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Coal combustion is the major contributor to global toxic selenium (Se) emissions. Inorganic elements in coals significantly affect Se partitioning during combustion. This work confirmed that the calcium (Ca) in ash had a stronger relationship with Se retention at 1300 °C than other major elements. Ca oxide chemically reacted with gaseous Se, and its sintering densification slightly affected Se adsorption capacities (44.45 −1840.71→35.17 −1540.15 mg/kg) at 300 − 1300 °C. Therefore, Ca in coals was identified as having potential for hindering gaseous Se emissions, and coals with increased Ca contents (2.74→5.19 wt%) were used in a 350 MW unit. The decreased Se mass distribution (3.54%→2.63%) in flue gas at air preheater inlet (320 −362 °C) confirmed the effectiveness of increased Ca content on gaseous Se emission reduction. More gaseous Se further condensed and was chemically adsorbed by fly ash when passed through an electrostatic precipitator, resulting in a significant increase in the Se content of fly ash. Additionally, the corresponding Se leaching ratio decreased from 4.88 − 35.74% to 1.87 − 26.31%, indicating enhanced stability of Se enriched in fly ash. This research confirmed the feasibility and environmental safety of sequestration of gaseous Se from flue gas to fly ash by increasing the Ca content in coals. [Display omitted] ● Se retention at 1300 °C is associated with Ca content in ash. ● Ca oxide removes more gaseous Se via chemical reaction during flue gas cooling. ● Coals with higher Ca contents will release less gaseous Se into flue gas. ● A higher Ca content in fly ash can enhance the stability of Se within. ● Electrostatic precipitator induces Se emission reduction with increased Ca content. [ABSTRACT FROM AUTHOR]

Currently, the severe fluctuation of steam temperature in double-reheat (DR) ultra-supercritical (USC) units has become a major problem that constrains their efficient and flexible operation, wherein the deviation of coal calorific value from the design value is one of the major reasons for this issue. However, the effect of coal calorific value variation on the steam temperature characteristics during the flexible operation of DR USC units remains unclear. A dynamic model of a 1000 MW DR USC unit is developed with primary principles and validated with real operation data. The effect of the coal calorific value variation on the steam temperature characteristics is revealed. Results show that as the calorific value of the coal increases, the main steam temperature can always maintain at the design value with attempering water adjustments, while the two reheat steam temperatures decrease. The steady-state deviations of the reheat steam temperatures are positively correlated with the magnitude of the coal calorific value variation, where the deviation of the secondary reheat steam temperature is larger. Meanwhile, as the load increases, the steady-state deviations of reheat steam temperatures increase, but the response time decreases. Finally, the effect of calorific value variation on the boiler efficiency is studied. As the coal calorific value increases, the boiler efficiency firstly rises caused by the decrease in the loss of the flue gas and then falls caused by the limited furnace temperature increase. This study provides guidance on the DR USC boiler operation under coal variation. • A dynamic model of a 1000 MW double-reheat ultra-supercritical unit is developed. • The effect of the coal calorific value variation on the steam temperature characteristics is revealed. • The effect of calorific value variation on the boiler efficiency is revealed. • This study provides guidance on the double-reheat ultra-supercritical boiler operation under coal variation. [ABSTRACT FROM AUTHOR]

Sodium, calcium, sulfur and chlorine in the flue gas of coal-fired boilers are the main cause of ash deposition/fouling in heat exchangers. However, there are still many debates on the mechanism of ash deposition/fouling due to the lack of direct micro evidence. This paper aims to clarify the initial deposition/fouling characteristics of a high-alkali coal by computer-controlled scanning electron microscopy (CC-SEM). The results demonstrate that temperature has an influence on the deposition/fouling compounds. The crystalline compounds in the samples at 900 °C were mainly CaSO 4 and Ca 2 Al 2 SiO 7 , while the compounds in the samples at 500 °C were mainly NaCl. Sulfur had a significant adsorption effect on sodium and calcium at 900 °C, while chlorine had a significant adsorption effect on sodium at 500 °C. A 3-layer microstructure of deposition/fouling was found after the samples were stripped by molecular force. Additionally, deposition/fouling could be divided into three stages: the formation of sodium salt precursors, the growth of loose structural aluminosilicates and the adhesion of particulates on the surface. Sulfates, especially Na 2 SO 4 and CaSO 4 formed a precursor for the initial deposition/fouling of high-alkali coal. • The initial deposition/fouling characteristics of a high-alkali coal were studied. • Samples were stripped by molecular force. • A 3-layer structure of deposition/fouling was observed. • The mechanism of the influence of sulfur and chlorine on deposition/fouling was proposed. • A series of detailed microscopic maps of the deposition/fouling process was obtained. [ABSTRACT FROM AUTHOR]