FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1997

Project Title: UTILIZATION OF LIGHTWEIGHT AGGREGATES MADE FROM COAL GASIFICATION SLAGS

ICCI Project Number: 95-05 A1

Principal Investigator: Vas Choudhry, Praxis Engineers, Inc.

Project Manager: Richard R. Shockley, ICCI

ABSTRACT

The integrated gasification combined-cycle (IGCC) coal conversion process has been demonstrated to be a clean, efficient, and environmentally acceptable method of generating power using high sulfur Illinois coals. However, it generates a solid waste consisting of vitrified mineral matter (slag) and small quantities of unburned carbon. A 400-MW power plant using 4,000 tons of 10% ash coal per day may generate over 440 tons/day of slag, whose disposal represents significant costs. As coal gasification technologies find increasing commercial applications for power generation or production of chemical feedstocks, it becomes imperative that slag utilization methods be developed and commercialized in order to minimize the need for disposal. Praxis Engineers, Inc. is working on a DOE-funded project with co-funding provided by the Electric Power Research Institute (EPRI) and the Illinois Clean Coal Institute (ICCI), along with considerable industry involvement, to demonstrate the technical and economic feasibility of commercial production of slag-based lightweight aggregates (SLA) and ultra-lightweight aggregates (ULWAs). The economic incentive for developing this technology is provided by the fact that conventional lightweight aggregates (LWAs) made from expansible clays sell for $40/ton and ULWAs made from expansible perlite sell for $150/ton.

In Phase I of the project, over 10 tons of SLA with unit weights varying between 20 and 50 lb/ft³ were produced using a direct-fired rotary kiln and a fluidized bed calciner. In addition, slag and expansive clays were extruded in blended form and used to produce expanded lightweight aggregates. This demonstrated that slag could be used in the existing plants as a partial substitute for expansible clays or shales. It was concluded that slag may be used to produce SLA which replicates conventional LWA as well as a near-ultra-lightweight material which may be used as a substitute for some applications of expanded perlite. In the current reporting period (Phase II), various samples of SLA that were produced earlier were evaluated as substitutes for conventional LWAs to make products including lightweight structural concrete, roof tiles, blocks, insulating concrete.

A preliminary economic analysis indicates that end-use products made from SLA would have considerably lower production costs due to the absence of raw material mining costs, the significantly lower temperature of expansion for slag (1400-1600^oF vs. 1800-2000^oF for conventional clays) and a possible credit for the avoided cost of slag disposal. Thus, SLA technology provides a good opportunity for developing value-added products from slag.

EXECUTIVE SUMMARY

This document represents a summary of work done during the reporting period between September 1, 1996, through August 31, 1997, for the project entitled "Utilization of Lightweight Materials Made from Coal Gasification Slags." The project consists of two phases. In Phase I, a 20-ton sample of slag (primary slag) was collected, processed for char removal, and pyroprocessed to produce expanded slag aggregates of various size gradations and unit weights, ranging from 12 to 50 lb/ft³. In Phase II, samples of the slag-based lightweight aggregates (SLA) are being tested for their suitability in manufacturing precast concrete products (e.g., masonry blocks and roof tiles) and insulating concrete, first at the laboratory scale and subsequently in commercial manufacturing plants. These products were evaluated using ASTM and industry test methods. Technical data generated during production and testing of the products are being used to assess the overall technical viability of expanded slag production. Relevant cost data for physical and pyroprocessing of slag to produce expanded slag aggregates were gathered for comparison with (i) the management and disposal costs for slag or similar wastes and (ii) production costs for the conventional materials which the SLA would replace. In addition, a market assessment was made to evaluate the economic viability of these utilization technologies. The major accomplishments for the current reporting period are summarized below.

Laboratory Testing of Slag III from a Gasifier Using an Illinois Basin Coal

A laboratory-scale study was performed to determine the feasibility of producing lightweight aggregates from a slag sample, termed Slag III, generated from an Illinois Basin coal. The study confirmed that a lightweight aggregate product with a unit weight of 40-50 lb/ft³ can be produced in both discrete particle form and in pelletized form after blending with 50% clay. A fired product with a low unit weight of 22.1 lb/ft³ could also be produced from Slag III particles at a lab furnace temperature of 1600F. It was concluded that, as is the case with other slag samples tested in the project, recovery of char is important before pyroprocessing this slag. Based on the laboratory test results, it was decided to procure a large sample for char removal and LWA production at the pilot scale. Char removal was completed successfully and LWA production in the pilot plant is scheduled for October 1997.

Testing of Expanded Slag for Production of Lightweight Blocks

The objective of this task is to use commercial-scale concrete block manufacturing equipment and techniques to produce blocks using expanded slag. This work is being done at the facilities of Harvey Cement Products, Inc., a major block manufacturer and distributor in the greater Chicago area. Harvey Cement was selected as they are located close to the recently commissioned Wabash River IGCC plant which is a potential long-term source of slag and hence slag-based LWA.

The mix designs used for laboratory testing was identical to that currently used at the plant. For lightweight blocks, the cement-to-aggregate ratio was 1:6.6, and for regular blocks it was 1:8.7. Water was added on an as-required basis depending on the overall workability of the aggregates and the cement paste in the mix. Test specimens (2" x 4" cylinders) were made from the concrete and stored in a curing chamber used to cure commercial blocks. A total of nine blocks were made for each batch, which allowed three blocks per compression test. These tests were conducted after 3, 7, and 28 days of curing. For the last batch, only six specimens were made, which were tested after 3 and 28 days of curing. The compression test results indicate that at the 1:8.7 and 1:6.6 cement-to-aggregate ratios, the 28-day strength was below the ASTM requirement of 2000 psi for load-bearing blocks. These strength values may be increased by adding a higher proportion of cement to the mix or by increasing the slag sand or limestone sand content. This work is in progress.

Testing of Expanded Slag for Production of Structural Concrete

Testing of expanded slag for the structural concrete application has been completed. The results indicate that structural concrete can be made to meet the ASTM compressive strength requirement of 2500-4000 psi at the corresponding unit weight of 105-115 lb/ft³ using 50/50 SLA (i.e., a pelletized expanded product made from 50/50 slag/clay). Tests conducted with 50/50 slag/clay aggregates using 6.0 sacks of cement/yd³ concrete resulted in 7- and 28-day concrete strength measurements of 2910 and 4210 psi respectively. The 28-day value exceeds the ASTM requirement of 4000 psi at a unit weight of 115 lb/ft³. These results were far superior to those of tests done using SLA (3/8" combined) at the same cement level, as well as to those for a control test using commercially manufactured aggregates which had a strength value of 3400 psi.

Tests conducted at a higher cement level (6.5 sacks/yd³ concrete) resulted in compressive strengths of 3480 and 4380 psi for the 7- and 28-day curing periods, respectively at a unit weight below 115 lb/ft³. Strengths for control tests using clay LWA were 4040 and 5100 psi, which are in a comparable range. The 28-day compressive strength of concrete made from commercial LWA at the 5/8" size designation was 3440 psi, which is lower than that of concrete made from the expanded slag/clay-based aggregate. These data indicate that blending slag and clay results in a high-quality product. Freeze/thaw tests are being conducted as part of this evaluation.

Evaluation of Expanded Slag for Horticultural Applications

Expanded slag was tested and evaluated for a number of horticultural/nursery applications by a major nursery in Tennessee. This involved preparation of various potting mixes measuring approximately 1 yd³ using expanded slag as a partial or total substitute for perlite and vermiculite which were added to improve porosity and water retention.

Three potting mix batches were formulated: (1) a control sample without expanded slag, (2) a sample where expanded slag partially replaces the perlite and vermiculite, and (3) a sample in which the slag completely replaces these materials. Each batch also contained 4 lb fertilizer (14/14/14) and 8 lb lime. Evaluation consisted of observation of the growth rate, general health, and appearance of fuchsia plants grown in a solarium. The performance of Batches 1 and 2 was identical in terms of plant growth rate, general health, and appearance. However, the mix for Batch 3 did not provide a good growing medium possibly due to its high water retention capacity. Also, the slag made the pots unacceptably heavy for 12-inch hanging baskets. It was concluded that expanded slag can be used as a partial substitute for perlite and vermiculite. The following general comments and conclusions were made by the nursery:

• The Batch 3 mix design is not suitable for expanded slag and needs to be modified based on Batch 2.
• Since slag has a good water retention property, more attention needs to be paid to understanding its water release rate requirements.
• Because expanded slag is heavier than perlite and vermiculite, it may be more suitable for use in pots larger than 3 gallons in size and in large containers used for growing tall plants where it would help improve stability.
  
  

Economic Analysis of Expanded Slag Production

The costs of producing SLA were developed both with and without including a credit for the avoided cost of slag disposal. These costs were then compared with those for conventional LWAs and ULWAs. SLA production costs include capital costs for a modern facility located at the gasifier site capable of handling approximately 440 tons per day of slag generated by a 400-MW integrated gasification combined-cycle (IGCC) facility. Operations include processing the raw slag for char removal and kiln processing of the slag to produce LWAs from the +50 mesh slag and pelletized LWAs from the minus 50-mesh fines after mixing with an expansible clay binder and extruding. Fifty percent of the energy required for pyroprocessing is assumed to be supplied by the char recovered from the slag, with the remaining 50% supplied by coal, which is easily available on site. The economic analysis was carried out for two different pyroprocesses, i.e., a rotary kiln and a fluidized bed expansion system. A 15% contingency is allowed for capital costs, and the facility is depreciated over 20 years. Necessary utilities are assumed to be available at prevailing market rates at the gasifier site. Production costs for the rotary kiln and the fluidized bed processes to $24.40 and $21.87 per ton of product, respectively. These costs compare very favorably with current LWA production costs of about $30/ton. When these numbers are modified to reflect the avoided costs of slag disposal, the economics of SLA production become even more attractive.

NOTE: DOE extended their portion of this study through June 1999. Praxis' final report to DOE will be included in the ICCI 1999 Annual Report.