Feature Report: Biofuels

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Refining Plant Biomass: Biofuels and Green Chemistry

In the same way that crude oil is refined to produce fuels, plastics and numerous chemical products, plant biomass can be converted into a wide variety of products ranging from biofuels to high value‑added molecules used in "green chemistry". Extremely active research efforts worldwide are ensuring regular progress.

Ethanol reservoirs at the Arcis-sur-Aube sugar refinery and distillery in eastern France.© AFP

Following suitable processing, various types of plant matter can provide extremely high-value derivative products – or may do so one day. Sources include sugar plants, oilseed crops, grains and starchy roots, as well as wood, farming waste, non-edible plants and seaweed.

There are two main steps to "refining" plant biomassIn the energy sector, biomass is defined as all organic matter of plant or animal origin...:

The four main conversion pathways – all at different stages of industrial development – concern sugar plants, oilseed crops, lignocellulose and microalgae1.

Sugar Plant and Grain Crop Conversion

1.3 metric tons: The amount of biodiesel that can be produced from a hectare of rapeseed.

Sugar plants are primarily either sugar beet or sugar cane, while grain crops, which are rich in starch, include wheat and maize. Conversion of these plants first consists in extracting the sugar using the "soft chemistry route", which does not break the molecules apart. Then, the sugar is fermented using yeast or genetically modified micro-organisms to provide a highly diverse range of molecules that can serve to make derivative products, notably bioethanol. A hectare of wheat or maize can produce 3,000 liters of bioethanol, while the same amount of sugar cane or beet yields around 7,000 liters. Bioethanol can be blended with fuel at varying concentrations. This conversion pathway results in several usable residues, including "bagasse" from sugar cane, which can serve as a fuel source, and "pulp" from sugar beet and "spent grains" from grain crops, which can be used as cattle feed.

Oilseed Crop Conversion

This pathway is used for crops such as rapeseed, sunflower, soybean and oil palm. The oils are first extracted before undergoing a chemical reaction known as esterification. The resulting biodiesel can be blended with regular dieselDiesel is the name of an internal combustion engine that works by compression-ignition.... A marketable byproduct of rapeseed and soybean conversion is the press cake, which can be used as animal feed. One hectare of rapeseed can supply 1.3 metric tons of biodiesel and 2 metric tons of press cakes, while one hectare of sunflower produces 0.7 metric tons of biodiesel and 0.7 metric tons of press cakes. Oil palm produces 5 metric tons of oil per hectare, while also yielding 17.5 metric tons of solid (empty fruit bunches) and liquid (effluents) waste, which is starting to be recycled into fertilizer.

BiofuelsA fuel produced from plant or animal matter. There are currently two types of biofuel... (bioethanol and biodiesel) produced from sugar plants, grains and oilseed crops are known as "first‑generation biofuels" and are only just beginning to be developed on an industrial scale (see Infographic: Conventional Biofuel Production Methods). 

Lignocellulose Conversion

Lignocellulose is composed of lignin, hemicellulose and cellulose. It is found in the cell walls of all plants, including non-edible plants suitable for use as energy crops such as miscanthus (commonly known as elephant grass), switchgrass and short rotation coppice willow and poplar, as well as wood and straw.

There are currently two main conversion pathways, both at the research stage:

Biofuels produced from lignocellulose are commonly referred to as "second-generation" or "advanced", and are only just in the early stages of development2 (see Infographic: Advanced Biofuels ).

It should be noted that some second-generation processes have already been implemented on an industrial scale, such as those that harness used cooking oil and residual oil to be used as feedstocks. However, they currently produce only extremely limited amounts of fuel.

Microalgae Conversion

Plant biomass can be converted into biofuels and numerous products used in "green chemistry".

Some microalgae can be used to produce oils, providing many molecules with applications in the pharmaceutical, cosmetics, food and animal feed industries, as well as in biofuels. However, many years are still needed before they can be mass-produced with sufficient reliability and at an economically viable cost (see Close-Up: "Microalgae Biomass Conversion").