Reforming Of Simulated Biogas With Added O₂in A Spark-shade Plasma

Biogas is a promising renewable and carbon-neutral energy resource. The efficient utilization of biogas is to convert biogas into syngas (CO+H2) via reforming for synthesizing liquid fuels and chemicals. Conventional catalytic method for reforming is hampered by catalyst deactivation due to carbon deposition and sulfur poisoning. An attractive alternative technique is plasma, which possesses significant advantages including no catalytic deactivation, fast response and high efficiency. Towards industrial application of biogas reforming by plasma, high reactant conversion, low energy cost and large-volume reaction space are required, but the existing plasma reactors can not meet the requirements. In this work, for biogas reforming with added O2to procuce syngas, a novel spark-shade plasma reactor with large-volume reaction space was designed and made for the first time. Using the plasma reactor, and reforming of simulated biogas (60%CH4+40%CO2) with added O2was investigated based on the thermaldynamic-equlibrium caculation. The main results are as follows:l.In chapter2, thermodynamic-equlibruium caculation using HSC7.0software was conducted for investigating the effects of temperature, O2/CH4ratio and O2content in oxygen source on reforming of simulated biogas.Within the investigated ranges of temperature and O2/CH4ratio, thermodynamic-equlibrium conversion of O2was100%. With increasing temperature, thermaldynamic-equlibrium conversions of CH4and CO2increased. With increasing O2/CH4ratio, CH4conversion was promoted but CO2conversion was restrained, and dry-basis concentration of syngas changed little. At temperature>750℃, O2content in oxygen source hardly affected the conversions of CH4and CO2.However, dry-basis concentration of syngas decreased significantly with decreasing O2content.2. In chapter4, reforming of simulated biogas with added O2in the novel spark-shade plasma was studied. The optimal electrode distance was determined at17mm and the effects of specific energy input (SEI), O2/CH4ratio and O2content in oxygen source was investigated. In the range of O2/CH4ratio from0.42to0.67, the conversions and dry-basis concentration of syngas first increased rapidly, and then turned to increase slowly. Moreover, V-shape profile for energy cost of syngas versus SEI was observed. Thereby, the optimal SEI was found at which the lowest energy cost of syngas and a very high dry-basis concentration of syngas were simultaneously achieved. The optimal SEI significantly decreased from128to84kJ/mol with increase O2/CH4ratio from0.42to0.67. Moreover, at their optimal SEI, conversions of O2and CH4and dry-basis concentration of syngas increased from86%,64%and64%to98%,83%and73%, respectively; energy cost of syngas decreased from1.6to1.0eV/molecule, respectively.O2content in oxygen source had a weak effect on the conversions of O2, CH4and CO2and selectivities of products, but had a significant effect on dry-basis concentration of syngas. At O2content>50%, dry-basis concentration and energy cost of syngas increased and decreased rapidly with O2. content, respectively; At O2content<50%, dry-basis concentration and energy cost of syngas increased and decreased slowly with O2content, respectively.It was confirmed that, by the comparion of reforming with added02in the feeding gas with and without CO2and the investigation on reverse water gas shift reaction, the main pathways for CH4and CO2conversion are partial oxidation and reverse water gas shift reaction, respectively.3. In chapter5, discharge characteristics and optical emission spectra of the spark-shade plasma in N2, H2+CO2and CH4+CO2+O2were examined.In the N2plasma, power factor was very low. Photocurrent was close to zero when discharge current was zero. Within the investigated SEI range, vibrational and rotational temperature of N2depended weakly on SEI and maintained at3100K and2200K, respectively.In the H2+CO2(H2/CO2=1and2) plasma, power factor was higher than that of N2plasma. Photocurrent maintained at a background value when discharge current was close to zero. With increasing SEI, the background value increased. Within the investigated SEI range and H2/CO2ratio, rotational temperature of OH and electron density depended weakly on SEI and H2/CO2ratio and maintained at2900K and9.6×1013cm-3, respectively.In the CH4+CO2+O2(CH4/CO2/O2=3/2/2) plasma, power factor was significantly higher than that of N2plasma, which is similar with the H2+CO2plasma. Photocurrent maintained at a background value when discharge current was close to zero. With increasing SEI, the background value increased significantly. With increasing SEI from63to105kJ/mol, rotational temperature of OH decreased from3400K to2900K, and then varied little with SEI. Electron density depended weakly on SEI and was about1.1×1014cm-3.