The evolution of the biofuel science

Due to their potential role in addressing the challenges of energy and environment, biofuels have been receiving significant attention from scientists in various disciplines and countries. Over the past few decades, the biofuel science has undergone major changes with regard to its rate of development, focus, and the contributing countries. We put the evolution the biofuel science into context by analyzing 49,000 relevant papers (comprising 0.26% of all publications across physical and life sciences) published between 1990 and 2014. We first determine the scientific contributions of different countries to the science on biofuels, and then analyze how the R & D expenditure and biomass availability is associated with their outputs. Subsequently, the structure of biofuel science in terms of the links between biomass feedstocks and final products are quantitatively discussed. We find that 28%, 16%, and 15% of all papers considered are published by researchers from the EU, China, and USA, respectively. Denmark, Sweden, and Finland have the highest number of biofuel-related publications per capita (i.e. 133, 113, and 103 papers per million capita and annum, respectively). The percentage share of studies on edible, lignocellulosic, and algal biomass are 46%, 40%, and 14%, respectively. Fermentation, hydrolysis, and esterification are the most widely studied conversion pathways, while liquefaction and life cycle assessment are the most cited subfields. Finally, using United States' statistics, it is shown that upon accounting for the time delay between the nominal budget release year and the calendar year in which corresponding papers would ultimately appear in print, the publication trend was closely correlated to the R & D expenditure (r=0.98).