Flame structure analysis and flamelet modeling of turbulent pulverized solid fuel combustion with flue gas recirculation

Abstract

In this work, carrier-phase direct numerical simulation (CP-DNS) is conducted for a pulverized coal flame in a temporally evolving turbulent jet with flue gas recirculation (FGR). Detailed gas phase kinetics are considered and heavy hydrocarbon molecules up to C20 are included to accurately represent tars in the volatile matter. The structure of the pulverized coal flame is analyzed with different flamelet models considering two mixing scenarios, namely the mixing of recirculated flue gases with the other fuel or oxidizer streams. In the first model approach, the mixing of recirculated flue gases with the gaseous fuels released from the coal particle is characterized with a fuel-split-based flamelet (FLT-FS) model. In the second approach, the mixing of flue gases with the transport air is described with an oxidizer-split-based flamelet (FLT-OS) model. In total, five trajectory variables are introduced in the flamelet table to represent the pulverized coal combustion states with FGR. The suitability of the flamelet models is evaluated through an a priori analysis for both the fully ignited state, as well as (more challenging) the igniting states. Comparisons show that both the FLT-FS model and the FLT-OS model perform well in predicting the thermo-chemical quantities for the fully ignited state. The FLT-OS model performs slightly better than the FLT-FS model in predicting the gas temperature and specific species mass fractions. This is due to the fact that the partial oxidization of the gaseous fuels by the hot flue gases outside the mixing layers cannot be reproduced by the FLT-FS model. While the state at the beginning of ignition can still be accurately predicted by the FLT-OS model, discrepancies can be observed for the tar species C20H10 and the intermediate species CO at a later stage when many particles ignite and the reason for this is explained. Further analysis in the flamelet solution spaces shows that the gaseous fuels are ignited on the fuel-lean side. The time evolution of the gas temperature from the beginning of ignition to the fully ignited state can be overall characterized by both flamelet models.