Effects of CO and CO₂ on the Removal of Elemental Mercury over Carbonaceous Surfaces

Coal gasification is a popular method for the optimization of coal utilization and the reduction of environmental pollutant emissions. However, the reductive atmosphere of its products is disadvantageous for removing elemental mercury (Hg⁰). Activated cokes (AC) was employed in this work for mercury capture in a reducing atmosphere. The high-temperature heating decreases the mercury-removal capability of carbon sorbents because the carbonaceous surface is becoming oxygen-depleted and micropore-decreased after the heating treatment. The mechanism of mercury adsorption in pure nitrogen follows the Mars-Maessen mechanism over the carbon sorbents. To identify the effects of carbon monoxide (CO) and carbon dioxide (CO₂) on Hg⁰ removal, the Hg⁰-adsorption and thermal desorption experiments were carried in a fixed-bed reaction system. CO inhibits both the chemisorption and physisorption of Hg⁰. CO₂ competes for the active sites, lactone groups and hydroxyl groups, and occupies the micropores, which is beneficial to adsorb Hg⁰ physically. When CO and CO₂ coexisted, the removal efficiencies show steadier than those in monocomponent gas (only CO or CO₂). CO₂ can resist the negative effect of CO on Hg⁰ removal, to some extent, because CO₂ can inhibit the oxidation and disproportionation of CO. This experimental study provides practical guidance for the development of mercury-removal technology with carbon materials in the coal gasification plant.