Apparatus for direct measurement of ash fusion and sintering behavior at elevated temperatures and pressures

Abstract

Ash fusion, sintering, and deposition may impose serious operational difficulties in conventional and advanced coal-combustion systems. Conventional ash fusion techniques (e.g., ASTM methods) determine the qualitative behavior of ash samples at atmospheric pressure. Presently, there is no known available technique that can measure the behavior of coal ash at elevated temperatures and pressures. In the literature, methods based on electrical resistance or shrinkage of coal ash have been reported at atmospheric pressure (elevated temperatures) conditions. A high-pressure microdilatometer (HPMD) has been developed to investigate ash fusion and sintering behavior at elevated pressures and temperatures by the simultaneous measurement of the temperature of initial contraction and electrical resistivity of samples. This novel technique facilitates the measurement of ash properties over a wide range of temperature, pressure, and gas atmosphere (oxidizing, reducing, or inert). The operating principle of the HPMD includes measuring the temperature at which there is a significant ‘‘shift’’ in the electrical resistivity (and/or sample volume) that represents ash sintering and fusion. Sintering occurs through the formation of solid-state, particle-to-particle ‘‘necks’’ or the appearance of a molten phase, which allows a path for electrical conductance. The ability to perform both resistivity and shrinkage measurements simultaneously or independently at elevated pressures makes the HPMD truly unique. The HPMD can also be used to investigate the swelling and softening behavior of pyrolyzing coal at elevated pressures and relatively rapid heating rates. The HPMD can provide insights into the sintering/fusion of coal ash or coal swelling at a range of conditions: (a) the influences of various gas atmospheres can be investigated, (b) the effects of pressure can be studied, (c) different temperature/heating rate schemes can be used (constant rates, isothermal holds below or above the sintering temperature, etc.), and (d) studies can be performed to investigate the influence of increased heating rate at elevated pressures (which were not performed previously) on coal swelling and plasticity. It can also be applied to other fossil fuels such as oil shale or carbonaceous materials (e.g., graphite) to measure their electrical conductivity or expansion and contraction behavior at ambient or elevated pressures and temperatures. The HPMD allows researchers to gather novel data and thereby facilitate a fundamental understanding of the behavior of coal or coal ash at actual processing conditions.