Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 148

The bioconversion platform is an industrial option (as might be used in a biorefinery) for producing fuels from biomass using biochemical reactions and/or biochemical agents. For example, fermentation or anaerobic digestion to produce fuels and chemicals from organic sources is a bioconversion platform. The bioconversion platform therefore has the ability to serve as the basis for wood-based biorefining operations, generating value-added bioproducts as well as fuel and energy for the forest sector.

The bioconversion platform typically uses a combination of physical or chemical pretreatment and enzymatic hydrolysis to convert lignocellulose into its component monomers. Once liberated, the carbohydrate components of wood may be processed into a number of chemical and fuel products.

Bioconversion technology is leading the way to new chemical products from the lignocellulose-based biorefinery, including bioethanol, lactic acid and polylactide, propanediol, and succinic acid. Other chemical products can be used to create consumer products such as bioplastics, or as platform chemicals in a number of industrial applications. The development of better ways to separate lignin from the lignocelluloses matrix during bioconversion has created the possibility of developing value-added lignin-based products as well.

The bioconversion platform uses biological agents to carry out a structured deconstruction of lignocellulose components and combines process elements of pretreatment with enzymatic hydrolysis to release carbohydrates and lignin from the wood. The first step is a pretreatment stage which must optimize the biomass feedstock for further processing. In the bioconversion platform, this step must be designed expose cellulose and hemicellulose for subsequent enzymatic hydrolysis, increasing the surface area of the substrate for enzymatic action to take place.

Bioconversion proceeds at lower temperatures and lower reaction rates and can offer high selectivity for products. Bioethanol production is a biochemical conversion technology used to produce energy from biomass. For ethanol production, biochemical conversion researchers have focused on a process model of dilute acid hydrolysis of hemicelluloses followed by enzymatic hydrolysis of cellulose. Biodiesel production is a biochemical conversion technology used to produce energy from oilseed crops.

As in traditional pulping, lignin is either softened or removed, and individual cellulosic fibers are released creating pulp. While bioconversion pretreatment is based on existing pulping processes, however, traditional pulping parameters are defined by resulting paper properties and desired yields, while optimum bioconversion pretreatment is defined by the accessibility of the resulting pulp to enzymatic hydrolysis.

Once pretreated, the cellulose and hemicellulose components of wood can be hydrolyzed. The majority of the commercial hydrolysis programs use enzymes to facilitate fast, efficient, and economic bioconversion of the wood. Enzymatic hydrolysis of lignocellulose materials uses cellulase enzymes most commonly produced by fungi such as Trichoderma, Penicillium, and Aspergillus. A cocktail of cellulase enzymes is required in order to break down the cellulosic microfibril structure into its carbohydrate components in an efficient manner, unlike the bioconversion of starch, which has a simpler chemical structure. The enzymatic hydrolysis step may be completely separate from the other stages of the bioconversion process, or it may be combined with the fermentation of carbohydrate intermediates to end-products.

Separate hydrolysis and fermentation (SHF) offers the platform more flexibility, and makes it easier in theory to alter the process for different end products; however a separate process requires additional engineering and will cost more to build and operate. Simultaneous saccharification and fermentation (SSF) has been found to be highly effective in the production of specific end products, such as bioethanol.

Separation techniques are being developed to isolate the base components of cellulose, hemicellulose, and lignin in order to facilitate industrial processing of these components. Sometimes, the most effective isolation may be carried out by combining correct pretreatments with enzymatic hydrolysis.

The advantage of the bioconversion platform is that it provides a range of intermediate products, including glucose, galactose, mannose, xylose, and arabinose, which can be relatively easily processed into value-added bioproducts. The bioconversion platform also generates a quantity of lignin or lignin components; depending upon the pretreatment, lignin components may be found in the hydrolysate after enzymatic hydrolysis, or in the wash from the pretreatment stage.

Once hydrolyzed, six-carbon sugars can be fermented to ethanol using age-old yeasts and processes. Five-carbon sugars, however, are more difficult to ferment; new yeast strains are being developed that can process these sugars, but issues remain with process efficiency and the length of fermentation. Other types of fermentation, including bacterial fermentation under aerobic and anaerobic conditions, can produce a variety of other products from the sugar stream, including lactic acid.

See also: Biochemical Conversion, Biofuels – Platform, Biorefinery, Thermochemical Platform.