PDF modeling and simulations of pulverized coal combustion – Part 2: Application
A transported probability density function (PDF) method developed for pulverized coal combustion is applied in simulations of a semi-industrial scale furnace. It is found that the simulation results are sensitive to the chosen model for unclosed terms in the gas-phase PDF transport equation due to the mass transfer from the solid to the gas phase through devolatilization and char combustion. A model which leads to a generation of mixture fraction fluctuations provides results which are in close agreement with the measurements. Two different models for the gas phase composition seen by the coal particles are investigated. One model is based on the local mean values of the gas composition and one model provides time correlated fluctuations of the seen composition. Small differences between the two model results are observed in the predictions of the char burnout. The simulation results are further used to analyze commonly adopted model assumptions in presumed PDF methods. It is found that both, the char combustion mixture fraction and the enthalpy are not statistically independent from the volatile mixture fraction. It is further shown that the marginal PDF of the volatile mixture fraction is not well approximated by a beta function PDF in regions of rapid devolatilization. The marginal enthalpy PDF indicates significant enthalpy fluctuations which are usually neglected in presumed PDF methods.