Encyclopedia of Renewable Energy. James G. Speight

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СКАЧАТЬ the current context, the other products are biofuels which have the potential to replace certain crude oil-derived fuels. Thus, biorefining offers a key method to accessing the integrated production of chemicals, materials, and fuels. The biorefinery concept is analogous to that of an oil refinery except that biomass (instead of crude oil) is the feedstock to the refinery.

      Biomass refers to (i) energy crops grown specifically to be used as fuel, such as fast-growing trees or switch grass, (ii) agricultural residues and by-products, such as straw, sugarcane fiber, and rice hulls, and (iii) residues from forestry, construction, and other wood-processing industries. These products can range from biomaterials to fuels such as ethanol or important feedstocks for the production of chemicals and other materials.

      Biomass is a renewable energy source, unlike the fossil fuel resources (natural gas, crude oil, and coal) and, like the fossil fuels, biomass is a form of stored solar energy but produced in lesser time than the fossil fuels. Thus, a biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, and chemicals from biomass. The concept is based on the modern crude oil refinery concept, which produces multiple fuels and products from crude oil. Industrial biorefineries have been identified as the most promising route to the creation of a new domestic biobased industry.

      Plants are effective chemical mini-factories or refineries insofar as they produce chemicals by specific pathways. The chemicals they produce are usually essential manufacture (called metabolites) which include sugars and amino acids that are essential for the growth of the plant, as well as more complex compounds.

In a manner similar to the crude oil refinery, a biorefinery would integrate a variety of conversion processes to produce multiple product streams such as motor fuels and other chemicals from biomass (Table B-30). In short, a biorefinery would combine the essential technologies to transform biological raw materials into a range of industrially useful intermediates. However, the type of biorefinery would have to be differentiated by the character of the feedstock.

Table B-30 Simplified representation of a biorefinery.

Feedstock	Process (primary)	Products (intermediates)	Process (final)	Products
Biomass	Gasification	Heat
		Power generation
	Gas cleaning	Hydrogen
	Synthesis gas		Fischer-Tropsch	Fuels
	Pretreatment	Cellulose	Hydrolysis	Sugars
Cellulose*	Hydrolysis	Sugars	Fermentation	Ethanol
				Bioproducts
				Chemicals
*Processes for the conversion of lignocellulose and lignin would also be included.

      For example, the crop biorefinery would use raw materials such as cereals or maize and the lignocellulose biorefinery would use raw material with high cellulose content, such as straw, wood, and paper waste.

      In addition, a variety of methods and techniques 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. As one example, the fermentation path to lactic acid shows promise as a route to bio-degradable plastics. An alternative is to employ thermochemical conversion processes which use pyrolysis or gasification of biomass to produce a hydrogen-rich synthesis gas which can be used in a wide range of chemical processes. Thus, a biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, and chemicals from biomass. The biorefinery concept is analogous to the crude oil refinery, which produce multiple fuels and products from crude oil.

A biorefinery can have different options for the production of biofuels from wood and other biomass materials. There is the (i) bioconversion, (ii) thermal conversion, and (iii) thermochemical conversion. Each of these options has merits, but selection is dependent on the feedstock and the desired product slate.

      By producing multiple products, a biorefinery can take advantage of the differences in biomass components and intermediates and maximize the value derived from the biomass feedstock. A biorefinery might, for example, produce one or several low-volume, but high-value, chemical products and a low-value, but high-volume, liquid transportation fuel, while generating electricity and processing heat for its own use and perhaps enough for the sale of electricity. The high-value products enhance profitability, the high-volume fuel helps meet national energy needs, and the power production reduces costs and avoids greenhouse-gas emissions.

      As a feedstock, biomass can be converted by thermal or biological routes to a wide range of useful forms of energy including process heat, steam, electricity, as well as liquid fuels, chemicals, and synthesis gas. As a raw material, biomass is a nearly universal feedstock due to its versatility, domestic availability, and renewable character. At the same time, it also has its limitations. For example, the energy density of biomass is low compared to that of coal, liquid crude oil, or crude oil-derived fuels. The heat content of biomass, on a dry basis (7,000 to 9,000 Btu/lb) is at best comparable with that of a low-rank coal or lignite, and substantially (50 to 100%) lower than that of anthracite, most bituminous coals, and crude oil. Most biomass, as received, has a high burden of physically adsorbed moisture, up to 50% by weight. Thus, without substantial drying, the energy content of a biomass feed per unit mass is even less.

      These inherent characteristics and limitations of biomass feedstocks have focused the development of efficient methods of chemically transforming and upgrading biomass feedstocks in a refinery. The refinery would be based on two “platforms” to promote different product slates.

      The sugar-base involves the breakdown of biomass into raw component sugars using chemical and biological means. The raw fuels may then be upgraded to produce fuels and chemicals that are interchangeable with existing commodities such as transportation fuels, СКАЧАТЬ