Progress on renewable synthetic fuels

Abstract

Since ancient times, energy has powered the development and transformation of human society. Every great progress of mankind has been accompanied by the improvement of energy utilization efficiency. In 2018, global energy consumption has reached 13.9 billion tons of crude oil equivalent, of which the proportion of fossil fuels is as high as 84.7%. Excessive exploitation of fossil fuels has led to energy shortage. On the other hand, utilization of fossil fuels has caused massive emission of pollutants such as carbon dioxide, SO x , NO x and heavy metals, which have raised serious environmental issues such as global warming, air pollution and seawater acidification. It is an urgent challenge to secure clean alternative fuel sources to ensure sustainable development of human society. In June 2019, the China Association for Science and Technology (CAST) released twenty crucial scientific and engineering problems which are keys to technological and industrial innovation, among which renewable synthetic fuels production is considered as one of crucial problems. It states “use of renewable energy sources such as solar, wind and biomass to convert carbon dioxide to synthetic fuel molecules suitable for efficient and clean combustion, achieving the molecular conversion CO2+H2O→C x H y and producing synthetic fuels such as methane, alcohol-ether fuels, alkane diesel, and aviation fuels”. There are a number of options for converting renewable energy sources to a variety of synthetic fuels; however, the optimal pathway for such a process has not yet to be defined. Synthetic fuels may be derived from direct routes such as electrochemical synthesis and photocatalysis with direct catalytic reduction of carbon dioxide. Alternatively, synthetic fuels can be sourced from indirect routes via biomass accumulation and conversion process. It is well known that photosynthesis, which is a multi-step process combing light and dark reactions catalyzed by enzymes and usually takes place in land-based plants and aquatic algae, offers a green route for carbon dioxide fixation whilst producing oxygen and accumulating organic materials such as glucose polymers in term of biomass. Biomass can be converted to synthetic fuels either via biological fermentation by microbes and enzymes under an ambient temperature and pressure within a few hours to days, or via thermal conversion with or without catalysts under a high temperature within a few seconds to minutes. In the middle of the 19th century, exploring renewable synthetic fuels was established by human beings via anaerobic digestion of biomass to produce biogas (methane-rich gaseous fuel). After over 160 years of development, various routes were proposed to produce multiple types of renewable synthetic fuels. This paper reviews the developments in synthetic fuels production from renewable energy sources, with the main focus on photocatalysis, electrochemical synthesis, thermochemical conversion and microbial conversion pathways. The progress and challenges of use of renewable energy for converting carbon dioxide to various synthetic gaseous and liquid fuels, such as methane, alcohol-ether fuels, alkane diesel, and aviation fuels, are summarized and discussed. The future trends and research directions are also highlighted. This paper may offer some ideas and guidelines for clean and sustainable energy supply in the future.