biofuel biomass 12 Technology: general issues 8 biodiesel 8 Research & information: general 7 biorefinery 6 History of engineering & technology 5 fermentation 5 sewage sludge 5 transesterification 5 Saccharomyces cerevisiae 4 furfural 4 hydrodeoxygenation 4 hydrogen 4 lignin 4 pretreatment 4 pyrolysis 4 renewable energy 4 response surface methodology 4 sustainability 4 torrefaction 4 5-hydroxymethylfurfural 3 antioxidant 3 bioenergy 3 bioethanol 3 biofuels 3 biogas 3 catalyst 3 cellulose 3 circular economy 3