An investigation of mercury emission from FBC systems fired with high-chlorine coals

Mercury is widely used in industry because of its diverse properties, which makes it an important part of industrial processes and an important ingred ient in many products. However, concern over health issues has led to a 75% decline in industrial demand for mercury over the past 11 years. The objec tive of this project was to study the reduction of mercury emission from co al-fired combustors by using HCl provided by high-chlorine coals to help co nvert elemental mercury to oxidized mercury at relatively low temperatures (500-600 degreesC). By oxidizing elemental mercury inside a fluidized bed c ombustion (FBC) system, total mercury emissions can be reduced with high ef ficiency and low cost while maintaining low emissions of other pollutants. The results of the study indicate that using high-chlorine coal in an FBC s ystem converted more than 99% of elemental mercury to an oxidized state, ma inly HgCl2. Without secondary air injection and after cooling the flue gas to 400 degreesC by using a convective heat exchange tube bank, the typical concentration of gas-phase mercury in FBC flue gas was 1500-3000 ng/Nm(3) f lue gas. Better results were obtained by using high-chlorine coals and a pr edetermined ratio of secondary air (secondary/primary air ratio > 0.15). On ly 0.5% of the total mercury input was emitted from the combustor in the el emental form. When a high-chlorine coal was used, close to 55% of the total mercury input was found in the solid phase (bed and fly ash). Of the mercu ry found in the solid phase, almost none was found in the bed ash because o f its high surrounding temperature (850 degreesC). The gas-phase mercury, w hich was around 45% of the total mercury input, was determined to be primar ily in the oxidized state (40%. of the total mercury input), while only a s mall portion (4.5% of total mercury input) still existed as elemental mercu ry in the flue gas even when a high-chlorine (0.42 wt %) coal was burned wi thout the benefit of secondary air injection. Our experimental results indi cate that the combustion temperature and secondary/primary air ratio are tw o major factors that influence mercury emissions in an FBC system when lime stone is used as sorbent.