The effects of carbon-in-ash on mercury capture from flue gas

Abstract

Mercury, although only a trace element in coal, is an important pollutant because coal burning is increasing around the world for manufacturing and energy production. In addition, coal-fired plants are the largest anthropogenic source of mercury emission to the environment. Mercury capture mostly occurs under heterogeneous conditions between sorbent particles and gaseous mercury, and this study is mainly devoted to understanding the role of carbon-in-ash on mercury capture from flue gas. Coal flyash samples are characterized by loss-on-ignition (LOI) carbon content, surface area (BET) tests, and scanning electron microscopy (SEM) morphology. Flyash sorbents are injected using an in-flight configuration and then mercury concentration is measured by Hg continuous emission monitoring system (CEMS). The unburned carbon in ash is found to be one main factor for capturing mercury. In addition, mercury capture increases when flyash samples have more surface area. Carbon in ash is also positively correlated with surface area within coal rank. This suggests that unburned carbon plays a formative or structural role in surface area increase on flyash. However, mercury uptake is shown to be relatively low when using flyash from high rank coal such as anthracite because of the small surface area coming from its non-porous structure. Therefore, when flyash is used as a sorbent for mercury capture, quantitative surface area should be compared and coal rank also should be considered. Carbon content in ash as mercury capture sorbents is a good indicator for mercury capture, but its surface area should also be considered for predicting mercury uptake.