Second-generation biofuels, also called advanced biofuels, are fuels made from non-food biomass. Biomass refers to plant materials and animal waste used mainly for energy.

First-generation biofuels are created from sugar and starch sources, such as sugarcane and corn, and from edible oils, like rapeseed and soybean oil. These are usually turned into bioethanol and biodiesel.

Second-generation biofuels use different materials and may need special technology to produce energy. These materials include tough plant matter like wood or crops grown on land not suitable for food, as well as leftover plant waste from farming.

The term "second-generation biofuels" can be confusing because it sometimes refers to both the advanced technology used to make biofuels and the use of non-food materials in standard fuel-making processes. This mix of meanings can cause misunderstandings. It is important to separate the types of materials used from the technology involved.

The development of second-generation biofuels has increased interest due to concerns about using farmland or crops for fuel production instead of food. Debates about whether biofuels affect food prices and availability have been discussed widely in scientific studies.

Introduction

Second-generation biofuel technologies were created to use non-food materials for making biofuels. This is because using food crops for biofuels can cause problems with food supplies and increase food prices. When food crops are used to make biofuels, it may lead to less food being available for people and more competition for land used to grow food.

First-generation bioethanol is made by fermenting sugars from plants, similar to how alcohol is made in beer and wine. This process uses food crops like corn, wheat, sugar cane, and sugar beet. Using these crops for biofuels can raise food prices and cause shortages in some areas. These crops also often need a lot of fertilizers, which can reduce the benefits of lowering greenhouse gases. Biodiesel, which is made from oils like rapeseed or palm oil through a chemical process, is also a first-generation biofuel.

Second-generation biofuels aim to make more biofuel in a sustainable way by using parts of plants that are not eaten, such as stems, leaves, and husks left after food is harvested. These fuels also use non-food plants like switchgrass, jatropha, and miscanthus, as well as waste materials like woodchips and fruit pulp. However, producing second-generation biofuels is challenging because it is often expensive and current technology is not advanced enough to make it widely available.

The main challenge for second-generation biofuels is breaking down tough plant materials, such as stems and stalks, which are mostly made of complex sugars called cellulose and hemicellulose. These sugars are trapped inside plant cell walls by a substance called lignin, which makes them hard to use directly. To make biofuel, these sugars are extracted using enzymes, heat, or other methods. The sugars are then fermented to create ethanol, just like in first-generation bioethanol production. A by-product of this process is lignin, which can be burned to create energy for power plants and nearby homes.

Thermochemical processes, which involve heating materials in water, can turn various plant materials into liquid fuels. These fuels could help replace or supplement other types of fuel. However, they do not meet the standards for diesel or biodiesel. Improving these fuels through physical or chemical treatments might make them better suited for use as energy sources.

Second-generation technology

There are several methods used to create second-generation biofuels. These methods involve heating carbon-based materials at high temperatures either without oxygen (pyrolysis) or with oxygen, air, and/or steam (gasification).

These heat-based processes create a mix of gases, including hydrogen, carbon monoxide, carbon dioxide, methane, and other hydrocarbons, as well as water. Pyrolysis also produces a solid material called char. The gas can be used to make fuels like ethanol, synthetic diesel, synthetic gasoline, or jet fuel through fermentation or chemical processes.

Other methods operate at lower temperatures, between 150–374 °C, and use water to break down biomass into sugars, sometimes with the help of additives.

Gasification is already widely used for materials like coal and crude oil. Second-generation gasification uses forest and agricultural residues, waste wood, energy crops, and black liquor. The main product is syngas, which can be further processed into fuels such as Fischer–Tropsch diesel, biomethanol, BioDME (dimethyl ether), gasoline through chemical conversion, or biomethane (synthetic natural gas). Syngas can also be used for heating or to generate power using gas motors or turbines.

Pyrolysis is a well-known method for breaking down organic materials at high temperatures without oxygen. In second-generation biofuels, it uses forest and agricultural residues, wood waste, and energy crops to produce bio-oil, which can be used as a fuel. However, bio-oil usually needs additional processing to be used in refineries.

Torrefaction is a type of pyrolysis that occurs at temperatures between 200–320 °C. It uses the same feedstocks and produces similar outputs as pyrolysis.

Hydrothermal liquefaction is a process similar to pyrolysis but can handle wet materials. It works at moderate temperatures up to 400 °C and higher pressures. This method is useful for processing a wide range of materials into fuels and chemical feedstocks.

Chemical and biological processes used in other industries are being adapted for second-generation biofuels. Biochemical processes often begin with pretreatment to speed up hydrolysis, which separates lignin, hemicellulose, and cellulose from biomass. After separation, cellulose can be fermented to produce alcohols.

Feedstocks for these processes include energy crops, agricultural and forest residues, food industry waste, municipal biowaste, and other biomass containing sugars. The products are alcohols like ethanol and butanol, as well as other hydrocarbons used for transportation.

Types of biofuel

Second-generation biofuels are being developed, but many are made from middle products like syngas using methods similar to those used for first-generation biofuels. The key difference is the technology used to create these middle products, not the final fuel produced.

A process that turns gas (usually syngas) into liquid fuel is called gas-to-liquid (GtL). If the gas comes from biomass, the process is also called biomass-to-liquids (BTL).

• Biomethanol can be used in methanol engines or mixed with gasoline up to 10–20% without changing existing fuel systems.
• BioDME can be made from biomethanol through a chemical process or directly from syngas. DME can be used in compression ignition engines.
• Bio-derived gasoline can be created from DME using a high-pressure chemical reaction. This gasoline is chemically the same as petroleum gasoline and can be blended into regular gasoline supplies.
• Biohydrogen can be used in fuel cells to generate electricity.
• Mixed alcohols (a mix of ethanol, propanol, butanol, and other alcohols) are made from syngas using different types of catalysts. Some catalysts are similar to those used for methanol. Molybdenum sulfide catalysts, studied by Dow Chemical, have shown promise. Adding cobalt sulfide to these catalysts improves their performance. These catalysts are studied by the U.S. Department of Energy’s Biomass Program. Noble metal catalysts can also produce mixed alcohols. Most research focuses on ethanol, but some fuels, like Ecalene and E4 Envirolene, are sold as mixed alcohols. Mixed alcohols have higher energy content than pure methanol or ethanol. They also improve gasoline and ethanol compatibility, reduce water absorption, and lower evaporative emissions. Higher alcohols have lower heat of vaporization, which helps engines start in cold weather.
• Biomethane (or Bio-SNG) can be made using the Sabatier reaction.

The Fischer–Tropsch (FT) process is a gas-to-liquid (GtL) method. When biomass is the gas source, it is called biomass-to-liquids (BTL). A drawback of this process is the high energy required for FT synthesis, making it currently uneconomical.

• FT diesel can be mixed with fossil diesel in any amount without changing infrastructure. Synthetic kerosene can also be produced.
• Biohydrogen can be made by some organisms that produce hydrogen under specific conditions. It is used in fuel cells to generate electricity.
• Butanol and isobutanol can be made using genetic engineering in organisms like E. coli and yeast. These alcohols can be produced through fermentation using glucose as a carbon and energy source.
• DMF (2,5-Dimethylfuran) can be made from fructose and glucose using a catalytic process. Recent advances have made this fuel more appealing.
• HTU (Hydro Thermal Upgrading) diesel is made from wet biomass. It can be mixed with fossil diesel in any amount without changing infrastructure.
• Wood diesel is a new biofuel developed by the University of Georgia using woodchips. Oil is extracted from the wood and added to standard diesel engines. New plants are grown to replace those used for fuel. Charcoal from the process is returned to the soil as fertilizer. According to Tom Adams, director of the project, this process can make the fuel carbon negative, meaning it removes more carbon dioxide from the air than it emits, which helps reduce the greenhouse effect.

Second Generation Feedstocks

To qualify as a second generation feedstock, a source must not be used for human food. Second-generation biofuel feedstocks include specially grown plants that are not eaten, oils that are not used for food, agricultural and city waste, used oils, and algae. However, crops like cereals and sugar plants are also used in second-generation processing methods. When deciding if biomass is suitable for energy, factors such as land use, existing industries that use biomass, and the technology used to convert biomass into fuel must be considered.

Plants are made of lignin, hemicellulose, and cellulose. Second-generation technology uses one, two, or all of these parts. Common plants used for energy include wheat straw, Arundo donax, Miscanthus spp., poplar and willow grown for short-term harvests. Each type of plant has different benefits, and no single plant is the best or worst choice.

When selecting which plants to grow for energy, their overall effect on the environment must be considered. For example, some grasses used for biofuel can spread uncontrollably in North America, so these should be avoided. Plants that are native or known not to spread uncontrollably are preferred, as native plants often support more ecosystem benefits. Other factors to consider include a life cycle analysis, which compares the total carbon impact of different crops. These analyses can also examine effects on soil and water nearby.

Municipal Solid Waste includes many different materials, and the total amount of waste is growing. In the UK, recycling programs reduce the amount of waste sent to landfills, and recycling rates are increasing each year. However, there are still many opportunities to turn this waste into fuel through methods like gasification or pyrolysis.

Household and business food waste can be turned into bioethanol and biomethane. Bioethanol can be used to power vehicles, while biomethane can replace natural gas, a fossil fuel.

Green waste, such as wood from forests or garden and park waste, can be used to make biofuel through various methods. Examples include biogas produced from biodegradable green waste, and gasification or hydrolysis to create syngas, which can then be processed into biofuels using chemical methods.

Black liquor, the leftover liquid from the kraft process that contains lignin and hemicellulose, can be gasified with very high efficiency to produce syngas. This syngas can then be used to make fuels like biomethanol or BioDME, which reduce greenhouse gas emissions.

The amount of crude tall oil produced during the process ranges from 30 to 50 kilograms per ton of pulp.

Greenhouse gas emissions

Lignocellulosic biofuels lower greenhouse gas emissions by 60–90% compared to fossil petroleum, according to a study by Börjesson.P. et al. (2013, Dagens och framtidens hållbara biodrivmedel). This reduction is similar to the best results from first-generation biofuels, which typically achieve 60–80% emission reductions. In 2010, the average reduction in emissions from biofuels used in the European Union was 60%, as reported by Hamelinck.C. et al. (2013, Renewable energy progress and biofuels sustainability, Report for the European Commission). In 2013, 70% of biofuels used in Sweden reduced emissions by 66% or more, according to a report by Energimyndigheten (2014, Hållbara biodrivmedel och flytande biobränslen 2013).

Commercial development

An operating lignocellulosic ethanol production plant is located in Canada and is managed by Iogen Corporation. The plant, which is a demonstration-scale facility, produces approximately 700,000 liters of bioethanol each year. A commercial plant is currently being built. Many additional lignocellulosic ethanol plants have been proposed in North America and other parts of the world.

In Sweden, the specialty cellulose mill Domsjö Fabriker in Örnsköldsvik is working on a biorefinery that uses Chemrec's black liquor gasification technology. When the biorefinery began operations in 2015, it was expected to produce 140,000 tons of biomethanol or 100,000 tons of BioDME annually. This production would replace 2% of Sweden's diesel fuel imports for transportation. However, in May 2012, it was announced that Domsjö Fabriker withdrew from the project, which halted its development.

In the United Kingdom, companies such as Ineos Bio and British Airways are developing advanced biofuel refineries. These facilities were planned to be completed by 2013 and 2014, respectively. Under favorable economic conditions and improved policy support, NNFCC projections indicate that advanced biofuels could supply up to 4.3% of the UK's transportation fuel by 2020. This would reduce carbon dioxide emissions by 3.2 million tons annually, equivalent to removing nearly one million cars from the road.

On February 1, 2012, in Helsinki, Finland, UPM announced plans to invest in a biorefinery that will produce biofuels from crude tall oil in Lappeenranta, Finland. This industrial-scale project is the first of its kind globally. The biorefinery will produce approximately 100,000 tons of advanced second-generation biodiesel annually for transportation. Construction will begin in the summer of 2012 at UPM's Kaukas mill site and be completed by 2014. UPM's total investment for the project is estimated at approximately EUR 150 million.

On April 30, 2012, in Calgary, Alberta, Iogen Energy Corporation agreed to a new plan with its joint owners, Royal Dutch Shell and Iogen Corporation, to adjust its strategy and activities. While Shell continues to explore methods for producing advanced biofuels on an industrial scale, the company will not proceed with its previously planned project to build a larger cellulosic ethanol facility in southern Manitoba.

In India, Indian Oil Companies have agreed to construct seven second-generation refineries across the country. The participating companies include Indian Oil Corporation (IOCL), HPCL, and BPCL. In May 2018, the Indian government introduced a biofuel policy, allocating INR 5,000 crores to establish second-generation biorefineries. Indian oil marketing companies were also in the process of building 12 refineries, with a total investment of INR 10,000 crores.