System integration of multi-grade exploitation of biogas chemical energy driven by solar energy

Solar-driven biogas reforming is an efficient way to produce hydrogen or other chemical product in the near and medium-term by multi-renewable energy complement. However, direct biogas reforming is prone to coke formation on the surface of the catalyst. Adding water steam to the reaction can ease the problem of carbon deposition; however, deionized water consumption and corresponding mid-temperature thermal energy are consumed sharply and inevitably, and addition of water restricts the conversion of carbon dioxide. To address these irreconcilable problems, the study develops a novel solar-driven two-stage biogas reforming process to produce syngas with a higher carbon monoxide/hydrogen molar ratio, which is also properer for chemical synthesis compared with the conventional single biogas steam reforming process. The carbon dioxide conversion rate can be promoted from 27.32% to 63.72% in the new design at a reaction temperature of 750 °C, and the total heating value of produced syngas in the new design has slight advantages. It is worth noting that the new design still exhibits excellent water and energy saving ability. For the reaction temperature from 600 °C to 750 °C, the safe range for critical biogas split ratio is from 0.58 to 0.68, and the optimal water/heat saving ratio can reach 32–42%. This study offers an integration idea for the future studies concerning the performance improvement of biogas reforming.