An ESCA and SEM Study of Changes in the Surface Composition and Morphology of Low-Calcium Coal Fly Ash as a Function of
- PDF / 2,813,727 Bytes
- 11 Pages / 417.6 x 639 pts Page_size
- 10 Downloads / 161 Views
AN ESCA AND SEM STUDY OF CHANGES IN THE SURFACE COMPOSITION AND MORPHOLOGY OF LOW-CALCIUM COAL FLY ASH AS A FUNCTION OF AQUEOUS LEACHING MYRA M. SOROCZAK*, H. C. EATON** and M. E. TITTLEBAUM College of Engineering, Louisiana State University, Baton Rouge, LA 70803 USA *Visiting from: Tennessee Valley Authority, National Fertilizer Development Center, Muscle Shoals, AL 35660 USA **Author to whom correspondence should be addressed. Received 20 October,
1986; refereed
ABSTRACT The reactivity of coal fly ash is dependent on the chemical composition of the surface. As reactions occur the ash particle size decreases and new material is available for reaction. This means that the near-surface chemistry can also be important. In the present study the surface chemistries of three ashes are determined by x-ray photoelectron spectroscopy both before and after exposure to a hydrating/leaching environment. Scanning electron microscopy is used to reveal ash morphology. The concentration of sulfur, found at the ash surfaces as a sulfate, and sodium decreased after leaching while the amount of iron and aluminum increased. Other elements, including calcium, increased and decreased with leaching depending on which ash was analyzed. Changes which occurred in the ash morphology after the removal of leachable elements are discussed. INTRODUCTION The production of steam by conventional coal-fired generators and large industrial boilers is expected to become increasingly important in the United States over the next decade. The amount of coal ash produced from conventional
combustion was 47.6 million metric tons in 1983 and is estimated to be 110 million metric tons by 1995. In 1983, about 85% of the ash produced in the U.S. was sent to disposal sites for ponding, landfilling (including disposal in surface mines), or interim ponding followed by landfilling [1]. Consequently, disposition of the ash produced as a byproduct of combustion will become a significant problem for industry. The mineral matter in coal is
composed of micrometer-size crystallites of
clays, carbonates, quartz and pyrite [2]. They are embedded in a carbonaceous matrix. Fly ash is produced at the combustion front in a coal-burning power plant. During combustion many of the minerals melt and coalesce producing the ash. The total amount of ash produced varies with the mineral content but can range from a few percent of the weight of the unburned coal to as much as 35%. Bottom ashes are also produced, but standard pulverized coal-fired boilers typically produce 80 to 90% of the ash as fly ash [1]. Low-calcium (Class F) bituminous coal fly ashes consist principally of five components:
crystalline,
substituted magnetite; hematite; amorphous
glass; crystalline, substituted mullite; and quartz [3]. As the particles or small spheres move through the stack, elements and compounds volatilized in the combustor condense onto the ash surfaces. Most of the phases in the bulk portion of the ash particles, e.g. aluminum and silicon oxides, pose no hazard to the environment, but
Data Loading...
