The migration and transformation behavior of selenium during coal chemical looping gasification (CLG) under the impact of a CuO/Bentonite (Ben) oxygen carrier (OC) were studied in a batch fluidized bed reactor. In the CLG process, the total percentage of selenium released in gaseous phase was 73.06%. In the conventional gasification process, 91.71% of the total selenium was released in a gaseous state. The addition of CuO/Ben OC apparently promoted the transformation from gaseous selenium to particulate selenium. The oxygen–carbon ratio played an important role in affecting the fraction of gaseous selenium released in the gasification process, with results showing that the amount of selenium adsorbed by CuO/Ben OC was added along with the increase in OC. By means of X-ray photoelectron spectroscopy characterization, we found that the reduced CuO/Ben OC contained a small amount of Cu2Se due to the oxidation and adsorption of selenium onto their porous surface. The regeneration performance of the CuO/Ben OC was favorable after 10 regeneration cycles of the CLG process. The increase in the pore volumes and specific surface areas contributed to the enhanced capacity of retaining selenium for CuO/Ben OC.

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Chemical looping combustion (CLC) is a key technology for capturing CO2. Different types of oxygen carrier (OC) particles are used in coal CLC. The migration and transformation behaviors of sulfur and nitrogen are basically the same when CaFe2O4 and Fe2O3/Al2O3 are used as OC. CLC can be divided into two reaction stages: coal pyrolysis and char gasification; SO2 and NO show bimodal release characteristics, both of which show a basic trend of rising first and then falling down. The contents of H2S and NO2 increased rapidly at the beginning of the reaction and then decreased slowly at the stage of char gasification. H2S is released rapidly during coal pyrolysis and discharged from the reactor with flue gas, and then part of H2S is converted to SO2 during the char gasification stage by OC particles. NO can be oxidized by OC particles and form NO2. The increase in the reaction temperature and oxygen-to-carbon ratio (O/C) contributes to the release of sulfur and nitrogen and higher reaction temperature and O/C can inhibit the formation of metal sulfide. O2 released by CuFe2O4 significantly increases the contents of SO2, H2S, NO and NO2 in flue gas. This work is helpful for improving control strategies for pollutants.

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Selenium is one of the most volatile toxic elements in coal, and its emissions must be strictly controlled. Chemical looping combustion (CLC) is a clean and efficient technology for coal. Herein, the iron-based oxygen carrier (OC) was used as an adsorbent to study the migration and emissions of selenium during the CLC of coal. Due to the oxidation and adsorption of selenium by ironbased OC, most of the selenium was retained in OC or distributed in the CO 2 stream. The proportion of gaseous selenium released into the atmosphere was less than 10%-significantly lower than that from the traditional combustion process of coal, which had a value of 91.79%. The presence of OC increased the distribution phase of selenium, promoted the conversion of gaseous selenium to solid selenium, and reduced selenium emissions in flue gas. During CLC of coal, the fuel reactor (FR) temperature and the number of OC re-oxidation cycles played an important role in the emissions and retention of selenium.The increasing FR temperature increased the gaseous selenium in the CO 2 stream, reduced the particulate selenium absorbed by OC, and reduced the selenium emissions in the atmosphere. After 10 continuous CLC cycles, the selenium concentration in OC increased from 0.889 to 8.20 mg kg À1 . The continuous cycling of CLC could realize the enrichment of selenium from coal to OC. Furthermore, the migration and transformation mechanism of selenium during CLC was deduced by experiments and thermodynamic simulation. This research provides a suitable reference for reducing selenium emissions and developing CLC technology. K E Y W O R D S chemical looping combustion, coal, iron-based oxygen carrier, selenium | INTRODUCTIONCoal is an important basic energy source that promotes rapid economic development and is a major source of carbon dioxide (CO 2 ) emissions. To reduce the impact of CO 2 emissions on global climate change, it is urgent to develop efficient and low-cost CO 2 capture technologies. [1] Chemical