Radiative properties of burning char particles are one of the key parameters for heat transfer in pulverized fuel boilers. Among the parameters which are suspected to change emissivity of burning ...char surfaces, particle temperature and the reaction atmosphere (air-fired vs. oxy-fuel) are two factors needing investigation. An experimental campaign is presented which studies the effect of both factors in the spectral range from 1.25 µm to 5.5 µm. An in-flight spectrometer test rig measuring thermal radiation emitted by single coal particles was employed. Particle temperature was adjusted by varying the oxygen content in the oxy-fuel atmosphere between 15 and 40%. Average char particle temperatures increase from 1952 to 2582 K with increasing oxygen content. The emissivity drops from ε = 0.6 to ε = 0.26 (λ = 2.4–5.5 µm) and ε = 0.33 to ε = 0.24 (λ = 1.25–2.25 µm) with increasing temperature. The influence of the reaction atmosphere was investigated by performing experiments in a N2/O2 atmosphere and comparing to the results to data from an oxy-fuel atmosphere, both containing 20% of oxygen. The difference εoxy-fuel-εN2/O2 is in the range of 0.1 and 0.2 for λ = 1.25–2.25 µm and λ = 2.4–5.5 µm. Effects of burnout and char structure are discussed to evaluate the results.
The investigation of burnout effects on the char particle emissivity in the spectral range from 1.25 to 5.5µm is presented. Single coal particles of a bituminous coal were combusted in a flat flame ...burner under oxy-fuel conditions (20.1mol%). The emissivity was determined using a test-rig, which also measures particle temperature and diameter, both being necessary input parameters for single particle emissivity, in the visual spectral range. The infrared radiation was measured spectrally resolved with a fiber spectrometer (1.25–2.25µm) and integrated with an InSb detector (2.4–5.5µm). The emissivity decreases clearly with progressing burnout: E.g. at 1255nm the emissivity decreases from 0.49 to 0.38. The effect is larger in the spectral range from 1.25 to 2.25µm, but still visible in the longer wave length range.
The combined “Blocked-Off & Discrete Ordinates” (BO/DO) approach is used within a DEM/CFD frame to model the radiative heating of stationary and moving (rotating with respect to the mesh) cubic ...particles. A single cube, placed inside a larger cubic box, is heated by radiation and its temperature evolution over time is investigated. Two different models for the particle heating are considered: the first one assumes a uniform cube temperature while the second model calculates the intra-particle heat transfer leading to a non-uniform temperature distribution as it occurs in thermally thick particles. Three different particle materials, wood, styrofoam and steel, are considered.
Initially, the radiative heating of stationary cubes, approximated by the BO approach in a CFD mesh, is compared to the heating of cubes enclosed by a corresponding body conformal mesh as a reference. The results show that deviations in cube temperature mainly result from the artificial increase of the particle surface in the BO. When applying a particle surface area ratio to compensate for the artificial surface area increase deviations in temperature can be reduced to 5.2% compared to body conformal mesh.
In a further step, the cube is constantly rotating which changes the orientation of the cube's surfaces relative to the radiation source. The evolution of the cube's surface temperatures, which is dependent on the orientation, is modelled by the BO/DO approach. As expected, materials with low thermal conductivity show larger temperature gradients over time than materials with high thermal conductivity. The temperature difference between different surfaces turns out to be in the ranges 50/14/0 K (for styrofoam/wood/steel) and 134/65/0 K between surface and core, thus reflecting the correct physical behavior. The evaluation of the BO/DO approach concerning its capability to account for radiative heat transfer in DEM simulations for particles with a regular (cubic) geometry, as examined in the current paper, forms the basis to extend the description to arbitrarily shaped particles, a great potential of the BO/DO approach as will be shown in an outlook example.
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•Transient simulations of radiative heating of moving, complex particles in DEM/CFD.•Using the Blocked-Off approach in combination with the DO model.•Due to the BO approach, temperature deviates by up to 5.2%.•Particle rotation has a large impact on the heating of the particle.•Low thermal conductivity materials have large intra-particle temperature gradients.
Cytomegalovirus (CMV) infection is a common, potentially life-threatening complication following allogeneic hematopoietic stem cell transplantation (allo-HSCT). We assessed prospectively the safety ...and efficacy of stem cell-donor- or third-party-donor-derived CMV-specific T cells for the treatment of persistent CMV infections after allo-HSCT in a phase I/IIa trial. Allo-HSCT patients with drug-refractory CMV infection and lacking virus-specific T cells were treated with a single dose of ex vivo major histocompatibility complex-Streptamer-isolated CMV epitope-specific donor T cells. Forty-four allo-HSCT patients receiving a T-cell-replete (D
repl; n=28) or T-cell-depleted (D
depl; n=16) graft from a CMV-seropositive donor were screened for CMV-specific T-cell immunity. Eight D
depl recipients received adoptive T-cell therapy from their stem cell donor. CMV epitope-specific T cells were well supported and became detectable in all treated patients. Complete and partial virological response rates were 62.5% and 25%, respectively. Owing to longsome third-party donor (TPD) identification, only 8 of the 57 CMV patients transplanted from CMV-seronegative donors (D
) received antigen-specific T cells from partially human leukocyte antigen (HLA)-matched TPDs. In all but one, TPD-derived CMV-specific T cells remained undetectable. In summary, adoptive transfer correlated with functional virus-specific T-cell reconstitution in D
depl patients. Suboptimal HLA match may counteract expansion of TPD-derived virus-specific T cells in D
patients.
This work presents first-of-its-kind high-speed planar laser-induced fluorescence measurements of the hydroxyl radical in the boundary layer of single coal particles. Experiments were performed in a ...laminar flow reactor providing an oxygen-enriched exhaust gas environment at elevated temperatures. Single coal particles in a sieve fraction of 90–125 µm and a significant amount of volatiles (36 wt%) were injected along the burner’s centerline. Coherent anti-Stokes Raman spectroscopy measurements were taken to characterize the gas-phase temperature. Time-resolved imaging of the OH distribution at 10 kHz allowed identifying reaction and post-flame zones and gave access to the temporal evolution of burning coal particles. During volatile combustion, a symmetric diffusion flame was observed around the particle starting from a distance of ~150 µm from the particle surface. For subsequent char combustion, this distance decreased and the highest OH signals appeared close to the particle surface.
Combining the Discrete Ordinates method with the so-called blocked-off approach to calculate radiative heat transfer is investigated for a prospective application in DEM/CFD simulations. Therefore, a ...single cubic DEM object is located within a prismatic enclosure. The temperature of all walls of the enclosure and of the surface of the DEM object are set to fixed values. The three-dimensional radiative transport is calculated and the heat transfer onto the individual surfaces of the DEM object is determined. A conventional CFD simulation employing a body-conformal mesh at the surface of the embedded DEM object, serves as a reference, an “exact” solution. The results obtained are compared to corresponding solutions with a blocked-off approach, where the shape of the DEM object and the required boundary conditions are enforced within the discretized intensity field while suppressing the solution process in the region obstructed by the object. Mesh-resolution and orientation of the cube in the enclosure are varied and two computationally cheap methods to obtain the radiative fluxes onto the embedded cube surfaces are employed.
The heat fluxes onto the cube's surfaces are generally overestimated. This is mainly due to the “artificial” increase of the cube's surface area by the blocked-off approach. The introduction of a simple surface area scaling factor improves the results significantly. The deviation in radiative heat from the exact solution depends on the cube's orientation in the prismatic box and on the position of the actual surface relative to the closest mesh line. The approximation deviates about 1.4% to 3.1% in radiative heat to the cube, values which seem to be acceptable.
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•3D CFD simulations of radiative heat transfer onto a cubic DEM object with the DOM.•Using the blocked-off approach for prospective application in DEM/CFD simulations.•Comparison between the blocked-off approach and a body-conformal mesh solution.•Application of a surface area ratio scaling improves the accuracy of the simulation.•The deviations for the blocked-off approach are in the range of 1.4% to 14.0%.
•New setup for measuring phase function on single particles and particle streaks.•Investigation of cold and burning coal particles.•Imaginary and real part of IOR inversely calculated utilizing Mie ...theory.•For cold coal particles: Re(IOR) = 1.4 and Im(IOR) = 0.1.•Low sensitivity of phase function wrt Im(IOR) complicates its determination.
This study investigates the scattering properties of cold and reacting (burning) particles with focus on the experimental quantification of the complex index of refraction (IOR). The scattering phase function for Colombian bituminous coal is measured experimentally using two different setups and the IOR is determined from these data using an inverse approach. The first experimental setup is designed for single cold fuel particle characterization, with the particle being spatially fixed contactless using an acoustic levitator. In the second setup, the scattering properties of particle streaks, provided by a flat flame burner, are measured, allowing also for reacting particles to be investigated. In both setups, a broadband light source irradiates the single particle/particle streak and the scattered radiation is quantified by a spectrometer placed at altering angular positions. Reacting particles are studied in an oxy-fuel atmosphere. For both setups, Mie theory is used for modelling the optical particle properties.
The goal of the current study was to investigate the influence of increased CO2 concentrations in oxy-fuel combustion on the products of coal devolatilization (gas, tar, soot and char). Experiments ...have been carried out in a laminar drop tube reactor (DTR) at conditions comparable to pulverized coal-fired boilers, in particular at a temperature of 1573K and heating rate of 104–105K/s. Atmospheres of N2, Ar, and CO2 as well as with O2/N2 and O2/CO2 mixtures (oxidizing oxy-fuel conditions) were applied.
The work focuses on the early stages of reaction of coal particles in a pulverized combustor, therefore, a residence time of 120ms was chosen, which assured the completion of pyrolysis while limiting the progress of char combustion and gasification. Gaseous, liquid and solid pyrolysis residues were extracted and analyzed by a multitude of techniques.
A remarkable result is the effect that CO2 has on the solid products of pyrolysis. A much larger production of soot is observed in CO2 conditions over Ar or N2 conditions (3:1). The combustion reactivity of both soot and char produced in CO2 is lower than that of the corresponding samples produced in Ar or N2 atmosphere. Differences in reactivity couple with differences in the CO complexes residing on the surface and measured by XPS. The effect of CO2 on gaseous products is to increase the concentration of acetylene, while abating most other hydrocarbon species.
When experiments are carried out in air and oxy-fuel atmospheres, soot and tar are consumed by combustion. Differences among chars are observed which can be mostly related to the attainment of different extents of burn out. In the oxy-fuel experiments, lower NO and NO2 and higher N2O concentrations are found in the gas compared to air experiments.
•Pyrolysis of bituminous coal was carried out with N2 or CO2 atmospheres•A DTR with very short residence time was used•Substituting N2 with CO2 atmospheres affects the reaction products severely•Composition of the volatiles as well as physico-chemical char properties change•A large production of soot is observed when coal is pyrolyzed in CO2 atmosphere
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•Effect of inert and CO2-rich atmospheres on the fast pyrolysis of coal.•Separation method of soot and char from carbon particulate formed in coal pyrolysis.•Structural ...characterization of coal-derived soot and char.•Soot and char formation mechanisms.
In a previous work Heuer et al., 2016 a large production of a fluffy carbon-rich material was observed to accompany the char formed during the early stages of a medium rank (bituminous) coal pyrolysis in a drop tube furnace (1573K, residence times < 130ms). This peculiar material was found to be much more abundantly formed in CO2 than in N2 flow. SEM analysis showed that it contains a large portion of submicron soot-like particles mixed with particles of tenths of microns in size with the typical char morphology. The present work reports on the separation of the two differently sized fractions produced in CO2 and N2 flow and their subsequent analysis. The separation was performed dispersing the material in ethanol by ultrasonic mixing, followed by settling, and decanting to produce top and bottom products enriched in the fine and coarse particle fractions, respectively. The procedure was repeated several times and the size separation effectiveness was checked by SEM and laser granulometry sizing. Thermogravimetry, elemental and spectroscopic analysis were applied to the coarse and fine fractions to provide insights on their structural features. The fine soot particulate was almost ash-free, suggesting that its formation occurs in the gas phase, as typically soot does, while the coarse fraction presented significant residues of coal inorganic matter typical of char.
Both fine and coarse particulate resulted less reactive, and somewhat smaller in size, when produced in CO2 in comparison to N2/Ar pyrolysis conditions. Their lower reactivity is associated with higher aromaticity and structural order as well as with a lower presence of hydrogen and aliphatic functionalities.
