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The reaction of SO 2 with fly ash in the presence of O 2 and H 2 O involves a series of reactions that lead to the formation of SO 3 and eventually H 2 SO 4 . Homogeneous experiments were conducted to evaluate the effects of the procedural variables, i.e., temperature, gas concentrations, and residence time, on the post-combustion conversion of SO 2 to SO 3 . The results were compared to existing global kinetics and found to be dependent upon SO 2 , O 2 , residence time, and temperature and independent of H 2 O content. For a residence time of 1 s, temperatures of about 900°C are needed to have an observable conversion of SO 2 to SO 3 . Literature suggested that the conversion of SO 2 to SO 3 is dependent upon the iron oxide content of the fly ash. Experiments using three different fly ash samples from Australian sub-bituminous coals were used to investigate the catalytic effects of fly ash on SO 2 conversion to SO 3 at a temperature range of 400−1000°C. It was observed that fly ash acts as a catalyst in the formation of SO 3 , with the largest conversion occurring at 700°C. A homogeneous reaction at 700°C, without fly ash present, converted 0.10% of the available SO 2 to SO 3 . When fly ash was present, the conversion increased to 1.78%. The catalytic effect accounts for roughly 95% of the total conversion. Average SO 3 /SO 2 conversion values between fly ash derived from air and oxy-fuel firing and under different flue gas environments were found to be similar.

This paper presents a comparison of techniques that can be used to obtain the sintering temperature of coal ash. These techniques are thermal conductivity analysis (TCA), thermomechanical analysis (TMA), the compressive strength test (CS), and a proposed new technique called the pressure-drop technique. Generally, the results from these four techniques compare well and showed sintering temperatures in the range of 600−1000 °C. These temperatures reflect the onset of strength development in coal ash deposits. An analysis of the results indicates the compressive strength and thermal conductivity techniques are believed to overestimate the sintering temperature of coal ash as measurements were conducted at temperature intervals and these measurements depend on the treatment time. Thermomechanical analysis and pressure-drop techniques are believed to provide a more accurate indication of the sintering temperature due to the constant monitoring of ash structural changes and the inherent sensitivity of the methods to structural changes.

A suite of coal maceral concentrates were prepared from a single coal using a water-based method for two discrete particle size fractions. Coal macerals were produced with the vitrinite content varying from 91.2% to 26.1% for the 106−212 μm particle size fraction and 96.0% to 38.2% for the 212−500 μm particle size fraction. Thermo-swelling of coal maceral concentrates were evaluated by the Computer Aided Thermal Analysis (CATA) technique with extended volumetric measurements. This novel CATA technique allows the apparent specific heat, thermal conductivity, and transient volumetric swelling of the coal sample to be measured simultaneously, as a function of temperature. The experimental results indicated that the maximum swelling (∼510 °C) and high-temperature contraction (600−1000 °C), as well as exothermic heat during primary devolatilization and thermal conductivity at maximum swelling all increased with vitrinite content, with the data inferring a linear relationship, and was independent of particle size, when coal maceral concentrates contain more than 63% vitrinite. A hypothesis for intragranular (within particles) and intergranular (between particles) swelling was used to explain the association of swelling with vitrinite content and particle size. Optical microscopic results of the final pyrolytic residues of different coal maceral concentrates support this hypothesis.

Ash produced during oxy-fuel combustion is expected to differ from ash produced during air combustion because of the higher CO 2 and SO 2 atmospheres in which it is generated. For a quantitative understanding of the sulfation behavior of fly ash in oxy-fuel combustion, fly ash from three commercial Australian sub-bituminous coals was tested and decomposed under an inert atmosphere. Thermal evolved gas analysis was completed for ash produced in both air and oxy-fuel environments. Pure salts were also tested under the same conditions to allow for identification of the species in the ash that capture sulfur, along with thermodynamic modeling using FactSage 6.3. Sulfur evolved during the decomposition of air and oxy-fuel fly ash was compared to the total sulfur in the ash to close the sulfur balance. Both total sulfur captured by the ash and sulfur evolved during decomposition were higher for oxy-fuel fly ash than their air counterparts. Correlations of capture with ash chemistry are presented.

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