4. The Change in Bed Materials Size Distribution and Its Effect on CFB Boiler Operation

When burning solid fuels in circulating fluidized bed boilers (CFB), it is important to ensure a large multiplicity of particle circulation along the loop of the furnace–cyclone–return system. The high flow rate of circulating particles determines the uniformity of the temperature field along the height of the furnace, ensures temperature equalization along the bed section, which is especially important for large CFB boilers with a furnace section of 200 and more m². The size of circulating particles is determined by the efficiency of capture in the cyclone, the higher it is, the smaller the average particle size. It is believed that a well-operated CFB boiler has an average size of circulating particles less than 0.2 and even 0.15 mm. Another factor determining the bed material size distribution is the effective removal of bottom ash, which is especially important for high-ash fuels. The first in Russia large CFB boiler of unit #9 of Novocherkassk TPP has some problems with overly large part of course particles in the bed. The report presents the characteristics of the fuel, bed material and circulating ash compositions. The estimate of circulating flow rate was made. The main characteristics of the boiler, including the temperature field and estimation of circulating flow rate are presented. Special attention is paid to the regimes with the addition of limestone and fly ash. The operating date of changes in the chemical and particle size distribution (PSD) of ash streams is presented. According to results of the study of changes in ash composition, it can be concluded that the composition of fly and circulating ash varies quite rapidly and reaches design values after 10 h (according to CaO). In bottom ash the PSD varies slowly (the order of magnitude is 50 h). It is possible that the fractional composition of bed material also varies slowly. The simplified model for calculation of changes in bed material size distribution after start-up of the boiler is presented. It takes into account the initial composition of sand and coal supplied. The calculation is carried out to determine the specific removal of particles from the bed. The capture efficiency is calculated by dependences of VTI and corrected taking into account published data and results of experiments. According to experimental data, the coefficients of bed material size distribution and the removed bottom ash are introduced. According to the results of calculations, it is shown that for the available fuel fractional composition, the number of large fractions increases over time. The addition of limestone leads to an increase in fractions with sizes of 0.1–0.2 mm. the Best results are obtained when feeding coal with sizes less than 5 mm.