Pyrolytic Mercury Removal from Coal and Its Adverse Effect on Coal Swelling

Abstract

Coal combustion is one of the largest anthropogenic contributors of mercury emissions to the environment. The high level of mercury toxicity and its potential to bioaccumulate have prompted strict regulations for mercury emission control in many countries. While end-of-pipe technologies for mercury control are currently undergoing increased international development, precombustion pyrolytic mercury removal has also been proposed as a possible option. Mercury is highly volatile at low temperatures, and its removal from coal can be controlled by low-temperature pyrolysis. This work investigated the effect of temperature on mercury release while monitoring the adverse effects of the thermal pretreatment process on coal using coal swelling as an indicative parameter for combustion quality and char burnout rate. Three coal samples were pyrolyzed for 20 min to selected temperatures ranging between 200 and 600 °C at a heating rate of 50 °C/min. Mercury content in the coal chars was determined by a cold vapor atomic fluorescence method. The total mercury in coal was found to decrease between 5 and 25% when the coal samples were pyrolyzed at 200 °C, while at 300 °C, the amount of mercury removed from coal ranged between 55 and 90%. Representative coal char samples were also observed by long-distance microscopy in a single-particle reactor in which they were pyrolyzed at a heating rate of 200 °C/s. The results showed a significant decrease in the maximum transient swelling behavior when the char samples were thermally treated at temperatures above 400 °C for one coal and 500 °C for the remaining two coal samples. The results suggested that mercury removal from coal by prior thermal upgrading to around a maximum temperature of 300 °C may provide an option for partial mercury control for some coals. However, a better understanding and quantification of the strongly bonded fraction of mercury in the coal is required to model a feasible mercury retention process using the option of thermal upgrading.