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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
The burning of pulverized coal in electric power plants produces sulfur dioxide (SO2) gas emissions. The 1990 Clean Air Act and its subsequent amendments mandated the reduction of power plant SO2 emissions. The Best Demonstrated Available Technology (BDAT) for reducing SO2 emissions is wet scrubber flue gas desulfurization (FGD) systems. These systems are designed to introduce an alkaline sorbent consisting of lime or limestone (primarily limestone) in a spray form into the exhaust gas system of a coal-fired boiler. The alkali reacts with the SO2 gas and is collected in a liquid form as calcium sulfite or calcium sulfate slurry. The calcium sulfite or sulfate is allowed to settle out as most of the water is recycled.
FGD scrubber sludge is the wet solid residue generated from the treatment of these emissions. The wet scrubber discharge is an off-white slurry with a solids content in the range of 5 to 10 percent. Because FGD systems are usually accompanied by or combined with a fly ash removal system, fly ash is often incorporated into the FGD sludge.
The relative proportion of the sulfite and sulfate constituents is very important in determining the physical properties of FGD sludges. Depending on the type of process and sorbent material used, the calcium sulfite (CaSO3) can contribute anywhere from 20 to 90 percent of the available sulfur, the remaining being calcium sulfate (CaSO4). FGD sludges with high concentrations of sulfite pose a significant dewatering problem. The sulfite sludges settle and filter poorly. They are thixotropic and generally not suitable for land disposal or management without additional treatment (a thixotropic material appears as a solid, but will liquify when vibrated or agitated). Treatment can include forced oxidation, dewatering, and/or fixation or stabilization.
Forced oxidation, which is a separate step after the actual desulfurization process, involves blowing air into the tank that holds calcium sulfite sludge, and results in the oxidation of the calcium sulfite (CaSO3) to calcium sulfate (CaSO4). The calcium sulfate formed by this reaction grows to a larger crystal size than calcium sulfite. As a result, the calcium sulfate can be filtered or dewatered to a much drier and more stable material than the calcium sulfite sludge.(1) Dewatering of FGD scrubber sludge is ordinarily accomplished by centrifuges or belt filter presses.
Fixation and stabilization are terms that are often used interchangeably when referring to FGD sludge treatment. In general, stabilization of FGD scrubber material refers to the addition of a sufficient amount of dry material, such as fly ash, to the dewatered FGD filter cake so that the stabilized material can be handled and transported by construction equipment without water seepage and can also support normal compaction machinery when placed into a landfill. Stabilization is primarily the result of physical reactions between the FGD sludge cake and the added drying agent.
Fixation ordinarily refers to the addition of sufficient chemical reagent(s) to convert the stabilized FGD scrubber material into a solidified mass and produce a material of sufficient strength to satisfy applicable structural specifications. This can involve the addition of Portland cement, lime, and/or self-cementing fly ash to induce both physical and chemical reactions between the stabilized sludge filter cake and the added reagents. The majority of the fixation processes currently in operation involve the addition of quicklime and pozzolanic fly ash, resulting in a pozzolanic reaction that provides added strength to dewatered FGD scrubber material.
Figure 6-1 presents an illustration of typical FGD processing, reuse, or disposal options.
As of December 1994, there were at least 157 coal-fired boiler units at 92 power plants that had operating wet scrubbing systems. These plants are located in at least 32 states.(2) Additional scrubbers are planned or under construction in order to achieve compliance with the Clean Air Act requirements. As of 1996, the operating scrubber systems at coal-fired power plants generated approximately 21.4 million metric tons (23.8 million tons) of FGD sludge annually.(3)
CURRENT MANAGEMENT OPTIONS
Fixated or stabilized calcium sulfite FGD scrubber material has been used as an embankment and road base material. Oxidized (calcium sulfate) FGD scrubber material, once it has been dewatered, has been sold to wallboard manufacturers as by-product gypsum.(2) This material has also been used as feed material, in place of gypsum, for the production of Portland cement. Oxidized FGD scrubber material (calcium sulfate) does not require fixation or stabilization for use as wallboard gypsum, but merely drying to a required solids content. Wallboard production represents the largest single market for FGD scrubber material.(2)
Although there is significant interest in using calcium sulfate FGD scrubber material in wallboard construction and in Portland cement production (as a gypsum source), relatively small amounts of calcium sulfate FGD scrubber material are presently being recycled. In 1996, approximately 0.8 million metric tons (0.9 million tons) of calcium sulfate FGD scrubber material were used to produce wallboard and approximately 0.06 million metric tons (0.07 million tons) of this material were used as feed material for cement production.(3) Also in 1996, approximately 0.04 million metric tons (0.05 million tons) of primarily fixated calcium sulfite FGD scrubber material were used for structural fill. Also, approximately 0.11 million metric tons (0.12 million tons) were used for road base construction.(3)
Figure 6-1. FGD sludge processing system.(4)
Almost all FGD scrubber sludge generated at the present time is disposed of in holding ponds or in landfills. Stabilization or fixation and placement in landfills is the most common method of disposal.
Fixated FGD scrubber material is generated, dewatered, and stabilized at utility coal-burning power plants that burn either medium- or high-sulfur coal that require scrubbing of the flue gas emissions to reduce sulfur dioxide levels. For FGD scrubber sludge to be a useable construction material that is suitable for recycling, it must first be dewatered, then stabilized or fixated. The scrubber sludge must also be dewatered and stabilized to enable its placement and compaction in a landfill.
The majority of the utility coal-fired power plants that are equipped with FGD scrubber systems dewater and stabilize the FGD scrubber sludge so that it can be placed in a landfill. The dewatering and stabilization (or fixation) steps are usually handled by the utility company, while the loading, transport, placement, and compaction of the stabilized or fixated FGD scrubber material are usually handled by an ash management contractor. Stabilized or fixated FGD scrubber material can be obtained from either the utility company or the ash management contractor at a particular power plant where FGD scrubbers are in operation.
Fixated or stabilized calcium sulfite FGD sludge filter cake can have a solids content from 55 to 80 percent, depending on the amount of fly ash in the blend. The resultant fixated FGD sludge product is a damp, gray, silty, compactable material capable of supporting normal construction equipment and developing compressive strength.(5)
HIGHWAY USES AND PROCESSING REQUIREMENTS
Stabilized or fixated FGD scrubber material has been used successfully for road base construction, as previously noted, at a number of different sites in Florida, Pennsylvania, Ohio, and Texas. (See references 6,7,8 and 9). Stabilization or fixation of FGD scrubber material (especially calcium sulfite sludge) can be accomplished by the addition of quicklime and pozzolanic fly ash, Portland cement, or self-cementing fly ash. Other activators may be used in place of quicklime. The FGD scrubber sludge is dewatered before the addition of stabilization or fixation reagents. Additional fixation reagents may need to be added for stabilized base construction in order to meet compressive strength or durability requirements.
Small amounts of fixated FGD scrubber material have been used for embankment construction in western Pennsylvania. (10) The material was reclaimed from a landfill and used in conjunction with a fly ash embankment project. No additional reagents were needed for embankment construction.
Dewatered FGD scrubber material is most frequently generated as calcium sulfite, although some power plant scrubbing systems are of forced oxidation design, resulting in a calcium sulfate (or by-product gypsum) material. Calcium sulfite FGD scrubber material is fixated and used for road base, while the calcium sulfate FGD scrubber material is frequently used for wallboard or as a cement additive. Table 6-1 shows the difference in typical physical properties (particle size and specific gravity) between calcium sulfite and calcium sulfate FGD scrubber material.(11) The oxidized material is coarser than the unoxidized material.
Table 6-1. Typical physical properties of FGD scrubber material.
The degree to which FGD scrubber material is treated influences its physical properties. Table 6-2 shows the physical characteristics of typical calcium sulfite FGD scrubber material in its dewatered, physically stabilized, and fixated conditions. Basic physical properties include solids content, moisture content, specific gravity, and wet and dry density.(7) When dewatered, the calcium sulfite FGD sludges become a soft filter cake with a solids content typically in the 40 to 65 percent range. Calcium sulfate FGD sludges can be dewatered much more easily and may achieve solids contents up to as high as 70 to 75 percent after dewatering.(12)
Dewatered and unstabilized calcium sulfite FGD scrubber sludge consists of fine silt-clay sized particles with approximately 50 percent finer than 0.045 mm (No. 325 sieve). It has a dry density in the range of 960 to 1,280 kg/m3 (60 to 80 lbs/ft3), with a specific gravity of solids in the 2.4 range.(13)
Table 6-2. Physical characteristics of typical calcium sulfite FGD scrubber material.
The solids contents of fixated FGD scrubber material ordinarily range from 60 to 80 percent. The specific gravity of fixated FGD sulfite scrubber material can range from 2.25 to 2.60, with an average of 2.38. Between 88 and 98 percent of the particles are in the silt size range. Depending on the amount of fly ash in the blend, maximum dry density values of fixated FGD scrubber material can range from 1,280 to 1,600 kg/m3 (80 to 102 lb/ft3) at optimum moisture contents ranging from 20 to 30 percent when tested using the standard Proctor (ASTM D698)(14) test method.(15)
The chemical composition of FGD scrubber material varies according to the scrubbing process, type of coal, sulfur content, and presence or absence of fly ash. Lime is the most commonly used reagent in the scrubbing process. Table 6-3 lists the major components of FGD scrubber material prior to dewatering or fixation.(11) This table indicates the normally expected percentage ranges of calcium sulfite, sulfate, or carbonate that result from the scrubbing of flue gases using various processes. Except for those subjected to forced oxidation, sludges from the scrubbing of bituminous coals are generally sulfite-rich, whereas forced oxidation sludges and sludges generated from scrubbing of subbituminous and lignite coals are sulfate-rich. Fly ash is a principal constituent of FGD scrubber material only if the scrubber serves as a particulate control device in addition to SO2 removal or if separately collected fly ash is mixed with the sludge.(11)
As shown in Table 6-3, the use of limestone as a sorbent with bituminous coals results in significantly lower percentages of calcium sulfite (CaSO3) and higher percentages of calcium sulfate (CaSO4) and calcium carbonate (CaCO3) than the use of lime as a sorbent with bituminous coals.
Table 6-3. Major components of FGD scrubber material from different coal types and scrubbing processes
(percent by weight).
Table 6-4 shows the expected range of mechanical properties (permeability, shear strength, and unconfined compressive strength) for dewatered, stabilized, and fixated calcium sulfite FGD scrubber material.
Table 6-4. Mechanical properties of typical calcium sulfite
FGD scrubber material.
Dewatered unstabilized calcium sulfite FGD scrubber sludges have a pastelike consistency with low shear strength and little bearing capacity. They are thixotropic, meaning that, when agitated, they revert to a liquid or slurry form. They have no unconfined compressive strength, an angle of internal friction around 20°, and a permeability in the range of 10-4 to 10-5 cm/sec.(9)
Stabilized or fixated calcium sulfite FGD scrubber material has unconfined compressive strength values in the range of 170 kPa (25 lb/in2) to 1380 kPa (200 lb/in2), an angle of internal friction of 35° to 45°, and coefficient of permeability values in the range of 10-6 to 10-7 cm/sec.(5)
If stabilized or fixated FGD scrubber sludge is to be used for road base construction, then the unconfined compressive strength is more likely to be in the range of 1720 kPa (250 lb/in2) to as high as 6900 kPa (1,000 lb/in2), depending on specification requirements and reagent addition rates. The flexural strength of stabilized FGD sludge road base materials is normally in the 690 to 1,720 kPa (100 to 250 lb/in2) range.(16) To achieve these strength ranges, additional fixation reagents (Portland cement, lime, fly ash, etc.) will usually be required.