Solid Waste/Byproducts of Gasification


A 300-MWe integrated gasification combined cycle (IGCC) power plant using 2,500 tons of 10% ash coal per day may generate 250 tons/day of slag or bottom ash, the disposal of which represents a significant operating cost. Commercial application of coal gasification technologies can be greatly enhanced if the solid byproduct can be utilized, rather than disposed of in a landfill. Gasification slag is similar to the material produced in wet-bottom PC plants and has as good or better leachability characteristics. It also has low bulk density, high shear strength, good drainage and filtering characteristics. Unfortunately, due to the relatively small quantities of boiler slag produced in the U.S. relative to fly ash and FGD material1, the markets for this type of material are not yet fully developed. There is also relatively little experience using coal gasification slag.

This section reviews the potential markets for utilizing slag material generated by IGCC power plants, examines the utilization experience for slag produced by wet-bottom pulverized coal (PC) plants, and discusses some of the limited utilization experience associated with currently operating IGCC plants.

Potential Markets for Gasifier Slag
Current large-volume markets for slag can mainly be found in those states that make use of wet bottom boilers, such as Ohio, New York, Illinois and Indiana. In the areas where slag is produced, it is utilized to a high extent. U.S. utilization of slag from coal-fired boilers was estimated to be about 84% in 2012 according to the American Coal Ash Association (ACAA).

Identified markets for IGCC slag include:

  • Construction structural backfill
  • Asphalt paving aggregate - hot mix and seal coat aggregate
  • Portland cement aggregate/additive; cement kiln feed
  • Asphalt shingle roofing granules
  • Pipe bedding material
  • Blasting grit or other abrasive application
  • Polishing media
  • Snow and ice control
  • Mineral filler
  • Landscaping
  • Road drainage media
  • Water filtering medium
  • Water-jet cutting - a new application for boiler slag
  • Building materials: Slag lightweight aggregate (SLA) and ultra-lightweight aggregate (ULWA)

Referring to Table 1, the use of boiler slag as blasting grit and roofing granules was its chief market, with structural fill also accounting for a significant fraction. From this it is reasonable to make the extrapolation that production of lightweight aggregates from slag, used to make roof tile, lightweight block, and structural concrete, appears to represent an excellent opportunity to develop a high-value market for IGCC slag.

Slag Lightweight Aggregates Utilization Project
A project funded by the DOE, the Electric Power Research Institute (EPRI), and the Illinois Clean Coal Institute (ICCI), along with considerable industry involvement, has demonstrated the technical and economic feasibility of commercial production and utilization of slag lightweight aggregates (SLA) and ultra-lightweight aggregates (ULWAs)2 having densities between 15 and 50 lb/standard cubic feet (scf). This project attempted to use all size components of the slag, including fines (the smallest bits) through binding with clay.

A sample from an Illinois basin coal slag generated at the Wabash River IGCC plant was the basis for evaluation in the project. For aggregate purposes, the presence of char is detrimental, so the char was removed. Rather than wasting it, the recovered char was upgraded from 50 to 70% carbon (by ash removal), which is suitable for use in kilns or a fluidized bed fuel. Specifically, it could substitute for 50% of rotary kiln fuel or 80% of the fuel in a fluidized bed.

The use of slag as aggregate for building and other purposes was tested according to American Society for Testing and Materials (ASTM) standards.

  • Three materials were tested for use as roof tiling material: expanded slag, a 50/50 slag-clay extruded pellet mix, and a commercial, lightweight-aggregate as a control. Aggregate and cement were mixed with five parts aggregate to two parts cement and then commercial accelerator and super-plasticizer were added. At 2,800 psi and over seven days, the compressive strength of the slag aggregate was 83% as strong as the control. In addition, its density of 93 lb/scf was 87% of the control and, if necessary, sand could be added to increase both strength and weight. The 50/50 slag-and-clay mix was similar to the 100% slag test specimen.
  • Insulating Concrete—SLA was then tried as an insulating concrete. It was found to have a density of 51 lb/scf and seven-day strength of 175 psi, but work is still in progress. In addition, SLA as a loose fill insulating material is being investigated.

Commercial Viability
The test study concluded that SLA can be produced with densities between 20 and 50 lb/scf using two different methods: rotary kiln and fluidized bed expansion. These products meet weight requirements for almost all lightweight and some ultra-lightweight aggregate uses. SLA also has some advantages:

  • No mining costs to obtain the slag (as the coal would have been mined regardless)
  • Energy savings, as the temperature for SLA creation is less than that for traditional clays and shales
  • Recovered char can provide between 50 and 80% of the fuel required for slag expansion depending on whether a kiln or fluidized bed is used
  • Slag can be used by itself or mixed with expansible clay, allowing for the use of slag fines. This could also allow slag to be used as a substitute for clays and shales in existing lightweight aggregate plants

Rotary Kiln Expansion
A direct-fired rotary kiln was used to lower the slag density—by expansion—for slag-based lightweight aggregate applications. After separating the slag into various size fractions, the coarse fraction (1/4" x 50-mesh) was fired as discrete particles. Density was able to be varied between 30 and 50 lb/scf by temperature control and could be lowered below 20 lb/scf but particle fusion became an issue. At a temperature of 1,450-1,500°C, plus 10-mesh slag was expanded to densities of 20-30 lb/scf. These temperatures are 300-400°F lower than typical temperatures for expanding clays and shales, which translates to significant savings in energy.

The slag fines are difficult to expand as is, so experiments were performed using a clay binder. Binding with clay was successful, as the clay was able to be blended with the slag fines for expansion. Minus 50-mesh slag was mixed with a 20-50% (by weight) expandable clay binder. The mix was then extruded and pelletized, where the size of the aggregate can be controlled by the extrusion process. Using a larger proportion of slag results in lower pellet moisture, which affects overall fuel requirements; more slag means less fuel costs.

The firing temperature for representative samples (80/20 and 50/50 slag-clay blends) ranges from 1,800-1,900°F. This temperature is higher than slag by itself but lower than clay alone. In addition, there was no sign of fusion with any of the mixtures up to 2,000°F. These expanded mixtures had densities between 27 and 33 lb/scf and the temperatures required were nearly 200°F lower than that for clay, which again saves energy.

Fluidized Bed Expansion
Various slag sizes were expanded as discrete particles to produce lightweight aggregates with densities between 18 and 26 lb/scf. Again using the extruded slag/clay pellets from minus 50-mesh fines, the pellets were granulated to 20-mesh aggregates for use in block and roof tile applications. These slag/clay aggregates had a minimum density of 30 lb/scf.

The economic incentive for developing this technology depends on the market prices of target applications: conventional LWAs made from expansible clays sell for $40/ton, and ULWAs made from expanded perlite sell for $150/ton. The results indicate that SLA is an excellent substitute for conventional LWA in roof tile, block, and structural concrete production. In addition, slag-based near-ultra-lightweight material may also be used as a partial substitute for expanded perlite in agricultural and horticultural applications. The preliminary economics indicate that SLA costs would be considerably lower than those of conventional materials due to the absence of mining costs and significantly lower temperature of expansion (1400-1600°F vs. 1800-2000°F for conventional clays). Production costs were calculated at $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 a possible $15/ton avoided costs of slag disposal, the economics of SLA production become even more attractive. The technology demonstrated under this project indicates a good opportunity for developing value-added products from IGCC slag.

References/Further Reading
  1. Coal-burning electric utilities produce over 100 million tons of CUBs annually in the United States. However, boiler slag production, as a percentage of total CUB production, dropped consistently from about 6-7% in 1988-93 to about 2.5% in 1996-99.
  2. Choudhry, V. and R. Shockley, “Utilization of Lightweight Aggregates Made from Coal Gasification Slags,” Final Technical Report, Praxis Engineers, Inc., August 1998.

Solid Waste/Byproducts of Gasification