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 mu m to 5.5 mu m. 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 epsilon = 0.6 to epsilon = 0.26 (lambda = 2.4-5.5 mu m) and epsilon = 0.33 to epsilon = 0.24 (lambda = 1.25-2.25 mu m) with increasing temperature. The influence of the reaction atmosphere was investigated by performing experiments in a N-2/O-2 atmosphere and comparing to the results to data from an oxy-fuel atmosphere, both containing 20% of oxygen. The difference epsilon(oxy-fuel) - epsilon(N2/O2) is in the range of 0.1 and 0.2 for lambda = 1.25-2.25 mu m and lambda = 2.4-5.5 mu m. Effects of burnout and char structure are discussed to evaluate the results.

The influence of minerals on solid fuel conversion is an important topic and has so far been addressed focusing mainly on the catalytic effects. Here, the impact of iron oxide on the emissivity of burning char particles was investigated. Synthetic, hydrothermally carbonized char was doped with SiO2 or with a mixture of SiO2 and Fe2O3. The sample was then exposed to an oxy-fuel atmosphere in a laboratory reactor. An established pyrometer/infrared (IR) spectrometer setup was used to measure the size, temperature, and IR radiation of single burning particles. Based on these measurements, the spectral and total emissivity in the near-IR (1.25-2.25 mu m) and the total emissivity in the mid-IR region (2.4-5.5 mu m) were calculated. The results show that the iron content directly influences the emissivity of char during combustion.

The radiative behaviour of coal and char particles is an important input parameter for simulations of coal combustion and gasification processes, as those typically feature elevated reactor temperatures. For consideration of radiative heat transfer, accurate knowledge of particle emissivity is a pre-requisite. Combining theoretical considerations and experimental data from literature, a temperature dependent relation for char emissivity is provided.

Char emissivity of burning particles is an important factor for heat transfer calculations in pulverized fuel boilers. As the chemical composition is known to influence the emissivity in general, a coal sample has been prepared by a leaching method to reduce the mineral content. A flat flame burner was used for the combustion of the particles in oxyfuel atmosphere, providing boundary conditions comparable to pulverized coal applications. The burnout-dependent emissivity of the sample was measured in a defined spectral range and compared with data for an unleached sample of the same coal, indicating that the mineral content has minor effect for the investigated conversion levels, although clear changes in the emissivity show that conversion in general is not negligible.

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.

Radiative heat transfer is a very important heat transfer mechanism in pulverized coal combustion. To identify the influence of parameters determining radiatve heat transfer and to give recommendations on the required accuracy of corresponding submodels, a 3D-periodic oxy-fuel pulverized coal combustion test case is investigated. Measurement values determined by the authors or elaborate submodels are applied for each parameter and compared to simplified models or empirical constants. To investigate the interaction between particle radiation and the strong spectral dependence of gas radiation in oxy-fuel scenarios, a comparison between spectrally averaged and spectrally resolved calculations performed. To the best knowledge of the authors, for the first time the contribution of the parameters determining radiative heat transfer are quantified and compared in one comprehensive study.The results indicate a strong influence of coal particle emissivity and scattering phase function as well as the projected particle surface on the radiative source term. For the wall heat flux, the largest influences were found for ash and coal particle emissivity, projected particle surface and the scattering phase function. Additionally, the difference between coal particle and gas temperature was found to have a significant influence on wall heat flux. A comparison of spectrally averaged to spectrally resolved results and the corresponding models for gas radiation (WSGGM and SNBM) yielded similar trends for the influence of each parameter. Thus, based on the models and parameters involved in this study, a spectrally averaged approach seems to be of sufficient accuracy to describe radiative heat transfer in oxy-fuel combustion systems. (C) 2016 Elsevier B.V. All rights reserved.

A newly designed experimental setup for the “in-flight” measurement of the emissivity of burning char particles in the spectral range from 1.25μm to 5.5μm is presented. Single coal particles (Colombian bituminous coal and Rhenish lignite) were burned in a flat flame burner under oxy-fuel conditions. As the spectral region from 850nm to 2.5μm is dominant for the radiative heat flux at temperatures typical for pulverized fuel combustion, the emissivity is measured spectrally resolved in this region by a fiber spectrometer. Additionally the total emissivity in the range from 2.4μm to 5.5μm is measured by an InSb-detector. As particle temperature and diameter are necessary for the experimental determination of the emissivity, two-color pyrometry with simultaneous particle size measurement was carried out in the visible wave length range. There are differences between the emissivities of both coal chars. These initial results represent the first emissivity measurements of pulverized coal char particles under combustion conditions.

It is widely believed that supernova remnants are the best candidate sources for the observed cosmic ray flux up to the knee, i.e. up to similar to PeV energies. Indeed, the gamma-ray spectra of some supernova remnants can be well explained by assuming the decay of neutral pions which are created in hadronic interactions. Therefore, fitting the corresponding gamma spectra allows us to derive the spectra of cosmic rays at the source which are locally injected into our Galaxy. Using these spectra as a starting point, we propagate the cosmic rays through the Galaxy using the publicly available GALPROP code. Here, we will present first results on the contribution of those SNRs to the total cosmic ray flux and discuss implications.