Geotechnical Properties of Flue Gas Desulfurization Material and Containment

Implications

Craig Schuettpelz1 and Todd Stong1 1Golder Associates, Inc., 44 Union Blvd, Suite 300, Lakewood, CO 80228 CONFERENCE: 2019 World of Coal Ash – (www.worldofcoalash.org) KEYWORDS: FGD material, geotechnical, material properties, impoundment

ABSTRACT Safe operation and efficient closure of coal combustion residual (CCR) containment facilities requires a detailed understanding of the geotechnical characteristics of the various materials contained. Flue Gas Desulfurization (FGD) material is a byproduct of an air pollution control technology where primarily sulfur dioxide is removed from flue gas via the calcium within a lime slurry. The purpose of this paper is to present detailed geotechnical material properties associated with FGD material produced at a coal-fired power plant with wet scrubber technology. Material properties discussed will include grain size analyses, density and moisture content, hydraulic conductivity, consolidation behavior, and shear strength. Potential test methods will be discussed, including the applicability of those test methods to better represent field conditions in support of permitting and design efforts. Laboratory results will be discussed and summarized with consideration given to how small-scale test results may be implemented to model material behavior in field-scale situations, including potential limitations of those models. In addition, the importance of changes to material properties over time will be discussed. INTRODUCTION Coal Combustion Residuals (CCRs) encompass a wide variety of materials produced during the process of burning coal to produce electricity. The most common materials produced include fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) material. In general, fly ash, bottom ash, and boiler slag, are direct physical products left over after burning coal and represent the solid materials unable to be consumed during the combustion process. These materials are either collected at the bottom of the boiler (bottom ash and boiler slag) or are carried through the top of the boiler with the flue gas (fly ash). Gaseous molecules produced during the combustion of coal make up the flue gas and the elemental composition of this flue gas depends on the coal source and the boiler configuration. Flue gas consists primarily of nitrogen since nitrogen makes up the largest percentage of the ambient air source used in most boiler configurations. Other significant molecular

components of flue gas include carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen oxides (NOX), and water vapor. Fly ash is removed from the flue gas, typically via an electrostatic precipitator or through use of fabric filters. Air pollution control technologies may be implemented either upstream or downstream of the fly ash collection equipment (depending on the technology) and may include a wet or dry FGD scrubber, dry sorbent injection (DSI), or selective noncatalytic reduction (SNCR) systems. The focus of this paper is to discuss the physical engineering properties of a FGD material produced via wet scrubber technology. A lime slurry mixture introduced to the flue gas is used to remove sulfur from the flue gas prior to emission from a facility. FGD materials usually consist of either calcium sulfite (CaSO3) or calcium sulfate (CaSO4) when the sulfur dioxide interacts with the lime. The additional oxygen is required to produce calcium sulfate, commonly referred to as gypsum, which is a marketable product that can be sold for production of wallboard and other construction materials. Calcium sulfite is less marketable and therefore is often contained in onsite disposal facilities, such as surface impoundments or landfills. This paper presents the results of physical testing on a calcium sulfite FGD material destined for disposal in onsite CCR containment facilities. As with all CCR products, FGD material is manufactured and is not a natural material for which standard geotechnical classifications and expectations necessarily apply. That is, the field of soil mechanics was generally developed to explain the behavior of natural materials (sand, silt, clay, etc.); however, the behavior of these man-made CCR materials may or may not be able to be easily described in accordance with these more typical, natural materials. This paper will hopefully provide useful material information for a FGD material and reinforce the importance of investigating material properties rigorously and frequently. SITE OVERVIEW AND BACKGROUND INFORMATION The FGD material tested comes from a site located in North Dakota. The facility is a coal-fired power plant that burns locally-sourced lignite coal. Historically, the site sluiced and comingled CCRs (fly ash, bottom ash, FGD material) into various ash ponds onsite for final disposal or in preparation for cleanout and disposal in nearby landfills. Since the early 2000s, CCRs were more strategically segregated and placed in specific locations according to their material properties and quantities. To more efficiently manage the environmental footprints of lined facilities, engineers and site personnel pursued vertical placement of CCRs in “raise” facilities. In particular, the “upstream raise” concept was borrowed from mine waste engineering. Generally, an “upstream raise” facility is one in which the vertical height increases by construction in an upstream direction. So, in the case of a composite-lined surface impoundment, the facility is expanded vertically toward the center of the surface impoundment and the next layer of the vertical expansion of the facility is built over and toward the center of the facility. Typically, weaker materials are placed in an upstream location (i.e. center of

a surface impoundment) and stronger or more resilient materials are placed on downstream slopes. Therefore, it is important to note that future layers of vertical construction in an “upstream raise” are occurring over weaker materials (see Figure 1).

Figure 1. Upstream raise facility (FGD material in the center with bottom ash and fly ash outer components). The “upstream raise” concept has been used in mining engineering for decades, but does require substantial upfront engineering as well as ongoing monitoring to ensure the facility is stable and is operating consistent with the design. In addition, “upstream raise” facilities are generally not constructed in areas prone to seismic activities. However, the plant, located in central North Dakota, is in an area with low historic seismic activity. No earthquakes of Magnitude V (i.e., Moderate-Strong) or greater (Mercalli intensity scale) have occurred in North Dakota during historical times [4]. For the site location, the peak (bedrock) ground acceleration (PGA) with a 2% probability of exceedance in 50 years is approximately 0.02g (1g equals 32.2 ft/sec2) [3]. Since construction is occurring over potentially weaker materials, confidence in the materials characteristics of the facility are important for a safe and effective design. Part of the engineering associated with such a facility includes physical testing associated with materials to be contained. Material characteristics obtained during physical testing can be used in seepage, consolidation, and slope stability models to determine whether the facility can be safely (i.e. slope stability) and effectively (i.e. site life, seepage and dewatering) operated. MATERIAL CONVEYANCE AND DEPOSITION The FGD material discussed in this document comes from a wet scrubber and is conveyed hydraulically to CCR surface impoundments. Hydraulic conveyance is an inexpensive and efficient means of transporting the FGD material; however, it does require that the site handle process water accordingly. FGD material is estimated to be conveyed at approximately 15% to 20% solids and at an approximate temperature of 60 degrees Celsius. With these parameters, the FGD material has a dynamic viscosity of between 3.78 and 1.90 milli-Pascal seconds (mPa-sec). For comparison, in this same temperature range, water has a viscosity of approximately 0.47 mPa-sec [2]. Figure 2 shows FGD material being deposited in one of the onsite “upstream raise” facilities. Material deposition is important and helps us to understand how the material may behave and how we might want to sample the material for testing.

Figure 2. FGD material deposition in an "upstream raise" facility. MATERIAL SAMPLING Obtaining representative samples of a material is the first step toward obtaining useful geotechnical material properties. Preferably, we’d like to perform laboratory tests on samples that are representative of field deposition (see previous section) and loading conditions. However, since CCR materials are manufactured and placed in containment facilities, it can be more difficult to obtain representative samples for a design without first placing those materials. In other situations, the materials may be difficult to sample in-situ due to inadequate or unsafe access over soft materials. Laboratory samples presented in this paper were acquired using several different methods, including grab samples from test ports along the hydraulic conveyance pipeline near the FGD scrubber tanks within the plant, grab samples from test pits where previous deposition of FGD material had occurred, and Shelby tube sampling of historically placed material. Laboratory samples were either remolded based on assumed densities (discussed in the following sections) or, in the case of Shelby tube samples, were considered “minimally disturbed” samples. Field tests were also performed directly on material recently exposed in the field. TESTING PERFORMED Physical testing was performed on FGD materials with the goal of evaluating the hydraulic, strength, and consolidation characteristics of the material. The material properties can be used as inputs to consolidation, seepage, and stability models. Index testing was also performed to understand the general characteristics of the material. A list of laboratory tests presented herein, and applicable ASTM standards is presented below: Grain Size Distribution (Sieve and Hydrometer) ............................................. ASTM D422

Atterberg Limits ............................................................................................ ASTM D4318 Specific Gravity .............................................................................................. ASTM D854 Moisture and Density...................................................................................... ASTM D698 Flexible Wall Hydraulic Conductivity............................................................. ASTM D5084 Column Settling ........................................................................................................... N/A Slurry Consolidation .................................................................................................... N/A Seepage Induced Consolidation Test and Analysis (SICTA) ...................................... N/A Consolidated Undrained Triaxial Compression ............................................ ASTM D4767 Laboratory Vane Shear ................................................................................ ASTM D4348 A list of field tests presented herein and applicable ASTM standards is presented below: Vane Shear .................................................................................................. ASTM D2573 GRAIN SIZE AND PLASTICITY Hydrometer analyses were performed on FGD samples collected between 2002 and 2012. The grain size distribution curves are presented in Figure 3. Grain size distributions are similar and show that samples are poorly graded, with the majority of particles falling between 0.009 and 0.03 mm in diameter. Atterberg limits tests were performed on a sample collected in 2010, but samples collected in 2011 and 2012 were “non-plastic.” The 2010 sample had a liquid limit of 64 and a plasticity index of 25. Based on the general lack of “cohesive” properties and the grain size distribution, the material is best described as silt based on standard geotechnical classification.

Figure 3. FGD Material grain size distribution. MOISTURE AND DENSITY Moisture and density information was obtained from both field and laboratory tests between 2003 and 2014. FGD material has a measured dry density ranging from 4.3 to 10.9 kN/m3 and a moisture content ranging between 53 and 188% based on column settling tests, field samples of in-situ FGD material, and laboratory samples consolidated to anticipated field loading conditions. In general, FGD material has a higher moisture content and lower dry density immediately after deposition. Based on anticipated final loading conditions, consolidation analyses estimate a dry density of between 6.0 and 8.0 kN/m3 at the end of deposition and a dry density of between 8.0 and 11 kN/m3 after closure. As the material is covered and consolidates, moisture content decreases and dry density increases. Figure 4 shows the relationship between moisture content and dry density for tested samples and the theoretical relationship between dry density and moisture content, assuming a specific gravity of 2.7 and degree of saturation equal to 1.0. The specific gravity of FGD material sampled between 2002 and 2012 ranges from 2.6 to 2.9 based on nine tests (Table 1). All specific gravity results except the sample taken in 2003 are approximately 2.7, with the 2003 sample indicating a value of 2.9.

Figure 4. FGD Material moisture-density relationship. Table 1. FGD Material specific gravity.

DATE SPECIFIC GRAVITY

2002 2.67

2003 2.92

2006 2.64

2008A 2.66

2008B 2.72

2010 2.72

2011A 2.70

2011B 2.68

2012 2.71

CONSOLIDATION ANALYSIS and HYDRAULIC CONDUCTIVITY Golder has performed two laboratory consolidation tests on FGD material since 2002. Both tests were performed on laboratory-prepared slurry forms of the FGD material to

approximate how these materials are deposited within CCR facilities. Results from the two tests are described in more detail below. Seepage Induced Consolidation Test Analysis (SICTA) Golder performed a Seepage Induced Consolidation Test and Analysis (SICTA) on FGD material collected in 2002. The SICTA offers a method of determining consolidation and desiccation parameters for materials deposited as slurries. Ultimately, the test allows for the development of a relationship between void ratio (dry density) and hydraulic conductivity. Based on a range of dry densities between 8.5 and 5.5 kN/m3 in the “upstream raise” facilities (respective void ratios 2.1 and 3.8, see Figure 5 and Figure 6), the saturated hydraulic conductivity is expected to range from 7x10-6 cm/s to 5x10-5, respectively (Figure 7). Column Settling and Slurry Consolidation Test Golder performed column settling tests in combination with a slurry consolidation test on FGD material collected in 2011. Column settling tests are performed to obtain an estimate of material density immediately after deposition (i.e. without surcharge load). Column settling tests produced dry densities of about 4.0 kN/m3, with a corresponding void ratio of approximately 5.4. Such densities and void ratios can be expected near the surface of the deposited FGD materials (nominal surcharge pressure). Slurry consolidation tests provide similar results to the SICTA, allowing a consolidation test to be performed on a material deposited as a slurry. The test allowed Golder to compare the 2002 SICTA results with additional information to better understand consolidation parameters. Based on a range of dry densities between 8.5 and 5.5 kN/m3 in the raise (respective void ratios 2.1 and 3.8, see Figure 5 and Figure 6), the saturated hydraulic conductivity is expected to range from 8x10-6 cm/s to 9x10-5, respectively (Figure 7). Comparison of SICTA and Slurry Consolidation Tests FGD material tested in 2002 compares relatively closely with material tested in 2011. Material property results may be different due to changes in sampling location or changes to plant processes between sampling dates. Densities based on field specimens and strength test specimens (consolidated undrained triaxial shear strength tests) were compared to consolidation tests results to evaluate the similarity between consolidation test specimens and field samples of the laboratory shear strength samples. Figure 6 compares density with effective confining stress for consolidation test specimens, field specimens, and triaxial test specimens. Flexible Wall Hydraulic Conductivity Two flexible wall hydraulic conductivity tests were performed on Shelby tube samples in 2014. Based on a range of dry densities between 10.1 and 7.2 kN/m3 (respective void

ratios 1.6 and 2.7), the saturated hydraulic conductivity ranged from 8x10-7 to 4x10-6 cm/s, respectively (Figure 7). The flexible wall hydraulic conductivity testing show potential variability in testing methods since the measured hydraulic conductivities were approximately one-half order of magnitude smaller than that which would have been expected based on the SICTA or slurry consolidation testing.

Figure 5. Void Ratio vs. Effective Stress.

Figure 6. Dry Density vs. Effective Stress.

Figure 7. Void Ratio vs. Hydraulic Conductivity. SHEAR STRENGTH FGD material shear strength is based on vane shear tests and laboratory triaxial tests performed on FGD material between 2003 and 2014. Vane Shear Testing Both laboratory and field vane shear testing were performed on FGD material between 2003 and 2005. Field testing was performed directly on material excavated from test pits, while laboratory tests were performed on remolded lab samples. Results from these tests represent a disturbed sample under little confining pressure. Shear strength results of seven vane shear tests varied between 19 kPa and 29 kPa, with an average of 24 kPa. Laboratory Triaxial Testing Consolidated undrained triaxial test specimens from between 2011 and 2014 are shown in Figure 8 and show that FGD materials have a clearly developed shear plane. Results of laboratory triaxial testing are shown in Figure 9 in terms of p-q space, where:

𝑝′ = 𝜎1

′ + 𝜎3′

2

𝑞 = 𝜎1 − 𝜎3

2

Failure criteria for these tests was defined as the principal stress ratio:

𝜎1′

𝜎3′

A best-fit linear trend was applied to both undrained (total stress) and drained (effective stress) laboratory test results for illustration only and to be able to discuss the results. As with any geotechnical laboratory testing results, interpretation should be performed in accordance with the design by a qualified person. Based on the test results, the undrained analyses produced an apparent cohesion of 15 kPa and a friction angle of 14 degrees and the drained analyses produced an apparent cohesion of 7 kPa and a friction angle of 36 degrees.

Figure 8. Post-Test consolidated undrained triaxial test photographs at different confining pressures from tests performed in 2011 and 2012.

Figure 9. FGD Material shear strength results. GENERAL IMPLICATIONS OF TEST RESULTS AND FIELD OBSERVATIONS The FGD material from this plant in North Dakota can be generally classified as a low plasticity silt (in terms of a standard geotechnical classification) made up of calcium sulfite particles. The material is consistent and grain size generally falls in the range between 0.009 mm and 0.03 mm. Due to the fine-grained nature of the material, the hydraulic conductivity is relatively low, ranging between approximately 1x10-6 cm/sec and 1x10-5 at closure conditions. Since the material is silty, the coefficients of consolidation observed during the SICTA and slurry consolidation testing are in the range of approximately 1x10-1 cm2/sec. For comparison, a typical lean clay may be on the order of 1x10-4 cm2/sec or even lower [1]. This indicates, that the FGD material would be anticipated to consolidate relatively rapidly compared with a clay material since the water available in the pore spaces is able to more freely drain. However, field observations would indicate that the FGD material is somewhat hydroscopic, or that it tends to take up water and retain that water, even when subjected to motion (i.e. vibration). With load, water appears to release from the pore spaces and drain away; however, without a surcharge the material remains

saturated. Figure 10 shows typical deposition of bottom ash over FGD material in an “upstream raise” facility. FGD material is anticipated to have some “apparent cohesion” based on triaxial testing and vane shear testing. Both test methods indicate the material has some nominal strength at low confining pressure. These observations seem to be confirmed in the field, as the material has some bearing capacity (ability to bear a small load if the surface is unsaturated) and can be stacked into piles without flowing. In addition, after consolidation has occurred, the strength of the FGD material was observed to be relatively high. A drained friction angle of 36 degrees (based on a linear best-fit regression) shows that the material particles appear to have shear strength characteristics more similar to a sand than a clay.

Figure 10. Deposition of bottom ash over FGD material in an "upstream raise." POTENTIAL ADDITIONAL TESTING This paper describes the results of several laboratory tests performed over more than a decade; however, additional testing and/or alternative test methods may be useful to help explain the material behavior. Below is a short list of potential additional testing that could be considered, but each site and material should be evaluated separately to determine applicable testing. A direct simple shear test may provide additional shear strength information at a different loading condition, one that may provide a more complete understanding of global stability. Cyclic shear testing is a valuable tool if the material were anticipated to be susceptible to seismic loading conditions. In the field, cone penetration testing (CPT) would provide valuable in-situ measurements of shear strength, pore pressure, and any number of additional physical properties of the material.

MATERIAL CHANGE WITH TIME FGD material, as with any other coal combustion residuals, is a manufactured material. As such, these materials do not necessarily conform to traditional material models developed for naturally occurring soils in geotechnical engineering (i.e. clay, silt, sand, etc.). In addition, these materials are subject to rapid changes in physical (and chemical) properties as plant processes change. The FGD material may change depending on the water source used for the system, the source of lime, or any number of modifications to air pollution control equipment installed within the plant. In addition, the FGD material may change in-situ since it may be subject to chemical and/or biological processes. Anecdotal evidence suggests that biological processes may affect the FGD material and change the material within the deposition location; however, a long-term evaluation of those processes and how they might affect the physical properties over time has not been evaluated. Engineers and owners of CCR containment facilities should strive to be aware of changes and should perform the appropriate level of due diligence to manage risks as materials change and state-of-the-art techniques to evaluate those materials change. Such due diligence that may be useful for understanding the risks associated with containing CCRs include continued testing of materials (or new testing, as available), updated modeling of CCR containment facilities, continued monitoring of the facilities (i.e. piezometers and inclinometers), or sampling and observations of historically deposited materials. CONCLUSIONS Material properties of Flue Gas Desulfurization (FGD) material produced at a lignite coal-fired power plant were evaluated over the course of approximately 12 years. Physical geotechnical material properties are important to our understanding of the structural stability of facilities built with these manufactured CCR materials, including containment landfills and surface impoundments. However, these material properties can also help us to understand issues more related to planning and operations of these facilities, such as dewatering, process water transport, and site life. This paper discusses detailed geotechnical material properties associated with a calcium sulfite FGD material. The physical nature of the silty FGD material discussed has been relatively consistent over time. Since the material is fine-grained, but without the presence of clay particles, the hydraulic properties are generally in between what would be expected for a sand and a lean clay. Consolidation properties and shear strength properties are closer to that of a sand (relatively rapid drainage and high effective strength parameters), likely due to the particle shape and depositional setting. REFERENCES [1] Bardet, J. Experimental Soil Mechanics. Prentice Hall, 1997.

[2] Munson, R.M., Young, D.F., and Okiishi, T.H. Fundamentals of Fluid Mechanics. John Wiley & Sons, Inc., 2002.

[3] United States Geological Survey. 2019. United Station Geological Survey – Unified Hazard Tool – Two percent probability of exceedance in 50 years map of peak ground acceleration, Retrieved March 7, 2019.

[4] United States Geological Survey. 2016. United Station Geological Survey – Earthquake Archives, http://earthquake.usgs.gov/earthquakes/search/, Retrieved June 13, 2016.