Study on ash deposition under oxyfuel combustion of coal/biomass blends

Combustion in an O₂/CO₂mixture (oxyfuel) has been recognized as a promising technology for CO₂capture as it produces a high CO₂concentration flue gas. Furthermore, biofuels in general contribute to CO₂reduction in comparison with fossil fuels as they are considered CO₂neutral. Ash formation and deposition (surface fouling) behavior of coal/biomass blends under O₂/CO₂combustion conditions is still not extensively studied. Aim of this work is the comparative study of ash formation and deposition of selected coal/biomass blends under oxyfuel and air conditions in a lab scale pulverized coal combustor (drop tube). The fuels used were Russian and South African coals and their blends with Shea meal (cocoa). A horizontal deposition probe, equipped with thermocouples and heat transfer sensors for on line data acquisition, was placed at a fixed distance from the burner in order to simulate the ash deposition on heat transfer surfaces (e.g. water or steam tubes). Furthermore, a cascade impactor (staged filter) was used to obtain size distributed ash samples including the submicron range at the reactor exit. The deposition ratio and propensity measured for the various experimental conditions were higher in all oxyfuel cases. The SEM/EDS and ICP analyses of the deposit and cascade impactor ash samples indicate K interactions with the alumina silicates and to a smaller extend with Cl, which was all released in the gas phase, in both the oxyfuel and air combustion samples. Sulfur was depleted in both the air or oxyfuel ash deposits. S and K enrichment was detected in the fine ash stages, slightly increased under air combustion conditions. Chemical equilibrium calculations were carried out to facilitate the interpretation of the measured data; the results indicate that temperature dependence and fuels/blends ash composition are the major factors affecting gaseous compounds and ash composition rather than the combustion environment, which seems to affect the fine ash (submicron) ash composition, and the ash deposition mechanismsThe research work reported in this paper was partly carried out with the financial support from the RFCS contract number RFCRCT- 2006-00010. The very fine work done by Peter Heere in carrying out the experiments is highly acknowledgedPublicad

Medium-scale 4-D ionospheric tomography using a dense GPS network

The ionosphere above Scandinavia in December 2006 is successfully imaged by 4-dimensional tomography using the software package MIDAS from the University of Bath. The method concentrates on medium-scale structures: between 100 km and 2000 km in horizontal size. The input consists of TEC measurements from the dense GPS network Geotrim in Finland. In order to ensure sufficient vertical resolution of the result, EISCAT incoherent scatter radar data from Tromsø are used as additional input to provide the vertical profile information. The TEC offset of the measurements is unknown, but the inversion procedure is able to determine this automatically. This auto-calibration is shown to work well. Comparisons with EISCAT radar results and with occultation results show that the inversion using EISCAT data for profile information is much better able to resolve vertical profiles of irregular structures than the inversion using built-in profiles. Still, with either method the intensities of irregular structures of sizes near the resolution (about 100 km horizontal size) can be underestimated. Also, the accuracy of the inversion worsens above areas where no receivers are available. The ionosphere over Scandinavia in December 2006 often showed a dense E-layer in early morning hours, which generally disappeared during midday when a dense F-layer was present. On 14 December, a strong coronal mass ejection occurred, and many intense irregularities appeared in the ionosphere, which extended to high altitudes