Numerical Analysis on Microscopic Characteristics of Pulverized Coal Moderate and Intense Low-Oxygen Dilution Combustion

Abstract

Moderate and intense low-oxygen dilution (MILD) combustion is regarded as a new clean combustion mode, which can simultaneously improve combustion efficiency and decease NO x emission. The computational fluid dynamics (CFD) modeling on pulverized coal MILD combustion was conducted to simulate the flue gas velocity, temperature and species concentration fields with two turbulence−chemistry interaction models, which were well-validated by the experimental data of International Flame Research Foundation (IFRF) furnace no. 1. Then, macro-and microscopic characteristics of pulverized coal MILD combustion were quantitatively analyzed to emphasize the unique MILD combustion regime compared to the conventional flame combustion. The eddy dissipation concept (EDC) model with a four-step global reaction can well reproduce turbulence−chemistry interaction behavior of pulverized coal MILD combustion. Strong turbulent mixing and entrainment of hot flue gases is found in the furnace because of high momentum secondary air jets. The internal flue gas recirculation ratio in the furnace is over 5, which makes local oxygen concentrations lower than 5% before the coal devolatilization proceeds and makes the fuel temperature higher than its autoignition temperature. Furthermore, the pulverized coal MILD combustion regime is first depicted with the calculated Damkoḧler number and Karlovitz number on the basis of different regimes of turbulent nonpremixed flames. The numerical results confirm that pulverized coal MILD combustion is in a slow chemistry regime (Da I < 10 and Ka ≫ 1) and disperses throughout the whole furnace volume. Pulverized coal MILD combustion is a unique combustion mode with strong turbulent mixing and entrainment, high internal flue gas recirculation ratio, and slow reaction rate under a low local oxygen concentration with respect to the traditional flame combustion mode.

MILD Combustion of Solid Fuels: Its Definition, Establishment, Characteristics, and Emissions

“…Many numerical studies examined the temperature distribution, species concentration distribution, heat transfer, and char gasification features during MILD combustion of pulverized coal. 66,67 Previous studies found that the volatile reaction in MILD conditions was influenced by the highly diluted atmosphere, 68 and the reaction mechanisms for methane MILD combustion were optimized. 69,70 Mei et al 62,71 numerically investigated the effect of initial conditions on the temperature distribution of the IFRF MILD furnace.…”

“…The MILD combustion of pulverized coal is characterized by volumetric reaction, uniform temperature field with a low peak temperature, and uniform species concentrations. Many numerical studies examined the temperature distribution, species concentration distribution, heat transfer, and char gasification features during MILD combustion of pulverized coal. , Previous studies found that the volatile reaction in MILD conditions was influenced by the highly diluted atmosphere, and the reaction mechanisms for methane MILD combustion were optimized. , Mei et al , numerically investigated the effect of initial conditions on the temperature distribution of the IFRF MILD furnace. When the primary air and secondary air jets were parallel or separated, the increase in primary air jet velocity from 26 to 67 m/s enhanced the entrainment and delayed the jets mixing, thus reducing the peak temperature by about 180 K and facilitating the establishment of MILD combustion.…”

Numerical study of further NO x emission reduction for coal MILD combustion by combining fuel‐rich/lean technology

“…The detailed configuration of the furnace is shown in Figure 1, which is first studied by the International Flame Research Foundation (IFRF) and subsequently adopted in many research works. [5][6][7][8][9]25,26 The burner consists of a central secondary air inlet (Ø 125 mm) and two primary air inlets (Ø 27.3 mm), through which flue gas from another combustor and nonpreheated air carrying coal particles are injected, respectively. Pure oxygen is added to the secondary air stream to maintain a similar O 2 concentration with the primary air stream.…”

Numerical investigation of the effect of constant velocity and constant residence time scaling criteria on the natural gas MILD combustion

“…There have been several studies on MILD combustion, both experimentally and numerically, such as; effects of geometry and operating condition, the effect of burner configuration (Nada et al, 2015;Lee et al, 2019a), furnace chamber configuration (Schaffel-Mancini et al, 2010;Tu et al, 2015a), jet velocity of reactant (Mi et al, 2011;Veríssimo et al, 2013), reactant temperature (Khoshhal et al, 2011;Huang et al, 2014), chemical composition of fuel and oxidizer (Dally et al, 2004;Tu et al, 2015b;Lee et al, 2019b), on NOx formation and reduction (Wünning and Wünning, 1997;Mancini et al, 2002) and microscopic characteristics (Jin and Zhou, 2015;Zhang et al, 2019;).…”