Recent Progress in the Steam Reforming of Bio-Oil for Hydrogen Production: A Review of Operating Parameters, Catalytic Systems and Technological Innovations

The present review focuses on the production of renewable hydrogen through the catalytic steam reforming of bio-oil, the liquid product of the fast pyrolysis of biomass. Although in theory the process is capable of producing high yields of hydrogen, in practice, certain technological issues require radical improvements before its commercialization. Herein, we illustrate the fundamental knowledge behind the technology of the steam reforming of bio-oil and critically discuss the major factors influencing the reforming process such as the feedstock composition, the reactor design, the reaction temperature and pressure, the steam to carbon ratio and the hour space velocity. We also emphasize the latest research for the best suited reforming catalysts among the specific groups of noble metal, transition metal, bimetallic and perovskite type catalysts. The effect of the catalyst preparation method and the technological obstacle of catalytic deactivation due to coke deposition, metal sintering, metal oxidation and sulfur poisoning are addressed. Finally, various novel modified steam reforming techniques which are under development are discussed, such as the in-line two-stage pyrolysis and steam reforming, the sorption enhanced steam reforming (SESR) and the chemical looping steam reforming (CLSR). Moreover, we argue that while the majority of research studies examine hydrogen generation using different model compounds, much work must be done to optimally treat the raw or aqueous bio-oil mixtures for efficient practical use. Moreover, further research is also required on the reaction mechanisms and kinetics of the process, as these have not yet been fully understood.

The impact of COVID-19 lockdown on atmospheric CO2 in Xi'an, China

Lockdown measures to control the spread of the novel coronavirus disease (COVID-19) sharply limited energy consumption and carbon emissions. The lockdown effect on carbon emissions has been studied by many researchers using statistical approaches. However, the lockdown effect on atmospheric carbon dioxide (CO₂) on an urban scale remains unclear. Here we present CO₂ concentration and carbon isotopic (δ¹³C) measurements to assess the impact of COVID-19 control measures on atmospheric CO₂ in Xi'an, China. We find that CO₂ concentrations during the lockdown period were 7.5% lower than during the normal period (prior to the Spring Festival, Jan 25 to Feb 4, 2020). The observed CO_2excess (total CO₂ minus background CO₂) during the lockdown period was 52.3% lower than that during the normal period, and 35.7% lower than the estimated CO_2excess with the effect of weather removed. A Keeling plot shows that in contrast CO₂ concentrations and δ¹³C were weakly correlated (R² = 0.18) during the lockdown period, reflecting a change in CO₂ sources imposed by the curtailment of traffic and industrial emissions. Our study also show that the sharp reduction in atmospheric CO₂ during lockdown were short-lived, and returned to normal levels within months after lockdown measures were lifted.