Toluene reforming by carbon dioxide recycled from biomass gasification into syngas production under microwave irradiation

Abstract: The quality of gaseous products can be deteriorated by tar, CO2 and other impurities during the process of biomass gasification. Based on the those impurities analysis, tar conversion by CO2 is performed in this paper. Toluene was chosen as a model compound in this study, and it was used for converting into syngas production over a biomass-derived char. Biomass char is obtained from the pyrolysis of corn straw at a microwave-assisted experimental system. The influences of heating method includes microwave heating (MH) and electrical heating (EH) as well as CO2 flow rate on toluene conversion, syngas yield and carbon loss. The results show that toluene conversion from microwave heating is significantly higher than that from electrical heating under the same circumstances. And it is revealed that a maximum difference of toluene conversion between microwave heating and electrical heating is reached up to 15.58% at CO2 flow rate of 80 mL/min. When a certain amount of CO2 is imported, toluene conversion can be improved effectively. The highest toluene conversion of 93.73% is achieved under microwave heating at CO2 flow rate of 80 mL/min, while toluene conversion under electrical heating is reached a peak of 82.13%, corresponding to CO2 flow rate of 40 mL/min. Moreover, the introduction of CO2 can regulate the conversion of toluene into syngas production with a suitable ratio of H2 and CO. At the same time, an excess of CO2 can result in a loss of carbon contained in biomass-derived char. The carbon consumed through the gasification of CO2 can be converted into part of syngas production, which can impose a direct contribution to total syngas yield. With the increase of CO2 flow rate, a higher syngas yield from carbon consumption is achieved. The maximum contribution of carbon consumption to syngas yield is 15.40% under microwave heating at CO2 flow rate of 120 mL/min. According to the results, it is found that the highest yield of syngas derived from toluene reforming by CO2 under microwave heating is 173.66 mL/min when CO2 flow rate is 80 mL/min. And the yield mentioned above is 5.68 times that of syngas obtained from toluene cracking in the absence of CO2. A decrease in the conversion of toluene cracking is revealed, with the advancement of cracking experiment. And continuous decrease in toluene conversion occurred in cracking experiment until the conversion of toluene cracking drops below 49.0%. Afterwards, a stable phase of toluene conversion is seen in toluene cracking. It is fond that the conversion obtained from toluene reforming is maintained at a higher level, compared to that from toluene cracking. After toluene reforming conducted for 140 min, a decrease in toluene conversion is emerged. At the same time, a gentle decrease in the yield of syngas produced from toluene reforming is observed after 140 min. The conclusions of this study have a significant effect on efficient disposal and utilization of tar from biomass gasification. This research can also provide beneficial reference to the emission of CO2.