Combustion Characteristics of Carbon:  Influence of the Zone I−Zone II Transition on Burn-Out in Pulverized Coal Flames

Abstract

Transition of reaction from Zone II to Zone I during char combustion in a pulverized coal (pc) flame is shown to result in significant and discontinuous drop, jointly, in temperature and in reaction rate, with a correspondingly significant increase in burn-out time. In the particular case studied, as example, the estimated increase in burn-out time is by a factor of at least 3. The reduction in reaction rate is partly due to a (300 °C) drop in temperature, but it is jointly due to the doubling of the (operational) activation energy associated with the Zone transition. The probability of such Zone transition on particles in pc flames is controlled by the thermal balance that, at the end of combustion, is shown to depend critically on the “seen” wall temperature. In the case of a boiler, particles seeing the relatively cold walls may “extinguish”, and this is a possible or probably cause of increased loss-on-ignition (LOI) that is increasingly being reported for pc combustion in boilers.

Soot Oxidation Kinetics: A Comparison Study of Two Tandem Ion-Mobility Methods

“…However, there are few data available for high-temperature oxidation. It is well known that, for the combustion of porous carbon particles, with increasing temperature, there is a transition from regime I to regime II (from reaction-controlled to diffusion-controlled). , It has also been shown that the reaction rate in the diffusion-controlled zone (regime II) is proportional to the square root of the rate constant . Because the intrinsic reaction activation energy is related to the reaction rate constant through the Arrhenius equation, the apparent activation energy for the reaction should be 1/2 the intrinsic activation energy of the reaction.…”

Modeling high-pressure char oxidation using langmuir kinetics with an effectiveness factor

“…In this study, the correlations for Sherwood number and Nusselt number are [17] Nu ‫ס‬ 2.0 ‫ם‬ 0.60 Re Pr (11) These correlations allow the HP-CBK model to be used for large-particle char oxidation. The boundary layer diffusion is modeled in a manner similar to that used by Mitchell et al [15], taking Stefan flow into account [18].…”

“…A more mechanistically meaningful representation of the intrinsic reaction rate is a Langmuir-Hinshelwood form [6,9], which in its simplest form becomes the Langmuir rate equation: In an attempt to treat effects of pressure, Essenhigh proposed a so-called second effectiveness factor [10] to account for the internal combustion. The second effectiveness factor was calculated from the power index of the normalized density-diameter relationship [5,10,11]:…”

Simulation Study on the Reaction-Diffusion Coupling in Simple Pore Structures

“…To study the reaction-diffusion processes within the complicated porous media, previous models have mostly based on dimensionless groups, such as Damköhler number, Thiele modulus, and the corresponding external/internal effectiveness factors, − which were defined based on Fick’s diffusion law using empirical effective diffusion coefficients and idealized pellet shapes. As gas diffusion in complicated pores does not follow Fick’s law, and actual pore microstructures are not considered directly, , these models and indexes, though helpful, are often used on a phenomenological and qualitative basis. , …”

“…As gas diffusion in complicated pores does not follow Fick's law, 17 and actual pore microstructures are not considered directly, 23,24 these models and indexes, though helpful, are often used on a phenomenological and qualitative basis. 26,27 Since the pore structure in actual porous media is too complicated to be described integrally and elaborately, it is meaningful to first focus on some typical structures at microscale and try to characterize the coupling of reaction and diffusion in a more quantitative way. For this purpose, molecular dynamics (MD) simulation can be used as a powerful tool in that the complicated diffusion and reaction processes are reproduced by the basic motion of the microscopic elements, rather than numerically computed from their material properties or statistical correlations, which are hard to obtain at this scale.…”