Effect of modified fly ash with hydrogen bromide on the adsorption efficiency of elemental mercury

Mercury is one of the most hazardous trace elements produced by coal-fired power plants. Mercury in the flue gas is predominately present as three different species: particulate mercury (Hg^p), oxidized mercury (Hg²⁺), and elemental mercury (Hg⁰). Of these three, elemental mercury is the most difficult to remove from flue gas streams due to its low reactivity and low solubility in water. With increasing production costs associated with activated carbon materials, and increasing restrictions on mercury emissions, the development of an alternative low cost absorbent to capture elemental mercury by using fly ash modified with bromide compounds is highly desirable. Modified fly ash is usually injected into the flue gas stream after the air pre-heater system of a coal-fired power plant to oxidize and subsequently absorb elemental mercury. Research on the quantity and method of modifying the bromide amended fly ash is needed to obtain the most efficient mercury capture rate. This study utilized the impregnation method to prepare three different fly ashes with hydrogen bromide (HBr). Adsorption capabilities of the modified fly ashes were then examined using a fixed bed reactor. Thermogravimetric (TG) analysis was employed to quantify the amount of hydrogen bromide in the modified fly ash, which was subsequently compared to the water extraction method using ion chromatography. TG-MS was also utilized to evaluate the release of HBr from the modified fly ash and elucidate the mechanism for mercury capture.