| Uncertain supply of petroleum-derived fuels makes it attractive to convert oil-designed boilers to coal firing. Among the questions to be addressed when considering such a conversion is the effect of ash and unburned char particles on performance of convective heat exchangers. Two candidate fuels, coal-water fuel and micronized coal (pulverized coal, finely ground), were burned in an oil-designed industrial boiler at Penn State's Energy and Fuels Research Center. Erosion of carbon steel by ash and unburned char particles was measured in the convective section of the boiler. The rate of erosion was enhanced by directing a small jet of clean gas (nitrogen, 3% oxygen in nitrogen, air, or oxygen) to accelerate the particles toward the surface of a test coupon. Each sample was exposed to the effects of the jet and particles for 2 hours with metal temperature at 450, 550, or 650 K.; A model for simultaneous erosion and oxidation of carbon steel provided the basis for determination of erosion rate coefficients from the experimental observations. In the model, metal undergoes ductile erosion, increasing with increasing temperature, while oxide exhibits brittle erosion, at a rate independent of temperature. Oxide and metal are removed in series, their proportions depending upon their relative resistance to erosion and the steady average thickness of the scale. Over most of the range of temperature investigated, oxide scale was more resistant to erosion than the carbon steel substrate.; Four parameters were adjusted to fit the model to the measurements: (1) Erosivity of the particles toward the metal; (2) Erosivity of the particles toward the metal oxide; (3) Effective order of the metal oxidation process with respect to oxygen; (4) Average area of scale removed by a single impact. Using the values for these parameters which fit the accelerated erosion measurements, the gas velocity dependence of the erosion rate was calculated for typical convection section conditions, corresponding to the coal-water fuel combustion and micronized coal combustion conditions investigated.; Under steady conditions the average thickness of oxide scale is that at which its rates of formation by oxidation and removal by erosion are equal. At low velocities, where material loss is slow, the oxide layer is thick and erosion behavior is that characteristic of oxide. As velocity is increased the steady thickness of oxide layer decreases. The onset of the transition from oxide- to metal-controlled erosion, where the rate of metal loss begins to increase rapidly, was predicted to occur at the following velocities: 12 m/s for coal-water fuel and 16 m/s for micronized coal at an impaction angle of {dollar}90spcirc{dollar} (on the upstream stagnation line of the tube); and 21 m/s for coal-water fuel and 25 m/s for micronized coal at an impaction angle of {dollar}30spcirc.{dollar} Erosion on the stagnation line was expected to be slower than 0.05 {dollar}murm m/h{dollar} at gas velocities less than approximately 8 m/s for coal-water fuel and 9.4 m/s for micronized coal, under respective conditions of particle size, particle composition, particle loading, metal temperature, and gas composition at which the measurements were made. (Abstract shortened by UMI.)... |
