What are the advanced alternatives to ethanol?

Such discussions are largely driven by the need to reduce reliance on fossil fuels. But biofuels are more viewed as one element in a diversified portfolio of renewable energy like solar, wind, geothermal, hydroelectric and tidal energy. 

Moreover, biofuel economics are often in direct conflict with food production and export. For example, Brazil earns more from soybean export than from producing biodiesel from soybean.

Let’s explore the alternatives to ethanol – Butanol and ABE biofuels, Biohydrogen, Biodiesel, Chemically synthesised liquid fuels, and Microalgal biodiesel – and examine both their benefits and challenges. 

Butanol and ABE biofuels

Butanol and acetone-butanol-ethanol (ABE) mixtures as fuel are superior to ethanol. These products are synthesised naturally by solventogenic Clostridia – bacteria capable of fermenting a broad spectrum of cellulosic and hemicellulosic substrates. Among them, Clostridium beijerinckii BA101 has been identified as a particular butanol tolerant, high-yield strain. 

However, ABE downstream processing for product recovery is more complex than a single product like ethanol, as the former involves separating multiple solvents (acetone-butanol-ethanol) while the latter needs water-ethanol separation. Researchers are also working on finding ways to make the ABE product recovery more viable. 

For instance, Studies of innovative feedstock strategies in Japan showed that supplementing municipal wastewater in treatment plants with simple glucose stimulated butanol production and simultaneously reduced sludge volumes destined for landfill. 

In addition, certain bacteria like Vibrio furnessi can accumulate saturated hydrocarbons such as octadecane as a distinct layer via fatty acid reduction, offering a direct microbial route to pump-ready fuels. Another common byproduct of ethanol fermentation and transesterification reaction is glycerol, which can also be used as a carbon source for fermentation and feed for the chemical synthesis of liquid alkanes via syngas through Fisher-Tropsch process at 300-450 °C using a platinum catalyst.

Biohydrogen

The International Energy Agency (IEA) envisions decarbonised transportation in which hydrogen is a key energy carrier. Hydrogen fuel cells can achieve electrical conversion efficiencies of approximately 50 per cent under practical conditions, outperforming most thermal power generation technologies. 

Bio-hydrogen for fuel cells can be produced in heterotrophic acid fermentation, where glucose is metabolised into either acetone-butanol-ethanol (ABE) solvent mixtures or carboxylic acids (e.g., butyric and acetic acids), with a maximum theoretical yield of 4 mol H₂ mol⁻¹ glucose. The key enzyme in this process is hydrogenase. 

Bacterial species like Clostridium are well-known high-yield producers, although Bacillus species have also been used to generate hydrogen from wastewater. The thermophilic bacterium Caldicellulosiruptor saccharolyticus can reach approximately 92 per cent of the theoretical hydrogen yield from glucose, and utilise waste carbon from the pulp and paper industry. In comparison, in vitro systems have higher yields and up to 11 moles of hydrogen can be generated per mole of glucose.

Photosynthetic biohydrogen production

Photosynthetic biohydrogen production is carbon-neutral and requires only chlorophyll, water, CO₂, and light, but the hydrogenases involved are sensitive to oxygen and CO₂. Some cyanobacteria overcome this through spatial segregation – vegetative cells produce oxygen while heterocysts carry out hydrogen evolution – others segregate the processes temporally between light phase during day and dark phase at night. 

Laboratory molecular evolution techniques, like random mutagenesis of hydrogenase, have been performed to render the enzyme insensitive to oxygen or other environmental factors. Alga Chlamydomonas reinhardtii, when grown in light and then shifted to sulphur deficient medium (which inactivates photosystem II) can sustain hydrogen synthesis for 100 hours. 

Thermophilic cyanobacteria are capable of photo-hydrogen production for three weeks in open air in the presence of carbohydrate – the requirement of very less light has hence led to the term photo-fermentation. Photosynthetic bacteria are also capable of water-gas shift reaction, a room temperature analogue of high temperature thermochemical reaction leading to hydrogen generation.

Biodiesel

Biodiesel is obtained through the transesterification reaction of plant and animal fat. These fats are esters of glycerol (headgroup) and fatty acids (which are long hydrocarbon chains) and can be saturated or unsaturated based on the presence of carbon-carbon double bonds in hydrocarbon chains. Multiple hydrocarbon chains are attached to a head group, mostly glycerol in animals, to form triglycerides. 

Transesterification replaces the headgroup with methanol in the presence of a catalyst, typically potassium hydroxide. Novel catalysts like amorphous carbon or SiO2-ZrO2, ion exchange resins are cleaner and recyclable alternatives. Use of high temperature and pressure at super critical conditions can further improve efficiency. 

Feedstocks with high free fatty acid content, such as non-refined oil from Jatropha curcas (15 per cent) cause saponification, leading to poor transesterification reaction. Enzymatic transesterification in the presence of lipases is a completely green solution, but enzymes are deactivated rapidly in methanol and are costly to produce. 

There is debate over whether biodiesel is completely carbon-neutral. The best estimates, however, suggest approximately 55 per cent reduction in CO2 emissions, alongside a reduction in SO2, CO, and particulate emissions. However, biodiesel combustion can cause higher NOx and hydrocarbon emissions. Also, there is a risk of mutagenicity due to soot from biodiesel combustion.

Chemically synthesised liquid fuels

Liquid diesel fuels can be synthesised through pyrolysis, which generates syngas – a mixture of CO and H2. In the presence of transition metal catalysts, this syngas can be converted to liquid fuels.

The use of lignocellulosic biomass for this purpose is highly attractive as it can potentially reduce 90 per cent of fossil fuel emissions, cause low SO2 generation and use of woody material from low-grade land, thereby reducing land-use pressure. 

However, production costs remain higher than fossil fuel, which reduces competitiveness. If these fuels can be produced economically, liquid fuels derived from pyrolysis offer better potential than E85 (a gasoline–ethanol blend) in overall reduction of CO2 and particulate matter.

Microalgal biodiesel

Microalgal biodiesel productivity is estimated to be up to 100 times greater than that of the best terrestrial oilseed crops. Microalgae can be grown in pond systems with CO2 delivered from flue gas of thermal power plants. A rough estimate suggests that microalgae diesel production from only 5 per cent of the land area of the US could meet the world’s requirement of petroleum without encroaching on arable land used for agriculture. 

Heterotrophic cultivation of algae like Chlorella protothecoides and Scenedesmus obliquus can accumulate high amounts of triglycerides using carbon sources like corn powder hydrolysate. The production of triglycerides in the heterotrophic cultivation of algae under dark conditions is equivalent to fermentation. 

Sustainability critique

Beyond cost, a larger issue lies in the sustainability of biofuels themselves. Biofuels can substitute fossil fuels only when two conditions are met: (i) all biological feedstocks are inherently renewable, and (ii) biomass supply is abundant. In practice, neither condition is fully achievable. From a thermodynamic perspective, a resource is sustainable only if it can be maintained indefinitely without loss of quality and without degrading the environment that supports it. 

Agricultural practices to generate biofuel feedstocks often violate both principles. Fertile topsoil takes centuries to regenerate (1 cm in approximately 300–400 years), groundwater recharge is slow, fertilizers are fossil-derived, and mechanised tilling compacts soils, accelerating fertility loss. Cropping also leaves land vulnerable to rapid moisture depletion and salt deposition. These changes are effectively irreversible and cannot be addressed through biotechnology. 

The assumption of biomass sufficiency is equally weak and lacks robust evidence. In reality, biofuels are more viewed as one element in a diversified portfolio of renewable energy like solar, wind, geothermal, hydroelectric and tidal energy. Moreover, biofuel economics are often in direct conflict with food production and export. For example, Brazil earns more from soybean export than from producing biodiesel from soybean.

Land use changes directed to convert natural forest land to cultivate fuel crops cause a spike in CO2 emissions, which exceed CO2 abatement leading to a net carbon debt. Even ethanol produced from corn grown on abandoned cropland has a net carbon debt of 48 years, while biodiesel from soybean cultivated on tropical rain forest land in Brazil has a carbon debt of 319 years.

Policy constraints also reflect sustainability concerns. The Environmental Protection Agency in the US, in its amendment of the Clean Air Act 1990, has put what is called a ‘blend wall’ on ethanol mixing by limiting a maximum increase of Reid Vapour Pressure to 1 pound per square inch due to blending of ethanol with gasoline. Ethanol has higher volatility than gasoline at 37˚C, leading to increased vaporisation that is a precursor to more serious air pollutants like ground ozone. 

In conclusion, compared to even the best biomass to biofuel conversion per unit area of cultivated land, afforestation of an equivalent area of land can sequester at least twice the amount of CO2 over a 30-year period than the emissions avoided by biofuel use.  

Post read questions

Why are butanol and ABE fuels considered superior to ethanol as biofuels in terms of physicochemical properties?

Evaluate the potential of municipal wastewater supplementation for butanol production in terms of both waste management and fuel synthesis.

From a thermodynamic perspective, why are current agricultural biofuel practices unsustainable?

What is the “food versus fuel” conflict? Explain with examples. 

What is meant by “carbon debt” in biofuels, and why can deforestation for biofuel crops result in a net increase in CO₂ emissions?

(Dr. Arunangshu Das is the Principal Project Scientist at the Centre for Atmospheric Sciences, Indian Institute of Technology, Delhi.)

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