1. Flue gas treatment

NOx treatment

​ NOx, mainly emitted from flue gases, causes serious environmental problems including acid rain and the depletion of the ozone layer; thus, the prevention of its emission is of prime importance. Selective catalytic reduction (SCR) is the most widely used technology for the removal of this particular pollutant, but there are several drawbacks such as high cost in catalyst, insufficient efficiency, and most critically, ammonia slip, which involves in the generation of particulate matter. As a solution, a wet-scrubber approach was proposed in our lab, which can possibly be applied as a simultaneous removal system for NOx and SOx. However, there are several problems to be concerned: slow aqueous solubility of NO (1.8 mM at room temperature), absorbent regeneration process, and generation of unwanted byproducts.

​ With the use of our group’s uniquely developed impeller-based scrubber system, these critical issues can somewhat be relieved by providing active contact and mixing between NO gas and absorbent. Use of proper absorbents is also the key of this approach, and our group applied ferrous-based chelating agent such as Fe2+(EDTA), Fe2+(DMPS)2, Fe2+(Cysteine)2, and so on. It increases the mass transfer of NO from gas to liquid phase, so it enhances the solubility of NO in water, binding with NO to form Nitrosyl compound.

Fe2+(Ligand)(aq)+ NO(g) → Fe2+(Ligand)-NO(aq) 

 In fact, there is a serious issue associated with iron-based absorption, which is the oxidation by oxygen. Ferrous is quickly oxidized to ferric, and it can no long bind with NO gas. Therefore, absorbent must be continuously regenerated, and it can be done by adopting activated carbon or electrochemical cell.

(Activated carbon) Fe3+(EDTA) → Fe3+ + EDTA

Fe3+ +e- →Fe2+ (E0 = 0.771V)

Fe2+ +EDTA → Fe2+(EDTA)

SOx treatment

​ Sulfur dioxide (SO2) is selected as a “Criteria Pollutant” due to its adverse effects of simultaneously causing various respiratory diseases and acid rain. Most SO2 is emitted from energy industries, thus a special SO2 treatment process called flue gas desulfurization (FGD) is commonly used.  Our approach involves using a wet scrubber, particularly impeller-based, for absorbing SO2 in flue gas via absorbent.

​ In NO capturing process, Fe2+(EDTA) is easily oxidized by O2, forming Fe3+(EDTA), which has no capacity to capture NO. Dissolved SO2 from the flue gas exists in the form of sulfite ion, which has low standard redox potential thus can act as reductant. By applying activated carbon at high temperature, sulfite ion is oxidized into sulfate ion on the surface of activated carbon, and Fe3+(EDTA) is reduced to Fe2+(EDTA)

​​Fe3+(Ligand)(aq) + SO3  (aq) + (Activated carbon) → Fe2+(Ligand)(aq) +SO4 (aq)

​ In addition, sulfite separates NO from Fe2+(EDTA)-NO for the reactivation of Fe2+(EDTA) as an absorbent.

​Fe2+(Ligand)-NO(aq) + SO3  (aq) + (Activated carbon) → Fe2+(Ligand)(aq) +N-S compund

​ The overall system proposes a way to obtain consistent NO gas removal, while disposing the SO2 pollutant at the same time.

Field application in pilot scale

​Our team has proceeded research projects with several corporations, applying scrubber system in pilot scale for removing flue gas from power plant and vessel engine. Simultaneous SOx/NOx removing system was installed at the actual power plant site, and the system was composed of impeller-based scrubber, activated carbon reactor, electrochemical cell, and mineralization tanks. High removal efficiency was obtained with sustainable operation, which was proven to be substantially better than other reported