Publications

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Apparent char kinetic rates are commonly used to predict pulverized coal char burning rates. These kinetic rates quantify the char burning rate based on the temperature of the particle and the oxygen concentration at the particle surface, thereby inherently neglecting the impact of variations in the penetration of oxygen into the char on the predicted burning rate. To investigate the impact of variable extents of penetration during Zone II burning conditions, experimental measurements were performed of char particle combustion temperature and burnout for a common U.S. subbituminous coal burning in an optical laminar entrained flow reactor with either helium or nitrogen diluents. The combination of much higher thermal conductivity and mass diffusivity in the helium environments resulted in substantially cooler char combustion temperatures than in equivalent N2 environments. Measured char burnout was similar in the two environments for a given bulk oxygen concentration but was approximately 60% higher in helium environments for a given char combustion temperature. Detailed particle simulations of the experimental conditions confirmed a 60% higher burning rate in the helium environments as a function of char temperature, whereas catalyst theory predicts that the burning rate in helium could be as high as 90% greater than in nitrogen, in the limit of large Thiele modulus (i.e. near the diffusion limit). For application combustion in CO2 environments (e.g. for oxy-fuel combustion), these results demonstrate that due to differences in oxygen diffusivity the apparent char oxidation rates will be lower, but by no more than 9% relative to burning rates measured in nitrogen environments.

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The Reacting Particle and Boundary Layer (RPBL) model computes the transient-state conditions for a spherical, reacting, porous char particle and its reacting boundary layer. RPBL computes the transport of gaseous species with a Maxwell-Stefan multicomponent approach. Mass transfer diffusion coefficients are corrected to account for a non-stagnant bulk flow condition using a factor based on the Sherwood number. The homogeneous gas phase reactions are modeled with a syngas mechanism, and the heterogeneous reactions are calculated with a six-step reaction mechanism. Both homogeneous and heterogeneous reaction mechanisms are implemented in Cantera. Carbon density (burnout) is computed using the Bhatia and Perlmutter model to estimate the evolution of the specific surface area. Energy equations are solved for the gas temperature and the particle temperature. The physical properties of the particle are computed from the fractions of ash, carbon, and voids in the particle. The void fraction is computed assuming a constant diameter particle during the reaction process. RPBL solves a particle momentum equation in order to estimate the position of the particle in a specific reactor. We performed a validation and uncertainty quantification study with RPBL using experimental char oxidation data obtained in an optically accessible, laminar, entrained flow reactor at Sandia National Laboratories. We used a consistency analysis to compare RPBL and experimental data (with its associated uncertainty) for three coal chars over a range of particle sizes. We found consistency for particle temperature and velocity across all experiments.

The proper consideration of the radiation correction for bare-wire thermocouple measurements requires consideration of the convective and radiative heat transfer of the thermocouple with its surroundings, as well as conductive heat transfer between the thermocouple bead and the connecting thermocouple wires. This has rarely been considered in the past, and to do so has involved complex simulation of the complete thermocouple energy balance. This paper reports on a new, easy-to-implement approach for calculating the proper radiant correction for thermocouples, subject to uncertainties associated with the relevant thermocouple and gas properties and limitations to characterizing convective heat transfer to the thermocouple bead and wires via standard correlations. Examples of the radiation correction computed with this new method as a function of temperature and bead and wire size are given, and are compared with traditional approaches considering heat transfer around either the thermocouple bead or the thermocouple wire.

In the formation of nano-particles during coal char combustion, the vaporization of inorganic components in char and the subsequent homogeneous particle nucleation, heterogeneous condensation, coagulation, and coalescence play decisive roles. However, conventional measurements cannot provide detailed information on the dynamics of nano-particle formation and evolution. In this work, a sophisticated intrinsic char kinetics model that considers ash effects (including ash film formation, ash dilution, and ash vaporization acting in tandem), both oxidation and gasification by CO2 and H2O, homogeneous particle nucleation, heterogeneous vapor condensation, coagulation, and coalescence mechanisms is developed and used to compare the temporal evolution of the number and size of nano-particles during coal char particle combustion as a function of char particle size, ash content, and oxygen content in O2/N2 and O2/CO2 atmospheres. Based on comparisons with measurements of char particle temperature, carbon conversion, mineral vaporization, and mean size of nano-particles at various residence times, the model can accurately predict the transient combustion of pulverized coal char particles and nano-particle formation and growth. Model results show that in either O2/N2 or O2/CO2 atmospheres, the char combustion temperature has a dominant effect on the formation and growth of nano-particles. High char burning temperatures result in a high mineral vaporization rate within the char particle, and subsequent high nucleation and condensation rate, and consequently more and larger nano-particles. As a result, high oxygen content, low ash content, and small sized char particles, all of which promotes high local char burning temperatures, yield more nano-particles and shift the nano-particle size distribution to larger sizes. In comparison to combustion in O2/N2, both the number density and size of the nano-particles formed in O2/CO2 are lower. Unlike condensation, which contributes to particle growth until the vapor molecules are fully consumed, nucleation ceases during the last stage of char combustion.

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Non-premixed oxy-fuel combustion of natural gas is used in industrial applications where high-intensity heat is required, such as glass manufacturing and metal forging and shaping. In these applications, the high flame temperatures achieved by oxy-fuel combustion increase radiative heat transfer to the surfaces of interest and soot formation within the flame is desired for further augmentation of radiation. However, the high cost of cryogenic air separation has limited the penetration of oxy-fuel combustion technologies. New approaches to air separation are being developed that may reduce oxygen production costs, but only for intermediate levels of oxygen enrichment of air. To determine the influence of oxygen enrichment on soot formation and radiation, we developed a non-premixed coannular burner in which oxygen concentrations and oxidizer flow rates can be independently varied, to distinguish the effects of turbulent mixing intensity from oxygen enrichment on soot formation and flame radiation. Local radiation intensities, soot concentrations, and soot temperatures have been measured using a thin-film thermopile, planar laser-induced incandescence (LII), and two-color imaging pyrometry, respectively. The measurements show that soot formation increases as the oxygen concentration decreases from 100% to 50%, helping to moderate a decrease in overall flame radiation. An increase in turbulence intensity has a marked effect on flame height, soot formation and thermal radiation, leading to decreases in all of these. The soot temperature decreases with a decrease in the oxygen concentration and increases with an increase in turbulent mixing intensity. Altogether, the results suggest that properly designed oxygen-enriched burners that enhance soot formation for intermediate levels of oxygen purity may be able to achieve thermal radiation intensities as high as 85% of traditional oxy-fuel burners utilizing high-purity oxygen.

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Truly quantifying soot concentrations within turbulent flames is a difficult prospect. Laser extinction measurements are constrained by spatial resolution limitations and by uncertainty in the local soot extinction coefficient. Laser-induced incandescence (LII) measurements rely on calibration against extinction and thereby are plagued by uncertainty in the extinction coefficient. In addition, the LII measurements are subject to signal trapping in flames with significant soot concentrations and/or flame widths. In the study reported here, a turbulent ethylene non-premixed jet flame (jet exit Reynolds number of 20,000) is investigated by a combination of LII and full-flame HeNe laser (633 nm) extinction measurements. The LII measurements have been calibrated against extinction measurements in a laminar ethylene flame. An extinction coefficient previously measured in laminar ethylene flames is used as the basis of the calibration. The time-Averaged LII data in the turbulent flame has been corrected for signal trapping, which is shown to be significant in this flame, and then the line-of-sight extinction for a theoretical 633 nm light source has been calculated acrob the LII-determined soot concentration field. Comparison of the LII-based extinction with that actual measured along the flame centerline is favorable, showing an average deviation of approximately 10%. This lends credence to the measured values of soot concentrations in the flame and also gives a good indication of the level of uncertainty in the measured soot concentrations, subject to the additional uncertainty in the previously measured extinction coefficient, estimated to be ±15%.

Oxy-fuel combustion is a well-known approach to improve the heat transfer associated with stationary energy processes. Its overall penetration into industrial and power markets is constrained by the high cost of existing air separation technologies for generating oxygen. Cryogenic air separation is the most widely used technology for generating oxygen but is complex and expensive. Pressure swing adsorption is a competing technology that uses activated carbon, zeolites and polymer membranes for gas separations. However, it is expensive and limited to moderate purity O₂ . MOFs are cutting edge materials for gas separations at ambient pressure and room temperature, potentially revolutionizing the PSA process and providing dramatic process efficiency improvements through oxy-fuel combustion. This LDRD combined (1) MOF synthesis, (2) gas sorption testing, (3) MD simulations and crystallography of gas siting in pores for structure-property relationship, (4) combustion testing and (5) technoeconomic analysis to aid in real-world implementation.

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Planar laser-induced incandescence (LII) imaging is reported at repetition rates up to 100 kHz using a burst-mode laser system to enable studies of soot formation dynamics in highly turbulent flames. To quantify the accuracy and uncertainty of relative soot volume fraction measurements, the temporal evolution of the LII field in laminar and turbulent flames is examined at various laser operating conditions. Under high-speed repetitive probing, it is found that LII signals are sensitive to changes in soot physical characteristics when operating at high laser fluences within the soot vaporization regime. For these laser conditions, strong planar LII signals are observed at measurement rates up to 100 kHz but are primarily useful for qualitative tracking of soot structure dynamics. However, LII signals collected at lower fluences allow sequential planar measurements of the relative soot volume fraction with a sufficient signal-to-noise ratio at repetition rates of 10-50 kHz. Guidelines for identifying and avoiding the onset of repetitive probe effects in the LII signals are discussed, along with other potential sources of measurement error and uncertainty.

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Final burnout of char particles from practical fuels such as coal and biomass occurs in the presence of a large ash component. Also, newly utilized coal resources, such as those from India, often contain much larger ash fractions than have traditionally been utilized. In the past, the inhibitory influence of ash on pulverized coal particle combustion has been most frequently modeled using an ash film model, though such films are rarely found when examining partially combusted particles. Conversely, some measurements have suggested that mineral components exposed on the surface of burning pulverized coal (pc) particles may diffuse back into the char matrix, the effect of which can be modeled as an ash dilution effect. To explore the implications of these different ash inhibition models on the temporal evolution of char combustion during burnout, we have developed a new computational model that considers the possibility of an ash film effect, an ash dilution effect, or some arbitrary combination of the two effects acting in tandem, which is the most realistic scenario. This new model predicts that restricted diffusion through the ash film has a significant impact on the char burnout rate throughout its lifetime, whereas char dilution only inhibits combustion significantly when most of the char has been consumed and the combustion mode shifts from predominantly external diffusion control to mixed diffusion control, with sensitivity to both external and internal diffusion resistance. The comparison of the model predictions with experimental results also confirms the previously suggested need to include gasification reaction steps when modeling coal char combustion.

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