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Title: Plasma-catalytic conversion of greenhouse gas into value-added fules and chemicals
Author: Mei, D.
ISNI:       0000 0004 6059 0831
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2016
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The huge demand for energy sources in the human race's development has resulted in a great energy challenge and climate change. The latter issue is mainly induced by greenhouse gas emissions (such as CO2) from the burning of fossil fuels - the world's primary energy sources. Investigation and development on the utilisation of CO2 (rather than considering it as a waste) are of significant importance not only to reduce the emission of greenhouse gases, but also to provide a new approach for the use of the derived carbon fuels in an environmental friendly and carbon neutral way. This study is performed in dielectric barrier discharge (DBD) reactors to gain a better understanding on the plasma processing of CO2, so as to help in the designing and optimisation of the plasma-catalytic system for CO2 utilisation. In the plasma-assisted decomposition of CO2 without catalyst, the effects of different processing parameters, including frequency, discharge power, feed flow rate, discharge length, discharge gap and dielectric thickness have been taken into consideration. Empirical expressions are obtained to relate the reaction performance (CO2 conversion and energy efficiency) to these different processing parameters. Through the sensitivity analysis, frequency is found to have negligible influence on both CO2 conversion and energy efficiency in our experimental range; while discharge gap and discharge power are the most important factors affecting CO2 conversion and energy efficiency of the process, respectively, compared with other processing parameters. Modified DBD reactors are proposed by using a screw-type inner electrode and/or an Al foil outer electrode to improve the CO2 decomposition performance. In the modified DBD reactor with the screw-type inner electrode, the distortion of the local electric field near the electrode surface intensifies the filamentary discharge and generates more energetic electrons and reactive species, thereby enhancing the conversion of CO2 with high energy efficiency. In the plasma-catalytic decomposition of CO2, the combination of plasma with BaTiO3 and TiO2 photocatalysts in the CO2 DBD slightly increases the gas temperature of the plasma by 6-11 oC compared to the CO2 discharge in the absence of a catalyst at a specific energy density (SED) of 28 kJ/l. The synergistic effect from the combination of plasma and photocatalysts (BaTiO3 and TiO2) at low temperatures contributes to a significant enhancement of both CO2 conversion and energy efficiency by up to 250%. The UV intensity generated by the CO2 discharge is significantly lower than that emitted from UV lamps used to activate photocatalysts in conventional photocatalytic reactions, which suggests that the UV emissions generated by the CO2 DBD only play a very minor role in the activation of the BaTiO3 and TiO2 catalysts in the plasma-photocatalytic conversion of CO2. The synergy of plasma-catalysis for CO2 conversion can be mainly attributed to the physical effect induced by the presence of catalyst pellets in the discharge and the dominant photocatalytic surface reaction driven by the plasma. In the packed-bed DBD reactor for CO2 conversion, both the physical and chemical effects on reaction performance have been investigated for the addition of BaTiO3 and glass beads into the discharge gap. The presence of these packing materials in the DBD reactor changes the physical characteristics of the discharge and leads to a shift of the discharge mode from a typical filamentary discharge with no packing, to a combination of filamentary discharge and surface discharge with packing. Highest CO2 conversion and energy efficiency are achieved when the BaTiO3 beads are fully packed into the discharge gap. It is found that adding the BaTiO3 beads into the plasma system enhances the average electric field and mean electron energy of the CO2 discharge by 86.9% and 75.0%, respectively, which significantly contributes to the enhancement of CO2 conversion, CO yield and energy efficiency of the plasma process. In addition, highly energetic electrons (> 3.0 eV) generated by the discharge could activate the BaTiO3 photocatalyst to form electronhole pairs on its surface, which contributes to the enhanced conversion of CO2. In the plasma-catalytic dry reforming of CH4, the effect of catalyst support on the performance of the plasma-catalytic reaction over the supported Ni catalysts is firstly investigated. It is found that due to the higher specific surface area and larger amount of basic sites, Ni/?-Al2O3 shows the higher conversion of reactants, the higher yield and selectivity of desired products and the higher carbon resistance compared with other catalysts (Ni/MgO, Ni/SiO2 and Ni/TiO2). Based on the Ni/?Al2O3 catalyst, the influence of the processing parameters (discharge power, total feed flow rate, CO2/CH4 molar ratio and Ni loading) and their interactions on the performance of the plasma-catalytic dry reforming reaction is evaluated using design of experiments (DoE). Quadratic polynomial regression models are established to reflect the relationships between these plasma processing parameters (different factors) and the performance of dry reforming process (different responses), in terms of the conversion of CO2 and CH4, the yield of CO and H2 as well as the fuel production efficiency (FPE) of the plasma process. The results indicate that the total feed flow rate is the most important factor affecting the conversion of CO2 and CH4 and the yield of CO and H2, while CO2/CH4 molar ratio has the most significant impact on FPE of the process. The interaction between discharge power and total feed flow rate plays a significant role in all the responses of the plasma-catalytic dry reforming process. The optimal process performance - CO2 conversion (31.7%), CH4 conversion (48.1%), CO yield (21.7%), H2 yield (17.9%) and FPE (7.9%) is achieved at a discharge power of 60.0 W, a total feed flow rate of 56.1 ml/min, a CO2/CH4 molar ratio of 1.03 and a Ni loading of 9.5%, as the highest global desirability of 0.854 is obtained at these conditions. The reproducibility of the experimental results successfully demonstrates the feasibility and reliability of the DoE approach for the optimisation of the plasma CO2 conversion process. Ni-based bimetallic catalysts have been designed and developed to further enhance the catalyst performance for plasma-catalytic dry reforming of CH4. The 10wt.%Ni+3wt.%Co/?-Al2O3 (10Ni3Co) catalyst shows the highest plasma-catalytic activity compared with other bimetallic catalysts (10wt.%Ni+3wt.%Cu/?-Al2O3 (10Ni3Cu) and 10wt.%Ni+3wt.%Mn/?-Al2O3 (10Ni3Mn)). This can be ascribed to the high specific surface area and larger amount of strong basic sites resulting from the interaction between Ni and Co in the Ni-Co/?-Al2O3 catalyst. It is suggested that the formation of Ni-Co alloy in the Ni-Co/?-Al2O3 catalysts contributes to the enhancement in the plasma-catalytic reforming performance when the Ni-Co/?-Al2O3 catalysts are combined with plasma. The specific surface area of the catalyst is decreased but the amount of strong basic sites on the catalysts is increased by increasing the Co loading in Ni-Co/?-Al2O3 catalyst. The compromise between the catalyst structure and the amount of basic sites on the catalyst favours the maximum enhancement in the performance of the plasma-catalytic dry reforming reaction when the 10Ni5Co catalyst is integrated with the plasma system. The maximum CH4 conversion of 50.7% and the maximum CO2 conversion of 30.9% are achieved for the plasma-catalytic dry reforming over the 10wt.%Ni+5wt.%Co/?-Al2O3 (10Ni5Co) catalyst at a discharge power of 50 W and a total gas flow rate of 50 ml/min. Moreover, the 10Ni5Co catalyst possesses the highest carbon resistance in the plasma-catalytic reforming process. It is worthy to note that the carbon deposition on the catalyst in our plasma-catalytic dry reforming reaction is significantly lower than that in the conventional thermal catalytic dry reforming of CH4 using similar NiCo/?-Al2O3 catalysts at high temperatures. In addition, the maximum FPE of 12.7% is obtained in the plasma-catalytic dry reforming of CH4 in this study, which is higher than most of the previous results obtained in the atmospheric non-thermal plasma reactors. The high reaction rate and fast attainment of steady state in plasma processes allow rapid start-up and shutdown of the process compared to thermal treatment, whilst plasma systems can also work efficiently with a rather small and compact size. This offers flexibility for plasma-catalytic processes to be integrated with renewable energy sources (such as waste energy from wind power and solar energy), and provide a promising approach to store and transport the surplus energy in a chemical form.
Supervisor: Tu, X. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral