Grantee Research Project Results
Final Report: Green plasma technology for siloxane removal and landfill gas upgrade
EPA Grant Number: SU839966Title: Green plasma technology for siloxane removal and landfill gas upgrade
Investigators: Hoque, Shamia , Berge, Nicole D , Farouk, Tanvir
Institution: University of South Carolina at Columbia
EPA Project Officer: Page, Angela
Phase: I
Project Period: October 1, 2019 through September 30, 2020 (Extended to December 31, 2021)
Project Amount: $24,992
RFA: P3 Awards: A National Student Design Competition Focusing on People, Prosperity and the Planet (2019) RFA Text | Recipients Lists
Research Category: P3 Awards , P3 Challenge Area - Sustainable and Healthy Communities
Objective:
This proposal addresses the challenge of removing the trace contaminant, volatile methyl siloxanes (VMSs) in a sustainable and economic way through the design and fabrication of a non-thermal plasma (NTP) system, CLnERG (Clean, Renewed, upgraded LFG) for siloxane removal and landfill gas (LFG) upgrade to syngas (H2 + CO). It will design a non-thermal plasma (NTP) system for siloxane removal and landfill gas (LFG) upgrade to syngas (H2 + CO). The specific objectives are:
1. Determine the operating parameters for complete removal of siloxane and maximum conversion of CH4 to synthesis gas (CO + 2H2).
2. Detailed characterization of the properties and composition of the solids produced and assess influence of operating parameters.
3. Assess economic feasibility of upscaling the process for industrial application.
4. Educate future scientists and engineers on designing sustainable solutions emphasizing both economic feasibility and environmental health.
Summary/Accomplishments (Outputs/Outcomes):
The prototype CLnERG for the removal of siloxane from the gaseous stream was designed, fabricated, and implemented. The project determined the optimum parameters for the conversion of siloxane to solid deposits of poly dimethyl siloxane under inert conditions (helium gas stream). Experiments further confirmed the feasibility of applying the technology in the presence of carbon dioxide. The results have shown that utilizing this technology improves average electricity generation of known landfills.
The optimum conditions for the removal of siloxanes specifically octamethylcyclotetrasiloxane (D4, C6H18O3Si3) has been established. < 80% of D4 was removed from the gas stream maintaining for the dielectric barrier discharge, peak to peak voltage of 24 kV and at a frequency of 23kHz. The results for octamethyltrisiloxane (L3, C8H24O2Si3) and decamethylcyclopentasiloxane (D5, C10H30O5Si5) are similar to D4. The linear and cyclic structure appears to influence the extent of deposition all else remaining constant. Gas flow rate does not appear to influence mass deposited, but exposure to plasma discharge could play a vital role. The DBD-NTP system has successfully achieved more than 80% removal of siloxane from the gas stream. The deposits are mainly polydimetylsiloxanes (PDMS). The possibility of tailoring those deposits based on exposure time to the plasma discharge is currently being assessed. Peaks of C, O and Si was identified.
Economic feasibility studies have shown that the cost of generating a plasma discharge is offset by the extra electricity generation possible since associated operation and maintenance costs with respect to siloxane is eliminated. This directly addresses CAA: Clean Air Act, Section 103 and SWDA: Solid Waste Disposal Act, Section 8001.
One graduate and five undergraduate students (one female, one African American, two Indians among them) have had the opportunity to learn about air quality, sustainability, and environmental impacts with respect to landfills. The students were from Civil and Environmental Engineering, Mechanical Engineering, Chemical Engineering, and Biochemistry. Two of them completed undergraduate research credits and two students received the McNair Juniors Fellowship which supported the undergraduate research efforts.
Conclusions:
Figure 1 shows the laboratory setup. Helium (Praxair UN1046) is bubbled through liquid D4 (99.8%, SigmaAldrich,[–Si (CH3)2O–]4) in a bubble column. The siloxane rich helium was passed through a tubular DBD reactor. The reactor is a borosilicate tube 120mm long, 6.5mm OD × 5.2mm ID. The plasma was formed in the helium-siloxane stream within the annular spacing of the reactor. The DBD was ignited and operated at steady state by a high voltage AC power supply (Information Unlimited, PVM 500). Voltage is measured with 1000:1 high voltage probe (North Star, PVM-4) connected directly to the powered electrode and current is measured with a Pearson current monitor (Model 6585). The effluent from the plasma reactor is passed through a cold trap with decane as a solvent for collecting the treated gas stream for further analysis. A mass flow controller (MKS Instruments) was employed to maintain flow rates of the gas stream. Figure 2a is the experimental setup in the lab. Figure 2b shows the plasma discharge formed in the helium-siloxane stream within the annular spacing of the reactor and Figure 2c shows the DBD discharge in D4 enriched He with 20% CO2.
Figure 1. Experimental setup
Figure 2 (a) Laboratory setup, (b) Plasma discharge in pure He-D4 and (c) Plasma discharge in He-20% CO2-D4.
For inert conditions the process can repeatedly achieve high levels of conversion of siloxane, D4 from the gaseous stream to the solid phase. More than 80% conversion has been achieved. First set of experiments have been done where the inert gas helium was mixed with varying concentrations of carbon dioxide. Figure 3a shows the calibration curve for D4 removal for different concentration levels. Figure 3b plots the removal efficiency of D4 and L3 for different gas flow rates.
Figure 3(a) Calibration curve, (b) Removal ratio of D4 and L3 and (c) Mass deposited for varying flowrates
Mass deposit appears to remain constant within the range of gas flow rate examined for the same duration of exposure as shown in Figure 3c. There is a slight increase of mass deposited from 100 to 300 sccm and then a significant drop at 400 sccm. The mass deposited at 500 sccm, 0.05 g is the maximum amount collected also at 300 sccm. The deposits was analyzed to confirm whether D4 was the source of the white crystals collected, Figure 4. The solids where also characterized using XRD (x ray diffraction) and NMR (nuclear magnetic resonance spectroscopy) which revealed the possibility that the residue is a cross linked chain with components of PDMS (poly di methyl siloxane). Peaks of C, O and Si was identified from SEM (scanning electron microscopy) analysis. Further NMR analysis will provide a more in-depth look for the determination of the type of compound and how it changes with changing experimental conditions.
Figure 4: 1000 times magnified images of the deposits collected from the reactor
Energy consumption assessments with and without using the DBD system showed that positive returns were obtained despite the electricity consumption required by the DBD system. Furthermore, applying current experimental data on possible electricity generation to select landfills showed that with the proposed system higher outputs are possible. This is due to reduced downtime for the plants which is anticipated for cleanup, consequence of silica build up due to presence of siloxane in the gas lines.
Fabrication
Figure 5 is the schematic DBD unit designed to avoid clogging from the formed residue and other parts. The setup has been applied successfully to conduct experiments. The advantage to the setup is the consistent plasma output obtained.
Figure 5. Schematic of the reactor assembly for the demo.
Journal Articles:
No journal articles submitted with this report: View all 5 publications for this projectProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.