In today’s world, there’s a pronounced emphasis on mitigating the carbon footprints and transitioning towards the sustainable energy sources. By 2022, aviation contributed 2% of global energy-related CO2 emission, experiencing a more rapid growth rate in recent years compared to rail, road, or shipping. Despite a temporary decline due to the COVID-19 pandemic, aviation emissions nearly reached pre-pandemic level in 2022, totalling close to 800 Mt CO2. This surge in emissions reflects the recovery in international travel demand, with projections suggesting a rapid increase in CO2 emission, likely surpassing 2019 level by around 2025 [1].
According to US department of energy, sustainable aviation fuels have the potential for reducing carbon footprint from the aviation transport. Several initiatives have been taken across the world to reduce the aviation emissions such as EU targets to cut greenhouse gas up to 40% of the level from 1990 by 2030. Likewise, under the 2050 low carbon economy, the target reduction is up 60 % in greenhouse gas by 2040 and up to 80% by 2050 including a 60% reduction in transport emissions. This requires to advance and innovate the technology to use sustainable biofuels as a potential candidate to meet the clean and green energy.
Global instabilities stemming from events like wars have led to significant fluctuations in oil prices, impacting both developed and developing economies worldwide. If we consider the context of India, the nation currently satisfies 80% of its oil consumption through imports, has set a goal to reduce this dependence, and wants to increase its share of non-fossil fuel-based energy sources. To achieve this objective, Government of India plans to increase the utilization of biofuels and biogas to meet its energy demands, which are renewable energy sources derived from organic materials. Government of India is targeting 20% blending of ethanol in petrol and 5% biodiesel in diesel by 2030 according to the Indian national policy on biofuels, 2018 and 2022.
Biofuels offer significant environmental advantages over fossil fuels. Unlike fossil fuels, which release CO2 into the atmosphere without recycling, biofuels are potentially carbon neutral as the CO2 emitted during combustion is reabsorbed during the growth of feedstock plants. Biofuels can be categorized into two main groups: 1) first-generation biofuels are produced from edible energy crops like sugar, starch, or oil-based crops through chemical processes or fermentation, and 2) second-generation biofuels are made from non-food crops or by-products considered waste, using biochemical conversion or thermochemical reactions.
The development of various techniques and production processes for converting biomass into bioenergy has progressed rapidly in recent years. There are many ways to process feedstocks and produce biofuels. Processes such as mechanical, chemical, and enzymatic extraction methods produce straight vegetable oil (SVO), while biodiesel is created through transesterification of vegetable oil. Hydrogenated vegetable oil is derived from SVO and animal fats through hydrogenation and isomerization processes. Pyrolysis of biomass yields bio-oil and synthesis gas, which can be used to produce Fischer-Tropsch (FT) fuel. Bioethanol, on the other hand, is produced from biomass via hydrolysis and fermentation. The diverse feedstock and production methods result in significant variations in biofuels’ physical and chemical properties, which subsequently impact combustion quality and performance in gas turbine and internal combustion engines for future use. Therefore, understanding these properties is crucial for ensuring system safety, designing fuel-flexible combustors, and optimizing gas turbine and internal combustion engine performance.
Table 1 Physical properties of various fuels
Despite having many advantages of biofuels, some difficulties are associated with using biofuels in engines, such as high viscosity, which leads to poor atomization with bigger droplets and carbon deposition in the combustion chamber, subsequently reducing combustion efficiency. Also, carbon deposition in the combustion chamber and fuel injection system leads to shorter life span of the engine, which increases maintenance cost. Biofuels such as bioethanol (ethanol) have viscosity comparable to Jet A-1 fuels as given in Table 1. It possesses a lower calorific value, approximately 29 MJ, in contrast to fossil fuels like kerosene (Jet A-1), which have a higher heat of combustion at 45 MJ. Also, we can see ethanol has a lower Cetane number (Cetane number is the measure of the auto-ignition quality of fuel. Fuel that has a higher Cetane number is easy to ignite), which makes it difficult to ignite, especially in extreme conditions. The most significant physical and practical aspects of biofuels are that the physical properties are very close to the jet aviation fuels. These properties of biofuels such as viscosity, volatility, and surface tension of blended fuel are crucial, which influence the important phenomena like droplets formation and evaporation process results in poor combustion. In recent years, plasma discharge shows promising results in enhancing the combustion performance and can help in developing the advance engine technologies burning the biofuels or blended biofuels.
Plasma, often referred to as the fourth state of matter, occurs when atoms and molecules lose their electrons, resulting in a highly energetic, electrically charged mixture of ions and free electrons. Plasma exists in two primary forms: equilibrium and non-equilibrium plasma. Equilibrium plasma, also known as thermal equilibrium plasma, is a state in which the various species within the plasma, including ions, electrons, and neutrals, have reached a state of thermal equilibrium, characterized by approximately equal temperatures. In this state, plasma exhibits a Maxwellian velocity distribution, akin to a gas in thermodynamic equilibrium. Examples of equilibrium plasmas include the core of a star, a candle flame, or a high-temperature laboratory plasma in thermal equilibrium. Conversely, non-equilibrium plasma, often referred to as non-thermal plasma, represents a state where different species within the plasma exhibit significantly different temperatures or energy distributions. In this state, not all particles within the plasma are in thermal equilibrium. Non-equilibrium plasmas are defined by the presence of high-energy electrons alongside cooler ions and neutrals. These high-energy electrons are often generated through external energy sources, such as electric fields, microwave radiation, or laser light.
Plasma can play a pivotal role in enhancing ignition and combustion processes through several pathways. Firstly, plasma can rapidly increase the mixture temperature through energy transfer from electrons to neutral molecules, thereby accelerating chemical reactions, which follow the Arrhenius law. The second enhancement pathway involves kinetic effects, where plasma generates high-energy electrons and ions, as well as electronically and vibrationally excited molecules. This, in turn, leads to the subsequent production of active radicals and reactive species, effectively expediting and diversifying the chain-initiation and branching pathways in combustion. The third enhancement pathway pertains to direct fuel decomposition through electron impact dissociation, whereby large fuel molecules are broken down and reformed into smaller ones. This modification increases fuel reactivity and enhances the diffusivity of the fuel mixture and so on [2].
In summary, the plasma-assisted combustion is a promising technology than can overcome the challenges associated with combustion enhancement with biofuels/sustainable aviation fuels, particularly in applications like aviation and ground transportation industries, and for meeting energy demands while reducing the carbon footprint. Therefore, low temperature plasma has the potential to enhance combustion and offer a viable solution for clean energy and efficient utilization of alternative energy sources.
[1] International Energy Agency
[2]Yiguang Ju, Wenting Sun, “Plasma assisted combustion: Progress, challenges and opportunities”, Combustion and flame 162: 529-532, 2015.
Author – Satender Singh, Pravendra Kumar, Department of Aerospace Engineering, IIT Madras, Chennai