Distribution and enrichment characteristics of arsenic in feed-coal and by-products of coal-fired power plants

The distribution and enrichment characteristics of arsenic in five circulating fluidized bed (CFB) units with capacity between 25 to 350 MW and five pulverized coal furnace (PC) units with capacity between 300 to 600 MW were investigated using microwave digestion and hydride generation-atomic fluorescence spectrometry. By comparing the conventional wet digestion method and three kinds of mixed-acid microwave digestion systems, the appropriate digestion method was determined to be HNO₃-HCl-HF acid solution mixed in a volume ratio of 6:2:2 with microwave digestion. The majority of the arsenic in coal evaporates during combustion and captured by the fly ash, the arsenic content in the bottom slag is only 1.95-9.75 μg/g. Most arsenic in the flue gas is adsorbed by the fly ash, most of the adsorbed arsenic is successively captured by the dust collector and desulfurization system. The arsenic contents in the fly ash and gypsum is 8.68-17.63 μg/g and 1.71-4.0 μg/g, respectively. Combustion temperature is the key factor affecting the release of arsenic. PC has a higher furnace temperature than CFB and makes more arsenic volatilize from coal and less arsenic remain in bottom slag. Meanwhile, a higher combustion temperature in PC unit produces more glassiness as aluminosilicate in the fly ash, which can capture the arsenic from the flue gas. Therefore, the arsenic content in the fly ash from PC unit is 12.08-17.63 μg/g, which is significantly higher than that from CFB, 8.68-13.84 μg/g. Moreover, the furnace temperature increases with the boiler load, which makes the ratio of arsenic content in the fly ash to the feed coal show an increasing trend. The ash content of the coal used for CFB and PC units is 33.96%-59.63% and 15.05%-41.67%, which makes the relative enrichment factor of arsenic in CFB higher than that in PC. Furthermore, more fine particles escaped from the dust collector should be captured by the desulfurization system, resulting that the arsenic concentration in the desulfurization gypsum of CFB unit is higher than that in PC unit.