In the past few decades, the global population has soared, urbanisation has accelerated, and the demand for energy in all its forms has increased. An increase in environmental pollution is associated with this increase in energy demand and consumption. As the world attempts to transition away from fossil fuel dependence, biodiesel manufacturing presents itself as a feasible alternative.
Elsevier's Handbook of Biofuels Production: Processes and Technologies offers a wide-ranging assessment of the need for biodiesel manufacturers to up their game. It also provides an overview of the trends, processes, and technologies dominating biofuels production, with biodiesel manufacturing being of particular interest to us in this article.
The need for more biodiesel plants in India
Most of the world's current energy needs are met by fossil fuels. In 2019, they satisfied 84% of global primary energy demand. Oil accounted for a third of this fossil fuel-based energy supply, while coal claimed 27% of it. Natural gas claimed 24.2%, hydroelectricity accounted for 6.4%, renewable energy 5%, and nuclear energy 4.3%. According to the Statistical Review of World Energy, 2020, the carbon emissions from energy production have gone up 0.5% for every 1% gain in global economic output since 2010.
While fossil fuels are not becoming more abundant for obvious reasons, global energy demand is charging ahead, with predictions that it will rise by over 50% by 2025. The World Bioenergy Association, 2020, also estimates that consumption will increase by over 90%. People today are more aware of humankind's dependence on fossil fuels, their steady depletion due to expansions in industrial activity and transportation, and their contribution to greenhouse gas (GHG) emissions. Add to this the macroeconomic concerns that emerge with geopolitical tensions, and we have a recipe for fuel price and supply fluctuations and climactic crises.
In such a scenario, the need to develop low-carbon systems like biofuels production, particularly biodiesel manufacturing for the transportation sector, has never been more urgent. There is a special case to be made for biodiesel plants like India. In a country which is home to the largest global population, growing urban centres, and increasing energy demands as the quality of life increases, biomass as a feedstock can be the perfect alternative to fossil oils.
What are biofuels?
Biofuels are liquid or gaseous fuels derived, as the name suggests, mainly from biological sources, referred to as biomass. Biomass can be used for biofuels production of various kinds; physical, chemical, biological, or a combination of these methods can be used for biomethanol, bioethanol, methane, hydrogen, or biodiesel manufacturing. Biomass, too, is of many different kinds - based on the chemical nature and complexity level of the biomass, the resultant biofuels are categorised into different generations.
First-generation biofuels include biodiesel and bioethanol produced from food-based feedstock like wheat, sugarcane, corn, or oil-bearing materials like palm, soybean, and canola.
To produce second-generation biofuels, bioethanol or biodiesel manufacturers use non-edible lignocellulosic materials as substrates. This includes agricultural by-products like cellulosic crop waste, sugarcane bagasse, and non-food or non-crop plants like jatropha, pongamia, and perennial grass. In Southeast Asia, edible palm oil has gained in popularity as a feedstock for biodiesel manufacturers. Meanwhile, the potential of jatropha's non-edible oilseeds is also being increasingly explored in biodiesel manufacturing.
Many first- and second-generation biofuels call for investing in more than just biodiesel plant machinery; vast swathes of land and other agricultural resources must also be dedicated to cultivation. The main challenge this presents is creating competition between food production and biofuels production, with first-gen biofuels directly impacting food prices as they rely on food crops for biodiesel manufacturing.
Enter third-generation biofuels, which use marine microalgae, seaweed, algal biomass, and cyanobacteria as feedstock to produce biodiesel, biogas, and bioethanol. Such feedstocks not only have high growth rates and lipid content, but they also need no agricultural land. They can be easily grown in controlled environments like nutrient-rich ponds or photobioreactors. As the most feasible and sustainable alternative to fossil oils, third-gen biofuels are ideal for future biodiesel plants in India, where land is a scarce resource and food security is a concern.
What is biodiesel?
Each biofuel has distinct physical and chemical properties. Biodiesel is biodegradable, non-explosive, and has a high combustion efficiency, cetane number (an indicator of its combustion speed and compression required for ignition), and flash point. It is used widely as a transportation fuel, either in its pure state or after mixing with gasoline, especially because of its low sulphur and aromatic content. It has the potential to be used to power vehicles, aeroplanes, and motorised engines.
Biodiesel manufacturing: Processes, trends, and technologies
In 2019, the highest biodiesel consumption was reported in the USA (and then Brazil), with the former alone consuming roughly 43 million barrels of biodiesel, according to the US Energy Information Administration. The USA and Brazil are not only the largest consumers but also among the largest biodiesel manufacturers in the world. In Europe, biodiesel manufacturing makes up over 80% of total biofuels production. The preferred feedstock here is rapeseed oil.
Currently, first-generation fuels dominate the market because they have a high conversion efficiency and high energy yield and, therefore, are more profitable. But given the threats of first-gen biodiesel manufacturing to issues of hunger and deforestation, explorations are in progress to develop feasible substitutes. A spotlight is being shone on advanced-generation biofuels, like those produced from low-value waste biomass and non-food materials.
Processes and production technologies for biodiesel manufacturing
Biodiesel largely comprises C14-C20 fatty acid methyl esters (FAMEs). These are obtained by the transesterification of fatty acids and/or glyceryl esters in organic oil-bearing materials like vegetable oils, oilseeds, used cooking oils, and animal fats. Transesterification uses strong sulphuric acid, strong alkalis, solid acids or alkalis, or suitable enzymes as catalysts to accelerate the reaction. Industrial biodiesel manufacturers primarily use alkaline catalysts because of their high conversion efficiency.
Production technologies for biodiesel vary across the manufacturing process. The pre-treatment methods employed depend on the structural properties of the feedstock, with the aim generally being to reduce particle size and remove recalcitrant components to aid conversion efficiency. Oils and lipids from sources like animal fats, oil plants, and algae undergo extraction and refining before the transesterification process is used to procure biodiesel. Lignocellulosic biomass is treated via gasification to become syngas, which is then converted to biodiesel through the Fischer-Tropsch reactions.
Trends in biofuels production
First-gen biofuels can be produced in biodiesel plant machinery from fatty acids, sugars, and starch via basic biochemical processes. But there are concerns about land use, deforestation, food shortages, and food price increases.
Second-gen biofuels are more efficient at reducing GHG emissions. But they have a comparatively low biofuel yield, have to adjust to unstable raw material supplies, and use terrestrial water for biodiesel manufacturing. Moreover, the technologies for their production are still under development to improve their efficiency.
Third-gen biofuels produced using microalgae like the lipid-rich Chlorella, meanwhile, offer the advantages of fast feedstock growth, high photoconversion capacity, batch or continuous cultivation all year round, use of saline water, and GHG reduction by coupling the process with carbon dioxide fixation. However, production technologies so far have low conversion efficiencies, limiting biofuels production from algae at industrial scales. Existing technologies also present barriers in terms of algal species selection, reactor designs, and downstream extractions - advances in these technologies could make third-gen biofuels production more economically viable.
Most recently, a fourth generation of biofuels is emerging. Biofuels production of this generation uses genetically modified algae, which research suggests has higher CO2 capture capacity and biofuel productivity. Finally, using waste materials for biodiesel manufacturing appears to be the most sustainable trend in the long run. This refers not only to using used cooking oil (which is already employed in biodiesel manufacturing) but also obtaining gasoline-range fuels synthesised from waste plastics via thermal catalytic cracking reactions and recycling factory CO2 emissions into gasoline via Fischer-Tropsch reactions.
