Refining Plant Biomass: Biofuels, Green Chemistry
10 min read
In the same way that is refined to produce fuels, plastics and numerous chemical products, plant 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 biomass:
- Converting the biomass into sugars, oils or gases.
- Using or thermochemical conversion techniques to process these sugars, oils or gases into intermediate molecules, which may then be used to create marketable products such as fuels (bioethanol, and biojet fuel), lubricants, plastics, rubbers, fertilizers, cosmetics and pharmaceuticals. Useful byproducts include animal feed and protein-rich dietary supplements for human consumption.
The four main conversion pathways – all at different stages of industrial development – concern sugar plants, oilseed crops, lignocellulose and microalgae.
Sugar Plant and Grain Crop Conversion
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 . 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.
Biofuels (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.
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:
- The biochemical pathway "breaks" the lignin to release the cellulose and hemicellulose. Using enzymatic , the cellulose is converted into sugar, or glucose, which is then fermented and distilled to form bioethanol. Research into the use of hemicellulose is currently underway.
- The thermochemical, or biomass-to-liquid (BtL) pathway, is mainly used for wood. Following or torrefaction, the wood is gasified at a high temperature to obtain a synthesis gas (or syngas) composed primarily of and . This gas can then be processed using a variety of techniques – some new, borrowed from biotechnology; others older, such as the invented in 1923 – and converted into several usable compounds (including diesel) and synthetic biojet fuel.
Biofuels produced from lignocellulose are commonly referred to as "second-generation" or "advanced", and are only just in the early stages of development.
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
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.
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