Analysis of particle seeding on the performance of a directly-fired furnace
A mathematical model was developed to predict physico-chemical phenomena occurring within the combustion space of a directly-fired natural gas furnace. The complete furnace model included submodels to solve momentum, species concentration, and energy conservation equations throughout the furnace volume. Supplementary equations were solved to predict turbulence, radiative heat transfer, nitric oxide formation, and the oxidation of soot particles. Both the submodels and the complete furnace model were validated by comparing predictions with experimental data from a variety of flows, flames, and furnaces. The furnace model was used to investigate particle seeding within a 12 MW directly-fired furnace. Particles were assumed to be non-scattering and to have the same optical properties as soot. An initial study of non-oxidizing particles seeded uniformly throughout the furnace volume indicated an optimum range of seeding concentration, afforded an understanding of how different regions of the furnace respond to particle seeding, and guided the selection of a particle injection rate for non-uniform seeding studies. Studies with non-uniform seeding concentrations revealed the effects of particle oxidation and furnace wall temperature. Non-oxidizing particles increased the total incident radiative heat rate to the furnace outer wall by more than 20%, and decreased nitric oxide formation by an order of magnitude due to reduced peak temperatures in regions of significant nitric oxide formation. Particle oxidation was found to destroy virtually all the particles near the edges of the flame, thereby preventing any significant heat transfer enhancement although nitric oxide formation was still reduced. Furnace wall temperature affected the heat transfer processes within the furnace and altered the way in which the furnace was affected by particle seeding. Increasing the wall temperature from 400 to 1200 K led to an increased density of radiative energy within the furnace volume. With (non-oxidizing) particle seeding, much of this energy was absorbed in the core of the flame where a high concentration of particles reside. The total incident radiative heat rate was increased by 11% and the net radiative heat transfer was increased by approximately 30%.
Ramadhyani, Purdue University.
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