The fate of mercury in fluidized beds under oxy-fuel combustion conditions

• Mercury speciation in an oxy-fuel fluidized bed combustor has been analyzed.
• Sulfated Ca-based sorbent enhances the Hg uptake as particle-bound mercury.
• The higher SO2 concentration the lower mercury oxidation from Hg0 to Hg2+.
• NO and H2O recycled hardly affect mercury speciation at these operating conditions.

Among the different pollutant gases released from oxy-coal combustion, mercury is the responsible for important operational issues in the CO2 processing unit, being able to cause material corrosion. Some studies concerning the fate of mercury during oxy-coal combustion are referred to pulverized coal (PC) boilers. However, the fate of mercury emissions in fluidized bed (FB) combustors has yet to be elucidated.In this work, mercury emissions from a 3 kWth bubbling FB combustor operating under oxy-fuel combustion conditions have been evaluated. For this purpose, two Spanish Ca-based sorbents and two Spanish coals were used. The effects of type of Ca-based sorbent, the Ca/S molar ratio, temperature and recycled gases (NO, SO2 and H2O) typical in oxy-fuel combustion process on the distribution of mercury species have been analyzed.It was observed that the presence of Ca-based sorbents in the combustor favored mercury fixation as particle-bound mercury which exhibited a maximum at a temperature about 925 °C corresponding to the highest degree of limestone sulfation. SO2 recirculation inhibited the Hg0 oxidation and thus the mercury fixation as particle-bound mercury decreased. However, neither NO nor steam recirculation affected mercury speciation.