Spotlight on Biofuels
The biorefinery concept
by Melvyn Askew, Central Science Laboratory, UK
Before the widespread adoption of the internal combustion engine at the beginning of the 20th century, and the subsequent growth of the petrochemical industry, agriculture was the primary source of raw materials for energy, food and a wide range of everyday commodities.
Over the last 60 years, in particular during the last two decades, food crops have become agriculture’s main focus. However, as policy priorities look towards sustainable growth using renewable raw materials, and decreasing our dependence on fossil-fuel derived feedstock, a return to a significant wider use of biomass is now possible.
All plants are effectively autonomous chemical mini-factories. The chemicals they manufacture (called metabolites) include sugars and amino acids that are essential for the growth of the plant, as well as more complex compounds. However, for many staple food crops, a potentially large economic resource is effectively being thrown away.
For example, the straw associated with the winter wheat crop in Europe is usually ploughed back into the soil, even though only a small proportion is needed to maintain the level of organic matter. In an average year, the EU-25 harvest of approximately 108 million tones of grain also yields a similar amount of straw: a huge renewable resource that is not being usefully exploited.
Wheat straw contains a range of potentially useful chemicals. These include: cellulose and related compounds (for production of paper or bioethanol); silica compounds (used as filter materials, including those for water purification); and long-chain lipids (fats that could be used in cosmetics or other speciality chemicals).
Biorefining for sustainable products
A key to accessing the integrated production of chemicals, materials and fuels is the development of biorefineries. The biorefinery concept is analogous to that of an oil refinery. These industrial refineries take crude oil and separate (fractionate) all of the potential molecular feedstocks for further processing and blending to produce the vast array of fossil-fuel based products that we use today. Could this approach be used with biomass in a biorefinery?
A biorefinery would integrate a variety of conversion processes to produce multiple product streams such as motor fuels, heat, electricity, and chemicals from biomass. Industrial-scale biorefineries have been identified as the most promising route to the creation of a sustainable bio-based economy. Plant biomass consists of carbohydrates, lignin, proteins and fats, with a variety of minor products such as vitamins, dyes, flavours, etc. Biorefineries combine the essential technologies to transform biological raw materials into a range of industrially useful intermediates.
By producing multiple products, a biorefinery maximizes the value derived from a biomass feedstock. A biorefinery could produce one or more low-volume, high-value chemical products together with a low-value, high-volume liquid transportation fuel, while generating electricity and process heat for its own use and/or export.
There are currently several biorefinery concepts being pursued in research and development. The ‘Whole Crop Biorefinery’ uses raw materials such as cereals or maize. The ‘Green Biorefinery’ can process materials with relatively high water content, such as green grass, clover or immature cereals, while the ‘Lignocellulose Feedstock Biorefinery’ uses ‘nature-dry’ raw material with high cellulose content, such as straw, wood and paper waste.
A variety of techniques (see below) can be employed to obtain different product portfolios of bulk chemicals, fuels and materials. Biotechnology-based conversion processes can be used to ferment the biomass carbohydrate content into sugars that can then be further processed. For instance, the fermentation path to lactic acid shows promise as a route to bio-degradable plastics and has been demonstrated commercially in the USA by Dow Cargill. An alternative is to employ thermochemical conversion processes (see Biofuels article) which use pyrolysis or gasification of biomass to produce a hydrogen-rich synthesis gas. This synthesis gas can then be used in a wide range of chemical processes.
Rapeseed oil is extracted mechanically before the residual meal is further treated and used as animal feed.
|Source: Gueguen, Personal Communication|
Whilst the concept of exploiting the wide range of chemicals from plants may appear novel, the published literature shows that large numbers of metabolites have already been identified and characterised from a wide variety of plant species. For example, from a study of the literature on the main forest trees grown in the UK, over 37 000 different potential and unexploited materials can be identified. These have a wide range of chemical, physical and biological properties and include phenolics, nitrogen containing compounds, and terpenes (terpenoids). The variety of molecular compounds is vast. For example, in the terpene group there are six sub-groupings of molecules with a large number of applications including use in anti-cancer drugs.
Extraction procedures can have a major impact on the availability of these chemicals and, to ensure optimal exploitation, some of the well-established extraction procedures may need to be revised. For example, in winter rapeseed the harvested seed is crushed and rapeseed oil extracted mechanically. The residual meal is then treated with hexane to extract the remaining oil, before being used as feed, primarily for ruminants. Rapeseed oil components have numerous applications including use in bio-diesel; and speciality chemicals.
However, innovative oil-extraction procedures could allow greater exploitation of protein-based metabolites in the rapeseed, which can comprise 25% or more of the rapeseed mass. Research from studies, such as the EC-funded Enhance project, has demonstrated that this separation would allow products to be produced for numerous applications (see diagram) with base cellulose material and some other metabolites remaining in the residual meal.
Biorefining for sustainable growth
Clearly, there is considerable potential to add value to agricultural output as land use changes due to agreed reforms such as the common agricultural policy (CAP) or as a result of other societal drivers like the Kyoto Protocol. It seems likely that in some Member States, land currently used for food production may be freed up for alternative uses. One such alternative might be the planting of grasslands with a range of grass species to yield a specific portfolio of chemicals extracted at a biorefinery. Another example would be at central fruit packing plants where biorefining technologies could take and process waste streams as feedstock, thereby transforming a costly problem into sustainable and, hopefully, profitable products.
A recent vision paper published by the Industrial Biotechnology section of the European Technology Platform for Sustainable Chemistry (http://www.suschem.org) foresees up to 30% of raw materials for the chemicals industry coming from renewable sources by 2025. The change of raw materials and process technologies will also alter our industrial landscape. Oil refineries situated near ports will gradually be replaced by biorefineries located in the countryside.
These factories of the future will integrate agriculture and a transformed chemical industry, converting biomass into a range of value-added products. Combining these with wide-scale energy production will result in better economy for the biorefineries but, which is much more important, also in taking a decisive step into transforming the European energy system into a sustainable one.
RENEWS would like to thank Mr Melvyn Askew for his contribution to this newsletter
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