Chemical Looping Combustion Pretreatment of Fuel Gas for a Novel Mercury Continuous Emissions Monitor by Cold Vapor Atomic Absorption Spectrometry

Abstract

The cold vapor atomic absorption spectrometry (CVAAS) technique has been commercially used in a Hg continuous emissions monitor (CEM) for flue gas and ambient air; however, it fails in detecting the Hg concentration in fuel gases containing interference components, such as aromatic hydrocarbons, because of the detection cell contamination and the overlap of adsorption peaks. Although the Hg separation method by gold trap was developed to eliminate the interference, it was only a semi-continuous measurement technology. Therefore, this study proposed novel Hg CEM to continuously measure the Hg concentration in fuel gases. The novel Hg CEM mainly consisted of a chemical looping combustion (CLC) pretreatment unit and a CVAAS Hg detector. The pretreatment unit was an electric-heated quartz tube reactor filled with an oxygen carrier (NiO), which was dispersed into quartz sand. The proposed Hg CEM worked in the following way: First, the gas with a controlled flow rate was fed into the reactor at an elevated temperature. All of the flammable gases were converted into CO2 and H2O, and all Hg species were transformed into elemental vapor. Then, the product gases were introduced into the condenser to remove water. Finally, the concentration of elemental Hg in CO2 was detected by CVAAS without any interference problem. With regard to the oxygen carrier, it was reduced to metal and then regenerated in air by oxidation. With two reactors, the real-time continuous Hg measurement via the proposed configuration can be realized by gas switch and oxygen carrier regeneration. Two types of gases containing Hg (toluene and coal pyrolysis gas) were continuously measured by the proposed Hg CEM. The results showed that satisfactory Hg recoveries were attained with CLC pretreatment, whereas the data without CLC pretreatment were totally distorted. Some technical challenges still remain unsolved, but it is believed that the proposed Hg CEM has a bright future for fuel gas.