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Erosion-Oxidation of Carbon Steel and Deposition of Particles on a Tube during Combustion of Coal-Based Fuels in an Industrial Boiler

  • James J. Xie
  • Peter M. Walsh

Abstract

An in situ test was used to assess the erosivity of particles in the convection section of an industrial boiler during combustion of micronized coal and while cofiring coal-water fuel with natural gas. Erosion was accelerated using a small jet of clean gas to increase the velocities of ash and unburned char particles at the surface of a carbon steel coupon. Because the jet alters the velocity distribution of the particles, a simulation of the system including particle behavior in the jet, erosion of both tube metal and oxide scale, and scale formation was needed to estimate erosion rates at lower velocities. Although the particles formed from coal-water fuel were more erosive than the particles from micronized coal in the accelerated test, calculations showed that the opposite would be the case on an isolated 51 mm diameter tube at typical convection section velocities. The change in relative erosion rates was due to differences in the size distributions and impaction efficiencies of the particles (effects of gas velocity and size of target). An accurate model is therefore essential to meaningful application of the results of the accelerated test. The calculations indicated that erosion of tube material at 550 K would be slower than 0.05.μ/hour at convection section velocities less than 8 m/s while firing coal-water fuel with natural gas and at velocities less than 10 m/s during combustion of micronized coal. At these velocities, under the conditions of gas and particle composition investigated, erosion is expected to be most rapid on the upstream stagnation line of an isolated tube and to remove material only from the oxide layer, not from the underlying carbon steel.

Deposition of particles, rather than erosion by particles, was the dominant process observed at the low velocities (3 to 4 m/s) present in the convection section of the boiler, in the absence of the accelerating jet. Deposition of particles from micronized coal decreased with increasing tube temperature, suggesting that thermophoresis made an important contribution to particle transport. At a tube temperature of 450 K, deposition of particles from micronized coal on the upstream stagnation line of a tube was 300 times greater than deposition of the particles from coal-water fuel. Erosion of the deposits by large particles was the explanation proposed for this behavior. The particles from coal-water fuel were larger (mean size of 65 μm vs. 43 μm for the particles from micronized coal) and had higher approach velocity (4.0 m/s vs. 3.1 m/s). The deposits were easily removed, but the sootblowing required to control flue gas temperature and maintain steam flowrate might lead to tube erosion. Comparison of the two fuels, which had nearly identical ash compositions, demonstrated the importance of fuel preparation and combustion to erosion and deposition, through their effects on carbon burnout and particle size distribution. The interaction between erosion and deposition is one of the areas most in need of additional work.

Keywords

Erosion Rate Carbon Steel Oxide Scale Metal Temperature Corrosion Engineer 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer Science+Business Media New York 1996

Authors and Affiliations

  • James J. Xie
    • 1
    • 2
  • Peter M. Walsh
    • 3
  1. 1.Department of Materials Science and EngineeringPennsylvania State UniversityUniversity ParkUSA
  2. 2.National Semiconductor CorporationSanta ClaraUSA
  3. 3.Energy and Environmental Research CorporationSandia National LaboratoriesLivermoreUSA

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