Short communication

Silica fines from included quartz in pulverized-coal combustion

Ash deposition on heat transfer surfaces in a boiler have been a significant operational challenge for coal-fired power plants. This chapter reviews the recent literature on ash deposition in coal-fired boilers, covering the nature of minerals and inorganic matter in coal, fundamentals of inorganic reactions and ash transformation, mechanisms of ash fouling and slagging, as well as ash deposition management. The influence of the furnace operating conditions on the formation and growth of ash deposit as well as the physical and chemical characteristics of the ash deposits are also discussed in detail. Practical examples are examined to illustrate how fundamental research has advanced and can lead to satisfactory solutions to practical problems. The ash characteristics and deposition formation during the co-firing of coal and biomass are also discussed. References on ash deposition in advanced ultra-supercritical power plants is scarce in the public domain and much of the literature covered in this chapter is based on the conventional subcritical coal-fired power plants and on laboratory based fundamental research. Therefore, the extension of the discussion to ultra-supercritical coal-fired boiler applications is drawn from the author’s own research and practical problem solving experiences.

The influence of a sand additive on the transport of the plastic mass during carbonization of two orthocoking coals was studied using X-rays. For this purpose, special markers were introduced into the coal charge. During heating the coal charge with the markers in it, X-ray pictures were taken. Based on the X-ray pictures, the data on the displacement of the charge and change in its volume were obtained at various carbonisation stages. It was stated that the addition of sand has a varying effect on mass transport processes in the coal plastic layer and on the volume of the coal charge. When coal Z is heated, an increase in the sand amount leads to a rise in the charge volume at the stage of coal grains swelling but it does not affect the processes of transport in the gas-saturated zone of the plastic layer. When heating coal P, a 1.8 times increase in sand reduces the distance over which the plastic mass displaces in the gas-saturated zone. The increase in sand in the two coals reduces the shrinkage of the semi-coke.

The material described as “mineral matter” in coal encompasses dissolved salts in the pore water and inorganic elements associated with the organic compounds, as well as discrete crystalline and non-crystalline mineral particles. A range of technologies, including but not restricted to low-temperature oxygen-plasma ashing, may be used to evaluate the total proportions of minerals and other inorganic constituents in a coal sample. The relative proportions of the individual minerals in the coal may be further determined by several different techniques, including Rietveld-based X-ray powder diffractometry, computer-controlled scanning electron microscopy (CCSEM), and normative interpretation of chemical analysis data. The mode of occurrence of particular minerals may be evaluated by optical or electron microscopy techniques.

The minerals in coal may represent transformed accumulations of biogenic constituents such as phytoliths and skeletal fragments, or they may be of detrital origin, introduced as epiclastic or pyroclastic particles into the peat bed. Other minerals are produced by authigenic precipitation, either syngenetically with peat accumulation or at a later stage in cleats and other pore spaces by epigenetic processes. They may represent solution and reprecipitation products of biogenic and detrital material, or they may be derived from solutions or decaying organic matter within the peat bed. Non-mineral inorganics may be derived from a range of subsurface waters, and possibly redistributed within low-rank seams by post-depositional ion migration effects. They may also be expelled in different ways from the organic matter with rank advance.

Quantitative analysis of minerals and other inorganics contributes significantly to defining coal quality. It may also be useful as an aid to stratigraphic correlation, either between seams in a coal-bearing sequence or between sub-sections within an individual coal bed. Mineralogical analysis may help in identifying the mode of occurrence and mobility of particular trace elements, including potentially toxic components such as arsenic and mercury. Knowledge of the mineral matter can also be used to evaluate the behaviour of particular coals in different utilization processes, including the processes that control the characteristics of fly ashes, slags and other combustion by-products.