November 22, 2020. By Kolemann Lutz
A one-step, low-temperature plasma-assisted catalytic process enhanced sulfur dioxide (SO2) conversion by 200% with H2 and 100% using CH4. This new technique could produce significantly less waste than current multistage flue-gas desulfurization (FGD) methods at much lower temperatures and power to convert sulfur dioxide to pure sulfur.
Challenge. On Earth, exposure to sulfur dioxides can cause significant environmental problems like acid rain and sea acidification. In 2015, exposure to the particulate SO2 was estimated to cause 4.2 million premature deaths and more than 100 million disability-adjusted life years—which measures years lost due to illness, disability or death—according to the Lancet Global Burden of Diseases Study. Close to all of the sources of sulfur dioxide come from the tail gas streams of electric utilities, especially those that burn coal, in addition to smaller amounts from petroleum refineries and cement manufacturing.
Current desulfurization methods on Earth can successfully remove sulfur dioxide from exhaust flue gases of fossil-fuel power plants but not without creating large quantities of waste in the form of metal sulfate and wastewater, which increases the cost of sulfur extraction.
Thermal catalysis represents a novel way to reduce sulfur dioxide to solid sulfur with a catalyst and reducing agents such as H2, CH4, or CO. However, the traditional catalytic process requires high power demand and temperatures to achieve high conversion and selectivity levels in addition to loss of catalyst activity.
On the Martian surface, sulfur dioxide (SO2) would be produced as a byproduct in the reduction of mineral hydrous sulfates. The heavy compound sulfur dioxide would be important for the chemical manufacturing of sulfuric acid (H2SO4), one of the most commercially important chemicals to manufacture everything from fertilizers and detergent to metallurgical processes and oxidizing agents. After several steps of reduction, the solid sulfur has been proposed to be used in industrial-scale 3D-printing to create Marscrete (concrete).
Experiment. In a 2020 study, Penn State researchers tested their idea by inserting an iron sulfide (FeS2) catalyst into a coaxial double dielectric barrier discharge (DBD) reactor, which generates plasma to decompose various gaseous compounds, inside an electric furnace to control the temperature. DBD reactors can also be used to produce plasma to modify or clean surfaces of materials, manufacture semiconductors, treat polymer surfaces, control aerodynamic flow, pollution control, and even weld or cut metal with high-power CO2 lasers.
After introducing a combination of feed gases (H2, SO2, or CH4) with the mass flow controllers into the catalyst bed, which was heated to 148.9° C, they turned on the nonthermal plasma and the reactions immediately occurred. “Through this process, the catalyst shows very excellent stability. When run for several hours, we do not see any deactivation. The activity and the selectivity stay the same." mentioned Xiaoxing Wang, an Associate Research Professor at the Penn State EMS Energy Institute.
Once the 10 watt reaction was complete, researchers collected and analyzed the solid sulfur at the bottom of the DBD reactor. They used a Molecular Sieve column to separate inert gases (H2, O2, N2, CH4, and CO) and a Plot Q column to separate sour gases and light hydrocarbons (CO2, H2S, SO2, COS, CS2, and C2+). By using an online micro-gas chromatograph equipped with thermal conductivity detectors (TCD), they analyzed the samples to see how much sulfur dioxide was in the gas and how much hydrogen or methane was consumed.
The researchers found that combining nonthermal plasma (NTP) with supported metal sulfide catalysts dramatically enhanced SO2 reduction and conversion at low temperatures by 148% to 200% using hydrogen and 87 to 120 percent using methane. The selectivity to elemental sulfur from SO2 reduction is over 98% with all the supported metal sulfides (Mo, Fe, Co, Ni, Cu, and Zn) with FeS2/Al2O3 catalyst exhibiting the highest activity under plasma condition.
The physiochemical changes of spent FeS2/Al2O3 catalysts are observed through seven techniques: N2 physical adsorption, FESEM, XPS, XRD, HRTEM, STEM/EDS, and EELS to understand plasma and thermal effects on the catalyst.
One of the primary challenges with thermal catalysis is oxidation of the catalyst, resulting in an oxygen gain and catalyst deactivation. Surprisingly, the plasma conditions not only prevents the thermal oxidation and preserves the surface FeS2 catalyst but also improves sulfidation.
"It's highly beneficial to energy and the environment. Our process saves energy, reduces waste and saves water. This is very transformational." Wang said.
For Mars. Plasma-enhanced catalysis are gaining increasing attention for CO2 utilization, methane conversion, ammonia synthesis, and more. In order to produce sulfuric acid for chemical manufacturing and sulfur for concrete, CH4 and CO can be produced in-situ on the Martian surface and liquid hydrogen and nitrogen can be imported.
Following the spatial analysis, sampling, extraction, and transportation of the most common hydrous sulfates on the surface, Fe2SO4, MgSO4, and CaSO4, the thermal decomposition of sulfates typically requires high heats of 500 - 1200°C in gaseous nitrogen environments.
Although the processing, production, and utilization of sulfur are likely to be integrated into later settlement expansion plans, it would be important to identify a settlement location with accessible mineral sulfates to make full use of the locally available resources. The in-situ production of sulfur and sulfuric acid might help bring early settlements closer to self-sustainability.
Mankind can improve the in-situ manufacturing processes for descendants on Mars by reducing environmental pollutants such as CO2 and SO2 to improve the quality of life on Earth first.
Research Paper. Mohammad S AlQahtani et al. One-Step Low-Temperature Reduction of Sulfur Dioxide to Elemental Sulfur by Plasma-Enhanced Catalysis Journal of Catalysis. April 8, 2020. DOI.org/10.1021/acscatal.0c00299