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CHARACTERIZATION OF COAL COMBUSTION PRODUCTS FOR BENEFICIAL USE IN MINE LAND RECLAMATION
ABSTRACT
Over 130 million tons of coal combustion products (CCPs) are produced each year in the U.S. Less than half of these CCPs will be utilized in beneficial use projects, such as mine land reclamation, an important practice in Pennsylvania. The remainder will be landfilled. Yet, CCPs are an ideal fill material due to their abundance and their desirable engineering characteristics. The main disadvantage of utilizing CCPs is the variability in material properties seen between differing CCP sources and the change in material behavior over time. This variability makes predicting CCP behavior as an engineered fill difficult. One approach for minimizing the risk associated with CCP variability is to catalog all available data on CCPs, including formation processes, chemical properties, material characteristics, and mechanical behavior. Once this catalog of data has been developed, common trends in material characteristics and mechanical behavior between CCP sources may be identified. Therefore, the purposes of this study are to collect and organize all existing data on Pennsylvania CCPs into an electronic database as well as contribute to the database by obtaining properties of two additional CCPs. An attempt was made to use this data gathered in the database and the results from additional CCP testing to identify trends in material properties and behavior. The testing of the additional CCPs followed a CCP testing framework developed in a previous study. This work focuses on the creation and utilization of this database along with the findings of this additional CCP testing.
The sources of the collected CCP data included published literature and results from years of the Pennsylvania Department of Environmental Protection (DEP) regulatory testing. The collected data was organized into a database designed in Filemaker Pro 11 and then launched as a website. For the additional CCP testing, two fly ash materials from two different power plants were selected for this study: a class F fly ash and an FBC fly ash. The testing methodology for the additional CCPs included general material characterization, such as compaction characteristics, particle size distribution, and specific surface area. Other tests focused on the mechanical behavior of these CCPs over time and included unconfined compression (UC) strength and hydraulic conductivity testing. The tests used to classify CCP mechanical behavior showed that the UC strength and hydraulic conductivity of the material may change, depending on the type of CCP tested. The results of quantitative x-ray diffraction and PHREEQCI modeling show that the formation of ettringite dictates CCP behavior. That is, as the amount of ettringite formed in FBC-PC increases, the strength of the CCP also increases. The lack of strength gain observed for the class F fly ash was due to the lack of formation of hydration products. In addition, ettringite formation over time reduced the void ratio of the FBC ash, and thus reduced the hydraulic conductivity.
One important finding of this study is that the UC strength trend observed in FBC-PC is very similar to the UC strength trend for another FBC ash investigated in a previous study. Both FBC ashes experienced the most significant increase in UC strength in the first 7 to 14 days of curing.
Another important finding of this study is the variability in the compaction characteristics of CCPs collected from the DEP. This variability prevents the identification of trends in maximum dry density and optimum moisture content based on CCP type.
One critical finding of this study is that the hydraulic conductivity data collected by the DEP are not reliable for comparison purposes. The methods and procedures followed by different laboratories contracted for this testing were found to be inconsistent. The most significant inconsistency is the time that laboratories allow test specimens to cure before testing. It is possible that through eliminating these inconsistencies, the regulatory hydraulic conductivity testing can be standardized and the resulting data will be more meaningful.
CCPs are a variable material and understanding how the mechanical behavior of the material changes, based on type and time, is essential for use in large volume applications such as mine land reclamation.
TABLE OF CONTENTS
LIST OF FIGURES………………………………………………………………………………………………………. viii
LIST OF TABLES…………………………………………………………………………………………………………. xi
ACKNOWLEDGEMENTS…………………………………………………………………………………………… xiii
Chapter 1 : INTRODUCTION………………………………………………………………………………………….. 1
1.1: Motivation…………………………………………………………………………………………………………. 3
1.2: Objectives…………………………………………………………………………………………………………. 3
Chapter 2 : LITERATURE REVIEW………………………………………………………………………………… 4
2.1: Geology of Pennsylvania Coal Fields……………………………………………………………………. 4
2.2: Coal Combustion Processes…………………………………………………………………………………. 7
2.2.1: Pulverized Coal Combustion………………………………………………………………………. 7
2.2.2: Clean Air Act and Coal Ash Production………………………………………………………. 8
2.2.3: Fluidized Bed Combustion (FBC)……………………………………………………………….. 9
2.2.4: CCP Particle Formation……………………………………………………………………………. 10
2.3: Beneficial Use of CCPs…………………………………………………………………………………….. 13
2.3.1: Roadway Embankment Design…………………………………………………………………. 13
2.3.2: Green Concrete……………………………………………………………………………………….. 21
2.3.3: Mine Land Reclamation…………………………………………………………………………… 25
2.4: Certification Process of CCPs for Beneficial Use…………………………………………………. 29
Chapter 3 : CCP DATABASE………………………………………………………………………………………… 33
3.1: Database Development……………………………………………………………………………………… 33
3.2: Data Acquisition………………………………………………………………………………………………. 40
3.2.1: Engineering and Environmental Data Acquisition……………………………………….. 40
3.2.2: CCP Production Data Acquisition……………………………………………………………… 41
3.3: Database Website……………………………………………………………………………………………… 43
Chapter 4 : CCP TESTING…………………………………………………………………………………………….. 45
4.1: CCP Selection………………………………………………………………………………………………….. 45
4.2: Testing Framework…………………………………………………………………………………………… 47
4.2.1: Chemical and Material Characterization Tests…………………………………………….. 47
4.2.2: Mechanical Characterization Tests (Plaks, 2010)………………………………………… 49
4.2.3: Thermodynamic Modeling (PHREEQCI)…………………………………………………… 51
Chapter 5 : RESULTS……………………………………………………………………………………………………. 52
5.1: Material Characterization………………………………………………………………………………….. 52
5.1.1: Standard Proctor Analysis………………………………………………………………………… 52
5.1.2: Modified Proctor Analysis………………………………………………………………………… 53
5.1.3: CCP Material Properties…………………………………………………………………………… 54
5.1.4: Energy Dispersive Spectroscopy (EDS) Mapping……………………………………….. 57
5.1.5: Zeta Potential………………………………………………………………………………………….. 60
5.2: Mechanical Behavior………………………………………………………………………………………… 61
5.2.1: Unconfined Compressive (UC) Strength Testing…………………………………………. 61
5.2.2: Hydraulic Conductivity……………………………………………………………………………. 67
5.3: Leaching Tests…………………………………………………………………………………………………. 72
5.3.1: Hydraulic Conductivity Effluent Chemistry………………………………………………… 72
5.3.2: Recommendations for the Modified MCC-3 Leach Test………………………………. 75
Chapter 6 : DISCUSSION AND ANALYSIS…………………………………………………………………… 77
6.1: Unconfined Compressive Strength Dependency on CCP Type………………………………. 77
6.2: Proctor Analysis and Hydraulic Conductivity Data Variability………………………………. 79
6.2.1: Proctor Analysis and CCP Type………………………………………………………………… 79
6.2.2: Hydraulic Conductivity and CCP Type………………………………………………………. 82
6.3: Recommendations for Improving DEP Required Testing………………………………………. 84
Chapter 7 : CONCLUSIONS………………………………………………………………………………………….. 85
Chapter 8 : RECOMMENDATIONS FOR FUTURE WORK…………………………………………….. 87
REFERENCES……………………………………………………………………………………………………………… 88
Appendix A: DATABASE IMAGES………………………………………………………………………………. 93
Appendix B: POWER PLANT SURVEYS…………………………………………………………………….. 102
Appendix C: DATABASE WEBSITE IMAGES…………………………………………………………….. 112
Appendix D: PROCTOR DATA……………………………………………………………………………………. 116
Appendix E: PARTICLE SIZE DISTRIBUTION AND SPECIFIC SURFACE………………….. 122
Appendix F: EDS MAPPING……………………………………………………………………………………….. 131
Appendix G: ZETA POTENTIAL DATA………………………………………………………………………. 137
Appendix H: UNCONFINED COMPRESSION TEST DATA…………………………………………. 140
Appendix I: QUANTITATIVE XRD RESULTS…………………………………………………………….. 148
Appendix J: HYDRAULIC CONDUCTIVITY DATA…………………………………………………….. 149
Appendix K: EFFLUENT CHEMICAL ANALYSIS………………………………………………………. 165
Appendix L: BASELINE CHEMICAL ANALYSIS………………………………………………………… 178
Appendix M: MODIFIED MCC-3 LEACH TEST…………………………………………………………… 182
Appendix N: COMPILED PROCTOR DENSITY DATA………………………………………………… 190
Chapter 1 : INTRODUCTION
Over the past 200 years, coal has been the source of fuel powering the US’s industrialization and rise as a world power. The most notable use of coal is as a fuel source in coal fired power plants. The Commonwealth of Pennsylvania has the benefit of abundant coal resources, comprised of both Bituminous and Anthracite coal fields. The mining of Pennsylvanian coal has a history spanning over 200 years and today is a $3.2 billion per year industry (PA Economy League, 2010). Despite its economic benefits, the mining and use of coal has many drawbacks, which include abandoned mine lands and waste products.
Decades of mining has left the commonwealth with close to 200,000 acres of abandoned mine land (AML) (Dalberto et al., 2004). AML poses many safety and environmental problems, such as subsidence and acid mine drainage (Siriwardane et al., 2003). Mine subsidence has the potential to cause millions of dollars in property damage in both the private and public sector. Acid mine drainage (AMD) has currently caused 3100 miles of polluted streams, wreaking havoc on Pennsylvania’s lush eco structure. The cost of fixing the state’s AML problem is estimated at $14.6 billion (Dalberto et al., 2004).
Another consequence of burning coal is the staggering amount of waste produced when coal is used to generate electricity. The different types of waste produced are collectively known as coal combustion products (CCPs). In 2009 alone, the United States power industry generated 134 million tons of CCPs from burning coal. Only roughly 41% of these CCPs were beneficially used (ACAA, 2009). The remaining 59%, or 79 million tons, of CCPs were landfilled. Figure 1.1 shows that the rate of CCP production exceeds the rate of beneficial use. In addition to the large quantities of CCPs produced, the types and properties of CCPs vary drastically from power plant to power plant due to variations in combustion and pollution control practices.
A potential solution to the problem of the residuals of the coal mining and power industries is large-volume beneficial use of CCPs as a fill material to reclaim AML. The Pennsylvania Department of Environmental Protection (DEP) has identified four uses by which to beneficially apply CCPs in mine land reclamation. These include use as an alkaline addition, a low permeability material, a soil additive, and as direct placement (Dalberto et al., 2004). The DEP requires a specific CCP source to undergo a certification process before it can be used beneficially in mine land reclamation. The DEP testing protocol requires bulk chemical analysis of the solids along with a leachate analysis, which is considered the most important test in the certification process. Other parameters required to be tested are the hydraulic conductivity, degree of compaction and neutralization potential of the CCP.
These parameters required by the DEP work well to determine how a specific CCP will behave in a mine land environment and ultimately if the material could pose as an environmental risk. However, these parameters do not fully characterize the material or predict how it will perform
Figure 1.1: Production and Use of CCPs from 1966 to 2009 (after ACAA, 2009)
structurally. In response to this issue, Plaks (2010) developed a testing framework that provides a thorough analysis of both the material and mechanical characteristics of CCPs.
Currently, there is a need to systematically collect and organize the data collected from this testing framework, along with any other available information on the material and mechanical characteristics of CCPs. A database that catalogues the source, chemical, environmental, and mechanical properties of Pennsylvania CCPs will fulfill this need. This database will potentially assist state officials, power generators, and engineers to track the properties of CCPs through time, further facilitating and improving the practice of CCP beneficial use in mine land reclamation.
1.1: Motivation
A summary of the motivation for this research is as follows:
- A great need exists to both identify a feasible use for the percentage of CCPs diverted to landfills and to further facilitate the reclamation of Pennsylvania mine lands.
- No centralized database exists to catalogue past, present, and future results of CCP testing and evaluation.
- The compilation of a Pennsylvania CCP database will facilitate data sharing between generators and the Department of Environmental Protection, promoting future studies into the behavior of coal ash in beneficial use projects.
1.2: Objectives
The objectives of this study are to:
- Develop a comprehensive database that catalogues the coal combustion products of Pennsylvania, specifically focusing on pulverized coal fly ash and fluidized bed combustion ash.
- Perform testing on two different sources of CCPs (class F fly ash and FBC ash) following the framework developed in Plaks (2010) for inclusion in the database.
- Identify preliminary trends of material characteristics, based on CCP types, through comparing the results of the coal ash tested in this study to the ash tested in Plaks (2010), as well as to any comparable information found pertaining to Pennsylvania ash compiled for the database.
- Based on an analysis of the Pennsylvania CCP data compiled in the database, make recommendations for improving DEP-required CCP testing for beneficial use certification.
CHARACTERIZATION OF COAL COMBUSTION PRODUCTS FOR BENEFICIAL USE IN MINE LAND RECLAMATION
