabrication of decorative materials of 2 inch diameter size, (c) enhance mechanical strength using variables such as type of fibers, mode of mixing, type of polymers, and temperature and pressure of curing, (d) upscale the decorative composites to size 4 inch by 4 inch and 6 inch by 6 inch, and (e) design materials with different patterns, colors, and textures.

Introduction: Flue gas desulfurization (FGD) technology commonly uses sorbents such as CaCO₃ or CaO to scrub SO₂ gas from the flue gases generated by coal burning power plants. Although FGD technology is successful in reducing the SO_x emission, it generates a large quantity of solid residue, called FGD scrubber sludge. FGD residue is generally composed of CaSO₄.2H₂O (gypsum) or CaSO₃.nH₂O, depending upon the FGD technology used. Currently, around 22 million tons of this residue are generated in the United States every year. The disposal of such a large quantity of scrubber sludge is a serious economic problem for the coal utilities.

Results and Conclusions: Three types of experiments were conducted. (1) We further explored how formation temperature, pressure, and water-to-sludge ratio affected the mechanical and structural properties of the hardened material from the CWLP sludge. (2) We established the feasibility of forming 2-inch sized composites with various colored patterns from CWLP scrubber sludge. Then we evaluated whether our technology could be adopted in the scale up of our material to sizes of 4" X 4", 6" X 6", and 8" X 2" without losing our ability of developing different patterns. (3) We systematically explored how the mechanical strength of our colored composites could be enhanced.

Based on the SEM, DSC, FTIR, and TMA experiments conducted on the materials fabricated at 24.2 MPa from sulfate-rich FGD scrubber sludge using four different temperatures and two water-to-scrubber sludge (w/s) ratio settings, we concluded the following:

(a) Mainly two phases of calcium sulfate, i.e., gypsum (CaSO₄.2H₂O) and hemihydrate (CaSO₄.0.5H₂O), were present in materials formed at high pressure. For samples fabricated at T < 130^oC, gypsum phase was the dominant phase, while the materials formed at T > 130^oC mainly contained hemihydrate.

(b) Water-to-scrubber (w/s) sludge ratio affected the crystal growth habits of the gypsum in our samples. For w/s ratio of 0.2, largely parallelogram shaped gypsum crystals were formed, while for w/s ratio of 0.6, gypsum crystallites exhibited needle-like shapes. The needle-shaped crystals showed orientational behavior within the material.

(c) The crystal growth rate was found to accelerate at higher temperatures (T < 130^oC) leading to the formation of interlocking needles of gypsum crystallites. This interlocking of the crystallites led to the higher fracture strength observed in the case of the materials fabricated at 90^oC as compared with those fabricated at 30^oC. The abrupt fall in the fracture strength observed in the case of the samples fabricated above 130^oC could be due to the dominance of hemihydrate phase in them.

(d) At 30^oC, the sample fabricated using w/s = 0.6 did exhibit slightly higher fracture strength as compared with the one fabricated with w/s = 0.2. However, the TMA results clearly showed that if the samples were fabricated at slightly higher temperatures of around 90^oC, then the materials have a greater than or equal fracture strengths irrespective of the w/s ratio used.

We also conducted systematic fabrication experiments in which we explored the effects of (1) type of polymer impregnation, (2) concentration of the polymer, (3) type of fiber, and (4) length of cure time, on the mechanical strength and elastic properties of the composites formed using the procedures developed from pressure-temperature-water content experiments. How various types of polymer affect the flexural strength of the materials formed from sludge is shown in figure 1. Once we had optimized the various aforementioned variable, we established that it is feasible to form larger sized composites, i.e., 4" X 4" X 0.25", 6" X 6" X 0.25", and 8" X 2" X 1", from CWLP scrubber sludge. This is demonstrated in figures 2 and 3. We believe the technology developed during the first year of the project is conducive for further scale up of our countertop and decorative materials from sludges.

The remainder of this report contains proprietary information and is not available for distribution except to the sponsor of this project.