Biofuels are renewable fuels produced from biomass feedstocks such as crops and their waste, grassy and woody biomass, municipal solid waste, and more. As fossil fuel supplies diminish and energy security and climate change concerns rise, research into advanced biofuels is booming. This paper will discuss the status and future of biofuel research based on scientific literature.
First generation biofuels such as corn ethanol and biodiesel from plant oils have received criticism for competing with food production and having relatively low energy returns. They play an important transition role as research moves biofuels to more sustainable feedstocks. Second generation cellulosic biofuels capable of using non-food biomass are the primary focus. Compared to first generation, cellulosic technologies are significantly less developed but have tremendous potential. Cellulosic ethanol could achieve 80% GHG reductions vs gasoline.
A crucial cellulosic challenge is breaking down plant cell walls composed of tough cellulose, hemicellulose, and lignin. Pretreatment is required before enzymatic hydrolysis and fermentation. Popular methods today include dilute acid, hot water, ammonia fiber explosion, lime, and ionic liquids. Combined Severity Factor modeling indicates optimal conditions while minimizing sugar degradation. Research aims to improve yield, selectivity and reduce costs of chemicals and enzymes.
After hydrolysis to monomeric sugars, microbial fermentation converts the sugars into fuels. Yeast, E. coli, thermophiles and Clostridium are common bacterial hosts engineered for improved yields. Research optimizes strains and cultivations to better utilize all sugars including pentoses and ferment lignocellulosic hydrolysates. Additional R&D improves tolerance to inhibitors including furfural, HMF and phenolic compounds that form during pretreatment and hydrolysis.
Gasification is another route applicable to diverse feedstocks, producing syngas that can be fermented or upgraded. Research maximizes carbon conversion efficiency while handling feedstock variability and ash issues. The feasibility of hybrid thermochemical-biological processing using syngas is also being explored. Catalytic upgrading of sugars and alcohols offers potential to diversify fuel options beyond ethanol. Drop-in hydrocarbons compatible with existing infrastructure are promising, though challenges remain around yield, selectivity and costs.
Beyond biochemical and thermochemical pathways, emerging concepts such as phototrophic biofuel production show early promise. Photosynthetic microbes could directly convert carbon dioxide and water into liquid fuels using solar energy more efficiently than plants. Success could enable carbon negative “solar fuels”. Challenges involve engineering robust, highly productive strains. Algae have also attracted attention but outdoor yields have fallen short of early projections. Work seeks to understand and optimize metabolic pathways for improved productivity.
Overall, research focus is shifting to fully realize the potential of cellulosic and advanced biofuels. Multi-scale research encompassing high-throughput screening, metabolic engineering, genomic analysis and synthetic biology aims to reduce costs through improved biomass deconstruction and targeted modifications of microbial hosts. At larger scales, industrial sustainability also requires advanced processes to recover and recycle water, nutrients and other resources. Progress integrating biorefinery systems will be crucial for advancing from pilot to commercial scale production. With continued research advances, advanced biofuels could play a major role in low carbon transportation.