Study on Kinetics of Anode Process of Chelated Iron Fuel Cell for Sulfur Recovery Unit of FLNG
Authors: Tang Xiao , Zhou Xiaoxiao , Ou Jiayi , Li Yan 
Published: February 2020

Floating liquefied natural gas (FLNG) refers to water-based liquefied natural gas (LNG) operations employing technologies designed to enable the development of offshore natural gas resources. For environmental protection and safety, a sulfur recovery unit must be designed on the FLNG to purify hydrogen sulfide (H2S) contained in natural gas. The chelated-iron sulfur recovery process was a promising technique for the removal of H2S for waste minimization and resource recovery. In this process, an iron (Ⅲ) complex was used as an oxidant for dissolved H2S in the desulfurization catalysis into elemental sulfur. The catalyst was reduced to iron（Ⅱ）complex, and then underwent separation for removal of elemental sulfur. Using oxygen in the air as the oxidant, iron（Ⅱ）complex was oxidized to iron (Ⅲ) complex, thus realizing the recycling of the chelated-iron solution.

However, in the traditional direct air regeneration process, the direct contact between air and chelated-iron catalyst can lead to degradation and subsequent failure of the chelated agent. Thus, the technology has encountered the problem of chelate degradation which made it difficult to ensure the reliable and economical operation. Furthermore, due to limitation of the FLNG space, the liquid chelated iron sulfur recovery process must also meet the requirements of a small footprint and light weight.

A chelated-iron fuel-cell process was designed to achieve rapid regeneration of chelated iron. Since the fuel cell separated the direct chemical reaction between the chelated iron catalyst and air, it can effectively avoid the failure of chelated agent. Therefore, the fuel-cell-assisted chelated-iron process became a preferred solution for sulfur recovery for FLNG because it achieved fast regeneration of chelated iron catalyst, while producing electric energy. The electrochemical reactions were expressed as

Anodic reaction: Fe
2+
L→Fe
3+
L+e
- (1)

Cathodic reaction: O
2+2H2O+4e
-→ 4OH
- (2)

During the sulfur recovery reaction cycle, the oxidation rate of sulfur ions was very fast, and the overall reaction rate was limited by the regeneration rate of chelated iron ions. Therefore, by increasing the regeneration rate of the iron complex, the amount of catalyst can be effectively reduced, thereby minimizing the volume and weight of the sulfur recovery device of FLNG.

In order to successfully complete the catalyst regeneration fuel cell design, it is necessary to study the kinetic mechanism of the anodic electrode reaction of different anodic materials in chelated iron catalysts fuel cell, so as to select the appropriate electrode materials. In this paper, cyclic voltammetry, polarization curves, and EIS measurements were used to study quantitatively the kinetic parameters of electrode processes for different electrode materials, including platinum, platinum plating titanium, graphite, ruthenium-titanium, and Harris alloy.

Results showed that:

(1) As the anode of the chelated iron catalyst fuel cell, reversible oxidation of ferrous ions as shown in reaction (1) occurs on the platinum, platinum-plated titanium, and graphite electrodes. The value of the electrode reaction rate constant K was almost the same on each of these electrodes, and the electrochemical reaction rate was fast. The rate-determining step was the diffusion of Fe2+.

(2) When using ruthenium-titanium and Harris alloys as anode materials of the chelated iron catalyst fuel cell, many side reactions occurred on the electrode, and the reaction rate was smaller. However, when the suitable oxidation potential was applied, the reaction rate was increased while the occurrence of side reactions can be reduced.

(3) The average current density of the platinum-plated titanium electrode used as the chelated-iron catalyst fuel cell anode was the largest of the six tested materials. Because the effective surface area was much larger than the apparent area, more reactive sites were available to carry out the electrode reactions. Therefore, the platinum-plated titanium electrode was the preferred anode material for chelated-iron catalyst fuel cells.