Pyrolysis-catalytic upgrading of bio-oil and pyrolysis-catalytic steam reforming of biogas: a review

Fast-rising population and economic growth are inducing a rapid increase in the energy demand. To date, fossil fuels constitute the primary energy source, but their combustion emits greenhouse gases responsible of global warming. Therefore, developing environment-friendly energy sources is critical. For instance, pyrolysis converts biomass into valuable compounds such as syngas, biochar, and bio-oil, yet produced bio-oil and biogas require further upgrading processes before their use. Current processes include gasification, purification, separation, and catalytic pyrolysis. For syngas synthesis, applying one or a couple of these processes leads to low product yields since they favor the production of bio-oil or biogas. Moreover, tar formation during biomass pyrolysis remains a major technological issue in reactor design. Catalytic reforming is an attractive and efficient method for tar removal via tar oxidation into valuable gases. More importantly, catalytic reforming of pyrolysis volatile products constitutes a promising approach for both tar removal and biogas conversion into syngas. Here, we review the two major catalytic approaches of biomass valorization: catalytic pyrolysis and pyrolysis-catalytic reforming. We demonstrate that the catalytic pyrolysis mainly upgrades bio-oil, while pyrolysis-catalytic efficiently converts biomass into syngas. We analyze the effects of temperature and steam-to-carbon ratio on the products derived from coupling the pyrolysis of agricultural biomass waste to catalytic reforming. Increasing the temperature and steam-to-carbon ratio enhances hydrogen yield. A complete conversion of pyrolysis volatile products into syngas was obtained at 600 °C with steam-to-carbon ratio of 4 for pinewood biomass and nickel catalyst. The use of nickel catalyst supported on lanthana–alumina Ni/La 2 O 3 - α Al 2 O 3 for the steam reforming of raw bio-oil produced from pine sawdust pyrolysis enhanced the yields of hydrogen and carbon monoxide, reaching 95% and 93%, respectively, at 700 °C and steam-to-carbon of 15.