Study on atomic sodium release from pulverised coal particles in a pre-mixed natural gas flame
Atomic sodium release during coal combustion has been shown to be a significant factor in fouling and corrosion of heat transfer surfaces within industrial coal-fired boilers. Although there has been significant research into the final forms of sodium from combustion, information about the release of sodium is limited. An initial study of the release of atomic sodium from pulverised coal particles in a pre-mixed natural gas flame was conducted using planar laser induced fluorescence, PLIF. Measurements were made of the intensity of sodium fluorescence, coal particle velocities and gas phase temperature within the flame in a purpose-designed laminar flame facility that provides similar time-temperatures history to coal particles in their early stages of combustion in an industrial coal-fired boiler. The velocity of the coal particles was measured to be about 6.1 m/s using particle streak velocimetry obtained from the natural luminescence of the hot particles and a 2.25 ms gate of the CCD camera. Temperatures were measured using thermocouples, and the maximum measured temperature was ~1500 °C. Calibration of the sodium fluorescence was not undertaken, but useful information was obtained of the release rate of sodium atoms from combusting pulverised coal particles.
The controlling chemistry in flame generated surface deposition of Na2SO4 and the effects of chlorine
This paper examines the behavior of sodium/sulfur flame deposition on cooled probe surfaces for cases where the sulfur concentration is equal to or in excess of that of sodium. In all cases, Na2SO4 is the predominant component deposited, quite pure in most cases, but occasionally indicating the presence of several percent of Na2CO3 and implying possible thermodynamic control by the surface. Rates of deposition are exactly first order with respect to sodium concentrations, and zero order with respect to sulfur. The surface shows a total indifference to the atomic or molecular state of the flame sodium, be it Na, NaOH, or NaCl. Moreover, it is indifferent to the nature of the flame, its fuel type, stoichiometry, temperature, or state of equilibration. The surface adsorption rate of sodium is very efficient for probes at 640–780 K (35–60%) and appears to be the rate-controlling kinetic step and is not governed by normal boundary layer or laminar/turbulent flow considerations. Deposition displays a linear growth rate with negligible incubation time and is irreversible. Up to at least the 100-ppm concentration level, salts such as Na2SO4, Na2SO3, Na2S2O3, and NaNO3 are shown to be equivalent sources of sodium or sulfur. For the range of probe temperatures (640 to 780 K) and with Inconel-600 probe material, negligible differences were noted for rates of deposition in a wide range of propane and hydrogen flames. Rather surprising, however, was the fact that deposits formed on top of the burner (350 K) reflected closely those on the probe and were not mixtures of other possible sodium/sulfur compounds. Preliminary experiments incorporating chlorine into the system indicate, for high levels of chlorine (0.4% Cl2), that the deposit consists of one-third Na2SO4 and two-thirds NaCl. At lower levels of chlorine no such halide component has been reported.
A review of factors controlling the gas turbine hot section environment and their influence on hot salt corrosion test methods
AbstractEnvironmental and operational factors that influence the formation and deposition of corrosive species on hot section components in gas turbine engines are reviewed. In addition to air and fuel impurities, combustion gas chemistry, velocity, pressure and temperature are identified as key operational factors affecting the formation and deposition of Na2SO4 salt (the primary corrosive species) when trace amounts of sodium and sulphur are entrained into combustors. Test methods for ranking the resistance of different materials to hot salt corrosion are reviewed and compared in terms of the type of damage they produce. The methods considered range from a simple furnace or crucible test to burner rig and engine tests. It is shown that high velocity burner rigs, running at atmospheric pressure, allow all the relevant operational factors to be simulated, and they produce realistic hot corrosion damage similar to that observed on engine parts in service.