Exploring the potential of plant-based oils as sustainable feedstocks for the biofuels industry

By Andrei Mihail

The shift towards sustainable and eco-friendly energy sources has been a major topic of discussion in recent years. The biofuels industry, in particular, has gained momentum due to its potential to reduce dependence on non-renewable energy sources and minimize carbon footprint.

One of the key components of the biofuels production process is the oil feedstock, and an increasing number of experts are exploring the potential of plant-based oils as a sustainable and cost-effective alternative to traditional feedstocks.

We cannot simply throw peanut oil in a truck engine and solve the issue of sustainable transportation.

Using plant oils in compression ignition engines is nearly as old as the engine itself, with Rudolf Diesel, the father of the diesel engine, having used peanut oil as a fuel for demonstration purposes as early as 1900.

However, due to the increased reliance of the modern world on fossil fuels for electricity, heating and transportation, and the ever-growing complexity of technology. We cannot simply throw peanut oil in a truck engine and solve the issue of sustainable transportation.

From the emergence of new technologies and innovative production processes to the role of government policies and consumer demand, this article aims to provide a comprehensive overview of the state of the biofuels industry and the role that plant-based oils play in its growth and success.

• What’s the issue with traditional fuels?
• What are biofuels?
• The role of plant-based oils
• Biofuel constraints
• Continuing biofuel research and development

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What’s the issue with traditional fuels?

The commonly used fossil fuels (e.g. coal, oil, and natural gas) are problematic because they release greenhouse gases, such as carbon dioxide, into the atmosphere when burned, contributing to climate change and global warming. Around two thirds of greenhouse gas emissions arise from the combustion of fuels, according to Climate Watch.

Burning of fossil fuels also releases noxious gases which can greatly affect human health and damage ecosystems. Additionally, these fuels are finite resources that are rapidly being depleted, leading to concerns about energy security and increasing reliance on politically unstable regions for oil supplies.

Extraction and transportation of these fuels can result in environmental damage, oil spills, and other negative impacts on communities and ecosystems. Combined, these factors make it clear that an alternative is needed.

What are biofuels?

Biofuels are renewable energy sources derived from organic matter, such as crops and waste materials, that can be used to power vehicles and generate electricity. Unlike traditional fossil fuels, biofuels are considered more environmentally friendly since they emit lower levels of greenhouse gases, such as carbon dioxide, into the atmosphere.

The production and use of biofuels can still result in net greenhouse gas emissions, depending on various factors such as the type of feedstock used, the production process, and the energy efficiency of the conversion process.

Sustainably produced biofuels are significantly more environmentally friendly than any fossil fuel. Some common types of biofuels include bioethanol, biomethanol, biodiesel, and biojet fuel.

Biofuels are produced through the processing of organic matter, such as crops like corn, sugarcane, and soybeans, or waste products like vegetable oil, animal fats, and municipal solid waste. The organic matter is usually first converted into a sugar, most often through a process called hydrolysis. Then, yeast or bacteria is added to the sugar solution to ferment it into alcohol. Finally, the alcohol is distilled to increase its purity and create biofuel. The most common types of biofuels created in this way is ethanol, which can be used to make biogas. Many alternatives exist, the most prominent being biodiesels.

According to the U.S. Energy Information Administration, biofuels can have lower lifecycle greenhouse gas emissions compared to fossil fuels, reduce dependence on foreign oil, and improve energy security, which is why the U.S. Department of Energy is increasingly aiming to expand this industry.

The European Commission also highlights the potential of biofuels to reduce greenhouse gas emissions, support rural development, and increase the use of renewable energy sources.

The role of plant-based oils

Plant-based oils can be converted into biofuels through several processes. The use of plant-based oils in the production of biofuels also increases the efficiency of the process and produces less waste, making biofuels an even more environmentally friendly and cost-effective source of energy. Some common byproducts, such as glycerol, can also be used in the production of other high-demand products, like soaps and detergents.

The main characteristic of plant oils that makes them an attractive option for biofuel production is their high energy content, despite their high content of chemically bound oxygen (which makes up 10-12% of the compound mass). The heating value of these natural oils is similar to diesel fuels and unlike diesel they are also low in sulfur and nitrogen, making emissions less of a health hazard.

Diesel-like biofuels: biodiesel, biooil and renewable diesel

Biodiesel, biooil, and renewable diesel are three major types of biofuels obtained from plant oils able to imitate traditional diesel fuel. They all use various plant oils as feedstock but are the products of different processes. The main conversion processes are:

• Alcoholysis and transesterification (reaction with alcohol), which produces fatty acid alkyl esters (FAME, or biodiesel)
• Pyrolysis, or cracking, to produce pyrolysis oil (biooil)
• Hydrotreatment to produce alkanes (renewable diesel or green diesel)

Each process, and the fuel it yields, have their own merits and challenges. The final products will have different chemical and physical properties depending on their feedstock, making certain feedstocks better suited for certain processes. 

Alcoholysis and biodiesel

Alcoholysis is a chemical process that involves the reaction of alcohols with oils or fats to form esters. The most common alcohol used in alcoholysis is methanol, and the resulting esters are commonly referred to as methyl esters. Alcoholysis is a simple and straightforward process that can be carried out using a variety of catalysts, including strong acids or alkali catalysts, and is generally performed at a temperature between 50-100°C.

Alcoholysis is a well-studied reaction, widely used method for biodiesel production due to its efficiency, simplicity, and the ability to use a wide range of feedstocks into biodiesel.

Transesterification is a specific type of alcoholysis in which the alcohol used is typically methanol or ethanol and the catalyst is typically sodium hydroxide (NaOH) or potassium hydroxide (KOH). The reaction between the alcohol and the oil results in the formation of methyl esters (in the case of methanol) or ethyl esters (in the case of ethanol). These esters can then be separated from the glycerol and used as biodiesel.

Advantages of biodiesel

Biodiesel can be used as a fuel for diesel engines and can also be blended with traditional diesel fuel to reduce dependence on fossil fuels. Biodiesel has several advantages over traditional diesel fuel, including lower emissions of pollutants such as sulfur oxides and particulate matter, as well as lower greenhouse gas emissions.

According to the U.S. Department of Energy, the use of biodiesel B100 can reduce greenhouse gas emissions by up to 74% compared to petroleum diesel. Moreover, it can be produced from a wide range of feedstocks, including waste cooking oil and agricultural by-products.

Challenges include the fact that biodiesel is primarily made from vegetable oils (over 95% globally), and this has led to controversy as it affects the global food market and food security. Castor oil is considered the best vegetable oil for producing biodiesel, but the end product has high viscosity, which can make it unusable with certain engines. Increased acidity compared to conventional fuels can also lead to engine damage.

Pyrolysis and biooil

Pyrolysis is a chemical process in which organic material is decomposed through heating in the absence of oxygen. It is conducted at high temperatures (450-900°C) and short residence times (seconds to minutes). The result is a complex mixture of various organic compounds, including liquids, gases, and solids. The liquid fraction can be further processed, such as through distillation, to obtain biooil which can be used as a substitute for conventional fossil fuels.

Biooil is the dark brown liquid condensate produced from this process. Historically, pyrolysis oils obtained from various sources were successfully used as fuel in different countries during the First and Second World Wars. The combustion heat of biooil obtained from the pyrolysis of palm oil and rapeseed oil is 38.9 and 38.4 MJ/kg, respectively, which is slightly lower than that of diesel fuel (43 MJ/kg), making an acceptable, but not ideal, fuel.

Biooil can be further processed to result in better properties, a process referred to as upgrading. Vegetable oil pyrolysis also produces useful byproducts such as paraffins and olefins, and the large-scale production of biooil is mainly done with wood and forest residues as feedstock, which means it does not compete with food production.

The main drawback of pyrolyzing plant-based oils is the formation of coke which leads to deactivation of the catalyst, but this effect can be reduced by improving the selectivity of the catalyst, optimizing the operation conditions, and recycling gases. Additionally, the lack of a universal standard for bio-oil and lack of public acceptance due to odor and environmental effects are also significant drawbacks of the process.

Hydrotreatment and renewable diesel

Hydrotreatment is a chemical process that involves the treatment of vegetable oils or other biooils at elevated temperatures (over 300C) and pressure (over 3MPa) with hydrogen gas in the presence of a catalyst. The hydrogen gas reacts with the compounds, breaking down impurities, reducing the viscosity, and improving the fuel’s overall quality. Hydrotreatment is an important step in the production of biofuels, as it helps to remove impurities that can negatively impact the fuel’s performance and stability. The process also reduces the amount of oxygen in the biofuel, which can increase the fuel’s energy density and improve its combustion characteristics.

Renewable diesel, also known as green diesel, is a diesel-like fuel produced from biological sources and is chemically different from biodiesel. Chemically similar to fossil diesel but is made from recently living biomass. It is also known as hydrotreated vegetable oil (HVO). Renewable diesel is the most similar to D2 fuel out of all biofuels, with a high calorific value (45 MJ/kg) and a very high cetane number (above 98), making it a highly efficient fuel. Compared to other biofuels, it has better commercial and practical properties such as excellent storage stability, reduced emissions of NOx, low sulfur content, and good cold flow properties.

Renewable diesel also reduces the acidity, viscosity, and composition of oxygen in the fuel. It does however have a drawback that cannot be ignored: hydrotreating requires hydrogen gas, which is not only expensive but also mainly obtained from unsustainable natural gas.

Biojet fuel

Another type of biofuel produced from plant-based oils is biojet fuel. Flights produced 781 Mt of CO2 in 2015, which amounted to more than 2% of all human-induced CO2 emissions and 12% of CO2 emissions in the transport sector. The aviation industry has set a goal of achieving carbon-neutral growth, and as the industry cannot rely on engine efficiency improvements alone to reach these targets, low-emission biojet fuel is seen as a key part of achieving this goal.

Biojet fuel is made by converting plant-based oils, such as algae oil, into a form that can be used in jet engines. At the moment, the most promising feedstocks seem to be algal oils, as well as non-edible oil crops, lignocellulose biomass (including timber, forestry residues and agricultural residues) and waste animal fats (WAFs) can be converted into jet fuel via various biochemical or thermochemical routes, such as the Fischer-Tropsch process. These feedstocks have the advantage of not competing with food crops and being cheaper.

The Fischer-Tropsch (FT) process is a thermochemical process that involves the conversion of syngas (a mixture of carbon monoxide and hydrogen) into longer-chain hydrocarbons, including jet fuel. In the FT process, syngas is produced from feedstocks, such as algal oils, non-edible oil crops, lignocellulose biomass and waste animal fats, through gasification or biomass pyrolysis. Other routes include transesterification and hydrotreating, which were previously explained. 

Biofuel constraints

As previously implied, one of the main technical issues with biofuels is their high viscosity, caused by the large molar mass and chemical structure of the vegetable oils they are produced from, which can result in viscosities ranging from 10 to 20 times greater than those of conventional diesel fuel (D2). This high viscosity leads to poor combustion and deposit formation in the fuel injector of diesel engines. Castor oil is an exception, with a viscosity that is more than 100 times greater than that of D2.

The other main constraint for biofuel production is cost, as between 70% and 95% of the production cost stems from the cost of feedstock. The use of biofuels made from feedstocks that could be used for food production or produced on arable land has also caused some controversy, as it can negatively affect food security and raise food prices. Availability of low-cost sustainable feedstocks that do not compete with food crops, such as waste cooking oil, is limited, but ongoing research is searching for alternative feedstocks. For example, in Finland, biofuels are being produced from food and forest industrial waste products and one forest company is producing biofuels from crude tall oil (CTO).

Continuing biofuel research and development

While research still needs to be carried out to further improve the efficiency and sustainability of plant based biofuels, their market share is growing, and they are slowly becoming more popular and easily available options.

The use of plant-based oils as a feedstock for the biofuels industry has the potential to reduce dependence on fossil fuels, decrease greenhouse gas emissions, and create jobs in the agricultural and biofuels industries.

It’s important to consider the potential negative impacts on the environment and to find sustainable ways of producing biofuels. This requires collaboration between governments, private sectors, and local communities to ensure the sustainability and scalability of biofuel production.

By continuing to research and develop new types of biofuels, finding ways to make biofuel feedstock procurement more sustainable, and working together to ensure no one is left behind due to this technology, we can create a green, low-carbon future. And while the use of plant oils within the biofuels industry is not a instant solution, and it should be part of a broader strategy that includes other forms of renewable energy, energy efficiency and sustainable transportation, it has the overall potential to significantly contribute to a more sustainable and secure future.