The biorefinery concept
3.1. Definition and perspectives
Among the several definition of biorefinery, the most exhaustive was recently performed by the IEA Bioenergy Task 42 “Biorefineries” [25]: “Biorefining is the sustainable processing of biomass into a spectrum of marketable products and energy”.
The biorefinery concept embraces a wide range of technologies able to separate biomass resources (wood, grasses, corn…) into their building blocks (carbohydrates, proteins, triglycerides…) which can be converted to value added products, biofuels and chemicals. A biorefinery is a facility (or network of facilities) that integrates biomass conversion processes and equipment to produce transportation biofuels, power, and chemicals from biomass. This concept is analogous to today’s petroleum refinery, which produces multiple fuels and products from petroleum.
A forward looking approach is the stepwise conversion of large parts of the global economy/industry into a sustainable biobased society having bioenergy, biofuels and biobased products as main pillars and biorefineries as the basis. Such a replacement of oil with biomass will require some breakthrough changes in the today’s production of goods and services: biological and chemical sciences will play a leading role in the generation of future industries and new synergies of biological, physical, chemical and technical sciences must be developed [24].
The efficient production of transportation biofuels is seen as one of the main promoting factors for the future development of biorefineries [26]. In fact, the transportation sector is growing steadily and the demand for renewable (bio-)fuels, which can only be provided from biomass, grows accordingly. As a consequence, the main challenge for biorefinery development seems to be the efficient and cost effective production of transportation biofuels, whereas from the coproduced biomaterials and biochemicals additional economic and environmental benefits can be gained.
The main biobased products are today obtained from conversion of biomass to basic products like starch, oil, and cellulose. In addition, chemicals like lactic acid and amino acids are produced and used in the food industry. Other already commercially available biobased products include adhesives, cleaning compounds, detergents, dielectric fluids, dyes, hydraulic fluids, inks, lubricants, packaging materials, paints and coatings, paper and box board, plastic fillers, polymers, solvents, and sorbents. Some examples of biorefinery and non-conventional biomass industries which are already competitive in the market are listed in Cherubini et al. [27], along with some existing pilot and demo plant.
Most of the existing biofuels and biochemicals are currently produced in single production chains and not within a biorefinery concept, and usually require materials in competition with the food and feed industry. Their exploitation is thereby limited. By contrast, lignocellulosic crops reduce the competition for fertile land, since they can be grown on land which is not suitable for agricultural crops. Moreover, in comparison with conventional crops that can contribute only with a small fraction of the above standing biomass, biorefineries based on lignocellulosic feedstocks can rely on larger biomass per hectare yields, since the whole crop is available as feedstock [24] and [28].
Concerning the conversion plant, consumption of non-renewable energy resources during biorefinery processing should be minimized, along with related environmental impacts, while the complete and efficient biomass use should be maximized. This ecological perspective requires:
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analyses of three important agricultural and forestry cycles, namely carbon (respiration, photosynthesis, and organic matter decomposition), water (precipitation, evaporation, infiltration, and runoff) and nitrogen (N fixation, mineralization, denitrification) and their interdependencies [29],
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system performance evaluations at plant scale [30],
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environmental impact evaluations carried out by means of Life Cycle Assessment [31].
Biorefinery industries are expected to develop as dispersed industrial complexes able to revitalize rural areas. Unlike oil refinery, which almost invariably means very large plants, biorefineries will most probably encompass a whole range of different-sized installations. In this context, several bio-industries can combine their material flows in order to reach a complete utilization of all biomass components: the residue from one bio-industry (e.g. lignin from a lignocellulosic ethanol production plant) becomes an input for other industries, giving rise to integrated bio-industrial systems. In addition, biomass resources are locally available in many countries and their use, may contribute to reduce national dependence on imported fossil fuels.
3.2. Feedstocks
The term “feedstock” refers to raw materials used in biorefinery. The biomass is synthesized via the photosynthetic process that converts atmospheric carbon dioxide and water into sugars. Plants use the sugar to synthesize the complex materials that are generically named biomass. An important stage in biorefinery system is the provision of a renewable, consistent and regular supply of feedstock. Initial processing may be required to increase its energy density to reduce transport, handling and storage costs.
Renewable carbon-based raw materials for biorefinery are provided from four different sectors:
1.
agriculture (dedicated crops and residues),
2.
forestry,
3.
industries (process residues and leftovers) and households (municipal solid waste and wastewaters),
4.
aquaculture (algae and seaweeds).
A further distinction can be done between those feedstocks which come from dedicated crops and residues from agricultural, forestry and industrial activities, which can be available without upstream concerns. The main biomass feedstocks can be grouped in 3 wide categories: carbohydrates and lignin, triglycerides and mixed organic residues.
3.2.1. Carbohydrates and lignin
Carbohydrates (from starch, cellulose and hemicellulose) are molecules of carbon, hydrogen, and oxygen and are by far the most common biomass component found in plant feedstocks. Six-carbon, single-molecule “monosaccharide” sugars (C6H12O6) include glucose, galactose and mannose, while the most common 5-carbon sugars (C5H10O5) are xylose and arabinose. The two most important sugar crops are sugar cane and sugar beet which, together with corn (a starch crop), supply almost all the ethanol that is produced today [32].
Starch (C6H10O5)n is a very large polymer molecule composed of many hundreds or thousands of glucose molecules (polysaccharides), which must be broken down into one or two molecule pieces prior to be fermented. The most widespread starch crops are wheat and corn. Once sugars have been depolymerized (for starch crops) or extracted (for sugar crops) they can be easily fermented to ethanol or used as a substrate for chemical reactions leading to a wide range of chemical products.
Lignocellulosic biomass has three major components: cellulose, hemicellulose and lignin. Cellulose (C6H10O6)n has a strong molecular structure made by long chains of glucose molecules (C6 sugar). The distinction with starch is given by the configuration of the bonds formed across the oxygen molecule that joins two hexose units. Starch can be readily hydrolyzed by enzymes or acid attack to the single sugar monomers, while cellulose (30–50% of total lignocellulosic dry matter) is much more difficult to hydrolyze and set free individual glucose monomers. Hemicellulose (C5H8O5)n is a relatively amorphous component that is easier to break down with chemicals and/or heat than cellulose; it contains a mix of C6 and C5 sugars. It is the second main component of lignocellulosic biomass (20–40% of total feedstock dry matter). Lignin (C9H10O2(OCH3)n), is essentially the glue that provides the overall rigidity to the structure of plants and trees and is made of phenolic polymers. While cellulose and hemicellulose are polysaccharides that can be hydrolyzed to sugars and then fermented to ethanol, lignin cannot be used in fermentation processes, but it may be useful for other purposes (chemical extraction or energy generation). Lignin (15–25% of total feedstock dry matter) is the largest non-carbohydrate fraction of lignocellulose.
Lignocellulosic biomass can be provided either as a crop or as a residue. Large amounts of cellulosic biomass can be produced via dedicated crops like perennial herbaceous plant species, or short rotation woody crops. Other sources of lignocellulosic biomass are waste and residues, like straw from agriculture, wood waste from the pulp and paper industry and forestry residues. The use of waste biomass offers a way of creating value for society, displacing fossil fuels with material that typically would decompose, with no additional land use for its production [33].
3.2.2. Triglycerides
Oils and fats are triglycerides which typically consist of glycerin and saturated and unsaturated fatty acids (their chain length ranges between C8 and C20, but 16, 18 and 20 carbons are the most common). The sources of oils and fats are a variety of vegetable and animal raw materials. Soybean, palm, rapeseed and sunflower oil are the most important in terms of worldwide production [34] and [35]. Vegetable oils are nowadays used for production of biodiesel by reacting with an alcohol, usually methanol. However, they can also be used as a substrate for chemical reactions thanks to two chemically reactive sites: the double bond in the unsaturated fatty acid chain and the acid group of the fatty acid chain [36]. Like sugar and starch crops, oilseed crops are characterized by low yield and high use of inputs. In the future, non-edible crops like Jatropha curcas and Pongamia pinnata, which require lower inputs and are suited to marginal lands, may become the most