Non-thermal plasma/catalysis for upgrading waste-derived pyrolysis oils and gases, and methanation of CO₂

This study explores the application of non-thermal dielectric barrier discharge (DBD) plasma reactor for upgrading the oil and gas derived from pyrolysis of waste and also conversion of CO₂ to a synthetic fuel, methane. As part of the global effort to reduce fossil fuel dependency and mitigate climate change, this work addresses critical challenges associated with carbon dioxide utilization, bio-oil upgrading, and waste-to-energy conversion by combining non-thermal plasma with catalysis and co-processing. First, co-pyrolysis of waste biomass and polystyrene waste was explored as a route to enhance bio-oil quality via the non-thermal plasma (NTP) reactor and the hydrogen donor feedstock. Bio-oil produced from the pyrolysis of biomass is chemically complex, viscous, highly acidic, and highly oxygenated. Co-pyrolysis of biomass and plastics can enhance oil quality by raising the H/C ratio, leading to improved biofuel properties. In the first stage, feedstocks were pyrolyzed at 650 °C, producing volatiles subsequently passed to a second-stage DBD non-thermal plasma reactor at 250 °C with the aim to further improve the product bio-oil via cracking and autohydrogenation reactions under non-thermal plasma conditions. Hydrogen yields rose from 0.04 to 0.28 mmol/g for polystyrene, from 0.67 to 1.28 mmol/g for biomass, and from 0.40 to 0.91 mmol/g for the biomass–polystyrene mixture at 70 W. There was a synergistic interaction between biomass and polystyrene in terms of overall oil and gas yield and composition even in the absence of the non-thermal plasma. However, the introduction of the non-thermal plasma produced a marked increase in monocyclic aromatic hydrocarbons (e.g., ethylbenzene from 62.94 mg/g in pyrolysis to 185.23 mg/g at 70 W), whereas polycyclic aromatic compounds decreased slightly in concentration (e.g., 2-phenyl-1,2,3,4-tetrahydronaphthalene decreased from 22.32 mg/g in pyrolysis alone to 6.11 mg/g at 70 W). Most notably, oxygenated compounds in the oil were almost completely removed at 50 and 70 W, with several species, including cyclopentanone and furfural, dropping below detection limits. It is suggested that the unique reactive environment produced by the plasma involving high-energy electrons, excited radicals, ions, and intermediates increases the interaction of the polystyrene and biomass pyrolysis volatiles. Increasing input plasma power from 50 to 70 W further enhanced the effects of the non-thermal plasma. Second, the study extended this approach to refuse-derived fuel (RDF) containing a high content of biomass and plastics to investigate pyrolysis/non-thermal plasma/catalysis as a method to produce de-oxygenated bio-oils and gases from RDF pyrolysis. The volatiles from the pyrolysis stage at 650 °C are passed directly to a non-thermal plasma/catalytic reactor at 250 °C where upgrading of the pyrolysis volatiles takes place in the presence of different catalysts (TiO₂, MCM-41, ZSM-5, and Al₂O₃). Even in the absence of a catalyst, the presence of the non-thermal plasma resulted in high yields of CO (from 0.94 mmol/g at 0 W to 1.71 mmol/g at 70 W) and CO₂ (from 1.49 mmol/g at 0 W to 1.85 mmol/g at 70 W) gases and reduced bio-oil oxygen content, confirming deoxygenation of the RDF pyrolysis volatiles. The addition of catalysts MCM-41 and ZSM-5 generated the highest yields of CO (2.61 mmol/g with MCM-41, 2.58 mmol/g with ZSM-5), CO₂ (2.34 mmol/g with MCM-41, 2.56 mmol/g with ZSM-5), and H₂ (3.06 mmol/g with MCM-41, 2.09 mmol/g with ZSM-5) due to the synergy between catalyst and plasma. The catalysts ranked in terms of total oxygenated oil yield are as follows: MCM-41 < ZSM-5 < TiO₂ < Al₂O₃. The non-thermal plasma which synergistically interact with the catalysts enables deoxygenation through decarboxylation and decarbonylation reactions. Third, bio-oil upgrading by deoxygenation has been investigated using a model bio-oil compound (furfural) and a model hydrocarbon (hexadecane) typically produced from plastics pyrolysis. Upgrading has been investigated using a non-thermal plasma reactor system with the presence of hexadecane as a hydrogen donor to improve bio-oil quality by raising the H/C ratio. The effect of input power on product yield, oil and gas composition has been investigated. There was little synergistic interaction between furfural and hexadecane in the absence of plasma. However, introduction of the non-thermal plasma, and increasing the input power to 70 W for the furfural: hexadecane mixture resulted in the highest feedstock conversion of 20.46 wt.%, comprising a gas yield of 10.30 wt.% and an oil yield of 11.32 wt.%. Positive synergy values for oil yield (+2.52%) and feed conversion (+1.96%) at this power level indicate advantageous interactions between furfural and hexadecane under plasma. Higher input power led to a greater yield of gas components, along with the production of single ring aromatic (a positive synergy of 0.49 % at 70 W for toluene) and mono-oxygenated oil compounds, while dual-oxygenated compounds in the oil were reduced (2-furanmethanol, 5-methyl- formation was suppressed in the presence of plasma). There was a positive synergy for most light hydrocarbons increasing from -0.15 % at 0 W to 31.14% at 70 W, with higher input plasma power leading to higher positive synergy percentages. Conversely, the synergistic effect for most heavy hydrocarbons was negative changing from of -4.70 at 0 W to -44.73 at 70 W, suppressing the formation of higher molecular weight oil compounds, which intensified with higher plasma power. Last, the hydrogenation of CO₂ to methane via the Sabatier reaction in the DBD non-thermal plasma/catalytic reactor was studied. A Ni/Al₂O₃ catalyst was used in the plasma/catalysis reactor and the process parameters investigated were the effect of input plasma power, catalyst temperature, catalyst weight hourly space velocity (WHSV), and H₂/CO₂ ratio in relation to the methanation of CO₂. In addition, the effect of the catalyst active metal type (ruthenium, cobalt, and lanthanum) supported on Al₂O₃ under the optimum reaction conditions was investigated. The optimised system, using Ni/ Al2O3, achieved a CO₂ conversion of 82.2 % with an energy efficiency of 22.5 gCO₂kWh-1, CH₄ selectivity of 90.2 % and energy efficiency of 7.4 gCH₄kWh-1 at the plasma input power of 70 W, catalyst temperature of 280 °C, catalyst WHSV of 768 ml/gcath, and H₂/CO₂ ratio of 4. The performance of the active catalyst metals in relation to CO₂ conversion to methane was Ru > Ni > Co > La.