Nano-metal oxides for elemental mercury removal from natural gas
Creators: name: Chalkidis, Anastasios
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340601
subject: Catalysis and mechanisms of reactions
classificationCode: 370104
subject: Atmospheric composition, chemistry and processes

description: Undoubtedly mercury pollution poses detrimental threats to the human health and environment. Global initiatives such as the Minamata Convention on Mercury (2017) and the United Nations Environment Programme (2018) have emphatically addressed the necessity to undertake drastic actions to reduce anthropogenic emissions. At the same time, the role of natural gas in mitigating greenhouse gas emissions and advancing renewable energy integration is certainly critical. It accounts for less than 25% of the world fuel consumption, with liquified natural gas (LNG) comprising around 10% of global gas trade. The intensive exploitation of low-cost shallow hydrocarbon reservoirs has directed exploration to deeper geological basins. However, drilling into deeper and hotter reservoirs increases the possibility of mercury contamination. This impacts on the industry sustainability from health and safety risk aspects due to corrosion and contamination of equipment, to catalyst poisoning and toxicity through emissions to the environment. In the past, catastrophic industrial events, such as the one occurred in Skikda LNG plant, Algeria in 1973 or the most recent 2004 New Year's Day Moomba gas plant explosion in South Australia, were attributed to mercury-induced corrosion. <br><br>Elemental mercury (Hg(0)) is the prevalent species in natural gas streams and it is characterised by its low water solubility and high mobility in the atmosphere. According to the industry practice, the mercury concentration of the gas stream entering the cryogenic section, must be reduced to fewer than 10 ng/m3. Thus, fixed-beds of non-regenerative and regenerative materials, such as sulfur-impregnated carbons, supported metal sulfides/oxides and silver-impregnated zeolites, are utilized to remove mercury by means of amalgamation, chemical or physical adsorption as well as reactive absorption. Since the 1970's, when the effect of mercury on natural gas cryogenic processing plants was realized, numerous patents have been published.<br><br> Nevertheless, the conventional materials are often problematic and alternative ones, are currently under research. These need to be efficient at ambient temperatures, regenerable at low temperatures, and effective at high-mercury concentration streams and reducing atmospheres. Given the extremely low mercury concentration specifications in LNG plants, the target efficiencies may as high as 99.9%, and when the efficiency drops below 99%, the sorbent may need to be replaced.<br><br>Metal oxides possess high adsorption capacity, strong metal ion affinity and the ability to remove heavy metal traces with the possibility of recovery and reuse. Among them, manganese oxides are low cost, environmentally friendly and have been tested as sorbents/catalysts for the elimination of various gaseous pollutants including Hg(0) vapor. A particular crystal phase of MnO2, alpha-phase MnO2, consists of 2x2 (4.6x4.6 Å) tunnels constructed from double chains of octahedra [MnO6] with a variety of stabilizing cations (e.g. K+, Ba2+) being situated inside its tunnels. Its distinctive properties which are related to the tunnel cavity, moderately acidic sites and easy release of lattice oxygen have rendered it promising for sorption and catalytic applications. Furthermore, various studies have involved the incorporation of CeO2 into sorbent/catalyst compositions with the aim of enhancing the Hg(0) removal activity, usually at temperatures over 150 °C. This is due to its unique properties, which are related to the reversible Ce3+/Ce4+ redox pair, the surface acid-base properties, and the defects that primarily consist of oxygen vacancies. <br><br>The target of this study was to develop a regenerable Hg(0) vapor composite sorbent based on nanostructured manganese and cerium oxides, for Hg(0) removal from natural gas streams at ambient conditions. The experimental work was divided into three parts. In the first part, different nanomorphologies (nanotubes, nanorods and nanowires) of  alpha-phase MnO2 were hydrothermally synthesized and evaluated for Hg(0) capture at CH4/N2 mixtures containing both CO2 and H2S. Alpha-phase MnO2 nanotubes manifested the highest uptake (1 wt%) and could be regenerated at only 250 °C. The enhanced activity of nanotubes was ascribed to the presence of abundant surface adsorbed oxygen species, that are believed to facilitate Hg0 adsorption. In the next part the aim was to improve the removal capacity of  alpha-phase MnO2 nanotubes by sustaining at the same time its regeneration ability. To achieve this, alpha-phase MnO2 nanotubes were decorated with CeO2 nanoparticles via a low-temperature hydrothermal method. The CeO2-saturated nanosorbent, which had a relative Ce/Mn atomic weight ratio of around 35%, exhibited a maximum Hg(0) uptake capacity exceeding 2 wt%, as determined from measurements of mercury breakthrough which corresponded to 99.5% Hg(0) removal efficiency. Its impressive activity was speculated to be related with the facile oxygen vacancy formation at alpha-phase MnO2 NTs as a result of CeO2 incorporation. Finally in the third part, the potential industrial application of the developed nanosorbent was demonstrated by coating a 3D-printed Ti-based support structure with the developed Mn-Ce nanocomposite. Engineered honeycomb-like structures are advantageous in gas separations because they can provide substantially uniform flow paths which reduce the pressure drop. The structured support was manufactured by a 3D-printing process (SLM) using as material source a titanium alloy, Ti-6Al-4V powder, which is attractive for application in oil and gas industry due to its corrosion resistance and high strength to weigh ratio. Despite the fact that the active phase, which was loaded on its surface by a simple dip-coating process, constituted less than 5 wt% of the total mass, the coated structure captured almost 100% Hg(0) vapor in the preliminary 7-h test. The future research plans concern the optimization of the coating process, the examination of alternatives synthetic routes to produce the developed Mn-Ce nanocomposite as well as the investigation of its tolerance against water vapor.