| Methane (CH4) is the main component of natural gas and biogas. Carbon dioxide (CO2) reforming of methane can produce hydrogen-rich synthesis gas, which is the feedstock for the production of chemical products, such as methanol and other liquid fuels. Thus, this reforming reaction plays an important role in adjusting current energy structure and mitigating greenhouse effect. Compared with catalytic process, non-thermal plasma can break through thermodynamic limitation and realize the efficient reforming of CH4at low temperature. However, how to achieve high reactants conversion at lower energy cost and how to overcome the disturbance of coke due to high CH4content in feed gas are two urgent problems to be solved. To resolve the above two problems, CO2reforming of methane and mixed reforming of simulated biogas in thermodynamic equilibrium were investigated firstly, then self-made spark-discharge plasma reactor with a rotary electrode was applied to the experimental study. The main research results are summarized as follows:1. The effect of temperature, pressure and CO2/CH4ratio on CO2reforming of CH4reaction in thermodynamic equilibrium was investigated, respectively. With increasing temperature, the thermodynamic-equilibrium reactant conversions, equilibrium syngas concentration increased, whereas the thermodynamic-equilibrium energy cost for syngas production decreased. Elevating pressure was not favorable for the forward reaction of CO2reforming of CH4in thermodynamic equilibrium. The maximum total-carbon conversion, equilibrium syngas concentration and the minimum energy cost for syngas production were achieved at CO2/CH4=1.The thermodynamic-equilibrium calculation results for mixed reforming of simulated biogas with O2addition showed that, in the temperature range of550-800℃, the equilibrium conversions of CH4and CO2increased and decreased respectively with the increase in O2/(CH4-CO2) ratio. The maximum total-carbon conversion was achieved at O2/(CH4-CO2)=0.5in the temperature range of800-1000℃. And the maximum syngas concentration on wet basis was achieved at O2/(CH4-CO2)=0.5in the temperature range of700-1000℃. The thermodynamic-equilibrium energy cost for syngas production decreased gradually with the increase in CO2/(CH4-CO2) ratio.2. CO2reforming of CH4reaction in spark-discharge plasma was investigated. Syngas was the main gas products of CO2reforming of CH4reaction in spark-discharge plasma. The screening results of electrode size showed that, the electrode of φ2×0.5mm (tube diameter×wall thickness) achieved the highest reactants conversion and energy efficiency as well as the lowest energy cost. Increasing specific energy input (SET) and power supply frequency improved reactant conversion and syngas concentration, but slightly affected the selectivity. With increasing electrode gap from3mm to9mm, reactant conversion and energy efficiency increased significantly, but energy cost decreased; however, the reactant conversion, energy efficiency and energy cost appeared opposite variation trend when the electrode gap further increased to12mm. The selectivity and syngas concentration were weakly affected by variation of electrode gap. With elevating pressure from0.1MPa to0.25MPa at the same SEI, the reactant conversion, syngas concentration and energy efficiency were improved, but energy cost decreased. Compared with other nonthermal discharge techniques, the kilohertz spark discharge exhibits the lowest energy cost and highest energy efficiency at high reactant conversion.The kinetic rate equation for carbon dioxide reforming of methane reaction in spark-discharge plasma was established and derived. The total carbon conversion (XTC), pd value (pressure×electrode gap) and SEI satisfied the formula: XTC/(1-XTC)=kn·(pd)SEI Where k" is the total carbon conversion rate constant. The effects of SEI, pressure and electrode gap on total carbon conversion rate verified the reliability of the kinetic rate equation in the pd value range of0.3-1.0MPa·mm. k"was positively correlated with pow supply frequency. With increasing pow supply frequency from5kHz to40kHz, k" increased from6.12×10-3mol·kJ-1·MPa-1·mm-1to8.02×10-3mol·kJ-1·MPa-1·mm-1.3. The effect of CO2/CH4ratio on CH4and CO2reforming reaction in spark-discharge plasma was investigated. Aiming at simulated biogas reforming reaction, the effects of pressurization on CO2reforming and mixed reforming of simulated biogas with O2addition in spark-discharge plasma were investigated, respectively. With the decrease of CH4content in feed gas, CH4conversion increased monotonically, CO2conversion showed a peak-shape curve. In the same range of conversion, the lowest energy cost and the highest energy efficiency were obtained at various CO2/CH4ratios:the lowest energy cost for converting CH4and the highest energy efficiency was achieved at CO2/CH4=0.5; the lowest energy cost for converting CO2was achieved at CO2/CH4=3; the lowest energy cost for syngas production was achieved at CO2/CH4=1.Elevating the pressure at the same SEI, not only reactant conversions and energy efficiency were improved, but also the energy cost was reduced. In particular, pressurization exhibited a significantly positive effect on increasing CO2conversion and decreasing energy cost for converting CO2. Syngas was the main gases products of simulated biogas reforming reaction, and its concentration increased with the elevation of pressure. As high as83%of syngas concentration was achieved in the wet based product-gas at0.2MPa and SEI=753kJ/mol. At0.2MPa, the effect of SEI was investigated by varying the power and flow rate, respectively. Compared with those at0.1MPa, with the increase in SEI, reactant conversions increased fast, energy cost rose slowly and energy efficiency decreased slowly at0.2MPa.Mixed reforming of simulated biogas with O2addition could effectively inhibit the formation of coke. Furthermore, H2+CO concentration increased and energy cost for syngas production reduced with appropriate O2addition. Compared thermodynamic-equilibrium with experimental results at various O2/(CH4-CO2) ratios, it can be concluded that the optimal O2/(CH4-CO2) ratio for really mixed reforming of simulated biogas in spark-discharge plasma was about0.7. When total-carbon conversion was relatively high (>75%), H2+CO concentration on wet basis was the highest and energy cost for syngas production was the lowest at O2/(CH4-CO2)=0.7, and their experimental results were closest to their thermodynamic-equilibrium values. |
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