Characterization of minerals in coal and interpretations of ash formation and deposition in pulverized coal fired boilers. (Volume 1: Text and Volume 2: Appendices). Ph. D. thesis

This Ph.D. dissertation entitled “Characterization of minerals in coal and interpretations of ash formation and deposition in pulverized coal fired boilers” reports research findings on the inorganic constituents in coal, ash formation, and deposition in pulverized coal-fired boilers. The dissertation includes two main parts: 1) methods for characterization of minerals in coal; and 2) evaluations of ash deposition based on full-scale trials conducted at power stations in Denmark.

Two new techniques based on scanning electron microscopy (SEM) and x-ray analyses were developed at the Geological Survey of Denmark and Greenland (GEUS) for characterization of minerals in coal, fly ash, and deposits: 1) Computer Controlled Scanning Electron Microscopy (CCSEM); and 2) Scanning Electron Microscopy Point Counting (SEMPC). The CCSEM technique is used for determination of the concentration, size, shape, and semi-quantitative composition of coal minerals and fly ash particles. The SEMPC technique measures the concentration and Chemical composition of individual points within a deposit. The repeatability errors associated with the CCSEM technique are mainly due to operator determined parameters, whereas sample and instrument errors have less influence. The repeatability of CCSEM analyses performed at GEUS is within the same range as observed at other SEM laboratories.

Six full-scale trials were conducted at three power stations in Denmark: Ensted, Funen, and Vendsyssel power stations. During these trials pulverized coal, bottom ash, fly ash, and deposits from cooled probes were sampled and analyzed with various techniques. Based on SEM analyses the deposits can be grouped into five textural types, which all posses distinet textural and Chemical characteristics. Likewise, the deposition mechanisms for these five types are characteristic and they may be used for constructing a model for the build-up and maturation of an ash deposit. The deposits collected on the probes were thin (maximum 2 mm) and the influence of operational parameters and probe temperatures on the magnitude of the deposits were minor. The probe temperatures had no influence on the composition of the ash deposits for coals with low ash deposition propensities, whereas the probe temperature did influence the composition of deposits for coals with medium ash deposition propensities. These results would indicate that coals with medium to high ash deposition propensities in existing boilers would tend to create inereasing problems with ash deposition in new boilers with higher steam temperatures.

Additionally, chars were collected from the bumer region during the full-scale tests at the Funen Power Station. CCSEM analyses of the ashes in the chars indicate that iron is redistributed into the clay-derived ash particles at an early stage of combustion, whereas calcium is less incorporated. This indicates that calcium vaporizes during combustion. The char analyses also revealed that the lighter color of fly ash from South Affican coals is due to more calcium incorporated into clay-derived ash particles compared to other coal types.

Observations of iron-sulfide crystals in selected deposits show how a local reduced environment in a deposit may lead to growth of reduced iron-sulfur compounds. These observations indicate that the influence of pyrite on ash deposition is not, as generally accepted, only related to the oxidation State of the pyrite on impact onto the heat transfer surface, but also to the post-depositional environment of the iron-compound.

In conclusion, this study provides new knowledge about the inorganic constituents in coal, transformation of the inorganic components during combustion, mechanisms controlling the ash deposition, and selection criteria with respect to slagging and fouling for coals to be bumed in the new boilers with higher steam data.