Can biofuel really replace fossil fuels?

Bioethanol can be easily generated from sugarcane, beet juice, rice, maize, and other grains. Notably, the use of maize for biofuel has turned India from a surplus producer and exporter to an importer of the feed grain. At the same time, debates are on over allowing imports of genetically modified maize for ethanol production.

But to what extent can bioethanol contribute to the transition to clean energy? Let’s understand.   

Biomass and its role in the carbon cycle

To put things into context, about 250 gigatonnes (Gt) of dry organic material move through the biosphere each year, of which 100 Gt is carbon, which is managed through processes like photosynthesis, respiration, feeding and consumption. Notably, a large part of this carbon is fixed through photosynthesis, which captures 2 × 1021 Joules of energy from sunlight each year across the planet. Humans manage approximately 0.5 per cent of global biomass in the form of food crops. 

Biomass is the predominant source of energy for the largest section of the human population, accounting for more than 10 per cent of total energy consumption. A large portion of this is directly used for heating. Biomass used as fuel can be classified into three major categories: thermochemical, biochemical and agrochemical. Ethanol, which has been steadily growing as a clean, sustainable alternative to fossil fuels, is a biochemical product. 

The appeal of biomass stems from the fact that net carbon fixed by photosynthesis is in a dynamic steady state, meaning carbon absorbed from the atmosphere is roughly balanced by carbon returned through respiration and decomposition. In contrast, fossil fuels release ancient carbon that is not part of this cycle. 

Hence, switching to biofuels involves channeling a portion of this existing carbon flux towards human use without affecting the steady state. In comparison, when fossil carbon is released, it increases radiative forcing and contributes to the rise in the planet’s overall temperature, which might be detrimental to the current diversity of flora and fauna.

Cleaner fuel and food security

Ethanol is produced through microbial metabolism, generally under acidic conditions (around pH 4-5). Microorganisms generate energy primarily through two major respiratory pathways: aerobic respiration, which requires molecular oxygen, and anaerobic respiration, which does not. 

In aerobic respiration, solar energy trapped by photosynthesis is stored in the chemical bonds of sugars, primarily glucose, and is released in a highly controlled, gradual process during respiration in the presence of oxygen, producing CO2 and water. This process releases nearly all the energy stored in glucose, which is used to perform biochemical and mechanical work. 

In contrast, anaerobic respiration is less efficient but faster. It releases only a portion of the total energy and generates byproducts such as lactic acid and ethanol, which contain significant energy. For instance, during sprinting (a short race run at top speed), when oxygen supply is insufficient, human muscle cells switch to anaerobic respiration, leading to the accumulation of lactic acid, which causes muscle fatigue.

Bioethanol can be easily generated from sugarcane, beet juice, corn, rice or other grains. These fuels are known as first generation biofuels. There is ongoing controversy among scientists and economists, particularly those who adopt a normative perspective, who argue that such diversion risks food supply shortages for populations living in poverty, especially in low and middle-income countries. 

Hunger continues to affect a significant portion of the global population, and has a disproportionate impact on women and children. Therefore, second generation biofuel projects aim to convert the large amounts of waste biomass, such as stalks, husks, wood and bagasse, into sources of fermentable sugars. 

However, the major challenge to this approach is the extraction of sugar from these highly complex and extremely stable biofibres. It requires pre-treatments using non-renewable and environmentally taxing chemicals like corrosive acid or alkali. In fact, the highest cost of ethanol production from waste biomass is often the cost of pretreatment only.

Turning plant waste into fuel

Biologically, cellulase – a combination of four enzymes – is widely distributed in the biosphere and can completely break down cellulose into glucose. However, there are three major practical constraints: cellulases have slow reaction rates, temperature instability, and susceptibility to cellulose structural heterogeneity. 

Ethanol fermentation involves microorganisms like fungi (yeast) and bacteria (E. coli), but ethanol produced is itself inhibitory and lethal to these organisms once its aqueous concentration exceeds 10 per cent. Therefore, producing fuel grade ethanol requires additional purification processes. Moreover, different microorganisms metabolise sugars differently, and cultivating a single species leaves many sugars unfermented in the liquor after fermentation. 

To address this, co-fermentation is employed to optimise the conversion of lignocellulosic biomass, contributing to a circular economy, waste valorisation and increased ethanol production. Overall, the production of fuel-grade bioethanol involves six major steps: biomass selection, pretreatment, saccharification, fermentation, distillation and dehydration, and by-product recovery. 

Advancements in biochemical engineering have provided alternatives to the use of live microbial cells. Enzymes are hence immobilised to enable reactions to occur without the need for living cells, using the continuous flow of substrates. These approaches further minimise sugar loss otherwise required to support vegetative growth and maintenance of microorganism biomass.

Ethanol as liquid fuel

Azeotropic ethanol contains approximately 4.4 per cent water, remains in liquid form between -114°C and 78°C, has a flash point of 9°C and self-ignites at around 423°C. Ethanol has all the essential characteristics of a liquid fuel. The energy density of liquid ethanol per unit volume is lower than liquid petroleum (24GJ/m3 vs 39GJ/m3). However, excellent combustion properties of ethanol nearly compensate for its low energy density, allowing comparable distance coverage per unit volume consumed. 

A 5 per cent blend with conventional petrol can be used in standard vehicles without any engine tuning. Additionally, the anti-knock properties of ethanol reduce engine vibration and give a smoother drive. E10 and E15, containing 10 per cent and 15 per cent ethanol respectively, can be used in standard spark-ignition petrol engines with minor tuning but generally require no major modification. 

However, water is not miscible with petrol and typically settles as sludge at the bottom of vehicle fuel tanks. When high ethanol blended fuel is added, water dissolves into the ethanol fraction, rendering the fuel unsuitable for use in unmodified engines.

India, a key player in global ethanol market

The US is the largest producer of ethanol, with more than 16 billion gallons produced in 2024. This is followed by Brazil. While ethanol produced in the US is primarily produced from corn, Brazil mainly produces its ethanol from sugarcane. In 2022, India also reached a record landmark of producing more than one billion gallons of ethanol and is now contributing around 5 per cent to the total global ethanol production. 

Compared to the US, ethanol produced in Brazil has a lower environmental impact due to integrated processing: waste bagasse from sugarcane is used to generate power for distilleries; molasses is used for cattle feed; surplus bagasse is pressed with binder to produce boards and construction material; and boiler ash serves as a source of phosphate. 

Along with the US, Brazil and the European Union, India has set a target of achieving 20 per cent ethanol blending in liquid fuels. The government has permitted sugar mills and distilleries to produce ethanol from sugarcane, as well as from heavy B-molasses and C-molasses. Additionally, the earlier ban on the sale of surplus grain has been lifted, allowing the Food Corporation of India (FCI) to sell up to 2.3 million tonnes of rice for grain-based ethanol production.

Hidden costs of biofuel expansion

Yet, the crucial question remains whether biofuels can truly substitute fossil fuels in terms of net energy consumption and fully abate radiative forcing. Sugarcane-based ethanol results in overall negative greenhouse gas (GHG) emissions, a benefit that most other biofuels unfortunately do not offer. 

Moreover, changes in land use patterns, the application of nitrogenous fertilizers and the decomposition of waste biomass generate nitrous oxide (N₂O) and methane (CH4), both of which have higher radiative forcing than carbon dioxide (CO2). 

The increasing use of biofuel has already led to some troublesome developments, such as proposals to clear large swathes of Amazon rainforest for energy crop cultivation, displacement of indigenous populations, and worsening of erratic weather and climate patterns. These developments bode poorly for the future of the planet. 

Commercial cultivation of energy crops often leads to monoculture practices and rapid local biodiversity loss, which can be extremely difficult to reverse once it occurs. Equally important is the vast amount of water required to cultivate these crops, which rapidly depletes the already scarce freshwater supplies worldwide.  

Post Read Questions

In what ways does ethanol, as a biochemical product of biomass, contribute to reducing dependence on fossil fuels?

How do the three categories of biomass fuels – thermochemical, biochemical, and agrochemical – differ in terms of efficiency and environmental impact?

Bioethanol can be easily generated from sugarcane, beet juice, corn, rice or other grains. However, some scientists and economists argue that such diversion risks food supply shortages. Evaluate. 

Given that biomass accounts for over 10% of global energy use, what implications does this have for energy equity and access in developing countries?

Could an overreliance on biomass energy create new environmental challenges? If so, what might they be?

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

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