FINAL TECHNICAL REPORT
September 1, 1996, through August 31, 1997
Project Title: MINERALOGICAL AND CHEMICAL COMPOSITION OF INORGANIC MATTER FROM ILLINOIS COALS
ICCI Project Number: 96-1/1.2B-1
Principal Investigator: Ilham Demir, Illinois State Geological Survey (ISGS)
Other Investigators: Randall E. Hughes, John M. Lytle, Rodney R. Ruch, Philip J. DeMaris, Chen-Lin Chou, each at the ISGS
Project Manager: Ken K. Ho, ICCI
ABSTRACT
The purpose of this project was to determine the mineralogical and chemical compositions of the marketed Illinois coals and of coal combustion residues from three types of power plants burning Illinois coals. These types of data are valuable when addressing two important issues of coal utilization: (1) the extent of the emissions of noxious elements at coal-fired power plants and the development of emission control techniques, and (2) increasing the use of coal combustion residues for making commercial products.
The project samples included 34 marketed Illinois coals collected for a previous project, 9 new coal samples, and 6 samples of coal combustion residues (3 fly and 3 bottom ashes). The combustion residue samples and 6 of the new coal samples were collected from fluidized bed combustion (FBC), cyclone combustion (CYC), and pulverized coal combustion (PC) plants. The remaining 3 new coal samples were collected from coal preparation plants. All project samples were mineralogically and chemically analyzed.
Average abundances of minerals in the marketed Illinois coals, with standard deviations (shown as values), were: quartz, 2.270 .87%; kaolinite, 2.190.58%; pyrite, 1.690.55%; mixed-layered illite/smectite, 1.680.74%; and illite, 1.390.52%. Marcasite, chlorite, calcite, and feldspars were also present in average concentrations of below 0.20%. Anhydrite and lime-portlandite were abundant in the FBC combustion residues, while the CYC and PC combustion residues contained mostly amorphous material (78-98%). Mullite, quartz, calcite, hematite, and magnetite contents of the combustion residues ranged from less than 0.1% to 8.4%.
Atmospheric emissions of 15 trace elements (As, Be, Cd, Co, Cr, F, Hg, Mn, Ni, P, Pb, Sb, Se, Th, and U ) from the three types of power plants were estimated using the analytical data on the feed coals and combustion residues. Fluorine and Mn emissions were high (>50%) for the FBC plant, and F, Hg, and Se emissions were high for the CYC and PC plants. The emission of the majority of the elements from the power plants was low (<25%).
Based on the mineralogical and chemical data, current and potential commercial uses of the fly and bottom ashes from the three types of power plants were evaluated and reported.
EXECUTIVE SUMMARY
This project determined and evaluated the mineralogical and chemical compositions of marketed Illinois coals and coal combustion residues from three types of power plants burning Illinois coals. The project results improved our understanding of (1) atmospheric emissions of hazardous air pollutant (HAP) elements at coal-fired power plants and (2) the use of coal combustion residues in making commercial products. Furthermore, the data can be used to improve the prediction and prevention of slagging and fouling at power plants.
Samples. The project samples included 34 marketed coals (collected and stored at the ISGS for a previous project), 9 new coal samples, 3 fly ash samples, and 3 bottom ash samples. Fly ash and bottom ash samples and 6 of the new coal samples were collected from fluidized bed combustion (FBC), cyclone combustion (CYC), and pulverized coal combustion (PC) plants. The other 3 new coal samples were collected from coal preparation plants. A limestone sample used in the FBC plant was also collected.
Chemical and mineralogical analysis. The newly collected coal samples and the fly ash and bottom ash samples were analyzed for their chemical compositions. Chemical analysis data on the 34 marketed coals stored at the ISGS were generated for a previous project and incorporated to the results of this project. All the project samples were analyzed for mineralogical composition. The mineralogical analysis of coal samples included three steps: (1) low temperature ashing, (2) extraction of the low temperature ashes (LTAs) with water, and (3) X-ray diffraction (XRD) analysis of the LTA solids after water extraction. The low temperature ashing process burns combustible materials at about 130oC but leaves the minerals in the LTA essentially unaltered from their original state in the coal.
Mineral content of marketed coals and mineral-HAPs correlations. On average, the clay mineral fraction (5.45% in whole coal) formed about 56% of the total mineral matter in the marketed Illinois coals. The average abundances of the individual minerals in the marketed coals were ( values are standard deviations): quartz, 2.270.87%; kaolinite, 2.190.58%; pyrite, 1.690.55%; mixed-layered illite/smectite, 1.680.74%; illite, 1.390.52%; chlorite, 0.190.14%; marcasite, 0.140.13%; calcite, 0.080.40%; plagioclase, 0.080.06%; K-feldspar, 0.010.02%. Using statistical analysis of the mineralogical and chemical data to establish correlations among individual minerals an HAP elements listed in the 1990 Clean Air Act Amendments was only partially successful. Chromium, F, and Th were positively correlated with mixed-layered illite/smectite and illite, and Mn was correlated with calcite. Also, there was a slight correlation between Hg and marcasite and between Co and chlorite.
Combustion behavior of minerals in coal. Among the minerals present in coal, only some of quartz survived the combustion process at the power plants. Part of the quartz and the clay minerals were converted largely to glass (amorphous silicates) and to a certain extent to mullite. Pyrite and marcasite were converted to hematite, magnetite, and amorphous Fe oxides. Some of calcite reacted with sulfur released from pyrite, marcasite, and organic matter to form anhydrite that was partially hydrated to gypsum. The rest of the calcite was converted to lime that partially or entirely changed back to calcite through reaction with atmospheric CO2. These mineral conversion reactions apparently took place in varying degrees depending on the types of combustion units, resulting in distinct differences between the mineralogical as well as chemical compositions of combustion residues from different types of coal-fired power plants. These differences have important implications for the effect of plant type on the HAPs emissions and on the suitability of the combustion residues for use in cement, ceramic, and other commercial applications.
HAPs emissions during coal combustion. Atmospheric emissions of 15 HAPs (As, Be, Cd, Co, Cr, F, Hg, Mn, Ni, P, Pb, Sb, Se, Th, and U ) from the three types of power plants were estimated using the analytical data on the feed and combustion residues from the plants:
| low emission (<25%> | moderate emission (25-50% | high emission (>50%) | |
| FBD plant | As, Be, Co, Cr, Hg, Ni, P, Pb, Sb, Se, Th, U | Cd | F, Mn |
| CYC plants | (As), Cd, Cr, (Ni), P, (Be), Co, Mn F, Hg, SePb, (Sb), Th, (U) | (Be), Co, Mn | F, Hg, Se |
| PC plant | As, Be, Cd, Cr, P, Pb, Sb, Th | Co, Mn, Ni, U | F, Hg, Se |
The emission of As, Ni, Sb, and U from two cyclone plants investigated was low to moderate, and that of Be was moderate to high. Overall lower emissions from the FBC plant than from the other plants resulted from either the lower FBC operating temperature compared with other combustion methods or from the creation of a favorable chemical environment for trace elements retention as a result of the addition of limestone. It is possible that the emission of HAPs, except highly volatile ones (F, Hg, Se), was overestimated. The reason is that the emission values were based on mass balance deficiencies, and the missing portion of an element could be adsorbed on surfaces of hardware on the cool side of a power plant instead of being emitted into the atmosphere.
Relative to their original amounts in the feed, most of the 15 HAPs were distributed normally or depleted in the combustion residues except in the CYC fly ash. Arsenic, Cr, Cd, Ni, P, Pb, Sb, Se and Th were enriched in the CYC fly ash. Other enrichments were: As, P, Sb, and U in the FBC bottom ash, Cd in the CYC bottom ash, and As and Pb in the PC fly ash.
Commercial uses of combustion residues. The fly and bottom ashes from the FBC plant contained large amounts of CaO and CaSO4 minerals (anhydrite, lime-portlandite) because of co-burning of coal with limestone to reduce SO2 emissions. Therefore, the FBC combustion ashes can potentially be used as feedstocks for making sulfate fertilizer and precipitated calcium carbonate and as acid neutralization agents in environmental and agricultural applications. Fly ashes from the CYC plants contained 12-20% unburnt carbon and 4-5% magnetite, both of which could be recovered and used as commercial adsorbent carbon and magnetite. The magnetite recovery, however, depends on whether this mineral occurs as discrete particles or is interlocked with hematite and other mineral particles. Most of the CYC bottom ashes was relatively coarse grained glass material that is suitable for use as frit on shingles. A fair amount of magnetite could be recovered also from the PC fly and bottom ashes, again depending on the nature of the magnetite particles (discrete vs. other forms). Because of their low carbon content (<3%), the PC ashes are suitable for use in making cement and ceramic products.