Biofuels Biotechnology: From Feedstocks to Advanced Fuels

Biofuels — liquid, gaseous, or solid fuels derived from biological materials — are the largest source of renewable energy in the transport sector. They are categorized by feedstock and technology generation. First-generation biofuels use food crops (sugarcane, corn, rapeseed, palm oil) and established conversion technologies. Second-generation biofuels use non-food lignocellulosic feedstocks (agricultural residues, forestry residues, dedicated energy crops). Third-generation biofuels use algal biomass. Advanced biofuels also include drop-in hydrocarbon fuels compatible with existing infrastructure.

The sustainability of biofuels depends critically on feedstock source, conversion technology, and land-use change effects. First-generation biofuels have been criticized for competing with food production and for indirect land-use change emissions. Second-generation biofuels address these concerns by using waste and residue feedstocks. Advanced biofuels (drop-in hydrocarbons, sustainable aviation fuel) address compatibility with existing engines and infrastructure. Each generation has technical challenges and research opportunities.

Hrisana Journal welcomes biofuels research across the full spectrum: feedstock characterization and pretreatment, enzymatic hydrolysis and fermentation, thermochemical conversion, algae cultivation and processing, fuel upgrading, process integration, and life-cycle assessment. Our peer-reviewed, open-access format ensures global visibility for your work.

Conventional bioethanol from sugarcane (Brazil) and corn (United States) is the largest biofuel by volume. Sugarcane ethanol, with its favourable energy return on investment (EROI) of 8-10, is among the most sustainable first-generation biofuels. Corn ethanol, with EROI of 1.3-1.6, is more marginal but has driven the development of the corn-to-ethanol industry and its associated infrastructure.

Lignocellulosic ethanol — from corn stover, wheat straw, switchgrass, bagasse, and other fibrous feedstocks — addresses the food-vs-fuel concern but faces the challenge of deconstructing recalcitrant lignocellulose. Pretreatment opens the lignocellulose structure; enzymatic hydrolysis releases fermentable sugars; microbial fermentation converts sugars to ethanol. Each step has been the subject of extensive research, and commercial-scale plants are now operating in several countries.

Key research areas include: development of more efficient and cheaper cellulase cocktails; engineering of yeast strains that ferment both hexose and pentose sugars (Saccharomyces cerevisiae naturally ferments only hexoses, leaving pentoses unutilized); consolidated bioprocessing (CBP) approaches that combine enzyme production, hydrolysis, and fermentation in a single step; and valorization of lignin co-product into chemicals and materials to improve overall process economics.

Biodiesel — fatty acid methyl esters (FAME) produced by transesterification of vegetable oils or animal fats with methanol — is the second-largest biofuel globally. The transesterification reaction can be catalyzed by base (NaOH, KOH), acid (H₂SO₄), or enzymes (lipases), with base catalysis dominating industrially due to its mild conditions and high conversion. Glycerol is produced as a co-product, and its valorization influences process economics.

Renewable diesel (also called hydrotreated vegetable oil, HVO) is produced by catalytic hydroprocessing of vegetable oils or animal fats to produce a hydrocarbon fuel chemically equivalent to petroleum diesel. Unlike biodiesel (FAME), renewable diesel is a drop-in fuel compatible with existing diesel engines and infrastructure at any blend level. It has emerged as a major product, with production capacity growing rapidly.

Feedstock diversification is a major research direction. Used cooking oil, animal fats, and other waste fats and oils offer better sustainability profiles than virgin vegetable oils but are limited in supply. Oilseed crops such as Camelina, pennycress, and Jatropha can be grown on marginal land with low inputs. Algal lipids, while promising, remain expensive to produce at scale. Microbial oils from oleaginous yeasts and bacteria offer a controlled-production alternative but require cheap sugar feedstocks to be economic.

Biogas — produced by anaerobic digestion of organic matter — is typically 55-65% methane, 35-45% CO₂, with trace hydrogen sulfide and other gases. It can be used directly for heat and power, or upgraded to biomethane (>95% methane) for injection into natural gas grids or use as vehicle fuel. Feedstocks include agricultural residues, animal manures, food waste, sewage sludge, and dedicated energy crops (maize, grass silage).

The microbiology of anaerobic digestion involves a syntrophic consortium: hydrolytic and fermentative bacteria break down complex organics to volatile fatty acids, acetate, hydrogen, and CO₂; syntrophic acetogens and hydrogen-producing acetogens further convert VFAs to acetate and hydrogen; methanogenic archaea convert acetate (acetoclastic methanogenesis) or hydrogen plus CO₂ (hydrogenotrophic methanogenesis) to methane. Process stability depends on the balance between acid-forming and methane-forming communities; acidification is the most common process upset.

Research directions include: co-digestion of multiple feedstocks to balance nutrients and improve methane yield; pretreatment of feedstocks (mechanical, thermal, chemical, biological) to improve biodegradability; trace element supplementation to support methanogenic activity; biological biogas upgrading using hydrogen-oxidizing methanogens to convert CO₂ in the biogas to additional methane using externally supplied hydrogen; and integration of biogas production with nutrient recovery from the digestate.

Advanced biofuels include drop-in hydrocarbon fuels, sustainable aviation fuel (SAF), and bio-based oxygenated fuels such as butanol. Drop-in hydrocarbons can be produced by hydrotreating of vegetable oils or animal fats (renewable diesel/jet), by Fischer-Tropsch synthesis from biomass-derived syngas, by alcohol-to-jet conversion (ATJ) of ethanol or butanol, or by direct microbial production of hydrocarbons (e.g., farnesene).

Sustainable aviation fuel has emerged as a critical application because aviation is difficult to electrify and air travel demand continues to grow. SAF can be blended with conventional jet fuel (currently up to 50%, with higher blends under certification) and used in existing aircraft. Production pathways approved under ASTM D7566 include HEFA (hydrotreated esters and fatty acids), FT (Fischer-Tropsch), ATJ (alcohol-to-jet), and SIP (synthesized isoparaffins from fermented sugars). Each pathway has different feedstock requirements, technology maturity, and economics.

Microbial production of advanced biofuels — engineered microorganisms producing higher alcohols (butanol, isobutanol), hydrocarbons (alkenes, alkanes, terpenes), or fatty acid ethyl esters — is an active research area. The attraction is the ability to use cheap sugar feedstocks and to produce fuels with properties matched to specific applications. The challenge is achieving the titres, rates, and yields needed for economic production. Hrisana Journal welcomes submissions across all advanced biofuels research.