Theoretical Analysis on the Criteria of MILD Coal Combustion

Abstract

MILD combustion is featured with a uniform heat flux and low NO x emission, and is thus a promising technology for clean coal utilization. Establishing the theoretical criteria of coal combustion modes is essential to guide the design and organization of MILD coal combustion. In this work, different coal combustion modes were classified theoretically based on the analysis of time scale, heterogeneous ignition, heat transfer, and flue gas entrainment. The predicted coal combustion modes agreed well with the experimental results from the literature. The effects of the structural and operational parameters on coal combustion modes were also discussed. As the nozzle diameter increases, the critical inlet Reynolds number switching from traditional combustion to MILD combustion increases, while the corresponding jet speed decreases. When the size of the furnace cross section increases, the critical inlet Reynolds number increases. As the coal particle diameter increases, the critical inlet Reynolds number decreases. As the secondary air temperature increases, the critical inlet Reynolds number decreases. The classification method of coal combustion modes was further applied to instruct the organization of MILD coal combustion on a Hencken burner. The experimental results proved that the theoretical analysis is reasonable.

Effect of Jet Velocity on the Formation of Moderate or Intense Low-Oxygen Dilution Combustion of Pulverized Coal

“…The calculation of time scale criterion in this work is the same as that in our previous work: 21 = ( )…”

“…The theoretical analysis of coal combustion modes was carried out on the basis of MILD coal combustion criteria proposed in our previous work . Based on the analysis of the time scale, flue gas entrainment, heat transfer, and ignition during combustion, the criterion of MILD combustion mode was proposed as shown in Table with reference to the MILD combustion conditions of gaseous fuel.…”

“…The calculation of time scale criterion in this work is the same as that in our previous work: t mix = L normalL true( 3 2 u normalf 2 true) 0.5 ≈ 2 3 l I u t normali = T normali − T 1 normald T normalp / normald t c = true( L normalL k 0.5 ν true) 0.5 where L L is the length scale of the integral vortex estimated by the characteristic length scale of the flow filed ( l ), u f is the fluctuation of velocity, I is the turbulent intensity, u is the mean velocity of jet inlet, T 1 is the temperature of the primary flow, d T p /d t is particle-heating rate, k is the turbulent kinetic energy, and ν is the fluid viscosity. The particle-heating rate can be simplified as follows when the radiative heat transfer is neglected: normald T normalp / normald t = 6 d normalp ρ normalc c normalp [ h false( T ...…”

“…The modified temperature increase is given as: normalΔ T = C coal H normalL C coal A ar c normalp + C coal ( 1 − A ar ) c normalg + 1 2 ( K i + K b ) c normalg where C coal is the mass concentration of coal in the primary flow; H L is the low heating value of coal; A ar is the proportion of ash in the received base; K i and K i are the flue gas entrainment ratio ignition point and when sufficient oxygen for complete combustion is entrained into the jet. The flue gas entrainment ratio, defined as the ratio of the total mass flow rate of a section in the jet region and the initial mass flow rate at the jet inlet, can be estimated by the distance from nozzle ( x ), the inner diameter of the nozzle ( D ) and the jet Reynolds number ( Re ) as follows: K = x D × { lefttrue 0.405 lg R e − 1.310 , R e < 15000 0.37 , R e > 15000 The parameters in the theoretical analysis were set according to the experimental conditions and the recommended values in our previous work …”

“…This is essential for the design and operation of MILD oxy-coal combustion technology because excessive jet velocity will give rise to heavy pumping power requirements. Feng et al 21 and Zhou et al 22 recently proposed theoretical criteria for MILD combustion mode of coal, but more detailed experimental data are still needed for improvement. More importantly, in addition to promoting the dilution and recirculation in the furnace, the influence of high jet velocity on the dispersion, heating, ignition of coal particles due to its strong shear, and strong turbulence mixing cannot be ignored, 23−26 but little attention has been paid to this in previous research.…”