Encyclopedia of Renewable Energy. James G. Speight
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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, oils, and hydrogen.
Although a number of new bioprocesses have been commercialized, it is clear that economic and technical barriers still exist before the full potential of this area can be realized. One concept gaining considerable momentum is the biorefinery which could significantly reduce production costs of plant-based chemicals and facilitate their substitution into existing markets. This concept is analogous to that of a modern oil refinery in that the biorefinery is a highly integrated complex that will efficiently separate biomass raw materials into individual components and convert these into marketable products such as energy, fuels, and chemicals.
By analogy with crude oil, every element of the plant feedstock will be utilized including the low-value lignin components. However, the different compositional nature of the biomass feedstock, compared to crude oil, will require the application of a wider variety of processing tools in the biorefinery. Processing of the individual components will utilize conventional thermochemical operations and state-of-the-art bioprocessing techniques. The production of biofuels in the biorefinery complex will service existing high-volume markets, providing economy-of-scale benefits and large volumes of by-product streams at minimal cost for upgrading to valuable chemicals. A pertinent example of this is the glycerol by-product produced in biodiesel plants. Glycerol has high functionality and is a potential platform chemical for conversion into a range of higher value chemicals. The high volume product streams in a biorefinery need not necessarily be a fuel but could also be a large-volume chemical intermediate such as ethylene or lactic acid.
A key requirement for delivery of the biorefinery concept is the ability to develop a process technology that can economically access and convert the five and six membered ring sugars present in the cellulose and hemicellulose fractions of the lignocellulosic feedstock. Although engineering technology exists to effectively separate the sugar containing fractions from the lignocellulose, the enzyme technology to economically convert the five ring sugars to useful products requires further development.
The construction of both large biofuel and renewable chemical production facilities coupled with the pace at which bioscience is being both developed and applied demonstrates that the utilization of non-food crops will become more significant in the near term. The biorefinery concept provides a means to significantly reduce production costs such that a substantial substitution of petrochemicals by renewable chemicals becomes possible. However, significant technical challenges remain before the biorefinery concept can be realized.
See also: Bioconversion, Bioconversion Platform, Biomass, Refining, Thermal Conversion, Thermal Conversion Platform.
Bio-SCOT Process
The Bio-SCOT process is a combination of the SCOT and Shell-Paques processes. This process can reduce the sulfur emissions from the sulfur recovery facilities to a low level thanks to the higher efficiency of the scrubber in the Shell-Paques technology.
The process eliminates the need to recycle hydrogen sulfide back to the inlet of the Claus unit. The hydrogen sulfide is converted to solid elemental sulfur in the form of a slurry, which can be melted and mixed with the sulphur from the Claus unit.
See also: Biodesulfurization, SCOT Process, Tail Gas Cleaning.
Bioscrubbing
Bioscrubber systems have been used for hydrogen sulfide removal from gas streams. The bioscrubber involves a two-stage process with an absorption tower and a bioreactor, in which the sulfide is oxidized to sulfur and/or sulfate. For example, the Shell-Paques THIOPAQ® process employs alkaline conditions to produce elemental sulfur. In the first step of this process,