03/02209 Activated carbons produced from unburned carbon in fly ash and their application for mercury capture: Zhang, Y. et al. Preprints of Symposia — American Chemical Society, Division of Fuel Chemistry, 2003, 48, (1), 32–33

Abstract

Introduction Following the current demand to develop technologies to utilize high carbon content fly ash from coal-fired utility combustors or gasifiers, a one-step activation protocol has been developed by the authors to produce activated carbons from unburned carbon in fly ash. Compared to the conventional two-step process that includes a devolatilization of the raw materials, followed by an activation step, unburned carbon only requires a one-step activation process, since it has already gone through a devolatilization process while in the combustor or gasifier. The produced activated carbons with a fine particle size are not only rich in micropores, but they also present a high content of mesopores, which leads to good mass transfer properties during the adsorption process. Mercury has been identified as a hazardous air pollutant of greatest potential public health concern by the Environmental Protection Agency (EPA), with coal-fired utility boilers being the largest source of anthropogenic mercury emissions. Based upon the EPA report, in 1997 it was estimated that 48 tons of mercury were emitted into atmosphere from coal-fired utilities, including 26 tons of elemental mercury and 20.5 tons of oxidized mercury, and where the state of Pennsylvania emitted around 5 tons. The injection of carbon adsorbent upstream of the electrostatic precipitator (ESP) or baghouse particulate collection devices is a promising technology to control mercury emissions from coal-fired combustion systems. A large excess of carbon adsorbent is normally required for injection upstream of the ESP or baghouse in order to obtain a high level of mercury capture. This is because the concentration of mercury in the flue gas is extremely low, (on the order of 1 ppb by volume), as well as the complexity of the flue gas composition and the poor selectivity of the carbon sorbent towards mercury. The typical carbon-tomercury mass ratios used in recent pilot-scale studies of mercury capture in power plant systems were between 1000:1 to 100,000:1. Therefore, the cost of the carbon sorbent plays an important role in the feasibility of the proposed technologies. Based on the low cost of fly ash carbon, its inherent porosity together with its fine particle size, the unburned carbon has been tested as a potential mercury sorbent candidate. However, its mercury capacity is lower than that of commercial carbons, probably due to its high ash content and low surface area. Accordingly, the present paper focuses on the activation of the unburned carbon after conventional demineralization process in order to increase the porosity of the unburned carbon. In addition, the prepared activated unburned carbon samples were tested for mercury adsorption using a fixed-bed with a simulated flue gas at 280F.